Polyurethane adhesive containing phosphorus-silicon synergistic flame-retardant structure and preparation method thereof

Abstract

The application discloses a polyurethane adhesive containing phosphorus-silicon synergistic flame-retardant structure, which is prepared from the following raw materials in parts by weight: polyol 50-80 parts, isocyanate 20-40 parts, end-hydroxyl phosphorus-silicon synergistic flame-retardant structure monomer 5-20 parts, chain extender 2-8 parts, and catalyst 0.1-1 part; the end-hydroxyl phosphorus-silicon synergistic flame-retardant structure monomer is a dihydric alcohol compound containing phosphorus and silicon, which is prepared from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and vinyl silane as main raw materials, and the general formula of the molecular structure is HO-R-Si(OR')3; wherein R is an organic chain segment containing a DOPO structural unit, and R' is methyl or ethyl. The application synthesizes a phosphorus-containing dihydric alcohol with a clear structure and a hydrolysable silane group, and prepares a main-chain phosphorus-silicon-containing intrinsic flame-retardant polyurethane through a copolymerization mode by taking the dihydric alcohol as a reactive monomer.

Description

Technical Field

[0001] This invention belongs to the field of polyurethane adhesive technology, specifically relating to a polyurethane adhesive with an intrinsic flame-retardant property containing a phosphorus-silicon synergistic flame-retardant structure and its preparation method. Background Technology

[0002] Polyurethane adhesives have excellent bonding properties and are widely used in construction, automotive, and electronics industries. However, traditional polyurethane materials are mainly composed of carbon, hydrogen, oxygen, and nitrogen elements, making them flammable polymers. Their limiting oxygen index (LOI) is usually below 19%, making them highly flammable in air when exposed to an open flame, accompanied by molten dripping, posing a serious safety hazard.

[0003] Currently, most flame-retardant adhesives on the market are produced using a physical blending method, which involves directly adding flame retardants (such as aluminum hydroxide, halogenated flame retardants, etc.) to ordinary adhesives to improve their flame-retardant properties. This "addition method" has significant drawbacks: 1. Poor compatibility: Small-molecule flame retardants have poor compatibility with the adhesive matrix, and phase separation easily occurs after long-term use or heating, leading to flame retardant precipitation, resulting in a sticky adhesive surface, reduced flame-retardant performance, and potential contamination of the adhered materials. 2. Deterioration of mechanical properties: Excessive addition of flame retardants can disrupt the microphase separation structure of polyurethane, leading to a significant decrease in the tensile strength, peel strength, and fatigue resistance of the adhesive. 3. Environmental concerns: Some halogenated flame retardants produce toxic gases during combustion, and solvent-based flame retardants contain VOCs (volatile organic compounds).

[0004] To overcome the shortcomings of additive flame retardants, intrinsic flame retardant technology has emerged. This technology uses chemical reactions to covalently integrate flame-retardant elements (such as P, N, and Si) into the main or side chains of polyurethane molecules, achieving molecular-level flame-retardant modification. 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is a highly efficient phosphorus-based flame-retardant intermediate. Its unique rigid phosphaphenanthrene structure not only provides gas-phase flame retardancy (capturing free radicals) but also promotes char formation. However, polyurethanes containing only DOPO often have excessively rigid molecular chains, leading to increased adhesive brittleness and impaired bonding performance.

[0005] Organosilicon compounds (such as amino silicone oil and hydroxyl silicone oil) have extremely low surface energy, excellent heat oxidation resistance and flexibility. Introducing them into polyurethane can improve the heat resistance and flexibility of the adhesive layer. However, polyurethane modified with siloxane alone has low flame retardancy efficiency and is difficult to achieve high flame retardancy ratings (such as UL-94 / V-0).

[0006] The phosphorus-silicon synergistic effect is a research hotspot in the field of flame retardancy. Theoretically, phosphorus can generate phosphoric acid or polyphosphoric acid at high temperatures, catalyzing the dehydration of polymers into char; silicon migrates to the surface of the char layer in the condensed phase, forming a dense Si-OC or Si-C cross-linked network to protect the underlying material. However, existing technologies mostly involve physically mixing phosphorus-containing monomers with silicon-containing monomers before polymerization, or preparing simple silane-terminated polyurethanes. This physical mixing or end-modification makes it difficult to ensure the precise positioning and chemical bonding of phosphorus and silicon atoms in the molecular chain, resulting in unstable synergistic effects, and the easy migration of siloxanes to the surface leads to a loss of cohesive strength.

[0007] Therefore, developing a novel hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer that simultaneously carries DOPO groups and hydrolyzable alkoxysilane groups, and embedding it as a hard or soft segment component into the polyurethane main chain through a specific synthesis process, is an effective way to solve the above-mentioned technical problems.

[0008] Patent document CN116144296B discloses a low-temperature resistant, formaldehyde-free waterborne adhesive. It synthesizes a branched modifier (introducing DOPO (phosphorus), siloxane, triazine ring, and aziridine into the system) to form an additive / crosslinking composite flame-retardant system. The aziridine group at the end of the modifier undergoes a crosslinking reaction with the carboxyl groups in the waterborne polyurethane, modified polyvinyl alcohol, and VAE emulsion in the system, forming a network structure to achieve reinforcement, toughening, and flame-retardant modification of the adhesive. However, the synthesis of this modifier involves five consecutive reactions, with multiple intermediates (unsaturated monomers, flame-retardant siloxanes, polysiloxanes), each requiring strict reaction conditions (nitrogen protection, specific temperature, pH value), and involving various highly reactive functional group monomers such as aldehyde siloxanes, DOPO, isocyanate silanes, and aziridine, making precise performance control difficult.

[0009] Patent document CN119859501A discloses a water-resistant plant starch formaldehyde-free adhesive. Using plant starch as a backbone, it employs a two-step chemical modification process: first, grafting DOPO flame retardant, then grafting hydrogen-containing silicone oil via hydrosilylation to obtain DOPO-silicone oil-modified starch. The modified starch is then used as a functional filler, physically blended into a polyurethane emulsion. Utilizing the char-forming properties of starch itself, combined with the grafted DOPO (phosphorus) and polysiloxane (silicon), an additive intumescent flame-retardant system is formed, while the polysiloxane imparts water resistance. However, this document involves a two-step chemical modification of starch, a natural polymer (esterification followed by hydrosilylation). The reaction mainly occurs on the hydroxyl groups in the amorphous regions of the starch molecule. Due to the polydispersity and complex supramolecular structure of starch, the grafting rates of these two reactions are typically low and uneven. Furthermore, the hydrosilylation uses a chloroplatinic acid catalyst. Not only is it costly, but the residual chloroplatinic acid may pose a potential risk of metal ion contamination. Its flame-retardant nature is achieved by adding P-Si-containing starch as a "filler" to polyurethane, where the flame-retardant elements are "diluted" by the large amount of starch and polyurethane matrix. To achieve a high flame-retardant rating, a very high addition amount may be required, which in turn impairs the adhesive's mechanical and application properties.

[0010] Patent document CN117363299A discloses a hot melt adhesive and its preparation method. Through a multi-step reaction, a quaternary ammonium salt (antibacterial), a silane coupling agent (KH560), and DOPO (flame retardant) are sequentially introduced to synthesize a polymeric additive containing quaternary ammonium salt, siloxane, and DOPO groups. This additive is added to the hot melt adhesive as a multifunctional additive. The siloxane group can bond with inorganic fillers to improve dispersibility; DOPO synergistically retards with N and Si; and the quaternary ammonium salt provides antibacterial properties. The hydroxyl groups on the additive can react with the polyurethane matrix to achieve a certain degree of chemical bonding. However, the additive molecule in this document combines quaternary ammonium salt (antibacterial), long-chain alkyl, silane coupling agent fragments, and DOPO, resulting in an extremely complex structure and a huge molecular weight. The large molecular structure may embed the antibacterial groups of the quaternary ammonium salt, reducing the antibacterial efficiency; the DOPO group may also have its gas-phase flame retardant mechanism (free radical capture) affected due to restricted molecular chain movement.

[0011] Patent document CN118085814A discloses a formaldehyde-free flame-retardant lignin adhesive. Using industrial lignin as raw material, it is activated and then reacted with self-made DOPO anhydride and hyperbranched silane to obtain modified lignin. The modified lignin is then used as a reactive macromolecular polyol to react with a polyisocyanate crosslinking agent, forming an adhesive with polyurethane properties. DOPO and hyperbranched silane are chemically bonded to the lignin backbone, constructing an NPC-Si integrated flame-retardant system and forming an interpenetrating network structure. However, this document describes how the "activation" (demethylation, hydroxyethylation) of lignin and the subsequent grafting of DOPO anhydride and hyperbranched silane are all carried out on lignin with complex structures and wide molecular weight distributions. The reaction sites and degrees are difficult to control precisely, resulting in a "modified lignin" that is actually an extremely complex mixture containing lignin molecules with different grafting degrees, unreacted small molecules, and self-polymers of hyperbranched silanes. This structural uncertainty poses significant challenges to the formulation design and performance control of the subsequent adhesive. Summary of the Invention

[0012] This invention provides a polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure and its preparation method. By chemically synthesizing a specific hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer, the flame-retardant element (phosphorus) and the heat-resistant element (silicon) are covalently embedded into the polyurethane molecular backbone, making them part of the adhesive. This achieves intrinsic flame retardancy, uniform distribution of flame-retardant elements without precipitation, high flame-retardant efficiency, excellent heat resistance and bonding strength, and is halogen-free and formaldehyde-free, meeting the requirements of green chemical industry.

[0013] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The present invention discloses a polyurethane adhesive with a phosphorus-silicon synergistic flame retardant structure, which is prepared from the following raw materials in parts by weight: 50-80 parts of polyol, 20-40 parts of isocyanate, 5-20 parts of hydroxyl-terminated phosphorus-silicon synergistic flame retardant monomer, 2-8 parts of chain extender, and 0.1-1 parts of catalyst. The hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer is a diol compound containing phosphorus and silicon, prepared mainly from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and vinylsilane. Its general molecular formula is HO-R-Si(OR')3; where R is an organic segment containing DOPO structural units, and R' is methyl or ethyl, that is, the terminal -Si(OR')3 of the monomer is a trimethoxysilane or triethoxysilyl group. The polyol is at least one of the following: polyether polyol, polyester polyol, reactive hydroxyl-terminated polyether modified polysiloxane compound (e.g., Yantai Debang DB-9521), and hydroxyl-terminated alkyl modified organosilicon compound (e.g., Shin-Etsu X-22 series) with a number average molecular weight of 500-3000 g / mol, and has a functionality of 2-3. The isocyanate is at least one selected from isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), 2,4-toluene diisocyanate (2,4-TDI), 2,6-toluene diisocyanate (2,6-TDI), 4,4-diphenylmethane diisocyanate (MDI), 1,5-pentane diisocyanate (PDI), terephthalic diisocyanate (PPDI), diphenylmethylene diisocyanate (XDI), tetramethylphenyl dimethyl diisocyanate (TMXDI), and 4,4-dicyclohexylmethane diisocyanate (HMDI). The chain extender is at least one selected from 1,4-butanediol, ethylene glycol, diethylene glycol, 1,6-hexanediol, and propylene glycol; The catalyst is an organotin catalyst (e.g., dibutyltin dilaurate, stannous octoate), an organobismuth catalyst (e.g., BiCAT8118, BiCAT8108), or a polyurethane amine catalyst (e.g., tertiary amine or quaternary ammonium salt).

[0014] In the structure of the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer, R is an organic bridging segment connecting phosphorus and silicon atoms. Specifically, this segment originates from the addition reaction of the PH bond of DOPO and a vinyl silane to form an alkylene structure (e.g., -CH2-CH2-, -CH(CH3)-). One end of this segment is connected to the DOPO skeleton via a PC bond, and the other end is connected to an alkoxysilane group [-Si(OR')3] via a Si-C bond. Therefore, the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer has a hydroxyl group (-OH) introduced by ring opening of an epoxy group at its molecule end, thus constituting a bifunctional monomer with a hydroxyl group at one end, a hydrolyzable silane at the other end, and a phosphorus-silicon synergistic structure in the middle.

[0015] Preferably, in the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant structural monomer, the molar ratio of phosphorus to silicon is 1:(1-3).

[0016] Preferably, the preparation method of the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant structural monomer includes the following steps: S1, pH addition reaction: DOPO is reacted with vinyltrimethoxysilane or vinyltriethoxysilane in the presence of a free radical initiator and under nitrogen protection at 90-120℃ for 4-8 hours; after the reaction is completed, intermediate A is obtained; the general molecular formula of intermediate A is: DOPO-P(O)-CH2-CH2-Si(OR')3. S2, Hydroxyl group introduction reaction: The intermediate A is reacted with epichlorohydrin at 80-110°C for 2-4 hours under the protection of a catalyst and nitrogen. During this process, the epoxy group of epichlorohydrin undergoes a ring-opening reaction. The reaction aims to introduce a chlorinated hydroxyl side chain at the end of the molecular chain, which is then converted to a dihydroxyl group or directly retains a monohydroxyl structure through hydrolysis. S3, Selective hydrolysis and purification: The reaction product of step S2 is dispersed in a weakly alkaline aqueous solution and hydrolyzed by stirring at 50-70°C for 1-2 hours. Under these conditions, taking advantage of the kinetic difference that the hydrolysis rate of chlorine atoms is much faster than that of alkoxysilanes, the reaction time and alkalinity are strictly controlled. The chlorine atoms (-Cl) on the side chain are rapidly hydrolyzed into hydroxyl groups (-OH), while the terminal alkoxysilane group [-Si(OR')3] remains stable, undergoing only trace amounts of hydrolysis or no hydrolysis at all. After the reaction is completed, the solvent is removed by separation, washing, drying and vacuum distillation to obtain the terminal hydroxyl phosphorus-silicon synergistic flame-retardant monomer. In step S1, the free radical initiator is at least one of benzoyl peroxide, dicumyl peroxide, azobisisobutyronitrile, and tert-butyl peroxide; in step S2, the catalyst is at least one of triethylamine, benzyltriethylammonium chloride, or stannous octoate; in step S3, the weakly alkaline aqueous solution is a Na2CO3 solution, NaHCO3 solution, NaOH solution, or a buffer solution thereof with a mass concentration of 1% to 5% and a pH value of 8.0 to 8.6.

[0017] The preparation method of the polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to the present invention includes the following steps: D1, the polyol, the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer, and the chain extender are added to a reaction vessel, stirred and mixed evenly, and then vacuum dehydrated at 100-120°C to reduce the water content to less than 0.03%; D2. Nitrogen gas is introduced into the reaction vessel for positive pressure protection, and the temperature is lowered to 60-80°C. The isocyanate is added, and after stirring evenly, the catalyst is added dropwise and reacted for 2-4 hours, while monitoring the NCO content of the system. When the absolute deviation of the NCO content of the prepolymer from the theoretical value is ≤0.5%, the reaction temperature is raised to 80-95°C, and the reaction is continued for 0.5-1 hour. When the NCO content stabilizes within ±0.3% of the theoretical value, the temperature is lowered and the reaction is stopped. D3, under nitrogen protection, the product of D2 is discharged while hot, sealed and stored to obtain the polyurethane adhesive with the phosphorus-silicon synergistic flame retardant structure.

[0018] The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure described in this invention is a moisture-curing, one-component polyurethane adhesive. In use, the polyurethane adhesive is applied to a clean, dry substrate surface, bonded, and then placed in the air. The NCO end groups in the polyurethane adhesive react with moisture in the air to generate urea bonds and release CO2. Simultaneously, the silane end groups (-Si(OR')3) undergo hydrolysis and condensation to form a three-dimensional cross-linked network, achieving curing.

[0019] Compared with the prior art, the beneficial effects of the present invention are: This invention designs and synthesizes a phosphorus-containing diol with a well-defined structure and hydrolyzable silane groups, and uses it as a reactive monomer to prepare an intrinsically flame-retardant polyurethane with a main chain containing phosphorus and silicon via copolymerization. By introducing a hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer, the flame-retardant element (phosphorus) and the heat-resistant element (silicon) are chemically bonded into the polyurethane molecular chain, avoiding the migration and precipitation of small-molecule flame retardants and achieving intrinsic flame retardancy. Phosphorus captures free radicals in the gas phase, interrupting the chain reaction; silicon promotes carbon formation in the condensed phase, forming a stable Si-OC crosslinking network. The synergistic effect of these two elements significantly improves the limiting oxygen index (LOI) and vertical flammability rating (UL-94) of the adhesive. The introduction of the siloxane structure also improves the thermal stability of the adhesive layer, increasing the thermal decomposition temperature by approximately 30–50 °C.

[0020] The adhesive system of this invention is completely free of halogens such as chlorine and bromine, produces low smoke and few toxic gases during combustion, exhibits no migration or volatilization of small molecules, and has extremely low VOC content, meeting the environmental protection requirements for green buildings and electronic and electrical products. The preparation process requires no toxic solvents, making it an environmentally friendly production process.

[0021] The hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer of this invention possesses universal hydroxyl and alkoxysilane functional groups, allowing it to react with various types of isocyanates and polyols. It is suitable for synthesizing a variety of materials, including polyurethane elastomers, coatings, foams, and sealants, and is not limited to adhesives. By changing the type and molecular weight of the polyol and the type of isocyanate, the ratio of hard and soft segments in the final product can be easily adjusted, thereby customizing its hardness, modulus, Tg, and other properties to meet different application requirements from flexible encapsulation to rigid structural bonding. It is suitable for high flame-retardant applications such as woodworking, electronics, and automotive interiors, and has broad market application prospects. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 A polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure is prepared from the following raw materials in parts by weight: 55 parts of polyether polyol (PPG-1000, molecular weight 1000, hydroxyl value 112 mgKOH / g), 44 parts of MDI, 15 parts of hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer (molar ratio of phosphorus to silicon is 1:1), 2.0 parts of 1,4-butanediol, and 0.5 parts of organotin catalyst; The preparation method of the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer includes the following steps: S1, pH addition reaction: 100 parts of DOPO and 82.5 parts of vinyltrimethoxysilane (molar ratio P:Si≈1:1.2, silane in excess to ensure complete reaction of DOPO) were reacted at 100°C for 5 h in the presence of a free radical initiator (benzoyl peroxide 0.5 parts) and under nitrogen protection; after the reaction, intermediate A (P:Si structural unit ratio of 1:1) was obtained. S2, hydroxyl group introduction reaction (epoxy ring opening): intermediate A and epichlorohydrin (WH-ECH from Wanhua Chemical Company) are mixed in a molar ratio of 1:1.1 (with a slight excess of epoxy groups), and the mixture is heated to 85°C for 3 hours under the protection of a catalyst (0.5 parts tetrabutylammonium bromide) and nitrogen. S3, Selective hydrolysis and purification: The reaction product of step S2 was dispersed in a weakly alkaline aqueous solution (3% Na2CO3 solution, pH controlled at 8.0-8.5), and hydrolyzed by stirring at 55-60℃ for 1.5h to introduce hydroxyl groups (-OH) into the free ends of the DOPO chain segments; after the reaction was completed, the solvent was removed by separation, washing, drying and vacuum distillation to obtain the hydroxyl-terminated phosphorus-silicon synergistic flame retardant monomer.

[0024] A method for preparing a polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure includes the following steps: D1, adding polyether polyol, hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer, and 1,4-butanediol to a reaction vessel, and vacuum dehydrating at 110°C for 1.5 h until the water content is below 0.03%; D2, after dehydration, stopping the vacuuming, introducing nitrogen gas into the reaction vessel for positive pressure protection, lowering the temperature to 70°C, adding accurately weighed MDI in one go while stirring, and then adding the catalyst dropwise using a microsyringe. The reaction was carried out under nitrogen protection for 3 hours. During this period, the NCO content was titrated every 30 minutes using the di-n-butylamine method, and the NCO content decreased from the initial theoretical value of 12.2% to 6.5%. Then the temperature was raised to 90℃ and the reaction was continued for 1 hour until the NCO content stabilized at 5.9% (theoretical design value of 5.8%). The reaction was then stopped by cooling. On day 3, heating was stopped, and under nitrogen protection, the pale yellow transparent viscous prepolymer was poured into a tin can pre-filled with nitrogen while it was still hot and immediately sealed to obtain a polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure.

[0025] Example 2 A polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure is prepared from the following raw materials in parts by weight: 68 parts of poly(1,4-butanediol adipate) (PBA-2000, molecular weight 2000, hydroxyl value 56 mg KOH / g), 27.5 parts of 1,5-pentanediisocyanate (PDI), 8 parts of hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer (molar ratio of phosphorus to silicon is 1:1), 3 parts of ethylene glycol, and 0.15 parts of organotin catalyst; wherein the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer is the same as in Example 1.

[0026] A method for preparing a polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure includes the following steps: D1, polyester polyol, hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer and ethylene glycol are added to a reaction vessel and dehydrated under vacuum at 105°C for 1.5 h to reduce the water content to less than 0.03%; D2. After dehydration is complete, stop the vacuuming process and introduce nitrogen gas into the reaction vessel for positive pressure protection. Lower the temperature to 65°C, and add an accurate amount of PDI in one go while stirring. Then, add the catalyst dropwise using a microsyringe. React under nitrogen protection for 3.5 hours. During this period, the NCO content is titrated every 30 minutes using the di-n-butylamine method. The NCO content decreases from the initial theoretical value of 12.8% to 6.4%. Then, raise the temperature to 85°C and continue the reaction for 1 hour until the NCO content stabilizes at 5.9% (theoretical design value of 5.7%). Then, lower the temperature to stop the reaction. D3. Stop heating. Under nitrogen protection, pour the light yellow, transparent, viscous prepolymer into a pre-filled nitrogen tin can while it is still hot, and seal it immediately to obtain a polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure.

[0027] Example 3 A polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure is prepared from the following raw materials in parts by weight: 60 parts of reactive hydroxyl-terminated polyether modified polysiloxane (PEO-PDMS-OH, DB-9521 from Yantai Debang, molecular weight 3000, hydroxyl value 37mgKOH / g); 31 parts of hydrogenated diphenylmethane diisocyanate (H12MDI, alicyclic); 12 parts of hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer (molar ratio of phosphorus to silicon is 1:2); 3.8 parts of 1,3-propanediol (PDO); and 0.3 parts of stannous octoate.

[0028] The preparation method of the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer (P:Si=1:2) includes the following steps: S1, pH addition reaction: 100 parts of DOPO and 137 parts of vinyltrimethoxysilane were reacted at 105 °C for 6 h in the presence of a free radical initiator (1.0 part of benzoyl peroxide) and under nitrogen protection; after the reaction was completed, intermediate A was obtained. S2, hydroxyl group introduction reaction (epoxy ring opening): intermediate A and epichlorohydrin (WH-ECH from Wanhua Chemical Company) are mixed in a molar ratio of 1:1 and reacted at 90°C for 4 hours under the protection of catalyst (0.6 parts benzyltriethylammonium chloride) and nitrogen. S3, Selective hydrolysis and purification: The reaction product of step S2 was dispersed in a weakly alkaline aqueous solution (3% NaHCO3 solution, pH controlled at 7.5-8.0), and hydrolyzed at 50-55℃ for 1.2 h with stirring to introduce hydroxyl groups (-OH) into the free ends of the DOPO chain segments; after the reaction was completed, the solvent was removed by separation, washing, drying and vacuum distillation to obtain a hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer (molar ratio of phosphorus to silicon is 1:2).

[0029] A method for preparing a polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure includes the following steps: D1: Reactive hydroxyl-terminated polyether-modified polysiloxane, hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer, and 1,3-propanediol were added to a reaction vessel and vacuum dehydrated at 100℃ for 2.0 h (siloxanes are sensitive to high temperatures, so the dehydration temperature was lowered) until the water content was less than 0.02%. D2: After dehydration is complete, stop the vacuuming process and introduce nitrogen gas into the reaction vessel for positive pressure protection. Reduce the temperature to 60°C and add an accurate amount of H12MDI (which needs to be preheated and melted) in one go while stirring. Then, add the catalyst dropwise using a microsyringe and react under nitrogen protection for 4 hours. During this period, the NCO content is titrated every 30 minutes using the di-n-butylamine method. The NCO content decreases from the initial theoretical value of 9.7% to 3.7%. Then, raise the temperature to 80°C and continue the reaction for 1.5 hours until the NCO content stabilizes at 3.2% (theoretical design value of 3.0%). Then, cool down to stop the reaction. D3: Stop heating, and under nitrogen protection, pour the colorless, transparent, viscous prepolymer into a pre-filled nitrogen tin can while it is still hot, and seal it immediately to obtain a polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure.

[0030] Example 4 A polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure is prepared from the following raw materials in parts by weight: 50 parts of a hydroxyl-terminated alkyl-modified organosilicon compound (dihydroxyl-terminated polydimethylsiloxane, brand: Shin-Etsu X-22-160B, molecular weight 3000, hydroxyl value 37 mgKOH / g); 27.5 parts of toluene diisocyanate (TDI-80, a mixture of 2,4- and 2,6- isomers); 18 parts of a hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer (molar ratio of phosphorus to silicon is 1:1); 2.5 parts of glycerol (partially participates in the reaction to control the degree of crosslinking); and 0.6 parts of dibutyltin dilaurate (DBTDL).

[0031] The preparation method of the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer includes the following steps: S1, pH addition reaction: 100 parts of DOPO and 80 parts of vinyltriethoxysilane were reacted at 110°C for 4.5 h in the presence of a free radical initiator (1.0 part of dicumyl peroxide DCP) and under nitrogen protection; after the reaction was completed, intermediate A was obtained. S2, hydroxyl introduction reaction (epoxy ring opening): intermediate A and epichlorohydrin (Yangnong Chemical Company's industrial grade ECH) are mixed in a molar ratio of 1:1 and reacted at 80°C for 3.5 h under the protection of catalyst (0.4 parts tetrabutylammonium bromide) and nitrogen. S3, Selective hydrolysis and purification: The reaction product of step S2 was dispersed in a weakly alkaline aqueous solution (borax-sodium hydroxide buffer solution with a pH of 8.2-8.6) and hydrolyzed at 58-62℃ for 2.0 h with stirring to introduce hydroxyl groups (-OH) into the free ends of the DOPO chain segments; after the reaction was completed, the solvent was removed by separation, washing, drying and vacuum distillation to obtain a hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer.

[0032] A method for preparing a polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure includes the following steps: D1: A hydroxyl-terminated alkyl-modified organosilicon compound, a hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer, and glycerol were added to a reaction vessel and dehydrated under vacuum at 105°C for 1.5 h until the water content was less than 0.03%. D2: After dehydration is complete, stop the vacuuming process and introduce nitrogen gas into the reaction vessel for positive pressure protection. Lower the temperature to 55℃ (TDI has high activity and requires low temperature to prevent rapid polymerization). Add an accurate amount of TDI-80 at once while stirring, and then add the catalyst dropwise using a microsyringe. React under nitrogen protection for 2.5 hours. During this period, the NCO content is titrated every 30 minutes using the di-n-butylamine method. The NCO content decreases from the initial theoretical value of 12.0% to 7.6%. Then raise the temperature to 75℃ and continue the reaction for 1 hour until the NCO content stabilizes at 7.1% (theoretical design value of 6.9%). Then lower the temperature to stop the reaction. D3: Stop heating. Under nitrogen protection, pour the light yellow, transparent, viscous prepolymer into a pre-filled nitrogen tin can while it is still hot, and seal it immediately to obtain a high-strength polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure.

[0033] Comparative Examples 1-2 Compared with Example 1, Comparative Example 1 uses an equal amount of ordinary DOPO derivative to replace the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer in Example 1, while the other raw materials and preparation methods are the same as in Example 1.

[0034] Compared with Example 2, Comparative Example 2 uses an equal amount of ordinary DOPO derivative to replace the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer in Example 2, while the other raw materials and preparation methods are the same as in Example 2.

[0035] Test Experiment Example The performance of the polyurethane adhesives prepared in Examples 1-4 and Comparative Examples 1-2 was determined.

[0036] Limiting Oxygen Index (LOI): According to the provisions of GB / T 2406.2-2009 and ISO 4589-2 "Determination of flammability by oxygen index method for plastics - Part 2: Test at room temperature", a sample with a size of 150mm×10mm×4mm was prepared for determination. Vertical flammability rating (UL-94): In accordance with the provisions of GB / T 2408-2008 and IEC 60695-11-10 "Determination of flammability of plastics - Horizontal and vertical methods", a vertical flammability test was conducted on a 3mm thick sample. Thermal decomposition temperature (°C): According to the provisions of GB / T 27761-2009 and ISO 11358-1 "Test method for weight loss and residual amount of thermogravimetric analyzer", under nitrogen atmosphere (flow rate 50 mL / min), the temperature at which 5% weight loss is measured is increased from room temperature to 800°C at a heating rate of 10°C / min. Bond strength: In accordance with the provisions of GB / T 7124-2008 and ISO 4587 "Determination of tensile shear strength of lap joints of rigid adhesives", aluminum alloy (LY12) was used as the adhesive, and the curing conditions were 25℃ / 7d or 80℃ / 2h. The test was conducted on a universal testing machine at a rate of 5mm / min. Resistance to damp heat aging: In accordance with GB / T 1740-2007 "Test Method for Resistance to Damp Heat of Coating Film" (refer to the standard) or ASTM D2247, the cured adhesive sample was placed in a constant temperature and humidity chamber at 50℃ and 95% relative humidity for 500h, and the appearance changes were observed after removal.

[0037] The measurement results are shown in Table 1.

[0038] Table 1. Performance test results of polyurethane adhesives in Examples 1-4 and Comparative Examples 1-2 As can be seen from Table 1, the polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure of the present invention significantly outperforms the comparative example in terms of flame retardant performance, heat resistance, and bond strength, demonstrating the superiority of the phosphorus-silicon synergistic flame-retardant structure. This product is particularly suitable for fields with stringent requirements for fire safety, bonding reliability, and long-term durability, such as high-speed rail interior bonding, new energy battery pack sealing, high-rise building curtain wall structural adhesives, and electronic device encapsulation.

Claims

1. A polyurethane adhesive containing a phosphorus-silicon synergistic flame-retardant structure, characterized in that, It is prepared from the following raw materials in parts by weight: 50-80 parts of polyol, 20-40 parts of isocyanate, 5-20 parts of hydroxyl-terminated phosphorus-silicon synergistic flame retardant monomer, 2-8 parts of chain extender, and 0.1-1 parts of catalyst. The hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer is a diol compound containing phosphorus and silicon, prepared by using 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and vinylsilane as the main raw materials. Its general molecular formula is HO-R-Si(OR')3; where R is an organic segment containing DOPO structural units and R' is methyl or ethyl.

2. The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to claim 1, characterized in that, The polyol is at least one of polyether polyol, polyester polyol, reactive hydroxyl-terminated polyether modified polysiloxane compound, and hydroxyl-terminated alkyl modified organosilicon compound with a number average molecular weight of 500-3000 g / mol, and has a functionality of 2-3.

3. The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to claim 1, characterized in that, The isocyanate is at least one selected from isophorone diisocyanate, hexamethylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4-diphenylmethane diisocyanate, 1,5-pentane diisocyanate, terephthalic diisocyanate, phenylenediamine diisocyanate, tetramethylphenyl dimethyl diisocyanate, and 4,4-dicyclohexylmethane diisocyanate.

4. The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to claim 1, characterized in that, The chain extender is at least one selected from 1,4-butanediol, ethylene glycol, diethylene glycol, 1,6-hexanediol, and propylene glycol.

5. The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to claim 1, characterized in that, The catalyst is an organotin catalyst, an organobismuth catalyst, or a polyurethane amine catalyst.

6. The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to claim 1, characterized in that, In the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant structural monomer, the molar ratio of phosphorus to silicon is 1:(1-3).

7. The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to claim 1, characterized in that, The preparation method of the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant structural monomer includes the following steps: S1, DOPO is reacted with vinyltrimethoxysilane or vinyltriethoxysilane in the presence of a free radical initiator and under nitrogen protection at 90-120°C for 4-8 hours; after the reaction is completed, intermediate A is obtained. S2, the intermediate A and epichlorohydrin are reacted at 80-110°C for 2-4 hours under the protection of a catalyst and nitrogen. S3, the reaction product of step S2 is dispersed in a weakly alkaline aqueous solution and hydrolyzed by stirring at 50-70°C for 1-2 hours; after the reaction is completed, the solvent is removed by separation, washing, drying and vacuum distillation to obtain the terminal hydroxyl phosphorus-silicon synergistic flame retardant monomer.

8. The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to claim 7, characterized in that, In step S1, the free radical initiator is at least one of benzoyl peroxide, dicumyl peroxide, azobisisobutyronitrile, and tert-butyl peroxide; in step S2, the catalyst is at least one of triethylamine, benzyltriethylammonium chloride, or stannous octoate.

9. The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to claim 7, characterized in that, In step S3, the weakly alkaline aqueous solution is a Na2CO3 solution, NaHCO3 solution, NaOH solution, or a buffer solution thereof with a mass concentration of 1% to 5% and a pH value of 8.0 to 8.

6.

10. The method for preparing the polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to any one of claims 1 to 9, characterized in that, Includes the following steps: D1, the polyol, the hydroxyl-terminated phosphorus-silicon synergistic flame-retardant monomer, and the chain extender are added to a reaction vessel, stirred and mixed evenly, and then vacuum dehydrated at 100-120°C to reduce the water content to less than 0.03%; D2. Nitrogen gas is introduced into the reaction vessel for positive pressure protection, and the temperature is lowered to 60-80°C. The isocyanate is added, and after stirring evenly, the catalyst is added dropwise and reacted for 2-4 hours, while monitoring the NCO content of the system. When the absolute deviation of the NCO content of the prepolymer from the theoretical value is ≤0.5%, the reaction temperature is raised to 80-95°C, and the reaction is continued for 0.5-1 hour. When the NCO content stabilizes within ±0.3% of the theoretical value, the temperature is lowered and the reaction is stopped. D3, under nitrogen protection, the product of D2 is discharged while hot, sealed and stored to obtain the polyurethane adhesive with the phosphorus-silicon synergistic flame retardant structure.

11. The application of the polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure according to any one of claims 1 to 9, characterized in that, The polyurethane adhesive with a phosphorus-silicon synergistic flame-retardant structure is a moisture-curing, single-component polyurethane adhesive. The polyurethane adhesive is applied to a clean and dry substrate surface, bonded together, and then placed in the air.

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