Thermal and photochemical curing of isocyanate and acrylate functionalized oligomers

Dual-cure formulations have been developed in order to broaden the range of applications of UV-radiation curing and achieve a sufficient cure of the non-illuminated areas of protective coatings, like the shadow areas of three-dimensional substrates or the deep-lying layers of thick pigmented coatings. In complement to the conventional acrylic photopolymerization, such systems involve a polyaddition reaction between isocyanate and hydroxyl functions upon heating, thus imparting satisfying mechanical properties to the coatings, even in the non-irradiated areas. For a quantitative study of the two curing reactions, infrared spectroscopy was chosen because it allows one to monitor accurately the chemical modifications induced in thin films by heat or light. This technique proved particularly useful to follow the polyaddition reaction through the decay of the characteristic IR band of the isocyanate group at 2271 cm−1. Several factors were shown to affect the reaction rate, in particular the chemical structure of the various functionalized compounds, the moisture content in the atmosphere, the curing temperature and the presence of amino catalysts. The viscoelastic properties of the resulting material have been evaluated by dynamic mechanical analysis and shown to be highly dependent on the curing conditions and the type of polymer network formed. For coatings which remain too soft in the dark areas, a thermal initiator was added to promote some further polymerization of the acrylate monomers.