The epoxy resin may be a polyglycidyl ether of a polyhydric alcohol such as 1,4-butanediol or 1,3- propanediol or, preferably, a polyglycidyl ether of a polyhydric phenol, for example a bisphenol such as bis (4-hydroxyphenyl) methane (bisphenol F) or 2, 2-bis- (4- hydroxyphenyl) propane (bisphenol A) or a novolak formed from formaldehyde and a phenol such as phenol itself or