EFFECTS OF MICROCELLS ON NOTCHED IMPACT STRENGTH AND FRACTURE SURFACE MORPHOLOGY OF HDPE

Abstract

The Izod notched impact strength of foamed and unfoamed high density polyethylene(HDPE)was tested systematically in a wide temperature range from-196℃ to 23℃,and the impact fracture surface morphology of samples was observed and analyzed by scanning electron microscopy(SEM).The size and size distribution of cells were analyzed by using the image-pro plus software.The samples were also characterized by dynamic mechanical measurements.The results show that the cell size is in the range of 35～40 μm having a normal distribution.The Izod notched impact strength of unfoamed HDPE is much higher than that of the foamed ones at room temperature.The impact strength of both samples decreases with the descent of test temperatures.As the decreasing rate of impact strength for unfoamed samples is high than that for the foamed samples,an inversion of impact strength values between the two samples occurs at around-35℃,below which the foamed HDPE samples exhibit higher impact strength than the unfoamed HDPE samples.For both samples a sudden drop of impact strength can be observed in the temperature range from-43℃ to-53℃ followed by a very slow decrease trend in the low temperature range,and the transition between the two regions is quite sharp.The effects of microcells on the impact properties of HDPE were analyzed.It is supposed that the existence of microcells may reduce the actual section area of samples under impact testing,on the other hand it may also favor the plastic deformation of the HDPE matrix and the inhibition of crack propagation under low temperatures.The contribution of these factors determines the different temperature dependence of the foamed and unfoamed HDPE samples under impact testing.