An anisotropic heat-insulating and flame-retardant guar gum aerogel and its preparation method

Abstract

The present invention discloses a heat-insulating and flame-retardant guar gum aerogel and a preparation method thereof, comprising: adding silicate and guar gum into water, stirring evenly, quantitatively dropping phytic acid to regulate the pH of the system, and stirring to obtain a guar gum composite solution; after the guar gum composite solution is subjected to vacuum degassing treatment, a guar gum precursor solution is obtained; after the guar gum precursor solution is subjected to solidification and shaping treatment, a guar gum pre-gel is obtained, and freeze-drying is carried out to obtain a heat-insulating and flame-retardant guar gum aerogel material with a three-dimensional network structure. The present invention utilizes the hydrogen bond interaction between the multi-hydroxyl structure of the flame-retardant additive phytic acid and guar gum and adjusts the pH of the system to decompose the embedded silicate. After freeze-drying to remove ice crystals, a three-dimensional network structure aerogel with an oriented lamellar structure is obtained, solving the problems that conventional cold-dry method aerogels are prone to collapse and have low compressive strength, etc., and obtaining an aerogel material with excellent mechanical properties, good heat insulation and flame retardancy. The product is expected to be applied in the fields of new lightweight heat-insulating and flame-retardant building materials, etc.

Description

Technical Field

[0001] The present invention relates to the field of preparation of biomass aerogels, and relates to an anisotropic heat-insulating and flame-retardant guar gum aerogel of natural biomass and a preparation method thereof. Technical Background

[0002] The consumption of traditional energy sources (including coal, oil, natural gas, etc.) has led to excessive global carbon dioxide emissions, and extreme harsh environments have gradually intensified. Developing renewable energy sources and improving energy efficiency have gradually become hot issues, and heat insulation is the key to improving energy conservation and reducing carbon emissions. Common heat-insulating materials such as mineral wool, polyurethane (PUR), and polystyrene (EPS) have low thermal conductivity but are prone to fire hazards. Therefore, it is of great significance to explore a heat-insulating and flame-retardant material that reduces energy consumption and is environmentally friendly.

[0003] For bio-based aerogels, they have the characteristics of low density, large porosity, and low thermal conductivity, and can be degraded by themselves without subsequent degradation and recycling problems. Based on this characteristic, a large number of reports on polysaccharide aerogels have emerged in recent years, including alginate-based aerogels, chitosan-based aerogels, pectin-based aerogels, etc. However, the internal structural design of most aerogel samples is relatively single and cannot meet the multifunctional applications of the materials in complex environments, and cannot meet their applications in fields such as architecture, energy chemical engineering, and aerospace.

[0004] Guar gum (GG) is a natural non-ionic linear polysaccharide and one of the most extensive sources of galactomannan. Guar gum has a high molecular weight and good water solubility, and the degree of entanglement between molecular chains is high at low concentrations, so the phenomenon of "low concentration and high viscosity" appears, that is, the number of molecular chains per unit volume is small, which is more conducive to realizing the lightweight of aerogels. Unfortunately, guar gum itself does not have flame-retardant properties. In order to avoid the occurrence of fire hazards and reduce the possibility of secondary pollution and secondary injuries, exploring a recyclable natural bio-based functional additive that can act as a flame retardant and a mechanical enhancer while stably binding to the guar gum matrix has become the core issue. Summary of the Invention

[0005] In view of the problems existing in the prior art, the object of the present invention is to provide a flame-retardant and heat-insulating guar gum aerogel and a preparation method thereof. Taking guar gum as a base material, other components such as silicate and phytic acid are selectively used. The multi-hydroxyl structure of the flame-retardant additive phytic acid forms hydrogen bonds with guar gum, and the pH of the system is adjusted to decompose the embedded silicate. After freeze-drying to remove ice crystals, a three-dimensional network structure aerogel with a directional lamellar structure is obtained, which solves the problems such as easy collapse and low compressive strength of conventional cold-drying method aerogels, and obtains an aerogel material with excellent mechanical properties, good heat insulation and flame retardancy. The product is expected to be applied in the fields of new lightweight heat-insulating and flame-retardant building materials, etc.

[0006] The technical solution of the present invention is as follows:

[0007] An anisotropic heat-insulating and flame-retardant guar gum aerogel and a preparation method thereof, comprising the following steps:

[0008] (1) Add silicate and guar gum into water, stir evenly, quantitatively drop phytic acid to regulate the pH of the system, and stir to obtain a guar gum composite solution;

[0009] (2) After the guar gum composite solution is subjected to vacuum degassing treatment, a guar gum precursor solution is obtained;

[0010] (3) After the guar gum precursor solution is subjected to solidification and shaping treatment, a guar gum pre-gel is obtained, and freeze-drying is carried out to obtain a guar gum aerogel.

[0011] In the above technical solution, the silicate in step (1) is sodium silicate, preferably one of powdered instant sodium silicate and sodium metasilicate nonahydrate, and preferably powdered instant sodium silicate. Powdered instant sodium silicate can be quickly dissolved in the system and can quickly obtain silicon dioxide in the system.

[0012] In the above technical solution, in step (1), the silicate accounts for 6-9% of the total mass of guar gum and phytic acid, preferably 6%.

[0013] In the above technical solution, in step (1), the mass ratio of guar gum to phytic acid in the spinning dope is: 89-67:11-33, preferably 80:20.

[0014] In the above technical solution, in step (1), the pH of the guar gum composite solution adjusted by phytic acid is between 2 and 6.

[0015] In the above technical solution, in step (1), the total mass of the guar gum, phytic acid solution and silicate in the guar gum composite solution is 2.5-3.2%, preferably 3.2%.

[0016] In the above technical solution, in step (2), the conditions for the vacuum degassing treatment are as follows: the temperature is 25 to 40 °C, preferably 25 °C, the system residual pressure is 500 Pa to 600 Pa, preferably 500 Pa, and the time is 20 h to 40 h, preferably 10 h.

[0017] In the above technical solution, in step (3), the conditions for the curing and shaping treatment are as follows: the curing and forming time is 30 to 60 min, preferably 60 min, and the shaping temperature is -40 to -80 °C, preferably -80 °C.

[0018] In the above technical solution, in step (4), the freeze-drying conditions are as follows: the temperature is -35 °C to -40 °C, preferably -40 °C, the freeze-drying time is 24 to 72 h, preferably 72 h, and the residual pressure is less than 2000 Pa.

[0019] The present invention uses guar gum as a substrate and selectively chooses other components such as silicate and phytic acid to construct a three-dimensional network structure aerogel with a directional lamellar structure, which exhibits anisotropic mechanical and thermal insulation properties in the radial and longitudinal directions, has good flame retardancy, and helps to develop new lightweight thermal insulation and flame retardant materials. Phytic acid, as a biomass-derived organic flame retardant, has good physiological and chemical properties, lower toxicity than table salt, and a high content of flame retardant element phosphorus and a highly symmetric molecular structure (negatively charged phosphate groups and hydroxyl groups that can react with groups such as carboxyl and amino groups), and also has great potential for improving the mechanical properties of materials. Silicate has natural flame retardancy and heat resistance. As a precursor, it can be transformed into a flame retardant material - silica with excellent heat resistance and high reactivity under acidic conditions. The intrinsic acidity of phytic acid can in-situ catalyze the decomposition of silicate to generate silica. The multiple hydrogen bond interactions between silica, phytic acid, and guar gum not only improve the mechanical properties and flame retardancy of the aerogel but also effectively avoid the shrinkage and collapse of the guar gum pre-gel during the freeze-drying process. The differentiation of the multiple hydrogen bond interactions realizes the anisotropy of the aerogel.

[0020] In the preparation method of the anisotropic heat-insulating and flame-retardant guar gum aerogel of the present invention, the components and the mechanism of action between the components are as follows: Guar gum, as a natural linear polysaccharide, contains a large number of hydroxyl groups. Its high molecular weight and hydrophilic properties enable it to form a high-viscosity solution at low concentrations. After freeze-drying, a material with a large volume and small mass can be obtained, providing conditions for the preparation of lightweight biomass aerogels; after the silicate is dissolved in water, it is alkaline. The presence of an appropriate amount of alkali can catalyze the conversion of the active primary hydroxyl groups on the guar gum branches into sodium alkoxides, and the silicate anions obtain hydrogen ions and can form hydrogen bond interactions with the hydroxyl groups on the main chain of guar gum, loading on the surface of guar gum; phytic acid, as an additive containing the flame-retardant element phosphorus, the main functions in the system are as follows: (1) Since the silicates we selected all show alkalinity after being dissolved in water, adding phytic acid can play a role in regulating the pH of the system. The sodium alkoxides on the guar gum branches are re-converted into primary hydroxyl groups, and at the same time, it catalyzes the decomposition of the silicate into secondary silica particles. The secondary particles have a spherical polyhydroxy structure and can form hydrogen bond interactions with guar gum and phytic acid, effectively improving the mechanical properties and flame-retardant properties of the aerogel and endowing the system with good heat-insulating properties; (2) containing a large amount of phosphorus elements, it forms an expanded carbon layer on the surface at the early stage of the combustion of the aerogel sample when heated, while preventing heat from entering the interior, endowing the aerogel with good flame-retardant properties; (3) The unique cyclic polyhydroxy structure of phytic acid, when undergoing a cross-linking reaction with the guar gum chain, because the primary hydroxyl groups on the branches are more active, phytic acid first undergoes a cross-linking reaction with the primary hydroxyl groups on the branches. Phytic acid, as a bridge between the main chains of guar gum, will induce the rearrangement of the macromolecular chains, making the chains more stable. Due to this microscopic adjustment, the viscosity of the aerogel precursor increases. During the freeze-drying process, the complex network formed by the hydrogen bond interactions between "silicate-guar gum, guar gum-phytic acid, phytic acid-silicate" synergistically hinders the growth of ice crystals. However, due to the difference in multiple hydrogen bonds, a lamellar structure is formed in situ, and a whisker-like secondary structure is formed between the lamellae, constructing a three-dimensional network structure of a directional lamellar structure guar gum aerogel, that is, the guar gum is arranged in a lamellar structure longitudinally, while in the radial direction, the hydrogen bond interaction between phytic acid and guar gum is macroscopically distributed in the form of microfibrils between the guar gum lamellae.

[0021] Advantages of the present invention:

[0022] The present invention uses guar gum as a raw material and prepares a three-dimensional network-structured guar gum aerogel with a directional lamellar structure through freeze-drying, solving the problems such as the easy collapse of conventional cold-drying method aerogels and low compressive strength, providing a feasible direction for the preparation of lightweight flame-retardant and heat-insulating materials. The present invention uses natural polysaccharide (guar gum) as a substrate and a bio-based material (phytic acid) as a flame-retardant additive. The multi-hydroxyl structure enables it to form hydrogen bonds with guar gum. At the same time, by regulating the pH of the system, the embedded silicate is decomposed, and finally a "silicate-guar gum, guar gum-phytic acid, phytic acid-silicate" multiple network structure is constructed, which synergistically hinders the growth of ice crystals during the freeze-drying process, and due to the difference in multiple hydrogen bond interactions, a three-dimensional network-structured guar gum aerogel with a directional lamellar structure is finally constructed.

[0023] The preparation method of the present invention is simple, green and environmentally friendly. The prepared guar gum aerogel has lightweight, anisotropic mechanical properties, good flame-retardant properties and heat-insulating properties. It has good application prospects in the fields of green buildings, energy chemical engineering and aerospace. Brief Description of the Drawings

[0024] Figure 1 Shows the preparation mechanism of the guar gum aerogel prepared in Examples 1-3.

[0025] Figure 2 Shows the infrared spectra of guar gum and the guar gum aerogel prepared in Examples 1-3.

[0026] Figure 3 Shows the physical pictures of the guar gum aerogel prepared in Examples 1-3.

[0027] Figure 4 Shows the thermal imaging pictures of the guar gum aerogel prepared in Examples 1-3.

[0028] Figure 5 Shows the flame-retardant performance pictures of the guar gum aerogel prepared in Examples 1-3.

[0029] Figure 6 Shows the SEM pictures of the combustion residues of the guar gum aerogel prepared in Examples 1-3.

[0030] Figure 7 Shows the SEM pictures of the guar gum aerogel prepared in Examples 1-3.

[0031] Figure 8 Shows the TGA pictures of the guar gum aerogel prepared in Examples 1-3.

[0032] Figure 9 Shows the DSC pictures of the guar gum aerogel prepared in Examples 1-3.

[0033] Figure 10 shows the mechanical property (radial) diagram of the guar gum aerogel prepared in Examples 1-3.

[0034] Figure 11 shows the mechanical property (longitudinal) diagram of the guar gum aerogel prepared in Examples 1-3.

[0035] Figure 12 Shows the stress-strain diagram of the guar gum aerogel when the deformation is 90% in Examples 1-3. Detailed implementation manners

[0036] In order to make the purpose, scheme, process and advantages of the present invention clearer, the process of this invention will be further described in detail with reference to the accompanying drawings and examples. It should be noted that the specific examples here are only for explaining the present invention and not for limiting the present invention. In the following examples, unless otherwise specified, the experimental methods used are all conventional methods, and the experimental equipment, materials, reagents, etc. used can all be purchased from chemical companies. In the following examples, unless otherwise specified, the experimental methods used are all conventional methods, and the materials, reagents, etc. used can all be purchased from biological or chemical companies.

[0037] The following are the raw materials used in the examples:

[0038] Guar gum: Purchased from Tianjin Guangfu Fine Chemical Research Institute, product standard number: Jin Q / HG NK204-2000;

[0039] Phytic acid solution: Purchased from Shanghai Macklin Biochemical Co., Ltd., product standard number: P816021; the phytic acid concentration in the phytic acid solution is 50 wt%.

[0040] Example 1

[0041] (1) Sodium silicate and guar gum powder were successively added to deionized water to prepare a mixed solution. After mixing evenly, the phytic acid solution was added dropwise and stirred for 8 h to obtain a guar gum composite solution. In this guar gum composite solution, the mass ratio of guar gum to phytic acid is 89:11, sodium silicate accounts for 9% of the total mass of guar gum and phytic acid, the mass fraction of the total solute mass is 2.5%, and the pH value of the solution is about 6.

[0042] (2) The guar gum composite solution in step (1) was degassed in a vacuum drying phase, where the temperature was 40 °C, the system residual pressure was 0.6 kPa, and the vacuum degassing time was 40 h to obtain a precursor solution.

[0043] (3) The precursor solution in step (2) was poured into a mold and transferred to a refrigerator for low-temperature curing at a temperature of -40 °C for 30-60 minutes to become a pre-gel.

[0044] (4) Transfer the solidified product (i.e., guar gum monomer) obtained in step (3) to a freeze dryer for freeze drying, where the freeze drying temperature is -35 °C, the freeze drying time is 24 hours, and the residual pressure is less than 2000 Pa, finally obtaining the finished product of guar gum aerogel.

[0045] Example 2

[0046] (1) Add sodium metasilicate and guar gum powder to deionized water in sequence to prepare a mixed solution. After mixing evenly, add phytic acid solution dropwise and stir for 8 h to obtain a guar gum composite solution. In this guar gum composite solution, the mass ratio of guar gum to phytic acid is 80:20, sodium silicate accounts for 7% of the total amount of guar gum and phytic acid, the total mass of solutes in the mass fraction of solvent water is 2.7%, and the pH value of the solution is about 4.

[0047] (2) Debubble the guar gum composite solution in step (1) in a vacuum drying phase, where the temperature is 30 °C, the system residual pressure is 0.55 kPa, and the vacuum debubbling time is 30 h to obtain a precursor solution.

[0048] (3) Pour the precursor solution in step (2) into a mold and transfer it to a refrigerator for low-temperature curing at -60 °C, and place it for 30 - 60 minutes to become a pre-gel.

[0049] (4) Transfer the solidified product (i.e., guar gum monomer) obtained in step (3) to a freeze dryer for freeze drying, where the freeze drying temperature is -38 °C, the freeze drying time is 48 h, and the residual pressure is less than 2000 Pa, finally obtaining the finished product of guar gum aerogel.

[0050] Example 3

[0051] (1) Add powdered instant sodium silicate and guar gum powder to deionized water in sequence to prepare a mixed solution. After mixing evenly, add phytic acid solution dropwise and stir for 8 h to obtain a guar gum composite solution. In this guar gum composite solution, the mass ratio of guar gum to phytic acid is 67:33, sodium silicate accounts for 6% of the total amount of guar gum and phytic acid, the total mass of solutes in the mass fraction of solvent water is 3.2%, and the pH of the solution is 2.

[0052] (2) Debubble the guar gum composite solution in step (1) in a vacuum drying phase, where the temperature is 25 °C, the system residual pressure is 0.5 kPa, and the vacuum debubbling time is 20 h to obtain a precursor solution.

[0053] (3) Pour the precursor solution in step (2) into a mold and transfer it to a refrigerator for low-temperature curing at -80 °C, and place it for 30 - 60 minutes to become a pre-gel.

[0054] (4) Transfer the cured product (i.e., guar gum monomer) obtained in step (3) to a freeze dryer for freeze drying, where the temperature is -40 °C, the freeze drying time is 72 h, and the residual pressure is less than 2000 Pa, and finally obtain the finished product of guar gum aerogel.

[0055] Example 4

[0056] Molecular structure characteristics of guar gum aerogel: Use an infrared spectrometer to detect the chemical structure characteristics of guar gum aerogel, Figure 1 showing the preparation mechanism of guar gum aerogel, Figure 2 showing the infrared spectra of guar gum (GG) and the guar gum aerogels prepared in Examples 1-3 (respectively represented as Example 1 to Example 3 in the figure). Figure 2 The infrared spectra of the aerogels of Examples 1 to 3 and GG can be seen, where GG has obvious absorption peaks at 3430 and 2925 cm -1 , and these two absorption peaks are attributed to the O-H vibration and the stretching vibration of C-H respectively. Compared with pure GG, in Examples 1, 2, and 3, the broad peak at 3430 cm -1 shifts to a lower wavenumber of 3413 cm -1 , which may be caused by the peak shift phenomenon due to hydrogen bonding between molecules. There are obvious doublets at 1071 cm -1 and 1044 cm -1 corresponding to the characteristic peaks of Si-O-Si, and there is an obvious absorption peak at 875 cm -1 corresponding to the characteristic peak of Si-OH. The absorption peak at 518 cm -1 may be the characteristic peak of PO4 of phytic acid 3- . As can be seen from the part indicated by the dotted box (a) in Figure 2 , with the increase of the phytic acid ratio, the corresponding Si-OH at 875 cm -1 gradually disappears, indicating that the addition of phytic acid can promote the decomposition of silicate and finally convert it into silica secondary particles.

[0057] Figure 3 showing the physical pictures of the guar gum aerogels prepared in Examples 1 to 3. The finished product is a white solid. Since guar gum can form a high-viscosity solution at low concentrations, a large-volume and small-mass material can be obtained after freeze drying, thus obtaining a lightweight aerogel. Especially in Example 3, the density of the aerogel is as low as 0.02747 g / cm 3 , and placing it on a dandelion will not cause it to collapse (the size of the aerogel in the figure: 3 cm × 3 cm × 1 cm).

[0058] Example 5

[0059] The performance detection method and detection results of the guar gum aerogel prepared by the present invention are as follows:

[0060] 1. Thermal insulation performance: Take the prepared guar gum aerogel with the same mass, place it on a hot stage, and gradually heat it to 40 °C. Use an infrared thermal imager to observe the surface temperature of the guar gum aerogel; place the guar gum aerogel sample in an oven at 50 °C for 6 h, then cut the sample into a cube with a diameter of 3 cm × 3 cm × 1 cm. At room temperature, place the sample between the two sensor probes of a thermal conductivity analyzer and cover the sample with a heat insulation cover, and conduct the test by extrusion. Each sample test is repeated 4 times. Determine the thermal insulation performance of the guar gum aerogel according to the temperature and thermal conductivity.

[0061] Table 1 and Figure 4 respectively show the thermal imaging detection results of the guar gum aerogels prepared in Examples 1 to 3 ( Figure 4 which are respectively represented as Example 1 to Example 3 in

[0062] Table 1 shows the thermal conductivity of the guar gum aerogel. As can be seen from Table 1, the thermal conductivity of the guar gum aerogel sample is between 0.0229 - 0.0301 W / m*K, indicating that the material has excellent heat preservation performance.

[0063] Figure 4 shows the thermal imaging diagram of the guar gum aerogel. As shown in Figure 4 (a) - (b) of , continuous heat transfer is carried out on the hot stage (ambient temperature: 16.7 °C, internal circulating liquid temperature of the hot stage: 40 °C) with the guar gum aerogel sample and commercial EPS foam. Due to the construction of the anisotropic structure, the guar gum aerogel sample shows differences in temperature conduction in the longitudinal and radial directions. The reason for the not obvious thermal performance lies in the regularly arranged lamellar internal structure, which has relatively large voids in the longitudinal direction, similar to a hollow structure. The hollow structure is not a particularly ideal thermal insulation structure because heat transfer can occur along the axial direction of the inner wall of the cavity, thus unable to effectively reduce heat transfer. At this time, the surface temperatures of Example 1, Example 2, Example 3, and EPS commercial foam are 22.4 °C, 22.3 °C, 21.9 °C, and 22.2 °C respectively; in contrast, in the radial direction, the surface temperatures of Example 1, Example 2, Example 3, and EPS commercial foam are 17.9 °C, 18.9 °C, 18.7 °C, and 17.5 °C respectively, indicating that the guar gum aerogel prepared by the present invention has anisotropic thermal insulation performance.

[0064] As shown in Figure 4In (c) to (d), the guar gum aerogel sample and the commercial EPS commercial foam were placed on a cold stage (ambient temperature: -22 °C, internal circulating liquid temperature of the cold stage: -25 °C) for continuous refrigeration. The surface temperatures of Example 1, Example 2, Example 3, and the EPS commercial foam in the longitudinal direction were 6.4 °C, 6.8 °C, 7.9 °C, and 6.7 °C respectively. In contrast, in the radial direction, the surface temperatures of Example 1, Example 2, Example 3, and the EPS commercial foam were 11.8 °C, 12.2 °C, 13.3 °C, and 12.3 °C respectively, indicating the anisotropic heat insulation performance of the guar gum aerogel prepared by the present invention, and the heat preservation performance of Example 3 is even better than that of the commercial EPS foam.

[0065] 2. Flame retardancy: The limiting oxygen index of the guar gum aerogel sample was tested according to the method for measuring the oxygen index in the national standard GB2406-2009. The aerogel sample was cut into a rectangular strip with a size of 8 cm × 0.4 cm × 1 cm. During the test, one end of the aerogel sample was clamped by a fixture and placed in a glass cover. The oxygen and nitrogen concentrations were adjusted. After the concentrations were stable for 3 minutes, a lighter was used to ignite one end of the aerogel and the combustion situation was observed. The nitrogen and oxygen concentrations were adjusted until the aerogel could burn 80% of the total length of the sample.

[0066] Table 1 shows the limiting oxygen index of the guar gum aerogel. As can be seen from Table 1, the LOI value of Example 1 was only 21.89%, while the addition of more phytic acid effectively increased the LOI value of the sample. The LOI value of the sample in Example 3 reached 30.77%, indicating that the guar gum aerogel has excellent flame retardancy.

[0067] Figure 5 (a) to (c) respectively show the flame retardant physical pictures of the guar gum aerogels prepared in Examples 1 to 3. It can be seen that the aerogel ignited by the lighter quickly self-extinguished; Table 1 shows the limiting oxygen index of the guar gum aerogels prepared in Examples 1 to 3, and the values are in the range of 24-30%, indicating that the prepared guar gum aerogel has good flame retardancy; Figure 6 (a) to (c) respectively show the microscopic morphology pictures of the guar gum aerogels prepared in Examples 1 to 3 after combustion. From Figure 6 (b), it can be seen that there is a tendency for the burned part of the aerogel to shrink inward, and a carbon layer is formed on the surface, which can prevent heat from being transferred to the inside of the sample and can prevent external air from contacting the burned part to supply oxygen. This structure is beneficial to the flame retardant effect of the aerogel.

[0068] 3. Surface morphology: The surface morphology of the combustion residue of the guar gum aerogel was observed by a scanning electron microscope (SEM). Figure 7(a) to (c) show the SEM images of the guar gum aerogels prepared in Examples 1-3, respectively. It can be seen that the pH of the system regulated by phytic acid has a great influence on the internal structure of the final guar gum aerogel. The more phytic acid is added, the stronger the anisotropy of the internal network.

[0069] 4. Thermodynamic analysis: The thermogravimetric analysis was performed using a thermogravimetric analyzer. The test conditions were a heating rate of 10°C / min, a temperature range of 40°C-700°C, a high-purity nitrogen atmosphere, and a flow rate of 20 mL / min. The thermal stability of the samples was measured using differential scanning calorimetry. The test conditions were a heating rate of 10°C / min from 30°C to 350°C in a N2 atmosphere (60 mL / min). Figure 8 , 9 Thermogravimetric analysis and differential scanning calorimetry of the guar gum aerogels prepared in Examples 1 to 3 are shown respectively. Figure 8 It can be seen that Examples 1 to 3 show a similar weight loss process, which consists of two obvious stages. The first stage is mainly caused by the volatilization of moisture absorbed from the air. The weight loss ratio in this stage is basically the same, indicating that the addition of phytic acid will not affect the storage of aerogels. However, the abundant hydroxyl groups on the guar gum macromolecular chain can form hydrogen bonds with phytic acid, construct a huge cross-linked network, and limit the volatilization of water molecules, so the endothermic peak shifts to the right; the initial decomposition temperature of the second stage gradually decreases with the increase of phytic acid content. This stage is mainly caused by the breakage of hydroxyl groups in the guar gum chain and the dissociation of the macromolecular chain. The reason for the decrease in decomposition temperature is that phytic acid itself is a cyclic small molecule structure with a certain steric hindrance effect. Part of the unreacted phytic acid hinders the cross-linking reaction between guar gum and phytic acid, and the decomposition temperature shifts to the left. In addition, since phytic acid has excellent flame retardant properties, when heated to 700°C, the residue content of Examples 1, 2 and 3 increases successively, and the residue in Example 3 is the largest, reaching 50.22%, indicating that the addition of phytic acid significantly improves the thermal stability of aerogels; Figure 9 It can be seen that the overall trend of the differential scanning calorimetry curve (DSC curve) is similar to Figure 8 The thermogravimetric curves (TGA curves) of the three aerogel samples are similar. Each DSC curve has an endothermic peak around 99-135°C, which is caused by the desorption of water, and an exothermic peak around 190-305°C, which is caused by the cracking of the molecular framework. Comparison shows that with the increase of phytic acid content, the endothermic peak gradually increases and the exothermic peak gradually decreases. Figure 8 and Figure 9 The results show that the overall thermal stability of guar gum aerogel is good.

[0070] 5. Mechanical properties: The mechanical properties were detected using a compression testing machine. First, the samples were prepared into cubes with dimensions of 3 cm × 3 cm × 3 cm, and then a compression test was carried out at a constant speed of 2 mm / min. Figures 10A to 10C , Figures 11A to 11C respectively show the stress-strain cyclic diagrams in the radial and longitudinal directions of the guar gum aerogels prepared in Examples 1 to 3. It can be seen that the mechanical properties of the guar gum aerogels are different in the radial and longitudinal directions, and it has good resilience in the radial direction ( Figures 10A to 10C ), and the resilience value of Example 3 reaches 76%. During the compression test, we found that the guar gum aerogel samples underwent anisotropic shrinkage. Taking Example 2 for detailed analysis: Since the gap between adjacent layers is relatively large, when the aerogel is compressed along the radial axis direction, the part that bears the force is the small filaments connecting between the compressed layers. While the small filaments undergo bending deformation, the cavities between the gaps collapse. This process is relatively rapid. Therefore, it is found that in the stress-strain curve diagram, there is no obvious plateau region, that is, the elastic region quickly changes to the compaction region. At this time, the initial elastic modulus is 0.0364 Mpa; when the aerogel is compressed along the longitudinal axis direction, the part that bears the force is the lamellar guar gum skeleton. Guar gum is a rigid chain and is not easily bent under the action of force and can withstand higher loads. Before the strain reaches 20% when the stress increases to 0.017 MPa, it is in the elastic region, and then there is an obvious plateau region. When the stress increases to 0.0369 Mpa and the strain reaches 58%, the curve starts to climb rapidly again. At this time, the internal network structure begins to be damaged, causing irreversible damage. Therefore, it cannot return to its original shape after the compression behavior ends.

[0071] Figure 12 Show the stress-strain diagrams of the guar gum aerogels prepared in Examples 1 to 3 when the longitudinal deformation is 90%. Figure 12 As can be seen, the maximum force borne by the guar gum aerogel samples in the longitudinal direction is between 0.58 MPa and 0.79 Mpa, indicating that the guar gum aerogel has good mechanical properties and can adapt to complex applications.

[0072] Table 1. Thermal conductivity and limiting oxygen index of the guar gum aerogels prepared in Examples 1 - 3

[0073]

[0074]

Claims

1. A preparation method of an anisotropic heat-insulating and flame-retardant guar gum aerogel, comprising the following steps: (1) Add silicate and guar gum into water, stir evenly, quantitatively drop phytic acid to regulate the pH of the system, and stir to obtain a guar gum composite solution; (2) After the guar gum composite solution is subjected to vacuum degassing treatment, a guar gum precursor solution is obtained; (3) After the guar gum precursor solution is subjected to solidification and shaping treatment, a guar gum pre-gel is obtained, and freeze-drying is carried out to obtain a guar gum aerogel; In step (1), the silicate accounts for 7-9% of the total mass of the guar gum and the phytic acid; In step (1), the mass ratio of the guar gum to the phytic acid in the guar gum composite solution is 89-67:11-33.

2. The preparation method according to claim 1, characterized in that, The silicate in step (1) is sodium silicate.

3. The preparation method according to claim 2, characterized in that, The sodium silicate is powdered instant sodium silicate or sodium metasilicate nonahydrate.

4. The preparation method according to claim 1, characterized in that, In step (1), the pH of the guar gum composite solution after being adjusted by phytic acid is between 2 and 6.

5. The preparation method according to claim 1, characterized in that, In step (1), the total mass fraction of the guar gum, phytic acid, and silicate in the guar gum composite solution is 2.5-3.2%.

6. The preparation method according to claim 1, wherein In step (2), the conditions of the vacuum degassing treatment are: the temperature is 25-40°C, the system residual pressure is 500 Pa-600 Pa, and the time is 20 h-40 h.

7. The preparation method according to claim 1, characterized in that, In step (3), the conditions of the solidification and shaping treatment are: the temperature is -40 to -80°C, and the time is 30-60 min.

8. The preparation method according to claim 1, wherein In step (4), the conditions of the freeze-drying are: carry out freeze-drying for 24-72 h at -35°C to -40°C and a residual pressure of less than 2000 Pa.

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