Molecular and morphological studies to understand slow crack growth (SCG) of polyethylene

Abstract

Slow crack growth (SCG) is a time-dependent brittle-type failure that polyethylene (PE) pipes suffer from when under low stress levels. This study discusses the relation of morphological, molecular weight and molecular weight distribution, rheological, thermal and tensile properties of different PE materials with SCG behavior obtained from the crack round bar (CRB) test, strain hardening (SH) test, and notched pipe (NPT) test. It was observed that increasing the molecular weight and its distribution, short-chain branches, co-monomer content and length, as well as lateral lamellar area increased the SCG resistance and the subsequent time to failure. This was attributed to the enhanced interlamellar entanglement and tie molecules that resisted deformation for a longer time. In addition, SCG resistance was found to decrease with decreasing lamella thickness and degree of crystallinity (within similar molecular weight range). A decrease in the SCG resistance was observed on increasing the onset degradation temperature. Despite the lack of correlation established from tensile tests in the elastic region, SH and CRB values were found to be directly related. As strain hardening modulus increased, time to failure also increased. Zero-shear viscosities also reflected the SCG resistance properties of the samples, as CRB values increased with increasing zero-shear viscosity.