For production and mechanical performance of carbon fiber reinforced polymers the coating of the reinforcing carbon fibers with a polymeric sizing film plays an important role. On the one hand, the sizing improves handling and textile processing properties of the fibers. On the other hand, it ensures an optimal wettability of the fibers by the polymeric resin system during composite production and improves fiber matrix adhesion in the final composite material. Both wettability and adhesion can be tuned by the surface properties of the carbon fibers and the application of functional sizings specifically adapted to the properties of carbon fiber and resin.
To investigate the influence of the sizing on the wetting behavior, we use two types of epoxy based sizing with different degrees of reactivity. Additionally, the surface activation of the carbon fibers is varied by different degrees of functionalization by anodic oxidation. The wetting behavior is investigated by contact angle measurements utilizing a tensiometer to realize capillary rise experiments with the carbon fibers aligned parallel in a glass tube. First, the wetting of the carbon fibers during the sizing process is investigated by contact angle measurements between carbon fiber and sizing solution. Secondly, the wetting of the sized fibers with the liquid epoxy resin, which is relevant for the resin infiltration process, is examined by the same method. Also the surface energies of the carbon fibers are identified before and after sizing.
The results of this comparative study show that the wetting behavior of carbon fibers can be controlled by a suitable choice of surface activation of the fibers and reactivity of the polymeric sizing solution.