Background

In aerospace and military applications, equipment is expected to work in a wide range of climatic conditions. Consequently, these applications provide a harsh testing ground for all components and particularly the adhesives that hold them together.

The benefits of adhesive bonding are well known and the specific advantage of weight saving is very important to the aerospace industry. However there are many problems associated with adhesive bonding and two typical areas under development are the durability of joints and their high temperature resistance. In the main, durability can be improved by making the interphase more stable and by increasing the moisture resistance of the adhesives. Most commercial adhesives have poor high temperature resistance for many high-speed applications so it has been necessary to develop adhesives to play this role.
Moisture Resistance

Water acts aggressively on bonded joints. For example, if a typical bond, in which two metals are epoxy joined, is immersed in water at 600°C for 1500 hours, it loses over 75% of its initial strength. The locus of failure moves from being cohesive in the adhesive to being purely interfacial. This test regime may seem severe when compared to other regimes which use a variety of time spans, temperatures and degrees of humidity. But this method can produce a well defined and successful adhesive. Experience has shown that unless an adhesive passes such a critical test it will cause problems during its lifetime.
Silanes

A primer system based upon an organosilane is often recommended to enhance the bonding of epoxy resins to steel substrates. Most commercially available organosilane coupling agents are based on the following generalised structure

R-Si-(X)3

where X is a hydrolysable group and R is an organofunctional group capable of some form of interaction with a given polymer matrix. It is generally believed that organosilanes impart a covalent bridge structure across the interfacial zone. This results in a structure that is more resistant to the effects of water than those solely reliant on secondary force interactions.

It is important, however, to apply the silane in the correct way. The silane must have time to react with the steel surface before the epoxy is applied. If such time is not allowed, perhaps by accelerated drying of the silane-coated surface, then little if any durability improvement will be seen. Three parameters affect the silane priming process: the age of the silane solution, the solvent used for the silane and the drying time/temperature. Only by optimising these conditions can the best durability be obtained.

The age of the silane solution when it is applied to the substrate critically influences the eventual durability. The durability of joints improves with age so that it reaches a maximum about one hour after the silane is mixed with water. When the majority of the water is replaced by ethanol there is little change in silane efficiency with time and the eventual durability is significantly lower than with the water based system. An equally important parameter is the drying temperature used on the substrates after priming. Higher temperatures reduce the effectiveness of the silane, probably by not allowing the silane complex to react with the substrate.