Photocatalytic antibacterial polylactic acid composite film, and preparation method and application thereof

Abstract

The application provides a photocatalytic antibacterial polylactic acid composite film and a preparation method and application thereof, and belongs to the technical field of polylactic acid.The polylactic acid is uniformly mixed with a modifier, a toughening agent and a condensing agent, and is subjected to stirring reaction, and the photocatalytic antibacterial polylactic acid composite film is prepared through a solution casting method or a melt extrusion method.The modifier is prepared by the following steps: mixing rare earth doped zinc oxide quantum dots and carbon nitride micropipes, ball milling, and then coating modification through polydopamine.The photocatalytic antibacterial polylactic acid composite film prepared by the application has good antibacterial property, self-cleaning property, ultraviolet resistance, weather resistance and other performances, and the mechanical property, such as tensile strength, is obviously improved, and the long-acting antibacterial effect is realized.

Description

Technical Field

[0001] This invention relates to the field of polylactic acid technology, specifically to a photocatalytic antibacterial polylactic acid composite membrane, its preparation method, and its application. Background Technology

[0002] Polylactic acid (PLA), also known as polylactide, is a polymer material made from renewable plant materials (corn, cassava, beet, straw cellulose, etc.) through microbial fermentation into small-molecule lactic acid, followed by polymerization. At room temperature, PLA is a colorless or pale yellow thermoplastic polymer with good gloss and transparency, and a density of 1.20-1.30 g / cm³. 3 Glass transition temperature (T) g The temperature range is 55-65℃, and the melting point (T) is... m With a temperature range of 160-180℃, polylactic acid (PLA) can be processed using common plastic processing techniques such as injection molding, extrusion, blow molding, vacuum forming, hot pressing, foaming, and spinning. PLA possesses mechanical properties similar to PET and biaxially oriented polystyrene (tensile strength up to 60 MPa), high gloss, good transparency, is non-toxic, and exhibits excellent biocompatibility. Furthermore, PLA has widely available and renewable raw materials and can completely degrade into water and carbon dioxide after use, making it an ideal "green plastic." However, PLA also has several drawbacks, such as its hard and brittle texture, low impact strength, poor crystallinity, poor heat resistance, insufficient hydrophilicity, long processing cycle, high cost, and unstable degradation cycle.

[0003] Plastics are widely used in people's lives due to their convenience and low cost. However, during use, ordinary plastics can be contaminated by various microorganisms, including pathogens that can harm human health. As people pay increasing attention to health and hygiene, the hygiene and safety of plastic products have also gained importance, leading to the development of antibacterial plastics. Antibacterial plastics possess antibacterial and bactericidal properties, capable of killing or inhibiting the growth and reproduction of microorganisms on their surfaces, and reducing microbial contamination during use. Antibacterial properties are typically enhanced by incorporating antibacterial agents into the plastic. Antibacterial agents are generally classified into three categories: inorganic antibacterial agents, organic antibacterial agents, and natural antibacterial agents. However, directly adding antibacterial agents to plastics presents compatibility issues that can affect material properties. Summary of the Invention

[0004] The purpose of this invention is to propose a photocatalytic antibacterial polylactic acid composite membrane, its preparation method and application, which has good antibacterial properties, self-cleaning properties, UV protection and weather resistance, and mechanical properties such as tensile strength are significantly improved, and a long-lasting antibacterial effect is achieved.

[0005] The technical solution of this invention is implemented as follows: This invention provides a method for preparing a photocatalytic antibacterial polylactic acid composite film. Polylactic acid is mixed evenly with a modifier, a toughening agent, and a condensing agent, and stirred to react. The photocatalytic antibacterial polylactic acid composite film is obtained by solution casting or melt extrusion. The modifier is prepared by ball milling rare earth-doped zinc oxide quantum dots and carbon nitride microtubes, followed by polydopamine coating modification.

[0006] As a further improvement of the present invention, the method for preparing the modifier is as follows: S1. Preparation of rare earth-doped zinc oxide quantum dots: Lithium hydroxide alcohol solution was added dropwise to a stirred alcohol solution containing zinc salt and rare earth salt, the reaction was heated and stirred, n-hexane was added, the mixture was allowed to stand, centrifuged, washed, and dried to obtain rare earth-doped zinc oxide quantum dots; Rare earth-doped zinc oxide quantum dots can extend the photoresponse of zinc oxide quantum dots from the ultraviolet to the visible light region, and reactive oxygen can be generated under indoor light. However, zinc oxide quantum dots are prone to aggregation, which greatly reduces their activity.

[0007] S2. Preparation of carbon nitride microtubes: Melamine and silk fibers were mixed and dissolved in water, subjected to hydrothermal reaction, centrifuged, washed, dried, and calcined in an inert gas atmosphere to obtain carbon nitride microtubes; TiO2 possesses good photocatalytic activity, antibacterial properties, and UV protection. However, its application is greatly limited because it can only utilize the ultraviolet light region. Therefore, the inventors found a similar substitute, graphitic carbon nitride, which has advantages such as visible light response, moderate band gap, safety and non-toxicity, strong thermal stability, and low cost. It can generate active oxygen under visible light excitation, thus playing a bactericidal role. Traditional bulk carbon nitride has low specific surface area, rapid photogenerated carrier recombination rate, and limited visible light utilization, which limits its application. This invention uses hydrothermal conditions to achieve pre-assembly of melamine and silk fibers, followed by high-temperature calcination to obtain tubular carbon nitride, which has an excellent three-dimensional structure, large specific surface area, and excellent electron transfer rate.

[0008] S3. Preparation of quantum dot / carbon nitride microtube composite: Rare earth-doped zinc oxide quantum dots and carbon nitride microtubes were mixed and ball-milled to obtain quantum dot / carbon nitride microtube composite; The zinc oxide quantum dots prepared by the sol-gel reaction of this invention are ball-milled and mixed with carbon nitride microtubes, thereby improving their dispersibility, avoiding their agglomeration, and better exerting their synergistic photocatalytic effect. After ball milling, they form a tight heterojunction with carbon nitride microtubes, inhibiting photogenerated electron-hole recombination, improving photocatalytic and antibacterial properties, and helping to degrade organic pollutants on the surface of materials, thus achieving self-cleaning.

[0009] S4. Modification: The quantum dot / carbon nitride microtube composite was added to Tris-HCl solution, dopamine hydrochloride was added, the mixture was heated and stirred, centrifuged, washed, dried, and ball-milled to obtain the modifier.

[0010] This invention coats the surface of a quantum dot / carbon nitride microtube composite with a polydopamine layer. The amino groups in the polylactic acid (PLA) matrix undergo a condensation reaction with the carboxyl groups in the PLA matrix under the catalysis of a condensing agent, thereby achieving grafting of the modifier into the PLA. This improves the dispersibility and compatibility of the modifier with PLA, reduces interfacial porosity, and thus improves the mechanical properties of PLA to a certain extent, such as tensile strength. Furthermore, the polydopamine coating provides a sustained-release, long-term antibacterial effect.

[0011] As a further improvement of the present invention, the mass ratio of lithium hydroxide, zinc salt and rare earth salt in step S1 is 2-2.5:3-3.7:0.05-0.1, the zinc salt is zinc chloride, zinc sulfate or zinc nitrate, and the rare earth salt is selected from at least one of cerium chloride, cerium nitrate, lanthanum nitrate and lanthanum chloride, preferably a mixture of lanthanum chloride and cerium chloride, with a mass ratio of 1:0.5-1.5, and the heating and stirring reaction temperature is 65-75°C and the time is 70-90 min.

[0012] As a further improvement of the present invention, the mass ratio of melamine to silk fiber in step S2 is 40-60:1, the temperature of the hydrothermal reaction is 170-190℃, and the time is 20-28h.

[0013] As a further improvement of the present invention, the mass ratio of rare earth-doped zinc oxide quantum dots to carbon nitride microtubes in step S3 is 8-10:20-30, and the ball milling time is 2-4 hours.

[0014] As a further improvement of the present invention, the pH value of the Tris-HCl solution in step S4 is 8.5-9.5, the mass ratio of the quantum dot / carbon nitride microtube composite to dopamine hydrochloride is 10:0.2-0.3, and the temperature of the heating and stirring reaction is 45-65℃, and the time is 3-5h.

[0015] As a further improvement of the present invention, the toughening agent is selected from at least one of ethylene / butyl methacrylate / glycidyl methacrylate terpolymer, polypropylene carbonate polyurethane, and the condensing agent is dicyclohexylcarbodiimide, 1-hydroxybenzotriazole and N-hydroxysuccinimide in a mass ratio of 5-7:1-2:2-3.

[0016] As a further improvement of the present invention, the mass ratio of polylactic acid to modifier, toughening agent and condensing agent is 100:5-10:4-7:1-2, and the stirring reaction time is 10-20h.

[0017] The present invention further protects a photocatalytic antibacterial polylactic acid composite membrane prepared by the above-mentioned preparation method.

[0018] This invention further protects the application of the above-mentioned photocatalytic antibacterial polylactic acid composite film in the preparation of antibacterial and food preservation materials.

[0019] The present invention has the following beneficial effects: 1. This invention achieves the pre-assembly of melamine and silk fibers under hydrothermal conditions, and obtains tubular carbon nitride by high-temperature calcination. It has excellent three-dimensional structure, large specific surface area and excellent electron transfer rate, and has good photocatalytic activity. Therefore, it can significantly improve the antibacterial properties, photocatalytic degradation of organic matter, UV protection and weather resistance of polylactic acid materials.

[0020] 2. The rare earth-doped zinc oxide quantum dots prepared by this invention can extend the photoresponse of zinc oxide quantum dots from the ultraviolet to the visible light region. They can generate active oxygen under indoor light. After ball milling, they form a tight heterojunction with carbon nitride microtubes, which inhibits photogenerated electron-hole recombination, improves photocatalytic and antibacterial properties, and helps degrade organic pollutants on the surface of materials, thus achieving self-cleaning.

[0021] 3. In this invention, a polydopamine layer is coated on the surface of a quantum dot / carbon nitride microtube composite. The modifier is grafted into polylactic acid through a condensation reaction, which improves the dispersibility of the modifier and its compatibility with polylactic acid, improves the mechanical properties of polylactic acid, such as tensile strength, and achieves a long-lasting antibacterial effect. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 SEM image of the modifier prepared in Example 3; Figure 2 Figure A is a SEM image of the photocatalytic antibacterial polylactic acid composite membrane prepared in Example 3, and Figure B is a SEM image of the photocatalytic antibacterial polylactic acid composite membrane prepared in Comparative Example 5. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Ethylene / butyl methacrylate / glycidyl methacrylate terpolymer (GEBMA), trade name KT-20, white granules, glycidyl methacrylate content 8%, specific gravity 0.94-0.98 g / cm³. 3 The melt index is 3 g / 10 min (190℃, 2.16 kg), and the number average molecular weight is 54200 g / mol.

[0026] Preparation Example 1 Modifier The preparation method is as follows: S1. Preparation of rare earth-doped zinc oxide quantum dots: 100 mL of ethanol solution containing 0.2 g lithium hydroxide was added dropwise to 150 mL of ethanol solution containing 0.3 g zinc nitrate and 0.005 g rare earth salt while stirring. The mixture was heated to 65 °C and stirred for 90 min. 200 mL of n-hexane was added, and the mixture was allowed to stand for 1 h. After centrifugation, washing, and drying, rare earth-doped zinc oxide quantum dots were obtained. The rare earth salt is a mixture of lanthanum chloride and cerium chloride in a mass ratio of 1:0.5.

[0027] S2. Preparation of carbon nitride microtubes: 40g of melamine and 1g of silk fiber were mixed and dissolved in 260mL of water, and the mixture was hydrothermally reacted at 170℃ for 28h. After centrifugation, washing, drying, and calcination at 550℃ for 4h in a nitrogen atmosphere, carbon nitride microtubes were obtained. S3. Preparation of quantum dot / carbon nitride microtube composite: 8g of rare earth-doped zinc oxide quantum dots and 20g of carbon nitride microtubes were mixed and ball-milled for 2h to obtain quantum dot / carbon nitride microtube composite. S4. Modification: 10g of quantum dot / carbon nitride microtube composite was added to 200mL of Tris-HCl solution with pH 8.5, 0.2g of dopamine hydrochloride was added, the mixture was heated to 45℃, stirred for 5h, centrifuged, washed, dried, and ball-milled for 1h to obtain the modifier.

[0028] Preparation Example 2 Modifier The preparation method is as follows: S1. Preparation of rare earth-doped zinc oxide quantum dots: 100 mL of ethanol solution containing 0.25 g lithium hydroxide was added dropwise to 150 mL of ethanol solution containing 0.37 g zinc sulfate and 0.01 g rare earth salt while stirring. The mixture was heated to 75 °C and stirred for 90 min. 200 mL of n-hexane was added, and the mixture was allowed to stand for 1 h. The mixture was then centrifuged, washed, and dried to obtain rare earth-doped zinc oxide quantum dots. Rare earth salts are a mixture of lanthanum chloride and cerium chloride in a mass ratio of 1:1.5.

[0029] S2. Preparation of carbon nitride microtubes: 60g of melamine and 1g of silk fiber were mixed and dissolved in 300mL of water, and the mixture was hydrothermally reacted at 190℃ for 20h. After centrifugation, washing, drying, and calcination at 550℃ for 4h in a nitrogen atmosphere, carbon nitride microtubes were obtained. S3. Preparation of quantum dot / carbon nitride microtube composite: 10g of rare earth-doped zinc oxide quantum dots and 30g of carbon nitride microtubes were mixed and ball-milled for 4h to obtain quantum dot / carbon nitride microtube composite. S4. Modification: 10g of quantum dot / carbon nitride microtube composite was added to 200mL of Tris-HCl solution with pH 9.5, 0.3g of dopamine hydrochloride was added, the mixture was heated to 65℃, stirred for 3h, centrifuged, washed, dried, and ball-milled for 1h to obtain the modifier.

[0030] Preparation Example 3 Modifier The preparation method is as follows: S1. Preparation of rare earth-doped zinc oxide quantum dots: 100 mL of ethanol solution containing 0.22 g lithium hydroxide was added dropwise to 150 mL of ethanol solution containing 0.34 g zinc chloride and 0.007 g rare earth salt while stirring. The mixture was heated to 70 °C and stirred for 80 min. 200 mL of n-hexane was added, and the mixture was allowed to stand for 1 h. After centrifugation, washing, and drying, rare earth-doped zinc oxide quantum dots were obtained. Rare earth salts are a mixture of lanthanum chloride and cerium chloride in a mass ratio of 1:1.

[0031] S2. Preparation of carbon nitride microtubes: 50g of melamine and 1g of silk fiber were mixed and dissolved in 260mL of water, and the mixture was hydrothermally reacted at 180℃ for 24h. After centrifugation, washing, drying, and calcination at 550℃ for 4h in a nitrogen atmosphere, carbon nitride microtubes were obtained. S3. Preparation of quantum dot / carbon nitride microtube composite: 9g of rare earth-doped zinc oxide quantum dots and 25g of carbon nitride microtubes were mixed and ball-milled for 3h to obtain quantum dot / carbon nitride microtube composite. S4. Modification: 10g of quantum dot / carbon nitride microtube composite was added to 200mL of Tris-HCl solution with pH 9, 0.25g of dopamine hydrochloride was added, the mixture was heated to 55℃, stirred for 4h, centrifuged, washed, dried, and ball-milled for 1h to obtain the modifier. Figure 1 The image shows a SEM image of the prepared modifier. As can be seen from the image, the modifier has a tubular structure and a coating layer on its surface.

[0032] Preparation Example 4 The difference from Preparation Example 3 is that the rare earth salt is a single lanthanum chloride.

[0033] Preparation Example 5 The difference from Preparation Example 3 is that the rare earth salt is a single cerium chloride.

[0034] Comparative Preparation Example 1 The difference from Preparation Example 3 is that no rare earth salts were added in step S1.

[0035] The preparation method is as follows: S1. Preparation of zinc oxide quantum dots: 100 mL of ethanol solution containing 0.22 g lithium hydroxide was added dropwise to 150 mL of ethanol solution containing 0.34 g zinc chloride while stirring. The mixture was heated to 70 °C and stirred for 80 min. 200 mL of n-hexane was added, and the mixture was allowed to stand for 1 h. The mixture was then centrifuged, washed, and dried to obtain zinc oxide quantum dots. S2. Preparation of carbon nitride microtubes: 50g of melamine and 1g of silk fiber were mixed and dissolved in 260mL of water, and the mixture was hydrothermally reacted at 180℃ for 24h. After centrifugation, washing, drying, and calcination at 550℃ for 4h in a nitrogen atmosphere, carbon nitride microtubes were obtained. S3. Preparation of quantum dot / carbon nitride microtube composite: 9g of zinc oxide quantum dots and 25g of carbon nitride microtubes were mixed and ball-milled for 3h to obtain quantum dot / carbon nitride microtube composite; S4. Modification: 10g of quantum dot / carbon nitride microtube composite was added to 200mL of Tris-HCl solution with pH 9, 0.25g of dopamine hydrochloride was added, the mixture was heated to 55℃, stirred for 4h, centrifuged, washed, dried, and ball-milled for 1h to obtain the modifier.

[0036] Comparative Preparation Example 2 The difference from Preparation Example 3 is that in step S3, the carbon nitride microtubes are replaced by bulk carbon nitride.

[0037] The preparation method is as follows: S1. Preparation of rare earth-doped zinc oxide quantum dots: 100 mL of ethanol solution containing 0.22 g lithium hydroxide was added dropwise to 150 mL of ethanol solution containing 0.34 g zinc chloride and 0.007 g rare earth salt while stirring. The mixture was heated to 70 °C and stirred for 80 min. 200 mL of n-hexane was added, and the mixture was allowed to stand for 1 h. After centrifugation, washing, and drying, rare earth-doped zinc oxide quantum dots were obtained. Rare earth salts are a mixture of lanthanum chloride and cerium chloride in a mass ratio of 1:1.

[0038] S2. Preparation of bulk carbon nitride: 10g of urea was added to 30mL of water, dried at 60℃ for 12h, then calcined at 180℃ for 3h, and ground to obtain bulk carbon nitride; S3. Preparation of quantum dot / carbon nitride microtube composite: 9g of rare earth-doped zinc oxide quantum dots and 25g of bulk carbon nitride were mixed and ball-milled for 3h to obtain quantum dot / carbon nitride composite; S4. Modification: 10g of quantum dot / carbon nitride complex was added to 200mL of Tris-HCl solution with pH 9, 0.25g of dopamine hydrochloride was added, the mixture was heated to 55℃, stirred for 4h, centrifuged, washed, dried, and ball-milled for 1h to obtain the modifier.

[0039] Comparative preparation example 3 The difference from preparation example 3 is that steps S1 and S3 were not performed.

[0040] The preparation method is as follows: S1. Preparation of carbon nitride microtubes: 50g of melamine and 1g of silk fiber were mixed and dissolved in 260mL of water, and the mixture was hydrothermally reacted at 180℃ for 24h. After centrifugation, washing, drying, and calcination at 550℃ for 4h in a nitrogen atmosphere, carbon nitride microtubes were obtained. S2. Modification: 10g of carbon nitride microtubes were added to 200mL of Tris-HCl solution with a pH of 9, and 0.25g of dopamine hydrochloride was added. The mixture was heated to 55℃ and stirred for 4h. After centrifugation, washing, drying, and ball milling for 1h, the modifier was obtained.

[0041] Comparative preparation example 4 The difference compared to Preparation Example 3 is that steps S2 and S3 were not performed.

[0042] The preparation method is as follows: S1. Preparation of rare earth-doped zinc oxide quantum dots: 100 mL of ethanol solution containing 0.22 g lithium hydroxide was added dropwise to 150 mL of ethanol solution containing 0.34 g zinc chloride and 0.007 g rare earth salt while stirring. The mixture was heated to 70 °C and stirred for 80 min. 200 mL of n-hexane was added, and the mixture was allowed to stand for 1 h. After centrifugation, washing, and drying, rare earth-doped zinc oxide quantum dots were obtained. Rare earth salts are a mixture of lanthanum chloride and cerium chloride in a mass ratio of 1:1.

[0043] S2. Modification: 10g of rare earth-doped zinc oxide quantum dots were added to 200mL of Tris-HCl solution with a pH of 9, and 0.25g of dopamine hydrochloride was added. The mixture was heated to 55℃, stirred for 4h, centrifuged, washed, dried, and ball-milled for 1h to obtain the modifier.

[0044] Comparative preparation example 5 The difference compared to preparation example 3 is that step S4 was not performed.

[0045] The preparation method is as follows: S1. Preparation of rare earth-doped zinc oxide quantum dots: 100 mL of ethanol solution containing 0.22 g lithium hydroxide was added dropwise to 150 mL of ethanol solution containing 0.34 g zinc chloride and 0.007 g rare earth salt while stirring. The mixture was heated to 70 °C and stirred for 80 min. 200 mL of n-hexane was added, and the mixture was allowed to stand for 1 h. After centrifugation, washing, and drying, rare earth-doped zinc oxide quantum dots were obtained. Rare earth salts are a mixture of lanthanum chloride and cerium chloride in a mass ratio of 1:1.

[0046] S2. Preparation of carbon nitride microtubes: 50g of melamine and 1g of silk fiber were mixed and dissolved in 260mL of water, and the mixture was hydrothermally reacted at 180℃ for 24h. After centrifugation, washing, drying, and calcination at 550℃ for 4h in a nitrogen atmosphere, carbon nitride microtubes were obtained. S3. Preparation of quantum dot / carbon nitride microtube composite: 9g of rare earth-doped zinc oxide quantum dots and 25g of carbon nitride microtubes were mixed and ball-milled for 3h to obtain the quantum dot / carbon nitride microtube composite, which is the modifier.

[0047] Example 1 This embodiment provides a method for preparing a photocatalytic antibacterial polylactic acid composite membrane. 10g of polylactic acid is dissolved in 200mL of chloroform and stirred until dissolved. 0.5g of the modifier obtained in Preparation Example 1 and 0.1g of the condensing agent are added, and the mixture is stirred at room temperature for 10 hours. Then, 0.4g of an ethylene / butyl methacrylate / glycidyl methacrylate terpolymer is added, and the mixture is stirred for 30 minutes. Finally, 10mL of polyethylene glycol is added, and the mixture is stirred at room temperature for 5 hours. The solution is then uniformly cast onto a smooth mold using a casting method and dried to obtain the photocatalytic antibacterial polylactic acid composite membrane. The condensing agent is dicyclohexylcarbodiimide, 1-hydroxybenzotriazole, and N-hydroxysuccinimide, with a mass ratio of 5:1:2.

[0048] Example 2 This embodiment provides a method for preparing a photocatalytic antibacterial polylactic acid composite film. 10g of polylactic acid is dissolved in 200mL of chloroform and stirred until dissolved. 1g of the modifier obtained in Preparation Example 2 and 0.2g of the condensing agent are added, and the mixture is stirred at room temperature for 20 hours. Then, 0.7g of an ethylene / butyl methacrylate / glycidyl methacrylate terpolymer is added, and the mixture is stirred for 30 minutes. Finally, 10mL of polyethylene glycol is added, and the mixture is stirred at room temperature for 5 hours. The solution is then uniformly cast onto a smooth mold using a casting method and dried to obtain the photocatalytic antibacterial polylactic acid composite film. The condensing agent is dicyclohexylcarbodiimide, 1-hydroxybenzotriazole, and N-hydroxysuccinimide in a mass ratio of 7:2:3.

[0049] Example 3 This embodiment provides a method for preparing a photocatalytic antibacterial polylactic acid composite film. 10g of polylactic acid is dissolved in 200mL of chloroform and stirred until dissolved. 0.7g of the modifier obtained in Preparation Example 3 and 0.15g of the condensing agent are added, and the mixture is stirred at room temperature for 15 hours. Then, 0.55g of an ethylene / butyl methacrylate / glycidyl methacrylate terpolymer is added, and the mixture is stirred for 30 minutes. Finally, 10mL of polyethylene glycol is added, and the mixture is stirred at room temperature for 5 hours. The solution is then uniformly cast onto a smooth mold using a casting method and dried to obtain the photocatalytic antibacterial polylactic acid composite film. The condensing agent is dicyclohexylcarbodiimide, 1-hydroxybenzotriazole, and N-hydroxysuccinimide, with a mass ratio of 6:1.5:2.5. Figure 2 Figure A shows the SEM image of the photocatalytic antibacterial polylactic acid composite membrane prepared in Example 3, and Figure B shows the SEM image of the photocatalytic antibacterial polylactic acid composite membrane prepared in Comparative Example 5. As can be seen from the figures, the modifier in Example 3 is uniformly distributed and has a smooth and flat surface. However, the modifier in Comparative Example 5, due to the lack of polydopamine coating modification, is unevenly dispersed, has poor compatibility, and exhibits significant agglomeration, resulting in numerous bulges on the membrane surface.

[0050] Example 4 The difference from Example 3 is that the modifier was prepared in Preparation Example 4.

[0051] Example 5 The difference from Example 3 is that the modifier was prepared from Preparation Example 5.

[0052] Comparative Example 1 The difference from Example 3 is that the modifier was prepared from Comparative Preparation Example 1.

[0053] Comparative Example 2 The difference from Example 3 is that the modifier was prepared from Comparative Preparation Example 2.

[0054] Comparative Example 3 The difference from Example 3 is that the modifier was prepared from Comparative Preparation Example 3.

[0055] Comparative Example 4 The difference from Example 3 is that the modifier was prepared from Comparative Preparation Example 4.

[0056] Comparative Example 5 The difference from Example 3 is that the modifier was prepared from Comparative Preparation Example 5 and no condensing agent was added.

[0057] This comparative example provides a method for preparing a photocatalytic antibacterial polylactic acid composite film. 10g of polylactic acid is dissolved in 200mL of chloroform and stirred until dissolved. 0.7g of the modifier prepared in Comparative Preparation Example 5 and 0.55g of ethylene / butyl methacrylate / glycidyl methacrylate terpolymer are added. After stirring and mixing for 30min, 10mL of polyethylene glycol is added and stirred at room temperature for 5h. The solution is then uniformly cast onto a smooth mold using a casting method and dried to obtain the photocatalytic antibacterial polylactic acid composite film.

[0058] Test Example 1 The photocatalytic antibacterial polylactic acid composite membranes prepared in Examples 1-5 and Comparative Examples 1-5, as well as a pure polylactic acid membrane (thickness 0.15 mm), were subjected to performance tests, and the results are shown in Table 1. The tensile strength and elongation at break of the composite membranes were tested using a microcomputer-controlled electronic universal testing machine. The water vapor permeability of the composite membranes was tested using a water vapor permeability meter.

[0059] Table 1

[0060] As shown in the table above, the photocatalytic antibacterial polylactic acid composite films prepared in Examples 1-3 of the present invention have good tensile strength and elongation at break. Due to the addition of the modifier, the path of water molecules through the film is extended, thereby hindering the permeation of water molecules through the film. Therefore, the exchange of water molecules can be reduced and the preservation effect can be improved.

[0061] Test Example 2 Antibacterial Test The antibacterial properties of the photocatalytic antibacterial polylactic acid composite membranes prepared in Examples 1-5 and Comparative Examples 1-5, as well as pure polylactic acid membranes (thickness 0.15 mm), were tested, and the results are shown in Table 2. Using the ASTM E2180-07 standard method, 40 μL of a 10% concentration was added to a 2 cm × 2 cm antibacterial membrane. 8CFU / mL bacterial suspension (containing Gram-negative Escherichia coli ATC25922 and Gram-positive Staphylococcus aureus ATCC25923) was used to cover the surface of the sample with a sterile glass slide. All samples were incubated in Petri dishes at 37°C for 1 hour. The slides and film surfaces were then rinsed with PBS, followed by a series of dilutions. Finally, viable colony counting was performed, and the samples were incubated at 37°C for 24 hours. Each experiment was repeated three times.

[0062] Antibacterial rate (%) = (CFU) A -CFU B ) / CFU A ×100% In the formula, CFU A Total bacterial count (CFU) in the control group B This represents the total number of colonies in the treatment group.

[0063] Table 2

[0064] As can be seen from the table above, the photocatalytic antibacterial polylactic acid composite membranes prepared in Examples 1-3 have good antibacterial properties.

[0065] Test Example 3: Preservation Experiment Fresh strawberries of similar size, weight, and quality were purchased for the experiment. The photocatalytic antibacterial polylactic acid composite membranes prepared in Examples 1-5 and Comparative Examples 1-5, as well as a pure polylactic acid membrane (0.15 mm thick), were used to preserve fresh strawberries. Strawberries were wrapped in each group of membranes; unwrapped strawberries were designated as the control group. Three parallel experiments were conducted for each group. All groups were stored at 55±2%RH and 25±2℃ for 2 days. The strawberries were weighed on days 0 and 3, and the weight loss rate was calculated. On day 3, the firmness of the strawberries was tested using a GY-4 fruit firmness tester, and the soluble solids content was read using a WY015R handheld refractometer. The results are shown in Table 3.

[0066] Table 3

[0067] As shown in the table above, the photocatalytic antibacterial polylactic acid composite membranes prepared in Examples 1-3 of this invention can effectively reduce the weight loss rate of strawberries, maintain the firmness and soluble solids content of strawberries, and thus have a good preservation effect on the quality of strawberries.

[0068] Compared to Example 3, in Examples 4 and 5, the rare earth salt used in the preparation of the modifier was either lanthanum chloride or cerium chloride. In Comparative Example 1, compared to Example 3, no rare earth salt was added during the preparation of the modifier. Its antibacterial and preservative effects decreased, with a more significant decrease in Comparative Example 1, and the water vapor transmittance also increased significantly. Rare earth-doped zinc oxide quantum dots can extend the photoresponse of zinc oxide quantum dots from the ultraviolet to the visible light region, generating active oxygen even under indoor light. After ball milling, it forms a tight heterojunction with carbon nitride microtubes, inhibiting photogenerated electron-hole recombination, improving photocatalytic and antibacterial performance, and helping to degrade organic pollutants on the material surface, achieving self-cleaning. In Comparative Example 2, compared to Example 3, the carbon nitride microtubes were replaced with bulk carbon nitride during the preparation of the modifier, resulting in decreased antibacterial and preservative effects, and a significant decrease in mechanical properties. Traditional bulk carbon nitride has a low specific surface area, a rapid photogenerated carrier recombination rate, and limited visible light utilization, which restricts its application. This invention utilizes hydrothermal conditions to pre-assemble melamine and silk fibers, followed by high-temperature calcination to obtain tubular carbon nitride. This tubular carbon nitride exhibits excellent three-dimensional structure, large specific surface area, and superior electron transfer rate, significantly improving the mechanical properties of the material. In Comparative Example 3, compared to Example 3, steps S1 and S3 were omitted during the preparation of the modifier. Antibacterial and preservative properties decreased. Zinc oxide quantum dots tended to agglomerate, resulting in a significant decrease in activity. The zinc oxide quantum dots prepared by the sol-gel reaction of this invention are ball-milled and mixed with carbon nitride microtubes, thereby improving their dispersibility, preventing agglomeration, and better exerting a synergistic photocatalytic effect. Rare-earth-doped zinc oxide quantum dots can extend the photoresponse of zinc oxide quantum dots from the ultraviolet to the visible light region, generating active oxygen even under indoor light. After ball milling, they form a tight heterojunction with carbon nitride microtubes, inhibiting photogenerated electron-hole recombination and improving photocatalytic and antibacterial properties. In Comparative Example 4, compared to Example 3, steps S2 and S3 were omitted during the preparation of the modifier. The antibacterial and preservative properties decreased, and the mechanical properties also decreased significantly, while the water vapor permeability increased. The tubular carbon nitride prepared by this invention has an excellent three-dimensional structure, a large specific surface area, and an excellent electron transfer rate. It has advantages such as visible light response, moderate band gap, safety and non-toxicity, strong thermal stability, and low cost. It can generate active oxygen under visible light excitation, which plays a bactericidal role. At the same time, it can also improve water vapor barrier properties and improve mechanical properties by forming hydrogen bonds with polylactic acid. Compared with Example 3, Comparative Example 5 did not perform step S4 in the preparation process of the modifier, and the various properties decreased significantly. This invention coats the surface of the quantum dot / carbon nitride microtube composite with a polydopamine layer. The amino groups and carboxyl groups in the polylactic acid matrix can undergo a condensation reaction under the catalysis of a condensing agent, thereby realizing the grafting of the modifier into polylactic acid. This improves the dispersibility of the modifier and its compatibility with polylactic acid, reduces interfacial voids, and thus improves the mechanical properties of polylactic acid to a certain extent, such as tensile strength. Furthermore, the polydopamine coating provides a sustained-release, long-term antibacterial effect.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a photocatalytic antibacterial polylactic acid composite membrane, characterized in that, Polylactic acid is mixed evenly with modifier, toughening agent and condensing agent, stirred and reacted, and photocatalytic antibacterial polylactic acid composite film is prepared by solution casting or melt extrusion. The modifier is prepared by ball milling rare earth doped zinc oxide quantum dots and carbon nitride microtubes and then coating and modifying them with polydopamine.

2. The preparation method according to claim 1, characterized in that, The preparation method of the modifier is as follows: S1. Preparation of rare earth-doped zinc oxide quantum dots: Lithium hydroxide alcohol solution was added dropwise to a stirred alcohol solution containing zinc salt and rare earth salt, the reaction was heated and stirred, n-hexane was added, the mixture was allowed to stand, centrifuged, washed, and dried to obtain rare earth-doped zinc oxide quantum dots; S2. Preparation of carbon nitride microtubes: Melamine and silk fibers were mixed and dissolved in water, subjected to hydrothermal reaction, centrifuged, washed, dried, and calcined in an inert gas atmosphere to obtain carbon nitride microtubes; S3. Preparation of quantum dot / carbon nitride microtube composite: Rare earth-doped zinc oxide quantum dots and carbon nitride microtubes were mixed and ball-milled to obtain quantum dot / carbon nitride microtube composite; S4. Modification: The quantum dot / carbon nitride microtube composite was added to Tris-HCl solution, dopamine hydrochloride was added, the mixture was heated and stirred, centrifuged, washed, dried, and ball-milled to obtain the modifier.

3. The preparation method according to claim 2, characterized in that, In step S1, the mass ratio of lithium hydroxide, zinc salt, and rare earth salt is 2-2.5:3-3.7:0.05-0.

1. The zinc salt is zinc chloride, zinc sulfate, or zinc nitrate. The rare earth salt is selected from at least one of cerium chloride, cerium nitrate, lanthanum nitrate, and lanthanum chloride. Preferably, it is a mixture of lanthanum chloride and cerium chloride with a mass ratio of 1:0.5-1.

5. The heating and stirring reaction is carried out at a temperature of 65-75°C for 70-90 minutes.

4. The preparation method according to claim 2, characterized in that, In step S2, the mass ratio of melamine to silk fiber is 40-60:1, the hydrothermal reaction temperature is 170-190℃, and the time is 20-28h.

5. The preparation method according to claim 2, characterized in that, In step S3, the mass ratio of rare earth-doped zinc oxide quantum dots to carbon nitride microtubes is 8-10:20-30, and the ball milling time is 2-4 hours.

6. The preparation method according to claim 2, characterized in that, In step S4, the pH value of the Tris-HCl solution is 8.5-9.5, the mass ratio of the quantum dot / carbon nitride microtube composite to dopamine hydrochloride is 10:0.2-0.3, and the heating and stirring reaction is carried out at a temperature of 45-65℃ for 3-5 hours.

7. The preparation method according to claim 1, characterized in that, The toughening agent is selected from at least one of ethylene / butyl methacrylate / glycidyl methacrylate terpolymer, polypropylene carbonate polyurethane, and the condensing agent is dicyclohexylcarbodiimide, 1-hydroxybenzotriazole and N-hydroxysuccinimide in a mass ratio of 5-7:1-2:2-3.

8. The preparation method according to claim 1, characterized in that, The mass ratio of polylactic acid to modifier, toughening agent and condensing agent is 100:5-10:4-7:1-2, and the stirring reaction time is 10-20h.

9. A photocatalytic antibacterial polylactic acid composite membrane prepared by the preparation method according to any one of claims 1-8.

10. The application of the photocatalytic antibacterial polylactic acid composite film as described in claim 9 in the preparation of antibacterial preservation materials.

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