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 fi ber were investigated using dynamic contact angle analysis. The surface' ac id-base property were characterized by calculating the work of acid-base in teraction according to Fowkes' and Good's theory. To evaluate the effect of sizing on carbon fiber surface properties, Ultem(R) polyimide and polyuret hane (PU) sized fibers were studied in comparison with unsized fibers. It w as found that the sizing on the carbon fiber tended to reduce the surface e nergy and cover acid-base sites. The surface of unsized and sized carbon fi bers were analyzed by XPS to investigate changes in surface chemistry. XPS analysis indicated that hydroxyl groups on the fiber surface decreased with sizing (Ultem(R) and polyurethane). These changes in chemistry are reflect ed in a decrease in the polar gamma(s)(p) and basic gamma(s)(-) contributio n of the surface energy of the fibers. Single fiber fragmentation testing o f unsized and Ultem(R) and polyurethane sized fibers in epoxy matrix showed that there is a direct effect of the surface chemical changes on the fiber /matrix adhesion. (C) 2000 Elsevier Science B.V. All rights reserved.