The adhesion of thermoplastic fibre composites

Abstract

Fibre composites based upon thermoplastic polymeric matrices containing continuous fibres of carbon or ‘ Kevlar' are being increasingly used in engineering structures. Engineering applications frequently require that they be joined to components fabricated from similar fibre composites, or to other types of materials, and the use of structural adhesives, typically based upon epoxy resins, offers many advantages compared with other methods of joining. The present paper describes in detail the mechanics and mechanisms of the adhesion of thermoplastic fibre composites. The surface topography and chemistry of the composites have been characterized using contact angle measurements and X-ray photoelectron spectroscopy, both before and after using various surface treatments. Joints have then been prepared using epoxy adhesives and the adhesive fracture energies, Gc, of the joints have been measured. Two major aspects of the observed results, with wide applicability to many adhesion problems, have been analysed in detail. First, the need to apply a critical intensity of surface treatment to the thermoplastic fibre composite to prevent interfacial failure, and hence give relatively high values of G has been interpreted by relating the chemical composition of the surface of the composite to the corresponding value of the polar force component of the surface free energy. It is thereby shown that the fundamental requirement is that a critical value of the surface polarity has to be attained. Secondly, after this critical value is reached, it is shown that the locus of joint failure may then be accurately predicted from a knowledge of the stress field in the joint and the experimentally measured interlaminar fracture stress of the fibre composite substrates.