a) Ac is R3 --CO and R3 is a linear or branched alkyl group containing from 6 to 20, preferably 6 to 12, more preferably 7 to 9 carbon atoms and wherein the longest linear alkyl chain extending from and including the carbonyl carbon contains from 5 to 18, preferably 5 to 10 carbon atoms, R3 optionally being substituted (preferably alpha to the carbonyl moiety) by Cl, Br, OCH3 or OC2 H5.