Patent ID: 7696304

Claim:
A copolymer which is reversibly crosslinkable with polyvalent metal ions at temperatures of above 150° C. and comprises: i) 80-90% by weight of one or more structural units of the formula A where R 1 is hydrogen or methyl, R 2 is C 2 -C 10 -alkylene, and Me + 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 3 and R 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 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.