EFFECT OF RAPID COOLING IN THE MELT SPINNING OF POLYPROPYLENE

Abstract

Polypropylene crystallizes in the monoclinic form in slow cooling, and in the pseudo-hexagonal (smectic) in rapid cooling. In melt spinning of polypropylene, we found out that the undrawn filament with high birefringence is obtained by rapid cooling before the crystallization begins in the filament (i. e. before the remarkable increase in the birefringence takes place.)
This rapid-cooled filament is consisted of the highly oriented small paracrystals (smectic modification).
When the rapid cooling was made, even in low take-up speed, the elongation decreases and the apparent strength becomes higher as compared with the filament made by the conventional spinning.
The cold-drawing of the rapid-cooled filament at low draw ratio is accompanied by further increase in the apparent strength and the decrease in the breaking elongation.
These rapid cooling effect of spun filament is influenced by the spinning stress and filament temperature just before the cooling, however, the melt temperature is the important in our experiment. At low melt temperature, we could not observe remarkable rapid cooling effect in spite of the occurrence of the smectic structure, and thus the melt temperature is considered to play an important role for the cooling effect.
From the result of the molecular degradation by increase of melt temperature, it is considered that the decrease in the molecular weight should be concerned with the movability of molecules, which should contribute to the high orientation of paracrystals.