Crystallisation during spinning

Residence times of the real drawing are extremely short some milliseconds compared with the normal half-time of crystallisation (f 1/2 of the order of 50s) it is clear that the crystallisation process during spinning with high-speed winding is many decades faster than that in the isotropic melt. 

As manufactured, PET is a relatively weak, amorphous (non-crystalline) polymer which has to be crystallised and orientated by stretching to give it its textile properties. Orientation is carried out during the spinning operation. Equally important nowadays is the use of PET as a replacement for glass in soft drink bottles. Although more expensive than the polyolefins or PVC, it has a distinctive appearance and can be readily separated from a waste stream. Its recovery for recycling will be discussed in Chapter 4. 

The crystallisation from strained melt as for instance in a blown film or in the jet during fibre spinning produces a row nucleated structure. " Linear nuclei are formed parallel to the strain direction. They contain more or less extended polymer chains. Secondary epitaxial nucleation on the surface of such linear row nuclei produces folded chain lamellae which are oriented perpendicular to the strain (Fig. 6). In such a case the sample exhibits a high uniaxial orientation of chain axes in the strain direction with random orientation of the a- and b-axes perpendicular to it. If the growing lamellae exhibit a helical twist the chain orientation in the strain direction is very soon replaced by the orientation of the axis of maximum growth rate (b-axis in the case of polyethylene) perpendicular to the strain direction and a more random orientation of the remaining two axes (a- and c-axes in the case of polyethylene) with a maximum in the strain direction. Such a row nucleated structure has parallel cylindrical spherulites (cylindrites) as its basic supercrystalline element. 

The structure of fibres is formed during the crystallisation, which occurs below spinneret orifices, in the cooling zone, by solidification of the extruded stream. The crystallization rate is comparable to the cooling rate of the polypropylene in the spinning line. Therefore, the formation of crystalline structure in the melt-spinning process depends strongly on the spinning conditions and polymer characteristics. 

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