It is preferred to use epoxy resins containing on average more than one group of formula X ##STR13## wherein each of Q and Q is a hydrogen atom and Q' is a hydrogen atom or a methyl group or Q and Q together are --CH2 CH2 -- or --CH2 CH2 CH2 -- and Q' is a hydrogen atom, which group of formula II is attached to a hetero atom, e.g. a sulfur atom and, preferably, to an oxygen or nitrogen atom.