SURFACE PROPERTIES OF CARBON FIBERS: IMPACT ON THE MECHANICAL PERFORMANCE OF SHORT FIBER REINFORCED POLYPROPYLENE COMPOSITES

Carbon fibers (CF) reinforced polymer composites are widely used in various application fields like aerospace, marine and automobile industries, as their high specific strength and stiffness offer the possibility for significant weight reduction. However, the full potential of CF reinforced polymers has not yet been fully exploited, as CF exhibit poor interfacial adhesion with matrix polymers. To overcome this shortage, CF are usually surface treated, in order to introduce functional groups that can interact with the polymer matrix or a coupling agent. The aim of this study was to evaluate the influence of CF surface properties and the amount of coupling agent on the mechanical performance of injection molded CF reinforced PP composites. Therefore, sized, de-sized and plasma treated CF were first characterized by X-ray photoelectron spectroscopy (XPS) and contact angle measurements and subsequently processed to CF reinforced PP composites. Information on fiber/matrix interaction was derived from mechanical composite performance and scanning electron microscopy (SEM) images of fracture surfaces. Contact angle measurements and XPS showed key differences in surface properties between the three CF samples. While for the sized fibers, highly polar surface energies and high amounts of surface functionalities were measured, the de-sized CF showed a mostly nonpolar character. Furthermore, low pressure plasma treatment could be used to induce functional groups on CF surfaces. Mechanical testing and SEM images of fracture surfaces proved that fiber surface properties and the amount of coupling agent have major impacts on fiber/matrix interaction, hence on mechanical composite performance. Composite performance was in accordance with the fiber surface properties, determined by XPS and contact angle measurements. The excellent performance of composites reinforced with de-sized CF is particularly remarkable, showing great potential for recycling of CF. The possibility to induce functional groups by low pressure plasma treatment further broadens the application spectrum of recycled CF.