Note: Descriptions are shown in the official language in which they were submitted.
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HIGH STIFFNESS NONWOVEN FILTER MEDIUM
RAGKGROUND OF THE INVENTION
This invention relates generally to a nonwoven fabric or web which is formed
from
spunbond fibers of a thermoplastic resin and laminates using such a web as a
component.
The fabric has a high stiffness and may be used as a filter.
i o Thermoplastic resins have been extruded to form fibers, fabrics and webs
for a number
of years. The most common thermoplastics for this application are potyolefins,
particularly
polypropylene. Other materials such as polyesters, polyetheresters, polyamides
and
polyurethanes are also used to form nonwoven spunbond fabrics.
Nonwoven fabrics or webs are useful for a wide variety of applications such as
diapers,
1s feminine hygiene products, towels, recreational or protective fabrics and
as geotextiles and
filter media. The nonwoven webs used in these applications may be simply
spunbond
fabrics but are often in the form of nonwoven fabric laminates like
spunbondlspunbond (SS)
laminates or spunbond/meltblown/spunbond (SMS) laminates.
As filter media, some of the desired characteristics of nonwoven fabrics are
that they be
2o permeable to the fluid being filtered yet have a high filtration
efficiency. Permeability to the
fluid being filtered is quite important as low permeability could result in a
high pressure drop
across the filter requiring a higher, and hence more costly, energy input into
the filtered fluid
and shortening filter life. Low permeability could also result in physical
damage to the filter
upon being clogged with filtered particles because of increased pressure drop
across. the
25 filter.
CA 02231507 2004-03-25
High filtration efficiency is, of course, the main purpose for a filter and
great
efficiency and ability to maintain the efficiency at an acceptable level are
key to
filter performance.
In many applications, filtration materials are required which have structural
integrity by themselves and can be converted into various shapes and which
will
then hold that shape. This convertibility is aided by stiffening the filter
medium.
Commonly, stiff filter media can be made into a pleated shape which gives far
more surface area for filtration than a non-pleated shape in the same space.
The present invention seeks to provide a spunbond polyolefin nonwoven
fabric or web for use as a filter medium which has a high permeability and
high
filtration efficiency. The present invention also seeks to provide a filter
medium
which is stiff and so can be successfully converted into a finished pleated
filter.
The present invention also seeks to provide a pleated filter made from the
filter
medium.
SUMMARY OF THE INVENTION
According to an aspect of the present invention there is provided a filter
medium which is a nonwoven web of conjugate fibers having a Frazier
permeability above 200 CFM, an NaCI efficiency above 75 percent, a Gurley
Stiffness above 700 mg, and a SEP value of above 80. The conjugate fibers are
made from polymers, more particularly polyolefins, still more particularly
polypropylene and polyethylene in a side-by-side configuration. The filter
medium
may be treated by a hot-air knife, through-air bonded and electret treated
during
the production process. The medium has sufficient stiffness to be converted
into a
filter by conventional means such as pleating. Such filters may be used in air
filtration for home or commercial heating, ventilating and air conditioning
(HVAC)
systems and may also be used in filtration of breathing air in transportation
applications like automobile cabin air filtration, airplane cabin air
filtration, and
train and boat air filtration. While the invention is directed mainly to air
filtration,
other gasses may be filtered as well.
According to another aspect of the present invention there is provided a
filter medium having high stiffness comprising a nonwoven web of crimped
polypropylene/polyethylene side-by-side conjugate fibers having a basis weight
between about 68 gsm and about 340 gsm and having a Frazier permeability
2
CA 02231507 2004-03-25
above 200 CFM, an NaCI efficiency above 75 percent, a Gurley Stiffness above
700 mg, and a SEP value of above 80, formed by a process wherein said web is
subjected to hot air knife treatment, through-air bonding and electret
treatment.
According to another aspect of the present invention there is provided a
pleated filter comprising a nonwoven Web of crimped polypropylene/polyethylene
side-by-side conjugate fibers having a basis Weight between about 68 gsm and
about 340 gsm, a Frazier permeability above 200 CFM, an NaCf efficiency above
75 percent, a Gurley Stiffness above 700 mg, and a SEP value of above 80,
formed by a process wherein said web is subjected to hot air knife treatment
immediately after formation, through-air bonding after said hot air knife
treatment,
electret treatment after said through-air bonding, and wherein said web is
pleated
to form a filter.
2a
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,~,IRIEF DESDRIPTION OF THE DRAWING
Figure 1 is a schematic drawing of a process line for making a filter medium
of this
invention.
DEFINITIONS
As used herein the term "nonwoven fabric or web" means a web having a
structure of
individual fibers or threads which are interlaid, but not in an identifiable
manner as in a
to knitted fabric. Nonwoven fabrics or webs have been formed from many
processes such as,
for example, meltblowing processes, spunbonding processes, and bonded carded
web
processes. The basis weight of nonwoven fabrics is usually expressed in ounces
of material
per square yard (osy) or grams per square meter (gsm) and the fiber diameters
useful are
usually expressed in microns. (Note that to convert from osy to gsm, multiply
osy by 33.91).
As used herein the term "microfibers" means small diameter fibers having an
average
diameter not greater than about 75 microns, for example, having an average
diameter of
from about 0.5 microns to about 50 microns, or more particularly, microfibers
may have an
average diameter of from about 2 microns to about 40 microns. Another
frequently used
expression of fiber diameter is denier, which is defined as grams per 9000
meters of a fiber
2o and may be calculated as fiber diameter in microns squared, multiplied by
the density in
grams/cc, multiplied by 0.00707. A lower denier indicates a finer fiber and a
higher denier
indicates a thicker or heavier fiber. For example, the diameter of a
polypropylene fiber given
as 15 microns may be converted to denier by squaring, multiplying the result
by .89 g/cc and
multiplying by .00707. Thus, a 15 micron polypropylene fiber has a denier of
about 1.42 (152
2s x 0.89 x .00707 = 1.415). Outside the United States the unit of measurement
is more
commonly the "tex", which is defined as the grams per kilometer of fiber. Tex
may be
calculated as denier/9.
3
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As used herein the term "spunbonded fibers" refers to small diameter fibers
which are
formed by extruding molten thermoplastic material as filaments from a
plurality of fine,
usually circular capillaries of a spinneret with the diameter of the extruded
filaments then
being rapidly reduced as by, for example, in U.S. Patent no. 4,340,563 to
Appel et al., and
s U.S. Patent no. 3,692,618 to Dorschner et al., U.S. Patent no. 3,802,817 to
Matsuki et al.,
U.S. Patent nos. 3,338,992 and 3,341,394 to Kinney, U.S. Patent no. 3,502,763
to Hartman,
and U.S. Patent no. 3,542,615 to Dobo et al. Spunbond fibers are generally not
tacky when
they are deposited onto a collecting surface. Spunbond fibers are generally
continuous and
have average diameters (using a sample size of at least 10) larger than 7
microns, more
io particularly, between about 10 and 20 microns.
As used herein the term "meltblown fibers" means fibers formed by extruding a
molten thermoplastic material through a plurality of fine, usually circular,
die capillaries as
molten threads or filaments into converging high velocity, usually hot, gas
(e.g. air) streams
which attenuate the ~taments of molten thermoplastic material to reduce their
diameter,
is which may be to microfiber diameter. Thereafter, the meltblown fibers are
carried by the
high velocity gas stream and are deposited on a collecting surface to form a
web of randomly
disbursed meltblown fibers. Such a process is disclosed, for example, in U.S.
Patent no.
3,849,241 to Butin. Meltblown fibers are microfibers which may be continuous
or
discontinuous, are generally smaller than 10 microns in average diameter
(using a sample
2o size of at least 10), and are generally tacky when deposited onto a
collecting surface.
As used herein the term "polymer" generally includes but is not limited to,
homopolymers, copolymers, such as for example, block, graft, random and
alternating
copolymers, terpolymers, etc. and blends and modifications thereof.
Furthermore, unless
otherwise specifically limited, the term "polymer' shall include but are not
limited to isotactic,
2s syndiotactic and random symmetries.
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As used herein, the term "machine direction" or MD means the length of a web
in the
direction in which it is produced. The term "cross machine direction" or CD
means the width
of web, i.e. a direction generally perpendicular to the MD.
As used herein the term "monocomponent" fiber refers to a fiber formed from
one or
s more extruders using only one polymer. This is not meant to exclude fibers
formed from one
polymer to which small amounts of additives have been added for coloration,
anti-static
properties, lubrication, hydrophilicity, etc. These additives, e.g. titanium
dioxide for color, are
generally present in an amount less than 5 weight percent and more typically
about 2 weight
percent.
to As used herein the term "conjugate fibers" refers to fibers which have been
formed
from at least two polymers extruded from separate extruders but spun together
to form one
fiber. Conjugate fibers are also sometimes referred to as multicomponent or
bicomponent
fibers. The polymers are usually different from each other though conjugate
fibers may be
monocomponent fibers. The polymers are arranged in substantially constantly
positioned
i5 distinct zones across the cross-section of the conjugate fibers and extend
continuously along
the length of the conjugate fibers. The configuration of such a conjugate
fiber may be, for
example, a sheath/core arrangement wherein one polymer is surrounded by
another or may
be a side by side arrangement, a segmented configuration or an "islands-in-the-
sea" I
arrangement. Conjugate fibers are taught in U.S. Patent 5,108,820 to Kaneko et
al., U.S.
2o Patent 5,336,552 to Strack et al., and U.S. Patent 5,382,400 to Pike et al.
For two
component fibers, the polymers may be present in ratios of 75/25, 50/50, 25/75
or any other
desired ratios.
As used herein, the term "compaction roll" means a set of rollers above and
below the
web to compact the web as a way of treating a just produced spunbond web in
order to give
25 it sufficient integrity for further processing, but not the relatively
strong bonding of secondary
bonding processes like through-air bonding, thermal bonding and ultrasonic
bonding.
Compaction rolls slightly squeeze the web in order to increase its self-
adherence and
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thereby its integrity. Compaction rolls perform this function well but have a
number of
drawbacks. One such drawback is that compaction rolls do indeed compact the
web,
causing a decrease in bulk or loft in the web which may be undesirable for the
use desired.
A second and more serious drawback to compaction rolls is that the web will
sometimes
wrap around one or both of the rollers, causing a shutdown of the web
production line for
cleaning of the rollers, with the accompanying obvious loss in production
during the down
time. A third drawback of compaction rolls is that if a slight impertection is
produced in
formation of the web, such as a drop of polymer being formed into the web, the
compaction
roll can force the drop into the foraminous belt, onto which most webs are
formed, causing
io an impertection in the belt and ruining it.
As used herein, the term "hot air knife' or HAK means a process of pre- or
primarily
bonding a just produced spunbond web in order to give it sufficient integrity,
i.e. increase the
stiffness of the web, for further processing, but does not mean the relatively
strong bonding
of secondary bonding processes like TAB, thermal bonding and ultrasonic
bonding. A hot air
knife is a device which focuses a stream of heated air at a very high flow
rate, generally from
about 1000 to about 10000 feet per minute (fpm) (305 to 3050 meters per
minute), or more
particularly from about 3000 to 5000 feet per minute (915 to 1525 m/min.)
directed at the
nonwoven web immediately after its formation. The air temperature is usually
in the range of
the melting point of at least one of the polymers used in the web, generally
between about
200 and 550°F (93 and 290°C) for the thermoplastic polymers
commonly used in
spunbonding. The control of air temperature, velocity, pressure, volume and
other factors
helps avoid damage to the web while increasing its integrity. The HAK's
focused stream of
air is arranged and directed by at least one slot of about 1/8 to 1 inches (3
to 25 mm) in
width, particularly about 3/8 inch (9.4 mm), serving as the exit for the
heated air towards the
2s web, with the slot running in a substantially cross-machine direction over
substantially the
entire width of the web. In other embodiments, there may be a plurality of
slots arranged
next to each other or separated by a slight gap. The at least one slot is
usually, though not
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essentially, continuous, and may be comprised of, for example, closely spaced
holes. The
HAK has a plenum to distribute and contain the heated air prior to its exiting
the slot. The
plenum pressure of the HAK is usually between about 1.0 and 12.0 inches of
water (2 to 22
mmHg), and the HAK is positioned between about 0.25 and 10 inches and more
preferably
s 0.75 to 3.0 inches (19 to 76 mm) above the forming wire. In a particular
embodiment the
HAK plenum's cross sectional area for cross-directional flow (i.e. the plenum
cross sectional
area in the machine direction) is at least twice the total slot exit area.
Since the foraminous
wire onto which spunbond polymer is formed generally moves at a high rate of
speed, the
time of exposure of any particular part of the web to the air discharged from
the hot air knife
io is less than a tenth of a second and generally about a hundredth of a
second in contrast with
the through air bonding process which has a much larger dwell time. The HAK
process has
a great range of variability and controllability of many factors such as air
temperature,
velocity, pressure, volume, slot or hole an-angement and size, and the
distance from the HAK
plenum to the web.
i5 As used herein, through air bonding or 'TAB" means a process of bonding a
nonwoven conjugate fiber web in which air which is sufficiently hot to melt
one of the
polymers of which the fiibers of the web are made is forced through the web.
The air velocity
is between 100 and 500 fpm (30-152 m/min.) and the dwell time may be as long
as 60
seconds. The air temperature may be between about 230 and 325°F (110-
162°C),
2 o depending on the melting points of the polymers used. The melting and
resolidification of
the polymer provides the bonding. Through air bonding is generally regarded a
second step
bonding process, and since TAB requires the melting of at least one component
to
accomplish bonding, it is restricted to webs with at least two components like
conjugate
fibers or those which include an adhesive.
z5 As used herein, "ultrasonic bonding" means a process pertormed, for
example, by
passing the web between a sonic hom and anvil roll as illustrated in U.S.
Patent 4,374,888 to
Bomslaeger.
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As used herein "thermal point bonding°' involves passing a fabric or
web of fibers to
be bonded between a heated calender roll and an anvil roll. The calender roll
is usually,
though not always, patterned in some way so that the entire fabric is not
bonded across its
entire surface and the anvil is usually flat. As a result, various patterns
for calendar rolls
s have been developed for functional as well as aesthetic reasons. One example
of a pattern
is the Hansen Pennings or "HIP" pattern with between about a 5 and 50% bond
area with
between about 50-3200 bondslsquare inch as taught in U.S. Patent 3,855,046 to
Hansen
and Pennings. One example of the H8~P pattern has square point or pin bonding
areas
wherein each pin has a side dimension of 0.038 inches (0.965 mm), a spacing of
0.070
io inches (1.778 mm) between pins, and a depth of bonding of 0.023 inches
(0.584 mm).
Another typical point bonding pattern is the expanded Hansen Pennings or "EHP"
bond
pattern which produces about a 159'° bond area with a square pin having
a side dimension of
0.037 inches (0.94 mm), a pin spacing of 0.097 inches (2.464 mm) and a depth
of 0.039
inches (0.991 mm). Another typical point bonding pattern designated
'°714" has square pin
is bonding areas wherein each pin has a side dimension of 0.023 inches, a
spacing of 0.062
inches (1.575 mrn) between pins, and a depth of bonding of 0.033 inches (0.838
mm). The
resulting pattern has a bonded area of about 15%. Yet another common pattern
is the G
Star pattern which has a bond area of about 16.9%. The C-Star pattern has a
cross-
directional bar or "corduroy" design interrupted by shooting stars. Other
common patterns
2o include a diamond pattern with repeating and slightly offset diamonds and a
wire weave
pattern looking as the name suggests, e.g. like a window screen. Typically,
the percent
bonding area varies from around 10% to around 30% of the area of the fabric
laminate web.
As in well known in the art, the spot bonding holds the laminate layers
together as well as
imparts integrity to each individual layer by bonding filaments and/or fibers
within each layer.
As used herein, the term "bonding window" means the range of temperature of
the
mechanism, e.g. calendar rolls or through-air bonding, used to bond the
nonwoven web
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together, over which such bonding is successful. For polypropylene spunbond,
this bonding
window is typically from about 270°F to about 310°F
(132°C to 154°C). Below about 270°F
- the polypropylene is not hot enough to melt and bond and above about
310°F the
polypropylene will melt excessively and can stick to the calender rolls.
Polyethylene has an
s even narrower bonding window at a lower temperature range, e.g. 235 to
260°F (113 -
127°C).
TEST METHODS
i o Frazier Permeability: A measure of the permeability of a fabric or web to
air is the
Frazier Permeability which is performed according to Federal Test Standard No.
191A,
Method 5450 dated July 20, 1978, and is reported as an average of 3 sample
readings.
Frazier Permeability measures the air flow rate through a web in cubic feet of
air per square
foot of web per minute or CFM. Convert CFM to liters per square meter per
minute (LSM) by
is multiplying CFM by 304.8.
NaCI Efficiency: The NaCI Efficiency is a measure of the ability of a fabric
or web to
stop the passage of small particles through it. A higher efficiency is
generally more desirable
and indicates a greater ability to remove particles. NaCI efficiency is
measured in percent
according to the TSI Inc., Model 8110 Automated Filter Tester Operation Manual
of February
20 1993, P/N 1980053, revision D, at a flow rate of 32 liters per minute using
0.1 micron sized
NaCI particles and is reported as an average of 3 sample readings. The manual
is available
from TSI Inc. at PO Box 64394, 500 Cardigan Rd, St. Paul, MN 55164.
Melt Flow Rate: The melt flow rate (MFR) is a measure of the viscosity of a
polymer.
The MFR is expressed as the weight of material which flows from a capillary of
known
2s dimensions under a specified load or shear rate for a measured period of
time and is
measured in grams/10 minutes at a set temperature and load according to, for
example,
ASTM test 1238-90b.
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Gurley Stiffness: The Gurley Stiffness test measures the bending resistance of
a
material. It is carried out according to TAPPI Method T 543 om-94 and is
measured in
milligrams and reported as an average of 5 sample readings. The sample size
used for the
testing herein was 1.5 inch (3.8 cm) in the MD by 1 inch (2.54 cm) in the CD.
DETAILED DESCRIPTION
The spunbond process generally uses a hopper which supplies polymer to a
heated
extruder. The extruder supplies melted polymer to a spinneret where the
polymer is fiberized
io as it passes through fine openings arranged in one or more rows in the
spinneret, forming a
curtain of filaments. The filaments are usually quenched with air at a tow
pressure, drawn,
usually pneumatically and deposited on a moving foraminous mat, belt or
"forming wire" to
form the nonwoven web. Polymers useful in the spunbond process generally have
a
process melt temperature of between about 400°F to about 610°F
(200°C to 320°C).
The fibers produced in the spunbond process are usually in the range of from
about 10
to about 50 microns in average diameter, depending on process conditions and
the desired
end use for the webs to be produced from such fibers. For example, increasing
the polymer
molecutar weight or decreasing the processing temperature results in larger
diameter fibers.
Changes in the quench fluid temperature and pneumatic draw pressure can also
affect fiber
2o diameter. The fibers used in the practice of this invention usually have
average diameters in
the range of from about 7 to about 35 microns, more particularly from about 15
to about 25
microns.
The fabric of this invention may be a multilayer laminate incorporating the
high stiffness
filter medium polymer fiber web and may be formed by a number of different
techniques
a5 including but not limited to using adhesive, needle punching, ultrasonic
bonding, thermal
calendering and any other method known in the art. Such a multilayer laminate
may be an
embodiment wherein some of the layers are spunbond and some meltblown such as
a
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spunbond/meltblownfspunbond (SMS) laminate as disclosed in U.S. Patent no.
4,041,203 to
Brock et al. and U.S. Patent no. 5,169,706 to Collier, et al. or as a
spunbond/spunbond
laminate. An SMS laminate may be made by sequentially depositing onto a moving
conveyor belt or forming wire first a spunbond web layer, then a meltblown web
layer and
s last another spunbond layer and then bonding the laminate in a manner
described above.
Alternatively, the three web layers may be made individually, collected in
rolls, and combined
in a separate bonding step.
Areas in which the web of this invention may find utility are in filtration.
More
particularly, webs produced according to this invention are useful in heavier
basis weight
io applications. Filter fabrics may have basis weights ranging from about 0.25
osy (8.5 gsm) to
about 10 osy (340 gsm).
The fibers used to produce the web of this invention are conjugate fibers,
such as side-
by-side (S/S) fibers. The polymers used to produce the fibers are may be
polyolefins,
particularly polypropylene and polyethylene. As these conjugate fibers are
produced and
is cooled, the differing coefficients of expansion of the polymers cause these
fibers to bend
and ultimately to crimp, somewhat akin to the action of the bimetallic strip
in a conventional
room thermostat. Crimped fibers have an advantage over uncrimped fibers in
that they
produce a more bulky web which therefore increases fabric or web permeability.
High
permeability is a very desirable characteristic for a filter and so crimped
fiber fitters are more
2 o desirable than uncrimped fiber filters.
Many polyolefins are available for fiber production, for example polyethylenes
such as
Dow Chemical's ASPUN~ 6811A linear low density polyethylene, 2553 LLDPE and
25355
and 12350 high density polyethylene are such suitable polymers. The
polyethylenes have
melt flow rates in g/10 min. at 190°F and a load of 2.16 kg, of about
26, 40, 25 and 12,
_ 2s respectively. Fiber forming potypropylenes include Exxon Chemical
Company's
ESCORENE~ PD 3445 polypropylene and Himont Chemical Co.'s PF-304. Many other
polyolefins are commercially available.
11
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After the fibers are crimped and deposited on the forming wire and create the
web of
this invention, the web may be passed through a hot air knife or HAK to very
slightly
consolidate it and provide it with enough integrity for further processing.
After deposition but
before HAK treatment, the crimped fiber web has low stiffness which makes its
difficult, if not
s impossible, to successfully pleat on commercially available pleating
equipment commonly
used to convert filter medium into finished filters. One aspect of this
invention provides a
means to utilize a nonwoven web having crimped fibers while also providing
sufficient
stiffness to convert the web into a finished filter by commercially available
pleating
equipment, by applying the HAK to the web. The application of the HAK allows
forming a
io web of crimped fibers to deliver high permeability and stiffness by melting
only a portion of
the lower melting component in the web, preferably only that lower melting
component on the
side facing the HAK air, in a pre- or primary bonding step. This HAK step
creates a zone of
pre-bonded crimped fibers located on one side of the web which then undergo a
second
melting when exposed to through-air bonding. The exposure of this zone to at
least two
15 heating and melting cycles is believed to create a zone of high stiffness
in the web from the
crystallization of 'the polymer, however, since the zone is comprised of a
small percentage of
the total web, the effect on permeability of the web is minimized. This
differs from the
commonly used method of increasing the integrity of a web known as compaction
rolls since
while compaction rolls increase the stiffness of a web they also reduce the
web permeability.
After treatment with the HAK, the web is sufficiently cohesive to move it to
the next step
of production; the secondary bonding step. The Secondary bonding procedure
which may
be used in the practice of this invention is through-air bonding because it
does not
appreciably reduce web pore size and therefore permeability. When used with
HAK pre-
2s bonding, through-air bonding very effectively produces high stiffness in
the web since it
provides a second heating of the polymer previously heated by the HAK and
provides
sufficient heat to bond fibers not bonded by the HAK. This creates bonds at
almost every
12
CA 02231507 2004-03-25
fiber cxossover point, thereby restric~ng movement of the majOtity of the
fibers of tha wob.
Thermal point bonding by contrast results in bonds at discrete points, thereby
allowing the
fibers between the bond points the freedom to band and rotate individually and
so producing
a much smaller increase in stiffness and so is not an acceptable bonding
process for this
s invention.
Another method of increasing web stiffness is by simply increasing the basis
weight of
the web. This technique, however, is undesirable since it also increases the
cost of the
nonwoven web. It is also undesirable because the overall permeability of the
web is again
reduced. The HAK in conjunction with through-air bonding allows for increasing
the stiffness
of a web without the cost penalty associated with increasing the basis weight
of the web and
without adversely affecting the permeability of the nonwoven web.
After through-air bonding ~e web may be eledret treated. Electret treatment
further
intxeases fittrat;on effidency by drawing partides to be filtered toward the
filter by virtue of
their electrical charge. Electret treatment can be carried out by a number of
different
is techniques. One technique is described in US Patent number 5,401;448 to
Tsai et al.
assigned to the Universityr of Tennessee Research Corporation .
Tsai describes a process whereby a web or film is sequer>tially
subjected to a series of electric fields such that adjacent elecVic fields
have substantially
opposite polarities with respell to each other. Thus, one side of the web or
film is initially
2o subjected to a positive charge white the other side of the web or film is
initially subjected to a
negative charge. Then, the first side of the web orfilm is subjected to a
negative charge and
the other side of the web or film is subjected to a positive charge. Such webs
are produced
with a relatively high charge density without an attendant surface static
electrical charge.
The process may be carried out by passing the web through a plurality of
dispersed non-
e s ardng electric fields which may be varied over a range depending on the
charge desired in
the web. The web may be charged at a range of about 1 kVDC/cm to 12 kVDC/cm or
more
13
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WO 97/14495 PCT/US96/14383
particularly 4 kVDC/cm to 10 kVDC/cm and still more particularly 7 kVDCJcm to
about 8
kVDCJcm.
Other methods of electret treatment are known in the art such as that
described in US
Patents 4,215,682 to Kubik et al, 4,375,718 to Wadsworth, 4,592,815 to Nakao
and
s 4,874,659 to Ando.
Turning to Figure 1, a process line 10 for preparing an embodiment of the
present
invention is disclosed. The process line 10 is arranged to produce conjugate
continuous
filaments, but it should be understood that the present invention comprehends
nonwoven
fabrics made with multicomponent filaments having more than two components.
For
1o example, the fabric of the present invention can be made with filaments
having three or four
components. The process tine 10 includes a pair of extruders 12a and 12b for
separately
extruding a polymer component A and a polymer component B. Polymer component A
is fed
into the respective extruder 12a from a first hopper 14a and polymer component
B is fed into
the respective extruder 12b from a second hopper 14b. Polymer components A and
B are
is fed from the extruders 12a and 12b through respective polymer conduits 16a
and 16b to a
spinneret 18. Spinnerets for extruding conjugate filaments are well-known to
those of
ordinary skill in the art and thus are not described herein detail. Generally
described, the
spinneret 18 includes a housing containing a spin pack which includes a
plurality of plates
stacked one on tap of the other with a pattern of openings arranged to create
flow paths for
2o directing polymer components A and B separately through the spinneret. The
spinneret 18
has openings arranged in one or more rows. The spinneret openings form a
downwardly
extending curtain of filaments which the polymers are extruded through the
spinneret. For
the purposed of the present invention, spinneret 18 may be arranged to form
side-by-side or
eccentric sheath/core conjugate filaments.
2s The process line 10 also includes a quench blower 20 positioned adjacent
the curtain of
filaments extending from the spinneret 18. Air from the quench air blower 20
quenches the
CA 02231507 2004-03-25
filaments extending from the spinneret 18. The quench air can be directed from
one side of
the filament arrtain as shown in Fig. 1, or both sides of the filament
curtain.
A fiber draw unit or aspirator 22 is positioned below the spinneret 18 and
receives the
quenched filaments. Fiber draw units or aspirators for use in melt spinning
polymers are
s well-known as discussed above. Suitable fiber draw units for use in the
process of the
present invention include a linear, fiber aspirator of the type shown in U.S.
Patent No.
3,802,817 and eductive guns of the type shown in U.S. Patent Nos. 3,892,818
and
3,423,266.
Generally described, the fiber draw unit 22 indudes an elongate vertical
passage
i o through which the filaments are drawn by aspirating air entering from the
sides of the
passage and flowing downwardly through the passage. A heater 24 supplies hot
aspirating
air to the fiber draw unit 22. The hot aspirating air draws the filaments and
ambient air
through the fiber draw unit.
An endless foraminous forming surface 26 is positioned below the fiber draw
unit 22
i 5 and receives the continuous filaments from the outlet opening of the fiber
draw unit. The
forming surface 28 travels around guide rollers 28. A vacuum 30 positioned
below the
forming surface 28 where the filaments are deposited draws the filaments
against the
forming surface.
The process line 10 as shown also indudes a hot-air knife 34 which receives
the web as
2 0 ' the web is drawn off of the forming surface 26. In addition, the process
line indudes a
bonding apparatus which is a through-air bonder 36. After passing through the
through-air
bonder, the web is passed between a charging wire or bar 48 and a charged
miler 42 and
then between a second charging wire or bar 50 and rOlier 44.
Lastly, the process line 10 includes a winding roll 42 for taking up the
finished fabric.
i s To operate the process line 10, the hoppers 14a and 14b are filled with
the respecctivve
polymer components A and B. Polymer components A and B are melted and extruded
by
the respective extruders 12a and 12b through polymer conduits 16a and 16b and
the
CA 02231507 1998-03-31
WO 97/14495 PCT/CTS96/14383
spinneret 18. Although the temperatures of the molten polymers vary depending
on the
polymers used, when polypropylene and polyethylene are used as components A
and B
respectively, the preferred temperatures of the polymers range from about
370° to about
530° F. and preferably range from 400° to about 450° F.
s As the extruded filaments extend below the spinneret 18, a stream of air
from the
quench blower 20 at least partially quenches the filaments to develop a latent
helical crimp in
the filaments at a temperature of about 45° to about 90° F. and
a velocity from about 100 to
about 400 feet per minute.
After quenching, the filaments are drawn into the vertical passage of the
fiber draw unit
io 22 by a flow of hot air from the heater 24 through the fiber draw unit. The
fiber draw unit is
preferably positioned 30 to 60 inches below the bottom of the spinneret 18.
The temperature
of the air supplied from the heater 24 is sufficient that, after some cooling
due to mixing with
cooler ambient air aspirated with the filaments, the air heats the filaments
to a temperature
required to activate the latent crimp. The temperature required to activate
the latent crimp of
i5 the filaments ranges from about 110°F to a maximum temperature less
that the melting point
of the lower melting component which for through-air bonded materials is the
second
component B. The temperature of the air from the heater 24 and thus the
temperature to
which the filaments are heated can be varied to achieve different levels of
crimp. Generally,
a higher air temperature produces a higher number of crimps. The ability to
control the
2o degree of crimp of the filaments is a particularly advantageous feature of
the present
invention because it allows one to change the resulting density, pore size
distribution and
drape of the fabric by simply adjusting the temperature of the air in the
fiber draw unit.
The crimped filaments are deposited through the outlet opening of the fiber
draw unit 22
onto the traveling forming surface 26. The vacuum 20 draws the filaments
against the
25 forming surface 26 to form an unbonded, nonwoven web of continuous
filaments. The web
36
CA 02231507 1998-03-31
WO 97/14495 PCT/US96/14383
is then given a degree of integrity by the hot-air knife 34 and through-air
bonded in the
through-air bonder 38.
- In the through-air bonder 36, air having a temperature above the melting
temperature of
component B and below the melting temperature of component B and below the
melting
s temperature of component A is directed from the hood 40, through the web,
and into the
perforated roller 38. Alternatively, the through-air bonder may be a flat
arrangement wherein
the air is directed vertically downward onto the web. The operating conditions
of the two
configurations are similar, the primary difference being the geometry of the
web during
bonding. The hot air melts the lower melting polymer component B and thereby
forms bonds
to between the conjugate filaments to integrate the web. When polypropylene
and
polyethylene are used as polymer components A and B respectively, the air
flowing through
the through-air bonder usually has a temperature ranging from about
230°F to about 325°F
(110°C to 162°C). and a velocity from about 100 to about 500
feet per minute. It should be
understood, however, that the parameters of the through-air bonder depend on
factors such
is as the type of polymers used and thickness of the web.
The web is then passed through the charged field between the charging bar or
wire 48
and the charging drum or roller 42 and then through a second charged field of
opposite
polarity created between charging bar or wire 50 and charging drum or roller
44. The web
may be charged at a range of about 1 kVDC/cm to 12 kVDC/cm.
2o Lastly, the finished web is wound onto the winding roller 42 and is ready
for further
treatment or use.
The three key attributes for the desired filter medium of this invention are
Frazier
Permeability (P), NaCI efficiency (E), and Gurley Stiffness (S). Note that in
calculating S, the
stiffness is normalized for basis weight by dividing the Gurley Stiffness in
milligrams by the
z 2s basis weight in grams per square meter (mg/gsm). It is believed that
filter medium produced
according to this invention should have a Frazier Permeability of greater than
about 200
CFM, an NaCI efficiency of greater than about 75 percent, and a Gurley
Stiffness above
17
CA 02231507 2004-03-25
about 700 mg. A convenient rule of thumb for combining these measuBmentS is
through the
SEP value. The SEP value is by: pog(PxE)](S)(3.73) and gives an overall number
for evaluation. It is believed that filter medium produced according to this
invention should
have an SEP value above about 80 and more particularly above about 80.
The filter medium of this invention may be made into a filter by any suitable
means
known in the art, though the preferred method is by rotary pleating. The
rotary pleating
process is quite dependent upon the stiffness of the filter medium. Guriey
Stiffness values
of at feast 600 mg are required to allow pleating on high speed rotary
pleating equipment
while other methods of pleating are not as sensitive to web stiffness but are
slower. Rotary
io pleating is desirable primarily from the perspective of the speed of the
process, which is
greater than other methods. A faster pleating process, of course, results in
lover production
costs and, ultimately, lower costs to the consumer. One such acceptable rotary
pleating
method is disdosed in US Patent number 5,389,175 to Mann and Hummel,
This method involves scoring the filter medium and then
is putting it between at least two cogbelts spaced apart from one another and
disposed above
and below the filter medium web and running with the filter web, with the
cogbelts
subsequently pleating the filter medium according to the scoring. The pleated
filter medium
is then advanced with a defined distance between pleats by at least one helix
adjoining the
cogbelts.
20 Filters made acxording to this invention may be used in a number of
different
applications. The filters may be used in air filtration for home or commercial
heating,
ventilating and air conditioning (HVAC) services. They may also be used in
filtra~on of
breathing air in transportation applications like automobile cabin air
filtration, airplane cabin
air filtration, and train and boat air filtration. While this invention is
directed mainly to air
2 s filtration, other fluids and other gasses may be filtered as well. Such
other gasses may
indude, for example, nitrogen when produced or when used e.g., in industrial
or medical
settings. Other fluids may indude liquids like oil or water.
18
CA 02231507 1998-03-31
WO 97!14495 PCT/US96/14383
The following sample data numbered 1-14 include Comparative Examples (1-10),
an
example of a web of the Invention (11) and Commercial product evaluations (12-
14) and
- show the characteristics of webs which satisfy the requirements of this
invention versus
those that do not.
s Samples 2-11 used a HAK. In these samples, the HAK air flowrate was about
3800 fpm
(1160 m/min.), the HAK temperature was 360°F (182°C) and the HAK
height above the web
was 1 inch (2.54 cm) except for Example 2 where it was 7/8 inch (2.22 cm).
Samples 2-10
were subjected to compaction rolls. Samples 2-11 used side-by-side conjugate
fibers while
sample 1 used sheath-core, and all samples 1-11 were extruded through
spinnerets having a
io diameter of 0.6 mm to produce fibers having diameters of from 16 to 19
microns. Samples
1-11 used a polypropylene marketed by the Exxon Chemical Company of Houston,
TX under
the trade designation ESCORENE~ PD 3445 and a linear low density polyethylene
marked
by the Dow Chemical Company of Midland, MI under the trade designation ASPUN~
6811A
which were processed at a melt temperature of about 448 °F (231
°C). Samples 1-11 were
is processed through a through-air bonder at a temperature of between about
265 and 295°F
(130-146°C) at an air rate of between about 200 to 300 fpm (61-91
m/min.) for a time period
of about 10 seconds. Samples 1-11 were treated according to the method of US
Patent
5,401,446 by passing the web between a conductive bar or wire and a curved
conductive
drum with a non-arcing electric field between the bar or wire and the dnrm of
about 8
2o kVDC/cm of separation between the bar and drum, and then passing the web
through a
second electric field generated by the same means and as the same strength as
the first but
with the field orientation being 180 degrees of the first relative to the web.
The comparative webs of samples 12-14 are from Reemay Inc., of Old Hickory,
TN, are
marketed under the trade designation REEMAY~ and are webs commonly used in
2 s manufacturing filters. The particular webs used for samples 12-14 were,
respectively,
REEMAY~ 6140, REEMAY~ 6240 and REEMAY~ 6260 fabrics.
19
CA 02231507 1998-03-31
WO 97/14495 PCT/LTS96/14383
After formation, the webs were tested for permeability, stiffness and
efficiency
according to the methods herein and the results are shown in Table 1.
CA 02231507 1998-03-31
WO 97/14495 PCT/US96/14383
m
m
O ODON (D1~N tpN~ ~ N t~M
47O Of~h CO1'COOi' O l0
r r
W
E r O 1~(fl00(Vtno rf~M tnM ~.
~ aiuiuiv ~f~fv ~tui~tc~c~ui
~
E
w
x M O NN N N M ~ C~M N ~ ~t~
0- st'~t'~!~ ~:~ et'ct'~t~i'~:M M M
O
O
r
~'
U
C O N ~t~Wit'r N i'N(D~ M COr-
Z
N O O 0~00N O O r (flCVO ~ O)CV
E O O 0000O aDO 00OD00t~M N M
~
O
d
v
J
m
N
O N M NM M a0f~1~(OO CO~
~ N N (DaDO O)M M Mr N ~ O pp
N N r ~-r r e-N M NN N
~
O
a~
O
_
N
N
C
'.~~dON ~t r (Otl71'O(flOD00M
(n r M I'~ t'O M 1~MCD0000N
E '
T M 1~I~tntt)tn~ ~ to~n1~o~1~N
N
z
7
C~
w.
L
O_
N tnCOO O 07O O tntl~tnCOC~M
E O N Mr r r r r NN N M M O
r r rr r r r r rr r r r N
R
m
m
fl.
E e-N MV'tn(~t~QOOO r N M et'
r r r c-r
R
29
CA 02231507 1998-03-31
WO 97/14495 PCT/fJS96/14383
The results show that the filter medium of this invention, sample 11, has a
good
combination of permeability, efficiency and stiffness. Sample 11 had a Frazier
Permeability
of greater than about 200 CFM, an NaCI effiiciency of greater than about 75
percent, and a
Guriey Stiffness above about 700 mg. Such attributes are very desirable in
combination,
s resulting in a SEP value much greater than webs of comparable basis weights.
Although only a few exemplary embodiments of this invention have been
described in
detail above, those skilled in the art will readily appreciate that many
modifications are
possible in the exemplary embodiments without materially departing from the
novel teachings
and advantages of this invention. Accordingly, all such modifications are
intended to be
1o included within the scope of this invention as defined in the following
claims. In the Gaims,
means plus funcl:ion claims are intended to cover the structures described
herein as
performing the recited function and not only structural equivalents but also
equivalent
structures. Thus although a nail and a screw may not be structural equivalents
in that a nail
employs a cylindrical surface to secure wooden parts together, whereas a screw
employs a
i5 helical surface, in the environment of fastening wooden parts, a nail and a
screw may be
equivalent structures.
n
22
