Lignin-based polyurethane and preparation method thereof

Abstract

The invention belongs to the technical field of polyurethane, and particularly discloses lignin-based polyurethane and a preparation method thereof. The preparation method comprises the following steps: firstly, carrying out acid treatment on a corrugated board to obtain high-activity lignin, then mixing polyol, isocyanate, a catalyst and an organic solvent, and carrying out prepolymerization reaction to obtain an intermediate product; and mixing the obtained high-activity lignin with the intermediate product, and reacting to obtain the lignin-based polyurethane. The high-activity lignin is used as a chain extender, polyhydric alcohol is replaced in the polyurethane synthesis process, the performance of polyurethane is remarkably enhanced by adding the high-activity lignin, and the obtained polyurethane can be used as a flexible substrate material to be applied to flexible electronics. The obtained high-activity lignin is small in molecular weight and high in activity, further modification treatment is not needed, the high-activity lignin can directly participate in subsequent reaction, and the preparation steps of lignin-based polyurethane are simplified.

Description

Technical Field

[0001] The invention relates to the technical field of polyurethane, and in particular to a lignin-based polyurethane and a preparation method thereof. Background Art

[0002] Polyurethane (PU) is a high molecular polymer with a characteristic structural unit, carbamate (-NH-COO-), in its main chain. It is usually prepared by the addition reaction between diisocyanates and polyols. Polyurethane (PU) is a high molecular polymer with a variety of structures, properties and applications. Among them, polyurethane elastomer is one of the most important categories of polyurethane materials. Since it was first reported in the 1930s, after more than 80 years of development, it has been widely used in home furnishing, construction, daily necessities, transportation, home appliances and other fields, becoming the sixth largest variety of polymer materials. Traditional polyurethane is mainly synthesized by stepwise addition polymerization reaction between isocyanate and hydroxyl groups, using polyisocyanates and polyols derived from fossil resources, such as diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI), polypropylene glycol (PPG), etc. With the continuous development of society, ordinary polyurethane can no longer meet the needs of modern society. The polyols used in synthetic polyurethane mainly come from petroleum resources. In the increasingly serious environmental resource problems, it is urgent to choose an environmentally friendly and renewable material to replace petroleum-based polyols. Some renewable materials containing hydroxyl groups or modified to obtain hydroxyl groups, including lignin, cellulose, starch, rosin, vegetable oil, etc., have attracted a lot of attention as potential polyurethane raw materials.

[0003] At present, the application prospect of using bio-based polyols to replace petroleum-based polyols to produce bio-based polyurethane is good. Most bio-based polyols come from plant oils or lignin and its derivatives. Lignin is the second most abundant biomass resource in nature after cellulose. Its surface contains a large number of hydroxyl groups, which makes it possible to replace polyols to prepare bio-based polyurethane materials.

[0004] Therefore, how to provide a lignin-based polyurethane and a preparation method thereof, use lignin to effectively replace polyols, and reduce the use of petroleum-based polyols is a difficult problem that needs to be solved urgently in this field. Summary of the invention

[0005] In view of this, the present invention provides a lignin-based polyurethane and a preparation method thereof to solve the problem that the existing polyurethane preparation method has excessive reliance on petroleum-based polyols, which is not conducive to environmental protection; and the problem of complex preparation process when other components are used to replace petroleum-based polyols.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A method for preparing lignin-based polyurethane comprises the following steps:

[0008] 1) Treating corrugated cardboard with acid to obtain highly active lignin;

[0009] Mixing polyol, isocyanate, catalyst and organic solvent to carry out prepolymerization reaction to obtain an intermediate product;

[0010] 2) The highly active lignin obtained in step 1) is mixed with the intermediate product and reacted to obtain lignin-based polyurethane.

[0011] Preferably, the acid treatment in step 1) is as follows: mixing the corrugated paperboard with an acid solution and performing a heat treatment to obtain highly active lignin.

[0012] Preferably, the mass volume ratio of the corrugated paperboard to the acid solution is 1:15-25;

[0013] The acid solution includes one or more of formic acid, acetic acid and toluenesulfonic acid;

[0014] The temperature of the heat treatment is 90-150°C, and the time of the heat treatment is 120-150 minutes.

[0015] Preferably, in step 1), the mass ratio of the polyol, isocyanate, catalyst and organic solvent is 12-36:10-30:0.5-1.5:48-144.

[0016] Preferably, the prepolymerization reaction temperature is 60-90° C., and the prepolymerization reaction time is 60-150 min.

[0017] Preferably, the polyol includes one or more of polytetramethylene glycol, 1,4-butanediol, polyethylene adipate and polycarbonate diol;

[0018] The isocyanate includes one or more of isophorone diisocyanate, hexamethylene diisocyanate, toluene diisocyanate and p-phenylene diisocyanate;

[0019] The catalyst is dibutyltin dilaurate;

[0020] The organic solvent includes one or more of N,N-dimethylformamide, acetone, toluene and ethyl acetate.

[0021] Preferably, in step 2), the mass ratio of the highly active lignin to the polyol added during the preparation of the intermediate product is 1.2-3.6:12-36.

[0022] Preferably, the reaction temperature in step 2) is 60-90° C., and the reaction time is 3-5 h.

[0023] Preferably, after the reaction in step 2) is completed, a drying step is further included; the drying temperature is 60 to 90° C., and the drying time is 12 to 24 hours.

[0024] Another object of the present invention is to provide a lignin-based polyurethane prepared by the preparation method.

[0025] It can be seen from the above technical solutions that, compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention mainly uses highly active lignin as a chain extender to replace polyols in the polyurethane synthesis process. The addition of lignin significantly enhances the performance of the polyurethane, and the polyurethane can be used as a flexible substrate material in flexible electronics. Since the molecular weight of lignin is large and the reactivity is low, the performance of the polyurethane obtained by adding lignin within a limited range in the present invention is relatively excellent, wherein the elongation at break is significantly improved. However, if the amount of lignin added is too high, it will cause its aggregation inside the polyurethane, resulting in a decrease in the performance of the polyurethane.

[0027] The three-dimensional network composed of lignin provides good mechanical ability and structural integrity for materials such as elastomers, coatings, adhesives and insulators. At the same time, lignin has the dual functions of chemical cross-linking and covalent cross-linking (hydrogen bonding), which can not only enhance the mechanical properties of polyurethane elastomers, but also give polyurethane some lignin-specific properties, such as antibacterial properties and biocompatibility, and has broad application prospects. Therefore, the application of lignin in polyurethane materials promotes the high-value utilization of lignin, which can not only reduce the consumption of petroleum resources, but also achieve green environmental sustainability.

[0028] The present invention also limits the extraction of high-activity lignin through corrugated cardboard. The high-activity lignin obtained by the extraction method of the present invention has a small molecular weight (2500-3500) and high activity, does not require further modification treatment, and can directly participate in subsequent reactions, thereby simplifying the preparation steps of lignin-based polyurethane; in addition, the corrugated cardboard in the present invention can be waste corrugated cardboard, which is beneficial to reducing the preparation cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0030] Figure 1 The mechanical properties test results of the lignin-based polyurethanes prepared in Examples 1 to 3 of the present invention and the polyurethanes prepared in Comparative Examples 1 to 2;

[0031] Figure 2 This is a SEM image of the lignin-based polyurethane prepared in Example 1 of the present invention;

[0032] Figure 3 This is a SEM image of the lignin-based polyurethane prepared in Example 2 of the present invention;

[0033] Figure 4 This is a SEM image of the lignin-based polyurethane prepared in Example 3 of the present invention;

[0034] Figure 5 This is a SEM image of the polyurethane prepared in Comparative Example 1 of the present invention;

[0035] Figure 6 This is a SEM image of the polyurethane prepared in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0036] The present invention provides a method for preparing lignin-based polyurethane, comprising the following steps:

[0037] 1) Treating corrugated cardboard with acid to obtain highly active lignin;

[0038] Mixing polyol, isocyanate, catalyst and organic solvent to carry out prepolymerization reaction to obtain an intermediate product;

[0039] 2) The highly active lignin obtained in step 1) is mixed with the intermediate product and reacted to obtain lignin-based polyurethane.

[0040] In the present invention, the acid treatment in step 1) is as follows: corrugated paperboard is mixed with acid solution and subjected to heat treatment to obtain highly active lignin.

[0041] In the present invention, the mass volume ratio of the corrugated paperboard to the acid solution is 1:15-25; preferably 1:18-22; and more preferably 1:20.

[0042] In the present invention, the acid solution includes one or more of formic acid, acetic acid and toluenesulfonic acid.

[0043] In the present invention, the heat treatment temperature is 90-150°C, specifically 100°C, 110°C, 120°C, 130°C, 140°C; the heat treatment time is 120-150min, specifically 125min, 130min, 135min, 140min, 145min.

[0044] In the present invention, the molecular weight of the highly active lignin is between 2500 and 3500, and the following embodiments are all within this range.

[0045] In the present invention, the mass ratio of the polyol, isocyanate, catalyst and organic solvent in step 1) is 12-36:10-30:0.5-1.5:48-144, preferably 15-30:15-25:0.8-1.2:50-120, and more preferably 20:20:1:80-100.

[0046] In the present invention, the temperature of the prepolymerization reaction is 60-90°C, specifically 65°C, 70°C, 75°C, 80°C, 85°C; the time of the prepolymerization reaction is 60-150min, specifically 70min, 80min, 90min, 100min, 110min, 120min, 130min, 140min.

[0047] In the present invention, the polyol includes one or more of polytetramethylene glycol, 1,4-butanediol, polyethylene adipate and polycarbonate diol; the isocyanate includes one or more of isophorone diisocyanate, hexamethylene diisocyanate, toluene diisocyanate and p-phenylene diisocyanate; the catalyst is dibutyltin dilaurate; and the organic solvent includes one or more of N,N-dimethylformamide, acetone, toluene and ethyl acetate.

[0048] In the present invention, the mass ratio of the highly active lignin in step 2) to the polyol added during the preparation of the intermediate product is 1.2-3.6:12-36, preferably 1.5-3:15-30, and more preferably 2:20.

[0049] In the present invention, the reaction temperature in step 2) is 60-90°C, specifically 65°C, 70°C, 75°C, 80°C, 85°C; the reaction time is 3-5h, specifically 3.2h, 3.5h, 3.8h, 4h, 4.2h, 4.5h, 4.8h.

[0050] In the present invention, after the reaction in step 2) is completed, a standing step is further included, in which the mixed solution after the reaction is completed is cooled along with the container, and the standing step time is 12 to 24 hours, specifically 14 hours, 15 hours, 16 hours, 18 hours, 20 hours, or 22 hours.

[0051] In the present invention, the standing step can allow the material to fully react inside, and through slight reconstruction, mainly dissolution and crystallization, the obtained polyurethane solution can be more uniform.

[0052] In the present invention, a drying step is further included after the standing step; the drying temperature is 60-90°C, specifically 65°C, 70°C, 75°C, 80°C, 85°C; the drying time is 12-24h, specifically 14h, 15h, 16h, 18h, 20h, 22h.

[0053] The present invention also provides a lignin-based polyurethane prepared by the preparation method.

[0054] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0055] Example 1

[0056] Weigh a certain amount of polytetrahydrofuran ether glycol and place it in a flask, then place the flask in a vacuum drying oven for 2 hours to remove moisture from the solution. Then place the flask in the drying oven and lower the temperature to 60°C before taking it out.

[0057] Preparation of highly active lignin: 1 part of waste corrugated cardboard and 20 parts of formic acid are put into a flask, a rotor is placed in the flask for stirring (80r / min), and a mixed solution is obtained at 100°C for 120min. Then, the mixture is filtered to obtain a supernatant containing lignin, and the supernatant is rotary evaporated (60°C, 40r / min) to obtain a concentrated solution containing lignin. An equal proportion of deionized water is added to the concentrated solution, lignin is precipitated, and the highly active lignin can be obtained by filtering and washing.

[0058] 20 parts of polytetramethylene glycol, 80 parts of N,N-dimethylformamide and 25 parts of isophorone diisocyanate were put into a three-necked flask connected with a stirring device and a water circulation device, and 0.8 parts of dibutyltin dilaurate were added dropwise, and the mixture was stirred and mixed in a water bath at 70°C for 60 minutes (500 r / min).

[0059] After the prepolymerization was completed in the water bath, the rotation speed was increased (700 r / min), and 1.3 parts of high-activity lignin was added dropwise to the reaction system. The reaction was continued to stir in a 70° C. water bath for 3 hours, and then the three-necked flask was taken out of the water bath.

[0060] The polytetrafluoroethylene mold was placed in a vacuum drying oven and heated to 70° C., then taken out, and the solution obtained in the three-necked flask was poured into the polytetrafluoroethylene mold and placed at room temperature for 12 hours; dried in a vacuum drying oven at 70° C. for 12 hours to obtain a lignin-based polyurethane material.

[0061] Example 2

[0062] Weigh a certain amount of polytetrahydrofuran ether glycol and place it in a flask, then place the flask in a vacuum drying oven for 2 hours to remove moisture from the solution. Then place the flask in the drying oven and lower the temperature to 60°C before taking it out.

[0063] Preparation of highly active lignin: 1 part of waste corrugated cardboard and 20 parts of formic acid are put into a flask, a rotor is placed in the flask for stirring (80r / min), and a mixed solution is reacted at 90°C for 150min. Then, the mixture is filtered to obtain a supernatant containing lignin, and the supernatant is then rotary evaporated (60°C, 40r / min) to obtain a concentrated solution containing lignin. An equal proportion of deionized water is added to the concentrated solution, lignin is precipitated, and the highly active lignin can be obtained by filtering and washing.

[0064] 19 parts of polytetramethylene glycol, 76 parts of N,N-dimethylformamide and 24.3 parts of isophorone diisocyanate were put into a three-necked flask connected with a stirring device and a water circulation device, and 0.9 parts of dibutyltin dilaurate were added dropwise, and the mixture was stirred and mixed in a water bath at 70°C for 60 minutes (500 r / min).

[0065] After the prepolymerization was completed in the water bath, the rotation speed was increased (700 r / min), and 1.6 parts of high-activity lignin was added dropwise to the reaction system. The reaction was continued to stir in a 70° C. water bath for 3 hours, and then the three-necked flask was taken out of the water bath.

[0066] The polytetrafluoroethylene mold was placed in a vacuum drying oven and heated to 70° C., then taken out, and the solution obtained in the three-necked flask was poured into the polytetrafluoroethylene mold and placed at room temperature for 12 hours; dried in a vacuum drying oven at 70° C. for 12 hours to obtain a lignin-based polyurethane material.

[0067] Example 3

[0068] Weigh a certain amount of polytetrahydrofuran ether glycol and place it in a flask, then place the flask in a vacuum drying oven for 2 hours to remove moisture from the solution. Then place the flask in the drying oven and lower the temperature to 60°C before taking it out.

[0069] Preparation of highly active lignin: 1 part of waste corrugated cardboard and 20 parts of toluenesulfonic acid are put into a flask, a rotor is placed in the flask for stirring (80r / min), and a mixed solution is obtained at 90°C for 150min. Then, the mixture is filtered and mixed to obtain a supernatant containing lignin, and the supernatant is then rotary evaporated (60°C, 40r / min) to obtain a concentrated solution containing lignin. An equal proportion of deionized water is added to the concentrated solution, lignin is precipitated, and the highly active lignin is obtained by filtering and washing.

[0070] 17 parts of polycarbonate diol, 68 parts of N,N-dimethylformamide and 25 parts of hexamethylene diisocyanate were put into a three-necked flask connected with a stirring device and a water circulation device, and 1 part of dibutyltin dilaurate was added dropwise. The mixture was stirred and mixed in a 90°C water bath for 80 minutes (500 r / min).

[0071] After the prepolymerization was completed in the water bath, the rotation speed was increased (700 r / min), and 2.3 parts of high-activity lignin were added dropwise to the reaction system. The reaction was continued in a 90° C. water bath with stirring for 4 hours, and then the three-necked flask was taken out of the water bath.

[0072] The polytetrafluoroethylene mold was placed in a vacuum drying oven and heated to 90° C., then taken out, and the solution obtained in the three-necked flask was poured into the polytetrafluoroethylene mold and placed at room temperature for 12 hours; dried in a vacuum drying oven at 90° C. for 12 hours to obtain a lignin-based polyurethane material.

[0073] Example 4

[0074] Weigh a certain amount of polytetrahydrofuran ether glycol and place it in a flask, then place the flask in a vacuum drying oven for 2 hours to remove moisture from the solution. Then place the flask in the drying oven and lower the temperature to 60°C before taking it out.

[0075] Preparation of highly active lignin: 1 part of waste corrugated cardboard and 20 parts of acetic acid are put into a flask, a rotor is placed in the flask for stirring (80r / min), and a mixed solution is obtained at 150°C for 120min. Then, the mixture is filtered and mixed to obtain a supernatant containing lignin, and the supernatant is then rotary evaporated (60°C, 40r / min) to obtain a concentrated solution containing lignin. An equal proportion of deionized water is added to the concentrated solution, lignin is precipitated, and the highly active lignin is obtained by filtering and washing.

[0076] 36 parts of polycarbonate diol, 120 parts of N,N-dimethylformamide and 25 parts of hexamethylene diisocyanate were put into a three-necked flask connected with a stirring device and a water circulation device, and 1.5 parts of dibutyltin dilaurate were added dropwise, and the mixture was stirred and mixed in a water bath at 60°C for 150 minutes (500 r / min).

[0077] After the prepolymerization was completed in the water bath, the rotation speed was increased (700 r / min), and 3 parts of high-activity lignin were added dropwise to the reaction system. The reaction was continued to stir in the water bath at 80° C. for 4 hours, and then the three-necked flask was taken out of the water bath.

[0078] The polytetrafluoroethylene mold was placed in a vacuum drying oven and heated to 80° C., then taken out, and the solution obtained in the three-necked flask was poured into the polytetrafluoroethylene mold and placed at room temperature for 12 hours; dried in a vacuum drying oven at 60° C. for 24 hours to obtain a lignin-based polyurethane material.

[0079] Comparative Example 1

[0080] The difference between this comparative example and Example 1 is that high-activity lignin is not added, and 1,4-butanediol containing hydroxyl groups in an equal mole to that of the high-activity lignin is added.

[0081] Comparative Example 2

[0082] The difference between this comparative example and Example 1 is that the high-activity lignin is replaced by alkali lignin (molecular weight: 15000-20000, from MacLean Chemical Reagent).

[0083] Comparative Example 3

[0084] The only difference between this comparative example and Example 1 is that no standing step is performed.

[0085] Experimental Example 1

[0086] (1) Mechanical properties test

[0087] The mechanical properties of the lignin-based polyurethanes prepared in Examples 1 to 3 of the present invention and the polyurethanes obtained in Comparative Examples 1 to 2 were tested using a universal testing machine (Instron 5500) at room temperature. Five samples were selected from each group with the specifications of rectangular polyurethane samples of 100 mm in length and 20 mm in width. The tensile speed was set to 10 mm / s and the gauge length was 30 mm. After testing all the samples, the average value was taken to obtain the final test results of the samples. The test results are shown in Figure 2. Figure 1 As shown, through Figure 1 It can be seen that the tensile properties and tensile stress of polyurethane with the addition of lignin are increased to varying degrees, but excessive lignin will cause the stress of polyurethane to decrease.

[0088] (2) Stretch recovery performance:

[0089] In order to observe the mechanical properties of lignin-based polyurethane more intuitively, the lignin-based polyurethane was quickly stretched with fingers at room temperature to observe that the polyurethane had excellent mechanical properties. When the external force was removed, the recovery of the polyurethane was observed.

[0090] Example 1: 45mm before stretching; stretched to 90mm; rebounded to 49mm; recovery rate 91%;

[0091] Example 2: 50 mm before stretching; stretched to 100 mm; rebounded to 52 mm; recovery rate 96%;

[0092] Example 3: 50 mm before stretching; stretched to 100 mm; rebounded to 55 mm; recovery rate 90%;

[0093] Comparative Example 1: 37mm before stretching; stretched to 80mm; rebounded to 43mm; recovery rate 84%;

[0094] Comparative Example 2: 35mm before stretching; stretched to 70mm; rebounded to 40mm; recovery rate 86%.

[0095] Through the tensile recovery performance test, it can be found that the lignin-based polyurethane prepared in the embodiment of the present invention has a better recovery rate.

[0096] (3) Scanning electron microscope test (SEM)

[0097] The morphology of Examples 1 to 3 and Comparative Examples 1 to 2 after drying was characterized by a scanning electron microscope (SU-8010, Japan). The microstructures of the polyurethanes are shown in FIG. Figures 2 to 6 As shown, Figures 2 to 4 Corresponding to Examples 1, 2, and 3, Figures 5-6 Corresponding to proportions 1 and 2 respectively. Figures 2 to 6 It can be seen that the surface of the polyurethane to which unmodified alkali lignin is added will have large accumulations, while the surface of the lignin-based polyurethane prepared by the present invention is smoother than that of alkali lignin, which is beneficial to subsequent processing. At the same time, it has excellent chemical resistance and stability, increases the chemical resistance and wear resistance of the product, and extends the service life of the product.

[0098] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0099] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a lignin-based polyurethane, characterized in that: The steps include: 1) Treating corrugated cardboard with acid to obtain highly active lignin; Mixing polyol, isocyanate, catalyst and organic solvent to carry out prepolymerization reaction to obtain an intermediate product; 2) The highly active lignin obtained in step 1) is mixed with the intermediate product and reacted to obtain lignin-based polyurethane.

2. The method for preparing a lignin-based polyurethane according to claim 1, characterized in that: The acid treatment steps in step 1) are as follows: mixing the corrugated paperboard with acid solution and performing heat treatment to obtain highly active lignin.

3. The method for preparing a lignin-based polyurethane according to claim 2, characterized in that: The mass volume ratio of the corrugated paperboard to the acid solution is 1:15-25; The acid solution includes one or more of formic acid, acetic acid and toluenesulfonic acid; The temperature of the heat treatment is 90-150°C, and the time of the heat treatment is 120-150 minutes.

4. The method for preparing a lignin-based polyurethane according to claim 3, characterized in that: The mass ratio of the polyol, isocyanate, catalyst and organic solvent in step 1) is 12-36:10-30:0.5-1.5:48-144.

5. The method for preparing a lignin-based polyurethane according to claim 4, characterized in that: The temperature of the prepolymerization reaction is 60-90° C., and the time of the prepolymerization reaction is 60-150 minutes.

6. The method for preparing a lignin-based polyurethane according to any one of claims 1 to 5, characterized in that: The polyol includes one or more of polytetramethylene glycol, 1,4-butanediol, polyethylene adipate and polycarbonate diol; The isocyanate includes one or more of isophorone diisocyanate, hexamethylene diisocyanate, toluene diisocyanate and p-phenylene diisocyanate; The catalyst is dibutyltin dilaurate; The organic solvent includes one or more of N,N-dimethylformamide, acetone, toluene and ethyl acetate.

7. The method for preparing a lignin-based polyurethane according to claim 6, characterized in that: The mass ratio of the high-activity lignin to the polyol added during the preparation of the intermediate product in step 2) is 1.2-3.6:12-36.

8. The method for preparing a lignin-based polyurethane according to claim 7, characterized in that: The reaction temperature in step 2) is 60-90° C., and the reaction time is 3-5 h.

9. The method for preparing a lignin-based polyurethane according to claim 7 or 8, characterized in that: After the reaction in step 2) is completed, a drying step is further included; the drying temperature is 60 to 90° C., and the drying time is 12 to 24 hours.

10. The lignin-based polyurethane prepared by the preparation method according to any one of claims 1 to 9.

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