Characteristics of hydrophobicity loss on silicone rubber surface during a dynamic drop test with direct current voltage application
© Seo et al; licensee Springer. 2012
Received: 10 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Dynamic drop test for studying the temporal lowering of hydrophobicity on the surface of silicone rubber with direct current voltage application was carried out. In this study, we evaluated the influence of the temporal lowering of hydrophobicity under various conductivities and dropping rates for water droplets. As a result, it was found that the dropping rate and the conductivity of water droplets greatly influenced the hydrophobicity loss time on the surface of silicone rubber.
The development of insulating materials used in electric-powered apparatuses plays an essential role in a stable power supply. Polymer materials, for example, have attracted attention in recent years. The use of polymer materials for housings of insulators and arresters has increased. The widely accepted polymer materials for housings are silicone rubber [SiR] and ethylene vinyl acetate. These polymer materials have some excellent properties such as being lightweight, hydrophobicity, and antiweatherability . Additionally, even if the hydrophobicity initially disappears by external stresses, the hydrophobicity recovers because of the migration of low molecular weight SiR in a short time [2–5].
Incidentally, electric-powered apparatuses are utilized not only by alternating current [AC] voltage applications, but also by direct current [DC] voltage applications. The polymer material is made from organic matters; therefore, the aged deterioration due to electric discharges and acidic products is worrying. Studies on evaluation methods of insulation deterioration of polymer materials are continuously made by the International Council on Large Electric Systems [CIGRE]. A dynamic drop test [DDT] which can easily evaluate the characteristics of temporal lowering of hydrophobicity on the surface of the polymer material was proposed through a continuous research effort . Here, it should be noted that the AC voltage application is a normal test condition for the DDT. The evaluation on the surface with the DC voltage application has almost not been reported.
In this study, we carried out DDT for evaluating the temporal lowering of hydrophobicity on the surface of silicone rubber with DC voltage application. Our results have contributed to the evaluation of polymer materials' reliability with DC voltage application.
Test condition of DDT
Test voltage [kV]
Deionized or distilled water with NaCl
0.1, 1, 4, 8, 12, and 16
Temperature of test liquid [°C]
23 ± 3
Dropping rate [drops/min]
12, 24, 36, 48, and 60
We evaluated the influence of the temporal lowering of hydrophobicity under various conductivities and dropping rates. The changes of hydrophobicity and discharges generated at the surface of the test sample were observed by a CCD camera and shown in Figure 1. The hydrophobicity loss time, when the leakage current of above 2 mA continuously flowed for 4 s from the voltage application, was defined as the same as that proposed by CIGRE.
Results and discussions
The increase of dropping rate after impressed voltage application
Dropping rate before impressed voltage application
Dropping rate after impressed voltage application (drops/min)
Thus, due to the influence of the high electric field, the increase of dropping rate was confirmed after impressed voltage was applied. Such increase of droplets is remarkable with the increase of the initial dropping rate of electrolyte. The increase of droplets promoted the lowering of hydrophobicity, and the hydrophobicity loss time became shorter. The surface of the test sample has a greater opportunity to attach NaCl and keeps electrification on the surface of SiR with the increase of the dropping rate.
We carried out DDT in investigating the temporal lowering phenomena of hydrophobicity of the SiR surface with DC voltage application. Here, the influence of the temporal lowering of hydrophobicity under various conductivities and dropping rates was evaluated. With the progress of the lowering of hydrophobicity, small discharges on the surface of SiR could be seen. Additionally, in the final stage, the hydrophobicity lowered, and an obvious water channel was confirmed. Such hydrophobicity loss was influenced by the conductivity and dropping rate of the electrolyte.
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