Carbon fiber‐reinforced composites: Effect of fiber surface on polymer properties

Abstract

AbstractDynamic mechanical measurements in a torsional (shear) mode have been used to characterize an unfilled epoxy (Epon 828/m‐phenylene diamine) and A series of uniaxial graphite fiber (Hercules types A and HM) composites. In unfilled resins containing an excess of the epoxy component, Mc—the average molecular weight between crosslinks—decreases with increasing temperature and duration of cure, suggesting a temperature‐dependent side reaction. In fiber‐reinforced composites, the dynamic mechanical response is sensitive to fiber type and curing schedule; elevation of Tg by as much as 45°C has been observed. Comparison of the dynamic data with properties predicted by micromechanical models shows only a fair agreement at room temperature, which rapidly worsens at higher temperatures. Surface treatment of type A fibers gives enhanced interlaminar shear strength (ILSS), both at ambient conditions and after hydrothermal aging. Dynamic data for surface‐treated systems during hydrothermal aging show a sharper drop in G′ and increase in tan δ. The dynamic data and ILSS results are interpreted in terms of a balance of polymer‐fiber interactions, a weak but widespread preferential adsorption of epoxy oligomers on the graphite basal planes at the fiber surface, and a low concentration of covalent bonds between polymer and fiber‐surface‐functional groups.