PROCESS FOR PRODUCING COMPOSITE SEMIPERMEABLE MEMBRANE

Abstract: A process for producing a composite semipermeable membrane, which comprises coating a microporous support membrane with a polyamide formed by the interfacial polycondensation between a polyfunctional aromatic amine and a polyfunctional acyl halide. The polyfunctional aromatic amine component contains, as the comonomer component, 1 to 20 wt %, based on said aromatic amine, of at least one member selected from among alkyldiamine, 1,3,5-trihydroxybenzene and 1,3-dihydroxybenzene, and hence the resulting composite semipermeable membrane has markedly improved oxidation resistance while maintaining the excellent highly selective separative function and high permeability of a crosslinked polyamide type composite semipermeable membrane. This membrane can be suitably used for desalting of brine or seawater.

Description: This invention relates to a process of producing a composite semipermeable membrane which is useful for selective separation of liquid mixtures, especially for desalination of brackish and sea water. Composite semipermeable membranes prepared by covering a microporous substrate with an ultra-thin membrane of a crosslinked polyamide obtained by interfacial polycondensation reaction between a polyfunctional aromatic amine and a polyfunctional acid halide are now drawing attention as reverse osmosis membranes with high permeability and high separation selectivity. Preparation of the composite membrane by the interfacial polycondensation reaction between the polyfunctional aromatic amine and the polyfunctional acid halide is disclosed in, for example, US-A-4277344, US-A-4520044, US-A-4529646, US-A-4626468, US-A-4643829, US-A-4661254, US-A-4749488 and US-A-4761234. EP-A-0227043 discloses a composite semipermeable membrane which is obtained by crosslinking on a porous support, (1) a polyhydroxy compound and (2) an amino compound having at least two secondary amino groups and no primary amino groups in the molecule with (3) a multifunctional compound capable of reacting with the hydroxyl group and a secondary amino group. The polyhydroxy compound may be resorcinol or phloroglucinol. On the other hand, a composite semipermeable membrane prepared by laminating an ultra-thin membrane of a polyamide obtained by interfacial polycondensation reaction between ethylenediamine and a polyfunctional acid halide on a microporous substrate is known as a reverse osmosis membrane with excellent oxidation resistance and high separation selectivity, which semipermeable membrane is disclosed in, for example, EP-A-0085111. Although the composite semipermeable membrane prepared by covering a microporous substrate with an ultra-thin membrane of the crosslinked polyamide obtained by interfacial polycondensation reaction between a polyfunctional aromatic amine and a polyfunctional acid halide has excellent permeability and separation selectivity, it does not have a satisfactory oxidation resistance which is required for a practical reverse osmosis membrane. That is, in a practical reverse osmosis process, the system is usually washed with an oxidant for removing organic substances fixed in the system, so that the reverse osmosis membrane is desired to have resistance against the oxidant, expecially against free chlorine. On the other hand, although the composite semipermeable membrane prepared by laminating an ultra-thin membrane of a polyamide obtained by interfacial polycondensation reaction between ethylenediamine and a polyfunctional acid halide on a microporous substrate has excellent oxidation resistance and separation selectivity, it does not have a satisfactory permeability required for a practical reverse osmosis membrane. Permeability is one of the most important factors governing economic efficiency of a practical reverse osmosis process. Thus, a reverse osmosis membrane which has both high permeability and high oxidation resistance is desirable. The object of the present invention is to provide a process of producing a composite semipermeable membrane which has both the high oxidation resistance and high permeability, thereby overcoming the above-mentioned problems of the prior art. That is, the present invention provides a process of producing a composite semipermeable membrane comprising the step of forming on a microporous substrate an ultra-thin membrane containing as a major component a crosslinked polyamide obtained by interfacial polycondensation reaction between a polyfunctional aromatic amine having not less than 2 amino groups in a molecule thereof and a polyfunctional acid halide having not less than 2 halogenated carbonyl groups in a molecule thereof, to form a composite semipermeable membrane, characterized in that at least one member selected from (a) an alkyldiamine of the formula (1): (wherein R¹ and R² are each, independently of one another, C₁ - C₃ alkyl, C₆ phenyl or hydrogen, R³ is a C₁-C₇ straight chain alkyl group which may have one or more side chains or a C₃ - C₇ cycloalkyl group which may have one or more side chains), (b) 1,3,5-trihydroxybenzene and (c) 1,3-dihydroxybenzene is added to the polyfunctional aromatic amine as a copolymerization component in an amount of not less than 1% by weight to not more than 20% by weight with respect to the polyfunctional aromatic amine when the interfacial polycondensation reaction is carried out. By the process of producing a composite, semipermeable membrane of the present invention, the oxidation resistance of the composite semipermeable membrane is largely improved while substantially retaining the high permeability and high separation selectivity of the crosslinked aramide-based composite semipermeable membrane. That is, by the process of the present invention, a composite semipermeable membrane having excellent oxidation resistance as well as high permeability and high separation selectivity can be produced. The composite semipermeable membrane produced by the process of the present invention has an ultra-thin membrane substantially having separation capability laminated on a microporous substrate which substantially does not have separation capability. The ultra-thin membrane comprises as a major component a crosslinked polyamide obtained by an interfacial polycondensation reaction of a polyfunctional aromatic amine and at least one copolymer component selected from the group consisting of alkyldiamines, 1,3,5-trihydroxybenzene and 1,3-dihydroxybenzene with a polyfunctional acid halide. As the polyfunctional aromatic amine, those having two or more amino groups in a molecule, for example, m-phenylenediamine, p-phenylenediamine and 1,3,5-triaminobenzene may be employed. The above-described aromatic amine may be employed individually or in combination. In particular, by employing 1,3,5-triaminobenzene and m-phenylenediamine in combination, a membrane with excellent permeability and separation selectivity may be obtained. The aromatic amine is usually dissolved in water and forms an ultra-thin membrane containing crosslinked polyamide as the major constituent by an interfacial polycondensation upon contact with an acid halide hereinbelow described. In the process of the present invention, at least one copolymer component selected from the group consisting of alkyldiamine, 1,3,5-trihydroxybenzene and 1,3-dihydroxybenzene is added to the polyfunctional aromatic amine in an amount of not less than 1% by weight to not more than 20% by weight, and thereby the oxidation resistance is prominently improved. If the amount of the alkyldiamine, 1,3,5-trihydroxybenzene or 1,3-dihydroxybenzene is less than 1% by weight, the improvement of the oxidation resistance is insufficient, and if it is more than 20% by weight, the permeability is poor. In view of the oxidation resistance and the permeability, the preferred range is 2 - 15% by weight. The alkyldiamine is represented by the formula (1): In the formula (1), R¹ and R² are each, independently of one another, C₁ - C₃ alkyl, C₆ phenyl or hydrogen, R³ is a C₁ - C₇ straight chain alkyl group which may have one or more side chains or a C₃ - C₇ cycloalkyl group which may have one or more side chains. Examples of the alkyldiamine include ethylenediamine, 1,2-propanediamine, 1,3-propanediamine, hexamethylenediamine, 1,2-cyclohexanediamine, 1,3-cyclohexanediamine, 1,4-cyclohexanediamine, N-methylethylenediamine, N,N'-dimethylethylenediamine and N-phenylethylenediamine. These alkyldiamines may be employed in combination as long as the total thereof is not less than 1% by weight and not more than 20% by weight with respect to the aromatic amine. In view of the cost, high permeability, high salt rejection and oxidation resistance of the produced membrane, ethylenediamine is most preferred. In place of the alkyldiamine, 1,3,5-trihydroxybenzene and 1,3-dihydroxybenzene may also be empolyed. Among these, in view of the permeability and separation selectivity of the membrane, 1,3,5-trihydroxybenzene is preferred. These hydroxybenzenes may be employed in combination with the alkyldiamine. In this case, the total thereof is not less than 1% by weight and not more than 20% by weight with respect to the aromatic amine. The polyfunctional acid halide is an acid halide which has two or more halogenated carbonyl groups in one molecule, which yields a polyamide by an interfacial polycondensation reaction with the above-described polyfunctional aromatic amine. Examples of the preferred polyfunctional acid halide include halides of alicyclic and aromatic acid such as cyclohexane-1,3,5-tricarboxylic acid, cyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid, benzene-1,3,5-tricarboxylic acid, benzene-1,3-dicarboxylic acid, and benzene-1,4-dicarboxylic acid. In view of the reactivity with the polyfunctional aromatic amine, the polyfunctional acid halide may preferably be a polyfunctional acid chloride and in view of the separation selectivity and heat resistance of the membrane, the polyfunctional acid halide may preferably be a polyfunctional aromatic acid chloride. Thus, it is most preferable to employ benzene-1,3,5- tricarboxylic acid chloride, benzene-1,3-dicarboxylic acid chloride and/or benzene-1,4-dicarboxylic acid chloride individually or in combination as the polyfunctional acid halide. The microporous substrate does not have substantial separation capability and is employed for reinforcing the ultra-thin membrane substantially having separation capability. The microporous substrate may have micro pores of a uniform size or may have micro pores the pore size of which is gradually enlarged from one surface to another surface, and the pore size at the surface may preferably be not larger than 100 nm. Such a microporous substrate may be selected from commercially available materials such as "Millipore Filter VSWP" (trade name) commercially available from Millipore Co., Ltd. and "Ultrafilter UK10" (trade name) commercially available from Toyo Roshi Co., Ltd. Alternatively, the microporous substrate may preferably be, for example, a polysulfone microporous substrate reinforced with a cloth containing as a major component at least one of polyesters and aromatic polyamides. Such a substrate may be prepared in accordance with "Office of Saline Water Research and Development Progress Report" No. 359 (1968). As the material for forming the substrate, a homopolymer or a blended polymers, such as, polysulfone, cellulose acetate, cellulose nitrate and polyvinyl chloride may be employed. Among these, polysulfone is most preferred since it has chemical, mechanical and thermal stability. In particular, a polysulfone having a repeating unit [A], by which pore size is easily controlled and which has a high dimensional stability is preferred: By casting a solution of this polysulfone in dimethylformamide (DMF) on a densely woven polyester cloth or a non-woven fabric to a prescribed thickness, and wet-coagulating the cast solution in an aqueous solution containing 0.5% by weight of sodium dodecyl sulfate and 2% by weight of DMF, a microporous substrate which has micro pores with a diameter of not larger than 10 nm for most of the area of the surface may be obtained. The process for producing the composite semipermeable membrane will now be described in detail. The ultra-thin membrane with substantial separation capability in the composite semipermeable membrane is formed by the interfacial polycondensation reaction using an aqueous solution containing the above-described polyfunctional aromatic amine and at least one compound selected from the group consisting of alkyldiamine, 1,3,5-trihydroxybenzene and 1,3-dihydroxybenzene, and an organic solution of the above-described polyfunctional acid halide in an organic solvent which is immiscible with water. The concentration of the aromatic amine in the polyfunctional aromatic amine solution may preferably be 0.1 - 10% by weight, more preferably 0.5 - 5.0% by weight. The concentration of at least one compound selected from the group consisting of alkyldiamine, 1,3,5-trihydroxybenzene and 1,3-dihydroxybenzene may preferably be 0.01 - 0.1% by weight, more preferably 0.03 - 0.15% by weight, and the compound is added in the amount of not less than 1% by weight and not more than 20% by weight of the aromatic amine. The aqueous solution may contain for example a surfactant, organic solvent, alkaline compound or anti-oxidant as long as it does not adversely affect the reaction between the amino compound and the polyfunctional acid halide, and the solution preferably has a pH of 6 - 13, more preferably 10 - 13 in view of the reactivity of the alkyldiamine. Further, the aqueous solution may contain a water-soluble macromolecular compound such as water-soluble polyvinyl alcohol in an amount not adversely affecting the performance. The application of the aqueous amine solution on the surface of the microporous substrate may be conducted by any appropriate method as long as the surface of the microporous substrate is covered with the aqueous amine solution uniformly and continuously. For example, the aqueous solution may be coated on the surface of the microporous substrate or the microporous substrate may be immersed in the aqueous solution. The aqueous amine solution applied in excess is removed in a liquid-removing step. The excess solution may be removed by, for example, vertically holding the membrane so that the excess solution naturally drops from the membrane. It is preferable not to leave a drop on the membrane. Although the membrane may be dried after the excess solution is removed, this does not always bring about preferred results. Then the organic solution of the above-described polyfunctional acid halide in the organic solution is applied to the membrane to form the ultra-thin membrane containing crosslinked polyamide as the major component by the interfacial polycondensation. The concentration of the polyfunctional acid halide in the organic solution may preferably be 0.01 - 10% by weight, more preferably 0.02 - 2% by weight. In some cases, it is preferred that the organic solution contain an acylation catalyst such as DMF since the interfacial polycondensation is enhanced. The organic solvent is required to be immiscible to water, to dissolve the acid halide and not to destroy the microporous substrate. Any organic solvent satisfying these requirements, which is inert to the amino compound and to the acid halide may be employed. Preferred examples of the organic solvent include hydrocarbons, for example, trichlorotrifluoroethane. In view of the reaction rate and the volatility of the solvent, n-hexane and trichlorotrifluoroethane are preferred, and in view of the inflammability of the solvent, trichlorotrifluoroethane is most preferred. The polyfunctional acid halide may be contacted with the aqueous phase of the amino compound in the same manner as the application of the aqueous amino compound solution on the microporous substrate. The thus prepared composite semipermeable membrane exhibits satisfactory performance as it is. However, by immersing the thus prepared composite semipermeable membrane in an aqueous solution containing chlorine with a pH of 6 - 13, the performance of the membrane, especially the salt rejection and the water flux may be improved. Examples of the chlorine-generating reagent include chlorine gas, bleaching powder, sodium hypochlorite, chlorine dioxide, Chloramine B, Chloramine T, Halazone, dichlorodimethylhydantoin, and isocyanuric acid chloride as well as salts thereof. The concentration thereof may preferably be determined based on the oxidation power thereof. Among the above-mentioned chlorine-generating reagents, aqueous sodium hypochlorite solution is most preferred in view of the ease of handling. There is an important relationship between the oxidation power and pH of the chlorine-containing aqueous solution. For example, in case of aqueous sodium hypochlorite solution, if the pH is less than 6, the solution does not have a sufficient oxidation power and if the pH is more than 13, the amide bond may be hydrolyzed. If such a chlorine treatment is carried out, a dechlorination step is performed thereafter. The dechlorination may be conducted by, for example, contacting the membrane with a reductant such as a bisulfite salt. The invention will now be described with reference to Examples thereof. In the examples, the solute rejection was calculated by the following equation: Solute Rejection [%] = (1 - Y X ) x 100 wherein X represents the concentration of solute in the supplied liquid and Y represents the concentration of the solute in the permeate. The water flux indicates the amount of water (m³) which passed through the membrane per 1 m² of the membrane per day. Reference Example 1 The fiber-reinforced polysulfone substrate used in the present invention was prepared by the following process: A taffeta sizing 30 cm (longitudinal direction) x 20 cm (lateral direction) consisting of polyester fibers (both of the warp and weft are multifilament yarn of 150 denier, warp density: 90 warps/inch, weft density: 67 wefts/inch, 160 µm thickness) was fixed on a glass plate and a 15% by weight solution of polysulfone (Udel-P3500 commercially available from Union Carbide) in dimethylformamide (DMF) was cast thereon at room temperature (20°C) to a thickness of 200 µm. The resultant film was immediately immersed in pure water and was left to stand for 5 minutes to prepare a fiber-reinforced polysulfone substrate (hereinafter referred to as "FR-PS substrate"). The permeation coefficient for water of the thus obtained FR-PS substrate (210 - 215 µm thickness) measured under a pressure of 1 kg/cm² and at room temperature (25°C) was 0.005 - 0.01 g/cm² sec atm. Example 1 The FR-PS substrate obtained in Reference Example 1 was immersed in an aqueous amine solution with a composition shown in Table 1 for one minute. The substrate was gradually drawn up in the vertical direction to remove the excess aqueous solution from the surface of the substrate. Then a polyfunctional acid chloride (again shown in Table 1) in trichlorotrifluoroethane containing 300 ppm of DMF was applied to the surface of the membrane so that the entire surface of the membrane is wetted completely, and the membrane was left to stand for one minute. After removing the excess solution by vertically holding the membrane, the membrane was immersed in 0.2% by weight aqueous sodium carbonate solution for 5 minutes. The thus prepared composite semipermeable membrane was subjected to a reverse osmosis test under a pressure of 15 kg/cm² at 25°C using 1500 ppm of aqueous sodium chloride solution of which pH was adjusted to 6.5. The membrane performance shown in Table 1 was obtained. The meaning of the abbreviations in the Table is as follows: ED: ethylenediamine TAB: 1,3,5-triaminobenzene m-PDA: meta-phenylenediamine TMC: benzene-1,3,5-tricarboxylic acid chloride (trimesoyl chloride) TPC: terephthaloyl chloride Example 2 A composite semipermeable membrane was produced in the same manner as in Example 1 except that 0.05% by weight of ethylenediamine was added. As a result, membrane performance shown in Table 1 was obtained. Example 3 After immersing the membrane in 0.2% by weight aqueous sodium carbonate solution for 5 minutes in Example 1, the membrane was further immersed in an aqueous solution with a pH of 7 containing 600 ppm of sodium hypochlorite and 0.2% by weight of potassium phosphate for 2 minutes, followed by washing with tapped water. The thus obtained composite semipermeable membrane was subjected to the reverse osmosis test as in Example 1 to obtain the membrane performance shown in Table 1. Examples 4 - 7 A composite semipermeable membrane was produced in the same manner as in Example 3 except that the amount of the amine component and the acid halide was changed to that shown in Table 1. As a result, the membrane performance shown in Table 1 was obtained. Comparative Example 1 A composite semipermeable membrane was produced in the same manner as in Example 3 except that the amount of the added ethylenediamine was 0.5% by weight. As a result, the membrane performance shown in Table 1 was obtained. Example 8 The membrane obtained in Example 1 was immersed in an aqueous solution containing 100 ppm of chlorine, pH 6.5, at 25°C for 45 hours and then the membrane was washed with citric acid. As a result, the performance of the membrane was changed as shown in Table 2. In Table 2, the "increase ratio of salt permeation" means "salt permeation after/salt permeation before" chlorine treatment, and "salt permeation (%)" means "(100 - salt rejection) (%)". Example 9 The membrane obtained in Example 4 was subjected to treatment in the presence of chlorine in the same manner as in Example 8. As a result, the performance was changed to that shown in Table 2. Example 10 The membrane obtained in Example 7 was subjected to treatment in the presence of chlorine in the same manner as in Example 8. As a result, the performance was changed to that shown in Table 2. Comparative Example 2 A composite semipermeable membrane was produced in the same manner as in Example 3 except that the ethylenediamine was not added. As a result, the membrane performance shown in Table 1 was obtained. Further, the membrane was subjected to treatment in the presence of chlorine as in Example 8. As a result, the membrane performance was changed to that shown in Table 2. Comparative Example 3 A composite semipermeable membrane was produced in the same manner as in Example 7 except that the ethylenediamine was not added. As a result, the membrane performance shown in Table 1 was obtained. Further, the membrane was subjected to treatment in the presence of chlorine as in Example 8. As a result, the membrane performance was changed to that shown in Table 2. Example 11 - 16 A composite semipermeable membrane was produced in the same manner as in Example 3 except that 0.05% by weight of 1,3-propanediamine, 1,2-cyclohexanediamine, N-methylethylenediamine, N,N'-dimethylethylenediamine, N-phenylethylenediamine or 1,3,5-trihydroxybenzene was used in place of ethylenediamine. As a result, the membrane performance shown in Table 3 was obtained. Further, the membrane was subjected to treatment in the presence of chlorine as in Example 8. As a result, the membrane performance was changed to that shown in Table 3. As is apparent from a comparison between Comparative Example 2 and Examples 1 - 6, 8, 9 and 11 - 16, and between Comparative Example 3 and Examples 7 and 10, by adding the alkyldiamine or 1,3,5-trihydroxybenzene to the aqueous amine solution, the production of a composite semipermeable membrane with excellent oxidation resistance was attained while maintaining a high desalination performance and permeability. Further, as is apparent from a comparison between Comparative Example 1 and Examples 1 - 6, if the amount of the added alkyldiamine exceeds 20% by weight, the permeability is reduced, so that a sufficient permeability for practical use was not obtained. [Industrial Applicability] As described above, by the process of producing composite semipermeable membranes embodying the present invention, composite semipermeable membranes with high separation selectivity and high permeation and which had excellent oxidation resistance were obtained. Thus, the process of the present invention can suitably be employed as a process of producing a practical reverse osmosis membrane.

Claims: A process of producing a composite semipermeable membrane comprising the step of forming on a microporous substrate an ultra-thin membrane containing as a major component a crosslinked polyamide obtained by interfacial polycondensation reaction between a polyfunctional aromatic amine having not less than 2 amino groups in a molecule thereof and a polyfunctional acid halide having not less than 2 halogenated carbonyl groups in a molecule thereof, to form a composite semipermeable membrane, characterized in that at least one member selected from (a) an alkyldiamine of the formula (1): (wherein R¹ and R² are each, independently of one another, C₁-C₃ alkyl, C₆ phenyl or hydrogen, R³ is a C₁-C₇ straight chain alkyl group which may have one or more side chains or a C₃ - C₇ cycloalkyl group which may have one or more side chains), (b) 1,3,5-trihydroxybenzene and (c) 1,3-dihydroxybenzene is added to the polyfunctional aromatic amine as a copolymerization component in an amount of not less than 1% by weight to not more than 20% by weight with respect to the polyfunctional aromatic amine when the interfacial polycondensation reaction is carried out. A process according to claim 1, wherein the polyfunctional aromatic amine contains at least one of m-phenylenediamine, p-phenylenediamine and 1,3,5-triaminobenzene. A process according to claim 1 or claim 2, wherein the polyfunctional acid halide comprises at least one of benzene-1,3,5-tricarboxylic acid chloride and benzene-1,4-dicarboxylic acid chloride. A process according to any preceding claim, wherein the microporous substrate is made of polysulfone. A process according to any preceding claim, wherein the copolymerization component is ethylenediamine. A process according to any preceding claim, which additionally comprises the steps of treating the composite semipermeable membrane obtained by the polycondensation reaction with an aqueous chlorine-containing solution having a pH of 6-13.

Inventors: FUJIMAKI HIDEO, IKEDA TOSHIHIRO, UEMURA TADAHIRO, FUJIMAKI, HIDEO, IKEDA, TOSHIHIRO, UEMURA, TADAHIRO
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1989
Main classification: B01D71
IPC codes: B01D69, B01D71