A transparent UV-blocking composite film, its preparation method and application

Abstract

This invention belongs to the field of thin film materials technology, specifically relating to a transparent UV-blocking composite film, its preparation method, and its application. The transparent UV-blocking composite film provided by this invention comprises acetylated lignin, halloysite nanomaterials, chitosan, and polyvinyl alcohol as raw materials; the acetylated lignin is loaded onto halloysite nanomaterials. This invention's specific transparent UV-blocking composite film ensures high transparency while maintaining a UV light transmittance of less than 5% in the 200-400nm wavelength range, and a visible light transmittance of over 60% in the 400-780nm wavelength range. It also possesses high mechanical properties, and exhibits good moisture and oxygen barrier properties, good biocompatibility, and biodegradability.

Description

Technical Field

[0001] This invention belongs to the field of thin film materials technology, specifically relating to a transparent UV-blocking composite film, its preparation method, and its application. Background Technology

[0002] With changing modern lifestyles, consumers increasingly demand transparency in product information and packaging. Packaging, as the most convenient physical method to slow down the oxidation and spoilage of materials, has been widely adopted. Ultraviolet radiation can be divided into three bands based on its wavelength: UVA (320-400nm), UVB (280-320nm), and UVC (200-280nm). UVA and UVB bands have strong penetrating power, harming not only humans and other organisms but also accelerating the oxidation rate of food and the aging, degradation, and fading of everyday materials. Therefore, as people gain a deeper understanding of the dangers of ultraviolet radiation, UV-resistant packaging materials have become a research hotspot. Transparent UV-resistant packaging films are packaging films composed of different types of polymers, designed for various applications. By using this type of packaging, while shielding against ultraviolet rays, moisture, and oxygen, food manufacturers can better showcase the freshness of their products to customers, thus increasing sales. There are many types of common barrier packaging. For example, aluminum foil packaging has good barrier properties, but poor extensibility, which makes it easy to form pinholes and reduce its barrier performance. Opaque packaging such as kraft paper makes it impossible to visually evaluate its quality. Ordinary plastic packaging has good air barrier properties and is transparent, but it comes from non-renewable petrochemical resources, is non-degradable, and has poor UV protection. Under long-term exposure to ultraviolet light, the contents of the packaging bag absorb the energy of ultraviolet light, triggering an auto-oxidation reaction and accelerating the oxidation rate.

[0003] Lignin-based composite materials, using lignin as a biodegradable filler, not only reduce the production cost of polymer materials but also improve the composite material's hydrophobicity, antioxidant properties, antibacterial properties, UV shielding (UV shielding includes absorbing and reflecting / scattering UV rays to reduce UV transmittance; lignin's physical structure provides UV shielding through reflection and scattering, while phenolic hydroxyl and ketone carbonyl functional groups in lignin absorb UV rays, working together to give lignin good UV resistance), and water vapor barrier properties. Chinese patent CN114474449B discloses a lignin / PBAT composite film material, its preparation method, and its applications. It uses biodegradable lignin filler to reduce PBAT production costs and improve the tensile properties of the composite material. However, due to the high-temperature blending and hot-pressing process used in this method, the condensation phenomenon during hot processing leads to lignin aggregation, resulting in a darker film color and poor appearance.

[0004] Although lignin-based materials have been extensively researched and applied, the aforementioned films still have shortcomings. Single-component natural lignin, with its complex and diverse functional groups, possesses excellent full-spectrum UV shielding properties, but lacks film-forming properties, exhibits poor barrier performance, and its high content of phenolic hydroxyl groups results in a darker color, significantly limiting its application in food packaging. While adding lignin can reduce polymer material costs and provide high UV shielding performance, it still cannot guarantee a visible light transmittance of over 60% (high transparency) in the 400-780nm range while maintaining a UV transmittance of less than 5% in the 200-400nm wavelength range, and simultaneously possessing high mechanical properties. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the shortcomings of existing lignin-added UV-protective composite films, which cannot guarantee that the transmittance of ultraviolet light in the wavelength range of 200-400nm is less than 5%, while achieving a visible light transmittance of more than 60% in the wavelength range of 400-780nm (high transparency) and high mechanical properties. Thus, the present invention provides a transparent UV-protective composite film, its preparation method and application.

[0006] This invention provides a transparent UV-blocking composite film, the raw materials of which include acetylated lignin, halloysite nanomaterials, chitosan and polyvinyl alcohol;

[0007] The acetylated lignin is loaded onto halloysite nanomaterials.

[0008] This invention provides a method for preparing a transparent, UV-blocking composite film, comprising the following preparation steps:

[0009] 1) Preparation of acetylated lignin;

[0010] 2) The acetylated lignin prepared in step 1) is loaded onto halloysite nanomaterials to obtain halloysite nanocomposites;

[0011] 3) The halloysite nanocomposite prepared in step 2) is mixed with chitosan solution and polyvinyl alcohol solution. After removing air bubbles from the solution, a film-forming treatment is performed to obtain the transparent UV-protective composite film.

[0012] Preferably, step 2) includes: mixing halloysite nanomaterials, acetylated lignin and solvent, and then separating them to obtain halloysite nanocomposite.

[0013] Preferably, in step 2), halloysite nanomaterials and water are first mixed to form solution A, then the acetylated lignin prepared in step 1) is dissolved in dioxane aqueous solution to form solution B, and then solution B is added to solution A and mixed. After ultrasonic treatment, the solid is separated to obtain halloysite nanocomposite.

[0014] Optionally, a drying step may be included after the solids separation step;

[0015] Optionally, an acidification step may be included before performing step 2);

[0016] Optionally, the halloysite nanomaterials may be further subjected to ultrasonic pretreatment in an aqueous hydrochloric acid solution before being mixed with water.

[0017] Optionally, when the halloysite nanomaterial is ultrasonically pretreated in hydrochloric acid aqueous solution, the ratio of halloysite nanomaterial to hydrochloric acid aqueous solution is (1-5):(400-500), in g:mL;

[0018] Optionally, the mass concentration of hydrochloric acid in the hydrochloric acid aqueous solution is 0.01-0.5 mol / L.

[0019] Optionally, the halloysite nanomaterials include, but are not limited to, different forms of natural or synthetic halloysite, such as sheet-like, block-like, or tubular forms; optionally, the halloysite nanomaterials are halloysite nanotubes.

[0020] Optionally, after ultrasonic pretreatment of halloysite nanomaterials in hydrochloric acid aqueous solution, a drying step may also be included;

[0021] Optionally, after ultrasonic pretreatment in hydrochloric acid aqueous solution, halloysite nanomaterials are dried at 25-100℃ to remove moisture.

[0022] Optionally, the ultrasonic power for ultrasonic pretreatment of halloysite nanomaterials in hydrochloric acid aqueous solution is 25-300W, and the ultrasonic pretreatment time is 0.5-2h.

[0023] Preferably, the ratio of halloysite nanomaterials to water in step 2) is 1:(10-100), in g:mL;

[0024] The ratio of the acetylated lignin to the dioxane aqueous solution is 1:(10-100), in g:mL;

[0025] In step 2), the volume ratio of dioxane to water in the dioxane aqueous solution is (1-9):(1-5).

[0026] The mass ratio of acetylated lignin in solution B to halloysite nanomaterials in solution A is (1-5):(1-5);

[0027] The ultrasonic power for the ultrasonic treatment in step 2) is 25-300W, and the ultrasonic treatment time is 0.5-2h.

[0028] The stirring speed for adding solution B to solution A is 50-600 r / min, and the mixing time is 0.5-24 h.

[0029] Preferably, in step 3), the halloysite nanocomposite obtained in step 2) is mixed with dioxane aqueous solution, homogenized, and then mixed with chitosan solution, polyvinyl alcohol solution and glycerol. After ultrasonic treatment to remove air bubbles in the solution, a film-forming treatment is performed to obtain the transparent UV-protective composite film.

[0030] The homogenization in this invention is to achieve better dispersion. In addition to dispersion, ultrasound also removes air bubbles from the solution, which is beneficial for film formation.

[0031] Preferably, the ratio of halloysite nanocomposite to dioxane aqueous solution in step 3) is 1:(40-60), in g:mL;

[0032] In step 3), the volume ratio of dioxane to water in the dioxane aqueous solution is (1-5):(1-5).

[0033] The homogenization process takes 5-10 minutes and the homogenization speed is 6000-24000 r / min.

[0034] Optionally, the homogenization process is carried out at a temperature of 10-55°C, for example, at room temperature.

[0035] Optionally, in step 3), the ultrasonic power for removing bubbles from the solution by ultrasonic treatment is 25-300W, and the ultrasonic treatment time is 0.5-2h.

[0036] The ratio of halloysite nanocomposite to chitosan solution is 1:(13-17), in g:mL;

[0037] The volume ratio of the chitosan solution, polyvinyl alcohol solution, and glycerol is (13-17):(28-32):(0.1-0.3).

[0038] Optionally, halloysite nanocomposites are mixed with dioxane aqueous solution by ultrasonication at a power of 200-300W for a duration of 0.5-1.5h.

[0039] Optionally, after homogenization, the mixture is stirred and mixed with chitosan solution, polyvinyl alcohol solution and glycerin at a stirring speed of 400-700 r / min, a stirring time of 10-14 h and a stirring temperature of 10-45℃.

[0040] Optional, mix by stirring at room temperature.

[0041] The chitosan solution contains 1-5% chitosan by mass.

[0042] The solvent in the chitosan solution is an aqueous acetic acid solution; the mass concentration of acetic acid in the aqueous acetic acid solution is 1-2%.

[0043] The polyvinyl alcohol content in the polyvinyl alcohol solution is 2-6% by mass;

[0044] The polyvinyl alcohol solution is an aqueous solution of polyvinyl alcohol;

[0045] Optionally, the film-forming process includes casting and drying.

[0046] The present invention does not specifically limit the film-forming process, including but not limited to hot pressing and melting, casting, etc. For example, the film-forming process may include casting and drying.

[0047] Preferably, in step 1), lignin is added to a propionic anhydride / pyridine solution and stirred to react. After the reaction is complete, water is added and the solid is separated to obtain acetylated lignin.

[0048] Preferably, the ratio of lignin to propionic anhydride / pyridine solution is 1:(11-15), in g:mL;

[0049] The volume ratio of propionic anhydride to pyridine in the propionic anhydride / pyridine solution is (2-5):10;

[0050] The stirring reaction rate is 400-600 r / min, and the stirring reaction time is 20-26 h;

[0051] Optionally, the volume ratio of water to propionic anhydride / pyridine solution is (200-500):(11-15).

[0052] The lignin includes, but is not limited to, naturally or synthetically obtained alkaline lignin, dealkalized lignin, modified lignin, or lignin salts, any of which can be commercially available. (Lignin salts include, for example, sodium lignin and lignin sulfonates, typically not limited to, calcium lignin sulfonate, sodium lignin sulfonate, etc.).

[0053] Optionally, after the solid separation step is completed, the method further includes washing and drying the solid with water;

[0054] Optionally, the water washing is performed 2-4 times;

[0055] Optionally, in the water washing and drying steps, the drying temperature is 50-80℃ and the drying time is 10-30h.

[0056] This invention provides a transparent UV-blocking composite film, which is prepared by the above-described method for preparing a transparent UV-blocking composite film.

[0057] This invention provides an application of the above-described transparent UV-blocking composite film or the transparent UV-blocking composite film prepared by the above-described preparation method in UV-blocking materials. For example, UV-blocking film materials.

[0058] This invention provides an application of the above-described transparent UV-blocking composite film or the transparent UV-blocking composite film prepared by the above-described preparation method in food packaging materials.

[0059] The technical solution of this invention has the following advantages:

[0060] The present invention provides a transparent UV-blocking composite film, the raw materials of which include acetylated lignin, halloysite nanomaterials, chitosan, and polyvinyl alcohol; the acetylated lignin is loaded onto halloysite nanomaterials. Acetylated lignin not only reduces the color of the original dealkalized lignin substrate, thereby significantly improving the transparency of the composite film; simultaneously, the grafting of acetyl groups increases the hydrophobicity and antioxidant properties of the film substrate. Furthermore, the presence of acetyl groups induces n→π* transitions, which, although weakening the absorption of ultraviolet light in the UVC band, enhances the absorption of ultraviolet light in the 320-400nm band. After ultraviolet light is absorbed by the film substrate, the transmittance of ultraviolet light is reduced, thus improving the ultraviolet shielding performance. Halloysite nanomaterials possess characteristics such as large specific surface area, good adsorption capacity, abundant hydroxyl groups, negatively charged outer surface, and inherent UV absorption and reflection properties. By loading acetylated lignin onto halloysite nanomaterials and combining it with positively charged chitosan, electrostatic interactions promote molecular redirection, increasing intermolecular order. Loading acetylated lignin effectively solves the problem of its tendency to aggregate, increasing compatibility and eliminating large lignin aggregates, further improving the transparency of the composite film. Simultaneously, the abundant hydroxyl groups in polyvinyl alcohol can form a strong hydrogen bond network with chitosan and halloysite nanomaterials, increasing intermolecular interactions. The resulting composite film structure is denser and more uniform, with a more significant UV shielding effect, and it also reduces lignin molecule aggregation, improving the mechanical properties of the composite film. Furthermore, the excellent short-wavelength UV absorption performance of halloysite nanomaterials synergistically enhances the short-wavelength UV absorption of the composite film. The transparent UV-blocking composite film of this invention exhibits high shielding properties for light with wavelengths in the 200nm-400nm range and high transmittance for visible light (wavelength). Overall, the specific transparent UV-blocking composite film of this invention ensures high transparency while maintaining a UV light transmittance of less than 5% in the 200-400nm wavelength range, and a visible light transmittance of over 60% in the 400-780nm wavelength range. It also possesses high mechanical properties, and exhibits good moisture and oxygen barrier properties, good biocompatibility, and biodegradability.

[0061] 2. The transparent UV-protective composite film obtained by the specific preparation method of this invention is biocompatible. It is not easily damaged when used as packaging material and will not react with the contents of the packaging. It is safe and environmentally friendly. Therefore, this transparent UV-protective composite film can be used as food packaging material that can be kept away from food or in contact with food. Attached Figure Description

[0062] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0063] Figure 1 SEM images of dealkalized lignin and acetylated lignin prepared in Example 1 of this invention;

[0064] Figure 2 Comparison of Fourier transform infrared spectroscopy (FT-IR) analysis of the composite films prepared in Example 1 and Comparative Examples 1-3 of this invention;

[0065] Figure 3 Comparison of the UV resistance properties of the composite films prepared in Example 1 and Comparative Examples 1-4 of this invention;

[0066] Figure 4 Images of the composite membrane prepared in Example 1 of this invention under bending test in different directions;

[0067] Figure 5 Comparison of tensile strength and elongation of the composite membranes prepared in Example 1 and Comparative Examples 1-3 of this invention;

[0068] Figure 6 Comparison of biodegradability experiments of the composite membranes prepared in Example 1 and Comparative Example 1 of this invention;

[0069] Figure 7 A comparative diagram of the biosafety experiments of the composite membranes prepared in Example 1 and Comparative Example 1 of this invention;

[0070] Figure 8 Comparison of the actual colors of the composite films obtained in Examples 1-3 and Comparative Examples 1-4 of this invention. Detailed Implementation

[0071] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0072] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0073] The halloysite nanotubes of the present invention are available commercially available. The halloysite nanotubes used in the embodiments and comparative examples of the present invention are from Guangzhou Runwo Materials Technology Co., Ltd., with item number 332-58-7.

[0074] The dealkalized lignin samples used in the embodiments and comparative examples of this invention were purchased from Maclean's, CAS: 9005-53-2.

[0075] Example 1

[0076] This embodiment provides a method for preparing a transparent UV-blocking composite film, comprising the following steps:

[0077] 1) Take 1g of dealkalized lignin sample dried to constant weight and add it to a 50mL beaker. Then add 13mL of propionic anhydride / pyridine solution (the volume ratio of propionic anhydride to pyridine is 3:10). Stir the mixture at 400r / min for 24 hours at room temperature. After the reaction is complete, add 400mL of distilled water and obtain the solid residue by centrifugation. Wash the solid residue three times with distilled water and vacuum dry it at 60℃ for 24 hours to obtain acetylated lignin. Prepare 1g of acetylated lignin according to this method for later use.

[0078] 2) Take 3g of halloysite nanotubes into a 500mL beaker, add 8mL of 11.9mol / L hydrochloric acid aqueous solution and 450mL of water, and sonicate at 150W for 30min to achieve pore expansion, impurity removal and reduction of agglomeration. Centrifuge the sonicated halloysite nanotubes and dry them at 80℃ for later use. Take 1g of acetylated lignin prepared in step 1), add 100mL of dioxane aqueous solution (dioxane to water volume ratio is 4:1), and sonicate at 150W for 1h to form solution B. Take 1g of the dried sonicated halloysite nanotubes and mix them with 100mL of water to form mixed solution A. Under magnetic stirring at 400r / min, add solution B to solution A and stir magnetically for 5h. After sonicating at 150W for 30min, centrifuge to separate the solid, remove unadsorbed lignin, and dry at 60℃ for 12h to obtain halloysite nanocomposite.

[0079] 3) Take 1g of the halloysite nanocomposite prepared in step 2), add 50mL of dioxane aqueous solution (dioxane to water volume ratio of 4:1), and sonicate at 150W for 1h. Then homogenize at 12000r / min for 5min at room temperature. After homogenization, add 15mL of chitosan solution, 30mL of polyvinyl alcohol aqueous solution, and 0.2mL of glycerol. Stir magnetically at 400r / min for 12h at room temperature until fully mixed. Then sonicate at 150W for 30min to remove air bubbles from the solution. Then cast into a 10cm×10cm mold and dry at 60℃ for 24h to form a film, thus obtaining the transparent UV-protective composite film. The chitosan solution was prepared by dissolving 3g of chitosan in 100mL of 1% acetic acid aqueous solution. The polyvinyl alcohol aqueous solution was prepared by dissolving 4g of polyvinyl alcohol in 100mL of water.

[0080] Example 2

[0081] This embodiment provides a method for preparing a transparent UV-blocking composite film, comprising the following steps:

[0082] 1) Take 1g of alkali-free lignin sample dried to constant weight and add it to a 50mL beaker. Then add 11mL of propionic anhydride / pyridine solution (the volume ratio of propionic anhydride to pyridine is 5:10). Stir the mixture at 600r / min for 20 hours at room temperature. After the reaction is complete, add 500mL of distilled water and obtain the solid residue by centrifugation. Wash the solid residue three times with distilled water and vacuum dry it at 50℃ for 30 hours to obtain acetylated lignin. Prepare 2g of acetylated lignin according to this method for later use.

[0083] 2) Take 3g of halloysite nanotubes into a 500mL beaker, add 8mL of 11.9mol / L hydrochloric acid aqueous solution and 450mL of water, and sonicate at 300W for 30min to achieve pore expansion, impurity removal and reduction of agglomeration. Centrifuge the sonicated halloysite nanotubes and dry them at 80℃ for later use. Take 2g of acetylated lignin prepared in step 1), add 160mL of dioxane aqueous solution (dioxane to water volume ratio is 4:1), and sonicate at 150W for 1h to form solution B. Take 1g of the dried sonicated halloysite nanotubes and mix them with 80mL of water to form mixed solution A. Under magnetic stirring at 600r / min, add solution B to solution A and stir magnetically for 1h to mix. After sonication at 300W for 1h, centrifuge to separate the solid, remove unadsorbed lignin, and dry at 60℃ for 12h to obtain halloysite nanocomposite.

[0084] 3) Take 1g of the halloysite nanocomposite prepared in step 2), add 60mL of dioxane aqueous solution (dioxane to water volume ratio of 4:1), and sonicate at 300W for 1.5h. Then homogenize at 24000r / min for 5min at room temperature. After homogenization, add 17mL of chitosan solution, 28mL of polyvinyl alcohol aqueous solution, and 0.2mL of glycerol. Stir magnetically at 700r / min for 10h at room temperature until fully mixed. Then sonicate at 25W for 2h to remove air bubbles from the solution. Then cast into a 10cm×10cm mold and dry at 60℃ for 24h to form a film, thus obtaining the transparent UV-protective composite film. The chitosan solution was prepared by dissolving 3g of chitosan in 100mL of 1% acetic acid aqueous solution. The polyvinyl alcohol aqueous solution was prepared by dissolving 4g of polyvinyl alcohol in 100mL of water.

[0085] Example 3

[0086] This embodiment provides a method for preparing a transparent UV-blocking composite film, comprising the following steps:

[0087] 1) Take 1g of dealkalized lignin sample dried to constant weight and add it to a 50mL beaker. Then add 15mL of propionic anhydride / pyridine solution (the volume ratio of propionic anhydride to pyridine is 2:10). Stir the mixture at 500r / min for 26 hours at room temperature. After the reaction is complete, add 200mL of distilled water and obtain the solid residue by centrifugation. Wash the solid residue three times with distilled water and vacuum dry it at 80℃ for 10 hours to obtain acetylated lignin. Prepare 3g of acetylated lignin according to this method for later use.

[0088] 2) Take 3g of halloysite nanotubes into a 500mL beaker, add 8mL of 11.9mol / L hydrochloric acid aqueous solution and 450mL of water, and sonicate at 150W for 30min to achieve pore expansion, impurity removal and reduction of agglomeration. Centrifuge the sonicated halloysite nanotubes and dry them at 80℃ for later use. Take 3g of acetylated lignin prepared in step 1), add 150mL of dioxane aqueous solution (dioxane to water volume ratio is 4:1), and sonicate at 150W for 1h to form solution B. Take 1g of the dried sonicated halloysite nanotubes and mix them with 100mL of water to form mixed solution A. Under magnetic stirring at 50r / min, add solution B to solution A and stir magnetically for 24h to mix. After sonication at 25W for 2h, centrifuge to separate the solid, remove unadsorbed lignin, and dry at 60℃ for 12h to obtain halloysite nanocomposite.

[0089] 3) Take 1g of the halloysite nanocomposite prepared in step 2), add 40mL of dioxane aqueous solution (dioxane to water volume ratio of 4:1), and sonicate at 200W for 0.5h. Then homogenize at 6000r / min for 10min at room temperature. After homogenization, add 13mL of chitosan solution, 32mL of polyvinyl alcohol aqueous solution, and 0.3mL of glycerol. Stir magnetically at 500r / min for 14h at room temperature until fully mixed. Then sonicate at 300W for 1h to remove air bubbles from the solution. Then cast into a 10cm×10cm mold and dry at 60℃ for 24h to form a film, thus obtaining the transparent UV-protective composite film. The chitosan solution was prepared by dissolving 3g of chitosan in 100mL of 1% acetic acid aqueous solution. The polyvinyl alcohol aqueous solution was prepared by dissolving 4g of polyvinyl alcohol in 100mL of water.

[0090] Comparative Example 1

[0091] This comparative example provides a method for preparing a transparent UV-blocking composite film, comprising the following steps:

[0092] Take 15 mL of chitosan solution, 30 mL of polyvinyl alcohol aqueous solution, and 0.2 mL of glycerol. Stir magnetically at 400 rpm for 12 hours at room temperature until thoroughly mixed. Then, sonicate at 150 W for 30 minutes to remove air bubbles. Cast the mixture into a 10 cm × 10 cm mold and dry at 60 °C for 24 hours to form a film, thus obtaining the transparent, UV-protective composite film. The chitosan solution was prepared by dissolving 3 g of chitosan in 100 mL of 1% acetic acid aqueous solution. The polyvinyl alcohol aqueous solution was prepared by dissolving 4 g of polyvinyl alcohol in 100 mL of water.

[0093] Comparative Example 2

[0094] This comparative example provides a method for preparing a composite membrane, comprising the following steps:

[0095] Take 1g of dealkalized lignin and add 50mL of dioxane aqueous solution (dioxane to water volume ratio 4:1). Mix ultrasonically at 150W for 1h, then homogenize at 12000r / min for 5min at room temperature. After homogenization, add 15mL of chitosan solution, 30mL of polyvinyl alcohol aqueous solution, and 0.2mL of glycerol. Stir magnetically at 400r / min for 12h at room temperature until fully mixed. Then, ultrasonically treat at 150W for 30min to remove air bubbles. Cast into a 10cm×10cm mold and dry at 60℃ for 24h to form a film, thus obtaining the composite membrane. The chitosan solution was prepared by dissolving 3g of chitosan in 100mL of 1% acetic acid aqueous solution. The polyvinyl alcohol aqueous solution was prepared by dissolving 4g of polyvinyl alcohol in 100mL of water.

[0096] Comparative Example 3

[0097] This comparative example provides a method for preparing a composite membrane, comprising the following steps:

[0098] 1) Take 1g of dealkalized lignin sample dried to constant weight and add it to a 50mL beaker. Then add 13mL of propionic anhydride / pyridine solution (the volume ratio of propionic anhydride to pyridine is 3:10). Stir the mixture at 400r / min for 24 hours at room temperature. After the reaction is complete, add 400mL of distilled water and obtain the solid residue by centrifugation. Wash the solid residue three times with distilled water and vacuum dry it at 60℃ for 24 hours to obtain acetylated lignin. Prepare 1g of acetylated lignin according to this method for later use.

[0099] 2) Take 1g of the acetylated lignin prepared in step 1), add 50mL of dioxane aqueous solution (dioxane to water volume ratio is 4:1), and sonicate at 150W for 1h. Then homogenize at 12000r / min for 5min at room temperature. After homogenization, add 15mL of chitosan solution, 30mL of polyvinyl alcohol aqueous solution, and 0.2mL of glycerol. Stir magnetically at 400r / min for 12h at room temperature until fully mixed. Then sonicate at 150W for 30min to remove air bubbles from the solution. Then cast into a 10cm×10cm mold and dry at 60℃ for 24h to form a film, thus obtaining the composite film. The chitosan solution was prepared by dissolving 3g of chitosan in 100mL of 1% acetic acid aqueous solution. The polyvinyl alcohol aqueous solution was prepared by dissolving 4g of polyvinyl alcohol in 100mL of water.

[0100] Comparative Example 4

[0101] This comparative example provides a method for preparing a composite membrane, comprising the following steps:

[0102] 1) Take 3g of halloysite nanotubes into a 500mL beaker, add 8mL of 11.9mol / L hydrochloric acid aqueous solution and 450mL of water, and sonicate at 150W for 30min to achieve pore expansion, impurity removal and reduction of agglomeration. Centrifuge the sonicated halloysite nanotubes and dry them at 80℃ for later use. Take 1g of the dried sonicated halloysite nanotubes and mix them with 100mL of water to form mixed solution A. Stir at 400r / min under magnetic stirring for 5h. After sonication at 150W for 30min, centrifuge to separate the solid and dry at 60℃ for 12h to obtain halloysite nano-treated material.

[0103] 3) Take 1g of the halloysite nanoparticles prepared in step 1), add 50mL of dioxane aqueous solution (dioxane to water volume ratio of 4:1), and sonicate at 150W for 1h. Then homogenize at 12000r / min for 5min at room temperature. After homogenization, add 15mL of chitosan solution, 30mL of polyvinyl alcohol aqueous solution, and 0.2mL of glycerol. Stir magnetically at 400r / min for 12h at room temperature until fully mixed. Then sonicate at 150W for 30min to remove air bubbles from the solution. Then cast into a 10cm×10cm mold and dry at 60℃ for 24h to form a film, thus obtaining the composite film. The chitosan solution was prepared by dissolving 3g of chitosan in 100mL of 1% acetic acid aqueous solution. The polyvinyl alcohol aqueous solution was prepared by dissolving 4g of polyvinyl alcohol in 100mL of water.

[0104] Test Example 1

[0105] The microstructures of dealkalized lignin and the acetylated lignin prepared in Example 1 were studied using scanning electron microscopy (SEM), and the results are as follows: Figure 1 As shown. Figure 1 Figure a in the image is a SEM image of dealkalized lignin. Figure 1 Figure b in the image is an SEM image of the acetylated lignin prepared in Example 1, which shows that the surface of the acetylated lignin becomes smooth and microspheres appear.

[0106] Test Example 2

[0107] The composite films obtained in Examples 1-3 and Comparative Examples 1-4 were tested. The transparent UV-blocking composite film obtained in Example 1 was designated PC-HALG, the transparent UV-blocking composite film obtained in Example 2 was designated PC-2HALG, the transparent UV-blocking composite film obtained in Example 3 was designated PC-3HALG, the composite film obtained in Comparative Example 1 was designated PC, the composite film obtained in Comparative Example 2 was designated PC-LG, the composite film obtained in Comparative Example 3 was designated PC-ALG, and the composite film obtained in Comparative Example 4 was designated PC-HNTS. Specific tests are as follows:

[0108] 1. Fourier Transform Infrared Spectroscopy (FT-IR) Analysis of Composite Membrane

[0109] The structure of the composite films prepared in Example 1 and Comparative Examples 1-3 was analyzed using Fourier Transform Infrared Spectroscopy (FT-IR), and the results are as follows: Figure 2 As shown. 3419cm -1 The broad peak at 3438 cm⁻¹ corresponds to the hydroxyl groups in phenolic and aliphatic groups, and the PC-HALG spectrum shows a peak at 3438 cm⁻¹. -1 The peak at 2943 cm⁻¹ is a low-intensity peak resulting from the acetylation reaction between lignin and the hydroxyl groups. -1 The peak at 1265 cm⁻¹ is attributed to the CH skeletal vibrations in the -CH₂ and -CH₃ groups, while the CH transformation associated with aromatic ring vibrations appears at 1265 cm⁻¹. -1 1423cm -1 and 1577cm -1 Acetyl groups were also observed at 1737 cm⁻¹. -1 (Aliphatic C=O stretching of acetyl groups) is attributed to the acetylation reaction that produces acetyl groups, which broadens the peak shape. Acetyl groups not only reduce the hydroxyl content in lignin chains, but also break the hydrogen bonds between lignin chains, thereby increasing the active space of lignin molecular chains and reducing the aggregation of lignin molecules.

[0110] 2. Study on the UV resistance of composite films

[0111] The composite films prepared in Examples 1-3 and Comparative Examples 1-4 were tested using a UV-Vis spectrophotometer. The transmittance of ultraviolet light in the 200nm-400nm range and the transmittance of visible light in the 400-780nm range are shown in Table 1. The UV-Vis spectra of the composite films prepared in Examples 1 and Comparative Examples 1-4 are compared below. Figure 3 As shown.

[0112] Table 1

[0113]

[0114] As shown in Table 1, the composite film obtained in Example 1 has a transmittance of only 2.90% in the 200nm-400nm range, which is less than 5%. This means that the composite film obtained in Example 1 has a shielding performance exceeding 95% in the ultraviolet band, demonstrating excellent UV resistance. The composite film of Example 1 has a transmittance of 78.39% in the visible light range of 400nm-780nm, exhibiting high transparency. The composite film of Comparative Example 2 has an ultraviolet transmittance of less than 5% in the 200-400nm range, but its visible light transmittance is low, resulting in poor transparency. The composite films of Comparative Examples 1 and 4 have visible light transmittance exceeding 60%, but their ultraviolet transmittance in the 200-400nm range is much higher than 5%. The composite film of Comparative Example 3 has an ultraviolet transmittance of 13.92% in the 200-400nm range, failing to meet the requirement of less than 5%, and its visible light transmittance is also less than 60%. That is, the composite film obtained in the embodiments of this application has a transmittance of less than 5% for ultraviolet light in the wavelength range of 200-400nm, and a transmittance of more than 60% for visible light in the wavelength range of 400-780nm (high transparency).

[0115] 3. Bending test of composite membrane in different directions

[0116] The transparent UV-blocking composite film prepared in Example 1 was bent in different directions, as shown in the following figures. Figure 4 As shown, the transparent UV-blocking composite film prepared in Example 1 exhibits good flexibility in all directions, and it completely maintains its original shape after the bending stress is released.

[0117] 4. Tensile strength and elongation tests

[0118] The composite films prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to tensile strength and elongation tests. The mechanical properties of the film samples were tested using an intelligent electronic tensile testing machine to determine the tensile strength (TS) and elongation at break (EAB). Before measurement, the films were cut into rectangular sheets of 70 mm × 10 mm. The initial distance was set to 25 mm, and the measurement speed was set to 50 mm / min. Five sets of samples were prepared for each test, and the tests were repeated to calculate the average value. The thickness of the films was measured using a handheld electronic digital micrometer, and the average value was taken, in mm. TS and EAB were calculated using the following formulas:

[0119]

[0120] In the formula, F represents the tension at membrane rupture / N; b is the membrane width / mm; d is the membrane thickness / mm; L is the length of the membrane at rupture / mm; and L0 is the initial length of the membrane / mm.

[0121] The test results are shown in Table 2, where the tensile strength and elongation of the composite films prepared in Example 1 and Comparative Examples 1-3 are compared. Figure 5 As shown.

[0122] Table 2

[0123] Tensile strength (mean) Elongation (mean) Example 1 75.84 104.23 Example 2 70.32 93.16 Example 3 67.58 90.44 Comparative Example 1 64.49 86.23 Comparative Example 2 37.51 58.65 Comparative Example 3 57.32 73.15 Comparative Example 4 65.23 89.24

[0124] When used as packaging, membrane materials should possess sufficient mechanical strength and ductility. As shown in Table 2, the composite membrane prepared by adding lignin in Comparative Example 2, compared to the composite membrane in Comparative Example 1, exhibits poorer mechanical properties due to the self-aggregation of lignin molecules, with a tensile strength of 37.51 MPa. The composite membrane obtained in Example 1 by adding modified acetylated lignin and halloysite nanotubes has a tensile strength of 75.84 MPa, which is 102% higher than the tensile strength of the composite membrane in Comparative Example 2 (P<0.05). Simultaneously, the elongation at break of the composite membrane in Example 1 also increased from 58.65% to 104.23% compared to the composite membrane in Comparative Example 2. This may be due to the negative charge on the outer surface of the halloysite nanofiller, which electrostatically interacts with the positively charged chitosan, increasing molecular order. Simultaneously, mixing with polyvinyl alcohol containing a large number of hydroxyl groups increases intermolecular interactions, forming a strong hydrogen bond network structure and reducing the aggregation of lignin molecules. This indicates that the specific transparent UV-protective film of the present invention has significantly enhanced tensile strength and elongation at break, and possesses excellent mechanical properties.

[0125] 5. Experimental diagram of biodegradability of composite membrane

[0126] The composite membrane (PC-HALG membrane, 5×5cm) prepared in Example 1 and the composite membrane (PC membrane, 5×5cm) prepared in Comparative Example 1 were placed in soil at a depth of approximately 5-10cm to simulate the degradation process of the composite films under natural conditions. Appropriate amounts of water were sprayed to maintain humidity, and samples were collected every 2 days. The degradability of the material was assessed by recording changes in the appearance of the composite membrane over time. The recorded degradation process was compared with that of the comparative example... Figure 6 As shown, the PC-HALG membrane degrades rapidly in soil due to the addition of acetylated lignin halloysite nanotubes, and water absorption and swelling accelerate its degradation rate. The PC-HALG membrane of Example 1 can be almost completely degraded within 6 days. The results indicate that the PC-HALG membrane of Example 1 has a relatively small negative impact on the environment.

[0127] 6. Biosafety test diagram of composite membrane

[0128] Bean sprout seeds were soaked in water under low light indoors for 12 hours, then transferred to mesh trays for cultivation. The treatment group used a solution prepared in Example 1 (PC-HALG membrane solution) mixed with water to form a PC-HALG membrane solution with a concentration of 1 mg / mL for water culture in the mesh trays. The control group used tap water for cultivation. Plant conditions were photographed at 0, 3, 6, and 9 days using a mobile camera, and bean sprout growth parameters, including average germination rate and average plant height, were calculated. Growth conditions are as follows: Figure 7 As shown, there were no significant differences in the growth and color of the two types of sprouts in water and the PC-HALG composite membrane solution of Example 1. The sprouts grew uniformly, proving that the PC-HALG composite membrane of Example 1 has good biocompatibility and is non-toxic and pollution-free.

[0129] 7. Colorimetric test of composite film

[0130] The composite films prepared in Examples 1-3 and Comparative Examples 1-4 were placed on a white standard plate and the L*, a*, and b* values ​​were measured using a handheld colorimeter (CM-2300d Konica Minolta), as shown in Table 3. L* (0-100) represents the trend from black to white; a* (-80-100) represents the trend from green to red; and b* (-80-70) represents the trend from blue to yellow. Three measurements were randomly performed on each composite film, and the color difference value E was calculated, as shown in Table 3. The calculation formula is as follows:

[0131]

[0132] Where ΔL*, Δa*, and Δb* are the color differences between the sample and the white standard (L* = 97.62, a* = -0.15, b* = 2.70 for the white standard). The actual color comparison diagrams of the composite films prepared in Examples 1-3 and Comparative Examples 1-4 are shown below. Figure 8 As shown.

[0133] Table 3

[0134] Recorded as L* a* b* ΔE Comparative Example 1 PC <![CDATA[92.95±2.76 a ]]> <![CDATA[3.29±1.06 c ]]> <![CDATA[15.64±4.99 c ]]> <![CDATA[14.24±5.24 d ]]> Comparative Example 2 PC-LG <![CDATA[53.78±0.62 d ]]> <![CDATA[12.65±0.28 a ]]> <![CDATA[29.68±0.18 ab ]]> <![CDATA[52.89±0.49 a ]]> Comparative Example 3 PC-ALG <![CDATA[73.71±5.86 c ]]> <![CDATA[3.13±3.29 c ]]> <![CDATA[33.85±2.43 a ]]> <![CDATA[39.57±5.68 b ]]> Comparative Example 4 PC-HNTS <![CDATA[91.35±1.25 a ]]> <![CDATA[3.56±1.65 c ]]> <![CDATA[18.10±2.25 bc ]]> <![CDATA[23.60±1.25 c ]]> Example 1 PC-HALG <![CDATA[88.09±2.05 b ]]> <![CDATA[3.66±0.11 c ]]> <![CDATA[23.80±4.27 b ]]> <![CDATA[23.50±4.77 c ]]> Example 2 PC-2HALG <![CDATA[86.71±3.24 ab ]]> <![CDATA[4.25±3.34 ab ]]> <![CDATA[24.15±2.11 b ]]> <![CDATA[25.53±3.68 c ]]> Example 3 PC-3HALG <![CDATA[84.25±3.86 ab ]]> <![CDATA[5.41±2.26 b ]]> <![CDATA[27.32±1.43 b ]]> <![CDATA[36.23±2.35 b ]]>

[0135] Table 3 shows the color parameters of each composite film. Since lignin is dark brown, even with only 1 wt% lignin addition, the color parameters (L*, a*, b*, and ΔE) of the composite films showed significant differences (P<0.05). As shown in the figure, it is evident that the composite film of Comparative Example 2 has lower transparency, possibly due to the aggregation of lignin molecules leading to the formation of larger structures that absorb higher wavelengths of light, thus increasing absorption and opacity in the visible light range. Compared to the composite film of Comparative Example 2, the composite film of Comparative Example 3, obtained by adding acetylated lignin, showed significantly lower L* and a* values, indicating that the acetyl groups successfully replaced the phenolic hydroxyl groups, reducing the color of the lignin, consistent with the aforementioned results. The composite film obtained in Example 1 showed a 63.79% increase in brightness (L*) and a 255.3% decrease in a* value compared to the composite film in Comparative Example 2. The color difference (ΔE) was significantly different. This difference is likely due to the lower apparent density of the acetylated lignin sample, which reduces the concentration of chromophores and auxochromes per unit volume at the macroscopic level. Furthermore, the loading of acetylated lignin onto halloysite nanotubes further reduced lignin molecule aggregation, resulting in a lighter color in the composite film obtained in Example 1. There was no significant difference in L* and a* values ​​between the composite film of Example 1 and the composite film of Comparative Example 1, indicating that the transparency of the lignin-based composite film obtained in Example 1 of this invention was greatly improved.

[0136] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A transparent, UV-protective composite film, characterized in that, The raw materials of the transparent UV-blocking composite film include acetylated lignin, halloysite nanomaterials, chitosan solution, and polyvinyl alcohol solution; The acetylated lignin is loaded onto halloysite nanomaterials; The method of loading acetylated lignin onto halloysite nanomaterials includes: first, mixing halloysite nanomaterials and water to form solution A; then, dissolving acetylated lignin in dioxane aqueous solution to form solution B; then, adding solution B to solution A and mixing; and finally, separating the solid after ultrasonic treatment to obtain halloysite nanocomposite. The ratio of halloysite nanocomposite to chitosan solution is 1:(13-17), in g:mL; The volume ratio of the chitosan solution to the polyvinyl alcohol solution is (13-17):(28-32); The chitosan solution contains 1-5% chitosan by mass. The polyvinyl alcohol content in the polyvinyl alcohol solution is 2-6% by mass.

2. A method for preparing a transparent, UV-blocking composite film, characterized in that, The preparation steps include the following: 1) Preparation of acetylated lignin; 2) The acetylated lignin prepared in step 1) is loaded onto halloysite nanomaterials to obtain halloysite nanocomposites; 3) The halloysite nanocomposite prepared in step 2) is mixed with chitosan solution and polyvinyl alcohol solution. After removing air bubbles from the solution, a film-forming treatment is performed to obtain the transparent UV-blocking composite film. In step 2), halloysite nanomaterials are first mixed with water to form solution A. Then, the acetylated lignin prepared in step 1) is dissolved in dioxane aqueous solution to form solution B. Solution B is then added to solution A and mixed. After ultrasonic treatment, the solid is separated to obtain halloysite nanocomposite. The ratio of halloysite nanocomposite to chitosan solution is 1:(13-17), in g:mL; The volume ratio of the chitosan solution to the polyvinyl alcohol solution is (13-17):(28-32); The chitosan solution contains 1-5% chitosan by mass. The polyvinyl alcohol content in the polyvinyl alcohol solution is 2-6% by mass.

3. The method for preparing the transparent UV-blocking composite film according to claim 2, characterized in that, After the solid separation step is completed, a drying step is also included.

4. The method for preparing the transparent UV-blocking composite film according to claim 2 or 3, characterized in that, The process includes an acidification step of the halloysite nanomaterials before step 2).

5. The method for preparing the transparent UV-blocking composite film according to claim 2 or 3, characterized in that, Before mixing the halloysite nanomaterials with water, the process also includes a step of ultrasonic pretreatment of the halloysite nanomaterials in an aqueous hydrochloric acid solution.

6. The method for preparing the transparent UV-blocking composite film according to claim 2 or 3, characterized in that, The ratio of halloysite nanomaterials to water in step 2) is 1:(10-100), in g:mL; The ratio of the acetylated lignin to the dioxane aqueous solution is 1:(10-100), in g:mL; In step 2), the volume ratio of dioxane to water in the dioxane aqueous solution is (1-9):(1-5). The mass ratio of acetylated lignin in solution B to halloysite nanomaterials in solution A is (1-5):(1-5); The ultrasonic power for ultrasonic treatment in step 2) is 25-300W, and the ultrasonic treatment time is 0.5-2h; In step 2), the stirring speed for adding solution B to solution A is 50-600 r / min, and the mixing time is 0.5-24 h.

7. The method for preparing the transparent UV-blocking composite film according to claim 2 or 3, characterized in that, In step 3), the halloysite nanocomposite prepared in step 2) is mixed with dioxane aqueous solution, homogenized, and then mixed with chitosan solution, polyvinyl alcohol solution and glycerol. After ultrasonic treatment to remove air bubbles in the solution, a film-forming treatment is performed to obtain the transparent UV-blocking composite film.

8. The method for preparing the transparent UV-blocking composite film according to claim 7, characterized in that, The ratio of halloysite nanocomposite to dioxane aqueous solution in step 3) is 1:(40-60), in g:mL; In step 3), the volume ratio of dioxane to water in the dioxane aqueous solution is (1-5):(1-5). The homogenization process takes 5-10 minutes and the homogenization speed is 6000-24000 r / min. The volume ratio of the chitosan solution, polyvinyl alcohol solution, and glycerol is (13-17):(28-32):(0.1-0.3). The solvent in the chitosan solution is an aqueous acetic acid solution; the mass concentration of acetic acid in the aqueous acetic acid solution is 1-2%. The polyvinyl alcohol solution is an aqueous solution of polyvinyl alcohol.

9. The method for preparing the transparent UV-blocking composite film according to claim 7, characterized in that, The film-forming process includes casting and drying.

10. The method for preparing the transparent UV-blocking composite film according to claim 2 or 3, characterized in that, In step 1), lignin is added to a propionic anhydride / pyridine solution and stirred to react. After the reaction is complete, water is added and the solid is separated to obtain acetylated lignin.

11. The method for preparing the transparent UV-blocking composite film according to claim 10, characterized in that, The ratio of lignin to propionic anhydride / pyridine solution is 1:(11-15), in g:mL; The volume ratio of propionic anhydride to pyridine in the propionic anhydride / pyridine solution is (2-5):10; The stirring reaction rate is 400-600 r / min, and the stirring reaction time is 20-26 h.

12. The method for preparing the transparent UV-blocking composite film according to claim 10, characterized in that, The volume ratio of water to propionic anhydride / pyridine solution is (200-500):(11-15).

13. The application of the transparent UV-blocking composite film according to claim 1 or the transparent UV-blocking composite film prepared by the preparation method according to any one of claims 2-12 in UV-blocking materials.

14. The application of the transparent UV-blocking composite film according to claim 1 or the transparent UV-blocking composite film prepared by the preparation method according to any one of claims 2-12 in food packaging materials.

Images