Effect of water and its activity on transport properties of glass/epoxy particulate composites

Silane coated glass microspheres embedded in an epoxy polymer matrix have been employed as a model system to investigate water sorption at three water activities. Composites containing soda-lime-silica (A) glass in the range 0-25% by volume were tested in deionized water, saturated NaCl and saturated MgCl2, at 40 degrees C. Increase in water activity led to a decrease in the effective water diffusivity due to trapping, especially at interfaces ruptured by osmotic pressure. Higher water activities favoured interfacial water transport, whereas lower water activities favoured water transport through the bulk of the polymer. Electrical resistance and water permeation tests suggested very rapid preferential water transport through areas of low cross-link density, even in the pure polymer, enhanced by the introduction of interfaces. Effective diffusivity calculations during water absorption and desorption, together with electrical resistance measurements and water uptake measurements in saturated NaCl, suggested the formation of a pecolating liquid channel network for composites containing more than 15% glass. Electrical resistance measurements using silver-coated A-glass/epoxy composites located the percolation threshold at 16% glass. Similar water sorption effects were observed with uncoated A-glass filled epoxy composites. (C) 1997 Elsevier Science Limited.