Effect of crystallization on drawdown force at capillary extrusion for polyethylene

Abstract

The effect of crystallization on the drawdown force, i.e., the force needed to stretch a molten polymer uniaxially, was studied using conventional polyethylene samples. It was found that the drawdown force of high-density polyethylene (HDPE) increases with increasing the die length. This is attributed to the rapid crystallization owing to the low density of entanglement couplings after passing through a long die. On the contrary, the die length has no/little influence on the drawdown force of linear low-density polyethylene (LLDPE), indicating that the molecular orientation applied in a long die is almost relaxed prior to the crystallization. Even for LLDPE, however, the drawdown force becomes sensitive to the die length by the addition of a small amount of HDPE, because the crystallization process is accelerated.