Dual modified lignin as well as preparation method and application thereof

Abstract

The invention discloses double modified lignin as well as a preparation method and application thereof, and belongs to the technical field of lignin modification. The preparation method of the double modified lignin comprises the following steps: S1, mixing lignin with an epoxidation reagent, and carrying out epoxy grafting under the action of a catalyst to obtain epoxidized lignin; and S2, mixing the epoxidized lignin with an acetylation reagent, and carrying out acetylation modification on the epoxidized lignin under the catalysis of lipase to obtain the double modified lignin. According to the preparation method disclosed by the invention, acetylation modification of the lignin can be realized while an epoxy group is reserved, and the prepared double-modified lignin has high compatibility, dispersity and reaction activity; the coating prepared by mixing the modified epoxy resin with epoxy resin has excellent mechanical properties, and also shows excellent water resistance, salt mist resistance, ultraviolet resistance and antibacterial adhesion performance.

Description

Technical Field

[0001] This invention relates to the field of lignin modification technology, and in particular to a dual-modified lignin, its preparation method, and its application. Background Technology

[0002] Lignin is the main component of lignocellulose and an important biomass resource. As a high-molecular-weight compound rich in aromatic ring structures, lignin is also a non-fossil resource in nature that can provide renewable aryl compounds. Due to its wide availability, structural diversity, and eco-friendliness, lignin has gained widespread recognition in the field of green materials. Its applications include composite biocoatings, stimulus-responsive drug delivery carriers, flexible strain sensors, and functionalized hydrogels, among which research on lignin-based composite coatings has developed rapidly.

[0003] However, natural lignin contains a large number of phenolic and alcoholic hydroxyl groups. These functional groups easily form a dense hydrogen bond network between molecules, causing lignin to easily aggregate within organic matrices. This results in poor compatibility, dispersibility, and reactivity, severely limiting its further application in high-performance materials. To improve the compatibility of lignin, chemical modification is necessary. Acetylation is a common method, which can reduce the hydroxyl content by introducing hydrophobic acetyl groups, thereby improving its compatibility and processing properties. However, in the traditional acetylation process, the introduction of acetyl groups destroys other important functional groups on lignin, such as epoxy groups, thus compromising the functional integrity of the modified lignin.

[0004] Therefore, developing a mild modification method that can retain key functional groups and improve the chemical reactivity, dispersibility, and material compatibility of lignin is of positive significance for expanding the application of lignin in the field of composite coating materials. Summary of the Invention

[0005] To address the aforementioned problems in existing technologies, this invention provides a dual-modified lignin, its preparation method, and its applications. This invention sequentially performs epoxidative grafting and acetylation modification on lignin to obtain a dual-modified lignin with high interfacial compatibility and reactivity.

[0006] The technical solution of the present invention is as follows: The first aspect of this invention protects a method for preparing dual-modified lignin, comprising the following steps: S1. Mix lignin with an epoxidizing agent and perform epoxidative grafting under the action of a catalyst to obtain epoxidized lignin. S2. The epoxidized lignin is mixed with an acetylation reagent, and the epoxidized lignin is acetylated under the catalysis of lipase to obtain a double-modified lignin.

[0007] Preferably, in S1, the lignin includes at least one of sodium lignin sulfonate, alkali lignin, dealkalized lignin, enzymatically hydrolyzed lignin, alcoholic cassava lignin, and pine lignin; And / or, the epoxidizing agent includes epichlorohydrin; And / or, the mass ratio of the lignin to the epoxidizing agent is (0.5~2):(4~10).

[0008] Preferably, in S1, the catalyst includes at least one of a phase transfer catalyst and an alkaline catalyst; The phase transfer catalyst includes at least one of tetrabutylammonium bromide, benzyltriethylammonium chloride, and tetrabutylammonium hydrogen sulfate; The alkaline catalyst includes sodium hydroxide; The mass ratio of lignin to phase transfer catalyst is (100~300):(0.5~2). The mass ratio of lignin to alkali catalyst is (3~8):(0.5~2).

[0009] Preferably, the specific steps of S1 include: mixing the lignin with the epoxidizing agent, adding the phase transfer catalyst and the base catalyst, reacting at temperature T1 for 2-6 hours, cooling to temperature T2 and then adding the base catalyst, then heating to temperature T3 and holding for 0.5-3.5 hours, filtering, washing the obtained solid to neutral, and freeze-drying at -100 to -60°C for 18-30 hours to obtain epoxidized lignin; The temperature T1 is greater than the temperature T2, the temperature T1 is 65~95℃, and the temperature T2 is 40~65℃; The temperature T2 is less than the temperature T3, and the temperature T3 is 50~70℃.

[0010] Preferably, in S2, the acetylation reagent includes at least one of ethyl acetate, methyl acetate, propyl acetate, and butyl acetate; the lipase includes at least one of Novozymes 435, Rhodococcus TrLipE lipase, Aspergillus niger lipase, Aspergillus oryzae lipase, and Mucor lipase. The mass ratio of the epoxidized lignin to the acetylation reagent is (0.5~2):(4~10). The mass ratio of the epoxidized lignin to the lipase is (10~50):(0.5~2).

[0011] Preferably, the specific steps in S2 include: mixing the epoxidized lignin with the acetylation reagent, adding the lipase, reacting at 30~80℃ for 18~30h, filtering, washing the obtained solid until neutral, and freeze-drying at -100~-60℃ for 18~30h to obtain double-modified lignin.

[0012] A second aspect of the present invention protects a dual-modified lignin, which is prepared by the preparation method described in the first aspect.

[0013] A third aspect of this invention protects the application of a dual-modified lignin in the field of coatings, the dual-modified lignin comprising the dual-modified lignin described in the second aspect, and / or the dual-modified lignin prepared by the preparation method described in the first aspect.

[0014] Preferably, the double-modified lignin is mixed with epoxy resin, then additives are added and mixed evenly before being sprayed onto the surface of the substrate to obtain a coating.

[0015] Preferably, the epoxy resin includes at least one of E51 and E44; And / or, the mass ratio of the dual-modified lignin to epoxy resin is (0.5~2):(7~9.5); And / or, the additives include at least one of curing agents and diluents.

[0016] The beneficial technical effects of this invention are as follows: (1) In this invention, lignin is first grafted with an epoxidizing agent to obtain epoxidized lignin. Then, lipase is used as a catalyst to acetylate the epoxidized lignin while retaining the epoxy group, thereby obtaining a dual-modified lignin with high compatibility, dispersibility and reactivity.

[0017] (2) The dual-modified lignin of the present invention can form hydrogen bonds or other intermolecular forces with epoxy resin, which enhances the compatibility and dispersibility between materials. The coating made from the two raw materials has improved mechanical properties and mechanical properties. At the same time, the coating also exhibits excellent water resistance, salt spray resistance, UV resistance and antibacterial adhesion properties.

[0018] (3) The lignin used in this invention is agricultural waste, which is widely available, non-toxic and harmless. It is a green and low-cost raw material that alleviates the environmental pressure caused by the large-scale application of plastics. It can not only make use of waste, but is also more environmentally friendly. The preparation method adopted in this invention has mild reaction conditions and does not produce harmful byproducts, which is in line with the principles of green chemistry. Attached Figure Description

[0019] Figure 1 This is a flowchart of the preparation of dual-modified lignin and coating according to the present invention.

[0020] Figure 2 The infrared spectra of alcoholic cassava lignin, epoxidized alcoholic cassava lignin, and double-modified alcoholic cassava lignin in Example 7 are shown.

[0021] Figure 3The graph shows the hardness test results of the coatings in Example 18 and Comparative Examples 1-3.

[0022] Figure 4 The figures show the water resistance and salt spray resistance test results of the coatings in Example 18 and Comparative Examples 1-3; In the figure, a represents the water resistance test result of the coating of Comparative Example 1; b represents the water resistance test result of the coating of Comparative Example 2; c represents the water resistance test result of the coating of Comparative Example 3; and d represents the water resistance test result of the coating of Example 18. d represents the salt spray resistance test result of the coating in Comparative Example 1; e represents the salt spray resistance test result of the coating in Comparative Example 2; f represents the salt spray resistance test result of the coating in Comparative Example 3; d represents the salt spray resistance test result of the coating in Example 18.

[0023] Figure 5 The image shows the UV resistance test results of the coatings in Example 18 and Comparative Examples 1-3.

[0024] Figure 6 The graph shows the antibacterial adhesion test results of the coatings in Example 18 and Comparative Examples 1-3. In the figure, a represents the antibacterial adhesion test of the coatings in Example 18 and Comparative Examples 1-3; b represents the optical density value of the bacterial solution in the coatings in Example 18 and Comparative Examples 1-3. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0026] A method for preparing dual-modified lignin includes the following steps: S1. Mix lignin with an epoxidizing agent and perform epoxidative grafting under the action of a catalyst to obtain epoxidized lignin. S2. The epoxidized lignin is mixed with an acetylation reagent, and the epoxidized lignin is acetylated under the catalysis of lipase to obtain a double-modified lignin.

[0027] In some embodiments, the mass ratio of lignin to the epoxidizing agent is (0.5~2):(4~10); preferably, the epoxidizing agent is epichlorohydrin.

[0028] In some embodiments, the mass ratio of lignin to the phase transfer catalyst is (100~300):(0.5~2); preferably, the phase transfer catalyst is tetrabutylammonium bromide.

[0029] In some embodiments, the mass ratio of lignin to alkali catalyst is (3~8):(0.5~2); preferably, the alkali catalyst is sodium hydroxide.

[0030] In some embodiments, the specific steps of S1 include: mixing the lignin with the epoxidizing agent, adding the phase transfer catalyst and the base catalyst, reacting at temperature T1 for 2-6 hours, cooling to temperature T2 and then adding the base catalyst, then heating to temperature T3 and holding for 0.5-3.5 hours, filtering, washing the obtained solid to neutral, and freeze-drying at -100 to -60°C for 18-30 hours to obtain epoxidized lignin; The temperature T1 is greater than the temperature T2, the temperature T1 is 65~95℃, and the temperature T2 is 40~65℃; The temperature T2 is less than the temperature T3, and the temperature T3 is 50~70℃.

[0031] In step S1, the alkali catalyst is added in two stages, with the mass ratio of lignin to the total amount of alkali catalyst added in both stages being (3~8):(0.5~2). Preferably, the alkali catalyst is added in two stages.

[0032] In some embodiments, the mass ratio of epoxidized lignin to the acetylation reagent is (0.5~2):(4~10); preferably, the acetylation reagent is ethyl acetate.

[0033] This invention first epoxidizes lignin to obtain epoxidized lignin, and then performs acetylation modification on this lignin. Compared with traditional acetylation strategies, the acetylation modification using lipase and ethyl acetate does not destroy the integrity of the epoxy groups. Ethyl acetate has the dual function of acetylation reagent and solvent, and it can be recovered and reused by distillation after the reaction, which not only meets the principles of green chemistry but also avoids the use of traditional highly polluting acetylation reagents. Moreover, the reaction conditions are mild, the epoxy groups in the system remain stable, do not destroy the functional integrity of the modified lignin, and do not produce harmful byproducts.

[0034] An application of dual-modified lignin in the field of coating involves mixing the dual-modified lignin with epoxy resin, adding additives and mixing evenly, and then spraying the mixture onto the surface of a substrate to obtain a coating.

[0035] In some embodiments, the mass ratio of the dual-modified lignin to the epoxy resin is (0.5~2):(7~9.5); preferably, the epoxy resin is E51.

[0036] In some embodiments, the additives include at least one of a curing agent and a diluent.

[0037] Preferably, the curing agent includes at least one of polyamide 650, polyamide 651, and polyamide 140; the amount of curing agent added is 50-60 wt% of the total mass of the dual-modified lignin and epoxy resin.

[0038] Preferably, the diluent includes at least one of ethyl acetate, propyl acetate, and butyl acetate; the amount of the diluent added is 30-40 wt% of the total mass of the dual-modified lignin, epoxy resin, and curing agent.

[0039] In some embodiments, the substrate to be sprayed includes at least one of steel plate, copper plate, and plastic, and the coating thickness is 50±5 μm; the curing temperature after spraying is 50~70℃ and the time is 10~14 h.

[0040] The dual-modified lignin of this invention can enhance intermolecular hydrogen bonds, exhibiting excellent dispersibility and interfacial compatibility in epoxy resins, thereby increasing crosslinking density and improving structural integrity. Acetylation modification improves the flexibility of molecular chains, giving the coating good buffering and deformation capabilities under stress; at the same time, the microphase separation structure that may form in the system helps with stress dispersion and energy dissipation. The synergistic effect of these factors significantly improves the impact resistance and toughness of the coating.

[0041] The acetyl groups and other auxochrome groups in the dual-modified lignin of this invention can form a large conjugated π system with the benzene ring, which can strongly absorb ultraviolet light. At the same time, acetylation significantly enhances the hydrophobicity of lignin, promotes the formation of a dense surface structure of lignin particles inside the coating, and reduces ultraviolet light transmission through reflection or scattering, so that the coating exhibits the best resistance to ultraviolet aging.

[0042] Maintaining the epoxy crosslinking network helps reduce bond strain energy, thereby enhancing the structural stability of ester bonds in the system and improving its resistance to hydrolysis. Simultaneously, ether bonds effectively quench free radicals and block oxidative degradation. Furthermore, acetylation modification oxidizes and inertizes the CH bonds on the lignin molecule side chains, further improving the coating's antioxidant properties. In addition, the reduced water absorption of the coating effectively suppresses plasticization and reduces molecular chain migration under high humidity conditions. The synergistic effect of covalent modification, free radical quenching, and interfacial hydrophobicity provides a multi-scale protection mechanism against hydrolysis, oxidation, and softening at the molecular level, in terms of chemical stability and interfacial properties, showing broad application prospects in environmentally friendly multifunctional composite coatings.

[0043] The raw materials used in the following embodiments and comparative examples of the present invention, such as lignin, lipase, and epoxy resin E51, are all commercially available raw materials that can be obtained by purchase.

[0044] Example 1 A method for preparing dual-modified lignin includes the following steps: S1. In a three-necked flask equipped with a spherical condenser, a nitrogen inlet tube, a stirrer, and a thermometer, add 100 g of sodium lignosulfonate, followed by 400 ml of epichlorohydrin solution, 1 g of tetrabutylammonium bromide, and 10 g of sodium hydroxide to catalyze the epoxidative grafting of lignin. The mixture is heated to 80°C and reacted for 4 hours. Then, the temperature is lowered to 55°C, and 10 g of NaOH is added. The temperature is then raised to 60°C and maintained for 2 hours. The mixture is filtered, and the lignin is washed with water until neutral. Finally, it is freeze-dried at -80°C for 24 hours to obtain sodium epoxidized lignosulfonate.

[0045] S2. Dissolve 50 g of sodium epoxidized lignin sulfonate in 400 ml of ethyl acetate solvent, and add 5 g of Novozymes 435 lipase. Acetylate modification of sodium epoxidized lignin sulfonate is catalyzed by lipase, and the temperature is maintained at 60℃ for 24 h. Filter, wash the lignin with water until neutral, and freeze-dry at -80℃ for 24 h to obtain double-modified sodium lignin sulfonate.

[0046] Example 2 A method for preparing dual-modified lignin is basically the same as that in Example 1, except that 100 g of alkali lignin is added and freeze-dried at -80°C for 24 h in a freeze dryer to obtain dual-modified alkali lignin; the rest is the same as in Example 1.

[0047] Example 3 A method for preparing dual-modified lignin is basically the same as that in Example 1, except that 100 g of dealkalized lignin is added and freeze-dried at -80°C for 24 h in a freeze dryer to obtain dual-modified dealkalized lignin; the rest is the same as in Example 1.

[0048] Example 4 A method for preparing dual-modified lignin is basically the same as that in Example 1, except that 100 g of enzymatically hydrolyzed lignin is added and freeze-dried at -80°C for 24 h in a freeze dryer to obtain dual-modified enzymatically hydrolyzed lignin; the rest is the same as in Example 1.

[0049] Example 5 A method for preparing dual-modified lignin is basically the same as that in Example 1, except that 100 g of alcoholic cassava lignin is added and freeze-dried at -80°C for 24 h in a freeze dryer to obtain dual-modified alcoholic cassava lignin; the rest is the same as in Example 1.

[0050] Example 6 A method for preparing dual-modified lignin is basically the same as that in Example 1, except that 100 g of pine lignin is added and freeze-dried at -80℃ for 24 h in a freeze dryer to obtain dual-modified pine lignin; the rest is the same as in Example 1.

[0051] Example 7 A method for preparing dual-modified lignin includes the following steps: S1. Add 100 ml of ethanol-coated cassava lignin to a three-necked flask equipped with a spherical condenser, nitrogen inlet tube, stirrer, and thermometer. Then add 400 ml of epichlorohydrin solution, 1 g of tetrabutylammonium bromide, and 10 g of sodium hydroxide to catalyze the epoxidative grafting of lignin. Heat to 80°C and react for 4 hours. Then cool to 55°C, add 10 g of NaOH, and heat to 60°C for another 2 hours. Filter, wash the lignin with water until neutral, and freeze-dry at -80°C for 24 hours to obtain epoxidized ethanol-coated cassava lignin.

[0052] S2. Dissolve 50 g of epoxidized alcoholic cassava lignin in 400 ml of ethyl acetate solvent, and add 5 g of Rhodococcus TrLipE lipase lyophilized powder. Acetylate modification of the epoxidized alcoholic cassava lignin is carried out using TrLipE catalysis at 60℃ for 24 h. After filtration, the lignin is washed with water until neutral and then freeze-dried at -80℃ for 24 h to obtain double-modified alcoholic cassava lignin.

[0053] The raw material ethanol cassava lignin, the intermediate product epoxidized ethanol cassava lignin, and the product doubly modified ethanol cassava lignin from the above preparation process were analyzed by Fourier transform infrared spectroscopy, and the results are as follows: Figure 2 As shown. From Figure 2 It can be seen that 916 cm -1 The absorption peak at 1750 cm⁻¹ is significantly enhanced, indicating that the hydroxyl groups of epichlorohydrin reacted with alcoholic cassava lignin under the catalysis of tetrabutylammonium bromide, successfully introducing epoxy groups; -1 The C=O stretching vibration peak was significantly enhanced at 1200 cm⁻¹. -1 With 1128 -1 The enhanced CO bond signal at cm confirmed the effective grafting of the acetyl group; furthermore, at 1600 cm... -1 1500 cm -1 and 1450cm -1 The presence of the absorption peak due to the vibration of the benzene ring skeleton indicates that the basic structure of lignin remained intact during the reaction.

[0054] Example 8 A method for preparing dual-modified lignin is basically the same as in Example 7, except that 5 g of Aspergillus niger lipase lyophilized powder is added. The lignin is acetylated by epoxidation of alcoholic cassava using Aspergillus niger lipase catalyzed at 60°C for 24 h. After filtration, the lignin is washed with water until neutral and then freeze-dried at -80°C for 24 h to obtain dual-modified alcoholic cassava lignin; the rest is the same as in Example 7.

[0055] Example 9 A method for preparing dual-modified lignin is basically the same as in Example 7, except that 5 g of Aspergillus oryzae lipase lyophilized powder is added, and the acetylation modification of alcoholic cassava lignin is carried out by catalytic epoxidation of Aspergillus oryzae lipase, and the temperature is maintained at 60°C for 24 h. After filtration, the lignin is washed with water until neutral, and then freeze-dried in a freeze dryer at -80°C for 24 h to obtain dual-modified alcoholic cassava lignin; the rest is the same as in Example 7.

[0056] Example 10 A method for preparing dual-modified lignin is basically the same as in Example 7, except that 5 g of *Rhizopus* lipase lyophilized powder is added, and the *Rhizopus* lipase catalyzes the epoxidation of alcoholic cassava lignin for acetylation modification, with the temperature maintained at 60°C for 24 h. After filtration, the lignin is washed with water until neutral, and then freeze-dried in a freeze dryer at -80°C for 24 h to obtain dual-modified alcoholic cassava lignin; the rest is the same as in Example 7.

[0057] Example 11 A method for preparing dual-modified lignin is basically the same as that in Example 7, except that in step S2, the temperature is maintained at 50°C for 24 hours; otherwise, it is the same as in Example 7.

[0058] Example 12 A method for preparing dual-modified lignin is basically the same as that in Example 7, except that in step S2, the temperature is maintained at 40°C for 24 hours; otherwise, it is the same as in Example 7.

[0059] Example 13 A method for preparing dual-modified lignin is basically the same as that in Example 7, except that in step S2, the temperature is maintained at 30°C for 24 hours; otherwise, it is the same as in Example 7.

[0060] Example 14 A method for preparing dual-modified lignin is basically the same as that in Example 7, except that in step S1, 500 ml of epichlorohydrin solution is added; in step S2, the temperature is maintained at 30°C for 24 h; the rest is the same as in Example 7.

[0061] Example 15 A method for preparing dual-modified lignin is basically the same as that in Example 7, except that in step S1, 600 ml of epichlorohydrin solution is added; in step S2, the temperature is maintained at 30°C for 24 h; the rest is the same as in Example 7.

[0062] Example 16 A method for preparing dual-modified lignin is basically the same as that in Example 7, except that in step S2, 50 g of epoxidized lignin is dissolved in 500 ml of ethyl acetate solvent; in step S2, the temperature is maintained at 30°C for 24 h; the rest is the same as in Example 7.

[0063] Example 17 A method for preparing dual-modified lignin is basically the same as that in Example 7, except that in step S2, 50 g of epoxidized lignin is dissolved in 600 ml of ethyl acetate solvent; in step S2, the temperature is maintained at 30°C for 24 h; the rest is the same as in Example 7.

[0064] Example 18 A method for preparing a dual-modified lignin coating includes the following steps: The dual-modified alcoholic cassava lignin obtained in Example 7 is mixed uniformly with epoxy resin E51 at a mass ratio of 1:9 using high-speed mixing; Polyamide 651 is added as a curing agent and dispersed uniformly; the amount of curing agent added is 55 wt% of the total mass of the dual-modified alcoholic cassava lignin and epoxy resin E51; finally, butyl acetate is added as a diluent; the amount of diluent added is 35 wt% of the total mass of the dual-modified alcoholic cassava lignin, epoxy resin E51, and curing agent polyamide 651, resulting in a coating EALC. The coating EALC is then sprayed onto a steel plate using a spraying method, with a thickness of 50 ± 5 μm; and cured at 60°C for 12 h to obtain the dual-modified lignin coating (EALC).

[0065] Comparative Example 1 A method for preparing a coating involves uniformly dispersing epoxy resin E51 and curing agent polyamide 651, with the amount of curing agent added being 55 wt% of the total mass of epoxy resin E51. Finally, butyl acetate is added as a diluent, with the amount of diluent added being 35 wt% of the total mass of epoxy resin E51 and curing agent polyamide 651, to obtain a coating. The coating is then sprayed onto a steel plate using a spraying method to obtain a coating (EC). The spraying specifications are the same as in Example 18.

[0066] Comparative Example 2 A method for preparing a coating is basically the same as in Example 18, except that the dual-modified alcoholic cassava lignin is replaced with the alcoholic cassava lignin in Example 7, thereby obtaining the coating (LC); the rest is the same as in Example 18.

[0067] Comparative Example 3 A method for preparing a coating is basically the same as in Example 18, except that the dual-modified alcoholic cassava lignin is replaced with the epoxidized alcoholic cassava lignin in Example 7, thereby obtaining the coating (ELC); the rest is the same as in Example 18.

[0068] Test case The coatings prepared in Example 18 and Comparative Examples 1-3 were applied to steel plate substrates with a coating thickness of 50±5μm and cured at room temperature for 7 days to obtain the coating.

[0069] 1. The thickness, pencil hardness, pendulum hardness, impact resistance, and toughness of the coatings in Example 18 and Comparative Examples 1-3 were tested according to the following methods. The test results are shown in Table 1 and... Figure 3 As shown.

[0070] Test methods: Coating thickness is tested according to GB / T 13452.2, pencil hardness is tested according to GB / T 6739, impact resistance is tested according to GB / T 1732, and toughness is tested according to GB / T 1731.

[0071] Table 1: Coating performance test results of Example 18 and Comparative Examples 1-3

[0072] According to Table 1 and Figure 3 As can be seen, with the same coating thickness, compared to the coatings of Comparative Examples 1-3, the coating of Example 18 shows a slight increase in pendulum hardness and pencil hardness, and significantly enhanced impact resistance and toughness. This indicates that the coating prepared with dual-modified lignin (Example 18) has better overall performance than the coating prepared without lignin (Comparative Example 1), the coating prepared with unmodified lignin (Comparative Example 2), and the coating prepared with epoxidized lignin (Comparative Example 3). This may be because the dual-modified lignin containing epoxy and acetyl groups prepared by the method of this invention can form hydrogen bonds or other intermolecular forces with epoxy resin, thereby enhancing the compatibility and dispersibility between materials. Excellent compatibility and dispersibility allow the dual-modified lignin to be more evenly distributed in the E51 matrix, reducing stress concentration problems caused by uneven distribution, thereby comprehensively improving the overall performance of the coating.

[0073] 2. The water resistance and salt spray resistance of the coatings in Example 18 and Comparative Examples 1-3 were tested using the following method. The test results are as follows: Figure 4 As shown.

[0074] Test methods: Water resistance was tested according to GB / T 1733-1993; salt spray resistance was tested according to GB / T 1771-2007.

[0075] according to Figure 4As shown in a, b, c, and d, rust spots of different shapes and sizes appeared on the coating surfaces of Comparative Examples 1 to 3. Among them, the coating LC (Comparative Example 2) made with unmodified lignin showed the most significant corrosion, with rust spots evenly distributed on the coating surface. This may be due to the fact that the polyhydroxy structure of lignin easily adsorbs water molecules, causing the coating to absorb water, swell, and lose its density. At the same time, its phenolic hydroxyl groups may react with Fe. 2+ / Fe 3+ Complexation accelerates the active dissolution of metals; the surface corrosion of the ELC coating (Comparative Example 3) prepared with epoxidized lignin is less than that of the LC coating; the surface of the EALC coating (Example 18) prepared with dual-modified lignin is almost free of corrosion.

[0076] according to Figure 4 As can be seen from g, f, g, and h, in the salt spray resistance test, especially in the coating cut area, although all coatings showed corrosion, the LC corrosion of the coating was the most significant. This is attributed to the poor dispersion of lignin in the coating and the high hydrophilicity of its hydroxyl groups, which easily adsorb moisture and Cl from the salt spray. - This exacerbates coating corrosion; the amount of corrosion products on the ELC coating surface is less than that on the LC coating. The EALC coating produces the fewest corrosion products; this is because the introduction of acetyl groups significantly reduces the coating's water absorption rate, maximizing the barrier against Cl. - It penetrates with H2O, while reducing redox activity and weakening the catalytic effect of the coating on substrate corrosion. In addition, the polar compatibility between the dual-modified lignin and E51 epoxy resin is improved, which improves material compatibility, reduces phase separation and interface defects, makes the coating structure denser, and enhances barrier properties.

[0077] 3. The UV resistance of the coatings in Example 18 and Comparative Examples 1-3 was tested using the following method, and the test results are as follows: Figure 5 As shown.

[0078] Test method: UV resistance was tested according to GB / T 1865-2009.

[0079] according to Figure 5 It can be seen that the coating LC (Comparative Example 2) made with unmodified lignin has a better protective effect than the coating EC (Comparative Example 1) without lignin. However, the unmodified lignin particles form micro-aggregates in the coating, resulting in uneven lignin distribution and incomplete surface coverage, allowing ultraviolet light to still penetrate the resin-rich areas. The coating EALC in Example 18 has a better protective effect than Comparative Examples 1-3. This is because the acetyl groups in the double-modified lignin reduce the polarity of the lignin, making it highly compatible with the hydrophobic segments of E51 epoxy resin, achieving molecular-level dispersion, and thus exhibiting the best shielding effect.

[0080] 4. The antibacterial adhesion properties of the coatings in Example 18 and Comparative Examples 1-3 were tested using the following method. The test results are as follows: Figure 6 As shown.

[0081] Test method: Biofilm formation was promoted by extending the incubation time of *Pseudomonas aeruginosa*. 1 mL of bacterial suspension (10...) 8 CFU / mL was added to 24-well plates containing samples, with three replicates per group. After incubating the bacteria at 30 °C for 72 hours, the bacterial solution was removed by pipette, and the plates were carefully rinsed three times with ASW to remove any unattached biofilm. The biofilm was then fixed onto the coating with methanol, stained with crystal violet, and excess crystal violet was rinsed off with ASW. The biofilm was then photographed with a digital camera. Finally, 33% AcOH was added to dissolve the dye on the biofilm, and the absorbance at 589 nm was measured using a ELISA reader (Tecan Infinite M200Pro). The survival rate of the treated biofilm relative to the control was calculated.

[0082] Optical density (OD) is positively correlated with biofilm quality. According to... Figure 6 As can be seen, the OD value of the LC coating in Comparative Example 2 is as high as 1.08. This is because the lignin aggregation effect leads to an increase in the particle size of the coating surface, providing attachment points for bacterial adsorption. The OD value of the EC coating in Comparative Example 1 is 0.916, which shows a certain degree of anti-biofilm adhesion ability, but it is worse than the ELC coating in Comparative Example 3 (OD value 0.667) and even worse than the EALC coating in Example 18 (OD value 0.572). That is, the EALC coating prepared with dual-modified lignin has the lowest biofilm adhesion on its surface and exhibits the best antibacterial adhesion performance, with a 62.4% improvement in anti-biofilm efficiency compared to EC. This is because the acetyl groups reduce the hydrophilicity of the coating, forming a hydrophobic barrier that is unfavorable to bacterial adsorption and growth. In summary, the coating prepared with dual-modified lignin can effectively inhibit biofilm adhesion and achieve a long-lasting antifouling effect.

[0083] In summary, the dual-modified lignin prepared by the method of this invention can enhance the compatibility and dispersibility between materials. The coating made from it has improved pencil hardness, impact toughness and pendulum hardness. At the same time, the coating also exhibits excellent water resistance, salt spray resistance, UV resistance and antibacterial adhesion properties.

[0084] The above description is merely a preferred embodiment of the present invention, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations that are directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.

Claims

1. A method for preparing dual-modified lignin, characterized in that, Includes the following steps: S1. Mix lignin with an epoxidizing agent and perform epoxidative grafting under the action of a catalyst to obtain epoxidized lignin. S2. The epoxidized lignin is mixed with an acetylation reagent, and the epoxidized lignin is acetylated under the catalysis of lipase to obtain a double-modified lignin.

2. The method for preparing dual-modified lignin according to claim 1, characterized in that, In S1, the lignin includes at least one of sodium lignin sulfonate, alkali lignin, dealkalized lignin, enzymatically hydrolyzed lignin, alcoholic cassava lignin, and pine lignin. And / or, the epoxidizing agent includes epichlorohydrin; And / or, the mass ratio of the lignin to the epoxidizing agent is (0.5~2):(4~10).

3. The method for preparing dual-modified lignin according to claim 1, characterized in that, In S1, the catalyst includes at least one of a phase transfer catalyst and an alkaline catalyst; The phase transfer catalyst includes at least one of tetrabutylammonium bromide, benzyltriethylammonium chloride, and tetrabutylammonium hydrogen sulfate; The alkaline catalyst includes sodium hydroxide; The mass ratio of lignin to phase transfer catalyst is (100~300):(0.5~2). The mass ratio of lignin to alkali catalyst is (3~8):(0.5~2).

4. The method for preparing dual-modified lignin according to claim 3, characterized in that, The specific steps of S1 include: mixing the lignin with the epoxidizing agent, adding the phase transfer catalyst and the base catalyst, reacting at temperature T1 for 2-6 hours, cooling to temperature T2 and then adding the base catalyst, then heating to temperature T3 and holding for 0.5-3.5 hours, filtering, washing the obtained solid to neutral, and freeze-drying at -100 to -60℃ for 18-30 hours to obtain epoxidized lignin; The temperature T1 is greater than the temperature T2, the temperature T1 is 65~95℃, and the temperature T2 is 40~65℃; The temperature T2 is less than the temperature T3, and the temperature T3 is 50~70℃.

5. The method for preparing dual-modified lignin according to claim 1, characterized in that, In S2, the acetylation reagent includes at least one of ethyl acetate, methyl acetate, propyl acetate, and butyl acetate; the lipase includes at least one of Novozymes 435, Rhodococcus TrLipE lipase, Aspergillus niger lipase, Aspergillus oryzae lipase, and Mucor lipase. The mass ratio of the epoxidized lignin to the acetylation reagent is (0.5~2):(4~10). The mass ratio of the epoxidized lignin to the lipase is (10~50):(0.5~2).

6. The method for preparing dual-modified lignin according to claim 1 or 5, characterized in that, The specific steps in S2 include: mixing the epoxidized lignin with the acetylation reagent, adding the lipase, reacting at 30~80℃ for 18~30h, filtering, washing the obtained solid until neutral, and freeze-drying at -100~-60℃ for 18~30h to obtain double-modified lignin.

7. A dual-modified lignin, characterized in that, The dual-modified lignin is prepared by the preparation method described in any one of claims 1 to 6.

8. An application of a dual-modified lignin in the field of coatings, characterized in that, The dual-modified lignin includes the dual-modified lignin of claim 7, and / or the dual-modified lignin prepared by any one of the preparation methods of claims 1 to 6.

9. The application according to claim 8, characterized in that, The double-modified lignin is mixed with epoxy resin, and then additives are added and mixed evenly. The mixture is then sprayed onto the surface of the substrate to obtain a coating.

10. The application according to claim 9, characterized in that, The epoxy resin includes at least one of E51 and E44; And / or, the mass ratio of the dual-modified lignin to epoxy resin is (0.5~2):(7~9.5); And / or, the additives include at least one of curing agents and diluents.

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