A tough lignin-based waterborne polyurethane nanocomposite material and its preparation and application

Abstract

The present invention discloses a tough lignin-based waterborne polyurethane nanocomposite material, as well as its preparation and application. The present invention fractionates or partially depolymerizes industrial lignin, using low-molecular-weight lignin to synthesize waterborne polyurethane, and high-molecular-weight lignin to prepare nanomicelles and reinforce the waterborne polyurethane, thereby achieving the rational utilization of lignin. The lignin-based waterborne polyurethane nanocomposite material prepared by the present invention exhibits excellent mechanical properties, with a tensile strength greater than 25 MPa and an elongation at break exceeding 700%. This is primarily due to the low-molecular-weight lignin forming a complete cross-linked network in the waterborne polyurethane, while strong hydrogen bonds are formed between the well-dispersed lignin nanoparticles and the waterborne polyurethane matrix. The composite material also exhibits excellent UV resistance, aging resistance, and photothermal conversion properties, and can be used as a coating for thermoelectric generators for photothermal power generation.

Description

Technical Field

[0001] The present invention belongs to the technical field of waterborne polyurethane materials, and in particular relates to a tough lignin-based waterborne polyurethane nanocomposite material and its preparation and application. Background Art

[0002] Waterborne polyurethane (PU) is an environmentally friendly polyurethane material using water as its dispersion medium. It boasts adjustable mechanical properties, low-temperature compliance, abrasion resistance, zero organic volatilization, and ease of modification. Waterborne polyurethane is primarily used in adhesives and coatings for applications such as leather, fabrics, fibers, flooring, and paper, providing adhesion, corrosion protection, wear resistance, and improved hand feel. The raw materials used to synthesize waterborne polyurethane are primarily derived from petrochemical resources and are non-renewable. Therefore, the development of biodegradable waterborne polyurethane materials using biomass is crucial.

[0003] Lignin is an aromatic polymer widely found in higher plants. Its molecular structure contains a variety of functional groups, such as alcoholic hydroxyl, phenolic hydroxyl, methoxyl, carbonyl, and carboxyl groups. It is a natural bio-based polyol that can partially replace petrochemical-based polyols in the synthesis of waterborne polyurethanes. However, the synthesis process typically requires high-boiling-point organic solvents, such as N,N-dimethylformamide and N-methylpyrrolidone, to dissolve the lignin. These organic solvents are difficult to recover by decompression after the reaction and can pollute the environment [Polymer Materials Science and Engineering, 2016, 32, 143; CN113201112A]. The amount of lignin used is also limited, typically not exceeding 10% of the replacement amount. Excessive lignin content can lead to excessively large emulsion particles and significantly reduced stability [J. Appl. Polym. Sci., 2013, 130, 1855]. Industrial lignin has low reactivity and poor compatibility with the polyurethane matrix. The mechanical properties of the prepared lignin-based waterborne polyurethane are poor. Although chemical modification can improve the reactivity of lignin, the modification process is complicated, costly, and difficult to industrialize [Polymers 2016, 8, 318].

[0004] Another application of lignin in waterborne polyurethanes is as a functional filler (reinforcement agent and antioxidant) added to waterborne polyurethane emulsions to create waterborne polyurethane / lignin composites. To address the issue of lignin dispersibility in emulsions, two types of lignin are commonly used: water-soluble lignin and lignin nanoparticles. Water-soluble lignin reduces the water resistance of waterborne polyurethanes [J.Appl.Polym.Sci., 2007, 106, 4257; Int.J.Polym.Sci., 2018, 2018, 1-7]. Upon contact with water, lignin migrates from the interior of the waterborne polyurethane into the water. Lignin nanoparticles are usually prepared by solvent transfer or nanoprecipitation. The solvent transfer method generally requires the addition of a large amount of poor solvent (water) or dialysis to reduce the solubility of lignin [ACS Sustainable Chem. Eng., 2023, 11, 17142]. The process is complex and time-consuming. The nanoprecipitation method mainly utilizes the characteristics of lignin's alkali solubility and acid precipitation to cause precipitation by adjusting the pH of the solution. It is difficult to obtain uniform nanoparticles by adjusting the process conditions, and the post-processing process is also cumbersome, making it difficult to industrialize [Front. Chem. Sci., 2018, 13, 59].

[0005] Lignin, used as both a raw material and a functional filler in waterborne polyurethanes, each has its own advantages and disadvantages. Further research is needed to efficiently utilize lignin resources, increase its usage in waterborne polyurethanes, and prepare high-performance lignin-based waterborne polyurethanes. Summary of the Invention

[0006] In order to address the shortcomings and deficiencies of the prior art, the primary purpose of the present invention is to provide a tough lignin-based waterborne polyurethane nanocomposite material.

[0007] Another object of the present invention is to provide a method for preparing the above-mentioned tough lignin-based waterborne polyurethane nanocomposite material.

[0008] Another object of the present invention is to provide an application of the above-mentioned tough lignin-based waterborne polyurethane nanocomposite material.

[0009] The purpose of the present invention is achieved through the following technical solutions:

[0010] A tough lignin-based waterborne polyurethane nanocomposite material is obtained by reacting the following components, calculated by weight: 70-95 parts of a long-chain diol, 5-30 parts of a low-molecular-weight lignin, 10-40 parts of lignin nanomicelles, an isocyanate (the amount is determined by the isocyanate index), 2-8 parts of a hydrophilic chain extender, 0.1-1 part of a catalyst, and 2-6 parts of a neutralizer.

[0011] The total amount of the low molecular weight lignin and the long chain diol is 100 parts; the isocyanate index R is 1.1-1.3.

[0012] Preferably, the tough lignin-based waterborne polyurethane nanocomposite material is obtained by reacting the following components, calculated by weight: 70 to 90 parts of long-chain diol, 10 to 30 parts of low-molecular-weight lignin, 10 to 40 parts of lignin nanomicelles, isocyanate (the amount is determined by the isocyanate index), 6 to 8 parts of a hydrophilic chain extender, 0.1 to 1 part of a catalyst, and 3 to 4.5 parts of a neutralizer;

[0013] The total amount of the low molecular weight lignin and the long chain diol is 100 parts; the isocyanate index R is 1.1-1.3.

[0014] Preferably, the long-chain diol is at least one of polytetramethylene glycol (PTMG), polyethylene glycol (PEG), polycaprolactone glycol (PCL) and polypropylene glycol (PPG), and has a number average molecular weight of 500 to 4000 g / mol.

[0015] Preferably, the low molecular weight lignin is obtained by fractionation and / or partial depolymerization of industrial lignin, and has a weight average molecular weight of 800 to 2000 g / mol.

[0016] More preferably, the low molecular weight lignin is added in the form of a 10-20 wt% solution, and the solvent of the solution is at least one of acetone and tetrahydrofuran.

[0017] Preferably, the lignin nano-micelles are obtained by ultrafiltration and / or dialysis of a high molecular weight lignin alkaline solution, and have a particle size of 100 to 200 nm; the lignin nano-micelles are added in the form of a 10 to 40 wt% aqueous solution.

[0018] More preferably, the lignin nano-micelles are obtained by the following method: industrial lignin is fully dispersed and dissolved in an organic solvent, filtered and then the filter residue is collected to obtain high molecular weight lignin; the high molecular weight is dissolved in a 0.5-1 wt% alkaline solution, the alkali is removed by ultrafiltration and / or dialysis, and the ultrafiltration and / or dialysis is terminated when the pH of the lignin solution drops to the range of 7-8 to obtain a lignin nano-micelle aqueous solution.

[0019] More preferably, the high molecular weight lignin is dissolved in a 0.5-1 wt% alkaline solution at a concentration of 50-300 g / L; the alkaline solution is at least one of a sodium hydroxide solution, a potassium hydroxide solution and an ammonia solution.

[0020] More preferably, the organic solvent in the method for obtaining lignin nanomicelles is at least one of ethanol, acetone and ethyl acetate.

[0021] Preferably, the isocyanate is at least one of isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), diphenylmethane diisocyanate (MDI) and toluene diisocyanate (TDI).

[0022] Preferably, the hydrophilic chain extender is at least one of 2,2-dimethylolpropionic acid (DMPA) and 2,2-dimethylolbutanoic acid (DMBA).

[0023] More preferably, the hydrophilic chain extender is added in the form of a 20-30 wt% solution; the solvent of the solution is at least one of N-methylpyrrolidone, N,N-dimethylformamide and dimethyl sulfoxide.

[0024] Preferably, the catalyst is an organotin catalyst; more preferably, dibutyltin dilaurate.

[0025] Preferably, the neutralizing agent is at least one of triethylamine, ammonia water, sodium hydroxide and potassium hydroxide.

[0026] A method for preparing a tough lignin-based waterborne polyurethane nanocomposite material comprises the following steps:

[0027] (1) Dispersing industrial lignin in an organic solvent, fully dissolving it at room temperature, filtering it, concentrating the filtrate, and drying it to obtain graded low-molecular-weight lignin; and drying the filter residue to obtain high-molecular-weight lignin;

[0028] (2) dissolving high molecular weight lignin in an alkaline solution, removing the alkali by ultrafiltration and / or dialysis, and ending the ultrafiltration and / or dialysis when the pH of the lignin solution drops to a range of 7 to 8 to obtain a lignin nanomicelle solution;

[0029] (3) dissolving industrial lignin in an alkaline solution, reacting at 140-180°C for 4-12 hours, cooling to room temperature after the reaction, adjusting the solution to acidity to precipitate lignin from the solution, filtering, washing, and drying to obtain partially depolymerized low molecular weight lignin;

[0030] (4) adding a long-chain diol, an isocyanate and a catalyst into a reactor at 70 to 80° C. and prepolymerizing for 1 to 2 hours, then adding a hydrophilic chain extender and reacting for 0.5 to 1 hour, then adding a graded and / or partially depolymerized low molecular weight lignin and reacting for 1 to 4 hours, and after the reaction is completed, lowering the temperature to room temperature, adding a neutralizer and stirring for 0.5 to 1 hour;

[0031] (5) The reaction system is transferred to an emulsifier, and the lignin nanomicelle solution is added under stirring for emulsification, and then water is added to adjust the solid content of the emulsion to obtain an aqueous polyurethane emulsion, which is solidified into a film to obtain a tough lignin-based aqueous polyurethane nanocomposite material.

[0032] Preferably, the weight average molecular weight of the industrial lignin in step (1) is 3000-6000 g / mol, and its concentration in the organic solvent is 200-500 g / L.

[0033] Preferably, the organic solvent in step (1) is at least one of ethanol, acetone and ethyl acetate.

[0034] Preferably, the operation of fully dissolving in step (1) is mechanically stirring for 2 to 5 hours.

[0035] Preferably, the drying temperature in step (1) is 50-70°C.

[0036] Preferably, the concentration of the alkaline solution in step (2) is 0.5-1 wt%.

[0037] Preferably, the pore size of the filter membrane used for ultrafiltration in step (2) is 500 to 1000 Da.

[0038] Preferably, the dialysis in step (2) is performed using a dialysis bag of 500 to 1000 Da.

[0039] Preferably, the concentration of the high molecular weight lignin in the alkaline solution in step (2) is 50 to 300 g / L, and the solid content of the obtained lignin nanomicelle solution is 10 to 40 wt%.

[0040] Preferably, the alkaline solutions in steps (2) and (3) are at least one of sodium hydroxide solution, potassium hydroxide solution and ammonia solution.

[0041] Preferably, the concentration of the alkaline solution in step (3) is 0.5-2 wt%.

[0042] Preferably, the adjustment to acidity in step (3) refers to adjusting the solution pH to 2-3 using dilute hydrochloric acid.

[0043] Preferably, the concentration of the industrial lignin in the alkaline solution in step (3) is 100-300 g / L.

[0044] Preferably, the hydrophilic chain extender in step (4) is added in the form of a 20-30 wt% solution, and the solvent is at least one of N-methylpyrrolidone, N,N-dimethylformamide and dimethyl sulfoxide; the low molecular weight lignin is added in the form of a 10-20 wt% solution, and the solvent is at least one of acetone and tetrahydrofuran.

[0045] Preferably, the stirring speed in step (5) is 800 to 1500 rpm.

[0046] Preferably, the solid content of the emulsion in step (5) is 20 to 45 wt%.

[0047] Preferably, after adding water to adjust the solid content of the emulsion in step (5), the organic solvent needs to be removed. The method for removing the organic solvent is to remove the organic solvent under reduced pressure at 200-300 rpm and 40-50°C.

[0048] Preferably, the film-forming curing in step (5) refers to casting the aqueous polyurethane emulsion into a polytetrafluoroethylene mold and evaporating it at room temperature to form a film.

[0049] Application of the above-mentioned tough lignin-based waterborne polyurethane nanocomposite material.

[0050] Preferably, the application refers to a coating for preparing a thermoelectric power generation sheet for photothermal power generation.

[0051] This invention addresses the issues of low lignin reactivity and the need for high-boiling-point solvents. By fractionating or partially depolymerizing industrial lignin, low-molecular-weight lignin with high reactivity is used as a bio-based polyol to synthesize waterborne polyurethane. High-molecular-weight lignin with low reactivity is used to prepare nanomicelles for incorporation into waterborne polyurethane, achieving 100% high-value utilization of lignin. The waterborne polyurethane nanocomposite prepared by this invention exhibits excellent mechanical properties, good elasticity, and superior UV and aging resistance. It can be used as a coating for thermoelectric generators, achieving light-to-heat-to-electricity conversion under illumination.

[0052] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0053] 1. The present invention uses low molecular weight lignin to participate in the synthesis of waterborne polyurethane, which has high reaction activity and uses acetone as a solvent, avoiding the use of high boiling point solvents, which is consistent with the production process of industrial waterborne polyurethane.

[0054] 2. The present invention prepares lignin nano-micelles by ultrafiltration and dialysis. Compared with the solvent transfer method and nano-precipitation method, the operation is simple, the cost is low, and it is easy to achieve large-scale industrial production.

[0055] 3. During the curing process of the composite emulsion, water gradually evaporates, and the lignin nanomicelles form evenly dispersed nanoparticles in the aqueous polyurethane matrix, which not only enhances the mechanical properties of the composite material, but also improves the UV resistance and anti-aging properties of the composite material.

[0056] 4. Based on the tight π-π stacking effect in lignin nanoparticles, lignin-based waterborne polyurethane nanocomposites exhibit excellent photothermal conversion performance and can be used as a coating for thermoelectric generators to convert light energy into electrical energy under illumination. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1The particle size distribution curves of lignin nanomicelles, lignin-based aqueous polyurethane emulsion (Comparative Example 2) and its nanocomposite emulsion (Example 6) are shown.

[0058] Figure 2 (a) and (b) are scanning electron microscopy images of the lignin-based waterborne polyurethane nanocomposite material of Example 6.

[0059] Figure 3 The surface temperature of lignin-based waterborne polyurethane nanocomposites changes with illumination time.

[0060] Figure 4 The voltage output by the thermoelectric generator prepared from the lignin-based aqueous polyurethane composite emulsion (Example 6) changes with light intensity. DETAILED DESCRIPTION

[0061] The present invention will be described in further detail below with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.

[0062] In the examples of the present invention, if the specific conditions are not specified, the experiments were carried out according to conventional conditions or the conditions recommended by the manufacturer. All raw materials and reagents used without specifying the manufacturer are conventional products that can be purchased from the market.

[0063] The weight-average molecular weight of the industrial lignin used in the following examples and comparative examples is 3500 g / mol. 200 g of industrial lignin was dispersed in 1 L of anhydrous ethanol, stirred for 2 hours, and then filtered. The filtrate and filter residue were dried separately to obtain graded low-molecular-weight lignin and high-molecular-weight lignin, of which the low-molecular-weight lignin accounted for approximately 30% of the total. 200 g of industrial lignin was dissolved in 1 L of a 1 wt% NaOH solution and added to a digester. The reaction was carried out at 160°C for 8 hours. After the reaction was completed, the solution was cooled to room temperature and the pH of the solution was adjusted to 3 with dilute hydrochloric acid. The solution was then filtered, washed, and dried to obtain partially depolymerized low-molecular-weight lignin. The molecular weight and functional group content of the different lignins described above were characterized, and the relevant parameters are shown in Table 1. After graded or partially depolymerized lignin, its polydispersity index (PDI) decreased, the molecular weight became relatively concentrated, and the low-molecular-weight lignin had a higher hydroxyl content.

[0064] Table 1 Characteristic parameters of industrial lignin and low molecular weight lignin

[0065]

[0066] In the following examples and comparative examples, the pore size of the filter membrane used for ultrafiltration is 800 Da.

[0067] In the following examples and comparative examples, the total amount of low molecular weight lignin and long chain diol is 100 parts, the number average molecular weight of the long chain diol used is 2000 g / mol; the amount of isocyanate used is determined according to the isocyanate index R; and the catalyst used is dibutyltin dilaurate.

[0068] Unless otherwise specified, the parts in the following examples and comparative examples are by mass.

[0069] Example 1

[0070] (1) Industrial lignin was dispersed in anhydrous ethanol at a concentration of 200 g / L, stirred at room temperature for 2 hours, and then filtered. The filtrate was concentrated, and the filter residue and the filtrate were dried under reduced pressure at 50°C to obtain graded low molecular weight lignin and high molecular weight lignin;

[0071] (2) dissolving high molecular weight lignin in a 0.5 wt% NaOH solution at a concentration of 100 g / L, and ultrafiltration was performed. When the pH of the solution dropped to a range of 7 to 8, ultrafiltration was terminated to obtain a lignin nanomicelle solution with a solid content of 30 wt%;

[0072] (3) 95 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 5 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0073] (4) The reaction system was transferred to an emulsifier, and 10 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50° C. to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0074] Example 2

[0075] (1) Same as Example 1;

[0076] (2) Same as Example 1;

[0077] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0078] (4) Same as Example 1.

[0079] Example 3

[0080] (1) Same as Example 1;

[0081] (2) Same as Example 1;

[0082] (3) 80 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 20 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0083] (4) Same as Example 1.

[0084] Example 4

[0085] (1) Same as Example 1;

[0086] (2) Same as Example 1;

[0087] (3) 70 parts of PTMG, a certain amount of IPDI and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 30 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index was controlled to be 1.1;

[0088] (4) Same as Example 1.

[0089] Example 5

[0090] (1) Same as Example 1;

[0091] (2) Same as Example 1;

[0092] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0093] (4) The reaction system was transferred to an emulsifier, and 20 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50° C. to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0094] Example 6

[0095] (1) Same as Example 1;

[0096] (2) Same as Example 1;

[0097] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0098] (4) The reaction system was transferred to an emulsifier, and 30 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50° C. to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0099] Example 7

[0100] (1) Same as Example 1;

[0101] (2) Same as Example 1;

[0102] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0103] (4) The reaction system was transferred to an emulsifier, and 40 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50° C. to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0104] Example 8

[0105] (1) Same as Example 1;

[0106] (2) Same as Example 1;

[0107] (3) Dissolving industrial lignin in a 1 wt% NaOH solution at a concentration of 200 g / L, reacting at 160°C for 8 hours, cooling to room temperature after the reaction, adjusting the solution pH to 3 with dilute hydrochloric acid, and then filtering, washing, and drying to obtain partially depolymerized low molecular weight lignin;

[0108] (4) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of partially depolymerized low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0109] (5) The reaction system was transferred to an emulsifier, and 30 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50°C to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0110] Example 9

[0111] (1) Same as Example 1;

[0112] (2) Same as Example 1;

[0113] (3) 80 parts of PCL, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours, and then 6 parts of DMBA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 20 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.3;

[0114] (4) The reaction system was transferred to an emulsifier, and 10 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50° C. to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0115] Example 10

[0116] (1) Same as Example 1;

[0117] (2) Same as Example 1;

[0118] (3) Dissolving industrial lignin in a 1 wt% NaOH solution at a concentration of 200 g / L, reacting at 160°C for 8 hours, cooling to room temperature after the reaction, adjusting the solution pH to 3 with dilute hydrochloric acid, and then filtering, washing, and drying to obtain partially depolymerized low molecular weight lignin;

[0119] (4) 80 parts of PEG, a certain amount of IPDI and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours, and then 6 parts of DMBA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 20 parts of partially depolymerized low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0120] (5) The reaction system was transferred to an emulsifier, and 10 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50°C to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0121] Example 11

[0122] (1) Same as Example 1;

[0123] (2) Same as Example 1;

[0124] (3) 90 parts of PTMG, a certain amount of HDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours, and then 4 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 3 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0125] (4) The reaction system was transferred to an emulsifier, and 20 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50° C. to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0126] Example 12

[0127] (1) Same as Example 1;

[0128] (2) Same as Example 1;

[0129] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours, and then 8 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 3 parts of KOH were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0130] (4) The reaction system was transferred to an emulsifier, and 40 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50° C. to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain the tough lignin-based aqueous polyurethane nanocomposite material.

[0131] Example 13

[0132] (1) Same as Example 1;

[0133] (2) High molecular weight lignin was dissolved in 0.5 wt% NaOH solution at a concentration of 100 g / L. The lignin alkaline solution was dialyzed using a dialysis bag with a pore size of 800 Da for 10 days until the pH of the solution dropped to the range of 7 to 8. The dialyzate was concentrated by rotary evaporation to obtain a lignin nanomicelle solution with a solid content of 30 wt%;

[0134] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0135] (4) The reaction system was transferred to an emulsifier, and 30 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50°C to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain a lignin-based aqueous polyurethane nanocomposite material.

[0136] Comparative Example 1 (Compared with Example 6, without adding low molecular weight lignin)

[0137] (1) Same as Example 1;

[0138] (2) Same as Example 1;

[0139] (3) 100 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours. 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were then added and reacted for 1 hour. After the reaction, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours. The isocyanate index R = 1.1;

[0140] (4) The reaction system was transferred to an emulsifier, and 30 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50°C to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain a lignin-based aqueous polyurethane nanocomposite material.

[0141] Comparative Example 2 (Compared with Example 6, without adding lignin nano-micelles)

[0142] (1) Same as Example 1;

[0143] (2) Same as Example 1;

[0144] (3) 90 parts of PTMG, a certain amount of IPDI and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; wherein the isocyanate index was controlled to be 1.1;

[0145] (4) The reaction system was transferred to an emulsifier, and deionized water was added at 1000 rpm for emulsification. The solid content of the emulsion was adjusted to 30 wt %. After the emulsification was completed, the rotation speed was reduced to 300 rpm. Then, the acetone was removed under reduced pressure at 50° C. to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain a lignin-based aqueous polyurethane material.

[0146] Comparative Example 3 (Compared with Example 6, industrial lignin was directly added instead of low molecular weight lignin)

[0147] (1) Same as Example 1;

[0148] (2) Same as Example 1;

[0149] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70°C for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of industrial lignin (dispersed in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0150] (4) The reaction system was transferred to an emulsifier, and 30 parts of lignin nanomicelles (in the form of a 30 wt% solution) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50°C to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain a lignin-based aqueous polyurethane nanocomposite material.

[0151] Comparative Example 4 (Compared with Example 6, lignin alkaline solution was directly added instead of lignin nano-micelles)

[0152] (1) Same as Example 1;

[0153] (2) dissolving high molecular weight lignin in a 0.5 wt% NaOH solution to obtain a lignin alkaline solution with a solid content of 30 wt%;

[0154] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0155] (4) The reaction system was transferred to an emulsifier, and 30 parts of high molecular weight lignin (in the form of a 30 wt% alkaline solution) was added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50°C to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain a lignin-based aqueous polyurethane nanocomposite material.

[0156] Comparative Example 5 (Compared with Example 6, an organic solvent was used instead of an alkaline solution to prepare lignin nanomicelles)

[0157] (1) Same as Example 1;

[0158] (2) High molecular weight lignin was dissolved in tetrahydrofuran at a concentration of 200 g / L, and 10 times the volume of deionized water was slowly added dropwise to the lignin solution for 8 hours. During this process, the lignin precipitated to form nanoparticles, and then the tetrahydrofuran and part of the water were removed by rotary evaporation to obtain lignin nanomicelles with a solid content of 30 wt%;

[0159] (3) 90 parts of PTMG, a certain amount of IPDI, and 0.4 parts of dibutyltin dilaurate were added to a reactor for prepolymerization at 70° C. for 2 hours, and then 6 parts of DMPA (dissolved in N-methylpyrrolidone, 20 wt%) were added and reacted for 1 hour. Then, 10 parts of graded low molecular weight lignin (dissolved in acetone, 20 wt%) were added and reacted for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and 4.5 parts of triethylamine were added and stirred for 0.5 hours; the isocyanate index R = 1.1;

[0160] (4) The reaction system was transferred to an emulsifier, and 30 parts of lignin nanomicelles (in the form of a 30 wt% dispersion) were added at 1000 rpm for emulsification. Deionized water was then added to adjust the solid content of the emulsion to 30 wt%. After the emulsification was completed, the rotation speed was reduced to 300 rpm. The acetone was then removed under reduced pressure at 50°C to obtain an aqueous polyurethane emulsion, which was cast into a polytetrafluoroethylene mold and evaporated at room temperature to form a film to obtain a lignin-based aqueous polyurethane nanocomposite material.

[0161] In the above Examples 1 to 8, lignin nano-micelles with a solid content of 30 wt% were prepared by ultrafiltration. The particle size of the lignin nano-micelles, the lignin-based aqueous polyurethane emulsion (Comparative Example 2) and the nano-composite emulsion (Example 6) were tested using the dynamic light scattering mode of the Malvern nano-particle size analyzer. The results are as follows: Figure 1 The particle size of lignin nanomicelles is in the range of 110-220 nm, and the particle size of lignin-based aqueous polyurethane emulsion (Comparative Example 2) is in the range of 38-130 nm. The composite nanomicelles prepared by combining the two (Example 6) have a particle size in the range of 30-250 nm, which is consistent with the regularity.

[0162] The nanocomposite material sample of Example 6 was tested by scanning electron microscopy and transmission electron microscopy. Figure 2 As shown, scanning electron microscopy shows that the cross-section of the composite material is very flat and no lignin agglomerates are visible, indicating that the lignin nanoparticles have a small particle size and good compatibility with the waterborne polyurethane matrix; transmission electron microscopy shows that very small nanoparticles are evenly dispersed in the waterborne polyurethane matrix, and the lignin nanomicelles lose water during the curing process, and their particle size becomes further smaller.

[0163] The lignin-based waterborne polyurethane nanocomposites from the examples and comparative examples were fabricated into dumbbell-shaped test pieces that meet the GB / T528-2009 standard. Mechanical properties such as tensile strength and elongation at break were tested using an MTS universal testing machine. The results are shown in Table 2. As can be seen from the table, the prepared lignin-based waterborne polyurethane nanocomposites (Examples 1-7) exhibited excellent mechanical properties, with tensile strengths exceeding 25 MPa and elongation at break exceeding 700%. The highly active low-molecular-weight lignin formed a complete chemically cross-linked network structure within the waterborne polyurethane. During the curing process, the lignin nanomicelles formed uniformly dispersed nanoparticles, which formed strong hydrogen bonds with the waterborne polyurethane matrix, reinforcing the waterborne polyurethane. Increasing the amount of low-molecular-weight lignin (Examples 1-4) enhanced the cross-linking effect within the waterborne polyurethane, gradually increasing the tensile strength and Young's modulus of the material, but gradually decreasing the elongation at break. Increasing the amount of lignin nanomicelles (Examples 2 and 5-7) also enhanced the reinforcing effect.

[0164] Comparing Example 6 with Comparative Examples 1 to 3, it can be found that the lack of chemical cross-linking structure or the lack of nanoparticles in the waterborne polyurethane will lead to a decrease in the mechanical properties of the material. Comparing Example 6 with Comparative Example 4, during the emulsion curing process, the lignin molecules in the lignin alkaline solution will agglomerate into large particles in the micron range, resulting in a greatly reduced reinforcing effect. Comparing Examples 6, 13 and Comparative Example 5, there is little difference in their mechanical properties. Lignin nanoparticles prepared by different methods have similar reinforcing effects on lignin-based waterborne polyurethane, but the ultrafiltration method is simple to operate, has high production efficiency, low cost, and is easy to industrialize. The dialysis method and the organic solvent method are only suitable for small-batch preparation in the laboratory. In addition, the ultrafiltration method and the dialysis method are more environmentally friendly than the organic solvent method.

[0165] Table 2 Mechanical properties of lignin-based polyurethane nanocomposites

[0166]

[0167] The lignin-based polyurethane nanocomposites from the Examples and Comparative Examples were aged for three days in a 100°C thermo-oxidative aging chamber. The changes in mechanical properties before and after aging were compared, as shown in Table 3. The samples from the Examples exhibited excellent aging resistance, with tensile strength retention rates exceeding 100% and elongation at break retention rates approaching 100%. Furthermore, the retention of Young's modulus gradually increased with increasing lignin nanoparticle content. In contrast, the samples from the Comparative Examples exhibited poor aging resistance, with tensile strength significantly decreasing after aging.

[0168] Table 3 Anti-aging properties of lignin-based polyurethane nanocomposites

[0169]

[0170]

[0171] Based on the natural photothermal conversion ability of lignin, the photothermal conversion performance of lignin-based waterborne polyurethane nanocomposites was studied. Figure 3 As shown. At 0.6W / cm 2 Under the irradiation of 808nm near-infrared light, the surface temperature of the nanocomposite material rises rapidly and reaches a stable state within 90 seconds, showing rapid photothermal conversion performance. Comparing Example 2, Example 6 and Example 7, the surface temperature of the nanocomposite material gradually increases with the increase of the content of lignin nanomicelles. The photothermal conversion performance of the material is greatly reduced when low molecular weight lignin or lignin nanomicelles are missing (Comparative Example 1, Comparative Example 2); and the composite material with the addition of lignin alkaline solution (Comparative Example 4) is not as good as Example 6 in photothermal conversion performance due to the larger particle size of the formed lignin particles.

[0172] The prepared lignin-based waterborne polyurethane composite emulsion (Example 6) was coated on the surface of a thermoelectric generator (model SP1848-27145) and cured into a coating. A xenon lamp light source was used to simulate sunlight to illuminate the surface, and a multimeter was used to record the change in the voltage output of the thermoelectric generator as the light intensity increased. Figure 4 As shown. It can be seen that the voltage value is linearly related to the light intensity. When the light intensity is 0.45W / cm 2 The output voltage can reach 0.6V. This nanocomposite material has potential application value in light-heat-electricity conversion.

[0173] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A tough lignin-based waterborne polyurethane nanocomposite material, characterized in that: The method comprises reacting the following components, calculated by weight: 70 to 95 parts of a long-chain diol, 5 to 30 parts of a low-molecular-weight lignin, 10 to 40 parts of lignin nano-micelles, an isocyanate, 2 to 8 parts of a hydrophilic chain extender, 0.1 to 1 part of a catalyst, and 2 to 6 parts of a neutralizer; The total amount of the low molecular weight lignin and the long chain diol is 100 parts; the isocyanate index R=1.1-1.3; The low molecular weight lignin is obtained by fractionation and / or partial depolymerization of industrial lignin, and has a weight average molecular weight of 800 to 2000 g / mol; The lignin nano-micelles are obtained by ultrafiltration and / or dialysis of a high molecular weight lignin alkaline solution, and have a particle size of 100 to 200 nm; The high molecular weight lignin is obtained by the following method: industrial lignin is fully dispersed and dissolved in an organic solvent, and the filter residue is filtered to obtain the high molecular weight lignin; The weight average molecular weight of the industrial lignin is 3000-6000 g / mol, and its concentration in the organic solvent is 200-500 g / L.

2. The tough lignin-based waterborne polyurethane nanocomposite material according to claim 1, characterized in that: The method comprises reacting the following components, calculated by weight: 70 to 90 parts of a long-chain diol, 10 to 30 parts of a low-molecular-weight lignin, 10 to 40 parts of lignin nano-micelles, an isocyanate, 6 to 8 parts of a hydrophilic chain extender, 0.1 to 1 part of a catalyst, and 3 to 4.5 parts of a neutralizer; The total amount of the low molecular weight lignin and the long chain diol is 100 parts; the isocyanate index R is 1.1 to 1.

3.

3. The tough lignin-based waterborne polyurethane nanocomposite material according to claim 1, characterized in that: The low molecular weight lignin is added in the form of a 10-20 wt% solution; The lignin nanomicelles are added in the form of a 10-40 wt% aqueous solution; The lignin nano-micelles are obtained by the following method: industrial lignin is fully dispersed and dissolved in an organic solvent, filtered, and then the filter residue is collected to obtain high-molecular-weight lignin; the high-molecular-weight lignin is dissolved in a 0.5-1 wt% alkaline solution, and the alkali is removed by ultrafiltration and / or dialysis. When the pH of the lignin solution drops to a range of 7-8, the ultrafiltration and / or dialysis is terminated to obtain a lignin nano-micelle aqueous solution; The high molecular weight lignin is dissolved in a 0.5-1 wt% alkaline solution at a concentration of 50-300 g / L; the alkaline solution is at least one of a sodium hydroxide solution, a potassium hydroxide solution and an ammonia solution.

4. The tough lignin-based waterborne polyurethane nanocomposite material according to claim 1, characterized in that: The long-chain diol is at least one of polytetramethylene glycol, polyethylene glycol, polycaprolactone glycol and polypropylene glycol, and has a number average molecular weight of 500 to 4000 g / mol; The isocyanate is at least one of isophorone diisocyanate, hexamethylene diisocyanate, diphenylmethane diisocyanate and toluene diisocyanate; The hydrophilic chain extender is at least one of 2,2-dimethylol propionic acid and 2,2-dimethylol butyric acid; The hydrophilic chain extender is added in the form of a 20-30 wt% solution; The catalyst is an organotin catalyst; The neutralizing agent is at least one of triethylamine, ammonia water, sodium hydroxide and potassium hydroxide.

5. The method for preparing a tough lignin-based waterborne polyurethane nanocomposite material according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) Dispersing industrial lignin in an organic solvent, dissolving it fully at room temperature, and filtering it. The filtrate is concentrated and dried to obtain graded low molecular weight lignin; the filter residue is dried to obtain high molecular weight lignin; (2) dissolving high molecular weight lignin in an alkaline solution, removing the alkali by ultrafiltration and / or dialysis, and ending the ultrafiltration and / or dialysis when the pH of the lignin solution drops to a range of 7 to 8 to obtain a lignin nanomicelle solution; (3) Dissolve industrial lignin in an alkaline solution and react at 140-180°C for 4-12 hours. After the reaction is completed, cool to room temperature and adjust to acidity to precipitate lignin from the solution. After filtering, washing, and drying, partially depolymerized low molecular weight lignin is obtained. (4) Add long-chain diol, isocyanate and catalyst into the reactor for prepolymerization at 70-80°C for 1-2 hours, then add hydrophilic chain extender and react for 0.5-1 hour, then add fractionated and / or partially depolymerized low molecular weight lignin and react for 1-4 hours. After the reaction, lower the temperature to room temperature, add neutralizer and stir for 0.5-1 hour; (5) The reaction system of step (4) is transferred to an emulsifier, and the lignin nanomicelle solution is added under stirring for emulsification, and then water is added to adjust the solid content of the emulsion to obtain an aqueous polyurethane emulsion, which is solidified into a film to obtain a tough lignin-based aqueous polyurethane nanocomposite material.

6. The method for preparing a tough lignin-based waterborne polyurethane nanocomposite material according to claim 5, characterized in that: The pore size of the filter membrane used for the ultrafiltration in step (2) is 500 to 1000 Da; and the dialysis in step (2) is performed using a dialysis bag with a pore size of 500 to 1000 Da.

7. The method for preparing a tough lignin-based waterborne polyurethane nanocomposite material according to claim 5, characterized in that: The stirring speed in step (5) is 800 to 1500 rpm; the solid content of the emulsion in step (5) is 20 to 45 wt%.

8. The method for preparing a tough lignin-based waterborne polyurethane nanocomposite material according to claim 5, characterized in that: The weight average molecular weight of the industrial lignin in step (1) is 3000-6000 g / mol, and its concentration in the organic solvent is 200-500 g / L; The organic solvent in step (1) is at least one of ethanol, acetone and ethyl acetate; The operation of fully dissolving in step (1) is mechanical stirring for 2 to 5 hours; The concentration of the alkaline solution in step (2) is 0.5-1 wt%; In step (2), the concentration of the high molecular weight lignin in the alkaline solution is 50 to 300 g / L, and the solid content of the obtained lignin nanomicelle solution is 10 to 40 wt%; The alkaline solution in steps (2) and (3) is at least one of sodium hydroxide solution, potassium hydroxide solution and ammonia solution; The concentration of the alkaline solution in step (3) is 0.5-2 wt%; The step (3) of adjusting the solution to acidity refers to adjusting the solution pH to 2-3 with dilute hydrochloric acid; The concentration of the industrial lignin in the alkaline solution in step (3) is 100 to 300 g / L.

9. The method for preparing a tough lignin-based waterborne polyurethane nanocomposite material according to claim 5, characterized in that: In step (4), the hydrophilic chain extender is added in the form of a 20-30 wt% solution, and the solvent is at least one of N-methylpyrrolidone, N,N-dimethylformamide, and dimethyl sulfoxide; the low molecular weight lignin is added in the form of a 10-20 wt% solution, and the solvent is at least one of acetone and tetrahydrofuran; After adding water to adjust the solid content of the emulsion in step (5), the organic solvent needs to be removed. The method for removing the organic solvent is to remove the organic solvent under reduced pressure at 200-300 rpm and 40-50°C; The film-forming curing in step (5) refers to casting the aqueous polyurethane emulsion into a polytetrafluoroethylene mold and evaporating it at room temperature to form a film.

10. Use of the tough lignin-based waterborne polyurethane nanocomposite material according to any one of claims 1 to 4 in preparing a coating for a thermoelectric power generation sheet.

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