Note: Descriptions are shown in the official language in which they were submitted.
A METHOD OF REDUCING SHRINKAGE DURING
DRYING OF A PAPER ~B 1340148
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to a method of reducing shrinkage
in the Z direction or thickness during drying of a paper web
formed from paper stock. More particularly, this invention
relates to a method of reducing shrinkage during drying of
a paper web by use of a surfactant added to the paper stock
prior to drying of the pressed web.
INFORMATION DISCLOSURE STATEMENT
In the production of various grades of paper and board,
it is necessary that such grades meet a specified thickness,
or caliper, standard. Furthermore, such grades must meet
certain density standards. By way of example, fine papers
may be controlled within a narrow range of caliper so that
the product will run well in machines designed for specific
paper thickness such as a copying machine or converting
machinery for making envelopes or the like.
Various boxboards and carton stocks are made to a bulk,
or density, speciflcation to maintain a required stiffness of
the product. For example, the sidewalls of a milk carton must
have sufficient stiffness to hold the contents within the car-
ton without excessive bulging of the carton. Control of the
caliper and density of paper and board grades is of great
importance in the manufacture of paper products because stiff-
ness can vary up to the cube of the thickness.
During recent years the extended nip press has been used
commercially in the production of board for removing more
water in the press section than was possible with the more
conventional press arrangements. The extended nip press, by
removing a larger portlon of water from the formed web, not
only results in savings of energy, particularly in the drying
section, but has also resulted in increased productivity.
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The additional water removal achieved at the extended nip
press section results from greater compaction of the web
during passage through the nips of the extended nip press.
This, in turn, produces a paper or board with a lower caliper
and a higher density. Such higher density is accompanied by
greater fiber-to-fiber bonding and a stronger sheet.
By way of example, if a caliper, or thickness of the web,
of .035 inches ( 089 centimeters) is required, it has been
found that after convent10nal pressing the wet thlckness of
the board may be .055 inches (.14 centimeters~. However,
during drying, the board shrinks significantly in the thick-
ness direction, or Z direction, to the target of .035 lnches
(.089 centimeters). Such shrinking occurs as a consequence
of Campbell forces. (Campbell forces are forces between
fibers due to the surface tension of water.) As the water
evaporates, the surface tension effects pull fibers closer
together and hydrogen bonding occurs. The forces become
stronger as the water present decreases. Thus, it is seen
that with the conventional method of pressing a formed web,
it is not uncommon for the board, or paper web, to shrink in
thickness, or caliper, by as much as 40% during the evaporative
drying process. Additionally, such shrinkage in the Z direction
occurs due to fiber collapse.
However, with the use of the extended nip pressing tech-
nique, the dryness of the resultant paper or board is improved
by 7 to 14 percentage points of dryness after the press section.
This represents reduction in the water to be evaporated of
between 25 and 50 percent. Although such reduction in water
to be evaporated during the drying section is very desirable
from the thermal energy saving viewpoint, a problem exists in
that the web emerging from the extended nip press is compacted
from .OSS inches (.14 centimeters) down to .04 inches ~.10
centimetersl after pressing. From these figures, it evident
-- 13401~8
that the web emerging from an exte~nded nip press has a caliper,
or thickness,that ls reduced during pressing from between 15
to 30 percent. At least a portion of this reduced thickness
may be carried through the drying process and it is not un-
common to achieve a further 40 percent reductlon ln the web
thickness as the web passes through the drylng section. For
example, with a web thickness of .04 inches (.lO centimeters)
enterlng the drylng sect10n, a drled web may result havlng a
thickness of .025 lnches ~.06 centimeters). Thus, the board
produced at the desired hlgher dryness after pressing has a
callper that falls below the target or deslred callper. Al-
though, as a result of the hlgher density, the web has ln-
creased strength propertles such as tenslle strength, such
increased strength ls not always needed or, in fact, deslred.
Consequently, the problem has been to find a method that
permlts the desired increase in water removal at the press
section but that does not result ln a dried web thlckness that
falls below the desired, or target, value.
Although it ls true that certain hardwood flbers, or
thermal-mechanical pulp furnishes (stock or stuff) recover
thickness after presslng to achieve a higher degree of caliper
than with stock produced from softwood, there does not appear
to be any way to achleve greater water removal by pressing ln
the press sectlon without also reducing the wet thlckness of
the paper or board. Apparently, the lncreased dryness is a
direct consequence of the lncreased compactlon.
The present inventlon seeks to overcome the aforementloned
problem by not attempting to reduce the compaction that occurs
during pressing, but rather by reducing the shrinkage that
occurs during evaporatlve drying of the sheet.
It has been discovered that since the caliper loss that
occurs during drying is greater than the loss due to lncreased
ll 1340148
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wet pressing, an attempt to recover the thickness by acting
upon the thickness of the web through the drying section
would be most effective. The present invention provides
means for dealing with the cause of greatest loss in thickness
which is the surface tension, or Campbell forces pulling fibers
sufficiently close together to permit hydrogen bonding during
evaporative drying.
An article by J. A. Yan den Akker published in TAPPI,
volume 35, N 1, page 13-lS (1952) reported on the effect of
drying paper by sublimation where surface tension forces are
minimized. The thickness or density values were not given
but tensile strength was greatly reduced and opacity increased,
indicating a smaller degree of hydrogen bonding.
Broughton and Wang, published in TAPPI, volume 38, N 7,
page 412 - 415 (1955~ found a reduced strength of paper made
and dried in low surface tension, non-polar liquids instead
of water.
J. W. Swanson, published in TAPPI 1974 Engineering Confer-
ence Proceedings (Seattle, WA) page 327 - 332, reported on the
effects of surfactants on fibers. Anionic surfactants were
found to reduce surface tension and correlated well to reduce
bonding in the sheets. Cationic surfactants were found to
give marked decreases in sheet properties that depend on
bonding.
The present invention seeks to overcome the aforementioned
inadequacies of the prior art methods of producing board and
paper by providing a method of producing board or paper using
an extended nip press and yet resul ting in a product having
the desired caliper and density.
Another ob~ect of the present invention is the provision
of a method of reducing shrinkage during drying of a paper web
formed from paper stock. The method including adding a sur-
factant to the paper stock.
13~0148
Another object of the present invention is the pro-
vision of a method of reducing shrinkage during drying of
a paper web by adding a cationic surfactant to the paper
stock for reducing both surface tension and bonding between
adjacent fibers within the formed, pressed web.
While the present invention is directed primarily to the
reduction of shrinkage in the Z direction of a web
emerging from an extended nip press, the present invention
also envisages reducing such shrinkage of a web produced
by a conventional high-load press.
Furthermore, although the present invention will be
described hereinafter with particular reference to adding
the surfactant to the stock, the present invention is not
limited to such application of the surfactant. The present
invent10n, as defined by the appended claims, includes
adding the surfactant as by spraying or the like onto the
formed web prior to drying thereof. Alternatively, the sur-
factant could be introduced at the wet end or at the press
section. Also, the surfactant may be introduced even at
the dryer section. In the production of a multi-ply sheet,
the surfactant could be introduced onto one or more of the
plys only.
Other objects and advantages of the present invention
will be readily apparent to those skilled in the art by a
consideration of the detailed description and the appended
claims.
SUMMARY OF THE INVENTION
The present invention relates to a method of reducing
shrinkage during drylng of a paper web formed from paper
stock. The method includes the steps of adding a surfactant
to the paper stock prlor to forming the stock into the paper
web. The surfactant ls dispersed throughout the stock such
that the surfactant is intimately mlxed with the stock. The
mixture of surfactant and stock is supplied to a headbox of
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a paper forming section and the stock and surfactant mixture
is ejected from the headbox onto a forming wire of the forming
section. A first portion of water is drained from the stock
such that the paper web is formed on the forming wire. The
formed web is led from the forming section to an extended nip
press where the formed web is passed through the extended
nip press such that a second portion of water is removed
from the web. The pressed web is conducted from the extended
nip press to a drying section for removing a third portion of
water from the formed, pressed web such that the surfactant
inhibits bonding between adjacent fibers within the pressed
web so that shrinkage of the pressed web during drying is
reduced.
In a more specific embodiment of the present invention,
the surfactant is a cationic surfactant which is effectlve for
reducing both surface tension and bondlng between adjacent
fibers within the formed, pressed web.
Although the present invention is particularly directed to
the use of a surfactant added to the paper stock, it will be
apparent to those skilled in the art that other chemicals can
be added to the stock for inh~biting bonding between adjacent
fibers during the drying process and that the use of such
chemicals does not depart from the spirit and scope of the
present invention as defined by the appended claims.
Additionally, while the present invention is primarily
directed to reducing shrinkage of a web produced by an extended
nip press, the present invention also includes application
of such surfactant to a web produced by a high-load conventional
press.
Also, the present invention includes adding the surfactant
to the web at either the wet end, the press section, or even
the drying section and further includes adding such surfactant
to one or more plys in the case of the manufacture of a
multi-ply sheet.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a comparative chart showing typical values
of caliper for a board after the press section and after
drying for sheets pressed with a conventional press section
and pressed with an extended nip press.
Figure 2 is a graph showing the effect of several
surface-active agents on sheet thickness.
Figure 3 is a comparative chart showing the resultant
thickness using conventional and extended nip pressing
techniques and showing the results on thickness by the
application of the method of the present invention.
DETAILED DESCRIPTION 1340148
Although surfactants are currently used in the paper
industry in various applications, applicant is unaware of
any prior use of surfactants in the paper stock for overcoming
the problem of reduced caliper when using an extended nip
press. Applicant is aware that surfactants have been used
by manufacturers of tissue to inhibit bonding in order to
produce a softer feeling product. A "fluff" pulp is made
using agents that interfere with bonding and the pulp is
then much easier to redisperse in water or ln air for air-
formed products such as disposable diapers.
However, the present 1nvention is directed to a method
of controlling the final caliper of a paper or board emerging
from a drying section. Laboratory experiments have shown
that by the use of surfactants in the paper stock, the final
caliper can be effectively controlled as shown by the results
indicated in figure 2 of the drawings. These results from
figure 2 indicate that the thickness of the resultant dry
web can be controlled by controlling the concentration of the
surfactant added to the stock or fiber water slurry.
As shown in figure 3, use of the surfactants or other
debonding agents can reduce the shrinkage occurring during
drying in order to meet target values. From figure 3, it is
evident that by adding the surfactant to the stock prior to
pressing, the thickness of the web emerging from the drying
section is greater than when no surfactant is used.
From the aforementioned method, particularly when the
surfactant is a cationic agent, such controlled addition of
the surfactant controls the final sheet, or board, thickness
to meet a target spec~fication while, at the same time, greatly
increasing the amount of water that may be removed during
passage through the extended nip press. Therefore the method
according to the present invention, enables great savings in
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the amount of steam used in the drxing section resulting in
increased productivity which more than offsets the cost of
adding such surfactants in low concentrations to the pulp
slurry.
Referring more particularly to the drawings, figure 1
shows by way of comparison, the change in board thickness
during pressing and during drying. More specifically the
thickness in thousands of an inch, is indicated by the
vertical axis 10. Using a conventional press section, the
thickness of the resultant pressed web is indicated by 12.
Furthermore, the thickness of the web emerging from an
extended nip press is indicated by 14. This comparison
shows the considerable reduction in resultant thickness
using the extended nip press which provides a pressed web
of greatly reduced water content and increased density.
Numeral 16 indicates the target, or required thickness of the
web after passing the web through the drying section. This
reduction in thickness of the web after passing through
the drying section from a conventional press represents
approximately a 40% reduction due to shrinkage. If an
extended nip press is utilized followed by conventional
drying without use of the surfactant according to the
present invention, the resultant dried web will have a
thickness indicated by 18 which, as can be seen, is less
than the required target thickness. HoweYer, when using a
conventional press, the web shrinks from the thickness
shown as 12 to the required thickness as indicated at 20.
Figure 2 is a graph showing the effect of using
different surfactants at various concentration levels within
the stock, or slurry. The vertical axis indicates the
percentage increase in thickness, or caliper, of the web
emerging from the drying section while the horizontal axis
indicates the percentage per unit volume of stock of the
surfactant added prior to the headbox. Graph 22 indicates
13401~8
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the relative increase in caliper.u~ing Arquad 2HT-75 while
graph 24 indicates the corresponding increase, particularly
with a lower concentration of surfactant using a~cErosol
C-61. Both graph 22 and 24 are applicable when using liner-
board pulp. Arquad is a registered trademark and Arquad 2HT-75
was supplied by Armak Corporation, a subsidiary of Armour
& Company.
Similarly, graph 26 and graph 28 respectively, indicate
the corresponding increases in caliper when producing a board
or paper web from bleached softwood kraft (B.S.W.K) using
various concentrations of surfactant.
Figure 3 is a comparison chart in which the vertical
axis represents the thickness of the paper or board web
after pressing and drying. 30 indicates the thickness after
pressing and 32 indicates the thickness after drying, both
utilizing conventional pressing and drying.
34 indicates the thickness after pressing and 36 indi-
cates the thickness after drying of the web, both using an
increased pressing technique obtained by utilizing an extended
nip press in conjunction with a conventional drylng system.
36 shows the thickness of the web being considerably less
than the target thickness indicated by the horizonta~ dashed
line 38.
indicates the thickness of the web emerging from an
extended nip press and 42 shown in dotted lines indicates the
acquired thickness resulting from the addition of sarfactant
to the stock according to the present invention.
When such a web is passed through a conventional drying
system, the thickness of the web decreases during passage
through the drying system to the target thickness 38 as indi-
cated by the after drying thickness 44.
According to the present invention, a method is provided
for reducing shrinkage during drying of a paper web formed
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1340148~
from paper stock. The method inc,l~udes the steps of adding
a surfactant, preferably a cationic surfactant to.the paper
stock prior to forming the stock into a paper or board web.
Preferably, the surfactant is added to the stock in fairly
small quantities as indicated in figure 2. The stock may
be linerboard or bleached softwood kraft as indicated in
figure 2. However, the present invention is equally appli-
cable to using a surfactant added to any type of stock and
the surfactant may be anionic, or in fact, any other type of
surface-active agent. The surfactant is dispersed throughout
the stock so the surfactant ls intimately mixed with the stock
prior to feeding the stock to the headbox of a paper forming
section. The surfactant and stock is supplied to the headbox
where the stock is ejected onto a forming wire of the forming
section. Water is drained from the forming wire, or forming
roll, in order to provide a formed web of paper or board.
The web is then passed through an extended nip press which
removes a second portion of water from the formed web, the
first portion of water having been drained from the stock
in the forming section. The second portion of water pressed
from the web is greater than the amount of water removed
in a conventional press section. Consequently, the thickness,
or caliper, of the resultant web is less than the caliper of
a web emerging from a conventional press. Furthermore, the
density of the emerging web will correspondingly be greater
than that resulting from a conventional press. The pressed
web is then led to a conventional drying system. However, such
conventional drying system will either require less drying
drums, or if the same number of drying drums are utilized,
less thermal energy will be required in the drying section
due to the larger amount of water removed in the extended
nip press. Due to the addition of the surfactant within
the stock, bonding of adjacent fibers within the web is
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inhibited because the surfactant ~ends to inhibit hydrogen
bonding which, without the use of a surfactant, would result
in a web caliper less than the target caliper.
Additionally, such shrinkage in the Z direction can be
reduced according to the present invention by introducing the
surfactant at either the wet end, the press section or even
the dryer section. Furthermore, such surfactant can be added
to one or more plys when a multi-ply board is being produced.
The present invention not only provides a resultant
pressed, dried web having the required thickness and density
but also provides a great savings in the cost of the provision
of drying steam and this more than offsets the cost of adding
the surfactant in low concentration to the pulp slurry.
