Preparation of Nanocellulose Reinforced Composites by In-situ Polymerization and Its Preparation Method

Abstract

This invention provides an in-situ polymerization method for preparing nanocellulose-reinforced composite materials and the same method. The nanocellulose-reinforced composite material includes a polymer as the core and modified nanocellulose coated or wrapped around the surface of the polymer. The modified nanocellulose is covalently linked to the molecular chains of the polymer to form an interfacial chemical cross-linking network. The nanocellulose nanocomposite particles have a regular spherical or near-spherical core-shell structure with an average particle size of 100 nm to 10 μm. After processing, the nanocellulose-reinforced composite material can be obtained, ultimately achieving good composite material composition.

Description

Technical Field

[0001] This invention relates to the field of polymer materials, and in particular to an in-situ polymerization method for preparing nano-cellulose-reinforced composite materials and the same method. Background Technology

[0002] Aromatic vinyl monomer and acrylic polymer composites are polymeric material systems prepared by homopolymerization or copolymerization of styrene, α-methylstyrene, acrylic acid, methacrylic acid and their ester monomers, followed by various functional reinforcing fillers. The matrix includes core varieties such as polystyrene (PS), polymethyl methacrylate (PMMA), polymethyl methacrylate (PMA), polyethyl methacrylate (PEMA) and their copolymers. The reinforcing fillers are mainly divided into two categories: one is fibrous fillers such as glass fiber and carbon fiber used to improve tensile strength and impact toughness; the other is inorganic fillers such as montmorillonite, alumina, and boron nitride used to improve heat distortion temperature and thermal stability. With their irreplaceable excellent weather resistance, easy processing, and chemical stability, these materials have become one of the most widely used polymeric materials globally, widely penetrating high-end optical devices, lightweight transparent automotive components, biomedical devices, electronic packaging, environmentally friendly coatings and adhesives, etc., with an overall market size exceeding hundreds of billions of US dollars.

[0003] However, while reinforcing fillers can improve the mechanical and thermal properties of materials to some extent, some reinforcing fillers have poor compatibility with the polymer matrix, easily leading to agglomeration and uneven distribution. This has seriously affected the improvement of mechanical properties and the guarantee of optical properties, becoming a core technical challenge that the industry has long faced. Improving the compatibility between fillers and polymer substrates through simple operating methods, achieving good dispersion of fillers in the polymer substrate, and thus significantly enhancing the mechanical properties of composite materials has become a global technical bottleneck restricting the high-end application of fiber-reinforced composite materials. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing nanocellulose-reinforced composite materials by in-situ polymerization. The method uses reactive active groups as reinforcing fillers and employs a green, low-cost, and scalable preparation process to solve the interfacial compatibility problem between nanocellulose containing reactive active groups and aromatic vinyl and acrylic polymer matrices, thereby achieving a synergistic improvement in the mechanical, thermal, and optical properties of the composite material.

[0005] To achieve the above objectives, this technical solution provides a method for preparing nano-cellulose-reinforced composite materials through in-situ polymerization, comprising the following steps: (1) Modified nanocellulose containing reactive functional groups on its surface is dispersed in a dispersion medium to obtain a nanocellulose dispersion, and monomers and initiators are added to the nanocellulose dispersion to construct a heterogeneous system, wherein the reactive functional groups are any one of unsaturated carbon-carbon double bonds or active hydrogen groups. (2) Initiate monomer polymerization and initiate in-situ chemical grafting reaction between monomers and reactive functional groups on the surface of modified nanocellulose to obtain a uniform nanocomposite particle dispersion. (3) The uniform nanocomposite particle dispersion is processed and shaped to obtain nanocellulose reinforced composite material.

[0006] This method specifically selects nanocellulose containing unsaturated carbon-carbon double bonds or active hydrogen groups on its surface as a reinforcing filler, and uses a one-step in-situ polymerization method to prepare nanocellulose-reinforced composite materials. The nanocellulose containing reactive functional groups in this method integrates three functions: droplet stabilization, physical network construction, and chemically active grafting. On the one hand, it stabilizes the emulsion in solid particle form, achieving green polymerization without emulsifiers, salts, or organic solvents. On the other hand, the reactive functional groups on the fiber surface can be covalently grafted in situ with monomers, allowing polymer molecular chains to grow directionally on the fiber surface, simultaneously constructing a physical entanglement network and a covalent chemical cross-linking network within the matrix, forming a dual physical-chemical network reinforcement structure. Therefore, the nanocellulose grafted with polymer molecular chains achieves thermodynamic compatibility and complete interfacial bonding with the polymer matrix, fundamentally eliminating filler agglomeration and phase separation, achieving efficient stress transfer, significantly improving the mechanical and thermal stability of the composite material, and simultaneously maintaining high optical transparency, demonstrating outstanding green chemical value and industrial scale-up potential.

[0007] That is, in some embodiments, the reactive functional group is selected from at least one of the following: (1) unsaturated carbon-carbon double bonds, including vinyl, acryloyl, methacryloyl or allyl; (2) groups containing active hydrogen, including mercapto, primary amine group, secondary amine group, etc.

[0008] Regarding the selection of nanocellulose containing reactive functional groups in this scheme: As a green nanomaterial derived from nature, renewable and biodegradable, nanocellulose is gradually becoming a promising new nano-reinforcement in the field of polymer reinforcement modification due to its multi-dimensional performance advantages. In particular, nanocellulose CNF has unique filler characteristics: nanoscale size (diameter usually 5-20 nm, length up to several micrometers), high aspect ratio, high specific surface area, and excellent inherent mechanical properties (CNF crystalline region tensile strength of about 2 GPa, modulus of about 150 GPa), etc., which make it a sustainable and effective reinforcement material that has attracted great attention. At the same time, the active hydroxyl groups of nanocellulose are easy to modify, which can better achieve good dispersion of nanocellulose in PMMA matrix.

[0009] However, most CNF-reinforced composite materials currently on the market are thermosetting composite materials prepared by solvent exchange and solvent casting methods. They are difficult to further process and mold, have complicated processes, and cause significant environmental pollution. Thermoplastic CNF-reinforced composite materials are generally prepared by emulsion polymerization or in-situ polymerization containing CNF. Although this solves the problem of difficult processing of composite materials, the preparation process usually involves the addition of additional organic solvents, inorganic salts, or small molecule surfactants, which makes the process cost high. In addition, it is necessary to remove and recover organic solvents and remove impurities such as inorganic salts through processes such as filtration, distillation, and drying. The process is complicated and generates waste liquid, causing environmental pollution.

[0010] Based on the pain points and difficulties in the preparation of nanocellulose-reinforced composite materials, our team conducted extensive theoretical analysis and systematic experimental verification, and found that the core problems of existing technologies are concentrated in two points: First, unmodified nanocellulose can only form weak physical interface bonds such as hydrogen bonds and van der Waals forces with aromatic vinyl and acrylic polymer matrices, and cannot build a stable stress transmission network. This makes it easy for filler agglomeration and phase separation to occur, which not only makes it difficult to exert the nano-reinforcement effect, but also degrades the optical and mechanical properties of the material. Second, common nanocellulose has poor self-emulsification ability and must rely on the addition of small molecule surfactants, inorganic salts or co-solvents to achieve the stability of the emulsion polymerization system, which leads to complicated subsequent impurity removal processes, high production costs and serious environmental pollution.

[0011] To address this, our team proposes using nanocellulose with surface-grafted reactive functional groups as a reinforcing filler. This nanocellulose containing reactive functional groups integrates three functions—droplet stabilization, physical reinforcement, and chemical interfacial bonding—into a single filler. Simultaneously, this invention develops a one-step heterogeneous free radical polymerization process that requires no external traditional additives. This process allows the nanocellulose containing reactive functional groups to covalently graft aromatic vinyl and acrylic polymer chains onto the nanocellulose during polymerization. Ultimately, this constructs a dual-reinforcing network of physical entanglement and chemical cross-linking within the matrix, fundamentally solving the long-standing industry challenges of poor interfacial compatibility, insufficient greenness of the process, and the inability to synergistically improve mechanical, thermal, and optical properties in nanocellulose-reinforced polyacrylic acid composites.

[0012] In some embodiments, the nanocellulose containing reactive active groups has a diameter of about 1.28–4.69 nm, a length of 0.93–2.61 μm, and a charge of 280–1000 μmol / g.

[0013] It should be noted that the nanometer-scale diameter of 1.28–4.69 nm ensures high visible light transmittance of the material and avoids light scattering; the length of 0.93–2.61 μm gives the fiber a suitable aspect ratio, which can overlap and entangle in the matrix to form an effective physical network; the high surface charge of 280–1000 μmol / g provides strong electrostatic repulsion, allowing the fiber to be uniformly dispersed without agglomeration, while also giving it excellent solid particle emulsification ability, stabilizing Pickering emulsion and ensuring the smooth construction of covalent grafting and double network structure, which can determine the dispersibility, emulsion stability, optical transparency and mechanical reinforcement effect of the composite material.

[0014] Regarding the preparation of nanocellulose with reactive functional groups on its surface according to this scheme: The modified nanocellulose is prepared by any of the following methods: (1) Acid anhydride esterification method: cellulose raw materials or nanocellulose are modified by esterification with acid anhydrides containing double bonds. The acid anhydrides containing double bonds include at least one of maleic anhydride, methacrylic anhydride, itaconic anhydride, acrylic anhydride, dimethylmaleic anhydride, pentene anhydride, octenyl succinic anhydride, and dodecenyl succinic anhydride. (2) Silane coupling agent modification method: The nanocellulose is obtained by surface modification with a silane coupling agent containing unsaturated bonds or mercapto groups. The silane coupling agent containing unsaturated double bonds includes at least one of vinyltrimethoxysilane KH-171, vinyltriethoxysilane KH-151, and γ-methacryloyloxypropyltrimethoxysilane KH-570; the silane coupling agent containing mercapto groups includes at least one of γ-mercaptopropyltriethoxysilane KH-580 and γ-mercaptopropyltrimethoxysilane KH-590. (3) Amide modification method: The carboxylated nanocellulose is modified by amidation with amine compounds containing unsaturated bonds or thiol groups, wherein the amine compounds containing unsaturated bonds or thiol groups are at least one of allylamine, 4-aminostyrene, 2-aminoethyl methacrylate, cysteine, and L-cysteine. (4) Epoxy ring-opening grafting modification method: modified by ring-opening reaction between epoxy groups containing unsaturated bonds and hydroxyl or carboxyl groups of cellulose. The epoxy groups containing unsaturated bonds include at least one of glycidyl methacrylate, vinyl glycidyl ether, glycidyl acrylate, limonene monoepoxide, and allyl glycidyl ether.

[0015] In a preferred embodiment, modified nanocellulose with reactive functional groups on its surface is obtained by esterification modification of cellulose raw materials or nanocellulose with acid anhydrides containing double bonds. The acid anhydrides containing double bonds include at least one of maleic anhydride, methacrylic anhydride, itaconic anhydride, octenyl succinic anhydride, and dodecenyl succinic anhydride. After pretreatment, mechanical processing yields nanocellulose with carbon-carbon double bonds on the nanofiber surface. During the reaction, the hydroxyl groups on the nanocellulose surface undergo ring-opening esterification with the molten acid anhydride, simultaneously introducing polymerizable carbon-carbon double bonds and an appropriate amount of carboxyl groups onto the surface of the nanocellulose while retaining its original nanostructure.

[0016] In some embodiments, the degree of substitution of reactive maleic anhydride in the maleic anhydride-modified nanocellulose is 24.7–51.8 mol.

[0017] In some embodiments, the nanocellulose is obtained by introducing double bonds through surface chemical modification, wherein the surface chemical modification includes one or more of the following: acid anhydride esterification modification containing double bonds, silane coupling agent modification, amidation modification, and epoxy ring-opening grafting, and reactive functional groups are grafted onto the surface of the nanocellulose through chemical bonding.

[0018] Regarding the selection of raw materials for nanocellulose containing reactive functional groups: In some embodiments, the modified nanocellulose containing reactive functional groups on the surface is L-CNFs with carbon-carbon double bonds on the surface, that is, nanocellulose containing carbon-carbon double bonds on the surface is directly prepared from biomass raw materials such as wood, bamboo, and agricultural waste. In this way, nanocellulose containing double bonds can retain the natural lignin components in the raw materials. The presence of a large amount of lignin allows L-CNFs to form stable droplets without further hydrophobic modification, addition of salts or co-solvents.

[0019] At this point, using bamboo fiber with high lignin content as raw material, lignin-rich cellulose nanoparticles (L-CNFs) were prepared through maleic anhydride esterification pretreatment and mechanical treatment. Because the chemical pretreatment of L-CNFs is relatively mild and retains a significant amount of lignin, it endows the polymer composite film with excellent UV shielding (anti-UV) function without sacrificing the high optical transmittance of the matrix. Furthermore, the introduction of lignin significantly improves the natural hydrophilicity of cellulose, giving L-CNFs suitable amphiphilicity, enabling them to function as highly efficient solid particle emulsifiers to construct highly stable Pickering emulsion systems.

[0020] The specific process is as follows: 2 kg of bamboo fiber and 10 kg of maleic anhydride were added to a 50 L non-pressurized reactor, and esterification was carried out for 3 hours at 95 ℃ and a stirring speed of 100 rpm. Subsequently, the resulting fibers (L-fiber) were soaked and washed with a mild alkaline solution until the pH value reached 7. The esterified and washed L-fiber was then nano-sized by stirring at approximately 30,000 rpm for 10 minutes using a high-speed mixer, and filtered through a 400-mesh sieve to remove any un-nanosized residues.

[0021] In some embodiments, the three main components in L-CNFs containing carbon-carbon double bonds on the surface are: lignin content of 2.1–24.0%; hemicellulose content of 4.0–23.9%; and cellulose content of 74.0–90.9%.

[0022] Regarding the selection of monomers: In some embodiments, the monomers include aromatic vinyl monomers and acrylic monomers.

[0023] In some embodiments, the aromatic vinyl monomers are selected from one or more of styrene, α-methylstyrene, p-methylstyrene, p-tert-butylstyrene, and divinylbenzene, preferably styrene; the acrylic monomers are selected from one or more of methyl methacrylate monomers, acrylate monomers, and methacrylate monomers, preferably methyl methacrylate monomers, preferably methyl methacrylate monomers, such as acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, ethyl methacrylate, ethyl methacrylate, butyl methacrylate, isooctyl acrylate, isobutyl acrylate, lauryl methacrylate, ethyl acetoacetate methacrylate, cyclohexyl methacrylate, hydroxyethyl methacrylate, and hydroxypropyl acrylate.

[0024] Correspondingly, the polymer matrix of the polymer containing reactive functional groups in the composite material formed by this scheme is selected from one or more of the following: polystyrene, poly-α-methylstyrene, poly-p-methylstyrene, poly-p-tert-butylstyrene, polyvinylbenzene, polyacrylic acid, polymethacrylic acid, polymethyl acrylate, polymethyl methacrylate, polyethyl acrylate, polyethyl methacrylate, polybutyl acrylate, polyisooctyl acrylate, polyisobutyl acrylate, polylauryl methacrylate, polyethyl acetoacetate, polycyclohexyl methacrylate, polyhydroxyethyl methacrylate, polyhydroxypropyl acrylate, and acrylate-methacrylate copolymers.

[0025] Process conditions for preparing nanocellulose-reinforced composite materials containing reactive functional groups: In some embodiments, the concentration of the nanocellulose dispersion is 0.01 wt% to 3 wt%.

[0026] This scheme uses an oil-soluble free radical initiator, selected from azo initiators or organic peroxide initiators. Azo initiators offer advantages such as stable decomposition, no acidic byproducts, and no damage to the surface functional groups of modified cellulose or emulsion stability. In some embodiments, the azo initiator is selected from one or a combination of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, and azodicyclohexylformitrile. Organic peroxide initiators offer higher activity; in some embodiments, the organic peroxide initiator is selected from one or a combination of benzoyl peroxide, tert-butyl peroxypentanoate, and di-tert-butyl peroxide.

[0027] Preferably, the oil-soluble free radical initiator is azobisisobutyronitrile (AIBN), which can decompose smoothly without acidic byproducts, gently initiate polymerization, and perfectly protect the stability of the substrate structure and the system (emulsion).

[0028] In some embodiments, the ratio of oil-soluble free radical initiator to monomer is 0.1 to 3 wt%.

[0029] In some embodiments, the amount of modified nanocellulose added is 0.1 wt% to 5 wt% of the monomer.

[0030] Preferably, the amount of double-bonded nanocellulose added is 2 wt% of the acrylate monomer.

[0031] In some embodiments, the monomer is added to the nanocellulose dispersion and then emulsified, wherein the emulsification method is selected from one or more of high-speed shearing, ultrasonication, high-pressure homogenization, microfluidization, and pipeline emulsification.

[0032] Preferably, the initial shearing condition is ULTRA TUUREX at 8000 rpm for 1.5 min.

[0033] In some embodiments, the conditions for ultrasonic emulsification are emulsification at 600W for 2 to 4 minutes.

[0034] Preferably, the shearing condition is ultrasonic treatment at 600W for 4 minutes using an ultrasonic disruptor.

[0035] It should be noted that this solution uses modified nanocellulose with reactive functional groups on its surface as a solid emulsifier, which can completely eliminate the dependence on traditional small molecule surfactants, inorganic salts and other auxiliaries, and realize green one-step polymerization. At the same time, it enables the nanofibers to be precisely positioned on the surface of monomer oil droplets, ensuring that the polymer chains and reactive functional groups on the fiber surface covalently graft during the polymerization process, constructing a uniform physical-chemical dual reinforcement network, fundamentally solving the interfacial compatibility problem. It can also ensure that the nanofibers are uniformly dispersed, avoiding light scattering and stress concentration caused by agglomeration, and ultimately achieving a synergistic improvement in the mechanical, thermal and optical properties of the composite material.

[0036] In some embodiments, the heterogeneous system is subjected to heterogeneous polymerization for 1 to 12 hours in an inert atmosphere and a high temperature environment, wherein the inert atmosphere is a nitrogen protective atmosphere and the high temperature environment is 65 to 85°C.

[0037] Preferably, the high-temperature environment is 70℃.

[0038] Preferably, the Pickering emulsion is placed in an inert atmosphere and a high-temperature environment for a constant temperature reaction for 7 hours.

[0039] It should be noted that during the in-situ polymerization process, the modified nanocellulose achieved uniform nanoscale dispersion in aromatic vinyl or acrylic polymer substrates. Simultaneously, the monomers can grow in situ on nanocellulose containing highly reactive groups during polymerization, resulting in stable interfacial chemical bonds between the nanocellulose and the polymer substrate molecular chains. This leads to a dual-network structure in the final nanocellulose-reinforced polymer composite material containing reactive functional groups. This dual-network structure significantly optimizes interfacial compatibility. When the nanocellulose-reinforced composite material containing reactive functional groups is subjected to external forces, it can achieve efficient stress transfer and dissipation between the matrix and the filler, thereby amplifying the mechanical reinforcement and toughening effects of nanocellulose.

[0040] In some embodiments, methanol is added to the reaction system after in-situ polymerization for demulsification, and the demulsified system is centrifuged and washed multiple times to remove unreacted impurities, ultimately obtaining a pure nanocellulose-reinforced composite material containing reactive functional groups.

[0041] Furthermore, the nanocomposite particle dispersion prepared in this scheme can be directly coated onto the substrate surface as an aqueous film-forming substance. Meanwhile, the nanocellulose-reinforced composite material containing reactive functional groups prepared in this scheme is a thermoplastic. The prepared powdered nanocellulose-reinforced composite material containing reactive functional groups can be hot-pressed to obtain a transparent composite film material.

[0042] In some embodiments, the processing and molding described in step (3) is a hot working process or a coating molding process; The hot working process includes the following steps; After drying, the nanocomposite particles are hot-pressed, injection-molded, or extruded at a temperature 10–50°C above their melting point to construct a three-dimensional reinforcing skeleton network that runs through the entire composite material. The coating process includes the following steps; The nanocomposite particle dispersion from step (2) is directly applied to the substrate surface as an aqueous film-forming substance.

[0043] Secondly, this solution provides a nanocellulose-reinforced composite material containing reactive functional groups prepared according to the first aspect, comprising a polymer as the core and modified nanocellulose coated or wrapped around the surface of the polymer, wherein the modified nanocellulose is connected to the molecular chain of the polymer by covalent bonds to form an interfacial chemical cross-linking network, and the nanocellulose-reinforced composite material has a core-shell structure with regular spherical or near-spherical knots, and the average particle size of the nanocomposite particles is 100 nm to 10 μm.

[0044] As mentioned earlier, the nanocellulose-reinforced composite material containing reactive functional groups in this scheme constructs a dual network structure in the polymer matrix. This network structure greatly optimizes the interfacial compatibility. When the composite material is subjected to external forces, it can achieve efficient stress transfer and dissipation between the matrix and the filler, thereby multiplying the mechanical reinforcement and toughening effect of nanocellulose.

[0045] In some embodiments, double-bond-containing nanocellulose is obtained by treating cellulose with anhydride esterification methods such as maleic anhydride, methacrylic anhydride, itaconic anhydride, octenyl succinic anhydride, and dodecenyl succinic anhydride, or by introducing double bonds through surface chemical modification of nanocellulose by methods such as double-bond-containing anhydride esterification, silane coupling agent modification, amidation modification, and epoxy ring-opening grafting modification.

[0046] In some embodiments, the monomers of the polymer include aromatic vinyl monomers or acrylate monomers. Preferably, the monomers are selected from one or more of styrene, α-methylstyrene, acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, and butyl acrylate.

[0047] The nanocellulose-reinforced composite material with reactive functional groups prepared by this method achieved good dispersion and mechanical reinforcement of the filler, providing an scalable preparation route for composite materials and revealing the mechanism by which sustainable interface design regulates the structure-property relationship in biocomposite systems.

[0048] Specifically, the glass transition temperature of the nanocellulose-reinforced composite material containing reactive functional groups in this scheme is increased from 120℃ to 125-126℃, and the maximum thermal degradation temperature is increased from 354℃ to 362-373℃; the Young's modulus in the tensile test is increased by 0.4-1.0 GPa; and the flexural modulus in the bending test is increased by 0.5-1.0 GPa.

[0049] Preferably, when the amount of nanocellulose containing reactive functional groups added is 2 wt% of methyl methacrylate monomer, the glass transition temperature of the nanocellulose-reinforced composite material containing reactive functional groups increases from 120°C to 126°C, and the maximum thermal degradation temperature increases from 354°C to 373°C. This demonstrates that the addition of nanocellulose improves the thermodynamic stability of the polymer, making the composite material more suitable for use in extreme environments. The Young's modulus in the tensile test increases by 0.8 GPa, an increase of nearly 40%, and the flexural modulus in the flexural test increases by 0.9 GPa, an increase of nearly 30%.

[0050] Compared with existing technologies, this technical solution has the following characteristics and beneficial effects: This scheme utilizes nanocellulose containing reactive functional groups as a reinforcing material and designs an in-situ polymerization process to prepare nanocellulose-reinforced composite materials containing reactive functional groups. The entire process requires no addition of small molecule surfactants, inorganic salts, or toxic organic solvents, generates no waste liquid, simplifies subsequent impurity removal processes, and possesses excellent industrial scale-up potential. Furthermore, this scheme constructs a dual reinforcing network of "physical entanglement and chemical crosslinking" through covalent grafting, solving the problem of weak interfacial bonding between nanocellulose and aromatic vinyl or acrylic polymer matrices, while simultaneously achieving a comprehensive and synergistic improvement in thermodynamic stability and mechanical properties. Attached Figure Description

[0051] Figure 1 These are the infrared spectra of the nanocellulose-reinforced composite material containing reactive functional groups prepared in Example 1 and the polymethyl methacrylate material prepared in Comparative Example 2.

[0052] Figure 2 These are microscopic morphology images of the nanocomposite particles prepared in Examples 1 and 2 and Comparative Examples 1 and 2.

[0053] Figure 3 These are cross-sectional structural diagrams of the nanocellulose-reinforced composite material containing reactive functional groups prepared in Example 1 and the polymethyl methacrylate material prepared in Comparative Example 2.

[0054] Figure 4 This is a schematic diagram illustrating the principle and process of the in-situ polymerization method for preparing nano-cellulose-reinforced composite materials according to this scheme. Detailed Implementation

[0055] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0056] Material: The bamboo was purchased from Guangde Buqiucao Bamboo Industry Co., Ltd. Preparation of anhydride-esterified nanocellulose containing double bonds: Bamboo fiber and anhydride were added to a sealed three-necked flask at a weight ratio of 1:20. Melt esterification was carried out at a certain temperature and a stirring speed of 100 rpm for 3 hours. Subsequently, the resulting fibers (L-fiber) were washed with a mild alkaline solution until the pH reached 7. The esterified and washed L-fiber was then nano-sized by stirring at approximately 30,000 rpm for 10 minutes using a high-speed mixer and filtered through a 400-mesh sieve to remove any un-nanosized residue.

[0057] Preparation of silane coupling agent modified nanocellulose: A certain concentration of nanocellulose aqueous suspension was centrifuged and sonicated, and repeatedly washed and replaced with ethanol until it was uniformly dispersed in a mixed solvent of ethanol / water (volume ratio, for example, 80:20 or 90:10). Glacial acetic acid was added dropwise to adjust the pH of the system to between 4.0 and 5.0. 10 wt% of the dry weight of nanocellulose in KH-570 (or KH-580) was added to the above system, and the mixture was pre-stirred at room temperature for 60 minutes. The reaction flask was placed in a constant temperature water bath, heated to 65°C, refluxed, and mechanically stirred for 46 hours. After the reaction, the mixture was cooled to room temperature. The mixture was centrifuged, and the precipitate was washed three times alternately with anhydrous ethanol and deionized water.

[0058] Regarding the preparation of Holo-CNFs: Lignin was removed using the peracetic acid method. Specifically, bamboo strips were soaked in a 4% peracetic acid solution and heated at 80°C to remove lignin, with a stirring speed of 100 rpm. During the reaction, 20 wt% sodium hydroxide solution was added to adjust the pH to 4.5–4.8. Each reaction cycle lasted 1.5–2 hours. After the reaction, the bamboo strips were filtered through a 100-mesh screen, and the above steps were repeated. The reaction was repeated 4 times to obtain lignin-free Holo-Fiber. The lignin-free and washed Holo-Fiber was then stirred at approximately 30,000 rpm for 10 minutes using a high-speed mixer to achieve nano-sizing, and then filtered through a 400-mesh sieve to remove any un-nanosized residues.

[0059] Preparation of TEMPO-oxidized cellulose nanofibers (TEMPO-CNFs): Holo-Fiber, TEMPO, and NaBr were dispersed in water, and then a 4 wt% NaClO solution was added. The weight ratio of Holo-Fiber:TEMPO:NaBr:NaClO was 5 g:0.078 g:0.514 g:1.861 g, and the solid-liquid ratio was 1:100. The reaction was carried out at room temperature for 3 h with a stirring speed of 300 rpm. During the reaction, the pH of the solution was maintained at around 10 by slowly adding NaOH. After the reaction, the TEMPO-Fiber was thoroughly washed with deionized water and then stirred at approximately 30,000 rpm for 10 minutes to achieve nano-sizing. The nanofiber was then filtered through a 400-mesh sieve to remove any un-nanosized residues.

[0060] Preparation of amidated modified nanocellulose: A TEMPO-oxidized nanocellulose aqueous suspension (concentration approximately 0.5–1 wt%) was placed in a flask and stirred at room temperature. EDC·HCl and NHS solids were added sequentially to the nanocellulose suspension according to a carboxyl group:EDC:NHS molar ratio of 1:2:2. The pH of the system was adjusted to between 4.5 and 5.5 using 0.1M HCl or 0.1M NaOH, and stirred at room temperature in the dark for 2 hours. Allylamine was added dropwise to the system; the molar amount of amine is typically twice that of the carboxyl group. After addition, the pH of the system was adjusted to 7.0. The reaction was continued to be stirred at room temperature for 24 hours. The resulting suspension was placed in a dialysis bag and dialyzed in deionized water for 5 days, changing the water twice daily, until the conductivity of the dialysate was consistent with that of pure water.

[0061] Preparation of epoxy-modified nanocellulose: Aqueous suspensions of nanocellulose were centrifuged and successively replaced with anhydrous ethanol, N,N-dimethylformamide (DMF), or dimethyl sulfoxide (DMSO) to obtain a DMF / DMSO suspension of nanocellulose. A catalyst was added to the suspension. An organic base, such as tetrabutylammonium bromide (TBAB), was typically used. Nitrogen gas was purged for 15 min to remove oxygen from the system. Under nitrogen protection, glycidyl methacrylate was slowly added dropwise at a specific molar ratio (1:1) of the hydroxyl groups in the nanocellulose. The reaction system was heated to 60°C and mechanically stirred for 8 h. After the reaction, the product was cooled to room temperature. A large amount of anhydrous ethanol was added to flocculate and precipitate the modified nanocellulose, which was then collected by centrifugation and washed three times alternately with ethanol and deionized water.

[0062] Example 1 S1: 2 wt% of maleic anhydride esterified modified nanocellulose was uniformly dispersed in water using Ultra turrax to obtain an aqueous dispersion of nanocellulose with a total volume of 90 mL. S2: Dissolve 0.5–3 wt% of AIBN initiator by monomer mass in 10 mL of methyl methacrylate to obtain an oil phase system; S3: Mix 90 mL of the aqueous dispersion of nanocellulose and 10 mL of the oil phase system in a glass bottle for preliminary emulsification, and then use an ultrasonic cell disruptor to emulsify the above mixed solution to obtain Pickering emulsion. S4: The above emulsion is put into a three-necked flask reactor and inert gas is continuously introduced to ensure that the reaction is carried out in an inert gas environment. The three-necked flask reactor is placed in an oil bath at 68-85℃ and stirred with a top-mounted mechanical stirrer at a speed of 200-300 rpm for 4-8 hours. S5: The reaction product was demulsified with methanol and then subjected to three rounds of centrifugation, washing and drying to obtain nanocomposite particles; S6: The nanocomposite particles obtained after post-processing can be hot-pressed to obtain a transparent composite film material.

[0063] Example 2 S1: 2 wt% of the monomer mass of silane coupling agent KH-570 modified nanocellulose was uniformly dispersed in water using ULTRA TUUREX to obtain an aqueous dispersion of nanocellulose with a total volume of 90 mL. S2: Dissolve 0.5–3 wt% of AIBN initiator by mass of monomer in 10 mL of styrene monomer to obtain an oil phase system; S3: Mix 90 mL of the aqueous dispersion of nanocellulose and 10 mL of the oil phase system in a glass bottle for preliminary emulsification, and then use an ultrasonic cell disruptor to emulsify the above mixed solution to obtain Pickering emulsion. S4: The above emulsion is put into a three-necked flask reactor and inert gas is continuously introduced to ensure that the reaction is carried out in an inert gas environment. The three-necked flask reactor is placed in an oil bath at 68-85°C and stirred with a top-mounted mechanical stirrer at a speed of 200-300 rpm for 4-8 hours. S5: The reaction product was demulsified with methanol and then subjected to three rounds of centrifugation, washing and drying to obtain nanocomposite particles; S6: The nanocomposite particles obtained after post-processing can be hot-pressed to obtain a transparent composite film material.

[0064] Example 3 S1: 2 wt% of allylamine-modified nanocellulose by monomer mass was uniformly dispersed in water using ULTRA TUUREX to obtain an aqueous dispersion of nanocellulose with a total volume of 90 mL. S2: Dissolve 0.5–3 wt% of AIBN initiator by monomer mass in 10 mL of acrylic acid monomer to obtain an oil phase system; S3: Mix 90 mL of the aqueous dispersion of nanocellulose and 10 mL of the oil phase system in a glass bottle for preliminary emulsification, and then use an ultrasonic cell disruptor to emulsify the above mixed solution to obtain Pickering emulsion. S4: The above emulsion is put into a three-necked flask reactor and inert gas is continuously introduced to ensure that the reaction is carried out in an inert gas environment. The three-necked flask reactor is placed in an oil bath at 68-85°C and stirred with a top-mounted mechanical stirrer at a speed of 200-300 rpm for 4-8 hours. S5: The nanocomposite particle dispersion obtained from the reaction is directly coated onto the substrate surface as an aqueous film-forming substance.

[0065] Example 4: S1: 2 wt% glycidyl methacrylate ring-opening graft modified nanocellulose was uniformly dispersed in water using ULTRA TUUREX to obtain an aqueous dispersion of nanocellulose with a total volume of 90 mL. S2: Dissolve 0.5–3 wt% of AIBN initiator by monomer mass in 10 mL of methyl methacrylate to obtain an oil phase system; S3: Mix 90 mL of the aqueous dispersion of nanocellulose and 10 mL of the oil phase system in a glass bottle for preliminary emulsification, and then use an ultrasonic cell disruptor to emulsify the above mixed solution to obtain Pickering emulsion. S4: The above emulsion is put into a three-necked flask reactor and inert gas is continuously introduced to ensure that the reaction is carried out in an inert gas environment. The three-necked flask reactor is placed in an oil bath at 68-85°C and stirred with a top-mounted mechanical stirrer at a speed of 200-300 rpm for 4-8 hours. S5: The reaction product was demulsified with methanol and then subjected to three rounds of centrifugation, washing and drying to obtain nanocomposite particles; S6: The nanocomposite particles obtained after post-processing can be hot-pressed to obtain a transparent composite film material.

[0066] Example 5: S1: 1 wt% of maleic anhydride-esterified nanocellulose was uniformly dispersed in water using ULTRA TUUREX to obtain an aqueous dispersion of nanocellulose with a total volume of 90 mL. S2: Dissolve 0.5–3 wt% of AIBN initiator by monomer mass in 10 mL of methyl methacrylate monomer to obtain an oil phase system; S3: Mix 90 mL of the aqueous dispersion of nanocellulose and 10 mL of the oil phase system in a glass bottle for preliminary emulsification, and then use an ultrasonic cell disruptor to emulsify the above mixture to obtain MA-CNF / MMA Pickering emulsion. S4: The above emulsion is put into a three-necked flask reactor and inert gas is continuously introduced to ensure that the reaction is carried out in an inert gas environment. The three-necked flask reactor is placed in an oil bath at 68-85°C and stirred with a top-mounted mechanical stirrer at a speed of 200-300 rpm for 4-8 hours. S5: The reaction product was demulsified with methanol and then subjected to three rounds of centrifugation, washing and drying to obtain nanocomposite particles; S6: The nanocomposite particles obtained after post-processing can be hot-pressed to obtain a transparent composite film material.

[0067] Example 6: S1: 3 wt% maleic anhydride-esterified nanocellulose was uniformly dispersed in water using ULTRA TUUREX to obtain an aqueous dispersion of nanocellulose with a total volume of 90 mL. S2: Dissolve 0.5–3 wt% of AIBN initiator by monomer mass in 10 mL of methyl methacrylate monomer to obtain an oil phase system; S3: Mix 90 mL of the aqueous dispersion of nanocellulose and 10 mL of the oil phase system in a glass bottle for preliminary emulsification, and then use an ultrasonic cell disruptor to emulsify the above mixture to obtain MA-CNF / MMA Pickering emulsion. S4: The above emulsion is put into a three-necked flask reactor and inert gas is continuously introduced to ensure that the reaction is carried out in an inert gas environment. The three-necked flask reactor is placed in an oil bath at 68-85°C and stirred with a top-mounted mechanical stirrer at a speed of 200-300 rpm for 4-8 hours. S5: The reaction product was demulsified with methanol and then subjected to three rounds of centrifugation, washing and drying to obtain nanocomposite particles; S6: The nanocomposite particles obtained after post-processing can be hot-pressed to obtain a transparent composite film material.

[0068] Comparative Example 1: S1: 2 wt% of TEMPO-CNFs by monomer mass was uniformly dispersed in water using ULTRA TUUREX to obtain an aqueous dispersion of nanocellulose (total volume of 90 mL). S2: Dissolve 1 wt% of AIBN initiator by monomer mass in 10 mL of methyl methacrylate monomer to obtain an oil phase system; S3: Mix 90 mL of the aqueous dispersion of nanocellulose and 10 mL of the oil phase system in a glass bottle for preliminary emulsification, and then use an ultrasonic cell disruptor to emulsify the above mixed solution to obtain Pickering emulsion. S4: The above emulsion is put into a three-necked flask reactor and inert gas is continuously introduced to ensure that the reaction is carried out in an inert gas environment. The three-necked flask reactor is placed in a 70°C oil bath and stirred with a top-mounted mechanical stirrer at a speed of 200-300 rpm for 7 hours. S5: The reaction product was demulsified with methanol and then subjected to three rounds of centrifugation, washing and drying to obtain nanocomposite particles; S6: The nanocomposite particles obtained after post-processing can be hot-pressed to obtain a transparent composite film material.

[0069] Comparative Example 2: S1: Place 0.8g PVA and 160mL water in an 80℃ water bath and heat and stir until the solution cools to 30℃-40℃.

[0070] S2: Dissolve 0.4g AIBN in 40mL of methyl methacrylate and pour into a 500mL three-necked flask. Then add the PVA solution prepared in S1 and 80mL of water to the three-necked flask, stir at 800rpm, raise the temperature to 70℃, and continue for 15min.

[0071] S3: Raise the temperature to 78℃ and react for 1.5h, then raise the temperature to 85℃ and hold for 1h.

[0072] S5: The reaction product was centrifuged, washed and dried three times to obtain nanocomposite particles; S6: The nanocomposite particles obtained after post-processing can be hot-pressed to obtain a transparent composite film material.

[0073] Test example: Structural testing: The composite material powder prepared in Example 1 was subjected to Soxhlet extraction (acetone as solvent, extraction for 48 hours), and then infrared spectroscopy was performed on the sample before and after extraction. The results are as follows: Figure 1 As shown, the extracted sample is at 1740 cm⁻¹ -1 (Ester group), 2995cm -1 The presence of the characteristic absorption peak of polymethyl methacrylate at the (methyl) position proves that there is a covalent bond between nanocellulose and polymethyl methacrylate.

[0074] The microstructures of the nanocomposite particles prepared in Examples 1 and 2 and Comparative Examples 1 and 2 are as follows: Figure 2 As shown, modified nanocellulose can be observed coating or wrapping around the surface of the polymer core to form a shell.

[0075] Figure 3 shows the microstructure of the nanocellulose-reinforced composite material prepared in Example 1 and Comparative Example 2. It can be seen that the nanocellulose is uniformly dispersed in the polymethyl methacrylate matrix with no obvious agglomeration.

[0076] Thermodynamic stability testing: The glass transition temperature was measured using differential scanning calorimetry (DSC), and the thermal degradation temperature was measured using thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 10 °C / min. DSC and TGA test results of the nanocellulose-reinforced composite materials prepared in Example 1 and Comparative Example 2: The glass transition temperature of Example 1 was 126 °C, and the maximum thermal degradation temperature was 373 °C.

[0077] Mechanical property testing: Tensile properties were tested using a universal testing machine according to GB / T1040-2018 standard, and flexural properties were tested according to GB / T 9341-2008 standard. The sample size was 5 mm × 10 mm × 0.5 mm, and the tensile rate was 5 mm / min. The Young's modulus and flexural modulus of the nanocellulose-reinforced composite materials prepared in Examples 1, 5, and 6 and Comparative Example 2 are shown in Table 1 below. The results show that the Young's modulus of Example 1 is 3.0 GPa, and the flexural modulus is 4.1 GPa.

[0078] Table 1. Mechanical test results of materials from Examples 1, 5, 6 and Comparative Example 2.

[0079] Those skilled in the art should understand that the technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0080] The above embodiments are merely illustrative of several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for preparing nanocellulose-reinforced composite materials containing reactive functional groups by in-situ polymerization, characterized in that, Includes the following steps: (1) Modified nanocellulose with reactive functional groups on its surface is dispersed in a dispersion medium to obtain a nanocellulose dispersion, and monomers and initiators are added to the nanocellulose dispersion to construct a heterogeneous system, wherein the reactive functional groups are any one of unsaturated carbon-carbon double bonds or active hydrogen groups. (2) Initiate monomer polymerization and initiate in-situ chemical grafting reaction between monomers and reactive functional groups on the surface of modified nanocellulose to obtain a uniform nanocomposite particle dispersion. (3) The uniform nanocomposite particle dispersion is processed and shaped to obtain nanocellulose reinforced composite material.

2. The method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization according to claim 1, characterized in that, The unsaturated carbon-carbon bond includes any choice of vinyl, acryloyl, methacryloyl, or allyl, and the group containing active hydrogen includes any choice of mercapto, primary amine, or secondary amine.

3. The method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization according to claim 1, characterized in that, The modified nanocellulose is prepared by any of the following methods: (1) Acid anhydride esterification method: cellulose raw materials or nanocellulose are modified by esterification with acid anhydrides containing unsaturated carbon-carbon double bonds. The acid anhydrides containing double bonds include at least one of maleic anhydride, methacrylic anhydride, itaconic anhydride, acrylic anhydride, dimethylmaleic anhydride, pentene anhydride, octenyl succinic anhydride, and dodecenyl succinic anhydride. (2) Silane coupling method: Nanocellulose is obtained by surface modification with a silane coupling agent containing unsaturated bonds, mercapto groups or amine groups. The silane coupling agent containing unsaturated double bonds includes at least one of vinyltrimethoxysilane KH-171, vinyltriethoxysilane KH-151, and γ-methacryloyloxypropyltrimethoxysilane KH-570; the silane coupling agent containing mercapto groups includes at least one of γ-mercaptopropyltriethoxysilane KH-580 and γ-mercaptopropyltrimethoxysilane KH-590; and the silane coupling agent containing amine groups includes at least one of 3-aminopropyltriethoxysilane KH-550 and 3-aminopropyltrimethoxysilane KH-540. (3) Amide modification method: The carboxylated nanocellulose is modified by amidation with amine compounds containing unsaturated bonds or thiol groups, wherein the amine compounds containing unsaturated bonds or thiol groups are at least one of allylamine, 4-aminostyrene, 2-aminoethyl methacrylate, cysteine, and L-cysteine. (4) Epoxy ring-opening grafting method: modified by ring-opening reaction between epoxy groups containing unsaturated bonds and hydroxyl or carboxyl groups of cellulose. The epoxy groups containing unsaturated bonds include at least one of glycidyl methacrylate, vinyl glycidyl ether, glycidyl acrylate, limonene monoepoxide, and allyl glycidyl ether.

4. The method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization according to claim 1, characterized in that, The monomers include aromatic vinyl monomers and acrylic monomers, wherein the monomers include one or more of styrene, α-methylstyrene, p-methylstyrene, p-tert-butylstyrene, divinylbenzene, acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, isooctyl acrylate, isobutyl acrylate, lauryl methacrylate, ethyl acetoacetate, cyclohexyl methacrylate, hydroxyethyl methacrylate, and hydroxypropyl acrylate.

5. The method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization according to claim 1, characterized in that, The initiator is an oil-soluble free radical initiator, including one of azobisisobutyronitrile (AIBN), azobisisoheptanenitrile (ABVN), benzoyl peroxide (BPO), etc.

6. The method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization according to claim 1, characterized in that, The concentration of the nanocellulose dispersion is 0.01 wt% to 3 wt%, and the amount of modified nanocellulose added is 0.1 wt% to 5 wt% of the monomer.

7. The method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization according to claim 1, characterized in that, The monomer is added to the nanocellulose dispersion and then emulsified, wherein the emulsification method is selected from one or more of the following: high-speed shearing machine, ultrasound, high-pressure homogenization, microfluidic jet, and pipeline emulsification.

8. The method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization according to claim 1, characterized in that, The heterogeneous system was subjected to heterogeneous polymerization for 1 to 12 hours in an inert atmosphere and a high temperature environment, wherein the inert atmosphere was a nitrogen protective atmosphere and the high temperature environment was 65 to 85°C.

9. The method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization according to claim 1, characterized in that, The processing and molding described in step (3) is a hot working process or a coating molding process; The hot working process includes the following steps; After drying, the nanocomposite particles are hot-pressed, injection-molded, or extruded at a temperature 10–50°C above their melting point to construct a three-dimensional reinforcing skeleton network that runs through the entire composite material. The coating process includes the following steps; The nanocomposite particle dispersion from step (2) is directly applied to the substrate surface as an aqueous film-forming substance.

10. A nanocellulose-reinforced composite material, characterized in that, The nano-cellulose-reinforced composite material prepared according to any one of claims 1 to 9 is prepared by a method for preparing nano-cellulose-reinforced composite materials by in-situ polymerization, comprising: The core polymer and the modified nanocellulose coated or wrapped around the surface of the polymer, wherein the modified nanocellulose is connected to the molecular chain of the polymer by covalent bonds to form an interfacial chemical cross-linking network, and the nanocomposite particles formed by the nanocellulose and the polymer have a regular spherical or near-spherical core-shell structure, and the average particle size of the nanocomposite particles is 100 nm to 10 μm.

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