Aging and recovery of HTV silicone and EPDM rubbers due to continuous wetting

Abstract

The present study is concerned with the aging of polymer materials by immersion in distilled water and their subsequent recovery in air. In this study, the changes in the specimen's weight, surface roughness, electrical conductivity and surface tension of liquids during aging are measured. During water immersion at 75/spl deg/C and 98/spl deg/C, the weights increased rapidly in first few days and then slowly but after a certain time the weights started to decrease. The change in the weight began earlier when the immersion temperature was higher. EPDM and HTV silicone rubbers showed almost the same behavior during the aging process. During the recovery in air at 75/spl deg/C and 98/spl deg/C, the water evaporated from the rubber. The amount of the loss in the weight indicated that not only the water in the rubber but also some of the rubber constituents decreased. The surface tension of the immersing liquid decreased by the dissolution of LMW (low molecular weight) fluid from the silicone and EPDM rubbers into the liquids. This decrease of the surface tension was larger when the immersion temperature was higher. The LMW fluid has a low surface energy, therefore, the surface tension of the immersing liquid was reduced by the dissolution of LMW. Due to this dissolution of the LMW fluid, an erosion of the sample surface occurred and, therefore, increased the surface roughness. However, when the samples absorbed water during immersion, the surface roughness decreased. During an immersion in hexane and a subsequent drying in air at 40/spl deg/C the loss of EPDM was about 5 times of the HTV silicone rubber.