Effect of acid-base properties of sized carbon fibers on fiber/epoxy matrix adhesion

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