Aqueous coating composition comprising dispersed acrylic copolymer

Abstract

An aqueous coating composition comprising a dispersed acrylic copolymer obtained by emulsion polymerization of an ethylenically unsaturated monomer component in the presence of an aqueous dispersion of an emulsified polymer, wherein the aqueous dispersion of the emulsified polymer is obtained by solution polymerizing a styrene-free first monomer mixture of 5-20% by weight of methacrylic acid, at least 15% by weight of methyl methacrylate, and optionally other ethylenically unsaturated monomers; at least partially neutralizing the acid-functional polymer thus obtained; and dispersing the neutralized polymer in water wherein the ethylenically unsaturated monomer component is styrene-free and comprises methyl methacrylate in an amount of at most 45% by weight, ethyl acrylate in an amount of at least 47% by weight, and optionally glycidyl methacrylate in an amount of at most 8.0% by weight. A method of coating a metallic can, a portion thereof or a metallic substrate for it with the coating composition and an article coated with the coating composition.

Description

Technical Field

[0001] This invention relates to an aqueous coating composition comprising a dispersed acrylic copolymer suitable for coating metal substrates of food and beverage packaging materials such as food and beverage cans or portions thereof, a method of coating metal food or beverage cans or portions thereof with the coating composition, and an article coated with the coating composition. Background Technology

[0002] Metal containers (cans) for food and beverages, and parts thereof, are typically coated on both the inside and outside to protect the metal substrate from deterioration and to protect the food or beverages contained therein from spoilage. Some canned foods and beverages are highly acidic and pose a harsh environment to the metal substrate. Therefore, the coating must meet stringent requirements in terms of adhesion to the substrate, chemical resistance, and food safety.

[0003] Current industry standard can coatings contain bisphenol or bisphenol-based compounds such as bisphenol A, bisphenol F, bisphenol S, aromatic glycidyl ether compounds, or their epoxides. These compounds are considered potentially harmful to human health. Therefore, there is a need to provide can coating compositions produced without the intentional use of such compounds.

[0004] Can coatings produced without the addition of bisphenol A, bisphenol F, bisphenol S, aromatic glycidyl ether compounds, or epoxy resins or other compounds based thereon are known. For example, coatings based on waterborne acrylic technology.

[0005] However, acrylic can coatings are relatively soft and have poor chemical resistance. Increasing the glass transition temperature of such coatings by using (meth)acrylic comonomers with higher glass transition temperatures may result in hard, brittle coatings that lack sufficient flexibility to withstand any deformation during production and transportation. However, a higher glass transition temperature is required to avoid so-called flavor adsorption. Flavor adsorption is a degradation of the quality of packaged food or beverages due to the absorption of volatile flavors by the packaging material or the absorption of undesirable flavors from the packaging material. Therefore, the requirements of minimizing flavor adsorption and possessing high flexibility are somewhat contradictory. Consequently, it is difficult to provide coating compositions that meet both requirements.

[0006] Acrylic can coatings typically contain styrene or other aromatic comonomers to increase the coating's hardness. It is desirable to avoid styrene or other aromatic comonomers in the production of polymers for food contact coatings.

[0007] WO 2018 / 013766 describes an aqueous coating composition for food and beverage containers comprising an emulsified acrylic latex polymer obtained by emulsion polymerization of an olefinically unsaturated monomer in the presence of an emulsifying polymer having a number average molecular weight of at least 8,500. The cured film of the coating composition of WO 2018 / 013766 has a glass transition temperature of at least 40°C. All coating compositions exemplified in WO 2018 / 013766 comprise acrylic polymers containing styrene monomers.

[0008] WO 2018 / 085052 describes an aqueous internal spray coating composition suitable for forming a substantially BPA-free food contact coating for food or beverage cans. This coating composition comprises a dispersed acid or anhydride-functionalized acrylic polymer and a nitrogen-containing carboxyl reactive crosslinking agent. The coating composition of WO 2018 / 085052 is substantially styrene-free.

[0009] There is a need for aqueous acrylic coating compositions for food and beverage cans that are styrene-free and preferably also free of other aromatic monomers, and possess sufficiently high hardness and flexibility. In particular, there is a need for aqueous acrylic coating compositions suitable for roll coating of metal substrates that can be used to form beverage can lids. Metal substrates for roll coating of beverage can lids cure for only a few seconds at typical baking temperatures (peak metal temperatures) in the range of 180-250°C. Summary of the Invention

[0010] It has been found that by carefully selecting (meth)acrylic monomers other than styrene in a two-step polymerization process, dispersed styrene-free acrylic copolymers can be provided, which, when used as a base polymer in aqueous coating compositions for food and beverage metal packaging materials, result in coatings with sufficient hardness, high flexibility, and good adhesion to metal substrates. Furthermore, this coating composition is suitable for coating metal beverage can lids using roll-to-roll coating or for coating metal substrates to be molded into beverage can lids.

[0011] Therefore, in a first aspect, the present invention provides an aqueous coating composition comprising a dispersed acrylic copolymer, wherein the acrylic copolymer is obtained by emulsion polymerization of an olefin unsaturated monomer component in the presence of an aqueous dispersion of an emulsified polymer. The aqueous dispersion of the emulsified polymer is obtained as follows: a) Solution polymerization of a first monomer mixture to obtain a solution-type acid-functionalized polymer, wherein the first monomer mixture comprises the following: (1i) Methacrylic acid used in amounts ranging from 5 to 20% by weight (1ii) The amount of methyl methacrylate is at least 15% by weight, and optionally (1iii) Other olefinic unsaturated monomers used in amounts of up to 80% by weight All weight percentages are based on the total weight of (1i), (1ii) and (1iii), and the first monomer mixture is styrene-free; b) Using a neutralizing agent to at least partially neutralize the acid-functionalized polymer obtained in a) to obtain an acid-functionalized polymer that is at least partially neutralized; and c) Disperse the at least partially neutralized acid-functionalized polymer in water. The olefinic unsaturated monomer component is a mixture of second monomers comprising the following: (2i) The amount used is up to 45% by weight of methyl methacrylate; (2ii) The amount of ethyl acrylate is at least 47% by weight; and optionally (2iii) The amount of glycidyl methacrylate used is up to 8.0% by weight. All weight percentages are based on the total weight of the olefinic unsaturated monomers in the second monomer mixture. Furthermore, the second monomer mixture does not contain styrene.

[0012] The present invention provides, in a second aspect, a method for coating a metal food or beverage can, a portion thereof, or a metal substrate to be formed into a metal food or beverage can or a portion thereof, comprising applying a coating composition according to the first aspect of the invention to at least a portion of the metal surface of the can, the portion thereof, or the metal substrate to be formed into a metal can or the portion thereof, and curing the applied coating composition.

[0013] The present invention provides, in a third aspect, an article coated with a coating composition according to the first aspect of the present invention, wherein the article is a beverage can lid or a beverage can body, preferably a beverage can lid. Detailed Implementation

[0014] The coating composition of the present invention is an aqueous coating composition comprising a dispersed acrylic copolymer. This acrylic copolymer is used as a film-forming base polymer that forms a coating film upon curing.

[0015] The coating composition is an aqueous coating composition. As used herein, an aqueous coating composition refers to a coating composition in which the acrylic copolymer is dispersed in a liquid carrier containing at least 35% by weight water, preferably at least 40% by weight water, more preferably at least 50% by weight water.

[0016] This acrylic copolymer is obtained by emulsion polymerization of an olefin unsaturated monomer component in the presence of an aqueous dispersion of an emulsified polymer.

[0017] The aqueous dispersion of the emulsified polymer was obtained as follows: a) Solution polymerization of a first monomer mixture of olefinic unsaturated monomers to obtain a solution-type acid-functionalized polymer; b) Using a neutralizing agent to at least partially neutralize the acid-functionalized polymer obtained in a) to obtain an acid-functionalized polymer that is at least partially neutralized; and c) Disperse the at least partially neutralized acid-functionalized polymer in water.

[0018] Therefore, the dispersed acrylic copolymer is obtained by a two-step polymerization process, wherein the first step includes solution polymerization of a first monomer mixture and the second step includes emulsion polymerization of an olefinic unsaturated monomer component (a second monomer mixture) in the presence of the emulsified polymer from the first step, which has been at least partially neutralized and dispersed in water.

[0019] In the solution polymerization step a) of preparing an aqueous dispersion of an emulsified polymer, a first monomer mixture of olefinic unsaturated monomers is polymerized.

[0020] The first monomer mixture consists of the following: (1i) Methacrylic acid used in amounts ranging from 5 to 20% by weight; (1ii) Methyl methacrylate used in an amount of at least 15% by weight; and optionally (1iii) Other olefinic unsaturated monomers used in amounts of up to 80% by weight.

[0021] All weight percentages are based on the total weight of (1i), (1ii), and (1iii). The first monomer mixture is styrene-free. Preferably, the first monomer mixture is free of any aromatic olefinic unsaturated monomers, such as aromatic (meth)acrylate monomers like benzoate (meth)acrylates.

[0022] In some embodiments, the first monomer mixture does not contain other olefinically unsaturated monomers (1iii). In these embodiments, the first monomer mixture comprises methacrylic acid and methyl methacrylate as the sole monomers, i.e., methacrylic acid in the range of 5-20% by weight and methyl methacrylate in the range of 80-95% by weight.

[0023] In other embodiments, the first monomer mixture comprises other olefinically unsaturated monomers (1iii). The other olefinically unsaturated monomers (1iii) may be a mixture comprising more than one other olefinically unsaturated monomer.

[0024] Preferably, the other olefinic unsaturated monomers (1iii) comprise (meth)acrylate monomers, more preferably, they comprise (meth)acrylate monomers selected from cyclohexyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and mixtures of two or more thereof. Even more preferably, the other olefinic unsaturated monomers (1iii) comprise (meth)acrylate monomers selected from cyclohexyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, and mixtures of two or more thereof.

[0025] Preferably, the other olefinic unsaturated monomers (1iii) are composed of one or more (meth)acrylate monomers selected from cyclohexyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-hydroxyethyl methacrylate and hydroxypropyl methacrylate, more preferably composed of one or more methacrylate monomers selected from cyclohexyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate and mixtures of two or more thereof.

[0026] Polymerization step a) is a solution polymerization step. Solution polymerization is well known in the art. Any process conditions known in the art can be suitably applied. In solution polymerization, monomers in a liquid carrier comprising an organic solvent or a mixture of organic solvents are polymerized in the presence of a free radical initiator. Preferably, the organic solvent is miscible with water. Examples of suitable solvents include glycol ethers such as dipropylene glycol methyl ether. The liquid carrier in polymerization step a) may contain water in an amount of less than 50% by weight, preferably less than 20% by weight, more preferably less than 10% by weight.

[0027] The weight ratio of the first monomer mixture to the liquid carrier is typically in the range of 40 / 60-80 / 20, preferably in the range of 45 / 65-70 / 30.

[0028] Solution polymerization is typically carried out at temperatures in the range of 80-150°C, preferably 100-130°C.

[0029] Initiators suitable for solution polymerization of olefinic unsaturated monomers are well known in the art. Any suitable initiator can be used in any suitable amount.

[0030] In solution polymerization step a), a solution-type acid-functionalized polymer is obtained. This acid-functionalized polymer preferably has an acid value of at least 40 mg KOH / g polymer, more preferably at least 50 mg KOH / g polymer, and even more preferably at least 70 mg KOH / g polymer. Particularly preferred are acid values ​​in the range of 8-125 mg KOH / g polymer. The acid value referred to herein is a calculated acid value (calculated from the amount of monomer used in solution polymerization).

[0031] Preferably, the acid-functionalized polymer has a number-average molecular weight in the range of 5,000-60,000 g / mol, more preferably 10,000-50,000 g / mol, and even more preferably 15,000-40,000 g / mol. Preferably, the polydispersity index of the acid-functionalized polymer is at most 3.0, more preferably at most 2.5. The number-average molecular weight referred to herein is the number-average molecular weight determined according to ISO 16014 using size exclusion chromatography with polystyrene standards. The polydispersity index (the ratio of number-average molecular weight to weight-average molecular weight) is calculated as the quotient of the number-average molecular weight and the weight-average molecular weight, both of which are determined according to ISO 16014 using size exclusion chromatography with polystyrene standards.

[0032] Preferably, the acid-functionalized polymer has a glass transition temperature (calculated Fox glass transition temperature) in the range of at least 40°C, more preferably in the range of 45-150°C, and even more preferably in the range of 50-130°C. To calculate the Fox glass transition temperature, the following glass transition temperatures are used for homopolymers of the following monomers: Methacrylic acid: 505K (228°C) Methyl methacrylate: 378K (105°C) Cyclohexyl methacrylate: 365K (92°C) n-Butyl methacrylate: 293K (20°C) tert-butyl methacrylate: 391K (118°C) Isobutyl methacrylate: 326K (53°C) 2-Hydroxyethyl methacrylate: 330K (57°C) Hydroxypropyl methacrylate: 349K (76°C) Ethyl acrylate: 249K (-24°C) Glycidyl methacrylate: 461K (188°C) For monomers not listed above, use the glass transition temperatures of their homopolymers given in J. Brandrup, EH Immergut and EA Gulle (eds.), Polymer Handbook, 4th edition, John Wiley & Sons, Inc., 1999.

[0033] In neutralization step b), the acid-functionalized polymer obtained in step a) is at least partially neutralized using a neutralizing agent to obtain a at least partially neutralized acid-functionalized polymer. The neutralizing agent can be any suitable neutralizing agent, preferably an amine. Any suitable amine can be used, preferably a tertiary amine. An example of a suitable amine is dimethylaminoethanol. Preferably, at least 40% of any acidic functional groups in the acid-functionalized polymer are neutralized, more preferably at least 50%.

[0034] In dispersion step c), the at least partially neutralized acid-functionalized polymer obtained in b) is dispersed in water to obtain an aqueous dispersion of the emulsified polymer. Preferably, the amount of water should be such that the solids content of the dispersion is in the range of 25-40% by weight, more preferably 30-35% by weight.

[0035] In the second step of polymerization, the olefinic unsaturated monomer component is emulsion polymerized in the presence of an aqueous dispersion of the emulsified polymer. Emulsion polymerization is typically carried out in water in the presence of a free radical initiator, preferably a water-soluble free radical initiator. Emulsion polymerization is well known in the art. Any conditions suitable for emulsion polymerization can be used.

[0036] Emulsion polymerization is preferably carried out at a temperature in the range of 30-100°C.

[0037] Any suitable free radical initiator can be used, such as peroxides, percarbonates, superphosphates, persulfates, or combinations thereof. The free radical initiator can be used alone or as an oxidizing component in redox systems that further include a reducing component (accelerator), such as ascorbic acid, malic acid, oxalic acid, lactic acid, or alkali metal sulfites. Additionally, promoters such as chlorides or sulfates of cobalt, iron, nickel, or copper, preferably ferric sulfate, can be used.

[0038] The olefinically unsaturated monomer component is a second monomer mixture comprising (2i) up to 45% by weight of methyl methacrylate, (2ii) at least 47% by weight of ethyl acrylate, and optionally, (2iii) up to 8.0% by weight of glycidyl methacrylate. All weight percentages are based on the total weight of the olefinically unsaturated monomers in the second monomer mixture. The second monomer mixture is styrene-free. Preferably, the second monomer mixture is free of any aromatic olefinically unsaturated monomers, such as aromatic (meth)acrylate monomers like benzoate (meth)acrylates.

[0039] The second monomer mixture may contain olefinic unsaturated monomers other than (2i), (2ii), and (2iii), preferably in an amount of up to 10% by weight, more preferably up to 5% by weight, or even up to 1% by weight, based on the total weight of the olefinic unsaturated monomers. In a particularly preferred embodiment, the second monomer mixture is free of olefinic unsaturated monomers other than (2i), (2ii), and (2iii) and is therefore composed of up to 45% by weight of methyl methacrylate, at least 47% by weight of ethyl acrylate, and optionally up to 8.0% by weight of glycidyl methacrylate.

[0040] Preferably, the second monomer mixture contains glycidyl methacrylate in an amount ranging from 0 to 5.0% by weight. In one embodiment, the second monomer mixture does not contain glycidyl methacrylate. In another embodiment, the second monomer mixture contains glycidyl methacrylate in an amount ranging from 1.0 to 5.0% by weight.

[0041] The second monomer mixture preferably comprises at least 20% by weight methyl methacrylate based on the total weight of the olefinically unsaturated monomers in the second monomer mixture. In a particularly preferred embodiment, the second monomer mixture comprises 20-45% by weight methyl methacrylate, 50-80% by weight ethyl acrylate, 0-5.0% by weight glycidyl methacrylate, and 0-1% by weight olefinically unsaturated monomers other than methyl methacrylate, ethyl acrylate, and glycidyl methacrylate.

[0042] The acrylic copolymer preferably comprises 20-60% by weight of the acid-functionalized polymer and 40-80% by weight of the polymer polymerized from the olefinic unsaturated monomer component, i.e., the second monomer mixture.

[0043] It has been found that aqueous coating compositions containing the dispersed acrylic copolymers prepared as described above provide coatings with high flexibility and high pencil hardness after curing.

[0044] The coating composition preferably contains an acrylic polymer in the range of 5-65% by weight (w / w solids based on solids) of the total solids of the coating composition, more preferably 10-55% by weight.

[0045] The coating composition may include a crosslinking agent. The crosslinking agent may be any crosslinking agent reactive to any functional group on the acrylic copolymer, such as any carboxyl or hydroxyl functional group on the acrylic copolymer. Alternatively or additionally, the crosslinking agent may be a self-crosslinking compound that forms a network independently of the acrylic copolymer.

[0046] Examples of suitable crosslinking agents include phenol-formaldehyde resins (also known as phenolic plastics or phenolic resins), amino plastic resins, polyisocyanates, and β-hydroxyalkylamides. Phenolic-formaldehyde resins are condensation products of aldehydes and phenols and include linear phenolic resins and methyl phenolic resins. Amino plastic resins are condensation products of aldehydes such as formaldehyde and compounds containing amino or amide groups such as urea, melamine, and benzoguanamine. Preferred amino plastic resins are benzoguanamine-formaldehyde resins, melamine-formaldehyde resins, and urea-formaldehyde resins. Suitable polyisocyanate crosslinking agents include blocked or unblocked diisocyanates, including their trimers.

[0047] Preferably, the crosslinking agent is a phenol-formaldehyde resin, especially a methyl phenolic resin, β-hydroxyalkylamide, or a mixture thereof. For coating compositions applied by roll coating, such as coatings for beverage can lids, the crosslinking agent is preferably a phenol-formaldehyde resin, more preferably a methyl phenolic resin.

[0048] The coating composition may contain a crosslinking agent in any suitable amount. Suitable amounts of crosslinking agent are known in the art and depend on the type of crosslinking agent. The coating composition typically contains a crosslinking agent in the range of 0.5-35% by weight based on the total solids content of the acrylic copolymer. Preferably, the coating composition contains a crosslinking agent in the range of 1.0-20% by weight based on the total solids content of the acrylic copolymer, more preferably in the range of 1.5-10% by weight.

[0049] In one embodiment, the second monomer is free of glycidyl methacrylate and the coating composition contains β-hydroxyalkylamide as a crosslinking agent. In this embodiment, it is preferred that both the first and second monomer mixtures are free of glycidyl methacrylate. The amount of β-hydroxyalkylamide in this embodiment is preferably in the range of 1-10% by weight.

[0050] The coating composition may further comprise pigments and / or one or more additives commonly used in coating compositions to facilitate the manufacture, processing, treatment, and application of the coating composition and / or to improve specific properties of the coating composition or the cured coating obtained therefrom. Pigments may include colored pigments and extender pigments (commonly referred to as fillers). Additives may include catalysts, dyes, lubricants, preservatives, flow control agents, thixotropic agents, dispersants, antioxidants, adhesion promoters, light stabilizers, surfactants, and mixtures thereof.

[0051] The coating composition is an aqueous coating composition and preferably contains water in the range of 25-90% by weight, more preferably in the range of 30-80% by weight, based on the total weight of the coating composition.

[0052] The coating composition may contain an organic solvent, preferably in an amount of up to 40% by weight, more preferably up to 35% by weight, based on the total weight of the coating composition.

[0053] The coating composition preferably has a total solids content in the range of 10-70% by weight, more preferably 20-50% by weight, based on the weight of the coating composition.

[0054] The coating composition is an acrylic-based polymer and preferably free of intentionally added bisphenol A, bisphenol F, aromatic glycidyl ether compounds (e.g., bisphenol A glycidyl ether, bisphenol F glycidyl ether, and linear phenolic epoxy resins), and any epoxide resins or other compounds based thereon. Furthermore, the acrylic copolymer is styrene-free and preferably free of any other aromatic monomers. "Styrene-free" means free of styrene in monomeric or polymeric form. Preferably, the coating composition is styrene-free.

[0055] The second aspect of the present invention relates to a method of coating a metal food or beverage can, a portion thereof, or a metal substrate to be formed into a metal food or beverage can or a portion thereof, comprising applying a coating composition according to one aspect of the present invention to at least a portion of the metal surface of the can, the portion thereof, or the metal substrate to be formed into a metal can or the portion thereof, and curing the applied coating composition.

[0056] The metal used to form rigid food or beverage cans or portions thereof is typically tin-plated steel, cold-rolled steel, or aluminum. The metal substrate may be pretreated before the coating composition is applied to it. If the coating composition is applied via coil coating, the metal substrate is typically pretreated to provide corrosion protection and improve coating adhesion. Coil pretreatment is well known in the art and includes pretreatment with an anti-corrosion washing solution.

[0057] The coating composition can be applied to the surface of the metal before or after optionally pre-treated metal substrates are formed into cans or portions thereof, such as beverage can lids or can bodies. In one embodiment of the method of the invention, the coating composition is applied to the metal substrate, cured, and then the coated metal is formed (e.g., via stamping) into cans or portions thereof (e.g., food or beverage cans or can lids). In this embodiment, the coating composition is preferably applied to the metal surface via a roll-to-roll coating process.

[0058] The applied coating composition can be cured in an oven. Curing conditions can vary depending on the application method and intended end use. Oven temperatures are typically in the range of 100-300°C, preferably 150-260°C. Oven time can vary from 5 seconds to 10 minutes.

[0059] For coating beverage can lids, the coating composition is typically applied via roll-to-roll coating. The roll-to-roll coating is then cured by heating the metal substrate at its peak metal temperature, typically 5-20 seconds, within the range of 180-250°C.

[0060] This coating composition is particularly suitable for coating food and beverage cans (e.g., two-piece cans, three-piece cans, etc.). Two-piece cans are manufactured by joining a can body (typically a drawn metal can body) to a can lid (typically a drawn metal can lid). The coating compositions of the present invention are suitable as food contact coatings and can be advantageously applied to the inside of food or beverage cans or portions thereof. They are particularly suitable for application to beverage can lids via roll coating operations.

[0061] The invention relates, in a final aspect, to an article coated with a coating composition according to the first aspect of the invention, wherein the article is a beverage can lid or a beverage can body, preferably a beverage can lid.

[0062] The present invention is further illustrated by the following non-limiting embodiments. Unless otherwise stated, all parts and percentages are by weight.

[0063] Example

[0064] Test methods

[0065] Pencil hardness

[0066] The film hardness of the dried coating on the beverage can lid, which is made from coated plates, was determined according to the pencil hardness test of ISO 15184.

[0067] resistance to boiling

[0068] The integrity of the coating after exposure to heat and pressure from liquids (water and citric acid) is determined by testing for whitening after such exposure.

[0069] water

[0070] The beverage can lids, fabricated from coated plates, were immersed in tap water at 131°C for 30 minutes. The whitening of the coated substrate was then tested. Whitening is the cloudy / whitish appearance of the coating when it has absorbed water. Whitening is undesirable. Undamaged coatings do not show whitening. Whitening was visually evaluated on a scale of 0 (best—no whitening) to 5 (worst—the film is completely white).

[0071] 2% citric acid

[0072] Beverage can lids made from coated plates were immersed in a 2% by weight citric acid aqueous solution at 121°C. The whitening of the coated substrate was then tested.

[0073] 4% citric acid

[0074] Beverage can lids made from coated plates were immersed in a 4% by weight citric acid aqueous solution at 121°C for 30 minutes. The whitening of the coated substrate was then tested.

[0075] Enamel Rater (ER)

[0076] Beverage can lids made from coated metal sheets were immersed in an electrolyte solution (1% NaCl in water) and subjected to a enamel rating test, in which the current passing through the metal sheet was measured when an electric potential was applied. Coatings with a enamel rating below 2 mA were considered good coatings (flexible and essentially defect-free).

[0077] Example 1 – Emulsified Polymer

[0078] Preparation of emulsified polymers

[0079] Various samples of the emulsified polymer were prepared by solution polymerization of a first monomer mixture consisting of methyl methacrylate (MMA), methacrylic acid (MAA), and a third monomer "X". Monomer "X" was different for each sample. The various monomers "X" used in the various samples are shown in Table 1. For all samples, the weight percentages of the three monomers in the monomer mixture were 23.2 wt% MMA, 13.9 wt% MAA, and 62.9 wt% monomer "X".

[0080] For all samples, solution polymerization was performed on dipropylene glycol methyl ether (Dowanol). TM The reaction was carried out using DPM (purchased from Dow) as the solvent (55 wt% monomer and 45 wt% solvent). The solvent was heated to 125°C. Trigonox 21 (tert-butyl peroxy-2-ethylhexanoate) was added as an initiator (based on monomer weight of 0.75 wt%). While maintaining the temperature at 125°C, the first monomer mixture was added at a constant rate over 90 minutes until the target solids content of 55 wt% was achieved. The resulting polymer solution was then cooled to 90°C.

[0081] The calculated acid value for all emulsified polymer samples was 90 mg KOH / g polymer. Batch size, feed rate, and all other parameters were kept constant throughout the preparation of all samples and controlled by a high-throughput experimental robot.

[0082] The calculated Fox glass transition temperatures of the emulsified polymers are given in Table 1. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the emulsified polymers were determined according to ISO 16014 using THF as the eluent (10% by weight of polymer in THF) and UV detection (254 nm). The number-average molecular weight and polydispersity index (PDI), i.e., the quotient of Mw and Mn, are given in Table 1.

[0083] Properties of emulsified polymers

[0084] To pre-screen the film-forming properties of the emulsified polymer itself, model tests were conducted in which the film is formed from a solution of the emulsified polymer.

[0085] Once the polymer solution is cooled to 90°C, other dipropylene glycol methyl ethers are added until a calculated solids content of 35% by weight is achieved, and 3% by weight (based on solids) of wax dispersant is added to obtain the coating composition.

[0086] The resulting solution is applied to an A4-sized aluminum plate using a wire-wound coating bar. The coated plate is then dried in an oven set to the metal peak temperature of 240°C for 10 seconds. The amount of polymer solution applied should achieve 10-12 g / m². 2 The weight of the film.

[0087] The pencil hardness, boiling resistance, and enamel rating of the cured films were determined as described above. The results of the pencil hardness and boiling resistance tests are shown in Table 1. The enamel rating is unacceptable for all films of the emulsified polymer itself (above 80 mA, most of which are even above 170 mA; a rating below 2 mA is considered acceptable).

[0088] Table 1. Properties of emulsified polymers and cured films of these polymers

[0089] a Fox calculates Tg

[0090] No relationship was found between the pencil hardness of the cured film and the calculated Fox Tg of the polymer. Nor was a relationship found between the pencil hardness of the cured film and the molecular weight of the polymer.

[0091] Example 2—Preparation of Dispersed Acrylic Copolymer (Latex)

[0092] The dispersed acrylic copolymers (latex) were prepared from the emulsified polymers EP 5, EP 11 and EP15 of Example 1 as follows.

[0093] The emulsified polymer solution (55 wt% non-volatiles) was cooled to 90°C. During cooling, dimethylaminoethanol (DMAE) was added at a solution temperature of 105–110°C to neutralize 55% of the carboxyl groups on the polymer, and the solution was stirred for 15 minutes. Once the temperature reached 90°C, a mixture of propylene glycol (PG) and water was slowly added while maintaining the temperature at 90°C with continuous stirring until a theoretical solids content of 18.3 wt% was achieved. The dispersion was further cooled to 50°C with stirring while bubbling nitrogen gas into it. The resulting dispersion was approximately water-viscosity and completely transparent. The amounts of emulsified polymer solution, DMAE, PG, and water are given in Table 2 (items 1–4).

[0094] Ascorbic acid, deionized water, other DMAEs, and FeSO4 were then added to the dispersion in the amounts shown in Table 2 (items 5-8). The mixture was maintained at 50°C for 15 minutes with stirring while bubbling nitrogen into it. The second monomer mixture (items 9-11 in Table 2) was then added, and stirring continued for 30 minutes. The remaining water and free radical polymerization initiator (items 12 and 13 in Table 2) were gradually added. The temperature rise due to the exothermic reaction was monitored, and the reaction mixture was cooled to room temperature after the exothermic reaction was no longer observed.

[0095] The second monomer mixture contains methyl methacrylate (MMA), optional glycidyl methacrylate (GMA), and monomer Y. Monomer Y is selected from various (meth)acrylic monomers. The type and amount of monomer Y are given in Table 3. The following monomers Y are used: n-butyl acrylate (BA); benzyl methacrylate (benzyl MA); ethyl acrylate (EA); and 2-hydroxypropyl acrylate (HPA).

[0096] Table 3 gives the weight percentage of each monomer (MMA, optional GMA, and monomer Y) in the second monomer mixture based on the total weight of the second monomer mixture. The second monomer mixture forms 60% by weight of the final latex solids. The emulsified polymer forms 40% by weight of the final latex solids. Each latex has a non-volatile content of 35% by weight and a VOC (volatile organic compound) content of 500 g / kg.

[0097] Table 2 Latex Preparation

[0098] Example 3—Coating Composition

[0099] To prepare the coating composition, the latexes prepared in Example 2 were added to a blending container. Dowanol DPM (dipropylene glycol monomethyl ether) and polyether-modified polydimethylsiloxane (BYK 333) were added in an amount such that the VOC was 750 g / kg and the Byk 333 concentration was 0.1% by weight based on the total weight of the (wet) coating composition. Finally, 3% by weight (based on solids) of wax was added.

[0100] The resulting coating composition is applied to an acid-washed A4-sized aluminum plate using a wire-wound coating bar. The coated plate is then dried in an oven set to a peak metal temperature of 240°C for 10 seconds. The amount of polymer solution applied should be 10-12 g / m². 2 The weight of the film.

[0101] The pencil hardness, retort resistance, and enamel rating of the cured films were determined as described above. The test results for pencil hardness, enamel rating (ER), and retort resistance are shown in Table 3. As can be seen from Table 3, except for coating compositions 4, 5, 10, 14, 28, and 31, the ER performance of all coating compositions was unacceptable (an ER rating less than 2 mA was considered acceptable). However, coating compositions 4 and 10 did not exhibit sufficient pencil hardness (a pencil hardness of at least 3H was considered sufficient). Only coating compositions 5, 14, 28, and 31 exhibited good ER ratings, good hardness, and acceptable retort resistance.

[0102] Table 3 Coating Compositions—Composition and Properties Comparative test Example 4 The emulsified polymer solution was prepared as described in Example 1 for EP 5. The dispersed acrylic copolymer (latex) was prepared from EP5 as described in Example 2. The second monomer mixture contained 25% by weight methyl methacrylate (MMA), 4% by weight glycidyl methacrylate (GMA), and 71% by weight ethyl acrylate (EA).

[0103] Two different coating compositions were prepared from this latex: one with phenol-formaldehyde resin as a crosslinking agent and the other without a crosslinking agent.

[0104] Crosslinking agent-free coating compositions

[0105] Add the latex to a blending container. Add Dowanol DPM (dipropylene glycol monomethyl ether) and polyether-modified polydimethylsiloxane (BYK 333) in an amount such that the VOC is 750 g / kg and the Byk 333 concentration is 0.1% by weight based on the total weight of the (wet) coating composition. Finally, add 3% by weight (based on solids) of wax.

[0106] Coating compositions containing crosslinking agents

[0107] The latex was added to a blending container. Dowanol DPM (dipropylene glycol monomethyl ether) and polyether-modified polydimethylsiloxane (BYK 333) were added in an amount such that the VOC was 750 g / kg and the Byk 333 concentration was 0.1% by weight based on the total weight of the (wet) coating composition. Finally, 3% by weight of wax and 4% by weight of phenol-formaldehyde resin (both based on solids) were added as crosslinking agents.

[0108] The coating composition is applied to an acid-washed A4-sized aluminum plate using a wire-wound coating bar. The coated plate is then dried in an oven set to the metal peak temperature of 240°C for 10 seconds. The amount of polymer solution applied should be 10-12 g / m². 2 The weight of the film.

[0109] The pencil hardness, retort resistance, and enamel rating (ER) of the cured film were determined as described above. The test results for pencil hardness, enamel rating (ER), and retort resistance are shown in Table 4. As can be seen from Table 4, the presence of the crosslinking agent leads to improved enamel rating (indicating flexibility and coating integrity) and improved retort resistance.

[0110] Table 4—Coating performance with and without crosslinking agent

[0111] Example 5

[0112] Preparation of emulsified polymer EP 17

[0113] The emulsified polymer was prepared by solution polymerization of a first monomer mixture consisting of 23.2 wt% methyl methacrylate (MMA), 13.9 wt% methacrylic acid (MAA), and 62.9 wt% n-butyl methacrylate (nBMA).

[0114] Solution polymerization in propylene glycol n-butyl ether (Dowanol) TM The reaction was carried out using PnB (purchased from Dow) as the solvent (55 wt% monomer and 45 wt% solvent). The solvent was heated to 125°C. Trigonox 21 (tert-butyl peroxy-2-ethylhexanoate) was added as an initiator (based on a monomer weight of 0.75 wt%). The first monomer mixture was added at a constant rate over 90 minutes while maintaining the temperature at 125°C until the target solids content of 55 wt% was achieved. The calculated acid value was 90 mg KOH / g polymer. Batch size, feed rate, and all other parameters were kept the same as those for the emulsified polymer prepared in Example 1 and were controlled by a high-throughput experimental robot.

[0115] Preparation of Dispersed Acrylic Copolymers (Latex)

[0116] Two different dispersion acrylic copolymers were prepared from emulsified polymer EP 17 as follows: one with GMA and one without GMA.

[0117] The emulsified polymer solution (55 wt% non-volatiles) was cooled to 90°C. During cooling, dimethylaminoethanol (DMAE) was added at a solution temperature of 105–110°C to neutralize 40% of the carboxyl groups on the polymer, and the solution was stirred for 15 minutes. Once the temperature reached 90°C, a mixture of propylene glycol (PG) and water was slowly added while maintaining the temperature at 90°C with continuous stirring until a theoretical solids content of 18.3 wt% was achieved. The dispersion was further cooled to 50°C with stirring while bubbling nitrogen gas into it. The resulting dispersion was approximately water-viscosity and entirely transparent or translucent. The amounts of emulsified polymer solution, DMAE, PG, and water are given in Table 2 (items 1–4).

[0118] Then, ascorbic acid, deionized water, other DMAEs, and FeSO4 were added to the dispersion in the amounts shown in Table 2 (items 5-8). The mixture was kept at 50°C for 15 minutes with stirring while bubbling nitrogen into it. The second monomer mixture (items 9-11 in Table 2) was then added and stirring continued for 30 minutes. The remaining water and free radical polymerization initiator (items 12 and 13 in Table 2) were gradually added. The temperature rise due to the exothermic reaction was monitored, and the reaction mixture was cooled to room temperature after the exothermic reaction was no longer observed.

[0119] For the first latex, the second monomer mixture contains 25.0 wt% methyl methacrylate (MMA), 4.0 wt% glycidyl methacrylate (GMA), and 71.0 wt% ethyl acrylate (EA). For the second latex, the second monomer mixture contains 27.0 wt% MMA and 73.0 wt% EA.

[0120] The second monomer mixture constitutes 60% by weight of the final latex solids. The emulsified polymer constitutes 40% by weight of the final latex solids. Each latex has a non-volatile content of 35% by weight and a VOC (volatile organic compound) content of 500 g / kg.

[0121] Coating composition

[0122] Various coating compositions were prepared from latex with and without GMA. The type and amount of crosslinking agent used in the coating compositions were varied.

[0123] The coating composition was prepared as follows. Latex was added to a blending container. Dowanol PnB (propylene glycol n-butyl ether) was added in an amount such that the VOC concentration was in the range of 780-1,000 g / kg. Finally, a crosslinking agent and wax (3% by weight wax solids based on total solids) were added. The type and weight percentage of the crosslinking agent (crosslinking agent solids based on resin solids) are given in Table 5.

[0124] The coating composition is applied to pickled A4-sized aluminum sheets using a wire-wound coating bar. The coated sheets are then dried in an oven set to the metal peak temperature of 240°C for 10 seconds. The amount of polymer solution applied should be 10-12 g / m². 2 The weight of the film.

[0125] The pencil hardness, boiling resistance, and enamel rating (ER) of the cured film were determined as described above. The results of the pencil hardness, enamel rating (ER), and boiling resistance tests are shown in Table 5.

[0126] Table 5 Coating Compositions—Composition and Properties

[0127] a Phenol-formaldehyde resin

[0128] b Primid QM-1260 (purchased from EMS), β-hydroxyalkylamide crosslinking agent

[0129] As shown in Table 5, coating compositions with acceptable hardness and acceptable flexibility can be obtained from both phenolic crosslinking agents and β-hydroxyalkylamide crosslinking agents, as well as latex with GMA monomer and latex without GMA monomer.

Claims

1. An aqueous coating composition comprising a dispersed acrylic copolymer, wherein the acrylic copolymer is obtained by emulsion polymerization of an olefin unsaturated monomer component in the presence of an aqueous dispersion of an emulsified polymer. The aqueous dispersion of the emulsified polymer is obtained as follows: a) Solution polymerization of a first monomer mixture to obtain a solution-type acid-functionalized polymer, wherein the first monomer mixture comprises the following: (1i) Methacrylic acid used in amounts ranging from 5 to 20% by weight (1ii) The amount of methyl methacrylate is at least 15% by weight, and optionally (1iii) Other olefinic unsaturated monomers used in amounts of up to 80% by weight All weight percentages are based on the total weight of (1i), (1ii) and (1iii), and the first monomer mixture is styrene-free; b) Using a neutralizing agent to at least partially neutralize the acid-functionalized polymer obtained in a) to obtain an acid-functionalized polymer that is at least partially neutralized; and c) Disperse the at least partially neutralized acid-functionalized polymer in water. The olefinic unsaturated monomer component is a mixture of second monomers comprising the following: (2i) The amount used is up to 45% by weight of methyl methacrylate; (2ii) The amount of ethyl acrylate is at least 47% by weight; and optionally (2iii) The amount of glycidyl methacrylate used is up to 8.0% by weight. All weight percentages are based on the total weight of the olefinic unsaturated monomers in the second monomer mixture, and the second monomer mixture contains no styrene.

2. The coating composition according to claim 1, wherein the first monomer mixture consists of 5-20% by weight methacrylic acid and 80-95% by weight methyl methacrylate.

3. The coating composition according to claim 1, wherein the first monomer mixture comprises other olefinically unsaturated monomers (1iii).

4. The coating composition according to claim 3, wherein the first monomer mixture does not contain aromatic olefin unsaturated monomers.

5. The coating composition according to claim 3 or 4, wherein the other olefinically unsaturated monomer (1iii) comprises a (meth)acrylate monomer, preferably selected from cyclohexyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and mixtures of two or more thereof.

6. The coating composition according to claim 5, wherein the other monomer (1iii) is selected from cyclohexyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, and mixtures of two or more thereof.

7. The coating composition according to any one of the preceding claims, wherein the second monomer mixture does not contain aromatic olefin unsaturated monomers.

8. A coating composition according to any one of the preceding claims, wherein the second monomer mixture does not contain any olefinic unsaturated monomers other than methyl methacrylate, ethyl acrylate and glycidyl methacrylate.

9. The coating composition according to any one of the preceding claims, wherein the content of glycidyl methacrylate in the second monomer mixture is in the range of 0-5.0% by weight, preferably 1.0-5.0% by weight.

10. A coating composition according to any one of the preceding claims, wherein the second monomer mixture comprises at least 20% by weight methyl methacrylate.

11. A coating composition according to any one of the preceding claims, wherein the acrylic copolymer comprises 20-60% by weight of the acid-functionalized polymer and 40-80% by weight of a polymer polymerized from a mixture of second monomers.

12. The coating composition according to any one of the preceding claims further comprises a crosslinking agent, preferably a phenol-formaldehyde resin, a β-hydroxyalkylamide, or a mixture thereof.

13. A method of coating a metal food or beverage can, a portion thereof, or a metal substrate to be formed into a metal food or beverage can or a portion thereof, comprising applying a coating composition according to any one of the preceding claims to at least a portion of the metal surface of the can, the portion thereof, or the metal substrate to be formed into the metal can or the portion thereof, and curing the applied coating composition.

14. The method of claim 13, wherein the method is a method of coating a metal substrate to be formed into a beverage can lid, and wherein the aqueous coating composition is applied by roll coating and cured by heating at the metal peak temperature of the metal substrate in the range of 180-250°C for 5-20 seconds.

15. An article coated with a coating composition according to any one of claims 1-12, wherein the article is a beverage can lid or a beverage can body, preferably a beverage can lid.

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