EP1024886A1

EP1024886A1 - A manufacturing method of composite membrane having hydrophilic coating layer on hydrophobic support membrane

Info

Publication number: EP1024886A1
Application number: EP98951774A
Authority: EP
Inventors: Jung Min Lee; Choong Kyun Yeom; Chul Ung Kim; Beom Sik Kim; Kwang Joo Kim
Original assignee: Korea Research Institute of Chemical Technology
Current assignee: Korea Research Institute of Chemical Technology
Priority date: 1997-10-20
Filing date: 1998-10-20
Publication date: 2000-08-09
Also published as: KR19990032690A; JP2001520111A; WO1999020378A1; KR100217311B1

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

CLAIMSWhat 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 ╬╝m,

which is prepared through proper adjustment of concentration in an aqueous

solution of both reacting agent and hydrophilic polymer.