Adhesion of Graphite Fibers to Epoxy Matrices. III. The Effect of Hygrothermal Exposure

Abstract

Abstract Surface treatments have been used to promote shear strength between graphite fibers and epoxy matrices by adding polar surface groups to increase the attraction between the fiber surface and the polar epoxy matrix. Because of the polar interactions between fiber and matrix, interfacial bonding may be susceptable to moisture attack resulting in permanent loss of fiber matrix bonding with a resultant loss in composite shear strength. Fiber "finishes" are also used to improve the processability of surface treated graphite fibers. These "finish" layers may not respond to hygrothermal exposure in the same manner as the matrix. Type AS1 surface treated graphite fibers and surface treated and "finished" (ASIC) for improved adhesion to epoxy matrices were used in this study. Single fibers were encapsulated in an epoxy-mPDA matrix so that interfacial shear strength and matrix response to shear loading could be quantified and observed microscopically with polarized light as a function of moisture exposure of the samples. Moisture was sorbed into the coupons at 20°C, 70°C, and 125°C until saturation was reached. The interfacial shear strength was measured under saturated conditions and after dehydration to the initial weight in an attempt to isolate matrix plasticization effects from interface effects. For epoxy matrices and fiber finishes which consist of a layer of epoxy without curing agent, the following conclusions can be made: At temperatures below the wet glass transition temperature of the matrix, matrix properties are reduced by the sorption of moisture but can be restored to their original values by reversibly removing the moisture. The fiber surface is not affected by hygrothermal exposure at these conditions. The interfacial shear strength loss is primarily due to the plasticization of the matrix. At temperatures above the wet glass transition temperature of the matrix, some irreversible changes in matrix properties occurred which affected the interfacial shear strength through matrix shear transfer. A substantial difference in interfacial shear strength alteration with hygrothermal exposure was detected at all conditions between the surface treated and the surface treated and "finished" fibers. In all cases the "finished" fibers retained a higher absolute value for the interfacial shear strength after hygrothermal exposure. This was attributed to the creation of an interphase region in the "finish" layer that was more brittle but had a lower Tg than the matrix. The presence of this layer at low temperatures promoted better stress transfer but at higher temperatures in the wet state acted like a compliant zone that reduced irreversible damage occurring at the fiber-matrix interface.