Note: Descriptions are shown in the official language in which they were submitted.
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ADVANCED FILTRATION DEVICES AND METHODS
RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application Nos.
60/542,409, entitled "Advanced Filtration Devices and Methods" to David S.
Soane and
Christopher D. Tagge, filed February 5, 2004, and 60/466,160, entitled "Rapid
Production
of Sheet Filter Medial By Direct Casting and Imprinting," to David S. Soave,
filed April
28, 2003, which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
The field of the invention is filtration, particularly filtration with
neutralization or
remediation of harmful particles, aerosols, vapors and gases.
BACKGROUND OF THE INVENTION
Harmful substances, such as toxins and pathogens, are present in the air,
which
may cause illness in excessive concentrations. There are many sources for
these harmful
substances, which may be carried long distances or may be confined to an
indoor
environment. Indoor air pollution is a serious concern. For example, toxic
mold has
become a concern for homeowners and businesses. In addition, recent events
make the
possible use of pathogens and toxins, as a weapon of terror, a serious
concern. The term
harmful substances is meant to include natural substances, such as dust, mold
toxins and
pollens, manmade substances, such as smoke and volatile organic compounds, and
terror
weapons, such as VX, sarin, ricin and anthrax. The chemical formulas for four
harmful
substances are shown in Figures lA-1D.
Some toxic chemicals are lughly reactive polar compounds. For example, four
categories of lmown chemical weapons include cholcing agents, blister agents,
blood
agents and nerve agents. Chol~ing agents, such as chlorine and phosgene,
attack the
lungs. Blister agents, such as mustard gas (HD), are volatile liquids that
blister organic
tissue when absorbed on the skin, in the eyes, or within the lungs. Blood
agents, such as
hydrogen cyanide (HCN), are absorbed through the lungs into the blood, or via
the
intestines in the case of water born HCN, where it irreversibly binds to
hemoglobin and
prevents the uptake of oxygen. Nerve agents, such as sarin (GB), soman (GD),
and VX,
are extremely potent and deadly when inhaled or adsorbed through the shin or
lungs.
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Chlorine, phosgene, and hydrogen cyanide are gases. GB, GD, VX, and HD are
semi-
volatile liquids that are commonly dispersed as an aerosol or a combination of
gas and
aerosol.
The decontamination of surfaces after exposure to toxic substances has been
studied. The predominant decontamination strategies are adsorption (followed
by
incineration) and neutralization by chemical reaction. While the structures
and functions
of these toxic chemicals vary widely (Figures lA-1D), all are highly reactive
materials,
which may be quickly neutralized by the right combination of reagents. For
example,
toxic chemicals may react rapidly with oxidizing agents, all~aline solutions,
or both.
Nerve agents undergo rapid oxidation to phosphoric acid in the presence of
strong
oxidants. HD also undergoes oxidation, although care must be taken to avoid
partial
oxidation to the substantially less toxic but still dangerous sulfone
derivative. Hydrogen
cyaude, chlorine, phosgene, HD, Sarin, Soman, and VX are all attacked rapidly
by
alkaline solutions to yield relatively benign products.
Volatile organic compounds (VOC) are prevalent throughout residential and
commercial buildings and have been found to cause serious health consequences
for
occupants. While national and international health-related agencies have
varied
classifications, VOCs are widely recognized as compounds that are gaseous or
have a
significant vapor pressure at room temperature. For example, products such as
particle
board, plywood, fabrics, coatings, and insulation release significant amounts
of gaseous
formaldehyde, a common VOC. Peak concentrations of formaldehyde in homes and
workplaces typically range from 0.04 - 0.4 ppm. Regular exposure to peak
concentrations greater than 0.06 ppm may result in serious health consequences
including
mucosa irritation, rash, severe allergic reactions, fatigue, headache, nausea,
depression,
and a significantly increased risk of throat cancer with chronic, long-term
exposure.
Other VOCs commonly released from products in the home are benzene, toluene,
styrene,
acetone, para-dichlorobenzene, chloroform, tetrachloroethylene, acrylic acid
esters, and
aliphatic lcetones and alcohol. In addition, building occupants are often
exposed to low
levels of other toxic gases including volatile sulfur complexes, and ammonia
gas. Most
VOC's may be neutralized by oxidizing agents, allcaline solutions or both.
Air filtration systems of air handling systems in buildings, vehicles and
personal
protective equipment are intended to improve the safety and/or quality of the
air we
breath. However, such filtration systems as those used in heating, ventilating
and air
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conditioning (HVAC) systems are not capable of preventing harmful
concentrations of
microbes, particles, vapors and gases from dispersing throughout a structure.
See Table I
for a comparison of the size of some common harmful substances. Air filtration
systems
in vehicles aren't capable of reducing ordinary velucle emission exhausts to
safe levels,
much less any unexpected release of harmful substances in the vicinity of a
vehicle. Even
personal protective equipment that is designed to prevent exposure to harmful
substances
has severe limitations on the duration of exposure permitted before
replacement of
filtration elements is required to guarantee continued protection.
In one example, concentrations of formaldehyde can exceed levels that may
cause
harm to human health merely from natural and manmade sources. Few HVAC systems
can afford to neutralize such a substance. W deed, HVAC systems are more
likely to
spread intentionally released toxins throughout a building rather than
affording occupants
any protection.
High Efficiency Particulate Air (HEPA) filters may be effective in trapping
airborne pathogens and particles. If used, these filters might offer some
limited, passive
defense against some harmful substances, such as anthrax.
HEPA filters are ineffective against many other harmful substances, including
most volatile organic compounds (VOC's) and other harmful substances in vapor,
aerosol
or gas form. Also, HEPA filters are highly efficient in capturing particles,
but HEPA
filters offer a concomitantly high cost of operation, wluch is related to the
large resistance
to airflow through the HEPA filter. The resistance to air flow, which may be
measured as
a pressure drop, requires more energy to be used in circulating air through
the filters,
which increases operating costs. Furthermore, the pressure drop across HEPA
filters
creates a high back pressure, which may lead to leaks in the HVAC conduits,
dramatically lowering overall capture efficiency of the system.
Granular activated carbon filters, such as those used in conventional gas
masks,
are lmown to provide short-term protection from harmful gases. Activated
carbon filters
suffer from several limitations that have prevented their widespread adoption
in HVAC
applications and that reduce their effectiveness in personal protection
applications, such
as filters in protective masks. Specifically, the mechanism of gas adsorption
in activated
carbon is reversible. Thus, the absorbed gas may be released by changes in
temperature,
humidity or the chemical composition of the air. For example, the presence of
a second
gas with a higher affinity for the granular activated carbon can cause release
of a
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previously absorbed gas. Additionally, activated carbon, which is non-polar,
shows
relatively poor adsorption of polar gases. These limitations mean that a
comparatively
high density of activated carbon is necessary to provide a reasonable filter
lifetime. As
discussed in relation to HEPA filters, this high density results in a large
pressure drop
across the filter, which is not desirable in a filtration system. Finally,
granular activated
carbon, which is held loosely in a filter bed, is susceptible to the formation
of open
channels, which significantly reduces the efficacy of the filter. U.S. Pat.
No. 6,435,184,
which is hereby incorporated by reference, discloses the structure of a
conventional
protective mask, for example.
U.S. Pat. No. 3,017,329 issued in 1962 and disclosed a germicidal and
fungicidal
filter using a conventional non-woven filter medium. The filter medium is
coated by
conventional process, such as by spraying or bathing the filter medium using
an active
ingredient. The active ingredient was selected from organo silver compounds or
organo
tin compounds, which have a neutral pH, but are highly toxic to mammals. The
treated
filter is then heated to drive off water, which was used as a solvent in the
coating process
and to cure a binder that fixes the active ingredient to the filter medium.
U.S. Pat. No. 3,116,969 describes a filter having an alkyl aryl quaternary
ammonium chloride antiseptic compound, which is held to conventional filter
fibers by a
tacky composition that includes a hygroscopic agent, a thickening agent and a
film
forming agent.
U.S. Pat. No. 3,820,308 describes a sterilizing filter having a wet oleaginous
coating containing a quaternary ammonium salt as the sterilizing agent.
M. Dever et al. Tappi .Iournal 1997, 80(3), 157 discloses the results of a
study of
the antimicrobial efficacy of an antimicrobial agent incorporated into fibers
of a melt-
blown polypropylene filter medium. Each of three different, mudentified agents
were
blended with polypropylene, which was then melt-blown conventionally to form a
filter
medium. Only two of the agents were detectable by FTIR after processing, and
these two
agents provided antimicrobial properties. However, the agents negatively
affected the
physical properties of the polypropylene, causing thickening of the fibers of
the filter
medium and reduced collection efficiencies than unblended polypropylene.
K. K. Foard and J.T. Hanley, ASHRAE Trans., 2001, v.107, p.156 discloses the
results of field tests using filters treated with one of three unidentified
antimicrobial
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agents. Known antimicrobial filter treatments produced little effect under the
test
conditions, showing growth on both untreated and treated counterparts alike.
A. Kanazawa et al., J. Applied Polyrne~ Sci., 1994, v.54, p.1305 discloses an
antimicrobial filter medium using covalently immobilized antimicrobial
phosphonium
chloride moieties onto a cellulose substrate. Phosphoniuln salts with longer
alkyl chains
tended to have a higher capacity for capturing bacteria.
M. Okamoto, Proceedings of the Institute of EfaviYOhrnental ,Sciences atad
Technology, 1998, p.122 discloses the use of silver zeolite as an
antimicrobial agent in lam
air handling filter. The silver zeolite is attached by a binder to one side of
the filter.
U.S. Pat. Publ. No. 2001/0045398 discloses a process for the preparation of
non-
woven porous material having particles immobilized in the interstices thereof.
The
particles are added by contacting the material with a suspension of particles
and forcing
the suspension through the material, capturing the entrained particles in the
interstices of
the porous material and providing an antimicrobial barner.
The English language abstract of International Publ. No. WO 00/64264 discloses
a
bactericidal organic polymeric material for filters that is made of a polymer
base
comprising a backbone and a polymeric pendant group bonded to the backbone.
The
material comprises units derived from an N-alkyl-N-vinylalkylamide and
triiodide ions
fixed to the polymeric material.
International Publication No. WO 02/058812 discloses a filter medium
containing
time release microcapsules of antimicrobial agent. The microcapsules contain
the agent
suspended in a viscous solvent, which slowly diffuses out of the porous shell
of the
microcapsule. The microcapsule may be held to conventional filter medium using
gum
Arabic as an adhesive.
Other methods of removing airborne pathogens includes percolating air through
a
liquid, electrostatic precipitation (e.g. U.S. Pat. No. 5,993,738),
ultraviolet light (e.g. U.S.
Pat. No. 5,523,075), but each of these uses significantly more energy than is
acceptable
for high volume HVAC applications.
All of the previous examples have shortcomings that prevent their widespread
adoption in air filtering systems, such as producing hazardous waste disposal
problems,
having high operating costs and having high costs for production and
maintenance of the
filtration systems. Thus, there is an urgent need for substantial improvement
in the
protective capabilities of low-cost and effective filters. Low energy
consumption is
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particularly needed in the capture and neutralization of harmful particles,
pathogens,
aerosols, vapors and gases.
Generating sub-micron patterns may be achieved using photolithographic
processes, as is known in the art of semiconductor device fabrication, such as
the process
disclosed by U.S. Pat. No. 5,110,697, which is incorporated herein by
reference.
Generating 1-50 micron patterns is easily achieved by well-known, conventional
processes using photolithography and other conventional techniques, such as
color
printing and embossed sheet printing. In color printing, processes for
registering precise
positioung of multiple layers is well known. Also, embossed sheet formation is
known
that provides durable patterned roller surfaces in processing molten viscous
fluids or
softened solids. For example, in forming embossed sheets, a polymer melt is
squeezed
through a narrow gap, known as the nip region, of a set of calender rolls with
embedded
surface features. These processes, which are not related to the fabrication of
conventional
filter medium, are referred to herein as conventional substrate printing
SUMMARY
A filter comprises a medium for capturing and neutralizing harmful substances.
A
low-pressure, high efficiency pre-filter may be used to capture particles
prior to entry into
a filter medium. The advanced filter medium comprises a filtration component
and a
neutralization component. The neutralization component is a film of viscous
organic
components and reactive components coated in a thin film on a substrate,
providing low-
pressure-drop, high-throughput remediation of harmful substances.
In one embodiment, a neutralization component is supported by a filtration
component, such as fibers, which supports the neutralization component. The
fibers are
distributed in the filter such that air passing through the filter must pass
through tortuous
channels around the fibers. Thus, harmful substances entrained in the air
contact the
neutralization component, which neutralizes one or more of the harmful
substances.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures lA -1D show the chemical formula for several toxic substances.
Figure 2 illustrates one embodiment of the invention having fibers coated by a
remediation layer.
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Figure 3 shows one embodiment of a broad spectrum filter for remediation of a
plurality of toxic or harmful substances.
Figure 4 shows filter fibers without a remediation layer.
Figure 5 illustrates fibers coated with a remediation layer that is sprayed
with a
solution containing mold spores after being placed in an incubator at
temperature aald
humidity suitable for mold growth.
Figure 6 illustrates the fibers of Figure 4 that are similarly coated and
incubated,
as in Figure 5, showing substantial mold growth compared to the embodiment
illustrated
in Figure 5.
Figure 7 shows actual fibers of a filter according to the prior art.
Figure 8 illustrates a filter comprising a coarse pre-filter, a neutralizing
component coating the coarse pre-filter, and a sheet filter medium.
Figure 9 illustrates the concept of fiber wrapping, which means placing a
layer or
layers of functional polymer on the surface of a fiber, providing a
remediation layer.
Figure 10 illustrates one embodiment of a sheet filter medium.
Figure 11 shows a method of preparing a sheet filter medium.
DETAILED DESCRIPTION
In one embodiment, a filter comprises a filtration component and a
neutralization
component. The filtration component may be comprised of several distinct
layers of
filtration medium or may be continuous. Regardless, the neutralization
component may
be a single active agent, a combination of active agents or may be a plurality
of active
agents striated into separate areas in the filtration component.
For example, the neutralization component is supported by the filtration
component, as shown in Figure 2, which acts as a substrate for the
neutralization
component. Conventional filtration media use randomly intertwined and/or
entangled
fibers, as shown in Figure 7, that cause air passing through the filter to
pass through
tortuous airways between the fibers. The fibers are distributed in the filter
such that air
passing through the filter must pass through tortuous channels around the
fibers. Thus,
harmful substances entrained in the air contact the neutralization component,
which
neutralizes one or more of the harmful substances.
In one example, a filter for neutralizing of harmful substances comprises a
conventional, non-reactive filter media, such as polymer fibers, coated with a
remediation
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layer. The remediation layer comprises a host material that harbors one or
more
neutralizing substances, such as acidic, basic or oxidizing substances. The
host material
may have a tacky surface that sticks to entrained particles that impact on the
tacky
surface, improving the efficiency of particle capture, and allowing additional
time for
neutralization of the particles, such as pathogens.
Coating fibrous filters with thin layers of viscous organic components in a
remediation layer is found to adsorb organic gases and aerosols. High-
throughput filter
media are equipped with reactive coatings tailored to efficiently capture and
neutralize
toxic and harmful substances, such as pathogens, VOC's and other chemicals.
The
reactive coatings comprise reactive components to neutralize toxic or harmful
substances
and a coating matrix that provides adhesion between the reactive component to
the filter
media.
Examples of reactive components are oxidizing agents, such as sodium
hypochlorite, calcium hypochlorite, and potassium permanganate; acidic
complexes, such
as acrylic acid derivatives and sulfonic acid derivatives; and basic complexes
such as
sodium alkoxides, tertiary amines, and pyridines and compatible mixtures
thereof.
The matrix component is typically a polymeric material such as polyvinyl
pyrrolidone), polyvinyl pyridine), poly(acrylic acid) (free acid or salt),
polystyrene
sulfonic acid) (free acid or salt), polyethylene glycol), polyvinyl alcohol),
polysiloxanes, polyacrylate derivatives, caxboxymethyl cellulose, and mixtures
and
copolymers thereof. hz some cases, such as polystyrene sulfonic acid) the
matrix
component may also act as the reactive component. The matrix component may
also
consist of non-volatile, low molecular weight complexes, such as glycerol and
ethylene
glycol oligomers, to promote affinity to the target toxic or harmful
substance.
The polymer component may also be crosslinked to prevent removal of the
reactive coating from the filter fiber under extreme conditions. The reactive
coating
technology is applicable to any fibrous filter media including glass and
synthetic polymer
fibers such as polyester and cellulose derivatives.
One advantage of these highly efficient filters is the high probability of
remediation of the toxic or harmful substance or substances. This is a
function of the filter
structure, which preferably causes a large number of collisions between the
toxic or
harmful substances and the filter media, and the probability that each
collision with the
filter media will result in adsorption or neutralization. No single set of
conditions is
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universally applicable to all substances. Thus, for a multiphasic filtration,
a multiple
environment, broad spectrum filter may be desirable as shown in Figure 3, for
example.
The following examples of the neutralizing component each provide protection
against specific harmful substances as reported; however, other harmful
substances may
also be neutralized that were not tested.
Example 1
Formation of an Oxidizing Gel Coating
An aqueous solution is made of the following: 20 wt.% tetraethoxysilane, 20
wt.% bis(triethoxysilyl)methane, 10 wt.% glycerol, and 0.05 wt.% citric acid.
A fiber
glass pad is dipped in the above solution, blotted, and then cured by steam
heating for 6 h.
The pad is then dipped in an aqueous solution of 2 % sodium hypochlorite and
0.5
cyanuric acid and then dried. The resulting filter was effective for the
neutralization of
diethyl sulfide.
Example 2
Formation of an Oxidizing Coating
An aqueous solution is made of 10 wt% poly(vinyl pyrrolidone). A fiber glass
pad is dipped in the above solution, blotted, and then irradiated under a UV
lamp for 6 h
to crosslink the polymer. The pad is then dipped in am aqueous solution of 2
wt%
calcium hypochlorite and air dried. The resulting filter was effective for the
neutralization of formaldehyde.
Example 3
Formation of an Oxidizing Coating
An aqueous solution is made of 10 wt% poly(vinyl pyrrolidone) and 2 wt%
potassium permanganate. A fiber glass pad is dipped in the above solution and
then air
dried. The resulting filter was effective for the neutralization of
formaldehyde.
Example 4
Fornation of an Oxidizing Coating
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An aqueous solution is made of 10 wt% poly(vinyl pyrrolidone). A fiber glass
pad is dipped in the above solution, blotted, and then irradiated under a UV
lamp for 6 h
to crosslinlc the polymer. The pad is then dipped in an aqueous solution of 2
% potassium
permanganate and air dried. The resulting filter was effective for the
neutralization of
formaldehyde.
Example 5
Formation of an Oxidizin _ Coating
An aqueous solution is made of 10 wt% poly(ethylene glycol) and 2 wt% calcium
hypochlorite. A fiber glass pad is dipped in the above solution and air dried.
The
resulting filter was effective for the neutralization of formaldehyde.
Example 6
Formation of an Oxidizing Coating
An aqueous solution is made of 10 wt% poly(acrylic acid) sodium salt and 2 wt%
calcium hypochlorite. A fiber glass pad is dipped in the above solution and
air dried.
The resulting filter was effective for the neutralization of formaldehyde.
Example 7
Formation of an All~aline Coating
An aqueous solution containing 30 wt% glycerol, 5 wt% polyethylenimine and
0.25 wt% glycerol propoxylate triglycidyl ether. A glass pad is immersed in
the solution,
blotted dry and cured at 100 C for 6 hours. The glass pad is then immersed in
an aqueous
sodium hydroxide solution of pH 12 or greater containing 30 wt% glycerol,
removed
from the solution, blotted dry and dried at 50 C for two hours. The resulting
filter was
effective for the neutralization of hydrogen cyanide gas.
Example 8
Formation of an Acidic Coating
An aqueous solution is made of the following: 30 wt% glycerol, 5 wt% styrene
sulfonic acid, 0.1 wt% divinylbenzene, 0.13 wt% 2,2'-azobisisobutyronitrile,
0.02 wt%
potassium persulfate, and 0.5 wt% sodium dodecyl sulfate. A fiberglass pad is
dipped in
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the above solution, padded dry, and then cured at 85 C for 2 h. The resulting
filter was
useful for the neutralization of ammonia gas.
Method for Rapid Production of Sheet Filter Medium
Figure 10 illustrates a partial, close-up view of a sheet filter medium having
a
uniform rectangular mesh of monodisperse pores. Monodisperse meaals that the
pore size
is substantially the same in at least a substantial portion of the sheet
filter medium. More
generally, the method may produce filter media having any desired distribution
of pores.
For example, Figure 8 shows another filter medium having a honeycomb
structure;
however, any structure may be formed by the method for rapid production of
sheet filter
media.
One method of producing a filter having an engineered distribution of pores
provides for rapid production of sheet filter medium by direct casting and
imprinting. A
conventional substrate printing method is used to form a two-dimensional or
three-
dimensional porous sheet filter medium. By "direct casting and imprinting," it
is meant
that fibers of a two-dimensional or three-dimensional filter medimn are in
situ from
precursors during the generation of the desired filter medium structure by a
printing
method. The process is distinguished from conventional embossing and
papermaking, for
example, by the formation of sub-micron and greater porous structures formed
by the in
sitr~ fibrous pattern generation. The structure of the resulting filter medium
is an
organized and intercomzected pattern of fibers. The interconnected pattern of
fibers may
be formed into a sheet filter medium that exhibits both strength and
flexibility unavailable
from conventional filter media.
It is believed, without being limiting in any way, that surface tension forces
are
responsible for causing a spontaneous segregation of a two-phase solution or
emulsion of
the polymer precursors and a solvent, for example, on a substrate with ridges
having one
surface tension and grooves having another surface tension. Thus, the pattern
of the
fibrous filter medium may be laid out by this separation on a printing
substrate that is
patterned using a conventional substrate printing method, such as
photolithography,
color-printing-like process or embossed sheet printing.
For example, an aqueous precursor emulsion is prepared for use with a
photolithographically prepared printing substrate having a pattern of ridges
and grooves
on its surface. The emulsion is prepared such that at least one precursor
phase surrounds
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the ridges and water fills the grooves. Alternatively, the opposite
orientation of the
precursor phase and the aqueous phase, or other solvent, containing the
precursor phase
may be selected by controlling the water-in-oil or oil-in-water nature of the
multi-phase
emulsion or solution. The precursor phase may contain substances that form
fibers by
polymerization, solidification, crystallization, catalytic growth from the
vapor or any
other consolidation process that retains the orientation provided by the
pattern of ridges
and grooves from the surface of the printing substrate.
For example, the precursor phase comprises polymer precursors that form
polymer fibers ih situ during phase separation on the ridges of the substrate
surface. Then,
the removal of the aqueous phase or solvent may be achieved by evaporation,
for
example, leaving well-defined voids of a particular size, shape and
distribution in
between strands of polymer that are intercoimected in a the pattern of the
ridges. A
network of intercomlected fibers is formed that define pores in the filter
medium. The
fibers may be coated by the neutralizing component to assist in capture an
neutralization
of harmful substance. In one example, the fibers are substantially coated by
the
neutralizing component, which provides for both efficient particle capture and
adequate
neutralization of harmful gases, such as volatile organic compounds (VOCs).
In another example, the precursor phase is dissolved or digested cellulosics
or a
proteinaceous polymer in an aqueous solution, and the aqueous solution is
mixed with a
solvent in an emulsion, which segregates to the ridges on the surface of a
printing
substrate. In this example, the precursor phase forms ih situ in the grooves.
The resulting
fibers retain the pattern of the grooves, which may be any pattern. For
example, the
pattern forms a mesh, a honeycomb or any other two-dimensional geometric
shape.
In one embodiment, the separation of the precursor phase from the aqueous or
solvent phase is not attributed to dissimilar materials at the ridges and
grooves on the
surface of the printing substrate. Instead, the separation is caused by
temperature
differences between the material at the ridges and within the grooves, which
drives
spontaneous assembly of a precursor phase. In this specific example, the
precursor feed
need not even be a multiphase system. For example, pre-polymers or polymers
may be
selected that will form around comparatively cold features from a metastable
solution,
leaving solvent rich and stable solution in the comparatively warmer spots.
Thus, a sheet
filter medium may be formed from a non-aqueous polymer solution.
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In another alternative, the surface of the printing substrate does not have
ridges
and grooves. Instead, the surface is substantially flat, and is comprised of a
pattern of
dissimilar materials, such as a pattern printed on the surface of a printing
substrate. The
dissimilar materials have differing surface tension forces with the emulsion
or solution,
which causes separation of the precursor phase and the aqueous or solvent
phase. In this
manner, a pattern of fibers may be formed from the printing substrate, as
before.
In yet another example, the precursor phase comprises a polyester dissolved in
an
acrylated monomer/crosslinker/photo-initiator formulation, such as a
polyethyleneteraphthalate plasticized by a benzylacrylate and bisphenol-A-
diacrylate
mixture. A roller surface is patterned by a conventional substrate printing
method, and the
roller is used to spontaneously initiate phase migration and self assembly
according to the
pattern on the roller surface. A patterned sheet is formed that is then
exposed to a strong
LTV light source, whereupon the acrylate formulation polymerizes to form an
interpenetrating molecular network within the polyester fibers. Residual water
or other
solvent, if present, evaporates, leaving organic fibers in a strongly
crossliu~ed and
flexible sheet filter medium.
The polyester fibers are anchored firmly by the interpenetrating acrylic
networlc,
which ensures that the porous pattern that is formed by the roller is stable
and permanent.
Thus, the sheet filter medium has a pattern of pores with a constant size,
distribution and
spacing based solely on the pattern provided on the surface of the roller and
the thickness
of the strands of organic polymers. The pattern on the roller may have any
pattern from
submicron size to 50 microns for filtering applications. The strand tluckness
may be
controlled by the amount of precursor component and the concentration of the
precursor
component in the diluent or solvent.
In another example, a partially polymerized fluorocarbon suspension in waster
is
subjected to contact with the surface of a roller having an embossed pattern
of
poly(tetrafluoroehtylene). The fluorocarbon material accumulates around the
poly(tetrafluoroethylene) patterns, leaving water (or aqueous solution) in
metal oxide
regions of the roller surface. The patterned fluorocarbon is polymerized by a
polymerization reaction, such as by applying heat or light, e.g. UV light. A
very fine, lace
lilce fluorocarbon filter medium is formed by his process.
Alternatively, polyester or nylon sheets are formed by the same process,
except
that partially polymerized precursors (or other percursors, such as blends of
dead
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polymers and yet unreacted or partially reacted precursors) are used that form
polyester or
nylon. The polyester and nylon sheets also form lace life filter media. After
a lace-life
architecture is formed and polymerization is completed while mechanically
stretching the
sheet filter media in both orthogonal directions simultaneously. The
stretching draws the
fibers, uniformly decreasing the fiber diameter and increasing the pore size.
This
technique, which is applicable to other precursors, as well, enables the
production of filter
media having sub-micron fiber diameters, improving the capture efficiency
versus
pressure drop characteristics compared to larger fiber diameters.
In another alternative embodiment, hybrid processes use pre-spun fibers
together
with direct casting and imprinting to form filter media. For example, pre-spun
parallel
fibers may be fed into a calender that has embossed lines perpendicular to the
direction of
parallel fiber travel. A precursor-containing feed is contacted with the
embossed surface,
forming patterns of the precursor material over the parallel fibers. The
precursor is
reacted, forming sheet filter media in combination with the pre-spun fibers.
h1 one
example, ample voids exist between the "warp" and the "fill" yarns, i.e. the
fibers formed
ih situ and the pre-spun fibers. The new fibers adhere to the pre-spun fibers,
imparting
strength to the hybrid filter medium. In addition, the focused overlap points
allow the
fibers to pivot, giving the material sufficient flexibility. As before, the
new fibers may be
porous or~ solid, synthetic or natural and simple in construction or a
composite structure.
The new fibers may possess a core-shell geometry by using organic-aqueous
systems
having at least two phases, where both phases contain polymers or precursors.
Also, the
pattern of the fibers may include wavy, curved or articulated lines deposited
on the pre-
spun fibers, which are not possible using conventional pre-spun fibers in the
absence of
casting and imprinting.
In yet another embodiment, an aqueous-organic-aqueous complex emulsion may
be used to engineer porous strands. These porous strands surround voids, but
also have
voids within the strands that are formed during the evaporation of water
formed within
the strands by chemical affinity with the water or surface tension forces, for
example. In
one example, hydrophilic and hydrophobic moieties are used to produce such
porous,
polymer strands.
Patterns may be generated using the process of this invention that are
difficult or
impossible to reproduce otherwise in a low-cost, mass-producible filter
medium. For
example, a sheet filter medium is produced having at least one area with a
monodisperse
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pore size. More preferably, the monodisperse pore size, shape and distribution
is uniform
over a large surface area of the sheet filter medium. The uniformity of the
pore size is
capable of greatly increasing the particle capture efficiency of the filter W
edium per fiber
volume as compared to conventional, heterogeneous filter medium. Thus,
compared to
conventional filter media, sheet filter media of the present invention will
have a lower
pressure drop for any desired efficiency of particle capture or a lugher
particle capture
efficiency for any desired pressure drop.
In alternative embodiments, many polymer blends possessing hydrophilic and
hydrophobic characteristics may be used to form sheet filter media in two-
phase or
multiphase systems. The polymer blends may be mixtures of amorphous or
crystalline
polymers, homopolyrners or copolyners, dead polymers mixed with reactive
components
(e.g. monomers, oligomers, crosslinkers and others), and viscous reactive
oligomers or
macromers. For example, reactions for formation of fiber networks may be of a
free-
radical or condensation nature. Also, the polymers or polymer precursors may
be
synthetic or natural. Synthetic polymers span not only hydrocarbons, such as
polyolefms,
polyesters, acetates, acrylics and nylons, but also fluorocarbons and
silicones.
When mechanical means are employed for thin, porous sheet filter media
formation, interfacial properties between the polymer/precursor systems and
the roller
may be exploited to make fleece-like structures. If strong but transient
adhesion exists
between the feed material and the shape-producing substrate surface, then fine
strings or
whiskers may be pulled from the filter sheet wltil the viscoelasticity of the
material
ruptures the thinning tethers between the substrate surface and the feed
material. The
numerous strings or whiskers thus produced may be fleece-like. In one example,
subsequent curing produces locks in this fleece-like, three-dimensional
architecture. Also,
a highly textured sheet filter medium is produced by promoting string
formation via a
deliberately abraded (i.e. roughened) substrate surface that has a plurality
of nanoscopic
or microscopic defects. Alternatively, a highly regular and clean sheet filter
medium is
assured by making a defect-free surface and using a mold release agent to
reduce the
surface tension between the surface of the substrate and the feed material.
In another example, two mating rollers have parallel grooves with one set
running
across the width of the first roller and a second set spanning the
circumference of the
second roller. When a feed containing the precursor component is squeezed
thrQUgh the
nip region, a crisscrossed fibrous pattern is produced. In one example, the
lines are not
CA 02524015 2005-10-27
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continuous across the width of the first roller or along the circumference of
the second
roller. Thus, a pattern is formed that has increased flexibility. In addition,
the roller
surfaces may be provided with indented dots in the grooves that results in a
sheet filter
medium having raised points, providing enhanced tactile properties.
In another embodiment, a sacrificial layer or carrier film, such as a soluble
film, is
used. Patterns may be deposited by any of the methods of the invention on one
or both
sides of the sacrificial layer or carrier film. For example, such patterns may
be multi-
layered, and connections may be formed between the patterns on opposite sides
of a
sacrificial layer by way of "vias" that are formed through the sacrificial
layer, such as by
forming preexisting holes therein. After the sheet filter media are formed,
the sacrificial
layer may be dissolved, providing interconnects between the sheet filter
medium only at
specific interconnection points that corresponded to the location of vias.
Thus, complex
three-dimensional geometry may be formed, which are not possible using
convention
filter media. Alternatively, a Garner film may be simply peeled from the
filter medium
and reused, perhaps, reducing cost of manufacture.
In another embodiment, a semisolid film containing at least one precursor is
squeezed through a set of embossed rollers having patterns embossed on the
surface of
each roller. For example, the patterns, such as lines and holes, are punched
through the
semisolid film, creating openings that may be enhanced by careful stretching
of the
semisolid film. Then, the precursors of the semisolid film are processed, such
as by
polymerization, curing, crystallization or solidification, to yield a fully
solid sheet filter
media, retaining the openings. For example, the semisolid film may be heated
or exposed,
such as to UV light, X-rays, microwaves, electron beam or gamma radiation, to
fully
react the semisolid film and form a solid sheet filter medium.
The term "semisolid film" refers to a film that exhibits the properties of a
film
under no shear forces or low shear forces, but that yields under high shear
forces. Thus,
the semisolid film may be handled as a solid film, but may be punched through
easily by
a raised surface on a roller or rollers. In one example, a semisolid film has
an elastic
modulus from 106 dynes/cm2 to 107 dynes/cma. In another example, the rollers
are
capable of applying greater force, and the upper range of the elastic modulus
was
increased to be no greater than 101° dynes/cm2. For example, the
elastic modulus of the
semisolid film may be modified by mixing high molecular weight polymers with
lower
molecular weight polymers and optional diluents, plasticized polymers and
partially
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swollen (by solvents) polymers. One or more of these constituents may be
reactive one
with the other, such as dead polymers having pendant functional groups that
may be
crosslinked by a crosslinking constituent or agent. Also, the diluents and/or
plasticizers
may be either completely or partially polymerized or crosslinked to form
interpenetrating
polymer networks (IFNs). The semi-solid film may also comprise reactive
oligomers or
macromers or mixtures thereof and other additives, such as antioxidants, fire
retaxdants,
mold release agents, flow aids, bioactive agents, activated charcoal,
microfibrils and/or
pre-existing natural andlor synthetic fibers.
In an alternative embodiment, the semisolid filin does not need to be punched
through mechanically by rollers. Instead, opaque regions may be deposited on
one or both
surfaces of the semisolid film prior to processing the film precursors to
induce the filin to
solidify, such as by exposure to UV light. Then, the opaque regions, which did
not
solidify, may be preferentially dissolved to open holes in the sheet filter
medium. Masks
may be used to provide the opaque regions, as in standard photolithography. A
mask may
be prepared by coating a mylax film with a patterned metal. The patterned
mylar fihn then
imparts a shadow on the semisolid film during exposure of the semisolid film
to the
radiation source. For example, the radiation source may solidify the unmasked
portion of
the semisolid film while traversing on a conveyor. Alternatively, rapid laser
scanning is
another alternative for creating a pattern in the semisolid film.
In alternative embodiments, the semisolid film may be developed independently,
or semisolid films may be one or more layers in a mufti-layered filter medium.
For
example, a mufti-layered film may be exposed on opposite sides simultaneously
or
sequentially. If one of the layers is a semisolid film, then the other layers
may be
deposited thereon, as for any substrate. For example, a central semi-solid
film may be
coated with liquid precursors on either or both sides to make a laminated,
open filter
structure in fewer steps than would be required for forming each layer
independently. In
one embodiment, the pore size progresses in size from one side of a mufti-
layered film to
the other. Thus, a layer on a first surface of the mufti-layered filter has
the largest pore
size, while the layer on the opposite surface has the smallest. Intermediate
layers
transitions from the largest to the smallest pore size in sequence from the
first surface to
the opposite surface, which can be used as a progressive microsieve that
filters out the
largest particles first. Such mufti-layered filters may have the pores
substantially aligned
or may have the pores offset, from one layer to the next, to increase the
toriuosity of the
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airflow path. A tortuous pathway may increase the efficiency of particle
capture, but also
increases the pressure drop across the filter. It is within the ordinary skill
in the art to
determine the arrangement that is optimal for a particular application.
In yet another method, ins jet printing is used to deposit a pattern of fiber
precursors on a surface, such as a flat surface or a curved surface. In one
embodiment,
multiple nozzles are simultaneously employed to rapidly deposit the precursor
component
on the surface. Any conceivable pattern may be printed in this manner, and
different
nozzles may deposit different precursors, which may be reacting or non-
reacting. In
addition, the method may cure the precursors, such as by using heat, light or
addition of a
catalyst, to fuse, polymerize or further polymerize the precursors during ink
j et printing
and/or after inlc jet printing is completed. In one embodiment, the surface is
a non-stick
surface, and the cured sheet filter medium is removed by simply peeling the
sheet filter
medium from the surface. For example, a Teflon~ surface, i.e.
polytetrafluoroethylene
(PTFE) or a substrate coated with a non-stick coating may be used.
In alternative embodiments, the nozzles are oscillated or moved in a
predetermined pattern to cause continuous fibrous material ejected from the
nozzles to be
deposited on the surface in a pattern. For example, the pattern may force the
strands to
cross over one another, fusing the strands together. Alternatively, the
strands may be
deposited in lines and then the table beneath the strands may be rotated, and
a second pass
made to cause a second pattern of lines to cross over the first pattern of
lines. The strands
may then be fused using heat or by any other method, such as crosslinl~ing by
photopolymerization. Alternatively, the process may be a continuous process
that uses a
conveyor belt to move the deposition surface from one set of nozzles to a next
set of
nozzles. Different precursor materials may be deposited by different nozzles
in layers
(and non-adjacent layers) that may be fused together. In one example,
crisscrossed
junction points are formed with some junctions being joined and other not
being joined,
allowing a three-dimensional structure to be formed by expanding a two-
dimensional
sheet filter into a third dimension. Very intricate two- and three-dimensional
patterns may
be formed by selectively fusing certain points between alternating layers of a
filter media
with multiple layers deposited by nozzles using this method.
~ Teflon is a registered trademark of the Dupont Corp.
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In another method, laser printing technology is used to deposit precursors on
a
surface, which are then polymerized. The precursors are deposited using plates
or rollers
that impart a static charge to a toner to form a pattern that is then
transferred to a transfer
surface. The toner patter may contain the precursors or may be used to form
the
precursors into patterns as previously related.
In yet another embodiment, the feed also contains a blowing or foaming agent.
Any conventional blowing or foaming agent may be incorporated into the fibers,
themselves. Once a lace-lilce structure is formed, the fibrous strands are
expanded,
creating hollow fibers, porous fibers or a combination of the two.
The methods and advanced filter media presented herein provide for engineered
micro-filter and sub-micron filter production having a specific pore size,
shape and
distribution, reducing pressure drop and increasing capture efficiency
compared to
conventional filters with randomly oriented fibers. Also, conventional
filters, with
random distribution of fibers, require more material to perform the same level
of filtering,
increasing the filter cost.
One or more of these filter media may be coated with a neutralizing component
that is capable of neutralizing one or more harmful substances, such as
harmful
particulates, aerosols, gases, vapors and pathogens.
Advanced filters of the present invention may be used in aaly and all
filtration
systems, such as HVAC, surgical masks, protective masks, vacuum clea~ier bags,
sieves,
medical isolation, clean rooms, transportation and industrial applications.
It is not possible to list all of the combinations that may be used to form
sheet
filter media. Although the present invention has been described in relation to
particular
embodiments and examples thereof, many other variations and modifications and
other
uses will be apparent to those skilled in the art. These are intended to be
included within
the scope of the claimed invention; therefore, it is preferred that the
present invention be
limited not by the specific disclosure herein, but only by the issued claims.
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Table I: Relative Size of Harmful Substances
Harmful Substance Size (microns) Capture Mechanism
Anthrax 1-10 interception
Virus sub-micron diffusion/interaction
Chemicals Aerosol: 1-10 interception
Gas: 0.005 diffusion/adsorption
