The Influence of CFRP-Sizings on Processability and Interphase Properties

Abstract

Sizing systems of carbon fibers have to fulfill several distinct tasks:

• enhancement of the adhesion between fiber and matrix and improvement of the mechanical properties at the interphase.

• optimization of the fiber wetting and fiber spreading during prepreg and composite fabrication.

• protection from damages during transport and handling of the fiber itself and of reactive groups on the fiber surface.

  The objective of the reported research work was to investigate the influence of sizing systems on the processability and interphase properties of CFRP.

  The first step was the analysis of two typical sizing systems. Physico/chemical methods were evaluated and optimized for the determination of the sizing composition. HPLC,GPC,titration and elemental analysis were found to be appropriate methods. Epoxy components and emulsifying agents were analyzed as basic ingredients of the sizings. ESCA analysis of fiber surfaces have indicated that a residue of the sizing still remains on the fiber, even when the fiber is prepared by multiple step solvent extraction to remove the sizing. On the carbon fiber surface, an interface layer mainly consisting of sizing components can be expected. The preceding statement tends to confirm the hypothesis of an interphase with a nonhomogeneous distribution of sizing and resin around the carbon fiber. To simulate a nonhomogeneous sizing/resin interphase we have prepared mixtures with different ratios of sizing and resin from two sizings and three commercial resin systems. The experiments with these mixtures showed the following results.

  The sizing can affect the generation of voids during the cure cycle, especially when highly flow controlled resin systems are used.

  Thermal analysis (TGA, DSC, TMA, DMA) of the cured mixtures revealed that, due to the sizing systems, a decrease in the glass transition temperature, changes in the cross link density as well as a decrease in the thermal stability can all be found.