Environmental stress cracking in thermoplastic polyurethanes
2012-11-29T13:06:53Z (GMT) by
This research has been concerned with investigating stress cracking phenomena which occur when thermoplastic polyurethanes (TPU's) are immersed in seawater at low temperatures. The investigation was concerned with testing the following hypothesis: a) that cracking of TPU was due to environmental stress cracking (ESC) and/or b) due to structure and morphology changes which occur during the ageing period. The combined effects of polyurethane chemical backbone, domain structure and crystallinity on stress cracking in TPU were studied. Further, the investigation concerned itself with studying, on stress cracking, the effects of storage conditioning the TPU granules prior to processing, the processing conditions and various postcuring treatments of TPU. Characterisation and analytical techniques employed to study the structure of TPU's investigated consisted of thermal analysis and X-ray diffraction. Molecular weight distribution was studied by gel permeation chromatography and solution viscosity techniques. Processing'was evaluated by melt flow index (MFI), injection moulding and extrusion. A new accelerated ageing test for environmental stress cracking has been developed which uses high pressures. Also, a new type of ESC chemical class of reagent which relates the hydrogen bonding parameter of the ESC agent to that of the TPU's has been discovered and a theory developed. A test method has been designed to measure, in active environments, the existence of a critical strain for all the TPU examined whether commercial or laboratory synthesised materials. Results show that the stress cracking in TPU's is due to the following events: (a) a large reduction in molecular weight due to the processing, and (b) the combined effects of applied stresses and the hydrolysis of the polymer. Molecular weight reduction in TPU's and their resistance to ESe was also found dependent on the preconditioning history of unprocessed TPU granules, as well as any postcuring operations applied to the processed materials. It has been established that, the ESe resistance of TPU's can be improved by (i) optimising processing conditions, (ii) by insertion of slight crosslinking or by selecting certain types of diisocyanates, specifically p~phenylene diisocyanate, and trans 1,4-cyclohexane diisocyanate. A mechanism to explain ESe of TPU's inactive and mild environments is also proposed.