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.