Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-03-29
2001-10-23
Woodward, Ana (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S409000, C523S411000, C523S413000, C523S423000, C524S504000
Reexamination Certificate
active
06306934
ABSTRACT:
This invention relates to aqueous coating compositions particularly useful as can coatings for beverage and food containers, a process for making the compositions and a process for using them.
Industrial coatings are surface protective coatings (paint coatings) applied to substrates and typically cured or crosslinked 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 theological properties to the fluid paint coating. Upon curing or crosslinking, 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 colour in addition to durability and hardness. Protective coatings which contain little or no opacifying pigments are described as clear coatings. The manufacture of protective 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 coatings materials as a vehicle or polymeric binder for the 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 are particularly useful for interior surfaces of containers. Coatings for the interior of soft drink and beer cans, for instance, are critical due to taste sensitivity wherein such can coatings must not alter the product taste of beverages in the containers. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by absorption of flavour by the coating, or sometimes by chemical reaction, or by perhaps some combination. The closure ends of cans are typically produced by stamping circular end caps from pre-coated flat metal where the top end ordinarily contains a stamped opening profile adapted to snap and roll back to provide an opening. The coating for ends must be resilient and avoid stringing while maintaining good interior resistance to the food or beverage contents.
Container coating technology frequently utilises 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/or 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 solvents 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. Also, epoxy coatings used for beverage or food containers raise a concern that such coatings may contain low molecular weight epoxy fragments which could leach out of the coating and into the contents of the container.
Epoxy based can coatings comprising a carbon grafted acrylic chain are disclosed in U.S. Pat. No. 4,212,781 which teaches a carbon grafting process involving solvent polymerisation 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. The acrylic grafted epoxy is particularly useful when utilised with a co-reactive crosslinking melamine crosslinker.
PCT application WO 96/10612 discloses a can coating composition containing both water and organic solvent base on a polymer comprising the reaction of a carboxyl addition polymer and an epoxy resin in the presence of a tertiary amine catalyst. The epoxy and carboxyl addition polymer are pre-formed by co-reacting in organic solvent and subsequently dispersed into water. Similarly, U.S. Pat. No. 5,296,525 discloses can coatings based on styrene monomer copolyerised with an epoxy ester copolymer to provide a carboxyl functional polymer for subsequent dispersing into water.
U.S. Pat. No. 4,683,273 teaches an epoxy coating composition based on a polymeric blend of an epoxy-acrylic graft copolymer combined with a low molecular weight polyester and an amine crosslinking component. Upon heat curing the coating, the amine crosslinks co-reactive groups on the epoxy-acrylic graft polymer and the low molecular weight polyester.
U.S. Pat. No. 5,532,297 pertains to can coatings based on an epoxy-acrylic graft copolymer dispersed into water and subsequently overpolymerised with mono-ethylenically unsaturated monomers and minor amounts of divinyl benzene for crosslinking during the emulsion polymerisation step. Similarly, U.S. Pat. No. 5,464,885 discloses a carboxyl functional epoxy-ester comprising an epoxy esterified with an unsaturated polyester where the epoxy-ester is dispersed into water and subsequently emulsion copolymerised with ethylenically monomers.
It now has been found that high quality can coatings can be produced with little or no organic solvent and without epoxy resin fragments by pre-forming an unsaturated polyester, dispersing the polyester into water with an epoxy-acrylic graft copolymer dispersant, and then aqueous emulsion copolymerising with ethylenically unsaturated monomers. The resulting aqueous dispersed copolymer provides a highly flexible polymeric coating film with considerable physical and taste resistance to acidic beverages and sterilisation often required for food containers. An emulsion copolymer of polyester with copolymerised acrylic or vinyl monomers produces a coating composition particularly suitable for metal containers capable of being tooled into container end caps. On metal can interior surfaces, the coating provides good flavor resistance and maintains good resistance to sterilisation. The emulsion copolymer of this invention does not require conventional surfactants and avoids blush film problems associated with surfactants used in conventional latex polymerisation processes. These and other advantages of this invention will become more apparent by referring to the details of the invention.
According to the present invention there is provided an aqueous dispersed, protective coating composition containing an emulsion copolymer polymeric binder substantially free of volatile organic compounds, the polymeric binder comprising by weight:
(a) between 0.1% and 40% of a preformed, low molecular weight unsaturated polyester having a weight average molecular weight between 1000 and 3000, the unsaturated polyester synthesised by esterifying excess molar equivalents of polyol with lesser molar equivalents of dicarboxylic acid, at molar percents between,
(i) excess of 50 mol percent polyol consisting of glycol,
(ii) 10 and 35 mol percent aliphatic dicarboxylic acid,
(iii) 5 and 35 mol percent aromatic dicarboxylic acid,
(iv) 5 and 25 mol percent unsaturated dicarboxylic acid,
(v) 0 and 3 mol percent polyol having three or more hydroxyl groups;
(b) at least 20%, preferably 20% to 80% of in-situ, emulsion copolymerised ethylenic monomers, where the ethylenic monomers comprise at least 20% styrene based on the weight of the ethylenic monomers copolymerised; and
(c) between 20% and 80% carboxyl functional epoxy-acrylic graft copolymer dispersant, the copolymer dispersant having an Acid No. greater than 30, the copolymer dispersant being by weight 5% to 80% epoxy resin and 20% and 95% copolymerised ethylenically unsaturated monomer including carboxylic acid monomer, the graft copolymer dispersan
Belton Martin
Bode Daniel
Bradley Christopher
Howard Deborah
Imperial Chemical Industries PLC
Schmitz Thomas M.
Woodward Ana
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