Microhardness profiles on aged rubber compounds

In a study of the long term natural ageing of rubbers, nineteen compounds were naturally aged for forty years at three sites classified as temperate (Shawbury, UK), hot dry (Cloncurry, Australia) and hot wet (Innisfail, Australia). The specimens were stored in specially constructed aluminium boxes which allowed free circulation of ambient air around the test pieces but excluded direct light and rain. After 40 years ageing, the use of microhardness measurements was investigated as a means of determining the degree of ageing as a function of material thickness. Compression set test pieces were sliced into 1 mm thick sheets and hardness determined. The procedure proved effective for determining hardness profiles. Significant differences were observed between skin and bulk suggesting that differential ageing had taken place. However, only the net effect of scission and crosslinking reactions is detected and differences can be due to differences in cure before exposure to ageing. After consideration of the known macro hardnesses of these materials and differences found at the centres of test pieces from the different exposure sites, it is suggested that the ageing process had not been greatly restricted by the rate of oxygen diffusion. (C) 2001 Elsevier Science Ltd. All rights reserved.