(a) a core/shell polymer wherein (i) the core comprises 45 to 99.9, parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer, and from 0.1 to 5 parts by weight of units derived from at least one crosslinker, and (ii) the shell comprises 80 to 99 parts by weight of units derived from at least one C1-C8 alkyl methacrylate monomer, and 1 to 20 by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl methacrylate monomer and wherein the shell has a Tg above 20° C.

(b) at least 1% by weight of an acrylic polymer that contains 1-15%, by weight of the acrylic polymer, of one or more monomers selected from the group consisting of (mono, di, tri alkoxysilyl) alkyl (meth)acrylate, methylol acrylamide, methylol methacrylamide, methyl acrylamidoglycolate methyl ether and acrylamidoglycolic acid.