Epoxy-phenolic durability

Adhesives recommended include modified epoxies, modified phenoUcs, epoxy-phenolics, neoprene-phenolics, second-generation acrylics, cyanoacrylates, silicone rubbers, and vinyl plastisols. Sell has ranked a number of adhesives in the order of decreasing durability with aluminum adherends as follows ... 

Cotter has reported the results of weathering conducted at three sites over a period of six years. All adhesives performed best at the desert site, although the epoxy/polyamide bonds were considered the least durable. At the hot, wet tropical site, the epoxy/polyamide bonds lost most of their strength after two years, in sharp contrast to the excellent strength retention of specimens bonded with novolac-epoxy, nitrile-phenolic, and vinyl-phenolic adhesives. It was also found that the combination of environmental exposure and stress was particularly harmful, the effects being more pronounced with certain adhesives. Thus stressed epoxy/polyamide bonds all failed after two years tropical exposure and stressed vinyl-phenolic bonds under the same conditions lost strength rapidly after two years and all specimens had failed after six years. The novolac-epoxy and nitrile-phenolic were less effected by stress, as were an epoxy-phenolic and a modified epoxy. 

Elastomer-epoxies are prepared mostly by nitrile rubber addition as the elastomeric component and are usually called modified or toughened epoxy. The bond strengths of elastomer-epoxies are lower in comparison to those of nylon epoxies. Their durability with respect to moisture resistance is better, but not as good as those of vinyl phenolics or nitrile-phenolics. A wide application is in films and tapes. Elastomer-epoxies typically cure at low pressures and temperatures, and over short cure periods, by adding a catalyst to the adhesive formulation. 

The alkyd resins are of value because of their comparatively low cost, durability, flexibility, gloss retention and reasonable heat resistance. Alkyd resins modified with rosin, phenolic resin, epoxy resins and monomers such as styrene are of current commercial importance. 

Another use of acetone in the chemical industry is for bisphenol A ( ). BPA results form the condensation reaction of acetone and phenol in the presence of an appropriate catalyst. BPA is used in polycarbonate plastics, polyurethanes, and epoxy resins. Polycarbonate plastics are tough and durable and are often used as a glass substitute. Eyeglasses, safety glasses, and varieties of bullet-proof glass are made of polycarbonates. Additional... 

FIGURE 15.17 Effect of adhesive on the durability of etched 6061-T6 aluminum alloy joints exposed to a marine environment. (1) Two-part epoxy, (2) one-part epoxy, (3) nitrile-phenolic, (4) vinyl-phenolic, (5) vinyl-phenolic.33... 

Strong chemical bonds between the adhesive and adherend help stabilize the interface and increase joint durability. Aluminum joints formed with phenolic adhesives generally exhibit better durability than those with epoxy adhesives. This is partially attributable to strongly interacting phenolic and aliphatic hydroxyl groups that form stable primary chemical bonds across the interface. 

All of the commercial epoxy adhesives presented in App. B bond well to aluminum and to a wide variety of other materials. Sell22 has ranked a number of aluminum adhesives in order of decreasing durability as follows nitrile-phenolics, high-temperature epoxies, elevated-temperature curing epoxies, elevated-temperature curing rubber-modified epoxies, vinyl epoxies, two-part room temperature curing epoxy paste with amine cure, and two-part urethanes. 

Phenolic resins are the oldest form of synthetic structural adhesives. Usage ranges from bonding automobile and other types of brake linings to aerospace applications. These adhesives have a reputation for providing the most durable structural bonds to aluminum. Because of volatiles, however, and the need for high pressures, the phenolic resins are used less as adhesives than the epoxy resins. 

Formation of durable chemical bonds is an obvious means to stabilize the interface and has been demonstrated for phenolic/alumina joints [25] and for silane coupling agents [26,27]. However, for most structural joints using epoxy adhesives and metallic adherends, moisture-resistant chemical bonds are not formed and mechanical interlocking on a microscopic scale is needed between the adhesive/primer and adherend for good durability. In these cases, even if moisture disrupts interfacial chemical bonds, a crack cannot follow the convoluted interface between the polymer and oxide and the joint remains intact unless this interface or the polymer itself is destroyed. 

Epoxy resins form a durable protective coating only when polymerized with amine, polyamide, or esterified fatty acids. Coal-tar epoxies are modified with coal-tar fiUer to improve moisture resistance. Coatings highly resistant to solvents, acids, and alkahs are based on phenoHc cross-linked epoxies. These coatings are used to protect process equipment. When the phenolformaldehyde constituent of phenol is used to cross-hnk epoxy resin instead of amine, the coating has improved resistivity to alkahs. 

Nylon-epoxy film adhesives have the tendency of picking up substantial amounts of water before use. They also tend to lose bond strength rapidly after use on exposure to water or moist air. After 18 months of exposure to 95% RH, conventional nitrile-phenolic adhesive loses only a fraction of its initial strength, going from 21 to 18 MPa in tensile shear. On the other hand, one of the best nylon-epoxy adhesives available degraded from about 34 to 6.8 MPa in just two months under the same test conditions. A considerable effort has been made to solve this moisture problem, but nitrile-epoxy or acetal-toughened epoxy film adhesives are still superior in durability. "" ... 

Although unmodified phenolic resole adhesives can give bonded structures that will exhibit excellent durability - both thermal oxidative resistauce aud resistance to harsh environments such as oil, sea water, natural weathering - they are inherently brittle. Much research work has been carried out over the years to impart some degree of toughness into these systems and at the same time to reduce the shrinkage on cure and, thus, provide some stress relief. However, toughening, in the sense understood with epoxy adhesives, is not really possible with these resins. 

The durability of the bonded joints was greatly influenced by the nature of the adhesive the best performers in all climates were epoxy-novolak and nitrile-phenolic formulations. A tropical, hot-wet climate was the most damaging to bonded stmctures and the combination of high humidity and applied stress was particularly deleterious. During exposure to natural environments, the failure mode of aluminium joints was found to change gradually from wholly cohesive, within the adhesive, to include increasing amounts of interfacial failure (see Stress distribution mode of failure). 

Unmodified materials are brittle, so when the phenolics high strength and environmental performance are required and there is any possibility of flexing or impact damage, then the modified versions must be used. With the development of the toughened epoxy and acrylic adhesives (see Section 5.1.12), the phenolics-even the modified versions - should only be used where their excellent durability and heat resistance is vital. 

In a review of previous work, DeLollis concluded that the nitrile-phenolic is the most durable adhesive, surviving all environmental exposures. The nylon-epoxy adhesive performed well on exposure to medium-humidity environments, but was susceptible to high humidities. The nitrile-epoxies showed good resistance to most outdoor exposures except the seacoast environment, where bonds failed after four years. Epoxy/polyamide bonds gave good results after four years exposure to an industrial environment and joints exposed to a tropical climate had retained about 33% of their initial strength after three years. This performance should be compared with that of some heat-cured adhesives, for which the bonds are completely degraded by tropical exposure. 

Extensive information on the durability of bonded aluminum joints is available in the reviews of Minford. " Figure 4 illustrates some typical results, showing the effect of adhesive variation on joint durability for a marine exposure. Vinyl-phenolics and nitrile-phenolics have an excellent history of joint durability and rank among the most resistant to environmental deterioration. In spite of this, however, the current trend is to use epoxy-based adhesives, which provide easier processing and higher peel... 

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