A method for preparing a layered, gradient foam layer coated filter material

Abstract

The present invention discloses a method for preparing a layered, gradient foam layer coated filter material. The present invention uses a foam coating finishing liquid with polytetrafluoroethylene (PTFE) as the main body, and after forming a PTFE film layer on the surface of the filter substrate, introduces polyvinyl alcohol (PVA) to modify PTFE, and innovates the finishing process after the foam film layer is coated. By finely controlling the thickness of the PTFE foam layer and the PVA@PTFE foam layer, they are sequentially coated on a non-woven filter substrate, and are firmly combined by heating treatment to form a layered, gradient foam pore layer structure, thereby achieving precise design of the pore size of the filter material. The present invention forms a dense microporous membrane layer on the surface of the filter substrate, presenting a gradient multi-layer filtration, effectively preventing the penetration of particulate pollutants, maintaining a high filtration efficiency, while improving the filtration accuracy of the substrate and extending the service life.

Description

Technical Field

[0001] The invention relates to a preparation method of a layered and gradient foam layer coated filter material, belonging to the field of post-finishing of high-temperature smoke filter materials. Background Art

[0002] Particulate matter (PM) is a major component of air pollution and can be categorized by diameter into PM10, PM2.5, and PM0.5. Smaller particles remain in the air longer, making them more likely to enter the lungs and immune system, leading to various health problems. Therefore, reducing particulate matter emissions is a key measure to effectively address air pollution. Bag filters are a highly effective emission reduction technology that significantly reduces particulate matter emissions by filtering particulate matter from industrial flue gases, thereby protecting the environment and reducing the impact of pollutants on human health.

[0003] Currently, in industrial production, bag filter systems typically utilize post-processing to treat the filter substrate. Polytetrafluoroethylene (PTFE) foam coatings are widely used in industrial bag filter systems due to their high thermal stability. However, for particles with a diameter of 0.5 to 2.5 µm, the average pore size of PTFE foam coatings is relatively large, resulting in low initial PM collection efficiency. For example, granted patent CN101479016B describes a method for preparing a filter medium by mixing a heat-resistant water-soluble resin, a foam stabilizer, a blowing agent, and a thickener to prepare a coating solution. This solution is then subjected to a series of drying, heat treatment, and cooling steps. The resulting filter medium has a single-layer microporous structure with an average pore size of 30 µm. These micropores are evenly distributed across the filter medium's surface, contributing to improved PM filtration efficiency. However, the patent does not specifically describe how to control the pore size, particularly with regard to capturing smaller particles (e.g., 0.5 to 2.5 µm). Authorized patent CN217188396U utilizes a multi-layer composite and gradient filtration design, including a front-loaded filter support layer, a PP meltblown intermediate filter layer, and a PP meltblown filter precision reinforcement layer, all bonded together using PU or PO adhesives or ultrasonic bonding technology. This structural design improves the filter media's filtration precision and dust holding capacity, while reducing its pressure drop. However, the bonding between the filter layers is susceptible to process conditions and material properties, which can lead to inconsistent bond strength, thus affecting the overall performance and durability of the filter media. Reference document DOI: 10.1177 / 15280837231188965 utilizes a mixture of PTFE emulsion and polyvinyl alcohol (PVA) solution. By adding a foaming agent and thickener, a coating is formed on a glass fiber mat using a foam coating technique. The coating is then dried and heat-treated to form a porous PTFE emulsion foam-coated membrane. This method allows the membrane's morphology, pore size, and distribution to be optimized by adjusting the PVA content. However, the single-layer microporous surface structure causes its pressure drop to be too large. After 30 cycles, the PM2.5 filtration efficiency is 98.93%, and the pressure drop after the cycle is 392Pa.

[0004] To further improve the performance of bag filter systems, especially the collection efficiency of particles with a diameter of 0.5 to 2.5 µm, in-depth research and optimization of the finishing process of the PTFE foam coating is needed. This is expected to reduce the average pore size of the coating, increase the coating density, and improve the surface properties of the coating, thereby achieving higher filtration efficiency for particles with a diameter of 0.5 to 2.5 µm. Summary of the Invention

[0005] The present invention provides a method for preparing a filter material coated with a layered, gradient foam layer. This method innovates the post-finishing process by preparing a foam coating finishing liquid primarily composed of PTFE to form a PTFE membrane layer on the surface of a filter substrate. By carefully controlling the thickness of the PTFE foam layer and the PVA@PTFE foam layer, the layers are sequentially coated onto a non-woven PTFE foam filter substrate. Heat treatment secures the bond, forming a layered, gradient foam pore structure. This allows for precise design of the filter material's pore size, resulting in a high-performance, high-temperature-resistant composite filter material with efficient filtration.

[0006] In order to solve the above problems, the present invention provides a method for preparing a layered, gradient foam layer coated filter material, characterized in that it comprises the following steps:

[0007] (1) Add 5% to 17% of solvent to the PTFE emulsion to prepare a foaming coating solution, and use a high-speed multi-dimensional stirring method to stir at a speed of 1000 to 3000 r / min for 10 to 30 minutes to prepare PTFE foam.

[0008] (2) Use an automatic coating machine to coat the PTFE foam on the filter substrate, and place it in an oven to heat and dry at 80-160°C for 3-5 minutes.

[0009] (3) PVA and PTFE were mixed at a ratio of 1:5 and stirred evenly. A magnetic stirrer was used to stir at a speed of 300-600 r / min for 1-3 min. Then, 5%-17% of solvent was added to prepare a PVA foaming coating solution. The PVA@PTFE foam was prepared by a high-speed multi-dimensional stirring method at a speed of 1000-3000 r / min for 15-35 min.

[0010] (4) Coat the PVA@PTFE foam on the PTFE foam film layer, place it in an oven and heat it at 80-160°C for 3-5 minutes to form a PVA@PTFE composite foam film layer structure.

[0011] Preferably, the foaming coating solution in step (1) contains, by mass percentage, 86% to 95% of a solid content PTFE emulsion, 1% to 2% of a foaming agent hexadecyl dimethylamine oxide, 2% to 6% of a foam stabilizer fatty acid polyoxyethylene ester, and 2% to 6% of a thickener methyl cellulose.

[0012] Preferably, the filter substrate in step (2) is polyimide needle-punched felt.

[0013] Preferably, the thickness of the coated foam layer in step (2) and step (4) is 0.3 mm to 1 mm, wherein the thickness ratio of the PTFE and PVA@PTFE foam layers is PTFE:PVA@PTFE=(3:7) to (7:3).

[0014] More preferably, the thickness of the foam layer coated in step (2) is 0.7 mm, forming a thicker foam layer with larger pores; the coating thickness in step (4) is 0.3 mm, which is smaller. When scraping, the larger foam breaks and splits into smaller microporous layers, and the surface film layer is denser and has a higher degree of continuity.

[0015] Preferably, the PVA foaming coating solution in step (3) contains, by mass percentage, 72% to 79% of a PTFE emulsion having a solid content of 60 wt%, 14% to 16% of a PVA solution having a solid content of 10 wt%, 1% to 2% of a foaming agent hexadecyl dimethylamine oxide, 2% to 6% of a foam stabilizer fatty acid polyoxyethylene ester, and 2% to 6% of a thickener methyl cellulose.

[0016] Preferably, the PVA@PTFE foam coating in step (4) is performed by superimposing the coating on the PTFE foam layer that has been dried and cooled to room temperature.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] This invention enables precise control of pore size: By precisely adjusting the thickness ratio of the PTFE foam layer and the PVA@PTFE foam layer, the pore size of the filter material is precisely controlled. This process involves precisely adjusting the solid content of the PTFE emulsion and PVA solution, as well as the ratio of the foaming agent, foam stabilizer, and thickener, and strictly controlling the coating and curing process parameters. This precise reagent ratio and process control enable the filter material to effectively adsorb particles of varying sizes, improving filtration efficiency and accuracy.

[0019] The present invention can optimize the filtration structure: using a layered coating process, first coating the PTFE foam layer, and then coating the PVA@PTFE foam layer thereon, to form a gradient foam pore layer structure. The layered structure design enables the filter material to maintain efficient filtration performance and good mechanical stability when facing different filtration conditions.

[0020] This invention reduces pressure drop by optimizing the thickness and structure of the foam layer, adjusting the thickness and density of the coating foam layer, and ensuring a reasonable pore size distribution within the filter material to reduce resistance to the passage of air or other fluids. This effectively controls pressure drop during the filtration process. Furthermore, precise control of the heating and drying process ensures a uniform pore structure within the foam layer, thereby reducing the overall resistance of the filter material. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of coating filter materials with PVA@PTFE / PTFE layered and gradient foam layers.

[0022] Figure 2 Schematic diagram of PVA@PTFE / PTFE layered and gradient foam layers.

[0023] Figure 3 Schematic diagram of coating a PTFE foam layer with filter material. DETAILED DESCRIPTION

[0024] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the specific implementation of the present invention will be further described below in conjunction with the accompanying drawings and specific implementation examples, but the implementation of the present invention is not limited thereto.

[0025] Example 1

[0026] like Figure 1 As shown, the filter material is coated in layers and gradient foam layers, and the PTFE membrane layer 2 and the PVA@PTFE membrane layer 1 are coated on the surface of the filter substrate 3 in sequence.

[0027] The specific preparation method comprises the following steps:

[0028] (1) Preparation of foaming liquid:

[0029] (1-1) Preparation of PVA solution: 10 g of PVA powder and 90 g of distilled water were added to a beaker and stirred in an 80°C water bath for 3 h to prepare a PVA solution.

[0030] (1-2) PTFE Foaming Solution: Add 86 g of 60 wt% solids PTFE emulsion to a stirring vessel, along with 2 g of hexadecyl dimethylamine oxide as a foaming agent, 6 g of fatty acid polyoxyethylene ester as a foam stabilizer, and 6 g of methylcellulose as a thickener. Stir at 1000 rpm for 30 min to prepare the PTFE foaming solution.

[0031] (1-3) PVA@PTFE foaming solution: 72 g of 60 wt% solids PTFE emulsion and 16 g of 10 wt% solids PVA solution were added to a stirring vessel. 2 g of hexadecyldimethylamine oxide was added as a foaming agent, 5 g of polyoxyethylene fatty acid ester was added as a foam stabilizer, and 5 g of methylcellulose was added as a thickener. A high-speed stirrer was used at 3000 rpm for 15 min to prepare the PVA@PTFE foaming solution.

[0032] (2) Preparation of foamed film layer: The coating was carried out using an automatic coating machine. First, the first-stage coating thickness was set to 0.5 mm, and the PTFE foam was coated on the polyimide needle-punched felt filter substrate. Then, a heat drying treatment was performed: heating at 80°C for 3 minutes and heating at 160°C for 2 minutes to obtain a filter material with a single-sided PTFE foam layer. Then, the second-stage coating thickness was set to 0.5 mm, and the PVA@PTFE foam was coated on the PTFE foam layer filter material. Then, a heat drying treatment was performed: heating at 80°C for 3 minutes and heating at 160°C for 2 minutes to obtain a filter material with a composite foamed film layer.

[0033] The above filter materials are Figure 1 The following three parts are shown from top to bottom: PVA@PTFE membrane layer 1, PTFE membrane layer 2, and filter substrate 3. The dust-facing side of the filter material is Figure 2 The composite foam film layer 12 is coated with Figure 1 The shown structure is composed of PVA@PTFE membrane layer 1 and PTFE membrane layer 2.

[0034] Example 2

[0035] like Figure 1 As shown, the filter material is coated in layers and gradient foam layers, and the PTFE membrane layer 2 and the PVA@PTFE membrane layer 1 are coated on the surface of the filter substrate 3 in sequence.

[0036] The specific preparation method comprises the following steps:

[0037] (1) Preparation of foaming liquid:

[0038] (1-1) Preparation of PVA solution: 10 g of PVA powder and 90 g of distilled water were added to a beaker and stirred in an 80°C water bath for 3 h to prepare a PVA solution.

[0039] (1-2) PTFE Foaming Solution: Add 88 g of 60 wt% solids PTFE emulsion to a stirring vessel, along with 2 g of hexadecyl dimethylamine oxide as a foaming agent, 5 g of fatty acid polyoxyethylene ester as a foam stabilizer, and 5 g of methylcellulose as a thickener. Stir at 1500 rpm for 15 minutes to prepare the PTFE foaming solution.

[0040] (1-3) PVA@PTFE foaming solution: 75 g of 60 wt% solids PTFE emulsion and 15 g of 10 wt% solids PVA solution were added to a stirring vessel. 1 g of hexadecyldimethylamine oxide was added as a foaming agent, 5 g of polyoxyethylene fatty acid ester was added as a foam stabilizer, and 4 g of methylcellulose was added as a thickener. A high-speed stirrer was used at 3000 rpm for 20 min to prepare the PVA@PTFE foaming solution.

[0041] (2) Preparation of foamed film layer: The coating was carried out using an automatic coating machine. First, the first-stage coating thickness was set to 0.7 mm, and the PTFE foam was coated on the polyimide needle-punched felt filter substrate. Then, a heat drying treatment was performed: heating at 80°C for 3 minutes and heating at 160°C for 2 minutes to obtain a filter material with a single-sided PTFE foam layer. Then, the second-stage coating thickness was set to 0.3 mm, and the PVA@PTFE foam was coated on the PTFE foam layer filter material. Then, a heat drying treatment was performed: heating at 80°C for 3 minutes and heating at 160°C for 2 minutes to obtain a filter material with a composite foamed film layer.

[0042] The above filter materials are Figure 1 The following three parts are shown from top to bottom: PVA@PTFE membrane layer 1, PTFE membrane layer 2, and filter substrate 3. The dust-facing side of the filter material is Figure 2 The composite foam film layer 12 is coated with Figure 1 The shown structure is composed of PVA@PTFE membrane layer 1 and PTFE membrane layer 2.

[0043] Example 3

[0044] like Figure 1 As shown, the filter material is coated in layers and gradient foam layers, and the PTFE membrane layer 2 and the PVA@PTFE membrane layer 1 are coated on the surface of the filter substrate 3 in sequence.

[0045] The specific preparation method comprises the following steps:

[0046] (1) Preparation of foaming liquid:

[0047] (1-1) Preparation of PVA solution: 10 g of PVA powder and 90 g of distilled water were added to a beaker and stirred in an 80°C water bath for 3 h to prepare a PVA solution.

[0048] (1-2) PTFE Foaming Solution: Add 90 g of 60 wt% solids PTFE emulsion to a stirring vessel, along with 1 g of hexadecyl dimethylamine oxide as a foaming agent, 4 g of fatty acid polyoxyethylene ester as a foam stabilizer, and 6 g of methylcellulose as a thickener. Stir at 2000 / min using a high-speed stirrer for 15 minutes to prepare the PTFE foaming solution.

[0049] (1-3) PVA@PTFE foaming solution: 77 g of 60 wt% solids PTFE emulsion and 15 g of 10 wt% solids PVA solution were added to a stirring vessel. 2 g of hexadecyldimethylamine oxide was added as a foaming agent, 3 g of polyoxyethylene fatty acid ester was added as a foam stabilizer, and 3 g of methylcellulose was added as a thickener. A high-speed stirrer was used at 3000 rpm for 20 min to prepare the PVA@PTFE foaming solution.

[0050] (2) Preparation of foamed film layer: The coating was carried out using an automatic coating machine. First, the first-stage coating thickness was set to 0.3 mm, and the PTFE foam was coated on the polyimide needle-punched felt filter substrate. Then, a heat drying treatment was performed: heating at 80°C for 3 minutes and heating at 160°C for 2 minutes to obtain a filter material with a single-sided PTFE foam layer. Then, the second-stage coating thickness was set to 0.7 mm, and the PVA@PTFE foam was coated on the PTFE foam layer filter material. Then, a heat drying treatment was performed: heating at 80°C for 3 minutes and heating at 160°C for 2 minutes to obtain a filter material with a composite foamed film layer.

[0051] The above filter materials are Figure 1 The following three parts are shown from top to bottom: PVA@PTFE membrane layer 1, PTFE membrane layer 2, and filter substrate 3. The dust-facing side of the filter material is Figure 2 The composite foam film layer 12 is coated with Figure 1 The shown structure is composed of PVA@PTFE membrane layer 1 and PTFE membrane layer 2.

[0052] Example 4

[0053] like Figure 3 As shown, the filter material is coated in layers and gradient foam layers, and the PTFE membrane layer 2 is coated on the surface of the filter substrate 3 in sequence.

[0054] The specific preparation method comprises the following steps:

[0055] (1) Preparation of foaming liquid:

[0056] (1-1) PTFE foaming solution: Add 93 g of 60 wt% solids PTFE emulsion to a stirring vessel, along with 1 g of hexadecyl dimethylamine oxide as a foaming agent, 3 g of fatty acid polyoxyethylene ester as a foam stabilizer, and 3 g of methylcellulose as a thickener. Stir at 3000 rpm for 10 min to prepare the PTFE foaming solution.

[0057] (2) Preparation of foam film layer: PTFE foam was coated on the polyimide needle-punched felt filter substrate using an automatic coating machine with a coating thickness of 1 mm. The filter substrate was then heat-dried at 80°C for 3 min and 160°C for 2 min to obtain a filter material with a single-sided PTFE foam film layer.

[0058] The above filter materials are Figure 3 The figure shows the PTFE membrane layer 2 and the filter substrate 3 from top to bottom. Figure 3 It is shown coated with a PTFE membrane layer 2.

[0059] Different film thicknesses are applied at different coating stages, and the film pore sizes are different, as shown in Table 1.

[0060] Table 1

[0061] Example Example 1 Example 2 Example 3 Example 4 First stage coating thickness 0.5mm 0.7mm 0.3mm 1mm Second stage coating thickness 0.5mm 0.3mm 0.7mm none Minimum aperture 1.83µm 1.84µm 1.83µm 2.81µm Average pore size 22.13µm 19.44µm 26.7µm 85.29µm Maximum aperture 205.45µm 198.6µm 214.16µm 299.86µm

[0062] According to the different coating stages, different film layer thicknesses are applied, and the filtration efficiency of the film layer for dust particles of different sizes is different, as shown in Table 2.

[0063] Table 2

[0064] Example Example 1 Example 2 Example 3 Example 4 First stage coating thickness 0.5mm 0.7mm 0.3mm 1mm Second stage coating thickness 0.5mm 0.3mm 0.7mm none 0.5µm particle filtration efficiency 32.3% 54.8% 21.5% 1.2% 2.5µm particle filtration efficiency 99.8% 99.9% 99.9% 99.9% Filtration pressure drop 71Pa 90Pa 146Pa 41Pa

[0065] It can be seen from Tables 1 and 2 that the pore size, filtration efficiency and filtration pressure drop of the filter material can be affected by adjusting the coating thickness and the formula of the foaming liquid. Table 1 shows the coating thickness and corresponding pore size of different embodiments. The pore size includes the minimum pore size, the average pore size and the maximum pore size. Although the minimum pore size of the layered, gradient foam layer coated filter material prepared in the embodiment is about 1.8µm, it still has filtration efficiency for 0.5µm particles. This phenomenon can be attributed to the combined effect of multiple filtration mechanisms and material properties. Deep filtration mechanisms such as inertia, interception, diffusion and gravity inside the filter material allow particulate matter to be captured by the fiber, even if the particle size is smaller than the pore size. The pore size can be adjusted by changing the coating thickness. The first-stage coating thickness of Example 2 is 0.7 mm, and the second-stage coating thickness is 0.3 mm. The average pore size of the filter material of the prepared composite foam membrane layer is the smallest. Compared with the composite filter material coated with only the first-stage PTFE membrane layer in Example 4 in Table 2, the layered and gradient foam layer coated filter material of Example 2 has a higher filtration efficiency for 0.5 µm particles, but the filtration pressure drop increases.

[0066] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Those skilled in the art may make various specific changes according to different actual needs without departing from the scope and spirit of the present invention, but they still fall within the scope of protection of this application.

Claims

1. A method for preparing a layered, gradient foam layer coated filter material, characterized in that: The preparation method comprises the following steps: (1) Preparation of foaming liquid: (1-1) Preparation of PVA solution: 10 g of PVA powder and 90 g of distilled water were added to a beaker and stirred in a water bath at 80° C. for 3 h to prepare a PVA solution; (1-2) PTFE foaming liquid: 88 g of PTFE emulsion with a solid content of 60 wt% was added into a stirring container, 2 g of hexadecyl dimethylamine oxide as a foaming agent, 5 g of fatty acid polyoxyethylene ester as a foam stabilizer and 5 g of methyl cellulose as a thickener were added, and a high-speed stirrer was used to stir for 15 min at 1500 r / min to prepare a PTFE foaming liquid; (1-3) PVA@PTFE foaming liquid: 75 g of PTFE emulsion with a solid content of 60 wt% and 15 g of PVA solution with a solid content of 10 wt% were added to a stirring container, 1 g of hexadecyl dimethylamine oxide as a foaming agent, 5 g of fatty acid polyoxyethylene ester as a foam stabilizer and 4 g of methyl cellulose as a thickener were added, and a high-speed stirrer was used at 3000 r / min for 20 min to prepare a PVA@PTFE foaming liquid; (2) Preparation of the foamed film layer: The coating was performed using an automatic coating machine. The first-stage coating thickness was set to 0.7 mm. The PTFE foaming liquid was coated on the polyimide needle felt filter substrate. The filter substrate was then subjected to a heat drying treatment by heating at 80°C for 3 min and at 160°C for 2 min to obtain a filter material having a single-sided PTFE foam layer. The second-stage coating thickness was set to 0.3 mm. The PVA@PTFE foaming liquid was coated on the PTFE foam layer filter material. The filter substrate was then subjected to a heat drying treatment by heating at 80°C for 3 min and at 160°C for 2 min to obtain a filter material having a composite foamed film layer. The filter material is composed of three parts from top to bottom: PVA@PTFE membrane layer, PTFE membrane layer and filter substrate.

2. The method for preparing a layered, gradient foam layer coated filter material according to claim 1, characterized in that: Compounding foam layers with different foam pore sizes to form a multi-layer gradient foam layer structure is beneficial to improving the filtration accuracy and filtration efficiency of the filter material.

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