The influence of crystalline morphology on the growth of water trees in PE

A small amount of phenolphthalein, acting as a nucleating agent, was added to low density polyethylene (LDPE) to modify the size and number of spherulites. The crystalline morphology was observed and analyzed with the aid of a chemical etching method. It was found that the growth of water trees is faster in LDPE samples in which spherulites are larger and the number of the spherulites is lower. In contrast, water trees grow more slowly in other LDPE samples in which spherulites are smaller in size and greater in number. On the other hand, three different kinds of additives, weak electrolyte and hydrophilic or hydrophobic substances, were added to the LDPE materials to investigate the influence of typical spherulite boundary regions on the growth of water trees. It was verified experimentally that during the process of crystallization, these three different additives are rejected and finally settled near the spherulite boundaries. Therefore, three kinds of boundary regions with different characteristics are formed. It has been concluded that hydrophillic boundary regions can strongly inhibit the growth of water trees, while the other two boundary regions (weak electrolyte, hydrophobic) have little influence on the water tree growth.