US20140023782A1

US20140023782A1 - Coating formulation for the interior surfaces of cans

Info

Publication number: US20140023782A1
Application number: US14/034,753
Authority: US
Inventors: Andreas Kunz; Patrick Podwoiski; Holger Endres; Wolfgang Schneider; Bernd Struck
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 2011-03-31
Filing date: 2013-09-24
Publication date: 2014-01-23
Also published as: CN103619974A; EP2691483A1; CA2831808A1; AU2012237371B2; EP2505625A1; WO2012130563A1; ES2416466T3; EP2505625B1; JP2014514392A; AU2012237371A1; PL2505625T3

Abstract

The invention relates to a water-based can inner coating comprising a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid in water-dispersed form, wherein the acid number of the copolymer or of the copolymer mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g, and at least one water-dispersed or water-soluble curing agent. Inventive can inner coatings are characterized in that due to the good crosslinking of the copolymer or of the copolymer mixture with the curing agent, the cured film on the inner surfaces of metal cans possesses excellent properties in regard to hardness, abrasion resistance and resistance towards hot liquids.

Description

This application is a continuation of PCT/EP2012/053830 filed 7 Mar. 2012, which claims priority to EP11160695.0 filed 31 Mar. 2011.
The present invention relates to a water-based can inner coating comprising a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid in water-dispersed form, wherein the acid number of the copolymer or of the copolymer mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g, and at least one water-dispersed or water-soluble curing agent selected from the group of aminoplasts and/or the group of carbodiimides. Inventive can inner coatings are characterized in that due to the good crosslinking of the copolymer or of the copolymer mixture with the curing agent, the cured film on the inner surfaces of metal cans possesses excellent properties in regard to hardness, abrasion resistance and resistance towards aqueous liquids. The present invention makes available an alternative to the conventional use of epoxides based on bisphenols in can inner coatings.
In the food industry, tin plate strip is valued as a suitable material for the production of packaging units for receiving aqueous liquids or preserved foods. This is because, even over a longer period of time, tin plate strip, due to its electrochemically noble tin layer, releases only low amounts of potentially toxic tin salts to the food product that is in contact with the tin surface. Tin plate strip is therefore an important starting material for food packaging, for example for the production of cans for receiving beverages. Aluminum strip, due to its passive oxide layer, is also a suitable starting material for the production of cans for filling with beverages. In addition, aluminum salts that are taken up in small amounts by the liquid are harmless to health.
The packaging industry, when producing cans, coats the inner surface of the can with an organic protective layer or alternatively uses a strip material pre-coated with an organic protective layer for producing cans. The organic finish that coats the inner surface prevents any direct contact of the metallic interior of the can with the liquid. This achieves first of all a significantly reduced corrosion of the base material and secondly minimizes the entry of metal salts, such that the taste of the foodstuff is not changed for the worse even after a lengthy storage or stockpiling of the beverage cans.
Another aspect in regard to the production of cans concerns the composition of the coating, which conventionally consists of epoxy resins based on Bisphenol A. Such epoxides with a Bisphenol A basic structure are suspected estrogens and are teratogens for males. Cured coating formulations that come into contact with acid-containing aqueous foodstuffs can release Bisphenol A from the coating into the stored foodstuff. In practice, the curing of the coating and the resulting crosslinking of the coating components is also never complete, such that unreacted Bisphenol A-based epoxides can also diffuse into the foodstuff. Consequently there exists a need for Bisphenol A-free formulations for the inner coating of cans for storing foodstuffs; various national legislation initiatives, driven inter alia by the EU Directive 2002/72/EU, exist that define the maximum limits for the migration of Bisphenol A from packaging into foodstuffs.
US 2008/0193689 discloses an epoxide-based coating composition that is suitable for use as a can coating and comprises, in addition to the epoxy resin, mono and difunctional low molecular weight organic compounds that can react with the epoxy resin. The coating is formulated in such a way that after curing, only very minor amounts of unreacted epoxides based on Bisphenol A remain in the coating, such that when the composition is used as a can inner coating only traces of Bisphenol A from the cured coating can migrate into the stored foodstuff.
On the other hand, EP 2031006 proposes can inner coatings based on specific alicyclic epoxides, so as to circumvent in this way the formulations that include epoxides based on Bisphenol A.
WO 2006/045017 provides a beverage can coating formulation that contains latices of ethylenically unsaturated monomers and an aqueous dispersion of an acid-functional polymer in the presence of amines, wherein the latices for the crosslinking are constructed at least partially from monomers having a glycidyl group. Such can inner coatings can be formulated free of epoxides based on Bisphenol A.
The object of the present invention consists in providing another alternative to an epoxide-based can inner coating, wherein the coating formulation can be deposited on the inner surfaces of the can in a spray process and after curing affords thin, homogeneous, highly flexible coating films with a simultaneously good coating adhesion and resistance towards aqueous compositions. Another object consists in being able as far as possible to obviate the use of organic solvents and emulsifiers in the formulation of stable and coatable can inner coatings.
This object is achieved by a water-based can inner coating comprising, in addition to water,

a) a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid in water-dispersed form, wherein the acid number of the copolymer or of the copolymer mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g, and
b) at least one water-dispersed or water-soluble curing agent selected from the group of aminoplasts and/or the group of carbodiimides.

Cans are inventively understood to mean metallic containers for filling, storing and holding stocks of foodstuffs, in particular of beverages.
In this context, a can inner coating is a coating formulation that for the formation of a coating layer on the inner surface of the can is deposited, made into a film and cured in order to prevent the direct contact of the foodstuff with the metallic material of the can during filling, storing and holding stocks of the foodstuff.
A water-based coating inventively contains a dispersion and/or emulsion of organic polymers in a continuous aqueous phase, wherein in the context of the present invention, an aqueous phase is also understood to mean a homogeneous mixture of water and a water-miscible solvent. The term “in water-dispersed form” therefore means that each polymer is dispersed as a solid or liquid in the continuous aqueous phase.
According to the invention, mixtures of chemically and/or structurally different copolymers of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid constitute a copolymer mixture. Thus, a copolymer mixture of an inventive coating formulation can for example comprise in parallel copolymers that comprise different alkenes or different α,β-unsaturated carboxylic acids as the comonomers or have a different number of otherwise identical comonomers in the copolymer.
The acid number is inventively an experimentally measurable characteristic number that reflects the number of the free acid groups in the copolymer or in the copolymer mixture.
The acid number is determined by dissolving a weighed quantity of the copolymer or the copolymer mixture in a solvent mixture of methanol and distilled water in the volume ratio 3: 1, and subsequently potentiometrically titrating the mixture with 0.05 mol/l KOH in methanol. The potentiometric measurement is carried out with a combination electrode (LL-Solvotrode® from Metrohm; reference electrolyte: 0.4 mol/l tetraethylammonium bromide in ethylene glycol). Here, the acid number corresponds to the added quantity of KOH in milligrams per gram copolymer or copolymer mixture at the inflection point of the potentiometric titration curve.
As a melted on, thin film on metal surfaces the copolymer or copolymer mixture of the aliphatic and acyclic alkane with an α,β-unsaturated carboxylic acid with the abovementioned acid number already shows a good coating adhesion, in particular on tin plate and aluminum surfaces. In addition, the acid groups impart the inherent characteristic to the copolymer or to the copolymer mixture of being self-emulsifying, such that in the aqueous phase, even in the absence of emulsifiers, microparticulate aggregates can be formed by using shear forces. The presence of the copolymer or copolymer mixture in the form of a microparticulate aggregate lends thixotropic properties to the inventive coating, such that a homogeneous wet film of the water-based coating can be deposited onto the inner surface of the can, the coating remaining there until a film is formed and cured, and does not run off inside the can due to the force of gravity.
If the acid number of the copolymer or copolymer mixture of alkenes and α,β-unsaturated carboxylic acids is less than 20 mg KOH/g, then a cured coating formulation according to the art of the present invention does not have sufficient adhesion to metal surfaces and consequently is not suitable as a film-forming component of can inner coatings. Conversely, an acid number of the copolymer or copolymer mixture of alkenes and α,β-unsaturated carboxylic acids greater than 200 mg KOH/g as the film-forming component in can inner coatings only brings about an inadequate barrier effect against corrosively acting ions in aqueous media and furthermore a coating that is comparatively less resistant against water at temperatures above 60° C.
The weight fraction of the aliphatic and acyclic alkenes in the copolymer or in the copolymer mixture is preferably at least 40 wt %, particularly preferably at least 60 wt %, but preferably not more than 95 wt %. This ensures that the ion-permeability of the cured coating on the can inner surface and the swelling of the coating in contact with aqueous media, with at the same time an adequate wettability and adhesion of the coating to the material of the can, is reduced as much as possible.
Preferred aliphatic and acyclic alkenes of the inventively obtained copolymer or copolymer mixture are selected from ethene, propene, 1-butene, 2-butene, isobutene, 1,3-butadiene and/or 2-methylbuta-1,3-diene, particularly preferably ethene.
Preferred α,β-unsaturated carboxylic acids of the inventively obtained copolymer or copolymer mixture are selected from cinnamic acid, crotonic acid, fumaric acid, itaconic acid, maleic acid, acrylic acid and/or methacrylic acid, particularly preferably acrylic acid and/or methacrylic acid, in particular acrylic acid.
Further comonomers that may be an additional component of the copolymer or the copolymer mixture in an inventive can inner coating are selected from esters of α,β-unsaturated carboxylic acids, preferably linear or branched alkyl esters of the acrylic acid and/or methacrylic acid containing not more than 12 carbon atoms in the aliphatic group. Such comonomers improve the adhesion of the cured inner coating of the can to metal surfaces due to an increased mobility of the polymer backbone which again facilitates the orientation of the acid groups that have a surface affinity to the metal surface. This effect is ensured in particular with low acid numbers of the copolymer below 100 mg KOH/g. It is generally the case that low acid numbers of the copolymer or copolymer mixture improve the barrier properties of the cured inventive coating formulation when exposed to aqueous media. Accordingly, copolymers or copolymer mixtures that additionally comprise the above described comonomers are inventively preferred with acid numbers below 100 mg KOH/g, particularly below 60 mg KOH/g.
The copolymer or the copolymer mixture of the inventive can inner coating preferably comprises less than 0.05 wt %, particularly preferably less than 0.01 wt of epoxidically bonded oxygen.
A good film-formation when curing the can inner coating requires that the water-dispersed copolymer or the water-dispersed copolymer mixture of the can inner coating is converted into a melted state after the aqueous phase has been driven off. This requirement is satisfied when copolymers or copolymer mixtures are preferred that as such have a glass transition temperature of not more than 80° C., particularly preferably not more than 60° C. Copolymers or copolymer mixtures with a weight average molecular weight M_wof not more than 20 000 u and which are based on alkenes and α,β-unsaturated carboxylic acids usually have glass transition temperatures that are significantly below 100° C., such that copolymers or copolymer mixtures with a weight average molecular weight of not more than 20 000 u, in particular not more than 15 000 u, are preferred in inventive can inner coatings.
In a preferred formulation of the inventive can inner coating, the acid groups of the water-dispersed copolymer or the water-dispersed copolymer mixture are at least partially neutralized. This measure increases the ability of the copolymer for self-emulsification in the aqueous phase, such that more stable coating formulations result with lower particle sizes of the dispersed copolymers. Accordingly, the can inner coating preferably additionally comprises a neutralizing agent. Preferred suitable neutralization agents that are additionally comprised in such a preferred formulation are ammonia, amines, metallic aluminum and/or zinc, preferably in powdered form, as well as water-soluble oxides and hydroxides of the elements Li, Na, K, Mg, Ca, Fe(II) and Sn(II). The person skilled in the art is aware here that the neutralization agents, corresponding to their function, enter into neutralization reactions with the components of the inventive coating, and therefore in these preferred formulations are optionally detectable as such only indirectly in the form of their reaction products. For example, metallic aluminum powder or zinc powder reacts in the aqueous phase, giving off hydrogen, to afford the corresponding hydroxides that again neutralize the acid groups of the copolymer or copolymer mixture, such that in the inventive coating finally only the cations of the elements aluminum or zinc can be detected. The neutralization agents are therefore understood to be solely as formulation aids of the inventive can inner coating. Ammonia and amines are particularly preferred neutralization agents, as they pass into the gas phase when the coating is cured at elevated temperatures and therefore do not remain in the cured can inner coating. Preferred amines that can be employed as the neutralization agent in inventive can inner coatings are morpholine, hydrazine, hydroxylamine, monoethanolamine, diethanolamine, triethanolamine, dimethylethanolamine and/or diethylethanolamine.
The degree of neutralization of the acid groups in the copolymer or copolymer mixture in the inventive can inner coating is such that at least 20%, particularly preferably at least 30% of the acid groups are neutralized. High degrees of neutralization above 70%, preferably above 60%, are to be avoided in a preferred embodiment of the can inner coating, as the almost completely neutralized copolymers are already dissolved in significant amounts in water, thereby resulting again in a high viscosity of the coating and average particle sizes of the dispersed copolymer or copolymer mixture in the sub-micrometer range, such that these kinds of formulations are less suitable as the can inner coating due to their rheological properties.
In this context, the neutralization agent to the can inner coating is preferably to be formulated in such an amount that, based on 1 g of copolymer or copolymer mixture, at least 4/z μmol, preferably at least 6/z μmol, each multiplied by the acid number of the copolymer or copolymer mixture, are comprised as the neutralization agent, but preferably not more than 12/z μmol, particularly not more than 10/z μmol, multiplied by the acid number of the copolymer or copolymer mixture. The divisor z is a natural number and corresponds to the equivalent number of the neutralization reaction. The equivalent number represents how many moles of acid groups of the copolymer or copolymer mixture are neutralized by one mole of the neutralization agent.
The inventive can inner coating comprises a water-dispersed or water-soluble curing agent from the group of aminoplasts and/or the group of carbodiimides. The curing agent enables the copolymer or the copolymer mixture to crosslink in a condensation reaction and thus to form a cured coating film on the inner surface of the can. The barrier properties of the cured inventive can inner coating as a film are comparable with those of cured epoxide-based coating films.
In the inventive coating the curing agent must have the characteristic that it crosslinks the copolymer or copolymer mixture through a condensation reaction only at temperatures above the glass transition temperature, preferably only above 100° C., as otherwise, curing would already occur before the dispersed polymeric components of the coating could form a complete film on the inner surface of the can, thus producing very heterogeneous coating films.
Particularly suitable aminoplast curing agents are based on melamine, urea, dicyandiamide, guanamines and/or guanidine. In inventive can inner coatings, the aminoplast curing agents are particularly preferably melamine-formaldehyde resins with a molar ratio of formaldehyde: melamine that is preferably greater than 1.5.
Alternatively or in addition, the curing agent of the inventive can inner coating is a carbodiimide. According to the invention, carbodiimides possess at least one diimide structural moiety of the —C═N═C— type. However, they are preferably polyfunctional with a diimide equivalent weight in the range of 300-500 grams of the polyfunctional compound per mole of diimide groups. Particularly preferred carbodiimides result from isocyanates with at least two isocyanate groups by decarboxylation, in particular those of the general Formula (I):
with n: a whole natural number between 1 and 20;

- R₁an aromatic, aliphatic or alicyclic residue with not more than 16 carbon atoms.

The isocyanate groups are additionally preferably blocked with hydrophilic protective groups that as such lend an improved water-dispersibility or water-solubility to the carbodiimide. The use of these preferred carbodiimides furnishes the additional advantage that the can inner coating can be formulated to be almost free of organic solvents as these carbodiimides are highly water-soluble without already crosslinking the copolymer or copolymer mixture in the aqueous formulation. In a preferred embodiment of an inventive can inner coating that at least partially comprises carbodiimides as the curing agent, the content of organic solvents is therefore less than 10 wt %, particularly preferably less than 4 wt %, in particular the can inner coating preferably comprises no solvent. Exemplary suitable protective groups with hydrophilic character are hydroxyalkyl sulfonic acids, hydroxyalkyl phosphonic acids, hydroxyalkyl phosphoric acids, polyalkylene glycols as well as quaternary aminoalkyl alcohols and aminoalkylamines. In a particularly preferred embodiment of the can inner coating, the curing agent is therefore selected from carbodiimides with blocked terminal isocyanate groups according to the general structural Formula (II):
with n: a whole natural number between 1 and 20;

- R₁: an aromatic, aliphatic or alicyclic residue with not more than 16 carbon atoms.
- X: —NH—R₁—N(R₁)₂, —O—R₁—N(R₁)₂, —NH—R₁—N(R₁)₃Y, —O—R₁—N(R₁)₃Y, —O—R₁—SO₃Z, —O—R₁—O—PO₃Z, —O—R₁—PO₃Z, —O—(C₂H₄)_p—OH, —O—(C₃H₆)_p—OH
  - with Y: hydroxide, chloride, nitrate, sulfate
  - with Z: hydrogen, ammonium, alkali metal or alkaline earth metal
  - with p: a whole natural number between 1 and 6

Preferred diisocyanates that are afforded by decarboxylation of the corresponding carbodiimides are for example hexamethylene diisocyanate, cyclohexane-1,4-diisocyanate, xylylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4-diisocyanate, methylcyclohexane diisocyanate and tetramethylxylylene diisocyanate, 1,5-naphthylene diisocyanate, 4,4-diphenylmethane diisocyanate, 4,4-diphenyldimethylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluenylene diisocyanate, 2,6-toluenyfene diisocyanate.
Basically, the weight average molecular weight M_wof the curing agent in the inventive can inner coating is preferably not more than 2500 u, particularly preferably not more than 1500 u, in order to ensure an adequate crosslinking with the copolymer or the copolymer mixture.
The flow properties of the inventive can inner coating are preferably to be adjusted, such that on the one hand to enable the coating to be applied in a spray process and especially in the airless process (which illustrates an airless atomization spray process) that is usually used in the beverage can industry. On the other hand, the wet film deposited on the inner surface of the can must not immediately run off due to gravitational forces, thereby causing an inhomogeneous coating. Optimum flow properties with good film formation of the dispersed constituents are obtained for inventive can coatings, whose dispersed polymeric constituents of the water-based coating preferably have a D₉₀value of not more than 100 μm, particularly preferably not more than 60 μm, wherein the D₅₀value is preferably not less than 1 μm, particularly preferably not less than 10 μm. The D₉₀-value, respectively the D₅₀-value, means that 90 vol % respectively 50 vol % of the dispersed particles of the can inner coating are smaller than the specified value.
The D₉₀-value, respectively the D₅₀-value, can be determined from volume weighted cumulative particle size distributions, wherein the particle size distribution curve can be measured with the help of light scattering methods.
The viscosity of the can inner coating is preferably such that a flow time between 20 and 40 seconds results, when measured with a 4 mm DIN flow cup of DIN EN ISO 2431. If the viscosity, measured as the flow time from the normalized flow cup, is in this range, then the coating, present as a thin film on the inside of the can, has a flow behavior that reduces any run off of the wet film and simultaneously ensures that the can inner coating is able to be applied in spray processes.
Emulsifiers that support the dispersion of the copolymer or the copolymer mixture can be added as an auxiliary to the inventive can inner coating. At least 0.1 wt % of emulsifiers are preferably added for this purpose. Preferably, non-ionic amphiphiles with an HLB value of at least 8 can be additionally comprised as the emulsifiers in the can inner coating.
According to the present invention, the HLB value is calculated by the following formula and can assume values of zero to 20 on an arbitrary scale:

HLB=20 (M_l/M)
with M_lmolecular weight of the lipophilic group of the amphiphile
- M: molecular weight of the amphiphile

The content of this added auxiliary emulsifier in the can inner coating is preferably not more than 5 wt %, particularly preferably not more than 2 wt %. However, the copolymer or the copolymer mixture used in the inventive can inner coating is characterized in that it already possesses self-emulsifying properties as a result of its acid groups. Moreover, it has been shown that the addition of emulsifiers frequently causes a decreased adhesion of the cured can inner coating to the tin plate and aluminum surfaces. Accordingly, in a preferred embodiment of the can inner coating, in the case that the acid number of the copolymer or copolymer mixture is greater than 60 mg KOH/g, preferably greater than 80 mg KOH/g, or the degree of neutralization of the copolymer or copolymer mixture with an acid number below 100 mg KOH/g is at least 30%, then less than 0.1 wt %, particularly preferably less than 0.01 wt % and especially preferably no emulsifiers are comprised that are based on the non-ionic amphiphiles with an HLB value of at least 8.
Alternatively or in addition to the added emulsifiers, the inventive can inner coating may comprise water-miscible organic solvents that decrease the polarity of the aqueous phase, so as to induce the emulsification of the copolymer or copolymer mixture. For this purpose, at least 1 wt % of water-miscible organic solvents are added. In this regard, the boiling point of the water-miscible solvent under standard conditions is preferably not more than 150° C.
Suitable solvents are glycol ethers, alcohols and esters. The content of the solvent in the can inner coating is preferably not more than 40 wt %, particularly preferably not more than 20 wt %.
Inventive can inner coatings may comprise wetting agents, leveling agents, defoamers, catalysts, film-formers, stabilizers and/or the already mentioned neutralizing agents as additional constituents. These kinds of auxiliaries are generally known to the person skilled in the art of coating objects, wherein film-formers in the present invention are understood to mean organic polymers that can crosslink with the curing agent present in the can inner coating. The content by weight of film-formers based on the copolymer or copolymer mixture is at most 20%, preferably at most 10%.
A preferred formulation of an inventive can inner coating comprises, in addition to at least 40 wt % water,

- a) 4-30 wt %, preferably 10-20 wt %, of the copolymer or of the copolymer mixture in dispersed form,
- b) 2-20 wt. %, preferably 4-12 wt. %, of the at least one curing agent,
- c) not more than 5 wt % of emulsifiers selected from non-ionic amphiphiles with an HLB value of at least 8;
- d) not more than 40 wt %, preferably at least 1 wt %, of water-miscible organic solvents;
- e) not more than 10 wt. % of auxiliaries selected from wetting agents, leveling agents, defoamers, catalysts, film-formers, stabilizers and/or neutralizing agents, preferably not more than 12/z μmol of neutralization agent multiplied by the acid number of the copolymer or copolymer mixture are comprised per gram of the copolymer or copolymer mixture where z is the equivalent number of the relevant neutralization reaction.

A particularly preferred reduced-solvent formulation of an inventive can inner coating comprises, in addition to at least 40 wt % water,

- a) 4-30 wt %, preferably 10-20 wt %, of the copolymer or of the copolymer mixture in dispersed form,
- b) 2-20 wt %, preferably 4-12 wt % of at least one resin, of which at least 40 wt % of a carbodiimide with terminal, blocked isocyanate groups based on the total content of the curing agent,
- c) not more than 5 wt % of emulsifiers selected from non-ionic amphiphiles with an HLB value of at least 8;
- d) not more than 10 wt %, preferably not more than 1 wt %, of water-miscible organic solvents;
- e) not more than 10 wt % of auxiliaries selected from wetting agents, leveling agents, defoamers, catalysts, film-formers, stabilizers and/or neutralizing agents, preferably not more than 12/z μmol of neutralization agent multiplied by the acid number of the copolymer or copolymer mixture are comprised per gram of the copolymer or copolymer mixture where z is the equivalent number of the relevant neutralization reaction.

Inventive can inner coatings are characterized in that due to the good crosslinking of the copolymer or of the copolymer mixture with the curing agent, the cured film on the inner surfaces of metal cans possesses excellent barrier properties. The metallic base material is consequently firstly effectively protected against corrosion and secondly the liquid stored in the can will not take up any extraneous substance. Therefore, the present invention makes available an alternative to the conventional use of epoxides in can inner coatings, in particular epoxides based on Bisphenol A. Consequently, the content of epoxidically bonded oxygen in inventive can inner coatings is preferably not more than 0.1 wt %, particularly preferably not more than 0.01 wt %. An inventive can inner coating particularly preferably comprises no organic constituents with epoxide groups.
Inventive can inner coatings can preferably be produced in closed processes in pressure reactors using shear forces, wherein all constituents of an inventive can inner coating are transferred into a pressure reactor, in order to be subsequently subjected to a shear rate of at least 1000 s⁻¹at temperatures in the range of 80-200° C. and a pressure of 1-6 bar, wherein the energy input is preferably in the range of 10³-10⁵J per second per liter of coating formulation. Alternatively, the solid constituents together with the usual components of the can inner coating are also dispersed in an open process, in which the melted copolymer or the melted copolymer mixture under the action of the abovementioned shear force is transferred into the aqueous composition of the usual can inner coating composition. However, the shear rate and residence time in each dispersion process is preferably adjusted such that the dispersed constituents of the can inner coating have a D₉₀value of not more than 100 μm, wherein the D₅₀value is preferably not below 1 μm, particularly preferably not below 10 μm.
The application of a wet film of the inventive can inner coating is preferably carried out in a spray process, particularly preferably in the “Airless Process”, in which the can inner coating is airlessly atomized and thus deposited onto the material surface. In this spray process, a defined quantity of the can inner coating is introduced into the cleaned and dried can interior by means of spray guns, while the can is rotated about its own Longitudinal axis in order to form a homogeneous film. The wet film on the can inner surface is then cured to a coating film in a drying oven at temperatures ranging between 120° C. and 200° C. (object temperature). The curing process includes the volatilization of the aqueous phase as well as the film formation and crosslinking of the polymeric constituents.
In another aspect, the present invention relates to the use of a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid in water-dispersed form, wherein the acid number of the copolymer or of the copolymer mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g, and the acid groups of the copolymer or of the copolymer mixture in the water-dispersed form are at least 20%, but not more than 70% neutralized, as a constituent of water-based can inner coatings, wherein preferred uses can be realized by above described corresponding embodiments of the copolymer or of the copolymer mixture.
In another aspect, the present invention relates to the use of an above described can inner coating that is deposited in a dry film thickness of at least 5 g/m², but preferably of not more than 50 g/m², on to the inner surface of a tin plate can and in a dry film thickness of at least 1.5 g/m², but preferably not more than 50 g/m², on to the inner surface of an aluminum can.

EXAMPLES

Table 1 lists the compositions of the inventive can inner coatings that were deposited as a wet film on to the inner surfaces of tin plate cans by means of spray processes, and then cured for 40 seconds at 180° C. to a dry coating with a coating weight of 6-7 g/m².
The water-based can inner coatings were manufactured in an open reactor by continuously metering the melted copolymer to an aqueous composition of the remaining constituents under a shear stress of 1500 s⁻¹at 95° C. After metering in the copolymer, the homogenization was continued in the open reactor until a constant viscosity of the coating formulation was achieved. The viscosity of the coating formulations, measured as the flow time from a DIN 4 mm flow cup according to DIN EN ISO 2431 lay in the range 25-28 seconds.
The coating formulations homogenized in this way were then deposited onto the inner surfaces of the tin plate can in a two-step spray process, wherein the tin plate can was rotated about an axis and initially the bottom of the can and lower part of the body was coated and then the can body and end were sprayed. The wet film was then cured.
From Table 2 it can be seen that the tin plate cans coated with the inventive coating possess an excellent flexibility (T-bend test) and water resistance (Koch test). Solely the hardness and solvent resistance tests showed differing results, which, however, all met the requirements of the beverage can industry.

TABLE 1

Exemplary formulations of inventive can inner coatings

	Constituents in wt
	% (rest is water)

Constituent	Compound	B1	B2	B3

Copolymer	Ethylene-acrylic acid; acid no. 37-44 mg KOH/g; Neutralization degree	17.1
	50% (dimethylethanolamine)
	Ethylene-acrylic acid; acid no. 37-44 mg KOH/g; Neutralization degree	—	21.3	21.5
	30% (ammonia)
Curing agent	Melamine/formaldehyde resin partially methylated of the imino type	9.0	—	4.5
	Polycarbodiimide with diimide eq. wt of 445 g/mol	—	4.0	4.0
Solvent	Monopropylene glycol monomethyl ether	11.0	—	6.0
	Butyl glycol	9.0	—	1.7
Defoamer	Polyether siloxane copolymer	0.63	0.5	0.5
Wetting agent	Bis(2-ethylhexyl)sulfosuccinate, Na salt	—	0.75	0.75

TABLE 2

Properties of the cured can inner coatings of Table 1 in tin plate cans

Property	Test	B1	B2	B3

Coating adhesion¹	T-bend acc. DIN ISO 17132	0	0	0
	Cross-hatch test acc. DIN	0	0	0
	53151 (24 h)
	Boil test acc. DIN 53151	0	0	0
	(30 min at 85° C.)
Coating hardness	Pencil hardness acc. DIN	HB	B	B
	ISO 15184
Solvent resistance	MEK Test acc. DIN EN	90	90	100
	13523-11

¹Classification according to coating dissolution in % based on the tested surface 0: no dissolution; 1: 5%; 2: 15%; 3: 35%; 4: <65%; 5: >65%

Claims

1. A water-based can inner coating comprising, in addition to water,

a) a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one α,β-unsaturated carboxylic acid in water-dispersed form, wherein the acid number of the copolymer or of the copolymer mixture is at least 20 mg KOH/g, but not more than 200 mg KOH/g, and

b) at least one water-dispersed or water-soluble curing agent selected from an aminoplast, a carbodiimide and combinations thereof.

2. The water-based can inner coating according to claim 1, wherein the acid groups of the copolymer or of the copolymer mixture in water-dispersed form are at least partially neutralized.

3. The water-based can inner coating according to claim 2, comprising a neutralization agent for neutralizing the acid groups of the copolymer or of the copolymer mixture in water-dispersed form, said neutralization agent being selected from the group consisting of ammonia, amines, metallic Al, metallic Zn, water-soluble oxides of Li, Na, K, Mg, Ca, Fe(II), Sn(II), water-soluble hydroxides of Li, Na, K, Mg, Ca, Fe(II), Sn(II) and combinations thereof.

4. The water-based can inner coating according to claim 3, wherein the neutralization agent is selected from ammonia, morpholine, hydrazine, hydroxylamine, monoethanolamine, diethanolamine, triethanolamine, dimethylethanolamine, diethylethanolamine, and combinations thereof.

5. The water-based can inner coating according to claim 1, wherein the copolymer or the copolymer mixture has a glass transition temperature of not more than 80° C.

6. The water-based can inner coating according to claim 1, wherein the aliphatic and acyclic alkene is selected from ethene, propene, 1-butene, 2-butene, isobutene, 1,3-butadiene, 2-methylbuta-1,3-diene and combinations thereof.

7. The water-based can inner coating according to claim 1, wherein the α,β-unsaturated carboxylic acids are selected from cinnamic acid, crotonic acid, fumaric acid, itaconic acid, maleic acid, acrylic acid, methacrylic acid and combinations thereof.

8. The water-based can inner coating according to claim 1, wherein a weight fraction of the aliphatic and acyclic alkenes in the copolymer or in the copolymer mixture is at least 40 wt. %, but not more than 95 wt. %.

9. The water-based can inner coating according to claim 1, wherein the copolymer or the copolymer mixture additionally comprises comonomers that are selected from esters of α,β-unsaturated carboxylic acids.

10. The water-based can inner coating according to claim 9, wherein the esters of α,β-unsaturated carboxylic acids comprise linear or branched alkyl esters of acrylic acid and/or methacrylic acid, said esters having an aliphatic group of not more than 12 carbon atoms, wherein the copolymer or the copolymer mixture has an acid number of less than 100 mg KOH/g.

11. The water-based can inner coating according to claim 1, wherein water dispersed polymeric constituents of the water-based can inner coating have a D90 value of not more than 100 μm and a D50 value of not less than 1 μm.

12. The water-based can inner coating according to claim 1, comprising at least 40 wt. % water and

a) 4-30 wt. % of the copolymer or of the copolymer mixture,

b) 2-20 wt. % of the at least one curing agent,

c) not more than 5 wt. % of emulsifiers selected from non-ionic amphiphiles with an HLB value of at least 8;

d) not more than 40 wt. % of water-miscible organic solvents;

e) not more than 10 wt. % of auxiliaries selected from wetting agents, leveling agents, defoamers, catalysts, film-formers, stabilizers and neutralizing agents.

13. A method of coating inner surfaces of a can comprising applying the water-based can inner coating according to claim 1 to inner surfaces of a can and drying said water-based can inner coating.

14. The method of claim 13, wherein the can is a tin plate can and the inner coating is deposited in a dry film thickness of at least 5 g/m², but not more than 50 g/m², on to the inner surface.

15. The method of claim 13, wherein the can is an aluminum can and the inner coating is deposited in a dry film thickness of at least 1.5 g/m², but not more than 50 g/m², on to the inner surface.

16. The method of claim 13, wherein the water-based can inner coating is deposited in a spraying process.