Degradable bio-based food packaging material and preparation method thereof

Abstract

The invention relates to the technical field of polymer compositions, and particularly discloses a degradable bio-based food packaging material and a preparation method thereof.The degradable bio-based food packaging material is prepared from polylactic acid, ITA esterified bacterial cellulose, CSL-ITA esterified bacterial cellulose, polycaprolactone glycol, acetyl tributyl citrate, a photoinitiator and tocopherol; the preparation method comprises the following steps: esterifying bacterial cellulose hydrogel with itaconic anhydride, grafting with sodium copper chlorophyllin, dehydrating with gradient ethanol, and drying with supercritical CO2 to obtain modified powder; according to the preparation method, the interfacial compatibility of the bacterial cellulose and the polylactic acid is enhanced through ITA esterification, the illumination antibacterial function is given by utilizing CSL grafting, the polycaprolactone glycol and the acetyl tributyl citrate are synergistically used for toughening and improving the brittleness, the photoinitiator is added, the photoinitiator is added for mixing, and ultraviolet crosslinking curing is performed after film forming. The tocopherol protects the stability of the antibacterial structure of the CSL, the water resistance is improved through photo-crosslinking, and the material is degradable and meets the food packaging requirement.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer compositions, and in particular to a degradable bio-based food packaging material and a preparation method thereof. Background Art

[0002] Currently, the majority of materials used in food packaging are non-degradable. These materials are difficult to degrade naturally after use, and their accumulation can cause severe white pollution and persistent damage to soil, water, and other ecological environments. With growing global environmental awareness and the advancement of sustainable development strategies, the development of biodegradable, bio-based food packaging materials has become a key approach to addressing the environmental challenges of traditional packaging materials and an inevitable trend in the future development of the food packaging industry.

[0003] Based on the application scenario of degradable food packaging materials, the prior art has proposed an antimicrobial PLA-based degradable food packaging material and its preparation method (Application No.: CN201811146791.3). This material utilizes activated carbon-loaded nano-titanium dioxide, polylactic acid, polyhydroxyalkanoate, and a plasticizer as raw materials. It offers advantages such as ease of handling, simple processing, excellent antimicrobial efficacy, and complete biodegradability. Ultrafine grinding of the activated carbon and its composite with a polymer material ensures a smooth and aesthetically pleasing packaging surface. Its large specific surface area, high activity, and well-developed micropores enhance its ability to load inorganic particles. Coating the nano-titanium dioxide with cetyltrimethylammonium bromide enhances its antimicrobial properties, with a significant bactericidal effect even under dim light. During preparation, the activated carbon is pretreated to improve its adsorption properties, then loaded with nano-titanium dioxide and mixed with other raw materials to form the packaging material.

[0004] However, the existing technology still has certain shortcomings. The PLA molecular chain is relatively rigid, with a glass transition temperature of approximately 60°C. Although PHA has good toughness, it crystallizes quickly. Although polyhydroxyalkanoate (PHA) is degradable, it has poor processing stability and poor compatibility with PLA. At the same time, when activated carbon is used as the filling phase, the interfacial bonding force between the micron-sized activated carbon and the polylactic acid / polyhydroxyalkanoate matrix is ​​weak, and the two have poor compatibility. Stress concentration is easily generated when subjected to force, resulting in insufficient toughness of the material and easy brittle cracking. Summary of the Invention

[0005] In view of the technical defects existing in the background technology, the present invention proposes a lightweight, high-strength, waterproof gypsum board and a preparation method thereof, which solves the above technical problems and meets practical needs. The specific technical solution is as follows: A biodegradable bio-based food packaging material comprises the following components, measured in parts by weight: 100-150 parts of polylactic acid, 15-25 parts of ITA-esterified bacterial cellulose, 5-8 parts of CSL-ITA-esterified bacterial cellulose, 3-5 parts of polycaprolactone diol, 8-12 parts of acetyl tributyl citrate, 0.3-0.5 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 0.3-0.5 parts of 1-hydroxycyclohexyl phenyl ketone, and 0.2-0.4 parts of tocopherol.

[0006] As a further technical solution of the present invention, the molecular weight of the polylactic acid is 100,000-150,000, and the crystallinity is ≥45%, and the molecular weight of the polycaprolactone diol is 2000-3000.

[0007] As a further technical solution of the present invention, the degree of esterification of the ITA-esterified bacterial cellulose is 40-60%, and the diameter of the ITA-esterified bacterial cellulose is 20-100 nm.

[0008] As a further technical solution of the present invention, the grafting amount of sodium copper chlorophyllin (CSL) in the CSL-ITA esterified bacterial cellulose is 1.5-3.0 wt%.

[0009] A method for preparing a biodegradable bio-based food packaging material comprises the following steps: (1) Mixing bacterial cellulose hydrogel with ITA (itaconic anhydride), adjusting the pH value of the reaction system to a weakly acidic condition, controlling the reaction temperature to carry out esterification reaction, and dialysis purification after the reaction is completed to obtain ITA esterified bacterial cellulose hydrogel; (2) activating sodium copper chlorophyllin (CSL) with an activator to cause a grafting reaction between CSL and the ITA-esterified bacterial cellulose hydrogel obtained in step (1) to obtain CSL-ITA-esterified bacterial cellulose hydrogel; (3) The ITA-esterified bacterial cellulose hydrogel and the CSL-ITA-esterified bacterial cellulose hydrogel were subjected to gradient ethanol dehydration and supercritical CO2 drying to obtain powdered ITA-esterified bacterial cellulose and CSL-ITA-esterified bacterial cellulose, respectively; (4) Melt blending: melt-mix polylactic acid and acetyl tributyl citrate, add polycaprolactone diol and continue mixing; (5) Solid phase composite: ITA esterified bacterial cellulose and CSL-ITA esterified bacterial cellulose are added to the melt of step (4), and the mixture is further kneaded and then rolled to form a film; (6) Photocrosslinking treatment: The film is placed under a wavelength of 365±5nm and an intensity of 48-52mW / cm 2 The cross-linking and curing is carried out under an ultraviolet light source for 50-60 minutes to obtain a degradable bio-based food packaging material.

[0010] As a further technical solution of the present invention, step (1) is specifically as follows: ITA is dissolved in acetone to prepare a 30-35wt% ITA / acetone solution, bacterial cellulose hydrogel is placed in a reactor, mechanically stirred at 200-300rpm to form a homogeneous suspension, the ITA / acetone solution is slowly added dropwise to the reactor through a constant pressure dropping funnel, the system temperature is maintained at ≤25°C in an ice-water bath, and immediately after the addition is completed, the pH is adjusted to 6.0-6.5 with 0.1M NaOH, nitrogen is passed through and the temperature is raised to 50-55°C for 15 minutes, and then 0.8wt% is added. 4-DMAP, heated to 75-80°C under nitrogen protection and reacted for 4-5 hours. After the reaction was completed, the bacterial cellulose hydrogel was dialyzed in deionized water using a dialysis membrane with a molecular weight cutoff of 8000 until the conductivity was ≤20μS / cm. The solid content of the bacterial cellulose hydrogel was 1.8-2.2wt%, and the molar ratio of ITA to hydroxyl groups of the bacterial cellulose in the added ITA / acetone solution was 1:0.8-1.0.

[0011] As a further technical solution of the present invention, step (2) is specifically as follows: dissolving sodium copper chlorophyllin (CSL) in a sufficient amount of MES buffer solution with a pH of 5.2 to prepare 12-18 mg / mL sodium copper chlorophyllin / MES buffer solution, adding EDC at a molar ratio of CSL:EDC=1:1.3, and then adding 0.5 wt% 4-DMAP, stirring in the dark for 30 min to activate, and the mixture is stirred at a molar ratio of 0.5 wt% 4-DMAP per mg of CSL. The proportion of 33-50 mg of ITA-esterified bacterial cellulose solid components is required, and the corresponding mass of the ITA-esterified bacterial cellulose hydrogel obtained in step (1) is calculated and added. MES buffer with pH = 5.2 is added to the system to make the total liquid volume reach 10-12 times the volume of the added hydrogel. Then, the reaction is carried out at 30°C in the dark for 4 hours under the assistance of ultrasound at 40 kHz and a power density of 80-100 W / L. After the reaction is completed, the dialysis membrane with a molecular weight cutoff of 3500 is dialyzed to a conductivity of ≤15 μS / cm in the dark to obtain CSL-ITA-esterified bacterial cellulose hydrogel.

[0012] As a further technical solution of the present invention, step (3) is specifically: S1. Sample pretreatment: ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel were cut into 2×2×2 mm 3 small pieces; S2. Gradient ethanol dehydration S2.1: Add 20 mL of 30% ethanol per gram of ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel, respectively. Stir magnetically at 200 rpm for 20 minutes at 25°C, then centrifuge at 3000 rpm for 5 minutes. Discard the supernatant and collect the gel at the bottom. S2.2: Add 20 mL of 50% ethanol to each of the bottom gels obtained in step S2.1, treat with ultrasound at 40 kHz and 100 W for 20 min at 25°C, centrifuge at 3000 rpm for 5 min at 4°C, discard the supernatant, and collect the bottom gels. S2.3: Using the same method as S2.2, treat the bottom gel obtained in step S2.2 with 70% ethanol and 90% ethanol, respectively, and collect the bottom gel separately; S2.4: Add 20 mL of anhydrous ethanol to the bottom gel obtained in step S2.3, soak at 4°C for 10 min, centrifuge at 3000 rpm for 5 min, discard the liquid, and repeat this operation three times to obtain ethanol-replaced ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel, respectively; S3. Supercritical CO2 drying The ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel after ethanol replacement were spread on a carrier plate and placed in an autoclave. The autoclave body was pre-cooled to 4°C and maintained for 30 minutes. Then, liquid CO2 was injected at a flow rate of 1.5L / min and slowly pressurized to 7.5MPa with a pressure increase rate of 1 MPa / min. Then, the temperature was raised to 40°C and the pressure was raised to 10MPa. The CO2 flow rate was adjusted to 2L / min and dynamic flushing was performed for 3 hours. The ethanol content in the outlet CO2 was sampled and tested every 30 minutes until the ethanol content was <50ppm. Then, the pressure was slowly released at a rate of 0.3MPa / min and the temperature was raised to 45°C. After the pressure release was completed, it was maintained at 45°C and normal pressure for 30 minutes. S4. Product collection and storage: Take out the dried sample from the nitrogen atmosphere glove box, immediately seal it in an aluminum foil light-proof bag, and store it in a dark box at -20℃ after filling it with nitrogen.

[0013] As a further technical solution of the present invention, step (4) specifically comprises the following steps: melt-blending 125 parts of polylactic acid and 10 parts of acetyl tributyl citrate in an internal mixer at 172-178°C for 5 minutes, cooling the temperature to 135-140°C, adding 4 parts of polycaprolactone diol and mixing for 2 minutes, further cooling the temperature to 110-115°C, sequentially adding 0.3 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 0.3 parts of 1-hydroxycyclohexyl phenyl ketone, and mixing for 3 minutes to obtain a melt, wherein the rotor speed of the internal mixer is 55-65 rpm.

[0014] As a further technical solution of the present invention, step (5) specifically comprises the following steps: first adding 20 parts of ITA esterified bacterial cellulose and 7.5 parts of CSL-ITA esterified bacterial cellulose to the melt of step (4) at 108-112° C. and a rotor speed of 55-65 rpm, mixing for 3-4 minutes, then adding 0.3 parts of tocopherol, mixing for 1 minute, and then transferring to a double-roll calender, and rolling into a film in the double-roll calender at 108-112° C., with a roller spacing of 0.20-0.30 mm and a rolling rate of 2.0-3.0 m / min.

[0015] The beneficial effects of the present invention are: The present invention uses polylactic acid, bio-based modified bacterial cellulose, etc. as main ingredients, which can be degraded in the natural environment. The bacterial cellulose is modified by esterification with itaconic anhydride, which improves its compatibility with polylactic acid and enhances its structural strength. The sodium copper chlorophyllin (CSL) in the CSL-ITA esterified bacterial cellulose can generate active oxygen under light, giving the material good antibacterial properties. The combination of two photoinitiators can improve the polymerization addition efficiency of double bonds introduced by itaconic anhydride, and further enhance the internal cross-linking degree of the material after ultraviolet light cross-linking. Combined with the plasticizing effect of polycaprolactone diol and acetyl tributyl citrate, the material has both strength and toughness. In addition, tocopherol can quench excess free radicals, avoid the destruction of the porphyrin ring of CSL, and ensure the stability of the antibacterial effect. DETAILED DESCRIPTION

[0016] The following describes the implementation of the present invention in conjunction with relevant embodiments. The implementation of the present invention is not limited to the following embodiments, and the present invention relates to relevant necessary components in this technical field and should be regarded as a well-known technology in this technical field, which can be known and mastered by technical personnel in this technical field.

[0017] A biodegradable bio-based food packaging material comprises the following components, measured in parts by weight: 100-150 parts of polylactic acid, 15-25 parts of ITA-esterified bacterial cellulose, 5-8 parts of CSL-ITA-esterified bacterial cellulose, 3-5 parts of polycaprolactone diol, 8-12 parts of acetyl tributyl citrate, 0.3-0.5 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 0.3-0.5 parts of 1-hydroxycyclohexyl phenyl ketone, and 0.2-0.4 parts of tocopherol.

[0018] The ITA-esterified bacterial cellulose of the present invention introduces ester functional groups through an esterification reaction between itaconic anhydride (ITA) and the hydroxyl groups of bacterial cellulose, forming good chemical affinity with the ester bond of polylactic acid, which can reduce the interfacial tension between bacterial cellulose and polylactic acid. At the same time, CSL-ITA-esterified bacterial cellulose is further grafted with sodium copper chlorophyllin (CSL) on the basis of ITA esterification. The polar groups in its molecular chain can form hydrogen bond interactions with polylactic acid, further strengthening the interfacial bonding force.

[0019] The present invention uses polylactic acid as the material to provide basic strength. ITA-esterified bacterial cellulose can form a three-dimensional network support in the matrix, enhancing the tensile strength and impact resistance of the material. Polycaprolactone diol and acetyl tributyl citrate serve as composite plasticizers, which can be inserted between polylactic acid molecular chains to reduce the rigidity of the molecular chains. The flexible chain segments of polycaprolactone diol can improve the brittleness of polylactic acid, while acetyl tributyl citrate improves the fluidity of the chain segments. The combination of the two effectively alleviates the problem of brittle cracking caused by the strong rigidity of polylactic acid.

[0020] The combination of CSL-ITA esterified bacterial cellulose and tocopherol plays a key role in the antimicrobial performance and stability of this invention. The porphyrin ring structure in CSL generates reactive oxygen species (such as singlet oxygen) under light, which can damage microbial cell membranes and achieve antimicrobial effects. Tocopherol, a natural antioxidant, quenches excess free radicals generated during the photocrosslinking process, preventing free radical damage to the CSL porphyrin ring, ensuring the stability of the antimicrobial components and prolonging the antimicrobial efficacy of the material. Furthermore, the dense structure formed by photocrosslinking reduces the internal erosion of moisture and oxygen by the material. Combined with the hydrophobicity of polycaprolactone diol, it enhances the material's water resistance, meeting the basic requirements for food packaging.

[0021] As one of the preferred embodiments of the present invention, the molecular weight of the polylactic acid is 100,000-150,000, and the crystallinity is ≥45%, and the molecular weight of the polycaprolactone diol is 2,000-3,000.

[0022] Specifically, the molecular weight of polylactic acid is preferably 120,000-130,000, the crystallinity is 48-52%, and the molecular weight of polycaprolactone diol is preferably 2400-2600.

[0023] The molecular weight and crystallinity of polylactic acid directly affect the basic performance of the material. The molecular weight of polylactic acid is preferably 12-13. When the molecular weight of polylactic acid is lower than 120,000, the molecular chains are insufficiently entangled, and the tensile strength and tear resistance of the material decrease; when it is higher than 130,000, the melt viscosity is too high, and uneven dispersion is likely to occur during melt blending, resulting in processing difficulties. A crystallinity of 48-52% can ensure the density of the material structure, improve rigidity and heat resistance, and avoid the weakening of mechanical properties due to insufficient crystallization.

[0024] The molecular weight of polycaprolactone diol is preferably 2400-2600. When the molecular weight of polycaprolactone diol is lower than 2400, the small molecules are easily migrated and precipitated from the matrix, reducing the toughening durability. When the molecular weight is higher than 2600, the segment matching with polylactic acid decreases, the compatibility becomes worse, and may even cause interface defects.

[0025] As one of the preferred embodiments of the present invention, the degree of esterification of the ITA-esterified bacterial cellulose is 40-60%, and the diameter of the ITA-esterified bacterial cellulose is 20-100 nm.

[0026] The esterification degree of ITA-esterified bacterial cellulose is preferably 50-55%. When the esterification degree is too low, too many hydroxyl groups remain on the surface of the bacterial cellulose, resulting in strong polarity, poor compatibility with polylactic acid (mainly non-polar), and easy agglomeration, leading to stress concentration. When the esterification degree is too high, excessive reaction of hydroxyl groups will weaken the hydrogen bonding of the fiber itself, resulting in a decrease in its structural rigidity, inability to form an effective three-dimensional support network, and reduced material strength.

[0027] The diameter of ITA-esterified bacterial cellulose is preferably 60 nm. If the diameter of ITA-esterified bacterial cellulose is too small (<20 nm), it is easy to entangle and agglomerate due to excessively high surface energy, resulting in poor dispersion uniformity. If the diameter is too large (>100 nm), the interfacial contact area with the polylactic acid matrix is ​​reduced, the stress transfer efficiency is reduced, and it is difficult to improve the toughness of the material.

[0028] As one of the preferred embodiments of the present invention, the grafting amount of sodium copper chlorophyllin (CSL) in the CSL-ITA esterified bacterial cellulose is 1.5-3.0 wt %.

[0029] The grafting amount of sodium copper chlorophyllin (CSL) in CSL-ITA esterified bacterial cellulose is preferably 1.5-3.0wt%. When the grafting amount is less than 1.5wt%, the number of porphyrin rings in CSL is insufficient, and the amount of active oxygen generated under light is limited, making it difficult to form an effective antibacterial effect and unable to meet the antibacterial requirements of food packaging. When the grafting amount is higher than 3.0wt%, excessive CSL tends to aggregate on the fiber surface, and its polar groups will increase the interfacial tension with the polylactic acid matrix, resulting in a decrease in compatibility. At the same time, it may destroy the fiber network structure of the bacterial cellulose and weaken the mechanical properties of the material. A method for preparing a biodegradable bio-based food packaging material comprises the following steps: (1) Mixing bacterial cellulose hydrogel with ITA (itaconic anhydride), adjusting the pH value of the reaction system to a weakly acidic condition, controlling the reaction temperature to carry out esterification reaction, and dialysis purification after the reaction is completed to obtain ITA esterified bacterial cellulose hydrogel; (2) activating sodium copper chlorophyllin (CSL) with an activator to cause a grafting reaction between CSL and the ITA-esterified bacterial cellulose hydrogel obtained in step (1) to obtain CSL-ITA-esterified bacterial cellulose hydrogel; (3) The ITA-esterified bacterial cellulose hydrogel and the CSL-ITA-esterified bacterial cellulose hydrogel were subjected to gradient ethanol dehydration and supercritical CO2 drying to obtain powdered ITA-esterified bacterial cellulose and CSL-ITA-esterified bacterial cellulose, respectively; (4) Melt blending: melt-mix polylactic acid and acetyl tributyl citrate, add polycaprolactone diol and continue mixing; (5) Solid phase composite: ITA esterified bacterial cellulose and CSL-ITA esterified bacterial cellulose are added to the melt of step (4), and the mixture is further kneaded and then rolled to form a film; (6) Photocrosslinking treatment: The film is placed under a wavelength of 365±5nm and an intensity of 48-52mW / cm 2 The cross-linking and curing is carried out under an ultraviolet light source for 50-60 minutes to obtain a degradable bio-based food packaging material.

[0030] The present invention uses controllable esterification and grafting reactions in steps (1) and (2). During ITA esterification, pH buffering, nitrogen protection, and temperature gradient control are used to ensure that the esterification reaction between bacterial cellulose hydroxyl groups and ITA proceeds uniformly, and a stable esterification degree of 40-60% is obtained. CSL grafting uses EDC activation and ultrasound assistance, which not only avoids the destruction of the porphyrin ring of CSL, but also improves the grafting uniformity, so that the CSL grafting amount is stabilized at 1.5-3.0wt%.

[0031] The gradient ethanol dehydration in step (3) gradually replaces water, avoiding the structural collapse of bacterial cellulose caused by rapid dehydration; supercritical CO2 drying uses low temperature and high pressure conditions to completely remove residual solvent while maintaining the nanofiber structure (diameter 20-100nm). The resulting powder has excellent dispersibility, solving the agglomeration problem caused by traditional drying and ensuring that it can be evenly distributed in the matrix during subsequent melt blending.

[0032] Steps (4) and (5) avoid high-temperature degradation of polylactic acid and ensure sufficient diffusion of plasticizer and polycaprolactone diol by gradually cooling down. When adding modified bacterial cellulose, the temperature is further controlled to 108-112°C to reduce damage to the fiber structure due to high temperature. The stable parameters of double-roll calendering are combined to make the film thickness uniform and improve the consistency of mechanical properties. Step (6) uses ultraviolet light of specific wavelength and intensity, combined with the synergistic effect of two photoinitiators, to efficiently induce double bond crosslinking and form a stable three-dimensional network. At the same time, tocopherol is mixed before crosslinking to ensure its uniform dispersion, effectively quench excess free radicals, avoid the destruction of the CSL antibacterial structure, and enable the material to obtain both mechanical enhancement and antibacterial stability.

[0033] As one of the preferred embodiments of the present invention, step (1) is specifically as follows: ITA is dissolved in acetone to prepare a 30-35wt% ITA / acetone solution, bacterial cellulose hydrogel is placed in a reactor, mechanically stirred at 200-300rpm to form a homogeneous suspension, the ITA / acetone solution is slowly added dropwise to the reactor through a constant pressure dropping funnel, the system temperature is maintained at ≤25°C in an ice-water bath, the pH is immediately adjusted to 6.0-6.5 with 0.1M NaOH after the addition is completed, nitrogen is passed through and the temperature is raised to 50-55°C for 15 minutes, and then 0.8wt% is added. 4-DMAP, heated to 75-80°C under nitrogen protection and reacted for 4-5 hours. After the reaction was completed, the bacterial cellulose hydrogel was dialyzed in deionized water using a dialysis membrane with a molecular weight cutoff of 8000 until the conductivity was ≤20μS / cm. The solid content of the bacterial cellulose hydrogel was 1.8-2.2wt%, and the molar ratio of ITA to hydroxyl groups of the bacterial cellulose in the added ITA / acetone solution was 1:0.8-1.0.

[0034] Step (1) is the esterification reaction between ITA and bacterial cellulose: hydroxyl groups (-OH) on the surface of bacterial cellulose and carboxyl groups (-COOH) on ITA undergo an esterification reaction under the action of a catalyst. 4-DMAP, as an esterification catalyst, activates the carboxyl groups and accelerates the reaction. Nitrogen protection prevents oxidation of the system. Preheating to 50-55°C prepares for the reaction, and 75-80°C is the optimal reaction temperature. 4-5 hours ensures a complete reaction, ultimately forming an ester bond (-COO-). Dialysis removes unreacted ITA and small molecular impurities to ensure product purity.

[0035] In step (1), ITA is prepared into a 30wt% acetone solution to ensure its uniform dispersion and stable dropwise addition; the bacterial cellulose hydrogel solid content is 2.2wt%, and stirring at 300rpm can form a homogeneous suspension to avoid fiber agglomeration. The ice water bath temperature is controlled at ≤25℃ to prevent ITA from prematurely reacting due to local overheating during dropwise addition, ensuring that the initial stage of the reaction is mild and controllable. After dropwise addition, the pH is adjusted to 6.0 (weakly acidic), nitrogen is passed through to deoxygenate and preheated to 55℃, and then 0.8wt% 4-DMAP (catalyst) is added and reacted at 80℃ for 5h to promote the full reaction of ITA and bacterial cellulose hydroxyl groups. Combined with a molar ratio of 1:0.9, a stable esterification degree of 50% can be obtained. Finally, dialysis is performed using an 8000 molecular weight cutoff membrane to effectively remove unreacted ITA and small molecular impurities. The conductivity is ≤20μS / cm to ensure that the impurities are completely removed to avoid residual substances affecting subsequent compatibility and material properties.

[0036] As one of the preferred embodiments of the present invention, step (2) is specifically as follows: dissolving sodium copper chlorophyllin (CSL) in a sufficient amount of MES buffer solution with a pH of 5.2 to prepare 12-18 mg / mL sodium copper chlorophyllin / MES buffer solution, adding EDC at a molar ratio of CSL:EDC=1:1.3, and then adding 0.5 wt% 4-DMAP, stirring in the dark for 30 min to activate, and the mixture is stirred at a molar ratio of 0.5 wt% 4-DMAP per mg of CSL. The proportion of 33-50 mg of ITA-esterified bacterial cellulose solid components is required, and the corresponding mass of the ITA-esterified bacterial cellulose hydrogel obtained in step (1) is calculated and added. MES buffer with pH = 5.2 is added to the system to make the total liquid volume reach 10-12 times the volume of the added hydrogel. Then, the reaction is carried out at 30°C in the dark for 4 hours under the assistance of ultrasound at 40 kHz and a power density of 80-100 W / L. After the reaction is completed, the dialysis membrane with a molecular weight cutoff of 3500 is dialyzed to a conductivity of ≤15 μS / cm in the dark to obtain CSL-ITA-esterified bacterial cellulose hydrogel.

[0037] Step (2) is the grafting reaction of CSL and ITA-esterified bacterial cellulose: EDC first activates the carboxyl group of CSL (forming an active intermediate), and 4-DMAP assists in enhancing the activation efficiency; the residual hydroxyl groups on the surface of ITA-esterified bacterial cellulose form covalent ester bonds with the activated CSL, ultrasound promotes the contact between the fiber and CSL, light-proofing the CSL structure, and dialysis removes the ungrafted CSL and activator, ultimately achieving stable CSL grafting.

[0038] Step (2) The stable grafting of CSL onto ITA-esterified bacterial cellulose was achieved by controlling the reaction conditions. CSL was dissolved in MES buffer at pH 5.2 and prepared into 12 mg / mL sodium copper chlorophyllin / MES buffer. EDC was added at a molar ratio of 1:1.3. Excess EDC can fully activate the carboxyl groups of CSL. 0.5 wt% 4-DMAP further catalyzed the activation. The mixture was stirred in the dark to prevent degradation of CSL due to light.

[0039] According to the ratio of 33 mg of ITA-esterified bacterial cellulose solid components per mg of CSL, the grafting amount can be controlled at 1.5 wt%; MES buffer was added to 12 times the volume to ensure uniform dispersion of the system. 40kHz ultrasonic assistance (100W / L) can promote contact between the fiber and CSL. The reaction was carried out at 30°C in the dark for 4 hours to ensure sufficient grafting and avoid damage to the CSL structure. Finally, it was dialyzed in the dark using a 3500 molecular weight cutoff membrane to a conductivity of ≤15μS / cm, which can effectively remove ungrafted CSL and small molecular impurities, avoid residual residues that affect subsequent compatibility and antibacterial properties, and protect CSL activity.

[0040] As one of the preferred embodiments of the present invention, step (3) is specifically as follows: S1. Sample pretreatment: ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel were cut into 2×2×2 mm 3 small pieces; S2. Gradient ethanol dehydration S2.1: Add 20 mL of 30% ethanol per gram of ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel, respectively. Stir magnetically at 200 rpm for 20 minutes at 25°C, then centrifuge at 3000 rpm for 5 minutes. Discard the supernatant and collect the gel at the bottom. S2.2: Add 20 mL of 50% ethanol to each of the bottom gels obtained in step S2.1, treat with ultrasound at 40 kHz and 100 W for 20 min at 25°C, centrifuge at 3000 rpm for 5 min at 4°C, discard the supernatant, and collect the bottom gels. S2.3: Using the same method as S2.2, treat the bottom gel obtained in step S2.2 with 70% ethanol and 90% ethanol, respectively, and collect the bottom gel separately; S2.4: Add 20 mL of anhydrous ethanol to the bottom gel obtained in step S2.3, soak at 4°C for 10 min, centrifuge at 3000 rpm for 5 min, discard the liquid, and repeat this operation three times to obtain ethanol-replaced ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel, respectively; S3. Supercritical CO2 drying The ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel after ethanol replacement were spread on a carrier plate and placed in an autoclave. The autoclave body was pre-cooled to 4°C and maintained for 30 minutes. Then, liquid CO2 was injected at a flow rate of 1.5L / min and slowly pressurized to 7.5MPa with a pressure increase rate of 1 MPa / min. Then, the temperature was raised to 40°C and the pressure was raised to 10MPa. The CO2 flow rate was adjusted to 2L / min and dynamic flushing was performed for 3 hours. The ethanol content in the outlet CO2 was sampled and tested every 30 minutes until the ethanol content was <50ppm. Then, the pressure was slowly released at a rate of 0.3MPa / min and the temperature was raised to 45°C. After the pressure release was completed, it was maintained at 45°C and normal pressure for 30 minutes. S4. Product collection and storage: Take out the dried sample from the nitrogen atmosphere glove box, immediately seal it in an aluminum foil light-proof bag, and store it in a dark box at -20℃ after filling it with nitrogen.

[0041] Step (3) retains the material structure and maintains activity through gradient dehydration and supercritical drying. Pretreatment and cutting into small pieces can increase the contact area, which is beneficial for subsequent mass transfer. Gradient ethanol dehydration from 30% to anhydrous ethanol gradually replaces the water content of the hydrogel to avoid direct contact with high-concentration ethanol, which may cause shrinkage of the fiber structure. Ultrasonic assistance enhances the uniformity of dehydration. Repeated anhydrous ethanol treatment ensures complete water replacement. Supercritical CO2 drying utilizes the surface tension-free property of its supercritical state (10MPa, 40℃) to avoid material pore collapse during drying. Precooling and slow pressurization ensure sufficient CO2 penetration. Dynamic flushing removes residual ethanol. Slow pressure relief and temperature increase prevent structural damage. Finally, the product is stored under nitrogen, away from light, and at low temperature. Since CSL is susceptible to light and heat degradation, this condition can prevent its oxidation or structural damage. The overall material structure integrity and activity retention are achieved through step-by-step control.

[0042] As one of the preferred embodiments of the present invention, step (4) is specifically as follows: 125 parts of polylactic acid and 10 parts of acetyl tributyl citrate are melt-blended in an internal mixer at 172-178°C for 5 minutes, the temperature is lowered to 135-140°C, 4 parts of polycaprolactone diol are added and mixed for 2 minutes, the temperature is further lowered to 110-115°C, 0.3 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 0.3 parts of 1-hydroxycyclohexyl phenyl ketone are added in sequence, and the mixture is mixed for 3 minutes to obtain a melt, wherein the rotor speed of the internal mixer is 55-65 rpm.

[0043] Step (4) achieves uniform dispersion of the components by staged temperature-controlled melt blending. Polylactic acid and acetyl tributyl citrate are added to an internal mixer and blended at 178°C. At this temperature, acetyl tributyl citrate can quickly penetrate between the polylactic acid molecular chains and reduce the melt viscosity by weakening the intermolecular force. Mixing for 5 minutes ensures the initial fusion of the two. Then the temperature is lowered to 140°C and polycaprolactone diol is added. Because this temperature is lower than its easy degradation temperature and can maintain melt fluidity, it is fully entangled with the polylactic acid chain segment. Mixing for 2 minutes achieves uniform dispersion of the toughening component. The temperature is further lowered to 115°C and the photoinitiator is added. This temperature can prevent it from prematurely decomposing due to high temperature. Mixing for 3 minutes ensures uniform distribution.

[0044] As one of the preferred embodiments of the present invention, step (5) is specifically as follows: at 108-112° C. and a rotor speed of 55-65 rpm, 20 parts of ITA esterified bacterial cellulose and 7.5 parts of CSL-ITA esterified bacterial cellulose are first added to the melt of step (4), mixed for 3-4 minutes, and then 0.3 parts of tocopherol are added and mixed for 1 minute. Subsequently, the mixture is transferred to a double-roll calender and rolled into a film in the double-roll calender at 108-112° C., with a roller spacing of 0.20-0.30 mm and a rolling rate of 2.0-3.0 m / min.

[0045] In step (5), a composite film is prepared by temperature-controlled mixing and rolling. At 112° C. and a rotor speed of 65 rpm in an internal mixer, 20 parts of ITA esterified bacterial cellulose and 7.5 parts of CSL-ITA esterified bacterial cellulose are first added to the melt of step (4) and mixed for 4 minutes. Then, 0.3 parts of tocopherol are added and mixed for 1 minute. The mixture is then transferred to a double-roll rolling mill and rolled into a film in the double-roll rolling mill at 112° C. with a roller spacing of 0.20 mm and a rolling rate of 3.0 m / min.

[0046] 112°C is the ideal processing temperature for the melt, which maintains the fluidity of the polylactic acid-based melt to encapsulate the cellulose while preventing high temperatures from damaging the cellulose structure. The addition of two esterified bacterial celluloses allows for mixing for 4 minutes, allowing the cellulose to be evenly dispersed through melt flow and utilizing its fiber structure to enhance the membrane's mechanical properties. Tocopherol is an antioxidant, and mixing for just 1 minute allows for rapid dispersion and prevents thermal decomposition, preventing material oxidation. The twin-roll calender is operated at the same temperature to maintain melt plasticity.

[0047] Example 1 (1) Preparation of ITA-esterified bacterial cellulose hydrogel ITA was dissolved in acetone to prepare a 32wt% ITA / acetone solution; a bacterial cellulose hydrogel with a solid content of 2.0wt% was placed in a reactor and mechanically stirred at 250rpm to form a homogeneous suspension. The ITA / acetone solution was slowly added dropwise to the reactor through a constant pressure dropping funnel at a molar ratio of ITA to bacterial cellulose hydroxyl group of 1:0.9, and the system temperature was maintained at ≤25°C in an ice-water bath; after the addition was completed, the pH was adjusted to 6.2 with 0.1M NaOH, nitrogen was passed through and the temperature was raised to 52°C for 15 minutes, 0.8wt% 4-DMAP was added, and the temperature was raised to 78°C under nitrogen protection for 4.5 hours; after the reaction was completed, the hydrogel was dialyzed in deionized water using a dialysis membrane with a molecular weight cutoff of 8000 until the conductivity was ≤20μS / cm, obtaining an ITA-esterified bacterial cellulose hydrogel with an esterification degree of 52% (fiber diameter 60nm).

[0048] (2) Preparation of CSL-ITA esterified bacterial cellulose hydrogel CSL was dissolved in MES buffer at pH 5.2 to prepare 15 mg / mL CSL / MES buffer, EDC was added at a molar ratio of CSL:EDC=1:1.3, and then 0.5 wt% 4-DMAP was added, and the mixture was stirred in the dark for 30 min for activation; the hydrogel obtained in step (1) was added at a ratio of 33 mg ITA esterified bacterial cellulose solid components per mg CSL, and MES buffer at pH 5.2 was added until the total liquid volume was 11 times the volume of the hydrogel; the reaction was carried out in the dark for 4 h at 30°C under the assistance of ultrasound at 40 kHz and a power density of 90 W / L; after the reaction was completed, the mixture was dialyzed in the dark using a dialysis membrane with a molecular weight cutoff of 3500 to a conductivity of ≤15 μS / cm, thereby obtaining a CSL-ITA esterified bacterial cellulose hydrogel with a CSL grafting amount of 2.2 wt%.

[0049] (3) Drying treatment S1. Cut the hydrogels obtained in steps (1) and (2) into 2×2×2 mm 3 small pieces; S2. Gradient ethanol dehydration: sequentially treated with 30%, 50%, 70%, and 90% ethanol as described in step (3), and finally replaced with anhydrous ethanol three times; S3. Supercritical CO2 drying: Spread the ethanol-displaced hydrogel onto a carrier plate and place it in an autoclave, precooling it to 4°C for 30 minutes. Inject liquid CO2 at 1.5 L / min and slowly increase the pressure to 7.5 MPa (pressurization rate 1 MPa / min). Raise the temperature to 40°C and pressurize it to 10 MPa. Dynamically flush the hydrogel for 3 hours at a CO2 flow rate of 2 L / min until the ethanol content is <50 ppm. Slowly release the pressure at 0.3 MPa / min and increase the temperature to 45°C. After decompression, maintain the hydrogel at 45°C and atmospheric pressure for 30 minutes. S4. Remove samples from the nitrogen glove box, seal them in aluminum foil light-proof bags (filled with nitrogen), and store them in a dark box at -20°C to obtain fluffy powders of ITA-esterified bacterial cellulose and CSL-ITA-esterified bacterial cellulose, respectively.

[0050] (4) Melt blending 125 parts of polylactic acid with a molecular weight of 125,000 (crystallinity 50%) and 10 parts of acetyl tributyl citrate were melt-blended in an internal mixer at 175°C (rotor speed 60 rpm) for 5 minutes; the temperature was lowered to 138°C, and 4 parts of polycaprolactone diol with a molecular weight of 2500 were added and mixed for 2 minutes; the temperature was further lowered to 112°C, and 0.3 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 0.3 parts of 1-hydroxycyclohexyl phenyl ketone were added in sequence and mixed for 3 minutes to obtain a melt.

[0051] (5) Solid-phase composite At 110°C and a rotor speed of 60 rpm, 20 parts of ITA-esterified bacterial cellulose and 7.5 parts of CSL-ITA-esterified bacterial cellulose were added to the melt of step (4) and mixed for 3.5 minutes; 0.3 parts of tocopherol were added and mixed for 1 minute; the mixture was transferred to a double-roll calender at 110°C and rolled into a film at a roller spacing of 0.25 mm and a rolling rate of 2.5 m / min.

[0052] (6) Photocrosslinking treatment The film was placed under a wavelength of 365 nm and an intensity of 50 mW / cm 2 The biodegradable bio-based food packaging material was obtained by cross-linking and curing under ultraviolet light for 55 minutes.

[0053] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that no CSL-ITA esterified bacterial cellulose was added, 7.5 parts of CSL-ITA esterified bacterial cellulose in the formula was replaced by an equal amount of ITA esterified bacterial cellulose (i.e., the amount of ITA esterified bacterial cellulose was increased to 27.5 parts), and the other components and preparation process remained unchanged.

[0054] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that no photocrosslinking treatment is performed, step (6) is omitted, and the remaining components and preparation process remain unchanged.

[0055] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that vacuum freeze drying is used instead of supercritical CO2 drying, and S3 in step (3) is replaced by: the hydrogel after ethanol replacement is placed at -40°C for pre-freezing for 4 hours, and then freeze-dried for 12 hours under the conditions of vacuum degree of 10 Pa and cold trap temperature of -50°C; the other components and preparation process remain unchanged.

[0056] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the esterification degree of ITA-esterified bacterial cellulose is reduced to 30% (achieved by adjusting the molar ratio of ITA to bacterial cellulose hydroxyl groups in step (1) to 1:1.5 and shortening the reaction time to 2 h), and the other components and preparation process remain unchanged.

[0057] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that no tocopherol is added, the operation of adding 0.3 parts of tocopherol in step (5) is omitted, and the other components and preparation process remain unchanged.

[0058] Performance Testing Tensile strength GB / T1040-2018 "Determination of tensile properties of plastics"; antibacterial rate refers to GB / T 31402-2015 "Test method for antibacterial properties of plastic surfaces", water vapor permeability refers to GB / T 1037-2021 "Plastic film and sheet - Determination of water vapor permeability - Cup method", biodegradation rate refers to HJ / T209-2005 "Technical requirements for environmental labeling products - Packaging products", and the biodegradation rate of the degradable film after 16 weeks is measured.

[0059] The tensile strength, antibacterial rate (E. coli), water vapor transmission rate, and maximum biodegradation rate of Example 1 and Comparative Examples 1-5 were tested, and the specific results are shown in the following table:

[0060] In the test data above, after replacing CSL-ITA esterified BC with ITA-esterified BC in Comparative Example 1, the transverse and longitudinal tensile strengths decreased from 56.2 MPa and 54.3 MPa to 49.8 MPa and 48.1 MPa, respectively, and the E. coli antibacterial rate dropped from 90.5% to 68.2%. This demonstrates that CSL-ITA esterified BC not only enhances interfacial adhesion and improves the mechanical properties of the material through hydrogen bonding with polylactic acid, but also that its CSL porphyrin ring structure is the key to achieving efficient antibacterial properties, which cannot be replaced by ITA-esterified BC alone.

[0061] Comparative Example 2 After omitting the photocrosslinking treatment, the transverse and longitudinal tensile strengths dropped to 45.3 MPa and 44.0 MPa, respectively, and the water vapor transmission rate dropped from 170.6 g / m 2 ・d increased to 195.3g / m 2 d. This is because photocrosslinking can form a dense three-dimensional network structure, enhancing the structural stability of the material to improve mechanical strength while reducing water penetration. In the absence of photocrosslinking, the material structure is not dense enough, resulting in a decrease in mechanical properties and weakened water resistance.

[0062] Comparative Example 3: After vacuum freeze drying was used instead of supercritical CO2 drying, the transverse and longitudinal tensile strengths dropped to 50.1 MPa and 48.5 MPa, respectively, and the water vapor transmission rate increased to 210.5 g / m 2 d. This is because supercritical CO2 drying can prevent the collapse and agglomeration of the bacterial cellulose structure, ensuring its uniform dispersion in the matrix and forming an effective support. Vacuum freeze-drying, on the other hand, can easily cause fiber agglomeration, reducing dispersion uniformity and thus affecting mechanical properties and water resistance.

[0063] In Comparative Example 4, the esterification degree of ITA-esterified bacterial cellulose was 30%, and the transverse and longitudinal tensile strengths dropped to 42.7 MPa and 41.2 MPa, respectively. This is because when the esterification degree is insufficient, too many hydroxyl groups remain on the surface of the bacterial cellulose, resulting in poor compatibility with polylactic acid, easy aggregation, and stress concentration, leading to weakened mechanical properties.

[0064] In Comparative Example 5, without the addition of tocopherol, the E. coli antibacterial rate dropped from 90.5% to 75.6%, with minimal changes in other properties. This is because tocopherol quenches excess free radicals generated by photocrosslinking, protecting the CSL porphyrin ring structure from destruction. In the absence of tocopherol, the CSL structure is susceptible to free radical damage, resulting in a decrease in antibacterial efficacy.

[0065] The above description is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this technical field can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A biodegradable bio-based food packaging material, characterized in that: The invention comprises the following components in parts by weight: 100-150 parts of polylactic acid, 15-25 parts of ITA-esterified bacterial cellulose, 5-8 parts of CSL-ITA-esterified bacterial cellulose, 3-5 parts of polycaprolactone diol, 8-12 parts of acetyl tributyl citrate, 0.3-0.5 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 0.3-0.5 parts of 1-hydroxycyclohexyl phenyl ketone, and 0.2-0.4 parts of tocopherol.

2. The biodegradable bio-based food packaging material according to claim 1, characterized in that: The molecular weight of the polylactic acid is 100,000-150,000, and the crystallinity is ≥45%. The molecular weight of the polycaprolactone diol is 2,000-3,000.

3. The biodegradable bio-based food packaging material according to claim 1, characterized in that: The esterification degree of the ITA-esterified bacterial cellulose is 40-60%, and the diameter of the ITA-esterified bacterial cellulose is 20-100 nm.

4. The biodegradable bio-based food packaging material according to claim 1, characterized in that: The grafting amount of sodium copper chlorophyllin (CSL) in the CSL-ITA esterified bacterial cellulose is 1.5-3.0 wt %.

5. A method for preparing the degradable bio-based food packaging material according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) Mixing bacterial cellulose hydrogel with ITA (itaconic anhydride), adjusting the pH value of the reaction system to a weakly acidic condition, controlling the reaction temperature to carry out esterification reaction, and dialysis purification after the reaction is completed to obtain ITA esterified bacterial cellulose hydrogel; (2) activating sodium copper chlorophyllin (CSL) with an activator to cause a grafting reaction between CSL and the ITA-esterified bacterial cellulose hydrogel obtained in step (1) to obtain CSL-ITA-esterified bacterial cellulose hydrogel; (3) The ITA-esterified bacterial cellulose hydrogel and the CSL-ITA-esterified bacterial cellulose hydrogel were subjected to gradient ethanol dehydration and supercritical CO2 drying to obtain powdered ITA-esterified bacterial cellulose and CSL-ITA-esterified bacterial cellulose, respectively; (4) Melt blending: melt-mix polylactic acid and acetyl tributyl citrate, add polycaprolactone diol and continue mixing; (5) Solid phase composite: ITA esterified bacterial cellulose and CSL-ITA esterified bacterial cellulose are added to the melt of step (4), and the mixture is further kneaded and then rolled to form a film; (6) Photocrosslinking treatment: The film is placed under a wavelength of 365±5nm and an intensity of 48-52mW / cm 2 The cross-linking and curing is carried out under an ultraviolet light source for 50-60 minutes to obtain a degradable bio-based food packaging material.

6. The preparation method according to claim 5, characterized in that Step (1) is specifically as follows: ITA is dissolved in acetone to prepare a 30-35wt% ITA / acetone solution, bacterial cellulose hydrogel is placed in a reactor, mechanically stirred at 200-300rpm to form a homogeneous suspension, the ITA / acetone solution is slowly added dropwise to the reactor through a constant pressure dropping funnel, the system temperature is maintained at ≤25°C in an ice-water bath, 0.1M NaOH is immediately used to adjust the pH to 6.0-6.5 after the addition is completed, nitrogen is passed through and the temperature is raised to 50-55°C for 15 minutes, then 0.8wt% 4-DMAP is added, the temperature is raised to 75-80°C under nitrogen protection, and the reaction is carried out for 4-5 hours. After the reaction is completed, the bacterial cellulose hydrogel is dialyzed in deionized water using a dialysis membrane with a molecular weight cutoff of 8000 to a conductivity of ≤20μS / cm, the solid content of the bacterial cellulose hydrogel is 1.8-2.2wt%, and the molar ratio of ITA to the hydroxyl group of bacterial cellulose in the added ITA / acetone solution is 1:0.8-1.

0.

7. The preparation method according to claim 5, characterized in that Step (2) is specifically as follows: dissolving sodium copper chlorophyllin (CSL) in a sufficient amount of MES buffer solution with a pH of 5.2 to prepare 12-18 mg / mL sodium copper chlorophyllin / MES buffer solution, adding EDC according to a molar ratio of CSL:EDC=1:1.3, and then adding 0.5 wt% 4-DMAP, stirring in the dark for 30 min to activate, calculating and adding the corresponding mass of the ITA-esterified bacterial cellulose hydrogel obtained in step (1) according to the ratio of 33-50 mg of ITA-esterified bacterial cellulose solid component required for each mg of CSL, adding MES buffer solution with a pH of 5.2 to the system so that the total liquid volume reaches 10-12 times the volume of the added hydrogel, and then reacting in the dark for 4 h at 30°C under the assistance of ultrasound at 40 kHz and a power density of 80-100 W / L. After the reaction is completed, dialyzing is carried out in the dark using a dialysis membrane with a molecular weight cutoff of 3500 to a conductivity of ≤15 μS / cm to obtain CSL-ITA-esterified bacterial cellulose hydrogel.

8. The preparation method according to claim 5, characterized in that Step (3) is as follows: S1. Sample pretreatment: ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel were cut into 2×2×2 mm 3 small pieces; S2. Gradient ethanol dehydration S2.1: Add 20 mL of 30% ethanol per gram of ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel, respectively. Stir magnetically at 200 rpm for 20 minutes at 25°C, then centrifuge at 3000 rpm for 5 minutes. Discard the supernatant and collect the gel at the bottom. S2.2: Add 20 mL of 50% ethanol to each of the bottom gels obtained in step S2.1, treat with ultrasound at 40 kHz and 100 W for 20 min at 25°C, centrifuge at 3000 rpm for 5 min at 4°C, discard the supernatant, and collect the bottom gels. S2.3: Using the same method as S2.2, treat the bottom gel obtained in step S2.2 with 70% ethanol and 90% ethanol, respectively, and collect the bottom gel separately; S2.4: Add 20 mL of anhydrous ethanol to the bottom gel obtained in step S2.3, soak at 4°C for 10 min, centrifuge at 3000 rpm for 5 min, discard the liquid, and repeat this operation three times to obtain ethanol-replaced ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel, respectively; S3. Supercritical CO2 drying The ITA-esterified bacterial cellulose hydrogel and CSL-ITA-esterified bacterial cellulose hydrogel after ethanol replacement were spread on a carrier plate and placed in an autoclave. The autoclave body was pre-cooled to 4°C and maintained for 30 minutes. Then, liquid CO2 was injected at a flow rate of 1.5L / min and slowly pressurized to 7.5MPa with a pressure increase rate of 1MPa / min. Then, the temperature was raised to 40°C and the pressure was raised to 10MPa. The CO2 flow rate was adjusted to 2L / min and dynamic flushing was performed for 3 hours. The ethanol content in the outlet CO2 was sampled and tested every 30 minutes until the ethanol content was <50ppm. Then, the pressure was slowly released at a rate of 0.3MPa / min and the temperature was raised to 45°C. After the pressure release was completed, it was maintained at 45°C and normal pressure for 30 minutes. S4. Product collection and storage: Take out the dried sample from the nitrogen atmosphere glove box, immediately seal it in an aluminum foil light-proof bag, and store it in a dark box at -20℃ after filling it with nitrogen.

9. The preparation method according to claim 5, characterized in that Step (4) is specifically as follows: 125 parts of polylactic acid and 10 parts of acetyl tributyl citrate are melt-blended in an internal mixer at 172-178°C for 5 minutes, cooled to 135-140°C, 4 parts of polycaprolactone diol are added and mixed for 2 minutes, the temperature is further cooled to 110-115°C, 0.3 parts of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide and 0.3 parts of 1-hydroxycyclohexyl phenyl ketone are added in sequence, and mixed for 3 minutes to obtain a melt. The rotor speed of the internal mixer is 55-65 rpm.

10. The preparation method according to claim 5, characterized in that Step (5) is specifically as follows: at 108-112° C. and a rotor speed of 60±5 rpm, 20 parts of ITA esterified bacterial cellulose and 7.5 parts of CSL-ITA esterified bacterial cellulose are first added to the melt of step (4), mixed for 3-4 minutes, and then 0.3 parts of tocopherol are added and mixed for 1 minute. The mixture is then transferred to a double-roll calender and rolled into a film in the double-roll calender at 108-112° C., with a roller spacing of 0.20-0.30 mm and a rolling rate of 2.0-3.0 m / min.

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