Citric acid-MMT-IL cross-linked flame-retardant antibacterial cellulose aerogel and preparation method thereof

Abstract

The invention discloses a flame-retardant antibacterial cellulose aerogel cross-linked by citric acid-MMT-IL and a preparation method of the flame-retardant antibacterial cellulose aerogel cross-linked by citric acid-MMT-IL. The invention aims to optimize the flame retardant property and mechanical property of the nanocellulose aerogel and endow the nanocellulose aerogel with certain antibacterial property and conductivity. CA and MMT-IL are selected to carry out synergistic modification treatment on CNF aerogel, a rigid sheet layer of MMT can disperse stress and reduce local stress of the material, meanwhile, an esterification reaction of CA and cellulose can enhance cellulose-MMT interface bonding, and the mechanical property of the material is further improved; cA and MMT-IL dilute combustible gas, capture free radicals and form a compact carbon layer to improve the flame retardant property of the CNF aerogel; and free ions in MMT-IL can endow the CNF aerogel material with antibacterial performance and conductivity, the application fields of CNF aerogel, such as medical and intelligent equipment fields, are expanded, the process has high efficiency and environmental protection, and a new thought is provided for green manufacturing.

Description

Technical Field

[0001] The present invention relates to the technical field of functional fiber materials, and in particular to a flame-retardant and antibacterial cellulose aerogel cross-linked with citric acid-MMT-IL and a preparation method thereof. Background Art

[0002] Due to its unique structure, excellent properties, and processability, nanocellulose has enormous potential for creating multifunctional foams or porous materials for a variety of 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 are three-dimensional porous materials characterized by low density, high strength, high porosity, high specific surface area, low thermal conductivity, low thermal expansion, and sustainability. They have applications in medicine, energy, the environment, and architecture.

[0003] However, the flammability of cellulose itself greatly limits its application. Therefore, improving flame retardancy is of great significance to further expand the application field of cellulose aerogel. In addition to overcoming its flammability, it is also very important to study and optimize other properties of cellulose aerogel materials to further expand its application range. For example, research on improving the mechanical properties of aerogels, overcoming the problem of large-sized aerogels used in the construction field prone to cracks, optimizing their antibacterial properties or conductive properties so that they can be used in medical and smart equipment fields, and at the same time, research on simplifying the preparation process and exploring green, economical and sustainable production methods to promote the industrialization of flame-retardant cellulose aerogels. Summary of the Invention

[0004] To address the above problems, the present invention provides a flame-retardant and antibacterial cellulose aerogel cross-linked with citric acid-MMT-IL and a preparation method thereof, thereby improving the flame retardancy, electrical conductivity, antibacterial properties and related mechanical properties of the cellulose aerogel material.

[0005] According to one object of the present invention, the present invention provides a method for preparing a flame-retardant and antibacterial cellulose aerogel cross-linked with citric acid-MMT-IL, comprising the following steps:

[0006] Step 1: Raw material extraction:

[0007] The prepared natural fiber powder was subjected to alkaline hydrolysis (70°C, 3h), then bleached with a bleaching agent (60-70°C, 1h), and finally acid hydrolyzed with an acid hydrolyzing agent to further purify the cellulose (4h). The resulting solution was then homogenized with a high-pressure homogenizer (3h) to obtain a uniformly dispersed nanofiber suspension.

[0008] Step 2: Aerogel molding:

[0009] The homogenized cellulose nanofiber (CNF) dispersion obtained in step 1 was placed in an ultra-low temperature freezer for deep freezing pretreatment 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 cellulose nanofiber (CNF) aerogel;

[0010] Step 3, MMT-IL preparation:

[0011] Montmorillonite (MMT) was dispersed in deionized water and stirred for 1 hour to fully expand its layered structure. Ionic liquid was weighed in proportion and completely dissolved in deionized water by ultrasonic treatment for 5 minutes. The swollen MMT suspension was mixed with the ionic liquid solution and stirred at 60°C for 60 minutes to promote the cations of the ionic liquid (such as imidazolium) to be embedded in the MMT interlayer through electrostatic interaction. The unbound ionic liquid was then removed by centrifugation (10,000 rpm, 5 minutes). The MMT-IL nanocomposite was then dried at 80°C.

[0012] Step 4: Cross-linking modification:

[0013] Pure CNF aerogel is soaked in a mixture of citric acid (CA), MMT-IL, 1 wt% catalyst and 1 wt% dispersant for 24 hours; the soaked CNF aerogel is then dried at 130°C in a high-temperature oven or curing furnace until completely solidified, and then washed with deionized water to remove excess acid and dried again to obtain citric acid cross-linked cellulose aerogel material.

[0014] Preferably, the alkaline hydrolysis agent in step 1 is a 2 wt % sodium hydroxide solution or potassium hydroxide solution, which is responsible for removing lignin and hemicellulose, swelling cellulose to loosen the fiber structure, and facilitating the subsequent separation of nanofibers.

[0015] Preferably, the bleaching agent in step 1 is a hydrogen peroxide solution or a peracetic acid solution, which is responsible for oxidizing the residual lignin and pigment, improving the purity of cellulose, and sterilizing and disinfecting to ensure the stability of the raw materials.

[0016] Preferably, the acidolysis agent in step 1 is a 12 wt % citric acid solution, which further purifies the cellulose in an acidic environment and promotes the separation of nanofibers.

[0017] Preferably, the ratio of the ionic liquid in step 3 is 2:1 by weight of MMT to the ionic liquid.

[0018] Preferably, the ionic liquid in step 3 is 1-methyl-3-octylimidazolium bromide, which produces imidazolium cations and Br- to form an ion conductive network between the MMT layers, reducing the surface resistance of the material. The imidazolium cations can destroy the bacterial cell membrane through electrostatic adsorption to enhance the antibacterial properties of the material. When the aerogel burns, on the one hand, Br- releases and captures highly active free radicals (such as H·, OH·), interrupting the chain combustion reaction. On the other hand, the imidazolium cations promote the formation of a dense carbon layer during the thermal decomposition of cellulose, further isolating the heat.

[0019] Preferably, the weight concentration of citric acid in step 4 is 16 wt %. The cross-linked structure of citric acid and CNF aerogel remains stable in a humid environment, and can significantly reduce the release of combustible gases (such as CO), thereby improving the flame retardant properties and water stability of CNF aerogel. The cross-linked structure can also improve the mechanical properties of the aerogel. At the same time, the esterification reaction of CA and cellulose can enhance the cellulose-MMT interface bonding, thereby further improving the mechanical properties of the material.

[0020] Preferably, the weight concentration of MMT-IL in step 4 is 5 wt %. On the one hand, the silicate layer of MMT forms a dense carbon-silicate composite layer at high temperature, which acts as a barrier, reflects thermal radiation and isolates oxygen; on the other hand, MMT can enhance the melt viscosity of the material and reduce melt dripping. At the same time, the rigid sheet layer of MMT can disperse stress, reduce local stress concentration in the material, and improve the mechanical properties of the material.

[0021] Preferably, the catalyst in step 4 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.

[0022] Preferably, the dispersant in step 4 is polyvinylpyrrolidone (PVP), which is used to reduce the agglomeration of MMT-IL in the aerogel material and improve the dispersibility.

[0023] According to another object of the present invention, the present invention provides a flame-retardant and antibacterial cellulose aerogel cross-linked with citric acid-MMT-IL prepared by the above method.

[0024] The beneficial effects of the present invention are:

[0025] 1. The crosslinking agent used in the present invention is citric acid. The crosslinked structure between citric acid and CNF aerogel remains stable in a humid environment and can significantly reduce the release of combustible gases (such as CO), improving the flame retardancy and water stability of CNF aerogel. The crosslinked structure can also improve the mechanical properties of the aerogel. At the same time, the esterification reaction between CA and cellulose can enhance the cellulose-MMT interface bonding, further improving the mechanical properties of the material.

[0026] 2. The synergistic flame retardant material selected in the present invention is MMT-IL, which further enhances the flame retardancy and mechanical properties of the aerogel material and provides it with excellent electrical conductivity and antibacterial properties, expanding the application areas of CNF aerogels. The ionic liquid 1-methyl-3-octylimidazolium bromide in MMT-IL generates imidazolium cations and Br-ions, forming an ionic conductive network between the MMT layers, reducing the surface resistance of the material. The imidazolium cations can disrupt bacterial cell membranes through electrostatic adsorption, enhancing the antibacterial properties of the material. During aerogel combustion, Br-ions release and capture highly reactive free radicals (such as H· and OH·), interrupting the chain combustion reaction. Furthermore, the imidazolium cations promote the formation of a dense carbon layer during cellulose pyrolysis, further insulating the material. At high temperatures, the silicate layer of MMT forms a dense carbon-silicate composite layer, acting as a barrier, reflecting thermal radiation and isolating oxygen. It also enhances the melt viscosity of the material and reduces melt dripping. The synergistic flame retardant effect of the two significantly enhances the material's flame retardancy. Simultaneously, the rigid sheet of MMT disperses stress, reducing local stress concentration and improving the material's mechanical properties.

[0027] 3. The present invention combines MMT with the ionic liquid 1-methyl-3-octylimidazolium bromide through ion exchange to form a nanocomposite material. Compared with traditional brominated flame retardants, it is more environmentally friendly, almost non-volatile, has a very low risk of release, and is multifunctional. Because the ionic liquid itself has an extremely low vapor pressure and is almost non-volatile, the risk of diffusion through the air is reduced. Its chemical structure is stable and does not easily decompose into free bromide ions or other harmful substances under normal conditions of use. At the same time, it is tightly bound to montmorillonite (MMT) through cation exchange to form a layered nanocomposite material. This structure can effectively limit the migration of bromide ions and reduce the probability of their release in the environment. In addition, the actual amount used in the present invention is low, and consumption is low.

[0028] 4. 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.

[0029] 5. The present invention optimizes the cross-linking modification process of CNF aerogel materials. Citric acid serves as both a cross-linking agent in the present invention and an acidolysis agent in the cellulose purification process. The MMT-IL nanocomposite material and the cross-linking agent are simultaneously applied to the cellulose aerogel material to achieve multifunctionality. This process avoids the traditional multi-step processing (such as pretreatment → antibacterial treatment → flame retardant treatment → conductive coating, etc.), significantly simplifies the process and reduces resource consumption. It is both efficient and environmentally friendly, providing new ideas for green manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart for preparing the citric acid cross-linked cellulose aerogel material of the present invention.

[0031] Figure 2 Schematic diagram of the MMT-IL modification principle. DETAILED DESCRIPTION

[0032] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] Example 1

[0034] like Figure 1 and Figure 2 As shown,

[0035] A method for preparing a flame-retardant and antibacterial cellulose aerogel cross-linked with citric acid-MMT-IL comprises the following steps:

[0036] 1. Raw material extraction:

[0037] The prepared natural fiber powder is hydrolyzed using an alkaline hydrolyzer (70°C, 3 hours), then bleached using a bleaching agent (60-70°C, 1 hour), and finally hydrolyzed using an acid hydrolyzer to further purify the cellulose (4 hours). The resulting solution is then homogenized using a high-pressure homogenizer (3 hours) to obtain a uniformly dispersed nanofiber suspension. In this step 1, the alkaline hydrolyzer is a 2wt% sodium hydroxide solution or potassium hydroxide solution; the bleaching agent is a hydrogen peroxide solution or peracetic acid solution; and the acid hydrolyzer is a 12wt% citric acid solution.

[0038] 2. Aerogel molding:

[0039] The homogenized CNF dispersion obtained in step 1 was placed in an ultra-low temperature freezer at -20°C for deep freezing pretreatment, 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.

[0040] 3. Preparation of MMT-IL:

[0041] Montmorillonite (MMT) was dispersed in deionized water and stirred for 1 hour to fully expand its layered structure. An appropriate amount of ionic liquid was weighed and sonicated for 5 minutes to completely dissolve it in the deionized water. The swollen MMT suspension was mixed with the ionic liquid solution and stirred at 60°C for 60 minutes to allow the cations of the ionic liquid (e.g., imidazolium) to electrostatically intercalate between the MMT layers. Unbound ionic liquid was then removed by centrifugation (10,000 rpm, 5 minutes). The mixture was then dried at 80°C to yield an MMT-IL nanocomposite. In this step 3, the ionic liquid ratio was 2:1 (by weight) and the ionic liquid was 1-methyl-3-octylimidazolium bromide.

[0042] 4. Cross-linking modification:

[0043] Pure CNF aerogels were immersed in a mixture of citric acid (CA), MMT-IL, 1 wt% catalyst, and 1 wt% dispersant for 24 hours. The soaked CNF aerogels were then dried in a high-temperature oven or curing furnace at 130°C until fully solidified. The aerogels were then washed with deionized water to remove excess acid and dried again to obtain a citric acid-crosslinked cellulose aerogel. In this step 4, the citric acid concentration was 16 wt%; the MMT-IL concentration was 5 wt%; the catalyst was disodium hydrogen phosphate; and the dispersant was polyvinylpyrrolidone (PVP).

[0044] Comparative Example 2

[0045] This comparative example is different from Example 1 in that the weight concentration of MMT-IL in step 4 is 0 wt %.

[0046] Comparative Example 3

[0047] This comparative example is different from Example 1 in that the weight concentration of MMT-IL in step 4 is 2 wt %.

[0048] Comparative Example 4

[0049] The difference between this comparative example and Example 1 is that the weight concentration of citric acid in step 4 is 12 wt %, and the weight concentration of MMT-IL is 0 wt %.

[0050] The flame retardant, conductive, antibacterial and mechanical properties of the prepared MMT-IL cross-linked cellulose aerogel material were tested.

[0051] As can be seen from Table 1, the flame retardancy of the aerogel material is significantly improved, especially after adding MMT-IL to the mixture. Both LOI and residual carbon rate are greatly improved (about 30%). This is attributed to the silicate layer of MMT forming a dense carbon-silicate composite layer at high temperature, which acts as a barrier, reflecting thermal radiation and isolating oxygen.

[0052] At the same time, Br- in MMT-IL releases and captures highly active free radicals (such as H·, OH·), interrupting the chain combustion reaction; imidazolium cations promote the formation of a dense carbon layer during cellulose pyrolysis, further isolating the heat.

[0053] Table 1 Comparison of flame retardant performance test results

[0054]

[0055] It can be seen from Tables 2, 3, and 4 that after adding MMT-IL to the mixture, the surface resistance of CNF aerogel is greatly reduced, the conductivity is improved, and the antibacterial property is obtained. At the same time, the mechanical properties of the aerogel material are also greatly improved.

[0056] This is because the ionic liquid 1-methyl-3-octylimidazolium bromide in MMT-IL can generate imidazolium cations and Br- to form an ion conductive network between the MMT layers, reducing the surface resistance of the material. In addition, the imidazolium cations can destroy the bacterial cell membrane through electrostatic adsorption, improving the antibacterial properties of the material. The rigid sheets of MMT can disperse stress, reduce local stress concentration in the material, and improve the mechanical properties of the material.

[0057] At the same time, the esterification reaction between CA and cellulose can enhance the cellulose-MMT interface bonding, further improving the mechanical properties of the material.

[0058] Table 2 Conductivity test results

[0059]

[0060] Table 3 Comparison of antibacterial performance test results

[0061]

[0062] Table 4 Comparison of mechanical properties test results

[0063]

[0064] This invention aims to optimize the flame retardancy and mechanical properties of nanocellulose aerogels while also imparting them with certain antibacterial and electrical conductivity. CA and MMT-IL are used to synergistically modify CNF aerogels. The rigid MMT layers disperse stress and reduce local stress in the material. The esterification reaction between CA and cellulose strengthens the cellulose-MMT interface, further enhancing the mechanical properties of the material. CA and MMT-IL dilute combustible gases, capture free radicals, and form a dense carbon layer, enhancing the flame retardancy of CNF aerogels. Furthermore, the free ions in MMT-IL impart antibacterial and electrical conductivity to the CNF aerogel material, expanding its application areas to areas such as medical and intelligent devices. The process is both efficient and environmentally friendly, providing new insights into green manufacturing.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing flame-retardant and antibacterial cellulose aerogel cross-linked with citric acid-MMT-IL, characterized in that: The following steps are involved: Step 1: Raw material extraction: The natural fiber powder was alkaline hydrolyzed at 70°C for 3 hours, bleached at 60-70°C for 1 hour, and then acid hydrolyzed for 4 hours using an acid hydrolyzed agent to further purify the cellulose. The resulting solution was homogenized with a high-pressure homogenizer for 3 hours to obtain a nanofiber suspension. Step 2: Aerogel molding: The homogenized cellulose nanofiber dispersion obtained in step 1 was deep-frozen at -20°C for pretreatment, and then freeze-dried at a vacuum pressure of 133 mbar to -133 mbar and a temperature of -54°C to obtain a pure cellulose nanofiber aerogel; Step 3, MMT-IL preparation: Montmorillonite was dispersed in deionized water and stirred for 1 hour to expand its layered structure. Ionic liquid was weighed at a weight ratio of MMT to ionic liquid of 2:1 and sonicated for 5 minutes to dissolve it in deionized water. The swollen MMT suspension was mixed with the ionic liquid solution and stirred at 60°C for 60 minutes to allow the ionic liquid cations to embed between the MMT layers. The unbound ionic liquid was then removed by centrifugation at 10,000 rpm for 5 minutes and dried at 80°C to obtain the MMT-IL nanocomposite. Step 4: Cross-linking modification: Pure CNF aerogel was immersed in a mixture containing 16 wt% citric acid, 5 wt% MMT-IL, 1 wt% catalyst and 1 wt% dispersant for 24 hours, dried at 130 ° C until completely solidified, washed with deionized water to remove excess acid and then dried again to obtain citric acid cross-linked cellulose aerogel material.

2. The preparation method according to claim 1, characterized in that The alkaline hydrolysis agent in step 1 is a 2 wt % sodium hydroxide solution or potassium hydroxide solution.

3. The preparation method according to claim 1, characterized in that The bleaching agent in step 1 is a hydrogen peroxide solution or a peracetic acid solution.

4. The preparation method according to claim 1, characterized in that The acidolysis agent in step 1 is a 12 wt% citric acid solution.

5. The preparation method according to claim 1, characterized in that The ionic liquid in step 3 is 1-methyl-3-octylimidazolium bromide.

6. The preparation method according to claim 1, characterized in that The catalyst in step 4 is one of disodium hydrogen phosphate, sodium dihydrogen phosphate or trisodium phosphate.

7. The preparation method according to claim 1, characterized in that The dispersant in step 4 is polyvinyl pyrrolidone.

8. A flame-retardant and antibacterial cellulose aerogel cross-linked with citric acid-MMT-IL, prepared by the method according to any one of claims 1 to 7.

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