SURFACE ENERGETICS OF CARBON-FIBERS AND ITS EFFECTS ON THE MECHANICALPERFORMANCE OF CF EP COMPOSITES/

To exploit the reinforcement potential of the fibers in advanced compo sites, it is necessary to reach a deeper understanding on the interrel ations between fiber surface chemical and energetic characteristics, w etting properties, and mechanical performance. In this study CF/EP was chosen as a model thermoset composite material, whereby a hot-curing epoxy (EP) system served as the matrix. The fibers selected were PAN-b ased high-tenacity carbon fibers (CF) of varying surface treatment lev el and/or coating. Surface free energies for the carbon fibers were de termined by dynamic contact angle measurements in a variety of test li quids of known polar and dispersive surface tension utilizing a micro- Wilhelmy wetting balance and following the methods proposed by Zisman and Owens and Wendt, respectively. Surface treatment resulted in an in crease of the polar fraction of the fiber surface free energy, whereas its dispersive part remained unaffected. The interfacial shear streng th (IFSS) as determined in the microdroplet pull-off test was enhanced both by intensification of the surface treatment and sizing the CF wi th an EP component. A linear relationship between IFSS and the polar f raction of the fiber surface free energy gamma(s)(upsilon) was found. Further attempts were made to find correlations between surface free e nergy of the CF and laminate strengths measured in shear and transvers e tension. (C) 1996 John Wiley & Sons, Inc.