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Over the past decade, UV-radiation curing has found an increasing number of industrial applications because of its unique advantages, mainly solvent-free formulations, high cure speed and ambient temperature processing. One of the important fields where UV-curing has been unable so far to compete with the existing technology is in the area of protective coatings used in outdoor applications, such as automotive finishes and wood varnishes. The main reason is that the presence of the unreacted photoinitiator was considered to have a deleterious effect on the weathering resistance of UV-cured polymers. Moreover, the UV-absorber which needs to be introduced in protective coatings to screen efficiently the harmful UV-solar radiation is causing an unwanted decrease of the cure speed, and of the adhesion as well. Earlier work has however shown that some UV-cured acrylate coatings can be remarkably resistant to accelerated weathering 1-6, in particular in the presence of adequate light stabilizers. The adhesion of the coating on organic substrates can be substantially increased by the creation of covalent bonds at the interface through a grafting reaction'. Such UV-cured coatings should therefore prove to be a valid alternative to solvent-based coatings in a number of outdoor applications, in particular as clearcoats to increase the durability of organic materials such as plastics, paper, wood or painted metals.
The objective of the present work was to compare the weathering resistance of UV-cured acrylic
coatings to that of thermosetting acrylate coatings. We have thus been able to show that, by selecting properly the various components of the formulation, it is possiblelo obtain UV-cured polymers which are substantially more resistant to accelerated weathering than the thermoset clearcoats currently used as automotive finishes. Moreover, the longer lifetime of the UV-absorber in photoset coatings is another advantage, for it extends the anti-UV protection and thus increases the service life of the coated material upon outdoor exposure.
