Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-01-27
2001-10-09
Sellers, Robert E. L. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C522S084000, C522S100000, C522S101000, C522S103000, C523S414000, C525S404000, C525S407000, C525S438000, C525S523000, C525S530000, C525S531000, C525S532000, C525S486000, C525S511000
Reexamination Certificate
active
06300428
ABSTRACT:
BACKGROUND OF THE INVENTION
Coating compositions and methods for coating substrates with cured coating compositions have been known for years. A coating composition is typically applied as a film which may include water and/or organic solvent. The coating is typically applied on a substrate by a process such as lamination, solution or dispersion coating, powder coating, electrocoating, spraying, roll coating, or reverse roll coating. Coating compositions can also be applied as a film by extrusion in melt form through an extrusion coating die onto a substrate. Once coated to the substrate, the composition must dry and cure without defects within a few seconds as modern high speed coating lines heat the coated substrates rapidly to temperatures of up to 490° F. (254° C.). Many different types of substrates have been coated using such techniques, including wood, plastics, and metal in the form of sheets, strips or coils. Metal coated substrates are especially useful in the packaging, processing, and holding of foods and beverages.
It is desirable to avoid the use of excess organic solvent in a coating method so that the environmental hazards of allowing organic solvent to evaporate into the atmosphere are minimized. However, a relatively large quantity of organic solvent is typically required in order to process a coating composition and provide a coalesced film after drying.
A number of attempts have been made in the prior art to prepare aqueous, solvent based, or solvent free coating compositions suitable for use in a process for forming cured coated substrates. For example, a water-borne coating composition prepared by combining an epoxy resin, a carboxylic acid polymer, and an excess of tertiary amine to form a reaction product has been reported. This excess of amine provides a resin coating composition wherein polymeric quaternary ammonium salt linkages almost completely predominate over ester linkages. This process is carried out in a mixed organic and aqueous medium. In another variation, the reaction of an &agr;,&bgr;-ethylenically unsaturated carboxylic acid with some of the oxirane groups prior to the formation of quaternary ammonium groups using a preformed carboxylic acid containing addition polymer has been described. Another solution based coating composition, which includes an aqueous epoxy ester emulsion prepared by forming a carboxyl bearing polymer in the presence of a defunctionalized epoxy resin in an organic solvent has been reported.
Solvent-free low-monomer or monomer-free hot melt compositions polymerizable by means of free radicals and suitable for use in coating various substrates have also been described. The compositions do not include quaternary ammonium compounds.
Conventional coating compositions containing quaternary ammonium salts typically suffer from the formation of extremely high molecular weight resins with high viscosity. Such compositions often possess such a high viscosity that application to a substrate can be hindered, resulting in problems of uneven flow and poor adhesion qualities. One other problem that commonly arises during attempted preparation of a water based coating composition is formation of an extremely high molecular weight product that will not dissolve in water. There is a continuing need for improved low viscosity coating compositions that can be applied to a substrate with excellent coating characteristics.
SUMMARY OF THE INVENTION
The invention provides a curable coating composition that may be formed in the absence of organic solvent or water. The invention also provides compositions that may be easily dispersed in water. The coating composition includes quaternized reaction product formed from epoxy resin, acid, and tertiary amine. The coating composition typically also includes reactive diluent and/or curing agent. As employed herein the term “quaternized reaction product” refers to compounds in which the oxirane groups originally present in the epoxy resin (“original oxirane groups”) have been partially or completely converted into quaternary ammonium groups. Typically, the quaternized reaction product includes quaternary ammonium groups derived from at least about 25% of the original oxirane groups. The coating solids portion of the composition typically includes at least about 30 wt. % quaternized reaction product, from about 5 to 70 wt. % reactive diluent, and up to about 30 wt. % curing agent.
The invention also provides a method of forming a cured coated substrate in a solvent free, solvent based, or aqueous environment. The method includes applying the coating composition onto any of a wide variety of substrates, including but not limited to metal (e.g., metal coils), wood, and polymeric materials and curing the coated substrate. The curing process is often carried out as a two stage process—(i) an initial radiation initiated or relatively low temperature cure to cross link (“polymerize”) the coating material; and (ii) a final bake to react the quaternary ammonium groups and bond the coating to the underlying substrate surface.
The coating composition can be applied in a solvent free, solvent based, or aqueous environment. For solvent free systems, the composition is applied to the substrate as a hot melt or liquid through the use of roll, slit, or slot coaters. For highly viscous compositions, a minimal amount of organic solvent may be employed to lower viscosity when using conventional application equipment such as roll coaters and the like. For aqueous based systems, water is added to produce a fully dissolved or dispersed aqueous coating composition capable of being applied to the substrate using conventional application equipment.
The method is generally completed by curing the coated substrate according to a one or two-step process. If desired, the coated substrate may initially be exposed to a radiation source, preferably ultra-violet light, causing any unsaturated carboxylic acids and any unsaturated diluents within the coated substrate to polymerize, to form a polymerized coated substrate. After the optional exposure to radiation, the substrate is typically baked to form the fully cured coated substrate. The bake can decompose the quaternary ammonium salts present in the quaternized reaction product, ensure adhesion of the coating to the substrate, and/or effect further crosslinking of the resinous or diluent components, e.g., through reaction of aminoplast or phenoplast curing agent(s) that may be present in the curing agent.
The invention also provides a cured coated substrate for the packaging of goods such as foods and beverages. The cured coated substrate is formed as described above, by applying the coating composition to the substrate and exposing the coated substrate to radiation, if necessary, and finally to heat.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
The terms related to coating compositions and methods for coating substrates are used in accordance with the understanding of one skilled in the art, unless otherwise noted. For example, the terms “coating solids” and “coating solids component” refer to the sum of the mass of components used in the composition exclusive of water or organic solvent—e.g., epoxy resin, acid, tertiary amine, and any other reactive curing agents, reactive diluent components, or initiators that are employed. As used herein, “solvent” refers to a volatile liquid component of the composition which does not react during the curing or baking steps. Most of the solvent is generally volatilized during a baking step and does not become incorporated into the cured coating. The organic solvent is typically made up of saturated organic compound(s) having a molecular weight of less than about 300.
“Polymerization” or “polymerizable” denotes the curing or cross-linking of the coating composition after a coated substrate is exposed to radiation (e.g., ultra-violet or electron beam), heat, or other means of initiating polymerization. “Thermosettable” or “thermoset” refers to a composition that is transformed into a gel structure by application of radiation and/or heat.
Brandenburger Larry
Mazza John
McCarty William
Niederst Jeffrey
Stevenson Thomas A.
Schwegman Lundberg Woessner & Kluth P.A.
Sellers Robert E. L.
The Valspar Corporation
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