Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A PROCESS FOR PRODUCTION OF A PAPER OR PAPERBOARD
PRODUCT.
Field of the invention
The present invention relates to a process for production of a paper or
paperboard product comprising a furnish which comprises starch and
microfibrillated cellulose.
Background
In papermaking processes there is an ongoing concern to find ways to
produce paper or paperboard at reduced costs without impairing the
properties of the product, e.g. without decreasing the strength of the
product.
One way to reduce the cost is to increase the filler content of a paper or
paperboard product and thus be able to reduce the amount of fibers in the
paper or paperboard. Besides being economically beneficial, fillers also
improve the opacity and printability properties of the product. However, large
amount of fillers in the product decreases the strength. Thus, there is a
balance between the possible amount of fillers added and the required
strength of the paper or paperboard produced.
Furthermore, during production of paperboard there is a desire to
produce a strong but yet low density product. When increasing the strength of
a paperboard the density normally increases. There is thus a balance
between the desired strength and the density of the paperboard product.
It is possible to compensate for the decrease in strength, caused for
example by addition of large amount of filler or by increased bulk, by
improving the fiber bonding properties between the fibers in the paper or
paperboard, thereby maintaining the strength. The predominant treatment for
improving paper or paperboard strength, particularly dry strength, has so far
been to add a strength agent, preferably cationic starch, to the furnish prior
to
the sheet forming operation. Cationic starch molecules added to the furnish
can adhere to the naturally anionic pulp fibers by electrostatic attraction
and
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thus be retained in the wet fiber mat and remain in the final paper or
paperboard.
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.
Furthermore, high amounts of starch often cause problems with runnability,
microbiology and foaming during the production process.
It has recently been found that addition of microfibrillated cellulose to a
paper or board will increase the strength of the product, probably due to the
improved fiber bonding.
However, there is still a need for a cost efficient product with good
strength properties.
Summary of the invention
It is an object of the present invention to provide a process which in an
easy and cost efficient way will be able to produce a paper or paperboard with
improved strength and density.
These objects and other advantages are achieved by the paper or
paperboard product according to claim 1. The present invention relates to a
process for producing a paper or paperboard product which process
comprises the steps of; providing a furnish comprising fibers, adding starch
to
the furnish, adding microfibrillated cellulose to the furnish and conducting
the
furnish to a wire in order to form a web, wherein the starch and
microfibrillated
cellulose is added separately to the furnish. It has been shown that a product
comprising both starch and microfibrillated cellulose (MFC) increases the
strength of the product since the amount of starch can be increased without
increasing the density of the paper or paperboard product.
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It is preferred that the furnish comprises 2-15% by weight of starch and
1-15% by weight of microfibrillated cellulose. The amount of starch
respectively MFC of the product depends on the end use and the
corresponded desired properties of the product. High amounts of starch will
increase the strength of the product and it has been shown that the
combination of MFC and starch makes it possible for the product to retain
larger amounts of starch.
The web being formed by said furnish comprising starch and
microfibrillated cellulose preferably forms a layer of the paper or paperboard
product. The paper or paperboard product is preferably a multilayer product
which comprises at least two layers. It may be preferred that the product
comprises at least three layers and that the layer located in the middle of
the
product comprises furnish comprising starch and microfibrillated cellulose,
i.e.
the web being formed by the furnish that comprises starch and MFC, forms a
middle layer of the paper or paperboard.
Detailed description
The invention relates to a process for production of a paper or
paperboard product comprising a furnish which comprises starch and
microfibrillated cellulose. The furnish comprises starch in an amount of 2-15%
by weight, preferably between 3-5% by weight. Consequently, the product
thus comprises starch of an amount of 2-15% by weight, preferably between
3-5% by weight.
The furnish preferably comprises cellulosic fibers. The cellulosic fibers
may be hardwood and/or softwood fibers. The cellulosic fibers may be
mechanically, chemimechanically and/or chemically treated. The fibers may
also be bleached or unbleached.
It has been found that the addition of starch and microfibrillated
cellulose (MFC) to a furnish has a very good effect on the strength of the
paper or paperboard. Products comprising both starch in large amounts and
MFC, makes it possible for the product to retain larger amount of starch. .
Surprisingly, it has also been found that the addition of both starch, in
large
amounts, and MFC do not increase the densification of the product. The
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combination of MFC and starch thus has a synergistic effect when it comes to
boosting the strength of a paper or paperboard product without increasing the
density of the product to the same extend. It is thus possible to decrease the
density of the paper or paperboard product but still be able to produce a
strong product. In this way a much cheaper product can be produced since
the fiber content of the product can be decreased and the strength can be
remained (compared to products produced according to prior art).
Furthermore, the weight of the product will be decreased which will reduce
costs during transportation and handling.
Microfibrillated cellulose (MFC) (also known as nanocellulose) is a
material made from wood cellulose fibers, where the individual microfibrils
have been partly or totally detached from each other. MFC is normally very
thin (-20 nm) and the length is often between 100 nm to 10 pm. However, the
microfibrils may also be longer, for example between 10-100 pm but lengths
up to 200pm can also be used. Fibers that has been fibrillated and which
have microfibrills on the surface and microfibrils that are separated and
located in a water phase of a slurry are included in the definition MFC.
MFC can be produced in a number of different ways. It is possible to
mechanically treat cellulosic fibers so that microfibrils are formed. The
production of nanocellulose or microfibrillated cellulose with bacteria is
another option. It is also possible to produce microfibrils from cellulose by
the
aid of different chemicals and/or enzymes which will break or dissolve the
fibers.
One example of production of MFC is shown in W02007091942 which
describes production of MFC by the aid of refining in combination with
addition of an enzyme.
It is also possible to modify the microfibrillated cellulose before addition
to the furnish. In this way it is possible to change its interaction and
affinity to
other substances. For example, by introducing more anionic charges to MFC
the stability of the fibril and fibril aggregates of the MFC are increased.
How
the modification of the microfibrillated fibers is done depends, for example
on
the other components present in the furnish.
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The furnish comprises 2-15% by weight of starch, preferably between
3-5% by weight, and it is preferred that the furnish further comprises MFC in
an amount of 1-15% by weight. The chosen amount of starch respectively
MFC added to the furnish depends on the final product produced and the
desired properties of the product. Higher amounts of starch will increase the
strength of the product. However, it is not possible to increase the amount of
starch too much since other problems then may occur. Also, the amount of
MFC must be regulated based on the amount of starch and of course also on
the end use of the product. Too high amounts of MFC may cause dewatering
problems since MFC is a very fine material which easily absorbs water and
increased content will make it more difficult to dewater the product.
The paper or paperboard product is preferably a multilayer product
comprising at least two layers. It may be preferred that the product comprises
at least three layers and that the layer located in the middle of the product
comprises furnish comprising starch and microfibrillated cellulose. However,
it
is also possible that at least one outer layer of the product or even all
layers
of the product comprises furnish comprising starch and MFC. For some
products it might be advantageous that at least one of the outer layers
comprises furnish comprising starch and MFC. In this way it is possible to
increase the strength and/or the bulk of this layer. Consequently, depending
on the end use of the product, it is decided which and how many of the layers
that will comprise furnish comprising starch and MFC.
It is not necessary that the entire furnish in a layer of the paper or
paperboard product comprises starch and MFC, but it is preferred that the
starch and MFC is added to the majority of the furnish of the layer. However,
the layer may also comprise other components, such as broke pulp which
does not comprise starch and MFC.
The furnish may also contain various amounts of fillers to increase for
example runnability and cost-efficiency of the process and the produced
substrate. Other commonly used additives used in the production of paper or
paperboard can also be added.
The paperboard product is preferably a high quality paperboard
product, such a liquid packaging board, graphical board or food service
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board. The paper product is preferably a high quality paper, such as copy
paper of grades A or B, graphical papers, LWC, SC or news paper for high
speed printing machines.
The process for producing a paper or paperboard product which
process comprises the steps of providing a furnish comprising fibers, adding
starch to the furnish, adding microfibrillated cellulose to the furnish and
conducting the furnish to a wire in order to form a web. The addition of
starch
and MFC is preferably done in the machine chest or before the fan pump. It
may also be possible to do the addition to the circulation water which later
on
is added to the furnish. However, all practical points of addition for the
starch
and MFC can be used as long as there is enough time and mixing of the
starch and MFC with the furnish before it is conducted to the wire.
The starch and MFC is added separately. It is preferred to first add
starch followed by addition of MFC. It could be possible to mix the starch and
MFC before addition to the furnish, however the result is then not as good as
when starch and MFC is added separately. It is then possible to mix starch
and MFC before addition to the furnish by cooking them, preferably by the
use of a jet-cooker. It has been shown that by cooking the mixture of starch
and MFC, the MFC shows less tendency to flocculate. Furthermore, by
altering the charge and charge density of e.g. starch, different degrees of
flocculation can be obtained and it would thus be possible to neutralize the
charge of MFC. This might affect dewatering and retention of the fibers and
eventual fillers in the product. It may also be possible to add a layer of
starch
on the MFC and then add this mixture to the furnish, i.e. a multilayering
effect
can be created.
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Examples
Material used:
Bleached chemothermomechanical pulp (BCTMP) at 570 CSF.
Microfibrillated cellulose (MFC) was prepared by refining bleached
hardwood sulphite pulp of 4% consistency with edge load of 2 Ws/m to 28
SR. The pulp were thereafter enzymatically treated with Endoglucanase
(Novozym 476) with the activity of 0,85 ECU/g. The enzymes were dosed to
the pulp and which thereafter was treated at 50 C for 2 hours, at pH 7. After
the enzymatic treatment, the pulp was washed and enzymes were
deactivated at 80 C for 30 min. The pulp was thereafter refined once more to
90-95 SR and the refined pulp was then fluidized (Microfluidizer, Microfuidics
corp.) by letting pulp of 3% consistency pass through a 400pm chamber
followed by a 100pm chamber wherein the MFC used were formed.
Starch used was cationized starch, Raisamyl 70021, Ciba (now BASF).
C-PAM used was Percol 292 NS, Ciba (now BASF).
BMA used was Eka NP495, Eka Chemicals.
Example 1:
The dried BCTMP were soaked in water over night and then dispersed
in hot water. The BCTMP suspension was thereafter diluted to a
concentration of 0.3%.
The produced MFC was also diluted to a concentration of 0.3% and
dispersed using a kitchen mixer.
A formette sheet former was used to prepare the sheets for testing.
The sheets were prepared according to the following procedure; Pulp
suspension measured to produce a 150gsm sheet was added to the stock
tank. During agitation, starch if used, and MFC if used, was added. After 30
seconds, 500g/t C-PAM was added and after another 30 seconds was 300g/t
BMA added to the stock and the sheet forming was thereafter started.
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The formed sheet was wet pressed and dried while the shrinkage was
constrained. The dried sheet was tested for structural density according to
SCAN P 88:01, z-strength according to SCAN P 80:88 and tensile strength
index according to ISO 1924-3.
Table 1: Results of strength and density tests.
Sample Structural z-strength Tensile strength
density (kPa) Index
(kg/m3) geometrical mean
(Nm/ )
Reference 570CSF 388 103 27
Addition of 5% MFC 459 218 40
Addition of 5% Starch 396 142 32
Addition of 5% MFC and 436 240 44
2,5% Starch
Addition of 5% MFC and 426 256 47
5% Starch
As can be seen from table 1 above it is clear that the addition of the
combination of MFC and starch strongly increases the strength, both the z-
strength and the tensile index. Furthermore, the densification of the sheets
has decreased compared to if only MFC was added.
Example 2:
Tests were also performed where MFC and cationic starch either was
pre-mixed or added separately to the stock.
When pre-mixed, the MFC and cationic starch was well mixed before
addition to the stock. When added separately, cationic starch was first added
and well mixed with the stock for 5 minutes followed by addition of MFC.
MFC in an amount of 25 kg/t and cationic starch in an amount of 20
kg/t were added in both samples, both the pre-mixed and the separately
added.
After addition of the MFC and cationic starch to the stock, a formette
sheet former was used in the same manner as described in Example 1.
As a reference the stock without any MFC and cationic starch was
used.
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Scott Bond was measured according to TAPPI UM-403.
Table 2: Results of strength and density tests.
Sample Structural z-strength Scott Bond (J/m2)
density (kPa)
(k /m3)
Reference 310 170 100
Pre-mixed sample 320 210 125
Separate) added sample 320 225 145
As can be seen from Table 2, both z-strength and Scott Bond is
increased when cationic starch and MFC is separately added to the stock.
In view of the above detailed description of the present invention, other
modifications and variations will become apparent to those skilled in the art.
However, it should be apparent that such other modifications and variations
may be effected without departing from the spirit and scope of the invention.
