CHARACTERIZATION OF FILLERS AND FIBERS BY WETTING AND ELECTROKINETIC MEASUREMENTS

The surface chemical properties of organosilane-treated silica powders and oxidized carbon fibres were studied by means of wetting and electrokinetic measurements. A capillary flow technique was successfully applied to characterize the influence of the surface treatment on the wetting behaviour of silica powders. By measuring the time dependence of the liquid rise in a loosely packed column of powder the advancing contact angles with water and methylene iodide were calculated. Comparison of the measurements of the rise height of the liquid front, with the weight gain caused by the penetrating liquid, shows that the calculated contact angles agree well when surface-treated silica powders are investigated. Different approaches for the determination of the solid surface tension were applied and provide comparable results regarding the differences in the wetting properties of various fillers. The results of the wetting experiments correlate with zeta potential measurements (electrophoresis, streaming potential). This method is a useful experimental technique to determine the nature (polar, non-polar) and the amount of functional groups on surface treated silica fillers and carbon fibres. By measuring the zeta potential vs the pH of a 10(-3) N KCl solution varying amounts of acidic groups at the surface can be proved to exist. 1. The content of acidic silanol groups is decreased when treating silica powders with organosilanes. This correlates with a decrease in the solid surface tension. 2. The surface oxidation of carbon fibres results in an increase in the number of acidic carboxylic groups which correlates with a higher solid surface tension. The decreased hydrophilicity of silane-treated fillers is also shown by a higher zeta potential maximum in KCl solutions and by lower Gutmann acceptors numbers (AN). Combination of the methods applied provides a good understanding of the surface chemical properties of silica powders and carbon fibres.