Copolymer (b) is a copolymer of one or more methacrylic acid esters of formula [A] and one or more nitrogen-containing monomers of the formula ##STR4## where R2 and R4 are a hydrogen atom or a methyl group, R3 is an alkylene group of 2-18 carbon atoms, n is an integer of 0 or 1, and X is amine moieties or heterocyclic moieties containing 1-2 nitrogen atoms and 0-2 oxygen atoms.