Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
PATENT
LAYERED TWO-PLY TISSUE PROCESS AND PRODUCT
Background of the Invention
In the manufacture of tissue products such as facial and
bathroom t;ssue, it is known to improve the softness of the tissue by
incorporating hard~ood fibers into the tissue furnish. This can be
accomplished by using a blended furnish or by making a layered
product in which the hardwood fibers are concentrated in the outer
10 layers. An example of a layered tissue process and product is ,~
provided by U.S. Patent No. 4,300,981 to Carstens, in which a tissue !
web having 60% or more short fibers in the outer layer is disclosed. '`
However, conventional wisdom in the tissue industry, as evidenced by I
Carstens, is that in order to obtain a softer, smoother tissue, it is
preferred to use the dryer side of the tissue web as the outwardly
facing surface of the final tissue product. ~his is because the
dryer side of the tissue web (the side that is in contact with the
dryer during drying and creping) experiences the greatest degree of
debonding during creping and hence is traditionally viewed as the
softer of the two sides of the tissue web. Hence commercially
available layered prior art tissue is made by placing the hardwood
layer against the dryer surface and positioning that layer as the
outwardly facing surface of the final produc~. However, it has now
been discovered that by producing a layered tissue web with a former
commonly known in the industry as a crescent former, in which a
tissue web is initially formed between a forming wire and a
papermaking felt, a superior product can be made with a lesser amount
of short fibers. In addition, such a process provides a nulnber of
process advantages which make it even more desirable.
Summary of the Inven-tion
In one aspect, the invention resides ln a method for making a
two-ply tissue product comprising: (a) depositing a stratified
aqueous slurry o-f papermaking fibers between a forming wire and a
felt to form a layered embryonic web, said embryonic web having a
first layer of fibers comprising a substantial amount of short fibers
in contacting relationship with -the felt and a second layer of
fibers, preferably of predominantly long fibers, which provides most
of the strength of the resulting tissue web and which is in
contacting relationship with the forming wire; (b) dewatering the
web, preferably to a consistency of about 30 weight percent or
greater; (c) adhering the dewatered web to the surface of a rotating
drying cylinder and drying the web, wherein the second layer is in
contacting relationship with the surface of the drying cylinder; (d)
creping the dried web to form a tissue web; and (e) converting the
tissue web into a two-ply tissue product wherein the first lay~r of
the tissue web is positioned as an outwardly facing surface of the
tissue product.
In another aspect, the invention resides in a two-ply tissue
product made by the method described herein.
It has been found that the method of this invention offers a
large number of advantages. First, the resulting product has
surprising softness and is preferred by users relative to untreated
commercially available tissues made in the conventional manner. In
addition, forming the embryonic web with the second layer, which is
preferably predominantly long fibers, in contact with the forming
wire provides a filter and forming base for the short fibers of the
first layer and results in higher fiber retention than forming with a
substantial amount of short fibers against the forming wire. Also,
applying a predominantly long fiber layer against the drying/creping
cylinder reduces the amount of dust generated during creping, both in
the tissue web and in the manufacturing environment. Further, by
winding two of these plies together with the dryer (dustier) side
inward to convert the plies into a two-ply tissue product, the dust
level in the manufacturing and converting areas of the mill are
further reduced. Furthermore, such a two-ply tissue product exhibits
reduced lint in use because the dustier sides of each of the two
tissue webs are plied toge-ther ~nwardly in the middle of the tissue
product. This is a significant benePit to the user, for which lint
is a common complaint when using tissues having a significant level
of hardwood fibers.
For purposes herein, a tissue web is a web suitable for use as a
facial or bath tissue and having a finished basis weight of from
about 5 to about 15 pounds per 2880 square feet per ply. Creped
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tissue web densities can be from about 0.1 to about 0.3 grams per
cublc cent;meter. Strengths in the machine direction can be from
about 100 to about 1000 grams per inch of width, preferably from
about 200 to about 350 grams per inch of width. Tensile strengths in
the cross-machine direction can be from about 50 to about 500 grams
per inch of width, preferably from about 100 to about 250 grams per
inch of width. Such webs are preferably made from natural cellulosic
fiber sources such as hardwoods, softwoods, and nonwoody species, but
can also contain significant amounts of synthetic fibers.
Brief Description of the Drawina
Figure 1 is a schematic flow diagram of the method of this
invention.
Figure 2 is a schematic flow diagram of the converting step of
this invention, depicting plying of two creped webs together such
that the layers containing a substantial amount of short fibers are
the outwardly facing surfaces of the final tissue product.
Detailed Description of the Invention
Referring to Figure 1, the invention will be described in
greater detail. Shown is a multi-layered headbox 1 which can have
one, two, or more internal d;viders for separating different
furnishes during formation of an embryonic web. ~eadboxes of this
kind are well known in the paper industry and a wide variety of
suitable designs are available. Not shown is the stock system for
supplying the multi-layered headbox with different furnishes used to
form the different layers within the tissue. In operation, the
headbox deposits a stratified aqueous slurry of papermaking fibers
between a forming wire 2 and a felt 3. Each of the strata consis-ts
of a fibrous sl~rry of different characteristics. The felt is
wrapped around the forming roll 4. The forming wire is also
partially wrapped around the forming roll, but to a lesser extent,
and simultaneously spans the gap between the breast roll 5 and the
wire take-off roll 6. Remaining rolls 7, ~, 9, and 10 provide means
to properly adjust the tension of the forming wire. The relative
positions of the forming roll, the breast roll, and the wire take-off
roll must be adjusted to optimize the forminy process.
The forming wire is characterized by a weave which provides
immediate water drainage while providing adequate support for the
fibers deposited thereon. Typically these fabrics are made of
polyester or nylon and have a mesh of at least about 80 and a count
of at least about 100. On the other hand, the felt is characterized
by a fabric construction which absorbs or wicks away water from the
embryonic web to assist in dewatering and which causes the embryonic
web to adhere to the felt. Typically felts are made of a woven base
with graded layers of batt needled into the base, with finished
weights of from at least about 3.6 ounces per square foot to about
5 ounces per square foot.
The layered embryonic web contains at least two layers or
strata. The first layer 15, which is in contacting relationship with
the felt, comprises a substantial amount of short fibers such as
hardwood fibers. Suitably, the amount of short fibers constitutes at
least 40 weight percent of the fiber content of the layer.
Preferably, the amount of short fibers is from about 50 to about 60
weight percent of the layer, although amounts up to 100 percent can
be used to further alter the surface characteristics of the web.
Because of the presence of substantial amounts of short fibers which
tend to form webs of low strength, the fiber composition of this
first layer generally exhibits a handsheet TAPPI burst factor of
about 15 meters2 per gram2, second layer 16, which is in contacting
relationship with the forming wire, can be of any composition which
provides sufficient strength to the resulting tissue web for its
intended use and preferably consists predominantly (50 dry weight
percent or greater) of long fibers such as softwood fibers. For
purposes herein, short fibers are those which have an average length
of from about 0.25 m;llimeters to about 1.50 millimeters and
primarily include hardwood fibers, such as eucalyptus f;bers. Long
fibers are those which have an average length greater than about 1.50
millimeters and pr1marily include softwood fibers. The strength
layer fiber composition exhibits a greater handsheet TAPPI burst
factor than the layer 15, wh;ch preferably ;s about 30 meters squared
per gram squared.
In the vicinity between the forming roll 4 and the wire take-off
roll 6; the forming wire is passively disengaged from the embryon;c
web, which remains with the felt. The embryonic web is dewatered by
the absorbent ac~ion of the felt as the felt carries the web to the
rotating drying cylinder 2~. Transfer of the dewatered web to the
drying cylinder takes place at the pressure roll 21, which presses
the dewatered web against the surface of the drying cylinder. The
degree of hardness of the pressure roll can vary, although pressure
rolls hav;ng a hardness of about 38 P&J (as measured on the Pusey and
~ones scale~ are preferred. Adhesion of the dewatered web to the
drying cylinder is accomplished by the presence of moisture in the
web and is preferably augmented by the presence of a creping
adhesive, which is applied by a suitable spray device 22 as shown.
The creping adhesive can be any creping adhesive which provides the
appropriate level of adhesion for the particular web composition and
basis weight. Such adhesives are well known in the papermaking
industry. Particular adhesives which have been found to work well are
those based on polyvinyl alcohol. The web is then dried to about 5
weight percent moisture and is dislodged from the drying cylinder by
contact with the doctor blade 23, which crepes the web. Because the
second layer of the web (dryer side) is against the dryer surface,
the formation of lint is minimized while the web is softened,
particularly if the second layer primarily contains long fibers. The
creped web is then wound onto a soft roll 24 with the aid of a reel
drum 25.
Figure 2 illustrates a method or combining two of the layered
webs made as described above to form a two-ply facial tissue in
accordance with this invention. Shown are two soft rolls 24 being
unwound and brought together in a nip formed between a first pair of
steel calender rolls 31 and 32 followed by a second pair of steel
calender rolls 33 and 34. The calendered webs are then crimped
together between an anvil roll 35 and a crimper wheel 36. The
crimped webs, now a two-ply basesheet, are then appropriately slit
between an anvil roll 37 and a slitter roll 38 and wound onto a
hardroll 40, from which the tissue web ls Further converted (folded,
inter~olded, packaged) into the final tissue product.
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Examples
Examplç l: Makinq the tissue web.
A tissue web was made as illustrated in ~igures 1 and 2. More
specifically, a layered headbox was used to form a two-layered
embryonic web between the felt and the forming wire. The layer
against the felt (first layer) consisted of 100 dry weight percent
eucalyptus fibers provided as an aqueous slurry having a consistency
of about 0.14 weight percent. The layer against the forming wire
(second layer) consisted of 100 dry weight percent northern softwood
kraft fibers provided as an aqueous slurry having a consistency o~
about 0.14 weight percent. The forming wire was an 84 mesh polyester
fabric manufactured by Appleton ~ire Company, Appleton, Wisconsin.
The felt was a Yankee pick-up felt also manufactured by Appleton Wire
Company. The embryonic layered web was dewatered to a consistency of
about 40 weight percent before being transferred to a Yankee dryer
with the second layer against the surface of the dryer. Transfer to
the dryer was effected with a soft pressure roll having a hardness of
about 38 P&J. A creping adhesive consisting of a mixture of
polyv;nyl alcohol, KYMENE, and Quaker release agent was used to
enhance the adhesion of the web to the Yankee dryer. The web was
creped at about 5 weight percent moisture and the resulting tissue
web was wound onto a softroll.
Two softrolls of tissue webs, each made as described above, were
plied together as described above in reference to Figure 2. The two-
ply basesheet was converted into two-ply fac~al tissue having a
finished basis weight of about 18.5 pounds per 2880 square feet with
the first layer of each ply positioned as the outwardly fac~ng
surface of the tissue product.
Example 2: Makinq the tissue web.
A two-ply facial tissue was made as described in Example 1,
except the furnish of the layer against the felt (first layer) was
about 50 dry weight percent eucalyptus fibers and about 50 dry weight
percent northern softwood kraft fibers and was unrefined. The
furnish of the layer against the forming wire (second layer) was the
same, but was highly refined to provide sufficient strength. The
finished basis weight of the two-ply facial tissue was about 19
pounds per 2880 square feet.
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Example 3: Making the tissue web.
A two-ply facial tissue was made as described in Example 1,
except the furnish of the layer against the felt (first layer) was
about 55 dry weight percent eucalyptus fibers and about 45 dry we;ght
percent northern softwood kraft fibers and was unrefined. The
furnish of the layer against the forming wire (second layer) was the
same, but was highly refined to provide sufficient strength. The
finished basis we;ght of the two-ply tissue product was about 19
pounds per 2880 square feet.
ExamDle 4: Consumer evaluations.
Two-ply facial tissues made as described in Examples l and 2
were placed with consumers in test cells of 100 subjects each. The
consumers were given unidenti~ied sample facial tissues to view and
handle and were asked to determine which sample was softer. In the
evaluations, tissue samples of Examples 1 and 2 were compared to a
commercially-available two-ply facial tissue (PUFFS), each ply of
which is layered and having an outer layer of about 100% eucalyptus
fibers which during manufacture is positioned against the creping
cylinder surface. In the test cells comparing facial tissues of
Example 1 versus PUFFS, on average about 58 percent selected Example
1 as being softer, about 20 percent selected PUFFS as being softer,
and about 22 percent had no preference. In the test cells comparing
facial tissues o~F Example 2 versus PUFFS, on average about 54 percent
selected Example 2 as being softer, about 39 percent selected PUFFS
as being softer, and about 7 percent had no preference. In the test
cells comparing facial tissues of Example 3 versus PUFFS, on average
about S1 percent selected Example 3 as being softer, about 30 percent
selected PUFFS as being softer, and about 19 percent had no
preference. These results are statistically signiFicant and
illustrate that the method of this invention produces tissue products
which are superior in softness, either with an equal percentage or
with a lesser percentage of eucalyptus fibers in the outer layer o-F
the tissue. This is not only an advantage from a product performance
standpoint, but is also an advanta~e from an economic standpoint
because of the relatively greater expense of eucalyptus fibers
compared to soFtwood fibers.
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It will be appreciated that the foregoing examples are given for
purposes of illustration and are not to be construed as limiting the
scope of this invention, which is defined by the following claims:
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