The effect of substituted poly(p-xylylene) on the quality of bonded joints when used as a primer replacement

Organic coatings are widely used in the aerospace industry as corrosion protective coatings or as primers in improving the adhesive bond in structural adhesive joints. However, these organic coatings may become unstable under exposure to high temperatures or in extreme hostile environments. This article presents a method to successfully coat metallic surfaces with an environmentally friendly and easy to apply coating which resists high temperatures. The coating consists of derivative of poly (p-xylylene) (PPX) polymers (so-called poly(o-amino-p-xylylene-co-p-xylylene) (amppx)). The surface modification after application of an amppx coating and the adhesion strength of bonded joints (on titanium, aluminium and stainless steel) were verified by means of contact angle, roughness measurements and interfacial shear strength tests. The adhesion of the amppx coating to three surfaces was compared with other surface treatment methods such as chromic acid anodising and atmospheric plasma in terms of adhesive bond strength performance. Lap-shear test results show that the strength of adhesive bonded titanium specimens, which were grit blasted and coated with an amppx coating is almost as good as after a chromic acid anodising treatment. The failure mode of the specimens is 100% cohesive. The method presented in this study opens new possibilities for the use of PPX and their derivative coatings for aerospace applications.