WO2010082710A1

WO2010082710A1 - Method for preparing a highly durable reverse osmosis membrane

Info

Publication number: WO2010082710A1
Application number: PCT/KR2009/001713
Authority: WO
Inventors: Yong Taek Lee; No Won Kim; Dong Ho Shin
Original assignee: Kyung Hee University
Current assignee: Kyung Hee University
Priority date: 2009-01-14
Filing date: 2009-04-02
Publication date: 2010-07-22
Anticipated expiration: 2011-07-14
Also published as: KR101114537B1; KR20100083700A

Abstract

Provided is a method for preparing a reverse osmosis membrane having improved durability, comprising the steps of: forming a polyamide thin film on a porous supporting layer; next, primarily coating the surface of the polyamide thin film with an aqueous solution of silane derivatives; then secondly coating the primarily coated polyamide thin film surface with an aqueous solution of glycidyl compounds and drying the coat; and washing the dried membrane with a basic aqueous solution. Also provided is a reverse osmosis membrane prepared thereby.

Description

METHOD FOR PREPARING A HIGHLY DURABLE REVERSE OSMOSIS MEMBRANE

The present invention relates to a method for preparing a reverse osmosis membrane with improved durability and a reverse osmosis membrane obtained thereby.

It is known that dissolved substances can be separated from their solvents by use of selective membranes. For example, there are various types of membranes such as microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes and the like. The reverse osmosis membranes are particularly useful in the desalination of brackish water or sea water. The desalination process of brackish water or sea water using reverse osmosis membranes involves a filtration process for separation by applying pressure to the feed water and forcing the water through the membrane, so that purified water passes through the membrane and the salts or other dissolved substances or molecules are filtered out. During such filtration process, osmotic pressure naturally occurs, and the more concentrated the feed water, the greater the osmotic pressure, which requires higher pressure applied from outside.

Tap water should be treated by chlorination for sterilization. Water for industrial use or desalinated water from sea water should also be treated with chlorine so as to prevent water from being contaminated by bacteria present on a reverse osmosis membrane, and then undergone a dechlorination process. Such dechlorination is generally done by use of activated charcoal or chemical agents, thus having many difficulties in their maintenance.

Reverse osmosis membranes prepared from a porous supporting layer and a polyamide membrane formed on said supporting layer have been widely used. The polyamide membrane can be obtained by interfacial polymerization between polyfunctional amines and polyfunctional acyl halides.

US patent No. 4,277,344 issued to Cadotte, et al., discloses an invention providing an aromatic polyamide thin film obtained by interfacial polymerization between aromatic polyfunctional amine having two primary amine substituents and aromatic acyl halide having three or more acyl halide groups. Although the film has made quite an achievement in terms of increased flow rate and high salt rejection, to date, various researches to make further improvement in flow rate and salt rejection characteristics still have been made.

US patent No. 4,872,984 issued to Tomashke, proposes a water permeable osmosis membrane prepared by the following steps: (a) coating a microporous supporting layer with an aqueous solution comprising an aromatic polyamine reactant having at least two amine functional groups and an amine salt, to form a liquid layer on said microporous supporting layer; (b) contacting said liquid layer with an organic solvent solution of an aromatic polyfunctional acyl halide or mixture thereof, wherein the aromatic polyfunctional acyl halide has, on the average, at least about 2.2 acyl halide groups which are amine-reactive functional groups; and (c) drying the product of step (b) so as to form a water permeable membrane. Tomashke's method of membrane preparation has significantly increased the permeation flow rate by introducing a step of a monomeric amine salt addition, into the Cadotte's membrane preparation method.

US patent No. 5,614,099 issued to Hirose, discloses a technique of adjusting a polyamide surface structure. The invention is to provide a process for preparing a membrane having at least a certain degree of surface roughness, from the finding that the surface roughness is closely related to a flow rate increase, based on the Tomashke's method of membrane preparation.

Although these polyamide type reverse osmosis membranes of prior arts show excellent desalination and permeation characteristics, membrane fouling occurred by adsorption of floats or dissolved fouling substances in a solution to the membrane surface cannot be avoided, resulting in decrease in the flow rate. Cleaning agents such as sodium hypochlorite(NaOCl) are used to prevent the membrane fouling. However, the use of cleaning agents poses another problem such as declination of the separation performance caused by residual chlorines which break the polyamide backbone.

To date, techniques to reduce fouling of polyamide membranes generally involve a double-coating process. However, such double coating increases membrane resistance, lowering the permeation property as compared to the water permeability of polyamide composite membrane without double coating.

The present inventors have continued researches to develop polyamide membranes having greater resistance to both chlorine and fouling, and excellent permeation performance. As a result, the present inventors have developed a method for preparing a highly durable reverse osmosis membrane having both satisfying chlorine resistance and fouling resistance by primarily coating the surface of a polyamide membrane with an aqueous solution of silane derivatives having chlorine resistance, followed by second coating process using an aqueous solution of glycidyl compounds having fouling resistance, as well as a composite reverse osmosis membrane prepared therefrom.

The present invention is to provide a method for preparing a highly durable reverse osmosis membrane having both fouling resistance and chlorine resistance by primarily coating the polyamide membrane surface with an aqueous solution of silane derivatives and then secondly coating the resulted surface with an aqueous solution of glycidyl compounds.

Further, the present invention is to provide a reverse osmosis membrane having improved durability prepared by the said method.

Moreover, the present invention is to provide a reverse osmosis membrane having excellent permeability.

In one aspect, the present invention provides a method for preparing a reverse osmosis membrane having improved durability, characterized by comprising the steps of: forming a polyamide thin film on a porous supporting layer; primarily coating the surface of the polyamide thin film with an aqueous solution of silane derivatives; secondly coating the primarily coated polyamide thin film surface with an aqueous solution of glycidyl compounds and drying the resulted membrane; and washing the dried membrane with a basic aqueous solution.

According to one embodiment of the present invention, the silane derivatives are alkoxyalkylsilanes represented by the following chemical figure 1:

ChemistryFigure 1

wherein R¹ is selected from the group consisting of substituted or unsubstituted C1~C18 alkyl group, substituted or unsubstituted vinyl group, substituted or unsubstituted aromatic group, and substituted or unsubstituted phenyl group; R2 is C1-C4 alkyl; and n is an integer of 1, 2 or 3.

According to one embodiment of the present invention, the glycidyl compounds may include 3~4 three-membered cyclic epoxy groups, and at least one functional group selected from the group consisting of ether group, glyceryl group and sorbitol group.

According to another embodiment of the present invention, the method further includes, after secondly coating the polyamide thin film with the aqueous solution of glycidyl compounds and drying it, a step of drying the resulted membrane with hot air at the temperature range of 25~100℃.

In Another aspect, the present invention provides a reverse osmosis membrane with improved durability prepared by the foregoing method.

The reverse osmosis membrane according to the present invention has improved resistance to chlorine and fouling, as well as excellent permeation performance, by primarily coating the surface of a polyamide thin film with an aqueous solution of silane derivatives having chlorine resistance, and secondly coating the surface with an aqueous solution of glycidyl compounds having fouling resistance.

The method for preparing a reverse osmosis membrane of the present invention includes the steps of:

forming a polyamide thin film on a porous supporting layer;

primarily coating the surface of the polyamide thin film with an aqueous solution of silane derivatives;

then secondly coating the primarily coated polyamide thin film surface with an aqueous solution of glycidyl compounds and drying the resulted membrane; and

washing the dried membrane with a basic aqueous solution.

Hereinafter, the present invention is further illustrated in detail.

The step of forming a polyamide thin film on a porous supporting layer may be carried out by well-known methods in this field of art. The polyamide thin film is generally formed by: spreading an aqueous polyfunctional amine solution over the microporous supporting layer; removing the excess solution therefrom; contacting the resulted surface with an organic solvent containing amine-reactive compounds selected from the group consisting of polyfunctional acylhalide, polyfunctional sulfonylhalide and polyfunctional isocyanate; and carrying out interfacial polymerization.

The porous supporting layer used herein refers to a microporous supporting layer. Any well-known materials and production methods for the porous supporting layer in this field of art may be used herein without being specifically limited. The dimension of a micropore should be large enough for the feed water to permeate, but not too much to hinder the crosslinking of a thin film formed on the micropores. The hole diameter of the porous supporting layer is desirably 1~500 nm, without being specifically limited to this. When the hole diameter of the porous supporting layer is more than 500 nm, the surface membrane obtained from the interfacial polymerization permeates through the holes on the supporting layer, being difficult to form a normal flat membrane.

There is no specific restriction on materials for the microporous supporting layer, any materials generally used in this field of art may be employed. For instance, polymers such as polysulfone, polyether sulfone, polyimide, polyamide, polyether amide, polyacrylonitrile, polymethylmethacrylate, polyethylene, polypropylene, polyvinylidene fluoride and the like may be desirably mentioned. Although thickness of the microporous supporting layer is not specifically limited, 25~130 ㎛ is desirable.

The species of polyfunctional amines are not specifically limited, and any types well-known in this field of art may be used, for example, such as one polyfunctional amine or mixtures thereof. Particularly, examples include aromatic primary diamines of methylphenyldiamine or para-phenyl diamine, or substituted derivatives thereof; analogues having N-alkyl or aryl substituents; and alkane diamines or cycloaliphatic primary amines such as cyclohexane diamine, cycloaliphatic secondary amines such as piperazine and alkyl derivatives thereof, and aromatic secondary diamines. The substituent for the substituted aromatic primary amines includes an alkyl group, an alkoxy group, a hydroxyalkyl group or a halogen atom.

The polyfunctional amine is used in the form of an aqueous solution. The amine may be contained in the aqueous solution at the amount of 0.1~30 wt%. The aqueous solution of the polyfunctional amine has a pH of 7~13. Acid acceptors such as hydroxide, carboxylate, carbonate, borate, alkali metal phosphate, trialkyl amine and the like may be further added to the aqueous solution of the polyfunctional amine, so as to neutralize acids (hydrochloric acid, etc.) generated during the interfacial polymerization.

The polyfunctional amine solution, i.e. aqueous polyamine solution may be used alone or with other additives described in conventional arts. For the improvement in flow rate, a polar protic solvent and a mixture thereof, a mixture of polar protic solvent and polar aprotic solvent may be used. Examples of the polar protic solvent include alcohol, dialcohol, alcohol-ether and the like, and examples of the polar aprotic solvent include diethyleneglycol, ether derivatives such as di(ethylene glycol) tertiary butylmethylether, and sulfoxide derivatives such as dimethylsulfoxide, butylsulfoxide, tetramethylenesulfoxide. Other additives containing monomeric amine salts may be used for increasing the flow rate.

The polyfunctional acyl halide used in the interfacial polymerization is an aromatic compound having 2~3 carboxylic acid halides, and for example, trimesoyl chloride, isophthaloyl chloride, terephthaloyl chloride and mixtures thereof may be widely used. The aromatic polyfunctional acylhalide may be used alone or as a mixture, for example such as trimesoyl chloride, a mixture of trimesoyl chloride and isophthaloyl chloride and a mixture of trimesoyl chloride and terephthaloyl chloride. The content of whole carboxylic acid halides in organic solvent may be 0.005~5 wt%.

After forming the polyamide thin film, the surface thereof is primarily coated with an aqueous solution of silane derivatives. As for the silane derivatives, alkoxyalkyl silane compounds represented by the following chemical figure 1 may be used.

[Chemical Figure 1]

wherein R¹ is selected from the group consisting of substituted or unsubstituted C1~C18 alkyl group, substituted or unsubstituted vinyl group and substituted or unsubstituted phenyl group; R² is C1~C4 alkyl group; and n is an integer of 1, 2 or 3.

The alkoxyalkylsilane compounds used in the present invention are preferably compounds represented by the chemical figure 1, wherein R¹ is a substituted or unsubstituted C8-C18 alkyl group, in terms of chlorine resistance. More preferred is alkoxyalkylsilane compounds represented by the chemical figure 1, wherein R¹ is a substituted or unsubstituted C8 alkyl group.

Alkoxyalkylsilane compounds used in the present invention may include trialkoxyalkyl silane, dialkoxydialkyl silane, alkoxytrialkyl silane and mixtures thereof.

Preferably, the alkyl group may further contain at least one functional group selected from the group consisting of amine, aldehyde, acid, ether, ketone, alcohol, ester, halogen and aromatic group.

The alkoxyalkyl silane compounds which can be used in the present invention is at least one selected from the group consisting of trimethylmethoxy silane, trimethylethoxy silane, trimethylpropoxy silane, dimethylethylmethoxy silane, dimethylethylethoxy silane, dimethylethylpropoxy silane, diethylmethylmethoxy silane, diethylmethylethoxy silane, diethylmethylpropoxy silane, methyltrimethoxy silane, ethyltrimethoxy silane, propyltrimethoxy silane, ethyltriethoxy silane, propyltriethoxy silane, methyltripropoxy silane, ethyltripropoxy silane, propyltripropoxy silane, pentyltriethoxy silane, octyltrimethoxy silane, octyltriethoxy silane, octadecyltrimethoxy silane, octadecyltriethoxy silane, (trifluoromethyl)trimethoxy silane, (3,3,3,-trifluoropropyl)trimethoxy silane, 3-aminopropyl-methoxy-diethoxy silane, penyltriethoxy silane, benzyltriethoxy silane, phenethyltrimethoxy silane, vinyltriethoxy silane, 3-aminopropyltrimethoxy silane, 3-aminopropyltriethoxy silane and mixtures thereof.

After preparing an aqueous solution of silane derivatives by mixing 0.1~4 wt% of above-described silane derivatives with 96~99.9 wt% of a coating solvent, it is applied to the surface of the polyamide thin film so as to form a primary coat thereon through a self-assembly method. Ethanol is preferably used as a coating solvent, without being limited to this. When the content of the silane derivatives is less than 0.1 wt%, a sol-gel reaction of silane on the membrane surface is so negligible that it cannot cover the entire polyamide surface, which may cause poor resistance to chlorine or fouling. In the meantime, when the content of the silane derivatives is more than 4 wt%, the sol-gel reaction of silane on the membrane surface occurs excessively, forming a thick coat, which may cause a significant decrease in the flow rate, making the resulting membrane inefficient, although resistance to chlorine and fouling can be obtained.

As a result of self-assembly coating of the aqueous solution of silane derivatives onto the surface of the polyamide thin film, it is possible to form a non-polar nanostructure on the surface of the polyamide thin film, improving the chlorine resistance. Specifically, the aqueous solution of silane derivatives forms a polymer through a sol-gel reaction in the way of self-assembly, being evenly distributed to the polyamide surface. The term, self-assembly used herein, refers that monomers are arranged to form a thermodynamically stable structure before the sol-gel polymerization reaction via n-alkyl silane coupling agents, wherein the polymerization is carried out by hydrolysis between adjacent silane monomers. At this time, one alkoxide of silane reacts with a polar atom on the surface of the polyamide thin film, achieving polyamide-polysiloxane immobilization. Since the non-polar functional groups are arranged before the polymerization reaction, it is easy to adjust the non-polar nanostructure.

As a result, during the permeation process through the polyamide thin film, permeation of chlorine radicals and substituted chlorine radicals into the polyamide thin film is alleviated, being possible to improve the durability to a chlorine-containing solution, i.e. chlorine resistance.

The coating process of the surface of a polyamide thin film with the aqueous solution of silane derivatives by using a self-assembly method, is well-known to persons who have ordinary knowledge in the field of art to which the present invention pertains, thus detailed description regarding the process is eliminated herein.

After finishing primary coating of the surface of the polyamide thin film with the aqueous solution of silane derivatives, the resulted surface is secondly coated with an aqueous solution of glycidyl compounds and dried. The glycidyl compounds contain 3~4 three-membered cyclic epoxy groups, and at least one functional group selected from the group consisting of ether, glyceryl and sorbitol group.

The glycidyl compounds containing 3 epoxy groups may be selected from the group consisting of glycerol triglycidyl ether, diglycerol triglycidyl ether, pentaerythritol triglycidyl ether, sorbitol triglycidyl ether, glycerolpropoxylate triglycidyl ether, trimethylolpropane triglycidyl ether, 1,1,1-tris(hydroxymethy)ethane triglycidyl ether, 1,1,1-tris(hydroxypenyl)ethane triglycidyl ether, tris-hydroxymethylnitromethane triglycidyl ether, tris-(2,3-epoxypropyl)isocyanurate, fluoroglucynol triglycidyl ether, N,N-diglycidyl-4-glycidyloxyaniline, a product obtained from a reaction of epichlorohydrin with 1,3,5-tris(2-hydroxyethyl)cyanuric acid and a product obtained from a reaction of epichlorohydrin with tris(hydroxymethyl)aminomethane.

The glycidyl compounds having 4 epoxy groups may be selected from the group consisting of sorbitol tetraglycidyl ether, pentaerythritol tetraglycidyl ether, polyglycerol tetraglycidyl ether, and 4,4'-methylenebis(N,N-diglycidylaniline).

The glycidyl compounds used herein may be water-soluble or soluble in a mixed solution of water and ethanol, wherein the water content is not less than 50 vol%.

The layer coated primarily with the aqueous solution of silane derivatives is again secondly coated with an aqueous solution of glycidyl compounds prepared by mixing 0.1~4 wt% of glycidyl compounds with 96~99.9 wt% of a coating solvent. Water is generally used as a solvent for the coating solution, without being limited to this.

The second coating of the nanostructure formed by primarily coating the surface of the polyamide thin film with an aqueous silane derivative solution, with an aqueous glycidyl compound solution rigidly fixes said nanostructure by being served as a bridge, and helps to reduce fouling of the membrane surface by allowing the hydrophilic groups located on the primarily coated polyamide thin film.

As described above, it is possible to provide a highly durable reverse osmosis membrane having improved resistance to both chlorine and fouling, by primarily coating the polyamide thin film with an aqueous solution of silane derivatives having chlorine resistance, and secondly coating the surface with an aqueous solution of glycidyl compounds.

According to one embodiment of the present invention, it is desirable to dry a sample by a hot-air or air drying process. The hot-air or air drying process increases the effective concentration of the materials for a sol-gel reaction.

Specifically, the present invention further comprises, after primarily coating the surface of the polyamide thin film with an aqueous silane derivative solution and then secondly coating the resulted surface with an aqueous glycidyl compound solution, a step of drying the resulted membrane with hot air at the temperature range of 25~100℃. The hot air drying process effectively fixes the first and the second coating layers formed by each silane derivatives and glycidyl compounds to the surface of the polyamide thin film. As such hot air drying process is generally well known in this field of art, further detailed description is eliminated herein.

Next, upon completion of the drying process, the resulted polyamide membrane is washed with a basic aqueous solution. Any types of basic aqueous solutions generally used in this field of art may be used without specific restriction. In this specification, a sodium carbonate or sodium hydroxide solution was used.

According to one embodiment of the present invention, the method for preparing a reverse osmosis membrane includes the steps of:

forming a polyamide thin film on a porous supporting layer;

next, primarily coating the surface of the polyamide thin film with an aqueous solution of glycidyl compounds;

after primarily coating the surface of the polyamide thin film with an aqueous solution of glycidyl compounds and washing the resulted surface with a basic aqueous solution, secondly coating the primarily coated polyamide thin film surface with an aqueous solution of silane derivatives and drying the resulted membrane; and

washing the dried membrane with a basic aqueous solution.

When coating the surface of the polyamide thin film primarily with an aqueous glycidyl compound solution and then secondly an aqueous silane derivative solution, adhesion of glycidyl compounds in the middle is improved and it is possible to obtain better coating of silane derivatives owing to increased reaction activity of glycidyl compounds with silane compounds contributed to the opening of glycidyl rings.

As it has been described so far, the present invention can provide a highly durable reverse osmosis membrane having both satisfying chlorine resistance and fouling resistance, prepared by primarily coating the surface of a polyamide thin film with an aqueous solution of glycidyl compounds and then secondly coating it with an aqueous solution of silane derivatives. The each step can be carried out as described in the above.

In another aspect, the present invention provides a reverse osmosis membrane prepared by said method. Specifically, the reverse osmosis membrane is comprised of: a porous supporting layer; a polyamide thin film formed on the porous supporting layer; and a silane derivative layer and a glycidyl compound layer both of which are coated on the surface of the polyamide thin film.

Hereinafter, the present invention is further illustrated in detail through the following examples. However, these examples are provided only to illustrate the present invention, without limiting the scope of the present invention.

Examples

The present invention used a disassembled commercial membrane as a substrate to minimize the changes in physical properties in the course of preparing a polyamide membrane, wherein the commercial membrane was originally manufactured as a module. A reverse osmosis membrane for brackish water (CPA2-4040), manufactured by Hydranautics corp. USA was selected. For the porous supporting layer, a thin film made of polyester unwoven fabric casted with polysulfone was used, wherein the thickness of the thin film was 100-150 ㎛ and the pore size was 0.01 ㎛. Specifically, the reverse osmosis membrane used herein was a polyamide reverse osmosis membrane for industrial use, made of about 90-95 ㎛ of a reinforcing layer made of polyester unwoven fabric, 40-50 ㎛ of a polysulfone support casted on said reinforcing layer, and about not more than 2 ㎛ of polyamide polymer coated on the polysulfone support.

The initial salt rejection rate and permeation flow rate in the present invention were measured by using 2,000 ppm of an aqueous NaCl solution. A system used for estimation of a reverse osmosis membrane was comprised of a permeation cell in the form of a flat panel, a high pressure pump, a reservoir and a cooling device. The structure of the permeation cell in the form or a flat panel employs a cross-flow type, having an effective permeation area being 27.01 cm². After installing a washed membrane to the permeation cell, warm-up operation of the system was carried out sufficiently for about 1 hour until the pressure and water permeation becomes regular. Water permeation degree was determined by measuring the amount of permeated water at an interval of 30 minutes. Salt rejection was determined by analyzing the salt concentration before and after permeation, by using a conductivity meter.

The resistance to chlorine was determined in a mixed aqueous solution of 2,000 ppm of NaCl and 2,000 ppm of NaOCl. In order to exclude the permeation of chlorine and salts through the unwoven fabric that is a porous supporting layer of the reverse osmosis membrane and a polysulfone layer, the mixed solution was allowed to flow through the inside of the system for 10 to 30 seconds and maintained as being a stationary phase so that the salt permeation only occurs through the polyamide surface. Changes in salt rejection and flow rate were practically measured by operating time.

For determination of fouling resistance, an aqueous solution of 2,000 ppm of NaCl and 100 ppm of casein was used. After determination of the initial permeation property, 100 ppm of the aqueous casein solution was directly fed to a tank, and then the permeation property was immediately determined. The changes in water permeation before and after the introduction of the casein-containing solution were plotted, and the slope at the time of change was calculated by using a computer program, determining the level of fouling.

Example 1

The surface of the above-mentioned polyamide membrane (CPA2-4040) was coated with 1.5 wt% of an aqueous solution of octyltriethoxy silane (OcTES), wherein ethanol was used as a solvent, and dried with hot air at 70℃ for 10 minutes, forming a primary coat. Next, the octyltriethoxy silane-polyamide reverse osmosis membrane surface was secondly coated with 1.0 wt% of sorbitoltriglycidyl ether, wherein water was used as a solvent, and dried with hot air at 70℃ for 10 minutes. The performance of the resulted membrane was measured by using 2,000 ppm of an aqueous NaCl solution under pressure of 225 psi, and the results were represented in Table 1.

Example 2

A reverse osmosis membrane was prepared as in Example 1, except that 1.0 wt% of polyglycerol triglycidyl ether was used for the secondary coating process. The performance of the resulted membrane was measured by using 2,000 ppm of an aqueous NaCl solution under pressure of 225 psi, and the results were represented in Table 1.

Example 3

A reverse osmosis membrane was prepared as in Example 1, except that 1.0 wt% of diglycerol triglycidyl ether was used for the secondary coating process. The performance of the resulted membrane was measured by using 2,000 ppm of an aqueous NaCl solution under pressure of 225 psi, and the results were represented in Table 1.

Comparative example 1

The polyamide membrane (CPA2-4040) was not subjected to any surface treatment, but only soaked in distilled water 1 hour or more. Then, its permeation performance was estimated under the same conditions as described in Examples 1 to 3, and the results were represented in Table 1.

Table 1

	initial salt rejection (%)	initial permeation flow rate (GFD*)
Example 1	99.08	8.454
Example 2	99.98	5.864
Example 3	98.61	5.866
Comparative example 1	98.10	17.17

*GFD: gallons/ft²day

Test examples 1 to 3: Estimation of chlorine resistance

For estimation of the chlorine resistance of the reverse osmosis membranes prepared by Examples 1 to 3, the initial permeation property of each reverse osmosis membrane was determined by operating the system with a mixture of 2,000 ppm of an aqueous NaCl solution and 2,000 ppm of an aqueous NaOCl solution under pressure of 225 psi, and the results were represented in Table 2.

After completion of the above estimation, the system was maintained with the permeation cell equipped with said membrane for 6 hours under the same conditions as described above, then operated 30 minutes under 225 psi, and the performance of the reverse osmosis membrane was measured again. After an elapse of 12 hours, the system was operated 30 minutes under 225 psi, and then the permeation property of the reverse osmosis membrane was measured. The results showing the changes in performance as time elapsed were represented in Tables 3 and 4.

Comparative test example 1

The polyamide membrane (CPA2-4040) was not subjected to any surface treatment, but only soaked in distilled water 1 hour or more. Then, it was estimated under the same conditions as described in test Examples 1 to 3.

Table 2

	initial salt rejection (%)(after addition of NaOCl)	initial permeation flow rate (GFD)(after addition of NaOCl)
Test example 1	98.96	10.54
Test example 2	98.87	6.88
Test example 3	98.49	7.53
Comparative test example 1	97.97	17.35

Table 3

	salt rejection (%) (after being exposed to NaOCl 6 hours)	salt rejection (%) (after being exposed to NaOCl 12 hours)
Test example 1	98.44	98.53
Test example 2	98.63	98.56
Test example 3	98.42	98.32
Comparative test example 1	97.15	97.06

Table 4

	flow rate (GFD) (after being exposed to NaOCl 6 hours)	flow rate (GFD) (after being exposed to NaOCl 12 hours)
Test example 1	12.22	13.01
Test example 2	8.44	9.49
Test example 3	9.38	9.55
Comparative test example 1	17.38	18.4

Test examples 4 to 6: Estimation of fouling resistance

The salt rejection and permeation flow rate of the reverse osmosis membranes obtained from Examples 1 to 3 were measured by using a mixed aqueous solution of 2,000 ppm NaCl and 100 ppm casein. The salt rejection and flow rate before and after the introduction of the casein-containing solution were determined and compared. The permeation flow rate was converted to MFI, an index of membrane fouling, and the results were disclosed in Table 5.

* Membrane Fouling Index (MFI)

MFI, defined by the following math figure, is mainly used for measuring a fouling level in a microfiltration membrane.

MathFigure 1

The MFI can be represented by the above math figure, provided that the thickness of a cake layer formed on the membrane surface (fouling) is in proportion to the filtered amount. To determine the MFI value, the flow rate (Q) is obtained by multiplying the measured permeation flux by the membrane area (A). The resulted value is multiplied by filtration time (t) to obtain the filtered amount (V). By assigning a filtered amount (V) to the horizontal axis (x) and 1/Q to the vertical axis (y), the slope of t/V to V curve is a MFI value. The symbols used in the above math figure refer to the following:

- ΔP (pressure difference before and after passing a membrane) = ΔP_m+ΔP_p+ΔP_c

ΔP_m: pressure drop occurred by passing through a membrane

ΔP_p: pressure drop owing to a concentration polarization layer

ΔP_c: pressure drop owing to a cake layer

- R_m: membrane resistance

- A: area of a membrane

- η: absolute viscosity

- C_b: concentration of a bulk layer/ C_c: concentration of a cake layer

- r_c: radius of cake particles

If MFI is applied to a reverse osmosis membrane process, the change in permeation before and after introduction of fouling materials (reduction in permeation) can be expressed as a change in the slope, helping to understand the level of fouling on the reverse osmosis membrane by comparing the slopes. In other words, according to the definition of MFI, the higher the MFI value of a reverse osmosis membrane, the greater the membrane fouling occurred.

Comparative test example 2

The polyamide membrane (CPA2-4040) was not subjected to any surface treatment, but only soaked in distilled water 1 hour or more. Then, its permeation performance was estimated under the same conditions as in test examples 4 to 6. The change in permeation flow rate represented by MFI was disclosed in Table 5.

Table 5

	initial salt rejection before addition of casein (%)	salt rejection in 2 hours after addition of casein (%)	initial flow rate before addition of casein (GFD)	flow rate in 2 hours after addition of casein (GFD)	MFI value
Test example 4	98.65	98.87	12.15	11.07	1.16×10³
Test example 5	98.07	98.35	8.35	8.00	1.53×10³
Test example 6	98.02	98.33	12.04	11.72	0.17×10³
Comparative test example 2	97.53	98.07	19.95	16.83	2.6×10³

From Tables 1 to 5, it can be seen that the reverse osmosis membranes of the present invention exhibit excellent chlorine and fouling resistance, at the same time. The reverse osmosis membranes according to the present invention also show strong resistance to cleaning agents such as sodium hypochlorite(NaOCl) that is generally introduced to reduce the membrane fouling, and are capable of reducing fouling of the membrane surface, thereby being advantageously applicable to a sea water desalination system or ultrapure water system.

Claims

A method for preparing a reverse osmosis membrane having improved durability, comprising the steps of:

forming a polyamide thin film on a porous supporting layer;

primarily coating the surface of the polyamide thin film with an aqueous solution of silane derivatives;

secondly coating the primarily coated polyamide thin film surface with an aqueous solution of glycidyl compounds and drying the resulted membrane; and

washing the dried membrane with a basic aqueous solution.
A method for preparing a reverse osmosis membrane having improved durability, comprising the steps of:

forming a polyamide thin film on a porous supporting layer;

primarily coating the surface of the polyamide thin film with an aqueous solution of glycidyl compounds;

after said primary coating and washing with a basic aqueous solution, secondly coating the primarily coated polyamide thin film surface with an aqueous solution of silane derivatives and drying the resulted membrane; and

washing the dried membrane with a basic aqueous solution.
The method for preparing a reverse osmosis membrane having improved durability according to claim 1 or 2, wherein the aqueous solution of silane derivatives is prepared by mixing 0.1~4 wt% of silane derivatives and 96~99.9 wt% of ethanol.
The method for preparing a reverse osmosis membrane having improved durability according to claim 1 or 2, wherein the silane derivatives are alkoxyalkyl silane compounds represented by the following chemical figure 1:

[chemical figure 1]

wherein R¹ is selected from the group consisting of substituted or unsubstituted C1~C18 alkyl group, substituted or unsubstituted vinyl group and substituted or unsubstituted phenyl group; R² is C1~C4 alkyl group; and n is an integer of 1, 2 or 3.
The method for preparing a reverse osmosis membrane having improved durability according to claim 4, wherein the silane derivatives are alkoxyalkyl silane compounds represented by the chemical figure 1 wherein R¹ is C8~18 alkyl group; R² is C1-C4 alkyl group; and n is an integer of 1, 2 or 3.
The method for preparing a reverse osmosis membrane having improved durability according to claim 4, wherein the alkyl group further comprises at least one functional group selected from the group consisting of amine, aldehyde, acid, ether, ketone, alcohol, ester, halogen and aromatic group.
The method for preparing a reverse osmosis membrane having improved durability according to claim 4, wherein the alkoxyalkyl silane compounds are selected from the group consisting of trialkoxyalkyl silane, dialkoxydialkyl silane, alkoxytrialkyl silane and mixtures thereof.
The method for preparing a reverse osmosis membrane having improved durability according to claim 4, wherein the alkoxyalkyl silane compounds are at least one selected from the group consisting of trimethylmethoxy silane, trimethylethoxy silane, trimethylpropoxy silane, dimethylethylmethoxy silane, dimethylethylethoxy silane, dimethylethylpropoxy silane, diethylmethylmethoxy silane, diethylmethylethoxy silane, diethylmethylpropoxy silane, methyltrimethoxy silane, ethyltrimethoxy silane, propyltrimethoxy silane, ethyltriethoxy silane, propyltriethoxy silane, methyltripropoxy silane, ethyltripropoxy silane, propyltripropoxy silane, pentyltriethoxy silane, octyltrimethoxy silane, octyltriethoxy silane, octadecyltrimethoxy silane, octadecyltriethoxy silane, (trifluoromethyl)trimethoxy silane, (3,3,3,-trifluoropropyl)trimethoxy silane, 3-aminopropyl-methoxy-diethoxy silane, penyltriethoxy silane, benzyltriethoxy silane, phenethyltrimethoxy silane, vinyltriethoxy silane, 3-aminopropyltrimethoxy silane, 3-aminopropyltriethoxy silane and mixtures thereof.
The method for preparing a reverse osmosis membrane having improved durability according to claim 1 or 2, wherein the aqueous solution of glycidyl compounds is prepared by mixing 0.1~4 wt% of glycidyl compounds and 96~99.9 wt% of water.
The method for preparing a reverse osmosis membrane having improved durability according to claim 1 or 2, wherein the glycidyl compounds comprise 3~4 three-membered cyclic epoxy groups, and at least one functional group selected from the group consisting of ether, glyceryl and sorbitol group.
The method for preparing a reverse osmosis membrane having improved durability according to claim 10, wherein the glycidyl compounds comprising 3 epoxy groups are selected from the group consisting of glycerol triglycidyl ether, diglycerol triglycidyl ether, pentaerythritol triglycidyl ether, sorbitol triglycidyl ether, glycerolpropoxylate triglycidyl ether, trimethylolpropane triglycidyl ether, 1,1,1-tris(hydroxymethy)ethane triglycidyl ether, 1,1,1-tris(hydroxypenyl)ethane triglycidyl ether, tris-hydroxymethylnitromethane triglycidyl ether, tris-(2,3-epoxypropyl)isocyanurate, fluoroglucynol triglycidyl ether, N,N-diglycidyl-4-glycidyloxyaniline, a product obtained from a reaction of epichlorohydrin with 1,3,5-tris(2-hydroxyethyl)cyanuric acid and a product obtained from a reaction of epichlorohydrin with tris(hydroxymethyl)aminomethane.
The method for preparing a reverse osmosis membrane having improved durability according to claim 10, wherein the glycidyl compounds having 4 epoxy groups are selected from the group consisting of sorbitol tetraglycidyl ether, pentaerythritol tetraglycidyl ether, polyglycerol tetraglycidyl ether, and 4,4'-methylenebis(N,N-diglycidylaniline).
The method for preparing a reverse osmosis membrane having improved durability according to claim 1 or 2, wherein the glycidyl compounds are water-soluble glycidyl compounds.
The method for preparing a reverse osmosis membrane having improved durability according to claim 1 or 2, wherein the glycidyl compounds are soluble in a mixed solution of water and ethanol, wherein the water content is not less than 50 vol%.
The method for preparing a reverse osmosis membrane having improved durability according to claim 1 or 2, further comprising, after secondly coating the surface of the polyamide thin film and drying it, a step of drying the resulted membrane with hot air at the temperature range of 25~100℃.
A reverse osmosis membrane having improved durability prepared by the method according to claim 1 or 2.