A method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids

Abstract

This invention discloses a method for preparing nano-sized ammoniated aramid by nitration followed by reduction in mixed acid, belonging to the fields of chemistry and materials technology. This invention uses waste aramid as raw material, utilizing the nitryl ions in mixed acid to convert aramid into nitrated aramid; then, using a relatively inexpensive tin dichloride catalyst under mild conditions, the nitro groups on the nitrated aramid are reduced to amino groups, successfully preparing ammoniated aramid. The method for preparing nano-sized ammoniated aramid by nitration followed by reduction in mixed acid has the following advantages: (1) simple operation, low cost, and conducive to industrial production; (2) low reaction temperature, mild experimental conditions, and sustainability; (3) fast reaction and short cycle; (4) convenient product collection, facilitating subsequent use.

Description

Technical Field

[0001] This invention relates to the fields of chemistry and materials technology, and in particular to a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids. Background Technology

[0002] Polymer nanofibers and their derivatives possess high porosity, large specific surface area, high aspect ratio, and unique nano-effects, making them promising candidates for applications in many emerging fields. Aramid nanofibers (ANFs), first prepared by Kotov and colleagues in 2011 through the deprotonation of macroscopic poly(p-terephthalic acid) (PPTA) fibers, are one-dimensional nanofibers with high strength modulus, high aspect ratio, high specific surface area, and good thermal and chemical stability. Considered a promising nanoscale building block, they have been widely used in composite reinforcement materials, electrical insulation materials, adsorption and filtration media, battery separators, flexible electrodes, and biological tissues. As a rigid rod polymer with excellent mechanical and thermal properties, they are primarily used as reinforcing agents for flexible polymers. However, rigid rod polymers are generally incompatible with flexible polymers.

[0003] Most reported methods involve incorporating aramid waste into flexible polymers using high-pressure homogenization. Therefore, phase separation is difficult to avoid when preparing these mixtures. Similar to nanomaterials requiring surface modification to achieve uniform dispersion in a polymer matrix, rigid rod macromolecules also require modification to prevent phase separation and achieve molecular-level dispersion within the matrix. Currently reported methods include synthetic and post-modification methods. The former has high production costs and cannot reuse waste aramid fibers; the latter involves grafting functional molecules onto macroscopic fibers through chemical or physical reactions, which is difficult to achieve at the nanoscale and exhibits poor stability. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing nano-sized ammoniated aramid by nitration followed by reduction using mixed acid. This method utilizes a mixed acid of concentrated sulfuric acid and concentrated nitric acid to nitrate waste aramid and then gently reduce it to obtain nano-sized ammoniated aramid with a high aspect ratio. This method has the advantages of low cost, simple operation, short synthesis cycle, and ease of large-scale industrial production.

[0005] To achieve the above objectives, this invention provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically comprising the following steps:

[0006] (1) Dispersion: The waste aramid washed with acetone was placed in a round-bottom flask, and concentrated sulfuric acid that had been pre-chilled was added under ice-water bath conditions to disperse the waste aramid, and finally a light yellow transparent dispersion was obtained.

[0007] (2) Nitration: Add a mixed acid solution of concentrated nitric acid and concentrated sulfuric acid to the dispersion obtained in step (1), stir the reaction under ice bath conditions to obtain a bright orange solution, continue to pour the bright orange solution into ice distilled water under ice bath conditions and stir to obtain an orange heterogeneous solution, vacuum filter and wash until the pH of the washing solution is 6.5-7.5, freeze dry to obtain nitrated aramid.

[0008] (3) Reduction: The nitrated aramid obtained in step (2) was ultrasonically dispersed using anhydrous ethanol, and then a concentrated hydrochloric acid solution of tin dichloride was added. The mixture was preheated under oil bath conditions, and then the temperature was raised to carry out the reduction reaction. After the reaction was completed, the mixture was transferred to anhydrous ethanol and stirred. After vacuum filtration, the mixture was washed until the filtrate was neutral. After vacuum drying of the filter cake, nano-sized nitrated aramid was obtained.

[0009] Preferably, in step (1), the refrigeration conditions for concentrated sulfuric acid are 2-4℃ and the refrigeration time is 15min.

[0010] Preferably, in step (1), the mass of the waste aramid is 2-3g; the volume of concentrated sulfuric acid is 50-70ml.

[0011] Preferably, in step (2), the mass fraction of concentrated nitric acid in the mixed acid solution is 5-20 wt%, and the mass fraction of concentrated sulfuric acid is 80-95%.

[0012] Preferably, in step (2), the mass fraction of concentrated nitric acid and concentrated sulfuric acid in the mixed acid solution is one of 5wt% concentrated nitric acid + 95wt% concentrated sulfuric acid, 10wt% concentrated nitric acid + 90wt% concentrated sulfuric acid, 15wt% concentrated nitric acid + 85wt% concentrated sulfuric acid, or 20wt% concentrated nitric acid + 80wt% concentrated sulfuric acid.

[0013] Preferably, in step (3), the volume of concentrated hydrochloric acid is 1-10 mL; and the mass of tin dichloride is 4-9 g.

[0014] Preferably, in step (3), the volume of concentrated hydrochloric acid is one of 4 ml, 6 ml, or 10 ml; and the mass of tin dichloride is one of 4 g, 6 g, 8 g, or 9 g.

[0015] Preferably, in step (3), the oil bath temperature is 55-60℃, the preheating time is 30min, the temperature is raised to 60-65℃, and the reduction reaction time is 2-4h.

[0016] Preferably, in step (3), the specific operation of washing after vacuum filtration is as follows: stir in anhydrous ethanol for 10 minutes and then perform vacuum filtration. Then, redisperse the obtained filter cake in anhydrous ethanol for washing and vacuum filtration. Repeat this process three times and then use distilled water for washing and vacuum filtration until the filtrate is neutral.

[0017] Therefore, the method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids provided by the present invention has the following beneficial effects:

[0018] (1) This invention successfully converts waste aramid fibers into ammoniated aramid by mixed acid nitration and tin dichloride reduction, which significantly improves their dispersibility in solvents and is also beneficial for incorporation into other flexible polymers. The resulting composite material has better mechanical properties and processability.

[0019] (2) The preparation method provided by the present invention is simple to operate, low in cost, and conducive to industrial production; at the same time, the reaction temperature is low, the experimental conditions are mild, it is sustainable, and the reaction speed is fast and the production cycle is short.

[0020] (3) The high aspect ratio nano-sized ammoniated aramid obtained by the present invention is easy to collect and is beneficial for subsequent use.

[0021] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0022] Figure 1 The following is a flowchart of the method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction in mixed acids according to the present invention:

[0023] Figure 2 These are TEM images of different regions of aramid, nitroaramid, and amination aramid in Example 5 of the present invention; wherein, a1-a2 are aramid; b1-b2 are nitroaramid; and c1-c2 are amination aramid.

[0024] Figure 3 These are FIR comparison images of aramid, nitroaramid, and ammoniated aramid in Example 5 of the present invention; where a is a comparison image over a wider wavenumber range; and b is a comparison image of details at low wavenumbers. Detailed Implementation

[0025] like Figure 1 As shown, this invention provides a method for preparing nano-sized ammoniated aramid by nitration followed by reduction in mixed acid. Using waste aramid as raw material, the aramid is converted into nitrated aramid by utilizing the nitryl cation in the mixed acid. Then, the nitro groups on the nitrated aramid are reduced to amino groups under mild conditions using a relatively inexpensive tin dichloride catalyst. The successful preparation of ammoniated aramid is demonstrated by characterization methods such as infrared spectroscopy.

[0026] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims and are all within the protection scope of the present invention.

[0027] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0028] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0029] Unless otherwise specified, the reagents, instruments, equipment, and performance testing methods used in this invention are all commonly used by those skilled in the art.

[0030] Example 1

[0031] This embodiment provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically including the following steps:

[0032] (1) Aramid dispersion process: Refrigerate concentrated sulfuric acid at 2-4℃ for 15 min. Before use, aramid needs to be soaked in acetone and then air-dried naturally. Take 2g of acetone-treated aramid, add it to 50ml of pre-refrigerated concentrated sulfuric acid and stir mechanically (the entire dispersion process is carried out in an ice-water bath), and finally obtain a light yellow transparent dispersion.

[0033] (2) Aramid nitration process: 20g of mixed acid (the ratio of concentrated nitric acid to concentrated sulfuric acid = 5wt% to 95wt%) was slowly added dropwise to the aramid dispersion in (1). The entire experiment was carried out in an ice bath with mechanical stirring. The reaction was rapid, and a bright orange solution was finally obtained. Then, the solution was slowly poured into ice-distilled water (this experiment was carried out in an ice bath) and stirred continuously to obtain an orange heterogeneous solution. The solution was then vacuum filtered, and the filter cake was repeatedly washed with distilled water. The filtration continued until the pH of the washing liquid in the filtration flask was 6.5. Finally, the filter cake was freeze-dried to obtain nitrated aramid.

[0034] (3) Reductive amination process: First, the nitrated aramid product from (2) was ultrasonically dispersed in 30 ml of anhydrous ethanol. Then, 4 g of tin dichloride was dissolved in 4 ml of concentrated hydrochloric acid. The concentrated hydrochloric acid solution of tin dichloride was then poured into a three-necked flask containing the ethanol dispersion of nitrated aramid. The mixture was then preheated in an oil bath at 55°C for 30 min, and then heated to 60°C for a reduction reaction for 2 hours. After the reaction was completed, the entire reaction mixture was poured into anhydrous ethanol and stirred for 10 min. The mixture was then vacuum filtered. The resulting filter cake was then redispersed in anhydrous ethanol for washing and vacuum filtration. This process was repeated about three times. The filter cake was then washed with distilled water and vacuum dried until the filtrate was neutral. Finally, the filter cake was vacuum dried to obtain the ammoniated aramid.

[0035] Example 2

[0036] This embodiment provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically including the following steps:

[0037] (1) Aramid dispersion process: Refrigerate concentrated sulfuric acid at 2-4℃ for 15 min. Before use, aramid needs to be soaked in acetone and then air-dried naturally. Take 2g of acetone-treated aramid, add it to 50ml of pre-refrigerated concentrated sulfuric acid and stir mechanically (the entire dispersion process is carried out in an ice-water bath), and finally obtain a light yellow transparent dispersion.

[0038] (2) Aramid nitration process: 15g of mixed acid (the ratio of concentrated nitric acid to concentrated sulfuric acid = 10wt% to 90wt%) was slowly added dropwise to the aramid dispersion in (1). The entire experiment was carried out in an ice bath with mechanical stirring. The reaction was rapid, and a bright orange solution was finally obtained. Then, the solution was slowly poured into ice-distilled water (this experiment was carried out in an ice bath) and stirred continuously to obtain an orange heterogeneous solution. The solution was then vacuum filtered, and the filter cake was repeatedly washed with distilled water and filtered until the pH of the washing liquid in the filtration flask was 6.5. Finally, the filter cake was freeze-dried to obtain nitrated aramid.

[0039] (3) Reductive amination process: First, the nitrated aramid product from (2) was ultrasonically dispersed in 30 ml of anhydrous ethanol. Then, 4 g of tin dichloride was dissolved in 4 ml of concentrated hydrochloric acid. The concentrated hydrochloric acid solution of tin dichloride was then poured into a three-necked flask containing the ethanol dispersion of nitrated aramid. The mixture was then preheated in an oil bath at 55°C for 30 min, and then heated to 60°C for a reduction reaction for 2 hours. After the reaction was completed, the entire reaction mixture was poured into anhydrous ethanol and stirred for 10 min. The mixture was then vacuum filtered. The resulting filter cake was then redispersed in anhydrous ethanol for washing and vacuum filtration. This process was repeated about three times. The filter cake was then washed with distilled water and vacuum dried until the filtrate was neutral. Finally, the filter cake was vacuum dried to obtain the ammoniated aramid.

[0040] Example 3

[0041] This embodiment provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically including the following steps:

[0042] (1) Aramid dispersion process: Refrigerate concentrated sulfuric acid at 2-4℃ for 15 min. Before use, aramid needs to be soaked in acetone and then air-dried naturally. Take 2g of acetone-treated aramid, add it to 50ml of pre-refrigerated concentrated sulfuric acid and stir mechanically (the entire dispersion process is carried out in an ice-water bath), and finally obtain a light yellow transparent dispersion.

[0043] (2) Aramid nitration process: 9g of mixed acid (the ratio of concentrated nitric acid to concentrated sulfuric acid = 15wt% to 85wt%) was slowly added dropwise to the aramid dispersion in (1). The entire experiment was carried out in an ice bath with mechanical stirring. The reaction was rapid, and a bright orange solution was finally obtained. Then, the solution was slowly poured into ice-distilled water (this experiment was carried out in an ice bath) and stirred continuously to obtain an orange heterogeneous solution. The solution was then vacuum filtered, and the filter cake was repeatedly washed with distilled water and filtered until the pH of the washing liquid in the filtration flask was 7. Finally, the filter cake was freeze-dried to obtain nitrated aramid.

[0044] (3) Reductive amination process: First, the nitrated aramid product from (2) was ultrasonically dispersed in 30 ml of anhydrous ethanol. Then, 4 g of tin dichloride was dissolved in 4 ml of concentrated hydrochloric acid. The concentrated hydrochloric acid solution of tin dichloride was then poured into a three-necked flask containing the ethanol dispersion of nitrated aramid. The mixture was then preheated in an oil bath at 55°C for 30 min, and then heated to 60°C for a reduction reaction for 2 hours. After the reaction was completed, the entire reaction mixture was poured into anhydrous ethanol and stirred for 10 min. The mixture was then vacuum filtered. The resulting filter cake was then redispersed in anhydrous ethanol for washing and vacuum filtration. This process was repeated about three times. The filter cake was then washed with distilled water and vacuum dried until the filtrate was neutral. Finally, the filter cake was vacuum dried to obtain the ammoniated aramid.

[0045] Example 4

[0046] This embodiment provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically including the following steps:

[0047] (1) Aramid dispersion process: Refrigerate concentrated sulfuric acid at 2-4℃ for 15 min. Before use, aramid needs to be soaked in acetone and then air-dried naturally. Take 2g of acetone-treated aramid, add it to 50ml of pre-refrigerated concentrated sulfuric acid and stir mechanically (the entire dispersion process is carried out in an ice-water bath), and finally obtain a light yellow transparent dispersion.

[0048] (2) Aramid nitration process: 6g of mixed acid (the ratio of concentrated nitric acid to concentrated sulfuric acid = 20wt% to 80wt%) was slowly added dropwise to the aramid dispersion in (1). The entire experiment was carried out in an ice bath with mechanical stirring. The reaction was rapid, and a bright orange solution was finally obtained. Then, the solution was slowly poured into ice-distilled water (this experiment was carried out in an ice bath) and stirred continuously to obtain an orange heterogeneous solution. The solution was then vacuum filtered, and the filter cake was repeatedly washed with distilled water and filtered until the pH of the washing liquid in the filtration flask was 7. Finally, the filter cake was freeze-dried to obtain nitrated aramid.

[0049] (3) Reductive amination process: First, the nitrated aramid product from (2) was ultrasonically dispersed in 30 ml of anhydrous ethanol. Then, 4 g of tin dichloride was dissolved in 4 ml of concentrated hydrochloric acid. The concentrated hydrochloric acid solution of tin dichloride was then poured into a three-necked flask containing the ethanol dispersion of nitrated aramid. The mixture was then preheated in an oil bath at 55°C for 30 min, and then heated to 60°C for a reduction reaction for 2 hours. After the reaction was completed, the entire reaction mixture was poured into anhydrous ethanol and stirred for 10 min. The mixture was then vacuum filtered. The resulting filter cake was then redispersed in anhydrous ethanol for washing and vacuum filtration. This process was repeated about three times. The filter cake was then washed with distilled water and vacuum dried until the filtrate was neutral. Finally, the filter cake was vacuum dried to obtain the ammoniated aramid.

[0050] Example 5

[0051] This embodiment provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically including the following steps:

[0052] (1) Aramid dispersion process: Refrigerate concentrated sulfuric acid at 2-4℃ for 15 min. Before use, aramid needs to be soaked in acetone and then air-dried naturally. Take 3g of acetone-treated aramid, add it to 50ml of pre-refrigerated concentrated sulfuric acid and stir mechanically (the entire dispersion process is carried out in an ice-water bath), and finally obtain a light yellow transparent dispersion.

[0053] (2) Aramid nitration process: 12g of mixed acid (the ratio of concentrated nitric acid to concentrated sulfuric acid = 15wt% to 85wt%) was slowly added dropwise to the aramid dispersion in (1). The entire experiment was carried out in an ice bath with mechanical stirring. The reaction was rapid, and a bright orange solution was finally obtained. Then, the solution was slowly poured into ice-distilled water (this experiment was carried out in an ice bath) and stirred continuously to obtain an orange heterogeneous solution. The solution was then vacuum filtered, and the filter cake was repeatedly washed with distilled water and filtered until the pH of the washing liquid in the filtration flask was 7. Finally, the filter cake was freeze-dried to obtain nitrated aramid.

[0054] (3) Reductive amination process: First, the nitrated aramid product from (2) was ultrasonically dispersed in 30 ml of anhydrous ethanol. Then, 6 g of tin dichloride was dissolved in 6 ml of concentrated hydrochloric acid. The concentrated hydrochloric acid solution of tin dichloride was then poured into a three-necked flask containing the ethanol dispersion of nitrated aramid. The mixture was then preheated in an oil bath at 60°C for 30 min, and then heated to 65°C for a reduction reaction for 4 hours. After the reaction was completed, the entire reaction mixture was poured into anhydrous ethanol and stirred for 10 min. The mixture was then vacuum filtered. The resulting filter cake was then redispersed in anhydrous ethanol for washing and vacuum filtration. This process was repeated about three times. Afterward, the mixture was washed with distilled water and vacuum dried until the filtrate was neutral. The filter cake was then vacuum dried to obtain the ammoniated aramid.

[0055] Example 6

[0056] This embodiment provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically including the following steps:

[0057] (1) Aramid dispersion process: Refrigerate concentrated sulfuric acid at 2-4℃ for 15 min. Before use, aramid needs to be soaked in acetone and then air-dried naturally. Take 3g of acetone-treated aramid, add it to 70ml of pre-refrigerated concentrated sulfuric acid and stir mechanically (the entire dispersion process is carried out in an ice-water bath), and finally obtain a light yellow transparent dispersion.

[0058] (2) Aramid nitration process: 9g of mixed acid (the ratio of concentrated nitric acid to concentrated sulfuric acid = 20wt% to 80wt%) was slowly added dropwise to the aramid dispersion in (1). The entire experiment was carried out in an ice bath with mechanical stirring. The reaction was rapid, and a bright orange solution was finally obtained. Then, the solution was slowly poured into ice-distilled water (this experiment was carried out in an ice bath) and stirred continuously to obtain an orange heterogeneous solution. The solution was then vacuum filtered, and the filter cake was repeatedly washed with distilled water and filtered until the pH of the washing liquid in the filtration flask was 7. Finally, the filter cake was freeze-dried to obtain nitrated aramid.

[0059] (3) Reductive amination process: First, the nitrated aramid product from (2) was ultrasonically dispersed in 50 ml of anhydrous ethanol. Then, 8 g of tin dichloride was dissolved in 10 ml of concentrated hydrochloric acid. The concentrated hydrochloric acid solution of tin dichloride was then poured into a three-necked flask containing the ethanol dispersion of nitrated aramid. The mixture was then preheated in an oil bath at 60°C for 30 min, and then heated to 65°C for a reduction reaction for 4 hours. After the reaction was completed, the entire reaction mixture was poured into anhydrous ethanol and stirred for 10 min. The mixture was then vacuum filtered. The resulting filter cake was then redispersed in anhydrous ethanol for washing and vacuum filtration. This process was repeated about three times. The filter cake was then washed with distilled water and vacuum dried until the filtrate was neutral. Finally, the filter cake was vacuum dried to obtain the ammoniated aramid.

[0060] Example 7

[0061] This embodiment provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically including the following steps:

[0062] (1) Aramid dispersion process: Refrigerate concentrated sulfuric acid at 2-4℃ for 15 min. Before use, aramid needs to be soaked in acetone and then air-dried naturally. Take 3g of acetone-treated aramid, add it to 70ml of pre-refrigerated concentrated sulfuric acid and stir mechanically (the entire dispersion process is carried out in an ice-water bath), and finally obtain a light yellow transparent dispersion.

[0063] (2) Aramid nitration process: 15g of mixed acid (the ratio of concentrated nitric acid to concentrated sulfuric acid = 10wt% to 90wt%) was slowly added dropwise to the aramid dispersion in (1). The entire experiment was carried out in an ice bath with mechanical stirring. The reaction was rapid, and a bright orange solution was finally obtained. Then, the solution was slowly poured into ice-distilled water (this experiment was carried out in an ice bath) and stirred continuously to obtain an orange heterogeneous solution. The solution was then vacuum filtered, and the filter cake was repeatedly washed with distilled water and filtered until the pH of the washing liquid in the filtration flask was 7. Finally, the filter cake was freeze-dried to obtain nitrated aramid.

[0064] (3) Reductive amination process: First, the nitrated aramid product from (2) was ultrasonically dispersed in 30 ml of anhydrous ethanol. Then, 9 g of tin dichloride was dissolved in 10 ml of concentrated hydrochloric acid. The concentrated hydrochloric acid solution of tin dichloride was then poured into a three-necked flask containing the ethanol dispersion of nitrated aramid. The mixture was then preheated in an oil bath at 60°C for 30 min, and then heated to 65°C for a reduction reaction for 4 hours. After the reaction was completed, the entire reaction mixture was poured into anhydrous ethanol and stirred for 10 min. The mixture was then vacuum filtered. The resulting filter cake was then redispersed in anhydrous ethanol for washing and vacuum filtration. This process was repeated about three times. The filter cake was then washed with distilled water and vacuum dried until the filtrate was neutral. Finally, the filter cake was vacuum dried to obtain the ammoniated aramid.

[0065] Example 8

[0066] This embodiment provides a method for preparing nano-sized ammoniated aramid fibers by nitration followed by reduction with mixed acids, specifically including the following steps:

[0067] (1) Aramid dispersion process: Refrigerate concentrated sulfuric acid at 2-4℃ for 15 min. Before use, aramid needs to be soaked in acetone and then air-dried naturally. Take 3g of acetone-treated aramid, add it to 70ml of pre-refrigerated concentrated sulfuric acid and stir mechanically (the entire dispersion process is carried out in an ice-water bath), and finally obtain a light yellow transparent dispersion.

[0068] (2) Aramid nitration process: 12g of mixed acid (the ratio of concentrated nitric acid to concentrated sulfuric acid = 15wt% to 85wt%) was slowly added dropwise to the aramid dispersion in (1). The entire experiment was carried out in an ice bath with mechanical stirring. The reaction was rapid, and a bright orange solution was finally obtained. Then, the solution was slowly poured into ice-distilled water (this experiment was carried out in an ice bath) and stirred continuously to obtain an orange heterogeneous solution. The solution was then vacuum filtered, and the filter cake was repeatedly washed with distilled water and filtered until the pH of the washing liquid in the filtration flask was 7. Finally, the filter cake was freeze-dried to obtain nitrated aramid.

[0069] (3) Reductive amination process: First, the nitrated aramid product from (2) was ultrasonically dispersed in 30 ml of anhydrous ethanol. Then, 9 g of tin dichloride was dissolved in 10 ml of concentrated hydrochloric acid. The concentrated hydrochloric acid solution of tin dichloride was then poured into a three-necked flask containing the ethanol dispersion of nitrated aramid. The mixture was then preheated in an oil bath at 60°C for 30 min, and then heated to 65°C for a reduction reaction for 4 hours. After the reaction was completed, the entire reaction mixture was poured into anhydrous ethanol and stirred for 10 min. The mixture was then vacuum filtered. The resulting filter cake was then redispersed in anhydrous ethanol for washing and vacuum filtration. This process was repeated about three times. The filter cake was then washed with distilled water and vacuum dried until the filtrate was neutral. Finally, the filter cake was vacuum dried to obtain the ammoniated aramid.

[0070] Comparative Example

[0071] Existing methods for preparing ammoniated aramid involve organic synthesis. Specifically, under nitrogen protection and in an ice-water bath environment, nitro-terephthalic acid and 2-nitro-1,4-phenylenediamine are used to synthesize nitro-aramid in a multi-solvent system of pyridine and dimethylacetamide under the synergistic catalysis of triphenylphosphine and hexachloroethane. Then, palladium on carbon is used as a catalyst and hydrogen as a reducing agent to reduce nitro-aramid to ammoniated aramid in an anhydrous dimethylacetamide solvent system, and the reduction reaction time is at least 24 hours.

[0072] Compared with the above preparation methods, the present invention realizes the sustainable utilization of waste aramid fibers, has low production costs, does not require multiple catalysts in the synthesis process, has a less harsh synthesis environment, does not require precious metal catalysts during reduction, does not require high temperatures, has a stable and non-flammable reducing agent, has a short reduction time, is safe and simple to operate in experiments, and is suitable for industrial production.

[0073] Figure 2 The images show TEM images of waste aramid, nitrated aramid, and amination aramid from Example 5. As can be seen from the images, neither the nitration nor the reductive amination process damaged the nanofiber structure of the aramid. The TEM images of the products obtained in other examples are largely the same as those in Example 5.

[0074] Figure 3 The image shows a comparison of FIR spectra of waste aramid, nitrated aramid, and ammoniated aramid in Example 5. As can be seen from the image, the reduced aramid clearly shows a double peak of stretching vibration at 3500-3300. In the 1600-1700 band, due to the nitration of aramid, the carbonyl group on the amide bond that originally peaked at 1644 was replaced by an electron-withdrawing nitro group on the benzene ring, causing the carbonyl group to red shift to 1683. After the nitro group was reduced, the electron-withdrawing group became an electron-donating group, and the carbonyl group blue shifted to 1639, which is slightly blue shifted compared to the original 1644. The reason why it is not a sharp peak is that the amino group grafted on the benzene ring can form P-π conjugate vibration with the benzene ring, resulting in a strong NH bending vibration peak. In the 1600-1450 nm band, there is a palm-shaped vibration peak of the benzene ring. Due to the nitration of the benzene ring, the bending vibration peak of the nitro group at 1540 nm is significantly enhanced. When the nitro group is reduced to an amino group, the bending vibration peak of the nitro group disappears. Due to the P-π conjugation vibration of the amino group and the benzene ring, the vibration peak of the benzene ring is significantly enhanced. At 1310 nm, there is a CN vibration peak on the aramid bond. When the aramid is nitrated, the CN vibration peak of the N in the nitro group and the C on the benzene ring is shown at 1340 nm. When the nitro group is reduced, the CN vibration peak formed by the N on the amino group and the C on the benzene ring is clearly shown at 1278 nm. The FIR spectra of the products obtained in other examples are roughly the same as those in Example 5.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing nanoscale aminofibers by mixed acid nitration followed by reduction, characterized in that, Specifically comprising the following steps: (1) dispersion: the waste aramid fiber washed with acetone is placed in a round-bottom flask, and pre-chilled concentrated sulfuric acid is added under ice water bath condition to disperse the waste aramid fiber, and finally a light yellow transparent dispersion liquid is obtained; (2) nitration: a mixed acid solution of concentrated nitric acid and concentrated sulfuric acid is added to the dispersion liquid obtained in step (1), and the bright orange solution is stirred under ice bath condition to obtain an orange heterogeneous solution, which is washed to a pH of 6.5-7.5 after vacuum filtration, and then freeze-dried to obtain a nitration aramid fiber; (3) reduction: the nitration aramid fiber obtained in step (2) is ultrasonically dispersed using anhydrous ethanol, and then a concentrated hydrochloric acid solution of tin dichloride is added, preheated under oil bath condition, and then heated for reduction reaction. After the reaction is completed, it is transferred to anhydrous ethanol for stirring, and then washed to neutral after vacuum filtration, and vacuum dried to obtain nano-sized ammoniated aramid fiber; In step (1), the concentrated sulfuric acid is chilled at 2-4℃ for 15 min; In step (2), the mass fraction of concentrated nitric acid in the mixed acid solution is 5-20wt%, and the mass fraction of concentrated sulfuric acid is 80-95%; In step (3), the oil bath temperature is 55-60℃, the preheating time is 30 min, the temperature is raised to 60-65℃, and the reduction reaction time is 2-4h.

2. The method for preparing nanoscale aminated aramid fiber by nitration with mixed acid and then reduction according to claim 1, characterized in that: In step (1), the mass of waste aramid fiber is 2-3g, and the volume of concentrated sulfuric acid is 50-70ml.

3. The method for preparing nanoscale aminated aramid fiber by nitration with mixed acid and then reduction according to claim 1, characterized in that: In step (2), the mass fraction of concentrated nitric acid and concentrated sulfuric acid in the mixed acid solution is one of 5wt% concentrated nitric acid + 95wt% concentrated sulfuric acid, 10wt% concentrated nitric acid + 90wt% concentrated sulfuric acid, 15wt% concentrated nitric acid + 85wt% concentrated sulfuric acid, and 20wt% concentrated nitric acid + 80wt% concentrated sulfuric acid.

4. The method for preparing nanoscale aminated aramid fiber by nitration with mixed acid and then reduction according to claim 1, characterized in that: In step (3), the volume of concentrated hydrochloric acid is 1-10mL, and the mass of tin dichloride is 4-9g.

5. The method for preparing nanoscale aminated aramid fiber by nitration with mixed acid and then reduction according to claim 1, characterized in that: In step (3), the volume of concentrated hydrochloric acid is one of 4ml, 6ml and 10ml, and the mass of tin dichloride is one of 4g, 6g, 8g and 9g.

6. The method for preparing nanoscale aminated aramid fiber by nitration with mixed acid and then reduction according to claim 1, characterized in that: In step (3), the specific operation of washing after vacuum filtration is as follows: after stirring in anhydrous ethanol for 10 min, vacuum filtration is carried out, then the obtained filter cake is dispersed in anhydrous ethanol for washing, vacuum filtration is carried out again, and after repeating three times, distilled water is used for washing and vacuum filtration until the filtrate is neutral.

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