A MANUFACTURING METHOD OF COMPOSITE MEMBRANE HAVING HYDROPHILIC COATING LAYER ON HYDROPHOBIC SUPPORT MEMBRANE

Abstract: This invention relates to a manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane and more particularly, to the manufacturing method of an improved composite membrane having excellent permeation rate and separation performances with easy adjustment of a coating membrane, fabricated in such a manner that when hydrophobic support membrane containing reacting agent over its surface and in its pores, is immersed in an aqueous solution containing a hydrophilic polymer, the hydrophilic polymer is thinly coated over the surface of the hydrophobic support membrane via reaction between the hydrophilic polymer and reacting agent.

Description: A Manufacturing Method of Composite Membrane Having Hydrophilic Coating Layer on Hydrophobic Support Membrane BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane and more particularly, to the manufacturing method of an improved composite membrane having excellent permeation and separation performances with easy adjustment of a coating membrane, fabricated in such a manner that when hydrophobic support membrane containing reactive material(hereinafter referred to as "reacting agent") over its surface and in its pores, is immersed in an aqueous solution containing hydrophilic polymer, the hydrophilic polymer is thinly coated over the surface of hydrophobic support membrane via reaction between the hvdrophilic polymer and reacting agent. Description of the Prior Art When a membrane for separation process is employed in the industrial field, the membrane should be essential to enhance the permeation rate and productivity. In general, a permeation rate is inversely proportional to the thickness of membrane but if decreasing the thickness of membrane for the enhancement of permeation rate, such membrane is easily broken due to a weaker mechanical strength of membrane. In case where a thin coating is attempted, a support membrane should be further prepared. The support membrane should simply support the non-porous active layer of membrane, while little affect on the flow of materials permeated into the active layer of membrane. Such type of membrane, which is called as a composite membrane, has been mainly used in the membrane process of reverse osmosis, pervaporation and gas separation. In general, main examples of support membrane include asymmetric ultrafiltration membranes including commercially marketable hydrophobic poly sulf one membranes and polyimde membranes. A composite membrane where a weak hydrophilic polymer is coated on the hydrophobic support membrane in the form of membrane by the method of interficial polymerization and coating is quite popular (J. Macromol. Sci. Chem., A15 (1981) 727-755, J. Membr. Sci., 48 (1990) 203). However, the active layer of polymer is required to separate any aqueous solution containing either organic compounds or ionic solutes. In spite of the fact that such hydrophobic support membrane has been recognized as an excellent support membrane due to remarkable mechanical strength, chemical resistance and thermal properties, its hydrophobic property makes it somewhat difficult to implement the coating of hydrophilic polymer on the membrane, since the surface energy of these hydrophobic polymers is relatively smaller than that of hydrophilic polymers. More specifically, a thin coating of hydrophilic polymer on the membrane cannot obtain an uniform layer due to their lump induced by poor wettability but if the thickness of J hydrophilic polymer layer is larger, smaller permeation rate of permeating materials may result in reducing the productivity. When a hydrophilic polymer is coated on the membrane, adhesion between the polymer and the support membrane may be enhanced to some extent via physicochemical surface treatment of such hydrophobic support membrane designed to increase the surface area of support membrane or to activate the surface of hydrophobic support membrane. However, if some physicochemical properties are extremely different between hydrophobic support membrane and hydrophilic polymer to be coated, such treatment cannot be recommended for a final choice to coat the hydrophilic polymer on the membrane. As an alternative, a method of coating a hydrophilic polymer on the hydrophilic support membrane may be easily carried out based on increasing adhesiveness at the interface and similarity of physicochemical properties. However, since a hydrophilic support membrane for use has to possess adequate mechanical, thermal and chemical-resisting properties, the scope of such available polymer is quite limited. The typical hydrophilic support membranes include cellulose acetate and polyacrylonitrile [Germany Patent No. 3,220,570 Al (1983) and Europe Patent No. 0,096,339 (1983)]. However, cellulose acetate has some shortcomings in chemical-resisting and biochemical properties, while polyacrylonitrile is quite expensive and its commercial marketing cannot be made in the form of membrane; even though the membrane is marketed, its use has been restricted due to awfully high price. Further, since the hydrophilic support membrane has to meet the mechanical, thermal and chemical-resisting properties as an adequate support membrane, any weak hydrophilic property becomes visible depending on some materials, when the coating of hydrophilic support membrane on the hydrophobic support membrane is to be made. Therefore, there is a urgent need for an easily purchasable and inexpensive support membrane derived from polysulfone or polyimde, which gives adequate physical property to the membrane SUMMARY OF THE INVENTION To overcome the above shortcomings, an object of this invention is to provide a manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane, so fabricated in such a manner that hydrophobic support membrane containing reacting agent over its surface and in its pores, are immersed in an aqueous solution containing hydrophilic polymer. Thus, the composite membrane of this invention has several advantages in that a) through better wettability of hydrophilic polymer coated on the hydrophobic support membrane, the coating of hydrophilic polymer on the membrane may be easily made, b) the affinity between hydrophobic support membrane and hydrophilic polymer may be enhanced, and c) via chemical reaction between hydrophilic polymer and reacting agent dispersed in hydrophobic support membrane, the limitedness of affinity and surface energy at interface may be overcome. Detailed Description of the Preferred Embodiments This invention is characterized by a manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane, which comprises: (i) the hydrophobic support membrane is immersed in an aqueous solution containing a reacting agent having reactivity to the hydrophilic polymer in such a manner that the reacting agent is distributed at the surface and in the pores of the hydrophobic support membrane; and (ii) the hydrophilic polymer is thinly coated on the hydrophobic support membrane via reaction between the hydrophilic polymer and the reacting agent at the surface of the hydrophobic support membrane. This invention is explained in more detail as set forth hereunder: When a hydrophobic support membrane is immersed in an aqueous solution containing a reacting agent having reactivity to the hydrophilic polvmer for a certain time, the reacting agent infiltrates into the pores present at the surface and within the hydrophobic support membrane. Through the drying process of the membrane, the reacting agent may be evenly dispersed over the whole hydrophobic support membrane. More specifically, irrespective of its affinity with the hydrophobic support membrane, the reacting agent may be evenly dispersed at the surface and within the hydrophobic support membrane via such processing steps. According to this invention, preferred examples of hydrophobic support membrane include polysulfone- or polyetherimide-based ultrafiltration membrane in consideration of price and mechanical property. The available shapes of such hydrophobic support membrane include a flat membrane or hollow fiber membrane. According to this invention, it is preferred that prior to coating a hvdrophilic polymer on the hydrophobic support membrane, a swelling agent filled in the pores of support membrane should be removed. The reason why such swelling agent has to be removed lies in the fact that since a swelling agent consists mainly of glycerin, and thus anv attempt to coat the hydrophilic polymer on the membrane without removal of the swelling agent cannot ensure the manufacture of a desired composite membrane, while any effective coating of polymer on the membrane cannot be made available due to the fact that the reacting agent is not infiltrated into the pores. The swelling agent can be removed in such a manner that the hvdrophobic support membrane is dipped in an aqueous solution containing isopropvl alcohol and water in a weight ratio of 1:1 for 1-5 hours. As such, the reacting agent is placed at the surface and inside the hydrophobic support membrane where the swelling agent is removed. Since the hydrophilic polymer used for this invention has to meet adequate membrane property and mechanical strength, it is preferred to use sodium alginate, chitosan or polyvinyl alcohol. The use of other hydrophilic polymer including the aforementioned polymers may have the same effects. Further, examples of the reacting agents used for this invention include some cross-linking agents, coagulating agents and reaction modifiers. It is preferred that if sodium alginate is employed as hydrophilic polymer, calcium chloride, aluminum nitrate, aluminum sulfate, copper sulfate or hydrochloric acid may be used; if chitosan is employed as hydrophilic polymer, sulfuric acid may be used; if polyvinyl is employed as hydrophilic polymer, glutalaldehyde may be used. According to this invention, the hydrophobic support membrane with no swelling agent is immersed for more than 5 minutes with aqueous solution containing the reacting agent whose concentration is adjusted to 0.5-20%. If the concentration of the reacting agent in aqueous solution is less than 0.5%, the insufficient diffusion of such reacting agent at the surface and within the pores of hydrophobic support membrane cannot ensure adequate coating of hvdrophilic polymer on the membrane, but in excess of 20%, the coating layer of the reacting agent in a larger amount is formed at the surface of membrane. In addition, if the impregnating time is less than 5 minutes, the reacting agent cannot be sufficiently infiltrated into the hvdrophobic support membrane. In general, a desired infiltration may be achieved within 5-10 minutes. After the reacting agent is diffused in the hydrophobic support membrane, the hydrophobic support membrane is immersed in an aqueous solution containing hydrophilic polymer, either without any drying process or with complete drying process. Then, the hydrophilic polymer is evenly coated on the hvdrophobic support membrane via chemical reaction between the reacting agent and hydrophilic polymer at the surface of the hydrophobic support membrane. The concentration of hydrophilic polymer in aqueous solution is adjusted to 0.1-1%; if the concentration is less than 0.1%, the hydrophilic membrane cannot be coated on the hydrophobic support membrane but in case of exceeding 1%, the coating layer thickness of hydrophilic membrane becomes quite thicker. The immersion time is determined as more than 5 minutes; if the impregnating time is less than 5 minutes, the polymer cannot be sufficiently coated due to insufficient reaction time. According to this invention, when the hydrophilic membrane is coated on the hvdrophobic support membrane, it is very important to adjust the thickness of coating layer of hydrophilic membrane. Such coating thickness is adjusted by the concentrations of reacting agent in aqueous solution and of hydrophilic polymer in aqueous solution. Hence, if both concentrations of reacting agent and hydrophilic polymer is getting higher, the thickness of membrane becomes larger. It is preferred that the thickness is adjusted in the range of 0.1-1.5 μm; if the thickness is less than 0.1 μm, the formation of coating layer with no defect is unavailable but in case of exceeding 1.5 μm, the permeation rate of material becomes smaller due to extremely large coating layer. After being immersed in the aqueous solution containing hydrophilic polymer, the support membrane is dried in the air and if deemed necessary, additional reaction may be performed in an oven at high temperature. Throughout such reaction, a composite membrane has adequate adhesion in layers between hydrophobic support membrane and hydrophilic polymer membrane. To investigate the coating state on the composite membranes, so prepared, the fragments of membrane are observed using an electron microscope. As a result, the coating layer having the normal thickness of 0.1-1.5 μm was observed. To investigate the membrane performance according to this invention, an anion-based emulsifier in aqueous solution was separated using the method of reverse osmosis. In an apparatus for reverse osmosis test, the separation performance of membrane was measured using a pre-designed membrane cell so as to minimize the concentration polarization. The following specific examples are intended to be illustrative of the invention and should not be construed as limited the scope of the invention as defined by appended claims. Example 1 A polysulfone ultrafiltration membrane was immersed in a mixture containing isopropanol and water in a weight ratio of 1: 1 for 5 hours, for removal of glycerin which was filled into the pores of membranes. The membrane was washed with distilled water and further impregnated with 0.5% calcium chloride as a crosslinking agent for sodium alginate in aqueous solution for 10 minutes. Then, the membrane was immediately immersed in 0.2% sodium alginate in aqueous solution and after being taken, immersed in 1.5% calcium chloride in aqueous solution for 5 minutes and dried in the air to prepare a membrane of this invention. The membrane performance test based on the method of reverse osmosis was carried out using a small amount of anion-based emulsifier in aqueous solution. The results were shown in the following table 1. Examples 2-4 In the same manner as in Example 1, a glycerin-removed polysulfone ultrafiltration membrane was immersed in each of calcium chloride in aqueous solution containing 1.0%, 1.5% and 2.0% to prepare a membrane of this invention. The membrane performance test based on the method of reverse osmosis was carried out using an anion-based emulsifier in aqueous solution. The results were shown in the following table 1. Table 1 The above table 1 showed the coating thickness and permeation property of the composite membrane, so prepared, using different concentrations of calcium chloride, a cross-linking agent for sodium alginate. Even though the thickness of a coating membrane became larger in parallel with higher concentration of a polyvalent ion cross-linking agent, such coating membrane having the thickness of less than 1 μm was prepared with relatively excellent permeation property in the applicable range of concentration. In addition, the composite membrane, so prepared from a process for separating membrane, demonstrate its remarkable adhesiveness in that the coating membrane was not separated from the support membrane. In particular, its high rejection rate reflected a defect-free coating layer and this was well ascertained by electron microscope. Example 5-7 In the same manner as in Example 3, a glycerin-removed polysulfone ultrafiltration membrane was immersed in each of sodium alginate in aqueous solution containing 0.1%, 0.3% and 0.5% to prepare a membrane of this invention. The membrane performance test based on the method of reverse osmosis was carried out using an anion-based emulsifier in aqueous solution. The results were shown in the following table 2. Table 2. As noted in the table 2, the coated thickness of composite membrane, so prepared, became thicker in parallel with higher concentration of sodium alginate in aqueous solution. In addition, it was ascertained that the composite membrane having the thickness of less than 1 μm could be prepared in a polymer concentration of less than 0.3%, which also reflected a high permeation rate and rejection rate. Example 8 In the same manner as in Example 3, a glycerin-removed polysulfone ultrafiltration membrane was immersed in an aqueous solution of 1 % hydrochloric acid, a coagulating agent, to prepare a membrane of this invention. The membrane performance test based on the method of reverse osmosis was carried out using an anion-based emulsifier in aqueous solution. The results were shown in the following table 3. Example 9-10 In the same manner as in Example 3, a glycerin-removed polysulfone ultrafiltration membrane was immersed in 5% glutalaldehyde as a cross-linking agent for poly(vinly alcohol) in aqueous solution and 0.15% hydrochloric acid as catalyst for 10 minutes and further immersed in each aqueous solution containing 0.5% and 1.0% polyvinyl alcohol, respectively, for 5 minutes. Then, the membrane was dried in the air and further cross-linked at 100 ° C for 10 minutes. The membrane performance test based on the method of reverse osmosis was carried out using an anion-based emulsifier in aqueous solution. The results were shown in the following table 3. Example 11 In the same mariner as in Example 3, a glycerin-removed polysulfone ultrafiltration membrane was immersed in 2% sulfuric acid as a crosslinking agent for chitosan for 10 minutes and further immersed in 0.3% chitosan in aqueous solution for 5 minutes. Then, the membrane was again immersed in 0.5% sulfuric acid in aqueous solution for an additional cross-linking reaction and dried in the air to prepare the composite membrane. The membrane performance test based on the method of reverse osmosis was carried out using an anion-based emulsifier in aqueous solution. The results were shown in the following table 3. Table 3. The above table 3 showed the coating thickness and permeation property of the composite membrane, so prepared, using a) a coagulating agent as reacting agent for sodium alginate, or b) a cross-linking agent as the reacting material derived from other hydrophilic polymers as a coating polymer except for sodium alginate. Despite the fact that a coating layer of membrane was formed in the case of chitosan, a poor permeation and rejection may be explained in that a) since chitosan is a typical cationic polymer, anionic solute was adhered to the surface of support membrane which resulted in causing a severe fouling with time, thus deteriorating the permeation rate and rejection rate as well. Comparative examples 1-4 In the same manner as in Example 1, a polysulfone ultrafiltration membrane was immersed in each reacting agent in aqueous solution, at the concentrations mentioned in the following table 4 and then, the composite membrane was prepared by coating each polymer on the membrane. The membrane performance test based on the method of reverse osmosis was carried out using an anion-based emulsifier in aqueous solution. The results were shown in the following table 4. Table 4. As shown in the above table 4, the permeation test was carried out using the separation membrane, fabricated in such a manner that a polysulfone ultrafiltration membrane was immersed in an aqueous solution containing reacting agent having each concentration of 0.2% and 0.3% and a polymer was coated on the membrane. The permeation rate of the above membrane with much porosity was higher than that of the separation membrane, so prepared in such manner that an ultrafiltration membrane of this invention was immersed in an aqueous solution containing a reacting agent followed with the coating process of each polymer, while the rejection rate of the former was lower than that of the latter due to the fact that any polymer was uncoated on the membrane. When an aqueous solution of the reacting agent having each concentration of 25% and 30%, the rejection of the former due to its large thickness of coating membrane was higher than that of the latter, while the permeation rate was extremely low. As described above, this invention has several advantages in that a) the composite membrane having a hydrophilic coating layer on a hydrophobic support membrane may be prepared based on the chemical reaction between a reacting agent dispersed in the hydrophobic support membrane and hydrophilic polymer, a coating polymer; b) the thickness of coating layer may be easily changed by the proper adjustment of concentration of a reacting agent or hydrophilic polymer in aqueous solution; and c) the composite membrane having a hydrophilic coating layer of membrane with excellent separation power may be prepared.

Claims: CLAIMS What is claimed is: 1. A manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane, which comprises: (i) the hydrophobic support membrane is impregnated with an aqueous solution of a reacting agent having reactivity to the hydrophilic polymer in such a manner that the reacting agent is dispersed at the surface and in the porosity of the hydrophobic support membrane; and (ii) the hydrophilic polymer is thinly coated on the hydrophobic support membrane via reaction between the hydrophilic polymer and reacting agent at the surface of the hydrophobic support membrane. 2. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1, wherein said hvdrophobic support membrane consists of polysulfone or polyetherimide. 3. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1 or 2, wherein the shape of said hydrophobic support membrane has a flat membrane or hollow fiber membrane. 4. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1, wherein said hydrophilic polymer consists of sodium alginate, chitosan or polyvinyl alcohol. 5. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1 or 4, wherein the concentration of said hydrophilic polymer is in the range of 0.1-1%. 6. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1, wherein the reacting agent having reactivity to said hydrophilic polymer include calcium chloride, aluminum nitrate, aluminum sulfate, copper sulfate or hydrochloric acid, when sodium alginate is employed as a hydrophilic polymer; sulfuric acid, when using chitosan as a hydrophilic polymer; glutalaldehyde, when using polyvinyl alcohol as a hydrophilic polymer. 7. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1 or 6, wherein the concentration of said reacting agent in aqueous solution is in the range of 0.5-20%. 8. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1, wherein said hydrophobic support membrane is immersed in an aqueous solution of the reacting agent for more than 5 minutes, followed by another immersed in an aqueous solution of the hydrophilic polymer for more than 5 minutes, whereby hydrophilic polymer is coated on the hydrophobic support membrane via chemical reaction between the reacting agent and hydrophilic polymer at the surface of the hydrophobic support membrane. 9. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1 or 8, wherein said hydrophobic support membrane, immersed in aqueous solution containing the reacting agent, is impregnated with an aqueous solution of hydrophilic polymer, either without any drying process or with complete drying process. 10. The manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane according to claim 1, wherein said hydrophilic coating layer has the thickness in the range of 0.1-1.5 μm, which is prepared through proper adjustment of concentration in an aqueous solution of both reacting agent and hydrophilic polymer.

Inventors: KIM BEOM SIK, KIM CHUL UNG, KIM KWANG JOO, LEE JUNG MIN, YEOM CHOONG KYUN, KIM, BEOM, SIK, KIM, CHUL, UNG, KIM, KWANG, JOO, LEE, JUNG, MIN, YEOM, CHOONG, KYUN
Applicants: KIM BEOM SIK, KIM CHUL UNG, KIM KWANG JOO, KOREA RES INST CHEM TECH, LEE JUNG MIN, YEOM CHOONG KYUN, KIM, BEOM, SIK, KIM, CHUL, UNG, KIM, KWANG, JOO, KOREA RESEARCH INSTITUTE OF CHEMICAL TECHNOLOGY, LEE, JUNG, MIN, YEOM, CHOONG, KYUN
Application year: 1998
Main classification: B01D67
IPC codes: B01D67, B01D69, B01D71, B05D7