Polyvinylidene fluoride fiber for filter fabric and preparation method of polyvinylidene fluoride fiber

Abstract

The invention discloses a polyvinylidene fluoride fiber for filter fabric and a preparation method thereof, and the preparation method comprises the following steps: mixing 0.2-3 wt.% of perfluorinated polymer compound, 0.5-4 wt.% of oleophylic and hydrophobic nucleating agent and 93-99 wt.% of polyvinylidene fluoride resin, and adding the mixture into a twin-screw extruder to prepare polyvinylidene fluoride composite master batch; feeding into a single-screw extruder for melting to form a melt; extruding the melt through a screw extrusion spinning machine to obtain a melt extrudate; the melt extrudate passes through a first air bath, a second air bath, an oil bath, a first stretching roller, a second stretching roller, a third stretching roller, a heat setting roller and an interlacer and then is wound to obtain the polyvinylidene fluoride fiber, and the obtained fiber is smooth in surface, free of holes, good in fineness uniformity, high in crystallinity, good in macromolecular chain orientation, good in mechanical property and high in tensile strength. The material has excellent aging resistance, can be continuously used in a temperature range of-40 to 150 DEG C, and has good chemical stability.

Description

Technical Field

[0001] The present invention relates to the field of fiber preparation, and particularly to a polyvinylidene fluoride fiber for a filter fabric and a preparation method thereof. Background Art

[0002] A liquid filter is an indispensable device on a pipeline for transporting a medium. From the perspective of market demand, it is widely used in fields such as semiconductors, chemical industry, medicine, food, metal processing, water treatment, and automobiles. In common plate-type pressure filtration, cartridge filtration and other devices, according to the liquid working conditions, polypropylene filaments and polytetrafluoroethylene monofilaments are mostly used. However, the former cannot adapt to the filtration environment of special working conditions due to poor corrosion resistance and aging resistance of the material; while polytetrafluoroethylene has excellent chemical stability and heat resistance, but its "insoluble and infusible" characteristic makes it impossible to use the melt spinning technology to manufacture fibers, and it is difficult to produce monofilaments and multifilaments with a fineness lower than 100D, which cannot meet the requirements of high-efficiency liquid filtration working conditions, and the fineness uniformity of polytetrafluoroethylene fibers prepared by the paste extrusion method or the film splitting method needs to be improved. Polyvinylidene fluoride (PVDF) is thermoplastic, and no organic reagent is required during the melt spinning process, the production process is environmentally friendly, and the prepared fibers have good fineness uniformity and can be adjusted. For the preparation of high-end liquid filter fabrics, PVDF fibers are indispensable. The industrialization of PVDF fibers will be beneficial to the improvement of the filtration level in fields such as medicine, food, water treatment, and chemical industry. Therefore, the development of PVDF fibers for filter fabrics has important research and application values.

[0003] Different from other conventional thermoplastic polymers, PVDF (polyvinylidene fluoride) is a fluorine-containing polymer with a relatively high melt viscosity, and it is prone to "melt fracture" during the spinning process. Moreover, PVDF has hydrophobicity and a relatively high fiber elongation at break, and it is difficult to hold together during the stretching and winding process, and it is extremely easy to generate phenomena such as hairiness and filament breakage, so the spinning is difficult. Therefore, most of the current reports on PVDF fibers are mainly research-oriented, and there is still a large gap from industrial production. Further, there are even fewer reports on PVDF fibers for filter fabrics. Most of the literature and patents only focus on studying the influence of spinning process parameters on the crystal structure of PVDF fibers, mainly for medical suture lines, artificial scaffolds, pressure sensors, flame retardant fabrics, fishing lines, fishing nets, etc. Patent CN106350879A discloses a method for preparing PVDF fibers by solution spinning technology, but this preparation process requires a large amount of organic solvents, which not only harms the environment but also greatly increases the production cost; Patent CN106012045A discloses a method for preparing PVDF melt-spun fibers, but the thinnest monofilament is only 9.6 tex, the maximum tensile strength is only 31.2 cN / tex, and the crystallinity is 52.3%. If it is to be applied to filter fabrics, the fineness, mechanical properties, surface wettability, etc. of the fibers still need to be improved. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention innovatively proposes a PVDF fiber for filter fabrics and its preparation method. The PVDF fiber for filter fabrics prepared has a smooth surface, no holes, good and adjustable fineness uniformity, high crystallinity, good orientation of macromolecular chains, good mechanical properties, a tensile strength of 3.5 cN / dtex, an elongation at break of 10-25%, a thermal shrinkage rate of <5% at 120 °C, a water contact angle of >110°, an oil contact angle close to 0°, excellent aging resistance, can be continuously used in the temperature range of -40 to 150 °C, and has good chemical stability to common chemical reagents, acid-base solutions, etc. The method has simple process, environmental protection process, high production efficiency, is easy to be industrially produced on a large scale, the fiber properties fully meet the processing requirements of filter fabrics, and the waste fibers or fabrics are convenient for recycling and reuse.

[0005] The technical solution of the present invention to solve the technical problems of the fiber is: to provide a PVDF fiber for filter fabrics, the fiber-forming system composition of which includes 0.2-3 wt.% perfluorinated high molecular compound, 0.5-4 wt.% lipophilic and hydrophobic nucleating agent, and 93-99 wt.% PVDF resin, and the sum of each component is 100 wt.%; the perfluorinated high molecular compound includes perfluoropolyether and tetrafluoroethylene-perfluoroalkoxy vinyl ether blend; the lipophilic and hydrophobic nucleating agent is a blend of three forms of additives, namely hydrophobic silica with a "spherical" average particle size of 20-100 nm, carbon nanotubes with a "linear" length of 1-10 μm, and graphene with a "flake" average flake diameter of 1-10 μm, in a mass ratio of 1:1:1; the PVDF resin is a resin with a melt index of 3-13 g / 10 min tested under the conditions of 230 °C / 2.16 kg; the average molecular weight of the perfluoropolyether is 500-6000, and tetrafluoroethylene-perfluoroalkoxy vinyl ether is a perfluorinated high molecular compound with a melt index of 1-10 g / 10 min tested under the conditions of 230 °C / 2.16 kg; and the PVDF fiber for filter fabrics prepared has a smooth surface, no holes, good and adjustable fineness uniformity, high crystallinity, good orientation of macromolecular chains, good mechanical properties, a tensile strength of 3.5 cN / dtex, an elongation at break of 10-25%, a thermal shrinkage rate of <5% at 120 °C, a water contact angle of >110°, an oil contact angle close to 0°, excellent aging resistance, can be continuously used in the temperature range of -40 to 150 °C, and has good chemical stability to common chemical reagents, acid-base solutions, etc.

[0006] The technical solution of the present invention to solve the technical problems of the preparation method is: to provide a preparation method of a PVDF fiber for filter fabrics, and the preparation method adopts the PVDF fiber-forming system of the present invention as follows:

[0007] 1) First, use a high-speed mixer to fully mix the components of the PVDF fiber-forming system for the filter fabric, and then use a twin-screw extruder to make it into PVDF composite masterbatch;

[0008] 2) Feed the dried PVDF composite masterbatch into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch successively passes through the first screw zone at a temperature of 155 - 190 °C, the second screw zone at a temperature of 185 - 220 °C, the third screw zone at a temperature of 195 - 230 °C, the fourth screw zone at a temperature of 215 - 250 °C, the fifth screw zone at a temperature of 230 - 265 °C, the screw flange at a temperature of 235 - 270 °C, and the melt pipe at a temperature of 230 - 265 °C to obtain a fully mixed and molten melt;

[0009] 3) The fully mixed and molten melt is quantitatively passed through the spinning box flange at a temperature of 230 - 265 °C and the spinning box body at a temperature of 235 - 270 °C under the action of a metering pump. After filtration, it is finally extruded to obtain a molten extrudate;

[0010] 4) The molten extrudate successively passes through multiple air baths, oil baths, the first drawing roll, the second drawing roll, the third drawing roll, the heat setting roll, and the networker, and then is wound to obtain the PVDF fiber for the filter fabric.

[0011] Compared with the prior art, the PVDF fiber for the filter fabric of the present invention has the advantages of smooth surface, no holes, good and adjustable fineness uniformity, high crystallinity, good orientation of macromolecular chains, good mechanical properties, a tensile strength of 3.5 cN / dtex, an elongation at break of 10 - 25%, a heat shrinkage rate of < 5% at 120 °C, a water contact angle > 110°, and an oil contact angle close to 0°. It has excellent aging resistance, can be continuously used in the temperature range of - 40 - 150 °C, and has good chemical stability to common chemical reagents, acid-base solutions, etc. The preparation method of the PVDF fiber for the filter fabric of the present invention has the characteristics of relatively simple process, environmental protection in the process, high production efficiency, easy industrial-scale production, the fiber properties fully meeting the processing requirements of the filter fabric, and the waste fibers or fabrics being convenient for recycling and reuse. Detailed Embodiments

[0012] The present invention will be further described below in conjunction with specific embodiments of the present invention.

[0013] A PVDF fiber (hereinafter referred to as fiber) designed for the present invention for a filter fabric is characterized in that the fiber-forming system composition of the fiber includes 0.2-3 wt.% of a perfluorinated high molecular compound, 0.5-4 wt.% of a lipophilic and hydrophobic nucleating agent, and 93-99 wt.% of PVDF resin, and the sum of each component is 100 wt.%; the perfluorinated high molecular compound includes a perfluoropolyether and a blend of tetrafluoroethylene-perfluoroalkoxy vinyl ether; the lipophilic and hydrophobic nucleating agent is a blend of three forms of additives, namely, "spherical" hydrophobic silica with an average particle size of 20-100 nm, "linear" carbon nanotubes with a length of 1-10 μm, and "flake-shaped" graphene with an average flake diameter of 1-10 μm, in a mass ratio of 1:1:1; the PVDF resin is a resin with a melt index of 3-13 g / 10 min tested under the conditions of 230°C / 2.16 kg; in the present invention, the average molecular weight of the perfluoropolyether is 500-6000, and tetrafluoroethylene-perfluoroalkoxy vinyl ether has a melt index of 1-10 g / 10 min tested under the conditions of 230°C / 2.16 kg; and the PVDF fiber for the filter fabric prepared has a smooth surface, no holes, good and adjustable fineness uniformity, high crystallinity, good orientation of macromolecular chains, good mechanical properties, a tensile strength of up to 3.5 cN / dtex, an elongation at break of 10-25%, a thermal shrinkage rate of <5% at 120°C, a water contact angle of >110°, an oil contact angle close to 0°, excellent aging resistance, can be continuously used in the temperature range of -40 to 150°C, and has good chemical stability to common chemical reagents, acid-base solutions, etc.

[0014] In the fiber-forming system of the present invention, the perfluorinated high molecular compound is a blend of a perfluoropolyether and tetrafluoroethylene-perfluoroalkoxy vinyl ether. The addition of the perfluorinated high molecular compound plays a role in hydrophobic modification on the one hand, thereby improving the surface wettability of the fiber and increasing the filtration efficiency. On the other hand, it also plays a lubricating role, improving the drawback that it is difficult to fully mix pure inorganic additives in the fiber matrix, and increasing the spinnability of the fiber. In addition, the perfluorinated high molecular compound has good affinity with the fiber matrix PVDF resin. While adjusting the surface wettability of the fiber, it also improves the spinnability and mechanical properties of the fiber.

[0015] In the fiber-forming system of the present invention, the lipophilic and hydrophobic nucleating agent is a blend of three types of additives, namely, "spherical" hydrophobic silica with an average particle size of 20 - 100 nm, "linear" carbon nanotubes with a length of 1 - 10 μm, and "flake-shaped" graphene with an average flake diameter of 1 - 10 μm, in a mass ratio of 1:1:1. The lipophilic and hydrophobic nucleating agent not only plays a role in lipophilic and hydrophobic modification together with the perfluorinated polymer compound to improve the filtration efficiency, but also serves as a nucleating agent for the crystallization of PVDF, promoting the formation of crystallization and improving the structure and properties of the fiber. Secondly, the synergistic effect among the three different forms of lipophilic and hydrophobic nucleating agents is more conducive to the improvement of the crystal structure and mechanical properties of the fiber.

[0016] The present invention also designs a preparation method for the PVDF fiber used in the filter fabric, and the preparation method is as follows:

[0017] 1) First, use a high-speed mixer to fully mix each component of the PVDF fiber-forming system of the present invention for the filter fabric, and then use a twin-screw extruder to make it into a PVDF composite masterbatch.

[0018] The twin-screw extruder has five-zone temperature control. The temperature of the first zone of the screw is 160 - 190 °C, the temperature of the second zone of the screw is 190 - 220 °C, the temperature of the third zone of the screw is 200 - 230 °C, the temperature of the fourth zone of the screw is 210 - 240 °C, the temperature of the fifth zone of the screw is 220 - 250 °C, and the temperatures of the pipeline and the die head are both 200 - 230 °C. Among them, the die head temperature is 20 °C lower than the temperature of the fifth zone of the screw. These parameter settings are mainly to reduce the influence of the heat treatment history on the molecular structure of the fluoropolymer while ensuring the full mixing of each component of the raw materials, and to avoid the occurrence of degradation or carbonization. The setting of the temperature of the twin-screw extruder provides technical support for the preparation of the composite masterbatch.

[0019] 2) Feed the dried PVDF composite masterbatch into the hopper of the single-screw extruder. Then, under the shearing action of the screw, the masterbatch sequentially passes through the first zone of the screw with a temperature of 155 - 190 °C, the second zone of the screw with a temperature of 185 - 220 °C, the third zone of the screw with a temperature of 195 - 230 °C, the fourth zone of the screw with a temperature of 215 - 250 °C, the fifth zone of the screw with a temperature of 230 - 265 °C, the screw flange with a temperature of 235 - 270 °C, and the melt pipeline with a temperature of 230 - 265 °C to obtain a fully mixed and molten melt.

[0020] The temperature of the second zone of the screw of the single-screw extruder is 30 °C higher than that of the first zone of the screw, the temperature of the third zone of the screw is 10 °C higher than that of the second zone of the screw, the temperature of the fourth zone of the screw is 20 °C higher than that of the third zone of the screw, the temperature of the fifth zone of the screw is 15 °C higher than that of the fourth zone of the screw, the temperature of the screw flange is 5 °C higher than that of the fifth zone of the screw, and the temperature of the melt pipe is the same as that of the fifth zone of the screw. Since the melt viscosity of the fluoropolymer is relatively large, in order to ensure the smooth progress of spinning, a relatively high spinning temperature is adopted starting from the fifth zone of the screw.

[0021] 3) The fully mixed and melted melt is quantitatively passed through the spinning box flange at a temperature of 230 - 265 °C and the spinning box body at a temperature of 235 - 270 °C under the action of a metering pump, and after filtration, it is finally extruded to obtain a molten extrudate.

[0022] The temperature of the single-screw spinning box body is 5 °C higher than that of the fifth zone of the screw, which is different from the setting in step 1) where the die head temperature of the twin-screw is 20 °C lower than that of the fifth zone of the screw. This is because the melt viscosity of the fluoropolymer is relatively large, and the "melt fracture" phenomenon is very likely to occur during the spinning process. To avoid this phenomenon, it is necessary to ensure good fluidity of the melt and reduce the time required for internal stress relaxation.

[0023] 4) The molten extrudate passes through multiple air baths, oil baths, a first drawing roll, a second drawing roll, a third drawing roll, a heat setting roll, and a networker in sequence, and then is wound to obtain the PVDF fiber for the filter fabric.

[0024] The air bath is a multiple air bath. There is one air bath at a position closer to the spinneret outlet, with a temperature of 120 - 140 °C and a length of 100 - 150 cm. The purpose is to extend the crystallization time and adjust the fiber structure to obtain fibers with a higher degree of crystallinity and good mechanical properties; then there are two air baths, with a temperature of 20 - 28 °C and a length of 50 - 100 cm. The purpose is to fully solidify the fibers to obtain fibers with uniform fineness.

[0025] The preparation method of the PVDF fiber for the filter fabric according to the present invention is characterized in that: the drying process of the PVDF composite masterbatch in step 2) is carried out in a vacuum drying oven at 40 - 50 °C for 10 - 12 h.

[0026] The preparation method of the PVDF fiber for the filter fabric according to the present invention is further characterized in that: the rotation direction of the oiling roll in the oil bath in step 4) is opposite to the fiber transmission direction. This structure can not only improve the oiling rate, but also help the multifilaments better hold together. At the same time, it slightly increases the resistance to the transmission of the fiber at the backward drawing part, thereby helping to reduce the influence of the disturbance of the filament bundle at the backward drawing part on the filament bundle at the spinneret drawing part, and is more conducive to the stability of the spinning process.

[0027] The preparation method of PVDF fibers for filter fabrics according to the present invention is further characterized in that: in step 4), the stretching temperature of the first stretching roller is 85 - 95 °C, the stretching temperature of the second stretching roller is 85 - 95 °C, the stretching temperature of the third stretching roller is 105 - 115 °C, and the temperature of the heat setting roller is 120 - 130 °C. This is because 90 °C is the temperature most conducive to the crystal form transformation of PVDF under post-stretching. In addition, considering the ambient temperature and the machine heating state, the temperatures of the first stretching roller and the second stretching roller are set to 85 - 95 °C. And 120 °C is the softening point of PVDF, so the temperature of the heat setting roller is set to 120 - 130 °C to appropriately release the internal stress of the stretched fibers and reduce the creep performance of the fibers. When the fibers are at the third stretching roller, the previous two-stage stretching has been completed and they start to enter the heat setting stage, so the temperature of the third stretching roller is set to 105 - 115 °C, which is between the post-stretching temperature and the heat setting temperature.

[0028] The preparation method of PVDF fibers for filter fabrics according to the present invention is further characterized in that: during the process of the melt being extruded from the spinneret to the final winding into fibers, the spinneret draw ratio is 30 - 150, the primary draw ratio is 1.5 - 3.5, the secondary draw ratio is 1.1 - 2, and the winding shrinkage rate is 90 - 97%. The settings of the spinneret draw ratio, post-draw ratio, and winding shrinkage rate will directly affect the crystal structure, fineness, mechanical properties, heat shrinkage properties, etc. of the fibers. In addition, due to the special linear molecular structure of the PVDF polymer, the fibers have good spinnability and are prone to creep, so the preparation method described in the present invention has a relatively large spinneret draw ratio and winding shrinkage rate.

[0029] The preparation method of PVDF fibers for filter fabrics according to the present invention is further characterized in that: the fineness of the prepared PVDF fibers is 40 - 2400 D, and the number of filaments is 6f, 8f, 10f, 24f, 48f, 50f, 96f, 144f, 192f, etc.

[0030] The following gives specific embodiments of the present invention. The specific embodiments are only used to further illustrate the present invention in detail and do not limit the protection scope of the claims of this application.

[0031] Example 1

[0032] The present invention prepares a PVDF fiber for a filter fabric. The process method is as follows:

[0033] 1) The PVDF resin with a melt index of 8 g / 10 min at 230 °C / 2.16 kg is used as the matrix phase, and the addition amount is 97 wt.%; the perfluoropolyether with an average molecular weight of 1800 and tetrafluoroethylene-perfluoroalkoxy vinyl ether with a melt index of 5 g / 10 min (230 °C / 2.16 kg) are mixed at a mass ratio of 2:3 as the perfluoropolymer compound, and the addition amount is 1 wt.%; the blend of three additives, namely hydrophobic silica with a "spherical" average particle size of 40 nm, carbon nanotubes with a "linear" length of 5 μm, and graphene with a "flake" average flake diameter of 5 μm, at a mass ratio of 1:1:1 is used as the lipophilic and hydrophobic nucleating agent, and the addition amount is 2 wt.%. Use a high-speed mixer to fully mix the PVDF resin, perfluoropolymer compound, and lipophilic and hydrophobic nucleating agent, and then use a twin-screw extruder to make it into a PVDF composite masterbatch. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first zone of the screw is 180 °C, the temperature of the second zone of the screw is 210 °C, the temperature of the third zone of the screw is 220 °C, the temperature of the fourth zone of the screw is 230 °C, the temperature of the fifth zone of the screw is 240 °C, and the temperatures of the pipeline and the die head are both 220 °C.

[0034] 2) Feed the PVDF composite masterbatch dried in a vacuum drying oven at 50 °C for 10 h into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch sequentially passes through the first zone of the screw at 180 °C, the second zone of the screw at 210 °C, the third zone of the screw at 220 °C, the fourth zone of the screw at 240 °C, the fifth zone of the screw at 255 °C, the screw flange at 260 °C, and the melt pipeline at 255 °C to obtain a fully mixed and molten melt.

[0035] 3) The fully mixed and molten melt is quantitatively passed through the spinneret flange of a screw extrusion spinning machine at 255 °C and the spinneret body at 260 °C under the action of a metering pump, and after filtration, it is finally extruded to obtain a melt extrudate.

[0036] 4) The melt extrudate sequentially passes through a first air bath (temperature 130 °C, length 125 cm), a second air bath (temperature 25 °C, length 75 cm), an oil bath (reversed), a first draw roll (temperature 90 °C), a second draw roll (temperature 90 °C), a third draw roll (temperature 110 °C), a heat setting roll (temperature 125 °C), and a netting device, and then is wound to obtain the PVDF fiber for the filter fabric. Among them, the spinneret draw ratio is 100, the primary draw ratio is 2.5, the secondary draw ratio is 1.5, and the winding shrinkage rate is 93%.

[0037] After testing, the fineness of the obtained PVDF fibers for the filter fabric is 500D / 50f, the crystallinity is 60% ± 2%, the tensile strength is 4.2 cN / dtex ± 0.2 cN / dtex, the elongation at break is 18% ± 2%, the thermal shrinkage rate at 120°C is 3% ± 1%, the water contact angle is 122° ± 2°, and the oil contact angle is close to 0°.

[0038] Comparative Example 1

[0039] Prepare PVDF fibers for a filter fabric. The process method is as follows:

[0040] 1) Take PVDF resin with a melt index of 8 g / 10 min at 230°C / 2.16 kg as the matrix phase, with an addition amount of 97 wt.%; take polytetrafluoroethylene as the perfluorinated polymer compound, with an addition amount of 1 wt.%; take the blend of three forms of additives, namely hydrophobic silica with an average particle size of 40 nm in the "spherical" shape, carbon nanotubes with a length of 5 μm in the "linear" shape, and graphene with an average sheet diameter of 5 μm in the "sheet" shape, in a mass ratio of 1:1:1 as the lipophilic and hydrophobic nucleating agent, with an addition amount of 2 wt.%. Use a high-speed mixer to fully mix the PVDF resin, perfluorinated polymer compound, and lipophilic and hydrophobic nucleating agent, and then use a twin-screw extruder to make it into a PVDF composite masterbatch. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first zone of the screw is 180°C, the temperature of the second zone of the screw is 210°C, the temperature of the third zone of the screw is 220°C, the temperature of the fourth zone of the screw is 230°C, the temperature of the fifth zone of the screw is 240°C, and the temperatures of the pipeline and die head are both 220°C.

[0041] 2) Feed the PVDF composite masterbatch dried in a 50°C vacuum drying oven for 10 h into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch sequentially passes through the first zone of the screw at 180°C, the second zone of the screw at 210°C, the third zone of the screw at 220°C, the fourth zone of the screw at 240°C, the fifth zone of the screw at 255°C, the screw flange at 260°C, and the melt pipeline at 255°C to obtain a fully mixed and molten melt.

[0042] 3) The fully mixed and molten melt is quantitatively passed through the spinneret flange at 255°C and the spinneret body at 260°C under the action of a metering pump, and after filtration, it is finally extruded to obtain a melt extrudate.

[0043] 4) The molten extrudate successively passes through a first air bath (temperature: 130°C, length: 125 cm), a second air bath (temperature: 25°C, length: 75 cm), an oil bath (inverted), a first drawing roll (temperature: 90°C), a second drawing roll (temperature: 90°C), a third drawing roll (temperature: 110°C), a heat setting roll (temperature: 125°C), a texturing device, and then is wound to obtain the PVDF fibers for the filter fabric. Among them, the spinneret draw ratio is 100, the primary draw ratio is 2.5, the secondary draw ratio is 1.5, and the winding shrinkage rate is 93%.

[0044] After testing, the fineness of the obtained PVDF fibers for the filter fabric is 500 D / 50 f, the crystallinity is 53% ± 2%, the tensile strength is 3.5 cN / dtex ± 0.2 cN / dtex, the elongation at break is 12% ± 2%, the thermal shrinkage rate at 120°C is 3% ± 1%, the water contact angle is 120° ± 2°, and the oil contact angle is close to 0°.

[0045] Comparative Example 2

[0046] Prepare PVDF fibers for a filter fabric. The process method is as follows:

[0047] 1) Take PVDF resin with a melt index of 8 g / 10 min under the conditions of 230°C / 2.16 kg as the matrix phase, with a content of 97 wt.%; take perfluoroethylene propylene as the perfluorinated polymer compound, with a content of 1 wt.%; take the blend of three forms of additives, namely hydrophobic silica with an average particle size of 40 nm in the form of "spherical", carbon nanotubes with a length of 5 μm in the form of "linear", and graphene with an average sheet diameter of 5 μm in the form of "flake", in a mass ratio of 1:1:1 as the lipophilic and hydrophobic nucleating agent, with a content of 2 wt.%. Use a high-speed mixer to fully mix the PVDF resin, perfluorinated polymer compound, and lipophilic and hydrophobic nucleating agent, and then use a twin-screw extruder to make it into a PVDF composite masterbatch. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first zone of the screw is 180°C, the temperature of the second zone of the screw is 210°C, the temperature of the third zone of the screw is 220°C, the temperature of the fourth zone of the screw is 230°C, the temperature of the fifth zone of the screw is 240°C, and the temperatures of the pipeline and the die head are both 220°C.

[0048] 2) Feed the PVDF composite masterbatch dried in a 50°C vacuum drying oven for 10 h into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch successively passes through the first zone of the screw at 180°C, the second zone of the screw at 210°C, the third zone of the screw at 220°C, the fourth zone of the screw at 240°C, the fifth zone of the screw at 255°C, the screw flange at 260°C, and the melt pipeline at 255°C to obtain a fully mixed and molten melt.

[0049] 3) The fully mixed and molten melt is quantitatively passed through the flange of the spinning box at 255°C and the spinning box body at 260°C under the action of a metering pump. After filtration, it is finally extruded to obtain a molten extrudate.

[0050] 4) The molten extrudate successively passes through a first air bath (temperature 130°C, length 125 cm), a second air bath (temperature 25°C, length 75 cm), an oil bath (inverted), a first drawing roll (temperature 90°C), a second drawing roll (temperature 90°C), a third drawing roll (temperature 110°C), a heat setting roll (temperature 125°C), a texturing device, and then is wound to obtain the PVDF fiber for the filter fabric. Among them, the spinneret draw ratio is 100, the primary draw ratio is 2.5, the secondary draw ratio is 1.5, and the winding shrinkage rate is 93%.

[0051] After testing, the fineness of the obtained PVDF fiber for the filter fabric is 500 D / 50 f, the crystallinity is 50% ± 2%, the tensile strength is 3.4 cN / dtex ± 0.2 cN / dtex, the elongation at break is 15% ± 2%, the heat shrinkage rate at 120°C is 5% ± 1%, the water contact angle is 115° ± 2°, and the oil contact angle is close to 0°.

[0052] Comparative Example 3

[0053] Prepare a PVDF fiber for a filter fabric. The process method is as follows:

[0054] 1) Use a PVDF resin with a melt index of 8 g / 10 min at 230°C / 2.16 kg as the matrix phase, with a content of 97 wt.%; mix perfluoropolyether with an average molecular weight of 1800 and tetrafluoroethylene-perfluoroalkoxy vinyl ether with a melt index of 5 g / 10 min (230°C / 2.16 kg) in a mass ratio of 2:3 as the perfluorinated high molecular compound, with a content of 1 wt.%; use carbon nanotubes with a "linear" length of 5 μm as the lipophilic and hydrophobic nucleating agent, with a content of 2 wt.%. Use a high-speed mixer to fully mix the PVDF resin, perfluorinated high molecular compound, and lipophilic and hydrophobic nucleating agent, and then use a twin-screw extruder to make it into a PVDF composite masterbatch. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first zone of the screw is 180°C, the temperature of the second zone of the screw is 210°C, the temperature of the third zone of the screw is 220°C, the temperature of the fourth zone of the screw is 230°C, the temperature of the fifth zone of the screw is 240°C, and the pipe temperature and die head temperature are both 220°C.

[0055] 2) The PVDF composite masterbatch dried in a vacuum drying oven at 50 °C for 10 h was fed into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch passed successively through the first screw zone at 180 °C, the second screw zone at 210 °C, the third screw zone at 220 °C, the fourth screw zone at 240 °C, the fifth screw zone at 255 °C, the screw flange at 260 °C, and the melt pipe at 255 °C, obtaining a fully mixed and molten melt.

[0056] 3) The fully mixed and molten melt was quantitatively passed through the spinning box flange at 255 °C and the spinning box body at 260 °C under the action of a metering pump. After filtration, it was finally extruded to obtain a molten extrudate.

[0057] 4) The molten extrudate passed successively through a first air bath (temperature 130 °C, length 125 cm), a second air bath (temperature 25 °C, length 75 cm), an oil bath (reversed), a first drawing roll (temperature 90 °C), a second drawing roll (temperature 90 °C), a third drawing roll (temperature 110 °C), a heat setting roll (temperature 125 °C), and a texturing device, and then was wound to obtain the PVDF fiber for the filter fabric. Among them, the spinneret draw ratio was 100, the primary draw ratio was 2.5, the secondary draw ratio was 1.5, and the winding shrinkage rate was 93%.

[0058] After testing, the fineness of the obtained PVDF fiber for the filter fabric was 500 D / 50 f, the crystallinity was 54% ± 2%, the tensile strength was 3.6 cN / dtex ± 0.2 cN / dtex, the elongation at break was 18% ± 2%, the thermal shrinkage rate at 120 °C was 3% ± 1%, the water contact angle was 117° ± 2°, and the oil contact angle was close to 0°.

[0059] Comparative Example 4

[0060] Prepare a PVDF fiber for a filter fabric. The process method is as follows:

[0061] 1) The PVDF resin with a melt index of 8 g / 10 min at 230 °C / 2.16 kg is used as the matrix phase, with a content of 97 wt.%. The perfluoropolyether with an average molecular weight of 1800 and the tetrafluoroethylene-perfluoroalkoxy vinyl ether with a melt index of 5 g / 10 min (230 °C / 2.16 kg) are mixed as the perfluoropolymer compound in a mass ratio of 2:3, with a content of 1 wt.%. The graphene with a "flake" average particle diameter of 5 μm is used as the lipophilic and hydrophobic nucleating agent, with a content of 2 wt.%. The PVDF resin, the perfluoropolymer compound, and the lipophilic and hydrophobic nucleating agent are fully mixed using a high-speed mixer, and then made into PVDF composite masterbatch through a twin-screw extruder. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first screw zone is 180 °C, the temperature of the second screw zone is 210 °C, the temperature of the third screw zone is 220 °C, the temperature of the fourth screw zone is 230 °C, the temperature of the fifth screw zone is 240 °C, and the pipe temperature and the die head temperature are both 220 °C.

[0062] 2) The PVDF composite masterbatch dried in a vacuum drying oven at 50 °C for 10 h is fed into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch sequentially passes through the first screw zone at 180 °C, the second screw zone at 210 °C, the third screw zone at 220 °C, the fourth screw zone at 240 °C, the fifth screw zone at 255 °C, the screw flange at 260 °C, and the melt pipe at 255 °C, obtaining a fully mixed and molten melt.

[0063] 3) The fully mixed and molten melt is quantitatively passed through the spinning box flange at 255 °C and the spinning box body at 260 °C under the action of a metering pump, and after filtration, it is finally extruded to obtain a molten extrudate.

[0064] 4) The molten extrudate sequentially passes through a first air bath (temperature 130 °C, length 125 cm), a second air bath (temperature 25 °C, length 75 cm), an oil bath (reversed), a first drawing roll (temperature 90 °C), a second drawing roll (temperature 90 °C), a third drawing roll (temperature 110 °C), a heat setting roll (temperature 125 °C), and a netting device, and then is wound to obtain the PVDF fiber for the filter fabric. Among them, the spinneret draw ratio is 100, the primary draw ratio is 2.5, the secondary draw ratio is 1.5, and the winding shrinkage rate is 93%.

[0065] After testing, the fineness of the obtained PVDF fiber for the filter fabric is 500 D / 50 f, the crystallinity is 56% ± 2%, the tensile strength is 3.7 cN / dtex ± 0.2 cN / dtex, the elongation at break is 14% ± 2%, the thermal shrinkage rate at 120 °C is 3% ± 1%, the water contact angle is 118° ± 2°, and the oil contact angle is close to 0°.

[0066] Comparative Example 5

[0067] Prepare PVDF fibers for a filter fabric. The process method is as follows:

[0068] 1) Use PVDF resin with a melt index of 8 g / 10 min at 230 °C / 2.16 kg as the matrix phase, with a content of 97 wt.%; mix perfluoropolyether with an average molecular weight of 1800 and tetrafluoroethylene-perfluoroalkoxy vinyl ether with a melt index of 5 g / 10 min (230 °C / 2.16 kg) in a mass ratio of 2:3 as the perfluoropolymer compound, with a content of 1 wt.%; use the blend of three forms of additives, namely hydrophobic silica with a "spherical" average particle size of 40 nm, carbon nanotubes with a "linear" length of 5 μm, and graphene with a "flake" average flake diameter of 5 μm, in a mass ratio of 1:1:1 as the lipophilic and hydrophobic nucleating agent, with a content of 2 wt.%. Use a high-speed mixer to fully mix the PVDF resin, perfluoropolymer compound, and lipophilic and hydrophobic nucleating agent, and then use a twin-screw extruder to make it into a PVDF composite masterbatch. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first screw zone is 180 °C, the temperature of the second screw zone is 210 °C, the temperature of the third screw zone is 220 °C, the temperature of the fourth screw zone is 230 °C, the temperature of the fifth screw zone is 240 °C, and the pipe temperature and die head temperature are both 220 °C.

[0069] 2) Feed the PVDF composite masterbatch dried in a 50 °C vacuum drying oven for 10 h into the feed hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch sequentially passes through the first screw zone at 180 °C, the second screw zone at 210 °C, the third screw zone at 220 °C, the fourth screw zone at 240 °C, the fifth screw zone at 255 °C, the screw flange at 260 °C, and the melt pipe at 255 °C to obtain a fully mixed and molten melt.

[0070] 3) The fully mixed and molten melt is quantitatively passed through the spinning box flange at 255 °C and the spinning box body at 260 °C under the action of a metering pump, and after filtration, it is finally extruded to obtain a molten extrudate.

[0071] 4) The molten extrudate passes through an air bath (temperature 25 °C, length 75 cm), an oil bath (reversed), a first drawing roller (temperature 90 °C), a second drawing roller (temperature 90 °C), a third drawing roller (temperature 110 °C), a heat setting roller (temperature 125 °C), and a texturing device, and then is wound to obtain the PVDF fibers for the filter fabric. Among them, the spinneret draw ratio is 100, the primary draw ratio is 2.5, the secondary draw ratio is 1.5, and the winding shrinkage rate is 93%.

[0072] After testing, the fineness of the PVDF fibers for the obtained filter fabric is 500D / 50f, the crystallinity is 45% ± 2%, the tensile strength is 3.0 cN / dtex ± 0.2 cN / dtex, the elongation at break is 20% ± 2%, the thermal shrinkage rate at 120 °C is 5% ± 1%, the water contact angle is 122° ± 2°, and the oil contact angle is close to 0°.

[0073] Example 2

[0074] Prepare PVDF fibers for a filter fabric. The process method is as follows:

[0075] 1) Use PVDF resin with a melt index of 8 g / 10 min at 230 °C / 2.16 kg as the matrix phase, with a content of 97 wt.%; use perfluoropolyether with an average molecular weight of 1800 and tetrafluoroethylene-perfluoroalkoxy vinyl ether with a melt index of 5 g / 10 min (230 °C / 2.16 kg) in a mass ratio of 2:3 as the perfluorinated high molecular compound, with a content of 3 wt.%. Use a high-speed mixer to fully mix the PVDF resin and the perfluorinated high molecular compound, and then use a twin-screw extruder to make it into PVDF composite masterbatch. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first screw zone is 180 °C, the temperature of the second screw zone is 210 °C, the temperature of the third screw zone is 220 °C, the temperature of the fourth screw zone is 230 °C, the temperature of the fifth screw zone is 240 °C, and the pipe temperature and die head temperature are both 220 °C.

[0076] 2) Feed the PVDF composite masterbatch dried in a 50 °C vacuum drying oven for 10 h into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch sequentially passes through the first screw zone at 180 °C, the second screw zone at 210 °C, the third screw zone at 220 °C, the fourth screw zone at 240 °C, the fifth screw zone at 255 °C, the screw flange at 260 °C, and the melt pipe at 255 °C to obtain a fully mixed and molten melt.

[0077] 3) The fully mixed and molten melt is quantitatively passed through the spinning box flange at 255 °C and the spinning box body at 260 °C under the action of a metering pump, and after filtration, it is finally extruded to obtain a molten extrudate.

[0078] 4) The molten extrudate successively passes through an air bath (temperature: 130°C, length: 125 cm), a second air bath (temperature: 25°C, length: 75 cm), an oil bath (inverted), a first drawing roll (temperature: 90°C), a second drawing roll (temperature: 90°C), a third drawing roll (temperature: 110°C), a heat setting roll (temperature: 125°C), a texturing device, and then is wound to obtain the PVDF fibers for the filter fabric. Among them, the spinneret draw ratio is 75, the primary draw ratio is 1.5, the secondary draw ratio is 1.2, and the winding shrinkage rate is 95%.

[0079] After testing, the fineness of the obtained PVDF fibers for the filter fabric is 700 D / 50 f, the crystallinity is 51% ± 2%, the tensile strength is 3.2 cN / dtex ± 0.2 cN / dtex, the elongation at break is 20% ± 2%, the thermal shrinkage rate at 120°C is 5% ± 1%, the water contact angle is 119° ± 2°, and the oil contact angle is 10° ± 2°.

[0080] Example 3

[0081] Prepare PVDF fibers for a filter fabric. The process method is as follows:

[0082] 1) Take PVDF resin with a melt index of 8 g / 10 min under the conditions of 230°C / 2.16 kg as the matrix phase, with a content of 95 wt.%; take perfluoropolyether with an average molecular weight of 1800 and tetrafluoroethylene-perfluoroalkoxy vinyl ether with a melt index of 5 g / 10 min (230°C / 2.16 kg) in a mass ratio of 2:3 as the perfluorinated high molecular compound, with a content of 1 wt.%; take the blend of three forms of additives, namely hydrophobic silica with a "spherical" average particle size of 40 nm, carbon nanotubes with a "linear" length of 5 μm, and graphene with a "flake" average flake diameter of 5 μm, in a mass ratio of 1:1:1 as the lipophilic and hydrophobic nucleating agent, with a content of 4 wt.%. Use a high-speed mixer to fully mix the PVDF resin, the perfluorinated high molecular compound, and the lipophilic and hydrophobic nucleating agent, and then use a twin-screw extruder to make it into a PVDF composite masterbatch. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first zone of the screw is 180°C, the temperature of the second zone of the screw is 210°C, the temperature of the third zone of the screw is 220°C, the temperature of the fourth zone of the screw is 230°C, the temperature of the fifth zone of the screw is 240°C, and the pipe temperature and the die head temperature are both 220°C.

[0083] 2) Feed the PVDF composite masterbatch dried in a vacuum drying oven at 50 °C for 10 h into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch successively passes through the first screw zone at 180 °C, the second screw zone at 210 °C, the third screw zone at 220 °C, the fourth screw zone at 240 °C, the fifth screw zone at 255 °C, the screw flange at 260 °C, and the melt pipe at 255 °C to obtain a fully mixed and molten melt.

[0084] 3) The fully mixed and molten melt is quantitatively passed through the spinning box flange at 255 °C and the spinning box body at 260 °C under the action of a metering pump. After filtration, it is finally extruded to obtain a molten extrudate.

[0085] 4) The molten extrudate successively passes through a first air bath (temperature 130 °C, length 125 cm), a second air bath (temperature 25 °C, length 75 cm), an oil bath (reversed), a first drawing roller (temperature 90 °C), a second drawing roller (temperature 90 °C), a third drawing roller (temperature 110 °C), a heat setting roller (temperature 125 °C), and a texturing device, and then is wound to obtain the PVDF fiber for the filter fabric. Among them, the spinneret draw ratio is 50, the primary draw ratio is 1.3, the secondary draw ratio is 1.2, and the winding shrinkage rate is 96%.

[0086] After testing, the fineness of the obtained PVDF fiber for the filter fabric is 800 D / 50f, the crystallinity is 56% ± 2%, the tensile strength is 3.8 cN / dtex ± 0.2 cN / dtex, the elongation at break is 10% ± 2%, the thermal shrinkage rate at 120 °C is 3% ± 1%, the water contact angle is 123° ± 2°, and the oil contact angle is close to 0°.

[0087] Example 4

[0088] Prepare a PVDF fiber for a filter fabric. The process method is as follows:

[0089] 1) The PVDF resin with a melt index of 8 g / 10 min at 230 °C / 2.16 kg is used as the matrix phase, with a content of 93 wt.%. The perfluoropolyether with an average molecular weight of 1800, tetrafluoroethylene-perfluoroalkoxy vinyl ether with a melt index of 5 g / 10 min (230 °C / 2.16 kg) are used as the perfluorinated high molecular compound in a mass ratio of 2:3, with a content of 3 wt.%. The blend of three morphological additives, namely hydrophobic silica with a "spherical" average particle size of 40 nm, carbon nanotubes with a "linear" length of 5 μm, and graphene with a "flake-like" average flake diameter of 5 μm in a mass ratio of 1:1:1 is used as the lipophilic and hydrophobic nucleating agent, with a content of 4 wt.%. The PVDF resin, perfluorinated high molecular compound, and lipophilic and hydrophobic nucleating agent are fully mixed using a high-speed mixer, and then made into PVDF composite masterbatch through a twin-screw extruder. Among them, the twin-screw extruder has five-zone temperature control. The temperature of the first screw zone is 180 °C, the temperature of the second screw zone is 210 °C, the temperature of the third screw zone is 220 °C, the temperature of the fourth screw zone is 230 °C, the temperature of the fifth screw zone is 240 °C, and the pipe temperature and die head temperature are both 220 °C.

[0090] 2) The PVDF composite masterbatch dried in a vacuum drying oven at 50 °C for 10 h is fed into the hopper of a single-screw extruder. Then, under the shearing action of the screw, the masterbatch sequentially passes through the first screw zone at 180 °C, the second screw zone at 210 °C, the third screw zone at 220 °C, the fourth screw zone at 240 °C, the fifth screw zone at 255 °C, the screw flange at 260 °C, and the melt pipe at 255 °C, obtaining a fully mixed and molten melt.

[0091] 3) The fully mixed and molten melt is quantitatively passed through the spinneret flange at 255 °C and the spinneret body at 260 °C under the action of a metering pump, and after filtration, it is finally extruded to obtain a molten extrudate.

[0092] 4) The molten extrudate sequentially passes through a first air bath (temperature 130 °C, length 125 cm), a second air bath (temperature 25 °C, length 75 cm), an oil bath (reversed), a first drawing roll (temperature 90 °C), a second drawing roll (temperature 90 °C), a third drawing roll (temperature 110 °C), a heat setting roll (temperature 125 °C), a netting device, and then is wound to obtain the PVDF fiber for the filter fabric. Among them, the spinneret draw ratio is 30, the primary draw ratio is 1.3, the secondary draw ratio is 1.2, and the winding shrinkage rate is 97%.

[0093] After testing, the fineness of the PVDF fibers for the obtained filter fabric is 900D / 50f, the crystallinity is 54% ± 2%, the tensile strength is 3.4 cN / dtex ± 0.2 cN / dtex, the elongation at break is 8% ± 2%, the thermal shrinkage rate at 120°C is 3% ± 1%, the water contact angle is 125° ± 2°, and the oil contact angle is close to 0°.

Claims

1. A preparation method of polyvinylidene fluoride fibers for filter fabrics, characterized in that: comprising the following steps, (1) Mix 0.2 - 3 wt.% of perfluorinated polymer compound, 0.5 - 4 wt.% of lipophilic and hydrophobic nucleating agent, and 93 - 99 wt.% of polyvinylidene fluoride resin, and add them into a twin-screw extruder to prepare a polyvinylidene fluoride composite masterbatch; wherein, the perfluorinated polymer compound is composed of perfluoropolyether and tetrafluoroethylene-perfluoroalkoxy vinyl ether; the lipophilic and hydrophobic nucleating agent includes one or more of hydrophobic silica, carbon nanotubes, and graphene; (2) Feed the polyvinylidene fluoride composite masterbatch into a single-screw extruder to melt and form a melt; (3) Extrude the melt through a screw extrusion spinning machine to obtain a molten extrudate; after passing the molten extrudate through a first air bath, a second air bath, an oil bath, a first drawing roll, a second drawing roll, a third drawing roll, a heat setting roll, and a texturing device, wind it up to obtain the polyvinylidene fluoride fiber.

2. The preparation method of the polyvinylidene fluoride fiber for the filter fabric according to claim 1, characterized in that: In step (1), the melt index of the polyvinylidene fluoride resin is 3 - 13 g / 10 min.

3. The preparation method of the polyvinylidene fluoride fiber for the filter fabric according to claim 1 or 2, characterized in that: In step (1), the mass ratio of perfluoropolyether to tetrafluoroethylene-perfluoroalkoxy vinyl ether is 2:3, the average molecular weight of perfluoropolyether is 500 - 6000, and the melt index of tetrafluoroethylene-perfluoroalkoxy vinyl ether is 1 - 10 g / 10 min.

4. The preparation method of the polyvinylidene fluoride fiber for the filter fabric according to claim 1 or 2, characterized in that: In step (1), the lipophilic and hydrophobic nucleating agent is composed of spherical hydrophobic silica with an average particle size of 20 - 100 nm, linear carbon nanotubes with a length of 1 - 10 μm, and flaky graphene with an average sheet diameter of 1 - 10 μm, which are blended in a mass ratio of 1:1:

1.

5. The preparation method of the polyvinylidene fluoride fiber for the filter fabric according to claim 1 or 2, characterized in that: In step (1), the twin-screw extruder includes five-zone temperature control. The temperature of the first zone of the screw is 160 - 190 °C, the temperature of the second zone of the screw is 190 - 220 °C, the temperature of the third zone of the screw is 200 - 230 °C, the temperature of the fourth zone of the screw is 210 - 240 °C, the temperature of the fifth zone of the screw is 220 - 250 °C, and the temperatures of the pipeline and the die head are both 200 - 230 °C, wherein the die head temperature is 20 °C lower than the temperature of the fifth zone of the screw.

6. The preparation method of the polyvinylidene fluoride fiber for the filter fabric according to claim 1 or 2, characterized in that: In step (2), feed the polyvinylidene fluoride composite masterbatch into a single-screw extruder, and successively pass through the first zone of the screw with a temperature of 155 - 190 °C, the second zone of the screw with a temperature of 185 - 220 °C, the third zone of the screw with a temperature of 195 - 230 °C, the fourth zone of the screw with a temperature of 215 - 250 °C, the fifth zone of the screw with a temperature of 230 - 265 °C, the screw flange with a temperature of 235 - 270 °C, and the melt pipeline with a temperature of 230 - 265 °C to obtain a fully mixed and molten melt.

7. The preparation method of the polyvinylidene fluoride fiber for a filter fabric according to claim 1 or 2, characterized in that: In step (3), pass the melt through the spinning box flange of the screw extrusion spinning machine with a temperature of 230 - 265 °C and the spinning box body with a temperature of 235 - 270 °C, and extrude it after filtration to obtain a molten extrudate.

8. The preparation method of the polyvinylidene fluoride fiber for the filter fabric according to claim 1 or 2, characterized in that: In step (3), the temperature of the first air bath is 120 - 140 °C and the length is 100 - 150 cm; the temperature of the second air bath is 20 - 28 °C and the length is 50 - 100 cm; the stretching temperature of the first stretching roller is 85 - 95 °C, the stretching temperature of the second stretching roller is 85 - 95 °C, the stretching temperature of the third stretching roller is 105 - 115 °C, and the temperature of the heat setting roller is 120 - 130 °C.

9. The preparation method of the polyvinylidene fluoride fiber for the filter fabric according to claim 1 or 2, characterized in that: For the winding, the spinneret draw ratio is 30 - 150, the primary draw ratio is 1.5 - 3.5, the secondary draw ratio is 1.1 - 2, and the winding shrinkage rate is 90 - 97%.

10. The polyvinylidene fluoride fiber for a filter fabric prepared by the method for preparing a polyvinylidene fluoride fiber for a filter fabric according to claim 1.