Epoxy adhesives moisture effects

It has also been shown in several studies that the combination of stress, temperature, and moisture can accelerate the hydrolytic instability of certain epoxy adhesives. In an FTIR study of the effect of moisture on DGEBA epoxy cured with nadic methyl anhydride, spectra changes were observed in stressed specimens aged for 155 days at 80°C and 100 percent RH.47 This was attributed to the slow, stress-induced hydrolysis of ester groups. In another study,... 

Adhesives commonly used on concrete must be formulated so that they cure well in the presence of moisture. The adhesive should have a viscosity that allows it to penetrate the concrete for mechanical bonding. Epoxy and epoxy-polysulfide adhesives are especially effective adhesives for concrete. The starting formulations given in Table 16.16 are for an ambient cure 100 percent solids epoxy and a waterborne epoxy adhesive. 

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. 

The effect of moisture on stress strain plots for epoxy adhesive samples tested at 90° C can be seen in O Fig. 31.16b. First, it is worth comparing the dry stress-strain plots at the two temperatures in Tigs. 31.16a andO 31.16b where it can be seen that increasing temperature has a similar effect to the absorbed moisture, that is, reduction in modulus and UTS and an increase in strain to failure and plastic strain energy density at failure. However, comparing... 

Effect of absorbed moisture on the bulk mechanical properties of a modified epoxy adhesive at (a) 22°C (b) 90°C... 

Not only is low tensile shear strength noticed on moisture aging, but also the mode of failure changes from one of cohesion to adhesion. Table 7.6 shows the effect of humidity and water immersion on an epoxy-nylon adhesive compared to a nitrile-phenolic adhesive. Substrate primers have been used with epoxy-nylon adhesives to provide improved moisture... 

The effect of electrical-grade fillers (e.g., silica) on the electrical properties of the adhesive is usually marginal. Generally fillers are not used to improve electrical resistance characteristics such as dielectric strength. The unfilled epoxy is usually optimal as an insulator. Also under conditions of high humidity, fillers may tend to wick moisture and considerably degrade the electrical resistance properties of the adhesive. 

The strength of these systems is typically adequate for most structural applications. However, the deleterious effect moisture has on the bonding between the metal and adhesive must be taken into account if epoxy resins are to be used for bonding steel in moist or humid environments. This places a severe limitation on the number of structural bonding applications where epoxide adhesives could be used. 

Organic coatings may vary from very dense to rather open for water vapour. A dense coating (ideally without flaws), based on epoxy, polyurethane or chlorinated rubber polymers, may block ingress of aggressive species. Nevertheless, the presence of such a layer strongly hinders the evaporation of the moisture that is present in the concrete at the time of treatment. This situation can lead to loss of adhesion to the concrete and thus to a loss of effectiveness of the coating. 

Cure cycles for adhesives should not be interrupted. Step curing should be performed in the same oven without exposing the adhesive to ambient air between steps. Moisture absorption in partially cured adhesives affects the curing process and alters properties. The effects of interrupting the cure cycle and exposing an epoxy encap-sulant to air moisture were shown by Naito and Todd. ... 

Brewis et al. studied the effects of moisture and temperature on the properties of epoxy-aluminum joints by measuring changes in the mechanical strength properties of the soaked adhesive [90]. The Tg of the wet adhesive and relative strengths of wet and dry joints were evaluated for up to 2500 hours. They concluded that the joint weakening effect of water was due to plasticization of the adhesive that, in turn, was dependent on the rate of water diffusion within the adhesive. 

In some cases, silicone is as effective when blended into an adhesive formulation as when it is applied separately as a primer. For silicone-couphng agents, moisture adsorbed on the substrate plays an important role in attaching the silicone molecule through hydrolysis. The opposite end of the molecule contains a chemical group such as a vinyl or amine, which is reactive with the epoxy, polyester, or other resin that is to be adhered to the... 

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