U.S. patent number 3,804,695 [Application Number 05/128,117] was granted by the patent office on 1974-04-16 for apparatus for making tobacco smoke filters.
This patent grant is currently assigned to Celanese Corporation. Invention is credited to Charles Herbert Keith, John Courtright Randall.
United States Patent |
3,804,695 |
Randall , et al. |
April 16, 1974 |
APPARATUS FOR MAKING TOBACCO SMOKE FILTERS
Abstract
Improved aerosol filters, particularly cigarette filters, are
prepared by imparting a patterned surface, preferably a plurality
of longitudinal grooves, to a web of a synthetic thermoplastic
fibrous material, preferably a tow of longitudinally aligned
crimped continuous cellulose acetate filaments, and forming the
resultant patterned web into a filter of the desired form.
Inventors: |
Randall; John Courtright
(Charlotte, NC), Keith; Charles Herbert (Charlotte, NC) |
Assignee: |
Celanese Corporation (New York,
NY)
|
Family
ID: |
26826284 |
Appl.
No.: |
05/128,117 |
Filed: |
March 25, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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737519 |
Jun 17, 1968 |
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Current U.S.
Class: |
156/441; 156/209;
131/343; 156/180; 156/200; 156/462 |
Current CPC
Class: |
B01D
39/18 (20130101); D04H 3/14 (20130101); A24D
3/0204 (20130101); Y10T 156/1008 (20150115); Y10T
156/1023 (20150115) |
Current International
Class: |
A24D
3/02 (20060101); A24D 3/00 (20060101); B01D
39/18 (20060101); D04H 3/14 (20060101); D04h
003/08 () |
Field of
Search: |
;156/441,180,209,219,220,200,553,462 ;131/267,265,266,268
;19/66T |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Fritsch; Daniel J.
Attorney, Agent or Firm: Morgan; Thomas J. Murphy; Stephen
D. Sayko; Andrew F.
Parent Case Text
This application is a continuation of copending application Ser.
No. 737,519, filed June 17, 1968, and now abandoned.
Claims
1. An apparatus for converting a tow of crimped continuous
synthetic filaments into improved tobacco smoke filter rods
comprising in sequence:
a. deregistering means for longitudinally shifting the individual
filaments of said tow relative to adjacent filaments;
b. opening means for laterally spreading said filaments;
c. plasticizing means for applying a plasticizer to the surface of
said tow;
d. patterning means for imparting a patterned configuration to at
least one surface of said tow, said patterning means being
comprised of a pair of parallel rolls in pressure engagement
defining a nip therebetween, at least one roll having a
circumferential or helical grooved surface, said rolls being
adapted to travel at the same surface speed as tow passing through
the nip thereof providing to said tow a series of parallel
essentially continuous permanent depressions providing pathways
along the longitudinal dimension of said tow, said grooves having a
frequency of five to 80 per inch and having a depth of 0.035 to
0.005 inches with intermediate lands of 0.03 to 0.005 inches width;
and
e. condensing means for condensing said tow into a tobacco smoke
filter
2. The apparatus of claim 1 wherein both rolls of said pair of
rolls have
3. The apparatus of claim 1 wherein said rolls are yieldably
mounted for movement so as to spread and define a nip up to about
0.02 inch.
Description
BACKGROUND OF THE INVENTION
Aerosol filters, and particularly cigarette filters, have
conventionally been produced from a great variety of fibrous
materials. Of the fibrous materials employed, however, only paper
and cellulose acetate tow have met with any significant commercial
acceptance.
Paper filters are generally characterized by higher filtration as
measured by smoke removal efficiency, but also adversely affect
taste and odor of the delivered smoke stream. Moreover, their
phenol selectivity is significantly lower than that available with
conventional cellulose acetate tow filters. Further, paper filters
are susceptible to collapse during smoking, primarily because of
their tendency to absorb moisture from the tobacco smoke stream and
smoker's mouth. Also, the compressibility of paper filters at a
given pressure drop, i.e., resistance of the filter to air flow, is
generally greater than that of conventional tow filters.
In comparison with paper filters, conventional cellulose acetate
two filters overcome all of the above disadvantages of paper
filters and for this reason, are more commercially acceptable in
spite of the fact that the smoke removal efficiency at a given
pressure drop is relatively lower than that of paper filters.
SUMMARY OF THE INVENTION
The primary object of the present invention is to provide an
aerosol filter, particularly a cigarette filter, which will exhibit
a smoke removal efficiency to pressure drop relationship up to and
even exceeding that normally associated with paper filters, without
exhibiting the above undesirable properties ordinarily associated
with such aerosol filters.
Another object is to provide an intermediate product suitable for
formation into filters of the above characteristics.
Further objects of the present invention reside in the provisions
for a process and apparatus for preparing filters exhibiting the
aforesaid properties.
Still other objects of the present invention, if not specifically
set forth herein, will be readily obvious to one skilled in the art
by reference to the detailed description of the invention and to
the drawings.
DRAWINGS
FIG. 1 is a diagrammatic view of an apparatus suitable for the
practice of the present invention.
FIGS. 2, 3, 4 and 5 are illustrations of patterned surfaces which
may be suitably employed in the present invention.
FIG. 6 is a 15.times. magnification of the preferred intermediate
product of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The steps of a preferred embodiment of the present invention are
set forth in the following detailed description and should be
considered in conjunction with the appended drawings of an
apparatus suitable for use in the practice of the present
process.
In the preferred practice of the present invention, which results
in a filter of substantially longitudinally aligned filaments with
continuous grooves substantially parallel to the filaments, a tow
12 of continuous cellulose acetate filaments, preferably having
about five to about 15 transverse crimps per inch, and acetyl value
of 38-41 percent, a regular or a non-circular, e.g., I, X, or Y
cross section, and a total denier of about 45,000 to about 120,000
or more is removed from tow bale 10 and passed over guide means 14
to opener 16. The purpose of opener 16 is to cause deregistration
of the crimps of the individual filaments, and thus provide a tow
having improved uniformity and bulkiness. In the drawings, opener
16 is a threaded roll opener of the type generally described in
U.S. Pat. No. 3,032,829 to Mahoney, et al., and 3,156,016 to
Dunlap, et al. Essentially, the threaded roll opener shown
comprises two pairs of rolls, with at least one roll of one pair
being driven. Desirably, at least one roll of each pair has a
patterned surface preferably composed of circumferential or helical
grooves. However, the roll pairs may be different, e.g., only one
roll of one pair need be grooved. When the tow passes through the
rolls, individual filaments of the tow are differentially
restrained causing a longitudinal shifting of the relative location
of the crimps of the individual filaments. It is to be understood,
of course, that other openers, for example, those producing
deregistration by air turbulence or flexing of the tow, may also be
suitably employed.
After passing through opener 16, tow 12 is commonly passed through
a banding jet 16 which spreads the tow by application of one or
more air streams into a flat band of about three to eight times its
original width and causes further separation of the individual
filaments. A suitable banding jet is shown, for example, in U.S.
Pat. No. 3,226,773. However, other means for achieving filament
separation such as equipment utilizing electrostatic forces are
known in the art and may also be used for this purpose.
The opened tow is then passed through plasticizer applicator 20
which treats the surface of the individual filaments with a
plasticizing liquid, preferably an organic ester such as triacetin,
to cause bonding of the filaments. Other suitable plasticizers
include, for example, triethyl citrate, dimethyl ethyl phthalate,
or the dimethyl ether of triethylene or tetraethylene glycol. In
the drawings, plasticizer applicator 20 is a centrifugal
plasticizer applicator of the type described in U.S. application
Ser. No. 555,647, filed June 2, 1966, now U.S. Pat. No. 3,387,992.
Essentially, applicator 20 comprises a housing, a rotating disc
located within the housing below the path of the tow and
substantially transverse thereto, means for applying a plasticizer
to the rotatable disc, and means for recycling plasticizer which
does not remain on the tow. Other applicators which are adapted to
apply plasticizer to a continuous web may also be used for this
purpose. For example, such applicators may utilize wicks or spray
nozzles in plasticizer application.
After treatment of the tow with pasticizer, the tow may optionally
be passed to heating means 22 and/or converging means 24. The
purpose of heating means 22 is to raise the temperature of the
thermoplastic web, thus rendering it more susceptible to bonding
upon compression. Heating means 22 may be used alone or in
conjunction with a heating source within patterned rolls 26 later
described. Converging means 24 serves to reduce the width of the
web, thus rendering the web more readily processable. More uniform
distribution of plasticizer is also obtained by passing tow 12
through converging means 24.
Downstream from plasticizer applicator 20, or heating means 22,
and/or converging means 24, if employed tow 12 is passed to and
through the nip of a pair of rolls 26, at least one of which has a
patterned surface. Essentially, these patterned, e.g., grooved
rolls 26 are adapted to afford a plurality of permanently depressed
areas in the substrate. Said rolls 26 are positioned substantially
transverse to the tow path and are arranged with parallel axes.
Ordinarily, one roll of the pair will be adjacent the upper surface
of the tow path, while the second roll will be mounted opposite
said roll and below the tow path. However, the web may also follow
a vertical path with a patterned roll mounted of either side of
such path. The rolls 26 may be mounted yieldably in contact or
slightly separated. The rolls should be of a proximity, however,
sufficient to cause at least some permanent depression of the tow
as it passes therebetween. Preferably, the separation of the rolls
is from 0 to about 0.02 inches, and even more desirably from 0 to
about 0.01 inches. The separation, of course, will depend upon a
thickness of the tow as determined by the total denier and width
thereof. Webs processed through rolls of the above separation will
have an overall thickness of from about 0.1 to about 2.5mm.
In order to obtain the advantages of the present invention, a
variety of patterns may be imparted to the surface of the web
material. Such patterns may comprise continuous depressed areas
and/or continuous lands. For example, a waffle or quilted surface
as illustrated in FIG. 2 may be imparted to the surface of the web.
In this pattern, either the continuous or discontinuous area may be
compressed. The waffle or quilted pattern may also be oriented so
that the edges of the pattern are at an angle to the longitudinal
axis of the web, in effect imparting a diamond-shaped pattern as
shown in FIG. 3 to the surface of the web. Generally, it has been
found that the preferred patterns of the present invention from the
standpoint of the greatest relative reduction in pressure drop
comprise grooves defining a path substantially paralle to the
longitudinal axis of the web. These longitudinal grooves preferably
form a straight line along the web, i.e., accordian pleats;
sinusoidal or zigzag grooves (as shown in FIGS. 4 and 5) are also
possible, however.
Desirably, the preferred rolls employed in the present invention
are circumferentially or helically grooved, and will have from
about five to about 80 and preferably from about 20 to about 45
grooves per inch. The lands of the rolls will ordinarily be of
about 0.03 to about 0.005 inch and more preferably from about 0.015
to about 0.008 inch in width. The grooves will ordinarily be about
0.035 to about 0.005 and preferably from about 0.002 to about 0.001
inch in depth. The lands of a given roll will ordinarily, but not
necessarily, be of uniform width. In fact, lands which
progressively decrease in width outwardly from the center of the
patterned area may aid in the construction of a more uniform
filter. Similarly, the depths of the grooves may be of differential
dimensions across the web.
Rod firmness can be improved by using rectangular or substantially
rectangular grooves, since such grooves tend to yield a material
which, upon gathering into rod form, has self-supporting,
triangular-shaped, difficulty compressible channels. The term
substantially rectangular grooves is intended to define a groove
wherein the angle from the vertical of the wall is from 0.degree.
to 45.degree. and preferably from 0.degree. to 30.degree.. It is,
of course, within the scope of the present invention to use other
grooved shapes, e.g., semi-circular, trapezoidal, or triangular
grooves.
In most instances, the use of heated patterned rolls has been found
to be of value in obtaining improved corrugation. In the preferred
embodiment of the present invention, therefore, patterned rolls
having an internal heat source are employed. In the preferred
embodiment, electrical means is used to heat the patterned rolls.
It is to be understood, of course, that other heating means such as
heated fluids and gases may be employed for this purpose. Roll
temperatures are generally from about 25.degree. to about
225.degree.C, with 110.degree. to about 160.degree.C being
preferred. Such treatment may be utilized to reduce the cross
sectional dimension of the substrate material or to impart enhanced
processability thereto, but conditions are adapted to ensure
substantial retention of filamentary character.
Preferably, corrugating rolls 26 are at least about 2 inches in
diameter and even more preferably from about 4 to about 8 inches in
diameter. The dimensions of the patterned portion of the rolls,
will, of course, be determined to some extent by the width of the
tow being structured. Generally, a total patterned width of from
about 8 to about 16 is sufficient for most operations. As an
alternative to or in conjunction with the aforementioned heating
means and/or heated patterned rolls, advantageous results may be
obtained by applying a heated plasticizer to the tow. Also,
desirable results may be obtained by treating the tow with
solvation agents such as acetone, methylene chloride or water prior
to structuring. Optionally, the patterned web is then passed over
forming means 28 which in essence constitutes a curved or other
shaped surface which tends to reduce the overall width of the web
and produce a more uniform ultimate product.
Thereafter, the tow may then be directed into a rod maker 30, which
shapes the patterned web into a filter rod. The rod issuing from
the rod maker will ordinarily be of about 8 mm. in diameter, and
will be severed into lengths of about 60-180 mm., a length
sufficient to yield 6 filter rods of 10-30 mm. each when ultimately
severed for attachment to tobacco sections.
While the present patterned webs are particularly adapted for use
in cigarette filter form, they may also be advantageously employed
in other filter forms, such as air conditioning or industrial gas
filters. Also, the patterned webs may be suitably utilized as
interliners and the like where a light weight coherent structure is
desired.
It has been previously noted that only a limited smoke removal
efficiency has been obtained with previous cellulose acetate tow
filters. This limited S.R.E. has been due primarily to pressure
drop considerations. That is to say, pressure drop limits the
amount of material that can be packed into a cigarette filter. A
linear relationship exists between smoke removal efficiency and the
surface area of the material employed in the filter. Since surface
area is related to the weight of material employed, smoke removal
efficiency is necessarily limited. In the present invention, it is
possible to utilize greater weights of filtration material at
acceptable pressure drops than possible by prior art techniques,
and thus obtain an increased smoke removal efficiency. To
illustrate, prior art filters of conventional dimensions, i.e.,
20mm. in length and 8mm. in diameter, have contained generally from
about 0.12 to about 0.14 gram of the tow material when a pressure
drop of from about 20 to about 90mm. H.sub.2 O, the normally
acceptable level, was obtained. In comparison, pressure drops
within this range can be obtained with the present invention while
using up to 0.30 gram or more of cellulose acetate tow. Thus,
instead of the 25 to 55 percent, S.R.E. observed with prior art
filters, an S.R.E. of as high as 65 percent or even higher is
possible with the filters of the present invention utilizing tows
of conventional denier for this art. Even further improvements are
obvious, with, for example, webs of lower individual denier per
filament.
In order to obtain the maximum filtration properties of a given
filter, falling within the scope of the present invention,
essentially four variables should be controlled. These variables
are: denier per filament, tip weight, sheet density, and
corrugation frequency. In discussing these parameters, the
limitations given are those applicable to the preparation of
cigarette filters having pressure drop characteristics within the
above specified ranges. It is to be realized that certain of these
parameters may be expanded somewhat in preparing filters, for
example, industrial gas filters, having a pressure drop without the
above ranges.
Generally, it has been determined that filtration properties
improve in proportion to the reduction in denier per filament. That
is to say, a reduction in denier per filament increases the surface
area of the filamentary material, and thus improves filtration
properties. The preferred denier per filament range in the present
invention is from about 0.1 to about 5, with a denier per filament
below about 3 being especially preferred. Products may be prepared
from fibers having a denier per filament up to 16, however.
Tip weight, of course, is a function of the total denier of the tow
being employed in preparing the filter. Generally the tows employed
in the present invention will have a total denier of from about
35,000 to about 200,000 or higher and preferably from about 60,000
to about 120,000. Filters of standard dimension, i.e., 20mm. in
length and 8mm. in diameter, prepared from tows of this total
denier will generally have a weight of from about 0.14 gram to
about 0.34 gram and when the preferred total denier is employed, a
weight of from about 0.19 gram to about 0.32 gram.
Sheet density is also a significant factor to be considered in
maximizing the filtration properties of the present corrugated
filter. While higher densities permit the inclusion into the filter
of greater weights of material at a given pressure drop, this
advantage is offset somewhat by a reduction in total available
surface area due to packing at these higher densities. Considering
these factors, a sheet density of from about 25 to about 175
g/m.sup.2 is generally employed, with a density of from about 50 to
about 125 g/m.sup.2 being preferred. Sheet density is used herein
in the sense employed in the paper art and is a measurement of the
weight of a portion of a sheet defined by the area of the
surface.
Corrugation frequency, that is the number of separate or land areas
per square inch of material, also influences the S.R.E. of the
filter. Generally, a frequency of about five to 80 is employed. The
particular pattern employed is, of course, a consideration in
determining the optimum frequency. For the preferred longitudinal
grooves of the present invention, the corrugation frequency will
correspond to the specifications previously given for preferred
rolls.
After a study of the present disclosure, one skilled in the art
will realize that the above parameters are to some extent dependent
upon each other. For example, a lower denier per filament will
yield improved filtration at a given sheet density. On the other
hand, one can hold the denier per filament as a constant and
improved filtration performance by increasing the sheet density.
The following examples are presented for the purpose of
illustration, and are not to be considered as limiting thereof.
EXAMPLES 1-6
A crimped tow of continuous acetate filaments, 3.3 denier per
filament, Y cross section, 48,000 total denier, was removed from a
tow bale and opened on a threaded roll opener, described above. A
sample of this opened tow comprised of approximately a 3 foot
length was placed on a table where it was plasticized with
triacetin sprayed from a hand held atomizer to a plasticizer level
of approximately 7 percent by weight. The plasticized tow sample
was folded to produce a tow band with a sheet density of
approximately 58 grams per square meter. The band was corrugated by
pressing briefly between two heated, grooved plates, 31/2 .times. 7
inches which had 10 grooves per inch; the grooves being 0.060
inches wide and 0.060 inches deep. The resulting corrugated sheet
was gathered into a rod, wrapped with a conventional filter paper
wrap, and cut to 20mm. length. The resulting 20mm. tips weighed
0.197 gram and had a pressure drop of 68mm. H.sub.2 O measured by
drawing air through the tip at an air flow rate of 18 cc/sec. When
mounted on standard cigarette columns and smoked on an apparatus
which took 35 ml. puffs over a 2 second interval on a 60 second
cycle, the filter tips removed 51.9 percent of the smoke
particulate matter. Other tips were prepared in a similar manner.
The results obtained and comparative data obtained using
conventional cellulose acetate tips of comparable pressure drop are
shown in the following table.
__________________________________________________________________________
S.R.E. SHEET NUMBER PLATE TIP TIP CONVENTIONAL S.R.E. EXAMPLE DPF
DENSITY GROOVES TEMP WT. .DELTA.P C. A. TIP SAMPLE
__________________________________________________________________________
1 3.3 58 10 220.degree.C 0.197 68 48.8 51.9 2 3.3 94 13 -- 0.176 74
50.1 53.4 3 2.4 81 13 -- 0.191 71 50.4 53.5 4 2.4 119 20 -- 0.237
63 48.7 53.5 5 1.6 70 30 -- 0.258 63 50.5 56.6 6 1.6 137 20 --
0.313 59 49.7 64.8
__________________________________________________________________________
EXAMPLE 7
A crimped tow of continuous cellulose acetate filaments, 2.3 denier
per filament, Y cross section, 99,000 total denier, was withdrawn
from a tow bale and continuously processed on a modified threaded
roll processing system. The threaded roll opened tow was passed
through a banding jet and passed through a continuous plasticizer
application system. The plasticized tow was passed through a driven
feed nip and supplied to the nip of a pair of corrugating rolls.
The plasticized tow was converted from a loose band of continuous
substantially longitudinally aligned fibers substantially free of
interfiber bonding to a coherent corrugated sheet by compressing
the tow in the nip of a pair of corrugating rolls. The rolls were 4
inches in diameter by 16 inches long and had 30 grooves/inch 0.0152
inches wide by 0.0165 inches deep with a 30.degree. included angle.
The rolls were heated with a surface temperature of 130.degree. and
were operated under a pressure of 100 pounds per inch of tow band
width in the nip. The continuously corrugated sheet having a
density of 75 g/m.sup.2 was fed through a convergence guide to the
garniture of a conventional cigarette filter rod maker and rods
were prepared. Rods 20mm. in length and 8mm. in diameter weighed
0.257 gram, had a pressure drop of 67mm. H.sub.2 O, and an S.R.E.
of 57.9. A conventional tip of comparable pressure drop had an
S.R.E. of 49.9.
While the foregoing description has dealt only with the preparation
of a filter from a single material, i.e., a cellulose acetate tow,
it is also possible to prepare satisfactory and often improved
filters by incorporation of one or more other filtration materials
into the web material prior to corrugation. Such materials include
carbon, silica gel or other high surface area absorbents, granular
polyurethanes, cellulose acetate flake, wood pulp, flock, liquid
additives and other gas adsorbents or selective absorbents.
Generally, up to about 20 percent of these materials based on the
weight of the filter may be employed, with from about 5 percent to
about 10 percent being preferably utilized. Obviously, a
multiplicity of, and tows comprising the same of different
filamentary materials could be combined to form suitable filter
structures as described herein.
Filters prepared by the above method may be used as the sole
filtration means on a cigarette. It is, of course, possible to use
filters prepared by the present invention as part of a dual or
segmented filter. In this context, the present filters are
particularly suitable in combination with paper filters and
conventional cellulose acetate filters.
While the invention has been described particularly with reference
to the processing of cellulose acetate tow, satisfactory filters
may also be prepared by imparting a pattern to the surface of webs
of other thermoplastic materials prior to corrugation. For example,
spray spun polyolefin webs prepared, for example, in accordance
with the teachings of commonly assigned application Ser. No.
581,075, filed Sept. 20, 1966 and now abandoned, may also be
corrugated in the presently described manner to yield products
which may be formed into improved filters. Webs of discontinuous
fibers substantially free of interfiber bonding prior to
corrugation, e.g., a carded staple roving, are also suitably
treated by the present invention. Suitable thermoplastic materials
envisaged for conversion into filters in accordance with this
invention include the cellulose esters, including the triesters,
with organic carboxylic acid having two to four carbon atoms, the
polyesters such as polyethylene terephthalate, the polyamides such
as nylon 6 and 66, the acrylics and especially those having an
acrylonitrile content of at least 85 percent, the polyolefins such
as polypropylene, polyethylene, poly 3 methyl butene or poly 4
methyl pentene. The polyacetals, especially those containing at
least 75 mol percent of recurring oxymethylene units, and
copolymers and mixtures of the foregoing in any suitable coherent
fibrous form.
As previously noted, the smoke removal efficiency of a filter at a
given pressure drop is directly proportional to the surface area of
the filtration material present within the filter. A conventional
20mm. cellulose acetate filter having an acceptable pressure drop,
i.e., 90mm. H.sub.2 O or less, comprises material having a total
surface area of below 500 cm.sup.2. On the other hand, a filter
prepared in accordance with the present disclosure contains
sufficient material to present a surface area of 550 to 1,000
cm.sup.2 within the acceptable pressure drop range of 20 to 90mm.
H.sub.2 O. When using the lower range of fiber denier per filaments
disclosed herein, surface areas within the range of 750-1,000
cm.sup.2, or up to about 150-200 percent that is obtainable in a
conventional filter, are produced.
The distinction of the present filter of a cellulose acetate tow is
also readily apparent in the relationship between smoke removal
efficiency and pressure drop. This relationship is illustrated in
the foregoing examples and by the following equations describing
the relationship of these two factors within a pressure drop range
of 40-90mm. in filter tips of 20mm. in length and 24.8mm. in
circumference:
CONVENTIONAL CELLULOSE ACETATE FILTER
S.R.E. = 28.9 + 0.295.DELTA. P .+-. 2.6
CORRUGATED CELLULOSE ACETATE FILTER
S.R.E. = 40.2 + 0.259.DELTA. P .+-. 7.7
It is to be realized that paper filters are conventionally
longitudinaly corrugated and that the prior art contains many
teachings of this fact. It is significant, however, that such
corrugation in the paper filter art has been employed for the
purpose of obtaining structural rigidity of the filter. In the
filtration art, both in paper technology and in filament
technology, it has been felt that channeling within the filter was
a most undesirable factor, since such channeling permitted the
smoke to flow along an unrestricted path without being subjected to
the resistance of the filter. Of course, longitudinal corrugation
effectively creates a multitude of channels for the smoke. It is
surprising, therefore, that treatment of filamentary material by a
procedure heretofore employed with paper for an entirely different
purpose will result in a product exhibiting improved filtration
properties, particularly in view of the fact that a structure is
produced that has always been considered to be disadvantageous in
the filtration art. Similarly, one would not have expected that
such procedure would produce a large decrease in pressure drop
while producing only a relatively small decrease in S.R.E.
It is to be understood that the foregoing detailed description is
given merely by way of illustration and that many variations may be
made therein without departing from the spirit and scope of our
invention.
* * * * *