Note: Descriptions are shown in the official language in which they were submitted.
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Application No. 2,706,323
Attorney Docket No. 17648-211
Structuring belt, press section and tissue papermaking machine for
manufacturing a high bulk
creped tissue paper web and method therefor
This invention relates to a structuring belt comprising a structuring layer
adapted to structure
a wet fibrous web during pressing in an extended press nip in a press section
of a tissue
papermaking machine for manufacturing a high bulk tissue paper. The invention
also relates
to a press section for a tissue papermaking machine and to a tissue
papermaking machine for
manufacturing a structured high bulk tissue paper web by means of pressing.
The invention also relates to a method of manufacturing a structured high bulk
tissue paper
web and to such a high bulk tissue paper web.
The invention relates furthermore to a method of converting or upgrading an
existing tissue
papermaking machine.
The term "tissue paper" as used herein refers to soft paper with a basis
weight usually of less
than 25 g/m2. Tissue paper web forms a base paper for certain single-ply and
multi-ply
products, e.g. napkins, towels and toilet rolls.
Tissue manufacturers wish to produce products with high bulk and softness. At
the same time
the energy costs for the process is important.
In the manufacture of creped tissue paper, there are two established
technologies for
dewatering the formed wet paper web of cellulose fibres before it is dried and
creped on a
Yankee cylinder. In the commercially predominant technology, the paper web
which is
carried by a felt is dewatered in one or two press nips with rolls against the
Yankee cylinder.
This process gives a tissue product with relatively low bulk and there is no
distinct structure
on the surface. The other technology is TAD (through air drying), in which the
paper web is
dewatered with the aid of a vacuum and then dried by through air drying before
it is
transferred to the Yankee cylinder for final drying and creping. The TAD
process gives a
high bulk and a distinct structure, but requires slightly more than twice as
much energy to
produce a tonne of paper.
It has been proposed to use a shoe press with an extended press nip against
the Yankee
cylinder in order to improve the quality of the tissue product. The aim has
been to provide an
improved quality, higher bulk and softness compared to conventional processes.
It has been
found that it is possible to achieve a certain improvement, but that the
product is still more
like a conventionally manufactured product than a TAD-manufactured product.
The
thickness or bulk of the paper is important for its capacity to absorb water,
as well as the feel
of the textile structure and softness. The TAD technique is therefore still
superior to the
pressing technique with respect to the quality of the paper web, but it has
the great
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disadvantage that it requires substantially higher energy consumption than is
the case with a
pressing technique.
Paper making machines which use the pressing technique to dewater and
simultaneously
structure the paper web with the aid of a structuring clothing have been
proposed. This
dewatering and structuring occurs in one or more press nips while the paper
web is
transferred from the felt to the structuring clothing. The structuring
clothing then carries the
paper web to the Yankee cylinder, where it is transferred with the aid of a
press roll which
only ensures the transfer of the paper web. Structuring clothings of this kind
may be belts or
fabrics. The present invention relates to a structuring belt, i.e. a non-woven
structuring
clothing. This means that 3D patterns are created, not by the woven structure,
but by other
means. The bulk of the paper is maintained in that cavities in the structure
of the belt receive
the fibrous network and prevent compression of the fibrous network during
dewatering in the
press nip.
The expression "structuring" of the paper as used herein refers to the fact
that a
three-dimensional pattern of the structuring layer is embossed into the wet
fibrous web
during a pressing process when the fibrous network structure fills the three-
dimensional
pattern of the structuring belt and that fibres in the wet fibrous web are
movable relative to
each other so that they are advantageously brought to new positions and
directions relative to
each other by the action of the elastically compressible press felt, which
presses the wet
fibrous web into the three-dimensional pattern of the structuring belt, and
this altogether
contributes to an increased bulk and softness with the same basis weight, and
to an improved
structure.
US 6,547,924 and US 6,340,413 describe a tissue papermaking machine in which a
structuring belt carries the fibrous web from the last press in the press
section to the drying
cylinder. However, the papermaking machine described in the said patent
specification
cannot produce a tissue paper of sufficiently high quality owing to the
plurality of press nips
passed through in accordance with the requirements and wishes of customers
today. There
were moreover problems with the runnability of the machine, as the press felt
was saturated
with water and could not absorb a sufficient quantity in the nip that led to
paper break.
Further examples of tissue papermaking machines provided with embossing or
structuring
clothings are EP 1 078 126, EP 0 526 592, US 6,743,339, EP 1 075 567, EP 1 040
223,
US 5,393,384, EP 1 036 880 and US 5,230,776.
Following comprehensive research, the present inventors have realised that the
structure of
the structuring layer of the structuring belt in contact with the web during
the pressing
process has great and probably crucial importance from the point of view of
being able to
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Application No. 2,706,323
Attorney Docket No. 17648-211
achieve a tissue paper with a higher bulk than that possible hitherto in a
papermaking
machine using the pressing technique and that the structure of this layer of
the structuring
belt can also be used as a parameter for controlling slip properties of the
web after the nip and
for achieving a high dryness of the web in connection with the pressing in the
press section in
which the actual structuring of the wet fibrous web occurs.
The object of the invention is to make it possible to manufacture a tissue
paper web with a
bulk of at least 8-20 cm3 /g, e.g. at least 10-16 cm3 /g, said bulk being
comparable to that of
TAD paper, that is 12-20 cm3/g, using the pressing technique and with low
energy costs. It
should be noted that conventional tissue paper manufactured by the pressing
technique
normally has a bulk in the range of 5-9 cm3 /g. The low energy costs are
achieved in that the
use of the structuring belt according to the invention gives a high dryness of
the fibrous web
after the press section, said dryness being in the range of 40-50 %. The high
dryness in turn
means that a smaller quantity of water has to be evaporated from the drying
surface in the
subsequent drying stage, which in turn means an energy saving. The energy-
intensive TAD
technique for removing water from the fibrous web can thus be avoided.
High bulk of a tissue paper web is important for the absorption capacity of
the web. After
manufacture, a tissue paper web can be rewound into finished products
consisting of a
plurality of paper layers, such as sanitary paper, napkins, towels and toilet
paper. The quality
of these products is determined, inter alia, by the absorption capacity of the
products and by
how soft consumers find the products.
The abovementioned object is obtained according to the invention by a
structuring layer of a
structuring belt according to the present invention.
The invention is described further with reference to the drawings.
Figures 1 to 10 show ten different tissue papermaking machines with a
structuring belt
according to the invention.
Figure 11 shows a structuring belt according to a first embodiment of the
invention.
Figure 12 is a section through the structuring belt according to Figure 11.
Figure 13 shows a structuring layer of a structuring belt according to a
second embodiment of
the invention.
Figure 14 shows a structuring layer of a structuring belt according to a third
embodiment of
the invention.
Figure 15 shows a structuring layer according to a fourth embodiment of the
invention.
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Figure 16 shows a structuring layer according to a fifth embodiment of the
invention.
Figure 17 is a section of a tissue paper web manufactured by a tissue
papermaking machine
according to the invention.
Figure 18 is a top view of a tissue paper web manufactured by a tissue
papermaking machine
according to the invention.
Figures 1-10 show schematically different embodiments of a tissue papermaking
machine for
manufacturing a tissue paper web 1 structured by means of pressing without the
use of
through air drying (TAD) for preliminary dewatering according to the present
invention.
Common to the different embodiments is the fact that they comprise a wet
section 2 for
forming a continuous paper web, a press section 3 for dewatering and
structuring the web and
a drying section 4 for final drying of the web. The wet section 2 of each
tissue papermaking
machine according to the embodiments shown comprises a forming section 5
including a
headbox 6 feeding a stock of fibres and water to a forming clothing, a forming
roll 7 enclosed
by a forming clothing for partial dewatering of the web, and a first forming
clothing 8
running about and in contact with the forming roll 7 and carrying the paper
web. In the
embodiments according to Figures 1-8, the forming section 5 also has a second
forming
clothing 9, that is a fabric, running in an endless loop about a plurality of
guide rolls 10 and
about the forming roll 7 in contact with the first clothing 8 so as to receive
a stock jet from
the headbox 6 between it and the first clothing, after which the stock is
dewatered through the
clothing 9 for the forming of a formed fibrous web 1'.
The press section 3 comprises a main press 11 including a first press element
12 and a second
press element 13 which cooperate with each other to form a press nip Ni
between them. The
main press 11 may be a roll press, a long nip press or a shoe press (not shown
in the figures).
The press section 3 furthermore comprises a structuring belt 14 running in an
endless loop
about a plurality of guide rolls 15, about a smooth transfer roll 16 located
in connection to the
drying section 4, and through the press nip Ni of the main press 11 together
and in contact
with the formed fibrous web l' in order to provide for dewatering and
structuring of the
formed fibrous web l' when it passes through the press nip Ni so that a
structured fibrous
web 1" will leave the press nip Ni. The structured fibrous web 1" is carried
by the structuring
belt 14 up to a transfer nip N2 between the transfer roll 16 and a drying
cylinder 19 of the
drying section 4, no pressing or dewatering taking place in the said nip N2,
but only the
transfer of the fibrous web 1" to the surface of the drying cylinder 19. In
this case, the drying
cylinder 19 is a Yankee cylinder, but other types of drying sections are
possible. The press
section 3 furthermore comprises a water-receiving press felt 17 elastically
formable and
compressible in the z-direction, running in an endless loop about a plurality
of guide rolls 18
and through the press nip Ni of the main press 11 together with the
structuring belt 14 and in
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contact with the formed fibrous web 1'. The first press element 12 is located
in the loop of the
structuring belt 14 and the second press element 13 is located in the loop of
the second press
felt 17. In the embodiments shown in Figures 1-10, both press elements 12, 13
are press rolls,
but they are alternatively rolls forming a long nip. The press felt 17 leaves
the structured
5 fibrous web 1" immediately after it has passed through the press nip Ni
in order to prevent
rewetting of the fibrous web 1".
Immediately before the first guide roll 18 after the main press 11, a spray
device 53 is
arranged on the inside of the press felt 17 for supplying fresh water to the
wedge-shaped
narrowing space between the press felt 17 and the guide roll 18, said water
being pressed into
the press felt 17 and displaces the contaminated water in the press felt 17
after pressing in the
main press 11 through and out of the press felt 17 when this runs about the
guide roll 18.
Upstream of the following guide roll 18, suction boxes 54 are arranged on the
outside of the
press felt in order to withdraw water out from the press felt.
Once the structuring belt 14 has left the transfer roll 16 and before it
reaches the main press
11, the structuring belt 14 passes through a cleaning station 30 for cleaning
the
web-contacting surface.
During its passage through the press section 3, the fibrous web l', 1" is
brought from a
dryness in the range of 15-30 % to a dryness in the range of 42-52 %.
The drying section 4 comprises said drying cylinder 19, which, in the
embodiments shown, is
the only drying cylinder, advantageously a Yankee drying cylinder.
Alternatively, the drying
section may consist of a plurality of drying cylinders or drying belts made of
metal. The
drying cylinder 19 with which the transfer roll 16 forms the said transfer nip
N2 has a drying
surface 20 for drying the structured fibrous web 1". A creping doctor 21 is
arranged
downstream of the drying surface 20 in order to crepe the dried fibrous web 1"
from the
drying surface 20 in order to obtain the tissue paper web 1 which is both
structured and
creped. The drying cylinder 19 is covered by a hood 22. The structuring belt
14 and the
structured fibrous web 1" run together through the transfer nip N2, but leave
the transfer nip
N2 separately in that the structured fibrous web 1" adheres to and is
transferred to the drying
surface 20 of the drying cylinder 19. The pressure in the transfer nip N2
formed by the roll 16
and the drying cylinder 19 is less than 1 MPa and no dewatering of the fibrous
web 1" occurs
in this nip. In order to ensure that the fibrous web 1" is transferred to the
drying surface 20,
an adhesive is advantageously applied to the drying surface 20 by means of a
spray device 23
at a point between the creping doctor 21 and the transfer nip N2 where the
drying surface 20
is free.
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The forming section 5 may be a so-called C-former, as shown in Figures 1, 2, 7
and 8, or a
so-called Crescent former, as shown in Figures 3-6, or a so-called suction
breast roll former,
as shown in Figures 9 and 10.
The main press 11 may be a roll press in which the two press elements 12, 13
are rolls with
smooth mantle surfaces, or preferably a long nip press, e.g. a shoe press, in
which the first
press element 12 is a smooth counter roll and the second press element 13
comprises a press
shoe and an endless belt or a jacket running through the press nip of the shoe
press in sliding
contact with the press shoe, which exerts a predetermined pressure on the
inside of the belt
and on the counter roll 12. The press shoe thus constitutes a device forming
an extended
press nip. In a further preferred embodiment of the main press 11, the first
press element 12
is a smooth counter roll and the second press element comprises a device for
forming an
extended press nip, said device including an elastic support body arranged to
press in the
direction towards the counter roll. In an alternative embodiment, the press
element 13 is a
smooth counter roll, while the second press element 12 comprises a device
forming an
extended nip of any one of the types known in paper making.
In the embodiment according to Figure 1, the press felt 17 of the main press
is also used as
the first inner forming clothing 8 of the forming section 5 so that the
forming roll 7 is also
located within the loop of the press felt 17. The wet section 2 in this case
also comprises a
predewatering device 24, namely a suction device. In this embodiment, the
device 24
comprises a suction roll 25 located within the loop of the press felt 17, and
a steam box 26
located on the outside of the loop of the press felt 17 in front of the
suction roll 25 for heating
the water in the fibrous network of the formed fibrous web 1'. The quantity of
water in the
fibrous structure of the formed fibrous web l' and in the press felt 17 is
decreased with the
aid of such a suction roll 25 and steam box 26, so as to give the formed
fibrous web 1' a
desired increased dryness before the main press 11. A high-pressure spray
device 55, that is a
needle-type spray device with a jet diameter of 1 mm, is arranged on the
outside of the
forming felt 8 upstream of the forming roll 7 in order to clean the forming
felt 8 before it
reaches the forming roll 7.
The embodiment according to Figure 2 is similar to that of Figure 1, except
that it is
additionally provided with a preheating device 27 downstream of the main press
11 in order
to increase the temperature of the structured fibrous web 1" in the press 11
before the fibrous
web 1" reaches the drying cylinder 19.
In the embodiment according to Figure 3, the structuring belt 14 is also used
as the first inner
forming clothing 8 of the forming section so that the forming roll 7 is also
located within and
surrounded by the loop of the structuring belt 14. In this case, the press
felt 17 of the main
press 11 runs in a single loop about a plurality of guide rolls 28 and the
second press
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element 13. The guide roll located upstream of the second press element 13 is
a suction roll
29 by means of which water is removed from the press felt 17 in order to
increase the
capacity of the press felt 17 to dispose of relatively large quantities of
water pressed out in
the nip Ni. One special effect with this embodiment, in which the structuring
belt 14 also
passes about the forming roll 7, is that it will be possible for the fibres of
the stock to
penetrate into and orient themselves in the z-direction in the depressions of
the structuring
belt 14 so that some of the formed fibrous web l' is already oriented in the
depressions before
pressing is started in the main press 11. Such a pre-orientation of fibres in
the depressions is
therefore advantageous in order to provide higher bulk. Immediately in front
of the first guide
roll 28 after the main press 11, a spray device 53 is arranged on the inside
of the press felt 17
for supplying fresh water into the wedge-shaped tapering space between the
press felt 17 and
the guide roll 28, said water being pressed into the press felt 17 and
displaces the
contaminated water in the press felt 17 after pressing in the main press 11
through and out of
the press felt 17 when this runs about the guide roll 28. Upstream of the
following guide roll
28, suction boxes 54 are arranged on the outside of the press felt 17 in order
to withdraw
water out from the press felt 17, as well as a high-pressure spray device 55
which cleans the
press felt 17 before it arrives at the suction roll 29, which deals with the
remaining water in
the press felt 17. The suction roll 29 removes water from the press felt 17
and thus increases
the capacity of the press felt to absorb the water in the nip Ni.
The embodiment according to Figure 4 is similar to that of Figure 3, except
that it is
additionally provided with a preheating device 27 corresponding to the
embodiment
according to Figure 2 and that a steam box 31 is arranged on the outside of
the press felt 17
immediately in front of the suction roll 29 in order to increase the
dewatering capacity
thereof
In the embodiment according to Figure 5, the first inner forming clothing 8,
the press felt 17
and the structuring belt 14 have their own loops, wherein the forming clothing
8 is a felt
running about a plurality of guide rolls 18'. The press section 3 in this case
comprises a pre-
press 32 including a first press element 33 located within the loop of the
press felt 17 and a
second press element 34 located within the first inner forming clothing 8,
said press elements
33, 34 forming a press nip N3 with each other through which the forming felt 8
carrying the
fibrous web l' runs in order to meet the press felt 17 which also runs through
the said press
nip N3 in order to receive the formed fibrous web l' and carry it on to the
main press 11. The
forming felt 8 thus also forms the second press felt of the pre-press 32. The
guide roll located
immediately upstream of the second press element 34 is a suction roll 35 by
means of which
water is removed from the forming felt 8. A steam box 36 is located on the
outside of the
forming felt 8 immediately in front of the suction roll 35 in order to make
the dewatering of
the felt 8 more effective. Immediately in front of the first guide roll 18'
after the pre-press 32,
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a spray device 53' is arranged on the inside of the forming felt 8 for
supplying fresh water
into the wedge-shaped tapering space between the forming felt 8 and the guide
roll 18', said
water being pressed into the forming felt 8 and displaces the contaminated
water in the
forming felt 8 after pressing in the pre-press 32 through and out of the
forming felt 8 when
this runs about the guide roll 18'. Upstream of the following guide roll 18',
suction boxes 54'
are arranged on the outside of the forming felt 8 in order to withdraw water
out from the
press felt 8, as well as a high-pressure spray device 55' which cleans the
forming felt 8 before
it reaches the forming roll 7.
The embodiment according to Figure 6 is similar to that of Figure 5, except
that it is
additionally provided with a preheating device 27 corresponding to the
embodiment
according to Figure 2.
In the embodiment according to Figure 7, the first inner forming clothing 8,
that is a forming
fabric, the press felt 17 and the structuring belt 14 have their own loops as
in the embodiment
according to Figure 5. In this case, the forming section 5 is thus a twin-wire
C-former. The
forming roll 7 may be a suction roll if desired. The press section 3 in this
case also comprises
a pre-press 32 including a first press element 33 located within the loop of
the press felt 17
and a second press element 34 located within a second press felt 37 running in
a loop about a
plurality of guide rolls 38, wherein the guide roll located immediately
upstream of the second
press element 34 is a suction roll 39 by means of which water is removed from
the second
press felt 37. A steam box 50 is located on the outside of the second press
felt 37
immediately in front of the suction roll 39 in order to improve dewatering of
the press felt 37.
The second press felt 37 runs in contact with the first inner forming fabric 8
in order to form
a transfer zone in which the press felt 37, the formed fibrous web 1' and the
forming fabric 8
form a sandwich structure. When the fibrous web l' leaves the transfer zone,
it is carried by
the second press felt 37. A suction device 51 may be located within the loop
of the second
press felt 37 after the transfer zone in order to ensure the transfer of the
fibrous web P.
Immediately in front of the first guide roll 38 after the pre-press 32, a
spray device 53' is
arranged on the inside of the press felt 37 for supplying fresh water into the
wedge-shaped
tapering space between the press felt 37 and the guide roll 38, said water
being pressed into
the press felt 37 and displaces the contaminated water in the press felt 37
after pressing in the
pre-press 32 through and out of the press felt 37 when this runs about the
guide roll 38.
Upstream of the following guide roll 38, suction boxes 54' are arranged on the
outside of the
press felt 37 in order to withdraw water out from the press felt 37, as well
as a high-pressure
spray device 55' which cleans the press felt 37 before it reaches the suction
device 51.
The embodiment according to Figure 8 is similar to that of Figure 7, except
that it is
additionally provided with a preheating device 27 after the main press
corresponding to the
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embodiment according to Figure 2 in order to increase the temperature and
dryness of the
paper web 1".
The embodiment according to Figure 9 is similar to that of Figure 7 except for
the wet
section 2 which in this case has a forming section of a type other than C-
former and Crescent
former as mentioned previously. The forming section according to Figure 9 is a
so-called
suction breast roll former including a headbox 6, a forming roll 7, that is a
suction breast roll,
and a forming clothing 8, that is a forming fabric, running in a loop about
the suction breast
roll 7 and guide rolls 18 and forming a transfer zone with the second press
felt 37
corresponding to the embodiment according to Figure 7. The suction breast roll
7 has a
suction zone 52 forming a forming zone across which the forming fabric 8
passes together
with stock emitted in a jet from the headbox 6 and dewatered within the
forming zone 52 in
order to form a formed fibrous web 1'.
The embodiment according to Figure 10 is similar to that of Figure 9, except
that it is
additionally provided with a preheating device 27 corresponding to the
embodiment
according to Figure 2.
The pre-press 32 used in the embodiments according to Figures 5-10 may be a
press selected
from the group of different presses described above in connection with the
main press 11.
The structuring belt 14 comprises a structuring layer 60 forming the side of
the structuring
belt carrying the paper web. The layer 60 has a web-contacting surface 61 with
a
three-dimensional structure formed by the depressions 63 in the form of
recesses or pockets
in the otherwise flat web-contacting surface 61, said depressions 63 being
regularly recurrent
and distributed in the longitudinal direction (MD) and cross direction (CD) of
the structuring
belt. The web-contacting surface 61 thus has a flat, continuous top surface
area 70 in which
said depressions 63 are formed. Each depression 63 in the web-contacting
surface 61 is thus
delimited by said continuous surface area 70. In addition to these depressions
63 further
patterns in the form of figures or text may be formed in the structuring layer
60.
All of the depressions 63 are preferably identical and are arranged in a
regular pattern.
Alternatively, one and the same structuring belt may comprise two or more
groups of
depressions, wherein the design of the depressions in the different groups
differs, but the
depressions within each group are identical.
Tests have shown that the form and extent of the depressions 63 is very
important with
respect to the runnability of the tissue machine and its ability to produce a
tissue paper web
of good quality, i.e. high bulk of 8-20 cm3/g and high softness.
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In order to achieve an optimum structure and dryness of the web, it is
important that the
structuring belt 14 allows the wet fibrous web l' to be formed into the
depressions 63 when
the fibrous web 1' passes through the press nip Ni together with the press
felt 17 and the
structuring belt 14 with the wet fibrous web 1' enclosed therebetween. It is
also important
5 that the press felt 17 can reach down into all of the depressions 63
during the pressing
process in order to build up a sufficiently high hydraulic pressure so that
water in the wet
fibrous web l' can move into the press felt 17 and not remain in the fibrous
web at the end of
the pressing operation. The depressions 63 must be sufficiently large to allow
the press felt
17 to penetrate into the depressions 63. Each depression 63 must have an
optimum depth
10 which allows water in the bottom of the depression 63 to be transported
away. In other
words, the depth of the depression 63 must not be too great, as an excessive
depth will
prevent the desired hydraulic pressure from building up.
The structuring layer 60 with this specific well-defined, structured, web-
contacting surface
61 is an important parameter for controlling the structure, thickness/bulk and
dryness that can
be expected in the structured and dewatered fibrous web 1" after the press nip
Ni before final
drying. It is taken for granted that the pressure in the press nip Ni is
within the normal ranges
conventionally used for pressing, normally a maximum of 6 MPa, and that the
press felt 17 is
of the conventional elastically compressible type, which, in addition to its
required
water-receiving capacity during compression, forms elastically into the web-
contacting
surface of the structuring layer with the wet fibrous web located therebetween
in the manner
and for the purpose specified above.
Each depression 63 has a predetermined dimension 1 in the machine direction
(MD) of the
structuring layer 60 and a predetermined dimension b in the cross direction
(CD) of the belt
14. The depressions 63 may be oriented in the machine direction, in which case
1> b, or in
the cross direction, in which case 1 <b. However, the depressions 63 are
preferably oriented
substantially in the machine direction, as this gives better creping and
results in a softer tissue
paper. It should be noted here that woven structuring clothings normally have
a pattern that is
MD-oriented.
Each depression 63 also has a predetermined depth d, a predetermined area a
and a
predetermined volume v. The depth d of the depressions may be constant over
substantially
all of the depression 63, in which case the depression 63 has a bottom surface
71 which is flat
and parallel to the surface area 70. The depth d may alternatively vary over
the surface of the
depression 63 and then an average depth or mean depth d is preferably used to
characterise
the extension of the depression 63 in the z-direction.
The depressions 63 are arranged at a predetermined distance from each other so
that they are
distributed in a uniform manner over the web-contacting surface 61 and cover a
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predetermined part thereof Thus, the abovementioned continuous top surface
area 70, which
delimits the depressions 63 and constitutes the part of the web-contacting
surface 61
cooperating with the drying surface 20 when the fibrous web 1" is transferred
to the drying
cylinder 19, constitutes the remaining part of the web-contacting surface 61.
The abovementioned parameters must therefore cooperate in order to obtain in
good
runnability and good quality of the tissue paper web 1. Tests have shown that
the following
parameters should be fulfilled in order to achieve this:
1 [mm] h [mm] d [mm] a [mm2] y
[mm]
0.25-2.5 0.25-2.0 0.05-0.6 0.3-4.0 0.05-
1.0
The abovementioned parameter value a should be measured in the plane of the
top surface
area 70. However, tests have shown that a should preferably be within the
range of
0.5-2.0 mm2.
It is recognized that the structuring belt 14 is compressed when it passes
through the nip Ni
between the press elements 12 and 13. The abovementioned range for d applies
when the
structuring belt 14 and therefore also the depressions 63 are in the
compressed state, i.e. when
the structuring belt 14 is passing through the nip Ni. The press pressure in
this nip has
normally a maximum of 6 MPa. When it is stated herein that the structuring
belt 14 is in the
compressed state, this refers to the fact that it is loaded with a pressure of
a maximum of 6
MPa. The depressions 63 in the non-compressed state can consequently have a
depth d
greater than 0.6 mm, but in the compressed state, i.e. in the nip Ni, d should
not exceed 0.6
mm. In the case where the depth of the depressions 63 varies, the value d
refers to the mean
depth of the depression. However, under no circumstances should the greatest
depth of the
depression exceed 0.6 mm when the depression is in the compressed state.
In addition to the abovementioned parameter values, the depressions should
altogether cover
between 20 % and 80 % of the total web-contacting surface 61.
A creped, reeled tissue paper having the following properties can be
manufactured in a tissue
papermaking machine provided with a structuring belt with a structuring layer
as above:
Basis weight 10-50 g/m2
Thickness 160-400 tm, preferably 200-300 [tm
Bulk 8-20 cm3/g
MD tensile strength 50-300 N/m
CD tensile strength 30-250 N/m and
Softness 70-90
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12
The above values refer to a paper conditioned at 20 C and 50 % atmospheric
humidity. The
softness value is measured according to EMTEC TSA (Tissue Softness Analyzer)
with a
measuring scale from 0 to 100. The above bulk and softness values should be
compared with
those for conventional creped tissue paper, which has a bulk in the range of 5-
9 cm3/g and a
softness in the range of 50-70.
More specifically, tissue paper of the qualities facial, i.e. facial tissues,
toilet paper and
household paper can be manufactured by a tissue papermaking machine according
to the
invention, said tissue paper having the following properties:
Facial Toilet paper Household paper
Basis weight [g/m2] 13-15 15-25 18-23
Bulk [cm3/g] 10-13 10-15 10-14
MD tensile strength [N/m] 70-120 50-150 170-300
CD tensile strength [N/m] 50-100 30-100 170-300
Figure 11 shows a first embodiment of a structuring belt 14 with a structuring
layer 60
according to the invention, said structuring layer 60 including reinforcing
means 57 and
being arranged on a wear layer 58. Figure 12 is a partial view of this belt 14
in a cross section
in the machine direction (MD). The web-contacting surface 61 of the
structuring layer 60 has
a plurality of identical depressions 63 in the form of recesses or pockets,
arranged in parallel
rows 72, extending in the machine direction of the belt 14. Adjacent rows 72
are displaced by
approximately half the length of a pocket relative to each other in the
machine direction.
Each depression 63 is substantially in the form of a square block with
cylindrical ends, said
square block extending in the machine direction of the belt 14. The bottom
surface 71 of each
depression 63 is flat and parallel to the continuous top surface area 70. The
side walls 73 of
the depression 63 form a substantially 90 angle relative to the bottom
surface 71 of the
pocket. The depressions 63 have a dimension 1 in the machine direction of 2.0
mm and a
dimension b in the cross direction of 1.0 mm. The depth d is 0.3 mm. The
depressions 63
have an area a in the range of 0.3-4.0 mm2, and preferably 0.5-2.0 mm2, e.g.
approximately
1.8 mm2, and a volume v of 0.05-1.0 mm3, preferably approximately 0.54 mm3.
The distance
between two adjacent depressions 63 in the machine direction s is
approximately 1.0 mm.
The distance between two adjacent rows 72 of depressions 63 in the cross
direction t is
approximately 0.5 mm. The depressions 63 cover approximately 40 % of the web-
contacting
surface 61.
Figure 13 shows a second embodiment of a structuring layer 60 of a structuring
belt 14
according to the invention. The structuring layer 60 of the belt 14 has
depressions 63 of
substantially the same form and arranged in the same manner as the depressions
described
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13
above. In this case, the depressions 63 have a dimension 1 in the machine
direction of 1.0
mm, a dimension b in the cross direction of 0.5 mm, a depth d of 0.2 mm, an
area a of
approximately 0.3-4.0 mm2, e.g. 0.45 mm2, and a volume v of approximately 0.09
mm3. The
distance between two adjacent depressions 63 in the machine direction s is 0.5
mm. The
distance between two adjacent rows 72 of depressions 63 in the cross direction
t is 0.5 mm.
Figure 14 shows a third embodiment of a structuring layer according to the
invention, said
structuring layer also having the depressions 63 of substantially the same
form and arranged
in the same manner as the depressions described in connection with Figure 11.
In this case,
the depressions 63 are slightly larger than the depressions shown in Figure 13
and have a
dimension 1 in the machine direction of 0.5 mm, a dimension b in the cross
direction of 1.0
mm, a depth d of 0.4 mm, an area of approximately 1.3 mm2 and a volume v of
approximately 0.51 mm3. The distance between two adjacent depressions 63 in
the machine
direction s is 0.5 mm. The distance between two adjacent rows 72 of
depressions 63 in the
cross direction t is 0.5 mm.
Figure 15 shows a further embodiment of a structuring layer according to the
invention. In
this case, the depressions 63 are formed by recesses or pockets, which, except
for rounded
inner corners, are substantially entirely rectangular or formed as square
blocks. The
depressions 63 are arranged in rows 72 extending in the machine direction of
the belt 14 and
columns 74 extending in the cross direction of the belt 14. In this
embodiment, the
depressions 63 have a dimension 1 in the machine direction of 2.0 mm, an
extent b in the
cross direction of 2.0 mm, a depth d of 0.2 mm, an area of approximately 3.9
mm2 and a
volume v of approximately 0.79 mm3. The distance between two adjacent
depressions 63 in
the machine direction s is 1.0 mm. The distance between two adjacent rows 72
of depressions
63 in the cross direction t is 1.0 mm.
Figure 16 shows an alternative embodiment of a structuring layer according to
the invention,
in which the structuring layer instead of recesses is provided with elevations
62 in the form of
projecting portions or "islands" in the otherwise flat, continuous lower
surface area 76. The
same parameter values specified above in the case of the structuring layer
with recesses also
apply to this variant of the structuring layer, with the difference that the
value d in this case
gives the height of the elevations. In the embodiment shown in Figure 16, the
elevations 62
are in the form of square blocks projecting approximately 0.2 mm from the
lower surface
area 76 and having slightly rounded outer corners. The square blocks are
approximately 1
mm long and 1 mm wide and are arranged in rows extending diagonally in the
machine
direction of the structuring belt 14. The elevations consequently have a
dimension 1 in the
machine direction and a dimension b in the cross direction of approximately
1.4 mm in each
case. Each elevation 62 has an area a of approximately 0.95 mm2 and a volume v
of
approximately 0.19 mm3. The distance between adjacent elevations is
approximately 0.5 mm
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14
and the elevations 62 consequently cover approximately 42 % of the web-
contacting surface
61. The upper surface areas 75 of the elevations 62 are preferably flat so
that they cooperate
with the drying surface 20 when the fibrous web 1" is transferred to the
drying cylinder 19.
The structuring layer according to the invention is preferably made of a
polymer material,
e.g. polyurethane, the depressions 63 or the lower surface area 76 preferably
being formed in
said structuring layer in that material is cut from the surface of the
structuring layer. The
structuring layer 60 may alternatively be made of a different material, e.g.
metal or carbon
fibre, and other techniques may be used to form the depressions or said lower
surface area.
The structuring layer 60 is preferably approximately 3-6 mm thick, but its
thickness may be
between 0.2 and 10 mm.
The structuring belt is preferably substantially water-impermeable as
mentioned for the tissue
papermaking machines shown. Alternatively, the structuring belt may be water-
permeable.
E.g. the structuring layer may be needled so that it has through holes. The
depressions 63 or
the surface area 70 surrounding the depressions, or both, may be needled. In a
similar
manner, the elevations 62 and/or said lower surface area 76 may be needled.
When it is stated
that the structuring belt is needled, this refers to the fact that the
structuring belt has small,
through openings, said openings allowing water, but not paper fibres to pass
therethrough.
In order to increase the service life of the structuring belt 14, as described
above in
connection with Figure 11, the structuring belt 14 may comprise a wear layer
58, e.g. in the
form of a felt layer arranged on the side of the structuring belt 14 directed
away from the
fibrous web 1'. Like the structuring layer 60, the wear layer 58 may be
needled.
In order to increase the strength of the structuring belt 14, the structuring
belt 14 may
comprise reinforcing means 57, e.g. in the form of reinforcement wires
arranged within the
structuring layer 60. The reinforcing means may alternatively be formed by a
metal strip or a
fabric arranged within the structuring layer 60.
With the aid of a structuring belt 14 according to the invention, it is thus
possible to
manufacture a tissue paper web which, after creping from the drying surface 20
and
conditioning at 20 C and an air humidity of 50 %, has a basis weight in the
range of
10-50 g/m2, a thickness in the range of 160-400 [im, preferably 200-300 lam, a
bulk in the
range of 8-20 cm3/g, an MD tensile strength in the range of 50-300 N/m, a CD
tensile
strength in the range of 30-250 N/m and a softness in the range of 70-90 as
measured
according to EMTEC TSA (Tissue Softness Analyzer) with a measuring scale of 0
to 100.
Figure 17 is a cross section through a tissue paper web 1 manufactured by a
structuring belt
including depressions according to the invention. By virtue of the three-
dimensional structure
of the structuring layer 60, the finished tissue paper web 1 has a varying
thickness, wherein
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the thickness of the tissue paper web 1 is greater in those portions 77 in
which the tissue
paper web 1 has been formed by the top surface area 70 than in those portions
78 in which
the tissue paper web 1 has been formed by the depressions 63 of the
structuring belt 14.
The fibrous web l', 1" preferably comprises a short-fibre layer and a long-
fibre layer,
5 wherein the fibrous web 1', 1" is transferred to the drying surface 20 in
the transfer nip N2
with the short-fibre layer directed towards the drying surface 20. The
finished tissue paper
web 1 thus preferably also has a short-fibre layer on one side 79, i.e. the
side which has been
in contact with the drying surface 20, and a long-fibre layer on its other
side 80, i.e. on the
side which has been in contact with the structuring belt 14. Figure 18 shows
the long-fibre
10 side 80 of the tissue fibre web 1.
The invention has been described above by way of a number of embodiments.
However, it
will be clear that other embodiments or variants are within the scope of the
invention. E.g. it
will be clear that alternative embodiments of the depressions or elevations
are possible
without going beyond the scope of the invention as defined in the claims.
Alternative
15 embodiments of this kind comprise, e.g. circular, rhombic or elliptical
depressions or
elevations, the longitudinal axes of which do not necessarily have to be
situated in the
machine or cross direction of the structuring belt, but may form an acute
angle therewith.
25
P2196PC TEl 090115
