The influence of temperature on water treeing in polyethylene

The influence of temperature on water treeing in polyethylene (PE) is investigated in this paper. Low-density polyethylene (LDPE) and peroxide cross-linked polyethylene (XLPE) are chosen as the test materials. The liquid needle electrode method is used for water tree development at 20, 40, 60 and 80 °C, and a metallographic microscope is used to observe water tree morphology. The sizes and initiation rate of water trees are measured. The results obtained from the water treeing experiments indicate that water treeing is significantly influenced by temperature. The initiation rates of water trees decrease first, and then increase as temperature rises. The sizes of water trees also exhibit an increasing trend with ascending temperature. The experiments conducted on LDPE and XLPE demonstrate similar results. However, XLPE is found to have better anti water treeing performance than LDPE. Mechanical tensile test results show that the mechanical properties of the two materials, such as elastic modulus, tensile strength, tensile breaking stress and tensile yield stress, drop significantly as temperature increases. The results of the crystallinity determination and crystal morphology observation show that the crystal blocks of XLPE are smaller and denser compared with those of LDPE, which are lager and relatively sparser. Based on the analysis, XLPE is found to have better anti water treeing performance than LDPE because of cross-linking limitation to the formation of large crystal blocks and the enhancement of the mechanical properties of the material. Moreover, the failure of the mechanical properties of the materials, pore expansion and the accelerated movement of moisture and ions are the reasons for the acceleration of water treeing at high temperatures.