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METHOD AND APPARATUS FOR MAKING SOFT TISSUE
Back~round of the Invention
- There are many characteristics of tissue products such as bath and facial tissue
that must be considered in producing a final product having desirable attributes that
- make it suitable and preferred for the product's intended purpose. Improved softness of
the product has long been one major objective, and this has been a particularly
10 significant factor for the success of premium products. In general, the major components
of softness include stiffness and bulk (density), with lower stiffness and higher bulk (lower
density) generally improving perceived softness.
While enhanced softness is a desire for all types of tissue products, it has been
especially challenging to achieve softness improvements in uncreped throughdried15 sheets. Throughdrying provides a relatively noncompressive method of removing water
from a web by passing hot air through the web until it is dry. More specifically, a wet-laid
web is transferred from the forming fabric to a coarse, highly permeable throughdrying
fabric and retained on the throughdrying fabric until dry. The resulting dried web is softer
and bulkier than a conventionally-dried uncreped sheet because fewer bonds are formed
20 and because the web is less compressed. Thus, there are benefits to eliminating the
Yankee dryer and making an uncreped throughdried product. Uncreped throughdried
sheets are typically quite harsh and rough to the touch, however, compared to their
creped counterparts. This is partially due to the inherently high stiffness and strength of
an uncreped sheet, but is also due in part to the coarseness of the throughdrying fabric
25 onto which the wet web is conformed and dried.
Therefore, what are lacking and needed in the art are a method and apparatus formanufacturing tissue products having improved softness, and in particular uncreped
throughdried tissue products having improved softness.
Summary of the Invention
It has now been discovered that an improved uncreped throughdried web can be
made by dewatering the web to greater than about 30 percent consistency prior totransferring the wet web from a forming fabric to one or more slower speed intermediate
35 transfer fabrics before further transferring the web to a throughdrying fabric for final
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drying of the web. In particular, increasing the consistency of the uncreped throughdried
web before the point of differential speed transfer has surprisingly been found to result
in: (1) both higher machine direction and cross direction tensile properties, contributing to
improved runnability of the web; and (2) reduced modulus, that is increased softness,
5 when the tensile strength is adjusted to the normal value. This discovery allows for the
manufacture of tissue products with lower modulus at given tensile strengths as
compared even to tissue products produced by undergoing differential speed transfer at
lower consistencies.
One particularly desirable means by which the web can be dewatered to greater
10 than about 30 percent consistency comprises an air press located just upstream of the
differential speed transfer. While pressurized fluid jets in combination with a vacuum
device have previously been discussed in the patent literature, such devices have not
been widely used in tissue manufacturing. Principally, this appears to be due to the fact
that it had not been previously recognized that dewatering the web to greater than about
15 30 percent consistency in advance of the differential speed transferwould result in the
improved product properties identified herein. Moreover, the disincentive to using such
equipment is also b~'.Evcd to be attributable to the difficulties of actual implementation,
including disintegration of the tissue web, pressurized fluid leaks, seal andlor fabric wear,
and the like. The air press used in the present method overcomes these difficulties and
20 provides one practical apparatus for achieving the desired consistency ahead of the
differential speed transfer.
Hence, in one aspect the invention resides in a method of making a soft tissue
sheet. The method includes the steps of: depositing an aqueous suspension of
papermaking fibers onto an endless forming fabric to form a wet web; dewatering the wet
25 web to a consistency of from about 20 to about 30 percent; supplementally dewatering
the wet web using noncompressive dewatering means to a consistency of greater than
about 30 percent; transferring the supplementally dewatered web to a transfer fabric
traveling at a speed of from about 10 to about 80 percent slower than the forming fabric;
transferring the web to a throughdrying fabric; and throughdrying the web to a final
30 dryness.
The air press desirably provides a pressure differential across the wet web of from
about 35 to about 60 inches of mercury. This may be achieved in part by an air plenum
of the air press maintaining a fluid pressure on one side of the wet web of from about 5
to about 60 pounds per square inch, and particularly from about 5 to about 30 pounds
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per square inch. The pressurized fluid may be air at ambient temperature, heated air,
steam, or the like. Overall, the air press may be operable to increase the consistency of
the wet web by at least about 3 and preferably at least about 5 percent. Optional steam
showers may be employed before the air press to increase post air press consistency.
Another aspect of the invention resides in an air press for dewatering a wet web.
In one embodiment, the air press includes an air plenum comprising a plenum cover
having a bottom surface and a vacuum box comprising a vacuum box cover having a top
surface positioned in close proximity to the bottom surface of the plenum cover. The air
press also includes means for supplying pressurized fluid to the air plenum and means
for applying vacuum to the vacuum box. Side seal members of the air press are adapted
to reside in contact with the air plenum and the vacuum box for minimizing the escape of
the pressurized fluid. The side seal members are attached to one of the air plenum and
the vacuum box, and are positioned in close proximity to side seal contact surfaces
defined by the other of the air plenum and the vacuum box. The side seal members are
adapted to flex into sealing contact with the side seal contact surface upon exposure to
the pressurized fluid to enhance the seal effectiveness.
Optionally, the air press may include a position control mechanism that functions
to maintain the air plenum in close proximity to the vacuum box. In particular, the
position control mechanism desirably includes a rotatably mounted lever attached to the
air plenum, and a counterbalance cylinder attached to the lever. The position control
mechanism is adapted to rotate the lever to counteract pressure changes within the air
plenum. In this way, the air plenum resides in close proximity to or in contact with the
fabrics passing between the air plenum and the vacuum box, without clal.l,-..lg the
fabrics therebetween.
In another embodiment, the air press includes an air plenum co~"prisi"g a plenumcover having a bottom surface, and means for supplying pressurized fluid to the air
plenum. The air press also includes a vacuum box comprising a vacuum box cover
having a top surface positioned in close proximity to the bottom surface of the plenum
cover, and means for applying vacuum to the vacuum box. An arm that is pivotallymounted on the air plenum comprises first and second portions, with the first portion of
the arm being disposed at least partially inside the air plenum. A sealing bar is formed
from or mounted on the first portion of the arm. The air press also includes means for
pivoting the arm in response to fluid pressure within the air plenum.
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In this embodiment, the sealing bar portion of the pivotable arm acts as an end
seal to prevent the escape of pressurized fluid from between the air plenum and the
vacuum box. The sealing bar may conform to fabric irregularities or misalignment of the
supporting structure. The end seals, which are also referred to as cross direction or CD
5 seals, improve containment of the pressurized fluid and thus result in more efficient
operation of the air press. The loading of the end seals is contrc'led to maintain the
sealing bar in contact with the underlying moving fabric, without causing undue wear of
the fabric.
The improved results can be obtained without sacrifice of efficiency. The present
10 method and apparatus are car~hl~ of functioning at commercially viable web speeds.
For example, the forming fabric may be controlled to travel at speeds of at least about
2000 feet per minute (fpm), and more desirably at speeds of at least about 4000 fpm.
The intermediate transfer fabric or fabrics are traveling at a slower speed than the
forming fabric during the transfer in order to impart stretch into the sheet. As the speed
15 differential between the forming fabric and the slower transfer fabric is increased
(sometimes referred to as "negative draw" or"rush transfer"), the stretch imparted to the
web during transfer is also increased. The transfer fabric can be relatively smooth and
dense compared to the coarse weave of a typical throughdrying fabric. Preferably the
transfer fabric is as fine as can be run from a practical standpoint. Gripping of the web is
20 accomplished by the presence of knuckles on the surface of the transfer fabric. In
addition, it can be advantageous if one or more of the wet web transfers, with or without
the presence of a transfer fabric, are achieved using a "fixed gap" or"kiss" transfer in
which the fabrics simultaneously converge and diverge, which will be hereinafterdescribed in detail. Such transfers not only avoid any significant compaction of the web
25 while it is in a wet bond-forming state, but when used in combination with a differential
speed transfer and/or a smooth transfer fabric, are observed to smoothen the surface of
the web and final dry sheet.
The speed difference between the forming fabric and the transfer fabric can be
from about 10 to about 80 percent or greater, preferably from about 10 to about 35
30 percent, and more preferably from about 15 to about 25 percent, with the transfer fabric
being the slower fabric. The optimum speed differential will depend on a variety of
factors, including the particular type of product being made. As previously mentioned,
the increase in stretch imparted to the web is proportional to the speed differential. For
an uncreped throughdried three-ply wiper having a basis weight of about 20 grams per
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square meter per ply, for example, a speed differential in the production of each ply of
from about 20 to about 25 percent between the forming fabric and a sole transfer fabric
produces a stretch in the final product of from about 15 to about 20 percent.
The stretch can be imparted to the web using a single differential speed transfer
or two or more differential speed transfers of the wet web prior to drying. Hence there
can be one or more transfer fabrics. The amount of stretch imparted to the web can
hence be divided among one, two, three or more differential speed transfers.
The transfer is desirably carried out such that the resulting "sandwich" (consisting
of the forming fabric/webltransfer fabric) exists for as short a duration as possible. In
10 particular, it exists only at the leading edge of the vacuum shoe or transfer shoe slot
being used to effect the transfer. In effect, the forming fabric and the transfer fabric
converge and diverge at the leading edge of the vacuum slot. The intent is to m;ni.~ e
the distance over which the web is in simultaneous contact with both fabrics. It has been
found that simultaneous convergence/divergence is the key to eliminating macrofolds
15 and thereby enhances the smoothness of the resulting tissue or other product.In practice, the simultaneous convergence and divergence of the two fabrics willonly occur at the leading edge of the vacuum slot if a sufficient angle of convergence is
maintained between the two fabrics as they approach the leading edge of the vacuum
slot and if a sufficient angle of divergence is maintained between the two fabrics on the
20 downstream side of the vacuum slot. The minimum angles of convergence and
divergence are about 0.5 degree or greater, more specifically about 1 degree or greater,
more specifically about 2 degrees or greater, and still more specifically about 5 degrees
or greater. The angles of convergence and divergence can be the same or different.
Greater angles provide a greater margin of error during operation. A suitable range is
25 from about 1 degree to about 10 degrees. Simultaneous convergence and divergence is
achieved when the vacuum shoe is designed with the trailing edge of the vacuum slot
being sufficiently recessed relative to the leading edge to permit the fabrics to
immediately diverge as they pass over the leading edge of the vacuum slot. This will be
more clearly described in connection with the Figures.
In setting up the machine with the fabrics initially having a fixed gap to further
minimize compression of the web during the transfer, the distance between the fabrics
should be equal to or greater than the thickness or caliper of the web so that the web is
not significantly compressed when transferred at the leading edge of the vacuum slot.
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Increased smoothness is achieved by use of the air press upstream of the
differential speed transfer. This is most preferably used in combination with a fixed gap
carrier fabric section following drying. Calendering of the web is not necessary to obtain
desirable levels of smoothness, but further processing of the sheet, such as by
calendering, embossing or creping, may be beneficial to further enhance the sheet
properties.
As used herein, "transfer fabric" is a fabric which is positioned between the
forming section and the drying section of the web manufacturing process. Suitable
transfer fabrics are those papermaking fabrics which provide a high fiber support index
10 and provide a good vacuum seal to maximize fabric/sheet contact during transfer from
the forming fabric. The fabric can have a relatively smooth sùrface contour to impart
smoothness to the web, yet must have enough texture to grab the web and maintaincontact during a rush transfer. Finer fabrics can produce a higher degree of stretch in
the web, which is desirable for some product applications.
Transfer fabrics include single-layer, multi-layer, or composite permeable
structures. Preferred fabrics have at least some of the following characteristics: (1) On
the side of the transfer fabric that is in contact with the wet web (the top side), the
number of machine direction (Ml~) strands per inch (mesh) is from 10 to 200 and the
number of cross-machine direction (CD) strands per inch (count) is also from 10 to 200.
20 The strand diameter is typically smaller than 0.050 inch; (2) On the top side, the distance
between the highest point of the MD knuckle and the highest point of the CD knuckle is
from about 0.001 to about 0.02 or 0.03 inch. In between these two levels, there can be
knuckles formed either by MD or CD strands that give the topography a 3-dimensional
characteristic; (3) On the top side, the length of the MD knuckles is equal to or longer
25 than the length of the CD knuckles; (4) If the fabric is made in a multi-layer construction,
it is preferred that the bottom layer is of a finer mesh than the top layer so as to control
the depth of web penetration and to maximize fiber retention; and (5) The fabric may be
made to show certain geometric patterns that are pleasing to the eye, which typically
repeat between every 2 to 50 warp yarns.
Specific suitable transfer fabrics include, by way of example, those made by
Asten Forming Fabrics, Inc., Appleton, Wisconsin and designated as numbers 934, 937,
939 and 959. Particular transfer fabrics that may be used also include the fabrics
disclosed in U.S. Patent 5,429,686 issued July 4, 1995, to Chiu et al., which is
- 6 -
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incorporated herein by reference. The void volume of the transfer fabric can be equal to
or less than the fabric from which the web is transferred.
The forming process and tackle can be conventional as is well known in the
papermaking industry. Such formation processes include Fourdrinier, roof formers (such
5 as suction breast roll), gap formers (such as twin wire formers, crescent formers), or the
like. Forming wires or fabrics can also be conventional, with the finer weaves with
greater fiber support being preferred to produce a more smooth sheet or web.
~eadboxes used to deposit the fibers onto the forming fabric can be layered or
nonlayered .
The method disclosed herein can be applied to any tissue web, which includes
webs for making facial tissue, bath tissue, paper towels, dinner napkins, or the like. Such
tissue webs can be single-ply products or multi-ply products, such as two-ply, three-ply,
four-ply or greater. One-ply products are advantageous l~ec~use of their lower cost of
manufacture, while multi-ply products are preferred by many consumers. For multi-ply
15 products it is not necessary that all plies of the product be the same, provided at least
one ply is in accordance with this invention. The webs can be layered or unlayered
(blended), and the fibers making up the web can be any fibers suitable for papermaking.
Suitable basis weights for these tissue webs can be from about 5 to about 70
grams per square meter (gsm), preferably from about 10 to about 40 gsm, and more20 preferably from about 20 to about 30 gsm. For a single-ply bath tissue, a basis weight of
about 25 gsm is preferred. For a two-ply tissue, a basis weight of about 20 gsm per ply is
preferred. For a three-ply tissue, a basis weight of about 15 gsm per ply is preferred. In
general, higher basis weight webs will require lower air flow to maintain the same
operating pressure in the air plenum. The width of the slots of the air press are desirably
25 adjusted to match the system to the available air capacity, with wider slots used for
heavier basis weight webs.
The drying process can be any noncompressive drying method which tends to
preserve the bulk or thickness of the wet web including, without limitation, throughdrying,
infra-red irradiation, microwave drying, or the like. Because of its commercial availability
30 and practicality, throughdrying is a well-known and preferred means for
noncompressively drying the web. Suitable throughdrying fabrics include, without- limitation, Asten 920A and 937A, and Velostar P800 and 103A. The throughdrying
fabrics may also include those disclosed in U.S. Patent 5,429,686 issued July 4, 1995, to
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Chiu et al. The web is preferably dried to final dryness without creping, since creping
tends to iower the web strength and bulk.
While the mechanics are not completely understood, it is clear that the transferfabric and throughdrying fabric can make separate and independent contributions to final
5 sheet properties. For example, sheet surface smoothness as determined by a sensory
panel can be manipulated over a broad range by changing transfer fabrics with the same
throughdrying fabric. Webs produced by the present method and apparatus tend to be
very two-sided unless calendered. Uncalendered webs may, however, be plied together
with smooth/rough sides out as required by specific product forms.
Numerous features and advantages of the present invention will appear from the
following description. In the description, reference is made to the accompanyingdrawings which illustrate preferred embodiments of the invention. Such embodiments do
not represent the full scope of the invention. Reference should therefore be made to the
claims herein for interpreting the full scope of the invention.
Brief Description of the Drawin~s
Fig. 1 representatively shows a schematic process flow diagram illustrating a
method and apparatus according to the present invention for making uncreped
throughdried sheets.
Fig. 2 representatively shows an enlarged top plan view of an air press from theprocess flow diagram of Fig. 1.
Fig. 3 representatively shows a side view of the air press shown in Fig. 2, withportions broken away and shown in section for purposes of illusl,dlion.
Fig. 4 representatively shows an enlarged section view taken generally from the
plane of the line 4 - 4 in Fig. 3.
Fig. 5 representatively shows an enlarged section view similar to Fig. 4 but taken
generally from the plane of the line 5 - 5 in Fig. 3.
Fig. 6 representatively shows a side view of an alternative sealing system for the
air press shown in Figs. 2 and 3, with portions broken away and shown in section for
purposes of illustration.
Fig. 7 representatively shows an enlarged side view of a vacuum transfer shoe
shown in Fig. 2.
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Fig. 8 representatively shows an enlarged side view similar to Fig. 7 but
illustrating the simultaneous convergence and divergence of fabrics at a leading edge of
a vacuum slot.
Fig. 9 is a generalized plot of load/elongation curve for tissue, illustrating the
determination of the MD Slope.
Detailed DescriPtion of the Invention
The invention will now be described in greater detail with reference to the Figures.
10 Similar elements in different Figures have been given the same reference numeral for
purposes of consistency and simplicity. In all of the embodiments, illustrated,
conventional papermaking apparatus and operations can be used with respect to the
headbox, forming fabrics, web transfers, drying and creping, all of which will be readily
understood by those sicilled in the papermaking art. Nevertheless, various conventional
15 components are illustrated for purposes of providing the context in which the various
embodiments of the invention can be used.
One embodiment of a method and apparatus for manufacturing a tissue is
representatively shown in Fig. 1. For simplicity, the various tensioning rolls schematically
used to define the several fabric runs are shown but not numbered. A paper",aking
20 headbox 20 injects or deposit~ an aqueous suspension of papermaking fibers 21 onto an
endless forming fabric 22 traveling about a forming roll 23. The forming fabric 22 allows
partial dewatering of the newly-formed wet web 24 to a consistency of about 10 percent.
After formation, the forming fabric 22 carries the wet web 24 to one or more
vacuum or suction boxes 28, which may be employed to provide additional dewatering of
25 the wet web 24 while it is supported on the forming fabric 22. In particular, a plurality of
vacuum boxes 28 may be used to dewater the web 24 to a consistency of from about 20
to about 30 percent. The Fourdrinier former illustrated is particularly useful for making
the heavier basis weight sheets useful as wipers and towels, although other forming
devices such as twin wire formers, crescent formers or the like can be used instead.
30 Hydroneedling, for example as disclosed in U.S. Patent No. 5,137,600 issued August 11,
1992 to Barnes et al., can optionally be employed to increase the bulk of the web.
Enhanced dewatering of the wet web 24 is thereafter provided by suitAhle
supplemental noncompressive dewatering means, for example selected from the group
consisting of an air press, infra-red drying, microwave drying, sonic drying, throughdrying,
I
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and ~isplacement dewatering. In the illustrated embodiment, the supplemental
noncompressive dewatering means comprises an air press 30, described in greater detail
hereinafter. The air press 30 desirably raises the consistency of the wet web 24 to
greater than about 30 percent, particularly greater than about 31 percent, more
5 particularly greater than about 32 percent, and even more particularly greater than about
33 percent. In particular embodiments, the wet web 24 has a consistency exiting the air
press 30 and prior to subsequent transfer of from about 31 to about 36 percent. In
particular embodiments, the air press 30 increases the consistency of the wet web 24 by
at least about 3 and preferably at least about 5 percent.
Desirably, a support fabric 32 is brought in contact with the wet web 24 in
advance of the air press 30. The wet web 24 is sandwiched between the support fabric
32 and the forming fabric 22, and thus supported during the pressure drop created by the
air press 30. Fabrics suitable for use as a support fabric 32 include almost any fabric
including forming fabrics such as Albany International 94M.
The wet web 24 is then transferred from the forming fabric 22 to a transfer fabric
36 traveling at a slower speed than the forming fabric in order to impart increased stretch
into the web. Transfer is preferably carried out with the ~csisPnce of a vacuum transfer
shoe 37 as described hereinafler with reference to Figs. 7 and 8. The surface of the
transfer fabric 36 is relatively smooth in order to provide smoothness to the wet web 24.
The openness of the transfer fabric 36, as measured by its void volume, is relatively low
and can be about the same as that of the forming fabric 22 or even lower.
The transfer fabric 36 passes over rolls 38 and 39 before the wet web 24 is
transferred to a throughdrying fabric 40 traveling at about the same speed, or a different
speed if desired. Transfer is effected by vacuum transfer shoe 42, which can be of the
same design as that used for the previous transfer. The web 24 is dried to final dryness
as the web is carried over a throughdryer 44.
Prior to being wound onto a reel 48 for subsequent conversion into the final
product form, the dried web 50 can be carried through one or more optional fixed gap
fabric nips formed between carrier fabrics 52 and 53. The bulk or caliper of the web 50
can be controlled by fabric embossing nips formed between rolls 54 and 55, 56 and 57,
and 58 and 59. Suitable carrier fabrics for this purpose are Albany International 84M or
94M and Asten 959 or 937, all of which are relatively smooth fabrics having a fine
pattern. Nip gaps between the various roll pairs can be from about 0.001 inch to about
0.02 inch (0.025 - 0.51 mm). As shown, the carrier fabric section of the machine is
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designed and operated with a series of fixed gap nips which serve to control the caliper
of the web and can replace or compliment off-line calendering. Alternatively, a reel
calender can be employed to achieve final caliper or complement off-line calendering.
The air press 30 is shown in greater detail by the top view of Fig. 2 and the side
5 view of Fig. 3, the latter having portions broken away for purposes of illustration. The air
press 30 generaily comprises an upper air plenum 60 in combination with a lower
vacuum or suction box 62. The terms "upper" and "lower" are used herein to facilitate
reference to and understanding of the drawings and are not meant to restrict the manner
in which the components are oriented. The sandwich of the wet tissue web 24 between
the forming fabric 22 and the support fabric 32 passes between the air plenum 60 and
the vacuum box 62.
The illustrated air plenum 60 is adapted to receive a supply of pressurized fluid
through air manifolds 64 operatively connected to a pressurized fluid source such as a
compressor or blower (not shown). The air plenum 60 is fitted with a plenum cover 66
which has a bottom surface 67 that resides during use in close proximity to the vacuum
box 62 and in close proximity to or contact with the support fabric 32 (Fig. 3). The
plenum cover 66 is formed with slots 68 (Fig. 5) extending perpendicular to the machine
direction across substantially the entire width of the wet web 24 to permit p~ssage of
pressurized fluid from the air plenum 60 through the fabrics and the wet web.
The vacuum box 62 is operatively connected to a vacuum source and fixedly
mounted to a support structure (not shown). The vacuum box 62 comprises a cover 70
having a top surface 72 over which the forming fabric 22 travels. The vacuum box cover
70 is formed with a pair of slots 74 (Figs. 3 and 5) that correspond to the location of the
slots 68 in the plenum cover 66. The pressurized fluid dewaters the wet web 24 as the
pressurized fluid is drawn from the air plenum 60 into and through the vacuum box 62.
The fluid pressure within the air plenum 60 is desirably maintained at about 5
pounds per square inch (psi) (0.35 bar) or greater, and particularly within the range of
from about 5 to about 30 psi (0.35 - 2.07 bar), such as about 15 psi (1.03 bar). The fluid
pressure within the air plenum 60 is desirably monitored and controlled to a
predetermined level.
The bottom surface 67 of the plenum cover 66 is desirably gently curved to
facilitate web control. The surface 67 is curved toward the vacuum box 62, that is curved
about an axis disposed on the vacuum box side of the web 24. The curvature of the
bottom surface 67 allows a change in angle of the combination of the supporting fabric
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32, the wet web 24, and the forming fabric 22 resulting in a net downward force that
seals the vacuum box 62 against the entry of outside air and supports the wet web 24
during the dewatering process. The angle of curvature allows the loading and unloading
of the air press 30 as required from time to time, based on process conditions. The
change in angle necessary is dependent on the pressure differential between the
pressure and vacuum sides and is desirably above 5 degrees, and particularly within the
range of 5 to 30 degrees, typically about 7.5 degrees.
The top and bottom surfaces 72 and 67 desirably have differing radii of curvature.
In particular, the radius of curvature of the bottom surface 67 is desirably larger than the
10 radius of curvature of the top surface 72 so as to form contact lines between the air
plenum 60 and the vacuum box 62 at the leading and trailing edges 76 of the air press
30. With proper attention to the position of the supporting fabric 32 and the forming
fabric 22 sandwich and loading and unloading mechanisms, the radii of curvature of
these surfaces may be reversed.
The leading and trailing edges of the air press 30 may also be provided with endseals 78 (Fig. 3) that are maintained in very close proximity to or contact with the support
fabric 32 at all times. The end seals 78 minimize the escape of pressurized fluid
between the air plenum 60 and the vacuum box 62 in the machine direction. Suitable
end seals 78 may be formed of resilient plastic compounds or the like.
With additional reference to Figs. 4 and 5, the air press 30is desirably provided
with side seal members 80 to prevent the loss of pressurized fluid along the side edges
82 of the air press. The side seal mernbers 80 comprise a semi-rigid material that is
adapted to deform or flex slightly when exposed to the pressurized fluid of the air plenum
60. The illustrated side seal members 80 define a slot 84 for attachment to the vacuum
25 box cover 70 using a clamping bar 85 and fastener 86 or other suitable means. In cross
section, each side seal member 80 is L-shaped with a leg 88 projecting upward from the
vacuum box cover 70 into a side seal slot 89 formed in the plenum cover 66.
Pressurized fluid from the air plenum 60 causes the legs 88 to bend outward into sealing
contact with the outward surface of the side seal slot 89 of the plenum cover 66, as
30 shown in Figs. 4 and 5. Alternatively, the position of the side seal members 80 could be
reversed, such that they are fixedly attached to the plenum cover 66 and make sealing
contact with contact surfaces defined by the vacuum box cover 70 (not shown). In any
such alternative designs, it is desirable for the side seal member to be urged into
engagement with the sealing contact surface by the pressurized fluid.
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A position control mechanism 90 maintains the air plenum 60 in close proximity to
the vacuum box 62 and in contact with the support fabric 32. The position control
mechanism 90 comprises a pair of levers 92 connected by crosspieces 93 and fixedly
attached to the air plenum 60 by suitable fasteners 94 (Fig. 3). The ends of the levers 92
5 opposite the air plenum 60 are rotatably mounted on a shaft 96. The position control
mechanism 90 also comprises a counterbalance cylinder 98 operably connecting a fixed
structural support 99 and one of the crosspieces 93. The counterbalance cylinder 98 is
adapted to extend or retract and thereby cause the levers 92 to rotate about the shaft 96,
which causes the air plenum 60 to move closer to or further from the vacuum box 62.
In use, a control system causes the counterbalance cylinder 98 to extend
sufficiently for the end seals 78 to contact the support fabric 32 and the side seal
members 80 to be positioned within the side seal slots 89. The air press 30 is activated
such that pressurized fluid fills the air plenum 60 and the semi-rigid side seal members
80 are forced into sealing engagement with the plenum cover 66. The pressurized fluid
also creates an upward force tending to move the air plenum 60 away from the support
fabric 32. The control system directs operation of the counterbalance cylinder 98 to
offset this upward force based on continuous measurements of the fluid pressure within
the air plenum 60 by the pressure monitoring system. The end seals 78 are thereby
maintained in very close proximity to or contact with the support fabric 32 at all times.
The control system counters random pressure drops or peaks within the air plenum 60 by
proportionately decreasing or increasing the force applied by the counterbalance cylinder
98. Consequently, the end seals 78 do not clamp the fabrics 32 and 22, which would
otherwise lead to excessive wear of the fabrics.
An alternative sealing system for the air press 30 is representatively shown in Fig.
6. The air plenum 100 is provided with a pivotable arm 102 defining or carrying a sealing
bar 104 that is adapted to ride on the support fabric 32 across the width of the wet web
24 to minimize escape of pressurized fluid in the machine direction. While only one arm
102 is illustrated in Fig. 6, it should be understood that a second arm at the opposite end
of the air plenum 100 may be employed and constructed in a similar manner. The sides
of the air plenum 100 may incorporate side seal members 80 as described in relation to
Figs. 2 - ~ or be fixedly mounted on the vacuum box 62 to minimize or eliminate side
- leakage of pressurized fluid.
The pivotable arm 102 desirably comprises a rigid material such as structural
steel, graphite composites, or the like. The arm 102 has a first portion 106 disposed at
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least partially inside the air plenum 100 and a second portion 108 preferably disposed
outside the air plenum. The arm 102 is pivotally mounted on the air plenum 100 by a
hinge 110. A hinge seal 112 impervious to the pressurized fluid is attached to both the
interior surface of a wall 114 of the air plenum 100 and the first portion 106 to prevent
5 escape of the pressurized fluid. The sealing bar 104 is desirably a separate element
mounted on the first portion 106 and motivated toward the support fabric 32 (not shown
in Fig. 6) by contact of the pressurized fluid on the first portion. Suitable sealing bars 104
may be formed of a low-resistance, low friction coefficient, durable material such as
ceramic, heat resistant polymers, or the like.
A counterbalance bladder 120 having an inflatable chamber 122 is mounted on
the second portion 108 of the arm 102 with brackets 124 or other suitable means. The
chamber 122 is operably connected to a source of pressurized fluid such as air to inflate
the chamber. The arm 102 and the bladder 120 are positioned so that the bladderwhen
inflated (not shown) presses against the exterior surface of the wall 114 of the air plenum
100 causing the arm to pivot about the hinge 110. Alternatively, a mechanism using
pressurized cylinders (not shown) could be used in place of the counterbalance bladder
as a means for pivoting the arm 102.
A control system is operable to inflate or deflate the bladder 120 proportionally in
response to the pressure of the fluid within the air plenum 100. For example, aspressure within the air plenum 100 increases, the control system is adapted to increase
pressure within or inflation of the counterbalance bladder 120 so that the sealing bar 104
does not clamp down excessively against the support fabric 32.
The design of the vacuum transfer shoe 37 used in the transfer fabric section ofthe process (Fig. 1) is more clearly illustrated in Figs. 7 and 8. The vacuum transfer shoe
37 defines a vacuum slot 130 (Fig. 7) connected to a source of vacuum and having a
length of "L" which is suitably from about 0.5 to about 1 inch (12.7 - 25.4 mm). For
producing uncreped throughdried bath tissue, a sl lit~hlC vacuum slot length is about 1
inch (25.4 mm). The vacuum slot 130 has a leading edge 132 and a trailing edge 133,
forming corresponding incoming and outgoing land areas 134 and 135 of the vacuumtransfer shoe 37. The trailing edge 133 of the vacuum slot 130 is recessed relative to the
leading edge 132, which is caused by the different orientation of the outgoing land area
135 relative to that of the incoming land area 134. The angle "A" between the planes of
the incoming land area 134 and the outgoing land area 135 can be about 0.5 degrees or
greater, more specifically about 1 degree or greater, and still more specifically about 5
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degrees or greater in order to provide sufficient separation of the forming fabric 22 and
the transfer fabric 36 as they are converging and diverging.
Fig~ 8 further illustrates the wet tissue web 24 traveling in the direction shown by
the arrows toward the vacuum transfer shoe 37. Also approaching the vacuum transfer
shoe 37 is the transfer fabric 36 traveling at a slower speed. The angle of convergence
between the two incoming fabrics is designated as "C". The angle of divergence
between the two fabrics is designated as "D". As shown, the two fabrics simultaneousiy
converge and diverge at point "P", which corresponds to the leading edge 132 of the
vacuum slot 130. It is not necessary or desirable that the web be in contact with both
10 fabrics over the entire length of the vacuum slot 130 to effect the transfer from the
forming fabric 22 to the transfer fabric 36. As is apparent from Figure 8, neither the
forming fabric 22 nor the transfer fabric 36 need to be deflected more than a small
amount to carry out the transfer, which can reduce fabric wear. Numerically, the change
in direction of either fabric can be less than 5 degrees.
As previously mentioned, the transfer fabric 36 is traveling at a slower speed than
the forming fabric 22. If more than one transfer fabric is used, the speed differential
between fabrics can be the same or different. Multiple transfer fabrics can provide
operational flexibility as well as a wide variety of fabric/speed combinations to influence
the properties of the final product.
The level of vacuum used for the differential speed transfers can be from about 3
to about 15 inches of mercury, preferably about 5 inches of mercury. The vacuum shoe
(negative pressure) can be supplemented or replaced by the use of positive pressure
from the opposite side of the web 24 to blow the web onto the next fabric in addition to or
as a replacement for sucking it onto the next fabric with vacuum. Also, a vacuum roll or
25 rolls can be used to replace the vacuum shoe(s).
Examples
The following EXAMPLES are provided to give a more detailed understanding of
30 the invention. The particular amounts, proportions, compositions and parameters are
meant to be exemplary, and are not intended to specifically limit the scope of the
invention.
As referenced in relation to the Examples, MD Tensile strength, MD Stretch, and
CD Tensile strength are obtained according to TAPPI Test Method 494 OM-88 "Tensile
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Breaking Properties of Paper and Paperboard" using the f~ ;ng parameters:
Crosshead speed is 10 0 in/min (254 mm/min); full scale load is 10 Ib (4,540 9); jaw span
(the distance between the jaws, sometimes referred to as the gauge length) is 2.0 inches
(50.8 mm); and specimen width is 3 inches (76.2 mm). The tensile testing machine is a
Sintech, Model CITS-2000 from Systems Integration Technology Inc., Stoughton,
Massachusetts, a division of MTS Systems Corporation, Research Triangle Park, North
Carolina.
The stiffness of the Example sheets can be objectively represented by either themaximum slope of the machine direction (MD) load/elongation curve for the tissue10 (hereinafter referred to as the "MD Slope") or by the machine direction Stiffness (herein
defined), which further takes into account the caliper of the tissue and the number of
plies of the product. Determining the MD Slope will be hereinafter described in
connection with Figure 9. The MD Slope is the maximum slope of the machine direction
load/elongation curve for the tissue. The units for the MD Slope are kilograms per 3
15 inches (7.62 centimeters). The MD Stiffness is calculated by multiplying the MD Slope by
the square root of the quotient of the Caliper divided by the number of plies. The units of
the MD Stiffness are (kilograms per 3 inches) -microns05.
Figure 9 is a generalized load/elongation curve for a tissue sheet, illustrating the
determination of the MD Slope. As shown, two points P1 and P2, the distance between
20 which is exaggerated for purposes of illustration, are selected that lie along the
load/elongation cur~e. The tensile tester is programmed (GAP ~General Applications
Program], version 2.5, Systems Integration Technology Inc., Stoughton, MA; a division of
MTS Systems Corporation, Research Triangle Park, NC) such that it cP'~ tes a linear
regression for the points that are sampled from P1 to P2. This calculation is done
25 repeatedly over the curve by adjusting the points P1 and P2 in a regular fashion along
the curve (hereinafter described). The highest value of these c 'cul~tions is the Max
Slope and, when performed on the machine direction of the specimen, wilî be referred to
herein as the MD Slope.
The tensile tester program should be set up such that five hundred points such as
30 P1 and P2 are taken over a two and one-half inch (63.5 mm) span of elongation. This
provides a sufficient number of points to exceed essentially any practical elongation of
the specimen. With a ten inch per minute (254 mm/min) crosshead speed, this translates
into a point every 0.030 seconds. The program c.~'cul~tes slopes among these points by
setting the 10th point as the initial point (for example P1), counting thirty points to the
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40th point (for example, P2) and performing a linear regression on those thirty points. It
stores the slope from this regression in an array. The program then counts up ten points
to the 20th point (which becomes P1) and repeats the procedure again (counting thirty
points to what would be the 50th point (which becomes P2), c~'c~ ti~l9 that slope and
also storing it in the array). This process continues for the entire elongation of the sheet.
The Max Slope is then chosen as the highest value from this array. The units of Max
Slope are kg per three-inch specimen width. (Strain is, of course, dimensionless since
the length of elongation is divided by the length of the jaw span. This calculation is taken
into account by the testing machine program.)
ExamPle 1 - 4. To illustrate the invention, a number of uncreped throughdried
tissues were produced using the method substantially as illustrated in Fig. 1. More
specifically, Examples 1 - 4 were all three-layered, single-ply bath tissues in which the
outer layers comprised disperged, debonded eucalyptus fibers and the center layer
comprised refined northern softwood kraft fibers. Cenebra eucalyptus fibers were pulped
15 for 15 minutes at 10% consistency and dewatered to 30% consistency. The pulp was
then fed to a Maule shaft disperger. The disperger was operated at 160~ F. (70~ C.) with
a power input of 2.2 HPDIT (1.8 kilowatt-days per tonne). Subsequent to disperging, a
softening agent (Witco C6027) was added to the pulp in the amount of 7.5 kg per metric
ton dry fiber (0.75 weight percent).
Prior to formation, the softwood fibers were pulped for 30 minutes at 3.2 percent
consistency, while the disperged, debonded eucalyptus fibers were diluted to 2.5 percent
consistency. The overall layered sheet weight was split 35%130%t35% for Examples 1, 2
and 4 and 33%/34%133% for Example 3 among the disperged eucalyptus/refined
softwood/disperged eucalyptus layers. The center layer was refined to levels required to
25 achieve target strength values, while the outer layers provided softness and bulk. For
added dry and temporary wet strength, a strength agent identified as Parez 631 NC was
added to the center layer.
These examples employed a four-layer Beloit Concept lll headbox. The refined
northern softwood kraft stock was used in the two center layers of the headbox to
30 produce a single center layer for the three-layered product described. Turbulence
generating inserts recessed about three inches (75 millimeters) from the slice and layer
dividers extending about six inches (150 millimeters) beyond the slice were employed.
The net slice opening was about 0.9 inch (23 millimeters) and water flows in all four
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headbox layers were comparable. The consistency of the stock fed to the headbox was
about 0.09 weight percent.
The resulting three-layered sheet was formed on a twin-wire, suction form roll,
former with forming fabrics being Appleton Mills 2164-B fabrics. Speed of the forming
fabric ranged between 11.8 and 12.3 meters per second. The newly-formed web was
then dewatered to a consistency of 25 - 26% using vacuum suction from below the
forming fabric without air press, and 32 - 33% with air press before being transferred to
the transfer fabric which was traveling at 9.1 meters per second (29 - 35% rush transfer).
The transfer fabric was Appleton Mills 2164-B. A vacuum shoe pulling about 6 - 15
10 inches (150 - 380 millimeters) of mercury vacuum was used to transfer the web to the
transfer fabric.
The web was then transferred to a throughdrying fabric traveling at a speed of
about 9.1 meters per second. Appleton Mills T124-4 and T124-7 throughdrying fabrics
were used. The web was carried over a Honeycomb throughdryer operating at a
15 temperature of about 350~ F. (175~ C.) and dried to a final dryness of about 94 - 98%
consistency.
The sequence of producing the Example sheets was as follows: Four rolls of the
Example 1 sheets were produced. The consistency data reported in Table 1 is based on
2 measurements, one at the beginning and one at the end of the 4 rolls. The other data
20 shown in Table 1 represents an average based on 4 measurements, one per roll. The air
press was then turned on. Data just prior to and just after activation of the air press is
shown in Table 3 (individual data points). This data shows that the air press caused
significant increases in tensile values. The process was then modified to decrease the
tensile values to levels comparable to the Example 1 sheets. After this process
25 adjustment period, four rolls of the Example 2 sheets (this invention) were produce~
Later, 4 rolls of the Example 3 sheets (this invention) were produced using a different
throughdrying fabric and with the air press activated. The air press was sl'lut off and the
process adjusted to regain tensile strength values comparable to the Example 3 sheets.
Four rolls of Example 4 sheets were then produced. The consistency data for each30 Example in Table 2 is an average based on 2 measurements, one at the beginning and
one at the end of each set of 4 rolls. The other data in Table 2 is based on an average
of 4 measurements per Example sheet, one per roll. In Table 2, the Example 4 data is
presented in the left column and the Example 3 data is presented in the right column to
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remain consistent with Tables 1 and 3, which show data without the air press in the left
column and data with the air press in the right column.
Tables 1 - 3 give more detailed descriptions of the process condition as well asresulting tissue properties for examples 1 - 4. As used in Tables 1 - 3 below, the column
5 headings have the following meanings: "Consistency @ Rush Transfer" is the
consistency of the web at the point of transfer from the forming fabric to the transfer
fabric, expressed as percent solids; "MD Tensile" is the machine direction tensile
strength, expressed in grams per 3 inches (7.62 centimeters) of sample width; "CD
Tensile" is the cross-machine tensile strength, expressed as grams per 3 inches (7.62
10 centimeters) of sample width; "MD Stretch" is the machine direction stretch, expressed as
percent elongation at sample failure; "MD Slope" is as defined above, expressed as
kilograms per 3 inches ~7.62 centimeters) of sample width; "Caliper" is the 1 sheet caliper
measured with a Bulk Micrometer (TMI Model 49-72-00, Amityville, New York) having an
anvil diameter of 4 1/16 inches (103.2 mm) and an anvil pressure of 220 grams/square
15 inch (3.39 Kilo Pascals), expressed in microns; "MD Stiffness" is the Machine Direction
Stiffness Factor as defined above, expressed as (kilograms per 3 inches) -microns05;
"Basis Weight" is the finished basis weight, expressed as grams per square meter; "TAD
Fabric" means throughdrying fabric; "Refiner" is power input to refine the center .ayer,
expressed as kilowatts; "Rush" is the difference in speed between the forming fabric and
20 the slower transfer fabric, divided by the speed of the transfer fabric and expressed as a
percentage; "HW/SW" is the breakdown of weight of hardwood (HW) and softwood (SW)
fibers in the three-layered, single-ply tissues, expressed as a percent of total fiber weight;
and "Parez" is the add-on rate of Parez 631 NC expressed as kilograms per metric ton of
the center layer fiber.
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Table 1
EXAMPLE 1 EXAMPLE 2
(No Air(With Air Press
Press) and Process
Adjustment)
Consistency @ Rush Transfer (%) 25.2 - 26.1 32.5 - 33.4
MD Tensile (grams/3") 933 944
CD Tensile (grams/3") 676 662
MD Stretch (%) 24.5 24.7
MD Slope (kg/3U) 4.994 3.778
Caliper (microns) 671 607
MD Stiffness (kg/3"-microns~5)129 93
Basis Weight (gsm) 34.6 35.2
TAD Fabric T-124-4 T-124-4
Refiner (kVV) 32 26
Rush (%) 32 29
HW/SW (%) 70/30 70/30
Parez (kg/mt) 4 0 3.2
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Table 2
E)<AMPLE 4 EXAMPLE 3
(No Air(With Air Press
Press) and Process
Adjustment)
Consistency @ RushTransfer(%)24.6 32.4
MDTensile (grams/3") 961 gO7
CD Tensile (grams/3") 714 685
MD Stretch (%) 23.5 24.4
MD Slope (kg/3") 5.668 3.942
Caliper (microns) 716 704
MD Stiffness (kg/3"-microns~ 5)152 105
Basis Weight (gsm) 35.0 35.1
TAD Fabric T-124-7 T-124-7
Refiner (kW) 40 34.5
Rush (%) 35 31
HWISW (%) 66/34 70/30
Parez (kg.mt) 2.5 2.5
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Table 3
(No Air (With Air
Press) Press)
Consistency @ Rush Transfer (%) 25.2 32.5
MD Tensile (grams/3") 915 1099
CDTensile (grams/3") 661 799
CD Wet Tensile 127 150
MD Stretch (%) 24.4 28.5
MD Slope (kg/3") 4.996 4.028
Caliper (microns) 665 630
MD Stiffness (kg/3"-microns~S)129 101
Basis Weight (gsm) 34.3 34.6
TAD Fabric T-124-4 T-124-4
Refiner (kW) 32 32
Rush (%) 32 32
HW/SW (%) 70130 70130
Parez (kg/mt) 4.0 4.0
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As shown by the previous Examples, the air press produces significantly higher
consistencies upstream of the differential speed transfer which result in smoother sheets
as evidenced by lower modulus values. Desirably, the modulus (MD Stiffness) of tissue
products is at least 20 percent less than that of a comparable tissue product made
without supplementally dewatering to a consistency of greater than about 30 percent.
Further, the machine direction tensile of the tissue products is at least 20 percent greater,
and the cross direction tensile of the tissue products is at least 20 percent greater, than
that of a comparable tissue product made without supplementally dewatering to a
consistency of greater than about 30 percent. Additionally, the machine direction stretch
10 of tissue products is at least 17 percent greater than that of a comparable tissue product
made without supplementally dewatering to a consistency of greater than about 30percent.
The foregoing detailed description has been for the purpose of illustration. Thus,
a number of modifications and changes may be made without departing from the spirit
15 and scope of the present invention. For instance, alternative or optional features
described as part of one embodiment can be used to yield another embodiment.
Additionally, two named components could represent portions of the same structure.
Further, various process and equipment arrangements as disclosed in PCT Patent
Application Publication WO 95/00706 dated January 5, 1995, and U.S. Patent
20 Application Serial No. 08/330,166 filed October 27, 1994, by Engel et al. and titled
"Method For Making Smooth Uncreped Throughdried Sheets", the ~lisclosures of which
are incorporated herein by reference, may be employed. Therefore, the invention should
not be limited by the specific embodiments described, but only by the claims.
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