Preferably, an N-dialkylamide of the formula: ##STR2## wherein R4 or R5 are the same or different or each being a saturated hydrocarbon group having from 1 to 10 carbon atoms, preferably 1 to 3 carbon atoms, and optionally an oxygen atom or a nitrogen atom in a carbon chain, and R6 is a hydrogen atom or the same hydrocarbon group as above, provided that two of the R4, R5 and R6 may together form a