Polylactic acid coated paper capable of being composted at home and manufacturing process of polylactic acid coated paper

Abstract

The invention discloses polylactic acid coated paper capable of being composted at home and a manufacturing process of the polylactic acid coated paper. The polylactic acid composite material is coated on the surface of the raw paper layer, and the 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 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 a polylactic acid-glycolic acid copolymer and polylactic acid. The weight part ratio of the polylactic acid-glycolic acid copolymer to 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-glycolic acid copolymer is cooperatively added to modify the polylactic acid, so that the prepared coated paper has the characteristic of high degradation efficiency, and is beneficial to household composting.

Description

Technical Field

[0001] The present invention relates to the field of pulp compositions, and particularly to a home-compostable polylactic acid coated paper and its manufacturing process, and also to the manufacturing process of the coated paper. Background Art

[0002] Currently, the baking paper used in the market in China is mostly polyethylene (PE) coated. PE coated paper has good oil and water resistance, and its usage has increased rapidly with the development of the catering industry. According to the market statistical analysis of the coated paper industry in China, the market demand for coated paper in China showed an upward trend from 2012 to 2021, reaching approximately 4.35 million tons in 2021. However, PE coated products have the disadvantages of being non-degradable, non-repulpable, and difficult to recycle.

[0003] The existing invention patent with reference number CN201410660641.X, a new polylactic acid material for paper coating, and the invention patent with reference number CN202211673758.2, a polylactic acid coated paper and its preparation process, both of the above two technical solutions use polylactic acid as the main raw material to make the coated paper. However, although polylactic acid is biodegradable, its degradation rate is greatly affected by the environment, and home composting cannot ensure a suitable degradation environment for polylactic acid, that is, it leads to slow degradation efficiency and long time consumption of the coated paper, and it cannot be effectively utilized. Summary of the Invention

[0004] In view of the above-mentioned disadvantages of the prior art, the purpose of the present invention is to provide a home-compostable polylactic acid coated paper and its manufacturing process to make a polylactic acid coated paper suitable for home composting and reuse, and with high degradation efficiency.

[0005] A home-compostable polylactic acid coated paper, in which a 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 weight parts composition of the polylactic acid composite material is as follows: 80 - 120 parts of modified polylactic acid, 10 - 15 parts of polyethylene glycol, 6 - 13 parts of chitosan, 3 - 8 parts of chain extender, 6 - 9 parts of lubricant, 4 - 8 parts of antistatic agent, and 3 - 7 parts of antioxidant.

[0006] As a further improvement of the above solution, the base paper layer is any one of kraft paper, white cardboard, and corrugated paper.

[0007] As a further improvement of the above solution, the modified polylactic acid is obtained by blending and modifying polylactic acid - glycolic acid copolymer and polylactic acid. In the present invention, the polylactic acid - glycolic acid copolymer is synergistically added to modify polylactic acid, aiming to introduce a branched structure, reduce the crystallinity, and increase the terminal group concentration to further accelerate the hydrolysis process.

[0008] As a further improvement of the above solution, the weight ratio of the poly(lactic-co-glycolic acid) copolymer to polylactic acid is 0.2 - 0.7:3 - 6. At this ratio, it is beneficial to play the role of the poly(lactic-co-glycolic acid) copolymer and obtain a better graft modification effect.

[0009] As a further improvement of the above solution, the preparation method of the modified polylactic acid is as follows: 1) Disperse the poly(lactic-co-glycolic acid) copolymer and polylactic acid in water and mix them under ultrasonic conditions to obtain a uniformly dispersed suspension; 2) First perform a primary stirring reaction on the uniformly dispersed suspension obtained in step 1), then perform a secondary stirring reaction, and cool the melt obtained from the reaction to obtain the product.

[0010] As a further improvement of the above solution, in step 1), the amount of water used is equivalent to 1 - 2 times the total mass of the poly(lactic-co-glycolic acid) copolymer and polylactic acid. In step 2), the temperature of the primary stirring reaction is 80 - 100°C and the time is 20 - 30 min; the temperature of the secondary stirring reaction is 60 - 80°C and the time is 15 - 30 min; the cooling is carried out by rapid cooling with liquid nitrogen. The main function of the primary stirring is to perform preliminary blending; after reducing the temperature, continue the secondary stirring to further remove bubbles and improve the homogenization effect; the purpose of rapid cooling is to inhibit the crystallization process and reduce the crystallinity of the modified polylactic acid.

[0011] As a further improvement of the above solution, the addition amount of chitosan accounts for 0.5% - 1.5% of polylactic acid. The purpose of adding chitosan is to facilitate the microbial degradation of the spraying paper in the household composting environment. Specifically, chitosan is introduced into the material as a sugar molecule, which can cause the polymer chain to break under light conditions and become oligomer fragments that are easy to hydrolyze, increasing hydrophilicity to promote hydrolysis. When composting and degrading, 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 of the above solution, the chain extender is at least one of poly(trimethylene carbonate), polyurethane vulcanizing agent, hydroquinone dihydroxyethyl ether, 1,4-butanediol, glycerol, diethylene glycol, and toluene diamine. The purpose of adding the chain extender is to extend the molecular chain, increase the molecular weight, reduce the molecular chain mobility, slow down the crystallization rate, and thus reduce the crystallinity of the material. The reduction of the material crystallinity means that the molecular arrangement is relatively loose, and thus it is more susceptible to the influence of moisture and other degradation factors, improving the degradation rate. At the same time, due to the increase in molecular weight, the interaction force between polymer chains is enhanced, improving the strength and toughness of the material.

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

[0014] As a further improvement of the above solution, the antistatic agent is any one of amide phosphate, sodium sulfate cellulose, alkyl quaternary ammonium salt and sodium alkyl sulfonate. The purpose of adding the antistatic agent is to prevent static electricity accumulation, facilitate the use of the coated paper, and avoid its adhesion and dust adsorption.

[0015] As a further improvement of the above solution, 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 is likely to trigger an oxidation reaction, generating free radicals, which will initiate a chain reaction, accelerate the aging and degradation of the polymer, and reduce the quality of the obtained coated paper. Therefore, in this embodiment, an antioxidant is added to capture these free radicals, block the chain reaction, reduce the oxidation rate, delay the oxidation process of the polymer, so that the polymer can be smoothly processed to obtain the coated paper.

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

[0017] S1 Take the modified polylactic acid, polyethylene glycol, chitosan, chain extender, lubricant, antistatic agent and antioxidant, and melt and knead them to obtain the polylactic acid composite material, and set it aside;

[0018] S2 Prepare an aqueous solution of cellulose nanocrystals and set it aside;

[0019] S3 Coating the polylactic acid composite material on the surface of the base paper layer through a casting machine, and then coating the aqueous solution of cellulose nanocrystals on the surface of the polylactic acid composite material based on spraying treatment;

[0020] S4 After rolling, perform liquid nitrogen quenching treatment to obtain the coated paper.

[0021] As a further improvement of the above solution, the melting and kneading to obtain the polylactic acid composite material has three heating stages. The first heating stage is to raise the temperature to 180°C at a rate of 20°C / min and melt and knead for 10 - 20 min; the second heating stage is to continue to raise the temperature to 260°C at a rate of 10°C / min and melt and knead for 20 - 30 min; the third heating stage is to continue to raise the temperature to 300°C at a rate of 5°C / min and melt and knead for 30 - 40 min. In the present 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, the three stages cooperate with each other to make the raw materials melt and knead evenly, improve the casting uniformity during film formation, and reduce leakage and coating defects;

[0022] As a further improvement of the above solution, the method for preparing the aqueous solution of cellulose nanocrystals is specifically operated as follows: In S21, bleached cellulose and sulfuric acid solution are mixed and hydrolyzed in a ratio of 1:5 to 8, and a suspension is obtained after centrifugation; in S22, the suspension is spray-dried to obtain powdery cellulose nanocrystals, and the cellulose nanocrystals are dissolved in water by ultrasonic waves to obtain an aqueous solution of cellulose nanocrystals, and the ratio of cellulose nanocrystals to water is 1:3 to 5. In the present invention, the method of coating cellulose nanocrystals on the material surface is adopted to restrict the migration of molecular chains and the formation of crystal nuclei, thereby promoting the hydrolysis-biodegradation effect of the coated paper and improving the degradation rate.

[0023] As a further improvement of the above solution, the coated paper has 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 to 0.10 mm, the thickness of the cellulose nanocrystal layer is 0.01 to 0.02 mm, and the thickness of the base paper layer is 0.18 to 0.22 mm.

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

[0025] In the present invention, polyethylene glycol is added to introduce polar groups to enhance the water absorption of polylactic acid and accelerate the hydrolysis reaction rate. The hydroxyl group in polyethylene glycol is a hydrophilic group with strong hydrophilicity, which can form stable hydrogen bonds with water molecules, thereby promoting the hydrolysis of the coated paper. In addition, poly(lactic-co-glycolic acid) is also added synergistically to modify polylactic acid, aiming to introduce a branched structure, reduce the crystallinity and increase the terminal group concentration to further accelerate the hydrolysis process.

[0026] In the present invention, when making the coated paper, the method of coating cellulose nanocrystals on the material surface is adopted to restrict the migration of molecular chains and the formation of crystal nuclei, thereby promoting the hydrolysis-biodegradation effect of the coated paper and improving the degradation rate. Detailed implementation mode

[0027] The following makes a detailed description of the specific embodiments of the present invention.

[0028] Example 1

[0029] This example provides a home-compostable polylactic acid coated paper, the polylactic acid composite material is coated on the surface of kraft paper, and cellulose nanocrystals are coated on the surface of the polylactic acid composite material to form the polylactic acid coated paper. The main component of the base paper layer in this example is cellulose, which is a natural polymer polysaccharide that can be decomposed by microorganisms into carbon dioxide and water, has biodegradable properties, and is environmentally friendly.

[0030] Among them, the weight composition of the polylactic acid composite material is as follows: 80 parts of modified polylactic acid, 10 parts of polyethylene glycol, 6 parts of chitosan, 3 parts of polytrimethylene carbonate, 6 parts of EVA wax, 4 parts of amide phosphate, and 3 parts of naphthylamine. In this example, 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, which can form stable hydrogen bonds with water molecules, thereby promoting the hydrolysis of the coated paper. In addition, in this example, the modified polylactic acid is obtained by blending and modifying polylactic acid-glycolic acid copolymer and polylactic acid.

[0031] In this example, polylactic acid-glycolic acid copolymer is synergistically added to modify polylactic acid, aiming to introduce a branched structure, reduce the crystallinity and increase the end group concentration 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) First carry out a primary stirring reaction on the uniformly dispersed suspension obtained in step 1), then carry out a secondary stirring reaction, and cool the melt obtained from the reaction to obtain the product.

[0032] In step 1), the amount of water used is equivalent to 1 time the total mass of the polylactic acid-glycolic acid copolymer and polylactic acid; in step 2), the temperature of the primary stirring reaction is 80°C and the time is 20 min; the temperature of the secondary stirring reaction is 60°C and the time is 15 min; the cooling is carried out by rapid cooling with liquid nitrogen.

[0033] The coated paper of this example has polylactic acid as the main component. The polylactic acid is made from starch raw materials extracted from renewable plant resources such as corn, and has good biodegradability. After use, it can be completely degraded by microorganisms in nature, and finally generate carbon dioxide and water without polluting the environment. Due to the biodegradability of the coated paper, it is suitable for home composting use. That is, after the coated paper of this example is used, it can be decomposed through home composting to be converted into humus and mineral nutrients to achieve the ecological recycling of turning garbage into organic fertilizer.

[0034] The addition amount of the chitosan accounts for 0.5% of the polylactic acid. In this example, the purpose of adding chitosan is to facilitate the microbial degradation of the spray paper in the home composting environment. Specifically, chitosan is introduced into the material as a sugar molecule. Under light conditions, it can cause the polymer chain to break and become oligomer fragments that are easy to hydrolyze, increasing the hydrophilicity to promote the hydrolysis effect. When composting and degrading, 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 is to prevent static electricity accumulation, facilitate the use of the coated paper, and avoid its adhesion and dust adsorption. The purpose of adding EVA wax is to improve the fluidity of the polylactic acid material during the molding process, so as to facilitate its combination with the base paper layer during calendering and lamination, avoid the appearance of voids, and improve the production quality of the coated paper.

[0036] Considering that in the molten state, the presence of high temperature and oxygen is likely to trigger oxidation reactions and generate free radicals, which will initiate chain reactions, accelerate the aging and degradation of polymers, and reduce the quality of the resulting coated paper. Therefore, in this embodiment, naphthylamine is added to capture these free radicals, block the chain reaction, reduce the oxidation rate, and delay the oxidation process of the polymer, so that the polymer can be smoothly processed to obtain the coated paper.

[0037] In this embodiment, the purpose of adding poly(trimethylene carbonate) is to extend the molecular chain, increase the molecular weight, reduce the molecular chain mobility, slow down the crystallization rate, and then reduce the material crystallinity. The reduction of the material crystallinity means that the molecular arrangement is relatively loose, and it is more susceptible to the influence of moisture and other degradation factors, thus increasing the degradation rate. At the same time, due to the increase in molecular weight, the intermolecular interaction force between polymer chains is enhanced, resulting in an increase in the strength and toughness of the material.

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

[0039] S1 Take modified polylactic acid, polyethylene glycol, chitosan, poly(trimethylene carbonate), EVA wax, amide phosphate, and naphthylamine and melt and mix them to obtain the polylactic acid composite material, and set it aside. The melting and mixing to obtain the polylactic acid composite material has three heating stages. The first heating stage is to heat up to 180 °C at a rate of 20 °C / min and melt and mix for 10 min; the second heating stage is to continue heating up to 260 °C at a rate of 10 °C / min and melt and mix for 20 min; the third heating stage is to continue heating up to 300 °C at a rate of 5 °C / min and melt and mix for 30 min.

[0040] S2 Prepare an aqueous solution of cellulose nanocrystals and set it aside. 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 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 ultrasonic treatment to obtain an aqueous solution of cellulose nanocrystals. The ratio of cellulose nanocrystals to water is 1:3.

[0041] S3 Coating the polylactic acid composite material on the surface of kraft paper through a casting machine, and then coating the aqueous solution of cellulose nanocrystals on the surface of the polylactic acid composite material based on spraying treatment.

[0042] After S4 is roll-pressed, it is subjected to rapid quenching treatment with liquid nitrogen to obtain a coated paper. The coated paper has a polylactic acid composite layer, a cellulose nanocrystal layer, and kraft paper. Among them, the thickness of the polylactic acid composite 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 example provides a home-compostable polylactic acid coated paper. The polylactic acid composite is coated on the surface of white cardboard, and cellulose nanocrystals are coated on the surface of the polylactic acid composite to form the polylactic acid coated paper.

[0045] Among them, the weight parts composition of the polylactic acid composite is as follows: 90 parts of modified polylactic acid, 12 parts of polyethylene glycol, 8 parts of chitosan, 5 parts of polyurethane vulcanizing agent, 7 parts of EVA wax, 5 parts of sodium cellulose sulfate, and 4 parts of diphenylamine.

[0046] In this example, poly(lactic-co-glycolic acid) is added synergistically to modify polylactic acid. The purpose is to introduce a branched structure, reduce the crystallinity, and increase the end group concentration to further accelerate the hydrolysis process. The preparation method of the modified polylactic acid is the same as that in Example 1 in terms of specific operation.

[0047] The addition amount of the chitosan accounts for 1.0% of the polylactic acid. In this example, the purpose of adding chitosan is as follows: it is beneficial to the microbial degradation of the coated paper in the home composting environment. Specifically, chitosan is introduced into the material as a sugar molecule. Under light conditions, it can cause the polymer chain to break, becoming oligomer fragments that are easy to hydrolyze, increasing the 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 example, the purpose of adding sodium cellulose sulfate is to prevent electrostatic accumulation, facilitate the use of the coated paper, and avoid its adhesion and dust adsorption. Considering that in the molten state, the presence of high temperature and oxygen is likely to trigger oxidation reactions, generating free radicals. These free radicals will trigger chain reactions, accelerating the aging degradation of the polymer and reducing the quality of the obtained coated paper. Therefore, this example adds diphenylamine to capture these free radicals, block the chain reaction, reduce the oxidation rate, and delay the oxidation process of the polymer, so that the polymer can be smoothly processed to obtain the coated paper.

[0049] In this embodiment, the purpose of adding the polyurethane vulcanizing agent is to extend the molecular chain, increase the molecular weight, reduce the mobility of the molecular chain, slow down the crystallization rate, and thus reduce the crystallinity of the material. The reduction of the material crystallinity means that the molecular arrangement is relatively loose, and thus it is more susceptible to the influence of moisture and other degradation factors, improving the degradation rate. At the same time, due to the increase in molecular weight, the intermolecular force between polymer chains is enhanced, improving the strength and toughness of the material.

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

[0051] S1 Take the modified polylactic acid, polyethylene glycol, chitosan, polyurethane vulcanizing agent, EVA wax, sodium cellulose sulfate, and diphenylamine and melt and knead them to obtain the polylactic acid composite material, and set it aside. The melting and kneading to obtain the polylactic acid composite material has three heating stages, the same as in Example 1.

[0052] S2 Prepare an aqueous solution of cellulose nanocrystals and set it aside. The method for preparing the aqueous solution of cellulose nanocrystals is as follows: S21 Take bleached cellulose and sulfuric acid solution and mix and hydrolyze them in a ratio of 1:6, and obtain a suspension after centrifugation; S22 Spray-dry the suspension to obtain powdery cellulose nanocrystals, and dissolve the cellulose nanocrystals in water by ultrasonic wave 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 home-compostable polylactic acid coated paper. The polylactic acid composite material is coated on the surface of the corrugated paper, and cellulose nanocrystals are coated on the surface of the polylactic acid composite material to form the polylactic acid coated paper. The main component of the corrugated paper in this embodiment is cellulose, which is a natural high molecular polysaccharide and can be decomposed by microorganisms into carbon dioxide and water, having biodegradable performance and good environmental protection.

[0057] Among them, the weight composition of the polylactic acid composite material is as follows: 100 parts of modified polylactic acid, 13 parts of polyethylene glycol, 12 parts of chitosan, 7 parts of diethylene glycol, 8 parts of glycerol, 6 parts of alkyl quaternary ammonium salt, and 5 parts of p-phenylenediamine. In this embodiment, poly(lactic-co-glycolic acid) is synergistically added to modify polylactic acid, aiming to introduce a branched structure, reduce the crystallinity and increase the terminal group concentration to further accelerate the hydrolysis process. The preparation method of the modified polylactic acid is specifically operated as follows: 1) Disperse poly(lactic-co-glycolic acid) and polylactic acid in water and mix them under ultrasonic conditions to obtain a uniformly dispersed suspension; 2) First perform a primary stirring reaction on the uniformly dispersed suspension obtained in step 1), then perform a secondary stirring reaction, and cool the melt obtained from the reaction to obtain the modified polylactic acid.

[0058] In step 1), the amount of water used is 1.5 times the total mass of poly(lactic-co-glycolic acid) and polylactic acid. In step 2), the temperature of the primary stirring reaction is 90°C and the time is 25 min. The temperature of the secondary stirring reaction is 70°C and the time is 25 min. The cooling is carried out by rapid cooling with liquid nitrogen.

[0059] The addition amount of the chitosan accounts for 1.2% of the polylactic acid. In this embodiment, the purpose of adding chitosan is as follows: it is beneficial to the microbial degradation of the spray-coated paper in the household composting environment. Specifically, chitosan is introduced into the material as a sugar molecule, which can cause the polymer chain to break under light conditions and become oligomer fragments that are easy to hydrolyze, increasing the 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 the alkyl quaternary ammonium salt is to prevent electrostatic accumulation, facilitate the use of the coated paper, and avoid its adhesion and dust adsorption. The purpose of adding glycerol is to improve the fluidity of the polylactic acid material during the forming process, so as to facilitate its combination with the base paper layer during calendering lamination, avoid the appearance of voids, and improve the production quality of the coated paper.

[0061] Considering that in the molten state, the presence of high temperature and oxygen is likely to trigger an oxidation reaction, generate free radicals, and these free radicals will initiate a chain reaction, accelerating the aging degradation of the polymer and reducing the quality of the obtained coated paper. Therefore, in this embodiment, p-phenylenediamine is added to capture these free radicals, block the chain reaction, reduce the oxidation rate, delay the oxidation process of the polymer, and thus enable the polymer to be smoothly processed to obtain the coated paper.

[0062] In this example, the purpose of adding diethylene glycol is to extend the molecular chain, increase the molecular weight, reduce the mobility of the molecular chain, slow down the crystallization rate, and then reduce the crystallinity of the material. The reduction of the material crystallinity means that the molecular arrangement is relatively loose, which is more susceptible to the influence of moisture and other degradation factors, thus increasing the degradation rate. At the same time, due to the increase in molecular weight, the intermolecular force between polymer chains is enhanced, improving the strength and toughness of the material.

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

[0064] S1 Take modified polylactic acid, polyethylene glycol, chitosan, diethylene glycol, glycerol, alkyl quaternary ammonium salt, and p-phenylenediamine, and perform melt blending to obtain the polylactic acid composite material for standby. The melt blending to obtain the polylactic acid composite material has three heating stages. The first heating stage is to heat up to 180 °C at a rate of 20 °C / min and perform melt blending for 20 min. The second heating stage is to continue heating up to 260 °C at a rate of 10 °C / min and perform melt blending for 30 min. The third heating stage is to continue heating up to 300 °C at a rate of 5 °C / min and perform melt blending for 40 min.

[0065] S2 Prepare an aqueous solution of cellulose nanocrystals for standby. 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 1:7 for hydrolysis, and obtain a suspension after centrifugation. S22 Perform spray drying on the suspension to obtain powdered cellulose nanocrystals, and dissolve the cellulose nanocrystals in water by ultrasonic treatment to obtain an aqueous solution of cellulose nanocrystals. The ratio of cellulose nanocrystals to water is 1:5.

[0066] S3 is the same as Example 1.

[0067] S4 is the same as Example 1.

[0068] Example 4

[0069] This example provides a home-compostable polylactic acid coated paper. The polylactic acid composite material is coated on the surface of the corrugated paper, and the cellulose nanocrystals are coated on the surface of the polylactic acid composite material to form the polylactic acid coated paper. The main component of the corrugated paper in this example is cellulose, which is a natural polymer polysaccharide that can be decomposed by microorganisms into carbon dioxide and water, has biodegradable properties, and is environmentally friendly.

[0070] Among them, the weight composition of the polylactic acid composite material is as follows: 120 parts of modified polylactic acid, 15 parts of polyethylene glycol, 13 parts of chitosan, 8 parts of toluene diamine, 9 parts of glycerol, 8 parts of alkyl sulfonate, and 7 parts of phosphite. In this example, poly(lactic-co-glycolic acid) is added synergistically to modify polylactic acid, aiming to introduce a branched structure, reduce the crystallinity, and increase the terminal group concentration to further accelerate the hydrolysis process.

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

[0072] The addition amount of the chitosan accounts for 1.5% of the polylactic acid. In this example, the purpose of adding chitosan is as follows: it is beneficial to the microbial degradation of the spray-coated paper in the household composting environment. Specifically, chitosan is introduced into the material as a sugar molecule. Under light conditions, it can cause the polymer chain to break, becoming oligomer fragments that are easy to hydrolyze, increasing the hydrophilicity to promote the hydrolysis effect. When composting degradation is carried out, 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 example, the purpose of adding sodium alkyl sulfonate is to prevent electrostatic accumulation, so as to facilitate the use of the coated paper and avoid its adhesion and dust adsorption. Considering that in the molten state, the presence of high temperature and oxygen is likely to trigger an oxidation reaction, generating free radicals, and these free radicals will trigger a chain reaction, accelerating the aging degradation of the polymer and reducing the quality of the obtained coated paper. Therefore, in this example, phosphite is added to capture these free radicals, block the chain reaction, reduce the oxidation rate, and delay the oxidation process of the polymer, so that the polymer can be smoothly processed to obtain the coated paper.

[0074] In this example, the purpose of adding toluenediamine is to expand the molecular chain, increase the molecular weight, reduce the activity of the molecular chain, slow down the crystallization speed, and then reduce the crystallinity of the material. The reduction of the material crystallinity means that the molecular arrangement is relatively loose, and then it is more susceptible to the influence of moisture and other degradation factors, improving the degradation rate. At the same time, due to the increase in molecular weight, the intermolecular force between polymer chains is enhanced, making the strength and toughness of the material improved.

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

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

[0077] S2 is the same as that in Example 3.

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

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

[0080] Comparative Example 1

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

[0082] Comparative Example 2

[0083] The difference between this comparative example and Example 3 is only that polyethylene glycol is not added, and the rest is the same as Example 3.

[0084] Comparative Example 3

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

[0086] Next, the coated papers of Examples 1-4 and Comparative Examples 1-3 were subjected to a home composting test. The specific operation of home composting is as follows: 100 g of vegetable leaves and 100 g of dry leaves were respectively taken and pulverized. A layer of sandy soil was laid in a composting container with a volume of 500 ml, and then the pulverized vegetable leaves and dry leaves were evenly added to the composting container. The coated papers (50 g) of Examples 1-4 and Comparative Examples 1-3 were taken and buried in the above-mentioned materials. Finally, 2-3 cm thick soil was laid to introduce microorganisms, and water was added. The amount of water added accounted for 10% of the volume of the container. It was turned over once every 24 h. After 7 days of composting treatment, the weight of the coated paper was weighed, and the degradation rate was obtained by conversion. The test results are as follows: the degradation rate of Example 1 was, 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] Analyzing the above results, it can be seen that the degradation rates of Comparative Example 1 and Comparative Example 2 were both about twice lower than that of Example 3. This shows that adding polyethylene glycol and adding poly (lactic-co-glycolic acid) to modify polylactic acid has a synergistic positive effect, which can greatly improve the degradation efficiency of the material. However, the degradation rate of Comparative Example 3 was also significantly lower than that of Example 3, which proves that the method of coating cellulose nanocrystals on the surface of the material can promote the hydrolysis-biodegradation effect of the coated paper and achieve the effect of improving the degradation efficiency.

[0088] The above embodiments are only the preferred embodiments of the present invention. Any simple modification, modification and substitution change made to the above embodiments based on the technical essence of the present invention all belong to the scope of the technical solution of the present invention.

Claims

1. A household compostable polylactic acid coated paper, characterized in that, The 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 weight parts composition of the polylactic acid composite material is as follows: 80-120 parts of modified polylactic acid, 10-15 parts of polyethylene glycol, 6-13 parts of chitosan, 3-8 parts of chain extender, 6-9 parts of lubricant, 4-8 parts of antistatic agent, 3-7 parts of antioxidant.

2. The compostable polylactic acid coated paper according to claim 1, wherein The base paper layer is any one of kraft paper, white cardboard and corrugated paper.

3. The compostable polylactic acid coated paper according to claim 1, characterized in that, The modified polylactic acid is obtained by blending and modifying 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.

4. The compostable polylactic acid coated paper according to claim 3, characterized in that, 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) First carry out a first stirring reaction on the uniformly dispersed suspension obtained in step 1), then carry out a second stirring reaction, and cool the melt obtained by the reaction to obtain the product. In step 1), the amount of water used is 1-2 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 80-100 °C and the time is 20-30 min; the temperature of the second stirring reaction is 60-80 °C and the time is 15-30 min; the cooling is carried out by rapid cooling with liquid nitrogen.

5. The compostable polylactic acid coated paper according to claim 4, wherein The addition amount of the chitosan accounts for 0.5%-1.5% of the polylactic acid.

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

7. According to the manufacturing process of the coated paper as described in any one of claims 1 to 6, it is characterized in that, It includes the following steps: S1 Take modified polylactic acid, polyethylene glycol, chitosan, chain extender, lubricant, antistatic agent and antioxidant, and melt and knead them to obtain the polylactic acid composite material, and set aside; S2 Prepare an aqueous solution of cellulose nanocrystals and set aside; S3 Coat the polylactic acid composite material on the surface of the base paper layer through a casting machine, and then coat the aqueous solution of cellulose nanocrystals on the surface of the polylactic acid composite material based on spraying treatment; S4 After rolling, carry out rapid cooling treatment with liquid nitrogen to obtain the coated paper.

8. The manufacturing process according to claim 7, wherein The melting and kneading to obtain the polylactic acid composite material has three heating stages. The first heating stage is to heat up to 180 °C at a rate of 20 °C / min and melt and knead for 10-20 min; the second heating stage is to continue to heat up to 260 °C at a rate of 10 °C / min and melt and knead for 20-30 min; the third heating stage is to continue to heat up to 300 °C at a rate of 5 °C / min and melt and knead for 30-40 min.

9. The manufacturing process according to claim 8, characterized in that, The method for preparing the aqueous solution of cellulose nanocrystals is specifically operated as follows: In S21, bleached cellulose and sulfuric acid solution are mixed and hydrolyzed in a ratio of 1:5 to 8, and a suspension is obtained after centrifugation; in S22, the suspension is spray-dried to obtain powdery cellulose nanocrystals, and the cellulose nanocrystals are dissolved in water by ultrasonic waves to obtain an aqueous solution of cellulose nanocrystals, and the ratio of cellulose nanocrystals to water is 1:3 to 5.

10. The manufacturing process according to claim 9, wherein, The described coated paper has a polylactic acid composite material layer, a cellulose nanocrystal layer, and a base paper layer; among them, the thickness of the polylactic acid composite material layer is 0.08 to 0.10 mm, the thickness of the cellulose nanocrystal layer is 0.01 to 0.02 mm, and the thickness of the base paper layer is 0.18 to 0.22 mm.