Characterisation of Elastomers for Dynamic Sealing Applications

Abstract

One of the most extreme applications for elastomer materials is in sealing applications within the oil and gas sector. These applications often feature high strains, high strain rates, wide temperature ranges, contact with water and organic solvent mixes all of which can take place at large pressures. Attempting to understand and model just one of these effects would be difficult enough but to understand their full complexity when they are all acting in parallel is even more challenging. However, this thesis aims to lay the foundational work to build an all-encompassing model to help understand the behaviour of the elastomer materials that are found in mud motor stators. The work presented here focusses on material characterisation, static fracture and cyclic fatigue analysis over a range of temperatures together before and after ageing at high temperatures and in organic solvents. The materials used are typical compound formulations used in the oil and gas industry that use hydrogenated acrylonitrile butadiene rubber (HNBR) as the base polymer. However, there is only limited published research on the response of HNBR compounds to the extreme environments that are seen in these applications, their fatigue and tear behaviour and in particular the origins of their high levels of mechanical toughness. This thesis develops appropriate material characterisation models and then explores the different cyclic fatigue properties for both unaged and aged samples. This shows a complicated fatigue response frequently incorporating an increase in toughness after ageing. By examining changes to the hysteresis with ageing as well as changes to tearing type crack growth behaviour and by utilising X-ray diffraction techniques, it is possible to start to propose possible mechanisms that explain the surprisingly good performance of HNBR compounds in these types of extreme environment.