ADHESIVE BONDINGOF WOOD

Technical Bulletin No. 1512

August 1975

U.S. Department of Agriculture Forest Service

ADHESIVE BONDINGOF WOOD

ByM. L. Selbo, retired, formerly Chemical Engineer,

Forest Products Laboratory-Forest ServiceU.S. Department of Agriculture

Technical Bulletin No. 1512

Washington, D. C. August 1975

Selbo, M. L.1975. Adhesive bonding of wood. U.S. Dep. Agr., Tech. Bull. No. 1512,

p. 124.

Summarizes current information on bonding wood into dependable, long-lasting products. Characteristics of wood that affect gluing are detailed, as well astypes of adhesives and processes to be used for various conditions.

KEY WORDS; Bonding wood; adhesives; glues; glue types; glued products;gluing techniques; glulam.

Oxford No. 824.8

For sale by the Superintendent of Documents, U.S. Government Printing Office,Washington, D.C., 20402. Price $1.55. Stock No. 001-000-03382

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FOREWORD

Mote than four decades ago Thomas R. Truax wrote USDA Bulletin No. 1500,“Gluing of Wood.” In this bulletin, Truax laid down sound principles that have stoodthe critical tests of time.

But adhesive technology has expanded enormously and there are many buildingblocks to be added to the solid foundation Truax laid down in the 1920’s.

When Truax’ bulletin was published, synthetic adhesives had not been introducedand practically all wood gluing was done with glues formulated or compounded fromnaturally occurring materials. Some of these glues (based on casein, blood, starch, andanimal extracts) are still being used, but in quantities far overshadowed by syntheticssuch as phenol-,, resorcinol-, urea-, and melamine-formaldehyde resins, as well as vinylresins of various types.

Furniture was the major glued wood product when Truax wrote his technical bulletin;softwood plywood, suitable only for interior use, was in its infancy. Currently, gluingis involved in practically all branches of the wood-using industry. In housing, gluingis employed extensively, particularly in prefabrication, but also on the building site;plywood is mass produced in more than half of the States of the Union. Structurallaminated timbers ate produced for spans well over 300 feet and for structures as di-vergent as churches and minesweepers.

The technology of adhesives and gluing has come a long way. With some syntheticresins, joints can be produced that withstand the ravages of the elements folly as wellas wood itself.

ACKNOWLEDGMENT

The author appreciates the cooperation of gluing firms, associations, and equipment suppliersin providing photographs and granting permission to Publish them. Photographs were Providedby Rilco Laminated Products; Industrial Woodworking Machine Company; American PlywoodAssociation; Ashdee Division, George Koch Sons, Inc.; Carter Products Company, Inc.; Chas.Smith Enterprises, Inc.; Newman Machine Company; James L. Taylor Manufacturing Company;Evans Division, Royal Industries; Globe Machine Manufacturing Company, Inc.; Black Brothers;and Töreboda Limträ (Sweden).

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Use of trade, firm, or corporation names in this publication is for the information and convenienceof the reader. Such use does not constitute an official endorsement or approval of any product orservice by the U.S. Department of Agriculture to the exclusion of others that may be suitable.

Mention of a chemical in this publication does not constitute a recommendation;only those chemicals registered by the U.S. Environmental Protection Agency maybe recommended, and then only for uses as prescribed in the registration-and inthe manner and at the concentration prescribed. The list of registered chemicalsvaries from time to time; prospective users, therefore, should get current informationon registration status from Pesticides Regulation Division, Environmental Protec-tion Agency, Washington, D.C. 20460.

Requests for copies of illustrations contained in this publication should be directed to the ForestProducts Laboratory, USDA Forest Service, P.O. Box 5130, Madison, Wis. 53705.

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CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Wood Properties Important in Adhesive Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Shrinking and Swelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Adhesives for Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Synthetic Adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Adhesives of Natural Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Improving Performance of Wood Through Gluing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Crossbanded Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Laminated Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .End and Corner Joint Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Preparing Wood for Gluing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Moisture Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Drying and Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Storage Before Gluing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Surfacing Wood For Gluing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Machining Special Types of Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Cutting and Preparing Veneer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Adhesives and Bonding Processes for Various products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Plywood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Laminated Timbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Furniture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Ship and Boat Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Doors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Sporting Goods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Particleboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Housing and Housing Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .New Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Gluing Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mixing Adhesive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Spreading Adhesive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Assembling Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Assembly Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Pressing or Clamping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Curing Adhesive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Conditioning Glued Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Adjustments in Adhesives and Gluing procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Gluing Treated Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Wood Treated With Oil-Soluble Preservatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Wood Treated With Waterborne Preservatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Wood Treated With Fire-Retardant Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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101101101102109120

ADHESIVE BONDINGOF WOOD

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Figure 1 .–These glued-laminated beams stretching skyward are but one str ikingexample of today’s profitable partnership between wood and adhesives. Soonthese beams will hold up the roof of a sports arena.

INTRODUCTION

Bonding of wood with glue is known to common (fig. 1)— but the gluing processdate back to the Pharoahs and in all likeli- has never become static.hood the first use of glue with wood was This publication brings together cur-much further back in antiquity. Since rent information on use of adhesives forthen, glued wood products have become bonding wood, so it can serve as a guide

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in production of more dependable gluedproducts. The more important types areemphasized because new synthetics areappearing almost daily and to discuss allsynthetic and “natural” adhesives wouldbe an impractical and almost impossibletask.

The information presented here is basedon research carried out at the Forest Prod-ucts Laboratory and elsewhere, as well ason the author’s experience both in researchand production gluing. A list of selectedreferences follows each major section.

Factors that affect the adequacy of theglue bonds are emphasized, rather thantheories of adhesive bonding which, un-fortunately, still remain in a somewhatnebuous state. Even the world-famousscientist Debye1 steps lightly when ap-proaching the subject of adhesion: “Theforces between two molecules are supposedto consist of a universal attraction, whichincreases with diminishing distance untilthe two molecules touch.”

Blomquist1 states that “. . . the actualadhesion is more probably due to chemicalor physical forces . . .” and “Adhesion isassumed first to require actual wetting ofthe adherend by the adhesive. . . .”

There seems to be general agreementthat a prime prerequisite for good bondingis that the adhesive must wet the surfacesto be joined. A related example is thatwater generally wets clean, freshly ma-chined wood surfaces and also forms strong

M 136 540

Figure 2 .–Gluing variables require morecontrol in plant production than mightbe indicated from laboratory experi-ments.

bonds between them when cooled to freez-ing temperatures. So, an adhesive appar-ently must “wet” wood surfaces and sub-sequently solidify to make a strong bondedjoint.

Putting a drop of water on wood andobserving the rate at which it is absorbedhas been proposed as a test for gluability.This theory holds in most cases; however,there are exceptions such as southern pinetreated with creosote to an 8 pounds percubic food retention. Actually the pinewas glued adequately to serve more than25 years in bridge stringers, yet the oilycreosote certainly would have made thewater absorption test misleading.

One of the more successful attempts toexplain adhesive bonding of wood wasmade in 1929 when Truax,1 Browne, andBrouse discussed the theories of mechanicaland specific adhesion. Further theoreticalclarification undoubtedly will evolve. Butin the meantime, some practical engineer-ing principles must be applied to assuredependability in glue joints.

It is well known that numerous factors(such as pressure, temperature, and assem-bly time) play an important part at sometime during the formation of a glue bond.If these factors are controlled within areasonable range about the optimum foreach, high-quality glue bonds will result.But if borderline conditions are used forone or more of these factors-in otherwords, if no substantial factors of safety areemployed-then the end results can becatastrophic. Also, since the interactionsbetween the various factors are often ill-defined, aiming toward optimum condi-tions is the safest practice.

In figure 2, good results are indicatedby the flat (horizontal) portion of the curveand decreasing joint quality by the down-ward sloping part at left. Under laboratoryconditions good results can consistently beobtained even when operating near thebreaking point of the curve. In plant pro-duction, the control of the factors is usu-ally less exact and variable results may

1See reference on Page 3.

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occur (indicated as out of control on thefigure), unless greater margins of safety areallowed.

In bonding wood with adhesives onemust be aware that wood is not a uniformsubstance, but a complex material thatvaries significantly in many properties—density, for instance, which may rangefrom lower than 0.30 to higher than 0.80—and it would be mere chance if the samebonding material and procedure would besuitable for the entire range of woodspecies.

Adhesives available today cover a widearea in properties and performance charac-teristics, and the producer of glued woodproducts must be keenly aware of thesefacts when switching from one woodspecies to another, from one adhesive toanother, and from one product to another.

Use of adhesives for bonding wood hasincreased enormously over the past decadesand glued products vary in size from tinywood jewelry to giant laminated timbersspanning hundreds of feet. No single ad-hesive has ever been formulated, and prob-ably none ever will be, that will meet thevarious requirements of all the innumer-able applications of adhesive bonding. It istherefore important that the user has theproper background information to chooseand evaluate the adhesive best suited for aparticular application.

In general, the serviceability of a gluedwood assembly depends upon (1) the kindof wood and its preparation for use, (2) thetype and quality of the adhesive, (3) com-patibility of the gluing process with thewood and adhesive used, (4) type of jointor assembly, and (5) moisture-excludingeffectiveness of the finish or protectivetreatment applied to the glued product.In addition, conditions in use naturallyaffect the performance of a glue bond. Foradequate performance, a glue joint shouldremain as strong as the wood under theservice conditions to which the glued prod-uct is exposed. If it does not, it becomesthe weakest link in the assembly and thepoint at which failure first will occur.

During the more than 30 years theauthor has been involved in wood gluing—in plywood production, industry adhesiveresearch, and Government research onadhesives and glued products-greatchanges and progress have occurred in thisfield. The plywood industry, by far thelargest user of wood adhesives, has grownto become an extremely important factorin the construction field. The structurallaminating industry has also shown ahealthy growth as improved glues and de-sign information have become available.Adhesives for furniture have shifted moreand more from those based on the natural-occurring materials to synthetics. Thebonding of wood with adhesives-gen-erally far more efficient than the use ofmechanical fasteners-has made possible awide range of products and uses for whichwood was considered unsuitable a few shortdecades ago.

SELECTED REFERENCES

Blomquist, Richard F.1963. Adhesives-past, present, and future.

Edgar Marburg Lect., Am. Sot. Test.Mater., Philadelphia, Pa. 34 p.

Browne, Fred L. and Brouse, Don1929. Nature of adhesion between glue and

wood. Ind. and Eng. Chem. 21:80-84.Jan.

Debye, P.J. W.1962. Interatomic and intermolecular forces

in adhesion and cohesion. In Adhesion andCohesion, edited by Philip Weiss, p. l-17. Elsevier Publ. Co., New York

Truax, Thomas R.1929. The gluing of wood. USDA Bull.

No. 1500. 78 p.U.S. Forest Products Laboratory, Forest Service

1974. Wood handbook: Wood as an engi-neering material. U.S. Dep. Agric.,Agric. Handb. 72, rev., 432 p.

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WOOD PROPERTIES IMPORTANTIN ADHESIVE BONDING

Various properties of wood affect itsgluing characteristics. Perhaps the mostimportant is wood’s density, but theamount of shrinking and swelling withchanges in moisture content is also an im-portant factor, particularly where long-term serviceability of glue joints is re-quired. In certain cases, pitch content,oiliness, and the presence of other exuda-tion products and extractives also havesome influence on gluability.

D E N S I T Y

Two blocks of wood of equal volumemay vary a great deal in weight, even if theblocks are of the same species. Weight ofwood is generally expressed either inpounds per cubic foot or as a comparisonwith the weight of an equal volume ofwater (specific gravity, or sp. g.).

Table 1 shows the great range in specificgravity among a number of the more im-portant commercial species of the UnitedStates. In general, strength properties ofwood increase with specific gravity. In a

Table 1.— Range in specific gravity values1

of some common species of wood

Species Sp. g.

Hickories (Carya sp.):Pignut (C. glabra ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.66Shagbark (C. ovata) .................................. .64Shellbark (C. laciniosa). . . . . . . . . . . . . . . . . . . . . . . .62Pecan (C. illinoensis) . . . . . . . . . . . . . . . . . . . . . . . . .60

White oak (Quercus sp.):White (Q. alba) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60Chestnut ( Q. prinus). . . . . . . . . . . . . . . . . . . . . . . . . . . . .57Overcup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57

Red oak (Quercus sp.):Cherrybark (Q. falcata var.

pagodaefolia) ..................................... .61Northern red (Q. rubra) .................................. .56Southern red (Q. falcata) . . . . . . . . . . . . . . . . . . . .52

Table 1.— Range in specific gravity values1

of some common species of wood (continued)

Species Sp. g.

Maple (Acer sp.):Sugar (A. saccharum) . . . . . . . . . . . . . . . . . . . . . . . . . . .56Black (A. nigrum) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52Red (A. rubrum) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49Silver (A. saccharinum) . . . . . . . . . . . . . . . . . . . . . . . . .44

Birch, yellow (Betula alleghaniensis) . . . . . . . . . . . . .55

Ash, white (Fraxinus americana) . . . . . . . . . . . . . . . . .55

Pine (Pinus sp.):Longleaf (P. palutris) . . . . . . . . . . . . . . . . . . . . . . . . . .54Loblolly (P. taeda) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47Shortleaf (P. echinata) . . . . . . . . . . . . . . . . . . . . . . . . . .47Ponderosa (P. ponderosa) . . . . . . . . . . . . . . . . . . . . . . .38Eastern white (P. strobus) . . . . . . . . . . . . . . . . . . . . . .34Sugar (P. lambertiana) . . . . . . . . . . . . . . . . . . . . . . . . .34

Elm, American (Ulmus americana) . . . . . . . . . . . . . .46

Larch, western . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

Tupelo, black (Nyssa sylvatica) . . . . . . . . . . . . . . . . . .46

Sweetgum (Liquidambar styraciflua) . . . . . . . . . . . . .46

Douglas-fir, Coast (Pseudotsuga menziesiivar. menziesii) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

Hemlock, western (Tsuga heterophylla) . . . . . . . . . .42

Yellow-poplar (Liriodendron tulipifera) . . . . . . . . . .40

Fir (Abies sp.)Pacific silver (A. amabilis) . . . . . . . . . . . . . . . . . . . .40White (A. concolor) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37California red (A. magnifica) . . . . . . . . . . . . . . . . . .36

Spruce (Picea sp.)Sitka (P. sitchensis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37Engelmann (P. engelmannii) . . . . . . . . . . . . . . . . . . .33

Alder, red (Alnus rubra) . . . . . . . . . . . . . . . . . . . . . . . . . .37

Cottonwood, eastern (Populus deltoides). . . . . . . . .37

Aspen, quaking (Populus tremuloides) . . . . . . . . . . . . .35

Redwood, young growth (Sequoiasempervirens ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4

Balsam poplar (Populus balsamifera) . . . . . . . . . . . . . 31

Cedar, Northern white-(Thujaoccidentalis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

1From 1974 Wood Handbook. Specific gravityvalues are based on ovendry weight and green volume.

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MOISTURE CONTENT (PERCENT)M 137 283

F igu re 3 .— Relation between air space in wood and specific gravity at various mois-ture contents.

5

similar manner, the glue-bond quality re-quired for a dense species is greater thanfor a lighter one. Hence, the chart indicatesthe relative glue-bond quality required forthe species listed and for other speciesfalling within the density range given.

The solid wood substance of all specieshas about the same specific gravity (1.45),but in high-density species less of the vol-ume in the capillary structure of dry woodis occupied by air. As moisture is added tothe wood, the air space decreases (fig. 3).

When wood of different species is ex-amined with the naked eye, the ratio ofwood substance to air space is not readilyseen. Under the microscope, the differencein such characteristics as cell wall thick-ness, cell diameters, and pore space iseasily noticed. Figures 4 to 8 are photo-micrographs of species covering a widerange in these characteristics. The samemagnification is used for each photo.

The enlarged cross section of westernredcedar (fig. 4) shows that slightly more

M 139 603

Figure 4 .— Cross section of western red-cedar, 3 3 × (wood substance about 2 2pct. and air space 78 pct.).

Figure 5

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Figure 5 .— Aspen (quaking), 33× (woodsubstance about 27 pct. and air space73 pct.).

M 139 602

Figure 6.— Douglas-fir, 33× (wood sub-stance about 32 pct. and air space68 pct.).

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M 139 604F i g u r e 7 . — S u g a r m a p l e , 3 3 × ( w o o d

substance about 42 pct. and air space58 pct.).

M 139 601

F i g u r e 8 . — H i c k o r y ( t r u e ) 3 3 × ( w o o dsubstance about 48 pct. and air space52 pct.).

than one-fifth of the area is wood substanceand the remainder is air space. A transversesection of aspen (fig. 5) indicates thisspecies contains slightly more than one-quarter wood substance and about three-quarters air space. Throughout the cellstructure of this wood, numerous vesselsate about evenly dispersed (diffuse-porous).

A transverse section of Douglas-fir (fig.6) shows this species has about one-thirdwood substance and two-thirds air space.In Douglas-fir latewood the cell walls arethick and the cell diameters relativelysmall. In the earlywood the cell openingsincrease and the cell wall thickness de-creases.

Another diffuse-porous wood, sugarmaple (fig. 7), has about 42 percent woodsubstance and 58 percent air space.

One of the most dense native species,hickory, is shown in cross section in figure8. Hickory surpasses practically all othercommercial native species in shock resis-tance and in some other strength proper-ties. Hickory averages about 50-50 airspace and wood substance. Since hickory isabout 50 percent wood substance, com-pared to 20 percent for western redcedar,it is reasonable to assume that “splicing”of hickory requires different bondingagents and procedures than “splicing” ofwestern redcedar.

SHRINKING AND SWELLING

In ordinary use, wood shrinks as it givesoff moisture and swells as it absorbs mois-ture. These dimensional changes generallyput stresses on joints in glued products,the higher the stresses, the stronger theglue joints must be to avoid bond failure.Figure 9 shows the approximate change involumetric shrinkage of wood of variousspecific gravities with changes in moisturecontent from bone-dry to the fiber satura-tion point (the point at which further in-crease in moisture content causes no swell-ing or change in volume). These data also

7

M 137 286

F i g u r e 9 . — Relation between volumetric shrinkage and specific gravity as the mois-ture content changes.

indicate the need for higher quality glue density and greatest shrinkage (white oak)and stronger glue joints as the density and developed the largest amount of failure inshrinkage potential of the wood increase. the glue joints; the joints in the lightest

Figure 10 illustrates how joints made species (Sitka spruce) developed the leastwith three types of glue performed on three glue failure. Obviously, the glue andspecies of various densities and shrinking gluing condition that have given excellentand swelling characteristics during three bonds on a light species such as Sitkasoak-dry cycles. With each adhesive type, spruce may not be adequate for a densethe joints in the species of the highest wood such as white oak.

8

SELECTED REFERENCES Stamm, A. J.

Northcott, P. L.1964. Specific gravity influences wood bond

durability. Adhes. Age 7(10):34-36.Perry, D. A., and Choong, E. T.

1968. Effect of surface aging and extractiontreatment on gluability of southern pineveneer. La. State Univ. Wood Util. NoteNo. 13, 3 p. La. Sch. Forest.

1946. Passage of liquids, vapors, and dis-solved materials through softwood. U.S.Dept. Agric., Tech. Bull. No. 929. 80 p.

Truax, T. R., Browne, F. L., and Brouse, D.1929. Significance of mechanical wood

joints for the selection of woodworkingglues. Ind. and Eng. Chem. 21:74-79.

U.S. Forest Products Laboratory, Forest Service1974. Wood handbook: Wood as an engi-

Rice, J. T.1965. Effect of urea-formaldehyde resin

viscosity on plywood wood bond dura-bility. For. Prod. J. 15(3):107-112.

neering material. U.S. Dept. Agric.,Agric. Handb. 72, rev. 432 p.

F igu re 10 .— Effect of species on glue-joint delamination in gusset-type assemblyjoints made of white oak, mahogany, and Si tka spruce framing members andDouglas-f i r plywood gussets. The specimens were exposed to three soak-drycycles (ASTM D 1101-59).

9

ADHESIVES FOR WOOD

Until nearly the middle of the 20th cen-tury, glues based on naturally occurringmaterials were the principal adhesivebonding agents for wood. The basic in-gredients for these generally were byprod-ucts of meat processing (for animal andblood glues), or casein, soybean, andstarch.

small proportions to improve workingproperties such as viscosity of the adhesive.Walnut shell flour is the most commonlyused filler.

In the early 1930’s, synthetic resin ad-hesives began to appear on the woodwork-ing scene; because of their versatility andother advantages, they found widespreaduse in the woodworking industry. Somesynthetic resin adhesives, when properlyused, will produce joints that remain asstrong as the wood even in unprotectedexposure to the weather. More of them,and most of the “natural” glues, will pro-duce adequate joints for normally dry in-terior use.

SYNTHETIC ADHESIVES

The more important adhesives for woodare currently produced by chemical syn-thesis. The general synthesis of resin gluesis discussed in numerous textbooks andother publications and will not be repeatedhere. Production details may vary amongmanufacturers and are usually not dis-closed except in the patent literature.

A hardener or setting agent is usuallyrequired to convert synthetic adhesivesfrom liquid to solid. These agents may befurnished separately for addition to theresin before use, or they may be present(particularly with spray-dried powderedresins) in the resin as supplied. Hardenerssometimes fall in the class of catalystswhich increase the rate of curing but arenot consumed in the reaction.

Use of fillers with synthetic adhesives israther common. Fillers are generally inertmaterials that are added to the resins in

Extenders are low-cost materials (wheatflour, for example) added to resins to re-duce the cost of the adhesive. Highly ex-tended urea resins are often used for ply-wood where low moisture resistance ordurability is adequate. When phenolresins are used for bonding interior-typeplywood, they are commonly extendedwith materials such as lignin, dried blood,and specially treated Douglas-fir bark.

The chief advantage of some syntheticresin adhesives is their excellent durability,making glued wood products serviceableunder more severe exposures than waspossible with the nonresin glues. Whenproperly used, most synthetic adhesivesare capable of producing side grain-to-sidegrain joints as strong in shear as the wooditself with most species native to conti-nental United States. Some are capable ofmaintaining their strength under prac-tically any condition of service where woodis a suitable material. Others have onlymoderate resistance to heat or moisture orboth and are not suitable for critical orsevere use conditions. Between these twoextremes in durability, a wide range ofsynthetic resin adhesives is available.

Development of synthetic resin ad-hesives has facilitated manufacture ofmany important glued wood products.Among these are laminated bridge tim-bers, ship keels and frames, and otherlaminated members for use under severeservice; plywood for boats, signs, railroadcars, and other exterior uses; and com-ponents for houses and similar structures.

Most of the synthetic woodworkingadhesives currently in use set or cure bychemical reaction. The rate of curing, like

10

M 131 639

Figure 11 .— Test fence for evaluation of glue-bond durability in plywood. The ForestService maintains four such test areas to determine durability of adhesives underdifferent climatic conditions. One test area is near Madison, Wis.; one south ofOlympia, Wash.; one at San Joaquin Experimental Range, Calif.; and the fourthat the Harrison National Forest, La. Various types of glued wood products areexposed at each site.

that of all chemical reactions, depends ontemperature, in this case the temperatureof the glue. Raising the glueline tempera-ture speeds the rate of curing as well asthe rate of strength development of thejoint. This property is used to advantage inhigh-frequency heating, steam-heatedplatens, and other means of heating inhigh-speed production processes.

One of the most important differencesamong the various types of resin adhesivesis their durability, or resistance to deterior-ation under various service conditions. Forsome types-the phenols, resorcinols,ureas, melamines, and polyvinyls—considerable data and service records indi-cate their durability. With other types,laboratory data and experience are muchmore limited and hardly justify long-termperformance forecasts.

11

The following generalizations are basedon numerous exposures ofglued specimens,both laboratory-controlled and exposed tothe weather (fig. 11), and on service rec-ords from various parts of the country(fig. 12).

While the synthetic resins can be con-sidered in a variety of groupings, they arediscussed here under general headings.These include phenolics, ureas, melamines,polyvinyl resin emulsions, hot melts,epoxies, contacts, mastics, and variouscombinations of specific adhesives.

Phenolic Resins

Phenolic resins are formed by reactingformaldehyde with phenol in what is calleda condensation reaction, These may be con-sidered in four categories: High-tempera-

M 124 522

Figure 12 .— Partial view of 11 creosoted laminated southern pine bridge stringersinstal led on the Texas & Pacif ic Rai l road near Woodlawn, Tex., 1944. No jointseparat ion or other s ign of deter iorat ion is apparent. The l ight gray mater ialshown on the stringers is sandy silt that has seeped down from the ballast above.

ture-set t ing phenolics , intermediate- of the phenolics is generally similar, thistemperature-setting phenolics, resorcinols, phase is summarized after the individualand phenol-resorcinols. Because durability types are discussed.

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HIGH-TEMPERATURE-SETTINGPHENOLICS

Phenol-formaldehyde adhesives werefirst introduced in film form (about 1920).In production of this type of adhesive, theresin is deposited on a tissuelike paper andthe solvent removed by drying. The filmform of phenolic was particularly con-venient for making plywood from thinveneers since no increase in moisturecontent was involved.

As softwood plywood approached massproduction status, phenolic resins becameavailable in liquid form, making exteriorplywood a reality. This development per-mitted application of adhesive by rollspreaders and variation in spreads asveneer quality required.

Phenolic resins are also available asspray-dried powders to be mixed in wateror water-alcohol solutions before use.These phenolic resins (both liquid and filmform) require heat for curing to completepolymerization; thus they prompted de-velopment of hot presses.

Neat phenolic resins as used in the earli-est production of exterior plywood had atendency to “bleed-through’ or penetratethe wood excessively, particularly in loose-cut veneer. Incorporation of fillers such aswalnut shell flour or powdered oat hullresidue produced a more workable adhesiveby reducing penetration. Small amountsof wheat flour or heat-treated dry bloodalso have been used with phenolic resins,reportedly decreasing the cure time. Re-cently, it has been reported that neatresins, with higher solid contents appliedwith thinner spread than the filled resins,are performing very satisfactorily and alsopermit higher moisture content in veneers.

The phenolic resins used for makingplywood are generally alkaline and requirehigh temperatures for proper curing. Acidphenolic glues were also developed to set atmoderate-to-room temperatures, but acidtypes have not found volume use. The ex-cellent durability characteristics of thealkaline phenolics prompted their wide-

spread use in structural plywood and otherapplications.

The alkaline phenol resin adhesivesnormally cure at 265° to 310° F. As aresult, their use is restricted almostentirely to gluing the more durable typesof plywood and related thin products thatcan be heated to these temperatures in apractical time period. Phenol resins havebeen formulated to cure at temperaturesas low as 240° F. for hot pressing, butsuch formulations are not in common usein the United States.

The major use of liquid phenol resin ad-hesives is for bonding exterior softwoodplywood, including boat hull plywood andproducts for other marine uses. Their usein hardwood plywood manufacture is morelimited. Liquid phenol resins formulatedspecially for softwood plywood productionare generally quite reactive; as a result theyhave relatively short storage lives. Wherelonger storage is required, the powderedphenol resins are often used. They are pre-pared for use by dissolving in water or inwater and alcohol and in some cases mayrequire addition of separate hardeners.

Phenol resins are commonly used withsome walnut shell flour or other filler, butwithout extenders where highest jointdurability is required. In recent years, con-siderable amounts of interior-type soft-wood plywood also have been made withphenol resin to which were added fairlyhigh proportions of extenders and fillerssuch as ground bark, wood or walnut shellflour, dry soluble blood, or certain otheragricultural residues. These phenol ad-hesives replaced conventional protein gluesused for interior plywood for severaldecades.

Phenol resins can be formulated, withinlimits, to suit the manufacturing opera-tions of the glued products. The curingcycle or length of the pressure period in thehot press can be controlled at least par-tially by the proper amount and type ofcatalyst and by the way in which the resinis made. Assembly periods depend some-what on reactivity of the adhesive, but they

13

can generally be controlled within practicallimits. With some formulations for high-speed, flat plywood production, assemblyperiods as long as 15 minutes at 70° to80° F. are permissible. For other formu-lations, an all-open assembly period ofseveral hours or days can be allowed, asmay be necessary in bag molding opera-tions where a long layup period is un-avoidable. Adhesives for bag molding (seePressing and Clamping) must permit as-sembly of neatly tack-free, glue-coatedveneers and yet later flow adequately whenheat and pressure are applied in the finalcuring operation.

The dry film form of phenol resin ad-hesive is well adapted to gluing thin, andparticularly crotch, veneers because thereis no problem involved in controllingspread. Moreover, the danger of bleed-through is almost nil. Since the filmweighs about 12-l/2 pounds per 1,000square feet and approximately one-thirdof this weight is paper, a relatively lightspread of resin is obtained when a singlesheet is used per glueline. If film gluesare to be used successfully, the veneermust be well cut, smooth, and uniform inthickness. Since the film glue containslittle or no water, all moisture needed forsoftening the resin and providing thenecessary flow during pressing must comefrom the veneers. For this reason, the con-trol of moisture content in the veneers iseven more critical with a phenol resin filmadhesive than with conventional gluesapplied in liquid form.

Film adhesives normally do not givegood results on veneer at moisture contentsof less than 6 percent. The most satisfac-tory moisture content of veneer for gluingwith phenol resin films varies somewhatwith the species and veneer thickness; ingeneral, good results are obtained in therange of 8 to 12 percent. Too high amoisture content may cause blisters, ex-cessive bleed-through, and starved joints;one that is too low usually results in driedjoints of low strength. For furniture andsimilar interior uses, optimum moisture

content with this type of glue is about 8percent.

lNTERMEDIATE-TEMPERATURE-SETTING PHENOLICS

The intermediate-temperature-settingtypes of phenol resin adhesives were de-veloped as durable glues that could becured at 210° F. or less, such as in heatedchambers or electrically heated jigs.Special formulations of phenol resins wereoffered for this purpose, being more reac-tive at the lower curing temperatures be-cause of rather highly acid catalysts. Thus,this type of adhesive has often been referredto as acid-catalyzed phenol resin. Someformulations are suitable for gluing ply-wood at temperatures as low as 80° F. ifthe pressure periods are overnight or longerand if several days of additional condition-ing are allowed before subjecting the ply-wood to severe service.

The acid-catalyzed phenol resin ad-hesives have been used to a limited extentfor gluing sandwich panels, prefabricatedhouse panels, and truck panels. They arenormally supplied as liquid resins with theacid catalyst furnished separately for addi-tion at the time of use. Acid-catalyzedphenol resins do not glue as well on woodat 6 percent moisture content as at 10 to12 percent.

Since the introduction of the resorcinoland phenol-resorcinol resin adhesives, theacid-catalyzed phenol resin adhesives havenot been extensively used, although theyare generally somewhat cheaper and arelighter colored than the phenol-resorcinoland resorcinol resins. The acid-catalyzedphenol resins are not considered as durableas the resorcinol resin types for long-timesevere service and elevated-temperatureexposures.

RESORCINOLS

Adhesives based on resorcinol-formalde-hyde resins were first introduced in 1943.

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Almost immediately they found wideapplication in gluing laminated memberssuch as keels, stems, and frames for navalvessels, and for assembly gluing andlaminating in wood aircraft where thecombinat ion of high durabil i ty andmoderate-temperature curing was ex-tremely important.

The resorcinol resins bear many resem-blances to phenol resins. A principaldifference is the greater reactivity of theresorcinol resins, which permits curing atlower temperatures. Resorcinols are sup-plied in two components as a dark reddishliquid resin with a powdered, or at timeswith a liquid, hardener. These glues cureat 70° F. or higher, but usually are notrecommended for use below 70° F. withsoftwoods and generally require somewhathigher cure temperatures with dense hard-woods. Straight resorcinol resin adhesiveshave storage lives of at least a year at 70°F. Their working lives are usually from 2to 4 hours at 70° F.

Assembly periods are not too criticalon softwoods as long as the glue is stillfluid when gluing pressure is applied. Ondense hardwoods, such as oak, the assem-bly period must be adjusted (usually ex-tended) to give a rather viscous glue at thetime the assembly is pressed. One- to 2-hour assembly periods have been used withgood results when gluing oak at 70° to80° F., but the actual assembly time fora particular formulation depends on theage and viscosity of the glue at the timeof spreading as well as the temperature,absorptiveness of the wood, and otherfactors. Resorcinol adhesives are ideal forlaminating large timbers that require con-siderable time to assemble and bring underpressure. Very short assembly periods withdense woods can result in “starved” jointsand should be avoided.

Resorcinol resin glues will cure ade-quately on thin plywood and other lightconstructions of medium- to low-densityspecies at about 70° F. For such high-density hardwoods as white oak, used forlaminating ship timbers and similar items,curing for several hours at about 150° F.

15

glueline temperature has been necessary.If facilities for raising the temperature ofthe gluelines to such levels are not avail-able, adequate bonds can be obtained byextending curing periods.

These glues, as well as phenol-resorcinolmodifications, have earned an outstandingreputation for performance under severeservice conditions. Bridge timbers lami-nated with them are still in excellent con-dition after more than a quarter century ofservice. Laminated oak timbers for mine-sweepers have gained a similar reputation.Resorcinol resin adhesives are not affectedby the commonly used preservative treat-ments, which permits treating the gluedtimbers for long-term service. They alsobond treated wood, making it feasible totreat the lumber and then glue the assem-blies to the desired size and shape. This isparticularly advantageous where large,curved members that cannot be treated incylinders are involved. Special formula-tions have also been developed for gluingfire-retardant-treated wood.

PHENOL-RESORCINOLS

Phenol-resorcinol resins are modifica-tions of straight recorcinol resin adhesivesproduced by polymerizing the two resins(phenol-formaldehyde and resorcinol-formaldehyde). The principal advantageof the copolymer resins over straight resor-cinol resin is their significantly lower cost,because the price of phenol is much lowerthan that ofresorcinol. This cost advantageapparently is achieved without any signifi-cant losses in joint performance. For woodgluing, the volume of phenol-resorcinolused now far exceeds that of straight resor-cinol. Proportions of the two resin com-ponents in the copolymer are not generallyrevealed by the manufacturers. Like theircomponents, the copolymer resins are darkreddish liquids and are prepared for use byadding powdered hardeners. The hard-eners generally consist of paraformaldehydeand walnut shell flour, mixed in equalparts by weight.

Phenol-resorcinol resins generally haveshorter storage lives than straight resorci-nol resins, usually somewhat under 1 yearat 70° F. Many manufacturers formulatephenol-resorcinols to tit prevailing tem-perature conditions-fast-setting resinsfor cool weather, somewhat slower settingones for warmer weather, and still slowerones for hot weather. This is particularlyhelpful for the laminating industry, wherethe curing time could become prohibit-ively long with a slow-setting resin duringcooler weather and the permissible assem-bly time could be difficult to meet withfast-setting resins during hot weather.

Research has shown that, as the curingtemperature is increased, the requiredcuring period decreases logarithmically.Figure 13 shows the relation betweencuring temperature and curing time forfive different resorcinol and phenol-resorcinol adhesives. With glues A, C, andE, about the same joint quality could beobtained in laminated white oak when theglueline was heated for about 6 minutes

or more at 150° F. as when it was heatedfor about 2,500 hours at 80° F.

Originally, resorcinol and phenol-resorcinol adhesives were rather costly,which limited their use almost exclusivelyto the most severe service conditions.Within recent years, however, the pricehas come to about a third of what wascommon several decades ago.

DURABILITY OF PHENOLIC RESlNS

The durability of moderately alkalinephenol resin, resorcinol resin, and phenol-resorcinol resin adhesives is essentiallysimilar. When these glues are properlyused, they are capable of producing jointsthat are about as durable as the wood itselfunder various severe service conditionsstudied. Properly made joints will with-stand, without significant delamination orloss in strength, prolonged exposures tocold and hot water, to alternate soakingand drying, to temperatures up to thosethat seriously damage the wood, to high

M 112 243

Figure 13 .— Minimum curing times required at different temperatures for five resor-cinol and phenol-resorcinol glues on white oak. A and B are phenol-resorcinols;C, D, and E are resorcinols.

16

relative humidities where many untreatedspecies decay, and to outdoor weatheringwithout protection from the elements.

The joints between lumber laminationsor plies of plywood made with these ad-hesives will not separate when exposed tofire. The glues are not weakened by fungi,bacteria, or other micro-organisms and areavoided by termites. These adhesives,however, do not offer any significant pro-tection to the adjacent wood. Conse-quently, wood products glued with theseadhesives should be considered no moredecay- or insect-resistant than solid woodsof the same species.

Glued wood is subject to shrinkagestresses and, even if the glue joints aredurable, splitting and checking mightoccur adjacent to or away from the gluejoints. For severe service, therefore, it isimportant to employ treatments that pro-tect against wood-degrading organismsand also impart water repellency, thus re-ducing shrinking and swelling stresses inthe glued member.

Completely cured phenolic-type gluejoints (made with neutral or moderatelyalkaline resins) are highly resistant to theaction of solvents, oils, acids, alkalies,wood preservatives, and fire-retardantchemicals. Thus, in general, well-madejoints bonded with phenol, phenol-resorci-nol, and resorcinol resin glues are difficultto destroy without destroying the wooditself. However, as with other types ofglues, joints poorly made with thesedurable adhesives may fail in service.

Acid-catalyzed phenol resins haveshown good durability in such applica-tions as bonding honeycomb paper core toplywood faces of sandwich panels. Withdense species, such as white oak, muchlower shear strength was obtained withacid-catalyzed phenol glue than withphenol-resorcinols. Under exposure toelevated temperatures such as 158° F.,the joint quality was reduced more than forthe conventional alkaline phenol resinadhesives.

Urea Resins

Urea-formaldehyde resin adhesivescame on the market in the middle to late1930’s. By using different types andamounts of catalyst, they can be formu-lated either for hot-pressing or for room-temperature curing. They are compatiblewith various low-cost extenders or fillers,thus permitting variation in both qualityand cost. Even the hot-press formulationsset at appreciably lower temperatures thanthe alkaline phenolic adhesives. Beinglight in color or slightly tan, urea adhesivesform a rather inconspicuous glueline. Butexposure to moist conditions, and particu-larly to warm, humid surroundings, leadsto deterioration and eventual failure ofurea resin adhesive bonds. Durability ofurea-resin adhesives is summarized at theend of this section.

Major uses for urea resins are in hard-wood plywood, particleboard, and furni-ture manufacture. They are also availablein the retail trade for home workshop use.

Urea resins are generally marketed inliquid form (as water suspensions) wherelarge-scale use is involved and shippingdistances are not excessive. They are avail-able with solids contents from about 40 to70 percent. They are also marketed as drypowders, with or without catalyst in-corporated. The powdered ureas areprepared for use by mixing with water orwith water and catalyst if the catalyst issupplied separately. In general, powderedurea resin adhesives with separate catalystshave longer storage lives than the liquidurea resins or the powdered types withcatalysts incorporated.

Urea resins are generally more versatilethan some other resin adhesives; the sameresin, as received from the manufacturer,can be used for either hot-pressing or room-temperature cure by addition of the propercatalyst. Some manufacturers, however,supply different resins for hot-pressing andfor room-temperature curing. Specialformulations have been developed for such

17

uses as high-frequency curing and for tape-less splicing of veneers.

Urea resin adhesives can be extendedwith cereal flour to reduce cost where thejoint strength and durability attainablewith unextended glue are not required,such as in mild exposures with low-shrinkage woods. Wheat and rye flours aremost commonly used for extenders, and ex-tensions up to 100 parts by weight offlour to 100 parts resin solids (100 pct.extension) are used with room-temperature-curing formulations for bonding hard-wood plywood. Extensions up to 150 partsflour per 100 parts resin solids (150 pctextension) are sometimes used in hot-pressing hardwood plywood.

Various grades of wheat flour affect theworking properties of the adhesive differ-ently, particularly consistency and ten-dency to foam, and may also influence theeffect of the catalyst used. Flour extensiongenerally makes the adhesive more viscous;the degree of change depends on theamount and type of flour and also on theprotein content of the flour (formaldehydereacts readily with protein).

A small amount of sodium bisulfite (1 to2 pct. by weight) is sometimes added tothe flour during mixing with the resin tohelp overcome differences in flours and toreduce the amount of additional waterthat might otherwise be needed to make aspreadable mixture. The addit ion ofsodium bisulfite may affect the catalystsystem of some adhesives, and the usershould obtain the recommendations of theglue supplier for the type and amount ofextension suitable for a particular product.Extended glues often require somewhatheavier spreads and shorter assemblyperiods than the corresponsing unextendedglues.

Urea resins cure much faster thanphenol resins at the same temperature.When this advantage of ureas is added totheir lower costs and lack of color, theyare attractive for gluing furniture andarchitectural plywood for interior usewhere the greater durability of the phenolresin glues usually is not required. Typicalrecommendations for curing hot-press urearesins are 2 to 5 minutes for panels witha total thickness of 3/16 inch or less and8 to 10 minutes for l-inch panels withabout 1/2-inch cores when the platentemperature is 260° F. and one panel isglued per press opening.

Urea resin adhesives for edge gluing,assembly, veneer splicing, and laminatingof furniture parts are not normally ex-tended with cereal flours, but they docontain some walnut shell flour as fillerto improve working properties.

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For certain products and service condi-tions, the durability of hot-press urearesins can be improved by adding moredurable resins or special resin-formingingredients. These additives are generallyreferred to as fortifiers and the resultantglues as fortified urea resin glues. The mostwidely used fortifiers are the melamineresins, but crystal resorcinol has also beenemployed. The amount of fortifier variesconsiderably. Under more severe condi-tions, including outdoor weathering of

Urea resins are normally not recom-mended for use on wood below 6 percentin moisture content . This l imitat ionappears to be related to the porosity of thespecies and to the rate at which moistureis absorbed from the adhesive by the wood.

HOT-PRESS UREA RESlNS

Hot-press urea resin adhesives arenormal ly cured a t 240° to 260° F.Assembly periods vary considerably for thedifferent formulations. Many typical ad-hesives are formulated for assembly periodsof 10 to 30 minutes, but special formula-tions may permit assembly periods of 24hours or more. Because of their relativelyhigh reactivity, some urea resin adhesivesprecure on hot cauls or platens before fullgluing pressure is applied. This can beavoided by use of cooled cauls and propersequence in the spreading and press-loading operations.

plywood, durability has generally im-proved as the amount of fortifier is in-creased. Because of the special interest inmelamine-urea resin adhesives, these aredescribed separately. No entirely adequateroom-temperature-setting fortified urearesin glues have yet been introduced forindustrial use.

ROOM-TEMPERATURE-SETTINGUREA RESINS

Urea resins classified as room-tempera-ture-setting are formulated to cure attemperatures of 70° F. or higher. Theywere the first synthetic adhesives devel-oped for practical use at normal roomtemperatures. They were extensively usedin assembly gluing of aircraft parts, truckbody parts, and similar items before theintroduction of the more durable room-temperature-setting resorcinol resin glues.

In addition to their use in cold-pressingplywood (with hydraulic presses, I-beams,and retaining clamps to maintain thepressure after removal from the press),room-temperature-setting ureas are nowused for edge gluing on clamp carriers,in various assembly operations, and forlaminating furniture parts. Their availabil-ity in small retail packages as dry powdersthat require only the addition of watermakes them very convenient for small joband home workshop uses.

The working life of a glue of this type isusually from 3 to 5 hours at 70° F. andless at higher temperatures. Special slow-acting catalysts increase the working lifeof room-temperature-setting urea resinsduring hot weather and make them motepractical for use during summer months inplywood and other commercial applica-tions.

Assembly periods with these adhesivesare fairly short, usually with maximumclosed assembly of 30 minutes at 70° F.for critical applications. The maximumpermissible assembly period depends on

temperature and somewhat on the mois-ture content of the wood, amount of ex-tension, and the amount of glue spread.

The minimum pressure period dependson the type of glued product and upon thetemperature of the wood and the room, forthese temperatures control the speed of thecuring reaction. Because of slow heat trans-fer through the wood, room-temperature-setting urea resin glues generally cure in-adequately if the glue is spread on coldwood and then clamped for only a shorttime at 70° to 80° F. At 70° F. apressing period of at least 4 hours is gener-ally required on thin, flat members, suchas plywood, and at least 6 to 8 hours isrequired on heavy or curved members.Longer pressing Periods are generally re-quired for heavy species than for lighterones. In no case should pressure be releaseduntil the squeezeout is hard.

Room-temperature-setting urea resinformulations are often used in special heat-curing operations with heated jigs andhigh-frequency curing to get faster settingthan is possible with conventional hot-press formulations. But if assembly periodsare excessive, the adhesive may precure be-fore gluing pressure is applied. A room-temperature-setting glue must be fluid atthe time pressure is applied to adequatelytransfer glue to the unspread surface.

These glues will harden at temperaturesbelow 70° F. but at a very slow rate,and joints with erratic strength and dura-bility may result. Curing below 70° F.is therefore generally not recommended.

In special applications where rapidstrength development at room tempera-ture is of primary importance, the normalroom-temperature-setting urea resin for-mulation without catalyst may be appliedto one wood surface and a strong acidiccatalyst applied to the mating surface.Sometimes the liquid catalyst is applied inadvance and air dried. The joint is thenassembled and pressure quickly applied.The separately applied catalyst is assumedto penetrate into the glue and to causerapid setting. Such strong catalysts cannot

19

be incorporated in the glue before spread-ing because they shorten pot life.

This technique is referred to as the“separately applied catalyst process.”When properly conducted, it results inrapid development of joint strength, thuspermitting a shorter pressing period thanwith adhesives having catalyst mixed withthe resin. The process has not been widelyaccepted, however, because it is difficultto obtain uniform mixture of catalyst andresin. Uneven penetration results in erraticjoint quality; moreover, the highly acidglueline does not seem to be as durable asgluelines made with adhesive of the sametype without separately applied catalyst.

DURABlLlTY OF UREA RESINS

In general, well-made urea resin gluejoints develop high original dry strengthand wood failures with almost all U.S.commercial species, good resistance to con-tinuous soaking in cold water, and fairresistance to continuously high relativehumidity and alternatively high and lowrelative humidity. Nevertheless, a com-bination of high relative humidity andhigh temperature deteriorates urea resinglue bonds in a relatively short time.

Resistance to cyclic soaking and dryingexposures is reasonably good if the testpieces are plywood or thin members, butonly moderate to low resistance is obtainedif the pieces are heavy laminations of densewood. This applies to short-term expos-ures. Over the long term, urea resin gluejoints deteriorate under the exposures men-tioned, and the rate of deterioration usuallyincreases with the density of the species.Urea resin glue bonds are generally des-troyed by boil tests (generally 4 hr. boil-ing, 20 hr . drying a t 145° ± 5° F. ,4 hr. boiling, cooling, and testing wet,Prod. Std. PS 1-66) as used for glue-jointevaluation of exterior-type plywood.

The fortified urea resins ate more dur-able under practically all of the exposureconditions named. They are followed, in

order of decreasing durability, by thehot-press urea resins, room-temperature-setting urea resins, and highly extendedurea resins. Tests made with hot-press urearesin glue extended with rye flour showedthat the wet joint strength falls off slowlyas more flour is added. No important de-crease in wet joint strengths was apparentuntil 50 to 100 percent of extender, basedupon the weight of dry resin, had beenadded. Under dry conditions, the dry jointstrength decreased still more slowly, andjoints containing twice as much rye flouras resin exhibited high strength.

Under conditions conducive to develop-ment of mold and other micro-organisms,joints made with extended urea resin gluesalso lost strength more rapidly than unex-tended glues. The loss was noticeable withas little as 10 percent flour and particularlyrapid for glues having greater extensions.Tests with preservatives showed that themold resistance of flour-extended urearesins could be increased by adding chlor-inated phenols to the glue in amountsequal to 5 percent of the weight of theflour. (Concentrations lower than 5 pct.appeared to offer less protection, but thereseemed to be little advantage in increasingthe concentration above 5 pct.) Thesepreservatives seem to delay the effect of themicro-organism damage, but they areunable to prevent it over long periods ofexposure to high moisture conditions.

Thus, except for highly fortified types,the urea resins as a group are low in dura-bility under conditions involving hightemperatures and humidities. At hightemperatures and extremely low relativehumidity the joints are more durable, butthis is mainly of academic interest becausesuch conditions rarely exist where gluedwood products are used. Gradual weaken-ing of room-temperature-setting and hot-press urea resin glue joints occurs underdry conditions at 160° F. A much lesssignificant weakening of room-tempera-ture-setting urea resin glue joints hasbeen observed in birch plywood undercontinuous exposure at 80° F. and 65

20

percent relative humidity. The rate ofstrength loss is increased by high humidityat 80° F.

Delamination usually occurs within afew hours in boiling water. Urea resinbonds tend to break down at tempera-tures that char wood; therefore, when cer-tain urea resin-bonded plywoods are ex-posed to fire, even for short periods, theplies may delaminate. Plywood panelsmade with unfortified room-temperature-setting and hot-press urea resin adhesiveshave shown considerable delaminationafter 2 to 3 years of outdoor exposure atMadison, Wis. Panels made with fortifiedurea resins have shown much less delami-nation in the same length of time. Underexterior exposure and where high tempera-tures with or without high relat ivehumidities are involved, urea resin gluesare markedly less durable than phenol,resorcinol, and melamine resin glues. (Thisdoes not imply that melamine glues havethe same durability characteristics asphenol and resorcinol resins.)

Admittedly, urea resin glue joints (inu n f i n i s h e d s p e c i m e n s - n o l a c q u e r ,varnish, or paint) have shown much largerdecreases in strength than phenol, resorci-nol, phenol-resorcinol, and melamineresin glue joints after several years’ expos-ure to less severe laboratory-controlledconditions. Nevertheless, high-qualityurea resin glue joints do appear to besufficiently durable for nonstructuralinterior applications within the humancomfort range of temperature and humid-ity conditions. On the other hand, particu-larly with high shrinkage, dense species,the more durable resin adhesives wouldassure longer trouble-free service life.

Melamine Resins

Melamine resin adhesives are normallyof the hot-press type, curing at 240° to260° F., similar to the hot-press urea-resin glues. Special formulations havesometimes been offered for curing attemperatures as low as 140° F., but

they have not been widely used. Some ofthe high-temperature-setting melamineswill cure adequately at temperatures from140° to 180° F. if the curing periodis extended to 10 hours or more. Lamin-ated Douglas-fir beams bonded with theseglues and cured overnight at 140° F.have shown excellent performance inoutside exposure for up to 20 years.

Most of the melamine resin glues aremarketed as powders that are prepared foruse by mixing with water and sometimeswith a separate hardener. Those usinghardeners or catalysts will set much morerapidly or at lower temperatures thanthose cured without hardeners. There havebeen indicat ions, however , that thecatalyzed melamines do not have the sameresistance to weather that the uncatalyzedones have.

Pure melamine resin adhesives are al-most white, but the addition of fillerusually gives them a light tan color similarto the urea resins. The filler is usuallywalnut shell flour, but occasionally woodflour is used.

Melamine resins have been used to alimited extent for gluing hardwood ply-wood where the darkness of phenol resinsis objectionable and durability approach-ing that of phenol resins is required. Mela-mine resins are considerably more ex-pensive than phenol or urea resins.

Uncatalyzed melamine resin glues alsohave been investigated for gluing heavylaminated ship timbers at curing tempera-tures of 140° to 190° F. On Douglas-fir they showed promising results, but onoak the glue bond deteriorated when thespecimens were soaked in salt water(simulating sea water) for 15 years.Current commercial applications of mela-mine resin glues in structural wood lamin-ating include bonding interior fingerjoints, laminated decking, and laminatedbeams with 60:40 melamine-urea combin-ations and high-frequency curing. Themelamine resins have been used success-fully in high-frequency edge gluing wherea durable, colorless glueline is required.

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As a group, melamine resin adhesives gen-erally have a pot life of at least 8 hoursat 70° F. and they tolerate rather longopen and closed assembly periods.

Slow-curing, uncatalyzed melamineresin glues have shown good durabilitycharacteristics on laminated Douglas-firbeams exposed for several decades to theweather. Similar glues used for laminatingwhite oak failed almost completely after15 years of soaking in salt water. Rapid-setting, catalyzed melamine resin glues,in limited tests, have not shown the samedurability as indicated with uncatalyzedmelamines on softwood species.

Melamine-Urea Resins

Melamine-urea resins are a specialgroup of hot-press adhesives produced byeither dry blending urea and melamineresins or by blending the two separateresins in liquid solution and then spray-drying the mixture. In either case, theresins are supplied by the manufacturer aspowders, to be prepared by adding waterand catalyst. Reportedly, the adhesive pro-duced by spray-drying a mixture of the tworesins produces somewhat more durablebonds than the one produced by blendingthe two powdered resins. At present, themost common combinations are said tocontain 40 to 50 percent by weight ofmelamine resin and 50 to 60 percent ofurea resin on a solid basis.

In finger-jointing lumber for structurallaminated timbers, a 60:40 melamine-to-urea ratio is used. Such joints, when prop-erly produced, are considered adequate fornormally dry interior service but are notrecommended where long-term exterioruse is involved. The melamine-urea com-binations are used in much the same wayas the hot-press ureas and melamine glues,curing at 240° to 260° F. in manufac-ture of plywood. In finger-jointing opera-tions, they are generally cured by high-frequency heating. The melamine-urearesin glues offer advantages for hardwoodplywood in that they are colorless, more

durable than urea resins, cheaper thanstraight melamine or resorcinol resins, andcapable of curing at lower hot-press tem-peratures than conventional phenol resinglues.

Polyvinyl Resin Emulsions

Polyvinyl resin emulsions are thermo-plastic, softening when the temperature israised to a particular level and hardeningagain when cooled. They are prepared byemulsion polymerization of vinyl acetateand other monomers in water under con-trolled conditions. Since individual typesof monomers are not identified by themanufacturer, this group is simply referredto as polyvinyl resins or PVA’s. In theemulsified form, the polyvinyl resins aredispersed in water and have a consistencyand nonvolatile content generally com-parable to the thermosetting resin glues.They are marketed as milky-white fluidsto be used at room temperature in theform supplied by the manufacturer ,normally without addition of separatehardeners.

The adhesive sets when the water of theemulsion partially diffuses into the woodand the emulsified resin coagulates. Thereis no apparent chemical curing reaction, aswith the thermosetting resins.

Setting is comparatively rapid at roomtemperature, and for some constructionsit may be possible to release the clampingpressure in half an hour or less. Limitedtests indicate that some of these gluesset in most wood joints at 75° F. at arate comparable to that of hot animal glue.

The polyvinyl resins have indefinitelylong storage (in tight containers) andworking lives (at normal room tempera-tures) and can be used as long as the resinremains dispersed. Coagulation in storageby evaporation or freezing must be avoided,although special emulsions have beenoffered that are said to withstand repeatedfreezing and thawing. The set resins arelight in color, often transparent, and resultin gluelines that are barely visible.

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A considerable amount of variationhas been observed in the performance ofthe different glues of this type. Some ofthe poorer ones gave considerably lowerjoint strengths and developed little or nowood failure compared to other types ofresin and nonresin glues. On the otherhand, joints produced with some of thenewer polyvinyl resins gave unusually highshear strengths although generally nothigh wood failures, particularly withdenser species. Such results might be ex-pected of rather elastic-type adhesives, be-cause the load probably will be distributedmore uniformly over the entire joint areaunder test than with brittle glues. Thepolyvinyl resin adhesives have little dullingeffect on the sharp edges of cutting tools,but a tendency to foul sandpaper has beenreported.

Some PVA’s soften and lose a portion oftheir strength as the temperature in-creases above normal room temperature,and the strength of many of them isappreciably reduced at about 160° F.They are also generally weakened more byhigher relative humidity conditions thanare the thermosetting resin glues.

Probably the most serious limitation inthe use of these adhesives in woodworkingis the lack of resistance to continuouslyapplied loads. Such “cold flow“ is thetendency for a glue to yield to, rather thanresist, the stresses exerted on the joint atnormal room temperature. This limitationhas been most serious when polyvinylresins have been used for edge-gluinglumber for solid stock, particularly high-density hardwoods, which will not be sub-sequently veneered. When such a stock isexposed to low humidities, moisture con-tent changes most rapidly through the endgrain, with resultant shrinkage stressesacross the ends of the panels. When thesestresses are of appreciable magnitude andduration, the glue often fails, resulting inopen joints.

This cold-flow limitation has encour-aged considerable reformulation of poly-vinyl resins so several currently available

glues appear to be suitable for edge-gluingapplications. However, not all glues showsuch improvement and no quick andsimple screening test is yet available forchecking this property. Therefore, the usermust exercise caution in selecting such aglue for edge-gluing, particularly of densespecies. Screening tests, by cycling panelsmade with different glues between highand low humidity conditions, might be ad-visable before using PVA’s in full-scaleproduction.

At the British Forest Products Labora-tory, joints made with 39 brands of PVAwere tested in an atmosphere of 25° C.(77° F.) and 60 percent relative humidityunder approximately one-third ultimateload and normal loading rate. Joints with37 brands failed within 6 months; jointswith the other two brands survived andwere still intact after 24 months.

Studies have indicated that some poly-vinyl resin emulsion glues are promisingin assembly joints such as dowel, mortiseand tenon, and lock-corner. Their fastsetting is of benefit and their elasticitymay be an additional advantage where thedimensional changes in the joint are nomi-nal.

In terms of durability, polyvinyl resinemulsion adhesives are considerably lessresistant to warm, moist, or humid condi-tions than the thermosetting resins. ThePVA’s lack resistance to water and highrelative humidities, and a number tend tosoften at temperatures as low as 110° F.Even at normal room temperatures, creepor yield of the bond (cold flow) mightbecome a problem if heavy stress is con-tinued on the joint. The low resistance ofpolyvinyl resin emulsions to water andmoisture limit PVA use primarily to non-structural interior applications, as in cer-tain types of furniture joints.

Thermosetting PolyvinylEmulsions

Thermosetting polyvinyl emulsions,also identified as catalyzed PVA emulsionsand cross-linked PVA’s, have been avail-

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able for a decade. They are modified PVAemulsions and generally have heat andmoisture resistance superior to ordinaryPVA’s, particularly when cured at ele-vated temperatures.

Room-temperature cure of these ad-hesives has been insufficient to preventcreep when glued specimens were stressedduring exposure at 150° F. and highrelative humidity. Therefore, they are not

recommended for structural applicationsbecause of creep (fig. 14).

On the other hand, in tests on hot-pressed plywood made with a cross-linkedPVA, the joints performed almost as wellas those made with phenolic glues.

Further research has shown that theseadhesives cannot be classed with resorcinolsas being room-temperature-curing andsuitable for general structural applications.

M 122 542

Figure 14.— Cross sect ion of laminated oak glued with thermosett ing or cross-linked PVA and subjected to vacuum-pressure soaking, steaming, and drying. Thebridging in the joint at the center might explain why PVA glues have performedwell in cyclic tests on mortise and tenon and dowel joints. Had a brittle glue beenused, fractures would probably have occurred either in the bond or adjacent tothe glueline. The elastic PVA yielded enough to retain the bond between thejoint surfaces.

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They are, however, markedly superior toordinary PVA’s in resistance to moistconditions, and there is reason to believethey would perform well in most non-structural interior uses. In common withsome other adhesives, they would not beexpected to perform as well on dense,high-shrinkage species as on lighter ones.

Hot Melts

Hot-melt adhesives for wood are furn-ished in solid form, usually as pellets,chunks, granules, or in cord form on reels.They involve a wide variety of thermo-plastic mixtures that are converted by heatto spreadable consistency and appliedwhile hoc and fluid; they set almost instan-taneously as the heat dissipates from thethin glue film to the greater mass of thesubstrate. Pressure is applied on the jointduring formation of the bond. The bondforms very rapidly, depending upon thetemperature difference between the glueand the parts being joined. Setting timesas brief as a fraction of a second have beenreported.

One of the primary uses of hot melts inwood gluing has been for edge banding ofpanel products. Machines are increasinglycommon in the furniture industry to applyedge banding to panels with hot melts atabout 60 to 100 linear feet per minute.The process is reported to lend itself toapplication of veneer and thicker edgebands to lumber and particleboard cores.Hot melts are also being used to someextent for bonding decorative overlays orfilms to particleboard for counter and furn-iture tops and shelves, and for coatingpanel products. Methods of application in-clude roll coating, blade coating, and cur-tain coating.

The composition of hot-melt adhesivesvaries a great deal and may include poly-mers, such as ethylene vinyl acetate copoly-mers, polyamides, polyolefins, and poly-esters, as well as other resins or copoly-mers. These are generally modified with

plasticizers and other ingredients toimprove working properties.

Hot melts have melting points coveringa rather wide range. Transition points fromsolid to a soft mass or liquid have beenreported from as low as 150° F. to ashigh as 390° F., although working tem-peratures in the range of 375° to 410°F. are supposed to be more common.

Some hot melts are reported to bewater resistant and provide somewhat elas-tic gluelines; however, their resistance toheat is generally poor. For best results,good control is required of wood and gluetemperatures as well as the rate of applica-tion.

Epoxy Resins

Epoxy resin adhesives became availablein the 1940’s and found a major use formetal bonding in the aircraft industry.However, epoxies do adhere to a variety ofsubstrates and in recent years have beenemployed as bonding agents in numerousspecial applications. They are probably themost versatile adhesives currently availablein that they adhere to more different sub-strates than other synthetic or naturallyoccurring bonding agents. They have not,however, found extensive use for bondingwood.

Epoxy resin refers in a broad sense to awide variety of polymers characterized intheir simplest form by an oxygen atomlinked to each of two adjacent carbonatoms on a chain, as in ethylene oxide. Theearlier epoxy resins used for metal bondingwere condensation products of bisphenol Aand epichlorohydrin. Curing agents forthese resins were various amines and acidanhydrides. Improvements in the workingcharacteristics of epoxies have been madeover the years and a wide variety of formul-ations are now available. They cure byadditional polymerization with very littlevolume change or shrinkage while theyharden.

An important advantage of epoxy ad-hesives is that they can be formulated to

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meet a variety of use conditions. They areavailable as elevated- and room-tempera-ture-setting; their pot life can be variedfrom a few minutes to an hour or more;they can be used with numerous types offillers; and can be modified with poly-sulfide and natural or synthetic rubber tochange their elasticity. As practically nosolvent or other product is given off duringthe setting of epoxy adhesives, they havevery little shrinkage; thus, they can toler-ate much thicker gluelines and are moregap-filling than ordinary adhesives.

For wood gluing, use of epoxy resins hasbeen limited mostly to such special appli-cations as repair work, sometimes in com-bination with glass fiber for reinforcement.Clean, sanded surfaces have providedbetter bonds for such applications (in theauthor’s experience) than smoothly planedsurfaces. In gluing white oak, Douglas-fir,and Alaska-cedar with a number of com-mercial epoxy adhesives, better resultswere invariably obtained on sawn surfacesthan on smoothly planed surfaces. In short-term soaking tests (vacuum-pressure im-pregnation), epoxy adhesive bonds failedon white oak, but several formulationsshowed promising results on the two soft-woods.

Since epoxy adhesives are available in somany varieties for many different applica-tions, and in consistencies from free flow-ing to thixotropic, close cooperation be-tween producer and user is necessary forbest results.

Contact Adhesives

Contact adhesives are generally based onnatural or synthetic rubber in organic sol-vents. Adhesives of this type based on neo-prene rubber have found wide use for bond-ing plastic laminates to plywood orparticleboard for counter-tops, restaurantand kitchen tables, and similar products.

Generally, both surfaces to be bondedare spread with glue, the solvent is allowedto evaporate, and only contact pressure isrequired to form the bond.

Rubber adhesives are unique in thatthey develop considerable strength imme-diately upon contact of the surfaces to bebonded. Full joint strength, however, de-velops rather slowly, and the ultimatestrength is generally much lower than forordinary woodworking glues.

Emulsion-type rubber-base adhesivesare also available and their performance issimilar to the solvent type in many res-pects. However, the emulsion types haveless resistance to moisture.

Mastic Adhesives

One of the definitions for mastic is “anyof various quick-drying pasty cements usedfor cementing tiles to a wall.” To theauthor’s knowledge, the term “mastic ad-hesive” was first used in connection withwood bonding to describe thick, pasty soy-bean glue for hot-press plywood. Variousadhesives of “mastic” consistency havebeen marketed over the years. Their basicingredient was often rubber, but latelycompositions based on materials such aspolyurethanes, polyesters, silicones, andepoxies have come into use. Mastics aresometimes marketed as “constructionadhesives,” which could be misleadingbecause they generally provide less rigidbonds than commonly used in laminatedtimbers and other structural applications.

Because of their gap-filling properties,they do not require close-fitted joints, andhave apparently performed well in gluingplywood flooring to joists and bonding anunderlayment such as particleboard tostructural plywood floors. Increased stiff-ness and strength have been reported forsuch bonded systems. But long-term dataon the initial benefits gained from suchmastic bonds appear to be lacking formost formulat ions. However , somemastic-type adhesives based on urethaneresin have remained elastic for several yearswhen exposed to weather.

The bond strength of the mastic ad-hesives based on rubber and various syn-thetic materials is generally much lower

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than that of conventional thermosettingwood adhesives, but this would not neces-sarily limit the usefulness of mastics inapplications where high joint strength isnot a prerequisite for good performance.Gap-filling properties and ability to retainresilency over the long term can be highlyimportant.

ADHESIVES OF NATURALORIGIN

Adhesives of natural origin-such asanimal, casein, soybean, starch, and bloodglues-are still being used to bond woodin some plants and shops, but are beingreplaced more and more by synthetics.Animal glue is probably the “natural”adhesive most widely used, althoughcasein glue is being used a great deal forstructural laminating.

Animal Glue

Animal glue is a gelatin adhesive ob-tained from waste or byproducts of themeat processing and tanning industries.The most common raw materials are hidesor trimmings of hides, sinews, and bonesof cattle and other animals. Trimmingsfrom the leather industry (from tannedhides) are also utilized.

Glue made from hides is generally ofhigher grade than glue derived from bonesand tendons. However, there is consider-able variation in the quality or grades ofglue from hides as well as from the othersources. Glues for woodworking, as wellas most other uses, are commonly blendsof two or more batches from the samestock or from different classes of stock.Source is important only insofar as itaffects grade.

Each class of glue is sold in cake, flake,ground, pearl, shredded, and other forms;but the form of the glue is no particularindication of quality. The chief differencebetween the various forms is in the timerequired to put the glue into solution. The

finely divided forms absorb water morerapidly and can be dissolved more easilythan the cake and flake forms. The highergrade glues in the flake form are usuallylight in color and nearly transparent.Lower grade glues tend to be dark andopaque.

Color and transparency, however, arenot dependable indications of quality be-cause low-grade glues are sometimesbleached. Also, foreign substances such aszinc white, chalk, and similar materials arefrequently added to transparent glues toproduce what are technically known asopaque glues. The added materials, whilethey apparently do no harm, do not in-crease the adhesive qualities. Aside fromthe fact that they give an inconspicuousglueline in l ight-colored woods, the“opaque” or whitened glues have no ap-parent advantage over otter glues of thesame grade.

Marked improvements have been madeover the years in the standardization ofmethods for grading animal glues forwoodworking. The definitely establishedtests and specifications give the user ofanimal glues means to insure uniformityand to secure a product suited to hisoperating needs.

PREPARING ANIMAL GLUE FORUSE

In preparing animal glues for use, anumber of precautions must be observed ifsatisfactory results are to be obtained. Theproportion of glue and water should bevaried to meet manufacturing conditions.When the right proportions have beenworked out, they should be used consis-tently. The glue and the water should beweighed out whenever a batch is prepared.Clean, cold water should be used and themixture thoroughly stirred at once to allowa uniform absorption of water by the dryglue and prevent the formation of lumps.The batch should then stand in a coolplace until the glue is thoroughly watersoaked and softened. The soaking may take

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only an hour or two or longer, the timedepending upon the size of the particles.The glue should then be melted over awater bath at a temperature not higher than150° F. High temperature and long, con-tinued heating reduce the strength of ani-mal glue solutions and are to be avoided.The glue pot should be kept covered asmuch as possible to prevent the formationof a skin or scum over the glue surface.

Strict cleanliness should be maintainedfor glue pots and spreading equipment aswell as tables and floors in the glue room.Old glue soon becomes foul and providesa breeding place for bacteria that causedecomposition, exposing the fresh batchesto the constant danger of becoming con-taminated. Glue pots should be washedevery day and only enough glue for a day’srun should be prepared at a time. If thesesimple sanitary precautions are not ob-served, poor joints are likely to result.

STRENGTH AND DURABILITY OFANIMAL GLUE JOINTS

Making uniformly strong joints dependsprimarily upon having the proper correla-tion of gluing pressure and glue viscosityat the moment pressure is applied. Withanimal glue solutions, the consistency de-pends on cooling and drying effects. Forthe first few minutes after the animal gluehas been spread on the wood, the coolingeffect is much more important than thedrying; this temperature-viscosity rela-tionship varies with the grade and with theconcentration of the glue solution. High-grade animal glues thicken to the properpressing consistency quicker and at highertemperatures than do low-grade glues ofequal concentration. Assuming glues ofequal grade, one mixed with less waterwill thicken more rapidly than one mixedwith a greater quantity of water.

Warm animal glues, as they normallyexist in the spreader, are too thin for press-ing and some thickening must occur beforepressure is applied. The best consistency

for pressing exists when the glue is thickenough to form short, thick strings whentouched with a finger, but not thickenough to resist an imprint or a depressionreadily. The thickening time or assemblyperiod is ususally fixed by the operatingconditions that dictate how much timeshall elapse between spreading and press-ing. The grade of glue and the proportionof water added in mixing become, there-fore, the variables by which the manufac-turer can fit the glue mixture to hisoperating conditions. When once estab-lished, the glue grade and proportion ofwater should be adhered to except whentemperature changes in the glue room orwood require a change in the mixture.

When the assembly period is fixed bythe operation, and the temperature in theglue room rises, an adjustment must bemade to accelerate the speed of thickening.This adjustment can usually be made mosteasily by mixing less water with the glue.

Strong joints may be made with a num-ber of grades of animal glue, but differentgluing conditions must be used accordingto the grade of the glue. If wood jointtests are made with glues of differentgrades under a uniform set of gluing condi-tions, the grade of glue that gives the bestresults is the one best adapted to the par-ticular gluing conditions employed. Thejoint test results are not necessarily anaccurate measure of the inherent strengthsof the other grades tested.

With respect to maintaining strengthover the long term, animal glue in three-ply birch plywood joints showed no signi-ficant loss in strength after 5 years’ ex-posure at 80° F. and 65 percent relativehumidity. Cycling of similar specimensbetween 65 percent relative humidity and30 percent relative humidity producedvery little strength loss in the joints. Thisis the approximate change in moisture con-tent that can be expected in interior wood-work in normal use in heated buildings inthe northern part of the United States.In this type of service, properly designedand well-made joints of animal glue should

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give long-term satisfactory performance,particularly if the glued products have areasonably good moisture-excluding finish.Such a finish retards moisture changes andthus reduces the rate of stresses induced byshrinking and swelling.

Furniture and other products glued withanimal glues often serve satisfactorily inspite of occasional exposures to relativehumidities up to 80 percent or more.Protection afforded by the finish usuallyprevents the moisture content of the woodfrom reaching equilibrium values, particu-larly if the exposure to dampness is notprolonged. Degradation of the joints ismore apt to occur with dense, high-shrink-age species than with lighter species thatexert lower stresses on the glue joints.

Casein Glue

Casein glue has been used in Europe forat least a century and in the United Statesfor more than two-thirds of a century. Thebasic constituent of casein glue is driedcasein which, when combined with al-kaline chemicals (usually lime and one ormore sodium salts), is water soluble.

For some uses, the principal require-ments of casein glue are water and moldresistance combined with adequate drystrengths. For other applications, it isdesirable to formulate a less expensivecasein glue that possesses low stainingtendencies, long working life, high drystrength, or good spreading characteristics,even at some sacrifice of water resistance.The glue supplier can produce, therefore,a variety of casein glues of different prop-erties from which the user may chooseaccording to his needs.

PREPARATION OF CASEIN

When milk becomes sour, it separatesinto curd, the chief protein constituent,and whey. The curd, after being washedand dried, is the casein of commerce.When formed in this way, it is known asnaturally soured casein. Casein is also

precipitated by mineral acids, such as hy-drochloric or sulfuric, and by rennet. Inpreparing the glue, caseins precipitated bydifferent methods require differentamounts of water to produce solutions ofsimilar viscosity. Satisfactory glues can beproduced from caseins precipitated by anyof these methods, provided the casein is ofgood quality.

The starting point in the manufacture ofcasein is skim milk-that is, whole milkfrom which the fat has been removed inthe form of cream. The usual steps in themanufacturing process are: (1) Precipita-tion of the casein; (2) washing the curd toremove the acid and other impurities;(3) pressing the. damp curd, wrapped incloth, to remove most of the water; (4) dry-ing the curd; and (5) grinding it to apowder. The care with which these stepsare carried out determines the quality ofthe finished product.

FORMULATION OFCASEIN GLUES

The principal ingredients of a caseinglue are casein, water, hydrated lime, andsodium hydroxide. A properly propor-tioned mixture of casein, water, and hy-drated lime will yield a glue of high waterresistance, but its working life will be veryshort. A glue can also be prepared ofcasein, water, and sodium hydroxide.When properly prepared, such a glue willhave excellent dry strength and a longworking life, but it will not have thewater resistance ordinarily associated withcasein glues. By adjusting the proportionsof sodium hydroxide and lime, glues ofhigh water resistance and convenient work-ing life may be obtained.

Casein glues containing sodium hy-droxide and hydrated lime cannot be mixedin dry (solid) form and shipped. The hy-groscopic properties of sodium hydroxideprevent storing a casein glue containing itwithout danger of decomposition. Thealkali can be introduced in an indirectmanner, however, so that the casein can

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be mixed with all the necessary ingredi-ents, except water, in the form of a drypowder that can be handled and storedconveniently. One way is to replace thesodium hydroxide with chemically equiv-alent amounts of calcium hydroxide and asubstance that, when dissolved in water,reacts with the calcium hydroxide to formsodium hydroxide. Any convenientsodium salt of an acid whose calcium saltis relatively insoluble may be used,provided it is not hygroscopic and doesnot react with the lime or the caseinwhen the mixture is dry.

Prepared casein glues. -Most manufac-turers of wood glues furnish casein gluescontaining the required ingredients inpowder form ready to mix with water.They are prepared for use by merely siftingthem into the proper amount of water andstirring the mixture. They usually containthe essential ingredients of casein, hy-drated lime, and sodium salt, and areoccasionally formulated to reduce staining,hardness, or to impart other properties.Many of the formulas were protected bypatents, most of which are now outdated.Directions for mixing these glues withwater are usually furnished by the manu-facturer

Wet-mixed casein glues. — Some glueusers may prefer to mix the ingredientsdirectly from the basic materials— casein,sodium hydroxide, and lime. Approxi-mately the following proportions ofingredients should be mixed in this order.

lngredientsCasinwaterSodium hydroxidewaterCalcium hydroxide

(hydrated lime)Water

Parts by weight100150

1150

2050

This glue remains usable for some 6 to 7hours at temperatures between 70° and75° F. It is capable of producing joints

that will have good dry strength andwater resistance.

Sodium silicate may be used in place ofsodium hydroxide or in place of drysodium salts, and a glue so prepared willdiffer from one prepared by the formulaabove. Particularly, a much longer work-ing life is obtained with a glue usingsodium silicate and having alkalinityequal to that obtained by the use of sodiumhydroxide or other sodium salts. There is aconsiderable range ofpermissible lime con-tent (above that necessary to react with thesodium silicate); however, the working lifedecreases as the proportion of calciumhydroxide increases.

A small amount of cupric chloride incasein glue has been found effective inincreasing the water resistance. This im-provement is most striking in glues thatdo not contain as much lime as requiredfor optimum water resistance. It is notalways advisable to use the maximumamount of lime because high-lime gluesalmost invariably have a short working life.In such cases, it may be expedient to ob-tain high water resistance by addingcopper chloride rather than the maximumamount of lime.

USE CHARACTERISTICS OFCASE/N GLUE

Casein glue sets as a result of chemicalreaction and loss of moisture to wood andair. Hence, its rate of setting is affected bythe temperature of the wood and surround-ing atmosphere, the moisture content ofthe wood, and other factors. Longer settingtime is required in a cold shop than in awarm one, and wood high in moisturecontent will retard the setting rate.

Casein glue will set at a temperaturealmost as low as the freezing point of water,but the setting period required to developstrong joints at such temperatures variesfrom several days to several weeks. Thetime depends also on the species glued.The wet strength developed at low tem-

30

peratures may never be as good as thatdeveloped at normal room temperatures.A pressing period of 4 hours at 70° F.is considered a minimum for straightmembers; for curved members, a some-what longer period is desirable.

Casein glue will produce adequate bondswith wood at a wide range in moisturecontent— from about 2 to 18 percent. Toavoid serious shrinking or swelling stresseson the joints, however, the moisture con-tent of the wood at the time of gluingshould be slightly lower than the averageexpected in service.

DURABILITY OF CASEIN GLUE

Well-made casein glue joints will de-velop the full strength of most low- andmedium-density woods in shear parallel tothe grain and will retain a large part oftheir strength even when submerged inwater for a few days. With dense woods,however, casein glue develops only me-dium to low wood failure percentageswhen the joints ate tested in shear (fig. 15)

To improve resistance to deteriorationcaused by molds or other micro-organisms,

preservatives ate sometimes added to caseinglues. Federal Specification MMM-A-125gives minimum requirements for water-and mold-resistant casein glues. Prolongedexposure to conditions favorable to moldgrowth or other micro-organisms, how-ever, will eventually result in failure evenin joints made of casein glue containingpreservative.

Outdoors or where high humidities,either continuous or intermittent, ate in-volved, casein glue joints ate not durable.Casein glues containing preservatives haveshown greater resistance to high humidi-ties than have unpreserved caseins, but thepreservative did not prevent eventualdestruction of the ‘glue bonds under dampconditions. Consequently, casein glue isnot considered suitable for glued productsintended for exterior use, or for interioruse where the moisture content of the woodmay exceed 18 percent for repeated orprolonged periods. Voluntary ProductStandard PS 56 for s t ructural gluedlaminated timbers limits casein-gluedmaterial to service where the equilibriummoisture content of the wood does notexceed 16 percent.

M 132 434

F igu re 15 .— Percentage failure in wood of various species glued with carein, urearesin, and phenol-resorcinol resin when joints were tested in block shear (ASTMD 905). With both resins, the joints were about as strong as the wood; with casein,a large percentage of the failures in the denser species were in the glue, indicatingthat the glue bond was the weakest link.

31

M 138 199–4

F igu re 16 .— Building erected in 1935 with casein glued-laminated arches. It is cur-rently used for packaging research at the Forest Products Laboratory. Arches arein excellent condition.

Casein glue joints have demonstrated or mote in the United States (fig. 16).good resistance to dry heat. Results of test In Europe, similar structures that ate muchexposures to temperatures as high as 158° older ate not uncommon. This should beF. for periods up to 4 years have indi- adequate basis for confidence in casein gluecated that the glue bonds in bitch ply- as a structural bonding agent for softwoodwood ate about as resistant as the wood to laminates used under normally dry interiorthis type of exposure. Temperatures that conditions.chat and burn wood cause decomposition In joints where the grain of the piecesof casein glue. Chatted wood exposed to bonded is not parallel, casein glue has notfire, however, conducts heat to its interior performed neatly as well, particularly withvery slowly so that softening of casein dense wood having high shrinkage.glue joints takes place only next to thezone of char. Soybean Glue

Laminated softwood structural mem-bets bonded with casein glue have given Soybean glue was introduced to theexcellent service when protected from ex- plywood industry in the Pacific Northwestterior and damp conditions for 35 years during the early 1920’s, and for many

32

years was the major glue used for makingsoftwood plywood (interior type— no prac-tical exterior glue had yet been developed).The protein constituents of soybean gluethat supply the adhesive properties atesomewhat similar to those of casein.

The basic adhesive material in this glueis the protein from soybeans. The oil isfirst removed from the bean by expeller orsolvent processes. The coarse meal is thenusually passed through a rol ler mil l(smooth tolls) to crush the shell loose fromthe kernel. The kernel is ground to thedesired particle size, usually in a hammer-mil l . The f lout is mixed with smallamounts of chemicals and is then ready forshipment (usually in 100-lb. bags) to theplywood plant.

Soybean glue almost always is used as awet-mix glue. Usually, the glue powder isfirst mixed with sufficient water to make asmooth dough free of dry lumps. Thenadditional water is added slowly with themixer running. If the requited additionalwater is added all at once, the doughmight break up into lumps, making itnearly impossible to obtain a final smoothmixture. Slaked lime, caustic soda solu-tion, and sodium silicate ate usually addedto the mix in that order with short periodsof mixing between the addition of eachingredient. To prevent or reduce foaming,a small amount of pine oil or other de-foaming agent is usually added to the gluemix.

Directions for mixing and the amountof each ingredient to be added ate furn-ished by the glue manufacturer.

Softwood plywood well glued with soy-bean glue is toughly comparable in waterresistance to bitch or similar density ply-wood bonded with casein glue, and isgenerally considered satisfactory for nor-mally dry interior service. Soybean gluehas not proved entirely satisfactory forgluing hardwoods, particularly the denserones.

Soybean glue is generally not recom-mended for hardwood plywood; if both

casein and soybean glues ate used for mak-ing plywood of the same species, soybeanglue generally shows the poorer waterresistance. However, it is appreciablysuperior to starch glues in resistance tomoisture and high humidity.

As the standards for performance ofplywood have become stricter over theyears, it increasingly has become commonpractice to fortify soybean glue with acertain amount of dried blood or occa-sionally casein— casein if the plywood iscold pressed and blood if it is hot pressed.Since softwood plywood is being producedmote and mote by hot pressing, the blood-fortified soybean glues ate predominant.

Blood Glue

Glues made of soluble dried blood orblood albumin have been used to someextent in the United States, but they atemote common in some European andAsiatic countries.

Blood albumin, a slaughterhouse by-product, coagulates and sets firmly whenheated to a temperature of about 160° F.It then shows a significant resistance to thesoftening effect of water. This character is-tic makes it a desirable material for glueto use in products such as plywood.

A number of patents have been grantedon glue formulations based on blood. Aswith other protein glues, alkalies such ascaustic soda, hydrated lime, sodium sili-cate, or combinations of these ate em-ployed in formulating blood glues. Ther-mosetting resins (usually phenolic) are alsosometimes incorporated to increase theresistance of the glue bonds to degradinginfluences.

Hot-press blood glues ate probably themost resistant of the protein-type glues toweathering and similar severe service butate not recommended for long-termexterior use as ate the phenols and resor-cinols and some other synthetics.

33

SELECTED REFERENCES Kreibich, R. E., and Freeman, H. G.1965. Testing adhesives for creep can provide

data on adhesive systems which will helpimprove structural bondants. Adhes. Age

American Society for Testing and MaterialsStandard method of test for strength prop-

erties of adhesive bonds in shear by com-pression loading. Des. D 905. (See currentedition) Philadelphia, Pa.

Blomquist, R. F.1964. Durability of fortified urea-resin glues

exposed to exterior weathering. For. Prod.J. 14(10):461-466.

Blomquist, R. F., and Olson, W. Z.1957. Durability of urea-resin glues at

elevated temperatures. For. Prod. J.7(3):266-272.

Carlson, Herbert E.1962. Bonding with hot melts. Adhes. Age

5(11):32-33.Carroll, M. N., and Bergin, E. G.

1967. Catalyzed PVA emulsions as woodadhesives. For. Prod. J. 17(11):45-50.

Carruthers, J. S.1958. The comparative durability of assem-

bly glues in England and Nigeria. Dep. ofSci. and Ind. Res., For. Prod. Res. Lab.,Princes Risborough, England

Cheo, Y. C.1969. Hot melt coatings for wood products.

For. Prod. J. 19(9):73-79.Cone, Charles N.

1959. Resin-blood glue and process ofm a k i n g t h e s a m e . U . S . P a t . N o .2,895,928. 8 p. July 21.

Fotsyth, Robert S.1962. New developments in synthetic resin

hot melts. Adhes. Age 5(8):20-23.Freeman, H. G., and Kreibich, R. E.

1968. Estimating durability of wood adhe-sive in vitro. For. Prod. J. 18(7):39-43.

Gillespie, R. H., Olson, W. Z., and Blom-quist, R. F.

1964. Durability of urea-resin glues modi-fied with polyvinyl acetate and blood. For.Prod. J. 14(8):343-349.

Hartman, Seymour1970. High temperature adhesive. For.

Prod. J. 20(12):21-23.Kopyscinski, Walter, Norris, F. H., andHerman, Stedman

1960. Synthetic hot melt adhesives. Adhes.Age 3(5):31-36.

Kreibich, R. E., and Freeman, H. G.1970. Effect of specimen stressing upon

durability of eight wood adhesives. For.Prod. J. 20(4):44-49.

8(8):29-34.Lee, Henry, and Neville, Kris

1967. Handbook of epoxy resins. McGraw-Hill Book Co., New York.

McGrath, J. J.1967. Hot melts featured at adhesive meet-

ing. For. Prod. J. 17(4):22.Northcott, P. L., and Hancock, W. V.

1966. Accelerated tests for deterioration ofadhesive bonds in plywood. Durability ofAdhesive Joints, Spec. Tech. Publ. No.401. 62-79. Am. Sot. Test. Mater.,Philadelphia.

Olson, W. Z.1955. Polyvinyl-resin emulsion woodwork-

ing glues. For. Prod. J. 5(4):219-226.Olson, W. Z., and Blomquist, R. F.

1962. Epoxy-resin adhesives for gluingwood. For. Prod. J. 12(2):74-80.

Picotte, Gordon L.1965. “Toughness” of structural adhesives.

Adhes. Age 8(2):21-23.Rundle, V. A.

1969. Hot-melt coatings for wood products.For. Prod. J. 19(9):73-80.

Selbo, M. L.1969. Performance of southern pine plywood

during 5 years’ exposure to weather. For.Prod. J. 19(8):56-60.

Selbo, M. L.1965. Performance of melamine resin adhe-

sives in various exposures. For. Prod. J.15(12):475-483.

Selbo, M. L.1958. Curing rates of resorcinol and phenol-

resorcinol glues in laminated oak mem-bets. For. Prod. J. 8(5):145-149.

Selbo, M. L.1949. Durability of woodworking glues for

dwellings. Proc. For. Prod. Res. Soc.3:361-380.

Selbo, M. L.1949. Glue joints durable in beams lami-

nated of common lumber. South Lumber-man 179(2244):60-62.

Selbo, M. L., Knauss, A. C., and Worth, H. E.1965. Glulam timbers show good perform-

ante after two decades of service. For.Prod. J. 15(11):466-472.

Selbo, M. L., and Knauss, A. C.1958. Glued laminated wood construction

in Europe. Proc. Am. Soc. Civil Eng.84(ST 7). 19 p. Nov.

34

Selbo, M. L., Knauss, A. C., and Worth, H. E.1966. Twenty years of service durability of

pressure-treated glulam bridge timbers.Wood Res. News 44(3):5-16, Part I;44(4): 10-14, Part II.

Selbo, M. L., and Olson, W. Z.1953. Durability of woodworking glues in

different types of assembly joints. For.Prod. J. 3(5):50-57.

Simpson, W. T., and Soper, V. R.1970. Tensile stress-strain behavior of flexi-

bilized epoxy adhesive film. USDA For.Serv. Res. Pap. FPL 126. 13 p. U.S.Dept. Agric. For. Serv. For. Prod. Lab.,Madison, Wis.

Skeist, Irving1964. Modern structural adhesives for use

in the building industry. Adhes. Age7(4):21-26.

Troughton, G. E.1967. Kinetic evidence for covalent bonding

between wood and formaldehyde glues.Inf . Rep. No. VP-X-26, 22 p . For .Prod. Lab., Vancouver, B.C.

Truax, T. R., and Selbo, M. L.1948. Results of accelerated tests and long-

term exposures on glue joints in laminatedbeams. Trans. Am. Sot. Mech. Eng.70:393-400. May.

Twiss, Sumner B.1965. Structural adhesive bonding. Part II:

Adhesive classification. Adhes. Age8(1):30-34.

U.S. Department of Commerce1973. Structural glued laminated lumber.

Voluntary Prod. Stand. PS 56-73. Natl.Bur. Stand., Washington, D.C.

U.S. Forest Products Laboratory, Forest Service1967. Casein glues: Their manufacture,

preparation, and application. U.S. For.Serv. Res. Note FPL-0158.15 p. U.S.Dept. Agric., For. Serv. For Prod. Lab.,Madison, Wis.

U.S. Forest Products Laboratory, Forest Service1963. Durability of water-resistant wood-

working glues. Rep. 1530. 41 p. U.S.Dept. Agric. For. Serv. For. Prod. Lab.,Madison, Wis.

U.S. General Services Administration1969. Adhesive, casein-type water and mold

resistant. Fed. Specif. MMM-A-125.Fed. Supply Serv., Washington, D.C.

Williamson, D. V. S.1965. Hot melt adhesives in Europe. Adhes.

Age 8(8):24-27.Williamson, F. L., and Nearn, W. T.

1958. Wood-to-wood bonds with epoxideresins-species effect. For. Prod. J. 8(6):182-189.

IMPROVING PERFORMANCEOF WOOD THROUGH GLUING

To produce high-quality, adhesive-bonded wood products, it is not only im-portant to know and understand the prop-erties and use characteristics of theadhesive, it is equally important to know bowto select, prepare, and use the wood so that itwill serve to the best advantage.

Wood has good stability and excellentstrength properties in the grain direction.Across the grain it is stable at constantmoisture content but shrinks with de-creases in moisture and swells with in-creases in moisture. Normal straight-grained wood of some species may alsosplit or check along the grain when sub-jected to rapid reductions in moisturecontent.

Both resistance to splitting and uni-formity in strength properties can be im-proved by gluing together sheets or layersof wood with the grain in adjacent layersat approximately 90° (plywood).

By gluing together layers all having thegrain approximately parallel (laminating),the strength in bending and in tension inthe direction of the grain can be improved.Two-by-fours laminated from low-gradeveneers can be produced with much moreuniform strength properties than solidstructural 2 by 4’s. This is accomplishedby dispersion of strength-reducing defects.

By end-joint gluing, material of anydesired length can be obtained; and byedge gluing, any desired width is obtain-able.

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CROSSBANDEDCONSTRUCTION

Crossbanded construction includes alarge variety of panel products consistingusually of an odd number of layers of woodglued together with the grain in adjacentlayers at an angle of about 90°. Ply-wood, the most common form of cross-banded construction, is defined as acrossbanded assembly made of layers ofveneer or veneer in combination withlumber or other core materials and joinedwith an adhesive. The plywood construc-tions probably most widely used are veneerplywood and lumber core plywood. Theterm “veneered panels” is often used forlumber core plywood. A veneered panelcould also have a particleboard core (fig.17), with or without crossbands. Flushdoors are generally of crossbanded con-struction.

The bulk of softwood plywood produc-tion goes to structural applications

M 138 529

Figure 17.— Three- and five-ply veneeredpanels with particleboard cores.

(housing, concrete forms, packaging).Hardwood plywood goes to furniture,paneling, and other uses. Several types ofplywood are shown in figures 18, 19, and20. Veneer plywood is most commonlymade and used as flat panels (figs. 18 and19). It may also be made as flat panelsand later bent or formed within limits asmay be required (fig. 20, A). Plywoodwith sharp or compound curves may beformed by pressing the veneers (coatedwith glue) in molds of the desired shapeand curing the glue with radio-frequencyenergy or other types of heat. As a rule,curved plywood formed to the desiredshape during manufacture is more stablethan plywood bent to form later.

Most plywood is three or five ply. Inthree-ply construction, the two outsideplies are called faces, or face and back,and are usually laid at right angles tothe grain of the center ply or core (fig.18, A). In five-ply panels the outside pliesare also called faces, or face and back, andthe center ply is the core (fig. 18, B). Thesecond and fourth plies are termed thecrossbands and are usually at right anglesto the grain of the face, back, and core.Plywood construction other than three-or five-ply may be used, but odd numbersof plies are generally symmetricallyarranged on each side of the core. Thecore may be veneer, lumber, or variouscombinations of veneer or lumber lamin-ated so that they act as a single ply (fig. 19,A, B, C, D). In recent years it has beenfound advantageous to make panels of fourveneers. The two center ones are gluedtogether with the grain parallel and serveas the core in a three-ply panel. In asimilar manner three or more laminatedveneers could serve as core. Panels mayrange in total thickness from less thanone-eighth inch to more than 3 inches.They may vary in number and thicknessof plies, kinds and combinations of woods,and durability of the glues required forvarious service conditions.

The chief advantages of plywood as com-pared with solid wood are: (1) Greater

3 6

M 138 743

Figure 18.— Three constructions of all-veneer plywood: A, three-ply; B, five-ply;and C, seven-ply.

M 138 746

Figure 19.— Three different constructions of lumber core plywood and one with lam-inated veneer core: A, three-ply; B, five-ply, with veneer edge banding; C, seven-ply, with laminated veneer core (used where showthrough must be avoided andgood stability is required); and D, extra thick lumber core plywood. Crossband atmiddle of core reduces dimensional changes of core.

37

M 138 744

Figure 20.— Three types of curved plywood: A, conventional five-ply, all-veneerplywood bent to form; B, laminated core with veneer crossbands and faces; andC, five-ply, spirally wrapped plywood tubing.

resistance to splitting and checking,(2) more nearly equal strength propertiesalong the length and width of the panel,(3) dimensional changes with changes inmoisture content that are more nearlyequal in length and width and distinctlyless than the changes of solid lumber inwidth, and (4) the plywood productionprocesses utilize wood more efficiently.

These advantages are present becausethe direction of the grain of each ply isgenerally at right angles to that of adjacentplies. The strength of a piece of lumber orveneer along the grain is much greaterthan the strength across the grain. Whenpieces are glued together with the direc-tion of their grain at right angles, the highstrength and dimensional stability of eachpiece along the grain resist the stresses andmovement of adjacent pieces across thegrain, and the strength and stability of thepanel in the two directions are, in effect,equalized. The result is a more nearlyhomogeneous product than solid wood.Because of the cross plies, plywood panelsare very resistant to splitting in planes at

right angles to the plies. Along planesparallel to the plies, the splitting charac-teristics resemble those of solid wood.

The glued, crossbanded product, there-fore, is more nearly constant in width andlength under varying moisture conditions.It is not necessary that the cross plies bethick or that they occupy a very large partof the total thickness of the crossbandedproduct. In a five-ply veneered panel witha core of nominal l-inch lumber and faceand back of 1/28-inch veneer, for example,the crossbands are frequently of 1/20-inchveneer. The choice of thickness depends onthe tensile strength of the species. Thecrossbands must be sufficiently strong intension parallel to grain to withstand,without breaking, the stresses developedby the core when it tends to expand orcontract as the moisture content changes.

To realize fully the advantages of cross-banded construction, the panels must beproperly designed and glued. The ten-dency of panels to cup and twist may begreater in improperly constructed plywoodthan in the average panel of solid wood of

3 8

the same thickness. Crossbanded panelsmay be considered to be relatively freefrom stress at the time the glue sets.When the moisture content changesthereafter, however, adjoining plies try toshrink or swell in directions at right anglesto each other but each ply restrains the plyor plies next to it. Since the moisture in thepanel during service is rarely distributedas it was when the glue set, plywoodpanels may be considered as continuallyunder stresses that tend to rupture the gluejoints or to distort the panel. The furtherthe moisture content departs from that ex-isting when the glue set, the greater willbe the stresses developed. The develop-ment of these stresses cannot easily beprevented but their magnitude and effectcan be largely controlled by choice ofspecies, proper design, well-glued joints,and control of moisture content at the timeof gluing.

In crossbanded products that are prop-erly designed, the forces exerted by theplies on one side of the core balance inmagnitude and in direction the forces ex-erted by the plies on the other side of thecore. This balance is partly accomplishedby the use of an odd number of plies soarranged that for any ply on one side of thecore there is a corresponding parallel plyon the other side at the same distancefrom the core.

In addition to being correctly spacedfrom the core, the wood in correspondingplies should have the same shrinkage anddensity properties to obtain a balancedeffect. The shrinkage of wood varies withthe species and the method of cutting,and the stresses developed vary with thedensity. In some cases, the difference inshrinkage between edge-grained wood andflat-grained wood of the same stock isgreater than between similarly cut wood ofdifferent species. Consequently, flat-grained and quartered material of the samewood may not balance so closely as woodsofdifferent species. For best results, there-fore, corresponding plies should be of the

same species, of similar density, and cutin the same manner.

Since the outer plies of a crossbandedconstruction are restrained on only oneside, changes in moisture content inducerelatively larger stresses in the outer gluejoints. The magnitude of stresses dependsupon such factors as thickness of plies,density, shrinkage of the woods involved,and the amount of the moisture contentchanges. In general, one-eighth inch is themaximum thickness of face plies that canbe held securely in place when moderatelydense woods are used and large moisturechanges occur. For panels where facechecking would be objectionable, such asin doors and furniture, thin face veneers(1/28 in. or 1/32 in.) are preferable tothicker ones.

Quality of flies

In thin plywood, the quality of all theplies affects the shape and permanence ofform of the panel. For greatest stability allplies should be straight grained, smoothlycut, and of sound wood that is of uniformgrowth and texture.

In thick, five-ply (lumber core) panelsthe crossbands in particular affect thestability and quality of the panel. Imper-fections in the crossbands, such as markeddifferences in the texture of the wood,irregularities in the surface, or even pro-nounced lathe checks, may show throughthin face veneers as imperfections in thesurface of the panel.

Figured veneer cut from burls, crotches,stumps, and similar irregular material isnot straight grained but is used because ofits attractive appearance. It shrinks bothwith the width and length of the sheet,whereas plain veneer shrinks chiefly inwidth. This difference in shrinkage be-tween the two types of veneer causes warp-ing when they are used as opposing pliesin thin panels. With combinations ofplain and figured veneer, it is not practicalto have a strictly balanced construction andthe effect of the unsymmetrical arrange-

3 9

ment must be compensated for in someother way. Ordinarily, by laying figuredveneer over a thick and properly cross-banded core, the construction is made stiffenough to prevent the unbalanced stressesexerted by the thin faces from excessivelywarping or distorting the panel. Thick,five-ply veneered panels (fig. 19, B and C)make it possible to use a figured veneer onthe panel face and a straight-grained veneeron the back.

Causes and Prevention ofWarping

In a panel that is symmetrical andbalanced about its central plane, opposingplies must have about the same moisturecontent when glued. Variations in mois-ture content of corresponding plies at thetime of gluing bring about shrinkagedifferences that may result in warping.Large changes in the moisture content ofthe wood after gluing should be avoidedbecause they induce internal stresses oflarge magnitude that could cause warping,checking, and weakening of joints.

Warping may be expected if the panelcontains veneer that is partially decayed, orveneer that has abnormal shrinkagecharacteristics, such as exhibited by com-pression wood or tension wood.

Cross grain or short grain that runssharply through the crossband veneer fromone surface to the other often causes thepanels to cup. Cross grain that runsdiagonally across the crossband veneer islikely to cause the panel to twist unlessthe two opposing crossbands are laid withthe grain parallel to each other. Failure toobserve this simple precaution is the causeof much warping in crossbanded construc-tion. While it is impractical to eliminateall crossgrained veneer, that showing anexcessive amount of cross grain should berejected for most plywood manufacture.It can be used for purposes where itseffect is not harmful (cabinet backs, forexample).

For certain types of panels that are heldsecurely in place by mechanical means,tendencies toward warping might be un-important. For others, such as lids anddoors that generally are mechanicallyfastened at one edge, even a small amountof warp might be objectionable. In panelproducts, twisting and cupping are themost common types of warping.

TWISTING

Corresponding plies on opposite sides ofthe core should not only swell or shrinkin the same direction, but the stressesshould be of the same magnitude. If thestresses are not balanced with respect todirection, the distortion that resultsfrequently is twist, a form of warpingin which the four corners of the panel willnot rest simultaneously on a flat surface.

Generally, twisting in plywood is re-lated to grain direction. While factorsother than grain direction can cause twist-ing, they are not so frequently encounteredin practice. If plywood or veneered tops,unattached to supporting members, aretwisted, it is most likely that grain direc-tion of the panel plies is at fault.

In five-ply veneered panels with com-paratively thick cores and thinner cross-bands and faces, the crossbands are themost essential element in maintaininga panel free from twist. In this construc-tion, the grain of the crossband on one sideshould be parallel to the grain of thecrossband on the other side of the core.The amount of variation from this condi-tion that may occur without twistingdepends upon such factors as thickness ofcore, density of core, moisture content ofthe core at the time of gluing, andchange in moisture content after gluing.With thin, experimental panels, a varia-tion of 5° in grain direction of opposingcrossbands has caused distinct twisting.Examination of commercial, five-plyveneered panels has often disclosed pro-nounced twisting with a variation of 15°.

40

While the crossbands of five-ply con-struction are usually the critical elements,the faces are critical in three-ply construc-tion. If the crossbands of five-ply, thickcore construction are properly laid, varia-tions in the direction of grain of the facesseldom cause objectionable distortion. Infive-ply, thin core construction, however,parallel grain is important in the facesas well as in the crossbands.

Ordinarily, the causes of twisting areeasily detected. To avoid or reduce twist-ing that occurs when grain direction ofplies is not parallel, changes in manufac-turing procedure are usually required. Oneof the simplest and least costly methodsof reducing twisting is to select for cross-bands such species as basswood, aspen, andyellow-poplar that generally producereasonably straight-grained stock. If thevalue of the product justifies the addedcost, the veneer should be clipped andtrimmed parallel and perpendicular tothe grain rather than parallel and per-pendicular to the axis of the veneer bolt.Since adjacent pieces of sliced veneer arevery similar in grain formation, twistingmay be reduced or avoided by using twoadjacent pieces of sliced veneer for the twocrossbands of one panel. They must be laidwith the grain parallel to each other inthe panel. The same principle could beapplied to rotary-cut veneer for crossband-ing by properly marking and arranging theveneer as it comes from the lathe to insurethat matching sheets would be used forthe two crossbands of a panel. Such pre-cautions, however, may or may not bepractical in a commercial operation, de-pending on the cost of the final product.

If a panel changes in moisture contentto a marked degree at the edge while thecenter changes very little, the stresses de-veloped may cause twisting. This condi-tion can be detected by determination ofthe moisture content at the edges and atthe center of the panel. Much of the twist-ing will probably disappear when the panelis reconditioned to a uniform moisturecontent. Twisting has also been observed

when plywood panels were fastenedrigidly (particularly if glued) to supportingmembers or frames whose shrinkagecharacteristics differed from those of theplywood panel.

CUPPING

Ordinarily, the exact cause of cuppingis much more difficult to establish thanthe cause of twisting. However, cuppingdifficulties are often more easily elimi-nated in commercial operations than arethe causes of twisting.

Cupping generally results from forcesthat restrain the core unequally on the twosides. If, for example, a crossband wasglued to only one side of a core, the corewould be greatly restrained on one sidein its movements with moisture changesbut not at all on the other side, and cup-ping would surely result. The direction ofthe cupping would depend on whether thecore increased above or dried below themoisture content it had when the glue set.If the crossband on one side differs dis-tinctly in shrinkage characteristics orstrength properties (along the grain) fromthe corresponding crossband on the otherside, cupping may be expected. A fewof the more common causes are:

1. Crossband on one side thicker thanon the other. When the core attempts tochange dimensions under changes in mois-ture content, the movement of the corewill be restricted more strongly on theside with the thicker crossband (assumingboth crossbands ate of the same or similarspecies) and cupping will result. Unequalsanding of the faces of a three-ply panelproduces an effect similar to that causedby the use of faces of unequal thickness.Minor variations in the thickness of thinfaces on five-ply, lumber-core panels willnot ordinarily cause objectionable dis-tortion.

2. Short-grained crossband on one sideand a straight-grained crossband on theother. When the grain of a sheet of

41

veneer dips abruptly through the sheetfrom one surface to the other, the sheetwill have greater shrinkage in length thana sheet in which the grain is parallel to theplane of the sheet. If such a short-grainedsheet is laid as one crossband and astraight-grained sheet as the opposingcrossband, the short-grained sheet will notoffer the same resistance to the movementof the core as the straight-grained one andcupping will result.

3. Partially decayed crossband on oneside and a sound crossband on the other.When the core shrinks or swells undermoisture changes, the decayed crossbandoffers less resistance to the dimensionalchanges of the core than the sound cross-band and cupping results.

4. Reaction wood in one crossband andnormal wood in the other. One of thecharacteristics of reaction wood is a highdegree of longitudinal shrinkage as com-pared with normal wood. If a sheet ofveneer containing reaction wood is laid asone crossband and a sheet of veneer ofnormal wood as the other, the sheet con-taining reaction wood will tend to shrinkor swell longitudinally while the corres-ponding crossband of normal wood willtend to remain more nearly fixed in thelengthwise direction. Consequently, thecore will be restrained unequally on thetwo sides and cupping will result. If acrossband contained both reaction woodand normal wood, distortion in the formof combined twisting and cupping mightresult.

In exterior flush doors it is conceivablethat longitudinal shrinkage of the innerface and longitudinal swelling of the outerface, during the cold season, could alsocontribute to cupping.

HANDLING ANDFABRICATION

It is quite possible that well balancedand properly constructed plywood willwarp because of methods used in handling

or storing the plywood or because of theway in which the plywood is built into thefinished item.

One rather common cause of warpingthat is not related to grain direction orquality of plies is permitting plywood todry (or to regain moisture) more rapidlyfrom one side than from the other. It hasbeen observed frequently that the top panelof a pile of panels will be warped because itdried more rapidly from the upper surfacethan from the bottom. When panels arepiled solidly, the top of the pile shouldbe kept covered to prevent rapid loss orregain of moisture by the top surface. Ifconsiderable change in moisture content isexpected, it is often desirable to protectthe ends and edges of the panels from rapidchanges in moisture content.

When a finish that is highly resistant tothe passage of moisture is applied to oneside of the panel and either no finish or onelow in resistance to moisture movement isapplied to the other, moisture will move inor out of one surface more rapidly than theother. In this case, cupping may resultjust as when panels ate allowed to drymore rapidly from one surface than fromthe other.

While plywood shrinks and swellsmuch less than normal wood does in eitherthe tangential or radial direction, itshrinks and swells more than normal wooddoes in a longitudinal direction. A ply-wood panel that is fastened firmly to alongitudinal supporting member, there-fore, may warp or pull loose from thefastenings under severe changes in mois-ture content. If the design requires afastening between plywood and framingmembers, provision should be madewherever possible to permit a slight move-ment of the plywood relative to thesupporting member just as a solid tabletopis ordinarily fastened to the frame to permita slight swelling or shrinking of the top.Softwood plywood glued to studs in wallsof houses or mobile homes usually doesnot change enough in moisture content tocause any warping problems.

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REQUIREMENTS FORCROSSBANDS

If the flatness of plywood is an im-portant consideration, as often is the casewith furniture plywood, one of theessential requirements of good crossband-ing veneer is straightness of grain. Of thestraight-grained species that have beenavailable in quantity and sizes suitable forveneer cutting operations, yellow-poplarhas been a favorite.

If the crossbanding is to be laid underthin face veneers, it should be uniform intexture and free from defects. Species thatshow marked contrast between the early-wood and latewood, such as Douglas-firand southern pine, are less desirable thanthose in which the contrast between early-wood and latewood is slight, such as bass-wood, aspen, and yellow-poplar. Thedefects that can be permitted in thecrossbanding depend upon the thicknessof the face veneer and upon the quality offinish demanded. A high-gloss finish willaccentuate minor surface irregularitiesmuch more than a matte finish. If thethickness of the face veneer is about onetwenty-eighth inch, as often used, and ifa finish that shows no irregularities underreflected light is desired, the crossbandingmust be essentially free from defects andthe edge joints must be tightly glued.

When the plywood panels are thin (one-fourth inch or less, they can be more easilydistorted by the shrinking or swelling ofthe crossbands, and low shrinkage charac-teristics and low specific gravity of cross-bands become desirable properties. Forlumber core panels, the specific gravityand the shrinkage characteristics of thecrossbands are probably less important solong as one crossband balances the otherand stresses do not cause rupture of ad-jacent glue bonds.

The limited supply of species that pos-sess all or nearly all of the desirable charac-teristics for crossbanding necessitates theuse of less desirable species. Sweetgumand tupelo, for example, are frequently

used for crossbanding although they arenot so inherently straight in grain as mightbe desired. Birch and maple have been usedalthough they are comparatively high inspecific gravity and somewhat irregular ingrain direction. Even though the lessdesirable species are used for crossbanding,satisfactory items can be produced if thecharacteristics of the species are recognizedand the operating procedures adjusted tocompensate for some of the deficiencies.For instance, a thinner veneer of hightensile strength can be substituted for athicker one lower in strength.

Table 2 shows the average shrinkage anddensity values of some woods commonlyused for plywood and veneered panels.Shrinkage data for quartered (radial) androtary-cut (tangential) stock are shownsince some species are manufactured andused extensively in both forms. The tablepermits selection of species that have aboutthe same density and percentage ofshrinkage. Differences in density betweentwo woods can be compensated for byvarying the thickness of the plies in in-verse proportion to their specific gravities.This method of compensation results inusing a proportionately thicker ply of thelighter species, which might be advan-tageous in some cases. The practice,however, requites thorough knowledge ofthe properties of the wood and is notcommon.

REQUIREMENTS FOR CORES

A high percentage of core total ply-wood thickness helps maintain a flat, un-warped surface. In general, the core shouldcomprise five- to seven-tenths of the totalthickness of a five-ply panel where flatnessis important.

When crossbands and face veneers arerelatively thin, the cores for high-gradepanels must be practically free from knots,knotholes, limb markings (local areas ofcross grain occurring in the region ofknots), and decayed wood. Unless re-

43

Table 2 — Average shrinkage and density values of wood commonly glued1

Common species name Shrinkage2 Density 3

Radial Tangential

HardwoodsAlder, red . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Ash, white . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Aspen, quaking . . . . . . . . . . . . . . . . . . . . . . . . . . . .Basswood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Beech . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bitch, yellow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Cottonwood, eastern . . . . . . . . . . . . . . . . . . . . . . .Elm, American . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mahogany (Swietenia sp.) . . . . . . . . . . . . . . . . . .Maple, red . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Maple, sugar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Oak, northern red . . . . . . . . . . . . . . . . . . . . . . . . .Sweetgum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Sycamore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Tupelo, black . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Tupelo, water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Walnut, black . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Yellow-poplar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SoftwoodsDouglas-fir, coast . . . . . . . . . . . . . . . . . . . . . . . . . .Fir, white . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Hemlock, western . . . . . . . . . . . . . . . . . . . . . . . . .Latch, western . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Pine, ponderosa . . . . . . . . . . . . . . . . . . . . . . . . . . . .Pine, shortleaf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Pine, sugar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Redwood, old-growth . . . . . . . . . . . . . . . . . . . . .Spruce, Sitka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Percent Percent

4.4 7.34.9 7.83.5 6.76.6 9.35.5 11.97.3 9.53.9 9.24.2 7.23.7 5.14 . 0 8.24.8 9.94 . 0 8.65.3 10.25.0 8.45.1 8.74.2 7.65.5 7.84 . 6 8.2

0.41.60.38.37.64.62.40.50—.54.63.63.52.49.50.50.55.42

4.8 7.6 .483.3 7.0 .394.2 7.8 .454.5 9.1 .523.9 6.2 .404 . 6 7.7 .512.9 5.6 .362.6 4 . 4 .404.3 7.5 .40

1Data from Wood Handbook.2Shrinkage from green to overdry condition expressed in percent of dimensions when green.3Density expressed as specific gravity based on ovendry weight and volume at 12 pct. moisture content.

moved, such defects may be visible on thefaces of panels after they have received afinish. The size of defects that may occurin cores without showing upon the finishedfaces depends largely upon the thicknessof the crossband and face veneers. Prevail-ing commercial practice varies as to themaximum size of knots and blemishespermitted, depending in part upon thequality of finish demanded from the itemand in part on how readily minorirregularities in the surface may be de-tected during the common use of the item.A veneered tabletop, for example, is often

viewed in reflected light that causes even aminor irregularity in the surface to showclearly; on the sides and ends of the samepiece, minor irregularities in the surfacemay be much less conspicuous. Decayedwood has a different shrinkage rate thansound wood and under moisture changesthis shrinkage difference may causenoticeable irregularities on highly finishedsurfaces.

Edge-grained cores are better than flat-grained cores because of their lower shrink-age in width. With most species a coremade of all quartersawed or of all-flat-

44

able for cores. As a general rule, however,the less desirable the species, the morecare will be required to fabricate satisfac-tory panels.

LAMINATED CONSTRUCTION

sawed material remains more uniform inthickness with moisture content changesthan one made by combining these twotypes of material. Use of narrow core stripsmakes surface irregularities caused bymixed grain (flat sawn and vertical) lessobvious. For high-grade cores of softwoodit is desirable to use quartersawed stock.If both edge- and flat-grained material oreven all flat-grained lumber is used insoftwood cores, the panels are mote likelyto show wavy and irregular surfaces than ifthe stock is all edge grained. Edge-grainedmaterial is more desirable than flat-grainedfor softwood core stock for the additionalreason that the hard bands of latewood areless likely to show through thin face ven-eers, Mixing species in the same coreinvites irregularities in the surface becauseof the differences in the shrinkage charac-teristics of the different species. It is im-portant, of course, that all pieces in anyone core be at the same moisture contentwhen the core stock is surfaced. Otherwise,when the moisture content of the stockequalizes in service, some pieces will swellor shrink mote than others and irregulari-ties in the surface will result.

The best core woods for high-gradepanels have low density, low shrinkagecharacteristics, slight contrast between theearlywood and latewood, and are easilyglued. Yellow-poplar and basswood as wellas several foreign species are desirable corewoods. Edge-grained redwood is verysatisfactory. Sweetgum and tupelo are usedextensively for cores. Ponderosa pine isused extensively for cores in doors. Suchcores are commonly built from smallpieces of wood which are byproducts of themanufacture of sash and other millworkand are faced with thick veneers. Pon-derosa pine, because of its lower densityand shrinkage characteristics, is less apt tocause showthrough than denser species.

Many other species are used successfullyfor core stock even though they may lacksome of the most desirable properties.Satisfactory panels can be fabricated evenif the most desirable species are not avail-

Glued-laminated or parallel-grainconstruction, as distinguished fromplywood or other crossbanded construc-tion, refers to two or more layers of woodjoined with an adhesive so that the grainof all layers or laminations is approximatelyparallel. The size, shape, number, andthickness of the. laminations may varygreatly. Glued-laminated constructionmay be used as a base or core for veneer,as in doors or tabletops and other furniturepanels, or it may be used without veneercovering for chair seats, tabletops, bleacherseats, and bench tops, structural beamsand arches (fig. 21), boat timbers, lamin-ated decking, airplane propellers, spars,spar flanges, or for sporting goods itemssuch as baseball bats and bowling pins(fig. 22).

While the properties of glued-laminatedproducts are generally similar to those of

M 118 468

Figure 21.— Model of laminated archesused in churches, gymnasiums, super-markets, and similar structures.

45

M 138 358Figure 22 .— laminated bowling pin (un-

finished).

solid wood of similar quality, manufac-ture by gluing permits production of long,wide, and thick items out of smaller andless expensive material and often with lesswaste of wood than if solid wood alonewas used. Curved members may be fabri-cated by simultaneously bending andgluing thin laminations to shapes thatwould be very difficult or impossible toproduce from solid wood. The essentiallyparallel direction of grain of the woodto the longitudinal axis of these laminatedproducts gives them strength that is oftenfar superior to solid wood cut to thesame size and shape. Boat timbers, for ex-ample, are laminated in sizes that are no

longer available in solid timbers (fig. 23).The manufacture of glued-laminatedbeams, arches, and trusses has become avery important segment of the wood in-dustry. Glued structural members thatmay be used in full exposure to weatheras well as those intended for dry serviceare being produced. Laminated utilitypoles for power transmission lines arefurther examples of expanding applicationof the laminating technique. It seemsprobable that increasing scarcity of largetimbers will lead to further expansion inthe use of laminated products even throughproduction of laminated items involvesskills and equipment not necessary in pro-ducing items from solid wood.

M 994 395

Figure 23.— Laminated white oak shipframe.

Selection of Species and Grades

For many glued-laminated items, theselection of species is partially limited bythe properties desired in the finishedarticle. White oak is desired in ship tim-bers, for example, because it combinesexcellent properties to hold fastenings withhigh strength and durability under wetexposures; Sitka spruce is often specifiedfor booms, spars, propellers for wind

4 6

tunnels, and masts because of its highstrength-weight ratio; and yellow birch isalso sometimes favored for small aircraftpropellers because of toughness.

Softwood species, principally Douglas-fir, southern pine, western larch, and hem-lock, are used largely in laminated archesand beams because of favorable cost, avail-ability, and adequate strength properties.These softwood species are the mainstayof the structural laminating industry whileothers find special applications where theirproperties are desired. Occasionally,gluing characteristics and resistance to ad-verse use conditions may affect the choiceof species, although tests have shown thatglue joints of long-term durability andhigh strength can be produced with prac-tically all domestic species. To attain thishigh joint strength and permanence,however, the gluing procedure must be ad-justed more carefully and the adhesivemust be of better quality for some speciesthan for others.

Product quality or grade requirementsare often established by design criteria orby use requirements. Defects permitted inspars or propellers, for example, aresharply limited. Severely curved parts ofhigh-strength laminated members gener-ally require clear and straight-grainedwood, free of significant defects, so thatthe laminations may be bent to the desiredcurvature without breaking. Defects suchas large holes, knots, and decay reduceeffective glue-joint area. Surfaces contain-ing pitch, cross grain, and knots do notglue so well as clear wood. Medium- tolarge-sized knots and knotholes aggravateglue-joint delamination when the exposureinvolves alternate wetting and drying.Lower grades of lumber, consequently, areless adapted to laminating timbers for ex-terior use than for interior use in whichthey are kept dry and undergo less severechanges in moisture content. If the mem-bers are well treated (after gluing) with anoil-borne preservative, lower grades mightbe used for exterior service if strength re-quirements are met.

Sapwood is as durable as heartwoodunder continuously dry conditions, butunder moist service conditions the sap-wood of even the durable species is sus-ceptible to attack by wood-degradingfungi and by insects. When the laminatedproduct must be durable under moistexposures, the wood should be treated witha suitable preservative.

Stresses in Laminated Members

Differences in shrinking or swelling arethe fundamental causes of internal stresses.Within a single member, adjacent lamina-tions should shrink and swell in about thesame amounts and in the same direction.Laminations, therefore, should have some-what similar shrinkage properties (table 2)and be at about the same moisture contentwhen glued.

Maximum lengthwise shrinkage in astraight-grained piece of normal wood isonly about one-third of 1 percent, but theshrinkage across the grain may be 10 to 30times more. Cross grain as well as knots,burls, and other growth characteristicsaffect the strength of laminations. For thisreason, slope of grain and knots arerestricted in laminated timbers. In bend-ing members, laminations with smallerknots and straighter slope of grain areusually placed in the outer laminationswith the grade decreasing toward thecenter of the member.

If two or more pieces having differentshrinkage values are glued together, eventhough they are straight grained, a mois-ture content change will cause them toshrink or swell in different amounts andthus set up stresses. Flat-grained or plain-sawed lumber shrinks or swells more inwidth with moisture changes than vertical-grained or quartersawed lumber. Ifinternal stresses are to be avoided, there-fore, flat-grained wood should not beglued to edge-grained wood. In softwoodspecies for structural use, matching of

47

grain in adjacent laminations is much lesscritical than with dense high-shrinkagespecies. The top and bottom laminationson a laminated member are less apt to facecheck if the pith side is turned out becauseits shrinkage is less than the sap side(fig. 24). Where the laminations comefrom small trees, this might be a worth-while practice to follow.

If adjacent laminations differ in mois-ture content at the time of gluing, stressesin the glue joint and irregularities in thesurface will develop when the laminationslater come to a common moisture content.The pieces should therefore be conditionedto about the same moisture content beforebeing glued. For structural members, arange in moisture content no greater than5 percent between laminations in a singleassembly is suggested. If exact trueness ofsurface is important, as it may be in furni-ture, even this range may prove excessive.Stresses will also be created if the interiorportion of any one board differs greatly inmoisture content from the outer portionor shell, and it is suggested that suchdifferences not exceed 5 percent.

Laminations up to about 2 inches thickare most commonly used in gluing straighttimbers, provided that suitably dry stockis available. Within this 2-inch limit, thethickness of the lamination does not affectthe performance or durability of well-glued joints, so that different thicknessesmay safely be glued in the same laminatedassembly. It may sometimes be desirableto use more than one thickness of stockin flat assemblies to attain maximumutility from the lumber supply or to fabri-cate a laminated member to close dimen-sions. For curved members, the maximumthickness of laminations is usually gov-erned by the curvature to which the lami-nations are bent. The minimum radius towhich dry, clear, straight-grained lumbercan be bent without breaking is about 100to 125 times its thickness and varies a greatdeal with the species of wood as well aswithin the same species. In general, hard-

woods can be bent more severely than soft-woods of the same thickness.

Relief of Stresses

The stresses that develop in glued-laminated members due to differences in

M 138 756

Figure 24.— End section of laminatedbeam having the pith side out in topand bottom laminations. Particularlyin boards from small trees, the pithface has less checking tendencies thanthe sap face. (Fourth lamination frombottom shows section through verticalfinger joint.)

4 8

the properties of the various laminationswill gradually disappear if the glued articleis kept for a long time at a constantmoisture content. This is because of stressrelaxation and creep characteristics of woodwith time. Stresses due to moisturedifference between laminations, or withinlaminations, at the time of gluing will notreappear after having once been relieved.Stresses due to cross grain or to differencesin the shrinkage properties of the ad-jacent members, however, will reappear ifthe moisture content is changed after thestresses have once been relieved.

END AND CORNER JOINTCONSTRUCTION

When the end grain surfaces of twopieces of wood are glued together, a buttjoint is formed. Mitered joints are usuallycut at a 45° angle with the grain and mustessentially be treated as butt joints for glu-ing purposes. If two pieces of plywood areglued edge to edge or if the edge of onepiece of plywood is glued to the face orsurface of another, only partially effectiveglue bonds are obtainable since edge grainof certain plies only is bonded to edgegrain of plies of adjoining pieces. Severaltypes of corner joints are shown in figure25. The importance of moisture controlwhere miter and other corner joints areinvolved is illustrated in figure 26. Asomewhat different corner joint wheremoisture control and choice of low-shrink-age material is important is shown inFigure 27.

No gluing technique has yet been de-vised to make square-end butt joints (fig.25, H) sufficiently strong and permanentto meet the requirements of ordinaryservice, and no adhesive has been offeredfor commercial bonding of such joints.

Figure 28 compares bending strength(by two methods) of several types of gluedcorner joints made of particleboard. Amiter joint with a plywood spline appearsthe most promising.

M 138 532

Figure 25.— Various types of cornerjoints: A, slip or lock corner; B, dadotongue and rabbet; C, blind dovetail;D, dovetail; E, dowel; F, mortise andtenon; G, shouldered corner; and H,butt end to side grain.

To obtain acceptable strength in piecesspliced together endwise, it is necessaryto make a scarf, finger, or other slopedjoint (fig. 29). The plain scarf with a lowslope generally develops the higheststrength, but is also the most wasteful ofmaterial and requires considerable careboth in machining and gluing to obtainconsistently high-quality joints. If thegrain of a board to be spliced makes anangle with the face of the board, thescarf should be cut with the slope of the

49

M 136 539

Figure 26.— Miter joints can open when high-shrinkage material is used and widevariations in moisture content occur. A glued spline or other reinforcement canreduce or prevent joint separation. Vertical-grained material, having lower shrink-age in width, will reduce chances for separation of this type of joint.

grain rather than against it to more nearlyapproach side grain on the scarfed surface.During the gluing operation, end slippageshould be prevented to keep the parts inproper alignment, and a slight overlap isdesirable to insure adequate pressure on thejoint (fig. 30). If the members slip end-wise during the pressing operation, thejoint will not receive sufficient and uniformpressure and erratic strengths may be ex-pected. Even plain scarf joints with a lowslope are not as strong as clear wood (of thesame quality) in tension parallel to thegrain. Tests on specimens containing scarfjoints stressed in tension indicated theaverage strength ratios given in this tabula-tion:

Strength ratioSlope of Scarf (jointed/nonjointed)

(Pct.)1 in 12 and less steep 901 in 10 851 in 8 801 in 5 65

Adequate data are lacking on the dura-bility of glued scarf joints with very steep

M 138 461

Figure 27.— Corner joint showing effectof serious moisture changes after fab-rication (somewhat exaggerated toemphasize need for control of mois-ture content). The advantage of edgegrain over flat grain in this type ofconstruction is also evident.

50

M 137 679

Figure 28.— Comparison of relativebending strength of different types ofcorner joints in particleboard. In ad-dition to the mechanical reinforcementin four of the joints, they were alsoglued.

slopes. If strength is important, therefore,it is probably advisable to avoid scarfssteeper than 1 in 10 for exterior use orother severe exposure.

Approximate tensile strengths (ex-pressed as a percentage of clear wood)of various types of end joints are shown infigure 31.

Finger joints (fig. 29, B and C) lendthemselves more readily to rapid produc-tion and uniform quality than do plainscarf joints and are more practical wheretheir strength is sufficient for the designinvolved. Figure 32 illustrates a suggestedmethod for applying pressure when gluingfinger joints. When pressure is appliedonly in the longitudinal direction, unre-liable bonds in the outer fingers may

M 138 527

Figure 29.— End joints for splicing lum-ber (occasionally also used for panelproducts): A, scarf joint (when usedwith a low slope, can develop almostthe full strength of wood); B, horizon-tal finger joint (cut parallel to wideface of board); C, vertical finger joint(cut perpendicular to wide face ofboard). Type C is the most commonlyused finger joint for structural pur-poses.

result. With short fingers, this effect is notas pronounced as with long lingers.

Figure 33 shows a number of lingerjoints where the slope (angle between axisof member and sloping joint) and tipthickness (tip of linger) are held constant,

51

but the pitch (distance center-to-center offinger tips) increases from three-sixteenthinch to one-half inch and consequentlythe length also increases.

The tensile strengths of such joints,made with various slopes, are shown infigure 34. The sloping joint area (persquare inch of section) is included forcomparison. Reasonably good correlationbetween strength and joint area is indi-cated. This is logical because wood isroughly 10 times as strong in tension as inshear, and the number of strength-reducing finger tips decreases with in-creased joint area (fig. 33).

The joint geometry is as important asgood glue and gluing practices to producehigh-strength finger joints.

Finger-joint machining lends itselfquite well to mechanized continuousoperation. Figure 35 shows equipmentthat trims (or squares) the ends of boards

M 41 387

Figure 30.— Scarf joints should be prop-erly alined for gluing.

or planks, cuts the fingers, and appliesglue to the fingers in rapid succession.

Where strength is important, jointssuch as the plain end to side grain (fig. 25,H) have proved entirely inadequate because

M 138 748

Figure 31.— Approximate percentage of the tensile strength of clear wood obtain-able with different types of end joints. (Nearly the full tensile strength of a low-density species has been obtained with butt joints in laboratory experiments butno practical glue or gluing procedure have been developed to do this commer-cially.)

52

M 138 526

Figure 32.— Suggested method for ap-plying pressure (P) to finger jointswhile glue is curing

they are commonly subjected to unusuallysevere stresses in service. Under changingmoisture conditions, the end-grain surfaceof the joint tends to swell or shrink in alldimensions of the joint while the side-grain surface of the joint swells or shrinksonly in one direction. Joints that are notsubjected to much external stress mayserve satisfactorily, for example jointsmade by gluing facing strips of veneer onthe end edges of a crossbanded tabletop(fig. 19, B). In furniture parts, however,stresses might easily exceed the strength ofend-to-side-grain joints in service. Ulti-mate failure of the joints usually resultswhen external stresses are combined with

internal stresses from moisture changes.In the manufacture of such parts, it istherefore necessary to reinforce the end-to-side-grain joints with devices such asdowels and tenons (fig. 25, E and F). Eventhen, the stresses that recur with seasonalchanges put a severe strain on the joints,which makes it very desirable to protectsuch joints against changes in moisturecontent. The strength and permanence ofall types of end-to-side-grain joints dependupon the type of glue and gluing tech-nique, the accuracy of the machining, andthe design and fit of the parts.

In manufacturing wood assemblies,such as furniture, it is often necessary tofasten together two parts that shrink andswell differently’ with moisture changes.Tables and desks, for example, are usuallydesigned so that the tops can be expectedto swell or shrink differently than theframes. Even if feasible, it would be un-desirable to glue the tops rigidly to theframes because the differences in shrinkagecharacteristics would tend to distort thetables. Chests are often designed so thatthe tops and bottoms can be expected toshrink or swell appreciably in width, whileon the ends the wood grain runs length-wise across the width and no significantchange in this dimension can be expected.If the tops and bottoms of such items are

M 120 740

Figure 33.— Finger joints with constant slope (1:14, angle between axis of memberand sloping joint) and tip thickness, but with increasing (from left to right) pitchand length of fingers.

53

M 123 420

Figure 34.— Tensile strength of Douglas-fir finger joints with constant tip thicknessand six different slopes. For each slope the pitch varied from three-sixteenth toone-half inch (see fig. 33).

glued or fastened rigidly to the ends, vices, such as slotted screw holes, thatsubsequent swelling or shrinking with permit normal shrinking and swellingmoisture changes can be expected to cause when two elements differing in shrinkagesplits and distortion, to break the joint, properties must be jointed together or toor to rupture the wood. In designing wood insure that all joining elements haveitems, it is important to provide for de- similar shrinkage properties.

SELECTED REFERENCES

Bohannan, Billy, and Selbo, M. L.1965. Evaluation of commercially made

joints in lumber by three test methods.U.S. For. Serv. Res. Pap. FPL 41, 41 p.U.S. Dept. Agric. For. Serv. For. Prod.Lab., Madison, Wis.

Bryant, Ben S., and Stensrud, R. K.1954. Some factors affecting the glue bond

quality of hard-grained Douglas-fir ply-wood. For. Prod. J. 4(4):158-161.

Cass, S. B., Jr.1961. A comparison of hot-pressed interior

plywood adhesives. For. Prod. J. 11(7):285-287.

Freas, A. D., and Selbo, M. L.1954. Fabrication and design of glued lami-

nated wood structural members. U.S.Dept. Agric., Tech. Bull. No. 1069.220 p.

Haskell, H. H., Bair, W. M., and Donaldson,William.

1966. Progress and problems in the southernpine plywood industry. For. Prod. J.16(4): 19-24.

Heebink, B. G.1963. Importance of balanced construction

in plastic-faced wood panels. U.S. For.Serv. Res. Note FPL-021. 5 p. U.S.

54

M 138 686

Figure 35.— Finger-jointing equipment: A, trim saw; B, cutter (or shaper) head; andC, glue applicator.

Dept. Agric. For. Serv. For. Prod. Lab.,Madison, Wis.

Heyer, O. C., and Blomquist, R. F.1964. Stressed-skin panel performance. U.S.

For. Serv. Res. Pap. FPL 18. 12 p. U.S.Dept. Agric. For Serv. For. Prod. Lab.,Madison, Wis.

Jarvi, R. A.1968. Laboratory test for prepressing of ply-

wood. For. Prod. J. 18(3):53-54.Lambuth, A. L.

1961. Prepressing plywood assemblies. For.Prod. J. 11(9):416-419.

Raknes, E.1967. Finger jointing with resorcinol glue at

high wood moisture content. NorwegianInst. of Woodworking and Wood Tech.Rep. No. 32.

Raknes, E.1969. Finger jointing structural timbers

with a high moisture content. NorskSkogind 23(6):184-190.

Roth, A.1970. A new type of finger joint. Pap. ja

Puu 52(1):25-28.

Selbo, M. L.1963. The effect of joint geometry on tensile

strength of finger joints. For. Prod. J.13(9):390-400.

Stensrud, R. K., and Nelson, J. W.1965. Importance of overlays to the forest

products industry. For. Prod. J. 15(5):203-205.

Strickler, M. D.1970. End gluing of lumber. For. Prod. J.

20(9):47-51.S t r i c k l e r , M . D . , P e l l e r i n , R . - F . , a n dTalbott, J. W.

1970. Experiments in proof loading struc-tural and end-jointed lumber. For. Prod.J. 20(2):29-35.

U.S. Forest Products Laboratory, Forest Service1966. Some causes of warping in plywood

and veneered products. U.S. For. Serv.Res. Note FPL-0136. 8 p. U.S. Dept.Agric. For. Serv. For. Prod. Lab., Madi-son, Wis.

U.S. Forest Products Laboratory, Forest Service1974. Wood handbook: Wood as an engi-

neering material. U.S. Dept. Agric.,Agric. Handb. 72 rev. 432 p.

55

PREPARING WOOD FOR GLUING

Glues vary in properties and use charac-teristics as well as in quality, but in mostinstances failure of glue joints can betraced to improper preparation of thewood. Among the various factors causinginadequate glue bonds, the most prevalentis lack of proper moisture control, bothbefore and after gluing.

of glued members. A change in moisturecontent generally develops stresses on theglue joints; the magnitude of these stressesis roughly proportional to the magnitudeof the change with a particular species.A certain percentage change in a densewood develops greater stresses than thesame change in a light species.

MOISTURE CONTENT

The moisture content of wood at thetime of gluing is important because itaffects the quality of the bond and theperformance of the glued product inservice.

Satisfactory adhesion to wood is ob-tained with most adhesives when the woodis at moisture contents of about 6 to 17percent, and with some glues well beyondthis range (up to 25 pct. has been re-ported for resorcinol adhesives). The pre-cise upper and lower limits vary with ad-hesive type and formulation. Satisfactoryjoints have been made experimentally withwood that was near the fiber saturationpoint (with resorcinol-type glues) and atvery low moisture contents (with caseinglue).

Glue joints will remain most nearly freefrom stresses if the moisture content of theglued parts (when the glue sets) equals theaverage moisture content the product willattain in service. The moisture content ofwood for gluing, when increased by thewater from the glue, should be as nearas possible to the average moisture contentthat the glued member will have inservice.

The moisture content of the woodaffects the rate of change of viscosity ofmany adhesives during the assemblyperiod. It also affects the rate of settingand, in hot pressing, it affects markedlythe tendency to form blisters (unbondedareas caused by formation of steam in thejoint when moisture content is too high).Consequently, the moisture content of thewood may necessitate adjustments com-patible with the gluing procedure.

The moisture content of dry wood inservice above grade in dwellings in theUnited States commonly varies from about4 percent to about 13 percent. The woodin a chair in a dwelling in northernMinnesota, for example, may have amoisture content as low as 4 percent in thewinter and as high as 10 percent in thesame room in the summer. Wood in a simi-lar item in a dwelling along the Gulf Coastmay maintain a moisture content varyinglittle from 13 percent throughout theyear. Except for the coastal areas andcertain arid inland areas, the moisturecontent of wood in heated buildings willaverage about 7 or 8 percent for the year.Dry wood in protected but unheatedshelters has an average of about 12 percentmoisture throughout a large part of theUnited States, but the average is less in thearid areas and more along the coasts.

Most importantly, moisture content ofthe wood at the time of gluing has muchto do with the final strength of the joints,the development of checks in the wood,and the stability (freedom from warping)

The amount of water added to the woodin gluing varies from less than 1 percentin lumber 1 inch thick to over 60 percentin thin plywood where the amount of woodis small in proportion to the amount ofglue. Calculated percentages of moisture

5 6

added to wood in gluing are given in table3 for a number of species in constructions.Thickness of the wood, number of plies,density of the wood, glue mixture, andquantity of glue spread all affect the in-crease in the moisture content of the woodwhen the glue is spread. If pertinent tablesare not readily available, the percentageof water added with the glue may be cal-culated from the following formula:2

0.000192WG L – 1P =

TS L

2Both this formula and the percentages listed intable 3 are based on the assumption that all waferadded by the glue is absorbed by the wood. Theassumption is somewhat erroneous, but the methodyields results sufficiently accurate to guide in select-ing suitable moisture contents.

Thin veneer, even if dried almost freeof moisture, will take up so much waterfrom wet glue that its moisture contentwill become higher than the probableaverage for the plywood in service. Verydry veneer is easily split or cracked and isdifficult to handle before and during the

where P = percentage of water addedw = pounds of water in 100

pounds of mixed glueG = pounds of mixed glue used

per thousand square feet ofglue-joint area

L = number of laminationsL – 1 = number of gluelines in glued

assemblyT = average lamination thickness

(in inches)S = specific gravity of wood (dry)

Table 3. ---Calculated percentages of moisture1 added to wood in gluing (jive-ply construction)

Adhesive

CaseinDo................Do................Do.... . . . . . . . . . . .

65956595

Urea resin 45Do.... . . . . . . . . . . . . 65Do.... . . . . . . . . . . . . 45Do.... . . . . . . . . . . . . 65

Resorcinol orintermediate-temperature-settingphenol

Do................Do.... . . . . . . . . . . . .Do.................

45 3/4 .4 .4 .6 .6 .565 3/4 .6 .6 .8 .8 .845 3/8 .9 .8 1.1 1.2 1.165 3/8 1.3 1.2 1.6 1.7 1.6

Gluespread2

Lami-nationor plythick-ness

Inch Pct.3/4 1.43/4 2.13/8 2.83/8 4.2

3/4 .63/4 .93/8 1.23/8 1.7

Hardmaple

Wood species

Whiteoak

Pct.1.32.02.73.9

.6

.81.11.6

Mahogany

Pct.1.82.73.65.3

.81.11.52.2

Douglas- Southernfir pine

Pct.1.92.73.75.5

.81.11.52.2

Pct.1.82.63.55.1

.71.01.42.1

1Moisture calculated on the basis of average specific gravity values as follows: Hard maple, 0.63; white oak,0.67; mahogany, 0.49; Douglas-fir, 0.48; southern pine, 0.51. Glue mixtures used were:Casein, 1 part solids to 2 parts wafer; urea resin, 1 Part solids to 0.65 part water; resorcinol and inter-mediate-temperature-setting phenol, 70 pct. solids.

2In pounds per square foot of joint area.

57

gluing operation. It also very quicklyreabsorbs moisture during handling.Therefore, it is not practical to dry itbelow 2 or 3 percent moisture content.Furthermore, experience has shown that amoisture content of 5 percent or less inthe veneer at the time of gluing is satis-factory for furniture and similar uses.

Lumber with a moisture content of 6 to8 percent is satisfactory for gluing intofurniture and similar items that will beused in most of the areas of the continentalUnited States. Lumber for use in unheatedbuildings or shelters, and in partially pro-tected installations, should generally con-tain about 11 percent moisture beforegluing. The moisture added in gluing willthen bring the total moisture to about 12percent. In moist, wet, or unprotected ex-terior exposures, glued members maydevelop moisture contents well above 12percent. A moisture content range of 12 to15 percent, however, is generally satisfac-tory for such gluing.

The manufacturer shipping glued arti-cles to various parts of the country andmaking products for various uses cannotprovide for all the variations to be metin service. He must, therefore, aim atapproximate averages. Wherever feasible,a finish that effectively excludes moisturewill guard against checking and jointfailure because it slows down the rate ofmoisture changes.

DRYING ANDCONDITIONING

Wood free from casehardening andother internal stresses will be best for glu-ing. Internal stresses, which generallyresult from drying, may prevent a goodfit of the mating surfaces and mean warp-ing and checking after the wood is glued.Dried wood should be tested for the pres-ence of such stresses before it is removedfrom the kiln. If wood is casehardened orotherwise stressed, it should be treated torelieve the stresses.

Lumber

In drying lumber, although the desiredaverage moisture content may be reached,individual differences may be considerable;variations can occur in the moisture con-tent of individual boards and even betweendifferent parts of the same board. It isdesirable to reduce these differences asmuch as possible before gluing. In laminat-ing nominal l-inch boards, for example,moisture content should vary no more than5 percent between members in a singleassembly or between different parts of thesame board.

To obtain such uniformity, a condition-ing period after kiln or air drying isusually desirable. Conditioning is best ac-complished in a storage room in which thetemperature and humidity are controlledto maintain the desired moisture content.The time required for conditioning de-pends upon the species, dryness, method ofpiling, and circulation of air, as well asupon the temperature and relative humid-ity of the storage room. A conditioningperiod of 1 week in open piling is benefi-cial. Dense species generally require longerconditioning than lighter ones and themoisture content equalizes more rapidlywith increased temperatures.

Veneer

Veneer is generally dried in mechanicaldryers of various types, but may occasion-ally be kiln or air dried. The internalstresses that develop in drying veneer aregenerally not as great a problem as withheavier stock. Occasionally, however,internal stresses do cause wrinkling, check-ing, or honeycombing of the veneer. Thesedefects are easily recognizable and usuallycan be avoided by improving the dryingconditions.

In plywood plants where veneer is cut,it is customary to glue it soon after drying.For general purposes, veneer is in conditionfor gluing if it is reasonably flat, tightlyand smoothly cut, free from defects not

58

permitted by the grade, and at a moisturecontent suitable for the glue and gluingprocess. For most cold-press gluing, themoisture content of veneers should be 5percent or less. For hot pressing (withliquid glues), thin veneers should be 5percent or less in moisture content; mois-ture content of thicker veneer may beslightly higher, depending upon thespecies, the amount of glue spread, and thetemperature of the press platens. If themoisture content is too high, steam blisterswill form when the pressure is released.Veneer may take on moisture in lengthyshipment or storage, and it is good prac-tice to redry it just before gluing. Finehardwoods are often redried in hot platedryers in which the veneer remains be-tween the plates until it is sufficiently dryand flat. In the best practice, the veneeris then piled so that it will stay flat andwill cool before gluing, but so it will notreabsorb much moisture from the air.

Fancy veneer, such as burl, crotch, orother short-grained pieces, is more likelyto wrinkle and check than straight-grainedveneer. Film glue is well suited for fancyveneers and they may be hot pressed withthe veneers at a moisture content of 8 to 10percent. Fragile figured veneers are some-times toughened by dipping them in aliquid sizing solution and then redrying.Several different sizing solutions havebeen used, but one satisfactory solution is:

Ingredient Parts by weight

W a t e r 63Animal glue 16Alcohol 16Glycerine 5

It is often desirable to reinforce highlyfigured veneer to reduce dimensionalchanges and facilitate handling. This issometimes done by bonding the figuredface veneers to a thin but strong veneerbacking (as 1/40-in. birch or maple) usinga film glue. Film glue adds no moisture tothe veneer and permits forming the bondwhen the moisture content of the face

veneer is about 8 percent, so that chanceof subsequent drying and checking isreduced or eliminated.

Wet veneer is likewise glued to paperfacings to form a paper-veneer combina-tion for containers.

STORAGE BEFORE GLUING

It is not sufficient only to dry and con-dition wood to the proper moisture con-tent for gluing. Because a storage periodis generally required between drying andfinal machining and gluing, provisionsmust be made to prevent appreciable mois-ture changes during this time.

Lumber

The ideal condition is to provide storagespace for dried lumber with humidity andtemperature control to maintain the mois-ture content in the stock at a level suitablefor gluing-such as 7 percent for most in-door use and about 12 percent for outdoorservice. If the temperature is controlled atabout 75° F. and the relative humidity at35 percent (wet-bulb depression about 16°to 17° F.), the moisture content of thewood will be suitable for interior wood-work.

If controlled temperature and humiditystorage is not possible, maintaining thetemperature in the storage room about20° F. above the outdoor temperature willprovide a moisture content. in the lumberof 6 to 8 percent in most parts of the UnitedStates. Similarly, if the lumber is storedunder roof in an unheated area, the mois-ture content will be fairly close to 12 per-cent when the lumber is stored for a suffi-cient time to come to equilibrium. It isvery important, however, that during coldweather the lumber be brought into awarm room at least 24 hours before ma-chining and gluing; cold lumber will slowdown the rate of cure and generally resultin inferior glue bonds. This is importantwhen using resin adhesives, but even a

59

protein-base glue such as casein has givenmore dependable results on lumberwarmed to room temperature before glu-ing.

Warming lumber of the denser speciesbefore gluing is especially important.

Veneer

Veneer is usually glued shortly after dry-ing to minimize the chance for appreciablemoisture changes. Gluing hot veneer,however, must be avoided to prevent theglue from becoming too dry before pres-sure is applied.

When 2 to 3 days lapse between dryingand gluing, provision must be made toprevent moisture changes. Even thoughthe veneer is solid stacked, the ends and thetop of the loads may gain moisture rapidly.If it is impossible to store veneer at theproper equilibrium moisture condition(less than 5 pct. when hot pressing withliquid phenolics), covering the loads com-pletely with moistureproof film (poly-ethylene) provides considerable protection.

SURFAClNG WOOD FORGLUING

Careful machining is essential in prepar-ing wood for gluing. For strongest joints,wood surfaces should be machined smoothand true with sharp tools, and be essen-tially free from machine marks, chipped orloosened grain, and other surface irregu-larities. To provide uniform distribution ofgluing pressure, each lamination or plyshould be of uniform thickness. A smallvariation in thickness among laminationsor plies may cause considerable variationin the thickness of the assembly. In theproduction of glued-laminated members,for example, the differences in thicknessthroughout a lamination consisting of asingle board should not exceed 0.016 inch.When the lamination consists of two ormore boards, laid side by side or end toend, differences in thickness at the edgeand end joints between any two boards

in the layer should not exceed 0.010 inch.Experience has indicated that cup in inchesin boards after final surfacing preferablyshould not exceed one ninety-sixth of theratio ofwidth to thickness. Thus, for lami-nations 6 inches wide, a maximum cup issuggested as one-sixteenth inch in a board1 inch thick, one-eighth inch in a 1/2-inch board, and one-fourth inch in a 1/4-inch board.

Preferably, machining should be donejust before gluing so that the surfaces arekept clean and are not distorted by mois-ture changes. Where the four sides of apiece are to be glued, it is best to glue intwo operations and machine just beforeeach operation.

Surfaces made by saws are usuallyrougher than those made by planers, joint-ers, and other machines equipped withcutter heads. Modern saws freshly sharp-ened, well alined, and skillfully operatedare capable of producing surfaces adequatefor gluing many products and provide asaving in time and labor. Except where thesaws are usually well maintained, how-ever, glue joints between sawed surfacesare weaker and more conspicuous thanthose between well-planed or jointed sur-faces. Consequently, if inconspicuousglue joints of maximum strength are re-quired, planed or jointed surfaces are gen-erally more reliable.

Machine marks, caused by feeding thestock through a planer too fast for thespeed of the knife, prevent complete con-tact of the joint faces when glued. Machinemarks in cores of thinly veneered panelsare likely to show through the finishedsurface. Unequal thickness or width,which cause unequal distribution of gluingpressure and usually result in weak joints,may be due to the grinding, setting orwearing of machine knives. Knives that aredull or improperly set or ground mayproduce a burnished surface that interfereswith gluing or formation of the strongestglue bonds.

Wood surfaces are sometimes inten-tionally roughened by tooth planing,

6 0

scratching, or sanding with coarse sand-paper in the belief that rough surfaces arebetter for gluing. However, comparativestrength tests at the Forest Products Labo-ratory failed to show better results withroughened than with smooth surfaces.Also, studies of the penetration of glue intowood have shown the theoretical benefit ofthe roughened surface to be improbable.Light sanding has proved an advantage inpreparing for gluing such surfaces as resin-impregnated wood, laminated paperplastic, plywood that has been pressed athigh temperatures and pressures, or woodthat has been glazed from dull tools or bybeing pressed excessively against smooth,hard surfaces.

Within recent years, significant devel-opments in sanding equipment have beenreported. Advantages of so-called abrasiveplaning in preparing wood for gluing arereported to be deeper cuts in a single pass,close tolerances, and improved surfacequality for gluing.

MACHINING SPECIAL TYPESOF JOINTS

Plain side-grain-to-side-grain joints aregenerally prepared with planers and joint-ers equipped with rotary cutter heads andknives. With such equipment, clean-cutsmooth surfaces for gluing are relativelyeasy to obtain when the machines are wellmaintained. With special or irregularlyshaped joints, ideal surfaces for gluing areoften more difficult to obtain.

Side-Grain Surfaces

Plain, tongue-and-groove, circulartongue-and-groove, and dovetail are fourof the more common types of edge jointsused in gluing boards into wider pieces(fig. 36). As knowledge of adhesives andgluing techniques has increased, the plainedge joint has become far more commonthan any of the others. With most speciesofwood, straight plain joints between side-

grained surfaces can be made substantiallyas strong as the wood itself in shear parallelto the grain, tension across the grain, andcleavage. The tongue-and-groove andother shaped joints have the theoreticaladvantage of larger gluing surfaces thanthe plain joint, but the extra surface is notneeded because adequate plain joints canbe produced easily by normal commercialgluing procedures. Furthermore, the

M 138 531

Figure 36.— Various types of edge joints.The plain (top) is the most commonlyused.

61

theoretical advantage is often lost, whollyor in part, because the shaped joints aremore difficult to machine to a perfect fitthan are plain joints.

Experience has shown that the lack of aperfect fit in tongue-and-groove or dove-tail construction often results in joints thatare weaker than plain joints. The principaladvantage of the tongue-and-groove jointis that the pieces of wood to be glued aremore easily aligned and held in placeduring assembling. This makes possiblefaster clamping and less slipping of theparts under pressure, which are advantagesso important that some form of tongue andgroove is often used in edge gluing wherepressure is applied by clamps. A shallowtongue and groove (one-eighth inch or less)is as useful in this respect as a deeper cutand is less wasteful of lumber.

End-Grain Surfaces

It has proved practically impossible tomake end-grain butt joints sufficientlystrong or permanent for ordinary service.With certain synthetic resins (epoxies andurethanes) it has been possible to approachthe tensile strength of weaker species, butsuch end gluing is of little practical valuebecause of low bending strength and cum-bersome gluing procedures. To approachthe tensile strength of various species byend jointing, use a plain scarf, finger joint,or other form of joint that exposes a certainamount of side-grain surface (fig. 29). Aserrated scarf appears advantageous inproviding greater gluing area than a plainscarf, but has never been extensively usedbecause of greater difficulties in machin-ing it. Even the plain scarf has essentiallybeen replaced by finger joints, except incases where maximum strength is needed.

Careful machining to insure an accuratefit of the surfaces is essential to developthe maximum potential strength of thejoint. With available glues the plain scarfwith a low slope generally will producethe highest strength. Even with very lowslopes the average strength of scarf joints,

well-machined and well-glued, is less thanthe average strength of solid wood intension. Presumably strength is lowerbecause of the difficulty inaccurately align-ing the structural wood elements of onepiece with those of the joining piece andbecause of stress risers at the tip of thescarfs.

End-to-Side Surfaces

End-to-side-grain joints are difficult tomachine properly and to glue adequatelyfor ordinary requirements. Such joints aresubjected in service to unusually severestresses as a result of unequal dimensionalchanges in the two members of the jointas the moisture content changes. It istherefore necessary to use dowels, tenons,or other devices to reinforce the joint bybringing side grain into contact with sidegrain (fig. 25, E and F). In dowel, mortise-and-tenon, dado, tongue-and-groove,rabbet, slip, and dovetail construction, animperfect fit of the parts often results inonly partial adhesion in the joints. In mostof these joints, complete contact overpoorly fitted portions cannot be obtainedby ordinary gluing. Furthermore, pressureis often applied but momentarily in gluingand the glue does not set during the pres-sure period. Careful machining of irregu-larly shaped joints is therefore highlyimportant to obtain maximum strengthand durability in service.

CUTTING AND PREPARINGVENEER

Veneer is commonly rotary cut, sliced,or sawn. The total quantity of rotary-cutveneer, however is much larger than thecombined amount of sawed and slicedveneer produced. Most rotary-cut veneeris produced in large sheets by revolving thelog against a knife. The flat-grained veneerproduced in this manner is peeled off in acontinuous sheet very much like unrollingpaper. The half-round and back-cuttingprocesses produce highly figured veneer

62

from stumps, burls, and other irregularparts of logs. These processes consist ofplacing a part of a log or bolt off centerin a lathe, usually with an auxiliary devicecalled a staylog, and rotary cutting the boltinto small sheets of veneer. As the veneeris bent away from the log during cutting,the open (knife) side often develops checks.This open side is likely to show defectsin finishing and should be the glue sidewhenever possible.

Rotary-cut veneer is produced in thick-nesses ranging from about five-sixteenthsto one one-hundredth of an inch.

Sliced veneer is cut to obtain a definitefigure and is produced in long, narrowsheets by moving a flitch or block againsta heavy knife. The veneer is forced abruptlyaway from the flitch by the knife, thuscausing fine checks or breaks on the knifeside. The checked or open side should bethe glue side whenever possible.

In book-matching face stock where theopen side of every other sheet must be thefinish side, the veneer must be well cut.Mahogany, walnut, and other prized hard-woods are commonly sliced for the furni-ture trade, and slicing softwood for verticalgrain stock is becoming common practice.Most sliced veneer is cut in thicknessesranging from about one-sixteenth to oneone-hundred twenty-fifth or an inch, butfor special orders veneer one-eighth inch orthicker can be cut.

Veneer is also sawed from flitches, usu-ally to get a desirable figure or grain. Itis produced in long, narrow strips whichare essentially of the same ‘quality andappearance on both sides. Being equallyfirm and strong on both sides, alternatepieces may be turned over to match themfor figure to serve as the faces of veneeredpanels. Sawed veneer usually ranges inthickness from one-fourth to one-thirtiethof an inch. Because sawing wastes material,many mills have discontinued productionof sawed veneer.

Sliced and sawed veneers are used princi-pally for faces in plywood and veneeredpanels. Rotary-cut veneer is used for face

stock, thin cores (five-sixteenth inch andless), for cross-banding, and for curvedlaminated members. Most veneer that isglued ranges in thickness from one-fourthto one thirty-second inch. Veneer thinnerthan about one thirty-second inch is dif-ficult to bond with liquid glues because thesheets are fragile and curl readily when theglue is spread. Thin veneers can be gluedwithout prohibitive difficulty, however,with film glues.

Veneer surfaces, particularly the loosesides, are often somewhat rough and ir-regular, but by heating the logs and care-ful cutting, veneer can be produced thatis comparatively smooth and firm on bothsides. Since veneer is seldom resurfacedbefore it is glued, the care with whichit is cut is important to good gluing. Ifit is equally well cut, veneer produced byany of the three processes can be gluedequally well.

In gluing operations where mull-sizedsheets of veneer are available, the sheetsmay be glued immediately after drying.Cutting to size is preferably done beforethe final drying. Cutting after dryingallows more opportunity for the veneer toreabsorb moisture from the air. Further,very dry veneer is easily damaged andshould be handled as little as possible.

Whenever a face for a high-gradeveneered panel is made of two or moresheets of veneer, careful edge jointing isnecessary to make the joints inconspicuous.This type of joint is made by placing thedried veneer in piles of several sheets andthen running the piles over a special veneerjointer which makes the individual veneeredges smooth and true. These sheets arethen laid in the desired position and eitherglued together in a special machine calleda tapeless veneer splicer or taped tightlytogether by taping machines. In recentyears, the taping process has been in-creasingly replaced by a machine that gluesa thread (usually glass fiber) in a zig-zagfashion across the veneer joint (fig. 37).The glue is a hot melt, permitting veryrapid edge bonding of veneers.

63

M 138 265-11

Figure 37.— Machine for edge-bonding veneers by gluing hot-melt saturatedthreads across the faces of adjacent strips. A zig-zag thread pattern is used whentighter edge bonding is required.

For cores, crossbands, and sometimeseven for faces, perfectly tight joints be-tween the edges of the veneer are not neces-sary. Such items may be jointed satisfac-torily on a veneer clipper. For generalutility plywood, where thick veneers areused, the veneer sheets are merely laid inposition without fastening of any kind.The spaces between the sheets or evenlapped edges, which often result, are ofminor importance in this grade of panels.

If a very high-quality finish or a veryhigh degree of resistance to severe serviceis desired, edge gluing of all veneer jointsis justified. If the tapeless splicer is usedfor this purpose, the amount of glue spreadmust be carefully controlled lest excessglue squeeze out on the surface of veneerand interfere with subsequent gluing. Ex-cessively high pressures or temperaturesof the shoe of the tapeless splicer mayburnish the edges of the veneer sheetsenough to cause a weakly bonded streak inthe plywood. If weather resistance is im-portant, the adhesive used to splice thesheets should be as durable as the adhesiveused in bonding the plywood.

SELECTED REFERENCESDavis, E. M.

1962. Machining and related characteristicsof United States hardwoods. U.S. Dep.Agric., Tech. Bull. No. 1267. 68 p.

McMillen, J. M.1963. Stresses in wood during drying. U.S.

For. Prod. Lab. Rep. 1652. 33 p. U.S.Dep. Agric. For. Serv. For. Prod. Lab.,Madison, Wis.

Peck, E.C.1932. Moisture content of wood in dwell-

ings. U.S. Dep. Agric. Circ. 239. 24 p.Rasmussen, E. F.

1961. Dry kiln operator’s manual. U.S. Dep.Agric., Agric. Handb. No. 188. 197 p.Mar.

Rietz, R. C., and Page, R. H.1971. Air drying of lumber: A guide to

industry practices. U.S. Dep. Agric.,Agric. Handb. 402. 110 p.

Simpson, W. T., and Soper, V. C.1970. Tensile stress-strain behavior of flexi-

bilized epoxy adhesive films. USDA For.Serv. Res. Pap. FPL 126. 13 p. U.S. Dep.Agric. For. Serv. For. Prod. Lab., Madi-son, Wis.

Sisterhenm, G. H.1961. Evaluation of an oil-fired veneer

dryer-its effect on glue bond quality.For. Prod. J. 11(5):207-210.

64

ADHESIVES AND BONDING PROCESSESFOR VARIOUS PRODUCTS

As indicated earlier, adhesives that areexcellent for bonding certain wood speciesmay not be equally well suited for others.In a similar manner, and particularly be-cause production procedures vary, an ad-hesive well suited for one product may beentirely impractical for another. As an ex-ample, alkaline phenolics have for yearsbeen the mainstay in production of ex-terior-type softwood plywood, but theirhigh-temperature-curing requirementskeep them from being practical for lami-nated timbers. Such timbers would ex-plode from steam formation in the interiorif heated to the temperatures required tocure phenolic-type adhesives used forplywood.

No detailed discussion will be attemptedon the manufacture and composition ofdifferent types of adhesives. The user ismore concerned with how well the adhesiveadapts to his process and the dependabilityof his products. Hence, good communica-tion between supplier and user is moreimportant than knowledge of the type ofhardener, filler, extender, solvent, andfortifier that may or may not be employedin formulating the adhesive.

A brief discussion of adhesives and pro-duction procedures for major segments ofthe wood gluing industry follows.

PLY WOOD

Softwood plywood is generally producedin two types, interior and exterior. Exteriorplywood is bonded with completely water-proof adhesive that must be able to with-stand temperatures that chat the woodwithout separating joints. Interior ply-wood is produced with three levels of glue-bond quality: (1) Interior or moistureresistant, (2) intermediate moisture resist-

ant (lower than exterior but greater thaninterior), and (3) exterior adhesive, withveneers that may be of lower grade thanrequired for exterior plywood.

Minimum requirements for each type,developed through correlation with resultsof long-term exposures, are detailed inProduct Standard PS1.

High-temperature-setting (alkaline)phenol resin adhesives are used almost ex-clusively for exterior softwood plywood.Formulations vary and what is suitable forone species may not necessarily be the idealadhesive for other species. Adhesives thathad been used successfully for years inproduction of Douglas-fir plywood did notgive the same troublefree performance onsouthern pine, and reformulation of phe-nolic adhesives for southern pine becamenecessary. Higher glue spreads weregenerally required for southern pine ply-wood than for Douglas-fir plywood.

For moisture-resistant glue bonds ininterior plywood, soybean glues, generallyfortified with some blood, or highly ex-tended phenol resin glues are usually em-ployed. For intermediate moisture-resistant interior plywood, extended phe-nol resin adhesives or blood glues fortifiedwith phenol resin are commonly used.

Both exterior and interior plywood gluebonds are cured in multiopening hotpresses; steam is employed to raise the pressplatens to the required temperature.Interior plywood is sometimes also madeby cold pressing. The glue is then usuallymixtures of soy flour and spray-driedblood, often of low solubility, or evenstraight blood. There is still some straightsoy flour glue used, but the quantity isquite small. Figures 38 to 43 illustratethe major steps customarily employed insoftwood plywood production. Withinrecent years more automated layup systems

65

M 138 193Figure 38.— Rotary cutting of Douglas-

fir veneer.

M 138 192

Figure 39.— Clipping softwood veneerto width and cutting out objectionabledefects.

M 138 197Figure 40.— Drying softwood veneer in

roller-conveyor dryer.

M 138 190

Figure 41.— Spreading glue on veneerwith a conventional double-roll gluespreader and assembling the veneersfor plywood.

M 138 352

Figure 42.— Equipment for applying ad-hesive to veneer by curtain coating.A, Veneer sheet entering coater; B,veneer sheet coated with adhesivegoing to layup table.

M 138 191

Figure 43.— Glued and assembled ve-neers being transferred to automaticpress loader. Arrow points to 20-opening steam-heated hot press.

66

have been explored where the core piecesare joined edge to edge with hot-melt-coated threads (fig. 37) and then cut topanel size. Glue is applied by rubber-covered roll spreader (fig. 41), spraysystems, or by curtain coating (fig. 42).Curtain coating is reported to result inmore uniform spread as well as less wasteof adhesive.

While softwood plywood finds its majoruses in structural applications such as hous-ing, most hardwood plywood is producedfor furniture, wall paneling, door skins,and similar uses. Hardwood plywood isalso classified according to glue-bondquality: Type I and technical, fully water-proof; type II, water resistant’ and type III,moisture resistant.

Adhesives used for type I and technicalare generally phenol resin, melamine, ormelamine-urea combinations. This doesnot imply that these two adhesive typesare considered equally resistant or durablein long-term exterior service. Type II isbonded with urea resin (sometimes mod-erately extended), as well as other bondingagents of moderate moisture resistance.Type III is generally bonded with highlyextended urea, but occasionally with caseinglue.

Hardwood plywood is produced eitherby hot pressing or cold pressing, depend-ing on the equipment available. Type Iand technical, because of the adhesives em-ployed, are hot pressed. Types II and IIImay be either hot or cold pressed depend-ing on the adhesive used (ureas are formu-lated both for hot and cold pressing; caseinis generally cold pressed).

Much hardwood plywood, particularlythat going into furniture, is made with athick core of lumber or particleboard, andoccasionally other material. A commonconstruction is made up of a nominal l-inch core, crossbands of veneer frequentlyone-twentieth inch thick, and veneer facesone twenty-fourth or one twenty-eighthinch thick. Lumber core is often made upof narrow strips edge glued to the requiredwidth and with the annual rings perpendi-

cular to the face (fig. 19); this will reduceor eliminate cupping tendencies that aremore apt to occur with wide, flat-grainedcore boards.

With particleboard cores (fig. 17), thecrossbands are sometimes eliminated,depending upon various factors such asshape of particles, size of the panel, andshrinking and swelling characteristics bothof the core and the veneer. For large panelssuch as tabletops, crossbands are usuallyemployed; for smaller panels, and partic-ularly if both the particleboard and theface veneers are from woods of low ormoderate shrinkage, crossbands are ofteneliminated. Fine particles on the core facesare important when only face veneer is em-ployed. Large particles are more likely tocause showthrough. On the other hand,boards made of thin flakes generally shrinkand swell less in width and length thanboards made of particles such as slivers,shavings, or sawdust.

Materials for lumber-core panels aregenerally selected to obtain stable, smoothmaterial that will not contribute to thewarping of the panel nor contain defectsthat might show through the faces. Woodswith a relatively low density, low shrink-age characteristics, uniform texture, and areputation for staying flat in service arepreferred.

Correct and uniform moisture contentin the core at the time of gluing is alsoimportant. About 7 percent is suitable formost of the United States.

A simple method of determiningchanges in moisture content that mighthave occurred in the core after the panelwas glued is illustrated in figure 44. Astrip is cut from the panel in the directionof the grain of the crossbands, and witha thin bandsaw the crossbands are releasedfrom the core for a distance of about 10inches. In figure 44, A, the core was intension across the grain before releasing thecrossbands, and had been at higher mois-ture content when the panel was glued. Infigure 44, B, the core had been in compres-sion before being released from the cross-

67

M 137 693

Figure 44.— If the panel had 7 to 8 percent moisture content when veneers (cross-bands and faces) were separated from the core, it is safe to assume that: A,moisture content of core was too high at time of gluing; B, moisture content incore was too low when panel was glued.

band and had picked up moisture since the Careful inspection ofpanel surfaces, par-panel was glued. By using the average ticularly for furniture panels, is also im-shrinkage value for the species, the ap- portant in various stages of production.proximate moisture content of the core at The sooner surface defects can be detected,the time the panel was glued can be calcu- the less labor is expended if the panel mustlated. be rejected. Incident lighting fixtures such

M 138 350

Figure 45.— Incident lighting fixture for detecting defects such as showthrough, check-ing, or other surface blemishes in panels.

6 8

M 138 747

Figure 46.— Lighting arrangement used in some plywood plants to facilitate inspec-tion of panel surfaces as panel emerges from sander.

as illustrated in figure 45 are very usefulfor such inspections. Figure 46 is a sche-matic sketch of a lighting device used atinspection stations in some plywoodplants.

LAMINATED TIMBERS

Adhesives that set at room temperaturesor at moderately elevated temperatures aremost practical for laminated timbers ofappreciable cross section. Casein andphenol-resorcinol are used for normallydry interior service (where moisturecontent of the wood is not expected to

exceed 16 pct. for an appreciable lengthof time) and phenol-resorcinol is usedwhere the moisture content in use gen-erally exceeds 16 percent.

For end jointing of laminations, mela-mine-urea (in a 60:40 ratio) is used formost interior laminates, and resorcinol orphenol-resorcinol is the preferred gluewhen the product is intended for exterioruse. Co-sprayed melamine-urea has shownbetter durability than the two resinsmechanically mixed.

Some of the major operations in theproduction of laminated timbers are illus-trated in figures 47 to 54.

M 138 313

Figure 47.— Laying out template for an M 136 847-2

arch on mold-loft floor. Figure 48.— Setting jig to fit template.

69

M 136 847-2

Figure 49.— Ribbon glue spreader. A,Plank receiving adhesive; B, ribbonsof glue extruded from equally spacedholes on spreader pipe.

M 138 390

Figure 50.— Ribbon spreading of glue.

FURNITURE

A greater variety of species and jointdesigns are used in making furniture thanin any other segment of the wood-usingindustries. Some of the denser species arehickory, oak, pecan, sugar maple, beech,birch, ash, walnut, elm, hackberry, andcherry. In the medium and lower densityranges are gum, poplar, ponderosa pine,alder, basswood, and mahogany.

The external load a furniture joint mustwithstand is usually difficult to determine.The internal stresses, induced by moisturechanges, generally increase with wood

M 137 811

Figure 51.— Phenol-resorcinol adhesiveapplied by ribbon spreader. In thelaminating industry, the laminationsare often placed on edge (wide facevertical) during layup and pressureapplication; hence, the adhesive mustbe thixotropic to avoid running to bot-tom edge.

M 138 312

Figure 52.— Tightening clamps on gluedassemblies with nut runner operatedby compressed air.

density and amount ofshrinkage and swell-ing the joint is exposed to. In the author’sopinion, more furniture joints fail becauseof internal stresses than because of externalload, or they weaken to a degree whereexternal load brings on failure. It is im-portant, therefore, that the furnituredesigner be intimately acquainted with theproperties of the different woods, and

7 0

M 136 844-6

Figure 53.— Surfacing sides of slightlycambered laminated beams. M 138 751

Figure 55.— Various types of furniturejoints used in a study on effect ofjoint design and exposure on the per-formance of different types of glues:A, Dowel; B, mortise and tenon; C,blocked; D, slip (or lock); E, end grainto side grain; F, side grain to sidegrain.

M 138 311

Figure 54.— Marking curved laminatedmember for trimming to outline oftemplate.

know enough about adhesives to make theproper combination of wood, joint design,and bonding agent. Finish also plays animportant part in the performance ofgluedwood products.

In a study on performance of certaintypes of furniture joints (fig. 55) bondedwith different adhesives, the side-grain-to-side-grain joint, as would be expected, was

least affected by cyclical high-low humid-ity exposures, and the block corner jointshowed the greatest deterioration (fig. 56).Of the adhesives evaluated, resorcinol andphenol-resorcinol generally performedbest. Animal glue and casein glue were inthe upper range on side-grain-to-side-grain joints but were generally the leastdurable in other types of joints. Acid-catalyzed phenol and urea resin glues weregenerally intermediate in most joints,although there was considerable variationin the performance of the three ureas in-cluded in the study. Polyvinyl emulsionadhesive performed well in dowel and slipor lock joints but poorly in side-grain-to-side-grain joints.

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M 138 928

Figure 56.— Comparison of percentages of control strength values retained by dif-ferent types of assembly joints after 36 months of exposure to a repeating cycleconsisting of 4 weeks in air at 90 percent relative humidity followed by 4 weeksin air at 30 percent relative humidity, both at 80° F. (Two different commercialanimal glues were tested, and urea adhesives from three different manufacturerswere tested.)

Figures 57 and 58 illustrate the per-formance of two types of joints in twolevels of exposure. Under the humidityexposure of 65-30 percent-approxi-mately that for normal interior furnitureuse--side-grain-to-side-grain joints heldup well with all glues. Where the grainwas crossed, however (mortise and tenon,fig. 58), the casein glue (and others) dete-riorated appreciably in the milder exposureand seriously in the more drastic humiditychanges. The evaluations were made on

sugar maple without any finish or surfacecoating and no load was applied duringcycling.

Urea resin is probably the adhesive mostwidely used for furniture. If good-qualityurea adhesive is employed, satisfactory per-formance can be expected in reasonablywell-maintained furniture.

For moist and tropical conditions, boil-proof adhesives such as resorcinol and phe-nol-resorcinol would provide the bestlong-term performance.

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M 138 929

Figure 57.— Performance of various glues in side-grain-to-side-grain joints exposedto repeating cycles between 65-30 and 90-30 percent relative humidity, all at80° F.

Polyvinyl resin emulsions, because oftheir flexibility, have performed well incertain types of joints (dowel, slip or lockjoints). But where joints are continuallystressed, polyvinyl resin emulsions shouldbe avoided because of tendencies to creep.Also, their moisture resistance is low,which makes them unsuitable where highhumidity prevails.

Thermosetting polyvinyls, particularlywhen cured at elevated temperature, arefat superior in moisture resistance to the

ordinary PVA emulsions. However, theyare reported to be subject to creep andprobably would not be desirable for jointsunder appreciable continued external load.

Animal glues are still used to someextent for assembling furniture; if theproper care is taken both in gluing andfinish upkeep, good performance can beexpected in normally dry interior use.Although their moisture resistance is lowand they deteriorate under high humidityexposure, in side-grain-to-side grain joints

7 3

M 138 930

Figure 58.— Performance of various glues in mortise-and-tenon joints exposed torepeating cycles between 65-30 and 90-30 percent relative humidity, all at 80° F.

animal glues hold up reasonably well forshort periods even at the higher humid-ities.

Use of hot melts is rapidly increasingbecause of the automated and increasedproduction feasible with them. Many typesand formulations are available, leavinglittle basis for any general statement onlong-term durability at this time.

In operations where high-frequencyheating or other means for elevated tem-perature curing are available, melaminesand melamine-ureas would certainlydeserve consideration in furniture manu-facture. Of the two, melamine-urea curesfaster and is lower in cost.

One of the older methods of gluingfurniture panels, but still a commonly used

7 4

M 136 800-2

Figure 59.— Edge gluing oak for backsof church pews. Systems of this type(glue wheel) have been in use fordecades.

M 136 800-9

Figure 60.— Stack of laminated curvedbacks for church pews made of two¾-inch layers of particleboard andfaced with oak veneer. Jig used forgluing is in background.

one, is illustrated in figure 59. It is applic-able to room-temperature-setting glues,but moderate heat can also be applied whilethe “glue wheel” completes a cycle.

Figure 60 shows slightly curved backsfor church pews made by laminating twolayers of particleboard faced with thinveneers, and figure 61 shows middle andend supports (up-rights) for church pewsproduced by edge gluing.

M 136 801-2

Figure 61.— Edge-glued middle and endsupports for church pews.

M 138 264-6

Figure 62.— Lay-up table for single-opening batch-process, edge-gluingpress. Adhesive is heat cured.

Figures 62 and 63 illustrate edge-gluingoperations for furniture panels produced byhot pressing. The panels are bonded withurea resin adhesive. Figure 64 shows abattery of cold presses for mass productionof furniture panels and figure 65 shows acontinuous-feed, steam-heated edge-

75

M 138 264-8

Figure 63.— Single-opening hot pressfor edge-gluing panels. The curedpanels are. ejected as the new batchgoes in.

M 139 072

Figure 64.— Cold-pressing panels forcabinet doors glued with modifiedPVA.

M 138 704Figure 65.— Continuous-feed, steam-

heated press for edge gluing. Edgesof lumber spread with glue travel byconveyor to operator’s position.

M 138 265-5

Figure 66.— Machine for applying edgebanding to furniture panels with hot-melt glue.

gluing press. Figure 66 shows a machinefor continuous edge banding of panels,employing hot-melt adhesive.

SHIP AND BOATCONSTRUCTION

Gluing operations for ship and boatbuilding fall essentially in three categories:(1) Laminating structural members (keels,frames, and deck beams-figs. 47 to 54

7 6

M 95398 F

Figure 67.— Jig for gluing U-shapedship frame. The smaller membersshown inside the frame were gluedand clamped elsewhere and broughtto the curing area by overhead crane.The enclosed heating unit and fan forcirculating air are shown in back-ground. A metal cover is placed ontop of jig forming a complete en-closure during curing.

and fig. 67), (2) assembly gluing (plywoodgussets for joining frames at chines and forjoining deck beams to frames— fig. 68),and (3) production of marine plywood forbulkheads, decks, outer skin or planking,and superstructure (figs. 38 to 43).

For marine plywood, hot-press phenolicadhesives similar to those used for exteriorplywood are employed. The only substan-tial difference between the two types ofplywood is that certain defects permittedin veneers for exterior plywood are notallowed in the marine grade.

In ship and boat component production,such as laminating and assembly gluing,phenol-resorcinol adhesives are used al-most exclusively. Since elevated tempera-tures for curing often are not feasible inassembly gluing, it is advisable to checkwith the glue manufacturer to determineif the adhesive is room temperature settingon the species involved. Rustproof screwsor bolts are generally used for applying

M 99446 F

Figure 68.— Construction of V-bottomboat with frames joined at keel, chine,and deck with glued and bolted ply-wood gussets.

gluing pressure in assembly gluing ofboats. Predrilled holes slightly reamed outon the contacting surfaces permit drawingthe glued surfaces into closer contact.

For laminated ship and boat members,white oak is often used because of its highimpact resistance and durability (fig. 69).White oak is one of the higher densitynative species and requires high-qualityadhesive, generally of the phenol-resorcinoltype. Sufficient assembly period should beused to allow the viscosity of the glue tobuild up to the proper level. Elevatedcuring temperatures, about 150° F. for6 hours, are generally requited withcurrent adhesives, but lower curingtemperatures may be used with extendedcuring periods (fig. 11).

DOORS

Wood doors vary in size from smallcabinet doors to large garage or warehouseunits, and in construction from flushpanels with hollow or solid cores (figs. 70to 72) to panels with frames, usually calledstiles and rails. Details will not be dis-cussed here, but a few precautions will bepointed out to aid in avoiding pitfalls that

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M 94561 F

Figure 69.— Laminated white oak frames used in construction of Navy minesweeper.

M 138 750

Figure 70.— Three types of flush doors. A, Five-ply, solid-core; B, seven-ply, solid-core. The three-ply faces (door skins) are usually preglued; C, seven-ply, hollow-core door. Core material in this case is wood shavings produced by special process.

7 8

M 138 749

Figure 71.— Flush doors with three-ply plywood faces. A, Expanded cell-type core;B, mineral composition core: C, particleboard core.

have resulted in unsatisfactory door per-formance.

Flush doors are often made with “doorskins” for faces. These door skins are gen-erally three-ply plywood about 1/8 inchthick. The inner ply or core of this ply-wood is generally of a lower grade than thefaces. When such plywood is used for doorskins, the core becomes very important incontrolling warping characteristics of thedoor; in the seven-ply construction thatconstitutes the door, the two center pliesin the door skins are the crossbands for theentire panel. The importance of straightand parallel grain in avoiding warping isdiscussed earlier in the section titled“Crossbanded Construction.”

Solid cores for flush doors are often madeofshort blocks glued edge to edge, but notend glued. If the outline of these blockscan be observed on the face of the door(showthrough), the cause is usually un-equal shrinking or swelling of adjacent

blocks. This can result from placingvertical-grain blocks adjacent to flat-grain blocks in the core or from usingblocks of unequal moisture content at thetime the door is glued. Low-shrinkagespecies such as ponderosa pine are lesslikely to cause showthrough of core blocksthan higher shrinkage species such asDouglas-fir. Thick crossbands are alsomore beneficial in preventing showthroughthan thinner ones.

In panel doors, straight-grained framingmaterial of moderate shrinkage is lesslikely to cause warp than higher densitymaterial, particularly if the latter containscross grain. If the panels are glued to theframe, they are also apt to warp withchanges in moisture content.

Recently panel doors have been producedwhere the edges of the panels are set in softplastic foam. This permits dimensionalchanges in the wood without air leakingthrough open joints and also eliminates

79

M 96381

Figure 72.— Typical core types used in hollow-core doors. A, lattice; B, ladder; C,tube; D, honeycomb.

warping caused by shrinkage and swelling casein). Exterior doors, unless completelyof the panels. protected by wide overhangs, should have

Adhesives in doors for interior use are waterproof glue bonds of the quality usedgenerally of the water-resistant types (urea, for exterior plywood (phenolics).

8 0

SPORTING GOODS

Glued products used for sports includebowling pins, tennis rackets, snow skis,water skis, hockey sticks, and various typesof gym equipment. Laminated baseballbats (fig. 73) have also been produced.

These items generally are subject torough usage and must be made of tough,strong woods. Such woods usually exerthigh stresses on the glue joints under lossor increase in moisture content. For long-lasting, safe products, adhesives of goodquality are needed.

For items such as water skis, a water-proof bond is definitely required to obtainreasonable service life for the product.

Bowling pins are subject to severe im-pacts and a tough adhesive would be ex-pected to give the best performance. Butbecause bowling pins never get wet, andgenerally have a heavy plastic coating, theultimate in water resistance is not needed.One manufacturer of laminated bowlingpins successfully used a separate applica-tion of urea resin (catalyst applied to oneface and the resin to the other) for manyyears.

M 84744 F

Figure 73.— Laminated baseball batswith center lamination of hickory forimproved impact strength and the re-maining sections of ash with edgegrain exposed on the surface of thebat. Edge-grained surfaces make thebat more resistant to shelling.

The risk involved in using a moisture-sensitive adhesive would be if such lami-nated equipment, intended for normallydry use, would be stored in a damp ware-house for an extended period.

Where facilities for curing at elevatedtemperatures are available, a melamine- orresorcinol-fortified urea would provide agreater margin of safety than straight urearesin as far as durability of glue bonds isconcerned.

Where steel or fiberglass are combinedwith wood, as in some snow skis, an epoxyformulated for this purpose may be the bestchoice.

PARTICLEBOARD

Urea resin is used almost exclusively asbinder for interior particleboard. Since thewood is broken down into small particles,the stresses on the minute bonds are prob-ably lower with changes in moisturecontent than in a solid wood-to-woodjoint. On the other hand, it is well knownthat urea-bonded particleboard deterioratesin a few years when exposed directly to theweather. This indicates that urea resin isnot a suitable binder for particleboardwhere damp or humid use conditions areinvolved.

For exterior boards, phenolic binder isemployed but no substantial use has beenmade of particleboard for exterior servicein this country.

Particleboard has also been made withmelamine resin binder, and at least on anexperimental basis, with extracts frombark. Binder generally is applied by airspraying or airless spraying with agitationof the particles.

When veneering particleboard or bond-ing particleboard to itself or to wood, anadhesive fully as durable as the binder usedin the boards should be used. For normallydry interior applications, urea resin shouldbe adequate; for uses such as light cabinetdoors, high-quality polyvinyl glue hasbeen reported to give good service. A goodmoisture-excluding finish reduces stresses

81

on the glue bonds and is a good safetyfactor, particularly when panels are usedin kitchens and bathrooms where inter-mittent high humidity often occurs.

HOUSING AND HOUSINGCOMPONENTS

Glues for floor and wall panel applica-tions (figs. 74 and 75) are generally of theelastomeric or mastic type and are based onrubber, polyurethanes, and other mate-rials. They are usually furnished ready foruse in small cartridges (cylinders) that fitcalking guns; they may be applied as a beadto studs and joists, or to smooth wallswhen wood paneling is applied to existingwalls (fig. 76). Pneumatic glue guns arealso available for more efficient application.

The glues are smoothed by the nailpressure (or by hand rollers where no nailsare used); but because of their gap-fillingproperties, a thin, uniform glue film asobtained with well-fitted joints formedunder pressure is not necessarily required.On an experimental basis, paneling has

M 138 196

Figure 74.— Application of mastic ad-hesive for bonding plywood to joists.The plywood is also nailed, whichfurnishes gluing pressure.

M 138 198

Figure 75.— Application of mastic ad-hesive to studs for bonding wallpanels.

also been applied merely by pressing thepanel (by roller) against the wall to smoothout the glue without the benefit of nails.

Wall panels are sometimes nailed onlyat top and bottom (where nail holes willbe covered by molding or baseboard) andthe remainder of the panel is broughtinto close contact with the studs with handrollers to establish the glue bond.

Advantages of using glue in applyingwall panels include providing rackingresistance to the walls and, where decora-tive panels are involved, avoiding un-sightly marring of beautiful panels bynailing and nail popping.

Nail-glued plywood floors reportedlypermit wider joist spacing or smaller joiststhan floors only nailed. Another importantadvantage claimed for this system is elimi-nation or reduction of floor squeaks.

8 2

M 138 597

Figure 76.— Application of press-dried lumber paneling to plastered wall withmastic-type adhesive. left: Fitting panel to the adjacent one. Right: Applyingadhesive to wall with calking gun.

Housing components such as trusses andwall and floor sections have been factory-made for many years. Because of adverseexposure that can often occur during ship-ment and erection, a waterproof adhesive(resorcinol or phenol-resorcinol) is recom-mended for gluing such components.Since combinations of lumber and plywoodare often involved, appreciable stresses onthe joints with seasonal moisture changesare almost unavoidable. This is anotherreason for advocating highly durable adhe-sives for housing component manufacture.

Various means for pressing and curingthe glue joints in components have beendevised. Low-voltage heating is one of thecommon methods. It is also possible to

produce adequate glue bonds in certainmembers by nail-gluing and allowing theresin adhesive to set at room temperatures.The nail-glued truss shown in figure 77is a typical building unit produced by thismethod.

Two major advantages of preglued com-ponents are reduced labor cost at the siteand higher quality building units (im-proved strength by gluing). In the factory,jigs can be employed to provide both uni-formity of dimensions and rapid assemblyof parts. The conditions for obtaininghigh-quality glue joints are also muchmore favorable in the plant where tem-perature of both materials and surround-ings can be controlled.

8 3

ZM 96227 F

Figure 77.— Light truss with plywood gusset plates glued to framing members. Thegussets were ½ -inch, five-ply, exterior-grade Douglas-fir plywood; framing mem-bers were 1 5/8 by 3 5/8 inches in cross section; and ninepenny nails (indicated by+ on sketch) were used to apply gluing pressure.

NEW PRODUCTS

With use of adhesives steadily on theincrease, new bonded wood products— orcombinations of wood and other mate-rials-are continuously coming on themarket.

Wood “jewelry” in many varieties isgenerally made by bonding the shapedwood parts to metal clips or similarfasteners with epoxy adhesive. When aclear epoxy is used, a complete coating ofthe wood part can also provide a durablefinish.

Laminated flooring of various construc-tions has been made for many years, butnew adhesive bonding techniques aredeveloped from time to time. A methodof obtaining two three-ply flooring boardsby gluing and pressing one five-ply plankis illustrated in figure 78. This type offlooring, made of softwoods with oak topface, is produced in Scandinavia.

Details of construction of a four-ply,laminated flooring produced for manyyears in Europe are shown in figure 79.

Oak-faced flooring for use in permanentconstruction should be bonded with anadhesive at least as durable as fortified urea.Where appreciable fluctuations in mois-ture content or where high humidity andtemperature conditions prevail, a phenol-resorcinol would provide greater assuranceof long-term satisfactory performance.

Experimentally, bonding various typesof overlays to wood has improved appear-ance, paintability, and other properties,and heavier decorative overlays for kitchentables and cabinet tops have been producedfor several decades. Quite a range of adhe-sives from ureas to resorcinols, dependingon the moisture resistance required, bondthese overlays to panel products. Contact

M 138 530

Figure 78.— Single gluing operation andresawing yield two three-ply flooringboards (top and bottom of sketch)from one five-ply unit (center). Oaklamination is sawn at dashed line andthe surface of each half becomes thetop flooring surface.

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M 138 745

Figure 79.— Sketch of glued flooring made with softwood lumber core faced withwood veneer and a top layer made up of narrow strips of oak laid in variousparquet designs. The finished boards are furnished in about 6- by 120-inch strips,tongued-and-grooved, and varnished on the top surface.

adhesives applied to both the overlay andthe panel products also are widely used forthis purpose; they are particularly con-venient for do-it-yourself and on-the-jobapplications where equipment for applyingpressure over large areas is usually notavailable.

Use of vinyl overlays for such items asmoldings and furniture parts has been in-creasing the past few years. These overlays(vinyl films) are produced with wood grainpatterns; thus woods not usually suitablefor molding can be given the appearanceof walnut or other high-quality wood.These films are furnished with or withoutadhesive applied to the film and the adhe-sive composition is usually not disclosed.

Figure 80 illustrates one type of equip-ment for applying flexible vinyl overlay.With adjustable soft rolls, the film can beapplied to molding and other items of avariety of profiles.

As with products of established per-formance, species, finish, use conditions,and expected service life must be con-sidered when choosing an adhesive for anew product.

M 138 351

Figure 80.— Machine for applying flexi-ble overlay (vinyl film) to molding andsimilar stock. Arrow points to over-laid stock coming through the ma-chine.

85

SELECTED REFERENCES

Adhesives Age1964. Fibrous overlay bonded to wood at

high speeds. Adhes. Age 7(5):25.Adhesives Age

1965. Glued walls for a new building.Adhes. Age 8(2):36-37.

Adhesives Age1965. Sprayable adhesive reduces drywall

lamination costs. Adhes. Age 8(6):27.Anderson, A. B., Breuer, R. J., and Nicholls,G. A.

1961. Bonding particleboards with barkextracts. For. Prod. J. 11(5):226-228.

Bergin, E. G.1969. The strength and durability of thick

gluelines. Can. For. Serv. Publ. No.1260, 24 p.

Carroll, Murray1963. Efficiency of urea and phenol formal-

dehyde in particleboard. For. Prod. J.13(3): 113-120.

Cass, Stephen, B., Jr.1961. A comparison of hot press interior ply-

wood adhesives. For. Prod. J. 11(7):285-287.

Clausen, Victor H., and Zweig, Arnold1969. Automatic plywood layup develop-

ment at Simpson Timber Company. For.Prod. J. 19(9):62-73.

Ettling, B. S., and Adams, M. F.1966. Quantitative determination of phe-

nolic resin in particleboard. For. Prod. J.16(6):25-28.

Freas, A. D., and Selbo, M. L.1954. Fabrication and design of glued lami-

nated wood structural members. U.S.Dep. Agric., Tech. Bull. 1069. 220 p.

Gatchell, C. J., and Heebink, B. G.1964. Effect of particle geometry on prop-

erties of molded wood-resin blends. For.Prod. J. 14(11):501-507.

Heebink, B. G., Kuenzi, E. W., and Maki,A. C.

1964. Linear movement of plywood andflakeboards as related to the longitudinalmovement of wood. U.S. For. Serv. Res.Note FPL-073. 34 p. U.S. Dep. Agric.For. Serv. For. Prod. Lab., Madison, Wis.

Jarvi, R. A.1967. Exterior glues for plywood, For. Prod.

J. 17(1):37-42.Klein, W. A.

1970. How to laminate particleboard withadhesive-coated vinyl film. Adhes. Age13(12):26-27.

Lehmann, W. F.1965. Improved particleboard through

better resin efficiency. For. Prod. J.15(4):155-161.

Lehmann, W. F.1968. Resin distribution in flakeboards

shown by ultraviolet light photography.For. Prod. J. 18(10):32-34.

Lehmann, W. F.1970. Resin efficiency in particleboard as

influenced by density, atomization, andresin content. For. Prod. J. 20(11):48-54.

Miller, D. G.1953. Curved plywood, its production and

application in the furniture industry. For.Prod. J. 3(2):22-26.

Neusser, H.1967. Practical testing of urea resin glues

for particleboard with a view to shorteningpressing time. Holzforsch. u. Holzver-wert. 19(3):37-40. Wien.

Page, W. D.1968. Controlled manufacture of plywood

structural components. For. Prod. J.18(11):19-21.

Pinion, L. C.1967. Estimation of urea formaldehyde and

melamine resins in particleboard. For.Prod. J. 17(11):27-29.

Rice, J. G., Snyder, J. L., and Hart, C. A.1967. Influence of selected resins and bond-

ing factors on flakeboard properties. For.Prod. J. 17(8):49-56.

Selbo, M. L.1967. Long-term effect of preservatives on

glue lines and laminated beams. For.Prod. J. 17(5):23-32.

Selbo, M. L.1954. Jig for alining scarf joints. Proc. For.

Prod. Res. Soc. J. 4(4):43A-45A.Selbo, M. L.

1961. Adhesives for structural laminatedlumber. Adhes. Age 4(2):22-25.

Shen, K. C.1970. Correlation between internal bond and

the shear strength measured by twistingthin plates of particleboard. For. Prod.J. 20(11):16-20.

Snider, Robert F.1960. What to look for in glues for furni-

ture production. Adhes. Age 3(1):38-40.Webb, D. A.

1970. Wood laminating adhesive system forribbon spreading. For. Prod. J. 20(4):19-23.

86

The gluing operation generally consistsof these steps: (1) Mixing the ingredientsthat make up the glue, when ready for use;(2) spreading the glue on one or both jointsurfaces to be bonded; (3) assembling theindividual parts in the order planned forthe bonded product; (4) allowing thespread glue to thicken and penetrate thewood surfaces for a certain period (usuallyreferred to as the open and closed assemblyperiods and as a rule specified by thesupplier); (5) applying pressure to bringthe spread surfaces into close contact; (6)retaining pressure until the bond gainssufficient strength to permit safe handlingof the glued product; and (7) condition-ing the glued stock to complete adhesivecure and allow any solvent to diffusethroughout the glued assembly.

Each step will be discussed in moredetail, but inasmuch as the gluing proce-dures vary considerably for different prod-ucts, the discussion must necessarily besomewhat general. It is suggested there-fore that the adhesive user follow themanufacturer’s instruction very closely,and that the manufacturer’s technicalserviceman familiarize himself with thecustomer’s process and product so he willbe able to give sound advice to the cus-tomer.

MIXING ADHESIVE

Some adhesives, such as the film typesand the straight polyvinyls, are furnishedready for use and hence require no mixing.Others, as the ready-to-use caseins andsome powdered ureas, need only to bemixed with water, as prescribed by theglue supplier.

In this operation, usually part of thewater is first run into the mixer, after

GLUING OPERATION

which the powder is added gradually withthe mixer running to prevent formation oflumps. After a smooth, homogeneous mix-ture is obtained, the remaining water isadded slowly with the mixer running. Ifall the remaining water is added at once,the doughlike mix might break into largelumps; such lumps, particularly with low-viscosity adhesives, may be extremelydifficult to break up even with vigorousmixing.

This procedure is also sometimes neces-sary when a powdered hardener (often is amixture of the actual hardener and an inertpowder such as walnut shell flour or woodflour) is mixed with a liquid resin. It isoften easier to obtain a homogeneous mixif the hardener is first mixed with part ofthe resin until a smooth mixture is ob-tained, and then the remainder of the resinis added gradually with stirring.

The glue supplier generally furnishesinstructions for mixing and weighing theingredients to be mixed. Mechanicalmixers of various types (figs. 81 and 82)are invariably used in industrial operations;in the home workshop, small amounts canbe mixed satisfactorily by hand, using aclean metal or glass container and a paddlefor stirring. Since many adhesives areeither mildly acid or alkaline, containersnot affected by acid or alkali should beused. Strict cleanliness of gluing equip-ment is important for extraneous materialscan easily lower the bonding quality of theadhesive. A mixer suitable both for labora-tory and small shop use is shown in figure83.

Some resins must be kept cool duringstorage and also at the time of mixing.Instructions to this effect are generallyfurnished by the supplier.

87

M 138 385

Figure 81.— Counter-rotating paddle-type mixer for protein and resin ad-hesives. Mixers are available in vari-ous sizes.

Sometimes, particularly during coolweather, resin adhesives should be allowedto mature for a short period between mix-ing and use. During hot weather the re-action period usually can be omitted. Toavoid exceeding the working life of themixed glue, mix smaller batches duringhot weather than in the cooler seasons.This will prevent shutdowns for cleaningspreaders and other equipment becauseglue has exceeded its working life. Jacketedmixers permit the temperature of the mixto be controlled by running water of therequired temperature through the jacket(fig. 82).

Automatic mixers are also available forcertain applications, as shown in figure 84.

M 138 389

Figure 82.— Blender-type mixer for resinadhesives. The-mixer is available withor without water jacket for cooling orwarming the mix.

M 48436 F

Figure 83.— Three-speed mixer with twosizes of paddles and mixing bowls,suitable for laboratory and shop use.

SPREADING ADHESIVE

Various methods are used to apply adhe-sive to joint surfaces when bonding wood,depending largely on the type and amountof glued product and also to some extenton the adhesive.

In the small workshop, application bybrush is often practical. When larger

8 8

Figure 84.— Glue mixing and spreadingequipment. A, Unit that automaticallymixes liquid urea and powdered cata-lyst in the right proportions; B, gluespreader.

surfaces are involved, a mohair paint rollerworks well with many adhesives and ismore efficient than a brush.

Mastic adhesives used for bondingpanels to studs and joists are applied inbeads by hand-operated calking guns (figs.74, 75, 76) or by compressed air-operatedguns for more efficient operation.

In furniture manufacture where poly-vinyl glues are sometimes used extensively,the liquid glue is distributed by pumps orgravity feed through pipes, often appliedfrom nozzles conveniently located withinthe workmen’s reach.

In larger gluing operations, such as inplywood and laminated timber produc-tion, application by double-roll spreaders(fig. 41) equipped with doctor rolls forclose control of the spread has beencommon for decades.

In recent years, curtain coating, amethod similar to the process used for pre-finishing plywood, has come into use inplywood manufacture (fig. 42). Curtaincoating is claimed to result in more uni-form spread and less waste of adhesive.Reportedly, adhesives are also applied byextruders in some softwood plywoodplants.

In the laminating industry, ribbonspreading or extrusion spreading (figs. 49

and 50) is said to save adhesive and resultin more uniform spread and greater rate ofproduction. Ribbon spreading permits thelaminations to travel under the extruderat a greater rate of speed than througha roll spreader. Since the spread surfacesare often placed in a vertical positionduring the assembly period, ribbon spread-ing requires a thixotropic adhesive thatwill not sag or run to the bottom edge ofthe laminations before gluing pressure isapplied (fig. 51). The adhesive also mustremain sufficiently fluid to smooth out ina uniform film when gluing pressure isapplied.

The amount of spread (usually expressedin pounds of wet glue per 1,000 squarefeet of glue-joint area) varies considerablywith the type of adhesive used, productbeing bonded, species, moisture content ofthe wood, and the temperature and humid-ity of the gluing area. In general, appre-ciably higher spread is required withcasein glue than with most synthetics.Small items that can be assembled rapidlycan often be bonded satisfactorily withless spread than large members requiringa long assembly period. Often adjustmentsin the adhesive (such as setting rate) mustbe made for different size products. Densewoods generally require heavier spreadsthan lighter ones (assembly time and otherfactors may also need adjustment). Woodat low moisture content absorbs the solventfrom the adhesive faster than wood athigher moisture content; this makes in-creased spread necessary, unless assemblytime is short.

High temperature and low humidity inthe gluing area also suggest increases inthe glue spread. This again depends onwhether the assembly period can beshortened to compensate for the faster dry-ing and “skinning” over of the glue film.

As a rule, only one of the mating sur-faces of a joint is spread with adhesive.With certain products, however, such aslarge laminated members that require con-siderable time to assemble, spreading bothsurfaces of each lamination can be advanta-

8 9

M 139 027-1

Figure 85.— Spreading resin glue onboth faces of board with a rubber-covered double-roll spreader. Spread-er has adjustable speed to accommo-date different types of resins.

geous. This is called “double spreading”(fig. 85).

Special joints, such as finger joints, re-quire spreading mechanisms of the sameprofile as the joint for uniform applicationof adhesive. Such a spreader is shown infigure 86.

ASSEMBLING PARTS

Because of the large variety of gluedwood products, only a few will be brieflymentioned to give a general idea of theassembly operations involved.

The key to success in most industriestoday is automation, and significant break-throughs have been made in recent yearsin fields such as plywood manufacturewhere layup efficiency has improved im-mensely. In a similar manner, layup oflarge assemblies in the more progressivelumber-laminating plants is being donewith hardly a piece of lumber beingtouched by hand.

In some plants the planer and gluespreader are arranged in tandem withsynchronized rates of speed. The lamina-tions, end-jointed to length, run conti-

M 136 846-8

Figure 86.— Glue spreading mechanismfor finger joints. Glue is dispensed atarrow.

nuously through both machines to the lay-up station.

ASSEMBLY TIME

The interval between spreading theadhesive and the application of full gluingpressure is called assembly time. If woodsurfaces coated with glue are exposed freelyto the air, solvent evaporation and changesin adhesive consistency occur much morerapidly than if the joint surfaces are incontact. Free exposure of the coated sur-faces is called “open assembly;” surfacesin contact, “closed assembly.”

Proper adjustment of the assembly timeis very important and often has significanteffect on the quality of the glue joints.A too-short assembly period often resultsin “starved” glue joints, particularly withlow-viscosity adhesives and dense speciesthat absorb moisture from the glue slowly.Too long an assembly period (particularlyopen assembly) can easily result in “skin-ning” over or drying out of the glue film.The result is inadequate transfer ofadhesivefrom the spread to the unspread surfaces.

90

PRESSING OR CLAMPING

Glue-joint surfaces must be broughtinto close contact to enable the adhesiveto form a bond between them. Hence theapplication of adequate and uniformly dis-tributed pressure to the joint at the propertime is essential in production of consist-ently high-quality bonded joints. Pressuremust smooth the adhesive to a continuous,fairly thin layer between the wood surfaces,and hold the parts in close contact whilethe adhesive is setting or curing.

The optimum thickness of glue films injoints varies with the type of adhesive andwood species. Cured films as thin as 0.002inch have resulted in good bonds with ureaadhesives, and those as thick as 0.010 inchresulted in good quality joints with resor-cinol adhesives used with dense species.For best results, pressure should be appliedevenly over the entire joint area. Fluidpressure, such as used in bag molding withthin veneers, comes closest to being com-pletely uniform.

In hot-pressing plywood, multi-opening hydraulic hot presses (fig. 43)apply pressure of about 175 pounds persquare inch to the veneers while the glueis curing. In laminating, retaining clampsof various types (figs. 52, 87, and 88)are commonly used. Caul boards are laid

M 57292

Figure 87.— Double-bolt rockerheadclamps used to apply gluing pressureto laminated assemblies. The rocker-head equalizes the pressure across theassembly.

M 87206 F

Figure 88.— C-type rockerhead clampwith adjustable span used for lami-nating and other gluing pressure ap-plications.

between the clamp and the glued assemblyto distribute pressure to the areas betweenclamps. These cauls must be thick enoughto distribute the pressure uniformly, aswell as being flat and smooth. The clampsmust be sufficiently close together toproduce adequate pressure between as wellas under the points of contact. Gluingpressure in the range of 100 to 200 poundsper square inch is usually adequate for mostoperations, with viscous adhesives anddense species generally requiring the topof the range.

When clamps are used to apply gluingpressure, torque wrenches and similardevices may be used to determine theamount of pressure applied. Figure 89shows a panel press where pressure is ap-plied by compressed air hoses.

Satisfactory gluing for certain construc-tions can also be accomplished with nailpressure, provided the nails are spaced anddriven properly and the proper precautionsare taken with regard to assembly time.The pressure obtained with nails is rela-tively low; hence the adhesive must befairly fluid when the nails are driven.

No general rules have been developedfor relating nail size and spacing to insureadequate pressure. The nailing pattern andspacing shown for the light plywood-lumber truss in figure 77, however, hasgiven adequate glue-bond quality. Theadhesive was a phenol-resorcinol.

91

M 138 266-7

Figure 89.— Compressed air hose lam-inating press for cold pressing panels.

Bag-molded plywood for aircraft andlight boats was produced during WorldWar II and later. Recently, application ofwood-grained vinyl film to irregularly

shaped articles such as wood carvings hascome into use. Both vacuum molding andfluid pressure molding (fig. 90) are suit-able for application of plastic films to up-grade wood surfaces.

CURING ADHESIVE

Curing requirements of adhesives com-monly used for wood range from normalroom temperatures to about 300° F. Someadhesives, such as the ureas, are formulatedboth for room-temperature and elevated-temperature curing. The hot-setting ureasgenerally cure in the range of 240° to260° F. The melamines cure in about thesame range but will also cure at lowertemperatures with extended curingperiods.

For assembly operations, adhesives suchas polyvinyls, ureas, resorcinols, andanimal glues are generally used at normalroom temperatures. The thermosettingpolyvinyls can be cured both at normalroom temperatures and at elevated tem-

M 142395

Figure 90.— Three methods of forming bag-molded plywood.

92

M 96845 F

Figure 91.— Curing glue in scarf jointswith low-voltage electric heating. Theheating elements are embedded insi l icone rubber pads and aluminumf o i l s e p a r a t e s t h e p a d s f r o m t h eboards to prevent contamination ofthe pads by glue squeezeout.

peratures, but provide more durable bondswhen heat cured. Resorcinols providedurable bonds on many species when curedat room temperatures, but denser speciessuch as oak require elevated temperaturesto provide bonds as durable as the woodwithin a reasonable time period (fig. 11).

The rate of cure of resin adhesivesdepends both on the type of catalyst usedand the curing temperature. The higherthis temperature for a given glue, the morerapid is the curing reaction and the shorterthe time required to complete the cure.Alkaline phenolic resins, the type usedalmost exclusively for exterior plywood,cure in the range of about 265° to 310° F.Acid-catalyzed phenols are formulated toset at temperatures as low as room tem-perature.

Curing equipment ranges from large,steam-heated hot presses for plywood(fig. 43) to small, low-voltage heatingpads where resistance wire is embedded insilicone rubber (figs. 91 and 92) to generateheat. Thin wires or other conductive mate-rial in the glueline also have been used asheating elements with low-voltage electriccurrent. Curing of a slightly curved lami-

M 96844 F

Figure 92.— Curing phenol-resorcinol ins c a r f j o i n t s w i t h c o n t i n u o u s r u b b e rpad heated with low-voltage electriccurrent. Thin sheets of aluminum sep-ara te pad f rom boards to p reventcontamination of pad by squeezed-out glue.

nated member by low-voltage heating isillustrated in figure 93. This method re-quires a transformer and somewhat heavyleads.

Preheating wood before spreading andthen using the stored heat to cure the adhe-

M 138 264-2

Figure 93.— Curing glue in slightly curvedlaminated member with low-voltageelectric current.

9 3

M 89990 F

Figure 94.— Various electrode arrangements for applying high-frequency electricalenergy to glued assemblies. A, Assembly between electrodes, with electric fieldperpendicular to plane of glue joints; B, sandwich method, with high-voltageelectrode between the two assemblies being glued; C, electrodes arranged forparallel or selective heating of glue joints; D, stray-field heating arrangements ofelectrodes.

sive is a technique sometimes used forspecial applications such as finger-jointinglumber and for laminating timber deckingin a continuous process.

For large, laminated members, en-closures formed with canvas or other mate-rials over the clamped assemblies (fig. 67)are supplied with heat from steam pipesor by other means for curing the glue.

Because wood is a good heat insulator,this process is somewhat time consuming.

High-frequency (H-F) heating (fig. 94)is probably the method most widely usedfor elevated-temperature curing the glue inmembers that do not lend themselves tohot pressing, particularly for smaller itemssuch as furniture parts. H-F heating hasbeen used extensively for such operations

94

M 136 846-6

Figure 95.— Finger-jointed lumber, A,spread with glue traveling on a con-veyor toward an H-F unit, B, forcuring.

as edge gluing of lumber and curing gluein finger joints (figs. 95 and 96). It is alsobeing used in Europe for laminating beamsby continuous operation (fig. 97). Curingthe glue in steps (each step equals thelength of press) in laminated beams by H-Fheating has been practiced by at least twolaminators in the United States for a num-ber of years.

The H-F curing cycle depends on suchfactors as generator capacity, type of glueand glue joint area, and the arrangementof the electrodes in reiation to the gluejoints. Parallel heating (fig. 94, C) is gen-erally the most efficient method since thelarger part of the energy is converted toheat in the gluelines. The level of moisturecontent in the wood is an important factor.The higher the moisture content the moreconductive the wood becomes; thus, themore energy is dissipated throughout thewood instead of being concentrated at theglue joints.

Close control of the variables involvedin the gluing operation is required for suc-cessful H-F curing. Accurate machining,uniform moisture content in the wood, anduniform glue spread are some of the moreimportant factors. Technical knowledge inthe generation and use of high-frequencycurrents is also of vital importance withthis type of curing.

M 136 846-6

Figure 96.— Finger joints stopped byelectronic memory system, A, betweenelectrodes of. high-frequency genera-tor. B, indicates location of electrodesand the curing area.

In high-frequency curing, the resinadhesives for wood are generally rated fromeasiest to use to most difficult in this order:Ureas, melamine-ureas, thermosettingpolyvinyls, melamines, resorcinols,phenol-resorcinols, and phenols.

Figure 97.— High-frequency curing ofadhesive in laminated beams by con-tinuous process. Gluing pressure is ap-plied in the electrode area by blocksfastened to belt moving along endlesstrack. Parallel heating is employedand the upper electrode can be seenon top of the beam.

95

CONDITIONING GLUEDPRODUCTS

It is usually not economical to maintaingluing pressure or continue curing underpressure until the adhesive joints havereached their ultimate strength. A condi-tioning period after gluing pressure hasbeen released is beneficial in many ways.It allows moisture, if introduced by theglue, to diffuse away from the glue jointsand equalize throughout the member. Itpermits the glue to continue to set andapproach its ultimate bond strength.Stresses set up in the glued article duringthe gluing and curing operation will tendto be relieved and die out.

Because hot pressing generally lowersthe moisture content of a panel and coldpressing increases the moisture content,conditioning panels and other productsunder controlled humidity and tempera-ture is generally desirable and also mostefficient.

A typical example of inadequate condi-tioning is represented by the “sunkenjoints” sometimes found in edge-gluedlumber panels. They are often caused bysurfacing the stock too soon after gluing.The wood adjacent to the joint absorbswater from the glue and swells. If the panelis surfaced before this excess moisture isdistributed, more wood is removed alongthe joints than at intermediate points.Then during equalization of the moisture,greater shrinkage occurs at the joints thanelsewhere, and permanent depressions areformed. This condition is illustrated infigure 98 where panels were surfaced im-mediately after gluing pressure was re-leased. When improperly conditionedpanels are veneered, showthrough ofsunken joints and similar defects marssurface appearance.

Based on research at the Forest ProductsLaboratory, the following conditionsmaintained in a room with good circula-tion should provide reasonable assurance

that sunken joints will be minimized oreliminated in edge-glued panels:

7 days at 80° F. and 30 percent rela-tive humidity4 days at 120° F. and 35 percent rela-tive humidity24 hours at 160° F. and 44 percentrelative humidity16 hours at 200° F. and 55 percentrelative humidity

These recommendations are based onthe appearance of edge-glued panels witha high-gloss finish. With panels given amatte finish or panels covered with veneer,shorter conditioning periods generallywould be sufficient. If there is an appre-ciable layover period between the surfacingand the sanding and finishing operations,the conditioning period can probably besomewhat shortened, since some surfaceirregularities are removed by the sanding.

For edge-glued furniture panels that aresubsequently covered both with crossbands(usually one-sixteenth or one-twentiethinch) and face veneers (usually one twenty-eighth inch), it is expected that the condi-tioning times shown above could be appre-ciably shortened at the different tempera-tures, perhaps to as much as one-half thetime indicated.

ADJUSTMENTS IN ADHESIVESA ND GLUING PROCEDURES

The strength and quality of a glue jointdepend not only on the type of wood orthe quality of the glue used but also onthe gluing procedure in making the joint.Often the same glue is entirely adequatefor a wide range of species provided thegluing conditions are adjusted to the re-quirements of the particular species in-volved.

A specific example from productionillustrates the type of adjustments thatare required at times. White oak ship

96

M 86081 F

Figure 98.— Yellow-poplar panels edge-glued with urea resin cured by high-frequency dielectric heating and with animal glue set at room temperature. Upperpanels (left, urea; right, animal glue) were surfaced immediately after gluingpressure was released. lower panels (left, urea; right, animal glue) were condi-tioned 7 days at room temperature before they were surfaced. Note sunken jointson panels surfaced without conditioning.

frames were laminated in a plant where the where the same adhesive was used to lami-temperature generally exceeded 90° F. nate similar frames, the temperatureduring summer days. The glue was freshly ranged from 60° to 70°F. To obtain accept-mixed shortly before spreading. The lami- able glue bonds under these conditions,nations were assembled and clamping pres- the mixed glue had to be aged at leastsure applied in rapid succession. The glue half an hour before spreading and fullbonds were excellent. In another plant, gluing pressure was not applied for at least

9 7

2 hours after spreading. The longer closedassembly time was required (at the lowertemperature) to allow the glue to penetratethe dense wood surface and reach the properviscosity before applying full gluingpressure.

When bonding a dense wood, it appearsthat the glue must be viscous at the timepressure is applied on the joint. With alight, porous species much more latitudein glue viscosity is permissible. A lightwood is generally more absorbent; hence,a starved joint condition (glue too thinat time of pressure application) is less aptto occur. Also, a lower gluing pressureusually can be employed than with densespecies.

A good correlation has been noted be-tween the viscosity of urea resin and bonddurability in plywood made from slicedhard maple, with the higher viscositiesgiving the higher durability. The wood-worker of years past touched the spreadanimal glue film with his fingertips; whenthe glue was sufficiently tacky to stick tothe fingers and pull off the strings, heknew it was the proper time to bring themating pieces together and apply gluingpressure.

SELECTED REFERENCESAdhesives Age

1964. Edge glue spreader with vertical axisapplication roll. Adhes. Age 7(9):29.

Bellosillo, S. B.1970. Nail-gluing of lumber-plywood

assemblies-a literature review. Inf. Rep.OP-X-29. 70 p. Can. For. Serv., Dep.of Fisheries and Forest., Ottawa, Canada.

Chow, S. Z., and Hancock, W. V.1969. Method for determining degree of

cure ofphenolic resin. For. Prod. J. 19(4):21-29.

Currier, Raymond A.1963. Compressibility and bond quality of

western softwood veneers. For. Prod. J.13(2):73-79.

Freeman, Harlan G.1970. Influence of production variables on

quality of southern pine plywood. For.Prod. J. 20(12):28-31.

Rayner, C. A. A.1965. Cascade gluing in plywood manufac-

turing. Adhes. Age 8(2):33-35.Rice, J. T.

1965. Effect of urea-formaldehyde resin vis-cosity on plywood wood bond durability.For. Prod. J. 15(3):107-112.

Selbo, M. L.1952. Effectiveness of different conditioning

schedules in reducing sunken joints inedge glued lumber panels. For. Prod.J. 2(1):110-112.

Webb, David A.1970. Wood laminating adhesive system for

ribbon spreading. For. Prod. J. 20(4):19-23.

GLUING TREATED WOOD

Production of laminated glued woodproducts suitable for unprotected exterioruse dates back to the development of resor-cinol and phenol-resorcinol adhesives-about 1943. Glues available before thattime either lacked necessary water resist-ance or required very high curing tempera-tures such as those used for exterior-typeplywood.

The ability of resorcinol or phenol-resorcinol adhesives to provide a highly

durable bond at moderate temperaturesmade possible the production of largelaminated timbers suitable for exterior usefrom the standpoint of the bonded joint.To impart durability to the wood underexterior service, however, preservativetreatment is sometimes required. In someinstances, the laminations are treatedbefore gluing, and this necessitated devel-opment of procedures for bonding preserv-ative-treated wood.

98

M 128 112

Figure 99.— Bridge on logging railroad near Longview, Wash., built with laminatedstringers pressure treated with creosote-oil mixture before erection.

Preservative-treated structures can alsobe produced by treating the glued mem-bers, and numerous structures of this typehave been built (fig. 99). Treatment ofglued members permits application ofpreservative after all cutting, boring, andother framing has been done, to assure aprotective coating on all exposed surfaces.Material handling at the treating plant isoften simplified when the finished mem-bers, rather than the lumber, are treated.Probably the most serious disadvantage ofthis method is the limited size of treatingcylinders, which precludes treatment oflarger timbers and particularly of largecurved ones. Preservative penetration isblocked by gluelines to some extent andthis, of course, is a disadvantage. Further-

more, when only the outer layer of a mem-ber is treated, checks that sometimesdevelop later in service may allow decayto start. On the other hand, bridge timbersproduced by this method (pressure-treatedwith creosote or creosote and oil mixturesafter gluing) have been found to be in ex-cellent condition after more than 25 yearsof service.

Wood pressure-treated with preserv-atives can be used to produce members ofpractically any size and shape that arethoroughly impregnated. By proper selec-tion of materials, thin laminations can begiven complete penetration with preserv-ative chemicals; this as a rule is not possiblewith larger timbers. Laminated mem-bers produced from such treated stock can

99

be safely shaped and bored without ex-posing untreated material.

When a plant stocks treated lumber atthe proper moisture content, it can usuallyfill an order for glued treated wood muchmore promptly than when the membersmust be laminated and then shipped toa treating plant.

Of the two methods, treatment aftergluing has been most used. However,when laminated members do not lendthemselves to treatment because of theirsize and shape, gluing treated material isthe only known method to produce ade-quately treated members.

Studies on gluing of wood treated withwood preservatives and fire-retardantchemicals were undertaken during thelatter part of World War II and years that

followed. This type of material was gluedcommercially as early as 1945 (fig. 100).Data show that certain combinations ofglue and preservative treatments are com-patible under prescribed conditions ofgluing, whereas others require furtherstudy-both on laboratory and commer-cial scale-before definite productionprocedures can be formulated. The adviceof the glue manufacturer should be soughtbefore gluing wood treated with a partic-ular preservative.

All combinations of preservatives andglues do not perform equally well and theconditions that lead to good, durablebonds on untreated wood do not alwaysapply to treated wood.

Certain basic principles that apply togluing untreated wood do hold true to a

M 124 523

Figure 100.— Laminated southern pine stringers in 60-foot, open-deck trestle onAtlantic Coastline Railroad south of Palmetto, Fla. Lumber used in stringers wastreated with fluor-chrome-arsenate-phenol (Wolman salt) before gluing. Thestringers on the opposite side of the trestle were glued from creosote-treatedsouthern pine.

100

certain extent for gluing treated wood. Forexample, in gluing untreated wood thereis usually considerable difference in thegluing properties of the different species-the denser woods in general requiringstronger adhesion to the wood and greatercohesive strength in the glue than thelighter ones. This also applies to treatedwood, although bonding treated woodfurther depends on the concentration ofpreservative on the surface at the time ofgluing and the chemical effect of the pre-servative on the glue.

In general, somewhat more curing(higher temperature or longer time) is re-quired when gluing treated than untreatedwood.

A reasonably clean joint surface is re-quired in the bonding of untreated wood,and this appears to apply also to treatedwood. There also seems to be fairly goodevidence that, where gluing of treatedlumber is involved, surfacing after treating(preferably shortly before gluing) is re-quired. Where the glued members areintended to withstand exterior exposure,the joints should pass the tests prescribedin Voluntary Product Standard PS 56-73for structural glued-laminated timber.Where the treatment is intended mainlyfor resistance to termites and similarhazards, and the glued members areprotected from the weather, the tests pre-scribed for interior service of glue jointsin laminated timbers might be sufficient.

WOOD TREATED WITHOIL-SOLUBLE PRESERVATIVES

Because wood treated with oil-solublepreservatives usually goes into exterior orsimilar types of service, only adhesives suit-able for severe exposure conditions, such asresorcinol and phenol-resorcinols, shouldbe used. As a general rule, woods that taketreatment well, such as southern pine, canalso be glued satisfactorily. Those difficultto treat are more problematic, and a “cleantreatment” (by steaming or other means)

is required for satisfactory bonding. Thetype of solvent also affects gluability.Volatile solvents such as naphtha andmineral spirits cause less interference withbonding than heavier solvents such as fueloil. Wood treated with pentachloro-phenol in liquefied fuel gas is reported tocause practically no gluing problem.

WOOD TREATED WITHWATERBORNE PRESERVATIVES

When wood is treated with waterbornechemicals, the moisture content of thewood is appreciably increased and redryingis necessary. Upon being redried, thelumber generally is somewhat distorted,covered with deposits of chemicals to someextent, and too variable in thickness to besuitable for good gluing. Resurfacing be-comes necessary. When the stock is re-surfaced immediately before gluing, thereappears to be, generally, less problem ingluing wood treated with waterborne pre-servatives than in gluing wood treatedwith oil-borne preservatives. Laminatedbridge timbers glued from lumber treatedwith waterborne preservatives have givensatisfactory service for about a quartetcentury.

Treating large laminated timbers withwaterborne preservatives after gluing isgenerally not recommended because ofchecking and dimensional changes thatoccur during drying.

WOOD TREATED WITHFIRE-RETARDANT CHEMICALS

Because fire-retardant-treated wood isoften used in relatively dry exposure forsuch purposes as veneered doors, wallpanels, and partitions, adhesives onlymoderately resistant to moisture mightoccasionally be suitable. For maximum fireresistance (as far as the glue is concerned),it probably is necessary to use phenol,

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resorcinol, and melamine resins that donot permit the wood to delaminate orseparate when it is charred. Many fire-retardant salts are hygroscopic, and woodtreated with them has higher equilibriummoisture content than untreated wood.This is another reason for using adhesiveswith high water resistance.

Fire-retardant formulas usually employvarious chemicals in mixture so a desiredcombination of properties is obtained. Itis therefore extremely difficult to providegeneral recommendations for gluing woodtreated with such chemical mixtures.Wood treated with some widely used fireretardants has been glued successfully,however, using a high-formaldehyde-con-tent resorcinol adhesive developed especi-ally for the purpose. Before gluing fire-retardant-treated material, it is advisableto consult the treating company and theglue supplier or both for specific recom-mendations on the particular brand ofadhesive to use.

SELECTED REFERENCES

Bergin, E. G.1963. Gluability of fire-retardant-treated

wood. For. Prod. J. 13(12):549-556.Blew, J. O., and Olson, W. Z.

1950. The durability of birch plywoodtreated with wood preservatives and fire-retarding chemicals. Proc. Am. WoodPreserv. Assoc. 46:323-338.

Selbo, M. L.1959. Summary of information on gluing of

treated wood. U.S. For. Prod. Lab. Rep.1789. 21 p. U.S. Dep. Agric. For. Serv.For. Prod. Lab., Madison, Wis.

Selbo, M. L.1960. The gluing of treated wood. Proc.

Am. Wood Preserv. Assoc. 56:70-73.Selbo, M. L.

1961. Effect of solvent on gluing of preserv-ative-treated red oak, Douglas-fir, andsouthern yellow pine. Proc. Am. WoodPreserv. Assoc. 57:152, 163.

Selbo, M. L., and Gronvold, O.1958. Laminating of preservative-treated

Scotch pine. For. Prod. J. 8(9):25A-26A.

QUALITY CONTROL

The author’s first experience with evalu-ation of glue-joint quality was gained bysplitting apart the edge and end trims ofplywood panels as the panels came throughtrim saws. If the failures were mostly inthe wood, it was a good indication that thesamples would pass the more stringent andsophisticated laboratory tests, but if thefailures were in the glue joints, there wasa good chance they would not pass.

The purpose of mentioning such crudetests as ripping apart edge trims of ply-wood and prying apart laminations fromthe end trim of beams and arches with achisel is to stress these points: (1) Thesooner defective joints are detected, thesooner corrections can be made, and theless loss will be involved; (2) even a crudequality-control test is better than no testat all.

Up to recent years, shear block tests(ASTM D 905, Standard Method of Testfor Strength Properties of Adhesive Bondsin Shear by Compression Loading) on hardmaple were a common requirement in gluespecifications. This test has merit in evalu-ating glues for furniture of maple andsimilar species, particularly if specimensare also subjected to high-low humiditycycling; however, its dependability is farfrom adequate to estimate the service-ability of glues on species such as oak inexterior service.

It is good practice to evaluate a glue onthe species and under the bonding condi-tions that will be employed in production;it is also desirable to use test specimenssomewhat similar in construction to theproduct. But inasmuch as the range indensity and gluability is often appreciable

102

certain extent for gluing treated wood. Forexample, in gluing untreated wood thereis usually considerable difference in thegluing properties of the different species-the denser woods in general requiringstronger adhesion to the wood and greatercohesive strength in the glue than thelighter ones. This also applies to treatedwood, although bonding treated woodfurther depends on the concentration ofpreservative on the surface at the time ofgluing and the chemical effect of the pre-servative on the glue.

In general, somewhat more curing(higher temperature or longer time) is re-quired when gluing treated than untreatedwood.

A reasonably clean joint surface is re-quired in the bonding of untreated wood,and this appears to apply also to treatedwood. There also seems to be fairly goodevidence that, where gluing of treatedlumber is involved, surfacing after treating(preferably shortly before gluing) is re-quired. Where the glued members areintended to withstand exterior exposure,the joints should pass the tests prescribedin Voluntary Product Standard PS 56-73for structural glued-laminated timber.Where the treatment is intended mainlyfor resistance to termites and similarhazards, and the glued members areprotected from the weather, the tests pre-scribed for interior service of glue jointsin laminated timbers might be sufficient.

WOOD TREATED WITHOIL-SOLUBLE PRESERVATIVES

Because wood treated with oil-solublepreservatives usually goes into exterior orsimilar types ofservice, only adhesives suit-able for severe exposure conditions, such asresorcinol and phenol-resorcinols, shouldbe used. As a general rule, woods that taketreatment well, such as southern pine, canalso be glued satisfactorily. Those difficultto treat are more problematic, and a “cleantreatment” (by steaming or other means)

is required for satisfactory bonding. Thetype of solvent also affects gluability.Volatile solvents such as naphtha andmineral spirits cause less interference withbonding than heavier solvents such as fueloil. Wood treated with pentachloro-phenol in liquefied fuel gas is reported tocause practically no gluing problem.

WOOD TREATED WITHWATERBORNE PRESERVATIVES

When wood is treated with waterbornechemicals, the moisture content of thewood is appreciably increased and redryingis necessary. Upon being redried, thelumber generally is somewhat distorted,covered with deposits of chemicals to someextent, and too variable in thickness to besuitable for good gluing. Resurfacing be-comes necessary. When the stock is re-surfaced immediately before gluing, thereappears to be, generally, less problem ingluing wood treated with waterborne pre-servatives than in gluing wood treatedwith oil-borne preservatives. Laminatedbridge timbers glued from lumber treatedwith waterborne preservatives have givensatisfactory service for about a quartercentury.

Treating large laminated timbers withwaterborne preservatives after gluing isgenerally not recommended because ofchecking and dimensional changes thatoccur during drying.

WOOD TREATED WITHFIRE-RETARDANT CHEMICALS

Because fire-retardant-treated wood isoften used in relatively dry exposure forsuch purposes as veneered doors, wallpanels, and partitions, adhesives onlymoderately resistant to moisture mightoccasionally be suitable. For maximum fireresistance (as far as the glue is concerned),it probably is necessary to use phenol,

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within a species, as an added safety feature,the glue might also be evaluated on adenser species than the one to be used inproduction.

The well-worn phrase that dense wood is“more difficult to glue” does not neces-sarily mean that the same adhesive devel-ops high wood failure in a light wood andlow wood failure in a dense wood. The gluemay not have been used under the opti-mum conditions required for the denserwood; or it may not be strong enough tocause failure in the denser material.

As in any manufacturing operation,control ofquality of glue joints is extremelyimportant, particularly since the rawmaterials-wood and glue— are character-istically somewhat variable.

The person in charge of quality controlmust be very knowledgeable, both in woodtechnology and in the physical and chemi-cal characteristics of adhesives. There areproduct standards, industry standards,commercial standards, ASTM standards,Federal specifications, and military specifi-cations that generally specify minimumperformance requirements under differenttests, and the procedures for carrying outthe tests are fairly routine. But the inter-pretation of the test results, including thevisual examination of the test specimens,often requires a great deal of knowledgeand experience to determine their meaningand consider improvements or changes.

Needless to say, quality control does notinvolve only tests and evaluations of the

ZM 73979 F

Figure 101.— Effect of three types of accelerated laboratory tests on glue bonds inplywood-to-lumber gusset joints made with three types of adhesives. Thevacuum-pressure, soak-dry cycles resulted in the largest amount of joint separa-tion with each type of glue. The data indicate that two of the glues are unsuitablefor severe exposures.

103

ZM 69813 F

Figure 102.— Standard block, A, and stair-step type, B, shear specimens for evaluat-ing glue-joint strength and quality. The stair-step is convenient for testing succes-sive joints in a laminated timber.

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final products, but must begin with eachingredient that goes into the glued prod-uct.

This is particularly important where anumber of wood species are used and avariety of products are made. The adhesiveused for one species may not necessarilybe adequate for another, or at least mightrequire modification in the gluing proced-ure. Also, modifications might be re-quired when changing from one productto another.

The standards and specifications for thedifferent products generally specify one,and more often several, test requirementsthat a product must meet.

Although test methods must be usedthat will result in accelerated degradationof glue joints that would eventually failunder normal service conditions, the testsmust be reasonable for the type of bondinvolved. Even the very best casein-gluedjoint, for instance, will fail after a fewcycles of vacuum-pressure, soaking, anddrying. Casein glue is not capable of form-ing exterior-type bonds; hence, testmethods designed for such bonds are notapplicable to casein-glued joints. Effectsof three different accelerated test methodson three types of glue bonds in gusset-

M 76703

Figure 103.— Block-shear testing of gluejoints in universal testing machine.

type assembly joints are illustrated infigure 101.

Tests such as the compression blockshear (figs. 102 and 103) and the plywoodtension shear (figs. 104 and 105) have beenemployed to evaluate glue joints fordecades. They give a good indication ofinitial quality and workmanship in pro-ducing the joints when tested dry. Fordetermination of long-term durability,harsher treatments are required, and two4-hour boil cycles interspersed with 20hours of drying of the specimens (generallyreferred to as the boil test) has been themost widely used test for exterior plywood.Detailed procedures for this and other testsare given in many standards and specifica-tions for plywood and adhesives.

Vacuum-pressure, soak-dry tests similarto ASTM 2559 have been used for about3 decades to evaluate glue bonds in lami-nated construction (fig. 106) and have

Figure 104.— Tension-shear specimenfrom three-ply plywood.

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M 138 763-12

Figure 105.— Hydraulically operated,quick acting, tension-shear test ma-chine for plywood.

within recent years also been adopted forplywood and particleboard. They are moreindicative of weatherability and soak-dryresistance of glue joints and are also lesstime consuming than soaking and dryingat atmospheric pressure.

Tension tests are generally consideredthe most reliable for glued end joints. Arectangular specimen shown in figure 107is easily prepared and rapidly tested, im-portant features in quality control.

Figure 108 illustrates an electronicuniversal testing machine capable ofplotting stress-strain curves and suitablefor use in compression and tension testingof a wide range of specimen types.

M 59284 F

Figure 106.— Laminated oak beam section after completion of vacuum-pressure,soak-dry cycles (ASTM D 2559). Glue joints are still intact although wood is badlychecked from severe drying stresses inflicted by the test.

106

SELECTED REFERENCES

American Institute of Timber ConstructionStandard specifications for structural glued

laminated timber of Douglas-fir, westernlarch, and California redwood. AIRC 203(see current edition). Englewood, Cola.

American Society for Testing and MaterialsStandard specification for adhesives for struc-

tural laminated wood products for useunder exterior (wet use) exposure condi-tions. ASTM D 2559 (see current edition).Philadelphia.

American Society for Testing and MaterialsStandard method of rest for strength prop-

erties of adhesive bonds in shear by com-pression loading. ASTM D 905 (seecurrent edition). Philadelphia.

Callahan, Richard C.1953. Quality control in furniture manufac-

ture. For. Prod. J. 3(5):19-24.Kreibich, R. E., and Freeman, H. G.

1968. Development and design of an accel-erated boil machine. For. Prod. J. 18(12):24-26.

National Woodwork Manufacturers Association1969. Industry Standard I.S. 1-69. Wood

flush doors; hardwood veneered includinghardboard and plastic faced flush doors.July.

Ripley, Robert M.1953. Effective quality control on end prod-

uct. For. Prod. J. 3(5):25-28. Dec.Selbo, M. L.

1962. A new method for testing glue jointsof laminated timbers in service. For. Prod.J. 12(2):65-67.

Selbo, M. L.1964. Rapid evaluation of glue joints in

laminated timber. For. Prod. J. 14(8):361-365.

Selbo, M. L.1964. Tests for quality of glue bonds in

end-jointed lumber. ASTM Special Tech.Publ. No. 353: 1962 Symposium onTimber , pp . 78-86 . Am. So t . Tes t .Mater. Philadelphia.

U.S. Department of CommerceProposed Product Standard for hardwood and

decorative plywood. PS 51 (see mostrecent issue).

U.S. Department of CommerceSoftwood plywood construction and indus-

trial. U.S. Prod. Stand. PS 1 (see latestedition).

M 101 231

Figure 107.— Finger-jointed strip-tensionspecimen in test grips for testing.

U.S. Department of CommerceStructural glued laminated timber. Commer.

Stand. CS 253 (see latest edition).U.S. Department of Commerce

Wood double-hung window units. Commer.Stand. CS 190 (see latest edition).

107

M 138 766-11

Figure 108.— Electronic universal testing machine capable of plotting stress-straincurves.

U.S. Department of DefenseAdhesive; phenol and resorcinol resin base

(for marine use only). Mil. Specif. MIL-A-22397. Def. Supply Agency (see latestedition).

U.S. Department of DefenseAdhesive; modified epoxy resin with poly-

amine curing agent. Mil. Specif. MIL-A-81253(1). Def. Supply Agency (see latestedition).

U.S. Department of DefenseAdhesive; epoxy resin with polyamide curing

agent. Mil. Specif. MIL-A-81235(2).Def. Supply Agency (see latest edition).

U.S. General Services AdministrationAdhesive; urea resin type (liquid and

powder). Fed. Specif. MMM-A-188b.Fed. Supply Serv. (see latest edition).

U.S. General Services AdministrationAdhesive; contact. Fed. Specif. MMM-A-

130a. Fed. Supply Serv. (see latest edi-tion).

U.S. General Services AdministrationAdhesive; animal glue. Fed. Specif. MMM-

A-100c. Fed. Supply Serv. (see latest edi-tion).

U.S. General Services AdministrationAdhesive; vinyl acetate resin emulsion. Fed.

Specif. MMM-A-193c. Fed. Supply Serv.(see latest edition).

U.S. General Services AdministrationAdhesive; natural or synthetic-natural

rubber. Fed. Specif. MMM-A-139. Fed.Supply Serv. (see latest edition).

U.S. General Services AdministrationAdhesive; synthetic, epoxy resin base, paste

form, general purpose. Fed. Specif.MMM-A-187a. Fed. Supply Serv. (seelatest edition).

West Coast Adhesive Manufacturers Associa-tion’s Technical Committee

1966. A proposed new test for acceleratedaging of phenolic resin bonded particle-board. For. Prod. J. 16(6): 19-23.

West Coast Adhesive Manufacturers Associa-tion

1970. Accelerated aging of phenolic resinbonded particleboard. For. Prod. J.20(10):26-27.

GLOSSARY

Absorptiveness. — The ability of a solid toabsorb a liquid or vapor, or the rate atwhich the liquid or vapor is absorbed.

Aged (Matured).— The condi t ion a twhich the reaction between the activeingredients of an adhesive has reachedthe proper stage for spreading.

Air seasoning (Air drying).— The processof drying green lumber or other woodproducts by exposure to prevailingatmospheric conditions outdoors or inan unheated shed.

Annual ring.— The growth layer put ona tree in a single growth year, includingearlywood and latewood.

Architecturalplywood.— Plywood havingesthetic appeal, attractive grain pattern.

Assembly joints.— Joints for bondingvariously shaped parts such as in woodfurniture (as opposed to joints in ply-wood and laminates that are all quitesimilar).

Assembly time.— Interval betweenspreading the adhesive on the surfacesto be joined and the application of pres-sure to the joint or joints.

Note— For assemblies involvingmultiple layers or parts, the assemblytime begins with the spreading of theadhesive on the first adherend.

(1) Open assembly time is the timeinterval between the spreading of theadhesive on the adherend and the com-pletion of assembly of the parts forbonding.

(2) Closed assembly time is the timeinterval between completion of assem-bly of the parts for bonding and theapplication of pressure to the assembly.

Bacteria.— One-celled micro-organismswhich have no chlorophyll and multiplyby simple division.

Bag molding.— A method of molding orbonding involving the application offluid or pressure, usually by means of

air, steam, water, or vacuum, to a flex-ible cover which, sometimes in conjunc-tion with a rigid die, completely en-closes the material to be bonded.

Baseboard.— A board placed against thewall around a room next to the floorto finish properly between floor andplaster or gypsum board.

Blade-coating — Application of a film of aliquid material (liquid resin) on a panelsurface by scraping the straight edge ofa steel blade, or other material, over thepanel.

Blistering.— Formation of vapor pocketin a plywood panel because of too wetveneer, too much solvent in adhesive,too high adhesive spread, or too highcure temperature for the adhesive used.

Blood albumin.— Complex protinaceousmaterial obtained from blood.

Boilproof adhesive— Adhesive that willnot fail after many hours of boiling.

Bond failure.— Rupture of adhesivebond.

Book matching.— Matching veneer byturning over alternate sheets.

Boom.— A spar extending from a mast tohold bottom of sail outstretched; alsoused for loading and unloading pur-poses.

Bowing. — Distortion whereby the faces ofa wood product become concave orconvex along the grain.

Burnished.— A glazed surface with whichit may be difficult to obtain a satisfac-tory bond.

Burl.— Burls come from a warty growthgenerally caused by some injury to thegrowing layer just under the bark. Thisinjury, perhaps due to insects orbacteria, causes the growing cells todivide abnormally, creating excesswood, that finds room for itself in manylittle humps. Succeeding growth fol-lows these contours. Cutting across

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these humps by the half-round methodbrings them out as little swirl knotsor eyes.

Butt joint.— An end joint formed bygluing together the squared ends of twopieces. Because of the inadequacy andvariability in strength of butt jointswhen glued, such joints are generallynot depended on for strength.

Calking gun.— Device for dispensing abead of calking material, mastic glue,etc.

Capillary structure.— An inclusive termfor wood fibers, vessels, and other ele-ments of diverse structure making upthe material wood.

Casehardening. — A stressed condition ina board or timber characterized by com-pression in the outer layers accompaniedby tension in the center or core, theresult of too severe drying conditions.

Catalyst.— A substance that markedlyspeeds up a chemical reaction such asthe cure of an adhesive when added inminor quantity as compared to theamounts of the primary reactants.

Cell wall.— Enclosing membrane for theminute units of wood structure.

Cereal flour.— Flour from grain used asfood.

Char.— To scorch or reduce to charcoalby burning.

Checking.— A lengthwise separation ofthe wood that usually extends acrossthe rings of annual growth and com-monly results from stresses set up inwood during seasoning.

Chemical synthesis.— The formation of acomplex chemical compound by com-bining two or more simpler compounds,radicals, or elements.

Condensation reaction.— A chemicalreaction in which two or more mole-cules combine with the separation ofwater or some other simple substance.If a polymer is formed, the process iscalled polycondensation.

Continuous feed press.— Press in whichpanels are moving ahead (under pres-sure) while glue is setting.

Convex.— Curved like a section of the out-side of a sphere.

Copolymer.— Substance obtained whentwo or more monomers polymerize.

Clamping pressure.— Pressure developedby clamps of various designs to bringjoint surfaces into close contact for gluebond formation.

Cleavage.— Splitting or dividing alongthe grain.

Closed side.— Side of veneer not touchingknife as it is peeled from log (also calledtight side of veneer).

Coagulation.— The process by which aliquid becomes a soft, semisolid mass.

Cohesion.— The state in which the par-ticles of a single substance are heldtogether by primary or secondary va-lence forces. As used in the adhesivefield, the state in which the particlesof the adhesive (or the adherend) areheld together.

Cold flow.— Tendency to yield or “flow”under stress at normal room tempera-ture (see also Creep).

Cold pressing.— Pressing panels or lami-nates without application of heat forcuring the glue.

Compressometer. — Device used for meas-uring pressure. Consists essentially ofa cylinder, piston, and a pressure gage.Oil in the cylinder transmits the pres-sure applied to the piston to the gage.

Compression wood.— Abnormal woodformed on the lower side of branchesand inclined trunks of softwood trees.Compression wood is identified by itsrelatively wide annual rings, usuallyeccentric, relatively large amount ofearlywood, sometimes more than 50percent of the width of the annual ringsin which it occurs, and its lack of de-marcation between earlywood and late-wood in the same annual rings. Com-pression wood shrinks excessivelylengthwise, as compared with normalwood.

Concave. — Curved like a section of theinside of a sphere.

110

Co–spray dried.— Dried by spraying tworesins simultaneously into the samedrying chamber from atomizing nozzles.(See also Spray dried. )

Creep.— The dimensional change withtime of a material under load, follow-ing the initial instantaneous elastic orrapid deformation. Creep at room tem-perature is sometimes called cold flow.

Creosote.— Oily liquid used, among otherthings, as preservative for wood.

Critical exposure.— Exposure to harshconditions (see also Severe exposure.).

Cross grain.— A general term for anygrain deviating considerably from thelongitudinal axis of a piece of timberand emerging at an angle from a faceor edge.

Cross-link. — An atom or group connect-ing parallel chains in a complex mole-cule.

Crotch veneer.— Veneer cut from fork oftree to provide pleasing grain, figure,and contrast.

Cup.— Distortion whereby a board be-comes concave or convex across thegrain.

Curing (Cure).— To change the physicalproperties of an adhesive by chemicalreaction, which may be condensation,polymerization, or vulcanization; usu-ally accomplished by the action of heatand catalyst, alone or in a combination,with or without pressure.

Curtain coating.— Applying adhesive towood by passing the wood under a thinfalling curtain of liquid.

Dado.— A rectangular groove across thewidth of a board or plank.

Delamination.— The separation of layersin laminated wood or plywood becauseof failure of the adhesive, either in theadhesive itself or at the interfacebetween the adhesive and the adherend.

Density.— Weight per unit volume,generally expressed in pounds per cubicfoot. For wood, since changes in mois-ture content affect its weight andvolume, it is necessary to specify themoisture condition of the wood at the

time weight and volume are deter-mined.

Design criteria.— Standard rules fordesign.

Diagonal-grain wood.— A form of crossgrain where the longitudinal elementsrun obliquely but parallel to the surface;i.e., the growth layers are not parallelto the edge of the piece as viewed on aquartersawed surface.

Doctor roll.— Smooth roll whose positionin relation to spreader roll is adjustablefor regulating amount of glue spread.

Door skins.— Thin plywood, usuallythree-ply, used for faces of flush doors.

Double spreading.— Applying adhesiveto both mating surfaces of a joint.

Dovetail.— Joint shaped like a dove’s tail.Dowel. — Wood peg fitted into corre-

sponding holes in two pieces to fastenthem together.

Earlywood.— The portion of the annualgrowth ring formed during the earlygrowth period. Earlywood is less denseand mechanically weaker than latewood.

Edge gluing.— Bonding veneers or boardsedge to edge with glue.

Elasticity.— The capacity of bodies toreturn to their original shape, dimen-sions, or positions on the removal of adeforming force.

Elastomer.— A material that at room tem-perature can be stretched repeatedly toat least twice its original length and,upon immediate release of the stress,will return with force to its approximateoriginal length.

Electrodes.— In radiofrequency heating,metal plates or other devices for apply-ing the electric field to the materialbeing heated.

Elevated temperature setting.— An adhe-sive that requires a temperature at orabove 31° C. (87° F.) to set (see alsoRoom temperature setting ).

Emulsion.— A mixture in which verysmall droplets of one liquid are sus-pended in another liquid.

111

End grain. — The grain of a cross sec-tion of a tree, or the surface of such asection.

End joint.— A joint made by gluing twopieces of wood end to end, commonlyby a scarf or finger joint.

Equilibrium moisture content.— Themoisture content at which wood neithergains nor loses moisture when sur-rounded by air at a given relativehumidity and temperature.

Expeller.— Device that removes oil frombean by crushing (See also Roller mill).

Extender. — A substance, generally havingsome adhesive action, added to an adhe-sive to reduce the amount of the primarybinder required per unit area.

Exterior service.— Service or use in theopen (exposed to weather).

External load.— Load applied externally.External stresses.— Stresses imposed by

external load.Extractives.— Any substance in wood,

not an integral part of the cellular struc-ture, that can be removed by solutionin hot or cold water, ether, benzene,or other solvents that do not react chem-ically with wood components.

Extrusion spreading.— Adhesive forcedthrough small openings in spreader head(see also Ribbon spreading ).

Exudation products.— Tars and similarproducts that migrate to the wood sur-face.

Fiber saturation point.— The stage in thedrying or wetting of wood at which thecell walls are saturated with water andthe cell cavities are free of water. Alsodescribed as the moisture level abovewhich no dimensional changes takeplace in wood. It is usually taken asabout 30 percent moisture content,based on the weight when ovendry.

Figured veneer.— General term for deco-rative veneer such as from crotches,burls, and stumps.

Filler.— A relatively nonadhesive sub-stance added to an adhesive to improveits working properties, permanence,strength, or other qualities.

Film adhesive.— Describes a class of adhe-sives furnished in dry film form with orwithout reinforcing tissuelike paper orfabric.

Finger joint.— An end joint made up ofseveral meshing fingers of wood bondedtogether with an adhesive.

Fire retardant.— A chemical or prepara-tion of chemicals used to reduce flamma-bility or to retard spread of fire.

Flat-grained lumber.— Lumber that hasbeen sawed in a plane approximatelyperpendicular to a radius of the log.Lumber is considered flat grained whenthe annual growth rings make an angleof less than 45° with the surface of thepiece.

Flow.— In gluing, the state of a substancesufficiently liquid to penetrate pores andminute crevasses when pressure isapplied.

Fluid pressure.— Pressure applied by aninflated bag or similar means.

Flush panels.— Flat panels as on a flushdoor (no contorted or shaped parts).

Fortifier.— Material improving certainqualities in adhesives, such as waterresistance and durability.

Fungi.— Simple forms of nongreen plantsconsisting mostly of microscopicthreads (hyphae) some of which mayattack wood, dissolving and absorbingsubstrate materials (cell walls, cellcontents, resins, glues, etc.) which thefungi use as food.

Gap-filling adhesive.— Adhesive suit-able for use where the surfaces to bejoined may not be in close or continuouscontact owing either to the impos-sibility of applying adequate pressure orto slight inaccuracies in matchingmating surfaces.

Glazed.— Worn shiny by rubbing.Glossy finish.— Shiny finish, reflects

light.Gluability.— Term indicating ease or

difficulty in bonding a material withadhesive.

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Glue laminating.— Production of struc- High-frequency curing.— Setting ortural or nonstructural wood members by curing adhesive with high-frequencybonding two or more layers of wood electric currents.together with adhesive. Hollow-core construction.— A panel con-

Glueline.— The layer of adhesive affect- struction with facings of plywood, hard-ing union (bond) between any two ad- board, or similar material bonded to ajoining wood pieces or layers in an framed core assembly of wood lattice,assembly. paperboard rings, or the like, which

Glue wheel.— Continuous, caterpillar- support the facing at spaced intervals.type device or machine used for edge Honeycomb core.— A construction of thingluing panels or laminating small items sheet material, such as resin impreg-such as table legs. nated paper or fabric, which has been

Gluing pressure.— Pressure to bring the corrugated and bonded, each sheet insurfaces spread with glue into close opposite phase to the phases of adjacentcontact for bonding. sheets, to form a core material whose

Grain direction.— Fiber direction (essen- cross section is a series of mutually conti-tially parallel to pith of tree). nuous cells similar to natural honey-

Gravity feed.— Moves ahead by virtue of comb.its own weight. Honeycombing. — Fissures in the interior

Gusset.— A flat piece of wood, plywood, of a piece of wood generally caused byor similar type member used to provide drying stresses resulting from case-a connection at the intersection of wood hardening.members. Most commonly used at Hot press.— A press in which the platensjoints of wood trusses. They are fastened are heated to a prescribed temperatureby nails, screws, bolts, or adhesives or by steam, electricity, or hot water.with adhesive in combination with Humidity cycling.— Exposure to highnails, screws, or bolts. humidity followed by low humidity (or

vice versa) for various periods.Hammermill.— Consists of horizontal or Hygroscopic.— Term used to describe a

vertical shaft rotating at high speed on substance, such as wood, that absorbswhich crushing elements, hammers, and loses moisture readily.bars, or rings, are mounted. Incident lighting.— Light rays falling

Hardener.— A substance or mixture of on a surface at a low angle or almostsubstances added to an adhesive to pro- parallel to the surface.mote or control the curing reaction by Interior service.— Used in the interior (oftaking part in it. The term is also used a building) protected from outdoorto designate a substance added to control weather.the degree of hardness of the cured film. Internal stress.— Stresses set up from in-

Hardwood.— A conventional term for the ternal conditions, such as differentialtimber of broad-leaved trees, and the shrinkage, aside from external loadstrees themselves, belonging to the applied to a member.botanical group Angiospermae. Inverse proportion.— A relation between

Heartwood.— The wood extending from variables in which one increases as thethe pith to the sapwood, the cells of other decreases.which no longer participate in the life Jacketed mixer.— Double-wall mixer per-processes of the tree. Heartwood may mitting cooling or heating liquid to cir-be infiltrated with gums, resins, and culate between the walls.other materials that usually make it Jig.— A device for holding an assembly indarker and more decay resistant than place during gluing or machiningsapwood. operations.

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Jointer.— Machine equipped with rotarycutter and flat bed permitting surfac-ing one side of a member at a time.

Joint geometry.— Shape or design of joint(for example, a finger joint).

Joist.— One of a series of parallel beams,usually nominally 2 inches thick, usedto support floor and ceiling loads, andsupported in turn by larger beams,girders, or bearing walls.

Keel.— The chief timber or steel memberextending along the entire length of thebottom of a boat or ship to which theframes are attached.

Kiln drying.— The process of dryingwood products in a closed chamber inwhich the temperature and relativehumidity of the circulated air can becontrolled.

Lacquer.— A clear finishing materialconsisting of shellac or gum resins dis-solved in alcohol and other quick-dryingsolvents, with or without nitrocellulose.

Laminated member.— A wood memberglued up from smaller pieces of wood,either in straight or curved form, withthe grain of all pieces essentially parallelto the length of the member.

Laminated timber.— Synonymous tolaminated member, but usually impliesstructural member.

Latewood.— The denser, smaller celledpart of the growth layer formed late inthe growing season.

Layup.— Assembled parts placed in posi-tion they occupy in final product.

Lignin.— The noncarbohydrate, struc-tural constituent of wood and someother plant tissues, which encrusts thecell walls and cements the cells together;now believed to consist of a group ofclosely related polymers of certainphenylpropane derivatives.

Low-voltage beating.— Heating by pass-ing low-voltage electric current throughresistance elements.

Marine plywood.— Plywood made ofveneers of grades specified for marineuse and bonded with waterproof adhe-sive (usually phenolic type).

Mastic adhesive.— A substance withadhesive properties, generally used inrelatively thick layers that can be readilyformed with a trowel or spatula.

Matte finish.— Dull finish.Mature.— (see Aged ).Mechanical adhesion.— Adhesion ef-

fected by the interlocking action of anadhesive that solidifies within thecavities of the adherend.

Mechanical fasteners.— Nails, screws,bolts, and similar items.

Mesh sieve.— The size of openings in asieve as designated by the number ofmeshes (openings) per linear inch.

Mitered joint.— Joint cut at a 45° anglewith fiber direction.

Mixed grain.— Mixture of flat-sawn andquartersawn pieces.

Mold.— A fungus growth on wood prod-ucts at or near the surface and, there-fore, not typically resulting in deepdiscolorations. Mold discolorations areusually ash green to deep green,although black is common.

Molding.— Shaping or forming to desiredpattern or form.

Monomer.— A relatively simple com-pound which can react to form apolymer.

Mortise.— A slot cut in a board, plank,or timber, usually edgewise, to receivetenon ofanother board, plank, or timberto form a joint.

Multiopening press. -Press having anumber of platens between which panelscan be pressed.

Nailed glued.— A laminate for whichgluing pressure is obtained by nailingtogether the pieces spread with glue.

Nail popping.— Protrusion of nailheadsbecause of shrinking and swelling ofwood.

Natural adhesive.— Adhesive producedfrom naturally occurring products suchas blood and casein.

Neoprene.— Synthetic rubber.Nominal lumber.— The rough-sawed

commercial size by which lumber is

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known and sold in the market; forexample, a 2 by 4.

Nonvolatile.— Portion of a solution thatis not easily vaporized.

Oil-borne preservative.— Preservativedispersed or dissolved in oil carrier orvehicle.

Oil-soluble preservative.— Preservativechemical dissolved in oil carrier.

Open assembly.— The time interval be-tween the spreading of the adhesive onthe adherend and the completion ofassembly of the parts for bonding. (Seealso Assembly time. )

Open piling.— Stacking wood productslayer by layer, separated by strips ofwood inserted between layers to permitair circulation.

Open side.— Side of veneer next to knifeas it comes off the log (also called looseside).

Organic solvents.— Solvents based oncarbon compounds.

Original dry strength.— Shear strengthdeveloped by glue joints when testeddry and before aging or exposure todeteriorating conditions.

Overhang.— Part of roof extending be-yond the outer wall.

Overlay.— Plastic film or one or moresheets of paper impregnated with resinand used as face material, mainly overplywood but also on lumber or otherproducts. Overlays can be classified asmasking, decorative, or structural,depending on their purpose.

Parallel beating.— Electric or high-frequency field parallel to adhesivejoints.

Particleboard.— A generic term for apanel manufactured from lignocellulosicmaterials— commonly wood-in theform of particles (as distinct from fibers)which are bonded together with a syn-thetic binder (or other) under heat andpressure by a process wherein the inter-particle bonds are created wholly by theadded binder.

Pearl glue.— Animal glue dried in theform of round pearls.

Pitch.— In finger joints, the distancebetween midpoint of one fingertip andthe midpoint of the adjacent fingertip.

Pith side.— Side nearest to pith (and usu-ally center of tree).

Planer.— Machine equipped with cutterrolls and feed rolls for surfacing orplaning wood.

Plasticizer.— A liquid or solid chemicaladded to a compound to impart soft-ness or flexibility, or both, to it.

Platens.— Steel plates constituting thepressure elements in a single- or multi-opening hot press.

Plywood.— A composite product made upof crossbanded layers of veneer only orveneer in combination with a core oflumber or of particleboard bonded withan adhesive. Generally the grain ofadjacent plies is roughly at right anglesand an odd number of plies is usuallyused.

Pneumatic.— Filled with compressed air.Polymerization.— A chemical reaction in

which the molecules of a monomer arelinked together to form large moleculeswhose molecular weight is a multiple ofthat of the original substance. Whentwo or more monomers are involved, theprocess is called copolymerization orheteropolymerization.

Polyurethane.— A versatile chemical usedfor adhesives, sealing compounds,finishes, and other purposes.

Porosity.— The ratio of the volume of amaterial’s pores to that of its solidcontent.

Pot life.— Usable life of adhesive aftermixing (see also Working life ).

Precipitated.— Separated out (addition ofacid to milk causes curds to separate outfrom whey).

Precuring.— Condition of too much cureor set of the glue before pressure isapplied, resulting in inadequate flowand glue bond.

Prefabricated. — Factory-built, standard-ized sections or components for ship-ment and quick assembly, as for a house.

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Preservative.— Any substance that, for areasonable length of time, is effectivein preventing the development andaction of wood-destroying fungi, borersof various kinds, and harmful insectsthat deteriorate wood when the woodhas been properly coated or impregnatedwith it.

Quartersawed.— Sawn so the annualrings are essentially perpendicular to thewide face of the board. Lumber is con-sidered quartersawed when the annualgrowth rings form an angle of 45° to90° with the wide surface of the piece.

Rabbet.— A type of joint for fitting onewood member to another (for example,planking to keel and stem of a boat).

Racking.— Application of pressure to theend of a wall anchored at the base butfree to move at top.

Radiofrequency energy.— Electricalenergy produced by electric fields alter-nating at radiofrequencies.

Rail.— Bottom or top horizontal memberof a door.

Reaction wood.— Common term fortension wood in hardwoods and com-pression wood in softwoods.

Reactive.— Adhesives that cure or setrather fast (opposite to sluggish or slowcuring).

Reconditioned.— Brought back to aprevious condition (for example, pre-vious moisture level).

Relative humidity.— Ratio of the amountof water vapor present in the air to thatwhich the air would hold at saturationat the same temperature. It is usuallyconsidered on the basis of the weightof the vapor but, for accuracy, shouldbe considered on the basis of vaporpressures.

Rennet— A preparation or extract used tocurdle milk (as in cheesemaking).

Resiliency.— The quality of being resil-ient or elastic.

Resin.— A solid, semisolid, or pseudo-solid organic material that has an in-definite and often high molecularweight, exhibits a tendency to flow

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when subjected to stress, usually hasa softening or melting range, and usu-ally fractures conchoidally.

Resurfacing. — Planing again to obtain afreshly clean surface for gluing.

Ribbon spreading.— Spreading a glue inparallel ribbons instead of a uniformfilm.

Roll coating.— Application of a film of aliquid material (liquid resin) on a sur-face with rolls.

Roller mill.— Device for crushing beansby passing them between smooth rolls,thereby separating oil.

Room-temperature-setting adhesive.—An adhesive that sets at temperaturesbetween 20° and 30° C. (68° to 86°F.)— the limits for standard room tem-perature specified in ASTM D 618.

Rotary cut.— Veneer cut on a lathe whichrotates a log or bolt, chucked in thecenter, against a fixed knife.

Sandwich panel.— A layered constructioncomprising a combination of relativelyhigh-strength, thin, facing materialsintimately bonded to and acting inte-grally with a low-density core material.

Sapwood.— The living wood of pale colornear the outside of the log. Under mostconditions the sapwood is more suscep-tible co decay than heartwood.

Sash.— A frame for holding the glass paneor panes for a window.

Scarf joints.— Sloping joint between endsof two wood members.

Setting.— Hardening (see also Curing).Severe exposure.— Exposure to harsh

weather conditions or to harsh tests suchas boiling and drying at low humidities.

Shear.— The relative displacement ofwoody tissues following fracture as aresult of shearing stress.

Shear block test (also called glue blockshear test).— A means of testing a gluejoint in shear (ASTM D 905).

Shear parallel to grain.— Stresses appliedin a manner to cause shear failure alongthe grain.

Shear strength.— The capacity of a bodyto resist shearing stress.

Shoe (Tapeless splicer).— Device forbonding veneers edge to edge with glue(no tape).

Short grain.— Term used for cross grainas when end grain is exposed on faceof veneer.

Showthrough.— Term used when effectsof defects within a panel can be seen onthe face.

Sizing.— The process of applying dilutedanimal glue or similar material to theface or faces of a panel to reinforce fuzzyfibers and facilitate sanding.

Skinning.— Formation of a skin on theadhesive surface due to evaporation ofsolvent.

Sliced veneer.— Veneer that is sliced offa log, bolt, or flitch with a knife.

Slip joint.— Type of corner joint withinterlocking “fingers.”

Slope of grain.— Angle between graindirection and axis of piece.

Soak-dry cycles.— Type of test wherespecimens are alternately soaked anddried.

Softwood.— A conventional term for bothtimber and the trees belonging to thebotanical group Gymnospermae.

Solids content.— The percentage byweight of the nonvolatile matter in anadhesive.

Solid core.-Core with no open spaces asoccur in hollow cores.

Solvent.— The medium within which asubstance is dissolved, most commonlyapplied to liquids. Used to bring partic-ular solids into solution.

Spar.— Round wood member used onships for loading and unloading, alsofor keeping sails outstretched.

Spat-flange. — Upper or lower member ofa spar made in the form of an I-beam.

Specific adhesion.— Adhesion effected byvalence forces (of the same type as thosethat effect cohesion) acting between theadhesive and the adherend.

Specific gravity.— In wood technology,the ratio of the ovendry weight of apiece of wood to the weight of an equalvolume of water at 4° C. (39° F.). Speci-

fic gravity of wood is usually based onthe green volume.

Spline.— Thin piece of wood or plywoodoften used to reinforce a joint betweentwo pieces of wood.

Spray dried.— Dried under vacuum ofatomized particles of a liquid resin.

Squeezeout.— Bead of glue squeezed outof a joint when gluing pressure isapplied.

Starved joint.— A joint that is poorlybonded because insufficient adhesive hasremained in it as a result of excessivepressure on the joint or too low vis-cosity, or both; the adhesive is forcedout from between the surfaces to bejoined.

Stem.— Continuation of the keel to formthe prow of a boat or ship.

Stiles.— Vertical pieces in a panel orframe, as of a door or window.

Storage life.— The period of time duringwhich a packaged adhesive can be storedunder specified temperature conditionsand remain suitable for use. Sometimescalled shelf life.

Straight-grained wood.— Wood in whichthe fibers run parallel to the axis of thepiece.

Stress.— The force (per unit area) devel-oped in resistance to loading or, undercertain conditions, self-generated in thepiece by internal variations of moisturecontent, temperature, or both.

Stress risers.— Points of concentratedstress.

Structural plywood.— Plywood for struc-tural use, such as flooring, siding, androof sheathing.

Stud.— One of a series of slender, verticalstructural members placed as support-ing elements in walls and partitions.

Sunken joint.— Depression in woodsurface at glue joint caused by surfacingedge-glued material too soon aftergluing. (Inadequate time allowed formoisture added with glue to diffuseaway from the joint.)

Synthetic adhesives.— Adhesives pro-duced by chemical synthesis.

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Tuck.— The property of an adhesive thatenables it to form a bond of measurablestrength immediately after adhesive andadherend arelow pressure.

brought into contact under

Tapeless splicer.— Machine for joiningveneers edge to edge with glue only andno tape.

Tenon.— A projecting part cut on the endof a piece of wood for insertion into acorresponding hole in another piece tomake a joint.

Tensile strength.— The capacity of a bodyto sustain tensile loading (resistance tolengthwise stress). In wood, tensilestrength is high along the grain and lowacross the grain.

Tension parallel to grain.— Stress on amaterial (wood) in the long direction ofits fibers.

Tension wood.— An abnormal form ofwood found in leaning trees of somehardwood species and characterized bythe presence of gelatinous fibers andexcessive longitudinal shrinkage.Tension wood fibers hold togethertenaciously, so that sawed surfaces usu-ally have projecting fibers and planedsurfaces often are torn or have raisedgrain. Tension wood may cause warping.

Texture.— The arrangement of the parti-cles or constituent parts of material,such as wood, metal, etc. (Uniformlytextured wood— not a great differencebetween earlywood and latewood.)

Thermal softening.— Softens with heat.Thermoplastic.— Softens or becomes

plastic with sufficient heat.Thixotropy.— A property of adhesive

systems to thin upon isothermal agita-tion and to thicken upon subsequentrest.

Tongue-and-groove. — A kind of jointin which a tongue or rib on one boardfits into a groove on another.

Tooth planing.— Planing resulting in aridged or toothed surface which wasthought to give a better anchorage forglue than a smooth surface.

Torque wrench.— Wrench equipped withindicating device for measuring torque.

Transverse section.— Wood cut in a direc-tion perpendicular to the grain, pro-ducing an end-grain surface.

Treating cylinder.— Cylindrical-shapedvessel equipped with vacuum andpressure pumps used in preservativepressure treatment of wood.

Truss.— A frame or jointed structuredesigned to act as a beam of long span,while each member is usually subjectedto longitudinal stress only, eithertension or compression.

Twist.— A distortion caused by the turn-ing or winding of the edges of a boardso that the four corners of any face areno longer in the same plane.

Uncatalyzed.— No catalyst employed oradded.

Underlayment.— A material placed underfinish coverings, such as flooring orshingles, to provide a smooth, evensurface for applying the finish.

Vacuum molding.— Process of moldinga thin plywood or laminate to desiredshape by use of rubber bag, etc., fromwhich air can be evacuated.

Vacuum pressure.— Term describingprocess of applying vacuum and pressurealternately.

Varnish.— A thickened preparation ofdrying oil or drying oil and resin suit-able for spreading on surfaces to formcontinuous, transparent coatings or formixing with pigments to make enamels.

Veneer.— Thin sheets of wood made byrotary cutting or slicing of a log.

Veneer clipper.— Machine for cuttingveneers into desired sizes.

Viscosity.— That property of a fluidmaterial by virtue of which, when flowoccurs inside it, forces arise in such adirection as to oppose flow.

Waterborne chemical.— In wood preserv-ing, a chemical dissolved in water tofacilitate penetration into wood.

Water soluble.— Substance that can bedissolved in water.

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Weathering.— The mechanical or chemi-cal disintegration of the surface of woodthat is caused by exposure to light, theaction of dust and sand carried by winds,and the alternate shrinking and swellingof the surface fibers with continual varia-tion in moisture content brought bychanges in the weather. Weatheringdoes not include decay.

Wet-bulb temperature.— The tempera-ture indicated by any temperature-measuring device, the sensitive elementof which is covered by a smooth, clean,soft, water-saturated cloth (wet-bulbwick).

Wet joint strength.— Shear stress resistedby joints after exposure to water soakingor in wet condition.

Whey.— The thin, watery part of milkwhich separates from the thicker parts

(curds) after coagulation, as in cheese-making.

Wood failure.— The rupturing of woodfibers in strength tests on bonded speci-mens, usually expressed as the percent-age of the total area involved whichshows such failure.

Wood flour.— Very finely divided wood,as produced by grinding in a ball mill.It is graded according to the mesh itmust pass.

Working life.— The period of time duringwhich an adhesive, after mixing withcatalyst, solvent, or other compoundingingredients, remains suitable for use.

Zinc white.— Zinc oxide used as a pig-ment.

Zone of char.— Zone burned to a char(see also Char).

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INDEX

Acid-catalyzed phenol resin adhe-sives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 93Adjustments in adhesives andgluing procedures . . . . . . . . . . . . . 96Alkaline phenol resins . . . . . . . . 13, 65, 93Animal glue . . . . . . . . . . . . . . . . . 27

Durability . . . . . . . . . . . . . . . 28Mixing . . . . . . . . . . . . . . . . . 27Used at room temperature.... 92Used in furniture manufacture . 71

Assembly time. . . . . . . . . . . . . . . . . . 90, 109Closed assembly. . . . . . . . . . . . . . 87, 90, 109For animal glues . . . . . . . . . . . . 28For phenol resin adhesives. . . . . 13For resorcinol resins . . . . . . . . . 15For room-temperature-settingurea resins . . . . . . . . . . . . . . . . . . 19Open assembly . . . . . . . . . . . . . . . 87, 90, 109, 115

Bag molding . . . . . . . . . . . . . . . . . . . 14, 109Blood glue . . . . . . . . . . . . . . . . . . . . 33, 65Bonding dense woods . . . . . . . . . . . 98Bonding laminated timbers ...... 69, 97Bonding light, porous species . . . . 98Bonding plywood . . . . . . . . . . . 65Butt joint . . . . . . . . . . . . . . . . . . 49, 110Calking gun . . . . . . . . . . . . . . . . 82, 89Casein glues . . . . . . . . . . . . . . . . . . 29

Durability . . . . . . . . . . . . . . . . . . 31, 32Formulation . . . . . . . . . . . . . . 29Preparation . . . . . . . . . . . . . 29Prepared . . . . . . . . . . . . . . . 30Use characteristics . . . . . . . . . 30Used for furniture bonding . . . . 71Used for laminated timbers . . . 69Wet-mixed casein glue . . . . . . . . . 30

Causes and prevention of warping . 40Causes of plywood cupping . . . 41Circular tongue-and-groove edgejoint . . . . . . . . . . . . . . . . . . . . . 61Clamping glue joints . . . . . . . . . . 91Closed assembly . . . . . . . . . . . . 87, 90, 109Conditioning glued products . . . . . 96Contact adhesives . . . . . . . . . . . . 26, 85Corner joints . . . . . . . . . . . . . . . 49Crossbanded construction . . . . . . 36Cupping of plywood ............... 41Curing adhesives . . . . . . . . . . . . . . . 92

Elevated temperature curing .. 92, 93Equipment . . . . . . . . . . . . . . . 93, 94Hot-setting ureas . . . . . . . . . . 92Low voltage heating . . . . . . . 83Melamines . . . . . . . . . . . . 92

Curing temperatures for adhe-sives . . . . . . . . . . . . . . . . . . . . 77

Acid phenolic glues . . . . . . . . 13Alkaline phenol resins. . . . . . . . . 13Blood glues . . . . . . . . . . . . . 33Intermediate-temperature-setting phenol resins . . . . . . 14Melamine resin adhesives . . . . 21Resorcinol resins . . . . . . . . 15Thermosetting polyvinyl emul-sions . . . . . . . . . . . . . . . . 24Urea resin adhesives . . . . . . 18

Cutting and preparing veneer . . . . . 62Density . . . . . . . . . . . . . . . 4, 111Double spreading . . . . . . . . . 89, 111Dovetail edge joint . . . . . . . . 62Drying lumber . . . . . . . . . . 58Drying veneer . . . . . . . . . . 58Durability of

Alkaline phenol resins .......... 16Animal glue joints .............. 28Casein joints . . . . . . . . . . . . . . . . 31, 32Melamine resins . . . . . . . . . . 22Phenol resorcinol resins . . . . . . . 16Polyvinyl resin emulsions .. . . . . 24Resorcinol resins . . . . . . . . . . . 16Urea resins . . . . . . . . . . . . . . . . 17, 19, 20

Durability of synthetic adhe-sives . . . . . . . . . . . . . . . . . . . . . 10, 24Edge gluing . . . . . . . . . . . . . . . 35, 63, 111Elevated temperature curing ofglue . . . . . . . . . . . . . . . . . . . . . . . 92End-joint gluing . . . . . . . . . . . . 35, 112

Circular tongue-and-groove . . . . . 61Dado . . . . . . . . . . . . . . . . 62Dovetail . . . . . . . . . . . . . 62Mortise-and-tenon . . . . . . . . 62Plain . . . . . . . . . . . . . . . . 62Rabbet . . . . . . . . . . . . . . . 62Slip . . . . . . . . . . . . . . . . . 62Tongue-and-groove . . . . . . . . . 62

End and corner joint construc-tion . . . . . . . . . . . . . . . . . 49, 50Epoxy resin adhesives . . . . . . . 25, 81Extenders used in synthetic adhe-sives . . . . . . . . . . . . . . . . . 10, 112Extruders used in applying adhe-sives . . . . . . . . . . . . . . . . . . 89Factors affecting gluing . . . . . . 2

Assembly time . . . . . . . . . 2Pressure . . . . . . . . . . . . . 2Temperature . . . . . . . . . . 2

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Figured veneer ..................... 39, 112Fillets used in synthetic adhe-sives ................................. 10, 13, 112Finger joints ........................ 18, 51, 94, 112Flush doors ......................... 79Fortifiers ............................ 18, 112Furniture ............................ 70

Glues used ....................... 29, 72, 73Species used ..................... 70

Gluability test ...................... 2Glue block shear test .............. 116Gluing doors ....................... 79Gluing hardwood plywood ......... 67Gluing housing and housing com-ponents .............................. 82, 83Gluing jewelry ..................... 84Gluing laminated flooring ........ 84Gluing operations .................. 87

Applying gluing pressure ...... 91Assembling parts ............... 90Assembly time .................. 90Conditioning glued stock ...... 96Mixing ........................... 87Spreading ........................ 88

Gluing particleboard .............. 81Gluing preservative-treated wood . 99

Treated with fire-retardantchemicals ......................... 101Treated with oil-soluble preser-vatives ............................ 101Treated with resorcinol-resinadhesives ......................... 17Treated with waterborne pre-servatives ......................... 101

Gluing pressure .................... 91Gluing spotting goods ............ 81Gluing treated wood .............. 98Gluing untreated wood ........... 101Gluing woods treated with oil-soluble preservatives ............... 101Gluing woods treated with water-borne preservatives ................. 101Hardener for synthetic adhesives . . 10Hardwood plywood ................ 67

Construction ..................... 67Moisture content when gluing . 67

High-frequency heating ........... 18, 74, 94, 113Hot-melt adhesives ................ 21, 25, 63, 74Hot-press urea resin adhesives . . . 18, 92Intermediate-temperature-settingphenol resins ....................... 14Jig ................................... 83, 113Laminated construction ........... 45

Bonding .......................... 69Properties ........................ 45Selection of species and grades . . 46Stresses in ........................ 47Uses .............................. 45

Laminated flooring ................ 84Laminated ship and boat mem-bers .................................. 76

Machine marks on lumber ........ 60Machining special types of joints 62

End-grain surfaces .............. 62End-to-side-grain surfaces ..... 62Side-grain surfaces .............. 61

Marine plywood .................... 77, 114Mastic adhesives ................... 26, 82, 114Melamine resin adhesives ......... 21, 22Melamine urea resins .............. 22, 74, 81, 92,

95, 102Mixing adhesives ................... 87, 88Moisture content of wood ........ 56

At time of gluing ............... 56Gluing furniture ................ 56Gluing hardwood plywood ..... 67Gluing lumber .................. 58Gluing veneer ................... 58In service ......................... 56Using casein glue ............... 30

Nail gluing ......................... 82, 83Natural origin adhesives .......... 27

Animal ........................... 27, 73Blood ............................. 33Casein ............................ 29soybean .......................... 32

Open assembly ..................... 87, 115Overlay ............................ 115Overlays bonded to wood ......... 85Pearl glue ........................... 115Phenol-formaldehyde adhesives ... 13Phenol resins ....................... 11, 65Phenol-resorcinol resins ........... 15, 16, 72, 77,

80, 84, 92,95, 101

Plain edge joint .................... 61Plywood advantages vs. solid woodconstruction ........................ 36Plywood construction ............. 36, 38

Requirements .................... 43Species preferred ................ 46

Plywood cores ...................... 43Requirements .................... 43

Polyvinyl resin emulsions ......... 22, 23, 73, 89,92

Pot life .............................. 115For epoxy adhesives ............. 26For melamine resin adhesives ... 22

Preparing wood for gluing ....... 56Pressing and curing glue joints .. 83Pressing or clamping .............. 91Properties important in bonding 4Quality control ..................... 102

Tension tests .................... 106Vacuum-pressure, soak-drytests ............................... 106

Rate of curing or setting ......... 10, 11Requirements for crossbands 43.....Resorcinol-resin adhesives ........ 14, 16, 72, 78,

83, 84, 93,95, 101, 102

Ribbon spreading .................. 116

121

Room-temperature-setting urearesins ................................ 17, 92, 116Rotary-cut veneer .................. 62, 63Scarf joints .......................... 49, 116Serrated scarf joint ................. 62Shear block test .................... 116Ship and boat construction ....... 76

Adhesives used .................. 77Gluing operations ............... 77Species desired .................. 46Use of melamine resin glues 21Use of resorcinol resins ......... 15

Shrinking and swelling ........... 7Sliced and sawed veneers .......... 63Softwood plywood ................. 65

Exterior .......................... 65Interior ........................... 65Uses .............................. 67

Solid cores for flush doors ........ 79Soybean glue ....................... 27, 32, 65Spreading adhesives ................ 88Starved glue joint .................. 90, 117Storage of lumber .................. 59Storage of veneer ................... 60Stress relief in laminated construc-tion .................................. 49

Sunken joints .......................Surfacing wood for gluing ........Synthetic adhesives ................

Advantages ......................Durability ........................Melamines .......................Phenols ...........................Polyvinyls ........................Resorcinols .......................Ureas .............................Used in manufacturing .........

Tenon ..............................Thermosetting polyvinyl emul-sions .................................Tongue-and-groove ................Twisting of plywood ..............Urea-formaldehyde resin adhe-sives .................................

Vacuum molding ................Vinyl overlays ......................Warping, causes and preventionWood “jewelry” ....................

966010101021, 221322, 2314, 15, 2417, 18, 19, 7210118

23, 7361, 11840

17, 18, 19, 20,72, 81, 84,92

118854084

*U.S. GOVERNMENT PRINTING OFFICE: 1975 O— 566–978

122