Hydrolytic stability of PC/GF composites with engineered interphase of varying elastic modulus

Abstract

Effects of elastic moduli of the interphase on the hydrolytic stability and mechanical properties of polycarbonate (PC) reinforced with unidirectional E-glass and S2-glass fibers was investigated. Interphase, approximately 100 nm thick, were deposited onto E- and S2-glass fibers using various deposition techniques. Elastic modulus, E i, of the interphase was determined using two experimental techniques, i.e., measurement of Rayleigh wave speed and vibrating piezoelectric crystal. In this work, E i varied from 0.1 to 6 GPa. Single-embedded fiber fragmentation test was utilized to determine the average shear strength of the interphase, τ a. Increasing E i resulted in an increase of τ a from 19 MPa for the soft interphase to 42 MPa for the rigid interphase, respectively. It was also found that the usual silane sizing agents used commercially to promote matrix–fiber bonding led to a reduction of hydrolytic stability of the interphace under extreme conditions of stress and moisture. Elastic modulus and strength of the laminate were measured in the direction parallel and perpendicular to the direction of fibers. Properties of both model single-fiber composites and 8-ply UD laminates were measured in the dry state and after exposure to 85 °C water for 100 h followed by 100 h drying at 100 °C. No significant reduction in longitudinal properties of the laminates resulted from the exposure to water. In some cases, significant reduction in transverse properties, controlled by the matrix and the interphase behavior, was observed. PC/bare fiber composite annealed at 275 °C for 10 min prior the immersion in water exhibited excellent retention of properties after long-term immersion in hot water. Similar results were found for composites with fibers treated with PC oligomer and plasma deposited MPTCS. This finding was attributed to the formation of a dense, high modulus interphase consisting of the deposited layers and immobilized PC chains near the fiber surface. In addition, improved wetting of these fibers by the PC contributed to less defective and, hence, more stable interphase.