Mechanism of environmental stress cracking in linear polyethylene

Abstract

By means of a constant stress test, the environmental stress-cracking behavior of linear polyethylene has been studied on a macro and micro scale in an effort to determine the mechanism of the process. Upon the application of stress, linear polyethylene develops a network of very fine, elliptical fissures, the edges of which are connected by cold-drawn material. In the absence of an active environment, these fissures slowly grow and interconnect, resulting ultimately in the formation of a “neck.” When exposed to an active environment, however, the cold-drawn material ruptures as it is formed at the tips of the fissures. Unsupported, these fissures grow rapidly and interconnect resulting in sample failure. Fissures form both around and through the centers of spherulites with less cold drawing occurring at the interspherulite boundaries. Macroscopic studies confirmed the observation that active environments attack stressed polyethylene specifically at microzones of cold drawing. The effect of low molecular weight hydrocarbon species on the stress-crack resistance of linear polyethylene was evaluated. The role of flaws in the process is also discussed. Attempts have been made to establish a criterion of environmental activity. All of the active stress-cracking agents studied were found to reach similar levels of absorption in polyethylene; however, the specific chemical nature of the environment and not merely its level of absorption determines its ability to cause stress cracking.