Preparation method of polyethylene microporous membrane, water treatment membrane and application

Abstract

The invention relates to a preparation method of a polyethylene microporous membrane, a water treatment membrane and application, the preparation method comprises the following steps: step 1, stirring, mixing and uniformly dispersing polyethylene, a branched compound containing at least three ethylene alkyl structures and a photoinitiator, and curing to obtain a mixture; step 2, adding the mixture into a screw extruder for shearing and mixing, and performing melt extrusion to obtain a polyethylene melt; 3, the polyethylene melt is conveyed into a static mixer through a melt pipeline, supercritical carbon dioxide fluid is guided into the melt pipeline, a uniform single-phase melt is prepared, then extrusion is conducted through an extrusion die head, casting and foaming are conducted, and a polyethylene microporous membrane is obtained; and 4, carrying out two-way stretching on the polyethylene microporous membrane, immediately carrying out ultraviolet irradiation while hot to carry out grafting crosslinking, and then washing off the unreacted branched compound by using a solvent to obtain the polyethylene microporous support membrane. The composite material is thin in thickness, high in porosity, good in thermal stability and low in requirement on a casting sheet mold, and can be used as a supporting layer of a water treatment membrane.

Description

Technical Field

[0001] This invention belongs to the field of polyethylene microporous membrane preparation technology, specifically relating to a method for preparing a polyethylene microporous membrane, a water treatment membrane, and its applications. Background Technology

[0002] Traditional water treatment membranes (RO membranes or nanofiltration membranes) typically use nonwoven fabric as a support layer, with a thickness usually ranging from 100 to 200 micrometers. This thickness limits the overall thickness of the finished membrane and consequently the number of membrane sheets that can be assembled in the membrane module. Furthermore, the pore size of the nonwoven support layer is mostly around 20 micrometers, resulting in a rough surface due to its large and uneven pore size. Therefore, during the fabrication of water treatment membranes, a polysulfone / polyethersulfone or polyvinyl alcohol interlayer is required to form a uniform separation layer on the surface, which further increases the thickness of the finished membrane. Polyethylene battery separators, due to their small thickness (typically only 5-40 micrometers) and small, uniform pore size, have been repeatedly tested as a support layer for water treatment membranes in recent years.

[0003] Polyethylene battery separators are typically made from polyethylene as raw material, with white oil as a pore-forming agent, through melt extrusion, casting, stretching to form pores, organic solvent extraction, secondary stretching, and heat setting. The problems with this process are: 1) The porosity of the polyethylene porous material prepared by bi-stretching with white oil as a pore-forming agent is low, only 30-40%; 2) To ensure improved mechanical strength and porosity of the separator, the stretching ratio during bi-stretching is usually 3-10 times, which leads to high residual stress during pore formation. This results in significant shrinkage and a reduction in membrane area when the membrane approaches its softening point. To address these issues, CN104681763B discloses a method utilizing supercritical carbon dioxide fluid... The foaming technology used to prepare polyethylene battery separators involves injecting supercritical fluid to create pores and reducing residual stress by decreasing the bistretch ratio. However, this only increases the membrane porosity to around 50%, and its increase is limited by the amount of supercritical fluid used and the stretch ratio. Therefore, it remains an unsuitable choice for a support layer in water treatment membranes. While it improves the membrane's thermal stability, reducing the heat recovery rate at 90°C to below 1%, it doesn't completely solve the problem of thermal shrinkage. This can lead to the separation of the support layer from its surface functional separation layer. These factors result in poor performance of polyethylene battery separators as support layers in water treatment membranes. Furthermore, when preparing polyethylene battery separators of the same thickness, the reduced bistretch ratio necessitates a reduction in sheet thickness during extrusion casting. However, if the extruded sheet is too thin, it can cause localized tearing into large holes, reducing the sheet's yield.

[0004] Therefore, developing a polyethylene microporous support membrane with high porosity, high bi-stretch ratio during production, and low thermal shrinkage rate, and its preparation method, is crucial for preparing thin, mechanically strong, highly porous, and uniformly pore-sized polyethylene microporous support membranes for use in water treatment membranes. Summary of the Invention

[0005] The purpose of this invention is to overcome the defects in the prior art and provide a method for preparing a polyethylene microporous membrane, a water treatment membrane and its application. The membrane is thin, has high porosity, good thermal stability and high casting yield, and can be used as a support layer for water treatment membranes.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a polyethylene microporous supported membrane includes the following steps: Step 1: Add polyethylene, a branched compound containing at least three ethylene alkyl groups, and a photoinitiator to a mixer, stir and mix them evenly, and then cure them to obtain a mixture; Step 2: Add the mixture to a screw extruder for shearing, mixing, and melt extrusion to obtain polyethylene melt; Step 3: The polyethylene melt is transported to the static mixer through the melt pipeline using a melt pump, and supercritical carbon dioxide fluid is introduced into the melt pipeline. The material is made into a homogeneous single-phase melt in the static mixer. Then the homogeneous single-phase melt is extruded through the extrusion die, and the casting foams to obtain a polyethylene microporous membrane. Step 4: Biaxially stretch the polyethylene microporous membrane, heat-set it after stretching, and then immediately irradiate it with ultraviolet light while it is still hot to graft crosslink the polyethylene with a branched compound containing at least 3 ethylene alkyl structures to obtain a polyethylene microporous support membrane.

[0007] As a further technical solution, the branched compound with at least three ethylene alkyl structures includes tetra-p-vinylphenylsilane, tetra(4- Vinylphenyl One or more of methane; As a further technical solution, the mass ratio of polyethylene, branched compound containing at least three ethylene alkyl structures, and photoinitiator is 100:(1-5):(0.1-0.5).

[0008] As a further technical solution, the polyethylene is one or both of high-density polyethylene and ultra-high molecular weight polyethylene; As a further technical solution, the high-density polyethylene has a density of 0.94–0.96 g / cm³ and a molecular weight of 200,000–500,000.

[0009] The photoinitiator includes one or more of fluorenone, acetylbenzene, methoxyxanthone, and benzophenone.

[0010] As a further technical solution, functional additives and / or auxiliaries are selectively added to the mixer.

[0011] As a further technical solution, the functional additives include one or more of anti-aging agents, antioxidants, and antibacterial agents; As a further technical solution, the additives include one or more of the following: diluents, nucleating agents, thickeners, and wetting agents.

[0012] As a further technical solution, in step 1, the stirring speed is 150-250 rpm; the maturation temperature is 35-45℃; and the maturation time is 1.5-2.5 h. In step 2, the screw extruder rotates at 80-100 rpm, and the extrusion temperature of the polyethylene melt is 160-210℃. As a further technical solution, in step 3, the supercritical carbon dioxide fluid is introduced at a pressure greater than or equal to 74 atm. As a further technical solution, the amount of supercritical carbon dioxide fluid introduced is: 3-5 mL of supercritical carbon dioxide fluid is introduced per kilogram of polyethylene. As a further technical solution, in step 3, the temperature of the static mixer is 145-150℃; the extrusion rate of the extrusion die is 0.5-1 m / min; As a further technical solution, in step 4, during biaxial stretching, the preheating temperature for longitudinal stretching is 60-90℃, the preheating time is 1.5-2.5min, the longitudinal stretching temperature is 105-115℃, and the longitudinal stretching ratio is 4-8 times; the preheating temperature for transverse stretching is 60-90℃, the preheating time is 1.5-2.5min, the transverse stretching temperature is 115-125℃, and the transverse stretching ratio is 4-8 times.

[0013] As a further technical solution, in step 4, the wavelength of the ultraviolet light is 250 nm, the irradiation time is 30 s, and the irradiation intensity is 0.3 J / cm². 2 ; As a further technical solution, in step 4, the heat setting temperature is 130℃ and the heat setting time is 3 minutes.

[0014] The polyethylene microporous support membrane prepared by the method described herein is used as a support layer in water treatment membranes, wherein the water treatment membrane is a reverse osmosis membrane or a nanofiltration membrane.

[0015] A water treatment membrane, wherein the water treatment membrane is a nanofiltration membrane, the nanofiltration membrane comprising a support layer, a polyvinyl alcohol intermediate layer and a functional separation layer arranged sequentially; the support layer is a polyethylene microporous support membrane prepared by the preparation method described above.

[0016] As a further technical solution, the nanofiltration membrane is a polyamide nanofiltration membrane, and the functional separation layer is a polyamide separation layer.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention utilizes supercritical carbon dioxide foaming technology to prepare polyethylene microporous membranes. Using polyethylene as the raw material, the porosity of the membrane is increased by increasing the amount of supercritical carbon dioxide. After bistretching, a branched compound containing at least three styrene molecules is used for crosslinking to repair the large pores formed by the extensive bistretching and stabilize the membrane's pore structure. This improves the uniformity of the membrane pores and avoids thermal shrinkage caused by residual stress from bistretching. Therefore, compared to existing technologies, this invention significantly improves the porosity, thermal stability, and casting yield of polyethylene microporous membranes at high bistretching ratios. It can produce thin, highly uniform, and thermally stable polyethylene microporous membranes, and the prepared polyethylene microporous membranes can replace nonwoven fabrics as a support layer for water treatment membranes.

[0018] 2. The vinyl-containing branched compound of this invention is tetra-p-vinylphenylsilane. The presence of silicon in it can also improve the mechanical strength of the membrane material and solve the problem of reduced mechanical strength caused by increased porosity.

[0019] 3. The more branched chains a vinyl-containing branched compound has, the better its support effect on the pore structure, resulting in a lower thermal shrinkage rate and better thermal stability of the polyethylene microporous membrane prepared from it. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] All raw materials used in this invention are commercially available.

[0022] In this invention, Polyethylene (HDPE): density 0.94–0.96 g / cm³, molecular weight 200,000–500,000, melt index 0.5–1.0 g / 10min, Shanghai Tinger Plastic Technology Co., Ltd. Antioxidant type: 1010 antioxidant; Manufacturer: BASF; Four pairs of vinylphenylsilanes: Shanghai Yukang Biotechnology Co., Ltd.; 4 (4- Vinylphenyl Methane: Wuhan Xinxin Jiali Biotechnology Co., Ltd. p-Divinylbenzene: Shanghai Huayuan Biochemical Technology Co., Ltd.; Unless otherwise specified, all raw materials used in this invention are commercially available.

[0023] Example 1 A polyethylene microporous support membrane, comprising the following raw materials in parts by weight: Polyethylene: 100 parts Four pairs of vinylphenylsilanes: 2.5 parts; Photoinitiator: 0.3 parts benzophenone Functional additives: 0.1 parts antioxidant; 4 mL of supercritical carbon dioxide fluid is injected per kilogram of polyethylene; Its preparation method includes the following steps: A polyethylene microporous support membrane, comprising the following raw materials in parts by weight: Step 1: Mix polyethylene, vinylphenylsilane, photoinitiator, and functional additives at 200 rpm for 10 min, then let stand at 40°C for 2 h to mature, and obtain the mixture. Step 2: Add the mixture to the screw extruder for shearing, mixing, melt extrusion into the melt pipe to obtain polyethylene melt; The screw extruder rotates at 90 rpm, and the extrusion temperature of the polyethylene melt is 190℃. Step 3: The polyethylene melt is transported to the static mixer through the melt pipeline using a melt pump, and supercritical carbon dioxide fluid is introduced into the melt pipeline at a pressure of 74 atm. The amount of supercritical carbon dioxide introduced is 4 mL per kilogram of polyethylene melt. The material is made into a homogeneous single-phase melt in the static mixer. Then the homogeneous single-phase melt is extruded through the extrusion die, and the casting foams to obtain a polyethylene microporous casting sheet. The temperature of the static mixer is 148℃; the die extrusion rate is 0.5-1 m / min. Step 4: Biaxially stretch the polyethylene microporous casting sheet, then heat set it at 130℃ for 3 minutes. Immediately after the heat setting is completed, irradiate it with 250 nm ultraviolet light for 30 seconds to carry out a crosslinking reaction to obtain a polyethylene microporous support film. For biaxial stretching, the preheating temperature for longitudinal stretching is 75℃, the preheating time is 2 minutes, the longitudinal stretching temperature is 110℃, and the longitudinal stretching ratio is 6 times; for transverse stretching, the preheating temperature is 75℃, the preheating time is 2 minutes, the transverse stretching temperature is 120℃, and the transverse stretching ratio is 6 times.

[0024] The intensity of ultraviolet light irradiation is 0.3 J / cm. 2 ; Under the same stretching ratio, this embodiment prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0025] Example 2 A polyethylene microporous support membrane, comprising the following raw materials in parts by weight: Polyethylene: 100 parts Four pairs of vinylphenylsilanes: 4 parts; Photoinitiator: 0.4 parts; Functional additives: 0.1 parts antioxidant; Inject 5 mL of supercritical carbon dioxide fluid per kilogram of polyethylene; Its preparation method includes the following steps: same as in Example 1; Under the same stretching ratio, this embodiment prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0026] Example 3 A polyethylene microporous support membrane, comprising the following raw materials in parts by weight: Polyethylene: 100 parts Four pairs of vinylphenylsilanes: 1.5 parts; Photoinitiator: 0.2 parts Functional additives: 0.1 parts antioxidant; 3 mL of supercritical carbon dioxide fluid is injected per kilogram of polyethylene; Its preparation method includes the following steps: Same as Example 1; Under the same stretching ratio, this embodiment prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0027] Example 4 A polyethylene microporous support membrane comprises the following raw materials in parts by weight: Same as in Example 1, except that tetra(4- Vinylphenyl Methane replaces the tetrap-vinylphenylsilane in Example 1; Its preparation method includes the following steps: same as in Example 1.

[0028] Under the same stretching ratio, this embodiment prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0029] Example 5 A nanofiltration membrane, the preparation method of which includes the following steps: Step 1, Preparation of hydrophilic intermediate layer: Using the Slotdie quantitative coating method, 0.5% polyvinyl alcohol solution was coated onto the surface of the polyethylene microporous support membrane prepared in Example 1, and dried to obtain the polyvinyl alcohol layer; Step 2, Preparation of the functional separation layer: A polyethylene microporous support membrane coated with a polyvinyl alcohol layer was immersed in an aqueous solution for 60 seconds. After removal, the membrane surface was dried with an air gun to ensure that there were no visible water droplets. Then, it was immersed in an organic solution for interfacial polymerization for 30 seconds to generate a polyamide separation membrane layer. After the reaction was completed, it was heat-treated in an oven at 70°C for 5 minutes to obtain a nanofiltration membrane. In the aqueous solution, the concentration of the multifunctional amine is 1 wt%, the concentration of the acid absorbent is 1 wt%, and the concentration of the surfactant is 0.1 wt%. The concentration of pyromellitic acid chloride in the organic phase solution is 0.1 wt%, and the solvent of the organic phase solution is n-hexane.

[0030] Example 6 A nanofiltration membrane, the preparation method of which includes the following steps: the same as in Example 5, except that the polyethylene microporous support membrane prepared in Example 4 is used.

[0031] Comparative Example 1 A polyethylene microporous support membrane comprises the following raw materials in parts by weight: Same as in Example 1, except that divinylbenzene is used instead of tetravinylphenylsilane in Example 1; Its preparation method includes the following steps: same as in Example 1.

[0032] Under the same stretching ratio, this comparative example prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0033] Comparative Example 2 A polyethylene microporous support membrane comprises the following raw materials in parts by weight: same as in Example 1; The preparation method includes the following steps: the same as in Example 1, except that in step 4, the ultraviolet light irradiation crosslinking reaction is carried out immediately while the film is hot after the film has cooled down, instead of the immediate action taken while the film is hot in Example 1.

[0034] Under the same stretching ratio, this comparative example prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0035] Comparative Example 3 A polyethylene microporous support membrane comprises the following raw materials in parts by weight: same as in Example 1; The preparation method includes the following steps: Same as Example 1, except that in step 4, the ultraviolet light irradiation crosslinking reaction is carried out before bistretching. Specifically, in step 4, before stretching the polyethylene microporous casting sheet, it is first kept at 125°C for 3 minutes, and then immediately irradiated with ultraviolet light for 1 minute while hot to carry out the crosslinking reaction, thereby obtaining a crosslinked polyethylene microporous casting sheet; then, after bistretching the crosslinked polyethylene microporous casting sheet, it is heat-set at 125°C for 3 minutes to obtain a polyethylene microporous membrane.

[0036] Under the same stretching ratio, this comparative example prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0037] Comparative Example 4 A polyethylene microporous support membrane comprises the following raw materials in parts by weight: Same as in Example 1, except that tetrap-vinylphenylsilane and photoinitiator are not added; Its preparation method includes the following steps: same as in Example 1; Step 1: Mix polyethylene and functional additives at 200 rpm for 10 minutes, then let stand at 40°C for 2 hours to mature, and obtain the mixture. Step 2: Add the mixture to a screw extruder for shearing, mixing, and melt extrusion to obtain polyethylene melt; The screw extruder rotates at 90 rpm, and the extrusion temperature of the polyethylene melt is 190℃. Step 3: The polyethylene melt is transported to the static mixer through the melt pipeline using a melt pump, and supercritical carbon dioxide fluid is introduced into the melt pipeline at a pressure of 74 atm. The amount of supercritical carbon dioxide introduced is 4 mL per kilogram of polyethylene melt. The material is made into a homogeneous single-phase melt in the static mixer. Then the homogeneous single-phase melt is extruded through the extrusion die, and the casting foam is obtained to obtain a polyethylene microporous membrane. The temperature of the static mixer is 148℃; the die extrusion rate is 0.5-1 m / min. Step 4: Biaxially stretch the polyethylene microporous casting sheet, and then heat-set it at 130℃ for 3 minutes to obtain a polyethylene microporous support film. For biaxial stretching, the preheating temperature for longitudinal stretching is 75℃, the preheating time is 2 minutes, the longitudinal stretching temperature is 110℃, and the longitudinal stretching ratio is 6 times; for transverse stretching, the preheating temperature is 75℃, the preheating time is 2 minutes, the transverse stretching temperature is 110℃, and the transverse stretching ratio is 6 times.

[0038] Under the same stretching ratio, this comparative example prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0039] Comparative Example 5 A polyethylene microporous support membrane comprises the following raw materials in parts by weight: same as comparative example 4; the difference being that 1 mL of supercritical carbon dioxide fluid is injected per kilogram of polyethylene. Its preparation method includes the following steps: Same as Comparative Example 4; Under the same stretching ratio, this comparative example prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0040] Comparative Example 6: A polyethylene microporous support membrane comprises the following raw materials in parts by weight: (Same as Comparative Example 4) The preparation method includes the following steps: the same as comparative example 4, except that in step 4, during bistretching, the transverse stretching ratio is 1.3 times and the longitudinal stretching ratio is 0.4 times; Under the same stretching ratio, this comparative example prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0041] Comparative Example 7: A polyethylene microporous support membrane comprises the following raw materials in parts by weight: the same as comparative example 4, except that 1 mL of supercritical carbon dioxide fluid is injected per kilogram of polyethylene. The preparation method includes the following steps: the same as comparative example 4, except that in step 4, during bistretching, the transverse stretching ratio is 1.3 times and the longitudinal stretching ratio is 0.4 times; Under the same stretching ratio, this comparative example prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0042] Comparative Example 8 A polyethylene microporous support membrane, comprising the following raw materials in parts by weight: Polyethylene: 100 parts Functional additives: 0.1 parts antioxidant; Step 1: Mix and disperse polyethylene and functional additives by high-speed stirring at 200 rpm for 10 min, then let stand at 40℃ for 2 h to obtain the mixture. Step 2: Add the mixture obtained in Step 1 into a twin-screw extruder for melt extrusion casting to obtain polyethylene sheets; Step 3: Biaxially stretch the polyethylene sheet and then heat-set it at 130℃ for 3 minutes to obtain a polyethylene microporous support film. For biaxial stretching, the preheating temperature for longitudinal stretching is 75℃, the preheating time is 2 minutes, the longitudinal stretching temperature is 110℃, and the longitudinal stretching ratio is 6 times; for transverse stretching, the preheating temperature is 75℃, the preheating time is 2 minutes, the transverse stretching temperature is 110℃, and the transverse stretching ratio is 6 times.

[0043] Under the same stretching ratio, this comparative example prepared polyethylene microporous support membranes with thicknesses of 10 micrometers, 20 micrometers, 30 micrometers and 40 micrometers by changing the thickness of the cast sheet.

[0044] Example of effect 1 The porosity, average pore size, maximum pore size, tensile strength, and thermal shrinkage rate of the prepared polyethylene microporous support membrane were tested, and the results are shown in Table 1. In addition, the unqualified polyethylene microporous membranes (M waste) and the finished polyethylene microporous support membranes (M finished) discarded during the production process were collected, weighed, and the pass rate of the polyethylene microporous membranes was calculated. The results are shown in Table 1. Porosity: Porosity was measured using the weighing density method.

[0045] Measurement of average and maximum pore size: The bubble point method (bubble test method) is used. The bubble point method is currently the most widely adopted standard measurement method in industry, based on the principle of gas-liquid displacement for pore size characterization. This method calculates pore size parameters by measuring the pressure required for gas to overcome the liquid blocking the pore. When the gas pressure gradually increases until the first continuous bubble appears on the sample surface, the pressure value at this point corresponds to the maximum pore size. According to the Young-Laplace equation, pore size is inversely proportional to pressure.

[0046] Tensile strength: Tested using a universal testing machine in accordance with Chinese national standard GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets". The specified specimen width is (15±0.1) mm, the initial clamping distance is (100±5) mm, and the test speed is (250±10) mm / min.

[0047] The test method for heat shrinkage rate is as follows: cut a 10×10mm polyethylene microporous support film, bake it in an oven at 110℃ for 1 hour, and measure its heat shrinkage by longitudinal and transverse stretching. The pass rate of polyethylene microporous membrane = 100% - Mwaste / (Mwaste + Mfinished) * 100%; Table 1

[0048]

[0049] From the data in Table 1, we can see that: 1) A comparison of the data from Example 1, Example 4, Comparative Example 1 and Comparative Example 4 shows that: tetra-p-vinylphenylsilane, tetra(4- Vinylphenyl Methane, p-divinylbenzene, and tetra(4-)-methane can all repair macropores in membranes, reducing pore size and maximum pore size; however, compared to p-divinylbenzene, tetra(4-)-methane and tetra(4-)-methane can also repair macropores in membranes, reducing pore size and maximum pore size. Vinylphenyl Methane has a large number of branches, and the presence of phenyl groups in these branches can improve the mechanical strength of the membrane and provide support for its pore structure, thus reducing its thermal shrinkage rate. Among them, tetra-p-vinylphenylsilane has the most significant effect on improving tensile strength due to the presence of silicon.

[0050] 2) As can be seen from the comparison of Example 1, Comparative Example 2 and Comparative Example 4, the ultraviolet radiation graft crosslinking of a branched compound containing at least 3 styrene molecules with polyethylene needs to be carried out under heating conditions. Under non-heating conditions, graft crosslinking cannot be achieved, the pore structure of the membrane cannot be supported and stabilized, and its thermal shrinkage rate cannot be reduced.

[0051] 3) As can be seen from the comparison of Example 1, Comparative Example 3 and Comparative Example 4: the ultraviolet radiation graft crosslinking of the branched structure compound containing at least 3 styrene molecules with polyethylene must be carried out after the bistretching process in order to greatly improve its thermal shrinkage rate and pore uniformity, prevent the pore tearing phenomenon during the bistretching process, and reduce the maximum pore radius; while thermal radiation crosslinking before bistretching can only improve its pore size and thermal shrinkage problem to a certain extent, but cannot completely solve its thermal shrinkage problem, and has little effect on the maximum pore size formed by tearing.

[0052] 4) Comparative data from Examples 4-8 show that adding a small amount of supercritical carbon dioxide and a small stretching ratio to Comparative Example 7 can improve porosity and reduce thermal shrinkage to a certain extent. However, further increasing the amount of supercritical carbon dioxide or increasing the stretching ratio based on Comparative Example 7 will not make the porosity, pore size and thermal shrinkage meet the requirements of the water treatment membrane support layer.

[0053] Example 2 The nanofiltration membrane prepared in Example 5 was soaked in water for 0.5 h. Using a membrane testing platform, the concentration of the original aqueous solution was tested at 2000 ppm MgSO4 aqueous solution, the operating pressure was 1.0 MPa, the temperature was 25 °C, and the test duration was 30 min. The water permeation flux and salt rejection rate of the membrane were tested. The results were as follows: the water flux of the nanofiltration membrane prepared in Example 5 was 25.47 GFD, and the salt rejection rate was 99.02%; the water flux of the nanofiltration membrane prepared in Example 6 was 25.04 GFD, and the salt rejection rate was 99.17%.

[0054] The embodiments described above are merely preferred embodiments of the present invention, and not an exhaustive list of all possible implementations of the present invention. Any obvious modifications made by those skilled in the art without departing from the principles and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a polyethylene microporous supported membrane, characterized in that, Includes the following steps: Step 1: Add polyethylene, a branched compound containing at least three ethylene alkyl groups, and a photoinitiator to a mixer, stir and mix them evenly, and then cure them to obtain a mixture; Step 2: Add the mixture to a screw extruder for shearing, mixing, and melt extrusion to obtain polyethylene melt; Step 3: The polyethylene melt is transported to the static mixer through the melt pipeline using a melt pump, and supercritical carbon dioxide fluid is introduced into the melt pipeline. The material is made into a homogeneous single-phase melt in the static mixer. Then the homogeneous single-phase melt is extruded through the extrusion die, and the casting foams to obtain a polyethylene microporous membrane. Step 4: Biaxially stretch the polyethylene microporous membrane, heat-set it after stretching, and then immediately irradiate it with ultraviolet light while it is still hot to graft crosslink the polyethylene with a branched compound containing at least 3 ethylene alkyl structures to obtain a polyethylene microporous support membrane.

2. The method for preparing a polyethylene microporous supported membrane according to claim 1, characterized in that, The branched compounds with at least three ethylene alkyl structures include tetra-p-vinylphenylsilane, tetra(4-... Vinylbenzene base One or more of methane.

3. The method for preparing a polyethylene microporous supported membrane according to claim 1, characterized in that, The mass ratio of polyethylene, a branched compound containing at least three ethylene alkyl groups, and a photoinitiator is 100:(1-5):(0.1-1). The polyethylene is one or both of high-density polyethylene and ultra-high molecular weight polyethylene. The photoinitiator includes one or more of fluorenone, acetylbenzene, methoxyxanthone, and benzophenone.

4. The method for preparing a polyethylene microporous supported membrane according to claim 1, characterized in that, The mixer also selectively contains functional additives and / or auxiliaries; The functional additives include one or more of the following: anti-aging agents, antioxidants, and antibacterial agents; The additives include one or more of the following: diluents, nucleating agents, thickeners, and wetting agents.

5. The method for preparing a polyethylene microporous supported membrane according to claim 1, characterized in that, In step 1, the stirring speed is 150-250 rpm; the maturation temperature is 35-45℃; and the maturation time is 1.5-2.5 h. In step 2, the screw extruder rotates at 80-100 rpm, and the extrusion temperature of the polyethylene melt is 160-210℃.

6. The method for preparing a polyethylene microporous supported membrane according to claim 1, characterized in that, In step 3, the supercritical carbon dioxide fluid is introduced at a pressure greater than or equal to 74 atm; The amount of supercritical carbon dioxide fluid introduced is 3-5 mL per kilogram of polyethylene. In step 3, the temperature of the static mixer is 145-150℃; the extrusion rate of the extrusion die is 0.5-1 m / min.

7. The method for preparing a polyethylene microporous supported membrane according to claim 1, characterized in that, In step 4, during biaxial stretching, the preheating temperature for longitudinal stretching is 60-90℃, the preheating time is 1.5-2.5 min, the longitudinal stretching temperature is 105-115℃, and the longitudinal stretching ratio is 4-8 times; the preheating temperature for transverse stretching is 60-90℃, the preheating time is 1.5-2.5 min, the transverse stretching temperature is 115-125℃, and the transverse stretching ratio is 4-8 times.

8. The method for preparing a polyethylene microporous supported membrane according to claim 1, characterized in that, In step 4, the wavelength of the ultraviolet light is 250 nm, the irradiation time is 30 s, and the irradiation intensity is 0.3 J / cm². 2 ; In step 4, the heat setting temperature is 125-135℃, and the heat setting time is 2-4 minutes.

9. The application of the polyethylene microporous support membrane prepared by the preparation method according to any one of claims 1-8 as a support layer in a water treatment membrane, characterized in that, The water treatment membrane includes a reverse osmosis membrane or a nanofiltration membrane.

10. A water treatment membrane, characterized in that, It includes a support layer, a polyvinyl alcohol intermediate layer, and a functional separation layer arranged sequentially; the support layer is a polyethylene microporous support membrane prepared by the preparation method according to any one of claims 1-8.

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