Effect of acid–base properties of unsized and sized carbon fibers on fiber/epoxy matrix adhesion

The surface energies and acid–base character of unsized and sized Zoltek fiber were investigated using dynamic contact angle analysis. The surface’ acid–base property were characterized by calculating the work of acid–base interaction according to Fowkes’ and Good’s theory. To evaluate the effect of sizing on carbon fiber surface properties, Ultem® polyimide and polyurethane (PU) sized fibers were studied in comparison with unsized fibers. It was found that the sizing on the carbon fiber tended to reduce the surface energy and cover acid–base sites. The surface of unsized and sized carbon fibers were analyzed by XPS to investigate changes in surface chemistry. XPS analysis indicated that hydroxyl groups on the fiber surface decreased with sizing (Ultem® and polyurethane). These changes in chemistry are reflected in a decrease in the polar γsp and basic γs− contribution of the surface energy of the fibers. Single fiber fragmentation testing of unsized and Ultem® and polyurethane sized fibers in epoxy matrix showed that there is a direct effect of the surface chemical changes on the fiber/matrix adhesion.