Evolution of Visco-elastic Properties and Crosslinking Density of Radiation-cured Coatings with Conversion

Within a constantly and rapidly developing technology, radiation curing systems involving ultra-violet (UV) or electron beam (EB) curing are finding increasing applications in the fields of printing inks and coatings. The main advantages of these 100% solid systems are energy savings, no or very low volatile organic compound (VOC) production, very short cure cycles, no heating required and one pack system. Practically speaking, a liquid radiation-curable formulation comprises an acrylated oligomer bringing the properties (mechanical, chemical) in the final cured material, an acrylated dilution monomer which helps to regulate the viscosity during application and a photo-initiating system which will initiate the radicals chain. Optimizing the properties of radiation-cured networks requires understanding the structure of said materials, especially in terms of the average molar mass between junctions as well as of the heterogeneity of said network since the latters are expected to predominantly influence mechanical as well as chemical resistance properties. Such correlations are an ongoing topic of investigation in the literature. Among the various oligomers currently used (epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates ...), urethane acrylates combine interesting properties such as high chemical resistance, adhesion properties as well as flexibility, hardness and toughness. Efficient photo-initiating (P1) systems usually contain a photo-cleavage initiator associated with a photo-abstraction intiator (typically benzophenone) combined to its amine synergist photoactivator (usually a tertiary copolymerizable amine). The former will give satisfactory degree of cure in depth while the latters will cure the surface of the coating in spite of the oxygen inhibition. The present paper will discuss the characterization of UV- and EB-cured urethane acrylate formulations, the aim being to gather valuable information at a practical level about the correlations between conversion, glass transition temperature (Tg, which accounts for segmental mobility of polymer chains) and crosslinking density.