Aviation flame-retardant polyurethane pressure-sensitive adhesive and preparation method thereof

Abstract

The invention relates to the technical field of adhesives, in particular to an aviation flame-retardant polyurethane pressure-sensitive adhesive and a preparation method thereof.Cyclophosphazene is modified into a dihydroxy compound, so that the excellent gas-phase dilution flame retardance and ceramic carbonization capacity of cyclophosphazene are reserved, and the problems of crosslinking and catalysis caused by polyfunctional groups of cyclophosphazene are solved; the PZ and DOPO-HQ are subjected to isocyanate copolymerization to prepare a flame-retardant polymer, and a flame-retardant structure of PZ-isocyanate-DOPO-HQ is formed; by utilizing the synergistic effect of PZ and DOPO-HQ, the efficient synergistic effect of multiple elements of phosphorus-nitrogen-phosphorus is realized; the flame-retardant polymer can form an interpenetrating network structure with a polyurethane prepolymer, the overall flame-retardant property of the material is remarkably improved, meanwhile, excellent adhesive property and flexibility are effectively kept, and the prepared pressure-sensitive adhesive is halogen-free and environment-friendly, meets the strict requirements of low smoke, no toxicity, high flame retardance and high safety of aeronautical materials, and has wide application prospects. And the adhesive is particularly suitable for scenes such as aircraft interior decoration, floor bonding, wire harness fixing and composite material bonding.

Description

Technical Field

[0001] This invention relates to the field of adhesive technology, and in particular to an aerospace flame-retardant polyurethane pressure-sensitive adhesive and its preparation method. Background Technology

[0002] In the aerospace field, pressure-sensitive adhesives are widely used in critical areas such as bonding aircraft interior components, bundling wire harnesses, fixing sensors, and splicing composite skins. Due to the confined space, high density of personnel, and concentrated electronic equipment inside aircraft, fires spread rapidly and escape windows are extremely short in the event of a fire. Therefore, extremely stringent requirements are placed on the flame-retardant properties of materials. While existing polyurethane pressure-sensitive adhesives possess excellent adhesion, flexibility, and aging resistance, they are inherently flammable organic polymers that readily combust under high temperatures or electric arcs, releasing large amounts of toxic fumes and posing serious safety hazards. Flame-retardant modification is necessary to meet the stringent standards for aerospace materials, such as strict flame retardancy, low smoke, and low toxicity.

[0003] Currently, commonly used flame retardant strategies mainly rely on additive flame retardants, such as halogenated or traditional phosphorus-based ones. However, halogenated flame retardants produce corrosive gases and highly toxic substances during combustion, seriously threatening occupant safety and failing to comply with environmental regulations. Additive phosphorus-based flame retardants are prone to migration and precipitation, exhibit poor long-term stability, and often require high dosages to meet flame retardant requirements, severely impacting the adhesive properties and temperature resistance of the colloid. While some reactive flame retardants can effectively improve compatibility and durability, the introduction of these single compounds makes it difficult to achieve efficient synergistic flame retardancy between the gas and condensed phases. Furthermore, the structure of some flame retardants can easily lead to brittleness of the colloid, reducing initial tack and peel strength, making it difficult to balance flame retardancy and adhesive properties.

[0004] Therefore, existing technologies still need to be improved and developed. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide an aerospace flame-retardant polyurethane pressure-sensitive adhesive and its preparation method, which aims to solve the problems that existing pressure-sensitive adhesives are difficult to achieve efficient gas-phase and gel-phase synergistic flame retardancy, and that the addition of flame retardants causes the adhesive to become brittle.

[0006] The technical solution of the present invention is as follows: A method for preparing an aerospace flame-retardant polyurethane pressure-sensitive adhesive, comprising the following steps: The cyclophosphonitrile compound was modified to obtain dihydroxycyclophosphonitrile diol; The dihydroxycyclophosphonitrile diol was mixed with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, diisocyanate, and organotin catalyst, and then copolymerized to obtain a flame-retardant polymer. The flame-retardant polymer is mixed with a polyurethane prepolymer and a curing agent to obtain a curable pressure-sensitive adhesive composition. The curable pressure-sensitive adhesive composition is coated onto a substrate and cured to obtain an aerospace flame-retardant polyurethane pressure-sensitive adhesive.

[0007] The method for preparing the aforementioned aerospace flame-retardant polyurethane pressure-sensitive adhesive, wherein the step of modifying the cyclophosphonitrile compound to obtain dihydroxycyclophosphonitrile diol includes: Hexachlorocyclotriphosphazene was mixed with a monofunctional hydroxyl compound, an acid-binding agent, and an organic solvent, and after partial end-capping reaction, a first product with one active chlorine atom was obtained. The first product, which has an active chlorine atom, is mixed with a glycerol compound and subjected to a nucleophilic substitution reaction to obtain dihydroxycyclophosphonitrile diol.

[0008] The method for preparing the aerospace flame-retardant polyurethane pressure-sensitive adhesive, wherein the monofunctional hydroxyl compound includes one or more of methanol, ethanol, and butanol; and the molar ratio of the hexachlorocyclotriphosphazene to the monofunctional hydroxyl compound is 1:5.

[0009] The method for preparing the aerospace flame-retardant polyurethane pressure-sensitive adhesive, wherein the organic solvent includes one or more of tetrahydrofuran, N,N-dimethylformamide, diethyl ether, dichloromethane, acetonitrile, and N-methylpyrrolidone; the amount of the organic solvent added is 0.5-10 times the mass of the hexachlorocyclotriphosphazene.

[0010] The method for preparing the aviation flame-retardant polyurethane pressure-sensitive adhesive, wherein the acid-binding agent is one or more of triethylamine, a derivative of triethylamine, diisopropylethylamine, a derivative of diisopropylethylamine, pyridine, and a derivative of pyridine; the amount of the acid-binding agent added is 0.1%-1% of the mass of the hexachlorocyclotriphosphazene.

[0011] The preparation method of the aforementioned aviation flame-retardant polyurethane pressure-sensitive adhesive, wherein the temperature of the partial end-capping reaction is 0℃-40℃, and the time of the partial end-capping reaction is 1h-3h.

[0012] The preparation method of the aforementioned aviation flame-retardant polyurethane pressure-sensitive adhesive, wherein the temperature of the nucleophilic substitution reaction is 50℃-75℃; and the time of the nucleophilic substitution reaction is 2h-4h.

[0013] The method for preparing the aerospace flame-retardant polyurethane pressure-sensitive adhesive, wherein the glycerol compound is a C3-C6 saturated polyol containing two or three hydroxyl groups; and the molar ratio of the first product having one active chlorine atom to the glycerol compound is 1:1.

[0014] The preparation method of the aforementioned aviation flame-retardant polyurethane pressure-sensitive adhesive, wherein the copolymerization reaction temperature is 60℃-90℃ and the copolymerization reaction time is 2h-5h.

[0015] The method for preparing the aerospace flame-retardant polyurethane pressure-sensitive adhesive, wherein the diisocyanate includes one or more of hexamethylene diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, and isophorone diisocyanate; and the organotin catalyst includes one of dibutyltin dilaurate and stannous octoate.

[0016] The preparation method of the aforementioned aerospace flame-retardant polyurethane pressure-sensitive adhesive, wherein the mass ratio of the 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, the dihydroxycyclophosphonitrile diol, the diisocyanate and the organotin catalyst is 1:(1-3):(1-3):(0.01-0.5).

[0017] The method for preparing the aforementioned aviation flame-retardant polyurethane pressure-sensitive adhesive, wherein the polyurethane prepolymer is obtained by dehydration polymerization of polyester polyol or polyether polyol with aromatic or aliphatic diisocyanate.

[0018] The method for preparing the aforementioned aerospace flame-retardant polyurethane pressure-sensitive adhesive, wherein the curing agent comprises one or more of hexamethylene diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate trimer, and isophorone diisocyanate trimer.

[0019] The method for preparing the aerospace flame-retardant polyurethane pressure-sensitive adhesive, wherein the amount of curing agent added is 1%-15% of the mass of the polyurethane prepolymer.

[0020] The method for preparing the aforementioned flame-retardant polyurethane pressure-sensitive adhesive for aviation, wherein the flame-retardant polymer has a mass fraction of 5%-15% in the curable pressure-sensitive adhesive composition.

[0021] An aerospace flame-retardant polyurethane pressure-sensitive adhesive is prepared using a method for preparing aerospace flame-retardant polyurethane pressure-sensitive adhesive.

[0022] Beneficial Effects: This invention provides an aerospace flame-retardant polyurethane pressure-sensitive adhesive and its preparation method. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive includes the following steps: modifying a cyclophosphonitrile compound to obtain dihydroxycyclophosphonitrile diol; mixing the dihydroxycyclophosphonitrile diol with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, diisocyanate, and organotin catalyst, and copolymerizing them to obtain a flame-retardant polymer; mixing the flame-retardant polymer with a polyurethane prepolymer and a curing agent to obtain a curable pressure-sensitive adhesive composition; coating the curable pressure-sensitive adhesive composition onto a substrate and curing it to obtain the aerospace flame-retardant polyurethane pressure-sensitive adhesive. This invention modifies cyclophosphonitrile into a dihydroxy compound (i.e., modified cyclophosphonitrile, PZ), retaining its excellent gas-phase dilution flame retardancy and ceramic charring capabilities while avoiding the crosslinking and catalytic problems caused by the multifunctional groups of cyclophosphonitrile. Then, it copolymerizes PZ with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-HQ) via isocyanate copolymerization to prepare a flame-retardant polymer, forming a "PZ-isocyanate-DOPO-HQ" flame retardant polymer. The invention utilizes the synergistic effect of PZ and DOPO-HQ to achieve a highly efficient multi-element synergistic effect of phosphorus-nitrogen-phosphorus. Furthermore, this flame-retardant polymer can form an interpenetrating network structure with the polyurethane prepolymer, significantly improving the overall flame-retardant performance of the material while effectively maintaining excellent adhesion and flexibility. The resulting pressure-sensitive adhesive is halogen-free and environmentally friendly, meeting the stringent requirements of low smoke, non-toxicity, high flame retardancy, and high safety for aerospace materials. It is particularly suitable for applications such as aircraft interiors, floor bonding, wire harness fixing, and composite material bonding. Moreover, the pressure-sensitive adhesive preparation method provided by this invention is process-controllable, simple to operate, and has promising application prospects. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the process flow for preparing an aerospace flame-retardant polyurethane pressure-sensitive adhesive according to the present invention. Figure 2 This is a synthetic route diagram for dihydroxycyclophosphonitrile diol; Figure 3 This is a synthetic route diagram for flame-retardant polymers. Detailed Implementation

[0024] This invention provides an aerospace flame-retardant polyurethane pressure-sensitive adhesive and its preparation method. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0025] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless specifically defined as herein.

[0026] like Figure 1 As shown, this invention provides a method for preparing an aerospace flame-retardant polyurethane pressure-sensitive adhesive, comprising the following steps: Step S10: Modify the cyclophosphonitrile compound to obtain dihydroxycyclophosphonitrile diol; Step S20: The dihydroxycyclophosphonitrile diol is mixed with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, diisocyanate, and organotin catalyst, and copolymerized to obtain a flame-retardant polymer. Step S30: Mix the flame-retardant polymer with the polyurethane prepolymer and the curing agent to obtain a curable pressure-sensitive adhesive composition; Step S40: The curable pressure-sensitive adhesive composition is coated onto a substrate and cured to obtain an aerospace flame-retardant polyurethane pressure-sensitive adhesive.

[0027] In this embodiment, by modifying cyclophosphonitrile to a dihydroxy compound (i.e., modified cyclophosphonitrile, PZ), its excellent gas-phase dilution flame retardancy and ceramic charring capabilities are retained, while avoiding the crosslinking and catalytic problems caused by the multifunctional groups of cyclophosphonitrile. Then, it is copolymerized with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-HQ) via isocyanate to prepare a flame-retardant polymer, forming “PZ-isocyanate-DOPO-HQ”. The invention presents a flame-retardant structure; utilizing the synergistic effect of PZ and DOPO-HQ, it achieves a highly efficient synergistic effect of phosphorus-nitrogen-phosphorus multi-element synthesis; furthermore, this flame-retardant polymer can form an interpenetrating network structure with polyurethane prepolymer, significantly improving the overall flame-retardant performance of the material while effectively maintaining excellent adhesion and flexibility. The resulting pressure-sensitive adhesive is halogen-free and environmentally friendly, meeting the stringent requirements of low smoke, non-toxicity, high flame retardancy, and high safety for aerospace materials, making it particularly suitable for applications such as aircraft interiors, floor bonding, wire harness fixing, and composite material bonding. Moreover, the pressure-sensitive adhesive preparation method provided by this invention is process-controllable, simple to operate, and has promising application prospects.

[0028] Specifically, this invention uses modified cyclophosphonitrile (PZ) and DOPO-HQ to polymerize a high-molecular flame retardant, synergistically constructing a reactive flame retardant system. Furthermore, the flame retardant polymer uses cyclophosphonitrile as a rigid core and isocyanate segments as bridges, combining the free radical quenching and char-forming functions of DOPO-HQ to construct a phosphorus-nitrogen-phosphorus synergistic flame retardant system, possessing the capabilities of gas-phase dilution, free radical capture, and dense char layer formation. Cyclophosphonitrile provides a large amount of inert gas to dilute the flame, while DOPO in DOPO-HQ captures free radicals and inhibits chain reactions, jointly promoting the formation of a dense and stable char layer, significantly improving flame retardant efficiency. This design not only meets the stringent requirements of the aerospace industry for low smoke, non-toxicity, and high flame retardancy, but also, through molecular structure design and the formation of an interpenetrating network structure with the polyurethane prepolymer, ensures the excellent viscoelastic properties of the polyurethane pressure-sensitive adhesive, providing a solution for the development of high-performance and environmentally friendly aerospace flame-retardant pressure-sensitive adhesives.

[0029] In some embodiments, in step S10, the cyclophosphonitrile compound is modified to obtain dihydroxycyclophosphonitrile diol. The synthetic route is shown below. Figure 2 As shown, the steps include: Step S11: Hexachlorocyclotriphosphazene (HCCP) is mixed with a monofunctional hydroxyl compound, an acid-binding agent, and an organic solvent, and after partial end-capping reaction, a first product with one active chlorine atom is obtained; Step S12: The first product having an active chlorine atom is mixed with a glycerol compound and subjected to a nucleophilic substitution reaction to obtain dihydroxycyclophosphonitrile diol.

[0030] In this embodiment, the hexachlorocyclotriphosphazene and the monofunctional hydroxyl compound are partially capped in an organic solvent in the presence of an acid-binding agent, so that some chlorine atoms on the cyclotriphosphazene skeleton are replaced by the monofunctional hydroxyl compound, retaining one active chlorine atom. Then, the first product with one active chlorine atom undergoes nucleophilic substitution with the hydroxyl group of glycerol, i.e., chain-growing dihydroxylation, forming two free hydroxyl groups at the end of the molecular chain, to obtain a linearized, dihydroxyl cyclophosphazene diol. By modifying the cyclophosphazene compound, the resulting dihydroxyl cyclophosphazene diol retains its excellent gas-phase dilution flame retardancy and ceramic charring ability, while avoiding the crosslinking and embrittlement problems caused by the multifunctionality of cyclophosphazene. It can then be copolymerized with DOPO-HQ via isocyanate to obtain a polymeric flame retardant, forming a "PZ-isocyanate-DOPO-HQ" flame retardant structure, which can utilize the synergistic effect of the two to achieve a high-efficiency synergistic effect of phosphorus-nitrogen-phosphorus multi-element.

[0031] In some embodiments, the monofunctional hydroxyl compound has the molecular formula R1-OH, where R1 is C n H 2nWhere n is 1, 2, 3, or 4; preferably, the monofunctional hydroxyl compound includes one or more of methanol, ethanol, and butanol; the molar ratio of the hexachlorocyclotriphosphazene to the monofunctional hydroxyl compound is 1:5. The monofunctional hydroxyl compound can react with the chlorine atoms on the hexachlorocyclotriphosphazene to cap it. By reacting five of the six chlorine atoms on the hexachlorocyclotriphosphazene, leaving only one to further react with the glycerol compound, modification of the hexachlorocyclotriphosphazene is achieved.

[0032] In some embodiments, the organic solvent includes, but is not limited to, one or more of tetrahydrofuran (THF), N,N-dimethylformamide (DMF), diethyl ether, dichloromethane, acetonitrile, and N-methylpyrrolidone (NMP); the amount of the organic solvent added is 0.5-10 times the mass of the hexachlorocyclotriphosphazene. The above-mentioned organic solvent provides a reaction environment for the partially capping reaction, allowing the hexachlorocyclotriphosphazene (HCCP) to fully contact the monofunctional hydroxyl compound and the acid-binding agent, thereby improving the reaction efficiency.

[0033] In some embodiments, the acid-binding agent is, but is not limited to, one or more of triethylamine, triethylamine derivatives, diisopropylethylamine, diisopropylethylamine derivatives, pyridine, and pyridine derivatives; the amount of the acid-binding agent added is 0.1%-1% of the mass of hexachlorocyclotriphosphazene. Using the above-mentioned acid-binding agent as a key auxiliary agent in the reaction of hexachlorocyclotriphosphazene (HCCP) with the monofunctional hydroxyl compound, it is used to neutralize the acidic byproducts generated during the reaction, maintain the alkaline environment of the reaction system, and thus ensure the smooth progress of the reaction. Controlling the amount of the acid-binding agent added between 0.1% and 1% can effectively maintain the pH of the reaction system within a stable range, avoid the damage of acid-sensitive functional groups by strong acids, and effectively neutralize the acids generated in the end-capping reaction, improving the end-capping efficiency.

[0034] In some embodiments, the amount of acid-binding agent added may be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1% of the mass of the hexachlorocyclotriphosphazene, but is not limited thereto.

[0035] In some embodiments, the partial capping reaction is carried out at a temperature of 0°C-40°C for 1-3 hours. At this temperature and time, the hexachlorocyclotriphosphazene reacts with the monofunctional hydroxyl compound in an organic solvent in the presence of an acid-binding agent, causing some chlorine atoms on the cyclotriphosphazene skeleton to be replaced by the monofunctional hydroxyl compound, retaining one active chlorine atom, to obtain a first product having one active chlorine atom.

[0036] In a preferred embodiment, the partial capping reaction is carried out at a temperature of 25±5°C for 1-3 hours. At this temperature, no special heating or freezing treatment is required, allowing the partial capping reaction to occur at room temperature to obtain a first product with one active chlorine atom.

[0037] In some embodiments, the nucleophilic substitution reaction is carried out at a temperature of 50°C-75°C and for a duration of 2-4 hours. By controlling the temperature and time of the nucleophilic substitution reaction within the above ranges, a cyclotriphosphazene with only one chlorine atom remaining can react with one hydroxyl group on glycerol, exposing the remaining two hydroxyl groups on glycerol in the resulting compound, thereby yielding dihydroxycyclophosphazene diol.

[0038] In a preferred embodiment, the nucleophilic substitution reaction is carried out at a temperature of 70±5℃; the reaction time is 3 hours; and the reaction is conducted under nitrogen protection. By modifying cyclophosphonitriles into dihydroxy compounds under these conditions, their excellent gas-phase dilution flame retardancy and ceramic charring capabilities are retained, while the crosslinking and embrittlement problems caused by the multifunctionality of cyclophosphonitriles are avoided.

[0039] In some embodiments, the glycerol compound is, but is not limited to, a C3-C6 saturated polyol containing two or three hydroxyl groups; the molar ratio of the first product having one active chlorine atom to the glycerol compound is 1:1. By modifying cyclophosphonitrile into a dihydroxy compound using the above-mentioned glycerol compound, its excellent gas-phase dilution flame retardancy and ceramic charring ability are retained, while avoiding the crosslinking and embrittlement problems caused by the multifunctionality of cyclophosphonitrile.

[0040] In a preferred embodiment, the glycerol compound is glycerol, and by using it with the first product having an active chlorine atom, the simplest structure, dihydroxycyclophosphonitrile diol, can be obtained. This is beneficial for subsequent copolymerization with DOPO-HQ via isocyanate to prepare a polymeric flame retardant, forming a flame retardant structure of "PZ-isocyanate-DOPO-HQ".

[0041] In some embodiments, in step S20, the dihydroxycyclophosphonitrile diol is mixed with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, diisocyanate, and organotin catalyst, and then copolymerized to obtain the flame-retardant polymer. The synthetic route is shown in the diagram. Figure 3As shown, a high-molecular-weight flame-retardant polymer was prepared by using diisocyanate as a linker and, under the action of an organotin catalyst, sequentially linking DOPO-HQ and the dihydroxycyclophosphamide diol. This flame-retardant polymer can be cross-linked with polyurethane prepolymer and curing agent to form an interpenetrating network structure, significantly improving the overall flame-retardant performance of the material while effectively maintaining excellent adhesion and flexibility.

[0042] In some embodiments, the copolymerization reaction temperature is 60℃-90℃, and the copolymerization reaction time is 2h-5h. At this copolymerization reaction temperature and time, the dihydroxycyclophosphonitrile glycol and the 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide can be polymerized using the diisocyanate as a linker under organotin catalyst conditions, thereby sequentially linking the dihydroxycyclophosphonitrile glycol and the 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide together to form a high molecular weight flame retardant polymer.

[0043] In a preferred embodiment, the copolymerization reaction is carried out at a temperature of 70±5℃ for 3 hours.

[0044] In some embodiments, the diisocyanate has the molecular formula NCO-R2-NCO; specifically, the structural formula of the diisocyanate includes, but is not limited to, one of the following: , , , , .

[0045] In some embodiments, the diisocyanate includes, but is not limited to, one or more of hexamethylene diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, and isophorone diisocyanate; the organotin catalyst includes, among, dibutyltin dilaurate and stannous octoate. The aforementioned diisocyanate can serve as a linker in the copolymerization reaction of the dihydroxycyclophosphonitrile diol and the 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, sequentially linking the two together to form a high-molecular-weight flame-retardant polymer.

[0046] In some embodiments, the mass ratio of the 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, the dihydroxycyclophosphamide diol, the diisocyanate, and the organotin catalyst is 1:(1-3):(1-3):(0.01-0.5). At this mass ratio, the copolymerization rate can be increased, and a flame-retardant polymer with a degree of polymerization between 1 and 100 is obtained.

[0047] In some embodiments, the chemical structural formula of the flame-retardant polymer is as follows: Where m is an integer between 1 and 100.

[0048] In some embodiments, the polyurethane prepolymer is obtained by dehydration polymerization of, but not limited to, polyester polyol or polyether polyol with aromatic or aliphatic diisocyanate. Alternatively, the polyurethane prepolymer can be a commercially available polyurethane prepolymer. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive provided by this invention is applicable to commercially available polyurethane prepolymers. Utilizing the interpenetrating network structure formed by the prepolymer and the flame-retardant polymer, the overall flame-retardant performance of the material is significantly improved, while effectively maintaining excellent adhesion and flexibility. Furthermore, the resulting pressure-sensitive adhesive is halogen-free and environmentally friendly, meeting the stringent requirements of low smoke, non-toxicity, high flame retardancy, and high safety for aerospace materials.

[0049] In some embodiments, the curing agent is an isocyanate-based curing agent, including but not limited to one or more of hexamethylene diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate trimer, and isophorone diisocyanate trimer. Using the above-mentioned curing agent allows the material to be transformed from a liquid / plastic state into a solid elastomer, forming a pressure-sensitive adhesive with pressure-sensitive bonding properties.

[0050] In some embodiments, the amount of curing agent added is 1%-15% of the mass of the polyurethane prepolymer. Controlling the amount of curing agent added within the above range ensures sufficient chemical crosslinking while preventing excessive crosslinking and embrittlement.

[0051] In some embodiments, the amount of curing agent added can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15% of the mass of the polyurethane prepolymer, but is not limited to integers.

[0052] In some embodiments, the flame-retardant polymer is present in a mass fraction of 5%-15% in the curable pressure-sensitive adhesive composition. Controlling the mass fraction of the flame-retardant polymer in the curable pressure-sensitive adhesive composition between 5% and 15% ensures that, after curing, the resulting aerospace flame-retardant polyurethane pressure-sensitive adhesive meets the UL-94 V-0 flame retardant rating and has a limiting oxygen index (LOI) ≥ 28%.

[0053] In some embodiments, the curing temperature is 80℃-130℃ and the curing time is 0.5h-2h; curing at this temperature and time yields a flame-retardant adhesive film with pressure-sensitive bonding properties, namely, aerospace flame-retardant polyurethane pressure-sensitive adhesive.

[0054] In addition, the present invention also provides an aerospace flame-retardant polyurethane pressure-sensitive adhesive, which is prepared using the preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive.

[0055] In this embodiment, the aviation flame-retardant polyurethane pressure-sensitive adhesive prepared by the above method is halogen-free and environmentally friendly, meeting the stringent requirements of low smoke and non-toxicity, high flame retardancy and high safety of aviation materials. It is particularly suitable for scenarios such as bonding of aircraft interior parts, bonding of floors, fixing of wire harnesses, installation of sensors and splicing of composite material skins.

[0056] Specifically, the advantages of the aerospace flame-retardant polyurethane pressure-sensitive adhesive include: highly efficient and long-lasting halogen-free flame retardancy; excellent resistance to flame retardant migration, ensuring long-term stability; virtually undamaged bonding performance, balancing functionality and reliability; and good thermal aging stability and process adaptability.

[0057] In summary, the aerospace flame-retardant polyurethane pressure-sensitive adhesive provided by this invention constructs a linear polymeric flame-retardant polymer of the "cyclophosphonitrile-isocyanate-DOPO-HQ" type. This structure uses the rigid core of cyclophosphonitrile as the thermal stability and char formation center, the aromatic ring structure of DOPO-HQ as the free radical quenching unit, and the isocyanate segment as the compatibility adjustment region, achieving multifunctional integration and solving the problems of easy migration and poor compatibility of traditional phosphorus-nitrogen flame retardants. Furthermore, this aerospace flame-retardant polyurethane pressure-sensitive adhesive can achieve a synergistic flame-retardant mechanism of "phosphorus-nitrogen-phosphorus" multi-effect spatiotemporal synergy. By utilizing the synergistic release of non-flammable gases (such as NH3, N2, PO· free radicals) by cyclophosphonitrile and DOPO-HQ structure at high temperatures, the flammable gases are diluted, and the free radical chain reaction is inhibited, achieving the effect of gas-phase flame retardancy. At the same time, the phosphorus element and benzene ring structure in DOPO-HQ promote the formation of a dense and stable phosphorus-rich char layer, and the cyclophosphonitrile structure enhances the thermal stability and barrier properties of the char layer, effectively preventing heat and oxygen transfer, achieving the effect of condensed phase flame retardancy. In addition, the "cyclophosphonitrile-isocyanate-DOPO-HQ" type linear polymer flame retardant polymer constructed in this invention can be directly added to ordinary polyurethane prepolymer systems and crosslinked with curing agents to form an interpenetrating network structure. While significantly improving the flame retardant rating, it effectively avoids problems such as decreased viscosity, making the flame retardant polymer convenient to use and highly efficient in balancing flame retardancy and adhesion performance.

[0058] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention.

[0059] Example 1 This embodiment provides an aerospace flame-retardant polyurethane pressure-sensitive adhesive, the specific preparation process of which is as follows: 1) Preparation of dihydroxycyclophosphonitrile diol Partial end-capping reaction: In a dry four-necked flask, hexachlorocyclotriphosphazene (HCCP, 1.0 mol), butanol (5.0 mol), and tetrahydrofuran (THF, 300 mL) were added as solvents. After stirring to dissolve, triethylamine (5.0 mol) was slowly added dropwise as an acid-binding agent. The reaction temperature was controlled at 25±5℃, and the reaction was carried out for 2 hours. The reaction progress was monitored by TLC. After the reaction was completed, the generated triethylamine hydrochloride was removed by filtration, and the solvent was removed by vacuum distillation to obtain the intermediate.

[0060] Chain-growing dihydroxylation reaction: The above intermediate was dissolved in DMF (200 mL), glycerol (1.0 mol) was added, the temperature was raised to 70 °C, and the reaction was carried out for 3 hours. HPLC monitoring was used to ensure complete reaction of the active chlorine atoms. After cooling, the product was washed successively with dilute hydrochloric acid and water until neutral, extracted with ethyl acetate, dried over anhydrous sodium sulfate, and distilled under reduced pressure to obtain a pale yellow viscous product, which was dihydroxycyclophosphonitrile diol, with a yield of approximately 85%.

[0061] 2) Preparation of flame-retardant polymers The dihydroxycyclophosphonitrile diol (20 g) obtained in step 1) was added to a reaction vessel along with DOPO-HQ (20 g) and HDI (hexamethylene diisocyanate, 20 g). A trace amount of dibutyltin dilaurate (DBTDL, 0.05 wt%) was added. Under nitrogen protection, the temperature was raised to 70±5℃ and the reaction was carried out for 3 hours to obtain the flame-retardant polymer, which was then sealed and stored for later use.

[0062] 3) Formulation of curable pressure-sensitive adhesive composition Take 80g of commercially available polyester polyurethane prepolymer as the main agent, add 20g of the flame retardant polymer obtained in step 2), stir evenly, then add 8g of HDI trimer curing agent, and continue stirring for 30 minutes to obtain a homogeneous curable pressure-sensitive adhesive composition.

[0063] 4) Coating and curing The curable pressure-sensitive adhesive composition obtained in step 3 is evenly coated onto the release PET film using a scraper, with the wet film thickness controlled at 1 mm. It is then placed in a 120°C forced-air drying oven for 1 hour to cure. After cooling, the aerospace flame-retardant polyurethane pressure-sensitive adhesive is obtained.

[0064] Example 2 This embodiment provides an aerospace flame-retardant polyurethane pressure-sensitive adhesive, the specific preparation process of which is as follows: 1) Preparation of dihydroxycyclophosphonitrile diol Partial end-capping reaction: In a dry four-necked flask, hexachlorocyclotriphosphazene (HCCP, 1.0 mol), methanol (5.0 mol), and tetrahydrofuran (THF, 300 mL) were added as solvents. After stirring to dissolve, triethylamine (5.0 mol) was slowly added dropwise as an acid-binding agent. The reaction temperature was controlled at 25±5℃, and the reaction was carried out for 2 hours. The reaction progress was monitored by TLC. After the reaction was completed, the generated triethylamine hydrochloride was removed by filtration, and the solvent was removed by vacuum distillation to obtain the intermediate.

[0065] Chain-growing dihydroxylation reaction: The above intermediate was dissolved in DMF (200 mL), glycerol (1.0 mol) was added, the temperature was raised to 70 °C, and the reaction was carried out for 3 hours. HPLC monitoring was used to ensure complete reaction of the active chlorine atoms. After cooling, the product was washed successively with dilute hydrochloric acid and water until neutral, extracted with ethyl acetate, dried over anhydrous sodium sulfate, and distilled under reduced pressure to obtain a pale yellow viscous product, which was dihydroxycyclophosphonitrile diol, with a yield of approximately 85%.

[0066] 2) Preparation of flame-retardant polymers The dihydroxycyclophosphonitrile diol (20 g) obtained in step 1) was added to a reaction vessel along with DOPO-HQ (20 g) and HDI (hexamethylene diisocyanate, 20 g). A trace amount of dibutyltin dilaurate (DBTDL, 0.05 wt%) was added. Under nitrogen protection, the temperature was raised to 70±5℃ and the reaction was carried out for 3 hours to obtain the flame-retardant polymer, which was then sealed and stored for later use.

[0067] 3) Formulation of curable pressure-sensitive adhesive composition Take 80g of commercially available polyester polyurethane prepolymer as the main agent, add 20g of the flame retardant polymer obtained in step 2), stir evenly, then add 8g of HDI trimer curing agent, and continue stirring for 30 minutes to obtain a homogeneous curable pressure-sensitive adhesive composition.

[0068] 4) Coating and curing The curable pressure-sensitive adhesive composition obtained in step 3 is evenly coated onto the release PET film using a scraper, with the wet film thickness controlled at 1 mm. It is then placed in a 120°C forced-air drying oven for 1 hour to cure. After cooling, the aerospace flame-retardant polyurethane pressure-sensitive adhesive is obtained.

[0069] In this embodiment, compared to Example 1, the butanol in part of the capping reaction is replaced with methanol.

[0070] Example 3 This embodiment provides an aerospace flame-retardant polyurethane pressure-sensitive adhesive, the specific preparation process of which is as follows: 1) Preparation of dihydroxycyclophosphonitrile diol Partial end-capping reaction: In a dry four-necked flask, hexachlorocyclotriphosphazene (HCCP, 1.0 mol), butanol (5.0 mol), and tetrahydrofuran (THF, 300 mL) were added as solvents. After stirring to dissolve, triethylamine (5.0 mol) was slowly added dropwise as an acid-binding agent. The reaction temperature was controlled at 25±5℃, and the reaction was carried out for 2 hours. The reaction progress was monitored by TLC. After the reaction was completed, the generated triethylamine hydrochloride was removed by filtration, and the solvent was removed by vacuum distillation to obtain the intermediate.

[0071] Chain-growing dihydroxylation reaction: The above intermediate was dissolved in DMF (200 mL), glycerol (1.0 mol) was added, the temperature was raised to 70 °C, and the reaction was carried out for 3 hours. HPLC monitoring was used to ensure complete reaction of the active chlorine atoms. After cooling, the product was washed successively with dilute hydrochloric acid and water until neutral, extracted with ethyl acetate, dried over anhydrous sodium sulfate, and distilled under reduced pressure to obtain a pale yellow viscous product, which was dihydroxycyclophosphonitrile diol, with a yield of approximately 85%.

[0072] 2) Preparation of flame-retardant polymers The dihydroxycyclophosphonitrile diol (20 g) obtained in step 1) was added to a reaction vessel along with DOPO-HQ (20 g) and IPDI (isophorone diisocyanate, 20 g). A trace amount of dibutyltin dilaurate (DBTDL, 0.05 wt%) was added. Under nitrogen protection, the temperature was raised to 70±5℃ and the reaction was carried out for 3 hours to obtain the flame-retardant polymer, which was then sealed and stored for later use.

[0073] 3) Formulation of curable pressure-sensitive adhesive composition Take 80g of commercially available polyester polyurethane prepolymer as the main agent, add 20g of the flame retardant polymer obtained in step 2), stir evenly, then add 8g of IPDI trimer curing agent, and continue stirring for 30 minutes to obtain a homogeneous curable pressure-sensitive adhesive composition.

[0074] 4) Coating and curing The curable pressure-sensitive adhesive composition obtained in step 3 is evenly coated onto the release PET film using a scraper, with the wet film thickness controlled at 1 mm. It is then placed in a 120°C forced-air drying oven for 1 hour to cure. After cooling, the aerospace flame-retardant polyurethane pressure-sensitive adhesive is obtained.

[0075] In this embodiment, compared to Example 1, HDI in the preparation of the flame retardant polymer and the formulation of the curable pressure-sensitive adhesive composition is replaced with isophorone diisocyanate (IPDI).

[0076] Example 4 This embodiment provides an aerospace flame-retardant polyurethane pressure-sensitive adhesive, which differs from Embodiment 1 in that the amount of flame-retardant polymer added is increased from 20g to 30g in the preparation step of the curable pressure-sensitive adhesive composition, while the other conditions are the same as in Embodiment 1.

[0077] Comparative Example 1 This comparative example provides a physically blended halogenated flame-retardant pressure-sensitive adhesive system, as detailed below: Instead of using any flame-retardant polymer components, 25 g of decabromodiphenyl ethane and 5 g of antimony trioxide were directly added to 100 g of polyurethane main agent, dispersed at high speed for 30 minutes, and then 8 g of HDI trimer was added. After mixing evenly, the mixture was coated and cured, with the process being the same as in Example 1.

[0078] Comparative Example 2 Mix DOPO-HQ (20 g) with ordinary polyurethane main agent (100 g), then add HDI trimer curing agent (8 g), stir evenly, and then coat and cure. The process is the same as in Example 1.

[0079] Comparative Example 3 Mix DOPO-HQ (40 g) with ordinary polyurethane main agent (100 g), then add HDI trimer curing agent (8 g), stir evenly, and then coat and cure. The process is the same as in Example 1.

[0080] Comparative Example 4 Mix 100 g of ordinary polyurethane main agent, then add 8 g of HDI trimer curing agent, stir evenly, and then coat and cure. The process is the same as in Example 1.

[0081] The performance of the pressure-sensitive adhesives prepared in Examples 1-4 and Comparative Examples 1-4 was characterized, and the specific results are shown in Table 1: Table 1

[0082] As shown by the characterization data in Table 1, the synergistic introduction of dihydroxycyclophosphonitrile diol and DOPO-HQ into the polyurethane backbone creates a novel halogen-free flame-retardant polyurethane pressure-sensitive adhesive system. This system not only achieves high-efficiency flame retardancy (UL-94 V-0 rating, LOI≥31%), but also significantly reduces smoke density and heat release rate during combustion, meeting the stringent requirements of aerospace materials for "high flame retardancy, low smoke, halogen-free, and low toxicity".

[0083] Furthermore, because the flame-retardant components are chemically bonded into the polyurethane backbone to form a stable covalent structure, no flame retardant migration, precipitation, or volatilization occurs during long-term thermal aging or high-temperature environments. Experiments show (see Table 1) that no flame retardant precipitation was detected in Examples 1-4 after 7 days of aging with double 85, which is significantly better than the physical blend system (comparative Example 1 showed obvious precipitation). This effectively ensures the flame-retardant stability and electrical safety of the material during long-term service, avoiding the risk of interface failure or short circuits caused by flame retardant migration.

[0084] Regarding adhesion performance, this invention, by rationally controlling the content of flame-retardant components (preferably 15-20 wt%) and the linearity of the molecular structure, introduces a flame-retardant structure while maximizing the retention of the flexibility and adhesion of the polyurethane matrix. In contrast, Comparative Examples 2 and 3 exhibit significantly deteriorated adhesion due to structural embrittlement.

[0085] Furthermore, the synergistic flame-retardant mechanism of cyclophosphamide and DOPO works simultaneously in the gas phase (free radical capture) and the condensed phase (char formation enhancement), forming a dense char layer that effectively isolates heat and oxygen, thus achieving high-efficiency flame retardancy and low heat release. Comparative Example 2 (DOPO only), lacking the polyhedral rigid structure of cyclophosphamide and the synergistic effect of the nitrogen source, exhibits significantly insufficient flame-retardant efficiency. Comparative Example 3, with the addition of a large amount of DOPO, while achieving satisfactory flame retardancy, significantly impacts adhesion.

[0086] In summary, this invention provides an aerospace flame-retardant polyurethane pressure-sensitive adhesive and its preparation method. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive includes the following steps: modifying a cyclophosphonitrile compound to obtain dihydroxycyclophosphonitrile diol; mixing the dihydroxycyclophosphonitrile diol with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, diisocyanate, and organotin catalyst, and copolymerizing them to obtain a flame-retardant polymer; mixing the flame-retardant polymer with a polyurethane prepolymer and a curing agent to obtain a curable pressure-sensitive adhesive composition; coating the curable pressure-sensitive adhesive composition onto a substrate and curing it to obtain the aerospace flame-retardant polyurethane pressure-sensitive adhesive. This invention modifies cyclophosphonitrile into a dihydroxy compound (i.e., modified cyclophosphonitrile, PZ), retaining its excellent gas-phase dilution flame retardancy and ceramic charring capabilities while avoiding the crosslinking and catalytic problems caused by the multifunctional groups of cyclophosphonitrile. Then, it copolymerizes PZ with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-HQ) via isocyanate copolymerization to prepare a flame-retardant polymer, forming a "PZ-isocyanate-DOPO-HQ" flame retardant polymer. The invention utilizes the synergistic effect of PZ and DOPO-HQ to achieve a highly efficient multi-element synergistic effect of phosphorus-nitrogen-phosphorus. Furthermore, this flame-retardant polymer can form an interpenetrating network structure with the polyurethane prepolymer, significantly improving the overall flame-retardant performance of the material while effectively maintaining excellent adhesion and flexibility. The resulting pressure-sensitive adhesive is halogen-free and environmentally friendly, meeting the stringent requirements of low smoke, non-toxicity, high flame retardancy, and high safety for aerospace materials. It is particularly suitable for applications such as aircraft interiors, floor bonding, wire harness fixing, and composite material bonding. Moreover, the pressure-sensitive adhesive preparation method provided by this invention is process-controllable, simple to operate, and has promising application prospects.

[0087] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for preparing an aerospace flame-retardant polyurethane pressure-sensitive adhesive, characterized in that, Including the following steps: The cyclophosphonitrile compound was modified to obtain dihydroxycyclophosphonitrile diol; The dihydroxycyclophosphonitrile diol was mixed with 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, diisocyanate, and organotin catalyst, and then copolymerized to obtain a flame-retardant polymer. The flame-retardant polymer is mixed with a polyurethane prepolymer and a curing agent to obtain a curable pressure-sensitive adhesive composition. The curable pressure-sensitive adhesive composition is coated onto a substrate and cured to obtain an aerospace flame-retardant polyurethane pressure-sensitive adhesive.

2. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive according to claim 1, characterized in that, The steps of modifying cyclophosphonitrile compounds to obtain dihydroxycyclophosphonitrile diol include: Hexachlorocyclotriphosphazene was mixed with a monofunctional hydroxyl compound, an acid-binding agent, and an organic solvent, and after partial end-capping reaction, a first product with one active chlorine atom was obtained. The first product, which has an active chlorine atom, is mixed with a glycerol compound and subjected to a nucleophilic substitution reaction to obtain dihydroxycyclophosphonitrile diol.

3. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive according to claim 2, characterized in that, The monofunctional hydroxyl compound includes one or more of methanol, ethanol, and butanol; the molar ratio of the hexachlorocyclotriphosphazene to the monofunctional hydroxyl compound is 1:5; And / or, the organic solvent includes one or more of tetrahydrofuran, N,N-dimethylformamide, diethyl ether, dichloromethane, acetonitrile, and N-methylpyrrolidone; the amount of the organic solvent added is 0.5-10 times the mass of the hexachlorocyclotriphosphazene.

4. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive according to claim 2, characterized in that, The acid-binding agent is one or more of triethylamine, a derivative of triethylamine, diisopropylethylamine, a derivative of diisopropylethylamine, pyridine, and a derivative of pyridine; the amount of the acid-binding agent added is 0.1%-1% of the mass of the hexachlorocyclotriphosphazene.

5. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive according to claim 2, characterized in that, The temperature of the partial end-capping reaction is 0℃-40℃, and the time of the partial end-capping reaction is 1h-3h; And / or, the temperature of the nucleophilic substitution reaction is 50℃-75℃; the time of the nucleophilic substitution reaction is 2h-4h.

6. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive according to claim 2, characterized in that, The glycerol compound is a C3-C6 saturated polyol containing two or three hydroxyl groups; the molar ratio of the first product having one active chlorine atom to the glycerol compound is 1:

1.

7. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive according to claim 1, characterized in that, The copolymerization reaction is carried out at a temperature of 60℃-90℃ for 2h-5h.

8. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive according to claim 1, characterized in that, The diisocyanate includes one or more of hexamethylene diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, and isophorone diisocyanate; the organotin catalyst includes one of dibutyltin dilaurate and stannous octoate. Preferably, the mass ratio of the 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, the dihydroxycyclophosphamide diol, the diisocyanate, and the organotin catalyst is 1:(1-3):(1-3):(0.01-0.5); Preferably, the polyurethane prepolymer is prepared by dehydration polymerization of polyester polyol or polyether polyol with aromatic or aliphatic diisocyanate; Preferably, the curing agent comprises one or more of hexamethylene diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate trimer, and isophorone diisocyanate trimer; Preferably, the amount of curing agent added is 1%-15% of the mass of the polyurethane prepolymer.

9. The preparation method of the aerospace flame-retardant polyurethane pressure-sensitive adhesive according to claim 1, characterized in that, The flame-retardant polymer has a mass fraction of 5%-15% in the curable pressure-sensitive adhesive composition.

10. An aerospace flame-retardant polyurethane pressure-sensitive adhesive, characterized in that, It is prepared using the preparation method of aerospace flame-retardant polyurethane pressure-sensitive adhesive as described in any one of claims 1-9.

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