A high-strength polyamide reverse osmosis membrane and its preparation method

Abstract

The present invention belongs to the technical field of porous film material preparation, specifically relating to a high-strength polyamide reverse osmosis membrane and its preparation method. A small amount of amine monomer is added to the casting solution, an initiator is added to the coagulation bath, and the amine monomer is polymerized during phase inversion to form a modified base membrane. This improves the base membrane's amine absorption capacity and enhances the bond between the base membrane and the desalination layer. Modified layered mineral clay kaolin is added during interfacial polymerization to provide water channels while improving the membrane's mechanical properties and desalination performance. The resulting composite polyamide nascent membrane is immersed in a strong alkaline solution. Drying and heating the modified kaolin and the strong base trigger an alkali-thermal activation effect, generating a partial molecular sieve, which enhances the reverse osmosis membrane's adsorption capacity and provides additional water channels, further improving the membrane's desalination rate. The resulting membrane has a flux of no less than 25 GFD and a desalination rate of no less than 99.5%. After abrasion resistance testing, scratches on the membrane surface are shallow, and repeated performance changes are minimal.

Description

Technical Field

[0001] The invention belongs to the technical field of porous film material preparation, and particularly relates to a high-strength polyamide reverse osmosis membrane and a preparation method thereof. Background Art

[0002] Polyamide reverse osmosis membranes are widely used in water treatment fields such as drinking water, municipal sewage, food and medicine, seawater desalination, wastewater treatment and material concentration due to their high selectivity, good chemical stability and small footprint. At present, the preparation of commercial polyamide reverse osmosis membranes mainly adopts the preparation method of interfacial polymerization, which is prepared by the polymerization reaction between organic phase monomers and aqueous phase monomers. This results in poor mechanical properties of polyamide reverse osmosis membranes. Usually, during the rolling process of the membrane assembly, the membrane surface is easily damaged, compacted, and other phenomena are caused, which reduces the performance of the membrane.

[0003] In view of the problem of weak mechanical properties of current polyamide reverse osmosis membranes, the introduction of inorganic nanofillers can improve the comprehensive performance of the membrane without changing the membrane manufacturing process. Layered mineral clay kaolin is widely used in water treatment research due to its abundant resources, low cost, negative charge of kaolin flakes, large specific surface area, abundant reserves, and good adsorption performance. For example, CN111298651B improves the anti-biological fouling performance and water flux of the composite reverse osmosis membrane by introducing modified clay minerals and hydrophilic linkers with both high hydrophilicity and high antibacterial properties into the aqueous solution of m-phenylenediamine. The patent document CN110665377B discloses a high-flux anti-fouling reverse osmosis membrane and its preparation method. The reverse osmosis membrane comprises, from bottom to top, a polysulfone-based membrane layer, a polyamide desalination layer, and a polyvinyl alcohol protective layer. The polyvinyl alcohol protective layer comprises the following components by weight: 1 part polyvinyl alcohol, 0.02-0.1 parts nano-titanium dioxide-modified clay mineral, and 0.2 parts 3-aminopropylsilanetriol. The above two patented technologies primarily focus on improving the membrane's anti-fouling properties, without addressing performance improvements such as salt rejection.

[0004] The present invention is based on consideration of the above existing problems, and improves the mechanical properties and desalination performance of the reverse osmosis membrane by improving the bonding degree between the base membrane and the separation layer and introducing modified layered clay kaolin. Summary of the Invention

[0005] In order to solve the above problems, the present invention provides a high-strength polyamide reverse osmosis membrane and a preparation method thereof.

[0006] This is achieved specifically through the following technical solutions:

[0007] 1. A method for preparing a high-strength polyamide reverse osmosis membrane, comprising the following steps:

[0008] (1) preparing a casting solution by mixing a base film material, an amine monomer and a solvent;

[0009] Furthermore, the base film material is one of polysulfone, polyethersulfone, polyacrylonitrile, polyimide, and polyvinylidene fluoride, preferably polysulfone;

[0010] Furthermore, the solvent is one of nitrogen-nitrogen dimethylformamide, nitrogen-nitrogen dimethylacetamide, and nitrogen-methylpyridine, preferably nitrogen-nitrogen dimethylacetamide;

[0011] Furthermore, the amine monomer is one of aniline, phenylenediamine, caprolactam, and acrylamide, preferably aniline;

[0012] Furthermore, the mass ratio of the base film material to the solvent is 1-3:7-9; the mass ratio of the amine monomer to the casting solution is 0.5-20:1000;

[0013] (2) curing the casting solution obtained in step (1) on the reinforcing material to form a base film, adding a polymer initiator and additives during the film scraping, the thickness of the base film being 2-8 mil, preferably 4-6 mil; then immersing the base film in a polyamine solution, and obtaining a composite film after the surface is dried;

[0014] Furthermore, the polymerization initiator and additive are one of potassium persulfate, sodium persulfate, ammonium persulfate and hydrochloric acid, sulfuric acid, dodecylbenzenesulfonic acid, and tetradecylbenzenesulfonic acid, preferably ammonium persulfate and hydrochloric acid;

[0015] Furthermore, the polyamine solution also contains an acid acceptor and carboxylated kaolin;

[0016] Furthermore, the polyamine solution is a m-phenylenediamine solution with a mass concentration of 0.1-10%, preferably 1-5%, and an addition amount of 2%; the acid acceptor is a NaOH solution with a mass concentration of 0.001-10%, preferably 0.01-1%, and an addition amount of 0.05%;

[0017] The added amount of the carboxylated kaolin is 0.001-10%, preferably 0.01-1%, of the total mass of the m-phenylenediamine aqueous solution.

[0018] Furthermore, the carboxylated kaolin is prepared by adding kaolin and a carboxyl modifier into deionized water, mixing them uniformly by ultrasonication, then adding potassium persulfate and NaHCO3 dropwise until the solution has a pH of 6, reacting at 80°C for 5 hours, then filtering, washing with deionized water until neutral, and freeze-drying.

[0019] Furthermore, the carboxyl modifier is an acrylic acid monomer, and the added amounts of kaolin and the carboxyl modifier are 0.05 and 0.025% of the mass of deionized water.

[0020] (3) immersing the composite membrane obtained in step (2) in an organic solvent containing trimesoyl chloride and kaolin chloride, and then immersing it in a NaOH solution with a mass concentration of 0.05%, and then washing it with water after heat treatment, and then immersing it in a 6% moisturizing agent water tank for moisturizing treatment, and drying it at 60° C. after moisturizing treatment to obtain a high-strength reverse osmosis membrane;

[0021] Furthermore, the concentration of trimesoyl chloride is 0.01-10%, preferably 0.01-5%; the organic solvent is n-hexane, and the addition amount of kaolin chloride is 0.001-10%, preferably 0.01-1%.

[0022] Furthermore, the acyl chloride kaolin is obtained by adding kaolin to a mixed solution of thionyl chloride and dimethylformamide, heating and stirring at 70° C. for 20 hours, then washing with dichloromethane, filtering and drying.

[0023] Furthermore, the heat treatment temperature is 40-100°C, preferably 60-90°C.

[0024] Furthermore, the moisturizing agent is one of propylene glycol, glycerol, butylene glycol, xylitol, and sorbitol.

[0025] In summary, the beneficial effects of the present invention are as follows: by adding a small amount of amine monomer to the casting solution and an initiator to the coagulation bath, the amine monomer is polymerized during the phase inversion process to form a modified base membrane, thereby improving the base membrane's amine absorption capacity and enhancing the bond between the base membrane and the desalination layer. The addition of modified layered mineral clay kaolin during the interfacial polymerization process provides water channels while improving the mechanical properties and desalination performance of the membrane. The composite polyamide nascent membrane after interfacial polymerization is immersed in a strong alkaline solution. Under the action of drying and heating, the modified kaolin contacts the strong base and undergoes an alkali thermal activation effect, generating a partial molecular sieve, thereby improving the adsorption capacity of the reverse osmosis membrane and providing additional water channels, further increasing the membrane's desalination rate.

[0026] The reason for using carboxylated kaolin and acyl chloride kaolin is that both the oil phase and the water phase are added during the interfacial polymerization process, and the carboxyl modification is to improve the compatibility with the water phase, and the acyl chloride modification is to improve the compatibility with the oil phase.

[0027] The pollution-resistant seawater desalination reverse osmosis membrane prepared by the present invention has a membrane flux of not less than 25 gfd and a desalination rate of not less than 99.5% under test conditions of 225 psi, 2000 ppm raw water concentration (NaCl), and 25°C. After abrasion resistance testing, the membrane surface has shallow scratches and the performance change is small after retesting. DETAILED DESCRIPTION

[0028] The specific embodiments of the present invention are further described in detail below, but the present invention is not limited to these embodiments. Any improvement or replacement based on the basic spirit of the present embodiment still falls within the scope of protection required by the claims of the present invention.

[0029] Example 1

[0030] 1. Prepare a polysulfone / nitrogen-nitrogen dimethylacetamide casting solution with a polysulfone content of 20%. During the preparation process, add 0.1% aniline monomer to it. After complete dissolution, let it stand at room temperature for degassing. At the same time, add 0.05% ammonium persulfate and 0.1% hydrochloric acid to the coagulation bath. Control the temperature at 15°C and scrape the base film in phases to control the base film thickness to 5 mil. After scraping, place it in pure water for standby use.

[0031] 2. Soak one side of the above-mentioned base film in an aqueous solution containing 2% m-phenylenediamine, 0.02% sodium hydroxide, and 0.25% carboxylated kaolin for 1 min. After soaking, blow dry the membrane surface, and then soak it in an organic solution containing 0.1% trimesoyl chloride and 0.25% kaolin chloride for 30 s. After the reaction is completed, blow dry the surface and immerse it in 5% NaOH solution for 30 s. After soaking, take it out and dry it at 90°C to prepare the membrane for testing.

[0032] Example 2

[0033] 1. Prepare a polysulfone / nitrogen-nitrogen dimethylacetamide casting solution with a polysulfone content of 20%. During the preparation process, add 0.1% aniline monomer to it. After complete dissolution, let it stand at room temperature for degassing. At the same time, add 0.05% ammonium persulfate and 0.1% hydrochloric acid to the coagulation bath. Control the temperature at 15°C and scrape the base film in phases to control the base film thickness to 5 mil. After scraping, place it in pure water for standby use.

[0034] 2. Soak one side of the above-mentioned base film in an aqueous solution containing 2% m-phenylenediamine, 0.02% sodium hydroxide, and 0.5% carboxylated kaolin for 1 min. After soaking, blow dry the membrane surface, and then soak it in an organic solution containing 0.1% trimesoyl chloride and 0.5% kaolin chloride for 30 s. After the reaction is completed, blow dry the surface and immerse it in 5% NaOH solution for 30 s. After soaking, take it out and dry it at 90°C to prepare the membrane for testing.

[0035] Example 3

[0036] 1. Prepare a polysulfone / nitrogen-nitrogen dimethylacetamide casting solution with a polysulfone content of 20%. During the preparation process, add 0.1% aniline monomer to it. After complete dissolution, let it stand at room temperature for degassing. At the same time, add 0.05% ammonium persulfate and 0.1% hydrochloric acid to the coagulation bath. Control the temperature at 15°C and scrape the base film in phases to control the base film thickness to 5 mil. After scraping, place it in pure water for standby use.

[0037] 2. Soak one side of the above-mentioned base film in an aqueous solution containing 2% m-phenylenediamine, 0.02% sodium hydroxide, and 1.0% carboxylated kaolin for 1 min. After soaking, blow dry the membrane surface, and then soak it in an organic solution containing 0.1% trimesoyl chloride and 1.0% kaolin chloride for 30 s. After the reaction is completed, blow dry the surface and immerse it in 5% NaOH solution for 30 s. After soaking, take it out and dry it at 90°C to prepare the membrane for testing.

[0038] Comparative Example 1

[0039] 1. Prepare a polysulfone / nitrogen-nitrogen dimethylacetamide casting solution with a polysulfone content of 20%. After complete dissolution, let it stand at room temperature for degassing. After degassing, pass it through a 20°C coagulation bath for phase separation and scraping to form a base film. The base film thickness is controlled to be 5 mil. After scraping, place it in pure water for standby use.

[0040] 2. Soak one side of the above-mentioned base film in an aqueous solution containing 2% m-phenylenediamine and 0.02% sodium hydroxide for 1 minute. After soaking, blow dry the film surface and then soak it in an organic solution containing 0.1% trimesoyl chloride for 30 seconds. After the reaction is completed, take it out and dry it at 70°C to prepare the membrane for testing.

[0041] Comparative Example 2

[0042] 1. Prepare a polysulfone / nitrogen-nitrogen dimethylacetamide casting solution with a polysulfone content of 20%. During the preparation process, add 0.1% aniline monomer to it. After complete dissolution, let it stand at room temperature for degassing. At the same time, add 0.05% ammonium persulfate and 0.1% hydrochloric acid to the coagulation bath. Control the temperature at 15°C and scrape the base film in phases to control the base film thickness to 5 mil. After scraping, place it in pure water for standby use.

[0043] 2. Soak one side of the above-mentioned base film in an aqueous solution containing 2% m-phenylenediamine and 0.02% sodium hydroxide for 1 minute. After soaking, blow dry the film surface and then soak it in an organic solution containing 0.1% trimesoyl chloride for 30 seconds. After the reaction is completed, take it out and dry it at 70°C to prepare the membrane for testing.

[0044] Comparative Example 3

[0045] 1. Prepare a polysulfone / nitrogen-nitrogen dimethylacetamide casting solution with a polysulfone content of 20%. During the preparation process, add 0.1% aniline monomer to it. After complete dissolution, let it stand at room temperature for degassing. At the same time, add 0.05% ammonium persulfate and 0.1% hydrochloric acid to the coagulation bath. Control the temperature at 15°C and scrape the base film in phases to control the base film thickness to 5 mil. After scraping, place it in pure water for standby use.

[0046] 2. Soak one side of the above-mentioned base film in an aqueous solution containing 2% m-phenylenediamine, 0.02% sodium hydroxide, and 0.25% carboxylated kaolin for 1 min. After soaking, blow dry the membrane surface, and then soak it in an organic solution containing 0.1% trimesoyl chloride and 0.25% kaolin chloride for 30 s. After soaking, take it out and dry it at 70°C to prepare the membrane for testing.

[0047] 1. Diaphragm abrasion resistance test

[0048] 1.1 Experimental Materials

[0049] The membranes were prepared using the methods of Examples 1-3 and Comparative Examples 1-3.

[0050] 1.2 Experimental methods

[0051] The membrane was placed on an electric friction color fastness tester and rubbed back and forth 5 times with a pressure of 6N. The changes in water flux and salt rejection before and after the membrane anti-rubbing test were recorded under the conditions of a pressure of 225psi, a raw water concentration (NaCl) of 2000ppm, a temperature of 25°C, and a pH of 7.0. The results are shown in Table 1.

[0052] 1.3 Experimental Results

[0053] Table 1 Test results of film abrasion resistance

[0054]

[0055]

[0056] From the experimental results in Table 1, it can be seen that in Comparative Example 1, due to the lack of modified kaolin added in the preparation process, the flux of the membrane increased due to scratches after the anti-scratch test, and the desalination rate dropped significantly. Comparative Example 3 and Examples 1-2 improved the mechanical properties of the membrane due to the addition of modified kaolin, and the flux and desalination rate dropped less after the anti-scratch test. Examples 1 and 2 further improved the flux and desalination rate due to the alkali thermal activation effect of the modified kaolin. After the content of modified kaolin in the oil phase and the water phase was increased to 1%, the performance of the membrane decreased significantly. This was due to the excessively high content of modified kaolin, which formed membrane defects.

Claims

1. A method for preparing a high-strength polyamide reverse osmosis membrane, characterized in that: The following steps are involved: (1) preparing a casting solution by mixing a base film material, an amine monomer and a solvent; The amine monomer is one of aniline, phenylenediamine, caprolactam, and acrylamide; (2) curing the casting solution obtained in step (1) on the reinforcing material to form a base film, adding a polymer initiator and additives during film scraping, and then immersing the base film in a polyamine solution, and obtaining a composite film after the surface is dried; The polyamine solution further contains an acid acceptor and carboxylated kaolin; the polymerization initiator is one of potassium persulfate, sodium persulfate, and ammonium persulfate; and the additive is one of hydrochloric acid, sulfuric acid, dodecylbenzenesulfonic acid, and tetradecylbenzenesulfonic acid; (3) immersing the composite membrane obtained in step (2) in an organic solvent containing trimesoyl chloride and kaolin chloride, and then immersing it in a NaOH solution with a mass concentration of 0.05%, and then washing it with water after heat treatment, and then immersing it in a 6% moisturizing agent water tank for moisturizing treatment, and drying it at 60° C. after moisturizing treatment to obtain a high-strength reverse osmosis membrane; The heat treatment temperature is 40-100°C.

2. The method for preparing a high-strength polyamide reverse osmosis membrane according to claim 1, wherein: The base film material is one of polysulfone, polyethersulfone, polyacrylonitrile, polyimide, and polyvinylidene fluoride; the solvent is one of nitrogen-nitrogen dimethylformamide, nitrogen-nitrogen dimethylacetamide, and nitrogen-methylpyrrolidone; The mass ratio of the base film material to the solvent is 1-3:7-9; the mass ratio of the amine monomer to the casting solution is 0.5-20:1000; and the mass ratio of the polymer initiator and additives to the casting solution is 0.1-20:1000.

3. The method for preparing a high-strength polyamide reverse osmosis membrane according to claim 1, wherein: The polyamine solution in step (2) is m-phenylenediamine solution, with a mass concentration of 0.1-10% and an addition amount of 2%; the acid acceptor is NaOH solution, with a concentration of 0.001-10% and an addition amount of 0.05%.

4. The method for preparing a high-strength polyamide reverse osmosis membrane according to claim 1, wherein: The carboxylated kaolin is prepared by adding kaolin and a carboxyl modifier into deionized water, mixing them uniformly by ultrasonication, then adding potassium persulfate and NaHCO3 dropwise until the solution has a pH of 6, reacting at 80°C for 5 hours, then filtering, washing with deionized water until neutral, and freeze-drying. The added amount of the carboxylated kaolin is 0.001-10% of the total mass of the m-phenylenediamine aqueous solution.

5. The method for preparing a high-strength polyamide reverse osmosis membrane according to claim 4, wherein: The carboxyl modifier is an acrylic acid monomer, and the addition amounts of kaolin and the carboxyl modifier are 0.05 and 0.025% of the mass of deionized water.

6. The method for preparing a high-strength polyamide reverse osmosis membrane according to claim 1, wherein: The concentration of trimesoyl chloride in step (3) is 0.01-10%; and the organic solvent is n-hexane.

7. The method for preparing a high-strength polyamide reverse osmosis membrane according to claim 1, wherein: The acyl chloride kaolin is obtained by adding kaolin to a mixed solution of thionyl chloride and dimethylformamide, heating and stirring at 70° C. for 20 hours, then washing with dichloromethane, filtering and drying. The addition amount of the acyl chloride kaolin is 0.001-10% of the total mass of the m-phenylenediamine aqueous solution.

8. A high-strength polyamide reverse osmosis membrane prepared by the method for preparing a high-strength polyamide reverse osmosis membrane according to any one of claims 1 to 7.

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