Adhesion of novel high performance polymers to carbon fibers: fiber surface treatment, characterization, and microbond single fiber pull-out test

The adhesion of carbon fibers to several high performance polymers, including a phosphorus-containing bismaleimide, a cyanate ester resin, and a pyridine-containing thermoplastic, was evaluated using the microbond single fiber pull-out test. The objective was to determine the chemical and mechanical properties of the fiber and the polymer which affect the fiber/polymer adhesion in a given composite system. Fiber/matrix adhesion is of interest since the degree of adhesion and the nature of the fiber/matrix interphase has a major influence on the mechanical properties of a composite.

The surface chemical composition, topography, tensile strength, and surface energy of untreated AU-4 and commercially surface treated AS-4 carbon fibers were evaluated using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), single fiber tensile tests, and dynamic contact angle analysis. The commercial surface treatment which converted the AU-4 to the AS-4 fiber oxidized the carbon fiber surface. The surface of the AS-4 carbon fiber was further modified using air, oxygen, ammonia, and ethylene plasmas. The AS-4 fiber tow was also characterized following exposure to the aqueous poly(amic acid) solution used to disperse the matrix powder during aqueous suspension prepregging of thermoplastic matrix composites.

The air and oxygen plasma treatments significantly oxidized and roughened the surface of the AS-4 carbon fibers. In addition, the air and oxygen plasma increased the the polar component of the AS-4 fiber surface energy. The ammonia plasma increased the concentration of nitrogen on the fiber surface, without significantly altering the fiber topography (at a nlagnification of 50,000X). The atomic oxygen present in the air and oxygen plasma treatments is capable of reacting with both the edge and basal planes in the carbon fiber structure. As a result, the oxygen-containing plasmas progressively ablated the organic material in the carbon fiber surface. Energetic species in the ammonia plasma cleaned the fiber surface and reacted with the carbon fiber surface, increasing the concentration of amine groups in the fiber surface. The ethylene plasma deposited a layer of plasma polymerized polymer on the carbon fiber surface. The AS-4 carbon fibers were coated with poly(amic acid) when the tow was wet with the aqueous suspension prepregging solution.

The carbon fiber adhesion of bis(3-maleimido phenoxy) triphenylphosphine oxide was compared to that of Ciba-Geigy's Matrimid 5292 A/B bismaleimide system. With both bismaleimides, the carbon fiber adhesion increased significantly when the fiber received an oxidative commercial surface treatment or was exposed to an air or ammonia plasma prior to bonding. In contrast, the poly(pyridine-bis A) microbond pull-out test results showed that the carbon fiber adhesion of poly(pyridine-bis A) was not affected by the fiber surface chemical composition, fiber surface energy, or topography.