Investigation on the thermo-oxidative stability of carbon fiber sizings for application in thermoplastic composites

The stability of the surface chemical composition of different polymer sizings on carbon fibers, designed as a reinforcing material for thermoplastic composites, was investigated in detail under conditions closely related to the standard processing ones, i.e. high temperature, short periods of processing time and presence of an oxidative atmosphere. X-ray photoelectron spectroscopy and contact angle measurements were used to characterize the surface properties of the sizings in dependence of treatment time and temperature. The obtained results clearly show a rapid thermo-oxidative altering of the initial surface functionalities, in particular of the polyether based component, which was found to be the major surface constituent of all types of investigated sizing systems. Moreover, the observed degradation of surface functionalities leads to significant changes in the surface free energies of the sizings determined by contact angle measurements. Finally, the surface analysis of single fibers, which were pulled out during fracture of model composite specimens, confirmed that the revealed surface degradation of the sizing also takes place during a standard polymer composite manufacturing process.