Improved cycloaliphatic epoxy systems for high voltage applications

In recent years it has become increasingly evident that cyclo-aliphatic epoxy resins offer unique advantages as a high voltage insulating material. Generally, these systems contain in addition to the epoxy resin, hardeners, modifiers, catalysts, and fillers. It is essential that all these formulating variables be examined carefully in order to optimize physical and electrical properties. Previously available cycloaliphatic epoxy resins generally produced rigid, castings with marginal toughness. Typical modifiers could improve cast toughness, but at some sacrifice in other properties. A new resin system is discussed that improves toughness while minimizing loss in other properties, particularly weathering and electrical properties. Since SO to 65 weight percent of these castings is filler, this portion of the system deserves attention also. Both silica and hydrated alumina have been recommended for outdoor high voltage applications. Silica offers possible advantages for cast physical properties, however inclined plane arc-track data clearly indicate that little, if any, can be tolerated in a cast formulation for high voltage applications. Even with cycloaliphatic epoxies, silica filler can cause carbon formation under an electrical arc. Thus to prevent this, hydrated alumina must be used. Laboratory studies using hydrated alumina filled systems have shown that both physical and electrical properties can be upgraded. Specifically, sodium, in the form of its various salts can significantly increase electrical losses. The data shows correlations between both the total and soluble sodium salt content of hydrated alumina and electrical properties. Data is also presented that shows that absorbed moisture on hydrated alumina degrades electrical properties. Finally, coupling agents, which can greatly enhance physical and electrical properties are discussed. The data show that a threefold increase in toughness is possible through coupling agent addition to a hydrated alumina filled cycloaliphatic system. Further, significant increases in inclined plane arc-track erosion resistance are possible through coupling agent use. Two techniques for incorporating these coupling agents, integral blends and filler pre-treatment are detailed and compared.