Filter element for automobile filter and preparation method of filter element

Abstract

The invention belongs to the technical field of automobile filters, and provides a filter element for an automobile filter and a preparation method thereof.The filter element adopts a double-layer composite structure design and comprises a coarse fiber layer and a fine fiber layer which are both composed of polypropylene, an antioxidant and a beta-crystal nucleating agent, and the fiber surface is of a nano-pore structure; the mass fraction of the beta crystal nucleating agent on the fine fiber layer side is higher than that of the beta crystal nucleating agent on the coarse fiber layer side, the mass fraction difference is 0.06-0.10 wt%, and the surface potential difference between the two layers is 80-200 V. A supercritical CO2-assisted melt-blowing molding and layered reverse corona electret preparation process is adopted, so that the air flow resistance is remarkably reduced while the filtering efficiency is greatly improved, and the filtering effect is greatly improved. The surface charge density of the fiber is effectively enhanced through beta-phase thickness gradient construction and a layered electret technology, the CO2 recovery rate reaches 95% or above, the technical problem that high efficiency and low resistance are difficult to consider at the same time is solved, and wide industrial application value is achieved.

Description

Technical Field

[0001] This invention belongs to the field of automotive filter technology, and relates to a filter element for automotive filters and its preparation method. Background Technology

[0002] With the rapid development of the automotive industry and increasingly stringent environmental regulations, the performance requirements for automotive air filters, as a key component of the engine intake system, are constantly rising. Modern automotive engines have extremely demanding requirements for intake air quality. Not only must the filter efficiently filter harmful substances such as particulate matter, dust, and pollen from the air to ensure engine cleanliness and extend its service life, but it must also have extremely low airflow resistance to guarantee engine power performance and fuel economy. In environments with frequent urban smog and aggravated industrial dust pollution, the filtration efficiency of the filter directly affects engine reliability and vehicle emissions levels, while excessive airflow resistance leads to decreased engine power, increased fuel consumption, and negatively impacts overall vehicle performance. Therefore, developing filter element materials for automotive filters that combine high filtration efficiency and low airflow resistance is of significant strategic importance for enhancing automotive product competitiveness, meeting stringent environmental standards, and promoting the sustainable development of the automotive industry, and has become an important research direction in the fields of materials science and automotive engineering.

[0003] Currently, there are still significant technical bottlenecks in achieving a balance between high efficiency and low resistance in automotive filter elements, mainly due to the inherent limitations of traditional filter element material structure design and manufacturing processes. For example, Chinese patent CN204041300U discloses an automotive air filter element, but it suffers from the inability to coordinate filtration efficiency and airflow resistance. Traditional single-layer or simple multi-layer filter element structures often employ uniform fiber distribution and pore size design, which inevitably increases material density and thickness in pursuit of high filtration efficiency, thus significantly increasing airflow resistance. Conversely, reducing resistance sacrifices filtration performance, creating a contradictory relationship between performance indicators. In addition, existing manufacturing processes lack the ability to precisely control the fiber microstructure, making it impossible to achieve directional design of fiber surface properties at the molecular level. This results in insufficient electrostatic adsorption capacity, further limiting the improvement of filtration efficiency. At the same time, traditional electret processes suffer from uneven charge distribution and poor stability, causing significant performance degradation of the filter element during long-term use. It is unable to maintain the excellent performance of the initial stage, seriously restricting further breakthroughs in the overall performance of automotive filters and the improvement of industry technology. Summary of the Invention

[0004] (1) Technical problems to be solved The purpose of this invention is to provide a filter element for automotive filters and its preparation method, thereby solving the problem that current automotive filter elements cannot simultaneously achieve high efficiency and low resistance.

[0005] (2) Technical solution To achieve the above objectives, the present invention provides the following technical solution: A filter element for an automotive air filter includes a double-layer composite structure consisting of a coarse fiber layer and a fine fiber layer arranged sequentially in the airflow direction. Both the coarse fiber layer and the fine fiber layer are composed of polypropylene, antioxidants and β-crystal nucleating agents; The fiber surfaces of both the coarse and fine fiber layers have nanoporous structures. The mass fraction of the β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side, and the mass fraction difference is 0.06~0.10wt%. There is a surface potential difference of 80~200 V between the coarse fiber layer side and the fine fiber layer side, wherein the coarse fiber layer side is positive potential and the fine fiber layer side is negative potential.

[0006] Furthermore, the thickness of the coarse fiber layer is 50~150μm, and the thickness of the fine fiber layer is 20~80μm; The coarse fiber layer has an average fiber diameter of 8~12μm, and the fine fiber layer has an average fiber diameter of 2~5μm.

[0007] Furthermore, the antioxidant has a mass fraction of 0.1~0.3 wt% in both the coarse and fine fiber layers, and the antioxidant is one or more of 2,6-di-tert-butyl-4-methylphenol, tetra[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane, and 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.

[0008] Furthermore, the β-crystal nucleating agent is selected from one or more of N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide, acetophenone acetonide, and N,N'-diphenylhexadiamide; The mass fraction of β-crystal nucleating agent in the fine fiber layer is 0.11~0.22 wt%.

[0009] This invention employs a dual-layer composite structure design primarily to enhance the filtration performance and low-resistance properties of automotive filter elements. By sequentially arranging coarse and fine fiber layers according to the airflow direction, a graded filtration strategy is achieved. The coarse fiber layer performs pre-filtration, while the fine fiber layer handles fine filtration; the synergistic effect of the two layers significantly improves overall filtration efficiency. The combined use of polypropylene matrix material and antioxidants effectively ensures the long-term stability of the filter element. The introduction of antioxidants such as 2,6-di-tert-butyl-4-methylphenol, tetra[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane, and 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene prevents oxidative degradation of the material during use. The core innovation of this invention lies in the differentiated distribution design of β-crystal nucleating agents such as N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide, acetophenone urea, and N,N'-diphenylhexadiamide. A higher mass fraction of β-crystal nucleating agents on the fine fiber layer side promotes the formation of specific crystal forms, improving the fiber's microstructure and surface properties. The construction of the nanoporous structure increases the fiber's specific surface area, improving the particle capture efficiency. The surface potential difference between the positive potential of the coarse fiber layer and the negative potential of the fine fiber layer further enhances the electrostatic adsorption capacity. The gradient design of different fiber diameters achieves an optimized balance between resistance and efficiency, and the synergistic effect among the components achieves a unity of high-efficiency filtration and low resistance.

[0010] This invention also discloses a method for preparing a filter element for automotive filters, comprising supercritical CO2-assisted melt-blown molding and layered reverse corona electret, specifically including the following steps: S1: Formulation Pretreatment and Supercritical Injection Preparation: Polypropylene, antioxidant, and β-crystal nucleating agent are dry-mixed and dried. Carbon dioxide is conveyed to the mixing zone of the extruder to achieve a mass fraction of 0.5~2.0 wt% relative to the melt. S2: Dual-layer meltblown deposition and β-phase thickness gradient construction: A dual-extrusion, dual-metering method was used to construct the thickness gradient, resulting in a higher β-crystal nucleating agent mass fraction on the fine fiber layer side than on the coarse fiber layer side, with a mass fraction difference of 0.06~0.10 wt%. S3: Layered reverse corona electret: Apply a positive voltage of +25~+45 kV to the coarse fiber layer side and a negative voltage of -25~-45 kV to the fine fiber layer side to obtain a thickness surface potential difference of 80~200 V.

[0011] Furthermore, in step S1, the barrel and die head are heated to 230~255℃, the die pressure is controlled to be 0.8~1.2 MPa by the back pressure valve, and the mixture is dry-mixed in a closed mixer for 10~20 min. Carbon dioxide is delivered at 31~50℃ and 7.5~12 MPa, and the residence time of CO2 in the melt is controlled to be 20~60 s.

[0012] Furthermore, in step S2, a coarse fiber layer is deposited first, followed by a fine fiber layer, with the hot air temperature set to 220~260℃ and the wind speed to 150~220 m·s. -1 The distance from the spinneret to the collecting net is 0.20~0.35 m, and the collecting linear velocity is 0.5~1.5 m·min. -1 .

[0013] Furthermore, the unit area mass of the coarse fiber layer is 10~15 g·m. -2 The unit area mass of the fine fiber layer is 6~10 g·m -2 By fine-tuning process parameters through online closed-loop control of basis weight and air permeability, CO2 condensation and compression recovery is implemented to achieve a recovery rate of ≥95%.

[0014] Furthermore, in step S3, the distance between the electrode and the material is 50-80 mm, a linear or serpentine scanning trajectory is used, the duty cycle is 40-80%, and the scanning speed is 0.1-0.5 m / s. -1 Then, it is heat-treated at 50~70℃ for 10~20 min, and further post-processing and molding are carried out: after the material is shaped, it is hot-pressed and laminated with polypropylene spunbond base fabric, and pleating is performed to make the pleat height 20~30 mm and the pitch 2.0~3.0 mm.

[0015] This invention employs a supercritical CO2-assisted melt-blown molding and layered reverse corona electret preparation method primarily to enhance the filtration efficiency and low-resistance performance of automotive filter elements. Supercritical CO2, introduced as a physical foaming agent into the polypropylene melt, improves melt flowability and fiber forming quality through its unique thermodynamic properties. Simultaneously, the rapid phase change process of supercritical CO2 contributes to the formation of finer fiber structures and increases specific surface area. The uniform dry mixing of polypropylene, antioxidants, and β-crystal nucleating agents in the formulation pretreatment stage lays the foundation for subsequent molding. The core innovation of the preparation process lies in the construction of the β-phase thickness gradient achieved through dual extrusion and dual metering. By precisely controlling the difference in the mass fraction of β-crystal nucleating agents between the fine and coarse fiber layers, the differentiated development of crystal structures between different layers is promoted, thereby affecting the microstructure and performance of the fibers. The layered reverse corona electret technology, through differentiated treatment by applying a positive voltage to the coarse fiber layer and a negative voltage to the fine fiber layer, establishes a stable electric field gradient within the material, significantly enhancing the electrostatic adsorption capacity of the fibers. Precise control of process parameters, such as barrel and die head temperature, die pressure, hot air parameters, and electrode spacing, in conjunction with the CO2 condensation, compression, and recovery system, not only ensures the stability of product quality but also achieves a green and environmentally friendly production process. The synergistic optimization among various process links ultimately enables the efficient preparation of high-performance filter elements.

[0016] Application of a filter element for automotive filters in automotive air filters.

[0017] (3) Beneficial technical effects 1. Significantly improves filtration efficiency and reduces airflow resistance: Through a dual-layer composite structure design, the coarse fiber layer undertakes the pre-filtration function, while the fine fiber layer is responsible for fine filtration, realizing a graded filtration strategy. At the same time, the nanoporous structure increases the specific surface area of ​​the fibers, improving the particle capture efficiency. The fiber diameter gradient design between the coarse and fine fiber layers effectively balances filtration efficiency and airflow resistance, solving the technical problem of traditional filter cartridges that are difficult to balance high efficiency and low resistance.

[0018] 2. Effectively enhances electrostatic adsorption capacity: By establishing a surface potential difference between the positive potential of the coarse fiber layer and the negative potential of the fine fiber layer through layered reverse corona electret technology, a stable electric field gradient is formed inside the material, which significantly enhances the electrostatic adsorption capacity of the fiber. At the same time, the differentiated distribution of β-crystal nucleating agents such as N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide, acetophenone urea, and N,N'-diphenylhexanediamide promotes the formation of specific crystal forms, improves the microstructure and surface properties of the fiber, and further enhances the electrostatic capture effect.

[0019] 3. Significantly improves the long-term stability of materials: By introducing antioxidants such as 2,6-di-tert-butyl-4-methylphenol, tetra[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane, and 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, the oxidative degradation of the polypropylene matrix material during use is effectively prevented, ensuring the stability of the filter element's long-term performance and extending the product's service life.

[0020] 4. Achieving green and environmentally friendly production processes: Supercritical CO2-assisted meltblown molding technology is adopted. Supercritical CO2, as a clean physical foaming agent, improves melt flowability and fiber forming quality. At the same time, a CO2 condensation and compression recovery system is implemented, with a recovery rate of over 95%, reducing environmental pollution and achieving a green and sustainable production process that meets the environmental protection requirements of modern industry.

[0021] 5. Enhanced precision control of the preparation process: By constructing a β-phase thickness gradient through a dual extrusion and dual metering method, the mass fraction difference of the β-crystal nucleating agent between the fine fiber layer and the coarse fiber layer is precisely controlled. Combined with an online basis weight and air permeability closed-loop control system, the process parameters are finely adjusted, achieving precise control of product quality and stable production, and improving the controllability and reproducibility of the preparation process. Attached Figure Description

[0022] Figure 1 This is a morphology diagram of the fine fiber layer prepared in Example 2 of the present invention.

[0023] Figure 2 This is a morphology diagram of the coarse fiber layer prepared in Example 2 of the present invention.

[0024] Figure 3The images show the XRD phase analysis of the fine fiber layer and the coarse fiber layer prepared in Example 2 of this invention.

[0025] Figure 4 This is a morphology diagram of the fine fiber layer in Comparative Example 5 of the present invention.

[0026] Figure 5 This invention relates to the effect of the mass fraction difference of the β-crystal nucleating agent on filtration efficiency and initial resistance.

[0027] Figure 6 This invention relates to the effect of surface potential difference on filtration efficiency and initial resistance.

[0028] Figure 7 This invention relates to the effect of the heating temperature of the barrel and die head on the filtration efficiency and initial resistance.

[0029] Figure 8 This invention relates to the effect of CO2 mass fraction on filtration efficiency and initial resistance.

[0030] Figure 9 This invention relates to the effect of hot air temperature on filtration efficiency and initial resistance. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Example 1

[0032] A filter element for an automotive air filter includes a double-layer composite structure consisting of a coarse fiber layer and a fine fiber layer arranged sequentially in the airflow direction. In this embodiment, both the coarse and fine fiber layers are composed of polypropylene, an antioxidant, and a β-crystal nucleating agent. The fiber surfaces of both the coarse and fine fiber layers have a nanoporous structure. The mass fraction of the β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side, with a mass fraction difference of 0.08 wt%. There is a surface potential difference of 130 V between the coarse and fine fiber layers, with the coarse fiber layer side having a positive potential and the fine fiber layer side having a negative potential. The thickness of the coarse fiber layer in this embodiment is 90 μm, and the thickness of the fine fiber layer is 50 μm. The average fiber diameter of the coarse fiber layer is 10 μm, and the average fiber diameter of the fine fiber layer is 3.5 μm. The antioxidant in this embodiment has a mass fraction of 0.2 wt% in both the coarse and fine fiber layers, and the antioxidant in this embodiment is 2,6-di-tert-butyl-4-methylphenol. In this embodiment, the β-crystal nucleating agent is selected from N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide; wherein, the mass fraction of the β-crystal nucleating agent in the fine fiber layer is 0.16 wt%. The preparation method includes supercritical CO2-assisted melt-blown molding and layered reverse corona electret, specifically including the following steps: S1 Formulation pretreatment and supercritical injection preparation: Polypropylene, antioxidant and β-crystal nucleating agent are dry-mixed and dried, and carbon dioxide is transported to the mixing zone of the extruder to make the mass fraction relative to the melt 1.2 wt%; S2 Bilayer melt-blown deposition and β-phase thickness gradient construction: A thickness gradient is constructed by using a dual extrusion dual metering method so that the mass fraction of the β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side and the mass fraction difference is 0.08 wt%; S3 Layered reverse corona electret: A positive voltage of +35 kV is applied to the coarse fiber layer side and a negative voltage of -35 kV is applied to the fine fiber layer side to obtain a thickness surface potential difference of 130 V. In step S1 of this embodiment, the barrel and die head are heated to 240°C, and the die pressure is controlled to 1.0 MPa by a back pressure valve. Dry mixing is performed for 15 minutes in a closed mixer. Carbon dioxide is supplied at 40°C and 10 MPa, and the CO2 residence time in the melt is controlled to be 40 seconds. In step S2 of this embodiment, a coarse fiber layer is deposited first, followed by a fine fiber layer. The hot air temperature is set to 240°C, and the air velocity is 180 m / s. -1 The distance from the spinneret to the collecting net is 0.28 m, and the collecting linear velocity is 1.0 m·min. -1 The mass per unit area of ​​the coarse fiber layer is 12 g·m³. -2 The unit area mass of the fine fiber layer is 8 g·m³. -2 The process parameters are fine-tuned through online closed-loop control of basis weight and air permeability, and CO2 condensation and compression recovery is implemented to achieve a recovery rate of 95%. In step S3 of this embodiment, the distance between the electrode and the material is 65 mm, a linear scanning trajectory is used, the duty cycle is 60%, and the scanning speed is 0.3 m·s. -1Then, it is heat-treated at 60℃ for 15 min and further post-processed and shaped: after the material is shaped, it is hot-pressed and laminated with polypropylene spunbond base fabric, and pleated to make the pleat height 25 mm and the pitch 2.5 mm.

[0033] This embodiment uses moderately conservative parameter configurations, which features good process stability and controllable product quality, and is suitable for mass production applications of standard passenger car air filters. Example 2

[0034] A filter element for an automotive air filter includes a double-layer composite structure consisting of a coarse fiber layer and a fine fiber layer arranged sequentially in the airflow direction. In this embodiment, both the coarse and fine fiber layers are composed of polypropylene, an antioxidant, and a β-crystal nucleating agent. The fiber surfaces of both the coarse and fine fiber layers have a nanoporous structure. The mass fraction of the β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side, with a mass fraction difference of 0.06 wt%. There is a surface potential difference of 80 V between the coarse and fine fiber layers, with the coarse fiber layer side having a positive potential and the fine fiber layer side having a negative potential. The thickness of the coarse fiber layer in this embodiment is 50 μm, and the thickness of the fine fiber layer is 20 μm. The average fiber diameter of the coarse fiber layer is 8 μm, and the average fiber diameter of the fine fiber layer is 2 μm. The mass fraction of the antioxidant in both the coarse and fine fiber layers is 0.1 wt%, and the antioxidant in this embodiment is tetrakis[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane. In this embodiment, the β-crystal nucleating agent is selected from acetophenone urea; wherein, the mass fraction of the β-crystal nucleating agent in the fine fiber layer is 0.11 wt%. The preparation method includes supercritical CO2-assisted melt-blown molding and layered reverse corona electret, specifically including the following steps: S1 Formulation pretreatment and supercritical injection preparation: Polypropylene, antioxidant and β-crystal nucleating agent are dry-mixed and dried, and carbon dioxide is transported to the mixing zone of the extruder to make the mass fraction relative to the melt 0.5 wt%; S2 Bilayer melt-blown deposition and β-phase thickness gradient construction: A thickness gradient is constructed by using a dual extrusion dual metering method so that the mass fraction of the β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side and the mass fraction difference is 0.06 wt%; S3 Layered reverse corona electret: A positive voltage of +25 kV is applied to the coarse fiber layer side and a negative voltage of -25 kV is applied to the fine fiber layer side to obtain a thickness surface potential difference of 80 V. In step S1 of this embodiment, the barrel and die head are heated to 230°C, and the die pressure is controlled to 0.8 MPa by a back pressure valve. Dry mixing is performed for 10 minutes in a closed mixer. Carbon dioxide is supplied at 31°C and 7.5 MPa, and the CO2 residence time in the melt is controlled to be 20 seconds. In step S2 of this embodiment, a coarse fiber layer is deposited first, followed by a fine fiber layer. The hot air temperature is set to 220°C, and the air velocity is 150 m / s. -1The distance from the spinneret to the collecting net is 0.20 m, and the collecting linear velocity is 0.5 m·min. -1 The mass per unit area of ​​the coarse fiber layer is 10 g·m³. -2 The unit area mass of the fine fiber layer is 6 g·m -2 By fine-tuning process parameters through online closed-loop control of basis weight and air permeability, CO2 condensation and compression recovery is implemented to achieve a recovery rate of 96%. In step S3 of this embodiment, the distance between the electrode and the material is 50 mm, a serpentine scanning trajectory is used, the duty cycle is 40%, and the scanning speed is 0.1 m·s. -1 Then, it is heat-treated at 50℃ for 10 min and further post-processed and shaped: after the material is shaped, it is hot-pressed and laminated with polypropylene spunbond base fabric, and pleated to make the pleat height 20 mm and the pitch 2.0 mm.

[0035] This embodiment uses a low-resistance optimized parameter configuration, which features low airflow resistance and good air permeability, making it suitable for high-power engines or sports vehicles with high requirements for low resistance. Example 3

[0036] A filter element for an automotive air filter includes a double-layer composite structure consisting of a coarse fiber layer and a fine fiber layer arranged sequentially in the airflow direction. In this embodiment, both the coarse and fine fiber layers are composed of polypropylene, an antioxidant, and a β-crystal nucleating agent. The fiber surfaces of both the coarse and fine fiber layers have a nanoporous structure. The mass fraction of the β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side, with a mass fraction difference of 0.10 wt%. There is a surface potential difference of 200 V between the coarse and fine fiber layers, with the coarse fiber layer side having a positive potential and the fine fiber layer side having a negative potential. In this embodiment, the thickness of the coarse fiber layer is 150 μm, and the thickness of the fine fiber layer is 80 μm. The average fiber diameter of the coarse fiber layer is 12 μm, and the average fiber diameter of the fine fiber layer is 5 μm. In this embodiment, the antioxidant has a mass fraction of 0.3 wt% in both the coarse and fine fiber layers. The antioxidant in this embodiment is a mixture of 2,6-di-tert-butyl-4-methylphenol and tetrakis[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane in a mass ratio of 1:1. The β-crystal nucleating agent in this embodiment is selected from N,N'-diphenylhexamethylenediamide; wherein, the mass fraction of the β-crystal nucleating agent in the fine fiber layer is 0.22 wt%. The preparation method includes supercritical CO2-assisted meltblown molding and layered reverse corona electret, specifically including the following steps: S1 Formulation pretreatment and supercritical injection preparation: Polypropylene, antioxidant and β-crystal nucleating agent are dry mixed and dried, and carbon dioxide is transported to the mixing zone of the extruder to make the mass fraction relative to the melt 2.0 wt%; S2 Bilayer meltblown deposition and β-phase thickness gradient construction: The thickness formulation difference is constructed by using a dual extrusion dual metering method so that the mass fraction of β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side and the mass fraction difference is 0.10 wt%; S3 Layered reverse corona electret: A positive voltage of +45 kV is applied to the coarse fiber layer side and a negative voltage of -45 kV is applied to the fine fiber layer side to obtain a thickness surface potential difference of 200 V. In step S1 of this embodiment, the barrel and die are heated to 255°C, and the die pressure is controlled to 1.2 MPa by a back pressure valve. Dry mixing is performed for 20 minutes in a closed mixer, with carbon dioxide supplied at 50°C and 12 MPa, and the CO2 residence time in the melt is controlled to be 60 seconds. In step S2 of this embodiment, a coarse fiber layer is deposited first, followed by a fine fiber layer. The hot air temperature is set to 260°C, and the air velocity is 220 m / s. -1 The distance from the spinneret to the collecting net is 0.35 m, and the collecting linear velocity is 1.5 m·min. -1 The mass per unit area of ​​the coarse fiber layer is 15 g·m³. -2 The unit area mass of the fine fiber layer is 10 g·m -2 The process parameters are fine-tuned through online closed-loop control of basis weight and air permeability, and CO2 condensation and compression recovery is implemented to achieve a recovery rate of 95%. In step S3 of this embodiment, the distance between the electrode and the material is 80 mm, a linear scanning trajectory is used, the duty cycle is 80%, and the scanning speed is 0.5 m·s.-1 Then, it is heat-treated at 70℃ for 20 min and further post-processed and shaped: after the material is shaped, it is hot-pressed and laminated with polypropylene spunbond base fabric, and pleated to make the pleat height 30 mm and the pitch 3.0 mm.

[0037] This embodiment uses a parameter configuration optimized for high-efficiency filtration, which features high filtration efficiency and strong electrostatic adsorption capacity. It is suitable for heavy vehicles and special working conditions in environments with severe sand and dust or with extremely high air quality requirements. Example 4

[0038] A filter element for an automotive air filter includes a double-layer composite structure consisting of a coarse fiber layer and a fine fiber layer arranged sequentially in the airflow direction. In this embodiment, both the coarse and fine fiber layers are composed of polypropylene, an antioxidant, and a β-crystal nucleating agent. The fiber surfaces of both the coarse and fine fiber layers have a nanoporous structure. The mass fraction of the β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side, with a mass fraction difference of 0.09 wt%. There is a surface potential difference of 170 V between the coarse and fine fiber layers, with the coarse fiber layer side having a positive potential and the fine fiber layer side having a negative potential. In this embodiment, the thickness of the coarse fiber layer is 120 μm, and the thickness of the fine fiber layer is 65 μm. The average fiber diameter of the coarse fiber layer is 9 μm, and the average fiber diameter of the fine fiber layer is 4 μm. In this embodiment, the antioxidant has a mass fraction of 0.25 wt% in both the coarse and fine fiber layers. The antioxidant in this embodiment is a mixture of 2,6-di-tert-butyl-4-methylphenol, tetra[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane, and 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene in a mass ratio of 2:1:1. The β-crystal nucleating agent in this embodiment is selected from a mixture of N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide and acetophenone acetonide in a mass ratio of 3:1; wherein the mass fraction of the β-crystal nucleating agent in the fine fiber layer is 0.19 wt%. The preparation method includes supercritical CO2-assisted meltblown molding and layered reverse corona electret, specifically including the following steps: S1 Formulation pretreatment and supercritical injection preparation: Polypropylene, antioxidant and β-crystal nucleating agent are dry mixed and dried, and carbon dioxide is transported to the mixing zone of the extruder to make the mass fraction relative to the melt 1.7 wt%; S2 Bilayer meltblown deposition and β-phase thickness gradient construction: The thickness formulation difference is constructed by using a dual extrusion dual metering method so that the mass fraction of β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side and the mass fraction difference is 0.09 wt%; S3 Layered reverse corona electret: A positive voltage of +42 kV is applied to the coarse fiber layer side and a negative voltage of -42 kV is applied to the fine fiber layer side to obtain a thickness surface potential difference of 170 V. In step S1 of this embodiment, the barrel and die head are heated to 248°C, and the die pressure is controlled to 1.1 MPa by a back pressure valve. Dry mixing is performed for 18 minutes in a closed mixer, with carbon dioxide supplied at 45°C and 11 MPa, and the CO2 residence time in the melt is controlled to be 50 s. In step S2 of this embodiment, a coarse fiber layer is deposited first, followed by a fine fiber layer. The hot air temperature is set to 250°C, and the air velocity is 200 m / s. -1 The distance from the spinneret to the collecting net is 0.32 m, and the collecting linear velocity is 1.2 m·min. -1 The mass per unit area of ​​the coarse fiber layer is 13 g·m³. -2 The unit area mass of the fine fiber layer is 9 g·m -2By fine-tuning process parameters through online closed-loop control of basis weight and air permeability, and implementing CO2 condensation and compression recovery, a recovery rate of 97% is achieved. In step S3 of this embodiment, the distance between the electrode and the material is 70 mm, a serpentine scanning trajectory is used, the duty cycle is 65%, and the scanning speed is 0.4 m·s. -1 Then, it is heat-treated at 65℃ for 18 min and further post-processed and shaped: after the material is shaped, it is hot-pressed and laminated with polypropylene spunbond base fabric, and pleated to make the pleat height 28 mm and the pitch 2.8 mm.

[0039] This embodiment adopts a balanced and optimized parameter configuration, which has the characteristics of excellent comprehensive performance and long service life. It is suitable for all working conditions of high-end passenger cars and commercial vehicles that require both filtration efficiency and low resistance.

[0040] Comparative Example 1: Basically the same as Example 1, except that the mass fraction difference of the β-crystal nucleating agent between the fine fiber layer side and the coarse fiber layer side is 0.03 wt%.

[0041] Comparative Example 2: Basically the same as Example 1, except that the surface potential difference between the coarse fiber layer side and the fine fiber layer side is 40 V.

[0042] Comparative Example 3: It is basically the same as Example 1, except that it adopts a single-layer structure, which only contains a fine fiber layer and removes the coarse fiber layer.

[0043] Comparative Example 4: It is basically the same as Example 1, except that no β-crystal nucleating agent is added to the coarse fiber layer and the fine fiber layer, and it is composed only of polypropylene and antioxidant.

[0044] Comparative Example 5: It is basically the same as Example 1, except that the fiber surface does not have a nanoporous structure and a smooth surface fiber is prepared by conventional melt-blowing process.

[0045] Comparative Example 6: It is basically the same as Example 1, except that the barrel and die head are heated to 200°C in step S1.

[0046] Comparative Example 7: It is basically the same as Example 1, except that supercritical CO2 is not added in step S1, and only conventional meltblown molding process is used.

[0047] Comparative Example 8: It is basically the same as Example 1, except that in step S3, a positive voltage of +35 kV is applied to both the coarse fiber layer side and the fine fiber layer side, and the layered reverse corona electret treatment is not used.

[0048] Comparative Example 9: It is basically the same as Example 1, except that the average fiber diameter of the fine fiber layer is 8 μm, which is the same as the average fiber diameter of the coarse fiber layer, and no fiber diameter gradient is formed.

[0049] Comparative Example 10: Basically the same as Example 1, except that the hot air temperature is set to 180°C in step S2.

[0050] Comparative Example 11: Basically the same as Example 1, except that the mass fraction of the antioxidant is 0.05 wt%.

[0051] Comparative Example 12: Basically the same as Example 1, except that the residence time of CO2 in the melt is controlled to be 5s in step S1.

[0052] Comparative Example 13: It is basically the same as Example 1, except that the thickness of the fine fiber layer is 100 μm and the thickness of the coarse fiber layer is 50 μm, so that the thickness of the fine fiber layer is greater than the thickness of the coarse fiber layer.

[0053] Comparative Example 14: Basically the same as Example 1, except that the distance between the electrode and the material in step S3 is 30 mm.

[0054] Comparative Example 15: Basically the same as Example 1, except that sodium benzoate was used as the β-crystal nucleating agent.

[0055] Performance testing: Filtration Efficiency Test Experiment: Test Object: Double-layer composite structure material of automotive air filter element. Test Objective: To evaluate the filtration efficiency of the filter element for particles of different sizes and verify its core performance in automotive air filtration applications. Test Principle: Standard test dust is passed through the filter element at a specified flow rate, and the upstream and downstream particle concentrations are measured to calculate the filtration efficiency. Experimental Method: The filter element sample is installed on a standard test bench. ISO 12103-1 A2 fine test dust is used as the challenge particle, and the test is conducted at a flow rate of 1000 L / min. A laser particle size analyzer is used to simultaneously monitor the upstream and downstream particle concentration distribution. Standard Basis: The test is conducted according to ISO 5011:2020 "Air filters for road vehicles - Test methods". Key Parameters: Test temperature: 23±2℃, relative humidity: 45-75%, test dust concentration: 200±10 mg / m³, test time continues until the dust holding capacity reaches the design value. Data processing: The filtration efficiency η is calculated using the formula η=(Cup-Cdown) / Cup×100%, where Cup is the upstream concentration and Cdown is the downstream concentration. The efficiency value is required to be statistically analyzed within the particle size range of 0.3-10μm.

[0056] Initial Resistance Test Experiment: The test object is a double-layer composite structure material for automotive air filter elements. Test Objective: To measure the airflow resistance of the filter element in a clean state and evaluate its impact on the engine intake system. Test Principle: Under standard flow conditions, the pressure difference before and after air passes through the filter element is measured, reflecting the degree of airflow obstruction by the filter element. Experimental Method: A clean filter element is installed in a sealed test chamber. A standard fan provides a constant airflow. A high-precision differential pressure sensor measures the pressure difference across the filter element, while simultaneously monitoring flow stability. Standard Basis: The resistance test method is performed according to ISO 5011:2020. Key Parameters: Standard test flow rate is 1000 L / min, ambient temperature is 23±2℃, relative humidity is 45-75%, and pressure difference measurement accuracy is not less than ±1 Pa. Data Processing: The initial resistance Δp is recorded in Pa. Five consecutive measurements are required, and the average value is taken. The test results should meet the technical requirements of the automotive manufacturer, generally not exceeding 250 Pa.

[0057] Dust Holding Capacity Test Experiment: Test Object: Double-layer composite structure material of automotive filter element. Test Purpose: To evaluate the dust holding capacity and service life of the filter element during use, and to determine its replacement cycle under actual working conditions. Test Principle: Standard test dust is continuously supplied to the filter element, and the resistance growth process is monitored. When the resistance reaches the set endpoint value, the test is stopped and the dust holding capacity is weighed. Experimental Method: ISO 12103-1 A2 test dust is continuously supplied at a constant flow rate. The filter element resistance change is monitored in real time. When the resistance reaches 5 times the initial resistance or the absolute value reaches 2500 Pa, the test is stopped, and the filter element is removed and the total dust holding capacity is weighed. Standard Basis: The dust holding capacity test procedure of ISO 5011:2020 standard is strictly followed. Key Parameters: Test flow rate 1000 L / min, dust concentration 200±10 mg / m³, test temperature 23±2℃, relative humidity 45-75%, weighing accuracy 0.1g. Data processing: Dust holding capacity is calculated in g / m², and the resistance growth curve is recorded to evaluate the dust holding characteristics and performance of the filter element. The dust holding capacity is required to be no less than the industry average for similar products.

[0058] Surface Potential Distribution Test Experiment: The test object is the surface of the coarse fiber layer and fine fiber layer of a double-layer composite structure used in automotive air filters. Test Objective: To measure the potential distribution on the filter element surface after electret treatment and verify the effectiveness of the layered reverse corona electret process. Test Principle: A non-contact surface potentiometer is used to measure the electrostatic potential of the material surface and evaluate the distribution and stability of the electret charge. Experimental Method: A gridded scanning measurement is performed on the filter element surface using a surface potentiometer, with a measurement point spacing of 5 mm. The surface potential is measured on both the coarse fiber layer side and the fine fiber layer side, and the potential distribution diagram and statistical data are recorded. Key Parameters: Test environment temperature 23±2℃, relative humidity 45-55%, probe-sample distance 2 mm, measurement accuracy ±5 V, scanning speed 10 mm / s. Data Processing: The average surface potential difference between the coarse and fine fiber layers is calculated, requiring a range of 80-200 V. The uniformity of the potential distribution is also evaluated, with the standard deviation less than 15% of the average value.

[0059] Thermal Stability Test Experiment: The test object is a double-layer composite structure material for automotive filter elements. Test Objective: To evaluate the thermal stability of the filter element material under high-temperature conditions, ensuring its reliability in automotive engine compartment applications. Test Principle: Thermogravimetric analysis (TGA) is used to monitor the mass change of the material during programmed temperature rise, analyzing thermal decomposition behavior and thermal stability temperature. Experimental Method: A 10 mg sample is placed in a thermogravimetric analyzer and heated from room temperature to 600℃ at a rate of 10℃ / min under nitrogen protection. Mass change and temperature are recorded simultaneously, and thermal decomposition temperature and weight loss behavior are analyzed. Standard Basis: The test is conducted according to ASTM E1131-08 (2014) "Thermogravimetric Analysis Procedure". Key Parameters: Sample mass 8-12 mg, nitrogen flow rate 50 mL / min, heating rate 10℃ / min, temperature accuracy ±1℃, balance accuracy 0.1 μg. Data processing: Determine the 5% mass loss temperature T5% and the maximum decomposition rate temperature Tmax, plot TGA and DTG curves, requiring T5% to be no less than 280℃, and evaluate the long-term stability of the material under engine compartment ambient temperature.

[0060] Dust Holding Capacity Test Experiment: The test object is a double-layer composite structure material for automotive air filter elements. Test Objective: To evaluate the dust holding capacity and service life of the filter element during use, and to determine its replacement cycle under actual working conditions. Test Principle: Standard test dust is continuously supplied to the filter element, and the resistance growth process is monitored. When the resistance reaches a set endpoint value, the test is stopped and the dust holding capacity is measured. Experimental Method: ISO 12103-1 A2 test dust is continuously supplied at a constant flow rate. The filter element resistance change is monitored in real time. The test is stopped when the resistance reaches 5 times the initial resistance or the absolute value reaches 2500 Pa. The filter element is removed and the total dust holding capacity is measured. Key Parameters: Test flow rate 1000 L / min, dust concentration 200±10 mg / m³, test temperature 23±2℃, relative humidity 45-75%, weighing accuracy 0.1 g. Data Processing: Dust holding capacity is calculated in g / m², and the resistance growth curve is recorded simultaneously to evaluate the dust holding characteristics and performance of the filter element. The dust holding capacity is required to be no less than the industry average for similar products.

[0061] Surface Potential Distribution Test Experiment: The test object is the surface of the coarse fiber layer and fine fiber layer of a double-layer composite structure used in automotive air filters. Test Objective: To measure the potential distribution on the filter element surface after electret treatment and verify the effectiveness of the layered reverse corona electret process. Test Principle: A non-contact surface potentiometer is used to measure the electrostatic potential of the material surface and evaluate the distribution and stability of the electret charge. Experimental Method: A gridded scanning measurement is performed on the filter element surface using a surface potentiometer, with a measurement point spacing of 5 mm. The surface potential is measured on both the coarse fiber layer side and the fine fiber layer side, and the potential distribution diagram and statistical data are recorded. Key Parameters: Test environment temperature 23±2℃, relative humidity 45-55%, probe-sample distance 2 mm, measurement accuracy ±5 V, scanning speed 10 mm / s. Data Processing: The average surface potential difference between the coarse and fine fiber layers is calculated, requiring a range of 80-200 V. The uniformity of the potential distribution is also evaluated, with the standard deviation less than 15% of the average value.

[0062] The performance of the examples and comparative examples is summarized in Table 1. The performance degradation of each comparative example mainly stemmed from the lack of key technical elements or deviations from the optimal parameter range, leading to an imbalance in the structure-performance relationship. In Comparative Example 1, insufficient gradient difference of the β-crystal nucleating agent resulted in incomplete β-phase crystal transformation, reducing the density of active sites on the fiber surface and causing a decrease in both filtration efficiency and dust holding capacity. In Comparative Example 2, the excessively low surface potential difference directly weakened the electrostatic adsorption capacity, resulting in a significant decrease in filtration efficiency but little change in resistance. In Comparative Example 3, the single-layer structure disrupted the hierarchical filtration mechanism, losing the pre-filtration function of the coarse fiber layer, leading to a significant decrease in filtration efficiency and dust holding capacity. At the same time, while resistance decreased, overall performance was severely compromised. In Comparative Example 4, the lack of a β-crystal nucleating agent caused the fiber microstructure to lose its control, resulting in decreased filtration performance and deteriorated thermal stability. In Comparative Example 5, the smooth fiber surface reduced the specific surface area and surface roughness, decreasing mechanical retention and inertial impaction effects, thus affecting both filtration efficiency and dust holding capacity. In Comparative Example 6, low-temperature heating resulted in insufficient polymer melt fluidity, decreased fiber forming quality, and affected β-phase crystallization, causing dual damage to filtration performance and thermal stability. In Comparative Example 7, the lack of supercritical CO2 assistance resulted in reduced fiber microstructure roughness, decreased specific surface area, and decreased filtration efficiency and dust holding capacity. In Comparative Example 8, the same-polarity electret treatment failed to establish an effective electric field gradient, almost completely eliminating electrostatic adsorption capacity and significantly reducing filtration efficiency. In Comparative Example 9, the same fiber diameter disrupted the gradient filtration structure, eliminating the synergistic effect of coarse and fine fibers, significantly reducing filtration efficiency and dust holding capacity while increasing resistance. In Comparative Example 10, low-temperature hot air affected the final fiber forming quality and surface structure, decreasing filtration performance and deteriorating thermal stability. In Comparative Example 11, the low antioxidant content primarily affected long-term stability; short-term filtration performance showed little change, but thermal stability was significantly reduced. In Comparative Example 12, insufficient CO2 residence time led to inadequate foaming, increased fiber density, decreased filtration efficiency, and reduced dust holding capacity. In Comparative Example 13, the incorrect layer thickness configuration violated the principle of graded filtration; an excessively thick fine fiber layer caused a sharp increase in resistance, while an excessively thin coarse fiber layer resulted in insufficient pre-filtration, severely deteriorating overall performance. In Comparative Example 14, the excessively small electrode spacing resulted in an uneven distribution of the electret electric field, a reduced surface potential difference, and a weakened electrostatic adsorption effect. In Comparative Example 15, the use of an inappropriate β-crystal nucleating agent failed to effectively promote the formation of the target crystal form, leading to the failure of fiber microstructure regulation, decreased filtration performance, and impact on thermal stability. These deviations from the technical elements demonstrate the synergistic importance of the various components of the present invention and the scientific nature of the optimized parameter range.

[0063]

[0064] from Figure 1 and Figure 2Morphological analysis clearly shows that the fine fiber layer prepared in Example 2 has a diameter of about 2 μm and a distinct nanoporous structure on its surface. This unique surface morphology significantly increases the specific surface area and surface roughness of the fiber, providing more active sites for the mechanical retention and electrostatic adsorption of particulate matter. The coarse fiber layer has a diameter of about 8 μm and also has a nanoporous structure, but its density is relatively low, forming a reasonable hierarchical filtration gradient. Figure 3 XRD phase analysis results show that by precisely controlling the thickness gradient distribution of the β-crystal nucleating agent, the differential regulation of the β-phase crystal structure was successfully achieved. The crystallinity of the β-phase in the fine fiber layer was significantly higher than that in the coarse fiber layer. This gradient distribution of the crystal structure not only optimizes the microstructure of the fibers but also enhances the mechanical strength and thermal stability of the material, providing a solid materials science basis for high-efficiency filtration performance. Figure 4 The morphology of the fine fiber layer in Comparative Example 5 clearly shows a smooth fiber surface lacking nanoporous structures. This difference in surface morphology directly led to a decrease in filtration efficiency from 94.2% in Example 2 to 91.8% in Comparative Example 5, fully verifying the crucial role of nanoporous structures in improving filtration performance. Single-factor experiments on five key parameters verified that the theoretical optimum of each parameter lies within 72-75% of the patent range, maintaining a sufficient safe distance from the boundary values. This internally optimal distribution pattern not only demonstrates the scientific rigor and forward-looking nature of the patent parameter range setting but, more importantly, ensures the stability and reproducibility of the technical solution in practical applications.

[0065] based on Figure 5-9 The single-factor variable experimental results of the five key parameters presented demonstrate the rationality, reliability, and effectiveness of the technical solution of this invention through systematic scientific verification. Figure 5 The study clearly demonstrated the significant impact of the mass fraction difference of β-crystal nucleating agent on filtration performance within the range of 0.06-0.10 wt%. When the mass fraction difference increased from 0.03 wt% to the theoretical optimum of 0.087 wt%, the filtration efficiency steadily increased from 93.5% to 97.5%, while the initial resistance showed a reasonable trend of first decreasing and then increasing, reaching a minimum resistance of 155 Pa at 0.06 wt%. Subsequently, it increased moderately with the increase of β-phase nucleation density. This variation pattern is fully consistent with the material science mechanism of β-phase crystal form regulating fiber microstructure, verifying the scientific basis for setting the parameter optimization range. Figure 6The surface potential difference effect shown indicates that within the 80-200 V technical range, the filtration efficiency exhibits a significant positive correlation with the increase of surface potential difference, reaching a peak of 97.6% near the theoretical optimum of 170 V. The trend of initial resistance variation is highly consistent with the theoretical distribution of electrostatic field strength. Too low a surface potential difference cannot provide sufficient electrostatic adsorption force, while too high a field strength leads to uneven distribution of electric field between fibers, increasing airflow resistance. This equilibrium point is located precisely in the upper-middle part of the technical range, fully demonstrating the rationality of optimizing the parameters of the layered reverse corona electret process. Figure 7 The temperature effect curves of the barrel and die head heating exhibit typical polymer melt processing characteristics. Within the technical range of 230-255℃, the filtration efficiency continuously improves with increasing temperature and reaches an optimal value of 97.8% near 248℃. The initial resistance is lowest at 230℃, and then increases moderately with the improvement of melt flowability and fiber forming quality. This variation pattern is completely consistent with the rheological properties and β-phase crystallization behavior of polypropylene, verifying the rationality of the temperature control strategy. Figure 8 The CO2 mass fraction influence mechanism reflects that the effect of supercritical CO2 on the microstructure of fibers in the technical range of 0.5-2.0 wt% exhibits obvious stage characteristics. The filtration efficiency reaches a peak of 97.8% near the theoretical optimum of 1.6 wt%. The initial resistance change curve clearly reflects the physicochemical process of bubble nucleation, growth and stabilization during supercritical foaming. Too low a CO2 concentration cannot form a sufficient microporous structure, while too high a concentration leads to the collapse of the pore structure and increases resistance. The optimal distribution at 73% of the technical range is completely in line with the theoretical expectations of supercritical fluids. Figure 9 The hot air temperature effect further confirms the key role of temperature control in the final fiber forming stage. Within the technical range of 220-260℃, the filtration efficiency reaches a maximum of 98.0% near 250℃ as the hot air temperature increases. The change in initial resistance reflects the dynamic balance process between the optimization of fiber surface structure and the evolution of airflow channels. This temperature dependence is highly consistent with the thermoforming mechanism of polymer fibers.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A filter element for an automotive filter, characterized in that, It includes a two-layer composite structure consisting of a coarse fiber layer and a fine fiber layer arranged sequentially in the direction of airflow. Both the coarse fiber layer and the fine fiber layer are composed of polypropylene, antioxidants and β-crystal nucleating agents; The fiber surfaces of both the coarse and fine fiber layers have nanoporous structures. The mass fraction of the β-crystal nucleating agent on the fine fiber layer side is higher than that on the coarse fiber layer side, and the mass fraction difference is 0.06~0.10 wt%. There is a surface potential difference of 80~200 V between the coarse fiber layer side and the fine fiber layer side, wherein the coarse fiber layer side is positive potential and the fine fiber layer side is negative potential.

2. The filter element for an automotive filter as described in claim 1, characterized in that, The coarse fiber layer has a thickness of 50~150μm, and the fine fiber layer has a thickness of 20~80μm; The coarse fiber layer has an average fiber diameter of 8~12μm, and the fine fiber layer has an average fiber diameter of 2~5μm.

3. The filter element for an automotive filter as described in claim 1, characterized in that, The antioxidant has a mass fraction of 0.1~0.3 wt% in both the coarse and fine fiber layers, and the antioxidant is one or more of 2,6-di-tert-butyl-4-methylphenol, tetra[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane, and 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.

4. The filter element for an automotive filter as described in claim 1, characterized in that, The β-crystal nucleating agent is selected from one or more of N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide, acetophenone acetal urea, and N,N'-diphenylhexanediamide; The mass fraction of β-crystal nucleating agent in the fine fiber layer is 0.11~0.22 wt%.

5. A method for preparing a filter element for an automotive filter as described in any one of claims 1 to 4, characterized in that, This includes supercritical CO2-assisted meltblown forming and layered reverse corona electret, specifically comprising the following steps: S1: Formulation Pretreatment and Supercritical Injection Preparation: Polypropylene, antioxidant, and β-crystal nucleating agent are dry-mixed and dried. Carbon dioxide is conveyed to the mixing zone of the extruder to achieve a mass fraction of 0.5~2.0 wt% relative to the melt. S2: Dual-layer meltblown deposition and β-phase thickness gradient construction: A dual-extrusion, dual-metering method was used to construct the thickness gradient, resulting in a higher β-crystal nucleating agent mass fraction on the fine fiber layer side than on the coarse fiber layer side, with a mass fraction difference of 0.06~0.10 wt%. S3: Layered reverse corona electret: Apply a positive voltage of +25~+45 kV to the coarse fiber layer side and a negative voltage of -25~-45 kV to the fine fiber layer side to obtain a thickness surface potential difference of 80~200 V.

6. The method for preparing a filter element for an automotive filter as described in claim 5, characterized in that, In step S1, the barrel and die head are heated to 230~255℃, and the die pressure is controlled to be 0.8~1.2 MPa by the back pressure valve. The mixture is dry-mixed in a closed mixer for 10~20 min. Carbon dioxide is transported at 31~50℃ and 7.5~12 MPa, and the residence time of CO2 in the melt is controlled to be 20~60 s.

7. The method for preparing a filter element for an automotive filter as described in claim 5, characterized in that, In step S2, a coarse fiber layer is deposited first, followed by a fine fiber layer. The hot air temperature is set to 220~260℃, and the wind speed is set to 150~220 m·s. -1 The distance from the spinneret to the collecting net is 0.20~0.35 m, and the collecting linear velocity is 0.5~1.5 m·min. -1 .

8. The method for preparing a filter element for an automotive filter as described in claim 5, characterized in that, The unit area mass of the coarse fiber layer is 10~15 g·m -2 The unit area mass of the fine fiber layer is 6~10 g·m -2 By fine-tuning process parameters through online closed-loop control of basis weight and air permeability, CO2 condensation and compression recovery is implemented to achieve a recovery rate of ≥95%.

9. The method for preparing a filter element for an automotive filter as described in claim 5, characterized in that, In step S3, the distance between the electrode and the material is 50-80 mm, a linear or serpentine scanning trajectory is used, the duty cycle is 40-80%, and the scanning speed is 0.1-0.5 m / s. -1 Then, it is heat-treated at 50~70℃ for 10~20 min, and further post-processing and molding are carried out: after the material is shaped, it is hot-pressed and laminated with polypropylene spunbond base fabric, and pleating is performed to make the pleat height 20~30 mm and the pitch 2.0~3.0 mm.

10. The application of the automotive filter element as described in any one of claims 1 to 4 in an automotive air filter for filtering particulate matter in intake air.

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