U.S. patent application number 10/463690 was filed with the patent office on 2003-11-20 for filtering material and device and method of its manufacture.
Invention is credited to Dubson, Alexander.
Application Number | 20030213218 10/463690 |
Document ID | / |
Family ID | 11069572 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030213218 |
Kind Code |
A1 |
Dubson, Alexander |
November 20, 2003 |
Filtering material and device and method of its manufacture
Abstract
A device and method for producing a porous fiber structure. One
or more points of high surface curvature is produced in a liquefied
polymer, such as a polymer solution or a polymer melt. The points
of high surface curvature may be produced by forcing the liquefied
polymer through narrow nozzles, or by wetting sharp protrusions
with the liquefied polymer. The liquefied polymer is charged to a
high negative electrical potential relative to a grounded moving
belt. Thin jets of liquefied polymer emerge from the points of high
surface curvature to impinge as fibers on the moving belt, thereby
forming an unwoven fiber structure of relatively uniform porosity.
A powdered aerosol is charged to a high positive electrical
potential relative to the moving belt. As the belt moves past the
aerosol, the aerosol particles are attracted to fill interstices in
the fiber structure, thereby creating a composite filtering
material.
Inventors: |
Dubson, Alexander; (Migdal
Haemek, IL) |
Correspondence
Address: |
G.E. EHRLICH (1995) LTD.
c/o ANTHONY CASTORINA
SUITE 207
2001 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
11069572 |
Appl. No.: |
10/463690 |
Filed: |
June 18, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10463690 |
Jun 18, 2003 |
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09326559 |
Jun 7, 1999 |
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6604925 |
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09326559 |
Jun 7, 1999 |
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PCT/IL97/00403 |
Dec 9, 1997 |
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Current U.S.
Class: |
55/527 ;
264/465 |
Current CPC
Class: |
B01J 20/28028 20130101;
B01D 39/1623 20130101; B01J 20/26 20130101; B01D 17/10 20130101;
Y10T 442/614 20150401; D01D 5/0069 20130101; B01J 20/28033
20130101; B01D 39/2017 20130101; B01J 20/30 20130101; B01J 20/28004
20130101; B01D 17/0202 20130101; B01D 17/085 20130101; B01J
20/28023 20130101 |
Class at
Publication: |
55/527 ;
264/465 |
International
Class: |
H05B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 11, 1996 |
IL |
119809 |
Claims
What is claimed is:
1. A device for transforming a liquefied polymer into a fiber
structure, comprising: (a) a precipitation electrode; (b) a first
mechanism for charging the liquefied polymer to a first electrical
potential relative to said precipitation electrode; and (c) a
second mechanism for forming a surface on said liquefied polymer of
sufficiently high curvature to cause at least one jet of the
liquefied polymer to be drawn by said first electrical potential to
said precipitation electrode; wherein said first and second
mechanisms are designed such that when a plurality of fibers are
precipitated on said precipitation electrode, a high efficiency
particulate air unwoven fiber structure, capable of filtering out
at least 99.97% of 0.3 .mu.m particulates in air flowing at 5
cm/sec is obtainable.
2. The device of claim 1, wherein said first mechanism for charging
the liquefied polymer to a first electrical potential relative to
said precipitation electrode includes in combination: (i) a source
of high voltage; and (ii) a charge control agent mixed with the
liquefied polymer.
3. The device of claim 2, wherein said first mechanism for charging
the liquefied polymer to a first electrical potential relative to
said precipitation electrode further includes: (iii) a source of
ionized air being in contact with said liquefied polymer.
4. The device of claim 1, wherein said second mechanism is effected
by at least one rotating wheel having a rim formed with a plurality
of protrusions.
5. The device of claim 4, wherein each of said protrusions is
formed with a liquefied polymer collecting cavity.
6. The device of claim 4, wherein each of said at least one wheel
is tilted with respect to said precipitation electrode.
7. The device of claim 4, wherein each of said at least one wheel
includes a dielectric core.
8. The device of claim 1, wherein said second mechanism is effected
by a gas bubbles generating mechanism.
9. The device of claim 1, wherein said second mechanism is effected
by a rotating strap formed with a plurality of protrusions.
10. The device of claim 1, wherein said precipitation electrode is
operative to move past said mechanism for forming said surface of
high curvature.
11. The device of claim 10, wherein said precipitation electrode
includes a belt.
12. The device of claim 1, wherein said mechanism for forming said
surface of high curvature includes at least one nozzle.
13. The device of claim 1, wherein said mechanism for forming said
surface of high curvature includes at least one protrusion made of
a material which is wetted by the liquefied polymer, said at least
one protrusion including a tip whereon said surface of high
curvature is formed.
14. The device of claim 13, wherein said at least one protrusion is
disposed on a rim of a wheel with said tip pointing radially
outward from said wheel.
15. The device of claim 13, further comprising: (d) a bath for
holding the liquefied polymer; wherein said at least one protrusion
is operative to reciprocate within said bath, said jets of the
liquefied polymer being formed at a closest approach of said at
least one protrusion to said precipitation electrode.
16. The device of claim 1, further comprising: (d) an additional
electrode, intermediate between said precipitation electrode and
said mechanism for forming said surface of high curvature.
17. The device of claim 16, wherein said additional electrode
includes a plate having an aperture, opposite said mechanism for
forming said surface of high curvature, where through said at least
one jet of the liquefied polymer emerge towards said precipitation
electrode.
18. The device of claim 1, further comprising: (d) an aerosol
generator operative to supply an aerosol to said precipitation
electrode at a second electrical potential difference from said
precipitation electrode opposite in sign to said first electrical
potential difference.
19. The device of claim 18, wherein said aerosol generator
includes: (i) a pressure chamber; and (ii) a partition between said
pressure chamber and said precipitation electrode; said pressure
chamber and said partition cooperating to fluidize a filler powder
which is drawn by said second electrical potential difference to
said precipitation electrode.
20. The device of claim 18, wherein said aerosol generator includes
a slot sprayer.
21. A method for forming a polymer into a high efficiency
particulate air unwoven fiber structure capable of filtering out
99.97% of 0.3 .mu.m particulates in air flowing at 5 cm/sec,
comprising the steps of: (a) liquefying the polymer, thereby
producing a liquefied polymer; (b) supplementing the liquefied
polymer with a charge control agent; (c) providing a precipitation
electrode; (d) charging said liquefied polymer to a first
electrical potential relative to said precipitation electrode; and
(e) forming a surface on said liquefied polymer of sufficiently
high curvature to cause at least one jet of said liquefied polymer
to be drawn to said precipitation electrode by said first
electrical potential difference, thereby forming the unwoven fiber
structure capable of filtering out 99.97% of 0.3 .mu.m particulates
in air flowing at 5 cm/sec on said precipitation electrode.
22. The method of claim 21, wherein charging said liquefied polymer
to said first electrical potential relative to said precipitation
electrode is followed by recharging said liquefied polymer to a
second electrical potential relative to said precipitation
electrode, said second electrical potential is similar in
magnitude, yet opposite in sign with respect to said first
electrical potential.
23. The method of claim 21, wherein said liquefying is effected by
dissolving the polymer in a solvent, thereby creating a polymer
solution.
24. The method of claim 23, further comprising the step of: (f)
providing vapors of said solvent proximate to said surface of high
curvature.
25. The method of claim 21, wherein said charge control agent is
selected from the group consisting of biscationic amides, phenol
and uryl sulfide derivatives, metal complex compounds,
triphenylmethanes, dimethylmidazole and ethoxytrimethylsians.
26. The method of claim 21, wherein said forming of said surface of
high curvature is effected by causing said liquefied polymer to
emerge from a nozzle, said surface of high curvature being a
meniscus of said liquefied polymer.
27. The method of claim 21, wherein said forming of said surface of
high curvature is effected by wetting a protrusion having a tip
with said liquefied polymer, said surface of high curvature being a
surface of said liquefied polymer adjacent to said tip.
28. The method of claim 21, further comprising the step of: (f)
moving said precipitation electrode so that the unwoven fiber
structure is formed on said precipitation electrode as a sheet.
29. The method of claim 21, further comprising the step of: (f)
vibrating said surface of high curvature.
30. The method of claim 29, wherein said vibrating is effected at a
frequency between about 5000 Hz and about 30,000 Hz.
31. The method of claim 21, further comprising the steps of: (f)
charging a filler powder to a second electrical potential relative
to said collection surface, said second electrical potential being
opposite in sign to said first electrical potential, thereby
creating a charged filler powder; and (g) exposing the unwoven
fiber structure on said precipitation electrode to said charged
powder, thereby attracting said charged filler powder to the
unwoven fiber structure.
32. The method of claim 31, wherein said liquefied polymer is
charged negatively relative to said precipitation electrode and
wherein said charged powder is charged positively relative to said
precipitation electrode.
33. The method of claim 21, further comprising the step of: (f)
supplementing the liquefied polymer with an additive selected from
the group consisting of a viscosity reducing additive, a
conductivity regulating additive and a fiber surface tension
regulating additive.
34. The method of claim 32, wherein said viscosity reducing
additive is polyoxyalkylein, said conductivity regulating additive
is an amine salt and said fiber surface tension regulating additive
is a surfactant.
35. A high efficiency particulate air filter comprising unwoven
fibers of a polymer, the filter being capable of filtering out at
least 99.97% of 0.3 .mu.m particulates in air flowing at 5 cm/sec,
having a pressure drop of about 0.75 mm H.sub.2O to about 13 mm
H.sub.2O, and having a dust load to filter weight per area ratio of
about 1 to about 1.8.
36. The high efficiency particulate air filter of claim 35, wherein
the filter is substantially electrically neutral.
37. The high efficiency particulate air filter of claim 35, wherein
said fibers have a diameter of about 0.1 .mu.m to about 10
.mu.m
38. A high efficiency particulate air filter comprising unwoven
fibers of a polymer, the filter being capable of filtering out at
least 99.97% of 0.3 .mu.m particulates in air flowing at 5 cm/sec,
having a pressure drop of about 0.75 mm H.sub.2O to about 13 mm
H.sub.2O, the filter is substantially electrically neutral.
39. The high efficiency particulate air filter of claim 38, wherein
the filter has a dust load to filter weight per area ratio of about
1 to about 1.8.
40. The high efficiency particulate air filter of claim 38, wherein
said fibers have a diameter of about 0.1 .mu.m to about 10
.mu.m
41. A high efficiency particulate air filter comprising unwoven
fibers of a polymer having a diameter of about 0.1 .mu.m to about
10 .mu.m, the filter being capable of filtering out at least 99.97%
of 0.3 .mu.m particulates in air flowing at 5 cm/sec, having a
pressure drop of about 0.75 mm H.sub.2O to about 13 mm
H.sub.2O.
42. The high efficiency particulate air filter of claim 41, wherein
the filter has a dust load to filter weight per area ratio of about
1 to about 1.8.
43. The high efficiency particulate air filter of claim 41, wherein
the filter is substantially electrically neutral
44. A high efficiency particulate air filter comprising unwoven
fibers of a polymer, the filter being capable of filtering out at
least 99.97% of 0.3 .mu.m particulates in air flowing at 5 cm/sec
and having a pressure drop of about 0.75 mm H.sub.2O to about 13 mm
H.sub.2O, wherein at least about 90% of said fibers having a
diameter in a range of X and 2X, where X is in a range of about 0.1
.mu.m and about 10 .mu.m.
45. A high efficiency particulate air filter comprising unwoven
fibers of a polymer, the filter being capable of filtering out at
least 99.97% of 0.3 .mu.m particulates in air flowing at 5 cm/sec
and having a pressure drop of about 0.75 mm H.sub.2O to about 13 mm
H.sub.2O, the filter featuring pores formed among said fibers,
wherein at least about 90% of said pores having a diameter in a
range of Y and 2Y, where Y is in a range of about 0.2 .mu.m and
about 10 .mu.m.
Description
[0001] This is a continuation-in-part of PCT/IL97/00403, filed Dec.
9, 1997.
FIELD AND BACKGROUND OF THE INVENTION
[0002] The present invention is related to filtering means, in
particular to composite polymeric fiber filters, and to the
technology for their manufacture.
[0003] The creation of filtering materials capable of trapping
particles of 0.1-10 microns in size and their increasing use is
related to increasingly stringent requirements for quality and
reliability of manufactured commodities, as well as to the rapid
development of modem technology and production processes, such as,
but not limited to, electronics, aviation, automobile industry,
electrochemical industry, biotechnology, medicine, etc.
[0004] The main industrial manufacturing methods for such materials
include production from polymer solutions (V. P. Dubyaga et al.,
Polymer Membranes, "Chemistry" Publishing House, Moscow, 1981 (in
Russian); V. E. Gul and V. P. Dyakonova, Physical and Chemical
Principles of Polymer Films Manufacture, "Higher School Publishing
House, Moscow, 1978 (in Russian); German patent DE 3,023,788,
"Cationic absorbent for removing acid dyes etc. From waste
water--prepared from aminoplast precondensate and amine-amide
compound"), from powders and powder polymer composites (P. B.
Zhivotinskiy, Porous Partitions and Membranes in Electrochemical
Equipment, "Chemistry" Publishing House, Leningrad, 1978 (in
Russian); Encyclopedia of Polymer Science and Engineering, Wiley,
New York, 1987, Vol. 8 p. 533), from macromonolithic films (I.
Cabasso and A. F. Turbak, "Synthetic membranes", Vol. 1, ACS
Symposium, Ser. 154, Washington D.C., 1981, p. 267), and from
fibers and dispersions of fibrous polymers (T. Miura, "Totally dry
unwoven system combines air-laid and thermobonding technology",
Unwoven World Vol. 73 (March 1988) p.46). The latter method is the
most widespread, since it facilitates the manufacture of materials
with the optimal cost-quality ratio.
[0005] Great interest is also being expressed in the extension of
the traditional uses of filtering materials, especially to
combination functions of trapping micro-particles in gaseous and
liquid media with the adsorption of molecular admixtures, for
example, in the removal of mercaptans, as substrate for catalytic
reactions, in the enhancement of the bactericidal effect of the
filtering material, etc. Fulfillment of these additional functions
is possible due to the introduction into the fiber matrix of
fillers of some sort or functional groups giving the formation of
additional solid phase, i.e., as a result of manufacturing of
composite filtering materials.
[0006] At present, high efficiency polymeric filtering materials
are manufactured from synthetic fibers by means of a technology
that is similar in many aspects to the traditional technology
applied in the pulp and paper industry. A long fiber thread is cut
into pieces of a given length, which are then subjected to some
basic and supplementary operations out of more than 50
possibilities, which may include chemical processing for
modification of surface properties, mixing with binding and
stabilizing compositions, calendaring, drying process, etc. (O. I.
Nachinkin, Polymer Microfilters, "Chemistry" Publishing House,
Moscow, 1985 (in Russian), pp. 157-158). The complexity of such a
technological process hampers the manufacture of materials with
stable characteristics for subsequent exploitation; results in the
high cost of manufactured filtering materials; and practically
excludes the manufacture of composites with fillers sensitive to
moist, thermal processing.
[0007] Low efficiency filtering materials (class ASHRAE) are
manufactured by melt blow or spun-bonded processes.
[0008] There is, however, a method for the manufacture of
ultra-thin synthetic fibers (and devices for their production),
which facilitates the combination of the process of fiber
manufacture with the formation of a microporous filtering material,
and thus reduces the number of technological operations, precludes
the necessity for aqueous reaction media, and increases the
stability of properties of the product being manufactured (see, for
example, U.S. Pat. No. 2,349,950). According to this method, known
as "electrocapillary spinning", fibers of a given length are formed
during the process of polymer solution flow from capillary
apertures under electric forces and fall on a receptor to form an
unwoven polymer material, the basic properties of which may be
effectively changed.
[0009] With this method, fiber formation takes place in the gaps
between each capillary, being under negative potential, and a
grounded anti-electrode in the form of a thin wire, i.e., in the
presence of a heterogeneous field, being accompanied by corona
discharge. However, the process of solvent evaporation takes place
very rapidly, and as a result the fiber is subjected to varying
electric and aerodynamic forces, which leads to anisotropy along
the fiber width and formation of short fibers.
[0010] Manufacture of high-quality filtering materials from such
fibers is thus impossible because the electric charge of the fibers
is low, such that the process of forming the filtering material is
not controlled by electrical force and consequently the filtering
material is not uniform.
[0011] Exploitation of a device for executing the method described
above is complicated by a number of technological difficulties:
[0012] 1. Capillary apertures become blocked by polymer films that
form under any deviation from the technological process
conditions--concentration and temperature of solution, atmospheric
humidity, intensity of electric field, etc.
[0013] 2. The presence of a large number of such formations leads
to a complete halt of the technological process or drops form as a
consequence of the rupture of the aforementioned films.
[0014] 3. The presence of high intensity electric field in the area
of the precipitation electrode limits the productivity of the
method.
[0015] Therefore, the manufacture of synthetic fibers by this
method is possible from only a very limited number of polymers, for
example, cellulose acetate and low molecular weight polycarbonate,
which are not prone to the defects described above.
[0016] It is necessary to take into account the fact that such an
important parameter of filtering materials as monodispersity of the
pores (and the resultant separation efficiency of the product) has,
in this case, a weak dependency on fiber characteristics and is
largely determined by the purely probabilistic process of fiber
stacking.
[0017] Modern filtering materials are subject to strict, frequently
contradictory, requirements. In addition to high efficiency of
separation of heterogeneous liquid and gas systems, they are
required to provide low hydro- (or aero-) dynamic resistance of the
filter, good mechanical strength and technical properties (e.g.,
pleatability), chemical stability, good dirt absorption capacity,
and universality of application, together with low cost.
[0018] The manufacture of such products is conditional on the use
of high-quality long and thin fibers with an isometric
cross-section, containing monodispersed pores and exhibiting high
porosity. The practical value of this product may be greatly
increased as possible applications are expanded due to the
formation of additional phases, i.e., in the manufacture of the
above-mentioned composite filtering materials.
[0019] At present there is a high demand to high efficiency
particulate air (HEPA) filters which are defined as capable of
filtering out 99.97% of 0.3 .mu.m particulates in air flowing at 5
cm/sec. Such a requirement is met, for example, by glass-fiber
based filters, however on the expense of a high pressure drop, in a
range of 30-40 mm H.sub.2O.
[0020] U.S. Pat. Nos. 4,874,659 and 4,178,157 both teach high
efficiency particulate air filters capable of filtering out 99.97%
of 0.3 .mu.m particulates in air flowing at 5 cm/sec, characterized
by lower pressure drop in a range of 5-10 mm H.sub.2O. These
filters are made of nonwoven web (U.S. Pat. No. 4,874,659) or
sliced films (U.S. Pat. No. 4,178,157) made of polyolefines, such
as polyethylene or polypropylene, which are partially melted by
heating to about 100.degree. C. and are thereafter subjected to an
immense electrical field which electrically charges the polymer.
The result is a filter media, characterized by thick fibers (10-200
.mu.m) in diameter, low porosity and being electrically charged.
The latter property, provides these filters with the high
efficiency particulate air (HEPA) qualities. However, such filters
suffer few limitations. First, being based on the electrical charge
for effective capture of particulates, the performances of such
filters are greatly influenced by air humidity, causing charge
dissipation. Second, due to their mode of action and to being
relatively thin, such filters are characterized by low dust load
(the weigh of dust per area of filter causing a two fold increase
in pressure drop) per filter weight per area ratio of about 0.8,
wherein typically the dust load of such filters is about 50-80
g/m.sup.2 and their weight per area is about 80-130 g/m.sup.2.
[0021] Therefore, the main objective of the proposed technical
solution is removal of the above-listed defects of known solutions
for filtering applications (primarily directed at the manufacture
of microfilters from polymer fibers) and other purposes, including
application as micro-filtering means, i.e., the creation of means
and the meeting of the above-listed requirements for technical
means for the manufacture of micro-filtering materials with new
consumer properties.
SUMMARY OF THE INVENTION
[0022] According to one aspect of the present invention there is
provided a device for transforming a liquefied polymer into a fiber
structure, including (a) a substantially planar precipitation
electrode; (b) a first mechanism for charging the liquefied polymer
to a first electrical potential relative to the precipitation
electrode; (c) a second mechanism for forming a surface on the
liquefied polymer of sufficiently high curvature to cause at least
one jet of the liquefied polymer to be drawn by the first
electrical potential to the precipitation electrode; wherein the
first and second mechanisms are designed such that when a plurality
of fibers are precipitated on the precipitation electrode, a high
efficiency particulate air unwoven fiber structure, capable of
filtering out 99.97% of 0.3 .mu.m particulates in air flowing at 5
cm/sec is obtainable.
[0023] According to further features in preferred embodiments of
the invention described below, the first mechanism for charging the
liquefied polymer to a first electrical potential relative to the
precipitation electrode includes in combination (i) a source of
high voltage; and (ii) a charge control agent mixed with the
liquefied polymer.
[0024] According to still further features in the described
preferred embodiments the first mechanism for charging the
liquefied polymer to a first electrical potential relative to the
precipitation electrode further includes. (iii) a source of ionized
air being in contact with the liquefied polymer.
[0025] According to still further features in the described
preferred embodiments the second mechanism is effected by at least
one rotating wheel having a rim formed with a plurality of
protrusions.
[0026] According to still further features in the described
preferred embodiments each of the protrusions is formed with a
liquefied polymer collecting cavity.
[0027] According to still further features in the described
preferred embodiments each of the at least one wheel is tilted with
respect to the precipitation electrode.
[0028] According to still further features in the described
preferred embodiments each of the at least one wheel includes a
dielectric core.
[0029] According to still further features in the described
preferred embodiments the second mechanism is effected by a gas
bubbles generating mechanism.
[0030] According to still further features in the described
preferred embodiments the second mechanism is effected by a
rotating strap formed with a plurality of protrusions.
[0031] The basic device of the present invention includes a
grounded moving belt that acts as a precipitation electrode, and an
electrode-collector for charging a polymer solution negatively with
respect to the moving belt and for producing areas of high surface
curvature in the polymer solution.
[0032] In one embodiment of the device, the areas of high surface
curvature are formed by forcing the polymer solution through a bank
of nozzles. The nozzles of the electrode-collector are inserted
lengthwise in cylindrical holes sited at intervals in a negatively
charged cover plate of the electrode-collector. The source of
solvent vapors is connected to the holes. In an alternative
configuration, the nozzles are connected by a system of open
channels to the solvent vessel.
[0033] In one of the implementations, the device is provided with
an additional grounded electrode (or alternatively an under
potential electrode, of the same polarity of the high voltage
electrode, but with lower voltage) which is placed in parallel to
the surface of the nozzles of the electrode-collector and which is
able to move in the direction normal to the plane of the
electrode-collector's nozzles.
[0034] In order to improve the manufacturing process, the
additional electrode may take the form of a single wire stretched
over the inter-electrode space.
[0035] The additional electrode may also take the form of a
perforated plate with flange, in which case the surface of the
additional electrode, the flange, and the electrode-collector form
a closed cavity, and the apertures of the perforated plate are
co-axial to the apertures of electrode-collector.
[0036] Preferably, a device of the present invention also includes
an aerosol generator, made in the form of a hollow apparatus
(fluidized bed layer) divided into two parts by a porous
electro-conducting partition, which is connected to a mainly
positive high-voltage source. The lower part of the cavity forms a
pressure chamber, which is connected to a compressor, and the upper
part of the cavity is filled with the dispersible filler, for
example, polymer powder.
[0037] Alternatively, the aerosol generator may be made in the form
of a slot sprayer, connected to a positive high-voltage source and
a dry fluid feeder, provided with an ejector for supplying powder
to the sprayer.
[0038] Secondly, the objective put forward in the current invention
is obtained by the suggested method of manufacturing of a composite
filtering material, stipulating the following operations (stages)
(a) preparation of a polymer solution from a polymer, an organic
solvent and solubilizing additives, for example, by mixing at
elevated temperatures; (b) pouring the polymer solution into the
electrode-collector and introducing the dispersible filler, for
example, from a polymer of the same chemical composition as that in
the solution, into the cavity of electrified aerosol generator; (c)
supply of negative high voltage to the electrode-collector, and
creation of hydrostatic pressure to facilitate ejection of the
polymer solution through the electrode-collector nozzles to produce
polymer fibers with a negative electric charge; (d) transfer of the
aforementioned fibers under the action of electric and, inertial
forces to the precipitation electrode and chaotic stacking of the
fibers on its surface to transform the fibers into an unwoven
polymer material; (e) displacement of above-described polymer
material with the help of the precipitation electrode, followed by
interaction of the polymer material with the electrified aerosol
cloud formed from the dispersible filler in the aerosol generator
under positive high voltage and air pressure, accompanied by
penetration of the aerosol cloud into the structure of the
negatively charged unwoven polymer material to form a homogeneous
composite filtering material.
[0039] Thus, according to another aspect of the present invention
there is provided a method for forming a polymer into a high
efficiency particulate air unwoven fiber structure capable of
filtering out 99.97% of 0.3 .mu.m particulates in air flowing at 5
cm/sec, comprising the steps of (a) liquefying the polymer, thereby
producing a liquefied polymer; (b) supplementing the liquefied
polymer with a charge control agent; (c) providing a precipitation
electrode; (d) charging the liquefied polymer to a first electrical
potential relative to the precipitation electrode; and (e) forming
a surface on the liquefied polymer of sufficiently high curvature
to cause at least one jet of the liquefied polymer to be drawn to
the precipitation electrode by the first electrical potential
difference, thereby forming the unwoven fiber structure capable of
filtering out 99.97% of 0.3 .mu.m particulates in air flowing at 5
cm/sec on the precipitation electrode.
[0040] According to further features in preferred embodiments of
the invention described below, the liquefying is effected by
dissolving the polymer in a solvent, thereby creating a polymer
solution.
[0041] According to still further features in the described
preferred embodiments the method further comprising the step of (f)
providing vapors of the solvent proximate to the surface of high
curvature.
[0042] According to still further features in the described
preferred embodiments the charge control agent is selected from the
group consisting of biscationic amides, phenol and uryl sulfide
derivatives, metal complex compounds, triphenylmethanes,
dimethylmidazole and ethoxytrimethylsians.
[0043] According to still further features in the described
preferred embodiments the forming of the surface of high curvature
is effected by causing the liquefied polymer to emerge from a
nozzle, the surface of high curvature being a meniscus of the
liquefied polymer.
[0044] According to still further features in the described
preferred embodiments the forming of the surface of high curvature
is effected by wetting a protrusion having a tip with the liquefied
polymer, the surface of high curvature being a surface of the
liquefied polymer adjacent to the tip.
[0045] According to still further features in the described
preferred embodiments the method further comprising the step of (f)
moving the precipitation electrode so that the unwoven fiber
structure is formed on the precipitation electrode as a sheet.
[0046] According to still further features in the described
preferred embodiments the method further comprising the step of (f)
vibrating the surface of high curvature.
[0047] According to still further features in the described
preferred embodiments the vibrating is effected at a frequency
between about 5000 Hz and about 30,000 Hz.
[0048] According to still further features in the described
preferred embodiments charging the liquefied polymer to a first
electrical potential relative to the precipitation electrode is
followed by recharging the liquefied polymer to a second electrical
potential relative to the precipitation electrode, the second
electrical potential is similar in magnitude, yet opposite in sign
with respect to first electrical potential. Preferably the charge
is oscillated between the first and second electrical potentials in
a frequency of about 0.1-10 Hz, preferably about 1 Hz.
[0049] According to still further features in the described
preferred embodiments the method further comprising the steps of
(f) charging a filler powder to a second electrical potential
relative to the collection surface, the second electrical potential
being opposite in sign to the first electrical potential, thereby
creating a charged filler powder; and (g) exposing the unwoven
fiber structure on the precipitation electrode to the charged
powder, thereby attracting the charged filler powder to the unwoven
fiber structure.
[0050] According to still further features in the described
preferred embodiments the method further comprising the steps of
(f) supplementing the liquefied polymer with an additive selected
from the group consisting of a viscosity reducing additive, a
conductivity regulating additive and a fiber surface tension
regulating additive.
[0051] According to still further features in the described
preferred embodiments the viscosity reducing additive is
polyoxyalkylein, the conductivity regulating additive is an amine
salt and the fiber surface tension regulating additive is a
surfactant.
[0052] According to still further features in the described
preferred embodiments the liquefied polymer is charged negatively
relative to the precipitation electrode and wherein the charged
powder is charged positively relative to the precipitation
electrode.
[0053] According to still another aspect of the present invention
there is provided a high efficiency particulate air filter
comprising unwoven fibers of a polymer, the filter being capable of
filtering out at least 99.97% of 0.3 .mu.m particulates in air
flowing at 5 cm/sec and having a pressure drop of about 0.75 mm
H.sub.2O to about 13 mm H.sub.2O.
[0054] According to still further features in the described
preferred embodiments the filter is substantially electrically
neutral.
[0055] According to still further features in the described
preferred embodiments the fibers have a diameter of about 0.1 .mu.m
to about 10 .mu.m
[0056] According to yet another aspect of the present invention
there is provided a high efficiency particulate air filter
comprising unwoven fibers of a polymer, the filter being capable of
filtering out at least 99.97% of 0.3 .mu.m particulates in air
flowing at 5 cm/sec and having a pressure drop of about 0.75 mm
H.sub.2O to about 13 mm H.sub.2O, wherein at least about 90% of the
fibers having a diameter in a range of X and 2X, where X is in a
range of about 0.1 .mu.m and about 10 .mu.m.
[0057] According to still another aspect of the present invention
there is provided a high efficiency particulate air filter
comprising unwoven fibers of a polymer, the filter being capable of
filtering out at least 99.97% of 0.3 .mu.m particulates in air
flowing at 5 cm/sec and having a pressure drop of about 0.75 mm
H.sub.2O to about 13 mm H.sub.2O, the filter featuring pores formed
among the fibers, wherein at least about 90% of the pores having a
diameter in a range of Y and 2Y, where Y is in a range of about 0.2
.mu.m and about 10 .mu.m.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] The invention is herein described, by way of example only,
with reference to the accompanying drawings, wherein:
[0059] FIG. 1 is a schematic diagram of a device of the present
invention, including two alternative electrified aerosol
generators;
[0060] FIG. 2a is a top view of the electrode-collector of the
device of FIG. 1;
[0061] FIG. 2b is a lateral cross section of the
electrode-collector of FIG. 2a;
[0062] FIGS. 3 and 4 are lateral cross sections of alternative
nozzle-based electrode-collectors;
[0063] FIG. 5 is a lateral cross section of an electrode-collector
based on a rotating wheel;
[0064] FIG. 6 is a lateral cross section of an electrode-collector
based on reciprocating needles;
[0065] FIG. 7 is an electron micrograph of a filter according to
the present invention;
[0066] FIG. 8 is a cross section of a preferred embodiment of the
device according to the present invention, adapted for
manufacturing a a layered filter having a support layer and a
prefilter layer surrounding a middle layer of high efficiency
particulate air filter;
[0067] FIG. 9a is a cross section of a preferred embodiment of the
device according to the present invention, including an air ionizer
to increase the charging of the liquefied polymer and thereby to
enable more homogenic precipitation thereof on a precipitation
electrode;
[0068] FIG. 9b is an enlarged view of circle I of FIG. 9a, showing
an air ionizer in greater detail;
[0069] FIG. 10 is a cross section of a mechanism for forming a
surface on the liquefied polymer of sufficiently high curvature to
cause at least one jet of the liquefied polymer to be drawn to the
precipitation electrode effected via generation of bubbles in the
liquefied polymer;
[0070] FIG. 11 is a cross section of a device according to the
present invention including a plurality of tilted circular
wheels;
[0071] FIGS. 12a-b are side view and cross section of a wheel
according to a preferred embodiment of the invention, including a
dielectric core;
[0072] FIG. 13 is a cross section of a device according to the
present invention including a plurality of tilted circular wheels
in a different configuration;
[0073] FIG. 14 is a side view of a wheel according to a preferred
embodiment of the invention, including liquefied polymer collecting
cavities; and
[0074] FIG. 15 is a perspective view of yet another mechanism for
forming a surface on the liquefied polymer of sufficiently high
curvature which includes a rotateable strap of a conductive
material formed with a plurality of protrusions rotating in
parallel to the precipitation electrode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0075] The present invention is of a high efficiency particulate
air filter, which is also referred to herein as unwoven polymer
structure and further of a device and process for the electrostatic
precipitation of fibers thereof. Specifically, the present
invention can be used to make a composite unwoven filter.
[0076] The principles and operation of the present invention may be
better understood with reference to the drawings and the
accompanying description.
[0077] Before explaining at least one embodiment of the invention
in detail, it is to be understood that the invention is not limited
in its application to the details of construction and the
arrangement of the components set forth in the following
description or illustrated in the drawings. The invention is
capable of other embodiments or of being practiced or carried out
in various ways. Also, it is to be understood that the phraseology
and terminology employed herein is for the purpose of description
and should not be regarded as limiting.
[0078] According to the present invention there is provided high
efficiency particulate air filter comprising unwoven fibers of a
polymer. The filter according to the present invention is capable
of filtering out at least 99.97% of 0.3 .mu.m particulates in air
flowing at 5 cm/sec and has a pressure drop of 13 mm H.sub.2O,
preferably of about 10 mm H.sub.2O, more preferably of about 5 mm
H.sub.2O, most preferably of about 2 mm H.sub.2O, optimally of
about 0.75 mm H.sub.2O, or less. Thus a pressure drop of any value
in a range of about 0.75 mm H.sub.2O and about 13 mm H.sub.2O is
within the scope of the present invention.
[0079] The filter according to the present invention preferably has
a dust load to filter weight per area ratio of about 1 to about
1.8. Any value within this range is within the scope of the present
invention. For example, a filter according to the present invention
weighting 100 grams/m.sup.2 and having a 1.5 dust load to filter
weight per area ratio suffers a two fold increase in its pressure
drop when loaded with 150 grams/m.sup.2 of dust.
[0080] It will be appreciated that the filters disclosed in U.S.
Pat. Nos. 4,874,659 and 4,178,157 described in the Background
section above are characterized by a dust load to filter weight per
area ratio of less than 0.8.
[0081] According to a preferred embodiment of the present invention
the filter is substantially electrically neutral and therefore its
characteristics as a filter are much less affected by air humidity
as compared with the filters disclosed in U.S. Pat. Nos. 4,874,659
and 4,178,157, described in the Background section above, which owe
their performances to the charges associated therewith. The filter
of the present invention becomes electrically neutral typically
within 5-10 minutes after its precipitation on a precipitation
electrode, as further described hereinunder.
[0082] According to another preferred embodiment of the present
invention the fibers have a diameter of about 0.1 .mu.m to about 20
.mu.m. Fibers having a diameter of about 0.1-0.5 .mu.m, about 0.5-2
.mu.m, about 2-5 .mu.m and about 5-20 .mu.m, are all within the
scope of the present invention, and are obtainable by selecting
appropriate process parameters as further detailed hereinunder. It
will be appreciated that the filters disclosed in U.S. Pat. Nos.
4,874,659 and 4,178,157, described in the Background section above
are characterized by diameters in a range more than 10 to about 200
.mu.m.
[0083] According to yet another preferred embodiment of the present
invention, at least about 90% of the fibers have a diameter in a
range of X and 2X, where X is any value in a range of about 0.1
.mu.m and about 10 .mu.m. According to still another preferred
embodiment of the present invention, the filter featuring pores
formed among the fibers, wherein at least about 90% of the pores
have a diameter in a range of Y and 2Y, where Y is any value in a
range of about 0.2 .mu.m and about 10 .mu.m. These latter features
of the filter according to the present invention are effected by
the preferred method of its manufacture, as further detailed
hereinunder. The filters disclosed in described in the Background
section above fail to enjoy the described homogeneity in fiber and
pore diameters.
[0084] FIG. 7 provides a 4000 fold magnification of the filter
described herein. Please note that many of the fibers shown have a
1 .mu.m thickness (equals to 4 mm in the electron micrograph) and
that the deviation is low. Such magnifications were employed to
extract the above listed features and ranges describing the
physical properties of the filter according to the present
invention, and which distinct the filter according to the present
invention from prior art filters.
[0085] According to a preferred embodiment of the present invention
the filter is further supplemented with a filler, which, as
described hereinabove and further detailed hereinunder, is useful
in the removal of mercaptans, as substrate for catalytic reactions,
in the enhancement of the bactericidal effect of the filtering
material, etc.
[0086] The technological process of preparation of the composite
filtering material according to the present invention includes two
basic stages, which take place simultaneously. The first consists
of the formation and precipitation on a constantly moving surface
(base) of ultra-thin fibers (typically in a range of 0.1-10 .mu.m)
from the polymer solution that flows out of the capillary apertures
under the action of an electric field.
[0087] The second operation is the introduction of micro-dispersed
particles of filler of a particular composition into the fiber
structure (matrix) formed previously in the first stage of
production.
[0088] A basic variant of the device of the present invention (FIG.
1) includes a high-voltage electrode-collector 1, manufactured as a
bath, filled with the polymer solution (or melted polymer) and
provided with a base 2 and a cover 2'. The electrode-collector is
connected to a feeder 3 (shown in FIG. 2b) by a flexible pipe,
installed so as to allow vertical movement, and a source 4 of high
voltage of negative polarity.
[0089] Spinnerets 5 with nozzles 6 having capillary apertures are
screwed into threaded openings formed in cover 2 of
electrode-collector as on a chess board (FIG. 2). Because the
height of the spinnerets is slightly less than the width of cover
2' and the length of each nozzle 6 exceeds the width of cover 2',
the nozzle section is placed above cover 2' on the axis of
cylindrical depressions 7, connected to each other by a system of
open channels 8 (FIG. 2a). The solvent is fed into this system of
channels from a vessel 9.
[0090] A precipitation electrode 10 is situated at a certain
distance (e.g., about 15-50 cm) above cover 2'. Precipitation
electrode 10 is manufactured in the form of a constantly moving
surface (when in the operating mode), for example, a belt made of
electrical conducting material. Precipitation electrode 10 is
grounded. Shafts 11 and 12, connected to an electrical motor (not
depicted on drawings), are responsible for driving precipitation
electrode 10, keeping precipitation electrode 10 under tension, and
preliminary compression of the material on precipitation electrode
10.
[0091] A part of precipitation electrode 10 is wound around shaft
13, which has a large diameter, and is thus immersed in the
rectangular cavity of the electrified aerosol generator. The cavity
of the electrified aerosol generator is divided into two sections
by a porous conducting partition 15. The latter is connected to a
high-voltage source 16 of positive polarity. The lower part 14 of
the electrified aerosol generator, forming pressure chamber 17, is
connected to a compressor (not shown on drawings). A
micro-dispersible filler is poured onto the surface of the porous
partition 15 in the upper part of the generator. The entire device
depicted in FIG. 1 is preferably contained in a hermetically sealed
container, provided with a suction unit and a settling chamber for
trapping and re-circulation of the solvent vapors (not shown on
drawings).
[0092] The electrified aerosol generator may also be implemented in
the form of a slot sprayer 18, connected by a pipe to a dry powder
ejection feeder 19 and a source of positive high voltage 16. The
use of the slot sprayer with a charging of aerosol in the field of
the corona discharge is preferred in the case of metallic powders
(including graphite powder) and powders that are not easily
fluidized.
[0093] It was experimentally found that in filters with high
pleatability performances are achievable by adding to the basic
layer of polymer a minute quantity (say about 2-3%) of a powder,
such as polypropylene powder, epoxy powder and/or
phenolformaldehyde powder, and further adding about 5-6% of a
second powder such as talc powder, zinc powder and/or titanium
oxide powder and thereafter heating the powders loaded filter to
about 70-80% of the melting temperature of the polymer employed in
the basic layer.
[0094] The heating rate of any of the above powders depends on the
powder's dispersion and specific heat characteristics. So, for
polymer powders with high dispersion (root mean square diameter of
1-5 .mu.m) heating is low. Coarser metallic and oxide powders
require relatively higher temperatures.
[0095] The direction of fiber feeding on the vertical surface may
be reversed, and the dimensions of the electrode-collector and the
number of capillaries may be minimized with the help of the device
depicted in FIG. 3. The device consists of an electrode-collector
frame 20, manufactured from a dielectric material and having a
central channel 21, for example, of cylindrical shape. This channel
is connected by a pipe to a feeder (not shown on the drawing) and
is provided with aperture 22 to facilitate exchange of gases with
the atmosphere. A busbar 23 with spinnerets 5 and nozzles having
capillary apertures is installed in the lower part of frame 20. The
nozzles are connected to a source of high voltage (not shown on the
drawing). Cover 24 with apertures 25 is placed before the busbar.
Nozzles 6 are placed in these apertures with coaxial clearance. The
internal surface of the cover and busbar form a cavity 26, which is
connected to a saturator (not shown on drawing) by a pipe.
[0096] In a number of cases, the process of manufacturing the
composite filtering material may be improved by implementation of
the device shown in FIG. 4. Here, a dielectric flange 28 serves as
a base for a perforated grounded plate 27 (or alternatively an
under potential plate, of the same polarity of the high voltage
electrode, but with lower voltage), which is installed, with a
certain clearance C, say about 0.5-3 cm, parallel to the surfaces
of the electrode-collector 20 and the busbar 23. Plate 27 rests on
the flange in such a way as to provide for vertical movement for
regulation of the size of the clearance C. Apertures 29 of the
perforated plate are co-axial to the apertures of
electrode-collector's nozzles. The internal surface of perforated
plate 27 and busbar 23 form a cavity 26, which is connected by a
pipe to a saturator.
[0097] The proposed device in its basic form functions as follows:
From feeder 3 (FIG. 2b), the polymer solution runs into
electrode-collector bath 1, and under the action of hydrostatic
pressure the polymer solution begins to be extruded through the
capillary apertures of nozzles 6. As soon as a meniscus forms in
the polymer solution, the process of solvent evaporation starts.
This process is accompanied by the creation of capsules with a
semi-rigid envelope, the dimensions of which are determined, on the
one hand, by hydrostatic pressure, the concentration of the
original solution and the value of the surface tension, and, on the
other hand, by the concentration of the solvent vapor in the area
of the capillary apertures. The latter parameter is optimized by
choice of the area of free evaporation from cover 2' and of the
solvent temperature. Alternatively or additionally it is optimized
by covering the device and supplementing its atmosphere with
solvent vapor (e.g., via a solvent vapor generator).
[0098] An electric field, accompanied a by unipolar corona
discharge in the area of nozzle 6, is generated between cover 2'
and precipitation electrode 10 by switching on high-voltage source
4. Because the polymer solution possesses a certain electric
conductivity, the above-described capsules become charged.
Coulombic forces of repulsion within the capsules lead to a drastic
increase in hydrostatic pressure. The semi-rigid envelopes are
stretched, and a number of point microruptures (from 2 to 10) are
formed on the surface of each envelope. Ultra-thin jets of polymer
solution start to spray out through these apertures. Moving with
high velocity in the inter-electrode interval, these jets start to
lose solvent and form fibers that are chaotically precipitated on
the surface of the moving precipitation electrode 10, forming a
sheet-like fiber matrix. Since the polymer fiber posses high
surface electric resistance and the volume of material in physical
contact with precipitation electrode surface is small, the fiber
matrix preserves the negative electric charge for a relatively long
time, about 5-10 minutes. It will be appreciated that the
electrical resistance can be regulated by special additives.
[0099] When compressed air is fed into pressure chamber 17 of
electrified aerosol generator 14 and high-voltage source 16 is
switched on, the micro-dispersible filler becomes fluidized and
acquires a positive electric charge. Under the action of electric
and aerodynamic forces, the filler particles move to the surface of
precipitation electrode 10, which holds the fiber matrix. As a
result of the action of Coulombic forces, the filler particles
interact with the fiber matrix, penetrate its structure, and form a
composite material.
[0100] When the belt of precipitation electrode 10 passes between
shafts 11, preliminary material compression takes place,
accompanied by redistribution of the filler particles in the matrix
volume. Spherical particles, attached to the fiber material solely
by electrical forces, move along paths of least resistance into
micro-zones having a minimum volume density of matrix material,
filling large pores, and thus improving the homogeneity of the
composite and the degree of micro-dispersity of the pores.
[0101] The micro-dispersible powders from the following materials
may be used as fillers: a polymer of the same chemical composition
as that in the matrix, polymer latexes, glass, or Teflon, as well
as active fillers that lead to the production of composite
microfiltering materials with new consumer properties. These new
materials may find application as adsorbents, indicators,
catalysts, ion-exchange resins, pigments bactericides, etc.
[0102] The use of an electrified aerosol generator, as described
above with the fluidized layer, facilitates high productivity of
the process and product homogeneity. However, several powders have
difficulty in forming a fluidized layer: metallic powders,
particularly catalytic metals, can be subjected to electric
precipitation only in the field of a unipolar corona discharge.
Therefore, in these cases, as well as in the case in which it is
necessary to measure out exact amounts of filler, it is worthwhile
to use a slot sprayer 18 as the electrified aerosol generator (FIG.
1).
[0103] When compressed air from a compressor is fed into the dry
powder feeder and the high voltage source is switched on, the
powdered filler is ejected into slot sprayer 18. The aerosol cloud
coming out of the sprayer apertures becomes charged in the unipolar
corona discharge field, and under the action of electric and
aerodynamic forces is transferred to the precipitation electrode,
where it interacts with the fiber matrix as described above.
[0104] The functioning of the device described in FIG. 3
corresponds, in the main aspects, with the operation of the basic
device. The main difference is as follows: solvent vapor from the
saturator under slight excess pressure is fed into cavity 26 and
exits via aperture 25, flowing over the edges of the apertures of
nozzles 6. Alternatively or additionally the device is covered and
its atmosphere supplemented with solvent vapor (e.g., via a solvent
vapor generator).
[0105] The advantage of this configuration lies in the facts that
it provides the possibility of easy spatial reorientation and fiber
feeding in any direction and that it can be manufactured in compact
form with a small number of capillaries. A device of this type is
not efficient in installations aimed at high throughput due to
difficulties in obtaining homogenous distribution of the vapor-air
mixture through a large number of apertures and to the possibility
of vapor condensation in pipes and subsequent falling of drops.
[0106] Intensification of the fiber matrix manufacturing process
and a reduction of fiber width in order to produce filtering
materials with a minimum pore size assumes, on the one hand, that
the intensity of the electric field should be increased to values
close to the level at which electrical discharges would begin to
form between the emerging fibers and precipitation electrode 10
and, on the other hand, that the concentration of solvent vapors in
the inter-electrode interval be increased in order to maintain the
capability of consolidating fiber formation. Increasing the solvent
vapors in the inter-electrode interval can be effected, for
example, by covering the device and supplementing its atmosphere
with solvent vapor (e.g., via a solvent vapor generator). The
optimal electric field strength, both between electrode-collector 1
and precipitation electrode 10, and between the electrified aerosol
generator and precipitation electrode 10, is between about 2.5
KV/cm and about 4 KV/cm.
[0107] An increase in the average intensity and heterogeneity of
the electric field, leading to corona discharge, may be realized by
installing, in the inter-electrode interval, one or more grounded
electrodes (or alternatively under potential electrodes, of the
same polarity of the high voltage electrode, but with lower
voltage) manufactured, for instance, in the form of wires. This
solution facilitates an increase in the productivity of the process
by 1.5-2 times, but it does not lead to formation of short fibers
with, varying strength and size parameters. The negative effect of
using a linear grounded electrode instead of a planar grounded
electrode, thereby producing a non-homogeneous electrical field,
may be reduced by increasing the solvent vapor concentration in the
fiber-formation area, which is difficult in open devices and
increases solvent consumption and in some cases danger of fire.
Increasing the solvent vapor concentration in the fiber-formation
area can be effected by, for example, covering the device and
supplementing its atmosphere with solvent vapor (e.g., via a
solvent vapor generator).
[0108] This deficiency may be overcome by application of the device
described above and depicted in FIG. 4.
[0109] Switching on the high-voltage source 4 in the C clearance
produces an homogeneous electric field, the intensity of which may
be easily increased to 10-15 KV/cm. Under these conditions, the
impact of the electric field upon the jet of polymer solution
increases significantly. The fiber comes out thinner and more
homogeneous along its length. The initial fiber velocity also
increases, and thereafter it comes through apertures 29 of
perforated plate 27 and is stacked on precipitation electrode
surface as described above. A change of the size of clearance C
facilitates regulation of fiber thickness and device productivity,
as well as the degree of material porosity.
[0110] The present invention may be used to produce the polymer
fiber structure from a much wider range of polymers than is
possible using the prior art of U.S. Pat. No. 2,349,950.
[0111] While reducing the present invention into practice, it was
found that for obtaining a high efficiency particulate air unwoven
fiber structure, capable of filtering out 99.97% of 0.3 .mu.m
particulates in air flowing at 5 cm/sec, and further having the
above described features, improved charging of the polymer is
required. Improved charging is effected according to the present
invention by mixing the liquefied polymer with a charge control
agent (e.g., a dipolar additive) to form, for example, a
polymer-dipolar additive complex which apparently better interacts
with ionized air molecules formed under the influence of the
electric field. It is assumed, in a non-limiting fashion, that the
extra-charge attributed to the newly formed fibers is responsible
for their more homogenous precipitation on the precipitation
electrode, wherein a fiber is better attracted to a local maximum,
which is a local position most under represented by older
precipitated fibers, which, as will be recalled, keep their charge
for 5-10 minutes. The charge control agent is typically added in
the grams equivalent per liter range, say, in the range of from
about 0.01 to about 0.2 normal per liter, depending on the
respective molecular weights of the polymer and the charge control
agent used.
[0112] U.S. Pat. Nos. 5,726,107; 5,554,722; and 5,558,809 teach the
use of charge control agents in combination with polycondensation
processes in the production of electret fibers, which are fibers
characterized in a permanent electric charge, using melt spinning
and other processes devoid of the use of an precipitation
electrode. A charge control agent is added in such a way that it is
incorporated into the melted or partially melted fibers and remains
incorporated therein to provide the fibers with electrostatic
charge which is not dissipating for prolonged time periods, say
months.
[0113] In sharp distinction, the charge control agents according to
the present invention transiently bind to the outer surface of the
fibers and therefore the charge dissipates shortly thereafter
(within minutes). This is because polycondensation is not exercised
at all such that chemical intereaction between the agent and the
polymer is absent, and further due to the low concentration of
charge control agent employed. The resulting filter is therefore
substantially charge free.
[0114] Thus, a mechanism for charging the liquefied polymer to a
first electrical potential relative to the precipitation electrode
according to the present invention preferably includes a source of
high voltage, as described above, and a charge control agent mixed
with the liquefied polymer.
[0115] Suitable charge control agents include, but are not limited
to, mono- and poly-cyclic radicals that can bind to the polymer
molecule via, for example, --C.dbd.C--, .dbd.C--SH-- or --CO--NH--
groups, including biscationic amides, phenol and uryl sulfide
derivatives, metal complex compounds, triphenylmethanes,
dimethylmidazole and ethoxytrimethylsians. Conductivity control
additives as further described below may also be employed.
[0116] The functionality of biscationic amides, for example, was
experimentally evaluated. To this end, a 14% solution of a branched
polycarbonate polymer (MW=ca. 110,000) in chloroform was prepared
(viscosity was 180 cP). The above solution, supplemented with
increasing concentration of bicationic acid amide was used in
combination with a device as depicted in, and as described with
relation to, FIG. 3 to precipitate filters, which were thereafter
inspected for physical and functional properties. The examination
included estimation of fiber diameter and uniformity of
distribution, as well as, pressure drop evaluations. The addition
of increasing amounts of bicationic acid amide did not alter fiber
diameter, however, it had a striking effect on uniformity of
distribution which resulted in lowering the pressure drop values
associated with such filters, as exemplified in Table 1, below:
1 TABLE 1 Concentration of bicationic Pressure drop for 100
g/m.sup.2 acid amide (N-10.sup.-2) filters (mm H.sub.2O) 0 22 0.1
22 0.2 18 0.3 6 0.5 5 0.6 5 0.7 6 1.0 5
[0117] It is evident from Table 1 that the added charge control
agent improves the filter product in terms of pressure drop. It is
further clear that the influence of the charge control agent
reaches its maximal effectiveness in a low concentration and that
increasing its concentration above that value fails to further
improve the quality of the product in terms of pressure drop.
[0118] In a similar experiment, the functionality of metal complex
compound (iron salicylic acid complex), for example, was
experimentally evaluated. To this end, a 12% solution of a
polysulfone polymer (MW=ca. 80,000) in chloroform was prepared
(viscosity was 140 cP, conductivity was 0.32 .mu.S). The above
solution, supplemented with increasing concentration of the metal
complex compound was used in combination with a device as depicted
in, and as described with relation to, FIG. 3 to precipitate
filters, which were thereafter inspected for physical and
functional properties. The examination included estimation of fiber
diameter and uniformity of distribution, as well as, pressure drop
evaluations. As before, the addition of increasing amounts of the
charge control agent did not alter fiber diameter, however, it had
a striking effect on uniformity of distribution which resulted in
lowering the pressure drop values associated with such filters, as
exemplified in Table 2, below:
2 TABLE 2 Concentration of iron salicylic Pressure drop for 100
g/m.sup.2 acid complex (N-10.sup.-2) filters (mm H.sub.2O) 0 18 0.1
9 0.2 3 0.3 3 0.5 3 0.6 3 0.7 3 1.0 3
[0119] It is evident from Table 2 that the added charge control
agent improves the filter product in terms of pressure drop. It is
further clear that the influence of the charge control agent
reaches its maximal effectiveness in a low concentration and that
increasing its concentration above that value fails to further
improve the quality of the product in terms of pressure drop.
[0120] This phenomenon can be explained by saturation of the
polymer fiber surface by the charge control agent and further by
loss of access charge to the surrounding atmosphere.
[0121] The charge (or its absence) can be measured by a dedicated
device namely a gauge for measuring electric field intensities. The
end value of the electric charge or rate of loss does not reflect
on homogenous fiber distribution. Only the initial rate of the
charge is important to this end. The time required for charge
dissipation is about few minutes.
[0122] The device and method according to the present invention
differ from those disclosed in U.S. Pat. Nos. 4,043,331 and
4,127,706 to Martin et al. and U.S. Pat. No. 1,975,504 to Anton
Formhals in that it enables manufacturing a high efficiency
particulate air unwoven fiber structure, capable of filtering out
99.97% of 0.3 .mu.m particulates in air flowing at 5 cm/sec and
which further enjoy the physical features described hereinabove.
The devices and methods disclosed in the above patents are only
capable of providing lower grade filters which fail to meet the
requirements of high efficiency particulate air filters as
described herein.
[0123] According to a preferred embodiment of the present
invention, charging the liquefied polymer to a first electrical
potential relative to the precipitation electrode is followed by
recharging the liquefied polymer to a second electrical potential
relative to the precipitation electrode, the second electrical
potential is similar in magnitude, yet opposite in sign with
respect to first electrical potential. Preferably the charge is
oscillated between the first and second electrical potentials in a
frequency of about 0.1-10 Hz, preferably about 1 Hz. The charge
oscillation results in process productivity, more homogeneous
distribution of precipitated fibers and yielding filters with
improved qualities as described hereinabove.
[0124] Polymers amenable to the present invention include
polysulfone, polyphenyl sulfone, polyether sulfone, polycarbonate
in general, ABS, polystyrene, polyvynilidene fluoride,
postchlorinated polyvinyl chloride and polyacrilonitrile. Suitable
solvents include, inter alia, chloroform, benzene, acetone and
dimethylformamide. The optimal concentration of the solution
depends on the specific polymer and solvent used. Generally, the
higher the concentration of polymer in the solution, the higher the
process yield and the lower the product porosity. Concentrations of
between about 10% and about 12% have been found optimal for the
polymer solution used in electrode-collector 1. Melted polymers
such as, but not limited to, polyolefins, including polyethylene
and polypropylene, are also amenable to the process according to
the present invention.
[0125] It has been found advantageous to add certain additives to
the solutions of these polymers. Amine salts such as tetraethyl
ammonium bromide and benzyltriethylammonium bromide, are used to
regulate the conductivity of the polymer solution, as described
above. Small amounts of high molecular weight (order of 500,000)
polyoxyalkylene additives, such as polyethylene glycol and
polyvinyl pyrrolidone promote the formation of the polymer solution
jets by reducing intermolecular friction. Surfactants such as
dimethylmidazole and ethoxytrimethylsilane enhance fiber thickness
and uniformity. Using additives reducing viscosity and surface
tension it is possible to increase the polymer concentration up to
about 17-18%.
[0126] More generally, the scope of the present invention includes
the manufacture of the polymer fiber structure from a liquefied
polymer, and not just from a polymer solution. By a liquefied
polymer is meant a polymer put into a liquid state by any means,
including dissolving the polymer in a solvent, as described above,
and melting the polymer.
[0127] Also more generally, the scope of the present invention
includes the formation of a surface on the liquefied polymer, of
sufficient curvature to initiate the process discussed above of the
charged capsules, leading to the formation of the jets of liquefied
polymer that turn into fibers and precipitate onto precipitation
electrode 10. As discussed above, if the liquefied polymer is a
polymer solution, the fibers are formed by evaporation of the
solvent. If the liquefied polymer is a melt, the fibers are formed
by solidification of the jets.
[0128] In the process of the present invention as described above,
the highly curved surfaces are the menisci of polymer solution
emerging from nozzles 6. Other mechanisms for forming these highly
curved surfaces are illustrated in FIGS. 5 and 6. FIG. 5
illustrates a variant of electrode-collector 1 in which the polymer
solution, stored in a tank 33, is pumped by a pump 32 through a
feed pipe 31 to a delivery chamber 36. Rotateably mounted in
delivery chamber 36 is a circular wheel 30 made of an electrically
conductive material. Mounted on rim 38 of wheel 30 are triangular
protrusions 40 made of a material that is wetted by the polymer
solution. Tips 42 of protrusions 40 point radially outward from
wheel 30. Wheel 30 is charged negatively by source 4. As the
polymer solution is delivered to chamber 36, wheel 30 rotates and
each of protrusions 40 is successively coated with a layer of the
polymer solution, which in turn acquires a negative charge. The
surface of the portion of this polymer solution layer that
surrounds tip 42 constitutes the highly curved surface whence the
charged jets emerge. Polymer solution not consumed in the course of
precipitating fibers onto precipitation electrode 10 is returned to
tank 33 via an outlet pipe 35 by a pump 34. The optimal
concentration of polymer solution used in this variant of
electrode-collector 1 generally has been between about 14% and
about 17%.
[0129] FIG. 6 is a partial illustration, in cross-section, similar
to the cross-section of FIG. 2b, of a variant of
electrode-collector 1 in which nozzles 6 are replaced by
reciprocating needles 40, made of an electrically conductive
material that is wetted by the polymer solution. Each needle 40 is
provided with a mechanism 42 for raising and lowering needle 40.
When a needle 40 is lowered, the sharpened tip 44 thereof is wetted
and coated by the polymer solution. The surface of the polymer
solution is highly curved at tip 44. When a needle 40 is raised
towards precipitation electrode 10, the high voltage difference
between needle 40 and precipitation electrode 10 causes jets of the
polymer solution to emerge from the polymer solution surrounding
tip 44 and to stream towards precipitation electrode 10. It should
be noted that in this variant of electrode-collector 1, only
needles 40, and hence the polymer solution thereon, are negatively
charged by source 4.
[0130] Also shown in FIG. 6 is a speaker 50 of a system for
producing acoustical vibrations in the air above
electrode-collector 1. Speaker 50 emits a tone of a single
frequency, preferably in the range between about 5000 Hz and about
30,000 Hz, towards needles 40. The vibrations thus induced in the
highly curved surfaces of the polymer solution on tips 44 have been
found to stimulate the emission of jets of polymer solution towards
precipitation collector 10.
[0131] FIGS. 8-15 teach additional preferred embodiments of the
device and method according to the present invention.
[0132] Thus, as shown in FIG. 8, for the formation of a
multilayered filter having a prefilter layer and a support layer
surrounding a middle layer of high efficiency particulate air
filter, a triple configuration of the device described above, with
some modifications described hereinunder is provided. Thus,
electrode-collector 1 is replaced according to this configuration
by three electrode-collectors 100a, 100b and 100c, each designed
for precipitation of one of the above layers of the layered filter.
Via a suitable source of high voltage, electrode-collectors 100a,
100b and 100c are provided with, for example, a negative potential
of, for example, -100 KV. Precipitation electrode 10 according to
this embodiment is replaced by a modified version having three
independent precipitation electrodes 102a, 102b and 102c and a
revolving belt 104, wound around revolving shafts 106. The location
of precipitation electrodes 102a, 102b and 102c is selected above
electrode-collectors 100a, 100b and 100c and via independent
sources of high voltage they are provided with positive, negative
and negative potentials, say (+1)-(+5), (-1)-(-2) and (-2)-(-5) KV,
respectively, generating, for example, 101-105, 98-99 and 95-98 KV
potential differences with their respective electrode-collectors
100a, 100b and 100c. These potential differences in combination
with the potential drop with distance and with variable polymer
solutions are sufficient to induce marked changes upon the
precipitated fibers as follows.
[0133] In electrode systems such as point-plate with abrupt
non-uniform electrical field the intensity drop in the area near
the plate electrode is small, so the relative potential can provide
sufficient accelerating or decelerating effect. Thus, fibers
resulting from pair 100a-102a form a prefilter structure or layer
made of relatively refined and coarse (e.g., 8-10 .mu.fibers),
having a large volume (porosity 0.96), low aerodynamic resistance
and high dust loading capacity (40-50% of total mass).
[0134] Fibers resulting from pair 100b-102b form a high efficiency
particulate air filter made of fine fibers (e.g. 1-3 .mu.m in
diameter), having lower porosity (e.g., about 0.85-0.88), higher
aerodynamic resistance, and a dust loading capacity of about, e.g.,
20-30%.
[0135] Whereas fibers resulting from pair 100c-102c form a support
film or layer for providing the multilayer filter with mechanical
strength and technical properties, such as pleatability,
characterized by coarse fibers (10-20 .mu.m in diameter), porosity
of 0.9-0.92 and dust loading capacity of about 20-30%.
[0136] In fact, this version of the device according to the present
invention combines three individual devices as described herein,
each with somewhat modified properties, into a single device
enabling the continuous manufacturing of three (or more) layered
filter structures, each of the three or more layers featuring
different properties and serving a different purpose. Any suitable
number, e.g., from 2 to 10, of combined devices in envisaged for
different application. In any case, according to this embodiment of
the present invention, each of the layers is completely
precipitated before turning to the precipitation of another layer,
therefore, the properties of the device are selected such that the
efficiency of precipitation is as high as required to complete a
layer's precipitation in each of the stations in a single round
(e.g., by controlling the length of each section or individual
device). The resulting filter 105 is rolled over an additional
rotating shaft 107.
[0137] As shown in FIGS. 9a-b, according to another preferred
embodiment of the present invention ionized air generated by an air
ionizer 110, including an air inlet 112, a grounded net structure
114, an ionizing electrode 116 generating a potential of e.g., 15
KV/cm, and an air outlet 117, as well known in the art, is used to
increase the charging of the liquefied polymer (or fibers) and
thereby to enable more homogenic precipitation thereof on a
precipitation electrode. To this end, a bath 118, in which the
liquefied polymer 119 is held, and from which aliquots thereof are
collected via a rotating wheel 120 featuring triangular protrusions
122, as further detailed above with respect to FIG. 5 (wheel 30) is
contained in a housing 122 supplemented with ionized air via air
ionizer 110. As before, increasing the solvent vapors in the
inter-electrode interval can be effected, for example, by covering
the device and supplementing its atmosphere with solvent vapor
(e.g., via a solvent vapor generator).
[0138] As shown in FIG. 10, according to another preferred
embodiment of the present invention, a mechanism for forming a
surface on the liquefied polymer of sufficiently high curvature to
cause at least one jet of the liquefied polymer to be drawn by an
electrical potential to the precipitation electrode is provided, in
which gas (preferably solvent saturated vapor) bubbles formed in
the liquefied polymer provide the required surfaces.
[0139] To this end, an electrode-collector or bath 130 in which the
liquefied polymer 132 (typically but not obligatory a melted
polymer in this case) is held is provided with a compressed gas
releasing mechanism 134, typically in a form of a pipe 136
supplemented with a plurality of bubbles 137 generating openings
138. When reaching the surface of the liquefied polymer, the
bubbles form a surface on the liquefied polymer of sufficiently
high curvature to cause at least one jet of the liquefied polymer
to be drawn by the electrical potential to the precipitation
electrode.
[0140] As shown in FIGS. 11 and 12a-b and 13, according to yet
another preferred embodiment of the present invention, rotateably
mounted in delivery chamber 146 is a plurality of circular wheels
140. Mounted on rim 148 of wheels 140 are triangular protrusions
150 made of a conductive material that is wetted by the polymer
solution. Tips 152 of protrusions 150 point radially outward from
wheels 140. Wheels 140 are charged negatively by a source 149.
Wheels 140 are provided in a tilted orientation with respect to a
precipitation electrode 160, such that as the polymer solution is
delivered to chamber 146, wheels 140 rotates and each of
protrusions 150 is successively coated with a layer of the polymer
solution, which in turn acquires a negative charge, yet, due to the
tilted configuration, in general, protrusions 150 which are not
dipped in the polymer solution are positioned more evenly apart
from electrode 160, as compared with the vertical configuration,
shown, for example, in FIG. 5. This, in turn, results in more
homogenous fiber precipitation and more homogenous fiber thickness
or diameter. In order to avoid electric field superposition effects
while implementing this configuration of a plurality of wheels 140,
cores 162 of wheels 140 is made of a dielectric substance, whereas
outer rims 148 thereof, including protrusions 150, are made of an
electric substance. In a somewhat different configuration shown in
FIG. 13 the superposition effect is eliminated by selecting an
appropriately non shielding wheels tilt arrangement.
[0141] As shown in FIG. 14, according to yet another preferred
embodiment of the present invention, each of protrusions 150 is
formed with a liquefied polymer collecting cavity 151, for
facilitating the collection of a measured amount of liquefied
polymer. The advantage of this embodiment of the present invention
is that it delays the process of fiber formation, such that a
protrusion will generate fibers only when about to reenter the
liquefied polymer, such that all fibers will be generated from a
similar location and distance with respect to the precipitation
electrode, thereby improved homogeneity is achievable.
[0142] As shown in FIG. 15, according to yet another preferred
embodiment of the present invention, a mechanism for forming a
surface on the liquefied polymer of sufficiently high curvature to
cause at least one jet of the liquefied polymer to be drawn by the
electrical potential to the precipitation electrode includes a
rotateable strap 170 of a conductive material, formed with a
plurality of protrusions 171, rotating around at least two shafts
172 and connected to a source 174. Protrusions 171 are pointed at a
direction of a precipitating electrode 176, such that when strap
170 is rotated through a reservoir 178 including a liquefied
polymer, aliquots thereof accumulate over protrusions 171 to
thereby generate the a surface on the liquefied polymer of
sufficiently high curvature to cause at least one jet of the
liquefied polymer to be drawn to precipitation electrode 176. Since
the field is oriented perpendicular to the direction of rotation of
strap 170, strap 170 can be rotated at higher speeds, resulting in
even more homogenous polymer fiber distribution over electrode 176.
According to a preferred embodiment, just before entering reservoir
178, strap 170 is wiped from remnants of polymer by a wiper 180,
made, for example, of an adsorbing material.
[0143] Thus, the distance between the rotating strap and the
precipitation electrode is constant at all locations, so that the
electric field intensity experienced at each location is similar,
resulting in more uniform fiber thickness. Furthermore, since there
is no centrifugal force in the direction of the precipitation
electrode, it is possible to increase the speed of the rotating
strap to thereby improve mass distribution and productivity.
[0144] Although the invention has been described in conjunction
with specific embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent to
those skilled in the art. Accordingly, it is intended to embrace
all such alternatives, modifications and variations that fall
within the spirit and broad scope of the appended claims.
* * * * *