WATER-ABSORPTION AND DISC CRACKING IN URETHANE METHACRYLATE RESINS

Water absorption at 23 and 100-degrees-C has been studied in three mat erials based on a single type of urethane-methacrylate resin: the neat resin, a rubber-toughened blend containing 15 wt % of core-shell modi fier, and a filled rubber-toughened resin containing 46 wt % silica pl us 8 wt % core-shell modifier. In all six absorption experiments, wate r diffusion followed Fick's law. However, in neat resin at 100-degrees -C, the period of Fickian diffusion, during which the material appeare d to reach saturation, was followed by an additional period of water u ptake, which was accompanied by formation of internal disc cracks up t o 1 mm in diameter. At both 23 and 100-degrees-C, fracture of water-sa turated neat resin was defect dominated: cracks were initiated at inor ganic impurities between 40 and 80 mum in diameter. The adverse effect s of water are attributed to hydration and consequent swelling of thes e insoluble impurities. No internal cracking was detected in either th e rubber-modified or hybrid resin, which have higher a K(IC)s than the neat resin. (C) 1994 John Wiley & Sons, Inc.