Improving Environmental Resistance

There are several ways by which the formulator can moderately improve the heat or chemical resistance of room temperature curing epoxy adhesives. Using an elevated-temperature cure or a postcure will, of course, improve the temperature resistance by virtue of improved crosslink density. However, this section describes formulations that have been developed for moderately improved heat resistance after only a cure at room temperature. Optimal (heat-curing) high-temperature and chemically resistant epoxy adhesives are discussed in Chap. 15. 

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An alternative route starts with a carboxy-terminated oligomer [36], This is reacted with glycidyl methacrylate to provide the methacrylate-terminated polymer. The resulting linkage is susceptible to hydrolysis, so the hydroxy group may be reacted with an isocyanate to improve environmental resistance (Scheme 3). 

For improved adhesion and improved environmental resistance the more reactive primers can exhibit advantages, such that in some tough applications, they are the only choice. 

B) Phosphoric acid anodizing (Boeing Co. patent appl.) (for improved environmental resistance) (ASTM D3933-80) ... 

The crosslink formed by amino resins with hydroxyl functional acrylic copolymers is sensitive to acid-catalyzed hydrolytic degradation (13,14). The urethane crosslink formed by the reaction of hydroxyl functional acrylics and polyisocyanates is much more stable to this condition (15). This means that aiming for improved environmental resistance requires the more expensive polyisocyanate crosslinkers. Other crosslinking mechanisms of functional acrylics and crosslinkers will provide a better compromise of cost and etch performance, but these are often accompanied by other compromises. 

Three approaches have been identified that reduce susceptibility of CA resists to airborne contamination. In the first, process engineering changes such as the addition of special activated carbon filters to the environmental chambers surrounding the exposure tools (76,79), overcoating the resist with a soluble protective film to isolate the resist from the environment (77,80,81), or modifications of the process flow to minimize the time interval between exposure and post-exposure bake have been shown to improve CA resist processibibty. 

Demands for improved efficiency in aircraft gas turbines led to the use of a family of age hardenable, controlled expansion superaHoys for engine seals and casings. INCOLOY aHoys 903 [61107-16-2] (UNS N19903), 907 [107652-23-3] (UNS N19907), and 909 evolved from a continuing effort to improve the environmental resistance of this Cr-free, Fe—Ni—Co based system. 

From this table it will be noted that in terms of the mechanical and thermal properties quoted the copolymers are marginally inferior to the homopolymers. They do, however, show a marked improvement in resistance to environmental stress cracking. It has also been shown that the resistance to thermal stress cracking and to creep are better than with the homopolymer.This has led to widespread use in detergent bottles, pipes, monofilaments and cables. 

The characteristics of the three most common thermoset resin systems used in pultrusion are compiled in Table 11.2 [3]. It is noteworthy that unreinforced polyesters and vinylesters shrink 7-9% upon crosslinking, whereas epoxies shrink much less and tend to adhere to the die. These epoxy characteristics translate into processing difficulties, reduced processing speed, and inferior component surface finish. It is normal practice to use resin additives to improve processability, mechanical properties, electrical properties, shrinkage, environmental resistance, temperature tolerance, fire tolerance, color, cost, and volatile evaporation. It is normally the resin, or rather its reactivity, that determines the pulling speed. Typical pulling speeds for polyesters tend to be on the order of 10-20 mm/s, whereas speeds may exceed lOOmm/s under certain circumstances. Apart from the resins characterized in Table 11.2, several other thermosets, such as phenolics, acrylics, and polyurethanes, have been tried, as have several thermoplastics (as will be discussed in Sec. 11.2.6). 

Nylons require modification or stabilization to improve their resistance to certain environmental effects. Unstabilized nylon is degraded by ultraviolet light. The most widely used stabilizer has been approximately 2% well-dispersed carbon black, which has proved effective in the absorption of ultraviolet light. The nylons are considered adequate for outdoor applications if they are not exposed to direct sun-hght,... 

EC-2320 (c) AF-lll, AF-126, and AF-125-2 film epoxy adhesives 1.3—5.1 pm Air drying up to 120°C Improves shear and peel strengths as well as environmental resistance... 

These low-temperature environmental effects can be significant factors that contribute to an adhesive system s durability and life. This section discusses the characteristics of epoxy adhesive joints exposed to low temperatures and to thermal cycling and suggests formulations for improving the resistance of adhesives and sealants to these conditions. 

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