Abstract
Hydrophobicity of polymeric insulating material surface such as silicone rubber (SIR) and ethylene propylene diene monomer rubber (EPDM) insulator was studied by using image data analysis of the sample surface. Using a very high-speed microscope CCD camcorder, we digitized the hydrophobic surface image under the spraying of the mist of water, Even the size of the ob-
served image area is only about 2 mm2, the effect of aging in water and its recovery in air on the hydrophobicity of specimens were measured by their surface images of water droplets on the sample surface.Using this microscopic image, we can determine the hydrophobicity class (STRI’s HC level) without the effect of gravity or inclination of surface to the formation of water droplets on the surface. Image data analysis can calculate the image indexes such as size and shape
factor of the droplets. They are also helpful to define the hydrophobicity class more accurately.
1. Introduction
Polymeric insulating materials such as EPDM and SIR are widely used in the manufacture of weather-sheds of outdoor insulators [1,2]. These materials have many advantages including an inherent property of a low surface free energy that renders a hydrophobic quality to the polymer surface,The hydrophobicity is reduced by water absorption during aging with increasing environmental temperature and contamination buildup [3]. However, it recovers during the rest time in air at room temperature, A part of recovery of hydrophobicity is related to the evaporation process of the water in the specimen. In addition,specimens show excellent recovery of hydrophobicity if they have a low molecular weight (LMW) material in
the matrix and it migrate to the sample surface during recovery, where LMW has a low surface free energy, Hydrophobicity is usually measured by a contact angle of a droplet of distilled water on the surface. However it sometimes shows hysteresis, time dependence and location dependence of the measured point. Therefore, hydrophobic image of the surface is used to determine the STR1`s hydrophobicity class (HC level l to 7) [4]. A digital image processing method for estimating the
level of hydrophobicity of specimens are reported in [5].The experiment showed that the water droplet patterns
depend not only on the hydrophobicity of the sample but also on the inclination of the surface. When HC is
smaller, therefore sample is very hydrophobic; the effect of inclination is larger. It means the gravity force cannot ignore compared to adhesion force when the size of the droplet is large.
This paper studies the image data analysis of hydrophobicity by observing at microscopic areas of the specimen, where the effect of gravity should be smaller compared to the effect of surface free energy. Image dataindexes such as size-distribution of the very small droplets and shape of the droplets are discussed for the HC indexes. Not only distilled water but also solutions having lower surface free energies are used to evaluate the hydrophobicity class of the specimen surface.
2. Experimental procedure
Specimen tablets of SIR and EPDM insulator weather shed of 46 kV class outdoor insulation were employed.12 specimens were cut out from each shed which having a diameter of l6O mmw and a thickness of 4 mm (outer) to 9 mm (inner). Each specimen was subjected to a water stress by immersing it in distilled water in a glass container at room temperature. The specimens
were periodically removed from the water and the contact angle and weights of the specimens were measured to evaluate the amount of water absorption and solution of LMW fluid from the specimen, The conductivity of distilled water is less than 3 l\/S/cm. The change in the surface free energy of the immersion water of specimens during aging was also measured to evaluate the solution of LMW in SIR.
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