A lignin-based nitrogen-phosphorus intumescent flame retardant, a preparation method and application thereof

Abstract

The application discloses a lignin-based nitrogen and phosphorus intumescent flame retardant as well as a preparation method and application thereof, and belongs to the technical field of biomass-based flame-retardant materials and flame-retardant polymer materials. The preparation method comprises the following steps: purifying lignin raw materials; adding the purified lignin, manganese dioxide and acetic acid into a closed high-pressure reaction container to perform an oxidation reaction, and then performing separation, concentration, precipitation, washing and drying to obtain oxidized lignin; dispersing the oxidized lignin in a reaction solvent, adding a nitrogen-containing compound containing primary amino groups and / or secondary amino groups to perform a reaction, and then obtaining nitrogen-containing modified lignin; adding phytic acid into the reaction system, combining the phytic acid with the nitrogen-containing modified lignin, and then performing washing, solid-liquid separation, drying and crushing. The application is favorable for improving the combination stability between flame-retardant components and between the flame retardant and a polyurethane matrix, promoting the formation of an intumescent protective carbon layer in a combustion process, and realizing high-value utilization of industrial lignin.

Description

Technical Field

[0001] This invention relates to the field of biomass-based flame retardant materials and flame retardant polymer materials, specifically to a lignin-based nitrogen-phosphorus intumescent flame retardant with industrial lignin as the carbon source, a compound containing primary and / or secondary amines as the nitrogen source, and phytic acid as the phosphorus source, as well as the preparation method thereof, and the application of the flame retardant in flame retardant polyurethane materials, particularly flame retardant rigid polyurethane foam. Background Technology

[0002] Rigid polyurethane foam (PU) possesses characteristics such as low density, low thermal conductivity, high specific strength, easy molding, and good thermal insulation performance, making it widely used in building exterior wall and roof insulation, cold chain transportation, refrigerators and freezers, pipe insulation, home insulation, and sandwich materials for decorative panels. However, as an organic polymer formed from polyols and isocyanates, rigid polyurethane foam has a large number of internal pores, making it prone to thermal decomposition when exposed to flames or high temperatures. This results in the production of flammable volatiles, exhibiting problems such as rapid flame propagation, concentrated heat release, and low char residue after combustion. Therefore, improving the flame retardant properties of rigid polyurethane foam is crucial for expanding its safe applications in construction, cold chain, and home furnishing. Currently, methods to improve the flame retardant properties of rigid polyurethane foam mainly include adding halogenated flame retardants, phosphorus-containing flame retardants, nitrogen-containing flame retardants, inorganic flame retardants, expanded graphite, and intumescent flame retardants. Traditional halogenated flame retardants may produce corrosive or irritating gases during combustion; some small-molecule phosphorus-based flame retardants may suffer from migration, precipitation, volatilization, and insufficient compatibility with the polyurethane matrix. Inorganic flame retardants typically require high addition amounts and may cause increased foam viscosity, uneven cell structure, and decreased mechanical properties. Intumescent flame retardant systems usually consist of an acid source, a carbon source, and a gas source. During heating, the acid source promotes the dehydration and carbonization of the carbon source, while the gas source releases gas upon heating and causes the carbon layer to expand, ultimately forming an expanded carbon layer on the material surface that provides heat insulation, oxygen barrier, and protection against combustible volatiles. Traditional intumescent flame retardants are mostly physically compounded from small-molecule or low-molecular-weight components such as ammonium polyphosphate, pentaerythritol, and melamine. The lack of stable bonding between the components in the system may lead to problems such as uneven dispersion, moisture absorption, migration, and insufficient interfacial bonding with the polymer matrix.

[0003] Lignin is an important component of lignocellulosic biomass and one of the main byproducts generated during pulping, papermaking, and biomass refining. Lignin possesses an aromatic polymer framework, a high carbon content, and a certain number of phenolic and aliphatic hydroxyl groups, exhibiting good char-forming properties when heated, thus making it a suitable biomass carbon source for intumescent flame-retardant systems. However, unmodified industrial lignin typically suffers from heterogeneous molecular structure, strong intermolecular forces, limited reactive sites, and insufficient dispersibility in polyurethane blends. Directly adding lignin as a flame-retardant filler to rigid polyurethane foam often fails to simultaneously achieve high nitrogen and phosphorus content, good intumescent char-forming properties, and stable matrix compatibility. Phytic acid, a phosphorus-containing compound from biomass containing multiple phosphate groups, can promote dehydration and carbonization of materials during heating. Nitrogen-containing compounds such as polyethyleneimine, ethylenediamine, diethylenetriamine, and triethylenetetramine contain primary and / or secondary amino groups and can serve as nitrogen-containing modifying components, forming ion associations and hydrogen bond networks with phytic acid. Existing studies have shown that phytic acid can combine with lignin hydroxyl groups, and the polyethyleneimine-phytic acid system can also form a stable nitrogen- and phosphorus-containing composite structure.

[0004] Therefore, it is necessary to provide a new method for preparing lignin-based nitrogen-phosphorus intumescent flame retardants. This method involves oxidizing and activating industrial lignin to increase the content of carbonyl and active hydroxyl groups in the lignin, and then sequentially introducing nitrogen-containing compounds and phytic acid. This integrates the lignin carbon source, phosphoric acid source, and nitrogen gas source into the same flame retardant system, thereby improving the bonding stability between flame retardant components and the dispersibility and interfacial reactivity of the flame retardant in the polyurethane matrix. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems in existing technologies, such as insufficient reactivity of industrial lignin, low efficiency of nitrogen and phosphorus functionalization, reliance on physical mixing of components in traditional intumescent flame retardants, and insufficient compatibility and interfacial bonding between flame retardants and rigid polyurethane foam matrices. This invention provides a lignin-based nitrogen-phosphorus intumescent flame retardant, its preparation method, and its applications. First, purified lignin is oxidized using manganese dioxide, causing partial ether bonds and side chain structures to undergo oxidative transformation, increasing carbonyl and active hydroxyl sites. Then, nitrogen-containing compounds containing primary and / or secondary amino groups are combined with the oxidized lignin, and phytic acid is introduced. Through chemical bonding, acid-base ion interactions, and hydrogen bonding, a nitrogen- and phosphorus-containing lignin composite structure is constructed, resulting in a lignin-based nitrogen-phosphorus intumescent flame retardant that functions as a carbon source, acid source, and gas source.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing a lignin-based nitrogen-phosphorus intumescent flame retardant, comprising the following steps: S1: Lignin purification involves dispersing or dissolving lignin raw materials in an alkaline aqueous solution, stirring to fully dissolve the lignin, filtering or centrifuging to remove insoluble impurities, adding an acid solution to the resulting lignin solution to adjust the pH of the system to 1.5–3.0, causing the lignin to precipitate. The lignin precipitate is then filtered or centrifuged and repeatedly washed with deionized water until the washing solution is nearly neutral. Subsequently, the lignin precipitate is dried, pulverized, and sieved to obtain purified lignin with a particle size of 50–200 mesh. The lignin raw material is selected from at least one of sulfate lignin, alkali lignin, enzymatically hydrolyzed lignin, and organic solvent lignin. The alkaline aqueous solution is preferably at least one of sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, and sodium carbonate aqueous solution, more preferably a sodium hydroxide aqueous solution with a mass fraction of 1%–10%. The acid solution is selected from at least one of hydrochloric acid, sulfuric acid, and phosphoric acid aqueous solution, preferably a hydrochloric acid aqueous solution. S2: Lignin oxidation activation. The purified lignin, manganese dioxide, and acetic acid obtained in step S1 are added to a closed high-pressure reaction vessel and oxidized at 140-180°C for 1-6 hours. Preferably, the mass ratio of purified lignin to manganese dioxide is 1:(0.1-1.5), and the mass-volume ratio of purified lignin to acetic acid is 1g:(5-30)mL. More preferably, the oxidation reaction temperature is 150-175°C, and the reaction time is 2-5 hours. After the reaction, the reaction vessel is cooled to room temperature. The manganese-containing solid is separated by filtration or centrifugation. The obtained lignin-containing liquid phase is concentrated under reduced pressure, and the oxidized lignin is precipitated by adding water and / or adjusting the pH. The obtained oxidized lignin is washed with deionized water until the washing solution is close to neutral and dried at 50-80°C for 6-24 hours to obtain oxidized lignin with carbonyl and / or hydroxyl active sites. The carbonyl content, hydroxyl exposure degree, and molecular weight of lignin are adjusted by oxidation treatment to provide reaction sites for subsequent nitrogen-containing modification. S3: Nitrogen-containing modification. The oxidized lignin obtained in step S2 is added to the reaction solvent and dispersed by mechanical stirring, ultrasonic treatment, or a combination of mechanical stirring and ultrasonic treatment. The reaction solvent is water, ethanol, or a mixture of water and ethanol. The mass-to-volume ratio of oxidized lignin to the reaction solvent is 1 g: (10-50) mL. A nitrogen-containing compound containing primary and / or secondary amino groups is added to the oxidized lignin dispersion. The pH of the reaction system is adjusted to 7.5-11.0, and the reaction is carried out at 60-100℃ for 2-8 hours. A nitrogen-containing compound is combined with oxidized lignin to obtain nitrogen-modified lignin. Preferably, the reaction temperature is 70-90℃ and the reaction time is 3-6h. The nitrogen-containing compound is selected from at least one of polyethyleneimine, ethylenediamine, diethylenetriamine and triethylenetetramine, preferably polyethyleneimine. During the nitrogen modification process, the amino group in the nitrogen-containing compound undergoes condensation or addition reaction with the carbonyl group in the oxidized lignin to form a carbon-nitrogen bonded structure. At the same time, the nitrogen-containing compound can also combine with the lignin structure through hydrogen bonding and electrostatic interaction. S4: Phosphorus modification. A 20%–70% (w / w) phytic acid aqueous solution is added to the nitrogen-modified lignin reaction system obtained in step S3, and the pH of the system is adjusted to 2.0–6.0. ​​The reaction is carried out at 70–110°C for 2–8 hours, preferably at 80–100°C for 3–6 hours. During the reaction, the phosphate groups of phytic acid react with the hydroxyl groups in lignin and / or the amino groups in nitrogen-containing compounds through at least one of chemical bonding, ionic bonding, and hydrogen bonding to form nitrogen- and phosphorus-containing lignin reaction products. After the reaction, the reaction solution is cooled to room temperature, and the reaction products are washed with water and / or ethanol until the conductivity of two adjacent washing solutions does not change significantly. If the concentration of the washing solution is greater than 5%, or the conductivity of the washing solution reaches a basically stable value, the washed product is filtered or centrifuged, vacuum dried at 50-90℃ for 8-36 hours, pulverized and passed through a 50-200 mesh sieve to obtain a lignin-based nitrogen-phosphorus intumescent flame retardant. The mass ratio of oxidized lignin, nitrogen-containing compounds and phytic acid (calculated as pure phytic acid) is 1:(0.2-2.0):(0.2-2.0). Preferably, the mass ratio of oxidized lignin, polyethyleneimine and phytic acid (calculated as pure phytic acid) is 1:(0.3-1.0):(0.4-1.2). It should be noted that when using an aqueous phytic acid solution for feeding, the mass of phytic acid fed is based on the mass of pure phytic acid contained in the aqueous phytic acid solution.

[0007] A lignin-based nitrogen-phosphorus intumescent flame retardant, comprising a lignin aromatic carbon skeleton, a nitrogen-containing structure, and a phosphorus-containing structure, wherein the nitrogen-containing structure is derived from a nitrogen-containing compound containing primary and / or secondary amines, and the phosphorus-containing structure is derived from phytic acid.

[0008] The present invention also provides a lignin-based nitrogen-phosphorus intumescent flame retardant prepared by the above method, the flame retardant comprising: an aromatic carbon skeleton provided by lignin, a nitrogen-containing structure provided by a compound containing primary and / or secondary amino groups, and a phosphorus-containing structure provided by phytic acid, wherein the amino group in the nitrogen-containing compound forms a carbon-nitrogen bonded structure with the carbonyl group in the oxidized lignin, and the phytic acid forms at least one of chemical bonding, ionic bonding, and hydrogen bonding with the lignin hydroxyl group and / or the amino group in the nitrogen-containing compound through a phosphate group.

[0009] Preferably, the flame retardant contains 2%–12% nitrogen and 2%–10% phosphorus by mass. Preferably, the char residue of the flame retardant when heated to 800°C under a nitrogen atmosphere is not less than 30%.

[0010] The nitrogen and phosphorus content and char residue rate mentioned above should be determined by elemental analysis, ICP-OES or XPS and thermogravimetric analysis, respectively, and supported by actual examples.

[0011] This invention also provides a method for preparing flame-retardant rigid polyurethane foam, the method comprising the following steps: A1: Preparation of polyol premixed component: Polyether polyol, lignin-based nitrogen-phosphorus intumescent flame retardant prepared in this invention, foaming agent, foam stabilizer and catalyst are mixed evenly to obtain polyol premixed component. Based on 100 parts by weight of polyether polyol, the polyol premixed component includes: lignin-based nitrogen-phosphorus intumescent flame retardant: 10-25 parts, water: 1-2 parts, foam stabilizer: 2-3 parts, catalyst: 0.2-0.5 parts. The polyether polyol is preferably at least one of glycerol-type and aromatic amine-type polyether polyols used for preparing rigid polyurethane foam. The foam stabilizer is preferably an organosilicon foam stabilizer. The catalyst can be selected from amine catalysts, organotin catalysts and combinations thereof. The water acts as a chemical foaming agent and reacts with isocyanate to produce carbon dioxide gas. A2: Foaming and curing: Isocyanate is added to the polyol premixed component obtained in step A1, and after rapid stirring and mixing, it is injected into a mold for free foaming. After curing at room temperature or by heating, flame-retardant rigid polyurethane foam is obtained. The isocyanate is preferably polymeric diphenylmethane diisocyanate, and the amount of isocyanate added is determined according to the isocyanate index of 1.0 to 1.3. The isocyanate index is calculated using the following formula: "Isocyanate index" = "Amount of isocyanate group actually added" / "Theoretical amount of isocyanate group required for complete reaction of active hydrogen in the system"; When determining the amount of isocyanate, in addition to the active hydrogen provided by polyether polyol and water, the hydroxyl and amino groups in the lignin-based nitrogen-phosphorus intumescent flame retardant that can react with isocyanate should also be included in the total amount of active hydrogen. Some of the hydroxyl and amino groups in the lignin-based nitrogen-phosphorus intumescent flame retardant can react with isocyanate groups to form polyurethane bonds and / or polyurea bonds, so that a part of the flame retardant can be embedded in the polyurethane crosslinking network through chemical reaction. The lignin-based nitrogen-phosphorus intumescent flame retardant of the present invention can be used to prepare flame-retardant polyurethane materials, and is suitable for rigid polyurethane foam products that require flame retardant performance and environmental friendliness in building insulation, cold chain insulation, home insulation and decorative panel sandwich materials.

[0012] Compared with the prior art, the beneficial effects of the present invention are: 1. To improve the reactivity of industrial lignin, this invention uses manganese dioxide to oxidize purified lignin, transforming some side chains and alcohol hydroxyl structures of lignin into carbonyl structures, and promoting the cleavage of some ether bonds and the exposure of active hydroxyl groups. This provides more reaction and binding sites for the subsequent introduction of nitrogen-containing compounds. Compared with the direct use of unactivated industrial lignin, oxidized lignin has higher functional group reactivity, which is beneficial to improving the binding stability of nitrogen-containing components in the lignin structure.

[0013] 2. By integrating carbon source, acid source, and gas source into the same flame retardant system, the lignin aromatic skeleton has a high char-forming tendency and can be used as a carbon source for the intumescent flame retardant system. Phytic acid contains multiple phosphate groups and can promote the dehydration and carbonization of lignin and polyurethane matrix when heated, and can be used as an acid source. Nitrogen-containing compounds such as polyethyleneimine can generate nitrogen-containing gases when heated and are beneficial to the expansion of the char layer, and can be used as a gas source. Thus, the flame retardant prepared by this invention can simultaneously introduce carbon source, acid source, and gas source into the same lignin matrix structure, reducing the problem of uneven dispersion and component migration caused by the complete reliance on physical mixing of each component in traditional intumescent flame retardants.

[0014] 3. Enhanced binding between nitrogen-containing components, phytic acid, and lignin: This invention first binds nitrogen-containing compounds with oxidized lignin, then introduces phytic acid. The phosphate groups of phytic acid can form acid-base ion associations and hydrogen bonds with the amino groups in the nitrogen-containing compounds, and can also bind with the hydroxyl groups of lignin. Compared with simply mixing lignin, polyethyleneimine, and phytic acid, the sequential modification method of this invention is beneficial for forming a more stable nitrogen- and phosphorus-containing lignin structure.

[0015] 4. Improve high-temperature charring ability: The aromatic structure of lignin can provide a stable carbon skeleton, phytic acid can promote the dehydration and carbonization of lignin and polyurethane during thermal decomposition, and the nitrogen-containing structure is conducive to the expansion of the char layer. The synergistic effect of the three can help improve the high-temperature char rate of flame retardants and their polyurethane composites, and form a continuous, expanded and relatively dense protective char layer, thereby reducing the transfer of heat, oxygen and combustible volatiles between the combustion zone and the interior of the material.

[0016] 5. Improve the interfacial bonding ability between flame retardant and polyurethane matrix. The lignin-based nitrogen-phosphorus intumescent flame retardant prepared by this invention retains active groups such as hydroxyl and amino groups, at least a portion of which can react with isocyanate to form polyurethane bonds and / or polyurea bonds. The flame retardant coexists in the polyurethane matrix through physical dispersion and chemical bonding, which is beneficial to improve its interfacial compatibility and reduce the tendency of flame retardant components to migrate and precipitate during use.

[0017] 6. To achieve high-value utilization of industrial lignin, this invention can use sulfate lignin, alkali lignin, enzymatically hydrolyzed lignin or organic solvent lignin as raw materials, which is conducive to improving the utilization value of lignin by-products of pulping and papermaking and biomass refining, and reducing dependence on petroleum-based carbon sources and some traditional flame retardants. Attached Figure Description

[0018] Figure 1 This is a schematic flowchart of the preparation method of the lignin-based nitrogen-phosphorus intumescent flame retardant of the present invention.

[0019] Figure 2 This is a schematic flowchart of the flame-retardant rigid polyurethane foam preparation method of the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example: Please refer to Figure 1 and Figure 2 This invention provides a technical solution: a method for preparing a lignin-based nitrogen-phosphorus intumescent flame retardant, the preparation method comprising the following steps: S1: Purify, dry, pulverize and sieve the lignin raw material to obtain purified lignin; S2: Purified lignin, manganese dioxide, and acetic acid are added to a closed high-pressure reaction vessel and oxidized at 140–180°C for 1–6 hours. After the reaction is completed, the mixture is cooled, and the unreacted manganese dioxide is separated. The lignin-containing liquid phase is concentrated and precipitated, washed, and dried. The carbonyl content, hydroxyl exposure, and molecular weight of the lignin are adjusted by oxidation treatment to provide reaction sites for subsequent nitrogen-containing modification. S3: Disperse oxidized lignin in a reaction solvent, add a nitrogen-containing compound containing primary and / or secondary amino groups, and react at 60–100 °C for 2–8 h to allow the nitrogen-containing compound to combine with the oxidized lignin to obtain nitrogen-modified lignin. S4: Phytic acid is added to the reaction system of nitrogen-modified lignin, and the reaction is carried out at pH 2.0-6.0 and 70-110℃ for 2-8 hours to form a phosphorus-containing composite structure with nitrogen-modified lignin, forming a nitrogen- and phosphorus-containing lignin reaction product; after cooling, washing, solid-liquid separation and drying, it is pulverized to obtain a lignin-based nitrogen-phosphorus intumescent flame retardant; wherein, the mass ratio of oxidized lignin, nitrogen-containing compound and phytic acid is 1:(0.2-2.0):(0.2-2.0).

[0022] As a further limitation of the present invention, in step S2, the degree of lignin degradation and hydroxyl activity are controlled by the mass ratio of lignin, manganese dioxide and acetic acid, reaction time and reaction temperature.

[0023] The reaction solvent used in step S3 is any one of water, ethanol, or a mixture of water and ethanol.

[0024] In step S3, the nitrogen-containing compound is selected from at least one of polyethyleneimine, ethylenediamine, diethylenetriamine, and triethylenetetramine, preferably polyethyleneimine.

[0025] A lignin-based nitrogen-phosphorus intumescent flame retardant, comprising a lignin aromatic carbon skeleton, a nitrogen-containing structure, and a phosphorus-containing structure, wherein the nitrogen-containing structure is derived from a nitrogen-containing compound containing primary and / or secondary amines, and the phosphorus-containing structure is derived from phytic acid.

[0026] A method for preparing flame-retardant rigid polyurethane foam, the method comprising the following steps: A1: Mix polyether polyol, lignin-based nitrogen-phosphorus intumescent flame retardant according to any one of claims 12 to 14, foaming agent, foam stabilizer and catalyst evenly to obtain polyol premixed component; A2: Isocyanate is added to the polyol premixed components, stirred and mixed, then injected into a mold for free foaming, and cured to obtain flame-retardant rigid polyurethane foam. The amount of lignin-based nitrogen-phosphorus intumescent flame retardant added is 10-25 parts per 100 parts of polyether polyol, and the amount of isocyanate added is determined according to an isocyanate index of 1.0-1.3.

[0027] The specific implementation method of this embodiment is as follows: Example 1: Purification of Lignin. 100.0 g of bamboo sulfate lignin was weighed and added to 1000 mL of a 4% sodium hydroxide aqueous solution. The solution was stirred at 500 rpm for 60 min at room temperature to fully dissolve the lignin. The resulting alkaline lignin solution was filtered through a Buchner funnel to remove insoluble fibers, ash, and other mechanical impurities. Under continuous stirring, a 12% hydrochloric acid aqueous solution was slowly added to the filtrate to adjust the pH of the system to 3.0. After stirring for another 30 min, the solution was allowed to stand for 12 h to allow the lignin to fully precipitate. The lignin precipitate was collected by centrifugation at 5000 rpm for 10 min. It was redispersed with deionized water and washed by centrifugation. This washing process was repeated until the pH of the supernatant reached 6.0–7.0. The washed lignin precipitate was dried in a vacuum drying oven at 60℃ for 24 h, pulverized, and passed through a 100-mesh sieve to obtain purified bamboo sulfate lignin. The lignin raw material is purified, dried, pulverized, and sieved to obtain lignin powder with uniform particle size. The lignin is selected from at least one of sulfate lignin, alkali lignin, enzymatic hydrolyzed lignin, and organic solvent lignin. Example 2: Preparation of Oxidized Lignin. 40.0 g of purified bamboo sulfate lignin and 20.0 g of manganese dioxide were weighed and added to a sealed high-pressure reactor with a polytetrafluoroethylene liner and a mechanical stirrer. 600 mL of glacial acetic acid was added, and the temperature was raised to 165°C and maintained for 3 hours. After the reaction, heating was stopped, and the reactor was allowed to cool naturally to below 50°C before being opened. The reaction mixture was centrifuged at 8000 r / min for 10 minutes to separate the manganese-containing solid, and the upper lignin-containing liquid phase was collected. The resulting liquid phase was concentrated under reduced pressure at 50–60°C to approximately one-quarter of its original volume. Five times the volume of deionized water was added to the concentrate, and the pH of the system was adjusted to 3.0 using a 10% sodium hydroxide aqueous solution or hydrochloric acid aqueous solution to precipitate the oxidized lignin. After standing for 6 hours, the oxidized lignin precipitate was collected by centrifugation at 8000 r / min for 10 minutes. The oxidized lignin was repeatedly washed with deionized water until the washing solution was nearly neutral. The washed oxidized lignin was then dried in a vacuum drying oven at 60℃ for 18 hours, pulverized, and passed through a 100-mesh sieve to obtain oxidized lignin, denoted as O-Lig, approximately 28.4g. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, hydroxyl value determination, and gel permeation chromatography were used to analyze the changes in functional groups, hydroxyl content, and molecular weight of the lignin before and after oxidation. The yield of oxidized lignin was approximately 71%. The molecular weight (Mw) of the lignin decreased from approximately 3250 to 2150 g / mol, a decrease of approximately 34%; the carbonyl content increased from 0.65 to 1.70 mmol / g, an increase of approximately 162%; the phenolic hydroxyl content increased from 2.45 to 2.95 mmol / g, an increase of approximately 20%; and the aliphatic hydroxyl content decreased from 2.30 to 1.25 mmol / g, a decrease of approximately 46%.

[0028] Example 3: Preparation of N / P-LNP1. 10.0 g of the obtained oxidized lignin was weighed and added to 300 mL of a mixed solvent of water and ethanol in a 1:1 volume ratio. The mixture was mechanically stirred for 30 min and ultrasonically dispersed for 20 min to obtain an oxidized lignin dispersion. 6.0 g of polyethyleneimine was weighed and dissolved in 50 mL of deionized water, then slowly added dropwise to the oxidized lignin dispersion. The pH of the system was adjusted to 9.5 using a 10% sodium hydroxide aqueous solution. The reaction was carried out at 80 °C and 500 r / min for 4 h with stirring, allowing the amino groups in polyethyleneimine to react with the carbonyl groups in oxidized lignin, and then bind to the hydroxyl groups in lignin through hydrogen bonding or electrostatic interactions, resulting in a nitrogen-modified lignin solution. 8.0 g of phytic acid was weighed and diluted to 50 mL. The diluted phytic acid solution was slowly added dropwise to the nitrogen-modified lignin reaction solution, controlling the reaction system temperature at 60–70 °C during the addition process. After the addition was complete, the pH of the system was adjusted to 4.0, the temperature was raised to 90℃, and the reaction was stirred at 500 r / min for 4 h. After the reaction was completed, the mixture was cooled to room temperature and centrifuged at 8000 r / min for 10 min to collect the solid reaction product. The solid product was first washed with deionized water, and then washed with a 50% (v / v) ethanol aqueous solution. The washing was repeated until the conductivity change between two adjacent washes was no more than 5%. The washed product was dried under vacuum at 70℃ for 24 h, pulverized, and passed through a 100-mesh sieve to obtain a lignin-based nitrogen-phosphorus intumescent flame retardant, denoted as N / P-LNP1. In this example, the mass ratio of oxidized lignin, polyethyleneimine, and pure phytic acid was 1:0.6:0.8.

[0029] Example 4: Preparation of N / P-LNP2. Except for adjusting the mass ratio of oxidized lignin, polyethyleneimine, and pure phytic acid to 1:0.4:0.6, the remaining steps were the same as in Example 3, yielding the lignin-based nitrogen-phosphorus intumescent flame retardant N / P-LNP2.

[0030] Example 5: Preparation of N / P-LNP3. Except for adjusting the mass ratio of oxidized lignin, polyethyleneimine, and pure phytic acid to 1:0.8:1.0, the remaining steps were the same as in Example 3, yielding the lignin-based nitrogen-phosphorus intumescent flame retardant N / P-LNP3.

[0031] N / P-LNP indicates a lignin-based nitrogen-phosphorus flame retardant; the numbers 1 to 3 represent different ratios of polyethyleneimine and phytic acid. The carbon, hydrogen, and nitrogen content in the flame retardant was determined using an elemental analyzer. The phosphorus content in the flame retardant was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). It is recommended to record the nitrogen and phosphorus test results as shown in Table 1 below.

[0032] Table 1. Results of nitrogen and phosphorus element tests The raw materials for preparing a flame-retardant rigid polyurethane foam are shown in Table 2. Based on 100 parts by weight of polyether polyol, the following raw materials are weighed: Table 2. Raw materials for preparing a flame-retardant rigid polyurethane foam. Example 6: Foam with 10-25 parts flame retardant. First, polyether polyol, water, foam stabilizer, catalyst, and N / P-LNP1 were added to a plastic beaker and stirred at 1500 rpm for 60 seconds to uniformly disperse the flame retardant, obtaining a polyol premix. A calculated amount of polymeric MDI was added to the polyol premix, and the mixture was rapidly stirred at 2500 rpm for 10 seconds. The mixture was immediately poured into an open mold for free foaming. The foam was allowed to stand at room temperature for 24 hours before demolding and continued to mature at room temperature for 72 hours to obtain flame-retardant rigid polyurethane foam RPUF-FR. The amount of lignin-based nitrogen-phosphorus intumescent flame retardant added was 10-25 parts per 100 parts by weight of isocyanate.

[0033] Comparative Example 1: Pure RPUF. Without adding lignin-based nitrogen-phosphorus intumescent flame retardants, the remaining formulation and preparation steps are the same as RPUF-FR, resulting in pure rigid polyurethane foam RPUF.

[0034] Comparative Example 2: Addition of purified lignin. The obtained purified lignin was used to replace the lignin-based nitrogen-phosphorus intumescent flame retardant, and the remaining formulation and preparation steps were the same as RPUF-FR to obtain RPUF-L.

[0035] The apparent density, average cell size, compressive strength, thermal conductivity, limiting oxygen index, vertical burning rating, heat release rate, total heat release, total smoke release, effective heat of combustion, thermogravimetric char residue, and char morphology after combustion of the rigid polyurethane foam prepared above were tested.

[0036] This invention's lignin-based nitrogen-phosphorus intumescent flame retardant exerts its effect primarily through the synergistic action of condensed-phase and gas-phase flame retardancy during the combustion of rigid polyurethane foam. In the condensed phase, the phosphate groups in phytic acid, upon heating, form phosphoric acid or polyphosphoric acid structures, promoting the dehydration, cross-linking, and carbonization of lignin and polyurethane molecular chains. The aromatic structure of lignin provides a carbon skeleton for the char layer, which is beneficial for improving the thermal stability of the char layer. During the gas phase and the expansion into char, nitrogen-containing structures such as polyethyleneimine decompose upon heating, releasing nitrogen-containing gases and other non-flammable or low-flammable gases, causing the softened char layer to expand. The resulting phosphorus- and nitrogen-containing intumescent char layer can construct a physical barrier on the material surface, reducing the transfer of external heat and oxygen to the material interior and inhibiting the release of flammable pyrolysis products from the material interior to the flame zone, thereby reducing the combustion intensity and fire hazard of rigid polyurethane foam.

[0037] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0038] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a lignin-based nitrogen-phosphorus intumescent flame retardant, characterized in that: The preparation method includes the following steps: S1: Purify, dry, pulverize and sieve the lignin raw material to obtain purified lignin; S2: The purified lignin, manganese dioxide and acetic acid are added to a closed high-pressure reaction vessel and oxidized at 140-180°C for 1-6 hours. After the reaction is completed, the mixture is cooled, the unreacted manganese dioxide is separated, the lignin-containing liquid phase is concentrated and precipitated, and after washing and drying, oxidized lignin with carbonyl and / or hydroxyl active sites is obtained. S3: Disperse the oxidized lignin in a reaction solvent, add a nitrogen-containing compound containing primary and / or secondary amino groups, and react at 60-100°C for 2-8 hours to allow the nitrogen-containing compound to combine with the oxidized lignin to obtain nitrogen-modified lignin. S4: Add phytic acid to the nitrogen-modified lignin reaction system and react at pH 2.0-6.0 and 70-110℃ for 2-8 hours to form a phosphorus-containing composite structure with the nitrogen-modified lignin, forming a nitrogen- and phosphorus-containing lignin reaction product; after cooling, washing, solid-liquid separation and drying, pulverize to obtain a lignin-based nitrogen-phosphorus intumescent flame retardant; wherein the mass ratio of the oxidized lignin, nitrogen-containing compound and phytic acid is 1:(0.2-2.0):(0.2-2.0).

2. The method for preparing a lignin-based nitrogen-phosphorus intumescent flame retardant according to claim 1, characterized in that: The lignin raw material in step S1 is selected from at least one of sulfate lignin, alkali lignin, enzymatically hydrolyzed lignin, and organic solvent lignin. The lignin raw material is dispersed or dissolved in an alkaline aqueous solution. After removing insoluble impurities, the pH of the system is adjusted to 1.5-3.0 using an acid solution to precipitate the lignin. The precipitate is then washed and dried. The acid solution is selected from at least one of hydrochloric acid, sulfuric acid, and phosphoric acid aqueous solution. The particle size of the purified lignin is 50-200 mesh.

3. The method for preparing a lignin-based nitrogen-phosphorus intumescent flame retardant according to claim 1, characterized in that: In step S2, the mass ratio of purified lignin to manganese dioxide is 1:(0.1-1.5), and the mass-to-volume ratio of purified lignin to acetic acid is 1g:(5-30)mL; the oxidation reaction temperature is 150-175℃, the oxidation reaction time is 2-5h, after the oxidation reaction is completed, the manganese-containing solid is removed by filtration or centrifugation, the obtained liquid phase is concentrated under reduced pressure, and the oxidized lignin is precipitated by adding water and / or adjusting the pH; the obtained oxidized lignin is washed with water until the washing liquid is neutral, and then dried at 50-80℃ for 6-24h.

4. The method for preparing a lignin-based nitrogen-phosphorus intumescent flame retardant according to claim 1, characterized in that: The nitrogen-containing compound in step S3 is at least one of polyethyleneimine, ethylenediamine, diethylenetriamine, and triethylenetetramine, preferably polyethyleneimine. The reaction solvent is water, ethanol, or a mixture of water and ethanol. The mass-volume ratio of the oxidized lignin to the reaction solvent is 1 g: (10-50) mL. The pH of the reaction system is 7.5-11.0, the reaction temperature is 70-90°C, and the reaction time is 3-6 h.

5. The method for preparing a lignin-based nitrogen-phosphorus intumescent flame retardant according to claim 1, characterized in that: In step S4, 11.43 g of a 70% phytic acid aqueous solution, of which 8.00 g is pure phytic acid, is weighed and diluted to 50 mL with deionized water. The mass ratio of nitrogen-modified lignin to phytic acid is 1:(0.3-1.5). The reaction temperature is 80-100℃, the reaction time is 3-6 h, and the pH of the reaction system is 2.0-6.

0. The nitrogen- and phosphorus-containing lignin reaction products are washed with water and / or ethanol until the conductivity of the washing solution is basically stable. The washed reaction products are vacuum dried at 50-90℃ for 8-36 h and then pulverized to 50-200 mesh.

6. The method for preparing a lignin-based nitrogen-phosphorus intumescent flame retardant according to claim 1, characterized in that: The mass ratio of oxidized lignin, polyethyleneimine, and phytic acid is 1:(0.3-1.0):(0.4-1.2).

7. A lignin-based nitrogen-phosphorus intumescent flame retardant, characterized in that: The lignin-based nitrogen-phosphorus intumescent flame retardant, prepared by any one of claims 1 to 6, comprises a lignin aromatic carbon skeleton, a nitrogen-containing structure, and a phosphorus-containing structure, wherein the nitrogen-containing structure is derived from a nitrogen-containing compound containing primary and / or secondary amines, and the phosphorus-containing structure is derived from phytic acid. The flame retardant contains 6.0%–7.5% nitrogen and 6.0%–8.5% phosphorus by mass. The reaction of the amino group of the nitrogen-containing compound with the carbonyl group of oxidized lignin may form a carbon-nitrogen double bond structure. The phytic acid forms at least one of chemical bonding, ionic bonding, and hydrogen bonding with the hydroxyl group of lignin and / or the amino group of the nitrogen-containing compound through a phosphate group.

8. A method for preparing flame-retardant rigid polyurethane foam, characterized in that, The preparation method includes the following steps: A1: A polyether polyol or diethylene glycol, the lignin-based nitrogen-phosphorus intumescent flame retardant as described in any one of claims 12-14, a foaming agent, a foam stabilizer, and a catalyst are mixed evenly to obtain a polyol premixed component; the hydroxyl value of the polyether polyol or diethylene glycol is 1050-1065 mgKOH / g; the catalyst is a tertiary amine catalyst; and the foam stabilizer is an organosilicon surfactant. A2: Isocyanate is added to the polyol premixed component, stirred and mixed, then injected into a mold for free foaming, and cured to obtain flame-retardant rigid polyurethane foam. The amount of lignin-based nitrogen-phosphorus intumescent flame retardant added is 10-25 parts per 100 parts of polyether polyol, and the amount of isocyanate added is determined according to an isocyanate index of 1.0-1.

3. The isocyanate is polymeric MDI with an NCO mass fraction of 32%. Based on 100 parts by weight of polyether polyol, the premixed polyol components include: 10-25 parts of lignin-based nitrogen-phosphorus intumescent flame retardant; 1-2 parts of water; 2-3 parts of foam stabilizer; and 0.2-0.5 parts of catalyst. When determining the amount of isocyanate, at least one active group among the hydroxyl and amino groups contained in the lignin-based nitrogen-phosphorus intumescent flame retardant is included in the calculation of the isocyanate index.

9. A flame-retardant rigid polyurethane foam, characterized in that, The lignin-based nitrogen-phosphorus intumescent flame retardant is prepared by the preparation method according to any one of claims 15 to 17, wherein the lignin-based nitrogen-phosphorus intumescent flame retardant is dispersed in a rigid polyurethane foam matrix, and at least a portion of the hydroxyl and / or amino groups contained in the lignin-based nitrogen-phosphorus intumescent flame retardant reacts with isocyanate to form a polyurethane and / or polyurea bonded structure.

10. The application of a lignin-based nitrogen-phosphorus intumescent flame retardant according to any one of claims 8 to 9 in the preparation of flame-retardant polyurethane materials, wherein the flame-retardant rigid polyurethane foam is used in building insulation materials, cold chain insulation materials, home insulation materials or decorative panel sandwich materials.

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