99_fractal - I EEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 6 No. 6, December 1999 781 Fractal Phenomena Dependence of Hydrophobicity on Surface Appearance and Structural Features of SIR Insulators X. Wang and N. Yoshimura Dept of Electrical & Electronic Eng Akita University, Akita, Japan ABSTRACT This paper reports the study of the dependence of hydrophobicity on the surface appearances and inherent chemical structure of the high-temperature vulcanized silicone rubber (HTVSR) insulator based on the fractal geometric principle. The fractal phenomena link the surface roughness with the hydrophobicity characteristics of silicone rubber (SIR) insulators. The rough surfaces of the insulators, no matter whether due to salt-fog aging or roughening by sand- paper, show the property of a fractal surface. According to the amount of hydrophobic material on the surface, surface roughness exerts different effects on the hydrophobicity of SIR insula- tors. Due to the behavior of fractal surfaces, the hydrophobicity of the appropriately roughened insulator can be enhanced, but depressed if dipped in hexane. Outdoor polymer insulating ma- terials could be aged by environmental stresses and contaminants so that their surfaces become rough. The permanent change of surface roughness could be an indicator of aging state of the insulation material. The hydrophobicity of SIR insulators aged in salt fog in a fog chamber are investigated. During this aging, the inorganic filler and the hydrophobic groups in SIR are worn away so that the insulator surface becomes rough. The surface of the aged insulator becomes hydrophilic. After a subsequent rest, the hydrophobic polymer liquid in bulk diffuses and mi- grates to the insulator surface, which becomes more hydrophobic. Such a unique dependency of hydrophobicity on the surface appearance and chemical structure of SIR material is available for SIR insulator to maintain good contamination performances in outdoor applications. 1 INTRODUCTION ILICONE rubber materials have been used widely in outdoor appli- S cation, due to their superior contamination performance compared to other composite insulating materials [l-31. The superior contamina- tion performance of silicone rubber (SIR) insulators partly is due to the contribution of its hydrophobic surface [4,5]. The hydrophobicity of the material plays an important part in the contamination performance of outdoor insulators. The loss and degradation of hydrophobicity of the insulator surface will influence the tracking and discharge phenomena, leading to insulation breakdown or flashover under severe conditions [6,7l. It is known that surface hydrophobicity of materials is related not only to the micro-physical morphology and chemical structure of the material, i.e. the binding energy and surface energy, but also to sur- face appearance such as roughness [5,8,9]. Outdoor polymer insulat- ing materials can be aged by environmental stresses and contaminants so that their surfaces become rough. The permanent change of the sur-
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99_fractal - I EEE Transactions on Dielectrics and...

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