Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2000-10-02
2002-04-23
Tucker, Philip (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C523S414000, C525S533000
Reexamination Certificate
active
06376081
ABSTRACT:
BACKGROUND
Metal containers are typically coated with a protective coating to prevent damage to the container surfaces or contamination of the material packaged inside. Conventional container coatings may be derived, for example, from a formulation that includes trimellitic anhydride, a diol crosslinker, and an epoxy resin. These epoxy formulations described are applied as dispersions in a volatile organic solvent and then baked to form a lacquer-like coating on a metal substrate. Unfortunately, however, these volatile organic compounds (VOCs) are released into the atmosphere during the baking process, and may remain in the coating and degrade a product stored in the container.
To reduce VOC emissions, a heat curable coating for metal food contact surfaces may be derived from a water dilutable dispersion containing an epoxy resin. An effective water based coating may be obtained by adducting sufficient bisphenol to a diglycidyl ether of a bisphenol to react with all epoxy groups. The resulting oxirane defunctionalized adducts are then made water-soluble by reaction with an anhydride. The resulting dispersions are made thermoset using a conventional aminoplast crosslinking agent, such as highly butylated urea formaldehyde. Unfortunately, however, condensation products and by-products of these crosslinking agents, such as methanol, butanol, and formaldehyde, are released into the atmosphere during the baking process.
SUMMARY
There is a need in the packaging coatings industry for improved coatings (e.g., packaging coatings) that protect the packaged goods from contamination and do not release harmful compounds into the atmosphere during the baking process. Changes in food processing and environmental regulations continue to prompt manufacturers to develop new coating formulations with superior safety and processing characteristics compared to existing formulations. The present invention provides such formulations. In particular, the present invention provides improved resin compositions that comprise the reaction product of an epoxy resin and an anhydride. In preferred embodiments the resin composition's epoxide equivalent weight (EEW) has not been significantly changed, compared to the unreacted epoxy resin, to an extent that would cause any undesirable gelling or crosslinking of the resin. The reaction may preferably be conducted in an aprotic solvent.
In certain embodiments, the resin and coating compositions of the present invention do not require additional crosslinkers to form a crosslinked network. In addition, the resin compositions of the invention may be emulsified in water to form water based coating compositions that have reduced VOC emissions during bake. The self-crosslinking coating compositions of the invention are stable in aqueous media, have stability and curing profiles that comport with production scale food and beverage packaging applications, adhere well to metal, and are resistant to leaching, corrosion, and other forms of degradation.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
DETAILED DESCRIPTION
In one embodiment, the invention includes a resin composition that is the reaction product of an epoxy resin and an anhydride, wherein the reaction product has oxirane groups available to participate in further crosslinking reactions. The resin composition's epoxide equivalent weight (EEW) preferably has not been significantly changed, compared to the unreacted epoxy resin, to an extent that would cause any undesirable crosslinking of the resin. This resin may be used as a coating, or may be formulated with other components to create a coating composition.
The epoxy resin includes a backbone with pendant oxirane and pendant hydroxyl functional groups. In general, the reaction of the epoxy resin and the anhydride takes place in an organic medium under conditions selected such that the oxirane groups remain substantially intact and the pendant hydroxyl groups react with the anhydride to form ester linking groups on the backbone. The ester linking groups have pendant carboxyl functional groups.
The epoxy resins may vary widely depending on the intended application. The epoxy resin includes a pendant oxirane group and a pendant hydroxyl functional group on a backbone. A representative epoxy resin is shown in Formula 1:
Where B represents the backbone, X is an oxirane group, Y is a hydroxyl functional group, and n and m are independently at least 1, preferably at least 2.
In one embodiment, the epoxy resin is a reaction product of an epoxide and a dihydroxy compound. The dihydroxy compound used to make the epoxy resin may vary widely depending on the intended backbone structure needed in the epoxy resin. Preferably, the dihydroxy compound is selected from bisphenol A, bisphenol F, biphenol, resorcinol and the like, and bisphenol A is particularly preferred. Commercially available epoxy resins that are suitable for the present invention include those available under the trade designations EPON 1001, 1004, 1007, 1009, and 2004 resins from the Shell Chemical Co., Houston Tex. Preferred epoxy resins have a number average molecular weight of about 1,000 to about 10,000 and an epoxy equivalent weight of about 500 to about 5,000. Most preferred epoxy resins have an average number molecular weight of about 1,000 to about 8,000 and an epoxy equivalent weight of about 500 to about 5,000.
The epoxy resin is reacted with an anhydride to form a resin composition. Suitable anhydrides may vary widely depending on the epoxy resin selected and the reaction conditions. Examples of useful anhydrides include succinic anhydride, methyl succinic anhydride, tricarballylic anhydride, phthalic anhydride, hexahydrophthalic anhydride, trimellitic anhydride, itaconic anhydride, and maleic anhydride. Dianhydrides, such as, for example, benzophenone tetracarboxylic dianhydride (BTDA) or pyromellitic dianhydride, may also be used, and may increase cure rate and form a more densely crosslinked reaction product. When dianhydrides are used care should be taken to ensure that no undesirable gellation of the resin occur.
To prepare the resin compositions of the invention, the epoxy resin and the anhydride are reacted in a liquid medium under reaction conditions such that the reaction between the anhydride and the hydroxyl functional groups is substantially preferred over the reaction between the anhydride and the oxirane groups. The progress of the reaction can be monitored through methods such as NMR, IR, gas chromatography, or other methods known in the art. The resulting reaction product has oxirane groups available for further crosslinking reactions, and the reaction product's epoxide equivalent weight (EEW) preferably has not been significantly changed, compared to the unreacted epoxy resin, to an extent that would cause any undesirable gelling or crosslinking of the resin. Preferably, the resulting reaction product's EEW is no more than 50% higher than that of the starting epoxy resin. More preferably, the resulting reaction product's EEW is no more than 30% higher than that of the starting epoxy resin. Most preferably, the resulting reaction product's EEW is no more than 15% higher than that of the starting epoxy resin.
The anhydrides react with the pendant hydroxyl groups on the epoxy resin to generate ester acids. This reaction is represented in formula 2 below:
Where B is the resin backbone, X is an oxirane group, Y is a hydroxyl group, L is an ester linking group, Q is a reactive carboxyl functional group, m and n are as previously defined, p is at least 1, preferably 2, and the sum m-p is at least 0.
The liquid medium used to prepare the resin compositions of the invention is preferably selected from aprotic solvents such as ketones, ethers, aryl ethers, ether esters and alkyl ethers, aromatic hydrocarbons (e.g., toluene, xylene, etc.), used alone o
Bohler Paul
Legleiter Jason M.
Parekh Girish G.
Wind Donald
Aylward D.
Tucker Philip
Valspar Corporation
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