Interphase modification in graphite-epoxy composites

An electrodeposition technique has been used for the modification of the fiber-matrix interphase in graphite fiber-epoxy composites so as to study the effect of the interlayer chemistry on the interfacial shear strength in graphite-epoxy composites. A coating of poly (styrene-co-maleic anhydride) (SMA) polymer was electrodeposited from an aqueous solution on AU graphite fibers used as electrodes in an electrolytic cell. Different electrocoating parameters were initially used to establish optimum conditions to achieve thin uniform coatings suitable for functioning as interphases in composites reinforced by the coated fibers. The interfacial shear strength (IFSS), evaluated by a single-fiber composite technique, showed that the SMA coating resulted in an improvement of about 50% in IFSS compared to the commercially treated fibers (AS). This was achieved without sacrificing impact strength. Evidence of good epoxy penetration into the coating was obtained by the use of electron microprobe line scans for bromine across the diameter of a filament in a single fiber composite--the bromine introduced through the use of a brominated epoxy resin. The observed improvement in fiber-matrix interfacial shear strengths is dependent on the comonomer ratio in electrodeposited SMA.; A phenomenological model has been developed for use in flexible interlayer fiber composites. Inclusion of the interlayer provides a third bulk phase, with interphasial regions on either side of the interlayer bulk phase. It is possible, through the use of this model, to obtain the adhesion and interaction parameters of different fiber-interlayer-matrix systems. Dynamic mechanical analysis permits the use of actual composite specimens to obtain these interaction coefficients. This will provide adhesion parameter data considering the effect of neighboring fibers ignored by both the single fiber pull-out test and the single filament fragmentation method. Known values of adhesion parameters and mesophase thickness can be used effectively to provide information of the coating thickness necessary to provide optimized bonding and toughness characteristics of an interlayer coated fiber composite.