. . . . . . "wherein R5 is hydrogen or linear or branched C1-C20 alkyl or alkenyl group, preferably methyl, ethyl, propyl, butyl, pentyl or hexyl, in particular methyl or ethyl, examples of suitable compounds according to formula (VI) being formic acid (HCOOH) and acetic acid (CH3COOH), polycarboxylic acids, halogenated carboxylic acids, and other derivatives of carboxylic acids." .