Cooling conditions, melt spinning

Fibers emerging from the spinneret are cooled under controlled conditions, passing over guides and rollers to a take-up spool or bobbin. Often a finish is applied before windup to control static electricity and friction (Stevens 1993). Large-scale production machinery produces fiber at a rate of thousands of feet a minute. A schematic diagram of a melt-spinning apparatus is drawn in Figure 8-10. 

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. 

The key variables of the melt spinning process are (1) take-up velocity, (2) length of the spinline, (3) cooling conditions along the spinline, (4) extmsion temperature, (5) mass throughput per spinneret orifice, and (6) the size and shape of the spinneret holes. These process variables interact... 

It is well known that the extent to which isothermal crystallization in polymers occurs can be profoundly influenced by molecular orientation. Recently Nakamura et al. (275) have performed a detailed theoretical analysis of the relationships between crystallization temperature, crystallinity, orientation and cooling conditions for nonisothermal processes. These investigators have also evaluated the crystallization rates under molecular orientation during melt spinning experiments and... 

Post-die processing includes a niunber of operations carried out at the exit of the extruder die in a free-siuface way. Examples of such processes are fiber spinning, film blowing, and sheet forming. The shape and dimensions of the extrudate material are determined by the rheological properties of the melt, the die dimensions, the cooling conditions, and the take-up speed (relative to the extrusion rate). 

Han, C. D. and Apte, S. M., Studies of melt spinning VIII. The effects of molecular structure and cooling conditions on severity of draw resonance, J. Appl. Polym. ScL, 24, 61-87 (1979). 

FIGURE 9.7 Calculated relative importance of the various forces in the melt-spinning process as a function of the take-up speed. The spinning conditions are spinneret radius, 125 p.m filament radius at take-up, 9.25 qm extrusion viscosity, 300 Pa s and stationary cooling air. (Reprinted by permission of the publisher from Ziabicki and Kawai, 1985.)... 

NEXAFS orientation analysis can also be applied to polymer semiconductors, hi this example, films of regioregular and regiorandom P3HT were spin-coated from chloroform to approximately 20 mn atop the native oxide of silicon, then melted at 250°C and slowly cooled under vacuum. NEXAFS carbon K-edge spectra were collected at the same conditions as for pentacene. The resultant spectra are shown in Figure 4.2.7(a-b). 

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