U.S. patent application number 11/295861 was filed with the patent office on 2006-06-29 for filtration media for filtering particulate material from gas streams.
Invention is credited to Michael Allen Bryner, Joseph Brian Hovanec, David Charles Jones, Hyun Sung Lim, B. Lynne Wiseman.
Application Number | 20060137317 11/295861 |
Document ID | / |
Family ID | 36177902 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060137317 |
Kind Code |
A1 |
Bryner; Michael Allen ; et
al. |
June 29, 2006 |
Filtration media for filtering particulate material from gas
streams
Abstract
A filtration medium is disclosed for use in air filters used in
heating, ventilating and air conditioning systems. The medium
contains at least one nanofiber layer of fibers having diameters
less than 1 .mu.m and at least one carrier layer, each nanofiber
layer having a basis weight of at least about 2.5 g/m.sup.2, and up
to about 25 g/m.sup.2. The medium has sufficient stiffness to be
formed into a pleated configuration.
Inventors: |
Bryner; Michael Allen;
(Midlothian, VA) ; Jones; David Charles;
(Midlothian, VA) ; Lim; Hyun Sung; (Midlothian,
VA) ; Wiseman; B. Lynne; (Richmond, VA) ;
Hovanec; Joseph Brian; (Richmond, VA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
36177902 |
Appl. No.: |
11/295861 |
Filed: |
December 7, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60639771 |
Dec 28, 2004 |
|
|
|
Current U.S.
Class: |
55/528 |
Current CPC
Class: |
B01D 2273/28 20130101;
D04H 3/16 20130101; B32B 2262/0261 20130101; B32B 2459/00 20130101;
B01D 46/22 20130101; B01D 46/521 20130101; B32B 5/022 20130101;
D04H 1/4374 20130101; B01D 39/1615 20130101; B01D 2275/10 20130101;
B32B 2307/544 20130101; B32B 5/024 20130101; D04H 1/559 20130101;
D04H 1/728 20130101; B01D 2239/10 20130101; B01D 2239/025 20130101;
B32B 2250/20 20130101; B01D 39/18 20130101; B01D 46/546 20130101;
B01D 39/163 20130101; B32B 2307/546 20130101; B32B 5/26 20130101;
B01D 2239/0681 20130101 |
Class at
Publication: |
055/528 |
International
Class: |
B01D 46/00 20060101
B01D046/00 |
Claims
1. A filtration medium comprising at least one nanofiber layer of
continuous polymeric fibers having diameters less than about 1000
nanometers, each nanofiber layer having a basis weight of at least
about 2.5 g/m.sup.2, and at least one scrim layer, wherein the
medium has a filtration efficiency of at least about 20% when
challenged with particles having an average diameter of 0.3 .mu.m
in air flowing at a face velocity of 5.33 cm/sec, and a
Handle-o-meter stiffness of at least about 45 g.
2. The filtration medium of claim 1, wherein the total nanofiber
layer basis weight is about 25 g/m.sup.2.
3. The filtration medium of claim 1, wherein the nanofiber layer
has a thickness of less than 100 .mu.m.
4. The filtration medium of claim 1, wherein the wherein the
continuous polymeric fibers of the nanofiber layer have diameters
between about 100 nanometers and about 700 nanometers.
5. The filtration medium of claim 1, wherein the wherein the
continuous polymeric fibers of the nanofiber layer have diameters
between about 300 nanometers and about 650 nanometers.
6. The filtration medium of claim 1, wherein the medium has a
filtration efficiency of at least about 30% and up to about 99.97%
when challenged with particles having an average diameter of 0.3
.mu.m in air flowing at a face velocity of 5.33 cm/sec.
7. The filtration medium of claim 1, further comprising a second
scrim layer wherein the nanofiber layer is sandwiched between the
two scrim layers.
8. The filtration medium of claim 1, wherein the medium is
substantially electrically neutral.
9. The filtration medium of claim 1, wherein the scrim layer is a
spunbond nonwoven web or carded nonwoven web.
10. The filtration medium of claim 1, which has an initial pressure
drop less than about 30 mm H.sub.2O.
11. The filtration medium of claim 1, which has an initial pressure
drop less than about 24 mm H.sub.2O.
12. The filtration medium of claim 1, which has a Frazier air
permeability of at least about 0.91 m.sup.3/min/m.sup.2.
13. The filtration medium of claim 1, which has a Frazier air
permeability of between about 0.91 m.sup.3/min/m.sup.2 and about 48
m.sup.3/min/m.sup.2.
14. The filtration medium of claim 1, wherein the filtration medium
is pleated.
15. A process for filtering particulate matter from an air stream
comprising passing an air stream containing particulate matter
through a filtration medium comprising at least one nanofiber layer
of continuous polymeric fibers and at least one scrim layer,
wherein the continuous polymeric fibers of the nanofiber layer have
diameters less than about 1000 nanometers and wherein each
nanofiber layer has a basis weight of at least about 2.5 g/m.sup.2
and a thickness of less than about 100 .mu.m, and wherein the
filtration medium has a Handle-o-meter stiffness of at least about
45 g, and filtering up to about 99.97% of particles having an
average diameter of 0.3 .mu.m in an air stream moving at a face
velocity of 5.33 cm/sec.
16. The process of claim 15, wherein at least 20% of particles 0.3
.mu.m and larger are filtered.
17. The process of claim 15, wherein the initial pressure drop of
the filtration medium is less than about 30 mm H.sub.2O.
18. The process of claim 15, wherein the total basis weight of the
nanofiber layer is about 25 g/m.sup.2.
19. A process of forming a filtration medium comprising providing
at least one scrim layer having a Handle-o-meter stiffness of at
least about 10 g on a moving collection belt, and depositing
nanofibers on the scrim layer to form a single nanofiber layer
having a basis weight of at least about 2.5 g/m.sup.2 to form a
filtration medium having a Handle-o-meter stiffness of at least
about 45 g and a pressure drop of less than about 30 mm
H.sub.2O.
20. The process of claim 19, wherein the total nanofiber layer
basis weight is about 25 g/m.sup.2.
21. The process of claim 20, wherein the nanofiber layer is formed
in a single pass of the scrim layer on the moving collection
belt.
22. The process of claim 19, wherein the nanofibers are formed by
electroblowing a polymer solution from a series of spinneret holes
at a rate of at least about 1 cm.sup.3/min/hole and the collection
belt moves at a rate of at least 5 m/minute.
23. The process of claim 22, wherein the polymer solution is
electroblown from a series of spinneret holes at a rate of at least
2 cm.sup.3/min/hole.
24. The process of claim 19, further comprising pleating the
filtration medium.
25. The process of claim 19, further comprising adhering at least a
second scrim layer onto said nanofiber layer.
26. The process of claim 19, wherein said scrim layer has a
Handle-o-meter stiffness of at least about 45 g.
27. The process of claim 25, wherein said scrim layers have a
combined Handle-o-meter stiffness of at least about 45 g.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to air filtration media, for
filtering particulate material from gas streams.
[0003] 2. Description of the Related Art
[0004] Filter media typically utilized for HVAC air filters that
perform at efficiencies less than 99.97% at a 0.3 micron challenge
are either glass, cellulose or polymer based. Filters made with
media in this performance range are typically referred to as
"ASHRAE filters" since the American Society of Heating,
Refrigerating and Air-Conditioning Engineers writes standards for
the performance of filter media in such applications. Polymer based
filter media are typically spunbond or meltblown nonwovens that are
often electrostatically enhanced to provide higher filtration
efficiency at lower pressure drop when compared to glass or
cellulose media manufactured by a wet laid paper-making
process.
[0005] Electrostatically enhanced air filter media and media
manufactured by the wet laid process, more specifically with the
use of glass fibers, currently have limitations. Electrostatically
treated meltblown filter media, as described in U.S. Pat. Nos.
4,874,659 and 4,178,157, perform well initially, but quickly lose
filtration efficiency in use due to dust loading as the media begin
to capture particles and the electrostatic charge thus becomes
insulated. In addition, as the effective capture of particulates is
based on the electrical charge, the performance of such filters is
greatly influenced by air humidity, causing charge dissipation.
[0006] Filtration media utilizing microglass fibers and blends
containing microglass fibers typically contain small diameter glass
fibers arranged in either a woven or nonwoven structure, having
substantial resistance to chemical attack and relatively small
porosity. Such glass fiber media are disclosed in the following
U.S. patents: Smith et al., U.S. Pat. No. 2,797,163; Waggoner, U.S.
Pat. No. 3,228,825; Raczek, U.S. Pat. No. 3,240,663; Young et al.,
U.S. Pat. No. 3,249,491; Bodendorf et al., U.S. Pat. No. 3,253,978;
Adams, U.S. Pat. No. 3,375,155; and Pews et al., U.S. Pat. No.
3,882,135. Microglass fibers and blends containing microglass
fibers are typically relatively brittle and can break when pleated,
and produce undesirable yield losses. Broken microglass fibers can
also be released into the air by filters containing microglass
fibers, creating a potential health hazard if the microglass were
to be inhaled.
[0007] It would be desirable to provide a means for achieving
ASHRAE level air filtration while avoiding the above-listed
limitations of known filtration media.
SUMMARY OF THE INVENTION
[0008] In a first embodiment, the present invention is directed to
a filtration medium comprising at least one nanofiber layer of
continuous polymeric fibers having diameters less than about 1000
nanometers, each nanofiber layer having a basis weight of at least
about 2.5 g/m.sup.2, and at least one scrim layer, wherein the
medium has a filtration efficiency of at least about 20% when
challenged with particles having an average diameter of 0.3 .mu.m
in air flowing at a face velocity of 5.33 cm/sec, and a
Handle-o-meter stiffness of at least about 45 g.
[0009] A second embodiment of the present invention is directed to
a process for filtering particulate matter from an air stream
comprising passing an air stream containing particulate matter
through a filtration medium comprising at least one nanofiber layer
of continuous polymeric fibers and at least one scrim layer,
wherein the continuous polymeric fibers of the nanofiber layer have
diameters less than about 1000 nanometers and wherein each
nanofiber layer has a basis weight of at least about 2.5 g/m.sup.2
and a thickness of less than about 100 .mu.m, and wherein the
filtration medium has a Handle-o-meter stiffness of at least about
45 g, and filtering up to about 99.97% of particles having an
average diameter of 0.3 .mu.m in an air stream moving at a face
velocity of 5.33 cm/sec.
[0010] Another embodiment of the present invention is directed to a
process of forming a filtration medium comprising providing at
least one scrim layer having a Handle-o-meter stiffness of at least
about 10 g on a moving collection belt, and depositing nanofibers
on the scrim layer to form a single nanofiber layer having a basis
weight of at least about 2.5 g/m.sup.2 to form a filtration medium
having a Handle-o-meter stiffness of at least about 10 g and a
pressure drop of less than about 30 mm H.sub.2O.
DEFINITIONS
[0011] The term "nanofibers" refers to fibers having diameters of
less than 1,000 nanometers.
[0012] The term "filter media" or "media" refers to a material or
collection of materials through which a particulate-carrying fluid
passes, with a concomitant and at least temporary deposition of the
particulate material in or on the media.
[0013] The term "ASHRAE filter" refers to any filter suitable for
use in heating, ventilation and air conditioning systems for
filtering particles from air.
[0014] The term "SN structure" refers to a multilayer nonwoven
material containing a spunbond (S) layer and a nanofiber (N)
layer.
[0015] The term "SNS structure" refers to a multilayer nonwoven
material containing a nanofiber layer sandwiched between two
spunbond layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is an illustration of a prior art electroblowing
apparatus for forming nanofibers suitable for use in the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The invention relates to a filter medium comprising at least
one nanofiber layer and at least one scrim layer. The nanofiber
layer comprises a collection of substantially continuous organic
polymeric nanofibers in a filtration medium layer, the nanofibers
having diameters less than about 1 .mu.m or 1000 nm. Such filter
media can be used in filtration applications for removing
particulate material from a fluid stream, in particular,
particulate material from a gaseous stream such as air.
[0018] Filtration media suitable for use in air filtration
applications, including ASHRAE filtration and vehicle cabin air
filtration, can be made by layering one or more nanofiber layer(s)
with a scrim layer to form an SN.sub.x structure, or by sandwiching
one or more nanofiber layers between two scrim layers to form a
SN.sub.xS structure, where x is at least one. Each nanofiber layer
has a basis weight of at least about 2.5 g/m.sup.2, and the total
basis weight of the nanofiber layers is about 25 g/m.sup.2.
Additionally, the filter medium can contain other layers such as
one or more meltblown (M) layers.
[0019] In the medium of the invention, the nanofiber layer has a
thickness of less than about 100 .mu.m; advantageously the
thickness of the nanofiber layer is greater than 5 .mu.m and less
than 100 .mu.m. The thickness of the nanofiber layer can vary
depending on the density of the nanofiber polymer. The thickness of
the nanofiber layer can be reduced without substantial reduction in
efficiency or other filter properties if the solids volume fraction
of the nanofiber layer is increased, such as by calendering or by
collecting the nanofiber layer under high vacuum. Increasing the
solidity, at constant layer thickness, reduces pore size and
increases particulate storage.
[0020] The nanofiber layer in the present invention may be made in
accordance with the barrier webs disclosed in U.S. Published Patent
Application No. 2004/0116028 A1, which is incorporated herein by
reference.
[0021] The nanofiber layer is made up of substantially continuous
polymeric fibers having diameters less than 1000 nm, advantageously
between about 100 nm and about 700 nm, or even more advantageously
between about 300 nm and about 650 nm. The continuous polymeric
fibers of the nanofiber layer can be formed by any process capable
of making continuous fibers in this diameter range, including
electrostatic spinning or electroblowing. A process for forming
nanofibers via electroblowing is disclosed in PCT Patent
Publication Number WO 03/080905A (corresponding to U.S. Ser. No.
10/477,882, filed Nov. 20, 2002), which is incorporated herein by
reference. WO 03/080905A discloses an apparatus and method for
producing a nanofiber web, the apparatus essentially as shown in
FIG. 1. The method comprises feeding a stream of polymeric solution
comprising a polymer and a solvent from a storage tank 100 to a
series of spinning nozzles 104 within a spinneret 102 to which a
high voltage is applied through which the polymeric solution is
discharged. Meanwhile, compressed air that is optionally heated in
air heater 108 is issued from air nozzles 106 disposed in the sides
or the periphery of spinning nozzle 104. 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 110 above a vacuum chamber 114, which has
vacuum applied from the inlet of air blower 112.
[0022] The filter medium of the invention may be made by adhesively
laminating the nanofiber layer to the carrier layer (also referred
to herein as a "scrim"), or by forming the nanofiber layer directly
on the carrier or scrim layer by placing the scrim layer on the
collection belt 110 in the above described process to form an SN
structure, in which case the nanofiber layer is adhered to the
scrim layer by mechanical entanglement. The medium of the invention
can be made by forming a nanofiber layer in a single pass or by
building up the nanofiber layer to the desired thickness or basis
weight using multiple passes, e.g., in an electroblowing process.
The electroblowing process allows a nanofiber layer of suitable
basis weight for use in an air filter medium to be formed in a
single pass because a higher throughput is possible than previously
known in the production of nanofibers. The nanofiber layer may be
formed with a collection belt speed of at least 5 m/minute, and
advantageously at least 10 m/minute. The polymer solution
throughput in the electroblowing process for forming nanofibers is
at least about 1 cm.sup.3/min/hole of the spinneret, and
advantageously at least about 2 cm.sup.3/min/hole. Therefore, by
configuring the spinneret to have a series of spinning nozzles or
holes along the length of the spinneret, and delivering the polymer
solution through each nozzle or hole at such high rates of flow, a
higher basis weight nanofiber layer than known to date can be
formed on a scrim layer in a single pass. Depending on the polymer
solution flow rate and the collection belt speed, single nanofiber
layers having basis weights of between about 2.5 g/m.sup.2 and even
up to 25 g/m.sup.2 can be formed in a single pass. In conventional
processes for forming nanofiber-containing filtration media,
forming a nanofiber layer of suitable basis weight on a scrim
requires repeated passes of the scrim through the nanofiber
formation process to build up to a basis weight of even 1
g/m.sup.2. By forming the nanofiber layer in one pass according to
the present invention, less handling is required, reducing the
opportunity for defects to be introduced in the final filter
medium. The higher polymer solution throughput of the
electroblowing process provides a more economical process than
previously known in the production of nanofibers. Of course, those
skilled in the art will recognize that under certain circumstances
it can be advantageous to adjust the spinning conditions to deposit
multiple nanofiber layers of at least about 2.5 g/m.sup.2 in
multiple passes in order to build-up the total nanofiber layer
basis weight to as much as about 25 g/m.sup.2. Variations in the
spinning conditions to modify the nanofiber laydown rate, and
therefore the basis weight of a single nanofiber layer, can be made
in the collection belt speed, polymer solution flow rate and even
by varying the concentration of the polymer in the solution.
[0023] The layers of the filter medium are made from organic
polymer materials. Advantageously, the scrim layers are spunbond
nonwoven layers, but the scrim layers can be carded webs of
nonwoven fibers and the like. The scrim layers require sufficient
stiffness to hold pleats and dead folds. The stiffness of a single
scrim layer is advantageously at least 10 g, as measured by a
Handle-o-meter instrument, described below. Particularly high
stiffness can be achieved by using an acrylic bonded carded or wet
laid scrim comprising coarse staple fibers. Spunbond nonwovens may
also be used. The filtration medium of the invention has a total
Handle-o-meter stiffness of at least 45 g. Advantageously, the
filtration medium has a structure of SNS in which at least two
scrim layers contribute to the stiffness.
[0024] The medium of the invention can be fabricated into any
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. The filtration medium of the invention can be used in
virtually any conventional structure including flat panel filters,
oval filters, cartridge filters, spiral wound filter structures and
can be used in pleated, Z-filter, V-bank, or other geometric
configurations involving the formation of the medium to useful
shapes or profiles. Advantageous geometries include pleated and
cylindrical patterns. Such cylindrical patterns are generally
preferred because they are relatively straightforward to
manufacture, use conventional filter manufacturing techniques, and
are relatively easy to service. Pleating of media increases the
media surface area within a given volume. Generally, major
parameters with respect to such media positioning are: pleat depth;
pleat density (typically measured as a number of pleats per inch
along the inner diameter of the pleated media cylinder); and
cylindrical length or pleat length. In general, a principal factor
with respect to selecting filter medium pleat depth, pleat length,
and pleat density, especially for barrier arrangements, is the
total surface area required for any given application or situation.
Such principles would apply, generally, to the medium of the
invention and preferably to similar barrier-type arrangements.
[0025] The filter medium of the invention can be used for the
removal of a variety of particulate matter from fluid streams. The
particulate matter can include both organic and inorganic
contaminants. Organic contaminants can include particulate natural
products, organic compounds, polymer particulate, food residue and
other materials. Inorganic residue can include dust, metal
particulate, ash, smoke, mist and other materials.
[0026] The initial pressure drop (also referred to herein as
"pressure drop" or "pressure differential") of the filter medium is
advantageously less than about 30 mm H.sub.2O, more advantageously
less than about 24 mm H.sub.2O. The pressure drop across a filter
increases over time during use, as particulates plug the filter.
Assuming other variables to be held constant, the higher the
pressure drop across a filter, the shorter the filter life. A
filter typically is determined to be in need of replacement when a
selected limiting pressure drop across the filter is met. The
limiting pressure drop varies depending on the application. Since
this buildup of pressure is a result of dust (or particulate) load,
for systems of equal efficiency, a longer life is typically
directly associated with higher load capacity. Efficiency is the
propensity of the medium to trap, rather than to pass,
particulates. In general the more efficient filter media are at
removing particulates from a gas flow stream, the more rapidly the
filter media will approach the "lifetime" pressure differential,
assuming other variables to be held constant. The filter medium of
the invention has an efficiency of at least about 20%, meaning that
the medium is capable of filtering out at least about 20% of
particles having a diameter of 0.3 .mu.m in air flowing at a face
velocity of 5.33 cm/sec. For use in ASHRAE filters, advantageously,
the medium of the invention is capable of filtering out at least
about 30% and up to about 99.97% of 0.3 .mu.m particles in air
flowing at a face velocity of 5.33 cm/sec.
[0027] The higher the air permeability of the filter medium, the
lower the pressure drop, therefore the longer the filter life,
assuming other variables are held constant. Advantageously, the
Frazier air permeability of the filter medium of the invention is
at least about 0.91 m.sup.3/min/m.sup.2, and typically up to about
48 m.sup.3/min/m.sup.2.
[0028] The filter medium of the present invention is advantageously
substantially electrically neutral and therefore is much less
affected by air humidity as compared with the filters disclosed in
U.S. Pat. Nos. 4,874,659 and 4,178,157, described above, which owe
their performances to the electrical charges associated therewith.
By "substantially electrically neutral" is meant that the medium
does not carry a detectable electrical charge.
Test Methods
[0029] Filtration Efficiency was determined by a Fractional
Efficiency Filter Tester Model 3160 commercially available from TSI
Incorporated (St. Paul, Minn.). The desired particle sizes of the
challenge aerosol particles were entered into the software of the
tester, and the desired filter flow rate was set. A volumetric
airflow rate of 32.4 liters/min and a face velocity of 5.33 cm/sec
were used. The test continued automatically until the filter was
challenged with every selected particle size. A report was then
printed containing filter efficiency data for each particle size
with pressure drop.
[0030] Pressure Drop was reported by the Fractional Efficiency
Filter Tester Model 3160 commercially available from TSI
Incorporated (St. Paul, Minn.). The testing conditions are
described under the Filtration Efficiency test method. Pressure
drop is reported in mm of water column, also referred to herein as
mm H.sub.2O.
[0031] Basis weight was determined by ASTM D-3776, which is hereby
incorporated by reference and reported in g/m.sup.2.
[0032] Thickness was determined by ASTM D177-64, which is hereby
incorporated by reference, and is reported in micrometers.
[0033] Fiber Diameter was determined as follows. Ten scanning
electron microscope (SEM) images at 5,000.times. magnification were
taken of each nanofiber layer sample. The diameter of eleven (11)
clearly distinguishable nanofibers were measured from the
photographs and recorded. Defects were not included (i.e., lumps of
nanofibers, polymer drops, intersections of nanofibers). The
average fiber diameter for each sample was calculated.
[0034] Stiffness was measured using a "Handle-o-meter" instrument
manufactured by Thwing Albert Instrument Co. (Philadelphia, Pa.).
The Handle-o-meter measures in grams the resistance that a blade
encounters when forcing a specimen of material into a slot of
parallel edges. This is an indication of the stiffness of the
material, which has an inverse relationship with the flexibility of
the material. The stiffness is measured in both the longitudinal
direction (machine direction) of the material and the transverse
direction (cross-machine direction).
[0035] Frazier Permeability is a measure of air permeability of
porous materials and is reported in units of ft.sup.3/min/ft.sup.2.
It measures the volume of air flow through a material at a
differential pressure of 0.5 inches (12.7 mm) water. An orifice is
mounted in a vacuum system to restrict flow of air through sample
to a measurable amount. The size of the orifice depends on the
porosity of the material. Frazier permeability is measured in units
of ft.sup.3/min/ft.sup.2 using a Sherman W. Frazier Co. dual
manometer with calibrated orifice, and converted to units of
m.sup.3/min/m.sup.2.
EXAMPLES
Example 1
[0036] Nanofiber layers were made by electroblowing a solution of
nylon 6,6 polymer having a density of 1.14 g/cc (available from E.
I. du Pont de Nemours and Company, Wilmington, Del.) at 24 weight
percent in formic acid at 99% purity (available from Kemira Oyj,
Helsinki, Finland). The polymer and solvent were fed into a
solution mix tank, the solution transferred into a reservoir and
metered through a gear pump to an electroblowing spin pack having
spinning nozzles, as described in PCT Patent Publication No. WO
03/080905. The spin pack was 0.75 meter wide and had 76 spinning
nozzles. The pack was at room temperature with the pressure of the
solution in the spinning nozzles at 10 bar. The spinneret was
electrically insulated and applied with a voltage of 75 kV.
Compressed air at a temperature of 44.degree. C. was injected
through air nozzles into the spin pack at a rate of 7.5
m.sup.3/minute and a pressure of 660 mm H.sub.2O. The solution
exited the spinning nozzles into air at atmospheric pressure, a
relative humidity of 65-70% and a temperature of 29.degree. C. The
polymer solution throughput of the nanofiber-forming process was
about 2 cm.sup.3/min/hole. The fibers formed were laid down 310 mm
below the exit of the pack onto a porous scrim on top of a porous
belt moving at 5-12 m/minute. A vacuum chamber pulling a vacuum of
100-170 mm H.sub.2O beneath the belt assisted in the laydown of the
fibers. A 40 g/m.sup.2 basis weight spunbond nonwoven material
obtained from Kolon Industries (S. Korea) was used as the scrim.
The scrim had a stiffness of 35 g in the longitudinal direction and
55 g in the transverse direction.
[0037] The SN structure produced was challenged at various particle
sizes for filtration efficiency using a TSI tester 3160, and the
results are given in Table 1. Efficiencies reported in the data
below are for 0.3 micrometer particle challenge only.
Example 2
[0038] An SN structure was made as described in Example 1, but at a
higher basis weight of the nanofiber layer. The resulting structure
was challenged at various particle sizes for filtration efficiency,
and the results are given in Table 1. TABLE-US-00001 TABLE 1
Nanofiber Nanofiber basis Pressure Frazier air diameter weight
Efficiency Drop permeability Ex. No. (nm)* (g/m.sup.2) (%) (mm
H.sub.2O) (m.sup.3/m.sup.2/min) 1 341/387 3 69.9 3.7 37 2 374/362 5
85 6.4 22 *first measurement/second measurement
Example 3
[0039] A filtration medium having an SNS structure was formed by
hand consisting a nanofiber layer having a basis weight of about 3
g/m.sup.2 sandwiched between two spunbond layers each having a
basis weight of about 21 g/m.sup.2 made from bicomponent
sheath-core fibers having a sheath of polyethylene (PE) and a core
of poly(ethylene terephthalate) (PET). The average diameter of the
nanofibers was about 651 nm. The nanofibers were nylon. The Frazier
air permeability, the pressure drop and the efficiency of the
filtration medium are listed in Table 2.
Examples 4-10
[0040] Filtration media were formed as in Example 3, with the
exception that the media of Examples 4-10 had various numbers of
nanofiber (N) layers and meltblown (M) layers sandwiched between
the two spunbond (S) layers. The meltblown layers were made from
side-by-side PET-PE bicomponent fibers, each meltblown layer having
a basis weight of about 17 g/m.sup.2. The structure of each medium,
basis weight of the nanofiber layer, basis weight of the filtration
medium, Frazier air permeability, pressure drop and efficiency of
the filtration medium are listed in Table 2.
Examples 11-15
[0041] Filtration media were formed by electroblowing layers of
nylon 6 nanofibers onto a spunbond nonwoven support. The average
diameter of the nanofibers was between about 300 and 400 nm. The
number of passes to form the nanofiber layer, the nanofiber layer
basis weight, the media Frazier air permeability, pressure drop and
filtration efficiency are listed in Table 2.
Examples 16-17
[0042] Filtration media were formed by electroblowing layers of
nylon 6,6 nanofibers onto a scrim. The media of Examples 16-17 had
SN structures including multiple passes of the scrim layer through
the electroblowing process. The scrim was a bilayer structure
containing a layer of carded nylon and a layer of carded polyester
which was subsequently thermally bonded, obtained from HDK
Industries, Inc., having a basis weight of about 62 g/m.sup.2. The
average diameter of the nanofibers was between about 300 and 400
nm. The number of passes to form the nanofiber layer, the nanofiber
layer basis weight, and the filtration media basis weight, Frazier
air permeability, pressure drop and filtration efficiency are
listed in Table 2.
Example 18
[0043] A filtration medium was formed by electroblowing a single
layer of nylon 6,6 nanofibers onto a moving collection belt in an
electroblowing process at low vacuum pinning pressure, resulting in
a lofty nanofiber layer. The medium had a nanofiber layer with a
basis weight of about 20 g/m.sup.2, formed in a single pass through
the electroblowing process. The average diameter of the nanofibers
was between about 300 and 400 nm. The Frazier air permeability,
pressure drop and filtration efficiency of the filtration medium
are listed in Table 2. TABLE-US-00002 TABLE 2 Frazier air Nanofiber
Medium perme- Pressure basis basis ability Drop Effi- Ex. Medium
weight weight (m.sup.3/m.sup.2/ (mm ciency No. Structure
(g/m.sup.2) (g/m.sup.2) min) H.sub.2O) (%) 3 SNS 3.05 45.7 10.8
3.94 62.76 4 SMNNS 6.1 65.7 4.58 8.17 89.07 5 SMNNNS 9.2 68.8 3.41
11.1 94.03 6 SMNNNNS 12.2 71.8 2.87 14.9 97.53 7 SMMNNS 6.1 82.7
3.57 11.6 92.48 8 SMMNNNS 9.2 85.7 2.86 15.0 97.22 9 SMMNNNNS 12.2
88.8 2.51 17.7 98.14 10 SMMNNNNS- 24.4 178 1.21 35.7 99.93 SMMNNNNS
11 SN (1 pass) 5 14.6 3.30 64.50 12 SNN 10 7.31 6.82 87.14 (2 pass)
13 SNNN 15 4.57 10.2 91.94 (3 pass) 14 SNNNN 20 3.35 14.0 96.00 (4
pass) 15 SNNNNN 25 3.35 16.2 96.99 (5 pass) 16 SNNNN 17.0 79.4 2.74
16.2 99.13 17 SN 3.63 66.0 4.57 10.6 95.50 18 N (1 pass) 20 n/a
3.57 10.2 96.40
Comparative Examples 19-26
[0044] Filtration media having an SNS structure were formed by hand
of nylon nanofiber layers having between about 0.3 and 0.5
g/m.sup.2 basis weight sandwiched between two scrim layers, each
scrim layer having a basis weight of about 17 g/m.sup.2. In
Comparative Examples 19-20, the scrim layers were spunbond PET on
both sides. In Comparative Examples 21-22, the scrim layer was a
spunbond PET on one side, and a spunbond nylon (obtained from Cerex
Advanced Fabrics) on the other. In Comparative Examples 23-24, the
scrim layer was a spunbond PET on one side, and a bilayer structure
containing a layer of carded nylon and a layer of carded polyester
which was subsequently thermally bonded (obtained from HDK
Industries, Inc.) on the other. In Comparative Examples 25-26, the
scrim layers were bilayer structures containing a layer of carded
nylon and a layer of carded polyester which was subsequently
thermally bonded (obtained from HDK Industries, Inc.) on both
sides. The nanofiber layer basis weight, average diameter of the
nanofibers, the media Frazier air permeability, pressure drop and
filtration efficency are listed in Table 3. TABLE-US-00003 TABLE 3
Nano- Nanofiber fiber Medium Pressure basis diam- Basis Frazier air
Drop Effi- Comp. weight eter Weight permeability (mm ciency Ex. No.
(g/m.sup.2) (nm) (g/m.sup.2) (m.sup.3/m.sup.2/min) H.sub.2O) (%) 19
0.3 917 17.66 199 1.58 15.05 20 0.5 20.16 90.8 2.30 28.35 21 0.3
947 16.5 173 1.58 14.20 22 0.5 956 16.5 77.1 2.70 23.28 23 0.3 852
19.5 178 1.57 8.61 24 0.5 930 19.0 86.2 2.33 18.41 25 0.4 1275 36.0
109 1.38 10.63 26 0.3 206 1.25 8.60
* * * * *