Stress relaxation behaviour in compression and some other mechanical properties of thermoplastic-elastomer
Thermoplastic elastomers have been found to have unsual properties, a consequence of composition and structure. The molecular composition comprises hard thermoplastic blocks which aggregate into domains, and flexible elastomer blocks in a linear or inter-penetrating structure. The mechanical properties such as stress relaxation, tensile strength, elongation, recovery and hardness of some thermopastic elatomers have been studied in some detail. The stress relaxation studies have been made possible with the development of the stress relaxation measuring equipment at the Institute of Polymer Technology. Highly accurate and reproducible results were obtained from the "ideal curve" measurements taken with the equipment, which permits continuous measurements of residual force and instantaneous modulus. It was noted that stress relaxation, while not only dependent on thermoplastic type and/or formulation (as expected) but also depended on the measuring technique (e.g. the strain rate, continuous loading, interrupted loading etc.) which have significant effects on subsequent stress relaxation. Temperature and environment also affect the results. The effect of thermal treatment, lubrication of surfaces and interrupted loading were investigated. An attempt was made to relate the modulus enhancement factor "MEF" to Hysteresis. An attempt was also made to relate the change in "MEF" to the continuous structural re-organization in the material and finally to stress relaxation.The commercial significance of stress relaxation and "MEF" in the performance of seals and gaskets is also explored. Some of the material supplied by industry for this project was prepared by "dynamic vulcanization". Attempt has been made using peroxide cross-linking agent to prepare EPDM/PP blends by this technique to explore structure-property relationship. As expected, the cured samples out-performed the uncured samples. Long term stress relaxation measurement (up to 10,000 hours) revealed the low premanent set and modest stress relaxation associated with thermoplastic elastomers in general.