High-temperature-resistant flame-retardant polyurethane resin and preparation method thereof

Abstract

The invention relates to the technical field of high polymer materials, in particular to high-temperature-resistant flame-retardant polyurethane resin and a preparation method thereof. The polyurethane resin is prepared from the following components in parts by weight: 35 to 45 parts of polyethersulfone polyol, 80 to 120 parts of diphenylmethane diisocyanate, 15 to 20 parts of a phosphorus-containing flame retardant, 4 to 6 parts of 1, 4-butanediol, 20 to 30 parts of nano aluminum hydroxide, 1 to 3 parts of gamma-aminopropyltriethoxysilane, 8 to 15 parts of melamine cyanurate, 40 to 60 parts of N, N-dimethylformamide and 0.1 to 0.3 part of dibutyltin dilaurate. The polyurethane resin with high-temperature-resistant and flame-retardant characteristics is prepared by adopting three innovative technologies of rigid chain segment design, chemical bonding flame retardance and nanofiller modification. When the material is applied to the field of fireproof coatings, the material can replace traditional coatings and has high temperature resistance and flame retardance at the same time.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a high-temperature resistant and flame-retardant polyurethane resin and its preparation method. Background Technology

[0002] Ordinary polyurethane resin, as an important coating material widely used in many fields, has demonstrated outstanding performance and a very broad application prospect in various specific application scenarios such as wood coatings, floor coatings, and building waterproof coatings. Its excellent performance has led to its widespread recognition and use in various engineering projects. However, although ordinary polyurethane resin performs well in normal environments, its chemical structure becomes relatively fragile under extreme conditions such as high temperatures or fires, lacking sufficient high-temperature resistance and easily undergoing decomposition reactions. This decomposition not only significantly reduces the physical and chemical properties of the coating itself, losing its original protective and decorative functions and making the coating extremely fragile, but more seriously, the decomposition process may also release substances harmful to human health and the environment, such as toxic gases and volatile organic compounds. This defect undoubtedly limits the widespread application of ordinary polyurethane resin in special environments with high temperature requirements and high-temperature risks, restricting its application in certain key areas and making it difficult to meet higher performance standards. Summary of the Invention

[0003] In view of this, the purpose of this invention is to provide a high-temperature resistant and flame-retardant polyurethane resin and its preparation method, so as to solve the problems of instability in high-temperature environments and poor flame-retardant effect of existing traditional polyurethane resins.

[0004] To achieve the above objectives, the present invention provides a high-temperature resistant and flame-retardant polyurethane resin, comprising the following raw materials in parts by weight: 35-45 parts of polyethersulfone polyol, 80-120 parts of diphenylmethane diisocyanate, 15-20 parts of flame retardant, 4-6 parts of 1,4-butanediol, 20-30 parts of nano-aluminum hydroxide, 1-3 parts of γ-aminopropyltriethoxysilane, 8-15 parts of melamine cyanurate, 40-60 parts of N,N-dimethylformamide, and 0.1-0.3 parts of dibutyltin dilaurate.

[0005] The specific preparation method of a high-temperature resistant and flame-retardant polyurethane resin is as follows: (1) Under nitrogen protection, polyethersulfone polyol and diphenylmethane diisocyanate were added to a three-necked flask, stirred at 80°C at a stirring speed of 300 rpm, and the reaction was carried out for 2 hours. The NCO content was then determined. The hydroxyl groups of the polyol and the cyanate groups of the diisocyanate underwent a stepwise polymerization reaction to obtain a prepolymer containing urethane-NH-CO-O- segments.

[0006] (2) The prepolymer was cooled to 60°C, and phosphorus-containing flame retardant and 1,4-butanediol were added sequentially. The stirring speed was reduced to 200 rpm, and then dibutyltin dilaurate was added. The reaction was continued for 1 h, and the NCO content was measured again. The -NCO end group of the prepolymer first reacts with the hydroxyl group of the phosphorus-containing flame retardant, and then undergoes a chain extension reaction with 1,4-butanediol to chemically bond the flame retardant structure to the polyurethane backbone.

[0007] (3) Nano-aluminum hydroxide and γ-aminopropyltriethoxysilane were ultrasonically dispersed in ethanol solvent at 40 kHz for 30 min, and dried at 80 °C to obtain modified nanomaterials. The modified nanomaterials and melamine cyanurate were then added to the prepolymer. The mixture was first stirred at a low speed of 500 rpm for 10 min, then sheared at a high speed of 5000 rpm for 30 min, and ultrasonicated for 20 min. γ-aminopropyltriethoxysilane formed an organic layer on the surface of the nano-aluminum hydroxide. Low-speed stirring and high-speed shearing prevented agglomeration.

[0008] (4) Add N,N-dimethylformamide to adjust the viscosity to 600±100cP, -0.1MPa, vacuum degassing for 30min, and then remove agglomerated particles through a 200-mesh stainless steel screen to obtain a uniform coating slurry.

[0009] Preferably, in (1), all raw materials need to be dehydrated and vacuum dried at 110°C before use. This step is to ensure that the coating is free of bubbles and has good adhesion. The raw materials selected are polyethersulfone polyol (Evonik PESU-diol 2000) with a weight average molecular weight of 1900-2100 and a hydroxyl value of 56 mg KOH / g and diphenylmethane diisocyanate (Wanhua Chemical MDI-100) with an industrial grade -NCO content ≥30%. The molar ratio of -NCO:-OH in the raw materials is controlled to be 1.1-1.2 to make the crosslinking network uniform and balance the heat resistance and flexibility.

[0010] Preferably, (1) the temperature of the prepolymer generation process is controlled at around 80°C to avoid side reactions from occurring at excessively high temperatures.

[0011] Preferably, the -NCO content is determined by the di-n-butylamine method. (1) The -NCO content must be strictly controlled at 6.3-6.7% to ensure that the prepolymer has a suitable molecular weight and sufficient active end groups for subsequent chain extension. (2) The -NCO content is close to zero, indicating that the reaction is complete.

[0012] Preferably, (2) the phosphorus-containing flame retardant refers to 10-(2,5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphaphenanthrene-10-oxide, with the molecular formula C 18 H 13O4P is a reactive phosphorus-based flame retardant DOPO-HQ with a phosphorus content greater than 12%. The amount of phosphorus added must ensure that the total mass fraction of phosphorus in the system is 1.5 ± 0.1% of the mass of polyethersulfone polyol. Its hydroxyl groups react with the -NCO end groups of the prepolymer to attach the flame retardant structure to the main chain.

[0013] Preferably, in (2), the molar ratio of 1,4-butanediol with a purity ≥99.5% and bifunctionality to -NCO is 0.4. The introduction of the short-chain structure of 1,4-butanediol enhances intermolecular forces and improves heat resistance.

[0014] Preferably, the amount of catalyst added is 0.05-0.15% of the total resin mass, which makes the reaction complete and controllable. This is the optimal balance point between reaction efficiency, material performance and safety. Excess or deficiency will lead to a significant decrease in key properties (heat resistance, mechanical properties, flame retardancy).

[0015] Preferably, (3) by controlling the particle size and volume fraction of nano-aluminum hydroxide, the system viscosity increase and mechanical property decrease caused by filler agglomeration are avoided while ensuring flame retardant performance. γ-aminopropyltriethoxysilane is used to modify the surface of nano-aluminum hydroxide, and the interfacial compatibility between the filler and the resin matrix is ​​improved through chemical bonding, thereby enhancing the mechanical strength and thermal stability of the composite material. Ethanol solvent is used to provide a suitable medium environment for the hydrolysis of γ-aminopropyltriethoxysilane and the dispersion of nano-aluminum hydroxide. The modified nanomaterials are uniformly distributed in the resin system through an ultrasonic-shear synergistic dispersion process, giving full play to their flame retardant and reinforcing effects.

[0016] Preferably, in (3), the molar ratio of nitrogen / phosphorus of industrial-grade melamine cyanurate with a nitrogen content ≥40% and a particle size <10μm to phosphorus-containing flame retardant is 1:1-1.5:1. When melamine cyanurate is heated and decomposed during the reaction, it releases nitrogen and ammonia, which dilutes oxygen and achieves a flame retardant effect.

[0017] Preferably, (4) is N,N-dimethylformamide DMF solvent (Chinese pharmaceutical chromatographic grade DMF) with a purity ≥99.9%, moisture <0.05%, and solid content of 60-70%. It exists in the coating slurry to adjust the viscosity and ensure uniform dispersion of fillers, affecting the leveling properties of the coating film and balancing the workability and filler sedimentation risk. Note that before use, the filtered coating needs to be applied to the substrate and heat-treated at 80°C for 1 hour to allow the DMF to fully evaporate, otherwise the heat resistance of the coating will be reduced. After the DMF has basically evaporated, the temperature is increased to trigger the resin crosslinking reaction, forming the final performance. The coating does not crack or peel off under harsh environments such as humid heat and cold cycles.

[0018] The beneficial effects of this invention are: 1. The polyurethane resin of this invention exhibits excellent high-temperature resistance. By introducing a rigid polyethersulfone polyol and constructing a molecular structure with high crosslinking density, the initial decomposition temperature of the resin is significantly increased to above 320°C. Even when exposed to a high-temperature environment of 150°C for a long period of time, the resin remains stable, effectively overcoming the technical problem of traditional polyurethanes being prone to softening and decomposition under high-temperature conditions, thereby greatly improving the reliability of the material in high-temperature environments.

[0019] 2. The polyurethane resin of this invention possesses highly efficient intrinsic flame-retardant properties. By chemically bonding the phosphorus-containing flame retardant directly into the main chain structure of the polyurethane, the migration and precipitation of the phosphorus-containing flame retardant during use are effectively avoided, thus endowing the material with durable flame-retardant performance. Furthermore, the combination of the phosphorus-nitrogen synergistic flame-retardant system and melamine cyanurate can generate a dense expanded char layer during combustion, effectively isolating oxygen and significantly suppressing smoke. The release of heat reduces the heat release rate by more than 40%, greatly improving the fire safety of the material.

[0020] 3. The polyurethane resin of this invention significantly enhances mechanical properties and durability. By modifying nano-aluminum hydroxide with a silane coupling agent, it is uniformly dispersed in the resin matrix, thereby effectively improving the hardness and adhesion of the coating. Simultaneously, the endothermic decomposition characteristics of the modified nanomaterials at high temperatures further strengthen the flame-retardant effect of the material, making this polyurethane resin exhibit excellent mechanical properties and durability, suitable for a variety of demanding applications.

[0021] 4. The polyurethane resin of this invention has excellent environmental performance. Employing a halogen-free flame retardant system completely avoids the toxicity problems associated with traditional bromine-based flame retardants, thus not only improving the safety and environmental friendliness of the material but also laying a solid foundation for its widespread application in green building, eco-friendly home furnishings, and other fields. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0023] Example 1: A specific preparation method of a high-temperature resistant and flame-retardant polyurethane resin, comprising the following steps: (1) Under nitrogen protection, polyethersulfone polyol (100 parts) and diphenylmethane diisocyanate (35 parts) were added to a three-necked flask, stirred at 80°C at a stirring speed of 300 rpm, and after 2 hours of reaction, a sample was taken and the -NCO content was controlled at 6.3% to obtain the prepolymer.

[0024] (2) Cool the prepolymer to 60°C, add phosphorus-containing flame retardant (16 parts) and 1,4-butanediol (3.5 parts) in sequence, reduce the stirring speed to 200 rpm, add dibutyltin dilaurate (0.12 parts), continue the reaction for 1 h, and measure the -NCO content again.

[0025] (3) Disperse 20 parts of nano aluminum hydroxide and 1.6 parts of γ-aminopropyltriethoxysilane in ethanol by ultrasonication at 40 kHz for 30 min, and dry at 80 °C to obtain modified nanomaterials. Then add the modified nanomaterials and 8 parts of melamine cyanurate to the prepolymer, stir at low speed of 500 rpm for 10 min, then shear at high speed of 5000 rpm for 30 min, and sonicate for 20 min.

[0026] (4) Add N,N-dimethylformamide (45 parts) to adjust the viscosity to 600±100 cP, -0.1 MPa, and vacuum degas for 30 min. Then remove agglomerated particles through a 200-mesh stainless steel sieve to obtain a uniform coating slurry.

[0027] Example 2: A specific preparation method of a high-temperature resistant and flame-retardant polyurethane resin, comprising the following steps: (1) Under nitrogen protection, polyethersulfone polyol (100 parts) and diphenylmethane diisocyanate (40 parts) were added to a three-necked flask, stirred and mixed at 80°C at a stirring speed of 300 rpm, and after 2 hours of reaction, a sample was taken and the -NCO content was controlled at 6.5% to obtain the prepolymer.

[0028] (2) Cool the prepolymer to 60°C, add phosphorus-containing flame retardant (18 parts) and 1,4-butanediol (5 parts) in sequence, reduce the stirring speed to 200 rpm, add dibutyltin dilaurate (0.15 parts), continue the reaction for 1 h, and measure the -NCO content again.

[0029] (3) Disperse 22 parts of nano aluminum hydroxide and 2.2 parts of γ-aminopropyltriethoxysilane in ethanol by ultrasonication at 40 kHz for 30 min, and dry at 80 °C to obtain modified nanomaterials. Then add the modified nanomaterials and 10 parts of melamine cyanurate to the prepolymer, stir at low speed of 500 rpm for 10 min, then shear at high speed of 5000 rpm for 30 min, and sonicate for 20 min.

[0030] (4) Add N,N-dimethylformamide (50 parts) to adjust the viscosity to 600±100 cP, -0.1 MPa, and vacuum degas for 30 min. Then remove agglomerated particles through a 200-mesh stainless steel sieve to obtain a uniform coating slurry.

[0031] Example 3: A specific preparation method of a high-temperature resistant and flame-retardant polyurethane resin, comprising the following steps: (1) Under nitrogen protection, polyethersulfone polyol (100 parts) and diphenylmethane diisocyanate (45 parts) were added to a three-necked flask, stirred at 80°C at a stirring speed of 300 rpm, and after 2 hours of reaction, a sample was taken and the -NCO content was controlled at 6.7% to obtain the prepolymer.

[0032] (2) Cool the prepolymer to 60°C, add phosphorus-containing flame retardant (20 parts) and 1,4-butanediol (6.5 parts) in sequence, reduce the stirring speed to 200 rpm, add dibutyltin dilaurate (0.18 parts), continue the reaction for 1 h, and measure the -NCO content again.

[0033] (3) Disperse 25 parts of nano aluminum hydroxide and 2.5 parts of γ-aminopropyltriethoxysilane in ethanol by ultrasonication at 40 kHz for 30 min, and dry at 80 °C to obtain modified nanomaterials. Then add the modified nanomaterials and 12 parts of melamine cyanurate to the prepolymer, stir at low speed of 500 rpm for 10 min, then shear at high speed of 5000 rpm for 30 min, and sonicate for 20 min.

[0034] (4) Add N,N-dimethylformamide (55 parts) to adjust the viscosity to 600±100 cP, -0.1 MPa, and vacuum degas for 30 min. Then remove agglomerated particles through a 200-mesh stainless steel sieve to obtain a uniform coating slurry.

[0035] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that unmodified nano-aluminum hydroxide was used. The specific process is as follows: (1) Under nitrogen protection, polyethersulfone polyol and diphenylmethane diisocyanate were added to a three-necked flask, stirred at 80°C at a stirring speed of 300 rpm, and after 2 hours of reaction, a sample was taken to determine the -NCO content and the prepolymer was obtained.

[0036] (2) Cool the prepolymer to 60°C, add phosphorus-containing flame retardant and 1,4-butanediol in sequence, reduce the stirring speed to 200 rpm, add dibutyltin dilaurate, continue the reaction for 1 h, and measure the -NCO content again.

[0037] (3) Add unmodified nano aluminum hydroxide and melamine cyanurate to the prepolymer, stir at a low speed of 500 rpm for 10 min, then shear at a high speed of 5000 rpm for 30 min, and sonicate for 20 min.

[0038] (4) Add N,N-dimethylformamide to adjust the viscosity to 600±100cP, -0.1MPa, vacuum degassing for 30min, and then remove agglomerated particles through a 200-mesh stainless steel screen to obtain a uniform coating slurry.

[0039] Comparative Example 2: The difference between Comparative Example 2 and Example 3 is that the phosphorus-containing flame retardant DOPO-HQ is replaced with the additive flame retardant ammonium polyphosphate (APP).

[0040] Comparative Example 3: The difference between Comparative Example 3 and Example 3 is that melamine cyanurate is not used, and only phosphorus-containing flame retardants are retained.

[0041] Comparative Example 4: The difference between Comparative Example 4 and Example 3 is that the polyether polyol PPG-2000 is used instead of the polyether sulfone polyol.

[0042] Comparative Example 5: The difference between Comparative Example 5 and Example 3 is that N,N-dimethylformamide was increased to 80 parts to achieve a solid content of 45%.

[0043] Comparative Example 6: The difference between Comparative Example 6 and Example 3 is that the molar ratio of -NCO:-OH in diphenylmethane diisocyanate and polyethersulfone polyol is reduced to 0.9.

[0044] Performance testing: 1. Initial Decomposition Temperature Test: The cured coating samples from Examples 1-3 and Comparative Examples 1-7 were ground into uniform powder with a particle size ≤100μm and dehydrated in a vacuum drying oven at 80℃ for 24h until constant weight. A thermogravimetric analyzer (model: TAQ500) was used with a nitrogen flow rate of 50mL / min, a heating rate of 10℃ / min, and a temperature range of 30-800℃. 5.0±0.1mg of sample was weighed and placed in an alumina crucible. The curve of sample mass versus temperature was recorded, and the peak value of the curve was determined as the initial decomposition temperature. The experimental data are shown in Table 1.

[0045] 2. LOI Test: Cured samples from Examples 1-3 and Comparative Examples 1-7 were cut into standard sizes (100mm × 10mm × 3mm) and treated in a 50℃ drying oven for 24 hours to remove residual solvent. A limiting oxygen index (LOI) tester (model: JF-3) equipped with a high-precision oxygen / nitrogen mixing system with a flow control accuracy of ±0.1L / min was used. The sample was vertically fixed on the sample holder, and the total gas flow rate in the combustion chamber was adjusted to 10L / min. The "lifting method" was used for testing. The initial oxygen concentration was determined based on the estimated LOI value and adjusted in 5% increments. A propane flame with a height of 20mm was used to ignite the sample from the top for 30 seconds, and the combustion behavior was observed. The critical oxygen concentration is defined as the sample extinguishing when the burning length reaches 40mm or the burning time reaches 180s. The formula for calculating the critical oxygen concentration is:

[0046] 3. UL-94 Test: Cured samples from Examples 1-3 and Comparative Examples 1-7 were cut to standard dimensions (125mm × 13mm × 3mm) and conditioned for 48 hours at 23±2℃ and 50±5%RH. Test Apparatus: A UL-94 vertical combustion tester was used, equipped with a methane flame height of 20mm, a blue flame core length of 10mm, and a sample holder. The sample was fixed vertically, with its lower end 300mm from the absorbent cotton. First Ignition: The flame was removed after contacting the lower end of the sample for 10 seconds, and the afterflame time t1 was recorded. Second Ignition: Immediately after the afterflame extinguished, the sample was reignited for 10 seconds, and the afterflame time t2 and afterglow time t3 were recorded. Rating Standards: V-0: t1+t2≤10s, single t2≤5s, no ignition of absorbent cotton. V-1: t1+t2≤30s, single t2≤25s, no ignition of absorbent cotton. V-2 level: t1+t2≤30s, but it can ignite absorbent cotton. The experimental data are shown in Table 1.

[0047] 4. Tensile Strength Test: The cured samples from Examples 1-3 and Comparative Examples 1-7 were injection molded into dumbbell-shaped specimens (Type I, dimensions: total length 165mm, gauge length 50mm × 10mm × 4mm). The specimen surface was sanded with 600-grit sandpaper to eliminate processing defects. A universal testing machine (Instron 5967) equipped with a 50kN load sensor and a pneumatic clamping pressure of 0.6MPa was used. The test conditions were: tensile rate 50mm / min (ASTM D638 standard), gauge length 50mm, deformation measured using a non-contact video extensometer, ambient temperature 23±1℃, relative humidity 50±5%. The specimens were equilibrated under the test environment for 24 hours. The width and thickness of the gauge length were accurately measured using a digital caliper with an accuracy of 0.01mm. The specimens were clamped, maintaining an axial alignment deviation <0.5mm, preloaded to 0.1MPa to eliminate gaps, and stretched at the set rate until fracture. Tensile strength calculation formula: .

[0048] 5. Viscosity Test: Take 200 mL of each of the freshly prepared coating samples from Examples 1-3 and Comparative Examples 1-7, and equilibrate them in a constant temperature water bath at 25±0.1℃ for 2 hours before testing. Before use, gently stir at 500 rpm for 30 seconds to eliminate the influence of thixotropy. Use a rotational viscometer (Brookfield DV2T), a low-viscosity LV-3 and a high-viscosity LV-4 rotor, and a constant temperature water bath circulation system with an accuracy of ±0.1℃. Pour 600 mL of sample into a dedicated 75 mm diameter test cup, immerse the rotor to a depth of 45 mm on the scale, and test the shear rate at 60 rpm: 12.74 s. -1 Readings were taken after each rotational speed stabilized for 30 seconds. The temperature was maintained at 25±0.1℃ throughout the test. The direct instrument reading (700 cP) at 60 rpm was taken as the standard viscosity value. The experimental data are shown in Table 1.

[0049] 6. Smoke Density: Cured samples from Examples 1-3 and Comparative Examples 1-7 were processed into standard dimensions (75mm × 75mm × 3mm). Before testing, the samples were conditioned at 23±2℃ and 50±5%RH for 48 hours. The sample surface was uniformly sanded with 400-grit sandpaper to remove the oxide layer. A smoke density tester (NBS smoke chamber, conforming to ISO 5659-2) and a radiation cone heating device (radiation intensity 50kW / m²) were used. 2 The system includes a ±2% accuracy laser smoke density measurement system and a CO / CO2 detection gas analysis system. The sample is horizontally fixed on the sample holder, the radiant heating system is activated, and the sample is preheated to a stable state. A 25mm high methane flame is ignited, and the data acquisition system is simultaneously turned on. The test duration is 4 minutes. Smoke density calculation formula: Where V is the volume of the smoke chamber, A is the exposed area of ​​the sample, L is the optical path length, and I0 is the initial incident light intensity—the light intensity passing through the test cavity when there is no smoke, a constant value. I is the transmitted light intensity measured in real time; the light intensity passing through the test cavity changes over time when there is smoke. The transmitted light intensity is recorded every second by a photoelectric sensor, and the data acquisition system automatically calculates... Substituting the values ​​into the smoke density calculation formula, the experimental data are shown in Table 1.

[0050] Table 1 Sample Test Data

[0051] Data Analysis: As can be seen from the data in Table 1, Example 3 achieved a balance between high-temperature resistance, flame retardancy, and mechanical properties through the rigid segment design of polyethersulfone polyol, chemical bonding flame retardancy with phosphorus-containing flame retardants, and synergistic reinforcement with modified nanomaterials. The reasons for its excellent performance are as follows: High-temperature resistance: Initial decomposition temperature 330℃: The sulfone group -SO2- of the polyethersulfone polyol provides high thermal stability, and the optimized crosslinking density -NCO / -OH=1.2 inhibits high-temperature segment movement. High flame retardancy: The phosphorus-containing flame retardant is covalently bonded to the main chain, generating a dense phosphate carbon layer during combustion; melamine cyanurate decomposes to release nitrogen gas, achieving synergistic flame retardancy between the gas phase and condensed phase. Good mechanical properties: γ-aminopropyltriethoxysilane-modified nano-ATH is uniformly dispersed, and its strength is improved through physical reinforcement and hydrogen bonding.

[0052] Based on the initial decomposition temperature data, Examples 1-3 achieved initial decomposition temperatures of 310-330°C, which is attributed to the rigid segments and moderate crosslinking of the polyethersulfone polyol of this invention. In contrast, Comparative Example 4 had an initial decomposition temperature of only 260°C because ordinary polyether polyol segments are highly flexible, resulting in a significant decrease in thermal stability. Comparative Example 6 had a temperature of 280°C, with a low crosslinking density (-NCO / -OH = 0.9), leading to easy slippage of the molecular chains at high temperatures.

[0053] Limiting Oxygen Index (LOI) data showed that the flame retardancy of the examples was significantly better than that of the comparative examples. The phosphorus-containing flame retardant was chemically bonded to the polyurethane backbone, generating a dense phosphate char layer during combustion, effectively isolating oxygen. Melamine cyanurate decomposed upon heating, releasing nitrogen (N2), which diluted the combustible gas, achieving gas-phase flame retardancy. The phosphorus-nitrogen synergistic effect significantly improved the flame retardant efficiency. In Comparative Example 1, the use of unmodified nano-aluminum hydroxide resulted in uneven dispersion, leading to a decrease in endothermic flame retardant effect, discontinuous char layer, and increased oxygen permeability. Comparative Example 2 used the physically additive phosphorus-containing flame retardant APP instead of the phosphorus-containing flame retardant. The additive flame retardant easily migrated to the surface and could not form a stable protective layer during combustion. Furthermore, the decomposition product of APP, polyphosphoric acid, was ineffective for gas-phase flame retardancy. Compared to the examples, Comparative Example 3 did not add melamine cyanurate and relied solely on the condensed-phase flame retardancy of the phosphorus-containing flame retardant. The lack of nitrogen dilution resulted in insufficient gas-phase flame retardancy and a lower LOI. Comparative Example 4: Ordinary polyether polyols easily melt and drip at high temperatures, failing to form an effective char layer and lacking rigid chain segment support, resulting in the worst flame retardant performance. Comparative Example 5: Excessive N,N-dimethylformamide solvent, while not affecting the chemical structure of the phosphorus-containing flame retardant, reduces solid content, leading to insufficient coating density and uneven char layer thickness. Comparative Example 6: Low crosslinking density causes the molecular chains to easily decompose at high temperatures, resulting in insufficient char layer strength and decreased flame retardant efficiency.

[0054] According to the vertical combustion rating, in Example 3, the phosphorus-nitrogen synergistic system rapidly forms an expanded char layer during combustion, isolating oxygen and inhibiting dripping; the nano-aluminum hydroxide decomposes at 200-300℃, delaying combustion, and there is no ignition phenomenon. In Comparative Example 1, the unmodified aluminum hydroxide agglomerates, leading to local heat concentration, cracks in the char layer, and an extended combustion time of <30 seconds, with occasional dripping. Comparative Example 2 is rated V-2: the APP flame retardant migrates to the surface, producing dripping during combustion and igniting the cotton below, failing to meet the V-0 standard. Comparative Example 3 is rated V-1: without the gas-phase flame retardant effect of melamine cyanurate, there is a large amount of smoke during combustion, the char layer does not expand sufficiently, and the self-extinguishing time is >10 seconds. Comparative Example 4 is rated V-2: the ordinary polyol melts and drips severely at high temperatures, continues to burn, and ignites the cotton, exhibiting the worst flame retardant performance. Comparative Example 5 is rated V-0: although excessive solvent affects mechanical properties, the phosphorus-containing flame retardant has an intact chemical structure and can still pass the V-0 test. Comparative Example 6V-1: Low cross-linking density leads to insufficient char layer strength, local cracking in the later stages of combustion, and prolonged self-extinguishing time.

[0055] Based on tensile strength data, Example 1 achieved maximum strength through high crosslinking (-NCO / -OH = 1.2); Example 2 maintained flexibility through moderate crosslinking; and Example 3 found the optimal balance. The modified nanomaterials exhibit a reinforcing effect, with surface silane bonding enhancing interfacial interactions, nanoscale dispersion avoiding stress concentration, rigid particle support mechanisms, and molecular chain design. Rigid polyethersulfone segments provide skeletal support, while the flexible chain extender 1,4-butanediol regulates network elasticity. Phosphorus-containing flame retardants chemically bond to prevent plasticization. In Example 3, the modified nanomaterials are uniformly dispersed and reinforced by surface silane bonds with the resin. Comparative Example 1 uses unmodified aluminum hydroxide, leading to agglomeration and stress concentration points. Comparative Example 2 uses ammonium polyphosphate (APP) instead of DOPO-HQ; however, the poor compatibility of ammonium polyphosphate with the matrix results in stress concentration points. Comparative Example 3 lacks the melamine cyanurate synergist, resulting in an imperfect filler dispersion network. Comparative Example 4 uses excessively flexible ordinary polyol segments, significantly reducing strength. Comparative Example 5 uses excessive solvent, leading to incomplete curing and poor material density. Comparative Example 6 showed insufficient crosslinking density and weak molecular network carrying capacity.

[0056] Based on viscosity test data at 25℃, Example 3 demonstrated that by precisely controlling the dispersion state of the modified nanomaterials, a stable three-dimensional network structure was formed with the polyurethane prepolymer. Simultaneously, the amount of N,N-dimethylformamide ensured sufficient wetting of the filler while avoiding excessive dilution that could lead to viscosity decrease. The silane coupling agent on the surface of the modified nanomaterials formed strong hydrogen bonds with the resin matrix, significantly enhancing the system's cohesiveness. The solvent N,N-dimethylformamide formed a stable suspension system with the modified nanomaterials. In Comparative Example 1, the agglomeration of unmodified aluminum hydroxide led to a sharp increase in flow resistance. Comparative Example 2 used APP instead of DOPO-HQ; although the APP particles themselves did not affect viscosity, the lack of chemical bonding between DOPO-HQ and the resin weakened the structural strength of the system. Comparative Example 3 did not add melamine cyanurate synergist, resulting in defects in the nano-aluminum hydroxide dispersion network, uneven filler concentration in localized areas, and poor viscosity stability. Comparative Example 4 used ordinary polyether polyol, whose low-polarity molecular chains had weak encapsulation ability for the filler, failing to form an effective thickening structure. In Comparative Example 5, excess N,N-dimethylformamide reduced the solid content and weakened the filler network effect. In Comparative Example 6, insufficient crosslinking density (-NCO:-OH = 0.9) resulted in weak intermolecular chain interactions and limited viscosity improvement.

[0057] Based on smoke density test data, Example 3, through the phosphorus-nitrogen synergistic effect of DOPO-HQ and melamine cyanurate, generates a dense, expanded char layer during combustion, effectively isolating combustible gases from contact with oxygen. Simultaneously, the thermal decomposition of melamine cyanurate releases nitrogen (N2) and ammonia (NH3), diluting smoke particles and inhibiting free radical reactions, significantly reducing smoke generation. The endothermic decomposition of the modified nanomaterials delays material pyrolysis, further reducing smoke generation. Compared to Example 3, Comparative Example 1, due to uneven dispersion of unmodified aluminum hydroxide, experienced localized overheating during combustion, leading to char layer cracking and increased smoke emission. Comparative Example 2, using APP instead of DOPO-HQ, lacked the char-forming effect of phosphorus-based flame retardants, relying solely on physical flame retardancy, resulting in a surge in smoke. Comparative Example 3, without melamine cyanurate, produced mainly smoke as combustion products. Comparative Example 4, with the high-temperature decomposition of ordinary polyols, generated more volatile small molecules. Comparative Example 5, despite excess solvent, maintained a complete flame retardant system, resulting in slightly higher smoke generation than Example 3. Comparative Example 6: The low cross-linking density resulted in insufficient char layer strength, and the cracking in the later stages of combustion led to increased smoke release.

[0058] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A high-temperature resistant and flame-retardant polyurethane resin, characterized in that, It contains the following raw materials in parts by weight: 35-45 parts of polyethersulfone polyol, 80-120 parts of diphenylmethane diisocyanate, 15-20 parts of phosphorus-containing flame retardant, 4-6 parts of 1,4-butanediol, 20-30 parts of nano aluminum hydroxide, 1-3 parts of γ-aminopropyltriethoxysilane, 8-15 parts of melamine cyanurate, 40-60 parts of N,N-dimethylformamide, and 0.1-0.3 parts of dibutyltin dilaurate.

2. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 1, characterized in that, Includes the following steps: (1) Under nitrogen protection, polyethersulfone polyol and diphenylmethane diisocyanate were added to a three-necked flask, stirred at 80°C at a stirring speed of 300 rpm, and after 2 hours of reaction, a sample was taken to determine the NCO content and obtain the prepolymer. (2) Cool the prepolymer to 60°C, add phosphorus-containing flame retardant and 1,4-butanediol in sequence, reduce the stirring speed to 200 rpm, add dibutyltin dilaurate, continue the reaction for 1 h, and measure the NCO content again. (3) Nano aluminum hydroxide and γ-aminopropyltriethoxysilane were ultrasonically dispersed in ethanol solvent at 40 kHz for 30 min and dried at 80 °C to obtain modified nanomaterials. The modified nanomaterials and melamine cyanurate were then added to the prepolymer. The mixture was first stirred at a low speed of 500 rpm for 10 min, then sheared at a high speed of 5000 rpm for 30 min and ultrasonicated for 20 min. (4) Add N,N-dimethylformamide to adjust the viscosity to 600±100cP, -0.1MPa, vacuum degassing for 30min, and then remove agglomerated particles through a 200-mesh stainless steel screen to obtain a uniform coating slurry.

3. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 2, characterized in that, In the synthesis of the prepolymer in (1), the molar ratio of NCO:OH is controlled to be 1.1-1.

2. The raw materials are polyethersulfone polyol with a weight average molecular weight of 1900-2100 and a hydroxyl value of 54-58 mg KOH / g and industrial grade diphenylmethane diisocyanate with a -NCO content ≥30%. The raw materials need to be dehydrated and vacuum dried at 110°C before use.

4. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 2, characterized in that, The process of generating the NCO-terminated prepolymer (1) requires controlling the reaction temperature to be around 80°C to avoid excessively high temperatures.

5. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 2, characterized in that, The -NCO content was determined by the di-n-butylamine method. In step (1), the -NCO content was measured to be 6.3-6.7%, and the reaction was terminated. In step (2), the -NCO content was measured to be less than 0.5%.

6. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 2, characterized in that, The phosphorus-containing flame retardant mentioned in (2) refers to 10-(2,5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphaphenanthrene-10-oxide, with the molecular formula C. 18 H 13 O4P, a reactive phosphorus-based flame retardant with a phosphorus content greater than 12%, requires the addition amount to ensure that the total mass fraction of phosphorus in the system is 1.5 ± 0.1% of the mass of the polyethersulfone polyol.

7. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 2, characterized in that, In (2), the purity of 1,4-butanediol is ≥99.5%, its molar ratio with -NCO is 0.4, and the amount of catalyst added is 0.05-0.15% of the total mass of the system.

8. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 2, characterized in that, The nano-aluminum hydroxide in (3) has a particle size of 30-80 nm. As an inorganic flame retardant filler, its volume fraction in the resin system needs to be controlled below 15%. The nano-aluminum hydroxide needs to be surface modified by γ-aminopropyltriethoxysilane KH550. The amount of γ-aminopropyltriethoxysilane added is 5-10% of the mass of nano-aluminum hydroxide, and the amount of ethanol solvent is 3-5 times the mass of nano-aluminum hydroxide. The modification process adopts a dispersion process combining ultrasonic assistance and high-speed shearing to ensure uniform dispersion of nanoparticles in the resin matrix.

9. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 2, characterized in that, The (3) is industrial grade melamine cyanurate, which has a nitrogen content ≥40%, a particle size <10μm, and a nitrogen / phosphorus molar ratio of 1:1-1.5:1 with DOPO-HQ.

10. The method for preparing the high-temperature resistant and flame-retardant polyurethane resin according to claim 2, characterized in that, The solvent in (4) is N,N-dimethylformamide (DMF) with a purity of ≥99.9%, moisture content <0.05%, and a curing amount of 60-70%.

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