Nylon-epoxy adhesives

Epoxy-nylon adhesives are limited to a maximum service temperature of 85°C, and they exhibit poor creep resistance. Possibly their most serious limitation is poor moisture resistance because of the hydrophilic nylon (polyamide) constituent.9 The degradation by exposure to moisture occurs with both the cured and uncured adhesives. 

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... 

Exposure time, months Epoxy-nylon adhesive Nitrile-phenolic adhesive ... 

A pressure of 25 psi and temperature of 175°C are required for 1 h to cure the epoxynylon adhesive. Because of their excellent filleting properties and high peel strength, epoxy-nylon adhesives are often used to bond aluminum skins to honeycomb core in aircraft structures. In these applications, climbing drum peel strengths in excess of 150 lb/in have been achieved. 

Epoxy-nylon adhesives were developed in the 1960s for high-peel-strength applications. The key to their development is the use of noncrystalline nylons that are soluble in alcohols and other epoxy-compatible solvents. A commercial example of adhesive-grade nylon includes DuPont s Zytel 61. Standard nylons are not practical because of their incompatibility with most other resins. 

Epoxy-nylon adhesives show exceptionally high tensile shear and peel strengths however, they have poor resistance to moisture and elevated temperatures. These adhesives can absorb significant amounts of water from the ambient environment before and after cure. Table 7.5 gives tensile shear and peel strengths for a series of adhesives made by dissolving various ratios of nylon and epoxy resins in a alcohol-water mixtures. 

The toughness of an epoxy-nitrile adhesive is nearly equivalent to that of an epoxynylon adhesive. However, the epoxy-nitrile system has much better hydrolytic stability. Also, the low-temperature properties of an epoxy-nitrile adhesive are superior to those of epoxy-nylon adhesive. Table 7.3 illustrates the effect of nitrile addition on tensile shear and peel strength. 

Epoxy-phenolic adhesive compositions of commercially available types Epoxy-nylon adhesive composition... 

Effect of water on the adhesive. The influence of water on the adhesive is generally reversible, so that any deterioration in, say, mechanical properties is recovered upon drying. The extent of this influence depends upon the adhesive s composition. All polymers absorb greater quantities of water when above their Tg, so that rubbery materials tend to show greater water absorption than rigid adhesives. Interestingly, the key position in structural metal-to-metal adhesives for airframe construction is occupied by epoxy-nylon adhesives, some of which display water uptakes of the order of 14%. 

Epoxy-nylon. Epoxy-nylon adhesives offer both excellent shear and peel strength. They maintain their physical properties at cryogenic temperatnres bnt are limited to a maximum service temperature of 180°F. 

A23°F is even more dramatic. Epoxy-nylon adhesives also retain flexibility and yield 5,000 Ib/in shear strength in the cryogenic-temperature range. 

The polyvinyl butyral-P/F adhesives show a higher water absorption than the polyvinyl formal-P/F combinations, but the behaviour of the latter under hot, wet tropical conditions, although not so catastrophic as the epoxy-nylon adhesive mentioned above, was such that it must be regarded as water sensitive (Cotter, 1977). 

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