U.S. patent application number 13/098872 was filed with the patent office on 2011-11-03 for ultrafine continuous fibrous ceramic filter and method of manufacturing same.
This patent application is currently assigned to KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY. Invention is credited to Jeong Joo CHOO, Sung-Yeon JANG, Seong Mu JO, Dong Young KIM.
Application Number | 20110266213 13/098872 |
Document ID | / |
Family ID | 44857437 |
Filed Date | 2011-11-03 |
United States Patent
Application |
20110266213 |
Kind Code |
A1 |
JO; Seong Mu ; et
al. |
November 3, 2011 |
ULTRAFINE CONTINUOUS FIBROUS CERAMIC FILTER AND METHOD OF
MANUFACTURING SAME
Abstract
An ultrafine continuous fibrous ceramic filter, which comprises
a filtering layer of a fibrous porous body, wherein the fibrous
porous body comprises continuous ultrafine fibers of metal oxide
which are randomly arranged and layered, and powdery nano-alumina
incorporated into the ultrafine fibers or coated thereon, the
ultrafine fibers being obtained by electrospinning a spinning
solution comprising a metal oxide precursor sol-gel solution, and
optionally, a polymer resin, and sintering the electrospun fibers,
in which the ultrafine fibers have an average diameter of
10.about.500 nm, and the fibrous porous body has a pore size of
maximum frequency ranging from 0.05 to 2 .mu.m, exhibits high
filtration efficiency at a high flow rate, and can be
regenerated.
Inventors: |
JO; Seong Mu; (Seoul,
KR) ; KIM; Dong Young; (Seoul, KR) ; JANG;
Sung-Yeon; (Daegu, KR) ; CHOO; Jeong Joo;
(Seoul, KR) |
Assignee: |
KOREA INSTITUTE OF SCIENCE AND
TECHNOLOGY
Seoul
KR
|
Family ID: |
44857437 |
Appl. No.: |
13/098872 |
Filed: |
May 2, 2011 |
Current U.S.
Class: |
210/496 ;
427/458; 977/777 |
Current CPC
Class: |
B01D 2239/086 20130101;
C04B 35/62813 20130101; B01D 2239/1225 20130101; B01D 2239/025
20130101; C04B 35/76 20130101; B01D 69/10 20130101; B82Y 30/00
20130101; C04B 2235/5268 20130101; D10B 2505/04 20130101; B01D
39/2079 20130101; B01D 71/024 20130101; B01D 2239/0478 20130101;
C04B 2235/3218 20130101; C04B 2235/443 20130101; B01D 69/141
20130101; C04B 2235/5264 20130101; B01D 2239/0258 20130101; C04B
2235/5228 20130101; C04B 2235/5252 20130101; C04B 2235/5296
20130101; B01D 39/2082 20130101; D10B 2101/08 20130101; C04B
35/63444 20130101; C04B 35/624 20130101; B01D 67/0041 20130101;
B01D 2239/1216 20130101; B01D 2323/39 20130101; C04B 35/6224
20130101; C04B 35/6264 20130101; D01D 5/0007 20130101; C04B
35/62892 20130101; B01D 39/2089 20130101; C04B 2235/441 20130101;
C04B 35/62236 20130101; B01D 71/025 20130101; C04B 35/62245
20130101 |
Class at
Publication: |
210/496 ;
427/458; 977/777 |
International
Class: |
B01D 39/20 20060101
B01D039/20; B05D 1/04 20060101 B05D001/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 3, 2010 |
KR |
10-2010-0041315 |
Claims
1. A ceramic filter comprising a filtering layer of a fibrous
porous body, wherein the fibrous porous body comprises continuous
ultrafine fibers of metal oxide which are randomly arranged and
layered, and powdery nano-alumina incorporated into the ultrafine
fibers or coated thereon, the ultrafine fibers being obtained by
electrospinning a spinning solution comprising a metal oxide
precursor sol-gel solution, and optionally, a polymer resin, and
sintering the electrospun fibers, in which the ultrafine fibers
have an average diameter of 10.about.500 nm, and the fibrous porous
body has a pore size of maximum frequency ranging from 0.05 to 2
.mu.m.
2. The ceramic filter of claim 1, wherein the electrospun fibers
are sintered at a temperature ranging from 250 to 1000.degree.
C.
3. The ceramic filter of claim 2, wherein, before sintered, the
ultrafine fibers are subjected to heat compression at a temperature
ranging from room temperature to 250.degree. C.
4. The ceramic filter of claim 1, wherein the fibrous porous body
comprises 1.about.90 wt % of nano-alumina based on the total weight
of the porous body.
5. The ceramic filter of claim 1, wherein the nano-alumina is a
nanoparticle selected from the group consisting of boehmite
(AlOOH), aluminum hydroxide (Al(OH).sub.3), gamma-alumina
(.gamma.-Al.sub.2O.sub.3) and a mixture thereof, which are provided
in the form of nanorods, nanotubes or nanofibers, having a diameter
of 1 nm or more and a diameter to length ratio (an aspect ratio) of
5 or more.
6. The ceramic filter of claim 1, wherein the ultrafine fibers are
made of an metal oxide selected from the group consisting of silica
(SiO.sub.2), gamma-alumina (.gamma.-Al.sub.2O.sub.3), and a mixture
thereof.
7. The ceramic filter of claim 1, wherein the polymer resin is
selected from the group consisting of polyvinylpyrrolidone,
polyvinylalcohol, polyvinylacetate, polyethylene oxide, and a
mixture thereof.
8. The ceramic filter of claim 1, wherein the polymer resin is
polyacrylonitrile or its copolymer.
9. The ceramic filter of claim 1, wherein said electrospinning is
melt-blowing, flash spinning, or electro-blowing.
10. A method for preparing the ceramic filter of claim 1, which
comprises the steps of: (1) electrospinning a metal oxide precursor
sol-gel solution or a mixture of a metal oxide precursor sol-gel
solution and a polymer resin to make a layer of continuous
ultrafine fibers randomly arranged; and (2) sintering the
electrospun ultrafine fibers at a temperature ranging from 250 to
1000.degree. C., wherein (A) in step (1), before the
electrospinning, the metal oxide precursor sol-gel solution or the
mixture of the metal oxide precursor sol-gel solution and the
polymer resin is additionally mixed with one-dimensional powdery
nano-alumina; (B) the sintered ultrafine fibers from step (2) are
impregnated or coated with a suspension of one-dimensional powdery
nano-alumina; or (A) and (B) both are performed.
11. The method of claim 10, wherein, before the sintering of step
(2), the ultrafine fibers are subjected to heat compression at a
temperature ranging from room temperature to 250.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to an ultrafine continuous
fibrous ceramic filter, which exhibits high filtration efficiency
at a high flow rate, and can be regenerated, and to a method of
manufacturing same.
BACKGROUND ART
[0002] There has been a recent upsurge in demand for highly
advanced techniques for water purification capable of removing not
only physical contaminants such as organic materials, heavy metals,
etc. but also biological impurities such as viruses. Such a water
purification system typically includes a membrane filter having
pores smaller than particles that are to be filtered out. Examples
of the membrane filter include a microfiltration filter (MF; pore
size 50.about.2000 nm), an ultrafiltration filter (UF; pore size
1.about.200 nm), and a reverse osmosis filter (RO; pore size
0.1.about.2 nm). The membrane-based liquid filter/separation
techniques are regarded as very important in water treatment fields
including oil/water emulsion separation and desalting, since they
are very effective in separating fine particles, bio
macromolecules, oil/water emulsions, salts, and ultrafine particles
such as viruses. The RO or UF membrane is capable of removing
particles larger than 60 nm, and hence is used to remove bacteria
or toxic viruses from water, air or blood. The size of pathogenic
viruses such as the SARS virus and avian influenza virus is in
ranges of 80.about.200 nm. However, in order to remove ultrafine
particles (virus) of 30 nm or less, the size of pore must be much
smaller. This results in a drastic pressure drop and reduces
process flow rates. In addition, during use, membranes are
susceptible to clogging which further degrades the flow rates, and
back washing must be used. Back washing markedly increases
operating costs and undesirably causes membrane damage or pore size
increase. Accordingly, there has been a demand for a filtering
device having a low operating pressure and an improved filtration
efficiency in a large scale plant.
[0003] Mesh filters or non-woven fabric filters are known to have
low pressure drop. A fibrous depth filter is a non-woven fabric
filter composed of layers of randomly oriented fibers (LROF). The
porous structure is defined by gaps between the fibers, and thus
pores become smaller in proportion to an increase in the thickness
of the filter layer. When having a proper thickness, the filter can
retain fine particles by size exclusion. This filter is capable of
filtering 85.about.95% by weight of fine particles but cannot
filter ultrafine particles such as viruses.
[0004] Melt-blown non-woven fabrics usually have a fiber diameter
of 1 .mu.m or more and thus a filter made thereof cannot filter
nanoparticles such as viruses. Even when ultrafine fibers having a
diameter distribution of 5.about.500 nm are used, fibers having a
larger diameter are present so that large pores are formed,
undesirably decreasing the level of filtering precision and making
it difficult to remove water-borne viruses having a size of
10.about.100 nm.
[0005] On the other hand, ultrafine fibers having a diameter
corresponding to 1/10.about. 1/1000 of the diameter of melt-blown
fibers may be manufactured using electrospinning. Non-woven fabric
filters manufactured using this type of fiber have an operating
pressure much lower compared to an MF filter using a porous
membrane. However, it is very difficult to increase the level of
filtering precision enough to remove nanoparticles such as viruses,
while maintaining low operating pressures and high flow rates. The
reason for this is that there is a limit in decreasing the pore
size sufficiently to filter ultrafine particles such as viruses, by
minimizing the fiber fineness, and also that a small pore size
drastically increases the operating pressure while undesirably
sharply decreasing the flow rate.
[0006] International Publication No. WO 07/054,040 discloses
various polymeric nanofiber filters. However, these polymeric
nanofiber filters suffer from a short lifespan, low thermal
stability, swelling properties in various solvents, and
difficulties in surface modification.
[0007] In contrast, a ceramic nanofilter mainly used to purify
wastewater has higher corrosion resistance and mechanical strength,
and a long lifespan. Specifically, whereas the polymer filter is
easily damaged during steam cleaning or chemical processes
periodically conducted to remove contaminants, the ceramic filter
is stable even at a high temperature of 500.degree. C. and is
chemically inactive, thereby enabling easier maintenance in terms
of washing and regeneration.
[0008] The ceramic filter is typically manufactured from a sol-gel
solution of a metal oxide precursor, and comprises a support layer
provided in the form of a thin film having pores with a size of 1
.mu.m and an uppermost layer having nano-sized pores. The pores of
the ceramic filter are formed by voids between ceramic particles,
in which the ceramic particles having different sizes are arranged
in a layer-by-layer deposition form, thus forming a ceramic
membrane having a gradation structure. However, in the sol-gel
process, it is often difficult to control the pore size because of
the particles having an irregular shape, and undesirable cracks or
pinholes may be formed in the uppermost layer during drying and
sintering processes. Also, when pore size is decreased to increase
selectivity, a serious loss in the permeation flow rate and
agglomeration of fine particles in the uppermost layer may occur,
and thus it is difficult to maintain high selectivity and high
permeation flow rate. Furthermore, a dead end pore structure which
does not contribute to filtration is formed, and thus the porosity
of the separation layer is very low to the extent of 36% or less.
Hence, it is very difficult to actually obtain a porous ceramic
filter having both superior selectivity and a sufficiently high
permeation flow rate.
[0009] U.S. Pat. No. 7,601,262 discloses a water treatment
composite filter that uses powdery aluminum hydroxide nanofibers in
order to remove nano-sized viruses or particles. This filter is
manufactured from an alumina sol bound to glass microfibers having
a length of 2.about.3 mm. Because the aluminum hydroxide nanofibers
are powdery and thus cannot form a filter, glass fibers are used to
increase mechanical strength and formability of the filter. In
order to increase the precision of filtering, the thickness of the
alumina filter is doubled but the permeation flow rate is thereby
cut by half. Briefly, increase in the mechanical strength of the
filter results in a loss in the permeation flow rate.
[0010] International Publication No. WO 08/034,190 discloses a
filter capable of removing ultrafine particles such as viruses
which is composed exclusively of powdery metal oxide nanofibers,
without a glass fiber support, manufactured by using a suspension
of metal oxide nanofibers having a length larger than a diameter
and has a pore size of 1.about.100 nm. In this case, however, there
is a limit to the length of the metal oxide nanofibers which can
form a homogeneous suspension, and a non-uniform suspension makes
it difficult to manufacture a homogeneous filter. Furthermore,
although the filtration efficiency of ultrafine particles such as
viruses is very high because of a pore size of 1.about.100 nm, the
permeation flow rate undesirably decreases.
[0011] As described above, conventional filters known to date are
still unsatisfactory in terms of filtration efficiency, permeation
flow rate, heat resistance, preparation and so on, the properties
being required of an excellent water treatment filter material.
DISCLOSURE OF INVENTION
[0012] It is, therefore, an object of the present invention to
provide a circulatory ceramic filter which exhibits a high
permeation flow rate because of low pressure drop upon filtration
while having high filtration efficiency enough to remove ultrafine
particles such as viruses, and also which is able to be
regenerated, thus having a long lifespan, and a method of
manufacturing same.
[0013] In accordance with one aspect of the present invention,
there is provided a ceramic filter comprising a filtering layer of
a fibrous porous body,
[0014] wherein the fibrous porous body comprises continuous
ultrafine fibers of metal oxide which are randomly arranged and
layered, and powdery nano-alumina incorporated into the ultrafine
fibers or coated thereon, the ultrafine fibers being obtained by
electrospinning a spinning solution comprising a metal oxide
precursor sol-gel solution, and optionally, a polymer resin, and
sintering the electrospun fibers,
[0015] in which the ultrafine fibers have an average diameter of
10.about.500 nm, and the fibrous porous body has a pore size of
maximum frequency ranging from 0.05 to 2 .mu.m.
[0016] In accordance with another aspect of the present invention,
there is provided a method for preparing the ceramic filter, which
comprises the steps of:
[0017] (1) electrospinning a metal oxide precursor sol-gel solution
or a mixture of a metal oxide precursor sol-gel solution and a
polymer resin to make a layer of continuous ultrafine fibers
randomly arranged; and
[0018] (2) sintering the electrospun ultrafine fibers at a
temperature ranging from 250 to 1000.degree. C.,
wherein (A) in step (1), before the electrospinning, the metal
oxide precursor sol-gel solution or the mixture of the metal oxide
precursor sol-gel solution and the polymer resin is additionally
mixed with one-dimensional powdery nano-alumina; (B) the sintered
ultrafine fibers from step (2) are impregnated or coated with a
suspension of one-dimensional powdery nano-alumina; or (A) and (B)
both are performed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The above and other objects and features of the present
invention will become apparent from the following description of
preferred embodiments given in conjunction with the accompanying
drawings, in which:
[0020] FIG. 1 illustrates a schematic view of an electrospinning
device used in the present invention;
[0021] FIGS. 2A and 2B depict a transmission electron microscope
(TEM) image and an X-ray diffraction (XRD) pattern of boehmite
nanofibers manufactured in Example 1, respectively;
[0022] FIG. 3 shows a scanning electron microscope (SEM) image of a
porous body comprising SiO.sub.2 nanofibers manufactured in
Comparative Example 1;
[0023] FIG. 4 is an SEM image of a porous body comprising
.gamma.--Al.sub.2O.sub.3 ultrafine fibers manufactured in
Comparative Example 3;
[0024] FIGS. 5A and 5B describe SEM images of a porous body
comprising alumina/silica ultrafine fibers manufactured in Example
2 before and after adsorption of boehmite nanoparticles,
respectively;
[0025] FIGS. 6A and 6B illustrate SEM images of a porous body
comprising alumina/silica ultrafine fibers manufactured in
Comparative Example 2 before and after sintering, respectively;
and
[0026] FIGS. 7A and 7B show SEM images of a porous body comprising
alumina/silica/boehmite ultrafine fibers manufactured in Example 4
before and after compression and sintering, respectively.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] The ceramic filter according to the present invention is
characterized by comprising a filtering layer of a fibrous porous
body, which comprises continuous ultrafine fibers of metal oxide
which are randomly arranged and layered, and powdery nano-alumina
incorporated into the ultrafine fibers or coated thereon, the
ultrafine fibers being obtained by electrospinning a spinning
solution comprising a metal oxide precursor sol-gel solution, and
optionally, a polymer resin, and sintering the electrospun fibers,
in which the ultrafine fibers have an average diameter of
10.about.500 nm, and the fibrous porous body has a pore size of
maximum frequency ranging from 0.05 to 2 .mu.m.
[0028] In the ceramic filter according to the present invention,
the porous body comprising ultrafine fibers is manufactured by
electrospinning the metal oxide precursor sol-gel solution or the
mixture of the metal oxide precursor sol-gel solution and the
polymer resin, thereby randomly arranging continuous ultrafine
fibers to form a layer, and sintering the electrospun fibers. Such
ultrafine continuous fibers of metal oxide are formed by
electrospinning the afore-mentioned solution through a nozzle under
a high-voltage electric field, into continuous ultrafine fibers
having a diameter ranging from several nm to several .mu.m and a
length ranging from several ten cm to several hundred m, unlike
powdery metal oxide-based nanofibers, nanorods, nanotubes,
nanoparticles and so on which are typically prepared using a
chemical synthesis process. In the present invention, the porous
body is formed by electrospun continuous fibers of metal oxide
randomly arranged and layered.
[0029] The principle of electrospinning to form ultrafine
continuous fibers of metal oxide according to the present invention
is well represented in various pieces of literature [G. Taylor.
Proc. Roy. Soc. London A, 313, 453 (1969); J. Doshi and D. H.
Reneker, J. Electrostatics, 35 151 (1995)]. As shown in FIG. 1,
unlike electrostatic spray in which a low-viscosity liquid is
sprayed in the form of ultrafine drops under a high-voltage
electric field not lower than a critical voltage, the metal oxide
precursor solution having sufficient viscosity is formed into
ultrafine fibers under a high-voltage electrostatic force, which is
called electrospinning. The electrospinning device includes a
barrel (10) for storing a metal oxide precursor solution, a
quantifying pump (20) for discharging the metal oxide precursor
solution at a predetermined rate, and a spinning nozzle (30)
connected to a high-voltage generator (40). The metal oxide
precursor solution is discharged via the quantifying pump (20) in
the form of ultrafine fibers while passing through the spinning
nozzle (30) electrically charged by the high-voltage generator
(40), and accumulates as porous ultrafine fibers on a grounded
metal collector plate (50) in the form of a conveyor that moves at
a predetermined rate (FIG. 1). When the metal oxide precursor
solution is electrospun in this way, ultrafine fibers having a size
ranging from several to several thousand nm may be produced and
simultaneously may be fused and layered in a three-dimensional
network structure, resulting in a desired porous web comprising
metal oxide ultrafine fibers. This porous body comprising ultrafine
fibers has a volume to surface area ratio much higher than that of
conventional fibrous filters, and higher porosity.
[0030] In the present invention, the term "electrospinning" is
understood as the broadened concept of electrospinning, since the
metal oxide ultrafine fibers may also be formed by using
melt-blowing, flash spinning, or electro-blowing which is a
modification of these processes that uses a high-voltage electrical
field and air spraying to manufacture ultrafine fibers. All of
these methods have in common the concept of electrospinning
including extrusion using a nozzle under an electric field, and
thus electrospinning in the present invention includes all such
methods.
[0031] The filtering precision, namely, filtration efficiency, and
also the permeation flow rate are greatly affected by the porosity
and the pore size of a filtering layer. According to the present
invention, the porous body serves as the filtering layer, and the
pore size and distribution, and porosity of the porous body
comprising metal oxide ultrafine fibers are mainly affected by the
average diameter and diameter distribution of the component fibers.
As the fiber diameter decreases, the pore size becomes smaller, and
the pore size distribution becomes narrower. Furthermore, the
specific surface area of the fibers is increased in proportion to
the decrease in the diameter of the fibers, and thus the ability to
collect fine particles contained in the filtering solution also
increases. Thus, the average fiber diameter of metal oxide
ultrafine fibers comprised in the porous body serving as the
filtering layer falls in the range of 10.about.500 nm, and
preferably 10.about.300 nm.
[0032] In the case of a membrane filter, the surface layer of the
membrane filter has a pore size and porosity different from those
of the structure below the surface layer because the evaporation or
dissolution-out rate of the solvent in the membrane preparation
process varies depending on the depth in the membrane, and also
because dead end pores which do not contribute to filtration are
present. However, fibrous fibers are entirely uniform in terms of
the pore size and porosity and do not have dead end pores. Although
the porosity is not a factor used to evaluate the filter
performance, high porosity results in a high permeation flow rate.
Therefore, in order to ensure high filtration efficiency and high
permeation flow rate of the filtering layer, the diameter of
component fibers is adjusted according to the present
invention.
[0033] In the present invention, the pore size of maximum frequency
in the pore size distribution of the fibrous porous body is
0.05.about.2 .mu.m as measured using a capillary flow porometer.
However, this does not mean that each and every pore has a single
size in the entire pore size distribution, and a filtering layer
having both small pores and large pores may be formed, as needed.
Specifically, this filtering layer is configured such that a lower
layer is composed of fibers having a larger diameter and thus
provides a porous layer having a large pore size, and an upper
layer is composed of thinner fibers and thus provides a porous
layer comprising pores having a smaller pore size, thereby forming
a multilayered structure or the gradation in structure.
[0034] Such a filtering layer having the multilayered or gradation
structure may be easily formed by layering fibers having a large
diameter and then layering thinner fibers during the
electrospinning process.
[0035] In the fibrous porous body manufactured using
electrospinning, the porosity and the pore size do not decrease in
proportion to the decrease in the diameter of the fibers. For
example, in case that the average fiber diameter is 2.3 .mu.m, 1.3
.mu.m, 0.7 .mu.m and 0.5 .mu.m, the size of pores which are the
major component (i.e., the pore size of maximum frequency) in the
pore size distribution is 6.7 .mu.m, 4.5 .mu.m, 2.2 .mu.m and 1.7
.mu.m, respectively, and the porosity is reduced from 90% to 80%.
Specifically, the porosity and the pore size are not greatly
reduced relative to the degree of reduction of the diameter of the
fibers. Although the pore size for filtering ultrafine particles
such as viruses is required to be 1.about.100 nm, it is very
difficult to reduce the pore size of the fibrous porous body using
electrospinning to this level. When a porous body having such a
small pore size is manufactured, high filtration efficiency may be
obtained but a permeation flow rate remarkably decreases due to a
high pressure drop.
[0036] Thus, in order to filter ultrafine particles such as
viruses, a porous body as the filtering layer is formed using
ultrafine fibers having an average diameter of 10.about.500 nm,
preferably 10.about.300 nm, and more preferably 10.about.100 nm,
and a porous layer comprising fibers having a larger diameter is
formed as the lower layer. The ultrafine continuous fibers of metal
oxide may be subjected to heat compression at a temperature ranging
from room temperature to 250.degree. C. so that the permeation flow
rate is not greatly lost, thus reducing the porosity and pore size
of the porous body. When a polymer resin is included, heat
compression may be performed at a temperature ranging from a glass
transition temperature of the polymer to a melting point
thereof.
[0037] Typically, when ultrafine fibers composed exclusively of a
polymer, which are manufactured using electrospinning, are heat
compressed as above, the porosity of a porous body comprising the
fibers may decrease to 20% or less. When the compression level
becomes higher, the porous structure itself may almost be broken
due to melting of the polymer resin component.
[0038] In the present invention, because the porous body comprising
ultrafine continuous fibers of metal oxide obtained by
electrospinning the metal oxide precursor sol-gel solution or the
mixture of the metal oxide precursor sol-gel solution and the
polymer resin has a relatively high porosity of about 70.about.95%,
the ultrafine fibers are preferably subjected to heat compression
so as to achieve an appropriate porosity of 10.about.80%. The metal
oxide sol-gel phase is provided after heat compression, thus
increasing the heat resistance of the polymer resin component,
thereby maintaining the porous structure.
[0039] Subsequently, the ultrafine continuous fibers of metal
oxide, whether heat compressed or not, are sintered at
250.about.1000.degree. C., thereby obtaining a desired ceramic
filter comprising a filtering layer of a fibrous porous body.
[0040] Conversion into the metal oxide ultrafine fibers is
completed by means of the above sintering treatment. In this
procedure, the organic product of the sol-gel reaction and the
polymer resin components are pyrolyzed and removed. Hence, the
specific surface area of the metal oxide ultrafine continuous
fibers increases, the average diameter of the fibers greatly
decreases, and the porosity of the fibrous porous body which was
greatly reduced after heat compression increases again.
[0041] Consequently, it is very difficult for the porous body
comprising ultrafine fibers composed exclusively of a polymer which
is manufactured using electrospinning to have a fiber diameter, a
pore size and porosity necessary for providing high filtration
efficiency and high permeation flow rate. However, the porous body
according to the present invention is composed of ceramic fibers
having a much narrower diameter and larger specific surface area,
and the pore size of the porous body is much smaller but the
porosity thereof is higher, thus achieving high filtration
efficiency and high permeation flow rate required to filter fine
particles.
[0042] The metal oxide precursor according to the present invention
is M(OR).sub.x, MR.sub.x(OR).sub.y, MX.sub.y, M(NO.sub.3).sub.y
(M=metal including Si, Al, etc.; R=alkyl group; X=F, Cl, Br, I; x
and y=each independently an integer of 1.about.4), or a mixture
thereof. The ultrafine continuous fibers prepared therefrom are
sintered at 250.about.1000.degree. C., yielding a porous body
comprising ultrafine ceramic fibers of metal oxide selected from
the group consisting of silica (SiO.sub.2), gamma-alumina
(.gamma.-Al.sub.2O.sub.3), and a mixture thereof.
[0043] In the present invention, the polymer resin used in
combination with the metal oxide precursor sol-gel solution
includes a polymer resin in which carbon components do not remain
after sintering at 250.about.1000.degree. C. Specific examples of
the polymer resin include polyvinylpyrrolidone (PVP),
polyvinylalcohol (PVA), polyvinylacetate (PVAc), polyethylene oxide
(PEO), and a mixture thereof.
[0044] However, the polymer resin is not necessarily limited to a
polymer resin in which carbon components do not remain after
sintering, and a polymer resin which is carbonized during sintering
to thus form carbon fibers may be used in the present invention.
For example, polyacrylonitrile or its copolymer is prepared into
fibers, which are then carbonized to yield carbon fibers which are
stable even at 1000.degree. C. or higher and have superior
mechanical properties. Thus, in case that a mixture of a metal
oxide sol-gel precursor and polyacrylonitride or its copolymer is
used, a filter formed of ceramic/carbon composite fibers is
obtained.
[0045] In order to filter ultrafine particles such as viruses at
high efficiency, the pore size of the filtering layer should be
about 1.about.100 nm, preferably about 1.about.60 nm. Although the
filtering layer having such ultrafine pores has very high
filtration efficiency, it is problematic because the pressure drop
is too large and the permeation flow rate is too low. In the
inventive porous body, the size of pores which are the major
component (i.e., the pore size of maximum frequency) in the pore
size distribution is in the range of 0.05.about.2 .mu.m.
[0046] In order for the fibrous porous body having such a pore size
structure to efficiently filter ultrafine particles (viruses, metal
ions, organic materials and inorganic particles), before the
electrospinning, the metal oxide precursor sol-gel solution or the
mixture solution of the metal oxide precursor sol-gel solution and
the polymer resin is additionally mixed with one-dimensional
powdery nano-alumina, so that nano-alumina is incorporated into the
ultrafine fibers; or the sintered ultrafine fibers are impregnated
or coated with a suspension of one-dimensional powdery nano-alumina
to adsorb the nano-alumina onto the surface of the fibers; or both
these processes are performed. The resulting porous body includes
1.about.90 wt % of nano-alumina based on the total weight of the
porous body.
[0047] The one-dimensional powdery nano-alumina may include
nanoparticles of boehmite (AlOOH), aluminum hydroxide
(Al(OH).sub.3), gamma-alumina (.gamma.-Al.sub.2O.sub.3) and a
mixture thereof, which are provided in the form of nanorods,
nanotubes or nanofibers, having a diameter of 1 nm or more and a
diameter to length ratio (an aspect ratio) of 5 or more.
[0048] The ceramic filter according to the present invention which
includes the filtering layer of the porous body comprising
ultrafine continuous fibers of metal oxide may be provided in
various forms, such as layered flat panels, pleats, spirals,
etc.
[0049] As described above, the ceramic filter according to the
present invention has high filtration efficiency enough to remove
ultrafine particles such as viruses in water and air, and has low
pressure drop upon filtration to show a high flow rate, and can be
regenerated and thus has a long lifespan. Therefore, it is very
useful as an environmentally friendly and excellent water treatment
filter.
[0050] The following Examples and Comparative Examples are given
for the purpose of illustration only, and are not intended to limit
the scope of the invention.
EXAMPLE
[0051] The properties of each of the fibers, porous bodies and
filters including same as a filtering layer, as manufactured in the
following examples and comparative examples, were measured by the
following methods.
[0052] Diameter of Metal Oxide Ultrafine Fibers in Porous Body
[0053] From scanning electron microscope (SEM) images of the
surface or the cross-section of the porous body comprising
ultrafine continuous fibers of metal oxide, the diameter of the
metal oxide ultrafine fibers was measured using Sigma Scan Pro 5.0
(SPSS), so that the average diameter and the diameter distribution
could be evaluated.
[0054] Pore Size of Porous Body comprising Metal Oxide Ultrafine
Continuous Fibers
[0055] The average pore size was measured in the pressure range of
0.about.30 psi using a capillary flow porometer available from PMI
(version 7.0), and the pore size was calculated from a wet flow
curve and a dry flow curve as measured. As such, perfluoropolyether
(oxidized and polymerized 1,1,2,3,3,3-hexafluoropropene) was used
as a wetting agent.
[0056] Porosity
[0057] The porosity of the porous body comprising ultrafine
continuous fibers of metal oxide was evaluated by butanol
impregnation as represented by Equation 1 below.
Porosity
(%)={(M.sub.BuOH.rho..sub.BuOH)/(M.sub.BuOH/.rho..sub.BuOH+M.su-
b.m/.rho..sub.p)}.times.100 Equation 1
[0058] wherein M.sub.BuOH is the weight of absorbed butanol,
M.sub.m is the weight of the porous body comprising metal oxide
fibers, .rho..sub.BuOH is the density of butanol, and .rho..sub.p
is the density of the metal oxide fibers.
[0059] Filtering Precision (Filtration Efficiency)
[0060] 30 ml of a 0.1 wt % suspension, prepared by diluting an
aqueous suspension of 10 wt % polystyrene latex particles (Magshere
Inc.) having a diameter of 90 nm with deionized water, was supplied
and passed through a porous body comprising ultrafine continuous
fibers of metal oxide using a vacuum system so that a difference in
pressure between the supplying solution and the permeated solution
was 35 kPa, and the concentration of latex nanoparticles contained
in the initial suspension and the permeated solution that passed
through the porous body comprising ultrafine continuous fibers of
metal oxide was determined by quantitatively evaluating the
intensity of absorbance at 200.about.205 nm using a UV-visible
spectrometer, and the filtration efficiency of the filter was
evaluated by Equation 2 below. Also, 5 .mu.l of the permeated
solution was placed on a slide glass and then dried in a vacuum,
after which the number of latex particles was counted to evaluate
the filtration efficiency of the filter.
Filtration Efficiency (%)=[1-(C.sub.t/C.sub.0)].times.100 Equation
2
wherein C.sub.t is the concentration of latex particles of the
permeated solution, and C.sub.0 is the concentration of latex
particles of the initial suspension.
[0061] Permeation Flow Rate
[0062] As in the measurement of the filtering precision, the filter
was mounted to a filter holder, and while deionized water at
25.degree. C. was supplied so as to achieve a pressure difference
of 35 kPa, the permeation time was measured for every 5 ml of the
permeated solution that passed through the filter, thus determining
the permeation flow rate.
Example 1
[0063] A mixture solution comprising 7 g of aluminum isopropoxide
(AIP), 40 ml of ethylalcohol, 10 ml of water, and 25 .mu.l of HCl
was sonicated for 1 hour and stirred at about 90.degree. C. for 3
hours, after which the reaction product was diluted with ethanol
and filtered to prepare boehmite nanofibers as powdery
nano-alumina. The TEM image and the XRD pattern of the boehmite
nanofibers are shown in FIGS. 2A and 2B, respectively,
Comparative Example 1
[0064] A mixture solution comprising 20.8 g of
tetraethoxyorthosilicate (TEOS), 9.2 g of ethylalcohol, 3.5 g of
water, and 0.1 g of aqueous hydrochloric acid was stirred at about
70.degree. C. for about 3 hours to prepare a silica sol-gel
solution, which was then discharged at a rate of 20 .mu.l/min under
a high-voltage electric field of 20 kV using the 30 G spinning
nozzle of the electrospinning device of FIG. 1, to manufacture a
layer of continuous ultrafine fibers randomly arranged having an
average diameter of 230 nm. The ultrafine fibers were heat
compressed at 150.degree. C. and then sintered at about 350.degree.
C., thereby manufacturing a porous body comprising silica ultrafine
fibers having an average fiber diameter of 170 nm (minimum 130
nm.about.maximum 270 nm) and a specific surface area of 187
m.sup.2/g, with a porosity of 86% and a pore size of 1.2 .mu.m. The
SEM image of the porous body comprising silica ultrafine fibers is
shown in FIG. 3.
[0065] Using the porous body as the filtering layer of the filter,
the filtering precision and the permeation flow rate were measured.
The results are shown in Table 1 below.
Example 2
[0066] 6 g of the powdery boehmite nanofibers of Example 1 was
mixed with the TEOS solution of Comparative Example 1, and 0.12 g
of polyvinylpyrrolidone (PVP, mw 1,300,000) was added thereto to
prepare a homogeneous mixture solution, which was then discharged
at a rate of 20 .mu.l/min under a high-voltage electric field of 20
kV using the 27 G spinning nozzle of the electrospinning device of
FIG. 1, to obtain a layer of continuous ultrafine fibers randomly
arranged having an average diameter of 230 nm. The ultrafine fibers
were heat compressed at 100.degree. C., and sintered at about
300.degree. C., thereby manufacturing a porous body comprising
silica/boehmite ultrafine fibers having an average fiber diameter
of 100 nm (minimum 85 nm.about.maximum 250 nm) with a porosity of
76% and a pore size of 0.8 .mu.m. This fibrous porous body had 53.5
wt % boehmite based on the total weight of the porous body.
[0067] Using the porous body as the filtering layer of the filter,
the filtering precision and the permeation flow rate were measured.
The results are shown in Table 1 below.
Example 3
[0068] A mixture (molar ratio of aluminum nitrate:aluminum
isopropoxide:TEOS=3:9:4) comprising 15 g of aluminum isopropoxide,
9.4 g of aluminum nitrate, 7 g of TEOS, 40 ml of ethylalcohol, 10
ml of water, and 50 ml of aqueous hydrochloric acid was mixed with
3 g of PVP and stirred at about 70.degree. C. for 2 hours to
prepare a mixture solution. This solution was discharged at a rate
of 20 .mu.l/min under a high-voltage electric field of 26.5 kV
using the 30 G spinning nozzle of the electrospinning device of
FIG. 1, to obtain a layer of continuous ultrafine fibers randomly
arranged having an average diameter of 151 nm (minimum 100
nm.about.maximum 205 nm). The ultrafine fibers were sintered at
about 500.degree. C., from which PVP was then removed, thus
manufacturing a porous body comprising alumina/silica ultrafine
fibers having an average fiber diameter of 85 nm (minimum 55
nm.about.maximum 125 nm) with a porosity of 89% and a pore size of
0.4 .mu.m.
[0069] The resulting fibers were impregnated with a solution
obtained by dispersing the boehmite nanoparticles of Example 1 in
an amount of 2 wt % in a mixture solution of water and ethanol, so
that boehmite nanoparticles were adsorbed on the porous body,
followed by drying. The adsorbed boehmite amount was 1.3 wt % based
on the total weight of the fibrous porous body on which boehmite
was adsorbed. The SEM images of the porous body comprising
alumina/silica ultrafine fibers before and after adsorption of
boehmite nanoparticles are respectively shown in FIGS. 5A and
5B.
[0070] Using the porous body having the adsorbed boehmite
nanoparticles as the filtering layer of the filter, the filtering
precision and the permeation flow rate were measured. The results
are shown in Table 1 below.
Comparative Example 2
[0071] The porous body comprising alumina/silica ultrafine fibers
of Example 3 was compressed to 1/2 of the original thickness at
30.degree. C. before being sintered at 500.degree. C., to
manufacture a porous body comprising ultrafine fibers having an
average fiber diameter of 88 nm (minimum 40 nm.about.maximum 130
nm) with a porosity of 70% and a pore size of 0.12 .mu.m. The SEM
images of the porous body comprising alumina/silica ultrafine
fibers before and after sintering are respectively shown in FIGS.
6A and 6B.
[0072] Using the porous body as the filtering layer of the filter,
the filtering precision and the permeation flow rate were measured.
The results are shown in Table 1 below.
Example 4
[0073] A mixture (molar ratio of aluminum nitrate:aluminum
isopropoxide:TEOS=3:9:4) comprising 6 g of the powdery boehmite
nanofibers of Example 1, 15 g of aluminum isopropoxide, 9.4 g of
aluminum nitrate, 7 g of TEOS, 40 ml of ethylalcohol, 10 ml of
water, and 50 ml of aqueous hydrochloric acid was mixed with 3 g of
PVP and stirred at about 70.degree. C. for 2 hours to prepare a
mixture solution. This solution was discharged at a rate of 20
.mu.l/min under a high-voltage electric field of 28 kV using the 27
G spinning nozzle of the electrospinning device of FIG. 1, to
obtain a layer of continuous ultrafine fibers randomly arranged.
The ultrafine fibers were compressed to 1/5 of the original
thickness at 40.degree. C. and then sintered at 350.degree. C.,
thus manufacturing a porous body comprising ultrafine fibers having
an average diameter of 89 nm (minimum 40 nm.about.maximum 130 nm)
with a porosity of 65% and a pore size of 0.05 .mu.m. This fibrous
porous body had 49.5 wt % boehmite based on the total weight of the
porous body. The SEM images of the porous body comprising
alumina/silica/boehmite ultrafine fibers before and after
compression/sintering are respectively shown in FIGS. 7A and 7B. It
can be seen from the image of FIG. 7A that boehmite with a very
rough shape is exposed on the fiber surface.
[0074] Using the porous body having the adsorbed boehmite
nanoparticles as the filtering layer of the filter, the filtering
precision and the permeation flow rate were measured. The results
are shown in Table 1 below.
Comparative Example 3
[0075] A mixture solution comprising 7 g of aluminum isopropoxide,
40 ml of ethylalcohol, 10 ml of water, and 25 ml of aqueous
hydrochloric acid was stirred to prepare an aluminum isopropoxide
sol-gel solution. A solution of 1.5 g of PVP dissolved in 5 ml of
ethylalcohol was added to the sol-gel solution and stirred at about
70.degree. C. for 2 hours to prepare a mixture solution. This
solution was discharged at a rate of 40 .mu.l/min under a
high-voltage electric field of 15.5 kV using the 24 G spinning
nozzle of the electrospinning device of FIG. 1, to obtain a layer
of continuous ultrafine fibers randomly arranged. The ultrafine
fibers were sintered at about 500.degree. C., from which PVP was
then removed, thus manufacturing a porous body comprising alumina
ultrafine fibers having an average fiber diameter of 600 nm with a
porosity of 85% and a pore size of 1.9 .mu.m. The SEM image of the
porous body comprising alumina ultrafine fibers is shown in FIG.
4.
[0076] Using the porous body having the adsorbed boehmite
nanoparticles as the filtering layer of the filter, the filtering
precision and the permeation flow rate were measured. The results
are shown in Table 1 below.
Comparative Example 4
[0077] A mixture (molar ratio of aluminum nitrate:aluminum
isopropoxide:TEOS=3:9:4) comprising 15 g of aluminum isopropoxide,
9.4 g of aluminum nitrate, 7 g of TEOS, 40 ml of ethylalcohol, 10
ml of water, and 50 ml of aqueous hydrochloric acid was mixed with
0.5 g of PVP and stirred at about 70.degree. C. for 2 hours to
prepare a mixture solution. This solution was discharged at a rate
of 30 .mu.l/min under a high-voltage electric field of 17 kV using
the 24 G spinning nozzle of the electrospinning device of FIG. 1,
to obtain a layer of continuous ultrafine fibers randomly arranged.
The ultrafine fibers were sintered at about 500.degree. C., from
which PVP was then removed, thus manufacturing a porous body
comprising alumina ultrafine fibers having an average fiber
diameter of 1.3 .mu.m (minimum 0.7 .mu.m.about.maximum 2.5 .mu.m)
with a porosity of 91% and a pore size of 4.5 .mu.m.
[0078] Using the porous body having the adsorbed boehmite
nanoparticles as the filtering layer of the filter, the filtering
precision and the permeation flow rate were measured. The results
are shown in Table 1 below.
Comparative Example 5
[0079] The procedure of Comparative Example 1 was repeated except
for using 0.3 g of PVP instead of 1.5 g of PVP, to obtain a layer
of continuous ultrafine fibers randomly arranged. The ultrafine
fibers were sintered at about 500.degree. C., from which PVP was
then removed, thus manufacturing a porous body comprising alumina
ultrafine fibers having an average fiber diameter of 2.0 .mu.m
(minimum 1.6 .mu.m.about.maximum 2.6 .mu.m) with a porosity of 90%
and a pore size of 6.0 .mu.m.
[0080] Using the porous body having the adsorbed boehmite
nanoparticles as the filtering layer of the filter, the filtering
precision and the permeation flow rate were measured. The results
are shown in Table 1 below.
TABLE-US-00001 TABLE 1 Pore Size (.mu.m) Average of Porous Body
Permeation Diameter (nm) comprising Metal Filtration Flow Rate of
Metal Oxide Oxide Ultrafine Efficiency (l/m.sup.2 Ultrafine Fibers
Fibers (%) hr pa) C. Ex. 1 170 1.2 94 0.06 Ex. 2 100 0.8 98 0.041
Ex. 3 85 0.4 100 0.040 C. Ex. 2 88 0.12 100 0.025 Ex. 4 89 0.05 100
0.011 C. Ex. 3 600 1.9 65 0.251 C. Ex. 4 1300 4.7 1.3 10.15 C. Ex.
5 2000 6.0 0.9 16.20
[0081] As is apparent from Table 1, the filters obtained in
Comparative Examples 1 and 2 and Examples 2 to 4 are able to filter
almost all of the particles having a diameter of 90 nm and can
exhibit a high permeation flow rate. However, with regard to the
ultrafine particles such as viruses smaller than 90 nm, the filters
of Comparative Examples 1 and 2 are expected to have much lower
filtration efficiency because nano-alumina is neither incorporated
nor adsorbed. Also, the filters of Comparative Examples 3 to 5 have
a high permeation flow rate, but the filtration efficiency of 90 nm
particles is low to the extent of 65%, or is very low to the extent
that almost all of the 90 nm particles pass therethrough.
* * * * *