NONUNIFORM COOLING IN MULTIFILAMENT MELT-SPINNING OF POLYPROPYLENE FIBERS - COOLING AIR SPEED LIMITS AND FIBER-TO-FIBER VARIATIONS

The cooling of the spinning stage in a commercial compact-spinning lin e has been studied. A rectangular fiber bundle is extruded from the sp inneret and cooled by air entering from one side. The speed of the coo ling air is considerably reduced through the fiber bundle. There are p ractical lower and upper limits for the cooling air entrance speed, co rresponding to filament breakage at the leeward and windward sides, re spectively. These limits are functions of the material and processing parameters. Due to the nonuniform cooling, fibers sampled at the windw ard side generally have higher molecular orientation, lower amorphous fraction, higher density, and higher tensile modulus and strength. For most combinations of spinning and material parameters, the structure is either bimodally oriented ct-crystalline or uniaxially oriented mes omorphic at all spinneret positions. Fibers with different structure t ypes show opposite windward/leeward side trends with regard to local o rder and melting behavior. The structure may be mesomorphic at the lee ward side and oc-crystalline at the windward side, if the average spin -line stress is close to a critical value for orientation induced crys tallization, and the air speed difference across the spinneret is larg e. The cooling air speed affects the spin-line stress. Hence, the fibe r-to-fiber variations due to nonuniform cooling are discussed in terms of the molecular orientation in the melt and the effective time avail able for arranging molecules into ordered structures. (C) 1995 John Wi ley & Sons, Inc.