Loss of hydrophobicity of high density polyethylene

This study reports on the loss of hydrophobicity of high density polyethylene (HDPE) after immersion in a saline solution. The conductivity of the solution was varied from 5 to 10⁵ μS/cm and its temperature from 23 to 73°C. The contact angle of a droplet of water on the polymer surface which gives a measure of the hydrophobicity was determined. The influence of applying either a radio frequency (RF) discharges or a high voltage dc stress on the loss and the recovery of the hydrophobicity was investigated. It is found that the contact angle decreased with increasing salinity level and increasing temperature of the solution. The time which it took for the surface energy and therefore, the hydrophobicity to drop depended on the combined stresses of salinity and heat. The contact angle decreased substantially after subjecting the surface to RF discharges and subsequently recovered very slowly after the removal of the stress. The increase in the weight and the surface roughness of the specimens due to absorption of the saline solution were also measured and correlated to the changes in the contact angle and thus to the hydrophobic property of the surface