Preparation method of polypropylene cation exchange membrane
By grafting sulfonic acid groups and quaternary ammonium groups onto a polypropylene substrate and reinforcing it with sulfonated graphene, a heterogeneous cation exchange membrane was prepared, solving the problems of insufficient mechanical strength and lifespan of cation exchange membranes and realizing a nanochannel network with high strength and high conductivity.
Patent Information
- Application Number
- CN202511071926.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-31
- Publication Date
- 2025-11-07
AI Technical Summary
Domestically produced cation exchange membranes are insufficient in terms of mechanical strength and service life, making it difficult to meet the needs of large-scale industrial applications.
Sulfonic acid groups and quaternary ammonium groups were grafted onto a polypropylene substrate using plasma treatment and photochemical reaction, and reinforced with sulfonated graphene. A porous framework was formed through thermally induced phase separation and stretching orientation, and finally, a heterogeneous cation exchange membrane was prepared by combining it with a cation exchange resin.
This improved the mechanical strength and chemical stability of the membrane, enhanced its ionic conductivity, and formed a low-torsity nanochannel network, meeting the requirements of industrial applications.
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Abstract
Description
TECHNICAL FIELD
[0001] The application belongs to the technical field of film material preparation, and relates to a polypropylene cation exchange film preparation method. BACKGROUND
[0002] Membrane separation technology is a new and efficient separation technology, which is very suitable for modern industry, has the characteristics of energy saving, improving production efficiency, material reuse and environmental friendliness, and ion exchange membrane as an important component in membrane separation technology has been widely used in resource recycling, water treatment, fuel cells and seawater desalination and other fields. By comparing the removal efficiency of different types of cation exchange membranes (homogeneous and heterogeneous) in long-term experiments, it is found that the removal efficiency of homogeneous membrane is slightly higher than that of heterogeneous membrane, but the preparation process of heterogeneous membrane is relatively simple, the cost is lower, the production efficiency is higher, and it does not involve complex solution mixing and film forming process, so it has potential in large-scale production, which is of great significance for industrial application.
[0003] However, the membrane performance of domestic cation exchange membrane is still insufficient, which makes domestic heterogeneous ion exchange membrane only used for relatively simple primary industrial production. Especially in terms of mechanical strength and service life of the existing cation exchange membrane, it is not enough to meet the demand of large-scale use, so a polypropylene cation exchange film preparation method is needed to improve the stability and mechanical strength of the membrane. SUMMARY
[0004] In view of the above problems, the application provides a polypropylene cation exchange film preparation method, which solves the problems in the prior art.
[0005] In order to achieve the above purpose, the technical scheme adopted by the application is as follows:
[0006] A polypropylene cation exchange film preparation method, comprising the following steps:
[0007] Step S1, plasma treatment is performed on a polypropylene base material, and sulfonic acid groups -SO3H and quaternary ammonium groups -N are simultaneously grafted under photochemical reaction conditions to obtain a polypropylene master batch; + R3, to obtain a polypropylene master batch;
[0008] Step S2, the surface functionalized polypropylene master batch is mixed and dispersed with sulfonated two-dimensional carbon material to form a reinforced composite;
[0009] Step S3, the composite is mixed with an aprotic polar solvent to form a porous skeleton through thermally induced phase separation, and the porous skeleton is subjected to stretching and orientation treatment;
[0010] Step S4, the porous skeleton obtained in step S3 is compounded with a cation exchange resin, and a heterogeneous cation exchange membrane is obtained through hot pressing.
[0011] Preferably, in step S1, the plasma treatment is carried out in an oxygen atmosphere, with a gas flow of 10-15 cc, a pressure of 10-40 Pa, a power of 40-130 W, and a treatment time of 20-40 min.
[0012] The photochemical reaction uses an ethanol solution containing a sulfonic acid monomer and a quaternary ammonium monomer, and is carried out at 60-90°C under strong light for 2-4 h.
[0013] Preferably, the sulfonic acid monomer is selected from sodium styrene sulfonate or 2-acrylamide-2-methylpropane sulfonic acid, the quaternary ammonium monomer is selected from methacryloyloxyethyl trimethyl ammonium chloride or diallyl dimethyl ammonium chloride, and the molar ratio of -SO3H to -N + The molar ratio of R3 is 1:(0.9-1.1).
[0014] Preferably, in step S2, the sulfonated two-dimensional carbon material is sulfonated graphene with 1-3 layers and a flake diameter of 1-5 μm, and the addition amount is 0.1-0.5 wt% of the mass of the polypropylene masterbatch.
[0015] Preferably, in step S3, the cooling rate of the thermally induced phase separation is 0.5-2°C / min, and a bicontinuous pore structure with an average pore size of 10-50 nm is formed; the stretching orientation is uniaxial or biaxial stretching, and the stretching ratio is 1:2 to 1:5.
[0016] Preferably, in the biaxial stretching, the longitudinal and transverse stretching ratios are 1:3, and the tortuosity of the channels of the obtained porous framework is <1.5.
[0017] Preferably, in step S4, the mass fraction of the cation exchange resin is 52-64%, the mass fraction of the porous framework is 20-32%, and 5-10 wt% of polyisobutylene is added as a compatibilizer.
[0018] Preferably, in step S3, the aprotic polar solvent is [BMIM][PF6] and supercritical CO2, and the mixing temperature is 150-210°C.
[0019] Compared with the prior art, the present application has the following beneficial effects:
[0020] 1. The present application has a dual functional group synchronous grafting process, which achieves the purposes of improving the grafting rate of the groups and strengthening the chemical stability of the film.
[0021] 2. The present application has a sulfonated graphene bulk phase reinforcement process, which achieves the purposes of constructing a continuous proton transport channel and increasing the ionic conductivity.
[0022] 3. The present application has a stretching and pore-forming process, which achieves the purposes of forming a low-tortuosity nanochannel network and meeting the appropriate burst strength and flux decay rate. DETAILED DESCRIPTION
[0023] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only 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 without creative work fall within the protection scope of the present application.
[0024] The specific embodiments of the present application will be further described in detail below.
[0025] The present application comprises the following steps:
[0026] Pre-treatment of polypropylene (PP) substrate: washing the polypropylene substrate for 3 hours with ethanol through a Soxhlet extractor, washing for 3 hours with distilled water, and then drying under vacuum (room temperature RT) for 16 hours;
[0027] Step S1, plasma treatment is performed on the polypropylene substrate (0.5 g), and sulfonic acid groups (-SO3H) and quaternary ammonium groups (-N + R3)(-N + R3) are simultaneously grafted under photochemical reaction conditions to obtain a polypropylene masterbatch;
[0028] The plasma treatment is performed in an oxygen atmosphere, with a gas flow of 10-15 cc, a pressure of 10-40 Pa, a power of 40-130 W, and a treatment time of 20-40 min; in this process, the optimal conditions are a gas flow of 12 cc, a pressure of 25 Pa, a time of 30 min, and a power of 70 W;
[0029] The photochemical reaction uses an ethanol solution containing sulfonic acid monomers (-SO3H) and quaternary ammonium monomers (-N + R3) with a concentration of 5%, a xenon lamp is used for irradiation, the xenon lamp parameters are 10-30 mA, the system temperature is controlled at 60-90°C under strong light source for 2-4 h, and then alcohol washing and water washing are performed, and the product is obtained by drying under vacuum at 60°C;
[0030] The sulfonic acid monomers are selected from sodium styrene sulfonate (SSS) or 2-acrylamide-2-methylpropanesulfonic acid (AMPS), the quaternary ammonium monomers are selected from methacryloyloxyethyl trimethyl ammonium chloride (DMC) or diallyl dimethyl ammonium chloride (DADMAC), and the molar ratio of -SO3H to -N + R3 in the ethanol solution is 1:(0.9-1.1).
[0031] Step S2, mixing and dispersing the surface-functionalized polypropylene masterbatch with sulfonated two-dimensional carbon material to form a reinforced composite;
[0032] wherein the sulfonated two-dimensional carbon material is sulfonated graphene with a layer number of 1-3 layers and a flake diameter of 1-5 μm, and the addition amount is 0.1-0.5 wt% of the mass of the polypropylene masterbatch, and the surface sulfonic acid group density thereof is ≥0.8 mmol / g;
[0033] Specifically, the surface-functionalized polypropylene masterbatch and the graphene are first manually premixed for 10 min to preliminarily mix the graphene and the polypropylene masterbatch, and then a high-speed mixer is used to fully mix the materials for 10 min to make the graphene fully coated on the surface of the polypropylene masterbatch; then, the uniformly mixed materials are subjected to melt blending in a co-rotating twin-screw extruder, and the graphene and the polypropylene masterbatch mixed materials are repeatedly mixed at high speed by the twin screw under the action of repeated mechanical shearing force to make the graphene uniformly dispersed in the polypropylene matrix. The composite material extruded by the twin screw is cooled in a water cooling tank and then conveyed to a pelletizer for granulation for standby use; after that, the composite material must be dried before being subjected to forming processing to remove the water, otherwise the internal water will form bubbles in the processing to form defects in the material, therefore, after granulation, the composite material granules are fully dried in a constant-temperature drying oven at 80°C for 4 hours, and the granules are timely turned over during the drying process to ensure that the granules are fully dried, and the dried granules are marked as the functionalized composite;
[0034] The following effects are achieved by mixing the graphene with the polypropylene:
[0035] (1) significantly improving chemical stability and durability: PP membranes are prone to chemical degradation under strong acid / strong base, oxidizing environment (such as fuel cells, electrolytic cells) or high free chlorine conditions (such as water treatment). The dense sheet structure of graphene can form a physical barrier to block the direct contact of active substances (such as ·OH free radicals, H2O2, Cl - ) with the surface of the PP membrane;
[0036] (2) optimizing ion selectivity and conductivity: charge regulation: graphene can carry negative charges by surface modification (such as introducing -SO3H, -COOH), enhancing the cation selectivity of the membrane surface (electrostatic repulsion of anions); interface conduction: the oxygen-containing groups of the functionalized graphene can form a proton transport channel to improve the proton conductivity;
[0037] (3) inhibiting swelling and improving dimensional stability: cation exchange membranes are prone to swelling after absorbing water, leading to performance degradation. The hydrophobicity and two-dimensional constraint effect of graphene can limit the movement of polymer chains and reduce water intrusion.
[0038] Step S3, mixing the composite with aprotic polar solvent to form a porous framework by thermal phase separation, and performing stretching orientation treatment;
[0039] Wherein, the aprotic polar solvent is high-boiling-point solvent [BMIM] [PF6] and supercritical CO2 (solvent-free system), and the mixing temperature is 150-210℃;
[0040] Wherein, the cooling rate of thermally induced phase separation is 0.5-2℃ / min, and then the solvent is removed by extraction (such as water or ethanol) or volatilization to form a bicontinuous pore structure with an average pore size of 10-50nm; the stretching orientation is uniaxial or biaxial stretching, and the temperature is controlled at 120-160℃, and the stretching ratio is 1:2 to 1:5;
[0041] Further, the longitudinal and transverse stretching ratios of biaxial stretching are 1:3, and the tortuosity of the obtained porous framework is <1.5.
[0042] Step S4, the porous framework obtained in step S3 is compounded with a cation exchange resin, and a heterogeneous cation exchange membrane is obtained by hot pressing;
[0043] Specifically, the cation exchange resin (52%-64%) with a water content of 4%-10% (by mass, the same below) and the stretched material (20%-32%) are put into a high-speed mixer together, and are mixed at room temperature to form a uniform premix. Subsequently, the premix and 5% polyisobutylene are put into a torque rheometer, and the processing temperature is controlled at 180℃. After the torque value is stable, 10% coupling agent and 0.5% antioxidant additive are added one by one for further mixing. The mixing is continued for 15-20min until the torque value is stable again, and then the mixture is taken out. Thereafter, the solidified mixture is crushed again by a plastic crusher for subsequent processing, and is properly sealed for storage;
[0044] Then, the prepared mixture is placed in a mold, preheated at 200℃ for 30min, and then hot-pressed at a pressure of 10-30MPa and a temperature of 220℃ for 10-20min. After hot pressing, it is cooled to room temperature and the pressure is released, and then a polypropylene cation exchange membrane with a biomimetic micro-nano structure is obtained.
[0045] Example 1:
[0046] Pre-treatment of polypropylene (PP) substrate: the polypropylene substrate is washed with ethanol for 3 hours by a Soxhlet extractor, washed with distilled water for 3 hours, and then dried under vacuum (room temperature RT) for 16 hours;
[0047] Step S1, the polypropylene substrate (0.5g) is subjected to plasma treatment, and simultaneously grafted with sulfonic acid groups (-SO3H) and quaternary ammonium groups (-N + R3) under photochemical reaction conditions to obtain a polypropylene masterbatch;
[0048] The plasma treatment is performed in an oxygen atmosphere, with a gas flow of 12 cc, a pressure of 25 Pa, a power of 70 W, and a treatment time of 30 min.
[0049] The photochemical reaction uses an ethanol solution containing 5% of a sulfonic monomer (-SO3H) and a quaternary ammonium monomer (-N + R3) and is irradiated using a xenon lamp with a current parameter of 20 mA, and the system temperature is controlled at 80 DEG C under a strong light source for 3 h, and then washed with alcohol and water, and dried at 60 DEG C under vacuum to obtain;
[0050] The sulfonic monomer is selected from sodium styrene sulfonate (SSS) or 2-acrylamide-2-methylpropane sulfonic acid (AMPS), and the quaternary ammonium monomer is selected from methacryloyloxyethyl trimethyl ammonium chloride (DMC) or diallyl dimethyl ammonium chloride (DADMAC), and the molar ratio of -SO3H to -N + R3 in the ethanol solution is 1:1.
[0051] In step S2, the surface-functionalized polypropylene masterbatch is mixed and dispersed with the sulfonated two-dimensional carbon material to form a reinforced composite.
[0052] The sulfonated two-dimensional carbon material is sulfonated graphene with 1-3 layers and a flake diameter of 1-5 microns, and the addition amount is 0.3 wt% of the mass of the polypropylene masterbatch, and the surface sulfonic acid group density is greater than or equal to 0.8 mmol / g.
[0053] Specifically, the surface-functionalized polypropylene masterbatch and the graphene are manually pre-mixed for 10 minutes to preliminarily mix the graphene and the polypropylene masterbatch, and then a high-speed mixer is used to fully mix the materials for 10 minutes to fully coat the graphene on the surface of the polypropylene masterbatch. Then, the uniformly mixed materials are melt-blended in a co-rotating twin-screw extruder, and the graphene and the polypropylene masterbatch mixed materials are repeatedly mixed at high speed by the twin screw under the action of repeated mechanical shearing force to uniformly disperse the graphene in the polypropylene matrix. The composite material extruded by the twin screw is cooled in a water cooling tank and then conveyed to a pelletizer for granulation; after granulation, the composite material must be dried before being processed to remove moisture, otherwise the internal moisture will form bubbles during processing to form defects in the material. Therefore, after granulation, the composite material pellets are fully dried in a constant temperature drying oven at 80 DEG C for 4 hours, and the pellets are turned over in time during the drying process to ensure that the pellets are fully dried, and the functionalized composite is marked.
[0054] In step S3, the composite is mixed with an aprotic polar solvent to form a porous framework by thermal phase separation, and is subjected to a stretching orientation treatment.
[0055] Wherein, the aprotic polar solvent is high-boiling-point solvent [BMIM] [PF6] and supercritical CO2 (solvent-free system), and the mixing temperature is 150-210℃;
[0056] Wherein, the cooling rate of thermally induced phase separation is 1℃ / min, and then the solvent is removed by extraction (such as water or ethanol) or volatilization to form a bicontinuous pore structure with an average pore size of 10-50nm; the stretching orientation is biaxial stretching, and the temperature is controlled at 120-160℃, and the stretching ratio is 1:3.
[0057] Step S4, the porous framework obtained in step S3 is compounded with cation exchange resin, and a heterogeneous cation exchange membrane is obtained by hot pressing;
[0058] Specifically, the cation exchange resin with a water content of 4% to 10% (by mass, the same below) and the stretching material 30% are put into a high-speed mixer together, and are mixed at room temperature to form a uniform premix. Subsequently, the premix and 5% polyisobutylene are put into a torque rheometer, and the processing temperature is controlled at 180℃. After the torque value is stable, 10% coupling agent and 0.5% antioxidant additive are added one by one for further mixing. The mixing is continued for 15-20 min until the torque value is stable again, and then the mixture is taken out. Thereafter, the solidified mixture is crushed again by a plastic crusher for subsequent processing, and is properly sealed and stored;
[0059] Then, the prepared mixture is placed in a mold, preheated at 200℃ for 30 min, and then hot-pressed at a pressure of 10-30 MPa and a temperature of 220℃ for 10-20 min. After hot pressing, it is cooled to room temperature and the pressure is released, and a polypropylene cation exchange membrane with a biomimetic micro-nano structure is obtained, which is the polypropylene cation exchange membrane of specific embodiment 1.
[0060] Embodiment 2:
[0061] In order to verify the lower limit of the plasma treatment power, on the basis of specific embodiment 1, the power of the plasma treatment in step S1 is lowered to 40W, and the other preparation methods are the same as in embodiment 1, and the polypropylene cation exchange membrane of embodiment 2 is obtained.
[0062] Embodiment 3:
[0063] In order to verify the upper limit of the plasma power treatment, on the basis of specific embodiment 1, the power of the plasma treatment in step S1 is raised to 130W, and the other preparation methods are the same as in embodiment 1, and the polypropylene cation exchange membrane of embodiment 2 is obtained.
[0064] Embodiment 4:
[0065] To verify the lower limit of graphene, on the basis of embodiment 1, the addition amount of sulfonated graphene in step S2 is lowered to 0.1wt%, and other preparation methods are the same as embodiment 1, to obtain the polypropylene cation exchange membrane of embodiment 4.
[0066] Embodiment 5:
[0067] To verify the upper limit of graphene, on the basis of embodiment 1, the addition amount of sulfonated graphene in step S2 is increased to 0.5wt%, and other preparation methods are the same as embodiment 1, to obtain the polypropylene cation exchange membrane of embodiment 5.
[0068] Embodiment 6:
[0069] To verify the influence of biaxial stretching ratio and temperature reduction plastic, on the basis of embodiment 1, the cooling rate of thermally induced phase separation in step S3 is lowered to 0.5℃ / min, and the stretching ratio is adjusted to 1:2, and other preparation methods are the same as embodiment 1, to obtain the polypropylene cation exchange membrane of embodiment 6.
[0070] It should be noted that:
[0071] In the thermally induced phase separation (TIPS) process of step S3, the cooling rate is a core parameter for regulating the microstructure of the polymer porous membrane, especially for the formation of pore size (10-50nm) and channel network. Its mechanism involves the synergistic effect of phase separation kinetics, molecular chain movement ability and solvent crystallization behavior, which can be seen in the following table:
[0072] Cooling rate Diffusion time window Coarsening of phase field Final pore size Slow Wide Sufficient -> large pores merge 50-100 nm Medium Medium Partial -> pores uniform 30-50 nm Fast Narrow Inhibition -> small pores lock 10-30 nm
[0073] That is: the cooling rate rises→the molecular chain freezing time rises→the diffusion coefficient DD falls→the phase region growth time falls→the pore size falls.
[0074] Related principle explanation:
[0075] In the stretching composite forming process of step S3, the stretching ratio is a core process parameter for controlling the final porous structure (especially the pore size and channel network). Its influence mechanism involves multiple physical processes such as plastic deformation, phase separation, void nucleation and growth of the material microstructure. The following is the detailed mechanism of the influence of the stretching ratio on the formation of the pore size and channel network:
[0076] The influence mechanism of the stretching ratio on the pore size is as follows:
[0077] 1. Micropore nucleation and initial pore size control
[0078] Low temperature stretching stage (below the melting point of the polymer): the stretching stress acts on the amorphous region and the interface of the crystalline region, causing local plastic yield in weak areas (such as spherulite boundaries, amorphous regions).
[0079] Increasing stretch ratio intensifies stress concentration, activates more nucleation sites, and increases the number of initial pores.
[0080] Therefore: at the same porosity, high stretch ratio tends to form more but smaller initial pores.
[0081] 2. Pore size expansion and plastic deformation
[0082] High-temperature stretching phase (close to melting point): plastic deformation of crystalline regions (lamella slip, lamella separation).
[0083] Increasing stretch ratio further separates polymer chains / lamella, leading to longitudinal expansion of pores along the stretching direction.
[0084] Mechanism: small stretch ratio (λsmall): limited pore expansion, small pore size and narrow distribution.
[0085] Large stretch ratio (λlarge): greater plastic deformation, leading to stretching and thinning of pore wall material, resulting in significant increase in pore size (especially in the stretching direction).
[0086] Excessive stretching can lead to pore wall rupture, forming irregular large pores.
[0087] 3. Pore size distribution uniformity
[0088] Low stretch ratio: uneven deformation, prone to form structures with wide pore size distribution (uneven size).
[0089] High stretch ratio + uniform stretching: promotes overall plastic deformation coordination, resulting in more uniform pore size distribution.
[0090] In addition, in step S3, the influence mechanism of stretch ratio on channel network is as follows:
[0091] 1. Channel connectivity
[0092] Low stretch ratio: most pores are isolated or locally connected, forming closed or semi-connected structures. High channel tortuosity, poor permeability.
[0093] High stretch ratio: plastic deformation thins and ruptures the pore walls between adjacent pores, leading to pore merging, resulting in through channels. Channel network connectivity is significantly improved, forming a three-dimensional interconnected structure.
[0094] 2. Channel orientation and anisotropy
[0095] Uniaxial stretching: channels are highly oriented along the stretching direction (MD) → forming elongated channels. Few transverse channels, resulting in high anisotropy (e.g. longitudinal permeability of battery separator > transverse).
[0096] Biaxial stretching: balanced MD / TD stretch ratio, resulting in bidirectional expansion of the channel network in the plane, resulting in reduced anisotropy, approaching an isotropic network.
[0097] 3. Channel size and tortuosity
[0098] Increased stretch ratio: channel length increases (extends along the stretch direction). Channel width is controlled by the transverse stretch ratio: high TD ratio in biaxial stretching can widen the transverse channels.
[0099] Tortuosity: low stretch ratio: channels meander, resulting in high tortuosity. High stretch ratio: channels orient along the stretch direction, resulting in reduced tortuosity, resulting in improved fluid / ion transport efficiency.
[0100] The following tests were performed on the finished films of the six examples:
[0101]
[0102] The test results are as follows:
[0103]
[0104]
[0105] Through the above test results, the present application can achieve the following technical effects:
[0106] 1. Through the dual-function group synchronous grafting process, the purpose of improving the grafting rate of groups and strengthening the chemical stability of the film is achieved;
[0107] 2. Through the sulfonated graphene bulk phase enhancement process, the purpose of constructing continuous proton transport channels and increasing ion conductivity is achieved;
[0108] 3. Through the stretching and pore-forming process, the purpose of forming a low-tortuosity nanochannel network is achieved, which meets the appropriate burst strength and flux decay rate.
[0109] Although the present application 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 make equivalent substitutions for part of the technical features, as long as they are within the spirit and principles of the present application. Any modification, equivalent substitution, improvement, etc. made shall be included in the protection scope of the present application.
Claims
1. A method for preparing a polypropylene cation exchange membrane, characterized by: The method comprises the following steps: Step S1, plasma treatment of the polypropylene substrate and simultaneous grafting of sulfonic groups -SO3H and quaternary ammonium groups -N under photochemical reaction conditions + R3, obtaining a polypropylene masterbatch; Step S2, mixing and dispersing the surface-functionalized polypropylene masterbatch with the sulfonated two-dimensional carbon material to form a reinforced composite; Step S3, mixing the composite with an aprotic polar solvent to form a porous framework by thermally induced phase separation, and performing a stretching orientation treatment; Step S4, compounding the porous framework obtained in step S3 with a cation exchange resin, and obtaining a heterogeneous cation exchange membrane by hot pressing.
2. The method for preparing a polypropylene cation exchange membrane according to claim 1, characterized in that: In step S1, the plasma treatment is performed in an oxygen atmosphere, with a gas flow of 10-15 cc, a pressure of 10-40 Pa, a power of 40-130 W, and a treatment time of 20-40 min; The photochemical reaction uses an ethanol solution containing a sulfonic acid group monomer and a quaternary ammonium group monomer, and is performed at 60-90℃ under a strong light source for 2-4 h.
3. The method for preparing a polypropylene cation exchange membrane according to claim 2, characterized in that: The sulfonic acid group monomer is selected from sodium styrene sulfonate or 2-acrylamide-2-methylpropane sulfonic acid, the quaternary ammonium group monomer is selected from methacryloyloxyethyl trimethyl ammonium chloride or diallyl dimethyl ammonium chloride, and the molar ratio of -SO3H to -N + The molar ratio of R3 is 1:(0.9-1.1).
4. The method for preparing a polypropylene cation exchange membrane according to claim 1, characterized in that: In step S2, the sulfonated two-dimensional carbon material is sulfonated graphene with 1-3 layers and a flake diameter of 1-5 μm, and the addition amount is 0.1-0.5 wt% of the mass of the polypropylene masterbatch.
5. The method for preparing a polypropylene cation exchange membrane according to claim 1, characterized in that: In step S3, the cooling rate of the thermally induced phase separation is 0.5-2℃ / min, and a bicontinuous pore structure with an average pore size of 10-50 nm is formed; the stretching orientation is uniaxial or biaxial stretching, and the stretching ratio is 1:2 to 1:
5.
6. The method for preparing a polypropylene cation exchange membrane according to claim 5, characterized in that: The longitudinal and transverse stretching ratios of the biaxial stretching are 1:3, and the channel tortuosity of the obtained porous framework is <1.
5.
7. The method for preparing a polypropylene cation exchange membrane according to claim 1, characterized in that: In step S4, the mass fraction of the cation exchange resin is 52-64%, the mass fraction of the porous framework is 20-32%, and 5-10 wt% of polyisobutylene is added as a compatibilizer.
8. The method for preparing a polypropylene cation exchange membrane according to claim 1, characterized in that: In step S3, the aprotic polar solvent is [BMIM][PF6] and supercritical CO2, and the mixing temperature is 150-210℃.