UV-curable polyurethane acrylate coatings with different acrylate monomers as reactive diluents

Two series of UV-curable polyurethane acrylate (PUA) based on polycaprolactone (PCL), tetraxylylene diisocyanate (TMXDI), and hydroxyethyl acrylate (HEA) or hydroxyethyl methacrylate (HEMA) were studied. These ones were considered with different acrylates as reactive diluents. The effect of the chemical nature and functionality of the reactive diluents on the thermal and dynamic mechanical properties (DMS) was investigated. From a thermodynamic point of view, the PUA seem to display a one phase structure by DMS. Nevertheless, the statistic heterogeneities due to the use of three monomers or more can explain the Tg values and the mechanical relaxations of the PUA. The Tg-onset of the PUA is slightly influenced by the nature of the reactive diluents but is dependent on the Tg of the oligomer confirming the description of the structure using a clusters model. The same conclusions could be done from the dynamic mechanical spectra of the PUA sandwiched and UV-cured between two glass plates.