Curable composition

A curable composition comprising (A) at least one organic polymer selected from the group consisting of a polyether, a polyester, an ether-ester block copolymer, a polybutadiene, a styrene-butadiene copolymer, an acrylonitrile-butadiene copolymer, an acrylic acid ester-butadiene copolymer, an ethylene-butadiene copolymer, a vinylpyridine-butadiene copolymer, an ethylene-propylene copolymer, an ethylene-vinylacetate copolymer, an ethylene-acrylic acid ester copolymer, a polyisoprene, a styrene-isobutylene copolymer, a styrene-isoprene copolymer, an isobutylene-isoprene copolymer, a polychloroprene, a styrene-chloroprene copolymer, an acrylonitrile-chloroprene copolymer, a polyisobutylene, a polyacrylic acid ester and polymethacrylic acid ester, said organic polymer containing at least one reactive silicon functional group crosslinkable to produce an elastomer by the formation of a siloxane bond in its molecule wherein said reactive silicon functional group is a group represented by the formula (2):

• ##STR16## wherein R.sup.3 is a monovalent hydrocarbon group having 1 to 20 carbon atoms selected from the group consisting of an alkyl group, as aryl group and an aralkyl group or a triorganosiloxy group represented by the formula (3):
• (R.sup.4).sub.3 SiO-- (3)
• in which each R.sup.4 is a nonvalent hydrocarbon group having 1 to 20 carbon atoms, and when more than one R.sup.3 are present, the groups R.sup.3 are the same or different;
• X is a hydroxyl group or a hydrolyzable group, and when more than one X are present, the group X are the same or different;
• a is O or an integer of 1 to 3 and b is 0, 1 or 2, provided that at least one X is present in the silicon functional group;
• and;
• m is 0 or an integer of 1 to 18;
• and (B) an organic silicon compound having a molecular weight of not less than 140 and represented by the formula (1):
• [CH.sub.3).sub.3 SiO].sub.n R.sup.1 ( 1)
• wherein R.sup.1 has a valence of n and is a residue of a substituted or unsubstituted aliphatic alcohol having 2 to 30 carbon atoms, a residue of a substituted or substituted or aromatic hydroxyl compound having 6 to 30 carbon atoms, a residue of a substituted or unsubstituted aliphatic or aromatic carboxylic acid having 2 to 30 carbon atoms, a residue of a polymeric compound having hydroxyl or carboxyl groups or a residue of an inorganic acid, and n is 1, 2 or 3.