U.S. patent application number 10/818091 was filed with the patent office on 2005-10-06 for pleated aligned web filter.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Brandner, John M., Shah, Rahul, Sundet, Douglas C., Wu, Tien T..
Application Number | 20050217226 10/818091 |
Document ID | / |
Family ID | 34961955 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050217226 |
Kind Code |
A1 |
Sundet, Douglas C. ; et
al. |
October 6, 2005 |
Pleated aligned web filter
Abstract
A filter element that has rows of folded pleats and includes a
nonwoven filtration web that contains thermoplastic fibers, a
majority of which fibers are aligned at 90.degree..+-.20.degree.
with respect to the row direction. The filter elements can be made
by forming rows of pleats in such a nonwoven web and cutting the
web to a desired size and shape. The filter elements can provide
improved mechanical and filtration properties.
Inventors: |
Sundet, Douglas C.; (Hudson,
WI) ; Shah, Rahul; (Woodbury, MN) ; Brandner,
John M.; (St. Paul, MN) ; Wu, Tien T.;
(Woodbury, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
34961955 |
Appl. No.: |
10/818091 |
Filed: |
April 5, 2004 |
Current U.S.
Class: |
55/521 ;
55/528 |
Current CPC
Class: |
Y10T 442/643 20150401;
Y10S 55/05 20130101; Y10T 428/24686 20150115; B01D 39/1623
20130101; Y10T 442/60 20150401; Y10T 428/24628 20150115 |
Class at
Publication: |
055/521 ;
055/528 |
International
Class: |
B01D 046/00 |
Claims
We claim:
1. A filter element that comprises: a nonwoven filtration web that
has rows of folded pleats and that contains thermoplastic fibers, a
majority of which fibers are aligned at 90.degree..+-.20.degree.
with respect to the row direction.
2. A filter element according to claim 1 wherein about 55 to about
90% of the fibers are aligned at 90.degree..+-.20.degree. with
respect to the row direction.
3. A filter element according to claim 1 wherein about 70 to about
85% of the collected fibers are aligned at 90.degree..+-.20.degree.
with respect to the row direction.
4. A filter element according to claim 1 wherein fibers having
lengths of about 2-5 cm can be teased from the web.
5. A filter element according to claim 1 wherein the fibers have an
average effective fiber diameter of about 8 to about 25 .mu.m.
6. A filter element according to claim 1 wherein the web has at
least a 2:1 ratio of the in-plane tensile strength in the direction
transverse to the row direction to the tensile strength in the row
direction using a 50 mm gauge length.
7. A filter element according to claim 1 wherein the web has at
least a 4:1 ratio of the in-plane tensile strength in the direction
transverse to the row direction to the tensile strength in the row
direction using a 50 mm gauge length.
8. A filter element according to claim 1 wherein the web has at
least a 2:1 ratio of the in-plane Taber Stiffness in the direction
transverse to the row direction to the Taber Stiffness in the row
direction.
9. A filter element according to claim 1 wherein the web has at
least a 2.2:1 ratio of the in-plane Taber Stiffness in the
direction transverse to the row direction to the Taber Stiffness in
the row direction.
10. A filter element according to claim 1 wherein the major
surfaces of the web exhibit striations corresponding to substantial
alignment of individual fibers transverse to the row direction.
11. A filter element according to claim 1 wherein a wetting fluid
placed on the web preferentially wicks transverse to the row
direction.
12. A filter element according to claim 1 wherein the web has been
annealed.
13. A filter element according to claim 1 wherein the web has been
corona-treated or hydrocharged.
14. A method of making a pleated filter element, which method
comprises: forming rows of pleats in a nonwoven filtration web that
comprises thermoplastic fibers, a majority of which fibers are
aligned at 90.degree..+-.20.degree. with respect to the row
direction; and cutting the pleated filter element to a desired size
and shape.
15. A method according to claim 14 wherein about 55 to about 90% of
the fibers are aligned at 90.degree..+-.20.degree. with respect to
the row direction.
16. A method according to claim 14 wherein about 70 to about 85% of
the collected fibers are aligned at 90.degree..+-.20.degree. with
respect to the row direction.
17. A method according to claim 14 wherein fibers having lengths of
about 2-5 cm can be teased from the web.
18. A method according to claim 14 wherein the fibers have an
average effective fiber diameter of about 8 to about 25 .mu.m.
19. A method according to claim 14 wherein the web has at least a
2:1 ratio of the in-plane tensile strength in the direction
transverse to the row direction to the tensile strength in the row
direction using a 50 mm gauge length.
20. A method according to claim 19 wherein the web has a filtration
quality factor Q.sub.F of at least about 0.6 using 100 ppm dioctyl
phthalate particles having a size range between 10 and 700 nm
traveling at a 7 cm/sec face velocity.
21. A method according to claim 14 wherein the web has at least a
4:1 ratio of the in-plane tensile strength in the direction
transverse to the row direction to the tensile strength in the row
direction using a 50 mm gauge length.
22. A method according to claim 14 wherein the web has at least a
2:1 ratio of the in-plane Taber Stiffness in the direction
transverse to the row direction to the Taber Stiffness in the row
direction.
23. A method according to claim 14 wherein the web has at least a
2.2:1 ratio of the in-plane Taber Stiffness in the direction
transverse to the row direction to the Taber Stiffness in the row
direction.
24. A method according to claim 14 wherein the major surfaces of
the web exhibit striations corresponding to substantial alignment
of individual fibers transverse to the row direction.
25. A method according to claim 14 wherein a wetting fluid placed
on the web preferentially wicks transverse to the row
direction.
26. A method according to claim 14 wherein the web has been
annealed.
27. A method according to claim 14 wherein the web has been
corona-treated or hydrocharged.
28. A method according to claim 20 wherein the web has a filtration
quality factor Q.sub.F of at least about 0.6 using 100 ppm dioctyl
phthalate particles having a size range between 10 and 700 nm
traveling at a 7 cm/sec face velocity.
Description
[0001] This invention pertains to pleated filters that have fiber
alignment in the direction of web formation.
BACKGROUND
[0002] Meltblown nonwoven fibrous webs are used for a variety of
purposes including filtration (e.g., flat web and pleated filters),
insulation, padding and textile substitutes. References relating to
meltblown nonwoven fibrous webs include U.S. Pat. No. 3,959,421
(Weber et al.), U.S. Pat. No. 4,622,259 (McAmish et al.), U.S. Pat.
No. 5,075,068 (Milligan et al.), U.S. Pat. No. 5,141,699 (Meyer et
al.), U.S. Pat. No. 5,405,559 (Shambaugh), U.S. Pat. No. 5,652,048
(Haynes et al.), U.S. Pat. No. 5,665,278 (Allen et al.), U.S. Pat.
No. 5,667,749 (Lau et al.), U.S. Pat. No. 5,772,948 (Chenoweth) and
U.S. Pat. No. 5,811,178 (Adam et al.). References relating to
pleated filters include U.S. Pat. No. 4,547,950 (Thompson), U.S.
Pat. No. 5,240,479 (Bachinski), U.S. Pat. No. 5,709,735 (Midkiffet
al.), U.S. Pat. No. 5,820,645 (Murphy, Jr.), U.S. Pat. No.
6,521,011 B1 (Sundet et al. '011) and U.S. Pat. No. D449,100 S
(Sundet et al. '100), and U.S. Patent Application Publication Nos.
U.S. 2003/0089090 A1 (Sundet et al. '090) and U.S. 2003/0089091 A1
(Sundet et al. '091).
SUMMARY OF THE INVENTION
[0003] Nonwoven web manufacture typically involves deposition of
fibers on a moving collector surface. Perhaps partly as a
consequence of this motion, the collected web may exhibit a small
degree of fiber alignment in the machine direction, and to a small
extent some anisotropic physical properties (e.g., tensile
strength) in the machine and transverse directions. Nonwoven web
manufacturers often strive however to make products having
well-balanced and generally isotropic physical properties.
[0004] We have found that by forming nonwoven webs having much
greater than normal fiber alignment in the machine direction and
forming the resulting webs into pleated filtration media having
pleats generally transverse to the machine direction, we can obtain
filters with improved mechanical properties or improved filtration
performance. The present invention provides, in one aspect, a
filter element having rows of folded pleats comprising a nonwoven
filtration web of thermoplastic fibers a majority of which are
aligned at 90.degree..+-.20.degree. with respect to the row
direction.
[0005] The invention provides, in another aspect, a method for
forming a pleated filter element, comprising forming rows of pleats
in a nonwoven filtration web comprising thermoplastic fibers a
majority of which are aligned at 90.degree..+-.20.degree. with
respect to the row direction and cutting the pleated filter element
to a desired size and shape.
[0006] These and other aspects of the invention will be apparent
from the detailed description below. In no event, however, should
the above summaries be construed as limitations on the claimed
subject matter, which subject matter is defined solely by the
attached claims, as may be amended during prosecution.
BRIEF DESCRIPTION OF THE DRAWING
[0007] FIG. 1 is a schematic side view of a meltblowing apparatus
for making nonwoven webs having fibers substantially aligned in the
machine direction.
[0008] FIG. 2 is an overhead view of a vacuum collector for use in
the apparatus of FIG. 1.
[0009] FIGS. 3-6 are radar plots showing fiber alignment.
[0010] FIG. 7 is a perspective view of pleated filtration
media.
[0011] FIG. 8 is a perspective view, partially in section, of a
pleated filter mounted in a frame.
[0012] FIG. 9 is a graph showing filter pressure drop vs. air
velocity.
[0013] FIG. 10 is a schematic illustration of an apparatus for
making the disclosed pleated filters.
[0014] Like reference symbols in the various figures of the drawing
indicate like elements. The elements in the drawing are not to
scale.
DETAILED DESCRIPTION
[0015] The phrase "nonwoven web" refers to a fibrous web
characterized by entanglement or point bonding of the fibers.
[0016] The phrase "filtration web" refers to a porous web capable
of removing at least particles having an average particle diameter
greater than 10 .mu.m from a stream of air flowing at a 0.5 m/sec
face velocity at an initial pressure drop no greater than about 50
mm H.sub.2O.
[0017] The phrase "attenuating the filaments into fibers" refers to
the conversion of a segment of a filament into a segment of greater
length and smaller diameter.
[0018] The word "meltblowing" refers to a method for forming a
nonwoven web by extruding a fiber-forming material through a
plurality of orifices to form filaments while contacting the
filaments with air or other attenuating fluid to attenuate the
filaments into fibers and thereafter collecting a layer of the
attenuated fibers.
[0019] The phrase "meltblown web" refers to a nonwoven web made
using meltblowing.
[0020] The phrase "nonwoven die" refers to a die for use in
meltblowing.
[0021] The phrases "meltblown fibers" and "blown microfibers" refer
to fibers made using meltblowing.
[0022] The phrase "machine direction" when used with respect to a
meltblown web or to a meltblowing apparatus for meltblown web
formation refers to the in-plane direction of web fabrication.
[0023] The phrase "transverse direction" when used with respect to
a meltblowing apparatus or a meltblown web refers to the in-plane
direction perpendicular to the machine direction.
[0024] The phrase "row direction" when used with respect to a
pleated filter element refers to a direction generally parallel to
the pleat ridges and valleys in a filter element having a folded
structure with parallel, generally sharp-edged creases, and to a
direction generally parallel to the pleat crowns and base regions
in a filter element having a corrugated structure with parallel,
generally smooth undulations.
[0025] The term "self-supporting" when used with respect to a web
refers to a web having sufficient coherency and strength so as to
be drapable and handleable without substantial tearing or rupture,
and when used with respect to a pleated filter refers to a filter
whose pleats have sufficient stiffness so that they do not collapse
or bow excessively when subjected to the air pressure typically
encountered in forced air ventilation systems.
[0026] A variety of polymers may be employed to make the disclosed
aligned fiber webs. Representative polymers are thermoplastic,
extrudable and can be processed using a meltblowing apparatus, and
include polyolefins such as polyethylene, polypropylene or
polybutylene; polyamides; polyesters such as polyethylene
terephthalate; and other materials that will be familiar to those
skilled in the art. Polyolefins are particularly preferred.
[0027] A variety of sorbent particles can be added to the nonwoven
webs if desired. Representative sorbent particles are disclosed in
U.S. Pat. No. 3,971,373 to Braun, U.S. Pat. No. 4,429,001 to Kolpin
et al. and U.S. Pat. No. 6,102,039 to Springett et al. Activated
carbon and alumina are particularly preferred sorbent particles.
Mixtures of sorbent particles can be employed, e.g., to absorb
mixtures of gases, although in practice to deal with mixtures of
gases it may be better to fabricate a multilayer pleated filter
employing separate sorbent particles in the individual layers.
[0028] A variety of primary and secondary fluid streams may be
employed to make the disclosed filtration webs. Air is an
especially convenient fluid for both purposes. The remainder of
this application will discuss the use of air, sometimes referred to
as "primary air" or as "secondary quench air" as the context may
require. Those skilled in the art can readily employ other fluids
(e.g., carbon dioxide, nitrogen or water) with appropriate
modification of the operating parameters described below.
[0029] FIG. 1 shows a schematic side view of meltblowing apparatus
10. Molten polymer enters meltblowing die 12 through inlet 14 and
passes through die cavity 16. Small orifices (not shown in FIG. 1)
in die tip 18 cause the molten polymer to form filaments 22 upon
exiting die 12. Primary air supplied through inlets 20 impinges
upon the filaments 22 and attenuates them into fibers 24. Fibers 24
land on flat collector 26 and form nonwoven web 28 which can be
drawn away from collector 26 in the direction of web formation
(viz., the machine direction) 30 by a suitable take-up apparatus
(not shown in FIG. 1). On route to collector 26, secondary quench
air supplied to ducts 32 arrayed across the width of web 28
impinges upon the filaments or fibers, causing the fibers to
oscillate to and fro generally in and against the machine
direction. The collected fibers in the resulting web 28 are
substantially more aligned in the machine direction than would be
the case without the secondary quench air supply. The web's machine
direction and transverse direction mechanical properties (e.g., its
machine direction and transverse direction stiffness and tensile
strength) also exhibit greater anisotropy than when a secondary
quench air supply is not employed.
[0030] Viewed from the side (or transverse direction) using
high-speed photography, fibers 24 are laid down on collector 26 in
a "paintbrush" fashion. Measured at the collector, the oscillations
can have a very large machine direction amplitude, e.g., more than
one fourth the die to collector distance ("DCD") and in some
instances more than half the DCD. Several operating conditions may
be especially desirable to achieve such paintbrush deposition. For
example, the oscillations may occur regularly, may have increasing
amplitude en route to the collector, and may have a wavelength for
one complete cycle that is less than the distance from the
secondary quench air outlets to the collector. Preferably the
distance from the secondary quench air outlets to the collector is
not overly long. The fibers may in some instances exhibit a
whip-like action at their peak machine direction displacement en
route to the collector, and may momentarily move towards the
meltblowing die rather than always moving toward the collector.
Apparent fiber breakage can sometimes be seen as such whip-like
action occurs.
[0031] We have been able to tease fibers with discrete lengths
(e.g., between about 1 and about 10 cm, along with occasional
shorter or longer fibers) from the collected webs using tweezers.
Ordinarily it is quite difficult to remove any fibers (or any
fibers of such lengths) from conventional nonwoven webs, as the
fibers typically are restrained in the web by fiber-to-fiber
bonding or by interfiber entanglement.
[0032] Web 28 can be pleated as is, or further treated. Preferably
a heat treatment (e.g., annealing) is employed to stiffen the web.
Heat treatments may however make it more difficult to tease fibers
from the web, as the fibers may tend to fracture and the web may
have greater inter-fiber bonding or entanglement. Preferred
annealing times and temperatures will depend on various factors
including the polymeric fibers employed. As a general guide,
annealing times and temperatures of about 100.degree. C. up to the
polymer melting point for a time less than about 10 minutes are
preferred.
[0033] A vacuum can optionally be drawn thorough orifice 34 to
assist in consolidating web 28. Overdensification (e.g., using
calendaring) may however destroy the web's filtration capability.
Electric charge can be imparted to the fibers by contacting them
with water as disclosed in U.S. Pat. No. 5,496,507 to Angadjivand
et al., corona-treating as disclosed in U.S. Pat. No. 4,588,537 to
Klasse et al., hydrocharging as disclosed, for example, in U.S.
Pat. No. 5,908,598 to Rousseau et al. or tribocharging as disclosed
in U.S. Pat. No. 4,798,850 to Brown. Additives may also be included
in the fibers to enhance the web's filtration performance,
mechanical properties, aging properties, surface properties or
other characteristics of interest. Representative additives include
fillers, nucleating agents (e.g., MILLAD.TM. 3988 dibenzylidene
sorbitol, commercially available from Milliken Chemical), UV
stabilizers (e.g., CHIMASSORB.TM. 944 hindered amine light
stabilizer, commercially available from Ciba Specialty Chemicals),
cure initiators, stiffening agents (e.g.,
poly(4-methyl-1-pentene)), surface active agents and surface
treatments (e.g., fluorine atom treatments to improve filtration
performance in an oily mist environment as described in U.S. Pat.
Nos. 6,398,847 B1, 6,397,458 B1, and 6,409,806 B1 to Jones et al.).
The types and amounts of such additives will be apparent to those
skilled in the art.
[0034] The completed webs may have a variety of effective fiber
diameter ("EFD") sizes, basis weights and solidity (ratio of
polymer volume to web volume) values. Preferred EFDs are about 8 to
about 25, more preferably about 10 to about 25 .mu.m. Preferred
basis weights are about 50 to about 100 g/m.sup.2. Preferred
solidity values are about 5 to about 15%.
[0035] The disclosed webs have substantial machine direction
(direction of motion or direction of web formation) fiber
alignment. As a general guide for pleated filters made from
polypropylene webs, preferably about 55 to about 90% of the fibers
are aligned at 90.degree..+-.20.degree. with respect to the row
direction, and more preferably about 70 to about 85%. For webs made
from other polymeric materials the numbers may be lower or higher.
For example, as a general guide for pleated filters made from
polyethylene terephthalate webs, preferably about 51 to about 80%
of the fibers are aligned at 90.degree..+-.20.degree. with respect
to the row direction, and more preferably about 60 to about 80%. As
a general guide for pleated filters made from nylon webs,
preferably about 51 to about 70% of the fibers are aligned at
90.degree..+-.20.degree. with respect to the row direction. Very
highly aligned webs can be formed, e.g., webs having at least 80%
of the collected fibers aligned at 90.degree..+-.20.degree. with
respect to the row direction.
[0036] The disclosed webs have one or more anisotropic mechanical
properties. One class of preferred webs may have at least a 2:1
ratio of the in-plane tensile strength in the direction transverse
to the row direction to the tensile strength in the row direction
using a 50 mm gauge length, and more preferably at least a 3:1
ratio. Another class of preferred webs may have at least a 2:1
ratio of the in-plane Taber Stiffness in the direction transverse
to the row direction to the Taber Stiffness in the row direction,
and more preferably at least about 2.2:1.
[0037] It is possible to construct the disclosed meltblowing
apparatus and operate it under conditions that do not provide the
disclosed aligned fiber webs, or under conditions that will provide
weak webs poorly suited to filtration. For example, if insufficient
secondary quench air is employed then the above-described
oscillations may not occur and the fibers may not align
substantially in the machine direction. Excessively high quench
velocities may provide loftier webs having less interfiber bonding
and entanglement and improved filtration performance, but having
severely diminished mechanical properties such as stiffness and
pleatability. Thus it generally will be preferable to employ
secondary quench air within a range of mass flow ratios or volumes.
As an example for webs made using polypropylene and secondary air
chilled to below ambient temperature, a ratio of about 500 to about
2000 grams of secondary quench air per gram of extruded polymer may
be preferred, as may be secondary quench air outlet velocities of
about 15 to about 60 m/sec. These ranges may need to be adjusted
empirically based on factors such as the meltblowing die and
polymer employed, the target basis weight, target web solidity and
target extent of fiber alignment and mechanical property
anisotropy. Pulsation of the secondary quench air may also be
employed but appears not to be necessary. Instead it appears to be
better simply to adjust the secondary quench air flow upwards or
downwards to a steady state value that provides collected webs
having the desired final properties.
[0038] FIG. 2 shows a schematic overhead view of collector 26. Die
12 is positioned close to the leading edge of collector 26, but can
be moved downweb to positions such as position 36 in order to alter
the properties of web 28. Such repositioning may for example
provide webs having reduced Taber Stiffness. If a conventional
cylindrical collector surface is employed instead of a flat
collector then it usually will be more difficult to obtain webs
whose fibers are substantially aligned in the machine direction,
and the webs may have lower Taber Stiffness. Excessive DCD lengths
or excessive distances from the secondary quench air outlets to the
collector may also be detrimental, e.g., by causing too many
oscillations en route to the collector, excessive fiber attenuation
or excessive fiber breakage.
[0039] Further details regarding the manner in which meltblowing
would be carried out using an apparatus like that shown in FIG. 1
and FIG. 2 will be familiar to those skilled in the art.
[0040] The nonwoven filtration media can be further stiffened if
desired using a variety of techniques. For example, an adhesive can
be employed to laminate together layers of the filter media, e.g.,
as described in U.S. Pat. No. 5,240,479 (Bachinski). The filter
media can also be made using conjugate fibers, e.g., as described
in U.S. Pat. No. 5,709,735 (Midkiff et al.). Further details
regarding the nonwoven filtration media can also be found in
copending U.S. patent application Ser. No. ______(attorney docket
no. 58214US002), filed even date herewith and entitled ALIGNED
FIBER WEB, the disclosure of which is incorporated herein by
reference.
[0041] FIG. 7 shows pleated filter media 100 having rows of pleats
102. The rows are aligned in the transverse direction, and the
substantially-aligned fibers in web 100 are aligned at
90.degree..+-.20.degree. with respect to the row direction, that
is, at .+-.20.degree. with respect to the machine direction. FIG. 8
shows pleated filter media 100 and an expanded metal support 110
mounted in frame 112 to provide filter 114. The increased stiffness
of pleated media 100 and the substantial machine direction fiber
alignment transverse to the row direction both are believed to
contribute to increased resistance of pleated media 100 to pleat
deformation at high filter face velocities.
[0042] FIG. 10 shows an apparatus 120 for making the disclosed
pleated filters. Aligned filtration media 122 is typically provided
on a continuous roll 124. Media 122 may be slit to a desired width
at slitting station 126. Media 122 may optionally be preheated at a
heating station 128 to perform annealing or to make web 122 more
flexible while passing into or through apparatus 120. In the
illustrated embodiment, the heating station 128 is an infrared
heater.
[0043] In one pleated filter embodiment, a reinforcing member 130
is applied to a rear face 132 of media 122 at location 134.
Reference to the rear face 132 (or to the front face 164, discussed
below) is for purposes of description only and does not indicate a
required airflow orientation of the completed pleated filter. For
example, the reinforcing member 130 may be positioned upstream or
downstream in the air flow. Reinforcing member 130 may be applied
in a variety of positions, e.g., as one or more continuous strips
oriented in the machine direction 136, as discrete reinforcing
members oriented transverse to the machine direction 136, or in
other configurations that will be familiar to those skilled in the
art. Reinforcing member 130 may be bonded to media 122 using a
variety of techniques, such as adhesive bonding, thermal bonding,
solvent bonding, or ultrasonic bonding. In this embodiment,
location 134 is upstream from rotary-score pleater 138 which scores
media 122 and reinforcing member 130 prior to pleating at pleat
folding station 140. Infrared heaters 142 may optionally be
provided for heat-setting the pleats 144. The pleats 144 are
retained in an accumulator 146 and then advanced to a pleat spacing
device 148 that retains the pleats 144 in the desired pleat
spacing. Pleat formation and pleat spacing may be performed by a
variety of methods, such as disclosed in U.S. Pat. No. 4,798,575
(Siversson '575), U.S. Pat. No. 4,976,677 (Siversson '677) and U.S.
Pat. No. 5,389,175 (Wenz).
[0044] The resulting pleated filter media 100 is expanded to the
desired pleat spacing in the pleat spacing device 148. One or more
elongated, planar reinforcing strips 162 may optionally be applied
to the pleat tips along the filter front face 164 at station 168 to
maintain the pleat spacing. The reinforcing strips 162 may be
bonded to the pleat tips by various techniques, such as adhesive
bonding, thermal bonding, solvent bonding, or ultrasonic bonding,
and can provide additional dimensional stability to the pleats 144.
The pleated filter media 100 and optional reinforcing strip 162 can
be cut to a desired size and shape. Pleated filter media 100 may be
used in filtration applications, with or without a frame structure,
or as an insert into a permanent or a reusable frame.
[0045] In another embodiment, a scrim 162 extending substantially
across the entire front face 164 may be employed. Scrim 162 may be
bonded to the pleat tips to provide additional dimensional
stability to pleated filter media 100.
[0046] In yet another embodiment, one or more elongated, planar
reinforcing strips 166 may optionally be bonded to the rear face
132 of pleated filter media 100 at station 168. In a further
embodiment, the reinforcing strips 166 may be located over the
reinforcing member 130 and opposite the reinforcing strips 162 to
form truss structures as shown in FIG. 6 of Sundet et al. '011.
[0047] In embodiments where the pleated filter media 100 is formed
without a surrounding frame, the pleated filter 172 exits the
system 120 after the cutting station 186.
[0048] In a framed filter embodiment, a continuous strip of frame
material 180 may be applied to the side edges of pleated filter
media 100 parallel to the machine direction 136. An adhesive, such
as a hot melt adhesive, may be applied to a first flange of a
U-shaped channel formed from frame material 180 at station 187. An
adhesive for sealing the ends of the pleats 144 may be applied at
station 188. An adhesive may be applied to a second flange of the
U-shaped channel at station 190. The frame material 180 may be bent
into a U-shaped configuration at station 194. The ends of U-shaped
channel may extend partially onto the front face 164 and rear face
132 of the pleated filter media 100. An assembly including the
endless web of pleated filter media 100 and attached frame material
180 may be cut at station 186 to desired lengths. The pleated
filter media 100 and reinforcing strips 162 may also be cut to size
before application of frame material 180.
[0049] The pleated filter media 100 and side frames members 192 may
be rotated 90.degree. at location 191 to permit application of end
frame members 196 at station 198 and formation of framed pleated
filter 174. Framed pleated filter 174 may also be formed by
configuring members 192, 196 as two box structures that are
positioned over the first and second faces 132, 164, respectively,
of pleated filter media 100, with overlapped circumferential
portions, such as disclosed in U.S. Pat. No. 5,782,944 (Justice).
In another embodiment, the frame member members 192, 196 can be
configured as a one-sided die-cut frame that is folded around
framed pleated filter 174.
[0050] The pleated filters 172, 174 are typically enclosed in
suitable packaging. For typical HVAC applications, the pleat
spacing is typically about 3 to about 6 pleats per 25.4 centimeters
(1 inch). The pleat depth and filter thickness is typically about
25 centimeters to about 102 centimeters (1 inch to 4 inches). The
filter length and width is typically about 30.5
centimeters.times.30.5 centimeters (12 inches.times.12 inches) to
about 50.8 centimeters.times.122 centimeters (20 inches.times.48
inches).
[0051] Further details regarding pleated filter manufacture can be
found in the above-mentioned Sundet et al. '011 and Sundet et al.
'100 patents or will be familiar to those skilled in the art.
[0052] The disclosed pleated filters may be employed in a variety
of applications including ventilation (e.g., furnace and clean room
filters), pollution control (e.g., baghouse filters), liquid
treatment (e.g., water filters), personal protection (e.g.,
protective suits with powered air supplies) and other applications
that will be familiar to those skilled in the art.
[0053] The invention will now be described with reference to the
following non-limiting examples, in which all parts and percentages
are by weight unless otherwise indicated. Several measurements were
carried out as follows:
Effective Fiber Diameter
[0054] Effective geometric fiber diameters were evaluated according
to the method set forth in Davies, C. N., "The Separation of
Airborne Dust and Particles," Institution of Mechanical Engineers,
London, Proceedings 1B, 1952.
Optical/Visual Web Properties
[0055] The overall visual web appearance was evaluated using a
Zeiss Instruments dissecting microscope equipped with a charge
coupled device camera having an 8 mm.times.14 mm magnification
window.
Taber Stiffness
[0056] Web stiffness was evaluated using a Model 150-B TABER.TM.
stiffness tester (commercially available from Taber Industries).
Square 3.8 cm.times.3.8 cm sections were carefully vivisected from
the webs using a sharp razor blade to prevent fiber fusion, and
evaluated to determine their stiffness in the machine and
transverse directions using 3 to 4 samples and a 15.degree. sample
deflection.
[0057] Stress-Strain
[0058] Stress-strain (or load vs. elongation) was measured using a
Model 5544 INSTRON.TM. universal testing machine (commercially
available from Instron Corp.). Rectangular 2.5 cm.times.6.3 cm
sections were cut from the webs using a sharp razor blade and
evaluated to determine the maximum force and elongation at maximum
force, using 6 to 10 samples, a 50 mm initial jaw separation and a
3 cm/min stretch rate.
[0059] Filtration Quality Factor
[0060] Filtration quality factors (Q.sub.F) were determined using a
TSI.TM. Model 8130 high-speed automated filter tester (commercially
available from TSI Inc.) and a dioctyl phthalate ("DOP") challenge
aerosol flowing at 42.5 L/min. Calibrated photometers were employed
at the filter inlet and outlet to measure the DOP concentration and
the % DOP penetration through the filter. An MKS pressure
transducer (commercially available from MKS instruments) was
employed to measure pressure drop (.DELTA.P, mm H.sub.2O) through
the filter. The equation: 1 Q F = - ln ( % DOP penetration 100 )
P
[0061] was used to calculate Q.sub.F. Q.sub.F values can be
reported as a curve plotting Q.sub.F vs. the DOP challenge total
mass after various time periods. However, the initial Q.sub.F value
usually provides a reliable indicator of overall performance, with
higher initial Q.sub.F values indicating better filtration
performance and lower initial Q.sub.F values indicating reduced
filtration performance. Initial filtration quality factors Q.sub.F
of at least about 0.6 (using 100 ppm dioctyl phthalate particles
having a size range between 10 and 700 nm traveling at a 7 cm/sec
face velocity), more preferably at least about 0.8 and most
preferably at least about 1 are preferred.
[0062] Filtration Performance
[0063] Filtration performance was evaluated according to ASHRAE
standard 52.2, "Method of Testing General Ventilation Air-Cleaning
Devices for Removal Efficiency by Particle Size". The ASHRAE
standard evaluates filtration of a test aerosol containing
laboratory-generated potassium chloride particles dispersed into an
airstream. A particle counter measures and counts the particles in
12 size ranges upstream and downstream from the filter. The results
can be reported as minimum composite efficiency values for
particles in various size ranges. The minimum composite efficiency
values correspond to the minimum percent particle retention (the
downstream particle count/upstream particle count.times.100 for the
size range in question) as the filter is loaded to a final pressure
drop of 25.4 mm H.sub.2O. A set of particle size removal efficiency
performance curves at incremental dust loading levels may also be
developed, and together with an initial clean performance curve may
be used to form a composite curve representing the minimum
performance in each size range. Points on the composite curve are
averaged and the averages used to determine the minimum efficiency
reporting value for the filter.
EXAMPLES 1-3 AND COMPARISON EXAMPLE 1
Polypropylene Webs
[0064] A conventional 20.5 cm wide meltblowing apparatus was
modified by addition of a secondary air quench system and a
flat-bed collector arranged as in FIG. 1. In a conventional blown
microfiber process, secondary quench air would not be employed and
the web would be collected on a rounded surface such as a porous
drum. The modified apparatus was used to make meltblown
polypropylene webs whose fibers were highly aligned in the machine
direction. The secondary air quench system employed two opposed
horizontally-disposed 76 cm wide.times.51 cm high air outlets
disposed approximately 6 cm below the meltblowing die tip,
dispensing 12-13.degree. C. chilled air flowing at various rates
(or not at all) through the air outlets. The flat-bed collector
employed a vacuum collection system located under the bed. The
meltblowing die was positioned over the leading edge of the
collector. FINA.TM. type 3960 polypropylene (commercially available
from Fina Oil and Chemical Co.) was melted in an extruder operated
at 265.degree. C. and fed at 9.1 kg/hr to the meltblowing die. The
die was maintained at about 265.degree. C. using resistance heaters
and supplied with 300.degree. C. primary air flowing at 4.2
m.sup.3/min. The DCD was adjusted to provide webs having a 0.5 mm
H.sub.2O pressure drop at a 32.5 L/min flow rate. For webs prepared
using secondary quench air, the DCD was approximately 20 cm. For
webs prepared without secondary quench air, the DCD was
approximately 34 cm. The collector vacuum was adjusted to provide
webs having 8-9% solidity. The collector vacuum was 3250 N/m.sup.2
for webs prepared using secondary quench air at 50 or 35 m/sec
outlet velocity, 5000 N/m.sup.2 for webs prepared using secondary
quench air at 17 m/sec outlet velocity, and zero for webs prepared
without secondary quench air. The collected webs had an 80
g/m.sup.2 basis weight and a 19 .mu.m EFD. The webs were
corona-treated as described in U.S. Pat. No. 4,588,537 to Klasse et
al., hydrocharged as described in U.S. Pat. No. 5,908,598 to
Rousseau et al. and evaluated to determine their mechanical
properties and filtration quality factor Q.sub.F. The webs were
also heat treated at 126.degree. C. for 5 minutes and reevaluated
to determine their mechanical properties.
[0065] Set out below in Table 1 are the Example No. or Comparison
Example No., secondary air velocity and mass flow ratio, the fiber
count and fiber alignment data, and the filtration quality factor
Q.sub.F for each web. Set out below in Table 2 are the machine
direction ("MD") and transverse direction ("TD") Taber Stiffness
and tensile strength values for the each web, and the ratio of MD
to TD Taber Stiffness and tensile strength. Set out below in Table
3 are the MD and TD Taber Stiffness and tensile strength values for
the heat treated webs, and the ratio of MD to TD Taber Stiffness
and tensile strength.
1TABLE 1 Example Secondary No. of Fibers No. of Fibers No. or Air
Mass Total No. Fibers within Fibers within Filtration Comp.
Velocity at Ratio, of within 0-10.degree. of within 0-20.degree. of
Quality Example Outlet Secondary Measured 0-10.degree. of MD
0-20.degree. of MD Factor, No. (m/sec) Air:Polymer Fibers MD (%) MD
(%) QF 1 50 1770 38 30 80 34 89 1.65 2 35 1450 33 20 61 27 82 1.5 3
17 720 32 16 50 32 56 0.85 Comp. 0 0 33 11 33 33 57 0.7 Ex.1
[0066]
2TABLE 2 Secondary Ratio, Tensile Tensile Ratio, Example No. Air
Velocity Mass Ratio, Taber Taber Taber Strength, Strength, Tensile
or Comp. at Outlet Secondary Stiffness, Stiffness, Stiffness MD TD
Strength Example No. (m/sec) Air:Polymer MD TD MD:TD (dynes)
(dynes) MD:TD 1 50 1770 1.3 0.6 2.2 1773 635 2.8 2 35 1450 1.9 0.7
2.7 2270 730 3.1 3 17 720 2.1 1.1 1.9 1374 1101 1.2 Comp. Ex. 1 0 0
1.5 1.3 1.2 1670 1370 1.2
[0067]
3TABLE 3 (Heat Treated) Secondary Ratio, Tensile Tensile Ratio,
Example No. Air Velocity Mass Ratio, Taber Taber Taber Strength,
Strength, Tensile or Comp. at Outlet Secondary Stiffness,
Stiffness, Stiffness MD TD Strength Example No. (m/sec) Air:Polymer
MD TD MD:TD (dynes) (dynes) MD:TD 1 50 1770 1.2 0.7 1.7 2298 780
2.9 2 35 1450 1.9 0.7 2.7 2303 765 3.0 3 17 720 2.5 1.6 1.6 1770
1100 1.6 Comp. Ex. 1 0 0 2.5 2.0 1.3 1700 1489 1.1
[0068] As shown in Table 1, webs made using secondary quench air
had significantly greater machine direction fiber alignment than
webs made without secondary quench air. This is further illustrated
in FIGS. 3-6, which are polar "radar" plots respectively showing
the number and orientation (in degrees with respect to the
0.degree. machine direction) of fibers in Examples 1-3 and
Comparison Example 1. Because fibers oriented at 0.degree. with
respect to the machine direction could also be said to be oriented
at 180.degree., the plots have symmetric lobes reflected about the
origin. FIG. 3 shows for example that 12 fibers were oriented at
0.degree. with respect to the machine direction, 9 fibers were
oriented at -10.degree., 8 fibers were oriented at +10.degree., 6
fibers were oriented at -20.degree., and so on. FIGS. 3-5 show that
the disclosed webs had considerably greater machine direction
alignment than the web prepared without secondary quench air
plotted in FIG. 6. In a further comparison, the FIG. 4 web shown in
the Lau et al. '749 patent was evaluated to determine its fiber
orientation and fiber count, and found to have only 50% of its
fibers aligned within .+-.20.degree. of the machine direction.
[0069] The FIG. 3-5 plots generally mirror the behavior of a
wetting fluid placed on the non-collector side of the web. If a
drop of a suitable wetting fluid (preferably colored to aid in
observation) is so placed it will tend to spread into the web in an
oblong pattern generally corresponding to the radar plot lobe
shapes, thus providing a convenient indicator of the primary
direction of fiber orientation and web formation. When a wetting
fluid is placed on the Comparison Example 1 web, it tends to spread
more or less evenly outward in an expanding circular pattern.
[0070] Table 1 also shows that as the secondary quench air volume
increased, the filtration quality factor Q.sub.F increased and then
decreased slightly.
[0071] Webs prepared using secondary air had visible striations
generally aligned in the machine direction, a surface with an
overall smooth sheen and slight fuzziness, and few or none of the
nodules that usually are found in conventional blown microfiber
webs. Fibers having approximately 2 to 5 cm lengths could be teased
from the webs using tweezers. Webs prepared without secondary
quench air visually resembled standard blown microfiber webs
collected on a round collector. Some relatively short (less than 1
cm) individual fibers could be removed from these webs using
tweezers, but only with great difficulty.
[0072] As shown in Table 2, webs made using secondary quench air
had significantly more anisotropic Taber Stiffness than webs made
without secondary quench air. Webs made using higher volumes of
secondary quench air also had significantly more anisotropic
tensile strength than webs made without secondary quench air.
[0073] As shown in Table 3, heat treating could be used to increase
web stiffness and tensile strength. For several of the webs this
could be done without causing a substantial change in the web's
overall mechanical anisotropy as measured using MD:TD property
ratios.
EXAMPLES 4-6
Smaller EFD Webs
[0074] Using the general method of Example 1, meltblown
polypropylene webs were prepared using 300.degree. C. primary air
flowing at 3.4 m.sup.3/min and secondary quench air and collector
vacuum adjusted to provide collected webs having an 80 g/m.sup.2
basis weight, 8-9% solidity and smaller effective fiber diameters
than were obtained in Example 1. Set out below in Table 4 are the
Example No., secondary air velocity and mass flow ratio, effective
fiber diameter and the fiber count and fiber alignment data for the
resulting webs.
4TABLE 4 Secondary Fibers Fibers Air Velocity Mass Ratio, Total No.
of No. of Fibers within 0-10.degree. No. of Fibers within
0-20.degree. Example at Outlet Secondary Measured within
0-10.degree. of MD within 0-20.degree. of MD No. (m/sec)
Air:Polymer EFD Fibers of MD (%) of MD (%) 4 22.5 930 13.5 38 24 63
29 76 5 20 830 13.4 41 20 49 26 63 6 14.5 620 7.4 43 18 42 26
60
[0075] As shown in Table 4, webs having substantial machine
direction fiber alignment could be made at a variety of effective
fiber diameters.
EXAMPLES 7-10 AND COMPARISON EXAMPLES 2 and 3
PET and Nylon Webs
[0076] Using the general method of Example 1, polyethylene
terephthalate ("PET") and nylon (ULTRAMID.TM. BS-400N nylon,
commercially available from BASF Corp.), were employed to prepare
nonwoven webs using 350.degree. C. primary air flowing at 2.9
m.sup.3/min and optional secondary quench air. A 12.7 cm DCD was
used to prepare the PET webs and a 16.5 cm DCD was used to prepare
the nylon webs. The collected PET webs had an 85 g/m.sup.2 basis
weight, 5-6% solidity and a 16 .mu.m EFD. The collected nylon webs
had a 70 g/m.sup.2 basis weight, 5-6% solidity and a 17-18 .mu.m
EFD. Set out below in Table 5 are the Example No. or Comparison
Example No., secondary air velocity and mass flow ratio, polymer
employed and the fiber count and fiber alignment data for the
resulting webs.
5TABLE 5 Secondary Example Air Total No. No. of Fibers No. of
Fibers No. or Velocity Mass Ratio, of Fibers within 0-10.degree.
Fibers within 0-20.degree. Comp. at Outlet Secondary Measured
within 0-10.degree. of MD within 0-20.degree. of MD Ex. No. (m/sec)
Air:Polymer Polymer Fibers of MD (%) of MD (%) 7 35 1450 PET 41 26
63 32 78 8 22.5 930 PET 36 21 58 25 69 Comp. 0 0 PET 38 9 23 12 31
Ex. 2 9 35 1450 Nylon 33 17 50 21 64 10 22.5 930 Nylon 45 16 35 23
52 Comp. 0 0 Nylon 38 10 26 19 50 Ex. 3
[0077] As shown in Table 5, webs having substantial machine
direction fiber alignment could be made using a variety of
polymers.
EXAMPLE 11
Additive
[0078] Example 2 was repeated using a 1.5% addition of the additive
poly(4-methyl-1-pentene). This increased the filtration quality
factor Q.sub.F from 1.5 without the additive to 1.7 with the
additive.
EXAMPLE 12
Additives
[0079] Example 11 was repeated using a 1.5% addition of the
additive poly(4-methyl-1-pentene) and a 0.5% addition of CHIMASSORB
944 hindered amine light stabilizer. The web was hydrocharged but
not corona-treated. The filtration quality factor Q.sub.F was 2.5,
and more than double that obtained using webs made from
conventional untreated polypropylene blown microfibers made without
secondary quench air.
EXAMPLE 13 AND COMPARISON EXAMPLE 4
Q.sub.F Evaluation Over Time
[0080] Using the general method of Example 1, meltblown
polypropylene webs were prepared with and without secondary quench
air flowing at a 1770 secondary quench air:polymer mass flow ratio,
corona-treated, hydrocharged, and evaluated to determine their
filtration quality factor Q.sub.F. The webs had an 85 g/m.sup.2
basis weight, 19-21 .mu.m EFD, and a pressure drop of 0.4-0.5 mm
H.sub.2O at 42.5 L/min. Set out below in Table 6 are the Q.sub.F
factors after various cumulative DOP exposure levels for the web
made with (Example 13) or without (Comparison Example 4) secondary
quench air.
6TABLE 6 Cumulative DOP Comparison Challenge, Example 13 Example 4
(mg) Web, Q.sub.F Web, Q.sub.F Initial 1.8 0.7 50 1 0.2 100 0.6
0.15 150 0.45 0.1 200 0.3 <0.1
[0081] As shown in Table 6, a web made using secondary quench air
and having substantial machine direction fiber alignment provided
significantly better filtration performance than a web made without
secondary quench air and having less fiber alignment.
EXAMPLES 14 AND 15 AND COMPARISON EXAMPLES 5-7
Pleated Furnace Filters
[0082] The corona-treated and hydrocharged Example 2 webs, the
corona-treated and hydrocharged Comparison Example 1 webs and a
sample of ACCUAIR# corona-treated spunbond
polyethylene/polypropylene twinned fiber web (71 g/m.sup.2 basis
weight, .about.20 .mu.m EFD, commercially available from Kimberly
Clark Corp.) were formed into 50.8 cm.times.63.5 cm.times.2.1 cm
high filter elements having pleats 102 like pleated media 100 shown
in FIG. 7. The pleats 102 were arranged so that the folds were
aligned with the transverse direction, with 87 pleats (13.8
pleats/10 cm) along the long dimension. The pleated media 100 was
sandwiched between and glued to expanded metal supports like
support 110 and mounted in a cardboard frame like frame 112 shown
in FIG. 8 to form a framed filter like filter 114. The finished
filters were evaluated for filtration performance according to
ASHRAE standard 52.2, "Method of Testing General Ventilation
Air-Cleaning Devices for Removal Efficiency by Particle Size" at a
1.5 m/sec face velocity. The results reported below in Table 7 show
the minimum composite efficiency values for particles in the size
ranges 0.3 to 1 .mu.m, 1 to 3 .mu.m and 3 to 10 .mu.m. Table 7 also
reports the total filter weight gain (total particulate weight
captured by the filter) after completion of the evaluation. Higher
minimum composite efficiency and total filter weight gain values
correspond to better filtration, longer service life or both better
filtration and longer service life.
7TABLE 7 Total Minimum Filter Composite Weight Example Initial
Efficiency (%) Gain, No. or Pressure 0.1 1 3 25.4 mm Comp. Web Web
Drop, mm to to to H.sub.2O Ex. No. Employed Treatment H.sub.2O 1
.mu.m 3 .mu.m 10 .mu.m (g) 14 Example 2 Corona- 4.9 43 81 91 35
treated 15 Example 2 Corona- 5.1 53 89 97 37.1 treated and
hydrocharged Comp. Comp. Ex. 1 Corona- 4.6 36 71 86 23.8 Ex. 5
treated Comp. Comp. Ex. 1 Corona- 4.6 42 81 93 33.9 Ex. 6 treated
and hydrocharged Comp. ACCUAIR Corona- 4.6 41 80 91 26.1 Ex. 7 web
treated
[0083] As shown in Table 7, corona-treated or corona-treated and
hydrocharged meltblown nonwoven webs made using secondary quench
air and having substantial machine direction fiber alignment
(Examples 14 and 15) provided much better minimum composite
efficiency at a 1 mm H.sub.2O pressure drop than otherwise similar
meltblown nonwoven webs made without secondary quench air and
having less fiber alignment (Comparison Examples 5 and 6). The
Example 14 and Example 15 webs also had comparable or better
minimum composite efficiency than a commercial spunbond nonwoven
web (Comparison Example 7). The Example 14 and Example 15 webs had
better particle capture (as evidenced by their higher total filter
weight gain values) than the Comparison Example 5-7 webs.
EXAMPLES 16-18 AND COMPARISON EXAMPLE 8
Pleated Furnace Filter Pressure Drop vs. Face Velocity
Evaluation
[0084] Using the general method of Examples 14 and 15, a meltblown
aligned fiber polypropylene web was corona-treated and hydrocharged
but not heat treated. This web had a 1.7 MD Taber Stiffness value
and is identified below as the web of Example 16. A stiffer web was
prepared using corona-treatment, hydrocharging and heat treatment.
This web had a 2.2 MD Taber Stiffness value and is identified below
as the web of Example 17. A yet stiffer web was prepared using a
1.5% addition of the additive poly(4-methyl-1-pentene),
corona-treatment, hydrocharging and heat treatment. This web had a
3.7 MD Taber Stiffness value and is identified below as the web of
Example 18. The Example 16-18 webs and a sample of ACCUAIR
corona-treated spunbond polyethylene/polypropylene twinned fiber
web (having a 2.1 MD Taber Stiffness and identified below as the
web of Comparison Example 8) were formed into 30 cm.times.27
cm.times.2.1 cm high filter elements having pleats 102 like filter
media 100 shown in FIG. 7. The filters had 13.8 pleats/10 cm along
the long dimension, and were sandwiched between and glued to
expanded metal supports like support 110 in FIG. 8. In a series of
runs, each such filter was mounted in a PLEXIGLAS.TM. plastic frame
whose transparent side plates permitted the pleat edges to be
photographed. The frame side plates touched the filtration media
edges but permitted pleat movement. The frame was mounted atop a
vacuum table and exposed to air from a downwardly-directed box fan.
The filters were loaded by sprinkling a synthetic dust made from a
50:50 mixture of SAE Fine Test Dust and talc into the air stream
until the filter pressure drop reached about 0.35 in (0.9 cm) of
water at an approximate 1.5 m/sec face velocity. This simulated a
substantial natural loading level. Set out below in Table 8 are the
filter descriptions, MD Taber Stiffness values and total filter
weight gain values.
8TABLE 8 Example No. or MD Taber Total Filter Weight Gain Comp. Ex.
No. Stiffness (g) 16 1.7 37.9 17 2.2 40.2 18 3.7 33.4 Comp. Ex. 8
2.1 36.2
[0085] The filters were next mounted in a duct equipped with an
anemometer and exposed to flowing air at velocities sufficient to
cause pressure drops between about 0.2 in. (0.5 cm) of water and
1.2 in. (3 cm) of water. The Example 16 (1.7 MD Taber Stiffness)
filter began to exhibit noticeable pleat deformation, manifested by
pinching together of the pleats at the filter air inlet side, at a
0.35 in (0.9 cm) pressure drop. The Example 17 (2.2 MD Taber
Stiffness) and Comparison Example 8 filters began to exhibit
noticeable pleat deformation at a 0.5 in (1.3 cm) pressure drop.
The Example 18 (3.7 MD Taber Stiffness) filter did not exhibit
noticeable pleat deformation even at a 1.2 in (3 cm) pressure drop.
FIG. 9 shows a plot of the pressure drop (in inches of water) vs.
anemometer reading (in nominal units) for the filters of Example 16
(curve 116), Example 17 (curve 117), Example 18 (curve 118) and
Comparison Example 8 (curve 119). As shown in FIG. 9, the Example
18 filter exhibited a linear increase in pressure drop as airflow
increased, whereas the other tested filters exhibited a non-linear
response, indicative of pleat distortion, as airflow increased.
[0086] Various modifications and alterations of this invention will
be apparent to those skilled in the art without departing from this
invention. This invention should not be restricted to that which
has been set forth herein only for illustrative purposes.
* * * * *