The invention relates to one or several copolymers A), wherein the copolymers contain one or several structural units, derived from a) 19.9 to 75.9 weight percent glycerin b) 0.1 to 30 weight percent of at least one dicarboxylic acid, and c) 24 to 80 weight percent of at least one monocarboxylic acid of formula I (I) R