Towards a Methodology for Designing RCP-resistant Polyethylenes Using Batch Scale Test Methods

Session 10A
2:40 pm

Patrick S. Leevers, Imperial College London

To qualify as a ‘PE100’ a polyethylene must demonstrate, via small-scale or full-scale RCP tests on extruded pipe, a high RCP critical pressure at 0°C. Because PE grades demonstrate a sharply-defined but somewhat batch-sensitive transition temperature, it is neither necessary nor relevant to measure the critical pressure accurately. It is much more meaningful to measure the critical (transition) temperature and, when developing a PE100, the target should be as low as possible.

However, even the small-scale test is too expensive in material and in processing time to be convenient for resin development. For this task, the developer requires:
(a) test methods which can provide appropriate, geometry-independent material property data from reactor-batch volumes of resin;
(b) a model to transform these data into a critical temperature prediction; and
(c) a set of ‘feedback paths’, based on empirical or theoretical structure-property relationships, from property data to synthesis variables.

This paper outlines a PE100 design methodology which gives central importance to the separate modelling and measurement of plane-strain and plane-stress dynamic fracture resistance. The former is difficult to measure but can be estimated from weight-average molecular weight; the latter is related to high-rate adiabatic tensile deformation which can be modelled using low-rate tensile test data. An overall dynamic crack resistance is predicted as a function of temperature and pipe wall thickness. Meanwhile, employing a semi-analytical RCP model, the crack driving force is calculated as a function of temperature, pressure and pipe size. The outcome is an estimate of critical pressure as a function of temperature and, hence, of the critical temperature. The entire scheme is demonstrated using a group of experimental resins in the –20 to +2°C transition temperature range. The results indicate the promise of the scheme while highlighting areas in which further research is needed.

Christos Argyrakis, Research Student
Patrick Leevers, Reader in Polymer Engineering

Department of Mechanical Engineering
Imperial College London