The ion-specific behavior of watertree growth in LDPE

The growth of water trees in low-density polyethylene (PE) has been studied. Trees were grown in solutions containing a fixed anion, Cl ^-, but different cations with valences of +1 (Li⁺), +2 (Mn²⁺) and +3 (Fe³⁺). An ion-specific effect on tree growth and tree appearance has been found. Generally, these results have been shown to be correlated with either the ion-water interactions or the interionic attractions. However, for the specific case of adding small amounts of 3+ cations into univalent ion solutions, such a correlation was not found: it is likely that the hydrogen ions, introduced by the multivalent ions through their electrostatic interaction with water molecules, play an important role. The abnormal transport behavior of such hydrogen ions in the solution and probably in the polymer, may expand the overall region affected by treeing but reduce the visibility of the individual trees themselves. A similar process may account for the particularly translucent appearance of trees formed with pure FeCl₃ solutions. This argument is further demonstrated through tree growth experiments using KCl solution and its mixtures with small amounts of HCl. It was found that this additive accelerated tree growth. It is argued that small amounts of KOH might be expected to show an effect similar to HCl because of the similar transport behavior of OH^- ions and H⁺ ions respectively and such enhanced tree growth was demonstrated, using small quantities of KOH added to KC1 solutions