An organic sol-gel coating for the interior walls of cans for acidic beverages and a method for its preparation

Abstract

The application discloses an organic sol coating for the inner wall of an acidic beverage can and a preparation method thereof, and belongs to the technical field of food packaging coating. The coating comprises an organic sol film former, a barrier agent, a plasticizer, a cross-linking curing agent, an adhesion promoter and an auxiliary agent; the barrier agent is prepared by compounding sodium magnesium lithium silicate, nano calcium carbonate, basic magnesium aluminum carbonate, a polyphenol-zirconium complex component and a silane coating component. The finished product is obtained through dispersion, dispersion sand milling, mixing, defoaming and filtering. The obtained coating has good acid immersion resistance, barrier property, adhesion and bending processing adaptability, and is suitable for the protection of the inner wall of an acidic beverage metal can.

Description

Technical Field

[0001] This invention relates to the field of food packaging coating technology, specifically to an organosol coating for the inner wall of acidic beverage cans and its preparation method. Background Technology

[0002] Metal beverage can interior coatings are functional protective coatings applied between the inner wall of the metal packaging container and the beverage contents. They primarily isolate the metal substrate from the contents, reducing the corrosion of the metal can by moisture, oxygen, acidic components, and corrosive ions. They are widely used in the packaging of carbonated beverages, fruit juices, tea drinks, energy drinks, and other acidic beverages. As food packaging materials evolve towards greater safety, resistance to various media, longer shelf life, and processing compatibility, organic coatings for can interiors, with their excellent film-forming properties, adhesion, acid resistance, and barrier properties, are receiving increasing attention.

[0003] Existing coatings for the inner walls of acidic beverage cans typically use organic resins as the main film-forming component, combined with plasticizers, crosslinking curing agents, adhesion promoters, wetting and dispersing agents, and other additives to form a coating system. These coatings can form a continuous film on the surface of a metal substrate and, to some extent, prevent direct contact between the beverage contents and the metal substrate. However, under long-term storage conditions of acidic beverages, acidic components such as citric acid, malic acid, phosphoric acid, and carbonic acid, as well as moisture, oxygen, and metal ions, can still migrate to the metal substrate through micropores, free volume, interface defects, or filler dispersion defects in the coating. This can lead to problems such as whitening, blistering, decreased adhesion, edge peeling, localized exposure of the substrate, or can corrosion.

[0004] In existing technologies, to improve the barrier performance of coatings on the inner wall of tanks, lamellar silicates, carbonates, oxides, or other inorganic fillers are typically compounded with organic resins to extend the diffusion path of acidic media and corrosive ions. Other technologies improve coating density and interfacial bonding by increasing crosslinking density, introducing adhesion promoters, or adding wetting and dispersing agents. However, most of these methods involve simple dispersion or single-interface modification of inorganic fillers and organic film-forming systems, making it difficult to simultaneously form a stable multi-level barrier structure and good organic-inorganic interfacial bonding. When inorganic fillers are insufficiently dispersed or have inadequate surface compatibility in the organic sol system, agglomeration, sedimentation, filtration difficulties, and film defects easily occur, affecting the coating's acid resistance, barrier properties, and application stability.

[0005] Therefore, it is necessary to develop an acidic beverage can inner wall coating system that combines sheet barrier, acid response buffer, polyphenol-metal complex protection, silane interface coating, and organosol film-forming compatibility to improve the dispersion stability and interfacial compatibility of the barrier agent in the organosol film-forming system, reduce the migration of acidic media, moisture, and corrosive ions to the metal substrate, and improve the coating's acid immersion resistance, adhesion, and bending processing adaptability. Summary of the Invention

[0006] The purpose of this invention is to address the technical problems in existing organic coatings for the inner walls of acidic beverage cans, such as insufficient compatibility between inorganic barrier fillers and organic sol film-forming systems, weak interfacial bonding, poor dispersion stability, and the tendency for coatings to whiten, bubble, lose adhesion, reduce barrier performance, bend and crack, or expose the substrate during long-term immersion in acidic beverages and can processing. The invention provides an organic sol coating for the inner walls of acidic beverage cans and its preparation method. This coating uses polyvinyl butyral and polycaprolactone diol as organosol film-forming agents, and introduces a barrier agent composed of sodium magnesium lithium silicate, nano calcium carbonate, basic aluminum magnesium carbonate, tannic acid, phytic acid, zirconium oxychloride, melamine, tetraethyl orthosilicate, γ-(2,3-epoxypropoxy)propyltrimethoxysilane, polyvinylpyrrolidone, and sodium hexametaphosphate. Through the multi-layer synergistic effect of the lamellar silicate barrier structure, acid-responsive inorganic sacrificial sites, polyphenol-zirconium complex layer, silane coating layer, and organosol film-forming system, the dispersion and fixation ability of the barrier agent in the coating system, interfacial compatibility, acid medium barrier effect, and film-forming processing adaptability are improved, thereby obtaining an organosol coating for the inner wall of acidic beverage cans with good acid immersion resistance, electrochemical barrier performance, adhesion performance, and bending processing adaptability.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: An organosol coating for the inner wall of acidic beverage cans, comprising, by weight, the following components: 35-55 parts organosol film-forming agent; 5-12 parts barrier agent; 6-16 parts plasticizer; 3-10 parts crosslinking curing agent; 0.5-3 parts adhesion promoter; 0.3-2 parts calcium stearate; 0.2-1.5 parts wetting and dispersing agent; 0.1-1 part oleamide; 0.1-0.8 parts tributyl phosphate; and 20-55 parts cyclohexanone.

[0008] Furthermore, the barrier agent has a composite structure consisting of alkali-layer dispersion, carbonate complex, polyphenol / phytic acid-zirconium complex, and alcohol-water silane coating.

[0009] Furthermore, the barrier agent is characterized by comprising the following raw materials in parts by weight: 12-18 parts of sodium magnesium lithium silicate, 3-5 parts of nano calcium carbonate, 2-4 parts of basic aluminum magnesium carbonate, 3-5 parts of tannic acid, 1-5 parts of phytic acid, 1-4 parts of zirconium oxychloride, 0.5-2 parts of melamine, 1-4 parts of tetraethyl orthosilicate, 1-5 parts of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, 0.2-1 part of polyvinylpyrrolidone, and 0.3-1.5 parts of sodium hexametaphosphate.

[0010] Furthermore, the preparation method of the barrier agent includes the following steps: S1. Add the sodium magnesium lithium silicate and sodium hexametaphosphate to 60-120 parts of deionized water according to the mass ratio, adjust the pH value of the system to 8.3-8.7 with 25-28wt% ammonia water, stir at 800-1200r / min for 30-60 minutes at 25-35℃, and then ultrasonically disperse for 30-60 minutes under ultrasonic power of 250-500W and frequency of 35-45kHz to obtain dispersion A.

[0011] S2. Add nano-calcium carbonate and basic aluminum magnesium carbonate to the dispersion A, and stir at 700-1100 r / min for 1-2 hours at 30-45℃ to obtain the sheet composite.

[0012] S3. Add the tannic acid, phytic acid, and polyvinylpyrrolidone to the sheet complex, adjust the pH of the system to 4.4-4.8 with glacial acetic acid, and stir at 500-900 r / min for 1.5-2.5 hours at 35-50℃; then disperse the zirconium oxychloride in deionized water to prepare a zirconium salt dispersion with a mass concentration of 8-12% based on zirconium oxychloride, add the zirconium salt dispersion to the system at a dropping rate of 0.2-1.0 parts / min based on the total mass of the zirconium salt dispersion, control the pH of the system to 3.9-4.3 during the dropping process, and stir at 600-1000 r / min for 2-4 hours at 40-60℃ to obtain a polyphenol-zirconium complex sheet dispersion.

[0013] S4. Add the melamine to the polyphenol-zirconium complex sheet dispersion, stir at 45-60℃ and 600-900 r / min for 1-2 hours, then add the anhydrous ethanol, and add the tetraethyl orthosilicate and γ-(2,3-epoxypropoxy)propyltrimethoxysilane premixed into the system at a dropping rate of 0.1-0.5 parts / min, adjust the pH of the system to 8.4-8.8 with 25-28 wt% ammonia, and react at 50-70℃ and 600-1000 r / min for 4-5 hours to obtain a slurry of polyphenol-zirconium complex sheet composite.

[0014] S5. The slurry of the polyphenol-zirconium complex layered composite is centrifuged at 6000-9000 r / min for 8-20 minutes. The precipitate is washed 2-4 times with deionized water and anhydrous ethanol, and then vacuum dried at 50-75℃ and a vacuum degree of -0.08 to -0.10 MPa for 8-16 hours. After drying, it is ground and passed through a 400-800 mesh sieve to obtain the barrier agent.

[0015] Furthermore, the organic sol film-forming agent is composed of polyvinyl butyral and polycaprolactone diol in a mass ratio of 20-30:10-15.

[0016] Furthermore, the plasticizer is composed of acetylated tributyl citrate and tributyl citrate in a mass ratio of 3-8:1-5.

[0017] Furthermore, the crosslinking curing agent is composed of hexamethoxymethyl melamine and p-toluenesulfonate amine salt in a mass ratio of 1-4:0.05-0.5.

[0018] Furthermore, the adhesion promoter is composed of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, 3-aminopropyltriethoxysilane and 2-hydroxyethyl methacrylate phosphate, with a mass ratio of 1-4:0.5-2:0.5-3. Furthermore, the wetting and dispersing agent is composed of sodium dodecyl sulfate, sodium citrate and sodium hexametaphosphate, with a mass ratio of 1-3:0.5-2:0.5-2.

[0019] The present invention also provides a method for preparing an organosol coating for the inner wall of an acidic beverage can, comprising the following steps: A1. Weigh 18-40 parts by weight of cyclohexanone, organosol film-forming agent and plasticizer, and stir at 500-900 r / min for 1-3 hours at 45-65℃ to obtain organosol base material.

[0020] A2. Add the barrier agent and wetting and dispersing agent to the organic sol base, disperse at 25-40℃ and 800-1500 r / min for 30-60 minutes, and then sand mill for 20-50 minutes to obtain the barrier agent dispersion base.

[0021] A3. Add the crosslinking curing agent, adhesion promoter, calcium stearate, oleamide, tributyl phosphate and 3-14 parts of cyclohexanone to the barrier agent dispersion base, and stir at 500-900 r / min for 30-60 minutes at 25-35℃ to obtain the coating mixture.

[0022] A4. Degas the coating mixture under a vacuum of -0.08 to -0.10 MPa for 10-30 minutes, and then filter it through a 100-300 mesh filter to obtain the organic sol coating for the inner wall of acidic beverage cans.

[0023] Furthermore, in A2, the sand milling process uses zirconia beads as the grinding medium, the sand milling speed is 1200-2000 r / min, and the fineness of the barrier agent dispersion base obtained after sand milling is not greater than 15 μm.

[0024] Furthermore, in A4, the coating mixture is degassed for 15-20 minutes under a vacuum of -0.08 to -0.10 MPa, filtered through a 150-250 mesh filter, and the resulting organic sol coating for the inner wall of acidic beverage cans has a viscosity of 80-150 s at 25°C using a Forte-4 cup.

[0025] The present invention also provides an organosol coating for the inner wall of acidic beverage cans, which can be used in the fields of coatings for ships, aircraft, high-speed rail, marine engineering and other fields.

[0026] This invention uses polyvinyl butyral and polycaprolactone diol as organic sol film-forming agents to form a coating base with film-forming properties, adhesion, and flexibility. It uses tributyl acetyl citrate and tributyl citrate as plasticizers to improve the coating's processing adaptability during coating, baking, and bending. Simultaneously, this invention introduces a barrier agent composed of sodium magnesium lithium silicate, nano-calcium carbonate, basic aluminum magnesium carbonate, tannic acid, phytic acid, zirconium oxychloride, melamine, tetraethyl orthosilicate, γ-(2,3-epoxypropoxy)propyltrimethoxysilane, polyvinylpyrrolidone, and sodium hexametaphosphate. Through layered barrier, acid-responsive buffer, polyphenol-zirconium complexation, and silane encapsulation, this agent reduces the migration of acidic media, moisture, and corrosive ions to the metal substrate. Further integration with crosslinking curing agents, adhesion promoters, wetting and dispersing agents, calcium stearate, oleamide, tributyl phosphate, and cyclohexanone ensures the barrier agent is stably dispersed in the organosol system, improving the density, interfacial bonding, and application stability of the cured coating. Through dispersion film formation, dispersion milling, mixing and degassing, and filtration processes, an organosol coating for the inner wall of acidic beverage cans is finally obtained, possessing acid immersion resistance, electrochemical barrier properties, adhesion properties, and adaptability to bending processes.

[0027] Compared with the prior art, the beneficial effects of the present invention are: 1. The barrier agent of the present invention constructs a layered barrier base using sodium magnesium lithium silicate, and introduces nano-calcium carbonate and basic aluminum magnesium carbonate as acid-responsive inorganic components distributed between the layers, so that the barrier agent forms a longer medium diffusion path in the coating, while providing buffering and sacrificial protection in acidic environments. This structure is beneficial to reducing the migration of acidic media, moisture and corrosive ions to the metal substrate, and improving the acid immersion resistance and electrochemical barrier performance of the coating on the inner wall of acidic beverage cans.

[0028] 2. The barrier agent of this invention introduces a polyphenol-zirconium complex layer formed by tannic acid, phytic acid, and zirconium oxychloride onto the surface of the layered composite structure, and cooperates with melamine to participate in interfacial interactions, thereby forming a complex structure with composite protective function on the surface of the barrier agent. This structure can enhance the bonding stability between the layered components, reduce the aggregation and interfacial defects of the barrier agent in the organosol system, and help improve the structural integrity of the coating under long-term contact conditions with acidic media.

[0029] 3. The barrier agent of this invention is silane-coated with tetraethyl orthosilicate and γ-(2,3-epoxypropoxy)propyltrimethoxysilane, and dispersed with polyvinylpyrrolidone and sodium hexametaphosphate, resulting in good interfacial compatibility between the barrier agent and the organosol film-forming system composed of polyvinyl butyral and polycaprolactone diol. When used in conjunction with crosslinking curing agents, adhesion promoters, plasticizers, and wetting and dispersing agents, this system is beneficial for improving the storage and dispersion stability of coatings, coating adhesion performance, and adaptability to bending processes. Attached Figure Description

[0030] Figure 1 Fourier transform infrared spectrum of the barrier agent obtained in Example 1 of this invention. Detailed Implementation

[0031] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Preparation Example 1 Preparation of the barrier agent: 1. Raw material weight parts: Sodium magnesium lithium silicate: 15 parts; Nano calcium carbonate: 4 parts; Basic aluminum magnesium carbonate: 3 parts; Tannic acid: 4 parts; Phytic acid: 3 parts; Zirconium oxychloride: 2.5 parts; Melamine: 1.2 parts; Tetraethyl orthosilicate: 2.5 parts; γ-(2,3-epoxypropoxy)propyltrimethoxysilane: 3 parts; Polyvinylpyrrolidone: 0.6 parts; Sodium hexametaphosphate: 0.8 parts.

[0033] The sodium magnesium lithium silicate was purchased from Shanghai McLean Biochemical Technology Co., Ltd., and its model number is S902403.

[0034] The nano-calcium carbonate was purchased from Hangzhou Zhenghe Nanotechnology Co., Ltd., model S60, with a specific surface area of ​​16-18 m². 2 / g.

[0035] Polyvinylpyrrolidone was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., model number: P816205, with an average molecular weight of 58,000.

[0036] 2. Preparation method: S1. Add 90.0 parts by weight of deionized water to the reaction vessel, control the system temperature at 25°C, and stir at 1000 r / min. Add sodium hexametaphosphate, stir for 5 minutes, and then add sodium magnesium lithium silicate at a feeding rate of 0.5 parts / min. After the feeding is completed, add 28 wt% ammonia water at a dropping rate of 0.10 parts / min to adjust the pH of the system to 8.5. Then raise the temperature to 30°C and stir at 1000 r / min for 45 minutes. After stirring, place the system under ultrasonic dispersion conditions for 45 minutes. The ultrasonic power is 400 W, the ultrasonic frequency is 40 kHz, and the ultrasonic water bath temperature is controlled at 35°C to obtain a dispersion.

[0037] S2. The dispersion is heated to 38°C and stirred at 900 rpm in air. Nano-calcium carbonate is added at a feeding rate of 0.4 parts / min, followed by basic aluminum magnesium carbonate at a feeding rate of 0.3 parts / min. After the feeding is completed, stirring is continued for 90 minutes at 38°C and 900 rpm, with the pH of the system maintained at 8.2-8.6 during the stirring process, to obtain the sheet-like composite.

[0038] S3. The sheet composite was heated to 45°C and stirred at 700 rpm in air. Tannic acid, 50 wt% phytic acid aqueous dispersion, and polyvinylpyrrolidone were added sequentially. After the addition was complete, glacial acetic acid was added dropwise at 0.05 parts / min to adjust the pH of the system to 4.6, and the mixture was stirred at 45°C and 700 rpm for 2 hours. Separately, zirconium oxychloride was dispersed in 22.5 parts of deionized water to prepare a zirconium salt dispersion with a mass concentration of 10 wt% (based on zirconium oxychloride). This zirconium salt dispersion was added to the above system at a dropping rate of 0.5 parts / min for a total dropping time of 50 minutes. The pH was measured every 10 minutes during the dropping process, and 28 wt% ammonia was added dropwise at 0.04 parts / min to control the pH of the system between 3.9 and 4.2. After the zirconium brine dispersion was added dropwise, the system was heated to 50°C and stirred for 3 hours at 800 r / min to obtain a polyphenol-zirconium complex sheet dispersion.

[0039] S4. Melamine was added to the polyphenol-zirconium complex sheet dispersion, and the mixture was stirred for 1.5 hours under air atmosphere, 55°C, and 750 r / min. Then, 65.0 parts of anhydrous ethanol were added at a rate of 1.5 parts / min, maintaining the system temperature at 55°C and reflux condensation during the addition process. Tetraethyl orthosilicate and γ-(2,3-epoxypropoxy)propyltrimethoxysilane were premixed for 10 minutes to obtain a silane premix; this silane premix was added to the above system at a dropping rate of 0.25 parts / min over a dropping time of 22 minutes. After the dropping was completed, 28 wt% ammonia was added dropwise at 0.10 parts / min to adjust the pH of the system to 8.6. The temperature was then raised to 60°C, and the mixture was reacted for 4.5 hours under air atmosphere and reflux condensation at 850 r / min to obtain a slurry of the polyphenol-zirconium complex sheet composite.

[0040] S5. The slurry of the polyphenol-zirconium complex layered composite was centrifuged at 8000 r / min for 15 minutes, and the supernatant was discarded to obtain a wet precipitate. 80.0 parts of deionized water were added to the wet precipitate, and the mixture was washed at 25°C and 500 r / min for 10 minutes, followed by centrifugation at 8000 r / min for 10 minutes. This washing process was repeated three times. Subsequently, 65.0 parts of anhydrous ethanol were added to the precipitate, and the mixture was washed at 25°C and 500 r / min for 10 minutes, followed by centrifugation at 8000 r / min for 10 minutes. This washing process was repeated three times, with the pH of the final wash solution being 6.8-7.2. The washed precipitate was placed in a vacuum drying oven and dried at 65°C and a vacuum of -0.09 MPa for 12 hours. After drying, the lumpy dried material was ground for 20 minutes and passed through a 600-mesh sieve to obtain the barrier agent.

[0041] Figure 1 The image shown is the Fourier transform infrared spectrum of the barrier agent sample obtained in Preparation Example 1. The spectrum is at 3400 cm⁻¹. -1 The presence of a broad absorption peak near the sample indicates the presence of hydroxyl groups and hydrogen bonding; at 3000 cm⁻¹... -1 The presence of a weak absorption peak nearby indicates that an organic segment has been introduced into the system; the absorption peak at 1700-1400 cm⁻¹... -1 The absorption peaks within the range are related to the polyphenol structure, melamine structure, and carbonate components; in the 1250-1000 cm⁻¹ range... -1 The presence of strong absorption peaks within the range can be attributed to vibrations such as Si-O-Si, Si-O, PO, or P=O, indicating the coexistence of silicate layers, phytic acid structures, and silane-coated structures. The absorption peaks in the low wavenumber region further indicate the presence of inorganic silicates and metal-oxygen bond structures in the sample.

[0042] Comparative Preparation Example 1 The barrier agent was prepared by referring to the preparation method in Preparation Example 1, except that 4.0 parts by mass of nano-calcium carbonate was replaced with 4.0 parts by mass of nano-silica, and everything else remained the same as in Preparation Example 1.

[0043] Comparative Preparation Example 2 The barrier agent was prepared by referring to the preparation method in Preparation Example 1, except that 3.0 parts by mass of basic aluminum magnesium carbonate was replaced with 3.0 parts by mass of talc powder, and the rest remained the same as in Preparation Example 1.

[0044] Comparative preparation example 3 The barrier agent was prepared by referring to the preparation method in Preparation Example 1, except that 4.0 parts by mass of tannic acid was replaced with 4.0 parts by mass of gallic acid, and the rest remained the same as in Preparation Example 1.

[0045] Comparative preparation example 4 The barrier agent was prepared according to the preparation method in Preparation Example 1, except that 2.5 parts by mass of zirconium oxychloride was replaced with 2.5 parts by mass of aluminum sulfate octadecylhydrate, and the rest remained the same as in Preparation Example 1.

[0046] Comparative preparation example 5 The barrier agent was prepared by referring to the preparation method in Preparation Example 1, except that 1.2 parts by mass of melamine was replaced with 1.2 parts by mass of urea, and the rest remained the same as in Preparation Example 1.

[0047] Comparative preparation example 6 The barrier agent was prepared by referring to the preparation method in Preparation Example 1, except that 2.5 parts by mass of tetraethyl orthosilicate was replaced with 2.5 parts by mass of methyltrimethoxysilane, and the rest remained the same as in Preparation Example 1.

[0048] Comparative preparation example 7 The barrier agent was prepared according to the preparation method in Preparation Example 1, except that 3.0 parts by mass of γ-(2,3-epoxypropoxy)propyltrimethoxysilane was replaced with 3.0 parts by mass of 3-aminopropyltriethoxysilane, and the rest remained the same as in Preparation Example 1.

[0049] Comparative Preparation Example 8 The acid-responsive sheet barrier was prepared by referring to the preparation method in Preparation Example 1, except that 0.6 parts by mass of polyvinylpyrrolidone was replaced with 0.6 parts by mass of polyethylene glycol, and everything else remained the same as in Preparation Example 1.

[0050] Example 1

[0051] Preparation of an organosol coating for the inner wall of acidic beverage cans: 1. Raw material weight parts: Organic sol film-forming agent: 44.0 parts, consisting of 30 parts polyvinyl butyral and 14.0 parts polycaprolactone diol.

[0052] Barrier agent: 8.0 parts, which is the barrier agent prepared in Preparation Example 1.

[0053] Plasticizer: 10 parts, consisting of 7.0 parts acetylated tributyl citrate and 3.0 parts tributyl citrate.

[0054] Crosslinking curing agent: 5 parts, consisting of 4.6 parts hexamethoxymethyl melamine and 0.4 parts p-toluenesulfonate amine salt.

[0055] Adhesion promoter: 1.5 parts, consisting of 0.7 parts γ-(2,3-epoxypropoxy)propyltrimethoxysilane, 0.3 parts 3-aminopropyltriethoxysilane and 0.5 parts 2-hydroxyethyl methacrylate phosphate.

[0056] Calcium stearate: 1.0 part.

[0057] Wetting and dispersing agent: 0.8 parts, consisting of 0.4 parts sodium dodecyl sulfate, 0.2 parts sodium citrate and 0.2 parts sodium hexametaphosphate.

[0058] Oleamide: 0.5 parts.

[0059] Tributyl phosphate: 0.3 parts.

[0060] Cyclohexanone: 36.0 parts.

[0061] The polyvinyl butyral was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., and its model number is P815775.

[0062] The polycaprolactone diol was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., and its model number is P750206.

[0063] 2. Preparation method: A1. Add 30 parts by weight of cyclohexanone to the reaction vessel, control the system temperature at 25°C under air atmosphere, and stir at 500 r / min for 5 minutes; then raise the temperature to 55°C, and add 30.0 parts of polyvinyl butyral at a feeding rate of 1.0 part / min to the cyclohexanone. After the feeding is completed, stir at 55°C and 700 r / min for 60 minutes; then add polycaprolactone diol, acetylated tributyl citrate and tributyl citrate, and continue stirring at 55°C and 700 r / min for 90 minutes to obtain the organosol base material.

[0064] A2. Cool the organic sol-based material obtained in step A1 to 32°C and stir at 1000 r / min in air. Add a barrier agent at a feeding rate of 0.4 parts / min, and continue stirring for 10 minutes after feeding. Then add sodium dodecyl sulfate, sodium citrate, and sodium hexametaphosphate in sequence, and disperse at 32°C and 1200 r / min for 45 minutes to obtain a pre-dispersed slurry. Subsequently, the pre-dispersed slurry is subjected to sand milling with zirconia beads as the milling medium at a speed of 1600 r / min for 35 minutes. During the sand milling process, the slurry outlet temperature is controlled not to exceed 40°C to obtain a barrier agent-dispersed base material.

[0065] A3. Add hexamethoxymethyl melamine to the barrier agent dispersion base obtained in step A2, and stir for 10 minutes at 30°C and 700 rpm; then add p-toluenesulfonate amine salt, and stir for 10 minutes at 30°C and 700 rpm; then premix γ-(2,3-epoxypropoxy)propyltrimethoxysilane, 3-aminopropyltriethoxysilane, and 2-hydroxyethyl methacrylate phosphate for 5 minutes to obtain an adhesion promoter premix, and add it to the system at a dropping rate of 0.1 parts / min. After the dropping is completed, add calcium stearate, oleamide, tributyl phosphate, and 6 parts of cyclohexanone, and continue stirring for 45 minutes at 30°C and 700 rpm to obtain a coating mixture.

[0066] A4. Degas the coating mixture obtained in step A3 under a vacuum of -0.09 MPa for 18 minutes, with the system temperature controlled at 25°C during the degassing process; after degassing, filter the coating mixture through a 200-mesh filter to obtain an organic sol coating for use on the inner wall of acidic beverage cans.

[0067] Example 2

[0068] The preparation of an organosol coating for the inner wall of acidic beverage cans is carried out by referring to the preparation method in Example 1, except that the mass fraction of the organosol film-forming agent is replaced with 55 parts, and the rest remains the same as in Example 1.

[0069] Example 3

[0070] The preparation of an organosol coating for the inner wall of acidic beverage cans is carried out by referring to the preparation method in Example 1, except that the mass fraction of the barrier agent is replaced with 12 parts, and the rest remains the same as in Example 1.

[0071] Example 4

[0072] The preparation of an organosol coating for the inner wall of acidic beverage cans is carried out by referring to the preparation method in Example 1, except that the mass fraction of plasticizer is replaced with 16 parts, and the rest remains the same as in Example 1.

[0073] Example 5

[0074] The preparation of an organosol coating for the inner wall of acidic beverage cans is carried out by referring to the preparation method in Example 1, except that the mass fraction of the crosslinking curing agent is replaced with 10 parts, and the rest remains the same as in Example 1.

[0075] Comparative Examples 1-8 An organosol coating for the inner wall of an acidic beverage can was prepared by referring to the preparation method in Example 1, except that the barrier agent was replaced with the barrier agent prepared in Comparative Preparation Examples 1-8, and everything else remained the same as in Example 1.

[0076] Comparative Example 9 The preparation of an organosol coating for the inner wall of acidic beverage cans is the same as in Example 1, except that no barrier agent is added.

[0077] Comparative Example 10 The preparation of an organosol coating for the inner wall of acidic beverage cans is carried out by referring to the preparation method in Example 1, except that the barrier agent is replaced with unmodified sodium magnesium lithium silicate, and everything else remains the same as in Example 1.

[0078] Comparative Example 11 The preparation of an organosol coating for the inner wall of an acidic beverage can is carried out according to the preparation method in Example 1, except that the barrier agent is replaced by a physical mixture of nano-silica and nano-calcium carbonate (the mass ratio of the two is the same as that in Comparative Preparation Example 1). This mixture is only physically mixed by high-speed stirring and is not subjected to the composite structure construction steps of the present invention. Otherwise, it is the same as in Example 1.

[0079] Comparative Example 12 An organic sol coating for the inner wall of acidic beverage cans was prepared according to the preparation method in Example 1, except that the adhesion promoter was not added, and everything else remained the same as in Example 1.

[0080] Performance testing: Preparation of test samples: Tinplate sheets were cut into 70mm × 150mm pieces, and the surfaces were successively wiped with anhydrous ethanol and deionized water, and dried at 60℃ for 20 minutes. The coating to be tested was applied to the surface of the tinplate sheets using a scraping method, baked at 200℃ for 10 minutes, and cooled to 25℃ to obtain the coated samples. The dry film thickness of the obtained coating was controlled to be 8-12μm.

[0081] 1. Acid Immersion Resistance Test: The coated sample was immersed in a 3wt% citric acid aqueous dispersion. The ratio of the citric acid aqueous dispersion to the effective contact area of ​​the coating on one side was 20 mL / cm². 2The soaking temperature was 60℃±2℃, and the soaking time was 72h±0.5h. After soaking, the sample was taken out, rinsed with deionized water for 30s±5s, and placed at 25℃±2℃ and relative humidity of 50%±10% for 2h±0.5h.

[0082] The test results are recorded in the following two items: (1) After soaking, assess the appearance of the coating to determine whether blistering, whitening, peeling or exposure of the substrate occurs; (2) Adhesion grade after soaking: cross-cut test and grade 0-5. The results are shown in Table 1.

[0083] 2. Electrochemical barrier performance test: Seal the edges of the coated sample with acid-resistant sealant, leaving only 1.0 cm. 2 The coating surface was used as the test area. The coated sample was used as the working electrode, and a platinum sheet (1.0 cm²) was used as the counter electrode. 2 A three-electrode testing system was constructed using a saturated calomel electrode as the reference electrode (filled with 3.5 mol / L potassium chloride dispersion). The test electrolyte was a mixed aqueous dispersion of 3.5 wt% sodium chloride and 1 wt% citric acid, where sodium chloride and citric acid were calculated based on the total mass of the electrolyte.

[0084] Before testing, the coated sample was immersed in the electrolyte for 2 h ± 0.2 h to stabilize the open circuit potential until the fluctuation did not exceed 5 mV within 10 min. Electrochemical impedance spectroscopy was then performed at a temperature of 25℃ ± 2℃ and a frequency range of 10 Hz. 5 Hz to 10 - 2 The AC disturbance voltage is 10mV. Record the low-frequency impedance modulus |Z| at 0.01Hz. 0.01 Hz, the results are shown in Table 1.

[0085] 3. Adhesion Test: A 6×6 grid is drawn on the coating surface using a cross-cutting tool, with a grid spacing of 1mm. The depth of the scratches is determined by penetrating the coating to the surface of the metal substrate. A pressure-sensitive adhesive tape with a width of 19mm±1mm is applied to the grid area, and compacted three times with a rubber roller. After a period of 90s±10s, the tape is peeled off at a 180° angle within 1s±0.5s. The area of ​​coating peeling off from the grid area is observed. The adhesion grade is evaluated as follows: Level 0 indicates that the grid edges are completely smooth and without any peeling. Level 1 indicates that there is a small amount of detachment at the intersection of the grid lines, and the detached area does not exceed 5%; Level 2 is defined as a detachment area greater than 5% but not exceeding 15%; Level 3 is defined as a detachment area greater than 15% but not exceeding 35%; Level 4 is defined as a detachment area greater than 35% but not exceeding 65%; Level 5 is defined as a detachment area greater than 65%, and the results are shown in Table 1.

[0086] 4. Bending resistance test: After placing the coated sample in an environment of 25℃±2℃ and relative humidity of 50%±10% for 24h±1h, bend it 180° along a 4mm diameter rod with the coated side facing outwards, and hold the bend for 10s±1s. Then observe the bent area with a 10x magnifying glass.

[0087] The test results were recorded after bending to determine the coating condition. Cracks, peeling, exposure of the substrate, or stress marks were observed and recorded in the bending area. The results are shown in Table 1.

[0088] Table 1. Results of performance test data

[0089] The data in the table show that the coatings obtained in Examples 1-6 maintained a relatively intact appearance after being soaked in a 3wt% citric acid aqueous dispersion. The adhesion grade after acid immersion was 0, and the low-frequency impedance modulus remained at 7.2 × 10⁻⁶. 8 -1.1×10 9 Ω·cm 2 Within the specified range, this invention demonstrates that when the amounts of organic sol film-forming agent, barrier agent, plasticizer, crosslinking curing agent, and adhesion promoter are adjusted within the defined proportions, the acid immersion resistance, interfacial bonding performance, and electrochemical barrier performance of the coating remain stable. This is because the sodium magnesium lithium silicate layer structure extends the diffusion path of acidic media, moisture, and corrosive ions; nano-calcium carbonate and basic aluminum magnesium carbonate provide buffering and sacrificial protection in acidic environments; the tannic acid / phytic acid-zirconium complex layer improves the surface structural stability of the barrier agent; and the silane coating layer and wetting and dispersion system improve the interfacial compatibility and dispersion stability between the inorganic barrier agent and the polyvinyl butyral / polycaprolactone diol organic sol system. In Comparative Examples 1-8, after only changing a single raw material in the barrier agent preparation process, the low-frequency impedance modulus decreased, and whitening, blistering, or localized detachment after immersion increased. This indicates that when the acid-response buffer structure, polyphenol-metal complex structure, silane coating structure, or dispersion and fixation effect are weakened, media migration channels and interfacial defects are more easily formed inside the coating. In Comparative Example 9, without the addition of a barrier agent, the coating lacks lamellar barrier and acid corrosion buffering effect, resulting in decreased resistance to acid immersion, electrochemical barrier, and integrity after bending. In Comparative Examples 10-12, when the plasticizing, cross-linking, or adhesion promoting effects are insufficient, the flexibility, density, or interfacial bonding of the coating with the metal substrate is affected, making it more prone to cracking, peeling, or exposure of the substrate after bending or acid immersion.

[0090] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An organosol coating for the inner wall of acidic beverage cans, characterized in that, By weight, it comprises the following components: 35-55 parts organic sol film-forming agent; 5-12 parts barrier agent; 6-16 parts plasticizer; 3-10 parts crosslinking curing agent; 0.5-3 parts adhesion promoter; 0.3-2 parts calcium stearate; 0.2-1.5 parts wetting and dispersing agent; 0.1-1 part oleamide; 0.1-0.8 parts tributyl phosphate; 20-55 parts cyclohexanone. The barrier agent has a composite structure consisting of alkaline sheet dispersion, carbonate complex, polyphenol / phytic acid-zirconium complex, and alcohol-water silane coating. The method for preparing the barrier agent includes the following steps: S1. Add the sodium magnesium lithium silicate and sodium hexametaphosphate to 60-120 parts of deionized water according to the mass ratio, adjust the pH value of the system to 8.3-8.7 with 25-28wt% ammonia water, stir at 800-1200r / min for 30-60 minutes at 25-35℃, and then ultrasonically disperse for 30-60 minutes under ultrasonic power of 250-500W and frequency of 35-45kHz to obtain dispersion A; S2. Add nano-calcium carbonate and basic aluminum magnesium carbonate to the dispersion A, and stir at 700-1100 r / min for 1-2 hours at 30-45℃ to obtain the sheet composite. S3. Add the tannic acid, phytic acid, and polyvinylpyrrolidone to the sheet complex, adjust the pH of the system to 4.4-4.8 with glacial acetic acid, and stir at 500-900 r / min for 1.5-2.5 hours at 35-50℃; then disperse the zirconium oxychloride in deionized water to prepare a zirconium salt dispersion with a mass concentration of 8-12% based on zirconium oxychloride, add the zirconium salt dispersion to the system at a dropping rate of 0.2-1.0 parts / min based on the total mass of the zirconium salt dispersion, control the pH of the system to 3.9-4.3 during the dropping process, and stir at 600-1000 r / min for 2-4 hours at 40-60℃ to obtain a polyphenol-zirconium complex sheet dispersion; S4. Melamine is added to the polyphenol-zirconium complex sheet dispersion, and the mixture is stirred at 600-900 r / min for 1-2 hours at 45-60°C. Then, anhydrous ethanol is added, and tetraethyl orthosilicate and γ-(2,3-epoxypropoxy)propyltrimethoxysilane are premixed and added to the system at a dropping rate of 0.1-0.5 parts / min. The pH of the system is adjusted to 8.4-8.8 with 25-28 wt% ammonia water, and the mixture is reacted at 600-1000 r / min for 4-5 hours at 50-70°C to obtain a slurry of polyphenol-zirconium complex sheet composite. S5. The slurry of the polyphenol-zirconium complex layered composite is centrifuged at 6000-9000 r / min for 8-20 minutes. The precipitate is washed 2-4 times with deionized water and anhydrous ethanol, and then vacuum dried at 50-75℃ and a vacuum degree of -0.08 to -0.10 MPa for 8-16 hours. After drying, it is ground and passed through a 400-800 mesh sieve to obtain the barrier agent.

2. The organosol coating for the inner wall of acidic beverage cans according to claim 1, characterized in that, The barrier agent is composed of the following raw materials in parts by weight: 12-18 parts of sodium magnesium lithium silicate, 3-5 parts of nano calcium carbonate, 2-4 parts of basic aluminum magnesium carbonate, 3-5 parts of tannic acid, 1-5 parts of phytic acid, 1-4 parts of zirconium oxychloride, 0.5-2 parts of melamine, 1-4 parts of tetraethyl orthosilicate, 1-5 parts of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, 0.2-1 part of polyvinylpyrrolidone, and 0.3-1.5 parts of sodium hexametaphosphate.

3. An organosol coating for the inner wall of an acidic beverage can according to claim 1, characterized in that, The organic sol film-forming agent is composed of polyvinyl butyral and polycaprolactone diol in a mass ratio of 20-30:10-15.

4. An organosol coating for the inner wall of an acidic beverage can according to claim 1, characterized in that, The plasticizer is composed of acetylated tributyl citrate and tributyl citrate in a mass ratio of 3-8:1-5.

5. An organosol coating for the inner wall of an acidic beverage can according to claim 1, characterized in that, The crosslinking curing agent is composed of hexamethoxymethyl melamine and p-toluenesulfonate amine salt in a mass ratio of 1-4:0.05-0.

5.

6. An organosol coating for the inner wall of an acidic beverage can according to claim 1, characterized in that, The adhesion promoter is composed of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, 3-aminopropyltriethoxysilane and 2-hydroxyethyl methacrylate phosphate, in a mass ratio of 1-4:0.5-2:0.5-3. The wetting and dispersing agent is composed of sodium dodecyl sulfate, sodium citrate and sodium hexametaphosphate, with a mass ratio of 1-3:0.5-2:0.5-2.

7. A method for preparing an organosol coating for the inner wall of an acidic beverage can as described in any one of claims 1-6, characterized in that, Includes the following steps: A1. Weigh 18-40 parts by weight of cyclohexanone, organosol film-forming agent and plasticizer, and stir at 500-900 r / min for 1-3 hours at 45-65℃ to obtain organosol base material; A2. Add the barrier agent and wetting and dispersing agent to the organic sol base, disperse at 25-40℃ and 800-1500r / min for 30-60 minutes, and then sand mill for 20-50 minutes to obtain the barrier agent dispersion base; A3. Add the crosslinking curing agent, adhesion promoter, calcium stearate, oleamide, tributyl phosphate and 3-14 parts of cyclohexanone to the barrier agent dispersion base, and stir at 500-900 r / min for 30-60 minutes at 25-35℃ to obtain the coating mixture; A4. Degas the coating mixture under a vacuum of -0.08 to -0.10 MPa for 10-30 minutes, and then filter it through a 100-300 mesh filter to obtain the organic sol coating for the inner wall of acidic beverage cans.

8. The method for preparing an organosol coating for the inner wall of an acidic beverage can according to claim 7, characterized in that, In A2, the sand milling process uses zirconia beads as the grinding medium, the sand milling speed is 1200-2000 r / min, and the fineness of the barrier agent dispersion base obtained after sand milling is not greater than 15 μm.

9. A method for preparing an organosol coating for the inner wall of an acidic beverage can according to claim 7, characterized in that, In A4, the coating mixture is defoamed for 15-20 minutes under a vacuum of -0.08 to -0.10 MPa, filtered through a 150-250 mesh filter, and the resulting organic sol coating for the inner wall of acidic beverage cans has a viscosity of 80-150 s at 25°C in a Forte 4 cup.

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