Study Of The Hydrophobic Recovery Mechanism Of Polymer Surface Modified By Plasma Technique

In recent years, polymer materials have been widely applied as insulators in electric power industry due to their unique hydrophobicity and hydrophobic recovery. However, polymers would suffer ageing effect and finally fail as insulator, when they are exposed in UV irradiation, corona discharge, plasma modification. Although the ageing effect of polymer insulating materials has been studied in many literatures, the conclusions are contradictory in some extent, due to the different ways for ageing and characterization. Especially, most studies only focused on the final structure of polymer insulator after ageing effect, and the reported results about the surface hydrophobic recovery of the polymer during early ageing process cannot be explained by public accepted mechanism.Therefore, the low temperature plasma is employed to simulate electrical ageing process. The polymer insulating materials, such as silicone rubber, high density polyethylene(HDPE), ethylene-vinyl acetate copolymer(EVA) are treated with plasma techniques. The influence of plasma ageing effect on the polymer surface was studied by contact angle measurement, microscopic infrared, scanning electron microscopy(SEM) and positron annihilation technique.1. When silicon rubber samples were treated by plasma treatment in different atmospheres, respectively, the surface would change from hydrophobic state to the hydrophilic state for all the samples. The correlation between the ageing effect and plasma modification was studied by varying the gas pressure, power and processing time. Microscopic infrared results revealed that there was no obvious change in the bulk. And SEM figures showed the samples modified in different atmospheres exhibit different surface morphology. Moreover, argon plasma treatment provided a faster hydrophobic recovery surface, while oxygen plasma treatment and nitrogen treatment provided a relatively slower hydrophobic recovery surface.2. RF plasma can improve the surface hydrophilicity, and the surface would recover back to the hydrophobic state in several days. The variation of water contact angle via storage time during the hydrophobic recovery process could be well fitted by a two-parameter decay model, which implies that the hydrophobic recovery of material surface may be caused by the diffusion of two different hydrophilic groups. The depth profile of polar groups in HDPE was characterized by positron annihilation doppler broadening technique coupled with a positron beam. The result showed that only the region that in the range of 700 nm depth from the surface was affected by plasma.3. DC glow discharge has been used to treat the EVA samples. The surface hydrophilicity dramatically improved by the plasma treatment, and the surface recover to the hydrophobic state in several hours. The variation of water contact angle via storage time during the hydrophobic recovery process could be well fitted by a two-parameter decay model.