HK1081981B - Compositions and methods for coating food cans - Google Patents

Description

Composition and method for coating food cans

Technical Field

The present invention relates to compositions and methods for coating metals. More particularly, the present invention relates to compositions and methods for coating food cans wherein the coating composition comprises a polyester and an acrylic polymer.

Background

The use of various treatment and pretreatment solutions for metals to prevent or inhibit corrosion is a well established technology. This is particularly the case in the field of metal food and beverage cans. A coating is applied to the interior of such containers to prevent the contents from contacting the metal of the container. Contact between the metal and the food or beverage can lead to corrosion of the metal container, which can contaminate the food or beverage. This is particularly the case when the contents of the can are acidic, such as tomato-type products and soft drinks. Coatings applied to the interior of food and beverage cans also help prevent corrosion of the can headspace of the can, the latter being the area between the fill line of the food product and the can lid; corrosion in the can headspace is especially problematic for food products with high salt content.

In the past, various epoxy-type coatings and polyvinyl chloride-type coatings have been used to coat the interior of metal cans to prevent corrosion. However, recycling of polyvinyl chloride-containing materials or related halogen-containing vinyl polymers can produce toxic by-products; moreover, these polymers are typically formulated with epoxy functionalized plasticizers. Additionally, epoxy-based coatings are prepared from monomers such as bisphenol a and bisphenol a diglycidyl ether ("BADGE"), which are reported to have adverse health effects. Although attempts have been made to remove residual unreacted epoxy groups, for example by acid functionalization of the polymer, this is not sufficient to solve the problem; some free BADGE or its by-products remain. Government regulatory agencies, especially in europe, are imposing more stringent limits on the amount of free BADGE or its by-products that can be accepted. Therefore, there is a need for food and beverage can liners that are substantially free of BADGE, epoxy and vinyl products.

Summary of the invention

The present invention relates to compositions and methods for coating the interior of food cans. The term "food can" is used herein to mean a can, container or any type of metal receptacle for holding any type of food or beverage. The method generally includes coating the can with a composition comprising a polyester and an acrylic polyol.

As is known in the art, polyester coatings have good flexibility but are susceptible to hydrolysis in an acid environment. In contrast, acrylic polymers may provide good resistance but not flexibility. Thus, the use of polyester or acrylic copolymers alone has drawbacks. However, their use together is sometimes problematic because polyesters and acrylic polymers are often incompatible. Their common use in the present invention therefore requires that they be made compatible in some way; methods of doing so are described herein and are another subject of the present invention.

Detailed description of the invention

The present invention relates to a composition for coating food cans comprising an acrylic copolymer; a polyester; and a crosslinking agent. The polyester and acrylic copolymer should be made compatible to form the composition of the present invention. This can be accomplished by any of the various methods known in the art or described herein, including, but not limited to, using blending techniques known in the art to prepare interpenetrating networks, or to form graft copolymers. In one embodiment, the composition is "epoxy-free". "epoxy-free" means that neither the polyester nor acrylic polymer portions of the composition contain epoxy rings, or residues of epoxy rings; bisphenol A; BADGE or an adduct of BADGE. The coating composition is also free of polyvinyl chloride or related halogen-containing vinyl polymers.

The polyester component used in the process of the present invention can be prepared by conventional methods such as polyesterification of polycarboxylic acids or anhydrides and polyols using techniques known to those skilled in the art. Typically, the polycarboxylic acids and polyols are aliphatic or aromatic dibasic acids and diols, although the invention is not so limited. Transesterification of the polycarboxylates is also possible using conventional techniques.

Typically, the weight average molecular weight ("Mw") of the polyester is 4,000 to 20,000, such as 5,000 to 13,000, or 7,000 to 11,000. The polyester generally has a hydroxyl number of 0 to 200mg KOH/g resin, such as 30 to 70, or about 40, and an acid number of less than about 10, such as less than 5.

Any polyol known to be suitable for use in the preparation of polyesters can be used to form the polyester component of the composition of the present invention. Examples include, but are not limited to, alkylene glycols such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, hexylene glycol, polyethylene glycol, polypropylene glycol, and neopentyl glycol; hydroxylated bisphenol A; cyclohexanediol; 1, 3-propanediol; ethylene glycol; 1, 4-butanediol; 1, 3-butanediol; butyl ethyl propylene glycol; trimethyl propylene glycol; cyclohexanedimethanol; caprolactone diols such as the reaction product of epsilon-caprolactone and ethylene glycol; a hydroxyalkylated bisphenol; polyether glycols such as poly (tetramethylene oxide) glycol and the like. Higher functionality polyols may also be used in limited amounts, provided they do not adversely affect flexibility. Examples include trimethylolpropane, trimethylolethane, pentaerythritol, tris-hydroxyethyl isocyanurate, and the like.

Similarly, any mono-or poly-acid known for use in making polyesters can be used to make the polyester polymer component of the present invention, for example, monomeric carboxylic acids or anhydrides having from 2 to 18 carbon atoms per molecule can be included. Examples include phthalic acid, isophthalic acid, 5-t-butylisophthalic acid, endomethylenetetrahydrophthalic acid, tetrachlorophthalic anhydride, chlorendic acid, naphthalenedicarboxylic acid, terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, methylhexahydrophthalic acid, 1, 3-cyclohexanedicarboxylic acid, 1, 4-cyclohexanedicarboxylic acid, adipic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid, itaconic acid, succinic acid, glutaric acid, sebacic acid, dodecanedioic acid and various types of other dicarboxylic acids. The polyester may include small amounts of monobasic acids such as benzoic acid, stearic acid, acetic acid and oleic acid. Also, higher carboxylic acids such as trimellitic acid and tricarballylic acid can be used. Although only acids are mentioned above, it goes without saying that their anhydrides present may be used instead of the acids. Also, lower alkyl esters of diacids, such as dimethyl glutarate and dimethyl terephthalate, can be used.

In one embodiment, the polyester component of the present invention is unsaturated. Although any unsaturated polyester can be used according to the present invention, particularly suitable polyesters are formed from butanediol, ethylene glycol, cyclohexanedicarboxylic acid, isophthalic acid and maleic anhydride. This embodiment is particularly suitable when a graft copolymer is prepared between the polyester and the acrylic copolymer; maleic anhydride, which is not normally incorporated into the polyester, facilitates grafting with the acrylic copolymer. Instead of maleic anhydride, or in addition to maleic anhydride, it is also possible to use maleic acid, fumaric acid and/or itaconic acid and/or anhydrides of these acids to prepare polyesters which also have components which are particularly suitable for promoting grafting. In certain instances, the polyester of this embodiment is also particularly desirable because all of the components of the polyester are approved by the U.S. food and drug administration ("FDA") for direct food contact; these components are also listed in the European inventory of Existing Commercial Subs series ("EINECS").

In one embodiment, the polyester is prepared with an excess of polyol relative to acid in order to obtain a polyester with hydroxyl functionality. Polyesters can also be prepared in such a way that they lack or have acid functionality.

Various acrylic monomers can be combined to prepare the acrylic copolymer used in the present invention. Examples include methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, (meth) acrylic acid, vinyl aromatics such as styrene and vinyl toluene, nitriles such as (meth) acrylonitrile, and vinyl esters such as vinyl acetate. Any other acrylic monomer known to those skilled in the art can also be used. The term "(meth) acrylate" and similar terms are used conventionally and herein to refer to both methacrylate and acrylate. Particularly suitable acrylic copolymers are formed from styrene, butyl acrylate, ethylhexyl acrylate and methacrylic acid, alone or in further combination with hydroxyethyl methacrylate and methyl methacrylate. Again, in some cases, the acrylic copolymer includes components approved by the FDA for use in food cans and is listed on EINECS. Typically, the Mw of the acrylic copolymer is about 10,000 to 250,000, such as 20,000 to 150,000, or 25,000 to 100,000.

As discussed above, the acrylic copolymer and polyester used in the present invention can be treated in any manner to render the two compatible. By "compatible" is meant that the polyester and acrylic copolymer can be combined together in the coating without phase separation, thus forming a homogeneous product. The compatibilized copolymers can simply be blended together. In this blending embodiment, the acrylate copolymer used according to the present invention has no pendant glycidyl groups when the polyester is acid terminated and no pendant hydroxyl groups when the polyester is hydroxyl terminated. Compatibilization can be achieved, for example, by using an acrylic copolymer having a Mw similar to the Mw of the polyester (i.e., in the range of about 1,000). Various functional groups can also be added to the acrylic copolymer and/or polyester to make the two compatible. For example, the acrylic copolymer can have N- (N-butoxymethyl) acrylamide ("NBMA") functionality. When the acrylic copolymer is functionalized with NBMA, it preferably has a Mw equal to or less than about 20,000. Other compatibilizing functional groups include acid functionality, hydroxyl groups, amide groups, and the like. Suitable solvents, known in the art as "coupling solvents", can also contribute to compatibilization. An example is ethylene glycol monobutyl ether, available as butyl cellosolve from Dow Chemical.

The acrylate copolymer and polyester can also be compatibilized, for example, by forming interpenetrating polymer networks. The preparation of such networks is described, for example, in US patent No.6,228,919, which is incorporated herein by reference.

Another method by which the polyester and acrylate copolymers can be compatibilized is by forming a graft copolymer. The graft copolymer can be formed using techniques standard in the art. In one method, the polyester is prepared according to a conventional method using the above materials. The acrylic monomer is then added to the polyester. The acrylic monomer can then be polymerized using standard free radical initiators. In this way, the acrylate copolymer is grafted into the polyester already prepared.

Alternatively, the polyester can be grafted to an acrylic copolymer that has been prepared. In this embodiment, the maleic anhydride groups can be polymerized in the acrylic copolymer, and the hydroxyl groups of the polyester can then be reacted with the acrylic copolymer to form a graft copolymer; the product is an acrylic copolymer having a polyester moiety grafted thereto.

In the grafting process according to the invention, the moieties to be incorporated into the polyester and the monomers to be incorporated together with the acrylate monomers which are reactive with one another are selected. Particularly suitable examples are the use of maleic anhydride in the formation of the polyester and the use of styrene as one of the acrylic monomers. In this embodiment, styrene will react with maleic anhydride; the acrylic copolymer grows styrene by forming radicals. The product is a polyester having an acrylic copolymer grafted thereto. It will be appreciated that not all acrylic copolymers and polyesters will be grafted; thus, there is some "pure" polyester and some "pure" acrylate copolymer in solution. However, sufficient acrylate copolymer and polyester will graft to compatibilize the two normally incompatible polymers.

It will be appreciated that maleic anhydride and styrene are provided as examples of two components to facilitate grafting between normally incompatible polymers, but the invention is not so limited. Other compounds such as fumaric acid/anhydride or itaconic acid/anhydride are incorporated into the polyester for grafting with the styrene-containing acrylic copolymer. Other moieties that promote grafting between the polyester and acrylic copolymer can also be used. Any group of compounds can be used for this purpose. All of these compounds are referred to herein as "graft promoting components". The amount of graft promoting component used in each polyester and/or acrylate moiety can affect the final product. If too much of these components is used, the product can gel or be unusable. The graft promoting component should therefore be used in an amount effective to promote grafting, but not to cause gelation. Sufficient grafting should be performed to compatibilize the polyester and acrylate polymers. In the maleic anhydride/styrene example, it is generally possible to use from 2 to 6% by weight of maleic anhydride and from 8 to 30% by weight of styrene, the% by weight being based on the weight of the polyester and the weight of the acrylic copolymer, respectively.

The Mw of the graft copolymer is generally from about 3000 to 250,000, such as from about 5000 to 125,000, or from about 30,000 to 50,000.

The weight ratio of polyester to acrylic polymer in the composition of the invention may vary widely. For example, the ratio of polyester to acrylic polymer may be 95: 5 to 20: 80. It has been determined that varying the amount of polyester in the composition will affect the amount of flexibility. A ratio of polyester to acrylic polymer of 70: 30 that is particularly suitable for coating food cans results in a product that is relatively flexible, but still has suitable acid resistance.

The acrylate copolymers and polyesters described above in blended or grafted form are further used in combination with a crosslinking agent. Suitable crosslinking agents can be determined according to the needs and desires of the user, and can include, for example, melamine crosslinking agents, and phenolic crosslinking agents. Melamine crosslinkers are commercially available, for example, as CYMEL 303, 1130, 325, 327 and 370 from Cytec Industries, inc. Phenolic crosslinkers include, for example, novolac resins, resole resins, and bisphenol a. Preferred for use in food cans are resoles not derived from bisphenol a.

The compositions of the present invention also include a solvent. Suitable solvents include esters, glycol ethers, glycols, ketones, aromatic and aliphatic hydrocarbons, alcohols, and the like. Particularly suitable are xylenes (zylenes), propylene glycol monomethyl acetate, and dibasic esters such as the dimethyl esters of adipic, glutaric and succinic acids. Typically, compositions having about 30 to 50 wt% solids are prepared.

The compositions of the invention can also contain any other conventional additives such as pigments, colorants, waxes, lubricants, defoamers, wetting agents, plasticizers, reinforcing agents and catalysts. Any mineral acid or sulfonic acid catalyst can be used. Particularly preferred for food can applications are phosphoric acid and dodecylbenzene sulfonic acid.

The present invention further relates to a method of coating a food can comprising applying any of the compositions described above to a food can. More specifically, these compositions comprise a polymer, an acrylic copolymer, a crosslinking agent, one or more solvents and optionally one or more conventional additives. The polyester and acrylic copolymer can be compatibilized by any of the means described above, such as using blending techniques known in the art, interpenetrating networks, or the novel graft copolymerization described herein. The coating composition can be applied to the food can by any means known in the art, such as roll coating, spray coating, and electrocoating. It will be appreciated that for two-piece food cans, the coating is typically sprayed after the can is prepared. For three-piece food cans, on the other hand, a coil or sheet is generally first roll coated with one or more compositions of the present invention, and the can is then formed.

After application, the coating is then cured. Curing is carried out by methods standard in the art. For coil coatings, the residence time at high heat (i.e., 485 peak metal temperature) is typically short (i.e., 9 seconds to 2 minutes); for coated metal sheets, curing is generally longer (i.e., 10 minutes), but at lower temperatures (i.e., 400 peak metal temperature).

Any material used to form food cans can be processed according to the method of the present invention. Particularly suitable substrates include tin-plated steel, tin-free steel, and black steel.

The coating of the present invention can be applied directly to steel without first applying any pretreatment or adhesion promoter to the metal. In addition, there is no need to apply any coating over the coating used in the method of the present invention.

In addition, the present invention provides a food can having a coating on a surface thereof, wherein the coating comprises any of the compositions described above. The invention also provides food cans having a coating on the surface, obtainable according to any of the above methods.

The composition of the present invention performs well in both flexibility and acid resistance as required. Notably, these results can be obtained with compositions that do not contain epoxy groups. Thus, the present invention provides particularly desirable compositions and methods for coating food cans that avoid the performance and health problems that result from other coatings and methods reported in the art.

In addition, the present invention provides a method for compatibilizing a polyester and an acrylic polymer. These methods are discussed above and include, for example, the use of acrylamide in the formation of the acrylic copolymer and the graft copolymerization of the acrylic polymer onto the polyester, or the graft copolymerization of the polyester onto the acrylic polymer.

As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word "about", even if the term does not expressly appear. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein. The term "polymer" as used herein refers to oligomers and both homopolymers and copolymers, and the prefix "poly" refers to two or more.

Examples

The following examples are intended to illustrate the invention and should in no way be considered as limiting.

Example 1

Polyester polymer "a" was prepared as follows:

TABLE 1

Composition (I)	Feed No. 1	Parts by weight
			2-methyl-1, 3-propanediol		2.4
Ethylene glycol		1.0
			1, 6-hexanediol		3.6
Terephthalic acid (TPA)		7.1
			Dibutyl tin oxide		0.035
	2# feedstock
			Isophthalic acid		3.0
Maleic anhydride		0.54
			Ionol		0.018
	3# feedstock
			Xylene		0.81
	4# feedstock
			Xylene		5.8

The feed No. 1 was charged to a 5L four-necked flask equipped with a motor driving a stainless steel stirring blade, a packed tower connected to a water-cooled condenser, and a heating mantle with a thermometer connected to a temperature feedback control device. The reaction mixture was heated to 195 ℃ and held for 6 hours, during which time 1.3 parts of water were distilled off. The mixture was briefly cooled to 180 ℃, 2# feedstock was added, and the mixture was heated again at 195 ℃ for 4 hours. After this hold period, the reaction was cooled. Feed # 3 was added, the packed column was replaced with a dean-Stark trap, and the mixture was heated to reflux (190 ℃). Heating was continued for 7 hours during which time additional water was azeotropically distilled off. When the acid number of the solution was below 1.5, the mixture was cooled to 150 ℃ and the resin diluted with # 4 feed.

Example 2

Polyester polymer "B" was prepared as follows:

TABLE 2

Composition (I)	Feed No. 1	Parts by weight
			1, 3-butanediol		10.0
Ethylene glycol		1.9
				2# feedstock
1, 4-Cyclohexanedicarboxylic acid		14.5
			Isophthalic acid		6.0
Maleic anhydride		1.0
			Dibutyl tin oxide		0.067
Methyl hydroquinone		0.0029
				3# feedstock
Xylene		1.5
				4# feedstock
Xylene		10.8

The feed No. 1 was charged to a 5L four-necked flask equipped with a motor driving a stainless steel stirring blade, a packed tower connected to a water-cooled condenser, and a heating mantle with a thermometer connected to a temperature feedback control device. The reaction mixture was heated to 125 ℃. Feed # 2 was added to the mixture and then heated to 155 ℃. Distillation of water was started and continued for 3.5 hours. The temperature was raised to 175 ℃ and held for 90 minutes, then to 195 ℃ and held for 4 hours. The reaction temperature was raised to 200 ℃ and held for 3.5 hours, where the distillation of water started to slow down significantly. The reaction mixture was cooled to 180 ℃, the packed column was replaced with a dean-stark trap and a nitrogen sweep was initiated. Feed # 3 was added and the reaction was heated to 195 ℃ and held for 7 hours at which time the acid number was below 2.0. The resin was cooled to 80 ℃ and then diluted with # 4 feed.

Example 3

An acrylic polyester copolymer "a" was prepared as follows:

TABLE 3

Composition (I)	Feed No. 1	Parts by weight
			Toluene		12.9
SOLVESSO 150		11.0
				2# feedstock
Xylene		6.0
			VAZO 67		2.0
	3# feedstock
			Acrylic acid butyl ester		12.0
2-hydroxyethyl methacrylate		11.2
			Methacrylic acid		1.0
Styrene (meth) acrylic acid ester		6.0
			2-ethylhexyl acrylate		4.0
Methacrylic acid methyl ester		5.8
			Polyester A of example 1		135.3
	4# feedstock
			VAZO 67		0.1
Xylene		0.4
				5# feedstock
SOLVESSO 150		17.9

¹Aromatic hydrocarbon mixtures boiling at 150 ℃ used as solvents, available from exxon chemical America.

²Azobis 2, 2' - (2-methylbutyronitrile), available from e.i. dupont de Nemours& Co.，Inc.。

The feed No. 1 was charged to a 3L four-necked flask equipped with a motor driving a stainless steel stirring blade, a water-cooled condenser and a heating mantle of a thermometer with a temperature feedback control device attached. The contents of the flask were heated to reflux (128 ℃). Addition of feed # 2 was started (over 190 minutes) and after 5 minutes feed # 3 was added (over 180 minutes). During the feed, the reflux temperature was gradually increased to 138 ℃. After the end of the addition, the reaction was held at 138 ℃ for 1 hour. The feed # 4 was added over 10 minutes and the mixture was held at 138 ℃ for an additional 1 hour. The resin was diluted with # 5 feed.

Example 4

An acrylic polyester copolymer "B" was prepared as follows:

TABLE 4

Composition (I)	Feed No. 1	Parts by weight
			SOLVESSO 150		8.0
	2# feedstock
			SOLVESSO 150		6.3
Di-tert-butyl peroxideArticle (A)		1.0
				3# feedstock
Acrylic acid butyl ester		12.0
			Methacrylic acid		1.0
Styrene (meth) acrylic acid ester		2.0
			2-ethylhexyl acrylate		5.0
Polyester B		67.3(46.8 solid)
				4# feedstock
SOLVESSO 150		0.45
			Di-tert-butyl peroxide		0.026
	5# feedstock
			SOLVESSO 150		0.45
Di-tert-butyl peroxide		0.026
				No.6 feedstock
SOLVESSO 150		0.45
			Di-tert-butyl peroxide		0.026
	7# feedstock
			SOLVESSO 150		0.45
Di-tert-butyl peroxide		0.026
				8# feedstock
Xylene		8.7

The feed No. 1 was charged to a 2L four-necked flask equipped with a motor driving a stainless steel stirring blade, a water-cooled condenser and a heating mantle of a thermometer with a temperature feedback control device attached. The contents of the flask were heated to reflux (150 ℃). Addition of both # 2 and # 3 feeds was started simultaneously and continued for 3 hours. After the end of the addition, the reaction was held at 150 ℃ for 30 minutes. The 4#, 5#, 6# and 7# feeds were then added to the mixture every 30 minutes. After addition of feed # 7, the mixture was held for an additional 30 minutes, cooled to 130 ℃ and feed # 8 was added.

Example 5

An acrylic polyester copolymer "C" was prepared as follows:

TABLE 5

Composition (I)	Feed No. 1	Parts by weight
			SOLVESSO 150		10.1
Toluene		10.1
				2# feedstock
Xylene		3.1
			VAZO 67		2.0
	3# feedstock
			Acrylic acid butyl ester		12.0
Methacrylic acid		1.0
			Styrene (meth) acrylic acid ester		6.0
2-ethylhexyl acrylate		4.0
			2-hydroxyethyl methacrylate		11.2
Methacrylic acid methyl ester		5.8
			Polyester B		14.4(10.0 solid)
	4# feedstock
			Xylene		0.31
VAZO 67		0.10
				5# feedstock
Xylene		6.7

The feed No. 1 was charged to a 2L four-necked flask equipped with a motor driving a stainless steel stirring blade, a water-cooled condenser and a heating mantle of a thermometer with a temperature feedback control device attached. The contents of the flask were heated to 128 ℃. Feed # 2 (over 190 minutes) was added and after 5 minutes feed # 3 (over 180 minutes) was added. After the end of the addition, the reaction was held at 150 ℃ for 30 minutes. During the addition, the temperature was gradually increased to reflux at 138 ℃. After the end of the addition, the reaction was held at 138 ℃ for 90 minutes. The feed # 4 was then added over 10 minutes, followed by 1 hour at 138 ℃. The resin was then diluted with # 5 feed and cooled.

Example 6

Three different samples were prepared by adding copolymers A, B and C prepared as described in examples 3, 4 and 5, respectively, to separate containers and mixing the following ingredients in the order shown under ambient conditions until homogeneous.

TABLE 6

Composition (I)	Sample 1	Sample 2	Sample 3
				Copolymer A	65.9g	0	0
Copolymer B	0	65.9g	0
				Copolymer C	0	0	65.9g
Phenolic crosslinkers	2.8	2.8	2.8
				Phenolic crosslinkers	8.3	8.3	8.3
Catalyst and process for preparing same	1.1	1.1	1.1
				Wax dispersion	3.3	3.3	3.3
Solvent(s)	9.3	9.3	9.3
				Solvent(s)	9.3	9.3	9.3
Total of	100	100	100

³GPRI 7590, a modified phenol-cresol-formaldehyde resin, available from Georgia Pacific.

⁴HARZ 6572LB, p-tert-butylphenol-formaldehyde resin, available from Bakelite.

⁵Solutions of ADDITOL XK-406, cresol-formaldehyde resin and phosphoric acid, available from Solutia.

⁶Luba-Print P1, a solution of lanolin wax, available from L.P.Bader& Co.GmbH。

⁷DOWANOL PM acetate, propylene glycol monomethyl ether acetate, available from Dow Chemical.

⁸SOLVESSO 150。

Coatings were prepared by drawing samples 1-3 and a commercially available epoxy inner wall coating for food cans (eurogld XF 12040, available from ppginindustries, Inc.) over a tin-plated steel (e.t.p.) panel with a #12 wire wound paint bar. The coating was baked at 400 deg.f for 10.5 minutes. The dry coating weight was 4.0 mgs/sq.in.

The flexibility of the coated sheet was evaluated by bending and stamping wedges (2.0 inches by 4.5 inches), stamping 300 food can ends, and by stretching the cup to 18mm and 26mm depths with one and two stages of stretching, respectively. For wedge bend and cup stretch, the percentage of the coating that remained crack free along the bend radius (for wedge bend) and along the stretched length (for cup) was determined. For a stamped 300 can end, the measured current (mA) was measured in a 4sec mode using an electrolyte solution of 7.0g potassium ferrocyanide, 5.4g sodium chloride, 0.5g sodium sulfosuccinate, and 1000g water with a WACO Enamel Rater (available from Wilkens-Anderson Company). The resistance of the coated drawn can ends and drawn cups was evaluated by treating in three food simulants (retort) and measuring their resistance to current flow (drawn can ends) and resistance to cracking (drawn cups) after 1 hour at 266/30psi in a sterilizer. The three simulants were tap water, a 1 wt% solution of sodium chloride in tap water, and a 1 wt% solution of lactic acid in tap water. All results are provided in table 7.

TABLE 7

	Commercial epoxy resins	Sample 1	Sample 2	Sample 3
					Flexibility test
1. Wedge bend (% crack free)	86％	93％	92％	73％
					2. 300 Ename Rate (mA) at bottom of tank	2mA	2mA	7mA	20mA
3. 18mm tensile cup (% crack free)	100％	100％	100％	100％
					4. 26mm tensile cup (% crack free)	100％	100％	100％	100％
					Resistance test (60min @ 130 ℃ C.)
1. Variation of 300 bottom Ename Rate in the following materials
					a. Water (W)	1mA	1mA	6mA	＞200mA
b.1% salt (aqueous solution)	2mA	2mA	7mA	＞200mA
					c.1% lactic acid (aqueous solution)	2mA	2mA	20mA	＞200mA
2. 18mm tensile cup (% crack free) tested in the following materials
					a. Water (W)	100％	100％	100％	100％
b.1% salt (aqueous solution)	100％	100％	100％	100％
					c.1% lactic acid (aqueous solution)	100％	100％	100％	100％
3. 26mm tensile cup (% crack free) tested in the following materials
					a. Water (W)	31％	100％	100％	19％
b.1% salt (aqueous solution)	38％	58％	100％	23％
					c.1% lactic acid (aqueous solution)	46％	38％	100％	19％

As can be seen from table 7, sample 1 has better results than the current epoxy-containing food can inner wall coating. Sample 2 also had very good results, especially with regard to acid resistance. Samples 1 and 2 each had a polyester to acrylic polymer ratio of about 70: 30. Sample 3, having a 20: 80 ratio of polyester to acrylic polymer, demonstrates that some flexibility can be lost with reduced polyester levels.

While specific embodiments of the invention have been described above for purposes of illustration, it will be apparent to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.

Claims (33)

1. A composition suitable for coating food cans comprising:

(a) a polyester;

(b) an acrylic copolymer;

(c) a crosslinking agent; and

(d) a solvent, a water-soluble organic solvent,

wherein the polyester and acrylic copolymer are blended together or grafted together using standard free radical initiators, but if the polyester and acrylic copolymer are blended together, the acrylic copolymer has no pendant glycidyl groups when the polyester is acid terminated and the acrylic copolymer has no pendant hydroxyl groups when the polyester is hydroxyl terminated.

2. The composition of claim 1, wherein the acrylic copolymer contains acrylamide functionality.

3. The composition of claim 2 wherein the acrylamide functionality is derived from N- (N-butoxymethyl) acrylamide.

4. The composition of claim 1, wherein the weight average molecular weight of the acrylic copolymer is within the range of ± 1000 of the weight average molecular weight of the polyester.

5. The composition of claim 1, wherein the solvent is xylene, propylene glycol monomethyl ether acetate, or a dibasic ester.

6. The composition of claim 1, wherein the polyester and acrylic copolymer are grafted together.

7. The composition of claim 6, wherein the acrylic copolymer is grafted to the polyester.

8. The composition of claim 6, wherein the polyester is grafted to the acrylic copolymer.

9. The composition of claim 1 wherein the polyester is unsaturated.

10. The composition of claim 9, wherein the polyester comprises maleic acid or maleic anhydride and the acrylic copolymer comprises styrene.

11. The composition of claim 1 wherein the polyester has a hydroxyl number of 0 to 200.

12. The composition of claim 11 wherein the polyester has a hydroxyl number of 30 to 70.

13. The composition of claim 1 wherein the polyester has an acid number of less than 10.

14. The composition of claim 13 wherein the polyester has an acid number of less than 5.

15. The composition of claim 6 wherein the graft copolymer has a weight average molecular weight of 3000 and 250,000.

16. The composition of claim 15, wherein the graft copolymer has a weight average molecular weight of 30,000 to 50,000.

17. The composition of claim 7, wherein the polyester comprises the reaction product of butanediol, ethylene glycol, cyclohexanedicarboxylic acid, isophthalic acid, and maleic acid and/or maleic anhydride.

18. The composition of claim 8 wherein the polyester comprises the reaction product of butanediol, ethylene glycol, cyclohexanedicarboxylic acid, isophthalic acid, and maleic acid and/or maleic anhydride.

19. The composition of claim 7, wherein the acrylic copolymer comprises styrene, butyl acrylate, ethylhexyl acrylate, and methacrylic acid.

20. The composition of claim 8, wherein the acrylic copolymer comprises maleic acid and/or maleic anhydride.

21. The composition of claim 17, wherein the acrylic copolymer comprises styrene, butyl acrylate, ethylhexyl acrylate, and methacrylic acid.

22. The composition of claim 18, wherein the acrylic copolymer comprises maleic acid and/or maleic anhydride.

23. The composition of claim 1 wherein the weight ratio of a: b is from 95: 5 to 20: 80.

24. The composition of claim 1 wherein the weight ratio of a: b is 70: 30.

25. The composition of claim 1 wherein the crosslinking agent is melamine or is derived from melamine.

26. The composition of claim 1, wherein the crosslinking agent is or is derived from a resole or bisphenol a-free novolac resin.

27. The composition of claim 1, wherein the polyester comprises maleic anhydride.

28. The composition of claim 1, wherein the composition is substantially free of epoxy groups.

29. The composition of claim 7, wherein the composition is substantially free of epoxy groups.

30. A method of coating a food can comprising applying to the can a composition comprising:

(a) a polyester;

(b) an acrylic copolymer;

(c) a crosslinking agent; and

(d) a solvent, a water-soluble organic solvent,

wherein the polyester and acrylic copolymer are blended together, grafted together using standard free radical initiators, or form an interpenetrating network, but if the polyester and acrylic copolymer are blended together, the acrylic copolymer does not have pendant glycidyl groups when the polyester is acid terminated and the acrylic copolymer does not have pendant hydroxyl groups when the polyester is hydroxyl terminated.

31. A method of coating a food can comprising applying the composition of claim 7 to the can.

32. Food can having a coating on its surface, wherein the coating comprises a composition according to any one of claims 1-29.

33. Food cans having a coating on their surface obtained by a process according to claim 30 or claim 31.