THE MECHANISM OF SLOW CRACK-GROWTH IN POLYETHYLENE BY AN ENVIRONMENTAL-STRESS CRACKING AGENT

Using single edge notch tensile specimens under plane strain conditions, the time to fail by slow crack growth was accurately measured in an ethylene-octene copolymer in Igepal and air. The primary result was the observation of a critical time for failure called the 'Igepal transition'. Only for failure times greater than the 'Igepal transition' did Igepal accelerate fracture. For failure times shorter than the 'Igepal transition' the temperature dependence and stress dependence of the failure times in air and Igepal were the same and secondary crazes were observed in Igepal but not in air. By relaxing the local stress the secondary crazes made the failure time in Igepal longer than in air. For times longer than the 'Igepal transition time' the Igepal accelerated fracture. The activation energy in Igepal was reduced to 69 kJ mol-1 as compared to 118 kJ mol-1 in air and the dependence of failure time on stress was changed by the Igepal. The Igepal has two distinct effects: it enhances crazing by plasticizing the amorphous region of the bulk polymer; and it enhances fracture of the craze by plasticizing the crystalline region of the fibrils. The 'Igepal transition time' was related to the difference between the diffusion coefficients in the crystalline and amorphous regions.