Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MAKING HIGH BULK WET-PRESSED TISSUE
Background of the Invention
In the manufacture of tissue products, such as facial tissue, bath
tissue, paper towels and the like, the tissue sheet is formed by
depositing an aqueous suspension of papermaking fibers onto a forming
fabric. The web is then transferred to a papermaking felt and dewatered
as it passes through a pressure nip created between a pressure roll and a
Yankee dryer as the wet web is transferred to the Yankee surface. Free
water expressed from the web in the pressure nip is absorbed and carried
away by the felt as the web transfers to the Yankee surface. The web is
then final dried on the surface of the Yankee and subsequently creped to
impart bulk and softness to the resulting tissue sheet. This method of
making.tissue sheets is commonly referred to as "wet-pressing" because of
the method used to dewater the wet web.
The wet-pressing method has a couple of distinct drawbacks. First,
pressing the tissue web while wet densifies the web significantly. As
the web is dried, the dried sheet retains this high density (low bulk)
until it is creped. Creping is necessary to attempt to undo what the
wet-pressing has done to the sheet. In response to this situation,
through-air-drying methods have been developed in which the newly-formed
web is partially dewatered to about 30 percent consistency using vacuum
suction. Thereafter the partially dewatered web is final dried without
- compression by passing hot air through the web while it is supported by a
throughdrying fabric. However, through-air-drying is expensive in terms
of capital and energy costs.
A second drawback, shared by conventional wet-pressing and through-
air-drying processes is the high energy costs necessary to dry the web
from a consistency of about 35 percent to a final dryness of about 95
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percent. This second drawback has recently been addressed in the
manufacture of high density paper products by the advent of the high
intensity extended nip press. This device employs an extended nip length
and heat to more efficiently dewater the wet web up to exit consistencies
of about 60 percent. Such devices have been successfully used for making
paperboard, but have not been used to make low density paper products
such as tissues because the high pressures and longer dwell times in the
extended nip press serve to further densify the sheet beyond that
experienced by conventional tissue wet-pressing methods. This increase
in density is detrimental to the quality of the resulting tissue products
because creping cannot completely overcome the added increase in sheet
density.
Therefore there is a need for a method of making wet-pressed tissue
sheets that minimizes or eliminates the high densities imparted to wet-
pressed tissue webs.
Summate of the Invention
It has now been discovered that the reduction in bulk associated
with wet-pressing can be substantially reduced by incorporating into the
web certain fibers which have been found to greatly diminish web
densification when subjected to the high pressures necessary for
dewatering with high intensity extended nip presses. As a consequence,
high intensity extended nip presses can be used to dewater tissue webs
without the heretofore adverse consequence of imparting a high degree of
densification to the web.
Hence in one aspect the invention resides in a method for making a
bulky tissue sheet comprising: (a) depositing an aqueous suspension of
papermaking fibers onto a forming fabric to form a wet tissue web, said
papermaking fibers comprising at least about 10 dry weight percent
modified wet-resilient fibers; (b) partially dewatering the wet web to a
consistency of about 15 percent or greater; (c) compressing the partially
dewatered web in a high intensity extended nip press to further dewater
the web to a consistency of about 35 percent or greater; and (d) final
drying the web, wherein the Bulk of the dewatered web prior to final
drying is greater than (-0.02C + 3.11), wherein "C" is the consistency of
the web leaving the high intensity extended nip press, expressed as
percent dryness, and Bulk is expressed as cubic centimeters per gram.
For a given consistency, the wet tissue webs of this invention have
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greater bulk than comparable wet tissue webs that have been dewatered by
conventional means. Furthermore, the consistency can be increased well
beyond that attainable by conventional tissue dewatering and, in most
instances, still have a higher bulk at higher consistencies than that of
conventional wet tissue webs at substantially lower consistencies.
In another aspect, the invention resides in the combination of
dewatering a tissue web using a high intensity extended nip press, which
greatly reduces the bulk of the tissue web, followed by rush transferring
the dewatered web to increase the bulk of the web back to levels suitable
for tissue. More specifically, the invention resides in a method for
making a bulky tissue sheet comprising: (a) depositing an aqueous
suspension of papermaking fibers onto a forming fabric to form a wet
tissue web; {b) partially dewatering the wet web to a consistency of
about 15 percent or greater; (c) compressing the partially dewatered web
in a high intensity extended nip press to further dewater the wet web to
a consistency of about 35 percent or greater; (d) transfering the
dewatered web to a first transfer fabric; {e) transfering the dewatered
web from the first transfer fabric to a second transfer fabric travelling
at a slower speed than the first transfer fabric (rush transfer) to
increase the bulk of the wet web; and {f) drying the web. The web can be
dried on a Yankee dryer and creped, or the web can be throughdried and
left uncreped or creped.
As used herein, "modified wet-resilient fibers" are fibers that have
been modified from their natural state and have the capability to recover
after deformation in the wet state, as opposed to fibers that remain
deformed and do not recover after deformation in the wet state. Examples
of modified wet-resilient fibers include, without limitation, chemically
cross-linked cellulosic fibers, heat-cured cellulosic fibers, mercerized
fibers and sulfonated pulp fibers. These fiber modification methods are
well known in the art. The amount of modified wet-resilient fibers in
the fiber furnish can be about 10 dry weight percent or greater, more
specifically from about 20 to about 80 percent, and still more
specifically from about 30 to about 60 percent. The bulk benefits
associated with using modified wet-resilient fibers increase as the
amount of the modified wet-resilient fibers increases. Consequently the
amount used must take into account the desireability for added bulk
versus other desired properties, such as tensile strength, that other
fibers may be better suited to provide.
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A "high intensity extended nip press", as used herein, is a water-
removing pressing apparatus wherein the wet web is compressed in an
extended nip formed between the arcuate surface of a backing roll and a
pressing fabric or blanket. Typically the pressing fabric is supported
by a press shoe having a concave surface. The backing roll can be heated
to elevated temperatures or remain at ambient temperature. The length of
the extended nip can be substantial, typically from about 5 to about 10
inches or more. Such devices permit the operator to vary conditions such
as d~nrell time, pressure and temperature to effect greater water removal
than can normally be obtained in a conventional roll press. Such an
apparatus can remove substantially all of the free water in the sheet and
a significant portion of the bound water as well. An example of such an
apparatus is disclosed and described in US Patent No. 4,973,384 issued
November 27, 1990 to Crouse et al. entitled "Heated Extended Nip Press
Apparatus". In operating the
high intensity extended nip press, the use of a heated press roll in the
extended nip is optional, although preferred for maximum water removal.
The consistency (weight percent fiber or percent dryness) of the
part~dally dewatered web entering the high intensity extended nip press
can be about 15 percent or greater, more specifically from about 15 to
about 30 percent. The consistency of the web leaving the high intensity
extended nip press can be about 35 percent or greater, more specifically
from about 40 to about 70 percent, and still more specifically from about
50 to about 65 percent. The final consistency may depend upon the
incoming web consistency, the speed of the web, the temperature of the
heated roll, the pressure within the nip, the length of the nip, the
properties of the fibers and the characteristics of the press felt, as
well as additional variables.
Depending upon the consistency to which the web is dewatered and
other factors, such as the temperature/pressure of the high intensity
extended nip press and the dwell time in the nip, the Bulk of the wet web
leaving the high intensity extended nip press can be from about 2.3 to
about. 3.5 cubic centimeters per gram or greater, more specifically from
about: 2.4 to about 3.0 cubic centimeters per gram. More specifically,
taking the consistency of the web into account, the Bulk of the wet web
leaving the high intensity extended nip press can be greater than (-
0.020 + 3.11), more specifically greater than (-0.032C + 3.78), still
more specifically greater than (-O.OZC + 3.52), and still more
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specifically greater than (-0.03C + 4.28), where "C" is the consistency
of the web. The origin of these values will be described in detail in
reference to the Drawings. Stated differently, the increase in Bulk
attained when using the high intensity extended nip press to dewater webs
containing modified wet-resilient fibers is from about 5 to about 50
percent, more specifically from about 10 to about 40 percent, and still
more specifically from about 20 to about 30 percent greater than the Bulk
of webs consisting of a 50/50 weight percent blend of eucalyptus and
northern hardwood kraft fibers produced under the same conditions.
As used herein, Bulk is determined by dividing the caliper of the
web by the basis weight. The caliper is measured for a single web or
sheet using a T.M.I. Model 549 micrometer (Testing Machines Inc.,
Amityvile, New York) using a circular pressure foot having an area of 200
square millimeters. The pressure foot lowering speed is about 0.8
millimeters per second. The pressure, when lowered, is about 0.50
kilogram per square centimeter. The dwell time is about 3 seconds. One
measurement is taken for each sheet and five sheets of each sample are
tested. The readings are taken near the end of the dwell time for each
test. The average of the five readings is the caliper of the sample.
In those embodiments of this invention in which a rush transfer is
utilized after the web has been dewatered, the speed of the first
transfer fabric (the fabric from which the web is being transferred) can
be about 5 to about 35 percent faster than the speed of the second
transfer fabric (the fabric to which the web is being transferred). More
specifically, the speed differential can be from about 10 to about 30
percent, and still more specifically from about 20 to about 30 percent.
As the speed differential is increased, the Bulk of the resulting web is
increased. Speed differentials greater than about 35 percent, however,
are not desirable because the sheet buckles to form macrofolds.
Brief Description of the Drawing
Figure 1 is a schematic diagram of a tissue making process in
accordance with this invention, illustrating the use of a high intensity
extended nip press.
Figure 2 is a schematic view of the high intensity extended nip
press, illustrating its function in more detail.
Figure 3 is a plot of Bulk as a function of web consistency for
handsheets produced under conditions simulating the operation of a high
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intensity extended nip press, illustrating the decrease in Bulk with
increasing exit consistency for a number of different fiber furnishes.
Detailed Description of the Drawin4
Referring to Figure l, shown is a schematic flow diagram of a tissue
making process utilizing a high intensity extended nip press in
accordance with this invention. Shown is a headbox 5 which deposits an
aqueous suspension of papermaking fibers between a papermaking felt 6 and
a forming fabric 7. Both fabrics converge and partially traverse the arc
of the forming roll 8, after which the web 9 is retained by the felt.
This forming geometry is commonly referred to as a crescent former.
However, other forming configurations can also be used, such as twin wire
formers. At this point in the process, the web typically will have a
consistency of about 15 percent.
While supported by the felt, the wet web is then passed through the
high intensity extended nip press 20 to further dewater the web to a
consistency of from about 35 to about 70 percent. The dewatered web
briefly transfers to the surface of the backing roll 21 of the high
intensity extended nip press before being further transferred to a first
transfer fabric 30.
The dewatered web 31 is then transferred to a second transfer fabric
40 with the aid of a vacuum box or transfer shoe 41. This transfer can
optionally be a rush transfer, in which the second transfer fabric is
travelling from about 5 to about 35 percent slower than the first
transfer fabric in order to partially debond the web to soften it and
introduce machine direction stretch. The web is thereafter applied to
the surface of a Yankee dryer 50 using pressure roll 51 to final dry the
web, which is thereafter creped with a doctor blade 52 and wound up into
a roll 53.
It will be appreciated that other drying/creping options are also
suitable in combination with high intensity extended nip press
dewatering. For example, the dewatered web 31 can be rush transferred as
described above and thereafter transferred to a throughdrying fabric and
throughdried, with or without subsequent creping. Alternatively, the
dewatered web 31 can be transferred to a Yankee dryer without a rush
transfer and creped.
Figure 2 illustrates the high intensity extended nip press of Figure
1 in more detail. Shown is the incoming web 9 supported by the felt 6
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centering the high intensity extended nip press 20. The nip is formed
between the backing roll 21 and a pressing fabric 56, which follows the
concave contour of the press shoe 55. The tissue web is briefly
transferred to the backing roll and thereafter transferred to a first
transfer fabric 31 using a vacuum roll 57.
iFigure 3 represents several plots of web Bulk versus consistency for
handsheets prepared to simulate webs exiting the high intensity extended
nip press and is discussed below in connection with the Examples.
Examples
Examine 1: Heat-cured fibers).
Southern pine softwood kraft pulp (CR-54) was fiberized in a Pallman
fiber~izer, preconditioned to a moisture content of 5fo and then heated in
a convection oven at 200°C. for 20 minutes crosslink and curl the
fibers.
(A cai~alyst can be used to reduce the temperature and length of the
treatr~nent.) After treatment, the fibers had a water retention value
(WRV) of 0.65g/g and a curl index of 0.15 (measured via Fiber Quality
Analy~~er) versus a WRV of 1.2g/g and a curl index of 0.09 before
treatment. This fiber was combined in a 50/50 blend with eucalyptus
kraft fiber that had been treated at high consistency and elevated
temperature in a disperser in accordance with US Patent No. 5,348,620
issued September 20, 1994 to Hermans et al. entitled "Method of Treating
Papermaking Fibers for Making Tissues".
More specifically, the eucalyptus fibers were dispersed in a
Maule shaft disperser at a temperature of about 150°F. at a
consistency
of about 30 percent with a power input of about 1.5 horsepower per day
per ton. The combined fiber furnish was then formed into handsheets and
subjected to dewatering conditions designed to simulate the operation of
a high intensity extended nip press.
More specifically, 25 grams of the softwood fibers and 25 grams of
the hardwood fibers were combined with 2000 grams of distilled water in a
British disintegrator and processed for 10 minutes. The appropriate
amount of slurry, based on its consistency, to form a 25 GSM handsheet
was poured into a standard square TAPPI handsheet mold. The handsheet
formation followed standard TAPPI methods for tissue. The wet handsheet
was couched off of the forming wire with only blotter paper and the
slightest amount of pressure provided manually. Each wet handsheet and
blotter paper were placed inside a sealable plastic bag, after which the
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blotter paper was carefully removed so as to not ruin the handsheet
formation. Each individual handsheet was therefore stored in a sealable
plastic bag at approximately 30% solids until they were to be tested on
the high intensity nip apparatus.
In order to simulate dewatering in a high intensity extended nip
press, two circles at a diameter of approximately 3 inches each were cut
out of each handsheet. An individual circular handsheet was placed in a
metal frame, which was a circular device consisting of a top and bottom
half, each half having a pattern of strings intended to hold the
handsheet in place during the test. Once in place in the frame, the
handsheet was visually saturated with water via a common household spray
gun. The frame was then placed on top of a pre-weighed circular felt
section in a stationery holder below the movable high intensity nip
platen. The platen then moved down and nipped the handsheet for a
defined impulse before returning to its original position. This impulse
was a replication of a production scale high intensity nip. The
capabilities of the impulse can be controlled through the temperature of
the platen, dwell time in the nip, and pressure. Temperatures in the nip
ranged from 72°F. to 350°F. The dwell time for all tests was 25
milliseconds. A standard pressure profile was used as described in the
Crouse et al. patent referenced above. The average pressure was about
600 psi. The pressed handsheets were then removed and weighed to
determine the exiting consistency for each of the conditions tested.
Example 2 (Chemically cross-linked fibers)
Same as Example 1, except the southern pine softwood kraft fiber was
treated in a disperses in accordance with U.S. Patent No. 5,348,620
described above. The fiber was then blended with ammonium zirconium
carbonate at a level of 1.2 pounds per pound and cured at 180°C. for
10 minutes. The well-blended pulp/crosslinker mixture was then fiberized
in a Pallman fiberizer. This fiber was combined in a 50/50 blend with
dispersed eucalyptus kraft fiber and made into handsheets and tested as
described in Example 1.
Example 3: (Chemically cross-linked fibers)
Same as Example 1, except Weyerhauser High Bulk Additive pulp was
substituted for the southern pine softwood kraft pulp fiber. This
cellulose pulp is impregnated with a urea-formaldehyde crosslinker and
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cured at elevated temperature.
Example 4: (Conventional tissue making fibers). Handsheets were prepared
and tested as described in Example 1, except the fibers used were a 50/50
blend of eucalyptus fibers and northern softwood kraft fibers.
Example 5: (Curled fibers). Handsheets were prepared and tested as
described in Example 1, except the fibers used were a 50/50 blend of
eucalyptus fibers and dispersed northern softwood kraft fibers. The
northern softwood kraft fibers were dispersed under the same conditions
as were the eucalyptus fibers as described in Example 1.
Example 6: (Conventional tissue making fibers with debonder L Handsheets
were prepared and tested as described in Example 1, except the fibers
were a 50/50 blend of eucalyptus fibers and northern softwood kraft
fibers to which 20 pounds per ton of fiber of a debonder had been added
(Berocell 596, manufactured by Eka Nobel Inc.).
The results of these six examples is summarized in Figure 3, which
is a plot of the Bulk as a function of the consistency of the wet tissue
sheet after being pressed under the conditions of the simulated high
intensity extended nip press. As shown, a line relating bulk to exit
consistency exists for each furnish tested in each of the Examples. In
all cases bulk decreases as exit dryness is increased. The increase in
bulk relative to the bottom "control" line represents the improvement due
to the treatment of the fibers. It is especially noteworthy that the
bulk of the modified wet-resilient fibers at 60 percent exit consistency
is for the most part greater than the control at 40 percent consistency.
This increase in bulk (or decrease in sheet density) allows the
production of high quality tissue despite pressing to 60 percent
consistency.
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.
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