Interfacial engineering of the interphase between carbon fibers and vinyl ester resin

Vinyl ester resins have been extensively used for the manufacture of low cost high performance composites. Carbon fibers are important reinforcement materials. The use of vinyl ester composites reinforced with carbon fibers requires an improvement in the fiber/matrix adhesion levels. The objectives of this study were to gain an understanding of the factors controlling interfacial adhesion between carbon fibers and vinyl ester resin; to model the contributions of the factors controlling fiber/matrix adhesion; and to provide an engineered and optimized interface between carbon fiber and vinyl ester for tailoring structurally efficient carbon fiber/vinyl ester composites. This work consists of three parts.; Part I. A partially cross-linked DGEBA epoxy polymer sizing placed onto carbon fiber surface was found to be a beneficial interphase between the carbon fiber and vinyl ester resin resulting in an increase in fiber-matrix adhesion. The adhesion was evaluated as interfacial shear strength (IFSS) with micro-indentation. Nano-indentation and nano-scratch technique were used to investigate the gradient between this epoxy sizing and vinyl ester resin. An optimized thickness of this sizing was found and the mechanism by which this sizing improved adhesion was also investigated. A set of 2-D non-linear finite element models was set up for simulation of the micro-indentation process and consistent results were found between the experimental data and numerical results. It was found that the epoxy sizing formed more chemical bonds with the surface of the carbon fiber reinforcement and an interpenetrating interphase with the vinyl ester resin. The resulting interphase between vinyl ester matrix and epoxy sizing reduced the residual stress caused by the volume shrinkage of the vinyl ester after curing.; Part II. Since it is known that the carbon fiber surface can interfere with the vinyl ester polymerization, the effects of preferential adsorption of the catalysts and styrene on the carbon fiber surface on fiber/matrix adhesion have been investigated. It was found that although the adsorption of the promoter and accelerator does occur on the carbon fiber surface, it does not substantially affect the fiber/matrix adhesion. The vinyl ester monomers and styrene monomers interact differently in the interphase. Optimum fiber-matrix adhesion can be obtained by selecting the initiator and adjusting the amount of initiator.; Part III. Vinyl ester resins undergo significant volume shrinkage upon cure. Cure volume shrinkage was measured by a lab-made dilatometer. The cure volume shrinkage was found to reduce fiber/matrix adhesion as a result of residual stress. A 3-D non-linear finite element model was used to analyze the residual stress distribution at the fiber/matrix interphase. Higher von Mises effective stress was found for large cure volume shrinkage which is consistent with the experimental results.