the group of copolymers (a) of two or more methacrylic acid esters of the formula ##STR1## where R1 is an alkyl group of 1-18 carbon atoms, and the group of copolymers (b) of one or more methacrylic acid esters of formula [A] and one or more nitrogen-containing monomers of the formula ##STR2## where R2 and R4 are a hydrogen atom or a methyl group, R3 is an alkylene group of 2-18 carbon atoms, n i