Improved silicone rubbers for the use as housing material in composite insulators

The paper gives a comparison of the tracking and erosion properties and the hydrophobicity behaviour between different kind of silicone rubbers: Unfilled gum, HCR silicone rubber filled with silica, silicone rubber containing silica and ATH and liquid silicone rubber containing silica filler only. The Inclined Plane Test was used to determine the tracking and erosion performance. It is shown on an example that the principle set-up of the Inclined Plane Test can be used to evaluate the hydrophobicity as well. It is found that the performance of gum in the Inclined Plane Test is very poor. The addition of silica with filler loadings around 20 - 25 wt% improves this behavior, but failures still occur at 6 kV in the Inclined Plane Test. Commercially available liquid silicone rubber shows a better performance than high consistency rubber, in the case that both systems contain only silica fillers of similar loadings. By the additional addition of high volumes of surface modified ATH, the Inclined Plane Test can be passed reliably at 6 kV. Special focus is put in the proper statistical evaluation of the results from Inclined Plane testing. In order to obtain statistically significant data for a precise comparison of the results, the sample size for the Inclined Plane Test should be at least 20 specimens. It was further found that the type and size of ATH particles have a substantial influence on the results. It was shown that a voltage of 6 kV in the Inclined Plane Test is better suitable for the identification of weak compositions of silicone rubber than a voltage of 4.5 kV. In order for high filler loadings not to be detrimental to the hydrophobicity, the use of surface modified particles is critically important.