Preparation method of linear reaction type phosphorus-nitrogen-silicon synergistic flame-retardant solvent-free polyurethane coating

Abstract

The invention relates to the field of polyurethane, and discloses a preparation method of a linear reaction type phosphorus-nitrogen-silicon synergistic flame-retardant solvent-free polyurethane coating, which comprises the following steps: S1, carrying out exchange reaction on phosphonate and alcohol amine ester to obtain a phosphorus-nitrogen polyol intermediate; s2, carrying out condensation polymerization on the phosphorus-nitrogen polyol intermediate and chlorosilane to obtain phosphorus-nitrogen-silicon polyol; s3, mixing polyester / polyether polyol, a chain extender and an amine / metal catalyst to obtain a component A; mixing and reacting isocyanate, part of polyester / polyether polyol and phosphorus-nitrogen-silicon polyol to obtain a component B; mixing the component A and the component B, spraying the mixture on release paper, and curing. Through a synergistic flame-retardant mechanism of phosphorus, nitrogen and silicon, the balance of efficient flame retardance, mechanical properties and excellent coating properties is realized. The solvent-free system realizes zero emission of volatile organic compounds, and the linear reaction type design ensures that the mechanical properties and durability of the coating are not influenced.

Description

Technical Field

[0001] The present invention relates to the field of polyurethane, and in particular to a method for preparing a linear reactive phosphorus-nitrogen-silicon collaborative flame-retardant solvent-free polyurethane coating. Background Art

[0002] Solvent-free polyurethane (SFPU) emits fewer harmful gases than solvent-based polyurethanes. Compared to water-based polyurethanes, it cures faster and exhibits superior low-temperature performance. Consequently, it has found widespread application in areas such as electronic and electrical protection, automotive interiors, and fire protection in buildings and transportation. However, the high energy density of SFPU makes it susceptible to combustion at high temperatures or under flames, releasing large amounts of toxic fumes and heat, significantly increasing the risk of fire. Therefore, the development of highly effective flame-retardant solvent-free polyurethane coatings has become a key research area in this field.

[0003] Currently, additive flame retardants commonly used in solvent-free polyurethanes suffer from shortcomings such as poor compatibility and migration. The development of intrinsically flame-retardant solvent-free polyurethanes containing green flame-retardant elements such as phosphorus, nitrogen, and silicon is attracting significant attention due to their ability to enhance performance through specific flame-retardant mechanisms while also offering durability and non-migration properties. Phosphorus catalyzes carbonization, promoting the formation of a uniform, dense carbon layer on the surface of solvent-free polyurethanes, effectively isolating them from oxygen and heat. Nitrogen decomposes upon heating to produce inert gases, diluting oxygen and inhibiting combustion. Silicon, upon combustion, forms a highly stable, dense carbon layer that prevents the volatilization of combustibles and enhances flame retardancy. However, existing intrinsically flame-retardant solvent-free polyurethanes still have numerous drawbacks. The synthesis process is complex, and the introduction of flame-retardant monomers requires more precise reaction conditions, resulting in more steps and increased costs. This also increases equipment requirements and maintenance and cleaning costs, while side reactions can affect product appearance and performance. In terms of performance, the overall flame-retardant properties are not well-balanced, and the mechanisms may not fully function under certain conditions. The flame-retardant effect may diminish with long-term use, impacting the polyurethane's flexibility, mechanical strength, and thermal stability. In terms of cost, functional monomer raw materials are expensive, and synthesis requires special catalysts or additives. The complex process leads to increased energy consumption and scrap rates. Regarding environmental protection and regulations, production and use may produce harmful byproducts. Improper disposal of waste products can pollute the environment, and the formulation process must be constantly adjusted to comply with increasingly stringent regulations. In terms of application, processing performance is limited, the melting temperature range is narrow, and compatibility with other materials is poor, making it difficult to meet the extreme performance requirements of specialized fields such as aerospace and defense. Summary of the Invention

[0004] To address the above-mentioned technical problems, the present invention provides a method for preparing a linearly reactive phosphorus-nitrogen-silicon synergistic flame-retardant solvent-free polyurethane coating. The present invention achieves a balance between high-efficiency flame retardancy, mechanical properties, and excellent coating performance through the synergistic flame retardancy mechanism of phosphorus, nitrogen, and silicon, combined with the environmentally friendly properties of solvent-free polyurethane. The multi-element synergistic effect of phosphorus, nitrogen, and silicon significantly improves flame retardancy while avoiding the problems of poor compatibility and easy migration of traditional additive flame retardants. The solvent-free system achieves zero volatile organic compound (VOC) emissions, meeting environmental protection requirements, while the linear reactive design ensures that the mechanical properties and durability of the coating are not affected.

[0005] The specific technical solution of the present invention is: a method for preparing a linear reactive phosphorus-nitrogen-silicon synergistic flame-retardant solvent-free polyurethane coating, which comprises the following steps:

[0006] S1. Dissolving phosphonate and alcohol amine in a solvent, and performing an ester exchange reaction to obtain a phosphorus-nitrogen polyol intermediate.

[0007] S2. Mixing the phosphorus nitrogen polyol intermediate with chlorosilane, sequentially performing polycondensation reaction, extraction, washing, drying, filtering, precipitation, and drying to obtain phosphorus nitrogen silicon polyol.

[0008] S3. Evenly mix polyester / polyether polyol, chain extender, and amine / metal catalyst to obtain component A; mix isocyanate, part of polyester / polyether polyol and phosphazene silicon polyol, and heat to react; then add the remaining polyester / polyether polyol, continue heating to react, and obtain a polyurethane prepolymer, which is component B; evenly mix component A and component B, and quickly spray them on release paper, and after reaction and curing, obtain a linear reactive phosphazene silicon synergistic flame retardant solvent-free polyurethane coating.

[0009] In S1, the present invention cleverly synthesizes a phosphorus-nitrogen polyol intermediate containing the elements P and N using phosphonates and olamines as raw materials. In S2, the present invention further synthesizes this phosphorus-nitrogen polyol intermediate with chlorosilane to obtain a phosphorus-nitrogen-silicon polyol containing P, N, and Si, which can be used as a reactive monomer in the subsequent synthesis of polyurethane. In summary, the present method achieves a balance between high-efficiency flame retardancy, mechanical properties, and excellent coating performance through the synergistic flame retardancy of phosphorus, nitrogen, and silicon, combined with the environmentally friendly properties of solvent-free polyurethane.

[0010] Furthermore, the polyurethane produced by this invention has a linear molecular structure. This linear reaction design, due to its stable molecular chain structure, strong reaction controllability, excellent compatibility, and high environmental stability, ensures that the mechanical properties and durability of the coating remain intact even after the addition of flame retardants. This design overcomes the performance degradation of traditional coatings caused by the addition of flame retardants by evenly dispersing the flame retardant and maintaining a balanced molecular structure.

[0011] To optimize the reactivity of the flame-retardant polyol, the present invention adjusts the timing of addition during the reaction process. Unlike a one-step reaction method (where all polyether polyol and flame-retardant polyol are added at once, potentially resulting in incomplete reaction of the flame-retardant polyol), the two-step addition of the flame-retardant polyol and polyether polyol offers significant advantages. First, the flame-retardant polyol preferentially reacts with the isocyanate, ensuring full utilization of its reactivity. This avoids the problem of incomplete reaction of the flame-retardant polyol in the one-step method due to the higher reactivity of the polyether polyol. Second, the two-step method effectively improves flame retardancy, enabling more uniform incorporation of the flame-retardant elements into the polyurethane matrix, reducing the combustion rate and increasing the carbon residue. Furthermore, this method helps form a more uniform polyurethane network structure, thereby enhancing the mechanical strength and flexibility of the material, reducing side reactions, and improving product purity and consistency. It also enhances process controllability, ensuring production reliability and repeatability.

[0012] Preferably, in S1, the phosphonate is an alkyl phosphonate with a carbon chain length of C1-C4 and / or a phosphonate containing a benzene ring structure; and the alcoholamine is an ethanolamine derivative containing 2-3 hydroxyl groups.

[0013] The present invention has discovered that the types of phosphonate and alcoholamine have a significant impact on the properties of the resulting polyurethane. Ultimately, preferred phosphonates include C1-C4 alkyl phosphonates and / or phosphonates containing a benzene ring structure. Dimethyl phosphite and diethyl benzylphosphonate are particularly desirable short-chain phosphonates. Dimethyl phosphite, due to its minimal steric hindrance, facilitates transesterification with the hydroxyl groups of the alcoholamine, shortening the reaction time (5-8 hours) and increasing the yield (≥90%). Short-chain phosphonates generate PO· radicals during combustion, inhibiting the chain combustion reaction. Diethyl benzylphosphonate, through the conjugation effect of the benzene ring, increases the thermal decomposition temperature, forming a graphitized carbon layer, and significantly reduces the heat release rate. The alcoholamine is preferably an ethanolamine derivative containing 2-3 hydroxyl groups, such as diethanolamine or N-phenyldiethanolamine. Diethanolamine can stabilize the hydroxyl value of polyols, reduce the residual unreacted monomers, and form a PN synergistic effect with phosphorus to release non-flammable gases such as NH3, reducing smoke density; N-phenyldiethanolamine stabilizes gas-phase free radicals through the benzene ring, prolongs the flame self-extinguishing time, has a strong gas-phase flame retardant effect, and increases the glass transition temperature.

[0014] Preferably, in S1, the molar ratio of the phosphonate to the alcoholamine is 1:1-1:2.

[0015] The present invention finds that when the above molar ratio is lower than 1:1, insufficient alcohol amine leads to incomplete reaction of phosphonate, and the purity and yield of intermediate are reduced; when it is higher than 1:2, excessive alcohol amine triggers side reactions, generates impurities and increases the difficulty of separation.

[0016] Preferably, in S1, the transesterification reaction uses a composite catalyst, which is a composite combination of an organic metal compound and an alcohol base, with a mass ratio of 2-4:1; further preferably, the organic metal compound is one or more of sodium methoxide, sodium ethoxide, sodium isopropoxide, and sodium butoxide; one or more of tetraethyl titanate, tetraisopropyl titanate, and tetrabutyl titanate.

[0017] Preferably, in S1, the added amount of the composite catalyst is 0.1-1.0 wt% of the reaction system.

[0018] When the catalyst content is lower than 0.1 wt%, the catalytic activity is insufficient and the reaction is slow; when it is higher than 1.0 wt%, the cost increases and side reactions are easily induced.

[0019] Preferably, in S1, the transesterification reaction is carried out at a temperature of 120-140° C. and for 5-10 h.

[0020] If the temperature is lower than 120°C, the reaction rate is slow, the time is long, and the energy consumption is high; if it is higher than 140°C, there are many side reactions, the product is easy to decompose, and the yield and purity are reduced.

[0021] Preferably, in S2, the chlorosilane is an alkylchlorosilane having a symmetrical substituent and / or a chlorosilane containing a benzene ring, wherein the number of carbon atoms of the alkyl substituent is within 3 and the number of phenyl substituents is not more than 2.

[0022] The present invention discovered that the type of chlorosilane has a significant impact on the properties of the resulting polyurethane. Chlorosilanes with symmetrical substituents or those containing benzene rings should be preferred. Dimethyldichlorosilane and dichlorodiphenylsilane are ideal choices. The number of carbon atoms in the alkyl substituent should be limited to three, and the number of phenyl substituents should be no more than two. Symmetrical alkylchlorosilanes (such as dimethyldichlorosilane) can uniformly distribute the polycondensation reaction sites, significantly increasing the reaction rate, accelerating the removal of the byproduct HCl, and shortening the reaction time to 3-5 hours. They also produce a narrower molecular weight distribution. Short-chain alkyl groups (C1-C3) with a polarity matching that of the polyurethane matrix effectively prevent phase separation and ensure a smooth coating surface. Chlorosilanes containing benzene rings (such as dichlorodiphenylsilane) can enhance the density of the carbon layer and improve oxygen permeability. The rigid structure of the benzene rings allows the thermal decomposition temperature of the siloxane segments to exceed 400°C. However, the number of carbon atoms in the alkyl group should not exceed three, as this reduces reactivity and impairs adhesion. The number of phenyl groups should also be controlled within 2. Excessive phenyl groups (such as triphenyl groups) will inhibit the reaction and increase the brittleness of the product, reducing the elongation at break.

[0023] Preferably, in S2, the molar ratio of the phosphorus-nitrogen polyol intermediate to the chlorosilane is 1:0.8-1:1.1.

[0024] The present invention has found that when the above molar ratio is lower than 1:0.8, the excess chlorosilane is easily hydrolyzed, triggering side reactions, resulting in high product viscosity and unstable performance; if it is higher than 1:1.1, the excess phosphorus-nitrogen polyol makes the system viscosity high, mass transfer difficult, and the reaction incomplete.

[0025] Preferably, in S2, the catalyst used in the polycondensation reaction is one or more of p-toluenesulfonic acid, pyridinium p-toluenesulfonic acid, blocked p-toluenesulfonic acid, and pyridinium p-toluenesulfonic acid.

[0026] Preferably, in S2, the amount of the catalyst used is 0.3-1.0 wt % of the reaction system. If the catalyst content is less than 0.3 wt %, the activity is insufficient and the reaction is slow; if it is greater than 1.0 wt %, the cost is increased and side reactions are easily induced.

[0027] Preferably, in S2, the polycondensation reaction is carried out at a temperature of 60-110°C for 3-6 hours, while removing the by-product HCl. A temperature below 60°C results in a slow reaction, a long reaction time, and high energy consumption. A temperature above 110°C results in numerous side reactions and the product is easily decomposed.

[0028] Preferably, in S2, the extraction uses chloroform.

[0029] Preferably, in S2, petroleum ether is used for the precipitation.

[0030] Preferably, in S3, the polyester / polyether polyol has an Mn of 800-4000 g / mol and is one or more of polytetramethylene ether glycol, castor oil, polycarbonate diol, polyethylene glycol, polyoxypropylene diol, polycaprolactone diol, tung oil polydimethylsiloxane hydroxyl-terminated, and hydroxy silicone oil.

[0031] Preferably, in S3, the chain extender is one or more of hexamethylenediamine, ethylene glycol, 1,4-butanediol, ethanolamine, diethylenetriamine, 1,3-butanediol, vinylaminosilane, and glycerol.

[0032] Preferably, in S3, the amine / metal catalyst is one or more of triethylenediamine, bisdimethylaminoethyl ether, N-methylmorpholine, dibutyltin dilaurate, stannous octoate, bismuth isooctanoate, and potassium acetate.

[0033] Preferably, in S3, the isocyanate is one or more of toluene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, phenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane diisocyanate, tetramethylxylylene diisocyanate, and dicyclohexylmethane diisocyanate;

[0034] Preferably, in S3, the mass ratio of polyester / polyether polyol and chain extender in component A is 100-23: 5-35; the amount of the amine / metal catalyst accounts for 0.01-0.5% by weight of the total reaction system.

[0035] A polyester / polyether polyol ratio below 100 or above 23 may result in insufficient or excessive coating hardness, impacting flexibility and adhesion. A chain extender ratio below 5 or above 35 may result in insufficient crosslinking density or increased coating brittleness. Amine / metal catalyst dosage: Below 0.01% results in insufficient catalytic activity and slow reaction rates. Above 0.5% increases costs and may trigger side reactions, impacting coating performance.

[0036] Preferably, in S3, the mass ratio of isocyanate to phosphazene silicon polyol in component B is 100:10-35.

[0037] The mass ratio of isocyanate to phosphazenium silicon polyol: When it is lower than 100:10, the isocyanate content in the prepolymer is insufficient, and the reaction with component A is insufficient, affecting the coating performance; when it is higher than 100:35, it may increase the toxicity risk, increase the side reactions, and affect the flexibility and adhesion of the coating.

[0038] Preferably, in S3, when preparing component B, the conditions for the first heating reaction are: temperature 40-85°C, time 2-5 hours; the conditions for the second heating reaction are: temperature 60-80°C, time 1-2 hours; after the reaction, a polyurethane prepolymer with an isocyanate content of 15-30wt% is obtained.

[0039] Preferably, in S3, the isocyanate index R (-NCO / -OH) in the component B and the component A is 0.9-1.5.

[0040] When the R value is lower than 0.9, the crosslinking density is insufficient, and the coating hardness and chemical resistance decrease; when it is higher than 1.5, the coating becomes more brittle and the adhesion decreases.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] (1) Combination of high-efficiency flame retardancy and environmental performance: The present invention not only achieves excellent flame retardancy through the synergistic flame retardant mechanism of phosphorus, nitrogen and silicon, but also effectively reduces the risk of combustion of materials under high temperature or flame conditions, reduces the release of toxic smoke and heat, and avoids the emission of VOCs in traditional solvent-based polyurethane coatings, complying with increasingly stringent environmental regulations and sustainable development requirements. At the same time, it avoids the problems of poor compatibility and easy migration of traditional additive flame retardants.

[0043] (2) Excellent mechanical properties and durability: The linear reaction design, due to its stable molecular chain structure, strong reaction controllability, good compatibility and high environmental stability, can ensure that the mechanical properties and durability of the coating are not damaged even after the addition of flame retardants. This design overcomes the problem of performance degradation caused by the addition of flame retardants in traditional coatings by evenly dispersing the flame retardant and maintaining the balance of the molecular structure, thereby achieving long-term stable protection, significantly reducing maintenance costs and extending service life, showing significant advantages and broad application prospects.

[0044] (3) The present invention further improves the reaction efficiency and the performance of the polyurethane by further optimizing phosphonates, alcoholamines and chlorosilanes. DETAILED DESCRIPTION

[0045] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0046] In the present invention, unless otherwise specified, all raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are all conventional methods in the art.

[0047] Example 1

[0048] (1) Preparation of phosphorus-nitrogen polyol intermediates: First, under N2 protection, dimethyl phosphite and diethanolamine were dissolved in 60 mL of dimethyl sulfoxide (DMSO) at a molar ratio of 1:2. 0.5 wt% of a composite catalyst (sodium methoxide and tetrabutyl titanate were mixed at a mass ratio of 3:1) was added, and the ester exchange reaction was carried out at 125 °C for 6 h to obtain a reactive polyol intermediate containing phosphorus and nitrogen elements.

[0049] (2) Preparation of phosphazenium silicon polyol: The phosphazenium silicon polyol intermediate and dichlorodimethylsilane were added to the reactor at a molar ratio of 1:0.9, and 0.4 wt% of the catalyst (p-toluenesulfonic acid) was added. The polycondensation reaction was carried out at 75 °C for 5 h, and the by-product HCl was removed at the same time. After the reaction was completed, it was extracted with 150 mL of chloroform. The organic phase was washed with deionized water, dried, filtered with anhydrous magnesium sulfate, and then precipitated with petroleum ether. Finally, it was dried under reduced pressure to obtain a flame retardant polyol with PN-Si synergistic effect, and its number average molecular weight (M n ) is 1500-3000 g / mol, functionality ≥2.5, and the molecular chain ends contain active hydroxyl groups (-OH) that can participate in polyurethane synthesis;

[0050] (3) Preparation of solvent-free polyurethane coating:

[0051] Polyester polyol (polycaprolactone diol, M n =2000 g / mol), a small molecule chain extender (1,3-butanediol), and an amine / metal catalyst (bis(dimethylaminoethyl)ether / dibutyltin dilaurate, accounting for 0.4% by weight of the total reaction system) are uniformly mixed by high-speed stirring to obtain component A;

[0052] Isocyanate, polytetramethylene glycol (10.3 g, Mn = 2000 g / mol), and a PN-Si synergistic flame-retardant polyol (18.3 g, Mn = 2000 g / mol) were mixed under a nitrogen atmosphere, heated to 65°C, and mechanically stirred for 2 h. Subsequently, another portion of polytetramethylene glycol (8.0 g, Mn = 2000 g / mol) was added, and the mixture was heated to 80°C under nitrogen and mechanically stirred for 1 h to prepare a polyurethane prepolymer with an isocyanate content of 22%, thus obtaining component B.

[0053] Components A and B were uniformly mixed by high-speed stirring in a low-pressure casting machine under the condition of R=1.1. The mixed liquid was then quickly sprayed onto release paper and vacuum-dried at 90°C for 2.5 h to obtain a linear reactive phosphorus-nitrogen-silicon synergistic flame-retardant solvent-free polyurethane.

[0054] The differences between the specific preparation methods of Examples 1 to 3 and Comparative Examples 1 to 3 and Example 1 are shown in Table 1.

[0055] Table 1

[0056]

[0057] Example 4

[0058] The difference from Example 3 is that the phosphonate in step S1 is diethyl benzylphosphonate.

[0059] Example 5

[0060] The difference from Example 3 is that the alcoholamine in step S1 is N-phenyldiethanolamine.

[0061] Example 6

[0062] The difference from Example 3 is that the chlorosilane in step S2 is dichlorodiphenylsilane.

[0063] Comparative Example 4

[0064] The difference from Example 3 is that the phosphonate in step S1 is dimethyl phosphate.

[0065] Comparative Example 5:

[0066] The difference from Example 3 is that the alcohol amine in step S1 is N-(2-hydroxyethyl)-1,2-ethylene glycol amine.

[0067] Comparative Example 6

[0068] The difference from Example 3 is that the chlorosilane in step S2 is dichloromethylsilane.

[0069] Comparative Example 7

[0070] The difference from Example 3 is that component B is prepared by a one-step method, specifically:

[0071] First, isocyanate, polytetramethylene glycol (18.3 g, Mn = 2000 g / mol), and a PN-Si synergistic flame-retardant polyol (18.3 g, Mn = 2000 g / mol) were mixed under a nitrogen atmosphere, heated to 65°C, and mechanically stirred for 3 hours to prepare a polyurethane prepolymer with a 22% isocyanate content, namely component B. Components A and B were then mixed at high speed using a low-pressure casting machine with an R ratio of 1.1. The resulting mixture was then quickly sprayed onto release paper and dried in a vacuum at 90°C for 2.5 hours to produce a linear reactive phosphorus-nitrogen-silicon synergistic flame-retardant solvent-free polyurethane.

[0072] Performance Testing

[0073] The mechanical properties, limiting oxygen index, and vertical combustion of the solvent-free polyurethanes prepared in the above examples and comparative examples were tested. The results are shown in Tables 2 and 3.

[0074] Table 2: Mechanical and limiting oxygen index test results of flame retardant polyurethane materials

[0075]

[0076] Note: Tensile strength and elongation at break are tested according to ASTM D638-14; limiting oxygen index is tested according to ISO4589-2. A high oxygen index indicates that the material is not easy to burn, while a low oxygen index indicates that the material is flammable. It is generally believed that an oxygen index of less than 22% is a flammable material, an oxygen index between 22% and 27% is a combustible material, and an oxygen index of more than 27% is a flame retardant material.

[0077] Table 3: Vertical burning test results of flame retardant polyurethane materials

[0078]

[0079] Note: Vertical burning is tested according to ANSI / UL-94-1985 standard;

[0080] V-0: After two 10-second combustion tests, the flame extinguishes within 30 seconds. No burning material may fall. V-1: After two 10-second combustion tests, the flame extinguishes within 60 seconds. No burning material may fall. V-2: After two 10-second combustion tests, the flame extinguishes within 60 seconds. Some burning material may fall.

[0081] As shown in Table 2, the tensile strengths of the linear reactive phosphanitrogen silicon synergistic flame-retardant solvent-free polyurethanes prepared in Examples 1-6 of the present invention were 56.4 MPa, 67.8 MPa, 78.1 MPa, 82.1 MPa, 85.3 MPa, and 80.8 MPa, respectively, while the tensile strengths of the solvent-free polyurethanes prepared in Comparative Examples 1-7 were 38.1 MPa, 51.5 MPa, 25.1 MPa, 70.9 MPa, 76.7 MPa, 73.6 MPa, and 71.5 MPa, respectively. Compared to Examples 1-2, the polyurethane synthesized using dicyclohexylmethane diisocyanate in Example 3 exhibited significant mechanical performance advantages. From a molecular structure perspective, the cyclohexyl group in the dicyclohexylmethane diisocyanate molecule imparts appropriate flexibility to the polyurethane molecular chain, which enables the material to effectively absorb and disperse stress when subjected to external forces, thereby improving the material's impact toughness and elongation at break. At the same time, its steric effect positively impacts the polyurethane synthesis process, promoting a more uniform and regular network structure and significantly reducing internal defects. Furthermore, polyurethanes synthesized with dicyclohexylmethane diisocyanate exhibit a moderate crosslink density, ensuring sufficient strength and rigidity while successfully avoiding the increased brittleness associated with excessive crosslink density. These multiple factors work together to achieve a delicate balance between strength and toughness, resulting in exceptional overall mechanical properties.

[0082] Further comparison revealed that compared to Example 3, the mechanical strength of Examples 4-6 showed an upward trend, but the elongation at break decreased slightly. This phenomenon is mainly attributed to the introduction of rigid benzene rings in the flame retardant polyol. While enhancing the strength of the material, it also restricted the movement of the molecular chain to a certain extent, resulting in a decrease in the elongation at break. Compared with Example 3, the mechanical properties of the linear reactive phosphorus nitrogen silicon synergistic flame retardant solvent-free polyurethanes prepared in Comparative Examples 1-3 declined significantly. The root cause lies in the limitations of the blending modification, which failed to form effective chemical bonds, resulting in damage to the overall mechanical properties of the material. Similarly, the mechanical properties of the flame retardant polyurethanes prepared in Comparative Examples 4-7 also declined slightly. On the one hand, the reaction efficiency of the synthesized flame retardant polyol was low and it failed to fully participate in the reaction; on the other hand, the isocyanate prepolymer synthesized by component B was not fully generated. These factors all had an adverse effect on the final performance of the polyurethane.

[0083] As shown in Tables 2 and 3, the limiting oxygen index values of the linear reactive phosphanitrogen-silicon synergistic flame-retardant solvent-free polyurethanes prepared in Examples 1-6 were 33.3%, 35.1%, 36.3%, 36.7%, 36.5%, and 37.6%, respectively, all achieving a V-0 rating. The LOI values of Comparative Examples 1-7 were 27.6%, 25.6%, 28.6%, 29.1%, 28.6%, 28.3%, and 31.2%, respectively. Compared to Examples 1-2, Example 3 synthesized the polyurethane using dicyclohexylmethane diisocyanate. The cyclohexyl group in the dicyclohexylmethane diisocyanate provides appropriate flexibility to the polyurethane molecular chain. This not only facilitates the formation of a dense carbonized layer during combustion, effectively isolating oxygen and heat transfer and improving flame retardancy, but also, due to its steric effect, makes the polyurethane network structure more uniform and regular, reducing heat conduction and the release of flammable volatiles during combustion, further improving the LOI value. Furthermore, Example 3 used dimethyl phosphite and diethanolamine as the phosphonate and alcoholamine. The low steric hindrance of dimethyl phosphite facilitated the transesterification reaction, and the resulting short-chain phosphonate effectively inhibited the combustion chain reaction. Furthermore, the introduction of dichlorodiphenylsilane enhanced the density and oxygen permeability of the carbon layer, achieving a LOI of 36.3%.

[0084] Due to the use of co-modification in Comparative Examples 1-3, the flame retardant failed to form effective chemical bonds with the polyurethane molecular chains, resulting in uneven dispersion. This resulted in a failure to exert a synergistic flame retardant effect during combustion, and the inability to form a complete and dense char layer to isolate oxygen and heat. The LOI values were 27.6%, 25.6%, and 28.6%, respectively, with a combustion rating of V-2, and the cotton burned. In Comparative Examples 4-7, the flame-retardant polyols had low reaction efficiency and did not fully participate in the reaction. Furthermore, the isocyanate prepolymer synthesized in Component B was imperfect, affecting the structural regularity and thermal stability of the polyurethane molecular chains. This resulted in LOI values of 29.1%, 28.6%, 28.3%, and 31.2%, respectively, with a combustion rating of V-1. Some melt dripping occurred, but the cotton did not burn.

[0085] Finally, it should be emphasized that while the above description merely illustrates preferred embodiments of the present invention, it is not intended to limit the scope of the present invention. Although the present invention has been described in detail through specific embodiments, it will be readily apparent to those skilled in the art that modifications or equivalent substitutions to the described technical solutions are still within the spirit and scope of the present invention.

Claims

1. A method for preparing a linear reactive phosphorus-nitrogen-silicon synergistic flame-retardant solvent-free polyurethane coating, characterized in that The following steps are involved: S1, dissolving a phosphonate and an alcohol amine in a solvent at a molar ratio of 1:1-1:2, and performing an ester exchange reaction to obtain a phosphorus-nitrogen polyol intermediate; S2, mixing the phosphorus nitrogen polyol intermediate with chlorosilane in a molar ratio of 1:0.8-1:1.1, performing polycondensation reaction in sequence, extracting, washing, drying, filtering, precipitating, and drying to obtain a phosphorus nitrogen silicon polyol; S3. Evenly mix polyester / polyether polyol, chain extender, and amine / metal catalyst to obtain component A; mix isocyanate, part of polyester / polyether polyol and phosphazene silicon polyol, and heat to react; then add the remaining polyester / polyether polyol, continue heating to react, and obtain a polyurethane prepolymer, which is component B; evenly mix component A and component B, and spray them on the substrate, and after reaction and curing, obtain a linear reactive phosphazene silicon synergistic flame retardant solvent-free polyurethane coating.

2. The preparation method according to claim 1, wherein: In S1, The phosphonate is an alkyl phosphonate with a carbon chain length of C1-C4 and / or a phosphonate containing a benzene ring structure; The alcoholamine is an ethanolamine derivative containing 2-3 hydroxyl groups.

3. The preparation method according to claim 2, wherein: In S1, The phosphonate is dimethyl phosphite and / or diethyl benzylphosphonate; The alcoholamine is diethanolamine and / or N-phenyldiethanolamine.

4. The preparation method according to claim 1, wherein: In S2, the chlorosilane is an alkylchlorosilane having a symmetrical substituent and / or a chlorosilane containing a benzene ring, wherein the number of carbon atoms of the alkyl substituent is within 3 and the number of phenyl substituents is not more than 2.

5. The preparation method according to claim 4, wherein: In S2, the chlorosilane is dimethyldichlorosilane and / or dichlorodiphenylsilane.

6. The preparation method according to claim 1, 4 or 5, characterized in that: In S2, The catalyst used in the polycondensation reaction is one or more of p-toluenesulfonic acid, pyridinium p-toluenesulfonic acid, blocked p-toluenesulfonic acid, and pyridinium p-toluenesulfonic acid; The amount of the catalyst used is 0.3-1.0 wt% of the reaction system; The conditions of the polycondensation reaction are: temperature 60-110°C, time 3-6 h, and removal of the by-product HCl; The extraction adopts chloroform; The precipitation adopts petroleum ether.

7. The preparation method according to claim 1, wherein: In S3, The polyester / polyether polyol has an Mn of 800-4000 g / mol and is one or more of polytetramethylene ether glycol, castor oil, polycarbonate diol, polyethylene glycol, polyoxypropylene diol, polycaprolactone diol, tung oil polydimethylsiloxane hydroxyl-terminated, and hydroxy silicone oil; The chain extender is one or more of hexamethylenediamine, ethylene glycol, 1,4-butanediol, ethanolamine, diethylenetriamine, 1,3-butanediol, vinylaminosilane, and glycerol; The amine / metal catalyst is one or more of triethylenediamine, bisdimethylaminoethyl ether, N-methylmorpholine, dibutyltin dilaurate, stannous octoate, bismuth isooctanoate, and potassium acetate; The isocyanate is one or more of toluene diisocyanate, diphenylmethane diisocyanate, naphthalene diisocyanate, phenylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane diisocyanate, tetramethylxylylene diisocyanate, and dicyclohexylmethane diisocyanate.

8. The preparation method according to claim 1 or 7, wherein: In S3, The mass ratio of polyester / polyether polyol to chain extender in component A is 100-23: 5-35; the amount of amine / metal catalyst used accounts for 0.01-0.5% by weight of the total reaction system; The mass ratio of isocyanate to phosphazene silicon polyol in the B component is 100:10-35.

9. The preparation method according to claim 1, wherein: In S3, when preparing component B, the conditions for the first heating reaction are: temperature 40-85°C, time 2-5 hours; the conditions for the second heating reaction are: temperature 60-80°C, time 1-2 hours; after the reaction, a polyurethane prepolymer with an isocyanate content of 15-30wt% is obtained.

10. The preparation method according to claim 1, wherein: In S3, the isocyanate index R (-NCO / -OH) in the B component and the A component is 0.9-1.5.

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