Biodegradable lactic acid-based food packaging composite material and preparation method thereof

Abstract

The invention is suitable for the technical field of food packaging materials, and provides a biodegradable lactic acid-based food packaging composite material and a preparation method thereof.The composite material is prepared from polylactic acid, polycaprolactone, tartaric acid-cysteine carbon dots, a bio-based plasticizer, an organic small molecule modifier and modified aerogel; tartaric acid-cysteine carbon dots and polycaprolactone are added to synergistically enhance the mechanical property, barrier property, antibacterial property and antioxidant property of the polylactic acid material; by adding the bio-based plasticizer, the processability and flexibility of the PLA are further improved; by adding the small organic molecule modifier, the dispersity of the carbon dots in a PLA matrix is improved, and the mechanical property, the oxidation resistance and the antibacterial property of the material are further enhanced; by adding the modified aerogel, excellent barrier property and adsorption capacity are provided, the mechanical property of the material is enhanced, and plant-derived antibacterial factors are uniformly distributed in aerogel pores, so that the growth of microorganisms is effectively inhibited.

Description

Technical Field

[0001] The present invention belongs to the technical field of food packaging materials, and in particular relates to a biodegradable lactic acid-based food packaging composite material and a preparation method thereof. Background Art

[0002] Traditional solutions in the food packaging field mainly use petroleum-based polymer materials such as polyethylene (PE) and polypropylene (PP). However, the negative impact of these materials on the environment due to their non-biodegradable nature is widely known. In recent years, research focus has shifted to biodegradable polymers such as polylactic acid (PLA) - this material is derived from renewable resources such as corn starch and has a smaller environmental footprint. However, PLA itself has inherent defects such as insufficient mechanical properties and limited barrier properties, which restrict its application in the food packaging field.

[0003] Although PLA-based composites have been developed to improve the performance of food packaging materials, major drawbacks remain. The mechanical properties of PLA are often insufficient to meet the requirements of certain food packaging applications, such as those requiring high durability and resistance to environmental stress. Furthermore, while the addition of plasticizers such as PCL can improve flexibility, it cannot completely solve the problem of low barrier properties to moisture, oxygen and other external factors, which may affect the shelf life of food. In addition, many existing composites often rely on inorganic fillers, which may lead to reduced biodegradability. In some cases, these solutions also fail to provide antibacterial or antioxidant properties, which can further enhance food preservation. Summary of the Invention

[0004] The present invention provides a biodegradable lactic acid-based food packaging composite material, aiming to solve the above-mentioned problems.

[0005] The present invention is achieved as follows: a biodegradable lactic acid-based food packaging composite material includes the following raw materials in parts by weight: 75-85 parts of polylactic acid, 15-25 parts of polycaprolactone, 2-6 parts of tartaric acid-cysteine ​​carbon dots, 5-15 parts of bio-based plasticizer, 3-7 parts of organic small molecule modifier, and 1-5 parts of modified aerogel.

[0006] Preferably, the following raw materials are included in parts by weight: 78-82 parts of polylactic acid, 17-23 parts of polycaprolactone, 3-5 parts of tartaric acid-cysteine ​​carbon dots, 8-13 parts of bio-based plasticizer, 4-6 parts of organic small molecule modifier, and 2-4 parts of modified aerogel.

[0007] Preferably, the following raw materials are included in parts by weight: 80 parts of polylactic acid, 20 parts of polycaprolactone, 4 parts of tartaric acid-cysteine ​​carbon dots, 10 parts of bio-based plasticizer, 5 parts of organic small molecule modifier, and 3 parts of modified aerogel.

[0008] Preferably, the preparation method of the tartaric acid-cysteine carbon dots is as follows: Dissolve tartaric acid and cysteine in deionized water at a molar ratio of 2:1, and stir thoroughly; Transfer to a high-pressure reaction kettle and perform hydrothermal reaction at 180°C for 8h; After cooling to room temperature, filter out unreacted large particle impurities, and collect the supernatant; Further fine purification by centrifugation and microporous filter membrane filtration to obtain a solution of tartaric acid-cysteine carbon dots with uniform size and good dispersion.

[0009] The carbon dots are dispersed in the polylactic acid matrix, both as functional components and structural components. The composite material presents a multiphase composite structure, with polylactic acid (PLA) as the continuous phase as the main matrix, and polycaprolactone (PCL) as the plasticizing phase uniformly distributed in the PLA matrix to form a blend system. TA-CDs carbon dots are uniformly dispersed in the polymer matrix at the nanoscale, and part of the TA-CDs form physical crosslinking points with the PLA molecular chains through surface functional groups.

[0010] TA-CDs and PCL synergistically enhance the performance of PLA materials: By adjusting the ratio of TA-CDs carbon dots and PCL, the performance of PLA / PCL / TA-CDs composite materials can be flexibly controlled.

[0011] Mechanical properties: The mechanical properties of this composite material are improved through the combined action of carbon dots and PCL plasticizers. TA-CDs carbon dots can enhance the strength of PLA, while PCL improves the flexibility of the material. The combination of the two makes the material maintain sufficient strength and not prone to brittle fracture.

[0012] Barrier properties: Carbon dots (TA-CDs) significantly improve the barrier properties of the composite material by filling the microvoids in the polylactic acid (PLA) matrix. This structural optimization effectively inhibits the penetration of environmental factors such as moisture and oxygen, thereby reducing their negative impact on the shelf life of food, providing longer-term protection for food packaging.

[0013] Antibacterial and antioxidant dual functional properties: Carbon dots (TA-CDs) synthesized from tartaric acid and cysteine have excellent antibacterial activity and antioxidant properties. Their antibacterial effect can effectively inhibit the growth of food spoilage microorganisms, while their antioxidant properties can delay food deterioration reactions such as lipid oxidation, thereby significantly extending the shelf life of food and providing a new strategy for the development of intelligent food packaging materials.

[0014] Biodegradability: The introduction of TA-CDs carbon dots and plasticizers does not affect the biodegradable environmental advantages of polylactic acid (PLA).

[0015] Preferably, the preparation method of the bio-based plasticizer is as follows: Mix castor oil, xylitol and citric acid in a mass ratio of 1:1.2-2:0.1-0.3; Heat the mixture to 140-160℃ in a closed reactor, and react for 2-3h under the assistance of microwave radiation (power 300-500W); Add plant-derived epoxy fatty acid methyl ester (3-6wt%) to the reaction system, and continue to react at 120-140℃ for 2-3h to make the product have better compatibility and thermal stability; After the reaction is completed, cool the product to room temperature, remove the impurities by centrifugal separation, then wash with ethanol for 2-3 times, and finally vacuum dry at 60-70℃ for 12-24h to obtain the bio-based plasticizer.

[0016] The bio-based plasticizer improves the processability and flexibility of PLA by reducing the interaction between the molecular chains of PLA and increasing the flexibility of the chain segments. The hydroxyl and ester groups in the composition can form hydrogen bonds and van der Waals forces with the molecular chains of PLA, enhancing the plasticizing effect. Meanwhile, the renewability and biodegradability of the bio-based plasticizer make it have significant advantages in environmental protection.

[0017] Preferably, the preparation method of the organic small molecule modifier is as follows: Mix L-arginine and chitosan in a mass ratio of 1:1-2, and dissolve in appropriate amount of deionized water; Under nitrogen protection, heat the mixed solution to 60-80℃, add appropriate amount of green oxidizing agent (such as hydrogen peroxide, 3-5wt%), and react for 4-6h. During the reaction, the functional groups of amino acids will undergo grafting reaction with the polysaccharide chain to form a modifier with special structure; After the reaction is completed, cool the product to room temperature, remove the unreacted small molecule substances by dialysis, then add vitamin C, and continue to react at room temperature for 2-3h to make the modifier have antioxidant function; Separate the product by centrifugation, wash with ethanol for 2-3 times, and finally vacuum dry at 40-50℃ for 12-24h to obtain the organic small molecule modifier.

[0018] The organic small molecule modifier improves the dispersibility of carbon dots in the PLA matrix by chemically reacting with the functional groups on the surface of carbon dots to form stable chemical bonds, thereby enhancing the mechanical properties and antioxidant properties of the material. The modifier gives the material better biocompatibility and environmental stability, while maintaining the antibacterial properties of the material.

[0019] Preferably, the preparation method of the modified aerogel is as follows: Silk fibroin and nanocellulose crystals are mixed at a mass ratio of 1-3:1, the nanocellulose crystals have a high aspect ratio and good mechanical properties, and can enhance the strength of the aerogel; The mixture is mixed with an appropriate amount of water, stirred uniformly, and then poured into a mold to form a wet gel; The wet gel is aged at 40-60°C for 24-48h to make the gel structure more compact; The aged wet gel is immersed in a precursor solution containing a temperature-sensitive polymer (such as polyethylene glycol, poly-N-isopropyl acrylamide, with a concentration of 5-15%), and under the synergistic action of ultrasound (frequency 30-50kHz) and magnetic field (strength 0.1-0.5T), the polymer grows in situ in the template pores to form an aerogel with a hierarchical porous structure; The in-situ grown aerogel is placed in an ethanol solution containing a plant-derived antibacterial factor (such as tea polyphenol, with a concentration of 5-10%), and directional adsorption is carried out under the assistance of microwave (power 200-400W), and finally modified aerogel is obtained after freeze-drying.

[0020] The modified aerogel provides excellent barrier properties and adsorption capacity for composite materials through its hierarchical porous structure and high specific surface area, and the nanocellulose crystals and temperature-sensitive polymers in the aerogel synergistically enhance the mechanical properties of the material, and the plant-derived antibacterial factor (such as tea polyphenol) is uniformly distributed in the pores of the aerogel, which can effectively inhibit the growth of microorganisms.

[0021] The application provides a preparation method of a biodegradable lactic acid-based food packaging composite material, which comprises the following steps: Prepare each raw material according to the proportion; Vacuum dry polylactic acid and polycaprolactone at 60°C and 45°C respectively for 4-6h to ensure that the water content is reduced to below 0.05%; Add polylactic acid and polycaprolactone into a high-speed mixer to obtain a premix; the mixing process adopts a segmented stirring program: first, pre-mix at a low speed of 200rpm for 2min to preliminarily disperse the raw materials, and then increase the speed to 800rpm for 5min; nitrogen gas is continuously introduced during the mixing process to prevent oxidation of the material; Add the premix into a screw extruder, and in the extrusion process, add tartaric acid-cysteine carbon dots, bio-based plasticizer, organic small molecule modifier and modified aerogel into the molten polymer through the side feeding port, control the screw speed at 40-45rpm, and make each component uniformly dispersed in the polymer matrix; The extruded molten material is formed by a water-cooled draw bar granulation system, first quickly cooled and solidified through a cooling water tank (temperature 20-25°C), and then cut into 3-5mm cylindrical particles by an underwater pelletizer; The obtained cylindrical particles were dried at 50℃ for 4h to remove surface moisture after removing debris by vibrating sieve, and finally the composite material was obtained.

[0022] Preferably, before the functionalized modified carbon dots are injected into the molten polymer, part (20-50% of the total amount) of the tartaric acid-cysteine carbon dots are pretreated with polycaprolactone to form composite particles with micro-nano structure, and then melt blended and extruded with polylactic acid. By melt blending part of the carbon dots with polycaprolactone (PCL) at high temperature, particles with micro-nano structure are formed. When melt blended with PLA, these particles can be more uniformly dispersed in the PLA matrix, reducing the agglomeration of carbon dots, enhancing the interaction between carbon dots and PLA molecular chains, and enhancing the mechanical properties and barrier properties of the material.

[0023] Preferably, the pretreatment method steps are as follows: Mix part of the tartaric acid-cysteine carbon dots with polycaprolactone uniformly; Put the mixture in a sealed container and heat to above the melting point of polycaprolactone (such as 80-100℃), and melt blend under stirring for 1-2h.

[0024] Pour the melt blended mixture into a mold and cool it naturally or use a cold water bath to quickly cool and solidify it; Grind the solidified block to obtain particles with micro-nano structure, and the particle size can be controlled within a certain range (such as 1-100μm) by sieving.

[0025] Compared with the prior art, the embodiments of the present application have the following beneficial effects: The biodegradable lactic acid-based food packaging composite material provided by the present application enhances the mechanical properties, barrier properties, antibacterial and antioxidant properties of polylactic acid material by adding tartaric acid-cysteine carbon dots and polycaprolactone; by adding a bio-based plasticizer, the interaction between polylactic acid molecular chains is reduced and the flexibility of the chain segment is increased, further improving the processing properties and flexibility of PLA; by adding an organic small molecule modifier, the dispersibility of carbon dots in the PLA matrix is improved, further enhancing the mechanical properties, antioxidant properties and antibacterial properties of the material; by adding modified aerogel, its hierarchical porous structure and high specific surface area provide excellent barrier properties and adsorption capacity for the composite material, enhancing the mechanical properties of the material, and the plant source antibacterial factor is uniformly distributed in the pores of the aerogel, further effectively inhibiting the growth of microorganisms. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a structural schematic diagram of a biodegradable lactic acid-based food packaging composite material provided by the present application. DETAILED DESCRIPTION

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting of the application; the use herein of terms such as "comprise", "comprising", "comprises", "including", "includes" or "contain" or "containing" is to be construed in a non-exclusive sense as meaning that other steps, features or components not specifically recited are optional and can be added. The use herein of terms such as "first", "second" and the like does not imply a particular order but is used for the purpose of nomenclature only.

[0028] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearances of the phrase that an embodiment in various places in the specification are not necessarily all referring to the same embodiment, nor are they necessarily mutually exclusive of one another. As will be apparent to those of ordinary skill in the art, embodiments described herein can be combined with one another.

[0029] Embodiment 1 The embodiment of the application provides a biodegradable lactic acid-based food packaging composite material, which comprises the following raw materials in parts by weight: 75 parts of polylactic acid (molecular weight of about 150,000), 15 parts of polycaprolactone (molecular weight of about 50,000), 2 parts of tartaric acid-cysteine carbon dots, 5 parts of a bio-based plasticizer, 3 parts of an organic small molecule modifier, and 1 part of modified aerogel, and a preparation method of the biodegradable lactic acid-based food packaging composite material is as shown in the figure, which comprises the following steps: Figure 1 The raw materials are prepared according to the ratio; The polylactic acid and the polycaprolactone are vacuum dried at 60 DEG C and 45 DEG C respectively for 4 hours to ensure that the water content is reduced to below 0.05%; The polylactic acid and the polycaprolactone are added into a high-speed mixer to obtain a premix; a segmented stirring program is adopted in the mixing process: the raw materials are preliminarily dispersed at a low speed of 200 rpm for 2 minutes, and then the speed is increased to 800 rpm for high-speed mixing for 5 minutes; nitrogen is continuously introduced during the mixing process to prevent oxidation of the materials; The premix is added into a screw extruder (zone 1, 165 DEG C; zone 2, 176 DEG C; zone 3, 178 DEG C; zone 4, 180 DEG C; die, 182 DEG C), and the tartaric acid-cysteine carbon dots, the bio-based plasticizer, the organic small molecule modifier and the modified aerogel are directly injected into the molten polymer through a side feeding port during the extrusion process, and the screw rotation speed is controlled at 40 rpm, so that the components are uniformly dispersed in the polymer matrix; ​The extruded molten material is shaped by a water-cooled drawing and pelletizing system, is rapidly cooled and solidified by a cooling water tank (temperature 20℃) first, and is then cut into 3mm cylindrical particles by an underwater pelletizer; The obtained cylindrical particles are dried at 50℃ for 4h after removing the debris by a vibrating sieve to remove the surface moisture, and finally the composite material is obtained.

[0030] The preparation method of the tartaric acid-cysteine carbon dots is as follows: Tartaric acid and cysteine are dissolved in deionized water at a molar ratio of 2:1, and stirred thoroughly; Transfer to a high-pressure reaction kettle, and perform hydrothermal reaction at 180℃ for 8h; After cooling to room temperature, remove the unreacted large particle impurities by filtration, and collect the supernatant; Further fine purification by centrifugation and microporous filter membrane filtration, and obtain a tartaric acid-cysteine carbon dot solution with uniform size and good dispersity.

[0031] Further, the preparation method of the bio-based plasticizer is as follows: Take castor oil, xylitol and citric acid and mix them at a mass ratio of 1:1.2:0.1; Heat the mixture to 140℃ in a sealed reactor, and perform reaction under the assistance of microwave radiation (power 300W) for 2h; Add plant-derived epoxy fatty acid methyl ester to the reaction system, and continue to perform reaction at 120℃ for 2h to make the product have better compatibility and thermal stability.

[0032] After the reaction is completed, cool the product to room temperature, remove the impurities by centrifugal separation, then wash it twice with ethanol, and finally vacuum dry it at 60℃ for 12h to obtain the bio-based plasticizer.

[0033] Preferably, the preparation method of the organic small molecule modifier is as follows: Mix L-arginine and chitosan at a mass ratio of 1:1, and dissolve them in a proper amount of deionized water; Under the protection of nitrogen, heat the mixed solution to 60℃, add a proper amount of green oxidizing agent, and react for 4h. During the reaction, the functional groups of amino acids will graft with the polysaccharide chains to form a modifier with a special structure.

[0034] After the reaction is completed, cool the product to room temperature, remove the unreacted small molecule substances by dialysis, then add vitamin C, and continue to react at room temperature for 2h to make the modifier have antioxidant function; Separate the product by centrifugation, wash it twice with ethanol, and finally vacuum dry it at 40℃ for 12h to obtain the organic small molecule modifier.

[0035] Further, the preparation method of the modified aerogel is as follows: The silk fibroin and nanocellulose crystals are mixed at a mass ratio of 1:1, the nanocellulose crystals have a high aspect ratio and good mechanical properties, and can enhance the strength of the aerogel; The mixture is mixed with an appropriate amount of water, stirred uniformly, and then poured into a mold to form a wet gel; The wet gel is aged at 40°C for 24h to make the gel structure more compact; The aged wet gel is immersed in a precursor solution containing a temperature-sensitive polymer (such as polyethylene glycol, PEG, molecular weight 6000), and under the synergistic action of ultrasonic waves (frequency 30kHz) and a magnetic field (intensity 0.1T), the polymer grows in situ in the template pores to form an aerogel with a hierarchical porous structure; The in-situ grown aerogel is placed in an ethanol solution containing a plant-derived antibacterial factor (such as tea polyphenol), and directional adsorption is carried out under the assistance of microwave (power 200W), and finally freeze-drying is carried out to obtain a modified aerogel.

[0036] Example 2 The embodiment of the present application provides a biodegradable lactic acid-based food packaging composite material, which comprises the following raw materials by weight: 78 parts of polylactic acid (molecular weight about 150,000), 17 parts of polycaprolactone (molecular weight about 50,000), 3 parts of tartaric acid-cysteine carbon dots, 8 parts of bio-based plasticizer, 4 parts of organic small molecule modifier, and 2 parts of modified aerogel. Figure 1 The preparation method of the biodegradable lactic acid-based food packaging composite material is as shown in the figure, which comprises the following steps: Prepare each raw material according to the ratio; Vacuum dry the polylactic acid and polycaprolactone at 60°C and 45°C respectively for 4h to ensure that the water content is reduced to below 0.05%; Put the polylactic acid and polycaprolactone into a high-speed mixer to obtain a premix; the mixing process adopts a segmented stirring program: first, pre-mix at a low speed of 200rpm for 2min to preliminarily disperse the raw materials, and then increase the speed to 800rpm for 5min; nitrogen gas is continuously introduced during the mixing process to prevent oxidation of the materials; Put the premix into a screw extruder (zone 1 165°C, zone 2 176°C, zone 3 178°C, zone 4 180°C, die 182°C), and during the extrusion process, the tartaric acid-cysteine carbon dots, bio-based plasticizer, organic small molecule modifier, and modified aerogel are directly injected into the molten polymer through the side feeding port, and the screw speed is controlled at 40rpm to make the components uniformly dispersed in the polymer matrix; The extruded molten material is formed by a water-cooled pull rod granulation system, first quickly cooled and solidified by passing through a cooling water tank (temperature 20°C), and then cut into 3mm cylindrical particles by an underwater pelletizer; The obtained cylindrical particles were vibrated and sieved to remove debris, and then dried at 50 °C for 4 h to remove surface moisture, finally obtaining a composite material.

[0037] The preparation method of the tartaric acid-cysteine ​​carbon dots is as follows: Dissolve tartaric acid and cysteine ​​in deionized water at a molar ratio of 2:1 and stir thoroughly; Transfer to a high-pressure reactor and perform hydrothermal reaction at 180 °C for 8 h; After cooling to room temperature, the mixture was filtered to remove unreacted large particles of impurities and the supernatant was collected; The product was then finely purified by centrifugation and microporous membrane filtration to obtain a tartaric acid-cysteine ​​carbon dot solution with uniform size and good dispersion.

[0038] Furthermore, the preparation method of the bio-based plasticizer is as follows: Mix castor oil, xylitol and citric acid in a molar ratio of 1:1.2:0.1; The mixture was heated to 140 °C in a sealed reactor and reacted for 2 h under the assistance of microwave irradiation (power 300 W); Plant-derived epoxy fatty acid methyl ester was added to the reaction system, and the reaction was continued at 120°C for 2 hours to make the product have better compatibility and thermal stability.

[0039] After the reaction, the product was cooled to room temperature, centrifuged to remove impurities, washed twice with ethanol, and finally dried in vacuum at 60°C for 12 h to obtain a bio-based plasticizer.

[0040] Preferably, the preparation method of the organic small molecule modifier is as follows: Mix L-arginine and chitosan in a mass ratio of 1:1 and add appropriate amount of deionized water to dissolve; Under nitrogen protection, the mixed solution is heated to 60°C, an appropriate amount of green oxidant is added, and the reaction is carried out for 4 hours. During the reaction, the functional groups of the amino acids will undergo grafting reaction with the polysaccharide chains to form a modifier with a special structure.

[0041] After the reaction is completed, the product is cooled to room temperature, and unreacted small molecules are removed by dialysis. Then vitamin C is added and the reaction is continued at room temperature for 2 hours to make the modifier have antioxidant function; The product was separated by centrifugation, washed twice with ethanol, and finally dried in vacuum at 40°C for 12 h to obtain an organic small molecule modifier.

[0042] Furthermore, the preparation method of the modified aerogel is as follows: Silk fibroin and nanocellulose crystals are mixed at a mass ratio of 1:1, the nanocellulose crystals have a high aspect ratio and good mechanical properties, and can enhance the strength of the aerogel; The mixture is mixed with an appropriate amount of water, stirred uniformly, and then poured into a mold to form a wet gel; The wet gel is aged at 40°C for 24h to make the gel structure more compact; The aged wet gel is immersed in a precursor solution containing a temperature-sensitive polymer (such as polyethylene glycol, PEG, molecular weight 6000), and under the synergistic action of ultrasound (frequency 30kHz) and magnetic field (intensity 0.1T), the polymer grows in situ in the template pores to form an aerogel with a hierarchical porous structure; The in-situ grown aerogel is placed in an ethanol solution containing a plant-derived antibacterial factor (such as tea polyphenol), and directional adsorption is carried out under the assistance of microwave (power 200W), and finally modified aerogel is obtained after freeze-drying.

[0043] Example 3 The embodiment of the present application provides a biodegradable lactic acid-based food packaging composite material, which comprises the following raw materials by weight: 80 parts of polylactic acid (molecular weight about 150,000), 20 parts of polycaprolactone (molecular weight about 50,000), 4 parts of tartaric acid-cysteine carbon dots, 10 parts of bio-based plasticizer, 5 parts of organic small molecule modifier, and 3 parts of modified aerogel. Figure 1 The preparation method of the biodegradable lactic acid-based food packaging composite material is as shown in the figure, which comprises the following steps: Prepare each raw material according to the ratio; Vacuum dry the polylactic acid and polycaprolactone at 60°C and 45°C respectively for 5h to ensure that the water content is reduced to below 0.05%; Add the polylactic acid and polycaprolactone into a high-speed mixer to obtain a premix; the mixing process adopts a segmented stirring program: first, pre-mix at a low speed of 200rpm for 2min to preliminarily disperse the raw materials, and then increase the speed to 800rpm for 5min; nitrogen gas is continuously introduced during the mixing process to prevent oxidation of the materials; Add the premix into a screw extruder (zone 1 165°C, zone 2 176°C, zone 3 178°C, zone 4 180°C, die 182°C), and during the extrusion process, the tartaric acid-cysteine carbon dots, bio-based plasticizer, organic small molecule modifier, and modified aerogel are directly injected into the molten polymer through the lateral feeding port, and the screw rotation speed is controlled at 43rpm to make the components uniformly dispersed in the polymer matrix; The extruded molten material is formed by a water-cooled pull rod granulation system, which is first rapidly cooled and solidified by a cooling water tank (temperature 22°C), and then cut into 4mm cylindrical particles by an underwater pelletizer; The obtained cylindrical particles were dried at 50℃ for 4h to remove surface moisture after removing debris by vibrating sieve, and finally the composite material was obtained.

[0044] The preparation method of the tartaric acid-cysteine carbon dots is as follows: Tartaric acid and cysteine were dissolved in deionized water at a molar ratio of 2:1, and stirred thoroughly; Transfer to a high-pressure reaction kettle, hydrothermal reaction at 180℃ for 8h; After cooling to room temperature, remove unreacted large particle impurities by filtration, and collect the supernatant; Further purified by centrifugation and microporous filter membrane, a uniform size, good dispersion of tartaric acid-cysteine carbon dots solution was obtained.

[0045] Further, the preparation method of the bio-based plasticizer is as follows: Take castor oil, xylitol and citric acid according to the mass ratio of 1:1.6:0.2; Heat the mixture to 150℃ in a sealed reactor, and react for 2.5h under the assistance of microwave radiation (power 400W); Add plant-derived epoxy fatty acid methyl ester to the reaction system, and continue to react at 130℃ for 2-3h to make the product have better compatibility and thermal stability.

[0046] After the reaction is completed, the product is cooled to room temperature, impurities are removed by centrifugal separation, then washed with ethanol for 3 times, and finally vacuum dried at 65℃ for 18h to obtain the bio-based plasticizer.

[0047] Preferably, the preparation method of the organic small molecule modifier is as follows: Mix L-arginine and chitosan according to the mass ratio of 1:1.5, and add appropriate amount of deionized water for dissolution; Under the protection of nitrogen, heat the mixed solution to 70℃, add appropriate amount of green oxidizing agent, and react for 5h. During the reaction, the functional groups of amino acids will graft with polysaccharide chains to form a modifier with special structure.

[0048] After the reaction is completed, the product is cooled to room temperature, and unreacted small molecule substances are removed by dialysis, then vitamin C is added, and the reaction is continued at room temperature for 2.5h to make the modifier have antioxidant function; The product is separated by centrifugation, washed with ethanol for 2 times, and finally vacuum dried at 45℃ for 18h to obtain the organic small molecule modifier.

[0049] Further, the preparation method of the modified aerogel is as follows: Silk fibroin and nanocellulose crystals are mixed at a mass ratio of 2:1, the nanocellulose crystals have a high aspect ratio and good mechanical properties, and can enhance the strength of the aerogel; The mixture is mixed with an appropriate amount of water, stirred uniformly, and then poured into a mold to form a wet gel; The wet gel is aged at 50°C for 36h to make the gel structure more compact; The aged wet gel is immersed in a precursor solution containing a temperature-sensitive polymer (poly-N-isopropyl acrylamide, PNIPAM, molecular weight 100,000), under the synergistic action of ultrasound (frequency 40kHz) and magnetic field (intensity 0.5T), the polymer grows in situ in the template pores, forming an aerogel with hierarchical porous structure; The in-situ grown aerogel is placed in an ethanol solution containing a plant-derived antibacterial factor (such as tea polyphenol), and directional adsorption is carried out under the assistance of microwave (power 300W), and finally modified aerogel is obtained after freeze-drying.

[0050] Example 4 The biodegradable lactic acid-based food packaging composite material provided by the embodiment of the application comprises the following raw materials by weight: 82 parts of polylactic acid (molecular weight about 150,000), 23 parts of polycaprolactone (molecular weight about 50,000), 5 parts of tartaric acid-cysteine carbon dots, 13 parts of bio-based plasticizer, 6 parts of organic small molecule modifier, and 4 parts of modified aerogel. Figure 1 The preparation method of the biodegradable lactic acid-based food packaging composite material is shown in the following steps: Prepare the raw materials according to the ratio; Vacuum dry the polylactic acid and polycaprolactone at 60°C and 45°C respectively for 6h to ensure that the water content is reduced to below 0.05%; Add the polylactic acid and polycaprolactone to a high-speed mixer to obtain a premix; the mixing process adopts a segmented stirring program: first, pre-mix at a low speed of 200rpm for 2min to preliminarily disperse the raw materials, and then increase the speed to 800rpm for 5min; nitrogen gas is continuously introduced during the mixing process to prevent oxidation of the materials; Add the premix to a screw extruder (zone 1 165°C, zone 2 176°C, zone 3 178°C, zone 4 180°C, die 182°C), and during the extrusion process, add tartaric acid-cysteine carbon dots, bio-based plasticizer, organic small molecule modifier, and modified aerogel directly into the molten polymer through the side feeding port, and control the screw speed at 45rpm to make the components uniformly dispersed in the polymer matrix; The extruded molten material is formed by a water-cooled pull rod granulation system, first quickly cooled and solidified by passing through a cooling water tank (temperature 25°C), and then cut into 5mm cylindrical particles by an underwater pelletizer; The obtained cylindrical particles were dried at 50℃ for 4h to remove surface moisture after removing debris by vibrating sieve, and finally the composite material was obtained.

[0051] The preparation method of the tartaric acid-cysteine carbon dots is as follows: Tartaric acid and cysteine were dissolved in deionized water at a molar ratio of 2:1, and stirred thoroughly; Transfer to a high-pressure reaction kettle, hydrothermal reaction at 180℃ for 8h; After cooling to room temperature, remove unreacted large particle impurities by filtration, and collect the supernatant; Further fine purification by centrifugation and microporous filter membrane filtration, get uniform size, good dispersion of tartaric acid-cysteine carbon dots solution.

[0052] Further, the preparation method of the bio-based plasticizer is as follows: Take castor oil, xylitol and citric acid according to the mass ratio of 1:2:0.3; The mixture was heated to 160℃ in a closed reactor, and the reaction was carried out under the assistance of microwave radiation (power 500W) for 3h; Add plant source epoxy fatty acid methyl ester to the reaction system, continue to react at 140℃ for 3h, so that the product has better compatibility and thermal stability.

[0053] After the reaction is completed, the product is cooled to room temperature, impurities are removed by centrifugal separation, then washed with ethanol for 3 times, and finally vacuum dried at 70℃ for 24h to obtain the bio-based plasticizer.

[0054] Preferably, the preparation method of the organic small molecule modifier is as follows: Mix L-arginine and chitosan according to the mass ratio of 1:2, and add appropriate amount of deionized water for dissolution; Under the protection of nitrogen, heat the mixed solution to 80℃, add appropriate amount of green oxidant, and react for 6h. During the reaction, the functional groups of amino acids will graft with polysaccharide chains to form a modifier with special structure.

[0055] After the reaction is completed, the product is cooled to room temperature, and unreacted small molecule substances are removed by dialysis, then vitamin C is added, and the reaction is continued at room temperature for 3h to make the modifier have antioxidant function; The product is separated by centrifugation, washed with ethanol for 3 times, and finally vacuum dried at 50℃ for 24h to obtain the organic small molecule modifier.

[0056] Further, the preparation method of the modified aerogel is as follows: Silk fibroin and nanocellulose crystals are mixed in a mass ratio of 3:1. Nanocellulose crystals have a high aspect ratio and good mechanical properties, which can enhance the strength of the aerogel. Mix the mixture with an appropriate amount of water, stir well, pour into a mold, and let it stand to form a wet gel; The wet gel was aged at 60 °C for 48 h to make the gel structure more compact; The aged wet gel is immersed in a precursor solution containing a thermosensitive polymer (such as polyethylene glycol, PEG, molecular weight 6000). Under the synergistic effect of ultrasound (frequency 50 kHz) and magnetic field (intensity 0.5 T), the polymer grows in situ in the template pores to form an aerogel with a hierarchical porous structure. The in situ grown aerogel was placed in an ethanol solution containing plant-derived antibacterial factors (such as tea polyphenols), and directional adsorption was carried out with the assistance of microwaves (power 400W). Finally, the modified aerogel was obtained by freeze-drying.

[0057] Example 5 The embodiment of the present invention provides a biodegradable lactic acid-based food packaging composite material, comprising the following raw materials in parts by weight: 85 parts of polylactic acid (molecular weight of about 150,000), 25 parts of polycaprolactone (molecular weight of about 50,000), 6 parts of tartaric acid-cysteine ​​carbon dots, 15 parts of a bio-based plasticizer, 7 parts of an organic small molecule modifier, and 5 parts of a modified aerogel. The preparation method of the biodegradable lactic acid-based food packaging composite material is as follows: Figure 1 As shown, the following steps are included: Prepare the raw materials according to the ratio; Polylactic acid and polycaprolactone were vacuum dried at 60°C and 45°C for 6 h, respectively, to ensure that the water content dropped below 0.05%. Polylactic acid and polycaprolactone were added to a high-speed mixer to obtain a premix. The mixing process adopted a staged stirring procedure: premixing at a low speed of 200 rpm for 2 minutes to initially disperse the raw materials, and then increasing the speed to 800 rpm for 5 minutes. Nitrogen protection was continuously introduced during the mixing process to prevent oxidation of the materials. The premix was fed into a screw extruder (zone 1: 165°C, zone 2: 176°C, zone 3: 178°C, zone 4: 180°C, and die head: 182°C). During the extrusion process, tartaric acid-cysteine ​​carbon dots, bio-based plasticizers, organic small molecule modifiers, and modified aerogels were directly injected into the molten polymer through a side feed port. The screw speed was controlled at 45 rpm to ensure that the components were evenly dispersed in the polymer matrix. The extruded molten material is formed by a water-cooled strand pelletizing system, firstly passed through a cooling water tank (temperature of 25°C) for rapid cooling and solidification, and then cut into 5mm cylindrical pellets by an underwater pelletizer; The obtained cylindrical particles were dried at 50℃ for 4h to remove surface moisture after removing debris by vibrating sieve, and finally the composite material was obtained.

[0058] The preparation method of the tartaric acid-cysteine carbon dots is as follows: Tartaric acid and cysteine were dissolved in deionized water at a molar ratio of 2:1, and stirred thoroughly; Transfer to a high-pressure reaction kettle, hydrothermal reaction at 180℃ for 8h; After cooling to room temperature, remove unreacted large particle impurities by filtration, and collect the supernatant; Further purified by centrifugation and microporous filter membrane, a uniform size, good dispersion of tartaric acid-cysteine carbon dots solution was obtained.

[0059] Further, the preparation method of the bio-based plasticizer is as follows: Take castor oil, xylitol and citric acid according to the mass ratio of 1:2:0.3; Heat the mixture to 160℃ in a sealed reactor, and react for 3h under the assistance of microwave radiation (power 500W); Add plant-derived epoxy fatty acid methyl ester to the reaction system, and continue to react at 140℃ for 3h to make the product have better compatibility and thermal stability.

[0060] After the reaction is completed, the product is cooled to room temperature, impurities are removed by centrifugal separation, then washed with ethanol for 3 times, and finally vacuum dried at 70℃ for 24h to obtain the bio-based plasticizer.

[0061] Preferably, the preparation method of the organic small molecule modifier is as follows: Mix L-arginine and chitosan according to the mass ratio of 1:2, and add appropriate amount of deionized water for dissolution; Under the protection of nitrogen, heat the mixed solution to 80℃, add appropriate amount of green oxidizing agent, and react for 6h. During the reaction, the functional groups of amino acids will graft with polysaccharide chains to form a modifier with special structure.

[0062] After the reaction is completed, the product is cooled to room temperature, and unreacted small molecule substances are removed by dialysis, then vitamin C is added, and the reaction is continued at room temperature for 3h to make the modifier have antioxidant function; The product is separated by centrifugation, washed with ethanol for 3 times, and finally vacuum dried at 50℃ for 24h to obtain the organic small molecule modifier.

[0063] Further, the preparation method of the modified aerogel is as follows: Silk fibroin and nanocellulose crystals are mixed at a mass ratio of 3:1, the nanocellulose crystals have a high aspect ratio and good mechanical properties, and can enhance the strength of the aerogel; The mixture is mixed with an appropriate amount of water, stirred uniformly, and then poured into a mold to form a wet gel; The wet gel is aged at 60°C for 48h to make the gel structure more compact; The aged wet gel is immersed in a precursor solution containing a temperature-sensitive polymer (such as poly-N-isopropyl acrylamide, PNIPAM, molecular weight 100,000), under the synergistic action of ultrasound (frequency 50 kHz) and magnetic field (intensity 0.5T), the polymer grows in situ in the template pores, forming aerogels with hierarchical porous structure; The in-situ grown aerogel is placed in an ethanol solution containing a plant-derived antibacterial factor (such as tea polyphenol), and directional adsorption is carried out under the assistance of microwave (power 400W), and finally modified aerogel is obtained after freeze-drying.

[0064] Example 6 The difference between this embodiment and Example 3 is that part (20-50% of the total) of the tartaric acid-cysteine carbon dots are pretreated with polycaprolactone before being injected into the molten polymer, forming composite particles with micro-nano structure, and then melt blending and extruding with polylactic acid. By melt blending part of the carbon dots with polycaprolactone (PCL) at high temperature, composite particles with micro-nano structure are formed. When melt blended with PLA, these composite particles can be more uniformly dispersed in the PLA matrix, reducing the agglomeration of carbon dots, enhancing the interaction between carbon dots and PLA molecular chains, and enhancing the mechanical properties and barrier properties of the material.

[0065] The pretreatment method is as follows: Part of the tartaric acid-cysteine carbon dots are mixed with polycaprolactone; The mixture is placed in a sealed container and heated to above the melting point of polycaprolactone (such as 80-100°C), and fully melt blended under stirring for 1-2h.

[0066] The melt blended mixture is poured into a mold and naturally cooled or rapidly cooled and solidified using a cold water bath or the like; The block-shaped material after cooling and solidification is crushed to obtain particles with micro-nano structure, and the particle size can be controlled within a certain range by screening or the like.

[0067] The designed comparison is as follows: Comparative Example 1: Traditional PLA packaging material.

[0068] Comparative Example 2: Lack of PCL, same as Example 3.

[0069] Comparative Example 3: tartaric acid-cysteine ​​carbon dots are missing, and the rest are the same as Example 3.

[0070] Comparative Example 4: lacks the bio-based plasticizer, and is otherwise the same as Example 3.

[0071] Comparative Example 5: lacks the organic small molecule modifier, and is otherwise the same as Example 3.

[0072] Comparative Example 6: lacks the modified aerogel, and is otherwise the same as Example 3.

[0073] Performance Testing The performance tests of the materials of Examples 1-6 and Comparative Examples 1-6 were carried out, and the results are shown in Table 1 below: Table 1 Performance test results

[0074] It can be seen from the above results that the composite material prepared by the present invention has excellent mechanical properties, barrier properties, antibacterial properties and antioxidant properties.

[0075] The above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on these embodiments, all other embodiments obtained by persons of ordinary skill in the art without inventive effort are also within the scope of protection of the present invention.

Claims

1. A biodegradable lactic acid-based food packaging composite material, characterized in that: The invention comprises the following raw materials in parts by weight: 75-85 parts of polylactic acid, 15-25 parts of polycaprolactone, 2-6 parts of tartaric acid-cysteine ​​carbon dots, 5-15 parts of bio-based plasticizer, 3-7 parts of organic small molecule modifier and 1-5 parts of modified aerogel.

2. The biodegradable lactic acid-based food packaging composite material according to claim 1, wherein: The invention comprises the following raw materials in parts by weight: 78-82 parts of polylactic acid, 17-23 parts of polycaprolactone, 3-5 parts of tartaric acid-cysteine ​​carbon dots, 8-13 parts of bio-based plasticizer, 4-6 parts of organic small molecule modifier, and 2-4 parts of modified aerogel.

3. The biodegradable lactic acid-based food packaging composite material according to claim 2, wherein: The invention comprises the following raw materials in parts by weight: 80 parts of polylactic acid, 20 parts of polycaprolactone, 4 parts of tartaric acid-cysteine ​​carbon dots, 10 parts of bio-based plasticizer, 5 parts of organic small molecule modifier, and 3 parts of modified aerogel.

4. The biodegradable lactic acid-based food packaging composite material according to claim 1, wherein: The preparation method of the tartaric acid-cysteine ​​carbon dots is as follows: Dissolve tartaric acid and cysteine ​​in deionized water at a molar ratio of 2:1 and stir thoroughly; The mixture was transferred to a high-pressure reactor and subjected to hydrothermal reaction at 180 °C for 8 h; After cooling to room temperature, the mixture was filtered to remove unreacted large particles of impurities and the supernatant was collected; The product was then finely purified by centrifugation and microporous membrane filtration to obtain a tartaric acid-cysteine ​​carbon dot solution with uniform size and good dispersion.

5. The biodegradable lactic acid-based food packaging composite material according to claim 1, wherein: The preparation method of the bio-based plasticizer is as follows: Mix castor oil, xylitol and citric acid in a molar ratio of 1:1.2-2:0.1-0.3; The mixture was heated to 140-160°C in a sealed reactor and reacted under microwave irradiation for 2-3 hours; Add plant-derived epoxy fatty acid methyl ester to the reaction system and continue the reaction at 120-140°C for 2-3 hours; After the reaction is completed, the product is cooled to room temperature, impurities are removed by centrifugation, and then washed with ethanol 2-3 times, and finally vacuum dried at 60-70°C for 12-24h to obtain a bio-based plasticizer.

6. The biodegradable lactic acid-based food packaging composite material according to claim 1, wherein: The preparation method of the organic small molecule modifier is as follows: Mix L-arginine and chitosan in a mass ratio of 1:1-2, and add appropriate amount of deionized water to dissolve; Under nitrogen protection, heat the mixed solution to 60-80°C, add an appropriate amount of green oxidant, and react for 4-6 hours; After the reaction is completed, the product is cooled to room temperature, and unreacted small molecules are removed by dialysis. Then vitamin C is added and the reaction is continued at room temperature for 2-3 hours. The product was separated by centrifugation, washed with ethanol 2-3 times, and finally dried in vacuum at 40-50° C. for 12-24 h to obtain an organic small molecule modifier.

7. The biodegradable lactic acid-based food packaging composite material according to claim 1, wherein: The preparation method of the modified aerogel is as follows: Mixing silk fibroin and nanocellulose crystals in a mass ratio of 1-3:1; Mix the mixture with an appropriate amount of water, stir well, pour into a mold, and let it stand to form a wet gel; Aging the wet gel at 40-60°C for 24-48h; The aged wet gel is immersed in a precursor solution containing a thermosensitive polymer. Under the synergistic effect of ultrasound and magnetic field, the polymer grows in situ in the pores of the template to form an aerogel with a hierarchical porous structure. The in situ grown aerogel was placed in an ethanol solution containing plant-derived antibacterial factors, subjected to microwave-assisted directional adsorption, and finally freeze-dried to obtain the modified aerogel.

8. The method for preparing the biodegradable lactic acid-based food packaging composite material according to any one of claims 1 to 7, characterized in that: The steps include: Prepare the raw materials according to the ratio; Polylactic acid and polycaprolactone were vacuum dried at 60°C and 45°C for 4-6 hours respectively to ensure that the water content dropped below 0.05%; Polylactic acid and polycaprolactone are added into a high-speed mixer, and nitrogen is continuously introduced for protection during the mixing process to obtain a premix; The premix is ​​added to a screw extruder. During the extrusion process, tartaric acid-cysteine ​​carbon dots, bio-based plasticizers, organic small molecule modifiers, and modified aerogels are directly injected into the molten polymer through a side feed port. The extruded molten material is formed by a water-cooled strand pelletizing system, first rapidly cooled and solidified by a cooling water tank, and then cut into 3-5mm cylindrical pellets by an underwater pelletizer; The obtained cylindrical particles were vibrated and sieved to remove debris, and then dried at 50 °C for 4 h to remove surface moisture, finally obtaining a composite material.

9. The method for preparing the biodegradable lactic acid-based food packaging composite material according to claim 8, wherein: Before the functionalized carbon dots are injected into the molten polymer, some of the tartaric acid-cysteine ​​carbon dots are pretreated with polycaprolactone and then melt-blended and extruded with polylactic acid.

10. The method for preparing the biodegradable lactic acid-based food packaging composite material according to claim 9, wherein: The pre-processing steps are as follows: Mixing a portion of tartaric acid-cysteine ​​carbon dots and polycaprolactone evenly; The mixture was placed in a sealed container, heated to above the melting point of polycaprolactone, and fully melted and blended under stirring conditions for 1-2 hours; The melt-blended mixture is poured into a mold and allowed to cool naturally or be rapidly cooled and solidified in a cold water bath; The cooled and solidified lumps are crushed to obtain particles with micro-nano structure.

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