A strong and tough antibacterial low-leaching polylactic acid composite material and a preparation method thereof

Abstract

The application discloses a kind of tough antibacterial low-leaching polylactic acid composite and preparation method thereof, the mass fraction composition of the composite is: polylactic acid 100 parts, quaternary ammonium LCNF 0.5-10 parts, ionic liquid 0.5-15 parts, compatibilizer 0.5-3 parts, lubricant 0.2-5 parts and stabilizer 0.2-3 parts;Quaternary ammonium LCNF has branched or network structure, and the quaternary ammonium group content thereof is 500-1500 μmol / g, ionic liquid is in situ coated on the surface of quaternary ammonium LCNF by spray drying process in advance, and the anion and cation of ionic liquid form stable coulomb force anchoring structure with hydroxyl group and grafted quaternary ammonium group on the surface of quaternary ammonium LCNF respectively.The composite realizes the synergistic promotion of plasticizing, reinforcing, anti-leaching and antibacterial performance, provides a new type of modification approach for PLA with multifunctional and high performance, and has obvious technical advantages and wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of biodegradable plastics technology, specifically relating to a strong, tough, antibacterial, low-emission polylactic acid composite material and its preparation method. Background Technology

[0002] Polylactic acid (PLA) is a bio-based biodegradable polymer derived from renewable resources such as corn starch, cassava, and wheat. It possesses excellent biocompatibility, high transparency, and high mechanical strength, making it one of the most widely used green biodegradable materials. PLA has a melting point of approximately 150-170℃ and a glass transition temperature of approximately 60℃. It exhibits high rigidity and good processability, and has been widely applied in food packaging, disposable products, medical devices, and 3D printing. Compared to traditional petroleum-based plastics, PLA can degrade into carbon dioxide and water under industrial composting conditions after use, making it environmentally friendly and meeting the current demands for green and sustainable materials development.

[0003] However, inherent defects in PLA severely limit its further promotion. First, the regular molecular chains and restricted segment movement of PLA result in high brittleness and low elongation at break (generally less than 10%), with impact toughness far lower than commonly used packaging materials such as polyethylene and polypropylene, making it prone to brittle fracture under compression and impact. Second, to address PLA's poor toughness, existing technologies generally involve adding exogenous plasticizers or flexible components for toughening, such as polyethylene glycol, citrate esters, and phthalate plasticizers. While traditional plasticizers can lower the glass transition temperature of PLA and improve chain segment flexibility, they also significantly reduce tensile strength and modulus. Furthermore, the bonding between plasticizer molecules and the PLA matrix relies mainly on van der Waals forces, resulting in weak adhesion. Under long-term storage or conditions of heat and moisture, migration and precipitation can easily occur, leading to a sticky, whitened surface, decreased mechanical properties, and potential food safety risks. Therefore, achieving low-precipitation or non-precipitation plasticization has become a significant technical bottleneck in the field of PLA toughening.

[0004] On the other hand, PLA inherently lacks antibacterial properties. In applications such as food packaging and medical consumables, PLA-based materials are susceptible to bacterial and mold contamination, leading to food spoilage or medical device malfunction. Existing antibacterial modification methods mostly achieve this by adding silver ions, metal oxides, or quaternary ammonium salt antibacterial agents. However, these antibacterial agents generally have poor compatibility with PLA, easily resulting in uneven dispersion in the matrix and affecting mechanical properties. Furthermore, some inorganic antibacterial agents may raise safety concerns, limiting their application in areas involving direct contact with food or the human body.

[0005] In recent years, some technologies have attempted to use ionic liquids to plasticize or modify PLA. Ionic liquids possess characteristics such as low volatility, high thermal stability, and good compatibility with polar polymers, which can improve the toughness of PLA to some extent (Xu et al., Green Chem., 2019, 21, 4449). However, existing research mainly focuses on laboratory methods such as solvent casting or assisted phase separation, which have failed to effectively solve the migration problem of ionic liquids in PLA, nor have they taken into account the antibacterial requirements. In addition, nanocellulose is often used to reinforce PLA due to its high specific surface area and high strength, but nanocellulose is highly hydrophilic, has poor compatibility with hydrophobic PLA, and is prone to aggregation, thus having limited improvement on mechanical properties (Niu et al., Journal of Hazardous Materials, 2021, 402: 124073). Some studies have proposed improving the dispersibility and imparting antibacterial properties through quaternization modification, but existing technologies are mostly focused on the dispersibility of nanocellulose itself and have not yet been used in conjunction with PLA systems to achieve a comprehensive function of toughening, strengthening, antibacterial and inhibiting plasticizer migration (Choi et al., Industrial Crops & Products, 2025, 228:120915; Xu et al., Carbohydrate Polymers, 2024, 340:122301).

[0006] In summary, existing technologies have significant shortcomings in improving PLA toughness, reducing plasticizer exudation, and enhancing antibacterial properties: toughening often leads to a decrease in strength; traditional plasticizers generally suffer from migration problems; the addition methods of antibacterial components are complex and have poor compatibility; and reinforcement and plasticization are difficult to coordinate. Therefore, there is an urgent need to develop a novel PLA-based composite material that can simultaneously achieve high toughness, low exudation, and antibacterial properties. Summary of the Invention

[0007] This invention aims to overcome the shortcomings of existing PLA materials, such as poor toughness, easy migration and exudation of traditional plasticizers, and lack of antibacterial properties, and provides a strong, tough, antibacterial, and low-exudation polylactic acid composite material and its preparation method. The composite material of this invention achieves a synergistic improvement in plasticizing, strengthening, anti-exudation, and antibacterial properties, providing a novel, multifunctional, and high-performance modification approach for PLA, with significant technical advantages and broad application prospects.

[0008] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: a strong, tough, antibacterial, and low-emission polylactic acid composite material, the mass fraction of which is: 100 parts polylactic acid, 0.5-10 parts quaternized lignocellulose nanofibers (i.e., quaternized LCNF), 0.5-15 parts ionic liquid, 0.5-3 parts compatibilizer, 0.2-5 parts lubricant, and 0.2-3 parts stabilizer.

[0009] This invention relates to a composite material based on polylactic acid (PLA), incorporating quaternized LCNF, an ionic liquid, a compatibilizer, a lubricant, and a stabilizer. The ionic liquid acts as a highly efficient, low-migration plasticizer, and a stable Coulombic force is formed between the quaternized LCNF and the ionic liquid, fundamentally inhibiting ionic liquid migration. Simultaneously, the quaternized LCNF itself possesses high strength, compensating for the strength reduction caused by plasticization, and its quaternary ammonium groups impart antibacterial properties to the material. The compatibilizer improves the interfacial compatibility between the components, while the lubricant and stabilizer ensure processing flowability and material stability. This composite material achieves a synergistic improvement in plasticization, reinforcement, anti-exudation, and antibacterial properties, providing a novel, multifunctional, and high-performance modification approach for PLA, exhibiting significant technical advantages and broad application prospects.

[0010] The quaternized lignocellulose nanofibers are quaternized lignocellulose nanofibers with branched or network structures. The quaternized lignocellulose nanofibers contain 500–1500 μmol / g of quaternized ammonium groups. The ionic liquid is pre-coated in situ onto the surface of the quaternized lignocellulose nanofibers by spray drying. The anions and cations of the ionic liquid form stable Coulombic anchoring structures with the hydroxyl groups and grafted quaternary ammonium groups on the surface of the quaternized lignocellulose nanofibers, respectively, so that the ionic liquid is in a restrained state in the polylactic acid matrix.

[0011] The quaternized LCNF used in this invention differs from ordinary rod-shaped cellulose nanocrystals (CNC). This quaternized LCNF possesses a unique branched or network structure, which provides a higher specific surface area and ideal sites for subsequent in-situ coating with ionic liquids, effectively improving the mechanical strength and impact resistance of the composite material. Furthermore, the quaternized LCNF contains 500–1500 μmol / g of quaternized ammonium groups to ensure effective Coulombic anchoring between the quaternized LCNF and the ionic liquid, and to prevent LCNF aggregation or decreased compatibility with the matrix due to excessive quaternary ammonium groups.

[0012] Preferably, the raw materials for the quaternized lignocellulose nanofibers are one or a combination of two or more of wood powder, bamboo powder, and straw powder. These raw materials are widely available, low in cost, and all contain abundant cellulose components. Stable quaternized LCNFs can be prepared through subsequent alkali treatment, bleaching, and quaternization modification.

[0013] Preferably, the ionic liquid is one or a combination of two or more of 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium tetrafluoroborate. These ionic liquids all exhibit low volatility, high thermal stability, and good compatibility with quaternized LCNF, making them suitable for this composite material system.

[0014] Preferably, the compatibilizer is at least one selected from silane coupling agents, titanate coupling agents, and aluminate coupling agents. These coupling agent molecules contain groups that can react with quaternized LCNF and groups that can react with polylactic acid, thus forming a bridging effect at the interface between the two phases, improving interfacial bonding, and thereby enhancing the mechanical properties of the composite material.

[0015] As a further preferred option, the silane coupling agent is KH540, KH550, KH580, or KH590, the titanate coupling agent is CS-101, and the aluminate coupling agent is DL-411. All of the above-mentioned coupling agents are commercially available mature products with well-defined chemical structures and stable quality, facilitating industrial application.

[0016] Preferably, the lubricant is one or a combination of two or more of the following: sodium cocoyl aminopropionate, polyvinyl acetate resin, ethylene bis-stearamide, oleamide, and erucamide. The lubricant's function is to reduce friction between the material and equipment during melt processing, improve fluidity, and enhance demolding performance. The aforementioned lubricant exhibits good compatibility with polylactic acid systems, facilitating smooth extrusion, injection molding, and other molding processes.

[0017] Preferably, the stabilizer is at least one of antioxidant 1010, antioxidant 1098, antioxidant 300, and antioxidant 168. All of the above antioxidants are hindered phenolic or phosphite antioxidants, capable of capturing free radicals generated by polylactic acid during high-temperature processing, inhibiting thermal oxidative degradation, and maintaining the material's mechanical properties and appearance.

[0018] The preparation method of the above-mentioned strong, tough, antibacterial, and low-exudation polylactic acid composite material includes the following steps: 1) Alkali treatment and bleaching of cellulose: Mix 1-10 parts of cellulose raw material powder with 20-150 parts of sodium hydroxide solution, heat and stir at 50-70℃ for 2-4 h, wash with pure water until neutral, filter, add 20-80 parts of sodium chlorite solution and 4-10 parts of glacial acetic acid, heat and stir at 50-70℃ for 0.5-2 h, wash with pure water until neutral, and dry to dryness to obtain purified cellulose that retains some residual lignin; 2) Quaternization modification: 4-8 parts of purified cellulose obtained in step 1) are dispersed in 90-100 parts of pure water, and 120-200 parts of 2,3-epoxypropyltrimethylammonium chloride are added. The mixture is heated and stirred at 60-65℃ for 4-6 h, and then mechanically sheared at 10000-15000 rpm for 1-2 h to obtain a reaction mixture containing quaternized lignocellulose nanofibers with branched or network structures. The reaction mixture is neutralized with a 5 wt% HCl solution and washed with pure water by centrifugation 4-6 times to prepare a 2 wt% quaternized lignocellulose nanofiber solution. 3) Spray drying: Add 0.5 to 15 parts of ionic liquid to 25 to 500 parts of quaternized lignocellulose nanofiber solution obtained in step 2), stir for 20 to 30 min, and spray dry to allow the quaternized lignocellulose nanofibers with branched or network structures to self-assemble into a physical entangled network, and allow the ionic liquid to coat the surface of the quaternized lignocellulose nanofibers in situ to form a Coulomb force anchoring structure, thus obtaining ionic liquid composite quaternized lignocellulose nanofibers; 4) High-speed premixing: The ionic liquid composite quaternized lignocellulose nanofibers obtained in step 3) are added to a high-speed mixer and mixed for 30-60 min at a speed of 300-800 rpm to obtain a premix. 5) Twin-screw melt extrusion granulation: The premix obtained in step 4) is added to a twin-screw extruder for extrusion and granulation. The processing temperature of the twin-screw extruder is 110-190℃, the screw length-to-diameter ratio is 30-60, and the rotation speed is 50-250 rpm to obtain polylactic acid / nanocellulose composite particles. 6) Final molding: The polylactic acid / nanocellulose composite material particles obtained in step 5) are processed into composite material products by injection molding machine, casting machine or hot press.

[0019] The preparation method of the composite material of this invention includes six steps: alkali treatment and bleaching of cellulose, quaternization modification, spray drying, high-mix premixing, twin-screw melt extrusion granulation, and final molding. Among them, the quaternized LCNF is prepared by a two-step method combining alkali treatment and bleaching pretreatment with quaternization grafting reaction, which has comprehensive advantages such as high efficiency, environmental protection, and controllability.

[0020] In the alkali treatment and bleaching steps of cellulose, the present invention can remove hemicellulose and most of the lignin in the raw material powder by alkali treatment with sodium hydroxide solution and oxidative bleaching with sodium chlorite solution, making the structure of cellulose purer and the reaction sites fully exposed, thereby improving the reactivity of cellulose in subsequent quaternization modification.

[0021] In the quaternization modification step, this invention utilizes a ring-opening reaction between 2,3-epoxypropyltrimethylammonium chloride and the hydroxyl groups in cellulose in an aqueous system to achieve efficient grafting of cationic quaternary ammonium groups. This process can be carried out at 60–65°C without organic solvents, reducing energy consumption and side reactions, thus improving the safety and environmental friendliness of the material. Simultaneously, combined with high-speed mechanical shearing, the fibers are refined, dispersibility is improved, and the exposure of quaternization groups on the fiber surface is promoted within the original reaction system, while grafting continues to enhance interfacial activity and antibacterial properties. Compared to existing technologies that only perform mechanical dissociation or chemical surface modification, the quaternized LCNF prepared by this invention through a dual mechanism of "chemical grafting + mechanical dissociation" exhibits high reactivity, high dispersibility, and stable antibacterial properties. Furthermore, by adjusting the feed amount of 2,3-epoxypropyltrimethylammonium chloride, reaction time, and shear time, the quaternization group content can be controlled within the range of 500–1500 μmol / g, thereby allowing for the design of the material's antibacterial properties, hydrophilicity, and interfacial compatibility according to actual needs. The quaternized LCNF solution obtained after neutralization and centrifugation exhibits excellent dispersibility, structural stability, controllable particle size, and high surface activity, significantly enhancing the interfacial bonding ability of PLA-based composite materials, imparting long-term stable antibacterial effects, and effectively reducing the risk of precipitation.

[0022] In the spray drying step, this invention utilizes a quaternized LCNF aqueous solution system to ensure thorough fiber dispersion and complete exposure of surface functional groups. During the mixing and stirring stage, the ionic liquid achieves uniform contact with the fiber at the molecular scale, resulting in a strong multi-site Coulombic anchoring effect: specifically manifested as the CN-C of the quaternary ammonium groups. + The quaternized LCNF, with its branched or network-like structure, forms an electrostatic trapping structure with the anionic centers of the ionic liquid and the -OH groups on the LCNF surface. Subsequently, during the rapid flash evaporation of the solvent in the spray-drying process, the quaternized LCNF self-assembles and constructs a physically entangled network, "locking" the ionic liquid in situ within the pores and fiber surface of this network, forming a Coulomb force anchoring structure. This synergistic restraining effect of chemical anchoring and physical coating not only effectively avoids the problems of local agglomeration and uneven coating that are easily caused by traditional dry mixing or spraying, forming a composite powder with uniform composition and good flowability, but also ensures the ultra-high dispersion and batch stability of the ionic liquid in subsequent twin-screw melt processing. It completely solves the problem of phase separation and migration of small molecule plasticizers in polylactic acid matrix from a mechanistic perspective, ultimately endowing the composite system with long-lasting and controllable toughening and antibacterial effects.

[0023] In the high-mixing premixing step, this invention involves high-mixing ionic liquid-composite quaternized LCNF with PLA, compatibilizers, lubricants, and stabilizers, which further strengthens the electrostatic interaction between the ionic liquid and the quaternized groups. The cations on the surface of the quaternized cellulose and the counterions of the ionic liquid can form a Coulombic binding structure, which not only enhances the interfacial affinity between LCNF and PLA but also keeps the ionic liquid in a "restrained" state within the system. This restraining effect effectively limits the free migration of small molecule additives such as plasticizers and lubricants, mechanistically solving the problems of easy precipitation and re-adhesion in traditional PLA composites, and significantly improving the long-term stability and durability of the composite material.

[0024] In the twin-screw melt extrusion granulation step, the premixed material is melt-blended in a twin-screw extruder. Quaternized LCNF maintains good dispersion with the help of the ionic liquid interfacial layer, forming a tighter interfacial bond with PLA. Specifically, the synergistic effect of the ionic liquid and quaternized LCNF matches the interfacial polarity, and the compatibilizer and PLA form a chemical or physical bridge at the interface, jointly constructing a stable interfacial network structure. This interfacial structure significantly improves the stress transfer efficiency between PLA and LCNF, enabling the material to achieve a significantly increased elongation at break while maintaining strength, achieving a combination of high strength and high toughness. Simultaneously, the quaternary ammonium groups remain firmly fixed on the LCNF surface under molten conditions, which can exert a charge-disrupting effect on microbial cell membranes, endowing the material with long-term, non-migrating antibacterial properties, significantly superior to the problem of easy failure of traditional additive antibacterial agents. The uniform distribution of LCNF not only improves the mechanical properties of the composite material but also, by improving stress transfer efficiency, enables the material to achieve better toughness and balanced mechanical performance.

[0025] In the final molding stage, due to the aforementioned design achieving flow stability, interfacial stability, and low additive migration in the material system, the composite material of this invention exhibits excellent processing fluidity and dimensional stability during injection molding or hot pressing. The material obtained by the method of this invention has no oil seepage or exudation on its surface, a wide processing window, and few molding defects. At the same time, the material combines high toughness, antibacterial properties, low exudation, and green biodegradability, with a performance integration level far exceeding that of existing systems that rely on multiple additives to achieve local performance improvements.

[0026] Preferably, in step 1), the raw material powder has a mesh size of 80-200 mesh, the sodium hydroxide solution has a concentration of 10-20 w / v%, and the sodium chlorite solution has a concentration of 10-20 w / v%. These parameter ranges ensure the effectiveness of the alkali treatment and bleaching treatment: excessively fine raw material powder increases costs, while excessively coarse powder results in insufficient treatment; excessively low concentrations of sodium hydroxide and sodium chlorite lead to poor impurity removal, while excessively high concentrations may cause excessive degradation of cellulose.

[0027] Compared with the prior art, the present invention has the following advantages: (1) The composite material of this invention utilizes ionic liquids as efficient and low-migration plasticizers. A stable Coulombic force can be formed between the quaternized LCNF and the ionic liquid, fundamentally inhibiting ionic liquid migration. Simultaneously, the quaternized LCNF itself possesses high strength, compensating for the strength reduction caused by plasticization, and its quaternary ammonium groups impart antibacterial properties to the material. Compatibilizers are used to improve the interfacial compatibility between components, while lubricants and stabilizers ensure processing fluidity and material stability. The composite material of this invention achieves a synergistic improvement in plasticization, reinforcement, anti-exudation, and antibacterial properties, providing a novel, multifunctional, and high-performance modification approach for PLA, with significant technical advantages and broad application prospects.

[0028] (2) The preparation method of the composite material of the present invention includes six steps: alkali treatment and bleaching of cellulose, quaternization modification, spray drying, high-mix premixing, twin-screw melt extrusion granulation, and final molding. This method achieves the flow stability, interfacial stability, and low migration of additives in the material system. The material obtained by the method of the present invention has no oil seepage or precipitation on its surface, a wide processing window, and few molding defects. At the same time, the material has both high toughness, antibacterial properties, low precipitation, and green biodegradability. Its performance integration is far higher than that of existing systems that rely on multiple additives to achieve local performance improvement. Attached Figure Description

[0029] Figure 1 Antibacterial properties of polylactic acid composites reinforced with different quaternized LCNF; Figure 2 Tensile properties of composite materials from Examples 1 to 5 with different amounts of quaternized LCNF; Figure 3 Impact properties of composite materials from Examples 1 to 5 with different amounts of quaternized LCNF; Figure 4 The exudation properties of composite materials from Examples 1 to 5 with different amounts of quaternized LCNF; Figure 5 Tensile properties of composite materials from Examples 6 to 10 with different amounts of ionic liquid added; Figure 6 Impact properties of composite materials from Examples 6 to 10 with different amounts of ionic liquid added; Figure 7 Precipitation properties of composite materials from Examples 6 to 10 with different amounts of ionic liquid added; Figure 8 Tensile properties of composite materials from Examples 11 to 14, which use different types of ionic liquids; Figure 9Impact properties of composite materials from Examples 11 to 14, which use different types of ionic liquids; Figure 10 Precipitation properties of composite materials from Examples 11 to 14, which use different types of ionic liquids; Figure 11 Tensile properties of composite materials of different quaternized LCNFs in Examples 15 to 17; Figure 12 Impact properties of composite materials from Examples 15 to 17 with different quaternized LCNFs; Figure 13 Precipitation properties of composite materials from Examples 15 to 17 with different quaternized LCNFs; Figure 14 TEM image of quaternized LCNF in Example 3; Figure 15 SEM image of the tensile fracture of the composite material in Example 3; Figure 16 SEM images of polylactic acid material under tensile fracture in Comparative Example 1. Figure 17 SEM images of tensile fracture of polylactic acid / ionic liquid composite material in Comparative Example 2; Figure 18 SEM images of tensile fracture of polylactic acid / unmodified LCNF composite material in Comparative Example 3; Figure 19 SEM image of tensile fracture of polylactic acid / unpre-sprayed LCNF composite material in Comparative Example 4. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0031] The mass fraction composition of the strong, antibacterial, and low-exudation polylactic acid composite materials of Examples 1 to 5 is shown in Table 1. The composite materials of Examples 1 to 5 are respectively represented by the numbers C1, C2, C3, C4, and C5, and single-factor experiments were conducted.

[0032] In Examples 1 to 5: the LCNF used was moso bamboo LCNF, the quaternary ammonium group content of the quaternized moso bamboo LCNF used was 1000 μmol / g, the ionic liquid used was 1-butyl-3-methylimidazolium tetrafluoroborate, the compatibilizer used was silane coupling agent KH580, the lubricant used was sodium cocoyl aminopropionate, and the stabilizer used was antioxidant 1010.

[0033] Table 1

[0034] The method for preparing the composite material in Example 1 includes the following steps: 1) Alkali treatment and bleaching of cellulose: 10 parts of cellulose raw material powder (bamboo powder) with a mesh size of 100 mesh were mixed with 100 parts of sodium hydroxide solution with a concentration of 10 w / v. The mixture was heated and stirred at 60°C for 4 h. After washing with pure water until neutral, the mixture was filtered. Then, 80 parts of sodium chlorite solution with a concentration of 15 w / v and 10 parts of glacial acetic acid were added. The mixture was heated and stirred at 60°C for 1 h. After washing with pure water until neutral, the mixture was dried to dryness to obtain purified cellulose that retains some residual lignin. 2) Quaternization modification: The 6 parts of purified cellulose obtained in step 1) were dispersed in 95 parts of pure water, and 160 parts of 2,3-epoxypropyltrimethylammonium chloride were added. The mixture was heated and stirred at 65°C for 5 h, and then mechanically sheared at 13000 rpm for 1.5 h to obtain a reaction mixture containing quaternized lignocellulose nanofibers with branched or network structures. The reaction mixture was neutralized with 5 wt% HCl solution and washed 6 times with pure water by centrifugation to prepare a 2 wt% quaternized LCNF solution with a quaternary ammonium group content of approximately 1000 μmol / g. 3) Spray drying: Add 5 parts of ionic liquid to the 25 parts of quaternized LCNF solution obtained in step 2), stir for 30 min, and spray dry to allow the quaternized lignocellulose nanofibers with branched or network structures to self-assemble and construct a physical entanglement network, and allow the ionic liquid to coat the surface of the quaternized lignocellulose nanofibers in situ to form a Coulomb force anchoring structure, thus obtaining ionic liquid composite quaternized LCNF. 4) High-speed premixing: Add the ionic liquid composite quaternized LCNF obtained in step 3) to 100 parts of polylactic acid, 1 part of compatibilizer, 3 parts of lubricant and 1 part of stabilizer into a high-speed mixer and mix for 45 min at a speed of 500 rpm to obtain a premix. 5) Twin-screw melt extrusion granulation: The premix obtained in step 4) is added to a twin-screw extruder for extrusion and granulation. The processing temperature of the twin-screw extruder is 110-190℃, the screw length-to-diameter ratio is 50, and the rotation speed is 200 rpm to obtain polylactic acid / LCNF composite particles. 6) Final molding: The polylactic acid / LCNF composite material particles obtained in step 5) are processed into composite material products through injection molding machine, casting machine or hot press.

[0035] The preparation methods of the composite materials in Examples 2 to 5 are basically the same as those in Example 1, except that in step 3) of the preparation of Examples 2 to 5, the amount of quaternized LCNF solution added is 50 parts, 150 parts, 300 parts and 500 parts respectively, and the composite materials of Examples 2 to 5 are prepared respectively.

[0036] For comparison, the following comparative examples 1 to 4 were prepared as polylactic acid materials or polylactic acid composites.

[0037] Comparative Example 1 is pure polylactic acid material, i.e., 100 parts of polylactic acid, without adding other materials.

[0038] Comparative Example 2 is a polylactic acid / ionic liquid composite material with the following composition by weight: 100 parts polylactic acid, 5 parts ionic liquid, 1 part compatibilizer, 3 parts lubricant, and 1 part stabilizer. Quaternized LCNF is not added. The ionic liquid used is 1-ethyl-3-methylimidazolium chloride. The compatibilizer is aluminate coupling agent DL-411. The lubricant is polyvinyl acetate resin. The stabilizer is antioxidant 1098. The preparation method of the composite material in Comparative Example 2 is as follows: a: Add 100 parts of polylactic acid, 5 parts of ionic liquid, 1 part of compatibilizer, 3 parts of lubricant, and 1 part of stabilizer to a high-speed mixer and mix for 45 minutes at a speed of 500 rpm to obtain a premix. b: The premix obtained in step a is added to a twin-screw extruder for extrusion and granulation. The processing temperature of the twin-screw extruder is 110-190℃, the screw length-to-diameter ratio is 50, and the rotation speed is 200 rpm to obtain polylactic acid / ionic liquid composite material.

[0039] Comparative Example 3 is an unmodified LCNF / polylactic acid / ionic liquid composite material, with the following mass fractions: 100 parts polylactic acid, 5 parts ionic liquid, 1 part compatibilizer, 3 parts lubricant, 1 part stabilizer, and 3 parts unquaternized LCNF. The LCNF was prepared from straw powder, the compatibilizer was silane coupling agent KH540, the lubricant was ethylene bis-stearamide, and the stabilizer was antioxidant 300. The preparation method of the composite material in Comparative Example 3 is basically the same as that in Example 1, except that 2,3-epoxypropyltrimethylammonium chloride is not added in step 2), and neutralization with HCl solution is not used.

[0040] Comparative Example 4 is an LCNF-reinforced polylactic acid / ionic liquid composite material prepared without spray pre-coating. Its mass fraction composition is: 100 parts polylactic acid, 5 parts ionic liquid, 1 part compatibilizer, 3 parts lubricant, 1 part stabilizer, and 3 parts quaternized LCNF. The LCNF used has a quaternary ammonium group content of 1000 μmol / g, the ionic liquid is 1-allyl-3-methylimidazolium chloride, the compatibilizer is silane coupling agent KH550, the lubricant is oleamide, and the stabilizer is antioxidant 168. The preparation method of the LCNF-reinforced polylactic acid / ionic liquid composite material of Comparative Example 4 without spray pre-coating is basically the same as that of Example 1, except that step 3) is not required. Instead, 3 parts of dried quaternized LCNF are directly added to a high-speed mixer along with 100 parts polylactic acid, 5 parts ionic liquid, 2 parts compatibilizer, 3 parts lubricant, and 3 parts stabilizer.

[0041] The composite materials of Examples 1 to 5 were prepared into tensile and impact test specimens according to the methods of GB / T 1040 and GB / T 1843 standards, and their tensile and impact mechanical properties were then tested in groups. The ionic liquid migration and precipitation rate of the composite materials of Examples 1 to 5 were tested in groups according to the method of HG / T 4454 standard. The surface antibacterial properties of the composite materials of Examples 1 to 5 were tested in groups according to the method of GB / T 31402 standard.

[0042] The antibacterial performance test sample was a square sheet, with a standard size of 50mm × 50mm, inoculated with a Staphylococcus aureus (ATCC6538) suspension (initial concentration 1.0 × 10⁻⁶). 5 (CFU / mL), covered with sterile polyethylene film and incubated at 37℃ and 90%RH for 24 h. The inhibition zones were observed after dilution and plate preparation. Results are as follows: Figure 1 As shown. The tensile test specimens were dumbbell-shaped, with a length of 75 mm, a gauge length of 30 mm, a mid-width of 4 mm, and a thickness of 2 mm. The tensile speed was 20 mm / min, and at least 5 specimens were tested in each group. The average tensile strength and elongation at break of the specimens were taken. The results are shown in the figure. Figure 2 As shown. The impact test used notched impact specimens, 80 mm long, 10 mm wide, and 4 mm thick, with a notch in the middle. The pendulum weight was 1.245 kg, and the angle was 150°. At least 5 specimens were tested in each group, and the average notched impact strength of the specimens was taken. The results are shown below. Figure 3 As shown. The ionic liquid migration and precipitation samples were square sheets, 50 mm in length and 1 mm in thickness. The ionic liquid migration and precipitation rate of each sample was tested in a 120℃ wind-heat aging test chamber. At least 5 sample points were tested in each group, and the average value was taken. The results are shown below. Figure 4As shown.

[0043] The mechanical properties and ionic liquid migration and precipitation samples of Comparative Examples 1 to 4 were prepared and tested in the same way as those of Examples 1 to 5, and their properties are shown in Table 2.

[0044] Table 2

[0045] from Figure 2 , Figure 3 and Figure 4 As can be seen, with the increase of the amount of quaternized LCNF, the tensile strength, elongation at break, and notched impact strength of the composite materials in Examples 1 to 5 all showed a trend of first increasing and then decreasing. When the amount of quaternized LCNF added was 3 parts, the elongation at break and the notched impact strength reached their maximum values, which were 100.5% and 29.6 kJ / m, respectively. 2 When the amount of quaternized LCNF added was 6 parts, the tensile strength reached a maximum of 68.1 MPa. The ionic liquid migration and precipitation rate gradually decreased with the increase of the amount of quaternized LCNF added, reaching 0.29% at 3 parts and then stabilizing.

[0046] In composite material systems, ionic liquids soften polylactic acid (PLA) segments, improving elongation at break and impact toughness. With the addition of quaternized LCNF, the LCNF provides high-modulus reinforcement and forms interfacial chemical bonds with compatibilizers, enhancing dispersion and interfacial bonding, thus achieving a strengthening and toughening effect and compensating for the strength reduction caused by the plasticizing effect of the ionic liquid. Excessive addition may lead to agglomeration, affecting the strengthening and toughening effect of the LCNF. Furthermore, with increasing quaternized LCNF, the number of binding sites for the ionic liquid on cellulose increases, reducing the free ionic liquid within the PLA matrix and decreasing its exudation potential.

[0047] Compared to Comparative Example 1, the tensile strength of the composite materials in Examples 1 to 4 was improved, and the elongation at break and notched impact strength were increased by approximately 19.7 times and 11.0 times, respectively. This is mainly due to the unique material composition and processing method of this invention, including quaternized LCNF, ionic liquid, and compatibilizer. Compared to Comparative Example 2, the tensile strength, elongation at break, and notched impact strength of Examples 1 to 5 were increased by approximately 1.7 times, 2.7 times, and 3.7 times, respectively. Comparative Example 2 itself is a toughening system with ionic liquid and has good toughness; however, it lacks quaternized LCNF and a unique processing method, so it obviously cannot achieve toughening. Moreover, the ionic liquid migration and precipitation rate is 12.8 times that of the material in Example 3. Compared to Comparative Example 3, the tensile strength, elongation at break, and notched impact strength of Examples 1 to 5 were increased by approximately 1.5 times, 3.2 times, and 3.7 times, respectively, while the ionic liquid migration and precipitation rate decreased to 0.07 times. Comparative Example 3 used unquaternized LCNF, demonstrating that the quaternary ammonium groups introduced onto the surface of the quaternized LCNF can effectively anchor the ionic liquid with coulombic forces, thereby enhancing interfacial bonding, improving mechanical properties, and effectively reducing the migration and precipitation rate of the ionic liquid. Compared to Comparative Example 4, the tensile strength, elongation at break, and notched impact strength of Examples 1-5 increased by approximately 1.4 times, 2.5 times, and 3.3 times, respectively, while the migration and precipitation rate of the ionic liquid decreased to 0.10 times. This shows that the pre-spray coating method of ionic liquid and quaternized LCNF can enhance the bonding between the two, preventing the ionic liquid from existing as a free phase, locking the uniform distribution of the ionic liquid on the cellulose surface, forming a stable and highly fluid composite powder, and fully utilizing the strengthening and toughening effects of both.

[0048] In addition, by Figure 1 It is evident that Example 3 exhibits good anti-Staphylococcus aureus activity, while samples without modified LCNF or without utilizing the special encapsulation process of this invention do not possess this antibacterial property. Figure 14 This is a TEM image of the quaternized LCNF from Example 3. Figure 15 This is a SEM image of the tensile fracture of the composite material in Example 3. Before transmission electron microscopy (TEM) observation, the sample was treated as follows: a 0.05 wt% aqueous suspension of the composite material sample was ultrasonically treated for 30 min, then stained in a 2 wt% uranium acetate solution for 3 min. The stained sample was then dropped onto a copper TEM grid, and excess water was absorbed with a paper towel. The microstructure of the sample could then be observed using TEM. Figure 14 The branched and network-like lignocellulose nanofibers of LCNF can be clearly seen in the images. Figure 15It can be seen that Example 3 exhibits a typical ductile fracture morphology, and it can be clearly observed that the branched LCNF forms a tight physical entanglement network in the polylactic acid matrix without obvious phase separation.

[0049] Figure 16 This is a SEM image of the tensile fracture of polylactic acid material in Comparative Example 1. Figure 17 The image shows the tensile fracture SEM image of the polylactic acid / ionic liquid composite material in Comparative Example 2. Figure 18 The image shows the tensile fracture SEM image of the polylactic acid / unmodified LCNF composite material in Comparative Example 3. Figure 19 The image shows a tensile fracture SEM image of the polylactic acid / unpre-spray-coated LCNF composite material from Comparative Example 4. Figures 16-19 The results show that: the polylactic acid material in Comparative Example 1 exhibits typical brittle fracture; in Comparative Example 2, the ionic liquid forms cavities within the PLA matrix; in Comparative Example 3, the interface between LCNF and the PLA matrix is ​​poor; and while the material in Comparative Example 4 shows ductile fracture, the interfacial bonding between LCNF and PLA is weak. Comparative analysis of Example 3 and Comparative Example 4 (without pre-spray coating) reveals that the material using the in-situ coating process exhibits approximately a 90% reduction in ionic liquid migration and precipitation, and an approximately 3.3-fold increase in notched impact strength. This strongly demonstrates that the branched entanglement structure of LCNF combined with the Coulomb force anchoring mechanism has a significant synergistic effect in achieving both strength and low precipitation in polylactic acid composites.

[0050] The mass fraction composition of the composite materials in Examples 6 to 10 is shown in Table 3. The composite materials in Examples 6 to 10 are respectively represented by the numbers S1, S2, S3, S4, and S5, and single-factor experiments were conducted.

[0051] In Examples 6 to 10: the LCNF used was straw LCNF, the quaternized LCNF used had a quaternary ammonium group content of 1000 μmol / g, the ionic liquid used was 1-butyl-3-methylimidazolium chloride, the compatibilizer used was titanate coupling agent CS-101, and the lubricant used was sodium cocoa oil-based aminopropionate.

[0052] Table 3

[0053] The preparation method of the composite material in Example 6 is basically the same as that in Example 1, except that in step 3) of the preparation of Example 6, the amount of quaternized LCNF solution added is 150 parts (i.e., the amount of quaternized LCNF added is 3 parts), the amount of ionic liquid added is 0.5 parts, the amount of compatibilizer is 0.5 parts, the amount of lubricant is 4 parts, and the amount of stabilizer is 2 parts.

[0054] The preparation methods of the composite materials in Examples 7 to 10 are basically the same as those in Example 6, except that in step 3) of the preparation of Examples 7 to 10, the amount of ionic liquid added is 1 part, 3 parts, 8 parts and 15 parts respectively, and the composite materials of Examples 7 to 10 are prepared respectively.

[0055] Using the same method as for the composite material in Example 1, tensile tests, impact tests, and ionic liquid migration and precipitation tests were performed on the composite materials of Examples 6 to 10. The test results for tensile strength and tensile elongation at break are as follows: Figure 5 As shown, the test results for notched impact strength are as follows: Figure 6 As shown, the test results for migration exudation are as follows: Figure 7 As shown.

[0056] from Figure 5 , Figure 6 and Figure 7 It can be seen that as the amount of ionic liquid added increases, the tensile strength of the composite materials in Examples 6 to 10 continuously decreases, while the elongation at break continuously increases. The notched impact strength shows a trend of first increasing and then decreasing, reaching a maximum value of 35.8 kJ / m when the amount of ionic liquid added is 3 parts. 2 Simultaneously, the migration and precipitation rate of the ionic liquid continuously increases. These phenomena are mainly due to the addition of the ionic liquid, which disrupts the crystal structure of polylactic acid (PLA), weakens its mechanical strength, and makes the PLA molecular chains easier to slide, thus playing a plasticizing role. The change in notched impact strength is due to the synergistic effect of the reinforcing effect of LCNF and the plasticizing effect of the ionic liquid. With the increase of ionic liquid content, the quaternized LCNF sites cannot effectively bind the free ionic liquid, leading to an increase in the ionic liquid precipitation rate and a decrease in tensile strength. Considering these performance values, Examples 6 to 10 all have significant advantages over Comparative Examples 1 to 4.

[0057] Compared to Comparative Example 1, the tensile strength of the composite materials in Examples 6-10 remained relatively stable or slightly increased when the ionic liquid content was low (Examples 6-8), but began to decrease when the content was high (Examples 9-10). Meanwhile, the elongation at break and notched impact strength increased significantly with increasing ionic liquid content, with the elongation at break increasing by a maximum of approximately 24.8 times and the notched impact strength by a maximum of approximately 13.3 times. This is mainly attributed to the effective toughening and interfacial reinforcement effect synergistically generated by the ionic liquid and quaternized LCNF. Compared to Comparative Example 2, the tensile strength of Examples 6-10 increased by a maximum of approximately 1.7 times, the elongation at break increased by a maximum of approximately 2.7 times, and the notched impact strength increased by a maximum of approximately 3.7 times. Furthermore, the ionic liquid migration and precipitation rate significantly decreased to 0.08 times that of Comparative Example 2. This is because the quaternized LCNF effectively anchored the ionic liquid through Coulomb forces, significantly inhibiting its free migration. Compared to Comparative Example 3, Examples 6-10 showed a maximum increase of approximately 1.5 times in tensile strength, approximately 3.2 times in elongation at break, and approximately 3.7 times in notched impact strength, while the migration and precipitation rate was significantly reduced to 0.07 times. This is because the quaternization modification significantly enhanced the interfacial compatibility and bonding force between LCNF and the ionic liquid and PLA matrix. Compared to Comparative Example 4, Examples 6-10 showed a maximum increase of approximately 1.4 times in tensile strength, approximately 2.5 times in elongation at break, and approximately 3.3 times in notched impact strength, while the precipitation rate was reduced to 0.10 times. This is because the pre-spray coating process achieved uniform fixation and dispersion of the ionic liquid on the LCNF surface, avoiding free phase precipitation and thus fully releasing the synergistic toughening effect of the composite system.

[0058] The mass fraction composition of the composite materials in Examples 11 to 14 is shown in Table 4. The composite materials in Examples 11 to 14 are respectively represented by the numbers E1, E2, E3, and E4, and single-factor experiments were conducted.

[0059] In Examples 11 to 14: the LCNF used was bagasse LCNF, the quaternized LCNF had a quaternary ammonium group content of 1000 μmol / g, the ionic liquids used were 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium tetrafluoroborate, the compatibilizer used was silane coupling agent KH590, the lubricant used was erucamide, and the stabilizer used was antioxidant 1098.

[0060] Table 4

[0061] The preparation method of the polylactic acid / LCNF composite material in Example 11 is basically the same as that in Example 1, except that in step 3) of the preparation in Example 11, the amount of quaternized LCNF solution added is 150 parts (i.e., the amount of quaternized LCNF added is 3 parts), the amount of ionic liquid (1-butyl-3-methylimidazolium chloride) added is 3 parts, the amount of compatibilizer is 3 parts, the amount of lubricant is 2 parts, and the amount of stabilizer is 1 part.

[0062] The preparation methods of the composite materials in Examples 12 to 14 are basically the same as those in Example 11, except that in step 3) of the preparation of Examples 12 to 14, the added ionic liquids are 1-ethyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium tetrafluoroborate, respectively, to prepare the composite materials of Examples 12 to 14.

[0063] Using the same method as for the composite material in Example 1, tensile tests, impact tests, and ionic liquid migration and precipitation tests were performed on the composite materials of Examples 11 to 14. The test results for tensile strength and tensile elongation at break are as follows: Figure 8 As shown, the test results for notched impact strength are as follows: Figure 9 As shown, the test results for migration exudation are as follows: Figure 10 As shown.

[0064] according to Figure 8 , Figure 9 and Figure 10 It can be seen that, with the change of ionic liquid type, tetrafluoroborate (Example 14) showed the most balanced performance, with its tensile strength (66.2 MPa), elongation at break (110.2%), and notched impact strength (32.3 kJ / m) being particularly high. 2 The tensile strength was at a relatively high level, while the migration and precipitation rate (0.25%) was relatively low. Specifically, the tensile strength varied significantly among different ionic liquids. Ionic liquids containing tetrafluoroborate anions provided better interfacial reinforcement due to the formation of more stable ion pairs with quaternized LCNF, while ionic liquids containing smaller anions such as chloride ions showed a more significant decrease in strength due to their stronger plasticizing effect. The trend of notched impact strength variation was directly related to the plasticizing ability and interfacial bonding stability of the ionic liquid, with the tetrafluoroborate system achieving the best balance between the two. Meanwhile, the migration and precipitation rate of ionic liquids showed a regular variation, with ionic liquids containing larger anions and stronger interactions with quaternary ammonium groups exhibiting a lower migration tendency. Overall, the chemical structure of the ionic liquid directly affects its bonding strength with LCNF, its dispersion state in the matrix, and its plasticizing efficiency, thus determining the final properties of the composite material.

[0065] Compared to Comparative Example 1, Examples 11 to 14 all achieved comprehensive performance improvements in different ionic liquid systems. The maximum increase in tensile strength was approximately 1.2 times (Example 11), the maximum increase in elongation at break was approximately 10.1 times (Example 14), and the maximum increase in notched impact strength was approximately 8.3 times (Example 13). This demonstrates that the selected ionic liquid and the quaternized LCNF synergistically achieved a strengthening and toughening effect. Compared to Comparative Example 2 (ionic liquid only), Examples 11 to 14 showed a maximum increase in tensile strength of approximately 1.6 times, a maximum increase in elongation at break of approximately 2.3 times, and a maximum increase in notched impact strength of approximately 3.1 times. Furthermore, the ionic liquid migration and precipitation rate was significantly reduced to a minimum of 0.11 times, highlighting the crucial role of the quaternized LCNF in anchoring the ionic liquid through Coulomb forces. Compared to Comparative Example 3 (using unquaternized LCNF), Examples 11-14 showed a maximum increase of approximately 1.5 times in tensile strength, a maximum increase of approximately 2.8 times in elongation at break, a maximum increase of approximately 3.5 times in notched impact strength, and a minimum decrease in migration and precipitation rate of 0.08 times. This demonstrates that quaternization modification is an effective way to enhance interfacial bonding and suppress precipitation. Compared to Comparative Example 4 (unencapsulated quaternized LCNF), Examples 11-14 showed a maximum increase of approximately 1.4 times in tensile strength, a maximum increase of approximately 2.8 times in elongation at break, a maximum increase of approximately 3.6 times in notched impact strength, and a minimum decrease in migration and precipitation rate of 0.09 times. This demonstrates the effectiveness of spray pre-encapsulation technology.

[0066] The mass fraction composition of the composite materials in Examples 15 to 17 is shown in Table 5. The composite materials in Examples 15 to 17 are respectively represented by the numbers D1, D2, and D3, and single-factor experiments were conducted.

[0067] Table 5

[0068] The method for preparing the composite material in Example 15 includes the following steps: 1) Alkali treatment and bleaching of cellulose: 10 parts of 80-mesh cellulose raw material powder were mixed with 100 parts of 15 w / v sodium hydroxide solution, heated and stirred at 60°C for 4 h, washed with pure water until neutral, filtered, and then 80 parts of 10 w / v sodium chlorite solution and 10 parts of glacial acetic acid were added. The mixture was heated and stirred at 60°C for 2 h, washed with pure water until neutral, and dried to dryness to obtain purified cellulose that retains some residual lignin. 2) Quaternization modification: The 4 parts of purified cellulose obtained in step 1) were dispersed in 100 parts of pure water, and 120 parts of 2,3-epoxypropyltrimethylammonium chloride were added. The mixture was heated and stirred at 60°C for 4 h, and then mechanically sheared at 10,000 rpm for 1 h to obtain a reaction mixture containing quaternized lignocellulose nanofibers with branched or network structures. The reaction mixture was neutralized with 5 wt% HCl solution and washed 6 times with pure water by centrifugation to prepare a 2 wt% quaternized LCNF solution with a quaternary ammonium group content of approximately 500 μmol / g. 3) Spray drying: Add 3 parts of ionic liquid to 500 parts of quaternized LCNF solution obtained in step 2), stir for 30 min, and spray dry to allow the quaternized lignocellulose nanofibers with branched or network structures to self-assemble and construct a physical entanglement network, and allow the ionic liquid to coat the surface of the quaternized lignocellulose nanofibers in situ to form a Coulomb force anchoring structure, thus obtaining ionic liquid composite quaternized LCNF. 4) High-speed premixing: Add the ionic liquid composite quaternized LCNF obtained in step 3) to 100 parts of polylactic acid, 3 parts of compatibilizer, 3 parts of lubricant and 3 parts of stabilizer into a high-speed mixer and mix for 45 min at a speed of 500 rpm to obtain a premix. 5) Twin-screw melt extrusion granulation: The premix obtained in step 4) is added to a twin-screw extruder for extrusion and granulation. The processing temperature of the twin-screw extruder is 190℃, the screw length-to-diameter ratio is 50, and the rotation speed is 200 rpm to obtain polylactic acid / LCNF composite particles. 6) Final molding: The polylactic acid / LCNF composite material particles obtained in step 5) are processed into composite material products through injection molding machine, casting machine or hot press.

[0069] The preparation method of the composite material in Example 16 is basically the same as that in Example 15, except that in step 2) of the preparation in Example 17, 6 parts of purified cellulose are dispersed in 95 parts of pure water, 160 parts of 2,3-epoxypropyltrimethylammonium chloride are added, and the mixture is heated and stirred at 65°C for 5 h, and then subjected to high-speed mechanical shearing at 13000 rpm for 1.5 h to obtain a reaction mixture containing quaternized lignocellulose nanofibers with branched or network structures. After neutralizing the reaction mixture with a 5 wt% HCl solution, it is washed 6 times by centrifugation with pure water to prepare a 2 wt% quaternized LCNF solution with a quaternary ammonium group content of about 1000 μmol / g.

[0070] The preparation method of the composite material in Example 17 is basically the same as that in Example 15. The difference is that in step (2) of the preparation in Example 17, 8 parts of purified cellulose are dispersed in 100 parts of pure water, 200 parts of 2,3-epoxypropyltrimethylammonium chloride are added, heated and stirred at 65°C for 6 h, and then mechanically sheared at 15000 rpm for 2 h to obtain a reaction mixture. The reaction mixture contains quaternized lignocellulose nanofibers with branched or network structures. After neutralizing the reaction mixture with 5 wt% HCl solution, it is washed 6 times by centrifugation with pure water to prepare a 2 wt% quaternized LCNF solution with a quaternary ammonium group content of about 1500 μmol / g.

[0071] Using the same methods as for the composite material in Example 1, tensile tests, impact tests, and ionic liquid migration and precipitation tests were performed on the composite materials of Examples 15 to 17. The test results for tensile strength and tensile elongation at break are as follows: Figure 11 As shown, the test results for notched impact strength are as follows: Figure 12 As shown, the test results for migration exudation are as follows: Figure 13 As shown.

[0072] according to Figure 11 , Figure 12 and Figure 13 It can be seen that the properties of the composite materials in Examples 15 to 17 show a regular change with the increase of quaternary ammonium group content in quaternized LCNF. When the quaternary ammonium group content increases from 500 μmol / g to 1000 μmol / g, the tensile strength, elongation at break, and notched impact strength of the material all increase simultaneously, reaching optimal values ​​at 1000 μmol / g; when further increased to 1500 μmol / g, the mechanical properties show a downward trend, while the ionic liquid migration and precipitation rate continuously decreases with the increase of quaternary ammonium group content. This is because an appropriate amount of quaternary ammonium groups (1000 μmol / g) forms an optimal density distribution on the LCNF surface, which can effectively anchor ionic liquids and enhance interfacial bonding through Coulomb forces, without causing a decrease in the dispersibility of LCNF in the matrix or self-aggregation due to excessive groups, thus achieving the optimal balance between mechanical properties and precipitation resistance. Excessive quaternary ammonium groups (1500 μmol / g) may cause LCNF to aggregate or reduce its compatibility with the matrix, which is detrimental to stress transfer and performance improvement.

[0073] Compared to Comparative Example 1, the composite materials of Examples 15-17 showed a maximum increase of approximately 1.3 times in tensile strength, approximately 15.2 times in elongation at break, and approximately 9.8 times in notched impact strength, demonstrating that quaternized LCNF can significantly enhance toughness at an appropriate quaternary ammonium content. Compared to Comparative Example 2 (with only ionic liquid added), Examples 15-17 showed a maximum increase of approximately 1.5 times in tensile strength, approximately 2.1 times in elongation at break, and approximately 2.9 times in notched impact strength, while the ionic liquid migration and precipitation rate decreased to a minimum of 0.09 times, indicating that quaternized LCNF can effectively lock in ionic liquids and inhibit migration. Compared to Comparative Example 3 (using unquaternized LCNF), Examples 15-17 showed a maximum increase of approximately 1.4 times in tensile strength, approximately 2.6 times in elongation at break, and approximately 3.2 times in notched impact strength, while the migration and precipitation rate decreased to a minimum of 0.06 times, further demonstrating that quaternization modification is key to enhancing bonding and reducing precipitation. Compared with Comparative Example 4 (without pre-spray coating process), the tensile strength of Examples 15 to 17 increased by a maximum of about 1.3 times, the elongation at break increased by a maximum of about 2.0 times, the notched impact strength increased by a maximum of about 2.8 times, and the migration and exudation rate decreased to a minimum of 0.08 times. This demonstrates that the pre-coating process helps the ionic liquid to be uniformly distributed on the cellulose surface and form a stable composite structure, thereby synergistically optimizing the performance.

Claims

1. A strong, tough, antibacterial, low-emission polylactic acid composite material, characterized in that, Its mass fraction composition is as follows: 100 parts polylactic acid, 0.5-10 parts quaternized lignocellulose nanofibers, 0.5-15 parts ionic liquid, 0.5-3 parts compatibilizer, 0.2-5 parts lubricant, and 0.2-3 parts stabilizer; The quaternized lignocellulose nanofibers are quaternized lignocellulose nanofibers with branched or network structures. The quaternized lignocellulose nanofibers contain 500–1500 μmol / g of quaternized ammonium groups. The ionic liquid is pre-coated in situ onto the surface of the quaternized lignocellulose nanofibers by spray drying. The anions and cations of the ionic liquid form stable Coulombic anchoring structures with the hydroxyl groups and grafted quaternary ammonium groups on the surface of the quaternized lignocellulose nanofibers, respectively.

2. The strong, tough, antibacterial, low-emission polylactic acid composite material according to claim 1, characterized in that, The raw materials for the quaternized lignocellulose nanofibers are one or more of wood powder, bamboo powder and straw powder.

3. The high-strength, antibacterial, low-exudation polylactic acid composite material according to claim 1, characterized in that, The ionic liquid is one or a combination of two or more of 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium tetrafluoroborate.

4. The strong, tough, antibacterial, low-exudation polylactic acid composite material according to claim 1, characterized in that, The compatibilizer is at least one of silane coupling agents, titanate coupling agents, and aluminate coupling agents.

5. The strong, tough, antibacterial, low-emission polylactic acid composite material according to claim 4, characterized in that, The silane coupling agent is KH540, KH550, KH580 or KH590, the titanate coupling agent is CS-101, and the aluminate coupling agent is DL-411.

6. The strong, tough, antibacterial, low-emission polylactic acid composite material according to claim 1, characterized in that, The lubricant is one or a combination of two or more of the following: sodium cocoyl aminopropionate, polyvinyl acetate resin, ethylene bis-stearamide, oleamide, and erucamide.

7. The strong, tough, antibacterial, low-emission polylactic acid composite material according to claim 1, characterized in that, The stabilizer is at least one of antioxidant 1010, antioxidant 1098, antioxidant 300 and antioxidant 168.

8. A method for preparing a strong, tough, antibacterial, low-exudation polylactic acid composite material according to any one of claims 1 to 7, characterized in that, Includes the following steps: 1) Alkali treatment and bleaching of cellulose: Mix 1-10 parts of cellulose raw material powder with 20-150 parts of sodium hydroxide solution, heat and stir at 50-70℃ for 2-4 h, wash with pure water until neutral, filter, add 20-80 parts of sodium chlorite solution and 4-10 parts of glacial acetic acid, heat and stir at 50-70℃ for 0.5-2 h, wash with pure water until neutral, and dry to dryness to obtain purified cellulose that retains some residual lignin; 2) Quaternization modification: 4-8 parts of purified cellulose obtained in step 1) are dispersed in 90-100 parts of pure water, and 120-200 parts of 2,3-epoxypropyltrimethylammonium chloride are added. The mixture is heated and stirred at 60-65℃ for 4-6 h, and then mechanically sheared at 10000-15000 rpm for 1-2 h to obtain a reaction mixture containing quaternized lignocellulose nanofibers with branched or network structures. The reaction mixture is neutralized with 5 wt% HCl solution and washed with pure water by centrifugation 4-6 times to prepare a 2 wt% quaternized lignocellulose nanofiber solution. 3) Spray drying: Add 0.5 to 15 parts of ionic liquid to 25 to 500 parts of quaternized lignocellulose nanofiber solution obtained in step 2), stir for 20 to 30 min, and spray dry to allow the quaternized lignocellulose nanofibers with branched or network structures to self-assemble into a physical entangled network, and allow the ionic liquid to coat the surface of the quaternized lignocellulose nanofibers in situ to form a Coulomb force anchoring structure, thus obtaining ionic liquid composite quaternized lignocellulose nanofibers; 4) High-speed premixing: The ionic liquid composite quaternized lignocellulose nanofibers obtained in step 3) are added to a high-speed mixer and mixed for 30-60 min at a speed of 300-800 rpm to obtain a premix. 5) Twin-screw melt extrusion granulation: The premix obtained in step 4) is added to a twin-screw extruder for extrusion and granulation. The processing temperature of the twin-screw extruder is 110-190℃, the screw length-to-diameter ratio is 30-60, and the rotation speed is 50-250 rpm to obtain polylactic acid / nanocellulose composite particles. 6) Final molding: The polylactic acid / nanocellulose composite material particles obtained in step 5) are processed into composite material products by injection molding machine, casting machine or hot press.

9. The method for preparing a strong, tough, antibacterial, low-exudation polylactic acid composite material according to claim 8, characterized in that, In step 1), the raw material powder has a mesh size of 80-200 mesh, the sodium hydroxide solution has a concentration of 10-20 w / v, and the sodium chlorite solution has a concentration of 10-20 w / v.

Images