A method for preparing a polyurethane flame-retardant foam material rich in phenolic hydroxyl lignin derivatives

Abstract

This invention relates to the field of polyurethane foam materials technology, and particularly to a method for preparing a flame-retardant polyurethane foam material rich in phenolic hydroxyl lignin derivatives. Industrial lignin is demethylated using an acidic molten salt system to obtain phenolic hydroxyl lignin, which is then reacted with 2NH₂DOPO via a Mannich reaction to prepare DOPO lignin flame retardant LF. The lignin flame retardant is further prepolymerized with PEG-400, followed by condensation polymerization with 4,4-MDI, foaming, and curing to prepare the polyurethane foam. This invention regulates the concentration of phenolic hydroxyl groups in the demethylated lignin derivatives using an acidic molten salt system, and further evaluates its Mannich addition ability with 2NH₂DOPO and its condensation ability as a reactive flame retardant based on the lignin phenolic hydroxyl content. The prepared polyurethane foam not only has flame-retardant and heat-insulating effects but also exhibits a certain degree of pressure deformation recovery. This not only expands the application prospects of polyurethane foam materials but also opens up new avenues for the renewable and sustainable high-value utilization of lignin.

Description

Technical Field

[0001] This invention relates to the field of polyurethane foam materials technology, and in particular to a method for preparing a polyurethane flame-retardant foam material rich in phenolic hydroxyl lignin derivatives. Background Technology

[0002] Polyurethane (PU) is a polymer synthesized through a condensation reaction between polyols (-OH) and isocyanates (-NCO). Due to its excellent thermal insulation properties, lightweight nature, and durability, polyurethane foam is ideally suited for use as insulation material in buildings. However, given the wide range of applications and low fire resistance of polyurethane, improving its fire resistance is crucial. Furthermore, since the polyols used in polyurethane production heavily rely on fossil resources, exploring sustainable methods for preparing renewable and eco-friendly polyurethane composites is essential for developing sustainable polyurethane.

[0003] With the depletion of petroleum resources, renewable resources will become the main foundation for materials and chemical production. Among them, lignin, with its low production cost, is one of the most abundant resources. Lignin is the second largest natural polymer after cellulose, possessing a three-dimensional network structure composed of phenylpropane chemical units. Lignin has a phenolic structure and can be cross-linked with other chemicals to prepare composite materials with sustainable development potential. Furthermore, lignin derivatives can provide high added value to composite materials by providing flame-retardant properties. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a flame-retardant polyurethane foam material rich in phenolic hydroxyl lignin derivatives. The method involves using a hydroxyl-rich lignin-amined modified polymer as a reactant, demethylating lignin using a molten salt hydrate system, and introducing a functional segment of 6-(1-(4-aminophenyl)-1-(p-tolyl)ethyl)dibenzo[c,e][1,2]phosphine oxychloride (2NH2DOPO) into the lignin molecular structure via the Mannich reaction to prepare a reactive flame retardant LF. Further, LF is used as a reactant in a condensation reaction with 4,4-MDI to generate polyurethane foam LFPU. Lignin demethylation enhances both its Mannich reactivity with 2NH2DOPO and the active sites for condensation reactions with isocyanates, thereby improving the flame-retardant and thermal barrier properties of the polyurethane foam. This green, energy-efficient, and sustainable manufacturing process, along with the cost-effectiveness of the final product, can open new avenues for producing stable and robust bioplastics from abundant, renewable, and sustainable biomass.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] This invention provides a method for preparing a phenol-hydroxyl lignin derivative-rich polyurethane flame-retardant foam material, comprising the following steps:

[0007] 1) Add alkali lignin to an acidic inorganic molten salt solution, stir under heating conditions to allow the reaction to proceed, add to an ethanol aqueous solution, precipitate lignin by antisolvent method, and then freeze dry to obtain demethylated lignin;

[0008] 2) Dissolve the demethylated lignin obtained in step 1) and 2NH2DOPO in N,N-dimethylformamide solution and stir to allow the reaction to proceed;

[0009] 3) Add formaldehyde aqueous solution to the solution obtained in step 2), stir, and carry out Mannich reaction. Through the Mannich reaction, formaldehyde undergoes an addition reaction with the ortho carbon atom of the lignin phenol hydroxyl group and the primary amine of 2NH2DOPO.

[0010] 4) Add the solution obtained in step 3) to an aqueous ethanol solution, precipitate out by antisolvent method, and then freeze dry to obtain lignin flame retardant;

[0011] 5) Dissolve the lignin flame retardant obtained in step 4) in PEG-400, add dibutyltin diisocyanate, water, silicone oil, Tween 80 and triethanolamine, and stir under heating to react and obtain reaction solution A;

[0012] 6) Add molten 4,4-MDI to reaction solution A, stir, then pour into a mold for foaming and curing to obtain the polyurethane foam; in this process, the polyol and isocyanate first undergo a condensation reaction, and then further utilize the chemical structure characteristics of the polyol to achieve cross-linking between molecular chains.

[0013] In the above technical solution, further, in step 1), the acidic inorganic molten salt hydrate is a mixed solution of LiBr aqueous solution and acid, wherein the concentration of LiBr aqueous solution is 60%, the concentration of acid is 0.2-1.0 mol / L, and the acid is hydrobromic acid;

[0014] The heating temperature is 60-140℃, and the reaction time is 30-150 min.

[0015] In the above technical solution, further, in step 2), the mass ratio of the demethylated lignin to 2NH2DOPO is 1:1.5.

[0016] In the above technical solution, further, in step 3), the mass ratio of the demethylated lignin to formaldehyde is 1:1.8;

[0017] The concentration of the formaldehyde aqueous solution is 30 wt%.

[0018] The conditions for the Mannich reaction are: a reaction temperature of 60-80℃ and a reaction time of 4-8 hours.

[0019] In the above technical solution, further, in step 5), the mass of the lignin flame retardant accounts for 15% of the total mass of the lignin flame retardant and PEG-400;

[0020] The mass percentages of the dibutyltin diacetate, water, silicone oil, Tween 80, and triethanolamine are 0.4%, 0.2%, 0.8%, 0.7%, and 0.4% of the mass of PEG-400, respectively.

[0021] The reaction temperature is 60-80℃.

[0022] In the above technical solution, further, in step 6), the mass ratio of the reaction solution A to 4,4-MDI is 1:1.5;

[0023] The stirring speed is 900-1200 r / min;

[0024] The foaming time is 1-5 minutes;

[0025] The curing temperature is 100-120℃.

[0026] The beneficial effects of this invention are as follows:

[0027] This invention increases the phenolic hydroxyl content of lignin through demethylation, yielding demethylated lignin with a high concentration of phenolic hydroxyl groups. Then, using the Mannich reaction, an addition reaction is performed between the primary amine group of 2NH₂DOPO and the ortho-carbon atom of the lignin benzene ring, introducing a reactive lignin flame retardant LF into the lignin molecular chain. Finally, the lignin is dissolved in PEG-400 and used as a reactant in a polyurethane mixture to form a flame-retardant polyurethane material through condensation polymerization. The Mannich reaction process introduces DOPO flame-retardant groups, enhancing the flame-retardant properties of the polyurethane foam and improving the char formation effect of lignin on top of the acid source flame retardant effect. Furthermore, the amino groups in the 2NH₂DOPO chain cause gas-phase flame retardancy in the polyurethane material and further encapsulate it within the lignin combustion char layer, resulting in a good multi-element intumescent flame-retardant effect. Its limiting oxygen index reaches 34.5%, achieving the UL94 flammability test V-0 rating. In addition, the lignin phenylpropane structure in the polyurethane molecular chain enhances its mechanical properties and thermal barrier effect.

[0028] This invention prepares lignin-derived polyurethane flame-retardant foam materials that not only exhibit flame-retardant properties but also demonstrate excellent thermal barrier and mechanical properties. This green energy manufacturing process, along with the cost-effectiveness of the final product, opens new avenues for producing stable and robust biomass materials from abundant, renewable, and sustainable biomass. Attached Figure Description

[0029] Figure 1 The infrared spectrum of the lignin flame retardant sample in Example 3;

[0030] Figure 2 The phosphorus NMR spectra of the demethylated lignin samples in Examples 1-6;

[0031] Figure 3 The 1H NMR spectrum of the lignin flame retardant sample from Example 3 is shown below.

[0032] Figure 4 XPS elemental analysis chromatograms of the lignin flame retardant samples from Examples 1-6;

[0033] Figure 5 Thermogravimetric curves of different polyurethane foam samples from Example 3, Comparative Example 1, and Comparative Example 2 are shown.

[0034] Figure 6 The thermal weight loss rates of different polyurethane foam samples in Example 3, Comparative Example 1, and Comparative Example 2 are shown.

[0035] Figure 7 This is a water contact angle diagram of the polyurethane foam sample from Example 3;

[0036] Figure 8 Infrared thermal imaging analysis of the polyurethane foam sample of Example 3;

[0037] Figure 9 The stress-strain curves of different polyurethane foam samples from Examples 1-6 and Comparative Example 1 are shown.

[0038] Figure 10 The limiting oxygen index values ​​are for different polyurethane foam samples of Examples 1-6 and Comparative Example 1.

[0039] Figure 11 The image shows the Raman spectrum of the combustion residue of the polyurethane foam sample in Example 3.

[0040] Figure 12 This is a SEM image of the carbon layer on the burning surface of the polyurethane foam sample from Example 3. Detailed Implementation

[0041] The present invention will be further described below with reference to specific embodiments, but this should not be construed as a limitation on the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention still fall within the scope of protection of the present invention.

[0042] Unless otherwise specified, all materials and reagents mentioned below are commercially available products well known to those skilled in the art; unless otherwise specified, all methods described are methods known in the art. Unless otherwise defined, the technical or scientific terms used should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0043] This invention provides a method for preparing a phenol-hydroxyl lignin derivative-rich polyurethane flame-retardant foam material, comprising the following steps:

[0044] 1) Add alkali lignin to an acidic inorganic molten salt solution, stir under heating conditions to allow the reaction to proceed, add to an ethanol aqueous solution, precipitate lignin by antisolvent method, and then freeze dry to obtain demethylated lignin;

[0045] 2) Dissolve the demethylated lignin obtained in step 1) and 2NH2DOPO in N,N-dimethylformamide solution and stir to allow the reaction to proceed;

[0046] 3) Add formaldehyde aqueous solution to the solution obtained in step 2), stir, and carry out the Mannich reaction;

[0047] 4) Add the solution obtained in step 3) to an aqueous ethanol solution, precipitate out by antisolvent method, and then freeze dry to obtain lignin flame retardant;

[0048] 5) Dissolve the lignin flame retardant obtained in step 4) in PEG-400, add dibutyltin diisocyanate, water, silicone oil, Tween 80 and triethanolamine, and stir under heating to react and obtain reaction solution A;

[0049] 6) Add molten 4,4-MDI to reaction solution A, stir, then pour into a mold for foaming and curing to obtain polyurethane foam rich in phenolic hydroxyl lignin derivatives.

[0050] In step 1), the acidic inorganic molten salt hydrate is a mixed solution of LiBr aqueous solution and hydrobromic acid, wherein the concentration of LiBr aqueous solution is 60% and the concentration of hydrobromic acid is 0.2-1.0 mol / L; the heating temperature is 60-140℃ and the reaction time is 30-150 min.

[0051] In step 2), the mass ratio of demethylated lignin to 2NH2DOPO is 1:1.5.

[0052] In step 3), the mass ratio of demethylated lignin to formaldehyde is 1:1.8; the concentration of the formaldehyde aqueous solution is 30wt%; and the conditions for the Mannich reaction are: reaction temperature of 60-80℃ and reaction time of 4-8 hours.

[0053] In step 5), the mass of the lignin flame retardant accounts for 15% of the total mass of the lignin flame retardant and PEG-400; the masses of dibutyltin diisocyanate, water, silicone oil, Tween 80, and triethanolamine are 0.4%, 0.2%, 0.8%, 0.7%, and 0.4% of the mass of PEG-400, respectively; and the reaction temperature is 60-80℃.

[0054] In step 6), the mass ratio of reaction solution A to 4,4-MDI is 1:1.5; the stirring speed is 900-1200 r / min; the foaming time is 1-5 min; and the aging temperature is 100-120℃.

[0055] This invention prepares a reactive lignin flame retardant LF containing phosphorus and nitrogen elements by reacting demethylated lignin (Lig) with 2NH2DOPO flame retardant compound via the Mannich reaction, and then polymerizes LF with 4,4-MDI to form a polyurethane foam rich in phenolic hydroxyl lignin derivatives.

[0056] The phenol-rich hydroxyl lignin derivative polyurethane flame-retardant foam material prepared by this invention can be applied to building flame-retardant and heat-insulating materials.

[0057] The phenolic hydroxyl lignin derivative polyurethane flame-retardant foam material prepared in this invention has a limiting oxygen index of 34.5% and reaches V-0 rating. Furthermore, due to its unique chemical structure, it exhibits a barrier property of 0.012 W / mK and a hydrophobic effect of 115.5°. By modifying the chemical structure of lignin, its flame-retardant application in polymer materials is enhanced, opening up new avenues for the high-value utilization of lignin.

[0058] Example 1

[0059] A method for preparing a phenol-hydroxyl lignin-rich polyurethane flame-retardant foam material includes the following steps:

[0060] 1) Add 4g of alkali lignin to 40mL of acidic inorganic molten salt solution (a mixed solution of 60% LiBr aqueous solution and 0.6mol / L HBr), stir the reaction at 80℃ for 150min, and after the reaction is completed, use 250mL of ethanol / water = 2 / 8 (v / v) as an antisolvent to obtain lignin particles, and then freeze-dry to obtain demethylated lignin, denoted as Lig2;

[0061] 2) Dissolve 4g of Lig2 obtained in step 1) and 6g of 2NH2DOPO in 100mL of N,N-dimethylformamide solution and stir to react;

[0062] 3) Add 24g of 30wt% formaldehyde aqueous solution to the solution obtained in step 2), stir at 80℃ for 4h to carry out the Mannich reaction;

[0063] 4) Pour the solution obtained in step 3) into 250 mL of ethanol / water = 2 / 8 (v / v) solution, precipitate by antisolvent method, and freeze dry to obtain lignin flame retardant, denoted as LF2;

[0064] 5) Dissolve 1.5g of lignin flame retardant LF in 8.5g of polyethylene glycol (PEG-400), and add 0.04g, 0.02g, 0.08g, 0.07g and 0.04g of dibutyltin diacetate, water, silicone oil, Tween 80 and triethanolamine respectively. Stir and react at 80℃ to obtain reaction solution A;

[0065] 6) Heat 15g of 4,4-MDI to 60℃ until melted, add the molten 4,4-MDI to 10g of reaction solution A, stir rapidly at 1000rpm / min, pour into a mold for 1min to foam, and then cure at 120℃ to obtain polyurethane foam, denoted as LF2PU.

[0066] Example 2

[0067] The preparation method of polyurethane foam is similar to that of Example 1, except that in step 1), the reaction is stirred for 30 minutes at 140°C. The demethylated lignin obtained in Example 2 is designated as Lig5, the lignin flame retardant as LF5, and the polyurethane foam as LF5PU.

[0068] Example 3

[0069] The preparation method of polyurethane foam is similar to that of Example 1, except that in step 1), the reaction is stirred at 110 min and 90 °C. The demethylated lignin obtained in Example 3 is denoted as Lig11, the lignin flame retardant is LF11, and the polyurethane foam obtained is denoted as LF11PU.

[0070] Example 4

[0071] The preparation method of polyurethane foam is similar to that in Example 1, except that in step 1), the acidic inorganic molten salt hydrate is a mixed solution of 60% LiBr aqueous solution and 0.2 mol / L HBr, and the reaction is carried out by stirring at 80°C for 90 min. The demethylated lignin obtained in Example 4 is designated as Lig13, the lignin flame retardant as LF13, and the polyurethane foam as LF13PU.

[0072] Example 5

[0073] The preparation method of polyurethane foam is similar to that of Example 1, except that in step 1), the reaction is stirred for 30 minutes and at 80°C. The demethylated lignin obtained in Example 5 is designated as Lig14, the lignin flame retardant as LF14, and the polyurethane foam as LF14PU.

[0074] Example 6

[0075] The preparation method of polyurethane foam is similar to that in Example 1, except that in step 1), the acidic inorganic molten salt hydrate (60% LiBr aqueous solution / 0.2 mol / L HBr) is stirred and reacted at 110°C for 30 min. The demethylated lignin obtained in Example 6 is designated as Lig15, the lignin flame retardant as LF15, and the polyurethane foam as LF15PU.

[0076] Comparative Example 1

[0077] (1) Add 0.04g, 0.02g, 0.08g, 0.07g and 0.04g of dibutyltin diacetate, water, silicone oil, Tween 80 and triethanolamine to 10g of polyethylene glycol (PEG-400) and stir at 80℃ to obtain a reaction solution;

[0078] (2) Heat 15g of 4,4-MDI to 60℃ until it melts, add the molten 4,4-MDI to 10g of reaction solution, stir rapidly at 1000rpm / min, pour into a mold for 1min to foam, and then cure at 120℃ to obtain polyurethane foam, denoted as PU.

[0079] Comparative Example 2

[0080] Using unmodified alkali lignin, the process includes the following steps:

[0081] 1) Dissolve 1.5g of alkali lignin (AL) in 8.5g of polyethylene glycol (PEG-400), and add 0.04g, 0.02g, 0.08g, 0.07g and 0.04g of dibutyltin diacetate, water, silicone oil, Tween 80 and triethanolamine respectively. Stir and react at 80℃ to obtain reaction solution A;

[0082] 2) Heat 15g of 4,4-MDI to 60℃ until melted, add the molten 4,4-MDI to 10g of reaction solution A, stir rapidly at 1000rpm / min, pour into a mold for 1min to foam, and then cure at 120℃ to obtain polyurethane foam, denoted as ALPU.

[0083] Figure 1 The image shows the infrared spectrum of the lignin flame retardant sample from Example 3. Figure 1 It can be seen that 2NH2DOPO is significantly different from DOPO at 3405 cm⁻¹ -1 and 2912cm -1 The -NH2- and -CH2- stretching peaks appear at the positions respectively, and 1240 cm⁻¹ is still present in 2NH2DOPO. -1 (P-Ph) and 912cm -1 The stretching peak of (P=O) was observed. LF showed a higher peak in the 3500-3000 cm⁻¹ range compared to other samples.-1 The stretching peaks within the range decrease and become sharper. At 1240 cm⁻¹ -1 (P-Ph) and 912cm -1 The (P=O) position shows a characteristic peak of the phosphenanthrene structure containing the DOPO structure. This indicates that during the synthesis of the lignin flame retardant, the lignin phenolic hydroxyl groups participate in the Mannich reaction to generate compounds with amino and phosphenanthrene characteristics. Furthermore, compared to other samples, the lignin flame retardant LF exhibits a higher peak at 1600 cm⁻¹ during the reaction. -1 -400cm -1 The range includes lignin benzene rings (1650 cm). -1 ) and -CH2- structure (2900cm) -1 Characteristic peaks, including 1240 cm⁻¹ -1 (P-Ph) and 912cm -1 The characteristic peaks of phosphorus phenanthrene (P=O). Additionally, such as... Figure 2 As shown in Table 1, the demethylated lignin Lig 31 The results of PNMR studies showed that the phenolic hydroxyl content of the demethylated lignin prepared in different examples was increased, and the concentration of phenolic hydroxyl in Lig11 prepared in Example 3 was 7.40 mmol / L.

[0084] Table 1 Demethylated Lignin 31 PNMR integral results

[0085] Types of functional groups Lig2 Lig5 Lig11 Lig13 Lig14 Lig15 Aliphatic-OH 0.43 0.89 0.82 0.56 0.62 0.56 S-OH 0.07 0.04 0.17 0.12 0.03 0.03 G-OH 5.17 3.32 7.38 6.28 0.78 1.58 H-OH 0.01 0.01 0.01 0.01 0.01 0.01 COOH 0.28 0.21 0.47 0.32 0.14 0.14 Total Ph-OH 5.25 3.37 7.40 6.42 0.82 1.62 Total OH 12.99 5.96 8.85 6.97 1.58 2.32

[0086] Figure 3 This is the lignin flame retardant sample from Example 3. 1 In HNMR, characteristic peaks of lignin benzene ring and aliphatic compounds are observed, along with enhanced chemical shifts in the phosphenanthrene structure. phosphenanthrene peak shifts are observed in the LF range of 6.94–8.0 ppm, and the main peak shifts of the p-amino hydrogen in 2NH₂DOPO appear at 5.06 ppm and 2.49 ppm.

[0087] Figure 4 XPS elemental analysis of lignin flame retardants LF with different phenolic hydroxyl contents revealed characteristic N1s and P2p peaks in LF compared to alkali lignin. This further indicates that the 2NH2DOPO flame-retardant segment is introduced into the hydroxyl-rich lignin oligomer molecular chain via the Mannich reaction.

[0088] like Figure 5 and Figure 6 As shown, in the thermogravimetric analysis conducted at LFPU, Comparative Example 1 began to degrade with increasing temperature, followed by Comparative Example 2 and Example 3. Compared to Comparative Example 1, Examples 2 and 3, which contained a certain amount of lignin, showed a lower temperature at T0. 5％(301.38℃, 294.43℃) and T 10％ Both (309.98℃, 314.42℃) increased. The temperature (T) in Example 3... 5％ The temperature of Example 3 was lower than that of Comparative Example 2, but as the temperature increased, the T value of Example 3... 10％ The thermal decomposition rate of Example 3 was higher than that of Comparative Example 2. Furthermore, the peak thermal decomposition rate of Example 3 was significantly lower than that of Comparative Examples 2 and 1. This is because the composite material in Example 2 contains some aminomethyl structures generated through the Mannich reaction, which undergo bond breakage and thermal degradation at around 300°C. The TG curves showed that lignin significantly improved the thermal stability of the composite material, and the PO-Ph and P=O components in the lignin flame retardant with the added organophosphorus phenanthrene structure, together with the lignin benzene ring, further enhanced the thermal stability of the composite material. The T3 of Example 3... 10％ and T max All were greater than those of Comparative Example 2 and Comparative Example 1. The char rates of Comparative Example 1, Comparative Example 2, and Example 3 were 10.85%, 11.68%, and 15.93%, respectively. During thermal decomposition, organophosphorus flame retardants promote the re-charring ability of lignin, resulting in flame-retardant polyurethane foam with a phosphorus-phenanthroline structure exhibiting a relatively high charring rate. Therefore, introducing nitrogen and phosphorus flame retardants through the Mannich reaction significantly improves the thermal stability of the composite material and enhances the high charring properties of the lignin-based composite material after thermal decomposition.

[0089] Table 2. T values ​​of polyurethane foam samples from Example 3, Comparative Example 1, and Comparative Example 2. 5％ T 10％ T max and residual carbon rate

[0090] Example Sample <![CDATA[T 5％ (℃)]]> <![CDATA[T 10％ (℃)]]> <![CDATA[T max (℃)]]> Carbon residue rate (%) Comparative Example 1 262.61 284.07 337.36 10.85 Comparative Example 2 301.83 309.98 344.37 11.68 Example 3 294.43 314.42 346.25 15.93

[0091] Figure 7 In the study, lignin flame retardant was observed as a soft chain within the polyurethane polymer chain. Due to the hydrophobic chemical groups present in the lignin after chemical modification, hydrophilic-hydrophobic testing revealed that the composite foam possessed a 115.5° hydrophobic structure. Figure 8 As shown, infrared imaging technology was used to scan and monitor the composite foam, and the xenon lamp provided heat to the surface of the composite material in Example 3. After 10 minutes of illumination, the internal temperature of the composite material showed a gradient change of 30℃-17℃. This is attributed to the fact that the dense porous structure of the composite material not only reduces the thermal conductivity, but also that the conjugated chemical bonds of the main phenylpropanoid compounds in the lignin component have anti-thermal conductivity against the ultraviolet light portion of the xenon lamp, reflecting and blocking the heat flow from the xenon lamp.

[0092] As shown in Table 3, the thermal conductivity test of polyurethane foam samples revealed that the lignin-based flame-retardant polyurethane foam prepared from phenol-rich hydroxyl lignin derivatives has a lower thermal conductivity and better thermal barrier properties.

[0093] Table 3. Thermal conductivity of polyurethane foams in Examples 1-6 and Comparative Examples 1-2

[0094] Example Sample Thermal conductivity (W / m·K) Comparative Example 1 0.0384 Comparative Example 2 0.0312 Example 1 0.0286 Example 2 0.0275 Example 3 0.0212 Example 4 0.0225 Example 5 0.0295 Example 6 0.0275

[0095] like Figure 9 As shown, stress-strain tests were performed on all example samples. The results showed that Example 3 had better compressive mechanical properties than other example samples, and the higher the concentration of phenolic hydroxyl groups, the stronger the compressive properties of the prepared LFPU.

[0096] The combustion behavior of LFPU polyurethane foam was evaluated using limiting oxygen index (LOI) and horizontal / vertical burning tests, as shown in Table 4. Figure 10 As shown. Comparative Example 1 exhibited rapid combustion with black smoke and dripping during combustion. The LOI of pure PU was low (20.7%), while the LOI of the composite material in Comparative Example 2, with added pure lignin, only increased to 22.5, but black smoke and dripping still occurred during combustion, achieving a vertical and horizontal ignition rating of V-2. Furthermore, it was found that the phenolic hydroxyl content of the depolymerized lignin varied depending on the processing conditions, resulting in different combustion phenomena in the prepared lignin flame retardant composite materials. Example 5 had a phenolic hydroxyl concentration of 0.98 mmol / L, and its ignition time was slow. Example 5 had an LOI of 22.9% and achieved a vertical and horizontal ignition rating of V-2.

[0097] Among all polyurethane foams, Example 3 exhibits excellent flame retardant properties. The composite material of Example 3 burns slowly, exhibiting a self-extinguishing effect after 5 seconds of combustion, without producing black smoke or dripping. During combustion, gases are generated on the substrate surface, but after extinguishing, a dense char structure forms on the substrate surface. Example 3 has an oxygen index of 34.5%, achieving a V-0 flammability rating. The combustion behavior of the composite material leads to the conclusion that lignin can improve the thermal stability of polyurethane materials. Different phenolic hydroxyl content in lignin results in different flame retardant properties in polyurethane foams prepared with amination lignin flame retardants. Higher concentrations of phenolic hydroxyl groups in demethylated lignin enhance flame retardancy by introducing 2NH₂DOPO flame-retardant groups via the Mannich reaction. During combustion, NH and OP=O condense with lignin fracture groups to form char, improving the char-forming properties of the composite material. This leads to changes in the chemical structure of depolymerized lignin, enabling its high-value utilization in flame-retardant polymer materials.

[0098] Table 4 Flame retardant performance indicators of polyurethane foam in Examples 1-6 and Comparative Examples 1-2

[0099]

[0100] like Figure 11As shown, Raman spectroscopy analysis was performed on the combustion residues of Comparative Example 1, Comparative Example 2, and Example 3. The ID / IG ratio represents the degree of sp2 graphitization of the carbon layer; a smaller ratio indicates a higher degree of graphitization. The ID / IG ratios for Comparative Example 1, Comparative Example 2, and Example 3 were 0.72, 0.63, and 0.48, respectively. The lignin flame retardant LF, as a reactant, enhances the density of the carbon layer, thereby improving the protective layer on the combustion surface and further diffusing the flame-retardant combustion.

[0101] Figure 12 This is a SEM image of the char layer surface after combustion in Example 3. The burned char layer is dense and flat, with some bubble protrusions. LF flame retardant is an intumescent flame retardant that exerts a synergistic effect of multiple elements, enhancing the flame retardant effect of lignin-based flame-retardant polyurethane foam.

[0102] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for preparing a phenol-hydroxyl lignin derivative-rich polyurethane flame-retardant foam material, characterized in that, Includes the following steps: 1) Add alkali lignin to an acidic inorganic molten salt solution, stir under heating conditions to allow it to react, after the reaction is complete, add it to an aqueous ethanol solution, precipitate the lignin by antisolvent method, and then freeze dry to obtain demethylated lignin; 2) Dissolve the demethylated lignin obtained in step 1) and 6-(1-(4-aminophenyl)-1-(p-tolyl)ethyl)dibenzo[c,e][1,2]phosphine 6-oxide in N,N-dimethylformamide solution and stir to allow the reaction to proceed; 3) Add formaldehyde aqueous solution to the solution obtained in step 2), stir, and carry out the Mannich reaction; 4) Add the solution obtained in step 3) to an aqueous ethanol solution, precipitate out by antisolvent method, and then freeze dry to obtain lignin flame retardant; 5) Dissolve the lignin flame retardant obtained in step 4) in PEG-400, add dibutyltin diisocyanate, water, silicone oil, Tween 80 and triethanolamine, and stir under heating to react and obtain reaction solution A; 6) Add molten 4,4-MDI to reaction solution A, stir, then pour into a mold for foaming and curing to obtain the polyurethane foam; In step 1), the acidic inorganic molten salt hydrate is a mixed solution of LiBr aqueous solution and hydrobromic acid, wherein the concentration of LiBr aqueous solution is 60% and the concentration of hydrobromic acid is 0.2-1.0 mol / L; The heating temperature is 90°C, and the reaction time is 110 min.

2. The preparation method according to claim 1, characterized in that, In step 2), the mass ratio of the demethylated lignin to 6-(1-(4-aminophenyl)-1-(p-tolyl)ethyl)dibenzo[c,e][1,2]phosphine 6-oxide is 1:1.

5.

3. The preparation method according to claim 1, characterized in that, In step 3), the mass ratio of the demethylated lignin to formaldehyde is 1:1.8; The concentration of the formaldehyde aqueous solution is 30 wt%; The conditions for the Mannich reaction are: a reaction temperature of 60-80℃ and a reaction time of 4-8 hours.

4. The preparation method according to claim 1, characterized in that, In step 5), the lignin flame retardant accounts for 15% of the total mass of the lignin flame retardant and PEG-400; The mass percentages of the dibutyltin diacetate, water, silicone oil, Tween 80, and triethanolamine are 0.4%, 0.2%, 0.8%, 0.7%, and 0.4% of the mass of PEG-400, respectively. The reaction temperature is 60-80℃.

5. The preparation method according to claim 1, characterized in that, In step 6), the mass ratio of reaction solution A to 4,4-MDI is 1:1.5; The stirring speed is 900-1200 r / min; The foaming time is 1-5 minutes; The curing temperature is 100-120℃.

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