Spinning of PET

Fot the producer of fibers vdio uses tlus process, the trouble begins with astrong drop of the mdt viscosity % due to the lower molecular weight and the intrinac viscosity of the polymer. Since, for regular spinning of PET, it is a requirement that melt viscosity should not be less than 1000 at 280 °C, a process othor than that based on the decrease of molecular weight may be mme convenient. 

A cartoon of this model of morphology and molecular chain topology development in melt spinning of PET is shown in Figure 1.6. 

Fig. 4.1. Elongation rate vs. processing distance in high-speed melt spinning of PET fibres at take-up speed 4800m/min [1]...

Figure 4.6 shows reduced steady-state elastic free energy of deformation, [/ei /ei( = 0)]/i NkT, vs. the processing time, t, computed from (4.28) for melt spinning of PET with the axial velocity gradient shown in Fig. 4.1. Temperature dependence of the chain relaxation time, r(T(t)), is accounted for assuming Arrhenius temperature dependence of the relaxation time and using the axial temperature profile reported in [1]. Reduced elongation rate.

Fig. 4.7. Reduced elongation rate, cst, vs. processing time, t, calculated for melt spinning of PET with axial velocity gradient presented in Fig. 4.1 and N = 100...

A great amount of work has been devoted to high speed spinning of PET because of its industrial importance (for reviews see References [75,77]). An increase of the take-up velocity first induces an orientation of the macromolecular chains, but the material remains amorphous no change in density is observed below 3000 m/ min. As an intermediate stage, many authors postulate the development of an oriented mesophase prior to the crystallization into the usual triclinic phase. Then,... 

Extensional flow is known to have more of an effect on chain extension than shear flow. We illustrate this idea by data obtained in the spinning of PET. Below spinning speeds of 3500 m/min there is very little crystallinity, but above this speed significant crystallinity is observed as shown in Figure 5.23. The processing time at these spinning speeds is a few milliseconds whereas = 50 s. Hence, the crystallization rate must be many decades faster than in the quiescent state. 

The simulation of nonisothermal fiber spinning of PET at intermediate spinning speeds by George (1982) is worth mentioning at this point. His model works well for spinning speeds from 1000 to 3000 m/min. For PET with an intrinsie viseosity (IV) equal to 0.675 dL/g (1 dL = 100 cm ), which is a measure of moleeular weight, extrusion temperatures... 

Dutta, A., Role of Quench Air Profiles in Multifilament Melt Spinning of PET Fibers , Textile Research Journal, 57, 13-19, 1987. 

Figure 5.23 Variation in the concentration of carboxylic end groups and intrinsic viscosity during the postcondensation of PET powder produced from DMT (1) and TPA (2) prepolymers (7, 240 °C) [49]. From Gerking, L., Modifications of fiber properties by polymer and within spinning line, presentation (Paper 52b) given at the 32nd International Man-Made Fibre Congress, 22-24 September, 1993, Dornbirn, Austria, and reproduced with permission of EMS Inventa-Fischer, GmbH Co. KG...

Bico fibers are a new class of fibers, rather than a sub-set of PET fibers. Such fibers are formed from two different polymers, which are melted separately, and then combined into a single fiber at the last moment before extrusion. In some cases, the fibers are actually extruded separately, and then combined beneath the spinneret while they are still molten, so that they fuse together after spinning. 

Ziabicki and Jericki reported the crystallization characteristics of PET as well as a theory of molecular orientation and oriented crystallization [5a], Besides these theoretical considerations, the rate of recrystallization understandably seems to play an important role, particularly in high-speed spinning. Little is known about the crystallinity gradient caused during melt spinning at high take-up speeds. 

Table 13.1 Intrinsic viscosity and molecular weight data for the three characterized polymers (cf. Figure 13.2) [13]. From Some effects of the rheological properties of PET on spinning line profile and structure developed in high-speed spinning , Perez, G., in High-Speed Fiber Spinning, Ziabicki, A. and Kawai, H. (Eds), 1985, pp. 333-362, copyright (1985 John Wiley Sons, Inc.). Reprinted by permission of John Wiley Sons, Inc.

Unlike nylon, which is highly crystalline, PET fibers are amorphous after spinning. They are like the molecules shown at the top of Figure 22-6 in Chapter 22. In order to make a usable textile yarn or staple fiber our of PET, it must be drawn under conditions that result in orientation and crystallinity. This is accomplished by drawing at temperatures of about 175°F with stretch 300-400%. As with nylon, the conditions of draw (especially... 

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