Molecular and rheological studies to understand slow crack growth in polyethylene pipe grade materials.

摘要

Slow crack growth (SCG) is a time-dependent brittle-type failure that polyethylene (PE) pipes suffer from when under low stress levels. In order to achieve a required lifetime of minimum 50 years, a deep understanding of material's properties is needed. The general accepted mechanism of SCG heavily depends on the degree of entanglement and tie molecules present in the material. Therefore, the long-term mechanical properties are influenced by the molecular properties such as molecular weight and distribution, comonomer content, branching, and degree of crystallinity. The purpose of this project is, thus, to understand and correlate the properties of 6 PE samples (materials A, B, C, P, E and F) with their SCG resistance property. Various characterization instruments were used to conduct the study; according to its objectives (rheological, thermal, molecular and morphological analyses). Normalized SCG properties were obtained from the crack round bar (CRB) test, strain hardening (SH) test and notched pipe test (NPT). From the different analyses conducted, the highest SCG resistance of material F was explained. Moreover, several correlations with SCG were determined; such as: increasing the molecular weight and its distribution, short chain branches, comonomer content and length, lateral lamellar area and zero shear viscosity will increase the SCG resistance and its subsequent time to failure. This was attributed to the enhanced interlamellar entanglement and tie molecules that resist 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). In order to elaborate more on this project, a list of future work actions were recommended by the end of this report.