ELSEVIER Composires Part A 28A (1997) 223.-236 ,P 1997 Elsevier Science Limited zyxwvutsrqpon S1359-835X(96)00113-3 Printed in Great Britain. All rights reserved 1359-835x/97/$17.00 Effect of water and its activity on transport properties of glass/epoxy particulate composites A. Lekatou”, S. E. Faidi, D. Ghidaoui, S. B. Lyon and R. C. Newman Corrosion and Protection Centre, UMIST, PO Box 88, Manchester M60 lQD, UK (Received 17 August 1995; revised 2 July 1996) 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 O-25% by volume were tested in deionized water, saturated NaCl and saturated MgC12, at 40°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. 0 1997 Elsevier Science Limited. All rights reserved. (Keywords: polymer matrix composites; water diffusion; electrical properties) INTRODUCTION At ambient temperature, low molecular weight substan- ces can easily migrate in polymer based materials. The most important of these substances is water. Polymer matrix composites (PMCs), for structural and coating applications, invariably make contact with water, especi- ally in marine applications such as small surface vessels, submersible structures and piping. The kinetics of water penetration into fibre-reinforced polymers have been widely studied’-14. However, there have been very few studies of water and aqueous solution effects on heterogeneous disordered model materials, such as glass microsphere-reinforced composites. These have long been used for fundamental studies of mechanical properties in the dry state15. The simplified geometry of the filler and its random three-dimensional distribution in the matrix can be used for a quantitative rationalization of the degradation behaviour of a whole class of engineering materials. By varying the input water activity, the degree of osmotic interfacial damage can be * To whom correspondence should be addressed at: Research Institute for Design, Manufacture and Marketing, University of Salford, Salford MS 4WT. UK controlled rather precisely. This paper focuses on the effect of water of various activities on a glass micro- sphere/epoxy composite, with respect to water sorption, permeation and electrical resistance changes. zyxwvutsrqponmlkjihgfe Water penetration into PMCs Water sorption into epoxy resins occurs by two modes’6m’8: 1) sorption by the free volume of the polymer, depending on the cross-link density; and 2) hydrogen bonding of water molecules into hydrophilic sites present in the polymer network. Water penetration into PMCs involves three mechan- isms’: 1) direct diffusion of water molecules into the matrix and, to a much less extent, into the filler material; 2) flow of water molecules along the filler-matrix interface, followed by diffusion into the bulk resin; and 3) transport of water by microcracks or other forms of microdamage, such as pores or small channels already present in the material or generated by water attack. Experimental studies have shown that water diffusion into PMCs initially follows the Fickian model, i.e. proportionality between mass gain and the square root of immersion time2-6, which corresponds to the first 223