Chitosan-based film and preparation method and application thereof

Abstract

The present application belongs to the technical field of high-toughness chitosan-based film, and relates to a chitosan-based film, a preparation method and application thereof. The preparation method of the high-toughness chitosan-based film comprises the following steps: uniformly mixing a chitosan solution and a water-soluble polymer solution to obtain a mixed solution A; casting the mixed solution A on a carrier surface; and contacting the cast mixed solution A with a fatty aldehyde to perform a cross-linking reaction, so as to obtain a gel film, and drying the gel film to obtain the high-toughness chitosan-based film. The chitosan-based film prepared by adopting the technical scheme of the present application has a tensile strength of up to 92.3 MPa, an elongation at break of up to 121.5%, and a toughness of up to 69.4 MJ / m 3 2, which effectively improves the toughness of the chitosan-based film. In addition, after chemical cross-linking and constraint drying, the barrier property of the film is significantly improved, the water vapor and oxygen transmission coefficients are reduced, and the film has higher flatness.

Description

Technical Field

[0001] This invention belongs to the field of high-toughness chitosan-based film technology, and more specifically, relates to a chitosan-based film, its preparation method, and its application. Background Technology

[0002] Chitosan-based films have good application potential as packaging materials due to their excellent antibacterial and biodegradable properties. However, the rigidity of chitosan molecular chains results in high brittleness, low elongation at break, and poor barrier properties in pure chitosan films, which limits their application. Existing chemical crosslinking methods are not effective in improving the toughness of materials. For example, Jabeen et al. used diglycidyl ether as a crosslinking agent to achieve crosslinking between chitosan molecules, and the resulting film had a tensile strength of 45 MPa and an elongation at break of 31% (Influence of chitosan and epoxy cross-linking on physical properties of binary blends. International Journal of Polymer Analysis and Characterization, 2016, 21(2), pp. 163-174). Patent 202410861613.8, after physically adding auxiliary film agents, emulsifiers, humectants and emulsifiers, the film had a tensile strength of 43.67 MPa and an elongation at break of only 4.78%. The low elongation at break achieved by this method failed to effectively improve the toughness of chitosan-based films. Summary of the Invention

[0003] To overcome the deficiencies of the prior art, the present invention provides a method for preparing chitosan-based films.

[0004] The present invention also provides a chitosan-based film.

[0005] The present invention also provides an application of chitosan-based films.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: A method for preparing a chitosan-based film includes the following steps: S1. After mixing the chitosan solution and the water-soluble polymer solution evenly, a mixed solution A is obtained; S2. Cast the mixed solution A onto the surface of the carrier; S3. The cast mixture A is brought into contact with aliphatic aldehyde to carry out a cross-linking reaction, resulting in a gel film. After drying, a high-toughness chitosan-based film is obtained. The fatty aldehydes include formaldehyde, glyoxal, and glutaraldehyde; The water-soluble polymers include polyacrylamide and acetylated polyvinyl alcohol.

[0007] Preferably, the fatty aldehyde is selected from at least one of formaldehyde, glyoxal, and glutaraldehyde; and the water-soluble polymer is selected from at least one of polyacrylamide and acetylated polyvinyl alcohol.

[0008] Preferably, the mass percentage of chitosan in the mixed solution A is 3% to 5%.

[0009] Preferably, the mass percentage of water-soluble polymers in the mixed solution A is 0.5% to 2%.

[0010] Preferably, the pH value of the mixed solution A is 3.5 to 4.5.

[0011] Preferably, in step S2, the casting process further includes casting onto a carrier surface after degassing; the carrier comprises glass; and the casting conditions include applying the mixed solution A at a concentration of 0.8~1.2 g / 3 cm⁻¹. 2 The amount used is cast onto the surface of the carrier.

[0012] Preferably, in step S3, the amount of fatty aldehyde added is calculated as pure aldehyde, and the mass ratio of fatty aldehyde to chitosan is (0.4~3.5):1; preferably, the mass ratio of fatty aldehyde to chitosan is (0.4~3.1):1; preferably, the mass ratio of fatty aldehyde to chitosan is (1~3):1; preferably, the mass ratio of fatty aldehyde to chitosan is (2~3):1.

[0013] Preferably, in step S3, the drying temperature is 30~50 ℃.

[0014] Furthermore, the cross-linking reaction of aliphatic aldehydes in step S3 includes the cross-linking reaction of aliphatic aldehydes with the cast mixed solution A through liquid-phase mass transfer or gas-phase mass transfer.

[0015] Furthermore, the gas-phase mass transfer method includes the following steps: placing the cast mixed solution A in a sealed environment, adding formaldehyde solution to the sealed environment and allowing it to evaporate naturally, contacting the formaldehyde vapor with the cast mixed solution A, and reacting at 65~75 ℃ for 0.5~4 h.

[0016] Preferably, the volume of the sealed environment is 18~22 L.

[0017] Furthermore, the liquid-phase mass transfer method includes the following steps: S11. Add fatty aldehyde to ethanol solution to obtain solution B; S22. Immerse the cast mixture A into solution B to react.

[0018] Preferably, the ethanol solution in step S11 is obtained by mixing ethanol and solvent C; the solvent C includes water and DMF; the volume fraction of ethanol in the ethanol solution is 50% to 90%; preferably, the volume fraction of ethanol is 50% to 70%; more preferably, the volume fraction of ethanol is 70%.

[0019] Preferably, in step S22, the reaction temperature is 65~75 ℃ and the reaction time is 0.5~20 h; preferably, the reaction time is 0.5~4 h; preferably, the reaction time is 0.5~2 h; preferably, the reaction time is 1~2 h.

[0020] Furthermore, step S3 also includes: S111. The gel membrane taken out after the crosslinking reaction is placed in a mixture of alcohol / amide organic solvent for solvent replacement; S222. The solvent-displaced wet film is placed between porous water-absorbing sheets and subjected to binding drying under vacuum conditions to obtain the chitosan-based high-toughness film.

[0021] Preferably, the solvent replacement is repeated 2-3 times, with each replacement lasting ≥8 hours; the alcohol / amide organic solvent mixture comprises an ethanol / N,N-dimethylformamide (DMF) mixture with a volume ratio of 1:(0.8-1.2). Preferably, the alcohol / amide organic solvent mixture comprises an ethanol / N,N-dimethylformamide (DMF) mixture with a volume ratio of 1:(0.9-1.1).

[0022] Preferably, the porous absorbent sheet includes at least one of cellulose filter paper, nonwoven fabric, or porous cellulose membrane; Preferably, the binding drying temperature is 30~50 ℃, the binding drying vacuum degree is -0.6-0.9 bar, and the binding drying time is 4~8 h.

[0023] Preferably, the binding drying temperature is 35~45 ℃; the binding drying vacuum degree is -0.7-0.9 bar; and the binding drying time is 5~7 h.

[0024] Furthermore, a chitosan-based film is prepared by the method described in this invention.

[0025] Furthermore, an application of a chitosan-based film in the packaging field.

[0026] Compared with the prior art, the beneficial effects of the technical solution of the present invention are: This invention employs an in-situ crosslinking reaction, avoiding the premature gelation problem caused by directly adding highly reactive crosslinking agents such as formaldehyde to the solution, thus forming a novel film-forming method. The films prepared using this invention can achieve a maximum tensile strength of 92.3 MPa, a maximum elongation at break of 121.5%, and a maximum toughness of 69.4 MJ / m. 3 This is consistent with the value reported in existing literature (23.5 MJ / m³). 3 Compared to (Effect of dialdehyde nanocellulose-tannin fillers on antioxidant, antibacterial, mechanical and barrier properties of chitosan films for cherry tomato preservation. Food Chemistry, 2024, 463(3), 141274), the improvement was 195%. Furthermore, solvent replacement and binding drying effectively solved the adhesion problem between the film and the substrate, resulting in a film exhibiting excellent flatness and barrier properties, with its water vapor permeability and oxygen permeability reduced to 4.2 × 10⁻⁶. -5 and 5.4 × 10 -6 mol·mm·m -2 ·d -1 ·kPa -1 This expands the application of chitosan-based films in the packaging field. Attached Figure Description

[0027] Figure 1 This is a photograph of the chitosan-based high-toughness film from Example 19.

[0028] Figure 2 This is a photograph of the chitosan-based high-toughness film from Example 21.

[0029] Figure 3 The image shows a sample of protocatechuic aldehyde that cannot be cross-linked, as shown in Comparative Example 2.

[0030] Figure 4 This is a photograph of the broken sample from Comparative Example 3.

[0031] Figure 5 The images show the mixed solution A of Comparative Example 8 before and after the addition of formaldehyde; a) before the addition of formaldehyde, b) after the addition of formaldehyde.

[0032] Figure 6 This is a photograph of the chitosan-based high-toughness film of Comparative Example 11.

[0033] Figure 7Comparative Example 1 and Example 21: Infrared data images; a) Full-band data image, b) Partial band magnified data image Figure 8 Examples 1-15, Comparative Example 1: Mechanical property data charts; a) Tensile strength data chart, b) Elongation at break data chart.

[0034] Figure 9 Mechanical property data charts for products of Examples 3, 16-24, and Comparative Example 1; a) tensile strength data chart, b) elongation at break data chart.

[0035] Figure 10 Barrier performance data graphs for Comparative Example 1, Comparative Example 13, and Example 21; a. Water vapor permeability coefficient, b. Oxygen permeability coefficient, c. Oil permeability.

[0036] Figure 11 Electron micrographs of the surface morphology of products from Comparative Example 1, Comparative Example 13, and Example 21. Detailed Implementation

[0037] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in this technical field.

[0038] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0039] Polyacrylamide (PAM), nonionic, molecular weight 2 million-14 million, Aladdin reagent.

[0040] Chitosan, degree of deacetylation ≥ 95%, viscosity 100-200 mPa·s, Aladdin reagent.

[0041] Formaldehyde 37 wt.% in H2O, containing 10-15% methanol stabilizer Maclean's reagent.

[0042] N,N-Dimethylformamide (DMF), 99.5% Maclean's reagent.

[0043] Protocatechuic aldehyde, salicylaldehyde, p-hydroxybenzaldehyde, vanillin, eugenol, cinnamaldehyde, gallic aldehyde, o-vanillin, and Maclean's reagent.

[0044] Polyvinyl alcohol 1799, degree of alcoholysis: 98-99% (mol / mol), Maclean's reagent.

[0045] tert-butyl acetoacetate, 95%, Maclean's reagent.

[0046] The present invention will be further described below: A method for preparing a chitosan-based film includes the following steps: S1. After mixing the chitosan solution and the water-soluble polymer solution evenly, a mixed solution A is obtained; S2. Cast the mixed solution A onto the surface of the carrier; S3. The cast mixture A is brought into contact with aliphatic aldehydes to undergo a cross-linking reaction, resulting in a gel film. After drying, a high-toughness chitosan-based film is obtained.

[0047] The fatty aldehydes include formaldehyde, glyoxal, and glutaraldehyde.

[0048] The water-soluble polymers include polyacrylamide and acetylated polyvinyl alcohol.

[0049] The mass percentage of chitosan in the mixed solution A is 3% to 5%.

[0050] The mass percentage of water-soluble polymers in the mixed solution A is 0.5% to 2%.

[0051] The pH value of the mixed solution A is 3.5~4.5.

[0052] In step S2, the casting process further includes casting the mixture onto a carrier surface after degassing; the carrier comprises glass; the casting conditions include applying the mixed solution A at a concentration of 0.8~1.2 g / 3 cm⁻¹. 2 The film is cast onto the surface of a carrier. The carrier serves only as a temporary support during the casting process; after subsequent drying or solvent replacement steps, the film is peeled off from the carrier.

[0053] In step S3, the amount of fatty aldehyde added is calculated as pure aldehyde, and the mass ratio of fatty aldehyde to chitosan is (0.4~3.5):1; preferably, the mass ratio of fatty aldehyde to chitosan is (1~3):1; preferably, the mass ratio of fatty aldehyde to chitosan is (2~3):1.

[0054] In step S3, the drying temperature is 30~50 ℃.

[0055] In some embodiments, the cross-linking reaction of aliphatic aldehydes in step S3 further includes the cross-linking reaction of aliphatic aldehydes with the cast mixed solution A via liquid-phase mass transfer or gas-phase mass transfer.

[0056] In some embodiments, the gas-phase mass transfer method further includes the following steps: placing the cast mixed solution A in a closed environment, adding formaldehyde solution to the closed environment and allowing it to evaporate naturally, and contacting the cast mixed solution A with the formaldehyde vapor, reacting at 65~75 °C for 0.5~4 h.

[0057] In some embodiments, the liquid phase mass transfer method further includes the following steps: S11. Adding an ethanol solution to a fatty aldehyde yields solution B. S22. Immerse the cast mixture A into solution B to react.

[0058] The ethanol solution in step S11 is obtained by mixing ethanol and solvent C; the solvent C includes water and DMF; the volume fraction of ethanol in the ethanol solution is 50%~90%; preferably, the volume fraction of ethanol is 50%~70%; more preferably, the volume fraction of ethanol is 70%.

[0059] In step S22, the reaction temperature is 65~75 ℃, and the reaction time is 0.5~20 h; preferably, the reaction time is 0.5~4 h; preferably, the reaction time is 0.5~2 h; preferably, the reaction time is 1~2 h.

[0060] In some implementations, step S3 further includes: S111. The gel membrane taken out after the crosslinking reaction is placed in a mixture of alcohol / amide organic solvent for solvent replacement; S222. The solvent-displaced wet film is placed between porous water-absorbing sheets and subjected to binding drying under vacuum conditions to obtain the chitosan-based high-toughness film.

[0061] The solvent replacement is repeated 2 to 3 times, with each replacement time ≥ 8 h; the alcohol / amide organic solvent mixture includes a volume ratio of 1:(0.8~1.2) of ethanol / N,N-dimethylformamide (DMF) mixture.

[0062] The porous absorbent sheet includes at least one of cellulose filter paper, nonwoven fabric or porous cellulose membrane; the binding drying conditions are: temperature 30~50 ℃, vacuum degree -0.6-0.9 bar, drying time 4~8 h.

[0063] In some embodiments, the preparation method of the acetylated polyvinyl alcohol (PVAA) includes the following steps: 20 g of PVA is dried in an oven at 60 °C for 30 min, then dissolved in 170 mL of dimethyl sulfoxide at 110 °C, and 12.0 g of tert-butyl acetoacetate is added dropwise at a rate of 2-3 drops / s while stirring at 200 rpm. t -BAA); and stirred at 200 rpm for 6 h. After the reaction was completed, PVAA was washed in anhydrous ethanol until free of impurities. t -BAA residue was found, and finally, PVAA was placed in a vacuum drying oven and completely dried in a vacuum environment at 60 ℃.

[0064] Example 1 S1. A chitosan solution with pH = 4 and mass percentage of 8 wt.% was prepared by adding water and chitosan to a 1.5 mol / L hydrochloric acid aqueous solution and stirring at 100 rpm.

[0065] Prepare a 2 wt.% PAM aqueous solution (dissolution method: add 20 g PAM to 980 g water and stir at 350 rpm at 50 ℃ until dissolved), and adjust the pH to 4 using hydrochloric acid.

[0066] A chitosan solution and a PAM solution were mixed at a mass ratio of 1:1 and stirred at 100 rpm to obtain a mixed solution A with pH = 4, containing 4 wt.% chitosan and 1 wt.% PAM. Mixed solution A was then centrifuged to remove bubbles (centrifugation speed: 8000 rpm, centrifugation time: 8 min). The degassed mixed solution A was then centrifuged at 1 g / 3 cm⁻¹. 2 The amount used is cast onto the glass plate.

[0067] S2. Prepare 400 mL of an ethanol aqueous solution with a volume fraction of 70%, adjust its pH to 4 using hydrochloric acid, then add 4.5 mL of formaldehyde solution (37 wt.%), mix thoroughly to obtain solution B. Immerse 100 g of the cast mixture A along with a glass slide into solution B, react at 70 ℃ for 0.5 h, remove the crosslinked gel film, and remove it along with the glass slide. Dry the wet film at 40 ℃ to obtain a chitosan-based high-toughness film. The mass ratio of formaldehyde to chitosan in step S2 is 0.45:1.

[0068] Examples 2-5 Examples 2-5 are similar to Example 1, except that in step S2, the mixed solution A after casting is immersed in solution B together with the glass slide, and the reaction time at 70 °C is different, as shown in Table 1.

[0069] Example 6 This embodiment is similar to Embodiment 1, except that in step S2, the ethanol volume fraction of the aqueous ethanol solution is 50%. Details are as follows: S1. A chitosan solution with pH = 4 and mass percentage of 8 wt.% was prepared by adding water and chitosan to a 1.5 mol / L hydrochloric acid aqueous solution and stirring at 100 rpm.

[0070] Prepare a 2 wt.% PAM aqueous solution (dissolution method: add 20 g PAM to 980 g water and stir at 350 rpm at 50 °C until dissolved), and adjust the pH to 4 using hydrochloric acid.

[0071] A chitosan solution and a PAM solution were mixed at a mass ratio of 1:1 and stirred at 100 rpm to obtain a mixed solution A with pH = 4, containing 4 wt.% chitosan and 1 wt.% PAM. Mixed solution A was then centrifuged to remove bubbles (centrifugation speed: 8000 rpm, centrifugation time: 8 min). The degassed mixed solution A was then centrifuged at 1 g / 3 cm⁻¹. 2 The amount used is cast onto the glass plate.

[0072] S2. Prepare 400 mL of an ethanol aqueous solution with a volume fraction of 50%, adjust its pH to 4 using hydrochloric acid, then add 4.5 mL of formaldehyde solution (37 wt.%), mix thoroughly to obtain solution B. Immerse 100 g of the cast mixture A along with a glass slide into solution B, react at 70 ℃ for 0.5 h, remove the cross-linked gel film, remove it along with the glass slide, and dry the wet film at 40 ℃ to obtain a chitosan-based high-toughness film.

[0073] Examples 7-11 Examples 7-11 are similar to Example 6, except that in step S2, the mixed solution A after casting is immersed in solution B together with the glass slide, and the reaction time at 70 °C is different, as shown in Table 1.

[0074] Example 12 This embodiment is similar to Embodiment 1, except that in step S2, the ethanol volume fraction of the aqueous ethanol solution is 90%. Details are as follows: S1. A chitosan solution with pH = 4 and mass percentage of 8 wt.% was prepared by adding water and chitosan to a 1.5 mol / L hydrochloric acid aqueous solution and stirring at 100 rpm.

[0075] Prepare a 2 wt.% PAM aqueous solution (dissolution method: add 20 g PAM to 980 g water and stir at 350 rpm at 50 ℃ until dissolved), and adjust the pH to 4 using hydrochloric acid.

[0076] A chitosan solution and a PAM solution were mixed at a mass ratio of 1:1 and stirred at 100 rpm to obtain a mixed solution A with pH = 4, containing 4 wt.% chitosan and 1 wt.% PAM. Mixed solution A was then centrifuged to remove bubbles (centrifugation speed: 8000 rpm, centrifugation time: 8 min). The degassed mixed solution A was then centrifuged at 1 g / 3 cm⁻¹. 2 The amount used is cast onto the glass plate.

[0077] S2. Prepare 400 mL of an ethanol aqueous solution with a volume fraction of 90%, adjust its pH to 4 using hydrochloric acid, then add 4.5 mL of formaldehyde solution (37 wt.%), mix thoroughly to obtain solution B. Immerse 100 g of the cast mixture A along with a glass slide into solution B, react at 70 ℃ for 0.5 h, remove the cross-linked gel film, remove it along with the glass slide, and dry the wet film at 40 ℃ to obtain a chitosan-based high-toughness film.

[0078] Examples 13-15 Examples 13-15 are similar to Example 12, except that in step S2, the mixed solution A after casting is immersed in solution B together with the glass slide, and the reaction time at 70 °C is different, as shown in Table 1.

[0079] Example 16 This embodiment is similar to Embodiment 1, except that in step S2, the cast mixture A and the glass slide are immersed together in solution B and reacted at 70 °C for 2 h; the S2 reaction is carried out under the condition of pH = 3.5, as detailed below: S1. A chitosan solution with pH = 3.5 and a mass percentage of 8 wt.% was prepared by adding water and chitosan to a 1.5 mol / L hydrochloric acid aqueous solution and stirring at 100 rpm.

[0080] Prepare a 2 wt.% PAM aqueous solution (dissolution method: add 20 g PAM to 980 g water and stir at 350 rpm at 50 ℃ until dissolved), and adjust the pH to 3.5 using hydrochloric acid.

[0081] A chitosan solution and a PAM solution were mixed at a mass ratio of 1:1 and stirred at 100 rpm to obtain a mixed solution A with a pH of 3.5 containing 4 wt.% chitosan and 1 wt.% PAM. Mixed solution A was then centrifuged to remove bubbles (centrifugation speed: 8000 rpm, centrifugation time: 8 min). The degassed mixed solution A was then centrifuged at 1 g / 3 cm⁻¹. 2 The amount used is cast onto the glass plate.

[0082] S2. Prepare 400 mL of an ethanol aqueous solution with a volume fraction of 70%, adjust its pH to 4 using hydrochloric acid, then add 4.5 mL of formaldehyde solution (37 wt.%), mix thoroughly to obtain solution B. Immerse 100 g of the cast mixture A along with a glass slide into solution B, react at 70 ℃ for 2 h, remove the crosslinked gel film, and take it out along with the glass slide. Dry the wet film at 40 ℃ to obtain a chitosan-based high-toughness film.

[0083] Example 17 This embodiment is similar to Embodiment 1, except that in step S2, the cast mixture A and the glass slide are immersed together in solution B and reacted at 70 °C for 2 h; the S2 reaction is carried out under the condition of pH = 4.5, as detailed below: S1. A chitosan solution with pH = 4.5 and a mass percentage of 8 wt.% was prepared by adding water and chitosan to a 1.5 mol / L hydrochloric acid aqueous solution and stirring at 100 rpm.

[0084] Prepare a 2 wt.% PAM aqueous solution (dissolution method: add 20 g PAM to 980 g water and stir at 350 rpm at 50 ℃ until dissolved), and adjust the pH to 4.5 with hydrochloric acid.

[0085] A chitosan solution and a PAM solution were mixed at a mass ratio of 1:1 and stirred at 100 rpm to obtain a mixed solution A with pH = 4.5 containing 4 wt.% chitosan and 1 wt.% PAM. Mixed solution A was then centrifuged to remove bubbles (centrifugation speed: 8000 rpm, centrifugation time: 8 min). The degassed mixed solution A was then centrifuged at 1 g / 3 cm⁻¹. 2 The amount used is cast onto the glass plate.

[0086] S2. Prepare 400 mL of an ethanol aqueous solution with a volume fraction of 70%, adjust its pH to 4 using hydrochloric acid, then add 4.5 mL of formaldehyde solution (37 wt.%), mix thoroughly to obtain solution B. Immerse 100 g of the cast mixture A along with a glass slide into solution B, react at 70 ℃ for 2 h, remove the crosslinked gel film, and take it out along with the glass slide. Dry the wet film at 40 ℃ to obtain a chitosan-based high-toughness film.

[0087] Example 18 This embodiment is similar to Example 1, except that in step S2, 10 mL of formaldehyde is added to solution B; the cast mixture A and the glass slide are immersed in solution B together and reacted at 70 °C for 2 h. The mass ratio of formaldehyde to chitosan in step S2 is 1:1.

[0088] Example 19 This embodiment is similar to Example 1, except that in step S2, 20 mL of formaldehyde is added to solution B; the cast mixture A, along with the glass slide, is immersed in solution B and reacted at 70 °C for 2 h. See Figure 1 In step S2, the mass ratio of formaldehyde to chitosan is 2:1.

[0089] Example 20 This embodiment is similar to Example 1, except that in step S2, 30 mL of formaldehyde is added to solution B; the cast mixture A and the glass slide are immersed in solution B together and reacted at 70 °C for 2 h. The mass ratio of formaldehyde to chitosan in step S2 is 3:1.

[0090] Example 21 This embodiment is similar to Embodiment 1, except for step S2, which is as follows: Prepare 400 mL of an ethanol-water solution with a 70% (v / v) concentration. Adjust the pH to 4 using hydrochloric acid, then add 20 mL of formaldehyde solution (37 wt.%) and mix thoroughly to obtain solution B. Immerse 100 g of the cast mixture A along with a glass slide into solution B and react at 70 °C for 2 h. Remove the cross-linked gel membrane, along with the glass slide, and place it in 400 mL of a 1:1 (v / v) ethanol / DMF mixture for solvent exchange for 8 h. Repeat the solvent exchange twice, each time for at least 8 hours, to obtain a chitosan-based wet membrane. Attach the wet membrane to the gel using two medium-speed filter papers (22 cm in diameter) and treat it using a Kaiser automated papermaking system at 40 °C and a vacuum of -0.8 bar for 6 h to obtain a chitosan-based high-toughness film. See [link to details]. Figure 2 .

[0091] Example 22 This embodiment is similar to Example 21, except that the components of mixed solution A are 1 wt.% PVAA and 4 wt.% chitosan.

[0092] Example 23 This embodiment is similar to Embodiment 1, except that in step S2, the mixed solution A (pH = 4) is cast and placed in a sealed environment at 70 ℃ (the volume of the sealed environment is 20 L), and 4.5 mL of formaldehyde solution is added to the sealed environment to allow the formaldehyde solution to evaporate naturally. The reaction is carried out for 2 h (that is, the liquid-liquid mass transfer of formaldehyde is introduced into gas-liquid mass transfer). The gel film formed after crosslinking is taken out and removed along with the glass slide. The wet film is dried at 40 ℃ to obtain a chitosan-based high-toughness film.

[0093] Example 24 This embodiment is similar to Embodiment 21, except that in step S2, solution B (70% ethanol aqueous solution by volume) is replaced with an ethanol / DMF solution by volume ratio of 1:1. Step S2 is as follows: Prepare 400 mL of a 1:1 ethanol / DMF solution, adjust its pH to 4 using hydrochloric acid, then add 20 mL of formaldehyde solution (37 wt.%) and mix thoroughly to obtain solution B. Immerse 100 g of the cast mixture A along with a glass slide into solution B and react at 70 °C for 2 h. Remove the cross-linked gel membrane, along with the glass slide, and place it in 400 mL of a 1:1 ethanol / DMF mixture for solvent replacement for 8 h. Repeat the solvent replacement twice, each time for at least 8 hours, to obtain a chitosan-based wet membrane. Attach the wet membrane to the gel using two medium-speed filter papers (22 cm in diameter) and treat it using a Kaiser automatic papermaking system at 40 °C and a vacuum of -0.8 bar for 6 h to obtain a chitosan-based high-toughness film.

[0094] Comparative Example 1 This comparative example is similar to Example 1, except that in step S2, the cast mixture A is dried directly to form a film without undergoing a reaction, as detailed below: S1. A chitosan solution with pH = 4 and mass percentage of 8 wt.% was prepared by adding water and chitosan to a 1.5 mol / L hydrochloric acid aqueous solution and stirring at 100 rpm.

[0095] Prepare a 2 wt.% PAM aqueous solution (dissolution method: add 20 g PAM to 980 g water and stir at 350 rpm at 50 ℃ until dissolved), and adjust the pH to 4 using hydrochloric acid.

[0096] A chitosan solution and a PAM solution were mixed at a mass ratio of 1:1 and stirred at 100 rpm to obtain a mixed solution A with pH = 4, containing 4 wt.% chitosan and 1 wt.% PAM. Mixed solution A was then centrifuged to remove bubbles (centrifugation speed: 8000 rpm, centrifugation time: 8 min). The degassed mixed solution A was then centrifuged at 1 g / 3 cm⁻¹. 2 The amount used is cast onto the glass plate.

[0097] S2. 100 g of the cast mixture A and the glass slide were dried at 40 °C to obtain a chitosan-based high-toughness film.

[0098] Comparative Example 2 This comparative example is similar to Example 1, except that in step S2, formaldehyde was replaced with one of the following aromatic aldehydes, added in an amount of 4.5 g: protocatechuic aldehyde (2-1), salicylaldehyde (2-2), p-hydroxybenzaldehyde (2-3), vanillin (2-4), eugenol (2-5), cinnamaldehyde (2-6), gallic aldehyde (2-7), and o-vanillin (2-8). During the preparation process, mixed solution A did not undergo a cross-linking reaction, could not be converted into a gel membrane, could not be formed, and a membrane product could not be obtained. The results are shown in Table 2. Figure 3 This is a graph showing experimental data on replacing formaldehyde with protocatechuic aldehyde.

[0099] Comparative Example 3 This comparative example is similar to Example 21, except that in step S2, solution B is replaced with water. During the preparation process, excessive swelling of mixed solution A led to rupture, resulting in extremely poor mechanical strength of the product, making it impossible to form and obtain a membrane product. See Figure 4 .

[0100] Comparative Example 4 This comparative example is similar to Example 21, except that in step S2, solution B is replaced with anhydrous ethanol. During the preparation process, chitosan and PAM in mixed solution A aggregate and precipitate in large quantities, which cannot be transformed into a gel membrane. The product has extremely poor mechanical strength, cannot be formed, and cannot obtain a membrane product.

[0101] Comparative Example 5 This comparative example is similar to Example 21, except that in step S2, the mixed solution A is not cross-linked (i.e., 100 g of the cast mixed solution A is not immersed in solution B together with the glass slide for reaction), but is directly placed in a 1:1 volume ratio ethanol / DMF solution for solvent replacement. During the preparation process, the mixed solution A cannot be formed in the 1:1 ethanol / DMF solution, and the membrane product cannot be obtained.

[0102] Comparative Example 6 This comparative example is similar to Example 21, except that in step S2, the solvent used for solvent replacement is anhydrous ethanol (not ethanol / DMF at a volume ratio of 1:1). During the preparation process, the resulting gel experienced severe aggregation and precipitation, making it impossible to form a membrane product.

[0103] Comparative Example 7 This comparative example is similar to Example 21, except that in step S2, the solvent used for solvent replacement is DMF (not ethanol / DMF with a volume ratio of 1:1). The resulting gel underwent severe swelling and cracking during the preparation process, making it impossible to form a membrane product.

[0104] Comparative Example 8 This comparative example is similar to Example 1, except that in step S1, after preparing mixed solution A, 4.5 mL of formaldehyde was directly added to mixed solution A (without using solution B). As a result, mixed solution A immediately gelled, making subsequent casting and film formation impossible, and a film product could not be obtained. See [link to example]. Figure 5 .

[0105] Comparative Example 9 This comparative example is similar to Example 16 (pH = 3.5), except that in step S1, after preparing mixed solution A, 4.5 mL of formaldehyde was directly added to mixed solution A (without using solution B). As a result, mixed solution A immediately gelled, making it impossible to carry out subsequent casting film formation operations and obtain a film product.

[0106] Comparative Example 10 This comparative example is similar to Example 17 (pH = 4.5), except that in step S1, after preparing mixed solution A, 4.5 mL of formaldehyde was directly added to mixed solution A (without using solution B). As a result, mixed solution A immediately gelled, making it impossible to carry out subsequent casting film formation operations and thus unable to obtain a film product.

[0107] Comparative Example 11 This comparative example is similar to Example 21, except that in step S2, the cross-linked gel membrane was removed along with the glass slide. The wet membrane was not subjected to solvent replacement; instead, two medium-speed filter papers (22 cm in diameter) were directly attached to the gel membrane. The membrane was then processed for 6 hours using a Kaiser automatic paper-making system at 40 °C and a vacuum of -0.8 bar. The result was that the dried membrane adhered severely to the filter paper, making non-destructive peeling impossible. See [link to Kaiser example]. Figure 6 .

[0108] Comparative Example 12 This comparative example is similar to Example 21, except that in step S2, the solvent used for solvent replacement is an ethanol / water solution with a volume ratio of 1:1, and the solvent replacement is performed for 8 hours. This solvent replacement is repeated twice, each time for at least 8 hours, to obtain a chitosan-based wet membrane. The wet membrane is then lined with two medium-speed filter papers (22 cm in diameter) on the top and bottom of the gel and treated using a Kaiser automated paper-making system at 40 °C and a vacuum of -0.8 bar for 6 hours. As a result, the dried film adhered severely to the filter paper, making non-destructive peeling impossible.

[0109] Comparative Example 13 This embodiment is similar to Embodiment 1, except that in step S1, the prepared mixed solution A is a pure chitosan solution, as detailed below: S1. A chitosan solution A with pH = 4 and a mass percentage of 5 wt.% was prepared by adding water and chitosan to a 1.5 mol / L hydrochloric acid aqueous solution and stirring at 100 rpm. The mixture A was then centrifuged to remove bubbles (centrifugation speed: 8000 rpm, centrifugation time: 8 min). The degassed mixture A was then centrifuged at 1 g / 3 cm⁻¹. 2 The amount used is cast onto the glass plate.

[0110] S2. 100 g of the cast mixture A was dried with a glass slide at 40 °C to obtain a pure chitosan film. The film was too brittle to measure its mechanical properties.

[0111] Analysis and detection Fourier Transform Infrared Spectrometer - ATR IS50 (USA); Thermogravimetric Analyzer - TG209F3 (USA); Ultra-High Resolution Field Emission Scanning Electron Microscope - SU8600 (Japan); Material Tensile Testing Machine - INSTRON 5565 (USA); Water Vapor Permeability Tester - Y413 (China); Oxygen Permeability Tester - Y310 (China); Surface Tensiometer - DCAT 21 (Germany). Results are shown in the table below: Table 1. Data of Examples and Comparative Examples

[0112] Table 2 Data table of products with different aldehydes in Comparative Example 2

[0113] Table 3 Data Tables for Example 21, Comparative Example 1, and Comparative Example 3

[0114] Results and Analysis: When toughening films using chemical methods, the crosslinking reaction rate is mainly affected by the size and functional group activity of the crosslinking agent. The smaller the crosslinking agent molecule, the faster the crosslinking reaction because its diffusion rate is higher. Therefore, using highly reactive small-molecule crosslinking agents such as formaldehyde easily leads to a large number of crosslinkings of polymers in a short time, causing the solution to gel and lose its film-laying ability. For this reason, existing research on chemical crosslinking of chitosan films generally avoids highly reactive small-molecule crosslinking agents such as formaldehyde, and is limited to using slow-acting crosslinking agents such as genipin, diglycidyl ether, and epichlorohydrin. The technical solution of this invention introduces aliphatic aldehyde crosslinking agents to form covalent bonds between molecular chains, thereby constructing a polymer system with a network structure. Since there are natural interactions such as hydrogen bonds between chitosan molecules, the covalent network formed by chemical crosslinking can work synergistically with these intermolecular interactions, thereby significantly improving the mechanical properties of the material.

[0115] Introducing multi-component crosslinking networks is an effective way to improve the mechanical properties of polymer materials. Chitosan, a highly deacetylated product of chitin, contains abundant free amino groups in its molecular structure, providing numerous effective reaction sites for chemical crosslinking. Existing research widely utilizes aldehyde crosslinking agents to modify chitosan, but the resulting products are mostly hydrogels or microspheres, primarily concentrated in areas such as metal ion adsorption and biomedicine. Due to the high susceptibility of crosslinking to material embrittlement, there are few reports of its successful application in preparing chitosan-based films with high tensile strength and high elongation at break. This invention uses water-soluble polymers containing active groups, such as polyacrylamide and acetylated polyvinyl alcohol, as reinforcing molecules, and formaldehyde as a bridging agent. Hydroxymethylation and condensation reactions occur between the active groups of the reinforcing molecules and the amino groups of chitosan, successfully constructing a dense covalently crosslinked three-dimensional network. This synergistic network not only retains the flexibility of the polymer but also significantly enhances the bonding force between molecular chains, thereby preparing a high-toughness chitosan-based film with excellent mechanical properties.

[0116] This invention achieves in-situ crosslinking by placing the cast polymer mixture solution into a reaction solution system containing aliphatic aldehydes (formaldehyde), thereby avoiding the premature gelation problem caused by directly adding highly active crosslinking agents such as formaldehyde to the solution, forming a new film-making method of "casting-in-situ crosslinking-solvent replacement-binding drying".

[0117] The technical solution of this invention overcomes the problem that the direct addition of small molecule fatty aldehydes to chitosan solution leads to instantaneous gelation and failure to form a film, and realizes the effective application of highly active crosslinking agents such as formaldehyde in chitosan-based films.

[0118] Constrained drying refers to the process of drying a wet film under vacuum conditions by placing it between porous, absorbent sheets. This invention solves the problems of edge curling and warping that occur during film preparation, while also reducing the surface roughness of the film. Figure 11 Furthermore, this invention performs an ethanol / DMF mixed solvent replacement treatment on the gel film before binding and drying, which solves the problem of the film adhering to the porous absorbent sheet during the binding and drying process, making it impossible to peel off without damage.

[0119] Experimental results show that Table 1 and Figures 8-9 The results show that the chitosan-based film prepared using the technical solution of this invention has a maximum tensile strength of 92.3 MPa, a maximum elongation at break of 121.5%, and a maximum toughness of 69.4 MJ / m. 3 The chitosan-based film prepared using the technical solution of this invention exhibits improved toughness, significantly superior to the uncrosslinked film (Comparative Example 1, 44.5 MPa / 21.6% / 5.6 MJ / m). 3 Furthermore, the introduction of chemical crosslinking and bound drying effectively filled the gaps between molecular chains, as shown in Table 3, reducing the water vapor and oxygen permeability coefficients of the film to 4.2 × 10⁻⁶. -5 and 5.4 × 10 -6 mol·mm·m -2 ·d -1 ·kPa -1 .

[0120] FT-IR test results are as follows Figure 7 As shown. In the spectrum of the uncrosslinked Comparative Example 1 sample, due to the amino group in the PAM side amide at 1609 cm⁻¹. -1 The bending vibration peak intensity at that location is relatively high, causing it to mask the peak at 1654 cm⁻¹. -1 The carbonyl stretching vibration peak is located nearby. As seen in Example 21, after crosslinking, the bending vibration peak intensity of the NH bond in the amide group significantly decreased, while the characteristic peak intensity of the carbonyl stretching vibration remained essentially constant. The two exhibited a partially overlapping shoulder peak state, proving that the amide group participated in the chemical reaction. Furthermore, the amino groups in chitosan showed a peak intensity at 1525 cm⁻¹. -1 The intensity of the bending vibration peak at that location also showed a significant decrease. The synergistic evolution of the above spectral characteristics confirms that the reaction occurred between the amide groups of PAM and the amino groups of chitosan, successfully constructing a covalently cross-linked network.

[0121] Furthermore, contact angle testing further confirmed the significant improvement in the water resistance of the film surface of the present invention. Table 3 shows that the initial water contact angles of the uncrosslinked Comparative Example 1 (chitosan / PAM blend film) and Comparative Example 13 (pure chitosan film) were both 90°, and rapidly decreased to 58° and 59° respectively after 10 minutes of water droplet residence, indicating that the uncrosslinked film surface is hydrophilic and readily absorbs water and swells. In contrast, the film of Example 21, prepared using the process of the present invention, showed an initial contact angle of 96°, exhibiting good surface hydrophobicity, and maintained a relatively high level of 79° after 10 minutes. This is mainly due to the in-situ crosslinking reaction consuming a large number of free hydrophilic amino and amide groups in the molecular chain, constructing a tight covalent three-dimensional network; simultaneously, combined with the vacuum-bound drying process, the film surface became more dense and smooth, effectively hindering the inward penetration of water molecules. The enhanced surface water resistance further broadens the potential of this high-toughness chitosan-based film in practical applications such as moisture-proof packaging.

[0122] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a chitosan-based thin film, characterized in that, Includes the following steps: S1. After mixing the chitosan solution and the water-soluble polymer solution evenly, a mixed solution A is obtained; S2. Cast the mixed solution A onto the surface of the carrier; S3. The cast mixture A is brought into contact with aliphatic aldehyde to carry out a cross-linking reaction, resulting in a gel film. After drying, a high-toughness chitosan-based film is obtained. The fatty aldehydes include formaldehyde, glyoxal, and glutaraldehyde; The water-soluble polymers include polyacrylamide and acetylated polyvinyl alcohol.

2. The method for preparing a high-toughness chitosan-based film according to claim 1, characterized in that, The pH value of the mixed solution A is 3.5~4.

5.

3. The method for preparing a high-toughness chitosan-based film according to claim 1, characterized in that, In step S3, the amount of fatty aldehyde added is calculated as pure aldehyde, and the mass ratio of fatty aldehyde to chitosan is (0.4~3.5):

1.

4. The method for preparing a high-toughness chitosan-based film according to claim 1, characterized in that, The cross-linking reaction of aliphatic aldehydes in step S3 includes the cross-linking reaction of aliphatic aldehydes with the cast mixed solution A through liquid-phase mass transfer or gas-phase mass transfer.

5. The method for preparing a high-toughness chitosan-based film according to claim 4, characterized in that, The gas-phase mass transfer method includes the following steps: placing the cast mixed solution A in a closed environment, adding formaldehyde solution to the closed environment and allowing it to evaporate naturally, contacting the cast mixed solution A with the formaldehyde vapor, and reacting at 65~75 ℃ for 0.5~4 h.

6. The method for preparing a high-toughness chitosan-based film according to claim 4, characterized in that, The liquid phase mass transfer method includes the following steps: S11. Add fatty aldehyde to ethanol solution to obtain solution B; S22. Immerse the cast mixture A into solution B to react.

7. The method for preparing a high-toughness chitosan-based film according to claim 6, characterized in that, The ethanol solution in step S11 is obtained by mixing ethanol and solvent C; the solvent C includes water and DMF; the volume fraction of ethanol in the ethanol solution is 50%~90%; in step S22, the reaction time is 0.5~20 h.

8. The method for preparing a high-toughness chitosan-based film according to claim 1, characterized in that, Step S3 also includes: S111. The gel membrane taken out after the crosslinking reaction is placed in a mixture of alcohol / amide organic solvent for solvent replacement; S222. The solvent-displaced wet film is placed between porous water-absorbing sheets and subjected to binding drying under vacuum conditions to obtain the chitosan-based high-toughness film.

9. A chitosan-based film, characterized in that, Prepared by the method described in any one of claims 1 to 8.

10. The application of the high-toughness chitosan-based film of claim 9 in the packaging field.

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