Preparation method of degradable photocatalytic film

Abstract

The application discloses a preparation method of a degradable photocatalytic film and belongs to the technical field of high polymer materials, and aims at solving the problems of easy agglomeration of photocatalysts in a polylactic acid matrix, low efficiency, and technical contradiction between utilization of ultraviolet light and shielding of ultraviolet light, the method is characterized in that: a lignin-chitosan graft copolymer is prepared, and nano-titanium dioxide is chemically anchored and modified to construct a ternary hybrid; and then the ternary hybrid is melt-blended with polylactic acid, nano-zinc oxide and the like to flow down and form a film. The film has excellent mechanical properties, high-efficiency photocatalysis, ultraviolet shielding and antibacterial functions, and excellent fresh-keeping effects are achieved in cooperation.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high polymer materials, and particularly relates to a preparation method of a degradable photocatalytic film. BACKGROUND

[0002] Under the background of global sustainable development, using biodegradable plastics (such as polylactic acid, PLA) to replace traditional petroleum-based plastics has become an important trend in the packaging industry. However, for the preservation packaging of fresh fruits and vegetables, only the degradability of the material is far from enough. Fresh fruits and vegetables face two major problems during postharvest storage and transportation: first, the ethylene gas released by their own respiration will accelerate their ripening and aging; second, microorganisms (bacteria, molds) in the environment will invade and cause them to spoil.

[0003] To solve the above problems, the prior art often introduces photocatalysts such as nano-titanium dioxide (TiO2) into the PLA matrix through physical blending. TiO2 can produce reactive oxygen species under ultraviolet light, effectively degrading ethylene. However, this simple physical blending method has many defects:

[0004] (1) The specific surface energy of nanoparticles is high, and they are prone to agglomeration in the PLA matrix, resulting in a significant decrease in the mechanical properties of the film, and most of the TiO2 is embedded and loses catalytic activity;

[0005] (2) TiO2 has poor interfacial compatibility with the PLA matrix, and the adhesion is weak, which can easily fall off from the surface of the film, causing functional failure and food safety risks.

[0006] In addition, the prior art also has technical biases and contradictions: on the one hand, TiO2 is used for photocatalysis, which requires the excitation of ultraviolet light; on the other hand, ultraviolet light itself is an important inducement for the degradation of nutrients such as vitamins and the oxidation of fats in food. The traditional PLA film itself has little ultraviolet shielding ability, therefore, the existing photocatalytic film may accelerate the photooxidation of food in the package while degrading ethylene, which fundamentally contradicts the original intention of "preservation".

[0007] Therefore, how to design a degradable film that can efficiently and stably exert photocatalytic effect, effectively shield ultraviolet light from damaging food, and overcome the brittleness problem caused by the introduction of fillers, achieving synergistic effect of multiple functions, is a key technical problem to be solved in the field. SUMMARY

[0008] The present application aims to overcome the defects and technical contradictions of the prior art, and provides a preparation method of a degradable photocatalytic film.

[0009] The object of the present application can be achieved by the following technical solutions:

[0010] A preparation method of a degradable photocatalytic film, comprising the following steps:

[0011] S1: Dissolve chitosan in 1% (v / v) acetic acid aqueous solution, then add alkali lignin, protect the system with nitrogen, heat the system to 50-65 DEG C, then add cerium ammonium nitrate-nitric acid solution, react at 50-65 DEG C for 4-6 hours, after the end, adjust the pH of the system to 8.5-9.0 with dilute NaOH solution, centrifugal collection of precipitate, wash the precipitate with deionized water and ethanol alternately until neutral, then vacuum drying, to obtain lignin-chitosan graft copolymer;

[0012] S2: Disperse nano-titanium dioxide in anhydrous ethanol, add KH-550, stir at 80-85 DEG C for 4-6 hours, after the end, centrifugal collection of solid, wash with anhydrous ethanol for three times, then vacuum drying, to obtain modified nano-titanium dioxide with surface grafted amino group;

[0013] S3: Dissolve lignin-chitosan graft copolymer in 1% (v / v) acetic acid aqueous solution, add modified nano-titanium dioxide with surface grafted amino group, ultrasonic dispersion for 20-30 minutes, adjust the pH of the system to 5-6 with 2M NaOH solution, then add 2.5% (v / v) glutaraldehyde aqueous solution dropwise under stirring, react at 50-60 DEG C for 3-4 hours, after the end, adjust the pH to neutral, centrifugal collection of product, after washing and freeze-drying, to obtain ternary hybrid;

[0014] S4: Mix polylactic acid, ternary hybrid, nano-zinc oxide and acetylated tributyl citrate for 10-20 minutes, then melt blend, extrude, cool, and cut into particles to obtain composite particles;

[0015] S5: Vacuum dry the composite particles at 40-45 DEG C for 12 hours, then cast into a film to prepare a degradable photocatalytic film.

[0016] Further, the amount ratio of chitosan, acetic acid aqueous solution, alkali lignin, cerium ammonium nitrate-nitric acid solution in S1 is 30g:2L:10g:50mL.

[0017] Further, the mass ratio of the chitosan to the alkali lignin in S1 is 3:1.

[0018] Further, the dosage ratio of the nano-titanium dioxide, the anhydrous ethanol and the KH-550 in S2 is 20g: 250-300mL: 2.4-3.0g.

[0019] Further, the dosage ratio of the lignin-chitosan graft copolymer, the acetic acid aqueous solution, the modified nano-titanium dioxide with surface grafted amino and the glutaraldehyde aqueous solution in S3 is 15g: 750mL: 12g: 15mL.

[0020] Further, the mass ratio of the lignin-chitosan graft copolymer to the modified nano-titanium dioxide with surface grafted amino in S3 is 5:4.

[0021] Further, the composite particles in S4 include the following raw materials in mass parts:

[0022] 100 mass parts of polylactic acid, 14-20 mass parts of the ternary hybrid, 3-4 mass parts of nano-zinc oxide and 10-12 mass parts of acetylated tributyl citrate.

[0023] Further, the thickness of the degradable photocatalytic film is 35-45mu m.

[0024] The beneficial effects of the present application are as follows:

[0025] (1) The technical scheme of the present application solves the technical contradiction between "utilizing ultraviolet" and "shielding ultraviolet", and realizes functional synergy: the natural ultraviolet absorber lignin is creatively introduced into the system. In the prepared ternary hybrid, the lignin unit can effectively absorb the ultraviolet light penetrating the film, protecting the food from photooxidation damage; and the nano-titanium dioxide anchored on the chitosan chain can utilize the ultraviolet light on the surface of the film for photocatalytic reaction. Both of them perform their respective functions in space, realizing the synergistic effect of "surface photocatalysis" and "bulk ultraviolet shielding".

[0026] (2) The technical scheme of the present application significantly improves the mechanical properties of the film, solving the inherent brittleness problem of PLA: by grafting the rigid lignin to the flexible chitosan molecular chain, a "rigid-flexible" copolymer reinforcing body is formed. The reinforcing body has good compatibility with the PLA matrix and is uniformly dispersed therein, which can not only improve the tensile strength of the film like a skeleton, but also absorb energy through the flexible movement of the molecular chain, greatly improving the elongation at break and toughness of the film, effectively overcoming the brittleness defect of pure PLA and traditional inorganic filler filled PLA.

[0027] (3) The technical scheme of the present application realizes efficient and stable loading of the photocatalyst, and has high catalytic efficiency: through the two-step method of "graft copolymerization-chemical anchoring", the nano titanium dioxide is firmly fixed on the lignin-chitosan graft copolymer (Lignin-g-CS) macromolecular carrier in the form of covalent bond, which fundamentally avoids its agglomeration in the matrix and falling off during use. The hydrophilicity of the chitosan segment provides a favorable microenvironment for the photocatalytic reaction, ensuring the high efficiency and long-term effectiveness of the photocatalytic degradation of ethylene.

[0028] (4) The technical scheme of the present application has triple fresh-keeping effect, and the comprehensive effect is outstanding: the film integrates three functions of photocatalytic degradation of ethylene (delaying ripening), broad-spectrum antibacterial (nanometer titanium dioxide, chitosan, and synergistic effect of photocatalytic active oxygen), and ultraviolet shielding (protecting nutritional ingredients), actively protects the food from multiple dimensions, and the fresh-keeping effect is much better than that of single-function packaging materials. DETAILED DESCRIPTION

[0029] The technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application. Meanwhile, the raw materials, reagents or devices used in the following embodiments can be obtained from conventional commercial channels or can be obtained by existing known methods, unless otherwise specified, wherein the raw materials used in the embodiments, such as polylactic acid (PLA, 4032D, NatureWorks), chitosan (degree of deacetylation 92%), alkali lignin (industrial grade), nano titanium dioxide (anatase type, P25), nano zinc oxide (particle size 50 nm), KH-550, glutaraldehyde, acetylated tributyl citrate (ATBC), etc., are all commercially available chemicals or industrial products.

[0030] Embodiment 1

[0031] A preparation method of a degradable photocatalytic film:

[0032] (1) Preparation of lignin-chitosan graft copolymer (Lignin-g-CS): 30 g of chitosan (degree of deacetylation 92%) was dissolved in 2 L of 1% (v / v) acetic acid aqueous solution, and 10 g of alkali lignin was added thereto, and stirred and dissolved to obtain a mixed solution. After nitrogen was passed through the solution for 20 minutes, the system was warmed to 50°C, and 50 mL of cerium ammonium nitrate-nitric acid solution (1 mol / L dilute nitric acid was added to 4.0 g of cerium ammonium nitrate to prepare 50 mL of cerium ammonium nitrate-nitric acid solution) was added thereto, and the reaction was performed at 50°C for 4 hours. After the reaction was completed, the pH of the system was adjusted to 8.5 using 2M NaOH solution, and a precipitate was separated. The precipitate was collected by centrifugation, and washed with deionized water and ethanol alternately until it became neutral, and dried at 60°C under vacuum to obtain a lignin-chitosan graft copolymer in powder form.

[0033] (2) Preparation of modified nano-titania with amino groups grafted on the surface (AP-TiO2): 20 g of nano-titania (anatase type, P25) was dispersed in 250 mL of anhydrous ethanol, and 2.4 g of silane coupling agent γ-aminopropyl triethoxysilane (KH-550) was added thereto, and the reaction was performed under reflux at 80°C for 4 hours with magnetic stirring. After the reaction was completed, the precipitate was collected by centrifugation, and washed with anhydrous ethanol three times, and then dried at 60°C under vacuum to obtain modified nano-titania with amino groups grafted on the surface in powder form.

[0034] (3) Preparation of "lignin-graft-chitosan-anchoring-titania" (LCS-TiO2) ternary hybrid: 15 g of the lignin-chitosan graft copolymer of step (1) was dissolved in 750 mL of 1% (v / v) acetic acid aqueous solution, and 12 g of the modified nano-titania with amino groups grafted on the surface of step (2) (the mass ratio of the lignin-chitosan graft copolymer to the modified nano-titania with amino groups grafted on the surface was 5:4) was added thereto, and ultrasonically dispersed for 20 minutes to uniformly disperse, and the pH of the system was adjusted to 5 to 6 using 2M NaOH solution. While stirring vigorously, 15 mL of 2.5% (v / v) glutaraldehyde aqueous solution was added dropwise as a crosslinking agent, and the reaction was performed at 50°C for 3 hours. After the reaction was completed, the pH was adjusted to neutral, and the product was collected by centrifugation, and washed repeatedly with deionized water, and finally freeze-dried to obtain a "lignin-graft-chitosan-anchoring-titania" ternary hybrid in powder form.

[0035] (4) Melt blending: 100 parts by mass of polylactic acid (PLA, 4032D), 14 parts by mass of the "lignin-graft-chitosan-anchor-titanium dioxide" ternary hybrid obtained in step (3), 3 parts by mass of nano-zinc oxide (particle size 50 nm), and 10 parts by mass of the plasticizer acetylated tributyl citrate (ATBC) were pre-mixed in a high-speed mixer for 10 minutes, and then the mixture was added to a twin-screw extruder, and melt blending and extrusion were performed in a temperature range of 170 to 195°C (zone temperatures: 170°C for zone 1, 180°C for zone 2, 195°C for zone 3, 190°C for zone 4, and 185°C for zone 5), and then water bath cooling and pelletization were performed to obtain composite particles.

[0036] (5) Film forming: the composite particles obtained in step (4) were vacuum dried at 40°C for 12 hours, and then cast film forming was performed by a single-screw casting machine to obtain a photocatalytic film with a thickness of 35 μm, which is a degradable photocatalytic film.

[0037] Example 2

[0038] A method for preparing a degradable photocatalytic film:

[0039] (1) Preparation of lignin-chitosan graft copolymer (Lignin-g-CS): 30 g of chitosan (degree of deacetylation 92%) was dissolved in 2 L of 1% (v / v) acetic acid aqueous solution, and then 10 g of alkali lignin (the mass ratio of chitosan to alkali lignin was controlled to be 3:1) was added thereto, and stirred and dissolved to obtain a mixed solution. Nitrogen was introduced into the system for 30 minutes, and then the system was heated to 60°C, and then 50 mL of cerium ammonium nitrate-nitric acid solution (4.0 g of cerium ammonium nitrate was added to 1 mol / L dilute nitric acid to prepare 50 mL of cerium ammonium nitrate-nitric acid solution) was added thereto, and constant temperature reaction was performed at 60°C for 6 hours. After the reaction was completed, the pH of the system was adjusted to 8.7 with 2M NaOH solution, and a precipitate was separated out. The precipitate was collected by centrifugation, and washed with deionized water and ethanol alternately until it was neutral, and then vacuum dried at 60°C to obtain a powder of lignin-chitosan graft copolymer.

[0040] (2) Preparation of amino group surface-grafted modified nano-titanium dioxide (AP-TiO2): 20 g of nano-titanium dioxide (anatase type, P25) was dispersed in 300 mL of anhydrous ethanol, and then 3.0 g of silane coupling agent γ-aminopropyl triethoxysilane (KH-550) was added thereto, and magnetic stirring reflux reaction was performed at 85°C for 6 hours. After the reaction was completed, the precipitate was collected by centrifugation, and then washed with anhydrous ethanol three times, and then vacuum dried at 60°C to obtain a powder of amino group surface-grafted modified nano-titanium dioxide.

[0041] (3) Preparation of "Lignin-graft-chitosan-anchor-titanium dioxide" (LCS-TiO2) ternary hybrid: 15 g of the lignin-chitosan graft copolymer of step (1) was dissolved in 750 mL of 1% (v / v) acetic acid aqueous solution, 12 g of the modified nano-titanium dioxide with surface-grafted amino groups of step (2) (mass ratio of lignin-chitosan graft copolymer to modified nano-titanium dioxide with surface-grafted amino groups was 5:4) was added thereto, and ultrasonic dispersion was performed for 30 minutes to uniformly disperse the same, and then the pH of the system was adjusted to 5-6 using a 2M NaOH solution. Under vigorous stirring, 15 mL of 2.5% (v / v) glutaraldehyde aqueous solution was added dropwise as a crosslinking agent, and reaction was performed at 60°C for 4 hours. After completion of the reaction, the pH was adjusted to neutral, the product was collected by centrifugation, washed repeatedly with deionized water, and finally freeze-dried to obtain a "lignin-graft-chitosan-anchor-titanium dioxide" ternary hybrid in the form of a powder.

[0042] (4) Melt blending: 100 parts by mass of polylactic acid (PLA, 4032D), 18 parts by mass of the "lignin-graft-chitosan-anchor-titanium dioxide" ternary hybrid obtained in step (3), 4 parts by mass of nano-zinc oxide (particle size 50 nm), and 12 parts by mass of a plasticizer acetylated tributyl citrate (ATBC) were pre-mixed in a high-speed mixer for 20 minutes, and then the mixture was introduced into a twin-screw extruder, and melt blending and extrusion were performed at a temperature range of 170-195°C (zone temperatures: 170°C for zone 1, 180°C for zone 2, 195°C for zone 3, 190°C for zone 4, and 185°C for zone 5), and then water-bath cooling and pelletization were performed to obtain composite particles.

[0043] (5) Film formation: the composite particles obtained in step (4) were vacuum-dried at 40°C for 12 hours, and then cast film formation was performed using a single-screw casting machine to produce a photocatalytic film having a thickness of 40 μm, which is a degradable photocatalytic film.

[0044] Example 3

[0045] A method for preparing a degradable photocatalytic film:

[0046] (1) Preparation of lignin-chitosan graft copolymer (Lignin-g-CS): 30 g of chitosan (degree of deacetylation 92%) was dissolved in 2 L of 1% (v / v) acetic acid aqueous solution, and 10 g of alkali lignin was added thereto, and stirred and dissolved to obtain a mixed solution. After nitrogen was passed through the solution for 30 minutes, the system was warmed to 65°C, and 50 mL of cerium ammonium nitrate-nitric acid solution (1 mol / L dilute nitric acid was added to 4.0 g of cerium ammonium nitrate to prepare 50 mL of cerium ammonium nitrate-nitric acid solution) was added thereto, and the reaction was performed at 65°C for 6 hours. After the reaction was completed, the pH of the system was adjusted to 9.0 with 2M NaOH solution, and a precipitate was separated. The precipitate was collected by centrifugation, and washed with deionized water and ethanol alternately until it became neutral, and dried at 80°C under vacuum to obtain a lignin-chitosan graft copolymer in powder form.

[0047] (2) Preparation of modified nano-titania with amino groups grafted on the surface (AP-TiO2): 20 g of nano-titania (anatase type, P25) was dispersed in 300 mL of anhydrous ethanol, and 3.0 g of silane coupling agent γ-aminopropyl triethoxysilane (KH-550) was added thereto, and the reaction was performed under reflux at 85°C for 6 hours with magnetic stirring. After the reaction was completed, the precipitate was collected by centrifugation, and washed with anhydrous ethanol three times, and then dried at 80°C under vacuum to obtain modified nano-titania with amino groups grafted on the surface in powder form.

[0048] (3) Preparation of "lignin-graft-chitosan-anchoring-titania" (LCS-TiO2) ternary hybrid: 15 g of the lignin-chitosan graft copolymer of step (1) was dissolved in 750 mL of 1% (v / v) acetic acid aqueous solution, and 12 g of the modified nano-titania with amino groups grafted on the surface of step (2) (the mass ratio of the lignin-chitosan graft copolymer to the modified nano-titania with amino groups grafted on the surface was 5:4) was added thereto, and ultrasonically dispersed for 30 minutes to uniformly disperse, and the pH of the system was adjusted to 5 to 6 with 2M NaOH solution. While stirring vigorously, 15 mL of 2.5% (v / v) glutaraldehyde aqueous solution was added dropwise as a crosslinking agent, and the reaction was performed at 60°C for 4 hours. After the reaction was completed, the pH was adjusted to neutral, and the product was collected by centrifugation, and washed repeatedly with deionized water, and finally freeze-dried to obtain a "lignin-graft-chitosan-anchoring-titania" ternary hybrid in powder form.

[0049] (4) Melt blending: 100 parts by mass of polylactic acid (PLA, 4032D), 20 parts by mass of the "lignin-graft-chitosan-anchor-titania" ternary hybrid obtained in step (3), 4 parts by mass of nano-zinc oxide (particle size 50 nm), and 12 parts by mass of the plasticizer acetylated tributyl citrate (ATBC) were pre-mixed in a high-speed mixer for 20 minutes, and then the mixture was fed into a twin-screw extruder, and melt blended and extruded in a temperature range of 170 to 195°C (zone temperatures: 170°C for zone 1, 180°C for zone 2, 195°C for zone 3, 190°C for zone 4, and 185°C for zone 5), and then water-bath cooled and pelletized to obtain composite particles.

[0050] (5) Film forming: the composite particles obtained in step (4) were vacuum dried at 45°C for 12 hours, and then cast into a film by a single-screw casting machine to produce a photocatalytic film with a thickness of 45 μm.

[0051] Comparative Example 1

[0052] A method for producing a photocatalytic film that is degradable:

[0053] Steps (1), (2), and (3) were omitted. In step (4), the individual original components that constitute the ternary hybrid were physically blended with polylactic acid and other materials. The specific proportions were as follows: 100 parts of polylactic acid (PLA, 4032D), 18 parts of "mixed powder" (the mixed powder was prepared by physically mixing 8.2 parts of lignin, 6.1 parts of chitosan (degree of deacetylation 92%), and 3.7 parts of nano-titania (anatase type, P25) in the same proportions as the components in the ternary hybrid in Example 2), 4 parts of nano-zinc oxide (particle size 50 nm), and 12 parts of acetylated tributyl citrate (ATBC) were melt blended. All other preparation conditions (extrusion temperature, film forming process, etc.) were identical to those in Example 2, and a photocatalytic film that is degradable was ultimately obtained.

[0054] Comparative Example 2

[0055] A method for producing a photocatalytic film that is degradable:

[0056] In step (1), no alkali lignin was added, and only chitosan was used. All other raw material amounts and preparation conditions were identical to those in Example 2, and a photocatalytic film that is degradable was ultimately obtained.

[0057] Comparative Example 3

[0058] A method for producing a photocatalytic film that is degradable:

[0059] Step (2) (ammonification of nano-titania surface) and the crosslinking agent glutaraldehyde in step S3 were cancelled. In step (3), the lignin-chitosan graft copolymer prepared in step (1) was simply mixed with unmodified nano-titania (anatase, P25) in acetic acid solution, ultrasonically dispersed and then directly dried to obtain a physically coated mixture. In step (4), 18 parts of the mixture were added. All other raw material dosages and preparation conditions were exactly the same as in Example 2, and a degradable photocatalytic film was finally obtained.

[0060] Comparative Example 4

[0061] A method for preparing a degradable photocatalytic film:

[0062] In step (4), no nano-zinc oxide (particle size 50 nm) was added. All other raw material dosages (polylactic acid (PLA, 4032D) 100 parts, "lignin-graft-chitosan-anchor-titania" ternary hybrid 18 parts, plasticizer acetylated tributyl citrate (ATBC) 12 parts) and preparation conditions were exactly the same as in Example 2, and a degradable photocatalytic film was finally obtained.

[0063] Test Example 1

[0064] The degradable photocatalytic films prepared in Examples 1-3 and Comparative Examples 1-4 were tested for performance, and the performance testing process was as follows, and the test results are shown in Table 1:

[0065] 1. Mechanical property test:

[0066] Test method: According to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: test conditions for films and sheets", each group of film samples was cut into dumbbell-shaped or long strip-shaped samples of 50 mm x 15 mm. An electronic universal testing machine was used to test at a temperature of 23 ± 2 °C and a relative humidity of 50 ± 5%, at a tensile rate of 100 mm / min. The tensile strength (MPa) and elongation at break (%) of each sample were recorded and calculated. Each group of samples was tested for not less than 5 valid data, and the results were taken as the arithmetic mean.

[0067] 2. Ethylene degradation performance test:

[0068] Test method: A 15 cm x 15 cm film sample (about 0.0225 m 2) The film sample was hung in a 10 L sealed glass container. Pure ethylene gas was injected through a micro-injector to make the initial ethylene concentration in the container 80 ppm. The whole device was placed under a UV lamp with a power of 30 W and a main wavelength of 365 nm, and the distance between the lamp and the surface of the film sample was kept at 25 cm. At 0 h, 3 h, 6 h, and 9 h of irradiation, 1 mL of gas was extracted from the sampling port of the container for analysis using a gas chromatograph with an FID detector to determine the ethylene concentration. The ethylene degradation rate (%) = [(initial concentration - t-time concentration) / initial concentration] x 100%.

[0069] 3. Ultraviolet shielding performance test:

[0070] Test method: Using an ultraviolet-visible spectrophotometer, the film sample was cut into a size suitable for the sample cell (e.g. 1 cm x 4 cm) and fixed on the sample holder. With air as the blank reference, the transmittance curve of the film in the wavelength range of 200-800 nm was scanned. The transmittance (T%) at the typical wavelength of 365 nm in the ultraviolet A region (UVA) was recorded.

[0071] 4. Comprehensive preservation performance test (taking strawberries as an example):

[0072] Test method: Fresh strawberries with uniform size and consistent maturity (about 80% mature) and no mechanical damage were selected. They were randomly divided into groups, with 9 strawberries in each group. Fresh-keeping bags (size 20 cm x 30 cm) made of Example 2, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, and commercially available ordinary PE fresh-keeping bags (blank control group) were used for packaging, and after sealing, they were placed in a refrigerator at 4 ± 1 ℃ for storage. Starting from the 0th day, the rotting rate and weight loss rate of each group of strawberries were observed and recorded every 2 days.

[0073] Rot rate (%): The number of strawberry grains with obvious mold, soft rot, or juice leakage / total number of grains x 100%.

[0074] Weight loss rate (%): (initial total weight - total weight on the day) / initial total weight x 100%.

[0075] Table 1 Test results

[0076] Item Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Tensile strength (MPa) 38.5 42.1 43.5 25.3 37.8 31.2 41.9 Elongation at break (%) 265 288 295 95 275 155 285 Ethylene degradation rate (6h, %) 85.6 92.5 94.1 21.3 91.8 45.7 92.2 UV transmittance (365 nm, %) 2.8 2.1 1.5 15.8 85.4 2.3 2.2 Strawberry rot rate (6th day, %) - 11.1 - 66.7 44.4 55.6 33.3 Strawberry weight loss rate (6th day, %) - 3.5 - 3.8 8.2 4.1 3.6

[0077] Data analysis from Table 1:

[0078] From the overall data of Table 1, it can be seen that the degradable photocatalytic film prepared by Examples 1-3 of the present application is significantly superior to each of the comparative examples in terms of mechanical properties (tensile strength, elongation at break), functionality (ethylene degradation rate, UV shielding performance), and actual application effect (strawberry preservation performance). This proves the scientificity and superiority of the "lignin-graft-chitosan-anchor-titanium dioxide" ternary hybrid structure constructed by the present application, as well as the overall formula design of the composite material.

[0079] The key role of each technical element will be analyzed in detail by grouping comparison as follows:

[0080] 1. Ternary hybrid structure vs. simple physical blending (Example 2 vs. Comparative Example 1):

[0081] Technical solution difference: Example 2 uses the method of the present application to pre-prepare a "lignin-graft-chitosan-anchor-titanium dioxide" ternary hybrid, which is then added as a functional filler to a polylactic acid (PLA) matrix. Comparative Example 1 omits the preparation step of the ternary hybrid, and directly physically mixes the original components (lignin, chitosan, and nano-titanium dioxide) that make up the hybrid in the same proportion, and then blends them with PLA and other materials.

[0082] Result analysis:

[0083] (1) Mechanical properties: The tensile strength (42.1 MPa) and elongation at break (288%) of Example 2 are 1.66 times and 3.03 times that of Comparative Example 1 (25.3 MPa, 95%), respectively. This indicates that the components are chemically linked to form an organic whole (ternary hybrid), which can be uniformly dispersed in the PLA matrix as an efficient reinforcing and toughening agent, effectively transmitting stress. In contrast, the simple physical mixing of powders in Comparative Example 1 easily agglomerates in the matrix due to poor compatibility, forming stress concentration points, thereby severely weakening the mechanical properties of the film and leading to brittle materials.

[0084] (2) Ethylene degradation performance: The ethylene degradation rate of Example 2 (92.5%) is much higher than that of Comparative Example 1 (21.3%). This is because in the ternary hybrid, nano-titanium dioxide is effectively dispersed and anchored on the surface of the carrier, ensuring the full exposure of its photocatalytic active sites. In Comparative Example 1, a large number of nano-titanium dioxide particles are wrapped inside the PLA matrix due to agglomeration, and cannot contact the ultraviolet light and ethylene, thereby losing catalytic activity.

[0085] (3) UV-shielding performance: The UV transmittance of Example 2 (2.1%) is much lower than that of Comparative Example 1 (15.8%). This indicates that the ternary hybrid structure helps the lignin, an ultraviolet absorber, to achieve uniform dispersion at the molecular or nanoscale level in the matrix, thereby constructing a dense UV shielding network. The aggregation of lignin in Comparative Example 1 leads to uneven shielding effect, with a large number of "leakage", and higher UV transmittance.

[0086] (4) Comprehensive preservation performance: The strawberry rot rate of Example 2 (11.1%) is much lower than that of Comparative Example 1 (66.7%). This directly reflects the comprehensive result of the above performance advantages: the excellent ethylene degradation ability delays the maturation and aging of strawberries, thereby inhibiting the occurrence of rot.

[0087] Conclusion: The comparison between Example 2 and Comparative Example 1 proves that the "ternary hybrid" structure of the core of the present application is the key to achieving high performance, rather than the simple addition of each component. Preparing the ternary hybrid fundamentally solves the problems of dispersion, stability and activity of functional fillers in the PLA matrix.

[0088] 2. The role of lignin (Example 2 vs. Comparative Example 2):

[0089] Technical solution difference: The technical solution of Comparative Example 2 is basically the same as that of Example 2, the only difference is that no alkali lignin is added when preparing the hybrid.

[0090] Result analysis:

[0091] (1) UV-shielding performance: This is the most divergent indicator between the two. The UV transmittance of Example 2 is only 2.1%, while the UV transmittance of Comparative Example 2, which lacks lignin, is as high as 85.4%, almost without UV shielding ability. It proves that lignin is the core component of the film of the present application to achieve UV shielding function.

[0092] (2) Mechanical properties: The tensile strength (42.1 MPa) and elongation at break (288%) of Example 2 are slightly higher than those of Comparative Example 2 (37.8 MPa, 275%). This indicates that the "rigid-flexible" copolymer structure formed by grafting rigid lignin onto flexible chitosan chains has a better reinforcing and toughening effect on the PLA matrix.

[0093] (3) Comprehensive preservation performance: The strawberry rot rate (11.1%) and weight loss rate (3.5%) of Example 2 were significantly better than those of Comparative Example 2 (rot rate 44.4%, weight loss rate 8.2%). With similar ethylene degradation rates (92.5% vs 91.8%), the huge difference in preservation effect was mainly due to the UV shielding function. The film of Comparative Example 2 could not block UV rays, causing the strawberries inside to be irradiated by UV rays, accelerating the degradation of nutrients and the destruction of tissue cells, and thus more prone to rot and water loss.

[0094] Conclusion: Comparing Example 2 and Comparative Example 2 proves that the introduction of lignin successfully solves the technical contradiction between "utilizing UV" and "shielding UV" in photocatalytic applications. It gives the film excellent intrinsic UV shielding ability without affecting the surface photocatalytic efficiency, which is crucial for protecting the quality of the food inside and is one of the key factors to achieve excellent preservation effect.

[0095] 3. Chemical anchoring vs. physical coating (Example 2 vs. Comparative Example 3):

[0096] Technical solution difference: Comparative Example 3 cancels step S2 (amino modification of nano-titanium dioxide) and the crosslinking agent glutaraldehyde in step S3. This means that nano-titanium dioxide is not connected to lignin-chitosan copolymer through chemical bonds, but only physically mixed or simply coated.

[0097] Result analysis:

[0098] (1) Mechanical properties: The tensile strength (42.1 MPa) and elongation at break (288%) of Example 2 are much higher than those of Comparative Example 3 (31.2 MPa, 155%). This shows that the chemical anchoring achieved by KH-550 modification and glutaraldehyde crosslinking forms a strong covalent bond between nano-titanium dioxide and the polymer carrier. This strong interfacial bonding is beneficial for stress transfer, avoiding inorganic particles as defect points. In Comparative Example 3, the physically mixed titanium dioxide has weak bonding force with the carrier and poor interfacial compatibility, which destroys the mechanical properties of the material.

[0099] (2) Ethylene degradation performance: The ethylene degradation rate of Example 2 (92.5%) is more than twice that of Comparative Example 3 (45.7%). This fully demonstrates that chemical anchoring can effectively prevent the agglomeration and shedding of nano-titanium dioxide during use, ensuring its efficient and stable catalytic activity. Physically coated titanium dioxide is prone to re-agglomeration or embedding during subsequent processing and use, resulting in a significant decrease in activity.

[0100] (3) Comprehensive preservation performance: The strawberry rot rate (11.1%) of Example 2 is much lower than that of Comparative Example 3 (55.6%). This is mainly due to its higher ethylene degradation efficiency, which can more effectively inhibit the ripening process of strawberries.

[0101] Conclusion: Comparing Example 2 and Comparative Example 3 proves that the anchoring method of "surface modification-chemical crosslinking" adopted in the present application is the key to achieve efficient and stable loading of photocatalysts. This method ensures the activity of the catalyst and the mechanical integrity of the film, which is the guarantee of long-term function.

[0102] 4. The role of nano zinc oxide (Example 2 vs. Comparative Example 4):

[0103] Technical solution difference: The technical solution of Comparative Example 4 is basically the same as Example 2, the only difference is that no nano zinc oxide is added in the melt blending step.

[0104] Result analysis:

[0105] (1) Mechanical, ethylene degradation, and UV shielding properties: Comparing the data of Example 2 and Comparative Example 4, there is almost no difference in tensile strength (42.1 vs 41.9 MPa), elongation at break (288 vs 285 %), ethylene degradation rate (92.5 vs 92.2 %), and UV transmittance (2.1 vs 2.2 %). This indicates that the effect of nano zinc oxide on these properties in the formulation system of the present application can be ignored.

[0106] (2) Comprehensive preservation performance: Although the above properties are similar, in the strawberry preservation experiment, the rot rate of Example 2 (11.1%) is only one third of that of Comparative Example 4 (33.3%). This is a significant difference.

[0107] Reasoning: After excluding the effects of mechanics, ethylene degradation, and UV shielding, the only variable that causes the difference in rot rate is nano zinc oxide. In addition to its own maturation and aging, the main cause of strawberry rot is also the invasion of microorganisms (such as mold). Therefore, it can be reasonably inferred that nano zinc oxide mainly plays an antibacterial and bacteriostatic role in the present application, effectively inhibiting the growth of microorganisms in the package, thereby significantly reducing the rot rate of strawberries.

[0108] Conclusion: Comparing Example 2 and Comparative Example 4 proves that the addition of nano zinc oxide provides a third preservation function for the film in addition to photocatalytic degradation of ethylene and UV shielding-antibacterial property. The synergistic effect of these three functions achieves excellent comprehensive preservation effect on strawberries.

[0109] In summary, by comparing and analyzing the test results in Table 1 with the technical solutions of each example and comparative example, the following conclusions can be drawn:

[0110] (1) The present application fundamentally solves the problems of filler agglomeration, poor compatibility and low catalytic efficiency in the traditional physical blending method by preparing a ternary hybrid of "lignin-graft-chitosan-anchor-titanium dioxide" in advance, which is the basis for obtaining excellent mechanical properties and functionality.

[0111] (2) The introduction of lignin is the key to realizing the synergy of "surface photocatalysis" and "bulk ultraviolet shielding" functions, effectively solving the internal contradictions of the prior art, which is crucial to improving the actual preservation effect.

[0112] (3) By chemically anchoring the nano-titanium dioxide to the polymer carrier, the photocatalyst can be effectively and stably used, and the mechanical integrity of the material is maintained.

[0113] (4) The compounding of nano-zinc oxide gives the film additional antibacterial function, which is complementary to the photocatalysis and ultraviolet shielding functions, realizing triple preservation effect.

[0114] It should be noted that in this paper, terms such as "include, contain" or any other variants are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or includes elements inherent to such process, method, article or device.

[0115] Although embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to the embodiments without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a biodegradable photocatalytic thin film, characterized in that, Includes the following steps: S1: Chitosan is dissolved in a 1% (v / v) aqueous acetic acid solution, then alkali lignin is added, and nitrogen gas is passed through for protection. The system is heated to 50-65℃, and then cerium ammonium nitrate-nitric acid solution is added. The reaction is carried out at 50-65℃ for 4-6 hours. After the reaction is completed, the pH of the system is adjusted to 8.5-9.0 with dilute NaOH solution. The precipitate is collected by centrifugation and washed with deionized water and ethanol alternately until it is neutral. Then it is dried under vacuum to obtain the lignin-chitosan graft copolymer. S2: Disperse nano-titanium dioxide in anhydrous ethanol, add KH-550, stir and reflux at 80-85℃ for 4-6 hours. After the reaction, collect the solid by centrifugation, wash it three times with anhydrous ethanol, and then dry it under vacuum to obtain modified nano-titanium dioxide with surface grafted amino. S3: The lignin-chitosan graft copolymer was dissolved in a 1% (v / v) aqueous acetic acid solution, and modified nano-titanium dioxide with surface-grafted amino groups was added. The mixture was ultrasonically dispersed for 20-30 minutes, and the pH of the system was adjusted to 5-6 with 2M NaOH solution. Then, a 2.5% (v / v) aqueous glutaraldehyde solution was added dropwise with stirring. The reaction was carried out at 50-60℃ for 3-4 hours. After the reaction was completed, the pH was adjusted to neutral, and the product was collected by centrifugation. The product was washed and freeze-dried to obtain a ternary hybrid. S4: Mix polylactic acid, ternary hybrid, nano zinc oxide and acetylated tributyl citrate for 10-20 minutes, then melt blend, extrude, cool and pelletize to obtain composite material particles; S5: The composite material particles are vacuum dried at 40-45℃ for 12 hours, and then cast into a film to obtain a biodegradable photocatalytic thin film.

2. The method for preparing a biodegradable photocatalytic thin film according to claim 1, characterized in that, The ratio of chitosan, aqueous acetic acid solution, alkali lignin, and cerium ammonium nitrate-nitric acid solution in S1 is 30g:2L:10g:50mL.

3. The method for preparing a biodegradable photocatalytic thin film according to claim 1, characterized in that, The ratio of nano-titanium dioxide, anhydrous ethanol, and KH-550 in S2 is 20g:250-300mL:2.4-3.0g.

4. The method for preparing a biodegradable photocatalytic thin film according to claim 1, characterized in that, The ratio of the lignin-chitosan graft copolymer, acetic acid aqueous solution, surface-grafted amino-modified nano-titanium dioxide, and glutaraldehyde aqueous solution in S3 is 15g:750mL:12g:15mL.

5. The method for preparing a biodegradable photocatalytic thin film according to claim 1, characterized in that, The composite material particles described in S4, by mass, include the following raw materials: 100 parts by weight of polylactic acid, 14-20 parts by weight of ternary hybrid, 3-4 parts by weight of nano zinc oxide, and 10-12 parts by weight of acetylated tributyl citrate.

6. The method for preparing a biodegradable photocatalytic thin film according to claim 1, characterized in that, The thickness of the biodegradable photocatalytic film is 35–45 μm.