Food paper based on plant fiber and PLA (polylactic acid) lamination and preparation method thereof

Abstract

The invention discloses food paper based on plant fibers and PLA (polylactic acid) lamination and a preparation method of the food paper, and belongs to the technical field of polylactic acid lamination papers.Acrylate monomers and organic silicon modified alkyl acrylate containing flexible chain segments are subjected to in-situ polymerization on TEMPO oxidized nano-cellulose under a controllable catalytic system, so that the food paper is obtained; the preparation method comprises the following steps: polymerizing a cellulose skeleton, anchoring a polymerized chain segment on the cellulose skeleton in a chemical manner, constructing an organic interface layer with certain flexibility on the surface of nano-cellulose to obtain a modified nano-cellulose filler with a stable structure, blending the modified nano-cellulose with polylactic acid to prepare a film spraying master batch, and combining the film spraying master batch with plant cellulose base paper through a film spraying process to prepare the plant cellulose composite film. The preparation method is simple and easy to operate, the food paper which is compact in lamination and uniform in attachment is obtained, stable attachment of the paper base and the PLA lamination is achieved while the bio-based and degradable characteristics of the material are kept, interface defects and delamination risks are reduced, the mechanical property and use reliability of the food paper are improved, and the food paper is suitable for food packaging application.

Description

Technical Field

[0001] This invention belongs to the field of polylactic acid coated paper technology, specifically food paper based on plant fiber and PLA coating and its preparation method. Background Technology

[0002] With the rapid growth in demand for takeout packaging, traditional plastic takeout bags are causing environmental pressure due to their difficulty in degradation. Developing widely available, green, and biodegradable packaging materials has become an industry trend. Plant fiber paper bases are widely used in food packaging due to their high renewability and biodegradability. Polylactic acid (PLA), as a typical biodegradable resin, has good film-forming properties and food contact safety, and is often used to coat paper bases. However, due to the high surface polarity of cellulose paper bases and the relatively weak polarity of PLA, the interfacial compatibility between the two is poor, which easily leads to insufficient adhesion of the coating layer, delamination, and peeling. This affects the overall mechanical properties and reliability of takeout bags during loading, handling, and compression.

[0003] Currently, common methods to address the problem of insufficient adhesion of PLA in paper-based coatings include adding compatibilizers, compatibilizers, or introducing nanofillers to improve the melt polarity and wettability of PLA. Among these, nanocellulose, due to its high specific surface area and good mechanical properties, is widely used to improve the interfacial interaction between PLA and plant fiber paper base.

[0004] In the prior art, Chinese patent application with publication number CN117845650A discloses a biodegradable PLA coating material and coating products based thereon. The coating material is obtained by sequentially citric acid esterification and acetylation of nanofiber crystals, followed by composite with polylactic acid matrix and then further prepared into coating paper.

[0005] In the above-mentioned technical solutions, small polar groups such as carboxyl and ester groups are introduced on the surface of nanocellulose to enhance the bonding between PLA and the paper substrate through hydrogen bonding and electrostatic attraction. However, this type of chemical reaction only occurs on the surface of nanocellulose and fails to construct a stable structure with adjustable chain segments or flexible shells. As a result, rigid nanocellulose is still prone to re-aggregation through inter-hydroxyl forces during melt blending, leading to increased local viscosity and uneven dispersion of the system. This makes the coated layer prone to delamination under bending or compression conditions. At the same time, the ester groups on the surface of nanocellulose may undergo thermal decomposition of ester bonds within the coating temperature range, further reducing the film's density and mechanical integrity. It is difficult to form a continuous and stable interfacial bonding structure, and thus cannot meet the requirements of food-grade coated paper for mechanical reliability and durability. Summary of the Invention

[0006] The purpose of this invention is to provide food-grade paper based on plant fibers and PLA coating, and its preparation method. By structurally regulating nanocellulose, it is made to have both dispersion stability and interface regulation ability in a polylactic acid matrix. Under the premise of good biodegradability, effective synergy and stable bonding between the polylactic acid coating and the plant fiber paper base are achieved, resulting in a food-grade paper with good interfacial bonding performance, processing stability and biodegradability, so as to meet the comprehensive requirements of environmental protection and performance in the food packaging field.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] This invention provides a food-grade paper material based on plant fiber and PLA coating, prepared through the following steps:

[0009] Step 1: Using TEMPO-oxidized nanocellulose as the reaction matrix, brominated initiating groups are introduced on its surface through acylation reaction to obtain bromine-containing nanocellulose with controllable initiation activity. At the same time, organosilicon-modified alkyl acrylate with flexible siloxane segments is prepared. Using bromine-containing nanocellulose as the initiation center, acrylate segments are grown in situ from the cellulose surface and synergistically grafted onto its surface to obtain modified nanocellulose filler.

[0010] Step 2: Modified nanocellulose filler and polylactic acid are melt-blended in a twin-screw extruder to obtain polylactic acid composite coating masterbatch. The masterbatch is then melt-coated onto the surface of plant fiber paper base through a coating extrusion process. After pressing and cooling, a continuous and dense PLA coating layer is formed, and finally, food paper material based on plant fiber and PLA coating is obtained.

[0011] This invention also provides a method for preparing food-grade paper based on plant fiber and PLA coating, comprising the following steps:

[0012] Polylactic acid composite coating masterbatch is melt-extruded through a coating extruder and coated onto the surface of plant fiber paper base. After being bonded by pressing rollers and cooling rollers and cooled and solidified, a dense coating layer is formed. After the coated paper roll is rolled up, it is placed in a drying oven and vacuum dried at 30-50℃ for 3-5 hours. This results in food-grade paper material based on plant fiber and PLA coating.

[0013] Furthermore, the barrel temperature of the coating extruder is 160-180℃, and the die temperature is 180-190℃.

[0014] Furthermore, the plant fiber paper base is plant cellulose-based kraft paper base paper.

[0015] Furthermore, the preparation process of the polylactic acid composite coating masterbatch is as follows:

[0016] 300-500g of polylactic acid (PLA), 30-50g of modified nanocellulose composite filler, 2-4g of antioxidant (antioxidant 1010), 3-6g of lubricant (calcium stearate), and 1-2g of multifunctional epoxy chain extender (Joncryl® ADR-4368-C) were placed in a high-speed mixer and stirred at high speed for 10-20 minutes. The mixture was then extruded and granulated using a twin-screw extruder. The temperatures of the first six zones of the extruder were set sequentially to 70-100℃, 140-160℃, 150-170℃, 160-180℃, 160-180℃, and 160-180℃, while the die head temperature was 160-180℃. After extrusion into strips, the mixture was cooled and granulated. The strips were then vacuum dried at 50-70℃ to a constant weight to obtain polylactic acid composite coating masterbatch.

[0017] Furthermore, the temperatures of each zone of the twin-screw extruder are 80-100℃, 100-120℃, 120-140℃, 140-160℃, 160-170℃, and 170-180℃ respectively, and the die temperature is 170-180℃.

[0018] Furthermore, the mass ratio of polylactic acid, modified nanocellulose composite filler, antioxidant, lubricant, and chain extender is 300-500:30-50:2-4:3-6:1-2.

[0019] Furthermore, the preparation process of the modified nanocellulose composite filler is as follows:

[0020] Methacrylate, organosilicon-modified alkyl acrylate, ethyl 2-bromoisobutyrate, bromine-containing nanocellulose, tetrahydrofuran, and N,N-dimethylformamide were placed in a reactor under nitrogen atmosphere and ultrasonically dispersed at 25-35℃ for 20-30 min. Cuprous bromide was added, and the reaction was carried out at 50-70℃ for 6-8 h. The reaction solution was added to anhydrous methanol to precipitate the mixture, filtered, and the filter cake was vacuum dried to constant weight to obtain the modified nanocellulose filler.

[0021] Furthermore, the ratio of methacrylate, organosilicon-modified alkyl acrylate, ethyl 2-bromoisobutyrate, bromine-containing nanocellulose, cuprous bromide, tetrahydrofuran, and N,N-dimethylformamide is 40-60g: 15-25g: 10-20g: 200-300mg: 100-150mL: 10-15mL.

[0022] This method uses bromine-containing nanocellulose as an initiator, which is activated in a catalytic system of cuprous bromide and ethyl 2-bromoisobutyrate, thereby enabling nanocellulose, methacrylate and organosilicon-modified alkyl acrylate to undergo free radical polymerization, resulting in a modified nanocellulose filler with stable structure and enhanced interfacial properties.

[0023] Furthermore, the preparation process of bromine-containing nanocellulose is as follows:

[0024] TEMPO-oxidized nanocellulose, 4-dimethylpyridine, and N,N-dimethylformamide were placed in a reaction vessel under a nitrogen atmosphere and stirred at 0-10℃ for 20-40 min. Then, 2-bromoisobutyryl bromide was added, and the reaction was carried out at 25-35℃ for 18-24 h. The mixture was filtered, washed, and vacuum dried to constant weight to obtain bromine-containing nanocellulose.

[0025] Furthermore, the ratio of TEMPO-oxidized nanocellulose, 4-dimethylpyridine, 2-bromoisobutyryl bromide and N,N-dimethylformamide is 15-35g: 1-2g: 4-8g: 200-300mL.

[0026] This method involves acylation of TEMPO-oxidized cellulose nanoparticles, causing the surface carboxyl groups to react with 2-bromoisobutyryl bromide, thereby introducing initiation sites on the cellulose surface that can be used for graft polymerization, forming bromine-containing cellulose nanoparticles that can initiate free radical polymerization.

[0027] Furthermore, the preparation process of organosilicon-modified alkyl acrylates is as follows:

[0028] Isophorone diisocyanate and monohydroxypropyl-terminated silicone oil were placed in a reaction vessel under a nitrogen atmosphere and stirred at 25-35°C for 10-30 min. Dibutyltin dilaurate was added, and the reaction was carried out at 35-45°C for 1-2 h. Then 4-hydroxybutyl acrylate was added, and the reaction was continued at 45-55°C for 1-2 h. Anhydrous ethanol was added for precipitation, the supernatant was removed, and the lower product was vacuum dried to constant weight to obtain organosilicon-modified alkyl acrylate.

[0029] Furthermore, the mass ratio of isophorone diisocyanate, monohydroxypropyl end-capped silicone oil, dibutyltin dilaurate, and 4-hydroxybutyl acrylate is 4-6:30-50:0.1-0.3:5-7.

[0030] This method utilizes an isocyanate group in isophorone diisocyanate to undergo an isocyanate-hydroxy addition reaction with monohydroxypropyl-terminated silicone oil. Subsequently, the remaining isocyanate group undergoes an addition reaction with 4-hydroxybutyl acrylate, introducing a free radical polymerizable olefin double bond into the alkyl acrylate, thereby obtaining a reactive organosilicon-modified alkyl acrylate.

[0031] The beneficial effects of this invention are:

[0032] 1. The food-grade paper material prepared by this invention, based on plant fiber and PLA coating, introduces brominated initiating groups on the surface of TEMPO-oxidized nanocellulose, making it a stable reaction core in the subsequent grafting polymerization process. Under the premise of not destroying the natural degradable skeleton of cellulose itself, the controlled growth of acrylate segments is achieved. The resulting modified nanocellulose maintains the homology and co-degradation characteristics with the plant fiber paper base, while its surface structure changes from a single high polarity state to a composite interface unit with gradient compatibility characteristics. This provides a continuous and synergistically degradable connection basis between the PLA coating layer and the paper base, structurally alleviating the problem of unstable interfacial bonding between traditional PLA and paper base caused by polarity differences, and laying the foundation for improving the overall mechanical properties of food-grade paper materials.

[0033] 2. The food-grade paper material based on plant fiber and PLA coating prepared by this invention introduces organosilicon-modified alkyl acrylate as flexible segments during the grafting process on the surface of bromine-containing nanocellulose. This allows the modified filler to exhibit good dispersion stability during PLA melt blending and coating extrusion. Simultaneously, bromine-containing nanocellulose is used as an initiator to achieve in-situ growth of segments, enabling the flexible segments to be more uniformly distributed on the cellulose surface. These flexible segments do not introduce non-degradable rigid structures and effectively weaken the strong polar attraction between cellulose molecules, thereby reducing the tendency of nanocellulose to aggregate in the PLA matrix. In this way, PLA maintains its bio-based and degradable properties while obtaining a more uniform and continuous coating structure. This allows the coating layer to stably adhere to the paper base surface during film formation and cooling, reducing delamination and mechanical property degradation caused by uneven melt flow and interface defects.

[0034] 3. The food-grade paper material prepared by this invention, based on plant fiber and PLA coating, uses modified nanocellulose as an interfacial functional filler distributed between the plant fiber paper base and the PLA coating layer. Its surface grafted segments are stably fixed to the cellulose backbone via covalent bonds. The acrylate backbone in the grafted segments can effectively form chain entanglement with PLA molecules. The alkyl chains introduced by the organosilicon-modified alkyl acrylate, together with the flexible silicon-oxygen segments, work together to reduce the surface energy of the modified nanocellulose, improving its wettability and spreading ability in the PLA melt. Furthermore, it forms a flexible buffer region at the interface, effectively reducing stress concentration under stress. Since the grafted structure is still mainly composed of cellulose and ester carbon chains, it does not form a recalcitrant cross-linked network. Moreover, the alkyl chains and silicon-oxygen segments only exist as flexible control units. Therefore, it does not hinder the overall degradability of the material during use and disposal. This allows the material to maintain food contact safety and degradability while simultaneously improving adhesion strength, peel resistance, and durability, making it more suitable for the practical application needs of food takeout packaging. Detailed Implementation

[0035] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0036] Example 1: This example provides a food-grade paper material based on plant fiber and PLA coating, prepared through the following steps:

[0037] S1: 4g of isophorone diisocyanate and 30g of monohydroxypropyl end-capped silicone oil were placed in a reaction vessel under a nitrogen protective atmosphere and stirred at 200r / min for 10min at 25℃. 0.1g of dibutyltin dilaurate was added and the reaction was continued at 35℃ with the same stirring rate for 1h. Then 5g of 4-hydroxybutyl acrylate was added and the reaction was continued at 45℃ with the same stirring rate for 1h. After the reaction was completed, the mixture was cooled to room temperature and anhydrous ethanol was added for precipitation. The supernatant was removed and the lower product was vacuum dried at 60℃ to constant weight to obtain organosilicon-modified alkyl acrylate.

[0038] S2: 15g of TEMPO-oxidized nanocellulose, 1g of 4-dimethylpyridine and 200mL of N,N-dimethylformamide were placed in a reaction vessel under nitrogen atmosphere protection. The mixture was stirred at 200r / min for 20min at 10℃. 4g of 2-bromoisobutyryl bromide was added, and the mixture was reacted at the same stirring rate at 25℃ for 18h. After the reaction was completed, the mixture was filtered, and the filter cake was washed twice with deionized water and anhydrous ethanol. The cake was then vacuum dried at 60℃ to constant weight to obtain bromine-containing nanocellulose.

[0039] S3: 40g of methacrylate, 15g of organosilicon-modified alkyl acrylate, 400mg of ethyl 2-bromoisobutyrate, 10g of bromine-containing nanocellulose, 100mL of tetrahydrofuran, and 10mL of N,N-dimethylformamide were placed in a reactor under nitrogen atmosphere and ultrasonically dispersed at 25℃ for 20min. 200mg of cuprous bromide was added, and the mixture was stirred at 50℃ at 200r / min for 6h. After the reaction was completed, the mixture was cooled to room temperature, and the reaction solution was added to anhydrous methanol to precipitate the precipitate. The precipitate was filtered, and the filter cake was vacuum dried at 50℃ to constant weight to obtain the modified nanocellulose filler.

[0040] S4: Place 300g of polylactic acid (PLA), 30g of modified nanocellulose composite filler, 2g of antioxidant (antioxidant 1010), 3g of lubricant (calcium stearate), and 1g of multifunctional epoxy chain extender (Joncryl® ADR-4368-C) in a high-speed mixer and stir at 500r / min for 10min. Extrude and granulate using a twin-screw extruder. Set the temperatures of the first six zones of the extruder to 80℃, 100℃, 120℃, 140℃, 160℃, and 170℃ respectively, and the die temperature to 160℃. After extrusion into strips, cool and granulate, and vacuum dry at 50℃ to constant weight to obtain polylactic acid composite coating masterbatch.

[0041] S5: Add polylactic acid composite coating masterbatch to the coating machine, add the dried resin to the coating machine hopper, and perform melt extrusion in the coating extruder. During the coating process, the extruder barrel temperature is 160℃ and the die temperature is 180℃. The molten resin film is directly coated onto the surface of plant cellulose-based kraft paper base paper after being extruded from the die. After being bonded by the pressing roller and the cooling roller, and cooled and solidified, a dense coating layer is formed. After the coated paper roll is wound up, it is placed in a drying oven and vacuum dried at 30℃ for 3 hours. This is a food paper material based on plant fiber and PLA coating.

[0042] Example 2: This example provides a food-grade paper material based on plant fiber and PLA coating, prepared through the following steps:

[0043] S1: 5g of isophorone diisocyanate and 40g of monohydroxypropyl end-capped silicone oil were placed in a reaction vessel under a nitrogen protective atmosphere and stirred at 250r / min for 20min at 30℃. 0.2g of dibutyltin dilaurate was added and the reaction was continued at 40℃ with the same stirring rate for 1.5h. Then 6g of 4-hydroxybutyl acrylate was added and the reaction was continued at 50℃ with the same stirring rate for 1.5h. After the reaction was completed, the mixture was cooled to room temperature and anhydrous ethanol was added for precipitation. The supernatant was removed and the lower product was vacuum dried at 70℃ to constant weight to obtain organosilicon-modified alkyl acrylate.

[0044] S2: 25g of TEMPO-oxidized nanocellulose, 1.5g of 4-dimethylpyridine and 250mL of N,N-dimethylformamide were placed in a reaction vessel under nitrogen atmosphere protection. The mixture was stirred at 250r / min for 30min at 5℃. 6g of 2-bromoisobutyryl bromide was added, and the mixture was reacted at the same stirring rate at 30℃ for 20h. After the reaction was completed, the mixture was filtered, and the filter cake was washed three times with deionized water and anhydrous ethanol. The cake was then vacuum dried at 70℃ to constant weight to obtain bromine-containing nanocellulose.

[0045] S3: 50g of methacrylate, 20g of organosilicon-modified alkyl acrylate, 500mg of ethyl 2-bromoisobutyrate, 15g of bromine-containing nanocellulose, 120mL of tetrahydrofuran, and 12mL of N,N-dimethylformamide were placed in a reactor under nitrogen atmosphere and ultrasonically dispersed at 30℃ for 25min. 250mg of cuprous bromide was added, and the mixture was stirred at 60℃ at 250r / min for 7h. After the reaction was completed, the mixture was cooled to room temperature, and the reaction solution was added to anhydrous methanol to precipitate the precipitate. The precipitate was filtered, and the filter cake was vacuum dried at 60℃ to constant weight to obtain the modified nanocellulose filler.

[0046] S4: Place 400g of polylactic acid (PLA), 40g of modified nanocellulose composite filler, 3g of antioxidant 1010, 5g of calcium stearate, and 1.5g of multifunctional epoxy chain extender (Joncryl® ADR-4368-C) in a high-speed mixer and stir at 700r / min for 15min. Extrude and granulate using a twin-screw extruder. Set the temperatures of the first six zones of the extruder to 90℃, 110℃, 130℃, 150℃, 165℃, and 175℃ respectively, and the die temperature to 170℃. After extrusion into strips, cool and granulate, and vacuum dry at 60℃ to constant weight to obtain polylactic acid composite coating masterbatch.

[0047] S5: Add polylactic acid composite coating masterbatch to the coating machine, add the dried resin to the coating machine hopper, and perform melt extrusion in the coating extruder. During the coating process, the extruder barrel temperature is 170℃ and the die temperature is 185℃. The molten resin film is directly coated onto the surface of plant cellulose-based kraft paper base paper after being extruded from the die. After being bonded by the pressing roller and the cooling roller, and cooled and solidified, a dense coating layer is formed. After the coated paper roll is wound up, it is placed in a drying oven and vacuum dried at 40℃ for 4 hours. This is a food-grade paper material based on plant fiber and PLA coating.

[0048] Example 3: This example provides a food-grade paper material based on plant fiber and PLA coating, prepared through the following steps:

[0049] S1: 6g of isophorone diisocyanate and 50g of monohydroxypropyl end-capped silicone oil were placed in a reaction vessel under a nitrogen protective atmosphere and stirred at 300r / min for 30min at 35℃. 0.3g of dibutyltin dilaurate was added and the reaction was continued at 45℃ with the same stirring rate for 2h. Then 7g of 4-hydroxybutyl acrylate was added and the reaction was continued at 55℃ with the same stirring rate for 2h. After the reaction was completed, the mixture was cooled to room temperature and anhydrous ethanol was added for precipitation. The supernatant was removed and the lower product was vacuum dried at 80℃ to constant weight to obtain organosilicon-modified alkyl acrylate.

[0050] S2: 35g of TEMPO-oxidized nanocellulose, 2g of 4-dimethylpyridine and 300mL of N,N-dimethylformamide were placed in a reaction vessel under nitrogen atmosphere protection. The mixture was stirred at 300r / min for 40min at 0℃. 8g of 2-bromoisobutyryl bromide was added, and the mixture was reacted at the same stirring rate at 35℃ for 24h. After the reaction was completed, the mixture was filtered, and the filter cake was washed four times with deionized water and anhydrous ethanol. The cake was then vacuum dried at 80℃ to constant weight to obtain bromine-containing nanocellulose.

[0051] S3: 60g of methacrylate, 25g of organosilicon-modified alkyl acrylate, 600mg of ethyl 2-bromoisobutyrate, 20g of bromine-containing nanocellulose, 150mL of tetrahydrofuran, and 15mL of N,N-dimethylformamide were placed in a reactor under nitrogen atmosphere protection and ultrasonically dispersed at 35℃ for 30min. 300mg of cuprous bromide was added, and the mixture was stirred at 70℃ at 300r / min for 8h. After the reaction was completed, the mixture was cooled to room temperature, and the reaction solution was added to anhydrous methanol to precipitate the precipitate. The precipitate was filtered, and the filter cake was vacuum dried at 70℃ to constant weight to obtain the modified nanocellulose filler.

[0052] S4: Place 500g of polylactic acid (PLA), 50g of modified nanocellulose composite filler, 4g of antioxidant 1010 agent, 6g of calcium stearate, and 2g of multifunctional epoxy chain extender (Joncryl® ADR-4368-C) in a high-speed mixer and stir at 800r / min for 20min. Extrude and granulate through a twin-screw extruder. Set the temperatures of the first six zones of the extruder to 100℃, 120℃, 140℃, 160℃, 170℃, and 180℃ respectively, and the die temperature to 180℃. After extrusion into strips, cool and cut into pellets, and vacuum dry at 70℃ to constant weight to obtain polylactic acid composite coating masterbatch.

[0053] S5: Add polylactic acid composite coating masterbatch to the coating machine, add the dried resin to the coating machine hopper, and perform melt extrusion in the coating extruder. During the coating process, the extruder barrel temperature is 180℃ and the die temperature is 190℃. The molten resin film is directly coated onto the surface of plant cellulose-based kraft paper base paper after being extruded from the die. After being bonded by the pressing roller and the cooling roller, and cooled and solidified, a dense coating layer is formed. After the coated paper roll is wound up, it is placed in a drying oven and vacuum dried at 50℃ for 5 hours. This is a food paper material based on plant fiber and PLA coating.

[0054] The food-grade paper materials based on plant fiber and PLA coating prepared in Examples 1-3 above were tested for total migration, specific migration, heavy metals, volatile organic compounds, and prohibited and restricted substances in accordance with standard GB / T 36392-2025. The results showed that the total migration was <10mg / dm², the migration of metals was <0.01mg / kg, organic solvents such as DMF and THF were not detected, and the residues of halogenated organic compounds and organotin compounds were all below the standard limits. All indicators met the requirements of GB / T 36392-2025 and can be used for food contact applications.

[0055] Comparative Example 1: Based on Example 2, commercially available 4-hydroxybutyl acrylate was used instead of the silicone-modified alkyl acrylate used in step S3, while the other steps remained unchanged.

[0056] Comparative Example 2: Based on Example 2, the organosilicon-modified alkyl acrylate prepared in step S1 was removed, while the remaining steps remained unchanged.

[0057] Comparative Example 3: Based on Example 2, nanocellulose was used instead of the modified nanocellulose filler prepared in step S3, while the other steps remained unchanged.

[0058] The monohydroxypropyl end-capping silicone oil purchased in the above examples and comparative examples was produced by Jinan Longcheng Organosilicon Co., Ltd., with an average viscosity of approximately 5000 mPa·s (25 ℃) and an average hydroxyl content of 7%; 4-hydroxybutyl acrylate was produced by Shanghai Maclean Biochemical Technology Co., Ltd., with CAS number 2478-10-6; TEMPO oxidized nanocellulose was produced by Nanjing Tianlu Nanotechnology Co., Ltd., model TEMPO oxidized nanocellulose TL-010-2, with a carboxyl content of 2%, and was used after freeze-drying; nanocellulose was produced by Nanjing Tianlu Nanotechnology Co., Ltd., model nanocellulose dry powder TLP002; and plant fiber paper base was produced by Baimao Paper Co., Ltd., which is plant cellulose-based kraft paper base paper, with natural wood pulp fiber as the main component.

[0059] The plant fibers prepared in Examples 1-3 and Comparative Examples 1-3 and the food paper material coated with PLA were subjected to performance tests. The test results are shown in Table 1.

[0060] Sample preparation: Referring to standard GB / T 450-2008, the food paper samples of the above examples and comparative examples were cut into 10mm×450mm food paper samples for mechanical property testing; the food paper samples were cut into 450mm×450mm length food paper samples for adhesion testing; and the food paper samples were cut into 10mm×10mm pieces for degradation performance testing.

[0061] Mechanical properties: Referring to standard GB / T 12914-2018, a tensile testing machine was used to conduct tensile tests on food-grade paper samples at a tensile speed of 20±5 mm / min. The tensile strength and elongation at break of the samples in the longitudinal direction were measured. Higher tensile strength and elongation at break indicate better mechanical properties of the film.

[0062] Adhesion degree: Referring to standard GB / T 36392-2025, the paper base of the sample is peeled off from the coating with a knife, and the exposed film surface is visually inspected to determine the percentage of the area on the film surface with paper fibers. This is repeated 3 times. The higher the percentage of the area, the higher the degree of interlayer adhesion between the paper base and the coating of the food paper sample.

[0063] Degradability: Referring to standard GB / T 39951-2021, the sample was uniformly mixed with compost substrate and placed in a vermiculite activation reactor. The cumulative carbon dioxide release of the sample was recorded at 142 days, and the biodegradation rate at the corresponding time point was calculated accordingly. The higher the biodegradation rate, the better the degradability.

[0064] Table 1 Performance Test Table for Food-Grade Paper

[0065] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Longitudinal tensile strength (MPa) 8.6 8.9 8.7 6.7 4.8 4.1 Longitudinal elongation at break (%) 3.4 3.6 3.3 1.9 1.2 0.9 Adhesion degree / (%) 95.4 96.1 95.8 82.3 65.0 46.9 Biodegradation rate at 142 days (%) 94.9 95.3 94.6 76.4 76.8 76.5

[0066] As shown in Table 1, the performance of Examples 1-3 is superior to that of Comparative Examples 1-3. This demonstrates that by introducing modified nanocellulose with a controllable grafting structure into the PLA coating system, the interfacial bonding state between the two is significantly improved. Without changing the composition of the plant fiber paper base and the PLA biodegradable system, the problem of poor adhesion and stress concentration caused by the polarity difference between the PLA coating and the paper base is effectively alleviated. This allows the load to be transferred more evenly at the interface, thereby significantly improving the mechanical properties and interlayer bonding reliability of food-grade paper. A more stable and continuous interfacial structure is formed between the coating layer and the paper base, achieving a good balance between mechanical properties, interlayer bonding reliability, and biodegradability in food-grade paper, which better meets the comprehensive requirements of food takeaway packaging for safety, durability, and environmental protection.

[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A method for preparing food-grade paper based on plant fiber and PLA coating, characterized in that, Includes the following steps: Step 1: Using TEMPO-oxidized nanocellulose as the reaction matrix, brominated initiating groups are introduced on its surface through acylation reaction to obtain bromine-containing nanocellulose with controllable initiation activity. At the same time, organosilicon-modified alkyl acrylate with flexible siloxane segments is prepared. Using bromine-containing nanocellulose as the initiation center, acrylate segments are grown in situ from the cellulose surface and synergistically grafted onto its surface to obtain modified nanocellulose filler. Step 2: Modified nanocellulose filler and polylactic acid are melt-blended in a twin-screw extruder to obtain polylactic acid composite coating masterbatch. The masterbatch is then melt-coated onto the surface of plant fiber paper base through a coating extrusion process. After pressing and cooling, a continuous and dense PLA coating layer is formed, and finally, food paper material based on plant fiber and PLA coating is obtained.

2. The method for preparing food-grade paper based on plant fiber and PLA coating according to claim 1, characterized in that, The preparation process of the modified nanocellulose filler in step one is as follows: Methacrylate, organosilicon-modified alkyl acrylate, ethyl 2-bromoisobutyrate, bromine-containing nanocellulose, tetrahydrofuran, and N,N-dimethylformamide were placed in a reactor under nitrogen atmosphere and ultrasonically dispersed at 25-35℃ for 20-30 min. Cuprous bromide was added, and the reaction was carried out at 50-70℃ for 6-8 h. The reaction solution was added to anhydrous methanol to precipitate the mixture, filtered, and the filter cake was vacuum dried to constant weight to obtain the modified nanocellulose filler.

3. The method for preparing food-grade paper based on plant fiber and PLA coating according to claim 2, characterized in that, The ratio of the amounts of methacrylate, organosilicon-modified alkyl acrylate, ethyl 2-bromoisobutyrate, bromine-containing nanocellulose, cuprous bromide, tetrahydrofuran, and N,N-dimethylformamide is 40-60g: 15-25g: 10-20g: 200-300mg: 100-150mL: 10-15mL.

4. The method for preparing food-grade paper based on plant fiber and PLA coating according to claim 3, characterized in that, The preparation process of the organosilicon-modified alkyl acrylate is as follows: Isophorone diisocyanate and monohydroxypropyl-terminated silicone oil were placed in a reaction vessel under a nitrogen atmosphere and stirred at 25-35°C for 10-30 min. Dibutyltin dilaurate was added, and the reaction was carried out at 35-45°C for 1-2 h. Then 4-hydroxybutyl acrylate was added, and the reaction was continued at 45-55°C for 1-2 h. Anhydrous ethanol was added for precipitation, the supernatant was removed, and the lower product was vacuum dried to constant weight to obtain organosilicon-modified alkyl acrylate.

5. The method for preparing food-grade paper based on plant fiber and PLA coating according to claim 4, characterized in that, The mass ratio of isophorone diisocyanate, monohydroxypropyl capped silicone oil, dibutyltin dilaurate, and 4-hydroxybutyl acrylate is 4-6:30-50:0.1-0.3:5-7.

6. The method for preparing food-grade paper based on plant fiber and PLA coating according to claim 3, characterized in that, The preparation process of the bromine-containing nanocellulose is as follows: TEMPO-oxidized nanocellulose, 4-dimethylpyridine, and N,N-dimethylformamide were placed in a reaction vessel under a nitrogen atmosphere and stirred at 0-10℃ for 20-40 min. 2-bromoisobutyryl bromide was added, and the reaction was carried out at 25-35℃ for 18-24 h. The mixture was then filtered, washed, and vacuum dried to constant weight to obtain bromine-containing nanocellulose.

7. The method for preparing food-grade paper based on plant fiber and PLA coating according to claim 6, characterized in that, The ratio of TEMPO-oxidized nanocellulose, 4-dimethylpyridine, 2-bromoisobutyryl bromide and N,N-dimethylformamide is 15-35g: 1-2g: 4-8g: 200-300mL.

8. The method for preparing food-grade paper based on plant fiber and PLA coating according to claim 1, characterized in that, In step two, the temperatures of each zone of the twin-screw extruder are 80-100℃, 100-120℃, 120-140℃, 140-160℃, 160-170℃, and 170-180℃ respectively, and the die temperature is 170-180℃.

9. The method for preparing food-grade paper based on plant fiber and PLA coating according to claim 1, characterized in that, In step two, the coating extrusion process involves a barrel temperature of 160-180℃ and a die temperature of 180-190℃ for the coating extruder.

10. A food-grade paper material based on plant fiber and PLA coating, characterized in that, It is prepared by the method for preparing food paper based on plant fiber and PLA coating as described in any one of claims 1-9.