Sulfur-containing phosphazene flame-retardant polymer, flame-retardant polyformaldehyde fiber aerogel as well as preparation method and application of flame-retardant polyformaldehyde fiber aerogel

Abstract

The invention relates to the technical field of radiation refrigeration materials, and particularly discloses a sulfur-containing phosphazene flame-retardant polymer, flame-retardant polyformaldehyde fiber aerogel as well as a preparation method and application of the flame-retardant polyformaldehyde fiber aerogel. The polyformaldehyde fiber aerogel is prepared by taking a sulfur-containing phosphazene polymer as a binder and silane coupling agent modified polyformaldehyde fiber as a skeleton through simple freeze drying, the solar reflectivity of the polyformaldehyde fiber aerogel is 90% or above, the atmospheric transparent window emissivity is 90% or above, and the polyformaldehyde fiber aerogel has necessary conditions for daytime radiation cooling. Besides, the composite binder is rapidly carbonized when being in contact with flames, air can be effectively isolated, the flames are prevented from further spreading into the aerogel, the polyformaldehyde fiber composite aerogel is endowed with good flame retardance, and the polyformaldehyde fiber composite aerogel has wide application prospects in the fields of building energy conservation, outdoor equipment or aerospace.

Description

Technical Field

[0001] The present invention relates to the technical field of radiation refrigeration materials, and in particular to a sulfur-containing phosphazene flame-retardant polymer, a flame-retardant polyoxymethylene fiber aerogel, and a preparation method and application thereof. Background Art

[0002] As a novel material that achieves cooling without consuming additional energy, radiative cooling materials have shown tremendous potential for applications in building energy conservation, outdoor equipment, aerospace, and other fields. By efficiently reflecting solar radiation and emitting long-wave infrared radiation, they can reduce surface temperatures without relying on traditional refrigeration equipment, significantly impacting energy conservation and environmental improvement. However, as safety requirements continue to increase across diverse applications, the flame retardancy of radiative cooling materials has become a significant bottleneck hindering their widespread adoption.

[0003] However, most existing radiation cooling materials have the problem of insufficient flame retardancy. In actual application scenarios, especially in the fields of construction and aerospace, once the materials encounter a fire, it is very easy to spread rapidly, causing serious safety hazards and economic losses. Although traditional flame retardants, such as halogen flame retardants, have good flame retardant effects, their use has been gradually restricted because they release toxic and harmful gases when burned, causing harm to the environment and human health. While other non-halogen flame retardants improve the flame retardant properties of the materials, they often have a negative impact on the optical and thermal properties of the radiation cooling materials, resulting in a decrease in their radiation cooling efficiency. Therefore, the development of a material that has both excellent flame retardant properties and high-efficiency radiation cooling properties has become a key issue that needs to be urgently addressed in this field. Summary of the Invention

[0004] This invention addresses the difficulty of balancing flame retardancy and radiative cooling performance in existing radiative cooling materials. A novel flame-retardant polyoxymethylene fiber aerogel material has been designed. This material utilizes a sulfur-containing phosphazene-based flame-retardant polymer as a binder, combined with polyoxymethylene fibers modified with a silane coupling agent. Not only does it exhibit high solar reflectivity and atmospheric window reflectivity, enabling efficient radiative cooling, but it also possesses excellent flame retardancy and mechanical properties. This material demonstrates broad application prospects in the radiative cooling field, potentially addressing the application bottlenecks of existing materials and meeting the stringent requirements of various fields, including building energy conservation and aerospace.

[0005] In order to solve the above technical problems, the technical solution provided by the present invention is:

[0006] In a first aspect, the present invention provides a sulfur-containing phosphazene flame-retardant polymer, which is prepared by reacting naphthalene hydroxysulfonate and hexachlorocyclotriphosphazene.

[0007] Polyoxymethylene fiber has inherently weak flame retardancy, and combustion easily triggers dripping and releases toxic gases such as formaldehyde, posing a serious safety hazard. Furthermore, the lack of active groups on its surface makes it chemically inert. This characteristic results in low interfacial bonding with other functional materials when preparing radiative cooling composite materials. During actual use, due to this weak interfacial bonding, the different components are prone to separation and shedding, resulting in the inability of the materials to synergistically achieve radiative cooling, ultimately causing a significant decline in the overall performance of the material.

[0008] Compared to the prior art, the present invention addresses the aforementioned issues with polyoxymethylene fibers by designing a sulfur-containing phosphazene flame-retardant polymer as a binder for polyoxymethylene fibers. The phosphorus, nitrogen, and sulfur elements in this polymer's molecular structure synergistically form a highly effective flame-retardant system, effectively improving the inherently weak flame retardancy of polyoxymethylene fibers, suppressing the phenomenon of dripping during combustion, and reducing the release of toxic gases such as formaldehyde, thereby imparting excellent flame retardancy to polyoxymethylene fibers. Furthermore, the active groups, such as hydroxyl and sulfonic acid groups, in this polymer molecule, when used as a binder for polyoxymethylene fibers, enhance the compatibility and bonding strength of polyoxymethylene fibers with other functional materials. Furthermore, this polymer binder does not negatively impact the optical properties of polyoxymethylene fibers. While improving their flame retardancy, it ensures that the polyoxymethylene fibers maintain high solar reflectivity and atmospheric window reflectivity, guaranteeing their radiant cooling efficiency. Furthermore, this sulfur-containing phosphazene flame-retardant polymer also enhances the mechanical properties of polyoxymethylene fibers, giving them improved structural stability and durability in practical applications, meeting the stringent requirements of various fields, such as building energy conservation and aerospace.

[0009] Furthermore, the hydroxy naphthalene sulfonate substance is 1,3-dihydroxy-6-sulfonate naphthalene.

[0010] While other non-halogen flame retardants often negatively impact the optical and thermal properties of polyoxymethylene fibers when enhancing their flame retardancy, the flame-retardant polymer produced by reacting 1,3-dihydroxy-6-sulfonatonaphthalene with hexachlorocyclotriphosphazene does not significantly affect the solar reflectivity and atmospheric window reflectivity of the polyoxymethylene fiber aerogel, ensuring the material's radiative cooling efficiency and achieving a good balance between flame retardancy and radiative cooling performance.

[0011] Furthermore, the preparation method of the naphthalene hydroxysulfonate substance comprises the following steps:

[0012] The hydroxy naphthalene substances are subjected to a sulfonation reaction with concentrated sulfuric acid to obtain the hydroxy naphthalene sulfonate substances.

[0013] Furthermore, the mass concentration of the concentrated sulfuric acid is 85% to 95%; and the molar ratio of the hydroxynaphthalene substance to the concentrated sulfuric acid is 1:(1.3 to 1.4).

[0014] Furthermore, the temperature of the sulfonation reaction is 40° C. to 60° C., and the reaction time is 2 h to 4 h.

[0015] In a second aspect, the present invention provides a method for preparing the above-mentioned sulfur-containing phosphazene flame retardant polymer, comprising the following steps:

[0016] With organic amine as catalyst, hexachlorocyclotriphosphazene and naphthalene hydroxysulfonate undergo nucleophilic substitution reaction in an organic solvent to obtain sulfur-containing phosphazene flame retardant polymer.

[0017] The preparation method of the sulfur-containing phosphazene flame-retardant polymer provided by the present invention is simple to operate, does not require special equipment, and is suitable for large-scale production and application.

[0018] Furthermore, the molar ratio of the hexachlorocyclotriphosphazene to the naphthalene hydroxysulfonate is (1.2-1.5):1.

[0019] The preferred ratio helps to form a stable molecular structure of sulfur-containing phosphazene flame-retardant polymer, in which flame-retardant elements such as phosphorus, nitrogen, and sulfur can be evenly distributed, effectively enhancing the synergistic flame-retardant effect, maximally suppressing the droplet phenomenon during combustion of polyformaldehyde fiber, and reducing the release of toxic gases such as formaldehyde.

[0020] Furthermore, the organic amine is triethylamine.

[0021] Furthermore, the amount of the organic amine added is 0.5% to 1.2% of the total mass of the reactants.

[0022] Furthermore, the temperature of the nucleophilic substitution reaction is 80° C. to 120° C., and the reaction time is 6 h to 12 h.

[0023] Furthermore, the organic solvent is N,N-dimethylformamide.

[0024] In a third aspect, the present invention further provides the use of the above-mentioned sulfur-containing phosphazene flame-retardant polymer as a binder for preparing flame-retardant polyoxymethylene fiber aerogel.

[0025] In a fourth aspect, the present invention further provides a flame-retardant polyformaldehyde fiber aerogel, which is prepared by freeze-drying polyformaldehyde fiber modified with a silane coupling agent and the above-mentioned sulfur-containing phosphazene flame-retardant polymer.

[0026] In a fifth aspect, the present invention also provides a method for preparing a flame-retardant polyoxymethylene fiber aerogel, comprising the following steps:

[0027] S1, plasma-treating polyformaldehyde fiber, adding it to a silane coupling agent solution, impregnating it, then adding acid to carry out hydrolysis reaction, and drying it to obtain polyformaldehyde fiber modified with the silane coupling agent;

[0028] S2, dispersing the polyformaldehyde fiber modified with the silane coupling agent and the sulfur-containing phosphazene flame-retardant polymer in an organic solvent, and freeze-drying to obtain a flame-retardant polyformaldehyde fiber aerogel.

[0029] The preparation method of the flame-retardant polyformaldehyde fiber aerogel provided by the present invention first uses plasma to pretreat the polyformaldehyde fiber. The plasma treatment can effectively break the chemical inertness of the polyformaldehyde fiber surface and greatly improve the hydrophilicity and reaction activity of the fiber surface by introducing a large number of active groups, creating favorable conditions for the subsequent reaction with the silane coupling agent. After impregnation, acid hydrolysis is added to promote the hydrolysis of siloxane groups in the silane coupling agent to generate silanol groups. The silanol groups and the active groups on the fiber surface form stable chemical bonds through dehydration condensation, thereby constructing a strong coupling agent modified layer on the fiber surface, greatly enhancing the interfacial bonding strength between the fiber and other materials, and effectively avoiding the problems of delamination and shedding during the use of the aerogel.

[0030] The silane coupling agent-modified polyoxymethylene fiber and the sulfur-containing phosphazene flame-retardant polymer are dispersed in an organic solvent. The organic solvent's excellent dispersibility ensures their uniform distribution throughout the system, allowing the sulfur-containing phosphazene flame-retardant polymer to fully wrap and penetrate the polyoxymethylene fiber, fully exerting its flame-retardant properties and effectively suppressing the melting droplet phenomenon and the release of toxic gases such as formaldehyde during the combustion of the polyoxymethylene fiber. Finally, the polyoxymethylene fiber aerogel is produced through freeze-drying. The aerogel's porous structure not only helps improve the material's radiative cooling efficiency, ensuring high solar reflectivity and atmospheric window reflectivity, but also gives the aerogel excellent thermal insulation and lightweight properties, making it more applicable and competitive in fields such as building energy conservation and aerospace.

[0031] Furthermore, in S1, the polyoxymethylene fiber has a diameter of 10 to 30 μm and a length of 500 μm to 5 mm.

[0032] Preferably, in S1, the polyoxymethylene fiber has a diameter of 10 to 30 μm and a length of 800 μm to 2 mm.

[0033] It should be noted that the polyoxymethylene fiber described in the present invention is a nano-scale fiber prepared by electrospinning. The electrospinning process can adopt the existing technology in this field and is not particularly limited in the present invention.

[0034] Furthermore, in S1, the power of the plasma treatment is 300W to 500W, and the treatment time is 10min to 30min.

[0035] Furthermore, in S1, the silane coupling agent solution is a KH-550 solution with a mass concentration of 1 wt% to 3 wt%; and the solvent of the silane coupling agent solution is water and anhydrous ethanol in a volume ratio of 1:(8-10).

[0036] Furthermore, in S1, the immersion temperature is room temperature, and the immersion time is 2 hours to 4 hours.

[0037] Furthermore, in S1, the acid is glacial acetic acid.

[0038] Furthermore, in S1, the molar ratio of the glacial acetic acid to the silane coupling agent is (0.1-0.5):1; the temperature of the hydrolysis reaction is 50° C.-60° C., and the time of the hydrolysis reaction is 4 h-6 h.

[0039] Acid-catalyzed hydrolysis causes the silane coupling agent to form Si-OH groups, which can further condense to form a three-dimensional siloxane network (-Si-O-Si-), enhancing the connection strength between fibers. In addition, the silicon-oxygen network structure can form a barrier layer during combustion, producing a superimposed flame-retardant effect with the P, N, and S synergistic system of the sulfur-containing phosphazene polymer, reducing the heat release rate and combustion level of the material.

[0040] Specifically, in S1, after the hydrolysis reaction is completed, the process further includes the steps of centrifugation and washing with deionized water.

[0041] Furthermore, in S2, the mass ratio of the silane coupling agent-modified polyoxymethylene fiber to the sulfur-containing phosphazene flame-retardant polymer is (2-5):1.

[0042] The preferred addition ratio of the flame retardant polymer can avoid adverse effects on mechanical properties and radiant cooling effects while significantly improving the flame retardant properties.

[0043] Furthermore, in S2, the organic solvent is dimethyl sulfoxide.

[0044] The solvent can fully dissolve the sulfur-containing phosphazene flame retardant polymer, so that the sulfur-containing phosphazene flame retardant polymer can fully wrap and penetrate the polyoxymethylene fiber, thereby facilitating full exertion of its flame retardant performance.

[0045] Furthermore, in S2, the ratio of the total mass of the polyoxymethylene fiber modified by the silane coupling agent and the sulfur-containing phosphazene flame retardant polymer to the organic solvent is 1 g: (8-10) mL.

[0046] It should be noted that, in S2, the vacuum freeze-drying may be performed by conventional non-directional freezing, unidirectional freezing or bidirectional freezing in the art, and the present invention does not impose any special limitation thereto.

[0047] Specifically, in S2, the vacuum freeze-drying temperature is -80°C to -90°C, the vacuum degree is 5Pa to 20Pa, and the time is 24h to 84h.

[0048] In a sixth aspect, the present invention also provides applications of the flame-retardant polyoxymethylene fiber aerogel in the fields of building energy conservation, outdoor equipment, or aerospace.

[0049] The polyformaldehyde fiber aerogel material provided by the present invention achieves a perfect combination of efficient flame retardancy and radiative cooling. During the combustion process, the phosphorus-nitrogen-sulfur cross-linked network generated by the reaction of hexachlorocyclotriphosphazene and naphthalene hydroxysulfonate substances will rapidly decompose. On the one hand, it releases phosphorus-containing free radicals, which capture key free radicals in the combustion chain reaction. At the same time, it releases nitrogen to dilute the oxygen concentration, inhibiting combustion from the gas phase. On the other hand, phosphorus oxide can catalyze the dehydration and carbonization of polyformaldehyde, forming a dense carbon layer that isolates heat and oxygen. Secondly, the cross-linked network is tightly combined with the silane-modified polyformaldehyde fiber, forming a three-dimensional skeleton inside the material, limiting the violent movement of molecules at high temperatures, making the carbon layer more stable and less prone to collapse. The silane coupling agent on the surface decomposes at high temperatures to produce a SiO2 ceramic layer, which further reflects heat and delays the spread of fire. In addition, the low-density porous structure of the aerogel itself can effectively block heat convection and heat conduction, thereby significantly delaying the occurrence of combustion.

[0050] The present invention provides important technical support for the technological development of polyformaldehyde fiber aerogels, can fill the application gap of polyformaldehyde in the field of aerogels, and solve the technical problems of polyformaldehyde fiber aerogels such as poor flame retardancy, weak interface bonding, and susceptibility to interference in radiation cooling efficiency, and has high potential application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 The SEM images of the polyoxymethylene fiber used in the embodiment of the present invention at different magnifications are as follows;

[0052] Figure 2 The axial and radial scanning electron micrographs of the polyoxymethylene fiber aerogel prepared in Example 3 of the present invention are shown, where (a) is the axial direction and (b) is the radial direction.

[0053] Figure 3 This is a comparison chart of the compressive strength of the polyoxymethylene fiber aerogels prepared in Example 3 of the present invention and Comparative Examples 1 and 2;

[0054] Figure 4 This is a comparison chart of the average reflectivity of the polyoxymethylene fiber aerogels prepared in Example 3 of the present invention and Comparative Examples 1-2 in the sunlight band (0.3-2.5 μm);

[0055] Figure 5 This is a comparison chart of the average emissivity of the polyoxymethylene fiber aerogels prepared in Example 3 of the present invention and Comparative Examples 1 and 2 in the mid- and far-infrared bands (6-16 μm);

[0056] Figure 6 This is a combustion test diagram of the polyoxymethylene fiber aerogel prepared in Example 3 of the present invention;

[0057] Figure 7 This is a comparison chart of microcalorimetric parameters of the polyoxymethylene fiber aerogels prepared in Example 3 of the present invention and Comparative Examples 1-2. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0059] In order to better illustrate the present invention, further examples are given below.

[0060] The test materials used in the examples of the present invention are all conventional test materials in the art and can be purchased through commercial channels.

[0061] The polyoxymethylene fibers used in the following examples and comparative examples have a diameter of 10 to 30 μm and a length of 800 μm to 2 mm.

[0062] Example 1

[0063] This embodiment provides a method for preparing a sulfur-containing phosphazene flame-retardant polymer, comprising the following steps:

[0064] Dissolve 1,3-dihydroxynaphthalene in 95% concentrated sulfuric acid at a molar ratio of 1,3-dihydroxynaphthalene to concentrated sulfuric acid of 1:1.4, heat to 55°C and react for 3 hours to obtain 1,3-dihydroxy-6-sulfonate naphthalene;

[0065] 1,3-Dihydroxy-6-sulfonatonaphthalene and hexachlorocyclotriphosphazene were dissolved in N,N-dimethylformamide at a molar ratio of 1:1.4, with the ratio of N,N-dimethylformamide to hexachlorocyclotriphosphazene being 5 mL:1 g. Then, triethylamine, a catalyst, was added in an amount of 0.8% of the total mass of the reactants. The reaction was carried out at 90° C. for 10 hours, followed by centrifugation, washing with deionized water, and drying to obtain a sulfur-containing phosphazene flame retardant polymer.

[0066] This embodiment provides a method for preparing a flame-retardant polyoxymethylene fiber aerogel, comprising the following steps:

[0067] Step a, soaking the polyoxymethylene fiber in anhydrous ethanol and ultrasonically cleaning it for 30 minutes, vacuum drying it, then placing it in a plasma reaction chamber and treating it at 300W for 30 minutes, then soaking it in a mixed solution of water and anhydrous ethanol with a mass concentration of 3% KH550, where the volume ratio of water to anhydrous ethanol is 9:1. After soaking for 2 hours, glacial acetic acid is added, where the molar ratio of glacial acetic acid to KH550 is 0.2:1, and hydrolyzing it at 50°C for 5 hours. The mixture is centrifuged, washed with deionized water, and dried to obtain polyoxymethylene fiber modified with a silane coupling agent;

[0068] Step b, dispersing the silane coupling agent-modified polyoxymethylene fiber and the sulfur-containing phosphazene flame retardant polymer prepared above in dimethyl sulfoxide at a mass ratio of 2:1, a solid-liquid ratio of 1 g:8 mL, ultrasonically dispersing for 40 minutes, and mechanically stirring for 1 hour to obtain a precursor dispersion; wherein the ultrasonic dispersion power is 450 W, the frequency is 30 Hz, and the mechanical stirring rate is 500 rpm;

[0069] Step c: injecting the precursor dispersion into a polytetrafluoroethylene mold and freeze-drying to obtain a flame-retardant polyoxymethylene fiber aerogel; wherein the vacuum freeze-drying temperature is -90°C, the vacuum degree is 20 Pa, and the time is 24 hours.

[0070] Example 2

[0071] This embodiment provides a method for preparing a sulfur-containing phosphazene flame-retardant polymer, comprising the following steps:

[0072] Dissolve 1,3-dihydroxynaphthalene in 85% concentrated sulfuric acid at a molar ratio of 1,3-dihydroxynaphthalene to concentrated sulfuric acid of 1:1.35, heat to 60°C and react for 2 hours to obtain 1,3-dihydroxy-6-sulfonate naphthalene;

[0073] 1,3-Dihydroxy-6-sulfonatonaphthalene and hexachlorocyclotriphosphazene were dissolved in N,N-dimethylformamide at a molar ratio of 1:1.5, with the ratio of N,N-dimethylformamide to hexachlorocyclotriphosphazene being 8 mL:1 g. Then, triethylamine, a catalyst, was added in an amount of 0.5% of the total mass of the reactants. The reaction was carried out at 80° C. for 12 h, followed by centrifugation, washing with deionized water, and drying to obtain a sulfur-containing phosphazene flame retardant polymer.

[0074] This embodiment provides a method for preparing a flame-retardant polyoxymethylene fiber aerogel, comprising the following steps:

[0075] Step a, soaking the polyoxymethylene fiber in anhydrous ethanol and ultrasonically cleaning it for 30 minutes, vacuum drying it, then placing it in a plasma reaction chamber and treating it at 500W for 10 minutes, then soaking it in a mixed solution of water and anhydrous ethanol with a mass concentration of 1% KH550, where the volume ratio of water to anhydrous ethanol is 10:1. After soaking for 4 hours, glacial acetic acid is added, where the molar ratio of glacial acetic acid to KH550 is 0.5:1, and hydrolyzing it at 60°C for 4 hours. The mixture is centrifuged, washed with deionized water, and dried to obtain polyoxymethylene fiber modified with a silane coupling agent;

[0076] Step b, dispersing the silane coupling agent-modified polyoxymethylene fiber and the sulfur-containing phosphazene flame retardant polymer prepared above in dimethyl sulfoxide at a mass ratio of 3:1, a solid-liquid ratio of 1 g:10 mL, ultrasonically dispersing for 30 minutes, and mechanically stirring for 3 hours to obtain a precursor dispersion; wherein the ultrasonic dispersion power is 500 W, the frequency is 50 Hz, and the mechanical stirring rate is 300 rpm;

[0077] Step c: injecting the precursor dispersion into a polytetrafluoroethylene mold and freeze-drying to obtain a flame-retardant polyoxymethylene fiber aerogel; wherein the vacuum freeze-drying temperature is -80°C, the vacuum degree is 5 Pa, and the time is 84 hours.

[0078] Example 3

[0079] This embodiment provides a method for preparing a sulfur-containing phosphazene flame-retardant polymer, comprising the following steps:

[0080] Dissolve 1,3-dihydroxynaphthalene in 90% concentrated sulfuric acid at a molar ratio of 1,3-dihydroxynaphthalene to concentrated sulfuric acid of 1:1.3, heat to 50°C and react for 3 hours to obtain 1,3-dihydroxy-6-sulfonate naphthalene;

[0081] 1,3-Dihydroxy-6-sulfonatonaphthalene and hexachlorocyclotriphosphazene were dissolved in N,N-dimethylformamide at a molar ratio of 1:1.3, with the ratio of N,N-dimethylformamide to hexachlorocyclotriphosphazene being 6 mL:1 g. Then, triethylamine, a catalyst, was added in an amount of 1% of the total mass of the reactants. The mixture was reacted at 100° C. for 8 h, centrifuged, washed with deionized water, and dried to obtain a sulfur-containing phosphazene flame retardant polymer.

[0082] This embodiment provides a method for preparing a flame-retardant polyoxymethylene fiber aerogel, comprising the following steps:

[0083] Step a, soaking the polyoxymethylene fiber in anhydrous ethanol and ultrasonically cleaning it for 30 minutes, vacuum drying it, then placing it in a plasma reaction chamber and treating it at 400W for 20 minutes, then soaking it in a mixed solution of water and anhydrous ethanol with a mass concentration of 2% KH550, where the volume ratio of water to anhydrous ethanol is 9:1. After soaking for 3 hours, glacial acetic acid is added, where the molar ratio of glacial acetic acid to KH550 is 0.3:1, and hydrolyzing it at 55°C for 5 hours. The mixture is centrifuged, washed with deionized water, and dried to obtain polyoxymethylene fiber modified with a silane coupling agent;

[0084] Step b, dispersing the silane coupling agent-modified polyoxymethylene fiber and the sulfur-containing phosphazene flame retardant polymer prepared above in dimethyl sulfoxide at a mass ratio of 4:1, a solid-liquid ratio of 1 g:9 mL, ultrasonically dispersing for 35 minutes, and mechanically stirring for 2 hours to obtain a precursor dispersion; wherein the ultrasonic dispersion power is 400 W, the frequency is 40 Hz, and the mechanical stirring rate is 400 rpm;

[0085] Step c: injecting the precursor dispersion into a polytetrafluoroethylene mold and freeze-drying to obtain a flame-retardant polyoxymethylene fiber aerogel; wherein the vacuum freeze-drying temperature is -85°C, the vacuum degree is 10 Pa, and the time is 48 hours.

[0086] Example 4

[0087] This embodiment provides a method for preparing a sulfur-containing phosphazene flame-retardant polymer, comprising the following steps:

[0088] Dissolve 1,3-dihydroxynaphthalene in 90% concentrated sulfuric acid at a molar ratio of 1,3-dihydroxynaphthalene to concentrated sulfuric acid of 1:1.4, heat to 40°C and react for 4 hours to obtain 1,3-dihydroxy-6-sulfonate naphthalene;

[0089] 1,3-Dihydroxy-6-sulfonatonaphthalene and hexachlorocyclotriphosphazene were dissolved in N,N-dimethylformamide at a molar ratio of 1:1.2, with the ratio of N,N-dimethylformamide to hexachlorocyclotriphosphazene being 7 mL:1 g. Then, triethylamine, a catalyst, was added in an amount of 1.2% of the total mass of the reactants. The reaction was carried out at 120° C. for 6 h, followed by centrifugation, washing with deionized water, and drying to obtain a sulfur-containing phosphazene flame retardant polymer.

[0090] This embodiment provides a method for preparing a flame-retardant polyoxymethylene fiber aerogel, comprising the following steps:

[0091] Step a, soaking the polyoxymethylene fiber in anhydrous ethanol and ultrasonically cleaning it for 30 minutes, vacuum drying it, then placing it in a plasma reaction chamber and treating it at 350W for 25 minutes, then soaking it in a mixed solution of water and anhydrous ethanol with a mass concentration of 2.5% KH550, with a volume ratio of water to anhydrous ethanol of 8:1. After soaking for 2.5 hours, glacial acetic acid was added, with a molar ratio of glacial acetic acid to KH550 of 0.1:1, and hydrolyzing it at 60°C for 6 hours. The mixture was centrifuged, washed with deionized water, and dried to obtain a polyoxymethylene fiber modified with a silane coupling agent;

[0092] Step b, dispersing the silane coupling agent-modified polyoxymethylene fiber and the sulfur-containing phosphazene flame retardant polymer prepared above in dimethyl sulfoxide at a mass ratio of 5:1, a solid-liquid ratio of 1 g:8 mL, ultrasonically dispersing for 40 minutes, and mechanically stirring for 2 hours to obtain a precursor dispersion; wherein the ultrasonic dispersion power is 300 W, the frequency is 45 Hz, and the mechanical stirring rate is 450 rpm;

[0093] Step c: injecting the precursor dispersion into a polytetrafluoroethylene mold and freeze-drying to obtain a flame-retardant polyoxymethylene fiber aerogel; wherein the vacuum freeze-drying temperature is -80°C, the vacuum degree is 15 Pa, and the time is 60 hours.

[0094] Comparative Example 1

[0095] This comparative example provides a method for preparing a flame-retardant polyoxymethylene fiber aerogel. The only difference from Example 3 is that 1,3-dihydroxy-6-sulfonatonaphthalene is replaced with ethylene glycol. The specific steps are as follows:

[0096] Step a, dissolving ethylene glycol and hexachlorocyclotriphosphazene in N,N-dimethylformamide at a molar ratio of 1:1.3, with the ratio of N,N-dimethylformamide to hexachlorocyclotriphosphazene being 6 mL:1 g, then adding triethylamine as a catalyst in an amount of 1% of the total mass of the reactants, reacting at 100° C. for 8 h, centrifuging, washing with deionized water, and drying to obtain a binder;

[0097] Step b, soaking the polyoxymethylene fiber in anhydrous ethanol and ultrasonically cleaning it for 30 minutes, vacuum drying it, then placing it in a plasma reaction chamber and treating it at 400W for 20 minutes, then immersing it in a mixed solution of water and anhydrous ethanol with a mass concentration of 2% KH550, with a volume ratio of water to anhydrous ethanol of 9:1. After immersing for 3 hours, glacial acetic acid was added, with a molar ratio of glacial acetic acid to KH550 of 0.3:1, and hydrolyzing it at 55°C for 5 hours. The mixture was centrifuged, washed with deionized water, and dried to obtain polyoxymethylene fiber modified with a silane coupling agent;

[0098] Step c, dispersing the silane coupling agent-modified polyoxymethylene fiber and the binder prepared above in dimethyl sulfoxide at a mass ratio of 4:1, with a solid-liquid ratio of 1 g:9 mL, ultrasonically dispersing for 35 minutes, and mechanically stirring for 2 hours to obtain a precursor dispersion; wherein the ultrasonic dispersion power is 400 W, the frequency is 40 Hz, and the mechanical stirring rate is 400 rpm;

[0099] Step d: injecting the precursor dispersion into a polytetrafluoroethylene mold and freeze-drying to obtain a flame-retardant polyoxymethylene fiber aerogel; wherein the vacuum freeze-drying temperature is -85°C, the vacuum degree is 10 Pa, and the time is 48 hours.

[0100] Comparative Example 2

[0101] This comparative example provides a method for preparing a flame-retardant polyoxymethylene fiber aerogel. The only difference from Example 3 is that 1,3-dihydroxy-6-sulfonatonaphthalene is replaced with hexamethylenediamine. The specific steps are as follows:

[0102] Step a, dissolving hexamethylenediamine and hexachlorocyclotriphosphazene in N,N-dimethylformamide at a molar ratio of 1:1.3, with the ratio of N,N-dimethylformamide to hexachlorocyclotriphosphazene being 6 mL:1 g, then adding triethylamine as a catalyst in an amount of 1% of the total mass of the reactants, reacting at 100° C. for 8 h, centrifuging, washing with deionized water, and drying to obtain a binder;

[0103] Step b, soaking the polyoxymethylene fiber in anhydrous ethanol and ultrasonically cleaning it for 30 minutes, vacuum drying it, then placing it in a plasma reaction chamber and treating it at 400W for 20 minutes, then immersing it in a mixed solution of water and anhydrous ethanol with a mass concentration of 2% KH550, with a volume ratio of water to anhydrous ethanol of 9:1. After immersing for 3 hours, glacial acetic acid was added, with a molar ratio of glacial acetic acid to KH550 of 0.3:1, and hydrolyzing it at 55°C for 5 hours. The mixture was centrifuged, washed with deionized water, and dried to obtain polyoxymethylene fiber modified with a silane coupling agent;

[0104] Step c, dispersing the silane coupling agent-modified polyoxymethylene fiber and the binder prepared above in dimethyl sulfoxide at a mass ratio of 4:1, with a solid-liquid ratio of 1 g:9 mL, ultrasonically dispersing for 35 minutes, and mechanically stirring for 2 hours to obtain a precursor dispersion; wherein the ultrasonic dispersion power is 400 W, the frequency is 40 Hz, and the mechanical stirring rate is 400 rpm;

[0105] Step d: injecting the precursor dispersion into a polytetrafluoroethylene mold and freeze-drying to obtain a flame-retardant polyoxymethylene fiber aerogel; wherein the vacuum freeze-drying temperature is -85°C, the vacuum degree is 10 Pa, and the time is 48 hours.

[0106] Morphology test

[0107] The SEM images of the polyoxymethylene fibers used in the examples and comparative examples of the present invention are as follows: Figure 1 As shown, it can be seen that the fiber diameter of the polyoxymethylene fiber is about 20 μm.

[0108] Figure 2 This scanning electron micrograph shows the polyoxymethylene fiber aerogel prepared in Example 3. It shows parallel channels in the axial direction and interconnected porous structures in the radial direction, demonstrating the successful preparation of an anisotropic polyoxymethylene fiber aerogel. Furthermore, a scanning electron micrograph of an axial cross-section shows polyoxymethylene fibers interspersed between the layers of the adhesive, demonstrating that the flame-retardant polymer prepared from hexachlorocyclotriphosphazene and 1,3-dihydroxy-6-sulfonatonaphthalene can effectively bond the polyoxymethylene fibers.

[0109] Compressive strength test

[0110] A sample with a size of 2×2×2 cm was selected, and the compressive strength of the flame-retardant polyoxymethylene fiber aerogel material prepared in Example 3 and Comparative Examples 1 and 2 was tested using a universal testing machine at a compression loading rate of 1.0 mm / min. The results are as follows: Figure 3When the compressive strain is 30%, the compressive strengths of Example 3, Comparative Example 1, and Comparative Example 2 are 75.6 kPa, 61.6 kPa, and 53.5 kPa, respectively, indicating that the bonding effect of hexachlorocyclotriphosphazene / 1,3-dihydroxy-6-sulfonatonaphthalene on polyoxymethylene fibers is significantly better than that of Comparative Examples 1 and 2.

[0111] Radiative cooling performance test

[0112] The radiative cooling performance of an object is directly affected by its absorption of sunlight. Therefore, achieving efficient radiative cooling requires samples with high reflectivity across the solar spectrum. Samples measuring 2 × 2 × 0.3 cm were selected, and their reflectivity was recorded in the 220–2500 nm range on a UV-Vis spectrophotometer with an integrating sphere. Figure 4 The reflectivity of Example 3, Comparative Example 1, and Comparative Example 2 in the ultraviolet-visible-near-infrared spectrum range is shown. It can be observed that the average reflectivities of Example 3, Comparative Example 1, and Comparative Example 2 are 93.6%, 88.3%, and 87.6%, respectively. The higher reflectivity can be attributed to the difference in refractive index between the aerogel skeleton and the air in the aerogel pores. The light will be refracted and reflected multiple times on the aerogel surface, and the absorption of polyoxymethylene fibers and binder chemical bonds is mainly concentrated in the mid-infrared band, with only a small amount of absorption in the ultraviolet-visible-near-infrared band.

[0113] An object can transfer its own heat to the outside space in the form of thermal radiation through the atmospheric window to achieve a cooling effect. A sample with a size of 2×2×0.3 cm was selected, and a Fourier infrared spectrometer with an integrating sphere was used to record the emissivity of the sample in the range of 2.5 to 25 μm. The emissivity of Example 3, Comparative Example 1, and Comparative Example 2 is as follows: Figure 5 As shown. The average emissivities of Example 3, Comparative Example 1, and Comparative Example 2 were 95.7%, 93.6%, and 92.3%, respectively. These high emissivities can be attributed to the fact that the light absorption bands of the C–O–C bonds in the polyoxymethylene fibers and the sulfonic acid groups and phosphazene rings in the binder are all within the atmospheric window band. Thus, the prepared polyoxymethylene fiber composite aerogel has high reflectivity within the solar radiation band and high emissivity in the mid-infrared band, meeting the basic conditions for radiative cooling and promising applications in the field of building energy conservation.

[0114] Flame retardant performance test

[0115] The flame retardant properties of polyoxymethylene fiber aerogel were tested through combustion experiments. Figure 6As shown, the polyformaldehyde fiber aerogel prepared in Example 3 was placed on a flame and burned twice. When the polyformaldehyde fiber composite aerogel came into contact with the flame, its surface was carbonized. The generated carbonized layer could effectively block the air and prevent the flame from further spreading into the interior of the aerogel. In addition, after the polyformaldehyde fiber composite aerogel was removed from the flame, the flame on its surface was quickly extinguished, and no melting droplets occurred during the entire process.

[0116] The combustion behavior of polyoxymethylene fiber aerogel was evaluated using a micro combustion calorimeter. 5 mg of polyoxymethylene fiber aerogel sample was heated from 100°C to 700°C with a N2 flux of 80.0 cc / min and an O2 flux of 20.0 cc / min. Figure 7 As shown, the total heat release (THR), heat release capacity (HRC), and peak heat release rate (PHRR) of Example 3 are 2.1 KJ / g, 23.4 W / g, and 27 J / g / k, respectively, indicating that it has excellent flame retardant properties. The THR, HRC, and PHRR of Comparative Example 1 are 5.7 KJ / g, 55.1 W / g, and 45 J / g / k, respectively; and the THR, HRC, and PHRR of Comparative Example 2 are 3.6 KJ / g, 32.6 W / g, and 35 J / g / k. This shows that the introduction of sulfonic acid groups significantly improves the flame retardant properties of the composite aerogel, and the flame retardant properties of the phosphorus-nitrogen-sulfur cross-linked network formed therefrom are better than those of Comparative Examples 1 and 2.

[0117] In summary, the present invention provides a polyformaldehyde fiber aerogel with radiative cooling and flame retardant properties. The aerogel is produced by simple freeze-drying using a sulfur-containing phosphazene-based polymer as a binder and a polyformaldehyde fiber modified with a silane coupling agent as a skeleton. The polyformaldehyde fiber aerogel exhibits a solar reflectivity exceeding 90% and an atmospheric transparent window emissivity exceeding 90%, meeting the necessary conditions for daytime radiative cooling. Furthermore, the composite binder rapidly carbonizes upon contact with flames, effectively isolating the air and preventing further flame spread into the aerogel. This imparts excellent flame retardancy to the polyformaldehyde fiber composite aerogel, promising broad application prospects in building energy conservation, outdoor equipment, and aerospace.

[0118] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A sulfur-containing phosphazene flame-retardant polymer, characterized in that: It is prepared by the reaction of naphthalene hydroxysulfonate and hexachlorocyclotriphosphazene.

2. The sulfur-containing phosphazene flame-retardant polymer according to claim 1, characterized in that The hydroxy naphthalene sulfonate substance is 1,3-dihydroxy-6-sulfonate naphthalene.

3. The sulfur-containing phosphazene flame-retardant polymer according to claim 1 or 2, characterized in that: The preparation method of the naphthalene hydroxysulfonate substance comprises the following steps: The hydroxy naphthalene substances are subjected to a sulfonation reaction with concentrated sulfuric acid to obtain the hydroxy naphthalene sulfonate substances.

4. The sulfur-containing phosphazene flame-retardant polymer according to claim 3, characterized in that The temperature of the sulfonation reaction is 40° C. to 60° C., and the reaction time is 2 h to 4 h.

5. The method for preparing the sulfur-containing phosphazene flame-retardant polymer according to any one of claims 1 to 4, characterized in that: The steps include: With organic amine as catalyst, hexachlorocyclotriphosphazene and naphthalene hydroxysulfonate undergo nucleophilic substitution reaction in an organic solvent to obtain sulfur-containing phosphazene flame retardant polymer.

6. Use of the sulfur-containing phosphazene flame-retardant polymer according to any one of claims 1 to 4 as a binder for preparing flame-retardant polyoxymethylene fiber aerogel.

7. A flame-retardant polyoxymethylene fiber aerogel, characterized in that: The invention is prepared by freeze-drying polyoxymethylene fiber modified with a silane coupling agent and the sulfur-containing phosphazene flame retardant polymer according to any one of claims 1 to 4.

8. The method for preparing the flame-retardant polyoxymethylene fiber aerogel according to claim 7, characterized in that: The steps include: S1, plasma-treating polyformaldehyde fiber, adding it to a silane coupling agent solution, impregnating it, then adding acid to carry out hydrolysis reaction, and drying it to obtain polyformaldehyde fiber modified with the silane coupling agent; S2, dispersing the polyformaldehyde fiber modified with the silane coupling agent and the sulfur-containing phosphazene flame-retardant polymer in an organic solvent, and performing vacuum freeze-drying to obtain a flame-retardant polyformaldehyde fiber aerogel.

9. The method for preparing the flame-retardant polyoxymethylene fiber aerogel according to claim 8, wherein: In S1, the power of the plasma treatment is 300W to 500W, and the treatment time is 10min to 30min; and / or In S1, the silane coupling agent solution is a KH-550 solution with a mass concentration of 1wt% to 3wt%; the solvent of the silane coupling agent solution is water and anhydrous ethanol in a volume ratio of 1:(8-10); and / or In S2, the mass ratio of the polyoxymethylene fiber modified with a silane coupling agent to the sulfur-containing phosphazene flame retardant polymer is (2-5):1; and / or In S2, the organic solvent is dimethyl sulfoxide.

10. Use of the flame-retardant polyoxymethylene fiber aerogel according to claim 8 in the fields of building energy conservation, outdoor equipment or aerospace.

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