Optionally, there may also be added a solution agent and/or a cross-linking agent of the formula: Me(OAlk)2-4 R'0-2 ( 5) Al(OAlk)2-3 R10-1 ( 6)wherein Me=Si or Ti, Alk has the same meaning mentioned above with regard to the formula (4), and R' is a methyl or ethyl group.