Flame-retardant composite aerogel as well as preparation method and application thereof

Abstract

The invention relates to a flame-retardant composite aerogel and a preparation method and application thereof, and the preparation method comprises the following steps: uniformly mixing a chitosan aqueous solution with an MXene dispersion liquid to obtain a mixed solution, dropwise adding an alkali liquid to obtain a black first suspension, and concentrating to obtain a second suspension; adding a cross-linking agent into the second suspension, and performing freezing cross-linking to obtain chitosan / MXene frozen gel; the chitosan / MXene frozen gel is subjected to unfreezing, solvent replacement and drying, and the flame-retardant composite aerogel is obtained. The composite aerogel obtained by the invention is good in mechanical stability and has excellent flame retardant property and oil absorption property; the preparation method is simple, no ultralow-temperature freeze drying equipment or supercritical drying equipment is needed, the preparation cost is low, the size is not limited, and large-scale production can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of composite aerogel materials, and in particular to a flame-retardant composite aerogel and its preparation method and application. Background Art

[0002] Physical adsorption is a cost-effective and environmentally friendly method for solving oil spill problems. However, due to the poor fluidity and high viscosity of crude oil, great challenges have been encountered in cleaning up crude oil spills. On the one hand, crude oil is very sensitive to temperature, and its viscosity can be greatly reduced with the increase of temperature. Increasing the temperature can effectively reduce the viscosity of crude oil, making it more conducive to being quickly adsorbed. But on the other hand, since oil is a flammable substance, the increase of temperature will greatly increase the risk of fire. Therefore, designing an adsorbent with rapid heat generation, flame retardancy and hydrophobicity is a feasible method to solve this problem.

[0003] Chitosan is widely sourced, low-cost, biodegradable and environmentally friendly. Two-dimensional transition metal carbide MXene has excellent photothermal conversion effects and easy processability in aqueous dispersions due to its hydrophilic functional groups. The composite aerogel composed of the two has a three-dimensional framework, low density, high porosity, compressibility, remarkable photothermal and flame retardancy, and is very suitable as a crude oil adsorbent. However, at present, the preparation of chitosan-based aerogels mostly uses freeze-drying (usually freezing at -50°C to -60°C for more than 48 hours) or supercritical drying methods. The preparation cost is high, and its size and scalable production are limited, which hinders its practical application. Therefore, there is an urgent need to develop a simple, economical and scalable preparation method to manufacture chitosan-based aerogels. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above technical deficiencies, and provide a flame-retardant composite aerogel and its preparation method and application, so as to solve the technical problems in the prior art that the preparation cost of chitosan-based aerogels is high, and their size and scalable production are limited.

[0005] To achieve the above technical purpose, the technical solution provided by the present invention is as follows: In the first aspect, the present invention provides a preparation method of a flame-retardant composite aerogel, including the following steps: mixing a chitosan aqueous solution and an MXene dispersion liquid evenly to obtain a mixed solution, then dropping an alkali solution to obtain a black first suspension, and concentrating to obtain a second suspension; adding a crosslinking agent to the second suspension, and performing freeze-crosslinking to obtain a chitosan / MXene cryogel; the chitosan / MXene cryogel is thawed, solvent-exchanged, and dried to obtain a flame-retardant composite aerogel.

[0006] In the second aspect, the present invention provides a flame-retardant composite aerogel prepared by the above preparation method.

[0007] In a third aspect, the present invention provides a hydrophobic composite aerogel, which is obtained by hydrophobically modifying the above-mentioned flame-retardant composite aerogel.

[0008] In a fourth aspect, the present invention provides an application of the above-mentioned flame-retardant composite aerogel or hydrophobic composite aerogel in the preparation of crude oil recovery materials, photothermal conversion materials or flame-retardant materials.

[0009] Compared with the prior art, the beneficial effects of the present invention include: 1. In the present invention, a flocculent suspension is prepared from a chitosan aqueous solution and an MXene dispersion liquid, and then uniformly mixed with a cross-linking agent and directly subjected to freeze cross-linking. After thawing and solvent replacement, a normal temperature and normal pressure drying method can be adopted, which is different from the commonly used drying methods (freeze drying or supercritical drying). The preparation process is not only simple and low-cost, but also can be used to prepare chitosan-based aerogels on a large scale, which is conducive to industrialization; 2. The composite aerogel prepared in the present invention has better mechanical properties than chitosan aerogel, and the strong interaction between MXene and chitosan after compounding helps to form a mechanically strong skeleton, improving mechanical stability, thereby avoiding structural collapse of the composite aerogel during normal pressure drying; 3. The present invention opens up a new way for manufacturing sustainable, durable and multifunctional normal pressure drying flame-retardant composite aerogels, which has great application potential in the fields of dye adsorption, oil-water separation, intelligent heaters, electromagnetic protection and aerospace, especially with excellent flame-retardant performance and oil absorption performance. Description of the Drawings

[0010] Figure 1 Transmission electron micrograph of the MXene dispersion liquid prepared in Example 1; Figure 2 Scanning electron micrograph of the flame-retardant composite aerogel prepared in Example 4; Figure 3 Static water contact angle test result graph of the flame-retardant composite aerogel prepared in Example 4 at room temperature; Figure 4 Schematic diagram of the adsorption of high-viscosity oil by the flame-retardant composite aerogel prepared in Example 4; Figure 5 Vertical burning test result graph of the flame-retardant composite aerogel prepared in Example 4 after adsorbing high-viscosity oil; Figure 6 Vertical burning test result graph of the pure chitosan aerogel prepared in Comparative Example 2 after adsorbing high-viscosity oil; Figure 7 Stress-strain curves of the aerogels prepared in Examples 1-4 and Comparative Examples 1-2 at a compressive strain of 80%; Figure 8The photothermal curves of the aerogels prepared in Examples 1-2, 4 of the present invention and Comparative Examples 1-2 under a light power density of 1.0 kW / m 2 are shown in Figure 1. Detailed Description of the Invention

[0011] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0012] Aiming at the deficiencies in the prior art that the preparation cost of chitosan-based aerogels is high, and the size and large-scale production are restricted, the present invention provides a flame-retardant composite aerogel, a preparation method and an application thereof. By means of atmospheric drying, a sustainable and multifunctional flame-retardant chitosan-based aerogel can be prepared in a large area, which has great application potential in the fields of dye adsorption, oil-water separation, intelligent heaters, electromagnetic protection and aerospace, and has a wide range of applications, especially when used as an oil-water separation material.

[0013] In a first aspect, the present invention provides a method for preparing a flame-retardant composite aerogel, comprising the following steps: Mix the chitosan aqueous solution and the MXene dispersion evenly to obtain a mixed solution, then add an alkali solution dropwise to obtain a black first flocculent suspension, and concentrate it to obtain a second flocculent suspension; Add a crosslinking agent to the second flocculent suspension, and freeze-crosslink to obtain a chitosan / MXene cryogel; The chitosan / MXene cryogel is thawed and solvent-exchanged, and dried at ambient temperature to obtain a flame-retardant composite aerogel.

[0014] Preferably, the mass ratio of MXene to chitosan in the mixed solution is 1: (1-4).

[0015] Preferably, the viscosity of chitosan is 800-1000 mPa·s; the chitosan aqueous solution is prepared by dissolving chitosan in an aqueous acetic acid solution with a mass fraction of 2.0-4.0%; the mass fraction of chitosan in the chitosan aqueous solution is 1.0-1.5%.

[0016] Preferably, the concentration of the MXene dispersion is 10-50 mg / mL.

[0017] More preferably, the MXene dispersion is prepared from concentrated hydrochloric acid solution, lithium fluoride and aluminum titanium carbide as raw materials and reacted at 30-35 °C for 22-24 h. MXene is a single-layer two-dimensional transition metal carbide.

[0018] Preferably, the chitosan aqueous solution and the MXene dispersion are mixed evenly. Specifically, the MXene dispersion is added to the chitosan aqueous solution and stirred for 2 - 4 h to mix evenly.

[0019] Preferably, the lye is an aqueous sodium hydroxide solution with a concentration of 0.5 - 1 mol / L.

[0020] Preferably, the concentration is achieved by filtering out part of the liquid, and the volume of the part of the liquid is 1 / 4 - 1 / 2 of the volume of the first flocculent suspension.

[0021] Preferably, the crosslinking agent includes epichlorohydrin.

[0022] Preferably, the volume of the crosslinking agent is 1.5% - 2% of the volume of the second flocculent suspension. If the addition amount of the crosslinking agent is too small, the crosslinking strength of the composite gel is insufficient, which will affect its molding; if it is too large, the resulting composite gel has high mechanical strength but insufficient elasticity.

[0023] Preferably, the freeze - crosslinking is carried out at - 15°C to - 30°C for 10 - 14 h. Among them, if the freezing time is too short, the solvent is not completely frozen and the crosslinking degree is insufficient, affecting its molding; if the freezing time is too long, the ice crystals grow excessively, the pore size increases, and the heat insulation performance is reduced. When the freezing temperature is relatively high (-15°C - 0°C), the ice crystal size is large, the pore structure of the composite aerogel becomes rough, reducing its specific surface area and mechanical strength. When the freezing temperature is too low, the growth time of the ice crystals decreases, but the crosslinking agent diffuses insufficiently, resulting in uneven local crosslinking.

[0024] After adding a crosslinking agent such as epichlorohydrin and dispersing it evenly in the present invention, the obtained mixed solution is directly added to a mold and placed in a common freezing device such as a refrigerator for freeze - crosslinking. The chitosan / MXene frozen gel obtained in this way has a small volume shrinkage during the subsequent environmental drying process, and the obtained aerogel has a small apparent density. In the present invention, the shape of the mold can be cylindrical, disc - shaped, cubic, etc. By customizing various different - shaped molds, the requirements for preparing various different - shaped chitosan aerogels can be met.

[0025] Preferably, the thawing is to thaw the chitosan / MXene frozen gel at ambient temperature.

[0026] Preferably, the solvents used for solvent replacement include ethanol (mass concentration not less than 99.8%), the number of solvent replacement times is 3 - 6 times, and the time for each replacement is 1 - 2 h.

[0027] Preferably, the ambient temperature is 20 - 30°C; the drying time is 48 - 72 h.

[0028] It can be understood that the drying method in the present invention is atmospheric drying.

[0029] Second aspect, the present invention provides a flame-retardant composite aerogel prepared by the above preparation method.

[0030] Third aspect, the present invention provides a hydrophobic composite aerogel, which is prepared by hydrophobic modification of the above flame-retardant composite aerogel.

[0031] Preferably, the flame-retardant composite aerogel is subjected to hydrophobic modification by chemical vapor deposition. The specific steps include: depositing methyltrimethoxysilane (MTMS) vapor on the surface of the flame-retardant composite aerogel at 75-80 °C to obtain a hydrophobic composite aerogel.

[0032] More preferably, the flame-retardant composite aerogel is placed in methyltrimethoxysilane (MTMS) vapor at 80 °C for 8-12 h.

[0033] Fourth aspect, the present invention provides an application of the above flame-retardant composite aerogel or hydrophobic composite aerogel in the preparation of crude oil recovery materials, photothermal conversion materials or flame retardant materials.

[0034] The present invention will be further described in detail below through specific examples.

[0035] Example 1 A preparation method of a flame-retardant composite aerogel, comprising the following steps: (1) Preparation of a single-layer MXene dispersion: Dissolve 3.2 g of lithium fluoride in 40 mL of 9 mol / L concentrated hydrochloric acid solution, stir magnetically for 15 min, and then slowly add 2 g of titanium aluminum carbide powder, and react at 35 °C in a water bath for 24 h. Then add deionized water to the reaction solution and centrifuge and wash multiple times until the pH = 7. Add ethanol to the centrifuged precipitate and ultrasonically treat it under nitrogen protection. Then centrifuge at a speed of 10,000 rpm for 10 min. Immediately add deionized water to the newly obtained precipitate and ultrasonically treat it under nitrogen protection. Finally, centrifuge at a speed of 5,000 rpm for 5 min to obtain a single-layer MXene dispersion with a concentration of 26.5 mg / mL; (2) Preparation of a chitosan / MXene mixed solution: Dissolve chitosan with a 2.0 wt% aqueous acetic acid solution to obtain a uniform aqueous chitosan solution with a mass concentration of chitosan in the aqueous chitosan solution of 1.25%; then add the MXene dispersion obtained in step (1) to the aqueous chitosan solution and stir well to obtain a chitosan / MXene mixed solution, and the mass ratio of MXene to chitosan is 2:8; (3) Preparation of a chitosan / MXene mixed suspension: Drop a 0.5 mol / L aqueous sodium hydroxide solution into the mixed solution obtained in step (2) under stirring to obtain a black first flocculent suspension, and then filter half of the liquid of the first flocculent suspension to obtain a second flocculent suspension with a higher concentration; (4) Freezing process: Add epichlorohydrin as a crosslinking agent with a volume ratio of 2% to the second flocculent suspension obtained in step (3), and then add it to a mold and freeze it at -25°C for 10 h to obtain a chitosan / MXene cryogel; (5) Thawing, solvent replacement, and drying process: Thaw the chitosan / MXene cryogel obtained in step (4) at ambient temperature and then replace the solvent with ethanol 6 times, with each replacement time being 1 h, and dry it at ambient temperature to obtain a flame-retardant chitosan / MXene composite aerogel; (6) Hydrophobic modification: The flame-retardant chitosan / MXene composite aerogel obtained in step (5) is subjected to hydrophobic modification by chemical vapor deposition. The modifier is methyltrimethoxysilane (MTMS). Its vapor will deposit on the surface of the flame-retardant chitosan / MXene composite aerogel at 80°C and be modified for 10 h to finally obtain a hydrophobic flame-retardant composite aerogel.

[0036] Example 2 This example is the same as Example 1, with the only difference being that: in step (2), the mass ratio of MXene to chitosan added is 3:7, and finally a hydrophobic flame-retardant composite aerogel is obtained.

[0037] Example 3 This example is the same as Example 1, with the only difference being that: in step (2), the mass ratio of MXene to chitosan added is 4:6, and finally a hydrophobic flame-retardant composite aerogel is obtained.

[0038] Example 4 This example is the same as Example 1, with the only difference being that: in step (2), the mass ratio of MXene to chitosan added is 5:5, and finally a hydrophobic flame-retardant composite aerogel is obtained.

[0039] Comparative Example 1 This example is the same as Example 1, with the only difference being that: in step (2), the mass ratio of MXene to chitosan added is 6:4.

[0040] It was found that when the amount of MXene used was too much, the chitosan / MXene mixed solution was non-uniform.

[0041] Comparative Example 2 A preparation method of a hydrophobic pure chitosan aerogel, comprising the following steps: (1) Preparation of chitosan aqueous solution: Dissolve chitosan with a 2 wt% aqueous acetic acid solution to obtain a uniform chitosan aqueous solution, and the mass concentration of chitosan in the chitosan aqueous solution is 1.25%; (2) Preparation of suspension: The solution obtained in step (1) was dropwise added with 0.5 mol / L aqueous sodium hydroxide solution under stirring to obtain a first flocculent suspension, and then half of the water in the first flocculent suspension was filtered to obtain a second flocculent suspension with a higher concentration; (3) Freezing process: Epichlorohydrin, a cross-linking agent with a volume ratio of 2% to the second flocculent suspension obtained in step (2), was added thereto, and then it was added to a mold and frozen at -25 °C for 10 h to obtain a chitosan cryogel; (4) Thawing, solvent replacement, and drying process: The cryogel obtained in step (3) was thawed at ambient temperature and then replaced with ethanol solvent 6 times, with each replacement time being 1 h, and dried at ambient temperature to obtain pure chitosan aerogel; (5) Hydrophobic modification: The dried pure chitosan aerogel obtained in step (4) was hydrophobically modified by chemical vapor deposition. The modifier was methyltrimethoxysilane (MTMS). Its vapor would deposit on the surface of the chitosan aerogel at 80 °C and was modified for 10 h to finally obtain a hydrophobic pure chitosan aerogel (CS).

[0042] Performance testing (1) Morphology detection of MXene dispersion: The MXene dispersion prepared in Example 1 was characterized using a transmission electron microscope (JEM-2100F) at 30,000 times magnification. The results are as Figure 1 shown, indicating that the prepared MXene is a single-layer flaky structure.

[0043] (2) Surface morphology detection of the flame-retardant composite aerogel prepared in Example 4: It was characterized using a scanning electron microscope (JSM-6510) at 55 times magnification. Its cross-sectional micro-morphology is as Figure 2 shown, and it has a typical porous structure of aerogel.

[0044] (3) Static water contact angle test: The contact angle of the sample was measured using a contact angle measuring instrument (JC2000C2E). Among them, the static water contact angle test results of the flame-retardant composite aerogel prepared in Example 4 at room temperature are as Figure 3 shown, and the contact angle is 134.8° ± 0.4°. It can be seen that the prepared chitosan aerogel has hydrophobic properties.

[0045] (4) Adsorption test of the flame-retardant composite aerogel for high-viscosity oil: A few drops of engine oil were dropped on a glass petri dish, and the sample was picked up with tweezers to contact the oil for adsorption. Among them, the adsorption results of the flame-retardant composite aerogel prepared in Example 4 for high-viscosity oil are as Figure 4 shown. When the sample contacts the oil droplet, it can adsorb the oil droplet, indicating that it has a good oil absorption effect, and its adsorption capacity is 32.8 g / g.

[0046] (5)Flame retardancy test: The vertical combustion test results of the flame-retardant composite aerogel prepared in Example 4 and the pure chitosan aerogel prepared in Comparative Example 2 after adsorbing high-viscosity oil and contacting a flame are as Figure 5 and Figure 6 shown. The results show that the flame-retardant composite aerogel prepared in Example 4 does not burn violently when contacting a flame after oil absorption, can quickly self-extinguish after removing the flame, and has good dimensional stability ( Figure 5 ); while the pure chitosan aerogel in Comparative Example 2 burns significantly when contacting a flame after oil absorption, still burns violently after removing the flame, and the size of the aerogel shrinks significantly after combustion ( Figure 6 ), indicating that the flame-retardant composite aerogel prepared by the present invention has excellent flame retardancy.

[0047] (6)Mechanical property test: The SUNS UTM6503 microcomputer-controlled electronic universal testing machine was used to conduct compression tests on Examples 1, 2, 3, 4 and Comparative Examples 1, 2 at a compression strain of 80%. As Figure 7 shown, when the addition amount of MXene (the mass ratio of MXene to the total mass of MXene and chitosan) is 50%, the prepared composite aerogel has the highest compression strength. Excessive or too low addition amount will affect its compression strength.

[0048] (7)Photothermal conversion performance test: The surfaces of the flame-retardant composite aerogels of Examples 1-2, Example 4 and the aerogels obtained in Comparative Examples 1-2 were irradiated with a xenon lamp light source system CEL-HXF300, and an infrared thermal imager (HIKMICRO, H13) was used to record their surface temperatures. See Figure 8 . When light irradiation is applied, the surface temperatures of each group of aerogels all rise rapidly and finally reach a relatively stable value. Under the simulated light irradiation of 1.0 kW / m 2 , the surface temperature of Comparative Example 1 can reach up to 100 °C at most, and the surface temperature of each aerogel has a linear relationship with the addition amount of MXene. In the case of lacking MXene, the highest temperature of the pure CS aerogel is only 54 °C, showing the positive effect of MXene on the photothermal conversion performance.

[0049] It can be seen from the above Examples 1-4 and Comparative Examples 1-2 that if the amount of MXene used is too much (the mass ratio of MXene to chitosan is greater than 1:1), a homogeneous solution cannot be obtained after mixing the aqueous solutions of the two, which affects the forming and mechanical properties of the composite aerogel; if the amount of MXene used is too little (the mass ratio of MXene to chitosan is less than 1:4), the mechanical strength of the composite aerogel is insufficient, and the photothermal conversion ability cannot be fully exerted. Therefore, the present invention preferably selects the mass ratio of MXene to chitosan to be 1: (1-4), that is, preferably the amount of MXene used accounts for 20-50% of the total mass of MXene and chitosan, and more preferably 40-50%.

[0050] In summary, the present invention provides a flame-retardant composite aerogel, a preparation method thereof, and an application. A flocculent suspension is prepared from a chitosan aqueous solution and an MXene dispersion liquid, and then directly subjected to freeze cross-linking after being uniformly mixed with a cross-linking agent. After thawing and solvent replacement, a drying method at normal temperature and pressure can be adopted to obtain the flame-retardant composite aerogel. The obtained composite aerogel has good mechanical stability, excellent flame-retardant performance and oil absorption performance; the preparation method of the present invention is simple, does not require ultra-low temperature freeze-drying equipment or supercritical drying equipment, has low preparation cost, and its size is not limited, and can be produced on a large scale.

[0051] The specific embodiments of the present invention described above do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and deformations made according to the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for preparing a flame-retardant composite aerogel, characterized in that: The following steps are involved: The chitosan aqueous solution and the MXene dispersion are uniformly mixed to obtain a mixed solution, and then an alkali solution is added dropwise to obtain a black first suspension, which is then concentrated to obtain a second suspension; adding a cross-linking agent to the second suspension to obtain a chitosan / MXene cryogel by freeze cross-linking; Chitosan / MXene cryogel was thawed, solvent replaced and dried to obtain flame-retardant composite aerogel.

2. The method for preparing the flame-retardant composite aerogel according to claim 1, characterized in that: The mass ratio of MXene to chitosan in the mixed solution is 1:(1-4).

3. The method for preparing the flame-retardant composite aerogel according to claim 1, characterized in that: The chitosan aqueous solution is prepared by dissolving chitosan in a glacial acetic acid aqueous solution with a mass fraction of 2.0 to 4.0%; the mass fraction of chitosan in the chitosan aqueous solution is 1.0 to 1.5%; and / or, The concentration of the MXene dispersion is 10 to 50 mg / mL.

4. The method for preparing the flame-retardant composite aerogel according to claim 1, characterized in that: The alkali solution is a 0.5-1 mol / L sodium hydroxide aqueous solution; and / or, The concentration is to remove part of the liquid by filtering, and the volume of the part of the liquid is 1 / 4 to 1 / 2 of the volume of the first suspension.

5. The method for preparing the flame-retardant composite aerogel according to claim 1, characterized in that: The cross-linking agent comprises epichlorohydrin; and / or, The volume of the cross-linking agent is 1.5% to 2% of the volume of the second suspension.

6. The method for preparing the flame-retardant composite aerogel according to claim 1, characterized in that: The freezing cross-linking is carried out by freezing at -15°C to -30°C for 10 to 14 hours.

7. The method for preparing the flame-retardant composite aerogel according to claim 1, characterized in that: The thawing environment temperature is 20-30°C; and / or, The solvent used in the solvent replacement includes ethanol, the number of solvent replacements is 3 to 6 times, and the time for each replacement is 1 to 2 hours; and / or, The drying environment temperature is 20-30° C. and the drying time is 48-72 hours.

8. The flame-retardant composite aerogel prepared by the preparation method according to any one of claims 1 to 7.

9. A hydrophobic composite aerogel, characterized in that: The hydrophobic composite aerogel is obtained by subjecting the flame-retardant composite aerogel according to claim 8 to hydrophobic modification.

10. Use of the flame-retardant composite aerogel according to claim 8 or the hydrophobic composite aerogel according to claim 9 in preparing crude oil recovery materials, photothermal conversion materials or flame-retardant materials.

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