Note: Descriptions are shown in the official language in which they were submitted.
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AERODYNAMIC METHOD FOR MAKING
TISSUE PAPER
FIELD OF THE INVENTION
The present invention relates to the field of papermaking, and more
particularly, to a method of manufacturing tissue paper featuring enhanced
absorbency.
BACKGROUND OF THE INVENTION
One of the main problems that one encounters when making tissue
paper by an aerodynamic method without using binders is the ability to
provide a tissue paper having both a high absorbency (hygroscopicity) and
sufficient strength. This is so because when the aerodynamic method is
used without binders, the bonding of fibers is obtained from hydrogen
bonds formed as a result of pressure processing and subsequent drying of
the moistened layer of fibers produced from aerosuspension. Pressing of
~5 the fibrous layer is necessary to provide a greater area of inter-fibrous
contact, while a drying is required to remove water molecules and form the
above-mentioned hydrogen bonds between the fibers. Thus, the greater
the pressure, the stronger the tissue produced and the lower its
absorbency, and vice versa.
2o One conventional aerodynamic method of paper making comprises
forming of a layer of cellulose fibers out of aerosuspension, impregnating
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this layer by a liquid reagent, and subsequently pressing and drying of this
layer. See, for example, USSR Author's Certificate 1594237, IPC 5 D21 H
23/00, 21/18, published August 23, 1990. This method is characterized by
the use of 2 - 8% water solution of resorcin that provides much better
swelling of fibers than water. Penetration of water into intercrystalline
space of pulp fibers facilitates enhancement of their plasticity, which
results
in more complete contact of fibers during pressing and drying, and, hence,
enhances the strength of fiber bonding. Since molecules of resorcin form
bonds of a "cellulose-resorcin-cellulose° type, resorcin also performs
the
function of a binder, which also facilitates the enhancement of the
produced tissue strength. Thus, when using a waterlresorcin solution for
moistening the fibrous layer, one can decrease the pressure applied at the
stage of pressing, thereby improving tissue absorbency while preserving
tissue strength. However, introduction of chemical additives makes tissue
~5 paper production more expensive.
Another conventional method for making high-absorbency products
out of fibrous materials comprises forming of a multi-layer structure of thin
paper layers and a layer of fibers produced out of aerosuspension and
placed between paper layers. All the layers arranged in the above manner
2o are pressed between rolls, one of which has a patterned surface. See, for
example, USA patent 3908653, IPC 2 A61 F 13/16, A61 L 15/00, published
September 30, 1975. Final formation of the product proceeds in the
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following manner. two structures obtained in the above-described way are
folded together facing each other with patterned surfaces and then the
edges are jet-molded or glued together. Since the filler in the product
comprises substantially non-pressed fibers, the final product offers high
hygroscopicity, but the manufacturing cost of such products is very high.
One conventional aerodynamic method of papermaking, which is
believed to be the closest to the present invention, comprises preparation
of aerosuspension of cellulose fibers, forming a fibrous layer on a moving
forming wire, moistening the moving fibrous layer with water, the amount of
which constitutes 20 - 60 % of fiber weight, and subsequently pressing and
drying of said fibrous layer. See, for example, USA patent 3949035, IPC 2
B29C 17/04, published April 6, 1976 - prototype. Pressing is performed
between two rotating rolls, one of which has a patterned surface made in
the form of ridges with flat faces of round (or circular) shape, and the
~5 distance between ridges doesn't exceed the average length of the cellulose
fibers. During pressing, compaction of the fibrous layer and formation of
greater contact area between the fibers take place in the ridge areas, while
no compaction occurs in the areas between the ridges (i. e., in valleys. As
a result, the final product obtained after drying has two types of areas:
2o areas of a pressed fibrous layer that determine the strength of the tissue
paper, and areas of non-pressed fibrous layer that determine tissue
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absorbency. Thus, this method enables one to produce tissue paper, the
structure of which concurrently provides tissue strength and hygroscopicity.
At the same time, to provide the formation of the above-mentioned
inter-fibrous bonds, the formed fibrous layer should be moistened with a
substantial amount of water. Besides, to provide better penetration of
water into the fibrous layer, this moistening is accompanied by rarefaction
of air produced underneath the wire carrying the fibrous layer. Such a
moistening process requires amounts of water that are excessive
compared to the amounts needed for the formation of inter-fibrous bonds.
In addition, it takes an extra time to moisten the entire fibrous layer. All
the
above results in extra expenditures of energy (mainly expended on
subsequent drying of the tissue paper web) and slows down the process of
tissue production. Besides, removal of great amounts of water through
drying leads to the shrinkage of the non-pressed part of the fibrous layer,
which results in a decrease in absorbency of the produced tissue paper.
SUMMARY OF THE INVENTION
The present invention advantageously decreases the costs related to
the manufacture of tissue paper and increases of the quality of the tissue
paper.
2o According to an embodiment of the present invention, a method of
making tissue paper comprises the following steps: preparing an
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aerosuspension of fibrous material; forming a layer of fibers on a forming
wire; transferring the layer of fibers to a profiling belt having a pressing
surface containing protruding elements for impressing first areas of the
fibrous layer in contact therewith; contacting the layer of fibers disposed on
5 the pressing surface of the profiling belt with a moistening belt; and
pressing the layer of fibers between the profiling belt and the moistening
belt. In addition, the moistening belt has a lower sorption capacity than a
sorption capacity of the first areas of the fibrous layer being impressed by
the protruding elements and higher than a sorption capacity than second
areas of the fibrous layer that are not impressed by the protruding
elements.
According to the present invention, a distance between protruding
relief elements on the pressing surface does not exceed an average length
of the fibers. The formed layer of fibers can be placed on a profiling belt
t5 that has a pressing surface that faces the layer of fibers. Moistening of
the
formed layer of fibers is performed concurrently with the pressing step, an
utilizes an additional belt such as, for example, a moistening belt. The
moistening belt is disposed such that a pressing force is exerted
concurrently on the profiling belt, the moistening belt, and the layer of
fibers
located therebetween. The moistening belt is preferably made of a
material having a sorption capacity that is lower than a sorption capacity of
those areas of the layer of fibers that are pressed due to the protruding
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relief elements, but higher than the sorption capacity of the areas of the
layer of fibers that are not pressed by the relief elements. The moistening
belt can be saturated with an appropriate fluid such as, for example, water,
in an area that is outside the pressing zone.
In one embodiment of the claimed invention, the fibrous layer placed
between the profiling and moistening belts during the step of pressing. In
the course of pressing, the sections of fibrous layer that are in the areas of
protruding relief elements get compacted, which results in an increase of
absorbency of the fibrous layer, due to the increase in the pressure of
capillary absorption. When the fibrous layer absorbency reaches a value
equal to the value of the same parameter of the moistening belt, the
sections of the fibrous layer being compacted begin to absorb water from
the moistening belt surface. With further compaction of the fibrous layer
the excess water is squeezed out from the compacted sections into the
~5 non-compacted sections, and due to the difference in capillary absorption
pressures, this water returns to the moistening belt. Part of the water
returned will subsequently be absorbed by new sections of the layer of
fibers being compacted. The moistening belt receives water required for
moistening outside the pressing zone, for example, absorbing it when
2o being passed through a tub with water.
The present invention provides concurrent moistening and pressing of
the fibrous layer in order to minimize the amount of water required at the
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pressing stage. Selective moistening of fibers only in the sections being
compacted eliminates the requirement for moistening the entire fibrous
layer, and excessive moistening. As a consequence, drying of the tissue
paper web after the pressing step requires significantly lower expenditures
of time and energy. Additionally, shrinkage of the tissue paper web is
eliminated because the non-pressed sections of the fibrous layer are not
moistened.
A wire made by means of interweaving threads can be used as the
profiling belt. In this instance interweaving nodes represent the protruding
relief elements of wire surface, and the shape of flat areas can be endowed
through the use of smoothing. This approach can significantly reduces the
cost of the process of the present invention.
Another simple and inexpensive embodiment of a moistening belt
comprises a fine-mesh wire. In this instance sorption properties of the
~5 moistening belt are determined by surface properties of the material of the
wire, as well as by relative sizes and geometrical configurations of threads
and openings of the wire.
Further, longitudinal twisting of fibers that significantly decreases the
area of contact between fibers in the areas pressed can be prevented if the
2o prepared aerosuspension has a moisture content sufficient for causing
saturation of fibers' walls with moisture.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig.1 is a process diagram illustrating the process of moistening and
pressing of a fibrous layer according to an embodiment of the present
invention.
Fig.2 is a top plan view illustrating a profiling belt made in the form of
wire, having smoothed surfaces of nodes produced by intersections of
threads.
DETAILED DESCRIPTION OF THE INVENTION
Referring to Fig. 1, a pressing unit of a paper-making machine that
uses the method of the present invention comprises a roller press having
two pressure rollers 1 and 2, a profiling belt 3, a moistening belt 4, and a
tub 5 with water into which a drum 6 is submerged. The drum 6 is intended
for transporting the moistening belt 4 through the tub 5. Fig. 1 also shows
a forming wire 7 on which fibrous layer 8 is formed, a drying drum 9, and
~5 wire-driving rollers and take-up suction rolls 10 and 11.
The profiling belt 3, a fragment of which is shown in Fig. 2, can be
made out of wire comprising interweaving threads 12 and wefts 13 of round
(circular) cross-section. The nodes of this wire on the side contacting the
fibrous layer are smoothed to such an extent that flat pressing surfaces 14
20 of elliptic shape are produced, and said flat pressing surfaces 14
determine
the fibrous layer sections to be pressed. Geometric size of the wire and
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the surfaces 14 are selected so that a distance between the surfaces 14 of
juxtaposed nodes of the wire doesn't exceed an average length of the
fibers. The strongest paper retaining good absorbing properties can be
produced when this distance is approximately equal to half of the average
length of the fibers. in this instance the individual fibers interconnect with
each other and transmit mechanical stresses arising in the conditions of
paper break from one pressed area to another.
Fibrous layer 8 formed out of aerosuspension (forming process is not
shown in Fig.1) through the use of the forming wire 7 and the profiling belt
3 is supplied to a zone of the suction roll 10 where the forming wire 7
breaks away, and the side of the fibrous layer 8 that has just lost contact
with the forming wire is covered by the moistening belt 4. Such a forming
belt can be made, for instance, in the form of a fine-mesh wire. The fibrous
layer 8, now positioned between the profiling belt 3 and the moistening belt
~5 4, is then fed to a pressing operation between rolls 1 and 2. Pressing of
the fibrous layer 8 proceeds as described in the Summary of the Invention
above. Subsequent to pressing, the belt 4 breaks away in the area of a
take-up suction roll 11, and the pressed fibrous layer is fed to the drying
drum 9, from which the finished paper web is subsequently obtained.
2o The possibility of implementing the claimed method was
experimentally tested in a following manner.
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Samples of tissue paper with specific weight of 40 - 45 g/m' were
produced. Softwood and hardwood sulfate bleached pulp {with the
average length of fibers 2.7mm and 1.4 mm, respectively) were used as
fibrous semi-finished material.
5 An aerosuspension of fibers was prepared out of fibrous semi-finished
material that has been moistened in advance to a moisture content of 50%.
This aerosuspension was fed to the forming wire where a fibrous layer
having a speck weight of 15 - 20 kglm3 was formed. A wire of interwoven
synthetic threads 0.25mm in diameter and with the distance of 0.25 mm
between core threads and 0.3 mm between weft threads was used as
forming wire.
The formed fibrous layer (the moisture content of which was 30 - 35%
at this technological stage) was transported to the profiling belt represented
by standard metal wires #1 or #2 of simple interweaving generally used in
~5 papermaking industry. Wire #1 is woven out of flat threads, and the shape
of its meshes is square. There are eight threads per 1 cm of running
length, thread width is equal to 0.6 mm; thread thickness = 0.15 mm;
distance between threads = 0.65 mm, and the area of threads constitutes
70% of the total wire area.
2o Wire #2 is woven out of threads of round section and of diameter 0.5
mm, and the shape of its meshes square. There are eight threads per 1
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cm of running length, and the distance between threads is 0.75 mm. One
surface of wire #2 was smoothed up to a depth of 0.25mm. Flat areas of
elliptical shape, the total area of which constitutes 40% of the total wire
area, were produced as a result of the smoothing. The fibrous layer was
placed on the smoothed wire surface at the pressing stage.
Two fine-mesh wires joined together were used as a moistening belt.
Each fine-mesh wire is made by simple interweaving of threads 0.25 mm in
diameter, and there are 24 threads per 1 cm of running length. Moisture
capacity defined as the amount of water retained by capillary forces in the
wire of area 1 m2 constitutes 0.08 kg/m2.
The formed fibrous layer placed between the profiling wire (#1 or #2)
and moistening wires was fed into the gap between the rolls of a roller
press developing a force of 10 or 18 kg on 1 cm of layer width. Subsequent
to pressing, the moistening wires were taken off the fibrous layer, while the
~5 layer being held by profiling wire was fed to drying unit, the surface of
which was heated to the temperature of 115 C.
Results of experimental testing are given in the Table below.
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SpecificMoistureof PressingMoisture Tensile Moisture
cavity content profilingforce, content strength capacity
o of wire of of of
the fibers fibers paper paper
layer on kg after sample, sample,
of fibers,the pressing,
forming NJm kg of water
.. wire, per kg of
kglm' % absolutely
dry fibers
1 0.04 35 1 ~ 10 52 600 2.2
2 0.045 35 1 10 I 54 550 2.1
3 0.04 30 1 18 43 r 850 1.9
4 0.04 32 1 18 I 45 ~ 870 1.9
0.04 35 1 ( 18 ( 47 I 960 ~ 1.9
6 0.045 30 1 18 I 45 840 I 1.8
7 ~ 0.04535 1 18 ~ 48 ~ 850 1.8
8 0.04 30 2 10 48 550 2.3
9 0.04 35 ~ 2 1 C~ _ ~~ 550 ' 2.2 '
0.045 30 2 10 I 49 I 620 1.8
11 0.045 35 2 10 50 640 1.9
12 0.04 30 2 18 48 670 2.2
13 0.04 35 2 18 51 700 2.1
The experiments performed confirm the possibility of implementing
the method of the present invention and verified the above-indicated
results. Using this method, it is possible to make tissue paper offering such
5 strength and hygroscopic properties that correspond to the current
specifications for tissue paper making. It should be pointed out that the
amounts of water expended with this method are significantly less
compared to the amounts spent when using other known methods. It can
SUBSTITUTE SHEET (RULE 26)
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be seen from the above Table that moisture content of the fibrous layer fed
to drying subsequent to pressing varies only slightly compared to the
moisture content of fibrous layer on the forming wire, which significantly
reduces the cost of drying and decreases the shrinkage of paper web
during drying.
Results of the experiments also indicate how parameters of
technological process and equipment units exert influence on the final
result. For example, when using wire #1 which is made out of flat threads
and which has "shallow" relief formed by the interwoven nodes of threads,
greater pressure should be applied to obtain required strength properties of
the final product.
On the other hand, to obtain a required strength of tissue paper, quite
high pressure is also needed when using wire #2 that is made of round
threads and that has lesser area of pressing zones compared to wire #1.
~5 However, it is just the lesser area of pressing zones that makes it
possible
to obtain tissue paper offering greater absorbency than the tissue paper
produced using wire #1.
Results of experiments given in the above Table in the 3ro, 4"', and 5~'
lines confirm that initial moisture content of a fibrous layer fed to a
pressing
20 operation also exerts impact on the strength of tissue paper being
manufactured. The greater the moisture content of the fibers, the softer
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and less twisted are they. Therefore, the contact area of such fibers is
greater in the course of pressing. This fact results in the formation of inter-
fibrous bonds on greater area, and, hence, in stronger tissue paper, while
absorbency of such tissue paper remains the same.
While this invention has been described in connection with what is
presently considered to be the most practical and preferred embodiments,
it is to be understood that the invention is not limited to the disclosed
embodiments, but, on the contrary, is intended to cover various
modifications and equivalent arrangements included within the spirit and
scope of the appended claims.
