UV curable coatings are one of the fastest growing sectors in the coatings industry. In recent years, UV technology has made inroads into a number of market segments like fiber optics, optical- and pressure-sensitive adhesives, automotive applications like UV cured topcoats, and UV curable powder coatings. The driving force of this development is mostly the quest for an increase in productivity of the coating and curing process. In automotive refinish applications where minor repairs need to be performed swiftly and at ambient temperature, UV technology promises to significantly increase the throughput of cars in a body shop. The development of refinish applications breaks new ground in UV technology. Safety concerns associated with the use of UV lamps in body shops as well as economic constraints will likely preclude the use of high intensity light sources. Relatively inexpensive low intensity lamps that emit only in the UV-A region of the electromagnetic spectrum are taking their place, thus posing new challenges to resin developers and formulators.
UV curable coating compositions are known in the art. See, e.g., U.S. Pat. Nos. 4,188,472; 4,340,497; 6,465,539; and 6,534,128; and published PCT applications WO 97/04881 and WO 03/027162. In addition, U.S. Pat. No. 5,777,024 is directed to the reaction product of a low viscosity isocyanurate group-containing polyisocyanate, which has been modified with allophanate groups to reduce its viscosity, with hydroxyl-functional olefinic compounds such as hydroxyalkyl (meth)acrylates and lactone modified versions of these (meth)acrylates. U.S. Pat. No. 6,306,504 is directed to a coating composition for polycarbonate which contains the reaction product of a low viscosity polyisocyanate, such as a low viscosity isocyanurate group-containing polyisocyanate, with a hydroxy acrylate.
Also known are compositions that are curable using UV radiation having a very low UV-B content and substantially no UV-C content (see, e.g., U.S. Patent Application Publication 2003/0059555 and U.S. Pat. No. 6,538,044). In order to be commercially useful as a clear coat, particularly for the refinish market, a coating composition must be curable in a relatively short time, preferably under relatively low intensity radiation. In addition, such coatings must exhibit a balance between hardness (as measured by pendulum hardness) and flexibility (as measured by impact resistance). None of the coating compositions described in the above literature meet these criteria.