Improved adhesion between stainless steel heating element and PPS polymer in resistance welding of thermoplastic composites

Abstract

Resistance welding of thermoplastic composites involves applying an electric current to a heating element, usually a stainless-steel (SS) mesh, which heats up by Joule effect, melts the surrounding polymer matrix and remains at the joint interface following the welding operation. In this study, the adhesion between a thermoplastic matrix, i.e., polyphenylene sulfide (PPS), and the SS mesh heating element is evaluated and improved through the application of surface coatings on the SS. The coatings are based on an organosilane, a bivalent molecule often used to create an organic coating on metals. The efficiency of the surface coatings is evaluated by measurement of the water contact angle on treated SS plates, characterization of the coating by ATR-FTIR, observations by SEM and TEM and mechanical testing of resistance-welded quasi-isotropic carbon fibre (CF)/PPS joints. A lap shear strength (LSS) improvement of 32% is reported for joints welded using the treated heating elements relative to joints welded using untreated heating elements. Double cantilever beam tests between a SS plate and the CF/PPS composite also show a much better adhesion between the PPS and SS following surface coating, with a higher load at failure and fracture surfaces showing polymer and fibre remaining attached to the SS plate.