SERVIC~A131~ITy OF GLUE JOINTS

SCHOOL OF FORESTR YOREGO1 STATE MIME

CORVALLIS, OREGO N

UNITED STATES DEPARTMENT OF AGRICULTUREL FOREST SERVIC E

L OREST PRODUCTS LABORATOR YMadison, Wisconsi n

In Cooperation with the University of Wisconsi nApril 1938

OF GLUE JOIaTSl

o sure tests give data_ vegetable, asein, and blood types-Trr3 . '

By

Bas se , Eprgsir e'or

Introduction

Tests have demonstrated that among the more important causes o ffailure of well-made glue joints in service are the following : (a) chemicalhydrolysis of the glue,,,(b) destruction of glue by microorganisms, and . (c )mechanical stresses developed on the glue joint as the wood changes cimen-siohs under the effect of changes in moisture content . The relative

different factors varies largely, depending on th etype of glued construction, the variety of glare,-an _service conditi-ens ..

The significance of :W 4i. is and. Sts rel.a-tila to ,a 'ham11tas been deve3apedd for casein glues (1)2 ant the ge p'! prine•idbelieved to app ,y to of ier<0ro'tre .llrl es . lager conditions f v al : edecay the destructive eeff'ft

epoorganisms on glues of corn osi ri'arthat serve as- fbed~ has b e

' . ; o be ?apid and _x r ,trti,•o- . • The ea fewt, how-ever, may be gre t .y redlaie '_by twIoe-o> atirrg tea~ic ri ztemilals in the glueor by adequate trea ent of tlt--e . entire. g1 ad-'a ffber with toxic. preserva-tives (2) . The-.lfect of micr•lergatiams on'the glue joint and deteriora-tion due to Yy!ol.ysis ray,, off' *o)tV .se, be ,eliminated by the selection ofadhesives that t not subject •tie. . attack by i .olds and ,that do not hydrolyze .

The effect of met +fi nicalaction, ii Wever, ¢ ia®rt be greatlyretarded by minor changes 14 formulation . t-4holag4D .1

1 iag mlekWwl ave_ a remarked-

t on b1 ate ,., ' hydro .ysis,

.w .ll the mechanicaleft-eats be changeO'to r ' it 1degroe ~ ~_ g . toxic terials t o~the glue ory tireatirrg~ , tide , I He'd I.Ambers wi .-

14 co

ly used fo rthe p<axe.sOvati,on of wood. 'The primary re•q• ' ; t, a :ttga t,.*e joints ar eto maintain the4-r strength under mereatefl ehah0p. in tmo; d

i c'eo

ns ,appears to be'a:-O.ue that ' i=w-fad-0s enough, eithi• in i ,we

c fir= nditionto. withstand 'this Ere *s c ®veiled udei,=sere conditions of service .

1Published' in Mechanical Engineering, April 1938 .2-.Number .s inn .parentheses refer to the Bibliogt'a y. at tflaoJINId o the WuBad

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Failure of we.1 ade. glue joints un,ela s

e e s deve

tchanges in moisture co n4e-i;1 r

be the 0t. 'e'€ as do 5a

li'rst ,magnitude of the stre .e 1 ,d 61Vped, a, ,d s Gnd, upoft 't`hlc ree to-TABthe glue softers and: ,weakens When it be*orse:s 4ed,st . The

a of thmedhaAtAOt ettageottdr velopeci-depends on the extaaat of the. arta4'e . eil rage sand how far the extremes of •moisture oontegt depart fx+e'rm the moiiM 99on-tent of the stock at the time the glue sets ; also upon the densi andshrinkage characteristics of the species used, .a . the thi I S of thelaminations . The resistance that the glue' lin -e offers *o ,ghe4swigMt;eeOBs ,may be thought of as depending oa. the ame it of water absorbed .

e glueline, and the degree to which the glue . softens- and ]cpses its s~ngth whenthis absorption occurs . Glues that are, water resiiant will wa standexposure to higher humidities than glis that. are laeking in tilt* proper ty.

Tests were conducted at the FO-r•eet Products -Laa , eI rmine the relative resistance of some of the commionly used we't~adhesives to service conditions involving succae•ssive exposii . 0 alternatingconditions of moisture that would develop umeohanical s',r ~ss' o- .nexeasingmagnitude on the glue joint . The purpose ,s gfur :here 4Q-1c._i.

7f'the conditions of service under whic} ~main permar~s

durable and the cone`,

eit

t o ~_ , 4

11.14

could be expected.

'

er

.,-

n pane ; 4 0 'r i roastM e

a and compthree plies of selected. 116-in . yellow birch veneer,

eachof seven glues . They were . (a) sodium-silicate-lime-casein glue

t1 ac-cording to . for-mina. )4-R of

Forest Products Laboratory, (b) and (c) twosodium-{h drextde-liaae-easein goes, (d) high-grade animal glue having ajelly st3 eatgth of 303 a and a viscosity .of 1,04 r llipoises, (e) low-gradeanimal glue ha ns a je1iy.O etgth of 190 g am* a 'v 'e 'st'

`, -

`poises, (f) c omtert_9.g v eg r it a .or starch glue a*d_

" _-.glue mixed, acrimrtli-n ; t 'o

paraterrnaldeh0cle formu7ld ' - •e 'Laboratory. In Table l,'rgich eves the compositioh. •of they

on aparts-byr.weight basis, the•ind idi 1 glues -a,re desi ■ted by' ,'e,Lettersused in the preso,edi-rag sentence ,

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In these tests the complications of species characteristics an dthickness combinations were eliminated by the use of a single species an da single thickness combination throughout, although it is recognized tha tthese factors must be considered when general applications of the result sare made . Further, the tests do not include the more recently develope dsynthetic resin adhesives . Other tests, however, have indicated thatjoints made with hot-pressed synthetic resins are highly resistant t oconditions favoring mold attack and hydrolysis and that they retain ahigh percentage of their dry strength when saturated with water or expose dto high humidities .

;4 .

1 T

Jj

Table 1 .--Composition of glues used in test s

Constituents

----G 1 u e--- -

(a)

(b)

(c)

(d)

(e)

: (f)

(g )

Ammonium hydroxide 5 . 5Animal glue 100 : 100Blood albumin 100

Casein 100 : 100 : 100Lime 2g . 15 : 15Paraformaldehyde 1 5

, processed" starch 100

Sodium hydroxide 12 . 4

3Sodium silicate : 70Pater 330 : 2 . 0 . 380 . 225 . 195 : 225 . 190

The veneer was conditioned to approximate equilibrium with 30percent relative humidity before gluing, which brought it to a moistur econtent of about 6-1/2 percent . The gluing conditions were adjusted t ofall within limits favorable to the production of good joints (3) with eachglue . The gluing operations with animal, casein, and vegetable glues wer ecarried out in the conventional way . The panels glued with blood albuminwere spread and pressed at room temperatures, allowed to remain unde rpressure overnight, and hot-pressed the following morning for about 1 0minutes at 260° F. and under a pressure of 200 pounds per square inch .

After gluing, the panels were again conditioned to approximat eequilibrium with 30 percent relative humidity and then cut into standar dplywood test specimens (3) . Each of the 17 panels yielded 30 test speci -mens, giving a total of 510 for each glue . Five specimens from eachpanel were tested dry, and any panels that gave low or erratic test value swere discarded .

$ped"imens rfom 't` f̀ie 'remains g panels of each glue were then mixe dtogether to insure random sampling and divided into 6 groups of 60 each ,one group for each of the tests listed in Table 2 . Air temperatures wer emaintained at approximately 80° F ., while the soaking water temperature sof test No . 6 varied with those of the laboratory. Five specimens wer ewithdrawn from each group and tested on a plywood-testing machine of con -ventional design (3) at intervals of 1 , 8, 12, 16, 20, 40, 6o, 80, 120 ,and 160 weeks . Specimens were unprotected, and their dimensions weresmall enough to permit the wood to attain approximate equilibrium wit hatmospheric conditions at each period of the exposure cycle .

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Table 2 .--Data on exposure test s

First stage Second stage

Type of : •Test

: exposure Time,

: Relative

. Time,

. Relativecycle days

: humidity,

:percent

days:

humidity ,percent

1

: Continuousa . . . . . . 30 302

: Alternating 14 60 14 30Alternating l4 g 0 14 30

Alternating 14 90 14 3 05

: Alternating 14b 97 b 14 306 Alternating 2- - 12 30

I7sed as controls .

kSoaked in water at room temperature .

In analyzing the data from the tests, the average test value o feach group that remained in 30 percent relative humidity was considered an control" average and other test values of specimens glued with the sam eglue were expressed as percentages of this . As the different exposure swere continued and test values were obtained at different intervals, thes ewere converted into percentages and plotted . After plotting, the point swere joined by straight lines to illustrate general trends in strengt hchanges . No attempt has been made to draw curves of mathematical exact -

. -ness because the number of specimens tested at each period was no tsufficient to justify such treatment . The tests have extended over asufficient period of time, however, to permit confidence in the genera ltendencies illustrated.

Animal and Vegetable or Starch Glue s

Both animal and vegetable or starch glues are lacking in wate rresistance, and the durability of joints made with them and exposed t owater or to damp atmospheres is so similar that the results were average dtogether and shown on the same curves, Figs . 1 and 2 . When exposed t otest No . 6, a soaking-drying cycle, all joints made with either anima lor vegetable glue separated completely either in the first soakin gperiod or in the drying that immediately followed (Fig . 2) . This be-havior is to be expected of glues that are not water-resistant .

Specimens, glued with animal or vegetable glue, exposed to tes tNo . 5 did not separate immediately, but at the first test period, the en dof the first cQnrlete cycle, the average joint strength was less than 40percent of the original . All specimens had failed before the end of thefourth cycle, or in somewhat less than 16 weeks . This test does no t

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involve exposure to free r a,tero but a relative Midity Qf 91 , parrs.

aissufficient "high to p:eimi t -devalo&rent of me L&s "and-to ? r d g ; a ur e

content of the wood to about 2 g percent during the "wet _l at" . Of tr i

cycle . Specimens exposed to test No . 4 failed less rapidly than Vh se ex -

posed to test No . 5 . They had decreased in strength, ho.Wover-,, by sl e •

50 percent by the end of the first test cycle, and all specimemshhad .

failed by the end of 36 weeks . Moisture content of the wa64 du-i_i g- tthe"wet half" of this circle probably reached about 22 pereen . (Fig . 1); .

In test No . 3, more than 20 weeks of exposure were requied -be -fore the average test values had fallen to 50 percent of the . origir,T.By the end of the eighty-first week, however, all •spetimaens . has f 401.6dindicating that animal- and vegetable-glued joints cannot be dep cOdupon to remain permanently durable when exposed to conditions where thewood may at times exceed about 17 percent moisture -erontant, which. i sapproximately the equilibrium e dition for g0 pegoant relative hu ity(Fig . 1) . These results may appear contrary to exile ience, for •f tat 00glued with starch and animal glues often serves sati fdeto ly i .n ;ipi -te ofoccasional exposures to relative humidities in excess of go p:eree:r t .In those cases, however, the proteetion afforded b the finiehh . mayprevent the . weed reaching the equilibrium moisture c'Ontent-tlf 17 percent ,particularly if the exposure , to dampness is not p :aloage4 ..

In tests Nos. 1 and 2, no evidence of signifieant loss instrength could be detected during the 160 weeks . that the tests were i nprogress . Test No . 2 approximates the changes t : moLst:ix o •onisenti tl attcan be expected in interior woodwork in normal uste in heated bujltnvgs- i nthe northern part of the United States (4) . In this type of .ore e,properly designed and well-made joints of animal .or vegetable glue shoaa]4prove permanently durable, and experimental evidence, other than thes e s .tests, supports that belief (5) .

Casein and Blood Glue s

When exposed to conditions that cause alterrna ting : achesthe moisture content of woo y,;,the casein glues used in these e-x .a iten:t sdisplayed no great differem et among themselves in Aur ab 1.itr of jo-t,a: ..Results from the three casein glues have been avetaged toget er, there-fore, and are shown in a single set of curves as Fig . 3 . Tiey are be-lieved to be typical of the general. tendencies that would dev~aI i. i-wbolany of the better grades of casein glues are subjected to• similar `I pigs'. .

Results of test No . 6 illustrate that joints made with gainglue cannot be termed waterproof . In this series of s'oakii -dryiA. ests ,all specimens glued with casein glue had failed by the end of 15:_voata. .In other similar tests (6), similar results have-been obtained . Mil ethe time of total failure has varied from 3 to 15 months, an eemiy weaken -ing of the joints was apparent in all similar tests .

Exposure to 97 percent relative humidity followed by d ng ,0

test No, 5, also causes rapid weakening of unprotee ed = s a g as _ flint .

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with total failure occurring in these testre r a t abo ut 1 . . oaOq or 12,complete cycles . Even the cycle of-2 weeks in 90 pers. rtt re] ' lark . talia tyfollowed by 2 weeks drying i 3.0: percent releegvi humidity, test .40,. :1 ,was severe enough to cause ultimate failure of unprotected., s~esi;

-ljoints . The rate of failure is slower than that displayed by 'joints o fvegetable or animal glue exposed to the same coaditi.ons, but at the- en dof some gl weeks, 20 complete cycles-, all casein joints had failed.

In test No . 3, hemmer, me waidence of significant wpakenpng ofjoint strength was disclosed during the ].60 weeks that the tests sage inprogress . The difference in resistance between casein and animal orvegetable glues is demonstrated in this test. Well-made ca:seln.gluajoints are apparently able. to . withstand repeated eaposur_es to-80 lerCen :trelative humidity while joints made with starch or animal glues u1tir` .t;eL,yfail in this exposure. Tests Nos . 1 and 2 failed to disclose any weakenr-ing of hell-made casein-glue joints while the tests were in progress an dnone would be expected, regardless of the length of time that the test smight be extended .

Until the development of artificial-resin glues, the "heteptft 1iparaformaldehyde blood glue was one of the most water-resistant of wealY xworking adhesives . In test No . 6, specimens held together for nearly2-1/2 years (Fig . 4) before total failure, and, in other similar test 's ,specimens have withstood the cycle for as long as 3 years, Even tli .sresistant glue, however, cannot be classified as waterproof, - far' :siremoisture changes repeated over a long time will cause failure in the.jei .

At the end of 160 weeks, blood-glue joints exposed to test- M-5had lost approximately 50 percent of their original strength . From thedownward trend in joint strength illustrated in Fig . 4, total failuremight be expected somewhere between 4 and 5 years . In other similar tests ,however, blood-glue joints have withstood this exposure for as long as 6years and, at the end of that time, the joints retained some 4o percentof their original strength . By that time, the wood which had been ex-posed to the cycle of 97 percent relative humidity followed by 30 percen twas beginning to show some evidence of decay .

From the trend illustrated in Fig . 4, ultimate failure might b eexpected in blood-glue joints exposed to test No . 4 . In view of the longresistance to the more severe test No . 5, however, it appears rea oiablysafe to assume that these joints might, on more extensive t-eets, prov epermanently durable as long as the moisture content of the wood did no texceed approximately 22 percent (Fig . 1) . These tests produced n oevidence that well-made blood-glue joints will fail when s- j,eet0d t otests Nos . 1, 2, or 3.

Destructive humidities appear to be between 60 and g0 percentrelative humidity, corresponding to a maximum equilibrium moisture con -tent in wood of about 17 percent, for animal and vegetable or starc h

41,

glues ; between g0 and 90 percent relative humidity, corresponding to a

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Omaximum equilibrium moisture content in wood of about 22 percent, fo r

casein glue ; and between 90 and 97 percent relative humidity, corres -ponding to a maximum equilibrium moisture content in wood of about 27percent, for blood glue .

Bibliography

(1) "Casein and Its Industrial Applications," by Edwin Sutermeister ,Chemical Catalog Co ., New York City, 1927, pp . 169-217 .

(2) "Increasing the Durability of Plywood," by Don Brouse, Mechanica lEngineering, Sept . 1931, pp . 664 +-66 .

(3) "The Gluing of Wood," by T . R . Truax, U. S . Dept . Agr . Bulletin1500 . (Available from the Supt . of Documents, Washington

D . C ., for 250 a copy . )

( L±) "Moisture Content of Wood in Dwellings," by E . C . Peck, U. S, 'Crept .

Agr . Circ . 239 . (Available from the Supt . of Documents, Washing -ton, D . C ., for 50 . )

(5) "Age and Strength of Glue Joints," by Don Brouse, Woodworkin gIndustries, June 1931 , pp . 32-33 .

(6) "Behavior of Casein and Blood Glue Joints Under Different Condition sof Exposure," by Don Brouse, Furniture Manufacturer, Sept . 193+ ,pp . 9-11 .

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