Note: Descriptions are shown in the official language in which they were submitted.
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Method for improving printability and coatability of paper and board
The invention relates to a method for improving the pn-intability and
coatability of
paper in connection with its production. First of all the method aims to
produce
paper, which after calendering, either machine finished (MF) or
super=calendered
(SC) has gained smoothness and gloss properties well suited for printing.
The invention concerns also calendered and especially super-calendered paper,
and
the use of the paper for gravure printing, besides the use for off set
printing. Espe-
cially the method produces paper having properties well suited for gravure
printing,
besides qualifying also the properties required for off set printing.
The invention relates also a composition suitable for the production of the
paper in
question.
The teen "paper" is used in this connection to mean paper and board, which is
produced using fiber from fiberizing methods which preserve lignin. Examples
of
this type of fiber are groundwood (GV~, pressure groundwood (PGV~, refiner
groundwood and thermo-mechanical pulp (TMP). The invention is applicable also
in paper production processes where chemically treated fiber is used. Such
fibers
include chemi-thenzno-mechanical pulp (CTMP), as well as sulphate and sulphite
pulps. The fiber may also have been processed only in mild chemical conditions
for
softening the lignin portion, such as NSSC-fiber and the like. The invention
can be
accomplished also using returned fiber, including de-inked fiber (DIP). The
invention is workable both on bleached and unbleached fiber.
The fibers of aforementioned kind and mixtures thereof, usually containing a
high
proportion of lignin, are widely used for several printing paper grades. One
exam-
ple to be named is magazine paper.
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Super-calendered (SC) magazine paper contains usually about 75 % of lignin-
rich
fiber, such as bleached groundwood. Unbleached sulphite fiber or semi-bleached
sulphate fiber is used as reinforcing fiber. One portion of the lignin-rich
fiber rnay
also consist of thermo-mechanical refiner fiber, whereby the amount of the
rein-
s forcing fiber can be lower. This paper may contain filler material in an
amount of
12 to 30%. The filler material promotes the achievement of good smoothness and
gloss properties to super-calendered paper. The filler material may consist of
ka-
olin, calcined kaolin, aluminosilicates, talc, calcium carbonate, both earth-
based
and precipitated (PCC), and the mixtures of the aforementioned materials. An
ad-
vantageous paper producing process according to the invention involves the use
of
filler material in amounts of, preferably over 5%, more preferably over 10%,
even
more preferably over 15% and most preferably over 20%.
A usual newsprint furnish consists of a fiber mixture having a chemical pulp
por
tion of about 10 to 20%, whereby the balance of fiber consists mainly of
mechani
cal pulp, such as groundwood (GW), pressure groundwood (PGW), refined
groundwood or thermo-mechanical pulp (TMP), but also de-inked waste paper
(DIP) is used as part of the furnish. The waste paper replaces a part of the
mechani-
cal pulp.
The furnish for light-weight coated papers (LWC) contains a higher percentage
of
reinforcing fiber, up to 50%, and the balance consists of lignin-rich thermo-
mechanical pulp or groundwood. The fibers produced in various methods are
light
bleached, the lignin-rich fiber using known lignin preserving methods, and
chemical pulp using semi-bleaching methods. The use of filler material in the
production of this paper grade is not customary. An exception also in this
case is
use of de-inked pulp bringing alongside usually unavoidable filler material,
which
has its own effects on the paper properties.
The paper disclosed in this application has at least machine-finishing,
preferably it
has been super-calendered, and most preferably it has undergone a finishing
treat-
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3
ment using modern calendering methods, including substrata moulding, which
produce paper quality equal to or exceeding the super-calendered quality.
The high percentage of lignin-rich fiber in paper depresses the strength
properties
of the paper. The problems are traditionally encountered by adding to paper,
in its
production stage where the fibers still form a stock, so called stock starch,
i.e.
starch having an unbroken chain structure, usually at least 5 kg/ton. The
starch
usually has slightly amended cationic, anionic or amphoteric electro-chemical
prop-
erties achieved by incorporating compounds to OH-groups in the starch monomer
structure, which compounds produce cationic, anionic or amphoteric properties.
The degree of substitution (DS) may be from 0.01 to 1, usually below 0.1,
whereby
the starch chain remains unbroken. The use of a proper stock starch improves
the
strength of the paper required for instance in printing and coating of the
paper. In
order to receive a high strength for the papers in question the starch usage
may be
up to 15 kg/ton. Especially a paper produced for off set printing is made with
a high
percentage of stock starch for achieving the required strength and suitable
liquid
penetration properties. The amount of the starch applied is typically over 3
kg/ton
of fiber.
A high percentage of starch in a paper, however, alters the paper properties
and
limits its usability. A high starch percentage renders the paper hard and
stiff,
whereby the compressibility is decreased. This has an adverse effect on the
workability of the paper surface in calendering. The paper is also less
suitable for
gravure printing, where a good printing quality presupposes, besides high
smooth-
ness, a certain degree of compressibility. A paper produced to be applicable
in off
set printing would possess, a fiber furnish composition suitable also for use
in
gravure printing, but the properties resulted from the use of starch prevent
the use
of the paper for this purpose. In the production of paper suitable for gravure
print-
ing, a stock starch addition of less than 1.5 kg/ton of fiber is usual.
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It is also known to use a highly thinned cationic starch as protective colloid
and
retention aid for hydrophobic size-dispersions (such as AKD). However, this
method does not produce strength and compressibility, which properties are
charac-
teristic to the paper produced by the method of the invention.
The problems encountered in papers produced from fibers having a high lignin
percentage, and where the production traditionally involves the use of
polysaccharide based size, such as starch for internal sizing, are, according
to the
invention obviated by adding to the fiber stock, besides a polysaccharide, as
a
hydrophobicity increasing agent, at least a dispersed polymer which contains
hydro-
phobic monomers.
The new composition according to the invention, being applicable in production
of
calendered and super-calendered paper grades for both off set and gravure
printing,
contains afore mentioned polysaccharide and polymer dispersion.
The film forming temperature of the polymer is preferably from -50 °C
to 200 °C,
more preferably from -25 ° C to 100 ° C and most preferably from
0 to ~0 ° C. The use
of a such polymer, besides a polysaccharide, or replacement of a part of the
polysaccharide with this polymer has resulted to a reduction in the stiffness
and an
improvement in the calendering behaviour of paper, and consequently a higher
smoothness in the calendered paper has been achievable, still keeping the
strength
properties of the paper unchanged. This has a general beneficial effect to the
paper
printability. Paper may be produced to suit for off set printing, and the
additional
improvement in the flexibility makes it suitable also in gravure printing.
Compounds applicable in the production of the polymer dispersion include vinyl-
acetate, butyl- and/or 2-ethyJhexylacrylate, methylmethacrylate, acrylnitrile,
sty-
rene, alfa-methylstyrene and/or butadiene. In the production of the dispersion
also
polymerable anionic and/or kationic monomers can be used, such as different
acids,
amines and amides. Examples are acrylic acid, methacrylic acid, and acrylic
amide.
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The polymer dispersion consists preferably of acrylate, styreneacrylate, or
styrenebutadiene copolymer. Preferably the polymer dispersion is produced by
using emulsion polymerisation techniques, where the polymerisation is
conducted
in a water solution. The production technology is described for instance in
the
handbook: Peter A. Lovell and Mohamed S. El-Aasser, Emulsion Polymerisation
and Emulsion Polymers, John Wiley and Sons; pp. 37 to 58.
Starch, mannan, carboxymethylcellulose, polyvinylacetate and/or emulgators can
be used as a stabilizing agent in the production of the polymer dispersion,
prefera-
bly cationic and/or oxidized starch is used as the stabilizing agent. The
production
of the polymer dispersion using starch as a stabilizing agent is described for
in-
stance in the WO publication 00/46264.
The polymer dispersion may be added in accordance with the invention in an
amount of 0.5 to 20 kg/ton of fiber calculated on the dry matter of the
dispersion
and the total dry matter of the fiber composition. A preferred addition amount
is
0.5 to 10 kg/ton of fiber, and a most preferred addition amount is 0.5 to 5
kg/ton of
fiber.
In an application of the invention the polysaccharide may be starch, mannan or
carboxymethyl cellulose (CMC), native, amphoteric or cationic, where the
substitution degree (DS) of the anionic and/or the cationic groups in the
polysaccharide chain is 0 to 2. The polysaccharide is preferably a cationic
starch,
where the substitution degree (DS) of the cationic groups in the starch chain
is 0 to
1, preferably 0.01 to 0.4, more preferably 0.01 to 0.2, even more preferably
0.01 to
0.1, and most preferably 0.01 to 0.05. The viscosity level of the
polysaccharide is
over 5 mPas (5%, 60°C, Brookfield), preferably over 100 mPas, more
preferably
over 300 mPas and most preferably over 400 mPas. Most preferably the
polysaccharide has undergone no substantial thinning (viscosity over 400
mPas),
and has a low cationic degree of substitution (DS 0.01 to 0.05). In the
process of the
invention the polysaccharide is added in an amount of about 0.1 to 15 kg/ton
of
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fiber, even 0.1 to 20 kg/ton, preferably 0.5 to 6 kg/ton, more preferably 1.5
to 5
kg/ton and most preferably 2 to 5 kg/ton of fiber.
When a polymer dispersion is used, which is stabilized with a synthetic
polymer or
with ionic monomers, it is preferred to use a cationic starch as
polysaccharide,
where the degree of substitution of the cationic groups is 0 to 2, preferably
0.02 to
1, more preferably 0.03 to 0.7, even more preferably 0.05 to 0.5 and most
prefer-
ably 0.1 to 0.4. The viscosity level of the polysaccharide is preferably over
5 mPas
(5%, 60°C, Brookfiled), mor preferably 50 to 2000 mPas and most
preferably 100
to 500 mPas. The most preferred polysaccharide in this embodiment is partly
thinned (viscosity 100 to 500 mPas) starch, mannan or carboxymethylcellulose
(CMC) having a relatively high cationic degree of substitution (DS 0.1 to
0.4),
especially starch. In exploitation of the invention the amounts of addition
for this
polysaccharide are within the range of 0.1 to 4 kg/ton fiber, preferably 0.1
to 3
kg/ton of f ber.
It has also been noticed that in practising the invention, the addition ranges
for
polysaccharides having the following degrees of substitution are:
Cationic polysaccharide, DS Minimum amount of addition,
kg/ton fiber
0.01 to 0.05 2
0.05 to 0.3 1
0.3 to 1 0.5
It is also beneficial to use two or more different polysaccharides, whereby
the addi-
tion shares are brought to comply with the aforementioned amounts.
The polymer dispersion and the polysaccharide may be added separately, but it
is
preferred that the addition on a paper machine is simultaneous, either as a
finished
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mixture, or together from the same addition point. The use of a finished
mixture is
most preferred.
The amount of the polysaccharide may also be divided in several parts, whereby
one part is added together with the polymer dispersion or in an admixture with
the
polymer dispersion. The addition of the polymer dispersion and the
polysaccharide
together guarantee that they will be well mixed and, consequently, that a
paper with
equal properties is produced. The simultaneous addition improves also the
effect of
the polymer dispersion, whereby also the smoothness of the paper is improved.
When practising the invention, the hydrophobic properties of the paper may be
increased by adding some other hydrophobic agent to the fiber stock in
addition to
the polymer dispersion. Preferably the addition is conducted simultaneously,
i.e.
from the same addition point or as a finished mixture. ASA, AKD or rosin
sizes, for
1 S instance, may be used as such hydrophobic agents.
The invention will be explained more detailed by means of the following
examples.
Example 1.
Paper (50 g/m2) was produced using 100% peroxide bleached thermo-mechanical
pulp (TMP) having a dewatering degree of 70 °SR. Anionic calcium
carbonate was
further added to the fiber stock as filler in an amount of 10% of the total
fiber com-
position. The fiber stock was admixed with cationic starch in each test point
in an
amount of 0.2%, the starch having a cationic substitution degree (DS) of 0.2.
In test
points 1, 2, 5 and 6 the fiber suspesion was further admixed with stock starch
in
amounts of 0.2 or 0.4 % on the f ber composition, the starch having a cationic
de-
gree of substitution of 0.032. The retention aid used was Percol 162 and
Hydrocol
O, in the amounts of 0.02% and 0.17%, respectively. The polymer dispersion
used
was styrene-acrylnitrile-bytyl-acrylate copolymer, which as a dispersion
stabilizing
agent contained cationic starch in an amount of 20% of the dispersion dry
matter,
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which starch had a degree of substitution of 0.2 in respect to the cationic
groups.
The polymer dispersion was added simultaneously with the starch as a mixture.
The
percentages of each of the added chemicals are calculated as dry matter on the
total
dry matter of the fiber composition. The paper was given a machine finishing
(MF)
by calendering.
Test Polymer Amount Geomet- Geomet- Scott Porosity,
point disper- of starchric ten-ric Bond, Bedtsen,
sion (DS site stiffnessJ/m2 ml/min
added, 0.035) in- index,
% added, dex, Nm/g
% Nm/g
1 0 0.2 30.7 4.07 268 118
2 0 0.4 32.6 4.83 306 117
3 0.4 31.9 4.36 220 93
4 0.8 35.8 4.66 230 107
5 0.4 0.2 32.1 4.00 313 102
6 0.8 0.2 33.0 4.01 376 97
The test results show that by using polymer dispersion a more flexible paper
can be
produced, the paper still possessing a similar improved strength which can be
achieved by using starch. Especially using a mixture of starch and polymer
disper-
sion, the lowest paper stiffness, which is beneficial for gravure printing,
and the
highest internal bond strength, beneficial for the off set printing, are
achieved. The
use of the polymer dispersion has also a beneficial effect to the porosity of
the
paper. A more dense paper prevents a coating colour to penetrate into the
paper
furnish, which improves the coating properties of a paper.
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Corresponding conclusions can be drawn also on the basis of the following
example
2, where the polymer dispersion, deviating from the previous example, is
stabilized
by a synthetic polymer. It may be noted from the test results, that when
polymer
dispersion is used, the porosity and the roughness, as well as the stiffiiess
of the
calendered paper are lower. The use of the polymer dispersion has a beneficial
effect also to the internal bonding and tensile strength of the paper.
Example 2.
Paper (50 g/m2) was produced using 100% peroxide bleached thermo-mechanical
pulp (TMP) having a dewatering degree of 70 °SR. The fiber stock was
additionally admixed with a stock starch in an amount of 0.2% or 0.4% , which
starch had a cationic substitution degree (DS) of 0.20, and with a retention
aid
Percol 162 and Hydrocol O, in the amounts of 0.02% and 0.17%, respectively. As
polymer dispersion was used styrene-acrylnitrile-butylacrylate-trimethylammo-
nium-propyl-metacryl-amidechloride copolymer including synthetic fatty-alcohol-
etoxylate as a stabilizing agent. The polymer dispersion was added as a
mixture
together with the cationic stock starch. The paper was finished to correspond
to
machine finishing (MF) by calendering.
Amount Amount Geomet- Scott Porosity,Rough- Stiffness
of poly-of stockric ten-Bond, Bendtsenness, index
mer dis-starch, sile J/m2 ml/min -
5 persion,% in- Bendtsen
% dex, ml/min
Nm/g
0.4 0.2 31.9 211 223 303 4.23
0.8 0.2 32.7 226 183 253 3.95
0.8 0.4 33.0 221 279 290 4.26
0.4 29.3 188 299 315 4.30
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Example 3.
Paper (60 g/m2) was produced using 70% thermo-mechanical pulp (TMP), which
was bleached with dithionite, and 30% pine kraft pulp having a dewatering
degree
of 70 °SR. To the paper furnish was further added anionic kaolin as
filler in an
5 amount of 30% of the total fiber furnish, stock starch having a cationic
degree of
substitution DS of 0.035 (Raisamyl 135) in an amount of 0.5 %, and Percol 162
as a
retention aid in an amount of 0.02 %. As polymeric dispersion was used styrene-
acrylnitrile-butylacrylate copolymer, which as a stabilizing agent contained
cationic
starch in an amount of 35% on the total dry matter of the dispersion, which
starch
10 had been substituted to a degree of substitution of 0.2 with cationic
groups. The
added amounts of each of the chemicals is calculated as dry matter on the
total dry
matter of the fiber composition. A super calendered (SC) finish was given to
the
paper, and the values of porosity, smoothness and surface strength were
measured,
whereby the following values were obtained.
Amount of Amount of Porosity, Smoothness,Scott Bond,
stock starch,polymer dis-PPS 10, PPS 10, J/m
persion, kPamz kPam2
%
0.5 0.169 1.15 180
1.0 0.171 1.18 263
1.0 0.3 0.108 1.13 397
0.5 0.6 0.009 1.11 271
The results indicate that the polymer dispersion essentially improves the
porosity
and smoothness in a calendered paper, which properties are advantageous in gra-
vure printing.
The use of a high amount of stock starch (10 kg/ton) in this example was
intended
to give to the paper as high as possible internal bonding strength which can
be
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achieved by a stock starch. The addition of the polymer dispersion still
improved
the internal bonding strength value, which means, that the previous strength
level
still can be reached, despite a lower amount of stock starch, when, besides
the star-
ch a polymer dispersion is added to the fiber stock. The paper produced is
thereby
S suitable also for gravure printing.
Example 4.
Paper (40g/m2) was produced using 100% of peroxide bleached thermo-mechanical
pulp (TMP). In addition, aniouc calcium carbonate in an amount of 10% on the
total fiber composition as filler, stock starch having a cationic degree of
substitution
DS of 0.35 in an amount of O.OS, as well as Percol 162 and Hydrocol O as
retention
aid in the amounts of 0.04% and 0.1 S, respectively, were used. The polymer
disper-
sion was styrene-acrylnitrile-butylacrylate copolymer, containing cationic
starch as
1S a dispersion stabilizing agent in an amount of 3S% on the dispersion dry
matter, the
starch having a degree of substitution of 0.2 relative to the cationic groups.
The
added amounts of each of the chemicals are calculated on dry matter basis on
the
total dry matter of the fiber composition. A machine finishing (MF) was given
to
the paper by calendering. The printing tests were conducted using Priifbau-
labora-
tory apparatus.
Amount Amount Amount Density Density Geomeric
of of of at at
polymer colour colour a colour a colour tensile
at a
dispersion,g/m2 at density amount amount strength
a of of of
2S % density 1.0 0.8 g/m2 1.0 g/mz index
of
0.8
0 0.94 1.37 0.73 0.82 11.16
0.1 0.88 1.36 0.76 0.87 11.33
0.3 0.86 1.3 0.77 0.88 12.74
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The results in the table indicate that when, besides starch a polymer
dispersion is
added, a print quality of a certain density level is achievable using a lower
amount
of colour and, correspondingly, a certain amount of colour produces a better
print
quality, than what is achievable when a calendered paper is used which is
produced
without an addition of polymer dispersion. When polymer dispersion was used
the
paper possessed also higher tensile strength values, which are also beneficial
for a
calendered paper used for printing.
Gloss of printed surface
Amount of Gloss, %, at Gloss, %, at Gloss, %, at
polymer disper-the the the
sion, % colour amount colour amount colour amount
of of of
1.0 g/m2 1.5 ghn2 2.0 g/m2
0 14.7 16.2 18.5
0.1 16.7 17.8 19.8
0.3 16.2 18.7 22
The gloss of paper is always higher when polymer dispersion is used in the
internal
sizing than what can be achieved using staxch only in the internal sizing.
The enclosed drawing figure illustrates the water penetration depending on
time on
calendered papers produced according to the Example 4. The measures were con-
ducted using a DPM (Dynamic Penetration Measurement) apparatus. A conclusion
can be drawn, that the polymer dispersion decreases the water penetration
speed,
which is beneficial both in printing and coating of calendered paper. The
beneficial
meaning of this paper feature for printing processes has been described in the
magazine: IPW, No. 5/99, pages 72 to 74, Future Demands on Printing Paper.
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The paper according to the invention, produced using a polysaccharide having a
degree of substitution relative to compounds with an electric charge in the
range of
0.01 to 1.2, and further the aforementioned polymer dispersion, which contains
hydrophobic monomers, has been proven to be especially suitable for use in
gravure
printing. By implementing the invention it was possible to increase the
percentage
of the polysaccharide in a paper suitable for gravure printing without a
negative
effect to properties of the paper, such as compressibility, required from a
paper
suitable for gravure printing. The paper is suited for gravure printing even,
when
the percentage of the polysaccharide is over 1.5 kg/ton of fiber, preferably
over 2
kg/ton, more preferably over 2.5 kg/ton, still more preferably over 3 kg/ton,
still
more preferably over 3.5 kg/ton, even more preferably over 4 kg/ton, most
prefera-
bly over 5 kg/ton, and even over 8 kg/ton of fiber.
A paper used in gravure printing must usually have a polysaccharide percentage
in
the range of 0.1 to 20 kg/ton of fiber, preferably of 0.5 to 10 kg/ton of
fiber and
most preferably of 1 to 5 kg/ton fiber. In certain applications it is
preferred to use at
least 3.7 kg/ton of fiber.
The degree of substitution of the polysaccharides relative to compounds with
an
electric charge has a relation to the amount of the use within the following
ranges:
Degree of substitution, Amount used, Preferred amount of use,
DS kg/ton of fiber kg/ton
0.01 to 0.05 2 to 15 3 to 8
0.06 to 0.29 1 to 12 1.5 to 7
0.3to0.7 O.lto4 O.Sto3
0.71to1.2 O.lto3 O.Stol.S
