A polyurethane composite nanofiltration membrane and its preparation method

Abstract

The present invention provides a method for preparing a polyurethane composite nanofiltration membrane, comprising: immersing a polyimide base membrane in a mixed solution of hexamethylenediamine and isopropanol for cross-linking treatment; coating by soaking in a buffer solution containing polyphenol monomers; soaking the polyphenol-coated membrane in an organic solvent for solvent exchange to remove residual moisture on the membrane surface, then taking it out and fixing it with a mold; adding an isocyanate organic phase solution to the mold for fixing the polyphenol-coated membrane, immersing the organic phase solution to submerge the polyphenol-coated membrane, and performing interfacial cross-linking on the membrane surface; a polyurethane composite nanofiltration membrane prepared according to the above method. By using the solvent exchange method to remove the moisture on the surface of the polyphenol-coated membrane during the interfacial cross-linking process, the present invention can effectively avoid the reaction of water with isocyanate to generate small molecule amine compounds and gases, slow down the generation of defects in the polyurethane selective layer, break through the "tradeoff" effect, and improve the flux on the premise of ensuring the rejection rate.

Description

Technical Field

[0001] The present invention belongs to the technical field of separation membranes, and particularly relates to a polyurethane composite nanofiltration membrane and a preparation method thereof. Background Art

[0002] Due to the discharge of industrial polluted wastewater and the continuous growth of the population, the problems of water resource pollution and shortage have attracted the attention of countries around the world. The rapidly developing nanofiltration technology in recent years plays a key role in treating wastewater and alleviating the global water resource shortage. Nanofiltration membranes have unique pore sizes (about 0.5 - 2 nm) and rejection molecular weights (about 200 - 1000 g / mol), and can be used to remove small molecules such as antibiotics, disaccharides, dyes, and high-valent or heavy metal salt ions. They are widely used in industrial fields such as medicine, food, and printing and dyeing, and have become a research hotspot in recent years. China started relatively late in the research field of high-performance separation membranes, and high-end technologies such as nanofiltration membranes are basically monopolized by foreign large companies. Therefore, developing high-performance nanofiltration membrane materials to fill the domestic gap has become an urgent problem to be solved.

[0003] Polyurethane materials have excellent mechanical and chemical stability and are usually prepared by the reaction of isocyanates with hydroxyl compounds. Polyurethane composite membranes have strong tolerance to strong acid and strong base solutions and organic solvents, and have great development prospects for separation applications in harsh environments. However, due to the easy side reaction of isocyanates with water during the interfacial polymerization process, which destroys the integrity of the selective layer, it is difficult for polyurethane selective layer composite membranes to overcome the "Trade-off" effect between the permeation flux and the rejection rate. Therefore, developing new composite nanofiltration membranes, optimizing the membrane preparation method, and adjusting the membrane structure and performance to break through the "Trade-off" effect of polyurethane composite membranes have become the key focus directions in this field. Summary of the Invention

[0004] In order to solve the problem that isocyanates are prone to side reactions with water during the interfacial polymerization process, which destroys the integrity of the selective layer, and to break through the "Trade-off" effect of polyurethane composite membranes, the present invention provides a polyurethane composite nanofiltration membrane and a preparation method thereof, specifically as follows:

[0005] A preparation method of a polyurethane composite nanofiltration membrane includes the following steps:

[0006] Step 1: Immerse a polyimide substrate membrane in a mixed solution of hexamethylenediamine and isopropanol for cross-linking treatment to improve the stability of the substrate membrane. The polyimide substrate membrane after hexamethylenediamine cross-linking is soaked in deionized water for standby;

[0007] Step 2: Immerse the polyimide substrate membrane treated in Step 1 in a buffer solution containing polyphenol monomers for coating, and then take it out and rinse it with deionized water;

[0008] Step 3. Immerse the polyphenol-coated film after being treated in Step 2 in an organic solvent for solvent exchange to remove the residual moisture on the film surface, and then take it out and fix it with a mold.

[0009] Step 4. Add the isocyanate organic phase solution to the mold fixing the polyphenol-coated film to submerge the polyphenol-coated film with the organic phase solution, perform interfacial crosslinking on the film surface, and then rinse it with an organic solvent and deionized water respectively to obtain a polyurethane composite membrane, which is stored in deionized water.

[0010] In Step 1, the concentration of the mixed solution of hexamethylenediamine and isopropanol is 5 - 20 g / L, and the crosslinking treatment time is 2 - 10 h.

[0011] In Step 2, the polyphenol monomer is one or a mixture of tannic acid, dopamine, or gallic acid, the concentration of the polyphenol monomer is 0.1 - 5 g / L, the buffer solution is an aqueous Tris solution, the pH value is 8.2 - 8.4, the polyphenol monomer coating time is 10 - 300 min, and the coating condition is static coating.

[0012] In Step 3, the organic solvent for solvent exchange is n-hexane or chloroform, and the solvent exchange time is 5 - 30 min.

[0013] In Step 4, the isocyanate is toluene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, or hexamethylene diisocyanate, the concentration of the isocyanate organic phase solution is 0.01% - 1% m / V, and the isocyanate crosslinking time is 15 - 240 s.

[0014] The polyurethane composite nanofiltration membrane prepared according to the above preparation method includes a polyimide substrate membrane and a polyurethane selective layer prepared by crosslinking polyphenols with isocyanate. The thickness of the polyurethane selective layer is 10 - 200 nm.

[0015] Advantages of the present invention:

[0016] 1. The present invention removes the moisture on the surface of the polyphenol-coated film during the interfacial crosslinking process by the solvent exchange method, which can effectively avoid the reaction of water with isocyanate to generate small molecule amine compounds and gases, and thus slow down the generation of defects in the polyurethane selective layer.

[0017] 2. The present invention introduces a large number of phenolic hydroxyl functional groups on the polyimide substrate membrane through polyphenol coating. The phenolic hydroxyl functional groups and isocyanate build a defect-free high-flux ultra-thin polyurethane selective layer through nucleophilic addition reaction to achieve efficient nanofiltration separation. The thickness of the prepared polyurethane selective layer is only about 10 nm, the mass transfer resistance is greatly reduced, and the permeation flux of the polyurethane composite membrane can reach 33.88 L·m -2 ·h -1 ·bar -1, which is about ten times higher than that of the polyurethane composite membrane prepared by the traditional interfacial polymerization method, and can completely retain small molecule dyes such as methylene blue and congo red.

[0018] 3. The present invention constructs a polyurethane selective layer composite nanofiltration membrane by using the nucleophilic addition reaction of polyphenol and isocyanate. The polyphenol monomer is green, environmentally friendly and low in price, and has better economic benefits compared with the traditional polyamide composite nanofiltration membrane.

[0019] 4. In addition, the inherent chemical stability of the polyurethane material endows the polyurethane composite membrane with excellent long-term separation stability, chlorine resistance and acid resistance. Description of the Drawings

[0020] Figure 1 is the scanning electron microscope photograph of the surface of the polyurethane composite nanofiltration membrane in Example 1 of the present invention;

[0021] Figure 2 is the transmission electron microscope photograph of the cross section of the polyurethane composite nanofiltration membrane in Example 1 of the present invention;

[0022] Figure 3 is the AFM photograph of the surface of the polyurethane composite nanofiltration membrane in Example 1 of the present invention;

[0023] Figure 4 is the schematic diagram of the long-term separation stability test result of the polyurethane composite nanofiltration membrane in Example 1 of the present invention. Detailed Embodiments

[0024] In the present invention, if there are no special requirements, the preparation raw materials used are all commercially available products well-known to those skilled in the art. Among them, the polyimide substrate membrane has a thickness of 200 um and a pore diameter of 0.05 um.

[0025] Next, the technical solutions in the present invention will be clearly and completely described in conjunction with the embodiments in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the present invention.

[0026] The present invention relates to a preparation method of a polyurethane composite nanofiltration membrane, which includes the following steps:

[0027] Step 1: Immerse the polyimide substrate membrane in a mixed solution of hexamethylenediamine and isopropanol for cross-linking treatment. After cross-linking with hexamethylenediamine, the polyimide substrate membrane is soaked in deionized water for standby. The concentration of the mixed solution of hexamethylenediamine and isopropanol is 5-20 g / L, and the cross-linking treatment time is 2-10 h.

[0028] Step 2: Immerse the polyimide-based film processed in Step 1 in a buffer solution containing polyphenol monomers for coating, then take it out and rinse with deionized water; the phenolic monomer is one or more mixtures of tannic acid, dopamine, or gallic acid, and the concentration of the polyphenol monomer is 0.1 - 5 g / L. The buffer solution is an aqueous Tris solution with a pH value of 8.2 - 8.4. The coating time of the polyphenol monomer is 10 - 300 min, and the coating condition is static coating.

[0029] Step 3: Immerse the polyphenol-coated film processed in Step 2 in an organic solvent for solvent exchange to remove the residual moisture on the film surface, then take it out and fix it with a mold; the organic solvent for solvent exchange is n-hexane or chloroform. The solvent exchange time is 5 - 30 min.

[0030] Step 4: Add the isocyanate organic phase solution to the mold fixing the polyphenol-coated film to submerge the polyphenol-coated film, conduct interfacial crosslinking on the film surface, and then rinse with an organic solvent and deionized water respectively to obtain a polyurethane composite membrane. The isocyanate is toluene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, or hexamethylene diisocyanate. The concentration of the isocyanate organic phase solution is 0.01% - 1% m / V, and the isocyanate crosslinking time is 15 - 240 s.

[0031] The present invention also includes a composite nanofiltration membrane obtained by the above preparation method. The composite nanofiltration membrane includes a polyurethane selective layer prepared from a polyimide-based film and an isocyanate-crosslinked polyphenol layer, and the thickness of the polyurethane selective layer is 10 - 200 nm.

[0032] The reaction mechanism of the present invention is as follows:

[0033] Immerse the polyimide-based film in a buffer solution containing polyphenol monomers for coating. The function is to make the polyphenol monomers adhere to the surface of the base film through Michael addition and Schiff base reactions to form a polyphenol coating layer, and provide a large number of phenolic hydroxyl reaction functional groups to further participate in the reaction.

[0034] Immerse the polyphenol-coated film in an organic solvent for solvent exchange. The function is to remove the residual moisture on the surface of the polyphenol-coated film and slow down the defects generated in the selective layer due to side reactions in the subsequent interfacial crosslinking reaction; immerse the organic phase solution of isocyanate on the surface of the polyphenol-coated film for interfacial crosslinking. The function is that the isocyanate functional groups in the isocyanate monomers will crosslink with the phenolic hydroxyl functional groups through nucleophilic addition reactions to construct a polyurethane selective layer.

[0035] Example 1

[0036] The preparation method of the polyurethane composite nanofiltration membrane in this example includes the following steps:

[0037] Step 1: Immerse the polyimide-based film in a mixed solution of hexamethylenediamine and isopropanol for crosslinking treatment. After crosslinking with hexamethylenediamine, the polyimide-based film is soaked in deionized water for standby. The concentration of the mixed solution of hexamethylenediamine and isopropanol is 20 g / L, and the crosslinking treatment time is 8 h.

[0038] Step 2: Immerse the polyimide-based film treated in Step 1 in a buffer solution containing polyphenol monomer for coating, then take it out and rinse with deionized water. The polyphenol monomer is tannic acid, and the concentration of the polyphenol monomer is 2 g / L. The buffer solution is an aqueous Tris solution with a pH value of 8.2. The coating time of the polyphenol monomer is 120 min, and the coating condition is static coating.

[0039] Step 3: Immerse the polyphenol-coated film treated in Step 2 in an organic solvent for solvent exchange to remove water molecules, then take it out and fix it with a mold. The organic solvent for solvent exchange is n-hexane. The solvent exchange time is 20 min. The isocyanate is toluene diisocyanate.

[0040] Step 4: Add the isocyanate organic phase solution to the mold fixing the polyphenol-coated film to submerge the polyphenol-coated film with the organic phase solution, conduct interfacial crosslinking on the film surface, and then rinse 3 times with the organic solvent and deionized water respectively to obtain the polyurethane composite membrane. The concentration of the isocyanate organic phase solution is 0.5% m / V, and the isocyanate crosslinking time is 180 s. The thickness of the polyurethane selective layer of the obtained polyurethane composite membrane is 10 nm.

[0041] Example 2

[0042] The preparation method of the polyurethane composite nanofiltration membrane in this example includes the following steps:

[0043] Step 1: Immerse the polyimide-based film in a mixed solution of hexamethylenediamine and isopropanol for crosslinking treatment. After crosslinking with hexamethylenediamine, the polyimide-based film is soaked in deionized water for standby. The concentration of the mixed solution of hexamethylenediamine and isopropanol is 5 g / L, and the crosslinking treatment time is 2 h.

[0044] Step 2: Immerse the polyimide-based film treated in Step 1 in a buffer solution containing polyphenol monomer for coating, then take it out and rinse with deionized water. The polyphenol monomer is gallic acid, and the concentration of the polyphenol monomer is 2 g / L. The buffer solution is an aqueous Tris solution with a pH value of 8.4. The coating time of the polyphenol monomer is 180 min, and the coating condition is static coating.

[0045] Step 3: Immerse the polyphenol-coated film treated in Step 2 in an organic solvent for solvent exchange to remove residual moisture, then take it out and fix it with a mold. The organic solvent for solvent exchange is chloroform. The solvent exchange time is 20 min.

[0046] Step 4: Add the isocyanate organic phase solution into the mold with the fixed polyphenol coating film, submerge the organic phase solution in the polyphenol coating film, conduct interfacial crosslinking on the film surface, and then rinse 3 times with an organic solvent and deionized water respectively to obtain a polyurethane composite membrane. The isocyanate is hexamethylene diisocyanate. The concentration of the isocyanate organic phase solution is 0.5% m / V, and the isocyanate crosslinking time is 240 s. The thickness of the polyurethane selective layer of the obtained polyurethane composite membrane is 24 nm.

[0047] Example 3

[0048] The preparation method of the polyurethane composite nanofiltration membrane in this example includes the following steps:

[0049] Step 1: Immerse the polyimide substrate membrane in a mixed solution of hexamethylenediamine and isopropanol for crosslinking treatment. The polyimide substrate membrane after hexamethylenediamine crosslinking is soaked in deionized water for standby; the concentration of the mixed solution of hexamethylenediamine and isopropanol is 15 g / L, and the crosslinking treatment time is 10 h.

[0050] Step 2: Immerse the polyimide substrate membrane treated in Step 1 in a buffer solution containing polyphenol monomers for coating, and then take it out and rinse with deionized water; the polyphenol monomer is dopamine, and the concentration of the polyphenol monomer is 4 g / L. The buffer solution is an aqueous Tris solution with a pH value of 8.3. The polyphenol monomer coating time is 300 min, and the coating condition is static coating.

[0051] Step 3: Immerse the polyphenol coating film treated in Step 2 in an organic solvent for solvent exchange to remove water molecules, and then take it out and fix it with a mold; the organic solvent for solvent exchange is n-hexane. The solvent exchange time is 20 min.

[0052] Step 4: Add the isocyanate organic phase solution into the mold with the fixed polyphenol coating film, submerge the organic phase solution in the polyphenol coating film, conduct interfacial crosslinking on the film surface, and then rinse 3 times with an organic solvent and deionized water respectively to obtain a polyurethane composite membrane. The isocyanate is toluene diisocyanate. The concentration of the isocyanate organic phase solution is 0.5% m / V, and the isocyanate crosslinking time is 180 s. The thickness of the polyurethane selective layer of the obtained polyurethane composite membrane is 52 nm.

[0053] Performance test

[0054] In the present invention, the separation performance of the membrane is carried out through a self-made dead-end filtration device in the laboratory. The test pressure is 5 bar, and the effective area of the filter element is 12.5 cm 2 , and before the test, the membrane needs to be pre-pressed with 200 mL of deionized water for 30 min to compact the membrane surface and make the permeation flux stable within a certain range. Subsequently, collect the aqueous solution permeated within a certain time, and calculate the pure water permeation flux of the membrane according to formula (1).

[0055] The calculation formula for the permeation flux is as follows:

[0056] In the formula, V is the volume of the permeated solution (L) within a certain time t; A is the effective area of the filter element (m 2 ); ΔP is the test pressure (bar); t is the filtration time (h);

[0057] P is the permeation flux (Lh -1 m -2 bar -1 ).

[0058] When measuring the rejection rate of different dye solutions, a 200 ppm dye solution is prepared with deionized water. Approximately 200 mL of the solution is added to the dead-end filtration device for filtration. After pre-pressurizing for about 20 minutes, the permeated solution within a certain time is collected. The concentration of the solution can be measured by an ultraviolet spectrophotometer, and the rejection rate of the membrane to the dye can be calculated according to formula (2). It should be noted that when measuring the dye rejection rate, a certain stirring speed should be maintained to prevent the formation of a filter cake layer due to concentration polarization of the dye on the membrane surface, which may affect the separation performance.

[0059] The calculation formula for the rejection rate of the membrane to the dye is as follows:

[0060] In the formula, C p —— is the concentration of the dye in the permeate; C f —— is the concentration of the dye in the feed solution;

[0061] R is the rejection rate of the dye.

[0062] The test results of Example 1 are as follows:

[0063] The surface scanning electron microscope photograph of the polyurethane composite membrane is as shown in Figure 1 ; the cross-section transmission electron microscope photograph of the polyurethane composite membrane is as shown in Figure 2 ; the AFM photograph characterizing the surface roughness of the polyurethane composite membrane is as shown in Figure 3 ; the long-term stability test of the polyurethane composite membrane is as shown in Figure 4 . It can be seen from Figures 1 to 4 that the polyurethane selective layer prepared by the method of polyphenol coating and isocyanate interfacial cross-linking coupling is complete, dense, has no obvious defects, has a smooth surface, and has a thickness of only about 10 nm. In addition, according to the long-term separation stability test results, the composite nanofiltration membrane prepared by the present invention can achieve complete rejection within the test time.

[0064] Control Example 1

[0065] It is basically the same as the preparation method of Example 1, except that the polyphenol-coated film does not undergo solvent exchange, and after removing the obvious water droplets on the film surface with a rubber roller, it is directly crosslinked at the interface with the organic phase solution of isocyanate.

[0066] In this comparative example, compared with the surface of the polyurethane composite nanofiltration membrane prepared after solvent exchange in Example 1, the surface of the polyurethane composite membrane prepared without removing the water on the membrane surface by solvent exchange has obvious defects due to the side reaction of water and isocyanate. The rejection rate of the composite nanofiltration membrane prepared in this comparative example for a 200 ppm methylene blue dye small molecule solution is 67.4%, and the pure water permeation flux is 44.65 L·m -2 ·h -1 ·bar -1 。

[0067] Comparative Example 2

[0068] It is basically the same as the preparation method of Example 1, except that the polyimide-based membrane crosslinked with hexamethylenediamine is not coated with polyphenol and does not undergo isocyanate interfacial crosslinking.

[0069] Comparative Example 3

[0070] It is basically the same as the preparation method of Example 1, except that the polyimide-based membrane crosslinked with hexamethylenediamine is coated with tannic acid to obtain a tannic acid-coated film, but does not undergo isocyanate interfacial crosslinking.

[0071] The membranes prepared in Comparative Examples 2 and 3 are the polyimide-based membrane crosslinked with hexamethylenediamine and the tannic acid-coated film respectively. Compared with the polyurethane composite nanofiltration membrane prepared in Example 1, the polyimide-based membrane in Control Group 2, as a porous substrate membrane, has a poor rejection effect on dyes. The rejection rate of the 200 ppm methylene blue dye is 39.8%, and the pure water permeation flux is 52.36 L·m -2 ·h -1 ·bar -1 。For the tannic acid-coated film prepared in Comparative Example 3, due to the short tannic acid coating time, a complete selective layer cannot be formed, and effective rejection of dyes cannot be achieved. The rejection rate of the 200 ppm methylene blue dye is 60.1%, and the pure water permeation flux is 48.37 L·m -2 ·h -1 ·bar -1 ,while the rejection rate of the polyurethane composite membrane prepared in Example 1 for the 200 ppm methylene blue dye is 99.8%, and the pure water permeation flux is 33.88 L·m -2 ·h -1 ·bar -1, on the premise of completely retaining methylene blue dye, it has a high permeation flux. It can be seen that an ultrathin high-flux polyurethane composite nanofiltration membrane can be prepared by the method of coupling tannic acid coating and isocyanate interfacial crosslinking. On the premise of maintaining the inherent chemical stability of the polyurethane material, the permeation flux of the polyurethane composite membrane is greatly improved.

[0072] It can be known from the test results that under the same test conditions, the polyphenol coating and isocyanate interfacial crosslinking in Example 1 constructed a polyurethane composite nanofiltration membrane with excellent performance. The rejection rate of small molecule dyes such as methylene blue and congo red is > 99%, and the pure water permeation flux can reach 33.88 L·m -2 ·h -1 ·bar-1.

[0073] The above content is only the preferred embodiment of the present invention and is not used to limit the implementation of the present invention. Those of ordinary skill in the art can easily make corresponding adaptations or modifications according to the main concept and spirit of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope required by the claims.

Claims

1. A preparation method of a polyurethane composite nanofiltration membrane, characterized in that, It includes the following steps: Step 1: Immerse the polyimide-based film in a mixed solution of hexamethylenediamine and isopropanol for crosslinking treatment. After crosslinking with hexamethylenediamine, the polyimide-based film is soaked in deionized water for standby. Step 2: Immerse the polyimide-based film treated in Step 1 in a buffer solution containing polyphenol monomers for coating, then take it out and rinse with deionized water. Step 3: Immerse the polyphenol-coated film treated in Step 2 in an organic solvent for solvent exchange to remove the residual moisture on the film surface and slow down the defects generated in the selective layer due to side reactions during the subsequent interfacial crosslinking reaction, then take it out and fix it with a mold. Step 4: Add the isocyanate organic phase solution into the mold fixing the polyphenol-coated film to submerge the polyphenol-coated film, perform interfacial crosslinking on the film surface, then rinse with an organic solvent and deionized water respectively to obtain a polyurethane composite membrane, and store it in deionized water.

2. The preparation method of a polyurethane composite nanofiltration membrane according to claim 1, characterized in that, In Step 1, the concentration of the mixed solution of hexamethylenediamine and isopropanol is 5 - 20 g / L, and the crosslinking treatment time is 2 - 10 h.

3. The preparation method of a polyurethane composite nanofiltration membrane according to claim 1, characterized in that, In Step 2, the polyphenol monomer is one or more of tannic acid, dopamine, or gallic acid, and the concentration of the polyphenol monomer is 0.1 - 5 g / L.

4. The preparation method of a polyurethane composite nanofiltration membrane according to claim 1, characterized in that, In Step 2, the buffer solution is an aqueous Tris solution with a pH value of 8.2 - 8.

4.

5. The preparation method of a polyurethane composite nanofiltration membrane according to claim 1, characterized in that, In Step 2, the coating time of the polyphenol monomer is 10 - 300 min, and the coating condition is static coating.

6. The preparation method of a polyurethane composite nanofiltration membrane according to claim 1, characterized in that, In Step 3, the organic solvent for solvent exchange is n-hexane or chloroform.

7. The preparation method of a polyurethane composite nanofiltration membrane according to claim 1, characterized in that, In Step 3, the solvent exchange time is 5 - 30 min.

8. The preparation method of a polyurethane composite nanofiltration membrane according to claim 1, characterized in that, In Step 4, the isocyanate is toluene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, or hexamethylene diisocyanate.

9. The preparation method of a polyurethane composite nanofiltration membrane according to claim 1, wherein, In Step 4, the concentration of the isocyanate organic phase solution is 0.01% - 1% m / V, and the isocyanate crosslinking time is 15 - 240 s.

10. A composite nanofiltration membrane obtained by the preparation method of the polyurethane composite nanofiltration membrane according to any one of claims 1 - 9, including a polyimide-based film and a polyurethane selective layer prepared by crosslinking polyphenols with isocyanate, and the thickness of the polyurethane selective layer is 10 - 200 nm.

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