Time-Temperature Strain and Molecular Weight Effects in the Environmental Stress Cracking of 0.95 Density Polyethylene

Abstract

Reduced variables of time-temperature and molecular weight are used to describe the stress relaxation failure of 0.95 density polyethylene in a stress cracking agent, Igepal CO-630. This method was previously shown to work for 0.96 density linear polyethylene. The 0.95 density polyethylene has an apparent activation energy of 38 kcal for stress cracking. This is appreciably higher than the 22 kcal found for 0.96 density linear polyethylene. A much higher dependence on molecular weight was also found. The 0.95 density polyethylene has a lifetime that is proportional to Mv5.8 while the 0.96 density polyethylene has a lifetime proportional to Mv3.4. Some data are also shown which indicate a difference in the sensitivity to strain for the two materials. This can lead to appreciably longer lifetimes for the 0.95 density polyethylene at low strain.