The influence of temperature on water treeing in polyethylene

In order to study the influence of temperature on water treeing in polyethylene, low density polythene and peroxide cross-linked polyethylene are chosen as the testing materials. At room temperature (20°C), 40°C, 60°C and 80°C, liquid needle electrodes method is used for water tree development, and metallographic microscope is used for observation of water tree morphology. Meanwhile, sizes and initiation rates of water trees are counted. Experimental results show that water treeing is significantly influenced by temperature when it is above 60°C. Water tree initiation rates first decrease and then increase with temperature rising, water tree sizes overall show an up-going trend with ascending temperature, experiments on LDPE and XLPE get the similar results, but XLPE has better anti-water treeing performances than LDPE. Meanwhile, in order to investigate the mechanism of water tree initiation and propagation at different temperatures, mechanical properties of the two materials were tested at four temperatures respectively. Experimental results show that mechanical properties of materials dropped significantly with the increasing temperature. After analysis, we consider that the limitation of cross-linking to the formation of large crystal block and the enhancement of the mechanical properties of materials are the two main reasons for better anti-water treeing performances of XLPE than LDPE. Meanwhile, we consider that mechanical properties of materials descending, pore expansion and the accelerated movement of water molecules and ions are the reasons for the acceleration of water tree at high temperatures.