A method for rubber toughening powder coatings

Abstract

This paper describes a new method for rubber toughening brittle powder coatings. The design involves the use of toughening agents with specific physical and chemical properties. The modifiers are low-to-medium molecular weight polymers with polymerizable end groups (macromonomers). To achieve the physical properties required for fabrication into fine powders for subsequent deposition using conventional powder coating equipment, semi-crystalline polymers with T_g's well below room temperature and melting points above ≈70°C were required. Upon copolymerization with a thermosetting resin, crystallization of the modifier was precluded provided that low ethylene oxide compositions were employed. This scheme, in principle, yields an amorphous low T_g modifier chemically bound to the cured network. Poly(ethylene oxide), PEO, was found to be an excellent candidate, since it has a T_g of -68°C and a T_m of 70°C, sufficient crystallinity for friability and functional end groups for copolymerization. Free radical polymerization of 2,2-bis[4-vinylbenzoyloxyphenyl] hexafluoropropane and step-growth polymerization of 4,4′-(hexafluoroisopropylidene)diphenyl cyanate were utilized to demonstrate the feasibility of this approach. An electrostatic powder coater was used to co-deposit the PEO macromonomers with either of the thermosetting precursor monomers, followed by thermal curing to produce the modified networks. The resulting networks showed multiphase morphologies with improved toughness.