Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR PRODUCING A PAPER OR BOARD AND A PAPER OR
BOARD PRODUCED ACCORDING TO THE PROCESS
The present invention relates to a process for producing a paper or board
product
wherein the furnish used is partially treated with polymers in steps.
Background of the invention
In paper making processes there is a desire in obtaining strong paper and
board
products having high bulk (low density). Paper and board products having high
bulk
require a smaller amount of fibers, which is desirable especially for
economical
reasons. Bulky board products with low fiber content have better bending
stiffness.
Paper and board products are typically produced by dewatering furnish on a
wire. The
furnish often contains a mixture of different pulps, including both chemical
pulps and
mechanical pulps. In order to produce a bulkier sheet with higher structural
stiffness
the interest in using mechanical pulps with high freeness has increased. To
obtain the
best result the mechanical pulp should contain long, intact fibers and as
little fine
material as possible. However, pulps that are rich in stiff, long fibers
unfortunately
show poor ability to produce sufficient fiber bonding of the paper or board.
The bulk
and strength properties of the resulting paper or board will therefore be a
compromise
between the ability of the pulp to increase the bulk and its ability to
increase the fiber
bonding properties of the paper or board.
The fiber bonding properties of mechanical pulp may also be improved by
treatment
with chemical additives. The predominant treatment for improving strength,
particularly dry strength, of paper or board has so far been to add cationic
starch to the
pulp fiber slurry prior to the sheet forming operation. It is however
difficult to adsorb
large amounts of starch to the fibers, especially when the fines amount is
small.
The cationic starch molecules added to the pulp slurries can adhere to the
naturally
anionic pulp fibers by electrostatic attraction and thus be retained in the
wet fiber mat
and remain in the final paper or board.
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However, there are two major problems with cationically modified starches as
additives to paper-making pulp slurries. The first is that the cationic starch
molecules
can overwhelm the anionic charge on the cellulose fibers, thus setting a limit
on the
amount of cationic starch which can be added to the slurry. If an excess of
cationic
starch is added, only a portion of the starch which is added will be retained
in the sheet
and the rest will circulate in the paper or board machine white water system.
A second
problems is that fibers which have been made cationic by excessive cationic
starch
addition, will not be able to adsorb other cationic additives which are
commonly added
the to the pulp slurry, such as sizing agents.
Another method to enhance the strength properties of paper is to treat the
fibers with
polymers. Such a method is described in WO 0032702 A1, in which particles,
such as
fibers, are provided with a multilayer coating of interacting polymers. The
particles are
treated in consecutive steps with solutions of the interacting polymers.
Before each
subsequent treatment step unabsorbed excess of the polymer of the previous
step must
be removed, unless the polymer amount added in each step is carefully
controlled,
such that substantially all polymer is absorbed to the particle surface and no
excess
remains.
By treating the pulp according to the method described in WO 0032702 more
polymers are added to and retained in the paper. Paper produced from the
treated pulp
has an improved strength. However, the polymer treatment according to this
method
has been found to have a negative effect on the bulk of the paper, i.e. the
density of the
paper becomes undesirably high. The polymer treatment is also rather polymer
consuming. There is thus a need for a method of producing paper or board,
which has
an improved strength without negatively affecting the bulk.
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Summary of the invention
The object of the invention is to provide a method of producing paper or
board, which
has an improved strength, without adversely affecting the bulk.
This object is achieved by the process according to claim 1. It has now been
found that
by subjecting only a portion of the furnish to polymer treatment, the strength
of the
resulting paper or board product is significantly improved whereas the bulk
remains
substantially unaffected. The above object is thus achieved by the present
invention as
defined by the appended independent claims. Preferred embodiments are set
forth in
the dependent claims and in the following description.
The present invention concerns a process for producing a paper or board
product,
which process comprises providing a furnish comprising fibers; subjecting a
first
portion (1) of the furnish to polymer treatment, in which polymers are added
to the
furnish in at least two steps; blending said polymer treated first portion
with an
untreated second portion (2) of the furnish; and dewatering the furnish on a
wire to
form a fiber web. By treating only a portion of the furnish, the strength of
the resulting
paper or board product is significantly improved whereas the bulk remains
substantially unaffected.
Said first furnish portion preferably comprises 20-80% by weight of the total
furnish
weight. In one embodiment said first furnish portion comprises the longest
fibers
present in the furnish. The polymer treatment is preferably performed on a
first furnish
portion which comprises sulphate or sulfite pulp from soft wood fibers. The
furnish
may comprise fillers and optionally other paper making additives, which
fillers and
additives may be added subsequent to the polymer treatment of said first
furnish
portion. Alternatively, fillers and additives may instead be added prior to
the polymer
treatment of said first furnish portion.
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The polymer used in each of the consecutive polymer treatment steps is
preferably
interacting with the polymer used in the subsequent step. The polymer
treatment
preferably includes one step in which cationic polymer is added, and at least
one step
in which anionic polymer is added. By alternating cationic and anionic polymer
additions one theory is that interacting polymer layers are obtained. The
cationic
polymer is preferably cationic starch and the anionic polymer is preferably
CMC. The
polymer treatment comprises two to seven steps. The polymer treatment
preferably
comprises three to seven consecutive steps.
The invention also relates to a paper or board product produced according to
the
process of the present invention.
Short description of the drawings
Figure 1 is a schematic view of one embodiment of the process of the present
invention;
Figure 2 is a diagram which shows a comparison of density and strength of
paper
produced according to the present invention and produced according to the
prior art
process.
Detailed description of the invention
The invention relates to a process for producing paper or board from a
furnish, of
which a portion has been treated with polymers in at least two steps. The
invention
also relates to a paper or board product produced according to the process of
the
present invention.
Treatment of the furnish according to the prior art as described in WO
0032702, the
entire furnish is treated with cationic and anionic polymers in consecutive
steps, the
amount polymers in the paper or board product can be increased. Thus, it is
possible
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for example to produce a paper or board with high amounts of cationic starch
and
consequently receive a very strong paper or board product.
It has now been found that by treating only a portion of the pulp with
polymers in two
or more polymer treatment steps, the amount of polymers added to the furnish
is
reduced seen to the whole pulp furnish, and the strength properties of the
paper or
board are surprisingly still very good in spite of the decreased polymer
addition. Even
more surprisingly, the bulk of the paper or board product is only slightly
increased as
compared to a paper or board product to which no polymers have been added. A
low
density increase as possible is important in order to obtain a board having
high
bending stiffness at certain strength value.
The present process for producing a paper or board product, comprises
providing a
furnish comprising fibers; subjecting a first portion (1) of the furnish to
polymer
treatment, in which polymers are added to the furnish in at least two steps;
blending
said polymer treated first portion with an untreated second portion (2) of the
furnish;
and dewatering the furnish on a wire to form a fiber web. By treating only a
portion of
the furnish, the strength of the resulting paper or board product is thus
surprisingly
significantly improved whereas the bulk remains substantially unaffected. Said
first
furnish portion comprises 20-80% by weight of the total furnish weight,
preferably 20-
50% of the total furnish weight. The percentage of the first portion is
adjusted in order
to achieve the desired strength and density of the product while keeping the
amount of
polymers added as low as possible. If said first portion is too small, the
increase in
strength of the product will not be sufficient, since the portion of the
furnish that is
treated with polymers is mixed with a larger portion untreated furnish. On the
other
hand, if the first portion is too large, the addition of polymer needed
increases more
than what the product gains in strength and also the density of the product
will
increase (i.e. decrease in bulk). The appropriate amount of the first portion
also
depends on the properties of the furnish of the first portion as well as of
the second
portion. The furnish typically contains a mixture of different kinds of pulp,
for
example chemical pulp (hardwood pulp, softwood pulp, sulphate pulp or sulfite
pulp)
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or mechanical pulp (CTMP or TMP). In one embodiment said first furnish portion
comprises the longest fibers present in the furnish. These fibers contribute
more to the
strength of the final product, than shorter fibers and the polymer treatment
will
therefore be particularly efficient when performed on a furnish portion
containing the
longest fibers. The polymer treatment is preferably performed on a first
furnish portion
which comprises sulphate or sulfite pulp, since these pulps comprise long
fibers as
compared to other pulps. Treatment of a furnish portion comprising sulphate or
sulfite
pulp is therefore a convenient way of selecting a long fiber portion of the
final blended
furnish. However, the above mentioned first furnish portion which is subjected
to
polymer treatment can comprise any part of the furnish. The furnish may also
comprise fillers and optionally other paper making additives, which fillers
and
additives may be added subsequent to the polymer treatment of said first
furnish
portion. Alternatively, fillers and additives may instead be added prior to
the polymer
treatment of said first furnish portion. Some paper and board qualities
contain filler or
other additives which improves the surface properties of the paper. However,
the filler
particles also affect the strength of the paper in a negative way. One theory
is that
when a furnish containing filler and additives is treated with polymers
according to the
invention, filler particles and additives bond stronger to other particles or
to the fibers,
and consequently a high filler content does not affect the strength of the
paper to the
same extent.
The polymer used in each of the consecutive polymer treatment steps is
preferably
interacting with the polymer used in the subsequent step, thereby enabling a
larger
amount of polymers to be attached to the fibers and thus results in increased
strength
of the final paper or board product. The polymer treatment preferably includes
one
step in which anionic polymer is added, and at least one step in which
cationic
polymer is added. By alternating anionic and cationic polymer coatings
interacting
polymer layers may be obtained.
The anionic polymer may be one or more chosen from the group consisting of:
carboxy methyl cellulose (CMC), polyvinyl sulphate, anionic
galactoglucomannan,
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anionic starch, polyphosphoric acid, alginate and polymethacrylic acid. The
anionic
polymer is preferably CMC, since it interact well with cationic polymers as
well as it
is economically beneficial, due to its low cost. The cationic polymer may be
one or
more chosen from the group consisting of: cationic starch, polyvinyl amine,
chitosan,
primary and secondary amines, polyethylene imines, polyvinyl pyrrolidone and
modified polyacryl amides. The cationic polymer is preferably cationic starch,
which
is advantageous because it results in a board or paper having enhanced
strength
properties and is economically beneficial, due to its low price and easy
availability.
The polymer treatment preferably comprises two to seven consecutive steps. The
optimal number of steps depends on what properties of the paper or board that
are
desired and on the properties of the furnish being treated. The more steps the
treatment
consists of, the bigger amount polymers are added and thus retained in the
paper or
board. Often it is a balance between the cost and the desired properties, for
example
strength of the paper or board. There is usually a limit when it is not longer
cost
effective to add more polymers seeing to the improvement of the desired
properties,
for example increase of strength of the paper or board manufactured. It is
preferred to
add cationic polymer in the first step of the polymer treatment and to add
anionic
polymer in the subsequent and to continue with alternate addition of cationic
and
anionic polymers until the desired amount of polymers have been added in a
desired
number of steps.
The furnish need not be washed between the polymer treatment steps. Any
unabsorbed
excess of polymer added in the preceding step can remain in the furnish and
need not
be rinsed away. Washing between the polymer treatment steps has not been shown
to
enhance the properties of the resulting product. It is thus advantageous to
subject the
first furnish portion to each of the polymer treatment steps without any
intermediate
washing. Thereby an unnecessary step may be avoided and a faster process will
be
achieved, having lower water consumption.
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g
After each polymer treatment step there should be sufficient time and mixing
for the
polymer to be absorbed to the fibers of the first furnish portion. A time
period of at
least 5 seconds between each polymer treatment"step is suitable. The optimal
time
depends on the capacity of the mixing of the equipment.
The polymer of each polymer treatment step can be added to the furnish in a
pulp
vessel, such as a pulp chest, or in-line in a furnish transport pipe, or a
combination
thereof. Where to add the polymer depends on the equipment available and where
it is
possible to malce the addition practically. When carrying out two or more
polymer
treatment steps by in-line addition of polymer it needs to be ensured that the
pipe is
long enough to allow both thorough blending of polymer and pulp, and to allow
enough time between the addition step for the polymer to be absorbed or an in-
line
mixer can be used to ensure good mixing in the pipe.
The amount of polymer to be added varies depending of the properties of the
pulp.
When cationic starch and CMC are used for the polyiner treatment the amount of
cationic starch added in each step is typically between 5-25 kg/ton and the
amount of
CMC added in each step is typically between 0.25-3 kg/ton. The amount polymers
added to the furnish may be controlled by measuring the z-potential or
cationic
demand measurement of the pulp. Even if an excess amount of added polymer is
not
detrimental to the final result, it is of course advantageous to add an amount
of
polymer that is close to what can be absorbed by the pulp, both for economical
and
environmental reasons.
Figure 1 is a schematic description of the process according to one embodiment
of the
invention. In this embodiment furnish for production of paper or board is
transferred to
pulp chest 1, 2 respectively. The furnish transferred and contained to the
first pulp
chest 1 is designated the first furnish portion, and the furnish transferred
and contained
in the second pulp chest 2 is designated the second furnish portion.
The first furnish portion is subjected to polymer treatment in three steps.
The second
portion of furnish is not subjected to any polymer treatment. In the first
polymer
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9
treatment step 4 a cationic polymer, such as cationic starch, is added to the
first furnish
portion in-line to the conducting furnish pipe, which is placed upstream the
first pulp
chest l. In the second step 5 an anionic polymer is added to the furnish in
the first pulp
chest 1. The second polymer treatment step 5 is initiated at least 5 seconds
after the
first polymer addition. The pulp chest is equipped with an agitator, making
sure that
the furnish is sufficiently blended with the added polymer. The furnish is
conducted
from pulp chest 1 to a headbox 3. The third polymer treatment step is carried
out by
the addition of a cationic polymer (e.g. cationic starch) 6 in-line through an
in-line
mixer 7 in the conducting furnish pipe, which in-line mixer is placed
downstream the
conducting pipe but upstream the headbox. Subsequent to the third polymer
treatment
step the polymer treated first portion of the furnish is blended with the
second furnish
portion, which is provided from pulp chest 2 at a point between the valve and
the
headbox. The furnish is then conducted from the headbox onto a wire where it
is
dewatered and further treated in a manner known in the art in order to produce
paper
or board.
The embodiment described in figure lcan also be utilized in a multilayer paper
or
board construction where only a part of the middle ply furnish has been
treated with
polymers, while the bottom and top plies of the paper or board consists of
furnish that
has not been treated.
Example
Twelve different furnish samples 1-12 were used to evaluate the process. Six
samples
were partially or entirely subjected to a three step polymer treatment. For
comparison,
six equivalent samples were left untreated. The furnish samples comprised CTMP
mixed with different amounts of unbleached sulphate softwood pulp.
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The three sequential steps of the polymer treatment were:
1. addition of cationic starch to the furnish sample;
II. addition of CMC to the furnish sample; and
III. addition of cationic starch to the furnish sample.
Each polymer treatment step was followed by washing. The polymer treatment
thus
resulted in a furnish having two additions of cationic starch with an
intermediate CMC
addition.
Samples 2, 4 and 6 were subjected as a whole to the polymer treatment in their
entirety. Samples 8, 10 and 12 were divided into a first portion, which
comprised only
sulphate pulp and a second portion, which comprised only CTMP. The first
portion of
each sample (sulphate pulp) was subjected to polymer treatment, whereas the
second
portion of each sample (CTMP) remained untreated. The first and second
portions
were then blended with each other. Samples 1, 3, 5, 7, 9, and 11 were not
subjected to
any polymer treatment.
Sheets were then prepared by dewatering the furnish and density (STFI density)
and
Scott-Bond were determined for all sheets. Scott-Bond is a measure of the
strength in
the z-direction of the sheet. The results are shown in Table I and in the
diagram in
Figure 2.
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11
Table I
Sample Symbol in %(wt) % % (wt) of the total STFI Scott Increase Increase
diagram 1 CTMP* (wt) fiber content Densit Bond density Scott Bond
sulph subjected to y J/m2 after after
ate polymer kg/m3 polymer polymer
pulp* treatment** treatment treatment
% %
1 100 0 0 510 110
2 100 0 100 540 190 5,6 42
3 ~ 2 80 20 0 550 155
4 , 80 20 100 640 365 14 58
3 20 80 0 720 250
6 0 20 80 100 780 500 7,7 50
7 +2 80 20 0 550 155
20 (sulphate pulp
8 80 20 570 280 3,5 45
portion only)
9 +4 70 30 0 565 145
30 (sulphate pulp
= 70 30 590 295 4,2 51
portion only)
11 *5 60 40 0 595 175
40 (sulphate pulp
12 ~ 60 40 625 430 4,8 59
portion only)
*percentage calculated on the total fiber content
** percentage calculated on the total fiber content
5
Table I shows the increase of Scott-Bond and density in percent comparing the
untreated sample with the sample that has been treated with polymers in three
steps.
When treating the entire furnish, consisting of 20% sulphate and 80% CTMP with
polymers, as done in sample 4, the density has increased with 14% and the
Scott-Bond
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12
value with 58%. When only the sulphate portion of the furnish (20%) has been
treated
with polymers, as done in sample 8, the density has only increased with 3,5%
and the
strength with 45%. Consequently, even though only 20% of the furnish is
treated with
polymers, the Scott-Bond value still has increased with 45%, compared to 58%
when
the whole furnish has been treated, and the bulk has only increased with 3,5%,
compared to 14%. ~
Figure 2 shows the Scott-Bond values of sheets made from the furnish samples.
Lines
have been drawn between the value for each untreated sample and its equivalent
corresponding polymer-treated sample. Solid lines in the diagram depict
samples
where the entire furnish was subjected to polymer treatment. Dashed lines
depict
samples where only the sulphate portion of the furnish was subjected to
polymer
treatment. A steeper inclination of the lines indicates a higher strength
increase at a
lower density increase.
As shown in Figure 2 the dashed lines have a steeper inclination than the
solid lines.
Subjecting only the sulphate portion of the furnish to polymer treatment thus
results in
a substantial strength increase while the density remains almost constant. As
shown
with solid lines, when the entire furnish is subjected to polymer treatment
the increase
in strength is greater than that observed after partial treatment, however the
density of
the sheets formed with the entirely treated furnish also increases to a larger
extent than
observed after partial treatment.
The results observed for sheets formed with samples 8, 10 and 12, which are
subjected
to polymer treatment according to the invention, thus show that a surprisingly
high
strength with remaining low bulk is achieved by a small polymer amount.
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Example
A board consisting of three layers was manufactured. The furnish forming the
top and
bottom layers of the board consisted of 50% hardwood pulp and 50% softwood
pulp
with a SR number of approximately 27.
The furnish forming the center ply of the board consisted of 55% CTMP and 45%
of a
mixture, hereinafter referred to as mixture portion, which mixture portion
consisted of
approximately 55% bleached softwood pulp, 15 % bleached hardwood pulp and 30%
CTMP. A board consisting of CTMP and said mixture portion in the center ply
represents a typical liquid packaging board.
Five different samples of board were manufactured. In two of the samples, the
entire
furnish used for the centre ply was treated with polymers in three consecutive
steps
(two steps with cationic starch additions with a step of CMC addition in
between).
Another two samples, were treated according to the invention, where only the
mixture
portion of the furnish (45% of the total furnish) was treated with polymers in
three
consecutive steps and the treated mixture portion were then blended with the
untreated
CTMP portion. One sample was used as a reference where no polymer treatment
was
done.
The amount of polymer added in each step was determined by measuring the z-
potential. First, small additions of cationic starch were added and the z-
potential was
continuously measured, when the z-potential was stabilized it indicated that
it would
be a suitable addition amount. Thereafter, anionic CMC was added in the same
way,
and when the z-potential was close to zero it indicated that that addition
amount would
be suitable. The amount of polymers added in the following step was determined
in the
same way. No wash of the furnish were performed between the polymer additions.
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The amounts of polymer added to the furnish, when treating the entire furnish
in three
consecutive steps, were; 15 kg/ton cationic starch in the first step, 1.5
kg/ton CMC in
the second step and 10 respectively 15 kg/ton cationic starch in the third
step.
When only treating the mixture portion of the furnish in three consecutive
steps, 15
kg/ton cationic starch was added in the first step, 1.5 kg/ton CMC was added
in the
second step and 15 respectively 20 kg/ton cationic starch were added in the
last step.
These amounts were calculated on the mixture portion, which is 45% of the
entire
furnish. Thus, the polymer amount added, seen to the entire furnish, were;
6.75 kg/ton
cationic starch in the first step, 0.675 kg/ton CMC in the second step and
6.75
respectively 9 kg/ton cationic starch in the last step.
When treating the pulp in three consecutive steps, the two first polymer
additions were
done in the pulp chest and the last addition of cationic starch was done in-
line to the
pipe.
The Scott Bond value of the board samples was measured and the results are
shown in
Diagram 1.
250
=
=
200
N
E
~ 150
= Reference
0 ~ Polymer in 3 steps
m 100 = Polymer in 3 steps on portion
-W
0
0
N
0
0 10 20 30 40 50
Starch, kg/ton (centre ply grammage)
20 Diagram 1. Scott Bond of the board compared to the added amount of starch.
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It is shown in Diagram 1 that the strength of the board, when only the mixture
part of
the furnish has been treated with polymers in three steps, is even higher than
the board
where the entire furnish has been treated with polymers in three steps, even
though the
amount of starch added is less.
The density of the boards was also measured. Diagram 2 shows the results of
the
measured Scott-Bond values compared to the density.
250
=
=
200
E ~
150
~ ~ = Reference
p = ~ Polymer in 3 steps
m 100 = Polymer in 3 steps on portion
:z
O
~
0
400 500 600 700
Density, STFI kg/m3
10 Diagram 2. Scott-Bond values compared to the density of the board samples.
It is shown in Diagram 2 that the density of the boards treated with polymers
is
retained, compared to the reference sample, even though the strength of the
boards are
increased.
Consequently, these tests presented in Diagram I and 2 show that it is
advantageous to
only treat one portion of the furnish since it results in a board having high
strength
with retained bulk at the same time as the polymer addition is decreased.
