Waterproof coating for paper bags and process for its preparation

Abstract

The application relates to the technical field of waterproof coating, and discloses a waterproof coating for paper bags and a preparation process thereof, which is prepared by water phase dispersion coating and drying of degradable resin, modified starch, modified nano silicon dioxide and additives. The obtained waterproof paper bag has a Cobb value of 8.2-11.1 g / m<2> 60 , and an oil-proof grade of 8-9. 2 The coating adopts a full-water dispersion process, is green and environment-friendly in the production process, and is simple and convenient to operate; the selected degradable resin such as polylactic acid, polybutylene adipate terephthalate and polybutylene succinate and the natural starch-based material have biodegradability, meet the low-carbon circular economy development requirement, and solve the problems of water leakage and oil leakage existing in the packaging field.

Description

Technical Field

[0001] This invention relates to the field of waterproof coating technology, specifically to a waterproof coating for paper bags and its preparation process. Background Technology

[0002] With increasingly stringent plastic bans and environmental regulations, paper bags are becoming increasingly widely used as green packaging. However, paper fibers are inherently hydrophilic and porous, making them prone to leakage when containing water- or oil-containing items, severely limiting their application in the food and catering industries. Currently, the main methods for imparting waterproof and oil-resistant properties to paper bags include polyethylene (PE) lamination, wax coating, and fluorinated finishing agent coating. While PE lamination offers excellent waterproofing, it is non-degradable, making the recycling of paper-plastic composites difficult. Incineration or landfilling both create an environmental burden, contradicting the environmentally friendly intentions of paper-based packaging. Waxing treatments tend to soften and fail at slightly higher temperatures, and their oil-resistant properties are also unsatisfactory. Fluorinated compounds provide good oil resistance, but their bioaccumulation and potential health risks are increasingly limiting their use. Furthermore, some solvent-based coatings emit volatile organic compounds during production, making it difficult to meet green manufacturing requirements. Therefore, developing a paper bag coating that combines excellent waterproof and oil-resistant properties with environmental friendliness has become a critical technical challenge that the industry urgently needs to address. Summary of the Invention

[0003] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a waterproof coating for paper bags and its preparation process.

[0004] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, a waterproof coating for paper bags is provided, wherein the waterproof coating is a biodegradable functional film layer applied to the surface of the paper bag base paper, and is formed by dispersing biodegradable resin, modified starch, modified nanomaterials and additives in an aqueous phase and then coating and drying to form a film. By weight, it contains 50-70 parts of biodegradable resin, 10-20 parts of modified starch, 2-8 parts of modified nanomaterials, 2-5 parts of additives, and the remainder is deionized water.

[0005] Furthermore, the biodegradable resin is one of polylactic acid, polybutylene terephthalate, and polybutylene succinate.

[0006] Furthermore, the method for preparing the modified starch includes the following steps: Weigh starch and add deionized water to prepare a starch suspension with a mass concentration of 30-40%. Stir well and slowly add 3-5% sodium hydroxide solution to the starch suspension while stirring to adjust the pH to 8.0-8.5. Maintain the temperature at 25-35℃ and slowly add 3-5% octenyl succinic anhydride (based on dry starch mass). During the reaction, maintain the pH of the system at 8.0-8.5 and the temperature at 30-35℃. After the addition is complete, continue the reaction for 1-2 hours. After the reaction is complete, adjust the pH of the system to 6.5-7.0 with citric acid. Centrifuge at 8000-10000 rpm for 10-15 minutes, wash 3-5 times with deionized water, and spray dry at an inlet air temperature of 180-200℃ and an outlet air temperature of 80-90℃. Crush and pass through an 80-120 mesh sieve to obtain modified starch.

[0007] Furthermore, the preparation method of the modified nanomaterial is as follows: Add 5-10 parts of nano-silica to 100-150 parts of anhydrous ethanol and disperse at 20-30℃ for 10-15 min to obtain a nano-silica-ethanol dispersion. Under stirring, slowly add 20-25% (w / w) of ammonia solution to adjust the pH of the system to 9-10. Slowly add 5-15 parts of hexadecyltrimethoxysilane to the reaction system and stir at 30-40℃ for 4-6 h. After the reaction is completed, centrifuge at 8000-10000 rpm for 10-15 min, discard the supernatant, and wash the precipitate alternately with anhydrous ethanol and deionized water 2-3 times each. Vacuum dry the washed product at 80-100℃ for 6-8 h, grind, and pass through a 400-600 mesh sieve to obtain the modified nanomaterial.

[0008] Further, the additives include plasticizers, antioxidants, and stabilizers in a mass ratio of 6-8:0.3-0.5:0.1-0.2; the antioxidant is antioxidant 1010 or tert-butylhydroquinone, the plasticizer is acetylated tributyl citrate or tributyl citrate, and the stabilizer is magnesium stearate or epoxidized soybean oil.

[0009] A second aspect of the present invention provides a method for preparing a waterproof coating for paper bags, comprising the following steps: S1. Weigh out the biodegradable resin, modified starch, modified nanomaterials and additives according to the mass ratio, add them to deionized water, adjust the viscosity to 400-800 mPa·s, stir at 500-1000 rpm for 30-60 min, and filter through an 80-120 mesh filter to obtain the coating liquid. S2. Apply the coating liquid evenly to the surface of the paper bag base paper using a roller coating method, with a coating amount of 5-15 g / m². 2 Dry at 80-100℃ for 5-15 minutes to form a film, thus obtaining a waterproof coating for paper bags.

[0010] A third aspect of the invention is a paper bag comprising paper bag base paper and a waterproof coating applied thereto to the surface of the paper bag.

[0011] Furthermore, a coating is applied to the inner wall, bottom, and outer surface of the paper bag.

[0012] (iii) Beneficial technical effects This invention discloses a waterproof coating for paper bags and its preparation method. A biodegradable resin is used as the continuous film-forming matrix. Octenyl succinic anhydride-modified starch is used to introduce hydrophobic long-chain groups, reducing the surface energy of the coating and preventing liquid water penetration. Simultaneously, hexadecyltrimethoxysilane is used to modify the surface of nano-silica, endowing the coating with excellent oleophobic and oil-repellent properties, significantly increasing the grease contact angle and making it difficult for the coating to wet and spread.

[0013] Compared with existing technologies, this invention has significant advantages: the coating adopts a fully water-based dispersion process, avoiding the use of organic solvents, making the production process green, environmentally friendly, and easy to operate; the selected biodegradable resins such as polylactic acid, polybutylene terephthalate, and polybutylene succinate, as well as natural starch-based materials, make the coating as a whole biodegradable, which meets the requirements of low-carbon circular economy development and solves the problems of water and oil leakage in the packaging field. Detailed Implementation

[0014] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0015] All parts used in this invention are parts by weight; Paper bag base paper: unbleached sulfate softwood pulp kraft paper, 80g / m² 2 ; Polylactic acid: film-forming grade, purchased from Natural Workshop; Polybutylene terephthalate (PET): purchased from BASF; Polybutylene succinate: purchased from Mitsubishi Chemical. Starch: Corn starch; Nano-silica: 20nm; Epoxidized soybean oil: epoxidation value 6.0%, iodine value ≤6.

[0016] Example 1 A waterproof coating for paper bags, wherein the waterproof coating is a biodegradable functional film layer applied to the surface of the paper bag base paper, and is formed by dispersing biodegradable resin, modified starch, modified nanomaterials and additives in an aqueous phase and then coating and drying to form a film. By weight, it contains 50 parts of biodegradable resin, 10 parts of modified starch, 2 parts of modified nanomaterials, 2 parts of additives, and the remainder is deionized water.

[0017] The biodegradable resin is polylactic acid.

[0018] The additives include plasticizer, antioxidant and stabilizer in a mass ratio of 6:0.3:0.1; the antioxidant is antioxidant 1010, the plasticizer is tributyl acetyl citrate and the stabilizer is magnesium stearate.

[0019] A paper bag includes paper bag base paper and a waterproof coating applied to its surface.

[0020] The inner wall, bottom and outer surface of the paper bag are coated.

[0021] The method for preparing the modified starch includes the following steps: Weigh starch and add deionized water to prepare a starch suspension with a mass concentration of 30%. Stir well and slowly add 3% sodium hydroxide solution to the starch suspension while stirring to adjust the pH to 8.0. Maintain the temperature at 25℃ and slowly add 3% octenyl succinic anhydride based on the dry weight of starch. During the reaction, maintain the pH of the system at 8.0 and the temperature at 30℃. After the addition is complete, continue the reaction for 1 hour. After the reaction is complete, adjust the pH of the system to 6.5 with citric acid, centrifuge at 8000 rpm for 15 minutes, wash three times with deionized water, spray dry at an inlet air temperature of 180℃ and an outlet air temperature of 80℃, pulverize, and pass through an 80-mesh sieve to obtain modified starch.

[0022] The preparation method of the modified nanomaterial: Five parts of nano-silica were added to 100 parts of anhydrous ethanol and dispersed at 20°C for 10 min to obtain a nano-silica-ethanol dispersion. Under stirring, 20% (w / w) of ammonia solution was slowly added dropwise to adjust the pH of the system to 9. Five parts of hexadecyltrimethoxysilane were slowly added to the reaction system and stirred at 30°C for 4 h. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 10 min, and the supernatant was discarded. The precipitate was washed twice each with anhydrous ethanol and deionized water. The washed product was vacuum dried at 80°C for 6 h, ground, and passed through a 400-mesh sieve to obtain the modified nanomaterial.

[0023] A method for preparing a waterproof coating for paper bags, characterized by comprising the following steps: S1. Weigh out the biodegradable resin, modified starch, modified nanomaterials and additives according to the mass ratio, add them to deionized water, adjust the viscosity to 400 mPa·s, stir at 500 rpm for 30 min, and filter through an 80-mesh filter to obtain the coating liquid. S2. The coating liquid is uniformly coated onto the surface of the paper bag base paper using a roller coating method, with a coating amount of 5g / m². 2 The film is dried at 80℃ for 5 minutes to obtain a waterproof coating for paper bags.

[0024] Example 2 A waterproof coating for paper bags, wherein the waterproof coating is a biodegradable functional film layer applied to the surface of the paper bag base paper, and is formed by dispersing biodegradable resin, modified starch, modified nanomaterials and additives in an aqueous phase and then coating and drying to form a film. By weight, it contains 55 parts biodegradable resin, 12 parts modified starch, 4 parts modified nanomaterials, 3 parts additives, and the remainder is deionized water.

[0025] The biodegradable resin is polybutylene terephthalate (PET).

[0026] The additives include plasticizer, antioxidant and stabilizer in a mass ratio of 7:0.4:0.15; the antioxidant is tert-butylhydroquinone, the plasticizer is tributyl acetyl citrate and the stabilizer is magnesium stearate.

[0027] A paper bag includes paper bag base paper and a waterproof coating applied to its surface.

[0028] The inner wall, bottom and outer surface of the paper bag are coated.

[0029] The method for preparing the modified starch includes the following steps: Weigh out starch and add deionized water to prepare a starch suspension with a mass concentration of 35%. Stir well and slowly add 4% sodium hydroxide solution to the starch suspension while stirring to adjust the pH to 8.2. Maintain the temperature at 30℃ and slowly add 4% octenyl succinic anhydride based on the dry weight of starch. During the reaction, maintain the pH of the system at 8.2 and the temperature at 32℃. After the addition is complete, continue the reaction for 1.5 hours. After the reaction is complete, adjust the pH of the system to 6.8 with citric acid, centrifuge at 9000 rpm for 15 minutes, wash 4 times with deionized water, spray dry at an inlet air temperature of 190℃ and an outlet air temperature of 85℃, pulverize, and pass through a 90-mesh sieve to obtain modified starch.

[0030] The preparation method of the modified nanomaterial: Eight parts of nano-silica were added to 120 parts of anhydrous ethanol and dispersed at 25°C for 10 min to obtain a nano-silica-ethanol dispersion. Under stirring, 22% (w / w) of ammonia solution was slowly added dropwise to adjust the pH of the system to 9.5. Ten parts of hexadecyltrimethoxysilane were slowly added to the reaction system and stirred at 35°C for 5 h. After the reaction was completed, the mixture was centrifuged at 9000 rpm for 10 min, the supernatant was discarded, and the precipitate was washed twice each with anhydrous ethanol and deionized water. The washed product was vacuum dried at 80°C for 6 h, ground, and passed through a 400-mesh sieve to obtain the modified nanomaterial.

[0031] A method for preparing a waterproof coating for paper bags, characterized by comprising the following steps: S1. Weigh out the biodegradable resin, modified starch, modified nanomaterials and additives according to the mass ratio, add them to deionized water, adjust the viscosity to 500 mPa·s, stir at 600 rpm for 40 min, and filter through a 100-mesh filter to obtain the coating liquid. S2. The coating liquid is uniformly coated onto the surface of the paper bag base paper using a roller coating method, with a coating amount of 10g / m². 2 The film is dried at 90℃ for 10 minutes to obtain a waterproof coating for paper bags.

[0032] Example 3 A waterproof coating for paper bags, wherein the waterproof coating is a biodegradable functional film layer applied to the surface of the paper bag base paper, and is formed by dispersing biodegradable resin, modified starch, modified nanomaterials and additives in an aqueous phase and then coating and drying to form a film. By weight, it contains 60 parts of biodegradable resin, 18 parts of modified starch, 5 parts of modified nanomaterials, 5 parts of additives, and the remainder is deionized water.

[0033] The biodegradable resin is polybutylene succinate.

[0034] The additives include plasticizer, antioxidant and stabilizer in a mass ratio of 8:0.5:0.2; the antioxidant is antioxidant 1010, the plasticizer is tributyl citrate and the stabilizer is epoxidized soybean oil.

[0035] A paper bag includes paper bag base paper and a waterproof coating applied to its surface.

[0036] The inner wall, bottom and outer surface of the paper bag are coated.

[0037] The method for preparing the modified starch includes the following steps: Weigh out starch and add deionized water to prepare a starch suspension with a mass concentration of 40%. Stir well and slowly add 3% sodium hydroxide solution to the starch suspension while stirring to adjust the pH to 8.5. Maintain the temperature at 30℃ and slowly add 5% octenyl succinic anhydride based on the dry weight of starch. During the reaction, maintain the pH of the system at 8.5 and the temperature at 30℃. After the addition is complete, continue the reaction for 1 hour. After the reaction is complete, adjust the pH of the system to 6.5 with citric acid. Centrifuge at 10,000 rpm for 10 minutes, wash three times with deionized water, spray dry at an inlet air temperature of 200℃ and an outlet air temperature of 80℃, pulverize, and pass through a 100-mesh sieve to obtain modified starch.

[0038] The preparation method of the modified nanomaterial: Eight parts of nano-silica were added to 120 parts of anhydrous ethanol and dispersed at 30°C for 10 min to obtain a nano-silica-ethanol dispersion. Under stirring, 20% (w / w) of ammonia solution was slowly added dropwise to adjust the pH of the system to 10. Twelve parts of hexadecyltrimethoxysilane were slowly added to the reaction system and stirred at 36°C for 5 h. After the reaction was completed, the mixture was centrifuged at 10,000 rpm for 10 min, and the supernatant was discarded. The precipitate was washed twice each with anhydrous ethanol and deionized water. The washed product was vacuum dried at 100°C for 6 h, ground, and passed through a 500-mesh sieve to obtain the modified nanomaterial.

[0039] A method for preparing a waterproof coating for paper bags, characterized by comprising the following steps: S1. Weigh out the biodegradable resin, modified starch, modified nanomaterials and additives according to the mass ratio, add them to deionized water, adjust the viscosity to 600 mPa·s, stir at 800 rpm for 40 min, and filter through a 100-mesh filter to obtain the coating liquid. S2. The coating liquid is uniformly coated onto the surface of the paper bag base paper using a roller coating method, with a coating amount of 8g / m². 2 The film is dried at 90℃ for 12 minutes to obtain a waterproof coating for paper bags.

[0040] Example 4 A waterproof coating for paper bags, wherein the waterproof coating is a biodegradable functional film layer applied to the surface of the paper bag base paper, and is formed by dispersing biodegradable resin, modified starch, modified nanomaterials and additives in an aqueous phase and then coating and drying to form a film. By weight, it contains 65 parts of biodegradable resin, 15 parts of modified starch, 6 parts of modified nanomaterials, 4 parts of additives, and the remainder is deionized water.

[0041] The biodegradable resin is polylactic acid.

[0042] The additives include plasticizers, antioxidants, and stabilizers in a mass ratio of 6:0.4:0.2; the antioxidant is antioxidant 1010, the plasticizer is tributyl acetyl citrate, and the stabilizer is epoxidized soybean oil.

[0043] A paper bag includes paper bag base paper and a waterproof coating applied to its surface.

[0044] The inner wall, bottom and outer surface of the paper bag are coated.

[0045] The method for preparing the modified starch includes the following steps: Weigh out starch and add deionized water to prepare a starch suspension with a mass concentration of 30%. Stir well and slowly add 5% sodium hydroxide solution to the starch suspension while stirring to adjust the pH to 8.0. Maintain the temperature at 30℃ and slowly add 4% octenyl succinic anhydride based on the dry weight of starch. During the reaction, maintain the pH of the system at 8.0 and the temperature at 30℃. After the addition is complete, continue the reaction for 1 hour. After the reaction is complete, adjust the pH of the system to 6.5 with citric acid, centrifuge at 8000 rpm for 15 minutes, wash 5 times with deionized water, spray dry at an inlet air temperature of 200℃ and an outlet air temperature of 90℃, pulverize, and pass through a 100-mesh sieve to obtain modified starch.

[0046] The preparation method of the modified nanomaterial: Ten parts of nano-silica were added to 140 parts of anhydrous ethanol and dispersed at 25°C for 15 min to obtain a nano-silica-ethanol dispersion. Under stirring, 25% (w / w) of ammonia solution was slowly added dropwise to adjust the pH of the system to 9. Ten parts of hexadecyltrimethoxysilane were slowly added to the reaction system and stirred at 30°C for 6 h. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 15 min, and the supernatant was discarded. The precipitate was washed three times each with anhydrous ethanol and deionized water. The washed product was vacuum dried at 80°C for 8 h, ground, and passed through a 500-mesh sieve to obtain the modified nanomaterial.

[0047] A method for preparing a waterproof coating for paper bags, characterized by comprising the following steps: S1. Weigh out the biodegradable resin, modified starch, modified nanomaterials and additives according to the mass ratio, add them to deionized water, adjust the viscosity to 500 mPa·s, stir at 800 rpm for 40 min, and filter through a 90 mesh screen to obtain the coating liquid. S2. The coating liquid is uniformly coated onto the surface of the paper bag base paper using a roller coating method, with a coating amount of 10g / m². 2 The film is dried at 90℃ for 10 minutes to obtain a waterproof coating for paper bags.

[0048] Example 5 A waterproof coating for paper bags, wherein the waterproof coating is a biodegradable functional film layer applied to the surface of the paper bag base paper, and is formed by dispersing biodegradable resin, modified starch, modified nanomaterials and additives in an aqueous phase and then coating and drying to form a film. By weight, it contains 70 parts of biodegradable resin, 20 parts of modified starch, 8 parts of modified nanomaterials, 5 parts of additives, and the remainder is deionized water.

[0049] The biodegradable resin is polylactic acid.

[0050] The additives include plasticizer, antioxidant and stabilizer in a mass ratio of 8:0.5:0.2; the antioxidant is tert-butylhydroquinone, the plasticizer is tributyl citrate and the stabilizer is magnesium stearate.

[0051] A paper bag includes paper bag base paper and a waterproof coating applied to its surface.

[0052] The inner wall, bottom and outer surface of the paper bag are coated.

[0053] The method for preparing the modified starch includes the following steps: Weigh out starch and add deionized water to prepare a starch suspension with a mass concentration of 40%. Stir well and slowly add 5% sodium hydroxide solution to the starch suspension while stirring to adjust the pH to 8.5. Maintain the temperature at 35℃ and slowly add 5% octenyl succinic anhydride based on the dry weight of starch. During the reaction, maintain the pH of the system at 8.5 and the temperature at 35℃. After the addition is complete, continue the reaction for 2 hours. After the reaction is complete, adjust the pH of the system to 7.0 with citric acid. Centrifuge at 10,000 rpm for 15 minutes, wash 5 times with deionized water, spray dry at an inlet air temperature of 200℃ and an outlet air temperature of 90℃, pulverize, and pass through a 120-mesh sieve to obtain modified starch.

[0054] The preparation method of the modified nanomaterial: Ten parts of nano-silica were added to 150 parts of anhydrous ethanol and dispersed at 30°C for 15 min to obtain a nano-silica-ethanol dispersion. Under stirring, 25% (w / w) of ammonia solution was slowly added dropwise to adjust the pH of the system to 10. 15 parts of hexadecyltrimethoxysilane were slowly added to the reaction system and stirred at 40°C for 6 h. After the reaction was completed, the mixture was centrifuged at 10,000 rpm for 15 min, and the supernatant was discarded. The precipitate was washed three times each with anhydrous ethanol and deionized water. The washed product was vacuum dried at 100°C for 8 h, ground, and passed through a 600-mesh sieve to obtain the modified nanomaterial.

[0055] A method for preparing a waterproof coating for paper bags, characterized by comprising the following steps: S1. Weigh out the biodegradable resin, modified starch, modified nanomaterials and additives according to the mass ratio, add them to deionized water, adjust the viscosity to 800 mPa·s, stir at 1000 rpm for 60 min, and filter through a 120 mesh filter to obtain the coating liquid. S2. The coating liquid is uniformly coated onto the surface of the paper bag base paper using a roller coating method, with a coating amount of 15g / m². 2 The film is dried at 100℃ for 15 minutes to obtain a waterproof coating for paper bags.

[0056] Comparative Example 1: Paper bags without any coating, the base paper is the same as the base paper used in Example 1.

[0057] Comparative Example 2: A paper bag coating, the formula and preparation method of which are the same as those of Example 1, except that the modified starch is replaced with an equal mass of unmodified starch.

[0058] Comparative Example 3: A paper bag coating, the formulation and preparation method of which are the same as those in Example 1, except that the modified nanomaterials are replaced with unmodified nanomaterials.

[0059] Test method: 1. Waterproofing test According to GB / T 1540-2002 "Determination of Water Absorption of Paper and Paperboard - Cobb Method", the water absorption of the coated surface (Comparative Example 1 is the inner surface) for 60 seconds was tested (Cobb method). 60 (value), unit g / m 2 The lower the value, the better the waterproof performance.

[0060] 2. Oil resistance test According to GB / T 22805.2-2008 "Determination of Grease Resistance of Paper and Paperboard - Part 2: Surface Repulsion Method", the Kit value method is used, and standard reagents of grades 1-12 are used for testing. The highest grade, where the coated surface is not wetted, is used. The higher the grade, the better the grease resistance.

[0061] 3. Food safety testing Referring to GB 4806.7-2023 "National Food Safety Standard - Plastic Materials and Articles for Food Contact" and GB9685-2016 "Additives for Food Contact Materials and Articles", the following tests were conducted: Total migration: Using 4% acetic acid, 10% ethanol, and refined olive oil as food simulants, the samples were immersed at 40°C for 10 days, and the total migration (mg / dm³) was determined. 2 ).

[0062] Potassium permanganate consumption: Using water as a simulant, the amount consumed (mg / kg) was determined after soaking at 60℃ for 2 hours.

[0063] Specific monomer migration: For PLA, PBAT, and PBS resins, the migration amounts (mg / kg) of monomers such as lactide, terephthalic acid, and succinic acid were measured. For PE membrane comparison samples, potentially migrating antioxidants were detected.

[0064] Table 1. Water and oil repellency test results

[0065] Table 2 Food safety test results

[0066] As shown in Table 1, the Cobb of Examples 1-5 60 The value is between 8.2 and 11.1 g / m³. 2 Between these values, the oil resistance rating is 8-9, indicating that the coating has excellent water and oil resistance. Comparative Example 1: Cobb 60 58.7g / m 2 The oil resistance rating is 0, indicating high water absorption and no oil resistance at all. This demonstrates that the untreated paper bag base paper cannot meet the waterproof and oil-resistant requirements for food packaging. Comparative Example 2: Cobb 60 Significantly increased to 28.4 g / m 2 The water resistance was significantly reduced, indicating that modified starch can effectively improve water resistance. Comparative Example 3: Cobb 60 It is 10.2 g / m 2 It has good water resistance, but the oil resistance level drops sharply to level 4, indicating that the modified nano silica improves the oil resistance of the coating.

[0067] As shown in Table 2, the total migration, potassium permanganate consumption, and specific monomer migration in the three food simulants of all examples and comparative examples meet the safety requirements of GB 4806.7-2023 and GB 9685-2016, indicating that the migration degree of coating components into food is extremely low.

[0068] 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. A waterproof coating for paper bags, characterized in that, The waterproof coating is a biodegradable functional film layer applied to the surface of the paper bag base paper. It is formed by dispersing biodegradable resin, modified starch, modified nanomaterials and additives in an aqueous phase, then coating and drying to form a film. By weight, it contains 50-70 parts of biodegradable resin, 10-20 parts of modified starch, 2-8 parts of modified nanomaterials, 2-5 parts of additives, and the remainder is deionized water.

2. The waterproof coating for paper bags according to claim 1, characterized in that, The biodegradable resin is one of polylactic acid, polybutylene terephthalate, and polybutylene succinate.

3. The waterproof coating for paper bags according to claim 1, characterized in that, The method for preparing the modified starch includes the following steps: Weigh starch and add deionized water to prepare a starch suspension with a mass concentration of 30-40%. Stir well and slowly add 3-5% sodium hydroxide solution to the starch suspension while stirring to adjust the pH to 8.0-8.

5. Maintain the temperature at 25-35℃ and slowly add 3-5% octenyl succinic anhydride (based on dry starch mass). During the reaction, maintain the pH of the system at 8.0-8.5 and the temperature at 30-35℃. After the addition is complete, continue the reaction for 1-2 hours. After the reaction is complete, adjust the pH of the system to 6.5-7.0 with citric acid. Centrifuge at 8000-10000 rpm for 10-15 minutes, wash 3-5 times with deionized water, and spray dry at an inlet air temperature of 180-200℃ and an outlet air temperature of 80-90℃. Crush and pass through an 80-120 mesh sieve to obtain modified starch.

4. The waterproof coating for paper bags according to claim 1, characterized in that, The preparation method of the modified nanomaterial: Add 5-10 parts of nano-silica to 100-150 parts of anhydrous ethanol and disperse at 20-30℃ for 10-15 min to obtain a nano-silica-ethanol dispersion. Under stirring, slowly add 20-25% (w / w) of ammonia solution to adjust the pH of the system to 9-10. Slowly add 5-15 parts of hexadecyltrimethoxysilane to the reaction system and stir at 30-40℃ for 4-6 h. After the reaction is completed, centrifuge at 8000-10000 rpm for 10-15 min, discard the supernatant, and wash the precipitate alternately with anhydrous ethanol and deionized water 2-3 times each. Vacuum dry the washed product at 80-100℃ for 6-8 h, grind, and pass through a 400-600 mesh sieve to obtain the modified nanomaterial.

5. A waterproof coating for paper bags according to claim 1, characterized in that, The additives include plasticizers, antioxidants, and stabilizers in a mass ratio of 6-8:0.3-0.5:0.1-0.2; the antioxidant is antioxidant 1010 or tert-butylhydroquinone, the plasticizer is acetylated tributyl citrate or tributyl citrate, and the stabilizer is magnesium stearate or epoxidized soybean oil.

6. A method for preparing a waterproof coating for paper bags according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Weigh out the biodegradable resin, modified starch, modified nanomaterials and additives according to the mass ratio, add them to deionized water, adjust the viscosity to 400-800 mPa·s, stir at 500-1000 rpm for 30-60 min, and filter through an 80-120 mesh filter to obtain the coating liquid. S2. Apply the coating liquid evenly to the surface of the paper bag base paper using a roller coating method, with a coating amount of 5-15 g / m². 2 Dry at 80-100℃ for 5-15 minutes to form a film, thus obtaining a waterproof coating for paper bags.

7. A paper bag comprising paper bag base paper and a waterproof coating for paper bags according to any one of claims 1-6 coated on its surface.

8. The paper bag according to claim 7, characterized in that, The inner wall, bottom and outer surface of the paper bag are coated.

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