Note: Descriptions are shown in the official language in which they were submitted.
WO 2020/223638
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FILTER MEDIA COMPRISING POLYAMIDE NANOFIBER LAYER
Priority Claim
100011 This application claims priority to US Prov. App. No. 62/841,485, filed
on May 1,
2019.
Technical Field
100021 The present invention relates to filter media comprising at least one
layer of
polyamide nanofiber nonwoven fabric, as well as to methods of making the at
least one layer
by melt blown process using a polyamide having a relative viscosity from 2 to
200.
Back2round
100031 The common filtration process removes particulate from fluids including
an air stream
or other gaseous stream or from a liquid stream such as a hydraulic fluid,
lubricant oil, fuel,
water stream or other fluids. Such filtration processes require mechanical
strength, chemical
and physical stability of the microfiber and the substrate materials. The
filter media can be
exposed to a broad range of temperature conditions, humidity, mechanical
vibration and
shock and both reactive and non-reactive, abrasive or non-abrasive
particulates entrained in
the fluid flow. Filters may be removed for service and cleaned in aqueous or
non-aqueous
cleaning compositions. Such media are often manufactured by spinning or melt
blowing fine
fiber and then forming an interlocking web of microfiber on a porous
substrate. In the melt
blowing process, the fiber can form physical bonds between fibers to interlock
the fiber mat
into an integrated layer. Such a material can then be fabricated into the
desired filter format
such as cartridges, flat disks, canisters, panels, bags and pouches. Within
such structures, the
media can be substantially pleated, rolled or otherwise positioned on support
structures.
100041 Existing filter media are described in the art. For example, U.S.
Patent No. 6,746,517
discloses the use of fine filters or fibers having a fiber diameter of about
0.0001 to 0.5
microns made by electrospinning fine fibers using conventional techniques.
100051 U.S. Patent No. 7,115,150 discloses filter arrangements for mist
removal include a
barrier media, usually pleated, and treated with a deposit of fine fibers.
Filter arrangements
may take the form of tubular, radially sealing elements; tubular, axial
sealing elements;
forward flow air cleaners; reverse flow air cleaners; and panel filters and
can have multiple
layers of fine fiber containing pleated media.
100061 U.S. Patent No. 6,716,274 discloses filter arrangements for industrial
air cleaners
include a barrier media, usually pleated, treated with a deposit of fine
fiber. The media is
particularly advantageous in high temperature (greater than 60 C) operating
environments.
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[0007] U.S. Patent Nos. 6,955,775; 7,070,640; 7,179,317; 7,270,693; 7,316,723;
8,366,797;
8,709,118; and 9,718,012 disclose improved polymer materials and fine fiber
materials that
can be made from the improved polymeric materials in the form of microfiber
and nanofiber
structures. The microfiber and nanofiber structures can be used in a variety
of useful
applications including the formation of filter materials.
[0008] U. S . Patent No. 8,512,432 discloses a composite filter media
structure which includes
a base substrate that includes a nonwoven synthetic fabric formed from a
plurality of fibers
with a spunbond process. The base substrate has a filtration efficiency of
about 35% to less
than 50%, measured in accordance with EN 1822 (1998) test procedure A
nanofiber layer is
deposited on one side of the base substrate. The composite filter media
structure has a
minimum filtration efficiency of about 70%, measured in accordance with EN
1822 (1998)
test procedure.
[0009] US20070074628 discloses a coalescing filtration medium is disclosed for
removing
liquid aerosols, oil and/or water from a gas stream. The medium contains a
nanofiber web of
at least one nanofiber layer of continuous, substantially poly olefin-free,
polymeric
nanofibers, each nanofiber layer having an average fiber diameter less than
about 800 nm and
having a basis weight of at least about 2.5 g/m2. Nanofiber webs of nanofiber
layers were
made by electroblowing a solution of nylon 6,6 polymer.
[0010] Various designs for filter media are also described in the art. For
example, U.S. Patent
No. 7,877,704 described replaceable filter elements including plural filter
media and related
filtration systems, techniques and methods. As disclosed the filter element
includes an outer
filter media and an inner filter media. The outer filter media is operable to
remove
particulates present in a flow of fluid and/or coalesce water contained in the
flow of fluid.
The inner filter media is operable to remove particulates from the flow of
fluid, separate
water from the flow of fluid, and remove particulates from the flow of fluid.
[0011] U. S . Patent No. 8,784,542 discloses a nanofiber membrane layer having
a basis
weight of 0.01-50 g/m2 and a porosity of 60-95%, comprising a nanoweb made of
polymeric
nanofibers with a number average fiber diameter in the range of 50-600 nm,
consisting of a
polymer composition comprising a semicrystalline polyamide having a C/N ratio
of at most
5.5. The invention also relates to water and air filtration devices comprising
such a nanofiber
membrane layer.
[0012] Multilayer structures have also been described. For example, U.S.
Patent No.
8,308,834 discloses composite filter media including a base substrate that
includes a
nonwoven synthetic fabric formed from a plurality of fibers with a spunbond
process. The
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base substrate has a minimum filtration efficiency of about 50%, measured in
accordance
with ASHRAE 52.2-1999 test procedure. A nanofiber layer is deposited on one
side of the
base substrate. The composite filter media structure has a minimum filtration
efficiency of
about 75%, measured in accordance with ASHRAE 52.2-1999 test procedure.
[0013] EP Patent No. 2321029 discloses a composite filter media comprising a
multi-
component filter media comprising at least two different materials, at least
one of the
materials being a low melt component; fine fibers carried by the multi-
component filter
media, the fine fibers formed of a polymeric material and having an average
fiber diameter
less than about 500 nm, wherein the fine fibers are heat bonded to the multi-
component filter
media by the low melt component.
[0014] U. S . Patent No. 8,679,218 discloses filter media having multiple
layers. As disclosed
the filter media include a nanofiber layer adhered to another layer. The layer
to which the
nanofiber layer is adhered is formed of multiple fiber types (e.g., fibers
that give rise to
structures having different air permeabilities and/or pressure drops). The
disclosed nanofiber
layer may be adhered to a single-phase or a multi-phase layer. The disclosed
the nanofiber
layer may be manufactured from a meltblown process. The filter media may be
designed to
have advantageous properties including, in some cases, a high dust particle
capture efficiency
and/or a high dust holding capacity.
[0015] Various methods of preparing filter components are disclosed in the
art. For example,
US Pub. No. 2015/0157971 discloses a filtration barrier comprising at least
one barrier layer
which includes polymeric nanofibers interlaced with microfibers, and at least
one substrate
layer which includes polymeric microfibers. The filtration barrier can be made
by
electrospinning processes.
[0016] As shown above, polymer membranes, including nanofiber and microfiber
nonwovens are known in the art and are used for a variety of purposes,
including in
connection with filter media and apparel. Known techniques for forming finely
porous
polymer structures include xerogel and aerogel membrane formation,
electrospinning, melt-
blowing, as well as centrifugal-spinning with a rotating spinneret and two-
phase polymer
extrusion through a thin channel using a propellant gas.
[0017] U.S. Pub. No. 2014/0097558 discloses to methods of manufacture of a
filtration
media, such as a personal protection equipment mask or respirator, which
incorporates an
electrospinning process to form nanofibers onto a convex mold, which may, for
example, be
in the shape of a human face. See, also, U.S. Pub. No. 2015/0145175. WO
2014/074818
discloses nanofibrous meshes and xerogels used for selectively filtering
target compounds or
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elements from a liquid. Also described are methods for forming nanofibrous
meshes and
xerogels, methods for treating a liquid using nanofibrous meshes and xerogels,
and methods
for analyzing a target compound or element using nanofibrous meshes and 10
xerogels.
[0018] Despite the variety of techniques and materials proposed, conventional
filter media
leave much to be desired in terms of manufacturing costs, processability and
product
properties.
Summary
[0019] In some embodiments, the present disclosure is directed to a filter
media comprising a
nanofiber nonwoven layer, wherein the nanofiber nonwoven layer comprises a
polyamide
with a Relative Viscosity from 2 to 200 which is spun into nanofibers with an
average fiber
diameter of less than 1 micron (1000 nanometers) and formed into the layer.
The nanofiber
nonwoven layer may comprise a polyamide which is spun into nanofibers with an
average
fiber diameter of less than 1 micron (1000 nanometers) and formed into the
layer, wherein the
layer has a melt point of 225 C or greater. Thee filter may be an air filter,
an oil filter, a bag
filter, a liquid filter, or a breathing filter, such as a face mask, surgical
mask or personal
protective equipment. In some aspects, the polyamide may be Nylon 6,6. In
further aspects,
the polyamide may be a derivative, copolymer, blend or alloy of Nylon 6,6 and
Nylon 6. In
some aspects, the polyamide is a high temperature nylon. In some aspects, the
polyamide is a
long chain aliphatic nylon selected from the group consisting of N6, N6T/66,
N612, N6/66,
N11, and N12, wherein "N" means Nylon and "T" refers to terephthalic acid. The
nanofiber
nonwoven layer may have an Air Permeability Value of less than 200 CFM/ft2. In
some
aspects, the nanofiber nonwoven layer has an Air Permeability Value of from 50
to 200
CFM/ft2. The nanofibers may have an average fiber diameter of from 100 to 907
nanometers,
e.g., from 300 to 700 nanometers or from 350 to 650 nanometers. The nonwoven
product
may have a basis weight of 150 grams/m2 (gsm) or less, e.g., from 5 to 150 gsm
or from 10 to
125 gsm. The filter media may further comprise a scrim layer. In some aspects,
the nanofiber
nonwoven layer may be spun onto the scrim layer. In further aspects, the
nanofiber nonwoven
layer may be spun onto a layer other than the scrim layer. In some aspects,
nanofiber
nonwoven layer is sandwiched between the scrim layer and at least one other
layer. In further
aspects, the nanofiber nonwoven layer is sandwiched between at least two
layers other than
the scrim layer. In still further aspects, the nanofiber nonwoven layer is an
outermost layer.
The filter media may further comprise at least one additional layer and the
nanofiber
nonwoven layer may be spun onto one of the at least one additional layers. The
Relative
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Viscosity of the polyamide in the nanofiber nonwoven layer may be reduced by
at least 20%
as compared to the polyamide prior to spinning and forming the layer.
[0020] In some embodiments, the present disclosure is directed to a method of
making filter
media comprising a polyamide nanofiber layer, the method comprising: (a)
providing a
spinnable polyamide polymer composition, wherein the polyamide has a Relative
Viscosity
of from 2 to 200; (b) melt spinning the polyamide polymer composition into a
plurality of
nanofibers having an average fiber diameter of less than 1 micron (1000
nanometers); and (c)
forming the nanofibers onto an existing filter media layer, wherein the
polyamide nanofiber
layer has an average nanofiber diameter of less than 1000 nanometers.
[0021] In some embodiments, the present disclosure is directed to a method of
making filter
media comprising a polyamide nanofiber layer, the method comprising: (a)
providing a
spinnable polyamide polymer composition; (b) melt spinning the polyamide
polymer
composition into a plurality of nanofibers having an average fiber diameter of
less than 1
micron (1000 nanometers); and (c) forming the nanofibers onto an existing
filter media layer,
wherein the polyamide nanofiber layer has an average nanofiber diameter of
less than 1000
nanometers and a melt point of 225 C or greater. The polyamide nanofiber
layer may be
melt spun by way of melt-blowing through a die into a high velocity gaseous
stream. The
polyamide nanofiber layer may be melt-spun by 2-phase propellant-gas spinning,
including
extruding the polyamide polymer composition in liquid form with pressurized
gas through a
fiber-forming channel. The polyamide nanofiber layer may be formed by
collecting the
nanofibers on a moving belt. In some aspects, the polyamide composition
comprises Nylon
6,6. In further aspects, the polyamide composition comprises a derivative,
copolymer, blend
or alloy of Nylon 6,6 and Nylon 6. In still further aspects, the polyamide
comprises a HTN.
In some aspects, the polyamide is a long chain aliphatic nylon selected from
the group
consisting of N6, N6T/66, N612, N6/66, N11, and N12, wherein "N" means Nylon
and "T"
refers to terephthalic acid. The polyamide nanofiber layer may have a basis
weight of 150
gsm or less. The filter media may further comprise a scrim layer. In some
aspects, the
polyamide nanofiber layer may be spun onto the scrim layer. In further
aspects, the
polyamide nanofiber layer may be spun onto a layer other than the scrim layer.
In still further
aspects, the polyamide nanofiber layer may be sandwiched between the scrim
layer and at
least one other layer. In yet further aspects, the polyamide nanofiber layer
may be
sandwiched between at least two layers other than the scrim layer. In some
aspects, the
polyamide nanofiber layer is an outermost layer. In some aspects, the filter
media may further
comprise at least one additional layer and the nanofiber nonwoven layer may be
spun onto
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one of the at least one additional layers. The Relative Viscosity of the
polyamide in the
polyamide nanofiber layer may be reduced by at least 20% as compared to the
polyamide
prior to spinning and forming the layer.
Brief Description of Drawings
[0022] The invention is described in detail below with reference to the
drawings wherein like
numerals designate similar parts and wherein:
[0023] Figure 1 and Figure 2 are separate schematic diagrams of a 2-phase
propellant-gas
spinning system useful in connection with the present invention;
[0024] Figure 3 is a photomicrograph of a nanofiber nylon 66 melt spun into a
nonwoven
having an RV of 7.3 at a magnification of 50X; and
[0025] Figure 4 is a photomicrograph of a nanofiber of a grade from Figure 3
of nylon 66
melt spun into a nonwoven having an RV of 7.3 at a magnification of 8000X.
Detailed Description
[0026] Overview
[0027] The present invention is directed, in part, to filter media comprising
a layer of
nanofiber nonwoven fabric comprising a polyamide. The polyamide may have a
melting
point of greater than 225 C. The polyamide may have a Relative Viscosity from
2 to 200,
e.g., from 2 to 100, from 2 to 60, from 20 to 50, from 20 to 13, from 13 to
20, or from 2 to 12.
The polyamide may be spun or melt blown into nanofibers with an average fiber
diameter of
less than 1000 nanometers (1 micron) and formed into the nonwoven product. The
nanofiber
nonwoven layer comprising a polyamide may then be incorporated into a filter.
The layer
may be made by providing a spinnable polyamide polymer composition that can be
melt
blown, wherein the polyamide has a Relative Viscosity of from 2 to 200; (b)
spinning or melt
blowing the polyamide polymer composition into a plurality of nanofibers
having an average
fiber diameter of less than 1 micron by way of a process directed to 2-phase
propellant-gas
spinning, including extruding the polyamide polymer composition in liquid form
with
pressurized gas through a fiber-forming channel, followed by (c) forming the
nanofibers into
the product. The general process for melt blowing technology is illustrated in
FIGS. 1 and 2.
[0028] Particularly preferred polyamides include:
Nylon 6,6
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0
N N
0 H n
As well as copolymers, blends, and alloys of Nylon 6,6 with Nylon 6
[0029] Other embodiments include nylon derivatives, copolymers, blends and
alloys
containing or prepared from Nylon 6,6 or Nylon 6 or copolymers with the repeat
units noted
above including but not limited to: N6T/66, N612, N6/66, Nil, and N12, wherein
"N" means
Nylon and "T" refers to terephthalic acid. Another preferred embodiment
includes High
Temperature Nylons ("HTN") as well as blends, derivatives or copolymers
containing them.
Furthermore, another preferred embodiment includes long chain aliphatic
polyamide made
with long chain diacids as well as blends, derivatives or copolymers
containing them.
[0030] The disclosure is appreciated by reference to FIGS. 1 and 2 which
illustrates a 2 phase
propellant gas spinning process useful for making the nanofiber, and a general
melt blowing
technique respectively. In particular, disclosed herein is a method of making
a nanofiber
nonwoven product wherein the nonwoven fabric is melt-spun by way of melt-
blowing
through a spinneret into a high velocity gaseous stream. More particularly,
the nonwoven
fabric is melt-spun by 2-phase propellant-gas spinning, including extruding
the polyamide
polymer composition in liquid form with pressurized gas through a fiber-
forming channel.
[0031] Definitions and Test Methods
[0032] Terminology used herein is given its ordinary meaning consistent with
the definitions
set forth below; gsm refers to basis weight in grams per square meter, and RV
refers to
Relative Viscosity and so forth.
[0033] Percentages, parts per million (ppm) and the like refer to weight
percent or parts by
weight based on the weight of the composition unless otherwise indicated.
[0034] Typical definitions and test methods are further recited in US Pub.
Nos.
2015/0107457 and 2015/0111019. The term "nanofiber nonwoven product " for
example,
refers to a web of a multitude of essentially randomly oriented fibers where
no overall
repeating structure can be discerned by the naked eye in the arrangement of
fibers. The fibers
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can be bonded to each other, or can be unbounded and entangled to impart
strength and
integrity to the web. The fibers can be staple fibers or continuous fibers,
and can comprise a
single material or a multitude of materials, either as a combination of
different fibers or as a
combination of similar fibers each comprising of different materials. The
nanofiber
nonwoven product is constructed predominantly of nanofibers. -Predominantly"
means that
greater than 50% of the fibers in the web are nanofibers. The term "nanofiber"
refers to fibers
having a number average fiber diameter less than 1000 nm or 1 micron. In the
case of
nonround cross-sectional nanofibers, the term "diameter" as used herein refers
to the greatest
cross-sectional dimension.
[0035] Basis Weight may be determined by ASTM D-3776 and reported in gsm
(g/m2).
[0036] "Consisting essentially of' and like terminology refers to the recited
components and
excludes other ingredients which would substantially change the basic and
novel
characteristics of the composition or article. Unless otherwise indicated or
readily apparent, a
composition or article consists essentially of the recited or listed
components when the
composition or article includes 90% or more by weight of the recited or listed
components.
That is, the terminology excludes more than 10% unrecited components.
[0037] To the extent not indicated otherwise, test methods for determining
average fiber
diameters, are as indicated in Hassan et al., J 20 Membrane Sci., 427, 336-
344, 2013, unless
otherwise specified.
[0038] Air permeability is measured using an Air Permeability Tester,
available from
Precision Instrument Company, Hagerstown, MD. Air permeability is defined as
the flow rate
of air at 23 1 C through a sheet of material under a specified pressure head.
It is usually
expressed as cubic feet per minute per square foot at 0.50 in. (12.7 mm) water
pressure, in
cm3 per second per square cm or in units of elapsed time for a given volume
per unit area of
sheet. The instrument referred to above is capable of measuring permeability
from 0 to
approximately 5000 cubic feet per minute per square foot of test area. For
purposes of
comparing permeability, it is convenient to express values normalized to 5 gsm
basis weight.
This is done by measuring Air Permeability Value and basis weight of a sample
(@ 0.5" H20
typically), then multiplying the actual Air Permeability Value by the ratio of
actual basis
weight in gsm to 5. For example, if a sample of 15 gsm basis weight has a
Value of 10
CFM/ft2, its normalized 5 gsm Air Permeability Value is 30 CFM/ft2.
[0039] As used herein, polyamide composition and like terminology refers to
compositions
containing polyamides including copolymers, terpolymers, polymer blends,
alloys and
derivatives of polyamides. Further, as used herein, a "polyamide" refers to a
polymer, having
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as a component, a polymer with the linkage of an amino group of one molecule
and a
carboxylic acid group of another molecule. In some aspects, the polyamide is
the component
present in the greatest amount. For example, a polyamide containing 40 wt.%
nylon 6, 30
wt.% polyethylene, and 30 wt.% polypropylene is referred to herein as a
polyamide since the
nylon 6 component is present in the greatest amount. Additionally, a polyamide
containing 20
wt.% nylon 6, 20 wt.% nylon 66, 30 wt.% polyethylene, and 30 wt.%
polypropylene is also
referred to herein as a polyamide since the nylon 6 and nylon 66 components,
in total are the
components present in the greatest amount. A suitable alloy may include, for
example, 20%
Nylon 6, 60% Nylon 6,6 and 20% by weight of a polyester.
[0040] Exemplary polyamides and polyamide compositions are described in Kirk-
Othmer,
Encyclopedia of Chemical Technology, Vol. 18, pp. 328371 (Wiley 1982).
[0041] Briefly, polyamides are products that contain recurring amide groups as
integral parts
of the main polymer chains. Linear polyamides are of particular interest and
may be formed
from condensation of bifunctional monomers as is well known in the art.
Polyamides are
frequently referred to as nylons. Although they generally are considered as
condensation
polymers, polyamides also are formed by addition polymerization. This method
of
preparation is especially important for some polymers in which the monomers
are cyclic
lactams (i.e. Nylon 6). Particular polymers and copolymers and their
preparation are seen in
the following patents: U.S. Patent Nos. 4,760,129; 5,504,185; 5,543,495;
5,698,658;
6,011,134; 6,136,947; 6,169,162; 7,138,482; 7,381,788; and 8,759,475.
[0042] A class of polyamides particularly preferred for some applications
includes High
Temperature Nylons (HTN's) as are described in Glasscock et al., High
Performance
Polyamides Fulfill Demanding Requirements for Automotive Thermal Management
Components, (DuPont),
http://www2.dupont.com/Automotive/en_US/assets/downloads/knowledg
e%20center/HTN-
whitepaper-R8.pdf available online June 10,2016. Such polymers typically
include one or
more of the structures seen in the following:
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H 0 0 H 0 0
I II \
N-I-CH2kN CII 0 C __ -ENICH2N 7C katr4-c
- x
6T Segment 66 Segment
6T/66
H 0 0 H 0 0
__ Ni-01-12 II7VV C 0 C {
CH2¨CH4CH2)¨ti4 g 0 IG
3
CH3 - Y
6T Segment
61/DT DT Segment
H H 0 0
/ I 0 II
N1-CH2kN C C __
-x
H 0 0
6T Segment ___________________________________ NIcH27-N __ C (c1-1 c
6
- z
H 0 0
LCH20 0 ij 66 Segment
-
61 Segment
6T16 V66
[0043] Relative viscosity (RV) of polyamides refers to the ratio of solution
or solvent
viscosities measured in a capillary viscometer at 25 C. (ASTM D 789). For
present purposes
the solvent is formic acid containing 10% by weight water and 90% by weight
formic acid.
The solution is 8.4% by weight polymer dissolved in the solvent.
[00441 The relative viscosity, (nr), is the ratio of the absolute viscosity of
the polymer
solution to that of the formic acid:
= (rip/rif) = (fr x dp x tp)/ rif
where: dp = density of formic acid-polymer solution at 25 C, tp = average
efflux time for
formic acid-polymer solution, s i1f = absolute viscosity of foimic acid, kPa x
s(E+6cP) and fr
= viscometer tube factor, min2/s (cSt)/s = hìr /t3.
100451 A typical calculation for a 50 RV specimen:
rir= (frxdpxtp)/rf
SUBSTITUTE SHEET (RULE 26)
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Where:
fr = viscometer tube factor, typically 0.485675 cSt/s dp = density of the
polymer - formic
solution, typically 1.1900 g/ml tp = average efflux time for polymer ¨ formic
solution,
typically 135.00 s
rif= absolute viscosity of formic acid, typically 1.56 cP
giving an RV of lir = (0.485675 cSt/s x 1.1900 g/ml x 135.00 s)/ 1.56 cP =
50.0
The term t3 is the efflux time of the S-3 calibration oil used in the
determination of the
absolute viscosity of the formic acid as required in ASTM D789.
100461 An additional embodiment of the present invention involves production
of a layer of
filter media comprising polyamide nanofibers having an average fiber diameter
of less than 1
micron, and having an RV of from 2 to 200. In this alternate embodiment,
preferable RV
ranges include: 2 to 200, 2 to 100, 2 to 60, and 5 to 60. The nanofibers are
subsequently
converted to nonwoven web. As the RV increases beyond about 20 to 30,
operating
temperature becomes a greater parameter to consider. At an RV above the range
of about 20
to 30, the temperature must be carefully controlled so as the polymer melts
for processing
purposes. Methods or examples of melt techniques are described in U.S. Patent
No.
8,777,599, as well as heating and cooling sources which may be used in the
apparatuses to
independently control the temperature of the fiber producing device. Non
limiting examples
include resistance heaters, radiant heaters, cold gas or heated gas (air or
nitrogen), or
conductive, convective, or radiation heat transfer mechanisms.
100471 Reducing the RV is generally not a desirable practice when spinning
nylon 6,6,
however, in the production of nanofibers, it may actually be an advantage. In
certain aspects
of this invention, it has been found to be advantageous to melt spin nylon 6,6
at the lowest
RV possible to achieve the smallest filament diameter in the production of
nanofiber
filaments. Raising process temperatures only slightly lowers viscosity.
Advantageously, it is
possible to lower the viscosity of nylon 6,6 by depolymerizing the polymer
with the addition
of moisture. This is an advantage over addition polymers like polypropylene.
In some
aspects, the RV of the polyamide nanofiber layer is at least 20% less than the
RV of the
polyamide prior to spinning or melt blowing and forming a layer, e.g., at
least 25% less, at
least 30% less, at least 35% less, at least 40% less, or at least 45% less.
[0048] Nonlimiting examples of polymers include polyamides, polypropylene and
copolymers, polyethylene and copolymers, polyesters, polystyrenes,
polyurethanes, and
combinations thereof. Thermoplastic polymers and biodegradable polymers are
also suitable
for melt blowing or melt spinning into nanofibers of the present invention. As
discussed
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herein, the polymers may be melt spun or melt blown, with a preference for
melt spinning or
melt blowing by 2-phase propellant-gas spinning, including extruding the
polyamide polymer
composition in liquid form with pressurized gas through a fiber-forming
channel.
[0049] Other polymer materials that can be used in the nanofiber of the
invention include
both addition polymer and condensation polymer materials such as polyolefin,
polyacetal,
polyamide (as previously discussed), polyester, cellulose ether and ester,
polyallcylene
sulfide, polyarylene oxide, polysulfone, modified polysulfone polymers and
mixtures thereof.
Preferred materials that fall within these generic classes include polyamides,
polyethylene,
polybutylene terephthalate (PBT), polypropylene, poly(vinylchloride),
polymethylmethacrylate (and other acrylic resins), polystyrene, and copolymers
thereof
(including ABA type block copolymers), poly(vinylidene fluoride),
poly(vinylidene
chloride), polyvinylalcohol in various degrees of hydrolysis (87% to 99.5%) in
crosslinked
and non-crosslinked forms. Addition polymers tend to be glassy (a Tg greater
than room
temperature). This is the case for polyvinylchloride and
polymethylmethacrylate, polystyrene
polymer compositions or alloys or low in crystallinity for polyvinylidene
fluoride and
polyvinylalcohol materials. Nylon copolymers embodied herein, can be made by
combining
various diamine compounds, various diacid compounds and various cyclic lactam
structures
in a reaction mixture and then forming the nylon with randomly positioned
monomeric
materials in a polyamide structure. For example, a nylon 6,6-6,10 material is
a nylon
manufactured from hexamethylene diamine and a C6 and a Cio blend of diacids. A
nylon 6-
6,6-6,10 is a nylon manufactured by copolymerization of E-aminocaproic acid,
hexamethylene diamine and a blend of a Co and a C to diacid material.
[0050] Block copolymers are also useful in the process of this invention. With
such
copolymers the choice of solvent swelling agent is important. The selected
solvent is such
that both blocks were soluble in the solvent. One example is an ABA (styrene-
EP-styrene) or
AB (styrene-EP) polymer in methylene chloride solvent. If one component is not
soluble in
the solvent, it will form a gel. Examples of such block copolymers are Kraton
type of
styrene-b-butadiene and styrene-b-hydrogenated butadiene (ethylene propylene),
Pebax
type of E-caprolactam-b-ethylene oxide, Sympatex polyester-b-ethylene oxide
and
polyurethanes of ethylene oxide and isocyanates.
[0051] Addition polymers like polyvinylidene fluoride, syndiotactic
polystyrene, copolymer
of vinylidene fluoride and hexafluoropropylene, polyvinyl alcohol, polyvinyl
acetate,
amorphous addition polymers, such as poly(acrylonitrile) and its copolymers
with acrylic
acid and methacrylates, polystyrene, poly(vinyl chloride) and its various
copolymers,
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poly(methyl methacrylate) and its various copolymers, are known to be solution
spun with
relative ease because they are soluble at low pressures and temperatures. It
is envisioned
these can be melt spun per the instant invention as one method of making
nanofibers.
[0052] There is a substantial advantage to forming polymeric compositions
comprising two
or more polymeric materials in polymer admixture, alloy format or in a
crosslinked
chemically bonded structure. We believe such polymer compositions improve
physical
properties by changing polymer attributes such as improving polymer chain
flexibility or
chain mobility, increasing overall molecular weight and providing
reinforcement through the
formation of networks of polymeric materials.
[0053] In one embodiment of this concept, two related polymer materials can be
blended for
beneficial properties. For example, a high molecular weight polyvinylchloride
can be blended
with a low molecular weight polyvinylchloride. Similarly, a high molecular
weight nylon
material can be blended with a low molecular weight nylon material.
[0054] In some embodiments, such as that described in U.S. Patent No.
5,913,993, a small
amount of polyethylene polymer can be blended with a nylon compound used to
form a
nanofiber nonwoven fabric with desirable characteristics. The addition of
polyethylene to
nylon enhances specific properties such as softness. The use of polyethylene
also lowers cost
of production, and eases further downstream processing such as bonding to
other fabrics or
itself. The improved fabric can be made by adding a small amount of
polyethylene to the
nylon feed material used in producing a nanofiber melt blown fabric. More
specifically, the
fabric can be produced by forming a blend of polyethylene and nylon 6,6,
extruding the blend
in the form of a plurality of continuous filaments, directing the filaments
through an die to
melt blow the filaments, depositing the filaments onto a collection surface
such that a web is
formed.
[0055] The polyethylene useful in the process of this embodiment of the
subject invention
preferably has a melt index between about 5 grams/10 min and about 200
grams/10 min and,
more preferably, between about 17 grams/10 min and about 150 grams/10 min. The
polyethylene should preferably have a density between about 0.85 grams/cc and
about 1.1
grams/cc and, most preferably between about 0.93 grams/cc and about 0.95
grams/cc. Most
preferably, the melt index of the polyethylene is about 150 and the density is
about 0.93.
[0056] The polyethylene used in the process of this embodiment of the subject
invention can
be added at a concentration of about 0.05% to about 20%. In a preferred
embodiment, the
concentration of polyethylene will be between about 0.1% and about 1.2%. Most
preferably,
the polyethylene will be present at about 0.5%. The concentration of
polyethylene in the
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fabric produced according to the method described will be approximately equal
to the
percentage of polyethylene added during the manufacturing process. Thus, the
percentage of
polyethylene in the fabrics of this embodiment of the subject invention will
typically range
from about 0.05% to about 20% and will preferably be about 0.5%. Therefore,
the fabric will
typically comprise between about 80 and about 99.95 percent by weight of
nylon. The
filament extrusion step can be carried out between about 250 C and about 325
C.
Preferably, the temperature range is about 280 C to about 315 C but may be
lower if nylon
6 is used.
100571 The blend or copolymer of polyethylene and nylon can be formed in any
suitable
manner. Typically, the nylon compound will be nylon 6,6; however, other
polyamides of the
nylon family can be used. Also, mixtures of nylons can be used. In one
specific example,
polyethylene is blended with a mixture of nylon 6 and nylon 6,6. The
polyethylene and nylon
polymers are typically supplied in the form of pellets, chips, flakes, and the
like. The desired
amount of the polyethylene pellets or chips can be blended with the nylon
pellets or chips in a
suitable mixing device such as a rotary drum tumbler or the like, and the
resulting blend can
be introduced into the feed hopper of the conventional extruder or the
spunbonding line. The
blend or copolymer can also be produced by introducing the appropriate mixture
into a
continuous polymerization spinning system.
[0058] Further, differing species of a general polymeric genus can be blended.
For example,
a high molecular weight styrene material can be blended with a low molecular
weight, high
impact polystyrene. A Nylon-6 material can be blended with a nylon copolymer
such as a
Nylon-6; 6,6; 6,10 copolymer. Further, a polyvinylalcohol having a low degree
of hydrolysis
such as a 87% hydrolyzed polyvinylalcohol can be blended with a fully or
superhydrolyzed
polyvinylalcohol having a degree of hydrolysis between 98 and 99.9% and
higher. All of
these materials in admixture can be crosslinked using appropriate crosslinking
mechanisms.
Nylons can be crosslinked using crosslinking agents that are reactive with the
nitrogen atom
in the amide linkage. Polyvinyl alcohol materials can be crosslinked using
hydroxyl reactive
materials such as monoaldehydes, such as formaldehyde, ureas, melamine-
formaldehyde
resin and its analogues, boric acids and other inorganic compounds,
dialdehydes, diacids,
urethanes, epoxies and other known crosslinking agents. Crosslinking
technology is a well-
known and understood phenomenon in which a crosslinking reagent reacts and
forms
covalent bonds between polymer chains to substantially improve molecular
weight, chemical
resistance, overall strength and resistance to mechanical degradation.
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[0059] The nanofiber can be made of a polymer material or a polymer plus
additive. One
preferred mode of the invention is a polymer blend comprising a first polymer
and a second,
but different polymer (differing in polymer type, molecular weight or physical
property) that
is conditioned or treated at elevated temperature. The polymer blend can be
reacted and
formed into a single chemical specie or can be physically combined into a
blended
composition by an annealing process. Annealing implies a physical change, like
crystallinity,
stress relaxation or orientation. Preferred materials are chemically reacted
into a single
polymeric specie such that a Differential Scanning Calorimeter (DSC) analysis
reveals a
single polymeric material to yield improved stability when contacted with high
temperature,
high humidity and difficult operating conditions. The nanofiber of this class
of materials can
have a diameter of about 0.01 to 5 microns. Preferred materials for use in the
blended
polymeric systems include nylon 6; nylon 66; nylon 6-10; nylon (6-66-610)
copolymers and
other linear generally aliphatic nylon compositions.
[0060] An embodiment of making the inventive nanofiber nonwovens is by way of
2-phase
spinning or melt blowing with propellant gas through a spinning channel as is
described
generally in U.S. Patent No. 8,668,854. This process includes two phase flow
of polymer or
polymer solution and a pressurized propellant gas (typically air) to a thin,
preferably
converging channel. The channel is usually and preferably annular in
configuration. It is
believed that the polymer is sheared by gas flow within the thin, preferably
converging
channel, creating polymeric film layers on both sides of the channel. These
polymeric film
layers are further sheared into fibers by the propellant gas flow. Here again,
a moving
collector belt may be used and the basis weight of the nanofiber nonwoven is
controlled by
regulating the speed of the belt. The distance of the collector may also be
used to control
fineness of the nanofiber nonwoven. The process is better understood with
reference to
Figure 1.
[0061] Figure 1 illustrates schematically operation of a system for spinning a
nanofiber
nonwoven including a polymer feed assembly 110, an air feed 1210 a spinning
cylinder 130,
a collector belt 140 and a take up reel 150. During operation, polymer melt or
solution is fed
to spinning cylinder 130 where it flows through a thin channel in the cylinder
with high
pressure air, shearing the polymer into nanofibers. Details are provided in
the aforementioned
U.S. Patent No. 8,668,854. The throughput rate and basis weight is controlled
by the speed of
the belt. Optionally, functional additives such as charcoals, copper or the
like can be added
with the air feed, if so desired.
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[0062] In an alternate construction of the spinneret used in the system of
Figure 1, particulate
material may be added with a separate inlet as is seen in U.S. Patent No.
8,808,594 to
Marshall et al.
[0063] Still yet another methodology which may be employed is melt blowing the
polyamide
nanofiber webs of the invention (Figure 2). Melt blowing involves extruding
the polymer into
a relatively high velocity, typically hot, gas stream. To produce suitable
nanofibers, careful
selection of the orifice and capillary geometry as well as the temperature is
required as is seen
in: Hassan et al., J Membrane Sci., 427, 336-344, 2013 and Ellison et al.,
Polymer, 48 (11),
3306-3316, 2007, and, International Nonwoven Journal, Summer 2003, pg 21-28.
[0064] In some aspects, the polyamide nanofiber is melt blown. Melt blowing is
advantageously less expensive than electrospinning. Melt blowing is a process
type
developed for the formation of fibers and nonwoven webs; the fibers are formed
by extruding
a molten thermoplastic polymeric material, or polymer, through a plurality of
small holes.
The resulting molten threads or filaments pass into converging high velocity
gas streams
which attenuate or draw the filaments of molten polymer to reduce their
diameters,
Thereafter, the melt blown fibers are carried by the high velocity gas stream
and deposited on
a collecting surface, or forming wire, to form a nonwoven web of randomly
disbursed melt
blown fibers. The formation of fibers and nonwoven webs by melt blowing is
well known in
the art. See, by way of example, U.S. Pat. Nos. 3,016,599; 3,704,198;
3,755,527; 3,849,241;
3,978,185; 4,100,324; 4,118,531; and 4,663,220.
[0065] US Patent No. 7,300,272 discloses a fiber extrusion pack for extruding
molten material
to form an array of fibers that includes a number of split distribution plates
arranged in a stack
such that each split distribution plate forms a layer within the fiber
extrusion pack, and features
on the split distribution plates form a distribution network that delivers the
molten material to
orifices in the fiber extrusion pack. Each of the split distribution plates
includes a set of plate
segments with a gap disposed between adjacent plate segments. Adjacent edges
of the plate
segments are shaped to form reservoirs along the gap, and sealing plugs are
disposed in the
reservoirs to prevent the molten material from leaking from the gaps. The
sealing plugs can be
formed by the molten material that leaks into the gap and collects and
solidifies in the reservoirs
or by placing a plugging material in the reservoirs at pack assembly. This
pack can be used to
make nanofibers with a melt blowing system described in the patents previously
mentioned.
[0066] Such melt blowing can form a polyamide nanofiber web having an
oxidative
degradation index ("ODI") from 214 to 4162 ppm. ODI is measured using gel
permeation
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chromatography (GPC) with a fluorescence detector. The instrument is
calibrated with a
quinine external standard. 0.1 grams of nylon is dissolved in 10 mL of 90%
formic acid. The
solution is then analyzed by GPC with the fluorescence detector. The detector
wavelengths
for ODI are 340 nm for excitation and 415 nm for emission. Additionally, such
melt blowing
can result in a thermal degradation index ("TDI") from 26 ¨ 1129 ppm. TDI is
measured the
same as ODI, except that the detector wavelengths for TDI are 300 nm for
excitation and 338
nm for emission. Melt blowing may also result in a relative viscosity as
described herein. TDI
and ODI test methods are also disclosed in US Patent No. 3,525,124.
100671 Filter Media
[0068] The polyamide nanofibers described herein are advantageously used in a
variety of
filter media applications, including air filters, oil filters, bag filters,
liquid filters, breathing
filters, fuel filters, hydraulic oil filters and others. While the polyamide
nanofibers are
typically not envisioned as being the sole layer in a filter, they are
envisioned as being used in
addition to traditional filters or to replace one or more layers in
traditional filters. The
polyamide nanofiber layer is also referred to as a nanofiber nonwoven layer
comprising a
polyamide.
[0069] Filtration Parameters
[0070] One common parameter characteristic of filter media is the "efficiency"
of the filter
media. Efficiency is the propensity of the media to trap particulates as
opposed to allowing
the particulates to not be filtered and instead pass through the media.
Another common
characteristic is pressure drop across the media, which often has
traditionally related to the
porosity of the media. The pressure drop relates to how restrictive the filter
media is to fluid
flow. Larger pore sizes typically have allowed for greater fluid flow, but
also unfortunately
typically result in more particulates being passed. As a result, often
efficiency is at odds with
pressure drop. In particular, while it is often desirable to trap a large
amount of particulates,
providing such a high efficiency often has had the undesirable effect of
increasing the
restrictiveness of the media and therefore the pressure drop across the media.
This shortens
the life of the filter.
[0071] Efficiency often means or refers to the initial efficiency, that is the
efficiency of the
filter media post manufacture but prior to usage and being loaded with
particulates. During
use, filter media traps and thereby picks up and traps particulates as a dust
cake and/or
otherwise within the media. These filtered-out particulates plug the larger
holes in the media
thereby preventing holes for smaller particles to pass and thereby increases
the efficiency of
the media over time to an operating efficiency greater than the initial
efficiency. However, by
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plugging fluid flow paths, such filtered out particulates also eliminate or
partially clog a fluid
passageway and thereby increase the pressure drop across the media making it
more
restrictive to fluid flow.
[0072] Usually, filter lifespan is determined by the pressure drop (delta P)
across the filter. In
one embodiment, the delta P may be from 0.5 to 10 mm H20, e.g., from 0.5 to 5
mm H20 or
from 0.5 to 3 mm H20. As more and more particles are filtered out of the fluid
flow and
trapped by the filter media, the filter media becomes more restrictive to
fluid flow. As a
result, the pressure drop across the filter media becomes higher. Eventually,
the media
becomes too restrictive, resulting in insufficient amount of fluid flow for
fluid needs of the
given application. Filter change intervals are calculated to coincide
approximately with such
an event (e.g. prior to reaching an insufficient fluid flow situation). Filter
change intervals
may also be determined through sensors that measure pressure drop load across
the media.
[0073] Generally, an electrospun nanofiber media is expected to provide
superior filtration
efficiency. This is because smaller diameter nanofibers can be packed together
without
increasing the overall solidity of the media, given the fact that smaller
fibers take up less
volume than larger fibers. Thus, an electrospun nanofiber media can
effectively capture fine
particles which a filter media formed of coarse fibers, such as a melt-blown
fiber filter media,
may not capture. Larger size particles, however, can quickly plug pores on the
upstream
surface of the electrospun nanofiber media, thereby increasing the pressure
drop of the filter
media to unacceptable levels to shorten the filter life. The multi-layer
filter media comprising
a polyamide nanofiber layer improves upon the such filter medias allowing for
smaller
particles to be captured within the depth of the melt spun polyamide nanofiber
layer, thereby
maintaining the high filtration efficiency while improving the filter life.
[0074] The polyamide nanofibers provide several advantages as compared to
traditional filter
materials, including filters containing polypropylene layers. It was
surprisingly and
unexpectedly discovered that forming a polyamide nanofiber layer by melt
blowing resulted
in increased strength, higher melt point, increased chemical resistance in
specific liquids,
smaller pore size, and a lower melt flow index as compared to traditional
filter materials,
including filters containing polypropylene layers. By incorporating polyamide
nanofibers into
a filter, the cost for producing the filter may be decreased due to the melt
spinning process as
compared to other processes, such as electrospinning, The polyamide nanofibers
also increase
filtering efficiency because of their small pore size compared to traditional
filter materials. A
filter having a polyamide nanofiber layer may also have reduced weight as
compared to
traditional filters and even the layers of construction of the filter may be
simplified due to the
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efficiency increases seen by using polyamide nanofibers. An additional
advantage of using a
polyamide nanofiber is that for filters that have pleating, as described
below, the polyamide
nanofibers can be combined with scrims or substrates that allow for less
energy to be used
during pleating due to less time and lower temperatures used during the
pleating process as
compared to traditional filters. Finally, including a layer of polyamide
nanofibers will
generally not require changes to the size of equipment specifically made for
working with the
filter, e.g., the canister surrounding the filter.
[0075] Filter Media Layers
100761 Filter media generally comprise several layers with each layer
providing different
filtration characteristics. One such layer is a scrim layer, e.g., a
reinforcing layer. In some
aspects, the scrim layer is selected to have a sizeable filtration capacity
and efficiency. In
other aspects, however, the scrim layer may have little or no filtration
capacity or efficiency.
The scrim layer may have a thickness from 0.1 to 0.81 mm, e.g., from 0.2 to
0.3 mm, or about
0.25 mm. The basis weight of the scrim layer may be from 5 to 203 gsm, e.g.,
from 5 to 60
gsm, from 15 to 45 gsm, or any values in between. The fibers of the scrim
layer may have a
median fiber diameter from 1 to 1000 micrometers, e.g., from 1 to 500
micrometers, from 1
to 100 micrometers, or any values in between. The thickness, basis weight, and
median fiber
diameter may be chosen based on the type of filter media in which the scrim is
used.
Generally, the scrim may have a Frazier air permeability at a differential
pressure of 0.5 inch
of water between 111 CFM and 1675 CFM, e.g., from 450 to 650 CFM, from 500 to
600
CFM, from 550 to 1675 or any values in between. Filtration efficiency of the
scrim layer can
be characterized by comparing the number of dust particulates with the
particle size ranging
from 0.3 p.m to 10 p.m on the upstream and downstream sides of the scrim
measured using
PALAS MFP-2000 (Geiniany) equipment. In one embodiment the filtration
efficiency of a
scrim selected for the scrim layer is measured using ISO Fine dust having 70
mg/m3 dust
concentration, a sample testing size of 1002 cm, and face velocity of 20 cm/s.
A suitable scrim
may be selected from generally commercially available scrims, or formed via
spun bonding
process or carding process or batting process or another process using a
suitable polymer. A
suitable polymer for the scrim includes but not limited to polyester,
polypropylene,
polyethylene and polyamide, e.g., a nylon or a combination of two or more of
these polymers.
For example, scrim suitable for the scrim layer is available in various
thicknesses from
suppliers including among others Berry Plastics formerly Fiberweb Inc, of Old
Hickory,
Tennessee or Cerex Advanced Fabrics, Inc. of Cantonment, Florida More than one
scrim
layer may be incorporated into the filter media.
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[0077] An additional layer in the filter media is the polyamide nanofiber
layer. In some
aspects, this layer is spun or melt blown directly onto the scrim layer or
scrim layers. In some
embodiments, the polyamide nanofiber layer has a thickness of at least 1 mm,
typically
between 1.0 mm and 6.0 mm, preferably between 0.07 mm and 3 mm, and in one
embodiment about 0.13 mm; and a basis weight less than 150 gsm (grams/m2),
e.g., a basis
weight less than 120 gsm, or basis weight of less than 100 gsm. In terms of
ranges, the basis
weight may be from 5 to 150 gsrn, e.g., from 10 to 150 gsm, from 10 to 120 gsm
or 10 to 100
gsm. The fibers of the polyamide nanofiber layer have a median fiber diameter
of between 1
nanometers and 1000 nanometers as described herein, and may be less than 1000
nanometers,
e.g., less than 907 nanometers, less than 900 nanometers, less than 800
nanometers, less than
700 nanometers, less than 600 nanometers, or less than 500 nanometers. In
terms of lower
limits, the average fiber diameter of the nanofibers in the fiber layer of the
nonwoven may
have an average fiber diameter of at least 100 nanometers, at least 110
milometers, at least
115 nanometers, at least 120 nanometers, at least 125 nanometers, at least 130
nanometers, at
least 150 nanometers, at least 300 nanometers or at least 350 nanometers. In
one
embodiment, filtration efficiency of the polyamide nanofiber layer can be
characterized by
comparing the number of dust particulates with the particle size ranging from
0.3 gm to 10
pm on the upstream and downstream sides of the media measured using PALAS MFP-
2000
(Germany) equipment.
[0078] As used here, the term "layer" does not require that the polyamide
nanofiber
completely cover the surface upon which it was spun. The layer may entirely
cover the
surface area of the underlying layer or may cover less than 99% of the surface
area, e.g., less
than 90%, less than 80%, less than 70% or less than 60%. In some aspects, the
polyamide
nanofiber layer may cover at least 5% of the surface area of the underlying
layer, e.g., at least
10%, at least 20%, at least 30%, or at least 40%. In terms of ranges, the
polyamide nanofiber
layer may cover from 5 to 100% of the layer upon which is was spun, e.g., from
5 to 99%,
from 10 to 90%, from 20 to 80%, from 30 to 70%, or from 40 to 60%. The same is
true for
layer spun onto the polyamide layer.
[0079] In addition to the scrim layer and the polyamide nanofiber layer,
conventional layers
may also be included. Such conventional layers may be formed by melt spinning
or electro
spinning.
[0080] Further descriptions of conventional filter media layers are disclosed
in several of the
references disclosed in the background of this application. In some aspects,
additional layers
may include polymers such as polyvinyl chloride (PVC), polyolefin, polyacetal,
polyester,
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cellulous ether, polyalkylene sulfide, polyarylene oxide, polysulfone,
modified polysulfone
polymers and polyvinyl alcohol, polyamide, polystyrene, polyacrylonitrile,
polyvinylidene
chloride, polymethyl methacrylate, and polyvinylidene fluoride.
[0081] Solvents used in polymeric solutions for electrostatic spinning fine
fibers may include
acetic acid, formic acid, m-cresol, tri-fluoro ethanol, hexafluoro isopropanol
chlorinated
solvents, alcohols, water, ethanol, isopropanol, acetone, and N-
methylpyrrolidone, and
methanol. The solvents are selected appropriately according to a polymer
solubility and a
desired fine fiber size. For example, a mixture of formic acid and acetic acid
may be used
with polyamide, which is also commonly known as nylon, to produce nylon fine
fibers that
can have an average fine fiber diameter less than 100 nanometers.
[0082] As described above, in some aspects, the polyamide nanofiber layer is
melt spun
directly onto a scrim layer. In specific embodiments, no solvent is used when
fabricating a
layer of nanofiber filaments. One or more additional layers may then be
deposited on top of
the polyamide nanofiber layer, e.g., two more layers, three more layers, four
more layers, or
five or more layers. In further aspects, additional layers may be deposited
onto the opposing
side of the scrim layer from the polyamide nanofiber layer. In even further
aspects, one or
more additional scrim layers may be included in the filter. The polyamide
nanofiber layer
may also be included more than one time. In still further aspects, the
polyamide nanofiber
layer is not melt spun directly onto the scrim layer, but instead is melt spun
onto a different
layer. In even further embodiments, the scrim layer is omitted and the filter
is comprised of
the polyamide nanofiber layer and other layers described herein. In each of
the aspects
described above, the polyamide nanofiber layer may be sandwiched between other
melt spun
layers, between electrospun layers, or between a melt spun and electrospun
layer.
[0083] In another aspect of the invention, one or more layers can be combined
to create a
filter media with higher thickness. The additional layers also increase the
dirt holding
capacity of the media Interestingly, the efficiency of the fabrics does not
increase much as
more layers are added. This is because the mean flow pore size does not change
substantially
with the addition of layers and the smaller particles that pass through the
first layer continue
to pass through the other layers. Layering the fabrics will provide a thicker
media increasing
the dirt holding capacity of the media but not dramatically increasing the
filtration efficiency.
A gradient filter can be created by adding another layer with higher
filtration efficiency. This
gradient filter will provide higher filtration efficiency.
[0084] Although the above description applies generally to various uses of
filter media,
further description of specific types of filters is provided below.
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Air filters
[0085] As described herein the polyamide nanofiber layer may be used in air
filters. Air
filters may be useful in application including air circulation systems in
buildings, vehicles,
vacuum cleaners, facemasks, respirator filters, and other applications
requiring filtered air.
Fluid streams such as air and gas streams often carry particulate material
therein. The
removal of some or all of the particulate material from the fluid stream is
needed. For
example, air intake streams to the cabins of motorized vehicles, air in
computer disk drives,
HVAC air, clean room ventilation and applications using filter bags, barrier
fabrics, woven
materials, air to engines for motorized vehicles, or to power generation
equipment; gas
streams directed to gas turbines; and, air streams to various combustion
furnaces, often
include particulate material therein. In the case of cabin air filters, it is
desirable to remove
the particulate matter for comfort of the passengers and/or for aesthetics.
With respect to air
and gas intake streams to engines, gas turbines and combustion furnaces,
removal of the
particulate material is needed because particulate can cause substantial
damage to the internal
workings to the various mechanisms involved. In other instances, production
gases or off
gases from industrial processes or engines may contain particulate material
therein. Before
such gases can be, or should be, discharged through various downstream
equipment to the
atmosphere, it may be desirable to obtain a substantial removal of particulate
material from
those streams.
[0086] A general understanding of some of the basic principles and problems of
air filter
design can be understood by consideration of the following types of filter
media: surface
loading media; and, depth media. Each of these types of media has been well
studied, and
each has been widely utilized. Certain principles relating to them are
described, for example,
in U.S. Pat. Nos. 5,082,476; 5,238,474; and 5,364,456.
[0087] In some aspects, the polyamide nanofibers may be formed on and adhered
to a filter
substrate. Natural fiber and synthetic fiber substrates, like spun bonded
fabrics, non-woven
fabrics of synthetic fiber and non-wovens made from the blends of cellulosics,
synthetic and
glass fibers, non-woven and woven glass fabrics, plastic screen like materials
both extruded
and hole punched, UF and MF membranes of organic polymers can be used. Sheet-
like
substrate or cellulosic non-woven web can then be formed into a filter
structure that is placed
in a fluid stream including an air stream or liquid stream for the purpose of
removing
suspended or entrained particulate from that stream. The shape and structure
of the filter
material is up to the design engineer. One important parameter of the filter
elements after
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formation is its resistance to the effects of heat, humidity or both. An
important aspect of the
filter media of the invention is the ability of the filter media to survive
contact with warm
humid air. In contact with such hot humid air streams, the polyamide
nanofibers should retain
greater than 50% of the fiber unchanged for filtration purposes after being
exposed to air
having a temperature of 60 C and 100% relative humidity for 16 hours. One
aspect of the
filter media of the invention is a test of the ability of the filter media to
survive immersion in
warm water for a significant period of time. The immersion test can provide
valuable
information regarding the ability of the polyamide nanofibers to survive hot
humid conditions
and to survive the cleaning of the filter element in aqueous solutions that
can contain
substantial proportions of strong cleaning surfactants and strong alkalinity
materials.
Preferably, the polyamide nanofibers of the invention can survive immersion in
hot water
while retaining at least 50% or even at least 75% of the fine fiber formed on
the surface of the
substrate as an active filter component. Retention of at least 50% of the
polyamide nanofibers
can maintain substantial fiber efficiency without loss of filtration capacity
or increased back
pressure. The thickness of the typical polyamide nanofiber filtration layer
ranges from 0.001
to 5 microns, e.g., from 0.01 to 3 microns with a polyamide nanofibers basis
weight ranging
from about 0.01 to 240 micrograrns/cm2. The polyamide nanofiber layer formed
on the
substrate in the filters should be substantially uniform in both filtering
performance and fiber
location. Substantial uniformity means that the fiber has sufficient coverage
of the substrate
to have at least some measurable filtration efficiency throughout the covered
substrate.
Adequate filtration can occur with a wide variation in fiber add-on.
Accordingly, the
polyamide nanofiber layers may vary in fiber coverage, basis weight, layer
thickness or other
measurement of fiber add-on and still remain well within the bounds of the
invention. Even a
relatively small add-on of fine fiber may add efficiency to the overall filter
structure.
[0088] The "lifetime" of a filter is typically defined according to a selected
limiting pressure
drop across the filter. The pressure buildup across the filter defines the
lifetime at a defined
level for that application or design. Since this buildup of pressure is a
result of load, for
systems of equal efficiency a longer life is typically directly associated
with higher capacity.
Efficiency is the propensity of the media to trap, rather than pass,
particulates. Typically the
more efficient a filter media is at removing particulates from a gas flow
stream, in general,
the more rapidly the filter media will approach the "lifetime" pressure
differential (assuming
other variables to be held constant). In this application the term "unchanged
for filtration
purposes" refers to maintaining sufficient efficiency to remove particulate
from the fluid
stream as is necessary for the selected application.
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[0089] Paper filter elements are widely used folins of surface loading media.
In general,
paper elements comprise dense mats of cellulose, synthetic or other fibers
oriented across a
gas stream carrying particulate material. The paper is generally constructed
to be permeable
to the gas flow, and to also have a sufficiently fine pore size and
appropriate porosity to
inhibit the passage of particles greater than a selected size therethrough. As
the gases (fluids)
pass through the filter paper, the upstream side of the filter paper operates
through diffusion
and interception to capture and retain selected sized particles from the gas
(fluid) stream. The
particles are collected as a dust cake on the upstream side of the filter
paper. In time, the dust
cake also begins to operate as a filter, increasing efficiency. This is
sometimes referred to as
"seasoning," i.e. development of an efficiency greater than initial
efficiency.
[0090] A simple filter design such as that described above is subject to at
least two types of
problems. First, a relatively simple flaw, i.e. rupture of the paper, results
in failure of the
system. Secondly, particulate material rapidly builds up on the upstream side
of the filter, as a
thin dust cake or layer, increasing the pressure drop. Various methods have
been applied to
increase the "lifetime" of surface-loaded filter systems, such as paper
filters. One method is
to provide the media in a pleated construction, so that the surface area of
media encountered
by the gas flow stream is increased relative to a flat, non-pleated
construction. While this
increases filter lifetime, it is still substantially limited. For this reason,
surface loaded media
has primarily found use in applications wherein relatively low velocities
through the filter
media are involved, generally not higher than about 20-30 feet per minute and
typically on
the order of about 10 feet per minute or less. The term "velocity" in this
context is the
average velocity through the media (i.e. flow volume per media area).
[0091] In general, as air flow velocity is increased through a pleated paper
media, filter life is
decreased by a factor proportional to the square of the velocity. Thus, when a
pleated paper,
surface loaded, filter system is used as a particulate filter for a system
that requires substantial
flows of air, a relatively large surface area for the filter media is needed.
For example, a
typical cylindrical pleated paper filter element of an over-the-highway diesel
truck will be
about 9-15 inches in diameter and about 12-24 inches long, with pleats about 1-
2 inches deep.
Thus, the filtering surface area of media (one side) is typically 30 to 300
square feet.
[0092] In many applications, especially those involving relatively high flow
rates, an
alternative type of filter media, sometimes generally referred to as "depth"
media, is used. A
typical depth media comprises a relatively thick tangle of fibrous material.
Depth media is
generally defined in terms of its porosity, density or percent solids content.
For example, a 2-
3% solidity media would be a depth media mat of fibers arranged such that
approximately 2-
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3% of the overall volume comprises fibrous materials (solids), the remainder
being air or gas
space.
[0093] Another useful parameter for defining depth media is fiber diameter. If
percent
solidity is held constant, but fiber diameter (size) is reduced, pore size or
interfiber space is
reduced, i.e. the filter becomes more efficient and will more effectively trap
smaller particles.
[0094] A typical conventional depth media filter is a deep, relatively
constant (or uniform)
density, media, i.e. a system in which the solidity of the depth media remains
substantially
constant throughout its thickness. By "substantially constant" in this
context, it is meant that
only relatively minor fluctuations in density, if any, are found throughout
the depth of the
media. Such fluctuations, for example, may result from a slight compression of
an outer
engaged surface, by a container in which the filter media is positioned.
[0095] Gradient density depth media arrangements have been developed. Some
such
arrangements are described, for example, in U.S. Pat. Nos. 4,082,476;
5,238,474; and
5,364,456. In general, a depth media arrangement can be designed to provide
"loading" of
particulate materials substantially throughout its volume or depth. Thus, such
arrangements
can be designed to load with a higher amount of particulate material, relative
to surface
loaded systems, when full filter lifetime is reached. However, in general the
tradeoff for such
arrangements has been efficiency, since, for substantial loading, a relatively
low solidity
media is desired. Gradient density systems such as those in the patents
referred to above,
have been designed to provide for substantial efficiency and longer life. In
some instances,
surface loading media is utilized as a "polish" filter in such arrangements.
[0096] A filter media construction according to the present invention includes
a first layer of
permeable coarse fibrous media or substrate having a first surface. A first
layer of polyamide
nanofiber media is secured to the first surface of the first layer of
permeable coarse fibrous
media and a second layer of polyamide nanofiber is secured to the substrate.
Preferably the
first layer of permeable coarse fibrous material comprises fibers having an
average fiber
diameter of at least 10 microns, typically and preferably about 12 (or 14) to
30 microns. Also
preferably the first and second layer of permeable coarse fibrous material
comprises a media
having a basis weight of no greater than about 200 gsm (grams/meter2 or g/m2),
preferably
about 0.50 to 150 gsm, and most preferably at least 8 gsm. Preferably the
first layer of
permeable coarse fibrous media is at least 0.0005 inch (12 microns) thick, and
typically and
preferably is about 0.001 to 0.030 inch (25-800 microns) thick.
[0097] In some arrangements, the first layer of permeable coarse fibrous
material comprises a
material which, if evaluated separately from a remainder of the construction
by the Frazier
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permeability test, would exhibit a permeability of at least 1 meter(s)/min,
and typically and
preferably about 2-900 meters/min (about 0.03-15 m-sec-i). Herein when
reference is made
to efficiency, unless otherwise specified, reference is meant to efficiency
when measured
according to ASTM-1215-89, with 0.78g monodisperse polystyrene spherical
particles, at 20
fpm (6.1 meters/min) as described herein.
[0098] In some aspects, the layer of polyamide nanofiber secured to the first
surface of the
layer of permeable coarse fibrous media is a layer of nano- and microfiber
media wherein the
fibers have average fiber diameters of no greater than about 2 microns,
generally and
preferably no greater than about 1 micron, and typically and preferably have
fiber diameters
smaller than 0.5 micron and within the range of about 0.05 to 0.5 micron.
Also, preferably the
first layer of fine fiber material secured to the first surface of the first
layer of permeable
coarse fibrous material has an overall thickness that is no greater than about
30 microns, more
preferably no more than 20 microns, most preferably no greater than about 10
microns, and
typically and preferably that is within a thickness of about 1-8 times (and
more preferably no
more than 5 times) the fine fiber average fiber diameter of the layer.
[0099] Certain aspects include filter media as generally defined, in an
overall filter
construction. Some preferred arrangements for such use comprise the media
arranged in a
cylindrical, pleated configuration with the pleats extending generally
longitudinally, i.e. in
the same direction as a longitudinal axis of the cylindrical pattern. For such
arrangements, the
media may be imbedded in end caps, as with conventional filters. Such
arrangements may
include upstream liners and downstream liners if desired, for typical
conventional purposes.
[0100] In some applications, media according to the present invention may be
used in
conjunction with other types of media, for example conventional media, to
improve overall
filtering performance or lifetime. For example, media according to the present
invention may
be laminated to conventional media, be utilized in stack arrangements; or be
incorporated (an
integral feature) into media structures including one or more regions of
conventional media.
It may be used upstream of such media, for good load; and/or, it may be used
downstream
from conventional media, as a high efficiency polishing filter.
[0101] Certain arrangements according to the present invention may also be
utilized in liquid
filter systems, i.e. wherein the particulate material to be filtered is
carried in a liquid. In
specific applications such as hot fluids, the melt point of nylon nanofiber
fabrics provides an
advantage. Melt points of nylon nanofiber fabrics may be from 223 C to 360 C,
e.g., from
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225 C to 350 C. Also, certain arrangements according to the present invention
may be used
in mist collectors, for example arrangements for filtering fine mists from
air.
[0102] Various filter designs are shown in patents disclosing and claiming
various aspects of
filter structure and structures used with the filter materials. U.S. Pat. No.
4,720,292, disclose
a radial seal design for a filter assembly having a generally cylindrical
filter element design,
the filter element being sealed by a relatively soft, rubber-like end cap
having a cylindrical,
radially inwardly facing surface. U.S. Pat. No. 5,082,476, disclose a filter
design using a
depth media comprising a foam substrate with pleated components combined with
the
microfiber materials of the invention. U.S. Pat. No. 5,104,537, relate to a
filter structure
useful for filtering liquid media. Liquid is entrained into the filter
housing, passes through the
exterior of the filter into an interior annular core and then returns to
active use in the
structure. Such filters are highly useful for filtering hydraulic fluids. U.S.
Pat. No. 5,613,992,
show a typical diesel engine air intake filter structure. The structure
obtains air from the
external aspect of the housing that may or may not contain entrained moisture.
The air passes
through the filter while the moisture can pass to the bottom of the housing
and can drain from
the housing. U.S. Pat. No. 5,820,646, disclose a Z filter structure that uses
a specific pleated
filter design involving plugged passages that require a fluid stream to pass
through at least
one layer of filter media in a "Z" shaped path to obtain proper filtering
performance. The
filter media formed into the pleated Z shaped format can contain the fine
fiber media of the
invention. U.S. Pat. No. 5,853,442 discloses a bag house structure having
filter elements that
can contain the fine fiber structures of the invention. U.S. Pat. No.
5,954,849 shows a dust
collector structure useful in processing typically air having large dust loads
to filter dust from
an air stream after processing a workpiece generates a significant dust load
in an
environmental air. Lastly, U.S. Design Patent No. 425,189, discloses a panel
filter using the Z
filter design.
[0103] The media can be a polyester synthetic media, a media made from
cellulose, or blends
of these types of materials. One example of usable cellulose media is: a basis
weight of about
45-55 lbs./3000 ft2 (84.7 g/m2), for example, 48-54 lbs./3000 ft2; a thickness
of about 0.005-
0.015 in, for example about 0.010 in. (0.25 mm); frazier permeability of about
20-25 ft/min,
for example, about 22 ft/min (6.7 m/min); pore size of about 55-65 microns,
for example,
about 62 microns; wet tensile strength of at least about 7 lbs/in, for
example, 8.5 lbs./in (3.9
kg/in); burst strength wet off of the machine of about 15-25 psi, for example,
about 23 psi
(159 kPa). The cellulose media can be treated with fine fiber, for example,
fibers having a
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size (diameter) of 5 microns or less, and in some instances, submicron. A
variety of methods
can be utilized for application of the fine fiber to the media, if it is
desired to use fme fiber.
Some such approaches are characterized, for example, in U.S. Patent No.
5,423,892, column
32, at lines 48-60. More specifically, such methods are described in U.S.
Patent Nos.
3,878,014; 3,676,242; 3,841,953; and 3,849,241. Enough fine fiber typically
would be
applied until the resulting media construction would have the individual test
between
50 to 90%, tested according to SAE J726C, using SAE fine dust, and an overall
efficiency
.of greater than 90%.
[0104] Example of usable filter constructions are described in U.S. Patent No.
5,820,646. In
another example embodiment the fluted construction (not shown) includes
tapered flutes. By
"tapered," it is meant that the flutes enlarge along their length such that
the downstream
opening of the flutes is larger than the upstream opening. Such filter
constructions are
described in U.S. App. Ser. No. 08/639,220. Details about fine fiber and its
materials and
manufacture is disclosed in U.S. App. Ser. No. 09/871,583.
[0105] Various filter designs are shown in patents disclosing and claiming
various aspects of
filter structure and structures used with the filter materials. U.S. Patent
No. 7,008,465
discloses filter designs which may be used in a wet-dry vacuum. U.S. Patent
No. 4,720,292
discloses a radial seal design for a filter assembly having a generally
cylindrical filter element
design, the filter element being sealed by a relatively soft, rubber-like end
cap having a
cylindrical, radially inwardly facing surface. U.S. Patent No. 5,082,476
discloses a filter
design using a depth media comprising a foam substrate with pleated components
combined
with the tnicrofiber materials of the invention. U.S. Patent No. 5,104,537
relates to a filter
structure useful for filtering liquid media, Liquid is entrained into the
filter housing, passes
through the exterior of the filter into an interior annular core and then
returns to active use in
the structure. Such filters are highly useful for filtering hydraulic fluids.
U.S. Patent No.
5,613,992 shows atypical diesel engine air intake filter structure. The
structure obtains air
from the external aspect of the housing that may or may not contain entrained
moisture. The
air passes through the filter while the moisture can pass to the bottom of the
housing and can
drain from the housing. U.S. Patent No. 5,820,646 discloses a Z filter
structure that uses a
specific pleated filter design involving plugged passages that require a fluid
stream to pass
through at least one layer of filter media in a "Z" shaped path to obtain
proper filtering
performance. The filter media formed into the pleated Z shaped format can
contain the fine
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fiber media of the invention. U.S. Patent No. 5,853,442, disclose a bag house
structure having
filter elements that can contain the fine fiber structures of the invention.
Berkhoel et al., U.S.
Pat. No. 5,954,849, show a dust collector structure useful in processing
typically air having
large dust loads to filter dust from an air stream after processing a
workpiece generates a
significant dust load in an environmental air. Lastly, Gillingham, U.S. Design
Pat. No.
425,189, discloses a panel filter using the Z filter design.
Oil Filters
[0106] Oil filters intended for use in combustion engines conventionally
comprise filter
media with fibers obtained from wood pulp. Such wood pulp fibers are typically
1 to 7
millimeters long and 15 to 45 microns in diameter. Natural wood pulp has
largely been the
preferred raw material for producing filtration media due to its relatively
low cost,
processability, various mechanical and chemical properties, and durability in
the end
application. The filter media are pleated to increase filtration surface area
transversally to the
direction of the oil flow.
[0107] U.S. Pat. No. 3,288,299 discloses a dual type of oil filter cartridge
wherein part of the
flow is through a surface type of filter element, such as pleated paper, and
the rest of the flow
is through a depth type of filter element such as a thick fibrous mass. An oil
filter and adapter
is disclosed in U.S. Pat. No. 3,912,631.
[0108] A typical oil filter includes pleated filter media (or filtration
media) a backing
structure. A conventional filter media exhibits low stiffness and has poor
mechanical strength
in terms of tensile strength and burst strength. The filter media is therefore
used together with
a metal mesh or other type of pleat shape when used in the end application.
[0109] Nevertheless, in view of the low mechanical strength the filter media
tend to burst
over time on exposure to engine oil at the temperatures encountered in a
combustion engine,
such as 125 to 135 C.
[0110] Although filter media products that are produced largely with wood pulp
are still an
excellent choice for most automotive and heavy duty oil filtration
applications, there is a
growing market demand for oil filtration products that exhibit increased
strength and
durability over time as the media is exposed to the various chemical, thermal,
and mechanical
stresses of the end application environment. This demand stems from both
harsher end
application conditions that the media is exposed to as well as increasing
demand for filter
media that can be safely used in the end application for increasingly longer
amounts of time
without rupturing or failing.
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101111 The long-standing and widely applied solution to this demand has been
to incorporate
some minor quantity of synthetic fiber, typically PET polyester, in the amount
of about 5-
20%. The result of fortifying the fiber furnish in this way is higher media
strength as well as
enhanced chemical and mechanical durability when the media is exposed to the
end
application environment, due to the superior chemical, thermal, and mechanical
durability of
the synthetic fibers themselves.
[0112] For air filters there are alternative technical solutions primarily
based on non-natural
fibers described in the art. U.S. Pat. No. 7,608,125 discloses a MERV filter
composed of a
wet laid fibrous mat comprising about 20-60 wt.% of glass fibers, about 15-60
wt.% of
polymer fibers, and about 15-40 wt.% of a binder for bonding of the fibers.
The binder of this
disclosure is latex modified with melamine formaldehyde.
[0113] U.S. Pub. No. 2012/0175298 discloses a HEPA filter comprising a
nonwoven web of
two different fiber components. The first fiber component is formed by fibers
of polyesters,
polyamides, polyolefin, polylactide, cellulose esters, polycaprolactone, up at
least 20% of the
weight of web. The second fiber can be composed of either cellulosic fibres
(Lyocell) or glass
or combination of the two. There is further a binder component formed by
acrylic polymers,
styrenic polymers, vinyl polymers polyurethanes, and combinations thereof.
[0114] U.S. Pub. No. 2013/0233789 discloses a glass-free non-woven fuel
filtration media
that is comprised of a blend of a staple synthetic fibers and fibrillated
cellulosic fibers.
[0115] U.S. Pat. Nos. 7,488,365, 8,236,082 and 8,778,047 disclose further
filtration media
containing 50 to 100% of synthetic fibers of the weight of the fibrous web. In
fact, the known
filtration media containing a high percentage of synthetic fibers are not
pleateable or self-
supporting as such, and they have to be co-pleated and reinforced with some
sort of
additional mechanical support layer, such as a plastic or wire mesh backing.
[0116] Media made with high levels of synthetic fiber typically tend to
exhibit drape and they
lack sufficient stiffness and rigidity causing the pleats to collapse without
an additional
support. A 100% synthetic media as disclosed in the art cannot maintain a
grooving pattern
like corrugation or a pleated structure due to the thermal and mechanical
properties of the
synthetic fibers. The fibrous media according to the present invention is
readily groovable,
i.e. corrugatable, and pleatable. And the material is capable of maintaining
most of its
original groove depth (or corrugation depth) even after long exposure times in
hot engine oil
having e.g. a temperature of 140 C. This feature also contributes to extended
operation life of
the present fibrous media.
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[0117] Some oil filters are able to omit expensive backing materials, allowing
for more easily
groovable (or corrugatable) and pleatable filters. The end result is the
ability to produce a
filter with the present fibrous media without a support backing material while
also achieving
significantly higher burst strength than possible with traditional style oil
filtration media that
contain wood pulps, excellent resistance to glycol assisted disintegration and
excellent dust
filtration capacity and particle removal efficiency.
[0118] By incorporating a polyamide nanofiber layer into oil filters, several
of the above
described problems may be alleviated due to the aforementioned benefits of the
polyamide
nanofiber layer.
[0119] As with other filter media described herein, the oil filter is
typically a multi-layer
filter. Exemplary thermoplastic fibers suitable for additional layers in an
oil filter include
polyesters (e.g., polyalkylene terephthalates such as polyethylene
terephthalate (PET),
polybutylene terephthalate (PBT) and the like), polyalkylenes (e.g.,
polyethylenes,
polypropylenes and the like), polyacrylonitriles (PAN), and additional
polyamide layers
(nylons, for example, nylon-6, nylon 6,6, nylon-6,12, and the like). Preferred
are PET fibers
which exhibit good chemical and thermal resistance which are properties of
importance for
the use of the media as oil filters.
[0120] In an embodiment, the thermoplastic synthetic fibers are selected from
fibers having
an average fiber diameter from 0.1 pm to 15 pm, such as 0.1 pm to 10 gm, and
an average
length from 1 to 50 mm, such as 1 to 20 mm. In general, fibers having a length
greater than 5
mm, in particular greater than 10 mm, are preferred for good burst strength.
In the present
context, "silicacious fibers" primarily stands for "glass" fibers such as
microglass fibers.
[0121] Such fibers generally have an aspect ratio (ratio of length to
diameter) of 1,000 to 1.
In one embodiment, the glass fibers have an average fiber diameter from 0.1 pm
to 5 pm, and
an aspect ratio of 1,000 to 1. In particular, the glass fibers may have an
average fiber diameter
of 0.4 to 2.6 p.m. Glass fibers are preferably included in a sufficient amount
to improve
efficiency of the fibrous media as a filter. In one embodiment, the synthetic
fibers comprise
up to 30 wt. %, preferably up to 20 wt.%, based on the total weight of the
fibers, of glass
fibers. Although the synthetic fibers comprise only up to 30 wt.% or up to 20
wt.% of glass
fibers, based on the total weight of the fibers, this amount is sufficient to
prepare a fibrous
media for filter examples. Typically, synthetic filter media of the prior art
include a high
amount of glass fibers for achieving a sufficient filtration efficiency of a
gas or a liquid, even
under high temperature conditions such as e.g. 150 C. However, by using less
glass fibers in
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the fibrous media as set forth in the claims, fibrous media may be provided
that have
excellent filtration properties in terms of particle removal efficiency and
hot oil burst
strength. In particularly preferred embodiments, there are at least two kinds
of glass fibers
present, viz, a first group of fibers having an average fiber diameter of less
than 1 pm and a
second group which having an average fiber diameter of 2 pm or more. The
weight ratio of
the two groups of fibers is typically 1:100 to 100:1, in particular about 1:10
to 10:1. The
synthetic fibers may also include up to 40% by weight, preferably up to 30% by
weight,
based on the total weight of the fibers, of a regenerated cellulosic material,
such as Lyocell or
viscose or combinations thereof.
[0122] The filter media may be contained in a canister, including a single or
dual canister.
Each canister may have an inlet and an outlet for introducing oil flow and
removing filter oil,
respectively. The filter media in each canister may differ to allow for
different filtration
capacities. For example, a first canister would contain a filter housing for a
full-flow path
filtration while a second canister would contain a filter housing for a
reduced-flow path
filtration. U.S. Pub. No. 2008/0116125 describes such dual canisters in
detail.
Ba2 Filters
[0123] Bag filters have been described in the art, including in U.S. Patent
No. 7,318,852 and
U.S. Pub. No. 2009/2055226. Dust collectors, also known as bag houses, are
generally used
to filter particulate material from industrial effluent or off-gas. Once
filtered, the cleaned off-
gas can be vented to the atmosphere or recycled. Such a bag house dust
collector structure
generally includes one or more flexible filter banks supported within a
cabinet or similar
structure. In such a filter cabinet and bank, the filter bag is generally
secured within the
cabinet and maintained in a position such that effluent efficiently passes
through the bag
thereby removing entrained particulates. The filter bag, secured within the
cabinet, is
typically supported by a structure that separates the upstream and downstream
air and
supports the filter bag to maintain efficient operation.
[0124] More specifically, in a so-called "baghouse filter", particulate
material is removed
from a gaseous stream as the stream is directed through the filter media. In a
typical
application, the filter media has a generally sleeve-like tubular
configuration, with gas flow
arranged so as to deposit the particles being filtered on the exterior of the
sleeve. In this type
of application, the filter media is periodically cleaned by subjecting the
media to a pulsed
reverse-flow, which acts to dislodge the filtered particulate material from
the exterior of the
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sleeve for collection in the lower portion of the baghouse filter structure.
U.S. Pat. No.
4,983,434 illustrates a baghouse filter structure and a prior art filter
laminate.
[0125] The separation of particulate impurities from industrial fluid streams
is often
accomplished using fabric filters. These textile based filter media remove
particulate from the
fluids. When the resistance to flow or pressure drop through the textile
caused by
accumulation of particulate on the filter becomes significant, the filter must
be cleaned, and
the particulate cake removed.
[0126] It is common in the industrial filtration market to characterize the
type of filter bag by
the method of cleaning. The most common types of cleaning techniques are
reverse air,
shaker and pulse jet. Reverse air and shaker techniques are considered low
energy cleaning
techniques.
[0127] The reverse air technique is a gentle backwash of air on a filter bag
which collects
dust on the interior. The back wash collapses the bag and fractures dust cake
which exits the
bottom of the bag to a hopper. Shaker mechanisms clean filter cake that
collects on the inside
of a bag as well. The top of the bag is attached to an oscillating arm which
creates a
sinusoidal wave in the bag to dislodge the dust cake. Pulse jet cleaning
techniques employs a
short pulse of compressed air that enters the interior top portion of the
filter tube. As the pulse
cleaning air passes through the tube venturi it aspirates secondary air and
the resulting air
mass violently expands the bag and casts off the collected dust cake. The bag
will typically
snap right back to the cage support and go right back into service collecting
particulate.
[0128] Of the three cleaning techniques, the pulse jet is the most stressful
on the filter media.
However, in recent years industrial process engineers have increasingly
selected pulse jet
baghouses.
101291 The need for high temperature (up to 200 C.), thermally stable,
chemically resistant
filter media in baghouses narrows the choice of filter media to only a few
viable candidates
for pulse jet applications. Common high temperature textiles comprise
polytetrafluoroethylene (PTFE), fiberglass, or polyimides (polyimides are
stable for
continuous use to 260 C.). When the effect of high temperature is combined
with the effect
of oxidizing agents, acids or bases, there is a tendency for fiberglass and
polyimide media to
fail prematurely. Thus, there is a preference for using PTFE. Commercially
available PTFE
fabrics are supported needlefelts of PTFE fiber. These felts usually weigh
from 20-26 oz/yd2
and are reinforced with a multifilament woven scrim (4-6 oz/yd2). The felts
are made up of
staple fibers, (usually 6.7 denier/filament, or 7.4 dtex/filament) and 2-6
inches in length. This
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product works similarly to many other felted media in that a primary dust cake
"seasons" the
bag. This seasoning, sometimes called in-depth filtration, causes the media to
filter more
efficiently but has a drawback in that the pressure drop increases across the
media during use.
Eventually the bag will blind or clog and the bags will have to be washed or
replaced. In
general, the media suffers from low filtration efficiency, blinding and
dimensional instability
(shrinkage) at high temperatures.
[0130] Another type of structure designed for high temperatures is described
in U.S. Pat. No.
5,171,339. A bag filter is disclosed that comprises a bag retainer clothed in
a filter bag. The
cloth of said filter bag comprises a laminate of a felt of poly(m-phenylene
isophthalamide),
polyester or polyphenylenesulfide fibers having a thin nonwoven fabric of
poly(p-phenylene
terephthalamide) fibers needled thereto, the poly(p-phenylene terephthalamide)
fabric being
positioned at the surface of the filter bag first exposed to the hot particle
laden gas stream.
The poly(p-phenylene terephthalamide) fabric can have a basis weight of from 1
to 2 oz/yd2.
[0131] A two layer product of porous expanded PTFE (ePTFE) membrane laminated
to
woven porous expanded PTFE fiber fabric has also been used. Commercial success
of this
product has not been realized due to several reasons, but primarily due to the
woven fiber
fabric backing not wearing well on the pulse jet cage supports. The woven yams
slide on
themselves and create excessive stress on the membrane, resulting in membrane
cracks.
[0132] Nonwoven fabrics have been advantageously employed for manufacture of
filter
media. Generally, nonwoven fabrics employed for this type of application have
been
entangled and integrated by mechanical needle-punching, sometimes referred to
as "needle-
felting", which entails repeated insertion and withdrawal of barbed needles
through a fibrous
web structure.
[0133] U.S. Patent No. 4,556,601 discloses a hydroentangled, nonwoven fabric,
which may
be used as a heavy-duty gas filter.
[0134] U.S. Patent No. 6,740,142 discloses nanofibers for use in baghouse
filters. A flexible
bag is at least partially covered by a layer having a basis weight of 0.005 to
2.0 grams per
square meter (gsm) and a thickness of 0.1 to 3 microns. The layer comprises a
polymeric fine
fiber with a diameter of about 0.01 to about 0.5 micron, but is limited in
basis weight due to
the limitations of the process used to produce it.
[0135] In some aspects, the filter may comprise a filtration medium including
a thermally-
stabilized nanoweb layer having a basis weight of greater than about 0.1 gsm,
or greater than
about 0.5 gsm, or greater than about 5 gsm, or even greater than about 10 gsm
and up to
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about 90 gsm. The filtration medium further comprises a substrate to which the
nanoweb is
bonded in a face-to-face relationship. Advantageously, the nanoweb layer is
positioned on the
upstream surface or side of the filter bag, i.e. on the surface which is first
exposed to the hot,
particle-laden gas stream.
[0136] In a further embodiment the filter comprises a composite of a first
substrate layer
having a thermally-stabilized nanoweb bonded thereto in a face-to-face
relationship, the
nanoweb being positioned on the upstream side of the filter bag, i.e. at the
surface of the filter
bag first exposed to the hot, particle-laden gas stream, wherein the nanoweb
has a basis
weight of greater than about 0.1 gsm, and a second substrate layer bonded to
the nanoweb
layer. In some cases it is advantageous that the second substrate layer is
positioned in
between the nanoweb and the first substrate layer, while in other cases it is
desirable that the
nanoweb layer be positioned between the first and second substrate layers.
[0137] Polymers useful for electroblowing or melt blowing nanofiber webs of
the present
invention are polyamides (PA), and preferably a polyamide selected from the
group
consisting of polyamide 6, polyamide 6,6, polyamide 6,12, polyamide 11,
polyamide 12,
polyamide 4,6, a semi-aromatic polyamide, high temperature polyamide and any
combination
or blend thereof The polyamides (PA) used in preparing the blending
composition of the
invention are well known in the art. Representative polyamides include
semicrystalline and
amorphous polyamide resins of a molecular weight of at least 5,000 as
described, for
instance, in U.S. Pat. Nos. 4,410,661; 4,478,978; 4,554,320; and 4,174,358.
[0138] In accordance with the invention, polyamides obtained by
copolymerization of two of
the above polymers, by terpolymerization of the above polymers or their
component
monomers, e.g., a copolymer of adipic acid, isophthalic acid and
hexamethylenediamine, or
blended mixtures of polyamides such as a mixture of PA 6, 6 and PA 6 may also
be used.
Preferably, the polyamides are linear and have melting points or softening
points above 200
C.
[0139] Such polyamides formed by electrospinning may be used in addition to
the inventive
polyamide nanofiber layer that is formed by melt spinning. The polyamide used
to spin the
fibers comprises a thermal stability additive, such as an antioxidant.
Suitable antioxidants for
use in the invention are any materials that are soluble in the spinning
solvent with the
polyamide if the polyamide is spun from solution. Examples of such materials
are copper
halides and hindered phenols. By "hindered phenol" is meant a compound whose
molecular
structure contains a phenolic ring in which one or both of the carbon atoms
cis to the
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hydroxyl moiety holds an alkyl group. The alkyl group is preferably a tertiary
butyl moiety
and both adjacent carbon atoms hold a tertiary butyl moiety.
[0140] Antioxidants include but are not limited to: phenolic amides such as
hexamethylene bis(3,5-di-(tert)-butyl-4-hydroxyhydrocinnamamide) (Irganox
1098); amines
such as various modified benzenamines (e.g. Irganox 5057); phenolic esters
such as
ethylenebis(oxyethylene)bis-(3-(5-tert-buty1-4-hydroxy-m-toly1)-propionate
(Irganox 245)
(all available from Ciba Specialty Chemicals Corp., Tarrytown, N.Y.); organic
or inorganic
salts such as mixtures of cuprous iodide, potassium iodide, and zinc salt of
octadecanoic acid,
available as Polyad 201 (from Ciba Specialty Chemicals Corp., Tarrytown,
N.Y.), and
mixtures of cupric acetate, potassium bromide, and calcium salt of
octadecanoic acid,
available as Polyad 1932-41 (from Polyad Services Inc., Earth City, Mo.);
hindered amines
such as 1,3,5-triazine-2,4,6-triamine,N,Nw41,2-ethane-diyl-bis[[[4,6-bis-
[butyl (1,2,2,6,6-
pentamethy1-4-piperidinypaminol-1,3,5-triazine-2-yllimino]-3,1-propanediyl]]
bis[N',N"-
dibutyl-M,N"-bis(1,2,2,6,6-pentamethy1-4-piperidinyl) (Chimassorb 119 FL), 1,6-
hexanediamine, N,M-bis(2,2,6,6-tetramethy1-4-piperidiny1)-polymer with 2,4,6-
trichloro-
1,3,5-triazine, reaction products with N-butyl-l-butanamine an N-buty1-2,2,6,6-
tetramethy1-
4-piperidinamine (Chimassorb 2020), and poly [[64(1,1,3,3-
tetramethylbutypamino]-1,3,5-
triazine-2,4-diyl][2,2,6,6-tetramethy1-4-piperidinypimino]-1,6-
hexanediy1[(2,2,6,6-
tetramethy1-4-piperidinyl)imino]]) (Chimassorb 944) (all available from Ciba
Specialty
Chemicals Corp., Tarrytown, N.Y.); polymeric hindered phenols such as 2,2,4
trimethyl-1,2
dihydroxyquinoline (Ultranox 254 from Crompton Corporation, a subsidiary of
Chemtura
Corporation, Middlebury, Conn., 06749); hindered phosphites such as bis(2,4-di-
t-
butylphenyl) pentaerythritol diphosphite (Ultranox 626 from Crompton
Corporation, a
subsidiary of Chemtura Corporation, Middlebury, Conn., 06749); and tris(2,4-di-
tert-butyl-
phenyl) phosphite (Irgafos 168 from Ciba Specialty Chemicals Corp., Tarrytown,
N.Y.); 3-
(3,5-di-tert-buty1-4-hydroxyphenyl)propionic acid (Fiberstab PA6, available
from Ciba
Specialty Chemicals Corp., Tarrytown, N.Y.), and combinations and blends
thereof.
[0141] The antioxidant agent used as stabilizer may be between 0.01 and 10% by
weight
relative to the polyamide layer formed by electrospinning and especially
between 0.05 and
5% by weight.
[0142] The substrate layers of the bag filter may be formed from a variety of
conventional
fibers including cellulosic fibers such as cotton, hemp or other natural
fibers, inorganic fibers
including glass fibers, carbon fibers or organic fibers such as polyesters,
polyimides,
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polyamides, polyolefins, or other conventional fibers or polymeric materials
and mixtures
thereof.
[0143] The substrate layers of the filter bag of the invention can be woven or
non-woven. In
woven bags, the fibers are typically formed into an interlocking mesh of fiber
in a typical
woven format. Non-woven fabrics are typically made by loosely forming the
fibers in no
particular orientation and then binding the fibers into a filter fabric. One
mode of constructing
the elements of the invention includes using a felt media as a substrate.
Felts are a
compressed, porous, non-woven fabric made by laying discrete natural or
synthetic fibers and
compressing the fibers into a felt layer using commonly available felt bonding
technology
that would be known to one skilled in the art.
[0144] Fibers are typically used which result in fabrics that exhibit
excellent resilience and
resistance to the effects of the passage of air and the entrapment of
particulates. The fabrics
can have stability with respect to chemical particulates, and can be stable
with respect to
varying temperatures of both the air passing through the bag house and the
temperature of the
particulate entrained on the filter surface.
[0145] The filter structures of the invention are typically maintained in
their useful open
shape by supporting the substrate plus nanoweb layer composite on a suitable
support
structure such as a retainer at the neck of a bag, or a support structure can
be located in the
interior of the bag. Such supports can be formed from linear members in the
form of a wound
wire or cage-like structure. Alternatively, the support can comprise a
perforated ceramic or
metal structure that mimics the shape of the bag. If the support structure
contacts the filter
substrate over a significant fraction of its surface area, the support
structure should be
permeable to the passage of air through the structure and should provide no
incremental
increase in pressure drop over the filter bag. Such support structures can be
formed such that
they contact the entirety of the interior of the filter bag and maintain the
filter bag in an
efficient filtration shape or confirmation.
[0146] A process for combining the nanoweb layers with the substrate to
produce the present
composite structure is not specifically limited. The nanofibers of the nanoweb
layer can be
physically entwined in the substrate layer, or they can be bonded by inter-
fusion of the fibers
of the nanoweb layer with those of the substrate, for example by thermal,
adhesive or
ultrasonic lamination or bonding.
[0147] Thermal methods for bonding the substrate layer to the nanoweb layer or
a nanoweb
plus substrate layer include calendering. "Calendering" is the process of
passing a web
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through a nip between two rolls. The rolls may be in contact with each other,
or there may be
a fixed or variable gap between the roll surfaces.
[0148] Advantageously, in the calendering process, the nip is formed between a
soft roll and
a hard roll. The "soft roll" is a roll that deforms under the pressure applied
to keep two rolls
in a calender together. The "hard roll" is a roll with a surface in which no
deformation that
has a significant effect on the process or product occurs under the pressure
of the process. An
"unpafterned" roll is one which has a smooth surface within the capability of
the process used
to manufacture them. There are no points or patterns to deliberately produce a
pattern on the
web as it passed through the nip, unlike a point bonding roll. The hard roll
in the process of
calendering used in the present invention can be patterned or unpatterned.
[0149] Adhesive lamination can be carried out in conjunction with calendering
or by
application of pressure by other means to the laminate in the presence of a
solvent based
adhesive at low temperatures, for example room temperature. Alternatively a
hot melt
adhesive can use used at elevated temperatures. One skilled in the art will
readily recognize
suitable adhesives that can be used in the process of the invention.
[0150] Examples of methods of entwining the fibers according to such a
physical bonding
are needle punch processing and water-jet processing, otherwise known as
hydroentangling
or spun lacing. Needle punching (or needling) consists essentially of tucking
a small bundle
of individual fibers down through a carded batt of fibers in such large
numbers of
penetrations that a cohesive textile structure is formed, as disclosed in U.S.
Patent Nos.
3,431,611 and 4,955,116.
[0151] For the process of manufacturing the filter of the present invention it
is desirable to
perform needle punch processing (or water-jet processing) on the high-density
layer
(substrate) side of the nonwoven fabric. Compared to the case where needle
punch processing
is performed on the low-density layer (nanoweb) side, needle punch processing
on the high-
density layer side can suppress collapse or deformation of the pores
accompanied by
intertwining, as well as undesirable widening of the pore size, thereby
suppressing lowering
of the initial cleaning efficiency with respect to smaller particles. It is
preferable to set the
number of needles (the number for penetration) per unit area in the range from
about 40 to
about 100 perforations/cm2, in order to suppress undesirable widening of the
pore diameter,
and to perform sufficient intertwining operation. Further, no more than about
25% of the
surface area of the low density layer should be perforated.
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[0152] The as-spun nanoweb may comprises primarily or exclusively nanofibers,
advantageously produced by electrospinning, such as classical electrospinning
or
electroblowing, and in certain circumstances, by meltblowing or other such
suitable
processes. Classical electrospinning is a technique illustrated in U.S. Pat.
No. 4,127,706,
wherein a high voltage is applied to a polymer in solution to create
nanofibers and nonwoven
mats. However, total throughput in electrospinning processes is too low to be
commercially
viable in forming heavier basis weight nanowebs.
[0153] The "electroblowing" process is disclosed in WO 03/080905. A stream of
polymeric
solution comprising a polymer and a solvent is fed from a storage tank to a
series of spinning
nozzles within a spinneret, to which a high voltage is applied and through
which the
polymeric solution is discharged. Meanwhile, compressed air that is optionally
heated is
issued from air nozzles disposed in the sides of, or at the periphery of the
spinning nozzle.
The air is directed generally downward as a blowing gas stream which envelopes
and
forwards the newly issued polymeric solution and aids in the formation of the
fibrous web,
which is collected on a grounded porous collection belt above a vacuum
chamber. The
electroblowing process permits formation of commercial sizes and quantities of
nanowebs at
basis weights in excess of about 1 gsm, even as high as about 40 gsm or
greater, in a
relatively short time period.
[0154] A substrate can be arranged on the collector so as to collect and
combine the
nanofiber web spun on the substrate. Examples of the substrate may include
various
nonwoven cloths, such as meltblown nonwoven cloth, needle-punched or spunlaced
nonwoven cloth, woven cloth, knitted cloth, paper, and the like, and can be
used without
limitations so long as a nanofiber layer can be added on the substrate. The
nonwoven cloth
can comprise spunbond fibers, dry-laid or wet-laid fibers, cellulose fibers,
melt blown fibers,
glass fibers, or blends thereof. Alternatively, the nanoweb layer can be
deposited directly
onto the felt substrate.
[0155] It can be advantageous to add known-in-the-art plasticizers to the
various polymers
described above, in order to reduce the Tg of the fiber polymer. Suitable
plasticizers will
depend upon the polymer to be electrospun or electroblown, as well as upon the
particular
end use into which the nanoweb will be introduced. For example, nylon polymers
can be
plasticized with water or even residual solvent remaining from the
electrospinning or
electroblowing process. Other known-in-the-art plasticizers which can be
useful in lowering
polymer Tg include, but are not limited to aliphatic glycols, aromatic
sulphanomides,
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phthalate esters, including but not limited to those selected from the group
consisting of
dibutyl phthalate, dihexl phthalate, dicyclohexyl phthalate, dioctyl
phthalate, diisodecyl
phthalate, diundecyl phthalate, didodecanyl phthalate, and diphenyl phthalate,
and the like.
The Handbook of Plasticizers, edited by George Wypych, 2004 Chemtec
Publishing,
discloses other polymer/plasticizer combinations which can be used in the
present invention.
Liquid Filters
[0156] Liquid filter media are often used for filtering microorganisms.
Biopharmaceutical
manufacturing is constantly looking for ways to streamline operations, combine
and eliminate
steps, and reduce the time it takes to process each batch of pharmaceutical
drug substances.
At the same time, market and regulatory pressures are driving
biopharmaceutical
manufacturers to reduce their costs. Since bacteria, mycoplasma and virus
removal account
for a significant percentage of the total cost of pharmaceutical drug
substance purification,
approaches that increase a porous membrane's filtration throughput and reduce
purification
processing time are very much in demand.
[0157] With the introduction of new prefiltration media and the corresponding
increases in
throughput of bacteria, mycoplasma and virus retentive filters, the filtration
of feed streams is
becoming flux-limited. Thus, dramatic improvements in the permeability of
bacteria,
mycoplasma and virus retentive filters will have a direct beneficial impact on
the cost of a
bacteria, mycoplasma and virus filtration step(s).
[0158] Filters used in liquid filtration can generally be categorized as
either fibrous non-
woven media filters or porous film membrane filters.
[0159] Porous film membrane liquid filters or other types of filtration media
can be used
either unsupported or in conjunction with a porous substrate or support.
Porous film liquid
filtration membranes, which typically have pore sizes smaller than porous
fibrous non-woven
media, can be used in: (a) microfiltration (MF), wherein particulates filtered
from a liquid are
typically in the range of about 0.1 micron (gm) to about 10 gm; (b)
ultrafiltration (UF),
wherein particulates filtered from a liquid, are typically in the range of
about 2 nanometers
(nm) to about 0.1 tun; and (c) reverse osmosis (RO), wherein particulate
matter filtered from
a liquid, are typically in the range of about 1 A to about 1 nm.
[0160] Retrovirus-retentive membranes are usually considered to be on the open
end of
ultrafiltration membranes.
Date Recue/Date Received 2022-11-25
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[0161] High permeability and high reliable retention are two parameters
desired in a liquid
filtration membrane. There is, however, a trade-off between these two
parameters, and for the
same type of liquid filtration membrane, greater retention can be achieved by
sacrificing
permeability. The inherent limitations of conventional processes for making
liquid filtration
membranes prevent membranes from exceeding a certain threshold in porosity,
and thus
limits the magnitude of permeability that can be achieved at any given pore
size.
[0162] Fibrous non-woven liquid filtration media include, but are not limited
to, non-woven
media formed from spunbonded, melt blown or spunlaced continuous fibers;
hydroentangled
non-woven media formed from carded staple fiber and the like, and/or
combinations thereof.
Typically, fibrous non-woven media filters used in liquid filtration have pore
sizes generally
greater than about 1 gm.
[0163] Non-woven materials are widely used in the manufacture of filtration
products.
Pleated membrane cartridges usually include non-woven materials as a drainage
layer (for
example, see U.S. Pat. Nos. 6,074,869, 5,846,438, and 5,652,050, each assigned
to Pall
Corporation; and U.S. Pat. No. 6,598,749 assigned to Cuno Inc, now 3M
Purification Inc.)
[0164] Non-woven microporous materials can also be used as a supporting screen
for an
adjacent porous membrane layer located thereon, such as Biomax
ultrafiltration membranes
by EMD Millipore Corporation, of Billerica, Mass.
[0165] Non-woven microporous materials can also be used as supporting
skeletons to
increase the strength of a porous membrane located on the non-woven
microporous structure,
such as MilligardTM filters also available from EMD Millipore Corporation.
[0166] Non-woven microporous materials can also be used for "coarse
prefiltration" to
increase the capacity of a porous membrane placed downstream of the non-woven
microporous material, by removing suspended particles having diameters that
are generally
greater than about 1 p.m. The porous membrane usually provides a critical
biosafety barrier or
structure having a well-defined pore size or molecular weight cut-off.
Critical filtration is
characterized by expected and validatable assurance of a high degree of
removal (typically
>99.99%, as defined by specified tests) of microorganisms and viral particles.
Critical
filtration is routinely relied upon to ensure sterility of liquid drug and
liquid
biopharmaceutical formulations at multiple manufacturing stages, as well as at
point of use.
[0167] Melt-blown and spunbonded fibrous media are often referred to as
"traditional" or
"conventional" non-wovens. Fibers in these traditional non-wovens are usually
at least about
1,000 nm in diameter, therefore the effective pore sizes in traditional non-
wovens are greater
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than about one micron. The methods of manufacturing traditional non-wovens
typically lead
to highly inhomogeneous fiber mats.
[0168] Historically, the random nature of conventional non-woven mat
formation, such as by
melt-blowing and spun-bonding, has led to the general assumption that non-
woven mats are
unsuitable for any critical filtration of liquid streams, and as such,
filtration devices
incorporating conventional non-wovens mats typically use these mats for
prefiltration
purposes only in order to increase the capacity of a porous critical
filtration membrane placed
downstream of the conventional non-wovens mats.
[0169] Another type of non-woven includes electronspun nanofiber non-woven
mats, which,
like "traditional" or "conventional" non-wovens have been generally assumed
unsuitable for
the critical filtration of liquid streams. (See for example, Bjorge et al.,
Performance
assessment of electrospun nanofibers for filter applications, Desalination,
249, (2009), 942-
948).
[0170] Electrospun polymeric nanofiber mats are highly porous, wherein the
"pore" size is
approximately linearly proportional to the fiber diameter, and the porosity is
relatively
independent of the fiber diameter. The porosity of an electrospun nanofiber
mat usually falls
in the range of about 85% to 90%, resulting in a nanofiber mat that
demonstrates dramatically
improved permeability when compared to immersion cast membranes having a
similar
thickness and pore size rating. The porosity advantages of electrospun
polymeric nanofiber
mats over porous membranes becomes amplified in the smaller pore size ranges
typically
required for virus filtration, because of the reduced porosity of UF membranes
discussed
supra.
[0171] Electrospun nanofiber non-woven mats are produced by spinning polymer
solutions
or melts using electric potential rather than meltblown, wetlaid or extrusion
manufacturing
processes used in making conventional or traditional non-wovens. The fiber
diameters
typically obtained by electrospinning are in the range of 10 nm to 1,000 nm,
and are one to
three orders of magnitude smaller than conventional or traditional non-wovens.
[0172] Electrospun nanofiber mats are formed by putting a dissolved or molten
polymer
material adjacent to a first electrode and applying an electrical potential
such that the
dissolved or molten polymer material is drawn away from the first electrode
toward a second
electrode as a fiber. In the process of manufacturing electrospun nanofiber
mats, the fibers are
not forced to lay down in mats by blown hot air or other mechanical means that
can lead to a
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very broad pore size distribution. Rather, electrospun nanofibers form a
highly uniform mat
because of the mutual electrical repulsion between the electrospun nanofibers.
101731 WO 2010/107503, assigned to EMD Millipore Corporation, teaches
nanofiber mats
having a specific thickness and fiber diameter offer an improved combination
of liquid
permeability and microorganism retention. The thinnest sample taught is 55 inn
thick with
permeability of 4,960 lmh/psi, however neither the method to determine
retention assurance
nor the achieved level of assurance is described. Generally, nanofiber mats
offer 2-10 times
better permeability than their porous membrane counterparts of comparable
retention, this is
thought to be a consequence of the nanofiber mats having a higher porosity
(90% vs. 70-
80% for a typical wet casting porous membrane).
[0174] Electrospun nanofiber mats can be manufactured by depositing fibers on
a
conventional spun-bonded non-woven fabric (examples of a face to face
interface of a non-
woven and a nanofiber layer are taught in WO 2009/010020 assigned to Elmarco
s.r.o.; and
in US Pub. App. No. 200910199717 assigned to Clarcor Inc. In each of these
approaches,
the roughness of the surface of the supporting non-woven fabric may propagate
into the
nanofiber layer causing potential non-uniformity of the nanofiber structure,
thereby
potentially compromising retention characteristics.
[0175] U.S. Pat. No. 7,585,437 issued to Jirsak et al. teaches a nozzle-free
method for
producing nanofibers from a polymer solution using electrostatic spinning and
a device for
carrying out the method.
101761 WO 2003/080905 assigned to Nano
Technics Co. LTD., teaches an electroblowing process, wherein a stream of
polymeric
solution comprising a polymer and a solvent is fed from a storage tank to a
series of spinning
nozzles within a spinneret, to which a high voltage is applied and through
which the
polymeric solution is discharged. Compressed air, which may optionally be
heated, is
released from air nozzles disposed in the sides of, or at the periphery of,
the spinning nozzle.
The compressed air is directed generally downward as a blowing gas stream
envelopes and
forwards the newly issued polymeric solution, thereby aiding in the formation
of a
nanofibrous web, which is collected on a grounded porous collection belt
located above a
vacuum chamber.
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[0177] U.S. Pub. No. 2004/0038014 to Schaefer et al. teaches a nonwoven
filtration mat
comprising one or more layers of a thick collection of fine polymeric
microfibers and
nanofibers formed by electrostatic spinning for filtering contaminants.
[0178] U.S. Pub. No. 2009/0199717 to Green teaches a method of forming
electrospun fiber
layers on a substrate layer, a significant amount of the electrospun fibers
have fibers with a
diameter of less than 100 nanometers (nm).
[0179] Bjorge et al., in Desalination 249 (2009) 942-948, teach electrospun
Nylon nanofiber
mats having a nanofiber diameter of about 50 nm to 100 nm, and a thickness of
about 120
gm. The measured bacteria LRV for non-surface treated fibers is 1.6-2.2.
Bjorge et al.
purportedly conclude that bacteria removal efficiency of nanofiber electrospun
mats is
unsatisfactory.
[0180] Gopal et al., in Journal of Membrane Science 289 (2007) 210-219, teach
electrospun
polyethersulfone nanofiber mats, wherein the nanofibers have a diameter of
about 470 nm.
During liquid filtration, the nanofiber mats act as a screen to filter out
particles above 1
micron (gm), and as a depth filter (e.g., prefilter) for particles under 1
micron.
[0181] Aussawasathien et al., in Journal of Membrane Science, 315 (2008) 11-
19, teach
electrospun nanofibers having a diameter of about 30 nm to 110 nm used in the
removal of
polystyrene particles having a diameter of about 0.5 gm to 10 plin.
[0182] One reason why researches investigated collecting electrode properties
is to control
the orientation of the collected nanofibers on that electrode. Li et al., in
Nano Letters, vol. 5,
no. 5 (2005) 913-916, described introducing an insulating gap into the
collecting electrode
and the effects of the area and the geometrical shape of that introduced
insulating gaps. They
demonstrated that assembly and alignment of the nanofibers could be controlled
by varying
the collecting electrode pattern.
[0183] A number of methods have been published that focus on geometrical
surface
properties, such as roughness. For example, US Pub. No. 2011/0305872 describes
changing
surface roughness of a substrate by grafting a polymer layer, in order to
change binding
properties of biologicals on that substrate. An optical profilometry method
was described to
determine surface roughness of the substrate using Olympus LEXT OLS4000 laser
confocal
microscope.
[0184] For critical filtration applications achieving high microorganism
retention by itself is
not enough but doing so in a reliable way with high assurance is required. In
order to predict
retention assurance statistical methods are often used, like censored data
regression, to
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analyze lifetime data for reliability, where lifetimes are truncated.
(Blanchard, (2007),
Quantifying Sterilizing Membrane Retention Assurance, BioProcess
International, v.5, No. 5,
pp. 44-51)
[0185] U.S. Pub. No. 2014/0166945 discloses a liquid filter comprising a
porous polymeric
nanofiber layer on a support, wherein at least on the surface of the support
facing the
polymeric nanofiber layer, the root mean square height of the surface is less
than about 70
micrometers. This publication discloses a variety of polymers that may be used
for the
nanofiber layer and for the support.
[0186] The electrospun nanofibers may be prepared from a broad range of
polymers and
polymer compounds, including thermoplastic and thermosetting polymers.
Suitable polymers
include, but are not limited to, nylon, polyimide, aliphatic polyamide,
aromatic polyamide,
polysulfone, cellulose, cellulose acetate, polyether sulfone, polyurethane,
poly(urea
urethane), polybenzimidazole (PBI), polyetherimide, poly acrylonitrile (PAN),
poly(ethylene
terephthalate), polypropylene, polyaniline, poly(ethylene oxide),
poly(ethylene naphthalate),
poly(butylene terephthalate), styrene butadiene rubber, polystyrene,
poly(vinyl chloride),
poly(vinyl alcohol), poly(vinylidene fluoride), poly(vinyl butylene),
polymethylmethacrylate
(PMMA), copolymers, derivative compounds and blends and/or combinations
thereof.
[0187] Non-limiting examples of single or multilayered porous substrates or
supports include
smooth non-wovens. In other non-limiting examples the smooth non-woven support
has a
substantially uniform thickness. Smooth non-wovens are produced from a variety
of
thermoplastic polymers, including polyolefins, polyesters, polyamides, etc.
[0188] The homogeneity of the non-woven substrate of the composite filtration
medium that
captures or collects the electrospun nanofibers may at least partially
determine the properties
in the resulting nanofiber layer of the final composite filtration structure.
For example, the
smoother the surface of the substrate used to collect the electrospun
nanofibers, the more
uniform the resulting nanofiber layer structure.
[0189] Smoothness of the supporting nonwoven pertains to geometrical
smoothness, or lack
of rough surface features that have dimensions greater than one fiber diameter
of the non-
woven, as well as low hairiness, i.e. a small number of fibers and/or loops
that protrude
beyond the surface. Geometrical smoothness can be easily measured by a number
of common
techniques, for example mechanical and optical profilometry, visible light
reflectivity (gloss
metering) and other techniques known to those skilled in the art.
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[0190] In some aspects, an electrospun nanofiber layer is bonded to a smooth
non-woven
support. Bonding may be accomplished by methods well known in the art,
including but not
limited to thermal calendering between heated smooth nip rolls, ultrasonic
bonding, and
through gas bonding. Bonding the electrospun nanofiber layer to the non-woven
support
increases the strength of the composite, and the compression resistance of the
composite,
such that the resulting composite filtration medium is capable of withstanding
forces
associated with forming the composite filtration platform into useful filter
shapes and sizes,
or when installing the composite filtration platform into a filtration device.
[0191] In other embodiments of the composite liquid filtration platform, the
physical
properties of the porous electrospun nanofiber layer such as thickness,
density, and the size
and shape of the pores may be affected depending on the bonding methods used
between the
nanofiber layer and the smooth nonwoven support. For instance, thermal
calendaring can be
used to reduce the thickness and increase the density and reduce the porosity
of the
electrospun nanofiber layer, and reduce the size of the pores. This in turn
decreases the flow
rate through the composite filtration medium at a given applied differential
pressure.
[0192] In general, ultrasonic bonding will bond to a smaller area of the
electrospun nanofiber
layer than thermal calendaring, and therefore has a lesser effect on
thickness, density and
pore size electrospun nanofiber layer.
[0193] Hot gas or hot air bonding generally has minimal effect on the
thickness, density and
pore size of the electrospun nanofiber layer, therefore this bonding method
may be preferable
in applications in which maintaining higher fluid flow rate is desired.
[0194] When thermal calendering is used, care must be taken not to over-bond
the
electrospun nanofiber layer, such that the nanofibers melt and no longer
retain their structure
as individual fibers. In the extreme, over-bonding will result in the
nanofibers melting
completely such that a film is formed. One or both of the nip rolls used is
heated to a
temperature of between about ambient temperature, e.g., about 25 C. and about
300 C. The
porous nanofiber medium and/or porous support or substrate, can be compressed
between the
nip rolls at a pressure ranging from about 0 lb/in to about 1000 lb/in (178
kg/cm).
[0195] Calendering conditions, e.g., roll temperature, nip pressure and line
speed, can be
adjusted to achieve the desired solidity. In general, application of higher
temperature,
pressure, and/or residence time under elevated temperature and/or pressure
results in
increased solidity.
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[0196] Other mechanical steps, such as stretching, cooling, heating,
sintering, annealing,
reeling, unreeling, and the like, may optionally be included in the overall
process of forming,
shaping and making the composite filtration medium as desired.
Breathing Filters
[0197] U.S. Pub. No. 2014/0097558 discloses that various types of breathing
filters are
known in the art. Personal protection equipment (PPE), specifically disposable
face masks,
may be required to conform to certain regulations during design and
manufacture. The user's
ability and ease of breathing while wearing the mask may be considered, as
well as the fit and
the comfort of the user who may wear the mask. Because of the disposable
nature of the
mask, a low cost manufacturing process may be desired. Certain regulatory
standards may
need to be met, such as EN149:2001 for Europe or 42 CFR part 84 for US or ISO
17420. PPE
under these regulations are class III products according to PPE directive in
Europe or other
parts of the world. PPE, such as disposable masks or reusable cartridges, may
comprise
filtration media, which may be made of melt-blown fibers and/or micro glass
material.
Filtration by a mask is accomplished when particles in the air are trapped in
the matrix of the
fibers contained in the filtration media of the mask.
[0198] Nanofibers formed by electrospinning a polymer solution may be
functionalized by
the addition of another material to the polymer solution. The additional
functionalizing
material may be operable to remove gases and may comprise one or more
chemicals that may
capture gases (where the gases might be volatile organic chemicals (VOCs),
acid vapors,
carbon dioxide (CO2), nitrogen monoxide (NO), nitrogen dioxide (NO2), ozone
(03),
hydrogen cyanide (HCN), arsine (AsH3), hydrogen fluoride (HF), chlorine
dioxide (C10C2),
ethylene oxide (C2H40), formaldehyde (CH20), methyl bromide (CH3Br), and/or
phosphine
(PH3)). In an embodiment, the functionalized material may comprise one of a
biocide (i.e. a
chemical substance or microorganism which can deter, render harmless, or exert
a controlling
effect on any harmful organism by chemical or biological means), a virucide
(i.e. a physical
or chemical agent that deactivates or destroys viruses) and/or a bactericide
(i.e. a substance
that kills bacteria, for example disinfectants, antiseptics, or antibiotics).
In other
embodiments, a functionalized nanofiber may be operable to remove humidity,
control
temperature, indicate end of service life, indicate clogged material, and/or
provide a fresh
odor inside the mask.
[0199] The filtration layer may be formed directly on the support layer rather
than being
formed in isolation. The filtration layer may contain one or more types of
fibers, made from
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the same or different polymeric fiber-forming materials. A majority of the
fibers in the
filtration layer are formed from fiber-forming materials capable of accepting
satisfactory
electret charge and maintaining adequate charge separation. Preferred
polymeric fiber-
forming materials are non-conductive resins having a volume resistivity of
1014 ohm-
centimeters or greater at room temperature (22 C). The resin may have a volume
resistivity
of about 1016 ohm-centimeters or greater. Resistivity of the polymeric fiber-
forming material
may be measured according to standardized test ASTM D 257-93. Some examples of
polymers which may be used include thermoplastic polymers containing
polyolefins such as
polyethylene, polypropylene, polybutylene, poly(4-methyl-1-pentene) and cyclic
olefin
copolymers, and combinations of such polymers. Other polymers which may be
used but
which may be difficult to charge or which may lose charge rapidly include
polycarbonates,
block copolymers such as styrene-butadiene-styrene and styrene-isoprene-
styrene block
copolymers, polyesters such as polyethylene terephthalate, polyamides,
polyurethanes, and
other polymers that will be familiar to those having ordinary skill in the
art. Some or all of
the filtration layer fibers may if desired be made from multicomponent fibers,
including
splittable fibers. Suitable multicomponent (e.g., bicomponent) fibers include
side-by-side,
sheath-core, segmented pie, islands in the sea, tipped and segmented ribbon
fibers. If
splittable fibers are employed, splitting may be carried out or encouraged
using a variety of
techniques that will be familiar to those having ordinary skill in the art
including carding, air
jets, embossing, calendering, hydroentangling or needle punching. The
filtration layer
preferably is prepared from monocomponent fibers of poly-4-methyl-1 pentene or
polypropylene, or from bicomponent fibers of poly-4-methyl-1 pentene and
polypropylene in
a layered or core-sheath configuration, e.g., with poly-4-methyl-1 pentene or
polypropylene
on the outer surface. Most preferably, the filtration layer is prepared from
polypropylene
homopolymer monocomponent fibers because of the ability of polypropylene to
retain
electric charge, particularly in moist environments. Additives may be added to
the polymer to
enhance filtration performance, electret charging capability, mechanical
properties, aging
properties, coloration, surface properties or other characteristics of
interest. Representative
additives include fillers, nucleating agents (e.g., MILLADTM 3988
dibenzylidene sorbitol,
commercially available from Milliken Chemical), electret charging enhancement
additives
(e.g., tristearyl melamine, and various light stabilizers such as CHIMASSORBTm
119 and
CHIMASSORB 944 from Ciba Specialty Chemicals), cure initiators, stiffening
agents (e.g.,
poly(4-methyl-1-pentene)), surface active agents and surface treatments (e.g.,
fluorine atom
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treatments to improve filtration performance in an oily mist environment as
described in U.S.
Pat. Nos. 6,398,847 Bl, 6,397,458 B1, and 6,409,806 B1 to Jones et al.). The
types and
amounts of such additives will be familiar to those having ordinary skill in
the art. For
example, electret charging enhancement additives are generally present in an
amount less
than about 5 wt. % and more typically less than about 2 wt. %. The polymeric
fiber-forming
material also preferably is substantially free from components such as
antistatic agents that
could significantly increase electrical conductivity or otherwise interfere
with the fiber's
ability to accept and hold electrostatic charge.
[0200] The filtration layer may have a variety of basis weights, fiber sizes,
thicknesses,
pressure drops and other characteristics, and by itself may be sufficiently
fragile so as not to
be roll-to-roll processable. The filtration layer may, for example, have a
basis weight in the
range of about 0.5 to about 300 g/m2 (gsm), about 0.5 to about 100 gsm, about
1 to about 50
gsm, or about 2 to about 40 gsm. Relatively low basis weights, e.g., of about
2, 5, 15, 25 or
40 gsm are preferred for the filtration layer. The fibers in the filtration
layer may have, for
example, a median fiber size less than about 10 gm, less than about 5 i.tm or
less than about 1
gm. The filtration layer thickness may, for example, be about 0.1 to about 20
mm, about 0.2
to about 10 mm, or about 0.5 to about 5 mm. Nanofiber filtration layers
applied at very low
basis weights to some support layers (e.g., rough-textured support layers) may
not change the
overall media thickness. The filtration layer basis weight and thickness can
be controlled or
adjusted, for example, by changing the collector speed or polymer throughput.
[0201] The support layer is sufficiently robust so that the filtration layer
may be formed on
the support layer and the resulting media may be further converted as needed
using roll-to-
roll processing equipment. The support layer may be formed from a variety of
materials, and
may have a variety of basis weights, thicknesses, pressure drops and other
characteristics. For
example, the support layer may be a nonwoven web, woven fabric, knit fabric,
open cell
foam or perforated membrane. Nonwoven fibrous webs are preferred support
layers. Suitable
fibrous precursors for making such nonwoven webs include the polymeric fiber-
forming
materials discussed above and other polymeric fiber-forming materials that do
not readily
accept or hold and electrostatic charge. The support layer may also be formed
from natural
fibers or from blends of synthetic and natural fibers. If made from a nonwoven
web, the
support layer may, for example, be formed from molten thermoplastic polymer
using
meltblowing, meltspinning or other suitable web processing techniques, be
formed from
natural fibers or from blends of synthetic and natural fibers using carding or
deposition from
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a Rando-Webber machine, or be formed using other techniques that will be
familiar to those
having ordinary skill in the art. If made from a woven web or knit fabric, the
support layer
may, for example, be formed from microdenier continuous filament or staple
fiber yarns
(viz., yarns having a denier per filament (dpi) less than about 1) and
processed into a woven
or knit support fabric using suitable processing techniques that will be
familiar to those
having ordinary skill in the art. The support layer may, for example, have a
basis weight in
the range of about 5 to about 300 gsm, more preferably about 40 to about 150
gsm. The
thickness of the support layer may, for example, be about 0.2 to about 40 mm,
about 0.2 to
about 20 mm, about 0.5 to about 5 mm or about 0.5 to about 1.5 mm.
[0202] In addition to the polyamide nanofiber layer, additional layers may be
added to the
disclosed media if desired. Representative additional layers will be familiar
to persons having
ordinary skill in the art, and include protective layers (e.g., anti-shedding
layers, anti-
irritation layers, and other cover layers), reinforcing layers and sorbent
layers. Sorbent
particles (e.g., activated carbon particles or alumina particles) may also be
introduced into the
media using methods that will be familiar to persons having ordinary skill in
the art.
[0203] Hydrocharging of the disclosed multilayer media may be carried out
using a variety of
techniques including impinging, soaking or condensing a polar fluid onto the
media, followed
by drying, so that the media becomes charged. Representative patents
describing
hydrocharging include the above-mentioned U.S. Pat. No. 5,496,507, and U.S.
Pat. Nos.
5,908,598; 6,375,886; 6,406,657; 6,454,986; and 6,743,464. Preferably water is
employed as
the polar hydrocharging liquid, and the media preferably is exposed to the
polar
hydrocharging liquid using jets of the liquid or a stream of liquid droplets
provided by any
suitable spray means. Devices useful for hydraulically entangling fibers are
generally useful
for carrying out hydrocharging, although the operation is carried out at lower
pressures in
hydrocharging than generally used in hydroentangling. U.S. Pat. No. 5,496,507
describes an
exemplary apparatus in which jets of water or a stream of water droplets are
impinged upon
the media at a pressure sufficient to provide the subsequently-dried media
with a filtration-
enhancing electret charge. The pressure necessary to achieve optimum results
may vary
depending on the type of sprayer used, the type of polymer from which the
filtration layer is
formed, the thickness and density of the media, and whether pretreatment such
as corona
charging was carried out before hydrocharging. Generally, pressures in the
range of about 69
to about 3450 kPa are suitable. Preferably, the water used to provide the
water droplets is
relatively pure. Distilled or deionized water is preferable to tap water.
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[0204] The disclosed media may be subjected to other charging techniques
before or after
hydrocharging including electrostatic charging (e.g., as described in U.S.
Pat. Nos. 4,215,682,
5,401,446 and 6,119,691), tribocharging (e.g., as described in U.S. Pat. No.
4,798,850) or
plasma fluorination (e.g., as described in U.S. Pat. No. 6,397,458 B1). Corona
charging
followed by hydrocharging and plasma fluorination followed by hydrocharging
are preferred
combination charging techniques.
[0205] Additional breathing filters are described, for example, in Fibrous air
filtration webs
are described, for example, in U.S. Pat. Nos. 4,011,067; 4,215,682; 4,592,815;
4,729,371;
4,798,850; 5,401,466; 5,496,507; 6,119,691; 6,183,670; 6,315,806 6,397,458;
6,554,881;
6,562,112 B2; 6,627,563; 6,673,136; 6,716,274; 6,743,273; and 6,827,764; and
in Tsai et al.,
Electrospinning Theory and Techniques, 14th Annual International TANDEC
Nonwovens
Conference, Nov. 9-11, 2004. Other fibrous webs are described, for example, in
U.S. Pat.
Nos. 4,536,361 and 5,993,943.
Embodiments
[0206] The present disclosure includes the following embodiments:
[0207] Embodiment 1: A filter media comprising a nanofiber nonwoven layer,
wherein the
nanofiber nonwoven layer comprises a polyamide with a Relative Viscosity from
2 to 200
which is spun into nanofibers with an average fiber diameter of less than 1
micron (1000
milometers) and formed into the layer.
[0208] Embodiment 2: An embodiment according to Embodiment 1, wherein the
nanofiber
nonwoven layer comprises a polyamide which is spun into nanofibers with an
average fiber
diameter of less than 1 micron (1000 nanometers) and formed into the layer,
wherein the
layer has a melt point of 225 C or greater.
[0209] Embodiment 3: An embodiment according to Embodiment 1 or 2, wherein the
filter is
an air filter, an oil filter, a bag filter, a liquid filter, or a breathing
filter.
[0210] Embodiment 4: An embodiment according to Embodiment 1 or 2, wherein the
polyamide is Nylon 6,6.
[0211] Embodiment 5: An embodiment according to Embodiment 1 or 2, wherein the
polyamide is a derivative, copolymer, blend or alloy of Nylon 6,6 and Nylon 6.
[0212] Embodiment 6: An embodiment according to Embodiment 1 or 2, wherein the
polyamide is a high temperature nylon.
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[0213] Embodiment 7: An embodiment according to Embodiment 1 or 2, wherein the
polyamide is a long chain aliphatic nylon selected from the group consisting
of N6, N6T/66,
N612, N6/66, N11, and N12, wherein "N" means Nylon, and "T" refers to
terephthalic acid.
[0214] Embodiment 8: An embodiment according to any of Embodiments 1-7,
wherein the
nanofiber nonwoven layer has an Air Permeability Value of less than 200
CFM/ft2.
[0215] Embodiment 9: An embodiment according to any of Embodiments 1-8,
wherein the
nanofiber nonwoven layer has an Air Permeability Value of from 50 to 200
CFM/ft2.
[0216] Embodiment 10: An embodiment according to any of Embodiments 1-9,
wherein the
nanofibers have an average fiber diameter of from 100 to 907 nanometers, e.g.,
from 300 to
700 nanometers or form 350 to 650 nanometers.
[0217] Embodiment 11: An embodiment according to any of Embodiments 1-10,
wherein the
nonwoven product has a basis weight of 150 GSM or less.
[0218] Embodiment 12: An embodiment according to any of Embodiments 1-11,
wherein the
filter media further comprises a scrim layer.
102191 Embodiment 13: An embodiment according to Embodiment 12, wherein the
nanofiber
nonwoven layer is spun onto the scrim layer.
[0220] Embodiment 14: An embodiment according to Embodiment 12, wherein the
nanofiber
nonwoven layer is spun onto a layer other than the scrim layer.
[0221] Embodiment 15: An embodiment according to Embodiment 12, wherein the
nanofiber
nonwoven layer is sandwiched between the scrim layer and at least one other
layer.
[0222] Embodiment 16: An embodiment according to Embodiment 12, wherein the
nanofiber
nonwoven layer is sandwiched between at least two layers other than the scrim
layer.
[0223] Embodiment 17: An embodiment according to Embodiment 12, wherein the
nanofiber
nonwoven layer is an outermost layer.
[0224] Embodiment 18: An embodiment according to any of Embodiments 1-11,
wherein the
filter media further comprises at least one additional layer and wherein the
nanofiber
nonwoven layer is spun onto one of the at least one additional layers.
[0225] Embodiment 19: An embodiment according to any of Embodiments 1-18,
wherein the
Relative Viscosity of the polyamide in the nanofiber nonwoven layer is reduced
by at least
20% as compared to the polyamide prior to spinning and forming the layer.
[0226] Embodiment 20: A method of making filter media comprising a polyamide
nanofiber
layer, the method comprising: (a) providing a spinnable polyamide polymer
composition,
wherein the polyamide has a Relative Viscosity of from 2 to 200; (b) melt
spinning the
polyamide polymer composition into a plurality of nanofibers having an average
fiber
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diameter of less than 1 micron (1000 nanometers); and (c) forming the
nanofibers onto an
existing filter media layer, wherein the polyamide nanofiber layer has an
average nanofiber
diameter of less than 1000 nanometers.
[0227] Embodiment 21: A method of making filter media comprising a polyamide
nanofiber
layer, the method comprising: (a) providing a spinnable polyamide polymer
composition; (b)
melt spinning the polyamide polymer composition into a plurality of nanofibers
having an
average fiber diameter of less than 1 micron (1000 nanometers); and (c)
forming the
nanofibers onto an existing filter media layer, wherein the polyamide
nanofiber layer has an
average nanofiber diameter of less than 1000 nanometers and a melt point of
225 C or
greater.
[0228] Embodiment 22: An embodiment according to Embodiment 20 or 21, wherein
the
polyamide nanofiber layer is melt spun by way of melt-blowing through a die
into a high
velocity gaseous stream.
[0229] Embodiment 21 An embodiment according to Embodiment 20 or 21, wherein
the
polyamide nanofiber layer is melt-spun by 2-phase propellant-gas spinning,
including
extruding the polyamide polymer composition in liquid form with pressurized
gas through a
fiber-forming channel.
[0230] Embodiment 24: An embodiment according to any of Embodiments 20-23,
wherein
the polyamide nanofiber layer is formed by collecting the nanofibers on a
moving belt.
[0231] Embodiment 25: An embodiment according to any of Embodiments 20-24,
wherein
said polyamide composition comprises Nylon 6,6.
[0232] Embodiment 26: An embodiment according to any of Embodiments 20-24,
wherein
said polyamide composition comprises a derivative, copolymer, blend or alloy
of Nylon 6,6
and Nylon 6.
[0233] Embodiment 27: An embodiment according to any of Embodiments 20-24,
wherein
said polyamide comprises a HTN.
[0234] Embodiment 28: An embodiment according to any of Embodiments 20-24,
wherein
said polyamide is a long chain aliphatic nylon selected from the group
consisting of N6,
N6T/66, N612, N6/66, N11, and N12, wherein "N" means Nylon, and "T" refers to
terephthalic acid.
[0235] Embodiment 29: An embodiment according to any of Embodiments 20-28,
wherein
the polyamide nanofiber layer has a basis weight of 150 GSM or less.
[0236] Embodiment 30: An embodiment according to any of Embodiments 20-29,
wherein
the filter media further comprises a scrim layer.
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[0237] Embodiment 31: An embodiment according to any of Embodiments 20-30,
wherein
the polyamide nanofiber layer is spun onto the scrim layer.
[0238] Embodiment 32: An embodiment according to Embodiment 31, wherein the
polyamide nanofiber layer is spun onto a layer other than the scrim layer.
[0239] Embodiment 33: An embodiment according to Embodiment 31, wherein the
polyamide nanofiber layer is sandwiched between the scrim layer and at least
one other layer.
[0240] Embodiment 34: An embodiment according to Embodiment 31, wherein the
polyamide nanofiber layer is sandwiched between at least two layers other than
the scrim
layer.
[0241] Embodiment 35: An embodiment according to Embodiment 31, wherein the
polyamide nanofiber layer is an outermost layer.
[0242] Embodiment 36: An embodiment according to any of Embodiments 20-31,
wherein
the filter media further comprises at least one additional layer and wherein
the nanofiber
nonwoven layer is spun onto one of the at least one additional layers.
[0243] Embodiment 37: An embodiment according to any of Embodiments 20-36,
wherein
the Relative Viscosity of the polyamide in the polyamide nanofiber layer is
reduced by at
least 20% as compared to the polyamide prior to spinning and forming the
layer.
[0244] The present disclosure is further understood by the following non-
limiting examples.
Examples
Example 1
102451 Utilizing the procedures and apparatus (as shown generally in Figure 1)
as described
in US 8,668,854, Nylon 6,6 were spun into a nonwoven at two basis weights by
melt spun
technology onto a moving drum. An extruder with a high compression screw,
running at 20
RPM, with a temperature profile of 245 C, 255 C, 265 C, and 265 C was used.
The polymer
temperature was 252 C and air was used as the gas. The sample with the higher
basis weight
was made by the same process, but spinning the nanofibers onto a scrim. Here,
the scrim is a
neutral article of the invention merely used for adding integrity to the
inventive nanofiber
web. The resins had a Relative Viscosity of 7.3. To ensure the viscosity of
the low RV resin
would remain constant, the polymer was made using an excess of about 5% adipic
acid.
[0246] The nonwovens were characterized for average fiber diameter, basis
weight, air
permeability in accordance with the Hassan et al. article noted above, J
Membrane Sci., 427,
336-344, 2013. Water vapor transmission rate was also measured (g/m2/24hr).
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[0247] Results and details appear in Table 1 and the nonwovens produced are
shown in the
photomicrographs of Figures 3 and 4. The nonwovens had an average fiber
diameter ranging
from 470 nm to 680 nm (575 nm average).
TABLE 1
Polyamide Nanofiber Nonwoven Product Properties
Air Water
vapor
Fiber Basis weight, permeability trans rate
Sample Resin RV diameter, nm gsm (CFM/ft2) g/m2/24 hr
1 7.3 470 118 182.8 1056
2 7.3 680 68 182.8 1140
[0248] It is appreciated from Table 1 that the melt spun nanofiber nonwovens
of the
invention had a fiber diameter averaging 570 for the RV of 7.3. Air
Permeability was about
182.8 CFM/ft2, while water vapor transmission rate averaged about 1100 g/m2/24
hrs.
[0249] Polyamides made into nanofibers having an RV above about 20-30 will
have a higher
molecular weight than those with a lower RV value. Resultant polymer
properties may be
different than those polymers of RV values less than 20, in particular in the
areas of elasticity,
strength, thermal or chemical stability, appearance, and or surface properties
among others. It
may be desired to use a mixture of lower and higher molecular weight polymers
in a
nonwoven web. The lower molecular weight polymer will fibrillate easier which
may result
in fibers having different diameters. If the polymers will not blend, separate
nozzles may be
utilized for the different molecular weight polymers.
[0250] In the case of polyamides having an RV above 20-30 and less than 200,
the average
fiber diameter of a significant number of fibers in the fiber layer of the
nonwoven can be less
than 1 micron, more preferably from about 0.1 to 1 micron, or more preferably
0.1 to about
0.6 microns. The resultant nonwoven will have an average fiber diameter less
than 1 micron.
[0251] In an embodiment of the invention, advantages are envisioned having two
related
polymers with different RV values (both less than 200 and having an average
fiber diameter
less than 1 micron) blended for a desired property.
Example 2
[0252] A melt blown process was used with the pack described in US Patent No.
7,300,272
to make nanofibers under the present invention. Nylon 6,6 with a 36 RV was
melt spun and
pumped to melt blown dies. The moisture levels of this resin ranged from about
0.2% to
about 1.0%. An extruder with four zones was used with temperature in the range
of 233 to
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310 C. A die temperature in the range of 286 to 318 C was used. Heated air
was used as the
gas in the melt blowing process. The nanofibers were deposited onto a 10 grams
per square
meter (gsm) thermally bonded, nylon spunbond scrim commercially available from
Cerex
Advanced Fabrics, Inc. under the trademark PBN-II . Other spunbond fabrics can
be used.
Other fabrics can be used such as a polyester spun bond fabric, a
polypropylene spunbond
fabric, a nylon melt blown fabric or other woven, knit, needlepunched, or
other nonwoven
fabrics. No solvents or adhesives were used. Various fabrics were made with a
layer of
nanofibers. The basis weight of the nanofiber layer ranged between about 0.7
gsm to about 23
gsm. The average fiber diameter of these nanofiber layers ranged between about
0.36 microns
to about 0.908 microns. The relative viscosity of these nanofiber layers
ranged from about 22
to about 31. The starting RV was in the range of 34 to 37 and about 36. The
efficiency of
these fabrics measured using a TSI 8130 with a challenge fluid of 0.3 microns
ranged
between about 2.71% to about 76.7%. The mean pore size of these fabrics ranged
from about
4.5 microns to about 84.1 microns. The air permeability of these fabrics
ranged from 21 to
1002 cfm/ft2.
Example 3
[0253] A resin with an RV in the range of 34 to 37 was used with the pack
described in US
Patent No. 7,300,272 to make nanofibers with an RV of about 16.8. This is a
reduction in RV
from resin to fabric of about 17.2 to 20.2 RV units. The resin contained about
1% moisture by
weight and was run on a small extruder with four zones ranging in temperature
from 233 to
310 C. A die temperature of about 308 C was used.
Example 4
[0254] A resin with an RV in the range of 34 to 37 with the pack described in
US Patent
7,300,272 to make nanofibers with an RV of about 19.7. This is a reduction in
RV from resin
to fabric of about 14.3 to 17.3 RV units. The resin contained 1% moisture by
weight and was
run on a small extruder with four zones ranging in temperature from 233 to 310
C. A die
temperature of about 277 C was used.
Example 5
[0255] A resin with an RV in the range of 34 to 37 was used with 2% nylon 6
blended in.
The pack described in US Patent 7,300,272 was used to make nanofibers with an
RV of about
17.1. This is a reduction in RV from resin to fabric of about 16.9 to 19.9 RV
units. The resin
contained 1% moisture by weight and was run on a small extruder with four
zones ranging in
temperature from 233 to 310 C. A die temperature of about 308 C was used.
Other blends or
copolymers of nylons can be used. In a preferred embodiment, blends of nylon 6
and nylon
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6,6 can be used. These nylon 6 and nylon 6,6 blends will have a melt point
between the melt
point of nylon 6 at about 220 C and the melt point of nylon 6,6 at about 260
C.
Example 6
102561 Three to six layers of nanofiber nonwoven fabric were combined to
create a media
with a higher basis weight and thickness. Each layer included a web of nylon
6,6 nanofibers
on a 10 gsm nylon spunbond scrim available under the tradename "PBN-II" from
Cerex
Advanced Fabrics, Inc. in Cantonment, Florida. Four different webs were used
with different
basis weights (13.3, 21.2, 13.2, and 20.2) as reported in Table 2. Table 2
shows the basis
weight, filtration efficiency as measured using a TSI 8130 with a challenge
fluid of 0.3
micron, mean flow pore size, and the average pressure drop (PD) as measured by
the TSI
8130. Two samples were measured to report an average for mean flow pore size,
efficiency
and pressure drop.
102571 The fabrics had basis weights ranging between 13.2 gsm and 127.2 gsm
and mean
flow pore sizes ranging between 3.9 to 5.8 microns and filtration efficiencies
as measured by
the TSI instrument as described previously, ranging between 63.5% to 80.2%.
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Table 2
Mean Flow Pore
Basis size (microns) Efficiency (%) Pressure Drop (Pa)
Average
Weight Sample Sample Sample
Penetration
Layers (gsm) Ave. 1 2 Ave. 1 2 Ave. 1 2 (%)
1 13.3 5.8 63.5 37.7 36.5
3 39.9 4.7 4.5 4.8 69 72.2 65.8 47.3 49.6 45.1 31
4 53.2 5.1 5.1 5.1 67.5 68.9 66.1 47.9 51.4 44.5 32.5
66.5 5.1 5.4 4.7 66.7 65.8 67.5 46.9 43.5 50.3 33.3
6 79.8 5.2 4.8 5.6 65 67.6 62.3 45.1 49.1 41.2 35
1 21.2 5 76.7 56.1 23.3
3 63.6 3.9 3.8 4 79.5 81.1 77.8 75.9 82.2 69.6 20.5
4 84.8 4.1 4.2 4 79.4 77.7 81.2 70 63.6 76.4 20.6
5 106 4.3 4 4.6
76.1 78.3 73.8 46.4 66.6 26.2 23.9
6 127.2 4.3 4.3 4.4 80.2 81.1 79.3 74.5 80.3 68.7 19.8
1 13.2 5.4 66 41.4 34
3 39.6 4.8 4.6 5 65.6 64.7 66.6 45.7 49.9 41.6 34.4
4 52.8 5 4.5 5.5 65.7 65.7 65.8 46.1 51.7 40.5 34.3
5 66
4.6 4.4 4.7 65.2 65 65.5 46 51.1 40.9 34.8
6 79.2 4.8 5.1 4.4 65.9 65.8 66 46.9 41.3 52.6 34.1
1 20.2 5 73.8 52.1 26.2
4 80.8 5.2 4.2 6.3 76.9 74.3 79.5 74.3 66.9 81.7 23.1
5 101 4.6 4.8 4.5
76.4 78.2 74.6 74.3 81.3 67.4 23.6
6 121.2 4.9 4.5 5.3 79.1 80.4 77.8 76 71.8 80.1 20.9
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Example 7 ¨ Bacterial and Particle Filtration Efficiency Tests
[0258] Two sample filters were prepared using webs of polyamide 66 nanofibers.
Filter 1 had
a basis weight of 8.2 gsm, and its nanofibers had an average fiber diameter of
612 nm and an
median fiber diameter of 440 nm. Air permeability was 72.1 cfm/ft2, mean flow
pore
diameter was 7.2, and bubble point was 28.1 microns. Filter 2 had a basis
weight of 11.1 gsm,
and its nanofibers had an average fiber diameter of 621 nm and an median fiber
diameter of
469 nm. Air permeability was 39.2 cfm/ft2, mean flow pore diameter was 5.9,
and bubble
point was 25.7 microns. The thickness of each filter was approximately 20 mm.
Each filter
had dimensions of about 174 mm by about 178 mm.
[0259] Filter 1 and Filter 2 were tested for Bacterial Filtration Efficiency
(BFE) and Particle
Filtration Efficiency (PFE). Filter 1 and Filter 2 were compared against a
standard filter made
of three layers of polypropylene-spunbond/meltblown/Spunbond.
[0260] The BFE test was performed to determine the filtration efficiency of
the test filters by
comparing the bacterial control counts upstream of the test filter to the
bacterial counts
downstream. A suspension of Staphylococcus aureus was aerosolized using a
nebulizer and
delivered to the test article at a constant flow rate (28.3 L/m) and fixed air
pressure (2.8 x 103
CFU). The conditioning parameters were 85% 5% relative humidity and 21 C 5
C for a
minimum of 4 hours. The challenge delivery was maintained at 1.7 - 3.0 x 103
colony
forming units (CFU) with a mean particle size (MPS) of 3.0 0.3 gm. The
aerosols were
drawn through a six stage, viable particle, Andersen sampler for collection.
This test method
complies with ASTM F2101-19 and EN 14683:2019, Annex B.
[0261] The pressure drop (delta P) test was performed to determine the
breathability of test
filter articles by measuring the differential air pressure on either side of
the test article using a
manometer, at a constant flow rate. The delta P test complies with EN
14683:2019, Annex C
and ASTM F2100-19.
[0262] The PFE testing was performed to evaluate the non-viable particle
filtration efficiency
(PFE) of the test filter articles (11.1 gsm, 8.2 gsm, and standard).
Monodispersed polystyrene
latex spheres (PSL) were nebulized (atomized), dried, and passed through the
test filter
article. The particles that passed through the test filter article were
enumerated using a laser
particle counter.
[0263] A one-minute count was performed, with the test filter in the system. A
one-minute
control count was performed, without a test filter article in the system,
before and after each
test article and the counts were averaged. Control counts were performed to
determine the
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average number of particles delivered to the test filter article. The
filtration efficiency was
calculated using the number of particles penetrating the test filter article
compared to the
average of the control values,
[0264] The procedure employed the basic particle filtration method described
in ASTM
F2299, with some exceptions; notably the procedure incorporated a non-
neutralized
challenge. In real use, particles carry a charge, thus this challenge
represents a more natural
state. The non-neutralized aerosol is also specified in the FDA guidance
document on
surgical face masks.
[0265] The results of the BFE and PFE testing are shown in Table 3. The
results shown in
Table 3 are average results. For 8.2 gsm meltblown polyamide and polypropylene
standard, 5
samples were averaged. For 11.1 and 11,1 gsm meltblown polyamide four samples
were
averaged.
[0266] Both the 11.1 gsm and 8.2 gsm meltblown polyamide 66 nanofiber
demonstrated
favorable PFE that were analogous with the standard. Advantageously, the 11.1
gsm also
excelled in BFE while improving (decreasing) the delta P over the standard.
This is a marked
and unexpected improvement. Similarly, although the BFE was slightly lower,
the 8.2 gsm
meltblown polyamide 66 nanofiber demonstrated a significantly lower delta P.
The
meltblown nanofibers of the present invention can provide functional
efficiencies with
improved performance over the polypropylene standard.
Table 3
Meltblown Polyamide 66 Nanofibers Polypropylene
11.1 gsm 8.2 gsm Standard
BFE 97.2% 86.4% 97.3%
PFE 97.5% 94.5% 98.1%
delta P (mm H20/cm2) 2.95 1.22 3.82
[0267] While the invention has been described in detail, modifications within
the spirit and
scope of the invention will be readily apparent to those of skill in the art.
Such modifications
are also to be considered as part of the present invention. In view of the
foregoing discussion,
relevant knowledge in the art and references discussed above in connection
with the
Background of the Invention, further description is deemed unnecessary. In
addition, it should
be understood from the foregoing discussion that aspects of the invention and
portions of various
embodiments may be combined or interchanged either in whole or in part.
Furthermore,
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those of ordinary skill in the art will appreciate that the foregoing
description is by way of
example only, and is not intended to limit the invention.
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