Elastomeric electron beam-cured coatings: Structure–property relationships. I. Oligomer structure

Abstract

Mechanical properties of electron beam-cured films were studied as a function of oligomer structure and monomer diluent. In films cast from polyester acrylourethane oligomers, increasing the chain length between the two acrylate groups on each end resulted in a decrease in breaking strength from 7000 psi to 1000 psi in Young's modulus from 200,000 psi to 700 psi, and in glass transition temperature from 50 to −25°C, while the ultimate elongation increased from 20% to 210%. These properties were virtually independent of dose above 1 Mrad. The elongations were compared with literature values at equivalent degrees of crosslinking and found to fall within the range, but on the low elongation side of the distribution. It was speculated that the crosslinks' functionality would equal the average degree of polymerization of the double bond at each end of the oligomer, resulting in multirayed, star-shaped crosslinks with less flexibility than those with the usual functionality of 3 or 4, and methods for reducing this functionality were investigated. Addition of active monomers effected only minor change, but diethylaminoethyl acrylate was found to be very helpful in increasing the extensibility of these films considerably. This behavior can be explained by assuming that this monomer has a significant chain transfer constant, which should reduce the degree of polymerization of acrylic endgroups, producing a looser structure.