Flame-retardant high-water-stability cellulose aerogel material and preparation method thereof

Abstract

The invention discloses a flame-retardant high-water-stability cellulose aerogel material and a preparation method thereof.The preparation method comprises the following steps that S1, raw material extraction is conducted, natural fiber powder is subjected to alkaline hydrolysis through an alkaline hydrolysis agent, then bleaching is conducted through a bleaching agent, finally hydrolysis is conducted through an acidolysis agent, then an obtained solution is subjected to homogenization treatment through a high-pressure homogenizer, and natural fiber powder is obtained; a nanofiber suspension is obtained; s2, aerogel forming: performing freezing pretreatment on the homogenized CNF dispersion liquid obtained in S1, and then drying by using a freeze dryer to obtain pure CNF aerogel; s3, cross-linking modification: soaking the pure CNF aerogel in a mixed solution of citric acid, phytic acid and a catalyst, drying the soaked CNF aerogel at 160 DEG C by using a high-temperature oven or a curing oven until the CNF aerogel is completely cured, cleaning with deionized water to remove excess acid, and performing secondary drying to obtain the citric acid cross-linked cellulose aerogel material. The flame retardance, water stability and related mechanical properties of the cellulose aerogel material in a building structure can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogels, and in particular to a flame-retardant and highly water-stable cellulose aerogel material and a preparation method thereof. Background Art

[0002] Today, with the development of society, the severe energy consumption situation has become extremely severe. Approximately 40% of global energy consumption is used for building energy consumption. Efficient thermal insulation materials play a key role in building structures. They maintain internal temperatures in hot and cold climates, providing a comfortable indoor environment for living and working, and prevent moisture condensation on walls, ceilings, windows, and other indoor surfaces. Therefore, research on new, environmentally friendly thermal insulation materials is essential.

[0003] Due to its unique structure, excellent properties, and processability, nanocellulose has great potential for fabricating multifunctional foams or porous materials for diverse applications, including packaging, thermal insulation, sound absorption, flame retardancy, oil-water separation, energy storage and harvesting, filtration, and biomedicine. Furthermore, nanocellulose is a renewable, recyclable, and environmentally friendly natural material with simple access, aligning with the current trend of sustainable development. Nanocellulose aerogels produced from nanocellulose exhibit excellent thermal insulation properties due to their high porosity and three-dimensional mesoporous structure. These porous materials are also common sound-absorbing materials and are widely used in noise control. However, the chemical structure of cellulose makes it highly flammable and spreads rapidly when exposed to open flames, and the hydrophilic nature of aerogels limits their application as building materials. Therefore, improving the stability of cellulose aerogels in humid environments is crucial. Furthermore, due to the high cost and environmental pollution of halogenated flame retardants, sustainable, environmentally friendly, and economically viable alternatives are needed to improve the flame retardancy of cellulose building materials.

[0004] Traditional methods of modifying cellulose with flame retardants usually involve many steps and often require the addition of additional flame retardants (such as halides) to improve flame retardant properties. They may also produce acidic waste liquids. Traditional cross-linking technologies usually rely on formaldehyde reagents (such as MF resins), which increase material production costs and are environmentally polluting. Therefore, optimization of the flame retardant cross-linking modification process is also very necessary. Summary of the Invention

[0005] The purpose of the present invention is to provide a flame-retardant and highly water-stable cellulose aerogel material and a preparation method thereof, which can be used to improve the flame retardancy, water stability and related mechanical properties of the cellulose aerogel material in building structures.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions: A method for preparing a flame-retardant and highly water-stable cellulose aerogel comprises the following steps: S1. Raw material extraction: The prepared natural fiber powder was alkaline hydrolyzed at 70°C for 3 hours, then bleached at 60-70°C for 1 hour, and finally hydrolyzed with an acid hydrolyzed agent to further purify the cellulose for 4 hours. The resulting solution was then homogenized using a high-pressure homogenizer for 3 hours to obtain a uniformly dispersed nanofiber suspension (CNF); S2. Aerogel Formation: The homogenized CNF dispersion obtained in S1 was placed in an ultra-low temperature freezer for deep freezing at -20°C, and then dried using a freeze dryer at a vacuum pressure range of 133 mbar to -133 mbar and a temperature of -54°C to obtain pure CNF aerogel; S3. Cross-linking modification: Soak the pure CNF aerogel in a mixture of citric acid (CA), phytic acid (PA) and 1 wt% catalyst for 24 hours. Then use a high-temperature oven or curing furnace to dry the soaked CNF aerogel at 160°C until it is completely cured. Then, wash it with deionized water to remove excess acid and dry it again to obtain citric acid cross-linked cellulose aerogel material.

[0007] Further preferably, the alkaline hydrolysis agent in S1 is a 2 wt % sodium hydroxide solution or potassium hydroxide solution.

[0008] Further preferably, the bleaching agent in S1 is a hydrogen peroxide solution or a peracetic acid solution.

[0009] Further preferably, the acidolysis agent in S1 is a 12 wt % citric acid solution.

[0010] Further preferably, the weight concentration of the citric acid in S3 is 20 wt %.

[0011] Further preferably, the weight concentration of the phytic acid (PA) in S3 is 5wt%, and the six phosphate groups of PA provide a phosphorus source, generating a polyphosphate char layer (POC bond) at high temperature, forming a P-CA-PA ternary synergistic flame retardant system with CA.

[0012] Further preferably, the catalyst in S3 is one of disodium hydrogen phosphate, sodium dihydrogen phosphate or trisodium phosphate, and the phosphate ions dissociated in the solution can provide an acidic environment for the cross-linking reaction, thereby promoting the protonation of cellulose hydroxyl (-OH) (forming R-OH2 + ), enhancing its electrophilicity, thereby accelerating the esterification reaction between the carboxylic acid group (-COOH) of citric acid and the hydroxyl group of cellulose to form a cross-linked structure.

[0013] The present invention also provides a flame-retardant and highly water-stable cellulose aerogel material, which is prepared according to the above-mentioned method for preparing a flame-retardant and highly water-stable cellulose aerogel material.

[0014] In summary, the present invention has the following beneficial effects: First, in order to optimize the performance of nanocellulose aerogel in building structures, the present invention uses citric acid (CA) and phytic acid (PA) to cross-link and modify nanocellulose aerogel to form a P-CA-PA ternary synergistic flame retardant system, thereby improving the flame retardancy, water stability and compressive resistance of nanocellulose aerogel. At the same time, the acidolysis agent and catalyst in the cross-linking modification process are optimized, the production efficiency and green environmental protection of the modified nanocellulose aerogel material are improved, the performance of CNF aerogel material in building structures is improved, and the application prospects of CNF aerogel material are expanded.

[0015] Secondly, the cellulose raw material of the present invention is extracted from common and cheap natural fibers, which greatly reduces the production cost of nanocellulose aerogel materials, improves the economic benefits and product competitiveness of aerogel materials, and conforms to the concept of sustainable development.

[0016] Third, the present invention uses one of disodium hydrogen phosphate, sodium dihydrogen phosphate or trisodium phosphate as a catalyst for the crosslinking reaction. The phosphate ions dissociated in the solution can provide an acidic environment for the crosslinking reaction, promoting the protonation of cellulose hydroxyl (-OH) (forming R-OH2 + ), enhancing its electrophilicity, thereby accelerating the esterification reaction between the carboxylic acid group (-COOH) of citric acid and the hydroxyl group of cellulose to form a cross-linked structure.

[0017] Fourthly, the present invention optimizes the cross-linking modification process of CNF aerogel materials. Citric acid is both the cross-linking agent of the present invention and the acidolysis agent in the cellulose purification process, eliminating the multi-step reagent replacement in the traditional process. Extraction and cross-linking are synchronized, which significantly simplifies the preparation process. At the same time, a weak acid catalyst is used to replace the traditional strong acid to avoid excessive hydrolysis of cellulose and reduce the use of corrosive reagents. The reagents are recyclable during the cross-linking process, and there are no toxic by-products. The production is green, environmentally friendly and sustainable. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a flow chart for preparing the citric acid cross-linked cellulose aerogel material of the present invention; Figure 2 This is a diagram showing the water stability test results of the present invention; Figure 3 This is a comparison chart of the combustion performance test of the CNF aerogel cross-linked using CA and PA mixtures with different mass fractions in the present invention. DETAILED DESCRIPTION

[0019] The present invention will be further described in detail below with reference to the accompanying drawings.

[0020] Example 1, a method for preparing a flame retardant and highly water-stable cellulose aerogel, such as Figure 1 As shown, the following steps are included: S1. Raw Material Extraction: The prepared natural fiber powder is hydrolyzed using an alkaline hydrolyzer at 70°C for 3 hours. In this example, the natural fiber powder is clover stalk powder. The alkaline hydrolyzer is a 2wt% sodium hydroxide solution or potassium hydroxide solution. This removes lignin and hemicellulose, swells the cellulose, and loosens the fiber structure, facilitating subsequent nanofiber separation. The powder is then bleached using a hydrogen peroxide solution or peracetic acid solution at 60-70°C for 1 hour. This oxidizes residual lignin and pigments, improves cellulose purity, and sterilizes the material to ensure raw material stability. Finally, the cellulose is hydrolyzed using an acid hydrolyzer (a 12wt% citric acid solution) for 4 hours to further purify the cellulose. The acidic environment further purifies the cellulose and facilitates nanofiber separation. The resulting solution is then homogenized using a high-pressure homogenizer for 3 hours to obtain a uniformly dispersed nanofiber suspension (CNF).

[0021] S2. Aerogel Formation: The homogenized CNF dispersion obtained in S1 was placed in an ultra-low temperature freezer and deep-frozen at -20°C for pretreatment. It was then dried using a freeze dryer at a vacuum pressure range of 133 mbar to -133 mbar and a temperature of -54°C to obtain pure CNF aerogel.

[0022] S3. Cross-linking modification: Pure CNF aerogel was immersed in a mixture of citric acid (CA), phytic acid (PA) and 1wt% catalyst for 24 hours. The weight concentration of citric acid was 20wt%. The cross-linking structure of citric acid and CNF aerogel remained stable in a humid environment and could significantly reduce the release of combustible gases (such as CO), thereby improving the flame retardant properties and thermal stability of CNF aerogel. At the same time, the cross-linking structure also improved the compressive strength of CNF aerogel. The weight concentration of phytic acid was 5wt%. The six phosphate groups of PA provided a phosphorus source, generating a polyphosphate char layer (POC bond) at high temperature, forming a P-CA-PA ternary synergistic flame retardant system with CA, further improving the flame retardant properties of CNF aerogel. At the same time, PA further strengthened the mechanical properties of CNF aerogel materials through hydrogen bonding and intercalation. The catalyst was one of disodium hydrogen phosphate, sodium dihydrogen phosphate or trisodium phosphate. The phosphate ions dissociated in the solution could provide an acidic environment for the cross-linking reaction, promoting the protonation of cellulose hydroxyl (-OH) (forming R-OH2 +), enhancing its electrophilicity, thereby accelerating the esterification reaction between the carboxylic acid groups (-COOH) of citric acid and the hydroxyl groups of cellulose, forming a cross-linked structure. The soaked CNF aerogel is then dried in a high-temperature oven or curing furnace at 160°C until fully solidified. It is then washed with deionized water to remove excess acid and dried again to obtain the citric acid-crosslinked cellulose aerogel material.

[0023] Example 2 is different from Example 1 in that the weight concentration of citric acid in step 3 is 16 wt %, and the weight concentration of phytic acid is 0 wt %.

[0024] Comparative Example 3 is different from Example 1 in that the weight concentration of citric acid in step 3 is 20 wt %, and the weight concentration of phytic acid is 0 wt %.

[0025] Example 4: The water stability, flame retardancy and compressive strength of the citric acid-phytic acid cross-linked cellulose aerogel material prepared above were tested.

[0026] from Figure 2 It can be seen from the figure that after the CNF aerogel is cross-linked and modified with CA and PA, the water stability of the aerogel material is significantly improved.

[0027] from Figure 3 It can be seen that with the increase of the CA mass fraction in the mixture, the flame retardant properties of CNF aerogel material are gradually improved, and when 5wt% PA is added to the mixture of 20wt% CA, the flame retardant grade reaches V-0.

[0028] Table 1 shows the cone calorimetry test results of cross-linking CNF aerogels using CA and PA mixtures with different mass fractions. As can be seen from Table 1, after adding 5wt% PA, the pHRR and THR of the CNF aerogel material are significantly reduced, and the flame retardant performance of the CNF aerogel material is further improved (by about 30%). This is attributed to the phosphorus source provided by the six phosphate groups of PA, which generates a polyphosphate char layer at high temperature and forms a P-CA-PA ternary synergistic flame retardant system with CA, which greatly improves the flame retardant performance of the CNF aerogel material.

[0029] Table 2 shows the compressive performance test results of CNF aerogel cross-linked with CA and PA mixtures of different mass fractions. It can be observed from Table 2 that the composite cross-linked network formed by CA-PA and CNF also improves the compressive properties of the aerogel material.

[0030] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. After reading this specification, those skilled in the art may make non-creative modifications to this embodiment as needed. However, as long as such modifications are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A method for preparing a flame-retardant and highly water-stable cellulose aerogel, characterized in that: The following steps are involved: S1. Raw material extraction: The prepared natural fiber powder was alkaline hydrolyzed at 70°C for 3 hours, then bleached at 60-70°C for 1 hour, and finally hydrolyzed with an acid hydrolyzed agent to further purify the cellulose for 4 hours. The resulting solution was then homogenized using a high-pressure homogenizer for 3 hours to obtain a uniformly dispersed nanofiber suspension (CNF); S2. Aerogel Formation: The homogenized CNF dispersion obtained in S1 was placed in an ultra-low temperature freezer for deep freezing at -20°C, and then dried using a freeze dryer at a vacuum pressure range of 133 mbar to -133 mbar and a temperature of -54°C to obtain pure CNF aerogel; S3. Cross-linking modification: Soak the pure CNF aerogel in a mixture of citric acid (CA), phytic acid (PA) and 1 wt% catalyst for 24 hours. Then use a high-temperature oven or curing furnace to dry the soaked CNF aerogel at 160°C until it is completely cured. Then, wash it with deionized water to remove excess acid and dry it again to obtain citric acid cross-linked cellulose aerogel material.

2. The method for preparing a flame-retardant and highly water-stable cellulose aerogel material according to claim 1, characterized in that: The alkaline hydrolysis agent in S1 is a 2 wt % sodium hydroxide solution or potassium hydroxide solution.

3. The method for preparing a flame-retardant and highly water-stable cellulose aerogel material according to claim 2, characterized in that: The bleaching agent in S1 is a hydrogen peroxide solution or a peracetic acid solution.

4. The method for preparing a flame-retardant and highly water-stable cellulose aerogel material according to claim 3, characterized in that: The acidolysis agent in S1 is a 12 wt % citric acid solution.

5. The method for preparing a flame-retardant and highly water-stable cellulose aerogel material according to claim 4, characterized in that: The weight concentration of the citric acid in S3 is 20 wt %.

6. The method for preparing a flame-retardant and highly water-stable cellulose aerogel material according to claim 5, characterized in that: The weight concentration of the phytic acid (PA) in S3 is 5wt%. The six phosphate groups of PA provide a phosphorus source, generating a polyphosphate char layer (POC bond) at high temperature, and forming a P-CA-PA ternary synergistic flame retardant system with CA.

7. The method for preparing a flame-retardant and highly water-stable cellulose aerogel material according to claim 6, characterized in that: The catalyst in S3 is one of disodium hydrogen phosphate, sodium dihydrogen phosphate or trisodium phosphate, and the phosphate ions dissociated in the solution can provide an acidic environment for the cross-linking reaction, promoting the protonation of cellulose hydroxyl (-OH) (forming R-OH2 + ), enhancing its electrophilicity, thereby accelerating the esterification reaction between the carboxylic acid group (-COOH) of citric acid and the hydroxyl group of cellulose to form a cross-linked structure.

8. The flame-retardant and highly water-stable cellulose aerogel material according to claim 1, characterized in that: The flame-retardant and highly water-stable cellulose aerogel material is prepared according to the method for preparing the flame-retardant and highly water-stable cellulose aerogel material according to any one of claims 1 to 6.

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