Lignin-containing anti-ultraviolet composite film and preparation method thereof

Abstract

The invention relates to the technical field of high-molecular compounds, and discloses a lignin-containing anti-ultraviolet composite membrane and a preparation method thereof. Lignin in vinasse is separated by adopting a choline chloride / lactic acid eutectic solvent and a catalyst ferric chloride, then demethylation, benzophenone graft modification and maleic anhydride esterification are performed in sequence to prepare benzophenone modified lignin ester, the benzophenone modified lignin ester is used as an anti-ultraviolet component to react with unsaturated polyester and o-diallyl bisphenol A epoxy resin, and the anti-ultraviolet performance of the epoxy resin is improved. According to the preparation method, the active monomer styrene and the photoinitiator are added and mixed, and the anti-ultraviolet composite film containing the lignin is prepared after film coating and photocuring, so that the problems that the lignin is easy to agglomerate, the anti-ultraviolet potential is difficult to release and a micromolecular ultraviolet absorbent is easy to photodecompose and migrate are solved, and the anti-ultraviolet performance and the mechanical performance of the composite film are improved.

Description

Technical Field

[0001] This invention relates to the field of polymer compound technology, specifically to a lignin-containing UV-resistant composite film and its preparation method. Background Technology

[0002] Unsaturated polyester resin is synthesized by the condensation polymerization of diacids (or anhydrides) and diols. Its main chain structure contains unique unsaturated bonds, which can undergo cross-linking reactions to form an unsaturated linear thermosetting resin. It has good physical properties and its processing technology is simple and easy to operate, making it one of the important film-forming resins for coatings.

[0003] However, the inherent ester bonds and other structures in the unsaturated polyester molecule cause yellowing and stress cracking on the material surface under ultraviolet radiation, which severely impairs mechanical properties and significantly shortens its service life. Current methods to improve the UV resistance of unsaturated polyesters involve introducing UV absorbers into the system; however, additive UV absorbers can leach from the unsaturated polyester matrix, affecting the product's lifespan. Reactive unsaturated polyesters, on the other hand, utilize their active groups to bond to the unsaturated polyester macromolecules, achieving long-term stability.

[0004] Due to its unique polyaromatic structure, low price, and wide availability, lignin has seen increasing research in recent years on its development as a UV-protective material. Lignin is a natural amorphous organic polymer with a three-dimensional network structure formed by a series of carbon-oxygen and carbon-carbon bonds, and it forms the cell wall of plants along with cellulose and hemicellulose. Lignin and hemicellulose, and cellulose in general, are primarily linked by hydrogen bonds, while lignin and hemicellulose are linked by covalent bonds in addition to hydrogen bonds. The compact and stable structure of lignin and cellulose makes it difficult for even small molecules like water to penetrate the crystalline regions of cellulose, significantly hindering the separation and utilization of lignin and cellulose. Furthermore, the lack of large conjugated systems in lignin molecules results in insufficient absorption of long-wave ultraviolet light with cumulative harmful effects. Lignin also tends to aggregate, and its disordered aggregation pattern further limits its UV protection and antioxidant properties, restricting its application as a UV absorber.

[0005] Existing technologies, such as Chinese patent application CN115785638A, disclose a high-strength, long-lasting UV-resistant bio-based UP composite material and its efficient preparation method and application. It uses itaconic acid, butanediol, and isosorbide as monomers to prepare a low-viscosity bio-based UP prepolymer system. At the same time, by adjusting the initiator content, adding nano-lignin, and controlling its addition amount, a composite material with high mechanical properties, thermal stability, and long-lasting stable UV resistance is finally obtained on the basis of introducing a small amount of nano-lignin. However, the UV absorption range of nano-lignin as a UV-resistant component is limited, and the UV resistance performance of the composite material needs to be improved. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a lignin-containing UV-resistant composite film and its preparation method. This invention separates lignin from distiller's grains using a choline chloride / lactic acid eutectic solvent and ferric chloride catalyst, then further modifies it to produce benzophenone-modified lignin ester, which is used as the UV-resistant component. This ester is reacted with unsaturated polyester and o-diallyl bisphenol A epoxy resin, and then mixed with the active monomer styrene and a photoinitiator. After coating and photocuring, a lignin-containing UV-resistant composite film is obtained. This invention solves the problems of lignin's easy aggregation, difficulty in releasing its UV-resistant potential, and easy photodegradation and migration of small-molecule UV absorbers, thus improving the UV resistance and mechanical properties of the composite film.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing a lignin-containing UV-resistant composite film includes the following steps:

[0009] Step (1): Add water to the lees, stir, filter, and dry to obtain pretreated lees;

[0010] Step (2): Add pretreated distiller's grains to the eutectic solvent of choline chloride / lactic acid, and add ferric chloride catalyst. After the reaction is completed, perform post-treatment to obtain lignin.

[0011] Step (3): Mix and dissolve lignin with dimethylformamide, add iodocyclohexane, reflux, and after the reaction is complete, purify to obtain demethylated lignin;

[0012] Step (4): Mix demethylated lignin with water, add a pH adjuster, add a tetrahydrofuran solution of potassium iodide and 5-bromo-2-hydroxybenzophenone, react, cool after the reaction is complete, purify, and obtain benzophenone-modified lignin.

[0013] Step (5): Mix and dissolve benzophenone-modified lignin and dimethyl sulfoxide, add maleic anhydride, react, cool after the reaction is complete, purify, and obtain benzophenone-modified lignin ester;

[0014] Step (6): Mix unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, and catalyst hexadecyltrimethylammonium bromide, react, and after the reaction is complete, add polymerization inhibitor hydroquinone and active monomer styrene, stir and mix, and cool down to obtain modified unsaturated polyester.

[0015] Step (7): Mix the modified unsaturated resin with the photoinitiator benzoin ether, coat the mixture onto the release film, and light-cur it. After curing, remove the release film to obtain a lignin-containing UV-resistant composite film.

[0016] Preferably, in step (1), the solid-liquid ratio of lees to water is 1g:5-10mL; the stirring conditions are: continuous mechanical stirring at a speed of 100-300r / min for 10-20min.

[0017] Preferably, in step (1), the lees include any one of yellow wine lees, white wine lees, and beer lees.

[0018] Preferably, in step (2), the eutectic solvent is prepared by the following steps: choline chloride and lactic acid are mixed at a molar ratio of 1:0.5-3 to form a choline chloride / lactic acid solution, and the solution is stirred at 100°C for 1 hour to remove water, thereby obtaining the eutectic solvent.

[0019] Preferably, in step (2), the mass ratio of choline chloride / lactic acid eutectic solvent, pretreated distiller's grains, and ferric chloride catalyst is 100:5:0.5-2; the reaction conditions are: reaction at 100-160℃ for 2-8 hours.

[0020] Preferably, in step (2), the post-processing operation includes: adding 200 mL of ethanol to the reaction product obtained after the reaction is completed, ultrasonically vibrating for 5 min, filtering with a 200-mesh filter bag, taking the filtrate, adding 800 mL of water, letting it stand for 12 h, filtration, taking the filter cake, washing with 10 wt% ethanol aqueous solution, and drying.

[0021] Preferably, in step (3), the solid-liquid ratio of lignin, dimethylformamide, and iodocyclohexane is 5g:40-80mL:10-20mL; the reflux reaction conditions are: reflux reaction at 140-150℃ for 12-18h in a nitrogen atmosphere.

[0022] Preferably, in step (3), the purification operation includes: adding n-hexane to extract and remove unreacted iodocyclohexane to obtain a crude reaction product, adding the crude reaction product to a saturated sodium metabisulfite aqueous solution to precipitate, filtering to collect the precipitate, washing, and drying.

[0023] Preferably, in step (4), the solid-liquid ratio of demethylated lignin, water, potassium iodide, and 5-bromo-2-hydroxybenzophenone is 2g:80-100mL:0.05-0.06g:0.18-0.28g; the tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone is prepared by mixing 5-bromo-2-hydroxybenzophenone and tetrahydrofuran at a solid-liquid ratio of 1:5-10.

[0024] Preferably, in step (4): the regulator is a 1 mol / L sodium hydroxide aqueous solution, and the pH value after adjustment is 10-11; the reaction conditions are: reaction at 70-90℃ for 6-9 hours.

[0025] Preferably, in step (4), the purification operation includes: rotary evaporation to remove tetrahydrofuran, extraction with dichloromethane, taking the aqueous phase, dialyzing in water for 3-5 days to remove inorganic salts, and freeze-drying.

[0026] Preferably, in step (5), the solid-liquid ratio of benzophenone-modified lignin, dimethyl sulfoxide, and maleic anhydride is 10g:80-120mL:2-2.4g; the reaction conditions are: reaction at a speed of 300-500r / min and a temperature of 90-100℃ for 8-10h.

[0027] Preferably, in step (5), the purification operation includes: adding ethyl acetate to precipitate, filtering to obtain the precipitate, adding dimethyl sulfoxide to dissolve, repeating the precipitation and filtration operation once, and drying.

[0028] Preferably, in step (6), the mass ratio of unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, hexadecyltrimethylammonium bromide, hydroquinone, and styrene is 80:40:5-10:0.15-0.2:0.2-0.3:15-20; the reaction conditions are: reacting at 110-120℃ for 2-3 hours; and the stirring and mixing conditions are: stirring and mixing at 80-90℃ for 1 hour.

[0029] Preferably, in step (7): the mass ratio of modified unsaturated resin to photoinitiator benzoin ether is 100:1.5-1.8; the photocuring conditions are: UV irradiation curing for 3-8 minutes at a wavelength of 365nm and an irradiation distance of 15-30cm.

[0030] Preferably, the lignin-containing UV-resistant composite film is prepared using the method described above for preparing a lignin-containing UV-resistant composite film.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] 1. This invention uses choline chloride / lactic acid eutectic solvent and ferric chloride catalyst to separate lignin from distiller's grains. The lignin is then subjected to demethylation, benzophenone grafting modification, and maleic anhydride esterification to obtain benzophenone-modified lignin ester. This benzophenone-modified lignin ester is used as an anti-UV component and reacted with unsaturated polyester and o-diallyl bisphenol A epoxy resin. After adding the active monomer styrene and a photoinitiator, a photocurable matrix resin is prepared. After photocuring the coating, an anti-UV composite film containing lignin is obtained. This invention solves the problems of lignin's easy agglomeration, difficulty in releasing its anti-UV potential, and easy photodegradation and migration of small molecule UV absorbers, thus improving the anti-UV performance of the composite film. Moreover, both the o-diallyl bisphenol A epoxy resin and the benzophenone-modified lignin ester contain carbon-carbon double bonds, increasing the carbon-carbon double bond content in the matrix resin, accelerating the photocuring process after coating, increasing the degree of crosslinking, and improving the mechanical properties of the composite film.

[0033] 2. This invention utilizes a eutectic solvent of choline chloride / lactic acid and a ferric chloride catalyst to separate lignin from distiller's grains. This method is both highly efficient and environmentally friendly. The solvent can be recycled, providing a new strategy for the resource utilization of agricultural waste. The resulting lignin is rich in aromatic rings and contains abundant chromophores such as carbonyl groups and double bonds, as well as auxochromes such as hydroxyl groups and methoxy groups, giving it ultraviolet absorption capabilities. Furthermore, the lignin is rich in phenolic hydroxyl groups, which can promptly capture free radicals induced by ultraviolet light and reduce oxidative damage caused by ultraviolet radiation.

[0034] 3. This invention employs an in-situ Lewis acid generation method to demethylate lignin, increasing the phenolic hydroxyl content and reactivity of lignin. Then, 5-bromo-2-hydroxybenzophenone undergoes a nucleophilic substitution reaction with the demethylated lignin, chemically linking the benzophenone-based small molecule UV absorber to the demethylated lignin. This solves the problems of lignin's easy aggregation, difficulty in releasing its UV resistance potential, and easy photodegradation and migration of small molecule UV absorbers.

[0035] Furthermore, maleic anhydride was used to modify benzophenone-modified lignin, and the resulting benzophenone-modified lignin ester could be linked with unsaturated polyester through a reaction between the carboxyl group and the epoxy group in o-diallyl bisphenol A epoxy resin. The epoxy resin, acting as a compatibilizer, improved the compatibility of the benzophenone-modified lignin ester in the matrix resin, while also improving the mechanical properties of the composite film. Attached Figure Description

[0036] Figure 1 These are bar charts showing the tensile strength of Examples 2-6 and Comparative Examples 1-2 in the performance test of this invention;

[0037] Figure 2 These are bar charts showing the ultraviolet light transmittance of Examples 2-6 and Comparative Examples 1-2 in the performance test of this invention;

[0038] Figure 3 This is a bar chart showing the tensile strength retention rate of Examples 2-6 and Comparative Examples 1-2 in the performance test of this invention;

[0039] Figure 4 This is the infrared spectrum of the lignin prepared in Example 1 of this invention;

[0040] Figure 5 This is a SEM image of the lignin-containing UV-resistant composite film prepared in Example 6 of this invention;

[0041] Figure 6 This is a photograph of the lignin-containing UV-resistant composite film prepared in Example 6 of this invention.

[0042] In the picture, Figure 5 The left image is a SEM image under a 100μm scale, and the right image is a SEM image under a 200μm scale. Detailed Implementation

[0043] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0044] Example 1

[0045] This embodiment discloses a method for preparing lignin, including the following steps:

[0046] Step (1): Add 500mL of water to 100g of lees, and continuously stir mechanically at 300r / min for 10min. Then filter and rinse with water to remove soluble impurities, and dry to constant weight to obtain pretreated lees.

[0047] Step (2): Mix choline chloride and lactic acid in a molar ratio of 1:3 to form a choline chloride / lactic acid solution. Stir at 100°C for 1 hour to remove water and obtain a eutectic solvent.

[0048] Add 5g of pretreated distiller's grains and 2g of ferric chloride catalyst to 100g of choline chloride / lactic acid eutectic solvent. React at 160℃ for 8h. After the reaction is complete, add 200mL of ethanol, sonicate for 5min, filter through a 200-mesh filter bag, take the filtrate, add 800mL of water, let stand for 12h, filter by suction, take the filter cake, wash with 10wt% ethanol aqueous solution, and dry to obtain lignin.

[0049] Example 2

[0050] This embodiment discloses a method for preparing a lignin-containing UV-resistant composite film, comprising the following steps:

[0051] Step (1): The lignin prepared in Example 1 was mixed and dissolved with dimethylformamide, and iodocyclohexane was added. The mixture was refluxed at 140°C for 18 hours under a nitrogen atmosphere. After the reaction was completed, n-hexane was added to extract and remove unreacted iodocyclohexane to obtain the crude product. The crude product was added to a saturated sodium metabisulfite aqueous solution to precipitate, the precipitate was filtered, washed, and dried to obtain demethylated lignin.

[0052] The solid-liquid ratio of lignin, dimethylformamide, and iodocyclohexane is 5g:40mL:15mL.

[0053] Step (2): Mix demethylated lignin with water, add 1 mol / L sodium hydroxide aqueous solution to adjust the pH to 11, add potassium iodide and tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone, react at 80℃ for 7 h, after the reaction is completed, cool to room temperature, remove tetrahydrofuran by rotary evaporation, extract with dichloromethane, take the aqueous phase, dialyze in water for 3-5 days to remove inorganic salts, freeze dry to obtain benzophenone modified lignin;

[0054] The solid-liquid ratio of demethylated lignin, water, potassium iodide, and 5-bromo-2-hydroxybenzophenone is 2g:80mL:0.05g:0.18g; the tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone is prepared by mixing 5-bromo-2-hydroxybenzophenone and tetrahydrofuran at a solid-liquid ratio of 1:10.

[0055] Step (3): Mix and dissolve benzophenone-modified lignin and dimethyl sulfoxide, add maleic anhydride at 100°C, and react at 100°C for 8 hours at 500 r / min. After the reaction is complete, cool to room temperature, add ethyl acetate to precipitate, filter to collect the precipitate, add dimethyl sulfoxide to dissolve, repeat the precipitation and filtration operation once, and dry at 50°C for 24 hours to obtain benzophenone-modified lignin ester.

[0056] The solid-liquid ratio of benzophenone-modified lignin, dimethyl sulfoxide, and maleic anhydride is 10g:100mL:2g.

[0057] Step (4): Mix unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, and catalyst cetyltrimethylammonium bromide, and react at 110°C for 3 hours. After the reaction is complete, add polymerization inhibitor hydroquinone and active monomer styrene, stir and mix at 80°C for 1 hour, and cool to room temperature to obtain modified unsaturated polyester.

[0058] The mass ratio of unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, hexadecyltrimethylammonium bromide, hydroquinone, and styrene is 80:40:5:0.15:0.2:15.

[0059] Step (5): Mix the modified unsaturated resin with the photoinitiator benzoin ether at a mass ratio of 100:1.5, coat the mixture onto the release film, and cure it under ultraviolet irradiation at a wavelength of 365nm and an irradiation distance of 20cm for 5 minutes. After curing, remove the release film to obtain a lignin-containing anti-ultraviolet composite film with a thickness of 0.5mm.

[0060] Example 3

[0061] This embodiment discloses a method for preparing a lignin-containing UV-resistant composite film, comprising the following steps:

[0062] Step (1): The lignin prepared in Example 1 is mixed and dissolved with dimethylformamide, and iodocyclohexane is added. The mixture is refluxed at 145°C for 15 hours under a nitrogen atmosphere. After the reaction is completed, n-hexane is added to extract and remove unreacted iodocyclohexane to obtain the crude product. The crude product is added to a saturated sodium metabisulfite aqueous solution to precipitate, filtered to obtain the precipitate, washed, and dried to obtain demethylated lignin.

[0063] The solid-liquid ratio of lignin, dimethylformamide, and iodocyclohexane is 5g:40mL:15mL.

[0064] Step (2): Mix demethylated lignin with water, add 1 mol / L sodium hydroxide aqueous solution to adjust the pH to 11, add potassium iodide and tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone, react at 80℃ for 7 h, after the reaction is completed, cool to room temperature, remove tetrahydrofuran by rotary evaporation, extract with dichloromethane, take the aqueous phase, dialyze in water for 3-5 days to remove inorganic salts, freeze dry to obtain benzophenone modified lignin;

[0065] The solid-liquid ratio of demethylated lignin, water, potassium iodide, and 5-bromo-2-hydroxybenzophenone is 2g:90mL:0.055g:0.2g; the tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone is prepared by mixing 5-bromo-2-hydroxybenzophenone and tetrahydrofuran at a solid-liquid ratio of 1:10.

[0066] Step (3): Mix and dissolve benzophenone-modified lignin and dimethyl sulfoxide, add maleic anhydride at 100°C, and react at 100°C for 8 hours at 500 r / min. After the reaction is complete, cool to room temperature, add ethyl acetate to precipitate, filter to collect the precipitate, add dimethyl sulfoxide to dissolve, repeat the precipitation and filtration operation once, and dry at 50°C for 24 hours to obtain benzophenone-modified lignin ester.

[0067] The solid-liquid ratio of benzophenone-modified lignin, dimethyl sulfoxide, and maleic anhydride is 10g:100mL:2.1g.

[0068] Step (4): Mix unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, and catalyst hexadecyltrimethylammonium bromide, and react at 115°C for 2.5 h. After the reaction is complete, add polymerization inhibitor hydroquinone and active monomer styrene, stir and mix at 80°C for 1 h, and cool to room temperature to obtain modified unsaturated polyester.

[0069] The mass ratio of unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, hexadecyltrimethylammonium bromide, hydroquinone, and styrene is 80:40:6:0.18:0.25:16.

[0070] Step (5): Mix the modified unsaturated resin with the photoinitiator benzoin ether at a mass ratio of 100:1.6, coat the mixture onto the release film, and cure it under ultraviolet irradiation at a wavelength of 365nm and an irradiation distance of 20cm for 5 minutes. After curing, remove the release film to obtain a lignin-containing anti-ultraviolet composite film with a thickness of 0.5mm.

[0071] Example 4

[0072] This embodiment discloses a method for preparing a lignin-containing UV-resistant composite film, comprising the following steps:

[0073] Step (1): The lignin prepared in Example 1 is mixed and dissolved with dimethylformamide, and iodocyclohexane is added. The mixture is refluxed at 145°C for 15 hours under a nitrogen atmosphere. After the reaction is completed, n-hexane is added to extract and remove unreacted iodocyclohexane to obtain the crude product. The crude product is added to a saturated sodium metabisulfite aqueous solution to precipitate, filtered to obtain the precipitate, washed, and dried to obtain demethylated lignin.

[0074] The solid-liquid ratio of lignin, dimethylformamide, and iodocyclohexane is 5g:40mL:15mL.

[0075] Step (2): Mix demethylated lignin with water, add 1 mol / L sodium hydroxide aqueous solution to adjust the pH to 11, add potassium iodide and tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone, react at 80℃ for 7 h, after the reaction is completed, cool to room temperature, remove tetrahydrofuran by rotary evaporation, extract with dichloromethane, take the aqueous phase, dialyze in water for 3-5 days to remove inorganic salts, freeze dry to obtain benzophenone modified lignin;

[0076] The solid-liquid ratio of demethylated lignin, water, potassium iodide, and 5-bromo-2-hydroxybenzophenone was 2g:90mL:0.055g:0.23g; the tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone was prepared by mixing 5-bromo-2-hydroxybenzophenone and tetrahydrofuran at a solid-liquid ratio of 1:10.

[0077] Step (3): Mix and dissolve benzophenone-modified lignin and dimethyl sulfoxide, add maleic anhydride at 100°C, and react at 100°C for 8 hours at 500 r / min. After the reaction is complete, cool to room temperature, add ethyl acetate to precipitate, filter to collect the precipitate, add dimethyl sulfoxide to dissolve, repeat the precipitation and filtration operation once, and dry at 50°C for 24 hours to obtain benzophenone-modified lignin ester.

[0078] The solid-liquid ratio of benzophenone-modified lignin, dimethyl sulfoxide, and maleic anhydride is 10g:100mL:2.2g.

[0079] Step (4): Mix unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, and catalyst hexadecyltrimethylammonium bromide, and react at 115°C for 2.5 h. After the reaction is complete, add polymerization inhibitor hydroquinone and active monomer styrene, stir and mix at 80°C for 1 h, and cool to room temperature to obtain modified unsaturated polyester.

[0080] The mass ratio of unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, hexadecyltrimethylammonium bromide, hydroquinone, and styrene is 80:40:7.5:0.18:0.25:18.

[0081] Step (5): Mix the modified unsaturated resin with the photoinitiator benzoin ether at a mass ratio of 100:1.6, coat the mixture onto the release film, and cure it under ultraviolet irradiation at a wavelength of 365nm and an irradiation distance of 20cm for 5 minutes. After curing, remove the release film to obtain a lignin-containing anti-ultraviolet composite film with a thickness of 0.5mm.

[0082] Example 5

[0083] This embodiment discloses a method for preparing a lignin-containing UV-resistant composite film, comprising the following steps:

[0084] Step (1): The lignin prepared in Example 1 is mixed and dissolved with dimethylformamide, and iodocyclohexane is added. The mixture is refluxed at 145°C for 15 hours under a nitrogen atmosphere. After the reaction is completed, n-hexane is added to extract and remove unreacted iodocyclohexane to obtain the crude product. The crude product is added to a saturated sodium metabisulfite aqueous solution to precipitate, filtered to obtain the precipitate, washed, and dried to obtain demethylated lignin.

[0085] The solid-liquid ratio of lignin, dimethylformamide, and iodocyclohexane is 5g:40mL:15mL.

[0086] Step (2): Mix demethylated lignin with water, add 1 mol / L sodium hydroxide aqueous solution to adjust the pH to 11, add potassium iodide and tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone, react at 80℃ for 7 h, after the reaction is completed, cool to room temperature, remove tetrahydrofuran by rotary evaporation, extract with dichloromethane, take the aqueous phase, dialyze in water for 3-5 days to remove inorganic salts, freeze dry to obtain benzophenone modified lignin;

[0087] The solid-liquid ratio of demethylated lignin, water, potassium iodide, and 5-bromo-2-hydroxybenzophenone is 2g:90mL:0.055g:0.25g; the tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone is prepared by mixing 5-bromo-2-hydroxybenzophenone and tetrahydrofuran at a solid-liquid ratio of 1:10.

[0088] Step (3): Mix and dissolve benzophenone-modified lignin and dimethyl sulfoxide, add maleic anhydride at 100°C, and react at 100°C for 8 hours at 500 r / min. After the reaction is complete, cool to room temperature, add ethyl acetate to precipitate, filter to collect the precipitate, add dimethyl sulfoxide to dissolve, repeat the precipitation and filtration operation once, and dry at 50°C for 24 hours to obtain benzophenone-modified lignin ester.

[0089] The solid-liquid ratio of benzophenone-modified lignin, dimethyl sulfoxide, and maleic anhydride is 10g:100mL:2.3g.

[0090] Step (4): Mix unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, and catalyst hexadecyltrimethylammonium bromide, and react at 115°C for 2.5 h. After the reaction is complete, add polymerization inhibitor hydroquinone and active monomer styrene, stir and mix at 80°C for 1 h, and cool to room temperature to obtain modified unsaturated polyester.

[0091] The mass ratio of unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, hexadecyltrimethylammonium bromide, hydroquinone, and styrene is 80:40:9:0.18:0.25:19.

[0092] Step (5): Mix the modified unsaturated resin with the photoinitiator benzoin ether at a mass ratio of 100:1.6, coat the mixture onto the release film, and cure it under ultraviolet irradiation at a wavelength of 365nm and an irradiation distance of 20cm for 5 minutes. After curing, remove the release film to obtain a lignin-containing anti-ultraviolet composite film with a thickness of 0.5mm.

[0093] Example 6

[0094] This embodiment discloses a method for preparing a lignin-containing UV-resistant composite film, comprising the following steps:

[0095] Step (1): The lignin prepared in Example 1 was mixed and dissolved with dimethylformamide, and iodocyclohexane was added. The mixture was refluxed at 150°C for 12 hours under a nitrogen atmosphere. After the reaction was completed, n-hexane was added to extract and remove unreacted iodocyclohexane to obtain the crude product. The crude product was added to a saturated sodium metabisulfite aqueous solution to precipitate, the precipitate was filtered, washed, and dried to obtain demethylated lignin.

[0096] The solid-liquid ratio of lignin, dimethylformamide, and iodocyclohexane is 5g:40mL:15mL.

[0097] Step (2): Mix demethylated lignin with water, add 1 mol / L sodium hydroxide aqueous solution to adjust the pH to 11, add potassium iodide and tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone, react at 80℃ for 7 h, after the reaction is completed, cool to room temperature, remove tetrahydrofuran by rotary evaporation, extract with dichloromethane, take the aqueous phase, dialyze in water for 3-5 days to remove inorganic salts, freeze dry to obtain benzophenone modified lignin;

[0098] The solid-liquid ratio of demethylated lignin, water, potassium iodide, and 5-bromo-2-hydroxybenzophenone is 2g:100mL:0.06g:0.28g; the tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone is prepared by mixing 5-bromo-2-hydroxybenzophenone and tetrahydrofuran at a solid-liquid ratio of 1:10.

[0099] Step (3): Mix and dissolve benzophenone-modified lignin and dimethyl sulfoxide, add maleic anhydride at 100°C, and react at 100°C for 8 hours at 500 r / min. After the reaction is complete, cool to room temperature, add ethyl acetate to precipitate, filter to collect the precipitate, add dimethyl sulfoxide to dissolve, repeat the precipitation and filtration operation once, and dry at 50°C for 24 hours to obtain benzophenone-modified lignin ester.

[0100] The solid-liquid ratio of benzophenone-modified lignin, dimethyl sulfoxide, and maleic anhydride is 10g:100mL:2.4g.

[0101] Step (4): Mix unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, and catalyst hexadecyltrimethylammonium bromide, and react at 120°C for 2 hours. After the reaction is complete, add polymerization inhibitor hydroquinone and active monomer styrene, stir and mix at 80°C for 1 hour, and cool to room temperature to obtain modified unsaturated polyester.

[0102] The mass ratio of unsaturated polyester, o-diallyl bisphenol A epoxy resin, benzophenone-modified lignin ester, hexadecyltrimethylammonium bromide, hydroquinone, and styrene is 80:40:10:0.2:0.3:20.

[0103] Step (5): Mix the modified unsaturated resin with the photoinitiator benzoin ether at a mass ratio of 100:1.8, coat the mixture onto the release film, and cure it under ultraviolet irradiation at a wavelength of 365nm and an irradiation distance of 20cm for 5 minutes. After curing, remove the release film to obtain a lignin-containing anti-ultraviolet composite film with a thickness of 0.5mm.

[0104] Comparative Example 1

[0105] This comparative example discloses a method for preparing a lignin-containing UV-resistant composite film, comprising the following steps:

[0106] Step (1): Mix unsaturated polyester, o-diallyl bisphenol A epoxy resin, lignin, and catalyst hexadecyltrimethylammonium bromide, and react at 110°C for 3 hours. After the reaction is complete, add polymerization inhibitor hydroquinone and active monomer styrene, stir and mix at 80°C for 1 hour, and cool to room temperature to obtain modified unsaturated polyester.

[0107] The mass ratio of unsaturated polyester, o-diallyl bisphenol A epoxy resin, lignin, hexadecyltrimethylammonium bromide, hydroquinone, and styrene is 80:40:5:0.15:0.2:15.

[0108] Step (5): Mix the modified unsaturated resin with the photoinitiator benzoin ether at a mass ratio of 100:1.5, coat the mixture onto the release film, and cure it under ultraviolet irradiation at a wavelength of 365nm and an irradiation distance of 20cm for 5 minutes. After curing, remove the release film to obtain a lignin-containing anti-ultraviolet composite film with a thickness of 0.5mm.

[0109] Comparative Example 2

[0110] This comparative example discloses a method for preparing a lignin-containing UV-resistant composite film, comprising the following steps:

[0111] Step (1): The lignin prepared in Example 1 was mixed and dissolved with dimethylformamide, and iodocyclohexane was added. The mixture was refluxed at 140°C for 18 hours under a nitrogen atmosphere. After the reaction was completed, n-hexane was added to extract and remove unreacted iodocyclohexane to obtain the crude product. The crude product was added to a saturated sodium metabisulfite aqueous solution to precipitate, the precipitate was filtered, washed, and dried to obtain demethylated lignin.

[0112] The solid-liquid ratio of lignin, dimethylformamide, and iodocyclohexane is 5g:40mL:15mL.

[0113] Step (2): Mix demethylated lignin with water, add 1 mol / L sodium hydroxide aqueous solution to adjust the pH to 11, add potassium iodide and tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone, react at 80℃ for 7 h, after the reaction is completed, cool to room temperature, remove tetrahydrofuran by rotary evaporation, extract with dichloromethane, take the aqueous phase, dialyze in water for 3-5 days to remove inorganic salts, freeze dry to obtain benzophenone modified lignin;

[0114] The solid-liquid ratio of demethylated lignin, water, potassium iodide, and 5-bromo-2-hydroxybenzophenone is 2g:80mL:0.05g:0.18g; the tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone is prepared by mixing 5-bromo-2-hydroxybenzophenone and tetrahydrofuran at a solid-liquid ratio of 1:10.

[0115] Step (3): Mix and dissolve benzophenone-modified lignin and dimethyl sulfoxide, add maleic anhydride at 100°C, and react at 100°C for 8 hours at 500 r / min. After the reaction is complete, cool to room temperature, add ethyl acetate to precipitate, filter to collect the precipitate, add dimethyl sulfoxide to dissolve, repeat the precipitation and filtration operation once, and dry at 50°C for 24 hours to obtain benzophenone-modified lignin ester.

[0116] The solid-liquid ratio of benzophenone-modified lignin, dimethyl sulfoxide, and maleic anhydride is 10g:100mL:2g.

[0117] Step (4): Mix unsaturated polyester, benzophenone-modified lignin ester, polymerization inhibitor hydroquinone, and active monomer styrene, stir and mix at 80°C for 1 hour, and cool to room temperature to obtain modified unsaturated polyester.

[0118] The mass ratio of unsaturated polyester, benzophenone-modified lignin ester, hydroquinone, and styrene is 120:5:0.2:15.

[0119] Step (5): Mix the modified unsaturated resin with the photoinitiator benzoin ether at a mass ratio of 100:1.5, coat the mixture onto the release film, and cure it under ultraviolet irradiation at a wavelength of 365nm and an irradiation distance of 20cm for 5 minutes. After curing, remove the release film to obtain a lignin-containing anti-ultraviolet composite film with a thickness of 0.5mm.

[0120] In the above embodiments and comparative examples: the lees are baijiu lees; choline chloride (C5H) 14 ClNO and lactic acid (C3H6O3) were both analytical grade AR; the unsaturated polyester was isophenylene-type neopentyl glycol unsaturated polyester resin; and the epoxy value of o-diallyl bisphenol A epoxy resin (DADGEBA) was 0.38.

[0121] Test case

[0122] The performance of the lignin-containing UV-resistant composite films prepared in Examples 2-6 and Comparative Examples 1-2 was tested, and the specific test results are shown in Table 1:

[0123] Table 1

[0124]

[0125] The tests for each indicator in Table 1 were conducted according to the following standards: Tensile strength was determined according to GB / T1040.3-2006 "Determination of Tensile Properties of Plastics - Part 3: Test Conditions for Films and Sheets"; UV resistance was expressed by UV transmittance and tensile strength retention rate after UV aging. UV transmittance was tested using a UV spectrophotometer at a wavelength of 320 nm; the specific test method for tensile strength retention rate was determined according to standard GB / T 14522-2008 "Laboratory Light Source Exposure Test Method", with an irradiation wavelength of 310 nm and a radiation intensity of 0.89 W / m². 2 The temperature is 60℃.

[0126] As can be seen from the test results in Table 1, the composite membrane prepared by the present invention has excellent UV resistance and mechanical properties. This is because the lignin prepared by the present invention is rich in aromatic rings and contains abundant chromophores such as carbonyl groups, double bonds, and auxochromes such as hydroxyl groups and methoxy groups, which endow it with UV absorption function. In addition, the lignin is rich in phenolic hydroxyl groups, which can capture free radicals induced by UV light in time and reduce oxidative damage caused by UV radiation. Demethylation of lignin increases its phenolic hydroxyl content and reactivity. Linking benzophenone-based small-molecule UV absorbers to demethylated lignin solves the problems of lignin's easy aggregation, difficulty in releasing its UV resistance potential, and easy photodegradation and migration of small-molecule UV absorbers. Maleic anhydride is used to modify benzophenone-modified lignin, and the resulting benzophenone-modified lignin ester can be linked with unsaturated polyester through a reaction between the carboxyl group and the epoxy group in o-diallyl bisphenol A epoxy resin. The epoxy resin acts as a compatibilizer, improving the compatibility of benzophenone-modified lignin ester in the matrix resin and simultaneously improving the mechanical properties of the composite film.

[0127] Comparative Example 1 directly added the lignin prepared in Comparative Example 1 without modification. This reduced the dispersibility and compatibility of lignin in the unsaturated resin, affecting the mechanical properties of the composite film. Furthermore, the lack of benzophenone-based small-molecule UV absorbers to enhance the UV absorption performance of lignin resulted in a decrease in both UV absorption and mechanical properties of Comparative Example 1. Comparative Example 2 did not add o-diallyl bisphenol A epoxy resin. The absence of epoxy resin as a compatibilizer to improve the compatibility of benzophenone-modified lignin ester in the matrix resin, coupled with a decrease in the carbon-carbon double bond content, reduced the degree of crosslinking during curing, affecting the mechanical properties of the composite film. Therefore, Comparative Example 2 also showed reduced mechanical properties.

[0128] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing an ultraviolet resistant composite film containing lignin, characterized by, Comprise the following steps: Step (1), the demethylated lignin is mixed with water, a regulator is added to adjust the pH value, potassium iodide and a tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone are added, and then reaction, cooling, purification and obtaining benzophenone modified lignin after the reaction is completed; Step (2), the benzophenone modified lignin is mixed and dissolved with dimethyl sulfoxide, maleic anhydride is added, and then reaction, cooling, purification and obtaining benzophenone modified lignin ester after the reaction is completed; Step (3), the unsaturated polyester, o-bisallyl bisphenol A epoxy resin, benzophenone modified lignin ester and cetyl trimethyl ammonium bromide are mixed, and then reaction, adding hydroquinone and styrene after the reaction is completed, stirring and mixing, cooling and obtaining modified unsaturated polyester; Step (4), the modified unsaturated resin is mixed with benzoin ether, coated on a release film, photocured, removing the release film after the curing is completed and obtaining an ultraviolet resistant composite film containing lignin.

2. The method for preparing a lignin-containing UV-resistant composite film according to claim 1, characterized in that, The demethylated lignin in the step (1) is prepared by the following steps: S1, adding water to the distiller's grains, stirring, suction filtration and drying to obtain pretreated distiller's grains; S2, adding the pretreated distiller's grains to the choline chloride / lactic acid eutectic solvent and adding a catalyst ferric chloride, and then reaction, post-treatment after the reaction is completed and obtaining lignin; S3, mixing and dissolving the lignin with dimethylformamide, adding iodine cyclohexane, refluxing and reaction, purification and obtaining demethylated lignin after the reaction is completed.

3. The method for preparing a lignin-containing UV-resistant composite film according to claim 2, characterized in that, When preparing the demethylated lignin in the step (1): In the S1, the solid-liquid ratio of the distiller's grains to water is 1g:5-10mL; the stirring condition is mechanical stirring at a speed of 100-300r / min for 10-20min; In the S2, the mass ratio of the choline chloride / lactic acid eutectic solvent, the pretreated distiller's grains and the catalyst ferric chloride is 100:5:0.5-2; the reaction condition is reaction at a temperature of 100-160℃ for 2-8h; The eutectic solvent is prepared by the following steps: mixing choline chloride and lactic acid at a molar ratio of 1:0.5-3 to form a choline chloride / lactic acid solution, stirring at a temperature of 100℃ for 1h to remove water and obtaining the eutectic solvent.

4. The method for preparing a lignin-containing UV-resistant composite film according to claim 2, characterized in that, When preparing the demethylated lignin in the step (1): In the S3, the solid-liquid ratio of the lignin, dimethylformamide and iodine cyclohexane is 5g:40-80mL:10-20mL; the refluxing reaction condition is refluxing at a temperature of 140-150℃ for 12-18h in a nitrogen atmosphere.

5. The method of claim 1, wherein the lignin-containing anti-UV composite film is prepared by the steps of: (a) mixing lignin, a polymer, and a plasticizer to prepare a mixture; (b) coating the mixture on a substrate; (c) drying the mixture; and (d) curing the mixture. In the step (1), the solid-liquid ratio of the demethylated lignin, water, potassium iodide and 5-bromo-2-hydroxybenzophenone is 2g:80-100mL:0.05-0.06g:0.18-0.28g; the tetrahydrofuran solution of 5-bromo-2-hydroxybenzophenone is prepared by mixing 5-bromo-2-hydroxybenzophenone and tetrahydrofuran at a solid-liquid ratio of 1:5-10.

6. The method of claim 1, wherein the lignin-containing anti-UV composite film is prepared by the steps of: (a) mixing lignin, a polymer, and a plasticizer to prepare a mixture; (b) coating the mixture on a substrate; (c) drying the mixture; and (d) removing the substrate. In the step (1), the regulator is 1mol / L sodium hydroxide aqueous solution, the adjusted pH value is 10-11; the reaction condition is reaction at a temperature of 70-90℃ for 6-9h.

7. The method of claim 1, wherein the lignin-containing anti-UV composite film is prepared by the steps of: (a) mixing lignin, a polymer, and a plasticizer to prepare a mixture; (b) coating the mixture on a substrate; (c) drying the mixture; and (d) removing the substrate. In the step (2), the solid-liquid ratio of benzophenone modified lignin, dimethyl sulfoxide and maleic anhydride is 10 g:80-120 mL:2-2.4 g; the reaction condition is that the reaction is carried out at a rotation speed of 300-500 r / min and a temperature of 90-100 °C for 8-10 h.

8. The method of claim 1, wherein the lignin-containing anti-UV composite film is prepared by the steps of: (a) mixing lignin, a polymer, and a plasticizer to prepare a mixture; (b) coating the mixture on a substrate; (c) drying the mixture; and (d) removing the substrate. In the step (3), the mass ratio of unsaturated polyester, o-bisallyl bisphenol A epoxy resin, benzophenone modified lignin ester, cetyl trimethyl ammonium bromide, hydroquinone and styrene is 80:40:5-10:0.15-0.2:0.2-0.3:15-20; the reaction condition is that the reaction is carried out at a temperature of 110-120 °C for 2-3 h; the stirring and mixing condition is that the stirring and mixing is carried out at a temperature of 80-90 °C for 1 h.

9. The method for preparing a lignin-containing UV-resistant composite film according to claim 1, characterized in that, In the step (4), the mass ratio of modified unsaturated resin and benzoin ether is 100:1.5-1.8; the photo-curing condition is that the ultraviolet irradiation curing is carried out at a wavelength of 365 nm and an irradiation distance of 15-30 cm for 3-8 min. 10.A lignin-containing anti-ultraviolet composite film prepared by a preparation method of the lignin-containing anti-ultraviolet composite film according to any one of claims 1-9.

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