U.S. patent application number 11/060080 was filed with the patent office on 2005-11-10 for particulate filter and method of use.
Invention is credited to Dahlgren, Andrew, Dikken, David A., Keleny, Lloyd G., Kiester, Mike, Nelson, Robert O..
Application Number | 20050247105 11/060080 |
Document ID | / |
Family ID | 34890495 |
Filed Date | 2005-11-10 |
United States Patent
Application |
20050247105 |
Kind Code |
A1 |
Dikken, David A. ; et
al. |
November 10, 2005 |
Particulate filter and method of use
Abstract
A particulate filter particularly suited for analytical
gravimetric weighing applications is disclosed. The particulate
filter includes a polytetrafluoroethylene (PTFE) media supported by
a ring and yields stable microgram and submicrogram weighing
results. The support ring may be PTFE, metal foil, or another
non-hygroscopic polymer such that the mass of the filter does not
vary with changes in atmospheric moisture. The filter simplifies
the discharge of electrostatic charge buildup such that when a
conductive filter media is combined with a conductive ring device,
the discharge may be accomplished by placing the filter on a
grounded weighing pan or other surface. The particulate filter can
simplify filter identification by including identification symbols
imprinted on each side of the filter media. Due to the chemically
inert qualities of its components, the particulate filter is
particularly suited for extraction techniques used during the
detection of polycyclic hydrocarbons as is conducted in emissions
and ambient air testing.
Inventors: |
Dikken, David A.;
(Bloomington, MN) ; Nelson, Robert O.; (Hastings,
MN) ; Keleny, Lloyd G.; (Champlin, MN) ;
Kiester, Mike; (Stillwater, MN) ; Dahlgren,
Andrew; (Chanhassen, MN) |
Correspondence
Address: |
PATTERSON, THUENTE, SKAAR & CHRISTENSEN, P.A.
4800 IDS CENTER
80 SOUTH 8TH STREET
MINNEAPOLIS
MN
55402-2100
US
|
Family ID: |
34890495 |
Appl. No.: |
11/060080 |
Filed: |
February 16, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60545296 |
Feb 16, 2004 |
|
|
|
60549076 |
Mar 1, 2004 |
|
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Current U.S.
Class: |
73/28.04 ;
73/61.72 |
Current CPC
Class: |
B01D 69/10 20130101;
B01D 63/021 20130101; B01D 39/1692 20130101; B01D 2313/025
20130101; G01N 1/2205 20130101; B01D 2325/02 20130101; B01D 2325/38
20130101; G01N 15/0618 20130101; G01N 35/00732 20130101; B01D 63/08
20130101; B01D 2325/00 20130101; B01D 2325/20 20130101; B01D 69/02
20130101; G01N 2035/00772 20130101; B01D 2325/04 20130101; G01N
5/02 20130101; B01D 71/36 20130101; G01N 2015/0046 20130101; G01N
1/2252 20130101 |
Class at
Publication: |
073/028.04 ;
073/061.72 |
International
Class: |
G01N 001/00 |
Claims
1. A particulate filter comprising: a fluoropolymer filter
membrane; an identification symbol printed on first and second
sides of the filter membrane; and a support ring fused to an outer
rim of the filter membrane, wherein the particulate filter is
substantially non-hygroscopic and moisture invariable, and
substantially inert to dissolution by organic solvents.
2. The particulate filter of claim 1, wherein the filter membrane
comprises a polytetrafluoroethylene (PTFE) material, and wherein
the support ring is welded to the filter membrane.
3. The particulate filter of claim 2, wherein a porosity of the
filter membrane ranges from about 70 percent to about 95
percent.
4. The particulate filter of claim 2, wherein the filter membrane
has an average pore size ranging from about 0.05 micrometers to
about 3.0 micrometers.
5. The particulate filter of claim 4, wherein the average pore size
ranges from about 1.0 micrometers to about 2.5 micrometers.
6. The particulate filter of claim 2, wherein the filter has an
airflow of at least about 1.0 cfm/ft.sup.2 at 0.5 inches of
water.
7. The particulate filter of claim 6, wherein the airflow is at
least 2.0 cfm/ft.sup.2 at 0.5 inches of water.
8. The particulate filter of claim 2, wherein the filter membrane
has a thickness in the range of about 0.0005 inches to about
0.0035.
9. The particulate filter of claim 8, wherein the filter membrane
has a thickness of about 0.0012 inches.
10. The particulate filter of claim 2, wherein the filter captures
at least 99 percent of 0.1 micrometer particles at about 10.5 fpm
air velocity.
11. The particulate filter of claim 10, wherein the filter captures
at least 99.99 percent of 0.1 micrometer particles at about 10.5
fpm air velocity.
12. The particulate filter of claim 2, wherein the support ring
comprises a material selected from the group consisting of PTFE;
polymers of tetrafluoroethylene, perfluorovinylether, and
perfluoroalkoxy; polymers of tetrafluoroethylene and ethylene;
polyvinylidine fluoride; polymers of tetrafluorothylene and
hexafluoropropylene; ethylene-chlorotrifluoroethyl- ene copolymer;
polymethylpentene; polyester; polypropylene; polyethylene; a metal
foil; and a moldable thermoplastic.
13. The particulate filter of claim 2, wherein the support ring is
overmolded to the filter membrane, and wherein the support ring
comprises a material selected from the group consisting of a
thermoplastic polyurethane resin; a thermoplastic elastomer; a
liquid crystal polymer; and a polyphenylene sulfide.
14. The particulate filter of claim 2, wherein the filter membrane
comprises a PTFE material and a material selected from the group
consisting of a carbon filler, a carbon powder, a carbon fiber, and
a ceramic material, to vary an electrical conductivity
characteristic of the particulate filter.
15. The particulate filter of claim 1, wherein the identification
symbol comprises a dot-coded identification symbol.
16. The particulate filter of claim 1, wherein the identification
symbol printed on the first side is offset from the identification
symbol printed on the second side.
17. The particulate filter of claim 16, wherein the identification
symbol comprises a unique start character to differentiate the
identification symbol on the first side from the identification
symbol on the second side.
18. The particulate filter of claim 1, wherein the identification
symbol comprises printed ink.
19. The particulate filter of claim 18, wherein the ink comprises a
known chemical profile.
20. The particulate filter of claim 18, wherein the ink is
insoluble.
21. The particulate filter of claim 1, wherein the particulate
filter is substantially circular and the support ring is welded
concentrically to the filter membrane.
22. The particulate filter of claim 21, wherein the particulate
filter has a diameter of less than about 100 millimeters and the
support ring has a diameter of less than about 10 millimeters.
23. The particulate filter of claim 22, wherein the particulate
filter has a diameter of about 47 millimeters and the support ring
has a diameter of about 3 millimeters.
24. The particulate filter of claim 1, wherein the particulate
filter has a hygroscopic absorption rate that is generally
proportional to a mass of the particulate filter.
25. The particulate filter of claim 24, wherein a particulate
filter having a mass of about 0.14 grams to about 0.16 grams has a
hygroscopic absorption rate of less than about 0.4 micrograms per
percent of relative humidity change.
26. The particulate filter of claim 1, wherein the particulate
filter is substantially inert to dissolution by organic solvents
selected from the group consisting of dichloromethane, acetone,
toluene, cyclohexane, hexane, a mixture of ethanol and toluene,
benzene, methylenechloride, methanol, and combinations of the
foregoing.
27. The particulate filter of claim 1, wherein the particulate
filter is less than about 7 millimeters from flat over an entire
surface.
28. The particulate filter of claim 27, wherein the particulate
filter is about 2 micrometers thick.
29. A method of analytical gravimetric weighing using a particulate
filter comprising the steps of: equilibrating a particulate filter
comprising a fluoropolymer filter membrane and a perfluoroalkoxy
polymer support ring to an ambient pre-test environment;
identifying the particulate filter by an identification symbol
printed on a first and a second side of the particulate filter;
dissipating electrostatic charge buildup on the particulate filter;
pre-weighing the particulate filter; conducting a testing
application using the particulate filter; equilibrating the
particulate filter to an ambient post-test environment; identifying
the particulate filter by the identification symbol printed on a
clean side of the particulate filter; post-weighing the particulate
filter; and determining a net load of particulate collected by the
particulate filter in the testing application based upon the
pre-weighing and the post-weighing.
30. The method of claim 29, wherein the fluoropolymer comprises a
polytetrafluoroethylene material.
31. The method of claim 29, further comprising the steps of:
exposing the particulate filter to a volume of air in the testing
application; and calculating a load of particulate within the
volume of air from the net load of particulate collected.
32. The method of claim 29, wherein the step of dissipating
electrostatic charge buildup on the particulate filter comprises
placing the particulate filter on a grounded surface.
33. The method of claim 29, wherein the step of dissipating
electrostatic charge buildup on the particulate filter comprises
using a Polonium 210 and an alpha emitter radioactive element.
34. The method of claim 29, wherein the steps of identifying the
particulate filter by an identification symbol further comprise
scanning the identification symbol.
35. The method of claim 29, wherein the step of conducting a
testing application using the particulate filter comprises
conducting an engine test to detect a polycyclic aromatic
hydrocarbon.
36. A method of detecting polycyclic aromatic hydrocarbons (PAHs)
using a particulate filter comprising the steps of: identifying a
particulate filter comprising a polytetrafluoroethylene filter
membrane and a perfluoroalkoxy polymer support ring by an
identification symbol printed on a first and a second side of the
filter membrane; collecting a particulate sample by the filter;
identifying the filter by the identification symbol printed on at
least one of the first and second side of the filter membrane;
extracting the particulate sample from the filter; and analyzing
the extracted sample for PAHs.
37. The method of claim 36, further comprising the step of
dissipating electrostatic charge buildup on the particulate
filter.
38. The method of claim 36, wherein the step of collecting a
particulate sample comprises exposing the particulate filter to a
combustion engine exhaust stream.
39. The method of claim 36, wherein the steps of identifying the
filter by an identification symbol further comprise scanning the
identification symbol.
40. The method of claim 36, wherein the step of extracting the
particulate sample comprises extracting the particulate sample
using an organic solvent.
41. The method of claim 36, wherein the step of extracting the
particulate sample comprises one of the extraction methods selected
from the group consisting of Soxhlet extraction, ultrasonic
extraction, and microwave extraction.
42. The method of claim 36, wherein the step of analyzing the
extracted sample comprises analyzing the extracted sample by gas
chromatography/mass spectrometry/selective ion monitoring in a
positive ion electron impact mode.
43. The method of claim 36, wherein the step of analyzing the
extracted sample comprises ignorning a component attributable to a
known chemical profile of an identification symbol ink.
44. A method of manufacturing a particulate filter used for
analytical gravimetric weighing comprising the steps of: welding a
support ring comprising a polymer of perfluoroalkoxy to a filter
membrane comprising a fluoropolymer; printing an identification
symbol on a first side of the filter membrane; and printing the
identification symbol on a second side of the filter membrane
offset from the identification symbol on the first side.
45. The method of claim 44, wherein the step of welding comprises
heat welding the support ring to the filter membrane.
46. The method of claim 44, further comprising the step of
utilizing an ink jet printer for printing the identification
symbols on the first side and the second side.
47. The method of claim 46, wherein the step of selecting an ink
comprises selecting an ink based at least in part upon a solubility
property of the ink.
48. The method of claim 46, wherein the step of selecting an ink
comprises selecting an ink having a known chemical profile.
49. The method of claim 44, wherein the fluoropolymer comprises
polytetrafluoroethylene.
50. The method of claim 44, further comprising the step of varying
a conductive electrical property of the particulate filter by
compounding the fluoropolymer with a material selected from the
group consisting of carbon filler, carbon power, carbon fiber, and
a ceramic material.
51. A particulate filter comprising: a fluoropolymer membrane; and
a polypropylene support scrim, wherein the polymeric support scrim
comprises a material selected from the group consisting of
polyester, polypropylene, polyethylene, and polyamide.
52. The particulate filter of claim 51, wherein the fluoropolymer
membrane comprises a polytetrafluoroethylene (PTFE) material.
53. The particulate filter of claim 52, wherein a porosity of the
membrane ranges from about 70 percent to about 95 percent.
54. The particulate filter of claim 53, wherein the membrane has an
average pore size ranging from about 0.05 micrometers to about 3.0
micrometers.
55. The particulate filter of claim 54, wherein the average pore
size ranges from about 1.0 micrometers to about 2.5
micrometers.
56. The particulate filter of claim 52, wherein the filter has an
airflow of at least about 1.0 cfm/ft.sup.2 at 0.5 inches of
water.
57. The particulate filter of claim 56, wherein the airflow is at
least 2.0 cfm/ft.sup.2 at 0.5 inches of water.
58. The particulate filter of claim 52, wherein the membrane and
support scrim have a thickness in the range of about 0.005 inches
to about 0.020 inches.
59. The particulate filter of claim 58, wherein the membrane and
support scrim have a thickness of about 0.010 inches.
60. The particulate filter of claim 52, wherein the filter captures
at least 99 percent of 0.1 micrometer particles at about 10.5 fpm
air velocity.
61. The particulate filter of claim 60, wherein the filter captures
at least 99.99 percent of 0.1 micrometer particles at about 10.5
fpm air velocity.
62. The particulate filter of claim 51, wherein the polymeric
support scrim has a thickness of less than about 0.0100 inches.
63. The particulate filter of claim 62, wherein the polymeric
support scrim has a thickness of less than about 0.0045 inches.
64. The particulate filter of claim 51, wherein the polymeric
support scrim has an air permeability of at least 770 cubic feet
per minute per square foot at 0.5 inches of water.
65. The particulate filter of claim 51, wherein the polymeric
support scrim has a moisture absorption of less than about 1
percent at 65 degrees Fahrenheit and 65 percent relative
humidity.
66. The particulate filter of claim 51, wherein a porosity of the
polymeric support scrim ranges from about 20 percent to about 80
percent.
67. The particulate filter of claim 66, wherein a porosity of the
polymeric support scrim ranges from about 40 percent to about 70
percent.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/545,296, filed Feb. 16, 2004, and U.S.
Provisional Application No. 60/549,076, filed Mar. 1, 2004, which
are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] Particulate filters have long been known and widely used.
Particulate filters of various types have been utilized for decades
in different applications, for example air filtration and
analytical measurements in laboratories for a wide range of testing
protocols.
[0003] Of recent importance is a method of gravimetric analysis.
Gravimetric analysis is founded on the principle that a weight of a
substance is the elementary measurement used for calculation. In
the case of gravimetric analysis of particulate filter
applications, the results of a weighing before (pre weighing) and
after (post weighing) exposure to an air stream are compared to
calculate the load of particulate within the volume of air to which
the filter was exposed.
[0004] It is important in this calculation that the net load based
upon the pre weighing and post weighing data is a result of the
trapped particulate and not a variation of the mass of the filter
itself. As smaller net loads have been of interest in industry, the
requirements for precision and accuracy of the particulate filter
gravimetric analysis have been becoming more stringent.
Requirements to accurately weigh to the microgram (0.000001 g)
level or sub-microgram level (0.0000001 g) are in place in the Code
of Federal Regulations for both laboratories responsible for
gravimetric fine particulate in the ambient air pollution market
comprised of federal, state, county, and local environmental
protection agencies, the Combustion Engine Manufacturers and their
regulating laboratories, as well as laboratories performing
chemical speciation via extraction techniques for the qualification
and quantification of the particulate deposited on the filter.
[0005] For all interests involved, accurate and precise measurement
of particulate filter net load is imperative. To date no known
particulate filter has been designed to address specifically
improved performance, stability, and usability at the microgram and
sub microgram level and for various applications.
SUMMARY OF THE INVENTION
[0006] The present invention is directed to a particle sampling
test filter. The filter can be used as a particle sampling filter
in diesel emissions testing, air quality monitoring and general lab
use.
[0007] According to one aspect of the invention, a particulate
filter comprises a fluoropolymer filter membrane; an identification
symbol printed on first and second sides of the filter membrane;
and a support ring fused to an outer rim of the filter membrane,
wherein the particulate filter is substantially non-hygroscopic and
moisture invariable, and substantially inert to dissolution by
organic solvents. In one embodiment, the filter membrane comprises
a polytetrafluoroethylene (PTFE) material.
[0008] According to another aspect of he invention, a method of
analytical gravimetric weighing using a particulate filter
comprises the steps of equilibrating a particulate filter
comprising a fluoropolymer filter membrane and a perfluoroalkoxy
polymer support ring to an ambient pre-test environment;
identifying the particulate filter by an identification symbol
printed on a first and a second side of the particulate filter;
dissipating electrostatic charge buildup on the particulate filter;
pre-weighing the particulate filter; conducting a testing
application using the particulate filter; equilibrating the
particulate filter to an ambient post-test environment; identifying
the particulate filter by the identification symbol printed on a
clean side of the particulate filter; post-weighing the particulate
filter; and determining a net load of particulate collected by the
particulate filter in the testing application based upon the
pre-weighing and the post-weighing.
[0009] According to yet another aspect of the invention, a method
of detecting polycyclic aromatic hydrocarbons (PAHs) using a
particulate filter comprises the steps of identifying a particulate
filter comprising a polytetrafluoroethylene filter membrane and a
perfluoroalkoxy polymer support ring by an identification symbol
printed on a first and a second side of the filter membrane;
collecting a particulate sample by the filter; identifying the
filter by the identification symbol printed on at least one of the
first and second side of the filter membrane; extracting the
particulate sample from the filter; and analyzing the extracted
sample for PAHs.
[0010] According to a further aspect of the invention, a method of
manufacturing a particulate filter used for analytical gravimetric
weighing comprises the steps of welding or overmolding a support
ring comprising a polymer to a filter membrane comprising a
fluoropolymer; printing an identification symbol on a first side of
the filter membrane; and printing the identification symbol on a
second side of the filter membrane offset from the identification
symbol on the first side. The polymer of support ring is preferably
a fluoropolymer such as perfluoroalkoxy.
[0011] The particulate filter and methods of the invention thereby
provide various advantages, such as one or more of improved
efficiency and permeability, improved moisture stability, improved
relative humidity stability, improved static stability, a ringed or
ring-less design for improved handling, and the optional
functionality of being printable with an identification symbol or
code to facilitate automated filter handling and weighing in some
embodiments. Embodiments of the particulate filter device of the
invention simplify the discharge of electrostatic charge buildup,
provide improved identification methods and means, and may be used
to accurately measure particulate pollutants and materials.
[0012] The above summary of the present invention is not intended
to describe each illustrated embodiment or every implementation of
the present invention. The figures and the detailed description
that follow more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0014] FIG. 1 depicts top and side schematic views of a particulate
filter according to one embodiment of the invention.
[0015] FIG. 2 depicts a top view of particulate filter media having
an identification symbol according to one embodiment of the
invention.
[0016] FIG. 3 is a flow chart of a test filter cycle according to
one embodiment of the invention.
[0017] FIG. 4 is a flow chart of a test filter cycle according to
one embodiment of the invention.
[0018] While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments described. On the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION
[0019] The present invention is directed to a particle sampling
test filter that can be used, for example, as a particle sampling
filter in diesel emissions testing, and the methodology of use of
the particle sampling test filter. The particle sampling filter of
the invention provides various advantages, such as one or more of
improved efficiency and permeability, improved moisture stability,
improved relative humidity stability, improved static stability, a
ringed or ring-less design for improved handling, and the optional
functionality of being printable with an identification symbol or
code to facilitate automated filter handling and weighing in some
embodiments. Embodiments of the particulate filter device of the
invention simplify the discharge of electrostatic charge buildup,
provide improved identification methods and means, and may be used
to accurately measure particulate pollutants and materials.
[0020] Embodiments of the invention have applicability in a wide
range of applications and testing methods, including combustion
vehicle emissions testing and analysis. Polycyclic aromatic
hydrocarbons (PAHs) are discussed in AGENCY FOR TOXIC SUBSTANCES
AND DISEASE REGISTRY, U.S. DEP'T OF HEALTH & HUMAN SERVS.,
TOXICOLOGICAL PROFILE FOR POLYCYCLIC AROMATIC HYDROCARBONS (1995),
which is herein incorporated by reference. PAHs are a group of
chemicals formed during the incomplete combustion of coal, oil,
gas, wood, garbage, and other organic substances and generally
occur as complex mixtures, e.g., as part of combustion products
such as soot. Many PAH forms are known carcinogens, one important
reason for monitoring their presence and levels in the
environment.
[0021] PAHs enter the environment most commonly as releases to air
from sources such as residential wood burning and exhaust from
automobiles and trucks. PAHs in diesel exhaust particulates are
dominated by three- and four-ring compounds, primarily
fluoranthene, phenanthrene, and pyrene. Diesel exhaust vapor
emissions are dominated by phenanthrene and anthracene.
Acenaphthene, fluorine, and phenanthrene have been found to be
predominant in total (particle- and vapor-phase) diesel emissions.
About 90-95% of particulate PAHs are associated with particle
diameters less than 3.3 micrometers, with peak distributions
localized between 0.4 and 1.1 micrometers. Coarse and nucleic PAH
particulate diameters can range overall from less than 0.1
micrometers to more than 5 micrometers. PAH detection and level
characterization are typically accomplished using filter sampling
methods. Current commercially available particulate filters,
however, do not provide the accuracy and ease of handling desired
in the industry. Further, known particulate filters do not account
for volatilization of support rings. As a result, the use of these
particulate filters for PAH sampling may not be entirely accurate.
One embodiment of the particulate filter and method of the
invention allow for extraction of PAH particulates collected in,
for example, vehicle emissions testing.
[0022] In one aspect of the invention, parameters have been
identified that affect the gravimetric performance of particulate
filters from automated laboratory measurements using state of the
art redundant weighing processes, robotic automation, and other
analytical instrumentation for measuring the environment and other
parameters such as electrostatic charge. These parameters include
hygroscopic and moisture variability, electrostatic charge, and
filter identification, which are discussed below.
[0023] With regard to hygroscopic and moisture variability, water
from the ambient air is absorbed or discharged from a filter
depending upon the ambient moisture content of the laboratory
environment in which the weighing occurs. Commercial particulate
filters that are currently available generally fail to meet the
desired and required accuracy and precision by a factor of five
times, resulting in test results that are useless if not
detrimental.
[0024] With regard to electrostatic charge, media typically used in
particulate filters is prone to accumulating electrostatic charge.
The effect of this charge is a positive bias on the filter being
weighed which can cause significant errors in excess of ten times
the desired and required accuracy and precision.
[0025] With regard to filter identification, existing filters have
either had no identification or an alphanumeric imprint. These
approaches force external "carrier tagging" if no identification is
present. This practice forces the use of the carriers that are
identified by the tagging, and the filter must then be stored or
carried in the identified carrier throughout its test life. Even in
the case of the alphanumeric imprinted filters, manual data entry
or writing is done extensively and is both tedious and prone to
errors. Furthermore, the edge printing is obscured when the filters
are placed in the cassettes typically used for insertion into air
streams.
[0026] Various embodiments of the particulate filter of the
invention provide reduced hygroscopic variability, improved
dissipation of electrostatic charge, enhanced filter
identification, and other benefits. These and other aspects of the
invention will be described in more detail below in various
contexts of the structure and composition of embodiments of the
particulate filter, and systems and methods of manufacture,
assemblage, and use.
[0027] A. Filter Media
[0028] The invention can include a fluoropolymer, preferably
polytetrafluoroethylene (PTFE), membrane with a polymeric support,
such as a polypropylene scrim support. In certain embodiments the
test filter has an airflow of at least 1.0 cfm/ft.sup.2 at 0.5
inches of water and a thickness of 0.010 inches at 0.5 psi, and
more typically at least 2.0 cfm/ft.sup.2 at 0.5 inches of water and
a thickness of 0.010 inches. The thickness can range from about
0.005 inches to about 0.02 inches. The test filter is generally
highly efficient, capturing at least 99 percent of 0.1 .mu.m
particles at 10.5 fpm air velocity, more typically at least 99.5
percent of 0.1 .mu.m particles at 10.5 fpm air velocity, and even
more desirably 99.99 percent of 0.1 .mu.m particles at 10.5 fpm air
velocity.
[0029] The polymeric support can comprise, for example, a
polypropylene material with a thickness of less than about 0.0100
inches, and typically less than about 0.0045 inches, an air
permeability of at least 770 cubic feet per minute per square foot
at 0.5 inches of water, and moisture absorption of less than about
1 percent at 65.degree. F. and 65 percent relative humidity.
[0030] A suitable polymeric film for use as the membrane includes
expanded polytetrafluoroethylene (PTFE) films, as described, for
example, in U.S. Pat. Nos. 3,953,566; 4,187,390; 4,945,125;
5,066,683; 5,157,058; and 5,362,553, each of which is incorporated
herein by reference, or available commercially, for example, as
Tetratec #1305 (Donaldson Membranes, Philadelphia, Pa.). An
expanded PTFE film typically comprises a plurality of nodes
interconnected by fibrils to form a microporous structure.
[0031] Expanded PTFE films for use in air filtering and other
applications often have a relatively good air permeability. One
measure of the air permeability of the expanded PTFE films is the
number of seconds required for the flow of 100 cubic centimeters of
air through the film. Typically, the air permeability of suitable
expanded PTFE films is not greater than about 20 seconds, as
measured using a Gurley densometer, Model No. 4110, Gurley
Precision Instruments, Troy, N.Y. Preferably, the air permeability
is not greater than about 6 seconds and, more preferably, not
greater than about 4 seconds.
[0032] The size of the pores contributes to determining the
effective range of particles that can be prevented or restricted
from flow through the test filter. Often the average pore size of
the membrane is about 2 .mu.m or less. For many filtering
applications, the average pore size ranges from about 0.05 .mu.m to
about 3.0 .mu.m, preferably, from about 0.2 .mu.m to about 3.0
.mu.m, and, more preferably, from about 1.0 .mu.m to about 2.5
.mu.m. However, larger or smaller average pore sizes may be
used.
[0033] Another factor in the flow through the filter is the
porosity of the membrane, (i.e., the percentage of open space in
the volume of the membrane, as determined by comparison of the
density of the membrane with respect to the density of nonporous
PTFE). Typically, the porosity of the membrane is about 20 percent
or greater and about 95 percent or less. Often the porosity of the
membrane of a test filter suitable for many filtering applications
ranges from about 70 percent to about 95 percent, preferably from
about 80 percent to about 95 percent, and more preferably from
about 85 percent to about 95 percent.
[0034] The support scrim is typically formed using a woven or
non-woven porous, polymeric material. Often the support scrim is
made using a fibrous material, however, other porous materials may
also be used. The average pore size of the support scrim is usually
larger than the average pore size of the membrane, although this is
not necessary in some applications. The porosity of support scrims
suitable for many filtering applications often ranges from about 20
percent to about 80 percent, preferably, from about 30 percent to
about 75 percent, and, more preferably, from about 40 percent to
about 70 percent.
[0035] Suitable polymeric materials for the support scrim include,
for example, stretched or sintered plastics, such as polyesters,
polypropylene, polyethylene, and polyamides (e.g., nylon). Examples
of commercially available non-woven materials for use as a support
scrim include Hollytex.TM. #3257 from Ahlstrom Filtration, Inc.
(Mount Holly Springs, Pa.) and Cerex.TM. #100 from Midwest
Filtration Company (Fairfield, Ohio) or Cerex Advanced Fabrics
(Pensacola, Fla.). These materials are often available in various
weights including, for example, 0.5 oz./sq.yd (about 17 g/m.sup.2),
1 oz./sq.yd. (about 34 g/m.sup.2), and 2 oz./sq.yd. (about 68
g/m.sup.2). Examples of commercially available woven materials for
use as a support scrim include a polyester film (Style 604, 150
denier) from Travis Textiles (New York, N.Y.). Additional examples
of support scrim materials are various stretched or sintered
polyethylene, polypropylene, and other plastics, including, for
example, Exxaire XBF-110W, XBF-116W, BF-303W, and BF-513K2 from
Exxon Corp. (Buffalo Grove, Ill.), AP3 materials from Amoco Corp.
(Atlanta, Ga.), X-7744 Porex T3 from Porex Technologies Corp.
(Fairbum, Ga.), and BR-300 from Clopay Building Products Co., Inc.
(Cincinnati, Ohio). These same materials may also be used, in some
embodiments, as a membrane instead of or in addition to the
expanded PTFE membrane.
[0036] B. Support Ring
[0037] Referring to FIG. 1, in one embodiment the particulate
filter 10 of the present invention comprises a fluoropolymer, for
example PTFE, filter membrane 12 supported by a support ring 14.
Support ring 14 can be fused or affixed to filter membrane 12 by
welding or other methods. Certain embodiments of filter 10 of the
present invention thereby provide improved methods of gravimetric
analysis, with stable microgram and sub-microgram weighing results.
Filter membrane 12 can have a thickness ranging from about 0.0005
inches to about 0.0035 inches, typically about 0.0012 inches.
[0038] According to a further implementation of the invention,
filter support ring 14 has been identified as one variable
affecting hygroscopic variability. Considered broadly, particulate
filters are comprised of two classes, including those that have a
support structure built into the whole surface as a substrate, as
described above, and those that have a membrane supported by a ring
for structural integrity. Due to some industry specific interests,
particularly in diesel emissions and air quality testing, the
supporting ring design is generally desirable. Experimental results
have identified that the support ring is one problematic component
affecting hygroscopic variability. It is not believed that this was
hereto before recognized.
[0039] In an embodiment, supporting ring 14 comprises a material
whose mass does not vary with changes in atmospheric moisture, for
example relative humidity and dew point. Supporting ring 14 may
comprise a PTFE or PFA material or other plastic material known to
those having skill in the art. In another embodiment, supporting
ring 14 comprises a metal foil, for example stainless steel or
aluminum. These supporting rings in conjunction with PTFE filter
membrane 12 provide a non-hygroscopic filter.
[0040] The PFA or other supporting ring material is preferably
affixed to the PTFE or other filter membrane by welding or another
non-adhesive bonding method. Welding of many fluoropolymers is
problematic. It has proven extremely difficult or impossible to
weld separate PTFE pieces together, for example, a PTFE filter
membrane and a PTFE ring. A PFA ring has proven to be much more
amenable to welding to a PTFE filter membrane and is suitably inert
and non-hygroscopic. Particular suitable welding techniques include
thermal welding and may include ultrasonic welding. Examples of
welding techniques for fluoropolymers in the context of tubing and
valves are illustrated in U.S. Patent Nos. 4,929,293 and 6,289,912,
both of which are incorporated by reference herein. The polymer of
the ring may also be overmolded onto the filter membrane creating a
very secure bond between the ring and membrane. The overmolding is
a process where the filter membrane, preferably expanded PTFE is
placed in a mold having a cavity for the ring. The molten polymer
is injected into the ring cavity and contacts the non-molten
expanded PTFE thereby bonding to it. The metal rings may be secured
to the filter membrane utilizing the adhesive characteristics of
these polymers.
[0041] Adhesive bonding methods can also be used in some
embodiments, for example those having applications in which
gravimetric stability and/or particulate extraction are of lesser
significance.
[0042] Alternative supporting ring materials can include, for
example, polymers of tetrafluoroethylene and perfluorovinylether,
perfluoroalkoxy (also known as PFA), polymers of
tetrafluoroethylene and ethylene (known as ETFE), polyvinylidine
fluoride (known as PVDF), polymers of tetrafluoroethylene and
hexafluoropropylene (known as FEP),
ethylene-chlorotrifluoroethylene copolymer (known as ECTFE);
polymethylpentene (known as PMP), polyetheretherketone (known as
PEEK); polyester, polypropylene, and polyethylene. Certain
proprietary polymers also appear suitable for the ring. Namely:
Isoplast 2530 from Dow Plastics; Dynaflex.RTM. D3204-1000-03
fromGLS Corporation; Vectra A130 from Ticona, and Ryton.TM. R-4
from Chevron Phillips Chemical Co.
[0043] C. Electrostatic Discharge
[0044] Specific embodiments of the particulate filter of the
invention simplify the discharge of electrostatic charge buildup.
The construction of the particulate filter, in particular the
construction and composition of the PTFE membrane, can be
engineered for electrical conductivity and dissipation in certain
embodiments.
[0045] A wide variety of materials suitable for compounding with
PTFE are generally available, allowing for conductive electrical
properties to be varied as needed for particular applications of
the invention. These materials include carbon filler, powder or
fiber, ceramic materials, and other material known in the art. When
a conductive filter membrane is combined with a conductive
supporting ring, the particulate filter will allow for
electrostatic dissipation when placed on a grounded surface,
eliminating the typical necessity of an additional discharging
step. For example, weighing pans on precision weighing equipment
are generally grounded. This eliminates the need to use Polonium
210 and an alpha emitter radioactive element for electrostatic
charge dissipation.
[0046] D. Filter Identification
[0047] Various embodiments of the invention also simplify filter
identification. Referring to FIG. 2, an identification symbol 16
can be printed on each side of filter membrane 12 near the center
of the exposed area. Filter applications in the industry typically
leave only one side visible at any time. For example, when filters
are placed against a backer plate for support when loaded into
airflow streams, the identification must be visible on the "load"
side; after loaded, the identification may become obscured by soot,
hence the need for a second, clean side identification.
[0048] In one embodiment, identification symbols 16 or information
codes are printed on both sides of filter membrane 12 and are
located off center of each other to preclude the other from
interfering with an accurate read. Additionally, to eliminate the
misreading of the mirror image information code on the opposite
side, the code is designed with a unique start character so as to
identify that the symbol or code being read is the target on the
facing side and not the mirror image on the opposing side. The
identification symbols or information codes are minimally sized so
as to provide a negligible effect on filter airflow and mass
variability. Identification symbol 16 can be two-dimensional dot
coding or bar coding labels, alpha-numeric labels, or other known
characters, images, or markings.
[0049] Identification symbols 16 are preferably ink printed, for
example by an ink-jet or similar printing device. The ink or other
material used for the identification symbols is selected for
stability and chemical properties, in particular for a known or,
preferably, lack of affect on gravimetric measurement, chemical
speciation via extraction, or other analysis. A known or absent
affect of the ink is preferable to streamline post-test extraction
and analysis steps and methods. For example, ink with a known or
desired chemical profile can be selected. Any ink artifacts
extracted with collected particulates or other collected matter can
then be easily ignored, aiding in the analysis of particulates of
interest. A particular insoluble or permanent ink can also be
selected such that the ink does not volatilize or otherwise react
with a filter stream, stream component, or collected particulate or
other collected matter to produce an extra gaseous or particulate
artifact and skew test results. In one embodiment, an ink is
selected which is not removed by an extraction solvent, preserving
the identification symbol on the filter membrane and maintaining
the accuracy and integrity of the test results. Thermal heating of
the printed filter membrane during manufacture or otherwise before
use can be used to drive off volatiles and ensure mass stability
over time.
[0050] E. Use of the Test Filter
[0051] FIGS. 3 and 4 are flow charts of exemplary life cycles of
filter 10, one embodiment of which is also shown in FIGS. 1 and 2.
Particulate filter 10 of the invention comprises a filter membrane
12 and non-hygroscopic supporting ring 14, wherein supporting ring
14 is arranged concentrically about filter membrane 12.
[0052] As previously described, filter 10 can comprise a PTFE
membrane or a similar material. Supporting ring 14 in this improved
design can comprise metal foil, PTFE, PFA, or other non-hygroscopic
plastic, or similar materials known to those skilled in the art.
The material composition, structure, scale, and/or other
characteristics of particulate filter 10 can be customized to meet
particular standards or requirements for flow rates, size of
particle trap, and efficiency, among other characteristics as
previously described. For example, filter 10 can be designed to
meet a particular industry's standards or to meet a specific set of
requirements for an individual application.
[0053] Similarly, filter 10 can comprise filter membranes 12 and
supporting rings 14 of various sizes and shapes. Filter 10 can have
various dimensions and shapes, but in certain embodiments is a
circle of less than 100 mm diameter, alternatively less than 75 mm
in diameter, and desirably less than 50 mm diameter. In one
embodiment, the filter has a diameter of 47 mm. A suitable size of
filter 10 is about 46.2 millimeters (mm) in diameter with a 3 mm
wide concentric supporting ring 14 around the rim. Supporting ring
14 can be welded, bonded, adhered, or fused onto one but preferably
both sides of filter membrane 12 around supporting ring 14, as
described in more detail above. Supporting ring 14 can be affixed
to filter membrane 12 by other methods and means according to a
desired use or application, for example by adhering, crimping, or
otherwise joining. The size and thickness of supporting ring 14 can
also vary for particular applications and embodiments.
[0054] Filter 10 is substantially flat in some embodiments,
although the thickness can vary overall or in portions depending
upon need or application. To facilitate weighing and other
applications known in the art, one example embodiment of
particulate filter 10 is less than about 7 millimeters from flat
over the entire surface. For example, filter membrane 12, excluding
supporting ring 14, is about 2 microns thick in one embodiment.
[0055] Filter 10 of the invention can also include identification
information. In use, only one side of filter 10 is typically
labeled with an identifier and visible. If the labeled side becomes
obscured during use by particulate that collects on filter 10, for
example soot or other debris, correctly identifying filter 10 after
use becomes difficult if not impossible.
[0056] Referring to the embodiment of FIG. 2, filter 10 is labeled
with an identification symbol 16, for example a two-dimensional
dot-coding identification symbol, on each side of filter membrane
12 before use. Identification symbol 16 can be located on membrane
12 so as to provide a negligible effect on airflow and mass
variability.
[0057] Particulate filter 10 preferably comprises a material whose
mass does not vary with changes in atmospheric moisture, for
example relative humidity and dew point. It is generally desired in
the industry that an air filter minimize the change in the amount
of retained moisture with respect to changing atmospheric moisture
conditions in order to provide accurate, precise, and stable
results. With such a filter, the difference between a filter's
weight after it has been used and its weight prior to being used
can therefore more accurately be attributed to the material
absorbed onto the filter, rather than to differences in atmospheric
moisture content between the pre-test and post-test weighings.
[0058] For example, moisture variation effect on mass may be less
than or approximately 0.4 micrograms per approximately 1% ambient
humidity change with constant temperature to about .+-.0.1.degree.
C. on filters in the mass range of approximately 0.140 grams to
approximately 0.160 grams, which are generally 47 mm filters. The
moisture variation for other filter sizes is generally proportional
to the filter mass.
[0059] Example embodiments of the particulate filter of the present
invention have an improved hygroscopic absorption rate. For
example, one embodiment of the particulate filter provided a
hygroscopic absorption rate of less than about 0.4 micrograms per
percentage relative humidity, as opposed to commercially available
filters which preferably have hygroscopic variation in excess of
ten times this absorption rate.
[0060] Accordingly, and referring in particular to FIG. 3, the
filter is first acclimated to the ambient weighing environment.
Next, in pre-weigh preparation, the filter is identified, for
example by scanning an identification symbol on the clean side of
the filter in an embodiment. Any electrostatic charge on the filter
is dissipated by placing the filter on a grounded surface, for
example a weighing pan, or by using a Polonium 210 source of alpha
particles. The filter then is weighed on a micro-balance and loaded
into a cartridge.
[0061] In a particular application, which can be an engine test, an
identification symbol on a load side of the filter is charged, the
engine test is run, and the filter is removed from the cartridge.
After the filter has acclimated to the ambient weighing
environment, the post-weigh procedure is performed. The filter is
identified by scanning the identification symbol on the clean side
of the filter, the filter is electrostatically discharged, and the
filter is weighed on a micro-balance for the post-test weight.
Another test method is shown in FIG. 4.
[0062] A particulate filter according to one embodiment of the
invention can have applicability in methods for detecting PAHs,
nitro-PAHs, and other relevant particulates. Sampling methods that
collect PAHs can include use of embodiments of the particulate
filter of the present invention, as the particulate filter allows
for improved, i.e., more accurate, extraction of PAH and nitro-PAH
emissions. In one embodiment, the method comprises the steps of
collecting particulate emissions by a non-hygroscopic particulate
filter having an inert or nearly inert filter ring, extracting the
particulate emissions from the particulate filter, cleaning the
extracts, and analyzing the extracts.
[0063] In one embodiment, the step of collecting particulate
emissions comprises exposing the particulate filter to an air
stream, more particularly to an exhaust stream and more
particularly still to an automotive exhaust stream. Embodiments of
the invention can therefore provide more accurate air testing
results, such as in diesel and other vehicle emissions testing for
PAHs, nitro-PAHs, and ambient air quality testing. In one
embodiment, the particulate filter comprises a PTFE filter membrane
and a PFA supporting ring. The step of extracting the particulate
emissions comprises Soxhlet extracting the particulate emissions
from the particulate filter. While numerous extraction methods are
available, for example Soxhlet, ultrasonic, and microwave, among
others, many involve the use of organic solvents. The PTFE filter
membrane and PFA support ring of this embodiment are nearly inert
to extraction process solvents, and an ink used to print
identification symbols on the filter can be selected for having a
known chemical profile, for being insoluble to a particular
solvent, and/or for having a nonvolatile affect on a test stream,
as described above. Typical organic solvents used for extraction
include dichloromethane, acetone, toluene, cyclohexane, hexane, an
ethanol/toluene mixture, benzene, methylenechloride, methanol, and
various mixtures of these as well as other solvents. The step of
analyzing the extracts can comprise analyzing by gas
chromatography/mass spectrometry/selective ion monitoring
(GC/MS/SIM) in positive ion electron impact mode for PAHs and
analyzing by GC/MS/SIM runs in negative ion chemical ionization
mode for nitro-PAHs. In the case of a soluble ink having a known
chemical profile, the step of analyzing the extracts further
comprises identifying and ignoring the extracted ink.
[0064] The invention may be embodied in other specific forms
without departing from the spirit of the essential attributes
thereof; therefore, the illustrated embodiments should be
considered in all respects as illustrative and not restrictive,
reference being made to the appended claims rather than to the
foregoing description to indicate the scope of the invention.
* * * * *