Membrane filter for liquids and filtering device using the same

Objects of the present invention are to provide a membrane filter for liquids which is used in microfiltration or ultrafiltration of liquids such as chemicals, water, and the like and to provide a filtering device using the membrane. The membrane is a membrane filter for liquids which has been rendered hydrophilic, in which the decrease of the pure water permeation flux of the membrane filter after being subjected to wetting treatment with a liquid having a low surface tension, subjected to replacement with pure water, and then exposed to an atmosphere of 25.degree. C. and 60% for 8 hours, to the pure water permeation flux of the membrane filter before the exposure is 20% or less. Such a membrane filter which has been rendered hydrophilic can be obtained by, for example, rendering the surface of a PTFE porous membrane hydrophilic by treating the surface with a low temperature plasma under conditions that a product of high frequency output density and treatment time is from 0.5 to 50 W.multidot.sec/cm.sup.2 and a total gas pressure is from 0.01 to 10 torr.

TECHNICAL FIELD 
The present invention relates to a membrane filter for liquids which is 
used in a microfiltration or an ultrafiltration of liquids such as 
chemicals and water, and to a filtering device using the membrane filter. 
Specifically, it relates to a membrane filter for liquids which is 
suitably used to filtrate chemicals for washing silicon wafers in 
semiconductor industries, and to a filtering device. 
BACKGROUND ART 
In the microfiltration or ultrafiltration treatment of liquids such as 
chemicals and water, fine particle-removing performance, permeation flux 
for liquid, resistance to chemicals, pressure resistance, heat resistance, 
and the like are important selection factors for membrane filter. For this 
reason, porous membranes comprising high molecular weight polymers such as 
fluororesins including polytetrafluoroethylene (hereinafter, abbreviated 
as "PTFE") and the like, polyethylene, polypropylene, polyolefins, and the 
like have conventionally been selected. 
However, membranes which have been rendered hydrophilic have become more 
and more necessary in recent years. That is, in the semiconductor 
industry, for example, washing of silicon wafers is being conducted using 
nitric acid, hydrofluoric acid, sulfuric acid, or the like, but when 
replacing the chemicals after circulating and washing, air flows into the 
cartridge filter for filtration by discharging the chemicals. Because of 
this, the surface of a hydrophobic membrane such as PTFE membrane or the 
like in the filter comes into contact with air and the effect that the 
membrane has been treated with an organic solvent or the like to render 
the membrane hydrophilic is lost and, hence, introduction of chemicals in 
the next operation results in a significantly reduced liquid permeation 
flux. 
Therefore, there has been a problem that the membrane should again be 
rendered hydrophilic with a liquid having a low surface tension. 
On the other hand, known methods for rendering a porous polymer membrane 
hydrophilic include, for example, surface treatment with a plasma or 
sputtering, exposure to radiation, and the like (Unexamined Published 
Japanese Patent Applications Nos. 59-186604 and 1-98640, and others). 
However, such methods have had a problem that the imparted hydrophilicity 
is insufficient, or if a membrane is to be rendered sufficiently 
hydrophilic, the membrane surface is modified excessively, causing to 
change the rejection performance the membrane originally possesses. 
DISCLOSURE OF THE INVENTION 
The present invention has been completed to solve such problems of the 
prior art techniques. By treating the surface of a fluorine-containing 
porous polymer membrane with a low-temperature plasma under specific 
conditions, the hydrophobic membrane is enabled to retain hydrophilicity 
without reducing the rejection performance the membrane originally 
possesses, whereby a membrane filter having a significantly improved 
permeation flux can be obtained. The present invention provides a membrane 
filter for liquids which does not show a decrease in permeation flux even 
if the liquid in the filtering device such as cartridge filter or the like 
is drained away, and also provides a filtering device in which the 
membrane filter has been incorporated. 
That is, the present invention relates to a membrane filter for liquids 
which has been rendered hydrophilic, characterized in that the decrease of 
the pure water permeation flux of the membrane filter after being 
subjected to wetting treatment with a liquid having a low surface tension 
(e.g., ethanol, methanol, isopropyl alcohol, or the like), subjected to 
substitution with pure water, and then exposed to an atmosphere of 
25.degree. C. and 60% for 8 hours, to the pure water permeation flux of 
the membrane filter before the exposure is 20% or less, preferably 10% or 
less. 
The present invention further relates to a membrane filter for liquids, 
characterized in that the surface of a fluorine-containing porous polymer 
membrane has been rendered hydrophilic by a low temperature plasma 
treatment under conditions that a product of 5 to 15 MHz high frequency 
output treating density and treatment time is from 0.5 to 50 
W.multidot.sec/cm.sup.2 and a total gas pressure is from 0.01 to 10 torr. 
The fluorine-containing porous polymer membrane in the present invention is 
a hydrophobic membrane comprising a fluororesin such as PTFE, 
poly(vinylidene fluoride), or the like, and PTFE is particularly preferred 
from the standpoints of resistance to chemicals, heat resistance, and 
others. Further, a membrane having a pore diameter of from 0.01 to 1 
.mu.m, particularly from 0.05 to 0.2 .mu.m, is suitably used. 
In the present invention, a hydrophilic membrane is obtained by rendering 
the surface of the above porous membrane hydrophilic by, for example, 
treating the porous membrane with a low temperature plasma under specific 
conditions. Such a plasma treatment is carried out by introducing a gas 
species (O.sub.2, H.sub.2, N.sub.2, H.sub.2 O, CO.sub.2, He, Ar, NO, or 
the like) under reduced pressure conditions, regulating the pressure 
generally at 0.01 to 10 torr, preferably at 0.01 to 1.0 torr, and treating 
the membrane at a radio frequency (5 to 15 MHz) such that the product of 
high frequency output treating density and treatment time becomes 0.5 to 
50 W.multidot.sec/cm.sup.2, preferably 1 to 10 W.multidot.sec/cm.sup.2. 
If this product of high frequency output treating density and treatment 
time is less than the above range, sufficient hydrophilicity is not 
imparted. On the other hand, if it is more than the range, small fibers 
(fibrils) in the membrane are broken and this results in a problem that 
the rejection performance is reduced and scattered fine particles come 
into permeation liquids to lower the membrane performance. 
Further, total gas pressures below 0.01 torr are insufficient for membrane 
modification, while the pressures above 10 torr are not preferred in that 
an apparent plasma intensity is increased due to an increased number of 
gas molecules, resulting in fibril breakage. 
In the treatment, if the membrane treated is directly placed on an 
electrode plate, there is a fear that the membrane surface is damaged and 
fibril breakage occurs, due to collision of accelerated electrons and 
charged particles. Normally, the membrane is placed at an 
electrode-to-membrane distance of from 1 to 20 cm, preferably from 2 to 10 
cm. 
It is preferable that the contact angle between the porous membrane thus 
rendered hydrophilic and 40 wt % ammonium fluoride aqueous solution is 
120.degree. or less. 
According to the present invention, the porous membrane which has been 
rendered hydrophilic by the plasma treatment as described above may be 
used to fabricate a filtering device such as a cartridge filter produced 
by processing the membrane into a disk or pleated form, bonding it to a 
support, and incorporating it into a container, a plate-type module in 
which the membrane in flat form is supported by a plate and a frame, a 
hollow fiber membrane module, or the like. 
Although the filtering device of the present invention is not particularly 
limited in application thereof, it can, for example, be advantageously 
used in the semiconductor industry as a cartridge filter for filtering 
washing chemicals for silicon wafers.

BEST MODES FOR CARRYING OUT THE INVENTION 
EXAMPLE 1 
A polytetrafluoroethylene porous membrane (manufactured by Nitto Denko 
Corporation; trade name, NTF 1111; nominal pore diameter, 0.1 .mu.m) was 
set in a plasma irradiator in parallel with an electrode plate at a 
distance of 10 cm therefrom, followed by evacuation, feeding of O.sub.2 
gas at a flow rate of 10 cc(STP)/min, and pressure regulation at 0.02 
torr. The membrane was treated at 13.56 MHz in a manner such that the 
product of high frequency output treating density and treatment time 
became 14.3 W.multidot.sec/cm.sup.2, and the membrane was then taken out 
to obtain a membrane filter of the present invention. 
The membrane was cut to 20 cm.sup.2, set in a cartridge-type filtering 
device, and rendered hydrophilic by a treatment that about 50 cc of 
ethanol was passed through the membrane. Pure water was then passed, with 
the degree of vacuum on the downstream side being 525 mmHg, and as a 
result, the initial pure water permeation flux was 7 cc/cm.sup.2 /min. 
This membrane had contact angles at 25.degree. C. of 125.degree. and 
114.degree. for pure water and 40 wt % NH.sub.4 F aqueous solution, 
respectively. Before the plasma treatment, they were 126.degree. and 
132.degree., respectively. Further, the permeation flux for 40 wt % 
NH.sub.4 F aqueous solution was 2.2 cc/cm.sup.2 /min. 
Suction was further continued to remove the liquid on the membrane surface. 
Thereafter, the relationship between air contact time and permeation flux 
was determined, and the results are shown in FIG. 1. 
Further, the membrane surface was analyzed by ESCA. As a result, it was 
ascertained that F had been eliminated and O had been incorporated. SEM 
examination (magnification 20,000) also revealed that there were no broken 
fibrils and the number of fine particles in the permeation liquid did not 
increase. 
Further, the membrane had a latex rejection, as measured through vacuum 
filtration of a 10 ppm aqueous solution of a 0.102 .mu.m latex, of 93%. 
COMATIVE EXAMPLE 1 
Initial pure water permeation flux measured in the same manner as in 
Example 1 except that the plasma treatment was omitted was found to be 6 
cc/cm.sup.2 /min. The results of the change in permeation flux after 
contact with air are also shown in FIG. 1. Further, the latex rejection 
was 94%. 
Further, the permeation flux for 40 wt % NH.sub.4 F aqueous solution was 
1.4 cc/cm.sup.2 /min. 
Although this membrane was equal to the membrane of Example 1 in rejection 
performance and pure water permeation flux, the membrane of Example 1 
which had undergone treatment for rending it hydrophilic showed higher 
permeation fluxes for high viscosity liquids such as the NH.sub.4 F 
aqueous solution. 
It can be clearly seen from FIG. 1 that the membrane filter of the present 
invention shows extremely good pure water permeation flux as compared with 
the membrane of the Comparative Example, although its latex rejection is 
almost the same as that of the membrane of the Comparative Example, and 
that the membrane filter of the present invention can retain such pure 
water permeation flux over a prolonged period of time even if the membrane 
surface is exposed to air. 
EXAMPLE 2 
A membrane was set in the same manner as in Example 1 and the plasma output 
was doubled (28.6 W.multidot.sec/cm.sup.2). As a result, the resulting 
membrane was equal in membrane performance to the membrane of Example 1. 
Further, even after contact with air, this membrane retained pure water 
permeation flux over a prolonged period of time, which is comparable to 
the membrane of Example 1. 
COMATIVE EXAMPLE 2 
The same treatment as in Example 1 was conducted except that the same 
membrane as used in Example 1 was placed directly on the electrode plate. 
As a result, the latex rejection of the membrane decreased from 93% to 
10%, while the pure water permeation flux increased 1.5 times. Moreover, 
this membrane was examined with an SEM and, as a result, fibril breakage 
was ascertained. 
COMATIVE EXAMPLE 3 
A membrane was set in the same manner as in Example 1 and the product of 
high frequency output treating density and treatment time was reduced (to 
0.3 W.multidot.sec/cm.sup.2). As a result, the resulting membrane showed 
membrane performance comparable to that in Comparative Example 1 and the 
membrane was not rendered hydrophilic. 
COMATIVE EXAMPLE 4 
The same operation as in Example 1 was conducted, with the product of high 
frequency output treating density and treatment time being increased (to 
57.2 W.multidot.sec/cm.sup.2). As a result, the latex rejection decreased 
from 93% to 30%, while the pure water permeation flux increased 1.3 times. 
Moreover, this membrane was examined with an SEM and, as a result, fibril 
breakage was ascertained. 
POSSIBILITY OF INDUSTRIAL APPLICATION 
According to the present invention, since a hydrophobic membrane is enabled 
to retain hydrophilicity without reducing the rejection performance the 
membrane inherently possesses, the hydrophilicity of the membrane in a 
filtering device such as a cartridge filter or the like is maintained even 
if the device is drained after liquid permeation because the membrane 
surface still retains the liquid and, hence, air does never come into 
contact with the inner of the membrane. Therefore, a membrane filter for 
liquids which shows no decrease in permeation flux even after discharge of 
the liquid in the filtering device can be obtained.