Note: Descriptions are shown in the official language in which they were submitted.
1
METHOD FOR IMPROVING PAPERMAKING OR BOARD MAKING PROCESS,
USE OF A POLYSACCHARIDE AND PAPER
The present invention relates to a method for improving papermaking or board
making process, use of a polysaccharide and a paper.
BACKGROUND OF THE INVENTION
Economical production of paper and board requires a good runnability of a
paper
machine. The paper machine runnability is often evaluated by the number of web
breaks in proportion to production speed. To attain good runnability, the
paper
must run well with a low number of web breaks in each sub-process along the
entire paper machine line. For example, the fluttering of the paper web in the
drying section should be minimised in order to avoid the possible web breaks.
In
order to avoid fluttering and web breaks, the paper web should preferably have
a
good tensile strength and good residual tension after strain.
Strength of wet paper web is one of the important factors in making of paper
or
board. Machines producing paper grades whose strength before drying is a
critical
factor may have high efficiency but their average production speed may be
significantly lower than their nominal speed. The speed of these paper
machines
could be raised if the strength of the wet paper web could be increased.
Common dry strength agents do not improve strength of the wet paper web. One
example of such dry strength agents is starch, which has a 1,4-a-anomeric
structure. Typical starches include amylose, which is a linear 1,4-a-glucan
polymer
and amylopectin, which has branched structure. The amylopectin backbone is 1,4-
a-glucan polymer and the branches are linked to the backbone with 1,6-a-
glycosidic bonds.
Fillers, such as clay, calcium carbonate, calcium sulphate or talc are used in
paper
and board making to reduce costs and to improve optical properties of paper or
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board. Fillers are added to the stock before the headbox of the paper machine.
For
coated paper grades coating pigments, which comprise the same minerals, may
partly enter to the paper via the broke, which is recycled back to paper
making
process. The content of fillers and coating pigments is typically measured
through
ash content measurement by burning the stock or paper sample in 525 C. The
base paper for uncoated fine paper and for coated fine paper is made from
softwood and hardwood and its ash content is typically 18 ¨ 24 A). The base
paper
for 100 A) softwood based uncoated fine paper and for coated fine paper has
an
ash content typically 10 ¨ 17 /0. An important limiting factor preventing the
increase of filler content in fine papers is the reduced strength properties
of the
paper and reduced web runnability.
An object of this invention is to minimise or even eliminate the disadvantages
existing in the prior art.
An object of the present invention is to provide an effective and simple
method for
improving tensile strength of a paper web or the like.
An object of the present invention is to increase filler content of paper in
order to
reduce papermaking costs.
These objects are attained with the present invention having the
characteristics
presented below in the characterising parts of the independent claims.
Typical method according to the present invention for improving papermaking or
board making process comprises
- forming a fibre stock,
- leading the fibre stock to a headbox and feeding it to a wire to form a wet
fibrous
web, and
- applying at least one polysaccharide having 1,4-8-anomeric configuration in
linkages between saccharide units of the polysaccharide backbone or the main
polysaccharide backbone to the fibre stock after machine chest or on the wet
fibrous web.
3
According to one embodiment, the invention relates to a method for improving
papermaking or board making process, comprising
- forming a fibre stock,
- using a filler in making of paper or board, which filler is selected from
clay,
calcium carbonate, calcium sulphate, titanium dioxide, talc, and their
mixtures,
- leading the fibre stock to a headbox and feeding the fibre stock to a wire
to form
a wet fibrous web,
the method comprising the step of:
- applying to the fibre stock after machine chest or on the wet fibrous web
when a
dryness of the web is < 50 %, at least one polysaccharide having 1,4-3-
anomeric
configuration in linkages between saccharide units of the polysaccharide
backbone or the main polysaccharide backbone, the at least one polysaccharide
being anionic carboxymethyl cellulose, in an amount of 0.3 to 3 kg/ton paper,
the
at least one polysaccharide comprising > 500 anhydroglucose units.
Typical paper according to the present invention is produced by using the
method
according to the invention.
Typical use according to the present invention of a polysaccharide which has
1,4-
f3-anomeric configuration in linkages between saccharide units of the
polysaccharide backbone or the main polysaccharide backbone is for increasing
filler content of the paper or board, for reducing basis weight, and/or
improving
runnability of a wet paper or board web.
Now it has been surprisingly found out that the tensile strength of the wet
paper or
board is clearly improved when a polysaccharide having 1,4-3-anomeric
configuration in the linkages between saccharide units is brought into a
contact
with fibres in the stock or with fibres in a wet paper web. The improved
tensile
strength of the wet web, as well as the improved dry tensile strength of the
paper
that may be achieved with the present invention enables an increase in the
filler
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content of paper. When the residual tension after strain is improved by the
use of
the present invention, a high filler content in the base paper may be used,
corresponding to ash content e.g. over 25 % both for uncoated fine paper and
for
coated fine paper base paper made from softwood and hardwood mixture.
Correspondingly, a high filler content in the base paper may be used for 100 %
softwood based uncoated fine paper and for coated fine paper base paper, the
high filler content corresponding to an ash content over 18 %. An improvement
in
tensile strength may enable an ash content increase also for other paper and
board grades, such as ash content increase to over 15 % for newsprint grades,
or
ash content increase over 12 % coated mechanical base paper, or ash content
increase over 34 % for SC paper. Improvement in tensile strength also may be
utilised by changing to a cheaper raw material mixture for the stock. For
example,
less old corrugated container (OCC) and more collected paper from households
to
make test liner or fluting board grade. The ash content of recycled fibre
based
fluting or test liner board may be increased over 15%.
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Fillers, which are used in making or paper or board, which are suitable for
use in
the present invention, and the content of which may be increased, are
preferably
clay, calcium carbonate, calcium sulphate, titanium dioxide or talc, or their
mixtures. Often the used filler has an anionic net charge.
According to one embodiment of the invention the polysaccharide having 1,4-6-
anomeric configuration in the linkages between saccharide units of the
polysaccharide backbone or the main polysaccharide backbone is selected from
the group comprising water soluble cellulose derivatives; galactomannans, such
as guar gum or locust bean gum; galactoglucomannans; carboxymethyl cellulose;
xylan and substituted glucans, such as xyloglucans; other suitable
hydrocolloids,
such as tamarind gum; chitosan; chitin; or their derivatives. According to one
preferred embodiment the polysaccharide having 1,4-6-anomeric configuration in
the linkages between saccharide units is selected from the group comprising
water
soluble cellulose derivatives; galactomannans, such as guar gum or locust bean
gum; galactoglucomannans; carboxymethyl cellulose; xylan and substituted
glucans, such as xyloglucans; other suitable hydrocolloids, such as tamarind
gum;
or their derivatives. According to another preferred embodiment the
polysaccharide, which has 1,4-6-anomeric configuration in the linkages between
saccharide units of the polysaccharide backbone or the main polysaccharide
backbone, is guar gum. In this context guar gum is understood as a
carbohydrate
polymer containing galactose and mannose structural building blocks,
especially
containing one galactose unit for every two mannose units. The backbone is a
linear chain of 6 1,4-linked mannose residues to which galactose residues are
1,6-
linked at every second mannose, forming short side-branches. Guar gum is
typically obtained as an extract of guar bean. It may be used in native form
or it
may be used in cationised or anionised form.
According to one embodiment of the present invention, anionised guar gum is
applied to the fibre stock or on the wet fibre web after application of
cationic
strength agent to the fibre stock and/or on the wet fibre web. The cationic
strength
agent may be cationic or amphoteric polyacrylamide, polyvinylamide,
polyamidoamine, epichlorohydrin, starch, cationic guar gum or derivative of
these.
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For example, cationic wet strength agent may be applied on the wet fibre web
by
spraying, after which anionised guar gum is applied by spraying. More
typically,
cationic wet strength agent is applied into the fibre stock, after which
anionised
guar gum is applied by spraying on the wet fibre web. Anionised guar gum has
5 typically a charge density < 2 meq/g.
According to another embodiment of the invention the polysaccharide having 1,4-
(3-anomeric configuration in the linkages between saccharide units of the
polysaccharide backbone or the main polysaccharide backbone is carboxymethyl
cellulose, CMC. Carboxymethyl cellulose is understood here as an anionic
polymer, which is produced by introducing carboxylmethyl groups to the
cellulose
chain, the degree of substitution and the chain length of the cellulose
backbone
affecting the properties of CMC, such as water solubility. When the degree of
substitution exceeds 0.3, carboxymethyl cellulose becomes water soluble.
According to another embodiment of the invention the polysaccharide having 1,4-
3-anomeric configuration in the linkages between saccharide units of the
polysaccharide backbone or the main polysaccharide backbone is a
polysaccharide with high degree of polymerisation (DP). This means
polysaccharides which comprise > 500 anhydroglucose units. Size exclusion
chromatography, SEC, may be used for determination of the polymerisation
degree. It has been observed that the tensile strength of the wet paper or
board is
further improved when these polysaccharides are used.
Polysaccharide having 1,4-I3-anomeric configuration in the linkages between
saccharide units of the polysaccharide backbone or the main polysaccharide
backbone and used in the present invention is water soluble. In case a
derivative
of the polysaccharide is used, the derivative is also water soluble. The
viscosity
(Brookfield) of the polysaccharide solution is < 5000 mPas, preferably < 2000
mPas. Solution may be diluted in order to achieve the desired concentration.
The polysaccharide having 1,4-3-anomeric configuration in the linkages between
saccharide units is applied as a solution to the wet fibre web in any suitable
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manner. Preferably the solution is obtained by dissolving the polysaccharide
in
powder form into a solvent, typically water. Preferably the polysaccharide
solution
is free from discrete polysaccharide particles. The polysaccharide solution
may
comprise one polysaccharide or it may comprise a mixture of different
polysaccharides, for example a mixture of two or three polysaccharides. Thus,
according to one embodiment a mixture of different polysaccharides may be
applied to the fibre stock after machine chest or on the wet fibrous web.
Typically
the concentration of the polysaccharide(s) in the polysaccharide solution is <
60
weight-%, more typically 0.02 ¨ 5 weight-%, preferably 0.05 ¨ 3 weight-%, more
preferably 0.05 ¨ 2 weight-%. Concentration of polysaccharides with high
degree
of polymerisation (DP) in the solution may be even < 1 weight-%, more
typically
0.05 ¨ 1 weight-%, even more typically 0.2 ¨ 0.6 weight-%.
According to one embodiment of the invention the polysaccharide is applied to
the
fibre stock between the last pump preceding the paper or board machine headbox
and the outlet of the paper or board machine headbox. Preferably, the
polysaccharide is added to the stock as near the headbox as possible, or the
polysaccharide may be added directly to the headbox, if adequate mixing to the
stock can be secured. Addition of the polysaccharide near the headbox improves
the bonding of the fibres together with the polysaccharide, as the
polysaccharide
remains in outstretched form due to short residence time in the stock and the
adsorption of the polysaccharide over the fibre surface is reduced. Also, when
the
polysaccharide is added to the stock after the last pump, the risk for
breaking the
flocks generated by the polysaccharide and fragmentation of the polysaccharide
backbone due to shear forces is minimised. Thus, the activity of the
polysaccharide remains in a high level, and it dosage may be reduced or better
tensile strength values may be obtained by using the same dosage.
According to one embodiment of the invention the polysaccharide is applied
into
the fibre stock together with a retention or drainage agent. The
polysaccharide and
the retention agent are added to the fibre stock typically near the headbox,
for
example by dosing at the machine filter. The retention or drainage agent may
be
any suitable retention agent. The retention agent may be selected from a group
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comprising anionic or cationic polyacrylamide, polyvinylamine,
polyethyleneimine,
cationic starch, bentonite or silica. Especially the retention agent may be
anionic or
cationic polyacrylamide, polyvinylamine or polyethyleneimine. The retention
agent
and the polysaccharide may be added as separate solutions, or they may be
added as single solution, comprising both the retention agent and the
polysaccharide. Polymeric retention agent dosage may be 50 ¨ 1000 g/t,
preferably 100 ¨ 600 g/t, given as dry polymer, and the polysaccharide dosage
may preferably be 200 ¨ 4000 g/t, preferably 500 ¨ 2500 g/t, given as dry
polymer.
According to another embodiment of the invention the polysaccharide is applied
into the fibre stock together with an anionic, cationic or amphoteric dry
strength
agent. The dry strength agent is selected from the group comprising
polyacrylamides, glyoxylated polyacrylamides, polyvinylamines, polyamine
epichlorohydrine co-polymers (PAAE), starch derivatives, and carboxymethyl
cellulose. The dry strength agent may be applied in amount of 0.1 ¨4 kg/t
paper,
typically in amount 0.2 - 2 kg/t, given as active substance.
According to one preferred embodiment of the invention the polysaccharide is
applied on the wet fibre web between the headbox and the last nip of a press
section. According to one preferred embodiment of the invention the
polysaccharide is applied on the wet fibre web by spraying, by coating, by
film
transfer or by foam layer application. It may be applied by using film
transfer to a
press belt, or by feeding of polysaccharide solution from a separate headbox.
Preferably the application of the polysaccharide solution is performed by
spraying.
It has been found out that the spraying of the polysaccharide solution onto
the
fibre web provides many surprising advantages. Spraying of the polysaccharide
solution does not influence the formation of the paper web, whereby there is
no
negative effects to be noticed in the final paper properties. On the other
hand, it
has also been noticed that the retention of the polysaccharide to the web is
improved. This means that the used amount of the polysaccharide can be kept
low, and chemical losses may be minimised. It has been observed that when the
polysaccharide solution is added by spraying, the polysaccharide is evenly
distributed through the whole web. No significant difference in amount of the
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polysaccharide can be observed between the surfaces and the core part of the
web.
Preferably, the polysaccharide is applied by spraying onto the wet paper web.
It
has been observed that the polysaccharide amount, which is applied, may be
reduced when the application is done by spraying, and still the improved
tensile
strength characteristics of the paper web are obtained. A polysaccharide
solution
suitable for use in the spraying may be obtained, for example, by dissolving a
polysaccharide in powder form into water in order to form a 0.2 ¨ 20 weight-%,
preferably 0.3 ¨ 3 weight-% solution.
According to still another embodiment, the polysaccharide is applied by foam
layer
application or foam coating. The polysaccharide may be applied by foam
coating,
whereby the polysaccharide is applied as a foam, which has an air content of
60 ¨
95 %, onto the wet paper web.
Irrespective of the method of application of the polysaccharide, the
polysaccharide
is applied in amount 0.1 ¨ 10 kg/(ton paper), preferably 0.3 ¨ 3 kg/(ton
paper).
When the polysaccharide is applied by spraying, it may be applied in amount <2
g/m2, typically 0.05 ¨ 1.5 g/m2, more typically 1 g/m2, most typically 0.05
¨ 1
g/m2, preferably 0.05 ¨ 0.5 g/m2, more preferably 0.05 ¨ 0.3 g/m2 on the wet
paper
web.
According to one embodiment of the invention the polysaccharide solution is
applied on the wet paper web when the dryness of the web is < 50 /0,
typically <
40 /0, more typically < 30 %, preferably 8 ¨ 15 %. When the pulp suspension
enters the headbox and thus the paper machine, its dryness level is typically
more
or equal to 0.3 % and less than 2 %. The first water removal from the web is
driven
by gravity when the web enters the wire section from the headbox. As paper
travels further in the wire section, water removal is assisted by different
vacuum
units. After the wire section, the dryness of the paper is typically 14 ¨ 22
%. The
dryness of paper increases to 40 ¨ 55 % during wet pressing. The applying of
the
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polysaccharide solution is preferably conducted before the last vacuum zone of
the wire section, preferably by spraying.
According to one embodiment of the invention two or more polysaccharides may
be applied on the wet fibre web after each other by spraying. Thus layers of
different polysaccharides may easily be applied on top of each other in order
to
obtain desired properties.
According to one embodiment an anionic or cationic polymer solution may be
.. applied to the wet paper web before or after the addition of the
polysaccharide.
The application of the anionic polymer is performed to the wet paper web
before
press section of a paper machine. For example, the application of the
polysaccharide to the wet paper web may be preceded or followed by application
of cationic or anionic polymer solution. This kind of sequential application
of
.. polysaccharide and one or more polymers to the wet paper web, preferably
through spraying, may produce a marked improvement of dry and wet paper web
strength. Anionic and cationic polymer solutions may also be pre-mixed
together
before their application, preferably by spraying, to the wet paper web.
.. The present invention is advantageous for improving strength of the wet
paper
web when producing wood-free uncoated and coated paper grades. The present
invention is also suitable for improving runnability of a wet paper or board
web by
improving strength of the wet paper web when producing paper grades including
wood-free uncoated paper, wood-free coated paper, super calendered (SC) paper,
.. ultralight weight coated (ULWC) paper, light weight coated (LWC) paper or
newsprint paper, but not limited to these. Especially paper webs that are to
be
used for making recording substrates for the inkjet printing are suitable to
be
treated according to the method of the present invention. The paper web may
comprise fibres from hardwood trees or softwood trees or a combination of both
fibres. The fibres may be obtained by any suitable pulping or refining
technique
normally employed in paper making, such as thermomechanical pulping (TMP),
chemimechanical (CM F), chemithermomechanical pulping (CTMP), groundwood
pulping, alkaline sulphate (kraft) pulping, acid sulphite pulping, and
semichemical
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pulping. The paper web may comprise only virgin fibres or recycled fibres or a
combination of both. The weight of the final paper web is 30 ¨ 800 g/m2,
typically
30 ¨ 600 g/m2, more typically 50 ¨ 500 g/m2, preferably 60 ¨ 300 g/m2, more
preferably 60 ¨ 120 g/m2, even more preferably 70 ¨ 100 g/m2.
5
In some embodiments the paper web may comprise fibres originating from non-
wood material, such as bamboo, sugar cane bagasse, hemp, wheat or rice straw.
According to one embodiment of the invention the filler content of the paper
or
10 board is increased, whereby the ash content in the wet paper or board
web is >25
A, for wood-free uncoated paper, >25 % for wood-free coated paper base paper,
>34 % for super calendered (SC) paper, >13 % for coated mechanical base paper,
>15 % for newsprint paper, fluting board or testliner board, the ash content
being
measured by burning the stock sample completely in 525 C.
One or more layers of chemical solutions may be applied to the wet paper web
before the press section or drying section. The addition of a cationic polymer
to the
stock of fibres is not compulsory, but it may be performed. The chemical
solutions
are preferably applied to the wet paper web by spraying, as described in the
application, but they may be applied by coating, film transfer, foam layer
application or feeding from a separate headbox. The chemical solution that is
applied to the web, e.g. by spraying, may be a solution of carboxymethyl
cellulose
(CMC), polyvinyl alcohol (PVA), chitosan or guar gum. Guar gum is here
understood as a galactomannan. It is a polysaccharide comprising galactose and
mannose. The backbone of the guar gum is a linear chain of 131,4-linked
mannose
residues to which galactose residues are 1,6-linked at every second mannose,
forming short side-branches. Guar gum may be applied to the web in form of
native guar gum, anionic guar gum or cationic guar gum. For example, native,
cationic or anionic guar gum may be applied to the wet paper web, which is
formed without using addition of a cationic polymer to the stock. In another
example, native or anionic guar gum may be applied to the wet paper web, which
is formed from stock into which cationic polymer, such as cationic guar gum,
is
added.
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EXPERIMENTAL
Example 1
Elementary Chlorine Free-bleached pine pulp was obtained from a Finnish pulp
mill. The pulp was refined and dewatered at the mill. The pulp was packed as
never-dried into airtight polyethylene bags, and kept at -18 C until used for
testing.
The Schopper-Riegler (SR) value of the pulp after dewatering and freezing was
20, measured according to ISO 5267-1. Native dissolved guar gum (Sigma
G4129Tm), carboxymethyl cellulose (DS 0.7, DP 140) and chitosan (Mw 400,000
g/mol) was added to the thick stock pulp 30 ¨ 90 min before sheet preparation
as
a 0.5 weight-% solution.
Wet and dry handsheets were prepared according to SCAN-C 26:76 standard.
.. Grammage of handsheets was 60 g/m2. After wet pressing the handsheets were
stored at cold storage room in airproof packages before measurements in order
to
maintain constant moisture in the sheets.
For the spraying of chemicals at laboratory, formed wet handsheets were placed
onto the wire and attached using vacuum. Vacuum usage enhanced also the
penetration of chemicals into the paper during spraying. The experimental unit
comprised a vacuum box, moving sample sledge with wire, and spraying unit. The
amount of the chemical sprayed was adjusted by the speed of the moving sample
sledge, while the spray remained constant and was immobilized. The samples
were wet pressed with 350 kPa and 50 kPa for 5+2 minutes after spraying. The
higher pressure gives higher dryness for test sheet. Chemical consistency
during
spray tests was 0.5%.
Measurements
Tensile strength was measured according to ISO 1924-2:2008. The dryness of the
paper samples was determined by using a Mettler Toledo HR73TM infra-red dryer.
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Results
Figure 1 shows results of laboratory tests (Example 1) for guar gum, which was
added to thick stock pulp or sprayed on wet web. It can be seen that it is
advantageous to add guar gum for wet web strength later at paper machine
process rather than to thick stock pulp.
Figure 2 shows results for laboratory tests (Example 1) for CMC, chitosan and
guar gum, each of which was sprayed on wet web. The effect of different
polysaccharides on wet web strength can be seen. Guar gum is most effective.
Chitosan and carboxymethyl cellulose (CMC) improved also wet web strength. As
reference were used a spraying of water or handsheet without any spraying.
Example 2
Pulp containing 70% hardwood with SR-value 24 and 30 % softwood with SR-
value 28 was acquired from a Finnish pulp mill. SR-value was measured
according
to ISO 5267-1. Precipitated calcium carbonate was used as filler. Filler was
added
to the pulp and target level for addition was 20 % filler content in the final
web.
Retention chemical was Fennopol K3400R Tv (Kemira Oyj) with dosage 200 g/t
and it was added to the headbox feed flow. Tests were made with a small
fourdrinier type of wire section. Grammage of formed web was 70 g/m2.
Chemicals
were sprayed on the wet web at the wire. The samples were wet pressed with 350
kPa and 50 kPa for 5+2 minutes after spraying.
Measurements
Tensile strength was measured according to ISO 1924-2:2008. The dryness of the
paper samples was determined by using a Mettler Toledo HR73TM infra-red dryer.
Results
Figure 3 shows results for tensile strength in semipilot test (Example 2) for
sprayed
guar gum and Figure 4 shows tensile energy adsorption in semipilot test
(Example
2) for sprayed guar gum. The effect of guar gum wet web spraying dosage levels
on wet web strength can be observed in the Figures. Spraying dosages were 0.1
g/m2 (1.4 kg/t), 0.3 g/m2 (4.2 kg/t) and 0.5 g/m2 (7.1 kg/t). Dosages of 0.1
g/m2 and
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0.3 g/m2 improved both wet web tensile and solids content after wet pressing.
Dosage of 0.5 g/m2 improves strength further, but solids content in wet
pressing is
reduced. Therefore optimum dosage may be between 0.1 g/m2 and 0.5 g/m2, at
least under these experimental conditions. Tensile strength is needed for web
to
.. keep enough tension at paper machine dryer section to allow high operation
speed. If the tension of the web is not high enough, the sheet does not follow
dryer
fabric and sheet fluttering may cause web break due to wind effect caused by
high
speed. Tensile energy adsorption T.E.A. improvement helps to avoid web break,
if
web has fault such as a hole, slime spot, sticky particle or locally lower
basis
weight, because higher strength reduces risk that web tears apart beging from
the
fault position.
Example 3
Test furnish was a mixture of softwood and hardwood kraft pulp for fine paper,
containing 40 weigth-% scalenohedric precipitated calcium carbonate (FCC)
filler.
In some tests additional PCC was applied. Chemicals were added to furnish
under
stirring with magnetic stirrer before sheet preparation. Dosing time of
cationic
polyacrylamide (C-PAM) retention aid and guar gum was typical for paper
machine
retention system, see Table 1. Guar gum and C-PAM were premixed as powder in
proportion 1:1, and then dissolved to 0.5 weight-% concentration with water,
giving
a final concentration of 0.25 weight-% guar gum and 0.25 weight-% C-PAM.
Chemicals used in the tests were: cationic potato starch (DS 0.035), guar gum
(Sigma G4129TM) and cationic polyacrylamide, C-PAM, Fennopol K3400RTM
(Kemira Oyj). All were dissolved to 0.5 weight-% solution except starch was
cooked to 1 weight-% solution.
Handsheets were prepared with Rapid-Kothen semi-automatic sheet former to 80
g/m2 basis weight according to ISO 5269-2:2004. Ash content was measured
according to ISO 1762:2001.
Wet web sheets were wet pressed according to ISO 5269-1:2005, but pressing
time was 1 min at 2 bar pressure between 2 plotters on top and 2 plotters
under. In
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tests D and E the wet pressing was 2 min at 4 bar pressure. Wet web tensile
measurements were performed at the dry content after wet pressing.
Dry tensile handsheets were vacuum dried according to Rapid-Kothen method.
Tensile indexes were measured according to ISO 1924-2:2008. The results can be
seen in the Table 1.
Table 1. The wet web tensile index and dry tensile index result for different
handsheets.
Dose -10min - 20 s -15 s -15 s
time
Starch added C- Guar sheet wet dry
PCC PAM gum ash, web tensile
525 C tensile index
index
Test kg/t g/t g/t Nm/g Nm/g
A 0 0 0 0 11 0,46 40
0 0 0 600 17 0,71 33
0 0 150 150 32 0,46 21
0 20 150 0 38 0,45 14
6 20 150 0 38 0,37 15
From the results it can be seen that the wet web tensile index was improved in
test
B compared to test A; also ash content increased, which reduced the dry
tensile
index. In test C significantly higher ash content was achieved with guar gum
and
C-PAM blend with similar wet web tensile compared to test A. In test E
cationic
starch was used as strength agent in high filler containing pulp. Starch
decreased
wet web strength compared to test D.
Even if the invention was described with reference to what at present seems to
be
the most practical and preferred embodiments, it is appreciated that the
invention
shall not be limited to the embodiments described above, but the invention is
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intended to cover also different modifications and equivalent technical
solutions
within the scope of the enclosed claims.
