Hot compaction of woven poly(ethylene terephthalate) multifilaments

Here we describe the development of a process, and the resulting mechanical properties, for hot-compacted sheets of woven poly(ethylene terephthalate) (PET) multifilaments. Investigation of the various processing parameters showed that a key aspect was the time spent at the compaction temperature, termed the dwell time. Molecular weight measurements, using intrinsic viscosity, showed that hydrolytic degradation occurred rapidly at the temperatures required for successful compaction, leading to embrittlement of the resulting materials with increasing dwell time. A dwell time of 2 min was found to be optimum because this gave the required percentage of melted material to bind the structure together, while giving only a small decrease in molecular weight. A combination of techniques, including mechanical tests, differential scanning calorimetry, and scanning electron microscopy, was used to examine the mechanical properties and morphology of the optimum compacted sheets. These tests reinforced the view from previous studies on hot-compacted polypropylene, of hot-compacted sheets as self-reinforced composites, whose behavior is a combination of the properties of the two components, that is, the original oriented multifilaments and the melted and recrystallized matrix. Other key findings from the research included a confirmation of the importance of obtaining high ductility in the melted and recrystallized phases, promoted by using a high molecular weight or by suppressing crystallinity during processing, and the proportionately high-impact performance of hot-compacted sheets, compared with that of other materials. (C) 2003 Wiley Periodicals, Inc.