19 сентября, 2015 
Pokraskin Epoxy resin Adhesives. Epoxies [47], [48] are among the most widely used structural adhesives. These chemically reactive systems include two-component systems that cure at room temperature or elevated temperature and one-component systems, which usually require heat for curing.
Epoxy thermoset adhesives, available in many formulations, can be used to join most materials. These adhesives have good strength, do not emit volatile substances during curing, and have low shrinkage. However, peel strength and flexibility are low, and epoxies are brittle.
A typical two-component system consists of a resin and a hardener, which are packaged separately. In a one-component system, the resin and hardener are packaged together. Other possible additives include accelerators, reactive diluents, plasticizers, resin modifiers, and fillers.
The most common epoxy resin is based on the diglycidyl ether of bisphenol A. Epoxy resins are sometimes esterified with fatty acids or modified with polyurethane intermediate chains for elastification.
The hardeners used for hot-setting formulations, i. e., for hardening temperatures above 100°C, are preferably dicarboxylic acid anhydrides, dicyandiamide, and certain aromatic amines. The hardening temperatures, which are normally in the range 150-200°C, can be lowered by adding accelerants [49].
The hardeners for cold-setting systems include aliphatic and cycloaliphatic amines and polyamines, adducts of polyamines and epoxy resins, phenol-amine combinations, and condensates of polyamines and dimerized fatty acids (polyaminoamides). Whereas amine hardeners must be used in a stoichiometric ratio to the reactive epoxy groups, polyaminoamides may be overdosed to a certain extent and thus used to elasticize the adhesive resin.
An exothermic reaction occurs during the curing of epoxies. This reaction can be minimized by lowering the temperature of the mixed components in two-component systems, by limiting the batch size, and by using shallow mixing containers.
Epoxies are brittle, especially if cured with an anhydride. Therefore, thermoplastics and rubbery modifiers are often added to decrease the brittleness.
Nylon epoxy, which first became available in 1960, has an approximate maximum service temperature of 180 °С, compared to 80 °С for unmodified epoxies. The main advantage of nylon epoxies is increased flexibility and much higher peel strength, compared to unmodified epoxies. Also this very tough material has excellent tensile lap-shear strength. Fatigue and impact resistance are also good. Limitations include poor peel strength at low temperatures and poor creep resistance. Perhaps the most serious limitation is extremely poor moisture resistance in both the uncured and cured material. Nylon epoxies are not as durable as elastomer epoxies or thermoplastic-modified epoxies. A typical application of nylon epoxies is in laminates.
Elastomer epoxies generally contain nitrile rubber as the elastomeric component. This system is also referred to as a modified or toughened epoxy. One of the applications of widest use is in films and tapes. Elastomer epoxies cure at low pressures and low temperatures over a short time interval. This is achieved by adding a catalyst to the adhesive formulation. Bond strengths of elastomer epoxies are lower than those of nylon epoxies. However, the major advantage of elastomer epoxies is their sub-zero peel strengths, which do not decrease as fast as those of nylon epoxies. In addition, the moisture resistance of elastomer epoxies is better than that of nylon epoxies but not as good as that of vinyl-phenolics or nitrile — phenolics. Limitations to the use of elastomer epoxies include poor water immersion resistance and poor properties when exposed to marine conditions.
In addition to liquid and pastelike epoxy resin adhesives, solid powdered products are also available commercially. In addition, epoxy resins are used in the form of adhesive films for certain applications. The most important are nylon epoxy resin films consisting of mixtures of relatively high molecular mass polyepoxides and polyamides, and epoxy resin-phenolic resin combinations.
Trade Names. In Germany: Araldite, Metallon, Pattex Kraft-Mix, UHU-pIus, Scotch weld, Terokal; in Japan: Sho-bond, KBK, Bond E, Sanyu Resin; in the USA: Scotch Weld, Resiweld, Ross Epoxy Glue, Ten-set Epoxy and Duro E-pox-e Glue.
Reactive Polyurethane Adhesives date back to the late 1930s and acquired their present economic significance over the past decades [50]-[52]. They are distinguished
by good adhesion to various substrates and, compared with other reactive adhesives, by high elasticity, even at low temperatures. Polyurethane adhesives are marketed both as solvent-free and as solvent-based adhesives.
The generic term polyurethane adhesives covers both adhesives that already contain polyurethane compounds or compounds with a urethane bond and also reactive adhesives in which urethane groups are formed only during the hardening process.
Chemically reactive polyurethanes include both one — and two-component systems. One-component systems are usually based on a polyether polyol treated with a polyisocyanate to give an isocyanate-terminated polymer. A one-component system cures when exposed to moisture at room temperature. One-component polyurethane hot — melt adhesives are also cured by moisture after application. Two-component systems result from the reaction of low molecular mass polyols and isocyanates or from isocyanate-terminated prepolymers with either polyols or polyamines. Two-component systems cure at room and/or elevated temperatures.
One-component heat-curable urethanes are also available. In these formulations, free isocyanate groups are typically blocked by the addition of phenol. The prepolymer is then blended with the polyglycol curing agent and packaged. The mixture is stable until heated to elevated temperatures, whereupon the phenol is released and the isocyanate groups are regenerated. A rapid cure occurs when these groups come into contact with the polyglycol.
Polyurethanes bond to most surfaces. They also have outstanding tensile lap-shear strength at lower temperatures. In fact, polyurethanes have better low-temperature strength than any other adhesive, even epoxy-nylons. Good flexibility, abrasion resistance, and toughness are other advantages of polyurethanes. Limitations include sensitivity to moisture in both uncured and cured adhesives, the toxicity of isocyanates, and poor tensile lap-shear strength at higher temperature. A major advantage, and also the reason for the wide range of applications of polyurethane adhesives, lies in their versatility in regard to raw materials, composition, and curing.
Normally, reactive polyurethane adhesives contain high molecular mass, still reactive polyurethane prepolymers with terminal hydroxyl or terminal isocyanate groups. Prepolymers with terminal hydroxyl groups can be hardened by the addition of polyisocyanates. Depending on the functionality of the prepolymers and polyisocyanate hardeners, curing may involve both cross-linking and linear enlargement of the molecule. High molecular mass polyurethane elastomers contain urethane groups that can still react with isocyanates and are thus cross-linked (contact cements).
Prepolymers with terminal isocyanate groups can react with and are hardened both with amino or hydroxyl compounds and with water. Hardening with water is preferred for one-pack systems, for which both atmospheric moisture and the film of water on the substrates act as hardeners for the prepolymer. Cross-linking with polyamines is particularly fast compared with curing by polyols.
One-pack polyurethane adhesives can also be manufactured by protecting the isocyanate groups of prepolymers with, for example, phenols or oximes, which are split off by the action of heat during bonding. Protected polyisocyanates can also be added to
reactive polyurethane adhesives as latent hardeners. Tertiary amines or organotin compounds can be added to accelerate isocyanate reactions.
Polyurethane adhesives should be stored in tightly sealed containers, because shelf life and adhesive quality can be affected adversely by the uptake of water.
Trade Names. In Germany: Liofol, Macroplast; in Japan: Diabond, Bostik; in the USA: Chemlok.
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