Surface crazing in PVC and other polymers

Abstract

Initiation and growth of surface crazes in rigid PVC, subjected to tensile creep loads, has been studied experimentally for a wide variety of conditions. The tests were carried out at room temperature, at stress levels ranging from 10 to 50 MPa, in air or in the vapour of natural gas condensate (a crazing agent). PVCs with different molecular weights ( K-values) and degrees of gelation were tested. Physical aging of the sample was considered by comparing quenched and aged samples. The behaviour under continuous loading was compared with that under cyclic loading. In cyclic loading, crazing behaviour differs markedly from that observed in continuous loading. The defects grow linearly with the number of load periods, i.e. linearly with the loading time, and they appear to be associated with cracks. In continuous loading, the craze length grows linearly with the logarithm of the loading time, after some initiation time has been passed. This logarithmic growth in constant loading is bounded by two processes of growth cessation. The first, at high stresses, coincides with gross yielding of the sample; the second, at low stress levels, is less well understood. The effects of stress level, crazing agent, K value, degree of gelation and physical aging on the processes of initiation and cessation of crazing under constant stress are discussed. The growth rate was shown to depend only on the stress; the crazing agent, molecular weight and degree of gelation have little influence. For other polymers, such as PS, PMMA and PC, the growth kinetics are essentially the same as for PVC. For these materials, the growth rate for crazing sharply decreases with increasing stress, according to the inverse second to inverse third power of the stress.