Toughening of polyester resins through blending with polyolefins

Abstract

The effect of blending low concentrations (1 to 10%) of polyolefins on the crystallization and irreversible deformation behaviour of polyester (PET) resins was investigated. The olefin particles did not nucleate crystallization of PET from the melt but did depress the rate of crystallization. Decreases in the cold crystallization temperature of PET during heating from the glassy state in blends with linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) were attributed to stress induced crystallization created by large volume expansions associated with melting of the olefin particles. A sharp transition in fracture strain for PET and the PET-olefin blends was observed with strain rate, where at low strain rates fracture occurred during work hardening, and as the strain rate increased, fracture occurred during cold drawing. The transition corresponded with a decrease in draw stress, a decrease in draw ratio and an increase in density of the neck. A shift in the transition to higher strain rates for the blend compositions was attributed to increased rates of crystallization and orientation due to strain induced crystallization at the stress fields surrounding the olefin particles. As the strain rate increased and the cold drawing process became more adiabatic, the mechanical behaviour was controlled by the kinetics of crystallization and orientation.