Rigid polyurethane foam, preparation method thereof and refrigerator

Abstract

The invention discloses rigid polyurethane foam, a preparation method thereof and a refrigerator, and relates to the technical field of foaming materials. The preparation method of the rigid polyurethane foam comprises the following steps: providing bio-based hydrolyzed lignin and a mixed liquefying agent, mixing, and liquefying to obtain liquefied hydrolyzed lignin; wherein the mixed liquefying agent comprises water and an alcohol reagent; providing epoxyalkane, mixing the epoxyalkane with the liquefied hydrolyzed lignin, and carrying out modification treatment to obtain bio-based polyol; and providing a foaming agent and an isocyanate compound, mixing the foaming agent and the isocyanate compound with the bio-based polyol, and carrying out foaming treatment to obtain the rigid polyurethane foam. The rigid polyurethane foam prepared by adopting the preparation method has excellent performances such as low heat conductivity coefficient, high compressive strength, high dimensional stability, high thermal stability and short curing time, and can meet the use requirements and production efficiency of a refrigerator thermal insulation layer.

Description

Technical Field

[0001] This application relates to the field of foaming materials technology, and in particular to a rigid polyurethane foam and its preparation method, and a refrigerator. Background Technology

[0002] A refrigerator is a refrigeration device that maintains a constant low temperature, and it is also a product that keeps food or other items at a constant low temperature. Foamed material is a closed-cell rigid foam that is chemically foamed and cured in the space between the refrigerator's outer shell and inner liner. The core function of foamed material is to create a continuous and uniform thermal insulation layer, minimizing the transfer of heat from the external environment to the refrigerator's interior, thereby maintaining a low-temperature environment and reducing energy consumption.

[0003] Polyurethane (PU) rigid foam is a core material for refrigerator insulation layers due to its excellent thermal insulation and mechanical properties. Currently, the production of PU rigid foam for refrigerators mainly relies on petroleum-based polyol raw materials. However, petroleum resources are increasingly depleted, prices fluctuate wildly, and the extraction and use of petroleum-based materials lead to increased greenhouse gas emissions, which is inconsistent with the global trend of low-carbon and environmentally friendly development. To address the raw material dependence issue, the industry has begun to explore using biomass resources to replace petroleum-based raw materials in the preparation of PU rigid foam.

[0004] Therefore, how to ensure or further improve the thermal insulation, compressive strength, and stability of rigid polyurethane foam while improving the environmental friendliness of its preparation still needs further development. Summary of the Invention

[0005] In view of this, this application provides a rigid polyurethane foam and a method for preparing the same, as well as a refrigerator, to solve at least one of the above-mentioned problems.

[0006] The embodiments of this application are implemented as follows: a method for preparing rigid polyurethane foam includes the following steps: A bio-based hydrolyzed lignin and a mixed liquefaction agent are provided, which are then mixed and liquefied to obtain liquefied hydrolyzed lignin; wherein the mixed liquefaction agent includes water and an alcohol reagent; An epoxy alkane is provided, which is then mixed with the liquefied hydrolyzed lignin and modified to obtain a bio-based polyol. A foaming agent and an isocyanate compound are provided, and after being mixed with the bio-based polyol, a foaming treatment is performed to obtain rigid polyurethane foam.

[0007] Optionally, in some embodiments of this application, the bio-based hydrolyzed lignin includes one or more of poplar-based hydrolyzed lignin, straw-based hydrolyzed lignin, bamboo-based hydrolyzed lignin, and pine-based hydrolyzed lignin.

[0008] The bio-based hydrolyzed lignin comprises: 50wt%~60wt% lignin, 25wt%~32wt% carbohydrates, 1.1wt%~1.5wt% ash, and ≤5wt% moisture.

[0009] Based on elemental content, the bio-based hydrolyzed lignin comprises: 60wt%~64wt% carbon, 5.8wt%~6.5wt% hydrogen, and 3.5wt%~4.5wt% nitrogen.

[0010] The alcohol reagents include one or more of ethanol, propanol, and ethylene glycol.

[0011] The epoxides include one or more of propylene oxide, ethylene oxide, and 1,2-epoxidebutane.

[0012] Optionally, in some embodiments of this application, the volume ratio of water to alcohol reagent in the mixed liquefaction agent is (30~50):(50~70).

[0013] The mass-to-volume ratio of the bio-based hydrolyzed lignin to the volume of the mixed liquefaction agent is (18~22) g: 100 mL.

[0014] The mass ratio of the liquefied hydrolyzed lignin to the epoxide is (50~70):(30~50).

[0015] The heating rate of the liquefaction process is 4℃ / min to 6℃ / min, the temperature of the liquefaction process is 240℃ to 260℃, and the time of the liquefaction process is 50min to 70min.

[0016] The heating rate of the modification treatment is 2℃ / min to 4℃ / min; the temperature of the modification treatment is 140℃ to 160℃; and the time of the modification treatment is 2h to 3h.

[0017] Optionally, in some embodiments of this application, the mixing of the epoxy alkane with the liquefied hydrolyzed lignin further includes the addition of an initiator and a solvent.

[0018] The initiator comprises a mixture of polyols and alkali metal hydroxides.

[0019] The polyol includes one or more of glycerol, ethylene glycol, and trimethylolpropane; the alkali metal hydroxide includes one or more of NaOH and KOH; the mass ratio of the polyol to the alkali metal hydroxide is (80~90):(10~20).

[0020] The mass ratio of the liquefied hydrolyzed lignin to the initiator is 100:(10~15).

[0021] The solvent includes one or more of acetone, ethanol, and isopropanol.

[0022] The mass ratio of the liquefied hydrolyzed lignin to the solvent is 100:(80~95).

[0023] Optionally, in some embodiments of this application, the weight-average molecular weight of the liquefied hydrolyzed lignin is 900 g / mol to 1100 g / mol; the number-average molecular weight of the liquefied hydrolyzed lignin is 600 g / mol to 700 g / mol; the dispersity of the liquefied hydrolyzed lignin is 1.5 to 1.8; and the hydroxyl value of the liquefied hydrolyzed lignin is 400 mg KOH / g to 500 mg KOH / g.

[0024] The bio-based polyol has a weight-average molecular weight of 1200 g / mol to 1900 g / mol; a number-average molecular weight of 700 g / mol to 1000 g / mol; a hydroxyl value of 210 mg KOH / g to 260 mg KOH / g; and a viscosity of 0.3 Pa·s to 0.7 Pa·s.

[0025] Optionally, in some embodiments of this application, the foaming agent includes one or more of the following: a compound foaming agent of acetone and cyclopentane, a compound foaming agent of water and cyclopentane, and a compound foaming agent of n-pentane and isopentane.

[0026] The isocyanate compounds include one or more of the following: isocyanate, 4,4'-diphenylmethane diisocyanate, 2,6-toluene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate, and phenylmethylene diisocyanate.

[0027] Optionally, in some embodiments of this application, the mixing of the foaming agent, the isocyanate compound, and the bio-based polyol further includes the addition of one or more of a crosslinking agent, a catalyst, a surfactant, and a viscosity modifier.

[0028] The crosslinking agent includes glycerol.

[0029] The catalyst includes one or more of amine catalysts and metal catalysts; the amine catalyst includes one or more of triethanolamine, diethanolamine, triethylamine, dimethylethanolamine, dimethylcyclohexylamine, triethylenediamine, tetramethylethylenediamine, tetramethylpropylenediamine, pentamethyldiethylenetriamine, N,N-dimethylbenzylamine, N,N-dimethylaniline, and triisopropanolamine; the metal catalyst includes one or more of stannous octoate, dibutyltin dilaurate, bismuth isooctanoate, bismuth neodecanoate, zinc naphthenate, zinc isooctanoate, cobalt isooctanoate, zirconium naphthenate, zirconium acetylacetonate, titanium acetylacetonate, organotin catalysts, and organobismuth catalysts.

[0030] The surfactant includes one or more of polyether-modified silicone and silicone oil.

[0031] The viscosity modifier includes water.

[0032] By weight, the bio-based polyol comprises 100 parts, the foaming agent comprises 15-25 parts, the crosslinking agent comprises 8-12 parts, the catalyst comprises 0.8-3 parts, the surfactant comprises 0.5-2 parts, the viscosity modifier comprises 0.3-2 parts, and the isocyanate compound comprises 130-150 parts.

[0033] Optionally, in some embodiments of this application, the foaming process includes a foaming stage, a curing stage, and a maturation stage performed sequentially.

[0034] The foaming stage lasts for 60-90 seconds; the curing stage lasts for 150-210 seconds; and the maturation stage lasts for 22-26 hours.

[0035] After the foaming process, the process further includes a curing process; the curing process is carried out at a temperature of 50℃ to 70℃ and for a time of 10h to 15h.

[0036] Accordingly, this application also provides a rigid polyurethane foam prepared by the above-described preparation method.

[0037] Accordingly, this application also provides a refrigerator, which includes a cabinet, and the cabinet has a layer of polyurethane rigid foam prepared by the above preparation method, or the above-mentioned polyurethane rigid foam.

[0038] The method for preparing rigid polyurethane foam provided in this application uses bio-based hydrolyzed lignin as raw material. Liquefaction is achieved through a mixed liquefying agent to obtain liquefied hydrolyzed lignin. Bio-based hydrolyzed lignin is environmentally friendly, avoiding the dependence of polyurethane foam on petroleum-based raw materials. The structure of hydrolyzed lignin is less affected by hydrolysis, retaining a complete aromatic ring skeleton and abundant hydroxyl functional groups. It exhibits excellent reactivity and can serve as a core reaction site for the synthesis of rigid polyurethane foam, enhancing foaming treatment and improving the strength of the rigid polyurethane foam. The mixed liquefying agent of water and alcohol is specifically designed for hydrolyzed lignin, which has a high aromatic ring content, strong polarity, and is difficult to liquefy with a single solvent. The design features of this product can solve the problems of low liquefaction rate and wide molecular weight distribution of hydrolyzed lignin. During the modification process, the hydroxyl groups on the liquefied hydrolyzed lignin molecular chain can react with epoxides to introduce side chains into the aromatic skeleton of lignin, thereby increasing the activity and content of hydroxyl groups. This makes it more suitable for reacting with isocyanate compounds to generate polyurethane. It can also reduce the viscosity of the system, improve the fluidity and processing performance of the liquefied system, and increase the flexibility of the molecules. This allows the modified product to have the structural characteristics of a polyol, which can be directly used as a bio-based polyol for the synthesis of rigid polyurethane foam, while improving the strength, heat resistance and dimensional stability of the rigid foam.

[0039] The polyurethane rigid foam preparation method provided in this application develops an efficient biomass raw material liquefaction system through liquefaction treatment of bio-based hydrolyzed lignin, modification treatment of epoxides, and foaming treatment with isocyanate compounds. This achieves efficient utilization of bio-based raw materials, improves the yield and purity of bio-based polyols, and optimizes their molecular weight distribution, meeting the structural requirements of polyurethane rigid foams for refrigerators. The modification process of bio-based polyols is improved, enhancing their reactivity with isocyanates and solving the problems of high thermal conductivity and insufficient compressive strength at room temperature and low temperature in existing bio-based polyurethane rigid foams, while balancing the relationship between bio-content and mechanical properties. The polyurethane rigid foam preparation formula and process are optimized, enhancing the wide-temperature-range dimensional stability and thermal aging performance of bio-based polyurethane rigid foams, ensuring low dimensional deformation rate in the range of -40℃ to 80℃ and good thermal stability below 200℃, while shortening the curing time to meet the efficiency requirements of refrigerator production lines. This method realizes the application of high-bio-content polyols in refrigerator polyurethane rigid foams, reducing dependence on petroleum-based raw materials and improving the environmental friendliness and cost advantages of the product.

[0040] The polyurethane rigid foam prepared in this application exhibits excellent properties such as low thermal conductivity, high compressive strength, high dimensional stability, high thermal stability, and short curing time, which can meet the usage requirements and production efficiency of refrigerator insulation layers. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 This is a flowchart of a method for preparing rigid polyurethane foam provided in an embodiment of this application. Detailed Implementation

[0043] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. Furthermore, it should be understood that the specific embodiments described herein are only for illustration and explanation of this application and are not intended to limit this application.

[0044] In this application, unless otherwise stated, directional terms such as "upper" and "lower" generally refer to the upper and lower positions of the device in its actual use or operating state, specifically the orientation shown in the accompanying drawings; while "inner" and "outer" refer to the outline of the device. Furthermore, in the description of this application, the term "comprising" means "including but not limited to". The terms first, second, third, etc., are used merely as illustrative purposes and do not impose numerical requirements or establish a numerical order.

[0045] In this application, "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural.

[0046] In this application, "at least one" means one or more, and "more than one" means two or more. "One or more", "at least one of the following", or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0047] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0048] In existing technologies, studies have been conducted on the preparation of bio-based polyols from biomass raw materials such as lignin and straw through processes such as liquefaction and modification. However, these methods have several drawbacks: First, the liquefaction efficiency of biomass raw materials is low. Under existing aqueous or single organic solvent liquefaction systems, the yield of the target product is low, and the molecular weight distribution is wide, making subsequent polyol modification difficult. Second, the reactivity of bio-based polyols with isocyanates is insufficient, resulting in polyurethane rigid foams with problems such as low compressive strength, significant degradation of low-temperature mechanical properties, and high thermal conductivity, which cannot meet the dual requirements of high-end refrigerators for extreme insulation and full-temperature-range mechanical support. Third, existing bio-based polyurethane rigid foams exhibit poor dimensional stability and significant degradation of mechanical properties after thermal aging in the wide temperature range environment (-40℃~80℃) of long-term refrigerator use, and the curing efficiency is low, limiting their application in large-scale refrigerator production lines.

[0049] This application can solve at least one of the following problems in existing rigid polyurethane foam for refrigerators: reliance on petroleum-based raw materials, insufficient thermal insulation and mechanical properties of bio-based foams, poor stability over a wide temperature range, and low curing efficiency.

[0050] The technical solution of this application is as follows: Firstly, please refer to Figure 1 This application provides a method for preparing rigid polyurethane foam, comprising the following steps: Step S11: Provide bio-based hydrolyzed lignin and a mixed liquefaction agent, mix them and then liquefy them to obtain liquefied hydrolyzed lignin; wherein, the mixed liquefaction agent includes water and alcohol reagents; Step S12: Provide epoxy alkane, mix it with the liquefied hydrolyzed lignin and then perform modification treatment to obtain bio-based polyol; Step S13: Provide a foaming agent and an isocyanate compound, mix them with the bio-based polyol, and then perform a foaming treatment to obtain rigid polyurethane foam.

[0051] It should be noted that bio-based hydrolyzed lignin refers to a natural aromatic polymer obtained from lignocellulosic biomass (such as straw and wood) through acid hydrolysis or hot water hydrolysis. Bio-based hydrolyzed lignin retains the lignin phenylpropane skeleton and a large number of active functional groups such as phenolic hydroxyl and methoxy groups, exhibiting excellent structural integrity and reactivity. It also has a narrower molecular weight distribution and better biodegradability, making it a high-quality renewable raw material for the preparation of polyurethane foam.

[0052] The method for preparing rigid polyurethane foam provided in this application uses bio-based hydrolyzed lignin as raw material. Liquefaction is achieved through a mixed liquefying agent to obtain liquefied hydrolyzed lignin. Bio-based hydrolyzed lignin is environmentally friendly, avoiding the dependence of polyurethane foam on petroleum-based raw materials. The structure of hydrolyzed lignin is less affected by hydrolysis, retaining a complete aromatic ring skeleton and abundant hydroxyl functional groups. It exhibits excellent reactivity and can serve as a core reaction site for the synthesis of rigid polyurethane foam, enhancing foaming treatment and improving the strength of the rigid polyurethane foam. The mixed liquefying agent of water and alcohol is specifically designed for hydrolyzed lignin, which has a high aromatic ring content, strong polarity, and is difficult to liquefy with a single solvent. The design features of this product can solve the problems of low liquefaction rate and wide molecular weight distribution of hydrolyzed lignin. During the modification process, the hydroxyl groups on the liquefied hydrolyzed lignin molecular chain can react with epoxides to introduce side chains into the aromatic skeleton of lignin, thereby increasing the activity and content of hydroxyl groups. This makes it more suitable for reacting with isocyanate compounds to generate polyurethane. It can also reduce the viscosity of the system, improve the fluidity and processing performance of the liquefied system, and increase the flexibility of the molecules. This allows the modified product to have the structural characteristics of a polyol, which can be directly used as a bio-based polyol for the synthesis of rigid polyurethane foam, while improving the strength, heat resistance and dimensional stability of the rigid foam.

[0053] The polyurethane rigid foam preparation method provided in this application develops an efficient biomass raw material liquefaction system through liquefaction treatment of bio-based hydrolyzed lignin, modification treatment of epoxides, and foaming treatment with isocyanate compounds. This achieves efficient utilization of bio-based raw materials, improves the yield and purity of bio-based polyols, and optimizes their molecular weight distribution, meeting the structural requirements of polyurethane rigid foams for refrigerators. The modification process of bio-based polyols is improved, enhancing their reactivity with isocyanates and solving the problems of high thermal conductivity and insufficient compressive strength at room temperature and low temperature in existing bio-based polyurethane rigid foams, while balancing the relationship between bio-content and mechanical properties. The polyurethane rigid foam preparation formula and process are optimized, enhancing the wide-temperature-range dimensional stability and thermal aging performance of bio-based polyurethane rigid foams, ensuring low dimensional deformation rate in the range of -40℃ to 80℃ and good thermal stability below 200℃, while shortening the curing time to meet the efficiency requirements of refrigerator production lines. This method realizes the application of high-bio-content polyols in refrigerator polyurethane rigid foams, reducing dependence on petroleum-based raw materials and improving the environmental friendliness and cost advantages of the product.

[0054] In step S11: In some embodiments, the bio-based hydrolyzed lignin includes one or more of poplar-based hydrolyzed lignin, straw-based hydrolyzed lignin, bamboo-based hydrolyzed lignin, and pine-based hydrolyzed lignin.

[0055] In some embodiments, the bio-based hydrolyzed lignin may be a byproduct of a cellulosic ethanol plant.

[0056] In some embodiments, the bio-based hydrolyzed lignin comprises: 50wt%~60wt% lignin, 25wt%~32wt% carbohydrates, 1.1wt%~1.5wt% ash, and ≤5wt% moisture, free from mechanical impurities. The bio-based hydrolyzed lignin has a high lignin content and can be directly liquefied without complex purification processes.

[0057] In some embodiments, the bio-based hydrolyzed lignin comprises, by elemental content: 60wt%~64wt% carbon, 5.8wt%~6.5wt% hydrogen, and 3.5wt%~4.5wt% nitrogen.

[0058] In some embodiments, the average particle size of the bio-based hydrolyzed lignin is ≤100 mesh, which facilitates liquefaction by allowing sufficient contact with the mixed liquefying agent.

[0059] In some embodiments, the alcohol reagent includes one or more of ethanol, propanol, and ethylene glycol.

[0060] In some embodiments, the volume ratio of water to alcohol reagent in the mixed liquefaction agent is (30~50):(50~70), for example, it can be 30:70, 35:65, 40:60, 45:55, 50:50, or any range between two of the above ratios. Within the above range, a suitable combination of water and alcohol reagent can effectively liquefy the structural characteristics of bio-based hydrolyzed lignin.

[0061] In some embodiments, the mass-to-volume ratio of the bio-based hydrolyzed lignin to the mixed liquefaction agent is (18~22) g:100 mL, for example, 18 g:100 mL, 19 g:100 mL, 20 g:100 mL, 21 g:100 mL, 22 g:100 mL, or any range between two of the above ratios. Within the above range, the bio-based hydrolyzed lignin can be sufficiently liquefied by the mixed liquefaction agent.

[0062] In some embodiments, the liquefaction process is carried out in an inert atmosphere. Further, the inert gas in the inert atmosphere may be nitrogen.

[0063] In some embodiments, the heating rate of the liquefaction process is 4°C / min to 6°C / min, for example, it can be 4°C / min, 5°C / min, 6°C / min, or any range between two of the above values; the temperature of the liquefaction process is 240°C to 260°C, for example, it can be 240°C, 245°C, 250°C, 255°C, 260°C, or any range between two of the above values; the time of the liquefaction process is 50 min to 70 min, for example, it can be 50 min, 55 min, 60 min, 65 min, 70 min, or any range between two of the above values. Within the above ranges, it is advantageous to perform liquefaction processes efficiently.

[0064] In some embodiments, the liquefaction process further includes: performing a post-liquefaction treatment.

[0065] Furthermore, the post-liquefaction treatment includes: cooling and depressurizing, rinsing with cleaning reagents, filtering to obtain filtrate, and distilling the filtrate.

[0066] Specifically, cooling and depressurization refers to reducing the liquefied system to room temperature and atmospheric pressure to facilitate the removal of the product.

[0067] The core function of cleaning reagents is to utilize their excellent organic solvent solubility to dissolve liquefied products and unreacted small molecules remaining on the walls of reaction equipment, thereby cleaning the equipment walls, preventing residual products from affecting subsequent batches of reactions, and ensuring easy removal by distillation without residual pollution. Cleaning reagents can include acetone, anhydrous ethanol, isopropanol, acetone-ethanol mixtures, ethyl acetate, etc.

[0068] The filtrate can be distilled using a rotary evaporator for vacuum distillation.

[0069] In some embodiments, the weight-average molecular weight (Mw) of the liquefied hydrolyzed lignin is 900 g / mol to 1100 g / mol, for example, it can be 900 g / mol, 950 g / mol, 1000 g / mol, 1050 g / mol, 1100 g / mol, or any range between two of the above values; the number-average molecular weight (Mn) of the liquefied hydrolyzed lignin is 600 g / mol to 700 g / mol, for example, it can be 600 g / mol, 620 g / mol, 650 g / mol, 680 g / mol, 700 g / mol, or any range between two of the above values; the dispersity (Mw / Mn) of the liquefied hydrolyzed lignin is 1.5 to 1.8, for example, it can be 1.5, 1.6, 1.7, 1.8, or any range between two of the above values; the hydroxyl value of the liquefied hydrolyzed lignin is 400 mg KOH / g to 500 mg KOH / g, for example, it can be 400 mg KOH / g. The KOH / g values ​​are 420 mg KOH / g, 450 mg KOH / g, 480 mg KOH / g, 500 mg KOH / g, or any range between two of the above values. Thus, the liquefied hydrolyzed lignin exhibits suitable properties, facilitating further modification treatment.

[0070] It should be noted that the liquefied hydrolyzed lignin mainly consists of a mixture of low molecular weight lignin derivatives, which retain the core aromatic ring structure of lignin and introduce a large number of hydroxyl groups. There are no obvious single pure components. The main components and their content ranges are as follows: Lignin liquefaction small molecules (75wt%~80wt%): The core effective components, including cleavage products of basic lignin structural units such as guaiacolylpropane and syringylpropane, with molecular weights concentrated in the range of 500g / mol to 1500g / mol. They contain a large number of phenolic and aliphatic hydroxyl groups and are the core substrates for subsequent hydroxypropylation modification; Small amount of carbohydrate degradation products (8wt%~12wt%): These are generated from the liquefaction and degradation of residual cellulose and hemicellulose in hydrolyzed lignin, including monosaccharides, disaccharides, and glutaraldehyde. Aldehyde derivatives can serve as auxiliary reaction sites to improve grafting efficiency with propylene oxide; hydroxylated small molecule polymers (5wt%~8wt%): mildly polymerized products of lignin liquefaction small molecules, with molecular weights of 1500g / mol~2000g / mol, which can optimize the viscosity and hydroxyl value distribution of liquefied hydrolyzed lignin; trace solvent residues and inorganic impurities (≤1wt%): small amounts of incompletely distilled mixed liquefying agents and dissolution products of trace ash in hydrolyzed lignin, which are inactive and have no significant impact on subsequent reactions.

[0071] In step S12: In some embodiments, the epoxide includes one or more of propylene oxide, ethylene oxide, and 1,2-epoxidebutane.

[0072] In some embodiments, the mass ratio of the liquefied hydrolyzed lignin to the epoxide is (50~70):(30~50), for example, it can be 70:30, 65:35, 60:40, 55:45, 50:50, or any range between two of the above values. Within the above range, it can be ensured that the reaction system of the modification treatment is homogeneous and the grafting reaction of the epoxide is sufficient.

[0073] In some embodiments, mixing the epoxy alkane with the liquefied hydrolyzed lignin further includes adding an initiator.

[0074] Furthermore, the initiator comprises a mixture of polyols and alkali metal hydroxides.

[0075] Furthermore, the polyol includes one or more of glycerol, ethylene glycol, and trimethylolpropane. The alkali metal hydroxide includes one or more of NaOH and KOH. In the initiator, the mass ratio of the polyol to the alkali metal hydroxide is (80~90):(10~20), for example, 80:20, 82:18, 85:15, 88:12, 90:10, or any range between two of the above ratios. Within the above range, the polyol and the alkali metal hydroxide cooperate to facilitate the initiation of the grafting modification reaction between epoxides and liquefied hydrolyzed lignin.

[0076] In some embodiments, the mass ratio of the liquefied hydrolyzed lignin to the initiator is 100:(10~15), for example, it can be 100:10, 100:11, 100:12, 100:13, 100:14, 100:15, or any range between two of the above ratios. Within the above range, it is beneficial for the initiator to fully initiate the grafting modification reaction between the epoxide and the liquefied hydrolyzed lignin.

[0077] In some embodiments, mixing the epoxy alkane with the liquefied hydrolyzed lignin further includes adding a solvent.

[0078] Furthermore, the solvent includes one or more of acetone, ethanol, and isopropanol. It is understood that the main function of the solvent is to dissolve and liquefy the hydrolyzed lignin, reduce the viscosity of the reaction system, and ensure uniform contact between the epoxide and the hydrolyzed lignin.

[0079] In some embodiments, the mass ratio of the liquefied hydrolyzed lignin to the solvent is 100:(80~95), for example, it can be 100:80, 100:85, 100:90, 100:95, or any range between two of the above ratios. Within the above range, it is beneficial to the complete dissolution of the liquefied hydrolyzed lignin and avoid the decrease in grafting efficiency of epoxides due to uneven local reactions.

[0080] In some embodiments, the heating rate of the modification treatment is 2℃ / min to 4℃ / min, for example, it can be 2℃ / min, 3℃ / min, 4℃ / min, or any range between two of the above values; the temperature of the modification treatment is 140℃ to 160℃, for example, it can be 140℃, 145℃, 150℃, 155℃, 160℃, or any range between two of the above values; the time of the modification treatment is 2h to 3h, for example, it can be 2h, 2.2h, 2.5h, 2.8h, 3h, or any range between two of the above values. Within the above ranges, it is advantageous to perform the modification treatment efficiently.

[0081] In some embodiments, the modification process further includes a post-modification process.

[0082] Furthermore, the post-modification treatment includes: cooling, rinsing with cleaning reagents, adding acid for neutralization, filtering to obtain filtrate, and distilling the filtrate.

[0083] Specifically, cooling, cleaning reagents, rinsing, filtering, distillation, etc. can all be referred to as post-liquefaction treatment, and will not be elaborated here.

[0084] The core function of acid neutralization is to neutralize unreacted alkali metal hydroxides in the system using an acid regulator, bringing the system to neutral, removing alkaline impurities, and preventing residual alkali metal hydroxides from affecting subsequent polyurethane foaming reactions. The acid regulator can be one or more of sulfuric acid, nitric acid, citric acid, acetic acid, sodium dihydrogen phosphate, tartaric acid, and phosphoric acid.

[0085] In some embodiments, the weight-average molecular weight (Mw) of the bio-based polyol is 1200 g / mol to 1900 g / mol, for example, it can be 1200 g / mol, 1400 g / mol, 1500 g / mol, 1600 g / mol, 1800 g / mol, 1900 g / mol, or any range between two of the above values; the number-average molecular weight (Mn) of the bio-based polyol is 700 g / mol to 1000 g / mol, for example, it can be 700 g / mol, 800 g / mol, 900 g / mol, 1000 g / mol, or any range between two of the above values; the hydroxyl value of the bio-based polyol is 210 mg KOH / g to 260 mg KOH / g, for example, it can be 210 mg KOH / g, 220 mg KOH / g, 230 mg KOH / g, 240 mg KOH / g, 250 mg KOH / g, 260 mg KOH / g. The KOH / g value or any value between any two of the above; the viscosity (80°C) of the bio-based polyol is 0.3 Pa·s to 0.7 Pa·s, for example, 0.3 Pa·s, 0.4 Pa·s, 0.5 Pa·s, 0.6 Pa·s, 0.7 Pa·s, or any value between any two of the above. Thus, the bio-based polyol exhibits excellent performance, making it suitable as a raw material for preparing rigid polyurethane foams with low thermal conductivity, high compressive strength, and high stability.

[0086] In step S13: In some embodiments, the foaming agent includes one or more of the following: a compound foaming agent of acetone and cyclopentane, a compound foaming agent of water and cyclopentane, and a compound foaming agent of n-pentane and isopentane.

[0087] Furthermore, the mass ratio of acetone to cyclopentane is 7:3, the mass ratio of water to cyclopentane is 3:7, and the mass ratio of n-pentane to isopentane is 1:1.

[0088] By combining the components and their quality in the compound foaming agent, three major problems specific to rigid polyurethane foam can be solved: cyclopentane or isopentane ensures low thermal conductivity (suitable for refrigerator insulation and energy saving); acetone, water, and n-pentane improve the compatibility of polar raw materials and regulate the foaming rate (suitable for complex filling of refrigerator cabinet layers); and an appropriate ratio balances thermal insulation performance and mechanical properties (avoiding phenomena such as delamination or excessively thin cell walls caused by excessively high or low ratios).

[0089] In some embodiments, the isocyanate compounds include one or more of isocyanates, 4,4'-diphenylmethane diisocyanate (MDI), 2,6-toluene diisocyanate (TDI), naphthalene diisocyanate (NDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), hydrogenated diphenylmethane diisocyanate (H12MDI), and phenylmethylene diisocyanate (XDI).

[0090] In some embodiments, the mixing of the foaming agent, the isocyanate compound, and the bio-based polyol further includes the addition of one or more of a crosslinking agent, a catalyst, a surfactant, and a viscosity modifier.

[0091] Furthermore, the crosslinking agent includes glycerol. The main function of the crosslinking agent is to improve the compressive strength and dimensional stability of the foam.

[0092] The catalyst includes one or more of amine catalysts and metal catalysts. The catalyst can catalyze the foaming reaction and regulate the foaming and gelation reaction rates. More specifically, the amine catalyst includes one or more of triethanolamine (TEOA), diethanolamine (DEA), triethylamine (TEA), dimethylethanolamine, dimethylcyclohexylamine, triethylenediamine, tetramethylethylenediamine, tetramethylpropylenediamine, pentamethyldiethylenetriamine, N,N-dimethylbenzylamine, N,N-dimethylaniline, and triisopropanolamine. The metal catalyst includes one or more of stannous octoate, dibutyltin dilaurate, bismuth isooctanoate, bismuth neodecanoate, zinc naphthenate, zinc isooctanoate, cobalt isooctanoate, zirconium naphthenate, zirconium acetylacetonate, titanium acetylacetonate, organotin catalysts, and organobismuth catalysts.

[0093] The surfactant includes one or more of polyether-modified silicone and silicone oil. The main function of the surfactant is to stabilize foam and refine the pores.

[0094] The viscosity modifier includes water. The main function of the viscosity modifier is to adjust the viscosity to a suitable level and to assist in controlling the reaction and cell structure.

[0095] In some embodiments, by weight, the bio-based polyol comprises 100 parts, the foaming agent comprises 15-25 parts, the crosslinking agent comprises 8-12 parts, the catalyst comprises 0.8-3 parts, the surfactant comprises 0.5-2 parts, the viscosity modifier comprises 0.3-2 parts, and the isocyanate compound comprises 130-150 parts. Selecting appropriate proportions is beneficial for preparing high-performance rigid polyurethane foam. Appropriate amounts of bio-based polyols ensure the basic construction of the polyurethane molecular chain and prevent excessively high system viscosity from causing uneven foaming. Appropriate amounts of crosslinking agents meet basic crosslinking requirements and prevent excessive crosslinking from increasing foam brittleness. Appropriate amounts of blowing agents ensure the foaming ratio and prevent excessive blowing agents from causing excessively thin cell walls and decreased mechanical properties. Appropriate amounts of catalysts ensure the reaction rate meets the target and prevent excessively fast reactions from causing bursting and uneven cell structure. Appropriate amounts of surfactants meet basic foam stabilization requirements and prevent excessive amounts from causing sticky foam surfaces and prolonged curing time. Appropriate amounts of viscosity modifiers prevent the system from becoming too viscous and avoid excessively high foam density and increased thermal conductivity.

[0096] It should be noted that all raw materials for the foaming process must be kept at a constant temperature of 20℃~25℃ before mixing to eliminate the influence of temperature on viscosity and reactivity.

[0097] In some embodiments, the foaming process includes a foaming stage, a curing stage, and a maturation stage performed sequentially.

[0098] Furthermore, the foaming stage lasts for 60-90 seconds, for example, 60, 70, 80, or 90 seconds, or any range between two of these values; the curing stage lasts for 150-210 seconds, for example, 150, 160, 170, 180, 190, 200, or 210 seconds, or any range between two of these values; and the maturation stage lasts for 22-26 hours, for example, 22, 23, 24, 25, or 26 hours, or any range between two of these values. It should be noted that the foaming, curing, and maturation stages can all be performed at room temperature (20°C-30°C).

[0099] In some embodiments, the foaming process is followed by a curing process.

[0100] Furthermore, the temperature of the post-curing treatment is 50℃~70℃, for example, it can be 50℃, 55℃, 60℃, 65℃, 70℃, or any range between two of the above values; the curing time is 10h~15h, for example, it can be 10h, 11h, 12h, 13h, 14h, 15h, or any range between two of the above values. Under these conditions, it is beneficial to eliminate internal stress.

[0101] Secondly, embodiments of this application also provide a rigid polyurethane foam, prepared by the above-described preparation method.

[0102] The polyurethane rigid foam prepared by the above method exhibits excellent properties such as low thermal conductivity, high compressive strength, high dimensional stability, high thermal stability, and short curing time, which can meet the usage requirements and production efficiency of refrigerator insulation layers.

[0103] Thirdly, embodiments of this application also provide a refrigerator, the refrigerator including a cabinet, wherein the interlayer of the cabinet is provided with rigid polyurethane foam prepared by the above preparation method, or the above-mentioned rigid polyurethane foam.

[0104] In addition to refrigerator insulation, the rigid polyurethane foam provided in this application embodiment can also be widely used in cold chain equipment such as cold chain containers and refrigerated trucks, and has broad market application prospects.

[0105] The present application will be specifically described below through specific embodiments. The following embodiments are only some embodiments of the present application and are not intended to limit the present application.

[0106] Example 1 This embodiment provides a rigid polyurethane foam, and the preparation method of the rigid polyurethane foam is as follows: Step S21: First, add 300 mL of mixed liquefaction agent (water and ethanol in a 50:50 volume ratio) to a stainless steel high-pressure reactor, start stirring (300 rpm), slowly add 60 g of poplar-based hydrolyzed lignin, and seal the reactor; purge the reactor with nitrogen three times (each purge pressure 0.5 MPa, held for 5 min), until the initial nitrogen pressure stabilizes at 2.0 MPa; heat to 250 °C at a heating rate of 5 °C / min for liquefaction treatment, hold for 60 min, and adjust the stirring speed during the holding period. Increase the speed to 500 rpm; after the liquefaction reaction is completed, quickly cool to room temperature (≤30℃) using a water-cooling coil, and slowly release the gas (pressure release rate ≤0.1MPa / min); add 100mL of acetone to the reactor, stir for 30min (400rpm), and rinse the reactor wall to remove residual product; use vacuum filtration (pressure -0.08MPa), separate the filtrate using filter paper, transfer the filtrate to a rotary evaporator, and distill at 50℃ and -0.09MPa vacuum to remove the solvent, and obtain liquefied hydrolyzed lignin; Step S22: Add 18.9g of liquefied hydrolyzed lignin, 18.9g of propylene oxide, 2.31g of initiator (glycerol and KOH mixed at a mass ratio of 89:11), and 16.8g of acetone solvent to a high-pressure reactor; start heating at atmospheric pressure, raise the temperature to 150℃ at a rate of 3℃ / min, maintain the stirring speed at 400rpm, and carry out the modification treatment. Keep the reaction at this temperature for 2.5h; after the modification treatment is completed, cool to room temperature (≤30℃), completely transfer the contents of the reactor to a beaker, rinse the reactor wall three times with 50mL of acetone, slowly add dilute sulfuric acid to the resulting mixture, and stir to neutralize to pH=7.0; use a Buchner funnel for vacuum filtration (pressure -0.08MPa) to remove salt impurities generated during neutralization; transfer the filtrate to a rotary evaporator and distill at 60℃ (bath temperature) and -0.095MPa vacuum for 1.5h to obtain bio-based polyol; Step S23: Add 100 parts of bio-based polyol, 10 parts of crosslinking agent (glycerol), 20 parts of foaming agent (acetone and cyclopentane compounded in a mass ratio of 14:6), 20 parts of catalyst (stannous octoate and triethylenediamine compounded in a mass ratio of 1:1), 2 parts of surfactant (silicone oil), and 2 parts of viscosity modifier (deionized water) to a plastic cup for premixing. Start a high-speed disperser and stir at 550 rpm for 10 seconds. Quickly add 130.5 parts of 2,6-toluene diisocyanate, increase the stirring speed to 800 rpm, and stir for 12 seconds. Observe the viscosity change of the mixture during this period. Stop stirring when a sudden increase in viscosity and whitening of color occur. Place the plastic cup on a horizontal platform (23℃, 50% humidity) and let it stand to foam. Then cure and mature for 24 hours. Transfer it to a 60℃ forced-air drying oven and mature for 12 hours (humidity ≤30%) to obtain rigid polyurethane foam.

[0107] Example 2 Example 2 is basically the same as Example 1, except that: In step S22, 18.9g of liquefied hydrolyzed lignin and 12.6g of propylene oxide are added.

[0108] Example 3 Example 3 is basically the same as Example 1, except that: In step S22, 18.9g of liquefied hydrolyzed lignin and 8.1g of propylene oxide are added.

[0109] Example 4 Example 4 is basically the same as Example 1, except that: In step S23, the foaming agent is replaced by a mixture of water and cyclopentane in a mass ratio of 3:7.

[0110] Example 5 Example 5 is basically the same as Example 1, except that: In step S23, the catalyst is replaced by a mixture of zinc isooctanoate and bismuth neodecanoate in a mass ratio of 1:2.

[0111] Example 6 Example 6 is basically the same as Example 1, except that: In step S21, poplar-based hydrolyzed lignin is replaced with bamboo-based hydrolyzed lignin.

[0112] Example 7 Example 7 is basically the same as Example 1, except that: In step S21, the mixed liquefying agent is replaced by a mixture of water and ethylene glycol in a volume ratio of 50:50.

[0113] Example 8 Example 8 is basically the same as Example 1, except that: In step S22, the initiator is replaced by a mixture of trimethylolpropane and NaOH in a mass ratio of 89:11.

[0114] Example 9 Example 9 is basically the same as Example 1, except that: In step S22, propylene oxide is replaced with 1,2-epoxybutane.

[0115] Example 10 Example 10 is basically the same as Example 1, except that: In step S21, the liquefaction conditions are: temperature 260℃, time 70min.

[0116] Example 11 Example 11 is basically the same as Example 1, except that: In step S21, the liquefaction conditions are: temperature 240℃, time 50min.

[0117] Comparative Example 1 Comparative Example 1 and Example 1 are basically the same, except that: Steps S21 and S22 were not performed; instead, the bio-based polyol in step S23 was replaced with a petroleum-based polyether polyol.

[0118] Comparative Example 2 Comparative Example 2 is basically the same as Example 1, except that: In step S21, poplar-based hydrolyzed lignin is replaced with corn cob enzymatically hydrolyzed lignin.

[0119] Comparative Example 3 Comparative Example 3 is basically the same as Example 1, except that: In step S21, the mixed liquefying agent is replaced with ethanol.

[0120] A dedicated foam cutting machine was used to cut rigid polyurethane foam to the standard size (40mm×40mm×20mm) for refrigerator insulation layers. The cuts were smooth and burr-free, ready for testing.

[0121] The weight-average molecular weight (Mw), number-average molecular weight (Mn), hydroxyl value, viscosity at 80°C, and observed state of the bio-based polyols in Examples 1-3 were tested, and the results are shown in Table 1.

[0122] Table 1

[0123] As shown in Table 1, with the increase of liquefied hydrolyzed lignin content, the weight-average molecular weight of bio-based polyols gradually decreases, the hydroxyl value gradually increases, and the viscosity gradually increases. This is because the higher the proportion of liquefied hydrolyzed lignin, the shorter the propylene oxide grafted chain, and the smaller the overall molecular chain length, thus reducing the weight-average molecular weight. Furthermore, the hydroxyl value of liquefied hydrolyzed lignin itself is higher than that of the propylene oxide grafted chain segment, so the higher the proportion of liquefied hydrolyzed lignin, the higher the hydroxyl value of the polyol. At the same time, the increased proportion of aromatic structures in liquefied hydrolyzed lignin enhances intermolecular forces, leading to an increase in viscosity.

[0124] The thermal conductivity (in W / (m·K)), room temperature compressive strength (test conditions: 25℃, 10% strain, in MPa), low temperature compressive strength (test conditions: -20℃, 10% strain, in MPa), dimensional stability at -40℃ (test conditions: 10 cycles at -40℃, in %), dimensional stability at 80℃ (test conditions: 10 cycles at 80℃, in %), thermal weight loss rate at 200℃ (the ratio of weight loss at 200℃, in %), density (in kg / m³), foaming rise time (in s), and curing time (in s) of the polyurethane rigid foams in Examples 1-10 and Comparative Examples 1-2 were tested. The test results are shown in Table 2.

[0125] The thermal conductivity was tested according to the GB / T 10294-2008 standard "Determination of Steady-State Thermal Resistance and Related Properties of Insulation Materials - Protective Hot Plate Method", and the testing equipment was an EKOHC-074-200 thermal conductivity meter.

[0126] The compressive strength at room temperature (25℃, 10% strain) was tested according to ASTM D695 standard, and the testing equipment was an Instron 68TM-50 universal testing machine (50kN sensor).

[0127] The low-temperature compressive strength (-20℃, 10% strain) was tested according to GB / T 20284-2006 standard. The testing equipment was an Instron 68TM-50 universal testing machine (50kN sensor) and a low-temperature environment chamber.

[0128] Dimensional stability at -40℃ / 80℃ indicates the degree of deformation of foam under specific temperature and humidity conditions, expressed as a percentage. It is tested according to the GB / T 8811-2008 standard. The lower the dimensional stability value, the more stable the foam.

[0129] The thermogravimetric analysis at 200℃ was performed using TG-DTG thermogravimetric analysis (10℃ / min, N2 atmosphere) on a Netzsch TG209 F3 Tarsus thermogravimetric analyzer.

[0130] Density was tested according to GB / T 6343-2009 standard.

[0131] The foaming rise time and curing time were measured using a stopwatch.

[0132] Table 2

[0133] From Table 2, we can obtain: The polyurethane rigid foam provided in this application embodiment has an effectively reduced thermal conductivity, directly reducing the load on the refrigerator's refrigeration system; its room temperature compressive strength is improved, which can reduce the refrigerator's damage rate during transportation, support the refrigerator's inner liner to remain unchanged for a long time, and avoid increased energy consumption due to cold leakage; its low temperature compressive strength is improved, solving the pain point of mechanical performance degradation of traditional foam at low temperatures, ensuring that the insulation layer's ability to resist door opening and closing and inner liner expansion does not decrease when the refrigerator freezer is used for a long time (-20℃), avoiding local damage; its wide temperature range dimensional stability is improved, solving the industry pain point of traditional foam low temperature shrinkage (leading to cold leakage at door gaps) and high temperature bulging (leading to cabinet deformation), extending the service life of the refrigerator's insulation layer; its thermal weight loss rate is improved, so that the foam will not cause significant degradation when the refrigerator compressor is abnormally heated, avoiding the release of harmful substances and improving safety; its curing time is shortened, which can improve the foaming efficiency of the refrigerator production line and reduce unit production costs.

[0134] This application uses hydrolyzed lignin, a byproduct of cellulosic ethanol plants, as raw material to achieve resource utilization of waste, increase biomass content, replace petroleum-based polyols, and reduce raw material costs. The thermal conductivity of the prepared polyurethane rigid foam is significantly lower than that of petroleum-based foam, effectively improving insulation efficiency and reducing refrigerator energy consumption, meeting the requirements for refrigerator energy efficiency upgrades. The compressive strength at room temperature (25℃) and at -20℃ is improved. Although there is a trend of "higher biomass content, slightly lower strength", all embodiments are effectively improved compared to traditional petroleum-based foam, which can effectively support the refrigerator cabinet structure and resist external impacts in scenarios such as transportation bumps and low-temperature use, avoiding damage to the insulation layer. Within the range of -40℃ (freezer) to 80℃ (high-temperature storage environment) used in the refrigerator, the dimensional stability is high, and the thermal weight loss rate below 200℃ is low.

[0135] The technical solutions provided by the embodiments of this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for preparing rigid polyurethane foam, characterized in that, Includes the following steps: A bio-based hydrolyzed lignin and a mixed liquefaction agent are provided, which are then mixed and liquefied to obtain liquefied hydrolyzed lignin; wherein the mixed liquefaction agent includes water and an alcohol reagent; An epoxy alkane is provided, which is then mixed with the liquefied hydrolyzed lignin and modified to obtain a bio-based polyol. A foaming agent and an isocyanate compound are provided, and after being mixed with the bio-based polyol, a foaming treatment is performed to obtain rigid polyurethane foam.

2. The preparation method according to claim 1, characterized in that, The bio-based hydrolyzed lignin includes one or more of poplar-based hydrolyzed lignin, straw-based hydrolyzed lignin, bamboo-based hydrolyzed lignin, and pine-based hydrolyzed lignin; The bio-based hydrolyzed lignin comprises: 50wt%~60wt% lignin, 25wt%~32wt% carbohydrates, 1.1wt%~1.5wt% ash, and ≤5wt% moisture; Based on elemental content, the bio-based hydrolyzed lignin comprises: 60wt%~64wt% carbon, 5.8wt%~6.5wt% hydrogen, and 3.5wt%~4.5wt% nitrogen. The alcohol reagents include one or more of ethanol, propanol, and ethylene glycol; The epoxides include one or more of propylene oxide, ethylene oxide, and 1,2-epoxidebutane.

3. The preparation method according to claim 1, characterized in that, In the mixed liquefaction agent, the volume ratio of water to alcohol reagent is (30~50):(50~70); The mass-to-volume ratio of the bio-based hydrolyzed lignin to the volume of the mixed liquefaction agent is (18~22) g: 100 mL; The mass ratio of the liquefied hydrolyzed lignin to the epoxide is (50~70):(30~50); The heating rate of the liquefaction treatment is 4℃ / min~6℃ / min, the temperature of the liquefaction treatment is 240℃~260℃, and the time of the liquefaction treatment is 50min~70min; The heating rate of the modification treatment is 2℃ / min to 4℃ / min; the temperature of the modification treatment is 140℃ to 160℃; and the time of the modification treatment is 2h to 3h.

4. The preparation method according to claim 1, characterized in that, The mixing of the epoxy alkane with the liquefied hydrolyzed lignin further includes: adding an initiator and a solvent; wherein... The initiator comprises a mixture of polyols and alkali metal hydroxides; The polyol includes one or more of glycerol, ethylene glycol, and trimethylolpropane; the alkali metal hydroxide includes one or more of NaOH and KOH; the mass ratio of the polyol to the alkali metal hydroxide is (80~90):(10~20); The mass ratio of the liquefied hydrolyzed lignin to the initiator is 100:(10~15). The solvent includes one or more of acetone, ethanol, and isopropanol; The mass ratio of the liquefied hydrolyzed lignin to the solvent is 100:(80~95).

5. The preparation method according to claim 1, characterized in that, The weight-average molecular weight of the liquefied hydrolyzed lignin is 900 g / mol to 1100 g / mol; The number-average molecular weight of the liquefied hydrolyzed lignin is 600 g / mol to 700 g / mol; The dispersion of the liquefied hydrolyzed lignin is 1.5~1.8; The hydroxyl value of the liquefied hydrolyzed lignin is 400 mg KOH / g to 500 mg KOH / g; The weight-average molecular weight of the bio-based polyol is 1200 g / mol to 1900 g / mol; The number-average molecular weight of the bio-based polyol is 700 g / mol to 1000 g / mol; The hydroxyl value of the bio-based polyol is 210 mg KOH / g to 260 mg KOH / g; The viscosity of the bio-based polyol is 0.3 Pa·s to 0.7 Pa·s.

6. The preparation method according to claim 1, characterized in that, The foaming agent includes one or more of the following: a compound foaming agent of acetone and cyclopentane, a compound foaming agent of water and cyclopentane, and a compound foaming agent of n-pentane and isopentane. The isocyanate compounds include one or more of the following: isocyanate, 4,4'-diphenylmethane diisocyanate, 2,6-toluene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate, and phenylmethylene diisocyanate.

7. The preparation method according to claim 1, characterized in that, The mixture of the foaming agent, the isocyanate compound, and the bio-based polyol further includes: the addition of one or more of a crosslinking agent, a catalyst, a surfactant, and a viscosity modifier; wherein... The crosslinking agent includes glycerol; The catalyst includes one or more of amine catalysts and metal catalysts; the amine catalyst includes one or more of triethanolamine, diethanolamine, triethylamine, dimethylethanolamine, dimethylcyclohexylamine, triethylenediamine, tetramethylethylenediamine, tetramethylpropylenediamine, pentamethyldiethylenetriamine, N,N-dimethylbenzylamine, N,N-dimethylaniline, and triisopropanolamine; the metal catalyst includes one or more of stannous octoate, dibutyltin dilaurate, bismuth isooctanoate, bismuth neodecanoate, zinc naphthenate, zinc isooctanoate, cobalt isooctanoate, zirconium naphthenate, zirconium acetylacetonate, titanium acetylacetonate, organotin catalysts, and organobismuth catalysts. The surfactant includes one or more of polyether-modified silicone and silicone oil; The viscosity modifier includes water; By weight, the bio-based polyol comprises 100 parts, the foaming agent comprises 15-25 parts, the crosslinking agent comprises 8-12 parts, the catalyst comprises 0.8-3 parts, the surfactant comprises 0.5-2 parts, the viscosity modifier comprises 0.3-2 parts, and the isocyanate compound comprises 130-150 parts.

8. The preparation method according to claim 6, characterized in that, The foaming process includes a foaming stage, a curing stage, and a maturation stage performed sequentially; wherein... The foaming stage lasts for 60 to 90 seconds. The curing stage lasts for 150s to 210s. The ripening stage lasts for 22 hours to 26 hours. After the foaming process, the process further includes a curing process; the curing process is carried out at a temperature of 50℃ to 70℃ and for a time of 10h to 15h.

9. A rigid polyurethane foam, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 8.

10. A refrigerator, characterized in that, The refrigerator includes a cabinet, and the interlayer of the cabinet is provided with rigid polyurethane foam prepared by the preparation method according to any one of claims 1 to 8, or with rigid polyurethane foam as described in claim 9.

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