KR20000070606A - Coating composition for food containers - Google Patents

Abstract

The present invention relates to a coating composition for food containers, the coating composition (a) 20 to 40 weight binder polymer selected from the group consisting of "water-soluble or water-dispersible polyester resin and water-soluble or water-dispersible acrylic polymer" %, (b) 5 to 20% by weight crosslinked urethane resin substantially free of free NCO groups, (c) 20 to 70% by weight water, (d) 0 to 18% by weight organic solvent, and (e) conventional adjuvant 0 To 5% by weight. At this time, the OH value of the binder polymer is 10 mg KOH / g or more and the content of the NCO group (blocked) of the urethane resin is 5% or more.

Description

Coating composition for food containers

Coatings made from coating compositions require specific properties depending on their use. When used as (eg external) coatings of food containers (eg cans), for example, food compatibility itself relates to the suitability of microbial removal steps in food processing (especially for coatings of internal and external cans). ). This kind of microbial removal step is generally carried out by pasteurization. In Pasteurization, the contents (along with the vessel) are carried out at temperatures of 60 ° C. to 80 ° C., but at some rates at temperatures below 100 ° C. and must be maintained at this temperature for a certain time. By this method, bacteria in food are damaged or sterilized. Pasteurization, however, cannot produce products that are germ-free; they only produce products that are germ-free. Therefore, as the storage period increases, especially with long term storage, it is necessary to carry out sterilization instead of pasteurization. In the sterilization step, the can is heated to a temperature higher than 120 ° C., in many cases higher than 130 ° C., for at least several minutes, for example at least 30 minutes, since the aim is to kill the bacteria substantially completely. If the coating is subjected to the pasteurization as well as the sterilization step, the coating should be designed to meet the requirements relating to hardness, solvent resistance and wearability before and after these concentrated heat treatment conditions. Solvent resistance can be tested simply with methyl ethyl ketone, MEK, for example. Finally, in the case of steam sterilization, the coating must not absorb the water to form decomposition content.

If the coating composition forms an outer coating on the sheet metal packaging, the coating must meet certain requirements for the fabrication and processing of the sheet-metal packaging, for example, in a Rutherford machine. For example, the coating must withstand molding operations such as folding, crimping, drawing, and the like. In addition, they must be good gloss, resistant to abrasion, easily printable, have good adhesion, smooth surface texture (ie, no craters) and good humidity effects. The above requirements apply in particular when used as a matte-free outer coating (ie without a final colorless protective coating).

Food container coating compositions of the base compositions based on organic solvents but specified in the disclosure are known in the art. In such compositions, the resin used to cure the hydroxyl-containing binder polymer is a melamine or benzoguanamine resin. Such known coating compositions are well known in food engineering and are also for containers comprising a container subjected to a sterilization step. However, from the viewpoint of environmental protection and workplace safety, it is preferable to replace the coating composition including the organic solvent as the solvent with the one containing water as the solvent. European Patent Publication No. EP 0006336 B1 discloses food container coating compositions based on hydroxyl-containing binder polymers cured with amino resins or phenolic resins and which are aqueous. Such known coating compositions are most suitable for the Pasteur process, but do not meet all requirements, especially hardness, before and after the sterilization step.

International Patent Publication No. PCT / EP90 / 00283 discloses a food container coating composition wherein the hydroxyl-containing binder polymer is cured with the aid of blocked di- or polyisocyanates. However, polyisocyanate resins are not used for this purpose. In addition, the solvent is a purely organic solvent. As a result, such known coating compositions have environmental concerns as already described above. German Patent Publication No. 2507884 discloses an aqueous coating composition based on hydroxyl-containing polymers, which can be cured by blocked polyisocyanates. Polyisocyanate resins are not used. Such known coating compositions do not meet the requirements of the food industry from a technical standpoint, in particular after exposure of the coatings produced therefrom under sterilization conditions.

German Patent Publication No. DE 4421823 A1 and European Patent Publication No. EP 0358979 B1 disclose aqueous coating compositions for the automotive industry based on hydroxyl-containing polymers which can be cured with polyisocyanates containing free NCO groups. Is starting. Polyisocyanates are not resins. Such coating compositions known in technically different fields cannot in any case be used as food containers in view of the processing properties, the properties of the coatings produced therefrom, and especially their food compatibility. This is because the coating technique is different.

The invention relates to a food container comprising a binder polymer, a crosslinking resin, a solvent and, if desired, conventional auxiliaries selected from the group consisting of "water soluble or water dispersible polyester resins; water soluble or water dispersible acrylic polymers" or mixtures thereof. It relates to the coating composition for. It can be adjusted to the optimum viscosity for the processing of the coating composition with water and / or organic solvent. It will be appreciated that the viscosity formed is also a function of the temperature of the coating composition. The binder polymer generally contains a significant number of OH groups capable of forming a bond, which can be crosslinked and cured with the crosslinking resin.

The present invention can be applied to the sterilization step and can be produced in a manner that meets all the requirements for hardness, elasticity and water-absorption mode while still being able to produce a coating which more closely meets the environmental requirements by using water as the solvent. It is based on the technical challenge of providing a replication composition.

In order to solve this problem, the present invention provides a binder polymer selected from the group consisting of (a) "a water-soluble or water-dispersible polyester resin; a water-soluble or water-dispersible acrylic polymer" and mixtures thereof, 20 to 70% by weight, (b 5 to 20% by weight of crosslinked urethane resin substantially free of free NCO groups, (c) 20 to 70% by weight of water, (d) 0 to 18% by weight of organic solvent and (e) 0 to 5% by weight of conventional auxiliaries. A coating composition comprising: wherein the sum of the weight ratios of components (a) to (e) is 100% by weight, the OH value of the binder polymer is at least 10 mg of KOH / g, and (blocking) NCO in the urethane resin The content of groups is at least 5%. The coating composition is completed by adding additional additives, for example fillers. The present invention provides a waterborne coating that meets all requirements when the urethane resin is bonded with the binder polymer in the above-described manner using a crosslinked urethane resin that is not common as the above-described aqueous binder polymer but is substantially free of free NCO groups. It is based on the unexpected finding that it is possible to provide a composition. This particular combination also ensures particularly sufficient crosslinking and ensures sufficient hardness of the final film even when the OH number and the content of NCO groups are in the aforementioned ranges. Crosslinkable urethane resins substantially free of free NCO groups are referred to as blocked polyisocyanate resins.

Advantages resulting from this novel combination include desirable environmental affinity and workplace stability and high hardness, sufficient elasticity and low moisture absorption of the novel coating compositions. The novel coating compositions are particularly applicable to loader-pod groups which are often used in the packaging industry. In this regard, the novel coating compositions produce a coating that meets all requirements despite the very short baking time in this case, for example 30 seconds (the maximum temperature of the product is about 190 ° C.). Has an advantage.

A particularly important advantage, however, is the combination of hardness, solvent resistance and adhesion (both substrates and any topcoats applied), even after exposure to the harsh conditions of the sterilization step, with coatings prepared from the novel coating compositions with the positive properties described above. Adherence to all requirements). Therefore, this invention provides the coating composition which can manufacture the film which has the technical characteristic calculated | required also in case of strict sanitary regulation.

Published German Patent Publication No. 4209248 discloses mixing a crosslinked urethane resin and a binder polymer substantially free of free NCO groups, wherein the coating composition is obtained in an aqueous solution or an aqueous dispersion. However, in this prior art the binder polymer is a copolymer specifically grafted onto the epoxy resin and / or phenoxy resin, wherein the amount of urethane resin used is dependent on the amount of binder polymer as a result of the specific structure of the graft copolymer. Very small amount in comparison. A disadvantage of this prior art is that not only is the suitability for the sterilization step in comparison with the present invention, but also the process of preparing the binder polymer is complicated and expensive.

The novel coating composition is preferably 25 to 35% by weight binder polymer, 8 to 15% by weight urethane resin, 61 to 34% by weight water, 1 to 15% by weight organic solvent, preferably based on the content of solids in the binder. 5 to 15 weight percent, and 0.3 to 1 weight percent of other adjuvants.

In particular, the novel coating compositions can be designed in a variety of ways in terms of binder polymers. On the other hand, the polyester resin used is an acrylic modified saturated polyester resin, which has an OH value of 30 to 120 mg KOH / g, preferably 35 to 45 mg KOH / g and an acid value of 40 to 80 mg KOH. / g, preferably 45-55 mg KOH / g. It is preferable that the viscosity of a polyester resin is 500-900 mPa.s when it measures at 23 degreeC with a density | concentration of 50% using butyl glycol as a solvent. Examples of suitable polyester resins will be selected from the group consisting of "hydroxyl-containing polyesters, hydroxyl-containing acrylic-modified polyesters, hydroxyl-containing epoxy-modified polyesters" or mixtures thereof. In addition, they may be used with epoxy resins and / or hydroxyl-containing acrylate polymers or copolymers, if desired. For example, polyester resins can be prepared by conventional techniques of polyesterification reactions, with aromatic and / or aliphatic dicarboxylic acids, dicarboxylic anhydrides, tricarboxylic anhydrides and / or aliphatic, cycloaliphatic and / or aromatic mono-, di- And / or esterification of mono- and dihydrides of tetracarboxylic acids with polyols. Examples of suitable carboxylic acids for polyester synthesis include phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid and hexahydrophthalic acid, dimethyl terephthalate, trimellitic acid, trimellitic anhydride, adipic acid, azelaic acid, sebacic acid, maleic acid, Glutaric acid, dimer and / or trimer fatty acids. Examples of alcohol components that may be used include aliphatic monools having 4 to 20 carbon atoms, 2,2-dimethyl-1,3-propanediol, ethylene glycol, diethylene glycol, 1,2- and 1,3-propylene glycol, butanediol , Pentanediol, neopentyl glycol, hexane-diol, 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butane-diol, dimethylolcyclohexane, glycerol, trimethylol-ethane, trimethylolpropane , Trimethylolbutane, pentaerythritol, dipentaerythritol, polycaprolactone-diol and -triol, and bisphenol A are mentioned. Or more complex polyesters based on reacting the epoxy resin with bisphenol A and further reacting with acrylic acid, styrene and butyl acrylate with or without an initiator such as tert-butyl perbenzoate It is also possible to use. On the other hand, the acrylic acid polymers used can be obtained by using methacrylic acid and ethyl acrylate as starting materials and carrying out the polymerization in the presence of butyl glycol and butanol, using styrene for copolymerization. In general, the OH value of the acrylic polymer is 10 to 100 mg KOH / g, preferably 25 to 35 mg KOH / g, and the degree of polymerization is 40 to 100 mg KOH / g, preferably 65 to 75 mg KOH / g. It is preferable. Acrylic polymers are essentially copolymers of carboxyl group-containing monomers and acrylic acid esters. Examples of acrylic esters that may be used are esters of acrylic or methacrylic acid with methanol, ethanol, butanol or ethylhexanol, or mixtures thereof, and esters with higher molecular weight (eg, 20 or more carbon atoms) alcohols. . Examples of carboxyl group-containing monomers that can be used are acrylic acid, methacrylic acid, crotonic acid, maleic acid or fumaric acid or mixtures thereof. In addition, polymerization may be further carried out using a reaction product of a hydroxyl-containing compound such as ethanediol, propanediol or butanediol with an ester or monoepoxide compound of acrylic or methacrylic acid or a mixture thereof and the acid. Can be done. Further suitable comonomers include styrene, methylstyrene, vinyltoluene, acrylamide, methacrylamide, and their unetherified or etherified N-methylol compounds or mixtures thereof. Preference is given to using acrylic acid, styrene, ethyl acrylate, 2-hydroxyethyl methacrylate and butyl diglycol. Polymerization can be carried out using conventional peroxy compounds as initiators, examples of which include tert-butyl peroxybenzoate.

Organic solvents used include, for example, aliphatic, cycloaliphatic and aromatic hydrocarbons, esters, ethers and ketones, for example 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, acetylacetone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, amyl acetate, methyl glycol acetate, ethyl glycol acetate and butyl diglycol acetate Can be mentioned.

As crosslinked urethane resins, for example, blocked polyisocyanates based on the product of the polymerization of monomeric diisocyanates having uretdione and / or biuret and / or isocyanurate and / or allophanate groups are used. It is possible to do As blocked polyisocyanates, the formed blocked polyisocyanates are resistant to hydroxyl and amino groups at room temperature, but any preferred polys in which isocyanate groups react with any compound to react at high temperatures (generally from about 80 to about 300 ° C.). It is possible to use isocyanates. In preparing blocked polyisocyanates, it is possible to use any organic polyisocyanate suitable for crosslinking. Preference is given to isocyanates containing from about 3 to 38 carbon atoms, in particular about 8 to 15 carbon atoms. Examples of suitable diisocyanates include hexamethylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate and 1-isocyanatomethyl-5-isocyanato-1,3,3-trimethylcyclo Hexane can be mentioned. Polyisocyanates of high molecular weight isocyanate functional groups can also be used. Examples thereof include trimmerized hexamethylene diisocyanate and trimerized isophorone diisocyanate. It is also possible to use mixtures of polyisocyanates. Organic polyisocyanates suitable as crosslinkers in the scope of the present invention may also be prepolymers derived for example from polyols such as polyetherpolyols or polyesterpolyols.

In blocking of polyisocyanates, preference is given to using any suitable aliphatic, cycloaliphatic or aromatic alkyl monoalcohols. Examples thereof include aliphatic alcohols such as methyl alcohol, ethyl alcohol, chloroethyl alcohol, propyl alcohol, butyl alcohol, amyl alcohol, hexyl alcohol, octyl alcohol, nonyl alcohol, 3,3,5-trimethylhexyl alcohol, decyl alcohol And lauryl alcohol; Alicyclic alcohols such as cyclopentanol and cyclohexanol; And aromatic alkyl alcohols such as phenylcarbinol and methylphenyl carbinol.

Other suitable blocking agents include hydroxy amines such as ethanolamine, oximes such as methyl ethyl ketone oxime and acetone oxime, and aliphatic diamines such as dibutylamine and diisopropylamine. In order to produce the partially blocked polyisocyanates described above, the polyisocyanates and blocking agents can be used in suitable amounts.

In a further aspect of the invention, a portion of the urethane resin, preferably 30 to 70% of the urethane resin used, is replaced with amino resins, in particular highly methylated melamine resins.

Typically suitable amino resins include melamine, benzoguanamine and ureaformaldehyde resins. They are preferably used in esterified form with lower alcohols, usually methanol and / or butanol. Suitable amino resins are commercially available, for example, under the trade names Cymel, Luwipal, Maprenal and Beetle. For example, hexamethoxymethylmelamine is a suitable amino resin.

In particular, the novel coating compositions can be formulated as colorless (ie transparent coatings) or as colored coating materials. In the latter case, conventional pigments can be added to the coating composition, for example TiO ₂ for the white coating material. Conventional lubricants are advisable as further additives. Pigments and additives of this kind are referred to as auxiliary.

In addition, it is also possible to use other pigments, of course, and examples thereof include iron oxide, colorants based on inorganic or organic substrates, bentonite, carbon black and the like.

Finally, other conventional auxiliaries may be added to the coating composition, such as antifoaming agents, stabilizers, plasticizers, light blocking additives and the like.

The present invention also relates to the use of a novel coating composition for producing a coating of a sterilizable can, preferably a metal can, preferably an outer coating, a novel coating consisting of mixing and homogenizing the respective components of the composition. A method of making a composition and a sterilizable can comprising at least some cans of the film cured with the novel coating composition. In particular, the can may be designed as a two-member can and / or may consist of aluminum sheets. In addition, steel cans are also available. In the following the invention is illustrated based on the coating composition, which merely represents an exemplary embodiment.

Example 1

The novel composition comprises 27.96 weight percent acrylic-modified saturated polyester resin, 9.32 weight percent butyl diglycol, 12.79 weight percent crosslinked urethane resin (substantially free of free NCO groups), 1.60 weight percent DMEA (dimethylethanolamine), It was prepared from a component of 0.30% by weight of an adjuvant comprising 48.03% deionized water and Additol XW 329 (silicone-containing coating adjuvant, at a concentration of 50% with butyl glycol as a solvent). The OH value of the polyester resin used was about 40 mg KOH / g, the degree of neutralization was 50 mg KOH / g, and the viscosity was about 700 mPa.s (at 23 ° C. at a concentration of 50% by weight using butyl glycol as solvent). Measured) and is available as Uradil (R) SZ250 G7G3-80. The foregoing amounts of polyester resins relate to solutions wherein the concentration of the resin in the epoxypropanol / butyl glycol is 80%. Polyester resins are readily soluble in water. The urethane resin used is a urethane baking resin based on hexamethylene diisocyanate and is commercially available from Desmodur® BL 3175. Such urethane resins are easily dissolved only in organic solvents. Its monogram weight is about 378 and the content of NCO is about 11.1% (blocked). The viscosity measured by DIN 53019/1 (concentration of solvent naphtha 100 at 75%) at 23 ° C. is about 3250 mPa · s. It is obvious that the above-described results and the results of the following examples for the materials used may vary, and numerical parameters may vary by less than or equal to 50% as long as the basic structural properties mentioned do not change.

A comparative example is that after coating the coating composition to an aluminum panel (aluminum alloy conventional in food technology), the cured coating is a conventional polyester / amino based coating (acrylate-modified, dilutable with water, It shows superior properties compared to saturated polyester resin / benzoguanamine resin, and the hardness of the film at 25 ° C. after the sterilization step at 129 ° C. for 30 minutes is 4H (in terms of hardness of conventional commercial pencils). Larger than the indicated film hardness) and hardly decreased substantially compared to the initial hardness (conventional coating decreased by about 1 hardness unit). The hardness of the film in hot water at 80 ° C. after the sterilization step of 129 ° C. for 30 minutes is greater than 2H and decreased by about 2 hardness units relative to the initial hardness (typically the film is reduced by more than 6 hardness units). The solvent resistance after 30 minutes of sterilization at 129 ° C. (the solvent resistance is indicated by the double stroke of cotton wool immersed in MEK) is 60 DS or less (the conventional coating is about 6 DS).

Example 2

Polyester resins and also urethane resins from Example 1 were used. The content of polyester resin was 32.40 wt% and the content of urethane resin was 14.83 wt%. Additionally, 0.40% by weight of Additol XW 329 (concentration 50% with butyl glycol as solvent), 7.62% by weight of butyl glycol, 8.51% by weight of butanol, 1.87% by weight of DMEA and BYK®-020 0.49% by weight (10% antifoaming agent for water-soluble systems based on the weight of the modified polysiloxane copolymer) is used. These components (also in other examples) are introduced into the mixing reactor and stirred sufficiently until uniform. Then 33.88% by weight of deionized water was added with stirring. After stirring for 30 minutes, the product obtained was filtered through a filter having a mesh size of 10 μm. The binder content of the product was 37.04 wt% and the solids content was 37.29 wt% (solid content from the additive was 0.25 wt%). The content of water was 33.88% by weight, and the content of the other solvents was 33.88% by weight. The viscosity according to DIN 425, GR.C. was 97 s. This product retained further improved properties compared to Example 1.

Example 3

30.856 weight% polyester resin and 14.121 weight% urethane resin from Example 1 were used. 0.377% by weight of the adjuvant from Example 1, Estole (trademark) 1447 (polyethylene glycol 400 dioleate) 4.762 weight%, bike (Byk, registered trademark) 0.20 0.472 weight%, butyl glycol 7.258 weight%, butanol 8.108% by weight, 1.784% by DMEA and 32.262% by deionized water are further used. The solids content of the resulting coating composition was about 40%. The film prepared from this coating composition (6.5 g / m 2, otherwise same as Example 1) showed a slight improvement in hardness compared to Example 1, and at the same time, solvent resistance was also greatly improved (in Example 1 Accordingly, the MEK after the sterilization step is greater than 80).

Example 4

53.887% by weight of the acrylic polymer obtained by the following method was used. 189.25 g acrylic acid, 578.3 g styrene, 1060.1 g ethyl acrylate, 136.8 g 2-hydroxyethyl methacrylate, and 40 g butyl glycol were stirred in the 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 (tert-butyl peroxybenzoate) and 20.0 g of butyl glycol were prepared. Then, a third mixture was added to the first mixture. 10% of the resulting first mixture was added back to the second mixture in the reactor. Since the temperature rises as a result of the exothermic reaction, the temperature must be maintained in the range of 120 ° C to 129 ° C. The remainder of the first mixture was then metered into the reactor and maintained at 129-131 ° C. for 3 hours. 80 g of butyl glycol was added as washing medium. The temperature was then maintained at 129-131 [deg.] C. for 0.5 hour, followed by the addition of 3.95 g of Trigonox® and 10 g of butyl glycol. The contents of the reactor were then held for an additional 2 hours at 129 ° C to 131 ° C. Finally, the contents of the reactor were cooled to 70 ° C. and 247.75 g of DMEA mixed with 203.7 g of deionized water was added for 0.5 h. Finally, 1541.35 g of deionized water was added and the resulting acrylic polymer solution was stirred until uniform.

In addition to the aforementioned acrylic polymer, in addition, 13.624% by weight of the urethane resin from Example 1, 0.366% by weight of the auxiliary agent from Example 1, 6.446% by weight of butyl carbitol, 3.617% by weight of material A, 4.692% by weight of material B and de 16.899% by weight of ionized water was used.

Material A is a lubricant based on polyethylene glycol dioleate and is commercially available from Priolube®. Material B is obtainable as follows. 56.016 kg of an epoxy resin, referred to as Araldit GY, was added to a batch containing 7.944 kg of butyl glycol-2-butoxyethanol and 8.613 kg of butanol, and mixed with 85% of phosphoric acid in 1.345 kg of butanol. To 3.655 kg of CZ FG (food grade) was added. Then, 1.0 kg of butanol, 1.36 kg of deionized water, 0.68 kg of deionized water, 0.68 kg of deionized water, 16.707 kg of butyl glycol-2-butoxyethanol and 2.0 kg of butyl glycol-2-butoxyethanol were added. Finally, 3.7 wt% DMEA and 22.23 wt% deionized water were added to 74.07 wt% of the resulting product and the mixture was stirred until uniform.

In the above-mentioned acrylic polymer, the OH value of 30 mg KOH / g of the acrylic polymer is changed to 50 mg KOH / g or more and 75 to 100 mg KOH / g, so that the increase in the OH value increases the viscosity of the coating composition. , It reduces the hardness of the coating and reduces the solvent resistance of the coating. From this, it became clear that the set OH number should not be too high.

The coating made of the coating composition described above was subjected to the following test. The autoclave was preheated to 95 ° C. The metal panel containing the coating was then introduced into the autoclave and applied to 1 L of water (23 ° C.). The autoclave was then performed while maintaining at 125 ° C. for 30 minutes. Then, the metal panel was recovered to evaluate the coating. Only a slight change was seen in a limited range, and furthermore this change disappeared completely after 1 hour. The film met all the requirements in terms of hardness and solvent resistance.

Example 5

57.4% by weight of acrylate resin (OH number: 30 mg 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, prio 2.3 wt% of Priolube® 1447, 0.4 wt% of Additol® XW 329 and 0.20 0.5% of bike were used.

Claims (10)

(a) 20 to 70% by weight of a binder polymer selected from the group consisting of "water soluble or water dispersible polyester resins; water soluble or water dispersible acrylic polymers" and mixtures thereof, and (b) crosslinked urethane resins substantially free of free NCO groups. 5 to 20% by weight, (c) 20 to 70% by weight of water, (d) 0 to 18% by weight of organic solvent, and (e) 0 to 5% by weight of conventional auxiliaries, The sum of the weight ratios of components (a) to (e) is 100% by weight, A coating composition for food containers, wherein the binder polymer has an OH value of 10 mg KOH / g or more and an NCO group (blocked) in the urethane resin. The method of claim 1, An acrylic modified saturated polyester resin is used as the polyester resin, and the polymerization of the resin is carried out under the condition that the OH value of the polyester resin is 30 to 120 mg KOH / g and the degree of neutralization is 40 to 80 mg KOH / g. Coating composition. The method according to claim 1 or 2, The coating composition has a viscosity of 50% by weight in a butyl glycol solvent and measured from 23 ° C. to 500 to 900 mPa · s. The method of claim 1, Acrylic polymers are obtainable by carrying out polymerization in the presence of butyl glycol and butanol, using methacrylic acid and ethyl acrylate as starting materials and styrene for copolymerization, wherein the OH number of the acrylic polymer is from 10 to 100 mg. A coating composition having a KOH / g and a neutralization degree of 40 to 100 mg KOH / g. The method according to any one of claims 1 to 4, A coating composition in which a portion of the urethane resin, preferably 30 to 70% of the urethane resin used is substituted with a highly methylated melamine resin. The method according to any one of claims 1 to 5, Coating composition with pigment added. The method according to any one of claims 1 to 6, Coating composition to which the lubricant was added. Use of the coating composition according to any one of claims 1 to 7 for producing a sterilizable can, preferably a metal can, preferably an outer coating. (a) 20 to 40% by weight of a binder polymer selected from the group consisting of "water-soluble or water-dispersible polyester resins and water-soluble or water-dispersible acrylic polymers," (b) crosslinked urethane resins 5 to 20 substantially free of free NCO groups. Any one of claims 1 to 7 homogenized by mixing by weight, (c) 20 to 70% by weight of water, (d) 0 to 18% by weight of organic solvent, and (e) 0 to 5% by weight of conventional adjuvant. A method for producing a coating composition according to claim. A sterilizable can, wherein at least a portion of the outer surface of the can is made of a cured coating of the coating composition according to any one of claims 1 to 7.