TECHNOLOGY

Phenolics lead structural adhesive market Epoxies also contribute to growing market, which may jump from today's $50 million to $140 million in 1970

Structural adhesives, which represent less than 10% of the total adhesive market today, could grow to between 15 and 20% of the adhesive market by 1970, according to combined figures from many of the major adhesive pro­ducers. In dollar value, this would mean an increase from less than $50 million up to about $140 million.

Epoxies and phenolics currently dominate the structural adhesives field, and have carved out substantial mar­kets in the automobile and aircraft in­dustries. And although new adhe­sives are continually being developed, the greatest growth in structural adhe­sives will probably come as a result of growing uses for epoxies and pheno­lics. Contributing to the overall struc­tural adhesive field but possessing only a small portion of total sales are such newer adhesives as polyurethanes, modified epoxies and phenolics, cyan-oacrylates, and adhesives based on various polymers.

The structural adhesive field is mainly a service industry, and most de­velopment work must be tailored to a specific application. For this reason development costs represent a large

share of the total sales cost. On the other hand, big-volume applications for adhesives are so competitive that they cannot carry the burdens of large development programs.

Thus, growth in adhesives technol­ogy generally responds to several fac­tors:

• Polymer producers, such as Bor­den, Shell, Union Carbide, and Du Pont, not satisfied with the introduc­tion of some of their newer polymers into the adhesive field, integrate pro­duction to become adhesive suppliers, and thus mount their own develop­ment effort.

• Producers of newer materials—en­gineering plastics, fibers, and elasto­mers, for example—often find it neces­sary to develop new adhesives which better suit their products than existing adhesives.

• In some fields, such as defense work, a suitable adhesive may be a critical factor in making a system op­erational. Under such circumstances, large development efforts can be justi­fied.

• Developments in allied fields such as paints, coatings, electroplating, and

ceramics often produce novel tech­niques which can be applied to adhe­sive systems. One example, devel­oped by National Cash Register, in­volves encapsulation of one compo­nent (usually the curing agent) of a two-part adhesive in gelatin capsules. The capsules are then suspended in the resin component. When pressure is applied to the adhesive-coated joint, the capsules burst, freeing the curing agent which then reacts with the resin.

Adhesives offer many advantages over conventional fastening methods. With adhesives, thinner-skin materials can be used because there is no con­centration of stresses around fastening points such as occur with rivets, sta­ples, or nails. Instead, stresses can be uniformly distributed along the glue line, and large areas of the substrate can absorb the applied load. The use of adhesives eliminates protruding fasteners which mar the smoothness of outer surfaces. And in honey­comb-type applications, cheaper core materials can be used in conjunction with a facing of more expensive mate­rial having special properties.

Another advantage of structural ad­hesives is that different metals and metal alloys can be bonded without galvanic corrosion occurring afterward. Such corrosion does not occur because the metal surfaces are separated by a nonconducting glue line. Thus, adhe­sive-bonded parts are electrically insu­lated from each other, an important factor in manufacturing electronic equipment and motors. Adhesive joints can also serve as thermal-stress absorbers, moisture sealants, and sound deadeners.

Adhesive joining has its limitations, however. For example, extensive sur­face preparation is often necessary to obtain optimum bond strengths. Proper curing also requires pressure, often requires a lengthy curing time, and may require heat. Any heat re­quirement can itself be a serious hand­icap. Even in some metal-to-metal applications, curing temperatures are limited by the fact that some alloys suffer losses in fatigue resistance or surface hardness when subjected to temperatures that are common for cur­ing adhesives.

Phenolic adhesives. Of the two major classes of structural adhesives,

Structural adhesives hard to define Describing the chemical nature and typical applications of today's struc­tural adhesives is far easier than defining the term itself. Some years ago, "structural adhesives" was generally used to describe thermoset­ting, high-modulus adhesives used in load-bearing applications. More recently, however, the term has taken on a more limited connotation. For instance, one definition holds that "structural" indicates only the intended application of the adhesive, but does not necessarily imply high mechanical quality. The required strength of the adhesive depends on the material being bonded and on the force which the bond must withstand. These requirements can differ widely for various industries— for example, from the bonding of wing sections of supersonic aircraft to such borderline applications as bonded abrasives, shoe soles, and furni­ture. Today, structural adhesives usually refer to adhesives for big-vol­ume applications in various industries such as the auto, aircraft, and construction industries. Structural adhesive bonds have one or more of the following characteristics: high modulus, tensile strengths up to 10,000 p.s.i., and lap shear strengths usually between 1000 and 5000 p.s.i. The adhesive not only contributes to the load-carrying capability of the structure but also can support a continuous load without excessive deformation or creep.

52 C&EN MARCH 27, 1967

WING FLAP. A worker at General Dynamics' Fort Worth plant helps guide a wing flap of the supersonic tactical fighter F - l l l into an autoclave. Sections are bonded in the autoclave with B. F. Goodrich's Plastilock epoxy adhesives

phenolic resins are the older and more widely used. Low cost has probably been the major factor for their exten­sive use. They are prepared by the condensation of formaldehyde with monohydric phenols or resorcinol; am­monia or amines are catalysts. Phe­nolic resins are usually modified for use as adhesives by compounding them with elastomers or other resins—vinyls, nitriles, neoprene, or nylon, for ex­ample—to improve the flexibility of the cured adhesive.

The largest outlet for phenolic ad­hesives is the building industry. Of the 410 million pounds of phenolic ad­hesives sold last year, about 150 mil­lion pounds were used for making ply­wood and particle board. Phenolic adhesives are also used for manufac­turing structural timber (beams made from laminating lengths of smaller pieces of lumber). These beams may be straight or curved, and are used to build structures of high strength-to-weight ratios—for example, domed churches, schools, auditoriums, and other high-ceiling buildings.

The first metal-to-metal application of phenolic structural adhesives came in the early 1940's in the aircraft in­dustry. A vinyl-modified phenolic was used to bond flanges of the primary wing structure of fighter aircraft. Eight years later, phenolics were intro­duced in the automobile industry— nitrile phenolics replaced the riveting method of bonding brake linings to brake shoes.

Today, bonding brake linings ac­counts for the bulk of the 5 million to 6 million pounds of phenolics used in the auto industry. In aircraft, phe­nolics—primarily nitrile-phenolics—are used in metal-to-metal applications such as structural sealing of fuel tanks, air frames, and helicopter rotor blades. Another type of modified phenolic—phenolic resins modified with polyvinyl formal or polyvinyl

butyral—is used in honeycomb sand­wich construction, both for metals and for nonmetals such as paper, glass fi­ber, polystyrene foam, and plastics.

Epoxy adhesives. Despite the large market which phenolic adhesives have captured, the fastest growing class of structural adhesives are the epoxies. In 1966, 150 million pounds of them were used in bonding and adhesive applications. Epoxies are prepared from the condensation products of epi-chlorohydrin and bisphenol A, using anhydrides and amines as catalysts and accelerators. Other curing agents and fillers are often added to modify the viscosity and work life of the raw adhesive and also to improve bond characteristics.

Like phenolic resins, epoxies are too brittle to be used alone, and so are mixed with other resins to attain flexi­bility in the cured adhesive. Resins most frequently used are polyamides, polysulfides, silicones, and even phe­nolics.

The main advantage that epoxies hold over phenolics is the fact that an epoxy system is 100% reactive. The parts can be mated immediately after coating, and the adhesive can be cured with only contact pressure. Epoxy adhesives can be compounded to cure from room temperature to 300° F. Phenolics, on the other hand, must be cured between 300° and 400° F. Fur­thermore, phenolics must be cured un­der pressure to counteract the disrup­tive force of water vapor that is formed as a by-product of the curing reaction between phenol and formaldehyde.

Not surprisingly, then, epoxies find widest use in metal-to-metal applica­tions, or for bonding such impervious substances as glass and ceramics. They are the most common type of adhesive used in aircraft construction. A typical supersonic aircraft uses more than 800 pounds of epoxy adhesives in its fabrication. The combined air-

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craft and aerospace industries prob­ably accounted for three fourths of all epoxy structural adhesives sold last year.

The automobile industry uses about 2.5 million pounds of epoxy adhesives per year. Some typical applications include fastening reinforcing spiders to hood and trunk lids, and installing roof bows. Other promising uses are for bonding components of transmissions, power trains, radiators, engines, and frames.

Although epoxies are more expen­sive than phenolics ( 50 to 60 cents per pound for epoxies vs. 25 cents per pound for phenolics) epoxies can ac­cept a high loading of inert fillers, thereby reducing the cost per joint. Besides, the versatility and ease of use of epoxies often justify their higher cost.

Newer adhesives. One of the newer structural adhesives that claims only a small part of today's adhesive market is polyurethanes. Isocyanates are highly reactive and can form the basis for two-part adhesive systems. A wide range of polyols can be used to form three-dimensional, cross-linked adhesive bonds. Urethane adhesives have been used for bonding metals to metals, elastomers, plastics, or foams. Polyurethanes are more expensive than phenolics and somewhat comparable in price to epoxies.

Several other types of new structural adhesives hope to carve a small share of the high-performance specialty mar­ket. Some of these boast simpler ap­plication methods and milder curing conditions but are, however, several-fold more expensive than either phe­nolics or epoxies. Polyhydroxyether-modified epoxies, for example, are single-component systems that have an indefinite shelf life and cure very rapidly. They have found limited use in critical aircraft and spacecraft ap­plications. Another single-component, rapid-curing system is a cyanoacrylate adhesive which is finding use in auto­mobile applications. Eastman Kodak produces the 2-cyanoacrylate from the reaction product of formaldehyde with the corresponding alkyl cyanoacrylate.

Other recent developments in struc­tural adhesives are aimed at higher service temperatures for the cured ad­hesives. Most current structural adhe­sives lose strength rapidly above 500° F. One example of modified epoxies that are useful at temperatures up to 700° F. are the epoxy-nocalacs-com-binations of epoxy with phenol-for­maldehyde resins, cross-linked with melamine and arsenic pentoxide. Other examples are epoxies based on polyethoxyphenylsiloxane, or modified with polyepoxides, monoglycidil ethers, or plasticizers such as dibutyl phthalate.

Other new high-temperature adhe­sive systems are being developed from polyimides, polybenzimidazoles, and semiorganic polymers based on ' bo­ron, silicon, phosphorus, phosphoni-trilic halides, phosphinoboranes, and ferrocenes.

Pfizer markets enzyme to replace rennet An enzyme suitable for replacing ani­mal rennet as a milk coagulant in mak­ing cheese has been introduced by Chas. Pfizer & Co. Tradenamed Sure-Curd, the enzyme has been approved by the Federal Standards of Identity for use in Cheddar, washed curd, Colby, granular, and Swiss cheeses.

Sure-Curd is derived from a strain of the microorganism Endothia para­sitica. The company discovered the species' activity after screening hun­dreds of enzyme-producing microor­ganisms from Pfizer's culture collec­tion at Groton, Conn.

The discovery is covered by U.S. Patent 3,275,453, issued to Dr. Joseph L. Sardinas, microbiologist at the Gro­ton laboratories. The patent claims the proteolytic milk-curdling enzyme, a process for producing the enzyme, and a process for making cheese using the enzyme.

For centuries, animal rennet has been used as a milk coagulant in cheese production. This substance is made from the fourth stomach of milk-fed calves. Supply varies seasonally and prices fluctuate widely. As a total or partial replacement for rennet, Sure-Curd could help free the cheese indus­try from depending on an unpredicta­ble animal source, Pfizer says.

Over the years, a number of poten­tial rennet substitutes have been tried by cheese makers. Of these, only pepsin has been commercially accept­able, and then only as a partial re­placement for rennet. Until now, Pfizer explains, vegetable and micro­bial enzymes have been unsatisfactory because they often produce bitter off-flavors.

But cheeses made with the new en­zyme have flavor, body, and texture comparable to rennet-made cheeses, the company claims. In addition, tests indicate that Sure-Curd acceler­ates curing in aged Cheddar cheese. Aged Cheddar made with Sure-Curd may, after six months, have the flavor, body, and texture of a cheese made the usual way and aged for 10 to 12 months.

Sure-Curd is a standardized, free-flowing powder which is dissolved in water immediately before using. Its use requires no change in normal cheese-making procedure, Pfizer notes.

54 C&EN MARCH 27, 1967