It is for instance possible to treat polymer particles containing epoxy groups with ethylene diamine to form --CH2 --NH--CH2 --CH2 --NH2 groups, or with ethylene oxide group-containing substances such as NH2 --R--(CH2 CH2 O)n R' or HOOCRCOO(CH2 CH2 O)n R', wherein R is a suitable aliphatic or aromatic group, R' is hydrogen or alkyl, and n is an integer from 1 to 500, to introduce --(CH2 --CH2 --O)