Degradable bio-based coating and method for its preparation

Abstract

The present application relates to the technical field of paint, in particular to a degradable bio-based paint and a preparation method thereof. The paint is a water continuous phase dispersion, and is prepared by using chain end itaconic acid half esterization poly L-lactic acid as a matrix, and combining sodium bicarbonate, oligomeric chitosan lactic acid salt, four-arm poly D-lactic acid seed and water-soluble chitosan lactic acid salt. By controlling the chain end carboxyl content, the partial neutralization degree and the adding time sequence of the three types of auxiliary components, the poly lactic acid dispersion has good water dispersion stability and low temperature film forming property. After being coated on food packaging base paper, the coating layer can form a compact structure containing stereocomplex crystals, reduces the Cobb60 water absorption and water vapor transmission rate of the coated paper, and improves the appearance stability after immersion, and is suitable for surface moisture-proof coating of food packaging paper.

Description

Technical Field

[0001] This invention relates to the field of coating technology, and in particular to a biodegradable bio-based coating and its preparation method. Background Technology

[0002] With the explosive growth of e-commerce logistics and the food delivery industry, the demand for disposable food packaging paper has surged. To impart waterproof, oil-proof, and barrier properties to paper, the industry commonly uses polyethylene (PE) or polypropylene (PP) lamination processes. However, these petroleum-based polymers are difficult to degrade in the natural environment, causing a serious "white pollution" problem. Although biodegradable materials such as polylactic acid (PLA) have been considered ideal alternatives in recent years, in practical processing applications, the inherent strong hydrophobicity of PLA makes it difficult to directly prepare stable water-based dispersion coatings. This usually requires the addition of large amounts of organic solvents or highly toxic emulsifiers, which contradicts the original intention of green environmental protection.

[0003] Even when aqueous polylactic acid (PLA) dispersions are successfully produced through physical blending or chemical modification, their film-forming properties under low-temperature drying conditions still face significant challenges. PLA has a slow crystallization rate; without high-temperature heat treatment, the coating's molecular chains are loosely arranged, failing to form a dense barrier layer. This results in the coated paper's water resistance and moisture-proof properties being far inferior to traditional plastic-coated paper. Furthermore, to improve the interfacial adhesion between the coating and paper, existing technologies often introduce small-molecule plasticizers or crosslinking agents. These substances are prone to migration and precipitation during use, not only reducing the coating's wet strength but also posing a safety hazard of migrating into food.

[0004] More importantly, existing bio-based coating technologies often compromise on several aspects, failing to simultaneously address the coating's storage stability, rapid low-temperature film formation, and biodegradability after disposal. Particularly in high-temperature, high-humidity food packaging environments, single-component polylactic acid coatings are prone to swelling, whitening, and even peeling, severely hindering the commercialization of fully biodegradable food packaging paper. Therefore, developing a fully biodegradable coating that is free of toxic additives, can rapidly form high-barrier, water-resistant, and stable coatings at low temperatures, is a crucial technological bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a biodegradable bio-based coating and its preparation method, so as to solve the problems of slow low-temperature film formation and crystallization, poor barrier properties and easy failure after immersion in water of polylactic acid aqueous dispersions without toxic additives, and to achieve the unity of high performance and full biodegradability of coating.

[0006] To achieve the above objectives, the present invention provides a biodegradable bio-based coating, wherein the biodegradable bio-based coating is an aqueous continuous phase dispersion, and based on dry weight parts, with 100 parts of chain-terminated itaconic acid semi-esterified poly-L-lactic acid, the biodegradable bio-based coating is prepared from raw materials comprising the following components: 100 parts of chain-terminated itaconic acid semi-esterified poly-L-lactic acid; Sodium bicarbonate 1.33-2.50 parts; The oligochitosan lactate formed from oligochitosan and D,L-lactic acid, wherein the oligochitosan content is 0.40-0.80 parts and the D,L-lactic acid content is 0.27-0.53 parts. 1.00-2.50 parts of four-armed poly-D-lactic acid seed crystals; 0.80-1.60 parts of water-soluble chitosan lactate; and water; The itaconic acid semi-esterified poly-L-lactic acid is obtained by semi-esterification of tetra-hydroxyl-terminated poly-L-lactic acid with itaconic anhydride. The content of the terminal carboxyl groups of the itaconic acid semi-esterified poly-L-lactic acid before neutralization is 0.30-0.52 mmol / g, and 45%-60% of the terminal carboxyl groups are neutralized by the sodium bicarbonate. The degree of neutralization of the chain-terminal carboxyl groups is calculated according to the following formula: Neutralization degree / %=[m(NaHCO3) / 84.01] / [m(PILA)×C(COOH)]×100%.

[0007] Wherein, m(NaHCO3) is the mass of sodium bicarbonate in g; m(PILA) is the mass of semi-esterified poly-L-lactic acid with itaconic acid at the chain end in g; and C(COOH) is the content of carboxyl groups at the chain end in mol / g. Controlling the degree of neutralization to 45%-60% allows for the formation of a sufficient carboxylate stabilizing layer on the particle surface, while avoiding excessive ionization that could lead to increased hygroscopicity of the coating.

[0008] The solid content of the biodegradable bio-based coating is 29%-35%.

[0009] Preferably, the oligomeric chitosan lactate is added before phase inversion emulsification, the four-armed poly-D-lactic acid seed crystals are added after the formation of the aqueous continuous phase dispersion and before the complete removal of ethyl acetate, and the water-soluble chitosan lactate is added after the addition of the four-armed poly-D-lactic acid seed crystals.

[0010] Preferably, the content of the terminal carboxyl groups of the chain-terminal itaconic acid semi-esterified poly-L-lactic acid before neutralization is 0.306-0.506 mmol / g.

[0011] Preferably, the four-arm hydroxyl-terminated poly-L-lactic acid is obtained by ring-opening polymerization of L-(-)-lactide, pentaerythritol and stannous octoate, wherein the mass ratio of L-(-)-lactide, pentaerythritol and stannous octoate is 900:11-20:0.160-0.220.

[0012] Preferably, the chain-terminated itaconic acid semi-esterified poly-L-lactic acid is obtained by reacting the four-arm hydroxyl-terminated poly-L-lactic acid, anhydrous ethyl acetate, itaconic anhydride and stannous octoate, followed by precipitation and drying, wherein the mass ratio of the four-arm hydroxyl-terminated poly-L-lactic acid, anhydrous ethyl acetate, itaconic anhydride and stannous octoate is 700:1400:25-42:0.070-0.090.

[0013] Preferably, the four-armed poly-D-lactic acid seed crystal is obtained by ring-opening polymerization of D-lactide, pentaerythritol and stannous octoate, wherein the mass ratio of D-lactide, pentaerythritol and stannous octoate is 60:1:0.020.

[0014] Preferably, the four-armed poly-D-lactic acid seed crystals are added in the form of an ethyl acetate solution, and the mass ratio of the four-armed poly-D-lactic acid seed crystals to anhydrous ethyl acetate is 6-15:54-120.

[0015] Preferably, the weight-average molecular weight of the oligochitosan is 800-1000, and the oligochitosan lactate is formed by mixing deionized water, oligochitosan and D,L-lactic acid, wherein the mass ratio of the deionized water, oligochitosan and D,L-lactic acid is 52-72:2.4-4.8:1.6-3.2.

[0016] Preferably, the sodium bicarbonate is added in the form of an aqueous sodium bicarbonate solution; based on 600 parts of chain-terminated itaconic acid semi-esterified poly-L-lactic acid, the aqueous sodium bicarbonate solution is prepared by 8-15 parts of sodium bicarbonate and 115-122 parts of deionized water to a total of 130 parts.

[0017] Preferably, the water-soluble chitosan lactate is added in the form of an aqueous solution, and the mass ratio of the water-soluble chitosan lactate to deionized water is 4.8-9.6:110.4-115.2.

[0018] This invention also provides a method for preparing a biodegradable bio-based coating, comprising the following steps: (1) L-(-)-lactide, pentaerythritol and stannous octoate are subjected to ring-opening polymerization to obtain four-arm hydroxyl-terminated poly-L-lactic acid; the four-arm hydroxyl-terminated poly-L-lactic acid is subjected to a half-esterification reaction with itaconic anhydride to obtain chain-terminated itaconic acid half-esterified poly-L-lactic acid. (2) D-lactide, pentaerythritol and stannous octoate are subjected to ring-opening polymerization to obtain four-armed poly-D-lactic acid seed crystals, and the four-armed poly-D-lactic acid seed crystals are dissolved in ethyl acetate to obtain a four-armed poly-D-lactic acid ethyl acetate solution. (3) Mix oligochitosan and D,L-lactic acid in water to obtain oligochitosan lactate pretreatment solution; (4) The chain-terminal itaconic acid semi-esterified poly-L-lactic acid is dissolved in ethyl acetate to obtain a polylactic acid oil phase. Sodium bicarbonate aqueous solution is added to the polylactic acid oil phase to obtain a partially neutralized polylactic acid oil phase. (5) Add the pretreated oligochitosan lactate solution to the partially neutralized polylactic acid oil phase, then add water and shear emulsify to obtain a water-continuous polylactic acid primary dispersion; (6) Add the four-arm poly-D-lactic acid ethyl acetate solution to the aqueous continuous phase polylactic acid primary dispersion, then add the water-soluble chitosan lactate aqueous solution, then remove the ethyl acetate, adjust the solid content and filter to obtain a biodegradable bio-based coating.

[0019] The beneficial effects of this invention are: (1) This invention precisely introduces a specific amount of terminal carboxyl groups through the semi-esterification reaction of four-arm hydroxyl-terminated poly-L-lactic acid with itaconic anhydride, and partially neutralizes them with sodium bicarbonate, so that the polylactic acid matrix retains its biodegradability and obtains excellent water dispersion stability. Test data show that when the solid content of the coating is controlled at 29%-35%, the viscosity at 25°C is stable at 168-356 mPa·s, which is convenient for coating processing; and the introduction of terminal carboxyl groups significantly improves the chemical bonding force between the coating and paper fibers. The water absorption of the coated paper Cobb60 in Examples 1-5 is as low as 11.82-17.38 g / m 2 Compared to the blank base paper's 68.6 g / m³ 2 The moisture-proof performance is significantly improved.

[0020] (2) This invention creatively employs a segmented addition strategy of oligomeric chitosan lactate and water-soluble chitosan lactate, combined with the introduction of four-armed poly-D-lactic acid seed crystals within a specific process window, successfully constructing a dense stereocomposite crystalline structure under low-temperature drying conditions. Performance data show that the stereocomposite melting peak enthalpy of Example 4 is as high as 6.8 J / g, and the water vapor transmission rate of the coated paper is reduced to 83.54 g / (m²). 2 • 24h); Meanwhile, the chitosan salt added in stages forms a gradient-enhanced interface barrier inside and outside the coating, so that after the coated paper is soaked in deionized water at 23℃ for 2 hours, the abnormal area rate of the coating is only 1.8%-5.1%, which completely solves the problem of traditional bio-based coatings easily swelling and falling off when exposed to water.

[0021] (3) The coating prepared by this invention is composed entirely of bio-based monomers and contains no petroleum-based polymers or non-degradable additives. According to GB / T 19277.1-2025 standard testing, the coatings of Examples 1-5 achieved an aerobic biodegradation rate of 70.6%-76.4% under controlled composting conditions after 60 days, fully demonstrating excellent environmental friendliness. This technology overcomes the limitation of existing bio-based coatings that "difficulty in achieving both performance and degradation," providing a practical and feasible technical solution for the green development of high-value-added food packaging materials. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0023] Raw material source and model: L-(-)-lactide was supplied by TCI (Shanghai) Chemical Industry Development Co., Ltd., product number L0115, CAS number 4511-42-6, with a purity greater than 98%. D-lactide was supplied by TCI (Shanghai) Chemical Industry Development Co., Ltd., product number L0366, CAS number 13076-17-0, with a purity greater than 98%. D,L-lactic acid was supplied by Merck Sigma-Aldrich, product number 69785, CAS number 50-21-5, with a content of 90%. Chitosan oligomers were supplied by Shanghai Maokang Biotechnology Co., Ltd., product number MS0806, with a weight-average molecular weight of 800-1000. Water-soluble chitosan lactate was supplied by Shanghai Maokang Biotechnology Co., Ltd., product number MS0803. The food packaging base paper was food-grade kraft paper from Dongguan Yiyu Paper Industry Co., Ltd., with a basis weight of 60 g / m³. 2 Before use, allow it to equilibrate for 24 hours at 23℃ and 50% relative humidity.

[0024] Example 1: Step 1: Add 900g of L-(-)-lactide, 15g of pentaerythritol and 180mg of stannous octoate to a dry reaction vessel. After evacuating to 500Pa, high-purity nitrogen is introduced. This process is repeated 3 times. The system is then heated to 135℃ and stirred at 200r / min for 5h under nitrogen protection. After the reaction is complete, the temperature is lowered to 120℃ and residual lactide is removed at 100Pa for 1h. After discharge, the product is vacuum dried at 45℃ and 200Pa for 12h to obtain hydroxyl-terminated low molecular weight poly-L-lactic acid. Step 2: Add 700g of hydroxyl-terminated low molecular weight poly-L-lactic acid obtained in Step 1 and 1400g of anhydrous ethyl acetate to a reactor equipped with a reflux condenser. Stir at 400r / min for 40min at 50℃. Then add 34g of itaconic anhydride and 80mg of stannous octoate. Heat to 72℃ and maintain reflux for 5h. After the reaction is completed, cool to 35℃. Add the reaction solution to a precipitate composed of 3360g of anhydrous ethanol and 840g of deionized water at 20g / min. Stir at 500r / min for 30min and filter. Wash the filter cake twice with a washing solution composed of 800g of anhydrous ethanol and 200g of deionized water. Then vacuum dry at 45℃ and 200Pa until the mass change does not exceed 1g for 2 consecutive hours to obtain chain-terminated itaconic acid semi-esterified polylactic acid. Step 3: Add 60g of D-lactide, 1g of pentaerythritol, and 20mg of stannous octoate to a dry reaction vessel. Vacuum the vessel to 500Pa and then purge with high-purity nitrogen. Repeat this process three times. Heat the system to 135℃ and stir at 200r / min for 4h under nitrogen protection. Then, devolatilize at 120℃ and 100Pa for 1h. Add the devolatilized product to 120g of anhydrous ethyl acetate and stir at 400r / min for 40min at 50℃. Add the resulting solution to a precipitate composed of 360g of anhydrous ethanol and 90g of deionized water. Filter and vacuum dry at 45℃ and 200Pa for 12h to obtain a four-armed poly-D-lactic acid seed precursor. Add 10g of the four-armed poly-D-lactic acid seed precursor to 80g of anhydrous ethyl acetate and stir at 600r / min for 40min at 50℃ to obtain a four-armed poly-D-lactic acid ethyl acetate solution. Step 4: Add 55g of deionized water, 3g of oligochitosan and 2g of D,L-lactic acid to a beaker, and stir at 400r / min for 30min at 25℃ to obtain oligochitosan lactate pretreatment solution. Step 5: Add 600g of the semi-esterified polylactic acid with chain-terminated itaconic acid obtained in Step 2 and 1200g of anhydrous ethyl acetate to the emulsification tank. Stir at 1000r / min for 40min at 50℃ to obtain the polylactic acid oil phase. Separately, dissolve 11g of sodium bicarbonate in 119g of deionized water to prepare a sodium bicarbonate aqueous solution. Add 130g of the sodium bicarbonate aqueous solution dropwise to the polylactic acid oil phase at 1g / min under non-closed exhaust conditions at 50℃. Maintain stirring at 1000r / min during the dropwise addition. After the dropwise addition is completed, continue stirring for 30min, and then let stand and exhaust for 10min to obtain a partially neutralized polylactic acid oil phase. Step 6: Add 60g of the oligochitosan lactate pretreatment solution obtained in Step 4 to the partially neutralized polylactic acid oil phase obtained in Step 5 at a rate of 1g / min. Stir at 1200r / min for 20min at 50℃. Preheat 1100g of deionized water to 45℃ and add it at a rate of 18g / min. Simultaneously, shear emulsify at 8000r / min. After all the deionized water has been added, continue shearing for 10min, then change to 600r / min and stir for 20min to obtain an aqueous continuous phase polylactic acid primary dispersion. Step 7: Add 90g of the four-arm poly-D-lactic acid ethyl acetate solution obtained in Step 3 to the aqueous continuous phase polylactic acid primary dispersion obtained in Step 6 at a rate of 3g / min. Maintain the temperature at 45℃ during the addition and shear dispersion at 2500r / min. After the addition is complete, switch to stirring at 600r / min for 60min. Dissolve 6g of water-soluble chitosan lactate in 114g of deionized water and stir at 400r / min for 30min at 25℃ to obtain a water-soluble chitosan lactate aqueous solution. Add 120g of the water-soluble chitosan lactate aqueous solution at a rate of 3g / min. After the addition is complete, stir at 600r / min for 40min at 45℃. Then, distill off the ethyl acetate under reduced pressure at 45℃ and 20kPa, maintaining stirring at 300r / min during the distillation process, until the residual ethyl acetate content is less than 500mg / kg as measured by gas chromatography. After solvent removal, adjust the solid content to 31% with deionized water and then filter through a 100-mesh sieve to obtain a biodegradable bio-based coating. Step 8: After stirring the biodegradable bio-based coating at 300 rpm for 10 minutes at 25°C, apply it to a 60 g / m² substrate using a wire rod coating method. 2 For food packaging base paper, the wet coating amount on one side should be controlled at 20g / m². 2 After coating, the paper is placed in a 45℃ hot air drying oven for 2 minutes, and then transferred to a 65℃ hot air drying oven for 8 minutes. The hot air speed is controlled at 1m / s. The coated paper is then equilibrated at 23℃ and 50% relative humidity for 24 hours to obtain a food packaging paper moisture-proof coating sample.

[0025] Example 2: Step 1: Add 900g of L-(-)-lactide, 12g of pentaerythritol and 160mg of stannous octoate to a dry reaction vessel. After evacuating to 500Pa, high-purity nitrogen is introduced. This process is repeated 3 times. The system is then heated to 135℃ and stirred at 200r / min for 5h under nitrogen protection. After the reaction is complete, the temperature is lowered to 120℃ and residual lactide is removed at 100Pa for 1h. After discharge, the product is vacuum dried at 45℃ and 200Pa for 12h to obtain hydroxyl-terminated low molecular weight poly-L-lactic acid. Step 2: Add 700g of hydroxyl-terminated low molecular weight poly-L-lactic acid obtained in Step 1 and 1400g of anhydrous ethyl acetate to a reactor equipped with a reflux condenser. Stir at 400r / min for 40min at 50℃. Then add 26g of itaconic anhydride and 70mg of stannous octoate. Heat to 72℃ and maintain reflux for 5h. After the reaction is completed, cool to 35℃. Add the reaction solution to a precipitate composed of 3360g of anhydrous ethanol and 840g of deionized water at 20g / min. Stir at 500r / min for 30min and filter. Wash the filter cake twice with a washing solution composed of 800g of anhydrous ethanol and 200g of deionized water. Then vacuum dry at 45℃ and 200Pa until the mass change does not exceed 1g for 2 consecutive hours to obtain chain-terminated itaconic acid semi-esterified polylactic acid. Step 3: Add 60g of D-lactide, 1g of pentaerythritol, and 20mg of stannous octoate to a dry reaction vessel. Vacuum the vessel to 500Pa and then purge with high-purity nitrogen. Repeat this process three times. Heat the system to 135℃ and stir at 200r / min for 4h under nitrogen protection. Then, devolatilize at 120℃ and 100Pa for 1h. Add the devolatilized product to 120g of anhydrous ethyl acetate and stir at 400r / min for 40min at 50℃. Add the resulting solution to a precipitate composed of 360g of anhydrous ethanol and 90g of deionized water. Filter and vacuum dry at 45℃ and 200Pa for 12h to obtain a four-armed poly-D-lactic acid seed precursor. Add 6g of the four-armed poly-D-lactic acid seed precursor to 54g of anhydrous ethyl acetate and stir at 600r / min for 40min at 50℃ to obtain a four-armed poly-D-lactic acid ethyl acetate solution. Step 4: Add 52g of deionized water, 2.4g of oligochitosan and 1.6g of D,L-lactic acid to a beaker, and stir at 400r / min for 30min at 25℃ to obtain oligochitosan lactate pretreatment solution. Step 5: Add 600g of the semi-esterified polylactic acid with chain-terminated itaconic acid obtained in Step 2 and 1200g of anhydrous ethyl acetate to the emulsification tank. Stir at 1000r / min for 40min at 50℃ to obtain the polylactic acid oil phase. Separately, dissolve 8g of sodium bicarbonate in 122g of deionized water to prepare a sodium bicarbonate aqueous solution. Add 130g of the sodium bicarbonate aqueous solution dropwise to the polylactic acid oil phase at 1g / min under non-closed exhaust conditions at 50℃. Maintain stirring at 1000r / min during the dropwise addition. After the dropwise addition is completed, continue stirring for 30min, and then let stand and exhaust for 10min to obtain a partially neutralized polylactic acid oil phase. Step 6: Add 56g of the oligochitosan lactate pretreatment solution obtained in Step 4 to the partially neutralized polylactic acid oil phase obtained in Step 5 at a rate of 1g / min. Stir at 1200r / min for 20min at 50℃. Preheat 1050g of deionized water to 45℃ and add it at a rate of 17g / min. Simultaneously, shear emulsify at 8000r / min. After all the deionized water has been added, continue shearing for 10min, then change to 600r / min and stir for 20min to obtain an aqueous continuous phase polylactic acid primary dispersion. Step 7: Add 60g of the four-arm poly-D-lactic acid ethyl acetate solution obtained in Step 3 to the aqueous continuous phase polylactic acid primary dispersion obtained in Step 6 at a rate of 3g / min. Maintain the temperature at 45℃ during the addition and shear dispersion at 2500r / min. After the addition is complete, switch to stirring at 600r / min for 60min. Dissolve 4.8g of water-soluble chitosan lactate in 115.2g of deionized water and stir at 400r / min for 30min at 25℃ to obtain a water-soluble chitosan lactate aqueous solution. Add 120g of the water-soluble chitosan lactate aqueous solution at a rate of 3g / min. After the addition is complete, stir at 600r / min at 45℃ for 40min. Then, distill off the ethyl acetate under reduced pressure at 45℃ and 20kPa, maintaining stirring at 300r / min during the distillation process, until the residual ethyl acetate content is less than 500mg / kg as measured by gas chromatography. After solvent removal, adjust the solid content to 29% with deionized water and then filter through a 100-mesh sieve to obtain a biodegradable bio-based coating. Step 8: After stirring the biodegradable bio-based coating at 300 rpm for 10 minutes at 25°C, apply it to a 60 g / m² substrate using a wire rod coating method. 2 For food packaging base paper, the wet coating weight on one side is controlled at 22 g / m². 2 After coating, the paper is placed in a 45℃ hot air drying oven for 2 minutes, and then transferred to a 65℃ hot air drying oven for 8 minutes. The hot air speed is controlled at 1m / s. The coated paper is then equilibrated at 23℃ and 50% relative humidity for 24 hours to obtain a food packaging paper moisture-proof coating sample.

[0026] Example 3: Step 1: Add 900g of L-(-)-lactide, 18g of pentaerythritol and 220mg of stannous octoate to a dry reaction vessel. After evacuating to 500Pa, high-purity nitrogen is introduced. This process is repeated 3 times. The system is then heated to 135℃ and stirred at 200r / min for 5h under nitrogen protection. After the reaction is complete, the temperature is lowered to 120℃ and residual lactide is removed at 100Pa for 1h. After discharge, the product is vacuum dried at 45℃ and 200Pa for 12h to obtain hydroxyl-terminated low molecular weight poly-L-lactic acid. Step 2: Add 700g of hydroxyl-terminated low molecular weight poly-L-lactic acid obtained in Step 1 and 1400g of anhydrous ethyl acetate to a reactor equipped with a reflux condenser. Stir at 400r / min for 40min at 50℃. Then add 42g of itaconic anhydride and 90mg of stannous octoate. Heat to 72℃ and maintain reflux for 5h. After the reaction is completed, cool to 35℃. Add the reaction solution to a precipitate composed of 3360g of anhydrous ethanol and 840g of deionized water at 20g / min. Stir at 500r / min for 30min and filter. Wash the filter cake twice with a washing solution composed of 800g of anhydrous ethanol and 200g of deionized water. Then vacuum dry at 45℃ and 200Pa until the mass change does not exceed 1g for 2 consecutive hours to obtain chain-terminated itaconic acid semi-esterified polylactic acid. Step 3: Add 60g of D-lactide, 1g of pentaerythritol, and 20mg of stannous octoate to a dry reaction vessel. Vacuum the vessel to 500Pa and then purge with high-purity nitrogen. Repeat this process three times. Heat the system to 135℃ and stir at 200r / min for 4h under nitrogen protection. Then, devolatilize at 120℃ and 100Pa for 1h. Add the devolatilized product to 120g of anhydrous ethyl acetate and stir at 400r / min for 40min at 50℃. Add the resulting solution to a precipitate composed of 360g of anhydrous ethanol and 90g of deionized water. Filter and vacuum dry at 45℃ and 200Pa for 12h to obtain a four-armed poly-D-lactic acid seed precursor. Add 14g of the four-armed poly-D-lactic acid seed precursor to 112g of anhydrous ethyl acetate and stir at 600r / min for 40min at 50℃ to obtain a four-armed poly-D-lactic acid ethyl acetate solution. Step 4: Add 63g of deionized water, 4.2g of oligochitosan and 2.8g of D,L-lactic acid to a beaker, and stir at 400r / min for 30min at 25℃ to obtain oligochitosan lactate pretreatment solution. Step 5: Add 600g of the semi-esterified polylactic acid with chain-terminated itaconic acid obtained in Step 2 and 1200g of anhydrous ethyl acetate to the emulsification tank. Stir at 1000r / min for 40min at 50℃ to obtain the polylactic acid oil phase. Separately, dissolve 15g of sodium bicarbonate in 115g of deionized water to prepare a sodium bicarbonate aqueous solution. Add 130g of the sodium bicarbonate aqueous solution dropwise to the polylactic acid oil phase at 1g / min under non-closed exhaust conditions at 50℃. Maintain stirring at 1000r / min during the dropwise addition. After the dropwise addition is completed, continue stirring for 30min, and then let stand and exhaust for 10min to obtain a partially neutralized polylactic acid oil phase. Step 6: Add 70g of the oligomeric chitosan lactate pretreatment solution obtained in Step 4 to the partially neutralized polylactic acid oil phase obtained in Step 5 at a rate of 1g / min. Stir at 1200r / min for 20min at 50℃. Preheat 1150g of deionized water to 45℃ and add it at a rate of 20g / min. Simultaneously, shear emulsify at 8000r / min. After all the deionized water has been added, continue shearing for 10min, then change to 600r / min and stir for 20min to obtain an aqueous continuous phase polylactic acid primary dispersion. Step 7: Add 126g of the four-arm poly-D-lactic acid ethyl acetate solution obtained in Step 3 to the aqueous continuous phase polylactic acid primary dispersion obtained in Step 6 at a rate of 3g / min. During the addition, maintain the temperature at 45℃ and shear dispersion at 2500r / min. After the addition is complete, switch to stirring at 600r / min for 60min. Dissolve 8.4g of water-soluble chitosan lactate in 111.6g of deionized water and stir at 400r / min for 30min at 25℃ to obtain a water-soluble chitosan lactate aqueous solution. Add 120g of the water-soluble chitosan lactate aqueous solution at a rate of 3g / min. After the addition is complete, stir at 600r / min at 45℃ for 40min. Then, distill off the ethyl acetate under reduced pressure at 45℃ and 20kPa, maintaining stirring at 300r / min during the distillation process, until the residual ethyl acetate content is less than 500mg / kg as measured by gas chromatography. After solvent removal, adjust the solid content to 33% with deionized water and then filter through a 100-mesh sieve to obtain a biodegradable bio-based coating. Step 8: After stirring the biodegradable bio-based coating at 300 rpm for 10 minutes at 25°C, apply it to a 60 g / m² substrate using a wire rod coating method. 2 For food packaging base paper, the wet coating weight on one side is controlled at 18 g / m². 2 After coating, the paper is placed in a 45℃ hot air drying oven for 2 minutes, and then transferred to a 65℃ hot air drying oven for 8 minutes. The hot air speed is controlled at 1m / s. The coated paper is then equilibrated at 23℃ and 50% relative humidity for 24 hours to obtain a food packaging paper moisture-proof coating sample.

[0027] Example 4: Step 1: Add 900g of L-(-)-lactide, 20g of pentaerythritol and 200mg of stannous octoate to a dry reaction vessel. After evacuating to 500Pa, high-purity nitrogen is introduced. This process is repeated 3 times. The system is then heated to 135℃ and stirred at 200r / min for 4.5h under nitrogen protection. After the reaction is complete, the temperature is lowered to 120℃ and residual lactide is removed at 100Pa for 1h. After discharge, the product is vacuum dried at 45℃ and 200Pa for 12h to obtain hydroxyl-terminated low molecular weight poly-L-lactic acid. Step 2: Add 700g of hydroxyl-terminated low molecular weight poly-L-lactic acid obtained in Step 1 and 1400g of anhydrous ethyl acetate to a reactor equipped with a reflux condenser. Stir at 400r / min for 40min at 50℃. Then add 30g of itaconic anhydride and 80mg of stannous octoate. Heat to 72℃ and maintain reflux for 5h. After the reaction is completed, cool to 35℃. Add the reaction solution to a precipitate composed of 3360g of anhydrous ethanol and 840g of deionized water at 20g / min. Stir at 500r / min for 30min and filter. Wash the filter cake twice with a washing solution composed of 800g of anhydrous ethanol and 200g of deionized water. Then vacuum dry at 45℃ and 200Pa until the mass change does not exceed 1g for 2 consecutive hours to obtain chain-terminated itaconic acid semi-esterified polylactic acid. Step 3: Add 60g of D-lactide, 1g of pentaerythritol, and 20mg of stannous octoate to a dry reaction vessel. Vacuum the vessel to 500Pa and then purge with high-purity nitrogen. Repeat this process three times. Heat the system to 135℃ and stir at 200r / min for 4h under nitrogen protection. Then, devolatilize at 120℃ and 100Pa for 1h. Add the devolatilized product to 120g of anhydrous ethyl acetate and stir at 400r / min for 40min at 50℃. Add the resulting solution to a precipitate composed of 360g of anhydrous ethanol and 90g of deionized water. Filter and vacuum dry at 45℃ and 200Pa for 12h to obtain a four-armed poly-D-lactic acid seed precursor. Add 15g of the four-armed poly-D-lactic acid seed precursor to 120g of anhydrous ethyl acetate and stir at 600r / min for 40min at 50℃ to obtain a four-armed poly-D-lactic acid ethyl acetate solution. Step 4: Add 72g of deionized water, 4.8g of oligochitosan and 3.2g of D,L-lactic acid to a beaker, and stir at 400r / min for 30min at 25℃ to obtain oligochitosan lactate pretreatment solution. Step 5: Add 600g of the semi-esterified polylactic acid with itaconic acid obtained in Step 2 and 1200g of anhydrous ethyl acetate to the emulsification tank. Stir at 1000r / min for 40min at 50℃ to obtain the polylactic acid oil phase. Separately, dissolve 10g of sodium bicarbonate in 120g of deionized water to prepare a sodium bicarbonate aqueous solution. Add 130g of the sodium bicarbonate aqueous solution dropwise to the polylactic acid oil phase at 1g / min under non-closed exhaust conditions at 50℃. Maintain stirring at 1000r / min during the dropwise addition. After the dropwise addition is completed, continue stirring for 30min, and then let stand and exhaust for 10min to obtain a partially neutralized polylactic acid oil phase. Step 6: Add 80g of the oligochitosan lactate pretreatment solution obtained in Step 4 to the partially neutralized polylactic acid oil phase obtained in Step 5 at a rate of 1g / min. Stir at 1200r / min for 20min at 50℃. Preheat 1100g of deionized water to 45℃ and add it at a rate of 18g / min. Simultaneously, shear emulsify at 8000r / min. After all the deionized water has been added, continue shearing for 10min, then change to 600r / min and stir for 20min to obtain an aqueous continuous phase polylactic acid primary dispersion. Step 7: Add 135g of the four-arm poly-D-lactic acid ethyl acetate solution obtained in Step 3 to the aqueous continuous phase polylactic acid primary dispersion obtained in Step 6 at a rate of 3g / min. During the addition, maintain the temperature at 45℃ and shear dispersion at 2500r / min. After the addition is complete, switch to stirring at 600r / min for 60min. Dissolve 9.6g of water-soluble chitosan lactate in 110.4g of deionized water and stir at 400r / min for 30min at 25℃ to obtain a water-soluble chitosan lactate aqueous solution. Add 120g of the water-soluble chitosan lactate aqueous solution at a rate of 3g / min. After the addition is complete, stir at 600r / min at 45℃ for 40min. Then, distill off the ethyl acetate under reduced pressure at 45℃ and 20kPa, maintaining stirring at 300r / min during the distillation process, until the residual ethyl acetate content is less than 500mg / kg as measured by gas chromatography. After solvent removal, adjust the solid content to 35% with deionized water and then filter through a 100-mesh sieve to obtain a biodegradable bio-based coating. Step 8: After stirring the biodegradable bio-based coating at 300 rpm for 10 minutes at 25°C, apply it to a 60 g / m² substrate using a wire rod coating method. 2 For food packaging base paper, the wet coating weight on one side is controlled at 17 g / m². 2 After coating, the paper is placed in a 45℃ hot air drying oven for 2 minutes, and then transferred to a 65℃ hot air drying oven for 8 minutes. The hot air speed is controlled at 1m / s. The coated paper is then equilibrated at 23℃ and 50% relative humidity for 24 hours to obtain a food packaging paper moisture-proof coating sample.

[0028] Example 5: Step 1: Add 900g of L-(-)-lactide, 11g of pentaerythritol and 180mg of stannous octoate to a dry reaction vessel. After evacuating to 500Pa, high-purity nitrogen is introduced. This process is repeated 3 times. The system is then heated to 135℃ and stirred at 200r / min for 5.5h under nitrogen protection. After the reaction is complete, the temperature is lowered to 120℃ and residual lactide is removed at 100Pa for 1h. After discharge, the product is vacuum dried at 45℃ and 200Pa for 12h to obtain hydroxyl-terminated low molecular weight poly-L-lactic acid. Step 2: Add 700g of hydroxyl-terminated low molecular weight poly-L-lactic acid obtained in Step 1 and 1400g of anhydrous ethyl acetate to a reactor equipped with a reflux condenser. Stir at 400r / min for 40min at 50℃. Then add 25g of itaconic anhydride and 70mg of stannous octoate. Heat to 72℃ and maintain reflux for 5h. After the reaction is completed, cool to 35℃. Add the reaction solution to a precipitate composed of 3360g of anhydrous ethanol and 840g of deionized water at 20g / min. Stir at 500r / min for 30min and filter. Wash the filter cake twice with a washing solution composed of 800g of anhydrous ethanol and 200g of deionized water. Then vacuum dry at 45℃ and 200Pa until the mass change does not exceed 1g for 2 consecutive hours to obtain chain-terminated itaconic acid semi-esterified polylactic acid. Step 3: Add 60g of D-lactide, 1g of pentaerythritol, and 20mg of stannous octoate to a dry reaction vessel. Vacuum the vessel to 500Pa and then purge with high-purity nitrogen. Repeat this process three times. Heat the system to 135℃ and stir at 200r / min for 4h under nitrogen protection. Then, devolatilize at 120℃ and 100Pa for 1h. Add the devolatilized product to 120g of anhydrous ethyl acetate and stir at 400r / min for 40min at 50℃. Add the resulting solution to a precipitate composed of 360g of anhydrous ethanol and 90g of deionized water. Filter and vacuum dry at 45℃ and 200Pa for 12h to obtain a four-armed poly-D-lactic acid seed precursor. Add 8g of the four-armed poly-D-lactic acid seed precursor to 64g of anhydrous ethyl acetate and stir at 600r / min for 40min at 50℃ to obtain a four-armed poly-D-lactic acid ethyl acetate solution. Step 4: Add 60g of deionized water, 3g of oligochitosan and 2g of D,L-lactic acid to a beaker, and stir at 400r / min for 30min at 25℃ to obtain oligochitosan lactate pretreatment solution. Step 5: Add 600g of the semi-esterified polylactic acid with chain-terminated itaconic acid obtained in Step 2 and 1200g of anhydrous ethyl acetate to the emulsification tank. Stir at 1000r / min for 40min at 50℃ to obtain the polylactic acid oil phase. Separately, dissolve 8.5g of sodium bicarbonate in 121.5g of deionized water to prepare a sodium bicarbonate aqueous solution. Add 130g of the sodium bicarbonate aqueous solution dropwise to the polylactic acid oil phase at 1g / min under non-closed exhaust conditions at 50℃. Maintain stirring at 1000r / min during the dropwise addition. After the dropwise addition is completed, continue stirring for 30min, and then let stand and exhaust for 10min to obtain a partially neutralized polylactic acid oil phase. Step 6: Add 65g of the oligochitosan lactate pretreatment solution obtained in Step 4 to the partially neutralized polylactic acid oil phase obtained in Step 5 at a rate of 1g / min. Stir at 1200r / min for 20min at 50℃. Preheat 1080g of deionized water to 45℃ and add it at a rate of 18g / min. Simultaneously, shear emulsify at 8000r / min. After all the deionized water has been added, continue shearing for 10min, then change to 600r / min and stir for 20min to obtain an aqueous continuous phase polylactic acid primary dispersion. Step 7: Add 72g of the four-arm poly-D-lactic acid ethyl acetate solution obtained in Step 3 to the aqueous continuous phase polylactic acid primary dispersion obtained in Step 6 at a rate of 3g / min. Maintain the temperature at 45℃ during the addition and shear dispersion at 2500r / min. After the addition is complete, switch to stirring at 600r / min for 60min. Dissolve 6.6g of water-soluble chitosan lactate in 113.4g of deionized water and stir at 400r / min for 30min at 25℃ to obtain a water-soluble chitosan lactate aqueous solution. Add 120g of the water-soluble chitosan lactate aqueous solution at a rate of 3g / min. After the addition is complete, stir at 600r / min at 45℃ for 40min. Then, distill off the ethyl acetate under reduced pressure at 45℃ and 20kPa, maintaining stirring at 300r / min during the distillation process, until the residual ethyl acetate content is less than 500mg / kg as measured by gas chromatography. After solvent removal, adjust the solid content to 30% with deionized water and then filter through a 100-mesh sieve to obtain a biodegradable bio-based coating. Step 8: After stirring the biodegradable bio-based coating at 300 rpm for 10 minutes at 25°C, apply it to a 60 g / m² substrate using a wire rod coating method. 2 For food packaging base paper, the wet coating weight on one side is controlled at 21 g / m². 2 After coating, the paper is placed in a 45℃ hot air drying oven for 2 minutes, and then transferred to a 65℃ hot air drying oven for 8 minutes. The hot air speed is controlled at 1m / s. The coated paper is then equilibrated at 23℃ and 50% relative humidity for 24 hours to obtain a food packaging paper moisture-proof coating sample.

[0029] Comparative Example 1: The difference from Example 1 is that the amount of pentaerythritol in step one is adjusted from 15g to 5g, and the amount of L-(-)-lactide is adjusted from 900g to 910g, while keeping the total mass of the feed in step one unchanged; the other conditions are the same as in Example 1.

[0030] Comparative Example 2: The difference from Example 1 is that: 34g of itaconic anhydride is not added in step two, and the amount of hydroxyl-terminated low molecular weight poly-L-lactic acid obtained in step one is adjusted from 700g to 734g in step two, while keeping the total mass of resin feed in step two unchanged; the other conditions are the same as in Example 1.

[0031] Comparative Example 3: The difference from Example 1 is that the amount of itaconic anhydride in step two is adjusted from 34g to 14g, and the amount of hydroxyl-terminated low molecular weight poly-L-lactic acid obtained in step one is adjusted from 700g to 720g, while keeping the total mass of resin feed in step two unchanged; the other conditions are the same as in Example 1.

[0032] Comparative Example 4: The difference from Example 1 is that the amount of sodium bicarbonate used in step five is adjusted from 11g to 4g, and the amount of deionized water used to dissolve sodium bicarbonate in step five is adjusted from 119g to 126g, keeping the total mass of the sodium bicarbonate aqueous solution at 130g; the other conditions are the same as in Example 1.

[0033] Comparative Example 5: The difference from Example 1 is that the amount of sodium bicarbonate used in step five is adjusted from 11g to 20g, and the amount of deionized water used to dissolve sodium bicarbonate in step five is adjusted from 119g to 110g, keeping the total mass of the sodium bicarbonate aqueous solution at 130g; the other conditions are the same as in Example 1.

[0034] Comparative Example 6: The difference from Example 1 is that: in step six, the oligochitosan lactate pretreatment solution obtained in step four is not added, but the 60g oligochitosan lactate pretreatment solution is added in step seven after the addition of the tetra-arm poly-D-lactic acid ethyl acetate solution and before the addition of the water-soluble chitosan lactate aqueous solution; the other conditions are the same as in Example 1.

[0035] Comparative Example 7: The difference from Example 1 is that the water-soluble chitosan lactate aqueous solution in step seven is not added after the tetra-arm poly-D-lactic acid ethyl acetate solution, but is instead added continuously in step six and the oligomeric chitosan lactate pretreatment solution obtained in step four; the other conditions are the same as in Example 1.

[0036] Comparative Example 8: The difference from Example 1 is that in step seven, instead of adding 90g of the four-armed poly-D-lactic acid ethyl acetate solution, a solution obtained by stirring 10g of the hydroxyl-terminated low molecular weight poly-L-lactic acid obtained in step one and 80g of anhydrous ethyl acetate at 50°C at 600r / min for 40min is added, keeping the total mass of the liquid and the mass of the solids added in step seven unchanged; the other conditions are the same as in Example 1.

[0037] Comparative Example 9: The difference from Example 1 is that 90g of four-armed poly(D-lactic acid) ethyl acetate solution is no longer added in step seven, but is instead added after the formation of the polylactic acid oil phase in step five and before the addition of sodium bicarbonate aqueous solution; the other conditions are the same as in Example 1.

[0038] Performance testing: The biodegradable bio-based coatings obtained in Examples 1-5 and Comparative Examples 1-9 were used as the coatings to be tested. All coated paper samples were coated on one side only. After coating, they were first dried in a 45°C hot air drying oven for 2 min, then dried in a 65°C hot air drying oven for 8 min, with a hot air velocity of 1 m / s. Subsequently, they were equilibrated for 24 h at 23°C and 50% relative humidity. Before testing, all paper samples were treated for 24 h under standard atmospheric conditions of 23°C and 50% relative humidity according to GB / T10739-2023. At least 3 parallel samples were taken for each test item, and 5 parallel samples were taken for items involving paper water absorption and water immersion appearance. The results were taken as the arithmetic mean.

[0039] Chain-terminal carboxyl content: The acid value determination was performed according to the methods outlined in GB / T6743-2008 and GB / T2895-2008. 1.0000 g of the dried product obtained in step two of Examples 1-5 and Comparative Examples 1-9 was taken respectively, and 50 g of a mixed solvent consisting of tetrahydrofuran and anhydrous ethanol in a mass ratio of 3:1 was added. The mixture was stirred at 400 r / min for 30 min at 25 °C until the sample was completely dissolved. Using phenolphthalein as an indicator, the solution was titrated with 0.0500 mol / L potassium hydroxide ethanol standard solution until a light pink color was achieved and maintained for 30 seconds. A blank test was performed simultaneously. The chain-terminal carboxyl content was calculated based on the acid value, expressed in μmol / g. The acid value was divided by 56.1 and then multiplied by 1000 during the calculation.

[0040] Non-volatile matter content of coatings: determined according to GB / T1725-2007. The coatings obtained in each example and comparative example were stirred at 300 r / min for 10 min at 25℃, and then samples were taken. 1.0000 g of the coating was placed in a pre-weighed aluminum dish, spreading it evenly into a thin layer no more than 1 mm thick. The aluminum dish was then heated in a 105℃ forced-air drying oven for 60 min, removed, cooled in a desiccator for 30 min, and weighed. Each sample was tested three times, and the non-volatile matter content was calculated.

[0041] Coating viscosity: determined according to GB / T2794-2022. Each coating sample was placed in a constant temperature water bath at 25℃ for equilibration for 30 min, and tested using a single-cylinder rotational viscometer with rotor No. 2 and a rotation speed of 60 r / min. The viscosity value was recorded after the reading stabilized for 60 s. Each sample was tested 3 times and the average value was taken.

[0042] Dry coating weight: The quantitative determination method is followed according to GB / T451.2-2023. Both the coated paper sample and the blank base paper sample are cut into 100mm × 100mm specimens. After cutting, they are equilibrated at 23℃ and 50% relative humidity for 24 hours. The unit area mass of the coated paper sample and the blank base paper sample are then weighed. The dry coating weight is calculated by subtracting the unit area mass of the blank base paper from the unit area mass of the coated paper, and the unit is g / m². 2 Ten tablets were tested for each sample, and the average value was taken.

[0043] Thermal behavior of the coating after low-temperature drying: Differential scanning calorimetry (DSC) was used to determine the thermal behavior according to GB / T 19466.3-2004. To avoid the influence of paper fibers on the thermal analysis, each coating was applied to the surface of a polytetrafluoroethylene (PTFE) plate according to the coating and drying regime described in step eight. The wet coating amount was the same as that in the corresponding examples or comparative examples. After drying, the coating was peeled off to obtain a free coating film. 6-8 mg of the free coating film was placed in an aluminum crucible, and the temperature was increased from 25°C to 240°C at a rate of 10°C / min under nitrogen flow of 50 mL / min. The enthalpy of the stereocomposite melting peak in the range of 205-230°C was recorded in J / g.

[0044] Cobb 60 water absorption: determined according to GB / T 1540-2002. The coated paper sample was cut into 125mm × 125mm specimens, with the coated side facing the test water surface. The test area was 100cm². 2 The contact time between water and the coating surface is 60 seconds; immediately after contact, the water is poured off, and the residual water on the surface is absorbed with absorbent paper under specified pressure. The change in sample mass is then measured. Each sample is tested 5 times, and the results are expressed in g / m³. 2 express.

[0045] Water vapor permeability: determined according to the cup weight gain method of GB / T 1037-2021. Coated paper samples were cut into circular specimens with a diameter of 74 mm, with the coated side facing the high-humidity side. 20 g of anhydrous calcium chloride, dried at 160℃ for 4 h, was placed in the permeation cup. The specimen and the permeation cup were sealed with a sealing ring and paraffin wax to ensure an effective permeable area of ​​33 cm². 2 The permeation cup was placed in a constant temperature and humidity chamber at 38℃ and 90% relative humidity. After equilibration for 2 hours, it was weighed, and then weighed every 2 hours thereafter for 24 hours. Each sample was tested 3 times, and the water vapor permeation was expressed as g / (m2·24h).

[0046] The stability of the coating appearance after immersion in water was determined according to the immersion test method in GB / T 1733-1993. Coated paper samples were cut into 50mm × 100mm specimens and completely immersed in deionized water at 23℃ for 2 hours. After removal, the specimens were allowed to drain vertically for 5 minutes without wiping the coating surface. Subsequently, the coating surface was photographed under a D65 light source. Image analysis software was used to statistically analyze the proportion of whitening, peeling, cracking, and flaking areas on the coating surface, which was recorded as the abnormal area rate of the coating after 2 hours of immersion, expressed as a percentage. Each sample was tested 5 times, and the average value was taken.

[0047] Aerobic biodegradation rate under controlled composting conditions for 60 days: determined according to GB / T 19277.1-2025. Each coating was applied to the surface of a polytetrafluoroethylene (PTFE) sheet according to the coating and drying regime described in step eight. After drying, the free coating film was peeled off and sheared to obtain sample particles with a diameter less than 2 mm. Mature compost was used as inoculum, the test temperature was set at 58℃, and the test period was 60 days. The aerobic biodegradation rate was calculated by measuring the amount of carbon dioxide released during the test, and the results are expressed as a percentage (%).

[0048] Table 1 Performance Test Results

[0049] As shown in Table 1, the water absorption and water vapor transmission rate of the blank base paper in Cobb 60 are 68.60 g / m³. 2 and 2795.00g / (m 2 (24h) This indicates that the uncoated food packaging base paper itself cannot provide a stable moisture barrier effect. The non-volatile content of the coatings obtained in Examples 1-5 is 29.2%-35.2%. After single-sided coating with a similar dry coating weight, the Cobb60 water absorption of the coated paper decreases to 11.82-17.38 g / m². 2 The water vapor permeability decreased to 83.54-137.82 g / (m³). 2 After immersion in water for 2 hours (24h), the abnormal area ratio of the coating was only 1.8%-5.1%, indicating that the coating formed by combining chain-terminal itaconic acid semi-esterified poly-L-lactic acid, oligomeric chitosan lactate, four-arm poly-D-lactic acid seed crystals and water-soluble chitosan lactate in the order of the present invention has good moisture barrier properties and water immersion appearance stability.

[0050] Compared to Example 1, in Comparative Example 1, reducing the amount of pentaerythritol decreased the carboxyl group content at the chain end to 164 μmol / g, the coating viscosity increased to 328 mPa·s, and the water absorption and water vapor transmission of Cobb60 increased to 30.58 g / m³. 2 and 248.64g / (m 2(24h) This indicates that insufficient reaction sites at the chain ends of the four-arm low molecular weight poly-L-lactic acid are detrimental to subsequent chain-end hydrophilication and stable dispersion; Comparative Example 2 did not undergo itaconic anhydride chain-end hemiesterification, and its chain-end carboxyl content was only 18 μmol / g. The water absorption, water vapor transmission rate, and abnormal coating area ratio of Cobb60 after immersion in water for 2 hours increased to 42.83 g / m². 2 438.92g / (m 2 The values ​​of Cobb60 and its water absorption rate were 35.6% and 34h, respectively. Even after reducing the amount of itaconic anhydride in Comparative Example 3, these indicators were still significantly higher than in Example 1, indicating that the introduction of carboxyl groups at the chain ends plays an important role in aqueous dispersion and coating continuity. Comparative Examples 4 and 5 reduced or increased the amount of sodium bicarbonate, respectively. Comparative Example 4 showed lower viscosity and reduced barrier properties due to insufficient neutralization. While Comparative Example 5 maintained a high biodegradability rate, its Cobb60 water absorption and abnormal surface area ratio after immersion were still higher than in Example 1, indicating that neither excessively low nor excessively high neutralization levels could simultaneously achieve dispersion stability and moisture barrier properties. Comparative Example 6 changed the addition sequence of oligomeric chitosan lactate, and Comparative Example 7 added water-soluble chitosan lactate earlier. After immersion for 2 hours, the abnormal surface area ratio of the coating increased to 13.6% and 11.8%, respectively. This indicates that oligomeric chitosan lactate participating in the initial interface stabilization first, followed by water-soluble chitosan lactate participating in the external interface fixation, is more beneficial for the coating's water resistance and stability. In Comparative Example 8, after replacing the four-armed polyD-lactic acid seed crystals with polyL-lactic acid, the stereocomposite melting peak enthalpy in the 205-230℃ range was only 0.3 J / g, and the water vapor permeability increased to 207.76 g / (m³). 2 •24h); In Comparative Example 9, after adding four-armed poly-D-lactic acid seed crystals to the oil phase in advance, the stereocomposite melting peak enthalpy was 2.9 J / g, which was still lower than 4.6 J / g in Example 1. This indicates that adding four-armed poly-D-lactic acid seed crystals after the formation of the aqueous continuous phase and before the complete removal of ethyl acetate is more conducive to its participation in the thermal behavior regulation of the polylactic acid dispersed particle surface.

[0051] A comprehensive comparison shows that, in Example 4, with a higher amount of four-armed poly-D-lactic acid seed crystals and a higher amount of water-soluble chitosan lactate, the water absorption and water vapor transmission of Cobb60 reached 11.82 g / m³. 2 and 83.54g / (m 2 • 24h), which is the best level among all groups; Example 3, due to the higher carboxyl groups at the chain ends and the higher chitosan content, maintained a biodegradability rate of 74.1%, but the moisture sensitivity index was slightly higher than that of Examples 1 and 4. This shows that the present invention does not simply rely on increasing the amount of a certain hydrophilic or hydrophobic component, but achieves a comprehensive balance between water dispersibility, coating moisture barrier properties, water immersion stability, and compostability and biodegradability under low-temperature drying conditions through partial neutralization of chain-end itaconic acid semi-esterified poly-L-lactic acid, segmented addition of oligomeric chitosan lactate and water-soluble chitosan lactate, and a specific addition window for four-armed poly-D-lactic acid seed crystals.

[0052] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A degradable bio-based coating, characterized in that, The biodegradable bio-based coating is an aqueous continuous phase dispersion. Based on dry weight, with 100 parts of chain-terminated itaconic acid semi-esterified poly-L-lactic acid, the biodegradable bio-based coating is prepared from raw materials comprising the following components: 100 parts of chain-terminated itaconic acid semi-esterified poly-L-lactic acid; Sodium bicarbonate 1.33-2.50 parts; The oligochitosan lactate formed from oligochitosan and D,L-lactic acid, wherein the oligochitosan content is 0.40-0.80 parts and the D,L-lactic acid content is 0.27-0.53 parts. 1.00-2.50 parts of four-armed poly-D-lactic acid seed crystals; 0.80-1.60 parts of water-soluble chitosan lactate; and water; The itaconic acid semi-esterified poly-L-lactic acid is obtained by semi-esterification of tetra-hydroxyl-terminated poly-L-lactic acid with itaconic anhydride. The content of the terminal carboxyl groups of the itaconic acid semi-esterified poly-L-lactic acid before neutralization is 0.30-0.52 mmol / g, and 45%-60% of the terminal carboxyl groups are neutralized by the sodium bicarbonate. The solid content of the biodegradable bio-based coating is 29%-35%.

2. The biodegradable bio-based coating according to claim 1, characterized in that, The oligomeric chitosan lactate is added before phase inversion emulsification; the four-armed poly-D-lactic acid seed crystals are added after the formation of the aqueous continuous phase dispersion and before the complete removal of ethyl acetate; and the water-soluble chitosan lactate is added after the addition of the four-armed poly-D-lactic acid seed crystals.

3. The biodegradable bio-based coating according to claim 1, characterized in that, The content of terminal carboxyl groups in the semi-esterified poly-L-lactic acid with itaconic acid before neutralization is 0.306-0.506 mmol / g.

4. The degradable bio-based coating of claim 1, wherein, The four-arm hydroxyl-terminated poly-L-lactic acid is obtained by ring-opening polymerization of L-(-)-lactide, pentaerythritol and stannous octoate, wherein the mass ratio of L-(-)-lactide, pentaerythritol and stannous octoate is 900:11-20:0.160-0.

220.

5. The degradable bio-based coating of claim 1, wherein, The chain-terminated itaconic acid semi-esterified poly-L-lactic acid is obtained by reacting the four-arm hydroxyl-terminated poly-L-lactic acid, anhydrous ethyl acetate, itaconic anhydride and stannous octoate, followed by precipitation and drying. The mass ratio of the four-arm hydroxyl-terminated poly-L-lactic acid, anhydrous ethyl acetate, itaconic anhydride and stannous octoate is 700:1400:25-42:0.070-0.

090.

6. The degradable bio-based coating of claim 1, wherein, The four-armed poly-D-lactic acid seed crystals are obtained by ring-opening polymerization of D-lactide, pentaerythritol and stannous octoate, wherein the mass ratio of D-lactide, pentaerythritol and stannous octoate is 60:1:0.

020.

7. The degradable bio-based coating of claim 1, wherein, The four-armed poly-D-lactic acid seed crystals were added in the form of an ethyl acetate solution, and the mass ratio of the four-armed poly-D-lactic acid seed crystals to anhydrous ethyl acetate was 6-15:54-120.

8. The degradable bio-based coating of claim 1, wherein, The weight-average molecular weight of the oligochitosan is 800-1000, and the oligochitosan lactate is formed by mixing deionized water, oligochitosan and D,L-lactic acid, with the mass ratio of deionized water, oligochitosan and D,L-lactic acid being 52-72:2.4-4.8:1.6-3.

2.

9. The degradable bio-based coating of claim 1, wherein, The sodium bicarbonate is added in the form of an aqueous sodium bicarbonate solution; based on 600 parts of chain-terminated itaconic acid semi-esterified poly-L-lactic acid, the aqueous sodium bicarbonate solution is prepared by 8-15 parts of sodium bicarbonate and 115-122 parts of deionized water to a total of 130 parts. The water-soluble chitosan lactate is added in the form of an aqueous solution, and the mass ratio of the water-soluble chitosan lactate to deionized water is 4.8-9.6:110.4-115.

2.

10. A method of producing a degradable bio-based coating according to any one of claims 1-9, characterized in that, Includes the following steps: (1) L-(-)-lactide, pentaerythritol and stannous octoate are subjected to ring-opening polymerization to obtain four-arm hydroxyl-terminated poly-L-lactic acid; the four-arm hydroxyl-terminated poly-L-lactic acid is subjected to a half-esterification reaction with itaconic anhydride to obtain chain-terminated itaconic acid half-esterified poly-L-lactic acid. (2) D-lactide, pentaerythritol and stannous octoate are subjected to ring-opening polymerization to obtain four-armed poly-D-lactic acid seed crystals, and the four-armed poly-D-lactic acid seed crystals are dissolved in ethyl acetate to obtain a four-armed poly-D-lactic acid ethyl acetate solution. (3) Mix oligochitosan and D,L-lactic acid in water to obtain oligochitosan lactate pretreatment solution; (4) The chain-terminal itaconic acid semi-esterified poly-L-lactic acid is dissolved in ethyl acetate to obtain a polylactic acid oil phase. Sodium bicarbonate aqueous solution is added to the polylactic acid oil phase to obtain a partially neutralized polylactic acid oil phase. (5) Add the pretreated oligochitosan lactate solution to the partially neutralized polylactic acid oil phase, then add water and shear emulsify to obtain a water-continuous polylactic acid primary dispersion; (6) Add the four-arm poly-D-lactic acid ethyl acetate solution to the aqueous continuous phase polylactic acid primary dispersion, then add the water-soluble chitosan lactate aqueous solution, then remove the ethyl acetate, adjust the solid content and filter to obtain a biodegradable bio-based coating.

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