The crosslinking agent has the structural formula OR1O PARALLEL PARALLEL CH2=C-C-O-R-O-C-C=CH2 CH3R2CH3 in which R is a 5 to 14 carbon atoms radical selected from the group consisting of alkyl, cycloalkyl and aralkyl; R1 and R2 are independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, aralkyl and aryl radicals and mixtures thereof; and the total number of carbon atoms in