U.S. patent application number 10/630520 was filed with the patent office on 2005-02-03 for high performance filter media with internal nanofiber structure and manufacturing methodology.
Invention is credited to Fallon, Stephen L., Haberkamp, William C., Pardue, Byron A., Verdegan, Barry M..
Application Number | 20050026526 10/630520 |
Document ID | / |
Family ID | 32508321 |
Filed Date | 2005-02-03 |
United States Patent
Application |
20050026526 |
Kind Code |
A1 |
Verdegan, Barry M. ; et
al. |
February 3, 2005 |
High performance filter media with internal nanofiber structure and
manufacturing methodology
Abstract
High performance filter media and manufacturing methodology
provides nanofibers of diameter less than 1 .mu.m incorporated and
processed into internal structure of a filter medium dominantly
composed of coarse fibers of diameter greater than 1 .mu.m, to
change the internal media structure.
Inventors: |
Verdegan, Barry M.;
(Stoughton, WI) ; Fallon, Stephen L.; (Madison,
WI) ; Pardue, Byron A.; (Cookeville, TN) ;
Haberkamp, William C.; (Cookeville, TN) |
Correspondence
Address: |
ANDRUS, SCEALES, STARKE & SAWALL, LLP
100 EAST WISCONSIN AVENUE, SUITE 1100
MILWAUKEE
WI
53202
US
|
Family ID: |
32508321 |
Appl. No.: |
10/630520 |
Filed: |
July 30, 2003 |
Current U.S.
Class: |
442/340 ;
210/295 |
Current CPC
Class: |
Y10T 442/615 20150401;
B01D 39/16 20130101; Y10S 428/903 20130101; Y10T 442/614 20150401;
Y10T 442/608 20150401; Y10S 264/48 20130101; Y10T 442/626 20150401;
B01D 39/20 20130101 |
Class at
Publication: |
442/340 ;
210/295 |
International
Class: |
B01D 039/00 |
Claims
What is claimed is:
1. High performance filter media comprising nanofibers of diameter
less than 1 .mu.m incorporated and processed into internal
structure of a filter medium dominantly composed of coarse fibers
of diameter greater than 1 .mu.m.
2. The filter media according to claim 1 wherein said nanofibers
and said coarse fibers are of different materials.
3. The filter media according to claim 1 wherein: said nanofibers
are selected from the group consisting of: polymeric materials;
ceramic materials; acrylic; nylon; polyvinyl alcohol; polymeric
halocarbon; polyester; polyaramid; polyphenylsulfide; cellulose;
titania; glass; alumina; and silica; and said coarse fibers are
selected from the group consisting of: polymeric materials; ceramic
materials; polyvinyl alcohol; cellulose; acrylic; polyester;
polyaramid; titania; glass; silica; nylon; polyphenylsulfide;
polymeric halocarbon; and alumina.
4. The filter media according to claim 1 wherein the ratio of
coarse fiber diameter to nanofiber diameter is between 10 and
5,000.
5. The filter media according to claim 1 wherein said nanofiber
diameter is less than 500 nm.
6. The filter media according to claim 1 wherein said nanofiber
diameter is greater than 50 nm.
7. The filter media according to claim 1 wherein said nanofibers
comprise less than 5% by weight of the weight of said filter
media.
8. The filter media according to claim 7 wherein said nanofibers
comprise less than 1% by weight of the weight of said filter
media.
9. The filter media according to claim 1 wherein said nanofibers
are distributed uniformly throughout the filter media.
10. The filter media according to claim 1 wherein said nanofibers
are distributed unevenly in the filter media such that said
nanofibers are concentrated in bundles providing pockets of
nanofibers in a matrix of coarse fibers, said pockets providing
spatially distinct areas of greater filtration efficiency in a
matrix of lesser filtration efficiency.
11. The filter media according to claim 10 wherein said nanofibers
are provided in low enough concentration and small enough diameter
that there is insubstantial difference in flow velocity, relative
to media without nanofibers, through said media across a face
thereof until said nanofiber bundles begin to plug, whereupon flow
is increasingly diverted through coarse fiber sections in said
matrix between said pockets such that filtration efficiency is
increased relative to media without nanofibers at the same flow
velocity and pressure drop, at least initially until said nanofiber
bundles begin to plug.
12. The filter media according to claim 1 wherein said filter media
has distally opposite upstream and downstream faces normal to flow
therethrough, and wherein said nanofibers are concentrated at one
of said faces and include a first set of nanofiber portions
extending parallel to said one face, and a second set of nanofiber
portions extending normal to said one face.
13. The filter media according to claim 1 wherein said filter media
has distally opposite upstream and downstream faces normal to flow
therethrough and defining a filter media thickness therebetween,
and wherein: said filter media has a macrostructure, defined as
viewed at magnification of 5 to 50.times., selected from the group
consisting of: macrostructure A wherein said nanofibers are
distributed uniformly throughout said filter media; macrostructure
B wherein said nanofibers are distributed unevenly in bundles
providing pockets of nanofibers in a matrix of coarse fibers; and
macrostructure C wherein said nanofibers are concentrated at one of
said faces; and wherein said filter media has a nanofiber/coarse
fiber interface providing a microstructure, defined as viewed at
magnification of 50 to 500.times., selected from the group
consisting of: microstructure 1 wherein said nanofibers form
bridges across pores between said coarse fibers; microstructure 2
wherein said nanofibers substantially collapse onto said coarse
fibers; and microstructure 3 wherein there is no significant
bridging of said nanofibers across said pores between said coarse
fibers and no significant collapse of said nanofibers onto said
coarse fibers, and instead said nanofibers clump together.
14. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure A and
microstructure 1.
15. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure A and
microstructure 2.
16. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure A and
microstructure 3.
17. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure B and
microstructure 1.
18. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure B and
microstructure 2.
19. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure B and
microstructure 3.
20. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure C and
microstructure 1.
21. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure C and
microstructure 2.
22. The filter media according to claim 13 wherein said filter
media is composed of the combination of macrostructure C and
microstructure 3.
23. The filter media according to claim 13 wherein said filter
media is composed of macrostructure A, and wherein said nanofibers
are distributed uniformly throughout said filter media in all three
dimensions.
24. The filter media according to claim 13 wherein said filter
media is composed of macrostructure B, and wherein each of said
bundles comprises one or more nanofibers twisted and intermingled
into an assemblage.
25. The filter media according to claim 24 wherein the longest
dimension of said bundle is less than said filter media
thickness.
26. The filter media according to claim 25 wherein said longest
dimension of said bundle is in the range of 10% to 50% of said
filter media thickness.
27. The filter media according to claim 13 wherein said filter
media is composed of macrostructure B, and wherein said bundles
cumulatively occupy less than 20% of the volume of said filter
media.
28. The filter media according to claim 13 wherein said filter
media is composed of macrostructure C, and wherein said nanofibers
are 3-dimensionally-randomly oriented at said one face such that
some nanofiber portions extend parallel to said one face, and other
nanofiber portions extend normal to said one face, such that the
normally extending nanofiber portions increase attachment strength
to said coarse fibers, reduce delamination risk of said nanofibers,
and reduce pressure drop due to increased orientation of said
nanofibers in the direction of flow.
29. The filter media according to claim 13 wherein said filter
media is composed of microstructure 1, and wherein said nanofibers
forming said bridges across said pores subdivide said pores into
subpores having a size dependent upon the relative numbers of said
nanofibers and coarse fibers.
30. The filter media according to claim 13 wherein said filter
media is composed of microstructure 2, and wherein the interface of
said nanofibers and said coarse fibers forms a composite fiber,
with said nanofibers lying along and across said coarse fibers and
creating channels for transport and drainage and providing an
artificially roughened collection surface with increased surface
area relative to coarse fibers alone.
31. The filter media according to claim 13 wherein said
microstructure is selected from the group consisting of
microstructure 1, microstructure 2 and microstructure 3, and
wherein said nanofibers are selected from the group consisting of
adsorptive materials and catalytic materials, to provide filter
media of increased surface area for one of adsorptive and catalytic
activity without substantially increasing restriction.
32. The filter media according to claim 13 wherein said
microstructure is selected from the group consisting of
microstructure 2 and microstructure 3 to bond said nanofibers to
said coarse fibers and provide increased strength of said filter
media and provide better retention of said nanofibers and of said
coarse fibers.
33. The filter media according to claim 1 wherein said nanofibers
have different triboelectric properties than said coarse fibers to
provide a triboelectric effect for removing particles from a fluid
to be filtered.
34. The filter media according to claim 33 wherein said nanofibers
and said coarse fibers comprise first and second fiber types,
respectively, and wherein: one of said first and second fiber types
is selected from the group consisting of: nylon, polyaramid; and
cellulose; and the other of said first and second fiber types is
selected from the group consisting of: acrylic; polyester;
polypropylene; and polymeric halocarbon.
35. The filter media according to claim 1 wherein said nanofibers
have different adsorption properties than said coarse fibers.
36. The filter media according to claim 1 wherein said nanofibers
have different surface charge characteristics than said coarse
fibers.
37. The filter media according to claim 36 wherein said different
surface charge characteristics provide a localized electric field
gradient within said filter media enhancing particle removal from
fluid to be filtered.
38. The filter media according to claim 1 wherein said nanofibers
and coarse fibers have different wettability.
39. The filter media according to claim 38 wherein said filter
media captures droplets from a liquid to be filtered, and wherein
said nanofibers are preferentially wetted by said droplets, and
said coarse fibers are preferentially non-wetted by said droplets,
whereby to create a capillary pressure gradient wicking droplets
off said coarse fibers, facilitating drainage.
40. The filter media according to claim 38 wherein said filter
media captures and coalesces droplets from a liquid to be filtered,
and wherein said nanofibers are preferentially non-wetted by said
droplets, and said coarse fibers are preferentially wetted by said
droplets, whereby to create a capillary pressure gradient wicking
droplets off said nanofibers, facilitating coalescence and
drainage.
41. The filter media according to claim 1 wherein said nanofibers
are composed of material selected from the group consisting of
catalytic materials, reactive materials, and adsorptive
materials.
42. The filter media according to claim 1 comprising a trimodal
distribution of fiber diameter comprising a first set of fibers in
the diameter range 50 to 500 nm, a second set of fibers in the
diameter range 1 to 5 .mu.m, and a third set of fibers in the
diameter range 10 to 50 .mu.m.
43. The filter media according to claim 42 wherein said first set
of fibers is supported by said second set of fibers, and said
second set of fibers is supported by said third set of fibers, said
first set of fibers providing said nanofibers, said second and
third sets of fibers providing said coarse fibers.
44. The filter media according to claim 43 wherein said first set
of fibers form bridges across pores between said second set of
fibers without substantial collapse onto said second set of
fibers.
45. The filter media according to claim 44 wherein said second set
of fibers comprise a fibrillated para-aramid polymer, and said
third set of fibers comprise a cellulose matrix.
46. The filter media according to claim 1 wherein said nanofibers
are flexible.
47. The filter media according to claim 1 wherein said nanofibers
are of non-glass material.
48. The filter media according to claim 1 wherein said filter media
filters a fluid selected from the group consisting of: gas,
including air, exhaust, and crankcase ventilation gas; and liquid,
including oil, fuel, coolant, water, and hydraulic fluid.
49. The filter media according to claim 1 wherein said nanofibers
are selected from the group consisting of: polymeric materials;
ceramic materials; acrylic; nylon; polyvinyl alcohol; polymeric
halocarbon; polyester; polyaramid; polyphenylsulfide; cellulose;
titania; glass; alumina; and silica; and said coarse fibers are
selected from the group consisting of: polymeric materials; ceramic
materials; polyvinyl alcohol; cellulose; acrylic; polyester;
polyaramid; titania; glass; silica; nylon; polyphenylsulfide;
polymeric halocarbon; and alumina; and wherein said filter media
has distally opposite upstream and downstream faces normal to flow
therethrough and defining a filter media thickness therebetween,
and wherein: said filter media has a macrostructure, defined as
viewed at magnification of 5 to 50.times., selected from the group
consisting of: macrostructure A wherein said nanofibers are
distributed uniformly throughout said filter media; macrostructure
B wherein said nanofibers are distributed unevenly in bundles
providing pockets of nanofibers in a matrix of coarse fibers; and
macrostructure C wherein said nanofibers are concentrated at one of
said faces; and wherein said filter media has a nanofiber/coarse
fiber interface providing a microstructure, defined as viewed at
magnification of 50 to 500.times., selected from the group
consisting of: microstructure 1 wherein said nanofibers form
bridges across pores between said coarse fibers; microstructure 2
wherein said nanofibers substantially collapse onto said coarse
fibers; and microstructure 3 wherein there is no significant
bridging of said nanofibers across said pores between said coarse
fibers and no significant collapse of said nanofibers onto said
coarse fibers, and instead said nanofibers clump together.
50. A method for manufacturing high performance filter media
comprising incorporating and processing nanofibers of diameter less
than 1 .mu.m into internal structure of a filter media dominantly
composed of coarse fibers of diameter greater than 1 .mu.m.
51. The method according to claim 50 comprising producing said
filter media with a bi-component fiber process initially providing
a precursor bi-component fiber which is reduced to a nanofiber upon
removal of a carrier.
52. The method according to claim 51 wherein said precursor
bi-component fiber process is selected from the group consisting of
islands-in-the-sea and segmented-pie processes.
53. The method according to claim 52 comprising producing said
filter media using said coarse fibers and said bi-component fibers,
producing said bi-component fibers with said islands-in-the-sea
process having a sea polymer as a carrier for an island polymer to
provide said nanofibers upon removal of the sea polymer carrier,
using a water soluble sea polymer and a water insoluble island
polymer, using said water as a carrier to disperse and suspend said
bi-component fibers and said coarse fibers to provide wet media and
to provide a solvent for the sea polymer such that the water is the
carrier for said bi-component fibers and said coarse fibers as well
as the solvent for the sea polymer.
54. The method according to claim 53 comprising dissolving the sea
polymer by heating the wet media.
55. The method according to claim 54 comprising performing said
heating step as a separate hot rinsing step.
56. The method according to claim 54 comprising drying said wet
media, and performing said heating step by applying heat during
said drying.
57. The method according to claim 54 comprising applying hot water
to said media, removing said hot water by a step selected from the
group consisting of vacuuming and draining, and applying heat to
dry the media and using such applied heat as said heating step.
58. The method according to claim 54 comprising performing said
heating step by increasing the temperature of said water and said
media to dissolve said sea polymer, leaving said nanofibers behind
and retained in said filter media.
59. The method according to claim 52 comprising producing said
filter media with said islands-in-the-sea process having a sea
polymer as said carrier, and dissolving said sea polymer with a
solvent comprising phenolic resin.
60. The method according to claim 52 comprising producing said
filter media with said islands-in-the-sea process having a sea
polymer as said carrier, and dissolving said carrier with a solvent
comprising a water-based resin.
61. The method according to claim 60 wherein said water-based resin
system is selected from the group consisting of acrylic and
water-based phenolic resin.
62. The method according to claim 60 comprising applying heat to
cure said resin, and using said heat to facilitate dissolution of
said sea polymer.
63. The method according to claim 50 comprising producing said
filter media with a bi-component fiber process having a carrier and
initially providing precursor bi-component fibers reduced to
nanofibers upon removal of said carrier, and comprising adding said
precursor bi-component fibers to said coarse fibers prior to
removal of said carrier.
64. The method according to claim 63 comprising dissolving said
carrier with a solvent, and heating said solvent.
65. The method according to claim 63 wherein said filter media has
distally opposite upstream and downstream faces normal to flow
therethrough and defining a filter media thickness therebetween,
and said filter media has a macrostructure C, defined as viewed at
magnification of 5 to 50.times., wherein said nanofibers are
concentrated at one of said faces, and comprising applying
dispersed said precursor bi-component fibers across said one
face.
66. The method according to claim 63 comprising separating said
nanofibers formed by dissolution of said carrier from said
precursor bi-component fibers by a step selected from the group
consisting of: adjusting pH; adding dispersant; adding ions;
altering wettability.
67. The method according to claim 63 wherein said filter media has
distally opposite upstream and downstream faces normal to flow
therethrough and defining a filter media thickness therebetween,
and said filter media has a macrostructure C, defined as viewed by
magnification of 5 to 50.times., wherein said nanofibers are
concentrated at one of said faces, and comprising using said
precursor bi-component fibers to create said macrostructure C.
68. The method according to claim 67 comprising using heat to
remove said carrier.
69. The method according to claim 50 comprising producing said
filter media with a macrostructure A having said nanofibers
distributed uniformly throughout said filter media.
70. The method according to claim 50 comprising producing said
filter media with a macrostructure B having said nanofibers
distributed unevenly in bundles providing pockets of nanofibers in
a matrix of said coarse fibers.
71. The method according to claim 50 wherein said filter media has
distally opposite upstream and downstream faces normal to flow
therethrough and defining a filter media thickness therebetween,
and comprising producing said filter media with a macrostructure C
having said nanofibers concentrated at one of said faces.
72. The method according to claim 50 comprising producing said
filter media with a microstructure 1 having said nanofibers forming
bridges across pores between said coarse fibers.
73. The method according to claim 50 comprising producing said
filter media with a microstructure 2 having said nanofibers
substantially collapsed onto said coarse fibers.
74. The method according to claim 50 comprising producing said
filter media with a microstructure 3 having no significant bridging
of said nanofibers across pores between said coarse fibers, and no
significant collapsing of said nanofibers onto said coarse fibers,
and instead with clumping of said nanofibers together.
75. The method according to claim 51 comprising removing said
carrier to yield bundles of nanofibers providing a macrostructure B
having said nanofibers distributed unevenly in said bundles
providing pockets of nanofibers in a matrix of said coarse
fibers.
76. The method according to claim 75 comprising reducing the length
of said precursor bi-component fibers to a desired length providing
shortened bi-component fibers, providing said shortened
bi-component fibers as less than 5% by weight of the weight of said
filter media, mixing said bi-component fibers with said coarse
fibers to form a suspension in a dispersing fluid, removing said
dispersing fluid, removing said carrier by a change in fluid
temperature or by a solvent, before, during or after the step of
removing the dispersing fluid, drying the media, and adding a
binder or resin at a designated step as part of the dispersing
fluid or separately following fluid or carrier removal.
77. The method according to claim 51 wherein said filter media has
distally opposite upstream and downstream faces normal to flow
therethrough and defining a filter media thickness therebetween,
and comprising using said precursor bi-component fibers to produce
filter media with a macrostructure C having said nanofibers
concentrated at one of said faces.
78. The method according to claim 77 comprising reducing the length
of said precursor bi-component fibers to a desired length providing
shortened bi-component fibers, dispersing said shortened
bi-component fibers in a fluid containing dispersants as needed to
provide a bi-component fiber suspension, dispersing said coarse
fibers in a fluid containing dispersants as needed to provide a
coarse fiber suspension, removing the dispersing fluid from the
coarse fiber suspension to provide a coarse fiber web, introducing
the bi-component fiber suspension over the coarse fiber web at a
time after the start of removal of the coarse fiber dispersing
fluid, removing the dispersing fluid from the bi-component fiber
suspension, removing said carrier by a change in fluid temperature
or by a solvent, before, during or after removal of the dispersing
fluid for the bi-component fibers, drying the media, applying a
binder or resin to the media at a designated step as part of the
dispersing fluid or separately following fluid or carrier removal.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
[0001] The invention relates to filter media, and more particularly
to filter media incorporating nanofibers of diameter less than 1
.mu.m for high performance.
[0002] Filter media with nanofibers is known in the prior art. A
nanofiber filter media layer is typically provided along an
upstream face surface of a bulk filter media including a layer of
coarse fibers. The nanofibers extend parallel to the face of the
bulk filter media layer and provide high efficiency filtering of
small particles in addition to the filtering of larger particles
provided by the coarse filter media. The nanofibers are provided in
a thin layer laid down on a supporting substrate and/or used in
conjunction with protective layers in order to attain a variety of
benefits, including increased efficiency, reduced initial pressure
drop, cleanability, reduced filter media thickness and/or to
provide an impermeability barrier to certain fluids, such as water
droplets. Prior approaches have several inherent disadvantages,
including the need for a supporting substrate, a risk of
delamination of the nanofiber layer from the substrate, more rapid
plugging of the filter by captured contaminants, and the alignment
of nanofibers parallel to the media face surface.
[0003] Also known in the prior art are filter media having
cellulose coarse fibers and a mixture of glass nanofibers and
microfibers in the media. These filters use stiff glass nanofibers,
and use polymeric microfibers to strengthen the media. They have
been used in fuel, air and hydraulic filters.
[0004] The present invention addresses and solves the above noted
problems. The invention provides a fibrous filter media with
nanofibers incorporated and processed into internal structure of a
filter medium. The invention may be used in a variety of
applications for filtering fluid, including gas such as air,
exhaust, and crankcase ventilation gas, and including liquid such
as oil, fuel, coolant, water, and hydraulic fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a schematic macrostructure illustration of filter
media in accordance with the invention.
[0006] FIG. 2 is like FIG. 1 and shows another embodiment.
[0007] FIG. 3 is like FIG. 1 and shows another embodiment.
[0008] FIG. 4 is a schematic microstructure illustration of filter
media in accordance with the invention.
[0009] FIG. 5 is like FIG. 4 and shows another embodiment.
[0010] FIG. 6 is like FIG. 4 and shows another embodiment.
[0011] FIG. 7 is a microphotograph of filter media in accordance
with the invention.
[0012] FIG. 8 is a Table of filter media characteristics.
[0013] FIG. 9 is a microphotograph of a test media sample.
[0014] FIG. 10 is like FIG. 9 and shows another test media
sample.
[0015] FIG. 11 is like FIG. 9 and shows another test media
sample.
[0016] FIG. 12 is like FIG. 9 and shows another test media
sample.
[0017] FIG. 13 is a graph of particle size vs. fractional
efficiency for test media samples.
[0018] FIG. 14 is a microphotograph of another test media
sample.
[0019] FIG. 15 is like FIG. 14 and shows another test media
sample.
[0020] FIG. 16 is like FIG. 14 and shows another test media
sample.
[0021] FIG. 17 is like FIG. 14 and shows another test media
sample.
[0022] FIG. 18 is a graph illustrating characteristics of the
sample filter media of the Table in FIG. 8.
[0023] FIG. 19 is another graph illustrating characteristics of the
sample filter media of the Table in FIG. 8.
[0024] FIG. 20 is like FIG. 14 and shows prior art.
DETAILED DESCRIPTION
[0025] FIG. 1 shows high performance filter media 30 having
nanofibers 32 of diameter less than 1 .mu.m incorporated and
processed into internal structure of a filter medium 34 dominantly
composed of coarse fibers 36 of diameter greater than 1 .mu.m. In
some embodiments, to be described, nanofibers 32 and coarse fibers
36 are of different materials. Nanofibers 32 are preferably
selected from the group consisting of: polymeric materials; ceramic
materials; acrylic; nylon; polyvinyl alcohol; polymeric halocarbon;
polyester; polyaramid; polyphenylsulfide; cellulose; titania;
glass; alumina; and silica. Coarse fibers 36 are preferably
selected from the group consisting of: polymeric materials; ceramic
materials; polyvinyl alcohol; cellulose; acrylic; polyester;
polyaramid; titania; glass; silica; nylon; polyphenylsulfide;
polymeric halocarbon; and alumina. The ratio of coarse fiber
diameter to nanofiber diameter is between 10 and 1,000. In some
embodiments, particularly for liquids, the nanofibers have a
diameter preferably less than 500 nm, and greater than 50 nm. In
further embodiments, particularly for air filtration, smaller
diameter nanofibers may be preferred. The nanofibers preferably
comprise less than 5% by weight of the weight of filter media 30,
and further preferably less than 1% by weight of the weight of the
filter media 30. In the embodiment of FIG. 1, nanofibers 32 are
distributed uniformly throughout filter media 30.
[0026] In another embodiment, FIG. 2, nanofibers 38 are distributed
unevenly in filter media 40 such that nanofibers 38 are
concentrated in bundles 42 providing pockets of nanofibers in a
matrix of coarse fibers 36. The bundles or pockets 42 provide
spatially distinct areas of greater filtration efficiency in a
matrix of lesser filtration efficiency. The nanofibers are provided
in low enough concentration and small enough diameter, to be
described, that there is insubstantial difference in flow velocity,
relative to media without nanofibers, as shown at arrow 44 through
media 40 across face 46 normal thereto, until nanofiber bundles 42
begin to plug, whereupon flow is increasingly diverted through
coarse fiber sections 48 in the matrix between the pockets, to
provide a net effect of overall higher efficiency than filter media
composed only of coarse fibers, and longer life than high
efficiency filters of higher concentration nanofibers. Filtration
efficiency is increased relative to media without nanofibers at the
same flow velocity and pressure drop, at least initially until
nanofiber bundles 42 begin to plug.
[0027] In a further embodiment, FIG. 3, nanofibers 50 are
concentrated at and distributed across one of the upstream and
downstream faces 52 and 54, preferably upstream face 52, of filter
media 56. Distally opposite upstream and downstream faces 52 and 54
are normal to the flow through filter media 56 as shown at flow
arrow 58. The nanofibers include a first set of nanofibers 60
extending substantially parallel to face 52, and a second set of
nanofibers 62 extending substantially normal to face 52. Nanofibers
60 and 62 may be separate distinct nanofibers or may be the same
nanofiber having differently oriented segments joined at a
bend.
[0028] In each of the noted embodiments, the coarse fibers
structurally support the nanofibers, without a separate supporting
substrate for the nanofibers. Depending upon application and media
thickness, a supporting substrate may be provided for the coarse
fibers.
[0029] Filter media 30, FIG. 1, has distally opposite upstream and
downstream faces 64 and 66 normal to flow therethrough, as shown at
flow arrow 68, and defining a filter media thickness 70
therebetween. Filter media 40, FIG. 2, has distally opposite
upstream and downstream faces 46 and 72 normal to flow
therethrough, as shown at flow arrow 44, and defining a filter
media thickness 74 therebetween. Filter media 56, FIG. 3, has
distally opposite upstream and downstream faces 52 and 54 normal to
flow therethrough, as shown at flow arrow 58, and defining a filter
media thickness 76 therebetween. Each of the filter medias has a
macrostructure, to be described, defined as viewed at magnification
of 5 to 50.times., namely: filter media 30, FIG. 1, has a
macrostructure A wherein nanofibers 32 are distributed uniformly
throughout the filter media; filter media 40, FIG. 2, has a
macrostructure B wherein nanofibers 38 are distributed unevenly in
bundles 42 providing pockets of nanofibers in a matrix of coarse
fibers 36; and filter media 56, FIG. 3, has a macrostructure C
wherein nanofibers 50 are concentrated near one of the faces 52 and
54, preferably face 52, in three-dimensionally spatially random
orientations. Each filter media 30, 40, 56 also has a
nanofiber/coarse fiber interface providing a microstructure, to be
described, defined as viewed at magnification of 50 to 500.times.,
namely: a microstructure 1, FIG. 4, wherein nanofibers 32 form
bridges 78 across pores 80 between coarse fibers 36; a
microstructure 2, FIG. 5, wherein nanofibers 82 substantially
collapse onto coarse fibers 36; and a microstructure 3, FIG. 6,
wherein pockets 84 of nanofibers 86 have no significant bridging
(FIG. 4) nor collapse (FIG. 5) of the nanofibers 86 onto the coarse
fibers 36 because pockets 84 contain only nanofibers 86 clumped
together and typically looped or folded onto each other. In various
embodiments, to be described, the filter media is composed of
combinations of: macrostructure A and microstructure 1;
macrostructure A and microstructure 2; macrostructure A and
microstructure 3; macrostructure B and microstructure 1;
macrostructure B and microstructure 2; macrostructure B and
microstructure 3; macrostructure C and microstructure 1;
macrostructure C and microstructure 2; macrostructure C and
microstructure 3.
[0030] In macrostructure A, FIG. 1, it is preferred that the
nanofibers are distributed uniformly throughout the filter media in
all three dimensions, i.e. first and second lateral dimensions
parallel to faces 64 and 66, namely into and out of the page and
left-right as viewed in FIG. 1, and the third dimension namely
vertically in FIG. 1 parallel to arrow 68.
[0031] In macrostructure B, FIG. 2, it is preferred that each
bundle 42 comprises one or more nanofibers 38, relatively short,
preferably less than 1 cm, and twisted and intermingled into a
knot, typically loose, or assemblage. The longest dimension of the
bundle is preferably less than filter media thickness, and further
preferably in the range of 10% to 50% of filter media thickness 74.
It is preferred that bundles 42 cumulatively occupy less than 20%
of the volume of filter media 40.
[0032] In macrostructure C, FIG. 3, it is preferred that nanofibers
50 are three-dimensionally-randomly oriented at face 52 such that
some nanofiber portions at 60 extend parallel to face 52, and some
nanofiber portions at 62 extend normal to face 52, such that the
normally extending nanofiber portions increase attachment strength
to the coarse fibers, reduce delamination risk of the nanofibers,
and reduce pressure drop due to increased orientation of nanofibers
in the direction of flow 58.
[0033] In microstructure 1, FIG. 4, it is preferred that the
nanofibers forming bridges 78 across pores 80 subdivide the pores
into subpores such as 88 and 90 having a size dependent upon the
relative numbers of nanofibers 32 and coarse fibers 36.
[0034] In microstructure 2, FIG. 5, it is preferred that the
interface 92 of nanofibers 82 and coarse fibers 36 form a composite
fiber 94, with the nanofibers 82 lying along and across the coarse
fibers 36 and creating channels for transport and drainage, and
providing an artificially roughened collection surface with
increased surface area relative to coarse fibers alone, and
providing strengthened bonding among fibers in the matrix
increasing media strength.
[0035] In microstructure 3, FIG. 6, flexible non-glass nanofibers
86 may not have sufficient strength to support themselves, and
hence may collapse onto themselves, in which case the media may
have less desirability for particulate filtration applications, but
would have desirable application for increased surface area for
adsorption filtration applications, or as a means to wick droplets
away from coarser fibers, e.g. in coalescer filtration
applications.
[0036] In a further embodiment, the nanofibers have different
triboelectric properties than the coarse fibers to provide a
triboelectric effect for removing particles from a fluid to be
filtered. The nanofibers and coarse fibers are provided by first
and second fiber types, respectively, preferably of different
materials far enough apart in the triboelectric series to produce a
charge when used together. One of the first and second fiber types
is selected from the group consisting of: nylon; polyaramid; and
cellulose. The other of the first and second fiber types is
selected from the group consisting of: acrylic; polyester;
polypropylene; and polymeric halocarbon. In another embodiment, the
nanofibers have different adsorption properties than the coarse
fibers. In another embodiment, the nanofibers have different
surface charge characteristics than the coarse fibers. The
different surface charge characteristics provide a localized
electric field gradient within the filter media enhancing particle
removal from fluid to be filtered. In another embodiment, the
nanofibers and coarse fibers have different wettability. For
example, in a fuel water separation application, the filter media
captures droplets dispersed in a liquid to be filtered, e.g. water
droplets from fuel in a fuel/water coalescer, wherein the
nanofibers are preferentially wetted by the droplets, and the
coarse fibers are preferentially non-wetted by the droplets,
whereby to create a capillary pressure gradient wicking droplets
off the coarse fibers, lowering pressure drop and facilitating
separation and drainage. In another example, including coalescer
applications, the filter media captures and coalesces droplets from
a liquid to be filtered, wherein the nanofibers are preferentially
non-wetted by the droplets, and the coarse fibers are
preferentially wetted by the droplets, whereby to create a
capillary pressure gradient wicking droplets off the nanofibers,
lowering pressure drop, and facilitating coalescence and drainage.
In microstructures 1, 2 and 3, desirable applications include
providing the nanofibers of adsorptive or catalytic materials to
increase and provide high surface area adsorptive or catalytic
activity without a substantial increase in restriction.
Microstructures 2 and 3 are also desirable for increased bonding of
the nanofibers to the coarse fibers and provide increased strength
of the filter media and provide better retention of the nanofibers
and the coarse fibers than possible with nanoparticulates or
nanopowders attached with adhesive or binder. In further desirable
applications, including microstructures 1, 2, 3, the nanofibers are
composed of material selected from the group consisting of
catalytic materials, reactive materials, and adsorptive
materials.
[0037] In the present invention, nanofibers, namely fibers having a
diameter less than 1 .mu.m, are incorporated into the structure of
filter media dominantly composed of coarser fibers larger than 1
.mu.m, prepared by a wet-laid process (the nanofibers and coarse
fibers can be mixed/blended together and wet-laid), vacuum-forming,
hydro-entanglement, or other processes. On a mass basis, the
nanofibers represent less than 5% by weight of the total media
weight, and preferably are present at less than 1% of the weight.
As noted above, the ratio of coarse fiber diameter to nanofiber
diameter is preferably between 10 and 5,000. The wettability
characteristics of the fibers can be selected to minimize the
adhesion of sludge and other semi-solids thus increasing filter
life, to reduce the pressure drop across a coalescer fuel/water
separator or other coalescer, and to achieve other desirable
performance characteristics.
[0038] In macrostructure A, the nanofibers are distributed
uniformly throughout the media matrix. The media is made using
conventional wet-laid processes using a mixture of nanofibers and
coarse fibers. In some embodiments, short nanofibers less than 1 cm
are used. In other embodiments, it may be desirable to use longer
nanofibers, including for the macrostructures A and B, to better
bridge coarse fiber pores and strengthen the media. On a macro
scale, the local filtration properties do not vary significantly
with spatial location. The net effect is a significantly higher
efficiency, lower pressure drop, and longer life and higher
capacity than comparable filters.
[0039] In macrostructure B, the nanofibers are distributed unevenly
throughout the media matrix. The nanofibers are concentrated in
bundles or patches or pockets throughout the matrix. A preferred
production process uses islands-in-the-sea technology, noted below.
This results in spatially distinct areas of greater and lesser
filtration efficiency within the matrix. Due to the low
concentration and small diameter of the nanofibers, relative flow
velocities through the various sections change as the nanofiber
bundles begin to plug. As this occurs, flow will be increasingly
diverted through the coarse fiber sections 48 between the pockets
42 of nanofibers. The net effect is overall higher efficiency as
compared to filters made only of coarse fibers, and longer life as
compared to other filters using nanofiber layers for high
efficiency.
[0040] In macrostructure C, nanofibers are produced using
islands-in-the-sea (IITS) technology, segmented-pie (SP)
technology, electrospinning, or the like, and concentrating the
nanofibers near the surface of filter media made from the coarse
fibers. The noted processes are known in the prior art, and
reference may be had to: "Advances in Sub-Micron Fiber Production".
John Hagewood, Arnold Wilkie, NonWovens World, April-May 2003,
pages 69-73; "The microfibre business in Japan", Max Golding,
Technical Textiles International, May 1992, pages 18-23, Elsevier
Science Publishers 1992. In some embodiments, short nanofibers less
than 1 cm in length are used. In further embodiments, even shorter
nanofibers in the millimeter range are used. The noted processes
produce nanofibers, but not necessarily short ones. Accordingly, it
may be necessary to chop or otherwise shorten the nanofibers so
produced. Each of the IITS and SP technologies uses a carrier for
the fibers, typically provided by a sea polymer carrier. By using a
sea polymer carrier that is slow-dissolving, relatively well
dispersed nanofibers are provided across the surface of the base
media, resulting in nanofibers with a more random, three
dimensional orientation, with some nanofiber portions 62 oriented
normal to the media face surface 52, rather than a flat
two-dimensional orientation with all fibers parallel to media face
surface 52. The noted three-dimensional orientation results in
increased surface area, better cleanability, and reduced
delamination risk of a nanofiber layer.
[0041] A benefit of the disclosed structures, particularly
macrostructures A and B, is that the incorporation of the
nanofibers into the internal structure of the media provides
structural support for the nanofibers. In macrostructure C, the
internally incorporated structure and the random orientation of the
nanofibers, including the three-dimensional orientation, minimizes
delamination. In all macrostructures A, B, C, more nanofiber per
unit media face area can be used as compared to conventional
nanofiber layer media, with less of a pressure drop or capacity
penalty. This is particularly advantageous in that nanofibers can
be used to increase the surface area of the media for adsorption
applications. It has been found that a small amount of nanofibers
boosts efficiency with minimal pressure drop, .DELTA.P, penalty, to
be described. The internally incorporated structure increases
removal efficiency of very small particles, e.g. less than five
microns, relative to large particle removal. The structure provides
increased adsorption and catalytic activity per unit volume. The
structure further provides improvements in strength and
processability of the media. In microstructure 1, nanofibers 32
serve as the noted bridges 78 across pores 80 formed by the coarse
fibers 36, giving rise to even smaller pores such as 88, 90 having
sides formed by a mix of the coarse fibers 36 and nanofibers 32. In
microstructure 2, nanofibers 82 collapse onto the coarse fibers 36
and the fiber interstices 96, producing the noted artificially
roughened collection surface with increased surface area relative
to the coarse fibers alone, and also strengthening the overall
media structure, e.g. higher Mullen burst strength, reducing the
amount of resin binder needed to finish the media.
[0042] As noted above, the nanofibers can be chosen with different
triboelectric properties relative to the coarse fibers in order to
use a triboelectric effect to enhance particle removal. The use of
triboelectric effect is of greatest benefit when the nanofibers are
formed by electrospinning. With this method, the generated
nanofibers are formed in an electrical field and are less subject
to contamination by chemicals that may moderate the triboelectric
effect. Nanofibers with different adsorption properties or surface
charge characteristics than the coarse fibers can also be used,
e.g. in oil or water filtration. This difference can be used to
enhance or create localized electrical field gradients within the
filter media to enhance particle removal. The nanofibers and coarse
fibers can be of different wetting characteristics, as noted
above.
[0043] In the various macrostructures and microstructures, it is
preferred that the lengths of the nanofibers be short enough for
incorporation into the structure, but long enough to bridge pores
80 between the coarse fibers or connect adjacent coarse fibers.
During initial development, it was preferred that the length of the
nanofibers be less than 1 cm. In continuing development, it has
been found that the length of the nanofibers for macrostructures A
and B need not be less than 1 cm, and in fact lengths greater than
1 cm may be desirable in order to better bridge coarse fiber pores
80 and strengthen the media. It was initially thought that the
short length less than 1 cm was needed in order to fit in the
bundles 42. However, it has since been found that the nanofibers
can wrap or clump together in the bundles 42 in a relatively
compact package, even with nanofiber lengths greater than 1 cm. If
the nanofiber length is too short, the nanofibers cannot span pores
80 in microstructure 1, FIG. 4. In macrostructure C, it is still
preferred that the nanofibers have a short length less than 1 cm,
and in some embodiments substantially shorter than 1 cm, e.g. in
the millimeter range.
[0044] The above noted IITS and SP technologies are bi-component
technologies, initially providing a precursor bi-component fiber
which is reduced to a nanofiber upon removal of the sea or carrier
polymer, as is known. A bi-component fiber is a fiber having two
different polymer constituents, one of which is removed, e.g. by a
solvent, which may be heated, leaving behind the nanofiber. The
bi-component fiber is initially a precursor fiber, typically
coarse. The above noted electrospinning process produces the
nanofibers directly, without the intermediate step of a precursor
bi-component fiber. In some cases, the nanofibers, including the
precursor bi-component fibers if used, should be well dispersed
prior to forming the composite filter media, and should be blended
with coarse fibers 36. If precursor bi-component fibers are used,
then appropriate solvents or processes should be used to remove the
sea polymer carrier from the parent IITS or SP fiber to create the
nanofibers. During production, the nanofibers, including the
precursor bi-component fibers if used, may need to be shortened,
particularly if short lengths less than 1 cm are desired for
macrostructure C, or to a desired length greater than 1 cm in
preferred embodiments of macrostructures A and B. The nanofibers,
including bi-component fibers if used, may tend to clump together,
and may need to be separated and dispersed prior to or upon
addition to the coarse fibers 36.
[0045] If the potential energy barrier of interaction between the
nanofibers and coarse fibers 36 as calculated using DLVO
(Derajaguin, Landau, Verwey, Overbeek) theory is low, the
nanofibers will tend to wrap around larger fibers and/or aggregate
at fiber interstices such as 92, making it difficult to obtain
microstructure 1, FIG. 4, without better dispersion/mixing of
fibers. The low potential energy barrier instead produces
microstructure 2, FIG. 5. To produce microstructure 1, enhanced
dispersion or blending of the nanofibers is desired. This is
achieved by increasing the potential energy barrier of interaction
between the nanofibers and coarse fibers 36 by adjusting the pH of
the dispersion fluid, or by adding surfactants or other
dispersants, or by adding adsorbing ions to increase the electrical
double layer repulsion between fibers, or by altering the wetting
characteristics of the fluid or the fibers. Upon removal of solvent
or dispersal liquid during drying or curing, nanofibers may
collapse onto larger fibers 36, making it difficult to obtain
microstructure 1. As noted, this collapsing of the nanofibers may
be reduced or controlled by altering or controlling the potential
energy barrier of interaction between the nanofibers and coarse
fibers 36 using the chemistry of the dispersion as noted. Further
alternatively, the noted collapsing may be reduced by increasing
the length of the nanofibers. Resin binders may be added to
strengthen the media and bind the nanofibers and coarse fibers 36
together.
[0046] When using the noted precursor bi-component fibers, e.g. the
IITS or SP technologies, the solvent or process for removal of the
sea polymer carrier should be compatible with the coarse fibers 36.
Once the sea polymer carrier is gone, the remaining nanofibers need
to be separated from one another if microstructure 1 is desired,
and prevented from clumping onto each other and from
clumping/wrapping around coarse fibers 36. As noted, this can be
achieved by increasing the potential energy barrier of interaction
between the nanofibers and coarse fibers 36 by adjusting the pH of
the dispersion fluid, by adding surfactants or other dispersants,
by adding adsorbing ions to increase the electrical double layer
repulsion between fibers, or by altering the wetting
characteristics of the fluid or the fibers.
[0047] In macrostructure A, the nanofibers are distributed
essentially as individual fibers, relatively uniformly throughout
the media, in all three dimensions. The nanofiber/coarse fiber
interfaces or associations are provided by microstructure 1 or
microstructure 2 or microstructure 3 or a combination thereof.
[0048] Macrostructure B may be produced from bulk nanofiber. This
is done by chopping bulk nanofiber, e.g. formed by electrospinning,
into appropriate shorter lengths, then mixing the chopped
nanofibers in small amounts, typically less than 1% of total media
mass, with an aqueous suspension containing the coarse fibers, then
dispersing the resultant suspension by mixing and, if needed, the
use of dispersants, such that the nanofibers largely remain present
as bundles, and then removing the dispersing fluid, e.g. filtering
the suspension through a supporting screen, and then drying the
media. An example of media produced in this manner is shown in FIG.
7, which is a microphotograph at magnification 500.times. and
includes a scale line showing a 10 .mu.m length. Binders and/or
resin may be applied to the media at appropriate steps to increase
the strength of the media.
[0049] Media with macrostructure B may alternatively be produced
using bi-component fibers, e.g. produced by the noted IITS or SP
processes, that, upon removal of the sea polymer carrier, results
in bundles of nanofibers. Typically, the following steps are
performed: the length of the bi-component fiber is reduced, if it
has not already been done so, by means of chopping or other
processes; small amounts, typically less than 5% of total fiber
mass, of shortened bi-component fibers are mixed with coarse fibers
to form a suspension; the suspended fibers are mixed and dispersed,
using an appropriate mixing device, with or without dispersants;
the dispersing fluid is removed, typically by filtering the
suspension through a supporting screen; the sea polymer carrier of
the bi-component fibers is removed by means of a change in fluid
temperature or through the use of a solvent, which may be done
before, during or after the dispersing fluid removal step; the
media is dried; and binders and/or resin may be applied to the
media at the appropriate step to increase the strength of the
media, wherein the binder may be applied as part of the dispersing
fluid, or separately following fluid or carrier removal.
[0050] In macrostructure C, the nanofibers are
randomly-three-dimensionall- y oriented, and do not lie solely flat
in a two-dimensional plane. This random three-dimensional
orientation has significant advantages, including: increased
strength of attachment of the nanofiber portion of the media to the
coarse fiber portion of the media and reduced delamination risk to
which nanofiber layers are otherwise subject; and reduced pressure
drop due to increased orientation of nanofibers in the direction of
flow 58. Media with macrostructure C may be produced using
bi-component fibers, by the following steps: the length of the
bi-component fibers may be reduced, if not already been done so, by
means of chopping or other processes; the shortened bi-component
fibers are dispersed in an appropriate fluid containing dispersants
as needed to provide a bi-component fiber suspension; the coarse
fibers are dispersed in an appropriate fluid containing dispersants
as needed to provide a coarse fiber suspension; the dispersing
fluid is removed from the coarse fiber suspension, typically by
filtering it through a supporting screen, to provide a coarse fiber
web; the bi-component fiber suspension is introduced over the
coarse fiber web, which may be done any time after the start of
removal of the coarse fiber dispersing fluid, wherein the sooner
the introduction, the more intermixed the two types of fibers will
be; the dispersing fluid is removed from the bi-component fiber
suspension, typically by filtering it through the coarse fiber web;
the sea polymer carrier of the bi-component fibers is removed by
means of a change in fluid temperature or through the use of a
solvent, which may be done before, during or after removal of the
dispersing fluid; the media is dried; and binders and/or resin may
be applied to the media at the appropriate step to increase the
strength of the media, which binder may be applied as part of the
dispersing fluid, or separately following fluid or carrier
removal.
[0051] A series of samples were made and tested, including five
samples for air filter testing, namely media A, media B, media C,
media D, media E, Table 1, FIG. 8. All five samples used similar
base cellulose fibers and amounts for the coarse fibers, but the
type and amount of secondary fibers varied as follows: media A
contains no secondary fibers, Table 1 and FIG. 9; media B contains
0.25 grams of 1,400-3,300 nm diameter melt-blown polyester
secondary fibers, Table 1; media C contains 1.00 grams of
1,400-3,300 nm diameter melt-blown polyester secondary fibers,
Table 1 and FIG. 10; media D contains 0.03 grams of 100-500 nm
diameter acrylic secondary nanofibers, Table 1 and FIG. 11; media E
contains 0.25 grams of 800 nm diameter glass secondary nanofibers,
Table 1 and FIG. 12. The physical properties of the media, as well
as their fractional efficiency and pressure drop, are summarized in
Table 1. FIGS. 9-12 are microphotographs at 500.times.
magnification and include scale lines showing the noted dimensional
length. FIG. 7 shows the upstream face of media D. The nanofibers
are present in the noted bundles or localized masses that are
dispersed throughout the thickness of the media. Table 1 shows that
the five media samples A-E have about the same basis weight and
thickness (caliper). The data in the Table shows that: efficiency
increased with the addition of secondary nanofibers with smaller
diameters than the coarse bulk fibers (pine pulp, cellulose); small
amounts of nanofibers used in the internal structure yielded a
large efficiency increase relative to the base media; about 30
times more coarse polyester melt-blown fiber on a mass basis was
required to achieve the efficiency obtained with the acrylic
nanofiber; about 4 to 10 times more coarse fiber polyester
melt-blown fiber on a mass basis was required to achieve the
efficiency obtained with the glass nanofiber; less nanofiber is
needed to obtain the efficiency increase than any of the coarser
secondary fibers. A comparison of air filter fractional
efficiencies for media samples A, B, C, D, E is shown in FIG. 13.
The incorporation of nanofibers into the media internal structure
is desirable for increased strength, and a concordant benefit of
reducing the amount of binder or resin required to strengthen the
media. For this benefit, macrostructures A or B are preferred using
well dispersed and relatively long nanofibers, e.g. greater than 1
cm. For purposes of increasing strength, microstructure 2 is
preferred, however microstructure 1 also can afford improved
strength, particularly if the bundles are relatively porous and
intermixed with the coarse fibers.
[0052] Another series of five samples were made and tested, namely
samples G, H, I, J, K, Table 1, FIG. 8. The properties of these
samples were chosen for fuel filter media. All five of these
samples used similar base cellulose coarse fibers and amounts, but
the type and amount of secondary fibers were varied as follows:
media G contains 0.5 grams of 800 nm diameter glass secondary
nanofiber, Table 1 and FIG. 14; media H contains 0.5 grams of
500-4,000 nm diameter fibrillated Kevlar secondary fiber, Table 1;
media I contains 1.00 grams of 500-4,000 nm diameter fibrillated
Kevlar secondary fiber, Table 1 and FIG. 15; media J contains 0.8
grams of 1,400-3,300 nm diameter melt-blown polyester secondary
fiber, Table 1 and FIG. 16; media K contains 0.06 grams of 200-600
nm diameter polyaramid secondary nanofiber, Table 1 and FIG. 17.
FIGS. 14-17 are microphotographs at magnification 1,000.times. and
include a scale line showing the noted length dimension. As in the
case of media samples A through E, only small amounts of nanofiber,
relative to the coarse fibers, in media samples G through K are
required to obtain significant reductions in mean flow pore size.
Media K, having macrostructure B, microstructure 2, was obtained
using polyaramid nanofibers. Media G, having macrostructure A,
microstructure 1, was obtained by increasing the amount of
nanofiber relative to coarse fiber.
[0053] FIG. 18 graphically shows mean flow pore size (MFP) in
microns vs basis weight in grams per square meter (g/m.sup.2) for
the media samples in Table 1. The first set of media samples A-E
are comparable to one another in terms of basis weight, namely
approximately 80 g/m.sup.2. Similarly, the second set of media
samples G-K are comparable to one another in terms of basis weight,
namely approximately 130 g/m.sup.2. In the first set, it is seen
that the addition of small amounts of nanofiber greatly decreases
MFP without noticeable increase in basis weight. This decrease in
MFP concordantly increases particle removal efficiency. In the
second set G-K, the addition of nanofibers, e.g. in media G,
decreased the MFP relative to a media without nanofibers, e.g.
media J, as was observed with the first set of media. However, the
addition of nanofibers in media K increased the MFP only to a small
degree relative to a media without nanofibers, e.g. media J. This
illustrates the significance of microstructure relative to MFP, and
hence removal efficiency. Media G is of microstructure 1 in which
the nanofibers subdivide the larger pores 80 (formed by the
intersection of coarse fibers 36) into smaller pores 88, 90. In
contrast, the nanofibers of media K collapsed onto the coarse
fibers, microstructure 2, FIG. 5, which did not significantly
influence either basis weight or MFP.
[0054] Microstructure 2 may be desirable in some applications for
improved media strength. Another advantage of microstructure 2 is
the ability to increase surface area within the structure without
dramatic increase in flow restriction or reduction in average pore
size. For example, adding finely divided powders to wet laid media
can increase surface area but it is very difficult to retain these
materials in the forming process. Nanofiber materials, on the other
hand, are easily retained because they entangle with the larger
fibers. The nanofibers still add substantial surface area just as a
fine nanoscale powder or nanoparticle would. Further, beneficially,
the nanoscale fibers may have chemical or catalytic properties,
e.g. can be composed of material including catalytic materials,
reactive materials, and adsorptive materials.
[0055] In comparing media I and media J, it is noted that they are
identical in terms of primary cellulosic components, but differ in
the use of secondary fibers. Media J uses only secondary
microfibers, e.g. diameter 1,400-3,300 nm. Media I uses largely
secondary microfibers, with some nanofibers, e.g. most of the
diameter range of 500-4,000 nm is secondary microfibers (greater
than 1 .mu.m) and a smaller portion of the range includes
nanofibers (less than 1 .mu.m). Concordantly, media I has a smaller
MFP of 13.4 .mu.m than media J having an MFP of 16.1 .mu.m.
[0056] In comparing media G and media H, it is noted that they are
identical in terms of primary cellulosic components, but media G
uses glass secondary nanofibers, while media H uses largely
secondary microfibers, with some nanofibers. Media G exhibits a
substantially smaller MFP than media H.
[0057] FIG. 19 graphically shows the relationship between mean flow
pore size (MFP) in microns and media thickness in millimeters for
the media samples in Table 1. In general, the results follow those
for basis weight in that the incorporation of nanofibers allows
smaller MFP to be obtained for the small media thickness. This is
an advantage in filtration because thinner media allows for the use
of a greater pleat density, which in turn increases dust holding
capacity in filter elements and reduces the fluid face velocity
through the filter.
[0058] The invention provides desirable methods for manufacturing
high performance filter media incorporating and processing
nanofibers of diameter less than 1 .mu.m into internal structure of
filter media dominantly composed of coarse fibers of diameter
greater than 1 .mu.m. The methods may use an electrospinning
process, as above noted, to directly provide the nanofibers, or may
use a bi-component processing technology, such as IITS or SP as
noted above, to provide the nanofibers through an intermediate step
with precursor bi-component fibers using a carrier, e.g. a sea
polymer carrier.
[0059] In one embodiment, the filter media is produced using coarse
fibers and bi-component fibers. The bi-component fibers are
produced by the IITS process using a water soluble sea polymer and
a water insoluble island polymer. Water is used as the carrier to
disperse and suspend the bi-component fibers and the coarse fibers
36 to provide wet media, and as the solvent for the sea polymer to
dissolve the sea polymer upon heating the wet media. The sea
polymer is the carrier for the island polymer, which later provides
the nanofibers, as is known. The water is the carrier for the
bi-component fibers and the coarse fibers 36, as well as the
solvent for the sea polymer. For example, a water soluble polymer,
such as polyvinyl alcohol or polyethylene oxide may be used as the
carrier or sea polymer, while a water insoluble island polymer,
such as polyester or nylon, may be used as the island or nanofiber
polymer. The heating step is performed as a separate hot rinsing
step. Alternatively, the heating step is performed by applying heat
during the drying. Further alternatively, hot water is applied to
the media, and the hot water is removed by vacuuming or draining,
and then applying heat to dry the media, and using such applied
heat as the heating step. The heating step is performed by
increasing the temperature of the water and/or the media to
dissolve the sea polymer, leaving nanofibers behind and retained in
the filter media.
[0060] In another embodiment, the filter media is produced with the
IITS process, and the sea polymer carrier is dissolved with a
phenolic resin solvent. In a further embodiment, the sea polymer
carrier is dissolved with a water-based resin, preferably an
acrylic and/or water-based phenolic resin. Heat may be applied to
cure the resin, and such heat may be used to facilitate dissolution
of the sea polymer carrier.
[0061] In desirable manufacturing implementations, the filter media
is produced with a bi-component process having a carrier and
initially providing precursor bi-component fibers which are reduced
to nanofibers upon removal of the carrier, wherein the bi-component
fibers are added to the coarse fibers prior to removal of the
carrier. The carrier is dissolved with a solvent, and preferably
the solvent is heated. In the case of macrostructure C, the
dispersed bi-component fibers are applied across the face of the
media without the need for electrospinning.
[0062] In a further embodiment, a trimodal distribution of fiber
diameter may be provided, including a first set of fibers 32, FIG.
4, in the diameter range 50 to 500 nm, a second set of fibers as
shown in dashed line at 98 in the diameter range 1 to 5 .mu.m, and
a third set of fibers 36 in the diameter range 10 to 50 .mu.m. The
first set of fibers is supported by the second set of fibers, and
the second set of fibers is supported by the third set of fibers.
The first set of fibers provides the nanofibers. The second and
third sets of fibers provide the coarse fibers. In one particular
embodiment, the first set of fibers form bridges across pores
between the second set of fibers without substantial collapse onto
the second set of fibers, and the first set of fibers are provided
by acrylic nanofibers, the second set of fibers are provided by a
fibrillated para-aramid polymer, and the third set of fibers are a
cellulose matrix.
[0063] In a further embodiment, the noted manufacturing methods
enable production of filter media with glass as well as flexible
non-glass nanofibers, in contrast to prior filter media. Table 1 in
FIG. 8 shows in the right-most column a commercial fuel filter
grade cellulose filter media (CF) known in the prior art and having
rigid glass nanofibers. FIG. 20 is a microphotograph at
1,000.times. magnification of the commercial fuel grade cellulose
media (CF) known in the prior art and includes a scale line showing
the noted length dimension. The structure of the fibers in FIG. 20
is a hybrid between microstructures 1 and 2, in that some of the
secondary fibers bridge larger pores while others have collapsed.
The CF media uses a phenolic resin binder, and, in addition to the
primary coarse cellulosic components, it contains a mixture of
nanofibers and microfibers. The macrostructure of media CF is
similar to macrostructure A and relies upon stiff glass fibers to
achieve such macrostructure.
[0064] The present system can use glass nanofibers and can
additionally or alternatively use flexible polymeric, e.g. acrylic,
nanofibers to achieve macrostructure A. To achieve macrostructure A
with flexible nanofibers, a significant aspect is to create a
stable suspension of the fibers that does not encourage clumping,
aggregation or collapse onto the coarse fibers 36. This may be done
by altering the solvent environment, adjusting the pH of the
suspending medium, or the use of surfactants or other additives to
increase the surface charge on the nanofibers, as above described.
The CF media uses polymeric microfibers to strengthen the media. In
contrast, for example with reference to sample K, and the use of
appropriate nanofiber material, e.g. polyaramid, smaller amounts of
nanofiber can instead be used. In the various embodiments of the
present invention, bi-component nanofiber technology, e.g. IITS or
SP, may be used to produce coarse bi-component fibers that can be
mixed in with the coarse bulk fibers 36, and then removal of the
sea polymer carrier to produce a high surface area filter media
with incorporated nanofibers. For example, a bi-component fiber
consisting of a water soluble sea or carrier polymer, such as
polyvinyl alcohol or polyethylene oxide, and a water insoluble
island or nanofiber polymer, such as polyester or nylon, may be
added to a suspension of water insoluble coarse fiber, such as
polyester, acrylic, cellulose. The present methodology is
particularly useful for producing macrostructures B and C, and
enables several alternative methods for producing the filter media
containing nanofibers. In one method, when a water soluble sea
polymer carrier is used, and water is used as the carrier to
disperse/suspend the fibers during the production of the filter
media, and dissolution of the sea polymer can be accomplished by
heating the wet media. This may be done as a separate hot rinsing
step or using the heat applied to dry the media while vacuuming or
draining off the hot water. In either case, the temperature of the
water and/or media is increased and the sea polymer carrier
dissolves, leaving nanofibers behind that are retained within the
media matrix. In another method, when a solvent-based phenolic
resin system is used to hold the fibers together, the solvent for
the resin system can be used to dissolve the sea polymer carrier,
eliminating the need for an additional processing step or
additional chemicals. In another method, when a water-based resin
system is used, such as acrylic and/or water-based phenolic, to
hold the fibers together, the water for the resin system can be
used to dissolve the sea polymer carrier, eliminating the need for
an additional processing step. Heat applied to cure the resin
system facilitates dissolution of the sea polymer. In order to more
uniformly distribute the nanofibers, the bi-component fibers can be
added to the coarse fibers during or before pulping operations and
the carrier water used to dissolve the water-soluble sea polymer.
Heat may be applied to facilitate this. In a desirable aspect in
production of macrostructure C, the invention enables the use of a
dual or multiple head box hopper, as known in the prior art, to
apply dispersed bi-component fibers. The sea polymer carrier can
then be removed using one of the above methods. This allows the
production of nanofiber filter media without having to create and
apply the nanofibers by electrospinning them onto a substrate.
[0065] It is recognized that various equivalents, alternatives and
modifications are possible within the scope of the appended
claims.
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