PTFE composite fiber membrane resistant to swelling in non-polar solvents and method for producing the same

Abstract

The application provides a PTFE composite fiber membrane resistant to non-polar solvent swelling and a preparation method thereof, and comprises the following steps: uniformly mixing a polyperfluoroethylene propylene resin dispersion liquid and isopropanol to obtain a microfiltration membrane finishing agent, immersing a PTFE fiber membrane in the microfiltration membrane finishing agent for cross-linking modification to obtain an immersed membrane, and performing heat treatment on the immersed membrane to obtain a PTFE microfiltration membrane resistant to non-polar solvent swelling; performing electrostatic centrifugal spinning on the PTFE microfiltration membrane by using an ultrafiltration membrane finishing agent to obtain a PTFE ultrafiltration membrane resistant to non-polar solvent swelling; sequentially stacking the PTFE ultrafiltration membrane or the PTFE microfiltration membrane and a PP spun-bond non-woven fabric, and then performing hot rolling to obtain a composite membrane sample; and transferring the composite membrane sample to a room temperature environment for cooling and solidification to obtain the PTFE composite fiber membrane resistant to non-polar solvent swelling.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of polytetrafluoroethylene (PTFE) membrane, in particular to a PTFE composite fiber membrane resistant to non-polar solvent swelling and a preparation method thereof. BACKGROUND

[0002] Polytetrafluoroethylene (PTFE) is known as "plastic king" due to its excellent chemical inertness, solvent resistance and thermal stability, and is an ideal membrane material for organic solvent separation, which is widely used in filtration and purification fields and has great application prospects in organic solvent separation.

[0003] However, the most widely used mechanically stretched PTFE fiber membrane has a unique microporous structure formed by the interconnection of fibrils and nodes, and the PTFE molecular chains are arranged in a spiral shape, with fluorine atoms uniformly distributed around the carbon chain, making the material as a whole non-polar. According to the principle of "like dissolves like", non-polar materials are more compatible with non-polar solvents. This characteristic makes pure PTFE fiber membrane encounter technical bottlenecks in practical application: due to the weak intermolecular force of PTFE, if the PTFE fiber membrane is placed in non-polar solvent for long-term immersion or pressure, the molecular chains of PTFE fiber are prone to slip, which makes the material exhibit significant creep characteristics, resulting in swelling of the fiber network, expansion of the pore size, and ultimately leading to a decrease in filtration precision, which seriously limits the promotion and application of pure PTFE fiber membrane.

[0004] Currently, most researches on improving the anti-creep performance of PTFE are filling modification methods, that is, by introducing rigid or high thermal conductivity fillers to inhibit the slip and entanglement of PTFE molecular chains. The filling modification strategy has made significant progress in improving the anti-creep performance of PTFE composite materials, but there are still obvious deficiencies in the research on the creep optimization of fiber membrane system, which are as follows: due to the network structure of PTFE fiber membrane formed by the interconnection of fibrils and nodes, if the size of rigid filler is close to or larger than the pore size of PTFE fiber membrane, the rigid filler will be embedded in the pore network, thereby destroying the original air permeability and filtration channel, and the rigid filler filled in the network structure will also destroy the flexible connection between the fibrils, resulting in brittle fracture of PTFE membrane in pressure filtration; in addition, under the long-term immersion of non-polar solvent, the rigid filler is prone to peel off from the surface of PTFE fiber, so the filling modification strategy is not suitable for the creep optimization of PTFE model, and a new technical solution for PTFE composite fiber membrane resistant to non-polar solvent swelling is urgently needed. SUMMARY

[0005] The present application aims to provide a PTFE composite fiber membrane resistant to non-polar solvent swelling and a preparation method thereof, which uses a step-by-step post-finishing technology with microfiltration membrane finishing agent or ultrafiltration membrane finishing agent to effectively improve the non-polar solvent swelling resistance of PTFE fiber membrane.

[0006] In a first aspect, the technical solution provides a preparation method of a PTFE composite fiber membrane resistant to non-polar solvent swelling, comprising the following steps:

[0007] S1: preparing a PTFE microfiltration membrane resistant to non-polar solvent swelling:

[0008] After uniformly mixing the polyperfluoroethylene propylene resin dispersion liquid and isopropanol to obtain a microfiltration membrane finishing agent, the PTFE fiber membrane is immersed in the microfiltration membrane finishing agent for cross-linking modification to obtain an immersed membrane, and the immersed membrane is subjected to heat treatment to obtain a PTFE microfiltration membrane resistant to non-polar solvent swelling;

[0009] S2: preparing a PTFE composite fiber membrane resistant to non-polar solvent swelling:

[0010] The PTFE microfiltration membrane and the PP spun-bonded non-woven fabric are cut, washed and dried, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric are sequentially stacked and subjected to hot rolling to obtain a composite membrane sample, and the composite membrane sample is transferred to a room temperature environment for cooling and solidification to obtain a PTFE composite fiber membrane resistant to non-polar solvent swelling.

[0011] The present solution focuses on the swelling problem of traditional PTFE fiber membranes in non-polar organic solvent separation, and introduces polyperfluoroethylene propylene resin FEP with strong resistance to swelling and hot melting characteristics. FEP has hot melting characteristics, so it can melt and flow during heat treatment or sintering, uniformly coating the surface of PTFE fibers and the junction of fibrils, forming a continuous fluorocarbon resin coating layer. This coating layer is tightly combined with the PTFE fiber through intermolecular van der Waals force, which is equivalent to building a "rigid constraint shell" outside the PTFE fiber network, which can effectively inhibit the slip and stretching of PTFE molecular chains under the infiltration of non-polar solvents, thereby improving the dimensional stability of the fiber network and reducing swelling. The high electronegativity of fluorine atoms in the FEP molecular chain makes it have strong chemical inertness and resistance to non-polar solvent swelling. When FEP coats PTFE fibers, the overall resistance to swelling of the composite system is dominated by FEP, further reducing the infiltration and swelling of non-polar solvents on the fiber network.

[0012] In step S1, the PTFE fiber membrane is fixed on a stainless steel frame, and the PTFE fiber membrane fixed on the stainless steel frame is immersed in the post-microfiltration finishing agent, so that the polyfluoroethylene propylene (FEP) resin can fully penetrate into the gaps of the PTFE fibril network. The PTFE fiber membrane itself has a certain flexibility and porous structure. If it is directly immersed in the post-microfiltration finishing agent, it may be wrinkled, shrunk or stretched due to liquid buoyancy, its own gravity or stirring, resulting in uneven distribution of the gaps between the fiber networks. In this scheme, the stainless steel frame can fix the shape of the PTFE fiber membrane, ensure that the membrane remains flat during immersion, and make the FEP resin dispersion uniformly penetrate into every gap of the PTFE fibril network, avoiding local insufficient penetration or excessive accumulation.

[0013] In step S1, the solid content of the polyfluoroethylene propylene resin dispersion is 60%, and the mass fraction of polyfluoroethylene propylene in the post-microfiltration finishing agent is 1wt%-4wt%. FEP and PTFE belong to fluorine-containing resins and have good compatibility, and excellent non-polar solvent swelling resistance. 1wt%-4wt% of FEP can form a thin and continuous coating layer or cross-linked structure on the surface of PTFE fibers and between the fibrils, thereby inhibiting the penetration and swelling of non-polar solvents on PTFE fibers through "physical barrier" and "enhanced intermolecular forces". If the mass fraction of FEP is too high, excessive FEP may melt and block the micropore channels of PTFE fibrils after heat treatment, resulting in a significant decrease in the porosity of the membrane and affecting its filtration performance. If the mass fraction is too low, it cannot form a complete coating layer, and the swelling resistance is insufficient.

[0014] In step S1, the average pore size of the PTFE fiber membrane is 0.1-0.3 μm. Preferably, the average pore size of the PTFE fiber membrane is 0.2 μm.

[0015] In step S1, the cross-linking modification time of the PTFE fiber membrane immersed in the post-microfiltration finishing agent for cross-linking modification is 15-25 min. Preferably, the cross-linking modification time is 20 min.

[0016] In step S1, the heat treatment time of the immersed membrane is 25-35 min, and the temperature is 280°C-360°C. Preferably, the heat treatment time is 30 min.

[0017] In step S2, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric are cut to the same size. In the examples of the present scheme, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric can be cut to 10 cm*10 cm.

[0018] In some embodiments, the grammage of the PP spun-bonded non-woven fabric is 40 g·m -2 -100 g·m -2.

[0019] In some embodiments, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric are cleaned and dried using anhydrous ethanol to ensure that the surface of the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric is clean and free of impurities.

[0020] In some embodiments, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric are sequentially stacked and hot-rolled for 2-4 minutes to obtain a composite membrane sample. Preferably, the hot-rolling is performed for 3 minutes.

[0021] In some embodiments, the hot-rolling temperature is 170-220°C.

[0022] In a second aspect, the present application provides a PTFE composite fiber membrane resistant to swelling in non-polar solvents, which is prepared according to the method for preparing a PTFE composite fiber membrane resistant to swelling in non-polar solvents mentioned in the first aspect.

[0023] In a third aspect, the present application provides a method for preparing a PTFE composite fiber membrane resistant to swelling in non-polar solvents, which comprises the following steps:

[0024] S1: preparing a PTFE microfiltration membrane resistant to swelling in non-polar solvents:

[0025] uniformly mixing a polyperfluoroalkoxy resin dispersion and isopropyl alcohol to obtain a post-finishing agent for microfiltration membranes, immersing the PTFE fiber membrane in the post-finishing agent for microfiltration membranes for cross-linking modification to obtain an immersed membrane, and heat-treating the immersed membrane to obtain a PTFE microfiltration membrane resistant to swelling in non-polar solvents;

[0026] S2: preparing a PTFE ultrafiltration membrane resistant to swelling in non-polar solvents:

[0027] uniformly mixing polyethylene oxide and deionized water to obtain a polyethylene oxide solution, uniformly mixing the polyethylene oxide solution and a polyperfluoroalkoxy resin emulsion to obtain a post-finishing agent for ultrafiltration membranes, injecting the post-finishing agent for ultrafiltration membranes into an electrostatic-centrifugal spinning machine spinneret, and using the electrostatic-centrifugal spinning machine spinneret to perform centrifugal spinning on the PTFE microfiltration membrane to obtain a spunlaid membrane, and performing solvent drying and sintering on the spunlaid membrane to obtain a PTFE ultrafiltration membrane resistant to swelling in non-polar solvents;

[0028] S3: preparing a PTFE composite fiber membrane resistant to swelling in non-polar solvents:

[0029] cutting, cleaning and drying the PTFE ultrafiltration membrane and the PP spun-bonded non-woven fabric, sequentially stacking the PTFE ultrafiltration membrane and the PP spun-bonded non-woven fabric, and hot-rolling to obtain a composite membrane sample, and transferring the composite membrane sample to a room temperature environment for cooling and solidification to obtain a PTFE composite fiber membrane resistant to swelling in non-polar solvents.

[0030] Different from the first embodiment, the present scheme adopts electrostatic centrifugal spinning, which combines the high electric field stretching of electrostatic spinning and the high centrifugal force spinning of centrifugal spinning, to spin the FEP emulsion into controllable micro-nano fibers. The diameters of these fibers are much smaller than the pore size of the PTFE base film, and can uniformly cover the surface of the PTFE microfiltration membrane prepared in S1 to form a loose fiber layer. A large number of nano-scale gaps are naturally formed between the fibers, avoiding the pore blockage caused by the "large block agglomeration" of FEP in the traditional melt filling. After sintering, the FEP micro-nano fiber layer is fused and bonded between the fibers to form a continuous three-dimensional interpenetrating network. This network is interpenetrated with the micro-pore structure of the PTFE base film, and a PTFE ultrafiltration membrane is obtained.

[0031] In step S1, the PTFE fiber membrane is fixed on a stainless steel frame, and the PTFE fiber membrane fixed on the stainless steel frame is immersed in the microfiltration membrane after finishing agent to ensure that the polyfluoroethylene propylene (FEP) resin can fully penetrate into the gaps of the PTFE fibril network. The PTFE fiber membrane itself has a certain flexibility and porous structure. If it is directly immersed in the microfiltration membrane after finishing agent, it may wrinkle, shrink or stretch due to liquid buoyancy, its own gravity or stirring, resulting in uneven distribution of fiber network gaps. The stainless steel frame used in the present scheme can fix the shape of the PTFE fiber membrane, ensuring that the membrane remains flat during immersion, so that the FEP resin dispersion can uniformly penetrate into every gap of the PTFE fibril network, avoiding local insufficient penetration or excessive accumulation.

[0032] In step S1, the solid content of the polyfluoroethylene propylene resin dispersion is 60%, and the mass fraction of polyfluoroethylene propylene in the microfiltration membrane after finishing is 1wt%-4wt%. FEP and PTFE belong to fluorine-containing resins and have good compatibility. In addition, FEP has excellent non-polar solvent swelling resistance. 1wt%-4wt% of FEP can form a thin and continuous coating layer or cross-linked structure on the surface of PTFE fibers and between the fibrils, thereby inhibiting the penetration and swelling of non-polar solvents on PTFE fibers through "physical barrier" and "enhanced intermolecular forces". If the mass fraction of FEP is too high, excessive FEP may melt and block the micro-pore channels of the PTFE fibril after heat treatment, resulting in a significant decrease in the porosity of the membrane and affecting its filtration performance. If the mass fraction is too low, it cannot form a complete coating layer, and the swelling resistance is insufficient.

[0033] In step S1, the average pore size of the PTFE fiber membrane is 0.1-0.3 μm. Preferably, the average pore size of the PTFE fiber membrane is 0.2 μm.

[0034] In step S1, the PTFE fiber membrane is immersed in the microfiltration membrane after-treatment agent for cross-linking modification for 15-25 min. Preferably, the cross-linking modification time is 20 min.

[0035] In step S1, the heat treatment of the immersed membrane is performed for 25-35 min at a temperature of 280-360°C. Preferably, the heat treatment time is 30 min.

[0036] In step S2, the mass fraction of polyethylene oxide in the polyethylene oxide solution is 4 wt%. In some embodiments, 4 g of polyethylene oxide (Mw=5000000) is mixed with 96 g of deionized water, and the mixture is stirred at room temperature for 6 h using a magnetic stirrer to obtain a polyethylene oxide solution.

[0037] In step S2, the polyethylene oxide solution and the polyperfluoroethylene propylene resin emulsion are magnetically stirred until the two phases are fully overlapped and there is no phase separation phenomenon, and the mixed solution is placed in the dark to remove bubbles to obtain an ultrafiltration membrane after-treatment agent. PEO (polyethylene oxide) is a water-soluble polymer, and the FEP emulsion is a water dispersion of a fluorine-containing resin (polyperfluoroethylene propylene). Although both belong to hydrophilic dispersion systems, there are large differences in molecular structure. If the mixture is not fully mixed, phase separation (such as layering and particle agglomeration) may occur. Magnetic stirring until there is no phase separation can force the breaking of interfacial tension, allowing PEO molecules to be uniformly dispersed in the FEP emulsion to form a stable and uniform system.

[0038] In some embodiments, the ratio of polyethylene oxide solution to polyperfluoroethylene propylene resin emulsion in the ultrafiltration membrane after-treatment agent is 4:96-10:90. Preferably, the ratio of polyethylene oxide solution to polyperfluoroethylene propylene resin emulsion is 4:96, 6:94, 8:92, or 10:90, respectively. In the ultrafiltration membrane after-treatment agent of the present scheme, the polyperfluoroethylene propylene resin emulsion plays a dominant role, ensuring that the spun fiber is mainly FEP, and after sintering (FEP melting and recrystallization), a stable fluorine-containing micro-nano fiber network is formed. This network, as the core structure of the ultrafiltration membrane, can resist non-polar solvent swelling through its own chemical inertness and achieve high-precision separation through the pores between the fibers.

[0039] In step S2, the ultrafiltration membrane after-treatment agent is injected into the electrostatic-centrifugal spinneret, and the PTFE microfiltration membrane is placed on the conveyor belt of the electrostatic-centrifugal spinneret. The electrostatic-centrifugal spinneret is used to perform centrifugal spinning on the PTFE microfiltration membrane to obtain a spinning membrane.

[0040] In some embodiments, the distance from the electrostatic-centrifugal spinneret needle to the conveyor belt of the electrostatic-centrifugal spinneret is 80-100 mm, and the spinning machine speed is set to 1400-1600 r·min -1Preferably, the electrostatic-centrifugal spinning machine nozzle of the electrostatic-centrifugal spinning machine is 90mm away from the conveying belt, and the spinning machine speed is set to 1500 r·min -1 .

[0041] In step S2, the spinning film is taken off from the conveying belt and dried at 70℃ for at least 6h before sintering to obtain a PTFE ultrafiltration membrane resistant to non-polar solvent swelling.

[0042] In some embodiments, the sintering temperature is 300℃-380℃.

[0043] In step S3, the PTFE ultrafiltration membrane, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric are cut to the same size. In the embodiments of the present scheme, the PTFE ultrafiltration membrane, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric can be cut to 10cm*10cm.

[0044] In some embodiments, the grammage of the PP spun-bonded non-woven fabric is 40g·m -2 -100 g·m -2 .

[0045] In some embodiments, the PTFE ultrafiltration membrane, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric are cleaned and dried with anhydrous ethanol to ensure that the surfaces of the PTFE ultrafiltration membrane, the PTFE microfiltration membrane and the PP spun-bonded non-woven fabric are clean and free of impurities.

[0046] In some embodiments, the PTFE microfiltration membrane, the PTFE ultrafiltration membrane and the PP spun-bonded non-woven fabric are sequentially stacked and hot-rolled for 2-4min to obtain a composite membrane sample. Preferably, the hot-rolling time is 3min.

[0047] In some embodiments, the hot-rolling temperature is 170℃-220℃.

[0048] In a fourth aspect, the present scheme provides a PTFE composite fiber membrane resistant to non-polar solvent swelling, comprising a PTFE ultrafiltration membrane and a PP spun-bonded non-woven fabric arranged in sequence, and is prepared according to the preparation method of the PTFE composite fiber membrane resistant to non-polar solvent swelling mentioned in the third aspect.

[0049] Compared with the prior art, the technical scheme has the following characteristics and beneficial effects:

[0050] The preparation method of the PTFE composite fiber membrane resistant to non-polar solvent swelling provided by the present scheme breaks through the performance by precise design of material selection and process innovation, aiming at the problems of swelling, creep and decline of filtration precision of pure PTFE fiber membrane in non-polar organic solvent separation due to molecular chain slippage:

[0051] In one aspect, the fluorine-containing resin FEP with excellent resistance to swelling and hot melting properties is introduced innovatively. By taking advantage of its good compatibility with PTFE, the connection strength of the PTFE fibril network is strengthened through melt coating, and the dimensional change of the fiber network under long-term immersion in a non-polar solvent is inhibited at the molecular level, significantly improving the resistance to swelling of the base film.

[0052] On the other hand, the electrostatic centrifugal spinning-sintering combined technology is created for the first time: a layer of FEP micro-nano fiber is spun on the surface of the PTFE microfiltration membrane prepared in S1, and a three-dimensional interpenetrating network structure is formed after sintering. This design avoids the pore blockage problem caused by traditional melt filling through the interpenetrating network formed by FEP micro-nano fibers, while ensuring the resistance to swelling of the PTFE base film, and improving the porosity and separation precision of the separation membrane. In addition, the composite structure of FEP interpenetrating network and PTFE base film further enhances the mechanical strength of the membrane, effectively resisting pressure impact and long-term use fatigue in actual application.

[0053] Finally, two systems of PTFE microfiltration composite membrane with resistance to non-polar solvent swelling and PTFE ultrafiltration composite membrane with high precision and high porosity and resistance to non-polar solvent swelling are constructed, providing a more optimal membrane material solution for the non-polar organic solvent separation field. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 is the surface morphology of the pure PTFE fiber membrane provided by the present scheme before and after immersion in n-hexane solvent.

[0055] Figure 2 is the surface morphology of the PTFE microfiltration composite membrane obtained in Example One provided by the present scheme before and after immersion in n-hexane solvent.

[0056] Figure 3 is the thermogravimetric analysis curve of the PTFE base film, PEO, and FEP particles in Example Three provided by the present scheme.

[0057] Figure 4 is the surface morphology of the PTFE ultrafiltration composite membrane obtained in Example Three provided by the present scheme before and after immersion in n-hexane solvent. DETAILED DESCRIPTION

[0058] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art belong to the scope of protection of the present application.

[0059] Example One: Preparation of PTFE microfiltration composite membrane with resistance to non-polar solvent swelling:

[0060] Step 1: 6 g of FEP resin dispersion liquid was uniformly mixed with 194 g of isopropyl alcohol to prepare a post-treatment agent for microfiltration membrane; a PTFE fiber membrane fixed on a stainless steel frame was immersed in the post-treatment agent for microfiltration membrane for 20 min for cross-linking modification, and then the immersed membrane was transferred to a drying device, and the heat treatment temperature was set to 320 ℃ to obtain a PTFE microfiltration membrane resistant to non-polar solvent swelling.

[0061] Step 2: The microfiltration membrane obtained in step 1 was cut into a size of 10 cm x 10 cm together with a 40 g / m2PP spun-bond non-woven fabric, and then the microfiltration membrane was stacked on the upper layer and the PP spun-bond non-woven fabric was stacked on the lower layer, and hot rolling was performed at a temperature of 180 ℃ for 3 min to obtain a PTFE microfiltration composite membrane resistant to non-polar solvent swelling. -2

[0062] Pore size analysis: The pore size of the membrane was measured by a pore size analyzer, and each sample was measured 5 times to obtain an average value. The test conditions were as follows: nitrogen, working pressure 0-100 Mpa, and the average pore sizes of the PTFE fiber membrane and the PTFE microfiltration composite membrane obtained in Example 1 were 289.1 nm and 221.7 nm, respectively.

[0063] Surface morphology: The surface morphologies of the pure PTFE fiber membrane before and after immersion in n-hexane solvent are shown in FIGS. 1(a) and 1(b), respectively. As shown in FIG. 1(a), the PTFE fiber membrane before immersion is composed of a “fibril + node” structure. As shown in FIG. 1(b), after immersion, the fibrils of the PTFE fiber membrane become thicker, the fibril network becomes disordered and fluffy, and the size stability is poor. The surface morphology of the PTFE microfiltration composite membrane obtained in Example 1 is shown in FIG. 2. As can be seen from the surface morphology, the composite membrane does not swell significantly, which indicates that the melting coating of FEP improves the stability of the fibril network, and the anti-swelling performance is obviously improved. Figure 1 Figure 1 Figure 2

[0064] Mechanical properties: The mechanical properties of the membrane samples before and after immersion were tested. The breaking strength of the PTFE membrane was 21.10 MPa, and the breaking elongation was 126.62%. After 7 days of immersion experiment, the breaking strength decreased significantly to 16.31 MPa, and the breaking elongation increased to 139.97%. This indicates that after the swelling of PTFE, the mechanical strength decreases significantly, the elastic modulus of the base film decreases, the flexibility increases, the fibrils are easily deformed, and the morphological structure stability is poor in the long-term organic solvent separation system. The breaking strength of the PTFE microfiltration composite membrane obtained in Example 1 decreased from 25.56 to 24.94 MPa after immersion in n-hexane solvent, and the breaking elongation change rate was only 2.5%, which indicates that the addition of FEP can enhance the swelling resistance of the base film.

[0065] ​​​​Swelling ratio: In order to test the swelling resistance of the PTFE microfiltration composite membrane obtained in Example 1, it was placed in n-hexane solvent for 7 days to observe the changes in its surface morphology and mechanical properties, and its swelling ratio after being immersed in three non-polar solvents (trichloromethane, n-hexane, tetrahydrofuran) for 7 days was tested. It was found that the swelling ratios of the PTFE fiber membrane after being immersed in n-hexane, tetrahydrofuran and trichloromethane for 7 days were all greater than 1%. However, the swelling ratios of the PTFE microfiltration composite membrane obtained in Example 1 were all less than 1%.

[0066] Preparation of PTFE microfiltration composite membrane resistant to non-polar solvent swelling

[0067] Step 1: 8 g of FEP resin dispersion was uniformly mixed with 192 g of isopropanol to prepare a microfiltration membrane finishing agent; the PTFE fiber membrane fixed on a stainless steel frame was immersed in the microfiltration membrane finishing agent for 20 min for crosslinking modification, and then the immersed membrane was transferred to a drying device, and the heat treatment temperature was set to 360 ℃ to obtain a PTFE microfiltration membrane resistant to non-polar solvent swelling.

[0068] Step 2: The microfiltration membrane obtained in Step 1 and a 70 g·m -2 PP spunbond non-woven fabric were cut into 10 cm×10 cm size, and then the microfiltration membrane was placed on the upper layer and the PP spunbond non-woven fabric was placed on the lower layer to stack, and hot rolling was performed at 200 ℃ for 3 min to obtain a PTFE microfiltration composite membrane resistant to non-polar solvent swelling.

[0069] Mechanical property testing was performed on the membrane samples before and after immersion. The breaking strength of the PTFE microfiltration composite membrane obtained in Example 2 was 25.22 MPa, and the breaking elongation was 94.71%. After 7 days of immersion, the breaking strength was 24.59 MPa, and the breaking elongation increased to 104.75%. This indicates that the swelling resistance of the PTFE microfiltration composite membrane obtained in Example 2 is also significantly better than that of pure PTFE fiber membrane.

[0070] Peeling strength test: The membrane sample was tested for peeling strength according to FZ / T 60039-2013 "Peeling strength test method for coated fabrics for membrane structures". The experimental instrument was a universal material testing machine, the tensile speed was set to 100 mm·min -1 , the distance between the clamps was 50 mm, and the median value in the region after displacement of 20 mm on the peeling curve of each sample was recorded. The average value of the median values of the five samples was taken as the peeling strength. The peeling strength of the PTFE microfiltration composite membrane obtained in Example 2 was 8.481 N.

[0071] Preparation of PTFE ultrafiltration composite membrane resistant to non-polar solvent swelling

[0072] Step 1: 6 g FEP resin dispersion liquid was uniformly mixed with 194 g isopropyl alcohol for preparing the post-treatment agent of microfiltration membrane; the PTFE fiber membrane fixed on the stainless steel frame was immersed in the post-treatment agent of microfiltration membrane for 20 min for cross-linking modification, then the immersed membrane was transferred to the drying equipment, and the heat treatment temperature was set to 320 ℃, thereby obtaining the PTFE microfiltration membrane resistant to non-polar solvent swelling.

[0073] Step 2: 4 g PEO was mixed with 96 g deionized water, and then stirred at room temperature by a magnetic stirrer at a constant speed for 6 h to obtain a 4 wt% PEO solution; the prepared PEO solution was mixed with the FEP emulsion, and the ratio of PEO solution to FEP was 4:96, and the mixture was uniformly stirred by a magnetic stirrer for 3 h to obtain a spinning solution; the spinning solution was injected into the spinneret of an electrostatic centrifugal spinning machine, and the microfiltration membrane prepared in step 1 was placed on the conveying belt to start centrifugal spinning; after the spinning was completed, sintering was performed at 350 ℃, thereby obtaining the PTFE ultrafiltration membrane resistant to non-polar solvent swelling.

[0074] The ultrafiltration membrane obtained in step 2 was cut into a size of 10 cm×10 cm together with a 40 g·m -2 of PP spun-bond non-woven fabric, then the ultrafiltration membrane was stacked on the upper layer and the PP spun-bond non-woven fabric was stacked on the lower layer, and hot rolling was performed at a temperature of 180 ℃ for 3 min, thereby obtaining the PTFE ultrafiltration composite membrane resistant to non-polar solvent swelling.

[0075] Pore size analysis: the average pore size of the PTFE ultrafiltration composite membrane obtained in Example Three was 87.7 nm, and the porosity was 66.47%, reaching the pore size of the ultrafiltration level.

[0076] Thermogravimetric analysis: the selection of sintering temperature is crucial for the formation of the ultrafiltration membrane. The FEP resin dispersion liquid was centrifugally dried to obtain FEP powder, and thermogravimetric analysis was performed on the PTFE base film, PEO and FEP particles, and the obtained TGA image is shown in Figure 3 The temperatures at which the masses of PEO and FEP began to lose were 331 ℃ and 454 ℃, respectively. In order to ensure the complete decomposition of PEO and the sufficient melting of FEP for bonding PTFE during the sintering process of the precursor fiber membrane, the sintering temperature should be greater than the decomposition temperature of PEO and less than the decomposition temperature of FEP. As can be seen from the figure, the sintering temperature should be selected to be greater than 340 ℃.

[0077] Surface morphology: the surface morphology of the PTFE ultrafiltration composite membrane obtained in Example Three is shown in Figure 4 As can be seen from the surface morphology image, the composite membrane did not swell significantly, which indicates that the attachment of FEP in the form of a fiber membrane or the coverage of FEP in the form of a large piece of membrane can ensure the morphological stability of the base membrane in a long-term solvent environment, and the anti-swelling performance is obviously improved.

[0078] Mechanical properties: The mechanical properties of the membrane samples before and after soaking were tested. The breaking strength of the PTFE ultrafiltration composite membrane obtained in Example Three decreased from 21.9 to 21.6 MPa after soaking in n-hexane solvent, and the elongation at break change rate was only 2.42%.

[0079] Swelling ratio: To detect the swelling resistance of the PTFE microfiltration composite membrane obtained in Example Three, it was placed in n-hexane solvent for 7 days to observe the changes in surface morphology and mechanical properties, and the swelling ratio after soaking in three non-polar solvents (chloroform, n-hexane, tetrahydrofuran) for 7 days was tested. The swelling ratio of the PTFE ultrafiltration composite membrane obtained in Example Three was less than 1% after soaking in n-hexane, tetrahydrofuran and chloroform for 7 days.

[0080] Example Four: Preparation of PTFE ultrafiltration composite membrane resistant to non-polar solvent swelling:

[0081] Step 1: 8 g of FEP resin dispersion was uniformly mixed with 192 g of isopropyl alcohol to prepare a microfiltration membrane finishing agent; the PTFE fiber membrane fixed on the stainless steel frame was immersed in the microfiltration membrane finishing agent for 20 min for crosslinking modification, then the immersed membrane was transferred to a drying device, and the heat treatment temperature was set to 360 ℃ to obtain a PTFE microfiltration membrane resistant to non-polar solvent swelling.

[0082] Step 2: 4 g of PEO was mixed with 96 g of deionized water and stirred at room temperature with a magnetic stirrer at a constant speed for 6 h to obtain a 4 wt% PEO solution; the prepared PEO solution was mixed with the FEP emulsion, and the ratio of PEO solution to FEP was 8:92, and the magnetic stirrer was uniformly stirred for 3 h to obtain a spinning solution; the spinning solution was injected into the spinneret of the electrostatic-centrifugal spinning machine, and the microfiltration membrane prepared in step 1 was placed on the conveyor belt to start centrifugal spinning; after spinning was completed, sintering at 370 ℃ was then performed to obtain a PTFE ultrafiltration membrane resistant to non-polar solvent swelling.

[0083] The ultrafiltration membrane obtained in step 2 was cut into a size of 10 cm x 10 cm together with a PP spun-bond non-woven fabric with a grammage of 870 g·m -2 -2, and then the ultrafiltration membrane was placed on the upper layer and the PP spun-bond non-woven fabric was placed on the lower layer, and hot rolling was performed for 3 min at a hot rolling temperature of 200 ℃ to obtain a PTFE ultrafiltration composite membrane resistant to non-polar solvent swelling.

[0084] The mechanical property test was carried out on the membrane samples before and after soaking. The breaking strength of the PTFE ultrafiltration composite membrane obtained in Example 4 was 22.16 MPa, and the breaking elongation was 98.16%. After 7 days of soaking experiment, the breaking strength was 21.69 MPa, and the breaking elongation increased to 102.25%. This shows that the swelling resistance of the PTFE ultrafiltration composite membrane obtained in Example 4 is also obviously better than that of the pure PTFE fiber membrane.

[0085] Peeling strength test: the peeling strength of the PTFE ultrafiltration composite membrane obtained in Example 4 was 8.507 N.

[0086] The above examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as a limitation on the scope of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are all within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A method for preparing a PTFE composite fiber membrane resistant to swelling in non-polar solvents, characterized in that, Includes the following steps: S1: Preparation of PTFE microfiltration membrane: A microfiltration membrane finishing agent is obtained by uniformly mixing a poly(perfluoroethylene propylene) resin dispersion with isopropanol. A PTFE fiber membrane is then impregnated in the microfiltration membrane finishing agent for crosslinking modification to obtain an impregnated membrane. The impregnated membrane is then heat-treated to obtain a PTFE microfiltration membrane resistant to swelling by non-polar solvents. S2: Preparation of PTFE ultrafiltration membrane: A polyethylene oxide solution is obtained by uniformly mixing polyethylene oxide and deionized water. An ultrafiltration membrane finishing agent is obtained by uniformly mixing the polyethylene oxide solution and polytetrafluoroethylene propylene resin emulsion. The ultrafiltration membrane finishing agent is injected into the spinneret of an electrostatic-centrifugal spinning machine, and the PTFE microfiltration membrane is centrifugally spun using the electrostatic-centrifugal spinning machine to obtain a spun membrane. The spun membrane is then solvent-dried and sintered to obtain a PTFE ultrafiltration membrane resistant to swelling by non-polar solvents. S3: Preparation of PTFE composite fiber membrane: The PTFE ultrafiltration membrane and PP spunbond nonwoven fabric were cut, cleaned and dried. The PTFE ultrafiltration membrane and PP spunbond nonwoven fabric were stacked in sequence and then hot-rolled to obtain a composite membrane sample. The composite membrane sample was transferred to a room temperature environment to cool and solidify to obtain a PTFE composite fiber membrane resistant to swelling by non-polar solvents.

2. The method for preparing a PTFE composite fiber membrane resistant to swelling by non-polar solvents according to claim 1, characterized in that, The polyethylene oxide solution contains 4 wt% polyethylene oxide, and the ratio of polyethylene oxide solution to polytetrafluoroethylene propylene resin emulsion in the ultrafiltration membrane finishing agent is 4:96~10:

90.

3. The method for preparing the PTFE composite fiber membrane resistant to swelling by non-polar solvents according to claim 1, characterized in that, The polyethylene oxide solution and the polytetrafluoroethylene propylene resin emulsion were magnetically stirred until the two phases were fully overlapped and there was no phase separation. The mixed solution was then allowed to stand in the dark to defoam and obtain the ultrafiltration membrane finishing agent.

4. The method for preparing a PTFE composite fiber membrane resistant to swelling by non-polar solvents according to claim 1, characterized in that, The PTFE microfiltration membrane was placed on the conveyor belt of the electrostatic-centrifugal spinning machine spinneret. The distance from the electrostatic-centrifugal spinning machine spinneret needle to the conveyor belt was 80-100 mm, and the spinning machine speed was set to 1400-1600 r·min. -1 .

5. A PTFE composite fiber membrane resistant to swelling by non-polar solvents, characterized in that, The membrane comprises a PTFE ultrafiltration membrane and a PP spunbond nonwoven fabric arranged in sequence, and is prepared by the method for preparing a PTFE composite fiber membrane resistant to swelling by nonpolar solvents according to any one of claims 1 to 4.

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