High-Speed Spinning and Structure Formation

The discussion of fiber spinning in the preceding sections referred to low and moderate spinning speeds in which very little degree of crystallinity can be generated in the spun fibers especially for slowly crystallizing polymers (e.g., PET, 

Crystallinity is important in fibers, because its degree affects the mechanical, diffusional, and solubility properties, as well the shrinkage of the fibers. In general, crystallization takes place under either quiescent or stress conditions. The effects of both these states on crystallinity were discussed in Section 5.5. When crystallization takes place during the spinning process the energy equation, Eq. 9.38, should be rewritten as 

TABLE 9.2 Kinetic Crystallization Characteristics of Various Polymers 

This equation adds one more unknown, the degree of crystallinity, 0c, to the system of equations continuity, constitutive, momentum, and energy. Thus, one additional equation is needed, which relates the crystallization rate to the spinning and physical characteristics of the system. This additional equation comes from a simplified approach to nonisother-mal crystallization under conditions of molecular orientation, which is discussed next. 

Nucleation and growth rates, and thus crystallization rates, are very sensitive to temperature. This sensitivity comes from the effect of the temperature on the energy terms in the respective equations of nucleation and growth rate. The experimentally observed crystallization half-time, ti/2 (time for 0c/0co = Vz), or its reciprocal rate constant, K(T) (t j2 = ( nl/K (T)) see also Eq. 5.159), was shown to obey an empirical relationship of the following form (Ziabicki, 1976)  

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