A process for the preparation of a flame-retardant modified polyisocyanate

Abstract

The application relates to a preparation method of a flame-retardant modified polyisocyanate, and belongs to the technical field of polyurethane elastomers. Nano-composite flame retardants are chemically grafted through the reaction of hydroxyl groups and isocyanate groups in polyisocyanate segments, can be uniformly dispersed in a hydrophilic system, and avoid agglomeration and sedimentation caused by physical mixing. Not only is the dispersibility of the nano-composite flame retardants in polymers improved, but also the interfacial bonding force between the two is improved. 2,2-dimethylol propionic acid is covalently grafted to the polyester side chain of the polyisocyanate through the esterification reaction of hydroxyl groups, pyromellitic anhydride and trimethylolpropane, is converted into a carboxylate after neutralization, and the dispersibility in water is improved. The branched main chain of the comb-shaped polyisocyanate and the side chain polyester unit form a large steric hindrance, further weaken the van der Waals force between molecules, enhance the repulsion between particles, avoid molecular aggregation, and make the flame-retardant modified polyisocyanate form a stable and uniform dispersion liquid in deionized water.

Description

Technical Field

[0001] This invention belongs to the field of polyurethane elastomer technology and relates to a method for preparing flame-retardant modified polyisocyanate. Background Technology

[0002] Traditional polyisocyanate-based materials (such as polyurethane foam) have significant flammability defects, with limiting oxygen index (LOI) typically below 21%, classifying them as flammable materials. To improve the flame-retardant properties of polyisocyanate-based materials, flame-retardant modification is mainly achieved through two approaches: First, additive flame retardancy, which involves incorporating phosphorus-based, halogen-based, or inorganic metal hydroxide flame retardants into the polymerization system of polyisocyanate and polyols. This method is simple to operate, but the large addition of flame retardants can easily lead to a decrease in the material's mechanical properties, hindered processing flow, and problems such as flame retardant migration and precipitation, resulting in insufficient stability of the flame-retardant effect during long-term use. Second, reactive flame retardancy, which involves introducing flame-retardant elements into the polyisocyanate molecular chain through chemical modification, making the flame-retardant groups part of the polymer structure.

[0003] Chinese invention patent application CN115322673B discloses a flame-retardant waterproof coating and its preparation method, comprising component A and component B. Component A includes flame-retardant polyether polyol, diphenylmethane diisocyanate, and chlorinated paraffin; component B includes flame-retardant polyether polyol, chlorinated paraffin, aromatic oil, zinc borate, dimethyl thiotoluene diamine, modified calcium carbonate, dispersant, and defoamer. The flame-retardant polyether polyol is used as a raw material for polyurethane, and it contains hydroxyl groups that can participate in the synthesis of polyurethane, so that the film-forming matrix of the coating itself has good flame-retardant properties. Compared with additive flame retardants, it can overcome the defects of easy seepage or migration of flame retardants, thereby improving the durability of flame-retardant effect and enhancing the effectiveness of flame-retardant active ingredients.

[0004] The production of traditional polyisocyanate coatings relies on volatile organic compounds, which not only pollute the environment but also harm people's health. Waterborne polyisocyanates are gradually being widely used in various industries due to their environmental advantages. However, the polyurethane prepolymer in the flame-retardant waterproof coatings mentioned above is highly hydrophobic and lacks hydrophilic groups. The auxiliary components are also mostly hydrophobic substances. They cannot be stably dispersed in the aqueous phase and will show obvious stratification after standing. The upper layer is a transparent aqueous phase and the lower layer is a viscous coating oil phase, making it difficult to form a homogeneous solution or a stable dispersion. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing flame-retardant modified polyisocyanate, which improves flame retardant performance and dispersibility in water by grafting a nanocomposite flame retardant onto polyisocyanate segments.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for preparing flame-retardant modified polyisocyanate includes the following steps:

[0008] Step 1: A nitrogen-containing intermediate is generated by grafting a nitrogen-containing diamine and an aldehyde donor through a Schiff base condensation reaction, and then intercalated between cobalt-based nano-montmorillonite layers through adsorption. A phosphorus-containing oxide is grafted onto the nitrogen-containing intermediate through a nucleophilic addition reaction to obtain a nanocomposite flame retardant.

[0009] Step 2: Graft the semi-closed diisocyanate prepolymer with trimethylolpropane, then introduce it into the polyester side chain through esterification with pyromellitic anhydride. The isocyanate groups of the diisocyanate and the terminal hydroxyl groups of the polyester side chain and the hydroxyl groups of the nanocomposite flame retardant undergo an addition reaction. After end-capping and neutralization, flame-retardant modified polyisocyanate is obtained.

[0010] Furthermore, the specific preparation process of the nanocomposite flame retardant is as follows:

[0011] N,N-dimethylformamide and a nitrogen-containing diamine were added to a reaction vessel and heated to 100-105°C. An aldehyde donor was then slowly added, and the mixture was refluxed and stirred for 6-8 hours to generate a nitrogen-containing intermediate. Cobalt-based nano-montmorillonite was then added, and the mixture was stirred at 800-1000 r / min for 6-8 hours. A phosphorus-containing oxide was then added, and the mixture was stirred for another 18-20 hours. The mixture was cooled to room temperature and then dropped into anhydrous ethanol. The product precipitated, and the product was filtered, washed, and dried to obtain a nanocomposite flame retardant.

[0012] Furthermore, the ratio of N,N-dimethylformamide, diamine containing a nitrogen source, aldehyde donor, cobalt-based nano-montmorillonite, phosphorus oxide and anhydrous ethanol is 1-1.2L:31-40g:22.7-26.7g:15-20g:108-120g:10-15L.

[0013] Furthermore, the nitrogen-containing diamine includes one of 2,4-diamino-6-phenyl-1,3,5-triazine, 4,4'-diaminodiphenyl sulfone, and 3,5-diamino-1,2,4-triazole.

[0014] Furthermore, one of 2,5-thiophene dicarboxaldehyde and terephthalaldehyde.

[0015] Further, one of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10 oxide and 10-hydroxy-9,10-dihydro-9-oxa-10-phosphaphenanthrene.

[0016] Furthermore, the specific preparation process of cobalt-based nano-montmorillonite is as follows:

[0017] Cobalt dichloride hexahydrate and deionized water were added to a reaction vessel and stirred to dissolve. Sodium-based nano-montmorillonite was then added and stirred at 500-600 r / min for 72-74 h. The mixture was then filtered, washed, and dried to obtain cobalt-based nano-montmorillonite.

[0018] Furthermore, the ratio of cobalt dichloride hexahydrate, deionized water, and sodium-based nano-montmorillonite is 58-70g: 5-6L: 300-400g.

[0019] Furthermore, the specific preparation process of diisocyanate prepolymers is as follows:

[0020] Diisocyanate compounds and N,N-dimethylacetamide were added to a reaction vessel. Methyl ethyl ketone oxime was added to the reaction vessel under nitrogen protection and at 50-55°C. The reaction was maintained at this temperature for 1-2 hours. The content of isocyanate groups was determined, and the liquid was washed and collected to obtain the diisocyanate prepolymer.

[0021] Furthermore, the ratio of diisocyanate compounds, N,N-dimethylacetamide, and methyl ethyl ketone oxime is 100-140g: 140-200mL: 39.2-60g.

[0022] Furthermore, the specific preparation process of flame-retardant modified polyisocyanate is as follows:

[0023] Trimethylolpropane and N,N-dimethylacetamide were added to a reactor, and the mixture was heated to 70-75°C under nitrogen protection. An organic bismuth catalyst was added dropwise, and a diisocyanate prepolymer was added dropwise over 20-30 minutes. The mixture was kept at 70-75°C for 2.5-3.5 hours, and the isocyanate content was determined to be 0. Pyromellitic anhydride and a 0.5 g / mL solution of 2,2-dimethylolpropionic acid and N,N-dimethylacetamide were added. The mixture was kept at 80-90°C for 2-3 hours, and then cooled to 70-75°C. A diisocyanate compound was added, and the mixture was reacted for 30-60 minutes. The isocyanate group content was measured. A nanocomposite flame retardant was added, and the mixture was reacted for 1-2 hours. Methyl ethyl ketone oxime was added, and the mixture was reacted for 1-2 hours. Triethylamine was added, and the mixture was stirred for 1-2 hours to obtain flame-retardant modified polyisocyanate.

[0024] Furthermore, the ratio of trimethylolpropane, N,N-dimethylacetamide, organic bismuth catalyst, diisocyanate prepolymer, pyromellitic anhydride, 2,2-dimethylolpropionic acid N,N-dimethylacetamide solution, diisocyanate compound, nanocomposite flame retardant, methyl ethyl ketone oxime and triethylamine is 60-100g: 100-180mL: 0.5-0.7mL: 380-600mL: 65-79g: 10-15mL: 60-70g: 12-20g: 21-31g: 60-70g.

[0025] Furthermore, diisocyanate compounds include one of isophorone diisocyanate and hexamethylene diisocyanate.

[0026] The beneficial effects of this invention are:

[0027] 1. In this invention, the nanocomposite flame retardant achieves chemical grafting through the reaction of hydroxyl groups with isocyanate groups in the polyisocyanate chain. It can be uniformly dispersed in a hydrophilic system, avoiding agglomeration and sedimentation caused by physical mixing. This not only improves the dispersibility of the nanocomposite flame retardant in the polymer, but also enhances the interfacial bonding force between the two. 2,2-Dimethylolpropionic acid covalently grafts carboxyl groups onto the polyester side chains of polyisocyanate through esterification reactions of hydroxyl groups with pyromellitic anhydride and trimethylolpropane. After neutralization, it is converted into carboxylate, improving its dispersibility in water. The branched main chain of the comb-type polyisocyanate and the side chain polyester units form a large steric hindrance, further weakening the van der Waals forces between molecules, enhancing the repulsive effect between particles, and preventing molecular aggregation. This allows the flame-retardant modified polyisocyanate to form a stable and uniform dispersion in deionized water, thereby extending the shelf life.

[0028] 2. The nanocomposite flame retardant in this invention contains nitrogen, phosphorus, and sulfur sources, which are intercalated between the layers of cobalt-based nano-montmorillonite. DOPO, upon thermal decomposition, produces polyphosphoric acid, whose strong dehydrating properties catalyze the rapid dehydration of the substrate into char, forming a dense, expanded char layer. The layered structure of the cobalt-based nano-montmorillonite further blocks heat and oxygen transfer and enhances the dispersion of each component between layers. The nitrogen source produces non-combustible gases such as nitrogen during combustion, diluting the concentration of combustible gases in the flame zone. The thiophene ring of 2,5-thiophene dicarboxaldehyde releases gases such as hydrogen sulfide and sulfur dioxide upon combustion, which can capture active free radicals such as H· and ·OH, interrupting the combustion chain reaction. Simultaneously, cobalt ions catalyze the oxidation of carbon monoxide to carbon dioxide, reducing the release of toxic gases. Finally, it is grafted into the polymer, resulting in a uniform distribution of flame-retardant particles and achieving a good flame-retardant effect.

[0029] 3. In this invention, the nanocomposite flame retardant is chemically grafted onto the polyisocyanate segments. This covalent bonding significantly enhances the interfacial bonding force between the nanoparticles and the polymer, becoming a concentrated absorption point for impact stress. Impact energy is absorbed through particle deformation and interfacial slippage. The branched main chain and side chain polyester units of the comb-type polyisocyanate form a rich molecular chain entanglement structure. Its flexible segments can deform under impact, dispersing local impact stress to the entire molecular network, avoiding stress concentration, improving the impact resistance of the flame-retardant modified polyisocyanate, avoiding mechanical degradation, and achieving a balance in flame-retardant mechanics. The polyester segments introduced by pyromellitic anhydride contain rigid benzene rings, which synergistically enhance the thermal stability of the flame-retardant modified polyisocyanate with the high thermal stability of the DOPO phosphaphenanthrene ring. Detailed Implementation

[0030] To further illustrate the technical means and effects of the present invention in achieving the intended purpose, the following detailed description of the specific implementation methods, features and effects of the present invention, in conjunction with preferred embodiments, is provided below.

[0031] Example 1: This example provides a method for preparing flame-retardant modified polyisocyanate, including the following steps:

[0032] S1: Add 58g of cobalt dichloride hexahydrate and 5L of deionized water to a reactor, stir to dissolve, add 300g of sodium-based nano-montmorillonite, stir at 500r / min for 72h, intercalate cobalt ions into the interlayer through ion exchange, filter, wash 3 times with distilled water, freeze dry for 24h, grind, and pass through a 200-mesh sieve to obtain cobalt-based nano-montmorillonite.

[0033] S2: 1 L of N,N-dimethylformamide and 31 g of 2,4-diamino-6-phenyl-1,3,5-triazine were heated to 100 °C, and then 22.7 g of 2,5-thiophene dicarboxaldehyde was slowly added. The mixture was refluxed and stirred for 6 h. The amino group in 2,4-diamino-6-phenyl-1,3,5-triazine and the aldehyde group in 2,5-thiophene dicarboxaldehyde underwent a Schiff base condensation reaction to generate a nitrogen-containing intermediate. Then, 15 g of cobalt-based nano-montmorillonite was added, and the mixture was stirred at 800 r / min for 6 h. Finally, 108 g of [unspecified substance] was added. 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10 oxide (DOPO) was stirred for 18 h. The pH bond of DOPO underwent a nucleophilic addition reaction with the C=N bond of the nitrogen-containing intermediate and intercalated into the interlayer of cobalt-based nano-montmorillonite. After cooling to room temperature, the reaction solution was dropped into 10 L of anhydrous ethanol, and the product precipitated. After centrifugation and filtration, the precipitate was washed three times with ethanol and dried under vacuum at 70 °C for 24 h to obtain the nano-composite flame retardant.

[0034] S3: Add 100g of isophorone diisocyanate and 140mL of N,N-dimethylacetamide to a reaction vessel. Under nitrogen protection and at 50℃, add 39.2g of methyl ethyl ketone oxime to the reaction vessel and keep the reaction at this temperature for 1h. The oxime group in the methyl ethyl ketone oxime undergoes a nucleophilic addition reaction with one isocyanate group in the isophorone diisocyanate. When the content of isocyanate groups in the system reaches the theoretical value (determined by dibutylamine titration), pour out the solution, rinse with 100mL of N,N-dimethylacetamide in small amounts several times, and collect the liquid to obtain a semi-closed diisocyanate prepolymer.

[0035] S4: Add 60g of trimethylolpropane and 100mL of N,N-dimethylacetamide to a reactor. Heat to 70℃ under nitrogen protection, then add 0.5mL of organic bismuth catalyst. Add 380mL of diisocyanate prepolymer to the reactor over 20 minutes. Maintain the temperature at 70℃ for 2.5 hours. When the isocyanate content reaches 0 (determined by dibutylamine titration), add 65g of pyromellitic anhydride and 10mL of a 0.5g / mL solution of 2,2-dimethylolpropionic acid and N,N-dimethylacetamide. Continue heating at 80℃. The mixture was kept at a constant temperature for 2 hours, then cooled to 70°C. 60g of isophorone diisocyanate was added, and the reaction was carried out for 30 minutes. The isocyanate group content was measured (using dibutylamine titration). When the isocyanate group content was about 30% of the initial value, 12g of nanocomposite flame retardant was added, and the reaction was carried out for 1 hour. Then, 21g of methyl ethyl ketone oxime was added, and the reaction was carried out for 1 hour. The isocyanate content was measured again. When there were no free isocyanate groups in the system, 60g of triethylamine was added for neutralization, and the mixture was stirred for 1 hour to obtain flame-retardant modified polyisocyanate.

[0036] Under the catalysis of an organobismuth catalyst, the hydroxyl groups of trimethylolpropane and the isocyanate groups in the diisocyanate prepolymer undergo an addition reaction to form urethane bonds, generating a diol intermediate with a trimethylolpropane backbone in the main chain and blocked NCO side chains. This provides hydroxyl sites for the subsequent introduction of polyester segments. Pyromellitic anhydride undergoes ring-opening and esterification with the terminal hydroxyl groups of the diol intermediate and the hydroxyl groups of 2,2-dimethylolpropionic acid, introducing functional polyester side chains into the comb-like structure. Isophorone diisocyanate undergoes an addition reaction with the terminal hydroxyl groups of the polyester segments and the hydroxyl groups of the nanocomposite flame retardant to generate urethane. Methyl ethyl ketone oxime is then used as a blocking agent to block the free isocyanate groups, eliminating the free isocyanate groups. Triethylamine is added to form hydrophilic carboxyl groups with carboxyl groups, enhancing the repulsive force between polymer particles and improving water dispersibility.

[0037] Example 2: This example provides a method for preparing flame-retardant modified polyisocyanate. The difference from Example 1 is that in step S2, 3,5-diamino-1,2,4-triazole is used instead of 2,4-diamino-6-phenyl-1,3,5-triazine to prepare flame-retardant modified polyisocyanate.

[0038] Example 3: This example provides a method for preparing flame-retardant modified polyisocyanate. The difference from Example 1 is that in step S2, terephthalaldehyde is used instead of 2,5-thiophene dicarboxaldehyde to prepare flame-retardant modified polyisocyanate.

[0039] Example 4: This example provides a method for preparing flame-retardant modified polyisocyanate. The difference from Example 1 is that in step S2, the ratio of N,N-dimethylformamide, 2,4-diamino-6-phenyl-1,3,5-triazine, 2,5-thiophene dicarboxaldehyde, cobalt-based nano-montmorillonite, and DOPO is 1.2L:40g:26.7g:20g:120g.

[0040] Example 5: This example provides a method for preparing flame-retardant modified polyisocyanate. The difference from Example 1 is that in step S4, the ratio of trimethylolpropane, N,N-dimethylacetamide, organic bismuth catalyst, diisocyanate prepolymer, pyromellitic anhydride, isophorone diisocyanate, nanocomposite flame retardant, methyl ethyl ketone oxime, and triethylamine is 100g:180mL:0.7mL:600mL:79g:70g:20g:31g:70g.

[0041] Example 6: This example provides a method for preparing flame-retardant modified polyisocyanate. The difference from Example 1 is that hexamethylene diisocyanate is used instead of isophorone diisocyanate in step S3.

[0042] Comparative Example 1: This comparative example provides a method for preparing flame-retardant modified polyisocyanate. The difference from Example 1 is that no nanocomposite flame retardant is added in step S4.

[0043] Comparative Example 2: This comparative example provides a method for preparing flame-retardant modified polyisocyanate. The difference from Example 1 is that cobalt-based nano-montmorillonite is not added in step S2.

[0044] Comparative Example 3: This comparative example provides a method for preparing flame-retardant modified polyisocyanate. The difference from Example 1 is that 2,2-dimethylolpropionic acid N,N-dimethylacetamide solution is not added in step S4.

[0045] The flame-retardant modified polyisocyanates prepared in Examples 1-6 and Comparative Examples 1-2 were subjected to performance tests:

[0046] Dispersion stability and storage stability test: Add 5.00g of flame-retardant modified polyisocyanate to 50.00g of deionized water, stir mechanically until homogeneous, and observe the dispersion of the solution.

[0047] Storage stability: Using an H1850 centrifuge, the sample was centrifuged at 4000 rpm for 15 minutes. The centrifuged sample was observed to see if any precipitation occurred. If no precipitation occurred, the storage stability of the flame-retardant modified polyisocyanate was considered to be more than 6 months.

[0048] The flame-retardant modified polyisocyanates prepared in Examples 1-6 and Comparative Examples 1-2 were mixed with waterborne hydroxyl acrylate resin (hydroxyl value 35, commercially available, purchased from Guangzhou Hengshike New Material Co., Ltd.) at a ratio of 1:5 and stirred for 5 minutes. The mixture was then coated onto tinplate using a 50 μm wire rod to prepare a coating. PTFE molds were used to fabricate specimens. After the coating and specimen surfaces were dried, they were baked at 140°C for 30 minutes to obtain cured samples. The samples were then subjected to the following tests:

[0049] Flame retardant performance testing: The limiting oxygen index (LOI) of the samples was tested using a JF-5 oxygen indexer (Nanjing Jiangning Instrument Factory, China) according to ASTM D2863-17a standard to analyze its LOI index at a specified oxygen concentration. The sample size was 80mm × 10mm × 4mm. The combustion of the samples was studied using a cone calorimeter (CCT) according to ISO 5660 standard. The sample size was 100mm × 100mm × 3mm, and the UL-94 rating in the vertical burning test was calculated.

[0050] Mechanical property testing: The mechanical properties of the samples were evaluated according to the testing specifications of GB / T 1040.2-2006 and GB / T 9341-2008. Notched impact testing was conducted on a PTM7000 pendulum impact testing machine with a constant impact energy of 55J. All test samples were 80mm × 10mm × 4mm in size. Each test was repeated 5 times, and the arithmetic mean of the 5 results was measured.

[0051] The test results are shown in the table below:

[0052] Table 1 Performance Test Overview

[0053] project Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 Comparative Example 3 Dispersion state in deionized water Clarifying liquid Clarifying liquid Clarifying liquid Clarifying liquid Clarifying liquid Clarifying liquid Clarifying liquid Clarifying liquid suspension Storage stability / month ＞6 ＞6 ＞6 ＞6 ＞6 ＞6 ＞6 ＞6 ＜6 LOI (vol%) 45 41 39 44 45 43 18 27 44 UL-94 rating V-0 V-0 V-0 V-0 V-0 V-0 NO V-1 V-0 <![CDATA[Impact strength (kJ / m 2 )]]> 15.93 15.85 15.91 15.81 15.86 15.87 1.25 9.84 14.54

[0054] As shown in Table 1, the flame-retardant modified polyisocyanates prepared in Examples 1-6 exhibited good dispersion and long storage time in deionized water, while Comparative Example 3 showed poor dispersion and short storage time in deionized water. This indicates that 2,2-dimethylolpropionic acid can increase the dispersibility of flame-retardant modified polyisocyanates in water. The grafting of nanocomposite flame retardants onto the polymer improves the dispersion uniformity. In addition, the comb-like structure can enhance the repulsive force between polymer particles, improve water dispersibility, and thus extend the storage time.

[0055] The LOI indexes in Examples 1 and 4-6 were higher than those in Examples 2-3 and Comparative Examples 1-2, indicating that the abundant nitrogen source in 2,4-diamino-6-phenyl-1,3,5-triazine, the sulfur source in 2,5-thiophene dicarboxaldehyde, and the synergistic effect of cobalt-based montmorillonite improved the flame retardant performance of the flame-retardant modified polyisocyanate, and there was no dripping (UL-94 rating of V-0). This may be because the cobalt-based montmorillonite in the nanocomposite flame retardant has a layered structure, which can isolate the diffusion of smoke particles, and the cobalt ions play a catalytic role in the conversion of carbon monoxide to carbon dioxide, thus reducing the amount of carbon monoxide released.

[0056] The impact strength in Examples 1-6 was greater than that in Comparative Examples 1-2, indicating that the hydroxyl groups of the nanocomposite flame retardant react with the isocyanate groups of the polyisocyanate backbone and are grafted onto the polymer, increasing the interfacial bonding force and improving dispersibility. The comb-shaped polyisocyanate backbone provides toughness, and the two work together to improve the mechanical impact properties of the flame-retardant modified polyisocyanate.

[0057] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing flame-retardant modified polyisocyanate, characterized in that, Includes the following steps: Step 1: A nitrogen-containing intermediate is generated by grafting a nitrogen-containing diamine and an aldehyde donor through a Schiff base condensation reaction, and then intercalated between cobalt-based nano-montmorillonite layers through adsorption. A phosphorus-containing oxide is grafted onto the nitrogen-containing intermediate through a nucleophilic addition reaction to obtain a nano-composite flame retardant. Step 2: Graft semi-closed diisocyanate prepolymers with trimethylolpropane, then introduce them into the polyester side chains through esterification with pyromellitic anhydride and 2,2-dimethylolpropionic acid. The isocyanate groups of the diisocyanate prepolymers and the terminal hydroxyl groups of the polyester side chains and the hydroxyl groups of the nanocomposite flame retardant undergo addition reactions. After end-capping and neutralization, flame-retardant modified polyisocyanate is obtained. The specific preparation process of the nanocomposite flame retardant mentioned in step one is as follows: Add 1-1.2 L N,N-dimethylformamide and 31-40 g of diamine containing a nitrogen source to a reaction vessel, heat to 100-105℃, then slowly add 22.7-26.7 g of aldehyde donor, reflux and stir for 6-8 h to generate a nitrogen-containing intermediate, then add 15-20 g of cobalt-based nano-montmorillonite, stir at 800-1000 r / min for 6-8 h, add 108-120 g of phosphorus oxide, continue stirring for 18-20 h, cool to room temperature, dropwise into 10-15 L of anhydrous ethanol, the product precipitates, filter, wash, and dry to obtain a nanocomposite flame retardant; The specific preparation process of the flame-retardant modified polyisocyanate in step two is as follows: Add 60-100g of trimethylolpropane and 100-180mL of N,N-dimethylacetamide to a reaction vessel. Under nitrogen protection, heat to 70-75℃. Add 0.5-0.7mL of organic bismuth catalyst dropwise. Over 20-30 minutes, add 380-600mL of diisocyanate prepolymer to the reaction vessel. Maintain the temperature at 70-75℃ for 2.5-3.5 hours. Once the isocyanate content is determined to be 0, add 65-79g of pyromellitic anhydride and 10-15mL of 0.5g / mL 2,2-dimethylolpropane. A solution of N,N-dimethylacetamide methylpropionic acid was kept at 80-90℃ for 2-3 hours, then cooled to 70-75℃. 60-70g of a diisocyanate compound was added, and the reaction was carried out for 30-60 minutes. The isocyanate group content was measured. 12-20g of a nanocomposite flame retardant was added, and the reaction was carried out for 1-2 hours. 21-31g of methyl ethyl ketone oxime was added, and the reaction was carried out for 1-2 hours. 60-70g of triethylamine was added, and the mixture was stirred for 1-2 hours to obtain a flame-retardant modified polyisocyanate. The specific preparation process of the diisocyanate prepolymer is as follows: Diisocyanate compounds and N,N-dimethylacetamide were added to a reaction vessel. Methyl ethyl ketone oxime was added to the reaction vessel under nitrogen protection and at 50-55°C. The reaction was maintained at this temperature for 1-2 hours. The content of isocyanate groups was determined. The liquid was washed and collected to obtain the diisocyanate prepolymer.

2. The method for preparing flame-retardant modified polyisocyanate according to claim 1, characterized in that, The nitrogen-containing diamine includes one of 2,4-diamino-6-phenyl-1,3,5-triazine, 4,4'-diaminodiphenyl sulfone, and 3,5-diamino-1,2,4-triazole; The aldehyde donor includes one of 2,5-thiophene dicarboxaldehyde and terephthalaldehyde; The phosphorus-containing oxide includes one of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10 oxide and 10-hydroxy-9,10-dihydro-9-oxa-10-phosphaphenanthrene.

3. The method for preparing flame-retardant modified polyisocyanate according to claim 1, characterized in that, The specific preparation process of the cobalt-based nano-montmorillonite is as follows: Cobalt dichloride hexahydrate and deionized water were added to a reaction vessel and stirred to dissolve. Sodium-based nano-montmorillonite was then added and stirred at 500-600 r / min for 72-74 h. The mixture was then filtered, washed, and dried to obtain cobalt-based nano-montmorillonite.

4. The method for preparing a flame-retardant modified polyisocyanate according to claim 3, characterized in that, The ratio of cobalt dichloride hexahydrate, deionized water, and sodium-based nano-montmorillonite is 58-70g: 5-6L: 300-400g.

5. The method for preparing a flame-retardant modified polyisocyanate according to claim 1, characterized in that, The ratio of the diisocyanate compound, N,N-dimethylacetamide, and methyl ethyl ketone oxime is 100-140g: 140-200mL: 39.2-60g.

6. The method for preparing a flame-retardant modified polyisocyanate according to claim 5, characterized in that, The diisocyanate compound is one of isophorone diisocyanate and hexamethylene diisocyanate.