The cold drawing of polyethylene terephthalate

Abstract

The cold-drawing behaviour of polyethylene terephthalate has been studied over a wide range of test conditions and polymer constitution. In addition to measurements at different temperatures and strain rates, the influence of pre-orientation, crystallinity and chemical cross-linking was investigated. Although crystallinity and cross-linking altered to some extent the details of the drawing process, the overall pattern of behaviour remained substantially unchanged despite rather severe changes in the microstructure of the polymer. Pre-orientation affected the yield stress and produced marked changes in the natural draw ratio. Previous studies by Pinnock and Ward have shown that pre-orientation is introduced during extrusion in a manner similar to stretching a rubber network and that the degree of stretch can be determined by the subsequent high-temperature shrinkage of the pre-oriented fibre. Measurements of the natural draw ratio and shrinkage of a series of pre-oriented fibres, described here, showed that the natural draw ratio may be regarded as equivalent to stretching a network to a limiting total extension. This limiting extension appeared to be a constant independent of the division of extension between the extrusion and drawing processes, and could provide a geometrical explanation for the large influence of pre-orientation on the natural draw ratio. An alternative view is that the degree of strain hardening achieved for a given elongation during the cold-drawing process will be affected by pre-orientation, and that the natural draw ratio could be very sensitive in this respect. Even with this explanation, the change in strain hardening may be associated with the limiting extension of a network. Measurement of the birefringences and moduli of the cold-drawn fibres showed that the development of optical and mechanical anisotropy could be fitted to a good approxi mation by an aggregate theory. This theory assumes that the drawn fibre consists of an aggregate of optically and mechanically anisotropic units which are aligned by the drawing process.