Hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel and preparation method thereof

Abstract

The present invention provides a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel and a preparation method thereof. The invention comprises the following steps: firstly adding aramid fiber to a potassium hydroxide solution, and then adding DMSO thereto to obtain an aramid nanofiber dispersion; then adding ultrapure water to the aramid nanofiber dispersion, and stirring to obtain an aramid nanofiber hydrogel; firstly placing the aramid nanofiber hydrogel in ultrapure water for a solvent exchange, and then placing it in a mixed solution of tert-butyl alcohol and ultrapure water for a second solvent exchange, and then sequentially adding an aqueous polyurethane and a hydrophobic agent to the hydrogel after the solvent exchange, and mechanically stirring to obtain a composite hydrogel; and finally freeze-drying to obtain the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel. In the preparation process of the present invention, the continuous changes of the cross-linked structure of aramid fiber, polyurethane molecules, and perfluorooctyltrichlorosilane are utilized, and ultimately, through the synergistic effect of the three, a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel with a special structure is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogel preparation, and in particular to a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel and a preparation method thereof. Background Art

[0002] The chemical name of aramid is poly(p-phenylene terephthalamide) (PPTA). It is a fiber with high strength, high modulus, high temperature tolerance and good chemical stability. It can be used to make various bulletproof vests, personal protection products, aerospace materials, etc. Aerogel has unique properties such as large specific surface area, high porosity, low thermal conductivity and low density. If aramid is prepared into nanofiber aerogel, it should theoretically have extremely high specific surface area and porosity, as well as low density, good high temperature stability and biocompatibility. It can be widely used in adsorption materials, catalyst carriers, gas separation, high temperature insulation materials, biomedicine and other fields. However, the aramid nanofiber aerogels prepared at this stage have low pore uniformity, relatively poor mechanical properties and insufficient long-term stability, which limits the application of aramid nanofiber aerogel materials in actual production.

[0003] Patent application number CN201710263470.0 discloses a method for preparing aramid nanofiber aerogel, which first extracts dried aramid fibers, then prepares an aramid nanofiber solution, then prepares an aramid nanofiber hydrogel, and then prepares the aramid nanofiber aerogel through vacuum filtration and freeze-drying steps. The disadvantage of this invention is that the pore distribution of the prepared aerogel is uneven, the pore size deviation is large, and local fiber network adhesion occurs, which will affect the performance of the obtained aerogel, reduce its stability in practical application, and thus affect its practical application effect. Patent application number 202010350965.9 discloses an aramid nanofiber aerogel and a method for preparing it, which first prepares a nanofiber dispersion, then adds water to it, mixes and filters to obtain an aramid nanofiber gel, and then prepares the aramid nanofiber aerogel through freeze-drying. The disadvantage of this invention is that the prepared aerogel sample has defects in morphology, irregular shape, and low mechanical properties, which may lead to limited use of the aerogel and reduced use effect during practical application. In addition, the functions of the aramid nanofiber aerogels currently prepared are relatively single, which further limits their applications.

[0004] In view of this, it is necessary to design an improved hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel and a preparation method thereof to solve the above problems. Summary of the Invention

[0005] The object of the present invention is to provide a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel and a preparation method thereof. The method comprises the following steps: subjecting aramid fibers to a specific process for degradation, preparing the obtained aramid nanofiber dispersion into a hydrogel, performing two solvent exchanges with a specific solvent, and then mixing the dispersion with aqueous polyurethane and perfluorooctyltrichlorosilane to prepare a composite hydrogel. The composite hydrogel is then freeze-dried, so that the cross-linking structure of the aramid fibers, polyurethane molecules, and perfluorooctyltrichlorosilane continuously changes. Ultimately, through the synergistic effect of the three, a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel with a special structure is obtained.

[0006] To achieve the above-mentioned object of the invention, the present invention provides a method for preparing a hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel, comprising the following steps:

[0007] S1. The aramid fiber was added to a potassium hydroxide solution and soaked for 15-25 seconds, and then DMSO was added thereto to obtain an aramid nanofiber dispersion of a preset concentration;

[0008] S2. adding ultrapure water to the aramid nanofiber dispersion prepared in step S1 and stirring to obtain an aramid nanofiber hydrogel; first placing the aramid nanofiber hydrogel in ultrapure water for a primary solvent exchange, then placing it in a mixed solution of tert-butyl alcohol and ultrapure water for a secondary solvent exchange, then adding aqueous polyurethane to the hydrogel after solvent exchange, mechanically stirring, and then adding a hydrophobic agent, and mechanically stirring to obtain a composite hydrogel;

[0009] S3. The composite hydrogel prepared in step S2 is pre-frozen at (-20)-(-50)°C, and finally placed in a freeze dryer for freeze drying to obtain a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel.

[0010] As a further improvement of the present invention, in step S1, the mass-volume fraction of the aramid nanofiber dispersion is 0.5%-1.5%.

[0011] As a further improvement of the present invention, when preparing the composite hydrogel in step S2, the mass ratio of the aramid nanofiber dispersion, the aqueous polyurethane and the hydrophobic agent is 97-99%:0.5-1.5%:0.5-1.5%.

[0012] As a further improvement of the present invention, the hydrophobic agent is perfluorooctyltrichlorosilane.

[0013] As a further improvement of the present invention, in step S2, the volume ratio of tert-butanol to ultrapure water in the mixed solution used in the secondary solvent exchange process is 45-55%:45-55%.

[0014] As a further improvement of the present invention, in step S2, the duration of the first solvent exchange is 20-30 hours; the duration of the second solvent exchange is 20-30 hours.

[0015] As a further improvement of the present invention, step S1 specifically comprises the following steps: first dissolving potassium hydroxide in ultrapure water to obtain a potassium hydroxide solution; soaking the aramid fiber in an alkali solution for 15-25 seconds at a mass ratio of potassium hydroxide solid to aramid fiber in the potassium hydroxide solution of 3:(1.5-2.5), then adding 100 mL of DMSO and mechanically stirring for 3-5 hours, ensuring that ultrapure water in the entire system accounts for 3%-5% of the volume of DMSO, so that the aramid fiber is fully dissolved, and obtaining an aramid nanofiber dispersion with a mass-volume fraction of 0.5%-1.5%.

[0016] As a further improvement of the present invention, in step S3, the freeze-drying time is 20-30 hours; the freeze-drying temperature is (-80)-(-45)°C, and the vacuum degree is 15-50Pa.

[0017] As a further improvement of the present invention, the aramid fiber is obtained by cutting aramid fiber filaments into short fibers of 4-6 mm.

[0018] To achieve the above-mentioned purpose of the invention, the present invention also provides a hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel, which is prepared by the preparation method of the hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel described in any one of the above.

[0019] The beneficial effects of the present invention are:

[0020] (1) The preparation method of the hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel provided by the present invention is to first soak the aramid short fibers in an alkaline solution, and then - (hydroxyl ions) react to destroy the fiber's epidermis, allowing DMSO to enter the fiber and provide more reaction sites. Adding DMSO is beneficial to accelerate the formation of aramid nanofiber dispersion and make the molecular chains of the degraded aramid nanofibers more orderly and shorter.

[0021] Ultrapure water is then added to the aramid nanofiber dispersion, and a network-structured aramid nanofiber hydrogel is formed through the bonding between the different amide groups on the aramid nanofiber molecular chains, the organic solvent, and water. Two solvent exchanges are then performed using specific solvents, causing the solvent in the pores of the aramid nanofiber hydrogel network structure to dynamically exchange with the primary solvent and the secondary exchange solvent, respectively. The organic solvent is continuously exchanged out of the hydrogel's network structure. Simultaneously, during the two dynamic exchanges, the bonding and intertwining structures between the aramid molecular chains, the organic solvent, and the water continuously change, ultimately allowing an appropriate amount of tert-butyl alcohol, water, and a very small amount of DMSO to reside in the pores, forming a secondary exchange hydrogel with a special structure. Subsequently, aqueous polyurethane and perfluorooctyltrichlorosilane are sequentially added to the secondary exchange hydrogel, causing the amide groups on the aramid fiber molecular chains, the amino and ester groups in the polyurethane molecular chains, and the perfluorooctyltrichlorosilane to crosslink and entangle, resulting in a composite hydrogel with a special structure. Freeze-drying causes the orientation of the polyurethane molecules and perfluorooctyltrichlorosilane bonded to the aramid molecular chains to change, and the polyurethane molecules and perfluorooctyltrichlorosilane molecules on the surface of the aramid nanofibers are bonded and arranged more neatly, obtaining a hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel with a special structure.

[0022] (2) The preparation method of the hydrophobic and oleophobic antifouling aramid nanofiber aerogel provided by the present invention is to introduce a water-based polyurethane (WPU) with corrosion resistance, weather resistance, wear resistance and good elasticity and durability, and add perfluorooctyl trichlorosilane (PFOTS) to improve the affinity of the aerogel surface to water and oil, thereby achieving the effect of hydrophobic and oleophobic antifouling, and preparing an aramid nanofiber aerogel with high specific surface area, good pore structure, excellent mechanical strength, certain elastic modulus and good wear resistance; at the same time, the obtained aramid nanofiber aerogel has high water dispersibility, high hydrophobic and oleophobic antifouling, high temperature stability and heat insulation and self-cleaning function, and the self-cleaning ability can reduce the cleaning workload and use cost, save resources, and extend the service life in practical applications. The controllability of this method is good, and it gives the aramid material unique properties and application advantages, and expands its scope of use under specific environments.

[0023] (3) The method for preparing the hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel provided by the present invention not only retains the excellent properties of the aramid nanofiber aerogel but also imparts a certain elasticity to the aerogel material. Furthermore, the aerogel material exhibits excellent hydrophobic and oleophobic properties, thereby extending its service life and expanding the application range of the aramid nanofiber aerogel material in various fields. This method has the advantages of energy conservation and environmental protection and is a relatively effective preparation method. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1This is a scanning electron microscope image of the hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel prepared in Example 1 of the present invention; the scale bar in 1a is 200 μm; the scale bar in 1b is 100 μm.

[0025] Figure 2 a is a water contact angle diagram of the hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel prepared in Example 1 of the present invention; Figure 2 b is the oil contact angle diagram of the hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel prepared in Example 1 of the present invention.

[0026] Figure 3 This is an optical image of the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0027] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] It should also be noted here that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the solutions of the present invention are shown in the drawings, while other details that are not closely related to the present invention are omitted.

[0029] In addition, it should be noted that the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0030] The present invention provides a method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel, comprising the following steps:

[0031] S1. Preparation of aramid nanofiber dispersion:

[0032] First, a certain mass of potassium hydroxide is dissolved in ultrapure water (UP water) to obtain a potassium hydroxide solution.

[0033] Then the aramid fiber filaments are cut into short fibers of 4-6 mm to obtain aramid staple fibers.

[0034] The aramid staple fibers were immersed in an alkaline solution at a mass ratio of 3:(1.5-2.5) of potassium hydroxide solid to aramid staple fibers. 100 mL of DMSO was then added and mechanically stirred for 3-5 hours, ensuring that ultrapure water accounted for 3%-5% of the DMSO volume in the entire system. Mechanical stirring was performed until the solution turned dark red, fully dissolving the aramid staple fibers and obtaining an aramid nanofiber dispersion with a mass-volume fraction of 0.5%-1.5%. A mass-volume fraction of 0.5%-1.5% means that 0.5-1.5 g of aramid staple fibers were dissolved in every 100 mL of DMSO.

[0035] The basic repeating unit of the molecular chain of aramid fiber is -[-CO-C6H4-CONH-C6H4NH-]-. The amide group is connected at the para position of the benzene ring, and hydrogen bonds are formed between the carbonyl and amino groups on different amide groups. At the same time, there is π-π stacking between different benzene rings. Therefore, there is a strong intermolecular interaction force between the molecular chains of aramid fiber, making it difficult to obtain nano-level aramid fiber.

[0036] In this process, the aramid fiber filaments are first cut into 4-6mm staple fibers to make them easier to disperse in alkali solution and DMSO. On this basis, the aramid staple fibers are soaked in alkali solution for 15-25s, and the deprotonation effect of KOH is used to initially destroy the hydrogen bonds between the molecular chains of the aramid staple fibers, providing conditions for the degradation of macro fibers into nanofibers; then DMSO is added thereto, and the alkaline environment provided by KOH and the strong polar environment of DMSO are used to continuously destroy the hydrogen bonds between the molecular chains of the aramid staple fibers to form nanofibers, and then the nanofibers are dissolved in the KOH / DMSO system to obtain an aramid nanofiber dispersion. Specifically, after the aramid staple fibers are soaked in KOH solution, the aramid staple fibers and the high concentration of OH - (hydroxide ions) reacted, destroying the fiber's epidermis. The roughness of the fiber surface and the contact area with DMSO increased, providing more reaction sites for subsequent DMSO to enter the fiber interior, which is beneficial to accelerate the formation of aramid nanofiber dispersion.

[0037] The invention dissolves potassium hydroxide particles (KOH) in ultrapure water (UP), soaks the aramid fiber in a KOH solution of a certain concentration, and then adds DMSO, thereby greatly shortening the preparation cycle of the aramid nanofiber dispersion.

[0038] S2. Preparation of composite hydrogel:

[0039] Ultrapure water is added to the aramid nanofiber dispersion prepared in step S1 and stirred thoroughly to obtain an aramid nanofiber hydrogel. In this process, different amide groups on the aramid nanofiber molecular chains, the organic solvent, and water form a network structure through hydrogen bonding.

[0040] The aramid nanofiber hydrogel is then placed in ultrapure water for a single solvent exchange, which lasts 20-30 hours. During this process, the organic solvent and water in the aramid nanofiber hydrogel network are dynamically exchanged with the water used in the primary solvent exchange, and the organic solvent is continuously exchanged out of the hydrogel network. Simultaneously, during this dynamic exchange, the bonding and intertwining structures between the aramid molecular chains, the organic solvent, and the water undergo dynamic changes, forming a single-exchange hydrogel with a specific structure.

[0041] The solvent is then filtered, and the resulting primary exchange hydrogel is placed in a mixed solution of tert-butanol and ultrapure water at a volume ratio of 45-55%:45-55% for a secondary solvent exchange. The secondary solvent exchange lasts for 20-30 hours. During this process, tert-butanol molecules and water molecules continuously enter the pores of the primary exchange hydrogel's network structure, and the DMSO and water originally present in the pores continuously enter the secondary solvent. Tert-butanol, with its unique structure, is evenly bonded to the pores of the bonded and intertwined structures formed by different aramid molecular chains, ultimately allowing an appropriate amount of tert-butanol, water, and a very small amount of DMSO to exist in the pores, forming a secondary exchange hydrogel with a special structure.

[0042] The obtained secondary exchange hydrogel and secondary exchange solvent are then placed in a beater, and the secondary exchange hydrogel is broken up and evenly dispersed; the evenly dispersed hydrogel undergoes further solvent exchange during the beating process to make the solvent exchange more thorough, and the secondary exchange solvent is filtered out; an appropriate amount of waterborne polyurethane (WPU) is then added to the secondary exchange hydrogel and mechanically stirred to uniformly disperse the waterborne polyurethane on the surface and internal pores of the secondary exchange hydrogel. At the same time, cross-linking occurs between different waterborne polyurethane molecules, and the amide groups on the aramid fiber molecular chains are hydrogen-bonded with the amino and ester groups in the polyurethane molecular chains, thereby increasing the mechanical strength of the hydrogel. A hydrophobic agent is then added thereto and mechanically stirred further to uniformly disperse the hydrophobic agent on the surface and internal pores of the hydrogel to obtain a composite hydrogel. The hydrophobic agent is preferably perfluorooctyltrichlorosilane (PFOTS), which further interacts and cross-links with the amino and ester groups in the polyurethane molecular chains and the amide groups on the aramid fiber molecular chains to obtain a composite hydrogel with a special three-dimensional network structure.

[0043] The mass ratio of the aramid nanofiber dispersion, the aqueous polyurethane and the hydrophobic agent is 97-99%: 0.5-1.5%: 0.5-1.5%.

[0044] S3. Preparation of hydrophobic, oleophobic and antifouling aramid nanofiber aerogel:

[0045] The composite hydrogel prepared in step S2 is pre-frozen at (-20)-(-50)°C for 5-7 hours, initially formed, and then placed in a freeze dryer for freeze drying at a temperature of (-80)-(-45)°C and a vacuum degree of 15-50 Pa for 20-30 hours to obtain a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel. During this process, the solvent in the pores of the composite hydrogel is continuously evaporated, and at the same time, the polyurethane molecules and perfluorooctyltrichlorosilane bonded to the aramid molecular chains undergo orientation changes under low vacuum. The polyurethane molecules and perfluorooctyltrichlorosilane molecules on the surface and inside of the aramid nanofibers are bonded and arranged more neatly, obtaining a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel with a special structure.

[0046] The present invention also provides a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel, which is prepared by using the above-mentioned method for preparing the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel.

[0047] The present invention is described in detail below through specific examples.

[0048] Example 1

[0049] A method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel comprises the following steps:

[0050] S1. Preparation of aramid nanofiber dispersion:

[0051] First, 1.5g of potassium hydroxide was dissolved in 4mL of ultrapure water (UP water); then, the aramid fiber filaments were cut into 5mm staple fibers to obtain aramid staple fibers. The aramid staple fibers were soaked in the aforementioned alkaline solution for 20 seconds, with a mass ratio of potassium hydroxide solids to aramid staple fibers of 3:2. 100mL of DMSO was then added and mechanically stirred for 4 hours until the solution turned dark red, fully dissolving the aramid staple fibers. This yielded an aramid nanofiber dispersion with a mass-volume fraction of 1.0%.

[0052] S2. Preparation of composite hydrogel:

[0053] Ultrapure water is added to the aramid nanofiber dispersion prepared in step S1, and the mixture is stirred thoroughly to obtain an aramid nanofiber hydrogel.

[0054] The aramid nanofiber hydrogel was then placed in ultrapure water for a single solvent exchange, lasting 25 hours, to obtain a primary exchange hydrogel. The solvent was then filtered, and the resulting primary exchange hydrogel was placed in a 50%:50% by volume mixture of tert-butyl alcohol and ultrapure water for a second solvent exchange, lasting 25 hours, to obtain a secondary exchange hydrogel.

[0055] The obtained secondary exchange hydrogel and secondary exchange solvent are then placed in a beater, and the secondary exchange hydrogel is broken up and evenly dispersed; then an appropriate amount of water-based polyurethane (WPU) is added to the secondary exchange hydrogel and mechanically stirred; then perfluorooctyltrichlorosilane (PFOTS) is added thereto and further mechanically stirred to make the hydrophobic agent evenly dispersed on the surface of the hydrogel to obtain a composite hydrogel.

[0056] The mass ratio of the aramid nanofiber dispersion, the aqueous polyurethane and the hydrophobic agent is 98%:1%:1%.

[0057] S3. Preparation of hydrophobic, oleophobic and antifouling aramid nanofiber aerogel:

[0058] The composite hydrogel prepared in step S2 was pre-frozen at -30°C for 6 hours to be initially formed, and then placed in a freeze dryer for freeze drying at a temperature of -60°C and a vacuum degree of 20 Pa for 30 hours to obtain a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel.

[0059] Figure 1 This is a scanning electron microscope image of the hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel prepared in Example 1. Figure 1 a and Figure 1 b It can be seen that the pore distribution of the obtained hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel is extremely uniform, and the porosity is relatively high.

[0060] Examples 2-3 and Comparative Examples 1-2

[0061] A method for preparing a hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel is provided. Compared with Example 1, the difference lies in that in step S1, the mass-volume fraction (abbreviated as concentration) of the aramid nanofiber dispersion is different. The rest is substantially the same as Example 1 and will not be repeated here.

[0062] The hydrophobic, oleophobic, and antifouling aramid nanofiber aerogels prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to performance tests, and the results are shown in Table 1:

[0063] The compressive strength refers to the pressure applied when the obtained hydrophobic, oleophobic and antifouling aramid nanofiber aerogel is compressed to 30% deformation.

[0064] Elastic recovery refers to the recovery effect within elastic deformation.

[0065] Table 1 Relevant properties of aerogels prepared in Examples 1-3 and Comparative Examples 1-2

[0066]

[0067] As shown in Table 1, within the specified limits, along with the increase (embodiment 1,2,3) of aramid nanofiber dispersion concentration, the porosity of gained nanofiber aerogel presents the trend of reducing, and compressive strength increases gradually, and water contact angle, oil contact angle and elastic recovery rate all first increase and then reduce.This may be because along with the increase of aramid nanofiber dispersion concentration, the aramid nanofiber hydrogel structure obtained after adding ultrapure water becomes compact, and therefore the nanofiber aerogel structure finally obtained is relatively compact, so that porosity is reduced, and compressive strength increases gradually.Meanwhile, along with the difference of aramid nanofiber hydrogel structure, and then affect solvent exchange process, thereby make aramid fiber molecular chain and the mutual crosslinking degree of polyurethane molecular chain and perfluorooctyl trichlorosilane different, and then affect hydrophobicity, oleophobicity and the elastic property of gained nanofiber aerogel.The pore distribution of gained nanofiber aerogel is relatively uniform, and overall performance is better.

[0068] As the concentration of the aramid nanofiber dispersion further increases (Comparative Example 1), the porosity and elastic recovery rate decrease significantly, which may be due to the fact that the structure of the obtained nanofiber aerogel is too dense.

[0069] When the concentration of the aramid nanofiber dispersion is low (Comparative Example 2), the cross-linking structure between the aramid fiber molecular chain, the polyurethane molecular chain and perfluorooctyltrichlorosilane is significantly affected, thereby affecting various properties.

[0070] Examples 4-5 and Comparative Examples 3-4

[0071] A method for preparing a hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel is provided. Compared with Example 1, the difference lies in that, in step S2, the volume ratio of tert-butanol to ultrapure water in the secondary solvent exchange is different. The other aspects are substantially the same as those in Example 1 and are not described herein.

[0072] The hydrophobic, oleophobic, and antifouling aramid nanofiber aerogels prepared in Examples 4-5 and Comparative Examples 3-4 were subjected to performance tests. The results are shown in Table 2:

[0073] Table 2 Relevant properties of aerogels prepared in Examples 4-5 and Comparative Examples 3-4

[0074]

[0075] As shown in Table 2, within a certain range, as the volume ratio of tert-butyl alcohol to ultrapure water in the secondary solvent exchange changes (Examples 1, 4, and 5), the porosity, compressive strength, water contact angle, oil contact angle, and elastic recovery rate of the resulting nanofiber aerogels all change, but the overall performance is good, and the pore uniformity is good. This may be because the change in the volume ratio of tert-butyl alcohol to ultrapure water within a certain range has little effect on the dynamic process of solvent exchange, resulting in little structural change in the resulting nanofiber aerogels and better performance.

[0076] When the content of tert-butanol or ultrapure water in the secondary solvent exchange is large (Comparative Examples 3 and 4), the performance of the resulting nanofiber aerogel has a large deviation. This may be because, with the large change in the volume ratio of tert-butanol to ultrapure water, the exchange of tert-butanol molecules, water molecules and the solvent in the pores of the primary exchange hydrogel is greatly affected, thereby affecting the amount of tert-butanol and water contained in the pores of the secondary exchange hydrogel. When WPU and PFOTS are further compounded, the distribution of WPU and PFOTS on the surface and inside of the hydrogel is affected, thereby affecting various bonding structures and ultimately affecting the performance of the nanofiber aerogel.

[0077] Examples 6-7 and Comparative Examples 5-8

[0078] A method for preparing a hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel is provided. Compared with Example 1, the difference lies in that in step S2, the mass ratio of the aramid nanofiber dispersion, aqueous polyurethane, and hydrophobic agent is different. The rest is substantially the same as Example 1 and will not be repeated here.

[0079] The hydrophobic, oleophobic, and antifouling aramid nanofiber aerogels prepared in Examples 6-7 and Comparative Examples 5-8 were subjected to performance tests. The results are shown in Table 3:

[0080] Table 3 Relevant properties of aerogels prepared in Examples 6-7 and Comparative Examples 5-8

[0081]

[0082]

[0083] As shown in Table 3, within the specified limits, along with the increase of aramid nanofiber dispersion liquid quality and the minimizing (embodiment 1,6,7) of aqueous polyurethane and hydrophobic agent quality, the porosity and the compressive strength of gained nanofiber aerogel first increase and then decrease, and water contact angle, oil contact angle and elastic recovery rate present the trend of reducing, and pore distribution is comparatively uniform.This is mainly because along with the difference of aramid nanofiber dispersion liquid, aqueous polyurethane and hydrophobic agent mass ratio, after composite aqueous polyurethane and hydrophobic agent, gained hydrogel surface and internal aqueous polyurethane and hydrophobic agent distribution are different, and the cross-linked structures of the three are different simultaneously, so performance is different.But aramid nanofiber dispersion liquid, aqueous polyurethane and hydrophobic agent three mass ratios can not cause too big impact to the performance of gained nanofiber aerogel in the variation on a small scale.

[0084] When the mass of the aramid nanofiber dispersion is too large (Comparative Example 5), the mass of the aqueous polyurethane and the hydrophobic agent is very small, and the hydrophobicity, oleophobicity and elastic recovery rate of the obtained nanofiber aerogel are significantly reduced. This is mainly because the aqueous polyurethane and hydrophobic agent wrapped on the surface of the nanofiber aerogel are less.

[0085] When the mass of the aramid nanofiber dispersion is too small (Comparative Example 6), the mass of the aqueous polyurethane and the hydrophobic agent is relatively large, and the porosity and compressive strength of the obtained nanofiber aerogel are significantly reduced. Excessive aqueous polyurethane and hydrophobic agent enter the pores of the hydrogel, reducing its porosity; at the same time, the reduction in the aramid nanofiber content affects the compressive strength to a certain extent.

[0086] When neither aqueous polyurethane (Comparative Example 7) nor hydrophobic agent (Comparative Example 8) is added, various properties of the nanofiber aerogel are affected, indicating that there is a synergistic effect between the three.

[0087] Comparative Example 9

[0088] A method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel is disclosed. Compared with Example 1, the difference is that in step S1, when preparing the aramid nanofiber dispersion, the alkali solution and DMSO are first mixed, and then the aramid staple fibers are added thereto. The other aspects are substantially the same as in Example 1 and will not be repeated here. The porosity of the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel obtained in Comparative Example 9 is 85.3%, the pore uniformity is worse than that in Example 1, the compressive strength is 36Kpa, the water contact angle is 117.5°, the oil contact angle is 117.1°, and the elastic recovery rate is 77%, which is worse than that in Example 1. This may be because the alkali solution and DMSO are directly mixed, and the system affects the degradation process of the aramid fiber, so that there are differences in the structure of the nanofibers and the length of the molecular chains in the obtained aramid nanofiber dispersion, thereby affecting the intertwined structure between the aramid fiber, the polyurethane molecules, and the hydrophobic agent, and the network structure obtained by cross-linking, thereby affecting the performance of the obtained aerogel.

[0089] Comparative Example 10

[0090] A method for preparing a hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel is described. This method differs from Example 1 in that, in step S2, the secondary solvent exchange is performed with a mixed solvent of ethanol and ultrapure water. All other aspects are substantially the same as in Example 1 and are not further described here. The hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel obtained in Comparative Example 10 exhibited a porosity of 71.4%, with pore uniformity inferior to that of Example 1. The aerogel also exhibited a compressive strength of 32 kPa, a water contact angle of 110.5°, an oil contact angle of 111.0°, and an elastic recovery rate of 70%, exhibiting inferior performance compared to Example 1. This indicates that the unique structure of tert-butyl alcohol significantly influences solvent exchange.

[0091] Comparative Example 11

[0092] A method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel is disclosed. Compared with Example 1, the difference is that in step S2, the secondary solvent exchange is a mixed solvent of 2-methyl-2-pentanol and ultrapure water. The other details are substantially the same as those in Example 1 and will not be repeated here. The porosity of the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel obtained in Comparative Example 11 is 70.6%, the pore uniformity is worse than that in Example 1, the compressive strength is 30Kpa, the water contact angle is 106.4°, the oil contact angle is 105.8°, and the elastic recovery rate is 68%, which is worse than that in Example 1. This may be because the steric hindrance of 2-methyl-2-pentanol is relatively large, which is not conducive to the solvent exchange in the pores of the secondary exchange hydrogel. At the same time, the interaction between 2-methyl-2-pentanol and the porous structure is poor, which affects its performance.

[0093] Comparative Example 12

[0094] A method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel is described. Compared to Example 1, the difference is that in step S2, only one solvent exchange is performed directly with a mixed solvent of tert-butyl alcohol and ultrapure water. The other details are substantially the same as in Example 1 and are not described here. The porosity of the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel obtained in Comparative Example 12 is 65.4%, the pore uniformity is worse than that of Example 1, the compressive strength is 23 kPa, the water contact angle is 95.2°, the oil contact angle is 94.3°, and the elastic recovery rate is 52%, which is worse than that of Example 1. This shows that the dynamic solvent exchange process performed through multiple solvents makes the final nano aerogel have a better structure and thus better performance.

[0095] Comparative Example 13

[0096] A method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel is disclosed. Compared with Example 1, the difference is that in step S2, the polyurethane used is a non-aqueous polyurethane. The other aspects are substantially the same as in Example 1 and are not described in detail here. The hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel obtained in Comparative Example 13 has a porosity of 73.2%, a pore uniformity worse than that of Example 1, a compressive strength of 18 kPa, a water contact angle of 85.2°, an oil contact angle of 84.9°, and an elastic recovery rate of 48%, which are worse than those of Example 1. This indicates that the special structure of the aqueous polyurethane makes the cross-linking and entanglement of the aramid nanofiber dispersion, aqueous polyurethane, and hydrophobic agent more uniform and stable, resulting in better performance.

[0097] Comparative Example 14

[0098] A method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel is described. Compared with Example 1, the difference is that in step S2, perfluorooctyltrichlorosilane (PFOTS) is first added to the secondary exchange hydrogel, and then the waterborne polyurethane (WPU) is added to the secondary exchange hydrogel after stirring. The other methods are roughly the same as those in Example 1 and will not be repeated here. The porosity of the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel obtained in Comparative Example 14 is 86.3%, the pore uniformity is worse than that in Example 1, the compressive strength is 35Kpa, the water contact angle is 53.2°, the oil contact angle is 54.1°, the elastic recovery rate is 76%, and the performance is worse than that in Example 1. This shows that only by compounding the waterborne polyurethane and perfluorooctyltrichlorosilane in a specific order can the obtained structure be more excellent.

[0099] Comparative Example 15

[0100] A method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel is described. Compared with Example 1, the difference is that in step S2, waterborne polyurethane (WPU) and perfluorooctyltrichlorosilane (PFOTS) are directly added to the secondary exchange hydrogel at the same time. The other steps are roughly the same as in Example 1 and will not be repeated here. The porosity of the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel obtained in Comparative Example 15 is 87.4%, the pore uniformity is worse than that in Example 1, the compressive strength is 36Kpa, the water contact angle is 62.5°, the oil contact angle is 63.1°, and the elastic recovery rate is 78%, which is worse than that in Example 1. It further illustrates that only by compounding waterborne polyurethane and perfluorooctyltrichlorosilane in a specific order can the obtained structure be more excellent.

[0101] Comparative Example 16

[0102] A method for preparing a hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel is described. This method differs from Example 1 in that, in step S2, the hydrophobic agent used is perfluorodecyltrichlorosilane. All other aspects are substantially the same as in Example 1 and are not further described here. The hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel obtained in Comparative Example 16 has a porosity of 89.6%, with pore uniformity inferior to that of Example 1. It also has a compressive strength of 37 kPa, a water contact angle of 113.2°, an oil contact angle of 113.8°, and an elastic recovery rate of 80%, exhibiting performance inferior to that of Example 1.

[0103] Comparative Example 17

[0104] A method for preparing a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel is disclosed. Compared with Example 1, the difference is that in step S2, the hydrophobic agent used is perfluorohexyltrichlorosilane. The other aspects are substantially the same as in Example 1 and are not described in detail here. The porosity of the hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel obtained in Comparative Example 17 is 85.7%, the pore uniformity is worse than that of Example 1, the compressive strength is 34 kPa, the water contact angle is 102.6°, the oil contact angle is 101.8°, and the elastic recovery rate is 74%, which is worse than that of Example 1. Comparative Examples 16 and 17 illustrate that changing the length of the hydrophobic agent molecular chain affects the structure of the resulting nanofiber aerogel, thereby affecting its performance.

[0105] Comparative Example 18

[0106] A method for preparing a hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel is described. This method differs from Example 1 in that, in step S2, the polyurethane used is a non-aqueous polyurethane, which is dissolved in an organic solvent and mixed with an aramid nanofiber dispersion to prepare the hydrogel. The remaining steps are substantially the same as in Example 1 and are not further described here. The hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel obtained in Comparative Example 18 has a porosity of 63%, which is inferior to the pore uniformity of Example 1. The compressive strength is 35 kPa, the water contact angle is 98.2°, the oil contact angle is 97.3°, and the elastic recovery rate is 61%, which is inferior to the performance of Example 1.

[0107] In summary, the present invention provides a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel and a preparation method thereof. The method comprises the following steps: degrading aramid fibers by a specific process, preparing the obtained aramid nanofiber dispersion into a hydrogel, performing two solvent exchanges with a specific solvent, and then mixing the dispersion with aqueous polyurethane and perfluorooctyltrichlorosilane to prepare a composite hydrogel. The composite hydrogel is then freeze-dried, so that the cross-linking structure of the aramid fibers, polyurethane molecules, and perfluorooctyltrichlorosilane continuously changes. Ultimately, through the synergistic effect of the three, a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel with a special structure is obtained. The obtained aramid nanofiber aerogel has the advantages of high specific surface area, good pore structure, a certain elastic modulus, and good wear resistance.

[0108] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel, characterized in that: The steps include: S1. The aramid fiber was added to a potassium hydroxide solution and soaked for 15-25 seconds, and then DMSO was added thereto to obtain an aramid nanofiber dispersion having a predetermined concentration; the mass-volume fraction of the aramid nanofiber dispersion was 0.5%-1.5%; S2. Adding ultrapure water to the aramid nanofiber dispersion prepared in step S1 and stirring to obtain an aramid nanofiber hydrogel; first placing the aramid nanofiber hydrogel in ultrapure water for a primary solvent exchange, then placing it in a mixed solution of tert-butyl alcohol and ultrapure water for a secondary solvent exchange, then adding aqueous polyurethane to the hydrogel after solvent exchange, mechanically stirring, and then adding a hydrophobic agent, and mechanically stirring to obtain a composite hydrogel; the mass ratio of the aramid nanofiber dispersion, aqueous polyurethane, and hydrophobic agent is 97-99%:0.5-1.5%:0.5-1.5%; the hydrophobic agent is perfluorooctyltrichlorosilane; and the volume ratio of tert-butyl alcohol to ultrapure water in the mixed solution used in the secondary solvent exchange process is 45-55%:45-55%; S3. The composite hydrogel prepared in step S2 is pre-frozen at (-20)-(-50)°C, and finally placed in a freeze dryer for freeze drying to obtain a hydrophobic, oleophobic, and anti-fouling aramid nanofiber aerogel.

2. The method for preparing the hydrophobic, oleophobic and antifouling aramid nanofiber aerogel according to claim 1, characterized in that: In step S2, the duration of the first solvent exchange is 20-30 hours; the duration of the second solvent exchange is 20-30 hours.

3. The method for preparing the hydrophobic, oleophobic and antifouling aramid nanofiber aerogel according to claim 1, characterized in that: Specifically, step S1 comprises the following steps: first dissolving potassium hydroxide in ultrapure water to obtain a potassium hydroxide solution; soaking the aramid fiber in an alkaline solution for 15-25 seconds at a mass ratio of potassium hydroxide solid to aramid fiber of 3:(1.5-2.5); then adding 100 mL of DMSO and mechanically stirring for 3-5 hours, ensuring that ultrapure water in the entire system accounts for 3%-5% of the volume of DMSO, so that the aramid fiber is fully dissolved, and obtaining an aramid nanofiber dispersion with a mass-volume fraction of 0.5%-1.5%.

4. The method for preparing the hydrophobic, oleophobic and antifouling aramid nanofiber aerogel according to claim 1, characterized in that: In step S3, the freeze-drying time is 20-30 hours; the freeze-drying temperature is (-80)-(-45)°C, and the vacuum degree is 15-50Pa.

5. The method for preparing the hydrophobic, oleophobic and antifouling aramid nanofiber aerogel according to claim 3, characterized in that: The aramid fiber is obtained by cutting aramid fiber filaments into short fibers of 4-6 mm.

6. A hydrophobic, oleophobic, and antifouling aramid nanofiber aerogel, characterized in that: The hydrophobic, oleophobic and anti-fouling aramid nanofiber aerogel is prepared by the preparation method of any one of claims 1 to 5.

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