Note: Descriptions are shown in the official language in which they were submitted.
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PAPER PRODUCTION PROCESS FEATURING ALTERNATING
CATIONIC AND ANIONIC POLYMER ADDITIONS
The present invention relates to a process for the production of a paper
product with
high filler content wherein the furnish used is treated with polymers in
steps.
Background of the invention
In papermaking processes there is an ongoing concern to find ways to produce
paper at reduced costs. Since fillers are cheaper than fibers, one way is to
increase
the filler content of the paper and thus be able to reduce the amount of
fibers in the
paper. Besides being economically beneficial, fillers also improve the opacity
and
printability properties of the paper. However, large amount of fillers in the
paper
decreases the strength of the paper. Thus, there is a balance between the
possible
amount of fillers added and the required strength of the paper produced.
Current
levels of fillers in paper vary depending on paper grade, for example the
filler
content of newsprint may be up to about 12-14%. The maximum filler content
today, considering available publication paper grades and fine paper grades,
is
about 35%, as it is in uncoated magazine paper.
One way to compensate for the decrease in strength caused when filler is added
is to
improve the fiber bonding properties between the fibers in the paper, thereby
maintaining the strength of the paper. With increased strength of the paper it
is
possible to increase the filler content. The predominant treatment for
improving
paper strength, particularly dry strength, of paper has so far been to add a
strength
agent, preferably cationic starch, to the pulp fiber slurry prior to the sheet
forming
operation. 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. It is however difficult
to
adsorb large amounts of cationic starch to the fibers.
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When adding large amounts of cationic starch to a papermaking furnish, in
order to
achieve high resulting paper strength, two major problems arise. The first is
that the
cationic starch molecules tend to saturate the anionic charge on the cellulose
fibers,
thus setting a limit to 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
added
will be retained in the sheet, and the rest will circulate in the paper or
board
machine white water system. A second problem is that fibers which are made
cationic by excessive cationic starch addition, will not be able to adsorb
other
cationic additives which are commonly added to the pulp slurry, such as sizing
agents and retention aids.
Another method to enhance the strength properties of paper is to treat the
fibers
with polymers in consecutive steps. The international application WO
2006041401
describes such method where a part of a furnish is treated with polymers in
consecutive steps, whereby a paper or board with improved strength and with
reduced amount of polymers is obtained.
Yet another method to enhance the strength properties of paper is described in
WO
0032702, in which particles (such as fibers or fillers) are provided with a
multilayer
coating of interacting polymers.
It is however still a need for a process by which paper products having a
maintained
or improved strength could be produced at a lower cost.
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Summary of the invention
The invention relates to a process by which a paper product having a high
strength can be
produced at low cost.
It has now surprisingly been found that by subjecting both fillers and fibers
of a furnish to
polymer treatment as defined herein the strength of the resulting paper
product is significantly
improved even though the filler content is high.
In one embodiment, the present invention concerns a process for the production
of a paper
product, which process comprises providing a furnish comprising fillers and
fibers; subjecting
the furnish to polymer treatment, in which polymers are added to the furnish
in at least three
steps; dewatering the furnish on a wire to form a fiber web, pressing said
fiber web and drying
the fiber web to form said paper product; said furnish containing fillers in
such an amount that
the paper product obtained by the process contains at least 15% by weight of
fillers. By
treating the furnish, which contains fibers and a high amount of fillers, with
polymers, a paper
product with high strength is obtained.
The paper product produced preferably contains 15-70% fillers by weight of the
total paper
weight.
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 it is believed
that interacting
polymer layers are obtained. The cationic polymer is preferably cationic
starch and the
anionic polymer is preferably CMC. The polymer treatment preferably comprises
three to
seven consecutive steps.
The invention also relates to a paper product produced according to the
process of the present
invention.
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Brief description of the drawings
Figure 1 shows the results of the z-strength measurement of the paper sheets 1-
10.
Figure 2 shows the results of the tensile index of the paper sheets 1-10.
Figure 3 shows the tensile index compared to the amount of starch in the paper
sheets 1-10.
Detailed description of the invention
The invention relates to a process for the production of a paper product from
a furnish
comprising fillers and fibers, wherein the furnish is treated with polymers in
at least three
steps and of which said furnish contains fillers in such an amount that the
paper product
produced contains at least 15% by weight of fillers. The invention also
relates to a paper
product produced according to the process of the present invention.
It is possible to produce a paper or board with high amounts of cationic
starch and
consequently receive a strong product. This is shown in both WO 0032702 and in
WO 2006041401.
It has now been found that by treating a furnish containing both fillers and
fibers with
polymers in three or more polymer treatments steps, the amount of fillers in
the produced
paper product can be increased and the strength of the paper is very good in
spite of the high
filler content of the paper. Surprisingly, the filler content can be increased
even more when
the furnish treated contains both fillers and fibers as compared to only
treating the fibers or
the fillers of the furnish with polymers in consecutive steps. One theory
behind this is that
when a furnish containing both fillers and fibers is treated with polymers
according to the
invention, the filler particles bind stronger to other filler particles or to
the fibers, and the filler
content can consequently be increased without affecting the strength of the
paper negatively.
By treating the entire furnish with polymers in consecutive steps and not only
one part of the
furnish as done in WO 2006041401, it has even more
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surprisingly been seen that the strength of the paper is maintained or even
increased,
even though the filler content of the paper is increased.
The present process for producing a paper or board product comprises,
providing a
furnish comprising fillers and fibers; subjecting the furnish to polymer
treatment, in
which polymers are added to the furnish in at least three steps; dewatering
the
furnish on a wire to form a fiber web; pressing said fiber web and drying the
fiber
web to form said paper product; said furnish containing fillers in such an
amount
that the paper product obtained by the process contains at least 15% by weight
of
fillers. By treating the entire furnish, which comprises both fillers and
fibers, the
filler content of resulting paper product, can be increased and the strength
of the
resulting paper is still surprisingly significantly good.
The filler content of the paper product is at least 15% by weight of the total
paper
sheet weight, preferably 15-70% and even more preferably 20-70% or 20-50% by
weight, since it has been seen that the strength of the paper still is high,
even if the
filler content is increased, and it thus makes it possible to produce a paper
product
at lower cost. The filler content is even more preferably between 30-50% by
weight
since the strength of the paper is good even at this high filler content, this
can bee
seen in table 1. The filler content is adjusted in order to achieve the
desired strength
of the paper product. It is desirable to have as high filler content as
possible without
decreasing the strength to unacceptable levels. The appropriate amount of
fillers
depends on the properties of the furnish and on the quality demands of the
paper
product. The amount of fillers added to the furnish, in order to receive a
paper
product with a desired filler content, depends on the filler retention of the
fiber web,
i.e. how much of the added fillers that are retained in the resulting paper
product.
The filler retention varies a lot and there are many factors that affect it,
some
examples are: the grammage of the paper, the formation unit of the paper
machine,
the fiber components of the furnish and the use and amount of added retention
agents. Consequently, there is a wide range of what the filler retention can
be,
normally the filler retention is about 20-70%, i.e. 20-70% of the added
fillers are
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retained in the paper product. To treat the furnish with polymers according to
the
invention might also affect the filler retention, however it is believed that
the
polymer treatment affects the filler retention in a positively way. This is
due to the
fact that both filler particles and fibers form complexes and that these
complexes
might improve the fillers ability to bind to the fibers in the paper and thus
improve
the fillers ability to remain in the paper.
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 fillers and fibers and thus result in
increased strength of the final paper 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 used may be one or more chosen from the group consisting
of:
carboxy methyl cellulose (CMC), polyvinyl sulphate, anionic
galactoglucomannan,
anionic starch, polyphosphoric acid, alginate and polymethacrylic acid. The
anionic
polymer is preferably CMC, since it has high charge density, which reduces the
amount of CMC needed in each addition, and which is therefore economically
beneficial. CMC also interacts very well with cationic polymers, and
especially
cationic starch. Thus, the use of CMC is very economically beneficial both due
to
its low price as well as the reduced amount needed. The cationic polymer used
may
be one or more chosen from the group consisting of: cationic starch, polyvinyl
amine, chitosan, primary and secondary amines, polyethylene imines, polyvinyl
pynolidone and modified polyacryl amides. The cationic polymer is preferably
cationic starch, which is advantageous because it results in a paper having
enhanced
strength properties and it is economically beneficial, due to its low price
and easy
availability. It is preferable to use cationic starch in combination with CMC
since
these two polymers have been found to interact well with each other, resulting
in
paper with good strength at a low cost.
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The polymer treatment preferably comprises three to seven consecutive steps.
The
optimal number of steps depends on what properties of the paper that are
desired
and on the properties of the furnish being treated. The more steps the
treatment
consists of, the larger amount of polymers are added and thus retained in the
paper.
Often it is a balance between the cost and the desired properties, for example
strength of the paper. There is usually a limit when it is no longer cost
effective to
add more polymers with respect to the improvement of the desired properties,
for
example increase of strength or filler content of the paper manufactured. It
is
preferred to add cationic polymer in a first step of the polymer treatment and
to add
anionic polymer in a subsequent and to continue with alternating additions of
cationic and anionic polymers, until the desired amount of polymers has been
added
in the desired number of steps.
The furnish need not be washed between each 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 furnish to each of the polymer treatment steps
without
any intermediate washing. Thereby, washing between the polymer treatment steps
is
unnecessary and may be excluded, thus a faster process is achieved which, in
addition, has lower water consumption.
After each polymer treatment step there should be sufficient time and mixing
for the
polymer to be absorbed to the fillers and fibers of the furnish. A time period
of at
least 5 seconds between each polymer treatment step is suitable. The optimal
time
period 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. The point of addition of the polymer depends on the equipment
available
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and where it is practically possible to make the addition. When carrying out
three 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,
filler and
fibers, and to allow enough time between each addition step for absorption of
the
polymer. Alternatively 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
furnish. When cationic starch and CMC are used for the polymer treatment the
amount of cationic starch added in each step is typically between 5-30 kg/ton
and
the amount of CMC added in each step is typically between 0.25-3 kg/ton. The
amount of polymers added to the furnish may be decided by measuring the charge
of the pulp or the process water. Even if an excess amount of added polymer is
not
detrimental to the final result, it may be advantageous to add an amount of
polymer
that is close to what can be absorbed by the pulp, both for economical and
environmental reasons. However, for some paper products it has been seen that
an
excess of polymers added to the furnish results in an improved paper product,
i.e. a
stronger product or a product with higher filler content with remained
strength. This
might be explained by that the polymers form polyelectrolyte complexes that
could
have favorable affects on the properties of the paper product.
The furnish typically contains a mixture of different kinds of pulp, for
example
chemical pulp (hardwood pulp, softwood pulp, sulphate pulp or sulfite pulp) or
mechanical pulp (CTMP or TMP).
Examples of fillers used in the furnish are; kaolin, calcium carbonate,
precipitated
calcium carbonate, talcum, gypsum and synthetic fillers.
The paper product produced can be of any paper grade at any grammage, for
example fine paper, magazine paper or newsprint.
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If necessary, additional generally known paper process steps, such as coating
and
calendering, can be used in order to produce the paper product according to
claim 1.
Example
Ten different furnish samples 1-10 were used to evaluate the process. The
furnish
used was a mixture of groundwood and kraft pulp in the ratio 2.5:1. In six of
the
samples only the fiber part of the furnish was treated with polymers and in
the
remaining four samples the entire furnish was treated with polymers. Clay was
used
as filler and the filler content of the paper product was either 30% or 50%.
For
comparison, some samples were treated with cationic starch in one step and the
other samples were treated with cationic starch and CMC in consecutive steps
according to the invention.
The three sequential steps of the polymer treatment were:
I. 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.
The charge of the furnish was measured after each addition, and the amount of
polymer added in each subsequent step was decided based on these measurements.
The polymers were added in excess as compared to the amount determined on
basis
of the charge measurement.
In samples 1-6 only the fiber part was subjected to polymer treatment and
thereafter
fillers were added, and the fibers and fillers were mixed into a furnish.
Samples 7-
10 were subjected as a whole to the polymer treatment, i.e. the entire furnish
comprising both fibers and fillers were subjected to the polymer treatment.
Sheets were then prepared by dewatering the furnish, pressing and drying the
fiber
web and the density (ISO 5270), tensile index (ISO 5270) and z-strength (SCAN
P
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80) were determined for all sheets. Prior to testing the sheets were
conditioned at
23 C/50% RH. The results are shown in Table 1.
Table 1
Paper Treatment Filler Starch- Density Tensile z- Starch
sheets content CMC- (kg/m3) index (Nm/g) strengt
analyzed
(%) Starch h (kPa)
in paper
(kg/ton) (%)
1 Fiber part 30 10-0-0 513 24,7 436
0,79
2 Fiber part 30 20-0-0 497 22,4 433
1,3
3 Fiber part 30 25-0-0 486 23,1 427
1,3
4 Fiber part 30 25-2-20 502 22,4 485
2,6
Fiber part 50 25-0-0 512 11,5 334 0,97
6 Fiber part 50 25-2-20 512 11,7 373
2,1
7 Entire furnish 30 22-0-0 544 35,7 540
1,8
8 Entire furnish 30 22-2-22 554 41,3
720 3,1
9 Entire furnish 50 20-0-0 582 20,9 454
1,6
Entire furnish 50 20-2-20 620 25,6 634 3,2 '
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Table 1 shows an increase in z-strength and tensile index for the resulting
paper
product when the entire furnish is subjected to polymer treatment, as compared
to if
only the fiber part of the furnish were treated. It can also be seen that
treatment with
polymers in consecutive steps improves both the z-strength and the tensile
index.
The density was also measured and it can be seen that in many cases there is
an
increase of density as the amount of starch in the sheet increases. However,
the
density increase is not large enough to explain the increase of tensile index.
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In order to get a better view of the results, they are also shown in Figure 1,
Figure 2 and
Figure 3. The abbreviation st, as can be seen in the Figures stands for
addition of cationic
starch in kg/ton.
Figure 1 shows that the z-strength of the paper sheets increases when the
entire furnish is
treated with polymers. The highest z-strength can be found when the furnish is
treated with
polymers in consecutive steps according to the invention. Surprisingly, the z-
strength of the
paper sheets with a filler content of 50% is very high.
Figure 2 shows that the tensile index of the paper sheets increases when the
entire furnish is
treated with polymers. The highest value of the tensile index can be found
when the furnish is
treated with polymers in consecutive steps according to the invention.
Figure 3 shows that the tensile index of the sheets is best for those sheets
for which the entire
furnish was treated with polymers, even if the amount of starch in the sheets
is not so high.
The tensile index of sheets with a filler content of 50% where the entire
furnish has been
treated, is comparable to the sheets with a filler content of 30% where only
the fiber part of
the furnish has been treated with polymers.
Consequently, these tests presented in table 1, and Figures 1, 2 and 3 show
that it is
advantageous to treat the entire furnish since it results in a paper having
high strength at the
same time as the filler content can be increased.
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