Epoxy adhesives environmental effects

Great caution must be exercised in exposing any adhesive joint to the simultaneous effects of environment and stress. The stress can act to accelerate the degradation caused by the environment, and vice versa. Joints that will be exposed to both high-humidity environments and high load at the same time are especially vulnerable, and prototype specimens need to be tested. This degradation mechanism and the performance of several epoxy adhesive systems to combined environmental stress conditions are discussed in Chap. 15. 

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. 

PI resins find use as coverlays. A coverlay is generally used to protect printed circuits during subsequent processing, primarily solder operations, or from environmental effects during use. The Pis can be used in combination with epoxies. In addition, additives, such as phosphorous-based flame retardants or adhesion promoters, may be used in the formulations. 

Custodio, J., et al. (2011). Rehabilitation of timber structures - Preparation and environmental service condition effects on the bulk performance of epoxy adhesives. Construction and Building Materials, 25, 3570-3582. 

With the increased usage of 120°C cured, rubber modified epoxy structural adhesives for aluminum airframes, certain service problems have been observed which have been attributed to environmental factors. The problems associated with the combined effects of sustained load, elevated temperature and high humidity upon the aluminum substrate, corrosion inhibiting primers, and the structural epoxy adhesive matrix are discussed. A particular type adhesive matrix, based on acrylonitrile/butadiene rubber modified bisphenol type epoxy systems is discussed in detail, and important advances in the preparation of more moisture resistant aluminum (oxide) surfaces are reviewed. 

Organic primers formulated with corrosion inhibitors are typically applied to pretreated metal surfaces to protect the surfaces prior to adhesive bonding and during environmental exposure. Pike [7-11] found that inorganic primers, such as sec-butyl aluminum alkoxide, improved the durability of aluminum-epoxy bonds when applied to both porous and nonporous aluminum oxide surfaces. It was shown that the effective thickness of the inorganic primer was directly related to the degree of oxide porosity and the depth of the porous oxide layer resulting from the normally used pretreatments for aluminum [10,11]. 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

Schematic representations of the typical response of cold-cure epoxies are depicted in Fig. 4.6. Naturally the relative importance of specific adhesive property data depends upon the application and the envisaged loading and environmental conditions that the real joint will be subjected to. Many analysts(32-34) advise that the strength of the adhesive equilibrated with the worst-case environment is the key to effective design. This implies laboratory tests conducted at high temperatures on specimens pre-equilibrated with high levels of water vapour or liquid water. For the application of adhesives to steel bridges in the USA. Albrecht era/.(34) selected a test environment of 49 °C and 90% r.h.

Water-based acrylic paints provide no harmful solvents. They are easy to apply and a wide variety of colors is available. Lacquers contain solvents that evaporate allowing the paint coating to dry without chemical reaction. The lacquer must be carefully chosen so the solvent is compatible with the plastic being painted. Enamels are based on a chemical reaction that causes the coating film to form and harden. The effect of this reaction on the parent plastic part must be carefiilly noted. Certain plastics may require a primer to help improve the adhesion of the paint to the plastic. After coating, a top coat may also be required to improve the environmental or wear resistance of the paint. Two part paints, such as epoxy or urethane coatings, provide excellent adhesion qualities and do not require a top coat. 

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. 

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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