FIELD OF THE INVENTION
This invention relates to a cartridge filter with the use
of a microporous filter membrane. More particularly, it relates to a cartridge filter
with the use of a hydrophilic microporous filter membrane having a high chemical
resistance which is appropriately usable in processes for producing, for example,
semiconductors and drugs.
BACKGROUND OF THE INVENTION
In the production of semiconductors, there have been recently
required filters which are highly resistant to liquid chemical agents such as organic
solvents, acids, alkalis and oxidizing agents and provide little eluate. It is a
present-day practice to use, in filtering these liquid chemical agents, filters
consisting of microporous microfilter membranes made of polytetrafluoroethylene
(PTFE) and other filter-constituting members made of fluoropolymers.
However, PTFE filter membranes suffer from a problem that
since they have a highly hydrophobic nature, they would undergo air-rock even upon
contamination with only a very small amount of bubbles thereby making filtration
impossible, even though they have been moistened with isopropanol at the initiation
of the filtration. When these fluoropolymer filters are disposed after using, there
arises another problem that toxic gases are generated by incineration.
SUMMARY OF THE INVENTION
Under these circumstances, an object of the invention is
to provide a filter cartridge which withstands the filtration of acids, alkalis,
oxidizing agents, alcohols, in particular, a liquid mixture of hydrochloric acid
with an aqueous solution of hydrogen peroxide (i.e., so-called HPM) and isopropanol
at a high temperature (60 to 80°C), as frequently used in processes of producing
semiconductors and drugs, and the filters of which can be easily disposed by incineration
The object of the invention has been achieved by the following
aspects of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
- (1) A microfilter cartridge wherein a microporous filter membrane, membrane
supports, a core, an outer cover and end-plates constituting the filter cartridge
are all made of a polysulfone polymer, the membrane supports are microporous membranes
provided with a number of fine grooves and/or projections, the microporous filter
membrane has a water bubble point of 0.3 MPa or more while the membrane supports
have a water bubble point of 0.15 MPa or less, measured in accordance with ASTM
F316, and the filter membrane is sandwiched between the membrane support layers
- (2) The microfilter cartridge as described in (1), wherein the polysulfone polymer
is polyether sulfone.
Fig. 1 illustrates a development of the whole structure
of a pleated filter cartridge commonly employed.
The simbols in Fig. 1 will hereinafter be explained.
DETAILED DESCRIPTION OF THE INVENTION
1: outer covering
2: upstream membrane
3: microfilter membrane
4: downstream membrane
8: fluid outlet
Known filter cartridges are generally classified into pleated
cartridges formed by laying a filter membrane on membrane supports for protecting
the membrane and then pleating the same, and flat laminate cartridges formed by
laminating plural filtration units each in the form of a flat sheet. The structure
of the pleated cartridges is described in, for example, JP-A-4-235722 and JP-A-10-66842,
while the structure of the flat laminate cartridges is described in, for example,
JP-A-63-80815, JP-A-56-129016 and JP-A-58-98111. Although filter cartridges of either
type are usable in achieving the object of the invention, pleated cartridges are
particularly useful therefor.
T. Gutowski et. al., Adv. Filtr. Sep. Technol. (1999),
13A, pp 521-528 disclose a highly asymmetric polysulfone filter, wherein the filter
hardware is also made of polysulfone. The filter has excellent chemical resistance
properties and a low pressure drop.
Now, the structure will be described in greater detail
by reference to a pleated filter cartridge by way of example.
Fig. 1 illustrates a development of the whole structure
of a pleated microfilter membrane cartridge filter commonly employed.
In the pleated filter cartridge as shown in Fig. 1, a microfilter
membrane 3 is pleated in a state of being sandwiched between two membrane supports
2 and 4 and wound around a core 5 provided a number of collection ports. An outer
covering 1 is provided outside to protect the microfilter membrane 3. The microfilter
membrane 3 is sealed at both ends of the cylinder with end-plates 6a and 6b. Each
end-plate is contacted with a sealing part of a filter housing (not shown) via a
gasket 7. In some cases, one of the end-plates is provided with an O-ring through
which it comes in contact with a filter housing. Since the gasket or O-ring can
be easily removed at disposal, it is not essentially required that these members
are also made of a polysulfone material. The filtered liquid is collected at the
collection ports of the core 5 and, via the hollow part of the cylinder, discharged
from a fluid outlet 8 located at the end of the cylinder. In some cases, two fluid
outlets are provided at both ends of the cylinder, while one fluid outlet is provided
at one end in others.
In such a pleated filter cartridge, it is also possible,
if necessary, to pass the liquid to be filtered and the filtrate each in the direction
opposite to the one described above. Namely, the liquid to be filtered is supplied
from the fluid outlet 8 into the filter cartridge and filtered through the microporous
filter membrane 3, while the filtrate is discharged outside the filter cartridge.
As the microfilter membrane 3, it is preferable to use
a membrane made of a non-halogen polymer, for example, a polyether sulfone such
as an aromatic polyaryl ether sulfone, a polyolefin or a polyamide. Among all, it
is preferable to use a hydrophilic membrane made of an aromatic polyaryl ether sulfone
(hereinafter referred to as a "polysulfone polymer") because of the excellent heat
resistance and chemical resistance thereof.
Chemical formulae (1) to (3) show typical chemical structures
of polysulfone polymers to be used in the invention. The polymer represented by
the chemical formula (1) has been marketed from Amoco Co. under the trade name UDEL
POLYSULFONE. The polyether sulfone represented by the chemical formula (2) has been
marketed from Sumitomo Chemical Co., Ltd. under the trade name SUMIKAEXCEL PES.
The microporous filter membrane to be used in the microfilter
cartridge of the invention can be produced by selecting and employing an appropriate
process from among the conventional processes for producing microporous filter membranes
with the use of polysulfone polymers.
Processes for producing hydrophilic microporous filter
membranes made of polysulfone polymers are described in detail in, for example,
JP-A-56-154051, JP-A-56-86941, JP-A-56-12640, JP-A-62-27006, JP-A-62-258707 and
Since there are marketed a number of microporous filter
membranes made of polysulfone polymers, it is also possible to select and employ
an appropriate marketed product therefor.
The pore size of microporous filter membrane to be used
in the invention generally ranges from 0.02 to 5 µm. In case of using in the
production of semiconductors, it is preferable to use a filter membrane having a
pore size of 0.02 to 0.45 µm. In case of using in the production of high-integrated
ICs, it is particularly preferable to use a filter membrane having a pore size of
from 0.02 to 0.2 µm. This membrane characteristic corresponds to a water bubble
point value, measured in accordance with ASTM F316, of 0.3 MPa or more and an ethanol
bubble point of 0.1 to 1 MPa. It is still preferable that the filter membrane has
an ethanol bubble point of from 0.3 to 0.7 MPa.
It is preferable that a membrane has a high porosity based
on the apparent volume, since the filtration resistance can be reduced thereby.
However, an excessively high porosity worsens the membrane strength thereby making
the membrane brittle. Therefore, it is preferable to use a membrane having a void
ratio of from 40 to 90%, still preferably from 57 to 85%.
The thickness of a membrane usually ranges from 30 to 220
µm. When the membrane is too thick, the membrane can be packed into the cartridge
only in a small area. When the membrane is too thin, on the other hand, the membrane
strength is worsened. It is, therefore, preferable that the membrane thickness ranges
from 60 to 160 µm, still preferably from 90 to 140 µm.
The microfilter membrane 3 is sandwiched between the membrane
supports 2 and 4 and then pleated by a publicly known method.
In the conventional pleated cartridges, it has been a practice
to use, for example, nonwoven fabrics, woven fabrics, nets or microporous membranes
as the upstream membrane support 2 and the downstream membrane support 4. These
membrane supports are employed to strengthen the filter membrane against changes
in filtration pressure, to allow the liquid to permeate from the liquid supply side
to the filtration side, and, at the same time, to introduce the liquid into the
inner parts of pleats in the direction parallel to the filter membrane. Accordingly,
it is necessary that these membrane supports have an adequate liquid-permeability
and a sufficient physical strength for protecting the filter membrane. Although
any sheet material is usable therefor so long as it satisfies these requirements,
there have been employed in most cases polyester or polypropylene non-woven fabrics
which are less expensive and excellent in performance.
The membrane supports usable in the invention should have
a heat resistance and a chemical resistance and be incineratable, in addition to
the general functions as described above. Thus, the inventors have conducted intensive
studies and, as a result, found out that it is preferable for achieving the object
as described above to use non-halogen polymers which are comparable or even superior
in heat resistance and chemical resistance to the material employed for the microporous
filter membrane 3, in particular, polysulfone polymers being excellent both in heat
resistance and chemical resistance. However, it is impossible to use non-woven or
woven fabrics made of polysulfone polymers, since no polysulfone polymer fiber is
In the microfilter cartridge filter according to the invention,
therefore, use is made of a microporous filter membrane made of a polysulfone polymer
as the membrane supports. These membrane supports are produced fundamentally in
the same manner as in the production of the microporous microfilter membrane as
described above. The microporous membrane to be used as the membrane supports generally
has a water bubble point of 0.15 MPa or less, preferably from 0.02 to 0.15 MPa and
still preferably from 0.04 to 0.15 MPa. It is preferable that the water permeability
in the direction perpendicular to the membrane support face is 150 ml/cm2
or more, more preferably 200 ml/cm2 or more, when expressed in the water
flow per minute under loading a differential pressure of 0.1 MPa. The Mullen bursting
strength of the membrane supports is preferably 80 kPa or more, still preferably
120 kPa or more.
Grooves and/or projections may be formed on the membrane
supports to be used in the invention by an arbitrary method without restriction.
This object can be achieved by using an embossing calender wherein a microporous
membrane is sandwiched between a metal roll provided with a number of projections
and a back-up roll having a smooth surface and continuously pressed. Grooves are
exclusively formed on a membrane support in case of using a hard back-up roll, while
projections are simultaneously formed on the opposite face of the grooves in case
of using a flexible back-up roll. Since pores are squashed and thus the water permeability
disappears in the grooves, it is favorable that these grooves are formed within
an area not more than a half of the whole support membrane.
In this case, the grooves and/or projections may be formed
either on one face or both faces of the membrane support. The depth/height of the
grooves/projections to be formed on the membrane support can range from 5 µm
to 0.25 mm, preferably from 20 µm to 0.15 mm and still preferably from 50 µm
to 0.1 mm. The width of the grooves and/or projections (hereinafter referred to
simply as grooves) to be formed on the membrane support can range from 5 µm
to 1 mm, preferably from 20 µm to 0.4 mm and still preferably from 50 µm
to 0.2 mm. It is not necessary that the width and depth of the grooves are uniform
anywhere. In case of forming grooves, it is not preferable to form round or polyhedral
grooves independent from each other. Namely, it is preferable to form grooves connected
to each other so as to allow a liquid to flow in the face direction. It is further
preferable that a number of grooves are formed lengthwise and crosswise and intersect
each other. It is preferable that intervals among grooves are 4 mm at the broadest,
still preferably from 0.15 to 2 mm.
The thickness of the microporous membrane to be used as
the membrane supports preferably ranges from 60 to 300 µm, still preferably
from 100 to 220 µm. When the microporous filter membrane is too thin, it can
achieve only a poor effect of reinforcing the filter membrane. On the other hand,
it is also unfavorable that the microporous filter membrane is too thick, since
the area of the membrane which can be packed in the cartridge is lessen in this
The microporous filter membrane is sandwiched between the
membrane supports and then subjected to pleating in a conventional manner. Either
one or more microporous filter membranes may be employed. Either one or more membrane
supports may be used in each side.
After pleating, both ends of the pleated filter material
are made uniform by cutting off unnecessary parts with, for example, a cutter knife
and rolled to form a cylinder. Then the pleats at the joint are liquid-tightly sealed
by heat-sealing or with the use of an adhesive. The six layers (i.e. the microporous
filter membrane and the membrane supports) may be all sealed together. Alternatively,
both ends of the filter membrane, excluding the support 2 or 4, may be overlapped
together and sealed. It is also possible that a polysulfone sheet is inserted into
the joint and heat-sealed. To achieve good adhesion, it is preferable that the adhesive
or the polysulfone sheet to be used herein is made of the same material as the filter
membrane. In case of using an adhesive, a polysulfone polymer is used in a state
of being dissolved in a solvent. For example, 10 parts of polyether sulfone is dissolved
in a liquid mixture of 30 parts of methylene chloride and 20 parts of diethylene
glycol and then 140 parts of diethylene glycol is slowly added thereto and mixed.
The solvent is not left on the filter cartridge but evaporated off by heating after
Next, the core 5 is inserted into the cylindrical filter
material thus obtained and the outer covering 1 is provided around the filter to
give a pleated matter. Methods usable in the step of heat-sealing wherein both ends
of the pleated matter are closely sealed to the end-plates 6 are roughly classified
into the hot-melt method and the solvent adhesion method. In the hot-melt method,
the sealing face of each end-plate is exclusively contacted with a hot plate or
irradiated with an infrared heater so that the surface thereof alone is molten.
Then one end face of the pleated matter is pressed to the molten face of the end-plate
and thus adhesion-sealed.
In the solvent adhesion method, it is important to select
an adequate solvent. It is recommended that about 1 to 7% by weight of a polymer
has been preliminarily dissolved in the solvent adhesive. As the polymer to be dissolved,
use is made of the same one as used in the end-plates or, at least, one which can
be easily adhered to the end-plates.
The materials to be used in the membrane supports 2 and
4, the core 5, the outer covering 1 and the end-plates 6 should also have a heat
resistance and a chemical resistance. It is also necessary that these materials
are incineratable. It is therefore preferable that these materials are selected
from among non-halogen polymer materials such as polysulfone polymers, polyolefins
and amides. Among all, polysulfone polymers are preferable and polyether sulfone
is still preferable because of being excellent in heat resistance and chemical resistance
and relatively less expensive. It is not always necessary that these members are
made of the same material, so long as these materials can be adhered to each other.
However, it is still preferable that these members are made of the same material,
since excellent adhesion can be established in such a case. It is particularly preferable
that all of these members are made of polyether sulfone from the viewpoint of broadening
the chemical resistance and achieving good adhesion sealing.
The microfilter cartridge according to the invention is
resistant to acids, alkalis, oxidizing agents and alcohols at high temperature.
After using, the filter cartridge can be easily incinerated.
The invention will be described in greater detail by reference
to the following Examples, but it should be understood that the invention is not
construed as being limited thereto.
A polysulfone membrane having an ethanol bubble point of
250 kPa was formed by the method described in Example 1 of JP-A-63-139930 and employed
as a microporous filter membrane (hereinafter referred to as the membrane A). On
the other hand, another polysulfone membrane having an ethanol bubble point of 50
kPa was formed by the method described in Example 3 of JP-A-63-139930 (hereinafter
referred to as the membrane B). On one face of the membrane B, grooves (width: about
0.15 mm, depth: about 55 µm) were formed at intervals of 0.15 to 0.3 mm by
emboss calendering. The membrane C thus obtained was employed as a membrane support.
The membrane A was sandwiched between two membranes C and
pleated in a conventional manner. The membranes C in the upstream and the downstream
were each contacted with the membrane A in the smooth face having no grooves. The
bundled membranes having been pleated about 120 times (pleat intervals: 10.5 mm,
membrane width: 240 mm) were cut and shaped into a cylinder. Then the pleats at
the joint were liquid-tightly sealed by heat-sealing. The bundled membranes and
the core were packed in a polysulfone outer covering and both ends were combined
together to form a pleated matter. The surface of an end-plate formed by cutting
a polysulfone round bar was irradiated with infrared light and thus molten at about
350°C. Then an end of the pleated matter, which had been sufficiently pre-heated,
was pressed against the end-plate surface and thus adhesion-sealed. Another end
of the pleated matter was also sealed to an end-plate in a molten state, thereby
completing the construction of a filter cartridge.
Comparative EXAMPLE 2
A filter cartridge was constructed as in Example 1 but
using the membranes B as in Example 1 as the upstream and downstream membrane supports
without emboss calendering.
Comparative EXAMPLE 3
A polyether sulfone film of 50 µm in thickness (SUMILITE
FS-1300, manufactured by Sumitomo Bakelite Co., Ltd.) was punched to give 3 pores
(diameter: 0.6 mm) per 2 cm x 2 cm. By using a flexible resin roll as a back roll,
the punched film was emboss calendered to thereby give grooves (width: about 0.2
mm) at intervals of about 0.2 mm. In this step, the surface temperature of the emboss
roll was 125°C and the pressing force was 100 kN/m.
A polyether sulfone microporous filter membrane of 0.1
µm in pore size (MICRO PES 1FPH, manufactured by Membrana, ethanol bubble point
value: 340 kPa) was sandwiched between two membranes formed above followed by pleating.
After pleating about 140 times (pleat intervals: 10.5mm, membrane width: 240 mm),
the bundled membranes were cut and shaped into a cylinder. Then the pleats at the
joint were liquid-tightly sealed by heat-sealing. A pleated matter was constructed
and sealed to end-plates in a molten state as in Example 1 to give a filter cartridge.
Comparative EXAMPLE 4
Filaments of 200 µm in diameter were spun from polyether
sulfone pellets (SUMIKAEXCEL PES3600G) and then twill-woven to give a net. A filter
cartridge consisting of bundled and pleated membranes (about 120 pleats) was constructed
as in Example 1 but using the net obtained above as the downstream support, the
same polyether sulfone membrane as in Comparative Example 2 as the microporous filter
membrane, and the same polyether sulfone film having been punched and emboss calendered
as in Comparative Example 3 as the upstream support.
COMPARATIVE EXAMPLE 1
A filter cartridge consisting of bundled and pleated membranes
(about 140 pleats) was constructed by using the membrane A as in Example 1 as the
microporous filter membrane, polypropylene non-woven fabric membranes (SYNTEX PS-160,
manufactured by Mitsui Petrochemical Industries, Ltd., fiber size: 2 D, basis weight:
30 g/m2) as the upstream and downstream supports, and molded polypropylene
articles as the core, the outer covering and the end-plates.
(1) Evaluation of chemical resistance
The filter cartridges of Example 1 and Comparative Examples
1 to 4 were compared and evaluated in chemical resistance. Table 1 shows the results.
As a result, polypropylene was seriously deteriorated when immersed in HPM, whereas
polysulfone and polyether sulfone were scarcely deteriorated. In the filter cartridge
of Comparative Example 1, the polysulfone membrane could not be supported by the
membrane supports any longer and suffered from cracking at the parts close to the
molded adhesion sites to the end-plates, thereby failing to maintain the integrity.
The chemicals employed were HPM and isopropanol. Each filter
cartridge was immersed in each chemical and heated to 80°C for 8 hours per
day, 480 hours in total. Next, the maintenance of the integrity and appearance of
the filter cartridge were observed. HPM was prepared by mixing conc. hydrochloric
acid, a 30% aqueous solution of hydrogen peroxide and ultrarpure water at a ratio
of 1:1:1. Since HPM would be deteriorated, it was replaced by a fresh one everyday.
(2) Evaluation of flow characteristics and pressure resistance
C. Ex. 2
C. Ex. 3
C. Ex. 4
C. Ex. 1
Maintenance of integrity
difference from untreated product.
were deteriorated and fell-off in pieces. End-plates, core and outer covering suffered
from serious cracking and color change.
Maintenance of integrity
difference from untreated product.
Fine marks were
formed on the surface of end-plates, core and outer covering.
Table 2 shows the results of the evaluation of the flow
characteristics and pressure resistances of the filter cartridges of Example 1 and
Comparative Examples 2 to 4. The filter cartridges of Example 1, and Comparative
Examples 3 and 4 are excellent both in the flow characteristics and pressure resistance.
In contrast, the filter cartridge of Comparative Example 2 with the use of the membrane
supports not emboss-calendered is much inferior in flow characteristics.
Flow characteristics were evaluated by feeding water with
a pump at a flow rate of 2 kl/min into a filter cartridge packaged in a filtering
unit and the filtration differential pressure was measured. Pressure resistance
was evaluated by supplying and discharging compressed air of 100 kPa alternately
from the upstream and the downstream of the filter cartridge, which had been moistened
with water so as to prevent air to permeate therethrough, repeatedly 3000 and 5000
times. Then the maintenance of the integrity was measured and the occurrence of
the filter breakage was evaluated. The maintenance of the integrity was judged by
applying an air pressure of 200 kPa to the upstream of the filter cartridge, then
blocking the supply of the compressed air and examining the occurrence of a decrease
in the air pressure over 3 minutes while allowing the filter cartridge to stand
during this period.
(3) Mullen bursting strength test
differential pressure (kPa)
After completing the immersion in HPM for evaluating the
chemical resistance as described in the above (1), the microporous filter membranes
of Example 1 and Comparative Example 3 were taken out and subjected to a Mullen
bursting strength test. The polysulfone membrane of Example 1 showed a Mullen bursting
strength of 95 kPa, thereby showing a slight decrease compared with the non-immersed
polysulfone membrane (110 kPa). The polyether sulfone membrane of Comparative Example
3 showed a Mullen bursting strength of 120 kPa, thereby showing no difference from
the non-immersed polyether sulfone membrane (120 kPa).
While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to one skilled in
the art that various changes and modifications can be made therein without departing
from the spirit and scope thereof.