A household compostable polylactic acid-coated paper and a manufacturing process thereof

Abstract

The application discloses a household compostable polylactic acid-coated paper and a manufacturing process thereof. The polylactic acid composite material is coated on the surface of a base paper layer, and the cellulose nanocrystal is coated on the surface of the polylactic acid composite material to form the polylactic acid-coated paper. The polylactic acid composite material comprises the following components in parts by weight: 80-120 parts of modified polylactic acid, 10-15 parts of polyethylene glycol, 6-13 parts of chitosan, 3-8 parts of a chain extender, 6-9 parts of a lubricant, 4-8 parts of an antistatic agent and 3-7 parts of an antioxidant. The modified polylactic acid is obtained by blending and modifying polylactic acid-hydroxyacetic acid copolymer and polylactic acid. The weight ratio of the polylactic acid-hydroxyacetic acid copolymer and the polylactic acid is 0.2-0.7:3-6. The polyethylene glycol is added to introduce a polar group to enhance the water absorption of the polylactic acid, and the polylactic acid is modified in cooperation with the polylactic acid-hydroxyacetic acid copolymer, so that the obtained coated paper has the characteristics of high degradation efficiency and is beneficial to household composting.

Description

Technical Field

[0001] This invention relates to the field of pulp compositions, and in particular to a compostable polylactic acid coated paper and its manufacturing process, as well as the manufacturing process of the coated paper. Background Technology

[0002] Currently, the vast majority of baking paper used in the Chinese market is polyethylene (PE) coated. PE coated paper has good oil and water resistance, and its usage has grown rapidly with the development of the catering industry. According to market statistics and analysis of my country's coated paper industry, the demand for coated paper in my country showed a continuous upward trend from 2012 to 2021, reaching approximately 4.35 million tons in 2021. However, PE coated products have disadvantages such as being non-degradable, non-re-pulping, and difficult to recycle.

[0003] The invention patents CN201410660641.X (a new polylactic acid material for paper coating) and CN202211673758.2 (a polylactic acid coated paper and its preparation process) both use polylactic acid as the main material to make coated paper. However, although polylactic acid is biodegradable, its degradation rate is greatly affected by the environment. Household composting cannot ensure a suitable degradation environment for polylactic acid, which results in slow degradation efficiency and long time consumption for coated paper, making it difficult to be effectively utilized. Summary of the Invention

[0004] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a polylactic acid coated paper suitable for home composting and its manufacturing process, which can produce a polylactic acid coated paper with high degradation efficiency suitable for home composting and reuse.

[0005] A compostable polylactic acid (PLA) coated paper is provided, wherein a PLA composite material is coated on the surface of a base paper layer, and cellulose nanocrystals are coated on the surface of the PLA composite material to form the PLA coated paper; wherein the PLA composite material has the following weight composition: 80-120 parts modified PLA, 10-15 parts polyethylene glycol, 6-13 parts chitosan, 3-8 parts chain extender, 6-9 parts lubricant, 4-8 parts antistatic agent, and 3-7 parts antioxidant.

[0006] As a further improvement to the above scheme, the base paper layer can be any one of kraft paper, white cardboard, and corrugated paper.

[0007] As a further improvement to the above scheme, the modified polylactic acid is obtained by blending polylactic acid-glycolic acid copolymer and polylactic acid. In this invention, the synergistic addition of polylactic acid-glycolic acid copolymer to modify polylactic acid aims to introduce a branched structure, reduce crystallinity, and increase the concentration of end groups, thereby further accelerating the hydrolysis process.

[0008] As a further improvement to the above scheme, the weight ratio of the polylactic acid-glycolic acid copolymer to polylactic acid is 0.2-0.7:3-6. This ratio is beneficial for maximizing the effect of the polylactic acid-glycolic acid copolymer and achieving a better grafting modification effect.

[0009] As a further improvement to the above scheme, the preparation method of the modified polylactic acid is as follows: 1) Disperse polylactic acid-hydroxyacetic acid copolymer and polylactic acid in water and mix them under ultrasonic conditions to obtain a uniformly dispersed suspension; 2) The uniformly dispersed suspension obtained in step 1) is first subjected to a stirring reaction, and then subjected to a second stirring reaction. The melt obtained from the reaction is cooled to obtain the final product.

[0010] As a further improvement to the above scheme, in step 1), the amount of water used is equivalent to 1 to 2 times the total mass of the polylactic acid-glycolic acid copolymer and polylactic acid. In step 2), the temperature of the first stirring reaction is 80 to 100°C and the time is 20 to 30 minutes; the temperature of the second stirring reaction is 60 to 80°C and the time is 15 to 30 minutes; the cooling is achieved by rapid cooling with liquid nitrogen. The main function of the first stirring is to perform preliminary blending; after lowering the temperature, the second stirring is continued to further eliminate air bubbles and improve the homogenization effect; the purpose of rapid cooling is to suppress the crystallization process and reduce the crystallinity of the modified polylactic acid.

[0011] As a further improvement to the above scheme, the amount of chitosan added accounts for 0.5% to 1.5% of polylactic acid. The purpose of adding chitosan is to facilitate the microbial degradation of the coated paper in a home composting environment. Specifically, chitosan, as a sugar molecule introduced into the material, can induce polymer chain breakage under light conditions, turning it into easily hydrolyzed oligomer fragments, thereby increasing hydrophilicity to promote hydrolysis. During composting degradation, a photo-biological synergistic degradation effect is formed to improve the degradation efficiency of the coated paper and shorten the degradation time.

[0012] As a further improvement to the above scheme, the chain extender is at least one selected from polytrimethylene carbonate, polyurethane vulcanizing agent, hydroquinone dihydroxyethyl ether, 1,4-butanediol, glycerol, diethylene glycol, and toluenediamine. The purpose of adding the chain extender is to extend the molecular chain, increase the molecular weight, thereby reducing the molecular chain mobility, slowing down the crystallization rate, and thus reducing the material's crystallinity. Lower crystallinity means a looser molecular arrangement, making it more susceptible to the effects of moisture and other degradation factors, thus increasing the degradation rate. Simultaneously, due to the increased molecular weight, the interaction forces between polymer chains are enhanced, resulting in improved material strength and toughness.

[0013] As a further improvement to the above solution, the lubricant is EVA wax or glycerin. The purpose of adding the lubricant is to improve the flowability of polylactic acid material during the molding process, so as to facilitate its bonding with the base paper layer during calendering and lamination, avoid voids, and improve the production quality of coated paper.

[0014] As a further improvement to the above solution, the antistatic agent is any one of amide phosphate, sodium cellulose sulfate, alkyl quaternary ammonium salt, and sodium alkyl sulfonate. The purpose of adding the antistatic agent is to prevent static electricity buildup, thereby facilitating the use of the coated paper and preventing it from sticking and attracting dust.

[0015] As a further improvement to the above scheme, the antioxidant is any one of naphthylamine, diphenylamine, p-phenylenediamine, and phosphite. Considering that in the molten state, the presence of high temperature and oxygen easily triggers oxidation reactions, generating free radicals. These free radicals can initiate chain reactions, accelerating polymer aging and degradation, and reducing the quality of the resulting coated paper. Therefore, this embodiment adds an antioxidant to capture these free radicals, thereby blocking the chain reaction, reducing the oxidation rate, and delaying the polymer oxidation process, thus allowing the polymer to be processed smoothly to obtain the coated paper.

[0016] The manufacturing process of the coated paper includes the following steps:

[0017] S1 is prepared by melt-blending modified polylactic acid, polyethylene glycol, chitosan, chain extender, lubricant, antistatic agent and antioxidant to obtain the polylactic acid composite material, for later use;

[0018] S2 is used to prepare an aqueous solution of cellulose nanocrystals for later use;

[0019] S3 coating polylactic acid composite material onto the surface of the base paper layer using a casting machine, and then coating the surface of polylactic acid composite material with an aqueous solution of cellulose nanocrystals based on a spraying process.

[0020] S4 is rolled and then subjected to rapid cooling with liquid nitrogen to obtain coated paper.

[0021] As a further improvement to the above scheme, the polylactic acid composite material obtained by melt mixing has three heating stages: the first heating stage is to heat to 180°C at a rate of 20°C / min and melt mix for 10-20 min; the second heating stage is to continue heating to 260°C at a rate of 10°C / min and melt mix for 20-30 min; the third heating stage is to continue heating to 300°C at a rate of 5°C / min and melt mix for 30-40 min. This invention, based on segmented heating—that is, rapid heating in the first stage, medium-speed heating in the second stage, and slow heating in the third stage—works in synergy to ensure uniform melting and mixing of the raw materials, improves the uniformity of film casting, and reduces leakage and coating defects.

[0022] As a further improvement to the above scheme, the method for preparing the aqueous solution of cellulose nanocrystals is specifically operated as follows: S21, bleached cellulose and sulfuric acid solution are mixed and hydrolyzed at a ratio of 1:5 to 8, and a suspension is obtained after centrifugation; S22, the suspension is spray-dried to obtain powdered cellulose nanocrystals, and the cellulose nanocrystals are dissolved in water by ultrasonication to obtain an aqueous solution of cellulose nanocrystals, with a ratio of cellulose nanocrystals to water of 1:3 to 5. This invention employs the method of coating the material surface with cellulose nanocrystals to restrict molecular chain migration and crystal nucleus formation, thereby promoting the hydrolysis-biodegradation effect of the coated paper and increasing the degradation rate.

[0023] As a further improvement to the above solution, the coated paper comprises a polylactic acid composite material layer, a cellulose nanocrystal layer, and a base paper layer; wherein the thickness of the polylactic acid composite material layer is 0.08–0.10 mm, the thickness of the cellulose nanocrystal layer is 0.01–0.02 mm, and the thickness of the base paper layer is 0.18–0.22 mm.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] This invention adds polyethylene glycol to introduce polar groups to enhance the water absorption of polylactic acid (PLA) and accelerate the hydrolysis reaction rate. The hydroxyl groups in polyethylene glycol are hydrophilic groups with strong hydrophilicity, capable of forming stable hydrogen bonds with water molecules, thereby promoting the hydrolysis of the coated paper. Furthermore, a polylactic acid-glycolic acid copolymer is synergistically added to modify PLA, aiming to introduce a branched structure, reduce crystallinity, and increase the concentration of end groups, further accelerating the hydrolysis process.

[0026] In the production of coated paper, this invention employs a method of coating the material surface with cellulose nanocrystals to restrict molecular chain migration and crystal nucleus formation, thereby promoting the hydrolysis-biodegradation effect of the coated paper and increasing the degradation rate. Detailed Implementation

[0027] The specific embodiments of the present invention will be described in detail below.

[0028] Example 1

[0029] This embodiment provides a compostable polylactic acid (PLA) coated paper. PLA composite material is coated onto the surface of kraft paper, and cellulose nanocrystals are coated onto the PLA composite material surface to form the PLA coated paper. The main component of the base paper layer in this embodiment is cellulose, a natural high-molecular-weight polysaccharide that can be decomposed by microorganisms into carbon dioxide and water, exhibiting biodegradability and good environmental friendliness.

[0030] The polylactic acid composite material has the following weight composition: 80 parts modified polylactic acid, 10 parts polyethylene glycol, 6 parts chitosan, 3 parts polytrimethylene carbonate, 6 parts EVA wax, 4 parts amide phosphate, and 3 parts naphthylamine. In this embodiment, the purpose of adding polyethylene glycol is to introduce polar groups to enhance the water absorption of polylactic acid and accelerate the hydrolysis reaction rate. The hydroxyl groups in polyethylene glycol are hydrophilic groups with strong hydrophilicity and can form stable hydrogen bonds with water molecules, thereby promoting the hydrolysis of the coated paper. Furthermore, this embodiment also uses polylactic acid-glycolic acid copolymer and polylactic acid blending modification to obtain the modified polylactic acid.

[0031] In this embodiment, polylactic acid-glycolic acid copolymer is synergistically added to modify polylactic acid, aiming to introduce a branched structure, reduce crystallinity, and increase the concentration of end groups to further accelerate the hydrolysis process. The preparation method of the modified polylactic acid is as follows: 1) Polylactic acid-glycolic acid copolymer and polylactic acid are dispersed in water and mixed under ultrasonic conditions to obtain a uniformly dispersed suspension; 2) The uniformly dispersed suspension obtained in step 1) is subjected to a first stirring reaction, followed by a second stirring reaction, and the resulting melt is cooled to obtain the final product.

[0032] In step 1), the amount of water used is equivalent to 1 times the total mass of polylactic acid-glycolic acid copolymer and polylactic acid; in step 2), the temperature of the first stirring reaction is 80°C and the time is 20 min; the temperature of the second stirring reaction is 60°C and the time is 15 min; the cooling is achieved by liquid nitrogen quenching.

[0033] The coated paper of this embodiment uses polylactic acid (PLA) as its main component. PLA is made from starch derived from renewable plant resources such as corn and has excellent biodegradability. After use, it can be completely degraded by microorganisms in nature, ultimately producing carbon dioxide and water without polluting the environment. Due to its biodegradability, the coated paper is suitable for home composting. That is, after use, the coated paper of this embodiment can be decomposed through home composting to transform into humus and mineral nutrients, thereby achieving the ecological reuse of waste into organic fertilizer.

[0034] The amount of chitosan added accounts for 0.5% of the polylactic acid. In this embodiment, the purpose of adding chitosan is to facilitate the microbial degradation of the sprayed paper in a home composting environment. Specifically, chitosan, as a sugar molecule introduced into the material, can induce polymer chain breakage under light conditions, turning into easily hydrolyzed oligomer fragments, thereby increasing hydrophilicity to promote hydrolysis. During composting degradation, a photo-biological synergistic degradation effect is formed to improve the degradation efficiency of the coated paper and shorten the degradation time.

[0035] In this embodiment, the purpose of adding amide phosphate ester is to prevent static electricity buildup, thereby facilitating the use of the coated paper and preventing it from sticking and attracting dust. The purpose of adding EVA wax is to improve the flowability of polylactic acid material during the molding process, so as to facilitate its bonding with the base paper layer during calendering and lamination, avoid voids, and improve the production quality of the coated paper.

[0036] Considering that in the molten state, the presence of high temperature and oxygen can easily trigger oxidation reactions and generate free radicals, which can trigger chain reactions, accelerate the aging and degradation of polymers, and reduce the quality of the resulting coated paper, this embodiment adds naphthylamine to capture these free radicals, thereby blocking the chain reaction, reducing the oxidation rate, and delaying the oxidation process of the polymer, so that the polymer can be processed smoothly to obtain coated paper.

[0037] In this embodiment, the purpose of adding polytrimethylene carbonate is to extend the molecular chain and increase the molecular weight, thereby reducing the molecular chain mobility, slowing down the crystallization rate, and thus reducing the crystallinity of the material. Lower crystallinity means a looser molecular arrangement, making the material more susceptible to the effects of moisture and other degradation factors, thus increasing the degradation rate. At the same time, due to the increased molecular weight, the interaction forces between polymer chains are enhanced, resulting in improved strength and toughness of the material.

[0038] The manufacturing process of the coated paper in this embodiment includes the following steps:

[0039] S1. Modified polylactic acid, polyethylene glycol, chitosan, polytrimethylene carbonate, EVA wax, amide phosphate, and naphthylamine are melt-blended to obtain the polylactic acid composite material, which is then set aside. The melt-blending process for obtaining the polylactic acid composite material involves three heating stages: the first heating stage is to heat to 180°C at a rate of 20°C / min and melt-blend for 10 min; the second heating stage is to continue heating to 260°C at a rate of 10°C / min and melt-blend for 20 min; and the third heating stage is to continue heating to 300°C at a rate of 5°C / min and melt-blend for 30 min.

[0040] S2. Prepare an aqueous solution of cellulose nanocrystals for later use. The specific operation of the method for preparing the aqueous solution of cellulose nanocrystals is as follows: S21. Mix bleached cellulose and sulfuric acid solution in a ratio of 1:5 for hydrolysis, and obtain a suspension after centrifugation; S22. Spray dry the suspension to obtain powdered cellulose nanocrystals, and dissolve the cellulose nanocrystals in water by ultrasonication to obtain an aqueous solution of cellulose nanocrystals, with a ratio of cellulose nanocrystals to water of 1:3.

[0041] S3 applies polylactic acid composite material to the surface of kraft paper using a casting machine, and then applies an aqueous solution of cellulose nanocrystals to the surface of the polylactic acid composite material based on a spraying process.

[0042] S4 is rolled and then subjected to liquid nitrogen rapid cooling to obtain coated paper, which comprises a polylactic acid composite material layer, a cellulose nanocrystal layer, and kraft paper. The thickness of the polylactic acid composite material layer is 0.08 mm, the thickness of the cellulose nanocrystal layer is 0.01 mm, and the thickness of the kraft paper is 0.18 mm.

[0043] Example 2

[0044] This embodiment provides a compostable polylactic acid (PLA) coated paper, wherein a PLA composite material is coated on the surface of white cardboard, and cellulose nanocrystals are coated on the surface of the PLA composite material to form the PLA coated paper.

[0045] The polylactic acid composite material is composed of the following parts by weight: 90 parts modified polylactic acid, 12 parts polyethylene glycol, 8 parts chitosan, 5 parts polyurethane vulcanizing agent, 7 parts EVA wax, 5 parts sodium cellulose sulfate, and 4 parts diphenylamine.

[0046] In this embodiment, polylactic acid-glycolic acid copolymer is synergistically added to modify polylactic acid. The purpose is to introduce a branched structure, reduce crystallinity, and increase the concentration of end groups to further accelerate the hydrolysis process. The preparation method of the modified polylactic acid is the same as in Example 1.

[0047] The amount of chitosan added accounts for 1.0% of the polylactic acid. In this embodiment, the purpose of adding chitosan is to facilitate the microbial degradation of the sprayed paper in a home composting environment. Specifically, chitosan, as a sugar molecule introduced into the material, can induce polymer chain breakage under light conditions, turning into easily hydrolyzed oligomer fragments, thereby increasing hydrophilicity to promote hydrolysis. During composting degradation, a photo-biological synergistic degradation effect is formed to improve the degradation efficiency of the coated paper and shorten the degradation time.

[0048] In this embodiment, the purpose of adding sodium cellulose sulfate is to prevent static electricity buildup, thereby facilitating the use of the coated paper and preventing it from sticking and attracting dust. Considering that in the molten state, the presence of high temperature and oxygen easily triggers oxidation reactions, generating free radicals. These free radicals can initiate chain reactions, accelerating polymer aging and degradation, and reducing the quality of the resulting coated paper. Therefore, in this embodiment, diphenylamine is added to capture these free radicals, thereby blocking the chain reaction, reducing the oxidation rate, and delaying the polymer oxidation process, thus allowing the polymer to be processed smoothly to obtain the coated paper.

[0049] In this embodiment, the purpose of adding a polyurethane vulcanizing agent is to extend the molecular chains and increase the molecular weight, thereby reducing the mobility of the molecular chains, slowing down the crystallization rate, and thus reducing the crystallinity of the material. Lower crystallinity means a looser molecular arrangement, making the material more susceptible to the effects of moisture and other degradation factors, thus increasing the degradation rate. At the same time, due to the increased molecular weight, the interaction forces between polymer chains are strengthened, resulting in improved strength and toughness of the material.

[0050] The manufacturing process of the coated paper in this embodiment includes the following steps:

[0051] S1. Modified polylactic acid, polyethylene glycol, chitosan, polyurethane vulcanizing agent, EVA wax, sodium cellulose sulfate, and diphenylamine are melt-blended to obtain the polylactic acid composite material, which is then set aside. The melt-blending process for obtaining the polylactic acid composite material involves three heating stages, as in Example 1.

[0052] S2. Prepare an aqueous solution of cellulose nanocrystals for later use. The specific operation of the method for preparing the aqueous solution of cellulose nanocrystals is as follows: S21. Mix bleached cellulose and sulfuric acid solution in a ratio of 1:6 and hydrolyze. After centrifugation, obtain a suspension. S22. Spray dry the suspension to obtain powdered cellulose nanocrystals. Dissolve the cellulose nanocrystals in water by ultrasonication to obtain an aqueous solution of cellulose nanocrystals. The ratio of cellulose nanocrystals to water is 1:4.

[0053] S3 is the same as in Example 1.

[0054] S4 is the same as in Example 1.

[0055] Example 3

[0056] This embodiment provides a compostable polylactic acid (PLA) coated paper. PLA composite material is coated onto the surface of corrugated paper, and cellulose nanocrystals are coated onto the surface of the PLA composite material to form the PLA coated paper. The main component of the corrugated paper in this embodiment is cellulose, a natural high-molecular-weight polysaccharide that can be decomposed by microorganisms into carbon dioxide and water, exhibiting biodegradability and good environmental friendliness.

[0057] The polylactic acid composite material has the following weight composition: 100 parts modified polylactic acid, 13 parts polyethylene glycol, 12 parts chitosan, 7 parts diethylene glycol, 8 parts glycerol, 6 parts alkyl quaternary ammonium salt, and 5 parts p-phenylenediamine. In this embodiment, the polylactic acid-glycolic acid copolymer is synergistically added to modify the polylactic acid, aiming to introduce a branched structure, reduce crystallinity, and increase the concentration of end groups to further accelerate the hydrolysis process. The preparation method of the modified polylactic acid is as follows: 1) Disperse the polylactic acid-glycolic acid copolymer and polylactic acid in water and mix them under ultrasonic conditions to obtain a uniformly dispersed suspension; 2) Perform a first stirring reaction on the uniformly dispersed suspension obtained in step 1), followed by a second stirring reaction, and cool the resulting melt to obtain the final product.

[0058] In step 1), the amount of water used is equivalent to 1.5 times the total mass of the polylactic acid-glycolic acid copolymer and polylactic acid. In step 2), the temperature of the first stirring reaction is 90°C and the time is 25 min. The temperature of the second stirring reaction is 70°C and the time is 25 min. Cooling is performed using liquid nitrogen quenching.

[0059] The amount of chitosan added accounts for 1.2% of the polylactic acid. In this embodiment, the purpose of adding chitosan is to facilitate the microbial degradation of the sprayed paper in a home composting environment. Specifically, chitosan, as a sugar molecule introduced into the material, can induce polymer chain breakage under light conditions, turning into easily hydrolyzed oligomer fragments, thereby increasing hydrophilicity to promote hydrolysis. During composting degradation, a photo-biological synergistic degradation effect is formed to improve the degradation efficiency of the coated paper and shorten the degradation time.

[0060] In this embodiment, the purpose of adding alkyl quaternary ammonium salt is to prevent static electricity buildup, thereby facilitating the use of the coated paper and preventing it from sticking and attracting dust. The purpose of adding glycerol is to improve the flowability of polylactic acid material during the molding process, so as to facilitate its bonding with the base paper layer during calendering and lamination, avoid voids, and improve the production quality of the coated paper.

[0061] Considering that in the molten state, the presence of high temperature and oxygen can easily trigger oxidation reactions and generate free radicals, which can trigger chain reactions, accelerate the aging and degradation of polymers, and reduce the quality of the resulting coated paper, this embodiment adds p-phenylenediamine to capture these free radicals, thereby blocking the chain reaction, reducing the oxidation rate, and delaying the oxidation process of the polymer, so that the polymer can be processed smoothly to obtain coated paper.

[0062] In this embodiment, the purpose of adding diethylene glycol is to extend the molecular chain and increase the molecular weight, thereby reducing the mobility of the molecular chain, slowing down the crystallization rate, and thus reducing the crystallinity of the material. Lower crystallinity means a looser molecular arrangement, making the material more susceptible to the effects of moisture and other degradation factors, thus increasing the degradation rate. At the same time, due to the increased molecular weight, the interaction forces between polymer chains are strengthened, resulting in improved strength and toughness of the material.

[0063] The manufacturing process of the coated paper in this embodiment includes the following steps:

[0064] S1. Modified polylactic acid, polyethylene glycol, chitosan, diethylene glycol, glycerol, alkyl quaternary ammonium salt, and p-phenylenediamine are melt-blended to obtain the polylactic acid composite material, which is then set aside. The melt-blending process for obtaining the polylactic acid composite material involves three heating stages: the first heating stage is to heat to 180°C at a rate of 20°C / min and melt-blend for 20 min; the second heating stage is to continue heating to 260°C at a rate of 10°C / min and melt-blend for 30 min; and the third heating stage is to continue heating to 300°C at a rate of 5°C / min and melt-blend for 40 min.

[0065] S2. Prepare an aqueous solution of cellulose nanocrystals for later use. The specific operation of the method for preparing the aqueous solution of cellulose nanocrystals is as follows: S21. Mix bleached cellulose and sulfuric acid solution in a ratio of 1:7 for hydrolysis, and centrifuge to obtain a suspension; S22. Spray dry the suspension to obtain powdered cellulose nanocrystals, and dissolve the cellulose nanocrystals in water by ultrasonication to obtain an aqueous solution of cellulose nanocrystals, with a ratio of cellulose nanocrystals to water of 1:5.

[0066] S3 is the same as in Example 1.

[0067] S4 is the same as in Example 1.

[0068] Example 4

[0069] This embodiment provides a compostable polylactic acid (PLA) coated paper. PLA composite material is coated onto the surface of corrugated paper, and cellulose nanocrystals are coated onto the surface of the PLA composite material to form the PLA coated paper. The main component of the corrugated paper in this embodiment is cellulose, a natural high-molecular-weight polysaccharide that can be decomposed by microorganisms into carbon dioxide and water, exhibiting biodegradability and good environmental friendliness.

[0070] The polylactic acid composite material has the following weight composition: 120 parts modified polylactic acid, 15 parts polyethylene glycol, 13 parts chitosan, 8 parts toluene diamine, 9 parts glycerol, 8 parts sodium alkyl sulfonate, and 7 parts phosphite. In this embodiment, the polylactic acid-glycolic acid copolymer is synergistically added to modify the polylactic acid, aiming to introduce a branched structure, reduce crystallinity, and increase the concentration of end groups to further accelerate the hydrolysis process.

[0071] The preparation method of the modified polylactic acid is the same as that in Example 3.

[0072] The amount of chitosan added accounts for 1.5% of the polylactic acid. In this embodiment, the purpose of adding chitosan is to facilitate the microbial degradation of the sprayed paper in a home composting environment. Specifically, chitosan, as a sugar molecule introduced into the material, can induce polymer chain breakage under light conditions, turning into easily hydrolyzed oligomer fragments, thereby increasing hydrophilicity to promote hydrolysis. During composting degradation, a photo-biological synergistic degradation effect is formed to improve the degradation efficiency of the coated paper and shorten the degradation time.

[0073] In this embodiment, sodium alkyl sulfonate is added to prevent static electricity buildup, thus facilitating the use of the coated paper and preventing it from sticking and attracting dust. Considering that in the molten state, the presence of high temperature and oxygen easily triggers oxidation reactions, generating free radicals. These free radicals can initiate chain reactions, accelerating polymer aging and degradation, and reducing the quality of the resulting coated paper. Therefore, in this embodiment, phosphite is added to capture these free radicals, thereby blocking the chain reaction, reducing the oxidation rate, and delaying the polymer oxidation process, thus allowing the polymer to be processed smoothly to obtain the coated paper.

[0074] In this embodiment, the purpose of adding toluene diamine is to extend the molecular chain and increase the molecular weight, thereby reducing the molecular chain mobility, slowing down the crystallization rate, and thus reducing the crystallinity of the material. Lower crystallinity means a looser molecular arrangement, making the material more susceptible to the effects of moisture and other degradation factors, thus increasing the degradation rate. At the same time, due to the increased molecular weight, the interaction forces between polymer chains are strengthened, resulting in improved strength and toughness of the material.

[0075] The manufacturing process of the coated paper in this embodiment includes the following steps:

[0076] S1 is the same as in Example 1.

[0077] S2 is the same as Example 3.

[0078] S3 is the same as in Example 1.

[0079] S4 is the same as in Example 1.

[0080] Comparative Example 1

[0081] The only difference between this comparative example and Example 3 is that the polylactic acid was not modified; otherwise, it is the same as Example 3.

[0082] Comparative Example 2

[0083] The only difference between this comparative example and Example 3 is that polyethylene glycol was not added; otherwise, they are the same as in Example 3.

[0084] Comparative Example 3

[0085] The only difference between this comparative example and Example 3 is that cellulose nanocrystals were not used for surface coating during the production of the coated paper; otherwise, it is the same as Example 3.

[0086] The following are home composting experiments conducted on the coated paper of Examples 1-4 and Comparative Examples 1-3. The specific operation of home composting is as follows: 100g of vegetable leaves and 100g of dried leaves were crushed and placed in a 500ml composting container. A layer of sand was then laid on the mixture, followed by the even addition of the crushed vegetable leaves and dried leaves. 50g of the coated paper from Examples 1-4 and Comparative Examples 1-3 was then placed inside the aforementioned material. Finally, a 2-3cm layer of soil was added to introduce microorganisms. Water was added, accounting for 10% of the container volume. The compost was turned over every 24 hours. After 7 days of composting, the weight of the coated paper was measured, and the degradation rate was calculated. The experimental results are as follows: The degradation rate of Example 1 was 66.3%, the degradation rate of Example 2 was 65.6%, the degradation rate of Example 3 was 67.1%, the degradation rate of Example 4 was 66.4%, the degradation rate of Comparative Example 1 was 30%, the degradation rate of Comparative Example 2 was 35%, and the degradation rate of Comparative Example 3 was 47%.

[0087] Analysis of the above results shows that the degradation rates of Comparative Examples 1 and 2 are twice that of Example 3. This indicates that the addition of polyethylene glycol and polylactic acid-glycolic acid copolymer to modify polylactic acid has a synergistic positive efficiency, which can significantly improve the degradation efficiency of the material. Comparative Example 3 also shows a significant decrease compared to Example 3. This proves that coating the material surface with cellulose nanocrystals can promote the hydrolysis-biodegradation effect of the coated paper, thereby improving the degradation efficiency.

[0088] The above embodiments are merely preferred embodiments of the present invention. Any simple modifications, alterations, and substitutions made to the above embodiments based on the technical essence of the present invention shall fall within the scope of the technical solution of the present invention.

Claims

1. A compostable polylactic acid coated paper, characterized in that, Polylactic acid composite material is coated on the surface of the base paper layer, and cellulose nanocrystals are coated on the surface of the polylactic acid composite material to form the polylactic acid coated paper; wherein, the polylactic acid composite material has the following weight composition: 80-120 parts modified polylactic acid, 10-15 parts polyethylene glycol, 6-13 parts chitosan, 3-8 parts chain extender, 6-9 parts lubricant, 4-8 parts antistatic agent, and 3-7 parts antioxidant; The base paper layer is any one of kraft paper, white cardboard, and corrugated paper; The modified polylactic acid was obtained by blending polylactic acid-glycolic acid copolymer and polylactic acid. The weight ratio of the polylactic acid-glycolic acid copolymer to polylactic acid is 0.2~0.7:3~6; The preparation method of the modified polylactic acid is as follows: 1) Disperse polylactic acid-glycolic acid copolymer and polylactic acid in water and mix them under ultrasonic conditions to obtain a uniformly dispersed suspension; 2) The uniformly dispersed suspension obtained in step 1) is first subjected to a stirring reaction, and then subjected to a second stirring reaction. The melt obtained from the reaction is cooled to obtain the product. In step 1), the amount of water used is equivalent to 1 to 2 times the total mass of polylactic acid-glycolic acid copolymer and polylactic acid; in step 2), the temperature of the first stirring reaction is 80 to 100°C and the time is 20 to 30 minutes; the temperature of the second stirring reaction is 60 to 80°C and the time is 15 to 30 minutes; the cooling is achieved by liquid nitrogen quenching.

2. The compostable polylactic acid coated paper according to claim 1, characterized in that, The amount of chitosan added is 0.5% to 1.5% of the polylactic acid.

3. The compostable polylactic acid coated paper according to claim 1, characterized in that, The chain extender is at least one selected from polytrimethylene carbonate, polyurethane vulcanizing agent, hydroquinone dihydroxyethyl ether, 1,4-butanediol, glycerol, diethylene glycol and toluene diamine; The lubricant is EVA wax or glycerin; the antistatic agent is any one of amide phosphate, sodium cellulose sulfate, alkyl quaternary ammonium salt, and sodium alkyl sulfonate; the antioxidant is any one of naphthylamine, diphenylamine, p-phenylenediamine, and phosphite.

4. The manufacturing process of the coated paper according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1 is prepared by melt-blending modified polylactic acid, polyethylene glycol, chitosan, chain extender, lubricant, antistatic agent and antioxidant to obtain the polylactic acid composite material, for later use; S2 is used to prepare an aqueous solution of cellulose nanocrystals for later use; S3 coating polylactic acid composite material onto the surface of the base paper layer using a casting machine, and then coating the surface of polylactic acid composite material with an aqueous solution of cellulose nanocrystals based on a spraying process. S4 is rolled and then subjected to rapid cooling with liquid nitrogen to obtain coated paper.

5. The manufacturing process according to claim 4, characterized in that, The polylactic acid composite material obtained by melt mixing has three heating stages: the first heating stage is to heat up to 180°C at a rate of 20°C / min and melt mix for 10-20 min; the second heating stage is to continue heating up to 260°C at a rate of 10°C / min and melt mix for 20-30 min; and the third heating stage is to continue heating up to 300°C at a rate of 5°C / min and melt mix for 30-40 min.

6. The manufacturing process according to claim 4, characterized in that, The method for preparing the aqueous solution of cellulose nanocrystals is as follows: S21 Take bleached cellulose and sulfuric acid solution and mix them in a ratio of 1:5~8 for hydrolysis, and obtain a suspension after centrifugation; S22 Spray dry the suspension to obtain powdered cellulose nanocrystals, and dissolve the cellulose nanocrystals in water by ultrasonication to obtain an aqueous solution of cellulose nanocrystals, wherein the ratio of cellulose nanocrystals to water is 1:3~5.

7. The manufacturing process according to claim 5, characterized in that, The coated paper comprises a polylactic acid composite material layer, a cellulose nanocrystal layer, and a base paper layer; wherein the thickness of the polylactic acid composite material layer is 0.08~0.10mm, the thickness of the cellulose nanocrystal layer is 0.01~0.02mm, and the thickness of the base paper layer is 0.18~0.22mm.