Powder lignin-based polyurethane capable of being subjected to thermoplastic processing as well as preparation method and application of powder lignin-based polyurethane

Abstract

The invention relates to the technical field of biomass polyurethane material preparation, in particular to powder lignin-based polyurethane as well as a preparation method and application thereof. The powder lignin-based polyurethane is prepared from lignin polyol, long-chain polyol, isocyanate, a chain extender, a catalyst and a hydroxyl-containing solvent. A polymerization process is innovated, a high-energy-consumption and long-time high-temperature curing process is omitted, the preparation process of powdery lignin-based polyurethane is shortened, the powdery material has repolymerization and hot working capabilities, and the hot working temperature and the forming time of the powdery material are obviously lower than those of a lignin-based polyurethane material formed through high-temperature curing at present; after being formed, the thermosetting lignin-based polyurethane shows excellent mechanical properties, and the problem that the thermosetting lignin-based polyurethane is difficult to be subjected to thermoplastic processing and forming is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of biomass polyurethane materials, and particularly to a powder lignin-based polyurethane and its preparation method and application. Background Art

[0002] Under the concept of green and low-carbon development, lignin resources, due to their non-grain biological properties, unique aromatic structure, and multiple hydroxyl reaction sites, can be used as biomass polyols to synthesize lignin-based polyurethanes, and endow polyurethanes with excellent strength, thermal stability, water stability, and various functionalities, which is of great significance for promoting the sustainable development of polyurethane materials.

[0003] Generally speaking, polyurethane materials often need to be processed into granular masterbatches or powders for downstream forming processing. Currently, after synthesizing lignin-based polyurethanes by the process, it is necessary to prepare masterbatches or powders through high-temperature curing and screw extrusion granulation processes. For example, the patents "CN201811334960.6", "CN200910193436.6", and "CN202010024715.6" disclose different polyurethane materials synthesized from lignin and their methods. In the above preparation routes, after lignin copolymerizes with isocyanate as a crosslinking agent or chain extender, the samples are all cured at high temperature to remove solvents and fully crosslink and cure to form a stable three-dimensional crosslinked network. Therefore, the prepared lignin-based polyurethane materials exhibit superior strength and stability. It should be noted that the existing lignin-based polyurethanes form a crosslinked three-dimensional structure after discharging solvents through high-temperature curing, showing thermosetting properties, and are difficult to process and form subsequently, which severely restricts the back-end application of lignin-based polyurethane materials.

[0004] To solve the problem of poor thermoplastic processability of crosslinked lignin-based polyurethanes, the patent "CN202211454567.7" discloses a preparation method of a thermoplastic processable lignin-based polyurethane. This invention utilizes the phenolic hydroxyl groups of low-molecular-weight lignin to construct dynamic phenolic hydroxyl-type urethane bonds, and constructs interfacial metal coordination bonds between polyurethane segments and lignin to form a dynamic double-crosslinked network structure, greatly improving the strength, toughness, and light-controlled self-healing performance of lignin-based polyurethanes. In addition, introducing reversible dynamic covalent bonds, hydrogen bonds, or coordination bonds into lignin-based elastomers can improve the thermoplastic processability of the materials. However, the above structural designs often require complex technical designs, and additional monomers with dynamic structures need to be introduced, and the synthesized materials still need to be prepared into powdery or granular masterbatches through processes such as screw extrusion and granulation processing, and the processing procedures are relatively complex. Therefore, how to innovate the polymerization and processing processes and prepare lignin-based polyurethane masterbatches (powders) that can be used for thermoplastic processing through simple and efficient methods has become a challenge. Summary of the Invention

[0005] In order to solve the above problems, the present invention provides a powdered lignin-based polyurethane and a preparation method and application thereof. The present invention designs a solvent-mediated polymerization technology to directly prepare a powdered lignin-based polyurethane that can be used for thermoplastic processing. Through solvent-mediated polymerization, terminal hydroxyl groups and amino functional groups are introduced into the lignin-based polyurethane structure, the degree of polymerization of the lignin-based polyurethane is regulated, and a chemical environment that is beneficial to the dynamic exchange of carbamate is constructed, and a powdered lignin-based polyurethane that is easy to be thermoformed is directly prepared. This invention application innovates the polymerization process, eliminates the high energy consumption and long-term high-temperature curing process, shortens the preparation process of powdered lignin-based polyurethane, and the powder has the ability to repolymerize and heat process. After molding, it exhibits excellent mechanical properties and functionality, solving the problem that thermosetting lignin-based polyurethane is difficult to thermoplastic process.

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

[0007] The present invention provides a thermoplastic processable powdered lignin-based polyurethane, which is prepared from the following raw materials in parts by weight:

[0008] 1-50 parts of lignin polyol, 30-50 parts of long-chain polyol, 20-40 parts of isocyanate, 5-10 parts of chain extender, 0.1-0.5 parts of catalyst and more than 150 parts of hydroxyl-containing solvent.

[0009] Preferably, the lignin polyol comprises at least one of deep eutectic solvent extracted lignin, commercially available dealkalized lignin, enzymatically hydrolyzed lignin and commercially available alkaline lignin;

[0010] The method for extracting lignin by self-extraction of low eutectic solvent is as follows: using poplar wood powder as raw material, preparing choline chloride-oxalic acid-based low eutectic solvent in a molar ratio of 1:1, then heating at 120°C for 8 hours, and then filtering, centrifuging and washing to purify and extract lignin, with a molecular weight of 1066Da, a total hydroxyl content of 8.47mmol / g, and a static contact angle of 107.4°. Specific methods are shown in the literature [Wang D, Liu L, Shen R, et al. Fascinating polyphenol lignin extracted from sawdust via a green and recyclable solvent route [J]. International Journal of Biological Macromolecules, 2023, 234: 123780.

[0011] Market dealkalized lignin: purchased from MacLean Reagent Co., Ltd., product CAS number 9005-53-2;

[0012] Enzymatic lignin: Purchased from Wuhan Lana White Pharmaceutical and Chemical Co., Ltd., product model 86855-54-1; Alkaline lignin: Purchased from Dahai Chemical Industry Co., Ltd. in Jiaxiang County, product model 8068-05-1;

[0013] The lignin is not limited to the above types, and is characterized in that the molecular weight is below 8000 Da, the content of reactive groups is higher than 3 mmol / g, and the static contact angle is greater than 90°.

[0014] Preferably, the long-chain polyol includes polyethylene glycol.

[0015] Preferably, the chain extender includes 1,4-butanediol;

[0016] The catalyst includes stannous octoate.

[0017] Preferably, the hydroxyl-containing solvent includes one or more of water, methanol, ethanol, propanol and isopropanol, and its hydroxyl functionality is 1.

[0018] The present invention also provides a method for preparing the powdery lignin-based polyurethane described in the above technical solution, including the following steps:

[0019] 1) Mix the lignin polyol, long-chain polyol, isocyanate and catalyst, and carry out a prepolymerization reaction to obtain a prepolymer product;

[0020] 2) Mix the prepolymer reactant obtained in step 1) with a chain extender and carry out a chain extension reaction to obtain a chain extension product;

[0021] 3) Mix the chain extension product obtained in step 2) with a hydroxyl-containing solvent, stir and react to obtain a reaction material, separate the solid and liquid of the reaction material, and dry the obtained solid to obtain the powdery lignin-based polyurethane.

[0022] Preferably, the conditions for the prepolymerization reaction in step 1) include: temperature 85°C, time 1 h.

[0023] Preferably, the conditions for the chain extension reaction in step 2) include: temperature 65°C, time 10 min.

[0024] Preferably, the conditions for the stirring reaction in step 3) include: rotation speed 300 rpm, time 10 min;

[0025] The drying temperature is 30-80°C.

[0026] The present invention also provides the application of the powdery lignin-based polyurethane described in the above technical solution in improving the mechanical properties of polyurethane materials.

[0027] The present invention directly prepares powder lignin-based polyurethane by replacing the traditional high-temperature curing process and screw extrusion granulation process with a solvent-mediated (hydroxyl-containing solvent) polymerization process. This solvent-mediated process is mainly reflected in three points: (1) The solvent mediates the end-group structure of the molecular network of lignin-based polyurethane, introducing terminal hydroxyl groups and amino groups to form a dynamic cross-linked network; (2) The solvent mediates the degree of polymerization of lignin-based polyurethane, endowing it with a low degree of polymerization and the ability of re-polymerization; (3) Utilizing the hydrophobic characteristics of the lignin segment, the solvent promotes the precipitation of lignin-based polyurethane to form powder. The above-mentioned mediation process endows the powder lignin-based polyurethane with excellent thermoplastic properties.

[0028] Advantages of the present invention:

[0029] 1. By replacing the traditional high-temperature thermal curing process through solvent mediation, powder lignin-based polyurethane material is directly prepared, eliminating the screw extrusion granulation process, which greatly saves energy consumption and processing time.

[0030] 2. The solvent-mediated method endows lignin-based polyurethane with a controllable degree of polymerization and a re-polymerizable end-group structure, overcoming the problem of poor thermo-processing performance of the cross-linked structure of lignin-based polyurethane.

[0031] 3. The powder lignin-based polyurethane is easy to process, has excellent mechanical properties after molding, and the material has advantages such as green environmental protection, and can be flexibly processed into various polyurethane products. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments.

[0033] Figure 1 The mediation reaction mechanism of the water solvent;

[0034] Figure 2 The physical photos of the powder lignin-based polyurethane and its molded objects;

[0035] Figure 3 The stress relaxation curves of the powder lignin-based polyurethane (left figure) and the control sample (middle figure) and the relaxation activation energy of both (right figure).

[0036] Figure 4 The variable-temperature infrared spectrogram of the powder lignin-based polyurethane;

[0037] Figure 5 The processing thermoplastic mechanism of the powder lignin-based polyurethane;

[0038] Figure 6 The stress-strain curve after hot pressing and molding of the powder lignin-based polyurethane; Detailed Embodiments

[0039] The present invention provides a powder lignin-based polyurethane, which is prepared from raw materials including the following parts by weight: 1 to 50 parts of lignin polyol, 30 to 50 parts of long-chain polyol, 20 to 40 parts of isocyanate, 5 to 10 parts of chain extender, 0.1 to 0.5 part of catalyst, and more than 150 parts of hydroxyl-containing solvent.

[0040] In the present invention, the lignin polyol preferably includes lignin extracted by a self-prepared deep eutectic solvent, with a molecular weight of 1066 Da, a total hydroxyl content of 8.47 mmol / g, and a static contact angle of 107.4°. The method for extracting this lignin is as follows: Using poplar wood powder as the raw material, a choline chloride-oxalic acid-based deep eutectic solvent is prepared according to a molar ratio of 1:1, and then heated at 120 °C for 8 h. Subsequently, the lignin is extracted through filtration, centrifugation, and washing and purification. For the specific method, refer to the literature [Wang D, Liu L, Shen R, et al. Fascinating polyphenol lignin extracted from sawdust via a green and recyclable solvent route[J]. International Journal of Biological Macromolecules, 2023, 234: 123780.].

[0041] In the present invention, the long-chain polyol preferably includes polyethylene glycol. In the present invention, the chain extender preferably includes 1,4-butanediol. In the present invention, the catalyst preferably includes stannous octoate. In the present invention, the hydroxyl-containing solvent preferably includes one or more of water, methanol, ethanol, propanol, and isopropanol.

[0042] The present invention also provides a preparation method of the powder lignin-based polyurethane described in the above technical solution, including the following steps:

[0043] 1) Mix the lignin polyol, long-chain polyol, isocyanate, and catalyst, and then carry out a prepolymerization reaction to obtain a prepolymer product;

[0044] 2) Mix the prepolymer reactant obtained in step 1) with the chain extender, and then carry out a chain extension reaction to obtain a chain extension product;

[0045] 3) Mix the chain extension product obtained in step 2) with the hydroxyl-containing solvent, stir and react to obtain a reaction material, separate the solid and liquid of the reaction material, and dry the obtained solid to obtain the powder lignin-based polyurethane.

[0046] In the present invention, the lignin polyol, long-chain polyol, isocyanate, and catalyst are mixed and then subjected to a prepolymerization reaction to obtain a prepolymer product. In the present invention, the conditions of the prepolymerization reaction preferably include: a temperature of 85 °C and a time of 1 h.

[0047] The prepolymer reactant obtained in the present invention is mixed with a chain extender and then subjected to a chain extension reaction to obtain a chain-extended product. In the present invention, the conditions of the chain extension reaction preferably include: a temperature of 65 °C and a time of 10 min.

[0048] The chain-extended product obtained in the present invention is mixed with a hydroxyl-containing solvent and subjected to a stirring reaction to obtain a reaction material. The reaction material is subjected to solid-liquid separation, and the obtained solid is dried to obtain powdered lignin-based polyurethane. In the present invention, the conditions of the stirring reaction preferably include: a rotation speed of 300 rpm and a time of 10 min. In the present invention, the drying temperature is preferably 30-80 °C. The present invention does not particularly limit the method of the solid-liquid separation, and those skilled in the art can follow the routine.

[0049] The present invention also provides the application of the powdered lignin-based polyurethane described in the above technical solution in improving the mechanical properties of polyurethane materials.

[0050] In order to further illustrate the present invention, the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0051] Example 1

[0052] A preparation method of powdered lignin-based polyurethane, the steps are as follows:

[0053] 50 parts of diisocyanate (HDI), 50 parts of long-chain polyol (polyethylene glycol, number-average molecular weight 1000), 0.5 part of catalyst (stannous octoate), 20 parts of lignin extracted by eutectic solvent (molecular weight 1066 Da, total hydroxyl content 8.47 mmol / g, static contact angle 107.4°). The method for extracting the lignin is as follows: Using poplar wood powder as the raw material, choline chloride-oxalic acid-based eutectic solvent is prepared according to a molar ratio of 1:1, and then heated at 120 °C for 8 h, and then lignin is extracted through filtration, centrifugation and washing purification. For the specific method, refer to the literature [Wang D, Liu L, Shen R, et al. Fascinating polyphenollignin extracted from sawdust via a green and recyclable solvent route[J]. International Journal of Biological Macromolecules, 2023, 234:123780.]. React at 85 °C for 1 h, then add 10 parts of chain extender (1,4-butanediol) and react at 65 °C for 10 min. Add 300 parts of water as the mediating solvent, react at a rotation speed of 300 rpm for 10 min, and then filter and separate using a 60-mesh sieve. The obtained solid is then dried at 60 °C to prepare powdered lignin-based polyurethane, and then hot-pressed at 140 °C to form square plates with a size of 10 cm × 10 cm, pentagram shapes and ring parts with a diameter of 5 cm.

[0054] For comparison, the present invention application synthesizes a lignin-based polyurethane comparative sample using a high-temperature curing process. The specific steps are as follows:

[0055] 50 parts of polyisocyanate (HDI), 50 parts of long-chain polyol (polyethylene glycol, number-average molecular weight 1000), 0.5 part of catalyst (stannous octoate), 20 parts of lignin extracted by eutectic solvent (molecular weight 1066 Da, total hydroxyl content 8.47 mmol / g, static contact angle 107.4°) are reacted at 85 °C for 1 h, then 10 parts of chain extender (1,4-butanediol) are added and reacted at 65 °C for 10 min, and then scraped into a 10 cm × 10 cm square film. Then, the lignin-based polyurethane film is placed in an oven at 110 °C for 2 h and cured at 80 °C for 12 h. After curing, the lignin-based polyurethane film is placed at 180 °C and hot-pressed for 20 min to form a 10 cm × 10 cm × 0.1 cm film material, named the control sample.

[0056] The proposed reaction mechanism mediated by the solvent water solvent is as Figure 1As shown, on the one hand, water molecules react with isocyanate to neutralize the excess HDI in the system and terminate the polymerization reaction. Subsequently, water molecules react with the isocyanate on the surface of lignin-based polyurethane, leaving excess hydroxyl groups on the surface and generating amino groups, thereby preparing powdery lignin-based polyurethane with controllable degree of polymerization and rich reactive groups on the surface. The photos of the actual product and its thermally processed and formed parts are as Figure 2 shown. It can be seen therefrom that the powdery lignin-based polyurethane has good flexible processing performance, can adapt to twin-screw extrusion spinning, and can be processed into complex shapes.

[0057] Such as Figure 3 shown, the powdery lignin-based polyurethane can reach relaxation equilibrium in only 6 s at 100 °C, and its chain segment relaxation activation energy is 91.9 kJ / mol. For the control sample cured at high temperature, its chain segment relaxation time at 100 °C is 1620 s, and its chain segment relaxation activation energy is as high as 121.7 kJ / mol. This result confirms that the molecular chain segments of the powdery lignin-based polyurethane constructed in the present invention have a higher thermally induced rearrangement rate, and its thermal processing is less dependent on temperature and is easy to process and form.

[0058] The variable-temperature infrared spectrum of the powdery lignin-based polyurethane is as Figure 4 shown, in which the absorption band peaks at 1685 - 1715 cm -1 , 1540 cm -1 and 1610 cm -1 respectively belong to the carbonyl absorption vibration, N-H bond stretching vibration and benzene ring skeletal vibration, indicating that lignin has been successfully polymerized into the polyurethane backbone structure. At the same time, characteristic absorption bands of amino and hydroxyl groups appear at 3440 and 3550 cm -1 respectively for the material, indicating the successful synthesis of the water solvent-mediated structure.

[0059] Figure 5 shows the thermal processing mechanism of this powdery lignin-based polyurethane, that is, the powdery lignin-based polyurethane has a low crosslinking network structure and a large number of free hydroxyl and amino groups, and can rearrange and re-polymerize under heating conditions until a stable crosslinking network is formed after molding.

[0060] The stress-strain curves of the formed powdery lignin-based polyurethane were measured using a universal testing machine, and the material fracture strength, fracture elongation, elastic modulus and toughness indexes were obtained based on the curves. Before testing, all the splines were cut into standard dumbbell-shaped splines, the distance between the tensile clamps was 40 mm, and the tensile rate was kept constant at 20 mm / min. Such as Figure 6 shown, the formed powdery lignin-based polyurethane maintains good mechanical properties. The same sample was tested three times and all maintained similar mechanical properties. Its strength is about 30 MPa, and the fracture elongation exceeds 400%.

[0061] Examples 2 - 5

[0062] Change the weight parts of lignin in Example 1 to 10, 15, 20, 25, and change the parts of long - chain polyol to 45, 40, 35, 30 accordingly, and the other conditions are the same as in Example 1.

[0063] Table 1 Influence of Lignin Mass Parts on the Mechanical Properties of Lignin - based Polyurethane

[0064]

[0065] Examples 6 - 9

[0066] Replace the mediated solvent in Example 1 with methanol, ethanol, propanol, and isopropanol, and the other conditions are the same as in Example 1.

[0067] Table 2 Influence of Mediated Solvent on the Mechanical Properties of Powder Lignin - based Polyurethane

[0068] Mediated solvent Breaking strength / MPa Elongation at break / % Elastic modulus / MPa <![CDATA[Toughness / (MJ / cm -3 )]]> Methanol 14.4 300.6 90.9 47.8 Ethanol 16.7 431.3 180.6 68.5 Propanol 15.4 487.8 142.9 70.3 Isopropanol 16.0 240.9 287.2 34.9

[0069] Examples 10 - 14

[0070] Change the low - temperature drying temperature in Example 1 to 30, 40, 50, 70, and 80 °C, and the other conditions are the same as in Example 1.

[0071] Table 3 Influence of Low - temperature Drying Temperature on the Mechanical Properties of Powder Lignin - based Polyurethane

[0072]

[0073]

[0074] Examples 15 - 19

[0075] Set the hot - pressing temperature of the powder lignin - based polyurethane in Example 1 to 100, 120, 160, 180, and 200 °C, and the other conditions are the same as in Example 1.

[0076] Table 4 Influence of Hot - processing Temperature on the Mechanical Properties of Powder Lignin - based Polyurethane

[0077] Hot working temperature / ℃ Breaking strength / MPa Elongation at break / % Elastic modulus / MPa <![CDATA[Toughness / (MJ / cm -3 )]]> 100 15.1 279.6 49.2 47.5 120 21.6 336.8 91.7 78.3 160 35.2 400.6 269.6 146.1 180 40.0 344.3 413.9 117.6 200 16.8 169.7 643.8 30.8

[0078] Although the above - mentioned examples have described the present invention in detail, they are only a part of the embodiments of the present invention, rather than all embodiments. People can also obtain other embodiments based on these embodiments without creative efforts, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A thermoplastically processable powder lignin-based polyurethane and its preparation method, characterized in that, Prepared from raw materials including the following parts by weight: 1 - 50 parts of lignin polyol, 30 - 50 parts of long - chain polyol, 20 - 40 parts of isocyanate, 5 - 10 parts of chain extender, 0.1 - 0.5 part of catalyst, and more than 150 parts of hydroxyl - containing solvent.

2. The powdered lignin-based polyurethane according to claim 1, wherein The lignin polyol is highly reactive lignin with a molecular weight of less than 8000 Da, a total hydroxyl content higher than 3.0 mmol / g, and a static contact angle greater than 90°.

3. The powdered lignin-based polyurethane according to claim 1, characterized in that, The long - chain polyol includes polyethylene glycol.

4. The powdered lignin-based polyurethane according to claim 1, characterized in that, The chain extender includes 1,4 - butanediol; The catalyst includes stannous octoate.

5. The powdered lignin-based polyurethane according to claim 1, wherein The hydroxyl - containing solvent includes one or more of water, methanol, ethanol, propanol, and isopropanol.

6. A method for preparing the powdered lignin-based polyurethane according to any one of claims 1 to 5, characterized in that, Including the following steps: 1) Mix the lignin polyol, long - chain polyol, isocyanate, and catalyst, and then carry out a prepolymerization reaction to obtain a prepolymer product; 2) Mix the prepolymer reactant obtained in step 1) with the chain extender and then carry out a chain - extension reaction to obtain a chain - extended product; 3) Mix the chain - extended product obtained in step 2) with a hydroxyl - containing solvent (hydroxyl functionality is 1), stir and react to obtain a reaction mixture. Separate the solid and liquid of the reaction mixture, and dry the obtained solid to obtain powdered lignin - based polyurethane.

7. The preparation method according to claim 6, characterized in that, The conditions for the prepolymerization reaction in step 1) include: temperature is 85 °C, time is 1 h.

8. The preparation method according to claim 6, characterized in that, The conditions for the chain - extension reaction in step 2) include: temperature is 65 °C, time is 10 min.

9. The preparation method according to claim 6, wherein The conditions for the stirring reaction in step 3) include: rotation speed is 300 rpm, time is 10 min; The drying temperature is 30 - 80 °C.

10. Use of the powdered lignin - based polyurethane according to any one of claims 1 - 5 in improving the mechanical properties of polyurethane materials.

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