Industrial coatings are surface protective coatings (paint coatings) applied to substrates and typically heat cured to form continuous films for decorative purposes as well as to protect the substrate. A protective coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives, where the polymeric binder acts as a fluid vehicle for the pigments and imparts rheological properties to the fluid paint coating. Upon curing, the polymeric binder hardens and functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paint coatings contain little or no opacifying pigments and are described as clear coatings. The manufacture of paint coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in the polymeric binder, and thinning to commercial standards.
Epoxy resins are particularly desirable for use in protective surface coating materials as a vehicle or polymeric binder for pigments, fillers, and other additives where the epoxy resins advantageously provide toughness, flexibility, adhesion, and chemical resistance. Water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions and particularly useful for interior surfaces. Comings for soft drink and beer cans, for instance, are critical due to taste sensitivity wherein such can coatings must not alter the product taste of canned beverages. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by adsorption of flavor by the coating, or sometimes by chemical reaction, or by some combination thereof.
Container coating technology frequently utilizes an epoxy resin which has been grafted with acrylic monomers, styrene, and methacrylic acid. This grafted epoxy resin is prepared in solvent, usually butyl cellosolve, and n-butanol, to maintain low processing viscosities and then reduced with water by a direct or inverse let down procedure. Although cured film properties are highly desirable, such coatings suffer from the fact that sizeable amounts of solvent are required to obtain good performance. High molecular weight epoxy resins typically require 50% to 90% solvent (based on total solids plus organic solvent) before reducing with amine and water.
Epoxy based can coatings comprising a carbon grafted acrylic chain produced in the presence of an extender resin are disclosed in U.S. Pat. No. 4,399,241 and U.S. Pat. No. 4,482,671 while U.S. Pat. No. 4,595,716, and U.S. Pat. No. 5,157,078 teach a carbon grafting process involving solvent polymerization at moderate temperatures with high levels of peroxide initiator to produce a carbon-graft polymer. The high solvent levels, however, invariably carry over to the aqueous dispersion when the resulting polymers are dispersed into water to produce a VOC (volatile organic compounds) level considerably above 2 and typically between 3 and 4 pounds volatile organic compounds per gallon of resin solids.
Commonly assigned U.S. Pat. No. 5,290,828 discloses an acrylic grafted epoxy polyester terpolymer produced by in-situ copolymerization of ethylenic monomers with low molecular weight epoxy and unsaturated polyester resins where carboxyl monomers esterify epoxy groups while monomer double bonds coreact with polyester double bonds to form the terpolymer. Commonly assigned Ser. No. 222,029 filed Apr. 4, 1994 discloses an acrylic grafted epoxy-ester produced by first esterifying epoxy resin with a carboxyl functional unsaturated polyester to form an unsaturated epoxy-ester. The unsaturated epoxy-ester is dispersed into water and followed by in-situ copolymerization of ethylenic monomers in the aqueous dispersion, where the copolymerized monomers partially graft to the preformed unsaturated epoxy-ester.
Coating compositions based on microgels are shown in U.S. Pat. No. 4,897,434 where epoxy esters are preformed, then dispersed into water, and thereafter further crosslink available epoxy and carboxyl groups on the preformed epoxy-ester.
It now has been found that a preformed, carboxyl functional, low molecular weight acrylic addition copolymer of copolymerized ethylenic monomers, including carboxyl functional ethylenic monomer, can be utilized advantageously as an aqueous dispersing or suspending agent for dispersing an epoxy resin into water to produce a very low VOC coating having a VOC (volatile organic compounds) less than about one pound per gallon of resin solids. The resulting water dispersed epoxy resin provides an emulsion polymerization medium, where the addition copolymer dispersing agent is esterified with the epoxy functional resin to form crosslinked microgel particles stably dispersed in water. The acid functional, low molecular weight addition copolymer, preferably an acrylic copolymer, (preferably 2,000 to 20,000 Mn) can be dispersed in ammonia water to form small particle size dispersions (ca 50 nm), while subsequent crosslinking with liquid epoxy resin converts these dispersions to microgels. The water dispersed, crosslinked small particle size provide good storage stability, rheology control, along with exceptional film properties including excellent water resistance, low temperature cure, excellent flexibility, and good resistance to odor adsorption. This invention avoids the use of acid functional polyesters, which can slowly hydrolyze causing gelation while rheology of very low viscosity products can only be controlled by adding rheological control agents. In this invention, the acrylic copolymer dispersant can be synthesized at a very low solvent level (&lt;1 lb/gal VOC), dispersed into ammonia water, and subsequently crosslinked with liquid epoxy resin to form stable microgel particles. Crosslinking of the addition copolymer with diepoxide while stably dispersed in water produces very small size crosslinked microgel particles, a physical property particularly useful for producing tough but resilient coating films applied to a substrate. Further advantages rely on an acid functional acrylic to produce small particle size dispersions (ca. 50 nm) with low applied shear which will not hydrolyze and provide excellent storage stability. Viscosity can be easily controlled by adjusting acrylic Mn, carboxylic acid concentration, comonomer type, and degree of reaction with epoxy resin. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.