Preferred ester starting materials are those esters in which R is a linear or branched chain alkyl group containing from about 6 to 30 carbon atoms, more particularly from 6 to about 20 carbon atoms, and R' is a linear or branched chain alkyl group containing from 1 to 20 carbon atoms, especially from 1 to 10 carbon atoms, particularly preferred being such esters wherein R' is methyl.