U.S. patent application number 09/759920 was filed with the patent office on 2002-01-31 for system and method for liquid filtration based on a neutral filter material.
Invention is credited to Goddard, Philip, Grant, Donald, Kelly, Wayne, Thom, Volkmar.
Application Number | 20020011450 09/759920 |
Document ID | / |
Family ID | 22644050 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011450 |
Kind Code |
A1 |
Kelly, Wayne ; et
al. |
January 31, 2002 |
System and method for liquid filtration based on a neutral filter
material
Abstract
The present invention teaches, that the removal of particles of
any charge from liquid stream is possible by mechanisms other than
sieving with good flow and low pressure drop. The use of a weakly
charged or neutral surface on the one or more filters within a
pre-selected pH range is used. Consequently, particles that are
much smaller than the filter pore size are captured with high
efficiency. The essentially neutral or weakly charged material
filter surface in the filtration liquid may be formed through the
proper selection of filter material or a surface modification of
the base filters(s) to create the essentially neutral or weakly
charged surface.
Inventors: |
Kelly, Wayne; (Prior Lake,
MN) ; Grant, Donald; (Excellsior, NM) ;
Goddard, Philip; (Nashua, NH) ; Thom, Volkmar;
(Chelmsford, MA) |
Correspondence
Address: |
MILLIPORE COPORATION
80 ASHBY RD
BEDFORD
MA
01730
US
|
Family ID: |
22644050 |
Appl. No.: |
09/759920 |
Filed: |
January 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60176363 |
Jan 14, 2000 |
|
|
|
Current U.S.
Class: |
210/767 ;
210/500.1 |
Current CPC
Class: |
B01D 61/14 20130101;
B01D 67/0093 20130101; B01D 61/147 20130101; B01D 69/02 20130101;
B01D 2323/30 20130101; B01D 61/145 20130101; B01D 2323/38 20130101;
B01D 67/009 20130101 |
Class at
Publication: |
210/767 ;
210/500.1 |
International
Class: |
B01D 039/00 |
Claims
What we claim:
1. A filter for liquids comprising one or more porous filters,
wherein the one or more filters each have surfaces that are
substantially neutral in a selected fluid.
2. The filter of claim 1 wherein the fluid is an aqueous fluid and
the surface is substantially neutral over a preselected pH
range.
3. The filter of claim 1 wherein the one or more filters have a
Zeta Potential of between about 10 and -10 millivolts in a selected
fluid.
4. The filter of claim 1 wherein the one or more filters have a
Zeta Potential of between about 5 and -5 millivolts in a selected
fluid.
5. A system for filtering fluids comprising a fluid and one or more
filters wherein the one or more filters each have surfaces that are
substantially neutral in a selected fluid.
6. The system of claim 5 wherein the fluid is selected from the
group consisting of aqueous fluids, non-aqueous fluids and blends
thereof and the one or more filters have a Zeta Potential of
between about 5 and -5 millivolts in the selected fluid.
7. The filter of claim 1 wherein the one or more filters have a log
reduction value (LRV) of at least 3 of particles in the selected
fluid.
8. The filter of claim 1 wherein the one or more filters have a log
reduction value (LRV) of from about 3 to about 20 of particles in
the selected fluid.
9. The filter of claim 1 wherein the one or more filters have a
Zeta Potential of between about 10 and -10 millivolts in the
selected fluid and a log reduction value (LRV) of at least 3 of
particles in the selected fluid.
10. The filter of claim 1 wherein the one or more filters have a
Zeta Potential of between about 5 and -5 millivolts in the selected
fluid and a log reduction value (LRV) of at least 3 of particles in
the selected fluid.
11. A method of filtration of liquids comprising selecting a fluid
containing one or more contaminants to be removed, forming one or
more porous filters, said one or more filters having surfaces which
are substantially neutral within the selected fluid and passing the
fluid through the one or more filters to remove the
contaminants.
12. The filter of claim 1 wherein the substantially neutral surface
is inherent in the nature of the one or more filters.
13. The filter of claim 1 wherein the substantially neutral surface
is formed by surface modification of the one or more filters.
14. The filter of claim 1 wherein the substantially neutral surface
is formed by surface modification of the filter and the surface
modification is selected from the group consisting of cross linking
and grafting of one or more monomers on surfaces of the one or more
filters.
15. The filter of claim 1 wherein the substantially neutral surface
is formed by surface modification of the filter and the surface
modification is by application of an energy source on surfaces of
the one or more filters.
16. The filter of claim 1 wherein the one or more filters are made
from a material selected from the group consisting of cellulose,
glass, ceramics and metals.
17. The filter of claim 1 wherein the one or more filters are made
from a material selected from the group consisting of cellulose,
regenerated cellulose and nitrocellulose.
18. The filter of claim 1 wherein the one or more filters are made
from a metal selected from the group consisting of stainless steel,
nickel, chromium and alloys and blends thereof.
19. The filter of claim 1 wherein the one or more filters are made
from a plastic selected from the group consisting of polyolefins;
copolymers or terpolymers of polyolefins; PVDF; PTFE resin; PFA;
perfluorinated thermoplastic resins; PVC; nylons; polyamides;
polysulphones; polyethersulphones; polyarylsulphones;
polyphenylsulphones; polyimides; polycarbonates; polyesters; and
blends thereof.
20. The filter of claim 1 wherein the one or more filters are made
from a polyolefin selected from the group consisting of
polyethylene, polypropylene and the like.
21. The filter of claim 1 wherein the one or more filters are made
from an ultrahigh molecular weight polyethylene.
22. A filter comprising one or more filters each having a surface
having an IEP within a selected operating range of pH such that the
filter surface either maintains a neutral or weak charge or does
not acquire a highly charged surface within the selected pH
range.
23. A filter comprising one or more filters each having a surface
having an IEP that matches the pH of the liquid in which it is
used.
24. A filter comprising two or more filters in a composite filter,
each with a different IEP.
25. The filter of claim 1 wherein the surface of the one or more
filters is treated with one or monomers selected from the group
consisting of acrylate or acrylamide monomers and methacrylate or
acrylamide monomers and blends thereof.
26. The filter of claim 1 wherein the surface of the one or more
filters is treated with acrylic acid.
27. The filter of claim 1 wherein the surface of the one or more
filters is treated with acrylic acid, a photoinitiator and a cross
linker.
28. The filter of claim 1 wherein the surface of the one or more
filters is treated with acrylic acid, a photoinitiator and a cross
linker, wherein the cross linker is
N,N'-methylenebisacrylamide.
29. The filter of claim 1 wherein the surface of the one or more
filters is treated with N,N'-methylenebisacrylamide.
30. The filter of claim 1 wherein the one or more filters have a
log reduction value (LRV) of at least 3 of particles of an average
diameter that is smaller than the nominal pore size of the filter
in the selected fluid.
Description
[0001] This invention relates to a system and method for the
filtration of particulates from fluids using porous media. More
specifically, it relates to a filter and method of filtration where
particles smaller than the pore size of the porous medium used as
the filter are effectively removed due to a non-sieving related
mechanism.
BACKGROUND OF THE INVENTION
[0002] The filtration of particulate materials from fluids has been
conducted for years relying mainly on sieving mechanisms, where
particles are predominantly removed based on size. Sieving by
itself in many cases has not been satisfactory for liquid
filtration as it provides poor or no retention for particles
smaller than the pore size of the filter, as well as low flow rates
requiring either large pressure gradients or large filter areas to
attain a reasonable flow. However, particles can be removed from
liquids relying on attractive particle-surface interactions, a
practice which has long been recognized in gas filtration, where
filtration mechanisms other than sieving dominate in most
applications and provide enhanced filter performance. See, Particle
Capture Mechanisms in Gases and Liquids: An Analysis of Operative
Mechanisms, by Grant, et. al., 1988 Proceedings of the Institute of
Environmental Sciences.
[0003] Particle-surface interactions in liquids are usually
governed by electrostatic interactions and Van der Waals Forces
assuming that no other highly specific interaction potentials are
present (as e.g. molecular recognition, chelating functionalities
etc.). Van der Waals forces are omnipresent attractive short ranged
forces acting between two materials of any kind and will thus
always represent attractive interactions. Electrostatic
interactions on the other hand, will be attractive, repulsive or
non-existent dependent on the sign of the electrostatic potential
of the two materials in question. It is common knowledge, that
like-charged materials will exhibit repulsive electrostatic
interactions between each other, whereas counter-charged material
will exhibit attractive electrostatic interactions between one
another. This knowledge has been exploited in the filtration of
liquids, where the filter material exhibiting electrostatic
potential is capable of attracting and successfully retaining
countercharged particles. The obvious limitation of these charged
filters are the repulsive electrostatic interactions of the filter
material with like-charged particles and the resulting poor
retention for the same.
[0004] The basis of this invention is the realization that when one
of the material surfaces in question is neutral in the liquid of
use (here the filter material), no adverse electrostatic
interactions will be present between the filter material and the
particle material and attractive Van der Waals forces will govern
the interaction between the filter material and particle material
leading to the retention of particles of any charge characteristic,
positive, negative, or neutral. Therefore, in providing a filter
material that exhibits no, or essentially no, electrostatic
potential in the liquid to be filtered (i.e. it is a neutral
surface), particles of any electrostatic character will be
attracted by the filter material leading to enhanced retention and
filtration beyond that currently obtained with conventional sieving
only filters.
[0005] The electrostatic potential of materials in liquids are
governed by two main phenomena, (i) the dissociation/association of
functional groups (acids or bases) leading to a charge, or (ii) the
adsorption of (charged) ionic species from the liquid. For example,
in aqueous media, materials without any functional groups at the
material surface usually exhibit a negative electrostatic potential
in basic or neutral solutions (pH>5-7) and positive
electrostatic potential in acidic solutions (pH<5-7) due to the
adsorption of hydroxyl (negatively charged: OH.sup.-) or hydronium
ions (positively charged: H.sup.+). Independent of the character of
the liquid (aqueous or non-aqueous) charged ionic species present
in the filtration liquid might also adsorb to the material surface
and therefore alter its electrostatic potential.
[0006] It is therefore important to realize, that a material
attains a different electrostatic potential in dependence on the
properties of the liquid it is immersed in. The electrostatic
potential of a material in a liquid will depend on properties of
the liquid such as (i) its proton donor/acceptor capability, (ii)
its dielectric constant, (iii) the concentration and kind of ionic
species present. Nevertheless, through the use of appropriate
materials or surface modifications of the same, one can adjust the
electrostatic potential of a material in a given liquid to be
essentially zero thereby creating an essentially neutral
surface.
[0007] Based on this finding, this invention teaches that a filter
material can be selectively adjusted for a given fluid in a given
pH range to attract particles of any electrostatic character and
can therefore effectively retain particles smaller than the pore
size of the filter based on attractive interactions between the
particle and the filter material in any given liquid.
SUMMARY OF THE INVENTION
[0008] The present invention provides a filter with high particle
capture (Log reduction value (LRV)>3, preferably >5, more
preferably >10) with low pressure drops and high flow. For
example, the present invention provides a filter with a traditional
pore size rating of 0.3 microns with an LRV of greater than 3 of
0.1 microns particles with a pressure drop which is less than
{fraction (1/10)} that of a 0.1 microns rated "sieving filter"
(i.e. a filter with a nominal or rated pore size of 0.1 microns).
At the same time, the 0.1 microns "sieving filter" has an LRV of
less than 3, generally about 2 of 0.1 microns particles.
[0009] The present invention uses a weakly charged or neutral
filter (surface). The present invention creates such a filter
surface by selecting a material (surface) with an isoelectric point
(IEP) or point of zero charge (PZC) in the liquid it filters, thus
maintaining a neutral or weakly charged surface which provides the
enhanced retention performance of the filter of the present
invention. Preferably, it can be used in aqueous and non-aqueous
fluids. In aqueous fluids, it can have a substantially neutral
surface over a broad pH range or if desired a selected narrow pH
range. In non-aqueous fluids, it is capable of maintaining a
substantially neutral surface over a wide range of ionic strength
fluids.
[0010] In one embodiment of the present invention, there is
provided a filter that has a surface that is substantially neutral
in charge in the fluid in which it is used.
[0011] In another embodiment of the present invention, there is
provided a filter that has a surface that is substantially neutral
in charge in an aqueous fluid over at least a range of pH and
preferably under all pH conditions.
[0012] A further embodiment of present invention is to utilize a
filter having an IEP within the selected or intended operating
range of pH or ionic strength such that the filter surface either
maintains a neutral or weak charge or does not acquire a highly
charged surface within the selected pH range or ionic strength.
[0013] Another embodiment of the present invention is to utilize a
filter having an IEP in the pH of the aqueous liquid in which it is
used. In this approach, the filter surface is matched to a specific
liquid in which the pH is fixed and the filter surface remains
neutral.
[0014] A further embodiment of the present invention is to utilize
a filter having an IEP at the ionic strength of the non-aqueous
liquid in which it is used. In this approach, the filter surface is
matched to a specific liquid in which the ionic strength is fixed
and the filter surface remains neutral.
[0015] An alternative embodiment of present invention is to utilize
two or more filters in a composite filter, each with a different
IEP. This allows for the use over a broad range of pH for aqueous
fluids and ionic strengths for non-aqueous fluids.
[0016] A further embodiment of present invention is to utilize a
filter having a Zeta Potential of between about 10 and -10
millivolts, preferably, between 5 and -5 millivolts within the
selected fluid such that the filter surface either maintains a
neutral or weak charge or does not acquire a highly charged surface
within the selected fluid.
[0017] In an additional embodiment of the present invention is a
method of filtration of liquids comprising selecting a fluid
containing one or more contaminants to be removed, forming one or
more porous filters, said one or more filters having surfaces which
are substantially neutral within the selected fluid and passing the
fluid through the one or more filters to remove the
contaminants.
IN THE DRAWINGS
[0018] FIG. 1 Test system for tests in Hydrofluoric (HF) Acid for
filters assembled in cartridges.
[0019] FIG. 2 The measured background particle concentrations in
the test system for cartridges.
[0020] FIG. 3 test system for tests in KCl for 47 mm filter
discs.
[0021] FIG. 4 shows the Zeta Potential of particles in KCl as
function of pH.
[0022] FIG. 5 shows the Zeta Potential of test filters in HF or KCl
as function of pH.
[0023] FIG. 6 shows test filter retentiveness in log reduction
values (LRV) for negatively charged PSL(-) particles in KCl or HF
of different pH versus the filter Zeta Potential in the respective
test liquid.
[0024] FIG. 7 shows test filter retentiveness in log reduction
values (LRV) for positively charged Si3N4(+) and PSL(+) particles
in KCl and HF of different pH versus their Zeta Potential in the
respective test liquid.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The present invention teaches, that the removal of particles
of any charge from liquid stream is possible by mechanisms other
than sieving when the electrostatic interaction between filter
material surface and particle surface are small enough so that
particles are captured by attractive Van Der Waals forces. The use
of a weakly charged or neutral surface stems from the concept that
the magnitude of the repulsive force between two surfaces is
related to the product of the charge densities on the two surfaces.
This means that if one surface is substantially neutral and
therefore has no charge, the repulsive force is eliminated,
regardless of the extent of charge on the particle surface.
Consequently, particles that are much smaller than the filter pore
size are captured with high efficiency by the omnipresent
attractive Van der Waals forces. This goal is achieved by providing
an essentially neutral or weakly charged material filter surface in
the filtration liquid.
[0026] Because such a "neutral" filter does not rely on sieving
alone, the pore size can be enlarged without loss of performance as
compared to a conventional, sieving only chemical filter, allowing
for high particle retention with high flow permeability.
[0027] Almost all materials in contact with a liquid acquire an
electrostatic potential, due to the presence of immobilized charged
groups or adsorbed charged ionic species at the surface of those
materials. The Zeta Potential (ZP) of a material surface is a
property used to characterize its electrostatic (surface) potential
or surface charge density. The ZP of a material (surface) has the
same sign as the overall surface charge of this material and is
generally proportional to the surface charge density. Consequently,
the presence or absence of charged groups on the surface of
materials such as revealed by their zeta potential will correlate
directly with their filter performance.
[0028] Different electrokinetic phenomena such as Electrophoresis
(the movement of charged colloidal particles in electric field) or
Streaming Potential (forcing a liquid through a capillary or porous
medium induces a difference of electric potentials) are commonly
used to determine the ZP of a material. The ZP of particles is e.g.
commonly measured by standard measurement systems (e.g. Zetasizer,
Malvern Instruments) as the stability of a particle suspension
(i.e. the ability to not agglomerate) depends directly on the ZP of
the particles in the liquid they are suspended in. When the
absolute value of the particle ZP is above a certain threshold
(e.g. 50 mV) a suspension is very stable due to electrostatic
repulsion between the particles; however, when the ZP is close to
zero particles will agglomerate (see. Robert Hunter, Zeta Potential
in Colloid Science, Academic Press, 1984).
[0029] The ZP of a non-conducting material is directly proportional
to the electrostatic potential of the material. Therefore, just as
the electrostatic potential of a material is negative, positive or
zero, the corresponding ZP of that material will be negative,
positive or zero. The condition under which a material exhibits no
electrostatic potential and therefore a ZP of zero is known as the
material's isoelectric point (IEP). Materials which are at their
IEP in a given liquid appear to have a neutral character, i.e. an
electrostatic potential of or near 0 mV.
[0030] In the present invention when a filter material exhibits its
IEP or a ZP close zero (i.e. ZP=-10 mV<IEP<+10 mV, preferably
ZP=-5 mV <IEP<+5 mV) in the liquid to be filtered, the filter
will retain particles of any electrostatic character which are
smaller than the filter pore size due to attractive Van der Waals
forces between the filter material surface and the particles. A
filter material which does not exhibit its IEP in the liquid to be
filtered, i.e. a filter which has a substantial electrostatic
potential in the liquid to be filtered (i.e. ZP=-10
mV<IEP<+10 mV, preferably ZP=<-5 mV or ZP>+5 mV) will
not retain particles smaller than its pore size which exhibit a
electrostatic potential (or ZP) of the same sign (like-charged
particles).
[0031] The electrostatic potential of a (filter) material in a
given liquid can be adjusted through known surface modification
techniques to become essentially neutral, i.e. to exhibit an IEP.
However, some filter materials may exhibit an IEP in a given liquid
without further adjustment through surface modification.
[0032] It is important to note, that when the liquid to be filtered
contains high amounts of dissolved ions (i.e. any liquid exhibiting
a high ionic strength, e.g. greater than 0.1-1 mol/l) almost any
material surface in contact with this liquid will be substantially
neutral (i.e. its ZP will be in the range of -10 mV<ZP>+10
mV, preferably -5 mV<ZP<+5 mV). Consequently, in such a
liquid any material surface will attract particles of any
electrostatic character. This phenomenon is often described as an
ion screening effect, i.e, dissolved ions can "screen" the
electrostatic potential of a material surface to the effect that
the potential decays more or less rapidly with distance from the
material surface depending on the amount of dissolves ions. So in
liquids with low concentrations of ions (weak electrolytes), the
electrostatic potential decays very slowly with distance and
reaches a long way into the surrounding liquid; in liquids with
high concentrations of dissolved ions (strong electrolyte), the
electrical potential decays very rapidly with distance leading to a
substantially neutral surface.
[0033] Liquids
[0034] The present invention will work in aqueous as well as
non-aqueous fluids such as organic solvents (alcohols, etc).
Especially in organic liquids with relatively high dielectric
constants, i.e. dielectric constants greater than 10, as these
liquids behave similarly as aqueous liquids. Additionally, it works
with fluids of different viscosities, ionic strengths and
dielectric constants. It is effective for removing any contaminants
such as organic and inorganic particles, ionic species, molecular,
oligomeric and polymeric materials as well as physically dissolved
gas from such fluids,
[0035] In aqueous fluids, one means for selecting the proper filter
material with a substantially neutral surface is the typical pH
range of the fluid as it is to be filtered. Thus one can ensure
that the filter remains substantially neutral in the fluid at that
given pH range. In non-aqueous fluids, pH is not a relevant value.
Here, the proton acceptor/donor properties of the filter material
and the liquid as well as the ionic strength of the liquid will
determine the electrostatic potential of the filter material.
[0036] Particles
[0037] The filters of the present invention are capable of removing
inorganic and organic particles, whether crystalline and
non-crystalline, elastic or non-elastic, in sizes ranging from 10
nm to 100 000 nm. More specifically, a filter of the present
invention is capable of removing solid particles as well as
colloidal particles such as liposomes, lipid containing colloids,
organelles, DNA aggregates, protein and protein aggregates, as well
as aggregates of any combination of colloidal particles.
[0038] Filter material
[0039] Filters useful in this present invention can be made from a
variety of materials including but not limited to, glass fiber and
fabric, metals such as stainless steel, nickel, chromium and alloys
and blends thereof, ceramics, plastics and combination thereof.
Preferably, the filter material is made of a plastic e.g. from
cellulosic materials such as regenerated cellulose or
nitrocellulose, thermoplastic materials such as polyolefins,
including polyethylene such as ultrahigh molecular weight
polyethylene, polypropylene and the like polyolefins; homopolymers,
copolymers or terpolymers of polyolefins; polyvinylidene fluoride
(PVDF), PTFE resin, PFA and other fluorinated resins, in
particular, perfluorinated thermoplastic resins; polyvinyl chloride
homopolymers (PVC) and copolymers; nylons; polyamides;
polysulphones; modified polysulphones such as polyethersulphones,
polyarylsulphones and polyphenylsulphones; polyimides;
polycarbonates; polyesters such as PET and the like and composites
thereof. Preferred are thermoplastic filter materials such as
polyolefins including polyethylene such as ultrahigh molecular
weight polyethylene and polypropylene, sulphones such as
polysulphones, polyethersulphone, polyarylsulphones and
polyphenylsulphones, PTFE, PVDF, PFA and other fluorinated
thermoplastic resins, nylons such as Nylon 66 and composites
thereof.
[0040] Filter Structure
[0041] The structure of the filter may be any that are typically
used in liquid filtration such as porous filter sheets, composite
filter sheets (two or more layers formed on each other to form an
integral sheet structure), any kind of porous membrane, woven or
non-woven mats or fabrics, depth filters, hollow fibers and the
like. Preferably, they are porous membranes or composite membranes
in flat sheet form, pleated form or spiral wound form.
[0042] Surface Modification of Filter Materials
[0043] The filter materials can be modified in a variety of manners
depending in large part upon the nature of the material from which
is it made. For all filters, the key is to ensure that the entire
inner and outer surface, including the one within the pores is
modified to ensure that there is adequate retention
characteristics. For example, metal filters may be modified by
passivation, oxidation, or coating of the metal surfaces of the
filter as is well known in the art. Ceramics may be modified by
oxidation or inorganic and organic surface chemistry modifications
or coatings. Polymers may be modified by grafting, oxidation,
adsorption, introduction of modifier materials into the polymer mix
before or during formation, post formation coatings, whether
crosslinked or otherwise, and the like. Such techniques are well
known to one of ordinary skill in the art.
[0044] One method of making a material with a substantially neutral
surface in a given liquid is to modify the surface of the material.
In doing so, the new surface substantially covers the underlying
filter material and becomes in effect the exposed surface of the
filter. The filter performance, i.e. its retentiveness for
particles smaller than the pore size of the filter, will thus
depend on the electrostatic character of the applied modifying
layer being exposed to the liquid.
[0045] Various types of energy are known to modify the surface
chemistry of polymeric filters. Any of these energy sources may be
used to form a substantially neutral surface. Included but not
limited in this group are energy sources such as gamma radiation,
x-ray radiation, Corona discharge, electron beam, laser excitation
and ultraviolet light.
[0046] One can use various acids or bases (depending upon the
filter material selected and the fluid in which it will be used) to
modify the filter surfaces. For examples, acids or bases can be
used to oxidize the surfaces of various metal, glass, ceramic or
plastic filters.
[0047] Preferably, the method for creating a substantially neutral
surface is to use surface modification chemistries. Typically,
these methods utilize one or more monomers that are applied to the
surface of the filter. In doing so, the monomers substantially
cover the underlying filter surface and become in effect the outer
surface of the filter. Most common methods include simply coating
the material onto the surfaces of the filter and relying upon a
mechanical holding of the coating to itself to maintain the outer
surface chemistry, cross-linking the monomers to the filter
surface, see for example U.S. Pat. No. 4,618,533 the teachings of
which are incorporated herein its entirety, or grafting the
monomers onto the filter surface, see for example U.S. Pat. Nos.
3,253,057; 4,151,225; 4,278,777 and 4,311,573 the teachings of
which are incorporated herein.
[0048] Selected monomers include those of the patents cited above.
More preferred are acrylate or acrylamide monomers such as acrylic
acid, 2-acrylamido-2-methyl-1-propanesulfonic acid,
sulfylpropylacrylate or N,N-dimethylacrylamide (DMAm) and
methacrylate or methacrylamide monomers such as methacrylic acid
and blends of two or more of the above.
[0049] Preferably these monomers are used in conjunction with a
photoinitiator, preferably a water soluble photoinitiator such as
Irgacure 2959 and a cross linker, A preferred cross linker is
N,N'-methylenebisacrylamide(MBAm).
[0050] A second preferred system uses no monomer. Instead it uses
the MBAm cross linker alone. It has been found that this surface
treatment alone is sufficient to provide a relatively neutral
surface useful in this invention.
[0051] One method of applying the surface treatment is as follows:
either system, the monomer alone or in combination with the
photoinitiator or preferably in combination with the photoinitiator
and/or cross linker, or the cross linker alone, is prepared in
water. The filter to be treated is pre-wet in alcohol, exchanged in
water and then soaked in the selected solution. It is then
subjected to a UV treatment to cause the surface treatment to cross
link and bond to the surfaces of the filter. The filter is then
washed and dried and is then ready for use.
[0052] An alternative method of applying the surface treatment is
to prepare a self wetting solution through the use of a water
miscible organic such as tert.-butyl alcohol or
2-methyl-2,4-pentanediol (10-20% by weight) with the selected
chemistry and apply it directly to the dry surface of the
filter.
[0053] In either method, the use of positive pressure or a vacuum
may be used to enhance the penetration rate and effective surface
area coated by the surface treatment.
[0054] An example of a modified filter can be made by taking an
ultra high molecular weight polyethylene membrane filter available
from Millipore Corporation of Bedford, Massachusetts and applying a
solution of 0.26 Kg Irgacure 2959 initiator, 6 Kg acrylic acid, and
1.62 Kg MBAm in 192.12 Kg deionized (DI) water. The membrane filter
is prewet in isopropanol, and exchanged in DI water for several
minutes. The membrane filter is then soaked in the solution for
several minutes, and after squeezing out the excess liquid,
irradiated with a Fusion "H" UV lamp at 30 ft./min under nitrogen.
The membrane filter is then rinsed in water in two successive baths
and dried under hot air.
[0055] The filter may be used as a flat sheet such as a cut disk of
25 or 47 mm diameter. It may also be used as a sheet in a cassette
cartridge as may be used in tangential flow or normal flow
filtration mode such as a Pellicon.RTM. cassette available from
Millipore Corporation of Bedford, Massachusetts.
[0056] Preferably, it is formed as one or more layers and made into
a pleated or spiral wound cartridge device. Such devices are well
known in the art.
[0057] In one embodiment of either the flat sheet or pleated, or
spiral wound device, the device contains a series of two or more
filters, each modified so that their surfaces have a low ZP or are
at IEP in the liquid in which they are used and arranged so that
there is a progression of smaller and smaller particle size
retention as the liquid progresses from the upstream side to the
downside side of the device.
[0058] In a further embodiment, they may be formed as hollow fiber
devices.
[0059] The pore size of the filter can vary widely from that
clearly within the ultraporous range (less than 0.01 microns
average or nominal diameter) to those in the traditional
microporous range (0.05 microns to 10 microns average or nominal
diameter, preferably 0.1 to 1 micron average or nominal diameter).
The advantage of the present invention is that one can use a larger
pore size and still obtain enhanced filtration of particles that
are smaller than selected pore size and can do so at enhanced flow
rates and reduced pressure drops. Thus one is no longer limited to
sieving or size exclusion as the primary or sole filtering
mechanism. For example, the present invention provides a filter
with a traditional pore size rating of 0.3 microns with an LRV of
greater than 3 of 0.1 microns particles with a pressure drop which
is less than {fraction (1/10)} that of a 0.1 microns rated "sieving
filter" (i.e. a filter with a nominal or rated pore size of 0.1
microns). The 0.1 microns "sieving filter" has an LRV of less than
3, generally about 2 of 0.1 microns particles.
[0060] Additionally, one can obtain enhanced filtration or LRV
levels for the "same pore size" membrane, e.g. one can obtain
higher levels of LRV (LRV>3, preferably 5 or greater) over the
same pore size membrane without the selected ZP in the same fluid
at the same pH.
[0061] Further, the filters of the present invention are capable of
not only filtering particles larger than their nominal pore size
but is also capable of removing a substantial amount of particles
that are smaller than the nominal pore size of the filter,
something not accomplished with sieving only filters. So for
example, filters of the present invention are capable of a particle
reduction of at least 3 LRV of particles having an average diameter
smaller than the nominal pore size of the filter,
EXAMPLES
[0062] Experiments were performed to illustrate the invention
described above. The ZP of differently charged model particles
(negatively charged polystyrene latex beads (PSL), positively
charged surface modified tertiary amine PSL, and positively charged
silicon nitride particles (Si3N4)) as well as different filter
materials that were surface modified as well as unmodified filter
materials were measured in different liquids (hydrofluoric acid
(HF) and potassium chloride solution (KCl)) at different pH values.
The enhanced retention of these charged model particles by
mechanisms other than sieving was demonstrated for these filters.
From the results, a clear correlation is seen between the ZP of the
respective filter material and its retentive properties for the
differently charged model particles.
[0063] Zeta Potential Measurements
[0064] A general description of ZP measurements can be found in the
text of Robert Hunter, Zeta Potential in Colloid Science, Academic
Press 1984, the teachings of which are incorporated herein by
reference.
[0065] The determination of the ZP of particles in different
liquids was determined according to U.S. Pat. No. 4,602,989.
[0066] The determination of the ZP of porous materials in different
liquids was determined according to WO8700285A1.
[0067] Measurement of Particle Retention
[0068] Two test systems were employed to measure particle retention
of test filters in different liquids. The first system uses HF of
different pH values as a test liquid and filters assembled in
cartridges. The second system uses KCl of different pH values as a
test for 47 mm diameter filter discs.
[0069] i) Test System for Filters Assembled in Cartridges for Tests
in Hydrofluoric (HF) Acid
[0070] A recirculating etch bath (REB) system as shown in FIG. 1
was used to test the performance of filters assembled in
cartridges. A centrifugal pump 1 circulated the liquid (HF) coming
from the recirculating etch bath 3 through the filter 5. The system
components were constructed of PFA
(poly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) or
poly(tetrafluoroethylene-co-hexafluoropropylene and polyvinylidene
fluoride (PVDF) and the entire system contained approximately 60
liters of liquid. The circulation rate varied with filter type;
being approximately 40 lpm for most filters. The particle
concentration in the recirculating etch bath 3 was monitored
continually with an HSLIS M65 optical particle counter 7 (Particle
Measuring Systems, Boulder, Colo.) Liquid was withdrawn from the
recirculating etch bath 3 by a peristaltic pump 9 and passed
through the particle counter 7. The M65 was a four-channel monitor
with channel sizes of 0.065 .mu.m, 0.10 .mu.m, 0.15 .mu.m and 0.20
.mu.m. A flow controller 10 was used downstream of the particle
counter 7 along with a pressure gauge 11. The sample was returned
to a weir 12 within the bath 3. The weir and bath were both
connected to the pump 1 for recirculating the fluid through the
bath. All testing was performed in a Class 100 cleanroom.
[0071] FIG. 2 shows typical background particle concentrations of
the system. The background particle concentration stemmed from the
shedding of particles from wetted parts of the system, i.e. when no
excess particles were present. Concentrations around 10
particles/ml.gtoreq.0.06- 5 .mu.m were routinely achieved. The data
in FIG. 2 were collected over an 18-hour time period.
[0072] Test Procedure:
[0073] Particle suspensions of known concentration were prepared by
dilution of a stock suspension and measured with the M65 particle
counter to determine the final concentration. Measured volumes of
these suspensions were placed into the weir 12 of FIG. 1 (up-stream
of the filter). The particle counter 7 monitored the concentration
of particles in the bath (down-stream of the filter). The peak
concentration of particles.gtoreq.0.065 .mu.m in the bath 3 was
used to calculate filter LRV. The concentration of particles
.gtoreq.0.065 .mu.m at the filter inlet was usually near 150,000
particles/ml. The concentration in the bath prior to adding
particles to the weir 12 was typically 50.+-.20
particles/ml.gtoreq.0.065 .mu.m. Hence, the minimum detectable and
substantial increase in concentration down-stream of the filter 5
was approximately 150 particles/ml.gtoreq.0.065 .mu.m. This
increase corresponds to an LRV of 3 (log.sub.10[150000/150]).
Although some filters 5 may have had better retention, the maximum
LRV detected by this method was 3.
[0074] The volumes of HF added were chosen to yield evenly spaced
molar concentrations on a logarithmic scale. The relationship
between molar concentration, weight %, dilution ratio, and
approximate solution pH are shown in Table I. The dissociation
constant used in calculating pH was 0.00035.
1TABLE I HF concentration relationships Molar Concentration HF
Concentration Approximate (m/l) (weight %) Dilution Ratio PH
10.sup.-3 0.0017 30,000:1 3.4 10.sup.-2 0.017 3,000:1 2.8 10.sup.-1
0.17 300:1 2.3 1 1.7 30:1 1.8 3 5.1 10:1 1.5
[0075] ii) Test System for Disc Flat Sheet Filters of 47 mm
Diameter in KCl
[0076] The experimental system is shown in FIG. 3. It consisted of
a high density polyethylene holding tank 20, a multistage
circulation pump 21, prefilters 22, static mixers 23, flow meters
24, pH meters 25. All parts as well as the piping in contact with
liquid after the prefilter are made from polyvinylidene difluoride
or glass to ensure low particle shedding and chemical resistance.
The holding tank was filled with ultrapure distilled water, and the
system was run either in recirculating mode for stabilization in
double distilled water or in drain mode when the filter was
challenged by particles. A 0.1 .mu.m pore size prefilter was used
after the circulation pump and after the first static mixer. The pH
was adjusted by continuously pumping either HCl or KOH from tank 26
into the main line before the first static mixer by using a
peristaltic pump 27 and again a prefilter 22. Similarly, particles
were introduced into the system before the second static mixer by
continuously pumping small amounts of highly concentrated particle
suspensions from a tank 28 into the main line upstream of the test
filter 29 yielding a challenge particle concentration of 300,000
particles per ml. Particle counts were monitored with a M65 Optical
Particle Counter 30. Also included were various valves 31 for
controlling flow and draining the system.
[0077] Test Procedure:
[0078] For each test, a new 47 mm disc filter was installed in the
test filter holder and flushed with recirculating ultrapure water
for several hours to achieve a low background particle
concentration. Particle retention by the filter was measured as a
function of pH. The filter was initially challenged with Si3N4(+),
PSL(+) or PSL(-) particles at the lowest desired pH (achieved by
adding HCl) and the pH was then incrementally increased by
continuously adding lower amounts of HCl and eventually increasing
amounts of KOH. Particle retention was measured for each pH value
after equilibration of the system was awaited (about 10 min). The
log reduction value (LRV) was calculated using the following
formula:
LRV=log(feed particle concentration/permeate particle
concentration)
[0079] Test Particles
[0080] A variety of particles used to challenge the filters
together with some of their properties are listed in Table II.
2TABLE II Challenge particles Specified Approximate IEP Material
particle size in pH units Supplier PSL(-) 102 nm 2 Duke Scientific
PSL(+) 90 nm 10 Bang Laboratories Si3N4(+) <325 mesh >6 Alfa
Aesar
[0081] Monodisperse polystyrene latex (PSL) particles with two
different surface characteristics were used: PSL(-) is a plain PSL
particle, and PSL(+) is a surface modified PSL particle exhibiting
tertiary-amine groups at the surface. Furthermore, silicon nitride
(Si.sub.3N.sub.4) suspensions were prepared from polydisperse
powders (Si3N4(+)). FIG. 4 shows the measured ZP of the PSL(-) and
the Si3N4(+) particles as function of the pH of the surrounding
liquid. From FIG. 4 we see, that PSL(-) particles are negatively
charged over the measured interval. They were therefore used as a
model particle representing a negatively charged entity. Similarly,
Si3N4(+) exhibits a positive Zeta potential over the measured pH
interval and was used as a model particle representing a positively
charged entity. The assumed ZP dependence on pH for PSL(+), i.e.
PSL particles exhibiting tertiary-amine groups at their surface, is
also depicted in FIG. 4. PSL(+) particles are expected to exhibit a
positive charge over the shown pH range due the protonation of the
amine group (see FIG. 4). No actual data for PSL(+) was available
however at the time being.
[0082] Filters Tested
[0083] A variety of microporous polymeric filter membranes listed
in Table III together with their properties was tested with respect
to their retentiveness towards the challenge particles listed in
Table II. The pore size of all filters was between 0.15 .mu.m and
0.3 .mu.m. Under sieving conditions, these filters would be
expected to have poor (30-50%) retention of 0.1 .mu.m PSL spheres.
The Millipore filters were surface modified according to U.S. Pat.
No. 4,944,879 using different functional monomers to influence the
surface properties of the membranes.
3TABLE III Properties of filters tested Pore Size Filter Name
Material (in .mu.m) Surface Modification Filter #1 N/A UPE.sup.a)
0.25 AA.sup.e), MBAm.sup.f), 12959.sup.g) Filter #2 Millipore
Corp.: UPE 0.15 DMAM.sup.h), MBAm, Guardian DEV 12959 Filter #3 US
Filter.: PSF.sup.b) 0.15 not known Mega-Etch Filter #4 Millipore
Corp.: UPE 0.25 AA, TAA.sup.i), 12959 Etchgard HP Filter #5 Pall:
PVDF.sup.c) 0.3 not known Superetch Filter #6 N/A UPE 0.25 no
surface modifi- cation Filter #7 N/A UPE 0.25 DMAM, MBAm, 12959
Filter #8 N/A PTFE.sup.d) 0.2 DMAM, MBAm, APMAm.sup.j), 12959
.sup.a)Ultra High Molecular Weight Polyethylene .sup.b)Polysulfone
.sup.c)Polyvinylidene difluoride .sup.d)Polytetrafluoroethylene
.sup.e)Acrylic Acid .sup.f)N,N-Methylenebis(Acrylamide)
.sup.g)4-(2-hydroxyethoxy)Phe- nyl-(2-Propyl)Ketone
.sup.h)N,N-Dimethylacrylamide .sup.i)Triallylamine
.sup.j)N-(3-Aminopropyl)methacrylamide HCL
[0084] Zeta Potential of Test Filters
[0085] FIG. 5 shows the ZP (in millivolts) of the filters listed in
Table III characterized in either KCl or HF as a function of pH. In
the case of filters characterized in KCl (open symbols) the test
liquid was a 0.001 mol/l KCl solution of double distilled high
purity water and the pH was adjusted by adding potassium hydroxide
(KOH) to attain pH values higher than 7, and hydrochloric acid
(HCL) to attain pH values of lower than 7. In the case of filters
characterized in HF (filled symbols), the pH was adjusted by adding
HF to double distilled high purity water. All filters show a trend
towards higher (more positive) ZP values for decreasing pH values
due to the adsorption of positively charged hydronium ions
(H.sup.+). Accordingly, most filters cross the line of zero ZP,
i.e. the IEP of point of zero charge at some pH value. At this pH
value the filter is essentially neutral and will therefore
according to this invention retain particles of all possible charge
characteristics, i.e. negatively charged, positively charged, or
neutral.
[0086] Filter Retention
[0087] FIG. 6 shows the LRV of negatively charged PSL particles
(PSL(-)) from HF and KCl respectively for the different test
filters listed in Table III as a function of the ZP of the
respective filters. Most likely due to electrostatic interactions
between the filter material and particle material, the negatively
charged particles, being much smaller than the filter pore size,
are retained at an LRV of greater than 3 by all filters for
positive filter ZP. However, the data also shows that negatively
charged particles are retained by up to 3 LRV also by filters with
a slightly negative ZP. Filter 4, e.g., is capable of retaining
PSL(-) with a LRV of greater than 3 at a filter ZP of approx. -15
mV. Similarly, the other filters retain PSL(-) with a LRV of up to
3 even though they exhibit a negative ZP. For small negative filter
ZP the attractive Van der Waals interaction overcomes the repulsive
electrostatic interaction between the filter material and the
particle material and the particle is retained by the filter.
[0088] FIG. 7 shows the LRV of positively charged PSL particles
(PSL(+)) and positively charged Si3N4(+) particles from HF and KCl
respectively for the different test filters listed in Table III as
a function of the ZP of the respective filters. Again, most likely
due to electrostatic interactions between the filter material and
particle material, the positively charged particles, being much
smaller than the filter pore size, are retained at an LRV of
greater than 3 (but at least at an LRV of 1.5) for all filters for
negative filter ZP. However, similarly to the data for negatively
charged particles in FIG. 6, the data shows again that positively
charged particles are retained by up to 3 LRV also by filters with
a slightly positive ZP. Again, for small positive filter ZP the
attractive Van der Waals interaction seem to overcome the repulsive
electrostatic interaction between the filter material and the
particle material and the particle is retained by the filter.
[0089] The data in FIGS. 6 and 7 shows, that particles of any
charge characteristic (positive or negative) can be efficiently
retained by filters which exhibit no or a relatively small ZP in
the respective filter liquid.
[0090] Other Particle Types
[0091] Several other types of particles were used to ensure that
the testing with PSL and Si.sub.3N.sub.4 particles was
representative of filter performance. The tests results showed that
Al.sub.2O.sub.3, a positively charged particle in the respective
filter liquids, was retained like Si.sub.3N.sub.4. Similarly, Si
particles, which are negatively charged in the respective filter
liquids, were retained like PSL(-) particles.
* * * * *