Pultruded Carbon Fiber/Vinyl Ester Composites Processed with Different Fiber Sizing Agents. Part II: Enviro-Mechanical Durability

Hexcel AS-4 12K carbon fibers were sized with two different thermoplastic sizing agents. The sizing agents tested were K-90 poly(vinylpyrrolidone) (PVP) and modified polyhydroxyether of bisphenol A (Phenoxy™). In addition, Hexcel AS-4 36K carbon fibers sized commercially with the standard G′ were tested. The sized carbon fibers were processed into composites utilizing a pilot scale pultruder and a pultrudable vinyl–ester resin. Tension-tension fatigue tests were performed on the composites at a frequency of 10 Hz . Residual stiffness and S–N curves were generated. The slopes of the normalized S–N were different for the G′ sizing and the thermoplastic sizings albeit the slopes of the two thermoplastic sizings were similar. Moisture absorption studies were performed on the composites by immersing the samples in tap water at 65°C . The diffusivity as well as maximum moisture content was observed to be sensitive to the type of sizing. In addition to the fiber-reinforced composites, blends of the sizing (PVP and phenoxy) and vinyl–ester resin were tested to evaluate the influence of sizing concentration on moisture uptake. These immersion tests were performed in a 65°C water bath. Preferential hydrogen bonding between the PVP and vinyl–ester was seen to affect the uptake behavior of the blends as opposed to the pure components. Finally, the residual tensile properties of the saturated composites were obtained and again the influence of the different sizings was seen to be prominent. The residual strengths for the Phenoxy™, PVP, and G′ composites were 0.85, 0.87, and 0.82, respectively.