east 1 when R is hydrogen and the sum of y and z being at least 1 when R is other than hydrogen, and ##STR26## wherein x has a value of from 1 to 6, n has a value of from 2 to 6, Z is an aliphatic hydrocarbon of the formula: Cy H2y + 2-N, in which y has a value of from 2 to 18 when n is 2 and a value of 3 to 6 when n is greater than 2, the value of y in all cases being equal to or greater than t