Note: Descriptions are shown in the official language in which they were submitted.
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Process for Improving Printing Papers
The invention relates to a process for improving the gravure and offset
printability of coated
and uncoated printing papers by increasing the filler share in these papers on
the top and
bottom sides or in the outer layers of the paper web by the use of retention
agents.
According to the prior art, so-called SC and LWC rotary printing papers, which
must meet
higher standards for printing results, are used for large print jobs. The term
"SC papers" refers
to rotary printing papers that contain up to 35% mineral filler and have high
gloss and
smoothness due to post-treatment in a super-calendar. The abbreviation "SC"
stands for
"super-calendared."
The term "LWC papers" refers to rotary printing papers that contain up to 15%
mineral filler
by mass and
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are provided inside or outside the paper-making machine with a pigmented
surface coating, the
coat, on both sides. After being coated, these printing papers are also
treated with a super-
calendar to attain high gloss and smoothness. The abbreviation LWC stands for
"light-weight
coated."
SC and LWC printing papers were initially produced on so-called endless-wire
paper-making
machines. These papers have the disadvantage known as "two-sidedness," i.e.,
due to one-sided
dewatering on the wire, filler and fine material were washed out on the bottom
side, so that
the wire side of the paper was always rougher than the top side, and the paper
web, seen in
the Z-direction, had an irregular or skewed ash cross-section. Varying filler
or ash contents
inevitably lead to irregular printing results on the top and wire sides, so
that such papers no
longer satisfy modern requirements.
To avoid such disadvantages, wire devices with two-sided dewatering, known as
hybrid
formers, were therefore developed.
The German publication "Paper Fabrication Weekly" [" Wochenblatt fur
Papierfabrikation"],
1987, pages 461 through 478, provides an overview of modern sheet formation
systems,
consisting of flow box and sheet formation devices. Section 3, in particular,
describes hybrid
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and twin-wire formers and their advantages compared with the classic endless-
wire models.
In addition, the "Paper Fabrication Weekly" [" Wochenblatt fur
Papierfabrikation"], 1987, pages
662 through 667, describes multi-layer printing papers formed by means of
multi-layer flow
box devices and twin-wire formers. The same reference also discloses that it
is possible to
deliberately influence the distribution of filler in the outer layers of the
paper web by adding
filler to the outer channels of a multi-layer flow box.
Finally, DE 42 37 309 discloses a process for producing a fibrous web, wherein
a fiber-bearing
liquid is evenly distributed, with the help of a flow box, on a wire or
between two wires. To
avoid fluctuations in the area weight cross-section, the retention of fiber-
bearing liquid on the
wire is sectionally influenced by the sectional dosing of retention agent in
the flow box.
Modern twin-wire formers for producing SC and LWC papers are operated at
working speeds
of up to 1,500 m/min. Known SC papers produced with these devices have filler
shares of up
to 34% with very even distribution in the Z-direction. These SC printing
papers currently
constitute
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the highest paper class with respect to printability.
Continued increases in production speed, including in twin-wire formers, are
urgently needed
to increase profitability. However, this raises the problem of intensifying
the dewatering of
the fiber-bearing liquid between the two wires along a given distance. This is
done by
applying a higher vacuum to individual dewatering elements, which engage with
the upper and
lower wires. However, the increased water extraction and the higher flow speed
that
accompanies it inevitably create the disadvantage that more filler is applied
in the Z-direction
of the formed wet paper web. This results in filler impoverishment on the
outer sides, and
thus in a substantial deterioration in printability.
DE 37 20 618 A1 attempts to offer an improvement by providing a process to
improve the
filler retention. The fiber-bearing liquid is supplied to a multi-layer flow
box after first being
divided by fractionation into a long-fiber portion and a short-fiber portion,
which also
contains all fillers. The long-fiber portion is subjected to additional
fibrillation by beating and,
after dilution and the simultaneous addition
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of retention agent, is supplied to the outer channels of a
mufti-layer flow box in front of the pulp pump by means of
the pulp pump. The portion containing short fibers and
filler is supplied to the middle or inner channels of the
5 mufti-layer flow box. Upon subsequent dewatering, the long
fibrillated fibers of a fine network are formed between the
two wires of a twin-wire former. The fine network, as a
filter layer, serves to retain the filler and fine material
of the middle layer and results in an increase in filler.
It is disadvantageous in this process that the anchoring of
the outer layer containing the long fibers is impaired by
the increased filler and fine material content of the middle
layer, so that the Z-strength is decreased. The paper tends
to split when stressed in the Z-direction, e.g., during
offset printing.
In one process aspect, the invention provides a process for
improving gravure and offset printability of coated and
uncoated printing papers, comprising the steps of:
increasing filler share in the paper on top and bottom sides
or in outer layers of a paper web using a retention agent;
and, providing outer layers of fiber-bearing liquid forming
the paper web with additional retention agent one of before
and during sheet formation one of on and between wires of a
paper-making machine, the step of providing additional
retention agent including injecting the additional retention
agent in a flow box downstream from a turbulence generator,
across a width of a suspension flow in top and bottom
boundary layers of the suspension flow.
In a further process aspect, the invention provides a
process for improving gravure and offset printability of
coated and uncoated printing papers, comprising the steps
of: increasing filler share in the paper on top and bottom
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5a
sides or in outer layers of a paper web using a retention
agent; and, providing outer layers of fiber-bearing liquid
forming the paper web with additional retention agent one of
before and during sheet formation one of on and between
wires of a paper-making machine, the step of providing
additional retention agent including spraying the additional
retention agent onto a wire gauze in a twin-wire former in a
returning drum of top and bottom wires downstream of a
turbulence generator.
Suitably, the step of providing additional retention agent
includes providing a retention agent having a high positive
charge. The additional retention agent may have a charge
opposite to that of the retention agent already present in
the fiber-bearing liquid. The additional retention agent
may consist of at least one of the group consisting of
polyethylene imines, polyacrylamides, epichlorhydrine
resins, and polyvinylamines. The additional retention agent
may consist of cationic hydrocolloids, cationic starch or
cationic guar. Suitably, the step of providing additional
retention agent includes injecting the additional retention
agent in a flow box with a multi-layer device, into the top
and bottom layers.
The process for improving the gravure and offset
printability of coated and uncoated printing papers by
increasing the filler share in these papers on their top and
bottom sides or in the pouter layers of the paper web by the
use of retention agents is characterized by the fact that,
before or during sheet formation on the wires or between the
wires
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of a paper-making machine, the outer layers of the fiber-bearing liquid
forming the paper web
are provided with additional retention agent.
The use of retention agents in producing paper has long been known and permits
good filler
and fibrous material retention on the sheet formation wire. Such retention
agents are
generally added to the fiber-bearing liquid before a mixing organ, e.g., the
flow box pump, so
that the fiber-bearing liquid is mixed evenly with the retention agent before
emerging from
the lip or nozzle of the flow box. Starting from the recognition that the
inner layer of a
paper web, due to the filter effect, already has sufficient retention for
filler and fine materials,
the inventors realized that an enrichment of the retention agent in the outer
layer is
advantageous. The problem was thus solved by the additional dosing of
retention agent.
Depending on the paper type, retention agents with cationic or anionic charges
of various
strengths can be used. Although strongly charged retention agents improve the
retention of
filler and fine materials, such agents can, depending on their reaction time
with the fiber-
bearing liquid, form large fiber flocks, which negatively affect the fibrous
formation in the
formed paper sheet. This
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is known as a cloudy look-through.
However, it has been recognized that this negative effect can be suppressed by
shortening the
reaction time. Therefore, in an advantageous embodiment of the process
according to the
invention, a retention agent having a high cationic charge is used.
In addition, so-called dual retention systems are known. In these, the fiber-
bearing liquid is
provided with a first retention agent, e.g., a cationic agent, before the flow
box, and mixed
well. Subsequently, but still before the flow box, a second retention agent,
in this case, an
anionic agent, is added to the fiber-bearing liquid and mixed. The retention
agents with
opposite charges react with each other and with the fibers, the filler and the
fine materials of
the fiber-bearing liquid, and contribute to substantially better overall
retention. In a further
advantageous embodiment of the invention, the charge of the additional
retention agent is
opposite to that of the retention agent already present in the fiber-bearing
liquid supplied to
the flow box.
The following known retention agents can be added individually or in mixtures:
polyethylene
imines, polyacrylamides, epichlorhydrine resins and/or polyvinylamines.
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Cationic hydrocolloids can also be advantageously used. These can consist of
cationic starch
or cationized guar. When cationic starch or guar is used, it is also possible
to influence the
charge and network formation via the degree of cationization and the chain
length of the
starch or guar.
In an advantageous process for providing the outer layers of the fiber-bearing
liquid forming
the paper web with additional retention agent, the retention agent is injected
into a flow box
downstream from the turbulence generator across the width of the fiber-bearing
liquid flow
in its upper or bottom boundary layer. Such a one-layer flow box is called a
step diffusor.
In a further advantageous process, the additional retention agent is injected,
in a flow box with
a mufti-layer device, into the chamber for the top and bottom layers.
In a further advantageous process, when a twin-wire former with a gap device
is used, the
additional retention agent is sprayed onto the wire gauze in the returning
drum of the upper
and lower wires.
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A flow box to implement the process according to the invention has, at a right
angle to the
flow direction of the fiber-bearing liquid, devices for the even dosing of the
additional
retention agent. In the simplest case, these devices comprise borings arranged
in the flow box
in a line that extends at a right angle to the flow direction, which borings
have inflow devices
for the retention agent.
A further device, especially for a mufti-layer flow box with three layers and
step diffusors, has
borings for injecting retention agent that are located upstream from the
respective step
diffusors of the top and bottom layers.
