A gradient composite air filter paper and a preparation method thereof

Abstract

The application discloses a gradient composite air filter paper and a preparation method thereof. A gradient composite structure composed of a support skeleton layer, a transition bonding layer and a fine fiber filter layer is constructed in the thickness direction of the filter paper, so that the material has good structural coordination while maintaining high filtering efficiency. Through the synergistic effect of low-melting-point binding fibers and flexible bonding agents, a stable and flexible connection structure is formed between the fibers and the interlayer interface, so that each layer can deform cooperatively under stress, avoiding stress concentration. The gradient structure forms a reasonable rigid-flexible distribution in the thickness direction of the filter paper, and can realize step-by-step transition of strain under subsequent pressure and bending conditions, thereby reducing the risk of local structural instability and improving the overall structural consistency and use stability.

Description

Technical Field

[0001] This application relates to the field of air filtration materials technology, and in particular to a gradient composite air filter paper and its preparation method. Background Technology

[0002] In existing technologies, air filter paper is formed by wet-processing after fiber dispersion, and a filter layer structure is constructed on the surface of the substrate to intercept particulate matter in the air. At the same time, in order to improve the filtration effect, the particle capture capacity is often enhanced by increasing the surface fine fibers or improving the material density. Such filter paper usually needs to be further processed into a folded structure to increase the filtration area.

[0003] However, the current structural design and manufacturing process of air filter paper focuses primarily on filtration performance, with insufficient consideration given to its structural adaptability in subsequent forming and processing stages. Especially during continuous folding or forming, the filter paper needs to withstand repeated compression and bending in a predetermined area. Because existing filter paper has a relatively uniform structure in the thickness direction and lacks a reasonable transition relationship between material layers, it is prone to uneven strain distribution under stress. In practical use, this manifests as inconsistent local deformation of the material, making it difficult for the filter paper to maintain a stable structural shape during folding.

[0004] When a fine fiber filter layer is set on the surface of filter paper to improve filtration efficiency, the bonding between the fine fiber layer and the matrix is ​​often relatively simple. When subjected to repeated bending or local pressure, stress concentration is likely to occur in the interface area, resulting in insufficient interlayer structural stability. This makes the filter paper more prone to problems such as local structural instability, uneven deformation, or poor forming consistency during subsequent processing or use. Summary of the Invention

[0005] The purpose of this application is to provide a gradient composite air filter paper and its preparation method, solving the technical problem that existing air filter papers struggle to balance filtration performance and molding stability. To address the above problem, this application adopts the following technical solution: A method for preparing gradient composite air filter paper, comprising: Plant fibers and synthetic short fibers are dispersed in water according to a preset mass ratio, and low-melting-point bonding fibers and wet strength agents are added. After stirring and dispersing, a transition slurry is prepared. The transition slurry is formed into a wet paper layer, and a fine fiber dispersion is applied to one side surface of the wet paper layer to form a fine fiber filter layer on the surface of the wet paper layer, thereby obtaining a wet composite filter paper. The wet composite filter paper is treated with a flexible binder and then subjected to pressing, dehydration, and heat drying to form fiber bonding points, thereby obtaining gradient composite air filter paper. The resulting gradient composite air filter paper sequentially forms a support skeleton layer, a transition bonding layer, and a fine fiber filter layer in the thickness direction.

[0006] Further, the step of dispersing plant fibers and synthetic short fibers in water at a preset mass ratio includes: Plant fibers are pre-wetted at 40-80℃, and synthetic short fibers are dispersed at 40-70℃. Plant fibers and synthetic short fibers are added to water and stirred according to a preset mass ratio to obtain an initial fiber mixture slurry. The initial fiber mixture slurry is pulped and homogenized to control the slurry concentration at 0.1% to 1.5%, thus obtaining the supporting skeleton slurry.

[0007] Furthermore, the plant fiber includes at least one of softwood pulp fiber, hardwood pulp fiber, bamboo pulp fiber, and hemp pulp fiber, and the synthetic short fiber includes at least one of polyester fiber, polypropylene fiber, polyamide fiber, and bicomponent composite fiber, and the mass ratio of the plant fiber to the synthetic short fiber is (55~90):(10~45).

[0008] Further, the step of adding low-melting-point bonding fibers and wet-strength agents, and then dispersing them by stirring to obtain a transition slurry includes: Add low-melting-point bonding fibers to the supporting skeleton slurry and stir at 300~1000r / min for 5~20min; Add a wet strength agent to the slurry containing low-melting-point bonding fibers and continue stirring for 3-15 minutes to obtain a uniformly mixed transition slurry. During the stirring process, the mass percentage of fiber solids in the transition slurry is controlled to be 0.1% to 1.2%.

[0009] Furthermore, the low-melting-point bonding fiber includes at least one of polyethylene / polyester bicomponent fiber, polypropylene / polyester bicomponent fiber, and low-melting-point polyester fiber, and the wet strength agent includes at least one of polyamide epichlorohydrin wet strength agent, melamine formaldehyde resin wet strength agent, and polyethyleneimine wet strength agent. The mass ratio of the plant fiber, synthetic short fiber, and low-melting-point bonding fiber is (55~85):(10~35):(3~15).

[0010] Further, the step of forming the transition slurry into a wet paper layer, applying a fine fiber dispersion to one side surface of the wet paper layer, and forming a fine fiber filter layer on the surface of the wet paper layer to obtain wet composite filter paper includes: The transition slurry is fed to a paper forming device for wet forming to obtain a wet paper layer, wherein the plant fibers and synthetic short fibers are interwoven to form a support skeleton layer, and the low melting point bonding fibers and wet strength agent are distributed in the support skeleton layer to form a transition bonding layer; The fine fibers are mixed with a dispersion medium, and a dispersion aid is added for dispersion treatment to obtain a fine fiber dispersion. The fine fiber dispersion is applied to one side surface of the wet paper layer to deposit the fine fibers and form a fine fiber filter layer, thus obtaining a wet composite filter paper. The basis weight of the fine fiber filter layer is controlled to be lower than that of the wet paper layer, so that the resulting wet composite filter paper forms a gradient structure in the thickness direction, from coarse fibers to fine fibers.

[0011] Further, the fine fibers include at least one of glass microfibers, ultrafine polyester fibers, ultrafine polypropylene fibers, and electret fine fibers; the dispersing agent includes at least one of nonionic surfactants, anionic dispersants, and alcohol wetting agents; and the basis weight of the fine fiber filter layer is 2~25 g / m³. 2 The basis weight of the wet paper layer is 20~120 g / m³. 2 .

[0012] Further, the step of applying a flexible binder to the wet composite filter paper includes: Prepare a flexible binder treatment solution and apply the flexible binder treatment solution to the wet composite filter paper obtained in step S3. The flexible binder treatment solution includes a flexible binder and water. By controlling the penetration of the flexible binder treatment liquid into the wet paper layer and the fine fiber filter layer, and forming a flexible connection at the fiber cross-linking point and the interlayer interface, a composite wet paper with interlayer reinforcement is obtained. The amount of the flexible binder treatment liquid applied is controlled within a range that maintains the interconnected pore structure of the fine fiber filter layer.

[0013] Furthermore, the step of pressing, dehydrating, and heating to dry and shape the low-melting-point bonding fibers to form fiber bonding points includes: The composite wet paper is pressed and dehydrated, and the pressed composite wet paper is heated and dried. The drying temperature is controlled at 80~180℃ and the drying time is controlled at 0.5~10min. During the heating and drying process, the low-melting-point bonding fibers are softened and form bonding points with the surrounding fibers to obtain gradient composite air filter paper. The resulting gradient composite air filter paper is cooled and then wound up to obtain the gradient composite air filter paper.

[0014] This application also discloses a gradient composite air filter paper, which is prepared by the method described in any of the above claims.

[0015] Compared with the prior art, this application has the following beneficial effects: This application constructs a gradient composite structure along the thickness of the filter paper, consisting of a supporting skeleton layer, a transition bonding layer, and a fine fiber filter layer. This structure allows the material to maintain high filtration efficiency while possessing good structural coordination. The supporting skeleton layer provides overall support, the transition bonding layer facilitates a smooth transition between different structural layers, and the fine fiber filter layer effectively intercepts fine particles, thus ensuring both filtration performance and structural stability during use. Simultaneously, the synergistic effect of low-melting-point bonding fibers and a flexible binder creates a stable and flexible connection structure between fibers and at interlayer interfaces. This allows each layer to deform collaboratively under stress, preventing stress concentration. This gradient structure enables a reasonable distribution of rigidity and flexibility along the thickness of the filter paper, allowing for gradual strain transitions under subsequent compression and bending conditions. This reduces the risk of local structural instability and improves overall structural consistency and operational stability. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] The structures, proportions, sizes, etc., shown in the accompanying drawings are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed in the specification, and are not intended to limit the implementation conditions of this application. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size should still fall within the scope of the technical content disclosed in this application, provided that they do not affect the effects and purposes that this application can produce.

[0018] Figure 1 This is a schematic diagram of the overall steps in the preparation method of a gradient composite air filter paper. Detailed Implementation

[0019] To make the inventive objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] In the description of this application, it should be understood that the terms "upper," "lower," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. It should be noted that when a component is considered to be "connected" to another component, it can be directly connected to the other component or there may be a component centrally located at the same time.

[0021] The technical solution of this application will be further described below with reference to the accompanying drawings and specific embodiments.

[0022] refer to Figure 1 This application provides a method for preparing gradient composite air filter paper, comprising: S1: Plant fibers and synthetic short fibers are added to water and dispersed according to a preset mass ratio, and low melting point bonding fibers and wet strength agent are added. After stirring and dispersing, a transition slurry is prepared. In step S1, two main fiber raw materials are prepared: plant fibers and synthetic short fibers. The plant fibers are preferably natural fibers with good water absorption and swelling capacity and web-forming ability. Their main function is to improve the dispersion stability of the slurry in water and the fiber interlacing ability during subsequent papermaking. The synthetic short fibers are mainly used to improve the dimensional stability, folding endurance, and overall skeleton strength of the formed substrate. Since plant fibers usually exhibit a certain degree of agglomeration in the dry state, they are preferably pre-wetted before being added to the main mixing system to allow them to gradually absorb water and expand, thereby improving the dispersion of fiber monomers. Simultaneously, the synthetic short fibers can be pre-dispersed in a separate container using mechanical stirring, hydraulic circulation, or a dispersion device to open up the fiber bundles and prevent the formation of flocs during subsequent mixing. After pretreatment, the plant fibers and synthetic short fibers are added to the main mixing tank according to a preset ratio and mixed under continuous stirring conditions, so that the two types of fibers gradually form a uniform suspension mixture in the aqueous phase. In this process, plant fibers preferentially form a continuous fiber network, while synthetic short fibers are dispersed and interspersed within the plant fiber network, thus providing a precursor for the subsequent formation of the supporting framework structure. After the basic mixing of plant fibers and synthetic short fibers is completed, low-melting-point binder fibers and a wet-strength agent are introduced into the mixture. The low-melting-point binder fibers do not need to melt at this stage; they are embedded in the fiber network composed of plant fibers and synthetic short fibers in a dispersed state. They can soften in the subsequent heat treatment stage and form thermal bonding points at the contact points of adjacent fibers. After the addition of the low-melting-point binder fibers, stable stirring is maintained to ensure uniform distribution along the volume direction of the pulp. The wet-strength agent is used to improve the retention of the wet paper layer during forming, transfer, and surface application of the fine fiber dispersion. It can be adsorbed or distributed on the fiber surface and fiber cross regions in the pulp system, thereby enhancing the integrity of the wet network. To ensure the forming performance of the pulp, the mixed pulp usually needs to undergo a certain degree of homogenization treatment, such as through circulating pulping, shear mixing, or gentle redispersion, to maintain a relatively stable fiber distribution, pulp concentration, and rheological state in the system. This ensures that a wet paper layer with a relatively uniform thickness and consistent structure can be formed during subsequent papermaking. After the above treatment, a transition pulp for subsequent forming is obtained. The transition pulp contains a composite pulp system of support network precursors, thermal bonding precursors, and wet-holding components.

[0023] S2: The transition slurry is formed into a wet paper layer, and a fine fiber dispersion is applied to one side surface of the wet paper layer to form a fine fiber filter layer on the surface of the wet paper layer, thereby obtaining wet composite filter paper; In step S2, the transition slurry is conveyed to paper forming equipment, such as a forming wire, a vacuum dewatering forming device, or other wet forming mechanism. Water in the slurry is gradually removed under gravity, vacuum suction, or wire filtration, leaving fibers trapped on the forming surface to form a continuous wet paper layer. During this forming process, plant fibers, due to their inherent good entanglement and web-forming capabilities, preferentially form the basic skeleton of the wet paper layer. Synthetic short fibers are dispersed within this skeleton, reinforcing its spatial stability and toughness. Low-melting-point bonding fibers and wet strength agents are diffusely distributed within the skeleton, providing conditions for subsequent interlayer bonding and thermal bonding. It should be noted that although the wet paper layer formed at this stage already has a layered shape, it is not a completely homogeneous solid layer. Instead, it is a wet fiber network with high water content, incompletely shrunk pores, and initial fiber cross-linking. Therefore, the wet paper layer at this stage has good surface bearing capacity and a certain degree of penetration and receptivity, creating conditions for subsequent application of fine fiber dispersion liquid to its surface. After the wet paper layer is formed, a fine fiber dispersion for surface filtration is prepared. The fine fibers in the dispersion can be fibrous materials capable of forming small-pore filtration structures. After the dispersion medium is added, they are fully separated using appropriate dispersion methods to avoid agglomeration due to surface interactions between the fine fibers. A suitable amount of additives can also be introduced during the dispersion process to ensure the fine fibers are suspended in the dispersion in a relatively uniform state. After dispersion is complete, the fine fiber dispersion is applied to one side of the aforementioned wet paper layer. The application method can be surface coating, spraying, coating, or secondary deposition—methods suitable for uniform liquid distribution—allowing the fine fibers to gradually deposit on the surface of the wet paper layer. Since the wet paper layer itself is still in a hydrated state, its surface and interior retain a certain amount of moisture. Therefore, after the fine fiber dispersion comes into contact with the wet paper layer, the fine fibers in the surface area are affected by moisture migration, surface tension, and the retention effect of the fiber network. Some fine fibers deposit on the outermost surface to form a denser filter layer, while others are slightly embedded in the shallow surface area of ​​the wet paper layer, thus forming a natural transition interface between the wet paper layer and the fine fiber filter layer. With the penetration of the wet interface and fiber embedding, the resulting wet composite filter paper begins to show a gradual transition from a coarse fiber network to a fine fiber filter layer in the thickness direction. In actual implementation, the amount of fine fiber layer applied can be controlled to be lower than the basis weight or fiber weight of the wet paper layer itself to ensure that the support layer still constitutes the main thickness, while the surface fine fiber layer mainly undertakes the filtration function. After this treatment, a wet composite filter paper with a preliminary composite structure is obtained.

[0024] S3: Apply a flexible binder to the wet composite filter paper and then press, dehydrate, heat, dry, and shape it to form fiber bonding points with low-melting-point bonding fibers, thereby obtaining gradient composite air filter paper. In step S3, while the wet composite filter paper is still in a wet or semi-wet state, a flexible binder is prepared into a suitable treatment solution and applied to the composite structure through surface spraying, roller coating, or light wetting. This allows the binder to preferentially penetrate into the interface region between the wet paper layer and the fine fiber filter layer under capillary action and interfacial moisture transfer. The flexible binder forms several flexible connection nodes at fiber contact points, fiber cross regions, and interfacial transition zones, thereby improving the integrity of the interface region without significantly disrupting the pore connectivity of the fine fiber layer. The flexible binder acts as a flexible bridge, creating a stress-bearing connection between the surface filter layer and the bottom wet paper layer. After the flexible binder is applied, the resulting composite wet paper is subjected to a pressing and dewatering treatment to reduce the overall moisture content for subsequent heat drying. Appropriate mechanical pressure further compacts the wet fiber network and improves the contact between different layers. During the pressing process, the larger pores within the wet paper layer shrink moderately, increasing the contact area between the surface fine fibers and the underlying fiber network. Simultaneously, the flexible binder is more easily distributed into the tiny interfacial pores under pressure. Therefore, the pressing stage is essentially a process of further adjustment and integration of the interlayer structure. It is important to note that the pressing degree should not be too high to avoid excessive embedding of the fine fiber filter layer or significant damage to the pore structure. Therefore, this step emphasizes achieving a balance between dehydration and structural preservation. After pressing, the composite wet paper is sent to a heated drying zone for heat drying and shaping. During this process, the internal moisture of the material gradually evaporates, the fiber network of the wet paper layer converges and becomes fixed, while the low-melting-point bonding fibers gradually soften under heat and form thermal bonding points at the contact points with adjacent plant fibers and synthetic short fibers. With controlled temperature and residence time, these thermal bonding points gradually establish themselves within the skeleton layer and near the interface, thereby fixing the previously loose wet composite structure into a stable dry composite structure. At the same time, the effective components in the flexible binder also gradually form a film or solidify during the drying process. However, this solidification is mainly manifested as point-like, bridging, or locally continuous flexible connections, rather than forming a completely dense barrier layer on the surface. Therefore, it can balance interfacial bonding and porosity retention.

[0025] As the heat drying process ends and the cooling stage begins, the internal connections of the material are finally fixed. At this point, the resulting filter paper forms a relatively clear three-layer progressive structure in the thickness direction: the lower layer is a support skeleton layer formed by interwoven plant fibers and synthetic short fibers, which plays a major supporting role; the middle layer is a transitional bonding layer formed by the combined action of low-melting-point bonding fibers, wet-strength agents, and flexible binders; and the upper layer is a fine fiber filter layer composed of fine fiber deposition, which plays a major fine filtration role. Because this three-layer structure is not formed by mechanical bonding, but is gradually constructed through continuous processes such as wet forming, surface deposition, interface penetration, and heat setting, the layers have a more natural transition relationship and higher integrity. Thus, the final gradient composite air filter paper maintains the skeleton stability of the support layer, takes into account the filtration function of the surface fine fiber filter layer, and achieves a coordinated connection between different structural layers through the intermediate transitional bonding layer. The filter paper produced in this way can be further wound up and used as an air filtration substrate. Because its thickness direction forms a gradient composite structure from coarse to fine and from support to filtration, and a controllable flexible bonding system is established inside, it can better adapt to the pressure and bending state of the folded area when it is subsequently processed into continuous M-shaped pleated filter paper, reducing the risk of instability caused by interface separation, surface damage or structural incompatibility.

[0026] In summary, the gradient composite air filter paper obtained in this application achieves the dispersion and buffering of external forces through the synergistic effect between structural layers with different thickness directions during use. When the filter paper is bent or compressed during subsequent processing or use, the bottom supporting skeleton layer first provides overall structural support to prevent material collapse. At the same time, the intermediate transition bonding layer buffers and disperses the force, allowing stress to be gradually transferred rather than concentrated at a single location. The fine fiber filter layer on the surface can deform in sync with the overall structure under the connection with the underlying structure, thereby avoiding local peeling or damage. Meanwhile, the connecting structure formed by low-melting-point bonding fibers and flexible binders ensures that the layers maintain good integrity and a certain degree of flexibility during stress, so that the filter paper can maintain structural stability under repeated bending conditions and is not prone to cracking, delamination, or springback. That is, when the filter paper is repeatedly folded, the layers can deform together without being pulled apart or broken, resulting in a more stable and durable shape.

[0027] It is worth noting that although the above embodiments do not list all possible raw material ratios and process parameter ranges, those skilled in the art can adjust the raw material ratios within the recommended range according to actual needs to optimize the performance of the composite material.

[0028] In another embodiment, referring to Table 1, comparative samples were selected for performance testing. The unified testing conditions were 23°C and 50% relative humidity. Filtration efficiency and initial resistance were measured using 0.3μm particles and a face velocity of 5.3cm / s. Continuous folding tests were conducted with the same folding distance, the same pressure line position, and 200 reciprocating bends. After folding, the number of cracks in the folded area, the interlayer peeling length, and the post-folding rebound rate were recorded to ensure comparability between different samples. This embodiment uses the gradient composite air filter paper preparation method described in this application. The filter paper forms a three-layer progressive structure in the thickness direction: a supporting skeleton layer, a transition bonding layer, and a fine fiber filter layer. A flexible interlayer connection is constructed through low-melting-point bonding fibers and a flexible binder. Comparative Example 1 uses a uniform fiber wet-formed filter paper with a common filter layer on the surface, but it lacks a clear thickness gradient structure and flexible interface reinforcement. Therefore, although it has a certain filtration capacity and relatively low initial resistance, the number of cracks at the fold lines increases significantly after continuous folding, and the interlayer peel length and resilience are also significantly higher than in this embodiment, indicating that its performance in this embodiment is significantly lower. When the folded area is subjected to pressure and bending, the stress distribution is uneven and the interlayer integrity is insufficient. Comparative Example 2 improves the filtration performance by increasing the amount of fine fiber deposited on the surface and the degree of surface densification on the basis of conventional filter paper. Its filtration efficiency is slightly higher than that of this embodiment, but the initial resistance is significantly increased. At the same time, since it still lacks the support, transition, filtration progression structure and flexible connection system described in this application, it also shows more cracks and a larger interlayer peeling length after continuous folding, and the rebound rate is high. This shows that although increasing the content of fine fiber on the surface or increasing the surface density can improve the filtration effect, it cannot effectively solve the structural stability problem during subsequent continuous folding. In contrast, this embodiment maintains high filtration efficiency and moderate initial resistance while minimizing the number of cracks, the shortest interlayer peeling length, and the lowest springback rate after continuous folding. This indicates that by constructing a gradient composite structure from coarse to fine and from support to filtration in the thickness direction of the filter paper, and forming a flexible buffer connection at the interface, this application enables the material to achieve more uniform stress transmission and coordinated deformation when it is compressed and bent in the folded area, thereby significantly improving the structural stability and shape retention in the subsequent M-shaped folding process.

[0029] Table 1: In one embodiment, the step of dispersing plant fibers and synthetic short fibers in water at a preset mass ratio includes: Plant fibers are pre-wetted at 40-80℃, and synthetic short fibers are dispersed at 40-70℃. Plant fibers and synthetic short fibers are added to water and stirred according to a preset mass ratio to obtain an initial fiber mixture slurry. The initial fiber mixture slurry is pulped and homogenized to control the slurry concentration at 0.1% to 1.5%, thus obtaining the supporting skeleton slurry.

[0030] In this embodiment, the selected plant fibers are pre-wetted at 40-80°C for 0.5-3 hours, allowing the dry fibers to fully absorb water and expand, gradually opening up the previously tangled fiber bundles, facilitating uniform mixing with other fibers. Pre-wetting also improves the surface hydrophilicity and dispersibility of the plant fibers, promoting the formation of a continuous fiber network during subsequent papermaking. Simultaneously, the synthetic short fibers are pre-dispersed in another dispersion container at room temperature or 40-70°C for 0.5-2 hours, transforming the synthetic short fibers from a bundled state to a relatively separated state, preventing agglomeration in the main mixing tank. The pre-wetted plant fibers and pre-dispersed synthetic short fibers are then added to the main mixing tank in a predetermined ratio and stirred at 300-1200 rpm for 5-30 minutes to form a uniform suspension mixture. Here, the plant fibers primarily serve as the web and framework, while the synthetic short fibers primarily improve dimensional stability, folding endurance, and framework toughness. After initial mixing, appropriate beating and homogenization are performed to refine and texturize the surface of the plant fibers, improving their mechanical entanglement and allowing the synthetic short fibers to be more evenly dispersed within the plant fiber network. The resulting supporting sizing material is a composite sizing system that combines the web-forming properties of natural fibers with the structural stability of chemical fibers.

[0031] In one specific embodiment, the plant fiber is softwood pulp fiber, the synthetic staple fiber is polyester staple fiber, and the mass ratio of plant fiber to synthetic staple fiber is 75:25. At this ratio, the plant fiber forms a continuous support network, and the polyester staple fiber is dispersed within the plant fiber network to improve the dimensional stability and folding endurance of the filter paper. Further, the concentration of the initial fiber mixture pulp is controlled at 0.8%.

[0032] In one embodiment, the step of adding low-melting-point bonding fibers and wet-strength agents, and then dispersing them by stirring to obtain a transition slurry includes: Add low-melting-point bonding fibers to the supporting skeleton slurry and stir at 300~1000r / min for 5~20min; Add a wet strength agent to the slurry containing low-melting-point bonding fibers and continue stirring for 3-15 minutes to obtain a uniformly mixed transition slurry. During the stirring process, the mass percentage of fiber solids in the transition slurry is controlled to be 0.1% to 1.2%.

[0033] In the above embodiments, after the supporting skeleton pulp has been stably formed, low-melting-point bonding fibers are gradually added to it. These fibers do not generate thermal bonding during the pulping stage; they are added while being stirred and dispersed, ensuring they are distributed throughout the entire pulp volume rather than concentrated in localized areas. After adding the low-melting-point bonding fibers, the pulp is gently stirred to allow them to penetrate the initial skeleton network formed by plant fibers and synthetic short fibers. Subsequently, a wet-strength agent is added. The wet-strength agent gradually disperses in the aqueous phase and adsorbs onto the fiber surfaces and fiber contact areas in the pulp, thereby improving the retention capacity of the wet paper layer during forming and post-processing stages. It should be noted that the wet-strength agent does not form a separate independent structure at this stage, but rather is dispersed throughout the entire pulp system, particularly focusing on the fiber contact areas within the future supporting skeleton layer and transition bonding zone. By controlling the proportions of each raw material, the final transition slurry can maintain sufficient forming fluidity while possessing high interlayer construction capability. Plant fibers form the main web framework, synthetic short fibers improve toughness and dimensional retention, low-melting-point bonding fibers provide conditions for subsequent thermal bonding, and wet-strength agents ensure that the wet paper layer is not easily damaged when receiving fine fiber deposition. Through this construction method, the transition slurry effectively contains all the precursor components that form the supporting framework layer and the transition bonding layer.

[0034] In another specific embodiment, the mass ratio of the plant fiber, synthetic staple fiber, and low-melting-point binder fiber is 72:20:8. The plant fiber is softwood pulp fiber, the synthetic staple fiber is polyester staple fiber, and the low-melting-point binder fiber is polyethylene / polyester bicomponent fiber. This ratio ensures both the web-forming properties of the supporting skeleton layer and the formation of a sufficient number of fiber bonding points after subsequent heat treatment. Furthermore, the wet-strength agent is added at 0.6% of the total mass of the plant fiber, synthetic staple fiber, and low-melting-point binder fiber.

[0035] In one specific embodiment, when adding low-melting-point bonding fibers to the supporting skeleton slurry, the stirring speed is controlled at 600 r / min and the stirring time is 10 min; after adding the wet strength agent, stirring is continued for 8 min, and the mass percentage of fiber solids in the transition slurry is controlled at 0.6%, so that the low-melting-point bonding fibers and wet strength agent are more evenly distributed in the slurry.

[0036] In one embodiment, the step of forming the transition slurry into a wet paper layer, applying a fine fiber dispersion to one side surface of the wet paper layer, and forming a fine fiber filter layer on the surface of the wet paper layer to obtain wet composite filter paper includes: The transition slurry is fed to a paper forming device for wet forming to obtain a wet paper layer, wherein the plant fibers and synthetic short fibers are interwoven to form a support skeleton layer, and the low melting point bonding fibers and wet strength agent are distributed in the support skeleton layer to form a transition bonding layer; The fine fibers are mixed with a dispersion medium, and a dispersion aid is added for dispersion treatment to obtain a fine fiber dispersion. The fine fiber dispersion is applied to one side surface of the wet paper layer to deposit the fine fibers and form a fine fiber filter layer, thus obtaining a wet composite filter paper. The basis weight of the fine fiber filter layer is controlled to be lower than that of the wet paper layer, so that the resulting wet composite filter paper forms a gradient structure in the thickness direction, from coarse fibers to fine fibers.

[0037] In the above embodiments, the transition slurry is fed into a wet forming apparatus. Water is gradually drained from the slurry through a forming mesh, vacuum suction, or a water-filtering support component, and fibers are deposited on the forming surface to form a continuous wet paper layer. During this process, plant fibers, due to their superior web-forming ability, preferentially form a continuous main framework, while synthetic short fibers interweave within this framework to form a reinforcing structure. Low-melting-point bonding fibers and wet-strength agents are dispersed within the fiber network, enabling the resulting wet paper layer to possess a certain degree of integrity and load-bearing capacity even while still containing a high moisture content. After the wet paper layer is completed, a fine fiber dispersion is prepared. Due to their smaller diameter and larger surface area, fine fibers are generally more prone to aggregation than coarse fibers in the substrate. Therefore, when preparing a fine fiber dispersion, a dispersion medium and appropriate additives are needed to keep it in a relatively stable suspension state. Subsequently, this dispersion is applied to one side of the wet paper layer. Under the action of liquid penetration and surface interception, the fine fibers gradually deposit on the surface of the wet paper layer. Some of the fine fibers remain on the outermost layer, forming a dense filter layer, while others are embedded in the shallow surface area of ​​the wet paper layer, forming a natural transition zone. In other words, the surface filter layer is formed at the wet interface through both deposition and embedding, thus exhibiting a more natural transition relationship with the underlying layer. By controlling the basis weight of the fine fiber layer to be lower than that of the wet paper layer itself, it can be ensured that the entire material remains dominated by the support layer, while the fine fiber layer mainly undertakes the task of fine filtration, thereby forming a gradient structure in the thickness direction that gradually transitions from coarse fibers to fine fibers.

[0038] In one specific embodiment, the fine fiber filter layer is formed of glass microfibers with a basis weight of 12 g / m³. 2 The basis weight of the wet paper layer is 65 g / m³. 2 Furthermore, the basis weight ratio of the fine fiber filter layer to the wet paper layer is 12:65, or approximately 1:5.4. With this basis weight ratio, the fine fiber filter layer primarily performs fine filtration, while the wet paper layer remains the main support layer, thus facilitating the formation of a stable gradient transition structure in the thickness direction.

[0039] In one embodiment, the fine fibers include at least one of glass microfibers, ultrafine polyester fibers, ultrafine polypropylene fibers, and electret fine fibers; the dispersing agent includes at least one of nonionic surfactants, anionic dispersants, and alcohol wetting agents; and the basis weight of the fine fiber filter layer is 2-25 g / m³. 2 The basis weight of the wet paper layer is 20~120 g / m³. 2 .

[0040] In another specific embodiment, the basis weight of the fine fiber filter layer is controlled to be 10% to 30% of the basis weight of the wet paper layer. For example, when the basis weight of the wet paper layer is 80 g / m³ 2 At that time, the quantitative amount of the fine fiber filter layer can be 8 g / m³. 2 15g / m 2 Or 20g / m 2 Preferably, the fine fiber filter layer has a basis weight of 15 g / m³. 2 By controlling the basis weight of the fine fiber filter layer within the aforementioned range of the basis weight of the wet paper layer, both filtration efficiency and initial resistance can be balanced.

[0041] In one embodiment, the step of applying a flexible binder to the wet composite filter paper includes: Prepare a flexible binder treatment solution and apply the flexible binder treatment solution to the wet composite filter paper obtained in step S3. The flexible binder treatment solution includes a flexible binder and water. By controlling the penetration of the flexible binder treatment liquid into the wet paper layer and the fine fiber filter layer, and forming a flexible connection at the fiber cross-linking point and the interlayer interface, a composite wet paper with interlayer reinforcement is obtained. The amount of the flexible binder treatment liquid applied is controlled within a range that maintains the interconnected pore structure of the fine fiber filter layer.

[0042] In this embodiment, a flexible binder treatment solution is formulated according to the required permeability and film-forming properties, ensuring sufficient fluidity for application without excessive viscosity that would cause it to remain on the surface and form a closed film. This treatment solution is applied to the wet composite filter paper. The application method can be selected based on equipment conditions, but the general principle is to allow the flexible binder to preferentially enter the interface region between the surface fine fiber layer and the base wet paper layer, rather than accumulating entirely on the outermost surface. Since the wet composite filter paper still has a certain moisture content and capillary structure, the flexible binder migrates with the liquid into the interfacial micropores, fiber intersections, and local voids, forming flexible bridging relationships in these areas. This connection adds several flexible connection points between fibers and layers, making the surface filter layer less prone to detaching from the matrix during subsequent pressing, drying, and use under stress. In other words, the interfacial relationship, which originally relied mainly on wet contact and subsequent thermal bonding, is further strengthened into an interlayer relationship with elastic buffering. By controlling the application amount, the fine fiber filter layer can maintain its interconnected pore structure, thus enhancing interlayer integrity without significantly sacrificing filtration channels.

[0043] In one specific embodiment, the flexible binder concentration in the flexible binder treatment solution is 1.5% by mass, with the remainder being water; the application rate of the flexible binder treatment solution is 5 g / m³. 2 Based on the amount of oven-dry flexible binder, the amount of flexible binder added to the finished filter paper is 0.075 g / m. 2 At this application rate, the flexible binder can penetrate into the interface region between the fine fiber filter layer and the wet paper layer, forming a flexible bridging structure without significantly blocking the pores.

[0044] In another specific embodiment, the amount of the flexible binder treatment liquid applied is controlled to be 10% to 40% of the quantitative amount of the fine fiber filter layer. For example, when the quantitative amount of the fine fiber filter layer is 12 g / m³... 2 At that time, the amount of flexible binder applied in the flexible binder treatment solution during oven-drying can be controlled to be 1.2 g / m³. 2 2.4g / m 2 Or 4.0g / m 2 Preferably, the amount of the flexible binder applied at room temperature is 2.4 g / m³. 2 When the amount of flexible binder applied dry exceeds 50% of the basis weight of the fine fiber filter layer, some pores of the fine fiber filter layer are easily covered by excessive binder, which is not conducive to maintaining the interconnected pore structure. The range for maintaining the interconnected pore structure is that the amount of flexible binder applied dry does not exceed 40% of the basis weight of the fine fiber filter layer, preferably not exceeding 30%.

[0045] In one embodiment, the step of pressing, dehydrating, and heating to dry and shape the low-melting-point bonding fibers to form fiber bonding points includes: The composite wet paper is pressed and dehydrated, and the pressed composite wet paper is heated and dried. The drying temperature is controlled at 80~180℃ and the drying time is controlled at 0.5~10min. During the heating and drying process, the low-melting-point bonding fibers are softened and form bonding points with the surrounding fibers to obtain gradient composite air filter paper. The resulting gradient composite air filter paper is cooled and then wound up to obtain the gradient composite air filter paper.

[0046] In this embodiment, after the composite wet paper completes interlayer reinforcement, it is first pressed and dehydrated to remove free water and some interstitial water from the material. Simultaneously, mechanical pressure promotes better adhesion between the surface fine fibers, the flexible interfacial bonding area, and the lower coarse fiber skeleton. The pressing step increases the interlayer contact while maintaining pore channels, allowing each layer to solidify synergistically during subsequent heat setting. The pressed material is then sent to a drying device for heating and drying. Under temperature, the low-melting-point bonding fibers gradually soften and form thermal bonding points at their contact points with adjacent plant fibers and synthetic short fibers. These bonding points are distributed within the supporting skeleton layer and near the interface, thus fixing the fiber network throughout its thickness. Simultaneously, the flexible binder introduced in the previous step gradually solidifies during drying, ensuring that the interface possesses both the strength formed by thermal bonding and a certain degree of flexible bonding capability. Through heating and subsequent cooling, the material is fixed into a filter paper with a stable three-layer progressive structure: a lower, primarily load-bearing skeleton layer; a middle, transitional and connecting bonding layer; and an upper, fine fiber layer responsible for fine filtration. The resulting filter paper is then cooled and rolled up, which not only facilitates storage and transportation but also allows it to be used as the base paper for subsequent continuous forming into M-shaped pleated filter paper.

[0047] It is worth noting that all the devices described in this application can be implemented using existing technology, the algorithms described are all mature algorithms based on existing technology, and the chemical substances and conditions used in the preparation process are all within safe limits and will not cause harm to operators or the environment.

[0048] The present invention also discloses a gradient composite air filter paper, which is prepared by a gradient composite air filter paper preparation method as described in any of the above claims. The air filter paper is formed by sequentially completing the processes of slurry dispersion, wet forming, surface fine fiber deposition, flexible binder interface reinforcement, and heat drying and shaping, forming a composite system with functional division in the thickness direction.

[0049] In this embodiment, the gradient composite air filter paper also possesses material characteristics suitable for use as a base paper in subsequent forming processes. Because the supporting skeleton layer provides good overall toughness and resistance to collapse, while the intermediate transition layer buffers stress transfer between the upper and lower layers, when the finished filter paper needs to be crimped, bent, or pleated in subsequent use, the external load will not be concentrated on a single surface layer or interface, but will be gradually dispersed along the thickness direction. Especially for applications requiring further continuous processing into M-shaped pleated filter paper, ordinary homogeneous filter paper is prone to surface cracking, interlayer peeling, or significant springback at fold lines. However, the gradient composite air filter paper obtained in this embodiment, because the lower layer provides support, the middle layer buffers and maintains interface integrity, and the upper fine fiber layer is stabilized through permeable interface reinforcement, allows each functional layer to deform relatively harmoniously when subjected to compression and bending in the fold line region, thus maintaining good structural continuity.

[0050] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing gradient composite air filter paper, characterized in that, include: Plant fibers and synthetic short fibers are dispersed in water according to a preset mass ratio, and low-melting-point bonding fibers and wet strength agents are added. After stirring and dispersing, a transition slurry is prepared. The transition slurry is formed into a wet paper layer, and a fine fiber dispersion is applied to one side surface of the wet paper layer to form a fine fiber filter layer on the surface of the wet paper layer, thereby obtaining a wet composite filter paper. The wet composite filter paper is treated with a flexible binder and then subjected to pressing, dehydration, and heat drying to form fiber bonding points, thereby obtaining gradient composite air filter paper. The resulting gradient composite air filter paper sequentially forms a support skeleton layer, a transition bonding layer, and a fine fiber filter layer in the thickness direction.

2. The method for preparing a gradient composite air filter paper according to claim 1, characterized in that, The step of dispersing plant fibers and synthetic short fibers in water at a preset mass ratio includes: Plant fibers are pre-wetted at 40-80℃, and synthetic short fibers are dispersed at 40-70℃. Plant fibers and synthetic short fibers are added to water and stirred according to a preset mass ratio to obtain an initial fiber mixture slurry. The initial fiber mixture slurry is pulped and homogenized to control the slurry concentration at 0.1% to 1.5%, thus obtaining the supporting skeleton slurry.

3. The method for preparing a gradient composite air filter paper according to claim 1, characterized in that, The plant fiber includes at least one of softwood pulp fiber, hardwood pulp fiber, bamboo pulp fiber, and hemp pulp fiber, and the synthetic short fiber includes at least one of polyester fiber, polypropylene fiber, polyamide fiber, and bicomponent composite fiber. The mass ratio of the plant fiber to the synthetic short fiber is (55~90):(10~45).

4. The method for preparing a gradient composite air filter paper according to claim 2, characterized in that, The step of adding low-melting-point bonding fibers and wet-strength agents, and then dispersing them by stirring to obtain a transition slurry includes: Add low-melting-point bonding fibers to the supporting skeleton slurry and stir at 300~1000r / min for 5~20min; Add a wet strength agent to the slurry containing low-melting-point bonding fibers and continue stirring for 3-15 minutes to obtain a uniformly mixed transition slurry. During the stirring process, the mass percentage of fiber solids in the transition slurry is controlled to be 0.1% to 1.2%.

5. The method for preparing a gradient composite air filter paper according to claim 1, characterized in that, The low-melting-point bonding fiber includes at least one of polyethylene / polyester bicomponent fiber, polypropylene / polyester bicomponent fiber, and low-melting-point polyester fiber. The wet strength agent includes at least one of polyamide epichlorohydrin wet strength agent, melamine formaldehyde resin wet strength agent, and polyethyleneimine wet strength agent. The mass ratio of the plant fiber, synthetic short fiber, and low-melting-point bonding fiber is (55~85):(10~35):(3~15).

6. The method for preparing a gradient composite air filter paper according to claim 1, characterized in that, The steps of forming the transition slurry into a wet paper layer, applying a fine fiber dispersion to one side surface of the wet paper layer, and forming a fine fiber filter layer on the surface of the wet paper layer to obtain wet composite filter paper include: The transition slurry is fed to a paper forming device for wet forming to obtain a wet paper layer, wherein the plant fibers and synthetic short fibers are interwoven to form a support skeleton layer, and the low melting point bonding fibers and wet strength agent are distributed in the support skeleton layer to form a transition bonding layer; The fine fibers are mixed with a dispersion medium, and a dispersion aid is added for dispersion treatment to obtain a fine fiber dispersion. The fine fiber dispersion is applied to one side surface of the wet paper layer to deposit the fine fibers and form a fine fiber filter layer, thus obtaining a wet composite filter paper. The basis weight of the fine fiber filter layer is controlled to be lower than that of the wet paper layer, so that the resulting wet composite filter paper forms a gradient structure in the thickness direction, from coarse fibers to fine fibers.

7. The method for preparing a gradient composite air filter paper according to claim 6, characterized in that, The fine fibers include at least one of glass microfibers, ultrafine polyester fibers, ultrafine polypropylene fibers, and electret fine fibers; the dispersing agent includes at least one of nonionic surfactants, anionic dispersants, and alcohol wetting agents; and the basis weight of the fine fiber filter layer is 2~25 g / m³. 2 The basis weight of the wet paper layer is 20~120 g / m³. 2 .

8. The method for preparing a gradient composite air filter paper according to claim 1, characterized in that, The step of applying a flexible binder to the wet composite filter paper includes: Prepare a flexible binder treatment solution and apply the flexible binder treatment solution to the wet composite filter paper obtained in step S3. The flexible binder treatment solution includes a flexible binder and water. By controlling the penetration of the flexible binder treatment liquid into the wet paper layer and the fine fiber filter layer, and forming a flexible connection at the fiber cross-linking point and the interlayer interface, a composite wet paper with interlayer reinforcement is obtained. The amount of the flexible binder treatment liquid applied is controlled within a range that maintains the interconnected pore structure of the fine fiber filter layer.

9. The method for preparing a gradient composite air filter paper according to claim 8, characterized in that, The steps of pressing, dehydrating, heating, drying, and shaping to form fiber bonds in low-melting-point bonding fibers include: The composite wet paper is pressed and dehydrated, and the pressed composite wet paper is heated and dried. The drying temperature is controlled at 80~180℃ and the drying time is controlled at 0.5~10min. During the heating and drying process, the low-melting-point bonding fibers are softened and form bonding points with the surrounding fibers to obtain gradient composite air filter paper. The resulting gradient composite air filter paper is cooled and then wound up to obtain the gradient composite air filter paper.

10. A gradient composite air filter paper, characterized in that, It is prepared by the method of any one of claims 1 to 9 for preparing gradient composite air filter paper.

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