The epoxy resin used is a commercially available epoxy resin such as those obtained by curing at ordinary temperature or by heating of the reaction product of bisphenol A and epichlorohydrin together with a curing agent such as amines, polyamines, amides, amide-amines, amine adducts, acid anhydrides (such as phthalic anhydride, maleic anhydride and trimellitic acid anhydride) etc.