High-efficient and low-resistance gradient microsphere composite micro-nano fiber air filtration material and preparation method thereof

Abstract

This invention belongs to the technical field of functional composite filter materials and discloses a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material and its preparation method. The composite membrane includes a first, second, and third fiber membrane layer with decreasing pore size from top to bottom. Several microspheres with progressively increasing particle size are embedded within the micro / nanofibers forming the first, second, and third fiber membrane layers. The mass fraction of microspheres in each fiber membrane layer is 0.5-2%, and the particle sizes of the microspheres in the first, second, and third fiber membrane layers are 1 μm, 3-4 μm, and 5-7 μm, respectively. At an airflow rate of 32.5 L / min, the filtration efficiency for 0.3-7.25 μm particles is >99.97%, and the air resistance is <132 Pa. During a continuous 15-day test, the filtration efficiency decreased by 0.21-0.268%, and the air resistance increased by 5-9 Pa. This invention achieves gradient filtration from "coarse filtration" to "fine filtration" by constructing a fiber membrane structure of "macro pore size gradient + micro protrusion modification", which further improves the air filtration performance and long-term operational stability of PA6 / PS nanofiber membranes.

Description

Technical Field

[0001] This invention belongs to the field of functional composite filter materials technology, and particularly relates to a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material and its preparation method. Background Technology

[0002] The purpose of air filtration is primarily to effectively capture and intercept harmful particles, gases, bacteria, and viruses from polluted gases using air filtration materials, thereby purifying the air or protecting human health. Nanofiber membranes, due to their high specific surface area, tunable porosity, and excellent mechanical properties, have become one of the most promising candidate materials for high-performance air filtration. Traditional nanofiber membranes mainly improve efficiency by increasing fiber density, increasing membrane basis weight, or using electret treatment. However, this often leads to problems such as increased air resistance due to overly dense structures or rapid performance degradation due to electrostatic decay. Current technologies introduce micron-sized particles to obtain more porous, three-dimensional fiber membranes, thereby improving efficiency and reducing air resistance.

[0003] However, this single, homogeneous pore structure lacks the ability to perform graded filtration of particulate matter, making it prone to deep clogging, resulting in shortened fiber membrane lifespan, increased resistance, and unfavorable long-term use. Inspired by biological graded filtration mechanisms (such as the respiratory tract of mammals), gradient pore structures can optimize particulate matter retention paths. However, traditional gradient membranes mainly rely on a single strategy of controlling the macroscopic fiber diameter to achieve graded filtration, failing to simultaneously optimize their pore structure and surface properties, thus hindering further improvements in their overall filtration performance. Currently, the existing technologies for composite materials used in air filtration are as follows:

[0004] CN109012218A discloses a four-layer composite micro / nanofiber air filter membrane. The four fiber layers include, from bottom to top, a nonwoven fabric substrate layer, an electrospun micron-sized fiber layer, an electrospun beaded nanofiber layer, and an electrospun ultrafine nanofiber layer. The preparation method involves sequentially depositing three layers of fiber filter membranes of different sizes and morphologies on the surface of a nonwoven fabric substrate using electrospinning technology. The fiber diameter and pore size of each layer gradually decrease from bottom to top, exhibiting a gradient distribution. This technology increases porosity by introducing a beaded structure into the nanofiber layers. However, the beads are formed due to fluid instability during electrospinning and are made of the same material as the fibers, resulting in significant randomness in their size and distribution. Furthermore, the beaded structure exists only in a single layer, while the remaining layers are smooth fibers, failing to achieve precise control over the microstructure of each layer. Particulate matter interception remains primarily single-layer, making localized clogging and rapid pressure drop likely during long-term use.

[0005] CN118788059A discloses a method for preparing a high-efficiency, low-resistance 3D microsphere composite air filter material with a beaded structure. The method involves dissolving a spinning polymer and microsphere particles in a solvent to obtain a spinning solution, followed by ultrasonic treatment and solution jet spinning to create a bead-like structure where microspheres and fibers are interwoven. While this technology makes the three-dimensional network structure of the nanofiber membrane more porous by adding microspheres, effectively reducing filtration resistance, the resulting filter material is a single-layer homogeneous structure, lacking the ability to grade and intercept particles of different sizes. In practical applications, it still suffers from uneven particle load distribution and limited dust holding capacity.

[0006] CN118543167A discloses a high-efficiency, low-resistance three-dimensional microsphere-structured micro / nanofiber composite air filter material. It employs a combination of airflow spraying and airflow spinning to embed nanoscale and micrometer-scale microspheres into a nanofiber framework via spraying, forming a compact two-dimensional and loose three-dimensional framework structure. This technology reduces pressure drop by increasing the overall porosity of the fibers through micrometer-scale microspheres and improves filtration efficiency by refining the interfiber gaps through nanoscale microspheres. However, in this technology, the microspheres are attached to the fiber framework surface via spraying, and the bonding strength between the microspheres and fibers needs improvement. Furthermore, the uniformity of microsphere distribution within the membrane is significantly affected by the spraying process, resulting in a single-layer homogeneous structure that fails to achieve a gradient pore size distribution along the thickness direction and hierarchical filtration.

[0007] As can be seen from the above patented technologies, there are currently many types of membrane materials used for air filtration, and their structures and preparation processes are also different. However, existing technologies may be single-layer homogeneous structures that lack hierarchical interception capabilities; or although they have multi-layer gradient structures, the gradient construction depends on the change of fiber diameter or the special morphology of a single layer, failing to achieve independent and precise control of the microstructure of each layer; or the combination of microspheres and fibers may have problems such as insufficient bonding strength and limited distribution uniformity.

[0008] In summary, how to achieve an effective pore size gradient distribution along the filtration direction while precisely controlling the microstructure (fiber fineness, network bulkiness, surface roughness) of each layer to efficiently intercept particles of different sizes in their respective matched layers, while maintaining overall low resistance and long-term stability, has become a pressing problem for engineers in the field of air filtration materials. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the technical problem to be solved by this invention is to provide a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material and its preparation method, which can achieve an effective pore size gradient distribution along the filtration direction, precisely control the microstructure of each layer, and efficiently intercept particles of different sizes in their respective matched layers, while maintaining overall low resistance and long-term stability.

[0010] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material, which is an integrated gradient structure composite membrane formed by the sequential deposition and stacking of three layers of micro / nanofiber membranes. The composite membrane includes a first fiber membrane layer as a coarse filtration layer, a second fiber membrane layer as a main filtration layer, and a third fiber membrane layer as a fine filtration layer, with pore sizes decreasing sequentially from top to bottom. Several microspheres with progressively increasing particle sizes are embedded within the micro / nanofibers forming the first, second, and third fiber membrane layers. The mass fraction of the microspheres in each fiber membrane layer is 0.5-2%, and the particle sizes of the microspheres in the first, second, and third fiber membrane layers are 1 μm, 3-4 μm, and 5-7 μm, respectively. At an airflow rate of 32.5 L / min, the filtration efficiency for 0.3-7.25 μm particles is >99.97%, and the air resistance is <132 Pa. During a continuous 15-day test, the filtration efficiency decreased by 0.21-0.268%, and the air resistance increased by 5-9 Pa.

[0011] The aforementioned high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material has a microsphere particle size of 1 μm in the first fiber membrane layer, a microsphere mass fraction of 1% in the fiber membrane layer, an average micro / nanofiber diameter of 601.37 nm, an average pore size of 6.4 μm in the fiber membrane, and a porosity of 62.5%.

[0012] The aforementioned high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material has a microsphere particle size of 3 μm in the second fiber membrane layer, a microsphere mass fraction of 1% in the fiber membrane layer, an average micro / nanofiber diameter of 568.68 nm, an average pore size of 4.8 μm in the fiber membrane, and a porosity of 70.8%.

[0013] The aforementioned high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material has a microsphere particle size of 6.5 μm in the third fiber membrane layer, a microsphere mass fraction of 1% in the fiber membrane layer, an average micro / nanofiber diameter of 532.10 nm, an average pore size of 3.7 μm in the fiber membrane, and a porosity of 79.3%.

[0014] The high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material described above uses any one of the following as the fiber-forming polymer material for the micro / nanofibers that form the first, second, and third fiber membrane layers: polyamide 6, polyamide 66, polyurethane, and polyvinyl alcohol.

[0015] The aforementioned high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material, wherein the microspheres are inorganic rigid microspheres, including any one of silicon dioxide, titanium dioxide, polystyrene, and polymethyl methacrylate.

[0016] A method for preparing a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material includes the following steps:

[0017] I. Preparation of spinning solution:

[0018] (1) Dissolve a certain amount of fiber-forming polymer in an 88% formic acid solution and repeat the operation three times to obtain three identical base solutions for later use.

[0019] (2) Add equal amounts of microsphere powder with different particle sizes to three portions of base solution, stir thoroughly at room temperature for 6 hours, and ultrasonically disperse for 1.5 hours to ensure that the fiber-forming polymer is completely dissolved and the microsphere powder is uniformly dispersed, so as to obtain spinning suspensions containing microspheres with three different particle sizes.

[0020] (3) The three spinning suspensions are labeled as spinning solution A, spinning solution B and spinning solution C according to the particle size from small to large.

[0021] II. Preparation of composite membranes:

[0022] (4) Let the spinning solution A, spinning solution B and spinning solution C obtained in step (3) stand for 30 minutes to remove bubbles, and set aside for later use;

[0023] (5) Using solution jet spinning, the spinning solution C is first delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, a continuous jet is formed for spinning. After spinning for a certain time, the third fiber membrane layer is obtained.

[0024] (6) The spinning solution B is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, the spinning solution B is uniformly deposited and stacked on the surface of the third fiber membrane for a certain period of time to obtain the second fiber membrane.

[0025] (7) The spinning solution A is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, the second fiber membrane layer is uniformly deposited and stacked on the surface for a certain period of time to obtain the first fiber membrane layer and a three-layer composite membrane.

[0026] (8) Take off the composite membrane obtained in step (7), put it in a vacuum drying oven at 60°C and dry it for 30 minutes to remove the residual formic acid solvent and obtain the gradient microsphere composite micro-nanofiber air filter material.

[0027] In the preparation method of the above-mentioned high-efficiency and low-resistance gradient microsphere composite micro / nanofiber air filter material, a dispersant stabilizer is added to the base solution in step (1), and the mass fraction of the dispersant stabilizer in the spinning suspension in step (2) is 0.1-0.3%.

[0028] In the preparation method of the above-mentioned high-efficiency and low-resistance gradient microsphere composite micro / nanofiber air filter material, the mass fraction of the fiber-forming polymer in the spinning suspension in step (2) is 8-10%, and the mass fraction of the microsphere powder is 1-1.5%.

[0029] In the preparation method of the above-mentioned high-efficiency and low-resistance gradient microsphere composite micro / nanofiber air filter material, in steps (5)-(7), the inner diameter of the spinning needle is 0.33 mm, the spinning solution propulsion speed is 1-2 mL / h, the compressed air pressure is 0.09-0.12 MPa, the receiving distance is 25-30 cm, the collector winding speed is 500-700 r / h, the ambient temperature is 30 °C, and the ambient humidity is 35%.

[0030] The advantages of the high-efficiency, low-resistivity gradient microsphere composite micro / nanofiber air filter material and its preparation method of this invention are:

[0031] 1. Achieved efficient and low-resistance synergistic filtration. This invention utilizes a dual-scale structural design to construct a high-porosity, fluffy three-dimensional network at the microscopic level by leveraging the spacing effect of microspheres and the induced fiber refinement effect, providing a structural basis for low airflow resistance. At the macroscopic level, by constructing a pore size gradient from large to medium to small, it achieves graded interception of particulate matter, avoiding rapid clogging of the homogeneous membrane.

[0032] 2. Excellent long-term stability. The gradient structure of this invention enables particulate matter to be retained in layers with different pore sizes in a graded manner, resulting in a more uniform load distribution and effectively delaying membrane fouling.

[0033] 3. Size-dependent hierarchical interception of particulate matter was achieved. Scanning electron microscopy confirmed that large particles were mainly trapped in the upstream macroporous layer, medium-sized particles were trapped in the midstream layer, and the finest particles were captured in the downstream microporous layer, which directly verified the "coarse filtration → main filtration → fine filtration" hierarchical filtration mechanism of this invention.

[0034] 4. Significant advantages compared to existing beaded fiber membranes. Existing beaded structures rely on fluid instability during electrospinning, resulting in highly random bead size and distribution. Furthermore, the beads are homogeneous with the fibers and easily deform under high wind speeds. In contrast, this invention uses pre-prepared rigid microspheres embedded in the fibers through physical encapsulation. The microspheres are precisely controllable in size and uniformly distributed, and they serve as a permanent scaffold, ensuring the long-term stability of the fiber network's loose structure. Moreover, this invention embeds microspheres in each layer, achieving full-layer microstructural control, whereas existing technologies only have one layer of beads, with the remaining layers being smooth fibers. The dual-scale synergistic effect of this invention is more significant.

[0035] 5. The preparation process is simple and controllable. This invention uses solution jet spinning, which allows for precise control of the microstructure and macroscopic gradient of the fiber membrane by simply adjusting the microsphere size and stacking sequence. The process is easy to scale up and suitable for industrial production. Attached Figure Description

[0036] Figure 1 This is a physical image of the high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material of the present invention.

[0037] Figure 2 A schematic diagram of the three-layer structure of the air filter material of this invention;

[0038] Figure 3 A schematic diagram of the preparation process of the air filter material of this invention;

[0039] Figure 4 These are electron microscope (EM) images and cross-sectional EEM images of the air filter material of this invention;

[0040] Figure 5 The images shown are electron microscope images and fiber diameter distribution diagrams of the first, second, and third fiber membrane layers in the air filter material prepared in Example 1.

[0041] Figure 6 This is a pore size distribution diagram of the fiber membranes containing microspheres of different sizes in the air filter material prepared in Example 1.

[0042] Figure 7 The porosity test diagrams are shown for each layer of fiber membrane containing microspheres of different particle sizes in the air filter material prepared in Example 1.

[0043] Figure 8 This is an electron micrograph of salt particles on the surface of the macroporous fiber membrane in the air filter material prepared in Example 1 after filtration.

[0044] Figure 9 This is an electron micrograph of salt particles on the surface of the mesopore fiber membrane in the air filter material prepared in Example 1 after filtration.

[0045] Figure 10 This is an electron microscope image of salt particles on the surface of the small-pore fiber membrane in the air filter material prepared in Example 1 after filtration.

[0046] Figure 11 The graph shows the filtration efficiency and air resistance of the fiber membrane containing microspheres of different particle sizes in the air filtration material prepared in Example 1. Detailed Implementation

[0047] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0048] In this invention, unless otherwise stated, directional terms such as "upper" and "lower" generally refer to the upper and lower positions of the device in its actual use or operating state, specifically the drawing directions in the accompanying drawings; while "inner" and "outer" refer to the outline of the device. Furthermore, in the description of this application, the term "comprising" means "including but not limited to". The terms first, second, third, etc., are used merely as illustrative purposes and do not impose numerical requirements or establish an order. The term "multiple" means "two or more".

[0049] like Figure 1 , 2 As shown, a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material is an integrated gradient structure composite membrane formed by the sequential deposition and stacking of three layers of micro / nanofiber membranes. The composite membrane includes a first fiber membrane layer as a coarse filtration layer, a second fiber membrane layer as a main filtration layer, and a third fiber membrane layer as a fine filtration layer, with pore sizes decreasing sequentially from top to bottom. Several microspheres with progressively increasing particle sizes are embedded within the micro / nanofibers forming the first, second, and third fiber membrane layers. The mass fraction of microspheres in each fiber membrane layer is 0.5-2%, and the particle sizes of the microspheres in the first, second, and third fiber membrane layers are 1 μm, 3-4 μm, and 5-7 μm, respectively. At an airflow rate of 32.5 L / min, the filtration efficiency for 0.3-7.25 μm particles is >99.97%, and the air resistance is <132 Pa. During a continuous 15-day test, the filtration efficiency decreased by 0.21-0.268%, and the air resistance increased by 5-9 Pa.

[0050] The first fiber membrane layer contains microspheres with a particle size of 1 μm, representing 1% of the total mass of the fiber membrane. The average diameter of the micro / nanofibers is 601.37 nm, the average pore size of the fiber membrane is 6.4 μm, and the porosity is 62.5%. The second fiber membrane layer contains microspheres with a particle size of 3 μm, representing 1% of the total mass of the fiber membrane. The average diameter of the micro / nanofibers is 568.68 nm, the average pore size of the fiber membrane is 4.8 μm, and the porosity is 70.8%. The third fiber membrane layer contains microspheres with a particle size of 6.5 μm, representing 1% of the total mass of the fiber membrane. The average diameter of the micro / nanofibers is 532.10 nm, the average pore size of the fiber membrane is 3.7 μm, and the porosity is 79.3%. The fiber-forming polymer material used to form the micro / nanofibers in the first, second, and third fiber membrane layers is any one of polyamide 6, polyamide 66, polyurethane, or polyvinyl alcohol. The microspheres are inorganic rigid microspheres, including any one of silica, titanium dioxide, polystyrene, and polymethyl methacrylate.

[0051] like Figure 3 As shown, the method for producing a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material according to the present invention includes the following steps:

[0052] I. Preparation of spinning solution:

[0053] (1) Dissolve a certain amount of fiber-forming polymer in an 88% formic acid solution and repeat the operation three times to obtain three identical base solutions for later use.

[0054] (2) Add equal amounts of microsphere powder with different particle sizes to three portions of base solution, stir thoroughly at room temperature for 6 hours, and ultrasonically disperse for 1.5 hours to ensure that the fiber-forming polymer is completely dissolved and the microsphere powder is uniformly dispersed, so as to obtain spinning suspensions containing microspheres with three different particle sizes.

[0055] (3) The three spinning suspensions are labeled as spinning solution A, spinning solution B and spinning solution C according to the particle size from small to large.

[0056] II. Preparation of composite membranes:

[0057] (4) Let the spinning solution A, spinning solution B and spinning solution C obtained in step (3) stand for 30 minutes to remove bubbles, and set aside for later use;

[0058] (5) Using solution jet spinning, the spinning solution C is first delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, a continuous jet is formed for spinning. After spinning for a certain time, the third fiber membrane layer is obtained.

[0059] (6) The spinning solution B is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, the spinning solution B is uniformly deposited and stacked on the surface of the third fiber membrane for a certain period of time to obtain the second fiber membrane.

[0060] (7) The spinning solution A is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, the second fiber membrane layer is uniformly deposited and stacked on the surface for a certain period of time to obtain the first fiber membrane layer and a three-layer composite membrane.

[0061] (8) Take off the composite membrane obtained in step (7), put it in a vacuum drying oven at 60°C and dry it for 30 minutes to remove the residual formic acid solvent and obtain the gradient microsphere composite micro-nanofiber air filter material.

[0062] In step (1), a dispersing stabilizer is added to the base solution. The dispersing stabilizer is polyethylene oxide, and its mass fraction in the spinning suspension in step (2) is 0.1-0.3%. In the spinning suspension in step (2), the mass fraction of the fiber-forming polymer is 8-10%, and the mass fraction of the microsphere powder is 1-1.5%. In steps (5)-(7), the inner diameter of the spinning needle is 0.33 mm, the spinning solution advance speed is 1-2 mL / h, the compressed air pressure is 0.09-0.12 MPa, the receiving distance is 25-30 cm, the collector winding speed is 500-700 r / h, the ambient temperature is 30℃, and the ambient humidity is 35%.

[0063] This invention successfully embeds PS microspheres of different particle sizes (1μm, 3μm, and 6.5μm) into nanofibers using an air-jet spinning process. This creates micron-sized protrusions on the fiber surface, significantly increasing the specific surface area of ​​individual fibers and enhancing the collision probability between particles and fibers. Furthermore, the larger PS microspheres (e.g., 6.5μm) can disrupt the airflow during spinning, enhancing the stretching and splitting of the jet, resulting in a decrease in the average fiber diameter from 582.45nm (1μm PS) to 532.10nm (6.5μm PS), forming a finer and more uniform fiber network. In addition, the addition of PS microspheres prevents dense fiber packing, creating a more porous three-dimensional structure. This increases the fiber membrane porosity from 62.5% to 79.3% and the air permeability from 28.5cm / s to 55.63cm / s, laying a structural foundation for high-efficiency, low-resistance filtration.

[0064] By spinning fiber layers containing PS microspheres of different sizes in the order of "large pore size (containing 1μm PS) → medium pore size (containing 3μm PS) → small pore size (containing 6.5μm PS)", a three-layer composite structure with obvious pore size gradient is formed. This pore size gradient structure enables graded filtration of particulate matter and optimized airflow: First, the upper large-pore layer (first fiber membrane layer) acts as a pre-filter, intercepting large particles and uniformly distributing airflow; the middle medium-pore layer (second fiber membrane layer) further captures medium-sized particles; and the lower small-pore layer (third fiber membrane layer) acts as a fine filter, efficiently capturing submicron particles and preventing excessive accumulation of particulate matter in a single layer. Second, this gradient structure ensures a smooth airflow transition, avoiding airflow bottlenecks at the fiber membrane inlet, resulting in a pressure drop of only 130 Pa, significantly lower than that of homogeneous membranes (153 Pa). Furthermore, the gradient structure disperses the particle load and delays clogging. After 15 days of continuous testing, the filtration efficiency of the pore size gradient membrane decreased by only 0.247%, and the pressure drop increased by only 9 Pa, demonstrating excellent long-term stability.

[0065] This invention achieves a filtration efficiency of 99.985% and a total filtration efficiency of 99.983% for 0.3 μm NaCl particles by constructing a PA6 / PS nanofiber membrane with a pore size gradient structure that can be regulated at both the macro- and micro-scales, and improves the quality factor to 0.0669 Pa. -1 This invention effectively solves the problems of traditional homogeneous membranes, such as the difficulty in balancing filtration efficiency and air resistance, rapid increase in air resistance, and short service life. To further improve the filtration performance and long-term operational stability of existing nanofiber filter membranes, this invention uses polycaprolactam (PA6, with excellent biocompatibility) as the substrate and polystyrene (PS) microspheres as the structure modifier. A PA-6 pore size gradient filter membrane containing 1, 3, and 6.5 μm PS microspheres was spun using an air-jet spinning method in a large-medium-small pore size arrangement. This gradient membrane achieves gradient filtration from "coarse filtration" to "fine filtration" by constructing a fiber membrane structure of "macroscopic pore size gradient + microscopic protrusion modification," further improving the air filtration performance and long-term operational stability of the PA6 / PS nanofiber membrane. This study not only demonstrates the great potential of dual-scale structure regulation in achieving synergistic optimization of "high efficiency and low resistance" in air filtration, but also provides an effective biomimetic path for designing next-generation high-performance, long-life air purification materials.

[0066] The present application will be specifically described below through specific embodiments. The following embodiments are only some embodiments of the present application and are not intended to limit the present application.

[0067] Example 1

[0068] A method for preparing a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material includes the following steps:

[0069] I. Preparation of spinning solution:

[0070] (1) Take a certain amount of polyamide 6 particles and polyethylene oxide powder and dissolve them in a formic acid solution with a mass fraction of 88%. Repeat the operation three times to obtain three identical base solutions for later use.

[0071] (2) Add equal amounts of polystyrene microspheres with particle sizes of 1, 3 and 6.5 μm to three base solutions respectively, stir thoroughly at room temperature for 6 h, and ultrasonically disperse for 1.5 h to ensure that the fiber-forming polymer is completely dissolved and the microsphere powder is uniformly dispersed, so as to obtain spinning suspensions containing 1, 3 and 6.5 μm polystyrene microspheres respectively; in the spinning suspension, the mass fraction of polyamide 6 is 8.0%, the mass fraction of polyethylene oxide is 0.2%, and the mass fraction of polystyrene microspheres is 1.0%.

[0072] (3) The three spinning suspensions were labeled as spinning solution A (1μm), spinning solution B (3μm), and spinning solution C (6.5μm) according to the particle size contained, from smallest to largest.

[0073] II. Preparation of composite membranes:

[0074] (4) Let the spinning solution A, spinning solution B and spinning solution C obtained in step (3) stand for 30 minutes to remove bubbles, and set aside for later use;

[0075] (5) Using solution jet spinning, the spinning solution C is first delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. The inner diameter of the spinning needle is 0.33 mm, the spinning solution propulsion speed is 1.5 mL / h, the compressed air pressure is 0.09 MPa, the receiving distance is 30 cm, the collector winding speed is 500 r / h, the ambient temperature is 30 ℃, and the ambient humidity is 35%. Under the stretching action of the high-pressure airflow, a continuous jet is formed for spinning. After spinning for a certain period of time, the third fiber membrane layer is obtained.

[0076] (6) The spinning solution B is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. The spinning conditions are the same as in step (5). The spinning solution B is uniformly deposited and stacked on the surface of the third fiber membrane for a certain period of time to obtain the second fiber membrane.

[0077] (7) The spinning solution A is delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. The spinning conditions are the same as in step (5). The spinning solution A is uniformly deposited and stacked on the surface of the second fiber membrane for a certain period of time to obtain the first fiber membrane and a three-layer composite membrane.

[0078] (8) Take off the composite membrane obtained in step (7), put it in a vacuum drying oven at 60°C and dry it for 30 minutes to remove the residual formic acid solvent and obtain the gradient microsphere composite micro-nanofiber air filter material.

[0079] The gradient microsphere composite micro / nanofiber air filter material prepared in this embodiment achieves a filtration efficiency of over 99.98% for 0.3μm sodium chloride particles and 100% for sodium chloride particles of 0.4μm and above at an air flow rate of 32.5L / min. The air resistance is less than 130Pa. Furthermore, during a continuous 15-day test, its filtration efficiency decreased by only 0.247%, while the air resistance increased by 9Pa, demonstrating slow performance degradation and long-term operational stability.

[0080] Example 2

[0081] A method for preparing a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material includes the following steps:

[0082] I. Preparation of spinning solution:

[0083] (1) Take a certain amount of polyamide 66 particles and polyethylene oxide powder and dissolve them in a formic acid solution with a mass fraction of 88%. Repeat the operation three times to obtain three identical base solutions for later use.

[0084] (2) Add equal amounts of silica microspheres with particle sizes of 1, 4 and 7 μm to three portions of the base solution respectively, stir thoroughly at room temperature for 6 h, and ultrasonically disperse for 1.5 h to ensure that the fiber-forming polymer is completely dissolved and the microsphere powder is uniformly dispersed, so as to obtain spinning suspensions containing silica microspheres of 1, 4 and 7 μm respectively; in the spinning suspension, the mass fraction of polyamide 66 is 9.0%; the mass fraction of polyethylene oxide is 0.3%; and the mass fraction of silica microspheres is 1.5%.

[0085] (3) The three spinning suspensions were labeled as spinning solution A (1μm), spinning solution B (4μm), and spinning solution C (7μm) according to the particle size contained, from smallest to largest.

[0086] II. Preparation of composite membranes:

[0087] (4) Let the spinning solution A, spinning solution B and spinning solution C obtained in step (3) stand for 30 minutes to remove bubbles, and set aside for later use;

[0088] (5) Using solution jet spinning, the spinning solution C is first delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. The inner diameter of the spinning needle is 0.33 mm, the spinning solution propulsion speed is 2 mL / h, the compressed air pressure is 0.1 MPa, the receiving distance is 25 cm, the collector winding speed is 700 r / h, the ambient temperature is 30 ℃, and the ambient humidity is 35%. Under the stretching action of the high-pressure airflow, a continuous jet is formed for spinning. After spinning for a certain period of time, the third fiber membrane layer is obtained.

[0089] (6) The spinning solution B is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. The spinning conditions are the same as in step (5). The spinning solution B is uniformly deposited and stacked on the surface of the third fiber membrane for a certain period of time to obtain the second fiber membrane.

[0090] (7) The spinning solution A is delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. The spinning conditions are the same as in step (5). The spinning solution A is uniformly deposited and stacked on the surface of the second fiber membrane for a certain period of time to obtain the first fiber membrane and a three-layer composite membrane.

[0091] (8) Take off the composite membrane obtained in step (7), put it in a vacuum drying oven at 60°C and dry it for 30 minutes to remove the residual formic acid solvent and obtain the gradient microsphere composite micro-nanofiber air filter material.

[0092] The gradient microsphere composite micro / nanofiber air filter material prepared in this embodiment has a filtration efficiency of 99.97% for 0.3-7.25μm sodium chloride particles at an air flow rate of 32.5L / min, with an air resistance of less than 125Pa. Furthermore, during a continuous 15-day test, its filtration efficiency decreased by only 0.21%, while the air resistance increased by 8Pa, demonstrating slow performance degradation and long-term operational stability.

[0093] Example 3

[0094] A method for preparing a high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material includes the following steps:

[0095] I. Preparation of spinning solution:

[0096] (1) Take a certain amount of polyurethane particles and polyethylene oxide powder and dissolve them in a formic acid solution with a mass fraction of 88%. Repeat the operation three times to obtain three identical base solutions for later use.

[0097] (2) Add equal amounts of polymethyl methacrylate microspheres with particle sizes of 1, 3 and 5 μm to three base solutions respectively, stir thoroughly at room temperature for 6 h, and ultrasonically disperse for 1.5 h to ensure that the fiber-forming polymer is completely dissolved and the microsphere powder is uniformly dispersed to obtain spinning suspensions containing polymethyl methacrylate microspheres of 1, 3 and 5 μm respectively; in the spinning suspensions, the mass fraction of polyurethane is 10%; the mass fraction of polyethylene oxide is 0.1%; and the mass fraction of polymethyl methacrylate microspheres is 1%.

[0098] (3) The three spinning suspensions were labeled as spinning solution A (1μm), spinning solution B (3μm), and spinning solution C (5μm) according to the particle size contained, from smallest to largest.

[0099] II. Preparation of composite membranes:

[0100] (4) Let the spinning solution A, spinning solution B and spinning solution C obtained in step (3) stand for 30 minutes to remove bubbles, and set aside for later use;

[0101] (5) Using solution jet spinning, the spinning solution C is first delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. The inner diameter of the spinning needle is 0.33 mm, the spinning solution propulsion speed is 1 mL / h, the compressed air pressure is 0.12 MPa, the receiving distance is 30 cm, the collector winding speed is 600 r / h, the ambient temperature is 30 ℃, and the ambient humidity is 35%. After spinning for a certain period of time, the third fiber membrane layer is obtained.

[0102] (6) The spinning solution B is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. The spinning conditions are the same as in step (5). The spinning solution B is uniformly deposited and stacked on the surface of the third fiber membrane for a certain period of time to obtain the second fiber membrane.

[0103] (7) The spinning solution A is delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. The spinning conditions are the same as in step (5). The spinning solution A is uniformly deposited and stacked on the surface of the second fiber membrane for a certain period of time to obtain the first fiber membrane and a three-layer composite membrane.

[0104] (8) Take off the composite membrane obtained in step (7), put it in a vacuum drying oven at 60°C and dry it for 30 minutes to remove the residual formic acid solvent and obtain the gradient microsphere composite micro-nanofiber air filter material.

[0105] The gradient microsphere composite micro / nanofiber air filter material prepared in this embodiment has a filtration efficiency of 99.985% for 0.3-7.25μm sodium chloride particles at an air flow rate of 32.5L / min, with an air resistance of less than 132Pa. Furthermore, during a continuous 15-day test, its filtration efficiency decreased by only 0.268%, while the air resistance increased by 5Pa, demonstrating slow performance degradation and long-term operational stability.

[0106] Comparative Example

[0107] The fiber composite membrane described in this comparative example differs from that in Example 1 only in that it uses microspheres of a specific particle size to prepare a homogeneous microsphere composite nanofiber membrane. The preparation method includes the following steps:

[0108] 1. Preparation of spinning solution.

[0109] Polyamide 6 particles and polyethylene oxide powder were dissolved in 88% formic acid solvent to obtain a base solution. Then, polystyrene microspheres were added to the base solution, and the mixture was stirred thoroughly at room temperature for 8-12 hours, followed by ultrasonic dispersion for 1-2 hours to ensure complete dissolution of the polyamide 6 particles and polyethylene oxide powder and uniform dispersion of the polystyrene microspheres, resulting in a spinning suspension containing polystyrene microspheres. The spinning solution concentration ratio was: polyamide 6 mass fraction 8.0%; polyethylene oxide mass fraction 0.2%; polystyrene microsphere mass fraction 1.0%; and a particle size of 6.5 μm.

[0110] 2. Preparation of homogeneous microsphere composite micro / nanofiber membranes.

[0111] The solution jet spinning method was employed. The spinning suspension containing 6.5 μm polystyrene microspheres was allowed to stand for 30 minutes to degas, and then transported to the nozzle of an air-jet spinning machine via a syringe and spinning hose. Under the stretching effect of a high-pressure airflow, a continuous jet was formed for spinning. The fibers were stacked on a collector to form a homogeneous microsphere composite micro / nanofiber membrane. The spun fiber membrane was removed and placed in a 60°C vacuum drying oven for 30 minutes to remove residual formic acid solvent, yielding a homogeneous microsphere composite micro / nanofiber membrane.

[0112] The spinning parameters are as follows: inner diameter of spinning needle is 0.33 mm, spinning solution feed speed is 1.5 mL / h, compressed air pressure is 0.09 MPa, receiving distance is 30 cm, collector winding speed is 500 r / h, ambient temperature is 30℃, and ambient humidity is 35%.

[0113] The homogeneous microsphere composite micro / nanofiber membrane obtained therefrom exhibits a filtration efficiency of 99.967% for 0.3-7.25 μm sodium chloride particles at a gas flow rate of 32.5 L / min, with an air resistance of 153 Pa. This air resistance is significantly higher than that of the gradient microsphere composite micro / nanofiber membrane in Example 1. Furthermore, during a continuous 15-day test, its filtration efficiency decreased by 0.903%, and its air resistance increased by 16 Pa, indicating rapid performance degradation and a lack of long-term operational stability.

[0114] Example 4

[0115] The similarities between this embodiment and Embodiment 1 will not be repeated here. The difference lies in the fact that, since the air filter material of this invention is a fiber membrane structure, even with a three-layer composite, its mechanical properties are still limited. Therefore, in order to meet the different industrial application environments, improve the mechanical properties of the composite membrane, and comprehensively enhance and extend the service life of the air filter material, the specific details are as follows:

[0116] 1. Add a small amount of SiO2 or TiO2 nanoparticles to the spinning solution as a reinforcing phase to improve the mechanical properties of the fiber. The specific scheme includes: adding a certain amount of SiO2 or TiO2 nanoparticles to the base solution in step (1), the amount of which is 0.5-1wt% relative to the mass of the fiber-forming polymer. In order to avoid agglomeration and clogging of the nozzle, the particle size of SiO2 or TiO2 nanoparticles is selected as 20-50nm. Ultrasonic dispersion for 2h, power 300W, intermittent operation: 5min on / 2min off.

[0117] Rigid particle reinforcement effect: SiO2 or TiO2 nanoparticles themselves possess high modulus and high strength. When uniformly dispersed as rigid fillers in a polymer matrix, they can effectively withstand external stress. When the fiber is stretched, the stress is transferred to the nanoparticles through the interface, thereby reducing excessive deformation of the polymer chain and improving the overall tensile strength and Young's modulus. Interfacial interaction enhancement: The surface of nanoparticles is rich in hydroxyl groups (-OH), which can form hydrogen bonds or covalent bonds with amide, carboxyl, or hydroxyl groups in fiber-forming polymers (such as polyamides, polyesters, etc.). Especially after surface modification with silane coupling agents (such as KH-550 treatment), the chemical bonding ability between the particles and the matrix is ​​enhanced, significantly improving the interfacial bonding force and preventing the propagation of microcracks caused by stress concentration. Crystallization nucleation and structural densification: Nanoparticles can act as heterogeneous nucleation sites, promoting the orderly arrangement of polymer molecular chains during crystallization, forming a denser and more regular crystal structure. This structural optimization not only improves mechanical properties but also enhances the fiber's thermal stability and durability, maintaining stable performance even under repeated bending or friction conditions. It is important to note that if the nanoparticle content exceeds 1 wt% or is unevenly dispersed, agglomeration can easily occur, becoming a source of defects and leading to a decrease in strength. Therefore, controlling the addition amount (0.5-1 wt%) and the dispersion process is crucial.

[0118] 2. During interlayer deposition, a brief low-intensity hot air treatment (<80℃) is applied to promote the melting and bonding of some micro / nanofibers between layers, improving the bonding strength between the first, second, and third fiber membrane layers and enhancing the overall integrity of the three-layer membrane structure. Details are as follows:

[0119] Low-intensity hot air treatment between layers promotes melt bonding. A movable hot air nozzle is set above the receiver and moves synchronously with spinning. The hot air temperature is 70-80℃, which is lower than the glass transition temperature of the polymer to prevent excessive deformation and affect the bulkiness. The treatment time is 10-15s, and it is carried out immediately after each layer is deposited. The air velocity is controlled at 0.5-1.0m / s to blow evenly and avoid fiber displacement.

[0120] Interlayer low-intensity hot air treatment can effectively improve the overall strength of the three-layer film. Its core mechanism lies in the significant enhancement of interlayer physical anchoring and bonding strength through controllable interfacial micro-melting and molecular chain diffusion. The specific mechanism is as follows:

[0121] 1) Surface micro-melting forms a physical interpenetrating network.

[0122] Under the action of hot air at 70-80℃, the polymer molecular chains on the surface of each fiber membrane layer gain sufficient kinetic energy and enter a highly elastic state or a partially molten state. At this time, the upper fiber will slightly embed into the softened surface of the lower layer during the deposition process, forming a microscale interpenetrating structure. After cooling, it solidifies into stable physical anchor points, significantly improving the interlayer bonding force.

[0123] 2) Molecular chain trans-interfacial diffusion and entanglement

[0124] Thermal energy induces short-range diffusion of polymer molecular chains at the interface between the upper and lower layers, resulting in chain segment entanglement in the interfacial region. This molecular-level fusion is equivalent to "self-adhesion," avoiding the interfacial weak layer problem caused by the reliance on adhesives in traditional composites, and making the three-layer structure closer to "integrated molding."

[0125] 3) Eliminate interfacial stress concentration and improve structural uniformity

[0126] Sequential deposition can easily lead to micro-voids or weak bonding zones between layers, becoming stress concentration points. Hot air treatment can smooth out interfacial micro-defects, promote densification, and reduce the risk of failures such as delamination and blistering, thereby improving the overall durability of the material under tensile and bending conditions. Key controls: The temperature must be below the polymer's glass transition temperature (Tg) to prevent excessive softening that could lead to fiber deformation or pore structure collapse; the time must be controlled within 10-15 seconds to ensure that the treatment only affects the surface and does not affect the main structure.

[0127] Example 5

[0128] To further improve filtration efficiency and performance based on the existing methods, the specific technical solutions are as follows:

[0129] 1. Add electrospinning aids (PVP or PEG) to spinning solution A to induce the formation of some nanoscale fiber branches during spinning, construct a multi-level structure of "microfiber trunk + nanofiber side branches", and significantly improve the specific surface area. The specific scheme is as follows: Add a certain amount of PVP / PEG to the spinning suspension after preparation in step (2). The amount added is 3-5wt% of the mass of the fiber-forming polymer. The model is selected as PVPK30 or PEG-1000. The molecular weight is moderate and conducive to fiber formation. Add 5-10% ethanol by mass to adjust the surface tension. When preparing the first fiber membrane, replace the solution jet spinning process with the electrostatic jet spinning process. Adjust the spinning parameters as follows: voltage: 8-10kV, induce branch formation, receiving distance: 25cm, and the fiber diameter can be reduced to 100-300nm. A networked nanostructure is formed in the final first fiber membrane.

[0130] The core mechanism by which this technology enhances filtration performance lies in achieving precise control over fiber morphology through solution property regulation and phase separation induction, thereby significantly improving the overall performance of air filter materials. The specific mechanism is as follows:

[0131] 1) Improves solution viscoelasticity and promotes nanofiber formation.

[0132] Both PVP (polyvinylpyrrolidone) and PEG (polyethylene glycol) are high-molecular-weight thickeners. When added to the spinning solution, they can significantly increase the viscosity and chain entanglement density of the solution. During the electric field stretching process, the higher viscoelasticity can effectively inhibit jet breakage, making it easier for droplets to be drawn into ultrafine nanofibers (the diameter can be reduced to 100-300 nm), which attach to the surface of the main microfiber as "side branches" to form a multi-level network structure.

[0133] 2) Induced phase separation to construct porous and branched structures.

[0134] PVP / PEG exhibits good hydrophilicity, but shows some compatibility differences with the fiber-forming polymer in formic acid solvent. During spinning, as the solvent rapidly evaporates, non-solvent-induced phase separation (NIPS) occurs, leading to the precipitation of locally rich PVP / PEG regions, forming microporous or finely branched structures. These nanoscale branches significantly increase the specific surface area of ​​the fiber membrane, improving the Brownian diffusion capture efficiency of fine particulate matter (such as PM0.3).

[0135] 3) Improve fiber uniformity and continuity, and reduce defects.

[0136] PVP / PEG can adjust the conductivity and surface tension of the spinning solution, making the Taylor cone more stable and the jet more uniform. It effectively reduces defects such as "beaded" structures, resulting in a continuous, smooth, and unbroken fiber network, thereby improving the mechanical integrity and airflow stability of the filter material.

[0137] 4) Enhance surface polarity and improve electrostatic adsorption capacity.

[0138] PVP and PEG molecules contain a large number of polar groups (such as -C=O, -OH), which can give the fiber surface a certain electrostatic potential energy. Without external electret treatment, they can still generate electrostatic adsorption for charged or polar aerosol particles in the air, further improving filtration efficiency, especially for submicron particles.

[0139] It is important to note that the amount of PVP / PEG added should be controlled between 3-5 wt%. Too high a content will result in residual hydrophilic components that affect the material's moisture resistance. It is recommended to remove these components partially or stabilize them through low-temperature heat treatment (<80℃) or cross-linking.

[0140] 2. Introduce submicron-sized porous microspheres (with added mesoporous SiO2) into the outermost layer (first fiber membrane layer) to enhance the inertial impaction capture efficiency of PM0.3. Specifically, add 1.0-1.5 wt% of mesoporous SiO2 microspheres relative to the total mass of spinning solution A to spinning solution A. The pore size of the mesoporous SiO2 microspheres is 10-50 nm, and the particle size is 300-500 nm. The dispersion process is: stirring at room temperature for 6 hours + ultrasonication for 1.5 hours (power 200W). This improves the inertial impaction efficiency and increases the PM0.3 filtration efficiency by 8-12%.

[0141] This technology significantly improves PM0.3 capture efficiency by introducing submicron-sized porous microspheres into the outermost layer. Its core mechanism lies in the synergistic effect of inertial collisions, diffusion interception, and surface adsorption induced by the porous structure, achieving highly efficient capture of ultrafine particles. The specific mechanism is as follows:

[0142] 1) Enhance inertial collision effect

[0143] Submicron-sized porous microspheres (e.g., 300-500 nm) act as three-dimensional barriers with high specific surface area, causing local acceleration and abrupt changes in direction of airflow as it passes through them. Due to their relatively large mass and strong inertia, PM0.3 particles are difficult to change direction with the airflow in time, thus making them more likely to collide with and remain on the surface or pore walls of the microspheres, significantly improving the initial capture efficiency.

[0144] 2) Extend the particle retention path and enhance diffusion interception.

[0145] The porous microspheres have nanoscale interconnected channels (10-50 nm) inside, forming a complex three-dimensional network structure.

[0146] The airflow generates micro-vortices and stagnation zones within the pores, prolonging the residence time of particles in the filter layer and increasing the probability of them contacting the fibers or pore walls. For particles like PM0.3 that are susceptible to Brownian motion, the diffusion and interception efficiency is significantly improved.

[0147] 3) High specific surface area and surface functional groups promote physical adsorption.

[0148] Mesoporous SiO2 or porous polymer microspheres have extremely high specific surface areas (up to 300-800 m²). 2 The microspheres, rich in polar groups such as hydroxyl (-OH), provide numerous adsorption sites. Their surfaces can adsorb polar aerosol particles via van der Waals forces, hydrogen bonds, or dipole interactions, exhibiting a strong affinity for organic or water-soluble particles.

[0149] 4) Gradient pore structure enables step-by-step filtration.

[0150] Porous microspheres are concentrated in the outermost layer (the third layer), forming a gradient structure of "coarse pores → fine pores". Large particles are initially intercepted by the outer microspheres, preventing clogging of the inner fine filtration structure; PM0.3 is efficiently captured in the porous microsphere layer, reducing the load on the inner layer and improving the overall dust holding capacity and service life.

[0151] It should be noted that the amount of microspheres added should be controlled between 1.0-1.5 wt%. Too high an amount will cause abnormal viscosity of the spinning solution or nozzle clogging. It is recommended to use microspheres with surface silanization treatment to improve compatibility with the polymer matrix.

[0152] The performance test results of the high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter materials prepared in Examples 1-3 of this invention are as follows:

[0153] 1. Regarding surface morphology:

[0154] like Figure 4 As shown, where, Figure 4 (a) Electron micrographs of a fiber membrane containing small-diameter microspheres (top) and medium-diameter microspheres (bottom); Figure 4 (b) Electron micrograph of the cross-section of the microsphere composite gradient filter membrane; Figure 4 (c) Electron micrograph of the fiber membrane containing large-diameter microspheres; The three-layer hierarchical structure of the fiber membrane can be clearly seen in the cross-sectional electron micrograph of the pore size gradient membrane. The fiber membrane achieves gradient filtration from large pore size to medium pore size and then to small pore size. Scanning electron microscopy (SEM) observation shows that the PS microspheres are tightly wrapped by PA6 fibers, and the pore size of each layer of the fiber membrane is different. It can be clearly seen from top to bottom in the image that the fiber membrane transitions from large pore size to medium pore size and then to small pore size. The first layer of the fiber membrane is fluffy, which can provide a low-resistance pre-filtration inlet for airflow and particulate matter during filtration. The bottom layer is a small-pore layer (containing 6.5 μm PS microspheres) which is relatively dense, realizing hierarchical filtration of particulate matter from coarse filtration to fine filtration.

[0155] like Figure 5The images shown are electron microscope (EM) images and fiber diameter distribution diagrams of PA6 / PS nanofiber membranes containing different PS microsphere particle sizes in the gradient microsphere composite micro / nanofiber air filter material prepared in Example 1: (a-1) is an EEM image of the first fiber membrane layer containing 1 μm microspheres, (b-1) is an EEM image of the second fiber membrane layer containing 3 μm microspheres, and (c-1) is an EEM image of the third fiber membrane layer containing 6.5 μm microspheres. (a-2) is a fiber diameter distribution diagram of the first fiber membrane layer containing 1 μm microspheres, (b-2) is a fiber diameter distribution diagram of the second fiber membrane layer containing 3 μm microspheres, and (c-2) is a fiber diameter distribution diagram of the third fiber membrane layer containing 6.5 μm microspheres. The EEM images show that the fibers have a "beaded" structure, with numerous micron-sized protrusions forming on the fiber surface. Furthermore, the large-diameter PS microspheres can disturb the airflow during spinning, enhancing the stretching and splitting of the jet, resulting in a decrease in the average fiber diameter and the formation of a finer and more uniform fiber network.

[0156] 2. Regarding pore size and porosity:

[0157] like Figure 6 , 7 As shown in the figure, with the increase of microsphere particle size, the stretching and splitting of the jet during spinning gradually increases, the fibers become significantly finer, and the microspheres gradually occupy more space, while the finer fiber portions fill the gaps. This structure greatly increases the packing density of the fiber network and significantly reduces the pore size of the fiber membrane. The presence of microspheres hinders the tight packing of fibers, expands the pore size, and enhances pore connectivity, making the fiber membrane more three-dimensional and fluffy, increasing the gaps between fibers, and improving porosity. Figure 5 Scanning electron microscopy (SEM) observations showed that the PS microspheres were successfully embedded within the PA6 fibers. With increasing PS microsphere size, the average fiber diameter decreased (PS-1μm: 601.37nm; PS-3μm: 568.68nm; PS-6.5μm: 532.10nm), the porosity increased (PS-1μm: 62.5%; PS-3μm: 70.8%; PS-6.5μm: 79.3%), and the average pore size decreased (PS-1μm: 6.4μm; PS-3μm: 4.8μm; PS-6.5μm: 3.7μm).

[0158] 3. Regarding filtration efficiency and air resistance:

[0159] To investigate the filtration performance of single-layer fiber membranes with microspheres of different sizes (i.e., the first, second, and third fiber membrane layers), this invention uses a Palas MFP Nano plus 4000 test stage to test the filtration performance of PA6 / PS nanofiber membranes with different microsphere sizes at an air flow rate of 32.5 L / min, as shown in the table below.

[0160]

[0161] Long-term operational stability: After 15 consecutive days of testing, the filtration efficiency decreased by 0.902%, and the air resistance increased by 16 Pa. This demonstrates that single-layer fiber membranes containing microspheres of different sizes all exhibited superior filtration performance. With increasing microsphere size, the filtration efficiency was significantly improved due to the reduced pore size and increased porosity, while the pressure drop also decreased significantly.

[0162] like Figure 8 As shown, large sodium chloride particles with a diameter ranging from 1 to 7.25 μm are mainly deposited on the surface and in the pores of the macroporous layer. These large particles are mostly trapped in the pore traps formed by the interwoven fibers or attached to the protrusions of the microspheres, while submicron-sized fine particles are rarely observed in this layer. This indicates that the layer primarily captures large particles through inertial impaction and direct interception mechanisms, achieving a "pre-filtering" function that effectively protects subsequent fine structures while ensuring low-resistance airflow.

[0163] like Figure 9 As shown, the medium-sized sodium chloride particles, ranging from 0.5 to 2 μm in diameter, are mainly deposited on the surface and in the pores of the mesopore layer fibers. The particles are evenly distributed, with some adhering to the fiber surface and others embedded in the pores of the fiber network. This layer bears the main filtration load through the synergistic effect of direct interception, inertial impaction, and diffusion of smaller particles, while also homogenizing the airflow, thus creating conditions for the fine filtration of the final layer.

[0164] like Figure 10 As shown, the surface of the small-pore fiber layer is mainly deposited with ultrafine sodium chloride particles with a diameter of less than 0.3 μm, especially the most easily penetrating particles in the 0.1-0.3 μm range. This layer, dominated by diffusion effects, combined with direct interception and van der Waals forces, efficiently captures ultrafine particles that penetrate the first two layers, ensuring overall filtration efficiency. At the same time, due to the light particle load, rapid clogging is avoided, providing a guarantee for long-term stability.

[0165] like Figure 11 As shown in the figure, it can be seen that as the particle size of the added microspheres increases, the pressure drop of the fiber membrane gradually decreases, while the filtration efficiency is also improved.

[0166] In summary, this invention uses microsphere size as a key structural control factor. First, it prepares a single-layer composite nanofiber membrane containing microspheres with different microporous structures. Then, following a descending order of pore size, it re-prepares a multilayer air filter material with a macroscopic pore size gradient. Through a novel dual-scale structural design concept, it organically unifies macroscopic pore size gradient engineering with microfiber structure engineering. This allows each layer to have precisely controlled fiber fineness, network bulkiness, and surface roughness determined by microspheres of different sizes. These layers are then continuously stacked according to the logical arrangement of the pore size gradient to form an integrated gradient structure. This achieves a synergistic improvement in size-dependent graded interception of particulate matter, high dust holding capacity, and excellent long-term stability, ultimately overcoming the technical bottleneck of existing air filter materials that struggle to achieve both simultaneously.

[0167] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should be protected by the present invention.

Claims

1. A high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filtration material, which is an integrated gradient structure composite membrane formed by the sequential deposition and stacking of three layers of micro / nanofiber membranes, characterized in that: The composite membrane comprises, from top to bottom, a first fiber membrane layer serving as a coarse filtration layer, a second fiber membrane layer serving as a main filtration layer, and a third fiber membrane layer serving as a fine filtration layer, with pore sizes decreasing sequentially. Within the micro-nano fibers forming the first, second, and third fiber membrane layers, a plurality of microspheres with progressively increasing particle sizes are embedded. The mass fraction of the microspheres in each fiber membrane layer is 0.5-2%, and the particle sizes of the microspheres in the first, second, and third fiber membrane layers are 1 μm, 3-4 μm, and 5-7 μm, respectively. At a gas flow rate of 32.5 L / min, the filtration efficiency for 0.3-7.25 μm particles is >99.97%, and the air resistance is <132 Pa. During a continuous 15-day test, the filtration efficiency decreased by 0.21-0.268%, and the air resistance increased by 5-9 Pa.

2. The high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material according to claim 1, characterized in that: The microspheres in the first fiber membrane have a particle size of 1 μm, the mass fraction of the microspheres in the fiber membrane is 1%, the average diameter of the micro / nanofibers is 601.37 nm, the average pore size of the fiber membrane is 6.4 μm, and the porosity is 62.5%.

3. The high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material according to claim 1, characterized in that: The microspheres in the second fiber membrane have a particle size of 3 μm, the mass fraction of microspheres in the fiber membrane is 1%, the average diameter of the micro / nanofibers is 568.68 nm, the average pore size of the fiber membrane is 4.8 μm, and the porosity is 70.8%.

4. The high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material according to claim 1, characterized in that: The microspheres in the third fiber membrane have a particle size of 6.5 μm, a mass fraction of 1% in the fiber membrane, an average diameter of 532.10 nm for the micro- and nanofibers, an average pore size of 3.7 μm for the fiber membrane, and a porosity of 79.3%.

5. The high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material according to claim 1, characterized in that: The fiber-forming polymer materials used to form the first, second, and third fiber membrane layers are any one of polyamide 6, polyamide 66, polyurethane, and polyvinyl alcohol.

6. The high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material according to claim 1, characterized in that: The microspheres are inorganic rigid microspheres, including any one of silica, titanium dioxide, polystyrene, and polymethyl methacrylate.

7. A method for preparing the high-efficiency, low-resistivity gradient microsphere composite micro / nanofiber air filter material as described in any one of claims 1-6, characterized in that, Includes the following steps: I. Preparation of spinning solution: (1) Dissolve a certain amount of fiber-forming polymer in an 88% formic acid solution and repeat the operation three times to obtain three identical base solutions for later use. (2) Add equal amounts of microsphere powder with different particle sizes to three portions of base solution, stir thoroughly at room temperature for 6 hours, and ultrasonically disperse for 1.5 hours to ensure that the fiber-forming polymer is completely dissolved and the microsphere powder is uniformly dispersed, so as to obtain spinning suspensions containing microspheres with three different particle sizes. (3) The three spinning suspensions are labeled as spinning solution A, spinning solution B and spinning solution C according to the particle size from small to large. II. Preparation of composite membranes: (4) Let the spinning solution A, spinning solution B and spinning solution C obtained in step (3) stand for 30 minutes to remove bubbles, and set aside for later use; (5) Using solution jet spinning, the spinning solution C is first delivered to the nozzle of the air jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, a continuous jet is formed for spinning. After spinning for a certain time, the third fiber membrane layer is obtained. (6) The spinning solution B is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, the spinning solution B is uniformly deposited and stacked on the surface of the third fiber membrane for a certain period of time to obtain the second fiber membrane. (7) The spinning solution A is delivered to the nozzle of the air-jet spinning machine through a syringe and a spinning hose. Under certain spinning conditions, the second fiber membrane layer is uniformly deposited and stacked on the surface for a certain period of time to obtain the first fiber membrane layer and a three-layer composite membrane. (8) Take off the composite membrane obtained in step (7), put it in a vacuum drying oven at 60°C and dry it for 30 minutes to remove the residual formic acid solvent and obtain the gradient microsphere composite micro-nanofiber air filter material.

8. The method for preparing the high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material according to claim 7, characterized in that: In the base solution of step (1), a dispersing stabilizer is added, and the mass fraction of the dispersing stabilizer in the spinning suspension of step (2) is 0.1-0.3%.

9. The method for preparing the high-efficiency, low-resistance gradient microsphere composite micro / nanofiber air filter material according to claim 7, characterized in that: In the spinning suspension of step (2), the mass fraction of the fiber-forming polymer is 8-10%, and the mass fraction of the microsphere powder is 1-1.5%.

10. The method for preparing the high-efficiency, low-resistivity gradient microsphere composite micro / nanofiber air filter material according to claim 7, characterized in that: In steps (5)-(7), the inner diameter of the spinning needle is 0.33 mm, the spinning solution propulsion speed is 1-2 mL / h, the compressed air pressure is 0.09-0.12 MPa, the receiving distance is 25-30 cm, the collector winding speed is 500-700 r / h, the ambient temperature is 30℃, and the ambient humidity is 35%.

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