Efficient fireproof aerogel steel structure coating and preparation method thereof

Abstract

The application provides a high-efficiency fireproof aerogel steel structure coating and a preparation method thereof, and belongs to the technical field of fireproof coatings. The application realizes excellent mechanical properties and heat-resistant stability by constructing a siliconized polyacrylonitrile fiber three-dimensional network through polyacrylonitrile fiber pre-oxidation, gas deposition and carbonization treatment. The application forms light-weight high-strength, heat-resistant and flame-retardant composite hybrid aerogel through the synergistic reaction of polyacrylonitrile fiber and tetraethyl orthosilicate and the complexation of boric acid, and further enhances the flame retardancy and structural stability through boric acid derivatives and two-step carbonization treatment. Lignosulfonate and melamine-phytic acid system are self-assembled to fill pores and improve the free radical quenching efficiency, and meanwhile, the supramolecular structure enhances the toughness and impact resistance of the coating. The outer carbon-silicon aerogel and the inner layer expansion coating form a double barrier, which significantly improves the fire-resistant and flame-retardant properties of the coating.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fireproof coatings, and relates to a high-efficiency fireproof aerogel steel structure coating and a preparation method thereof. BACKGROUND

[0002] Steel structures are widely used in modern buildings due to their excellent strength and load-bearing capacity. However, steel structures are prone to lose their load-bearing capacity during a fire due to the significant decrease in strength and stiffness at high temperatures. Therefore, effective fireproof treatment of steel structures is particularly important. Steel structure fireproof coatings, as an important fireproof protection measure, can form an insulating layer to slow down the heating of steel and ensure the safety of the structure during a fire.

[0003] Currently, there are various types of steel structure fireproof coatings on the market, including inorganic coatings, organic coatings, and composite coatings. These coatings have good fireproof performance to some extent, but there are still some obvious shortcomings in practical application. First, many traditional fireproof coatings have insufficient fire resistance and thermal stability, often failing to provide sustained and effective protection in high-temperature environments. For example, inorganic fireproof coatings are prone to cracking and peeling at high temperatures, leading to failure of protection, while organic fireproof coatings are prone to volatilization at high temperatures, affecting their durability.

[0004] Secondly, the adhesion and mechanical strength of existing fireproof coatings also have problems. In the construction process, insufficient adhesion of the coating may cause the coating to fall off, affecting the long-term use effect. In addition, traditional coatings are relatively brittle and are easily broken by external impact, leading to a decrease in fireproof performance. In order to ensure the long-term effectiveness of the coating under harsh conditions, it is urgent to develop fireproof coatings with higher toughness and durability. SUMMARY

[0005] To solve the above problems, the application aims to provide a high-efficiency fireproof aerogel steel structure coating and a preparation method thereof. The application introduces active groups into the siloxane interface layer through the oxidation treatment and KH-560 modification of polyacrylonitrile fibers, combines gas deposition and carbonization treatment, and constructs a three-dimensional network of siliconized polyacrylonitrile fibers to effectively disperse stress and reduce the risk of cracking and brittleness. Through the synergistic reaction of polyacrylonitrile fibers and tetraethyl orthosilicate and the complexation of boric acid, a lightweight, high-strength, heat-resistant, and flame-retardant composite hybrid aerogel is formed, and the boric acid derivative and two-step carbonization treatment further enhance the flame retardancy and structural stability. Lignosulfonate and melamine-phytic acid system self-assemble to fill pores and improve the efficiency of free radical quenching, while the supramolecular structure enhances the toughness and impact resistance of the coating. The outer carbon-silicon aerogel and the inner expanded coating form a double barrier, significantly improving the fire resistance and flame retardancy of the coating.

[0006] To achieve this purpose, the application adopts the following technical solutions:

[0007] In a first aspect, the present application provides a high-efficiency fireproof aerogel steel structure coating, which comprises an outer layer aerogel coating and an inner layer fire-retardant coating.

[0008] The outer layer aerogel coating is composed of siliconized polypropylene fibers, surface carbonized polypropylene fiber / silica aerogel, supramolecular network, pretreated binder, cerium dioxide powder, dispersant and deionized water.

[0009] The inner layer fire-retardant coating is composed of water-based epoxy resin, deionized water, modified halloysite, bisphenol A bis(diphenyl phosphate), dipentaerythritol, melamine cyanurate, wetting agent, defoamer BYK-028.

[0010] The outer layer aerogel coating comprises the following components:

[0011]

[0012]

[0013] The inner layer fire-retardant coating comprises the following components:

[0014]

[0015] In a second aspect, the present application provides a preparation method of a high-efficiency fireproof aerogel steel structure coating, which is as follows:

[0016] The preparation method of the outer layer aerogel coating is as follows:

[0017] S11: Pre-oxidizing polyacrylonitrile fibers to obtain pre-oxidized polyacrylonitrile fibers, and soaking the pre-oxidized polyacrylonitrile fibers in an ethanol aqueous solution of KH-560 to obtain pretreated polyacrylonitrile fibers; and then treating the pretreated polyacrylonitrile fibers in an inert atmosphere and performing gas deposition to obtain siliconized polyacrylonitrile fibers;

[0018] In the present application, polyacrylonitrile fibers are selected. Polyacrylonitrile fibers have excellent thermal stability, high carbon yield and abundant active functional groups, which provide excellent chemical reactivity and structural advantages for modification. Pre-oxidized polyacrylonitrile fibers are obtained by oxidation treatment. The oxidation process can introduce high-density hydroxyl groups, carboxyl groups and ketone groups, and part of the nitrile groups undergo cyclization reaction to form a higher heat-resistant conjugated structure, which improves the thermal stability and morphology stability of the fibers in a high-temperature environment.

[0019] The silanol groups generated after KH-560 hydrolysis form chemical bonds with the hydroxyl and carboxyl groups on the surface of pre-oxidized polypropylene fibers through a condensation reaction. Simultaneously, the nitrile groups may undergo ring-opening reactions with the epoxy groups in KH-560, further forming a stable chemical bond. This process constructs an interfacial layer rich in siloxanes and epoxy groups on the fiber surface, providing more active sites for the subsequent bonding of inorganic silicon components during vapor deposition.

[0020] Preferably, the temperature for the pre-oxidation treatment of the polyacrylonitrile fiber is 200-300℃, for example, it can be 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃ or 300℃, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0021] Preferably, the volume ratio of ethanol to water in the ethanol-water solution is 7:3-9:1, for example, it can be 7:3, 8:2, or 9:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0022] Preferably, the pretreated polyacrylonitrile fiber is treated at a temperature of 300-350°C under an inert atmosphere. For example, it can be 300°C, 305°C, 310°C, 315°C, 320°C, 325°C, 330°C, 335°C, 340°C, 345°C, or 350°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] Preferably, the pretreated polyacrylonitrile fibers are treated in an inert atmosphere for 1-1.5 hours, for example, 1.00 hours, 1.05 hours, 1.10 hours, 1.15 hours, 1.20 hours, 1.25 hours, 1.30 hours, 1.35 hours, 1.40 hours, 1.45 hours, or 1.50 hours, but are not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0024] Preferably, the gas composition of the vapor deposition is a mixture of nitrogen, hydrogen and silicon tetrachloride;

[0025] Preferably, the temperature of the vapor deposition is 670-690℃, for example, it can be 670℃, 672℃, 674℃, 676℃, 678℃, 680℃, 682℃, 684℃, 686℃, 688℃ or 690℃, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0026] Preferably, the pressure of the vapor deposition is 0.5-1 Pa, for example, it can be 0.50 Pa, 0.55 Pa, 0.60 Pa, 0.65 Pa, 0.70 Pa, 0.75 Pa, 0.80 Pa, 0.85 Pa, 0.90 Pa, 0.95 Pa or 1.00 Pa, but not only limited to the listed values, other values not listed in the range are also applicable.

[0027] The pre-processed polyacrylonitrile fiber is heat treated in an inert atmosphere, which not only removes residual moisture and small molecule impurities, reduces the risk of side reactions, but also further promotes the cyclization and carbonization of nitrile groups to form a preliminary carbon skeleton structure, significantly improving the thermal stability of the fiber. The escape of volatile substances during the heat treatment process introduces micron-sized pores in the fiber, increasing the specific surface area and providing more reaction sites for subsequent vapor deposition.

[0028] Through the vapor deposition process, a silicon-based skeleton is deposited on the surface and internal pores of the fiber, constructing a micro-nano scale rough structure, achieving the "anchoring" effect of the fiber surface, and significantly improving the interfacial bonding force with the substrate. The siliconized polyacrylonitrile fiber can form a three-dimensional reinforcing network structure in the coating, dispersing external stress to reduce crack formation, improving the mechanical properties of the material, reducing the risk of brittleness, while maintaining good heat resistance and structural stability, providing excellent comprehensive performance for the coating.

[0029] S12: Disperse polyacrylonitrile fiber, tetraethyl orthosilicate and boric acid in deionized water, add ammonia water and ultrasonic treatment to obtain a pre-gel, inject it into a mold and place it in a freeze dryer to obtain a polyacrylonitrile fiber / silica hybrid aerogel; calcine and carbonize it in an inert atmosphere to obtain a surface-carbonized polyacrylonitrile fiber / silica aerogel;

[0030] Preferably, the molar ratio of the polyacrylonitrile fiber to tetraethyl orthosilicate is 1:3.5-4.5, for example, it can be 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9, 1:4.0, 1:4.1, 1:4.2, 1:4.3, 1:4.4 or 1:4.5, but not only limited to the listed values, other values not listed in the range are also applicable.

[0031] Preferably, the molar ratio of boric acid to polyacrylonitrile fiber is 0.05-0.08:1, for example, it can be 0.05:1, 0.06:1, 0.07:1, 0.08:1, but not only limited to the listed values, other values not listed in the range are also applicable.

[0032] Preferably, the molar ratio of the ammonia water to the polyacrylonitrile fiber is 0.3-0.5:1, which can be 0.30:1, 0.32:1, 0.34:1, 0.36:1, 0.38:1, 0.40:1, 0.42:1, 0.44:1, 0.46:1, 0.48:1 or 0.50:1, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0033] In the present application, the polyacrylonitrile fiber and tetraethyl orthosilicate undergo a synergistic reaction under the catalysis of ammonia water: tetraethyl orthosilicate hydrolyzes and condenses to form a three-dimensional silica network; the active groups such as nitrile groups and hydroxyl groups and carboxyl groups introduced in the oxidation process of the polyacrylonitrile fiber surface can interact with the silanol groups through chemical bonds or hydrogen bonds, and are embedded in the inorganic network to form an interpenetrating structure, realizing the molecular-level compounding of the organic phase and the inorganic phase.

[0034] The introduced boric acid forms a stable tetrahedral coordination structure by complexing with the active groups on the fiber surface, effectively bridging adjacent molecular chains, and improving the stability of the gel network; in the subsequent carbonization process, the boric acid decomposes and converts into boron oxide, which is embedded in the carbon skeleton in a uniform distribution form, inhibits thermal decomposition by using the physical barrier effect, and further improves the flame retardancy and high-temperature stability of the material by catalyzing the carbonization.

[0035] The polyacrylonitrile fiber provides a flexible support skeleton and interface bonding site in the system, the silica network gives the material a rigid support, and enhances the mechanical properties; the boric acid derivative further realizes multiple reinforcement at the molecular and nanometer scales through chemical bonding and phase change at high temperature. Finally, a light-weight high-strength, heat-resistant and flame-retardant composite hybrid aerogel system is constructed, which has excellent structural stability and functional performance.

[0036] Preferably, the first temperature of the freeze-drying is -20°C, and the first time is 2-3h, which can be 2.0h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h or 3.0h, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0037] Preferably, the second temperature of the freeze-drying is -40°C, and the second time is 2-3h, which can be 2.0h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h or 3.0h, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0038] Preferably, the third temperature of the freeze-drying is -80°C, and the third time is 10-12h, for example, it can be 10.0h, 10.2h, 10.4h, 10.6h, 10.8h, 11.0h, 11.2h, 11.4h, 11.6h, 11.8h or 12.0h, but not limited to the listed values, other values not listed in the range are also applicable.

[0039] The solvent is gradually frozen by stepwise cooling, which reduces the damage to the gel skeleton and maximizes the preservation of the pore structure, and then the solvent is sublimated in the freeze-drying process to obtain a three-dimensional porous aerogel, which presents a high specific surface area and a low density.

[0040] Preferably, the first stage temperature of the calcination-carbonization is 180-220°C, for example, it can be 180°C, 184°C, 188°C, 192°C, 196°C, 200°C, 204°C, 208°C, 212°C, 216°C or 220°C, but not limited to the listed values, other values not listed in the range are also applicable.

[0041] Preferably, the first stage time of the calcination-carbonization is 1.5-2.5h, for example, it can be 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2.0h, 2.1h, 2.2h, 2.3h, 2.4h or 2.5h, but not limited to the listed values, other values not listed in the range are also applicable.

[0042] Preferably, the first stage atmosphere of the calcination-carbonization is a nitrogen atmosphere;

[0043] Preferably, the second stage temperature of the calcination-carbonization is 500-600°C, for example, it can be 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C or 600°C, but not limited to the listed values, other values not listed in the range are also applicable.

[0044] Preferably, the second stage time of the calcination-carbonization is 0.8-1.2h, for example, it can be 0.80h, 0.84h, 0.88h, 0.92h, 0.96h, 1.00h, 1.04h, 1.08h, 1.12h, 1.16h or 1.20h, but not limited to the listed values, other values not listed in the range are also applicable.

[0045] Preferably, the second stage atmosphere of the calcination-carbonization is a nitrogen-hydrogen mixed atmosphere;

[0046] On this basis, the hybrid aerogel formed by polyacrylonitrile fiber and silicon dioxide is subjected to two-step carbonization treatment at low and high temperatures. In the low-temperature stage, residual water, small molecules and part of the organic functional groups are removed, and at the same time, the nitrile group of polyacrylonitrile is cyclized to form a preliminary conjugated structure. In the high-temperature stage, the organic part is further converted into a carbon surface layer, and the impurity oxides are reduced by hydrogen to generate a more pure carbonized surface, thereby obtaining a surface-carbonized polyacrylonitrile fiber / silicon dioxide aerogel. This semi-carbonization treatment generates a carbon layer with low thermal conductivity and high-temperature resistance on the surface of the aerogel skeleton, while retaining the strength of the inorganic silica skeleton, thereby constructing a composite heat-resistant material with synergistic effect of organic and inorganic, and excellent thermal stability and mechanical properties.

[0047] S13: melamine and phytic acid are added to deionized water to obtain a melamine dispersion and a phytic acid dispersion, the phytic acid dispersion is added to the melamine dispersion, a phosphate buffer solution is used to adjust the pH, a sodium lignosulfonate solution is added in portions, and an ultrasonic treatment is performed to obtain an ultramolecular network after standing;

[0048] Preferably, the molar ratio of melamine to phytic acid is 1:1.8-2.2, for example, it can be 1:1.80, 1:1.84, 1:1.88, 1:1.92, 1:1.96, 1:2.00, 1:2.04, 1:2.08, 1:2.12, 1:2.16 or 1:2.20, but not limited to the listed values, other values not listed in this range are also applicable.

[0049] Preferably, the phosphate buffer solution is used to adjust the pH to 6-7, for example, it can be 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9 or 7.0, but not limited to the listed values, other values not listed in this range are also applicable.

[0050] Preferably, the mass fraction of the sodium lignosulfonate solution is 5-8wt.%, for example, it can be 5.0wt.%, 5.3wt.%, 5.6wt.%, 5.9wt.%, 6.2wt.%, 6.5wt.%, 6.8wt.%, 7.1wt.%, 7.4wt.%, 7.7wt.% or 8.0wt.%, but not limited to the listed values, other values not listed in this range are also applicable.

[0051] Preferably, the amount of sodium lignosulfonate is 10-15% of the mass of the supramolecular network, for example, it can be 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5% or 15.0%, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0052] Phytic acid contains multiple phosphate groups, which can form a supramolecular network with melamine through hydrogen bonding, and improve the flame retardant performance by utilizing the characteristics of phosphorus elements. Adjusting the pH to near neutral helps to partially dissociate the acidic groups on phytic acid, making the amino groups of melamine easily positively charged, thereby interacting with phytic acid and forming a stable network.

[0053] Sodium lignosulfonate not only increases viscosity and dispersion in this process, but also forms more hydrogen bond or ionic bond networks with phytic acid and melamine, and produces a carbon layer when heated due to its aromatic structure and high carbon content, thereby improving the overall flame retardant effect.

[0054] At the same time, the introduction of supramolecular structure helps to form a more flexible coating, reduces the tendency of brittle fracture when the material is stressed, and improves its impact resistance.

[0055] S14: After crushing the surface carbonized polyacrylonitrile fiber / silica aerogel, dry mixing with the cut siliconized polyacrylonitrile fiber to obtain a mixture; mixing and aging the silica sol with aluminum phosphate to obtain a pretreated binder, adding the mixture, cerium dioxide powder and dispersant BYK-2155 and ultrasonic treatment to obtain an outer layer aerogel coating;

[0056] The surface carbonized polyacrylonitrile fiber / silica aerogel is crushed to a particle size of ≤50 μm;

[0057] The siliconized polyacrylonitrile fiber is cut to a length-diameter ratio of 10:1-20:1, for example, it can be 3.0:1, 3.1:1, 3.2:1, 3.3:1, 3.4:1, 3.5:1, 3.6:1, 3.7:1, 3.8:1, 3.9:1 or 4.0:1, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0058] Preferably, the molar ratio of silica sol to aluminum phosphate in the pretreated binder is 3-4:1, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0059] After aging, the silica particles form a network with active hydroxyl groups, and the aluminum phosphate can also undergo condensation reaction with these hydroxyl groups to generate aluminum-oxygen-silicon framework structure, which is finally converted into a stable composite oxide bonding layer under high temperature conditions, thereby giving the coating good high temperature strength.

[0060] Cerium dioxide utilizes its catalytic and antioxidant properties to help stabilize the carbon layer and enhance the overall antioxidant ablative capacity of the coating at high temperatures.

[0061] Preferably, the temperature for aging the mixture of the silica sol and aluminum phosphate is 20-25℃, such as 20.0℃, 20.5℃, 21.0℃, 21.5℃, 22.0℃, 22.5℃, 23.0℃, 23.5℃, 24.0℃, 24.5℃ or 25.0℃, but not limited to the listed values, other values not listed in the range are also applicable.

[0062] Preferably, the time for aging the mixture of the silica sol and aluminum phosphate is 20-24h, such as 20.0h, 20.4h, 20.8h, 21.2h, 21.6h, 22.0h, 22.4h, 22.8h, 23.2h, 23.6h or 24.0h, but not limited to the listed values, other values not listed in the range are also applicable.

[0063] The preparation method of the inner layer flame-retardant coating is as follows:

[0064] S21: the halloysite nanotube is treated by immersing in an ethanol aqueous solution of KH-560 to obtain a modified halloysite; the modified halloysite is mixed with bisphenol A bis(diphenyl phosphate), dipentaerythritol and melamine cyanurate by ball milling to obtain a flame-retardant powder;

[0065] Halloysite is a natural layered silicate, and the surface hydroxyl group can form a chemical bond or strong adsorption with KH-560 to obtain a modified nanotube with better organic-inorganic compatibility, which improves the dispersibility and interfacial bonding force of halloysite in an organic resin or a flame-retardant system.

[0066] Bisphenol A bis(diphenyl phosphate) is a phosphorus-containing flame retardant, dipentaerythritol contains multiple hydroxyl groups and is a typical carbon source promoter; melamine cyanurate generates non-combustible gas through exothermic decomposition, and at the same time, generates a flame-retardant intumescent carbon layer through synergism with the phosphorus source. Through ball milling, the above flame-retardant components are fully mixed with the modified halloysite and are refined into a uniformly dispersed powder. The phosphorus source, the polyhydroxyl polyol and the nitrogen source synergize to form a dense carbon layer at high temperatures, block heat and oxygen, and release non-combustible gas; the modified halloysite can further provide physical blocking to improve the mechanical strength and stability of the carbon layer.

[0067] S22: the water-based epoxy resin Dow DER3210 is mixed with deionized water, the flame-retardant powder, the wetting agent BYK-349 and the defoaming agent BYK-028 are added, and mixing is performed to obtain an inner layer flame-retardant coating;

[0068] Compared with the prior art, the present application has the following beneficial effects:

[0069] The polyacrylonitrile fiber is selected in the application, active groups and conjugated structures are introduced by oxidation treatment, the thermal stability and morphology retention are improved, a siloxane interface layer is formed after modification by KH-560, and micropores and carbon skeletons are generated by carbonization under heat treatment, so that the reactive sites are increased; the silicon-based skeleton is constructed by gas deposition, the surface "anchoring" effect is realized, and the interface bonding force is enhanced. The siliconized polyacrylonitrile fiber forms a three-dimensional reinforcing network, effectively disperses stress and reduces cracks, and endows the material with excellent mechanical properties and thermal stability;

[0070] In the application, the organic-inorganic interpenetrating network is formed by the synergistic reaction of polyacrylonitrile fiber and tetraethyl orthosilicate, the gel stability is improved by the complexation of boric acid, and the boron oxide is converted in the carbonization process, so that the flame retardance and high-temperature stability are enhanced. The polyacrylonitrile fiber provides a flexible skeleton, the silica endows a rigid support, and the boric acid derivative realizes multi-scale reinforcement. Through two-step carbonization treatment at low and high temperatures, impurities are removed and a high-temperature-resistant carbon layer is generated, while the strength of the inorganic skeleton is retained, so that a light-weight high-strength, heat-resistant and flame-retardant composite hybrid aerogel is constructed, which has excellent thermal stability and mechanical properties;

[0071] In the application, the sodium lignosulfonate and the melamine-phytic acid system are self-assembled, the pores are effectively filled, the free radical quenching efficiency is improved, and the high-temperature stability is enhanced; at the same time, the introduction of the supramolecular structure helps to form a more flexible coating, reduces the brittle fracture tendency of the material under stress, and improves the impact resistance of the material;

[0072] In the application, the carbon-silicon skeleton of the outer layer aerogel and the inner layer intumescent coating form a physical-chemical double barrier, and the fire resistance and flame retardance of the coating are improved. DETAILED DESCRIPTION

[0073] The technical solutions of the application will be described in detail below in combination with specific embodiments. The embodiments described herein are specific specific embodiments of the application, which are used to illustrate the concept of the application; all the descriptions are explanatory and exemplary, and should not be understood as limiting the embodiments of the application and the protection scope of the application. In addition to the embodiments described herein, those skilled in the art can also employ other technical solutions that are obvious based on the content disclosed in the claims and the description of the application, which include technical solutions that make any obvious substitutions and modifications to the embodiments described herein.

[0074] The chemical reagents used in the embodiments and comparative examples of the application are all commercially available goods, which are not further purified or treated.

[0075] Example 1

[0076] The embodiment provides a high-efficiency fireproof aerogel steel structure coating and a preparation method thereof, and the preparation method of the high-efficiency fireproof aerogel steel structure coating specifically comprises the following steps.

[0077] The preparation method of the outer layer aerogel coating is as follows:

[0078] S11: polyacrylonitrile fibers are subjected to pre-oxidation treatment at 260 DEG C to obtain pre-oxidized polyacrylonitrile fibers; the polyacrylonitrile fibers are soaked in an ethanol aqueous solution of KH-560 to obtain pretreated polyacrylonitrile fibers, the volume ratio of ethanol to water being 7:3; the pretreated polyacrylonitrile fibers are subjected to treatment in an inert atmosphere and then subjected to gas phase deposition to obtain siliconized polyacrylonitrile fibers, wherein the treatment temperature in the inert atmosphere is 300 DEG C, and the treatment time is 1.2 h; the gas phase deposition temperature is 680 DEG C, and the pressure is 0.8 Pa;

[0079] S12: polyacrylonitrile fibers, tetraethyl orthosilicate and boric acid are dispersed in deionized water, wherein the molar ratio of polyacrylonitrile fibers to tetraethyl orthosilicate is 1:4; the molar ratio of boric acid to polyacrylonitrile fibers is 0.07:1; after adding ammonia water, ultrasonic treatment is performed to obtain a pre-gel, wherein the molar ratio of ammonia water to polyacrylonitrile fibers is 0.4:1; the pre-gel is injected into a mold and subjected to treatment in a freeze dryer to obtain polyacrylonitrile fiber / silica hybrid aerogel, wherein the first freezing temperature is-20 DEG C, the first freezing time is 2.5 h, the second freezing temperature is-40 DEG C, the second freezing time is 2.8 h, the third freezing temperature is-80 DEG C, and the third freezing time is 11 h; the polyacrylonitrile fiber / silica hybrid aerogel is subjected to calcination and carbonization in an inert atmosphere to obtain surface-carbonized polyacrylonitrile fiber / silica aerogel, wherein the first calcination and carbonization temperature is 200 DEG C, and the first calcination and carbonization time is 1.5 h; the second calcination and carbonization temperature is 560 DEG C, and the second calcination and carbonization time is 1 h;

[0080] S13: melamine and phytic acid are added into deionized water respectively to obtain a melamine dispersion and a phytic acid dispersion; the phytic acid dispersion is added into the melamine dispersion, wherein the molar ratio of melamine to phytic acid is 1:2; a phosphate buffer solution is used to adjust the pH to 6; a sodium lignosulfonate solution with a mass fraction of 7 wt.% is added in portions; after ultrasonic treatment, the supermolecular network is obtained after standing; wherein the amount of the sodium lignosulfonate solution added is 13% of the mass of the supermolecular network;

[0081] S14: the surface-carbonized polyacrylonitrile fiber / silica aerogel is crushed and dry-mixed with the siliconized polyacrylonitrile fibers after cutting to obtain a mixture, wherein the surface-carbonized polyacrylonitrile fiber / silica aerogel is crushed to a particle size of ≤50 μm; the siliconized polyacrylonitrile fibers are cut to have a length-diameter ratio of 15:1; a silica sol and aluminum phosphate are mixed and aged at a molar ratio of 3.6:1 to obtain a pretreated binder, wherein the aging temperature is 24 DEG C, and the aging time is 22 h; the mixture, cerium dioxide powder and a dispersant BYK-2155 are added and ultrasonically treated to obtain the outer layer aerogel coating.

[0082] The outer layer aerogel coating comprises the following components by mass fraction:

[0083]

[0084] The preparation method of the inner layer flame-retardant coating is:

[0085] S21: The halloysite nanotubes are treated by immersion in an ethanol aqueous solution of KH-560 to obtain modified halloysite; the modified halloysite is mixed with bisphenol A bis(diphenyl phosphate), dipentaerythritol, and melamine cyanurate by ball milling to obtain a flame-retardant powder;

[0086] S22: The water-based epoxy resin Dow DER3210 is mixed with deionized water, and the flame-retardant powder, wetting agent BYK-349, and defoaming agent BYK-028 are added to obtain an inner layer flame-retardant coating;

[0087] The inner layer flame-retardant coating comprises the following components by mass fraction:

[0088]

[0089]

[0090] Example 2

[0091] The present embodiment provides an efficient fireproof aerogel steel structure coating and a preparation method thereof. The preparation method of the efficient fireproof aerogel steel structure coating specifically comprises the following steps:

[0092] The preparation method of the outer layer aerogel coating is:

[0093] S11: The polyacrylonitrile fiber is pre-oxidized at 200℃ to obtain pre-oxidized polyacrylonitrile fiber; the polyacrylonitrile fiber is treated by immersion in an ethanol aqueous solution of KH-560 to obtain pretreated polyacrylonitrile fiber, and the volume ratio of ethanol to water is 8:2; the pretreated polyacrylonitrile fiber is treated in an inert atmosphere and then subjected to vapor deposition to obtain siliconized polyacrylonitrile fiber, wherein the treatment temperature in the inert atmosphere is 320℃, and the treatment time is 1.3h; the vapor deposition temperature is 685℃, and the pressure is 0.7Pa;

[0094] S12: polyacrylonitrile fibers, tetraethyl orthosilicate and boric acid are dispersed in deionized water, wherein the molar ratio of polyacrylonitrile fibers to tetraethyl orthosilicate is 1:4.2; the molar ratio of boric acid to polyacrylonitrile fibers is 0.06:1, ammonia water is added to obtain a pre-gel after ultrasonic treatment, wherein the molar ratio of ammonia water to polyacrylonitrile fibers is 0.3:1, which is injected into a mold and treated in a freeze dryer to obtain polyacrylonitrile fiber / silica hybrid aerogel, the first temperature of freeze drying is-20℃, the time is 2.8h, the second temperature of freeze drying is-40℃, the time is 2.6h, the third temperature of freeze drying is-80℃, the time is 10h; the surface carbonized polyacrylonitrile fiber / silica aerogel is obtained by calcining and carbonizing under an inert atmosphere, wherein the first stage temperature of calcining and carbonizing is 180℃, the time is 2h; the second stage temperature of calcining and carbonizing is 600℃, the time is 0.8h;

[0095] S13: melamine and phytic acid are added to deionized water respectively to obtain a melamine dispersion and a phytic acid dispersion, the phytic acid dispersion is added to the melamine dispersion, wherein the molar ratio of melamine to phytic acid is 1:1.8, the pH is adjusted to 6.5 using a phosphate buffer solution, a 5wt.% sodium lignosulfonate solution is added in portions, and an ultrasonic treatment is performed to obtain a supramolecular network after standing;

[0096] S14: the surface carbonized polyacrylonitrile fiber / silica aerogel is crushed and dry-mixed with the cut siliconized polyacrylonitrile fibers to obtain a mixture, wherein the surface carbonized polyacrylonitrile fiber / silica aerogel is crushed to a particle size of ≤50μm; the siliconized polyacrylonitrile fibers are cut to a length-diameter ratio of 17:1; a silica sol and aluminum phosphate are mixed in a molar ratio of 3:1 to obtain a pretreated binder after aging, wherein the aging temperature is 20℃ and the aging time is 20h; the mixture, cerium dioxide powder and dispersant BYK-2155 are added and ultrasonic treatment is performed to obtain an outer layer aerogel coating;

[0097] The outer layer aerogel coating comprises the following components by mass:

[0098]

[0099] The preparation method of the inner layer flame-retardant coating is as follows:

[0100] S21: the halloysite nanotubes are soaked in an ethanol aqueous solution of KH-560 to obtain modified halloysite; the modified halloysite is ball-mixed with bisphenol A bis(diphenyl phosphate), dipentaerythritol and melamine cyanurate to obtain a flame-retardant powder;

[0101] S22: mixing the aqueous epoxy resin Dow DER3210 with deionized water, adding the flame-retardant powder, the wetting agent BYK-349, and the defoaming agent BYK-028 to obtain the inner layer flame-retardant coating;

[0102] The inner layer flame-retardant coating comprises the following components by mass fraction:

[0103]

[0104] Example 3

[0105] The embodiment provides a high-efficiency fireproof aerogel steel structure coating and a preparation method thereof, and the preparation method of the high-efficiency fireproof aerogel steel structure coating specifically comprises the following steps:

[0106] The preparation method of the outer layer aerogel coating is as follows:

[0107] S11: pre-oxidizing polyacrylonitrile fibers at 300 DEG C to obtain pre-oxidized polyacrylonitrile fibers; soaking the polyacrylonitrile fibers in an ethanol aqueous solution of KH-560 to obtain pretreated polyacrylonitrile fibers, wherein the volume ratio of ethanol to water is 7:3; treating the pretreated polyacrylonitrile fibers in an inert atmosphere, and then performing gas phase deposition to obtain siliconized polyacrylonitrile fibers, wherein the temperature for treating in the inert atmosphere is 340 DEG C, and the time is 1 h; the temperature for gas phase deposition is 670 DEG C, and the pressure is 0.5 Pa;

[0108] S12: dispersing polyacrylonitrile fibers, tetraethyl orthosilicate and boric acid in deionized water, wherein the molar ratio of polyacrylonitrile fibers to tetraethyl orthosilicate is 1:4.5, and the molar ratio of boric acid to polyacrylonitrile fibers is 0.05:1; after adding ammonia water, ultrasonic treatment is performed to obtain a pre-gel, wherein the molar ratio of ammonia water to polyacrylonitrile fibers is 0.5:1; the pre-gel is injected into a mold and treated in a freeze dryer to obtain polyacrylonitrile fiber / silica hybrid aerogel, wherein the first temperature for freeze drying is -20 DEG C, the time is 2 h, the second temperature for freeze drying is -40 DEG C, the time is 2 h, the third temperature for freeze drying is -80 DEG C, and the time is 11.5 h; the polyacrylonitrile fiber / silica aerogel with surface carbonization is obtained by calcination and carbonization in an inert atmosphere, wherein the first stage temperature for calcination and carbonization is 210 DEG C, and the time is 2.2 h; the second stage temperature for calcination and carbonization is 580 DEG C, and the time is 1.2 h;

[0109] S13: melamine and phytic acid are added into deionized water respectively to obtain a melamine dispersion and a phytic acid dispersion, the phytic acid dispersion is added into the melamine dispersion, the molar ratio of melamine to phytic acid is 1:2.2, a phosphate buffer solution is used to adjust the pH to 6.8, a 6wt.% sodium lignosulfonate solution is added in portions, and an ultrasonic treatment is performed to obtain a supramolecular network after standing, wherein the amount of sodium lignosulfonate added is 12% of the mass of the supramolecular network;

[0110] S14: surface carbonized polyacrylonitrile fiber / silica aerogel powder is crushed and dry-mixed with cut siliconized polyacrylonitrile fiber to obtain a mixture, wherein the surface carbonized polyacrylonitrile fiber / silica aerogel powder is crushed to a particle size of ≤50μm; the siliconized polyacrylonitrile fiber is cut to a length-diameter ratio of 10:1; a silica sol is mixed with aluminum phosphate at a molar ratio of 3.8:1 to obtain a pretreated binder after aging, wherein the aging temperature is 22℃ and the aging time is 23h; the mixture, cerium dioxide powder and dispersant BYK-2155 are added and ultrasonically treated to obtain an outer layer aerogel coating;

[0111] The outer layer aerogel coating comprises the following components by mass:

[0112]

[0113] The preparation method of the inner layer flame-retardant coating is as follows:

[0114] S21: halloysite nanotubes are soaked in an ethanol aqueous solution of KH-560 to obtain modified halloysite; the modified halloysite is ball-mixed with bisphenol A bis(diphenyl phosphate), dipentaerythritol and melamine cyanurate to obtain a flame-retardant powder;

[0115] S22: an aqueous epoxy resin Dow DER3210 is mixed with deionized water, and the flame-retardant powder, wetting agent BYK-349 and defoaming agent BYK-028 are added to obtain an inner layer flame-retardant coating;

[0116] The inner layer flame-retardant coating comprises the following components by mass:

[0117]

[0118] Example 4

[0119] The present embodiment provides a high-efficiency fireproof aerogel steel structure coating and a preparation method thereof, and the preparation method of the high-efficiency fireproof aerogel steel structure coating specifically comprises the following steps:

[0120] The preparation method of the outer layer aerogel coating is as follows:

[0121] S11: polyacrylonitrile fibers are pre-oxidized at 280℃ to obtain pre-oxidized polyacrylonitrile fibers; the polyacrylonitrile fibers are soaked in an ethanol aqueous solution of KH-560 to obtain pre-processed polyacrylonitrile fibers, the volume ratio of ethanol to water being 9:1; the pre-processed polyacrylonitrile fibers are treated in an inert atmosphere and then subjected to vapor deposition to obtain siliconized polyacrylonitrile fibers, wherein the temperature for treatment in the inert atmosphere is 350℃, and the time is 1.5h; the temperature for vapor deposition is 690℃, and the pressure is 1Pa;

[0122] S12: polyacrylonitrile fibers, tetraethyl orthosilicate and boric acid are dispersed in deionized water, wherein the molar ratio of polyacrylonitrile fibers to tetraethyl orthosilicate is 1:3.5; the molar ratio of boric acid to polyacrylonitrile fibers is 0.08:1; after adding ammonia water, a pre-gel is obtained by ultrasonic treatment, wherein the molar ratio of ammonia water to polyacrylonitrile fibers is 0.45:1; the pre-gel is injected into a mold and treated in a freeze dryer to obtain polyacrylonitrile fiber / silica hybrid aerogel, wherein the first temperature for freeze drying is -20℃, the time is 3h; the second temperature for freeze drying is -40℃, the time is 3h; the third temperature for freeze drying is -80℃, the time is 12h; the surface carbonized polyacrylonitrile fiber / silica aerogel is calcined and carbonized in an inert atmosphere to obtain a surface carbonized polyacrylonitrile fiber / silica aerogel, wherein the first stage temperature for calcination and carbonization is 220℃, the time is 2.5h; the second stage temperature for calcination and carbonization is 500℃, the time is 1.1h;

[0123] S13: melamine and phytic acid are added to deionized water respectively to obtain a melamine dispersion and a phytic acid dispersion; the phytic acid dispersion is added to the melamine dispersion, wherein the molar ratio of melamine to phytic acid is 1:2.1; the pH is adjusted to 7 using a phosphate buffer solution; a sodium lignosulfonate solution with a mass fraction of 8wt.% is added in portions; after ultrasonic treatment, a supramolecular network is obtained by standing, wherein the amount of sodium lignosulfonate added is 15% of the mass of the supramolecular network;

[0124] S14: the surface carbonized polyacrylonitrile fiber / silica aerogel is crushed and dry-mixed with the cut siliconized polyacrylonitrile fibers to obtain a mixture, wherein the surface carbonized polyacrylonitrile fiber / silica aerogel is crushed to a particle size of ≤50μm; the siliconized polyacrylonitrile fibers are cut to a length-diameter ratio of 20:1; a silica sol and aluminum phosphate are mixed and aged at a molar ratio of 4:1 to obtain a pre-processed binder, wherein the temperature for aging is 25℃, and the time is 24h; the mixture, cerium dioxide powder and dispersant BYK-2155 are added and ultrasonically treated to obtain an outer layer aerogel coating;

[0125] The outer layer aerogel coating comprises the following components by mass:

[0126]

[0127]

[0128] The preparation method of the inner layer flame-retardant coating is:

[0129] S21: treating halloysite nanotubes immersed in an ethanol aqueous solution of KH-560 to obtain modified halloysite; mixing the same with bisphenol A bis(diphenyl phosphate), dipentaerythritol, melamine cyanurate by ball milling to obtain a flame-retardant powder;

[0130] S22: mixing the water-based epoxy resin Dow DER3210 with deionized water, adding the flame-retardant powder, wetting agent BYK-349, and defoaming agent BYK-028, and mixing to obtain an inner layer flame-retardant coating;

[0131] The inner layer flame-retardant coating comprises the following components by mass:

[0132]

[0133] Comparative Example 1

[0134] This comparative example provides a high-efficiency fireproof aerogel steel structure coating, which differs from Example 1 in that the step of vapor deposition in S11 is omitted, and the carbonized polyacrylonitrile fiber treated in an inert atmosphere is directly used, and other operation steps and process parameters are completely the same as those of Example 1.

[0135] Comparative Example 2

[0136] This comparative example provides a high-efficiency fireproof aerogel steel structure coating, which differs from Example 1 in that, in S12, the polyacrylonitrile fiber / silica hybrid aerogel is directly subjected to one-step carbonization treatment, and other operation steps and process parameters are completely the same as those of Example 1.

[0137] Comparative Example 3

[0138] This comparative example provides a high-efficiency fireproof aerogel steel structure coating, which differs from Example 1 in that S13 is omitted, and no supramolecular network is added in the outer layer aerogel coating, and other operation steps and process parameters are completely the same as those of Example 1.

[0139] The high-efficiency fireproof aerogel steel structure coatings of Examples 1-4 and Comparative Examples 1-3 are tested for performance, and the specific process is as follows:

[0140] The prepared inner layer flame-retardant coating is sprayed on the surface of the steel structure, which is placed at a temperature of 80℃ for curing for 1h, and after it is cooled, the outer layer aerogel coating is scraped and cured to form a high-efficiency fireproof aerogel steel structure coating layer.

[0141] The thermal conductivity of the sample is tested according to GB / T10297-2015;

[0142] The adhesion of the sample was tested according to GB / T5210-2006;

[0143] The fire resistance of the sample was tested according to GB / T14907-2018;

[0144] The test results are shown in Table 1.

[0145] Table 1: Test results of high-efficiency fireproof aerogel steel structure coating performance of Examples 1-4 and Comparative Examples 1-3

[0146]

[0147]

[0148] From the test results of Example 1 and Comparative Example 1, it can be seen that when the gas deposition step is omitted and carbonized polyacrylonitrile fibers are directly used, the adhesion of the coating is significantly reduced, because the surface of the carbonized fibers lacks the mechanical anchoring effect of silicon dioxide, and cannot form effective physical interlocking with the substrate; the thermal conductivity increases due to the increase in the connectivity of the aerogel pores, which increases the proportion of gas convection conduction; the pure carbon layer has a high oxidation rate at high temperatures, and the stability of the thermal barrier layer is insufficient, which cannot effectively block the penetration of heat flow, and the fireproof protection time is shortened.

[0149] From the test results of Example 1 and Comparative Example 2, it can be seen that after using a one-step carbonization process, the thermal conductivity of the coating increases, because the rapid carbonization causes the collapse of the pore structure, the proportion of solid-phase heat transfer path increases, and the heat insulation performance decreases; excessive carbonization increases the graphitization degree of the fiber surface, which reduces the chemical compatibility with the binder and weakens the interfacial bonding force, resulting in a decrease in adhesion; the fire resistance is significantly reduced due to the uneven thickness of the carbon layer, which causes stress concentration and accelerates the penetration of heat flow.

[0150] From the test results of Example 1 and Comparative Example 3, it can be seen that the high-temperature stability of the coating decreases after removing the supramolecular network, because the absence of sodium lignosulfonate causes the loss of free radical quenching function, and the oxidation and degradation rate of the aerogel skeleton increases; the decrease in fire resistance reflects the loosening of the carbon layer structure, which weakens the heat radiation reflection ability; the decrease in adhesion is due to the formation of micro-cracks in the interface area due to the absence of network filling, which aggravates the stress concentration phenomenon and reduces the bonding strength of the coating and the substrate.

[0151] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and those skilled in the art should understand that any changes or replacements within the technical scope disclosed by the present application can be easily thought of by those skilled in the art, and all fall within the protection scope and disclosure scope of the present application.

Claims

1. A highly fire resistant aerogel steel structure coating, characterized in that, The high-efficiency fireproof aerogel steel structure coating comprises an outer aerogel coating and an inner fire-retardant coating. The outer aerogel coating comprises the following components: surface carbonized polyacrylonitrile fiber / silica aerogel 50-60%, siliconized polyacrylonitrile fiber 10-15%, supramolecular network 5-8%, pretreated binder 18-22%, cerium dioxide powder 2-4%, dispersant 1-2% and deionized water 1-3%; the pretreated binder is obtained by mixing and aging silica sol and aluminum phosphate; The preparation method of the siliconized polyacrylonitrile fiber is as follows: S11: polyacrylonitrile fiber is subjected to pre-oxidation treatment to obtain pre-oxidized polyacrylonitrile fiber; the pre-oxidized polyacrylonitrile fiber is soaked in an ethanol aqueous solution of KH-560 to obtain pretreated polyacrylonitrile fiber; and then subjected to vapor deposition after treatment in an inert atmosphere to obtain siliconized polyacrylonitrile fiber; the temperature of vapor deposition is 670-690℃, and the pressure is 0.5-1Pa, The preparation method of the surface carbonized polyacrylonitrile fiber / silica aerogel is as follows: S12: polyacrylonitrile fiber, tetraethyl orthosilicate and boric acid are dispersed in deionized water, and after adding ammonia water, the pre-gel is obtained by ultrasonic treatment; the pre-gel is injected into a mold and treated in a freeze dryer to obtain polyacrylonitrile fiber / silica hybrid aerogel; the polyacrylonitrile fiber / silica hybrid aerogel is calcined and carbonized in an inert atmosphere to obtain surface carbonized polyacrylonitrile fiber / silica aerogel; the first stage of calcination and carbonization is carried out at a temperature of 180-220℃ for 1.5-2.5h in a nitrogen atmosphere; the second stage of calcination and carbonization is carried out at a temperature of 500-600℃ for 0.8-1.2h in a nitrogen-hydrogen mixed atmosphere; The preparation method of the supramolecular network is as follows: S13: melamine and phytic acid are added to deionized water to obtain a melamine dispersion and a phytic acid dispersion, respectively; the phytic acid dispersion is added to the melamine dispersion, and the pH is adjusted using a phosphate buffer solution; a sodium lignosulfonate solution is added in portions, and the supramolecular network is obtained by ultrasonic treatment and standing; The inner fire-retardant coating is composed of the following components: water-based epoxy resin 35-40%, deionized water 1-3%, modified halloysite 5-8%, bisphenol A bis(diphenyl phosphate) 25-30%, dipentaerythritol 8-12%, melamine cyanurate 8-12%, wetting agent 1-2% and defoaming agent 0.5-1.5%.

2. A process for the preparation of a high efficiency fire resistant aerogel steel structure coating as claimed in claim 1, wherein, The preparation method of the outer aerogel coating is as follows: S14: the surface carbonized polyacrylonitrile fiber / silica aerogel is crushed and dry-mixed with the cut siliconized polyacrylonitrile fiber to obtain a mixture; the pretreated binder is obtained by mixing and aging silica sol and aluminum phosphate, and then adding the mixture, cerium dioxide powder and dispersant and ultrasonic treatment to obtain the outer aerogel coating.

3. A process for the preparation of a high efficiency fire resistant aerogel steel structure coating as claimed in claim 2, wherein, In S11: The pretreated polyacrylonitrile fiber is treated in an inert atmosphere at a temperature of 300-350℃; The pretreated polyacrylonitrile fiber is treated in an inert atmosphere for 1-1.5h.

4. The method of claim 2, wherein the method further comprises the step of adding a fire retardant to the mixture of the silica sol, the binder, and the water. In S12: The molar ratio of the polyacrylonitrile fiber to tetraethyl orthosilicate is 1:3.5-4.5; The molar ratio of the boric acid to the polyacrylonitrile fiber is 0.05-0.08:1; The molar ratio of the ammonia water to the polyacrylonitrile fiber is 0.3-0.5:

1.

5. The method for preparing a highly efficient fire-retardant aerogel steel structure coating according to claim 2, characterized in that, In S13: The molar ratio of the melamine to the phytic acid is 1:1.8-2.2; The feeding amount of the sodium lignosulfonate is 10-15% of the mass of the supramolecular network.

6. The method for preparing a highly efficient fire-retardant aerogel steel structure coating according to claim 2, characterized in that, In S14: The surface-carbonized polyacrylonitrile fiber / silica aerogel is crushed to a particle size of ≤50 μm; The siliconized polyacrylonitrile fiber is cut to a length-diameter ratio of 10:1-20:1; The molar ratio of the silica sol to the aluminum phosphate in the pretreatment binder is 3-4:1; The temperature for aging after mixing the silica sol and the aluminum phosphate is 20-25 ℃; The time for aging after mixing the silica sol and the aluminum phosphate is 20-24 h.

7. The method for preparing a highly efficient fire-retardant aerogel steel structure coating according to claim 2, characterized in that, The preparation method of the inner layer flame-retardant coating is: S21: Laponite nanotubes are treated by immersion in an ethanol aqueous solution of KH-560 to obtain modified Laponite; the modified Laponite is mixed with bisphenol A bis(diphenyl phosphate), dipentaerythritol, and melamine cyanurate by ball milling to obtain a flame-retardant powder; S22: An aqueous epoxy resin is mixed with deionized water, and the flame-retardant powder, a wetting agent, and a defoaming agent are added to obtain an inner layer flame-retardant coating.

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