Coating compositions for metal cans, their preparation methods and applications

Abstract

This invention discloses a coating composition for metal cans, its preparation method, and its application, relating to the field of food packaging coating technology. The invention uses acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloyloxypropyltrimethoxysilane, and polyester resin copolymerization modification to prepare a water-soluble polyester resin with excellent abrasion resistance, adhesion, and stability. Combined with non-amino resin curing agents and additives, a coating composition with excellent high-temperature cooking resistance, abrasion resistance, and adhesion can be prepared. Furthermore, because no solvents or other raw materials that could cause the migration of harmful substances are used, there is no migration of harmful substances, preventing them from migrating into the packaged food and harming health. This invention meets the requirements for coatings used in food-grade metal can packaging.

Description

Technical Field

[0001] This invention belongs to the field of food packaging coating technology, and particularly relates to coating compositions for metal cans, their preparation methods, and applications. Background Technology

[0002] Aluminum cans are common food packaging containers, frequently used for packaging various canned foods and beverages. During the production process, aluminum cans typically undergo a coating process, where a layer of varnish is applied to the printed outer surface of the can to form a protective layer. This makes the surface ink more vibrant and glossy, protects the ink from scratches, and enhances the surface's appearance and durability. In some aluminum can food production processes, the contents of the can need to be sterilized by boiling. This requires the outermost varnish to possess excellent chemical stability and abrasion resistance during the production, storage, and transportation of the can, as well as resistance to boiling and high temperatures.

[0003] Common varnishes are mainly composed of acrylic resin, wax, additives (leveling agents, defoamers), and solvents, with acrylic resin being the matrix of the entire varnish solution. Patent (CN111320902B) provides a low-formaldehyde migration varnish for the outer packaging of printed tin food cans and its preparation method. The raw materials for preparing the varnish, by weight, include 60-70 parts acrylic resin, 4-8 parts wax paste, 5-15 parts curing agent, 10-15 parts solvent, and 0.1-1 parts additives. By using a suitable curing agent and controlling the selection of acrylic resin, wax paste, and additives, while ensuring low formaldehyde content to avoid adverse effects on food hygiene and safety and improving food safety, the varnish's properties such as anti-sticking, resistance to boiling, and adhesion can also meet industry requirements during coating and can-making processes. However, this technical solution, due to the use of organic solvents, still cannot prevent the migration and spillage of formaldehyde and low-molecular-weight organic substances, and it does not address abrasion resistance.

[0004] Patent (CN115044282B) discloses a safe, non-migrating UV varnish for food cans and its preparation method. The UV varnish, by weight, comprises 55-75 parts modified epoxy acrylate resin, 10-30 parts reactive monomers, 5-10 parts photoinitiator, 2-5 parts wax, and 0.3-1 parts other additives. The UV varnish features a simple printing process that requires no baking, low energy consumption during UV curing, and low VOC emissions. However, the stability of this technology in the boiling test is only 100℃, and it does not address the abrasion resistance of the varnish.

[0005] Patent (CN111454631B) discloses a cool color-changing effect varnish and its preparation method. The varnish, by weight, comprises 60-70 parts thermosetting acrylic resin, 5-10 parts phenylamino resin, 2-8 parts filler, and 12-20 parts solvent; the filler includes angle-dependent color pigments. When used in product packaging materials, such as food packaging cans, the resulting paint film makes the can's color brighter and produces angle-dependent color changes; it also exhibits good scratch resistance and processing performance during coating and can-making processes, and has high adhesion to food packaging cans, meeting processing and usage requirements. However, this technical solution does not address the varnish's resistance to boiling.

[0006] Patent (CN104356891A) discloses a high-temperature resistant deep-drawing varnish for tinplate can manufacturing and its preparation method. The varnish raw material formula, by weight, is as follows: 55-70 parts polyester resin, 5-12 parts amino resin, 2-5 parts blocked isocyanate, 15-30 parts solvent, 2-5 parts wax, 0.1-0.3 parts acid catalyst, 0.1-0.3 parts leveling agent, and 0.01-0.1 parts defoamer. The coating exhibits excellent deep-drawing resistance. After the deep-drawn can is manufactured, it can be boiled at 125℃. By improving the adhesion and tear resistance of the varnish, the coating can be ensured not to crack, and the pattern remains clear and perfect. It can be widely used in various deep-drawn two-piece food cans. However, this patent does not verify the coating's boiling performance, abrasion resistance, etc.

[0007] It is evident that existing metal can varnishes are primarily solvent-based, and generally do not simultaneously address the varnish's abrasion resistance, boiling resistance, and the migration of harmful substances. Based on this, this invention proposes a coating composition for metal cans. It utilizes functional acrylic monomers to graft-modify polyester, synthesizing a water-soluble polyester resin. Combined with non-amino resin curing agents and additives, a water-based varnish with excellent high-temperature boiling resistance, abrasion resistance, adhesion, and low migration of harmful substances can be prepared. Summary of the Invention

[0008] This invention addresses the problems existing in the prior art by providing a coating composition for metal cans, its preparation method, and its application. This invention uses acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloyloxypropyltrimethoxysilane, and polyester resin for copolymer modification. This allows for the preparation of a water-soluble polyester resin with excellent abrasion resistance, adhesion, and stability. Combined with non-amino resin curing agents and additives, a coating composition with excellent high-temperature cooking resistance, abrasion resistance, and adhesion can be prepared. Furthermore, because no solvents or other raw materials that could cause the migration of harmful substances are used, there is no migration of harmful substances, preventing them from entering the packaged food and harming health. This invention meets the requirements for coatings used in food metal can packaging.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] In a first aspect, the present invention provides a coating composition for metal cans, comprising, by weight: 35-70 parts of water-soluble polyester resin, 3-15 parts of blocked water-based isocyanate, 0.5-1.5 parts of catalyst, 10-30 parts of water, and 0.1-2 parts of additives.

[0011] In a preferred embodiment, the water-soluble polyester resin is obtained by copolymerization of polyester resin, acrylic acid, isobornyl acrylate, and phosphate-containing acrylate using a free radical initiator.

[0012] In a preferred embodiment, the mass ratio of the polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane is 10:2-4:1-3:0.5-2:0.5-1.

[0013] In a preferred embodiment, the water-soluble polyester resin is prepared using the following process:

[0014] Polyester resin and solvent are added to a reaction vessel, the temperature is raised, and a mixed solution of acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloyloxypropyltrimethoxysilane and free radical initiator is added. After the reaction is kept at a certain temperature, the solvent is removed, a neutralizing agent is added for neutralization, and water is added to disperse the resin to obtain a water-soluble polyester resin.

[0015] Preferably, the heat preservation reaction conditions are: heat preservation reaction at 110-140℃ for 2-6 hours; the solvent is selected from at least one of alcohol organic solvents, ether organic solvents, ester organic solvents and benzene organic solvents, and the amount of solvent is 1.5-4 times the mass of polyester resin.

[0016] Preferably, the neutralizing agent is selected from at least one of N,N-dimethylethanolamine and / or AMP-95, and the amount of neutralizing agent added is such that the pH of the water-soluble polyester resin is 7.5-8.5.

[0017] Preferably, in the water dispersion step, the amount of water added is 1-3 times the mass of the polyester resin, and the water is selected from deionized water and / or distilled water.

[0018] This invention utilizes acrylic acid, isobornyl acrylate, phosphate-containing acrylates, γ-methacryloyloxypropyltrimethoxysilane, and polyester resin copolymerization modification. On one hand, isobornyl acrylate contains a bridged ring, is hard and tough, which can impart appropriate hardness to the coating and improve toughness modification. It also exhibits good adhesion to various metal substrates, good resistance to solvents and acids / alkalis, and good thermal stability, thus improving the toughness, adhesion, and thermal stability of the polyester resin. On the other hand, phosphate-containing acrylates and γ-methacryloyloxypropyltrimethoxysilane are commonly used as adhesion promoters and have a certain chain length, exhibiting good adhesion to various metal substrates and improving adhesion, flexibility, and stability. However, phosphate-containing acrylates have a certain color, which can affect transparency, and excessive use of phosphate-containing acrylates can lead to excessive hydrophilicity, affecting high-temperature retort resistance. The compatibility between γ-methacryloxypropyltrimethoxysilane and polyester resin is not very good. Therefore, excessive addition of either will lead to problems with transparency, water resistance, and compatibility, further affecting the overall performance. By using a combination of the two, adhesion, flexibility, and stability can be comprehensively improved with a smaller addition amount. Thirdly, by introducing acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloxypropyltrimethoxysilane copolymerization, good water solubility can be imparted to the polyester resin. Furthermore, the introduced carboxyl and hydroxyl groups can further react with the blocked waterborne isocyanate, promoting the curing of the polyester resin and increasing the crosslinking network density, thereby improving wear resistance and stability. Through the above modifications, water-soluble polyester resin can be prepared, and the wear resistance, adhesion, and stability of the coating can be improved.

[0019] In a preferred embodiment, the free radical initiator is selected from at least one of benzoyl peroxide, benzoyl tert-butyl peroxide, and methyl ethyl ketone peroxide, and is used in an amount of 2-4% of the total amount of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane.

[0020] In a preferred embodiment, the phosphate-containing acrylate is selected from 2-hydroxyethyl methacrylate phosphate and / or alkyl acrylate phosphate.

[0021] In a preferred embodiment, the polyester resin is an unsaturated polyester, which is selected from at least one of orthophthalic unsaturated polyester, isophthalic unsaturated polyester, terephthalic unsaturated polyester, and vinyl ester unsaturated polyester.

[0022] In a preferred embodiment, the catalyst comprises at least one of blocked p-toluenesulfonic acid, blocked dodecylbenzenesulfonic acid, and blocked dinonylnaphthalenedisulfonic acid.

[0023] In a preferred embodiment, the additive is selected from at least one of dispersants, defoamers, leveling agents, and thickeners.

[0024] Secondly, the present invention provides a method for preparing the above-mentioned coating composition for metal cans, comprising the following steps:

[0025] Weigh the raw materials according to the formula, and add the water-soluble polyester resin, additives, blocked water-based isocyanate and catalyst to the water in sequence, and mix evenly.

[0026] Thirdly, the present invention provides the application of the above-described coating composition for metal cans in metal can packaging coatings.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. This invention uses acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloyloxypropyltrimethoxysilane and polyester resin copolymerization modification to prepare a water-soluble polyester resin with wear resistance, adhesion and stability. Combined with non-amino resin curing agents and additives, a coating composition with excellent high-temperature cooking resistance, wear resistance and adhesion can be prepared.

[0029] 2. The coating composition for metal cans described in this invention is an aqueous system. Since no solvents or other raw materials that may cause the migration of harmful substances are used, there is no migration of harmful substances, and harmful substances will not migrate into the packaged food and harm health.

[0030] 3. The coating composition for metal cans of the present invention has excellent adhesion, abrasion resistance, high temperature cooking resistance, solvent resistance and light transmittance, as well as extremely low formaldehyde migration rate, which can meet the application requirements of coatings for food metal can packaging. Detailed Implementation

[0031] It is worth noting that the raw materials used in this invention are all commercially available products, and their sources are not specifically limited.

[0032] Polyester resin: 196 unsaturated polyester resin, purchased from Shanghai Fengzhan Chemical Technology Co., Ltd.;

[0033] Phosphate-containing acrylate: EM39, purchased from Changxing Chemical Materials (Zhuhai) Co., Ltd.;

[0034] Defoamer: BYK-024;

[0035] Dispersant: BYK-190;

[0036] Blocked aqueous isocyanate: F-70D, purchased from Wuhan Shiquanxing New Material Technology Co., Ltd.;

[0037] Catalysts: Blocked acid catalysts BD7735 and BD7743, purchased from Guangzhou Oupeng Chemical Co., Ltd.

[0038] Free radical initiator: Benzoyl peroxide, commercially available;

[0039] Solvent: Ethyl acetate, commercially available;

[0040] Neutralizing agents: N,N-dimethylethanolamine, AMP-95, commercially available;

[0041] All other raw materials are commercially available.

[0042] Example 1

[0043] The coating composition for metal cans comprises the following parts by weight: 70 parts of water-soluble polyester resin, 15 parts of blocked water-based isocyanate, 1.5 parts of catalyst (BD7735), 30 parts of water, 0.5 parts of defoamer, and 0.5 parts of dispersant.

[0044] The water-soluble polyester resin is obtained by copolymerization of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane in a mass ratio of 10:3:2:2:1 using a free radical initiator. The amount of solvent used is three times the mass of the polyester resin, and the amount of free radical initiator added is 3% of the total amount of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane. The specific preparation process is as follows:

[0045] Polyester resin and solvent (ethyl acetate) were added to the reaction vessel, and the temperature was raised to 130°C. A mixed solution of acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloyloxypropyltrimethoxysilane, and free radical initiator was added dropwise over 40 minutes. After reacting at 130°C for 3 hours, the solvent was removed by vacuum distillation, and the temperature was lowered to 60°C. A neutralizing agent (N,N-dimethylethanolamine) was added dropwise while stirring. After the temperature stabilized, distilled water with a mass twice that of the polyester resin was added and dispersed at high speed to obtain a water-soluble polyester resin (solid content of about 47%, pH 7.9).

[0046] Example 2

[0047] The coating composition for metal cans comprises the following components by weight: 55 parts water-soluble polyester resin, 10 parts blocked water-based isocyanate, 1 part catalyst (BD7743), 20 parts water, 0.2 parts defoamer, and 0.3 parts dispersant.

[0048] The water-soluble polyester resin is obtained by copolymerization of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane in a mass ratio of 10:4:3:0.5:0.5 using a free radical initiator. The amount of solvent used is twice the mass of the polyester resin, and the amount of free radical initiator added is 4% of the total amount of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane. The specific preparation process is as follows:

[0049] Polyester resin and solvent (ethyl acetate) were added to the reaction vessel, and the temperature was raised to 110°C. A mixed solution of acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloxypropyltrimethoxysilane, and free radical initiator was added dropwise over 40 minutes. After reacting at 110°C for 5 hours, the solvent was removed by vacuum distillation, and the temperature was lowered to 50°C. A neutralizing agent (N,N-dimethylethanolamine) was added dropwise while stirring. After the temperature stabilized, distilled water with a mass twice that of the polyester resin was added and dispersed at high speed to obtain a water-soluble polyester resin (solid content of about 47%, pH 7.8).

[0050] Example 3

[0051] The coating composition for metal cans comprises the following components by weight: 35 parts water-soluble polyester resin, 5 parts blocked water-based isocyanate, 0.5 parts catalyst (BD7735), 10 parts water, 0.05 parts defoamer, and 0.05 parts dispersant.

[0052] The water-soluble polyester resin is obtained by copolymerization of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane in a mass ratio of 10:2:0.5:1.5:1, initiated by a free radical initiator. The amount of solvent used is three times the mass of the polyester resin, and the amount of free radical initiator added is 3% of the total amount of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane. The specific preparation process is as follows:

[0053] Polyester resin and solvent (ethyl acetate) were added to the reaction vessel, and the temperature was raised to 130°C. A mixed solution of acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloyloxypropyltrimethoxysilane, and free radical initiator was added dropwise over 40 minutes. After reacting at 130°C for 3 hours, the solvent was removed by vacuum distillation, and the temperature was lowered to 60°C. A neutralizing agent (AMP-95) was added dropwise while stirring. After the temperature stabilized, 1.5 times the mass of distilled water was added and dispersed at high speed to obtain a water-soluble polyester resin (solid content of about 50%, pH 8.0).

[0054] Comparative Example 1

[0055] Except for replacing KH570 with an equal amount of EM39, the rest is the same as in Example 1.

[0056] Comparative Example 2

[0057] Except for replacing EM39 with an equal amount of KH570, the rest is the same as in Example 1.

[0058] Comparative Example 3

[0059] Except for replacing N,N-dimethylethanolamine with triethylamine, the rest is the same as in Example 1.

[0060] Comparative Example 4

[0061] Except for replacing isobornyl acrylate with an equal amount of styrene, the rest is the same as in Example 1.

[0062] Comparative Example 5

[0063] Except for the different process for the water-soluble polyester resin, the rest is the same as in Example 1, and the specific process is as follows:

[0064] Polyester resin and solvent (ethyl acetate) were added to the reaction vessel, and the temperature was raised to 130°C. A mixed solution of acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloyloxypropyltrimethoxysilane, and one-third of the free radical initiator were added dropwise over 40 minutes. One-third of the free radical initiator was added after 1 hour and 2 hours of reaction at 130°C. After a total of 3 hours of reaction, the solvent was removed by vacuum distillation, and the temperature was lowered to 60°C. A neutralizing agent was added dropwise while stirring. After the temperature stabilized, distilled water with a mass of twice that of the polyester resin was added and dispersed at high speed to obtain water-soluble polyester resin.

[0065] Test Example Coating Performance Test

[0066] The coatings were prepared using conventional coating spraying processes. The coating compositions of Examples 1-3 and Comparative Examples 1-5 were uniformly sprayed onto aluminum plates, with a target dry film weight of 8 g / m². 2 Bake in an oven at 180℃ for 4 minutes until fully cured.

[0067] 1. The performance testing method is as follows:

[0068] 1) Using the Taber test method, the rotating disk rotates at 60 r / min and the pressure arm bears a 1 kg load. The paint film surface is worn down with a hard rubber friction wheel embedded with diamond abrasive. The number of rotations required to wear down the paint film and expose the substrate is recorded.

[0069] 2) Butyl ketone (MEK) resistance test: Wrap a 1kg iron hammer head in a cotton cloth, immerse it in liquid butyl ketone (MEK) solvent, and move it back and forth to wipe the surface of the coating being tested. One back and forth motion is counted as one time, until the coating is damaged and the metal substrate is exposed. The number of wiping times is the MEK value.

[0070] 3) High-temperature boiling resistance: Boil the coated aluminum plate sample in water at 121℃ for 60 minutes. Observe whether there is any bubbling, whitening, water droplets, fading, or displacement during boiling. If so, it is considered qualified; otherwise, it is unqualified. At the same time, the adhesion before and after boiling is verified by the cross-cut adhesion test. Grade 0 is the best, and grade 5 is the worst. The adhesion of the coated sample without the above treatment is recorded as F0, and the adhesion after treatment is recorded as F1.

[0071] 4) Light transmittance: The coating compositions of Examples 1-3 and Comparative Examples 1-5 were uniformly sprayed onto glass plates, with the film thickness controlled at 200-300 μm. The light transmittance of the blank glass plate and the coated glass plate was tested at 400-800 nm using a Cary 5000 UV-Vis spectrophotometer (Agilent Technologies, USA). The average value was taken. The average light transmittance of the coating = the average light transmittance of the coated glass plate / the average light transmittance of the blank glass plate × 100%.

[0072] 5) Formaldehyde migration: The coated sample was immersed in a 10% ethanol aqueous solution at 121 degrees Celsius for 30 minutes. The formaldehyde migration was then tested according to the acetylacetone spectrophotometric method in GB31604.48-2016, with units of μg / dm³. 2 .

[0073] 2. The effect data of Examples 1-3 and Comparative Examples 1-5 are shown in Table 1.

[0074] Table 1

[0075]

[0076] As can be seen from Table 1, the coating compositions of Examples 1-3 have excellent adhesion, abrasion resistance, high-temperature boiling resistance, solvent resistance and light transmittance, which are significantly better than those of Comparative Examples 1-5. Furthermore, no formaldehyde was detected in Examples 1-3 and Comparative Examples 1-5, indicating extremely low formaldehyde migration rates.

[0077] Comparing Example 1 and Comparative Examples 1-2, it can be seen that using a large amount of phosphate-containing acrylate EM39 or γ-methacryloyloxypropyltrimethoxysilane KH570 alone is detrimental to abrasion resistance, solvent resistance, high-temperature boiling resistance, and light transmittance. However, Example 1, which uses a combination of both, achieves ideal overall performance. Comparing Example 1 and Comparative Example 3, it can be seen that the type of neutralizing agent affects the physical properties of the coating composition. Using triethylamine as a neutralizing agent resulted in a decrease in the overall physical properties of the prepared coating composition. Comparing Example 1 and Comparative Example 4, it can be seen that isobornyl acrylate has better overall performance than styrene. This is because isobornyl acrylate contains a bridged ring, which provides better adhesion and toughness compared to methyl methacrylate or styrene, thus improving the overall performance of the coating. Comparing Example 1 and Comparative Example 5, it can be seen that the present invention gradually adds the free radical initiator to the polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane after mixing. This is beneficial to improving the polymerization efficiency, the degree of modification of the polyester resin, and its water solubility and dispersibility, thereby improving the overall performance of the coating.

[0078] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A coating composition for metal cans, characterized in that, By weight, it includes: 35-70 parts of water-soluble polyester resin, 3-15 parts of blocked water-based isocyanate, 0.5-1.5 parts of catalyst, 10-30 parts of water, and 0.1-2 parts of additives; The water-soluble polyester resin is obtained by copolymerization of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane initiated by a free radical initiator; the mass ratio of the polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane is 10:2-4:1-3:0.5-2:0.5-1. The catalyst includes at least one of blocked p-toluenesulfonic acid, blocked dodecylbenzenesulfonic acid, and blocked dinonylnaphthalenedisulfonic acid.

2. The coating composition according to claim 1, characterized in that, The preparation process of the water-soluble polyester resin is as follows: Polyester resin and solvent are added to a reaction vessel, the temperature is raised, and a mixed solution of acrylic acid, isobornyl acrylate, phosphate-containing acrylate, γ-methacryloyloxypropyltrimethoxysilane and free radical initiator is added. After the reaction is kept at a certain temperature, the solvent is removed, a neutralizing agent is added for neutralization, and water is added to disperse the resin to obtain a water-soluble polyester resin.

3. The coating composition according to claim 2, characterized in that, The heat preservation reaction conditions are as follows: heat preservation reaction at 110-140℃ for 2-6 hours; the solvent is selected from at least one of alcohol organic solvents, ether organic solvents, ester organic solvents and benzene organic solvents, and the amount of solvent used is 1.5-4 times the mass of polyester resin; the neutralizing agent is selected from at least one of N,N-dimethylethanolamine and / or AMP-95, and the amount of neutralizing agent added is based on making the pH of the water-soluble polyester resin 7.5-8.5; the water dispersion is carried out, and the amount of water added is 1-3 times the mass of polyester resin.

4. The coating composition according to claim 1, characterized in that, The free radical initiator is selected from at least one of benzoyl peroxide, benzoyl tert-butyl peroxide, and methyl ethyl ketone peroxide, and is used in an amount of 2-4% of the total amount of polyester resin, acrylic acid, isobornyl acrylate, phosphate-containing acrylate, and γ-methacryloyloxypropyltrimethoxysilane; the phosphate-containing acrylate is selected from 2-hydroxyethyl methacrylate phosphate and / or alkyl acrylate phosphate; the polyester resin is an unsaturated polyester, and the unsaturated polyester is selected from at least one of orthophthalic unsaturated polyester, isophthalic unsaturated polyester, terephthalic unsaturated polyester, and vinyl ester unsaturated polyester.

5. The coating composition according to claim 1, characterized in that, The additive is selected from at least one of dispersants, defoamers, leveling agents, and thickeners.

6. The method for preparing a coating composition for metal cans according to any one of claims 1-5, characterized in that, Includes the following steps: Weigh the raw materials according to the formula, and add the water-soluble polyester resin, additives, blocked water-based isocyanate and catalyst to the water in sequence, and mix evenly.

7. The use of the coating composition for metal cans as described in any one of claims 1-5 in metal can packaging coatings.

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