Method for extracting light-color lignin based on ternary complex solvent and application of light-color lignin in sunscreen cream

Abstract

The invention discloses a method for extracting light-color lignin based on a ternary complex solvent and application of the light-color lignin in sunscreen cream, and belongs to the field of lignin separation and high-value utilization thereof. The method comprises the following specific steps: mixing choline chloride, lactic acid and acetic anhydride to prepare a ternary composite solvent, heating plant fiber raw materials and the ternary composite solvent to react, and extracting the light-color lignin. According to the method disclosed by the invention, the molar ratio of choline chloride to lactic acid to acetic anhydride is 1: 1: (3-5), and the extracted lignin is faint yellow and shows a lighter color compared with other disclosed technologies. And after the prepared light-color lignin is mixed with the sunscreen cream according to the doping amount of 8wt%, the sunscreen index SPF reaches 38 and is close to the requirements of commercial sunscreen cream, and the light-color lignin does not generate a stain phenomenon on the skin when being smeared and used, has certain practical value and effectively expands the application potential of the lignin in the fields of cosmetics, ultraviolet absorbers and the like.

Description

Technical Field

[0001] The present invention belongs to the field of lignin separation and its high-value utilization. Specifically, the present invention relates to a method for extracting light-colored lignin based on a ternary composite solvent and its application in sunscreen. Background Art

[0002] Lignin is a natural polymer with a three-dimensional network structure formed by three phenylpropane structural units, namely guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H), connected by carbon-carbon bonds and ether bonds. It has broad-spectrum ultraviolet absorption characteristics, excellent antioxidant activity, and low cytotoxicity, and is regarded as a candidate for natural sunscreen. Using natural sunscreens to replace or partially replace chemical sunscreens helps to alleviate problems such as skin penetration and ecological pollution caused by chemical sunscreens. However, lignin, usually derived from industrial by-products, has a relatively dark color, which becomes the main obstacle to its application in sunscreen cosmetics.

[0003] The anti-ultraviolet properties of lignin are mainly attributed to the conjugated structure formed by benzene rings and carbonyl groups, carbon-carbon double bonds, etc., and rich chromophores, including phenolic hydroxyl groups, methoxy groups, and p / o-semiquinones formed by them. Chromophores can cause further red shift of the ultraviolet absorption spectrum of lignin, which may be the reason for lignin coloring. During the lignin separation process, strong acids or high temperatures will cause the cleavage of β-O-4 bonds, accompanied by condensation reactions to form carbon-carbon bonds and introduce various chromogenic and auxochromic groups, resulting in its darker color. In the field of cosmetic applications, people are more inclined to accept light-colored lignin. Literature reports that methods such as solvent fractionation, chemical modification, or regulation of lignin micro-nano structures can reduce the color of industrial lignin to a certain extent.

[0004] However, to realize the application of lignin in sunscreen cosmetics, high purity and light color of lignin have always been the research goals. In recent years, the "lignin-first" strategy has shown good prospects in suppressing the condensation problem of lignin. Acidic solvents can promote the selective cleavage of the linkage bonds in the lignin-carbohydrate complex (LCC) of lignocellulose and the unstable ether bonds (e.g., β-O-4) in the lignin structural units, showing excellent delignification ability and high lignocellulose separation efficiency. However, in most separation processes based on acidic solvents, strong acidity or high reaction temperature still triggers the condensation reaction of lignin, forming stubborn carbon-carbon bonds. Therefore, researchers combined the acidolysis and structural stabilization functions of lignin into the acidic solvent system to inhibit the condensation reaction of the lignin structure. Some literature proposed introducing a series of diols into the choline chloride / oxalic acid system (LIU Y Z, DEAK N, WANG Z W, et al. Tunable and functional deep eutectic solvents for lignocellulose valorization[J]. Nat Commun, 2021, 12(1): p. 5424.), to achieve the capture of reactive benzyl carbocations, thus inhibiting the condensation pathway, providing the required "stabilization" function for the β-O-4 bond during the lignin extraction process, and significantly increasing the yield of phenolic monomers after hydrogenolysis or acidolysis and the yield of glucose after enzymatic hydrolysis. However, the extracted lignin still has a relatively dark color, which is not conducive to practical applications in fields such as cosmetics. Summary of the Invention

[0005] The purpose of the present invention is to develop a method for extracting light-colored lignin based on a ternary composite solvent, realize the highly light-colored utilization of lignin, and apply the extracted lignin as a sunscreen formula.

[0006] The present invention adopts the following technical solutions:

[0007] A method for extracting light-colored lignin based on a ternary composite solvent, comprising the following steps:

[0008] (1) Mix choline chloride, lactic acid and acetic anhydride, stir evenly to prepare a ternary composite solvent;

[0009] (2) After uniformly mixing the plant fiber raw material with the ternary composite solvent in step (1), carry out a heating reaction;

[0010] (3) After the reaction is completed, add an ethanol / water solution and mix evenly;

[0011] (4) Filter the mixture obtained in step (3), wash it with an ethanol / water solution, concentrate the filtrate and drop it into water to precipitate lignin, and obtain a light-colored lignin product after freeze-drying.

[0012] Further, the molar ratio of choline chloride, lactic acid and acetic anhydride in step (1) is 1:1:3-5.

[0013] Preferably, the molar ratio of choline chloride, lactic acid and acetic anhydride in step (1) is 1:1:3.

[0014] Further, the temperature of the stirring in step (1) is 60-100 °C.

[0015] Further, the plant cellulose raw material in step (2) is one or more of poplar, eucalyptus and rice straw.

[0016] Preferably, the plant cellulose raw material in step (2) is poplar.

[0017] Further, the mass ratio of the plant cellulose raw material to the ternary composite solvent in step (2) is 1:5-1:50, preferably 1:10-1:20.

[0018] Further, the heating reaction temperature in step (2) is 80-100 °C, and the reaction time is 4-6 h.

[0019] Further, the ethanol concentration of the ethanol / water solution in step (3) is 50%-70%, preferably 70%.

[0020] A kind of light-colored lignin extracted by the above method, the brightness (L*) of the light-colored lignin is 80-90, and the whiteness index is 70-80.

[0021] The application of the above light-colored lignin in the preparation of sunscreen, the addition amount of the light-colored lignin in the sunscreen is 2-8 wt%.

[0022] The present invention provides a ternary composite solvent composed of choline chloride, lactic acid and acetic anhydride to extract lignin. Based on the existing technology, an excessive amount of acetic anhydride is innovatively added to the solvent. The carbonyl oxygen (C=O) of acetic anhydride has lone pair electrons, which can form hydrogen bonds with the hydroxyl group of lactic acid, thereby enhancing the hydrogen bond network structure in the system, improving the ability to form hydrogen bonds with lignin, and further increasing the dissolution rate of lignin. At the same time, the acetic anhydride that does not participate in the formation of hydrogen bonds in the system reacts with the separated lignin, protects the lignin hydroxyl group, blocks the occurrence of condensation reaction, and realizes light-colored extraction.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] The invention provides a method for extracting light-colored lignin based on a ternary composite solvent. Choline chloride, lactic acid and acetic anhydride are used to synthesize the ternary composite solvent according to a specific molar ratio. The system can improve the lignin extraction efficiency. The extracted lignin is light yellow, the brightness (L*) reaches above 80, the whiteness index reaches above 70, and it has good compatibility with cream. The light-colored lignin prepared by the invention is added to sunscreen in an amount of 8wt%, and the SPF value reaches nearly 40, which is close to the requirement of commercial sunscreen, and will not cause staining to the skin, so it has practical application significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The infrared spectra corresponding to the composite solvent systems of Inventive Examples 1, 2, 3 and Comparative Examples 1 and 2 are shown.

[0026] Figure 2 It is a graph showing the lignin extraction rate of the composite solvent system of Inventive Examples 1, 2, 3 and Comparative Examples 1 and 2.

[0027] Figure 3 This is a color comparison chart of the light-colored lignin and the industrial lignin of Example 1 of the present invention.

[0028] Figure 4 This is the infrared spectrum of light-colored lignin and industrial lignin extracted in Invention Example 1.

[0029] Figure 5 This is a graph showing changes in ultraviolet transmittance of the light-colored lignin of Example 1 of the present invention after being mixed with facial cream and commercial sunscreen at addition amounts of 2wt%, 3wt%, 4wt%, 6wt% and 8wt%.

[0030] Figure 6 (a) is a picture of the lignin-based sunscreen obtained by mixing the light-colored lignin of Example 1 of the present invention with commercial sunscreen in amounts of 2wt%, 3wt%, 4wt%, 6wt% and 8wt%.

[0031] Figure 6 (b) is a diagram showing the color performance of a lignin-based sunscreen obtained by mixing the light-colored lignin of Example 1 of the present invention with commercial sunscreen in amounts of 2wt%, 3wt%, 4wt%, 6wt%, and 8wt%, after being applied to human skin. DETAILED DESCRIPTION

[0032] The present invention is described in detail below in conjunction with examples, but the implementation manner and protection scope of the present invention are not limited to the following examples.

[0033] To describe the technical solution of the present invention more clearly and completely, the following further details are provided through specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Various changes can be made within the scope defined by the claims of the present invention.

[0034] The choline chloride, lactic acid, and acetic anhydride used in the following examples and comparative examples are all commercially available products that can be directly purchased.

[0035] Test indicators and methods for examples and comparative examples:

[0036] (1) Whiteness of lignin: Use an X-Rite spectrophotometer to characterize the color of the lignin sample, and record the color parameters L * , a * , b * . The whiteness index calculation formula is:

[0037] WI = 100 - [(100 - L * ) 2 + a *2 + b *2 1 / 2

[0038] In the formula, the L* value represents the brightness of the color (positive for white, negative for black); the a* value represents the red-green attribute of the color (positive for red, negative for green); the b* value represents the yellow-blue attribute (positive for yellow, negative for blue); WI represents the degree of closeness to white.

[0039] (2) Sun protection factor (SPF) of lignin: Apply the mixed cream evenly on the quartz plate according to the standard dose of 2 mg / cm 2 . After drying in the dark room, test the ultraviolet transmittance of the mixed cream, and calculate the SPF value of the sun protection factor based on this:

[0040]

[0041] In the formula, E λ is the erythema effect, S λ is the solar spectral irradiance. The numerical values of E λ and S λ are all well-known data in the art; T λ is the transmittance of the sample.

[0042] Example 1

[0043] (1) Mix choline chloride, lactic acid, and acetic anhydride in a molar ratio of 1:1:3, and stir at 80 °C to form a homogeneous and transparent liquid.

[0044] (2) Add poplar powder and the ternary composite solvent obtained above to a flask at a mass ratio of 1:10, set the reaction temperature to 100 °C, and mechanically stir for 4 h using a paddle stirrer.

[0045] (3) After the reaction, quickly cool to room temperature, and add an ethanol / water solution with an ethanol concentration of 70% to the solid-liquid mixture obtained in step (2) and stir.

[0046] (4) Filter the mixture in step (3), recover the washing liquid by rotary evaporation of the filtrate, add the concentrated liquid obtained after rotary evaporation to 10 times the volume of deionized water for precipitation separation, and obtain light-colored lignin by freeze-drying.

[0047] (5) Prepare lignin-based sunscreen by mixing the light-colored lignin obtained in step (4) with commercial sunscreen at addition amounts of 2 wt%, 3 wt%, 4 wt%, 6 wt%, and 8 wt%.

[0048] Example 2

[0049] (1) Mix choline chloride, lactic acid, and acetic anhydride in a molar ratio of 1:1:1, and stir at 80 °C to form a homogeneous and transparent liquid.

[0050] (2) Add poplar powder and the prepared ternary composite solvent to a flask at a mass ratio of 1:10, set the reaction temperature to 100 °C, and mechanically stir for 4 h using a paddle stirrer.

[0051] (3) After the reaction, quickly cool to room temperature, and add an ethanol / water solution with an ethanol concentration of 70% to the solid-liquid mixture obtained in step (2) and stir.

[0052] (4) Filter the mixture in step (3), recover the washing liquid by rotary evaporation of the filtrate, add the concentrated liquid obtained after rotary evaporation to 10 times the volume of deionized water for precipitation separation, and freeze-dry.

[0053] (5) Prepare lignin-based sunscreen by mixing the lignin obtained in step (4) with commercial sunscreen at 2 wt%, 3 wt%, 4 wt%, 6 wt%, and 8 wt%.

[0054] Example 3

[0055] (1) Mix choline chloride, lactic acid, and acetic anhydride in a molar ratio of 1:1:2, and stir at 80 °C to form a homogeneous and transparent liquid.

[0056] (2) Add poplar powder and the prepared ternary composite molten solvent to a flask at a mass ratio of 1:10, set the reaction temperature to 100 °C, and mechanically stir for 4 h using a paddle stirrer.

[0057] (3) After the reaction is completed, quickly cool it to room temperature, and add an ethanol / water solution with an ethanol concentration of 70% to the solid-liquid mixture obtained in step (2) and stir.

[0058] (4) Filter the mixture in step (3), recover the washing solution by rotary evaporation for the filtrate, and add the concentrated solution obtained after rotary evaporation to 10 times the volume of deionized water for precipitation separation, followed by freeze-drying.

[0059] (5) Mix the lignin obtained in step (4) with commercial sunscreen at 2wt%, 3wt%, 4wt%, 6wt%, and 8wt% to prepare lignin-based sunscreen.

[0060] Example 4

[0061] (1) Mix choline chloride, lactic acid, and acetic anhydride in a molar ratio of 1:1:5, and stir at 80 °C to form a homogeneous transparent liquid.

[0062] (2) Add poplar wood powder and the obtained ternary composite solvent to a flask at a mass ratio of 1:10, set the reaction temperature to 100 °C, and use a stirrer paddle for mechanical stirring for 4 h.

[0063] (3) After the reaction is completed, quickly cool it to room temperature, and add an ethanol / water solution with an ethanol concentration of 70% to the solid-liquid mixture obtained in step (2) and stir.

[0064] (4) Filter the mixture in step (3), recover the washing solution by rotary evaporation for the filtrate, and add the concentrated solution obtained after rotary evaporation to 10 times the volume of deionized water for precipitation separation, and obtain light-colored lignin by freeze-drying.

[0065] (5) Mix the light-colored lignin obtained in step (4) with commercial sunscreen at 2wt%, 3wt%, 4wt%, 6wt%, and 8wt% to prepare lignin-based sunscreen.

[0066] Comparative Example 1

[0067] (1) Mix choline chloride and lactic acid in a molar ratio of 1:1, and stir at 80 °C to form a homogeneous transparent liquid.

[0068] (2) Add poplar wood powder and the prepared ternary composite solvent to a flask at a mass ratio of 1:10, set the reaction temperature to 100 °C, and use a stirrer paddle for mechanical stirring for 4 h.

[0069] (3) After the reaction is completed, quickly cool it to room temperature, and add an ethanol / water solution with an ethanol concentration of 70% to the solid-liquid mixture obtained in step (2) and stir.

[0070] (4) Filter the mixture in step (3). The filtrate is rotary evaporated to recover the washing solution, and the concentrated solution obtained after rotary evaporation is added to 10 times its volume of deionized water for precipitation separation, followed by freeze-drying.

[0071] (5) Mix the lignin obtained in step (4) with commercial sunscreen at 2 wt%, 3 wt%, 4 wt%, 6 wt%, and 8 wt% to prepare lignin-based sunscreen.

[0072] Comparative Example 2

[0073] (1) Mix choline chloride, lactic acid, and acetic acid in a molar ratio of 1:1:3 and stir at 80 °C to form a homogeneous transparent liquid.

[0074] (2) Add poplar powder and the prepared ternary composite solvent to a flask at a mass ratio of 1:10. Set the reaction temperature to 100 °C and use a stirring paddle for mechanical stirring for 4 h.

[0075] (3) After the reaction is completed, quickly cool to room temperature. Add an ethanol / water solution with an ethanol concentration of 70% to the solid-liquid mixture obtained in step (2) and stir.

[0076] (4) Filter the mixture in step (3). The filtrate is rotary evaporated to recover the washing solution, and the concentrated solution obtained after rotary evaporation is added to 10 times its volume of deionized water for precipitation separation, followed by freeze-drying.

[0077] (5) Mix the lignin obtained in step (4) with commercial sunscreen at 2 wt%, 3 wt%, 4 wt%, 6 wt%, and 8 wt% to prepare lignin-based sunscreen.

[0078] The good deconstruction of lignocellulose and the dissolution of lignin are mainly attributed to its strong hydrogen bond network. Therefore, in this invention, the hydrogen bond interactions in different solvent systems were analyzed through Examples 1-3 and Comparative Examples 1-2, as Figure 1 shown. Compared with the choline chloride and lactic acid binary composite solvent in Comparative Example 1 and the choline chloride, lactic acid, and acetic acid ternary composite solvent in Comparative Example 2, the position of the hydroxyl absorption peak of the ternary composite solvent composed of choline chloride, lactic acid, and acetic anhydride in the examples further shifts to lower wavenumbers, indicating the strengthening of its hydrogen bond interaction. This reveals that the addition of acetic anhydride participates in the formation of a new hydrogen bond network. The carbonyl oxygen (C=O) of acetic anhydride has a lone pair of electrons and can form a hydrogen bond with the hydroxyl group of lactic acid, enhancing the hydrogen bond network structure in the system. In particular, in the solvent system of Example 1, a carbonyl characteristic peak of acetic anhydride was observed at 1824 cm -1 −1, which means that there is acetic anhydride that has not formed a hydrogen bond in the system. These excess acetic anhydrides may participate in chemical reactions during the lignin extraction process.

[0079] Table 1 quantitatively evaluated the hydrogen bond acidity, hydrogen bond basicity, and solvent polarity of different composite solvent systems through solvatochromic parameters. Apparently, the addition of acetic anhydride decreased the polarity of the ternary composite solvent system, and as the proportion of acetic anhydride increased, the solvent polarity further decreased until it stabilized. A solvent system with a larger difference between hydrogen bond acidity and hydrogen bond basicity represents a stronger net hydrogen bond contribution ability of the composite solvent system and a stronger ability to form hydrogen bonds with lignin. As can be seen from the table, the difference between the hydrogen bond acidity and hydrogen bond basicity of the ternary composite solvent system in Example 1 is the largest, which means that during the lignin separation process, both the reaction rate and solvent permeability will be enhanced.

[0080] Table 1 Performance data of examples and comparative examples

[0081]

[0082]

[0083] The yields of lignin extracted in Examples 1, 2, 3 and Comparative Examples 1, 2 are as Figure 2 shown. The figure shows that the lignin extraction rate of Comparative Example 1 is 3.5%, and the lignin extraction rate of Comparative Example 2 is 5.7%. However, the lignin yield in Example 1 increased to 16.4%, which is 3.7 times and 1.9 times higher than that of Comparative Example 1 and Comparative Example 2, indicating the improvement in the efficiency of the ternary composite solvent for lignin extraction in Example 1. At the same time, the modification effect of acetic anhydride on the hydroxyl groups of lignin can reduce the intermolecular hydrogen bonds and polar interactions of lignin, reduce its binding force with cellulose / hemicellulose, and thus improve its extraction rate. In addition, the addition of acetic anhydride decreased the polarity of the ternary composite solvent system, thereby promoting the dissolution of separated lignin in the system. Therefore, the addition of acetic anhydride in Example 1 promoted the lignin extraction effect through participating in the formation of a new stronger hydrogen bond network structure and its chemical action. Moreover, the separated lignin in Example 1 had a reduced chromophore due to the hydroxyl group blocking effect, and its color became significantly lighter. Therefore, the ternary composite system in Example 1 has both high extraction efficiency and the ability to endow the separated lignin with good color characteristics.

[0084] The color comparison diagram of the light-colored lignin in Example 1 and industrial lignin is as Figure 3 shown. Apparently, industrial lignin is dark brown, while the light-colored lignin prepared in Example 1 of the present invention is light yellow, and its color is significantly lighter than that of industrial lignin.

[0085] The infrared spectra of the light-colored lignin in Example 1 and industrial lignin are as Figure 4 shown. The infrared spectrum of the light-colored lignin shows a characteristic saturated ester carbonyl absorption peak corresponding to 1747 cm -1 and a peak at 1370 cm -1The carboxylic acid ester group -O-(C=O)-CH corresponding to the position 3 The C-H bending vibration peak of and 1230 cm -1 The C-O stretching vibration peak of the acetyl group corresponding to the position indicates that the lignin extracted in Example 1 is acetylated lignin. The results show that the addition of acetic anhydride participates in the chemical reaction during the lignin extraction process, closes the lignin hydroxyl groups, and realizes the light-colored extraction of lignin. Light-colored lignin was also obtained in Example 4, but the effect was not as good as that of Example 1, so it is not that the more acetic anhydride, the better. In the present invention, the molar ratio of choline chloride, lactic acid, and acetic anhydride is preferably 1:1:3.

[0086] Table 2 Comparison of the brightness and whiteness indices of lignin extracted in each example and comparative example with industrial lignin

[0087] Sample Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Industrial lignin Brightness (L*) 80.39 32.92 37.80 46.51 41.55 27.43 Whiteness index (WI) 74.79 31.32 36.29 43.65 39.78 25.03

[0088] According to Table 2, the brightness and whiteness indices of the light-colored lignin extracted in Example 1 are much greater than those of the comparative example and industrial lignin. The brightness (L*) of the lignin extracted by this system reaches above 80, and the whiteness index reaches above 70, having certain practical value in the field of daily sunscreen skin care.

[0089] Table 3 Improvement effect of light-colored lignin on the sun protection performance of commercial sunscreen

[0090] Sample SPF value Commercial sunscreen (Qichu baby water-sensitive moisturizing sunscreen) 16 2 wt% + commercial sunscreen 29 3 wt% + commercial sunscreen 30 4 wt% + commercial sunscreen 32 6 wt% + commercial sunscreen 35 8 wt% + commercial sunscreen 38

[0091] Table 3 shows the sun protection index (SPF) of the lignin-based sunscreen prepared by mixing the light-colored lignin of Example 1 with a commercial sunscreen. It can be seen that adding 2 wt% of lignin can increase the SPF value of the commercial sunscreen by 81%. Combining Figure 5 with the ultraviolet transmittance in, the ultraviolet transmittance of the lignin-based sunscreen is reduced compared with the commercial sunscreen in the range of 290 - 400, showing a broad-spectrum sun protection performance, indicating that lignin is an additive with a full ultraviolet band protection effect. When the lignin addition amount is 8%, the SPF value of the lignin-based sunscreen reaches 38, approaching the requirements of sunscreens on the market, and having practical application value in the field of daily sunscreen skin care products.

[0092] The pictures of the lignin-based sunscreens obtained by mixing the light-colored lignin of Example 1 with a commercial sunscreen at addition amounts of 2 wt%, 3 wt%, 4 wt%, 6 wt%, and 8 wt% and the color performance effect pictures after applying them to human skin are as shown in Figure 6 (a) and (b) in. The sunscreen added with lignin is light yellow, and even when the addition amount reaches 8 wt%, it will not cause staining to human skin and is easily acceptable to consumers, further illustrating the practical application value of the light-colored lignin of the present invention in sunscreen products.

[0093] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for extracting light-colored lignin based on a ternary composite solvent, characterized in that: The following steps are involved: (1) mixing choline chloride, lactic acid and acetic anhydride, stirring evenly, and preparing a ternary composite solvent; (2) uniformly mixing the plant fiber raw material with the ternary composite solvent of step (1), and heating the mixture for reaction; (3) After the reaction is completed, add ethanol / water solution and mix well; (4) filtering the mixture obtained in step (3) and washing it with an ethanol / water solution; concentrating the filtrate and dropping it into water to precipitate lignin; and freeze-drying the filtrate to obtain a light-colored lignin product.

2. The method according to claim 1, characterized in that The molar ratio of choline chloride, lactic acid and acetic anhydride in step (1) is 1:1:3-5.

3. The method according to claim 1, characterized in that The molar ratio of choline chloride, lactic acid and acetic anhydride in step (1) is 1:1:

3.

4. The method according to claim 1, characterized in that: The plant fiber raw material in step (2) is one or more of poplar, eucalyptus and rice straw.

5. The method according to claim 1, characterized in that The mass ratio of the plant fiber raw material to the ternary composite solvent in step (2) is 1:5 to 1:

50.

6. The method according to claim 1, characterized in that The heating reaction temperature in step (2) is 80-100° C., and the reaction time is 4-6 hours.

7. The method according to claim 1, characterized in that The ethanol concentration of the ethanol / water solution in step (3) is 50% to 70%.

8. The light-colored lignin extracted by the method according to any one of claims 1 to 7, characterized in that: The brightness of the light-colored lignin is 80-90, and the whiteness index is 70-80.

9. Use of the light-colored lignin according to claim 8 in the preparation of lignin-based sunscreen.

10. The use according to claim 9, characterized in that: The amount of light-colored lignin added to the sunscreen is 2 to 8 wt%.

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