Method for preparing seamless tube membrane by adopting closed-loop coating method

Abstract

The invention discloses a method for preparing a seamless tube membrane by adopting a closed-loop coating method, which comprises the following steps: dissolving a functional polymer with flexibility in a solvent to prepare a low-concentration membrane solution, and concentrating to obtain a concentrated coating solution; forming a closed-loop structure on the surface of a tubular support body by using the concentrated coating liquid in a brush coating or dip coating manner, airing to obtain a seamless layer, performing post-treatment on the seamless layer, and finally separating the seamless layer from the surface of the tubular support body to obtain the uniform seamless tubular membrane. Compared with the existing dialysis tubular membrane, the seamless tubular membrane has the advantages of various types, uniform thickness, high tensile strength, strong anti-pressure ability and difficulty in damage in a wet state, can be applied to separation of acid-containing or alkali-containing feed liquid in a diffusion dialysis process, has the advantages of high stability, strong water penetration resistance and good separation effect in a dialyzer, and is suitable for large-scale popularization and application. The seamless tube membrane can also be uniquely applied to separation of an alcohol-water solution in a permeation and volatilization process, shows the advantage of natural volatilization, and has a good separation effect.

Description

Technical Field

[0001] The present invention belongs to the field of membrane technology, and particularly relates to a method for preparing a seamless tube membrane by a closed-loop coating method. Background Art

[0002] Diffusion dialysis is a membrane process that uses the concentration difference across a membrane as the driving force to separate a feed solution. One side of the membrane is a high-concentration feed solution, and the other side is a low-concentration recovery solution. Driven by the concentration difference, active components such as H + or OH - ions can selectively permeate through the membrane into the recovery solution. After a period of time, the active components in the feed solution side are separated. Currently, common polymer membranes are in a flat shape. The flat membrane diffusion dialysis device is mainly of a plate-frame type. The device includes a certain number of repeating units, and each unit includes a dialysis chamber and a diffusion chamber separated by an ion membrane. The components mainly consist of an ion membrane, a liquid distribution plate, a reinforcing plate, a liquid flow plate frame, etc., and there are limitations such as many accessories, large weight, and narrow flow channels. The emerging tubular membrane is formed by bonding the two sides of a rectangular flat membrane to form a tubular structure, which can overcome various limitations of the flat membrane in the dialyzer. However, the bonding area of the tubular membrane has defects such as insufficient strength and possible leakage.

[0003] Pervaporation is to utilize the selectivity of the membrane for two different gas components. Driven by a pressure difference, the gas selectively permeates through the membrane to achieve the separation effect. A common separation system is an alcohol aqueous solution. Since the solution is in a liquid state at room temperature, it needs to be heated to vaporize. After vaporization, the alcohol and water are in a gaseous state. After contacting one side of the membrane, they selectively permeate through. In order to generate a pressure difference, usually, a vacuum is applied on the other side of the membrane to promote the permeation of the gaseous alcohol or water through the membrane. After permeating through the membrane, the alcohol or water is condensed and then transformed back into a liquid state. Therefore, the pervaporation process involves processes of vacuum reduction and phase change, consumes a large amount of energy, and the device is complex. Such a separation process cannot be implemented in a natural environment. For this reason, the present invention proposes a pervaporation process, that is, separating the alcohol aqueous solution without the processes of phase change and vacuum reduction.

[0004] The Chinese invention patent with the application number CN202210397895.1 discloses a mixed matrix membrane for acid diffusion dialysis, its preparation method and application. Specifically, UiO-66-(COOH)2 MOF nanoparticles centered on Zr metal are first prepared, and then quaternized polyphenylene oxide (QPPO) is prepared. The nanoparticles and the QPPO matrix are blended, and after coating and drying, a flat membrane is obtained. The advantage of this flat membrane is that high selective separation performance can be obtained with a low amount of nanoparticles used, improving the acid recovery efficiency. However, like most current optimization methods for the diffusion dialysis process, this solution only changes the membrane material, and problems such as the still too small effective area of the membrane and the complex installation of the flat membrane dialysis device have not been improved yet.

[0005] The Chinese invention patent with the application number CN202210465153.8 discloses a baffle frame type rapid diffusion dialysis membrane module and its application. The membrane module includes an ion membrane, a redirecting partition plate, and clamping plates arranged on both sides. Dialysis chamber compartments or diffusion chamber compartments are formed between the redirecting partition plate and the ion membranes on both sides, and the two types of compartments are arranged alternately. Through the redirecting partition plate, the dialysis chamber compartments are connected in series and the diffusion chamber compartments are connected in series. In this way, the flow path lengths of the raw liquid and the recovered liquid are significantly increased, improving the separation efficiency and the recovery rate. However, this solution still adopts the structure of a plate and frame type dialyzer, and does not simplify the structure of the dialyzer in essence. The problem of the bulky and cumbersome dialyzer has not been effectively solved.

[0006] The Chinese utility model patent with the application number CN202221750693.2 discloses a spiral wound diffusion dialysis module. An inlet and a feed port are installed at the end of the module. The internal membrane bag and the feed liquid flow path network are bonded to the central tube. The membrane bag and the feed liquid flow path network are arranged at intervals and wound around the central tube, so that the feed liquid evenly flows through the surface of the membrane bag and gives disturbance to the water flow. The inlet is connected to the central tube, and the feed port is connected to the area outside the central tube, thus solving the defect of leakage or dripping of acid-base liquids. This module is lighter than the traditional plate and frame type dialyzer, but has the defects of great difficulty in curling during assembly and being difficult to disassemble after forming the module.

[0007] The Chinese invention patent with the application number CN202210924758.9 discloses a preparation method and application of ultra-long tubular membranes by water bonding. The ultra-long tubular membranes are fixed on 3D-printed porous cylinders and then placed in a rectangular dialysis cell for diffusion dialysis. The preparation process of the tubular membranes is as follows: First, ordinary flat membranes are prepared, the flat membranes are cut into rectangular strips, the rectangular strips are wound around a cylindrical support, water is coated on one side edge of the rectangular strip to restore its adhesiveness, and after bonding the other side edge, tubes are formed. Multiple tubes are then connected to each other, and after heat treatment, ultra-long tubular membranes are obtained. The ultra-long tubular membranes are installed in a 3D-printed dialyzer and have the advantages of light weight, high packing density, and large processing capacity. However, there are three limitations to this type of tubular membrane: (1) The bonding layer is prone to rupture. The bonding layer is formed by the overlapping of two side edges. When the membrane swells in hot water or acid, the bonding layer may rupture. Especially when the pressure inside the tube increases, the bonding layer is more likely to burst. (2) It is difficult to bond polymers in non-PVA systems. Although the water bonding method is effective for PVA, it is difficult to prepare tubular membranes by bonding for polymers in non-PVA systems such as brominated polyphenylene ether (BPPO), ammoniumated polyphenylene ether (QPPO), and sulfonated polyphenylene ether (SPPO). (3) The separation ability of PVA-based tubular membranes is limited. Due to the large swelling degree of PVA, the swelling degree is even greater in hot water and high-concentration acidic environments, and the selectivity decreases during separation, resulting in insufficient membrane stability. Summary of the Invention

[0008] The purpose of the present invention is to overcome the above-mentioned deficiencies of the prior art and provide a preparation method for seamless tube membranes by using a closed-loop coating method. The seamless tube membranes prepared by this method have a dense and pore-free structure, no overlapping bonding layers, and are macroscopically in the shape of long tubes. They can replace traditional flat membranes and tubular membranes with bonding layers and be applied to the diffusion dialysis process to separate acid-containing or alkali-containing feed solutions, and can also be applied to the pervaporation process to separate alcohol aqueous solutions.

[0009] To achieve the above purpose, the present invention provides the following technical solutions:

[0010] A preparation method for seamless tube membranes by using a closed-loop coating method, comprising the following steps:

[0011] (1) Dissolve a flexible functional polymer in a solvent to form a low-concentration membrane solution with a mass concentration of 3-12 wt%, and concentrate the low-concentration membrane solution to obtain a concentrated coating membrane solution with a mass concentration of 13-35 wt%.

[0012] (2) Form a closed-loop structure on the surface of a tubular support by brushing or dipping the concentrated coating membrane solution, and after drying, obtain a seamless layer. Perform post-treatment on the seamless layer to obtain a seamless tube membrane.

[0013] In step (1), the flexible functional polymer is selected from polyvinyl alcohol (PVA), brominated polyphenylene oxide (BPPO), ammoniumated polyphenylene oxide (QPPO), sulfonated polyphenylene oxide (SPPO), polysilicon copolymer, blended SPPO / PVA, blended QPPO / PVA, blended PVA / polysilicon copolymer, or blended tetramethoxysilane (TMOS) / graphene oxide / PVA.

[0014] Preferably, in step (1), the solvent is selected from one or a combination of two or more of water, chlorobenzene, methanol, ethanol, or N,N-dimethylformamide (DMF) in any proportion.

[0015] Preferably, in step (1), the concentration method is to stir the low-concentration membrane solution at 45 - 90 °C or perform rotary evaporation on the low-concentration membrane solution.

[0016] Preferably, in step (2), the tubular support is a hollow polytetrafluoroethylene tube, plastic tube, or metal tube, and the outer diameter of the tube is 10 - 18 mm.

[0017] Preferably, in step (2), the brushing method is as follows: place the tubular support vertically, use a brush to brush the outer surface of the tubular support from bottom to top to form a closed-loop first layer. After natural evaporation until the surface has no stickiness, invert the tubular support and then place it vertically again, and continue to brush the second layer on the first layer. Repeat this brushing process 3 - 12 times to obtain a seamless layer.

[0018] Preferably, in step (2), the dipping method is as follows: place the tubular support vertically and insert its upper end into a cup-shaped container with a circular through-hole at the bottom. The circular through-hole is equivalent to the outer diameter of the tubular support. Add the concentrated coating solution into the cup-shaped container, and pull the cup-shaped container from top to bottom so that the concentrated coating solution is distributed around the tubular support in a closed-loop manner. After natural evaporation until the surface has no stickiness, form the first layer of coating. On the basis of the first layer, continue to dip the second layer of coating. Repeat this dipping process 3 - 6 times to obtain a seamless layer.

[0019] Application of the seamless tube membrane prepared by the above method in the diffusion dialysis separation of acid-containing or alkali-containing feed liquid.

[0020] Application of the seamless tube membrane prepared by the above method in the pervaporation separation of aqueous alcohol solution.

[0021] The diffusion dialysis is static, semi-dynamic, or dynamic diffusion dialysis:

[0022] When it is static diffusion dialysis, seal one end of the seamless tube membrane, vertically suspend it in the container, add the feed liquid into the seamless tube membrane, and add water into the container. Neither the liquid in the tube nor the mother liquid in the container flows, maintaining a static dialysis process;

[0023] In the case of semi-dynamic diffusion dialysis, a seamless tube membrane is installed in a dialysis device with a long flow channel. The two ports of the seamless tube membrane are respectively the inlet and outlet of the feed liquid, and the mother liquid does not flow in the dialysis device.

[0024] In the case of dynamic diffusion dialysis, a seamless tube membrane is installed in a dialysis device with a long flow channel. The two ports of the seamless tube membrane are respectively the inlet and outlet of the feed liquid, and the mother liquid flows in from the inlet of the dialysis device and flows out from the outlet of the dialysis device.

[0025] In the permeation evaporation process, an aqueous alcohol solution is filled into the seamless tube membrane, and the two ends of the seamless tube membrane are sealed. In a natural ventilation environment, water preferentially permeates through the membrane and reaches the outer surface, and then volatilizes into the air, while the alcohol remains concentrated in the seamless tube membrane.

[0026] Advantages of the present invention: The present invention uses pure polymers PVA, BPPO, QPPO, SPPO and blend polymers QPPO / PVA, SPPO / PVA, PVA / polysilicon copolymer, TMOS / graphene oxide / PVA as raw materials, and adopts a closed-loop coating method to prepare a uniform seamless tube membrane, greatly increasing the types of tube membranes. And because the tube membrane has a seamless structure, the mechanical properties and compressive capacity of the tube membrane are enhanced, avoiding the problem that the bonding part of the existing tubular membrane is easily damaged, and extending the service life of the tube membrane. Applying the seamless tube membrane of the present invention to a diffusion dialysis device, except for a peristaltic pump, only includes a seamless tube membrane and a container for holding the mother liquid, without the need for additional accessories, greatly simplifying the structure of the dialyzer. Using the seamless tube membrane of the present invention in combination with a dialysis device with a long flow channel has good separation effect on acids or alkalis, low water permeation, and large feed liquid treatment capacity. Applying the seamless tube membrane of the present invention to the permeation evaporation separation of an aqueous alcohol solution can concentrate the aqueous alcohol solution in a natural environment without the need for energy consumption provided artificially, and theoretically can overcome the limitations of large energy consumption and complex device of traditional pervaporation. Description of the Drawings

[0027] Figure 1 Photo of the seamless tube membrane prepared in Example 1;

[0028] Figure 2 Structural schematic diagram of a ridge-shaped dialysis device;

[0029] Figure 3 Photo of the seamless tube membrane prepared in Examples 4 and 5;

[0030] Figure 4 Photo of the seamless tube membrane prepared in Examples 6-9. Detailed Embodiments

[0031] The technical solution of the present invention will be described below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0032] Example 1: Preparation of PVA seamless tube film by brushing method

[0033] A preparation method for preparing a seamless tube film by a closed-loop coating method includes the following steps:

[0034] (1) Add 40 g of PVA to 760 mL of water. Under stirring, heat it to 96 °C at a heating rate of 15 °C / h, and continue stirring for 2.5 h to obtain a low-concentration film solution with a concentration of 5 wt%. Heat and stir the low-concentration film solution at 90 °C to volatilize the solvent, and then cool it to 55 °C to obtain a viscous PVA concentrated coating solution with a mass concentration of 14.5 wt%.

[0035] (2) Keep the temperature of the PVA concentrated coating solution at 55 °C. Place a hollow polytetrafluoroethylene tube with an outer diameter of 16 mm vertically on the ground. Then, use a brush with a width of 2.5 cm to dip the concentrated coating solution and brush it from bottom to top along the outer surface of the polytetrafluoroethylene tube to form a closed-loop first layer. After naturally volatilizing to no stickiness on the surface in a ventilated environment, invert the polytetrafluoroethylene tube and continue to place it vertically, and then continue to brush the second layer on the first layer. Repeat the brushing in this way until the 7th layer is brushed to complete the seamless layer. After complete drying, the thickness is 0.08 mm. Put the seamless layer together with the polytetrafluoroethylene tube into a forced-air oven, gradually heat it from 60 °C to 130 °C, control the heating rate to be 20 °C / h, and keep it at 130 °C for 4 h, and then cool it naturally. Finally, completely immerse the seamless layer together with the polytetrafluoroethylene tube in water for 6 h to make the seamless layer absorb water and swell. Then, pull the seamless layer with a force of 50 N. When the seamless layer moves relative to the polytetrafluoroethylene tube, reduce the pulling force to 10 N to make the seamless layer detach from the surface of the polytetrafluoroethylene tube to obtain the PVA seamless tube film.

[0036] After testing, the water content of the PVA seamless tube film prepared in this example is 118.4%, the swelling degree after soaking in water at 65 °C for 8 days is 297.5%, the tensile strength in the wet state is 10.4 MPa, and the elongation at break is 270%. Compared with the tubular film prepared by water bonding published in Chinese Patent CN202210924758.9 (the test results in the wet state are 0.94 MPa, 268%), the seamless tube film prepared in this example greatly improves the tensile strength.

[0037] Figure 1 The photo of the seamless tube film prepared for Example 1, by Figure 1It can be seen that the PVA seamless tube membrane can form a complete closed loop on the support. After heat treatment and water immersion, the seamless tube membrane swells and wrinkles, but can be completely detached from the support to obtain a long tubular structure.

[0038] The PVA seamless tube membrane prepared in Example 1 was used for dynamic diffusion dialysis to separate the H2SO4 / FeSO4 feed solution (2.98 M / 0.25 M): The structure of the ridge-shaped dialysis device was designed by Rhino 7 and Shapr3D software, and the ridge-shaped dialysis device was obtained after 3D printing. The structure is as Figure 2 shown. The dialysis device has a long flow channel. The dialysis device is 80 cm long, 4 cm wide, and 4 cm high. The outer diameter of the tube at both ends of the interface is 1.6 cm, and the inner diameter is 1.4 cm. The seamless tube membrane was installed into the ridge-shaped dialysis device. The feed solution was input from one port of the seamless tube membrane, and the residual liquid was output from the other port. Water in the ridge-shaped dialysis device flowed in from the inlet and flowed out from the outlet to recover the liquid. The H2SO4 / FeSO4 feed solution was input from one port of the seamless tube membrane at a rate of 80 mL / h, and the residual liquid was output from the other port. The outlet of the residual liquid was raised by 24 cm to reduce water penetration. Water in the 3D printed dialyzer flowed in from the inlet at a rate of 80 mL / h and flowed out from the outlet. The initial volume of water contained in the dialysis device was 500 mL.

[0039] The results of dynamic diffusion dialysis separation showed that the average volume increase of the mother liquor per hour within 6 h was 5.83 mL, indicating that water penetration could be effectively inhibited by increasing the pressure on the feed solution side, and even reverse osmosis of water could occur. After running for 6 h, [H + in the recovered liquid was 0.684 M, [Fe 2+ was 0.0033 M. The calculated recovery rate was 23.6%, and the rejection rate was 98.6%. At a relatively high water column pressure (water column height difference of 24 cm), the morphology of the seamless tube membrane remained unchanged and no leakage occurred, indicating high compressive capacity.

[0040] Example 2: Preparation of a PVA homogeneous seamless tube membrane by the dipping method

[0041] A preparation method for preparing a homogeneous seamless tube membrane by the closed-loop coating method includes the following steps:

[0042] (1) A low-concentration PVA membrane solution with a concentration of 5 wt% was heated and stirred at 90 °C to volatilize the solvent, and then cooled to 50 °C to obtain a viscous PVA concentrated coating solution with a mass concentration of 15 wt%.

[0043] (2) Keep the temperature of the PVA concentrated coating solution at 50 °C. Vertically place a seamless steel pipe with a diameter of 16 mm. Insert the upper end of the steel pipe into a plastic beaker with a volume of 1 L and a circular through-hole at the bottom. The diameter of the circular through-hole is equivalent to the outer diameter of the seamless steel pipe. Add 700 mL of the concentrated coating solution into the plastic beaker. Pull the plastic beaker from top to bottom at a speed of 50 cm / min, so that the concentrated coating solution is distributed around the steel pipe in a closed-loop manner, and naturally volatilize until the surface has no stickiness to form the first layer of coating. On the basis of the first layer, continue to dip-coat the second closed-loop coating layer, and repeat the dipping 5 times to obtain a seamless layer. After complete drying, pull the seamless layer with a force of 75 N to gradually separate it from the support. Put the seamless layer into a blast drying oven, gradually heat it from 60 °C to 130 °C, control the heating rate at 20 °C / h, and keep it at 130 °C for 4 h to obtain a PVA uniform seamless tube film.

[0044] After testing, the thickness of the PVA seamless tube film prepared in this example is 1.2 mm, the tensile strength in the wet state is 18.3 MPa, and the elongation at break is 277.8%. The PVA seamless tube film prepared in Example 2 was used for dynamic diffusion dialysis to separate the HCl / FeCl2 feed solution (1.19 M / 0.252 M). The results showed that at a flow rate of 225 mL / h, the acid recovery rate after stabilization was 69.3%, and the rejection rate was 90.4%.

[0045] Example 3: Preparation of a uniform TMOS / graphene oxide / PVA seamless tube film by brush coating

[0046] A preparation method for preparing a uniform seamless tube film by a closed-loop coating method, comprising the following steps:

[0047] (1) Pour 500 mL of a 5 wt% PVA solution into a three-necked flask, heat it to 80 °C, and stir mechanically at the same time. Mix 5 mL of graphene oxide (provided by Suzhou Carbon Feng Technology Co., Ltd.) and 20 mL of DMF, ultrasonically vibrate for 20 min, add the vibrated solution into the PVA solution in the flask within 0.5 h, and dropwise add 25 g of tetramethoxysilane (TMOS). Continue to heat and stir for 24 h, then pour the blend in the flask into a beaker, stir and volatilize at 80 °C, and concentrate the volume of the solution to 220 mL to obtain a concentrated coating solution with a mass concentration of 16 wt%.

[0048] (2) Keep the temperature of the TMOS / graphene oxide / PVA concentrated coating solution at 45 °C. Vertically place a hollow polytetrafluoroethylene tube with an outer diameter of 14 mm on the ground. Then, dip a brush with a width of 2 cm into the concentrated coating solution and brush it upward along the outer surface of the polytetrafluoroethylene tube in a circular motion to form the first closed-loop layer. After natural volatilization in a ventilated environment until the surface is non-sticky, invert the polytetrafluoroethylene tube and continue to place it vertically. Then, brush the second layer on the first layer. Repeat this brushing process until the fifth layer is completed to obtain a seamless layer. After complete drying, place the seamless layer together with the polytetrafluoroethylene tube in a forced-air oven and gradually heat it from 60 °C to 130 °C, controlling the heating rate at 20 °C / h, and keep it at 130 °C for 4 h. Then, let it cool naturally. Finally, completely immerse the seamless layer together with the polytetrafluoroethylene tube in water for 12 h to allow the seamless layer to absorb water and swell. Then, pull the seamless layer with a force of 40 N. When the seamless layer moves relative to the polytetrafluoroethylene tube, reduce the pulling force to 10 N to make the seamless layer detach from the surface of the polytetrafluoroethylene tube, obtaining the TMOS / graphene oxide / PVA seamless tube membrane.

[0049] After testing, the thickness of the TMOS / graphene oxide / PVA seamless tube membrane prepared in this example is 0.15 mm, the water content is 18.3%, the tensile strength in the wet state is 6.9 MPa, and the elongation at break is 129.3%.

[0050] Vertically suspend the TMOS / graphene oxide / PVA seamless tube membrane, fill 50 mL of an ethanol aqueous solution with a volume fraction of 50% into the tube, seal both ends of the seamless tube membrane, and sample at regular intervals in an environment with a room temperature of 2 - 6 °C and good ventilation to measure the change in the concentration of the solution in the tube. The results show that after 24 h, the concentration of the solution in the tube increases to 71.9%, after 48 h, the concentration increases to 80.3%, after 72 h, the concentration increases to 86.2%, and after 96 h, the concentration of the solution in the tube continues to increase to 93.3%. The ethanol rejection rate is 88%, indicating that through the process of pervaporation, the ethanol aqueous solution can be effectively concentrated.

[0051] Example 4: Preparation of a PVA / VBC polysilicon copolymer seamless tube membrane by the brushing method

[0052] A preparation method for preparing a seamless tube membrane by the closed-loop coating method, comprising the following steps:

[0053] (1) Preparation of VBC polysilicon copolymer with reference to the literature (J. Phys. Chem. B 2011, 115: 6474 - 6483), where the molar ratio of monomer vinylbenzyl chloride (VBC) to 3 - trimethoxysilylpropyl methacrylate (γ - MPS) is 3:7. After ammoniation, the ammoniated VBC polysilicon copolymer is added to a 3.2 wt% PVA solution, and the mass ratio of VBC polysilicon copolymer to PVA is 1:4. After reaction, a PVA / VBC polysilicon copolymer membrane solution with a total solute mass concentration of 4 wt% of PVA and VBC polysilicon copolymer is obtained; the PVA / VBC polysilicon copolymer membrane solution is stirred and concentrated at 85 °C to obtain a concentrated coating solution with a mass concentration of 16 wt%.

[0054] (2) Keep the temperature of the concentrated coating solution at 35 °C. Place a hollow polytetrafluoroethylene tube with an outer diameter of 16 mm vertically on the ground. Then, use a brush with a width of 5 cm to dip into the concentrated coating solution and brush it from bottom to top along the outer surface around the polytetrafluoroethylene tube to form a closed - loop first layer. After natural evaporation in a ventilated environment until the surface has no stickiness, invert the polytetrafluoroethylene tube and continue to place it vertically, and then continue to brush the second layer on the first layer. Repeat the brushing process until the 12th layer is completed to obtain a seamless layer. After complete drying, the thickness is 0.2 mm. Place the seamless layer together with the polytetrafluoroethylene tube in a forced - air oven, gradually heat from 60 °C to 130 °C, control the heating rate at 20 °C / h, and keep it at 130 °C for 4 h, then cool naturally. Finally, soak the seamless layer together with the polytetrafluoroethylene tube completely in water overnight (10 h) to make the seamless layer absorb water and swell. Then, pull the seamless layer with a force of 30 N. When the seamless layer moves on the surface of the support, adjust the pulling force to 15 N to make the seamless layer detach from the surface of the polytetrafluoroethylene tube, and obtain the PVA / VBC polysilicon copolymer seamless tube membrane.

[0055] After testing, the water content of the PVA / VBC polysilicon copolymer seamless tube membrane prepared in this example is 64.3%, and the swelling degree after 6 days in water at 65 °C is 283.1%; after soaking the seamless tube membrane in 1.68 M HCl solution for 60 h, the tensile strength is tested to be 15.96 MPa, and the elongation at break is 265.5%.

[0056] Static diffusion dialysis test: Seal the lower end of the PVA / VBC polysilicon copolymer seamless tube membrane prepared in Example 4, add 80 mL of H2SO4 / FeSO4 (2.85 M / 0.248 M) feed liquid into the tube, vertically suspend the seamless tube membrane in a graduated cylinder, add 500 mL of water into the graduated cylinder, and diffuse at a temperature of 10 °C for 6 h. Take samples from the graduated cylinder every 1 h to measure the change in ion concentration.

[0057] Test results: After 6 h, the concentration of H +The ion concentration is 0.34M, Fe 2+ The ion concentration is 0.0026M. The acid recovery rate is calculated to be 74.6%, and the rejection rate is 93.6%.

[0058] Example 5: Preparation of PVA / SSS polysilicon copolymer seamless tube membrane by brushing method

[0059] A preparation method for preparing a seamless tube membrane by a closed-loop coating method, comprising the following steps:

[0060] (1) The preparation of the SSS polysilicon copolymer refers to the reference (Desalination 2012, 304: 25–32). The molar ratio of the monomers sodium styrene sulfonate (SSS) and γ-MPS is 4:6. The SSS polysilicon copolymer is added to a PVA solution with a concentration of 5.5 wt%. The mass ratio of the SSS polysilicon copolymer to PVA is 1:4. After the reaction, a PVA / SSS polysilicon copolymer coating solution with a total solute mass concentration of 6.5 wt% of PVA and SSS polysilicon copolymer is obtained. The PVA / SSS polysilicon copolymer coating solution is stirred and concentrated at 85°C to obtain a concentrated coating solution with a mass concentration of 14 wt%.

[0061] (2) Keep the temperature of the concentrated coating solution at 35°C. Place a hollow polytetrafluoroethylene tube with an outer diameter of 16 mm vertically on the ground. Then, use a brush with a width of 3 cm to dip the concentrated coating solution and brush it from bottom to top along the outer surface of the polytetrafluoroethylene tube to form the first closed-loop layer. After natural evaporation in a ventilated environment until the surface has no stickiness, invert the polytetrafluoroethylene tube and continue to place it vertically, and then continue to brush the second layer on the first layer. Repeat the brushing until the 10th layer is completed to obtain a seamless layer. After complete drying, put the seamless layer together with the polytetrafluoroethylene tube into a blast drying oven, gradually heat it from 60°C to 130°C, control the heating rate at 10°C / h, and keep it at 130°C for 3 h, and then cool it naturally. Finally, soak the seamless layer together with the polytetrafluoroethylene tube in water overnight to make the seamless layer absorb water and swell. Under a tensile force of 18 N, the seamless layer can be detached from the surface of the polytetrafluoroethylene tube to obtain a PVA / SSS polysilicon copolymer seamless tube membrane.

[0062] Tests show that the water content of the seamless tube membrane prepared in this example is 103.9%, the tensile strength is 8.3 MPa, and the elongation at break is 160.0%.

[0063] Figure 3 Photos of the seamless tube membranes prepared for Example 4 and Example 5 are shown by Figure 3 It can be seen that the seamless tube membrane prepared from PVA and polysilicon copolymer has a completely seamless tubular appearance.

[0064] Static diffusion dialysis test: Vertically place the seamless tube membrane prepared in Example 5 in a graduated cylinder, seal the lower end, add 80 mL of NaCl / NaOH (1.68 M / 0.59 M) feed solution into the tube, add 1000 mL of water into the graduated cylinder, and conduct diffusion under static conditions.

[0065] Static diffusion dialysis shows that the recovery rate after 5 h is 70.6%, and the rejection rate is 73.8%.

[0066] Example 6: Preparation of QPPO seamless tube membrane by brush coating method

[0067] A preparation method for preparing seamless tube membrane by closed-loop coating method, comprising the following steps:

[0068] (1) Dissolve 20 g of brominated polyphenylene oxide (BPPO, provided by Shandong Tianwei Membrane Co., Ltd.) in 150 mL of chlorobenzene, and then perform rotary evaporation on the membrane solution at 60 °C to obtain a concentrated coating solution with a mass concentration of 30 wt%.

[0069] (2) Keep the temperature of the concentrated coating solution at 25 °C, vertically place a plastic tube with an outer diameter of 12 mm, dip a brush with a width of 3.0 cm into the concentrated coating solution, and brush the concentrated coating solution along the outer surface of the plastic tube in a bottom-up manner to form the first layer. After freely volatilizing in a fume hood until the surface has no stickiness, invert the plastic tube and continue to place it vertically, and then brush the second layer on the basis of the first layer. Repeat this process until the fourth layer is brushed to form a seamless layer on the surface of the plastic tube. Volatilize the dry solvent of the seamless layer in a naturally ventilated environment, and then immerse the seamless layer together with the plastic tube in a 10 wt% aqueous solution of trimethylamine for 2 days. After that, take it out, soak it in water for 18 h, change the water 4 times during the soaking period to remove the excess trimethylamine, and then pull the seamless layer away from the plastic tube with a force of 8 - 15 N to obtain an ammoniumated polyphenylene oxide (QPPO) seamless tube membrane.

[0070] Tests show that the water content of the seamless tube membrane prepared in this example is 15.9%, and the thickness in the wet state is 0.42 mm.

[0071] Static diffusion dialysis test: Load 30 mL of HCl / FeCl2 feed solution (1.20 M / 0.25 M) into the QPPO seamless tube membrane, then vertically suspend the seamless tube membrane in a graduated cylinder filled with 700 mL of water. Neither the feed solution in the tube nor the mother liquor in the graduated cylinder flows, and static diffusion dialysis is carried out at 30 °C.

[0072] Test results: After 5 h of the diffusion dialysis experiment, the concentration of H + ions in the mother liquor in the graduated cylinder is 0.037 M, and the concentration of Fe 2+ ions is 0.0014 M. Calculate that the recovery rate of acid is 71.9%, and Fe2+ The ion rejection rate is 87.2%.

[0073] Example 7: Preparation of QPPO / PVA seamless tube membrane by brushing method

[0074] A preparation method for preparing seamless tube membrane by closed-loop coating method, comprising the following steps:

[0075] (1) Preparation of QPPO / PVA concentrated coating solution: The preparation method of QPPO / PVA membrane solution refers to the reference (Journal of Membrane Science 2013, 428: 95–103), specifically as follows: Take 7 g of ammoniumated polyphenylene oxide (QPPO, produced by Shandong Tianwei Membrane Co., Ltd.), dissolve it in a mixed solvent composed of 50 mL of ethanol and 20 mL of N,N-dimethylformamide (DMF), add 0.78 g of tetraethoxysilane (TEOS), 0.90 g of phenyltriethoxysilane (EPh) and 1.44 g of water to the solution, stir and react at a temperature of 65 °C for 12 h, transfer the solution into a dropping funnel, and drop it into a PVA solution with a temperature of 65 °C, a volume of 450 mL and a concentration of 5 wt% within 1 h while maintaining stirring. After the dropping is completed, continue to stir and react for 12 h to obtain a membrane solution with a concentration of 5.5 wt%. Heat and stir the membrane solution at 80 °C to volatilize the solvent until the concentration of the membrane solution is 15 wt%.

[0076] (2) Keep the temperature of the concentrated coating solution at 35 °C, place a hollow polytetrafluoroethylene tube with an outer diameter of 16 mm vertically on the ground, then use a brush with a width of 5 cm to dip the concentrated coating solution and brush it from bottom to top along the outer surface around the polytetrafluoroethylene tube to form a closed-loop first layer. After naturally volatilizing to the surface without stickiness in a ventilated environment, invert the polytetrafluoroethylene tube and continue to place it vertically, and then brush the second layer on the first layer. Repeat the brushing in this way until the fourth layer is completed to obtain a seamless layer. After complete drying, put the seamless layer together with the polytetrafluoroethylene tube into a forced-air oven, gradually heat from 60 °C to 130 °C, control the heating rate at 20 °C / h, and keep it at 130 °C for 4 h, then cool naturally. Finally, soak the seamless layer together with the polytetrafluoroethylene tube completely in water overnight (9 h) to make the seamless layer absorb water and swell, so as to detach from the surface of the polytetrafluoroethylene tube to obtain the QPPO / PVA seamless tube membrane.

[0077] It is measured that the tensile strength of the QPPO / PVA seamless tube membrane prepared in this example in the dry state is 14.8 MPa, and the elongation at break is 85.5%.

[0078] Static diffusion dialysis test: Seal the lower end of the QPPO / PVA seamless tube membrane, add 25 mL of H2SO4 / FeSO4 (3.0 M / 0.25 M) feed solution into the tube, then vertically suspend the seamless tube membrane in a graduated cylinder filled with 500 mL of water, and conduct diffusion at a temperature of 30 °C to measure the change in ion concentration.

[0079] After measuring the static diffusion dialysis for 4 h, the concentration of H + ions in the recovered solution in the graduated cylinder is 0.12 M, and the concentration of Fe 2+ ions is 0.0021 M. The acid recovery rate is calculated to be 80.3%, and the rejection rate is 83.6%.

[0080] Example 8: Preparation of SPPO seamless tube membrane by brushing method

[0081] A preparation method for preparing a seamless tube membrane by a closed-loop coating method, comprising the following steps:

[0082] (1) Dissolve 20 g of sulfonated polyphenylene oxide (SPPO, sodium form, provided by Shandong Tianwei Membrane Co., Ltd.) in 186 mL of methanol, and then stir and volatilize at room temperature in a fume hood to obtain a concentrated coating solution with a mass concentration of 20 wt%.

[0083] (2) Keep the temperature of the concentrated coating solution at 25 °C, vertically place a plastic tube with an outer diameter of 16 mm, dip a brush with a width of 3.0 cm into the concentrated coating solution, and brush the concentrated coating solution around the outer surface of the plastic tube from bottom to top to form the first layer. After freely volatilizing in a fume hood until the surface is non-sticky, invert the plastic tube and continue to place it vertically, and then brush the second layer on the basis of the first layer. Repeat this process until the fourth layer is brushed to form a seamless layer on the surface of the plastic tube. Volatilize the dry solvent of the seamless layer in a naturally ventilated environment, then heat-treat it in an oven at 60 °C for 4 h, and after natural cooling, immerse the seamless layer together with the plastic tube in water for 8 h. Then pull the seamless layer with a force of 20 N. When the seamless layer moves on the plastic tube, reduce the pulling force to 10 N until the seamless layer detaches from the plastic tube to obtain the SPPO seamless tube membrane.

[0084] Tests show that the water content of the SPPO seamless tube is 88.0%, the length of the tubular membrane is 25 cm, the thickness in the wet state is 0.12 mm, the tensile strength is 7.9 MPa, and the elongation at break is 44.6%. There are bubbling phenomena after the membrane is immersed in water, which may be due to the introduction of a small amount of air bubbles during the brushing process.

[0085] Example 9: Preparation of SPPO / PVA seamless tube membrane by brushing method

[0086] A preparation method for preparing a seamless tube membrane by a closed-loop coating method, comprising the following steps:

[0087] (1) Take 30 g of sodium-type SPPO solid and soak it in 1.2 M HCl solution at room temperature. After 12 h, update the acid solution and continue soaking for another 12 h. After taking out the SPPO, soak it in deionized water and update the water 5 times within one day to obtain hydrogen-type SPPO (SPPO-H). Spread SPPO-H in a ventilated environment and let it dry naturally for 3 days. Stir and dissolve it in 100 mL of DMF at 65 °C. Drop 5.03 mL of tetraethoxysilane and 4.72 mL of phenyltriethoxysilane into the SPPO-H / DMF solution, stir at 60 °C for 6 h, and then drop it into 1800 mL of 5 wt% PVA solution at 60 °C within 1 h. Continue to stir at 60 °C for 12 h to obtain SPPO-H / PVA membrane solution. Put the membrane solution into a beaker, heat and stir it at 80 °C until it evaporates, until the concentration rises to 13 wt% to obtain SPPO-H / PVA concentrated coating solution.

[0088] (2) Keep the temperature of the concentrated coating solution at 35 °C. Place a hollow polytetrafluoroethylene tube with an outer diameter of 16 mm vertically on the ground. Then, use a brush with a width of 5 cm to dip the concentrated coating solution and brush it from bottom to top along the outer surface of the polytetrafluoroethylene tube to form a closed-loop first layer. After naturally evaporating in a ventilated environment until the surface has no stickiness, invert the tetrafluoroethylene tube and continue to place it vertically, and then continue to brush the second layer on the first layer. Repeat the brushing process until the sixth layer is completed to obtain a seamless layer. After complete drying, the thickness is 0.2 mm. Put the seamless layer together with the polytetrafluoroethylene tube into a blast drying oven, gradually heat it from 60 °C to 130 °C, control the heating rate at 20 °C / h, and keep it at 130 °C for 4 h, and then cool it naturally. Finally, soak the seamless layer together with the polytetrafluoroethylene tube completely in water overnight (12 h) to make the seamless layer absorb water and swell. Then, pull the seamless layer with a force of 30 N. When the seamless layer moves relative to the surface of the tetrafluoroethylene tube, reduce the pulling force to 15 N to make the seamless layer detach from the surface of the tetrafluoroethylene tube to obtain the SPPO / PVA seamless tube membrane.

[0089] Tests show that the length of the SPPO / PVA seamless tube membrane prepared in this example is 48 cm, the water content is 98.8%, the tensile strength in the wet state is 16.3 MPa, and the elongation at break is 363.0%, indicating that the membrane has good flexibility.

[0090] Figure 4 is a photo of the seamless tube membrane prepared for Examples 6-9, by Figure 4 It can be seen that using QPPO and SPPO as functional polymer raw materials can successfully prepare tube membranes, overcoming the defect that it is difficult to prepare PPO series tube membranes by the previous bonding method. The color of the prepared tube membrane is significantly darker than that of the PVA tube membrane, and the surface smoothness of the tube membrane with both PPO and PVA is better, without obvious wrinkling.

[0091] Finally, it should be noted that the above are only preferred examples of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements on some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a seamless tube film by a closed-loop coating method, characterized in that: The following steps are involved: (1) dissolving a flexible functional polymer in a solvent to form a low-concentration film solution with a mass concentration of 3 to 12 wt%, and concentrating the low-concentration film solution to obtain a concentrated coating solution with a mass concentration of 13 to 35 wt%; (2) applying the concentrated coating liquid to the surface of the tubular support by brushing or dipping to form a closed loop structure, drying to obtain a seamless layer, and post-treating the seamless layer to obtain a seamless tubular film; In step (1), the functional polymer with flexibility is selected from polyvinyl alcohol, brominated polyphenylene ether, ammonium polyphenylene ether, sulfonated polyphenylene ether, polysilicone copolymer, blended sulfonated polyphenylene ether / polyvinyl alcohol, blended ammonium polyphenylene ether / polyvinyl alcohol, blended polyvinyl alcohol / polysilicone copolymer or blended tetramethoxysilane / graphene oxide / polyvinyl alcohol.

2. The method for preparing a seamless tube film by a closed-loop coating method as claimed in claim 1, characterized in that: In step (1), the solvent is selected from one or a combination of two or more of water, chlorobenzene, methanol, ethanol or N,N-dimethylformamide in any proportion.

3. The method for preparing a seamless tube film by a closed-loop coating method as claimed in claim 1, characterized in that: In step (1), the concentration method is to stir the low-concentration membrane liquid at 45-90° C. or to perform rotary evaporation on the low-concentration membrane liquid.

4. The method for preparing a seamless tube film by a closed-loop coating method as claimed in claim 1, characterized in that: The tubular support in step (2) is a hollow polytetrafluoroethylene tube, a plastic tube or a metal tube, and the outer diameter of the tube is 10 to 18 mm.

5. The method for preparing a seamless tube film by a closed-loop coating method as claimed in claim 1, characterized in that: In step (2), the brushing method is: placing the tubular support body vertically, using a brush to brush the outer surface of the tubular support body from bottom to top to form a closed-loop first layer, and after naturally evaporating until the surface is non-sticky, the tubular support body is inverted and continued to be placed vertically, and then the second layer is continued to be brushed on the first layer, and the brushing is repeated for 3 to 12 layers to obtain a seamless layer.

6. The method for preparing a seamless tube film by a closed-loop coating method as claimed in claim 1, characterized in that: In step (2), the coating method is: placing the tubular support body vertically and inserting its upper end into a cup-shaped container having a circular through hole at the bottom, wherein the circular through hole is equivalent to the outer diameter of the tubular support body, adding concentrated coating liquid into the cup-shaped container, pulling the cup-shaped container from top to bottom, so that the concentrated coating liquid is distributed around the tubular support body in a closed loop manner, and after naturally evaporating until the surface is non-sticky, a first layer of coating is formed, and a second layer of coating is continuously coated on the basis of the first layer, and the coating is repeated for 3 to 6 layers to obtain a seamless layer.

7. Use of the seamless tubular membrane prepared by the method according to any one of claims 1 to 6 in the diffusion dialysis separation of acidic or alkaline liquids.

8. Use of the seamless tubular film prepared by the method according to any one of claims 1 to 6 in the separation of alcohol-water solution by osmotic volatilization.

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