Acid-base character of carbon fiber surfaces

Abstract

The acid-base character of the surfaces of commercially available carbon fibers used in advanced composites is determined using both pulsed and continuous flow microcalorimetry techniques. The carbon fibers investigated include unsized versions of AS4, IM6, IM7X, T300, and several Apollo fibers with different levels of surface treatment. All of these carbon fiber surfaces are amphoteric and energetically heterogeneous. In general, the heats of preferential adsorption in the pulsed flow mode of bases dissolved in n-heptane are larger than for acids. While most of the adsorbates used are reversibly and physically adsorbed onto the carbon fibers, some primary and secondary amines exhibit irreversible binding to a portion of the surface. In continuous flow experiments the adsorption heat isotherms for several bases on T300 display a sharp jump at low probe concentrations, reflecting the energetically heterogeneous nature of these surfaces. Comparisons between the flow microcalorimetry data and other measures of the surface chemistry are made. Pulsed flow adsorption heats correlate with the amount of oxidized carbon species on the fiber surfaces as detected using ESCA, and recently reported results of inverse gas chromatography and programmed thermal desorption (S. Wesson, Textile Research Institute). Calorimetry results are also compared with fracture mechanical measures of fiber-resin adhesion in manufactured composites. Adsorption heats of both acids and bases on selected carbon fibers correlate with edge delamination and 90° flexural strengths of composites composed of these fibers in both epoxy and bismaleimide resins. This supports a causal connection between carbon fiber surface adsorption heats and a practical measure of fiber-resin adhesion.

Keywords:

• Carbon fiber
• adhesion
• acid-base
• flow microcalorimetry
• surface chemistry