阅读说明：本技术 柔性阻燃气凝胶、其制备方法与应用 (Flexible flame-retardant aerogel, and preparation method and application thereof ) 是由 刘丽芳 降帅 张美玲 李萌萌 徐秋玉 于 2020-12-21 设计创作，主要内容包括：一种柔性阻燃气凝胶、其制备方法与应用,属于气凝胶技术领域。该柔性阻燃气凝胶采用以下原料制成：纳米纤维素、海藻酸钠、有机硅烷、硼酸、十水四硼酸钠、碳酸钙和D-(+)-葡萄糖酸δ-内酯,其中,纳米纤维素包括纤维素纳米线和表面富羧基的纤维素纳米微纤。本发明利用纤维素纳米线高长径比的形态结构特点提高单根纳米纤维素之间的关联度,以增加气凝胶的柔韧性,并利用了海藻酸钠本质阻燃性能以及硼系阻燃剂协同阻燃,在航空航天、建筑、异形件阻燃和隔热等领域具有广泛的应用前景。(A flexible flame-retardant aerogel, a preparation method and application thereof belong to the technical field of aerogels. The flexible flame-retardant aerogel is prepared from the following raw materials: the nano-crystalline cellulose comprises nano-cellulose, sodium alginate, organosilane, boric acid, sodium tetraborate decahydrate, calcium carbonate and D- (+) -gluconic acid delta-lactone, wherein the nano-cellulose comprises cellulose nano-wires and cellulose nano-microfibers rich in carboxyl on the surface. The invention utilizes the morphological structure characteristics of the high length-diameter ratio of the cellulose nanowires to improve the degree of association between single nanocelluloses so as to increase the flexibility of the aerogel, utilizes the intrinsic flame retardant property of sodium alginate and the synergistic flame retardant property of boron flame retardants, and has wide application prospects in the fields of aerospace, buildings, flame retardance of special-shaped parts, heat insulation and the like.)

1. The flexible flame-retardant aerogel is characterized by comprising nanocellulose and sodium alginate, wherein the nanocellulose comprises cellulose nanometer microfiber and cellulose nanowire, the nanocellulose and the sodium alginate form a composite matrix, and the composite matrix is provided with primary air holes with the pore diameter of tens to hundreds of micrometers;

in the composite matrix, nano-cellulose and sodium alginate are modified by silanol, boric acid and calcium ion crosslinking to form a continuous phase consisting of sodium alginate and cellulose nano-microfiber and a dispersed phase consisting of cellulose nano-wires; dispersed pores are formed between the continuous phase and the dispersed phase and between the dispersed phase and the dispersed phase, and the pore diameter is from several nanometers to tens of nanometers; the dispersion holes are secondary air holes, and the primary air holes are formed by communicating part of the secondary air holes.

2. The flexible flame-retardant aerogel according to claim 1, which comprises the following raw materials in parts by weight:

10 parts of nano-cellulose suspension liquid is prepared from nano-cellulose, and the mass fractions of cellulose nano-microfiber and cellulose nano-wire in the nano-cellulose suspension liquid are respectively 0.3% -1%; preferably, the weight ratio of the cellulose nano microfiber to the cellulose nano wire is 1: 2-5: 1;

preparing 5-20 parts of sodium alginate solution, wherein the mass fraction of sodium alginate in the sodium alginate solution is 0.1-1%; the weight ratio of the sodium alginate solution to the nano cellulose suspension is 1: 2-2: 1;

3.75-60 parts of organosilane, wherein the weight ratio of the organosilane to the total weight of the nano-cellulose and the sodium alginate is 1: 4-2: 1;

1.5-15 parts of a boron flame retardant, wherein the boron flame retardant comprises boric acid and sodium tetraborate decahydrate, and the weight ratio of the boron flame retardant to the total weight of the nano-cellulose and the sodium alginate is 1: 10-1: 2; preferably, the weight ratio of the boric acid to the sodium tetraborate decahydrate is 1: 2-2: 1;

3.75-15 parts of calcium carbonate, wherein the weight ratio of the calcium carbonate to the total weight of the nano-cellulose and the sodium alginate is 1: 4-1: 2;

1.875-15 parts of D- (+) -gluconic acid delta-lactone; preferably, the weight ratio of the D- (+) -gluconic acid delta-lactone to the calcium carbonate is 1: 2-1: 1.

3. The flexible flame-retardant aerogel according to claim 2, wherein the cellulose nano-microfibers and the cellulose nanowires are extracted from cellulose-containing biomass materials, wherein the cellulose nano-microfibers are obtained by purifying the cellulose-containing biomass materials and then combining a catalytic oxidation method with a mechanical method, the cellulose nano-microfibers have a diameter of 2-30 nm, a length of 300 nm-10 μm, and are rich in carboxyl groups on the surface; the cellulose nanowires are obtained by purifying cellulose-containing biomass materials and then performing a mechanical method, and the cellulose nanowires have the diameter of 5-80 nm and the length of 20 mu m-2 mm.

4. The flexible flame retardant aerogel according to claim 2, wherein the organosilane is at least one of methyltrimethoxysilane and gamma-methacryloxypropyltrimethoxysilane.

5. A method for preparing flexible flame-retardant aerogel, which is based on any one of claims 1 to 4, and comprises the following steps:

s1, preparing a nano cellulose suspension and a sodium alginate solution respectively, and then blending and stirring to obtain a biopolymer mixed solution;

s2, adding organosilane into the biopolymer mixed solution prepared in the step S1, adding an acidic pH regulator to regulate the pH to 4-5, and stirring to obtain a cross-linking solution;

s3, adding boric acid and sodium tetraborate decahydrate into the crosslinking liquid prepared in the step S2, adding an inorganic alkali solution to adjust the pH value to 10-11, and stirring to obtain a mixed liquid;

s4, adding an acidic pH regulator into the mixed solution prepared in the step S3, regulating the pH to 7-8, adding calcium carbonate, stirring at a high speed until the calcium carbonate is uniformly dispersed, adding D- (+) -gluconic acid delta-lactone, continuously stirring, pouring into a mold, and completing gelation after 2-8 hours to obtain the composite hydrogel;

s5, placing the composite hydrogel prepared in the step S4 in an environment with the temperature of 4-12 ℃ for precooling for 10-30 min, then freezing for 10-50 min by using liquid nitrogen, then carrying out freeze drying in a vacuum freeze dryer for 24-72 h, taking out, and standing for 6-24 h at the room temperature with the temperature of 20-30 ℃ and the relative humidity of 40-80% to obtain the aerogel.

6. The method for preparing a flexible flame-retardant aerogel according to claim 5, wherein the acidic pH regulator in steps S2 and S4 comprises hydrochloric acid and/or acetic acid solution, and the molar concentration of the solution is 0.3-0.5 mol/L.

7. The method for preparing the flexible flame-retardant aerogel according to claim 5, wherein the alkaline pH regulator in step S3 is sodium hydroxide solution, and the molar concentration of the solution is 0.3-0.5 mol/L.

8. The preparation method of the flexible flame-retardant aerogel according to claim 5, wherein the stirring speed in the step S3 is 150-400 r/min.

9. Use of the flexible flame retardant aerogel according to claim 1 in flame retardant materials, thermal insulation materials, flame retardant and thermal insulation materials.

Technical Field

The invention relates to the technology in the field of aerogel, in particular to flexible flame-retardant aerogel, and a preparation method and application thereof.

Background

Aerogel is an ultra-light porous material with a porosity of more than 90%. The aerogel with cellulose as a skeleton structure is a third-generation aerogel following inorganic aerogel and organic aerogel. Cellulose aerogels are typically obtained from nanocellulose precursors by supercritical or freeze drying. The nano-cellulose can be divided into cellulose nano-whisker, cellulose nano-microfiber and cellulose nano-wire, and the length-diameter ratios of the three are increased in sequence. The cellulose nano whisker has low length-diameter ratio and high rigidity, the prepared gel has high brittleness and is fragile, and the cellulose nano wire with higher length-diameter ratio can form an entanglement structure in the aerogel, so that the bending flexibility of the aerogel is improved. The aerogel with extremely high porosity has extremely low heat conductivity coefficient and can be used as heat-insulating materials of buildings, pipelines, spacecrafts and the like. The cellulose aerogel has the characteristics of ultralight weight, high porosity, low thermal conductivity and the like, and also has the advantages of wide raw material source, good biocompatibility, environmental protection, sustainability and the like, and the aerogel with good flexibility can be applied to the wider fields of heat insulation of special-shaped workpieces and the like. However, the inherent flammability of cellulose makes its use limited and aerogels prepared from pure cellulose do not meet the use standards of the product. Although the traditional phosphorus flame retardant and the traditional halogen flame retardant can effectively improve the flame retardant property of the material, harmful gases released after combustion still have potential safety hazards.

The present invention has been made to solve the above-mentioned problems occurring in the prior art.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides the flexible flame-retardant aerogel, the preparation method and the application thereof, the cellulose nano microfiber, the cellulose nano wire and the sodium alginate are used as main raw materials, and the additive is added for crosslinking modification to obtain the aerogel which is eco-friendly, has excellent flame retardant property, bending flexibility, good heat insulation property and hydrophobic property, and has wide application prospect in the fields of aerospace, building, flame retardance of special-shaped workpieces, heat insulation and the like.

The invention relates to a flexible flame-retardant aerogel, which comprises nanocellulose and sodium alginate, wherein the nanocellulose comprises cellulose nanometer microfiber and cellulose nanometer line, the nanocellulose and the sodium alginate form a composite matrix, and the composite matrix is provided with a primary air hole with the aperture of tens to hundreds of micrometers;

in the composite matrix, nano-cellulose and sodium alginate are modified by silanol, boric acid and calcium ion crosslinking to form a continuous phase consisting of sodium alginate and cellulose nano-microfiber and a dispersed phase consisting of cellulose nano-wires; dispersed pores are formed between the continuous phase and the dispersed phase and between the dispersed phase and the dispersed phase, and the pore diameter is from several nanometers to tens of nanometers; the dispersion holes are secondary air holes, and the primary air holes are formed by communicating a part of the secondary air holes.

The invention relates to a flexible flame-retardant aerogel which comprises the following raw materials in parts by weight:

10 parts of nano-cellulose suspension liquid is prepared from nano-cellulose, and the mass fractions of cellulose nano-microfiber and cellulose nano-wire in the nano-cellulose suspension liquid are respectively 0.3% -1%; preferably, the weight ratio of the cellulose nano microfiber to the cellulose nano wire is 1: 2-5: 1;

preparing 5-20 parts of sodium alginate solution, wherein the mass fraction of sodium alginate in the sodium alginate solution is 0.1-1%; the weight ratio of the sodium alginate solution to the nano cellulose suspension is 1: 2-2: 1;

3.75-60 parts of organosilane, wherein the weight ratio of the organosilane to the total weight of the nano-cellulose and the sodium alginate is 1: 4-2: 1;

1.5-15 parts of a boron flame retardant, wherein the boron flame retardant comprises boric acid and sodium tetraborate decahydrate, and the weight ratio of the boron flame retardant to the total weight of the nano-cellulose and the sodium alginate is 1: 10-1: 2; preferably, the weight ratio of the boric acid to the sodium tetraborate decahydrate is 1: 2-2: 1;

3.75-15 parts of calcium carbonate, wherein the weight ratio of the calcium carbonate to the total weight of the nano-cellulose and the sodium alginate is 1: 4-1: 2;

1.875-15 parts of D- (+) -gluconic acid delta-lactone; preferably, the weight ratio of the D- (+) -gluconic acid delta-lactone to the calcium carbonate is 1: 2-1: 1.

In some technical schemes, the cellulose nanometer microfiber and the cellulose nanowire are extracted from cellulose-containing biomass materials, wherein the cellulose nanometer microfiber is obtained by purifying the cellulose-containing biomass materials and then combining a catalytic oxidation and mechanical method, the cellulose nanometer microfiber has the diameter of 2-30 nm and the length of 300 nm-10 microns, and the surface of the cellulose nanometer microfiber is rich in carboxyl; the cellulose nanowires are obtained by purifying cellulose-containing biomass materials and then performing a mechanical method, and the cellulose nanowires have the diameter of 5-80 nm and the length of 20 mu m-2 mm.

In some embodiments, the organosilane is at least one of methyltrimethoxysilane, gamma-methacryloxypropyltrimethoxysilane.

The invention relates to a preparation method of a flexible flame-retardant aerogel, which comprises the following steps:

s1, preparing a nano cellulose suspension and a sodium alginate solution respectively, and then blending and stirring to obtain a biopolymer mixed solution;

s2, adding organosilane into the biopolymer mixed solution prepared in the step S1, adding an acidic pH regulator to regulate the pH to 4-5, and stirring to obtain a cross-linking solution;

s3, adding boric acid and sodium tetraborate decahydrate into the crosslinking liquid prepared in the step S2, adding an inorganic alkali solution to adjust the pH value to 10-11, and stirring to obtain a mixed liquid;

s4, adding an acidic pH regulator into the mixed solution prepared in the step S3, regulating the pH to 7-8, adding calcium carbonate, stirring at a high speed until the calcium carbonate is uniformly dispersed, adding D- (+) -gluconic acid delta-lactone, continuously stirring, pouring into a mold, and completing gelation after 2-8 hours to obtain the composite hydrogel;

s5, placing the composite hydrogel prepared in the step S4 in an environment with the temperature of 4-12 ℃ for precooling for 10-30 min, then freezing for 10-50 min by using liquid nitrogen, then carrying out freeze drying in a vacuum freeze dryer for 24-72 h, taking out, and standing for 6-24 h at the room temperature with the temperature of 20-30 ℃ and the relative humidity of 40-80% to obtain the aerogel.

Preferably, the acidic pH regulator in the steps S2 and S4 comprises hydrochloric acid and/or acetic acid solution, and the molar concentration of the solution is 0.3-0.5 mol/L.

Preferably, the alkaline pH regulator in step S3 is a sodium hydroxide solution, and the molar concentration of the solution is 0.3-0.5 mol/L.

Preferably, the stirring speed in the step S3 is 150-400 r/min.

The invention relates to application of the flexible flame-retardant aerogel in flame-retardant materials, heat-insulating materials and flame-retardant heat-insulating materials.

Technical effects

Compared with the prior art, the invention has the following technical effects:

1) the prepared aerogel gum base has excellent bending flexibility, ultralow density and good heat insulation effect on a biopolymer, and can effectively resist flame;

2) in the system, the cellulose nanowires with high length-diameter ratio are used for increasing the degree of correlation among single nanocelluloses, so that the aerogel with good flexibility is obtained;

3) the carboxyl on the carboxylated cellulose nano microfiber is utilized to enhance the compatibility with sodium alginate, and a more stable composite structure is formed when calcium ions are crosslinked;

4) the sodium alginate based on intrinsic flame retardance and boron flame retardants with different flame retardance mechanisms realize the synergistic flame retardance effect on the cellulose aerogel, so that the aerogel is ensured to have excellent flame retardance;

5) after the organosilane is hydrolyzed into silanol, the silanol and hydroxyl on a cellulose molecular chain generate a crosslinking reaction, the mechanical strength of the aerogel is enhanced, and the hydrophobic group on the organosilane is used for carrying out hydrophobic modification on the aerogel.

Drawings

FIG. 1 is a scanning electron micrograph (resolution: 1mm) of the flexible flame-retardant aerogel prepared in example 2;

FIG. 2 is a high-power scanning electron microscope (resolution: 1 μm) of the flexible flame-retardant aerogel prepared in example 2;

FIG. 3 is a diagram of a flexible flame-retardant aerogel prepared in example 2 in a bent state;

FIG. 4 is a graph of the cone-shaped heat release rate of combustion of the flexible flame retardant aerogel prepared in example 1;

FIG. 5 is a graph of total heat release from conical combustion for the flexible flame retardant aerogel prepared in example 1;

FIG. 6 is a graph of the cone-shaped heat release rate of combustion of the flexible flame retardant aerogel prepared in example 2;

FIG. 7 is a graph of total heat release from conical combustion for a flexible flame retardant aerogel prepared in example 2;

FIG. 8 is a water contact angle graph of a flexible flame retardant aerogel prepared in example 1;

fig. 9 is a water contact angle graph of the flexible flame retardant aerogel prepared in example 2.

Detailed Description

The invention is described in detail below with reference to the drawings and the detailed description. The experimental procedures, in which specific conditions are not specified in the examples, were carried out according to the conventional methods and conditions.

The rice straws used in the examples were purchased from a farm in Jiangsu; sodium hydroxide, sodium chlorite, sodium bromide, sodium hypochlorite and hydrochloric acid are purchased from chemical reagents limited of national drug group; toluene, acetic acid, potassium hydroxide, commercially available from Shanghai Linkuang Chemicals Co., Ltd; ethanol is purchased from Hongyi fine chemical industry Co., Ltd; dimethyl sulfoxide and 2,2,6, 6-tetramethylpiperidine oxide, sodium alginate, methyltrimethoxysilane, boric acid, sodium tetraborate decahydrate, calcium carbonate and D- (+) -glucono delta-lactone were purchased from alatin reagent (shanghai) ltd.

The digital display constant temperature water bath box used in the embodiment is manufactured by Shanghai imperial science and instruments, Inc., and has the model number of DK-S28; the multifunctional grinder is produced by Jiangxi Ganyun food machinery Co Ltd, and has the model of GY-FS-06; the electric temperature-regulating electric jacket is produced by Zhejiang Li Fang instruments science and technology Limited and has the model number of DZTW; the magnetic stirrer is produced by Shanghai plum Yingpu instrument manufacturing company Limited, and has a model number of 84-1; the pH meter is produced by Shanghai Jing Kelei magnetic instrument factory, and the model is PHS-3C; the homogeneous disperser is produced by IKA laboratory technology Co., Ltd, and has a model of T25 Ultra-turrax; the centrifuge is produced by Hunan instrument laboratory Instrument development Co., Ltd, and has the model of H650; the ultrasonic cell crusher is produced by Ningbo Xinzhi Biotechnology GmbH, and has a model of JY 92-IIN; the vacuum freeze drying machine is produced by Ningbo Xinzhi Biotechnology corporation, and has a model of SCIENTZ-18N; the refrigerator is manufactured by Qingdao Haier Co., Ltd, and has the model of BCD-160 TMPO.

Firstly, the embodiment of the invention extracts the rice straw purified cellulose powder from the rice straw, and the specific process is as follows:

preparing a sodium hydroxide solution with the mass fraction of 5% in a beaker, putting the beaker into a water bath box with the temperature of 90 ℃, immersing the rice straws in the sodium hydroxide solution, heating for 2 hours, washing and drying; crushing the dried rice straw by using a multifunctional crusher and sieving the crushed rice straw by using a 80-mesh sieve to obtain pretreated rice straw powder;

extracting the powder with toluene/ethanol mixture at a volume ratio of 2:1 in a Soxhlet extractor at a bath ratio of 1:20, and reacting in an electric jacket at 90 deg.C for 6 h; preparing a sodium chlorite solution with the mass fraction of 2%, adjusting the pH value to 4 by using acetic acid, washing and drying the extracted powder, immersing the powder into the solution according to the bath ratio of 1:50, and placing the powder into a water bath box with the temperature of 70 ℃ for reaction for 5 hours; preparing a potassium hydroxide solution with the mass fraction of 2%, washing and drying the powder treated by the sodium chlorite, immersing the powder into the solution according to the bath ratio of 1:50, putting the solution into a water bath tank with the temperature of 90 ℃ for reaction for 2 hours, and washing and drying the solution; the sodium chlorite and potassium hydroxide treatment process is repeated for 3 times in sequence, wherein the mass fraction of the solution in the last potassium hydroxide treatment is 5 percent;

finally obtaining the rice straw purified cellulose powder.

Then, preparing cellulose nano microfiber based on rice straw purified cellulose powder, which comprises the following specific steps:

immersing purified cellulose powder into dimethyl sulfoxide at a bath ratio of 1:30, heating to 60 ℃ in a water bath box, treating for 5h, washing and drying;

dispersing the dried cellulose powder into deionized water at a bath ratio of 1:100, respectively adding sodium bromide and 2,2,6, 6-tetramethylpiperidine oxide at a mass ratio of 1:0.636 and 1:0.032 to the cellulose powder under magnetic stirring at a rotation speed of 800r/min, and uniformly stirring;

dropwise adding a mixture of the cellulose powder and the cellulose powder in a mass-volume ratio of 1 g: 40mL of sodium hypochlorite solution and adjusting the pH value to 11 by using 0.5mol/L hydrochloric acid; when the pH value of the system is automatically reduced to 10.5, the reaction is continued for 4 hours and is carried out in a cold water bath at the temperature of 10 ℃; adding the mixture into the cellulose powder in a mass-volume ratio of 1 g: stopping the reaction by 10mL of ethanol; centrifuging and washing the reacted mixed solution in a centrifuge with the rotating speed of 5000r/min for 10min for 6 times;

dispersing again, and standing for 24h at room temperature in deionized water; layering appears after standing, and the lower layer sediment is treated for 5min by an ultrasonic cell crusher with the power of 450W to obtain the cellulose nano microfiber.

And preparing cellulose nanowires based on the rice straw purified cellulose powder, wherein the specific process is as follows:

a preparation method of cellulose nanowires comprises the following specific steps:

dispersing purified cellulose powder in deionized water to obtain a dispersion liquid with the mass fraction of 0.5%; treating the dispersion with an ultrasonic cell crusher with power of 800W for 30 min; and dispersing the dispersion liquid after ultrasonic treatment for 10min by using a homogeneous dispersion machine with the rotating speed of 10000r/min to obtain the cellulose nanowire.

Example 1

A preparation method of flexible flame-retardant aerogel comprises the following specific steps:

s1, preparing the cellulose nano microfiber and the cellulose nano wire by deionized water respectively to obtain suspension with the mass fraction of 0.5%, mixing the two suspensions according to the mass ratio of 3:1, and weighing 150g of mixed suspension; preparing a sodium alginate solution with the mass fraction of 0.25%, weighing 150g of the sodium alginate solution, mixing the mixed suspension and the sodium alginate solution, putting the mixture into a 500ml beaker, and magnetically stirring the mixture at the rotation speed of 1000r/min at the temperature of 25 ℃ to obtain a biopolymer mixed solution;

s2, weighing 0.5625g of methyltrimethoxysilane, dropwise adding the methyltrimethoxysilane into the biopolymer mixed solution obtained in the step S1, and adjusting the pH to 4 by using 0.5mol/L acetic acid; magnetically stirring for 2h at 25 ℃ at the rotating speed of 1000r/min for crosslinking to obtain crosslinking liquid;

s3, weighing 0.1125g of boric acid and 0.1125g of sodium tetraborate decahydrate, adding into the crosslinking liquid obtained in the step S2, and adjusting the pH to 10 by using 0.5mol/L of sodium hydroxide; magnetically stirring at 25 deg.C at rotation speed of 200r/min for 24 hr to obtain mixed solution.

S4, adjusting the pH value of the mixed solution obtained in the step S3 to 7 by using 0.5mol/L acetic acid, adding 0.5625g of calcium carbonate, stirring for 5min by using a homogeneous dispersion machine at the rotating speed of 6000r/min, then continuously stirring by using a magnetic stirrer at the rotating speed of 1000r/min, adding 0.5625g of D- (+) -gluconic acid delta-lactone, pouring into a polypropylene mould when the pH value is reduced to 7.5, and gelling for 6h at 25 ℃ to obtain composite hydrogel;

s5, pre-cooling the composite hydrogel obtained in the step S4 in a refrigerator at 4 ℃ for 20min, freezing the hydrogel for 30min by using liquid nitrogen, and freeze-drying the hydrogel in a vacuum freeze dryer for 72 h; taking out and standing for 12h at the room temperature of 25 ℃ and the relative humidity of 60% to obtain the flexible flame-retardant aerogel.

Tests show that the limit oxygen index of the prepared flexible flame-retardant aerogel is 34.3%, and the maximum heat release rate of conical combustion is 17.3kW/m²(as shown in FIG. 4), the total heat release was 1.0MJ/m²(as shown in FIG. 5), the density was 12.4kg/m³And a thermal conductivity at 25 ℃ of 0.028Wm^-1K^-1The water contact angle was 131 ° (as shown in fig. 8).

Example 2

A preparation method of flexible flame-retardant aerogel comprises the following specific steps:

s1, preparing the cellulose nano microfiber and the cellulose nano wire by deionized water respectively to obtain suspension with the mass fraction of 0.5%, mixing the two suspensions according to the mass ratio of 3:1, and weighing 150g of mixed suspension; preparing a sodium alginate solution with the mass fraction of 0.5%, weighing 150g of the sodium alginate solution, mixing the mixed suspension and the sodium alginate solution, placing the mixture in a 500ml beaker, and magnetically stirring the mixture at the rotation speed of 1000r/min at the temperature of 25 ℃ to obtain a biopolymer mixed solution;

s2, weighing 0.75g of methyltrimethoxysilane, dropwise adding the methyltrimethoxysilane into the biopolymer mixed solution obtained in the step S1, and adjusting the pH to 4 by using 0.5mol/L acetic acid; magnetically stirring for 2h at 25 ℃ at the rotating speed of 1000r/min for crosslinking to obtain crosslinking liquid;

s3, weighing 0.15g of boric acid and 0.15g of sodium tetraborate decahydrate, adding the boric acid and the sodium tetraborate decahydrate into the crosslinking liquid obtained in the step S2, and adjusting the pH to 10 by using 0.5mol/L of sodium hydroxide; magnetically stirring at 25 deg.C at rotation speed of 200r/min for 24 hr to obtain mixed solution.

S4, adjusting the pH of the mixed solution obtained in the step S3 to 7 by using 0.5mol/L acetic acid, adding 0.75g of calcium carbonate, stirring for 5min by using a homogeneous dispersion machine at the rotating speed of 6000r/min, then continuously stirring by using a magnetic stirrer at the rotating speed of 1000r/min, adding 0.75g of D- (+) -gluconic acid delta-lactone, pouring into a polypropylene mould when the pH is reduced to 7.5, and gelling for 6h at 25 ℃ to obtain the composite hydrogel.

And S5, pre-cooling the composite hydrogel obtained in the step (4) in a refrigerator at 4 ℃ for 20min, freezing the composite hydrogel with liquid nitrogen for 30min, freeze-drying the composite hydrogel in a vacuum freeze dryer for 72h, taking out the composite hydrogel, and standing the composite hydrogel at room temperature of 25 ℃ and relative humidity of 60% for 12h to obtain the flexible flame-retardant aerogel shown in the figure 3, wherein a scanning electron microscope is shown in the figures 1 and 2.

As shown in FIG. 3, the prepared flexible flame-retardant aerogel can still keep integrity under the condition of being folded and bent in time, and has excellent flexibility.

Tests show that the limit oxygen index of the prepared flexible flame-retardant aerogel is 38.5%, and the maximum heat release rate of conical combustion is 31.1kW/m²(as shown in FIG. 6), the total heat release of the conical combustion is 1.9MJ/m²(as shown in FIG. 7), the density was 16.6kg/m³Thermal conductivity at 25 ℃ of 0.030Wm^-1K-¹The water contact angle was 127 ° (as shown in fig. 9).

It is to be emphasized that: the above embodiments are only preferred embodiments of the present invention, and are not intended to limit the present invention in any way, and all simple modifications, equivalent changes and modifications made to the above embodiments according to the technical spirit of the present invention are within the scope of the technical solution of the present invention.