Improved crack resistant liquid epoxy resins

Recent trends in the electrical industry indicate a growing demand for liquid epoxy castings that combine a high usage temperature (i.e. high glass transition temperature of the cured resin) with good crack resistance.It is well-known that the use of liquid instead of solid epoxy resin results in an increase in the glass transition temperature, but this is usually accompanied by a reduction in crack resistance. Frequently, such liquid epoxy resin based castings have to be flexibilized to improve crack resistance, but then the glass transition temperature is inherently lowered.The present paper describes some approaches to a combination of a high glass transition temperature with excellent crack resistance. A study of a number of criteria showed that this can be achieved by careful control of the crosslink density.The proper ratio of epoxy resin to curing agent, in combination with a suitable catalyst ensures castings with a glass transition temperature ranging from 95 to 125°C, and which pass standard crack tests even at temperatures below -80°C and temperature cycling from +150°C to -75°C.