Epoxy resin-based curable compositions

An epoxy resin-based curable composition which comprises:

• (a) 100 parts by weight of a curable epoxy resin blend composed of an epoxy resin and a crosslinking or curing agent therefor in such a proportion as to effect curing of the epoxy resin;
• (b) from 5 to 100 parts by weight of a block copolymer having at least one hydroxyl group or epoxy group reactive with the component (a) and composed of:
• (b-1) at least one segment of a moiety of a substituted or unsubstituted epoxy novolac resin or a phenol novolac resin containing aromatic groups bonded through divalent linking units expressed by the formula --(--CR.sub.2 --).sub.5 --, in which R is a hydrogen atom or a monovalent hydrocarbon group having 1 to 5 carbon atoms and wherein t is a positive integer from 1 to 6;
• and
• (b-2) at least one segment of an organopolysiloxane moiety having 37 to 200 silicon atoms in the polysiloxane linkage and expressed by the average formula R.sup.1.sub.a SiO.sub.(4-a)/2, in which R.sup.1 is a monovalent organic group, at least 70% in number of the groups R.sup.1 being alkyl groups and a part of the groups R.sup.1 optionally being hydrogen atoms, and a is a positive number in the range from 1.8 to 2.7, said segments (b-1) and (b-2) being bonded together through a silicon-to-carbon linkage;
• and
• (c) an inorganic filler in an amount of 100 to 500 % by weight based on the total amount of the components (a) and (b).