The present invention relates to heat-curable compositions containing a mixture of reactive epoxy resins that contains, based on the mass of all epoxy resins, a) at least 10 wt % of an epoxy group-containing reaction product of epichlorohydrin with polypropylene glycol which has an epoxy equivalent weight of at least 250 g/eq, and b) at least 10 wt % of an epoxy group-containing reaction product of epichlorohydrin with a novolac resin which has an epoxy equivalent weight of at least 175 g/eq, and at least one latent hardener.