U.S. patent application number 11/665229 was filed with the patent office on 2008-01-31 for process for producing a paper or board and a paper or board produced according to the process.
Invention is credited to Mats Fredlund, Hans Hallgren, Hans Hoglund, Gunilla Pettersson, Lars Wagberg.
Application Number | 20080023164 11/665229 |
Document ID | / |
Family ID | 36148578 |
Filed Date | 2008-01-31 |
United States Patent
Application |
20080023164 |
Kind Code |
A1 |
Fredlund; Mats ; et
al. |
January 31, 2008 |
Process for Producing a Paper or Board and a Paper or Board
Produced According to the Process
Abstract
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. The invention also concerns a paper or
board product produced according to the process.
Inventors: |
Fredlund; Mats; (Karlstad,
SE) ; Wagberg; Lars; (Lidingo, SE) ; Hallgren;
Hans; (Bromma, SE) ; Hoglund; Hans; (Matfors,
SE) ; Pettersson; Gunilla; (Sundsvall, SE) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR
25TH FLOOR
CHICAGO
IL
60606
US
|
Family ID: |
36148578 |
Appl. No.: |
11/665229 |
Filed: |
October 13, 2005 |
PCT Filed: |
October 13, 2005 |
PCT NO: |
PCT/SE05/01520 |
371 Date: |
August 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60619370 |
Oct 15, 2004 |
|
|
|
Current U.S.
Class: |
162/164.2 ;
162/164.1 |
Current CPC
Class: |
D21H 11/20 20130101;
D21H 17/29 20130101; D21C 9/002 20130101; D21H 23/04 20130101; D21H
17/26 20130101 |
Class at
Publication: |
162/164.2 ;
162/164.1 |
International
Class: |
D21H 11/20 20060101
D21H011/20; D21H 17/26 20060101 D21H017/26; D21H 17/29 20060101
D21H017/29; D21H 23/06 20060101 D21H023/06 |
Claims
1. 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; dewatering the furnish on a wire to form a
fiber web.
2. The process as claimed in claim 1, wherein said first furnish
portion comprises 20-80% by weight of the total furnish weight.
3. The process as claimed in claims 1 or 2, wherein said first
furnish portion comprises the longest fibers present in the
furnish.
4. The process as claimed in any of the preceding claims, wherein
said first furnish portion comprises sulphate or sulfite pulp of
soft wood fibers.
5. The process as claimed in any of the preceding claims, wherein
the furnish comprises fillers and optionally other paper making
additives, which fillers and additives are added subsequent to the
polymer treatment of said first furnish portion.
6. The process as claimed in any of claims 1-4, wherein the furnish
comprises fillers and optionally other paper making additives,
which fillers and additives are added prior to the polymer
treatment of said first furnish portion.
7. The process as claimed in any of the preceding claims, wherein
the polymer used in each of the consecutive polymer treatment steps
is interacting with the polymer used in the subsequent step.
8. The process as claimed in any of the preceding claims, wherein
at least one of the polymer treatment steps includes the addition
of an anionic polymer, and at least one of the polymer treatment
steps includes the addition of a cationic polymer.
9. The process as claimed in claim 8, wherein the anionic polymer
is CMC.
10. The process as claimed in claim 8, wherein the cationic polymer
is cationic starch.
11. The process as claimed in any of the preceding claims, wherein
the polymer treatment comprises two to seven steps.
12. The process as claimed in claim 10, wherein the polymer
treatment comprises three to seven steps.
13. A board product produced according to the process of claim
1.
14. A paper produced according to the process of claim 1.
Description
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
SUMMARY OF THE INVENTION
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] The invention also relates to a paper or board product
produced according to the process of the present invention.
SHORT DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic view of one embodiment of the process
of the present invention;
[0014] FIG. 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
[0015] 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.
[0016] 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 for example to produce a paper or board with high
amounts of cationic starch and consequently receive a very strong
paper or board product.
[0017] 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.
[0018] 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)
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.
[0019] 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.
[0020] The anionic polymer 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 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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 make 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.
[0025] The amount of polymer to be added varies depending of the
properties of the pulp. When cationic starch and CMC are used for
the polymer 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.
[0026] FIG. 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 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 1. 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.
[0027] The embodiment described in FIG. 1 can 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
[0028] 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.
[0029] The three sequential steps of the polymer treatment were:
[0030] I. addition of cationic starch to the furnish sample; [0031]
II. addition of CMC to the furnish sample; and [0032] 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.
[0033] 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.
[0034] 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 FIG. 2. TABLE-US-00001 TABLE I Increase Increase % (wt) of the
total density Scott Bond fiber content after after % (wt) subjected
to STFI Scott polymer polymer Symbol in % (wt) sulphate polymer
Density Bond treatment treatment Sample diagram 1 CTMP* pulp*
treatment** kg/m.sup.3 J/m.sup.2 % % 1 .diamond-solid. 1 100 0 0
510 110 2 .box-solid. 100 0 100 540 190 5.6 42 3 .diamond-solid. 2
80 20 0 550 155 4 .tangle-solidup. 80 20 100 640 365 14 58 5
.diamond-solid. 3 20 80 0 720 250 6 .diamond. 20 80 100 780 500 7.7
50 7 .diamond-solid. 2 80 20 0 550 155 8 .DELTA. 80 20 20 (sulphate
pulp 570 280 3.5 45 portion only) 9 .diamond-solid. 4 70 30 0 565
145 10 .cndot. 70 30 30 (sulphate pulp 590 295 4.2 51 portion only)
11 .diamond-solid. 5 60 40 0 595 175 12 .quadrature. 60 40 40
(sulphate pulp 625 430 4.8 59 portion only) *percentage calculated
on the total fiber content **percentage calculated on the total
fiber content
[0035] 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 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%.
[0036] FIG. 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.
[0037] As shown in FIG. 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.
[0038] 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.
EXAMPLE
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] The density of the boards was also measured. Diagram 2 shows
the results of the measured Scott-Bond values compared to the
density.
[0048] 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.
[0049] Consequently, these tests presented in Diagram 1 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.
* * * * *