U.S. patent application number 15/209808 was filed with the patent office on 2017-04-06 for electrospinning of ptfe with high viscosity materials.
The applicant listed for this patent is Zeus Industrial Products, Inc.. Invention is credited to Bruce L. Anneaux, Robert L. Ballard, David P. Garner.
Application Number | 20170096755 15/209808 |
Document ID | / |
Family ID | 42340323 |
Filed Date | 2017-04-06 |
United States Patent
Application |
20170096755 |
Kind Code |
A1 |
Anneaux; Bruce L. ; et
al. |
April 6, 2017 |
ELECTROSPINNING OF PTFE WITH HIGH VISCOSITY MATERIALS
Abstract
An improved process for forming a PTFE mat is described. The
process includes providing a dispersion with PTFE, a fiberizing
polymer and a solvent wherein said dispersion has a viscosity of at
least 50,000 cP. An apparatus is provided which comprises a charge
source and a target a distance from the charge source. A voltage
source is provided which creates a first charge at the charge
source and an opposing charge at the target. The dispersion is
electrostatically charged by contact with the charge source. The
electrostatically charged dispersion is collected on the target to
form a mat precursor which is heated to remove the solvent and the
fiberizing polymer thereby forming the PTFE mat.
Inventors: |
Anneaux; Bruce L.;
(Lexington, SC) ; Ballard; Robert L.; (Orangeburg,
SC) ; Garner; David P.; (Lexington, SC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zeus Industrial Products, Inc. |
Orangeburg |
SC |
US |
|
|
Family ID: |
42340323 |
Appl. No.: |
15/209808 |
Filed: |
July 14, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14918877 |
Oct 21, 2015 |
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15209808 |
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13446300 |
Apr 13, 2012 |
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14918877 |
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12689334 |
Jan 19, 2010 |
8178030 |
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13446300 |
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61145309 |
Jan 16, 2009 |
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61256349 |
Oct 30, 2009 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B29K 2995/0077 20130101;
D01D 10/02 20130101; Y10T 442/656 20150401; B32B 5/022 20130101;
B32B 9/005 20130101; C08J 3/05 20130101; B29B 13/023 20130101; C08J
2471/02 20130101; B29K 2105/0094 20130101; Y10T 428/1314 20150115;
D04H 1/728 20130101; Y10T 428/1362 20150115; B29K 2027/18 20130101;
D01F 6/12 20130101; D04H 1/4382 20130101; D01D 5/0007 20130101;
B29L 2031/755 20130101; D06N 7/00 20130101; D04H 3/02 20130101;
Y10T 442/60 20150401; B29B 11/06 20130101; B32B 15/085 20130101;
D04H 1/413 20130101; D10B 2321/042 20130101; B32B 1/08 20130101;
D04H 1/4326 20130101; B05D 1/007 20130101; D01D 5/0038 20130101;
B29K 2995/0063 20130101; Y10T 442/681 20150401; Y10T 442/674
20150401; D04H 1/42 20130101; D01F 6/48 20130101; D04H 1/4318
20130101; C08J 3/005 20130101; D04H 1/74 20130101; C08J 2327/18
20130101; D01D 5/003 20130101; B32B 27/08 20130101; Y10T 428/1355
20150115; C08J 9/232 20130101; D04H 1/54 20130101; C08F 114/26
20130101; B29C 55/04 20130101 |
International
Class: |
D04H 1/728 20060101
D04H001/728; D01D 10/02 20060101 D01D010/02; D04H 1/413 20060101
D04H001/413; D04H 1/42 20060101 D04H001/42; C08J 3/00 20060101
C08J003/00; D04H 1/4326 20060101 D04H001/4326; D04H 1/4382 20060101
D04H001/4382; D04H 1/54 20060101 D04H001/54; D04H 1/74 20060101
D04H001/74; D04H 3/02 20060101 D04H003/02; D01D 5/00 20060101
D01D005/00; D04H 1/4318 20060101 D04H001/4318 |
Claims
1. A polymeric material comprising: a membrane comprised of
deposited polymeric fibers, the polymeric fibers having been
expanded in a first direction after the fibers are deposited.
2. The polymeric material of claim 1, wherein the deposited fibers
comprise sintered fibers.
3. The polymeric material of claim 1, wherein the deposited fibers
are exposed to an elevated temperature.
4. The polymeric material of claim 1, wherein the deposited fibers
are generally aligned in the first direction.
5. The polymeric material of claim 1, wherein the membrane is more
resistant to creep in the first direction after the membrane is
expanded.
6. The polymeric material of claim 1, wherein the deposited
polymeric fibers have been expanded in a second direction.
7. The polymeric material of claim 1, wherein the deposited
polymeric fibers comprise serially deposited polymeric fibers.
8. A deposited fiber mat comprising a portion of a device, the
deposited fiber mat comprising deposited fibers, wherein all of the
fibers have diameters of 400 nm to 3200 nm.
9. The deposited fiber mat of claim 8, wherein the deposited fiber
mat comprises spun polytetrafluoroethylene fibers.
10. The deposited fiber mat of claim 8, wherein all of the fibers
have diameters from 800 nm to 2.4 .mu.m.
11. The deposited fiber mat of claim 8, wherein the deposited
fibers have an average fiber density from 135 to 326,400 fibers per
mm.sup.2.
12. The deposited fiber mat of claim 8, wherein the largest pore
diameter is from 2.54 .mu.m to 9.96 .mu.m.
13. The deposited fiber mat of claim 8, wherein the fibers define
between 2 and 2.9.times.10.sup.12 intersections per mm.sup.2.
14. The deposited fiber mat of claim 8, wherein the deposited
fibers comprise serially deposited fibers.
15. A method of manufacturing a polymeric material comprising:
obtaining a membrane comprising a mat of polymeric fibers;
sintering the membrane; and expanding the membrane in a first
direction to at least partially elongate the membrane in the first
direction.
16. The method of claim 15, wherein obtaining the membrane
comprises: depositing polymeric fibers on a collection surface to
form a membrane.
17. The method of claim 15, wherein the membrane is sintered at a
temperature of 288.degree. C. to 482.degree. C.
18. The method of claim 15, wherein sintering the membrane
comprises heating the membrane to at least the crystalline melt
temperature of the polymeric fibers.
19. The method of claim 15, wherein expanding the membrane
comprises expanding the membrane such that the polymeric fibers
tend to align in the first direction.
20. The method of claim 15, wherein expanding the membrane
comprises expanding the membrane on the order of 103 to 200% of its
original length in the first direction.
21. The method of claim 15, further comprising expanding the
membrane in a second direction.
22. The method of claim 15, further comprising constraining the
membrane, wherein the sintering is conducted while the membrane is
constrained in place on a surface on which it was collected.
23. The method of claim 15, wherein obtaining the membrane
comprises: mixing a PTFE-water dispersion comprising from 50% to
80% PTFE solids by weight with PEO to create a mixture having 0.032
to 0.052 gm PEO/mL of total mixture; spinning the mixture;
collecting spun fibers on a cylindrical surface; and wherein the
sintering is at a temperature of 288.degree. C. to 482.degree.
C.
24. The method of claim 15, wherein expanding the membrane in a
first direction comprises expanding the membrane in a first
direction after the fibers are deposited.
25. The method of claim 15, wherein the membrane is sintered in
place on a surface on which it was collected.
26. The method of claim 15, wherein obtaining the membrane
comprises serially depositing polymeric fibers on a collection
surface to form a membrane.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a continuation application of
U.S. application Ser. No. 14/918,877, filed Oct. 21, 2015; which
application is a continuation application of U.S. application Ser.
No. 13/446,300, filed Apr. 13, 2012; which application is a
continuation of U.S. application Ser. No. 12/689334, filed Jan. 19,
2010; now U.S. Pat. No. 8,178,030, Issued May 15, 2012; which
application claims priority to pending U.S. Provisional Patent
Appl. No. 61/145,309, filed Jan. 16, 2009, and to pending U.S.
Provisional Patent Appl. No. 61/256,349, filed Oct. 30, 2009; and
all of the foregoing are incorporated herein in their entirety by
this reference.
BACKGROUND
[0002] The present invention is specific to a process of
electrospinning polytetrafluoroethylene (PTFE). More particularly,
the present invention is related to electrospinning high viscosity
PTFE dispersions and products manufactured thereby.
[0003] The process of electrostatic spinning is well known in the
art as represented in U.S. Pat. Nos. 2,158,416; 4,043,331;
4,143,196; 4,287,139; 4,432,916; 4,689,186; 6,641,773 and 6,743;273
each of which is incorporated herein by reference thereto. U.S.
Pat. Nos. 4,323,525, 4,127,706 and 4,044,404, all of which are
incorporated herein by reference, provide information related to
processing and electrostatic spinning of PTFE from an aqueous or
other dispersion.
[0004] Electrostatic spinning, also referred to in the art as
electrospinning, involves a charged polymer moving towards a
charged surface. In one embodiment the polymer is discharged
through a small charged orifice, such as a needle, towards a target
wherein the needle and target have opposing electrical charge. As
would be realized, the nature of the polymer is critical. It has
long been considered necessary in the art to maintain a relatively
low viscosity of less than about 150 poise with viscosity being
relatively higher for lower molecular weight polymers and
relatively lower for higher molecular weight polymers. If the
combination of viscosity and molecular weight were too high the
fiberization was considered to be inadequate.
[0005] It has long been considered undesirable to increase the
viscosity of the polymer solution over about 150 poise due to
thixotropic limitations which cause orifice clogging, poor fiber
formation, and the like. Furthermore, when a charged orifice is
used the polymer fibers repel during flight which has long been
believed to limit the number of fibers within a given volume of
spray. Through diligent research the present inventors have
determined, contrary to prior understandings, that a significant
increase in viscosity to well above that previously considered
feasible, actually improves the resulting material and provides
additional properties and advantages not previously considered
possible.
SUMMARY
[0006] It is an object of the invention to provide an improved
process for electrospinning PTFE.
[0007] It is another object of the invention to provide a method to
provide superior products based on electrospun PTFE.
[0008] A particular feature of the present invention is the ability
to utilize existing electrospinning techniques, and facilities,
while providing an improved product.
[0009] These and other advantages, as will be realized, are
provided in a process for forming a PTFE mat. The process includes
providing a dispersion with PTFE, a fiberizing polymer and a
solvent wherein said dispersion has a viscosity of at least 50,000
cP. An apparatus is provided which comprises a charge source and a
target a distance from the charge source. A voltage source is
provided which creates a first charge at the charge source and an
opposing charge at the target. The dispersion is electrostatically
charged by contact with the charge source. The electrostatically
charged dispersion is collected on the target to form a mat
precursor which is heated to remove the solvent and the fiberizing
polymer thereby forming the PTFE mat.
[0010] Yet another advantage is provided in a process for forming a
PTFE mat. The process includes providing a dispersion comprising
PTFE with a particle size of at least 0.1 microns to no more than
0.8 microns; 1 wt % to no more than 10 wt % of polyethylene oxide
with a molecular weight of at least 50,000 to no more than
4,000,000; and a solvent wherein said dispersion has a viscosity of
at least 50,000 cP. An apparatus is provided comprising an orifice
and a target a distance from the orifice. A voltage source is
provided to create a first charge at the orifice and an opposing
charge at the target. The dispersion is forced through the orifice
wherein the dispersion is electrostatically charged by contact with
the orifice. Electrostatically charged dispersion is collected on
the target to form a mat precursor which is heated to remove the
solvent and the fiberizing polymer thereby forming the PTFE
mat.
BRIEF DESCRIPTION OF THE D WINGS
[0011] FIG. 1 schematically illustrates electrodeposition.
[0012] FIG. 2 schematically illustrates the inventive process.
DETAILED DESCRIPTION
[0013] The present invention is directed to a process for the
electrostatic spinning of polytetrafluoroethylene (PTFE) into
continuous fibers for the formation of non-woven sheets, membranes,
tubes, and coatings with potential for multiple other applications
and forms. In particular, the present invention is directed to
electrospinning PTFE at a very high viscosity relative to the prior
art in direct contrast to that which was previously considered
feasible.
[0014] An electrostatic spinning apparatus is illustrated
schematically in FIG. 1. In FIG. 1 a reservoir, 10, is loaded with
a high viscosity dispersion as further described herein. A delivery
system, 11, delivers the dispersion from the reservoir to a charge
source, 12, which may be an orifice. A target, 15, is set some
distance from the charge source, 12. A power source, 16, such as a
DC power supply establishes an electrical charge differential
between the charge source and target such that polymeric material,
14, is electrically charged opposite the target. The polymeric
material is electrostatically attracted to the target and is
deposited thereon. The target may be static, in motion or it may be
a continuous, or near continuous, material which moves through the
zone of polymer impact, such as by movement on transport rollers,
17, or the like. In one embodiment the electrical charge, or ground
as illustrated, is applied to the roller which is in electrically
conductive contact with the target. The target may be a continuous
loop or it may initiate on a delivery device, such as a supply
spool and be taken up by a collector, such as a receiver spool. In
an alternative embodiment the charge source and target may be in a
common dispersion bath.
[0015] The instant process requires a dispersion or suspension of a
sufficient percentage of PTFE solids to aid in the post processing
of the collected fibrous mat into a form that has some mechanical
integrity. If the PTFE solid content in the dispersion is too low,
there will be no, or poor, mechanical integrity to the resulting
material. Second, the selection of the polymer used to increase the
viscosity of the solution, suspension or dispersion, also referred
to as a fiberization polymer, to be spun must be selected
carefully. We have found that too low of a molecular weight
fiberization polymer added to the PTFE will cause poor performance
and poor handling characteristics. It is also believed that too
high of a molecular weight will cause an increase in the viscosity
without enough of the polymer being present to actually bind the
PTFE powder together during the electrospinning and curing process.
Additionally, the process used to sinter the PTFE powder together
must be finely controlled such that the resulting product has good
mechanical integrity.
[0016] It is preferred that the PTFE have a molecular weight of
10.sup.6 to 10.sup.8.
[0017] It is preferred that the PTFE have a particle size of at
least 0.1 microns to no more than 0.8 microns. More preferably, the
PTFE has a particle size of at least 0.2 microns to no more than
0.6 microns. Below a particle size of 0.1 microns the materials
create manufacturing difficulties. Above a particle size of 0.8
microns the particle size approaches the target fiber diameter and
becomes a defect in the fiber. For other applications larger sizes
may be suitable for use.
[0018] The process for producing a non-woven PTFE material will be
described with reference to FIG. 2. An aqueous dispersion of a
narrow particle size distribution PTFE powder is prepared, 20. A
fiberizing polymer is added, 22, to the dispersion. Preferably, the
fiberizing polymer is added in an amount of between 1 and 10 wt %,
more preferably about 2 to 7 wt % with about 4 -5 wt % being most
preferred. The fiberizing polymer preferably has a high solubility
in the solvent, which is preferably water, with a solubility of
greater than about 0.5 wt % being preferred. It is preferable that
the fiberizing polymer has an ash content of less than about 5 wt
%, when sintered at about 400.degree. C., with even lower being
more preferred.
[0019] Particularly preferred fiberization polymers include
dextran, alginates, chitosan, guar gum compounds, starch,
polyvinylpyridine compounds, cellulosic compounds, cellulose ether,
hydrolyzed polyacrylamides, polyacrylates, polycarboxylates,
polyvinyl alcohol, polyethylene oxide, polyethylene glycol,
polyethylene imine, polyvinylpyrrolidone, polyacrylic acid,
poly(methacrylic acid), poly(itaconic acid), poly(2-hydroxyethyl
acrylate), poly(2-(dimethylamino)ethyl methacrylate-co-acrylamide),
poly(N-isopropylacrylamide),
poly(2-acrylamide-2-methyl-1-propanesulfonic acid),
poly(methoxyethylene), poly(vinyl alcohol), poly(vinyl alcohol) 12%
acetyl, poly(2,4-dimethyl-6-triazinylethylene),
poly(3-morpholinylethylene), poly(N-1,2,4-triazolyethylene),
poly(vinyl sulfoxide), poly(vinyl amine), poly(N-vinyl
pyrrolidone-co-vinyl acetate), poly(g-glutamic acid),
poly(N-propanoyliminoethylene), poly(4-amino-sulfo-aniline),
poly[N-(p-sulphophenyl)amino-3-hydroxymethyl-1,4-phenyleneimino-1,4-pheny-
lene)], isopropyl cellulose, hydroxyethyl, hydroxylpropyl
cellulose, cellulose acetate, cellulose nitrate, alginic ammonium
salts, i-carrageenan, N-[(3'-hydroxy-2',3'-dicarboxy) ethyl]
chitosan, konjac glocomannan, pullulan, xanthan gum,
poly(allyammonium chloride), poly(allyammonium phosphate),
poly(diallydimethylammonium chloride), poly(benzyltrimethylammonium
chloride), poly(dimethyldodecyl(2-acrylamidoethyly) ammonium
bromide), poly(4-N-butylpyridiniumethylene iodine),
poly(2-N-methylpridiniummethylene iodine), poly(N
methylpryidinium-2,5-diylethenylene), polyethylene glycol polymers
and copolymers, cellulose ethyl ether, cellulose ethyl hydroxyethyl
ether, cellulose methyl hydroxyethyl ether, poly(1-glycerol
methacrylate), poly(2-ethyl-2-oxazoline), poly(-hydroxyethyl
methacrylate/methacrylic acid) 90:10, poly(2-hydroxypropyl
methacrylate), poly(2-methacryloxyethyltrimethylammonium bromide),
poly(2-vinyl-1-methylpyridinium bromide), poly(2-vinylpyridine
N-oxide), poly(2-vinylpyridine),
poly(3-chloro-2-hydroxypropyl-2-methacryloxyethyldimethylammonium
chloride), poly(4-vinylpyridine N-oxide), poly(4-vinylpyridine),
poly(acrylamide/2-methacryloxyethyltrimethylammonium bromide)
80:20, poly(acrylamide/acrylic acid), poly(allylamine
hydrochloride), poly(butadiene/maleic acid),
poly(diallyldimethylammonium chloride), poly(ethyl acrylate/acrylic
acid), poly(ethylene glycol) bis (2-aminoethyl), poly(ethylene
glycol) monomethyl ether, poly(ethylene glycol)-bisphenol A
diglycidyl ether adduct, poly(ethylene oxide-b-propylene oxide),
poly(ethylene/acrylic acid) 92:8, poly(1-lysine hydrobromide),
poly(1-lysine hydrobromide), poly(maleic acid), poly(n-butyl
acrylate/2-methacryloxyethyltrimethylammonium bromide),
poly(N-iso-propylacrylamide),
poly(N-vinylpyrrolidone/2-dimethylaminoethyl methacrylate),
dimethyl sulfatequaternary, poly(N-vinylpyrrolidone/vinyl acetate),
poly(oxyethylene) sorbitan monolaurate (Tween 20.RTM.),
poly(styrenesulfonic acid), poly(vinyl alcohol),
N-methyl-4(4'-formylstyryl)pyridinium, methosulfate acetal,
poly(vinyl methyl ether), poly(vinylamine) hydrochloride,
poly(vinylphosphonic acid), poly(vinylsulfonic acid) sodium salt
and polyaniline.
[0020] A particularly preferred fiberizing polymer is
polyethyleneoxide (PEO) with a molecular weight of 50,000-4,000,000
and more preferably a molecular weight of about 250,000 to 350,000
and most preferably a molecular weight of about 300,000.
[0021] With further reference to FIG. 2, after mixing the PTFE and
fiberizing polymer dispersion is preferably allowed to homogenize,
24. In a particularly preferred method the polymer dispersion is
allowed to form slowly, without agitation, followed by transfer to
a jar roller that will turn it at a constant rate for several more
days. It is preferred to create a uniform solution that has little
to no air trapped in the resulting highly viscous mixture. Once the
dispersion is of uniform consistency it is preferably filtered to
remove any clumps or gels. The filtered dispersion with the desired
viscosity is then loaded, 26, in a controlled pumping device with a
fixed conductive element which acts as the charge source. A
particularly preferred conductive element is an orifice such as a
16 gauge needle that has been cut blunt and sanded to remove any
burs. The ejection volume from the pumping device is set to a
predetermined rate that is dependent on the form being made and the
desired fiber diameters. The charge source is preferably connected
to the positive side of a precision DC power supply. The negative
side of the power supply is preferably connected to the collection
surface or target. The polarity can be reversed but this is not
preferred.
[0022] The target surface can be a drum, device or sheet. The
surface can be a metal, ceramic or polymeric material with
particularly preferred materials selected from stainless steel,
cobalt chrome, nickel titanium (nitinol), magnesium alloys
polyactides, polyglycolides, polyhydroxyl butyrates,
polyhydroxyalkynoates, polydioxinine, polyetheretherketone (PEEK),
polyurethanes, polycarbonates and polyethyleneoxide. The voltage on
the power supply is increased to the desired voltage to uniformly
draw out the polymer/PTFE solution.
[0023] The applied voltage is typically from 2,000-80,000 volts.
The charge induced by the connection of the power supply repels the
charged polymer away from the charge source and attracts them to
the collection surface.
[0024] The collection target is preferably placed perpendicular to
the pump and orifice system and is moved in at least one direction
such that the entire surface is uniformly covered, 28, with the
fibers drawn towards the target. Once the collection surface has
been adequately covered the material is preferably cured/sintered,
30, either in place, by placing the entire collection surface in an
oven, or by removing the sheet tube or other form from the
collection surface and sintering it in an oven.
[0025] Electrospun PTFE fabrics undergo shrinkage upon sintering.
While not limited to any theory the shrinkage is believe to occur
in two steps. Initially, the fibers and fabrics, as spun, contain
both water and a fiberizing polymer, preferably polyethyleneoxide.
Upon completion of spinning the samples dry and undergo a small
degree of fiber rearrangement. The samples are sintered by exposing
the fibers and fabrics to temperatures of 550.degree. to
900.degree. F. for a period of time such that the water and
fiberizing polymer are evaporated. The evaporation is hypothesized
to generate a second, more significant, shrinkage. Cracking of the
fabric and breaking of the fibers is believed to occur during this
second shrinkage when the fabric has not been allowed to relax.
[0026] To accommodate for shrinkage, the fiber and fabrics can be
spun onto an expanded structure. The structure can then be removed
or contracted. This allows the fabric to shrink during sintering
without cracking. Another method involves spinning the fibers and
fabrics onto a structure which can then be expanded or contracted
prior to sintering. The range of contraction or expansion and
contraction is preferably on the order of 3 to 100% and depends
upon the thickness and size of the electrodeposited fabric mat.
[0027] For a sheet of fabric, if the direction of the deposition is
given as the perpendicular to the plane of the fabric then
contraction or expansion/contraction must occur in at least one or
more of the directions in the plane of the fabric. For a fabric
deposited upon a cylindrical surface the fabric must be contracted
or contracted/expanded radially and/or longitudinally. For a
spherical surface the fabric must be contracted or
contracted/expanded radially. These basic concepts of contraction
and/or expansion/contraction can be applied to any electrospun
fabric independent of the shape of the surface upon which it was
spun. Thus, very complex fabric shapes based upon PTFE fabric
become possible.
[0028] In a particularly preferred embodiment a high viscosity
material is used. It is surprising that superior properties are
observed by electrospinning a material with a viscosity of at least
50,000 cP to no more than 300,000 cP. More preferably the viscosity
is at least 100,000 cP to no more than 250,000 and most preferably
the viscosity is at least 150,000 cP no more than 200,000 cP. Above
a viscosity of 300,000 cP it becomes increasingly difficult to
generate a fiber.
[0029] In one embodiment electrospinning dispersions are based upon
Daikin D 210 PTFE and Sigma Aldrich polyethylene oxide with a
molecular weight of 300,000. Daikin D 210 PTFE is representative of
a material suitable for demonstrating the invention. Daikin D 210
PTFE has about 59-62 wt % solids, 6.0-7.2% wt % surfactant, a pH of
8.5 to 10.5, a specific gravity of 1.5 to 1.53 and a Brookfield
viscosity maximum of 35 cP.
[0030] The dispersion has a preferred PTFE percent solids, by
weight, of 50%-80%, more preferably 55-65 wt %, and even more
preferably 59-61 wt %. The specific gravity is preferably 1.5 to
1.54 and more preferably 1.51. By way of example, a 1000 ml
dispersion would have a weight range of 1500 gm to 1530 gm with 885
gm to 933.3 gm of PTFE.
[0031] A particularly preferred embodiment has 60% PTFE solids, a
specific gravity of 1.51, with 909 gm of PTFE per 1000 ml of
dispersion.
[0032] A particularly preferred example is prepared with from 32 gm
to 52 gm fiberizing polymer, most preferably PEO, per 1000 ml of
the Daikin D 210 dispersion which provides a ratio of fiberizing
polymer solids to PTFE dispersion (such as PEO/PTFE) of from 0.032
to 0.052 gm/ml PEO in the 1000 ml of dispersion. Fiberizing polymer
ratios, below 0.03 gm/ml by weight, results in very poor quality
and non-uniform fiber mat formation. Poor quality is defined as the
existence of high levels of fiber breakage, >20%, and the
formation of non-uniform fiber diameters which are also referred to
in the art as "beading". The existence of broken fibers and/or
non-uniform fibers results in non-uniform porosity within the
fibrous mat. The presence of broken fibrils, especially short
fibrils, leads to decreased efficiency over time as the
non-continuous fibrils are pulled from the mat.
[0033] A PTFE dispersion of 60% PTFE solids and PEOs ranging
between 200,000 and 4,000,000 Mw is representative. Fiberizing
polymer to PTFE ranges of 0.03 to 0.06 for fiberizing polymer being
PEO with a molecular weight of 300,000 is particularly
representative.
[0034] Viscosities for different formulations of PEO/PTFE at a
constant spindle speed setting of 2.5 for a #25 spindle at
25.degree. C. taken in a Brookfield LV Viscometer are provided in
Table 1.
TABLE-US-00001 TABLE 1 Sample Torque (%) Viscosity (cp) 0.052 gm/ml
PEO 88.5 171,000 0.048 gm/ml PEO 76.8 147,000 0.044 gm/ml PEO 79.2
152,000 0.040 gm/ml PEO 58.5 112,000 0.036 gm/ml PEO 40.1 77,000
0.032 gm/ml PEO 34.5 66,000
[0035] Assuming 909 gm PTFE in 1000 ml of the Diakin D 210
dispersion the preferred percent PEO/PTFE dispersion range is from
0.032 to 0.060 gm/ml. About 0.048 gm/ml is particularly preferred
for demonstrating the invention.
[0036] For deposition a charged needle and a grounded target is
preferred and relied on herein unless otherwise specified. This has
been accepted as a standard practice within the industry partially
for safety reasons. A grounded needle and a charged target can be
used but this is not preferred since the result is inferior
material.
[0037] Voltages of 11, 14, or 17 kV with corresponding distances
from tip to top of target (TTT) of 4.5'', 5.5'', and 6.5'' were
relied on for convenience for the samples set forth herein.
Voltages and distances are design choices based on experimental
apparatus employed the determination of which is well known to
those of skill in the art. For the purposes of demonstration,
samples were deposited onto foil, dried and then sintered at
725.degree. F. for five minutes then examined by SEM. The process
produced a smooth, heavy, wide web indicating a significant
improvement in material transfer efficiency. Test results using
reverse polarity were inferior. A mat was deposited that was about
5'' wide.
[0038] Visual observation showed various levels of degradation in
samples produced by reverse polarity. In addition, the
photomicrographs showed breakage of fibers as well as a twisting of
fibers to create fiber bundles. There was also a wide distribution
of fiber and fiber bundle diameters. All of these fiber
characteristics will result in an inconsistent and poor quality
fiber mat. These fiber characteristics are consistent with poor
fiber mat quality observed with our attempts to espin from low
PEO/PTFE concentration dispersions. High voltages and tip-to-target
distances 5.5 and 6.5'', showed the most fiber breakage whereas the
4.5'' TTT distance showed the most bundling.
[0039] When "normal" e-spun PTFE was examined under high
magnification there was no apparent fiber breakage, all fibers were
of uniform diameter and fiber mats survived the sintering
process.
[0040] Representative results are provided in Tables 2-4 with
standard deviations reported in parenthesis. In the Tables the air
flow, pore diameter, and bubble point were measured using a Porous
Materials, Inc. Capillary Flow Porometer Model CFP-1100-AEXL using
test type "Dry Up/Wet Up". Density was measured by a gas pycnometer
using ISO 1183-3. Tensile, elongation, and modulus were measured
using ASTM D882 with diecut ASTM D638 Type V dogbone sample
geometry. Viscosities for different formulations of PEO/PTFE were
done at the constant spindle speed setting listed for a #25 spindle
at 25.degree. C. taken in a Brookfield LV Viscometer.
[0041] A particular advantage offered by the present process is
that the resulting material has significantly fewer broken fibrils
upon sintering than a sample of the prior art. A decrease in fibril
breakage increases manufacturing productivity due to a decrease in
the material which is inferior. This is achieved without a loss of
product characteristics.
[0042] The invention has been described with reference to the
preferred embodiments without limit thereto. One of skill in the
art would realize additional embodiments and improvements which are
within the meets and bounds of the invention which are more
specifically set forth in the claims appended hereto.
TABLE-US-00002 TABLE 2 Physical Properties of PTFE Membranes 60%
PTFE, 0.048 gm/ml PEO/1000 ml PTFE dispersion. Examples 1 2 3 4 5
Viscosity (cP) 147,000 147,000 147,000 147,000 147,000 Thickness
(mil) 0.2 (0.08) 0.5 (0.05) 1.1 (0.24) 1.02 3.33 Spindle setting
2.5 2.5 2.5 2.5 2.5 Density (g/cm3) 0.3549 0.3227 0.4342 0.4239
Basis Weight 2.9605 8.0807 10.773 33.7 35.888 (g/cm2) BP: Pore
diameter 9.17 (0.787) 3.18 (0.157) 3.01 (0.123) 3.41 (0.235) 2.63
(0.088) (microns) Bubble point: 0.72 (0.062) 2.08 (0.102) 2.2
(0.089) 1.94 (0.138) 2.52 (0.082) Pressure (psi) Air Flow (micron)
1.4217 Density(g/cm3) 2.0871 2.2334 2.2022 2.2255 2.1985 Tensile
Strength 190 (58.1) 339 (15) 348 (86.9) 1450 (290) 208 (19.9) (psi)
Modulus (psi) 1420 (395) 1560 (45.8) 1910 (143) 3650 (1100) 1594
(235) Elongation (%) 110 (23) 190 (13) 140 (28) 255 (38) 130
(14.6)
TABLE-US-00003 TABLE 3 Physical Properties of PTFE Membranes 67%
PTFE, 0.015 to 0.040 gm/ml PEO/1000 ml PTFE dispersion. Examples 6
7 8 9 10 Viscosity(cP) 129,000 249,000 <500,000 Spindle setting
2.5 2.5 2.5 2.0 2.5 PEO/PTFE 0.015 0.02 0.025 0.030 0.040 (g/ml)
Thickness (mil) 2.7 2.2 2.5 1.5 1.0 Density (g/cm3) 0.4949 0.5973
0.5331 0.3854 0.2521 Basis Weight 18.745 12.385 24.371 12.767 4.681
(g/cm2) BP: Pore 4.14 3.95 5.09 4.05 6.7 diameter (microns) Bubble
point: 1.59 1.68 1.30 1.63 0.935 Pressure (psi) Air Flow 1.4806
1.5747 1.9938 1.8346 3.0425 (micron) Density (g/cm3) 2.2139 2.1978
2.2077 2.1904 Tensile Strength 371.37 626.76 321 109 114.47 (psi)
Modulus (psi) 2430 3381.8 1380 2300 1220 Elongation (%) 106.33
114.27 126.77 121.92 68
TABLE-US-00004 TABLE 4 Physical Properties of PTFE Membranes 72%
PTFE, 0.020 to 0.030 gm/ml PEO/1000 ml PTFE dispersion. Examples 11
12 13 PEO/PTFE (g/ml) 0.02 0.025 0.030 Thickness (mil) 1.8 2.3 1.5
Density (g/cm3) 0.3504 0.2972 0.4110 Basis Weight (g/cm2) 13.795
13.320 10.442 BP: Pore diameter (microns) 7.14 9.15 5.7223 Bubble
point: Pressure (psi) 0.92 0.76 1.09 Air Flow (micron) 3.3837
3.1946 2.6542 Density (g/cm3) 2.2236 2.1845 2.2182 Tensile Strength
(psi) 218.57 103.99 335 Modulus (psi) 1610 764 2790 Elongation (%)
86.42 48 110
* * * * *