Long term performance of polyethylene

All polymers display time-dependent failure, with time to failure dependent on the applied stress and temperature. Three distinct regions with different failure processes can be recognized as a function of the applied stress: (I) plasticity controlled (or ductile) failure, (II) crack growth controlled (or brittle) failure and (III) failure caused by molecular degradation. The timescale at which these failure processes cause catastrophic failure is of great importance for applications where a long service time is demanded, such as for polyethylene (PE) pipes used in the supply of water and gas.

PE pipe grades are classified by the stress level at room temperature at which a time to failure of 50 years is reached, called the minimum required strength (MRS). When this stress level is over 10 Mpa, the grade is ranked PE100. In recent years, applications required the development of PE with an MRS of 12.5 Mpa (PE125). However, no molecular or morphological parameter has yet been identified through which PE125 could be achieved.

The goal of this research is to study how morphology and molecular parameters influence the long term behaviour of PE in order to develop a PE grade with an MRS of 12.5 Mpa. The main focus will be on region (I) and (II) failure, as due to improvement of stabilisation techniques, region (III) failure is no longer a limiting factor for lifetime. It is expected that morphology will play a more significant role in region (I) failure, while molecular parameters have a greater influence on region (II) failure.