Pultruded carbon fiber vinyl ester composites processed with different fiber sizing agents. Part I: Processing and static mechanical performance

Hexcel AS-4 12K carbon fibers were sized with two different thermoplastic sizing agents. The sizing agents tested were K-90 poly (vinylpyrrolidone) and modified polyhydroxyether of bisphenol A (Phenoxy (TM)). Hexcel AS-4 36K carbon fibers sized commercially with the standard G' was also examined as reference. The sized carbon fibers were processed into composites utilizing a pilot scale pultruder and a pultrudable vinyl-ester resin. Both the thermoplastic sizing materials especially the Phenoxy (TM) sizing had improved processability over the traditionally utilized G' sizing material. These improvements in processability translated into improvements in final composite surface finish and quality. In addition, the thermoplastic sized fibers entered the dip bath stiff, thus minimizing fraying and damage thereby ensuring a better product. The thermoplastic sized composites outperformed the standard sizing G' by at least 25% in static tensile strength, by at least 11% in longitudinal flexure strength, by at least 30% in short beam shear strength, and 31% in static compressive strength. The tensile, longitudinal flexure, and compressive modulus appeared to be unaffected by variations in the sizing. Therefore, the mechanical performance of pultruded, carbon fiber/vinyl-ester composites is greatly affected by the nature of the sizing agent present and is believed to be the result of both improved fiber processability and altered interfacial properties. The affect of improved fiber/matrix adhesion is also believed to be a contributing factor but is secondary to the affects of processability and altered interphase properties.