U.S. patent application number 16/941364 was filed with the patent office on 2020-11-26 for filter media ribbons with nanofibers formed thereon.
The applicant listed for this patent is Ultra Small Fibers, LLC. Invention is credited to Collin Anderson, William H. Hofmeister, Robert A. Van Wyk.
Application Number | 20200368656 16/941364 |
Document ID | / |
Family ID | 1000004986434 |
Filed Date | 2020-11-26 |
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United States Patent
Application |
20200368656 |
Kind Code |
A1 |
Hofmeister; William H. ; et
al. |
November 26, 2020 |
FILTER MEDIA RIBBONS WITH NANOFIBERS FORMED THEREON
Abstract
Nanofiber filter media ribbons are flexible elongate strips of
polymeric material having a surface on which is formed an array of
nanofibers. Ribbons are formable into woven or non-woven mats. The
array of nanofibers can be configured to filter a predetermined
contaminant from a fluid stream passing through the mats. Filter
ribbons are formable by applying a moldable polymer to a first
angular location of a rotating cylindrical roll having an array of
nanoholes formed in a circumferential surface thereof so that the
polymer covers the surface of the roll and infiltrates the
nanoholes; cooling the polymer while rotating the polymer-covered
roll to a second angular position; and removing the cooled polymer
from the roll as an elongate film having an array of nanofibers
formed on a surface thereof by the polymer that infiltrated the
nanoholes.
Inventors: |
Hofmeister; William H.;
(Nashville, TN) ; Van Wyk; Robert A.; (St.
Petersburg, FL) ; Anderson; Collin; (Arlington
Heights, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ultra Small Fibers, LLC |
Wartrace |
TN |
US |
|
|
Family ID: |
1000004986434 |
Appl. No.: |
16/941364 |
Filed: |
July 28, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16875067 |
May 15, 2020 |
|
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16941364 |
|
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62852970 |
May 24, 2019 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 39/1623 20130101;
B01D 2239/1233 20130101; B01D 2239/1208 20130101; D04H 3/016
20130101; B01D 2239/0618 20130101 |
International
Class: |
B01D 39/16 20060101
B01D039/16; D04H 3/016 20060101 D04H003/016 |
Claims
1. A method for making polymeric objects having a surface on which
is formed an array of nanofibers, the method comprising: providing
a first cylindrical roll with an array of nanoholes formed in a
circumferential surface thereof; providing a source of moldable
polymer; rotating the first cylindrical roll; applying the moldable
polymer to the rotating first cylindrical roll at a first angular
location so that the moldable polymer covers at least a portion of
the circumferential surface of the rotating first cylindrical roll
and infiltrates at least a portion of the nanoholes; cooling the
moldable polymer while rotating the covered first cylindrical roll
to a second angular position; and removing the cooled polymer
applied to the first cylindrical roll from the first cylindrical
roll as an elongate film; wherein the polymer that infiltrated the
nanoholes forms the array of nanofibers on a surface of the
elongate film.
2. The method of claim 1, wherein the first cylindrical roll is
maintained at a temperature that causes the moldable polymer to
solidify in the nanoholes along with a portion of the moldable
polymer covering the circumferential surface of the first
cylindrical roll after a predetermined angular rotation of the
first cylindrical roll.
3. The method of claim 1, further comprising a second cylindrical
roll with an axis parallel to the first cylindrical roll, wherein
the second cylindrical roll is positioned adjacent to the first
cylindrical roll such that after a predetermined angular rotation
of the first cylindrical roll, the polymer covering the first
cylindrical roll is compressed in a space between the
circumferential surface of the first cylindrical roll and a
circumferential surface of the second cylindrical roll.
4. The method of claim 3, wherein the second cylindrical roll is
maintained at a temperature that causes the moldable polymer to
solidify in the nanoholes along with a portion of the moldable
polymer covering the circumferential surface of the first
cylindrical roll after the polymer passes through the space.
5. The method of claim 3, further comprising: providing a source of
polymer film; and feeding the polymer film into the space between
the circumferential surface of the first cylindrical roll and the
circumferential surface of the second cylindrical roll while
rotating the covered first cylindrical roll to the second angular
position so that the polymer film is compressed in the space with
the moldable polymer covering the first cylindrical roll to join
the polymer film to at least a portion of the moldable polymer
covering the circumferential surface of the first cylindrical roll;
wherein removing the cooled polymer from the first cylindrical roll
as an elongate film is removing the joined polymer film and cooled
polymer from the first cylindrical roll as an elongate layered
construct after the joined polymer film and cooled polymer pass
through the space; and wherein the polymer that infiltrated the
nanoholes forms the array of nanofibers on a surface of the
elongate layered construct.
6. The method of claim 5, wherein the elongate layered construct
comprises: a first layer having a first surface on which is formed
the array of nanofibers; and a second layer fixed to a second
surface of the first layer; wherein: the first layer is formed of
the cooled polymer removed from the first cylindrical roll, and the
second layer is formed of the polymer film.
7. The method of claim 5, wherein: the moldable polymer is formed
from a first polymeric material; and the polymer film is formed
from a second polymeric material different from the first polymeric
material.
8. The method of claim 3, wherein: the moldable polymer is a first
polymer film; and the method further comprises heating the polymer
film to a temperature that causes the heated polymer film covering
the first cylindrical roll to flow into at least a portion of the
nanoholes when the heated polymer film is compressed in the space
between the circumferential surface of the first cylindrical roll
and a circumferential surface of the second cylindrical roll.
9. The method of claim 8, further comprising: providing a source of
second polymer film; and feeding the second polymer film into the
space between the circumferential surface of the first cylindrical
roll and the circumferential surface of the second cylindrical roll
while rotating the covered first cylindrical roll to the second
angular position so that the first and second polymer films are
compressed in the space to join the second polymer film to at least
a portion of the first polymer film covering the circumferential
surface of the first cylindrical roll; wherein removing the cooled
polymer from the first cylindrical roll as an elongate film is
removing the joined second polymer film and cooled first polymer
film from the first cylindrical roll as an elongate layered
construct after the first and second polymer films pass through the
space; and wherein the polymer that infiltrated the nanoholes forms
the array of nanofibers on a surface of the elongate layered
construct.
10. The method of claim 1, further comprising: providing a means
for slitting the cooled polymer covering the first cylindrical
roll; forming a plurality of slits in the cooled polymer covering
the first cylindrical roll before removing the cooled polymer from
the first cylindrical roll; and removing the cooled polymer from
the first cylindrical roll as a plurality of elongate ribbons;
wherein the polymer that infiltrated the nanoholes forms the array
of nanofibers on a surface of each elongate ribbon of the
plurality.
11. The method of claim 1, further comprising: forming a plurality
of slits in the elongate film; and separating the elongate film
into a plurality of ribbons.
12. A method for making polymeric objects having a surface on which
is formed an array of nanofibers, the method comprising: providing
a rotating mold with an array of nanoholes formed in a
circumferential surface thereof; applying a flowable polymer to the
rotating mold at a first angular location so that the flowable
polymer coats at least a portion of the circumferential surface of
the rotating mold and infiltrates at least a portion of the
nanoholes; cooling the flowable polymer while the coated rotating
mold rotates to a second angular position; and removing the cooled
polymer from the rotating mold as an elongate film; wherein the
polymer that infiltrated the nanoholes forms the array of
nanofibers on a surface of the elongate film.
13. The method of claim 12, wherein the rotating mold is maintained
at a temperature that causes the flowable polymer to solidify in
the nanoholes along with a portion of the flowable polymer coating
the circumferential surface of the rotating mold after a
predetermined angular rotation of the mold.
14. The method of claim 12, further comprising a compressing
element positioned adjacent to the rotating mold such that after a
predetermined angular rotation of the rotating mold, the polymer
coating the mold is compressed in a space between the
circumferential surface of mold and a surface of the compressing
element.
15. The method of claim 14, wherein the compressing element is
maintained at a temperature that causes the flowable polymer to
solidify in the nanoholes along with a portion of the polymer
coating the circumferential surface of the rotating mold after the
polymer passes through the space.
16. A method for making polymeric objects having a surface on which
is formed an array of nanofibers, the method comprising: providing
a mold with an array of nanoholes formed in a surface thereof;
applying a malleable polymer to the mold so that the malleable
polymer covers at least a portion of the surface of the mold and
infiltrates at least a portion of the nanoholes; compressing the
malleable polymer against the surface of the mold with a
compressing element maintained at a temperature that causes the
malleable polymer to solidify in the nanoholes along with a portion
of the polymer covering the surface of the mold; removing the
solidified polymer from the mold as a film; wherein the polymer
that infiltrated the nanoholes forms the array of nanofibers on a
surface of the film.
17-29. (canceled)
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 62/852,970, filed May 24, 2019 and entitled
"Unitary Multiscale Filter Media," the entire disclosure of which
is hereby incorporated by reference.
[0002] A portion of the disclosure of this patent document contains
material that is subject to copyright protection. The copyright
owner has no objection to the reproduction of the patent document
or the patent disclosure, as it appears in the U.S. Patent and
Trademark Office patent file or records, but otherwise reserves all
copyright rights whatsoever.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0003] Not Applicable
REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING
APPENDIX
[0004] Not Applicable
BACKGROUND OF THE INVENTION
[0005] The present disclosure relates generally to filter media and
filter devices, and more specifically to filter media and filter
devices which combine user-defined arrays of nanofibers and
elongate ribbon-like elements to create filter media that provide
the benefits of nanofibers in a form that can be utilized like in
conventional filter media.
[0006] Fibrous filter media are used in various types of filter
devices to trap large and small particles in liquid and gas
streams. Such filter media are typically formed from multiple
layers of coarse and fine fibers extending parallel to an upstream
surface of the filter media. An outer layer of coarse fibers forms
a bulk filtration layer for filtering of larger particles, while an
inner or underlying layer of fine fibers provides filtering of
small particles. Fine fibers are often provided in a thin layer
laid down on a supporting permeable substrate or used with one or
more permeable protective layers to obtain a variety of benefits,
including increased efficiency, reduced initial pressure drop,
cleanability, reduced filter media thickness, and to provide a
selectively impermeable barrier to various liquids, such as water.
However, prior approaches have several inherent disadvantages,
including the need for a supporting substrate, a risk of
delamination of the fine fiber layer from the substrate, more rapid
loading of the filter by captured particles, the alignment of fine
fibers parallel to the media face surface, and an inability to
control spacings between fine fibers.
[0007] In addition to filtering mechanisms, on the molecular level,
fibrous materials also trap contaminants with electrostatic forces,
including ionic bonding, hydrogen bonding, and Van der Waals
forces. These electrostatic interactions occur on the fiber
surface. Because these interactions are known to increase
non-linearly at sub-micron length (diameter) scales, functional
improvement in fibrous filter media is largely based on minimizing
denier (linear mass density or fiber diameter). Although the
production of filter media comprising very fine fibers having a
high surface-to-volume ratio, such as microfibers and nanofibers,
has recently been emphasized in the industry, processing
limitations associated with traditional methods of producing such
fibers limit the utility of these materials in filtration
applications.
[0008] The benefits realized through the use of nanofibers for
filtering contaminants from a fluid stream are well known, and the
technology is widely used. As currently commonly practiced, a thin
layer of electrospun or melt-blown nanofibers is deposited on a
porous substrate. Nanofibers deposited using these processes form a
non-woven mat that lacks physical strength. This makes handling of
prior art nanofiber mats without a suitable permeable substrate
impractically difficult for filter manufacturing. The unique
filtering properties of a nanofiber mat derive from the diameter of
the nanofibers, and these properties are currently only obtainable
with fibers formed into these non-woven constructs. Filter media
formed of micro-fibers are easily handled during filter
manufacturing, but because of their larger diameter of the fibers
lack the enhanced filtering abilities of nanofibers. Accordingly,
to achieve these enhanced properties in a filter, nanofibers are
commonly deposited onto microfiber media in the manner previously
described.
[0009] Nanofibers for prior art filter applications are commonly
made by electrospinning, a method that requires the use of high
voltages and a flowing polymer solution containing solvents that
evaporate during production. Ensor, et al. in U.S. Pat. No.
8,652,229 describe methods for electrospinning nanofibers and
forming filter elements therefrom. In the methods described, the
electrospinning process requires electrical potentials in the 25 kV
to 30 kV range and the close control of several process parameters.
The rates of nanofiber production are low in the examples given. It
is not an environmentally friendly process due to the solvents
required. Electrospinning produces an interconnected web (or mat)
of continuous small fibers with length to diameter ratios generally
1,000,000:1 or greater.
[0010] When forming nanofibers by electrospinning, the nanofiber
materials are limited to polymers that can be mixed with a solvent
to achieve the properties required for the process.
[0011] In electrospinning the fibers of a closely controlled
diameter are deposited onto a substrate. The substrate may be a
flat plate oriented normal to the axis of the origin of the
solution stream. Alternatively, the substrate may be a rotating
element with a cylindrical, conical or other radially symmetric
shape, the axis of rotation being perpendicular to the axis of the
solution stream. Or the substrate may be a rotating disc with the
axis of rotation parallel to the axis of the solution stream. Each
of these substrate forms allow the forming of fiber mats configured
to achieve specific design objectives through optimizing the
deposition pattern of the fibers. If translation of the substrate
in a plane normal to the solution stream is added to any of the
substrate configurations, the deposited fiber may be given a
directionality. Indeed, the fiber mat may be formed with a
predetermined pattern to achieve design objectives for a given
application. Microfiber or nanofiber mats with a particular
preferential orientation of the fibers are frequently referred to
as "ordered", and in some cases an "ordered matrix", or "ordered
construct". The "order" to which this refers, then, is that the
elongate continuous fibers forming the mat do not have a random
directionality, but rather have a greater portion oriented parallel
to a first axis than to a second axis. This is a two-dimensional
effect only since the fiber mat forms a thin sheet, frequently
membrane-like.
[0012] Prior art nanofiber mats cannot withstand tensile loading.
Because nanofibers forming the mat have very low structural
strength, increasing the number nanofibers does not appreciably
increase the thickness of the mat, but simply creates a denser mat
with decreased porosity. When the nanofiber mat manufactured by the
electrospinning method is used to form an air filter, nanofibers
(fibers) can be easily clogged (that is, packing can easily occur),
resulting in a decrease in air permeability and an increase in
pressure loss. Since clogging can easily occur, there have been
problems in that the pressure loss may easily increase and the
service life of an air filter may be shortened.
[0013] To address these drawbacks, Konishi, in US Application
2018/0353883 discloses an alternate method (not electrospinning)
for forming a non-woven mat of nanofibers. Konishi's method forms a
mat of fibers that have a range of diameters that average less than
one micron, but also that also contains fibers of larger diameters
so as to give the mat increased thickness and spacing between the
nanofibers. The number of fibers having fiber diameters ranging
from 2 times up to 10 times the average fiber diameter of the
constituent fibers is in a range of 2 to 20% of a total number of
the constituent fibers. The fiber mat is deposited onto a non-woven
fabric using a complex process. Although the thickness of the mat
is somewhat increased, the long continuous fibers are randomly
deposited in a two-dimensional construct similar to electrospun
mats.
[0014] Microfibers for filters and other applications may be made
by melt blowing, a fiber making process in which melted polymer is
extruded through a plurality of small orifices surrounded by
streams of a high velocity gas. A plurality of randomly oriented
fibers are deposited onto a substrate so as to form a non-woven mat
or fabric. The process does not require the use of solvents or high
voltages, and the fiber deposition rates can be orders of magnitude
greater than those possible by electrospinning. Melt blown fibers
are generally in the range of two to five microns with a wide
diameter distribution. Because the fibers are not drawn to a
substrate by an electrostatic charge as in electrospinning, fiber
mats formed by melt blowing are not membrane-like, but rather have
fibers that are spaced one from another in the direction parallel
to the blowing direction. The fibers are long and continuous with a
random orientation. In some applications the mat is subsequently
compressed to form a non-woven fabric. Melt blowing nanofibers is
difficult since extremely small orifices are required and the
molten plastic must flow through these orifices and remain in fiber
form as they travel to the substrate. Surface tension in the molten
fiber tends to cause the material to become droplets rather than
fibers so as to reduce the surface energy. Accordingly, the
polymers that can be successfully melt blown into nanofibers is
limited and the process has not yet been scaled up sufficiently for
commercial use. The process remains an efficient method for forming
microfiber mats and non-woven fabrics for filters and other
applications.
[0015] In another approach, increasing the nanofiber content of a
filter is accomplished through the use of a stratified filter
construction with layers of nanofibers interspersed between
microfiber substrate layers.
[0016] Whether a nanofiber mat is formed by electrospinning,
Konshi's method, or another means, the mat is a thin construct,
frequently membrane-like. Because of this, the mat is oriented
essentially normal to the flow stream direction. The density of the
mat is limited by the backpressure that the filtering process can
tolerate.
[0017] The beneficial effects of including nanofibers in a filter
may be temporarily enhanced by electrostatically charging the
nanofibers. For instance, it has been demonstrated that charging
nanofiber mats interspersed between insulating separating permeable
layers causes a significant increase in the filter efficiency. This
is described in detail in US application 2019/0314746 by Leung.
However, the applied electrostatic charge degrades over time so
that filters of this type have a finite shelf life, making them
impractical for some applications.
[0018] Polymeric materials have an inherent electrostatic charge
that creates an attractive force, the force at any given point on a
surface being inversely dependent on the radius of curvature of the
external surface at that point. When the radius of curvature is
large the electrostatic attractive force is weak. As the radius is
decreased the attractive force increases, a factor exploited in
nanofiber filter media. The small diameter of the nanofibers
results in an attractive force that is orders of magnitude greater
than that of microfibers allowing nanofibers to draw contaminant
particles with greater force for removal from a fluid stream. This
electrostatic charge is intrinsic to the material and does not
degrade in the manner of an applied electrostatic charge.
[0019] Filters for use in personal protective equipment (PPE) may
also benefit from the inclusion of nanofibers. Specifically, face
masks that form a tight seal to the face, also referred to as
respirators, are commonly used to prevent contaminants from
entering the airway of the wearer. These devices reduce the wearers
exposure to particles including small particle aerosols and large
droplets. Face masks of this type must remove contaminants while
minimizing the pressure drop across the filter element. The
filtering element forming the mask my also be pliable so as to
allow the mask to form a seal with the face of the wearer.
Typically a wearable filter of this type will have a permeable
hydrophobic outer protective layer, a coarse filter media layer for
removing large particulate, a fine filter medial layer for removing
smaller particulate, and an inner soft permeable fabric layer for
contacting the face of the wearer.
[0020] Leung in U.S. Pat. No. 8,303,693 teaches a face mask that
incorporates a filtration medium a fine filter layer having a
plurality of nanofibers and a coarse filter layer having a
plurality of microfibers attached to the fine filter layer. Flow
passes through the coarse filter to the fine filter layer. The
polymer nanofibers in the fine filter layer may be obtained in a
variety of ways including electrospinning or by melt-blowing.
Accordingly, the nanofibers are long and continuous with a random
orientation. The thickness of this nanofiber layer may have a
thickness of about 0.01 to about 0.2 millimeters. Because the
nanofiber fine filter layer is a thin layer, the layer may tend to
clog easily and increase the resistance to air flow. The coarse and
fine layers together form a "well-bonded laminate structure", the
layers being bonded one to another. Indeed, it is necessary for the
nanofiber layer to be bonded to the microfiber layer for handling
purposes during manufacture of a filter since the nanofiber layer
lacks physical strength. In one embodiment the nanofibers are
deposited onto the microfiber layer during electrospinning or melt
blowing so that they adhere to the microfiber layer. In another
embodiment the nanofibers are deposited onto a liquid in which the
microfibers are submerged so that the nanofibers are not adhered to
the microfibers. When forming of a nanofiber layer is complete, the
liquid is removed leaving the nanofiber layer atop the microfiber
layer but not adhered thereto. The nanofiber layer and microfiber
layer are then compressed mechanically together with a small amount
of compatible adhesive to form a rigid structure. The manner in
which Leung's layered filter assembly is formed illustrates the
difficulty and limitations of forming filter assemblies
incorporating electrospun and melt blown nanofibers due to their
mechanical properties. As with other applications that incorporate
electrospun nanofibers, the fiber making process is difficult to
scale up and is environmentally undesirable due to the solvents
used. The integration of nanofibers into a mask assembly is
similarly difficult.
[0021] Hofmeister, et al. in U.S. Pat. No. 10,159,926 teaches media
and devices for filtering or separating a contaminant from a fluid
liquid or gas stream. The Hofmeister devices incorporate flow
passages formed by layered laminas comprising tunable topographies
of user-defined arrays of nanofibers and, optionally, nanoholes.
These tunable nanofiber topographies selectively remove
contaminants from the fluid stream as it flows through spaces
between adjacent laminas, parallel to the surface of the laminas,
with at least one of these surfaces having nanofibers formed
thereon. Contaminants are drawn to the nanofibers by electrostatic
forces in the manner previously described. Nanofiber filters
constructed in accordance with the Hofmeister patent can be tuned
to remove specific contaminants such as pathogens, chemical
contaminates, biological agents, and toxic or reactive compounds
from a fluid to be filtered by selecting a suitable nanofiber
diameter, height, distance between nanofibers, interlaminar gap and
material.
[0022] The Hofmeister filter construction requires a rigid housing
to maintain the orientation and alignment of the laminas making up
the filter so that a continuous flow path is created between an
inlet and outlet formed in the housing, the flow passing through
interlaminar spaces formed therein.
[0023] Accordingly applications for the Hofmeister filter with its
tuned topography are limited to those in which the fluid stream is
directed through spaces formed between adjacent, aligned laminas,
the alignment being maintained by a rigid housing structure.
Because of this, the benefits of filter elements comprising a tuned
topography formed of nanofiber arrays cannot be realized in
filtering devices that do not/cannot include a rigid housing and
flow between adjacent parallel laminas.
[0024] There is a need for filter media that exploit the inherent
electrostatic properties of nanofibers in optimized configurations
that do not require a rigid housing and laminar construction. Such
media are the subject of this invention.
[0025] Accordingly, it is an object of the present invention to
provide nanofiber filter media that can withstand tensile
loading.
[0026] It is also an object of the present invention to provide
nanofiber filter media that achieve high collection efficiency and
reduced clogging (packing) between fibers.
[0027] It is also an object of the present invention to provide
nanofiber filter media that does not require deposition on a
substrate during manufacture.
[0028] It is also an object of the present invention to provide
nanofiber filter media wherein the nanofibers are configured to
optimally exploit the electrostatic properties of the
nanofibers.
[0029] It is also an object of the present invention to provide
nanofiber filter media wherein the nanofibers cannot be easily
expelled from the filter media.
[0030] It is also an object of the present invention to provide
nanofiber filter media wherein the nanofibers are integrated in a
heterostructure containing nanofibers and support.
[0031] It is further an object of the present invention to provide
nanofiber filter media at lower cost than current nanofiber
media.
[0032] It is further an object of the present invention to provide
nanofiber filter media that may be produced without the need for
high voltages or environmentally detrimental solvents.
[0033] It is also an object of this invention to provide a method
for increasing the wettability of a fluid on a surface of filter
media through the formation of nanofibers on one or more surfaces
of the media.
[0034] It is further an object of this invention to provide a
method for decreasing the wettability of a fluid on a surface of
filter media through the formation of nanofibers on one or more
surfaces of the media.
[0035] It is an object of this invention to provide a method of
selectively increasing the wettability of a surface of a filter
media for a first flow stream component while decreasing the
wettability for a second flow stream component.
[0036] It is finally an object of this invention to provide
nanofiber filter media that can remove biological contaminants
including viruses from an air stream
BRIEF SUMMARY
[0037] These and other objects are achieved in devices and methods
of the present invention which addresses filter media, filtering
devices formed therefrom, and methods for their use wherein the
filter media is formed of flexible, elongate ribbon-like polymeric
elements having arrays of nanofibers formed thereon. These ribbon
elements and ribbon segments may be formed by cutting, slicing,
chopping, or slitting elongate film elements on which are formed
nanofiber arrays. Ribbons so formed have a planar portion of
predetermined thickness and width that may be formed to other
non-planar shapes in subsequent processing. These media ribbons may
also be formed by embossing of the nanofiber arrays on monofilament
fibers as well as on woven and non-woven fiber assemblies. Devices
and methods of the present invention are not limited by the method
of manufacture of the elongate ribbon-like elements.
[0038] The elongate ribbons of filter media of the present
invention are formed of a suitable polymeric film, have a flexible
planar portion of predetermined thickness and width, and have an
array of nanofibers formed on at least one surface of the film. In
a preferred embodiment the nanofibers are arranged in rows spaced a
first distance apart, with the nanofibers within each row spaced a
second distance apart. In some embodiments the first and second
distances are equal. In others they are not. The diameter of each
nanofiber generally decreases along the nanofiber's length from a
first diameter at its base, and the lengths of the fibers in an
array fall within a predetermined range. The form of a fiber is
largely determined by the ratio of the length of the fiber to its
diameter. At low ratios the fiber may have a post-like appearance,
while at high ratios the fiber may be tendrilous. Between these
extremes is a continuum of nanofiber configurations that share the
common characteristic of decreasing diameter over their finite
length. Because the electrostatic force at a point on a surface is
inversely related to the radius of curvature of the surface at that
point, the electrostatic force on a nanofiber of filter media of
the present invention is not constant along its length. The
electrostatic force increases with the distal reduction in
diameter, reaching its maximum at the fiber's distal end. In
certain embodiments the ends of the nanofibers are configured to
further enhance the electrostatic potential. The electrostatic
force of nanofibers formed on ribbon media of the present invention
has maximal intensity at the distal portions of the nanofibers--the
portion that is most exposed to the fluid stream. This
concentration results in much higher attractive forces to
contaminants in the fluid stream than the uniform-diameter,
continuous fibers of non-woven nanofiber mats previously herein
described and currently in use in filter applications. Because of
this, nanofiber arrays formed on filter ribbons of the present
invention are able to draw contaminants from a flow stream with
higher field gradients than other, prior art, nanofiber filter
elements.
[0039] As with suitably constructed prior art filters, an
electrostatic charge may be imparted to the filter media of the
present invention to increase the attractive force of the nanofiber
arrays formed on ribbons. In certain embodiments, filter ribbons of
the present invention are formed from a polymer or polymer blend
with suitable electret properties. Among these materials are
polypropylene, poly(phenylene ether) (PPE) and polystyrene (PS) and
others. In certain embodiments these ribbons have a lamellar
construction wherein a first layer on which are formed nanofiber
arrays of the present invention is bonded to a second layer with
optimal physical and/or electrical properties, the first layer
being formed of a suitable electret material. Charging of the media
may be accomplished by corona discharge, triboelectrification,
polarization, induction, or another suitable method. The imparted
electrostatic charge may be dissipated by particle loading, and/or
by quiescent or thermal stimulation decay.
[0040] In certain embodiments filter media ribbons of the present
invention are formed of an antimicrobial plastic. One such
material, MICROBAN.RTM. by Microban, Inc. (Huntersville, N.C.) is a
synthetic polymer material containing an integrated active
ingredient which makes it effective against microbial growth. In
certain embodiments these ribbons have a lamellar construction
wherein a first a layer on which are formed nanofiber arrays of the
present invention is bonded to a second layer with optimal physical
properties, the first layer being formed of a antimicrobial
plastic. Antimicrobial agents may be blended with polymers with
optimal properties for forming nanofiber arrays in methods herein
described to create filter ribbons of the present invention that
not only have the ability to efficiently remove microbes from a
fluid stream, but also to kill those microbes.
[0041] The non-random placement of nanofiber tips in a nanofiber
array represents a significant enhancement over nanofiber
structures produced by other methods, such as electrospinning,
because each fiber forming an array of nanofibers described herein
has an independent "end" or "tip." The "ends" or "tips" of the
nanofibers have stronger field gradients than the body of the
fibers because gradients are enhanced with curvature and the
curvature is highest at the tip. Thus, the use in filter devices of
nanofiber arrays having millions of tips per square centimeter of
lamina surface preserves and enhances the local fiber field
gradient far better than traditional fibrous filter media and
devices which rely on layered mats of fibers laid down on a
substrate.
[0042] Because the electrostatic forces are generated by nanofibers
formed on the surface of media ribbons of the present invention,
the width and thickness of the ribbon on which the nanofiber arrays
are formed may be selected based on physical strength, handling,
flow or other factors since it does not affect the electrostatic
properties of the nanofibers formed thereon. Because the ribbons
have appreciable physical strength, structures formed of them may
be handled independent of a substrate, and indeed, make practical
woven and non-woven mats that may be incorporated in a wide range
of filter configurations. Non-woven mats formed of the ribbons may
be integrated into a single assembly by bonding of the ribbons one
to another using a suitable bonding method. For applications in
which the filter must flexibly conform to an external surface, a
non-woven mat of bonded or loose ribbons may be positioned between
first and second porous or permeable sheet materials and secured
there by fastening means between the porous sheets. The sheets may
be joined by stitching, needling or other mechanical means, thermal
bonding, chemical bonding or other suitable joining method. In a
preferred embodiment a quilted assembly is formed by the permeable
sheets and the nanofiber mat positioned therebetween, stitching
serving to maintain the positions of the elements. In a preferred
embodiment one or both of the permeable sheets are formed of filter
media. In a preferred embodiment one or both sheets themselves
incorporate nanofiber arrays so as to impart specific wettability
properties. For instance, a permeable sheet may be nominally wetted
by a first selected liquid or vapor while nominally not wetted by a
second selected liquid or vapor. Filter media ribbons of the
present invention may be weaved to create flexible filter
structures. Individual ribbons may be weaved to form the structure,
or the ribbons may be formed into a yarn prior to weaving. The
tightness of the yarn and of the woven structure may be optimized
to achieve desired flow characteristics.
[0043] Elongate ribbons or the present invention with the
nanofibers formed thereon may be subsequently processed in the same
manner as other conventional fibrous media. Because of this,
nanofiber filter media of the present invention may be formed into
or integrated into filter elements at much lower cost and with much
greater design flexibility than prior art, conventionally formed
nanofibers made by electrospinning or other similar process.
[0044] While prior art nanofiber mats formed by electrospinning or
other methods form a thin, membrane-like structure, mats formed of
filter ribbons of the present invention are three-dimensional
constructs. Ribbons may be piled on top of other ribbons to create
mats of a desired thickness, or may fill a cavity through which the
fluid stream flows. Mats formed of filter media ribbons of the
present invention are flexible and resilient. Their pliable nature
and low resistance to fluid flow make mats of the present invention
ideally suited for use in personal protective filtering devices
used in medical and industrial applications.
[0045] A respirator mask of the present invention has a layered
filter construct includes filter ribbons of the present invention
and benefits from the unique properties of the ribbons. A first,
external (distal) layer is a thin woven or non-woven matt (fabric)
formed of filter ribbons of the present invention, the ribbons
being made of a hydrophobic polymeric material. On a surface of
each of these ribbons are formed arrays of nanofibers configured to
optimally increase the hydrophobic characteristics of this exterior
fabric. Proximal to this first layer is a second layer formed of
microfibers configured to remove large particulate. Optionally this
second layer may also contain nanofiber bearing filter ribbons of
the present invention with the nanofiber arrays configured to
optimally remove contaminants of a first composition or size.
Proximal to this second layer is positioned a third layer. This
layer is a non-woven mat formed of nanofiber bearing filter ribbons
of the present invention. Because the ribbons from which this layer
are formed have structural strength, the non-woven mat has a
predetermined thickness and flow characteristics selected for
optimal removal of contaminants while preserving airflow at low
pressure drop and resistance to clogging. The arrays of nanofibers
on these ribbons are optimally configured for the removal of small
particles. In certain embodiments nanofiber arrays of ribbons
forming this third layer may be configured to preferentially remove
specific contaminants. Indeed, additional layers of ribbon mats of
the present invention may be positioned proximal to this third
layer, the nanofiber arrays of each layer being optimized to remove
specific contaminants. Proximal to the previously described filter
layers is a permeable fabric, woven or non-woven that may, in some
embodiments, be comfortably pressed against the face of the wearer.
In production, the layers forming the filter assembly may be
produced as continuous sheets of material, laid up in the proper
order, and maintained in their relative position. Elements of the
construct may be fastened together in selected locations thermally,
by a glue or solvent bonding, by stitching, or by needle punch, a
joining method for non-woven fabrics. Because nanofibers of the
present invention are integrally formed on the surface of ribbons
of the present invention, the nanofibers cannot become loose and be
inhaled by the wearer as is possible with respirators made with
prior art filter assemblies.
[0046] In certain embodiments the film portion of nanofiber media
ribbons of the present invention remain smoothly, flexibly planar
or curvilinear depending on forces applied thereto. In other
embodiments the film portion may be crinkled, that is, may have
wrinkles or ripples formed therein so as create flow spaces between
ribbons when they are assembled into a woven or non-woven mat.
Alternatively, a ribbon may be twisted so as to ensure that there
are flow spaces between adjacent ribbons in a mat. While heretofore
nanofiber media have been described with reference to elongate
ribbons, in certain embodiments, the ribbons are chopped into short
segments prior to forming a bed of loose or bonded ribbons for
integration into a filter assembly.
[0047] The orientation of media ribbons of the present invention
relative to the fluid stream in a filter assembly may be random or
may have a degree of preferential orientation. That is, the ribbon
surfaces with nanofiber arrays formed thereon may be randomly
presented to the fluid flow, or may be oriented so that
preferentially the surfaces primarily face the oncoming flow, or
are primarily oriented parallel to the flow direction.
[0048] Filter media ribbons of the present invention with their
nanofiber arrays are formed without the use of solvents or high
voltage. Specifically, nanofiber arrays of the present invention
are formed in a casting process in which a suitable polymer heated
to a temperature sufficient to allow flow, is extruded onto a first
surface of a mold with an array of nanoholes formed therein, and
subsequently flows into the nanoholes of the mold. A surface of a
second compressing or quenching element may be used. Subsequently,
the polymeric material is cooled sufficiently so that when the
compressing element is removed, the polymer with the attached
molded nanofibers can be stripped from the mold surface. The result
is a planar polymeric film portion with an array of nanofibers
integrally formed on a first surface thereof, the form of the
nanofiber array being complementary to nanohole array in the mold.
The first surfaces of the mold and compressing element may be
planar with the polymeric material introduced therebetween as a
film prior to heating and material flow into the mold nanoholes.
Alternatively, the mold and second element may be rotating
cylinders, the polymer in molten form being introduced onto the
circumferential surface of the mold, and subsequently compressed
between the mold and the cylindrical surface of the second element.
This compression enhances the cooling the material so that it can
be subsequently peeled from the mold. Whether formed in discrete
segments as when using a mold of planar geometry, or formed as
elongate strips using the rotating cylindrical mold, the resulting
film with arrays of integral nanofibers formed thereon may be cut,
slit, chopped or otherwise divided into filter media ribbons of the
present invention.
[0049] In some embodiments the filter media ribbons are formed of a
single polymeric material. Others have a layered construction
comprising two or more polymeric materials that together give the
filter ribbons an optimal combination of filtering properties for a
given application, and physical properties for manufacture of the
ribbons. For instance, nanofiber arrays of a first material may be
laminated to a film of a second material with optimal mechanical
properties that is formed separately. In a variation of the
previously described casting method for producing film whereon are
formed arrays of nanofibers, rather than applying molten polymer to
the mold, a polymer film is applied to the mold. The film is then
heated to a temperature sufficient to melt or sufficiently soften
the material so as to allow the material to flow into nanoholes in
the mold. The surface of a compressing element may increase flow of
the material into the nanoholes. The polymer is then cooled
sufficiently to allow the film with nanofibers formed thereon to be
stripped from the mold. As with the previously described casting
process, nanofiber bearing films for fiber ribbons of the present
invention formed using this method may have a layered construction,
a second film being compressed against the first, nanofiber forming
film by the compressing element so that the films are bonded one to
another
[0050] Numerous other objects, advantages and features of the
present disclosure will be readily apparent to those of skill in
the art upon a review of the following drawings and description of
exemplary embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] Non-limiting and non-exhaustive embodiments are described
with reference to the following figures, wherein like reference
numerals refer to like parts throughout the various drawings unless
otherwise specified. In the drawings, not all reference numbers are
included in each drawing, for the sake of clarity.
[0052] FIG. 1A is a perspective schematic representation of a chill
roll casting system for making nanofiber filter elements of the
present invention.
[0053] FIG. 1B is an expanded view of the objects of FIG. 1A at
location A.
[0054] FIG. 1C is an expanded view of the objects of FIG. 1A at
location B.
[0055] FIG. 1D is a second perspective schematic representation of
the chill roll casting system of FIG. 1A.
[0056] FIG. 2A is a perspective view of a ribbon element of
nanofiber filter media of the present invention.
[0057] FIG. 2B is a plan view of the objects of FIG. 1.
[0058] FIG. 3 is a side elevational view of the objects of FIG.
1.
[0059] FIG. 4 is an expanded view of the objects of FIG. 2A at
location C.
[0060] FIG. 5 is an expanded view of the objects of FIG. 2B at
location B.
[0061] FIG. 6 is an expanded view of the objects of FIG. 3 at
location A.
[0062] FIG. 7 is an expanded view of the objects of FIG. 6 at
location D.
[0063] FIG. 8 is a side elevational view of an alternate embodiment
nanofiber of filter media of the present invention.
[0064] FIG. 9 is a side elevational view of the nanofiber of FIG. 7
depicting the electrostatic field surrounding the nanofiber.
[0065] FIG. 10 is a side elevational view of the nanofiber of FIG.
8 depicting the electrostatic field surrounding the nanofiber.
[0066] FIG. 11 is a side elevational sectional view of a planar
polymeric element wherein a first portion of a planar surface
comprises a nanofiber array of the present invention and a second
portion does not.
[0067] FIG. 12A depicts the polymeric element of FIG. 11 with
liquid applied to each portion wherein the nanofibers increase the
wettability of the surface.
[0068] FIG. 12B depicts the polymeric element of FIG. 11 with
liquid applied to each portion wherein the nanofibers decrease the
wettability of the surface.
[0069] FIG. 13 depicts a nonwoven mat of nanofiber bearing ribbon
elements of the present invention.
[0070] FIG. 14A depicts a personal filter mask formed with
nanofiber filter media of the present invention.
[0071] FIG. 14B depicts the layered structure of the mask of FIG.
14A at location A.
[0072] FIG. 15 is a sectional view of a composite filter media
assembly including elongate filter ribbons of the present
invention.
[0073] FIG. 16A is a perspective depiction of an alternate
embodiment filter ribbon of the present invention.
[0074] FIG. 16B is an expanded view of the objects of FIG. 16A at
location A.
[0075] FIG. 17A is a perspective depiction of another alternate
embodiment filter ribbon of the present invention.
[0076] FIG. 17B is an expanded view of the objects of FIG. 17A at
location A.
[0077] FIG. 18 is a perspective schematic representation of a chill
roll casting system for making nanofiber filter elements of the
present invention configured for directly producing filter ribbons
of the present invention.
[0078] FIG. 19 is an expanded view of the objects of FIG. 18 at
location A.
[0079] FIG. 20 is a perspective schematic representation of a chill
roll casting system for making nanofiber filter elements of the
present invention configured for directly producing filter ribbon
segments of the present invention.
[0080] FIG. 21 is an expanded view of the objects of FIG. 20 at
location A.
[0081] FIG. 22 is a perspective view of a filter ribbon segment of
the present invention.
[0082] FIG. 23 is a plan view of the objects of FIG. 22.
[0083] FIG. 24 is a side elevational view of the objects of FIG.
22.
[0084] FIG. 25 is a perspective view of an alternate embodiment
filter ribbon segment of the present invention.
[0085] FIG. 26 is a plan view of the objects of FIG. 25.
[0086] FIG. 27 is a side elevational view of the objects of FIG.
25.
[0087] FIG. 28 is a perspective schematic representation of an
alternate embodiment chill roll casting system for making nanofiber
filter elements of the present invention, configured for producing
nanofiber bearing film wherein a nanofiber bearing layer of a first
material is bonded to a film of a second material.
[0088] FIG. 29 is a side elevational view of the objects of FIG.
28.
[0089] FIG. 30 is an expanded view of the objects of FIG. 29 at
location B.
[0090] FIG. 31 is an expanded view of the objects of FIG. 28 at
location A.
[0091] FIG. 32 is a perspective view of an alternative method
system for producing nanofiber bearing filter ribbons of the
present invention.
[0092] FIG. 33 is a side elevational view of the objects of FIG.
32.
DETAILED DESCRIPTION
[0093] The details of one or more embodiments of the presently
disclosed subject matter are set forth in this document.
Modifications to embodiments described in this document, and other
embodiments, will be evident to those of ordinary skill in the art
after a study of the information provided herein. The information
provided in this document, and particularly the specific details of
the described exemplary embodiments, is provided primarily for
clearness of understanding and no unnecessary limitations are to be
understood therefrom. In case of conflict, the specification of
this document, including definitions, will control.
[0094] The present disclosure relates to filter media and devices
for removing a contaminant from a fluid stream. In a general
embodiment, the nanofiber filters disclosed herein are designed to
filter a substance or contaminant from a fluid stream using one or
more user-defined arrays of nanofibers, such as those described in
U.S. 2013/0216779 which is incorporated herein by reference in its
entirety.
[0095] While the terms used herein are believed to be well
understood by one of ordinary skill in the art, definitions are set
forth herein to facilitate explanation of the subject matter
disclosed herein.
[0096] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which the subject matter disclosed
herein belongs. Although any methods, devices, and materials
similar or equivalent to those described herein can be used in the
practice or testing of the presently disclosed subject matter,
representative methods, devices and materials are now
described.
[0097] The terms "a", "an", and "the" refer to "one or more" when
used in this application, including the claims. Thus, for example,
reference to "a contaminant" includes a plurality of particles of
the contaminant, and so forth. The use of the word "a" or "an" when
used in conjunction with the term "comprising" in the claims and/or
the specification may mean "one," but it is also consistent with
the meaning of "one or more," "at least one," and "one or more than
one."
[0098] All references to singular characteristics or limitations of
the present disclosure shall include the corresponding plural
characteristic(s) or limitation(s) and vice versa, unless otherwise
specified or clearly implied to the contrary by the context in
which the reference is made.
[0099] All combinations of method or process steps as used herein
can be performed in any order, unless otherwise specified or
clearly implied to the contrary by the context in which the
referenced combination is made.
[0100] The methods and devices of the present disclosure, including
components thereof, can comprise, consist of, or consist
essentially of the essential elements and limitations of the
embodiments described herein, as well as any additional or optional
components or limitations described herein or otherwise useful.
[0101] This description and appended claims include the words
"below", "above", "side", "top", "bottom", "upper", "lower",
"when", "upright", etc. to provide an orientation of embodiments of
the invention to allow for proper description of example
embodiments. The foregoing positional terms refer to the apparatus
when in an upright orientation. A person of skill in the art will
recognize that the apparatus can assume different orientations when
in use. It is also contemplated that embodiments of the invention
may be in orientations other than upright without departing from
the spirit and scope of the invention as set forth in the appended
claims. Further, it is contemplated that "above" means having an
elevation greater than, and "below" means having an elevation less
than such that one part need not be directly over or directly under
another part to be within the scope of "above" or "below" as used
herein.
[0102] The phrase "in one embodiment," as used herein does not
necessarily refer to the same embodiment, although it may.
Conditional language used herein, such as, among others, "can",
"might", "may", "e.g.," and the like, unless specifically stated
otherwise, or otherwise understood within the context as used, is
generally intended to convey that certain embodiments include,
while other embodiments do not include, certain features, elements
and/or states.
[0103] Unless otherwise indicated, all numbers expressing physical
dimensions, quantities of ingredients, properties such as reaction
conditions, and so forth used in the specification and claims are
to be understood as being modified in all instances by the term
"about". Accordingly, unless indicated to the contrary, the
numerical parameters set forth in this specification and claims are
approximations that can vary depending upon the desired properties
sought to be obtained by the presently disclosed subject
matter.
[0104] As used herein, the term "about," when referring to a value
or to an amount of mass, weight, time, volume, concentration,
percentage or a physical dimension such as length, width, or
diameter, is meant to encompass variations of in some
embodiments+-40% or more, in some embodiments+-20%, in some
embodiments+-10%, in some embodiments+-5%, in some embodiments+-1%,
in some embodiments+-0.5%, and in some embodiments+-0.1% from the
specified value or amount, as such variations are appropriate to
perform the disclosed methods.
[0105] As used herein, ranges can be expressed as from "about" one
particular value, and/or to "about" another particular value. It is
also understood that there are a number of values disclosed herein,
and that each value is also herein disclosed as "about" that
particular value in addition to the value itself. For example, if
the value "10" is disclosed, then "about 10" is also disclosed. It
is also understood that each unit between two particular units are
also disclosed. For example, if 10 and 15 are disclosed, then 11,
12, 13, and 14 are also disclosed.
[0106] As used herein, the term "fluid" is defined as any liquid or
gas which can be passed through the filter media and filter devices
disclosed herein. Multiple fluids having different specific
gravities and viscosities can be used as well as gas and vapor
streams, depending on the application.
[0107] As used herein, the term "nanofiber" refers to a fiber
structure having a diameter of less than 1000 nanometers for more
than half the length of the structure. In some embodiments, the
nanofibers disclosed herein can comprise a tapered base portion and
a relatively longer fiber portion which extends from the base
portion. In such embodiments, the fiber portion has a diameter of
less than 1000 nm and a length greater than that of the base
portion, and the base portion can have a diameter of from about 10
micron to less than 1.0 micron. Additionally, in some embodiments,
the base portion can also have a length of from about 1.0 micron to
about 10 microns, and the fiber portion can have a length of from
about 10 to 100 times greater than the length of the base portion.
Nanofibers having larger diameter base portions in the range of
from about 2.0 microns to about 10 microns are best suited for
applications wherein the bases must provide stiffness to the
nanofiber in the fluid stream.
[0108] In some preferred embodiments, nanofibers suitable for use
in the nanofiber filter media and filter devices disclosed herein
can have an overall length of from about 10 to about 100 microns.
Accordingly, suitable nanofibers can also have a length to diameter
ratio of from 10:1 to about 1000:1. In one embodiment, the length
to diameter ratio is from about 10:1 to about 100:1. By contrast,
nanofibers known in the art, including electrospun nanofibers,
melt-blown nanofibers and microfiber-derived nanofibers (i.e.,
microfibers split during processing to obtain sub-micron diameter
structures), typically have much greater length to diameter ratios
in the range of 1,000,000:1 to 100,000,000:1. As a result, the
nanofibers used in nanofiber filter media and filter devices
disclosed herein can have from about 10 to about 1000 times more
tips per unit length than electrospun nanofibers, melt blown
nanofibers and microfiber derived nanofibers.
[0109] The related terms "nanofiber array" and "array of
nanofibers," which are used interchangeably herein, collectively
refer to a plurality of freestanding nanofibers of user-defined
physical dimensions and composition integrally formed on and
extending from a backing member, such as a film, according to
user-defined spatial parameters. In some embodiments, the nanofiber
arrays disclosed herein include nanofibers which extend from a
surface of the backing member at an angle substantially normal to a
plane containing the surface of the backing member from which the
nanofibers extend. By contrast, electrospun nanofibers, melt-blown
nanofibers, and microfiber-derived nanofibers are neither
integrally formed on nor do they extend from a backing member.
[0110] User-tunable physical characteristics of the nanofiber
arrays disclosed herein include fiber spacing, diameter (also
sometimes referred to herein as "width"), height (also sometimes
referred to herein as "length"), number of fibers per unit of
backing member surface area (also referred to herein as "fiber
surface area density"), fiber composition, fiber surface texture,
and fiber denier. For example, nanofiber arrays used in the filter
media and filter devices disclosed herein can comprise millions of
nanofibers per square centimeter of backing member, with fiber
diameter, length, spacing, material composition, and texture
configured to perform a filtration function. In some embodiments,
one or more of fiber surface area density, diameter, length,
spacing, composition, and texture are controlled and optimized to
perform a filtration function. In certain embodiments, the
nanofiber arrays can be optimized or "tuned" to perform a specific
filtration function or target a preselected substance or specific
retentate. In further embodiments, an array of nanofibers disposed
on a portion of a filter lamina forming a flow passage of a filter
device disclosed herein is configured to filter a substance from a
fluid containing the substance when the fluid is flowed through the
flow passage.
[0111] The nanofiber arrays disclosed herein, when formed on a
substantially planar surface of a backing member, can include
nanofibers spaced along an X-axis and a Y-axis at the same or
different intervals along either axis. In some embodiments, the
nanofibers can be spaced from about 100 nm to 200 micron or more
apart on the X-axis and, or alternatively, the Y-axis. In certain
embodiments, the nanofibers can be spaced from about 1 micron to
about 50 micron apart on one or both of the X-axis and the Y-axis.
In a preferred embodiment, the nanofibers can be spaced from about
2 micron to about 7 micron apart on one or both of the X-axis and
the Y-axis.
[0112] In some embodiments, an array of nanofibers can include
nanofibers having an average length of at least 25 micron. In
certain embodiments, the nanofibers can have a length of from about
10 micron to about 100 micron. In certain embodiments, the
nanofibers can have a length of from about 15 micron to about 60
micron. In an exemplar embodiment, the nanofibers can have an
average length of from about 20 micron to about 30 micron. In
specific embodiments, the nanofibers can have a length of about
15.00 micron, 16.00 micron, 17.00 micron, 18.00 micron, 19.00
micron, 20.00 micron, 21.00 micron, 22.00 micron, 23.00 micron,
24.00 micron, 25.00 micron, 26.00 micron, 27.00 micron, 28.00
micron, 29.00 micron, 30.00 micron, 31.00 micron, 32.00 micron,
33.00 micron, 34.00 micron, 35.00 micron, 36.00 micron, 37.00
micron, 38.00 micron, 39.00 micron, 40.00 micron, 41.00 micron,
42.00 micron, 43.00 micron, 44.00 micron, 45.00 micron, 46.00
micron, 47.00 micron, 48.00 micron, 49.00 micron, 50.00 micron,
51.00 micron, 52.00 micron, 53.00 micron, 54.00 micron, 55.00
micron, 56.00 micron, 57.00 micron, 58.00 micron, 59.00 micron, or
60.00 micron.
[0113] The nanofiber backing member surface area density can range
from about 25,000,000 to about 100,000 nanofibers per square
centimeter. In some embodiments, the nanofiber surface area density
can range from about 25,000,000 to about 2,000,000 nanofibers per
square centimeter. In specific embodiments, the nanofiber surface
density is about 6,000,000 nanofibers per square centimeter. In an
exemplar embodiment, the nanofiber surface area density is about
2,000,000 nanofibers per square centimeter.
[0114] In some embodiments, an array of nanofibers can include
nanofibers having an average denier of from about 0.001 denier to
less than 1.0 denier. In an exemplar embodiment, the nanofibers
forming a nanofiber array disclosed herein can be less than one
denier and have a diameter ranging from about 50 nm to about 1000
nm.
[0115] Nanofiber arrays and methods for producing nanofiber arrays
suitable for use in the filter media and filter devices disclosed
herein are described by the present inventors in U.S. 2013/0216779,
U.S. 2016/0222345, and White et al., Single-pulse ultrafast-laser
machining of high aspect nanoholes at the surface of SiO2, Opt.
Express. 16:14411-20 (2008), each of which is incorporated herein
by reference in its entirety.
[0116] A preferred method for manufacturing herein described
ribbons and ribbon segments of the present invention with nanofiber
arrays for filter elements of the present invention is hot
pressing, a method in which a suitable polymeric film is positioned
between a temperature controlled compressing plate and a
substrate/mold formed of silica or another suitable material in
which patterns of nanoholes have been formed, the pattern of the
nanoholes being complementary to the pattern of nanofibers to be
produced. Methods for making molds with patterns of nanoholes
formed therein by single-pulse femto-second laser machining are
described in detail in US 2015/0093550, herein incorporated by
reference in its entirety. The compressing plate, mold and film are
heated to a predetermined temperature and a force is applied to the
compressing plate so as to press the film against the silica mold.
When the temperature of the film material reaches a sufficient
level, the softened film material flows into the nanoholes in the
mold. In some embodiments with certain materials the softened
polymer infiltrates the nanoholes due to surface tension effects
only. In other embodiments with films formed of the same or
different materials, infiltration of the nanoholes is accomplished
by a combination of hydrostatic pressure and surface tension.
Thereafter the system is cooled sufficiently to allow the film to
be peeled off of the substrate with the molded nanofibers attached
to its first surface. The hot-pressing method for producing filter
ribbons with nanofiber arrays is described in detail by Hofmeister,
et al. in US 2016/0222345, herein incorporated by reference. While
hot pressing is a preferred method for forming ribbons for filters
of the present invention, solution casting may also be used. The
solution casting method for producing filter ribbons with nanofiber
arrays is described in detail by Hofmeister, et al. in US
2015/0093550.
[0117] Another preferred method for manufacturing ribbons of the
present invention has the ability to produce continuous elongate
strips of film with arrays of nanofibers formed on at least one
surface thereof. In method 800, a variation of a film producing
technique referred to as "chill roll casting" and depicted in FIGS.
1A through 1D, polymer 820 is supplied via tubular member 822 to
extrusion head 808. Polymer 820 is heated above its melt point by
heater 824 and the melted polymer 810 is then applied to rotating
cylindrical roll 802 (referred to as a "chill roll") formed of
silica or another suitable material. An array of nanoholes 806 is
formed in the circumferential surface 804 of roll 802 so as to form
a mold, the nanohole array being complementary to the array of
nanofibers to be formed. The nanoholes are formed using methods
previously described herein. Molten polymer 810 flows into
nanoholes 806 as it is applied to circumferential surface 804 of
rotating chill roll 802. Chill roll 802 is maintained at a
temperature such that during a predetermined portion of the roll
rotation of chill roll 802, polymer 810 in nanoholes 806 solidifies
along with the portion of polymeric material 810 coating
circumferential surface 804 of roll 802. A cylindrical metallic
roll 812, commonly referred to as a "anvil roll" or "quench roll"
functions as the compressing element and is positioned adjacent to
chill roll 802 such that after a predetermined angular rotation of
chill roll 802 polymeric material 810 coating the surface of chill
roll 802 is compressed between surface 804 of chill roll 802 and
surface 814 of the quench roll 812. As implied by the name "quench
roll" polymeric material 810 undergoes rapid cooling during contact
with quench/anvil roll 812 so that it may be subsequently stripped
from the surface of chill roll 802 as a continuous elongate strip
of film 818. When the polymer strip 818 is removed from chill roll
802, material 810 that had previously flowed into nanoholes 806
forms molded nanofibers 816 on the surface of film strip 818. In
subsequent processing elongate strips 818 may be slit, cut, chopped
or otherwise formed into filter ribbons of the present invention.
As with the previously described hot pressing method, polymer 820
is not contained in a solution so the use of environmentally
undesirable solvents is not required.
[0118] Under certain conditions, with suitable polymers, quench
roll 812 is eliminated. The thickness of film strip 818 is
determined by process parameters, These may include properties of
polymer 820, the temperature of polymer 810 as it is deposited on
surface 804 of chill roll 802, the temperature and rotational speed
of chill roll 802, and other factors that affect the cooling of
film strip 818. Under these conditions, material is drawn into
nanoholes 806 of surface 804 of chill roll 802 by surface
tension.
[0119] In the methods of manufacture previously herein described,
reference is made to molds made of silica or another suitable
material. Among these suitable materials are transparent materials
like borosilicate glass, soda lime glass, BK7 optical glass,
plastic, single-crystal quartz, diamond and sapphire. All have been
successfully micromachined with femtosecond laser pulses. Fused
silica is a preferred material since it offers a combination of
properties like wide range of spectral transparency, low
autofluorescence, good biocompatibility, chemical inertness, near
zero thermal expansion, excellent thermal shock resistance, and low
dielectric constant and losses.
[0120] Any alternate method capable of producing integral arrays of
nanofibers of predetermined lengths, diameters, and profiles formed
on a surface of a film and substantially perpendicular to a first
surface of a film, and further, wherein the spatial arrangement of
the fibers has a predetermined pattern, may be used. All fall
within the scope of this invention.
[0121] Using the foregoing methods, nanofiber arrays with a variety
of mechanical, electrical and chemical properties, Debye moments,
tailored affinities, and functional binding sites can be produced
from almost a wide variety of polymers without the use of solvents
or high voltage electrical fields.
[0122] Nanofibers forming nanofiber arrays disclosed herein can be
composed of virtually any thermoplastic polymer, polymer resin, or
similar material. Non-limiting examples of suitable polymers
include poly(.epsilon.-caprolactone) (PCL), polyethylene oxide
(PEO), polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polyvinyl
formal (PVF), polyisoprene, trans (PI), polypropylene (PP),
low-density polyethylene (LDPE), high-density polyethylene (HDPE),
PIP castline (PiPc), PIP natural (PiPn), polyvinylidene fluoride
(PVDF), poly-lactic acid (PLA), and poly-L-lactic acid (PLLA). It
should be understood that a blend of two or more such polymers can
also be used. It should also be understood that a blend or block
co-polymer of two or more such polymers can also be used. For
example, in one embodiment, a blend of block co-polymer comprising
PCL-block-PEO can be used to alter the functionality of the backing
member and nanofibers.
[0123] As used herein "ribbon" or "ribbon-like structure" refers to
an elongate strip of flexible polymeric material having an array of
nanofibers formed on at least a portion of one of its planar
surfaces. Nanofibers are formed on a functional backing material in
web form. In a primary embodiment the webs are post processed by
chopping or slitting to form the ribbon or ribbon like structures.
However, for the purposes of the patent, the entire web may be
considered a ribbon or ribbon like structure.
[0124] FIGS. 2A through 7 diagrammatically depict a segment of a
filter media ribbon 100 of the present invention. Ribbon 100 has an
elongate planar film portion 102 of width 104 and thickness 106,
with a first surface 108 on which are formed nanofibers 110. In
some embodiments, width 104 is between 2.times. and 10.times.
thickness 106. In others width 104 is between 10.times. and
40.times. thickness 106. And in yet others width 104 is between
40.times. and 100.times. thickness 106. Nanofibers 110 have a
length 112 and are spaced distance 114 apart in the longitudinal
direction and 116 in the transverse direction. Nanofibers 110 have
a first diameter 120 near the base of the fiber and decrease in
diameter toward the distal end of the fiber. As defined herein the
term "nanofiber" refers to a fiber structure having a diameter of
less than 1000 nanometers for more than half the length of the
structure. In some embodiments, the nanofibers of filter media of
the present invention may have a tapered base portion and a
relatively longer fiber portion which extends from the base
portion. Referring now to FIG. 8, nanofiber 210 has a tapered
proximal base portion 201 of diameter 220 with elongate distal
portion 203 of diameter 222 formed thereon, and a length 212.
[0125] Ribbon 100 is depicted with longitudinal distance 114 and
transverse distance 116 between adjacent nanofibers 110 constant
over surface 108. In other embodiments, either distance 114 or
distance 116 or both may vary along the length of ribbon 100.
Nanofibers 110 are shown in ordered parallel rows. In other
embodiments other arrangements are used depending on the particular
filtering process requirements. Similarly, height 112 and diameter
120 of nanofibers 110 are constant across the surface of ribbon
100. In other embodiments height 112 and diameter 120 of nanofibers
on a first portion of surface 108 of ribbon 100 may have first
values, while on a second portion of surface 108, height 112 and
diameter 120 may have second values.
[0126] The process used to produce nanoholes 806 in chill roll 802
uses the energy of a single laser pulse to vaporize material so as
to form the nanohole. The vaporized material of chill roll 802 is
expelled to form a nanohole 806. The process is well controlled
within limits, however the precise geometry of a nanohole 806 is
determined by the flow of superheated vaporized material at the
site. Accordingly, there may be minor variations in the form of
nanoholes 806, and in the nanofibers 110 that are molded therein.
Also, nanofibers 110, particularly those with long, tendrilous
forms, may stretch somewhat during extraction from nanoholes 806.
Therefore it will be understood that when it is stated that
nanofibers 110 in an array have a height 112, height 112 is a
nominal height, and individual fibers 110 may have a height that is
somewhat greater or less than nominal height 112. Similarly, when
considering diameters 120 of nanofibers 110, diameter 120 is a
nominal value and there may be natural variations in the diameters
120 in nanofibers 110 within an array.
[0127] Nanofibers of the present invention may be broadly
characterized by the ratio of their length (112 in FIGS. 7 and 212
in FIG. 8) to their average diameter. Typically nanofibers of
filter media of the present invention have length to diameter
ratios between 10:1 and 1,000:1. Nanofibers with length to diameter
ratios at the lower end of the range may be used in applications in
which the fibers require a degree of stiffness to optimally affect
a fluid stream flowing thereby.
[0128] The nanofiber arrays formed on filter ribbons of the present
invention may form a tuned topography. That is ribbons may be
optimally configured to remove specific contaminants such as
pathogens, chemical contaminates, biological agents, and toxic or
reactive compounds from a fluid to be filtered. By selecting
specific values for longitudinal distance 114 and transverse
distance 116 between adjacent nanofibers (FIGS. 2 through 5), and
diameters 120 and 220, and lengths 112 and 212 of nanofibers 110
and 200 (FIGS. 7 and 8) ribbons may be formed that preferentially
remove a specific contaminant. Indeed, filtering devices may be
formed in which ribbons of a first configuration optimally designed
for removal of a first contaminant are combined with ribbons
designed to remove a second contaminant. Additional ribbons with
tuned topographies for removing specific contaminants may be added
to remove these substances from the flow stream. The ribbons may be
mixed in a filter device, or formed in discrete layers each
containing a single ribbon configuration or a combination of two or
more configurations.
[0129] Filter media ribbons with nanofibers of the present
invention may be formed from virtually any polymeric material.
These polymeric materials have inherent electrostatic properties
and exert an electrostatic force at a point on the surface of an
object formed therefrom that is inversely related to the radius of
curvature of the surface at that point. As the radius of the
surface at a given point is reduced, the electrostatic attractive
force at that point increases. Accordingly, the electrostatic force
exerted by a nanofiber is much greater than that exerted by a
microfiber. This is of particular importance in filter applications
in which contaminants smaller than the pore size of the filter must
be removed from a fluid stream. Electrostatic forces draw
contaminants to fibers for removal from the fluid stream. As the
diameter of the fibers is decreased, the electrostatic force
exerted by the fibers increases. The attractive force of a
nanofiber is generally orders of magnitude greater than that of a
microfiber, and therein lies the incentive for creating nanofiber
filters. The high level of electrostatic force exerted by
nanofibers allows them to efficiently remove contaminants from a
fluid stream.
[0130] FIGS. 9 and 10 depict field lines 130 and 230 depicting the
intensity of an electrostatic force field line surrounding
nanofibers 110 and 210 respectively. As described previously, the
field intensity at a point on the surface of a fiber is inversely
proportional to the radius of curvature of the fiber at that point.
This is reflected in the field line depicted. It should be noted
that the field intensity is maximal at the distal end of the
fibers. In prior art nanofiber filter medial formed by
electrospinning or other conventional methods the nanofibers are
virtually continuous with length to diameter ratios ranging from
1,000,000:1 to 100,000,000. Accordingly, for a given cumulative
nanofiber length, fibers of the present invention will have from
about ten to about one thousand times as many fiber ends. The
associated higher electrostatic potential of nanofiber media formed
in accordance with the present invention allows the construction of
filters with efficiencies not attainable using nanofibers formed by
electrospinning or other conventional methods.
[0131] The arrangement of nanofibers in an array can impact
filtration specificity and efficiency by modulating the strong
gradients in the electrical and chemical potential fields of
normally highly reactive sub-micron length scale structures.
Control of these gradients at process length scales can enhance
efficiency of transport or flow. However, if two nanofibers are in
close proximity and the potential fields overlap, then the gradient
of the potential field is reduced and the advantages of the
nanoscale topography are reduced. The arrangement of nanofibers in
a nanofiber array of the proper scale and spacing will preserve the
separation of nanofibers thus optimizing the potential field
gradient.
[0132] An electrostatic charge may be imparted to the filter media
of the present invention to increase the attractive force of the
nanofiber arrays formed on ribbons. Filter ribbons of the present
invention may be formed from a polymer or polymer blend with
suitable electret properties. Among these materials are
polypropylene, poly(phenylene ether) and polystyrene. In certain
embodiments these ribbons may have a lamellar construction that has
a first layer formed of an electret material on which are formed
nanofiber arrays of the present invention, and a second layer
bonded thereto with desirable physical and/or electrical
properties. The materials selected for each layer may be optimized
for a specific filtering application. Charging of the media may be
accomplished by corona discharge, triboelectrification,
polarization, induction, or another suitable method. Over time the
imparted electrostatic charge may be dissipated by particle
loading, and/or by quiescent or thermal stimulation decay.
[0133] Nanofiber arrays on media ribbons of the present invention
also advantageously affect the wetting of the surface of the ribbon
by water vapor. Many polymers are hydrophobic, or have low wetting
ability. The presence of nanofiber arrays of the present invention
on the surface of a polymeric filter element increases the
wettability of the surface so that vapor precipitates and collects
on the filter media. Nano-textured nucleation of the liquid from
the vapor is triggered by the tips of the nanofibers. Droplets grow
to cover the surface of the media once a critical radius is
reached. This wetting of the nanofiber array covered surface
enhances the collection efficiency of the element. This is
diagrammatically illustrated in FIGS. 11 and 12. FIG. 11 is a
sectional view of a polypropylene element 300 with a substrate 302
on which a first portion 305 of the upper surface has formed
thereon an array of nanofibers 310 of the present invention, and a
second surface portion 303 does not have nanofibers. In FIG. 12A
oil droplet 307 partially wets portion 303, while on the portion
305 with nanofibers 310 wetting of the surface by oil 309 is
complete. By contrast, in FIG. 12B, the wetting of first portion
303 by oil droplet 307 is low, and the wetting of second portion
305 by oil 309 is lessened by nanofibers 310 as shown by spherical
droplet 309 formed on nanofibers 310.
[0134] Methods for modifying the wettability of surfaces by forming
nanofiber arrays thereon are discussed in detail in co-pending
application U.S. 2020/0039122 herein incorporated by reference in
its entirety.
[0135] Referring now again to FIGS. 2 through 6, because the
nanofibers on media ribbons of the present invention are not
structural members, but rather formed on a surface of a ribbon that
serves as a structural member, width 104 and thickness 106 of
planar film portion 102 of ribbon 100 may be selected for ease of
processing and filter flow considerations. The thickness 106 and
width 104 of planar film portion 102 must be sufficient to allow
subsequent processing, and must allow for the efficient formation
of nanofibers 110 on film 102. Within these constraints it may be
desirable to minimize 104 so as to reduce the resistance to fluid
flow through the filter element formed. Ribbon 100 is formed as a
continuous elongate element that may be cut to length as required
during processing.
[0136] Woven filter media may be created from ribbons 100. The
ribbons may be weaved individually in the structure, or may be
formed into a multi-strand yarn prior to weaving. Alternatively,
ribbons 100 can be formed into a non-woven mat 400 as depicted in
FIG. 13. The orientation of ribbons 402 in mat 400 may be random,
or may have a preferential orientation in which the nanofiber
bearing surfaces of ribbons 402 lie at low angles to the direction
of flow, the orientation being established during manufacture of
mat 400.
[0137] Referring now to FIG. 14A, respirator mask 420 achieves high
filtering efficiency and low pressure drop through use of filter
ribbons of the present invention. As depicted in FIG. 14B, mask 420
has a first outer layer 422 formed of ribbons of a hydrophobic
polymer like, for instance polypropylene. The ribbons forming first
layer 422 have formed on them arrays of nanofibers configured to
maximize the hydrophobic nature of the material. First layer 422 is
preferably a woven or non-woven fabric. Second layer 424 is formed
of microfibers configured to filter micron-sized coarse particles
and some submicron-particles in the fluid stream. Microfibers of
second layer 424 may be made of polyethylene, glass, cellulose
acetate, activated carbon fiber or combinations thereof. Optionally
second layer 424 may also contain nanofiber bearing filter ribbons
of the present invention with the nanofiber arrays configured to
optimally remove contaminants of a first composition or size. Third
layer 426 is a non-woven mat formed of nanofiber bearing filter
ribbons of the present invention. Because the ribbons from which
this layer are formed have structural strength, the non-woven mat
has a predetermined thickness and flow characteristics selected for
optimal removal of contaminants while preserving airflow at low
pressure drop and resistance to clogging. The arrays of nanofibers
on these ribbons are optimally configured for the removal of small
particles. Because of the higher attractive electrostatic forces of
the nanofiber arrays compared to other filter elements with
continuous random fibers, filter layer 426 is able to draw
contaminants greater distances for removal from the fluid stream.
In certain embodiments nanofiber arrays of ribbons forming third
layer 426 may be configured to preferentially remove specific
contaminants. Indeed, additional layers of ribbon mats of the
present invention may be positioned proximal to this third layer,
the nanofiber arrays of each layer being optimized to remove
specific contaminants. Proximal to the previously described filter
layers is permeable layer 428 formed of a fabric, woven or
non-woven that may, in some embodiments, be comfortably pressed
against the face of the wearer. In masks of the present invention,
the filter layers are not bonded to each other. In production, the
layers forming the filter assembly may be produced as continuous
sheets of material, laid up in the proper order, and then bonded
together in selected locations thermally, by a glue or solvent
bonding, by stitching, or by needle punch, a joining method for
non-woven fabrics. Because nanofibers of the present invention are
integrally formed on the surface of ribbons of the present
invention, the nanofibers cannot become loose and be inhaled by the
wearer as is possible with respirators made with prior art filter
assemblies.
[0138] Advantageously, for certain applications like mask 420,
nanofiber bearing ribbons of the present invention may be formed of
an antimicrobial plastic. Representative of these materials is
MICROBAN.RTM. by Microban, Inc. (Huntersville, N.C.). MICROBAN.RTM.
is a synthetic polymer material containing an integrated active
ingredient which makes it effective against microbial growth. The
MICROBAN.RTM. additive may be blended with polymers with optimal
properties for forming nanofiber arrays in methods herein described
to create filter ribbons of the present invention that not only
have the ability to efficiently remove microbes from a fluid
stream, but also to kill those microbes. In certain embodiments
these ribbons have a lamellar construction wherein a first layer,
on which are formed nanofiber arrays of the present invention, is
bonded to a second layer with optimal physical properties, the
first layer being formed of an antimicrobial plastic.
[0139] Prior art filter media formed of nanofibers are primarily
made by electrospinning or a similar method that forms a thin,
membrane-like fiber mat. Flow through the structure is
substantially normal to the plane of the mat, and, because the
fibers are not substantially distanced one from another in a
direction normal to the plane of the mat, clogging may limit the
filter life and efficiency. In contrast, filter media of the
present invention comprises ribbons with cross-sections orders of
magnitude greater than nanofibers. This allows the construction of
filters wherein the media ribbons are spaced one from another so as
to create a resilient three-dimensional structure. Because the
fibers are so spaced, flow through the filter media is not
restricted to a single direction. Indeed, a suitable housing may be
filled with ribbons of the present invention and flow may proceed
from a defined inlet to a defined outlet with the path therebetween
being undefined. Indeed, baffles may be added to lengthen the path
for flow through the media. In filter media of the present
invention the nanofibers are not structural members but rather are
features on a structural members, these features being configured
to create attractive electrostatic forces that are orders of
magnitude greater than those created by filter elements wherein the
nanofibers are structural members. Particles suspended in a fluid
exhibit random motion resulting from their collisions with
fast-moving fluid molecules, an effect known as "Brownian Motion".
Filter elements formed of ribbons of the present invention create
flow paths that are orders of magnitude longer than those of prior
art membrane-like nanofiber filter elements. These longer flow
paths take advantage of the Brownian Motion effect to allow the
building of filters that have a high filtering efficiency combined
with a low pressure drop, and the added benefit of an increased
resistance to clogging.
[0140] FIG. 15 depicts a filter 500 wherein media 502, formed of
media ribbons of the present invention, is contained within a
housing 504 with an inlet 506 and an outlet 508. Housing 504 has
formed therein baffles 510 that form a labyrinthian flow path
between inlet 506 and outlet 508. The long flow path through media
502 exploits the Brownian Motion effect to maximize interaction
between contaminants and the electrostatic field created by
nanofiber arrays on ribbons of media 502. Filter 500 illustrates
the design flexibility that is enabled by filter ribbons of the
present invention. While prior art nanofiber filter elements have
only a simple flow path with the limitations previously herein
described, filter media formed of ribbons of the present invention
may be utilized in substantially the same manner as conventional
microfiber media.
[0141] While filter element ribbons and ribbon segments of the
present invention have been previously described and depicted with
flat film portions, other shapes are contemplated and fall within
the scope of this invention. For instance, ribbon 600 depicted in
FIGS. 16A and 16B has a film portion 602 that is folded
longitudinally during manufacture. As depicted, the fold remains
closed with the film halves 603 being essentially parallel. In
other embodiments film halves 603 are angularly oriented one to
another due to spring-back of the material after folding. This
angular orientation may be as great as 150 degrees in certain
embodiments.
[0142] FIGS. 17A and 17B depict ribbon 700 in which the film
portion 702 is given a form similar to a hollow fiber during
manufacture. Nanofibers 710 protrude radially outward from film
portion 702. In some embodiments edges 705 of film portion 702 may
meet to form a complete cylindrical body. In others they may be
separated by distances that approach the inner diameter of the
formed ribbon.
[0143] Unlike prior art processes for producing nanofibers filter
media, the chill roll casting process previously herein described
is scalable and may be automated to enable production of quantities
of nanofiber-bearing filter ribbons rapidly and at low cost. For
instance, referring now to FIGS. 18 and 19 depicting chill roll
casting system 800 (FIGS. 1A through 1D), slitting of film strip
818 may accomplished automatically by adding a slitting means as
depicted in FIG. 18. Subsequent to the removal of film strip 818
from chill roll 802, a plurality of slits 842 are formed in strip
818 so as to form a plurality of filter ribbons 819 of the present
invention as depicted in region 840 of FIG. 18. Ribbons 818 are
analogous in form and function to elongate ribbons 100 depicted in
FIGS. 2 through 7. However, in other embodiments (not shown), the
slitting means may be placed adjacent the chill roll 802 and
configured to form a plurality of slits in the cooled polymer
covering the chill roll 802 before removing the film strip 818 from
chill roll 802. The slitting may be accomplished by mechanical
means using a rotating cylindrical cutting element with a plurality
of sharpened circumferential cutting edges formed on its
cylindrical surface, and a second rotating cylinder. The axes of
both cylinders are parallel to the axis of chill roll 804, and are
positioned such that the cutting edges of the cutting element
contact or are in very close proximity to the surface of the second
rotating cylinder. Strip 818 passes between this rotating cutting
element and the second cylinder so that each cutting edge forms a
continuous longitudinal slit 842 in strip 818. Slitting of film
material in this manner is well known in the art.
[0144] In the casting system of FIGS. 18 and 19 longitudinal slits
842 are formed in strip 818 automatically as strip 818 is produced.
In other methods of the present invention, slitting of strip 818 is
done as a secondary process remote from the system 800. Strip 818
can be wound onto a spool for storage and subsequent slitting. In
the previous example, longitudinal slits 842 were formed in strip
818. In other methods for making filter ribbons of the present
invention, lateral slits are made to form ribbons. Indeed, any
method of cutting, slitting or chopping a film strip on which
nanofiber arrays are formed may be used to form filter ribbons of
the present invention. All fall within the scope of this
invention.
[0145] In some embodiments, filter media ribbons of the present
invention are divided into segments of predetermined length. These
segments may be formed into non-woven mats or placed in a housing
as previously described.
[0146] FIGS. 20 and 21 depict the chill roll system of FIGS. 18 and
19 with a means added for automatically cutting ribbons 819 into
short segments 900 depicted in FIGS. 22 through 24. Lateral cuts
852 are formed in ribbons 819 by a rotating cylindrical cutting
element with axially oriented cutting edges formed on the
circumferential surface of the elements. As with the forming slits
842 in strip 818, lateral cuts are formed by cooperative action
between the cylindrical cutting element and a second cylinder as
previously described. Transection of strip 818 by lateral cuts 852
creates a plurality of segments 900, length 905 of segment 900
being determined by the spacing of cutting edges on the cylindrical
cutting element.
[0147] Segment 900 is identical to ribbon 100 in all aspects except
as specifically hereafter described. Like ribbon 100, segment 900
has arrays of nanofibers 910 formed on first surface 908 of film
portion 902. However, segment 900 has a predetermined length 905.
In some embodiments length 905 is 100.times. or greater than width
904 of ribbon 900. In other embodiments, length 905 is between
10.times. and 100.times. width 904 of ribbon 900. In yet other
embodiments, length 905 is between 1.times. and 10.times. width 904
of ribbon 900. The length of a segment for an application may be
optimized based on filtering requirements and on the method of
manufacturing the filter. For instance, if the filter will
incorporate a non-woven mat formed of segments 900, it may be
advantageous to make length 905 a higher multiple of width 904 than
would be the case if segments 900 were to fill a cavity in a
housing.
[0148] In certain embodiments ribbon segments may have a shape
imparted to the film portion so that when the segments are
assembled in a non-woven mat or into a filter housing, natural flow
paths between segments are created. Referring now to FIGS. 25
through 27, segment 1000 is like segment 900 with an array of
nanofibers 1010 formed on first surface 1108 of film portion 1002.
Film portion 1002 is not flexibly planar as in previously described
embodiments, but rather has a form imparted thereto during
manufacture. Forming of film portion 1002 in the manner depicted
for segment 1000 may also be advantageously applied to elongate
filter ribbons of the present invention so as to aid in the
creation of flow paths through the assembled filter element.
[0149] When viewed in a plan view, ribbon segments 900 and 1000
have a rectangular shape imparted by the orthogonal cuts that
formed them. In other embodiments formed by other slitting, cutting
or chopping methods, the shape of the ribbon segments may have
other predetermined shapes, or may be randomly formed segments with
irregular shapes. All fall within the scope of this invention.
[0150] FIGS. 28 through 31 depict a chill casting system of the
present invention for making a layered film for forming filter
ribbons of the present invention. Polymer 1120 is supplied via
tubular member 1122 to extrusion head 1108. Polymer 1120 is heated
above its melt point by heater 1124 and the melted polymer 1110 is
then applied to rotating chill roll 1102. Molten polymer 1110 flows
into nanoholes 1106 as it is applied to circumferential surface
1104 of rotating chill roll 1102. Polymer film 1130 is drawn into
the juncture between quench roll 1112 and cylindrical surface 1108
of chill roll 1104 upon which melted polymer 1110 has been
deposited. Quench roll 1112 cools molten polymer 1110 in the manner
previously described, but also forms a bond between film 1130 and
polymer 1110 so that when film strip 1118 is removed from chill
roll 1104 as a layered construct with a first layer on which are
formed nanofiber arrays of the present invention, and a second
layer formed of film 1130. In this manner film and the filter
ribbons from which they are formed may have nanofiber arrays formed
of a first polymeric material 1110 with optimal filtering or
wetting properties for a given application, bonded to a second
polymeric material forming film 1130. Forming a construct in this
manner allows polymers with optimal properties for nanofiber
formation and/or filtering to be bonded to polymer films that have
optimal properties for filter production.
[0151] In an alternate system for making films with nanofiber
arrays for producing filter ribbons of the present invention,
nanofibers are embossed on an existing film of polymeric material,
the embossing being accomplished in a process similar to the chill
casting method previously herein described. In previous embodiments
a molten polymer is applied to the mold. In the embossing
embodiment film is applied to the mold; the film is sufficiently
heated to allow the material to flow into the mold nanoholes, then
cooled so that the film with its newly formed nanofibers can be
peeled from the mold. Referring now to FIGS. 32 and 33 depicting an
embossing system 1200 of the present invention, film 1280 wraps
around circumferential surface 1204 of mold 1202 wherein are formed
nanoholes 1206. Film 1280 is heated by airflow 1272 from nozzle
1270 sufficiently to melt or sufficiently softened to allow film
material to flow into nanoholes 1206. Quench roll 1212 applies a
compressive force to softened film 1280 that assists with the flow
of film material into nanoholes 1206. Chilled air 1278 from nozzle
1276 cools film 1280 so that chilled film 1282 with nanofibers 1284
can be peeled from cylindrical surface 1204 of chill roll 1202.
Film 1282 is like film 818 with nanofibers 816 formed by casting
system 800 (FIGS. 18 to 21) in all aspects of form and function.
Layered films with embossed nanofibers may also be made by a method
similar to that previously described and depicted in FIGS. 28
through 31. In the embossing method a second film is drawn into the
juncture between quench roll 1212 and film 1280 so as to bond film
1280 to the second film. System 1200 uses heated airflow to
increase the temperature of film so that film material can flow
into nanoholes. In other embodiments film 1280 is heated by a
radiant heater or other suitable means.
[0152] In other embodiments, film 1280 is formed of a malleable
polymer that is applied to surface 1204 of mold/chill roll 1202
such that the malleable polymer film 1280 infiltrates at least a
portion of nanoholes 1206. Roll 1212 is maintained at a temperature
such that compressive force applied by roll 1212 to film 1280
causes further infiltration of film 1280 into nanoholes 1206 and
solidification of that material and of material covering surface
1204 of chill roll 1202. Thereafter, film 1282 with nanofibers 1284
formed thereon is removed from roll 1202 in the manner previously
described.
[0153] Filter media of the present invention provide the benefits
of nanofibers in elongate ribbons that can be subsequently
processed in largely the same manner as conventional fibrous filter
media. Filter media of the present invention are not deposited on a
substrate during manufacturing and are configured to maximally
exploit the electrostatic properties of the materials from which
they are formed. Along with enhanced electrostatic properties, the
nanofibers arrays of ribbons may affect the wettability of the
ribbon surface on which they are formed. Wettability for selected
liquids may be preferentially enhanced while decreasing the
wettability for other liquids thereby increasing filter efficiency.
Because the nanofiber arrays are integral with the ribbon they
cannot be expelled from the filter media. Ribbons or the present
invention with nanofibers integrally formed thereon may be produced
at reduced cost compared to conventionally produced nanofiber
media, and without the use of high voltage or environmentally
detrimental solvents.
[0154] According to the principles of the present invention, any
flexible elongate ribbon-like polymeric structure having arrays of
nanofibers formed on at least one surface falls within the scope of
this invention regardless of the method of manufacture of the
structure.
[0155] Although embodiments of the present invention have been
described in detail, it will be understood by those skilled in the
art that various modifications can be made therein without
departing from the spirit and scope of the invention as set forth
in the appended claims.
[0156] This written description uses examples to disclose the
invention and also to enable any person skilled in the art to
practice the invention, including making and using any devices or
systems and performing any incorporated methods. The patentable
scope of the invention is defined by the claims, and may include
other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they
have structural elements that do not differ from the literal
language of the claims, or if they include equivalent structural
elements with insubstantial differences from the literal languages
of the claims.
[0157] It will be understood that the particular embodiments
described herein are shown by way of illustration and not as
limitations of the invention. The principal features of this
invention may be employed in various embodiments without departing
from the scope of the invention. Those of ordinary skill in the art
will recognize numerous equivalents to the specific procedures
described herein. Such equivalents are considered to be within the
scope of this invention and are covered by the claims.
[0158] All of the compositions and/or methods disclosed and claimed
herein may be made and/or executed without undue experimentation in
light of the present disclosure. While the compositions and methods
of this invention have been described in terms of the embodiments
included herein, it will be apparent to those of ordinary skill in
the art that variations may be applied to the compositions and/or
methods and in the steps or in the sequence of steps of the method
described herein without departing from the concept, spirit, and
scope of the invention. All such similar substitutes and
modifications apparent to those skilled in the art are deemed to be
within the spirit, scope, and concept of the invention as defined
by the appended claims.
[0159] Thus, although there have been described particular
embodiments of the present invention, it is not intended that such
references be construed as limitations upon the scope of this
invention except as set forth in the following claims.
* * * * *