The invention relates to a copolymer containing: i) 80-90% by weight of one or more structural units of the formula A, where R.sup.1 is hydrogen or methyl, R.sup.2 is C.sub.2-C.sub.10-alkylene, and Me.sup.+ is an ammonium or an alkali metal ion; ii) from 1 to 10% by weight of one or more structural units of the formula B where R.sup.3 and R.sup.4 are each independently hydrogen, methyl or ethyl; iii) from 1 to 10% by weight of one or more structural units of the formula C, where n is a number from 1 to 6; iv) from 0.1 to 5% by weight of structural units of the formula D where X is hydrogen, an ammonium ion or an alkali metal ion; and optionally, v) from 0 to 10% by weight of one or more structural units of the formula E, where R.sup.5 is hydrogen, methyl and/or ethyl, with the proviso that the content in the copolymers of structural units B) and C) is from 6 to 15% by weight. ##STR00001## ##STR00002## ##STR00003## ##STR00004## ##STR00005##