This invention relates to a method for the filtration of
particulates from fluids using porous media. More specifically, it relates to a
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
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.
Particle Capture Mechanisms in Gases and Liquids: An Analysis of Operative
Mechanisms, by Grant, et. al., 1988 Proceedings of the Institute of Environmental
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.
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.
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-) or hydronium ions (positively charged: H+).
Independent of the character of the Equid (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.
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.
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.
EP-A-0 888 810
discloses a reverse osmosis composite membrane that has a high salt rejection,
a high water permeability, and a high fouling tolerance, and permits practical desalination
at a relatively low pressure. Said reverse osmosis composite membrane is prepared
by coating the surface of a reverse osmosis membrane of aromatic polyamide with
polyvinyl alcohol (PVA), for example, and controlling the surface zeta potential
of the separation layer within ± 10mV at pH 6. This reverse osmosis composite
membrane is electrically neutral and controls the electrical adsorption of membrane-fouling
substances having a charge group present in water.
EP-A-0 087 228
discloses surface-modified, skinless, hydrophilic, microporous, polyamide
membranes with controlled surface properties which are prepared by the steps of
preparing a casting solution comprised of (A) a casting resin system comprising
of (a) an alcohol-insoluble polyamide resin, and (b) a water-soluble membrane surface
modifying polymer having functional polar groups and a molecular weight of 10,000
or greater, and (B) a solvent system in which the casting resin system is soluble;
inducing nucleation of the casting solution by controlled addition of a non-solvent
for the casting resin system under controlled conditions to obtain a visible precipitate
of casting resin system particles, thereby forming a casting composition; spreading
the casting composition on a substrate to form a thin film; contacting and diluting
the film of the casting composition with a liquid non-solvent system for the casting
resin system, thereby precipitating the casting resin system from the casting composition
in the form of a thin, skinless, hydrophilic, surface-modified, microporous, polyamide
membrane; and washing and drying the membrane. The membranes are characterized by
having fine pore ratings, the surface properties thereof being substantially controlled
by functional polar groups of a membrane surface modifying polymer and having the
capability, through the functional polar groups of the modifying polymer, of reacting
or interacting with particulates and/or non-particulates in a fluid.
US-A-4 617 124
discloses microfibrous, polymeric filter sheets and filter elements prepared
therefrom which are comprised of a normally hydrophobic, microfibrous, polymeric
web, wherein the surfaces of the microfibers making up the web are coated with a
cured, precipitated, cationic, thermosetting binder resin or polymer. The filter
sheet is further characterized by being hydrophilic and having a positive zeta potential.
The process for preparing such filter sheets comprises four steps:
- (1) applying a first solution or dispersion of a precipitating agent to a hydrophobic
web to at least partially wet the web;
- (2) applying a second solution of the binder resin or polymer to the wetted
web of step (1);
- (3) working the wetted web of step (2) to mix the first and second solutions,
thereby facilitating the precipitation of the binder resin or polymer and the distribution
of the precipitated binder resin or polymer as a coating on the surface of the microfibers
making up the worked web; and
- (4) drying and curing the coated web of step (3) to form the desired filter
The filter sheets have enhanced mechanical strength, are
hydrophilic, have positive zeta potentials over the pH range of from about 3 to
about 10 and typically have absolute pore ratings of from about 0.5 to about 80
US-A-4 645 567
discloses a process for production of anionically charged filter media
sheet including pretreatment of filter elements with cationic charge modifier, preferably,
employing inorganic colloidal silica charge modifiers. The resulting filters are
used for the removal of haze or haze formers from beverages.
US-A-4 431 545
discloses a microporous filter system which comprises two types of hydrophilic,
microporous filter media operating in series. The two filter media have opposite
zeta potentials with the upstream or first filter medium preferably having the positive
zeta potential and the downstream or second filter having the negative zeta potential.
The first filter medium typically has an absolute pore rating of from about 0.1
to about 1.0 micrometer and the second or downstream filter medium typically has
an absolute pore rating of from about 0.02 to about 0.1 micrometer. The downstream
or second filter has a finer absolute pore rating than the upstream or first filter.
Fluids contaminated with ultrafine particles can be purified with an essentially
absolute efficiency to remove 99.99 percent or more of the particulate matter in
the contaminated fluid.
SUMMARY OF THE INVENTION
The present invention provides a process of filtering a
fluid containing hydrofluoric acid, positively charged particles and negatively
charged particles to remove said positively charged particles and said negatively
charged particles from said fluid as claimed in claim 1. According to the present
invention a filter is used with high particle capture (Log reduction value (LRV)
> 3, preferably > 5, more preferably > 10) with low pressure drops and
high flow. For example, according to the present invention a filter is used 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 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 2 of 0.1 microns particles.
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. It is used in a fluid
containing hydrofluoric acid. In aqueous fluids, it can have a substantially neutral
surface over a broad pH range or if desired a selected narrow pH range.
According to the present invention, a filter is used that
has a surface that is substantially neutral in charge in the fluid in which it is
According to the present invention, a filter is used 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.
According to the present invention a filter is utilized
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.
According to the present invention a filter is utilized
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. According to the present invention a filter
is utilized having a Zeta Potential of 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.
According to the present invention there is provided a
method of filtration of a fluid comprising hydrofluoric acid and 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
Figure 1 Test system for tests in Hydrofluoric (HF) Acid
for filters assembled in cartridges.
Figure 2 The measured background particle concentrations
in the test system for cartridges.
Figure 3 test system for tests in KCI for 47 mm filter
Figure 4 shows the Zeta Potential of particles in KCI as
function of pH.
Figure 5 shows the Zeta Potential of test filters in HF
or KCI as function of pH.
Figure 6 shows test filter retentiveness in log reduction
values (LRV) for negatively charged PSL(-) particles in KCI or HF of different pH
versus the filter Zeta Potential in the respective test liquid.
Figure 7 shows test filter retentiveness in log reduction
values (LRV) for positively charged Si3N4(+) and PSL(+) particles in KCI and HF
of different pH versus their Zeta Potential in the respective test liquid.
DETAILED DESCRIPTION OF THE INVENTION
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.
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.
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.
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
Robert Hunter, Zeta Potential in Colloid Science, Academic Press, 1984
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.
In the present invention when a filter material exhibits
its IEP or a ZP close zero (i.e. 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).
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.
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 -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
dissolved 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.
The present invention will work in a fluid containing hydrofluoric
acid. 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.
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.
The filters used according to 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 used according to 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.
Filters useful in this present invention can be made from
ultrahigh molecular weight polyethylene.
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.
Surface modification of filter materials
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.
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.
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
US Patent 4,618,533
the teachings of which are incorporated herein its entirety, or grafting
the monomers onto the filter surface, see for example
US patents 3,253,057
the teachings of which are incorporated herein.
Selected monomers include acrylic acid.
These monomers are used in conjunction with a photoinitiator,
preferably a water soluble photoinitiator such as Irgacure 2959 and a cross linker,
which is N,N'-methylenebisacrylamide(MBAm).
It has been found that this surface treatment alone is
sufficient to provide a relatively neutral surface useful in this invention.
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.
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.
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.
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.
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®
cassette available from Millipore Corporation of Bedford, Massachusetts.
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
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.
In a further embodiment, they may be formed as hollow fiber
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 1micron 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 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.
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.
Further, the filters used according to 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 used according to 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.
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 (KCI)) 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.
Zeta Potential measurements
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.
The determination of the ZP of particles in different liquids
was determined according to
The determination of the ZP of porous materials in different
liquids was determined according to
Measurement of particle retention
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 KCI
of different pH values as a test for 47 mm diameter filter discs.
i) Test system for filters assembled in cartridges for tests in Hydrofluoric
A recirculating etch bath (REB) system as shown in Figure
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 Ipm 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, CO) 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 µm, 0.10 µm, 0.15 µm and 0.20 µm.. 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.
Figure 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 ≥ 0.065 µm were routinely achieved.
The data in Figure 2 were collected over an 18-hour time period.
Test procedure: 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 Figure 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 ≥ 0.065 µm in the bath 3 was used to calculate
filter LRV. The concentration of particles ≥ 0.065 µm 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 ≥
0.065 µm. Hence, the minimum detectable and substantial increase in concentration
down-stream of the filter 5 was approximately 150 particles/ml ≥ 0.065 µm.
This increase corresponds to an LRV of 3 (log10[150000/150]). Although
some filters 5 may have had better retention, the maximum LRV detected by this method
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.
ii) Test system for disc flat sheet filters of 47 mm diameter in KCl
Table I: HF concentration relationships
Molar Concentration (M/l)
HF Concentration (weight %)
Approximate Dilution Ratio
The experimental system is shown in Figure 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 µm 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.
Test procedure: 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:
A variety of particles used to challenge the filters together
with some of their properties are listed in Table II.
Table II: Challenge particles
Specified particle size
Approximate IEP in pH units
< 325 mesh
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 (Si3N4) suspensions
were prepared from polydisperse powders (Si3N4(+)). Figure 4 shows the measured
ZP of the PSL(-) and the Si3N4(+) particles as function of the pH of the surrounding
liquid. From Figure 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 Figure 4. PSL(+) particles
are expected to exhibit a positive charge over the shown pH range due the protonation
of the amine group (see Figure 4). No actual data for PSL(+) was available however
at the time being.
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 If. The pore size of all filters
was between 0.15 µm and 0.3 µm. Under sieving conditions, these filters
would be expected to have poor (30 - 50%) retention of 0.1 µm PSL spheres.
The Millipore filters were surface modified according to
US Patent 4,944,879
using different functional monomers to influence the surface properties
of the membranes.
Filters #2 to #8 do not fall within the scope of protection.
Zeta Potential of test filters
Table III: Properties of filters tested
Pore Size (in µm)
Millipore Corp.: Guardian DEV
DMAMh), MBAm, 12959
Filter # 3
US Filter.: Mega-Etch
Millipore Corp.: Etchgard HP
Filter # 5
no surface modification
Filter # 7
DMAM, MBAm, 12959
Filter # 8
DMAM, MBAm, APMAmj), 12959
a) Ultra High Molecular Weight
c) Polyvinylidene difluoride
e) Acrylic Acid
j) N-(3-Aminopropyl)methacrylamide HCL
Figure 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 KCI (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+). 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.
Figure 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.
Figure 7 shows the LRV of positively charged PSL particles
(PSL(+)) and positively charged Si3N4(+) particles from HF and KCI 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 Figure 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.
The data in Figure 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.
Other Particle Types
Several other types of particles were used to ensure that
the testing with PSL and Si3N4 particles was representative
of filter performance. The tests results showed that Al2O3,
a positively charged particle in the respective filter liquids, was retained like
Si3N4. Similarly, Si particles, which are negatively charged
in the respective filter liquids, were retained like PSL(-) particles.