Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD AND A SYSTEM FOR MAKING A FIBRE-CONTAINING
PRODUCT
Field of the invention
The invention relates to a method for making a fibre-containing product. The
invention also relates to a system for making a fibre-containing product.
Background of the invention
Fibre-containing sheets with a relatively large content of nanofibrillated cel-
lulose (NFC) has been found to have properties of particular interest. The
relative proportion of the surface of a piece with respect to the volume of
the
piece increases when the piece becomes smaller, for which reason the prop-
erties of nanoparticles often differ to a great extent from the properties of
macroscopic pieces. Properties which can be achieved with nanofibrillated
cellulose and are of interest to the industry, include, for example,
particularly
high tensile strength, very low porosity and partial translucency. Among other
things, these properties make nanofibrillated cellulose a sought-after raw
material, and products containing it would have a variety of industrial uses.
For the present, the application of nanofibrillated cellulose as a raw
material
for a fibrous product, such as paper or paperboard, is still limited by
various
problems in practice. Nanofibrillated cellulose is a gel-like material even at
low consistencies, and the removal of water from fibre materials with a high
content of nanofibrillated cellulose is often slow. Furthermore,
nanofibrillated
cellulose sticks easily to the wire used, because small particles of
nanofibril-
lated cellulose adhere particularly strongly to the wire cloth, which may
cause
problems in the manufacturing process of the fibrous product containing
nanofibrillated cellulose; due to the strong adherence of the particles, the
detachment of the web from the wire requires a relatively great force. Prob-
lems caused by the adherence to the wire are often pronounced because wet
nanofibrillated cellulose has a relatively low internal strength, for which
rea-
son the force required for detaching the web from the wire may be greater
than the internal strength of said wet fibre suspension. Particularly in the
case that the content of nanofibrillated cellulose in the pulp suspension is
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relatively high, the internal wet strength of the web may easily remain
so low in relation to the forces between the web and the wire that the
detachment of the intact web from the wire is particularly problematic.
In papermaking processes of prior art, the web is normally detached from the
wire at a very early stage when the web still has a low dry matter content,
for
which reason the addition of nanofibrillated cellulose into the pulp
suspension
in web forming machines may lead to particularly low running speeds of the
web forming machines, which, in turn, may reduce substantially the cost effi-
ciency of the grade to be run. Furthermore, this problem may be pronounced
because dewatering of the web that contains nanofibrillated cellulose may be
slower than normal.
In addition to the problems mentioned above, an additional problem in the
use of nanofibrillated cellulose may be the fact that the present wires com-
monly used in production are normally relatively coarse for small particles,
wherein the degree of retention of nanofibrillated cellulose may remain at a
particularly low level when such wires are used.
Under laboratory conditions, said problem of detaching the pulp from the wire
and the problem of the density of the wire have been resolved, among other
things, by using a particular small mesh wire, a so-called fine mesh wire,
whose density may be, for example, 200 mesh. In this way, a sufficiently high
degree of retention of nanofibrillated cellulose is normally achieved. In addi-
tion to first making the fibre sheets with a high content of nanofibrillated
cel-
lulose on the fine mesh wire, said fibre sheets have also been dried, under
laboratory conditions, on top of the fine mesh wire until the dry matter
content
of the sheets has increased. The dry strength of sheets that substantially
contain nanofibrillated cellulose is considerably higher than their wet
strength, so that a sheet dried to a sufficient degree can normally be
detached from the long wire without problems.
The fibrous web tends to shrink when it dries. Because the web-like product
is typically kept tight in the longitudinal direction of the web, problems in
the
drying shrinkage normally appear in the cross direction of the web. These
problems may occur both in the manufacture of conventional fibrous products
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and in the manufacture of fibrous products that contain substantially
nanofibrillated cellulose. Because the drying shrinkage takes place when the
web dries, it might be possible to overcome the problem by operating in such
a way that the web would be longer than normally in contact with the wire
cloth, on which the web is formed.
In large scale, for example in paper factories, there has been no practical
solution to these problems mentioned above. Thus, there would be a need
for solutions, by which a fibrous product could be made by better approaches
in the product processes in various web forming machines or corresponding
machines.
Brief summary of the invention
The aim of the present invention is to find a solution to the above-mentioned
problem caused by the low dry matter content in such a way that the fibrous
product is kept for a sufficiently long time in contact with the wire cloth,
on
which the web is formed. According to an advantageous embodiment, the
aim is to alleviate the problem of detaching the fibrous web that contains
nanofibrillated cellulose, from the wire in the manufacture of a web-like
fibrous product by drying the web to a sufficient extent before the web is
detached from said wire. In another advantageous embodiment, the aim is to
reduce and/or to control the drying shrinkage of the web in the cross direc-
tion.
In the solution according to the invention, the detachment of the web from the
wire can be performed at such a stage where the dry matter content of the
web reaches a predetermined limit value. In an advantageous embodiment,
this is achieved when the internal strength of the web is greater than the
forces effective between the web and the wire. In this way, the web can
remain intact when detached from the wire. According to an advantageous
embodiment, considerable amounts of nanofibrillated cellulose can be
included in the product in such a way that the web can still be detached from
the wire without breaking the web. According to another advantageous
embodiment, the web can be detached from the web forming wire when the
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dry matter content of the web is such that the drying shrinkage of the web
in the cross direction has been reduced to a desired extent.
According to an embodiment of the invention, the web is supported on at
least one wire from the formation of the web to at least the press section,
for
examplepast at least the first press nip of the press, preferably to the
drying
section of the web forming machine. In this case, the web is supported by the
same at least one wire substantially continuously all the way from the web
formation to at least said stage. In one embodiment, the web to be made is
supported by at least one and the same wire substantially to at least half-way
of the press section. According to one embodiment, the web is supported by
the same wire from the formation of the web at least past the first drying
group in the drying section, preferably substantially to half-way of the
drying
section. In one embodiment, the web is supported by the same wire past the
whole drying section, for example to the point where the web is close to the
first calender, the coating unit or the machine roll of the web forming
machine.
In one embodiment, the web is supported by only one long wire, that is, on
only one side of the web. In an advantageous embodiment, in which a long
wire is used, the mesh size of said long wire is advantageously selected in
such a way that it is possible to improve the retention of the fibre material
that
contains nanofibrillated cellulose. In one embodiment of the invention, the
web is supported by wires that are denser than the wires commonly used in
papermaking, so-called fine mesh wires. In one embodiment example, these
fine mesh wires are provided on both sides of the web.
In one embodiment of the invention, the long wire in contact with the web is
supported by separate wires which may be coarser than the long wire. In this
case, the long wire is preferably placed between the pulp and the normal
wire. Several of these so-called normal wires can be placed along the whole
path travelled by the long wire, or they can be placed, for example, in the
drying section only. In an advantageous embodiment, the web is supported
by long wires on both sides at least all the way to the drying section, and
normal wires are either not applied at all or they are applied in the drying
section only.
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The method for making a fibre product according to the present invention is
primarily characterized in a method for producing paper, paperboard or
corresponding fibrous product, in which method a pulp suspension is supplied
onto a first wire (8) in such a way that a web (10) is formed, and the formed
5 web (10) is dried, wherein the method the web (10) is kept in contact
with said
same first wire (8) after the formation of the web (10) continuously from the
web forming section (3) to the first press nip in the press section,
characterized
in that nanofibrillated cellulose is included in the web and that the content
of
nanofibrillated cellulose in the web, calculated in dry matter content, is at
least
10 wt-%.
Description of the drawings
In the following, the invention will be described in more detail with
reference
to the appended drawings, in which
Fig. 1 is a side view showing an example embodiment of a web forming
machine with a wire solution according to an example, and
Fig. 2 shows an advantageous detail of the wire solution according to
the embodiment shown in Fig. 1.
Detailed description of the invention
In this application, a web forming machine refers to a machine for making a
web that contains fibrous material. Such a machine may be, for example, a
paper machine, a paperboard machine, or another corresponding machine.
A web refers to a planar product running in the form of a continuous web
whose basic structure is a mesh formed at least partly of fibres or parts of
fibres. At least some of the fibres contain cellulose. In addition, the web
may
contain various quantities of other substances, such as various fillers, sur-
factants and additives. Furthermore, water is normally used in the manufac-
turing process. The final product of the manufacturing process may be, for
example, paper, paperboard, thin wood-plastic composite, or a corresponding
product.
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5a
A fine mesh wire refers, in the present application, to a dense wire with a
mesh
number of at least 130, 140 or 150. The fine mesh wire may also be
denser, such as, for example, a wire with a mesh number of at least 180, at
least 200 or at least 220. A normal wire refers, in the present application,
to a
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wet wire or a drying wire of prior art, normally used in a web forming
machine. The mesh number indicates the number of filaments in a filter, such
as in a wire, per one inch.
A long wire refers, in the present application, to a wire that extends substan-
tially from the web forming section (wire section) to at least the press
section,
preferably to the first press nip in the press section.
Normal wires 9 refer to commonly used wires, including both wet wires and
dry wires. Normal wires 9 can also be called second wires 9 in the present
application.
Nanofibrillated cellulose (NFC) refers to such fibrous material in which
single
microfibrils have been separated from each other. Nanofibrillated cellulose
particles typically have a length not greater than 1 pm, and their diameter is
smaller than 1 pm, typically from about 2 nm to 200 nm. The dimensions of
nanofibrillated cellulose particles are dependent on the method of production
of the nanofibrillated cellulose. Contrary to conventional grinding of
cellulose,
in the production of nanofibrillated cellulose the aim is to crush the
cellulose
fibres. In its appearance, nanofibrillated cellu.lose is typically
transparent, gel-
like material.
Figure 1 shows an advantageous embodiment of the invention. Figure 2
shows an advantageous detail of the wire solution according to the embodi-
ment of Fig. 1. Figures 1 and 2 show a web forming machine 1, a head box 2,
a web forming section (wire section) 3, a press section 4, a drying section 5,
a calender 6, a reel-up 7, a long wire 8, normal wires 9, and a web 10.
In the solution according to the invention, one or more normal wires 9 may be
provided, but it is also possible that the solution according to the invention
does not have any normal wires 9. The normal wires 9 can be left out par-
ticularly from those sections of the machine, in which there is at least one
long wire 8. In an advantageous example, the long wire 8 is preferably a fine
mesh wire. According to an advantageous example of the invention, normal
wires 9 can be used in the web forming machine substantially in the same
positions as the long wires 8, in such a way that the normal wire is placed at
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least partly between the long wire 8 and the rolls of the web forming
machine. In this way, the normal wire 9 may prevent the wear of the long wire
8 or long wires 8. This may be advantageous because the long wire 8 may
be a relatively expensive wire, in which case it is often less expensive to
replace the normal wires 9 than the long wires 8.
The web 10 is typically supplied from the headbox 2 to the web forming sec-
tion 3. In the machine according to the invention, there is at least one long
wire 8, and the suitable number of long wires is normally dependent on the
type of the web 10 forming machine 1. Normally, the number of long wires 8
is exactly one or exactly two, depending on the type of the web forming
machine 1. The long wire 8 can be supported by a normal wire 9(a) in the
web forming section 3. In this case, the normal wire 9(a) can delay the wear
of the long wire 8.
From the web forming section 3, the web 10 is carried, supported by at least
one long wire 8, forward in the drying process typically to the press section
4,
where the long wire 8 can be supported by normal wires 9b, 9c of the press
section, in which case said normal wires 9b, 9c can delay the wear of the
long wire 8. In the press section, one or both of the normal wires 9b, 9c can
also be eliminated from the solution according to the invention, if they have
been replaced by the long wire 8. In an advantageous embodiment, the long
wire 8 is placed to support the web 10 from the web forming section 3 past
the first press nip of the press section 4, after which the long wire is led
to the
web forming section again. According to another advantageous embodiment,
the long wire 8 is placed to support the web past the second press nip and/or
the third press nip in the press section 4, after which the web is detached
from the long wire 8, which long wire 8 is then led to the web forming section
3.
From the press section 4, the web 10 is advantageously led further to the
drying section 5. In an advantageous embodiment, the web 10 is led, sup-
ported by at least one and the same long wire 8, from the web forming sec-
tion 3 to the drying section 5. In an advantageous embodiment, the long wire
is placed in such a way that the web 10 is in contact with the same long wire
8 from the web forming section 3 all the way to the desired drying group in
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the drying section 5, such as the first drying group, the second drying
group, the third drying group, the fourth drying group, or the fifth drying
group. The web 10 is then detached from the long wire 8, after which the web
is led forward in the process, wherein it can be, for example, in contact with
normal wires 9, as in conventional papermaking. The drying section 5 may be
provided with one or more normal wires 9d-9g in addition to the long wire 8.
At least some of the normal wires 9 may be provided substantially in the
same location as the long wire 8 in the drying section 5 of the web forming
machine 1, being placed between the long wire 8 and one or more rolls of the
web forming machine 1, wherein said normal wires 9 may delay the wear of
the long wire 8.
One long wire 8 may be provided, in which case the long wire 8 may be
placed on one side of the web 10, or, for example, two long wires 8 may be
provided in such a way that the first long wire 8 is placed on one side of the
web 10 and the other long wire is placed on the other side of the web. If two
long wires 8 are provided, normal wires 9 may be placed, for example, in
connection with one long wire 8, if desired, or in connection with both long
wires 8. In some cases, the placement of normal wires 9 in connection with
both long wires 8 may be preferable, for example to prevent the wear of the
long wire 8. If there are two long wires 8 in the machine, that is, preferably
one long wire 8 on each side of the web, the detachment of the web from the
long wires 8 may take place either substantially at the same time or substan-
tially at different times, that is, one by one in such a way that the web 10
is
first detached from one long wire 8 and then from the other long wire 8. In
many cases, if only possible in view of the functionality of the process, it
is
more desirable to use only one long wire 8, to minimize the costs. In some
cases, however, it is necessary to use two long wires 8 on at least part of
the
drying length of the web. This kind of a situation may come up, for example,
in connection with a so-called gap former.
The long wire 8 is placed in the web forming machine 1 advantageously in
such a way that the web 10 is in continuous contact with the long wire 8 at
least from the web 10 forming section 3 to the press section 4 of the web 10,
for example to half-way of the press section, or all the way to the drying sec-
tion 5. If the machine 1 is provided with normal wires 9 in contact with the
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long wire 8, the long wire 8 is placed preferably in such a way that it is
between at least one normal wire 9 and the web 10 on at least one side of
the web 10. The long wire 8 and the web 10 are preferably kept in contact
with each other until the web 10 is sufficiently dry to be detached from the
long wire 8. The desired dry matter content of the web can be determined, for
example, in such a way that the web is intact when detaching from the wire,
and/or in such a way that the drying shrinkage in the cross direction of the
web can be prevented. After this, the web 10 can be supported by normal
wires 9, if necessary.
The number and locations of possible normal wires 9 may vary even to a
great extent from some advantageous embodiments of the invention
described above. An essential aspect is that the long wire 8 is placed in the
web forming machine 1 in such a way that said long wire supports the web
10 in a continuous manner from at least the web forming section 3 to the
press section 4, more preferably to the drying section, or, for example, all
the
way to a possible first calender or machine roll. Usually, inserting the long
wire 8 through the calender 6 is not necessary for the desired final result,
and
in view of the desired final result it may be preferable not to convey the
long
wire 8 through the calender 6.
Preferably, the long wire 8 is provided with cleaning members for cleaning
the long wire 8 or part of it during its each lap. This can be implemented, in
practice, for example by installing suitable cleaning members of prior art,
such as jets, for cleaning the long wire 8. Preferably, this cleaning of each
point of the long wire 8 always takes place after the web 10 has been
detached from the long wire 8 but before said point of the long wire 8 is at
that point of the web forming machine again where the web 10 is supplied to
the web forming section (wire section) of the web forming machine.
To secure the efficient drying of the web 10, it is often advantageous to pro-
vide the web forming section 3 and/or the press section 4 with some separate
means or devices for accelerating the drying. These may be means or
devices of prior art, such as, for example, suction boxes and/or suction
rolls.
Particularly in the case of two wires on top of each other, there may be a
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need to provide more such devices than normally, and/or these devices
may be more efficient than normally.
The web 10 can be detached from the long wire 8 at the stage where the dry
5 matter content of the web 10 is sufficient. The dry matter content may be
suf-
ficient, for example, when it is at least 8 wt-')/0, 9 wt-')/0, 10 wt-')/0, 11
wt-%,
12 wt-%, 13 wt-%, or 14 wt-%, but e.g. due to the structure of the wire, it
may
also be advantageously at least 15 wt-%, 18 wt-%, 20 wt-%, 25 wt-%,
30 wt-%, 40 wt-%, or at least 60 wt-%. The suitable dry matter content is
10 influenced, among other things, by the apparatus for drying the web and
par-
ticularly by the properties of the wire in contact with the web.
In one example, thanks to the invention, the content of nanofibrillated cellu-
lose in the pulp suspension can be set high, wherein it is possible to
achieve,
for example, the desired properties in the final product. Thus, by applying
the
long wire solution according to the invention, it is also possible to include
relatively large quantities of nanofibrillated cellulose in the pulp, for
example
in such a way that at least 10 wt-%, at least 20 wt-%, at least 30 wt-%, at
least 40 wt-%, at least 50 wt-%, at least 60 wt-%, or at least 70 wt-% of the
dry matter of the web 10 consists of nanofibrillated cellulose. If necessary,
for
example when required by the properties of the final products, it is possible
to
include even so much nanofibrillated cellulose that at least 80 wt-% or even
at least 90 wt-% of the dry matter of the web 10 consists of nanofibrillated
cellulose. In some cases, the solution of the invention may be more useful
when the content of nanofibrillated cellulose in the web increases, because in
this case the detachment of the web from the wire may become more difficult
than before. By the solution according to the invention, it may be possible to
prevent partly or even totally a situation in which the machine technical prop-
erties would become a hindrance to increasing the quantity of nanofibrillated
cellulose.
According to an example solution, supporting the web to the wire by signifi-
cant forces may prevent the drying shrinkage of the web in the cross direc-
tion, when the supporting is continued for a sufficiently long time or even
substantially along the whole drying path of the web. Particularly in this
case,
the invention may also be useful in the manufacture of a so-called normal
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fibrous web, among other things for controlling the drying shrinkage in the
cross direction. In the manufacture of a web that contains nanofibrillated
cellulose, an additional advantage can be achieved in that the addition of
nanofibrillated cellulose has often been found to have an effect that
increases
the drying shrinkage, particularly in the cross direction of the web. When
solutions of prior art are used, the drying shrinkage of a web of said kind in
the cross direction may thus even exceed 10%. Most of the drying shrinkage
often takes place at the end of drying, in which case keeping the web on the
long wire 8 may prevent the drying shrinkage substantially, particularly when
it takes place from the web forming section substantially to the end of the
drying section.
Even though the invention is particularly important for pulps that contain
nanofibrillated cellulose, the invention can also be applied in connection
with
such fibrous pulps which do not contain nanofibrillated cellulose. In this way
it
may be possible to substantially reduce web breaks, for example in the press
section. Furthermore, the control of the shrinkage of the web in the cross
section may be facilitated. The invention is suitable for application in
various
web forming machines, and the above-presented examples are not intended
to restrict the invention. Thus, the invention is not limited solely to the
exam-
ples presented in Figs. 1 and 2 and in the above description, but the inven-
tion is characterized in what will be presented in the following claims.
