Chitosan-based thymol-loaded emulsion coating, method for its preparation and use in the preparation of hydrophobic and grease-resistant packaging paper

Abstract

The application belongs to the technical field of high polymer materials, and particularly relates to a thymol-loaded chitosan-based emulsion coating, a preparation method thereof and application of the thymol-loaded chitosan-based emulsion coating in preparation of hydrophobic oil-proof packaging paper. The preparation method of the thymol-loaded chitosan-based emulsion coating comprises the following steps: dissolving thymol in cinnamyl aldehyde or cinnamyl oil to form an oil phase; uniformly mixing polylactic acid grafted waxy maize starch nanocrystal solution and chitosan solution to obtain an aqueous phase; and mixing and stirring the oil phase and the aqueous phase to obtain the thymol-loaded chitosan-based emulsion coating. In the application, polylactic acid grafted waxy maize starch nanocrystal is used as a particle emulsifier to stabilize the thymol Pickering emulsion, and modified chitosan is used as a film-forming base, so that the emulsion coating with hydrophobic oil resistance and antibacterial property is prepared in one step. After the emulsion coating prepared by the application is coated on the surface of paper and dried to form a film, the penetration of water and oil into the paper can be reduced, and paper material packaging with antibacterial, hydrophobic oil-resistant and biodegradable properties can be quickly and effectively obtained.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a chitosan-based emulsion coating loaded with thymol, its preparation method, and its application in the preparation of hydrophobic and oleophobic packaging paper. Background Technology

[0002] Food packaging materials are ubiquitous in modern life, but the market is currently dominated by petroleum-based materials, which cause serious environmental pollution after disposal. Paper-based packaging has gained popularity due to its superior biodegradability and eco-friendliness, attracting widespread research. However, pure paper exhibits poor water and oil resistance due to its hydrophilic cellulose-based framework and porous microstructure. Therefore, paper modification is necessary to improve its water and oil resistance.

[0003] Meanwhile, foodborne bacterial contamination has become a global safety concern. Food packaging incorporating antimicrobial agents can not only reduce pathogens but also control spoilage microorganisms. Essential oils extracted from plants exhibit excellent antimicrobial and antioxidant properties and can be added to active packaging as antimicrobial and antioxidant agents to extend the shelf life of food. Among them, thymol can kill a wide range of foodborne pathogens and antibiotic-resistant (AMR) strains. However, thymol's volatility and insolubility limit its antimicrobial effect in packaging materials. Currently, the traditional method of encapsulating plant essential oils by directly mixing surface-active compound-stabilized plant essential oil droplets with film-forming solutions cannot prevent the rapid evaporation of plant essential oils, which significantly reduces the long-term antimicrobial effect of plant essential oils when used in active films.

[0004] Therefore, it is necessary to develop a paper-based packaging material that combines good water and oil resistance with long-lasting antibacterial properties. Summary of the Invention

[0005] The first objective of this invention is to provide a method for preparing a chitosan-based emulsion coating loaded with thymol. The second objective of this invention is to provide a chitosan-based emulsion coating loaded with thymol prepared by the method. The third objective of this invention is to provide the application of the chitosan-based emulsion coating loaded with thymol.

[0006] According to a first aspect of the present invention, a method for preparing a chitosan-based emulsion coating loaded with thymol is provided, comprising the following steps:

[0007] Thymol is dissolved in cinnamaldehyde or cinnamon oil to form an oil phase;

[0008] A solution of polylactic acid-grafted glutinous rice starch nanocrystals and a chitosan solution were mixed evenly to obtain an aqueous phase.

[0009] The oil phase and water phase are mixed and stirred to obtain the final product.

[0010] This invention uses polylactic acid-grafted glutinous rice starch nanocrystals as a particle emulsifier to stabilize thymol-pickerlin emulsion and chitosan as a film-forming substrate to prepare a hydrophobic, oleophobic, and antibacterial emulsion coating in one step. After the emulsion coating prepared by this invention is applied to the surface of paper and dried, it forms a thin film, which can reduce the penetration of water and oil into the paper, quickly and effectively obtaining antibacterial, hydrophobic, oleophobic, and biodegradable paper packaging materials.

[0011] This invention uses polylactic acid (PLA) to modify glutinous rice starch nanocrystals, resulting in PLA-grafted glutinous rice starch nanocrystals with good wettability. These PLA-grafted glutinous rice starch nanocrystals are then used as solid particle emulsifiers to stabilize oil-in-water thymol-pickerlin emulsions. This not only makes the prepared emulsion highly stable, maintaining its basic structure during the drying and film-forming process, thus facilitating thymol retention, but also allows for better control of the thymol size distribution in the PLA-grafted glutinous rice starch nanocrystal-stabilized thymol-pickerlin emulsion. By controlling the amount of stabilizing particles, "valve" mechanisms of varying density can be formed between cinnamon oil / cinnamaldehyde and chitosan to control thymol release, thereby achieving a long-lasting antibacterial effect.

[0012] In some embodiments, before mixing the chitosan solution with the polylactic acid-grafted glutinous rice starch nanocrystal solution, the chitosan solution and cashew phenol glycidyl ether are mixed, heated at 50-100°C for 3-5 hours, then cooled to room temperature, and then stirred at 300-600 rpm for 3-5 hours at room temperature to obtain a chitosan-based cashew phenol glycidyl ether solution; wherein the molar ratio of the epoxy group in the cashew phenol glycidyl ether to the amino group in the chitosan solution is 1:1 to 1:4.

[0013] In some embodiments, the mass ratio of thymol to cinnamaldehyde or cinnamon oil is 1:5 to 3:5.

[0014] In some embodiments, a polylactic acid-grafted glutinous rice starch nanocrystal solution is mixed with a chitosan solution or a chitosan-based cashew phenol glycidyl ether solution at a volume ratio of 1:1 to 1:4.

[0015] In some embodiments, the oil phase and the water phase are mixed at a volume ratio of 1:4 to 2:3 and then placed in a homogenizer and stirred at 8000 to 12000 rpm for 2 to 4 minutes.

[0016] In some embodiments, the mass concentration of polylactic acid-grafted glutinous rice starch nanocrystals in the polylactic acid-grafted glutinous rice starch nanocrystal solution is 15-25 wt%.

[0017] In some embodiments, the chitosan concentration in the chitosan solution is 1.5–3 wt%.

[0018] In some embodiments, the preparation method of polylactic acid-grafted glutinous rice starch nanocrystal solution includes the following steps: adding polylactic acid-grafted glutinous rice starch nanocrystals to water, stirring at 300-500 rpm for 20-60 min at 80-100°C, and then cooling to 30-50°C to obtain polylactic acid-grafted glutinous rice starch nanocrystal solution.

[0019] In some embodiments, the preparation method of chitosan solution includes the following steps: dissolving chitosan powder in a 1-2 v / v% aqueous acetic acid solution and stirring at 600-1000 rpm for 3-5 h at room temperature to obtain chitosan solution.

[0020] In some embodiments, the preparation method of polylactic acid-grafted glutinous rice starch nanocrystals includes the following steps:

[0021] Glutinous rice starch nanocrystals and lactic acid aqueous solution were mixed, then heated to 70-100℃ and stirred at 300-600 rpm for 1-3 hours. Subsequently, a catalyst was added, and stirring was continued at 300-600 rpm for 12-15 hours. Finally, the reaction product was separated, purified, and dried to obtain polylactic acid grafted glutinous rice starch nanocrystals.

[0022] In some embodiments, the mass concentration of lactic acid in the aqueous lactic acid solution is 50–85 wt%.

[0023] In some embodiments, the mass ratio of glutinous rice starch nanocrystals to the volume ratio of the lactic acid aqueous solution is 1:4 to 1:5.

[0024] In some embodiments, the catalyst is stannous octoate, and the amount of catalyst added is 3 to 10 wt% of the mass of glutinous rice starch nanocrystals.

[0025] In some embodiments, the preparation method of glutinous rice starch nanocrystals includes the following steps: adding glutinous rice starch to concentrated sulfuric acid, stirring at 300-600 rpm for 3-7 days at 30-50℃, then centrifuging the reaction product, washing the centrifuged solution with water until the solution pH is neutral, and finally freeze-drying the obtained neutral solution to obtain glutinous rice starch nanocrystals.

[0026] In some implementations, the concentration of concentrated sulfuric acid is 3–4 mol / L.

[0027] In some embodiments, the mass ratio of glutinous rice starch to the volume ratio of concentrated sulfuric acid is 1:5 to 2:5.

[0028] According to a second aspect of the present invention, a chitosan-based emulsion coating loaded with thymol prepared by the above-described preparation method is provided.

[0029] According to a third aspect of the present invention, the application of the above-described chitosan-based emulsion coating loaded with thymol in the preparation of hydrophobic and oleophobic packaging paper is provided.

[0030] According to a fourth aspect of the present invention, a hydrophobic and oleophobic packaging paper is provided, which is prepared by the following method:

[0031] The base paper is immersed in the above-mentioned chitosan-based emulsion coating loaded with thymol for 2-10 minutes, then removed to obtain dip-coated paper. The dip-coated paper is then dried at room temperature overnight to allow the solvent to evaporate, thus obtaining the final product.

[0032] This invention constructs a chitosan-based waterproof and oleophobic packaging paper coating loaded with thymol plant essential oil using the Pickering emulsion template method, which can quickly and effectively obtain a chitosan-based coating with hydrophobic, oleophobic, and long-lasting antibacterial properties.

[0033] Compared with the prior art, the beneficial effects of the present invention include:

[0034] (1) This invention uses inexpensive and highly water-resistant cashew phenol glycidyl ether to modify chitosan, forming chitosan-based cashew phenol glycidyl ether. Using this chitosan-based cashew phenol glycidyl ether as a film-forming substrate, and leveraging the excellent film-forming properties, oil resistance, and biodegradability of chitosan, along with the water resistance, biodegradability, and UV resistance of cashew phenol glycidyl ether, a one-pot reaction effectively combines hydrophobic and oil-resistant properties. The emulsion coating prepared according to this invention, after being applied to the surface of paper and dried, forms a thin film, which reduces the penetration of water and oil into the paper and exhibits good biodegradability, UV resistance, and mechanical properties.

[0035] (2) In this invention, polylactic acid grafted glutinous rice starch nanocrystals are used as particle emulsifiers to stabilize thymol-pickerlin emulsion. The chitosan-based waterproof and oil-proof packaging paper coating loaded with thymol plant essential oil is constructed by using the Pickerlin emulsion template method. This method can quickly and effectively obtain a chitosan-based coating with long-lasting antibacterial, hydrophobic and oil-proof properties.

[0036] (3) The raw materials used in this invention are all bio-based and biodegradable materials. Therefore, the coating and coated paper prepared by the emulsion coating of this invention have excellent biodegradability. Moreover, the raw materials chitosan, thymol and polylactic acid grafted glutinous rice starch nanocrystals are renewable biomass resources with good biocompatibility, safety and biodegradability, and the degradation products do not cause harm to the environment and human body. Attached Figure Description

[0037] Figure 1 The images show the infrared spectra of glutinous rice starch before and after modification in Example 3.

[0038] Figure 2The images show X-ray diffraction patterns of glutinous rice starch, glutinous rice starch nanocrystals, and polylactic acid-grafted glutinous rice starch nanocrystals from Example 3.

[0039] Figure 3 Photographs and optical microscope images of the chitosan-based emulsion coating loaded with thymol prepared in Example 3.

[0040] Figure 4 The results show the water absorption rate of the uncoated filter paper and the coated paper used in Examples 1-4.

[0041] Figure 5 The results of the oil resistance test of the coated paper used in Examples 1-4 are presented using a titrant of grade 8.

[0042] Figure 6 Photographs showing the antibacterial activity of the coated paper used in Application Example 3 and Comparative Examples 1-3 against Gram-positive bacteria (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli).

[0043] Figure 7 These are photographs showing the antibacterial effects of filter paper soaked in thymol and cinnamon oil solution and coated paper from Application Example 3 on Staphylococcus aureus and Escherichia coli at different storage times. Detailed Implementation

[0044] The present invention will now be described in further detail with reference to the accompanying drawings. It should be noted that the following embodiments are merely for better explanation of the invention and do not limit the scope of protection of the invention. Process steps not disclosed in the embodiments are prior art. Unless otherwise specified, all raw materials are commercially available.

[0045] In the following examples, room temperature refers to 25±5℃.

[0046] Example 1

[0047] The preparation method of the chitosan-based emulsion coating loaded with thymol in this embodiment includes the following steps:

[0048] (1) Preparation of polylactic acid-grafted glutinous rice starch nanocrystals:

[0049] Prepare a 3.16 mol / L concentrated sulfuric acid solution. Then, weigh 30 g of dried glutinous rice starch and add it to 150 mL of the prepared concentrated sulfuric acid solution. Stir the solution at 400 rpm at 40 °C for 5 days. Then, centrifuge the resulting solution continuously at 10,000 rpm for 8 min. Wash the centrifuged solution with distilled water until the pH of the solution is neutral. Finally, freeze-dry the resulting neutral solution to obtain glutinous rice starch nanocrystals.

[0050] 7 g of glutinous rice starch nanocrystals and 30 mL of 85 wt% lactic acid (LA) aqueous solution were placed in a glass reactor equipped with a mechanical stirrer and a vacuum pump system. The reactor was evacuated at 90 °C to remove water, which was necessary to advance the reaction. The reactor was then placed in a 90 °C oil bath, and the mixture was stirred at 300 rpm for 1.5 h. Subsequently, 0.32 g of stannous octoate catalyst (Sn(Oct)₂) was added to the reactor, and stirring was continued at 300 rpm for 15 h. The reaction product was then extracted with 50 mL of acetone to dissolve the LA monomer, catalyst, and ungrafted polymer chains (homopolymer) residues. After acetone extraction, the resulting material was dried under vacuum to constant weight to obtain polylactic acid-grafted glutinous rice starch nanocrystals.

[0051] (2) Preparation of Pickering emulsion using polylactic acid-grafted glutinous rice starch nanocrystals:

[0052] Dissolve 2g of thymol in 10mL of cinnamaldehyde to form an oil phase.

[0053] Accurately weigh 4g of polylactic acid-grafted glutinous rice starch nanocrystals obtained in step (1) and add them to 20mL of deionized water. Stir thoroughly at 500rpm for 20min at 80℃, and then cool to 50℃ to obtain a polylactic acid-grafted glutinous rice starch nanocrystal solution.

[0054] Chitosan powder was dissolved in a 1 v / v % aqueous acetic acid solution and stirred at 600 rpm for 3 h at room temperature to prepare a 2 wt% chitosan solution. The chitosan solution and cashew phenol glycidyl ether (CGE) were then mixed thoroughly to obtain a mixture in which the molar ratio of epoxy groups from CGE to amino groups from chitosan was 1:4. The mixture was heated at 70 °C for 3 h, then cooled to room temperature, and stirred at 600 rpm for 3 h at room temperature to obtain a chitosan-based cashew phenol glycidyl ether solution.

[0055] 3 mL of polylactic acid-grafted glutinous rice starch nanocrystal solution and 11 mL of chitosan-based cashew phenol glycidyl ether solution were mixed to obtain an aqueous phase.

[0056] 6 mL of oil phase and 14 mL of aqueous phase were stirred in a high-speed homogenizer at 10,000 rpm for 2 min to obtain a chitosan-based emulsion coating loaded with thymol.

[0057] Example 2

[0058] The preparation method of the chitosan-based emulsion coating loaded with thymol in this embodiment includes the following steps:

[0059] (1) Preparation of polylactic acid-grafted glutinous rice starch nanocrystals:

[0060] Prepare a 3.16 mol / L concentrated sulfuric acid solution. Then, weigh 30 g of dried glutinous rice starch and add it to 150 mL of the prepared concentrated sulfuric acid solution. Stir the solution at 400 rpm at 40 °C for 5 days. Then, centrifuge the resulting solution continuously at 10,000 rpm for 8 min. Wash the centrifuged solution with distilled water until the pH of the solution is neutral. Finally, freeze-dry the resulting neutral solution to obtain glutinous rice starch nanocrystals.

[0061] 7 g of glutinous rice starch nanocrystals and 30 mL of 85 wt% lactic acid (LA) aqueous solution were placed in a glass reactor equipped with a mechanical stirrer and a vacuum pump system. The reactor was evacuated at 90 °C to remove water, which was necessary to advance the reaction. The reactor was then placed in a 90 °C oil bath, and the mixture was stirred at 300 rpm for 1.5 h. Subsequently, 0.32 g of stannous octoate catalyst (Sn(Oct)₂) was added to the reactor, and stirring was continued at 300 rpm for 15 h. The reaction product was then extracted with 50 mL of acetone to dissolve the LA monomer, catalyst, and ungrafted polymer chains (homopolymer) residues. After acetone extraction, the resulting material was dried under vacuum to constant weight to obtain polylactic acid-grafted glutinous rice starch nanocrystals.

[0062] (2) Preparation of Pickering emulsion using polylactic acid-grafted glutinous rice starch nanocrystals:

[0063] Dissolve 2g of thymol in 10mL of cinnamaldehyde to form an oil phase.

[0064] Accurately weigh 4g of polylactic acid-grafted glutinous rice starch nanocrystals obtained in step (1) and add them to 20mL of deionized water. Stir thoroughly at 500rpm for 20min at 80℃, and then cool to 50℃ to obtain a polylactic acid-grafted glutinous rice starch nanocrystal solution.

[0065] Chitosan powder was dissolved in a 1 v / v % aqueous acetic acid solution and stirred at 600 rpm for 3 h at room temperature to prepare a 2 wt% chitosan solution. The chitosan solution and cashew phenol glycidyl ether (CGE) were then mixed thoroughly to obtain a mixture in which the molar ratio of epoxy groups from CGE to amino groups from chitosan was 1:2. The mixture was heated at 70 °C for 3 h, then cooled to room temperature, and stirred at 600 rpm for 3 h at room temperature to obtain a chitosan-based cashew phenol glycidyl ether solution.

[0066] 3 mL of polylactic acid-grafted glutinous rice starch nanocrystal solution and 11 mL of chitosan-based cashew phenol glycidyl ether solution were mixed to obtain an aqueous phase.

[0067] 6 mL of oil phase and 14 mL of aqueous phase were stirred in a high-speed homogenizer at 10,000 rpm for 2 min to obtain a chitosan-based emulsion coating loaded with thymol.

[0068] Example 3

[0069] The preparation method of the chitosan-based emulsion coating loaded with thymol in this embodiment includes the following steps:

[0070] (1) Preparation of polylactic acid-grafted glutinous rice starch nanocrystals:

[0071] Prepare a 3.16 mol / L concentrated sulfuric acid solution. Then, weigh 30 g of dried glutinous rice starch and add it to 150 mL of the prepared concentrated sulfuric acid solution. Stir the solution at 400 rpm at 40 °C for 5 days. Then, centrifuge the resulting solution continuously at 10,000 rpm for 8 min. Wash the centrifuged solution with distilled water until the pH of the solution is neutral. Finally, freeze-dry the resulting neutral solution to obtain glutinous rice starch nanocrystals.

[0072] 7 g of glutinous rice starch nanocrystals and 30 mL of 85 wt% lactic acid (LA) aqueous solution were placed in a glass reactor equipped with a mechanical stirrer and a vacuum pump system. The reactor was evacuated at 90 °C to remove water, which was necessary to advance the reaction. The reactor was then placed in a 90 °C oil bath, and the mixture was stirred at 300 rpm for 1.5 h. Subsequently, 0.32 g of stannous octoate catalyst (Sn(Oct)₂) was added to the reactor, and stirring was continued at 300 rpm for 15 h. The reaction product was then extracted with 50 mL of acetone to dissolve the LA monomer, catalyst, and ungrafted polymer chains (homopolymer) residues. After acetone extraction, the resulting material was dried under vacuum to constant weight to obtain polylactic acid-grafted glutinous rice starch nanocrystals.

[0073] (2) Preparation of Pickering emulsion using polylactic acid-grafted glutinous rice starch nanocrystals:

[0074] Dissolve 2g of thymol in 10mL of cinnamaldehyde to form an oil phase.

[0075] Accurately weigh 4g of polylactic acid-grafted glutinous rice starch nanocrystals obtained in step (1) and add them to 20mL of deionized water. Stir thoroughly at 500rpm for 20min at 80℃, and then cool to 50℃ to obtain a polylactic acid-grafted glutinous rice starch nanocrystal solution.

[0076] Chitosan powder was dissolved in a 1 v / v % aqueous acetic acid solution and stirred at 600 rpm for 3 h at room temperature to prepare a 2 wt% chitosan solution. The chitosan solution and cashew phenol glycidyl ether (CGE) were then mixed thoroughly to obtain a mixture in which the molar ratio of epoxy groups from CGE to amino groups from chitosan was 1:1. The mixture was heated at 70 °C for 3 h, then cooled to room temperature, and stirred at 600 rpm for 3 h at room temperature to obtain a chitosan-based cashew phenol glycidyl ether solution.

[0077] 3 mL of polylactic acid-grafted glutinous rice starch nanocrystal solution and 11 mL of chitosan-based cashew phenol glycidyl ether solution were mixed to obtain an aqueous phase.

[0078] 6 mL of oil phase and 14 mL of aqueous phase were stirred in a high-speed homogenizer at 10,000 rpm for 2 min to obtain a chitosan-based emulsion coating loaded with thymol.

[0079] Example 4

[0080] The preparation method of the chitosan-based emulsion coating loaded with thymol in this embodiment includes the following steps:

[0081] (1) Preparation of polylactic acid-grafted glutinous rice starch nanocrystals:

[0082] Prepare a 3.16 mol / L concentrated sulfuric acid solution. Then, weigh 30 g of dried glutinous rice starch and add it to 150 mL of the prepared concentrated sulfuric acid solution. Stir the solution at 400 rpm at 40 °C for 5 days. Then, centrifuge the resulting solution continuously at 10,000 rpm for 8 min. Wash the centrifuged solution with distilled water until the pH of the solution is neutral. Finally, freeze-dry the resulting neutral solution to obtain glutinous rice starch nanocrystals.

[0083] 7 g of glutinous rice starch nanocrystals and 30 mL of 85 wt% lactic acid (LA) aqueous solution were placed in a glass reactor equipped with a mechanical stirrer and a vacuum pump system. The reactor was evacuated at 90 °C to remove water, which was necessary to advance the reaction. The reactor was then placed in a 90 °C oil bath, and the mixture was stirred at 300 rpm for 1.5 h. Subsequently, 0.32 g of stannous octoate catalyst (Sn(Oct)₂) was added to the reactor, and stirring continued at 300 rpm for 15 h. The reaction product was then extracted with 50 mL of acetone to dissolve the LA monomer, catalyst, and ungrafted polymer chains (homopolymer) residues. After acetone extraction, the resulting material was dried under vacuum to constant weight to obtain polylactic acid-grafted glutinous rice starch nanocrystals.

[0084] (2) Preparation of Pickering emulsion using polylactic acid-grafted glutinous rice starch nanocrystals:

[0085] Dissolve 2g of thymol in 10mL of cinnamaldehyde to form an oil phase.

[0086] Accurately weigh 4g of polylactic acid-grafted glutinous rice starch nanocrystals obtained in step (1) and add them to 20mL of deionized water. Stir thoroughly at 500rpm for 20min at 80℃, and then cool to 50℃ to obtain a polylactic acid-grafted glutinous rice starch nanocrystal solution.

[0087] Chitosan powder was dissolved in a 1 v / v % aqueous acetic acid solution and stirred at 600 rpm for 3 h at room temperature to prepare a 2 wt% chitosan solution.

[0088] 3 mL of polylactic acid-grafted glutinous rice starch nanocrystal solution and 11 mL of chitosan solution were mixed to obtain an aqueous phase.

[0089] 6 mL of oil phase and 14 mL of aqueous phase were stirred in a high-speed homogenizer at 10,000 rpm for 2 min to obtain a chitosan-based emulsion coating loaded with thymol.

[0090] Comparative Example 1

[0091] This comparative example uses chitosan solution directly as the coating, and its preparation method is as follows:

[0092] Chitosan powder was dissolved in a 1 v / v % aqueous acetic acid solution and stirred at 600 rpm for 3 h at room temperature to prepare a 2 wt% chitosan solution.

[0093] Comparative Example 2

[0094] This comparative example uses chitosan-based cashew phenol glycidyl ether solution as a coating, and its preparation method is as follows:

[0095] Chitosan powder was dissolved in a 1 v / v % aqueous acetic acid solution and stirred at 600 rpm for 3 h at room temperature to prepare a 2 wt% chitosan solution. The chitosan solution and cashew phenol glycidyl ether (CGE) were then mixed thoroughly to obtain a mixture in which the molar ratio of epoxy groups from CGE to amino groups from chitosan was 1:1. The mixture was heated at 70 °C for 3 h, then cooled to room temperature, and stirred at 600 rpm for 3 h at room temperature to obtain a chitosan-based cashew phenol glycidyl ether solution.

[0096] Comparative Example 3

[0097] This comparative example uses a blend of modified starch and modified chitosan as a coating, and its preparation method includes the following steps:

[0098] (1) Preparation of polylactic acid-grafted glutinous rice starch nanocrystals:

[0099] Prepare a 3.16 mol / L concentrated sulfuric acid solution. Then, weigh 30 g of dried glutinous rice starch and add it to 150 mL of the prepared concentrated sulfuric acid solution. Stir the solution at 400 rpm at 40 °C for 5 days. Then, centrifuge the resulting solution continuously at 10,000 rpm for 8 min. Wash the centrifuged solution with distilled water until the pH of the solution is neutral. Finally, freeze-dry the resulting neutral solution to obtain glutinous rice starch nanocrystals.

[0100] 7 g of glutinous rice starch nanocrystals and 30 mL of 85 wt% lactic acid (LA) aqueous solution were placed in a glass reactor equipped with a mechanical stirrer and a vacuum pump system. The reactor was evacuated at 90 °C to remove water, which was necessary to advance the reaction. The reactor was then placed in a 90 °C oil bath, and the mixture was stirred at 300 rpm for 1.5 h. Subsequently, 0.32 g of stannous octoate catalyst (Sn(Oct)₂) was added to the reactor, and stirring was continued at 300 rpm for 15 h. The reaction product was then extracted with 50 mL of acetone to dissolve the LA monomer, catalyst, and ungrafted polymer chains (homopolymer) residues. After acetone extraction, the resulting material was dried under vacuum to constant weight to obtain polylactic acid-grafted glutinous rice starch nanocrystals.

[0101] (2) Preparation of a blend of modified starch and modified chitosan:

[0102] Accurately weigh 4g of polylactic acid-grafted glutinous rice starch nanocrystals obtained in step (1) and add them to 20mL of deionized water. Stir thoroughly at 500rpm for 20min at 80℃, and then cool to 50℃ to obtain a polylactic acid-grafted glutinous rice starch nanocrystal solution.

[0103] Chitosan powder was dissolved in a 1 v / v % aqueous acetic acid solution and stirred at 600 rpm for 3 h at room temperature to prepare a 2 wt% chitosan solution. The chitosan solution and cashew phenol glycidyl ether (CGE) were then mixed thoroughly to obtain a mixture in which the molar ratio of epoxy groups from CGE to amino groups from chitosan was 1:1. The mixture was heated at 70 °C for 3 h, then cooled to room temperature, and stirred at 600 rpm for 3 h at room temperature to obtain a chitosan-based cashew phenol glycidyl ether solution.

[0104] 3 mL of polylactic acid-grafted glutinous rice starch nanocrystal solution and 11 mL of chitosan-based cashew phenol glycidyl ether solution were mixed to obtain a modified starch and modified chitosan blend.

[0105] The following performance tests were performed on the polylactic acid-grafted glutinous rice starch nanocrystals prepared in this invention.

[0106] 1. Fourier Transform Infrared (FTIR) Spectroscopy Detection

[0107] Detection method: The chemical structure of the particles was characterized using a Bruker VERTEX 70 Fourier transform infrared (FTIR) spectrometer. Samples were obtained in 32 scans, with a wavenumber range of 400 cm⁻¹. -1 Up to 600cm -1 The resolution is 4cm. -1 .

[0108] Figure 1These are the infrared spectra of glutinous rice starch before and after modification in Example 3. From... Figure 1 It can be seen that the Fourier transform infrared spectrum of pure glutinous rice starch nanocrystals shows that the -OH groups are visible at 1650 cm⁻¹. -1 and 3300cm -1 The characteristic peaks of stretching and bending vibrations at wavenumbers are observed. Furthermore, the stretching vibrations of CO bonds in the COH and COC groups of pure glutinous rice starch nanocrystals appear at 1150, 1077, and 990 cm⁻¹, respectively. -1 The FTIR spectrum of glutinous rice starch nanocrystals grafted with polylactic acid at a wavenumber of 1730 cm⁻¹ was obtained at 1730 cm⁻¹. -1 An additional sharp peak at 1250 and 1055 cm⁻¹. This peak is attributed to the ester carbonyl stretching group (=CO) present in lactic acid. Furthermore, there are additional peaks at 1250 and 1055 cm⁻¹. -1 Two new peaks were observed at 1650 cm⁻¹, which can be attributed to the CO vibration in the ester group. Furthermore, after grafting polylactic acid onto the surface of glutinous rice starch nanocrystals, a peak was observed at 1650 cm⁻¹. -1 and 3300cm -1 The peak intensity of the -OH functional group near the wavenumber decreased. These results indicate that polylactic acid was successfully grafted onto the surface of glutinous rice starch nanocrystals.

[0109] Fourier transform infrared spectroscopy was performed on the glutinous rice starch of Examples 1-2 and 4 before and after modification. The results were similar to those of Example 3. To save space, they will not be described in detail here.

[0110] 2. X-ray diffraction analysis

[0111] Detection method: A Na I crystal scintillation counter was used for measurement. The parameters selected were: diffraction angle range of 5° to 40°, scanning speed of 4° / min, power of 1200W (40kV×30mA), and copper target Cu Kα (λ=0.15406nm). Crystallinity was calculated using Jade.

[0112] Figure 2 These are X-ray diffraction images of glutinous rice starch, glutinous rice starch nanocrystals, and polylactic acid-grafted glutinous rice starch nanocrystals from Example 3. From... Figure 2 It can be seen that glutinous rice starch exhibits diffraction peaks at 2θ = 15°, 17°, 18°, and 23°, which is a typical A-type starch crystalline structure. Compared to the original glutinous rice starch, the glutinous rice starch nanocrystals prepared by sulfuric acid hydrolysis have more obvious crystallization diffraction characteristic peaks. This is mainly due to the hydrolysis of the amorphous regions in glutinous rice starch by sulfuric acid, leaving the acid-resistant crystalline portion as the glutinous rice starch nanocrystals. The crystallization characteristic diffraction peaks of glutinous rice starch nanocrystals did not change significantly before and after polylactic acid modification. This is because the polylactic acid groups are mainly grafted onto the surface of the glutinous rice starch nanocrystals rather than the interior; therefore, polylactic acid has almost no effect on the crystallization structure of the glutinous rice starch nanocrystals.

[0113] X-ray diffraction analysis was performed on the glutinous rice starch of Examples 1-2 and 4 before and after modification. The results were similar to those of Example 3. To save space, they will not be described in detail here.

[0114] Furthermore, the morphology of the prepared chitosan-based emulsion coating loaded with thymol was observed under a 40x objective lens using a Phenix BMC500 microscope equipped with a digital camera.

[0115] Figure 3 These are photographs and optical microscope images of the chitosan-based emulsion coating loaded with thymol prepared in Example 3. Figure 3 It can be seen that the chitosan-based emulsion coating loaded with thymol prepared in Example 3 has good droplet size dispersion and excellent emulsion stability.

[0116] The microstructure of the chitosan-based emulsion coatings loaded with thymol prepared in Examples 1-2 and 4 was observed. The test results were similar to those in Example 3, and will not be repeated here for the sake of brevity.

[0117] Then, the chitosan-based emulsion coatings loaded with thymol prepared in Examples 1-4 and the coatings prepared in Comparative Examples 1-3 were used to prepare coated paper.

[0118] Application Example 1

[0119] The cut filter paper was immersed in the chitosan-based emulsion coating loaded with thymol prepared in Example 1 for 10 minutes, and then removed to obtain dip-coated paper. The dip-coated paper was then dried at room temperature overnight to allow the solvent to evaporate, thus obtaining coated paper.

[0120] Application Example 2

[0121] The cut filter paper was immersed in the chitosan-based emulsion coating loaded with thymol prepared in Example 2 for 10 minutes, and then removed to obtain dip-coated paper. The dip-coated paper was then dried at room temperature overnight to allow the solvent to evaporate, thus obtaining coated paper.

[0122] Application Example 3

[0123] The cut filter paper was immersed in the chitosan-based emulsion coating loaded with thymol prepared in Example 3 for 10 minutes, and then removed to obtain dip-coated paper. The dip-coated paper was then dried at room temperature overnight to allow the solvent to evaporate, thus obtaining coated paper.

[0124] Application Example 4

[0125] The cut filter paper was immersed in the chitosan-based emulsion coating loaded with thymol prepared in Example 4 for 10 minutes, and then removed to obtain dip-coated paper. The dip-coated paper was then dried at room temperature overnight to allow the solvent to evaporate, thus obtaining coated paper.

[0126] Application Comparative Example 1

[0127] The cut filter paper was immersed in the coating prepared in Comparative Example 1 for 10 minutes, then removed to obtain dip-coated paper. The dip-coated paper was then dried at room temperature overnight to allow the solvent to evaporate, resulting in coated paper.

[0128] Application Comparative Example 2

[0129] The cut filter paper was immersed in the coating prepared in Comparative Example 2 for 10 minutes, then removed to obtain dip-coated paper. The dip-coated paper was then dried at room temperature overnight to allow the solvent to evaporate, resulting in coated paper.

[0130] Application Comparative Example 3

[0131] The cut filter paper was immersed in the coating prepared in Comparative Example 3 for 10 minutes, then removed to obtain dip-coated paper. The dip-coated paper was then dried at room temperature overnight to allow the solvent to evaporate, resulting in coated paper.

[0132] The following performance tests were conducted on the coated papers prepared in Application Examples 1-4 and Comparative Examples 1-3.

[0133] 1. Water absorption rate

[0134] Test method: Weigh the paper in a dry state, then immerse the paper in deionized water. After immersion for 2, 6, 18, 24, and 45 hours, remove the paper, remove excess water from the surface of the paper with a paper towel, and weigh the paper after water absorption. Calculate the water absorption rate: Water absorption rate (%) = (Weight of paper after water absorption - Weight of paper before water absorption) / Weight of paper before water absorption × 100%.

[0135] Figure 4 These are the water absorption test results for the uncoated filter paper and the coated paper used in Examples 1-4. Figure 4 It can be observed that the water absorption rate of the coated paper in Application Examples 1-4 is significantly lower than that of the uncoated base paper, indicating that the emulsion coating obtained after curing the chitosan-based emulsion coating loaded with thymol of the present invention has a significant advantage in improving the water resistance of paper-based packaging materials. It can also be seen that the water absorption rate of the coated paper decreases significantly with the increase of the cashew phenol glycidyl ether (CGE) grafting rate. Among them, Application Example 3, where the molar ratio of the epoxy groups of CGE to the amino groups of chitosan is 1:1, exhibits the best water resistance. This result may be attributed to the excellent hydrophobicity imparted to the emulsion coating by the long-chain alkyl groups in CGE.

[0136] 2. Oil resistance

[0137] Test method: Castor oil, toluene, and n-heptane were mixed in specific volume ratios to prepare 12 titrants with different surface tensions, which were then classified into 12 grades based on their surface tension. The titrants were then applied to coated paper for 15 seconds, and the solution was wiped off with a paper towel. The highest grade of titrant, where no oil stain was left on the paper, was used to determine the paper's oil resistance level. In the oil resistance test, a higher titrant grade indicates better oil resistance.

[0138] Figure 5 The results show the oil-repellent properties of the coated paper used in Examples 1-4, tested with a titrant of grade 8. Figure 5 It can be seen that no oil stains were left on the surface of the coated paper in Application Example 3, while the coated paper in Application Examples 1-2 and 4 all had oil stains covering a certain area. This shows that the coated paper in Application Example 3 has the best oil resistance, that is, when the molar ratio of the epoxy group of CGE to the amino group of chitosan is 1:1, the coated paper exhibits the best oil resistance.

[0139] 3. Antibacterial properties

[0140] Detection method: The antibacterial properties of the coated paper against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) were determined using the inhibition zone method. Both bacteria were cultured separately in sterile nutrient broth at 37°C on a rotating vibrating sieve at 150 rpm for 12 hours. The culture media were then diluted with sterile nutrient broth to obtain approximately 10... 7 CFU / mL inoculum suspension. Then, 0.1 mL of the inoculum suspension was evenly spread onto a petri dish containing nutrient agar medium. Simultaneously, a 6 mm diameter coated paper sample was placed on the surface of the nutrient agar medium, and after incubation at 37°C for 24 hours, the diameter of the inhibition zone around the sample was measured.

[0141] Figure 6 These are photographs illustrating the antibacterial activity of the coated paper used in Application Example 3 and Comparative Examples 1-3 against Gram-positive bacteria (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli). Figure 6 It can be observed that the inhibition zones of the coated paper used in Comparative Examples 1-3 are not obvious, indicating that the antibacterial ability of the coated paper used in Comparative Examples 1-3 is poor. The inhibition zone of the coated paper used in Example 3 is obvious, indicating that the coated paper used in Example 3 has excellent antibacterial ability. This is mainly because thymol can be effectively released from the coating of the coated paper and induce the death of pathogens through the accumulation of intracellular reactive oxygen species (ROS) and membrane lipid peroxidation.

[0142] 4. Sustained-release properties

[0143] Test method: The sustained-release performance of thymol in coated paper was tested using paper diffusion (inhibition zone).

[0144] First, Staphylococcus aureus and Escherichia coli were cultured separately in sterile nutrient broth at 37°C on a rotating vibrating sieve at 150 rpm for 12 hours. Then, they were diluted with sterile nutrient broth to obtain approximately 10... 7 CFU / mL inoculum suspension. Then, 0.1 mL of the inoculum suspension was evenly spread onto a petri dish containing nutrient agar medium.

[0145] Then, uncoated filter paper with a diameter of 6 mm was immersed in a 20 wt% thymol-cinnamon oil solution for 10 min. After removal, it was placed on a surface containing nutrient agar medium for 2 h, 4 h, and 16 h, respectively, and the diameter of the surrounding inhibition zone of the samples was measured at different time points. Simultaneously, coated paper samples with a diameter of 6 mm from Application Example 3 were placed on a surface containing nutrient agar medium for 2 h, 4 h, and 16 h, respectively, and the diameter of the surrounding inhibition zone of the samples was measured at different time points.

[0146] Figure 7 These are photographs showing the antibacterial effects of filter paper soaked in thymol and cinnamon oil solution, and the coated paper from Application Example 3, on Staphylococcus aureus and Escherichia coli after being left for different periods. Figure 7 It can be seen that when the filter paper soaked in thymol-cinnamon oil solution was placed on a surface containing nutrient agar medium for 2 hours, a clear inhibition zone was observed. After 4 hours, the inhibition zone decreased in size, and after 16 hours, the inhibition zone essentially disappeared. This is because the thymol in the filter paper volatilized, causing the filter paper to gradually lose its bactericidal effect. In contrast, when the coated paper of Example 3 was placed on a surface containing nutrient agar medium for 2 hours, a clear inhibition zone was observed. After 4 and 16 hours, the inhibition zone remained clear, and its size was essentially unchanged compared to the 2-hour observation. This is because the thymol in the coated paper of Example 3 had not completely volatilized after 16 hours, thus maintaining a strong bactericidal effect. Therefore, it can be seen that the thymol in the coated paper of the present invention has excellent slow-release properties, thereby giving the coated paper of the present invention excellent long-lasting antibacterial properties.

[0147] The above embodiments are merely examples for clear illustration and are not intended to limit the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing chitosan-based emulsion coatings loaded with thymol, characterized in that, Includes the following steps: Thymol is dissolved in cinnamaldehyde or cinnamon oil to form an oil phase; A polylactic acid-grafted glutinous rice starch nanocrystal solution and a chitosan solution were mixed evenly to obtain an aqueous phase. Before mixing the chitosan solution with the polylactic acid-grafted glutinous rice starch nanocrystal solution, the chitosan solution and cashew phenol glycidyl ether were mixed and heated at 50-100 °C for 3-5 h, then cooled to room temperature, and then stirred at 300-600 rpm for 3-5 h at room temperature to obtain a chitosan-based cashew phenol glycidyl ether solution. The molar ratio of the epoxy groups in the cashew phenol glycidyl ether to the amino groups in the chitosan solution was 1:1 to 1:

4. The oil phase and water phase are mixed and stirred to obtain the final product.

2. The method for preparing the chitosan-based emulsion coating loaded with thymol according to claim 1, characterized in that, The mass ratio of thymol to the volume of cinnamaldehyde or cinnamon oil is 1:5 to 3:5 g / mL. The polylactic acid-grafted glutinous rice starch nanocrystal solution and the chitosan-based cashew phenol glycidyl ether solution are mixed at a volume ratio of 1:1 to 1:

4. Mix the oil phase and water phase at a volume ratio of 1:4 to 2:3, and place them in a homogenizer and stir at 8000 to 12000 rpm for 2 to 4 minutes.

3. The method for preparing the chitosan-based emulsion coating loaded with thymol according to claim 1 or 2, characterized in that, The polylactic acid-grafted glutinous rice starch nanocrystal solution has a mass concentration of 15-25 wt% for polylactic acid-grafted glutinous rice starch nanocrystals. The chitosan solution contains 1.5 to 3 wt% chitosan.

4. The method for preparing the chitosan-based emulsion coating loaded with thymol according to claim 3, characterized in that, The preparation method of the polylactic acid grafted glutinous rice starch nanocrystal solution includes the following steps: adding polylactic acid grafted glutinous rice starch nanocrystals to water, stirring at 300-500 rpm for 20-60 min at 80-100 ℃, and then cooling to 30-50 ℃ to obtain the polylactic acid grafted glutinous rice starch nanocrystal solution. The method for preparing the chitosan solution includes the following steps: dissolving chitosan powder in a 1-2 v / v% aqueous acetic acid solution and stirring at 600-1000 rpm for 3-5 h at room temperature to obtain the chitosan solution.

5. The method for preparing the chitosan-based emulsion coating loaded with thymol according to claim 4, characterized in that, The preparation method of the polylactic acid-grafted glutinous rice starch nanocrystals includes the following steps: Glutinous rice starch nanocrystals and lactic acid aqueous solution were mixed, and then heated to 70~100 ℃ and stirred at 300~600 rpm for 1~3 h. Then a catalyst was added and stirring was continued at 300~600 rpm for 12~15 h. Finally, the reaction product was separated, purified and dried to obtain polylactic acid grafted glutinous rice starch nanocrystals.

6. The method for preparing the chitosan-based emulsion coating loaded with thymol according to claim 5, characterized in that, The lactic acid aqueous solution contains 50-85 wt% lactic acid. The mass ratio of the glutinous rice starch nanocrystals to the volume ratio of the lactic acid aqueous solution is 1:4 to 1:5; The catalyst is stannous octoate, and the amount of catalyst added is 3-10 wt% of the mass of glutinous rice starch nanocrystals. The preparation method of the glutinous rice starch nanocrystals includes the following steps: adding glutinous rice starch to concentrated sulfuric acid, stirring at 300-600 rpm for 3-7 days at 30-50℃, then centrifuging the reaction product, washing the centrifuged solution with water until the solution pH is neutral, and finally freeze-drying the obtained neutral solution to obtain glutinous rice starch nanocrystals; wherein, the concentration of the concentrated sulfuric acid is 3-4 mol / L; and the mass ratio of glutinous rice starch to the volume of concentrated sulfuric acid is 1:5-2:

5.

7. A chitosan-based emulsion coating loaded with thymol prepared by the preparation method according to any one of claims 1-6.

8. The application of the chitosan-based emulsion coating loaded with thymol as described in claim 7 in the preparation of hydrophobic and oleophobic packaging paper.

9. A hydrophobic and oleophobic packaging paper, characterized in that, It is prepared by the following method: The base paper is immersed in the chitosan-based emulsion coating loaded with thymol as described in claim 7 for 2-10 minutes, then removed to obtain dip-coated paper, which is then dried at room temperature to obtain the final product.

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