The Effect of an Oxidative Surface Treatment on the Adhesion of Carbon Fibres in an Epoxy Resin Matrix.

A comprehensive investigation of the adhesion at the interface of a carbon fibre in an epoxy resin has been made. The fibre surfaces were modified to increase their adhesion to resin, by an electrolytic surface treatment which was applied at various current densities. Subsequent changes in the fibre properties relating to possible mechanical, physical and chemical contributions to adhesion have been monitored. Tensile tests on single fibres indicated that the treatment altered the strength of the fibre, which was found to be highest and least variable at the optimum adhesion level. A method was developed to determine the strength of fibres in the resin and these results confirmed the single fibre data. It was found that physical contributions to the bond strength were not significant and wettability of the resin on both untreated and treated fibre was adequate. In order to study the surface functional groups, a novel method of labelling the acidic sites and producing adsorption isotherms was developed. Surface acidity correlated well with adhesion levels. An additional study of the resin system indicated that the bond strength of the untreated fibre was only affected by shrinkage after cure. Single fibre model tests were used to measure the adhesion improvement. Pull-out tests, modelled using a new combination failure criterion and fragmentation tests, indicated that the optimum adhesion level for this fibre/resin system was at 25 coulombs/m of the electrolytic treatment. Fractography studies indicated that failure in an untreated fibre/resin system involves shearing of an outer layer of the fibre, whereas for the treated fibre it is cohesive within the resin. The principle effects of the treatment were concluded to be due to chemical modification of the fibre surface coupled with the removal of a loosely adherent surface layer