Nano cellulose film and preparation method and application thereof

Abstract

The invention provides a nano cellulose film as well as a preparation method and application thereof. The preparation method comprises the following steps: preparing an aqueous dispersion from carboxylated cellulose nanofibrils and carboxylated cellulose nanocrystals; the mass ratio of the carboxylated cellulose nanofibrils to the carboxylated cellulose nanocrystals is 1: (0.25-4); adding a plasticizer into the aqueous dispersion, and uniformly stirring to obtain a composite solution; on the basis that the mass content of the carboxylated cellulose nanofibrils and the carboxylated cellulose nanocrystals is 100%, the mass content of the plasticizer is 10-30%; the composite solution is subjected to ultrasonic treatment to remove bubbles, and a film coating solution is obtained; pouring film covering liquid to uniformly distribute the film covering liquid; and drying to obtain the nano cellulose film. The technical problem to be solved is how to realize the synergistic improvement of the mechanical property and the high barrier property of the nano-cellulose film and realize the environment-friendly, efficient and low-cost large-scale preparation, so that the nano-cellulose film is more suitable and practical.

Description

Technical Field

[0001] This invention relates to the field of packaging materials technology, and in particular to a nanocellulose film, its preparation method, and its application. Background Technology

[0002] Nanocellulose, as a widely available renewable bio-based material, has great potential for application in the field of barrier materials due to its excellent mechanical properties, low density, high specific surface area, and good film-forming properties. However, existing nanocellulose film preparation technologies have key shortcomings in terms of material systems and preparation processes, making it difficult to meet the requirements of biodegradable packaging materials for comprehensive performance and production feasibility.

[0003] In terms of material systems, existing films mostly use aqueous dispersions of single cellulose nanofibers (CNF) or cellulose nanocrystals (CNC) as raw materials, which are wet-formed and then dehydrated to obtain self-supporting films. However, while single CNF films can form a network and have a certain degree of toughness, they are significantly hydrophilic, and their barrier properties and dimensional stability are easily affected by environmental humidity. Single CNC films, although highly crystallinity and rigid, are prone to stress concentration during particle accumulation and drying shrinkage, leading to film brittleness. Although some researchers have attempted to optimize the combination of the two, the interfacial compatibility problem has not yet been solved, making it difficult to balance mechanical properties and high barrier properties.

[0004] In terms of preparation processes, mainstream technologies have obvious drawbacks: vacuum filtration membrane formation relies on permeation dehydration, and increasing membrane thickness leads to increased filtration resistance and decreased membrane formation efficiency. During scale-up preparation, it is difficult to control the consistency of membrane thickness and structure, resulting in fluctuations in barrier performance. Electrospinning membrane formation is a complex process, sensitive to parameters such as voltage and flow rate, and difficult to control consistency. Furthermore, fiber network membranes require additional composite base membranes or densification treatment, increasing process costs. Although solution casting / casting is suitable for large-area membrane formation, it is highly sensitive to drying conditions, easily causing uneven shrinkage and structural defects. Moreover, high barrier performance depends on multi-component fillers, and the formulation and process window are complex, which is not conducive to low-cost large-scale production.

[0005] In addition, although existing studies have adopted strategies such as chemical crosslinking, surface modification, and nanofiller composites to optimize performance, they generally suffer from problems such as complex preparation processes, high costs, and insufficient controllability. Summary of the Invention

[0006] The main objective of this invention is to provide a nanocellulose film, its preparation method, and its application. The technical problem to be solved is how to address the issues of existing nanocellulose film material systems, such as difficulty in balancing mechanical properties and high barrier properties, low film formation efficiency, poor consistency, numerous structural defects, and high cost, as well as the complexity and insufficient controllability of existing performance optimization strategies. The invention aims to achieve a synergistic improvement in the mechanical properties and high barrier properties of nanocellulose films, while simultaneously enabling environmentally friendly, efficient, and low-cost large-scale preparation, thus making it more suitable for practical applications.

[0007] The objective of this invention and the technical problem it solves are achieved through the following technical solution. A method for preparing a nanofiber cellulose film according to this invention includes the following steps: S1 prepares an aqueous dispersion of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals; the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals is 1:0.25~4; S2 adds a plasticizer to the aqueous dispersion and stirs until homogeneous to obtain a composite solution; based on the mass content of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals being 100%, the mass content of the plasticizer is 10-30%; S3 involves ultrasonically treating the composite solution to remove air bubbles, resulting in a coating solution; S4 coating solution is poured to ensure uniform distribution; then dried to obtain a nanocellulose film.

[0008] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0009] Preferably, in the preparation method, the carboxylated cellulose nanofibers are prepared by TEMPO oxidation, potassium permanganate oxidation, and carboxymethylation, with a content of 0.5~2.5 mmol / g; the carboxylated cellulose nanocrystals are prepared by sodium hypochlorite oxidation, TEMPO oxidation, and carboxymethylation, with a content of 0.5~2.5 mmol / g.

[0010] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0011] Preferably, in the preparation method, the carboxylated cellulose nanofibers have a diameter of 1-5 nm and a length of 400-600 nm; the carboxylated cellulose nanocrystals have a diameter of 1-5 nm and a length of 100-200 nm.

[0012] Preferably, in the preparation method, the preparation of the aqueous dispersion includes: dispersing carboxylated modified cellulose nanofiber dispersion and carboxylated modified cellulose nanocrystal dispersion in deionized water in a certain proportion, stirring until completely dispersed to obtain an aqueous dispersion; the mass concentration of the aqueous dispersion is 0.1~3%.

[0013] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0014] Preferably, in the preparation method, the plasticizer in step S2 is selected from at least one of glycerol, sorbitol, polyethylene glycol, propylene glycol and xylitol; the stirring speed is 5000~10000 rpm and the stirring time is 0.5~2 hours.

[0015] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0016] Preferably, in the preparation method, the ultrasonic treatment time in step S3 is 0.5 to 5 hours.

[0017] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0018] Preferably, in the preparation method, the drying in step S4 involves allowing the wet film of the coating solution to stand at 40-80°C until the film is completely dry.

[0019] The objective of this invention and the technical problem it solves are achieved by the following technical solution. A nanocellulose film according to this invention is composed of carboxylated cellulose nanofibers, carboxylated cellulose nanocrystals, and a plasticizer; the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals is 1:0.25~4; based on a mass content of 100% for carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals, the mass content of the plasticizer is 10~30%.

[0020] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0021] Preferably, the nanocellulose film has a thickness of 0.08~0.12 mm; the nanocellulose film has a tensile strength ≥43 MPa, an elongation at break of 1.9%~9.2%, and a water vapor permeability of 4.4~5.2×10⁻⁶. -10 g / (m·s·Pa), oxygen permeability 21.86~60.01cm 3 / (m 2 It can degrade naturally in soil within 60 days (24 h, 0.1 MPa).

[0022] The objective of this invention and the technical problem it solves are achieved through the following technical solution: An application of the aforementioned nanocellulose film in the field of packaging materials technology, according to this invention.

[0023] By employing the above technical solutions, the nanocellulose film, its preparation method, and its application proposed in this invention have at least the following beneficial effects: This invention proposes a nanocellulose film, its preparation method, and its application. Starting from the intrinsic properties of the material, it imparts good interfacial compatibility between cellulose nanofibers (CNF) and cellulose nanocrystals (CNC) solely through carboxylation modification. Furthermore, it regulates the synergistic effect of the two components with a precise mass ratio of 1:0.25~4. Combined with a plasticizer comprising 10~30% of the total mass of the carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals, and employing a simplified aqueous phase preparation, ultrasonic defoaming, and casting-drying process, a pure bio-based system consisting of only three components is constructed. This achieves simultaneous breakthroughs in performance, preparation efficiency, and environmental friendliness. The approach is concise and specifically addresses the technical problems existing in current technologies, completely overcoming the limitations of existing techniques. Details are as follows: This invention uses only carboxylated CNF and carboxylated CNC as core raw materials to prepare an aqueous dispersion at a mass ratio of 1:0.25~4. This completely solves the problems of strong hydrophilicity, poor barrier properties and dimensional stability of single CNF films, strong rigidity of single CNC films but easy stress concentration and brittleness due to drying shrinkage, and the inability of existing simple composite systems to solve the problem of poor interfacial compatibility between the two. This invention achieves intrinsic compatibility between CNF and CNC through carboxylation modification. By adjusting the ratio, the toughness and film-forming properties of CNF are effectively mitigated, thus alleviating the brittleness problem caused by CNC particle accumulation. At the same time, the high crystallinity and rigidity of CNC compensate for the shortcomings of CNF's strong hydrophilicity and insufficient barrier properties, forming a positive complementarity. Furthermore, the hydroxyl functional groups in the plasticizer interact with the carboxyl functional groups on the surface of the carboxylated CNC and CNF of this invention through hydrogen bonding. This interaction, combined with the interfacial compatibility of carboxylation modification and the synergistic ratio of CNC and CNF, creates a synergistic effect, ultimately achieving a synergistic improvement in the mechanical properties and high barrier properties of the film. This design approach is completely different from the passive optimization logic of existing technologies.

[0024] Meanwhile, the preparation process design of this invention is highly compatible with the material system, simple and efficient, and completely solves many drawbacks of existing preparation processes. This invention completes film preparation through simple water dispersion, plasticizing mixing, ultrasonic defoaming, and casting drying. The entire process uses an aqueous system, eliminating the need for complex steps such as vacuum filtration for permeation dehydration and electrospinning for layering and densification, and requiring no organic solvents. This effectively solves the pain points of existing processes, such as low film formation efficiency, parameter sensitivity, poor consistency, and high cost. Specifically, the ultrasonic defoaming step thoroughly removes air bubbles from the composite solution, preventing pores in the film after casting, ensuring film density, and further improving barrier properties. The drying method after casting is well-suited to the shrinkage characteristics of the material system of this invention, effectively avoiding structural defects such as uneven shrinkage, warping, and cracks caused by improper drying conditions in existing casting methods. This significantly improves film quality and consistency while reducing equipment investment and preparation costs, possessing strong industrial feasibility and truly achieving efficient, low-cost, and easily scalable green preparation.

[0025] Furthermore, the material system constructed in this invention consists only of carboxylated CNF, carboxylated CNC, and plasticizer, without introducing any polymer matrix or multi-component functional filler. This completely solves the problems of decreased film degradability and environmental pollution caused by the use of organic solvents in the prior art. It not only fully retains the environmental advantages of nanocellulose being renewable and biodegradable, but also achieves high performance through intrinsic material synergy, breaking through the technical bottleneck of the prior art where high performance and environmental protection are difficult to balance.

[0026] More importantly, this invention achieves controllable material system performance by precisely controlling the ratio of carboxylated CNF and CNC, as well as the amount of plasticizer. Combined with a simplified preparation process, this creates a virtuous cycle where intrinsic material optimization drives process simplification, process simplification ensures cost control, and cost control promotes application implementation. The nanocellulose film prepared by this invention precisely matches the core requirements of the packaging materials field for mechanical strength, barrier properties, and environmental degradability, filling the technological gap in the field of degradable high-barrier packaging materials where high performance and ease of industrialization are difficult to achieve simultaneously. This propels nanocellulose materials from laboratory research to practical industrial applications. This invention requires no complex equipment or redundant external components; it solves multiple pain points that existing technologies have long been unable to address simply through the bidirectional adaptation of intrinsic material synergy and process simplification. Its original design concept and concise, efficient technical solution possess significant creativity, practicality, industrial value, and environmental significance.

[0027] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0028] Figure 1 These are mechanical property diagrams of the nano-cellulose films in Examples 1-3 of this invention; Figure 2 These are mechanical property diagrams of the nano-cellulose films of Comparative Examples 1 and 3-6 of the present invention; Figure 3 The appearance changes of the films of Example 2 and Comparative Example 3 in a soil environment; Figure 4 This is a photograph of the brittle / warped film of Comparative Example 5 of the present invention. Detailed Implementation

[0029] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following, in conjunction with the appended tables and preferred embodiments, details the specific implementation methods and effects of a nanocellulose film, its preparation method, and its application according to the present invention. In the following description, different "embodiments" or "embodiments" do not necessarily refer to the same embodiment. Furthermore, the results of one or more embodiments can be combined in any suitable manner. These embodiments are provided to make the invention thorough and complete, and to fully express the scope of the invention to those skilled in the art. It should be noted that, unless otherwise specifically stated, the relative arrangement of components and steps, material composition, numerical expressions, and values ​​described in these embodiments should be interpreted as merely exemplary and not as limiting.

[0030] This invention proposes a method for preparing nanofiber cellulose films, which includes the following steps: First, carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals are prepared into an aqueous dispersion. The carboxylated cellulose nanofibers and nanocrystals can be obtained according to conventional methods in the art. Alternatively, the cellulose nanofibers and nanocrystals can be modified by carboxylation. Carboxylation modification methods are standard practices in the art, and will not be specifically described in this invention.

[0031] In some specific embodiments of the present invention, cellulose nanofibers are preferably modified by TEMPO oxidation, potassium permanganate oxidation, or carboxymethylation to obtain carboxylated cellulose nanofibers; the reaction solution obtained by the above modification reaction does not require post-treatment and is used as a raw material in the form of a dispersion.

[0032] In some specific embodiments of the present invention, cellulose nanocrystals are preferably modified by sodium hypochlorite oxidation, TEMPO oxidation, or carboxymethylation to obtain carboxylated cellulose nanocrystals; the reaction solution obtained by the above modification reaction does not require post-treatment and is used as a raw material in the form of a dispersion.

[0033] A moderate degree of carboxylation modification ensures sufficient carboxyl groups are formed on the surfaces of both materials, effectively improving their interfacial compatibility and avoiding the delamination and aggregation problems caused by weak interfacial interactions in existing composite systems. However, it also avoids excessive modification that could damage their intrinsic structures, preserving the strength and toughness of CNF and the high rigidity of CNC, thus laying the foundation for their synergistic performance and the fabrication of high-performance films. To better control the interfacial interactions between CNC and CNF, this invention preferably controls the carboxyl content of the carboxylated cellulose nanofibers to be 0.5–2.5 mmol / g and the carboxyl content of the carboxylated cellulose nanocrystals to be 0.5–2.5 mmol / g, thereby ensuring good compatibility.

[0034] To balance the performance advantages of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals, fully utilize the toughness and film-forming properties of carboxylated cellulose nanofibers, alleviate the stress concentration and brittleness problems caused by the accumulation of carboxylated cellulose nanocrystal particles, and at the same time, leverage the high crystallinity and rigidity of carboxylated cellulose nanocrystals to compensate for the shortcomings of strong hydrophilicity and insufficient barrier properties of carboxylated cellulose nanofibers, thereby achieving a synergistic improvement in the performance of both, ensuring that the film can achieve both excellent mechanical properties and high barrier properties, and avoiding the inherent defects of single-component films, this invention preferably controls the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals to be 1:0.25~4.

[0035] To further optimize the dispersion and stacking effect of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals, and avoid agglomeration problems caused by excessive size or stress concentration problems caused by excessive size, the aspect ratio of carboxylated cellulose nanofibers (400~600nm in length) and the short rod-shaped structure of carboxylated cellulose nanocrystals (100~200nm in length) are matched to form a dense and uniform film structure during film formation, effectively reducing film porosity, improving the mechanical strength and dimensional stability of the film, enhancing its barrier properties, and ensuring the integrity of film formation, avoiding structural defects such as warping and cracks. In this invention, the diameter of carboxylated cellulose nanofibers is preferably controlled to be 1~5nm and the length to be 400~600nm, and the diameter of carboxylated cellulose nanocrystals is preferably controlled to be 1~5nm and the length to be 100~200nm.

[0036] In some specific embodiments of the present invention, the preparation of the aqueous dispersion includes: dispersing carboxylated modified cellulose nanofiber dispersion and carboxylated modified cellulose nanocrystal dispersion in deionized water in a certain proportion, stirring until completely dispersed, to obtain the aqueous dispersion.

[0037] In some specific embodiments of the present invention, the preparation of the aqueous dispersion includes: dispersing a carboxylated modified cellulose nanofiber dispersion in deionized water and stirring until completely dispersed to obtain a first aqueous dispersion; dispersing a carboxylated modified cellulose nanocrystal dispersion in deionized water and stirring until completely dispersed to obtain a second aqueous dispersion; and then mixing the first and second aqueous dispersions with deionized water in a certain proportion and stirring evenly to obtain an aqueous dispersion.

[0038] In the above technical solution, the key to the preparation of the aqueous dispersion lies in strictly controlling the precise ratio of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals to ensure that the mass ratio of the two always meets the core requirement of 1:0.25~4, which lays the foundation for the synergistic performance of the two in the future. At the same time, it is necessary to ensure that the two carboxylated cellulose nanofiber dispersions are fully stirred in deionized water until completely dispersed, without agglomeration or stratification.

[0039] In some specific embodiments of the present invention, in order to ensure that the carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals are uniformly dispersed in the aqueous dispersion without obvious agglomeration, and to adapt to the subsequent plasticizing and mixing, ultrasonic defoaming and casting film-forming processes, so as to ensure that the final film structure is dense and the performance is stable, and to take into account both film-forming efficiency and product qualification rate, the mass concentration of the aqueous dispersion is preferably controlled at 0.1~3%. If the concentration is too high, the dispersion viscosity will be too high, and the carboxylated cellulose nanofibers and nanocrystals will easily agglomerate, making it difficult to achieve complete dispersion through stirring. Furthermore, subsequent ultrasonic defoaming will not completely remove air bubbles, and the coating solution will not spread evenly during casting. The final film is prone to structural defects such as porosity, uneven thickness, and warping. It will also exacerbate stress concentration during film formation, leading to increased film brittleness and decreased barrier properties. If the concentration is too low, the content of carboxylated cellulose nanofibers and nanocrystals in the dispersion will be too low, resulting in an excessively thin wet film after casting. This film is prone to breakage during drying, making it difficult to form a continuous and complete self-supporting film. It will also reduce film formation efficiency, and the film structure will be loose with increased porosity, failing to achieve excellent mechanical strength and high barrier properties, thus failing to meet the requirements of packaging materials. Through extensive experimental verification, this invention further optimizes the mass concentration of the aqueous dispersion to be 0.5~2%; even more preferably, it optimizes it to be 0.8~1.2%.

[0040] After the aqueous dispersion is prepared, a plasticizer is added to the aqueous dispersion and stirred evenly to obtain a composite solution.

[0041] In some specific embodiments of the present invention, the plasticizer is preferably at least one of glycerol, sorbitol, polyethylene glycol, propylene glycol, and xylitol. The purpose of this configuration is to adapt to the aqueous preparation system of the present invention, ensuring that the plasticizer can be uniformly mixed with the aqueous dispersion of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals without stratification or precipitation. At the same time, these plasticizers all have good biocompatibility and water solubility, and have a strong interaction with the carboxyl groups on the surface of the carboxylated modified cellulose nanofibers. They can effectively exert a plasticizing effect, alleviate the stress concentration generated by the cellulose nanofibers during the drying and film formation process, improve the brittleness of the film, enhance the flexibility and integrity of the film, and avoid defects such as cracks and brittle fractures in the film. Furthermore, this type of plasticizer is environmentally friendly and biodegradable, does not introduce harmful components, and can fully retain the renewable and degradable advantages of the nanocellulose film of this invention. It does not affect the mechanical strength and barrier properties of the film, effectively avoiding the problem of decreased film degradability caused by the use of non-environmentally friendly plasticizers in the prior art. At the same time, this type of plasticizer is widely available and low in cost, which is suitable for the low-cost and large-scale preparation requirements of this invention. It is highly consistent with the core concept of the invention of minimalist process and green environmental protection. Moreover, its plasticizing effect is mild and moderate. It will not cause the film structure to become loose and the barrier properties to decrease due to excessive plasticizing, nor will it fail to alleviate the brittleness defect of CNC due to insufficient plasticizing. It can accurately match the synergistic system of carboxylated CNF and CNC, providing a guarantee for subsequent ultrasonic defoaming, casting film formation and film performance stability.

[0042] In the above technical solution, the amount of plasticizer added is crucial. Preferably, the plasticizer content is 10-30% based on a mass content of 100% for carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals. If the amount of plasticizer added is too low, it cannot effectively alleviate the stress concentration generated during the drying and film formation process of carboxylated cellulose nanocrystals, resulting in a brittle film prone to structural defects such as cracks and brittle fracture. Simultaneously, it is difficult to improve the interfacial bonding state between carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals, failing to fully utilize their synergistic effects, leading to insufficient film toughness and difficulty in forming a continuous and complete self-supporting film, thus failing to meet the requirements of packaging materials. The basic mechanical requirements are as follows: if the content is too high, it will lead to excessive residual plasticizer in the film and easy precipitation, which will destroy the dense structure of the film, increase the porosity inside the film, and significantly reduce its oxygen and water vapor barrier performance. At the same time, excessive plasticizer will weaken the interaction between carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals, resulting in a decrease in the core mechanical properties of the film such as tensile strength. It will also increase the preparation cost, and defects such as excessive shrinkage and warping are prone to occur during the drying and film formation process, affecting the dimensional stability of the film and making it impossible to achieve a synergistic balance between mechanical properties and barrier performance. Through a large number of experiments, the present invention further optimizes the plasticizer mass content to be 20-30%.

[0043] To ensure thorough and uniform mixing of the plasticizer with the aqueous dispersion of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals, allowing the plasticizer to fully contact the surfaces of the two types of cellulose nanofibers to exert its plasticizing effect and optimize the interfacial bonding state, while avoiding defects such as porosity, warping, and uneven performance in subsequent film formation due to uneven local concentration of the plasticizer, and balancing mixing efficiency with avoiding insufficient dispersion due to insufficient stirring or excessive air bubbles introduced by excessive stirring, which would increase the burden on subsequent ultrasonic defoaming, this invention preferably controls the stirring speed during plasticizer addition to 5000~10000 rpm and the stirring time to 0.5~2 hours.

[0044] The composite solution is then ultrasonically treated to remove air bubbles, resulting in a coating solution. In this step, the frequency, power, and other process parameters of the ultrasonic treatment can be set using conventional methods in the art, as long as they effectively remove bubbles; this invention does not impose specific limitations on these parameters.

[0045] In some specific embodiments of the present invention, the ultrasonic treatment time is 0.5 to 5 hours. The purpose of this setting is to ensure that free air bubbles and micro-nano-sized fine air bubbles introduced during the stirring process in the composite solution are completely removed, so as to avoid the formation of structural defects such as pores and pinholes after the coating solution is cast into a film, which would damage the density of the film. This ensures that the film has excellent oxygen and water vapor barrier properties and mechanical strength. At the same time, this time range takes into account both the defoaming effect and the preparation efficiency. It avoids the problem of insufficient defoaming due to too short ultrasonic time, which would lead to structural defects and uneven performance in the subsequent film formation. It also prevents the increase in energy consumption and decrease in preparation efficiency due to too long ultrasonic time, and even the destruction of the uniform dispersion state and interfacial bonding effect of carboxylated CNF and CNC due to long-term ultrasonic disturbance. This ensures the stability of the coating solution and lays the foundation for the formation of a continuous, flat and dense wet film in the subsequent casting.

[0046] Finally, the coating solution is poured in to ensure even distribution; then dried to obtain a nanocellulose film.

[0047] The above technical solution uses a solution casting method to form a film, which is simple to operate and fits the core concept of the minimalist preparation process of this invention. Specifically, the coating liquid can be slowly poured into a clean and flat mold (such as a polytetrafluoroethylene mold). Under laboratory conditions, it can be poured into a clean petri dish. The coating liquid can be spread evenly by scraping with a scraper or by natural leveling. The thickness of the wet film can be controlled to meet the requirements of the final product. Then, the mold containing the wet film is placed in a constant temperature environment of 40~80℃ to dry until the moisture is completely evaporated, the film is formed and does not stick to the mold. The nanocellulose film can then be peeled off. The drying temperature range is precisely adapted to achieve slow and uniform evaporation of moisture, effectively avoiding the problem of stress concentration caused by inconsistent shrinkage rates between the film surface and interior due to excessively fast drying speed in existing casting methods. At the same time, it can match the interfacial bonding characteristics of carboxylated cellulose nanofibers (CNF) and carboxylated cellulose nanocrystals (CNC) and the action rules of plasticizers, allowing the two types of carboxylated cellulose nanofibers to accumulate in an orderly manner during the drying process, forming a dense and uniform network structure. The plasticizer is simultaneously and uniformly dispersed in this network, further alleviating internal stress during the drying process, optimizing film flexibility, and ensuring that the synergistic performance advantages of carboxylated CNF and CNC are fully utilized. This invention employs a solution casting method that eliminates the need for complex film-forming equipment. Combined with a constant-temperature static drying process at 40-80°C, it significantly improves film-forming efficiency and consistency while ensuring the prepared film is continuous, intact, and has a smooth surface free from structural defects such as warping, cracks, and pores. Simultaneously, it fully preserves the intrinsic advantages of the two types of nanocellulose, achieving a synergistic improvement in mechanical strength and high barrier properties. Furthermore, the low-temperature drying process is energy-efficient and simple to operate, requiring no additional densification treatment. It is well-suited for low-cost, large-scale preparation needs, and eliminates the need for organic solvent evaporation, thus ensuring environmental friendliness. This invention completely solves the pain points of existing solution casting / casting methods, such as sensitivity to drying conditions, susceptibility to structural defects, and complex processes, laying the foundation for the industrial application of thin films.

[0048] In some specific embodiments of the present invention, the drying temperature is preferably 40~80℃ and the drying time is 6~12h; through extensive experimental verification, the drying temperature is further preferably 65~80℃ and the drying time is 6~8h.

[0049] The present invention also proposes a nanocellulose film, which is composed of carboxylated cellulose nanofibers, carboxylated cellulose nanocrystals and plasticizer; the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals is 1:0.25~4; and the mass content of plasticizer is 10~30% based on the mass content of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals being 100%.

[0050] The aforementioned nanocellulose film is a continuous and complete self-supporting film with a thickness that can be controlled between 0.08 and 0.12 mm according to packaging requirements. It has a smooth surface, no pores or cracks, uniform texture, and good toughness. It can be flexibly cut to fit different packaging specifications. Its component combination and structural formation are based on the core technology concept of this invention. There is no need to add polymer matrix, multi-component functional filler, or organic solvent. It can achieve the dual advantages of high performance and environmental protection through the precise ratio and synergistic effect of the three components. Its core mechanism lies in the fact that carboxylation modification endows carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals with sufficient carboxyl groups on their surfaces. The two achieve good interfacial compatibility through the interaction of these carboxyl groups. By precisely controlling the mass ratio of 1:0.25~4, the aspect ratio of carboxylated cellulose nanofibers and the short rod-like structure of carboxylated cellulose nanocrystals are matched to each other, forming a dense and uniform three-dimensional network structure after film formation. The carboxylated cellulose nanofibers exert their advantages of toughness and film-forming properties, alleviating the stress concentration caused by the accumulation of carboxylated cellulose nanocrystal particles. The carboxylated cellulose nanocrystals, with their high crystallinity and rigidity, compensate for the shortcomings of carboxylated cellulose nanofibers in terms of strong hydrophilicity and insufficient barrier properties. The two form a positive complementary property. The plasticizer is uniformly dispersed in the network structure, exerting a gentle plasticizing effect, further alleviating the internal stress of the network, improving the brittleness of the film, and at the same time not destroying the intrinsic structure and performance advantages of the two types of nanocellulose, ensuring the stability and density of the network structure. The resulting technological advantages are significant, completely solving the industry pain points of existing nanocellulose films, which either struggle to balance mechanical and barrier properties or rely on complex modifications or additives leading to decreased degradability and increased costs. This new film not only possesses excellent mechanical properties and high barrier performance, with a tensile strength exceeding 43 MPa, an elongation at break maintained at 1.9%–9.2%, and a water vapor permeability as low as 4.4–5.2 × 10⁻⁶. -10 g / (m·s·Pa), oxygen permeability 21.86~60.01cm 3 / (m 2 With a pressure of 0.1 MPa (24 h), it can effectively block the penetration of water vapor and oxygen, ensuring the storage stability of the contents of the packaging. It can also fully retain the renewable and biodegradable characteristics of nanocellulose, and can naturally degrade in the soil environment within 60 days, which is in line with the development trend of green and environmentally friendly packaging materials. At the same time, its components are simple and widely available, and the preparation process does not require complex equipment and cumbersome procedures. It is suitable for low-cost and large-scale production, and the product has uniform and stable performance with no harmful residues. It can be safely used in food, medicine and other fields with high requirements for the environmental protection and safety of packaging materials. It fills the technical gap in the field of biodegradable high-barrier packaging materials where it is difficult to achieve "high performance, easy industrialization and all-round environmental protection", which significantly enhances the application value and market competitiveness of nanocellulose materials in the packaging field.

[0051] The aforementioned nanocellulose film is preferably prepared by the preparation method described above in this invention.

[0052] This invention also proposes an application of the aforementioned nanocellulose film in the field of packaging materials technology. The core of this invention is to apply the nanocellulose film to various packaging scenarios, such as food and pharmaceuticals. This film combines biodegradability, high barrier properties, and excellent mechanical properties, and can be mass-produced at low cost. It can directly replace traditional non-degradable packaging materials or degradable packaging materials with insufficient performance. It meets the requirements of packaging materials for impermeability, tear resistance, and dimensional stability, while also aligning with the industry's development needs for green environmental protection and low-cost industrialization. It fully leverages the comprehensive advantages of the film—high performance, environmental friendliness, and ease of mass production—to achieve a precise match between material performance and packaging application requirements.

[0053] The present invention will be further described below with reference to specific embodiments, but this should not be construed as a limitation on the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above description of the present invention still fall within the scope of protection of the present invention.

[0054] Unless otherwise specified, all materials and reagents mentioned below are commercially available products well known to those skilled in the art; unless otherwise specified, all methods described are methods known in the art. Unless otherwise defined, the technical or scientific terms used should have the ordinary meaning understood by those skilled in the art to which this invention pertains. Example 1

[0055] This embodiment prepares a nanocellulose film, including the following steps: 1) Carboxylation modification of cellulose nanofibers Carboxylation modification of cellulose nanofibers was performed using TEMPO oxidation to obtain a carboxylated cellulose nanofiber dispersion. The specific reaction conditions were as follows: pulp fibers were dispersed in an aqueous system containing NaClO and TEMPO, and the pH of the reaction system was maintained at 10 by adding NaOH solution. The dispersion was carried out at 10,000 rpm for 1 hour using a high-speed disperser. After centrifugation and washing with water until neutral, the dispersion was homogenized five times at 100 MPa using a high-pressure homogenizer. Analysis showed that the carboxylated CNFs had a diameter of 1–5 nm, a length of 400–600 nm, and a carboxyl functional group content of 2.1 mmol / g.

[0056] 2) Carboxylation modification of cellulose nanocrystals Carboxylation modification of cellulose nanocrystals was performed using sodium hypochlorite oxidation to obtain a carboxylated cellulose nanocrystal dispersion. The specific reaction conditions were as follows: pulp fibers were dispersed in an aqueous system containing NaClO and CuSO4, and the pH of the reaction system was maintained at 9-10 by dropwise addition of NaOH solution. The reaction was carried out at 60-80℃ with continuous stirring for 2-6 hours. After the reaction, the mixture was neutralized with hydrochloric acid, and soluble impurities and residual reagents were repeatedly removed by centrifugation and washing with water. The mixture was then circulated five times using a high-pressure homogenizer at 100 MPa. Analysis showed that the carboxylated CNCs had a diameter of 1-5 nm, a length of 100-200 nm, and a carboxyl functional group content of 1.5 mmol / g.

[0057] 3) Under stirring conditions, the carboxylated cellulose nanofiber dispersion was added to deionized water and magnetically stirred until it was completely dispersed and the solution was uniform and free of lumps; then, the carboxylated cellulose nanocrystal dispersion was added and stirring was continued until it was uniformly dispersed to obtain an aqueous dispersion; wherein, the mass ratio of carboxylated cellulose nanofiber to carboxylated cellulose nanocrystal solid content was 4:1, and the mass solid content of the aqueous dispersion was 1%.

[0058] 4) Add glycerol to the aqueous dispersion and stir further until completely dissolved to obtain a composite solution. The amount of glycerol added is 30% based on a 100% solids content of the aqueous dispersion.

[0059] 5) Use an ultrasonic cleaner to ultrasonically treat the composite solution for 1 hour to obtain the coating solution; the power is 200W.

[0060] 6) Pour the coating solution into a plastic petri dish and cast it into a film, ensuring that the solution is spread evenly.

[0061] 7) Then place it in a 75°C oven to dry for 6 hours until completely dry, to obtain a transparent nanocellulose film.

[0062] The thin film prepared in this embodiment was subjected to performance tests according to conventional testing methods in the art. The tensile strength and elongation at break of the thin film are as follows: Figure 1 The results are shown in Table 1; the oxygen permeability (OTR) and water vapor permeability (WVP) of the film are shown in Table 2. Example 2

[0063] Same as Example 1, except that the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals is 1:1.

[0064] The tensile strength and elongation at break of the film are as follows: Figure 1 As shown in the figure; the film thickness results are shown in Table 1; the oxygen permeability (OTR) and water vapor permeability (WVP) results of the film are shown in Table 2. The appearance changes of the film in the soil environment are shown in... Figure 3 As shown. Example 3

[0065] Same as Example 1, except that the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals is 1:4.

[0066] The tensile strength and elongation at break of the film are as follows: Figure 1 The results are shown in Table 1; the oxygen permeability (OTR) and water vapor permeability (WVP) of the film are shown in Table 2.

[0067] Depend on Figure 1 As can be seen, the tensile strength of the film in Example 1 is 43.32 MPa, exhibiting high load-bearing capacity. The tensile strength of the film in Example 2 is 44.11 MPa, similar to that of Example 1, indicating that the composite system can form an effective reinforcing network and maintain high strength under different proportions. The film in Example 3 also exhibits a high mechanical level (tensile strength 57.83 MPa), indicating that the contribution of the rigid reinforcing component is more prominent under this formulation. The elongation at break varies among the three examples, ranging from 1.99% to 9.17%. This difference is related to the division of labor between the two components in the composite network: the fiber entanglement and network structure of the carboxylated CNF is beneficial to improving ductility. The rigid skeleton of the carboxylated CNC tends to improve the rigidity and strength of the material, but stress concentration may occur when there is local agglomeration or interface inhomogeneity, resulting in a decrease in elongation. Overall, all three examples can achieve high strength, but they show some differences in strength-toughness matching, which provides a basis for subsequent formulation selection based on application requirements. Example 4

[0068] Similar to Example 1, the difference lies in that the carboxylation modification method for cellulose nanofibers is the potassium permanganate oxidation method. The specific reaction conditions are as follows: pulp fibers are dispersed in a dilute nitric acid solution, potassium permanganate (KMnO4) is added as an oxidant, and the reaction is carried out under acidic conditions (pH=3~4) and at a high temperature (usually 60~80℃) with stirring for 2~6 hours. After the reaction, excess KMnO4 is reduced with sodium sulfite or hydrogen peroxide, and then repeatedly centrifuged and washed with water until neutral. The mixture is then circulated 5 times under a high-pressure homogenizer at 100 MPa. The carboxylated CNFs were found to have a diameter of 1~5 nm, a length of 400~600 nm, and a carboxyl functional group content of 1.2 mmol / g. The oxygen permeability (OTR) and water vapor permeability (WVP) results of the carboxylated films are shown in Table 2. The mass ratio of cellulose nanofibers to carboxylated cellulose nanocrystals was 1:1; the plasticizer was propylene glycol, with an addition amount of 20%; and the drying process involved placing it in an oven at 65°C for 8 hours.

[0069] The thickness results of the film are shown in Table 1; the oxygen permeability (OTR) and water vapor permeability (WVP) results of the film are shown in Table 2. Example 5

[0070] Same as Example 1, except that the carboxylation modification method for cellulose nanocrystals is TEMPO oxidation; the specific reaction conditions are as follows: pulp fibers are dispersed in an aqueous system containing TEMPO and NaBr, NaClO is added under stirring, and NaOH is added to maintain the system at pH=10, followed by reduction with NaBH4 and dilution, centrifugation and dialysis purification; the carboxylated CNCs are found to have a diameter of 1~5nm, a length of 100~200nm, and a carboxyl functional group content of 1.4mmol / g; the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals is 1:1; the plasticizer is polyethylene glycol, with an addition amount of 25%; drying is performed by placing it in an oven at 80℃ for 6 hours.

[0071] The thickness results of the film are shown in Table 1; the oxygen permeability (OTR) and water vapor permeability (WVP) results of the film are shown in Table 2.

[0072] Table 1 Comparison of film thickness and standard deviation in Examples 1-5 sample Average thickness (mm) Standard deviation Number of measurements (n) Example 1 0.0962 0.00585 5 Example 2 0.1036 0.00397 5 Example 3 0.0884 0.00498 5 Example 4 0.1068 0.00356 5 Example 5 0.1097 0.00642 5 Table 1 shows the thickness test results of the films prepared in Examples 1-5. Five points were randomly selected for measurement on each sample. The average thickness of the samples ranged from 0.0868 to 0.1098 mm, generally within the same order of magnitude with minimal difference in film thickness. This relative consistency in thickness indicates that the casting and drying processes of this invention provide good control over the film formation process. This helps reduce defects such as film warping, shrinkage, and cracking caused by uneven solvent evaporation rates in traditional casting methods, and also reduces internal stress accumulation, thereby improving the uniformity and stability of the film quality.

[0073] Table 2. Results of Oxygen Transmission Rate (OTR) and Water Vapor Permeability (WVP) of the Thin Films in the Examples sample WVPg / (m·s·Pa) <![CDATA[OTRcm 3 / (m 2 ·24 h·0.1 MPa)]]> Example 1 <![CDATA[5.14×10 -10 ]]> 60.01 Example 2 <![CDATA[4.40×10 -10 ]]> 21.86 Example 3 <![CDATA[4.75×10 -10 ]]> 45.98 Example 4 <![CDATA[4.70×10 -10 ]]> 28.36 Example 5 <![CDATA[4.60×10 -10 ]]> 35.36 Table 2 shows the oxygen transmission rate (OTR) and water vapor permeability (WVP) test results for each embodiment. The test data shows that there are certain differences in the OTR of the carboxylated CNF / CNC composite films among the different embodiments. These differences are related to the microstructure and diffusion paths formed by the two components in the composite film: the CNF fiber network, while providing support, may form relatively more micropores or interconnected channels, making it easier for oxygen molecules to pass through. CNC, on the other hand, has a relatively dense packing characteristic, which helps increase the tortuosity of gas diffusion and reduce effective transport channels, thereby lowering the OTR. The dispersion uniformity of the carboxylated CNF / CNC and plasticizer under different formulations, the interfacial bonding, and the presence of local agglomeration / defects may also lead to the formation of local "fast channels," thus causing differences in OTR. Overall, all five embodiments demonstrate good oxygen barrier performance, showing only certain differences in barrier level, providing formulation selection space for different application needs.

[0074] Based on the oxygen barrier test, the water vapor barrier performance was further tested, and the results are shown in Table 2. The WVP values ​​of the composite films in different embodiments were generally on the same order of magnitude, approximately 4.4~5.2×(10-10 g / (m•s•Pa)). The differences between the embodiments may be related to the density of the composite film, the number of pores / defects, and the formation of hydrophilic channels: the fibrous network structure of carboxylated CNF may, to some extent, introduce micropores or interconnected channels, while carboxylated CNC helps to improve structural density and increase the tortuosity of the water vapor diffusion path. Meanwhile, different ratios of carboxylated CNF / CNC to plasticizer affect the uniformity of solution dispersion, further affecting the interfacial bonding and transport channels of the composite film, thus causing differences in WVP. Overall, all embodiments achieved relatively stable water vapor barrier performance, showing only certain differences in barrier level, providing formulation selection space for different application needs.

[0075] By adjusting the ratio of CNF, CNC, and plasticizer, different performance optimizations can be achieved in terms of mechanical strength, barrier properties, and flexibility to meet various application requirements. The composite film of this invention has high adjustability, and different ratios can be flexibly selected according to needs in practical applications, exhibiting superior overall performance, especially in terms of environmental protection, biodegradability, and mechanical properties.

[0076] Comparative Example 1 (Single Carboxylated Cellulose Nanofibers) Same as Example 1, except that: the aqueous dispersion is composed of carboxylated cellulose nanofibers with a solid content of 1% by mass, and no carboxylated cellulose nanocrystals are added.

[0077] The tensile strength and elongation at break of the film are as follows: Figure 2As shown in the figure; the oxygen permeability (OTR) and water vapor permeability (WVP) results of the membrane are shown in Table 3.

[0078] Table 3. Results of oxygen permeability (OTR) and water vapor permeability (WVP) of comparative films. sample WVPg / (m·s·Pa) <![CDATA[OTRcm 3 / (m 2 ·24 h·0.1 MPa)]]> Comparative Example 1 <![CDATA[7.42×10 -10 ]]> 44.87 Comparative Example 3 <![CDATA[8.81×10 -13 ]]> 75.77 Comparative Example 4 <![CDATA[6.95×10 -10 ]]> 74.93 Comparative Example 5 <![CDATA[3.48×10 -10 ]]> 15.62 Comparative Example 6 <![CDATA[7.28×10 -10 ]]> 49.59 The test results of Comparative Example 1 show that the overall performance of the single carboxylated CNF film is poor: mechanically, its tensile strength and load-bearing capacity are insufficient, and it is more prone to early failure under stress. In terms of barrier properties, its OTR value is high, and its water vapor barrier capability is weak. Compared with the carboxylated CNF / CNC composite film, the single carboxylated CNF film has significant disadvantages in both mechanical and barrier properties, thus proving the necessity and effectiveness of introducing carboxylated CNC to construct a composite structure to improve the overall performance of the film.

[0079] Comparative Example 2 (Single Carboxylated Cellulose Nanocrystals) Same as Example 1, except that: the aqueous dispersion is composed of carboxylated cellulose nanocrystals with a solid content of 1% by mass, and no carboxylated cellulose nanofibers are added.

[0080] The results show that the single carboxylated CNC system cannot form a continuous and complete self-supporting film under the preparation conditions of Example 1 of this invention. After drying, the film layer exhibits significant cracking, making it difficult to obtain a complete sample and thus impossible to conduct stable mechanical and barrier property tests. This comparative example illustrates that the single carboxylated CNC system has insufficient film-forming ability. However, the introduction of cellulose nanofibers and cellulose nanocrystals into the composite structure of this invention significantly improves film integrity and forms a continuous film, thereby providing a structural basis for subsequent improvements in mechanical and barrier properties.

[0081] Comparative Example 3 (Traditional polyvinylidene chloride film) Commercially available traditional PVDC films were selected as control samples, and their barrier properties were compared under the same test conditions. Results... Figure 2 As shown in Table 3, the test results indicate that the PVDC film has a strong water vapor barrier capability, and its water vapor permeability is superior to that of the composite film of this invention. However, in terms of oxygen barrier performance, the oxygen permeability (OTR) of the PVDC film is not as good as that of the composite film of this invention.

[0082] Figure 3 The image shows a comparison of the appearance changes of the films from Example 2 and Comparative Example 3 in a soil environment. Figure 3As can be seen, under the same soil burial conditions, the film of Example 2 showed a clear degradation / disappearance trend over time. The sample size and integrity decreased significantly on day 30, and it was practically unidentifiable within the photographed area on day 60, indicating that the composite film has good degradability in the soil environment. In contrast, Comparative Example 3 maintained a relatively intact sheet-like appearance from day 1 to day 60, with only surface contamination or changes in attachments, and its overall morphology remained basically stable. The above comparative results show that, compared to Comparative Example 3, the composite film of the present invention possesses superior environmental friendliness while maintaining a usable film material morphology, and can be considered as a candidate for degradable packaging films or green barrier materials. While traditional PVDC films have advantages in water vapor barrier performance, they are insufficient in oxygen barrier performance and environmental friendliness. The TOCNF / CNC composite film of the present invention can achieve better oxygen barrier performance while possessing better degradability characteristics, thus demonstrating the comprehensive advantages of the present invention in the field of green high-barrier packaging materials.

[0083] Comparative Example 4 (CNF / CNC film without carboxylation surface modification) Composite films were prepared using CNF prepared by mechanical method and CNC prepared by sulfuric acid hydrolysis, with the same formulation and process conditions as in Example 2. Results Figure 2 Compared with Table 3, the test results show that the mechanical properties and barrier properties of this comparative example are inferior to those of Example 2. This is because the sulfate half-ester group of CNC in Comparative Example 4 has a higher surface charge and a stronger hydration layer. Although this is beneficial for dispersion, it also makes it easier to absorb water, form continuous aqueous migration channels, and reduce the dense stacking and effective hydrogen bond network of the film. In contrast, the carboxylated CNF and carboxylated CNC in the examples have better compatibility, making it easier to form tighter hydrogen bonds / dipole interactions and a denser, continuous multi-scale network. This makes load transfer more efficient and the gas diffusion path more tortuous, thus better for strength and oxygen barrier, while the driving force for the "dissolution-diffusion" of water vapor is also relatively lower.

[0084] Comparative Example 5 (without plasticizer) Same as Example 2, except that no plasticizer was added.

[0085] The only difference in this comparative example is the absence of the plasticizer glycerin. Subsequently, the film's appearance integrity (brittleness / warping, etc.), mechanical properties, and barrier properties were tested, as shown in Table 3 and Appendix. Figure 4 As shown, the film integrity and mechanical properties of the glycerol-free sample were significantly worse. The film was more prone to increased brittleness, decreased ductility, and reduced elongation at break, making it more susceptible to cracking or defects during actual testing and use. Furthermore, its barrier properties were inferior to the composite film with added glycerol. This comparative example illustrates that glycerol, as a plasticizer, plays a crucial role in improving the toughness of the nanocellulose composite system, ensuring film integrity, and achieving stable overall performance.

[0086] Comparative Example 6 (High Plasticizer Content) Same as Example 2, except that 150% plasticizer is added.

[0087] Test Results Figure 2 As shown in Table 3, excessive glycerol leads to overplasticization of the film, resulting in decreased mechanical strength or rigidity and adversely affecting barrier properties, particularly oxygen / water vapor barrier capacity. Therefore, compared to composite films with an appropriate amount of glycerol, the high-glycerol sample exhibits lower overall performance. This comparative example demonstrates that there is a reasonable optimal range for glycerol addition; appropriate addition improves the overall film performance, while excessive addition weakens mechanical and barrier properties.

[0088] The technical features in the claims and / or specification of this invention can be combined, and the combination is not limited to the combinations obtained through reference in the claims. Technical solutions obtained by combining the technical features in the claims and / or specification are also within the scope of protection of this invention.

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a nanocellulose film, characterized in that, It includes the following steps: S1 prepares an aqueous dispersion of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals; the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals is 1:0.25~4; S2 adds a plasticizer to the aqueous dispersion and stirs until homogeneous to obtain a composite solution; based on the mass content of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals being 100%, the mass content of the plasticizer is 10-30%; S3 involves ultrasonically treating the composite solution to remove air bubbles, resulting in a coating solution; S4 coating solution is poured in to ensure uniform distribution; then dried to obtain a nanocellulose film.

2. The preparation method according to claim 1, characterized in that, The carboxylated cellulose nanofibers were prepared by TEMPO oxidation, potassium permanganate oxidation, and carboxymethylation, with a content of 0.5~2.5 mmol / g; the carboxylated cellulose nanocrystals were prepared by sodium hypochlorite oxidation, TEMPO oxidation, and carboxymethylation, with a content of 0.5~2.5 mmol / g.

3. The preparation method according to claim 2, characterized in that, The carboxylated cellulose nanofibers have a diameter of 1-5 nm and a length of 400-600 nm; the carboxylated cellulose nanocrystals have a diameter of 1-5 nm and a length of 100-200 nm.

4. The preparation method according to claim 2, characterized in that, The preparation of the aqueous dispersion includes: dispersing carboxylated modified cellulose nanofiber dispersion and carboxylated modified cellulose nanocrystal dispersion in deionized water in a certain proportion, stirring until completely dispersed to obtain an aqueous dispersion; the mass concentration of the aqueous dispersion is 0.1~3%.

5. The preparation method according to claim 1, characterized in that, The plasticizer mentioned in step S2 is selected from at least one of glycerol, sorbitol, polyethylene glycol, propylene glycol and xylitol; the stirring speed is 5000~10000 rpm and the stirring time is 0.5~2 hours.

6. The preparation method according to claim 1, characterized in that, The ultrasonic treatment time in step S3 is 0.5 to 5 hours.

7. The preparation method according to claim 1, characterized in that, The drying process described in step S4 involves placing the wet film coated with the coating solution at 40-80°C until the film is completely dry.

8. A nanocellulose film, characterized in that, It is composed of carboxylated cellulose nanofibers, carboxylated cellulose nanocrystals and plasticizers; the mass ratio of carboxylated cellulose nanofibers to carboxylated cellulose nanocrystals is 1:0.25~4; based on the mass content of carboxylated cellulose nanofibers and carboxylated cellulose nanocrystals being 100%, the mass content of plasticizers is 10~30%.

9. The nanocellulose film according to claim 8, characterized in that, The thickness of the nanocellulose film is 0.08~0.12mm; the tensile strength of the nanocellulose film is ≥43MPa, the elongation at break is 1.9%~9.2%, and the water vapor permeability is 4.4~5.2×10⁻⁶. -10 g / (m·s·Pa), ranging from 21.86 to 60.01 cm. 3 / (m 2 It can degrade naturally in soil within 60 days (24 h, 0.1 MPa).

10. An application of the nanocellulose film according to claim 8 or 9 in the field of packaging materials technology.

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