A method for preparing an asymmetrically structured cationic membrane

Abstract

The application discloses a preparation method of a cation exchange membrane with an asymmetric structure. A negatively charged cation exchange membrane material and a water-soluble polymer additive are dissolved in an organic solvent to form a casting solution, which is then coated on a substrate. Subsequently, solvent evaporation-induced phase separation and immersion in water are performed to obtain a cation exchange membrane with an asymmetric structure. The method overcomes the problems of the traditional non-solvent phase inversion method for preparing a cation exchange membrane with an asymmetric structure, such as high requirements for the selection of a non-solvent bath, difficulty in accurately controlling the asymmetric structure and micro-phase separation structure, etc. Meanwhile, the obtained cation exchange membrane with an asymmetric structure has a low membrane surface resistance, which can meet the demand of processes such as diffusion dialysis and electrodialysis for high-performance cation exchange membranes.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of membrane technology, and particularly relates to a preparation method of asymmetric structure cation exchange membrane, and the obtained asymmetric structure cation exchange membrane can be used in application fields such as diffusion dialysis, electrodialysis, and electrodialfiltration, etc. with cation exchange membrane as the process core. BACKGROUND

[0002] It is well known that the thickness of the separation layer, the surface properties and the pore structure of the membrane have important influence on the flux of the membrane. Generally speaking, the thinner the separation layer is, the greater the flux is, but the too thin separation layer often leads to the decline of the mechanical properties of the membrane. Among them, the asymmetric separation membrane containing the porous layer support and the dense separation skin layer structure is an important method to solve the above problems. Moreover, this method has been widely used in the preparation of various different types and purposes of high performance separation membrane products, such as gas separation membrane, nanofiltration membrane, ultrafiltration membrane, reverse osmosis membrane, etc.

[0003] In the field of cation exchange membrane, in order to pursue high ion conductivity, researchers have also made a lot of attempts in the preparation of asymmetric cation exchange membrane, and the current reported preparation method of asymmetric cation exchange membrane is mainly non-solvent phase inversion method (NIPS). For example: Chen Dongju et al. reported a method for preparing asymmetric cation exchange membrane based on solvent phase inversion method, and the obtained cation exchange membrane has obvious asymmetric structure and high ion conductivity, which is a kind of excellent fuel cell separator material (application number: 201210541959.7). Wen Liping et al. reported a preparation method of asymmetric ion exchange membrane based on polyether sulfone main chain, which includes: polyether sulfone layer containing acidic groups in side chain and polyether sulfone layer containing basic groups in side chain, wherein the polyether sulfone layer containing acidic groups in side chain is prepared by non-solvent phase inversion method, and the above asymmetric ion exchange membrane is particularly suitable for reverse electrodialysis (Chinese patent application, application number: 202010567494.7). Yongsheng Chen et al. (Journal of Membrane Science, 2015, 473, 210-217.) prepared an asymmetric cation exchange membrane based on sulfonated polyphenyl ether by two-step phase inversion method, which showed good application prospect in salt differential energy utilization. Although the above non-solvent phase inversion method for preparing asymmetric cation exchange membrane shows good universality, the above solvent phase inversion method has the following problems in preparing asymmetric cation exchange membrane, that is, the functionalized cation exchange membrane material is generally hydrophilic, and the selection of non-solvent bath in the phase inversion process is relatively high (especially under the condition of high ion exchange capacity). In addition, this kind of method often also faces the problems of difficult accurate control of asymmetric structure and microphase separation structure. Therefore, the development of a new, simple and controllable asymmetric structure and microphase separation structure preparation method of asymmetric cation exchange membrane is proposed, which has important significance for the practical application and innovation of related technologies in this field. SUMMARY

[0004] The purpose of the present application is to provide a preparation method of asymmetric structure cation exchange membrane.

[0005] To achieve the above-mentioned task, the technical solutions adopted by the present application are as follows:

[0006] The preparation method of asymmetric structure cation exchange membrane is characterized in that: a negatively charged cation exchange membrane material is selected as a base material, a water-soluble polymer is selected as an additive, an organic solvent which is compatible with the negatively charged cation exchange membrane material and the additive is selected, and the asymmetric structure cation exchange membrane is prepared according to the following steps:

[0007] 1) First, the negatively charged cation exchange membrane material is fully stirred and dissolved in the organic solvent to obtain a first casting solution L-1;

[0008] 2) Then add the appropriate proportion of additives to the first casting solution L-1, fully stirred and dissolved to get the second casting solution L-2;

[0009] 3) The second casting solution L-2 is filtered, vacuum or ultrasonic defoaming to get the third casting solution L-3;

[0010] 4) The third L-3 is coated on the substrate, the solvent is evaporated at a suitable temperature, and finally fully immersed in water to obtain an asymmetric structure cation exchange membrane.

[0011] According to the application, the negatively charged cation exchange membrane material is a high molecular material containing negatively charged functional groups, and the polymer main chain structure is one or several of polyarylether sulfone, polyarylether ketone, polyphenyl ether, polystyrene, polyvinyl chloride, polyimide, polyamide, polyester, polyolefin, polybenzimidazole, and the negatively charged functional group is -R-SO3H. Among them, the negatively charged sulfonate is connected to the polymer main chain through R, and R represents a fatty chain structure with a length of 0-6 C, O backbone atoms.

[0012] Specifically, the organic solvent is a polar aprotic solvent, which is any one or several of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) or N-methyl pyrrolidone (NMP); and has the following characteristics:

[0013] (1) Good solubility for negatively charged cation exchange membrane material and additives;

[0014] (2) High boiling point, which is conducive to the control of the phase inversion speed of the cation exchange membrane material during solvent evaporation.

[0015] Further, the water-soluble polymer is a natural water-soluble polymer, which is starch, protein, alginic acid, gelatin; or a semi-synthetic water-soluble polymer, which is carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose; or a synthetic water-soluble polymer, which is one or several of polyacrylamide, polyacrylic acid and salt, polyvinylpyrrolidone, polyvinyl alcohol, polymaleic anhydride, polyether / polyether polyol, polyquaternary ammonium salt, polyethylene glycol, polyether amine, polyamide, polyethyleneimine, with a mass average molecular weight of 200-100000.

[0016] The concentration of the first casting solution L-1 is 3wt%-40wt%; the mass ratio of the additive to the negatively charged cation exchange membrane material is 0.05:1-2:1.

[0017] The substrate is a metal plate, a glass plate, a ceramic, a polymer plate or a fabric.

[0018] The temperature range for the evaporation of the solvent is 25-180 DEG C, the evaporation time is 5 min-48 h, and the soaking time in water is 1-120 h.

[0019] The preparation method of the asymmetric structure cation exchange membrane of the application realizes the preparation of the asymmetric structure cation exchange membrane with an ultra-thin separation layer and a uniform macroporous support layer by the difference in physical and chemical properties between the cation exchange membrane material and the water-soluble polymer additive and the good compatibility between the two through solvent evaporation induction and water etching of the additive.

[0020] Compared with the prior art, the technical innovation of the application lies in that:

[0021] 1. The method has the characteristics of universality, simplicity, easy operation, low cost and easy large-scale industrial application.

[0022] 2. The method overcomes the problems in the preparation of the asymmetric structure cation exchange membrane by the traditional non-solvent phase inversion method, such as high requirement for the selection of the non-solvent bath, difficulty in accurate control of the asymmetric structure and micro-phase separation structure.

[0023] 3. The obtained asymmetric structure cation exchange membrane has a low membrane surface resistance, which can meet the demand of the process with the cation exchange membrane as the core, such as diffusion dialysis and electrodialysis, for high-performance cation exchange membranes. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The morphological diagram of the asymmetric structure cation exchange membrane prepared for Example 1, wherein (a) is the surface morphology of the dense layer, and (b) is the overall cross-sectional morphology of the membrane.

[0025] Figure 2 The morphological diagram of the asymmetric structure cation exchange membrane prepared for Example 2, wherein (a) is the surface morphology of the dense layer, and (b) is the overall cross-sectional morphology of the membrane.

[0026] Figure 3 The morphological diagram of the asymmetric structure cation exchange membrane prepared for Example 3, wherein (a) is the surface morphology of the dense layer, and (b) is the overall cross-sectional morphology of the membrane.

[0027] Figure 4 The morphological diagram of the asymmetric structure cation exchange membrane prepared for Example 4, wherein (a) is the surface morphology of the dense layer, and (b) is the overall cross-sectional morphology of the membrane.

[0028] Figure 5The morphology of the asymmetric structure cation exchange membrane prepared in Example 5 is shown in the figures, wherein, (a) is the surface morphology of the dense layer, and (b) is the overall cross-section morphology of the membrane.

[0029] Figure 6 The morphology of the asymmetric structure cation exchange membrane prepared in Example 6 is shown in the figures, wherein, (a) is the surface morphology of the dense layer, and (b) is the overall cross-section morphology of the membrane.

[0030] Figure 7 The morphology of the asymmetric structure cation exchange membrane prepared in Example 7 is shown in the figures, wherein, (a) is the surface morphology of the dense layer, and (b) is the overall cross-section morphology of the membrane.

[0031] The application is further described in detail below in combination with the accompanying drawings and examples. DETAILED DESCRIPTION

[0032] In the following examples, the negatively charged cation exchange membrane material (P) is a high molecular material containing negatively charged functional groups. The main chain structure of these high molecular materials is mainly one or several of polyarylether sulfone, polyarylether ketone, polyphenyl ether, polystyrene, polyvinyl chloride, polyimide, polyamide, polyester, polyolefin, and polybenzimidazole, and the negatively charged functional group is mainly -R-SO3H. The negatively charged sulfonic acid group is connected to the polymer main chain through R, and R represents a fatty chain structure with a length of 0-6 C, O backbone atoms.

[0033] The water-soluble polymer is a natural water-soluble polymer, which is starch, protein, alginic acid, or gelatin; or a semi-synthetic water-soluble polymer, which is carboxymethyl cellulose, methyl cellulose, ethyl cellulose, or hydroxyethyl cellulose; or a synthetic water-soluble polymer, which is one or several of polyacrylamide, polyacrylic acid and salt, polyvinylpyrrolidone, polyvinyl alcohol, polymaleic anhydride, polyether / polyether polyol, polyquaternary ammonium salt, polyethylene glycol, polyether amine, polyamide, and polyethylene imine, with a mass average molecular weight of 200-100000.

[0034] The solvent is an organic solvent that has good compatibility with the negatively charged cation exchange membrane material and the additive.

[0035] The organic solvent is a polar aprotic solvent, which is any one or several of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or N-methyl pyrrolidone (NMP); and has the following characteristics:

[0036] (1) good solubility for the negatively charged cation exchange membrane material and the additive;

[0037] (2) High boiling point, which is beneficial to control the phase inversion speed of the cation exchange membrane material during solvent evaporation.

[0038] The embodiment provides a preparation method of an asymmetric structure cation exchange membrane, and the steps are as follows:

[0039] Step 1: The above negatively charged cation exchange membrane material is dissolved in the above polar aprotic solvent to prepare a first casting solution L-1 with a mass fraction of 3wt%-40wt%. The concentration of the first casting solution L-1 is preferably 5wt%-20wt%.

[0040] Step 2: Add an additive to the first casting solution L-1.

[0041] The additive has the following requirements:

[0042] First, the solubility in the polar aprotic solvent is much greater than that of the negatively charged cation exchange membrane material; in this way, the rapid phase inversion of the negatively charged cation exchange membrane material on the surface can be realized during the dissolution and evaporation process, and the additive is not.

[0043] Second, the additive contains charged hydrophilic groups such as carboxylic acid groups, sulfonic acid groups, phosphoric acid groups and quaternary ammonium groups, or contains hydroxyl groups, amino groups and amide groups which can easily form hydrogen bond donors or acceptor groups or elements, which can form a certain interaction between the molecules of the positively charged anion exchange membrane material through electrostatic interaction or hydrogen bond force, which is beneficial to the highly uniform dispersion of the negatively charged cation exchange membrane material and the additive in the solvent, and further beneficial to the formation of a relatively uniform microphase separation structure during the solvent evaporation phase inversion process.

[0044] Third, the physical and chemical properties of the additive and the negatively charged cation exchange membrane material are obviously different, which is beneficial to the microphase separation between the molecules of the negatively charged cation exchange membrane material and the additive in the later solvent evaporation stage.

[0045] Fourth, the additive has good solubility in water and can be removed by soaking in water solution in the later stage, further forming a uniform asymmetric macroporous structure.

[0046] In the embodiment, the additive can be selected from water-soluble polymers, preferably any one or several of carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, polyacrylamide, polyvinylpyrrolidone, polyvinyl alcohol, polyether (polyether polyol), polyquaternary ammonium salt, polyethylene glycol, polyether amine, polyamide and the like, and the mass average molecular weight of the additive is preferably between 200 and 60000.

[0047] The mass ratio of the additive to the negatively charged cation exchange membrane material is 0.05:1-2:1, preferably 0.1:1-1.8:1, and the specific additive amount can be adjusted according to the chemical structure of the negatively charged cation exchange membrane material and the physical and chemical structure of various different types of additives.

[0048] After the addition of the additive is completed, the second casting solution L-2 is obtained by fully stirring at 20-80°C for 1-24h until it is completely dissolved.

[0049] Step 3: The second casting solution L-2 is filtered and vacuum or ultrasonic degassed for 1min-24h to obtain the third casting solution L-3.

[0050] Step 4: The third casting solution L-3 is uniformly coated on any one of the substrates of metal plate, glass plate, ceramic, high polymer plate or fabric. Evaporation is carried out at 25-150°C for 5min-48h, preferably at 50-100°C for 3h-24h. The important purpose of solvent evaporation is to realize phase inversion of the negatively charged cation exchange membrane material (formation of dense separation layer and preliminary asymmetric structure) and microphase separation between the negatively charged cation exchange membrane material and the additive molecules.

[0051] Step 5: After the solvent evaporation step, the above-mentioned substrate membrane sheet is soaked in RO water or UP water at room temperature for 1h-120h. Preferably, the soaking time in water is 1h-48h. The main purpose of sufficient soaking in water is to remove the additive molecules in the membrane to form a uniform asymmetric macroporous support layer structure.

[0052] The following is a specific example given by the inventor.

[0053] Example 1:

[0054] This example gives a method for preparing an asymmetric ion exchange membrane from a polymer: this example starts from a main chain containing polyarylether sulfone, and the ion exchange capacity is 1.51mmol / g, and the polymer structural formula is as follows:

[0055]

[0056] 0.5g of the above polymer is dissolved in 5mL of organic solvent DMAc, and a magnetic stirrer is used for stirring at room temperature for 1h to obtain the first casting solution L-1.

[0057] 0.3g of polyethylene glycol 1000 is added to the first casting solution L-1, and the stirring and dissolution are continued at room temperature for 6h to obtain the second casting solution L-2. The second casting solution L-2 is filtered using a G2 type sand core funnel, and vacuum degassing is carried out for 30min to obtain the third casting solution L-3.

[0058] The third casting solution L-3 was poured on the glass plate at a constant speed, dried in an oven at 100°C for 6h, and after the solvent was evaporated, the glass plate was immersed in deionized water for 12 hours to obtain the asymmetric cation exchange membrane.

[0059] The morphology of the asymmetric cation exchange membrane was observed by scanning electron microscopy.

[0060] The surface morphology of the dense layer of the asymmetric cation exchange membrane prepared in this example is shown in Figure 1 (a), and Figure 1 (b) is the overall cross-sectional morphology of the asymmetric cation exchange membrane. It can be seen that the asymmetric cation exchange membrane prepared in this example exhibits a clear asymmetric structure, and the porous layer prepared can be seen from the cross-section to be a relatively uniform sponge-like porous structure.

[0061] Example 2:

[0062] The difference between this example and Example 1 is that the polymer used in this example has the following structure:

[0063]

[0064] The additive used is polyethylene glycol 1000 with an addition mass of 0.5g, and other conditions are the same as in Example 1. The morphology characterization method of the asymmetric cation exchange membrane prepared in this example is the same as in Example 1. The ion exchange capacity of the obtained membrane is 1.85mmol / g.

[0065] The surface morphology of the dense layer of the asymmetric cation exchange membrane prepared in this example is shown in Figure 2 (a), and Figure 2 (b) is the overall cross-sectional morphology of the asymmetric cation exchange membrane.

[0066] Example 3:

[0067] The difference between this example and Example 1 is that the polymer used in this example has the following structure:

[0068]

[0069] The additive used is polypropylene glycol 400 with an addition mass of 0.7g, and other conditions are the same as in Example 1. The morphology characterization method of the asymmetric cation exchange membrane prepared in this example is the same as in Example 1. The ion exchange capacity of the obtained membrane is 1.47mmol / g.

[0070] The surface morphology of the dense layer of the asymmetric cation exchange membrane prepared in this example is shown in Figure 3 (a), and Figure 3 (b) is the overall cross-sectional morphology of the asymmetric cation exchange membrane.

[0071] Example 4:

[0072] The difference between this embodiment and embodiment 1 is that the structure of the polymer used in this embodiment is as follows:

[0073]

[0074] The additive used is fatty alcohol polyoxyethylene ether (Mw ~ 590, CAS: 68131-39-5) with an added mass of 0.7 g, and other conditions are the same as in embodiment 3. The morphology characterization method of the asymmetric cation exchange membrane prepared in this embodiment is the same as in embodiment 1. The ion exchange capacity of the obtained membrane is 1.44 mmol / g.

[0075] The surface morphology of the dense layer of the asymmetric cation exchange membrane prepared in this example is as shown in Figure 4 (a), and Figure 4 (b) is the overall cross-sectional morphology of the asymmetric cation exchange membrane.

[0076] Embodiment 5:

[0077] The difference between this embodiment and embodiment 1 is that the structure of the polymer used in this embodiment is as follows:

[0078]

[0079] The additive used is polyethylene glycol 600 with an added mass of 0.5 g, and other conditions are the same as in embodiment 1. The morphology characterization method of the asymmetric cation exchange membrane prepared in this embodiment is the same as in embodiment 1. The ion exchange capacity of the obtained membrane is 1.77 mmol / g.

[0080] The surface morphology of the dense layer of the asymmetric cation exchange membrane prepared in this example is as shown in Figure 5 (a), and Figure 5 (b) is the overall cross-sectional morphology of the asymmetric cation exchange membrane.

[0081] Embodiment 6:

[0082] The difference between this embodiment and embodiment 1 is that the structure of the polymer used in this embodiment is as follows:

[0083]

[0084] The additive used is polyoxyethylene polyoxypropylene ether (L64) with an added mass of 0.5 g, and other conditions are the same as in embodiment 1. The morphology characterization method of the asymmetric cation exchange membrane prepared in this embodiment is the same as in embodiment 1. The ion exchange capacity of the obtained membrane is 1.62 mmol / g.

[0085] The surface morphology of the dense layer of the asymmetric cation exchange membrane prepared in this example is as shown in Figure 6 (a), and Figure 6(b) is the whole cross-section morphology of the asymmetric cation exchange membrane.

[0086] Example 7

[0087] The difference between this example and Example 1 is that the polymer structure used in this example is as follows:

[0088]

[0089] The additive used is polyvinylpyrrolidone K15, and the added mass is 1 g. Other conditions are the same as in Example 1, and the morphology characterization method of the asymmetric cation exchange membrane prepared in this example is the same as in Example 1. The ion exchange capacity of the obtained membrane is 1.37 mmol / g.

[0090] The surface morphology of the dense layer of the asymmetric cation exchange membrane prepared in this example is as shown in Figure 7 (a), and Figure 7 (b) is the whole cross-section morphology of the asymmetric cation exchange membrane.

Claims

1. A method for producing an asymmetrically structured cation exchange membrane, characterized by, The asymmetric structure cation exchange membrane is prepared by selecting a negatively charged cation exchange membrane material as a base material, a water-soluble polymer as an additive, and an organic solvent having good solubility to the negatively charged cation exchange membrane material and the additive, and by following the steps below: 1) first, the negatively charged cation exchange membrane material is fully stirred and dissolved in the organic solvent to obtain a first casting solution L-1; 2) then, the additive is added to the first casting solution L-1 in a proper proportion, and the second casting solution L-2 is obtained after fully stirring and dissolving; 3) the second casting solution L-2 is filtered, vacuumed or ultrasonically defoamed to obtain a third casting solution L-3; 4) the third L-3 is coated on the base, the solvent is evaporated at a suitable temperature, and finally the asymmetric structure cation exchange membrane is obtained after fully soaking in water. The water-soluble polymer is a natural water-soluble polymer, which is starch, protein, alginic acid or gelatin; or a semi-synthetic water-soluble polymer, which is carboxymethyl cellulose, methyl cellulose, ethyl cellulose or hydroxyethyl cellulose; or a synthetic water-soluble polymer, which is polyacrylamide, polyacrylic acid and salt, polyvinylpyrrolidone, polyvinyl alcohol, polymaleic anhydride, polyether / polyether polyol, polyquaternary ammonium salt, polyethylene glycol, polyether amine, polyamide or polyethylene imine, or one or more of them, and the mass average molecular weight is between 200 and 100000.

2. The method of claim 1, wherein, The negatively charged cation exchange membrane material is a high molecular material containing a negatively charged functional group, the polymer main chain of which is one or more of polyarylether sulfone, polyarylether ketone, polyphenyl ether, polystyrene, polyvinyl chloride, polyimide, polyamide, polyester, polyolefin or polybenzimidazole, and the negatively charged functional group is -R-SO3H; wherein the negatively charged sulfonate is connected to the polymer main chain through R, and R represents a fatty chain structure with a length of 0-6 C and O skeleton atoms.

3. The method of claim 1, wherein, The organic solvent is a polar aprotic solvent, which is any one or more of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) or N-methyl pyrrolidone (NMP); and has the following characteristics: (1) good solubility to the negatively charged cation exchange membrane material and the additive; (2) high boiling point, which is conducive to the control of the phase inversion speed of the negatively charged cation exchange membrane material during solvent evaporation.

4. The method of claim 1, wherein, The concentration of the first casting solution L-1 is 3wt%-40wt%, and the mass ratio of the additive to the negatively charged cation exchange membrane material is 0.05:1-2:

1.

5. The method of claim 1, wherein, The base is a metal plate, a glass plate, a ceramic, a polymer plate or a fabric.

6. The method of claim 1, wherein, The temperature range for evaporating the solvent is 25℃-180℃, the evaporation time is 5min-48h, and the soaking time in water is 1h-120h.

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