U.S. patent application number 13/049051 was filed with the patent office on 2011-07-07 for remote fluorination of fibrous filter webs.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Andrew W. Chen, Marvin E. Jones, Seth M. Kirk, William P. Klinzing, Steven J. Pachuta, Patrick J. Sager.
Application Number | 20110162653 13/049051 |
Document ID | / |
Family ID | 42825156 |
Filed Date | 2011-07-07 |
United States Patent
Application |
20110162653 |
Kind Code |
A1 |
Kirk; Seth M. ; et
al. |
July 7, 2011 |
REMOTE FLUORINATION OF FIBROUS FILTER WEBS
Abstract
A method of making a fluorinated fibrous web, which method
includes providing a nonwoven web 22 that contains polymeric
fibers, creating a plasma that contains fluorine atoms at a first
location 14, and contacting the nonwoven web with products from the
plasma at a second location 26 remote from the first location 14.
The method avoids exposure of the web to the plasma and hence
expands the manufacturing processing window. Webs so fluorinated
have a different C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+
ratio when compared to locally fluorinated webs having similar
levels of surface fluorination. The remote fluorinated webs can be
subsequently charged electrically to provide a good performing
electret filter 40 suitable for use in an air purifying respirator
30. Webs fluorinated in accordance with this invention also may
exhibit good performance even after being "aged" at high
temperatures.
Inventors: |
Kirk; Seth M.; (Minneapolis,
MN) ; Jones; Marvin E.; (Grant, MN) ; Pachuta;
Steven J.; (Eagan, MN) ; Chen; Andrew W.;
(Woodbury, MN) ; Klinzing; William P.; (Woodbury,
MN) ; Sager; Patrick J.; (Hastings, MN) |
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
42825156 |
Appl. No.: |
13/049051 |
Filed: |
March 16, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12418290 |
Apr 3, 2009 |
|
|
|
13049051 |
|
|
|
|
Current U.S.
Class: |
128/206.19 ;
428/219; 442/117; 442/60; 55/524 |
Current CPC
Class: |
B01D 39/1623 20130101;
B03C 7/006 20130101; B01D 2239/0414 20130101; B01D 2239/0435
20130101; B01D 2239/0627 20130101; B01D 2239/025 20130101; B03C
3/64 20130101; B01D 2239/0622 20130101; B01D 2239/10 20130101; C23C
16/50 20130101; B03C 3/28 20130101; Y10T 442/2008 20150401; Y10T
442/608 20150401; D06M 10/025 20130101; Y10T 442/2475 20150401;
D06M 2200/00 20130101; B01D 46/0032 20130101; D06M 10/06 20130101;
B03C 3/30 20130101; Y10T 442/68 20150401; B03C 2201/26
20130101 |
Class at
Publication: |
128/206.19 ;
442/117; 442/60; 55/524; 428/219 |
International
Class: |
A62B 7/10 20060101
A62B007/10; D04H 1/56 20060101 D04H001/56; D04H 13/00 20060101
D04H013/00; B01D 39/16 20060101 B01D039/16 |
Claims
1. An electret article that comprises: a nonwoven web that
comprises melt-blown fibers that contain polypropylene and that
have fluorine atoms on the surfaces of the melt-blown fibers in the
web such that (a) an atomic % fluorine is greater than 40%, and (b)
a ToF-SIMS C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio that
is greater than 0.3 and that is above a Remote Fluorination
Threshold RFT1 line for the atomic % fluorine.
2. The electret article of claim 1, wherein the polypropylene
melt-blown fibers are microfibers that have an effective fiber
diameter of 1 to 10 .mu.m and that have a volume resistivity of
10.sup.14 ohm-cm or greater, and wherein the article exhibits a
Q.sub.0 of at least about 2 and a Q9 of at least about 1.5 mm
H.sub.2O.
3. The electret article of claim 2, wherein the ToF-SIMS
C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio is above an
RFT2 line.
4. The electret article of claim 2, wherein the ToF-SIMS
C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio is above an
RFT3 line.
5. A filter that comprises the electret article of claim 1.
6. A respirator that comprises the filter of claim 5.
7. The electret article of claim 1, wherein the article exhibits a
Q9 of at least 1.6/mm H.sub.2O.
8. The electret article of claim 1, wherein the article exhibits a
Q9 of at least 1.8/mm H.sub.2O.
9. The electret article of claim 1, wherein the article has an
atomic % fluorine of greater than 40%.
10. The electret article of claim 1, wherein the article has an
atomic % fluorine of greater than 45%.
11. The electret article of claim 1 having a ToF-SIMS
C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio of at least
about 0.4.
12. The electret article of claim 1 having a basis weight of 20 to
150 g/m.sup.2, a solidity of 3 to 10%, and a thickness of 0.5 to 2
mm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. Ser. No.
12/418,290, filed Apr. 3, 2009, the disclosure of which is
incorporated by reference in its entirety herein.
[0002] The present invention pertains to a new method of
fluorinating a nonwoven fibrous web. The method uses
fluorine-containing plasma products created from a plasma formed at
a location remote from a point where the fluorine atoms are
transferred to the fibrous web. Webs fluorinated in accordance with
the present invention also may be subjected to electrical charging
so that they can subsequently be used as electret filters.
BACKGROUND
[0003] Electret articles--that is, dielectric articles that exhibit
at least quasi-permanent electric charge--are known to exhibit good
filtration properties. The articles have been fashioned in a
variety of constructions, but for air filtration purposes, the
articles commonly take the form of a nonwoven polymeric fibrous
web. An example of such a product is the Filtrete.TM. brand furnace
filter sold by the 3M Company. Nonwoven polymeric electret filters
are also used in personal respiratory protection devices--see, for
example, U.S. Pat. No. 5,307,796 to Kronzer et al., U.S. Pat. No.
5,804,295 to Braun et al., and U.S. Pat. No. 6,216,693 to Rekow et
al.
[0004] A variety of methods have been used to make electrets,
including fiber/electric particle bombardment (U.S. Pat. No.
4,215,682 to Kubik et al.), direct current "DC" corona charging
(see, U.S. Pat. Re. 30,782 and 32,171 to van Turnhout and U.S. Pat.
No. 4,592,815 to Nakao), hydrocharging (see, U.S. Pat. Nos.
5,496,507, 6,119,691, 6,375,886, and 6,783,574 to Angadjivand et
al., U.S. Pat. No. 6,406,657 to Eitzman et al., and U.S. Pat. No.
6,743,464 to Insley et al.), and from exposure to polar liquids
(U.S. Pat. No. 6,454,986 to Eitzman et al.). The electric charge
that is imparted to the dielectric article is effective in
enhancing particle capture.
[0005] During use, electret filters frequently become loaded with
particles and contaminants that interfere with the filtering
capabilities of the electret filter. Liquid aerosols, for example,
particularly oily aerosols, may cause electret filters to lose
their electret-enhanced filtering efficiency (see, U.S. Pat. No.
6,627,563 to Huberty).
[0006] Numerous methods have been developed to counter this
filtering efficiency loss. One method includes adding additional
layers of nonwoven polymeric web to the filter. This approach,
however, can increase the pressure drop across the electret filter
and can add to its weight and bulk. When the electret filter is
used in a personal respiratory protection device, these drawbacks
can be particularly troublesome. Increased pressure drop, for
example, results in increased breathing resistance, making the
respirator more uncomfortable to wear.
[0007] Another method for improving resistance to oily-mist
aerosols, includes adding a melt processable fluorochemical
additive such as a fluorochemical oxazolidinone, a fluorochemical
piperazine, or a perfluorinated alkane to the polymer during the
creation of the polymeric fibrous article--see, for example, U.S.
Pat. Nos. 5,025,052 and 5,099,026 to Crater et al. and U.S. Pat.
Nos. 5,411,576 and 5,472,481 to Jones et al. The fluorochemicals
are melt processable, that is they suffer substantially no
degradation under the melt processing conditions that are used to
form the fibers in the electret web--see also U.S. Pat. No.
5,908,598 to Rousseau et al. In addition to a melt-processing
method, fluorinated electrets also have been made by placing a
polymeric article in an atmosphere that contains a
fluorine-containing species and an inert gas and then applying an
electrical discharge to modify the surface chemistry of the
polymeric article. The electrical discharge may be in the form of a
plasma such as an AC corona discharge. The plasma fluorination
process causes fluorine atoms to become present on the surface of
the polymeric article. The fluorinated polymeric article may be
electrically charged using, for example, the hydrocharging
techniques mentioned above. The plasma fluorination process is
described in a number of U.S. Pat. Nos. to Jones/Lyons et al.:
6,397,458, 6,398,847, 6,409,806, 6,432,175, 6,562,112, 6,660,210,
and 6,808,551. Other publications that disclose fluorination
techniques include: U.S. Pat. Nos. 6,419,871, 6,238,466, 6,214,094,
6,213,122, 5,908,598, 4,557,945, 4,508,781, and 4,264,750; U.S.
Publications US 2003/0134515 A1 and US 2002/0174869 A1; and
International Publication WO 01/07144. U.S. Pat. Nos. 7,244,291 to
Spartz et al. and 7,244,292 to Kirk et al. describe fluorinated
electret articles that exhibit improved thermal stability.
[0008] U.S. Pat. No. 5,147,678 assigned to the University of
Western Ontario describes the use of remote plasmas to modify the
surfaces of polymeric articles. Remote plasma treatments are
different from direct plasma treatments in that the sample surface
is positioned away from the plasma creation region. The remote
location causes the sample to be exposed to only the longest lived
plasma species, which are able to reach the sample, unlike a
broader ranger of species that are present in a direct plasma
process. Remote N.sub.2, H.sub.2, and O.sub.2 plasmas are used to
incorporate nitrogen and oxygen on a polymer surface.
[0009] U.S. Pat. No. 6,197,234, assigned to Conte SA, discloses the
uses of a remote nitrogen plasma to treat polymeric powders or
articles and describes introducing NF.sub.3, either upstream or
downstream of the plasma zone, to increase "the anti-wettability of
a body."
[0010] Inagaki, from Shizuoka University in Japan, has authored
several publications that describe the use of remote plasma sources
to create modified polymer surfaces. One Inagaki article (N.
Inagaki, S. Tasaka, and S. Shimada, J. Appl. Polym. Sci. 79,
808-815 (2001)) describes surface modification of PET film by an
argon plasma, and examines the surface modification created as a
function of the distance from the "argon plasma zone". Reported
surface analysis finds oxygen added to the surface of PET treated
in the plasma, but less oxygen added to the surface treated by the
remote plasma. Another Inagaki article (Y. W. Park, N. Inagaki, J.
Appl. Polym. Sci. v. 93, pp. 1012-1020 (2004)) describes the
surface modification of fluorinated polymer films using remote
plasmas fed with Ar, H.sub.2, and O.sub.2. On three different
fluoropolymer substrates (PTFE, ETFE, and PVDF), these remote
plasma treatments reduced the surface fluorine concentration and
increased the surface oxygen concentration.
[0011] A number of additional patents and publications describe
plasma devices and methods, including remote plasma
fluorination--see, for example, U.S. Pat. Nos. 5,147,678,
6,197,234, 6,477,980, 6,649,222, 6,819,096, 7,005,845, 7,161,112,
7,245,084, 7,445,695, and 7,468,494. U.S. Patent Publication
2007/0028944A1 describes a method of using NF.sub.3 and a remote
plasma for removing surface deposits. International Publication
WO03/051969A2 describes a plasma treatment to fluorinate porous
articles. The following non-patent related publications also
describe remote plasma techniques: Renate Foerch et al. Oxidation
of Polyethylene Surfaces by Remote Plasma Discharge: A Comparison
Study with Alternative Oxidation Methods, JOURNAL OF POLYMER
SCIENCE: PART A: POLYMER CHEMISTRY, v. 28, pp. 193-204 (1990); N.
Inagaki et al., Comparative Studies on Surface Modification of
Poly(ethylene terephthalate) by Remote and Direct Argon Plasmas,
JOURNAL OF APPLIED POLYMER SCIENCE, v. 79, pp. 808-815 (2001); and
Brigitte Mutel, Polymer Functionalization and Thin Film Deposition
by Remote Cold Nitrogen Plasma Process, JOURNAL OF ADHESION SCIENCE
AND TECHNOLOGY, v. 22, pp. 1035-1055 (2008).
SUMMARY OF THE INVENTION
[0012] Although there are a number of documents that describe the
use of remote plasma discharge techniques to fluorinate various
articles, there is no known technique for using a remote plasma to
deliver fluorine atoms to the surface of a nonwoven web that
contains polymeric fibers. Fluorine atoms have been delivered to
fibrous webs using plasmas that are created in the presence of the
nonwoven web. It was not expected, however, that fluorine atoms
could be created at a remote location and then delivered to a
fibrous web through a conduit and a distribution means that would
allow the fluorine atoms to remain in reactive condition long
enough to sufficiently penetrate into the pores of the webs so as
to cause the fluorine atoms to be distributed over the fiber
surfaces within the interior and exterior of the nonwoven web.
Applicants have been able to demonstrate that adequate fluorination
can occur using a remote plasma source to enable the resulting
product, after being electrically charged, to demonstrate good
filtration performance in an oily mist environment. Such good
performance can be achieved even after being thermally aged.
[0013] The present invention provides a new method of making a
fluorinated fibrous web, which method comprises: providing a
nonwoven web that contains polymeric fibers; creating a plasma that
contains fluorine atoms at a first location; and contacting the
nonwoven web with products from the plasma at a second location
remote from the first location so as to allow fluorine atoms to be
transferred to surfaces of the polymeric fibers.
[0014] The present invention also provides a new method of making a
nonwoven fibrous electret, which method comprises: fluorinating a
nonwoven web, which contains polymeric fibers, with plasma products
from a plasma that had been created at a location remote from where
the fluorination occurs; and electrically charging the fluorinated
nonwoven fibrous web.
[0015] The present invention further provides a new electret
article that comprises a nonwoven web that comprises melt-blown
fibers that contain polypropylene and that have fluorine atoms on
the surfaces of the melt-blown fibers in the web such that (a) an
atomic % fluorine is greater than 40%, and (b) a ToF-SIMS
C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio that is greater
than 0.3 and that is above a Remote Fluorination Threshold RFT line
for the atomic % fluorine.
[0016] Conventional web fluorination techniques have exhibited
problems in creating and sustaining a uniform plasma over a large
area. The uniformity of the discharge is sensitive to variations in
the electrode spacing, variations in the process gas flow, and the
variations in the physical characteristics of the product that is
being treated. The sensitivity of the discharge to these factors
increases as the size of the discharge increases. Using the
inventive method, however, a plasma is used to create reactive
fluorine-atom-containing species, but the zone where the plasma is
created is separated from the zone where the web is exposed to
plasma products. This separation of the plasma reaction and the
web-treatment zones allows greater process control, by allowing
independent optimization of the two reaction processes. One
advantage of spatially separating the plasma creation from the web
reaction is that the desired uniformity of the delivered treatment
is influenced mainly by the distribution of the plasma products and
not by electrical distribution of the plasma discharge. When the
plasma is made remote from the location of fluorine atom transfer,
the manufacturing process window can be expanded because the
nonwoven web does not interfere with the plasma creation.
[0017] Another advantage of the present invention is that higher
power levels can be used in the plasma reaction, allowing more
complete disassociation of the fluorine-containing feed gases. In
known methods of plasma fluorinating fibrous webs, the higher power
levels needed for full disassociation of the reactant gas are more
difficult to achieve over a large electrode area. There is no
concern for risking damage to the web when it is not present at the
location where the plasma is made.
[0018] The present invention also is beneficial in that it allows
for NF.sub.3 use rather than fluorine (F.sub.2) gas or
fluorocarbons like C.sub.3F.sub.8. Surprisingly applicants
discovered that nitrogen is not incorporated in the nonwoven web
substantially when NF.sub.3 is used as the starting material for
producing the plasma. The avoidance of fluorinated carbons is
particularly beneficial in that their deposits do not need to be
cleaned off the plasma fluorination equipment or the fluorinated
web.
[0019] Articles of the present invention exhibit a different
C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio when compared
to similar fibrous webs that are locally fluorinated. The
C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio is greater in
nonwoven webs of the present invention for a given atomic %
fluorine. This ratio may be distinguished from known articles using
a Remote Fluorination Threshold line described below. Nonwoven webs
produced in accordance with the present invention will have a
C.sub.3F.sub.4H.sup.+:C.sub.2F.sub.5.sup.+ ratio that is above the
local fluorination line. Articles of the invention may have a ratio
so much greater that it is above a remote fluorination threshold
line RFT1, and even may be above a further remote fluorination line
RFT2, and even above a still further remote fluorination threshold
line RFT3. Thus, the C.sub.3F.sub.4H.sup.+:C.sub.2F.sub.5.sup.+
ratio that is manifested under a ToF-SIMS analysis is remarkably
distinct from the ratio provided in local fluorination,
particularly above atomic % fluorine levels of 40% or greater, more
particularly above about 42% atomic fluorine. The inventive
nonwoven webs that exhibit such fluorine content have been able to
demonstrate good performance after being aged under accelerated
conditions.
GLOSSARY
[0020] "comprises (or comprising)" means its definition as is
standard in patent terminology, being an open-ended term that is
generally synonymous with "includes", "having", or "containing".
Although "comprises", "includes", "having", and "containing" and
variations thereof are commonly-used, open-ended terms, this
invention also may be suitably described using narrower terms such
as "consists essentially of", which is semi open-ended term in that
it excludes only those things or elements that would have a
deleterious effect on the fluorinated article, the fluorinated
electret, or its method of being produced;
[0021] "electret" means a dielectric article that exhibits at least
quasi-permanent electric charge;
[0022] "electric charge" means that there is charge separation;
[0023] "fluorine atoms" means atomic fluorine and/or any molecular
fragment or molecule that contains fluorine;
[0024] "fluorinated" or "fluorinating" means placing fluorine atoms
on the surface of an article;
[0025] "manifold" means a device or combination of parts that
distributes a fluid;
[0026] "nonwoven" means a structure or portion of a structure in
which the fibers or other structural components are held together
by a means other than weaving;
[0027] "plasma" means an ionized gas;
[0028] "plasma products" or "products of the plasma" mean molecular
fragments (and combinations thereof) of the molecules subjected to
the plasma;
[0029] "polymer" means a material that contains repeating chemical
units, regularly or irregularly arranged;
[0030] "polymeric" and "plastic" each mean a material that mainly
includes one or more polymers and may contain other ingredients as
well;
[0031] "remote from" means not at the same place--that is,
different from location-wise;
[0032] "web" means a structure that is significantly larger in two
dimensions than in a third and that is air permeable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a schematic view of a method of making a nonwoven
fibrous fluorinated electret web.
[0034] FIG. 2 is a schematic illustration of an apparatus 10 for
making a fluorinated nonwoven fibrous fluorinated web 22 in
accordance with the present invention.
[0035] FIG. 3 is a front perspective view of a disposable
respiratory mask 30 that may use electret filter media in
accordance with the present invention.
[0036] FIG. 4 is a cross-section of the mask body 32 illustrated in
FIG. 3, showing a fluorinated fibrous electret filter layer 40.
[0037] FIG. 5 is a partial cross-section of an apparatus used for
making a nonwoven fibrous web 55 suitable for use in conjunction
with the present invention.
[0038] FIG. 6 illustrates ToF-SIMS spectra of locally and remotely
fluorinated polypropylene blown microfiber (BMF) webs.
[0039] FIG. 7 is a graph of the SIMS C.sub.3F.sub.4H.sup.+ to
C.sub.2F.sub.5.sup.+ ratio to atomic % fluorine.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0040] FIG. 1 illustrates an example of steps that may be used to
make a fibrous nonwoven web that has fluorine atoms on the surface
of the fibers. To create such a product, a plasma may be created at
a location remote from the fluorine atom delivery chamber. The
plasma is used to create a reactive fluorine-atom containing
species. As illustrated, the zone where the plasma is created is
separate from the zone where the nonwoven web is treated with
plasma products. The separation of the plasma reaction from the web
treatment zone allows for greater process control. There can be
independent optimization of the reactant generation and web
treatment steps. Reactive species generation and reaction can be
independently optimized. The fluorine delivery chamber also enables
improvements in the ability to develop a robustly large area for
uniform treatment of nonwoven webs by the fluorinated plasma
products. The plasma may be created using known remote plasma
sources including Litmas.RTM. machines from Advanced Energy
Industries, Inc. of Fort Collins, Colorado. Examples of suitable
remote plasma devices from Advanced Energy include the Litmas.RTM.
RPS1501 and 3001 remote plasma source products. These products are
suited to deliver reactive gas species and thus may be used to
create fluorine atom-containing plasma products in accordance with
the present invention. Other reliable sources for creating fluorine
gas species include Astron.RTM. plasma machines from MKS
Instruments Inc. Once the plasma is created, the plasma products
can be delivered via a conduit to the treatment chamber. The
treatment chamber preferably is an evacuated chamber into which the
nonwoven web-to-be-treated is placed. The plasma products that
contain the reactive species enter the chamber whereby rapid
reaction occurs with the nonwoven web. At the point where
fluorination occurs, there is no longer a plasma glow--that is,
there are molecular fragments and combinations of such fragments,
which resulted from the plasma but not the plasma itself. The
fluorinated web can then be removed from the chamber and
subsequently subjected to an electric charging step to create a
fluorinated electret nonwoven fibrous web. Electric charging of the
nonwoven fibrous web may be carried out using a number of known
techniques described below.
[0041] FIG. 2 schematically illustrates an example of a plasma
fluorination system 10 that comprises the treatment chamber 12 and
a plasma source 14. Within the treatment chamber 12 is a means 16
for distributing the plasma products, preferably evenly and
uniformly across the face of the web. Such a means 16 may include a
spray bar or spray head, or other manifold such as a pipe that
contains a series of orifices 18 or a slot(s), a showerhead plate,
a two or slightly three-dimension array of openings, or any other
suitable apparatus that can cause the plasma products to be
adequately delivered or evenly distributed across the web face.
Typically, the distribution means is spaced about 2 to 30
centimeters from the web. The distribution means traverses the web
normal to the web transport direction. The plasma products may be
delivered to the chamber 12 at about, for example, 3 cm.sup.3 per
minute per centimeter (cc/min/cm) of manifold length to about 400
cc/min/cm. The plasma products are delivered from the plasma source
14 to the distribution means 16 via a conduit 20. The conduit 20
may be a simple pipe or a series of pipes and elbows that
effectively enable the plasma products to be delivered to the
chamber interior 12. The distribution means may be fashioned to
have a continuous array of openings, that is a slot, up to about
one opening every 8 cm of closed area between openings. The
delivery means may be fashioned to have about 0.1 to 1.5 units of
inlet area to the total area of the outlet(s). The distribution
means may be made from a variety of materials that are suitable to
operate in the plasma product environment. Such materials typically
are thermally stable and noncorrosive and may be made from
aluminum, stainless steel, nickel, fluoropolymers, or other
materials suitable to survive the environment and permit the
delivery of plasma products to the reactant chamber. Preferably a
vacuum system is associated with the chamber 12 to remove molecules
from the chamber so that the plasma products react most effectively
with the nonwoven web 22. Suction manifolds may be disposed at one
or more locations within the vacuum chamber 12 to further manage
plasma product flow within the chamber 12. To direct flow through
the web, the suction manifold may be disposed in the vicinity of
the exposed portion of the web, on a side opposing the plasma
product distribution manifold. Web fluorination 22 may be carried
out as a bulk operation by delivering a roll 24 of the web to the
chamber interior 26. The web 22 can be unwound from the first roll
24 onto a second roll 28 while the plasma products are being
delivered from the plasma source 14 to the chamber interior 26 via
the conduit 20 and distribution means 16. The exposed web area
subject to plasma product impingement between the rolls may be
about 0.02 to 10 square meters (m.sup.2), more typically 0.1 to 0.5
m.sup.2. The web speed may be about 1 to 100 meters/minute (m/min),
more typically 2 to 30 m/min. Once the complete roll 24 has been
unwound from the fresh roll 24 to the second roll 28 and has been
adequately exposed to the plasma products during the process of web
transport, plasma products delivery to the fluorination chamber 12
may be ceased so that the fluorinated web can be removed from the
chamber 12. A new untreated web may then be introduced into the
chamber 12 and may be treated in the same manner. Using the method
of the present invention, the full width of a nonwoven web can be
fluorinated. Webs that are wider than 1 to 2 meters and even
greater than 4 meters may be fluorinated in accordance with the
present invention. Manifolds used to deliver plasma products to the
delivery chamber may have a length similar to the web width.
Manifolds used in connection with the present invention may be
fashioned to provide a substantially uniform distribution from a
pipe that has a linear array of ports. The manifold may be
constructed with specific geometric properties, including
port-shape orientation and cross-sectional area of the ports and
inlet cross-sectional area. See, for example, U.S. Patent
Publication 2006-0265169A1, entitled Manifolds for Delivering
Fluids Having a Desired Mass Flow Profile and Methods for Designing
the Same. For filtering applications, the webs typically are at
least 0.25 millimeters (mm) thick and even up to 5 mm or more
thick. Very large rolls may be placed within the fluorine delivery
chamber. Such rolls may have a diameter of at least about 0.5
meters, and even greater than 3 meters. The total volume of the
fluorination chamber may be about 1 to 60 cubic meters (m.sup.3),
more typically about 4 to 30 m.sup.3. The fluorine delivery chamber
also may include one or more windows so that the progress of
fluorination can be visually examined. This may be accomplished by
visually examining the transfer of web 22 from the first roll 24
onto the second roll 28. A suitable alarm or other means may also
be used to inform an operator of completion of the fluorination
process. The vacuum chamber may further include one or more doors
that allow the roll 28 to be removed from the vacuum chamber and to
allow another roll 24 to be introduced into the chamber.
[0042] Fluorinated electrets are suitable for many filtration
applications. Some filters, however, require enhanced thermal
stability to meet product specifications, for example, military
specifications and NIOSH requirements--see NIOSH, Statement of
Standard for Chemical, Biological, Radiological, and Nuclear (CBRN)
Air-Purifying Escape Respirator, Attachment A, Sep. 30, 2003 and
NIOSH, Statement of Standard for Chemical, Biological,
Radiological, and Nuclear (CBRN) Full Facepiece Air Purifying
Respirator (APR), Appendix A, Apr. 4, 2003.
[0043] The filtering performance of an electret article is commonly
characterized using a parameter that is referred to in the art as
"quality factor" or "Q value" or "QF". QF characterizes filtration
performance as a blend of the particle penetration and pressure
drop parameters. As indicated above, some filters require enhanced
thermal stability to meet filtration product specifications.
Applications exist where the electret filter media should be
resistant to charge degradation at high temperatures. Extraordinary
quality factor data can be achieved when testing the inventive
electret articles after an accelerated high-temperature aging
exposure. Specifically, extraordinary quality factor data can be
achieved after 9 hours of storage at 100.degree. C. The QF that
results from this test is referred to as "Q9". Nonwoven webs
fluorinated remotely in accordance with the present invention can
exhibit increased thermal stability--as measured by the Q9 value.
The inventive electret articles thus can maintain good filtration
efficiency despite being "aged" at high temperatures for an
extended time period.
[0044] The higher the Q9 value, at a given flow rate, the better
the filtering performance of the electret after high-temperature
storage. Electrets of the present invention can have a Q9 value of
at least about 1.5/mmH.sub.2O, preferably at least about
1.6/mmH.sub.2O, more preferably at least about 1.8/mmH.sub.2O. Q9
values may be determined according to the test set forth below.
[0045] The fluorination process may be performed at less than
atmospheric pressure, or under "reduced pressure" and also possibly
at atmospheric pressure. The fluorination process is preferably
performed in a controlled atmosphere to prevent contaminants from
interfering with the addition of fluorine atoms to the surface of
the article. The term "controlled" means the apparatus has the
ability to control the composition of the atmosphere in the chamber
where fluorination occurs. The atmosphere preferably is
substantially free of oxygen and other undesired components. The
atmosphere typically contains less than 1% oxygen or other
undesired components, preferably less than 0.1%, by volume.
[0046] The fluorine containing species present in the atmosphere
can be derived from fluorinated compounds that are gases at room
temperature, that become gases when heated, or that are capable of
being vaporized. Examples of useful sources of fluorine-containing
species include fluorine atoms, elemental fluorine, inorganic
fluorides such as fluorinated sulfur (e.g., SF.sub.6), fluorinated
nitrogen (e.g., NF.sub.3), and PF.sub.3, BF.sub.3, SiF.sub.4, and
combinations thereof. The atmosphere of fluorine containing species
can also include inert diluent gases such as the noble gases
helium, argon, etc, and combinations thereof. Nitrogen can also be
used as a diluent.
[0047] The electrical discharge that is created during plasma
formation is capable of creating a variety of fluorine containing
species. The plasma may be in the form of, e.g., glow discharge
plasma, corona plasma, silent discharge plasma (also referred to as
dielectric barrier discharge plasma and alternating current ("AC")
corona discharge), and hybrid plasma, e.g., glow discharge plasma
at atmospheric pressure, and pseudo glow discharge--see U.S. Pat.
Nos. 6,808,551, 6,660,210, 6,562,112, 6,432,175, 6,409,806,
6,398,847 and 6,397,458 to Jones/Lyons et al. Preferably, the
plasma is an AC plasma at reduced pressure. "Reduced pressure"
means pressure less than 700 Pa, preferably less than 140 Pa.
Examples of useful surface modifying electrical discharge processes
are described in U.S. Pat. Nos. 5,244,780, 4,828,871, and 4,844,979
to Strobel et al.
[0048] The fluorine surface concentration may be ascertained using
electron spectroscopy for chemical analysis (ESCA), also known as
X-ray photoelectron spectroscopy or XPS. The surface of the
inventive electret articles exhibits greater than about 40 atomic %
fluorine, more typically greater than about 45 atomic % fluorine
when analyzed by XPS. XPS analyzes the elemental composition of the
outermost surface (i.e., approximately 30 to 100 .ANG.) of a
specimen.
[0049] The C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio is
measured using Time of Flight Secondary Ion Mass Spectrometry
(ToF-SIMS) as described below. The ratio of C.sub.3F.sub.4H.sup.+
to C.sub.2F.sub.5 for the inventive articles is at least about 0.3,
and more typically is at least about 0.4. This ratio also is above
the Remote Fluorination Threshold (RFT1) line illustrated in FIG.
7. The RFT1 line is defined by the equation (1) set forth in the
Examples below. The C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+
ratio may also be above the RFT2 line, and more preferably above
the RFT3 line. The RFT2 and RFT3 lines are defined by the equations
(2) and (3), respectively, in the Examples section below.
[0050] Fibrous webs suitable for use in this invention can be made
from a variety of techniques, including air laid processes, wet
laid processes, hydro-entanglement, spun-bond processes, and melt
blown processes such as described in Van A. Wente, Superfine
Thermoplastic Fibers, 48 INDUS. ENGN. CHEM. 1342-46 and in Report
No. 4364 of the Naval Research Laboratories, published May 25,
1954, entitled Manufacture of Super Fine Organic Fibers by Van A.
Wente et al. The fibrous webs can be made using combinations of
these techniques and combinations of such fibers. Microfibers,
particularly meltblown microfibers, are particularly suitable for
use in fibrous webs that are used as filters. As used in this
document, "microfiber" means fiber(s) that have an effective
diameter of about 35 micrometers or less. Effective fiber diameter
can be calculated using equation number 12 in Davies, C. N., The
Separation of Airborne Dust and Particles, INST. MECH. ENGN.,
LONDON PROC. 1B (1952). For filtering applications, the microfibers
typically have an effective fiber diameter of less than 20
micrometers, more typically, about 1 to about 10 micrometers.
Fibers made from fibrillated films may also be used--see, for
example, U.S. Pat. RE30,782, RE32,171, 3,998,916 and 4,178,157 to
Van Turnout. Nonwoven webs that are made by the process of the
present invention may exhibit quality factors QF that exceed 2,
2.1, 2.2, and 2.3.
[0051] Staple fibers also may be combined with the microfibers to
improve web loft, that is, to reduce its density. Reducing web
density can lower the pressure drop across the web, making it
easier for air to pass through the filter. Lower pressure drops are
particularly desirable in personal respiratory protection devices
because they make the respirator more comfortable to wear. When the
pressure drop is lower, less energy is needed to draw air through
the filter. A respirator wearer who dons a negative pressure
mask--that is a respirator that requires negative pressure from the
wearer's lungs to draw air through the filter--does not have to
work as hard to breathe filtered air. Lower energy requirements
also can be beneficial in powered filtering systems to reduce costs
associated with powering the fan and to extend the service life of
a battery in a battery powered system. In a typical nonwoven
fibrous filter, no more than about 90 weight percent staple fibers
are present, more typically no more than about 70 weight percent.
Often, the remainder of the fibers are microfibers. Examples of
webs that contain staple fibers are disclosed in U.S. Pat. No.
4,118,531 to Hauser.
[0052] Active particulate also may be included in webs for various
purposes, including sorbent purposes, catalytic purposes, and
others. U.S. Pat. No. 5,696,199 to Senkus et al., for example,
describes various types of active particulate that may be suitable.
Active particulate that has sorptive properties--such as activated
carbon or alumina--may be included in the web to remove organic
vapors during filtration operations. The active particulate may be
present in the web at amounts up to about 95 volume percent.
Examples of particle-loaded nonwoven webs are described, for
example, in U.S. Pat. Nos. 3,971,373 to Braun, 4,100,324 to
Anderson, and 4,429,001 to Kolpin et al.
[0053] Polymers that may be suitable for use in producing nonwoven
fibrous webs suitable for electrets include thermoplastic organic
nonconductive polymers. These polymers are generally capable of
retaining a high quantity of trapped charge and are capable of
being processed into fibers, such as through a melt-blowing
apparatus or a spun-bonding apparatus. The term "organic" means
that the backbone of the polymer comprises carbon atoms. Preferred
polymers include polyolefins, such as polypropylene,
poly-4-methyl-1-pentene, blends or copolymers containing one or
more of these polymers, and combinations of these polymers. Other
polymers may include polyethylene, other polyolefins,
perfluoropolymers, polyvinylchlorides, polystyrenes,
polycarbonates, polyethylene terephthalate, other polyesters, such
as polylactide, naturally occurring polymers, and combinations of
these polymers and optionally other nonconductive polymers.
[0054] The fibrous electrets used in connection with the present
invention also may be extruded or otherwise formed to have multiple
polymer components--see U.S. Pat. No. 4,729,371 to Krueger and
Dyrud and U.S. Pat. Nos. 4,795,668, and 4,547,420 to Krueger and
Meyer. The different polymer components may be arranged
concentrically or longitudinally along the length of the fiber to
create, for example, a bicomponent fiber. The fibers may be
arranged to form a "macroscopically homogeneous" web, namely, a web
that is made from fibers that each have the same general
composition.
[0055] Fibers made from polymeric materials also may contain other
suitable additives. Possible additives include thermally stable
organic triazine compounds or oligomers, which compounds or
oligomers contain at least one nitrogen atom in addition to those
in the triazine ring--see U.S. Pat. Nos. 6,268,495, 5,976,208,
5,968,635, 5,919,847, and 5,908,598 to Rousseau et al. Another
additive known to enhance electrets charged by jets of water is
Chimassorb.TM. 944 LF
(poly[[6-(1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][[(2,2,6,6--
tetramethyl-4-piperidyl)imino]hexamethylene
[(2,2,6,6-tetramethyl-4-piperidyl)imino]]), available from Ciba
Specialty Chemicals, Inc. The additives may be N-substituted amino
aromatic compounds, particularly tri-amino substituted compounds
that are, for example, of the formulas (1) or (2) set forth
below:
##STR00001##
where Ar is a trivalent aromatic group substituted by zero to 3
nitrogen atoms, n is an integer of 1 to 20, and each R
independently may be a group that has less than about 20
non-hydrogen non-metal atoms. Each R, for example, may
independently be: hydrogen; halogen, for example, fluorine;
hydroxyl; alkyl having up to 20 carbon atoms, for example methyl,
ethyl, propyl, butyl, etc; halogen substituted alkyls such as
trifluoromethyl; alkoxy having 1 to 20 carbon atoms such as
methoxy; ester having 2 to 20 carbon atoms such as methoxycarbonyl;
substituted amines that contain 2 to 20 carbon atoms such as
methylamino; and nitro. Further examples of charge-enhancing
additives are provided in U.S. Patent Application Ser. No.
61/058,029, entitled Charge-Enhancing Additives For Electrets and
U.S. Patent Application Ser. No. 61/058,041, entitled Electret Webs
With Charging-Enhancing Additives. Typically, the additives are
present in the polymeric article at about 0.1 to 5% by weight, more
typically at about 0.25 to 2% by weight.
[0056] Other additives include light stabilizers, primary and
secondary antioxidants, metal deactivators, hindered amines,
hindered phenols, fatty acid metal salts, triester phosphites,
phosphoric acid salts, fluorine-containing compounds, melamines,
and the additives mentioned in U.S. Pat. No. 7,390,351 to Leir et
al., U.S. Pat. No. 5,057,710 to Nishiura et al., Japanese
Publication No. 2002-212439, Japanese Publication No.
2005-131485.
[0057] Fibers that contain additives can be quenched after shaping
a heated molten blend of the polymer and additive--followed by
annealing and charging steps--to create an electret article.
Enhanced filtration performance may be imparted to the article by
making the electret in this manner--see U.S. Pat. No. 6,068,799 to
Rousseau et al. The electret articles also can be made to have a
low level of extractable hydrocarbon (<3.0 weight %) to improve
loading performance--see U.S. Pat. No. 6,776,951 to Rousseau et
al.
[0058] The polymeric material that is used to produce a fibrous
electret according to the inventive method may have a volume
resistivity of 10.sup.14 ohmcm or greater at room temperature. The
volume resistivity may also be about 10.sup.16 ohmcm or greater.
Resistivity of the polymeric fiber-forming material can be measured
according to standardized test ASTM D 257-93. The polymeric
fiber-forming material used to make the fibrous electrets such as
the melt blown fibers also should be substantially free from
components such as antistatic agents, which agents could increase
the electrical conductivity or otherwise interfere with the ability
of the electret article to accept and hold electrostatic
charges.
[0059] Electrets that comprise nonwoven polymeric fibrous webs for
respiratory filters typically have a "basis weight" of about 2 to
500 grams per square meter (g/m.sup.2), more typically about 20 to
150 g/m.sup.2. The basis weight is the mass per unit area of filter
web. The thickness of such nonwoven polymeric fibrous web is
typically about 0.25 to 20 millimeters (mm), more typically about
0.5 to 2 mm. Multiple layers of fibrous electret webs are commonly
used in filter elements. The solidity of the fibrous electret web
typically is about 1 to 25%, more typically about 3 to 10%.
Solidity is a unitless parameter that defines the solids fraction
in the article.
[0060] The inventive electret articles may be used as filters in
filtering face masks or other air-filtering personal respiratory
protection devices, which are adapted to cover at least the nose
and mouth of a wearer. The inventive electret articles also can be
used in filter cartridges for half- and full-face respirators.
[0061] FIG. 3 illustrates an example of a filtering face mask 30
that may be constructed to contain an electrically-charged nonwoven
web that is produced according to the present invention. The
generally cup-shaped body portion 32 may be molded into a shape
that fits over the nose and mouth of the wearer. The body portion
32 is porous so that inhaled air can pass through it. The electret
filter medium is disposed in the mask body 32 (typically over
substantially the whole surface area) to remove contaminants from
the inhaled air. A conformable nose clip 33 may be placed on the
mask body to assist in maintaining a snug fit over the wearer's
nose. The nose clip can be an "M-shaped" clip as described in U.S.
Pat. Des. 412,573 and 5,558,089 to Castiglione. A strap or harness
system 34 may be provided to support the mask body 32 on the
wearer's face. Although a dual strap system is illustrated in FIG.
1, the harness 34 may employ only one strap 36, and it may come in
a variety of other configurations--see, for example, U.S. Pat. Nos.
4,827,924 to Japuntich et al., 5,237,986 to Seppalla et al.,
5,464,010 to Byram, 6,095,143 to Dyrud et al., and 6,332,465 to Xue
et al. An exhalation valve can be mounted to the mask body to
rapidly purge exhaled air from the mask interior--see U.S. Pat.
Nos. 5,325,892, 5,509,436, 6,843,248, 6,854,463, 7,117,868, and
7,311,104 to Japuntich et al.; U.S. Pat. RE37,974 to Bowers; and
U.S. Pat. Nos. 7,013,895, 7,028,689, and 7,188,622 to Martin et
al.
[0062] FIG. 4 illustrates an example of a cross-section of a mask
body 32. Mask body 32 may have a plurality of layers, as indicated
by numerals 38, 40, and 42. The electret filter media may be
supported by other layers, such as shaping layers that are made
from thermally bonded fibers, such as bicomponent fibers that have
an outer thermoplastic component that enables the fibers to bond to
other fibers at points of fiber intersection. Layer 38 can be an
outer shaping layer, layer 40 may be a filtration layer, and layer
42 may be an inner shaping layer. Shaping layers 38 and 42 support
filtration layer 40 and provide shape to mask body 32. Although the
term "shaping layers" is used in this description, shaping layers
also may have other functions, which in the case of an outermost
layer may even be a primary function, such as protection of the
filtration layer and prefiltration of a gaseous stream. Also,
although the term "layer" is used, one layer may in fact comprise
several sublayers, assembled to obtain desired thickness or weight.
In some embodiments only one, generally inner, shaping layer is
included in a face mask, but shaping may be accomplished more
durably and conveniently if two shaping layers are used, for
example, one on each side of the filtration layer as shown in FIG.
2. Shaping layer examples are described in the following patents:
U.S. Pat. Nos. 4,536,440 to Berg, 4,807,619 to Dyrud et al.,
5,307,796 to Kronzer et al., 5,374,458 to Burgio, and 4,850,347 to
Skov. Although the illustrated mask body shown in FIGS. 1 and 2 has
a generally round, cup-shaped configuration, the mask body may have
other shapes--see for example U.S. Pat. No. 4,883,547 to Japuntich.
Further, the mask body may comprise an inner and/or outer cover web
to provide a smooth and comfortable contact with the wearer's face
and/or to preclude fibers from the shaping and filtration layers
from coming loose from the mask body--see U.S. Pat. No. 6,041,782
to Angadjivand et al. The respiratory mask also may have a
flat-folded mask body (rather than a molded mask body)--see, for
example, U.S. Pat. Nos. 6,394,090 to Chen and 6,484,722 to Bostock
et al.
[0063] Nonwoven melt-blown microfiber webs useful in the present
invention may be prepared using, for example, an apparatus as shown
in FIG. 5. Such an apparatus includes a die 50 that has an
extrusion chamber 51 through which liquefied fiber-forming material
is advanced. Die orifices 52 may be arranged in line across the
forward end of the die and through which the fiber-forming material
is extruded. A gas, typically heated air, may be forced at high
velocity through cooperating gas orifices 53. The high velocity
gaseous stream draws out and attenuates the extruded fiber-forming
material, whereupon the fiber-forming material solidifies as
microfibers during travel to a collector 54 to form web 55.
[0064] When staple fibers are present in the web, they may be
introduced through use of a lickerin roll 56 disposed above the
microfiber blowing apparatus as shown in FIG. 5. A web 57 of staple
fibers, typically a loose, nonwoven web such as prepared on a
garnet or RANDO-WEBBER apparatus, is propelled along table 58 under
drive roll 59 where the leading edge engages against the lickerin
roll 56. The lickerin roll 56 picks off fibers from the leading
edge of web 57, separating the fibers from one another. The picked
fibers are conveyed in an air stream through an inclined trough or
duct 60 and into the stream of blown microfibers where they become
mixed with the blown microfibers. When particulate matter is to be
introduced into the web it may be added using a loading mechanism
similar to duct 60. In addition to melt-blowing techniques, fibrous
webs suitable for filtration applications can be made using other
methods such as spun-bond manufacturing processes. Further,
nanofibers could be used in the filter media in connection with the
present invention--see, for example, U.S. Patent Application
61/017,994 to Eaton et al., entitled Fluid Filtration Articles And
Methods Of Making And Using Same.
[0065] The electret charge can be imparted to the polymeric
articles using various known (or later developed) apparatus and
methods including hydrocharging systems. Documents that describe
known hydrocharging systems include U.S. Pat. Nos. 5,496,507,
6119,691, 6,375,886, and 6,783,574 to Angadjivand et al., U.S. Pat.
No. 6,406,657 to Eitzman et al., and U.S. Pat. No. 6,743,464 to
Insley et al.
[0066] Hydrocharging methods deposit both positive and negative
charge onto the fibers such that the positive and negative charge
is randomly dispersed throughout the web. Random charge dispersal
tends to produce an unpolarized web. Thus, a nonwoven fibrous
electret web produced by charging with a polar liquid like water
may be substantially unpolarized in a plane normal to the plane of
the web. Fibers that have been charged in this manner ideally
exhibit the charge configuration shown in FIGS. 5C of U.S. Pat. No.
6,119,691 to Angadjivand et al. If the fibrous web also is
subjected to a corona treatment operation, it would exhibit a
charge configuration similar to the configuration shown in FIG. 5B
of that patent. A web, formed from fibers charged solely using
hydrocharging, typically has unpolarized trapped charge throughout
the volume of the web. "Substantially unpolarized trapped charge"
refers to a fibrous electret web that exhibits less than 1
.mu.C/m.sup.2 of detectable discharge current using
thermally-simulated discharge current (TSDC) analysis, where the
denominator is the electrode surface area. This charge
configuration can be shown by subjecting the web to TSDC. One
example of a useful hydrocharging process includes impinging jets
of water or a stream of water droplets onto the article at a
pressure and for a period sufficient to impart a filtration
enhancing electret charge to the web, and then drying the
article--see U.S. Pat. No. 5,496,507 to Angadjivand et al. The
pressure necessary to optimize the filtration enhancing electret
charge imparted to the article will vary depending on the type of
sprayer used, the type of polymer from which the article is formed,
the type and concentration of additives to the polymer, and the
thickness and density of the article. Pressures in the range of
about 10 to about 500 psi (69 to 3450 kPa) are generally suitable.
The jets of water or stream of water droplets can be provided by
any suitable spray device.
[0067] Suitable spray means for use in the method of the present
invention include nebulizers where the aqueous liquid, provided
through fluid line, and pressurized air, provided through air line,
are supplied to a nozzle to provide a spray mist to impact the
article-to-be-charged and pump action sprayers where a pump handle
forces liquid provided by the supply means through the nozzle to
provide a spray mist. Further description of this method of
providing water contact is provided in U.S. Pat. No. 6,119,691 to
Angadjivand et al. Alternatively, the article to be charged can be
contacted with aqueous liquid using a variety of other methods,
including those described in U.S. Pat. Nos. 6,406,657 to Eitzman et
al., 6,375,886 to Angadjivand et al., 6,454,986 to Eitzman et al.,
and 6,824,718 to Eitzman et al.
[0068] Hydrocharging may be carried out by contacting the web with
an aqueous liquid sufficient to provide the web with filtration
enhancing electret charge. The pH and conductivity of the aqueous
liquid may be selected based on the zeta potential of the
article--see U.S. patent application Ser. No. 12/131,770 to
Sebastian et al. The aqueous liquid contact may be achieved by
spraying, soaking, condensing, etc., the aqueous liquid on the
polymeric fibrous web to be charged. If a sprayer is used, the
pressure necessary to achieve optimum results may vary depending on
the type of sprayer used, the type of polymer from which the
article is formed, the thickness and density of the article, and
whether pretreatment such as corona discharge treatment was carried
out before hydrocharging. Generally, pressures in the range of
about 10 to 500 psi (69 to 3450 kPa) are suitable. The aqueous
liquid may be selected to have a conductivity of about 5 to 9,000
microS/cm, when the zeta potential of the article is -7.5 mV or
less. When the zeta potential is greater than -7.5 mV, then the
contacting liquid may have a conductivity of about 5 to 5,500
microS/cm. Under either situation, the conductivity typically would
be about 7 to 3,000 microS/cm, and still more typically about 10 to
1,000 microS/cm. Distilled or deionized water is preferable to tap
water. The aqueous liquid may be purified water, made through, for
example, distillation, reverse osmosis, or deionization rather than
simply tap water. The aqueous liquid may contain at least 40%, more
typically 60%, and still more typically 80% water by volume.
Preferably, the aqueous liquid will contain essentially 100% water
by volume.
[0069] Corona pretreatment or post-treatment also may be used to
charge the webs alone or in conjunction with the hydrocharging
systems described above--see U.S. Pat. RE 30,782; 31,285 and 32,171
to van Turnhout, and U.S. Pat. Nos. 4,375,718 to Wadsworth et al.
and 5,401,446 to Tsai et al., U.S. Pat. No. 4,588,537 to Klasse et
al., and U.S. Pat. No. 4,592,815 to Nakao.
EXAMPLES
Test Methods
Quality Factor (QF) Testing Method
[0070] The samples were tested for % DOP aerosol penetration (%
Pen) and pressure drop (.DELTA.P), and the Quality Factor (QF) was
calculated. The filtration performance (% Pen and .DELTA.P) of the
nonwoven microfiber webs were evaluated using an Automated Filter
Tester AFT Model 8130 (available from TSI, Inc., St. Paul, Minn.)
using dioctylphthalate (DOP) as the challenge aerosol. The DOP
aerosol is nominally a monodisperse 0.3 micrometer mass median
diameter having an upstream concentration of 70-125 mg/m.sup.3. The
aerosol was forced through a sample of filter medium at a
calibrated flow rate of 42.5 liters/minute (face velocity of 6.9
cm/s) with the aerosol TSI Model 8113 Aerosol Neutralizer turned
off. The total testing time was 23 seconds (rise time of 15
seconds, sample time of 4 seconds, and purge time of 4 seconds).
Simultaneously with % Pen, the pressure drop (.DELTA.P in mm of
water) across the filter was measured by the instrument. The
concentration of DOP aerosol was measured by light scattering both
upstream and downstream of the filter media using calibrated
photometers. The DOP % Pen is defined as: % Pen=100.times.(DOP
concentration downstream/DOP concentration upstream). For each
material, typically 7 to 9 separate measurements were made at
different locations on the BMF web, and the results were
averaged.
[0071] The % Pen and .DELTA.P were used to calculate a QF by the
following formula:
QF=-In(% Pen/100)/.DELTA.P,
where In stands for the natural logarithm. A higher QF value
indicates better filtration performance and decreased QF values
effectively correlate with decreased filtration performance.
X-Ray Discharge Test
[0072] The Quality Factor and % Penetration of sample webs to be
tested were determined before exposure to X-ray radiation using the
test method described above. The Initial Quality Factor is
designated as "QF.sub.0". The sample web was exposed on each side
to x-rays using the system described below, ensuring that the
entire sample was uniformly exposed to the x-ray radiation. After
x-ray exposure, the filter medium sample was tested again to
measure its filter performance (QF and % Pen). The procedure was
repeated after 5 minutes of x-ray exposure, after 30 minutes of
x-ray exposure, and after 60 minutes of x-ray exposure. The %
Penetration Ratio (% Pen Ratio) is also reported. The % Pen Ratio
was calculated from the % Pen at 0 minutes and 60 minutes using the
equation where In stands for the natural logarithm:
% Pen Ratio = ln ( % Pen ( 0 min ) / 100 ) ln ( % Pen ( 60 min ) /
100 ) .times. 100 % . ##EQU00001##
[0073] X-ray exposure was carried out using a Baltograph 100/15 CP
(Balteau Electric Corp., Stamford, Conn.) X-ray exposure system
consisting of a constant potential end grounded generator rated at
100 KV at 10 mA with a beryllium window (0.75 mm inherent
filtration) with an output of up to 960 Roentgen/min at 50 cm from
the focal spot of 1.5 mm.times.1.5 mm. The voltage was set to 80
KV, with a corresponding current of 8 mA. A sample holder was set
up at an approximate distance of 57.2 centimeters (22.5 inches)
from the focal spot to produce an exposure of about 580
Roentgen/min.
Q9 Aging Test (100.degree. C. for 9 Hours)
[0074] To assess the thermal stability of the charged filter
medium, samples are placed in an oven at 100.degree. C. for 9 hours
then tested by the method described under Filtration Testing. The
samples were tested for % DOP aerosol penetration (% Pen) and
pressure drop (.DELTA.P), and the Quality Factor (Q9) was
calculated. This data is called Q9 and is compared with % DOP
aerosol penetration (% Pen) and pressure drop (.DELTA.P), and
Quality Factor (QF) collected on web samples made under the same
conditions but stored at ambient conditions instead of 100.degree.
C. Typically seven to nine samples of each example were tested, and
the results were averaged.
Q100 Test
[0075] While it is desirable to have a high degree of thermal
stability in the filtration performance, another valuable property
of fluorinated electret media is its superior performance against
an oily mist aerosol. Test samples of the example webs were tested
in a similar fashion to that used in the Filtration Testing Method,
except that the sample is exposed continuously to the flow of DOP
aerosol until the sample has been exposed to at least 100 mg of DOP
aerosol. The samples tested are in the form of 5.25 inch disks with
4.5 inch diameter circular sections exposed to the aerosol. The
samples are weighed before and after to check the exposures. With
the measured % Pen, known flow rate time of exposure, and initial
and final weights of the sample, the actual DOP exposure can be
calculated. Throughout the DOP exposure the % Pen and .DELTA.P are
monitored by a computer at about 60 second intervals. A useful
point in the exposure is 100 mg of DOP because it is one of the
points of interest in having a two cartridge respirator meet the
NIOSH 42CFR-84 certifications for R and P type respirators. From
this data Q100 is calculated by picking the first data point after
a sample exposure to 100 mg of DOP and selecting the value of % DOP
penetration at this point as % Pen at 100 mg (% Pen@100). From this
and the .DELTA.P measured at the beginning of the test we can
calculate Q100 by
Q100=-In(% Pen@100/100)/.DELTA.P.
In order to usefully compare samples, it is desirable that the
starting pressure drop (.DELTA.P) be similar between samples.
Method for Determining Surface Concentration of Fluorine Using
X-Ray Photoelectron Spectroscopy(XPS)
[0076] X-ray photoelectron spectroscopy (XPS) is a surface analysis
technique that uses a beam of soft x-rays (Al K.alpha., 1486.6 eV)
as a probe. The x-rays irradiate the material to generate
photoelectrons that are characterized by their kinetic energy and
intensity. The kinetic energies of the photoelectrons can provide
quantitative information concerning the elements and their chemical
states. XPS probes the outermost .about.30 to 100 .ANG. of a sample
surface. It is sensitive to essentially all elements except
hydrogen and helium, with detection limits down to approximately
0.1 atomic %.
[0077] XPS measurements were carried out on the sample materials
using a Kratos Axis Ultra Spectrometer (Kratos Analytical,
Manchester, England), which spectrometer was equipped with a
monochromatic Al K.alpha. x-ray excitation source and a spherical
mirror analyzer. The spectrometer had an x-ray power equal to or
near 120 Watts (W) (10 kV, 12 mA). The photoelectron take-off angle
for all recorded spectra was 90 degrees, measured with respect to
the sample surface. The Kratos system has a sampling area of
approximately 800 micrometers (.mu.m).times.600 .mu.m. The pressure
in the vacuum system during analysis was at or below
7.0.times.10.sup.-6 Pascals (Pa).
[0078] Using the XPS method, wide scan surveys were obtained from
the sample materials. A wide scan survey spectrum contains
photoelectron peaks that are characteristic of the elements present
on the surface of the material. The surface composition (in atomic
%) is derived from the relative areas of the core-level
photoelectron peaks, with linear background subtraction and
corrections to account for the instrument's atomic sensitivity
factors. The Kratos spectrometer's performance was verified by
analysis of a poly(tetrafluoroethylene) (PTFE) sample, which showed
experimental values of atomic % fluorine (APF)=65-67 and atomic %
carbon=33-35. These values are in excellent agreement with the PTFE
theoretical stoichiometry.
[0079] Typical instrumental settings that were used are given in
Table 1 below:
TABLE-US-00001 TABLE 1 Pass Scan eV/data Time/data Number Energy
Length point point of Analysis (eV) (eV) (eV) (ms) Sweeps Survey
160 0 to 1100 0.4 65 or 87 5 or 6
Method for Determining C.sub.3F.sub.4H.sup.+/C.sub.2F.sub.5.sup.+
Ratio Using ToF-SIMS:
[0080] Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is
a surface analysis technique in which a pulsed beam of kilovolt
energy ions (primary ions) is used to bombard a specimen, resulting
in sputtering of its surface. During sputtering, neutral and
ionized atomic and molecular species are emitted from the surface.
The ionized species are referred to as "secondary ions" to
distinguish them from the bombarding primary ions. Secondary ions
of one polarity are accelerated in an electric field to a mass
spectrometer, where they travel through a flight tube and arrive at
the detection and counting system. As a result of the fragments
departing from the sample surface at the same time, and being
subject to the same accelerating voltage, the lighter fragments
arrive at the detection system before the heavier ones. The
"time-of-flight" of a fragment is proportional to the square root
of its mass, so that different masses are separated during the
flight and can be detected individually. The quantity measured in
the analysis is the fragment mass divided by the charge amount on
the fragment (m/z).
[0081] ToF-SIMS analysis was performed on samples using an ION-TOF,
Gmbh (Munster, Germany) TOF.SIMS.5 instrument, with a 25 kilovolt
(keV) Bi.sup.+ primary ion beam rastered over a 500.times.500 .mu.m
sample target area. ToF-SIMS provides chemical information on the
outermost 10 to 20 .ANG. of a material and produces mass spectra in
both positive and negative ion modes, extending out to a mass of
1000 atomic mass units (u) and beyond. Analysis of positive ion
spectra of local-fluorinated webs and remote-fluorinated webs
showed three types of ions as follows:
[0082] Pure hydrocarbon ions of type CxHy.sup.+, where x and y have
values of 1 or greater (examples: C.sub.2H.sub.3.sup.+,
C.sub.3H.sub.5.sup.+)
[0083] Pure fluorocarbon ions of type CxFy.sup.+, where x and y
have values of 1 or greater (examples: C.sub.2F.sub.5.sup.+,
C.sub.3F.sub.7.sup.+)
[0084] Mixed hydrofluorocarbon ions of type CxFyHz.sup.+, where x,
y, and z have values of 1 or greater (examples:
C.sub.3F.sub.4H.sup.+, C.sub.5F.sub.4H.sup.+).
[0085] As an illustration of the spectra type, FIG. 6 shows
ToF-SIMS spectra of two fluorinated polypropylene blown microfiber
(BMF) webs, more specifically, the spectrum record for the m/z
region from 100 u to 150 u. FIG. 6 shows the spectrum of a
local-fluorinated web, and the spectrum of a remote-fluorinated web
having approximately the same level of fluorination as the
local-fluorinated web. FIG. 6 shows that there is a difference in
the relative abundance of the C.sub.3F.sub.4H.sup.+ ion at m/z 113
between the two spectra. In the material treated by remote plasma,
this ion is more intense relative to its neighbors. This has been
found to be a general characteristic of remote plasma treatment.
For local and remote plasma-fluorinated materials having similar
fluorination levels, as measured by XPS, the ToF-SIMS spectrum of
the remote-treated material will show a higher relative abundance
of C.sub.3F.sub.4H.sup.+.
[0086] In ToF-SIMS, it is convenient to use a reference ion to make
relative quantitative comparisons between spectra. The ratio
[integrated counts of the ion of interest]/[integrated counts of
the reference ion] provides a basis for comparison. In this case,
the C.sub.2F.sub.5.sup.+ fluorocarbon ion at m/z 119 provides a
suitable reference ion. Table 2 contains a relative comparison of
the abundance of the C.sub.3F.sub.4H.sup.+ ion for a series of
local and remote plasma-fluorinated BMF webs, together with the
atomic % F as determined by XPS. The ToF-SIMS peak integration
ranges were 112.5 u to 113.5 u for m/z 113, and 118.5 u to 119.5 u
for m/z 119.
Local Plasma Treatment Examples 1-9
[0087] A roll of nonwoven meltblown microfiber web having a nominal
basis weight of 65 grams/m.sup.2, an Effective Fiber Diameter of
7.5 micrometers, a web solidity of 6%, and a width of about 50
inches was used. The polypropylene resin used to make the web was
Total PP3941W available from Total Petrochemicals USA, Houston
Tex.
[0088] For Local Plasma Treatment Examples 1-9, the webs were
transported through a plasma by a roll-to-roll web conveying system
within a vacuum chamber. The vacuum chamber contained large-area
flat-plate electrodes spaced 25.4 mm apart, and a simple
speed-controlled system transported a continuous web from a source
roll, through the center of the space between the electrodes, to a
collection roll. The total down-web path length for the web between
the electrodes was 91 cm. Compressed F.sub.2 gas (Air Products,
>97% purity) and argon (Oxygen Service Co. St. Paul Minn.,
industrial grade, <5 ppm O.sub.2, <10 ppm H.sub.2O) were
metered through separate mass flow control devices and then
combined in a gas manifold. The gas mixture was introduced through
an array of 1.6 mm diameter exit holes located across the face of
each plate electrode. The electrodes were connected to a 13.56 MHz
power supply (RF Power Products Inc., Model RF50SWC) coupled
through a matching network (RF Power Products Inc., Model
7621020020) in order to sustain the plasma. The power supply was
operated at a level that provided 0.18 Watts/cm.sup.2 of electrode
area or 0.07 Watts/cm.sup.3 of plasma volume. The treatment chamber
was evacuated using a vacuum pump stack, consisting of a Roots
blower and a mechanical "dry" pump.
[0089] For the reported local plasma experiments, the reactor was
typically evacuated to a base pressure of not greater than
approximately 3 Pascals (Pa). The F.sub.2/argon gas mixture was
then introduced to the chamber at total flow rate of 8
liters/minute, which produced a steady pressure of approximately 67
Pa during the execution of each experiment. Approximately 15 m of
web was treated in each condition before a representative sample
was collected.
Remote Plasma Treatment Examples 1, 3-6
[0090] For Remote Plasma Treatment Example 1 and Examples 3-6, the
webs were treated using the same vacuum chamber and web transport
system that was used for the local plasma treatments. For these
remote plasma treatments, the flat-plate electrodes were removed
and the 13.56 MHz power supply was disconnected. A remote plasma
source (Astron hf-s model from MKS Instruments, Andover Mass.) was
mounted on the outside of the vacuum chamber and the output of this
source was connected to a port on the vacuum chamber. On the
interior side of the vacuum chamber, stainless steel tubing was
connected to the inlet port and to both ends of two slotted
reactive fluorine distribution manifolds (RFDM). The RFDM system
consisted of two 1-inch diameter aluminum tubes each with a row of
0.0156.times.0.6875 inch slots spaced 0.0625 inches apart. The
tubes were mounted approximately 5 inches away on the same side of
the target web, with the slot surface normal opposing and parallel
to the web surface normal. Compressed NF.sub.3 gas (Advanced
Specialty Gases, Reno Nev.) was metered through a mass flow control
system and introduced to the inlet of the remote plasma source.
When activated, the remote plasma source had an operating power
level between 6600 and 7500 Watts.
[0091] For the reported remote plasma experiments, the reactor was
typically evacuated to a base pressure of not greater than
approximately 3 Pascals (Pa). The NF.sub.3 gas was then introduced
to the remote plasma source at total flow rate of 4.8
liters/minute, which produced a steady pressure of approximately 53
Pa during the execution of each experiment. Approximately 15 m of
web was treated in each condition before a representative sample
was collected.
Remote Plasma Treatment Example 2
[0092] For Remote Plasma Treatment Example 2, the web was treated
as in Remote Plasma Treatment Example 1, with the following
exceptions. For Remote Plasma Treatment Example 2 a remote plasma
sources (Xstream 3151806 model from Advanced Energy Fort Collins
Colo.) was mounted on the outside of the vacuum chamber and the
output of this source was connected to a port on the vacuum
chamber. On the interior side of the vacuum chamber, the RFDM
system consisted of two 2-inch diameter aluminum tubes each with a
row of 0.062.times.0.625 inch slots spaced 0.125 inch apart and the
tubes were placed approximately 4 inches away from opposite sides
of the target web, with the slot surface normal opposing and
parallel to the web surface normal.
[0093] The data in Table 3 compare surface analysis for samples
treated by the described remote plasma fluorination process and
comparative examples treated with a local plasma. The data in this
table show that the two plasma treatment techniques produce web
that has similar performance in the Q testing, but have distinctly
different surface character seen in the SIMS and XPS analysis.
Sample Charging Procedure
[0094] After the webs were fluorinated, samples about 30 cm. wide
were cut cross-web. These cross-web samples were hydrocharged by
directing a fine spray of distilled water at the web from a pair of
nozzles operating at a pressure of about 790 kiloPascals (kPa) (115
psig). The distilled water was delivered to the webs using Teejet
Model 9501 spray heads available from Spraying Systems; Wheaton,
Ill. Spray heads were placed 10 cm apart and 10 cm away from the
webs and operated at a pressure of about 790 kPa. Webs passed under
the spray heads at a rate of 5.1 cm/sec while a vacuum was applied
to a slot positioned opposite the spray heads under the open mesh
carrier belt. Located under the conveying belt opposite the spray
heads was a vacuum slot, 25 cm long and 0.5 cm wide that was
attached to a Dayton Electric wet dry vacuum, model 2Z974B (Dayton
Electric, Chicago, Ill.). Each sample web was run through the
hydrocharger twice (sequentially once on each side) while spraying,
and then twice without spraying with just the vacuum to remove any
excess water. The webs were allowed to dry completely in air
overnight before filter testing.
TABLE-US-00002 TABLE 2 X-Ray Discharge Data % Pen % Pen QF at % Pen
Sample # QF at 30 at 60 60 min Ratio Local example 1 1.96 63.90
73.20 0.11 1883 Local example 2 2.07 43.80 58.00 0.17 1252 Local
example 3 2.08 42.90 67.60 0.13 1699 Local example 4 2.28 52.40
56.80 0.19 1248 Local example 5 Local example 6 Local example 7
2.41 5.19 25.60 0.43 584 Local example 8 2.37 8.64 28.60 0.36 663
Local example 9 2.44 5.82 22.40 0.53 473 Remote example 1 Remote
example 2a Remote example 2b Remote example 3 2.27 17.20 42.10 0.26
868 Remote example 4 2.39 8.61 27.80 0.39 634 Remote example 5 2.38
5.89 19.70 0.48 513 Remote example 6 2.30 16.90 36.40 0.34 683
The data in Table 2 show that the samples exhibit suitable levels
of electric charge when compared to comparative samples of known
techniques. Some of the better samples still had significant levels
of electret enhanced filtration, i.e. QF.sub.60 greater than 0.2
(mm of H.sub.2O).sup.-1 even after 60 minutes of x-ray
exposure.
Surface Analysis and Performance Results
[0095] The samples were tested to measure the atomic % fluorine,
the C.sub.3F.sub.4H.sup.+:C.sub.2F.sub.5.sup.+ ratio, Q.sub.0, Q9,
and Q100. Data are presented below in Table 3:
TABLE-US-00003 TABLE 3 Web ToF-SIMS speed XPS C3F4H+/ Sample #
(ft/min) % F C2F5+ Q.sub.0 Q9 Q100 Local example 1 42 38.7 1.663
1.99 1.32 0.23 Local example 2 35 42 1.361 2.12 1.62 0.29 Local
example 3 28 40.3 1.523 2.13 1.42 0.60 Local example 4 28 43 1.331
2.22 1.70 0.65 Local example 5 28 47.3 0.781 2.41 1.98 1.22 Local
example 6 14 49 0.493 2.79 2.06 1.42 Local example 7 14 49 0.664
2.35 1.48 1.25 Local example 8 14 49.3 0.632 2.35 1.67 1.30 Local
example 9 7 52.3 0.317 2.37 1.59 1.50 Remote example 1 51 43.3
1.695 2.45 2.04 1.03 Remote example 2a 51 43.3 1.691 1.79 1.60 0.77
Remote example 2b 51 45 1.699 1.79 1.60 0.77 Remote example 3 35 48
1.365 2.25 1.78 0.69 Remote example 4 28 50.3 1.403 2.25 1.76 1.00
Remote example 5 14 55 0.888 2.29 1.59 1.39 Remote example 6 7 57
0.949 2.18 1.84 1.44
[0096] The date in Table 3 compare surface analysis for samples
treated by comparative local plasma and the remote plasma
fluorination processes. The data in this table show that the two
plasma treatment techniques produce web that has similar
performance in the QF testing but have distinctly different surface
character seen in the ToF-SIMS and XPS analysis.
[0097] FIG. 7 shows a plot of XPS % F vs. ToF-SIMS
C.sub.3F.sub.4H.sup.+/C.sub.2F.sub.5.sup.+ ratio data. A regression
line has been plotted through the local plasma points to give the
Local Fluorination Line. Similarly, a regression line has been
plotted through the remote plasma points to give the Remote
Fluorination Line. The R.sup.2 value (a measure of goodness-of-fit,
where R.sup.2=1 for a perfect fit) for the Local Fluorination Line
is 0.985, and the equation of the line is y=-0.1037x+5.7098.
[0098] The R.sup.2 value for the Remote Fluorination Line is 0.940,
and the equation of the line is y=-0.0610x+4.364.
[0099] To distinguish between webs fluorinated locally or remotely,
three Remote Fluorination Threshold (RFT) lines have been
constructed from the ratio and atomic % fluorine data, as
illustrated in FIG. 7. Line RFT1 runs parallel to the Local
Fluorination Line and has the following equation:
y=-0.1037x+5.9188. (1)
[0100] At a fluorination level above 54% F, line RFT1 takes on a
constant value of y=0.32. The additional offset on line RFT1 is
based on statistical prediction of the maximum likely value for a
local-fluorinated web at a given % F level. BMF webs having a
C.sub.3F.sub.4H.sup.+/C.sub.2F.sub.5.sup.+ ratio that falls above
the RFT1 line at a given % F are distinguishable from webs that
were fluorinated locally.
[0101] Line RFT3 runs parallel to the Remote Fluorination Line and
has the following equation:
y=-0.0610x+4.191. (3)
[0102] The negative offset on line RFT3 is based on statistical
prediction of the minimum likely value for a remote-fluorinated web
at a given % F level. For a BMF web having a
C.sub.3F.sub.4H.sup.+/C.sub.2F.sub.5.sup.+ ratio that falls above
the RFT3 line at a given % F, there is a very high probability that
the web was fluorinated remotely.
[0103] Line RFT2 has the following equation:
y=-0.08235x+5.0549. (2)
[0104] Line RFT2 is the line that bisects lines RFT1 and RFT3. At a
fluorination level above 56% F, line RFT2 takes on a constant value
of y=0.44. For a BMF web having a
C.sub.3F.sub.4H.sup.+/C.sub.2F.sub.5.sup.+ ratio that falls above
the RFT2 line at a given % F, there is a high probability that the
web was fluorinated remotely.
[0105] This invention may take on various modifications and
alterations without departing from its spirit and scope.
Accordingly, this invention is not limited to the above-described
but is to be controlled by the limitations set forth in the
following claims and any equivalents thereof.
[0106] This invention also may be suitably practiced in the absence
of any element not specifically disclosed herein.
[0107] All patents and patent applications cited above, including
those in the Background section, are incorporated by reference into
this document in total. To the extent there is a conflict or
discrepancy between the disclosure in such incorporated document
and the above specification, the above specification will
control.
* * * * *