Aerospace lightweight high-temperature-resistant gradient filtration nonwoven fabric and a preparation method thereof

Abstract

This invention discloses a lightweight, high-temperature resistant gradient filter nonwoven fabric for aerospace applications and its preparation method. P84 polyimide short fibers and basalt short fibers are divided into 4-7 groups according to their average fineness. These groups are then opened, carded, and laid in a cross-laying pattern with fineness increasing from the dust-facing side to the clean air side. The fineness difference between adjacent groups is 0.3-0.8 dtex, forming a continuous gradient fiber web. The web is then progressively needle-punched at a higher density on the dust-facing side than on the clean air side, forming a felt. Finally, it is hot-rolled and set at a low temperature of 180-220℃. The product has no woven base fabric, no chemical impregnation or coating, a surface density of 320-380 g / m², no interlayer interfaces, a heat aging strength retention rate of ≥88% at 260℃ for 100 hours, a filtration efficiency of ≥99.8% at 0.3 μm, and a resistance of 390-450 Pa. This invention also relates to a gradient filter nonwoven fabric prepared by this method, suitable for high-temperature airflow filtration in aerospace applications.

Description

Technical Field

[0001] This invention relates to the field of high-temperature resistant filter materials technology, specifically a lightweight, high-temperature resistant gradient filter nonwoven fabric for aerospace applications and its preparation method. Background Technology

[0002] In the environmental control systems, engine bleed air systems, and auxiliary power units of aerospace vehicles, long-term and stable filtration and purification of high-temperature, dust-laden airflow is required. These operating conditions share common characteristics: high airflow temperature (180-260℃ long-term, and short-term impact temperatures exceeding 300℃), high airflow scouring intensity, and continuous vibration. Furthermore, the aircraft is extremely sensitive to every gram of weight, requiring filter materials to simultaneously meet standards in four dimensions: temperature resistance, precision, air permeability, and lightweight.

[0003] The current mainstream high-temperature resistant filter felt technology in the industry is as follows: High-temperature resistant short fibers (PPS fiber continuous operating temperature 160-190℃, aramid 1313 approximately 204℃, P84 polyimide fiber 240-260℃, PTFE fiber 260-280℃, basalt fiber long-term operating temperature 350-450℃) are selected, and then processed through opening and mixing, carding into a web or air-laid web, cross-laying, layering with a woven base fabric, pre-needling and main needle-punching to form a felt, followed by heat setting, singeing and calendering, PTFE emulsion impregnation, and coating with a membrane or water-repellent and oil-resistant coating to produce the finished product. To achieve gradient filtration accuracy, existing technologies generally employ three methods: first, multi-layer web laying with an ultra-fine fiber surface layer on the dust-facing side; second, layering fibers of different fineness; and third, using a progressive needle-punching density.

[0004] Chinese invention patent application CN105688511A discloses an ultra-low emission ultrafine polyimide fiber composite needle-punched felt and its preparation method. It uses a woven base fabric interwoven with polyimide fiber warp yarns and glass fiber weft yarns, with symmetrical composite needle-punched felt layers, up to 7-8 layers (including optional protective layers), and a 1.5D-2.5D ultrafine island fiber surface layer on the dust-facing side. In terms of process, each layer is carded into a web and laid in layers, and then composited by pre-needling, main needle-punching, secondary needle-punching and ultrasonic spot welding. The dust-facing side is then chemically impregnated with a waterproofing agent, an oil-repellent agent and PTFE emulsion and hot-pressed. This solution involves the layering of coarse and fine fibers rather than a continuous gradient, resulting in numerous interlayer interfaces. Under high-temperature-vibration coupling conditions, the impregnated adhesive layer may peel off after aging. The chemical impregnation process requires pre-drying at 215-355℃ for 6-15 minutes followed by baking at 205-325℃ for 10-15 minutes, which is energy-intensive and accompanied by fluorine emissions. Furthermore, its design goal is industrial flue gas dust removal, resulting in high weight, without considering the lightweight requirements of aerospace applications.

[0005] Chinese invention patent CN103691207B discloses a basalt fiber composite high-temperature resistant filter felt and its preparation method. It uses an interwoven base fabric of aramid warp and basalt weft as a reinforcing skeleton, with basalt fiber layers of the same specification laid on both the top and bottom surfaces. The felt is formed through pre-needling and main needle punching, achieving a running resistance of less than 700 Pa, suitable for filtering high-temperature acidic media below 350℃. However, this design features a homogeneous and symmetrical structure on both the top and bottom surfaces, lacking pore size or fineness gradients. Filtration accuracy relies entirely on deep filtration, making it difficult to balance accuracy and permeability. The short basalt fibers are brittle, with a breaking elongation of only about 3%, making them prone to breakage and shedding during needle punching. The finished product relies on a woven base fabric for strength, with a basis weight as high as 950g / m²-1000g / m², thus limiting its application to industrial flue gas.

[0006] In summary, existing technologies have the following problems: First, the basis weight of commercially available high-temperature resistant filter felts is generally too high, with PPS felt at 500g / m²-550g / m², P84 felt at 500g / m²-600g / m², and basalt felt reaching as high as 950g / m²-1000g / m². Moreover, they generally rely on woven base fabrics for load-bearing, which directly conflicts with the weight reduction requirements of aviation. Second, the interlayer interfaces of multi-layered structures are mechanically weak points under high-temperature and vibration conditions, resulting in frequent problems such as degumming, membrane detachment, and interlayer peeling, leading to short filter media lifespan. Third, homogeneous structures either have high resistance (basalt filter media has an operating resistance of about 700Pa) or insufficient precision, making it impossible to balance precision and air permeability. Fourth, post-processing such as chemical impregnation and membrane coating introduces VOC and fluorine emissions, and the baking temperature is as high as 205-355℃, resulting in high energy consumption and long processes. Therefore, developing a lightweight, high-temperature resistant gradient filter nonwoven fabric with no base fabric, no chemical coating, and an integrated structure is a technical challenge to be solved in the field of aerospace filtration. Summary of the Invention

[0007] To address the shortcomings of the existing technologies, the main technical problem to be solved by this invention is to provide a method for preparing a lightweight, high-temperature resistant gradient filter nonwoven fabric for aerospace applications. This method eliminates the need for woven base fabric and chemical impregnation finishing, enabling the nonwoven fabric to simultaneously possess 260°C-level high-temperature resistance, high precision, and low resistance at a low areal density of 320g / m²-380g / m². Furthermore, the fineness gradually changes along the thickness direction without interlayer interfaces, structurally eliminating the risk of interlayer delamination under high-temperature and vibration conditions. This resolves the contradiction of "lightweight, precision, breathability, and heat aging resistance" that is difficult to achieve simultaneously in existing multi-layer or homogeneous structures.

[0008] The technical solution adopted by this invention to solve its technical problem is as follows: A method for preparing a lightweight, high-temperature resistant gradient filter nonwoven fabric for aerospace applications, comprising the following steps: S1. Grouping and Opening Mixing: The fiber raw materials composed of organic high-temperature resistant short fibers and basalt short fibers are divided into multiple groups according to the average fineness from small to large, and each group of fibers is opened and mixed independently. S2, Grouping and carding into web: Each group of fibers is carded into a web in parallel by multiple carding machines that correspond one-to-one with the multiple groups, to obtain each group of thin web; S3. Stacking in order of increasing fineness: Stack the thin webs from bottom to top in order of decreasing average fineness, so that the group with the smallest average fineness is on the outermost layer. This outermost layer is used as the dust-facing surface, forming a gradient web with a continuously changing fineness along the thickness direction. S4. Progressive needle punching and one-time entanglement into felt: The gradient fiber web is sequentially and continuously subjected to one pre-needling and multiple main needle punchings. The penetration depth of each needle punch decreases gradually along the needle punching process, and the cumulative needle punching density of each needle punching from the dust-facing side is greater than the cumulative needle punching density of each needle punching from the clean air side, so that the gradient fiber web is entangled into felt in one step. S5. Low-temperature hot rolling and shaping: The felt body after being entangled into a felt is subjected to low-temperature hot rolling and shaping, wherein the temperature of the rolls for low-temperature hot rolling is lower than the glass transition temperature of the organic high-temperature resistant short fiber. The preparation method does not involve a woven base fabric throughout the entire process, nor does it involve chemical impregnation, coating, or lamination.

[0009] Preferably, in step S1, the organic high-temperature resistant short fiber accounts for 70wt%-90wt% of the total fiber mass, and the basalt short fiber accounts for 10wt%-30wt% of the total fiber mass.

[0010] Preferably, in step S1, the fiber raw material is divided into 4-7 groups, and the average fineness difference between two adjacent groups of fibers is 0.3dtex-0.8dtex; the organic high-temperature resistant short fiber is P84 polyimide fiber, including three specifications: fine denier fiber with a fineness of 1.1dtex-1.7dtex, medium denier fiber with a fineness of 2.0dtex-2.5dtex, and coarse denier fiber with a fineness of 3.0dtex-3.5dtex.

[0011] Preferably, the basalt short fibers have a diameter of 13μm-17μm and a length of 38mm-65mm.

[0012] Preferably, in step S3, each group of thin webs is folded and laid to a set surface density multiple times, and the total surface density of the gradient fiber web after stacking is 320g / m²-380g / m².

[0013] Preferably, in step S4, the pre-needling density is 60 needles / cm²-120 needles / cm², and the penetration depth is 11mm-13mm; the multiple main needles consist of three main needles: the first main needle is inserted from the dust-facing side, with a needle density of 120 needles / cm²-160 needles / cm² and a penetration depth of 9mm-11mm; the second main needle is inserted from the clean air side, with a needle density of 140 needles / cm²-180 needles / cm² and a penetration depth of 7mm-9mm; and the third main needle is inserted from the dust-facing side, with a needle density of 180 needles / cm²-260 needles / cm² and a penetration depth of 5mm-7mm; the net feeding speed is 0.8m / min-1.5m / min; and the ratio of the cumulative needle density of all needles inserted from the dust-facing side to the cumulative needle density of all needles inserted from the clean air side is 1.5 times-3.0 times.

[0014] Preferably, in step S5, the surface temperature of the rolls used for low-temperature hot rolling is 180-220℃, the linear pressure is 30N / mm-60N / mm, the machine speed is 2m / min-4m / min, and the dust-facing surface and the clean air surface are each hot-rolled once, with the dust-facing surface hot-rolled first and the clean air surface hot-rolled later; in step S1, the opening and mixing process involves feeding with a cotton grabber, mixing with a multi-bin cotton blender, and opening with a cotton opener, with the cotton opener beater speed being 400r / min-600r / min and the mixing bin air pressure being 0.4MPa-0.6MPa; in step S2, the carding machine cylinder linear speed is 800m / min-1200m / min, and the doffer speed is 30m / min-60m / min.

[0015] A lightweight, high-temperature resistant gradient filter nonwoven fabric for aerospace applications is prepared by any of the above-described methods. The nonwoven fabric is an integral felt made of organic high-temperature resistant short fibers and basalt short fibers through needle punching and entanglement. The average fineness of the fibers in the felt increases continuously and gradually from the dust-facing side to the air-cleaning side along the thickness direction, and the needle punching and entanglement density on the dust-facing side is greater than that on the air-cleaning side. The felt does not contain a woven base fabric, and the surface of the felt has no impregnation layer, coating layer, or film layer.

[0016] Preferably, the nonwoven fabric has a surface density of 320g / m²-380g / m² and a thickness of 1.6mm-2.1mm.

[0017] Preferably, the nonwoven fabric retains a longitudinal breaking strength of not less than 89% after heat aging at 260℃ for 100h, has a filtration efficiency of not less than 99.8% for 0.3μm particles, and a filtration resistance of not more than 450Pa at a filtration rate of 1.0m / min.

[0018] The advantages of this invention are: 1. This invention eliminates the woven base fabric, using a rigid basalt fiber skeleton and progressive needle-punched entanglement for load-bearing. The finished product's areal density is reduced from 950g / m²-1000g / m² of existing basalt filter felts to 320g / m²-380g / m², resulting in a weight reduction of over 60%. The filtration resistance is reduced from 687Pa (Comparative Example 1) of existing basalt filter felts to 390Pa-450Pa, a reduction of 38%-42%. Based on a single-unit filtration area of ​​20m², the filter material weight reduction can reach over 11kg, directly addressing the weight reduction requirements of aerospace applications. 2. The present invention features a continuous gradient in fineness and a single-layer entanglement into a felt, eliminating physical interlayer interfaces within the felt. After 260℃×100h heat aging, the longitudinal tensile strength retention rate reaches 89.7%-93.6%, while the existing process using woven base fabric with 8 layers and chemical impregnation (Comparative Example 2) only achieves 71.4%, and significant interlayer delamination occurs after aging. The present invention eliminates the delamination risk from a structural principle perspective, extending the service life of the filter material. 3. This invention achieves a filtration efficiency of 99.847%-99.962% for 0.3μm aerosols while maintaining an air permeability of 100mm / s-135mm / s; while Comparative Example 3, which homogenously mixes the same fibers into a web, has an efficiency of only 99.238% under similar resistance. This demonstrates that the coupling of fineness gradient and needle punching density gradient produces a synergistic effect that cannot be achieved by simply increasing the density or fineness of the fibers. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention 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 the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a process flow diagram of the preparation method of the present invention; Figure 2 A bar chart comparing the areal density and filtration resistance of each embodiment and comparative example; Figure 3 A bar chart showing the retention rate of longitudinal fracture strength after 260℃×100h thermal aging for each embodiment and comparative example. Figure 4 A biaxial graph showing the relationship between air permeability and filtration efficiency for each embodiment and comparative example; Figure 5 The bar chart shows the comparison of filtration efficiency before and after thermal aging for each embodiment and comparative example. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Please refer to Figure 1 A method for preparing a lightweight, high-temperature resistant gradient filter nonwoven fabric for aerospace applications is disclosed. The fiber system comprises organic high-temperature resistant short fibers and basalt short fibers, wherein the organic high-temperature resistant short fibers account for 70wt%-90wt% of the total fiber mass, and the basalt short fibers account for 10wt%-30wt% of the total fiber mass. The organic high-temperature resistant short fibers are selected from P84 polyimide fibers, including three specifications: fine denier fibers with a fineness of 1.1dtex-1.7dtex, medium denier fibers with a fineness of 2.0dtex-2.5dtex, and coarse denier fibers with a fineness of 3.0dtex-3.5dtex. The basalt short fibers have a diameter of 13μm-17μm (corresponding to a linear density of 3.5dtex-6.0dtex). All fibers are then averaged... The fiber raw materials are divided into 4-7 groups with fineness ranging from small to large. Each group is opened, mixed, and carded into a web separately. Then, they are cross-layered in order of increasing fineness along the thickness direction from the dust-facing side to the air-cleaning side. The average fineness difference between adjacent groups of fibers is controlled at 0.3dtex-0.8dtex, forming a gradient fiber web with a continuous gradual change in fineness along the thickness direction. Subsequently, a progressive needle punching process with a progressive needle punching density along the thickness direction is used to entangle the gradient fiber web into a felt in one go. The cumulative needle punching density on the dust-facing side is 1.5-3.0 times that on the air-cleaning side. Finally, it is hot-rolled and shaped at a low temperature of 180-220℃ to obtain a gradient filter nonwoven fabric with an areal density of 320g / m²-380g / m². No woven base fabric, chemical impregnation, coating, or lamination treatment is used in the entire process.

[0023] The core difference between this overall solution and existing technologies lies in the synergy of three technical approaches: First, the fiber system features an "organic-inorganic, multi-fineness" blend. Fine denier P84 fibers have a large specific surface area and a trilobal cross-section, resulting in small and uniform micropores after stacking, providing high-precision surface / shallow filtration on the dust-facing side. Basalt fibers have a long-term operating temperature of 350-450℃ and extremely low thermal shrinkage (almost no shrinkage at 260℃). Arranging them on the air-cleaning side forms a high-temperature resistant rigid skeleton, inhibiting the shrinkage and creep of organic fibers at high temperatures. This is key to maintaining dimensional stability and breaking strength even after eliminating the woven base fabric. Medium and coarse denier P84 fibers act as a transitional and entanglement bridge between the two. The three organic finenesses plus one inorganic fiber constitute four fineness steps, providing the material basis for a continuous gradient.

[0024] Second, the "grouping and fiber-laying" method creates a continuous gradient, rather than layered stacking. Existing technologies (such as CN105688511A) involve stacking functional layers after each layer is independently webbed, resulting in abrupt changes in fineness and physical interfaces between layers, requiring adhesives or ultrasonic spot welding for maintenance. This invention divides fibers into 4-7 groups based on fineness, with each group forming a separate web and then stacked sequentially. The average fineness difference between adjacent groups is compressed to 0.3-0.8 dtex. During stacking, each group of fiber webs is itself composed of multiple folded thin webs. Adjacent fiber groups are repeatedly carried up and down and interwoven by the needles during subsequent needle punching, ultimately forming a continuous gradient structure without clear interfaces in the thickness direction, thus eliminating the carrier for interlayer delamination in principle.

[0025] Third, the progressive needle-punching density gradient, linked to the web-laying rhythm, is entangled into a felt in one go. Needling is no longer a uniform reinforcement with a constant density, but rather a high needle-punching density on the dust-facing side with progressively decreasing penetration depth, and a low needle-punching density on the air-cleaning side. This densifies the surface fibers to intercept fine particles, while keeping the interior of the felt loose to maintain air permeability. The needle-punching density gradient and the fineness gradient are coupled in the same direction, entangled into a whole felt in one go, without the need for a base fabric for support or chemical bonding.

[0026] In conjunction with the above three methods, the finishing process only uses low-temperature hot rolling at 180-220℃: the roll temperature is lower than the glass transition temperature of P84 fiber (about 315℃), the hot rolling does not melt the fiber, only eliminates the residual stress of needle punching, locks the entanglement points and precisely controls the thickness, thereby completely replacing the chemical impregnation and hot pressing processes, cutting off VOC and fluorine emission sources.

[0027] The following section provides further definitions and explanations for each step in the process sequence: Step S1: Grouping and Mixing. According to the gradient design described above, all fibers are divided into 4-7 groups (preferably 5-6 groups). Each group of fibers is fed separately by a cotton picker, mixed by a multi-bin blender, and opened by a cotton opener. The opening machine beater speed is 400-600 r / min (preferably 480 r / min), and the blending bin air pressure is 0.4 MPa-0.6 MPa (preferably 0.45 MPa). The purpose of this step is to ensure that the fibers in each group are thoroughly and evenly mixed within the group, while preventing premature mixing between groups of different fineness, thus ensuring the controllability of the subsequent gradient boundaries. Basalt fibers are brittle; the opening beater speed should not exceed 600 r / min, otherwise the fiber breakage rate will increase significantly.

[0028] Step S2: Grouping and carding into a web. A number of carding machines, the same number as the number of fiber groups, are set up and run in parallel, with each carding machine corresponding to one group of fibers. The cylinder linear speed is 800m / min-1200m / min (preferably 1050m / min), and the doffer speed is 30m / min-60m / min (preferably 45m / min). Each carding machine outputs a uniform thin web with a basis weight of 8g / m²-15g / m². The purpose is to obtain a single-group fiber web with uniform basis weight, laying the foundation for accurate layering.

[0029] Step S3: Cross-laying in increasing fineness order. The thin webs from each carding machine are cross-laid from bottom to top using a cross-laying machine, with the air-cleaning side group first and the dust-facing side group last. That is, the group with the highest fineness is laid at the bottom layer, and the group with the lowest fineness is laid at the top layer (the top layer is the dust-facing side during use). Each group of thin webs is folded repeatedly to achieve its set areal density. The sum of the areal densities of all groups, i.e., the total areal density, is 320g / m²-380g / m² (preferably 340g / m²-365g / m²). The average fineness difference between adjacent groups of fibers must be 0.3dtex-0.8dtex (preferably 0.4dtex-0.6dtex). This step is the core of gradient forming: the stacking order cannot be reversed, and the fineness difference cannot be too large—if it exceeds 0.8dtex, a identifiable interlayer interface will appear between adjacent groups, losing the continuous gradient characteristic.

[0030] Step S4: Progressive needle punching is used to entangle the fiber web into a felt in one pass. The gradient fiber web passes through one pre-needling pass and three main needle punching passes in sequence. All four needle punching passes are completed continuously on the same needle punching production line without stopping the machine or transferring the web. Pre-needling: Needling density 60 needles / cm²-120 needles / cm² (preferably 90 needles / cm²), insertion depth 11mm-13mm, using No. 42 forked needles, the purpose is to initially compress and entangle the loose fiber web and fix its stacked shape; the first main needle is inserted from the dust-facing side, needle density 120 needles / cm²-160 needles / cm² (preferably 140 needles / cm²), insertion depth 9mm-11mm; the second main needle is inserted from the clean air side, needle density 140 needles / cm²-180 needles / cm² (preferably 170 needles / cm²), insertion depth 7mm-9mm; the third main needle is inserted from the dust-facing side, needle density 180 needles / cm²-260 needles / cm² (preferably 220 needles / cm²), insertion depth 5mm-7mm, then a No. 43 fine hook needle is used for finishing. Therefore, the ratio of the cumulative needled density on the dust-facing side to the cumulative needled density on the air-cleaning side is 1.5 to 3.0 times (preferably 2.2 to 2.8 times), the needled depth decreases gradually along the needled process, and the feeding speed is 0.8 m / min to 1.5 m / min (preferably 1.2 m / min). The function of this step is: the high-density needled needled needled needled the fine denier fibers into a dense microporous layer, while the low-density deep needled ...

[0031] Step S5, low-temperature hot rolling and setting. The needle-punched felt is continuously passed through a two-roll hot rolling mill. The roll surface temperature is 180-220℃ (preferably 200℃), the linear pressure is 30N / mm-60N / mm (preferably 45N / mm), and the speed is 2m / min-4m / min (preferably 3m / min). The front and back sides are hot rolled once each, with the dust-facing side rolled first and the air-protected side rolled last. The purpose of this step is to eliminate the internal stress of the needle punches, lock the entanglement points by thermal shrinkage, and control the thickness tolerance of the finished product within ±3%. The temperature must be lower than the glass transition temperature of P84 fiber (approximately 315℃). Temperatures exceeding 220℃ will cause the surface of the fine denier fibers on the dust-facing side to fuse and the air permeability to drop sharply, while temperatures below 180℃ will result in insufficient setting and thickness rebound.

[0032] Step S6: Roll up, cut, and inspect to obtain the finished product.

[0033] The resulting nonwoven fabric has a surface density of 320g / m²-380g / m², a thickness of 1.6mm-2.1mm, a longitudinal breaking strength ≥700N / 5cm, a longitudinal breaking strength retention rate ≥88% after heat aging at 260℃×100h, an air permeability of 100mm / s-135mm / s (200Pa pressure difference), a filtration efficiency of ≥99.8% for 0.3μm aerosols, a filtration resistance of 390Pa-450Pa at a filtration rate of 1.0m / min, and a limiting oxygen index ≥40%.

[0034] Another aspect of the present invention provides a lightweight, high-temperature resistant gradient filter nonwoven fabric for aerospace applications prepared by the above-described preparation method. The nonwoven fabric is an integral felt made of organic high-temperature resistant short fibers and basalt short fibers through needle punching and entanglement. The average fineness of the fibers in the felt increases continuously and gradually from the dust-facing side to the air-cleaning side along the thickness direction, and the needle punching entanglement density on the dust-facing side is greater than that on the air-cleaning side. The needle punching entanglement density is formed and characterized by the cumulative needle punching density of the progressive needle punching in step S4. The felt does not contain a woven base fabric, and the surface of the felt has no impregnation layer, coating layer, or film layer. Preferably, the nonwoven fabric has a surface density of 320g / m²-380g / m² and a thickness of 1.6mm-2.1mm; more preferably, after heat aging at 260℃ for 100h, the nonwoven fabric retains a longitudinal breaking strength of not less than 89%, a filtration efficiency of not less than 99.8% for 0.3μm particles, and a filtration resistance of not more than 450Pa at a filtration rate of 1.0m / min.

[0035] The following is combined Figures 2-5 The technical solution of the present invention will be further described in detail through specific embodiments and comparative examples. The raw materials used in each embodiment are commercially available: P84 polyimide fiber (trilobal cross-section, Lenzing GmbH, Austria), and basalt staple fiber (Zhejiang Shijin Basalt Fiber Co., Ltd., fiber density 2.65 g / cm³, lengths 38 mm / 51 mm / 65 mm). The equipment used includes: a Trützschler opening and cleaning line (including a cotton grabber, multi-bin blender, and cotton opener), a Trützschler TC10 high-speed carding machine, an Autefa Topliner cross-laying machine, a Dilo DI-LOOM needle punching machine, and a Kuster two-roll hot rolling mill.

[0036] The products of each embodiment and comparative example were tested using the following methods, and the data from the same batch were measured under the same instruments and conditions: Surface density: GB / T 24218.1-2009, unit g / m²; Thickness: GB / T 24218.2-2009, presser foot pressure 0.5 kPa, unit mm; Tensile strength: GB / T 24218.3-2010 Specimen method, specimen width 50mm, spacing 200mm, tensile speed 100mm / min, measured in the longitudinal and transverse directions respectively, unit N / 5cm; Thermal aging test: The sample is placed in a 260℃ forced-air oven for continuous aging for 100 hours. After being taken out and cooled to room temperature, the longitudinal breaking strength is measured, and the percentage of the longitudinal breaking strength before aging is calculated, i.e. the strength retention rate. Air permeability: GB / T 5453-1997, test pressure difference 200Pa, test area 20cm², unit mm / s; Filtration efficiency and filtration resistance: TSI 8130 automatic filter media tester was used, 0.3μm NaCl aerosol, filtration rate 1.0m / min; Limiting oxygen index: GB / T 5454-1997.

[0037] Example 1

[0038] Raw materials and dosage (divided into 6 groups according to gradient, total feed 300kg):

[0039] The total weight of basalt short fibers was 65 kg, accounting for 21.7 wt% of the total fiber mass. The average fineness difference between adjacent groups was 0.34 dtex, 0.55 dtex, 0.52 dtex, 0.66 dtex, and 0.68 dtex, respectively, all within the range of 0.3 dtex to 0.8 dtex.

[0040] Preparation steps: S1. Group opening and mixing: Six groups of fibers are fed in independently by a cotton grabber, mixed by a multi-compartment cotton mixer (compartment air pressure 0.45MPa), and opened by a cotton opener (beater speed 480r / min). The conveying pipes of each group are isolated from each other, and the output is uniform cotton.

[0041] S2. Grouping and carding into web: Six TC10 carding machines are connected in parallel, each corresponding to a group of fibers. The cylinder speed is 1050m / min, the doffer speed is 45m / min, and each machine outputs a thin web with a basis weight of 10g / m².

[0042] S3. Cross-laying: The thin webs from six carding machines are cross-laid from bottom to top in the order of F group → E group → D group → C group → B group → A group. That is, group A (the finest) is located on the outermost layer as the dust-facing surface, and group F is located on the bottom layer as the air-filtering surface. Each group of thin webs is folded and laid repeatedly to a set surface density: group A 50g / m², group B 55g / m², group C 55g / m², group D 60g / m², group E 65g / m², group F 63g / m², with a total surface density of 348g / m², resulting in a gradient fiber web with a continuous gradual change in fineness along the thickness direction.

[0043] S4. Progressive Needling: The gradient fiber web undergoes four consecutive needle punching stages on the Dilo needle punching production line without interruption—pre-needling begins on the dust-facing side with a density of 90 needles / cm² and a penetration depth of 12mm, using a No. 42 forked needle; the first main needle punch begins on the dust-facing side with a density of 140 needles / cm² and a penetration depth of 10mm; the second main needle punch begins on the clean air side with a density of 170 needles / cm² and a penetration depth of 8mm; the third main needle punch begins on the dust-facing side with a density of 220 needles / cm² and a penetration depth of 6mm, after which a No. 43 fine hook needle is used for finishing. The cumulative needle punching density on the dust-facing side is 450 needles / cm², and the cumulative needle punching density on the clean air side is 170 needles / cm², with a gradient ratio of 2.65; the web feeding speed is 1.2m / min.

[0044] S5. Low-temperature hot rolling: The needle-punched felt is continuously passed through the Kuster two-roll hot rolling mill with a roll surface temperature of 200℃, a linear pressure of 45N / mm, and a speed of 3m / min. The dust-facing side is rolled once, and the air-cleaning side is rolled once.

[0045] S6. Winding, slitting, and inspection. Finished product thickness: 1.86mm, thickness tolerance: ±2.7%.

[0046] Test results: Surface density 348 g / m²; thickness 1.86 mm; longitudinal tensile strength 826 N / 5 cm, transverse tensile strength 773 N / 5 cm; longitudinal tensile strength retention rate after 260℃×100 h heat aging 93.6%; air permeability 118.6 mm / s; 0.3 μm aerosol filtration efficiency 99.962%, filtration resistance 396 Pa; filtration efficiency after heat aging 99.881%; limiting oxygen index 41.6%. The dust-facing surface of the finished product feels dense and smooth, and no interlayer boundaries are visible when viewed against the light.

[0047] Example 2

[0048] Raw materials and dosage (divided into 5 groups according to gradient, total feed 300kg):

[0049] The total weight of basalt short fibers was 45 kg, accounting for 15.0 wt% of the total fiber mass; the average fineness difference between adjacent groups was 0.34 dtex, 0.55 dtex, 0.59 dtex, and 0.57 dtex, respectively.

[0050] Preparation steps: The process sequence is the same as in Example 1, except that five carding machines are connected in parallel (cylinder speed 1100m / min, doffer speed 42m / min); the areal density of each group is 55g / m² for group A, 65g / m² for group B, 70g / m² for group C, 80g / m² for group D, and 95g / m² for group E, with a total areal density of 365g / m²; the progressive needle punching parameters are: pre-needle punching 80 needles / cm², depth 12mm; first main needle punching (dust-facing surface) 130 needles / cm², depth 10mm; second main needle punching (clean air surface) 160 needles / cm², depth 8mm; third main needle punching (dust-facing surface) 190 needles / cm², depth 6mm; the cumulative needle punching density ratio of dust-facing surface to clean air surface is 2.5; the web feeding speed is 1.0m / min; the hot rolling temperature is 185℃, the linear pressure is 55N / mm, and the machine speed is 2.5m / min.

[0051] Test results: Surface density 365 g / m²; thickness 2.03 mm; longitudinal tensile strength 764 N / 5 cm, transverse tensile strength 719 N / 5 cm; longitudinal tensile strength retention rate after heat aging at 260℃ for 100 h 91.8%; air permeability 106.3 mm / s; filtration efficiency 99.913%, filtration resistance 428 Pa; filtration efficiency after heat aging 99.762%; limiting oxygen index 40.8%.

[0052] Example 3

[0053] Raw materials and dosage (divided into 7 groups according to gradient, total feed 260kg):

[0054] The total weight of basalt short fibers was 75 kg, accounting for 28.8 wt% of the total fiber mass; the average fineness difference between adjacent groups was 0.57 dtex, 0.55 dtex, 0.74 dtex, 0.44 dtex, 0.70 dtex, and 0.76 dtex, respectively, none of which exceeded 0.8 dtex.

[0055] Preparation steps: The process sequence is the same as in Example 1, except that: seven carding machines are connected in parallel (cylinder speed 1200m / min, doffer speed 38m / min), and the opening machine beater speed is 420r / min; the areal density of each group is: Group A 38g / m², Group B 42g / m², Group C 45g / m², Group D 48g / m², Group E 52g / m², Group F 52g / m², Group G 55g / m², with a total areal density of 332g / m²; progressive needle punching. The parameters are as follows: pre-needle punching 120 needles / cm², depth 13mm; first main needle punching (dust-facing surface) 160 needles / cm², depth 11mm; second main needle punching (clean air surface) 180 needles / cm², depth 9mm; third main needle punching (dust-facing surface) 260 needles / cm², depth 7mm; cumulative needle punching density ratio of dust-facing surface to clean air surface 3.0; wire feeding speed 1.5m / min; hot rolling temperature 220℃; linear pressure 30N / mm; machine speed 4m / min.

[0056] Test results: Surface density 332 g / m²; thickness 1.72 mm; longitudinal tensile strength 701 N / 5 cm, transverse tensile strength 655 N / 5 cm; longitudinal tensile strength retention rate after heat aging at 260℃ for 100 h 89.7%; air permeability 131.4 mm / s; filtration efficiency 99.847%, filtration resistance 441 Pa; filtration efficiency after heat aging 99.531%; limiting oxygen index 42.7%.

[0057] Comparative Example 1

[0058] Raw materials and dosage: A woven base fabric (area density 260g / m²) interwoven with aramid warp yarns and basalt weft yarns is used as the reinforcing layer; 354g / m² of basalt short fibers (15μm×51mm, Zhejiang Shijin Basalt Fiber Co., Ltd.) are laid on both the top and bottom.

[0059] Preparation steps: Basalt short fibers are opened and mixed by a cotton grabber, a cotton blender, and a cotton opener (beater speed 480 r / min). Two carding machines (cylinder linear speed 900 m / min) are used to form webs, which are then cross-laid to a set weight. The woven base is placed between the upper and lower fiber webs and stacked. Pre-needling (density 60 needles / cm², depth 12 mm) and main needle punching (density 220 needles / cm², depth 10 mm, one pass on each side) are then added to form a felt. The felt is then heat-set at 280℃ for 5 min to obtain a homogeneous symmetrical filter felt with a total areal density of 968 g / m².

[0060] Test results: Surface density 968 g / m²; thickness 3.15 mm; longitudinal tensile strength 1123 N / 5 cm, transverse tensile strength 1042 N / 5 cm; longitudinal tensile strength retention rate after heat aging at 260℃ for 100 h 83.7% (due to aging shrinkage of aramid fiber base fabric); air permeability 38.5 mm / s; filtration efficiency 99.412%, filtration resistance 687 Pa; filtration efficiency after heat aging 98.723%; limiting oxygen index 49.3%.

[0061] Comparative Example 2

[0062] Raw materials and dosage: Woven base fabric with polyimide fiber warp and glass fiber weft; 8 layers in total, with 4 layers of needle-punched felt on each of the top and bottom surfaces, and a 1.5D ultra-fine island fiber surface layer on the dust-facing side; The impregnation solution formula is 15wt% waterproofing agent, 8wt% oil repellent, 20wt% PTFE emulsion, and the balance is water.

[0063] Preparation steps: The fibers of each layer are combed into a web and laid in layers. Pre-needling (needling frequency 400 needles / min) is followed by main needleding (needling frequency 600 needles / min) and secondary needleding (needling frequency 800 needles / min). The composite is ultrasonically spot-welded. After chemical impregnation of the dust-facing surface, it is pre-dried at 280℃ for 10 min and baked at 260℃ for 12 min. Then it is hot-pressed at 230℃ and 1.8MPa to obtain a composite needle-punched felt with a total surface density of 612g / m².

[0064] Test results: Surface density 612 g / m²; thickness 2.58 mm; longitudinal tensile strength 945 N / 5 cm, transverse tensile strength 886 N / 5 cm; after 260℃×100 h heat aging, the longitudinal tensile strength retention rate was 71.4%. After aging, visible interlayer delamination appeared between the ultrafine surface layer on the dust-facing side and the adjacent felt layer, and the impregnated layer locally powdered and detached; air permeability 52.7 mm / s; filtration efficiency 99.978%, filtration resistance 543 Pa; after heat aging, the filtration efficiency decreased to 97.856%; limiting oxygen index 42.3%. The fluorinated impregnation waste liquid from this process requires special collection and treatment.

[0065] Comparative Example 3

[0066] Raw materials and dosage: The same fiber types and total amount as in Example 1 (235 kg of P84 fiber, 65 kg of basalt short fiber, totaling 300 kg), but without grouping, all fibers were put into the same cotton blending bin for homogenization.

[0067] Preparation steps: Homogeneous mixed fibers are formed into a web by a single carding machine and cross-laid to a total areal density of 348 g / m². The needle punching and hot rolling process parameters are exactly the same as in Example 1.

[0068] Test results: Surface density 355 g / m²; thickness 1.94 mm; longitudinal breaking strength 698 N / 5 cm, transverse breaking strength 641 N / 5 cm; longitudinal breaking strength retention rate after 260℃×100 h heat aging 92.2%; air permeability 124.8 mm / s; filtration efficiency only 99.238%, filtration resistance 512 Pa; filtration efficiency after heat aging 98.416%; limiting oxygen index 41.2%. Comparing with Example 1, it can be seen that with the same fiber system and similar surface density, eliminating the fineness gradient resulted in a 0.72 percentage point decrease in filtration efficiency and an increase in resistance of 116 Pa, making it impossible to simultaneously achieve both precision and air permeability.

[0069] Comparative Example 4

[0070] Raw materials and dosage: Same as in Example 1, divided into 6 groups and laid in the order of Example 1 to form a gradient fiber web.

[0071] Preparation steps: The web laying process is exactly the same as in Example 1; the needle punching is changed to four constant density needle punches, each with a density of 170 needles / cm² and a penetration depth of 9mm, and the needles are alternately punched on the dust-facing side, the clean air side, the dust-facing side, and the clean air side, with a web feeding speed of 1.2m / min; the hot rolling is the same as in Example 1.

[0072] Test results: Surface density 351 g / m²; thickness 2.11 mm; longitudinal tensile strength 587 N / 5 cm, transverse tensile strength 532 N / 5 cm; longitudinal tensile strength retention rate after 260℃×100 h heat aging 86.5%; air permeability 97.6 mm / s; filtration efficiency 99.694%, filtration resistance 476 Pa; filtration efficiency after heat aging 98.974%; limiting oxygen index 41.5%. The dust-facing surface lacks sufficient densification and has uneven internal entanglement, resulting in lower strength and precision compared to Example 1.

[0073] Comparative Example 5

[0074] Raw materials and dosage: The fiber grouping method, web density, needle punching and hot rolling processes are the same as in Example 1, except that the proportion of basalt short fibers in each group is increased proportionally, so that basalt short fibers account for 55 wt% of the total fiber mass (exceeding the upper limit of the range of 10 wt%-30 wt%).

[0075] Preparation steps: Same as in Example 1. During production, the number of broken fibers increased significantly during the opening process, and the combing process resulted in severe fiber shedding. Basalt short fiber fragments were visible on the surface of the fiber web.

[0076] Test results: Surface density 362 g / m²; thickness 2.28 mm; longitudinal tensile strength 512 N / 5 cm, transverse tensile strength 466 N / 5 cm; longitudinal tensile strength retention rate after 260℃×100 h heat aging 79.8%; air permeability 88.9 mm / s; filtration efficiency 99.311%, filtration resistance 488 Pa; filtration efficiency after heat aging 97.623%; limiting oxygen index 45.6%. Excessive brittle fibers in basalt lead to a decrease in the load-bearing capacity of entanglement points, resulting in a significant deterioration in overall strength and durability.

[0077] The summary table of test data is shown in Table 1:

[0078] Table 1

[0079] As shown in the table above, the three embodiments simultaneously achieved a combination of high strength, high heat aging retention rate, high filtration efficiency, and low resistance at a low areal density of 320g / m²-380g / m², exhibiting the best overall performance. Among them, Embodiment 1 showed the best filtration efficiency, strength retention rate, and filtration resistance. Although Comparative Examples 1 and 2 each had individual advantages (Comparative Example 1 had high strength, and Comparative Example 2 had high initial efficiency), their areal density, resistance, or heat aging durability were significantly inferior to the embodiments. Comparative Examples 3 and 4, after lacking the fineness gradient and needle-punch density gradient, respectively, showed a significant decline in precision or strength. Comparative Example 5, after exceeding the specified ratio, showed overall performance degradation. The above comparisons demonstrate that there is a synergistic relationship between the various technical means of this invention, and the absence or exceeding of any key feature will prevent the achievement of the technical effects of this invention.

[0080] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0081] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for producing a lightweight high-temperature-resistant gradient filtration nonwoven fabric for aerospace, characterized by, Includes the following steps: S1. Grouping and Opening Mixing: The fiber raw materials composed of organic high-temperature resistant short fibers and basalt short fibers are divided into multiple groups according to the average fineness from small to large, and each group of fibers is opened and mixed independently. S2, Grouping and carding into web: Each group of fibers is carded into a web in parallel by multiple carding machines that correspond one-to-one with the multiple groups, to obtain each group of thin web; S3. Stacking in order of increasing fineness: Stack the thin webs from bottom to top in order of decreasing average fineness, so that the group with the smallest average fineness is on the outermost layer. This outermost layer is used as the dust-facing surface, forming a gradient web with a continuously changing fineness along the thickness direction. S4. Progressive needle punching and one-time entanglement into felt: The gradient fiber web is sequentially and continuously subjected to one pre-needling and multiple main needle punchings. The penetration depth of each needle punch decreases gradually along the needle punching process, and the cumulative needle punching density of each needle punching from the dust-facing side is greater than the cumulative needle punching density of each needle punching from the clean air side, so that the gradient fiber web is entangled into felt in one step. S5. Low-temperature hot rolling and shaping: The felt body after being entangled into a felt is subjected to low-temperature hot rolling and shaping, wherein the temperature of the rolls for low-temperature hot rolling is lower than the glass transition temperature of the organic high-temperature resistant short fiber.

2. The method of claim 1, wherein: In step S1, the organic high-temperature resistant short fiber accounts for 70wt%-90wt% of the total fiber mass, and the basalt short fiber accounts for 10wt%-30wt% of the total fiber mass.

3. The preparation method according to claim 1, characterized in that: In step S1, the fiber raw material is divided into 4-7 groups, and the average fineness difference between two adjacent groups of fibers is 0.3dtex-0.8dtex; the organic high-temperature resistant short fiber is P84 polyimide fiber, including three specifications: fine denier fiber with a fineness of 1.1dtex-1.7dtex, medium denier fiber with a fineness of 2.0dtex-2.5dtex, and coarse denier fiber with a fineness of 3.0dtex-3.5dtex.

4. The preparation method according to claim 1 or 2, characterized in that: The basalt short fibers have a diameter of 13μm-17μm and a length of 38mm-65mm.

5. The method of claim 1, wherein: In step S3, each group of thin webs is folded and laid to the set surface density of each group multiple times. After laying, the total surface density of the gradient fiber web is 320g / m²-380g / m².

6. The preparation method according to claim 1, characterized in that: In step S4, the pre-needling density is 60 needles / cm²-120 needles / cm², and the penetration depth is 11mm-13mm; the multiple main needles consist of three main needles: the first main needle is inserted from the dust-facing side, with a needle density of 120 needles / cm²-160 needles / cm² and a penetration depth of 9mm-11mm; the second main needle is inserted from the clean air side, with a needle density of 140 needles / cm²-180 needles / cm² and a penetration depth of 7mm-9mm; and the third main needle is inserted from the dust-facing side, with a needle density of 180 needles / cm²-260 needles / cm² and a penetration depth of 5mm-7mm; the net feeding speed is 0.8m / min-1.5m / min; and the ratio of the cumulative needle density of all needles inserted from the dust-facing side to the cumulative needle density of all needles inserted from the clean air side is 1.5 times-3.0 times.

7. The preparation method according to claim 1, characterized in that: In step S5, the surface temperature of the rolls in the low-temperature hot rolling process is 180-220℃, the linear pressure is 30N / mm-60N / mm, the machine speed is 2m / min-4m / min, and the dust-facing side and the clean air side are each hot rolled once, with the dust-facing side hot rolled first and the clean air side hot rolled later. In step S1, the opening and mixing process involves feeding the cotton with a cotton grabber, mixing with a multi-bin cotton blender, and opening with a cotton opener. The speed of the cotton opener beater is 400r / min-600r / min, and the air pressure in the cotton blending bin is 0.4MPa-0.6MPa. In step S2, the linear speed of the carding machine cylinder is 800m / min-1200m / min, and the speed of the doffer is 30m / min-60m / min.

8. A lightweight, high-temperature resistant gradient filter nonwoven fabric for aerospace applications, prepared by the method described in any one of claims 1 to 7, characterized in that: The nonwoven fabric is an integral felt made of organic high-temperature resistant short fibers and basalt short fibers through needle punching and entanglement. The average fineness of the fibers in the felt increases continuously and gradually from the dust-facing side to the air-cleaning side along the thickness direction, and the needle punching and entanglement density on the dust-facing side is greater than that on the air-cleaning side. The felt does not contain a woven base fabric, and the surface of the felt has no impregnation layer, coating layer, or film layer.

9. The nonwoven fabric according to claim 8, characterized in that: The nonwoven fabric has a surface density of 320g / m²-380g / m² and a thickness of 1.6mm-2.1mm.

10. The nonwoven fabric according to claim 8 or 9, characterized in that: The nonwoven fabric retains a longitudinal breaking strength of no less than 89% after heat aging at 260℃ for 100h, has a filtration efficiency of no less than 99.8% for 0.3μm particles, and a filtration resistance of no more than 450Pa at a filtration rate of 1.0m / min.

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