PTFE folded filter element with high dirt holding capacity and low flow resistance, preparation method and application thereof

Abstract

The present invention relates to the field of preparation of foldable filter elements, and in particular to a PTFE foldable filter element with high dirt holding capacity and low flow resistance, and a preparation method and application thereof. The method comprises: (1) melting and mixing PTFE resin and fiber, and then sintering and forming to obtain a precursor; (2) stretching the precursor to obtain a PTFE base membrane; (3) modifying the PTFE base membrane with an organic modifier to obtain an organic modified PTFE base membrane; (4) preparing a filtration matrix layer with the organic modified PTFE base membrane, and folding it into fins to obtain a PTFE foldable filter element with high dirt holding capacity and low flow resistance. The PTFE foldable filter element provided by the present invention solves the problem of low support strength between the existing ordinary foldable filter element membranes, and at the same time can strengthen the deep filtration capacity of the foldable filter element and improve the dirt holding capacity. In addition, the special structure constructed in the scheme of the present invention can effectively reduce the filtration pressure difference of the filter element during use and extend its service life.

Description

Technical Field

[0001] The invention relates to the field of folded filter element preparation, and in particular to a PTFE folded filter element with high dirt holding capacity and low flow resistance, and a preparation method and application thereof. Background Art

[0002] Filter element filtration is widely used in water treatment and various process, and filter elements are divided into melt-blown filter elements, wire-wound filter elements and pleated filter elements. Pleated filter elements are composed of a film combination with a certain gap structure made of ultra-fine fibers, non-woven fabrics and plastic mesh layers, etc., and are made by advanced hot-melt and welding processes through stretching, pleating, filter material edge sealing, fixed inner core outer cover, and melt-sealed end caps. This filter element has the characteristics of high filtration accuracy, large filtration area, high pollution holding capacity, multiple backwashing and high-pressure sterilization. It is currently the fastest-growing filtration technology and is widely used in a variety of precision filtration processes in the fields of chemical industry, medicine, energy, environmental protection, etc.

[0003] A pleated filter element is generally composed of a fiber filter membrane, a plastic mesh layer and a filter element frame made of plastic material. The plastic mesh layer is used on the water inlet and outlet sides respectively to guide and distribute water. Due to the pleated design, the water inlet and outlet pass through the gaps provided by the plastic mesh layer, and the flow channels are narrow. In the prior art, Chinese invention patent CN92244291.6 provides a multifunctional pleated filter element, in which a cylindrical thickened guide layer is arranged on the inner side of the pleated membrane stack, and a guide groove is provided on the outer surface of the central water collecting pipe to increase the filtrate flow and reduce resistance. Such a design partially improves the water flow distribution and fluid resistance problems at the entrance of the pleats on the water inlet side, but does not solve the problem of narrow flow channels in the pleats. Chinese invention patent CN200810138261.4 provides a method for making a high filtration flux pleated filter element, wherein a layer of elastic support grid is attached to each side of a plate filter membrane, which is folded into an arc-shaped filter membrane by a folding machine, and then the two ends of the filter membrane are bonded together to form a cylinder, and the filter membrane is fixed with an inner stainless steel mesh and an outer stainless steel mesh, and finally the top and bottom ends of the cylindrical filter membrane are sealed and fixed with a filter element fixing head to make a cylindrical pleated filter element. The elastic effect of the support grid can cause the filter membrane to be folded into an arc shape, avoiding the dead angle caused by the previous folding shape, and greatly increasing the filtration flux of the filter membrane. However, the filter element of the invention has a narrow water flow channel, a high filtration pressure difference, and the filter membrane needs to be backwashed, which is difficult to wash, shortening the service life of the filter element. Summary of the invention

[0004] In order to solve the problems of high filtration pressure difference, narrow water flow channel and shortened service life of existing filter elements during use, the present invention provides a PTFE folded filter element with high dirt holding capacity and low flow resistance, and a method for preparing the PTFE folded filter element with high dirt holding capacity and low flow resistance.

[0005] The purpose of the present invention is: 1. To solve the problem of low support strength between existing common folded filter element membranes.

[0006] 2. Strengthen the deep filtration capability of foldable filter element.

[0007] 3. Increase the dirt holding capacity of the foldable filter element.

[0008] 4. Reduce the filtration pressure difference of the filter element during use and extend the service life of the filter element.

[0009] To achieve the above objectives, the present invention adopts the following technical solutions.

[0010] A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance, the method comprising: (1) melting and mixing PTFE resin and fiber, and then sintering and forming the mixture to obtain a precursor.

[0011] (2) The obtained precursor is stretched to obtain a PTFE base membrane.

[0012] (3) The PTFE base membrane is modified with an organic modifier to obtain an organic modified PTFE base membrane.

[0013] (4) The filter matrix layer is prepared with an organic modified PTFE base membrane, and after folding into fins, a PTFE folded filter element with high dirt holding capacity and low flow resistance is obtained.

[0014] Preferably, the amount of fiber in step (1) is 40 to 60 wt % of the PTFE resin.

[0015] Preferably, the fiber in step (1) is a mixed fiber of polyacrylonitrile fiber and polyvinyl pyrrolidone; the polyacrylonitrile fiber and polyvinyl pyrrolidone in the mixed fiber are mixed in a mass ratio of 1: (1.2-1.6); and the average molecular weight of the polyvinyl pyrrolidone is 32000-360000.

[0016] Preferably, the sintering molding in step (1) is carried out in stages at temperatures of 110-130°C and 190-210°C for 5-10 min.

[0017] Preferably, in step (2), the precursor is stretched at 150-210° C., and the stretching ratio of the precursor is controlled to be 150-300% during the stretching process.

[0018] Preferably, the organic modifier in step (3) is n-butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and 2-chloro-4-alkylpyridine, which are uniformly mixed in a mass ratio of 1: (1.8-2.2): 2: (0.6-0.8): 1: 4.

[0019] The organic modifier is mixed uniformly in a temperature environment of 35-45°C, preferably in a temperature environment of 40°C.

[0020] The organic modifier is used in an amount of 3 to 5 wt % of the mass of the PTFE base film.

[0021] Preferably, in step (3), the organic modifier is coated on one side of the PTFE base membrane by spraying, and the membrane is allowed to stand horizontally for 13 to 16 minutes to complete the modification. After the modification, the membrane is cleaned and dried to obtain the organic modified PTFE base membrane.

[0022] Preferably, the filtration matrix layer in step (4) is composed of a double-layer organically modified PTFE base membrane; the modified surfaces of the double-layer organically modified PTFE base membrane are facing the same side, that is, the modified surface of one layer of the organically modified PTFE base membrane is in contact with the unmodified surface of the other layer of the organically modified PTFE base membrane.

[0023] A PTFE pleated filter element with high dirt holding capacity and low flow resistance.

[0024] The invention discloses an application of a PTFE folded filter element with high dirt holding capacity and low flow resistance. The folded filter element is used for a cartridge filter and surrounds the outer surface of an inner support cylinder of the cartridge filter. An outer support cylinder is then sleeved on the outer side of the folded filter element to fix it.

[0025] In actual application, PTFE folded filter element can achieve good filtration efficiency, excellent chemical stability, heat resistance, cold resistance and chemical corrosion resistance, as well as good electrical insulation, low surface tension and low friction coefficient, non-flammability, high and low temperature adaptability and high mechanical properties. However, due to the strong hydrophobicity of PTFE membrane, when filtering water, it requires a large amount of energy for the fluid to pass through the filter base layer, resulting in high filtration pressure difference, high energy consumption and low fluid flux; in addition, if the content of macromolecules in the water increases, the pores of the filter matrix will be blocked, resulting in a decrease in fluid flux. In order to solve the above problems, the present invention has modified the PTFE base membrane and made structural changes. When some PTFE membranes with simple hydrophilic treatment are used to prepare filter elements, due to the fact that the actual folded structure produces a certain amount of evenness, some pores are stretched and expanded, and the filtration pressure difference is low, the energy consumption is small, and the flow rate is large when it is actually used, but the actual filtration effect has a very significant decrease.

[0026] For the technical solution of the present invention, firstly, a spatial structure is constructed through the interaction of other fiber materials, so that the precursor material has an internal structure mainly composed of pores. At the same time, the gradient change of the pore size forms a special functional structure, which increases the effective filtration area, improves the dirt holding capacity, and improves the structural stability of the PTFE membrane. It acts as a skeleton to a certain extent to prevent structural distortion after folding, which can extend the service life of the filter element. In the technical solution of the present invention, polyacrylonitrile fiber and polyvinyl pyrrolidone are added to polytetrafluoroethylene resin by multi-stage melt sintering to prepare a precursor. During the first stage of sintering, polyvinyl pyrrolidone is heated to produce directional winding, and a three-dimensional network structure is constructed with polyacrylonitrile fiber and polytetrafluoroethylene resin as anchor points to form dense micropores. At the same time, polyvinyl pyrrolidone, as a high molecular surfactant, performs the first structural modification on polytetrafluoroethylene resin. By grafting with polytetrafluoroethylene, a hydrophilic polymer is formed near the connection anchor point, and the combination becomes an anti-fouling coating with excellent performance, which effectively reduces the adhesion of biological macromolecules such as proteins. During the second stage of sintering, the polyvinyl pyrrolidone that does not participate in the structural modification will curl inward when heated, and the polyacrylonitrile fiber is affected by the curling force and heat, making the pores of the formed filter element continuous. The body is curved, and the combination of the two structures enables the PTFE resin of the present invention to produce a better internal molecular manufacturing effect while forming hydrophilicity, and can effectively avoid the occurrence of adhesion between filter layers caused by the complexation between metal ions and biological macromolecules during the filtration of water. The dense microporous structure and the anti-fouling layer formed by the present invention support the structure of the PTFE resin and enrich the metal ions inside, so that the metal ions will not enrich the complex on the surface to produce inter-membrane constraints, but will increase the filtration area of ​​the filter element and enhance the pollution-receiving capacity of the filter element, so that the structure of the PTFE folded filter element of the present invention is more stable and has a longer service life.

[0027] In addition, during the use of conventional PTFE folded filter elements, due to the stacking of PTFE separation membranes, a large amount of filtrate is enriched at the stacking interface of the PTFE separation membrane, resulting in a large amount of impurities enriched, causing the flow rate of the filter element to drop rapidly, and the filtration pressure difference also increases accordingly, making it difficult to achieve effective filtration. At the same time, the enrichment of impurities may cause the filter element to produce certain deformation, further leading to a weakening of the filtration effect.

[0028] For the technical solution of the present invention, modification is carried out by chemical grafting. Under weak alkaline conditions, n-butylamine and methyl acrylate are used as raw materials to synthesize a branched macromolecule centered on n-butylamine through Michael addition reaction, and then the branched macromolecule is subjected to amidation condensation reaction with ethylenediamine to synthesize a branched macromolecule with n-butylamine as the core. The specific reaction formula is as follows:

[0029] Wherein: R is an alkane with C≥6.

[0030] Through the above modification process, a double tetradentate polyamine structure ( ), has good complexing ability for heavy metal ions, and reacts with metal ions in water to synthesize bridged metal complexes with different steric hindrance substituents. The reaction equation is as follows. The double tetradentate polyamine structure composed of the metal complex has a large number of hydrogen bonds, which makes the surface structure highly hydrophilic, increases the water flux of the modified filter element, and reduces the filtration pressure difference of the modified filter element. Through multiple modifications to construct a new functional structure, the filtration pressure difference is reduced, the deep filtration capacity of the folded filter element is enhanced, and the dirt holding capacity of the folded filter element is increased, thereby extending the service life of the folded filter element and improving the filtration rate and filtration effect of the filter element. The complexation reaction is as follows.

[0031]

[0032] Wherein: M is a metal cation.

[0033] In the technical scheme of the present invention, the chemical grafting is mainly carried out by organic combination method, and n-butylamine is adsorbed on the surface of polytetrafluoroethylene membrane. After heating, the n-butylamine free radical captures the hydrogen on the polytetrafluoroethylene skeleton, and the generated macromolecular group reacts with methyl acrylate in the water phase to obtain the surface of the polytetrafluoroethylene membrane grafted with triethylamine, which reduces the contact angle and improves the macromolecular interception rate, showing good hydrophilicity and anti-pollution properties. Tetrahydrofuran is used as an exchange agent to carry out interfacial polycondensation reaction on the surface of the polytetrafluoroethylene membrane to obtain a modified membrane. If polytetrafluoroethylene is prepared with only a small amount of n-butylamine, the properties of the polytetrafluoroethylene membrane cannot be enhanced. The best polytetrafluoroethylene membrane is a modified membrane with a cross-linked network structure formed by reacting a polymer fiber mixture with n-butylamine, which has high hydrophilicity, no pollution and high liquid flux. Due to the action of the functional group on the n-butylamine, selective directional adsorption is formed on the interface where the water body and the surface of the polytetrafluoroethylene membrane are connected, so that the properties of the interface are significantly changed. The bidentate structure will form a dense hydrophilic layer on the interface with the PTFE membrane pore surface, thereby improving the hydrophilicity of the interface. The ionic surfactant contains charges that can electrostatically repel the adsorption of the same substances on the interface.

[0034] At the same time, under the modification of the organic modifier, the fiber pore size is changed. The micropores formed by the polyacrylonitrile fiber are emulsified with the functional groups, and attached to the mesh resin near the micropores to form a fiber internal structure with polytetrafluoroethylene resin as the skeleton, making the internal structure of the polytetrafluoroethylene membrane more stable, but the surface micropores will merge and expand, and the micropores are interconnected to form a grape chain shape. The outer pores are large and small in number, and the gaps become smaller when extending to the inner layer, appearing densely, and the pore size is dense and uniform, forming a gradient change in pore size. During use, particles will produce a "bridge" phenomenon in the filter layer pores. In the technical solution of the present invention, it is necessary to strictly control the organic modification time. If the time is too short, the surface emulsification will be incomplete, the skeleton structure will be weak, and the compressive strength of the filter element will be reduced, shortening the service life of the filter element; if the time is too long, the single-sided coating effect will not be obvious, and the separation membrane will become a large pore structure as a whole, losing the gradient change of pore size and the special functional structure.

[0035] The advantages of the present invention are: (1) The PTFE folded filter element provided by the present invention solves the problem of low support strength between membranes of existing common folded filter elements.

[0036] (2) The preparation method provided by the present invention can enhance the deep filtration capability of the folded filter element and increase the dirt holding capacity of the folded filter element.

[0037] (3) The special structure constructed in the scheme of the present invention can effectively reduce the filtration pressure difference of the filter element during use and extend the service life of the filter element. DETAILED DESCRIPTION

[0038] The present invention is further described in detail below in conjunction with specific embodiments. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention involved in the following description are generally only embodiments of a part of the present invention, rather than all embodiments. Therefore, based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work should fall within the scope of protection of the present invention.

[0039] Unless otherwise specified, the raw materials used in the examples of the present invention are all commercially available or available to those skilled in the art; unless otherwise specified, the methods used in the examples of the present invention are all methods known to those skilled in the art.

[0040] Unless otherwise specified, the thickness of the PTFE base membrane is 0.15 mm; the thickness of the filter matrix layer is 0.3 mm.

[0041] Example 1: A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance, (1) polyacrylonitrile fiber and polyvinyl pyrrolidone are uniformly mixed in a ratio of 1:1.2 to prepare a mixed fiber, 40wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered, the first sintering is at 110°C for 5 min, and the second sintering is at 190°C for 5 min to prepare a precursor.

[0042] (2) The PTFE base membrane was stretched at 150° C. and the stretching ratio was controlled to be 150% during the stretching process to prepare the PTFE base membrane.

[0043] (3) A mixture of n-butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and 2-chloro-4-alkylpyridine was uniformly mixed in a mass ratio of 1:1.8:2:0.6:1:4, and 3 wt% of the mass of the PTFE base film was applied on one side of the PTFE base film by spraying. The mixture was left to stand horizontally at a temperature of 40 °C for 13 min, and then rinsed with clean water and dried.

[0044] (4) The unmodified surface of one of the two organically modified PTFE base membranes is fixed together with the modified surface of the other base membrane to form a filter matrix layer, which is folded into a fin shape to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance. The modified surface of the PTFE folded filter element with high dirt holding capacity and low flow resistance is aligned with an inner support cylinder and is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is sleeved on the outside of the filter matrix layer, i.e., a cartridge filter based on the PTFE folded filter element with high dirt holding capacity and low flow resistance of the present invention.

[0045] The prepared PTFE folded filter element was subjected to the following tests, and the specific characterization results are as follows.

[0046] Structural integrity test: FBP-II porous material tester is used to test the first bubble point pressure p1 of PTFE folded filter element with high dirt holding capacity and low flow resistance. The maximum pore size value can be calculated from the bubble point pressure. If the gas pressure continues to increase, boiling bubbles will appear in the liquid, which is called group bubble point pressure p2. The difference between p2 and p1 should be as small as possible. If the difference is less than 10% of p1, it is considered that the filter element has not been damaged during the preparation process and the joints are reliably sealed.

[0047] Water flux test: Place the cartridge filter in the filtration device, add 20 mL of water, and circulate until the amount of water filtered within 5 minutes is stable. Then add 20 mL of water to the filtration device, measure the amount of water filtered naturally without external pressure, and calculate the volume of water passing through per unit area per unit time, that is, the water flux [L / (m 2 ·h)].

[0048] Unit area dirt holding capacity test: When the test liquid passes through the tested filter element at a flow rate of 8 mm / s (effective filter area is 10 cm 2 ), add test powder with a particle size of 10 μm to the upstream of the filter at a rate of 10 mg / min until the pressure difference of the filter element reaches 0.4 MPa. Stop the test and calculate the cumulative amount of test dust added. The calculation formula is as follows: M=G×q×t÷S÷1000.

[0049] Where: M——pollution holding capacity (g / cm 2 ).

[0050] G——Average mass contamination in the dust adding device (mg / L).

[0051] q——average injection flow rate (L / min).

[0052] t——actual test time when the final pressure difference is reached (min).

[0053] S——Effective filtration area of ​​​​filter element.

[0054] Filtration efficiency test: use 10 μm standard particles, and the coefficient of variation of particle size distribution is controlled within 5%. Take an appropriate amount of standard particles and clean water to prepare the test solution. Connect the prepared PTFE folded filter element to the detection system, start the detection system, and allow the test solution to pass through the filter element at a pressure difference of 0.15 MPa. Filter for 2 h, take liquid samples before and after filtration, characterize the weight of standard particles in the liquid sample, and calculate the particle retention rate. The formula is as follows: Particle retention rate = (1-N0 / N) × 100%.

[0055] Where: N0 – weight of standard particles per unit volume of filtrate (g).

[0056] N——weight of standard particles per unit volume of test fluid (g).

[0057] Metal ion removal rate: Collect non-ferrous metal mine dressing wastewater, especially copper smelting wastewater, as the standard test liquid, connect the prepared PTFE folded filter element to the detection system, start the detection system, and pass the standard test liquid through the prepared folded filter element at a flow rate of 8 mm / s for 2 hours. Take liquid samples before and after filtration, determine the copper ion concentration in the standard test liquid by titration, and calculate the metal ion removal rate. The formula is as follows: Metal ion removal rate = 1-(C·V / N·F) / (C·V0 / N·F)×100%.

[0058] Where: C is the concentration of titration reagent (mol / L).

[0059] V is the volume of titrant consumed by the reaction of the filtered liquid sample.

[0060] N is the amount of ionic substance consumed in the titration reaction.

[0061] F – the amount of substance of the ion.

[0062] V0 - the volume of titrant consumed by the liquid sample reaction before titration filtration (L).

[0063]

[0064] In addition, the antibacterial performance of the PTFE filter element substrate prepared in step (3) after immersion modification was tested by an oscillation method suitable for contact antibacterial materials. Staphylococcus aureus was used as the antibacterial target. The test results showed that the antibacterial rate reached 99%. The antibacterial performance of the PTFE folded filter element prepared in step (4) was also tested. The test results showed that the antibacterial rate reached 95.2%. It can be seen that the PTFE filter element substrate of the present invention has very excellent antibacterial performance, and at the same time, after stacking and folding, it still has good antibacterial properties and can effectively kill common harmful bacteria.

[0065] From the above characterization results, the pre-filtering PTFE folded filter element of the present invention can effectively filter and remove particles in water, and the removal rate of 10 μm particles is more than 90%. At the same time, the pollution holding capacity of the filter element is also higher than that of ordinary filter elements. After groundwater exploitation, the PTFE folded filter element of the present invention can effectively filter and remove solid particulate impurities. At the same time, the PTFE folded filter element of the present invention also has relatively good water flux and antibacterial ability.

[0066] Example 2: A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance, (1) polyacrylonitrile fiber and polyvinyl pyrrolidone are uniformly mixed in a ratio of 1:1.5 to prepare a mixed fiber, 50wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered, the first sintering is at 120°C for 8 min, and the second sintering is at 200°C for 8 min to prepare a precursor.

[0067] (2) The PTFE base membrane was stretched at 200° C. and the stretching ratio was controlled to be 200% during the stretching process to prepare the PTFE base membrane.

[0068] (3) n-Butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and a mixture of 2-chloro-4-alkylpyridine were uniformly mixed in a mass ratio of 1:2:2:0.7:1:4, and 4 wt% of the mass of the PTFE base film was applied on one side of the PTFE base film by spraying. The mixture was left to stand horizontally at 40 °C for 15 min, and then rinsed with clean water and dried.

[0069] (4) The unmodified surface of one of the two organically modified PTFE base membranes is fixed together with the modified surface of the other base membrane to form a filter matrix layer, which is folded into a fin shape to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance. The modified surface of the PTFE folded filter element with high dirt holding capacity and low flow resistance is aligned with an inner support cylinder and is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is sleeved on the outside of the filter matrix layer, i.e., a cartridge filter based on the PTFE folded filter element with high dirt holding capacity and low flow resistance of the present invention.

[0070] The prepared PTFE folded filter element was subjected to the following tests, and the specific characterization results are as follows.

[0071] Structural integrity test: FBP-II porous material tester is used to test the first bubble point pressure p1 of PTFE folded filter element with high dirt holding capacity and low flow resistance. The maximum pore size value can be calculated from the bubble point pressure. If the gas pressure continues to increase, boiling bubbles will appear in the liquid, which is called group bubble point pressure p2. The difference between p2 and p1 should be as small as possible. If the difference is less than 10% of p1, it is considered that the filter element has not been damaged during the preparation process and the joints are reliably sealed.

[0072] Water flux test: Place the cartridge filter in the filtration device, add 20 mL of water, and circulate until the amount of water filtered within 5 minutes is stable. Then add 20 mL of water to the filtration device, measure the amount of water filtered naturally without external pressure, and calculate the volume of water passing through per unit area per unit time, that is, the water flux [L / (m 2 ·h)].

[0073] Unit area dirt holding capacity test: When the test liquid passes through the tested filter element at a flow rate of 8 mm / s (effective filter area is 10 cm 2 ), add test powder with a particle size of 10 μm to the upstream of the filter at a rate of 10 mg / min until the pressure difference of the filter element reaches 0.4 MPa. Stop the test and calculate the cumulative amount of test dust added. The calculation formula is as follows: M=G×q×t÷S÷1000.

[0074] Where: M——pollution holding capacity (g / cm 2 ).

[0075] G——Average mass contamination in the dust adding device (mg / L).

[0076] q——average injection flow rate (L / min).

[0077] t——actual test time when the final pressure difference is reached (min).

[0078] S——Effective filtration area of ​​​​filter element.

[0079] Filtration efficiency test: use 10 μm standard particles, and the coefficient of variation of particle size distribution is controlled within 5%. Take an appropriate amount of standard particles and clean water to prepare the test solution. Connect the prepared PTFE folded filter element to the detection system, start the detection system, and allow the test solution to pass through the filter element at a pressure difference of 0.15 MPa. Filter for 2 h, take liquid samples before and after filtration, characterize the weight of standard particles in the liquid sample, and calculate the particle retention rate. The formula is as follows: Particle retention rate = (1-N0 / N) × 100%.

[0080] Where: N0 – weight of standard particles per unit volume of filtrate (g).

[0081] N——weight of standard particles per unit volume of test fluid (g).

[0082] Metal ion removal rate: Collect non-ferrous metal mine dressing wastewater, especially copper smelting wastewater, as the standard test liquid, connect the prepared PTFE folded filter element to the detection system, start the detection system, and pass the standard test liquid through the prepared folded filter element at a flow rate of 8 mm / s for 2 hours. Take liquid samples before and after filtration, determine the copper ion concentration in the standard test liquid by titration, and calculate the metal ion removal rate. The formula is as follows: Metal ion removal rate = 1-(C·V / N·F) / (C·V0 / N·F)×100%.

[0083] Where: C is the concentration of titration reagent (mol / L).

[0084] V is the volume of titrant consumed by the reaction of the filtered liquid sample.

[0085] N is the amount of ionic substance consumed in the titration reaction.

[0086] F – the amount of substance of the ion.

[0087] V0 - the volume of titrant consumed by the liquid sample reaction before titration filtration (L).

[0088]

[0089] In addition, the antibacterial performance test of the PTFE filter element substrate prepared in step (3) after immersion modification was carried out, and the test was carried out using an oscillation method suitable for contact antibacterial materials. Staphylococcus aureus was used as the antibacterial target. The test results showed that the antibacterial rate reached 99%. The antibacterial performance test of the PTFE folded filter element prepared in step (4) was also carried out, and the test results showed that the antibacterial rate reached 97.5%. It can be seen that the PTFE filter element substrate of the present invention has very excellent antibacterial performance, and at the same time, after stacking and folding processing, it still has good antibacterial properties and can effectively kill common harmful bacteria.

[0090] Judging from the above characterization results, it also has good dirt holding capacity and complete filter element structure.

[0091] Example 3: A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance, (1) polyacrylonitrile fiber and polyvinyl pyrrolidone are uniformly mixed in a ratio of 1:1.6 to prepare a mixed fiber, 60wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered, the first sintering is at 130°C for 10 min, and the second sintering is at 210°C for 10 min to prepare a precursor.

[0092] (2) The PTFE base membrane was stretched at 210° C. and the stretching ratio was controlled to be 300% during the stretching process to prepare the PTFE base membrane.

[0093] (3) A mixture of n-butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and 2-chloro-4-alkylpyridine was uniformly mixed in a mass ratio of 1:2.2:2:0.8:1:4, and 5 wt% of the mass of the PTFE base film was applied on one side of the PTFE base film by spraying. The mixture was left to stand horizontally at 40 °C for 16 min, and then rinsed with clean water and dried.

[0094] (4) The unmodified surface of one of the two organically modified PTFE base membranes is fixed together with the modified surface of the other base membrane to form a filter matrix layer, which is folded into a fin shape to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance. The modified surface of the PTFE folded filter element with high dirt holding capacity and low flow resistance is aligned with an inner support cylinder and is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is sleeved on the outside of the filter matrix layer, i.e., a cartridge filter based on the PTFE folded filter element with high dirt holding capacity and low flow resistance of the present invention.

[0095] The prepared PTFE folded filter element was subjected to the following tests, and the specific characterization results are as follows.

[0096] Structural integrity test: FBP-II porous material tester is used to test the first bubble point pressure p1 of PTFE folded filter element with high dirt holding capacity and low flow resistance. The maximum pore size value can be calculated from the bubble point pressure. If the gas pressure continues to increase, boiling bubbles will appear in the liquid, which is called group bubble point pressure p2. The difference between p2 and p1 should be as small as possible. If the difference is less than 10% of p1, it is considered that the filter element has not been damaged during the preparation process and the joints are reliably sealed.

[0097] Water flux test: Place the cartridge filter in the filtration device, add 20 mL of water, and circulate until the amount of water filtered within 5 minutes is stable. Then add 20 mL of water to the filtration device, measure the amount of water filtered naturally without external pressure, and calculate the volume of water passing through per unit area per unit time, that is, the water flux [L / (m 2 ·h)].

[0098] Unit area dirt holding capacity test: When the test liquid passes through the tested filter element at a flow rate of 8 mm / s (effective filter area is 10 cm 2 ), add test powder with a particle size of 10 μm to the upstream of the filter at a rate of 10 mg / min until the pressure difference of the filter element reaches 0.4 MPa. Stop the test and calculate the cumulative amount of test dust added. The calculation formula is as follows: M=G×q×t÷S÷1000.

[0099] Where: M——pollution holding capacity (g / cm 2 ).

[0100] G——Average mass contamination in the dust adding device (mg / L).

[0101] q——average injection flow rate (L / min).

[0102] t——actual test time when the final pressure difference is reached (min).

[0103] S——Effective filtration area of ​​​​filter element.

[0104] Filtration efficiency test: use 10 μm standard particles, and the coefficient of variation of particle size distribution is controlled within 5%. Take an appropriate amount of standard particles and clean water to prepare the test solution. Connect the prepared PTFE folded filter element to the detection system, start the detection system, and allow the test solution to pass through the filter element at a pressure difference of 0.15 MPa. Filter for 2 h, take liquid samples before and after filtration, characterize the weight of standard particles in the liquid sample, and calculate the particle retention rate. The formula is as follows: Particle retention rate = (1-N0 / N) × 100%.

[0105] Where: N0 – weight of standard particles per unit volume of filtrate (g).

[0106] N——weight of standard particles per unit volume of test fluid (g).

[0107] Metal ion removal rate: Collect non-ferrous metal mine dressing wastewater, especially copper smelting wastewater, as the standard test liquid, connect the prepared PTFE folded filter element to the detection system, start the detection system, and pass the standard test liquid through the prepared folded filter element at a flow rate of 8 mm / s for 2 hours. Take liquid samples before and after filtration, determine the copper ion concentration in the standard test liquid by titration, and calculate the metal ion removal rate. The formula is as follows: Metal ion removal rate = 1-(C·V / N·F) / (C·V0 / N·F)×100%.

[0108] Where: C is the concentration of titration reagent (mol / L).

[0109] V is the volume of titrant consumed by the reaction of the filtered liquid sample.

[0110] N is the amount of ionic substance consumed in the titration reaction.

[0111] F – the amount of substance of the ion.

[0112] V0 - the volume of titrant consumed by the liquid sample reaction before titration filtration (L).

[0113]

[0114] In addition, the antibacterial performance of the PTFE filter element substrate prepared in step (3) after immersion modification was tested by an oscillation method suitable for contact antibacterial materials. Staphylococcus aureus was used as the antibacterial target. The test results showed that the antibacterial rate reached 99%. The antibacterial performance of the PTFE folded filter element prepared in step (4) was also tested. The test results showed that the antibacterial rate reached 98.1%. It can be seen that the PTFE filter element substrate of the present invention has very excellent antibacterial performance, and at the same time, after stacking and folding, it still has good antibacterial properties and can effectively kill common harmful bacteria.

[0115] Judging from the above characterization results, it also has good dirt holding capacity and complete filter element structure.

[0116] Comparative Example 1: A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as Example 2, except that the organic modifier unique to the present invention is not used. The folded filter element is prepared. The specific operations are as follows: (1) Polyacrylonitrile fiber and polyvinyl pyrrolidone are evenly mixed in a ratio of 1:1.5 to prepare mixed fibers, 50 wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered. The first sintering is carried out at a temperature of 120°C for 8 min, and the second sintering is carried out at a temperature of 200°C for 8 min to prepare a precursor.

[0117] (2) The PTFE base membrane was stretched at 200°C, and the stretching ratio was controlled to be 200% during the stretching process to prepare the PTFE base membrane.

[0118] (3) Two PTFE base membranes are stacked together to form a filter matrix layer, which is folded into a fin shape. The filter matrix layer is wrapped around a cylinder to form an inner support cylinder, and then an outer support cylinder is placed outside the filter matrix layer to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance.

[0119] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0120]

[0121] By comparing the above characterization results with those of Example 2, it was found that the water flux and dirt holding capacity of the filter element were significantly reduced. After observation by the invention team, chemical grafting of organic modifiers can make the filter element highly hydrophilic, pollution-free and have a high liquid flux. At the same time, under the modification effect of the organic modifier, the fiber pore size is changed, and the micropores are interconnected to form a grape chain shape. The pores in the outer layer are large and small in number, and the gaps become smaller and denser when extending to the inner layer. The pore size is dense and uniform, forming a gradient change in pore size. During use, particles will produce a "bridging" phenomenon in the pores of the filter layer, increasing the dirt holding capacity of the filter element.

[0122] In addition, it can be clearly seen from the above results that the modification treatment of the present invention is of great significance for removing metal ions and treating heavy metal wastewater. In the case of no modification treatment, the metal ion removal rate has a very significant decrease. This shows that the formation of a double tetradentate polyamine structure in the modification process plays a key role in the chelation removal of metal ions.

[0123] Comparative Example 2: A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as Example 2, except that the method for using the unique organic modifier of the present invention is changed, and the PTFE base membrane is sprayed on both sides to prepare the folded filter element. The specific operations are as follows: (1) Polyacrylonitrile fiber and polyvinyl pyrrolidone are evenly mixed in a ratio of 1:1.5 to form a mixed fiber, and 50 wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered. The first sintering is carried out at a temperature of 120°C for 8 min, and the second sintering is carried out at a temperature of 200°C for 8 min to prepare a precursor.

[0124] (2) The PTFE base membrane was stretched at 200°C, and the stretching ratio was controlled to be 200% during the stretching process to prepare the PTFE base membrane.

[0125] (3) n-Butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and a mixture of 2-chloro-4-alkylpyridine were uniformly mixed in a mass ratio of 1:2:2:0.7:1:4, and 4 wt% of the mass of the PTFE base film was applied on one side of the PTFE base film by spraying. The mixture was left to stand horizontally at 40 °C for 15 min, and then rinsed with clean water and dried.

[0126] (4) The unmodified surface of one of the two organically modified PTFE base membranes is fixed together with the modified surface of the other base membrane to form a filter matrix layer, which is folded into a fin shape to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance. The modified surface of the PTFE folded filter element with high dirt holding capacity and low flow resistance is aligned with an inner support cylinder and is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is sleeved on the outside of the filter matrix layer, i.e., a cartridge filter based on the PTFE folded filter element with high dirt holding capacity and low flow resistance of the present invention.

[0127] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0128]

[0129] By comparing the above characterization results with those of Example 2, it was found that the water flux of the filter element prepared in the comparative example was significantly improved, but the dirt holding effect was significantly weakened. The invention team speculated that due to the coating of the modifier on both sides, the difference in pore size on both sides of the separation membrane was not obvious, and an effective spatial structure with decreasing pore size from the outside to the inside could not be formed, thereby weakening the dirt holding effect of the filter element.

[0130] Comparative Example 3: A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as that in Example 2, except that the unique extraction time of the present invention is changed to prepare the folded filter element. The specific operations are as follows: (1) Polyacrylonitrile fiber and polyvinyl pyrrolidone are uniformly mixed in a ratio of 1:1.5 to prepare mixed fibers, 50 wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered, the first sintering is carried out at 120°C for 8 min, and the second sintering is carried out at 200°C for 8 min to prepare a precursor.

[0131] (2) The PTFE base membrane was stretched at 200° C. and the stretching ratio was controlled to be 200% during the stretching process to prepare the PTFE base membrane.

[0132] (3) n-Butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and a mixture of 2-chloro-4-alkylpyridine were uniformly mixed in a mass ratio of 1:2:2:0.7:1:4, and 4 wt% of the mass of the PTFE base film was applied on one side of the PTFE base film by spraying. The mixture was left to stand horizontally at 40 °C for 30 min, and then rinsed with clean water and dried.

[0133] (4) The unmodified surface of one of the two organically modified PTFE base membranes is fixed together with the modified surface of the other base membrane to form a filter matrix layer, which is folded into a fin shape to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance. The modified surface of the PTFE folded filter element with high dirt holding capacity and low flow resistance is aligned with an inner support cylinder and is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is sleeved on the outside of the filter matrix layer, i.e., a cartridge filter based on the PTFE folded filter element with high dirt holding capacity and low flow resistance of the present invention.

[0134] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0135]

[0136] By comparing the above characterization results with those of Example 2, it was found that the water flux and dirt holding capacity of the filter element prepared in the comparative example were reduced, and even the difference between the bubble point pressures p2 and p1 was greater than 10% of p1. The prepared filter element was damaged and the joints were not completely sealed, and it was impossible to prepare a folded filter element that met the technical requirements.

[0137] Comparative Example 4: A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as that in Example 2, except that the unique extraction time of the present invention is changed to prepare the folded filter element. The specific operations are as follows: (1) Polyacrylonitrile fiber and polyvinyl pyrrolidone are uniformly mixed in a ratio of 1:1.5 to prepare mixed fibers, 50 wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered, the first sintering is carried out at 120°C for 8 min, and the second sintering is carried out at 200°C for 8 min to prepare a precursor.

[0138] (2) The PTFE base membrane was stretched at 200° C. and the stretching ratio was controlled to be 200% during the stretching process to prepare the PTFE base membrane.

[0139] (3) n-Butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and a mixture of 2-chloro-4-alkylpyridine were uniformly mixed in a mass ratio of 1:2:2:0.7:1:4, and 4 wt% of the mass of the PTFE base film was applied on one side of the PTFE base film by spraying. The mixture was left to stand horizontally at 40 °C for 5 min, and then rinsed with clean water and dried.

[0140] (4) The unmodified surface of one of the two organically modified PTFE base membranes is fixed together with the modified surface of the other base membrane to form a filter matrix layer, which is folded into a fin shape to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance. The modified surface of the PTFE folded filter element with high dirt holding capacity and low flow resistance is aligned with an inner support cylinder and is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is sleeved on the outside of the filter matrix layer, i.e., a cartridge filter based on the PTFE folded filter element with high dirt holding capacity and low flow resistance of the present invention.

[0141] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0142]

[0143] From the comparison of the above characterization results with Example 2, it is found that the particle retention rate of the foldable filter element prepared by the comparative example is higher, but the water flux and the dirt holding capacity are reduced. By comparing comparative examples 3 and 4 together, the invention team observed that the organic modification time needs to be strictly controlled in the technical scheme of the present invention. If the time is too short, it will lead to incomplete surface emulsification and weak skeleton structure, which will reduce the compressive strength of the filter element and shorten the service life of the filter element; if the time is too long, the single-sided coating effect is not obvious, and the separation membrane as a whole becomes a large pore structure, losing the gradient change of the pore size and the special functional structure.

[0144] Comparative Example 5: A commercially available PTFE folded filter element was characterized for the same performance as that of Example 2. The specific characterization results are as follows.

[0145]

[0146] It can be clearly seen from the analysis of the above characterization results that the commercially available PTFE folded filter element basically does not have a high dirt holding capacity and a short service life, but the water flux and particle retention rate are higher than the folded filter element prepared by the technical solution of the present invention.

[0147] Comparative Example 6: A method for preparing a PTFE folded filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as that in Example 2, except that the heat treatment method unique to the present invention is changed to prepare the folded filter element. The specific operations are as follows: (1) Polyacrylonitrile fiber and polyvinyl pyrrolidone are uniformly mixed in a ratio of 1:1.5 to prepare mixed fibers, 50 wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered, and sintered at 120°C for 16 min to prepare a precursor.

[0148] (2) The PTFE base membrane was stretched at 200°C, and the stretching ratio was controlled to be 200% during the stretching process to prepare the PTFE base membrane.

[0149] (3) n-Butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and a mixture of 2-chloro-4-alkylpyridine were uniformly mixed in a mass ratio of 1:2:2:0.7:1:4, and 4 wt% of the mass of the PTFE base film was applied on one side of the PTFE base film by spraying. The mixture was left to stand horizontally at 40 °C for 15 min, and then rinsed with clean water and dried.

[0150] (4) The unmodified surface of one of the two organically modified PTFE base membranes is fixed together with the modified surface of the other base membrane to form a filter matrix layer, which is folded into a fin shape to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance. The modified surface of the PTFE folded filter element with high dirt holding capacity and low flow resistance is aligned with an inner support cylinder and is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is sleeved on the outside of the filter matrix layer, i.e., a cartridge filter based on the PTFE folded filter element with high dirt holding capacity and low flow resistance of the present invention.

[0151] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0152]

[0153] Analysis of the above characterization results shows that the dirt holding capacity per unit area and the metal ion removal rate of the foldable filter element prepared in the comparative example are significantly reduced. The invention team observes and speculates that the second sintering in the technical scheme of the present invention will cause the polyvinyl pyrrolidone to curl inward when heated, and the filter element pores will appear conjoined and curved. The combination of these two structures allows the PTFE resin to form a dense microporous structure and an anti-fouling layer to support the structure of the PTFE resin while forming hydrophilicity, thereby enriching the metal ions inside, increasing the filtration area of ​​the filter element, and enhancing the dirt holding capacity of the filter element.

[0154] Comparative Example 7: A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as that in Example 2, except that the heat treatment method unique to the present invention is changed to prepare the folded filter element. The specific operations are as follows: (1) Polyacrylonitrile fiber and polyvinyl pyrrolidone are uniformly mixed in a ratio of 1:1.5 to prepare mixed fibers, 50 wt% of the mass of polytetrafluoroethylene resin is mixed with polytetrafluoroethylene resin and melt-sintered, and sintered at 200 °C for 16 min to prepare a precursor.

[0155] (2) The PTFE base membrane was stretched at 200°C, and the stretching ratio was controlled to be 200% during the stretching process to prepare the PTFE base membrane.

[0156] (3) n-Butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and a mixture of 2-chloro-4-alkylpyridine were uniformly mixed in a mass ratio of 1:2:2:0.7:1:4, and 4 wt% of the mass of the PTFE base film was applied on one side of the PTFE base film by spraying. The mixture was left to stand horizontally at 40 °C for 15 min, and then rinsed with clean water and dried.

[0157] (4) The unmodified surface of one of the two organically modified PTFE base membranes is fixed together with the modified surface of the other base membrane to form a filter matrix layer, which is folded into a fin shape to obtain a PTFE folded filter element with high dirt holding capacity and low flow resistance. The modified surface of the PTFE folded filter element with high dirt holding capacity and low flow resistance is aligned with an inner support cylinder and is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is sleeved on the outside of the filter matrix layer, i.e., a cartridge filter based on the PTFE folded filter element with high dirt holding capacity and low flow resistance of the present invention.

[0158] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0159]

[0160] Analysis of the above characterization results shows that the dirt holding capacity per unit area and the metal ion removal rate of the foldable filter element prepared in the comparative example are significantly reduced. The invention team observes and speculates that during the first sintering in the technical scheme of the present invention, the polyvinyl pyrrolidone is heated to produce directional entanglement, build a three-dimensional network structure, and form dense micropores. At the same time, the surface of the polytetrafluoroethylene resin is activated, and a hydrophilic polymer is formed by grafting with polytetrafluoroethylene, which is the first structural modification of the polytetrafluoroethylene resin.

Claims

1. A method for preparing a PTFE foldable filter element with high dirt holding capacity and low flow resistance, characterized in that: The method comprises: (1) Melting and mixing PTFE resin and fiber, and then sintering to obtain a precursor; (2) stretching the obtained precursor to obtain a PTFE base membrane; (3) modifying the PTFE base membrane with an organic modifier to obtain an organic modified PTFE base membrane; (4) The filter matrix layer is prepared by using an organic modified PTFE base membrane, and after folding into fins, a PTFE folded filter element with high dirt holding capacity and low flow resistance is obtained; The fiber in step (1) is a mixed fiber made by uniformly mixing polyacrylonitrile fiber and polyvinyl pyrrolidone in a certain proportion; The polyacrylonitrile fiber and polyvinyl pyrrolidone in the mixed fiber are mixed in a mass ratio of 1: (1.2-1.6); The average molecular weight of the polyvinyl pyrrolidone is 32000-360000; The sintering molding in step (1) is sequentially placed at 110-130°C and 190-210°C for 5-10 min in stages; The organic modifier in step (3) is n-butylamine, methyl acrylate, ethylenediamine, triethylamine, tetrahydrofuran and 2-chloro-4-alkylpyridine, which are uniformly mixed in a mass ratio of 1: (1.8-2.2): 2: (0.6-0.8): 1: 4; The organic modifier is used in an amount of 3 to 5 wt% of the mass of the PTFE base film; In step (3), the organic modifier is coated on one side of the PTFE base membrane by spraying, and the membrane is left to stand horizontally for 13 to 16 minutes to complete the modification. After the modification, the membrane is cleaned and dried to obtain the organic modified PTFE base membrane.

2. The method for preparing a PTFE folded filter element with high dirt holding capacity and low flow resistance according to claim 1, characterized in that: The fiber dosage in step (1) is 40 to 60 wt % of the PTFE resin.

3. The method for preparing a PTFE folded filter element with high dirt holding capacity and low flow resistance according to claim 1, characterized in that: In step (2), the precursor is stretched at 150-210° C., and the stretching ratio of the precursor is controlled to be 150-300% during the stretching process.

4. The method for preparing a PTFE folded filter element with high dirt holding capacity and low flow resistance according to claim 1, characterized in that: The filtration matrix layer in step (4) is composed of a double-layer organic modified PTFE base membrane; The modified surfaces of the double-layer organically modified PTFE base membranes are oriented toward the same side, that is, the modified surface of one layer of the organically modified PTFE base membrane is attached to the unmodified surface of the other layer of the organically modified PTFE base membrane.

5. A PTFE folded filter element with high dirt holding capacity and low flow resistance prepared by the method according to any one of claims 1 to 4.

6. An application of the PTFE folded filter element with high dirt holding capacity and low flow resistance as claimed in claim 5, characterized in that: The folded filter element is used for a cartridge filter, and surrounds the outer surface of an inner support cylinder of the cartridge filter. An outer support cylinder is then sleeved on the outer side of the folded filter element to fix it.

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