The monitoring of property changes in insulating materials containing silicone binder

Abstract

The construction of electrical devices operating at extremely high temperatures requires insulating systems with appropriate thermal properties. Insulating materials based on mica and silicone binder are usually used for this purpose. For these materials, it is necessary to understand their behavior under operating temperatures, as well as the specific properties which characterize their behavior with respect to temperature, e.g. glass transition temperature Tg and the thermal expansion coefficients in the glassy phase oil and the rubbery phase alpha2. It is a well-known fact that silicone binders have a glass transition temperature Tgin the negative range of values (below 0degC). This causes a problem when knowledge of the behavior of the insulating material in the positive range (above 0 degC) of values is required, since it is not possible to take the temperature Tg into account. This paper deals with examining the behavior of materials containing silicone binders at positive temperatures, and with searching for an alternate parameter, which would (thanks to its predicative ability) partly supply the above-mentioned glass transition temperature Tg. A material specifically designed for operating at high thermal stresses has been chosen for experimental purposes. The composition of the material is as follows: noncalcinated mica (91%) and thermally resistant silicone binder (9%). The maximum operating temperature given by the manufacturer is 500 degC. This material was first treated in an ageing oven at a temperature of 320 degC for 500 hours. After this thermal exposure, a voltage exposure followed. The intensity of the exposure has been selected based on our experience in the range from 3 kV for 80 hours to 5 kV for 14 hours. Afterwards, the analysis of the degradation effects at positive temperatures was realized by thermomechanical (TMA) analysis