Photocationic-curable powder coatings

The UV-curable powder coatings allow the finishing of temperature-sensitive substrates such as wood, MDF (medium-density fibreboard), plastics and paper. The binder system is mostly comprised of unsaturated polyester, a vinyl ether and acrylic polyurethane. The curing mechanism consists of free radical polymerisation of the double bonds. In comparison with free radical UV curing, the cationic-initiated UV-curing technology offers the advantages of insensitivity to oxygen and lower film shrinkage. Epoxides are one of the most important and widely used classes of resins in the field of cationic UV-curable powder coatings. Epoxy systems do not require any additional curing agent, they cure within seconds and have unlimited working times. For photocationic curable powder coatings, solid epoxy resins have been used. However, such a system has some disadvantages: the epoxy value is too low to obtain sufficient cross-link density. This problem may be overcome by the use of a binder having more epoxide rings in the oligomer structure (eg epoxypolyesters). In the present study, a series of solid epoxypolyesters were prepared by polycondensation of glycols, tetrahydrophthalic anhydride and dicyclopentadiene, followed by the epoxidation of the double bonds in unsaturated polyester. The epoxypolyesters were subject to pho-toinitiated cationic polymerisation. The evaluation of the film performance was also carried out. The coatings prepared from these epoxypolyesters exhibited good hardness, impact strength and adhesion.