Epoxy modified coating for food packaging as well as preparation method and application of epoxy modified coating

Abstract

The invention relates to the technical field of packaging coatings, in particular to an epoxy modified coating for food packaging and a preparation method and application thereof. The epoxy modified coating for food packaging is prepared from the following components in parts by mass: 20 to 40 parts of modified epoxy resin, 10 to 20 parts of phenolic resin, 40 to 75 parts of an organic solvent A, 4 to 7 parts of a wax slurry auxiliary agent, 0.1 to 0.5 part of a de-foaming agent and 0.1 to 0.5 part of a flatting agent. The epoxy modified coating disclosed by the invention has excellent leveling performance, the thickness of a paint film in a single-coating state can reach 12g / m < 2 >, meanwhile, the surface of the paint film has no defects of orange peel, poor leveling and the like, and the appearance of the paint film is far superior to the single-coating effect of a common epoxy system coating; when the epoxy modified coating is in a single-coating state, the appearance and performance parameters of a paint film are consistent with the double-coating effect of a common epoxy system coating, the can making efficiency is effectively improved, the energy consumption of equipment is greatly reduced, and the can making cost is saved.

Description

Technical Field

[0001] This invention relates to the field of packaging coating technology, and in particular to an epoxy-modified coating for food packaging, its preparation method, and its application. Background Technology

[0002] Metal cans are widely used in food and beverage packaging and many other fields due to their excellent mechanical properties, outstanding barrier properties, and significant advantages such as safety, environmental friendliness, and recyclability. However, a serious problem exists in the use of metal cans: electrochemical corrosion occurs when the contents come into contact with the metal substrate. This corrosion not only leads to contamination of the contents but may also cause damage to the can, affecting human health and increasing transportation costs.

[0003] To address the corrosion problem of metal cans, can coatings were developed. Can coatings are applied by spraying or coating the inner and outer layers of the metal can, and then cured by heating to form a protective film, effectively preventing corrosion damage to the metal can body from its contents and external moisture.

[0004] Among various can coatings, epoxy phenolic coatings are frequently used for the inner coating of metal cans due to their unique performance advantages. The cured film of epoxy phenolic coatings exhibits excellent resistance to boiling and acid, as well as good adhesion and processability. Metal cans coated with epoxy phenolic coatings are commonly used for filling beverages, beer, etc. One test in the can coating industry is the sulfur resistance test. For protein-rich contents, a single-layer coating on the inner surface of the can is insufficient for effective protection because the coating thickness is insufficient, resulting in sulfur spots after the sulfur resistance test. Therefore, for protein-rich beverages or high-protein foods, a double-layer coating method is often required. Double-layer coatings provide a thicker film and offer more effective protection, making them a safer option. However, this high-specification packaging with double coatings requires a second coating and a second baking process during can manufacturing, which consumes more time and energy than a typical single-layer coating, significantly increasing can manufacturing costs.

[0005] In addition, the film thickness of epoxy phenolic coatings after double coating is relatively high, generally ranging from 9 to 12 g / m². 2 A thicker film provides better protection for the metal can from corrosion by its contents. Although ordinary epoxy coatings can achieve 9-12 g / m² in a single-coat application using a large scraper... 2 While epoxy coatings can achieve high film thickness, defects such as poor leveling and orange peel texture can occur on the surface when the film is dry. Therefore, it is currently difficult for epoxy coatings to simultaneously meet the requirements of high film thickness and a defect-free surface when applied as a single coat.

[0006] In summary, existing technologies have shortcomings in addressing corrosion protection of metal cans, reducing can manufacturing costs, and improving coating effects, and a new technical solution is urgently needed to overcome these problems. Summary of the Invention

[0007] In order to solve the above-mentioned technical problems in the prior art, the present invention provides an epoxy modified coating for food packaging, its preparation method and application.

[0008] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: The first aspect of the present invention is to provide an epoxy-modified coating for food packaging, comprising, by weight parts: 20-40 parts modified epoxy resin, 10-20 parts phenolic resin, 40-75 parts organic solvent A, 4-7 parts wax paste additive, 0.1-0.5 parts defoamer, and 0.1-0.5 parts leveling agent.

[0009] Based on the above technical solution, the present invention can also be improved as follows: Furthermore, the modified epoxy resin is a dicarboxylic acid modified epoxy resin.

[0010] The epoxy-modified coating for food packaging provided by this invention modifies epoxy resin by incorporating flexible long-chain diacids. This introduces flexible long-chain segments into the rigid epoxy resin, creating a novel epoxy resin with a combined rigid and flexible structure. The flexible segments in this structure significantly enhance the mobility of intermolecular chains. During film formation, the long-chain segments entangle with each other, forming a continuous, dense, and cohesive three-dimensional network film. This fundamentally eliminates defects such as poor leveling and orange peel caused by uneven local internal stress. This structure also significantly improves the flexibility and impact resistance of the final cured coating. During roller coating, shear force breaks up this entangled network, resulting in a reversible deentanglement process, leading to a decrease in coating viscosity and providing better spreading and wetting capabilities. After coating, the shear force disappears, and the entanglement between long molecular chain segments is restored. The recovery time of entanglement between the molecular chain segments of the epoxy resin before modification is extended, allowing the coating to have sufficient time to eliminate defects such as orange peel before setting, thereby obtaining a surface with excellent leveling, comparable to the coating effect of traditional two-coat process.

[0011] Furthermore, the dicarboxylic acid is selected from any one or more of dodecanoic acid, octadecanoic acid, docosahexadecanoic acid, and docosahexadecanoic acid; the epoxy resin is a bisphenol A type epoxy resin.

[0012] Furthermore, the preparation method of the modified epoxy resin includes the following steps: organic solvent B is added to the reaction vessel in advance, and after stirring, epoxy resin and dicarboxylic acid are added to the reaction vessel, then benzyltriethylammonium chloride (TEBAC) is added, the temperature is raised to 130-140°C, and the reaction is carried out for 3-6 hours. After the reaction is completed, the mixture is cooled and filtered to obtain the modified epoxy resin.

[0013] The benzyltriethylammonium chloride (TEBAC) is used as a catalyst, and its dosage is 1‰ to 3‰ of the amount of epoxy resin, preferably 2‰.

[0014] Furthermore, the molar ratio of the epoxy resin to the dicarboxylic acid is (2-3):1, preferably 2.25:1.

[0015] Furthermore, the organic solvent B comprises ethylene glycol ethyl ether acetate and cyclohexanone, wherein the mass ratio of ethylene glycol ethyl ether acetate to cyclohexanone is 1:1.

[0016] Furthermore, the dicarboxylic acid is dodecanoic acid, and the bisphenol A type epoxy resin is E-06.

[0017] The reaction mechanism for preparing modified epoxy resin using dodecanoic acid and E-06 is shown below: .

[0019] Furthermore, the organic solvent A comprises any one or more of ether solvents, ester solvents, aromatic hydrocarbon solvents, alcohol solvents, and ketone solvents, wherein the ether solvent is propylene glycol methyl ether, the ester solvent is ethylene glycol ethyl ether acetate, the aromatic hydrocarbon solvent is xylene, the alcohol solvent is n-butanol, and the ketone solvent is cyclohexanone; and the phenolic resin is butanol etherified bisphenol A propane formaldehyde resin.

[0020] Furthermore, the mass ratio of propylene glycol methyl ether, ethylene glycol ethyl ether acetate, xylene, n-butanol, and cyclohexanone in the organic solvent A is 3:2:3:1:1.

[0021] Furthermore, the wax paste additive is German Lubrizol wax powder 1778, the defoamer is BYK-088, and the leveling agent is BYK-310.

[0022] The second aspect of the present invention is to provide a method for preparing the above-mentioned epoxy modified coating for food packaging, comprising the following steps: adding organic solvent A into a reaction vessel, starting stirring and adding modified epoxy resin, heating to 60-100°C and then adding phenolic resin, reacting for 1-3 hours, naturally cooling down and adding wax paste additive, defoamer and leveling agent, continuing stirring for 1-3 hours, and filtering to obtain the epoxy modified coating.

[0023] Another aspect of the present invention is to provide the application of the above-mentioned epoxy modified coating for food packaging on the inner coating of metal cans.

[0024] Compared with the prior art, the present invention has the following technical effects: The epoxy-modified coating prepared by this invention, while possessing excellent adhesion, resistance to boiling, sulfur resistance, and acid resistance, also exhibits superior leveling properties, achieving a film thickness of 12 g / m² in a single-coat application state. 2 Meanwhile, the paint film surface is free of defects such as orange peel and poor leveling, and the appearance of the paint film is far superior to the single-coat effect of ordinary epoxy system coatings. The epoxy modified coating prepared by this invention, when applied in a single coat, exhibits the same appearance and performance parameters as ordinary epoxy coatings in a two-coat application, effectively improving can-making efficiency and significantly reducing equipment energy consumption, thus saving can-making costs. Detailed Implementation

[0025] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0026] Example 1 Preparation of modified epoxy resin: 15 parts each of ethylene glycol ethyl ether acetate and cyclohexanone were added to the reactor beforehand. After stirring, 27 parts of E-06 and 0.7 parts of dodecanoic acid were added to the reactor, followed by 3.1‰ of benzyltriethylammonium chloride. The mixture was heated to 130°C and reacted for 4 hours. After cooling, the mixture was filtered through a 10µm filter bag to obtain dodecanoic acid-modified epoxy resin.

[0027] Preparation of epoxy modified coatings: Add 40 parts of organic solvent A to the reactor, start stirring and add 20 parts of dodecanoic acid modified epoxy resin. Heat to 80℃ and add 10 parts of butanol etherified bisphenol propane formaldehyde resin. After reacting for 2 hours, allow to cool naturally and add 4 parts of 1778, 0.1 parts of BYK-088 and 0.1 parts of BYK-310. Continue stirring for 3 hours. After filtration with a 10µm filter bag, the epoxy modified coating is obtained.

[0028] Example 2 Preparation of epoxy modified coatings: 73 parts of organic solvent A were added to the reactor. After stirring, 40 parts of the dodecanoic acid modified epoxy resin from Example 1 were added. The temperature was raised to 80°C and 15 parts of butanol etherified bisphenol A propane formaldehyde resin were added. After reacting for 2 hours, the temperature was allowed to drop naturally and 7 parts of 1778, 0.5 parts of BYK-088, and 0.5 parts of BYK-310 were added. Stirring was continued for 3 hours. After filtration using a 10µm filter bag, the epoxy modified coating was obtained.

[0029] Example 3 Preparation of epoxy modified coatings: 65 parts of organic solvent A were added to the reactor. After stirring, 30 parts of the dodecanoic acid modified epoxy resin from Example 1 were added. The temperature was raised to 80°C and 20 parts of butanol etherified bisphenol propane formaldehyde resin were added. After reacting for 2 hours, the temperature was allowed to drop naturally and 5 parts of 1778, 0.3 parts of BYK-088, and 0.3 parts of BYK-310 were added. Stirring was continued for 3 hours. After filtration using a 10µm filter bag, the epoxy modified coating was obtained.

[0030] Comparative Example 1 Preparation of unmodified epoxy coatings: Add 40 parts of organic solvent A to the reactor, start stirring and add 20 parts of E-06. Heat to 80℃ and add 10 parts of butanol etherified bisphenol A propane formaldehyde resin. After reacting for 2 hours, allow to cool naturally and add 4 parts of 1778, 0.1 parts of BYK-088 and 0.1 parts of BYK-310. Continue stirring for 3 hours. After filtration with a 10µm filter bag, the unmodified epoxy coating is obtained.

[0031] Comparative Example 2 Preparation of modified epoxy resin: 15 parts each of ethylene glycol ethyl ether acetate and cyclohexanone were pre-added to the reactor. After stirring, 27 parts of E-06 and 1.55 parts of dodecanoic acid were added to the reactor at a molar ratio of 1:1. Then, 3.1‰ of benzyltriethylammonium chloride was added. The mixture was heated to 130°C and then cooled. After reacting for 4 hours, the mixture was filtered through a 10µm filter bag to obtain dodecanoic acid-modified epoxy resin.

[0032] Preparation of epoxy modified coatings: Add 40 parts of organic solvent A to the reactor, start stirring and add 20 parts of dodecanoic acid modified epoxy resin. Heat to 80℃ and add 10 parts of butanol etherified bisphenol propane formaldehyde resin. After reacting for 2 hours, allow to cool naturally and add 4 parts of 1778, 0.1 parts of BYK-088 and 0.1 parts of BYK-310. Continue stirring for 3 hours. After filtration with a 10µm filter bag, the epoxy modified coating is obtained.

[0033] Preparation of test samples The epoxy-modified coatings prepared in the examples and the epoxy coatings prepared in the comparative examples were both applied in a single-coat manner on tin-plated or chromium-plated thin steel plates, with film thicknesses ranging from 9 to 12 g / m. 2 Select the appropriate scraper and bake at 200℃ for 12 minutes for testing; apply the epoxy coating prepared in the comparative example to a tin-plated or chrome-plated thin steel plate in a double-coat manner, with the double-layer film thickness also controlled at 9-12 g / m². 2 The corresponding scraper combination was selected, and the first baking was carried out at 190℃ for 12 minutes, and the second baking was carried out at 200℃ for 12 minutes, pending testing.

[0034] Apparent test According to GB / T 41899-2022, observe whether there is poor leveling on the surface of the coated tin-plated or chrome-plated thin steel sheet with the coatings of the examples and comparative examples. It is required that the coating film on the coated tin-plated or chrome-plated thin steel sheet is free from unevenness or orange peel.

[0035] acid resistance test According to GB / T 41899-2022, citric acid solution (20 g / L) and acetic acid solution (30 mL / L) were prepared respectively. The test procedure was as follows: The coatings of the examples and comparative examples were coated onto tin-plated or chrome-plated thin steel plates. The coated steel plates were repeatedly folded into squares with a spacing of 3 mm (the spacing between folded surfaces was 3 mm). These squares were then immersed in inert containers containing citric acid solution (20 g / L) and acetic acid solution (30 mL / L), respectively, and sealed. The test containers were then placed in an autoclave at 121°C for 30 minutes. After natural pressure reduction and cooling, the samples were removed, washed, dried, and the coating condition was observed. The inner coating should be free of bubbles, peeling, discoloration, whitening, and corrosion spots after testing.

[0036] Sulfur resistance test According to GB / T 41899-2022, a mixed solution of L-cysteine ​​hydrochloride (0.5 g / L), potassium dihydrogen phosphate (3.6 g / L), and disodium hydrogen phosphate (7.2 g / L) was prepared. The test procedure was as follows: Tin-plated or chrome-plated thin steel plates coated with the examples and comparative examples were repeatedly folded into squares with a spacing of 3 mm. These squares were then immersed in an inert container containing the mixed solution of L-cysteine ​​hydrochloride (0.5 g / L), potassium dihydrogen phosphate (3.6 g / L), and disodium hydrogen phosphate (7.2 g / L), and sealed. The test container was then placed in an autoclave at 121°C for 30 minutes. After natural depressurization and cooling, the samples were removed, washed, dried, and the coating condition was observed. The requirement was that after the test, the inner coating should show no obvious sulfide corrosion, peeling, discoloration, or whitening.

[0037] High-temperature sterilization test The experimental procedure was as follows: A thin steel sheet coated with the paints of the examples and comparative examples was folded into a 10cm V-shape (the total length of the steel sheet was 20cm; folding it lengthwise resulted in a V-shape with a single side length of 10cm). This V-shape was placed in an inert container filled with 5-7cm of pure water and sealed. The test container was then placed in a pressure cooker. Simultaneously, two thin steel sheets coated with the paints of Example 1 and the comparative examples were placed side-by-side (paint films facing outwards) and laid flat in the pressure cooker. The test conditions were 127℃ for 60 minutes. After natural pressure reduction and cooling, the samples were removed, washed, dried, and the coating condition was observed. The inner coating was required to be free of bubbles, peeling, discoloration, whitening, and water spots after the test.

[0038] Deep drawing test According to GB / T 41899-2022, a copper sulfate-hydrochloric acid solution [20% (mass fraction) copper sulfate, 10% (volume fraction) hydrochloric acid] is prepared. The test procedure is as follows: A thin steel plate sample coated with the paints of the examples and comparative examples, with its side facing down, is inserted into an impact tester and dropped from a height of 1 m with a 1 kg hammer. The sample is then immersed in the copper sulfate-hydrochloric acid test solution. After 2 minutes, the sample is removed, washed, and dried. The changes in paint film cracks at the impact site are observed using a 4x magnifying glass. The requirement is that after the test, the coating film should have no dense corrosion spots.

[0039] Adhesion test Using the cross-cut test, eleven cuts were made on the surface of the tin-plated or chrome-plated thin steel sheet coated with the paint of the examples and comparative examples, in both horizontal and vertical directions, with the force just enough to cut into the substrate, forming a total of 100 small squares. Adhesive tape with an adhesion strength of (47±3) N / 100mm was used to stick the tape onto the cross-cut test. The tape was then pressed flat with an eraser 2 to 3 times, leaving a 15 to 20 mm long strip of tape for gripping and then quickly peeled off from the sample surface at a 45° angle. The rating method is shown in Table 1.

[0040] Table 1 Adhesion Test Levels

[0041] The test results of the above embodiments and comparative examples are summarized and shown in Table 2.

[0042] Table 2. Comparison of performance characteristics of coated samples from the examples and comparative examples.

[0043] Note: The standard film thickness for single-coating is 7–8 g / m. 2 The coating thickness in this invention is 9-12 g / m. 2 This ensures that the coating surface is defect-free after curing; the required concentration is 9-12 g / m³. 2 This means that a single coat can achieve all thicknesses within this range, but this may result in orange peel on the surface of the coating after curing, requiring higher performance from the coating.

[0044] According to Table 2, in the examples and comparative examples, the different raw material ratios led to variations in various properties. The ranges of the raw material ratios in the formulations were determined through extensive experimental testing data. Specifically, in Example 2, the whitening in the two acid resistance tests, the sulfur spots in the sulfur resistance test, and the water spots in the high-temperature sterilization test were due to the excessive amount of modified epoxy resin compared to phenolic resin, resulting in a lower crosslinking density and a decrease in the density of the cured network of the paint film. During the tests, acids, sulfur, and water were more easily absorbed into the paint film, causing a decline in its performance. In Example 3, the decrease in apparent leveling performance was due to the excessive amount of phenolic resin compared to modified epoxy resin, which accelerated the curing speed during high-temperature baking. This meant that the paint film surface was cured before it was fully leveled, preventing further leveling. The decrease in adhesion was due to the excessive amount of phenolic resin, which increased the crosslinking density and improved the density of the cured network. The paint film appeared hard and brittle, and the area of ​​breakage after damage during the test increased, resulting in decreased adhesion. In the double coating of Comparative Example 1, since a double coating operation is used, which is the conventional operation of the prior art, the leveling effect of the two coatings is inevitably better. The purpose of this invention is to achieve a single coating effect comparable to the traditional double coating effect without compromising other performance.

[0045] By comparing the above data, the results show that the epoxy-modified coating for food packaging provided by this invention, while possessing excellent adhesion, resistance to boiling, sulfur resistance, and acid resistance, also exhibits excellent leveling properties, and the film thickness in a single-coat application state can reach 12 g / m². 2Meanwhile, epoxy-modified coatings with no orange peel or flow on the paint film surface, when applied in a single coat, exhibit the same appearance and performance parameters as ordinary epoxy coatings applied in a two-coat state. This effectively improves can-making efficiency, greatly reduces equipment energy consumption, and saves can-making costs.

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An epoxy-modified coating for food packaging, characterized in that, The product comprises the following components by weight: 20-40 parts modified epoxy resin, 10-20 parts phenolic resin, 40-75 parts organic solvent A, 4-7 parts wax paste additive, 0.1-0.5 parts defoamer, and 0.1-0.5 parts leveling agent.

2. The epoxy-modified coating for food packaging according to claim 1, characterized in that, The modified epoxy resin is a dicarboxylic acid modified epoxy resin.

3. The epoxy-modified coating for food packaging according to claim 2, characterized in that, The dicarboxylic acid is selected from any one or more of dodecanoic acid, octadecanoic acid, docosahexadecanoic acid, and docosahexadecanoic acid; the epoxy resin is a bisphenol A type epoxy resin.

4. The epoxy-modified coating for food packaging according to claim 2 or 3, characterized in that, The preparation method of the modified epoxy resin includes the following steps: organic solvent B is added to the reaction vessel in advance, and after stirring, epoxy resin and dicarboxylic acid are added to the reaction vessel, then benzyltriethylammonium chloride is added, the temperature is raised to 130-140°C, and the reaction is carried out for 3-6 hours. After the reaction is completed, the mixture is cooled and filtered to obtain the modified epoxy resin.

5. The epoxy-modified coating for food packaging according to claim 4, characterized in that, The molar ratio of the epoxy resin to the dicarboxylic acid is (2-3):

1.

6. The epoxy-modified coating for food packaging according to claim 4, characterized in that, The organic solvent B includes ethylene glycol ethyl ether acetate and cyclohexanone.

7. The epoxy-modified coating for food packaging according to claim 3, characterized in that, The dicarboxylic acid is dodecanoic acid, and the bisphenol A type epoxy resin is E-06.

8. The epoxy-modified coating for food packaging according to claim 1, characterized in that, The organic solvent A comprises any one or more of ether solvents, ester solvents, aromatic hydrocarbon solvents, alcohol solvents, and ketone solvents. The ether solvent is propylene glycol methyl ether, the ester solvent is ethylene glycol ethyl ether acetate, the aromatic hydrocarbon solvent is xylene, the alcohol solvent is n-butanol, and the ketone solvent is cyclohexanone. The phenolic resin is butanol etherified bisphenol A propane formaldehyde resin.

9. A method for preparing an epoxy-modified coating for food packaging according to any one of claims 1 to 8, characterized in that, The process includes the following steps: adding organic solvent A into the reactor, starting the stirrer and adding modified epoxy resin, heating to 60-100℃ and then adding phenolic resin, reacting for 1-3 hours, naturally cooling down and adding wax paste additives, defoamers and leveling agents, continuing to stir for 1-3 hours, and then filtering to obtain epoxy modified coating.

10. The application of the epoxy modified coating for food packaging as described in any one of claims 1 to 8 on the inner coating of metal cans.

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