X stands fora. ##STR37## wherein R4 is a C8 -C12 alkyl group and R5 and R6 are C1 -C3 alkyl groups, the total number of alkyl carbon atoms in R4, R5 and R6 being less than 15; b. halogen; c. an alkoxyl radical selected from the group consisting of a residue of a saturated or unsaturated straight chain or branched aliphatic alcohol, a saturated or unsaturated cycloaliphatic alcohol and an aromatic