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 whereby epoxy resins advantageously provide toughness, flexibility, adhesion, and chemical resistance. Water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity wherein such sanitary 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 25% to 50% 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 whereby the aqueous polymer dispersion typically produces a VOC (volatile organic compounds) level considerably above 2 lbs. per gallon VOC and typically between 3 and 4 VOC levels.
It now has been found that organic solvent can be substantially reduced or even eliminated from epoxy copolymer coatings by producing a graft epoxy copolymer in conjunction with a grafted unsaturated polyester and copolymerized ethylenic monomers to produce an acrylic grafted epoxy-polyester interpolymer. The unsaturated polyester functions as a non-volatile solvent for the epoxy resin during processing but contains minimal double bonds to enable the polyester to eventually coreact with copolymerizing ethylenic monomers and produce an acrylic grafted polyester. The epoxy esterifies with carboxyl functional monomer and/or undergoes hydrogen abstraction during addition polymerization to produce the epoxy graft. The epoxy, polyester, and acrylic addition polymer form a terpolymer interconnected by polymeric crosslinks to enable good compatibility of the hybrid polymer structures, subsequently dispersed into water while minimizing the need for volatile organic solvents. The resulting water dispersed polymers exhibit long term stability and are particularly useful as interior coatings for beer, beverage and food containers. Food container coatings must resist different types of acidic foods and ordinarily require high molecular weight resins which in turn inherently require higher levels of organic processing solvents.
It has been found that the interior can coatings of this invention exhibit excellent wetting and water resistance properties and can be made at very low VOC's by grafting ethylenic monomers with a blend of epoxy and polyester resins. Very low VOC coatings can be made by the process of this invention. It has been found that polyester resins (about 200 to 20,000 number average molecular weight) can be used to dissolve high molecular weight epoxy resins and can effectively replace most if not all of the organic volatile solvent ordinarily required for processing high molecular weight epoxy resins. Ethylenic monomers grafted onto both the epoxy resin and the polyester form a three way graft terpolymer that exhibits excellent stability and coating properties. The polyester resin greatly improves the processability of the epoxy resin without detracting from film properties such as gloss, clarity, and water resistance. An added advantage of this invention is lower cost because most polyester resins are less expensive than epoxy resins, while the cost of organic solvent is reduced or eliminated. The use of flexible polyesters in combination with epoxy and acrylic, particularly where the polyester is grafted onto the acrylic backbone by virtue of an unsaturated graft site, contributes to a lower viscosity and allows the removal of organic solvent to achieve VOC's well below 2 lbs/gal and advantageously below 1 lbs/gal. This polyester modified system has excellent formability and resistance to food storage, and also has advantages over the current state of the art in application and cleanup. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.