Impact of different fillers and filler treatments on the erosion suppression mechanism of silicone rubber for use as outdoor insulation material

The paper presents the influence of various fillers having different sizes (0.3 μm to 18 μm) and surface modifications (unmodified, modified by the material supplier and in-situ modified during compounding) to the erosion resistance of HTV Silicone Rubber (SR) composites. The particles used were Aluminium-Trihydrate (ATH), Alumina (Al₂O₃) and Silica (SiO₂), while the main research focus was on ATH fillers as it has the ability to release water at elevated temperature. First, by a simple water storage test under defined conditions the water uptake for different composites was analyzed to prove the effectiveness of an in-situ modification compared with the modification by the particle supplier. The Inclined Plane Test (according IEC 60587, further named IPT) and the high voltage, low current dry arc test (according IEC 61621, further named arc test) were used to determine the erosion resistance of the different samples. For the IPT, an adapted evaluation model was applied: This test is known to have a wide scatter in the case of material formulations, which are on the borderline to pass or to fail the test and therefore show substantial erosions on certain samples. The scatter could be reduced by evaluating the eroded volume by using samples only, which showed a limited erosion length. It was found for ATH, that larger particles show slightly better results than smaller particles. This can be explained by the formation of boehmite [AlO(OH)] for the larger particles, which causes a release of the bound water over a wider temperature range. This effect could be confirmed by thermo gravimetric analysis (TGA). The surface modification of the particles with Vinyltrimethoxysilane (VTMS) and Methyltrimethoxsilane (MTMS) did not improve the erosion resistance significantly, but reduces the water-uptake to a large extent, which is advantageous for the retention of the hydrophobicity. In order to achieve a low erosion rate, high- filler loadings are essential.