Residual stress in radiation-cured acrylate coatings

Radiation-cured acrylate-based materials are used in a wealth of applications. For coatings in particular, adhesion is a key property and remains challenging for some substrates such as plastics and metals. Stress build-up during and after resin polymerization is a handicapping issue that may lead to defect formation or debonding which results in bad coating performances. Using the substrate deflection method, stress development has been investigated in ultraviolet(UV)-cured acrylate coatings based on an epoxy acrylate combined with mono-, di- and triacrylate diluents. The coatings were prepared in conditions relevant to industrial practice using high UV intensity and short processing times. Stress measurements were conducted according to the prescriptions of a recent ASTM standard. A new set of data is provided which shows that photo-cured epoxy acrylates develop low stress as compared to other dilution acrylates (monomers). When formulated with most of the reactive diluents, the stress does not exceed 4 MPa as long as the dilution factor stays below 50%. This is fairly low given the high conversion rates. For vitrified coatings, it appears that in addition to polymerization stress, thermally-induced stress represents a non-negligible contribution to the overall stress. In an industrial UV-curing process however, the latter contribution is estimated to be less than 2-3 MPa. Lastly, an estimate of the interfacial fracture toughness between trimethylolpropane triacrylate and carbon steel is inferred from the stress data.