Note: Descriptions are shown in the official language in which they were submitted.
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Application of a layer of admixture in the web former
unit of a board machine
5
The invention concerns a method for application of a layer of admixture in the
web
former unit of a board machine.
i0 As is well known, board is almost always made of two or more separate
layers. In
such a case, the webs formed by two or more wire parts are combined one on top
of the other before the press section. In the case of webs produced by means
of a
conventional fourdrinier wire technique, combining of the surface sides of the
webs
that are not in contact with the wire does, as a rule, not involve problems,
and in
15 such a case the strength between the layers in the Z-direction of the web
thus
obtained, i.e. the "ply bond", is usually adequate. However, when the
conventional
fourdrinier wire technique is employed, the running speeds are clearly lower
than
1000 metres per minute. As the running speeds of the present-day, newer board
machines rise to a level higher than 1000 metres per minute, it is necessary
to
20 employ other techniques, for example the gap former technique. In such a
case, it
becomes a problem that the combination of two or more webs formed in a gap
former does not meet the requirements of strength that have been imposed. The
bond
strength between the different layers in the web is reduced. What is concerned
in
particular is the Z-direction strength represented by the what is called
"scott bond" .
25 The poor strength is contributed to by an inadequate concentration of fines
on the
faces of the layers to be combined. An adequate concentration of fines on the
faces
to be combined promotes the ply bond between the faces. It has proved
problematic
to produce board by means of a machine in which the so-called wire sides are
combined one against the other. This would be the case always if all the web
30 formers were gap formers. This problem sometimes also occurs in combining
of
webs formed both by means of a hybrid former and by means of a fourdrinier
wire.
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Attempts have been made to solve the problem, among other things, by spraying
starch as a solution or by spraying some other auxiliary agent that increases
the bond
strength directly onto the web or by introducing a thin layer of extensively
ground
cellulosic pulp, for example, from a separate headbox, i.e. from a secondary
headbox, onto one of the webs to be combined. One mode has been to grind the
stock to be passed to one of the layers to an extent clearly higher than
normal.
In the following, some prior-art solutions will be described in more detail.
In the FI Patent No. 71,377, a method for manufacture of mufti-layer board is
described, in which method the stock suspension that forms one of the surface
layers
is fed onto the horizontal portion of the first wire. Between the layers, a
third stock
suspension, which forms the intermediate layer, is fed, ahead of the wedge-
like gap
that is formed by the second wire, onto the web layer that was already formed.
After
this, the first wire and both of the web layers are passed into contact with
the third
wire, onto whose horizontal portion the stock suspension is fed which forms
the
other surface layer. The consistency of the stock suspension that forms the
inter-
mediate layer is considerably higher than the consistencies of the stock
suspensions
that form the surface layers.
In the US Patent No. 5, 607, 555, a paper machine for manufacture of mufti-
layer
paper is described. In this machine, there are at least two twin-wire formers,
which
produce a mufti-layer paper web, in which connection a paper is obtained whose
properties on both faces are equal.
In the Finnish Patent No. 92, 729, a stock feed system for a mufti-layer
headbox and
a method in the operation of a mufti-layer headbox are described. In this
solution,
into each inlet header in the mufti-layer headbox, a stock produced out of the
same
fresh stock is introduced, to which stock the necessary chemicals and fillers
have
been added.
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The prior-art solutions, however, involve a number of significant drawbacks. A
separate headbox that sprays starch or fibre stock is an expensive solution,
and its
use is confined to relatively low speeds, which remain lower than 1000 metres
per
minute. The starch jets deteriorate the runnability considerably, they block
the wires
5 and felts and contaminate the machines, in which case the equipments have to
be
stopped and cleaned rather frequently. Thus, the necessary auxiliary agents
cannot
be added selectively either. In most systems, a number of separate stock lines
are
needed, in which case the system is rather heavy and expensive to operate.
Grinding
of one of the stock layers deteriorates the draining of water clearly, and
thereby it
10 deteriorates the runnability and the efficiency of production and increases
the costs.
In an attempt to attain speeds higher than 1000 metres per minute, such
restrictions
cannot be tolerated, and a separate jet of admixture that promotes the binding
together of the different layers would not be even possible at these speeds.
15 The method in accordance with the invention for application of a layer of
admixture
in the web former unit of a board machine, the method for manufacture of
board,
and the board are mainly characterized in what is stated in the patent claims.
The invention will be described in the following in more detail with reference
to
20 some preferred embodiments of the invention illustrated in the figures in
the accom-
panying drawings, the invention being, however, not supposed to be confined to
said
embodiments alone.
Fig. 1 illustrates a preferred embodiment of the invention, which is in
particular
25 suitable for manufacture of board and in which the stock flow produced out
of the
same fresh stock is divided into two component flows. To the face that will be
placed against the face of the layer to be combined, the necessary admixtures
are
added in order to increase the fines content in the layer and to promote the
bond
strength between the faces to an optimal extent at an optimal point. The
component
30 flows are passed into a multi-layer headbox and from it into a gap former.
In this
way, good properties of formation and strength and good internal bond strength
are
obtained for the board. Thus, out of the same fresh stock 11, two component
flows
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121 and 122 are produced. To the component flow 121, out of which the face
will be
formed that will be placed against the face of the layer to be combined, at
the point
141 before the pump 22, at the point 142 after the pump 22, and/or at the
point 143
after the machine screen 33, the admixtures are added in order to increase the
fines
5 content in the layer and the bond strength between the faces. After this the
compo-
nent flows 121 and 122 are passed into the multi-layer headbox 44, which
comprises,
in the conventional way, inlet headers, a tube manifold connected with each
inlet
header, an intermediate chamber connected with the tube manifold, a turbulence
generator and turbulence tubes, a slice cone after the turbulence generator,
and vanes
10 that separate the layers in said slice cone. From the headbox the web is
transferred
into the former, in which 551 represents the first forming wire, 552 the
second
forming wire, 661 represents a former roll, and 662 the forming roll.
Fig. 2 illustrates a second preferred embodiment of the invention, in which
the stock
15 flow produced out of the same fresh stock is divided into three component
flows, of
which flows, onto the faces that will be fitted against the faces of the
layers to be
combined, the necessary admixtures will be added in order to increase the
fines
contents in the layers and to promote the bond strength between the faces to
an
optimal extent and at an optimal point. After that, the component flows are
passed
20 into a multi-layer headbox and further into a gap former. In this solution,
out of the
same fresh stock 11, three separate component flows 121, 122 and 123 are
formed.
To the faces that are formed on the component flows 121 and 123 and that will
be
placed against the faces of the layers to be combined, at the points 141
before the
pumps 221 and 223, at the points 142 after the pumps 221 and 223, and/or at
the
25 points 143 after the machine screens 331 and 333, the admixtures are added
in order
to increase the fines contents in the layers and the bond strength between the
faces.
After this the component flows 121, 122 and 123 are passed into the multi-
layer
headbox 44 and further into the gap former. As an alternative solution, the
admix-
tures can be added to one component flow 121, 122 or 123 only.
30
Fig. 3 shows a preferred combination in accordance with the invention, in
which the
layer 13 that contains an admixture and that is to be combined is introduced
through
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a twin-layer headbox 441 into a gap former, and the web thus obtained is
combined
with a second web, which is derived from a normal headbox 442 and from a
second
gap former. The numeral 551 represents the first forming wire, 552 the second,
553
the third, and 554 represents the fourth forming wire.
5
Fig. 4 shows a second preferred combination in accordance with the invention,
in
which the layers 131 and 132 that contain an admixture and that are to be
combined
are introduced into two separate mufti-layer headboxes, herein twin-layer
headboxes
443, 444, and the webs that contain admixtures are passed into gap formers and
then
10 combined with each other. When two layers that contain admixtures are
combined,
a stronger bonding layer is obtained, which is necessary in applications that
require
higher strength.
Fig. 5 shows a solution in which the outermost layers 133 and 134 of a three-
layer
15 headbox 445, which outer layers contain admixtures, operate as bonding
layers, with
which the stock layers derived from two normal headboxes 446 and 44~ are com-
bined. The numerals 551...556 represent the forming wires.
In the solutions in accordance with the invention, it is an essential feature
that in the
20 manufacture of board it is possible to employ a gap former in a mufti-layer
technique
at high speeds of levels higher than 1000 metres per minute. In such a case,
the
admixture that increases the fines content and/or the bond strength is applied
as a
layer onto the face that will be placed against the face of the layer to be
combined.
Thus, a higher fines content is obtained on the face that will be placed, for
example,
25 against the face of a middle layer, in which connection the binding
strength and the
internal bond strength between the layers are increased. When the solution in
accordance with the invention is employed in a 3-layer headbox, it is
preferable to
feed the admixtures onto the outermost faces. The agents that can be applied
in this
way as layers are starch, fillers, fines, retention agents, such as
polyacrylamide,
30 polyacrylimide, special chemicals, hydrophobifying sizes, and equivalent.
In this
solution it is also favourably possible to use the same fresh stock and one
basic stock
line. Further, it is possible to amend the poor properties of a gap former, to
improve
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the runnability and to help the machine to remain clean. In the solution in
accord-
ance with the invention, the distribution of the fibre-based fines, fillers
and starch in
the surface, frame and/or bottom layers of the board is regulated so that the
concen-
tration of said agents is higher at the face that will be placed against the
middle
5 layer. The amount of admixture can be optimized, and so also the location at
which
it is added, in which way an optimal bond between the layers is obtained. This
is
also reflected in the printing quality of the product. The admixture or
admixtures can
be applied as layers on the face to be combined only, or alternatively also on
several
layers consisting of component flows.
10
By means of the invention, the face of the web to be combined that will enter
into
contact with a second web can be made favourably in view of the bond strength.
Thus, said face is already formed in the headbox such that it is favourable in
view
of bonding properties by means of fines or concentration of starch or
equivalent. By
15 means of the order and amount of adding, it is also possible to have a
substantial
effect on the properties that are desirable in the product or in the process.
Besides
the application of admixtures as layers in the way described in the present
invention,
of course, it is also possible to carry out conventional metering of
admixtures in the
normal way. Since the present invention permits the use of gap formers in the
20 manufacture of board in stead of the conventional fourdrinier wire
technique, no
large wire pit is needed, the equipment requires less space, it is possible to
use
smaller amounts of water, fewer dewatering elements, and it is also possible
to run
at a higher consistency than in the case of a fourdrinier wire. The method in
accordance with the present invention can be employed both in an upper-wire
unit
25 and on the what is called frame layer. In the manufacture of some special
products,
this method can also be substituted for a secondary headbox altogether.
