Kinetics of environmental stress cracking in high density polyethylene

Abstract

The observation of environmental stress crack ( ESC) growth in high density polyethylene (HDPE) in a 10% lgepal CO-630 solution is reported using double-edge- notched specimens, which allow a fracture mechanics approach. Below the initial stress intensity factor K 1 value of 0.4 MPa m 1/2, the cracking process consisted of both an incubation time for cracking, t d′ and a crack growth stage. The incubation time is stress dependent (decreasing with increasing stress), while the crack growth exhibits a root time (t 1 2 ) dependence and is relatively stress independent. The incubation time is the time necessary to generate a dry void craze structure sufficient to allow the PE to absorb the aggressive liquid. As a consequence of the liquid transport in the craze structure, the crack growth is believed to be controlled by the velocity of the liquid entering the void/fibril structure where capillary pressure is the driving force. The incubation times were determined to be more significant than the actual average crack growth rates for the PE samples tested. Injection moulding orientation increases the average crack growth rate without significantly changing the incubation time.