A method for preparing powdered aramid nanofiber filler

Abstract

The application belongs to the field of polymer nanomaterials, and particularly relates to a preparation method of a powdery aramid nanofiber filler, which comprises the following steps: cutting aramid filaments into aramid fibers, and performing drying treatment to obtain dried aramid fibers; mixing concentrated sulfuric acid, phosphoric acid, hydrogen peroxide, a surface modifier and deionized water to prepare a shearing reaction solution; adding the dried aramid fibers into the shearing reaction solution to perform chemical cutting treatment, and obtaining a mixed emulsion; pouring the mixed emulsion into ice water to perform dilution, and adjusting a pH value; after filtration and washing, neutral wet-state aramid microfibers are obtained; performing wet ball milling and ultrasonic treatment on the wet-state aramid microfibers to obtain an aramid nanofiber dispersion solution; and performing freeze-drying treatment on the aramid nanofiber dispersion solution to obtain the powdery aramid nanofiber filler. The application can improve process efficiency, ensure good dispersibility and structural integrity of a product, and effectively maintain the microstructure stability of aramid fibers after nanocrystallization.

Description

Technical Field

[0001] This invention belongs to the field of polymer nanomaterials, specifically relating to a method for preparing powdered aramid nanofiber filler. Background Technology

[0002] Aramid fibers are a class of high-performance aromatic polyamide fibers with excellent mechanical properties and chemical stability, mainly including para-aramid and meta-aramid. Their molecular chains consist of highly oriented rigid benzene rings and amide groups, exhibiting high crystallinity, high melting point, and strong intermolecular hydrogen bonding. These structural features endow aramid with excellent tensile strength, high-temperature resistance, and corrosion resistance, making them widely used in electronic devices and filter materials. However, precisely because of the strong interactions between their molecular chains and extremely low solubility, aramid is difficult to achieve uniform dissociation and dispersion at the nanoscale during processing, severely limiting its further application in high-performance nanocomposites.

[0003] Currently, research on aramid nanofibers mainly focuses on mechanical and chemical methods. While mechanical methods are simple to operate, they often struggle to effectively break the strong hydrogen bond network between aramid molecules, making it difficult to uniformly refine the fibers to the nanoscale. Chemical methods, although they can enhance dispersibility by breaking hydrogen bonds with strong acids or oxidants or introducing hydrophilic groups, often suffer from harsh reaction conditions, numerous byproducts, and significant environmental pollution. Furthermore, improper handling can easily lead to backbone degradation, reducing the intrinsic properties of the fiber. Therefore, existing technologies still face several bottlenecks in the preparation of aramid nanofibers: on the one hand, it is difficult to achieve efficient dissociation without damaging the molecular structure; on the other hand, the resulting nanofibers exhibit poor dispersibility and insufficient stability, easily agglomerating, thus restricting their widespread application in high-end functional materials. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing powdered aramid nanofiber filler. This method achieves selective destruction of the dense crystalline structure of aramid fibers by preparing a shear reaction liquid with a strong acid oxidation system and a surface modifier, thereby improving the overall process efficiency and ensuring good dispersibility and structural integrity of the product. The mixed emulsion after the shear reaction is treated by ice water quenching, which effectively maintains the stability of the microstructure of the nano-sized aramid fibers and prevents irreversible collapse or aggregation.

[0005] Technical solution to achieve the purpose of this invention:

[0006] A method for preparing powdered aramid nanofiber filler, the method comprising:

[0007] S1: Cut the aramid filament into aramid fibers, and dry the cut aramid fibers to obtain dried aramid fibers;

[0008] S2: Prepare a shear reaction solution by mixing concentrated sulfuric acid, phosphoric acid, hydrogen peroxide, surface modifier and deionized water;

[0009] S3: Add the dried aramid fibers to the shearing reaction solution for chemical cutting treatment to obtain a mixed emulsion;

[0010] S4: Dilute the mixed emulsion in ice water and adjust the pH value using a pH adjuster; after filtration and washing, neutral wet aramid microfibers are obtained.

[0011] S5: Wet aramid microfibers are placed in a ball mill for wet ball milling to obtain a ball mill slurry; the ball mill slurry is ultrasonically treated to obtain an aramid nanofiber dispersion;

[0012] S6: Freeze-dry the aramid nanofiber dispersion to obtain powdered aramid nanofiber filler.

[0013] Furthermore, in step S1, the aramid precursor is para-aramid, the length of the cut aramid fiber is 1~3mm, and its initial molecular weight is 5×104~10×104Da; the drying parameters are: the cut aramid fiber is vacuum dried at 60~90℃ for 4~6h.

[0014] Further, in step S2, the mass ratio of concentrated sulfuric acid, phosphoric acid, hydrogen peroxide, surface modifier and deionized water in the prepared shear reaction solution is (60~75): (15~25): (1~5): (0.5~1): (5~10).

[0015] Further, in step S2, the concentration of concentrated sulfuric acid is 98%; the concentration of phosphoric acid is 85%; the concentration of hydrogen peroxide is 30%; the surface modifier is selected from one or more combinations of anionic, nonionic, cationic, or amphoteric surfactants; anionic surface modifiers include alkyl sulfates, alkyl sulfonates, sulfosuccinates, or alkyl phosphates; nonionic surface modifiers include fatty alcohol polyoxyethylene ethers, sorbitan fatty acid esters and their polyoxyethylene derivatives, alkyl polysaccharides, or polyether-modified silicone oils; cationic surface modifiers include quaternary ammonium salts, benzalkonium chloride, pyridinium salts, or imidazoline quaternary ammonium salts; and amphoteric surface modifiers include cocamidopropyl betaine, stearamidopropyl dimethyl betaine, or phospholipids.

[0016] Furthermore, in step S3, the mass percentage of the dried aramid fiber in the mixed emulsion is 0.5-3%.

[0017] Furthermore, in step S3, the temperature of the chemical cutting treatment is 60~80℃, and the time is 6~10h.

[0018] Furthermore, in step S4, the volume ratio of the mixed emulsion to ice water is 1:5~15.

[0019] Further, in step S4, the pH value is adjusted to 6-7 using a pH adjuster; the pH adjuster is selected from one or more of carbonates, sodium hydroxide, potassium hydroxide, ammonium hydroxide, borates, phosphates, or organic amine buffers; filtration is performed using a vacuum filtration method; the parameters of the vacuum filtration method are: vacuum degree -0.08~-0.1 MPa, filter membrane pore size 0.45 μm.

[0020] Further, in step S5, the parameters for wet ball milling are as follows: the grinding medium is zirconia balls with a diameter of 0.3~0.5 mm, the liquid phase for wet ball milling is deionized water, the solid-liquid ratio is 1:(5~10), the ball milling time is 8~12 h, and the rotation speed is 300~500 rpm; the parameters for ultrasonic treatment are as follows: the power is 300~500 W, the frequency is 20~40 kHz, and the ultrasonic treatment time is 30~60 min.

[0021] Furthermore, in step S6, the freeze-drying temperature is -50°C and the time is 12~24h.

[0022] The beneficial technical effects of this invention are as follows:

[0023] 1. This invention utilizes a strong acid oxidation system to pretreat aramid fibers, achieving selective destruction of their dense crystalline structure. This significantly reduces the energy and time costs required for subsequent mechanical dissociation, improving overall process efficiency. Simultaneously, the introduction of a surface modifier with stable adsorption capacity effectively prevents the aggregation and recrystallization of nanoscale aramid fragments under high acid conditions, ensuring good product dispersibility and structural integrity. The optimized chemical shearing-physical synergistic fiber dissociation process can stably prepare short fiber structures with lengths ranging from tens to hundreds of nanometers, exhibiting uniform particle size distribution and controllable morphology, greatly enhancing their application potential in composite materials, functional coatings, and flexible electronics.

[0024] 2. The ice-water quenching method used in this invention not only rapidly reduces the system temperature, effectively curbing the safety risks caused by excessive degradation or localized intense exothermic reactions due to residual heat, but also helps maintain the stability of the microstructure of the nano-sized fibers, preventing irreversible collapse or aggregation. Furthermore, by adding an appropriate amount of pH adjuster to slowly adjust the system from strongly acidic conditions to near neutral (pH 6-7), the activity of residual acidic substances can be further passivated, preventing the continuous degradation of the aramid backbone during subsequent processing. This provides a gentler operating environment for subsequent washing, centrifugation, and drying steps, while also reducing the corrosive impact of wastewater treatment on equipment, thus improving the overall safety and environmental friendliness of the process. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to the embodiments.

[0026] This invention provides a method for preparing powdered aramid nanofiber filler, specifically including the following steps:

[0027] S1: Aramid filament is cut into aramid fibers with a length of 1~3 mm and dried to obtain dried aramid fibers;

[0028] In step S1, the aramid precursor is para-aramid with an initial molecular weight of 5 × 10⁻⁶. 4 ~10×10 4 The drying parameters are as follows: the cut aramid fibers are vacuum dried at 60~90℃ for 4~6 hours.

[0029] S2: Prepare a shear reaction solution by mixing concentrated sulfuric acid, phosphoric acid, hydrogen peroxide, surface modifier and deionized water in a mass ratio of (60~75):(15~25):(1~5):(0.5~1):(5~10);

[0030] In step S2, the concentration of concentrated sulfuric acid is 98%; the concentration of phosphoric acid is 85%; the concentration of hydrogen peroxide is 30%; and the surface modifier can be one or a combination of two or more of anionic, nonionic, cationic, or amphoteric surfactants. Preferred anionic surface modifiers include alkyl sulfates (such as sodium dodecyl sulfate, sodium lauryl sulfate), alkyl sulfonates (such as sodium dodecyl sulfonate), sulfosuccinates (such as sodium dioctyl sulfosuccinate), or alkyl phosphate salts. Preferred nonionic surface modifiers include fatty alcohol polyoxyethylene ethers, sorbitan fatty acid esters and their polyoxyethylene derivatives, alkyl polysaccharides, or polyether-modified silicone oils. Preferred cationic surface modifiers include quaternary ammonium salts (such as hexadecyltrimethylammonium bromide, tetradecyltrimethylammonium chloride), benzalkonium chloride, pyridinium salts, or imidazoline quaternary ammonium salts. Preferred amphoteric surface modifiers include cocamidopropyl betaine, stearamide propyl dimethyl betaine, or phospholipids.

[0031] S3: Add the dried aramid fibers to the shear reaction solution and perform chemical cutting treatment at 60~80℃ for 6~10h to obtain a mixed emulsion;

[0032] In step S3, the mass percentage of the dried aramid fiber in the mixed emulsion is 0.5-3%, preferably 1-1.5%.

[0033] S4: Dilute the mixed emulsion in ice water and adjust the pH of the system to 6-7 using a pH adjuster; after filtration and washing, neutral wet aramid microfibers are obtained.

[0034] In step S4, the volume ratio of the mixed emulsion to ice water is 1:5~15 (the mixed emulsion is poured into ice water at a volume ratio of 1:5~15 to dilute it), preferably 1:10, in order to achieve rapid cooling and dilute the acidity, and prevent secondary aggregation or structural collapse of the nanofibers.

[0035] In step S4, the pH adjuster is selected from one or more combinations of carbonates, sodium hydroxide, potassium hydroxide, ammonium hydroxide, borates, phosphates, or organic amine buffers; carbonates and organic amine buffers are preferred to achieve gentle neutralization of acidic residues and avoid damage to the aramid molecular backbone. Carbonates include sodium bicarbonate, sodium carbonate, potassium bicarbonate, and potassium carbonate; organic amine buffers include triethanolamine, tris(hydroxymethyl)aminomethane (Tris), and 2-amino-2-methyl-1-propanol.

[0036] In step S4, the diluted mixture is filtered using vacuum filtration (vacuum degree -0.08~-0.1 MPa, filter membrane pore size 0.45 μm) to achieve solid-liquid separation. Then, it is washed with deionized water 3~5 times at room temperature, with the amount of water used for each wash being 5~10 times the sample volume, until the pH of the filtrate is 6~7 and the conductivity is close to that of deionized water, which is the washing endpoint, thereby obtaining neutral wet aramid microfibers.

[0037] S5: Wet aramid microfibers are placed in a ball mill for wet ball milling to obtain a ball mill slurry; then the ball mill slurry is ultrasonically treated to obtain an aramid nanofiber dispersion.

[0038] In step S5, zirconia balls with a diameter of 0.3~0.5 mm are used as the grinding medium. The liquid phase of the wet ball mill is deionized water, the solid-liquid ratio is 1:(5~10), the ball milling time is 8~12h, and the rotation speed is 300~500 rpm.

[0039] In step S5, the parameters for ultrasonic treatment are: power of 300~500 W, frequency of 20~40 kHz, and ultrasonic treatment time of 30~60 min.

[0040] S6: The aramid nanofiber dispersion was freeze-dried to obtain powdered aramid nanofiber filler.

[0041] In step S6, the freeze-drying temperature is -50℃ and the time is 12~24h.

[0042] The powdered aramid nanofiber filler prepared by the method of the present invention has an average particle size of 80~150 nm, a length of <1 μm, and a zeta potential ζ that satisfies: |ζ|>30 mV.

[0043] The present invention also provides a powdered aramid nanofiber filler, which is prepared by the above preparation method.

[0044] The chemical shearing mechanism in this invention is as follows: under the synergistic effect of strong acid and oxidant, a series of chemical processes disrupt the hydrogen bond network and inter-chain binding sites of aramid fibers, thereby effectively controlling their microstructure. Specifically, concentrated sulfuric acid mainly acts as a swelling agent, causing the crystalline regions between aramid molecular chains to expand and gradually loosen, thus improving the accessibility of the fiber interior and facilitating the penetration and reaction of subsequent oxidants. Simultaneously, in an acidic environment, the amide groups on the aramid molecular chains are protonated, enhancing their reactivity and promoting the breakage or partial degradation of the main chain. Furthermore, sulfuric acid can also sulfonate the aromatic rings, introducing polar sulfonic acid groups (–SO3H), significantly improving the hydrophilicity of the material and its stability in the dispersion medium.

[0045] Phosphoric acid, as an auxiliary acid, together with sulfuric acid to form a mixed acid system, has multiple functions: on the one hand, it can improve the diffusivity and wettability of the acid solution and promote its penetration into the fiber; on the other hand, phosphoric acid helps to regulate the viscosity of the system and avoid excessive sulfonation or disordered degradation of molecular chains caused by the use of sulfuric acid alone; at the same time, phosphoric acid also has a certain buffering capacity and can stabilize the pH value of the system during the reaction, thereby achieving controllable regulation of the reaction rate.

[0046] Hydrogen peroxide generates highly reactive free radicals (such as ·OH and ·OOH) in a strongly acidic environment. These free radicals can directionally attack the active sites on the aramid molecular chain, especially weak links such as the amide bonds connecting aromatic rings, achieving selective cleavage of the aramid backbone at room temperature and providing a structural basis for subsequent nanoscale dissociation. This process has a certain degree of directionality and controllability, enabling preliminary nanostructuring of fibers without completely destroying the basic fiber structure.

[0047] The role of the surface modifier is to adsorb onto the surface of the chemically sheared nano-sized aramid particles, forming a stable protective layer to prevent secondary aggregation or recrystallization in a high-acid environment. At the same time, in the subsequent dispersion process, the surface modifier can induce the nano-fragments to form short fibrous structures rather than randomly aggregated spherical particles, thereby further improving the degree of fiberization.

[0048] Example 1

[0049] This embodiment provides a method for preparing powdered aramid nanofiber filler, specifically including the following steps:

[0050] S1: The para-aramid precursor filament is cut into aramid fibers with a length of 1-3 mm, and vacuum dried at 60-90℃ for 4-6 hours to remove surface-adsorbed moisture, thereby obtaining dried aramid fibers. The initial molecular weight of the aramid precursor filament is 5×10⁻⁶. 4 ~10×10 4 Da.

[0051] S2: Prepare a shear reaction solution by mixing 98% concentrated sulfuric acid, 85% phosphoric acid, 30% hydrogen peroxide, sodium dodecyl sulfate, and deionized water in a mass ratio of 60:20:3:1:5.

[0052] S3: The dried aramid fibers were added to the above shear reaction solution and chemically cut at 60°C for 8 hours to obtain a mixed emulsion. The mass percentage of the dried aramid fibers in the mixed emulsion was 1%.

[0053] S4: Pour the mixed emulsion into ice water to dilute it, and use NaHCO3 solution to adjust the pH of the system to 7. Then, remove residual acid and other impurities by filtration and multiple washings to obtain neutral wet aramid microfibers.

[0054] S5: Wet aramid microfibers were placed in a ball mill for wet ball milling. Zirconia balls with a diameter of 0.3 mm were used as the grinding medium. The solid-liquid ratio was 1:5, the ball milling time was 10 h, and the rotation speed was 500 rpm to obtain a uniform ball mill slurry. Subsequently, the ball mill slurry was subjected to ultrasonic treatment with an ultrasonic power of 500 W, a frequency of 40 kHz, and a treatment time of 60 min to obtain an aramid nanofiber dispersion.

[0055] S6: The aramid nanofiber dispersion was freeze-dried at a temperature of −50℃ for 24 h to obtain powdered aramid nanofiber filler.

[0056] The powdered aramid nanofiber filler prepared in this embodiment has an average particle size of 80~150 nm, a length of <1 μm, and a zeta potential ζ that satisfies |ζ|>30 mV, exhibiting good dispersion stability.

[0057] Example 2

[0058] In this embodiment, except for the following parameters which are different from those in Example 1, the other parameters and preparation methods are the same as in Example 1.

[0059] In step S2, concentrated sulfuric acid, phosphoric acid, hydrogen peroxide, hexadecyltrimethylammonium bromide, and deionized water are mixed in a mass ratio of 68:20:3:0.75:7 to prepare a shear reaction solution.

[0060] In step S3, the mass percentage of the dried aramid fiber in the mixed emulsion is 2%; the parameters for the chemical cutting treatment are: temperature 70℃, time 8 h.

[0061] In step S4, the pH of the system is adjusted to 6.5 using 2-amino-2-methyl-1-propanol.

[0062] In step S6, the freeze-drying temperature is −50℃ and the time is 24 h.

[0063] The powdered aramid nanofiber filler prepared in this embodiment has an average particle size of 80~150 nm, a length of <1 μm, and a zeta potential ζ that satisfies |ζ|>30 mV, exhibiting good dispersion stability.

[0064] Example 3

[0065] In this embodiment, except for the following parameters which are different from those in Example 1, the other parameters and preparation methods are the same as in Example 1.

[0066] In step S2, concentrated sulfuric acid, phosphoric acid, hydrogen peroxide, fatty alcohol polyoxyethylene ether, and deionized water are mixed in a mass ratio of 75:25:5:1:5 to prepare a shear reaction solution.

[0067] In step S3, the mass percentage of the dried aramid fiber in the mixed emulsion is 3%; the parameters for the chemical cutting treatment are: temperature 80℃ and time 10 h.

[0068] In step S4, sodium carbonate is used to adjust the pH of the system to 7.

[0069] In step S6, the freeze-drying temperature is −50℃ and the time is 18 h.

[0070] The powdered aramid nanofiber filler prepared in this embodiment has an average particle size of 80~150 nm, a length of <1 μm, and a zeta potential ζ that satisfies |ζ|>30 mV, exhibiting good dispersion stability.

[0071] The present invention has been described in detail above with reference to the embodiments. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. All contents not described in detail in the present invention can be derived from existing technologies.

Claims

1. A method for preparing a powdered aramid nanofiber filler, characterized by, The method comprises: S1: cutting aramid filaments into aramid fibers, and drying the cut aramid fibers to obtain dried aramid fibers; S2: mixing concentrated sulfuric acid, phosphoric acid, hydrogen peroxide, a surface modifier and deionized water to prepare a shearing reaction solution; the mass ratio of the concentrated sulfuric acid, the phosphoric acid, the hydrogen peroxide, the surface modifier and the deionized water is (60-75):(15-25):(1-5):(0.5-1):(5-10); S3: adding the dried aramid fibers into the shearing reaction solution to perform chemical cutting treatment, and obtaining a mixed emulsion; the chemical cutting treatment is performed at a temperature of 60-80 DEG C for 6-10 hours; S4: diluting the mixed emulsion in ice water, and adjusting the pH value to 6-7 by using a pH regulator; after filtration and washing, neutral wet-state aramid microfibers are obtained; S5: placing the wet-state aramid microfibers into a ball mill to perform wet ball milling, and obtaining a ball milling slurry; performing ultrasonic treatment on the ball milling slurry to obtain an aramid nanofiber dispersion solution; S6: performing freeze-drying treatment on the aramid nanofiber dispersion solution to obtain a powder aramid nanofiber filler.

2. The method for preparing powdered aramid nanofiber filler according to claim 1, characterized in that, The aramid fiber after cutting has a length of 1-3 mm and an initial molecular weight of 5x10 4 ~10x10 4 Da, and the drying treatment parameters are: vacuum drying the aramid fiber after cutting at 60-90°C for 4-6h.

3. The method for preparing powdered aramid nanofiber filler according to claim 1, characterized in that, In the step S2, the concentration of the concentrated sulfuric acid is 98%; the concentration of the phosphoric acid is 85%; the concentration of the hydrogen peroxide is 30%; the surface modifier is selected from one or a combination of two or more of anionic, non-ionic, cationic or amphoteric surfactants; the anionic surface modifier includes alkyl sulfate, alkyl sulfonate, sulfosuccinate or alkyl phosphate; the non-ionic surface modifier includes fatty alcohol polyoxyethylene ether, sorbitan fatty acid ester and its polyoxyethylene derivative, alkyl polyglycoside or polyether modified silicone oil; the cationic surface modifier includes quaternary ammonium salt, benzalkonium chloride, pyridinium salt or imidazoline quaternary ammonium salt; and the amphoteric surface modifier includes cocamidopropyl betaine, stearamidopropyl dimethyl betaine or phospholipid.

4. The method for preparing powdered aramid nanofiber filler according to claim 1, characterized in that, In the step S3, the mass ratio of the dried aramid fibers in the mixed emulsion is 0.5-3%.

5. The method for preparing powdered aramid nanofiber filler according to claim 1, characterized in that, In the step S4, the volume ratio of the mixed emulsion to the ice water is 1:5-15.

6. The method for preparing powdered aramid nanofiber filler according to claim 1, characterized in that, In the step S4, the pH regulator is selected from one or a combination of two or more of carbonate, sodium hydroxide, potassium hydroxide, ammonium hydroxide, borate, phosphate or organic amine buffer; the filtration is performed by vacuum filtration; and the parameters of the vacuum filtration are: vacuum degree -0.08-0.1 MPa and filter membrane pore size 0.45 μm.

7. The method for preparing powdered aramid nanofiber filler according to claim 1, characterized in that, In the step S5, the parameters of the wet ball milling are: the grinding medium is zirconia ball with a diameter of 0.3-0.5 mm, the liquid phase of the wet ball milling is deionized water, the solid-liquid ratio is 1:(5-10), the ball milling time is 8-12 hours, and the rotation speed is 300-500 rpm; and the parameters of the ultrasonic treatment are: power 300-500 W, frequency 20-40 kHz, and ultrasonic treatment time 30-60 minutes.

8. The method for preparing powdered aramid nanofiber filler according to claim 1, characterized in that, In the step S6, the temperature of the freeze-drying is -50 DEG C, and the time is 12-24 hours.