EP0956321A1

EP0956321A1 - Coating composition for food containers

Info

Publication number: EP0956321A1
Application number: EP97953778A
Authority: EP
Inventors: Gerrit Jungheim; Jürgen Erwin LEMKE
Original assignee: PPG Industries Ohio Inc
Current assignee: PPG Industries Ohio Inc
Priority date: 1997-01-29
Filing date: 1997-12-10
Publication date: 1999-11-17
Also published as: KR20000070606A; DE19703091A1; AU5756098A; BR9714327A; WO1998032807A1; CA2278351A1

Abstract

The invention relates to a coating composition for food containers, which comprises: a) 20-40 % by weight of a binder polymer from the group 'water-soluble or water-dispersible polyester resin; water-soluble or water-dispersible acrylic polymer'; b) 5-20 % by weight of a cross-linking urethane resin which is virtually free from free NCO groups; c) 20-70 % by weight of water; d) 0-18 % by weight of an organic solvent; and e) 0-5 % by weight of customary auxiliaries. Here, the binder polymer has an OH number of at least 10 mg of KOH/g and the urethane resin has a content of NCO groups (blocked) of at least 5 %.

Description

Coating composition for food containers

Description

The invention relates to a coating composition for food containers, comprising a binder polymer from the group 'Water-soluble or water-dispersible polyester resin; water-soluble or water-dispersible acrylic polymer" or mixtures thereof, a crosslin ing resin, a solvent and, if desired, customary auxiliaries. Adjustment to a viscosity optimal for the processing of the coating composition can be made with the water and/or with the organic solvent. It is understood that the resulting

viscosity is also a function of the temperature of the coating composition. The binder polymer generally has a considerable number of OH groups capable of bond formation, by means of which, in conjunction with the crosslinking resin, it is possible to achieve crosslinking and, accordingly, curing.

Depending on their use, coatings produced from a coating composition are required to have particular properties. When used as an (external, for example) coating on food containers, such as cans, for example, suitability for foods per se must be accompanied by suitability for germ elimination stages in food processing, in particular for both interior and exterior can coatings. A germ elimination stage of this kind usually operates by pasteurization. In pasteurization, the contents (together with the container) are brought to temperatures in the range from 60°C to 80°C, but at any rate below 100°C, and are held at these temperatures for a certain time. By this means, microbes in the foods are damaged or killed. Pasteurization does not, however, lead to a microbe-free product but leads merely

to a product low in microbes. As a consequence of increased storage life, especially long-term storage life, however, it is increasingly becoming necessary to operate with sterilization instead of with pasteurization. In a sterilization stage, for the purpose of virtually complete killing of microbes, a can is heated at temperatures of more than 120°C, in many cases even more than 130°C, for at least several minutes, for example 30 minutes. If a coating is to be exposed not to pasteurization but to a sterilization stage, then the coating must be so designed that

even under and after the thus intensified heat treatment conditions it still meets all requirements with regard to hardness, solvent resistance and adhesion. The solvent resistance can be tested, for example, with methyl ethyl ketone, MEK for short. Finally, in the course of steam sterilization, there must be no absorption of water by the coating to a disruptive extent.

If the coating composition is intended for producing external coatings on sheet- metal packaging, then the coatings must meet the particular requirements of the manufacturing processes of sheet-metal packaging, for example on Rutherford machines. For instance, the coatings are required to withstand shaping procedures such as folding, crimping, drawing, etc. In addition they should be of high gloss, resistant to abrasion and readily printable, and should show good adhesion, a smooth surface texture (i.e. no craters) and a good humidity effect. What has been said applies especially in the case of use as a non-varnish exterior coating, i.e. without a final colourless protective coat.

Food container coating compositions of the basic composition specified at the outset, albeit based on organic solvents, are known from the art. In such compositions the resin used for curing the hydroxyl-containing binder polymer is a melamine or benzoguanamine resin. These known coating compositions have become well established in food technology, and indeed for containers including those subjected to a sterilization stage. On environmental protection grounds, and grounds of workplace safety, however, it is desirable to replace coating compositions comprising organic solvents by those comprising water as solvent. The document EP 0006336 Bl discloses a food container coating composition based on a hydroxyl-containing binder polymer which is likewise cured with an amino resin or phenolic resin, but which is aqueous. This known coating composition is suitable at best for pasteurization stages, but does not meet all requirements, especially with regard to hardness before and after sterilization stages.

The document PCT EP90/00283 discloses a food container coating composition in which the curing of the hydroxyl-containing binder polymer takes place with the aid of blocked di- or polyisocyanates. Polyisocyanate resins, however, are not employed for this purpose. Moreover, the solvent is purely organic. Consequently, this known coating composition is subject to the environmental concerns already addressed above. The document DE-A 2507884 discloses aqueous coating composition based, inter alia, on hydroxyl-containing polymers which can be cured by means of blocked polyisocyanates. Polyisocyanate resins are not employed. These known coating compositions do not meet the requirements of the food industry from a technical standpoint, especially after exposure under sterilization conditions of the coatings produced therefrom.

The documents DE 4421823 Al and EP 0358979B1 disclose aqueous coating compositions for the automotive industry which are based on hydroxyl-containing polymers that can be cured with polyisocyanates containing free NCO groups. The polyisocyanates are not resins. These coating compositions, known from a different sector of technology, can in any case not be used for food containers in terms of their processing properties, the properties of the coatings produced therefrom, and, in particular, their food suitability. The coating technology is a different one.

The invention is based on the technical problem of providing a coating composition with which it is possible to produce coatings which, even subsequent to sterilization stages, meet all requirements in terms of hardness, elasticity and water absorption behaviour but which nevertheless meet heightened environmental requirements through the use of water as solvent. To solve this problem the invention provides a coating composition for food containers, comprising a) 20-70% by weight of a binder polymer from the group 'Water-soluble or water-dispersible polyester resin; water-soluble or water- dispersible acrylic polymer" or mixtures thereof, b) 5-20% by weight of a crosslinking urethane resin which is virtually free from free NCO groups, c) 20-

70% by weight of water, d) 0-18% by weight of an organic solvent and e) 0-5% by weight of customary auxiliaries, the sum of the proportions by weight of components a) to e) being 100% by weight, the binder polymer having an OH number of at least 10 mg of KOH/g and the urethane resin having a content of NCO groups (blocked) of at least 5%. The coating composition can be completed by further additives, for example fillers. The invention is based on the surprising finding that, with the use of crosslinking urethane resins which per se are uncommon for the above-mentioned water-based binder polymers, and which are virtually free from free NCO groups, it is possible to provide a water-based coating composition which meets all requirements when the urethane resin is combined with the binder polymer in the manner indicated. This specific combination also, in particular, ensures sufficient crosslinking and thus sufficient hardness of the finished coating if the OH number and the content of NCO groups are within the stated ranges. Crosslinking urethane resins which are virtually free from free NCO groups are also referred to as blocked polyisocyanate resins.

Advantages which result from the novel combination are particular environmental friendliness and workplace safety of the novel coating composition and yet high hardness, sufficient elasticity and low water absorption. A novel coating

composition can, moreover, be applied particularly well with the Rutherford

machines which are often employed in the packaging industry. In this connection it is advantageous that a novel coating composition produces a coating which meets all of the requirements despite the very short baking times in this case, for example 30 s (at a maximum article temperature of about 190°C).

A particularly important advantage, however, is that in conjunction with the above positive properties a coating produced from a novel coating composition, even after exposure to the severe conditions of sterilization stages, meets all requirements in terms of hardness, solvent resistance and adhesion (both to the

substrate and to any topcoat that might be applied). Thus with the invention a coating composition is provided for which coatings produced therefrom have the required technical properties even in the case of more stringent hygiene regulations.

The literature reference DE-A-4209248 does indeed disclose mixing a binder polymer with a crosslinking urethane resin which is virtually free from free NCO groups, the coating composition being obtained in aqueous solution or dispersion. In this prior art, however, the binder polymer is a very specific graft copolymer on

epoxy resins and/or phenoxy resins, the amount of the urethane resin employed, moreover, compared to the amount of the binder polymer, being very small as a result of the specific structure of the graft copolymer. A disadvantage of this prior art, in addition to its poorer suitability for sterilization stages relative to the

invention, is the complex and expensive preparation of the binder polymer.

A novel coating composition preferably contains 25-35% by weight of the binder polymer, based on the solids content of the binder, 8-15% of urethane resin, 61- 34% of water, 1-15% by weight, preferably 5-15% by weight, of an organic solvent and 0.3-1% by weight of other auxiliaries.

In particular, the novel coating composition can be designed in various ways in terms of the binder polymer. On the one hand, the polyester resin used can be an acrylic-modified, saturated polyester resin whose preparation is conducted with the proviso that the polyester resin has an OH number of 30 - 120 mg of KOH/g, preferably of 35 - 45 mg of KOH/g, and an acid number of 40 - 80 mg of KOH/g, preferably of 45 - 55 mg of KOH/g. It is advantageous if the polyester resin has a viscosity of 500 - 900 mPa.s, measured at 23°C and 50% by weight in butyl glycol as solvent. Examples of suitable polyester resins are those from the group 'hydroxyl-containing polyesters, hydroxyl-containing acrylic-modified polyesters, hydroxyl-containing epoxy-modified polyesters" or mixtures thereof. They can also be employed, if desired, in combination with epoxy resins and/or hydroxyl- containing acrylate polymers or copolymers. The polyester resins are prepared, for example, by customary techniques of polyesterification reactions, by esterifying 9

aromatic and/or aliphatic dicarboxylic acids, dicarboxylic anhydrides, tricarboxylic anhydrides and/or tetracarboxylic mono- and dianhydrides with aliphatic, cyclo- aliphatic and/or aromatic mono-, di- and/or polyols. Examples of carboxylic acids suitable for the synthesis of the polyesters are phthalic, isophthalic, terephthalic, tetrahydrophthalic and hexahydrophthalic acids, dimethyl terephalate, trimellitic acid, trimellitic anhydride, adipic, azelaic, sebacic, maleic and glutaric acids and also dimeric and/or trimeric fatty acids. Examples of alcohol components which can be employed are aliphatic monools having 4 to 20 C atoms, 2,2-dimethyl-l,3- propanediol, ethylene glycol, diethylene glycol, 1,2- and 1,3-propylene glycol, butanediols, pentanediols, neopentyl glycol, hexane-diols, 2-methyl-l,5-pentane- diol, 2-ethyl-l,4-butane-diol, dimethylolcyclohexane, glycerol, trimethylol-ethane, trimethylolpropane, trimethylolbutane, pentaerythritol, dipentaerythritol, polycaprolactone-diols and -triols, and bisphenol A. Alternatively, it is also possible to employ more complex polyesters based, for example, on the reaction of epoxy resins with bisphenol A and on further reaction with acrylic acid, styrene and butyl acrylate, with or without an initiator such as tert-butyl perbenzoate, for example. On the other hand, the acrylic polymer employed can be one obtainable by using methacrylic acid and ethyl acrylate as starting materials, and styrene for copolymeriza-tion, and carrying out polymerization in the presence of butyl glycol and butanol. In general it is advantageous if the acrylic polymer has an OH number of 10 - 100 mg of KOH/g, preferably of 25 - 35 mg of KOH/g, and a neutralization number of 40 - 100 mg of KOH/g, preferably of 65 - 75 mg of KOH/g. Acrylic polymers are essentially copolymers of acrylic esters with carboxyl-containing monomers. Examples of acrylic esters which can be used are esters of acrylic or methacrylic acid with methanol, ethanol, the butanols or ethylhexanol, or mixtures thereof, and esters with higher alcohols (having for example up to 20 C atoms). Examples of carboxyl-containing monomers which can be used are acrylic, methacrylic, crotonic, maleic or fumaric acid or mixtures thereof. Polymerization can also be carried out with further hydroxyl-containing compounds, such as esters of acrylic or methacrylic acid with ethanediol, propanediol or butanediol or reaction products of these acids with monoepoxide compounds or mixtures thereof. Examples of further suitable comonomers are styrene, methylstyrene, vinyltoluene, acrylamide, methacrylamide and the non-etherified or etherified N- methylol compounds thereof, or mixtures of these. Preference is given to the use of acrylic acid, styrene, ethyl acrylate, 2-hydroxyethyl methacrylate and butyl diglycol. Polymerization can be carried out using customary peroxy compounds as initiators, an example being tertiary-butyl peroxybenzoate.

Organic solvents employed are, for example,. aliphatic, cycloaliphatic and aromatic hydrocarbons, esters, ethers and ketones, examples being butanol, xylene, various

white spirits, tetralin, decalin, solvent naphtha, various Solvesso® grades, various

Shellsol® grades, butyl glycol, ethylene glycol dibutyl ether, ethylene glycol diethyl

ether, ethylene glycol dimethyl ether, methyl ethyl ketone, methyl n-amyl ketone, diethyl ketone, ethyl butyl ketone, diisopropyl ketone, diisobutyl ketone, Ό acetylacetone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, amyl

acetate, methyl glycol acetate, ethyl glycol acetate and butyl diglycol acetate.

As the crosslinking urethane resin it is possible to employ blocked polyisocyanates based, for example, on polymerization products of monomeric diisocyanates having uretdione and/or biuret and/or isocyanurate and/or allophanate groups. As blocked polyisocyanates it is possible to use any desired polyisocyanates in which the isocyanate groups have been reacted with a compound so that the blocked polyisocyanate formed is resistant at room temperature to hydroxyl and amino groups but reacts at elevated temperatures, generally in the range from about 80°C to about 300°C. In preparing the blocked polyisocyanates it is possible to use any organic polyisocyanates which are suitable for crosslinking. Preference is given to the isocyanates which contain about 3 to 38, in particular about 8 to 15, carbon atoms. Examples of suitable diisocyanates are hexamethylene diisocyanate, 2,4- tolylene diisocyanate, 2,6-tolylene diisocyanate and l-isocyanatomethyl-5- isocyanato-l,3,3-trimethylcyclohexane. Polyisocyanates of higher isocyanate functionality can also be used. Examples of these are trimerized hexamethylene diisocyanate and trimerized isophorone diisocyanate. Mixtures of polyisocyanates can also be utilized. The organic polyisocyanates suitable as crosslinking agents in the context of the invention can also be prepolymers derived, for example, from a polyol, including a polyetherpolyol or a polyesterpolyol. U

For blocking of the polyisocycanates it is possible to use any suitable aliphatic, cycloaliphatic or aromatic alkyl monoalcohols. Examples of these are aliphatic

alcohols such as methyl, ethyl, chloroethyl, propyl, butyl, amyl, hexyl, octyl, nonyl, 3,3,5-trimethylhexyl, decyl and lauryl alcohols; cycloaliphatic alcohols such as cyclopentanol and cyclohexanol; and aromatic alkyl alcohols, such as phenyl carbinol and methylphenyl carbinol.

Other suitable blocking agents are hydroxy amines, such as ethanolamine, oximes, such as methyl ethyl ketone oxime and acetone oxime, and aliphatic diamines,such as dibutylamine and diisopropylamine. In suitable proportions, these polyisocyanates and blocking agents can also be used to prepare the partially blocked polyisocyanates mentioned above.

In a further embodiment of the invention a portion of the urethane resin, preferably 30-70% of the amount of urethane resin employed, is replaced by amino resins, in particular a highly methylated melamine resin.

Typical suitable amino resins are melamine, benzoguanamine and urea- formaldehyde resins. They are preferably used in a form etherified with lower alcohols, mostly methanol and/or butanol. Suitable amino resins are available commercially under the tradenames Cymel, Luwipal, Maprenal and Beetle, for example. Hexamethoxymethylmelamine, for example, is a suitable amino resin. / 2_

In principle the novel coating composition can be formulated as a colourless, i.e. as

a clearcoat, or as a coloured coating material. In the latter case a customary pigment is added to the coating composition, for example TiO₂ for a white coating material. A customary lubricant may be advisable as a further additive. Pigments and additives of this kind are referred to as auxiliaries.

In addition it is of course also possible to employ other pigments, examples being iron oxides, colorants on an inorganic or organic basis, bentonite, carbon black,

etc.

Finally, other customary auxiliaries can be added to the coating composition, examples being antifoams, stabilizers, plasticizers, light protection additives, etc.

The invention also relates to the use of a novel coating composition according to Claim 8, to a process for preparing a novel coating composition according to Claim 9, and to a sterilizable can according to Claim 10. The can can be designed, in particular, as a two-piece can and/or may consist of sheet aluminium. Use for steel cans is likewise possible. In the text below the invention is illustrated on the basis of coating compositions which represent merely exemplary embodiments.

Example 1. A novel coating composition was prepared from the following components: 27.96% by weight of an acrylic-modified saturated polyester resin, 9.32% by weight of butyl diglycol, 12.79% by weight of a crosslinking urethane resin which is virtually free from free NCO groups, 1.60% by weight of DMEA (dimethylethanolamine), 48.03%> by weight of demineralized water and 0.30% by weight of an auxiliary with the designation Additol XW 329 (silicone-containing coatings auxiliary, 50% strength in butyl glycol). The polyester resin employed has an OH number of about 40 mg of KOH/g and a neutralization number of about 50 mg of KOH/g and also a viscosity of about 700 mPa.s (the viscosity measured at 23°C and 50% by weight in butyl glycol as solvent) and is obtainable under the

proprietary name Uradil® SZ250 G7G3-80. The above-mentioned amount of the

polyester resin relates to an 80% strength solution of the resin in ethoxypropanol/butyl glycol. This polyester resin is readily soluble in water. The urethane resin employed is a urethane baking resin based on hexamethylene

diisocyanate and is obtainable under the proprietary name Desmodur® BL 3175.

This urethane resin is per se readily soluble only in organic solvents. It has an equivalent weight of about 378 and an NCO content of about 11.1% (blocked). The viscosity at 23°C and in accordance with DIN 53019/1 (75% strength in solvent naphtha 100) is about 3250 mPa.s. It is self-evident that the data given above and in the subsequent examples for the materials employed can be varied, it being possible for the variations in the numerical parameters to be up to 50% or more, provided the fundamental structural properties indicated are unchanged. Comparative experiments revealed that a coating of the above coating composition

applied to an aluminium panel (an aluminium alloy customary in food technology), after curing, has the following improved properties relative to a customary

polyester/amino based coating (acrylate-modified, water-dilutable, saturated polyester resin/benzoguanamine resin): the hardness of the coating at 25°C after a sterilization stage at 129°C for 30 minutes was more than 4 H (film hardness in units of the hardness of customary commercial pencils) and was reduced virtually not at all relative to the initial hardness (customary coating: slight reduction by about 1 hardness unit). The hardness of the coating in hot water at 80°C after a sterilization stage at 129°C for 30 minutes was more than 2 H and was reduced relative to the initial hardness by only about 2 hardness units (customary coating: reduction by more than 6 hardness units). The solvent resistance (indicated as double strokes of a cotton wool pad soaked with MEK) after a sterilization stage at 129°C for 30 minutes was up to 60 DS (customary coating: about 6 DS).

Example 2.

The polyester resin and also the urethane resin from Example 1 were employed. The proportion of the polyester resin was 32.40% by weight and that of the urethane resin 14.83% by weight. Additionally employed were: 0.40% by weight of Additol XW 329 (50% strength in butyl glycol), 7.62% by weight of butyl /S glycol, 8.51% by weight of butanol, 1.87% by weight of DMEA and 0.49% by

weight of BYK®-020 10% (antifoam for water-soluble systems, based on a

modified polysiloxane copolymer). These components (as in the other examples)

were introduced into a mixing reactor and stirred until sufficient homogeneity was reached. Then 33.88% by weight of demineralized water was added with stirring. After stirring for 30 minutes, the product obtained was filtered through a filter having a mesh size of 10 microns. The product had a binder content of 37.04% by weight and a solids content of 37.29% by weight (0.25% by weight of solids from the additive). The proportion of water was 33.88% by weight, while the proportion of other solvents was likewise 33.88% by weight. The viscosity in accordance with DIN 425, GR.C, was 97 s. This product had further improved properties relative to Example 1.

Example 3.

30.856% by weight of the polyester resin and 14.121%) by weight of the urethane resin from Example 1 were employed. Additionally employed were 0.377% by

weight of the auxiliary from Example 1, 4.762% by weight of Estol® 1447

(polyethylene glycol 400 dioleate), 0.472% by weight of Byk® 020, 7.258% by

weight of butyl glycol, 8.108% by weight of butanol, 1.784% by weight of DMEA and 32.262% by weight of demineralized water. The resulting coating composition had a solids content of about 40%. Coatings produced from this coating composition (6.5 g/m², otherwise as in Example 1) showed a slight improvement in

hardness relative to Example 1 while at the same time having greatly improved solvent resistance (MEK more than 80 after sterilization stage in accordance with Example 1).

Example 4.

53.887% by weight were employed of an acrylic polymer which is obtainable in the following way. 189.25 g of acrylic acid, 578.3 g of styrene, 1060.1 g of ethyl acrylate, 136.8 g of 2-hydroxyethyl methacrylate and 40 g of butyl glycol are stirred in a reactor to form a first mixture. Separately, a second mixture of 600.1 g of butyl glycol and 229.75 g of butyl diglycol, and a third mixture of 58.95 g of Trigonox C (tertiary-butyl peroxybenzoate) and 20.0 g of butyl glycol, are prepared. The third mixture is then added to the first mixture. 10% of the resulting first mixture are then added to the second mixture in a reactor. The temperature rises as a result of exothermic reaction and should be maintained within the range from 120°C to 129°C. The remainder of the first mixture is then metered into the contents of the reactor over 3 h at from 129°C to 131°C. 80 g of butyl glycol are added as the rinsing medium. Then the temperature is maintained at from 129°C to

131°C for 0.5h, and subsequently 3.95 g of Trigonox® C and 10 g of butyl glycol

are added. The contents of the reactor are then held at from 129°C to 13 FC for a further 2 h. Finally, the contents of the reactor are cooled to 70°C, and 247.75 g DMEA mixed with 203.7 g of deminera 7lized water are added over the course of 0.5 h. At the end, 1541.35 g of demineralized water are added and the resulting acrylic polymer solution is stirred until homogeneous.

In addition to the above acrylic polymer the following substances are employed: 13.624%) by weight of the urethane resin from Example 1, 0.366% by weight of the auxiliary from Example 1, 6.446% by weight of Butyl Carbitol, 3.617% by weight of a substance A, 4.692%> by weight of a substance B and 16.899% by weight of demineralized water.

The substance A is a lubricant based on polyethylene glycol dioleate and is

obtainable under the proprietary name Priolube® 1447. Substance B is obtainable

as follows. 56.016 kg of an epoxy resin with the proprietary name Araldit® GY

2600 are made up into a batch with 7.944 kg of butyl glycol - 2-butoxyethanol - and 8.613 kg of butanol. 3.655 kg of phosphoric acid 85% CZ FG (food grade) in 1.345 kg of butanol are added. There are then added in succession: 1.0 kg of butanol, 1.36 kg of demineralized water, 0.68 kg of demineralized water, 0.68 kg of demineralized water, 16.707 kg of butyl glycol - 2-butoxyethanol - and 2.0 kg of butyl glycol - 2-butoxyethanol. Finally, 3.7% by weight of DMEA and 22.23% by weight of demineralized water are added to 74.07% by weight of the resulting product and the mixture is stirred until homogeneous. *S>

Variations of the acrylic polymer in terms of the OH number of 30 mg of KOH/g, in accordance with the above acrylic polymer, to more than 50 and from 75 to 100 mg of KOH/g showed that an increasing OH number leads to a rise in the viscosity of the coating composition, to a reduction in the hardness of the coating and to a reduction in the solvent resistance of the coating. From this it is evident that the OH number established should also not be too high.

A coating produced with the above coating composition was subjected to the following test. An autoclave was preheated to 95°C. Then a metal panel bearing the coating was introduced into the autoclave and subjected to 1 1 of water (23°C). The autoclave was then run up to 125°C, where it was held for 30 minutes. The metal panel was then removed and the coating was assessed. Only very slight changes were evident in limited regions, and, moreover, these changes had completely disappeared again after 1 h. The coating meets all requirements with respect to hardness and solvent resistance as well.

Example 5

The following were employed: 57.4% by weight of the acrylate resin from Example 4, (OH number: 30 mg of KOH/g; 41%), 13.6% by weight of urethane resin from Example 1, 7.8% by weight of butyl diglycol, 18.0% by weight of

distilled water, 2.3% by weight of Priolube® 1447, 0.4% by weight of Additol®

XW 329 and 0.5% by weight of Byk® 020.

Claims

Patent Claims

1) Coating composition for food containers, comprising

a) 20-70% by weight of a binder polymer from the group 'Water-soluble or water-dispersible polyester resin; water-soluble or water-dispersible acrylic polymer" or mixtures thereof,

b) 5-20% by weight of a crosslinking urethane resin which is virtually free

from free NCO groups,

c) 20-70% by weight of water,

d) 0-18% by weight of an organic solvent and

e) 0-5% by weight of customary auxiliaries,

the sum of the proportions by weight of components a) to e) being 100% by weight, the binder polymer having an OH number of at least 10 mg of ?.l

KOH/g and the urethane resin having a content of NCO groups (blocked)

of at least 5%.

2) Coating composition according to Claim 1, where an acrylic-modified, saturated polyester resin is used as polyester resin and where its polymerization is carried out with the proviso that the polyester resin has an OH number of 30 - 120 mg of KOH/g and a neutralization number of 40 - 80 mg of KOH/g.

3) Coating composition according to Claim 1 or 2, where the polyester resin has a viscosity of 500 - 900 mPa.s, measured at 23°C and 50% by weight in butyl glycol as solvent.

4) Coating composition according to Claim 1, where the acrylic polymer is obtainable by employing methacrylic acid and ethyl acrylate as starting materials, and styrene for copolymerization, and carrying out polymerization in the presence of butyl glycol and butanol, and where the acrylic polymer has an OH number of 10 - 100 mg of KOH/g and a neutralization number of 40 - 100 mg of KOH/g. £ 2- 5) Coating composition according to one of Claims 1 to 4, where a portion of the urethane resin, preferably 30-70% of the amount of urethane resin employed, is replaced by a highly methylated melamine resin.

6) Coating composition according to one of Claims 1 to 5, where a pigment has been added.

7) Coating composition according to one of Claims 1 to 6, where a lubricant has been added.

8) Use of a coating composition according to one of Claims 1 to 7 as a coating composition for producing a coating, preferably an exterior coating, on sterilizable cans, especially metal cans.

9) Process for preparing a coating composition according to one of Claims 1 to 7, where

a) 20-40% by weight of a binder polymer from the group 'Water-soluble or water-dispersible polyester resin; water-soluble or water-dispersible acrylic polymer", b) 5-20%) by weight of a crosslinking urethane resin which is virtually free from free NCO groups,

c) 20-70% by weight of water,

d) 0-18% by weight of an organic solvent and

e) 0-5% by weight of customary auxiliaries,

are mixed and homogenized.

10) Sterilizable can, where at least the external surfaces of the can have been provided with a cured coating of a coating composition according to one of Claims 1 to 7.