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Water absorption experiments have been carried out on three DGEBA type epoxy resins. A reactive diluent added to the resin increased water permeation, while a monomer that expanded on cure decreased it. Exposure at 97% relative humidity (RH) gave results that closely paralleled those obtained by water immersion, though the resin had higher solubility for water and lower activation energy for diffusion when immersed, over the temperature range investigated, that is, 23 to 75°C. The activation energy for diffusion was independent of relative humidity, but the diffusion constant was directly proportional to it. The solubility of water was not determined by Henry's law, but instead was determined by relative humidity to the 1.4th power. The water does not appear to be bound to polar groups in the resin, or hydrogen bonding sites, although dielectric tests indicate that it does not behave as free water, since its polarizability is much reduced. The amount of reduction in effective dielectric constant of the water was only about 55 to 77% indicating that there was some clustering of the water molecules in the polymer, rather than complete separation of the molecules.
Professor, University of Toronto, Toronto, Ontario
Materials science engineer, Digital Equipment Canada Ltd., Kanata, Ontario
Stock #: CTR10249J