Note: Descriptions are shown in the official language in which they were submitted.
2117687 PATENT
METHOD OF TREATING PAPERMAKING FIBERS FOR MAKING TISSUE
Background of the Invention
In the manufacture of certain paper sheet products, such as
tissue and paper towels, one method of drying the sheet after
formation and dewatering is to pass heated air through the wet sheet
in a process well known in the papermaking art as throughdrying.
Throughdrying is advantageous in that it minimizes compaction of the
web and thereby produces a bulkier product compared to conventional
wet press manufacturing processes, which rely on high levels of
compression of the wet web and on a Yankee dryer to dry the web.
However, throughdrying has some disadvantages in that it requires a
substantial amount of expensive equipment and energy to carry out the
drying process. In particular, the drying efficiency of the
throughdrying process is in large part dependent upon the air
permeability of the wet sheet which permits the hot air to pass
therethrough. Air permeability is especially a problem for sheets
made from fiber furnishes containing secondary (recycled) fibers,
which inherently impart poor air permeability to the wet sheets into
which they are incorporated. With the continual efforts to utilize
more secondary fibers in paper products for environmental reasons,
there is a need to improve the throughdryability of secondary
papermaking fibers.
SummarY of the Invention
It has been surprisingly discovered that the ease in which
secondary papermaking fibers can be throughdried can be improved by
pre-treating the fibers of the papermaking furnish in a wet
mechanical process in which the fibers are appropriately worked while
suspended in a high consistency aqueous slurry. The effectiveness of
the pretreatment is manifested by an increase of the
Throughdryability Index (hereinafter defined and referred to as the
~TD Index") of the fibers. An increase in the TD Index translates
into faster throughdrying machine speeds for a given furnish and
21 ~ 7687
basis weight, which results in improved operating efficiency. The
method of this invention can be utilized for any papermaking fibers,
but is especially advantageous for improving the TD Index of
secondary fiber furnishes and furnishes containing a significant
amount of fines and/or short fibers, such as hardwood fibers. (As
used herein, "short fibers" are fibers having a population average
length of from 0.5 to about 1 millimeter as determined by using a
Kajaani FS-200 fiber length analyzer.) In some cases, the TD Index
of secondary fiber furnishes can be surprisingly improved beyond that
of untreated virgin furnishes. As used herein, ~fines~ are particles
that pass through a 200 mesh screen having a 76 micrometer diameter
opening. They can be measured in accordance with TAPPI Test Method
T261 pm-80 (1989).
In addition, while the fiber treatment of this invention is
particularly advantageous for throughdrying processes and products,
product improvements can also be realized when the treated fibers of
this invention are used for making wet pressed tissue products as
well. More specifically, it has been found that substituting the
treated fibers of this invention for a portion of the fibers of a
given tissue furnish, the softness of the resulting tissue can be
increased without loss of strength. This is especially effective
when treating hardwood fibers and combining the treated hardwood
fibers with other fibers, such as untreated softwood fibers, either
bler,ded or layered. The treated hardwood fibers improve the softness
of the resulting product while the untreated softwood fibers retain
the strength. Softness can be further enhanced by adding a softening
agent to the treated fibers either before treatment or after
treatment. Certain softening agents also provide an unexpected
increase in bulk as well as enhancing the softness of the tissue.
Hence in one aspect, the invention resides in a method of making
a tissue comprising: (a) forming an aqueous suspension of papermaking
fibers having a consistency (dry weight percent fibers in the aqueous
suspension) of about 20 or greater; (b) passing the aqueous
suspension through a shaft disperser at a temperature of 150-F. or
greater with a power input of at least about 1 horsepower-day per ton
of dry fiber, wherein the TD Index of the fibers is increased about
25 percent or greater, preferably about 50 percent or greater, and
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more preferably about 75 percent or greater; (c) feeding the fibers
through a tissue making headbox to form a wet web; and (d) drying the
web, such as by throughdrying, to form a dried tissue.
In another aspect, the invention resides in a method of making
tissue comprising: (a) forming an aqueous suspension of papermaking
fibers having a consistency of about 20 or greater; (b) passing the
aqueous suspension through a shaft disperser at a temperature of
150F. or greater with a power input of at least about 1 horsepower-
day per ton of dry fiber, wherein the TD Index of the fibers is about
0.15 or greater, preferably about 0.2 or greater, more preferably
about 0.3 or greater, and most preferably about 0.5 or greater;
(c) feeding the fibers through a tissue making headbox to form a wet
web; and (d) drying the web, such as by throughdrying, to form a
dried tissue.
In a further aspect, the invention resides in a throughdried
sheet made from a furnish comprising at least about 25 dry weight
percent secondary fibers, and/or having at least about 7 weight
percent fines and/or having at least about 20 percent short fibers,
said furnish having a TD Index of about 0.15 or greater, preferably
about 0.20 or greater, more preferably about 0.30 or greater, and
most preferably about 0.5 or greater. If present, the amount of
secondary fibers in the furnish can be anywhere within the range of
about 25 to about 50, 75, or 100 dry weight percent. In general,
secondary fibers inherently have a high proportion of fines and/or
short fibers. If present, the amount of fines can be about 7 weight
percent or greater, more particularly about 10 weight percent or
greater, still more particularly from about 10 to about 25 weight
percent. If present, the amount of short fibers can be about
20 weight percent or greater, more specifically about 50 weight
percent or greater, still more specifically about 75 percent or
greater.
Papermaking fibers useful for purposes of this invention include
any cellulosic fibers which are known to be useful for making paper,
particularly those fibers useful for making relatively low density
tissue papers such as facial tissue, bath tissue, dinner napkins,
paper towels, and the like. As used herein, the term ~tissue" is
used generically and is intended to include all such products. Such
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products can be creped or uncreped. The most common papermaking
fibers include virgin softwood and hardwood fibers, as well as
secondary or recycled cellulosic fibers. As used herein, "secondary
fiber" means any cellulosic fiber which has previously been isolated
from its original matrix via physical, chemical or mechanical means
and, further, has been formed into a fiber web, dried to a moisture
content of about 10 weight percent or less and subsequently
reisolated from its web matrix by some physical, chemical, or
mechanical means. Fibers which have been passed through a shaft
disperser as described herein are sometimes referred to as ~dispersed
fibers".
The basis weight of the throughdried sheet can be from about
5 to about 50, preferably from about 10 to about 40, and more
preferably from about 20 to about 30 grams per square meter. It will
be appreciated that lower basis weight sheets inherently have greater
permeability for a given furnish. Hence the greatest advantage of
this invention is obtained with relatively higher basis weights where
the sheets are normally more difficult to throughdry. The invention
is particularly suitable for making throughdried single-ply bath
tissue having a basis weight of about 25 grams per square meter.
The consistency of the aqueous suspension which is subjected to
the treatment of this invention must be high enough to provide
significant fiber-to-fiber contact or working which will alter the
surface properties of the treated fibers. Specifically, the
consistency can be at least about 20, more preferably from about
20 to about 60, and most preferably from about 30 to about 50 dry
weight percent. The consistency will be primarily dictated by the
kind of machine used to treat the fibers. For some rotating shaft
dispersers, for example, there is a risk of plugging the machine at
consistencies above about 40 dry weight percent. For other types of
shaft dispersers, such as the Bivis machine (commercially available
from Clextral Company, Firminy Cedex, France), consistencies greater
than 50 can be utilized without plugging. It is desirable to utilize
a consistency which is as high as possible for the particular machine
used.
The temperature of the fibrous suspension can preferably be
about 150F. or greater, more preferably about 210-F. or greater, and
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more preferably about 220F. or greater. In general, higher
temperatures are better for increasing the TD Index. The upper limit
on the temperature is dictated by whether or not the disperser is
pressurized, since the aqueous fibrous suspensions within apparatus
operating at atmospheric pressure cannot be heated beyond the boiling
point of water. A suitable temperature range is from about 170-F. to
about 220-F.
The amount of power applied to the fibrous suspension also
impacts the resulting TD Index. In general, increasing the power
input will increase the TD Index. However, it has also been found
that the improvement (increasing) of the TD Index falls off upon
reaching a power input of about 2 horsepower-days per ton (HPD/T) of
dry fiber in suspension. A preferred range of power input is from
about 1 to about 3 HPD/T, more preferably about 2 HPD/T or greater.
In working the fibers within the disperser, such as by shearing
and compression, it is necessary that the fibers experience
substantial fiber-to-fiber contact by rubbing or shearing in addition
to rubbing or shearing contact with the surfaces of the mechanical
devices used to treat the fibers. Some compression, which means
pressing the fibers into themselves, is also desireable to enhance or
magnify the effect of the rubbing or shearing of the fibers. The
desired fiber-to-fiber contact can in part be characterized by
apparatus having a relatively high volume-to-working surface area
ratio which increases the likelihood of fiber-to-fiber contact. The
working surface for purposes herein is defined as that surface of the
apparatus which contacts the majority of the fibers passing through.
For example, disc refiners have a very low volume-to-working surface
area (approximately 0.05 centimeters) because there is a relatively
small volume or space between the opposed rotating discs (working
surfaces). Such devices work the fibers primarily by contact between
the working surfaces and the fibers. However, the apparatus
particularly useful for purposes of this invention, such as the
various types of shaft dispersers, have a much higher volume-to-
working surface area. Such volume-to-working surface area ratios can
be about 1 centimeter or greater, preferably about 3 centimeters or
greater, and more specifically from about 5 to about 10 centimeters.
21~ 76~ 7
These ratios are orders of magnitude greater than those of disc
refiners.
The measure of the appropriate amount of shearing and
compression to be used lies in the end result, which is the
achievement of an increased TD Index. A number of shaft dispersers
or equivalent mechanical devices known in the papermaking industry
can be used to achieve varying degrees of the desired results.
Suitable shaft dispersers include, without limitation, nonpressurized
shaft dispersers, such as the Maule shaft disperser, and pressurized
shaft dispersers, such as the Bivis machines and the like.
If softening agents are used to enhance the softness of the
final tissue product, suitable agents include, without limitation,
fatty acids, waxes, quaternary ammonium salts, dimethyl
dihydrogenated tallow ammonium chloride, quaternary ammonium methyl
sulfate, carboxylated polyethylene, cocamide diethanol amine, coco
betaine, sodium lauroyl sarcosinate, partly ethoxylated quaternary
ammonium salt, distearyl dimethyl ammonium chloride, and the like.
Examples of suitable commercially available chemical softening agents
include, without limitation, Berocell 564 and 584 manufactured by Eka
Nobel Inc., Adogen 442 manufactured by Whitco Sherex Chemical
Company, Quasoft 203 manufactured by Quaker Chemical Company, and
Arquad 2HT-75, manufactured by Akzo Chemical Company.
Brief DescriDtion of the Drawinq
Figure 1 is a schematic flow diagram of the apparatus used for
determining the TD Index.
Figure 2 is an exploded perspective view of the sample holder of
the apparatus of Figure 1, including the sliding sample tray used to
place the sample holder into the drying apparatus.
Figure 3 is a representative depiction of a typical plot of
pressure drop versus moisture ratio as generated while testing a
sample using the apparatus described in Figure 1 above.
Figure 4 is a schematic process flow diagram of a method of
treating fibers in accordance with this invention using a shaft
disperser to work the fibers.
Figure 5 is a cut-away perspective view of the shaft disperser
of Figure 4.
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Figure 6 is an alternative schematic process flow diagram of a
method in accordance with this invention using a pair of Bivis shaft
dispersers in series.
Figure 7 is a sectional view of a Bivis shaft disperser useful
for purposes of this invention.
Figure 8 is a sectional view, viewed in the axial direction, of
the reverse-flighted screws of the Bivis shaft disperser,
illustrating the cut-out notches in the flights.
Figure 9 is a sectional view, viewed in the axial direction, of
the forward-flighted screws.
Figure 10 is a sectional view of a reversed-flighted section of
the machine, illustrating the flow of the fibrous suspension.
Detailed Description of the Invention
The ThroughdrYabilitY Index
During throughdrying, it is generally understood that the drying
rate is high and relatively constant at high moisture ratios
(constant rate period). The drying rate begins decreasing rather
rapidly after reaching a certain critical moisture ratio (falling
rate period) in the sheet. If a constant air permeation rate is
maintained throughout the-drying period, the pressure drop is also
expected to decrease as the moisture ratio decreases (or as the
drying process continues). The manner in which the pressure drop
varies during the throughdrying process under a constant air
permeation rate is of primary interest for purposes of this invention
because it provides a quantitative means for measuring the air
permeability of the sheet while being dried. If one can accurately
measure the instantaneous absolute humidity of the outlet air after
drying a tissue sample, one can readily calculate the instantaneous
moisture ratio from the humidity of the outlet air and the initial
and the final moisture ratios of the tissue sample as shown below:
Xm(t) = Xo~ (Xo~Xend) ~out(t) ~in~ dt
wherein ~XoN= the moisture ratio of the test sample at the
beginning of the test, expressed as kilograms of
water per kilogram of bone dry fiber;
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"Xend~= the moisture ratio of the test sample at the end of
the test, expressed as kilograms of water per
kilogram of bone dry fiber;
~Xmll= the instantaneous moisture ratio of the test sample,
expressed as kilograms of water per kilogram of bone
dry fiber;
"Yjnn= the humidity of the drying air immediately prior to
reaching the sample, expressed as kilograms of water
per kilogram of dry air;
"Y~t"= the humidity of the drying air immediately after
passing through the sample, expressed as kilograms of
water per kilogram of dry air; and
"t"= elapsed time, expressed in seconds.
Calculating the moisture ratio X~ from the humidity data for the
entire drying period data enables one to plot pressure drop as a
function of the instantaneous moisture ratio. The inverse of the
area under the resulting curve is referred to herein as the TD Index,
expressed as kilopascals-~. This index is a measure of the air
permeability of the wet sheet and reflects the ease in which a paper
sheet made from a particular furnish can be throughdried. Higher TD
Index values reflect greater ease in throughdrying, whereas lower
values reflect greater difficulty in throughdrying.
As will be described below, measurement of the TD Index requires
that the fibers in question be formed into a handsheet having a basis
weight of 24 grams per square meter. This is accomplished by
diluting a fiber sample in water to a consistency of 2.5 weight
percent in a British Pulp Disintegrator and allowing the dispersed
sample to soak for 5 minutes. The sample is then pulped for
10 minutes at ambient temperature, diluted to 0.04 percent
consistency, and formed into a handsheet on a British Handsheet Mold
(The Hermann Mfg. Co., Lancaster, Ohio). The handsheet is couched
off of the mold by hand using a blotter without applying any
pressure. The handsheet is dried for 2 minutes to absolute dryness
using a Valley steam hotplate and a standard weighted canvas cover
having a lead filled (4.75 pounds) brass tube at one end to maintain
uniform tension. The TD Index for the resulting dried handsheet is
then determined as described herein and the measured TD Index value
21~687
is assigned to the fibers or furnish from which the handsheet was
made.
Referring now to Figure 1, the apparatus for determining the TD
Index will be described in greater detail. Unless otherwise
specified, conduit in the mainstream of air flow has a 1.5 inch
inside diameter. Air for drying the samples is provided by two "oil
free" compressors 1, each rated for 29.9 cubic feet per minute at
90 psi. (Model DN 1024H-3DF, Atlas Copco, Cleveland, Ohio). The
outlet of the compressors is suitably connected to the inlet of a
condensed water separator 2 (Model WS0-08-000, Wilkerson, Engelwood,
Colorado), which serves to remove any liquid water entrained in the
air stream. The outlet of the separator is suitably connected to the
inlet of a molecular sieve 3 (Model M530, Wilkerson) which serves to
eliminate particulate matter in the air having a particle size
greater than about 0.05 microns. The outlet of the molecular sieve
is suitably connected to a compressed air dryer 4 (Wilkerson model
DHA-AE-000) with an outlet flow of 49 cubic feet per minute. The
outlet of the dryer is connected to the inlet of a surge tank 5
(approximately 75 cubic feet capacity). The outlet of the surge tank
is suitably connected to two additional oil heat exchangers 6 and 7
(5 liter capacity/250-C. max. temp./6 bar max. pressure, Apparatebau
Wiesloch GmbH, Weisloch, Germany) in series which serve to further
heat the air to the desired throughdrying temperature. In between
the surge tank and the two heat exchangers are a moisture monitor 8
(Aquanel, Gerhard GmbH, Blankebach, Germany), a gate-type control
valve 9 (DIN R65, 1.5 inch, PN 16, Henose, Hamburg, Germany) for
controlling the flow rate of the air, an orifice plate 10
(25 millimeter diameter opening, RST 37-2 PN6 DIN 2527-32-5784,
University of Karlsruhe, Karlsruhe, Germany), and a manometer 11
(Betz, Gottingen, Germany), which together are used to determine the
air flow rate. Other valves and piping (not shown) which are not
essential to the operation of the apparatus can be present for
convenience at various places to isolate or by-pass gauges and other
devices. The outlet of the second heat exchanger is directly
connected to a sample housing 12, which is designed to receive and
hold a slidable sample tray 13 (see Figure 2) into which a sample
holder (see Figure 2) is placed. All of the air passes through the
2I47687
sample placed in the sample holder. An inflatable gasket mounted
within the sample housing assures a positive seal between the sample
housing and the slidable sample tray when the gasket is activated. A
diffuser 14 is suitably connected to the outlet of the sample housing
such that the cross-sectional flow area is expanded to 11,600 square
millimeters in order to slow down the air flow to facilitate more
accurate moisture measurement. The diffuser is suitably connected to
a vent tube which carries the air to a suitable exhaust system. A
differential temperature sensor 16 (resistance type differentiation)
is suitably connected to measure the temperature of the air before
and after the sample. A differential pressure sensor 17 (PU 1000,
0-1000 millibar, llOV AC) is suitably connected to measure the
pressure drop across the sample. A second, more sensitive moisture
monitor 18 (infrared; twenty-five measurements per second; made by
Paderborn University, Paderborn, Germany) measures the moisture
content of the air leaving the sample. All three sensors are linked
to a Schlumberger data acquisition system 19 which is linked to a
computer 20 (RMC 80286 processor) for correlating the data.
Figure 2 illustrates the sample holder and the manner in which
the sample is mounted within the sample holder, including the sliding
sample tray 13 adapted to hold the sample holder and slide it in and
out of the sample housing. Shown in Figure 2 is the top of the
sample holder 21, which is the upstream portion, and the bottom of
the sample holder 22, which is the downstream portion. The bottom of
the sample holder contains a support fabric 23 (Asten 937, Asten
Corporation, Appleton, WI) upon which the sample rests. Sandwiched
in between the top and bottom is the handsheet sample 24 which has
been cut to the appropriate size. A thin line of grease (not visible
in this view) positioned around the inside edge of the top of the
sample holder provides a seal between the sample and the top of the
sample holder when the sample is secured. The top of the sample
holder contains two registration pins 25 which become inserted into
registration holes 26 in the bottom of the sample holder and the two
halves are secured by means of six screws 27 with appropriate
threaded holes.
Having identified the apparatus for determining the TD Index,
the procedure for determining the TD Index can now be described.
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21~7687
Generally speaking, to determine the TD Index for a given test sample
(24 gsm handsheet), the sample is carefully wetted to a particular
moisture level and dried in the apparatus described above under
controlled conditions of constant air mass flow rate. The moisture
level in the sample is continuously calculated by a computer, based
on the humidity measurements of the air before and after the sample
during the test. The measured pressure drop across the sample is
plotted as a function of the calculated moisture level of the sample,
and the area under the curve is the TD Index.
More specifically, the handsheet sample to be tested is cut into
a 10.25 centimeter diameter circle which fits into the sample holder
of the apparatus and is only slightly larger than the sample holder
opening. During the test, only a 10 centimeter diameter circle of
the sample is exposed to air flow. Accordingly, the portion of the
circular sample outside the sample holder opening is impregnated with
grease (Compound 111 Valve Lubricant and Sealant, Dow Corning
Corporation, Midland, Michigan) before being wetted for the test to
prevent any moisture from being wicked outside the circle as well as
establishing a more positive seal between the sample and the sample
holder. This is accomplished by applying and rubbing the grease
around the inside edge of the top half (upstream half) of the sample
holder. The sample is then placed onto the bottom half of the sample
holder which contains a piece of throughdrying fabric (Asten 937) for
supporting the sample. The top half of the sample holder is then
clamped onto the bottom half of the sample holder, thereby
impregnating the outer edge of the sample with the grease. The
supported sample is then wetted by spraying with a fine mist until a
moisture level of 3.0 kilograms of water per kilogram of bone dry
fiber is reached. During spraying, a cover guard should be placed
over the sample holder to prevent the sample holder from being
sprayed and to keep all of the sprayed water directed at the
10 centimeter circle. The moisture in the sample is accurately
determined by the weight difference before and after wetting.
While the sample is being prepared, the air system is turned on
at a predetermined constant flow rate of 3.0 kilograms per square
meter per second and heated to a temperature of 175C. When steady
state is reached, the air temperature will be steady and constant,
- 11 -
21~ 7~87
the humidity will be zero, and the pressure drop will be zero. After
the reference steady state conditions are achieved, the sample holder
with the wetted test sample is placed into the sliding sample tray
and slid into the sample housing of the apparatus. The humidity and
the pressure drop are continuously monitored by the instruments and
the computer, which provides a plot of pressure drop versus moisture
ratio. A typical plot is illustrated in Figure 3. (Note that time
increases from right to left in this plot.) As shown, the pressure
drop shows a very rapid initial increase and thereafter quickly
starts decreasing, typically reaching a constant level in about four
or five seconds. The computer then integrates the inverse of the
area under the curve and calculates the TD Index as earlier
described.
Figure 4 is a block flow diagram illustrating overall process
steps for treating fibers in accordance with this invention. Shown
is the paper furnish 28 to be treated being fed to a high consistency
pulper 29 (Model ST6C-W, Bird Escher Wyss, Mansfield, MA) with the
addition of dilution water 30 to reach a consistency of about 15
percent. Prior to being pumped out of the pulper, the stock is
diluted to a consistency of about 6 percent. The pulped fibers are
fed to a scalping screen 31 (Fiberizer Model FT-E, Bird Escher Wyss)
with additional dilution water in order to remove large contaminants.
The input consistency to the scalping screen is about 4 percent. The
rejects from the scalping screen are directed to waste disposal 32.
The accepts from the scalping screen are fed to a high density
cleaner 33 (Cyclone Model 7 inch size, Bird Escher Wyss) in order to
remove heavy contaminants which have escaped the scalping screen.
The rejects from the high density cleaner are directed to waste
disposal. The accepts from the high density cleaner are fed to a
fine screen 34A (Centrisorter Model 200, Bird Escher Wyss) to further
remove smaller contaminants. Dilution water is added to the fine
screen feed stream to achieve a feed consistency of about 2 percent.
Rejects from the fine screen are directed to a second fine screen 34B
(Axiguard, Model 1, Bird Escher Wyss) to remove additional
contaminants. The accepts are recycled to the feed stream to the
fine screen 34A and the rejects are directed to waste disposal. The
accepts from the fine screen, with the addition of dilution water to
2l 476~7
reach a consistency of about 1 percent, are then passed to a series
of four flotation cells 35, 36, 37 and 38 (Aerator Model CF1, Bird
Escher Wyss) to remove ink particles and stickies. Rejects from each
of the flotation cells are directed to waste disposal. The accepts
from the last flotation cell are fed to a washer 39 (Double Nip
Thickener Model 100, Black Clawson Co., Middletown, OH) to remove
very small ink particles and increase the consistency to about
10 percent. Rejects from the washer are directed to waste disposal.
The accepts from the washer are fed to a belt press 40 (Arus-Andritz
Belt Filter Press Model CPF 20 inches, Andritz-Ruthner Inc.,
Arlington, TX) to reduce the water content to about 70 percent.
Pressate is directed to waste disposal. The resulting partially
dewatered fibrous material is then fed to a shaft disperser 41
(GR II, Ing. S. Maule & C. S.p.A., Torino, Italy), described in
detail in Figure 5, in order to work the fibers to increase the TD
Index in accordance with this invention. Steam 42 is added to the
disperser feed stream to elevate the temperature of the feed
material. The resulting treated fibers 43 can be directly used as
feedstock for papermaking or otherwise further treated as desired.
Figure 5 is a cut-away perspective view of a preferred apparatus
for treating fibers in accordance with this invention as illustrated
in Figure 4. The particular apparatus is a shaft disperser, type
GR II, manufactured by Ing. S. Maule & C. S.p.A., Torino, Italy.
This apparatus has a volume-to-working surface area of about
8.5 centimeters. Shown is an upper cylindrical housing 51 and a
lower cylindrical housing 52 which, when closed, enclose a rotating
shaft 53 having a multiplicity of arms 54. The upper housing
contains two rows of knurled fingers 55 and three inspection ports
56. At one end of the upper housing is an inlet port 57. At the
inlet end of the rotating shaft is drive motor 58 for turning the
shaft. At the outlet end of the rotating shaft is a bearing housing
59 which supports the rotating shaft. The inlet end of the rotating
shaft contains a screw feed section 60 which is positioned directly
below the inlet and serves to urge the feed material through the
disperser. The outlet 61 of the disperser comprises a hinged flap 62
having a lever 63 which, when the disperser is closed up, is engaged
by hydraulic air bags 63 mounted on the upper housing. The air bags
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provide controllable resistance to the rotation of the hinged flap
and hence provide a means of controlling the back pressure within the
disperser. Increasing the back pressure increases the degree to
which the fibers are worked and thereby increases the TD Index.
During operation, the knurled fingers interdigitate with the arms of
the rotating shaft to work the feed material therebetween.
Figure 6 is a block flow diagram of an alternative process of
this invention utilizing a pair of Bivis shaft dispersers. As
illustrated, papermaking pulp, preferably secondary fiber pulp, at a
consistency of about 50 percent, is fed to a screw feeder. The screw
feeder meters the feedstock to the first of two Bivis shaft
dispersers in series. Each Bivis shaft disperser typically has three
or four compression/expansion zones. Steam is injected into the
first Bivis shaft disperser to raise the temperature of the fibers to
about 212-F. The worked pulp is transferred to the second Bivis
shaft disperser operating at the same conditions as the first
disperser. The worked pulp from the second machine can be quenched
by dropping it into a cold water bath and thereafter dewatered to a
suitable consistency.
Figure 7 is a sectional elevational view of a twin screw Bivis
shaft disperser useful for purposes of this invention. Shown are the
inlet 71, feed screw 72, forward flighted screws 73, 74, 75, and 76,
reverse-flighted screws 77, 78, 79 and 80, outlet 81, injection ports
82, 83, 84 and 85, optional extraction ports 86, 87, 88 and 89, and
thermocouples 90. In operation, pulp is introduced into the Bivis
through the inlet. The pulp then encounters the short feed screw,
which serves to introduce the pulp into the first working zone. The
working zones consist of a pair of slightly overlapping screws
encased in cylinders with less than 1 millimeter clearance between
the screw flights and the cylinder walls. The twin screws rotate in
the same direction, and at the same speed. Shaft rotation transports
the pulp axially through the machine. Key to the fiber property
modification within the machine are the reverse-flighted screw
sections which have small slots machined into the flights and are
positioned periodically along the length of both screws. These
reverse-flighted sections serve to reverse the flow of stock through
the machine, thereby introducing backpressure to the stock stream.
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Thus the stock travels forward axially until it encounters a
backpressure zone. The pressure builds in this zone, but because of
the slots in the reverse flights, the pressure behind is greater than
the pressure ahead. In this manner the stock is forced through the
slots where it encounters the next (lower pressure) forward-flighted
section of the screws. It is theorized that this
compression/expansion action further enhances the modification of
fiber properties. Typically the Bivis shaft disperser is set up to
include three or four working zones. The injection ports can be used
to inject different chemicals into each of the individual working
zones. The extraction ports associated with each working zone can be
used to extract liquid if desired. Although-not measured, the
volume-to-surface area ratio of the Bivis shaft disperser is believed
to be slightly less than that of the Maule shaft disperser.
Figure 8 is an axial view of a reverse-flighted section of the
twin screws of the apparatus of Figure 7. Shown are screws 92 and
93, each having slots 94 machined out of their flights. As shown,
the flights of each screw overlap.
Figure 9 is an axial view of a forward-flighted section of the
twin screws of the apparatus of Figure 7, illustrating the overlap of
the screw flights 95 and 96.
Figure 10 is an expanded sectional view of a working zone of the
apparatus of Figure 7, showing the upstream forward-flighted screw
section "AH, the reverse-flighted screw section "B~, and the
downstream forward-flighted screw section HCn. This figure serves to
illustrate the flow of stock (represented by the arrows) through the
reverse-flighted screw section.
EXAMPLES
Having described the TD Index and the method of carrying out
this invention, the invention will now be further described in detail
with reference to the following examples.
ExamDle 1.
Secondary fiber (office waste grade) was treated by the process
described in Figure 4. Specifically, the secondary fiber was
repulped at a consistency of 15%, cleaned, pressed to a pre-disperser
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consistency of 22.9%, then fed to a Maule shaft disperser as
illustrated in Figure 5. The fines content of the pulp fed to
disperser was 8.5 weight percent. The disperser was maintained at a
temperature of 175-F. Power input to the disperser was approximately
1.39 horsepower-day per ton of fiber (HPD/T). Pre- and post-
disperser samples were taken and made into dry 24 gsm. handsheets per
the previously defined method. Measurement of the TD Index was
carried out as previously described. The results are set forth in
Table 1 below. (The ~TD~ is the Throughdryability Index, expressed
as kilopascals ~. The "Tensile" is the tensile strength, expressed
as grams per inch of sample width. The "Tear" is the Elmendorf tear
strength, expressed as grams-force per four sheets. The "Caliper~ is
thickness, expressed as inches. The "TEA" is the tensile energy
absorbed, expressed as gram-centimeters per square centimeter.)
TABLE 1
Sample TDTensile Tear Caliper TEA
Pre-disperser 0.111274 18.0 0.0058 8.1
Post-disperser 0.30739 17.2 0.0076 3.8
Example 2.
The same secondary fiber material was treated as described in
Example 1, except the disperser temperature was 175-F., the pre-
disperser consistency was 34.7 percent, and the power input to the
disperser was 2.12 HPD/T. Pre- and post-disperser samples were taken
as before and formed into handsheets as previously described. The
pre-disperser fines content was 7.4 weight percent. The results~are
set forth in Table 2.
TABLE 2
TDTensile Tear Caliper TEA
Pre-disperser 0.121278 18.0 0.0060 6.6
Post-disperser 0.53585 14.0 0.0082 2.5
Example 3.
The same secondary fiber material was treated as described in
Example 1, except the disperser temperature was 150-F., the pre-
disperser consistency was 34.6%, and the power input to the disperser
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was 2.15 HPD/T. Pre- and post-disperser samples were taken as before
and formed into handsheets as previously described. The results are
set forth in Table 3.
TABLE 3
TD Tensile Tear Caliper TEA
Pre-disperser 0.14 1334 19.2 .0060 7.9
Post-disperser 0.37 884 18.0 .0070 4.9
10Example 4.
The same secondary fiber material was treated as described in
Example 1, except the disperser temperature was 81-F., the pre-
disperser consistency was 26.8% and the power input to the disperser
was 2.44 HPD/T. Pre- and post-disperser samples were taken as before
and formed into handsheets as previously described. The pre-
disperser fines content was 7.2 weight percent. The results are set
forth in Table 4.
TABLE 4
TD Tensile Tear Caliper TEA
Pre-disperser 0.10 1409 19.6 .0055 8.3
Post-disperser 0.17 1125 20.0 .0062 6.7
Example 5.
The same secondary fiber material was treated as described in
Example 1, except the disperser temperature was 79-F., the pre-
disperser consistency was 34.6% and the power input to the disperser
was 1.18 HPD/T. Pre- and post-disperser samples were taken as before
and formed into handsheets as prev;ously described. The pre-
disperser fines content was 6.9 weight percent. The results are set
forth in Table 5.
TABLE 5
TD Tensile Tear CaliDer TEA
Pre-disperser 0.10 1322 - .0055 5.7
35Post-disperser 0.14 1189 22.4 .0060 6.7
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Example 6.
The same secondary fiber material was treated as described in
Example 1, except the disperser temperature was 78-F., the pre-
disperser consistency was 37.1% and the power input to the disperser
was 2.95 HPD/T. Pre- and post-disperser samples were taken as before
and formed into handsheets as previously described. The pre-
disperser fines content was 7.8 percent. The results are set forth
in Table 6.
TABLE 6
TD Tensile Tear CaliDer TEA
Pre-disperser 0.10 1435 20.0 .0055 8.1
Post-disperser 0.19 1155 21.6 .0064 6.1
ExamDle 7.
Well-washed secondary fiber at approximately 50% consistency was
fed in crumb form to a Bivis twin screw shaft disperser (Clextral
Company, Firminy Cedex, France) via a screw feeder as illustrated in
Figures 7-10. The fines content of the washed fiber was 5.2 weight
percent. Steam was introduced into the interior compartment to raise
the pulp temperature to approximately 220-F. (105-C.J. The product
from the first Bivis disperser was sent to a second Bivis disperser
operated under the same conditions. From the second disperser, the
pulp was sent to a quench tank at 10-C. The pulp was then gravity
dewatered and made into 24 gsm handsheets as previously described and
the TD Index was determined as previously described. The results are
set forth in Table 7.
TABLE 7
TD Tensile Tear Caliper TEA
Pre-Bivis 0.08 1072 18.5 .0061 5.65
Post-Bivis 0.71 388 10.5 .0079 1.28
The results from the foregoing examples illustrate that the
Throughdryability Index of secondary fibers can be dramatically
increased (and hence the throughdrying behavior of the fibers can be
improved) by subjecting the fibers to appropriate working forces at
high consistency and high temperature in a shaft disperser. Also,
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the application of high consistency working forces improves the
tissue making character of the fibers by decreasing the tensile and
tearing strengths while increasing bulk.
ExamPle 8
Cenibra eucalyptus fibers were pulped for 15 minutes at 10%
consistency and dewatered to 30% consistency. A softening agent
(Berocell 584) was added to the pulp in the amount of 10 lb. Berocell
per ton dry fiber, and the pulp was then fed to a Maule shaft
disperser as illustrated in Figure 5. The disperser was operated at
160-F with a power input of 2.2 HPD/T.
The resulting dispersed eucalyptus fibers were made into a two-
layered tissue having a softwood fiber layer and a eucalyptus fiber
layer. Prior to formation, the northern kraft softwood fibers
(LongLac-l9) were pulped for 60 minutes at 4% consistency, while the
dispersed eucalyptus fibers were pulped for 2 minutes at 4%
consistency. Each layer was independently formed on separate forming
fabrics at 0.05% consistency at a speed of about 50 feet per minute
and the resulting two webs were couched together at approximately 10%
consistency to form a two-layered web. The resulting layered web was
transferred to a papermaking felt and thereafter pressed onto the
surface of a Yankee dryer, where the web was dried and creped at a
1.3 crepe ratio. The dryer side of the layered web was composed
entirely of the softwood fibers and had a basis weight of 7.25
lb./2880 ftZ (dryer basis weight). The air side of the layered web
was composed entirely of the dispersed eucalyptus fibers of equal
basis weight. After creping, the tissue was wound into bath rolls
under minimum draw.
The resulting tissue had the following properties: tensile
strength = 858 grams per 3 inches width (machine direction) and 488
grams per 3 inches width (cross-machine direction); stretch = 30.4%
(machine direction) and 5.8% (cross-machine direction); Panel
Softness = 7.70. (Panel Softness is determined by a trained sensory
panel which rates tissues for softness on a scale of from 0 to about
9.5.) For comparison, a typical softness value for throughdried
material at similar strength is 7.15.
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ExamPle 9
Southern hardwood kraft fibers (Coosa River-59) were pulped for
15 minutes at 10% consistency and dewatered to 28% consistency. A
debonder (Berocell 584) was added to the pulp in the amount of 10 lb.
Berocell per ton dry fiber. The pulp was then fed to a Maule shaft
disperser. The disperser was operated at 160-F with a power input of
2.2 HPD/T.
The resulting dispersed hardwood fibers were made into a two-
layered tissue having a softwood layer and a hardwood layer.
Specifically, northern softwood kraft fibers (LongLac-l9) were pulped
for 60 minutes at 4% consistency, while the dispersed hardwood fibers
were pulped for 2 minutes at 4% consistency. Each layer was
independently formed on separate forming fabrics at 0.05% consistency
and the resulting webs were couched together at about 50 feet per
minute at approximately 10% consistency to form a single layered web.
The resulting layered web was transferred to a papermaking felt and
thereafter pressed onto the surface of a Yankee dryer, where the web
was dried and creped at a 1.3 crepe ratio. The dryer side of the
layered web was composed entirely of softwood fibers and had a basis
weight of 7.25 lb./2880 ft2 (dryer basis weight). The air side of
the web was composed entirely of the dispersed hardwood fibers of
equal basis weight. After creping, the tissue was wound into bath
tissue rolls under minimum draw.
The resulting tissue had the following properties: tensile
strength = 689 grams per 3 inches width (machine direction) and
466 grams per 3 inches width (cross-machine direction); stretch = 32%
(machine direction) and 5.6% (cross-machine direction); Panel
Softness = 7.65. For comparison, a typical softness value for
throughdried material at similar strength is 7.30.
Examples 8 and 9 both illustrate the softness advantages of
treating virgin hardwood fibers with a disperser in accordance with
the method of this invention for making wet-pressed tissues.
Example 10
A secondary fiber blend (45 percent laser ledger, 45 percent
laser computer print-out, 10 percent white ledger) was pulped for
15 minutes at 40- C and 15 percent consistency. The resultant slurry
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was screened at ambient temperature and 3.0 percent consistency. The
slurry was then dehydrated to 30 percent consistency using a belt
press and screw fed directly to a shaft disperser (J. M. Voith GmbH,
Heidenheim, Germany) similar to that illustrated in Figure 5. The
disperser was maintained at a temperature of 176- F. Power input to
the disperser was approximately 4.17 HPD/T. Pre- and post-disperser
samples were taken and made into 24 gsm handsheets per the previously
defined method. As the improvement in TD Index occurs under all
drying conditions, the TD Index of this sample was measured under
different flow conditions than the previous examples. Specifically,
the air flow rate was 1.0 kg./m2 sec., the air temperature was 90 C,
and the initial moisture ratio was 3.5. Since each of these factors
affects the TD Index, the TD Index values in this example are not
directly relatable to those of the previous examples. Still, the
15 improvement in the TD Index is readily apparent. The results are set
forth in Table 8 below.
Table 8
Sample TD Fines
Pre-disperser 0.07 22.0
Post-disperser 0.18 23.3
Example 11
The same secondary fiber blend was treated as described in
Example 10, except the disperser was maintained at a temperature of
166- F and the power input to the disperser was approximately 5.28
HPD/T. Pre- and post-disperser samples were taken and made into
24 gsm handsheets as previously described. Measurement of the TD
Index was carried out as for Example 10. The results are set forth
in Table 9 below.
Table 9
SamDle TD Fines
Pre-disperser 0.07 22.0
Post-disperser 0.18 20.6
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Example 11
The same secondary fiber blend was treated as described in
Example 10, except the disperser was maintained at a temperature of
147 F and the power input to the disperser was approximately
5.55 HPD/T. Pre- and post-disperser samples were taken and made into
24 gsm handsheets as previously described. Measurement of the TD
Index was carried out as for Example 10. The results are set forth
in Table 10 below:
Table 10
Sample TD Fines
Pre-disperser 0.07 22.0
Post-disperser 0.17 20.6
As illustrated by Examples 10, 11 and 12, the method of this
invention greatly improves the TD Index even at very high fines
levels.
It will be appreciated that the foregoing examples, given for
purposes of illustration, are not to be construed as limiting the
scope of this invention, which is defined by the following claims and
all equivalents thereto.
