Curable composition for hard rubber and the use thereof

A curable composition comprising (1) at least one rubber selected from the group consisting of natural rubber, a diene rubber and a diene copolymer rubber, (2) 0.1 to 10 parts by weight per 100 parts by weight of the rubber of a sulfide resin as a curing agent having the following general formula:

• ##STR8## wherein X is methylene group, ethylene group, --S-- or --S--S--, R.sup.1 and R.sup.2 are independently a C.sub.1-10 alkyl or alkylene group, Y.sup.1 and Y.sup.2 are independently the residue of a phenol compound, p is an integer of 2 to 8, and n is an integer of 2 to 5, (3) 5 to 40 parts by weight per 100 parts by weight of the rubber of a phenolic thermosetting resin and (4) 0.5 to 6 parts by weight per 100 parts by weight of the rubber of a hardener for hardening said thermosetting resin, said hardener being a combination of an oxazolidine derivative of the formula:
• ##STR9## with another hardener suitable for hardening said thermosetting resin wherein said oxazolidine derivative is present in a ratio of oxazolidine derivative to said another hardener of at least 1:2.