Influence of morphology on electrical treeing in polyethylene blends

The effect of morphology on the development of electrical trees in semicrystalline polymers has been investigated using a blend of high and low-density polyethylene and for comparison, a low-density polyethylene. In the blend system, which contained 20% high-density polyethylene and 80% of the low-density polyethylene, thermal processing was used to generate a variety of morphologies. Quenching directly from the melt gave a fine-scale banded spherulitic morphology, isothermal crystallisation at 115°C gave a continuous, coarse-banded spherulitic morphology, whereas crystallisation at 124°C and above produced discrete and relatively compact lamellar aggregates of high-density polyethylene within a low-density matrix. The morphology of the low-density polyethylene was characterised by fine-scale banded spherulites. The growth of electrical trees was studied in these materials as a function of the applied AC electrical stress. In the low-density material and the blend materials, the rate of development of these structures was found to follow the well-known sigmoidal time dependence, where the increasing fractal dimension (branch density) of the growing tree structure is coupled with a decrease in the tree growth rate with increasing applied voltage. This leads to a local maximum and minimum in the average growth rates as a function of applied voltage. At all applied voltages, the tree growth rates were found to depend not only on the degree of crystallinity but also on the uniformity of the crystalline phase. Tree growth rates were greatest for LDPE and the quenched blend material and a minimum for the blend material crystallised at 115°C. The high-density inclusions formed at crystallisation temperatures of 124 and 125°C, acted as barriers to the tree growth and resulted in average tree growth rates between the quenched and the 115°C blend materials.