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In both aerospace and military applications, equipment is expected to continue to work in a wide range of climatic conditions. In order to ensure their stability under these conditions, these applications often provide a harsh testing ground for all components, particularly the adhesives that hold them together.
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 testing regime may seem severe as compared to other regimes that use a variety of time spans, temperatures, and degrees of humidity; however, this method has been shown to produce a well-defined and successful adhesive. Experience has shown that unless an adhesive passes this type of critical test, it will cause problems during its lifetime.
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 generalized structure of R-Si-(X)3, in which X is a hydrolyzable 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, which results in a structure that is more resistant to the effects of water as compared to those which are solely reliant on secondary force interactions.
Despite the potential improvement associated with the use of organosilane adhesives, it is imperative for the material to be placed correctly in order for the water-resistant properties to be achieved. In particular, the correct position of silane material provides a sufficient amount of time for it to react with the steel surface prior to application of the epoxy. If such time is not allowed, such as through an accelerated drying of the silane-coated surface, then little if any durability improvement will be seen. Three parameters that have been shown to affect the silane priming process include the age of the silane solution, the solvent used for the silane and the drying time and temperature. When these conditions are optimized, users will be able to achieve optimal durability in their final product.
The age of the silane solution, when applied to the substrate, plays a crucial role in determining its eventual durability. The durability of joints improves with age, in which it reaches a maximum strength about one hour after the silane has been mixed with water. When the majority of the water is replaced by ethanol, little change in the silane efficiency will be achieved, thereby resulting in significantly lower durability as compared to the water-based system. An equally important parameter is the drying temperature used on the substrates after priming. Higher temperatures have been shown to reduce the effectiveness of the silane, which has been hypothesized to result from the inability of the silane complex to react with the substrate.
As governments and private organizations have become increasingly interested in investing their time and money in the aerospace and defense industries, a growing demand for improving aerospace adhesives and sealants have subsequently followed. In addition to the organosilanes discussed here, various other materials have been investigated for their durability in aerospace and military applications.
Classic cyanoacrylates, for example, offer significant advantages in their bond strength to metals, which is primarily attributed to their ethyl and methyl chemistries. More specifically, any contact between the surface of cyanoacrylate adhesives and moisture allows for an ionic polymerization reaction to occur and ultimately cure with ambient water. Additionally, companies like ACC Silicones Europe have improved the formulations of their silicone adhesive sealants to not only withstand the most severe environmental conditions but also eliminate the production of any harmful byproducts when these sealants are in use.
Sources and Further Reading
This article was updated on 11th March, 2019.