Membrane casting solution, composite membrane and preparation method of composite membrane

Abstract

The invention discloses a membrane casting solution, a polyamide composite membrane and a preparation method of the polyamide composite membrane, and the membrane casting solution for preparing a polysulfone-based membrane comprises polysulfone, a pore-foaming agent and nanocellulose, and the diameter of the nanocellulose is 10-50 nm. Therefore, by adding the nanocellulose into the membrane casting solution for preparing the polysulfone-based membrane, after the membrane casting solution forms the membrane, the mechanical property of the prepared polysulfone-based membrane can be improved.

Description

Technical Field

[0001] The present application relates to the technical field of separation membranes, and in particular, to a membrane casting solution, a composite membrane and a preparation method thereof. Background Art

[0002] Polyamide composite membrane is a semipermeable membrane with selective permeability. The selective permeability of the composite membrane is reflected in the ability to intercept salt and most organic matter while maintaining a certain molecular flux. Therefore, it can be used for a variety of fluid separations and is now widely used in many fields such as petrochemicals, pharmaceuticals, food, and urban sewage treatment. Polyamide composite membranes are usually composed of a multilayer structure. The pore size, porosity, mechanical strength, flexibility, and surface hydrophilicity of the composite membrane will have an important influence on the desalination performance and mechanical properties of the polyamide composite membrane. Therefore, it is urgent to develop a base membrane with both high porosity and high strength to effectively improve the comprehensive performance of the composite membrane. Summary of the invention

[0003] In a first aspect of the present application, the present application proposes a casting solution for preparing a polysulfone-based membrane, comprising: polysulfone, a porogen, and nanocellulose, wherein the diameter of the nanocellulose is 10 nm-50 nm.

[0004] By adding nanocellulose to the casting solution for preparing the polysulfone-based membrane, the mechanical properties of the prepared polysulfone-based membrane can be improved after the casting solution forms a membrane.

[0005] In some embodiments, the mass fraction of the polysulfone in the casting solution is 15%-17%.

[0006] When the mass fraction of polysulfone in the casting solution is within the aforementioned range, the pore size and porosity of the prepared polysulfone-based membrane are both within a relatively suitable range, and the polysulfone-based membrane can not only serve as a place for the polymerization reaction to form polyamide, but also provide skeleton support while reducing the adverse effects on water permeation resistance and composite membrane flux. Within this range, the polysulfone-based membrane made from the casting solution has good permeability.

[0007] In some embodiments, the mass fraction of the porogen in the casting solution is 9%-15%.

[0008] When the mass fraction of the porogen in the casting solution is within the aforementioned range, the porosity of the prepared polysulfone-based membrane is more appropriate.

[0009] In some embodiments, the mass fraction of the nanocellulose in the casting solution is 0.5%-1%.

[0010] When the mass fraction of nanocellulose in the casting solution is within the aforementioned range, the pressure resistance of the polysulfone-based membrane prepared from the casting solution can be appropriately improved without excessively increasing the water permeation resistance of the polysulfone-based membrane, and the effect on the flux of the composite membrane is relatively small.

[0011] In some embodiments, the organic solvent in the casting solution includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide.

[0012] The selected organic solvent has good compatibility and can be used as a main solvent or as an auxiliary solvent, so that the amount of the organic solvent added can be adjusted according to the concentration requirements of the casting solution to meet the requirements of the preparation concentration.

[0013] In some embodiments, the porogen includes at least one of polyethylene glycol, polyvinyl pyrrolidone, ethylene glycol monomethyl ether, and ethylene glycol dimethyl ether.

[0014] The porogen is a water-soluble substance. When the casting solution is formed into a membrane, the porogen will be mixed into the membrane layer structure, and after being dissolved in water, a porous structure will be formed, so that the formed polysulfone-based membrane has a suitable membrane flux and structural strength.

[0015] In the second aspect of the present application, the present application proposes a method for preparing a casting liquid, comprising: adding nanocellulose and ionic liquid to an organic solvent under heating conditions to obtain a nanocellulose solution, wherein the diameter of the nanocellulose is 10nm-50nm; adding a porogen and polysulfone to the nanocellulose solution under heating conditions to obtain a casting liquid.

[0016] In some embodiments, the ionic liquid includes at least one of N-methylmorpholine-N-oxide (NMMO), 1-butyl-3-methylimidazolium chloride, and 1-allyl-3-methylimidazolium chloride.

[0017] Nanocellulose, ionic liquid and the aforementioned organic solvent selected can form a non-aqueous nanocellulose solvent system (i.e. nanocellulose solution), wherein, nanocellulose can fully swell. Under heating, porogen, polysulfones are added to the nanocellulose solution, to obtain a film casting solution. In the polysulfone film casting solution, nanocellulose is added as a skeleton support, nanocellulose contains a large amount of hydroxyls in the structure, and hydrophilicity is better, thereby the hydrophilicity and the structural strength of the polysulfone-based membrane formed can be improved, and then the hydrophilicity, mechanical property and the pliability of the composite membrane can be promoted.

[0018] In the third aspect of the present application, the present application proposes a method for preparing a composite membrane, comprising: coating a polysulfone-based membrane on a non-woven fabric to obtain a polysulfone-based membrane, wherein the polysulfone-based membrane is generated using a casting liquid, or, generated using the prepared casting liquid; forming a polyamide layer on the polysulfone-based membrane to obtain the composite membrane.

[0019] The composite membrane is obtained by coating the aforementioned casting solution on a non-woven fabric to form a polysulfone-based membrane and forming a polyamide layer on one side of the polysulfone-based membrane. Thus, the composite membrane has all the characteristics and advantages of the aforementioned casting solution and preparation method, which will not be described in detail here.

[0020] In some embodiments, the forming of the polyamide layer comprises: contacting the polysulfone-based membrane with an aqueous solution containing amine substances and an oil solution containing acyl chloride substances, and performing a heating treatment to form the polyamide layer; and performing an acid washing treatment on the polyamide layer.

[0021] The polyamide layer is a semipermeable membrane with selective permeability. Polyamine and acyl chloride substances undergo interfacial polycondensation reaction on the surface of the polysulfone base membrane to form a polyamide layer. The acid washing process can adjust the pore size distribution and pore size of the polyamide layer to obtain a porous and rough surface polyamide layer.

[0022] In some embodiments, the acidic solution in the pickling treatment includes at least one of citric acid, oxalic acid, hydrochloric acid, sulfuric acid, and phosphoric acid.

[0023] The acidic solution used in the pickling treatment can play a good pore-forming role, and a high-flux composite membrane structure can be obtained at a suitable reaction rate.

[0024] In some embodiments, the concentration of the acid solution in the pickling treatment is 1%-20%.

[0025] When the concentration of the acidic solution is within the aforementioned range, the polyamide layer can be treated to make its surface structure rougher and increase the specific surface area, while reducing the side reactions between the acidic solution and other parts of the composite membrane.

[0026] In a fourth aspect of the present application, the present application proposes a composite membrane obtained by using the method for preparing the composite membrane.

[0027] The composite membrane proposed by the present invention has the characteristics of high pressure resistance and high flux. Among them, the polysulfone-based membrane has the characteristics of high porosity, high strength and high flexibility. The nanocellulose added in the preparation process has a wide source, low price, simple preparation technology, easy operation, and is suitable for industrial promotion and application. DETAILED DESCRIPTION

[0028] The embodiments of the present application are described in detail below. Examples of the embodiments are shown, but unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of actually identical structures may be omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the following description is provided for those skilled in the art to fully understand the present application and is not intended to limit the subject matter described in the claims.

[0029] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by technicians in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; unless otherwise specified, the numerical values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0030] The terms "including" and "having" in the specification and claims of the present application and any modifications thereof are open expressions, that is, including the contents specified in the present application but not excluding other contents.

[0031] In the description of this application, regardless of whether the words "about" or "approximately" are used, all the numbers disclosed herein are approximate values. The value of each number may differ by less than 10% or a reasonable difference considered by those skilled in the art, such as 1%, 2%, 3%, 4% or 5%.

[0032] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0033] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0034] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0035] Among the common separation membranes, the reverse osmosis membrane can be a polyamide composite membrane, which can effectively separate ions and water molecules in water by constructing a non-porous polyamide separation layer. The salt water separation performance is an important indicator to measure the filtration performance of the reverse osmosis membrane, which is often controlled by the composition and structure of the polyamide layer. However, the separation performance of the polyamide layer does not only depend on its own structural characteristics. The structure of the base membrane and the combination form of the polyamide layer and the base membrane will also affect the overall separation performance of the composite membrane. The polyamide composite membrane is usually composed of a three-layer membrane structure, which are non-woven fabrics, polysulfone base membranes and polyamide layers stacked in sequence. The polysulfone base membrane is made of polysulfone casting liquid as the raw material and is coated on a non-woven fabric substrate. The polysulfone base membrane is the reaction site for generating the polyamide layer through interfacial polycondensation technology. Its pore size, porosity, mechanical strength, flexibility, and surface hydrophilicity will ultimately have an important influence on the desalination performance and mechanical properties of the polyamide composite membrane.

[0036] On the one hand, the surface hydrophilicity, pore size and porosity of the polysulfone-based membrane will affect the resistance of the polysulfone-based membrane to the separated substances, thereby affecting the permeability of the composite membrane. It will also affect the distribution state of the aqueous monomer on the surface of the polysulfone-based membrane during the interfacial polycondensation to generate polyamide, as well as the diffusion process of the aqueous monomer to the oil phase, thereby affecting the desalination performance of the polyamide layer. On the other hand, the mechanical properties and flexibility of the polysulfone-based membrane have a direct impact on the mechanical properties of the polyamide composite membrane. For example, if the porosity of a porous support layer of a simple polysulfone component is too large, it will lead to poor mechanical strength and flexibility, which will cause the composite membrane prepared from the high-porosity polysulfone-based membrane to exhibit poor pressure resistance.

[0037] In a first aspect of the present application, the present application proposes a casting solution for preparing a polysulfone-based membrane, comprising: polysulfone, a porogen, and nanocellulose, wherein the diameter of the nanocellulose is 10 nm-50 nm.

[0038] There are a large amount of hydroxyl groups on the cellulose molecular chain, which is easy to form hydrogen bonds and has high hydrophilicity. In the casting solution for preparing the polysulfone-based membrane, the nanocellulose of the aforementioned diameter range is added, and the mechanical strength of the nanocellulose itself is utilized to improve the structural stability of the porous structure in the polysulfone-based membrane. The aforementioned casting solution helps to improve the porosity of the formed polysulfone-based membrane, and when improving the membrane flux of the polysulfone-based membrane, also helps to improve the mechanical property and the hydrophilicity of the polysulfone-based membrane.

[0039] In some embodiments, the mass fraction of the polysulfone in the casting solution is 15%-17%.

[0040] When the mass fraction of polysulfone in the casting solution is within the aforementioned range, polysulfone and nanocellulose are evenly mixed, nanocellulose can provide mechanical support for the pores formed in the formed polysulfone-based membrane, the pore size and porosity of the prepared polysulfone-based membrane are also within a relatively appropriate range, and the membrane flux of the polysulfone-based membrane is better.

[0041] In some embodiments, the mass fraction of the porogen in the casting solution is 9%-15%.

[0042] The proper addition of porogens helps to control the formation of pores in the membrane structure, obtain the desired porosity, and obtain a polysulfone-based membrane with high permeability and necessary mechanical strength. When the mass fraction of the porogen in the casting solution is within the aforementioned range, the porogen plays a place-occupying role in the structure, helps the formation of the pore structure in the polysulfone-based membrane structure, helps to make the pore size distribution of the pores more uniform, and the distribution of the pores in the membrane structure more uniform.

[0043] It is understandable that the continuous increase in porosity does not lead to a continuous increase in the permeability of the polysulfone-based membrane, but rather affects the dissolution and solidification process of the casting solution. Therefore, adding a proper amount of porogen can obtain the desired porosity and obtain a polysulfone-based membrane with high permeability and necessary mechanical strength. Those skilled in the art can adjust according to actual conditions.

[0044] In some embodiments, the mass fraction of the nanocellulose in the casting solution is 0.5%-1%.

[0045] When the mass fraction of nanocellulose in the casting solution is within the aforementioned range, the nanocellulose mixed into the casting solution can be used as the skeleton support of the polysulfone-based membrane to form a porous structure with a larger pore size. The mechanical support of nanocellulose can effectively improve the stability of the porous structure, so that the polysulfone-based membrane has both high membrane flux and excellent mechanical strength. Nanocellulose also has high strength and high toughness and excellent hydrophilicity, which can improve the pressure resistance and mechanical strength of the polysulfone-based membrane and improve its hydrophilicity. In some embodiments, the organic solvent in the casting solution includes at least one of N, N-dimethylformamide, N, N-dimethylacetamide, and dimethyl sulfoxide.

[0046] The aforementioned organic solvents have good compatibility with cellulose and polysulfone, and can be used as the main solvent or as the auxiliary solvent, so that the concentration of the casting solution can be adjusted according to the concentration requirements of the casting solution to achieve the conditions required for completing the curing process of the polysulfone-based membrane.

[0047] In some embodiments, the porogen includes at least one of polyethylene glycol, polyvinyl pyrrolidone, ethylene glycol monomethyl ether, and ethylene glycol dimethyl ether.

[0048] The porogen is a water-soluble substance that assists the polysulfone and nanocellulose to form a pore structure when the casting solution forms a membrane. After the rough surface polysulfone-based membrane containing a porous structure is formed, the porogen can be dissolved in water and the polysulfone-based membrane structure can be formed by washing with water, etc., so that the porous structure is formed in the polysulfone-based membrane, so that the formed polysulfone-based membrane has a suitable membrane flux.

[0049] In a second aspect of the present application, the present application proposes a method for preparing a casting solution, comprising:

[0050] S100: Under heating conditions, adding nanocellulose and ionic liquid into an organic solvent to obtain a nanocellulose solution, wherein the diameter of the nanocellulose is 10nm-50nm

[0051] In some embodiments, the ionic liquid includes at least one of N-methylmorpholine-N-oxide, 1-butyl-3-methylimidazolium chloride, and 1-allyl-3-methylimidazolium chloride.

[0052] In some embodiments, the diameter of the nanocellulose is 10 nm-50 nm.

[0053] Cellulose nanofibrils (CNF) in nanocellulose contain crystalline, quasi-crystalline and amorphous regions of cellulose, with an aspect ratio greater than 10, and may contain bifurcations, intertwined or network structures. When the diameter of nanocellulose is within the aforementioned range, when preparing the polyamide layer, the length of the nanocellulose itself will not affect the penetration and uniform spreading of the casting solution on the polysulfone-based membrane. In the process of preparing a high-porosity polysulfone-based membrane by controlling the polysulfone content and the porogen content, the mechanical strength, flexibility and hydrophilicity of the porous support layer are improved by blending nanocellulose into the polysulfone casting solution, thereby improving the inter-layer bonding force and compressive mechanical properties of the final polyamide composite membrane product.

[0054] In some embodiments, the swelling time of nanocellulose in the organic solvent is greater than or equal to 30 min.

[0055] There are a large number of hydrogen bonds on the cellulose molecular chain, which can form crystal regions. Heating helps solvent molecules enter the crystal regions, allowing the nanocellulose to fully swell.

[0056] In some embodiments, after the nanocellulose is swollen, an ionic liquid is added to obtain a nanocellulose solution.

[0057] The amount of organic solvent added should cover the nanocellulose. Under heating, the organic solvent enters the crystalline and amorphous regions of the cellulose, and the cellulose is fully swollen. The addition of ionic liquid further promotes the dissolution of cellulose to form a cellulose solution.

[0058] S200: Under heating conditions, adding a porogen and polysulfone to the nanocellulose solution to obtain a casting solution.

[0059] In some embodiments, the heating temperature in step S200 is 85°C-95°C.

[0060] When the preparation operation is within the temperature range of the aforementioned heating conditions, the porogen, polysulfone and nanocellulose are fully miscible with the organic solvent, dissolve quickly, and will not induce other chemical reactions.

[0061] In some embodiments, when preparing the casting solution, stirring may be performed simultaneously, and the stirring speed is 300r / min-500r / min.

[0062] Stirring is beneficial to the full mixing of the solutes in the organic solution to form a mixed solvent that is evenly dispersed and fully mixed.

[0063] In some embodiments, polysulfone particles are added in small amounts and multiple times in step S200 .

[0064] Adding polysulfone particles in small amounts and multiple times is beneficial to the full dissolution of polysulfone in the organic solvent and to the full mixing of polysulfone in the organic solvent with nanocellulose.

[0065] In a third aspect of the present application, the present application proposes a method for preparing a composite membrane, comprising:

[0066] S1: coating a polysulfone-based membrane on a non-woven fabric to obtain a polysulfone-based membrane, wherein the polysulfone-based membrane is produced by using a casting solution, or the prepared casting solution is used to produce

[0067] In some embodiments, the nonwoven fabric comprises a polyester nonwoven fabric.

[0068] The polyester nonwoven fabric, as the bottom support layer of the composite membrane, can provide high mechanical strength support for the polysulfone-based membrane coated thereon.

[0069] In some embodiments, the casting solution is coated on the non-woven fabric by slot extrusion, and then solidified to form a polysulfone-based membrane.

[0070] By the coating mode of slit extrusion, the thickness of the polysulfone based membrane formed by coating is more uniform, and in the operating process, the thickness of the polysulfone based membrane can also be regulated more easily. Further, by controlling the polysulfone content and the porogen content in the casting solution, the porosity and the permeation flux of the polysulfone based membrane can be improved. By controlling the content of the nanocellulose in the casting solution, the pressure resistance, mechanical strength, flexibility and the hydrophilicity of the polysulfone based membrane can be improved.

[0071] In some embodiments, the nonwoven fabric coated with the casting solution is immersed in a 18° C. pure water coagulation bath to solidify into a film, and finally washed with 40° C. and 60° C. pure water in sequence to obtain a polysulfone-based membrane.

[0072] In the pure water coagulation bath, the casting solution coated on the non-woven fabric solidifies to form a polysulfone-based membrane. In the coagulation bath, the temperature and humidity of the polysulfone curing are relatively stable, which is conducive to the full completion of the polysulfone curing process. During the curing process, the porogen in the casting solution dissolves and detaches from the formed polysulfone-based membrane, leaving a porous structure. Pure water cleaning can further remove uncured chemicals from the polysulfone-based membrane, which is conducive to the unobstructed pores and improves the membrane flux of the polysulfone-based membrane.

[0073] S2: Forming a polyamide layer on the polysulfone-based membrane

[0074] In some embodiments, the polyamide layer is formed by interfacial polycondensation of an aqueous solution and an oily solution.

[0075] The reaction temperature of the interfacial polycondensation reaction is low and the film-forming speed is fast. The formed polyamide is insoluble in aqueous solvents and oily solvents and can undergo polycondensation reaction well along the interface where the reaction occurs.

[0076] In some embodiments, the aqueous phase solution is obtained by mixing appropriate amounts of m-phenylenediamine / piperazine and triethylamine hydrochloride dissolved in pure water.

[0077] Adding amino groups to the aqueous solution can increase the hydrophilicity of the generated polyamide layer and improve the water flux of the polyamide layer.

[0078] In some embodiments, the pH value of the aqueous solution can be adjusted to alkaline using an alkaline substance, wherein the pH value of the aqueous solution can be adjusted to about 11 using sodium hydroxide.

[0079] In the polycondensation reaction of polyamide, alkaline substances act as acid acceptors to remove acidic substances generated during the polycondensation reaction to maintain stable reaction conditions, which is conducive to the smooth progress of the polycondensation reaction.

[0080] In some embodiments, the oil phase solution is obtained by mixing an appropriate amount of trimesoyl chloride and an organic solvent.

[0081] As a reaction monomer for synthesizing the polyamide layer, trimesoyl chloride is mixed in an organic solvent, which is beneficial to the dispersion of the monomer and facilitates the smooth progress of the polycondensation reaction on the interface later.

[0082] In some embodiments, the polysulfone-based membrane is contacted with an aqueous solution containing amine substances and an oil-phase solution containing acyl chloride substances by contacting the polysulfone-based membrane with the aqueous solution by dipping or slit extrusion for 20 seconds, and then removing excess aqueous solution; and contacting the oil-phase solution by dipping or slit extrusion for 15 seconds, and then removing excess oil-phase solution.

[0083] The aqueous solution is coated on the polysulfone-based membrane by dipping or slit extrusion. The operation is simple and the addition amount of the aqueous solution and the oil solution is easy to control.

[0084] In some embodiments, the aqueous phase solution is contacted with the oil phase solution under the condition of applying an external electrostatic field.

[0085] Applying an external electrostatic field during the interfacial polymerization reaction can affect the degree of polymerization reaction, optimize the structure of the generated polyamide layer, and help improve the desalination and flux performance of the polyamide reverse osmosis membrane.

[0086] In some embodiments, the composite film with the polyamide layer formed thereon is treated in an oven at 50° C. for 3 minutes to obtain a composite film semi-finished product.

[0087] The heating operation promotes the curing of the polyamide layer into a film and removes the volatile substances generated by the reaction, which is conducive to obtaining a composite membrane with a complete pore structure.

[0088] In some embodiments, the polyamide layer is subjected to an acid-washing treatment under the condition of applying an external electrostatic field.

[0089] Applying an external electrostatic field during the pickling process of the polyamide layer can promote the neutralization reaction between the acid solution and the unreacted amine, improve the pickling efficiency, and achieve a better effect of removing the unreacted amine at room temperature.

[0090] In some embodiments, the acidic solution in the pickling treatment includes at least one of citric acid, oxalic acid, hydrochloric acid, sulfuric acid, and phosphoric acid.

[0091] The acid solution of the pickling treatment acts on the polyamide layer, making the surface of the polyamide layer rough, while not introducing substances that are easily attached to the membrane surface, so as to obtain a high-flux composite membrane.

[0092] In some embodiments, the concentration of the acid solution in the pickling treatment is 1%-20%.

[0093] When the concentration of the acidic solution is within the aforementioned range, a better amine washing effect can be achieved at room temperature, and the generated composite membrane semi-finished product will not be damaged due to excessive acidity of the acidic solution.

[0094] In some embodiments, the acid washing treatment is to immerse the semi-finished composite membrane in an acid solution for 1 min-10 min, and then wash it with pure water to obtain a polyamide composite membrane.

[0095] Acid treatment makes the surface structure of polyamide rougher, increases the specific surface area, and improves the hydrophilicity of the polyamide layer, which is beneficial to improving the membrane flux of the composite membrane.

[0096] In some embodiments, the polyamide composite membrane is immersed in a 2%-15% glycerol solution for 20 seconds to 100 seconds, and then dried in an oven at 60 degrees Celsius for 2 minutes to 10 minutes to obtain a dry polyamide composite membrane.

[0097] The polyamide composite membrane is soaked in glycerol and then dried. During the drying process, glycerol can occupy the pores and reduce the shrinkage and collapse of the pores in the polyamide composite membrane caused by the drying process.

[0098] In the fourth aspect of the present application, the present application proposes a composite membrane obtained by the method for preparing the composite membrane. The composite membrane proposed by the present invention has the characteristics of high pressure resistance and high flux. Among them, the improvement of the porosity of the porous support layer can also improve the bonding strength between the polysulfone-based membrane and the polyamide layer to a certain extent, and improve the overall strength of the composite membrane. Because the bonding between the polysulfone-based membrane and the polyamide layer relies only on a physical "plug-in" connection. The strength of the overall "plug-in" connection depends on the "number and uniform distribution of inserted components" × "connection strength of a single plug-in". Increasing the porosity of the polysulfone-based membrane is equivalent to increasing the "number of inserted components", which is beneficial to improving the overall characteristics of the composite membrane.

[0099] The scheme of the present application is described below by specific examples. It should be noted that the following examples are only used to illustrate the present application and should not be considered as limiting the scope of the present application. If no specific technology or conditions are specified in the examples, the technology or conditions described in the literature in this area or the product instructions are used. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained commercially.

[0100] Example 1

[0101] (1) Dissolution of nanocellulose

[0102] An appropriate amount of nanocellulose dry powder with a fiber diameter ranging from 10nm to 50nm is swollen in dimethyl sulfoxide at 90°C for 1h. The amount of dimethyl sulfoxide added is just enough to make the nanocellulose fully swell. At 90°C, NMMO solvent is added while stirring until the nanocellulose is completely dissolved to obtain a nanocellulose solution.

[0103] (2) Preparation of polysulfone casting solution

[0104] Add polyethylene glycol 200 and dimethyl sulfoxide to the nanocellulose solution obtained in step (1), start stirring, and the stirring speed is 400r / min; add polysulfone particles in small amounts and multiple times during the stirring process, and avoid the polysulfone particles from sinking to the bottom during the addition process; after the polysulfone particles are added, stir at room temperature for 0.5h, then raise the temperature to 70°C, and continue stirring until the polysulfone particles are completely dissolved. The mass fractions of nanocellulose dry powder, polyethylene glycol 200, and polysulfone particles in the casting solution are 0.8%, 10%, and 16%, respectively.

[0105] (3) Preparation of polysulfone-based membrane

[0106] The casting solution obtained in step (2) is coated on a polyester non-woven fabric by slit extrusion, immersed in a 18°C ​​pure water coagulation bath to solidify into a film, and finally washed with 40°C and 60°C pure water in turn to obtain a polysulfone-based membrane.

[0107] (4) Preparation of composite membrane

[0108] 1) Dissolve an appropriate amount of m-phenylenediamine and triethylamine hydrochloride in pure water to prepare an aqueous solution, and add NaOH to adjust the pH of the aqueous solution to about 11 for later use. Dissolve an appropriate amount of trimesic acid chloride in an ethylcyclohexane solution to prepare an oil solution for later use. 2) Contact the polysulfone-based membrane with the aqueous solution for 20 seconds to remove excess droplets on the membrane surface. 3) Contact the membrane of step 2) with the oil solution for 15 seconds to remove excess oil on the surface. 4) Place the membrane of step 3) in a 50°C oven for 3 minutes to obtain a nascent polyamide reverse osmosis membrane. 5) Soak the nascent polyamide reverse osmosis membrane in a 25°C, 1% sulfuric acid solution for 3 minutes. 6) Soak the membrane of step 5) in an 8% glycerol solution for 1 minute, and finally dry it in a 60-degree Celsius oven for 5 minutes to obtain a dry film sample.

[0109] Example 2

[0110] Example 2 is consistent with Example 1, except that, in the step of preparing the polysulfone casting solution, the mass fractions of nanocellulose dry powder, polyethylene glycol 200, and polysulfone particles in the casting solution are 0.8%, 8%, and 18%, respectively.

[0111] Example 3

[0112] Example 3 is consistent with Example 1, except that, in the step of preparing the polysulfone casting solution, the mass fractions of nanocellulose dry powder, polyethylene glycol 200 and polysulfone particles in the casting solution are 0.5%, 10% and 16% respectively.

[0113] Example 4

[0114] Example 4 is consistent with Example 1, except that, in the step of preparing the polysulfone casting solution, the mass fractions of nanocellulose dry powder, polyethylene glycol 200, and polysulfone particles in the casting solution are 1%, 10%, and 16%, respectively.

[0115] Comparative Example 1

[0116] (1) Preparation of polysulfone-based membrane

[0117] 16% polysulfone, 10% polyethylene glycol 200, and 74% dimethyl sulfoxide were mixed, heated, and stirred to obtain a completely dissolved casting solution. The casting solution was applied to a polyester non-woven fabric by slit extrusion, immersed in a 18°C ​​pure water coagulation bath to solidify the film, and finally washed with 40°C and 60°C pure water in turn to obtain a polysulfone-based membrane.

[0118] (2) Preparation of composite membrane

[0119] 1) Dissolve an appropriate amount of m-phenylenediamine and triethylamine hydrochloride in pure water to prepare an aqueous solution, and add NaOH to adjust the pH of the aqueous solution to about 11 for later use. Dissolve an appropriate amount of trimesic acid chloride in an ethylcyclohexane solution to prepare an oil solution for later use. 2) Contact the polysulfone-based membrane with the aqueous solution for 20 seconds to remove excess droplets on the membrane surface. 3) Contact the membrane of step 2) with the oil solution for 15 seconds to remove excess oil on the surface. 4) Place the membrane of step 3) in a 50°C oven for 3 minutes to obtain a nascent polyamide reverse osmosis membrane. 5) Soak the nascent polyamide reverse osmosis membrane in a 25°C, 1% sulfuric acid solution for 3 minutes. 6) Soak the membrane of step 5) in an 8% glycerol solution for 1 minute, and finally dry it in a 60-degree Celsius oven for 5 minutes to obtain a dry film sample.

[0120] Comparative Example 2

[0121] Comparative Example 2 is consistent with Comparative Example 1, except that the casting solution is obtained by mixing 18% by mass of polysulfone, 10% by mass of polyethylene glycol 200, and 74% by mass of dimethyl sulfoxide.

[0122] The polyamide composite membranes in the aforementioned examples and comparisons were subjected to the following tests, and the test results are shown in Table 1.

[0123] 1. Pressure resistance test of membrane:

[0124] Cut the membrane into the shape and size specified by the test board and place it in the test equipment. For specific test methods, refer to the 7th pressure resistance test in "Reverse Osmosis Membrane Test Method" GB / T 32373-2015.

[0125] 2. Membrane flux and rejection rate test:

[0126] Cut the membrane into the shape and size specified by the test board and place it in the test equipment. Prepare the test solution as 250ppm sodium chloride test solution. The test conditions are test pressure 0.48 MPa, test water temperature 25℃, and test recovery rate 15%. Use an electronic scale to test the water production per unit time per unit membrane area to calculate the water flux, and use a conductivity meter to test the conductivity of the produced water and raw water to calculate the retention rate. For specific methods, refer to "Reverse Osmosis Membrane Test Method" GB / T 32373-2015.

[0127]

[0128]

[0129] According to Comparative Examples 1 and 2, the polyamide composite membrane prepared from the polysulfone-based membrane with low polysulfone content and high porogen content can have a higher permeation flux while maintaining high desalination performance, but the pressure resistance is poor. After the polysulfone-based membrane is added with nanocellulose, the pressure resistance is significantly improved. It can be seen from Comparative Example 1 and Example 1 that after adding 0.8% nanocellulose, the flux attenuation rate of the 1.5 MPa pressure test is reduced from 24.7% to 5%; the flux attenuation rate of the 3 MPa pressure test is reduced from 39.8% to 10%. Moreover, when the flux performance of Example 1 is obviously dominant, the 3 MPa pressure resistance performance has exceeded that of Comparative Example 2 with a high polysulfone content.

[0130] According to Examples 1 to 4, the flux of the polyamide composite membrane prepared by the polysulfone-based membrane with high polysulfone content is low. After adding nanocellulose, the water seepage resistance will be further increased. For the polyamide composite membrane prepared by the polysulfone-based membrane with low polysulfone content, when the amount of nanocellulose added is increased from 0.5% to 1%, the pressure resistance will be improved accordingly, but it will also cause the water seepage resistance of the polysulfone-based membrane to be too large, affecting the flux performance of the composite membrane. In practical applications, the appropriate membrane formulation can be selected according to different application scenarios.

[0131] In the description of the present application, “plurality” means two or more.

[0132] In the description of the present application, "A and / or B" may include the case of A alone, the case of B alone, or any of the cases of A and B, where A and B are only used for examples and may be any technical feature connected by "and / or" in the present application.

[0133] In the present application, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process. The specific execution order of each step should be determined by its function and possible internal logic. If there is no special explanation, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed in sequence, or may include steps (b) and (a) performed in sequence. For example, the method may also include step (c), indicating that step (c) can be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0134] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A casting solution for preparing a polysulfone-based membrane, characterized in that: include: Polysulfone, porogen, nanocellulose, wherein the diameter of the nanocellulose is 10nm-50nm.

2. The casting solution according to claim 1, characterized in that: The mass fraction of the polysulfone in the casting solution is 15%-17%.

3. The casting solution according to claim 1 or 2, characterized in that: The mass fraction of the porogen in the casting solution is 9%-15%.

4. The casting solution according to claim 1 or 2, characterized in that: The mass fraction of the nanocellulose in the casting solution is 0.5%-1%.

5. The casting solution according to claim 1 or 2, characterized in that: The organic solvent in the casting solution includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide and dimethyl sulfoxide; and / or, The porogen includes at least one of polyethylene glycol, polyvinyl pyrrolidone, ethylene glycol monomethyl ether and ethylene glycol dimethyl ether.

6. A method for preparing the casting solution according to any one of claims 1 to 5, characterized in that: include: Under heating conditions, adding nanocellulose and ionic liquid into an organic solvent to obtain a nanocellulose solution, wherein the diameter of the nanocellulose is 10nm-50nm; Under heating conditions, porogen and polysulfone are added into the nanocellulose solution to obtain a casting solution.

7. A method for preparing a composite membrane, characterized in that: include: Coating a polysulfone-based membrane on a non-woven fabric to obtain a polysulfone-based membrane, wherein the polysulfone-based membrane is produced by using the casting solution described in any one of claims 1 to 5, or by using the casting solution prepared by the method described in claim 6; A polyamide layer is formed on the polysulfone-based membrane to obtain the composite membrane.

8. The method according to claim 7, characterized in that The forming of the polyamide layer comprises: The polysulfone-based membrane is contacted with an aqueous solution containing amine substances and an oil solution containing acyl chloride substances, and subjected to a heating treatment to form the polyamide layer; The polyamide layer is subjected to an acid-washing treatment.

9. The method according to claim 8, characterized in that The acid solution in the pickling treatment includes at least one of citric acid, oxalic acid, hydrochloric acid, sulfuric acid, and phosphoric acid; and / or The concentration of the acid solution in the pickling treatment is 1%-20%.

10. A composite membrane, characterized in that The method is prepared by any one of claims 7 to 9.

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