SIZING RESIN STRUCTURE AND INTERPHASE FORMATION IN CARBON-FIBER COMPOSITES

The single-embedded filament fragmentation test has been used to study the effect of fibre coatings on the adhesion of surface treated (oxidized) Type A and HS carbon fibres to an epoxy matrix. The presence of a sizing resin on the as-received fibres reduced the interfacial shear strength of the composite. For the unsized fibres, which were coated in the laboratory from commercial aqueous based sizing emulsions, a molecular weight dependence was observed. This suggests that compatibility of the deposited size with the matrix determines the adhesive bond between fibre and matrix and the formation of an interphasal region. On the other hand, deposition of a sizing resin from solution led to the differing conclusion that chemical interaction with the fibre surface had occurred. During composite fabrication these sizing resins will therefore have to act as 'coupling agents' to the matrix. Solvent extraction of emulsion-deposited sizing resins, particularly at elevated temperatures, appeared to promote their interaction with the fibre surface. The same trends in interfacial shear strength were observed in a second epoxy resin matrix of higher modulus, albeit at an increased magnitude. In this way, the plasticizing role of the 'low' molecular weight emulsion based size could be identified. Maximum likelihood statistics have been used to estimate the standard deviation on the value of interfacial shear strength.