Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method, paper machine and base paper for the
manufacture of LWC printing paper coated once
The invention relates to a method for the manufacture of LWC printing paper
coated once according to the preamble of claim 1.
The invention also relates to a paper machine for the manufacture of LWC
printing paper coated once according to the preamble of claim 22.
Further, the invention relates to a base paper according to the preamble of
claim
43.
FI patent application 991096 discloses a method and a paper machine line, in
particular for the manufacture of fine paper. By fine paper is meant uncoated
or
coated fine paper. The basis weight of uncoated fine paper is normally in a
range
of 40-230 g/m2 and the basis weight of coated fine paper is in a range of 60-
250
g/m2. The proportion of mechanical pulp in fine paper is generally below 10
and fillers are added to the stock in an amount of about 15-30 %. Recycled
fibres
can also be used in the pulp. The paper machine line comprises a short
circulation
the stock volume of which has been minimized, a headbox, a gap former, a press
section comprising at least one extended nip press, a dryer section of which
at
least a portion is based on impingement drying, a pre-calender, a two-side pre-
coater and a dryer section after that, an on-line coating station/stations and
an
after-dryer section/sections mainly based on contact-free drying after the on-
line
coating station/stations, an on-line calender in which the linear loads can be
regulated separately in each nip, and a reel-up. The pre-calender can be a
hard nip
calender, a soft calender or an extended nip calender. Relatively low linear
loads
are used in the pre-calender, for example, below 80 kN/m. Pre-coating can be
accomplished by means of a film transfer coating device marketed under the
name
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SymSizer or OptiSizer. The function of pre-coating is to make the pores in the
surface structure of the base paper smaller in a suitable manner in order that
the
actual surface coating shall remain on the surface and is not absorbed into
the
structure of the paper. In coating it is possible to use a suitable coating
station, for
example, a blade coater, a coating device of the jet or spray type,
advantageously
a coating device marketed under the name OptiCoat Jet. Here, the paper machine
in question is thus a paper machine in which coating is accomplished twice.
FI patent 104100 discloses an integrated paper machine. The paper machine
comprises, in the running direction of the web, a mufti-layer headbox, a gap
former having at least one pre-press, a press section having at least one
extended
nip press, a pre-dryer section in which the web is dried by a high-capacity
drying
unit, a dryer section comprising at least one drying group that applies single-
wire
draw, and a surface treatment device for the web. The paper machine has a
closed
draw at least to the end of the dryer section. The surface treatment device
may be
formed of a pre-calender, which is provided between a drying cylinder and a
roll.
FI patent application 981331 discloses a method and a paper machine for the
manufacture of paper. The method and the paper machine are most suitable for
the
manufacture of glossy and porous paper for colour powder printing. The paper
machine comprises a headbox, a wire section, a press section, a dryer section,
a
coating section, an after-dryer section, a calender, and a reel-up. The
headbox and
the wire section are designed such that paper is provided with a desired
composition layering in the z-direction and that the calender is a calender
device
that maintains or at least substantially preserves the porosity of the paper
web
existing before calendering.
In this patent application, by LWC printing paper (Light Weight Coated) which
is
coated once is meant a printing paper the basis weight of which is in a range
of
35-70 g/m2 and in which the basis weight of the coating is in a range of 2-12
g/m2/side. Each side of the base paper is coated once. In the arrangements of
the
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state of the art intended for the manufacture of this kind of printing paper,
the base
paper is subjected to light pre-calendering after the dryer section before
coating.
The pre-calendering according to the state of the art is carried out by a one-
nip
machine calendar such that the thermo roll temperature is in a range of 60-100
°C
and the linear load is in a range of 10-80 kN/m. Pre-calendering is performed
in
the state-of the-art arrangements with low linear loads in order not to lose
too
much bulk in pre-calendering. This kind of state-of the-art light pre-
calendering
provides base paper having a PPS-s10 smoothness in a range of 4-5.7 ~,m, Cobb
Unger oil absorption in a range of 13-26 g/m2 and bulk in a range of 1.7-1.9
cm3/g.
Out of this kind of state-of the-art base paper it is possible to manufacture
LWCO
printing paper (Light Weight Coated Offset), i.e. lightly coated offset
printing
paper using a film coating method or a non-contact coating method. On the
other
hand, out of this kind of state-of the-art base paper it is not possible to
manufacture LWCR printing paper (Light Weight Coated Rotogravure), i.e.
lightly coated rotogravure paper using a film coating method or a non-contact
coating method. This is due to the fact that the film coating method as well
as the
non-contact coating methods do not cover the uneven surface structure of the
base
paper in single coating, but this uneven surface structure of the base paper
is also
visible in the surface structure of the coated paper. Therefore, LWCR printing
paper manufactured out of state-of the-art base paper must be made by a blade
coating method, whereby the uneven surface structure of the base paper is
covered
in coating and the finished coated paper attains the surface properties
required
from LWCR printing paper. The blade coating method in turn requires that the
base paper contains at least 40-50 % chemical pulp and that there is hardly
any
recycled fibres in the base paper. The above-mentioned amount of chemical pulp
provides the base paper with the strength required in blade coating. The blade
coating method limits the speed of the paper machine in on-line coating to a
value
of about 1800 m/min.
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Since the film coating method is, by a rough estimate, about 5 % more
efficient
than the blade coating method and since the film coating method does not
impose
limitations on the speed of the machine in on-line coating, it would be
desirable to
replace the blade coating method with a film coating method or with a non-
contact coating method also in the manufacture of LWCR printing paper. In
order
that LWCR printing paper might be manufactured by a film coating method or
non-contact coating method, the PPS-s10 roughness of the base paper shall be
below 3.5 ~,m and its Cobb-Unger oil absorption shall be below 15 g/m2. A
reduction in roughness and oil absorbency in turn requires more efficient pre-
calendering, with the result that the base paper is densified and some bulk is
lost.
The loss of bulk of the base paper has been thought to also lead to a loss of
bulk
of equal magnitude in the coated end product and thus, in the opinion of the
person skilled in the art, the manufacture of LWCR printing paper has not been
possible by using a film coating method or a non-contact coating method.
However, in the invention it has been surprisingly found that the bulk lost in
reinforced pre-calendering does not necessarily lead to a corresponding loss
of
bulk in the end product. By means of the method in accordance with the
invention
it is possible to manufacture LWC printing paper which is coated once and the
bulk of which is at least as good as the bulk of LWC printing paper
manufactured
according to the state of the art.
The method according to the invention is mainly characterized in what is
stated in
the characterizing part of claim 1.
The paper machine according to the invention is in turn mainly characterized
in
what is stated in the characterizing part of claim 22.
The base paper according to the invention is in turn mainly characterized in
what
is stated in the characterizing part of claim 43.
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The arrangement in accordance with the invention employs a film coating method
or a non-contact coating method and reinforced pre-calendering. The reinforced
pre-calendering in accordance with the invention provides a base paper out of
which it is possible to manufacture LWCR printing paper using a film coating
5 method or a non-contact coating method.
The reinforced pre-calendering in accordance with the invention is based on
the
gradient calendering principle known per se, in which the surfaces of the web
are
densified and the middle part of the web remains bulky. When a web with a
suitable moisture content is brought under pressure in a pre-calendering nip
into
contact with a hot backup surface, the surface temperature of the web can be
raised above the glass transition temperature of fibres, which is dependent on
the
moisture of the fibres. The intention is to bring the surface of the web into
a state
in which the plastic deformation of the fibres is permanent. In that
connection, the
fibres in the surface of the web do not return to their original round shape
and
position in the treatment stages which take place subsequent to pre-
calendering
and increase the moisture content of the surface of the web. The relaxation of
fibres, i.e. return to the original form, causes non-desirable re-roughening
of the
web surface.
By the thermal effect of the pre-calender thermo roll, the surface of the web
lying
against the thermo roll is closed. The pre-calender also improves the
smoothness
of the surfaces of the web and attaches loose fibres or other stock components
to
the surface of the web. The pre-calender also makes it possible to correct the
two-
sidedness associated with the smoothness and oil absorbency of the web. This
can
be done in a pre-calender provided with two nips by regulation of the moisture
content of the web and/or by regulation of the temperatures of the thermo
rolls. In
each pre-calendering nip, the calendering effect is strongest on the surface
of the
web lying against the thermo roll. In the first pre-calendering nip, one
surface of
the web will be against the thermo roll and, in the second pre-calendering
nip, the
opposite surface of the web will be against the thermo roll. Since the web
dries in
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the first pre-calendering nip, the calendering effect of the second pre-
calendering
nip is smaller than that of the first one if the same temperatures are used in
the
thermo rolls. The calendering effect of the second pre-calendering nip can be
enhanced by moisturizing the web between the pre-calendering nips. The
calendering effect of the second pre-calendering nip can also be enhanced by
using a higher temperature in the second pre-calendering nip than in the first
pre-
calendering nip. Of course, the magnitude of the linear load used in each pre-
calendering nip also has an effect on the calendering efficiency of said nip.
Some bulk is lost in the base paper in the reinforced pre-calendering in
accordance with the invention but, surprisingly, the bulk of the end product
remains on at least the same level as that of the LWCR printing paper
manufactured by the blade coating method in accordance with the state of the
art.
One explanation for this phenomenon is that, because of reinforced pre-
calendering, end calendering can be made lighter. Thus, the loss of bulk in
end
ealendering remains smaller than in the arrangements of the state of the art.
Another explanation for this phenomenon is probably found in the pulp used in
the manufacture of paper. The film coating method or the non-contact coating
method used in the arrangement in accordance with the invention makes it
possible to drop the proportion of chemical pulp below 30 %. The film coating
method and the non-contact coating method do not require the same strength
from
the base paper as the blade coating method. The proportion of mechanical pulp
can in turn be raised to at least 70 %. In addition, recycled fibres can be
used in
the pulp, in which connection the chemical fibres contained in the recycled
fibres
are included in the above-mentioned proportion of chemical pulp and the
mechanical fibres contained in the recycled fibres are included in the above-
mentioned proportion of mechanical pulp. The ability of mechanical pulp fibres
to
recover their original shape after compression taking place during calendering
is
considerably better than that of chemical pulp fibres and thus a larger
portion of
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mechanical pulp helps the bulk of the middle part of the web to be preserved,
although the surfaces of the web are densified.
Increasing the proportion of mechanical pulp also improves the formation of
the
web, i.e. small-scale variation of the basis weight in the base paper
decreases.
Mechanical pulp, which comprises shorter fibres than chemical pulp, forms less
flocs in the forming section, whereby the web becomes more even. This also
leads
to the fact that the porosity of the surface layers of the web is already
reduced in
the forming section, with the result that the surfaces of the web become
dense.
The light scattering coefficient of the base paper is also improved when the
proportion of mechanical pulp is increased because the light scattering
coefficient
of mechanical pulp is 60 and the light scattering coefficient of chemical pulp
is
25. As a result of this, the opacity of the paper produced as the end product
is
improved.
A reduction in the proportion of chemical pulp also leads to savings in costs
because chemical pulp is generally more expensive than mechanical pulp. The
use
of recycled fibres also makes the pulp less expensive as compared with the
traditional pulp used in LWCR printing paper.
A higher than normal moisture content of the web when entering the pre-
calender
can be made use of in the pre-dryer section preceding the pre-calender. The
web
simply need not be dried as much in the pre-dryer section, which means that
the
energy demand of the pre-dryer section is reduced. Pre-calendering also
reduces
the need for end calendering. End calendering can be accomplished with a
smaller
number of nips or with lower linear loads. When the two-sidedness possibly
found
in the web in PPS roughness and Cobb-Unger oil absorption is corrected in the
pre-calender, end calendering is facilitated.
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By the method in accordance with the invention it is possible to manufacture
both
LWCO and LWCR printing paper. The greatest advantage is achieved in the
manufacture of LWCR printing paper because the state-of the=art blade coating
method can be replaced with a more efficient and less expensive film coating
method or non-contact coating method.
In the following, components suitable for the paper machine according to the
invention are described with reference to arrangements known per se and
disclosed in patent literature or other publications. With these references,
said
publications are incorporated in this application.
The short circulation process arrangement can be accomplished in the manner
described in WO publication 99/64668 and marketed under the trademark
OptiFeed T"? The mixing of component stocks takes place immediately after the
proportioning chests of the component stocks in a closed mixing volume, after
which the stock is passed in a closed volume to a headbox. By minimizing the
volume of the short circulation and by using an abundance of automation, the
time
taken by a grade change can be shortened from one hour to a few tens of
seconds.
From the point of view of the invention, it is also possible to use a
traditional
stock feed arrangement that is based on a proportioning chest, a machine
chest,
and a wire pit.
Fillers, fines and additives can be fed into the stock before the headbox or
only in
the headbox. Here, it is also possible to use a short circulation and/or a
headbox
that allows layering of additives and/or fillers and/or fines. One stock feed
arrangement of this kind is disclosed in EP patent 651 092.
As the headbox it is possible to use a single- or multilayer headbox. One
multilayer headbox is described in PCT patent application FI97/00713 and in
the
paper by M. Odell: Multilayering, Method of~ Madness?, XI halrraet Paper'
Technology Days 1998. The web can be provided with desired layer structures by
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means of a multilayer headbox by feeding the stock in layers between wires.
The
single- or multilayer headbox can be a dilution headbox, which is marketed
under
the trademark OptiFloT"". In this headbox, the basis weight profile can be
regulated
by consistency regulation and in it it is possible to affect the fibre
orientation by
adjusting the profile. A dilution headbox allows a uniform profile to be
imparted
to the web both in the machine direction and in the cross direction.
From the viewpoint of the invention, it is advantageous that the forming
section is
based on a gap former, for instance, on a roll gap former, a blade-shoe gap
former
or a roll blade-shoe gap former. The gap former is preferably provided with
loading dewatering elements. In the gap former, a slice jet produced by the
headbox is passed between two wires and most of the water is removed between
said wires in two directions, thereby producing a symmetric web. US patent
5, 798, 024 discloses one advantageous roll blade-shoe gap former applied in
the
invention. The paper by L. herkasalo: E~cient forming at High Speeds, XI
Yaln2et Paper Technology Days 1998 describes one advantageous gap former sold
under the trademark OptiFormer T"' The fibre and filler distribution in the
thickness direction of the web can be controlled to some extent by placement
and
vacuums of the dewatering elements of the gap former. Fillers are often
accumulated on the surfaces of the web in the dewatering stages. A gap former
allows higher speeds than those allowed by other types of formers and it
provides
good formation for the web that is being formed. The dewatering blades fitted
immediately after a forming roll reduce the layer thickness of the web before
loading blades. This in turn has a positive effect on the formation of the web
that
is being formed. The formation of the web is advantageously below 3.0 g/m2.
The
blades can be loaded with a relatively high force because the internal bond
strengths of the web that is being formed are not of high significance when
coated
paper is manufactured. In this kind of gap former, the surface of the web can
be
closed, so that the penetration of the coating agent into the web is reduced.
The
good formation, closed surfaces, good symmetry, uniform profile and sufficient
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smoothness attained by the gap former create good opportunities for the
further
treatment of the web.
The press section must have a closed and supported draw through the entire
press
5 section in order that it might be run at high speeds, typically over 1800
m/min.
The solids content of the web shall be raised in the press section to a value
of over
45 %, which is in itself possible in a press section provided with both roll
nips and
extended nips. A high solids content is required in order that the tensile
stress
directed at the web between the press section and the dryer section may be
10 minimized. The speed difference of the web between the press section and
the
dryer section is advantageously below 2 % because the porosity of the web
increases with higher differences of draw. Good bulk, a high solids content
and a
web that is as symmetric as possible are achieved by using a press section
provided with one or more double-felted extended-nip presses. WO publicatiofz
99/60202 discloses in one embodiment a press section provided with two
separate
double-felted extended nips. A press section is marketed under the trademark
OptiPressT"~with one of its embodiments comprising two separate extended nips
which are both double-felted. This kind of press section provides symmetric
dewatering and a web which is symmetric in its surface properties. One of the
felts can also be replaced with a transfer belt that does not receive water
and
transfers the web well. The smoothness of the web and its two-sidedness as
well
as the absorbency of the web can be controlled by means of transfer belts and
press felts.
In the pre-dryer section it is possible to use cylinder drying and/or blowing
drying,
for example, impingement drying and/or through drying. The upstream end of the
pre-dryer section in particular is important in order that the speed
difference of the
web between the press section and the pre-dryer section should remain as low
as
possible. Efficient drying is achieved and the speed difference is minimized
in the
blowing dryer portion situated at the beginning of the pre-dryer section. The
blowing drying also speeds up grade change because the regulation of the
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temperature of the blowing drying units is quick. The pre-dryer section can,
start
from a planar dryer portion that applies blowing drying, which is followed by
a
cylinder drying portion. In the cylinder drying groups it is also possible to
use
suction cylinders which are placed in the basement spaces, which have a large
diameter and in connection with which impingement units are placed for drying
the web running on the outer surface of the suction cylinder. Impingement
drying
also enables more efficient control of the moisture profile as compared with
mere
cylinder drying. PCT patent application FI98/00945 discloses a dryer section
that
applies impingement and which is marketed under the trademark OptiDry TM The
pre-dryer section can also be provided with steaming or moisturizing devices
known in themselves to allow curl of the paper web to be controlled and
regulated.
After the pre-dryer section the web is passed to a pre-calender, which can be
formed of a soft calender or an extended nip calender. The extended nip
calender
can be a shoe calender or a belt calender. In a shoe calender, the extended
nip is
formed between a shoe roll and a hard surface thermo roll. The shoe roll
comprises a stationary support structure and an elastic belt shell disposed
rotatably around it. The belt shell is loaded against the thermo roll by means
of a
loading shoe which is supported on the support structure of the shoe roll and
which forms an extended nip with the thermo roll. The belt calender can be
formed by a thermo roll, a belt loop and a backup roll, which can be either a
hard
surface or a soft surface roll. The belt runs over the backup roll and
guide/tension
rolls. In this kind of belt caler~der, the extended nip is formed between the
belt and
the hot thermo roll, in which connection, for example, a metal belt can be
used.
One such belt calender is disclosed in USpatent 5,483,873. As the pre-calender
it
is also possible to use a one-nip calender if both calendering backup surfaces
are
heatable. FI patent application 971342 discloses one belt calender application
in
which the web runs in a nip between two elastic surface rolls between metal
belts
running around said rolls. If both metal belts are heated, a one-nip calender
is
provided in which both surfaces of the web can be subjected to the treatment
of a
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hot calendering backup surface. The pre-calendar is, however, advantageously a
two-nip or mufti-nip calendar in which both surfaces of the web can be
subjected
to the treatment of a thermo roll. One advantageous calendar suitable for a
pre-
calender is the soft calendar which is marketed under the trademark
OptiSoftT"'in
which a nip is formed between a roll provided with an elastic cover and a hard
surface thermo roll. One OptiSoftTMCalender application is disclosed in FI
patent
application 99221. Another advantageous calendar suitable for a pre-calendar
is
the shoe calendar which is marketed under the trademark Optil7well TM and of
which one application is disclosed in US patent 6,158, 333.
The pre-calendar is followed by coating of the web, by which the surface
structure
of the base paper is evened out. Coating can be performed by applying a film
coating method or a non-contact coating method. The non-contact coating
methods include, among other things, a spray coating method, a curtain coating
method and a dry coating method. FI patent 9727 discloses a spray coating
method marketed under the trademark OptiSpray T"? FI patent application 991863
describes a curtain coating method. It is common to these coating methods that
the
coating layer becomes substantially equally thick. The coating layer conforms
to
the surface profile of the base paper, with the result that an uneven web
surface
causes an uneven coating surface. Coating is performed advantageously in a
compact coating station, in which both surfaces of the web can be coated
simultaneously. A film coating method is very suitable for such coating. The
closed and smooth surface of the web achieved by pre-calendering provides good
conditions for coating. In that connection, it is preferable to use a
profiling coating
device, which can be controlled automatically based on profile measurements.
This ensures a good cross-direction profile for the coated web and a uniform
quality for the coated paper. As an applicator device it is possible to use a
two-
side film coating device which is marketed under the trademark OptiSizerT~'and
in
which both surfaces of the web can be treated simultaneously with a starch or
pigment suspension. When needed, very light coating can be performed by this
kind of applicator device, in which connection the basis weight of coating is
at
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least about 2 g/m2/page. One coating device of this type is described in FI
patent
81734.
The coating station is followed by an after-dryer section which starts with a
dryer
S portion applying contact-free drying. The contact-free drying is followed by
a
short cylinder group, by which the stabilizing of the running of the web, the
draw
and tension of the web are primarily affected at the same time as drying is
continued. The cylinder group advantageously applies single-wire draw. In
connection with contact-free drying, it is possible to apply the drying which
is
marketed under the trademark Tuf~nDryT"" and in which the paper web is dried,
supported in a contact-free manner and turned by the same device, for example,
by a combination of a turning device and an airborne web-dryer. This enables a
quick grade change and, at the same time, stable running of the web is
assured.
One such drying method is described in FI patent 98944.
After that, there is on-line end calendering, the aim of which is primarily to
improve the gloss of the coated web because the required smoothness of the web
has already been achieved in the pre-calender. The two-sidedness associated
with
the roughness of the web has also been corrected on the pre-calender, so that
it is
not any more necessary to make so much correction in the end calender. As a
result of this, end calendering can be made lighter. End calendering can be
performed by a soft calender or a mufti-nip calender. As a soft calender it is
possible to use a calender marketed under the trademark OptiSoftT"'having a
nip
which is formed between a roll provided with an elastic cover and a hard
surface
thermo roll. One OptiSoftTM calender application is disclosed in FI patent
application 992214. The calender described in FI patent 96334 and marketed
under the trademark OptiLoaa"~"~ can be used as a mufti-nip calender, the
rolls of
the roll stack in said calender being relieved such that there is the same
linear load
in each nip. By the inulti-nip calender is here meant a calender which
comprises at
least three rolls in contact with one another such that two nips are formed
between
the rolls. The roll stack of the mufti-nip calender can be located in a
vertical plane
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or the roll stack can form an angle with a vertical plane. The mufti-nip
calendar
can also be formed of several separate roll stacks mounted on the same frame
or
on different frames, so that each roll stack forms, in a way, a mufti-nip
calendar of
its own. A soft calendar having 2-4 nips and a mufti-nip calendar having 4-7
nips
provide the gloss and the smoothness required from the end product in the
arrangement in accordance with the invention. In that connection, the surface
temperature of the thermo rolls in the soft calendar is at least 150
°C, the linear
load is in a range of 50-500 kN/m, and the surface temperature of the thermo
rolls
in the mufti-nip calendar is at least 120 °C and the linear load is in
a range of 150-
600 kN/m. The moisture content of the web before end calendering is regulated
to
be in a range of 5-11 %, advantageously in a range of 5-9 %.
The paper machine ends in a reel-up, for example, a reel-up which is marketed
under the trademark OptiReelTM and of which one application is described in FI
patent 91383. This kind of reel-up allows the amount of bottom broke to be
minimized and a high-quality reel to be achieved, so that further processing
of the
reel takes place without problems.
Appropriate automation and measurement devices are combined with the method
and the paper machine for manufacturing LWC printing paper coated once in
accordance with the invention, for example, for the purpose of determining and
correcting longitudinal and cross-direction profiles of the web or for
accomplishing a quick grade change. As a measurement device is used, for
example, a cross beam having several sensors or scanners and, at the same
time, it
is possible to measure machine-direction variation, for example, by scanning
devices.
In summary, it may be stated that high-quality LWC printing paper which is
coated once can be produced efficiently by the paper machine according to the
invention.
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By using profiling devices, profile variations found in the web can be
corrected in
different parts of the manufacturing line. The basis weight can be profiled by
regulating the consistency of the headbox. In the press section, a steam box
can be
used for increasing and profiling the solids content. Blowing drying enables
the
5 drying to be profiled and in the dryer section it is also possible to use a
moisturizing device for profiling the solids content. Surface size/coating
amount
can be profiled by coaters of the OptiSizer TMtype.
In connection with the invention, it is also possible to use arrangements
suitable
10 for control of curl of a paper web, these arrangements being described in
FI patent
applications 906216, 950434, 964830 and 972080.
In the following, the invention will be described with reference to the
figures in
the appended drawings, to the details of which the invention is, however, not
15 meant to be exclusively confined.
Figure 1 shows a forming section and a press section of a paper machine in
accordance with the invention.
Figure 2 shows the upstream end of a pre-dryer section.
Figure 3 shows the downstream end of the pre-dryer section.
Figure 4 shows a pre-calender, a coating station and an after-dryer section.
Figure S shows an end calender and a reel-up.
Figure 6 shows the PPS-s10 roughness of base paper as a function of bulk for
an
uncalendered base paper, a conventionally pre-calendered base paper and a base
paper subjected to reinforced pre-calendering.
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Figure 7 shows the Cobb-Unger oil absorption of base paper as a function of
bulk
for an uncalendered base paper, a conventionally pre-calendered base paper and
a
base paper subjected to reinforced pre-calendering.
Figure 8 shows the PPS-s10 roughness of coated paper as a function of bulk for
an
LWCO printing paper which has been pre-calendered in accordance with the state
of the art and coated by a blade coating method and a film coating method, and
for an LWCR printing paper which has been coated by a blade coating method.
Figure 9 shows, as a function of bulk, the roughness of LWCO printing paper
which has been film-coated once and pre-calendered in accordance with the
state
of the art and the roughness of an LWCO printing paper which has been film-
coated once and subjected to reinforced pre-calendering.
Figure 10 shows, as a function of bulk, the roughness of an LWCR printing
paper
which has been film coated once and pre-calendered in accordance with the
state
of the art and the roughness of an LWCR printing paper which has been film
coated once and subjected to reinforced pre-calendering.
The paper machine shown in Figs. 1-5 comprises, in the running direction of a
web W, a headbox 100, a gap former 200, a press section 300, a pre-dryer
section
400, a pre-calender 500, a coating station 600, an after-dryer section 700, an
end
calender 800, and a reel-up 900.
Fig. 1 shows the upstream end of the paper machine, i.e. the headbox 100, the
gap
former 200 and the press section 300. The headbox 100 is advantageously a
dilution headbox and it may also include layering of fibres and/or fillers
and/or
fines and/or additives. The gap former 200 comprises a first wire loop 201 and
a
second wire loop 202, between which loops a substantially vertical forming
zone
is formed. Stock is fed from the headbox 100 into a gap formed by the first
201
and the second 202 wire loop between a forming suction roll 203 and a breast
roll
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204. In the forming zone, a first dewatering unit 206 is arranged inside the
first
wire loop 201 and a second dewatering unit 207 comprising loading dewatering
members is arranged inside the second wire loop 202. Water is removed from the
web and the formation of the web that is being formed is improved by means of
the dewatering units 206, 207. At the end of the forming zone, the running
direction of the formed web W is turned by means of the vacuum of a suction
roll
205 placed inside the second wire loop 202, by the suction of which vacuum the
web W is separated from the first wire 201 and caused to adhere to the second
wire 202. After that, the web W is transferred on support of the second wire
202
to a pick-up point P, at which the web W is separated from the second wire 202
by
a pick-up suction roll 303 and transferred on support of a first press felt
301, i.e. a
pick-up felt, to the press section 300.
In the press section 300, the web W is passed between the first upper press
felt
301 and a second lower press felt 302, where the web W runs to a first press
nip
NP1. The first press nip NPl is an extended nip formed by a lower shoe roll
306
provided with a loading shoe and a belt shell and by an upper recessed surface
counter roll 305. After the first press nip NPl, the web W is separated from
the
first press felt 3 O l at a first transfer point S 1 by means of the vacuum of
a first
transfer suction roll 304 situated inside the second press felt loop 302 and
caused
to adhere to the second press felt 302. 'After that, the web W is passed on
support
of the second press felt 302 to a second transfer point S2, at which the web
is
separated from the second press felt 302 by means of the vacuum of a second
transfer suction roll 313 situated inside a third press felt loop 311 and
caused to
adhere to the third press felt 311. After that, the web W is passed on support
of the
third press felt 311 to a second press nip N2. The web W runs in the second
press
nip N2 between the third upper press felt 311 and a fourth lower press felt
312.
The second press nip NP2 is an extended nip, which is formed by an upper shoe
roll 316 provided with a loading shoe and a belt shell and by a lower recessed
surface counter roll 315. After the second press nip NP2, the web W is
separated
from the third press felt 311 and transferred on support of the fourth press
felt 312
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to a third transfer point S3, at which the web W is separated from. the fourth
press
felt 312 by means of the vacuum of a fourth transfer suction roll 410 situated
inside a drying wire loop 419 of a first drying group Rl in the pre-dryer
section
400. After that, the web W is transferred on support of said drying wire 419
to the
pre-dryer section 400.
Here it is also possible to use a press section in which one of the press
felts 311,
312 of the second press nip NP2 is replaced with a substantially non-water-
receiving transfer belt. By the transfer belt it can be ensured that, after
the second
press nip NP2, the web W follows the transfer belt on whose surface the web W
is
passed to the pre-dryer section 400.
Fig. 2 shows the upstream end of the pre-dryer section 400 illustrating first
three
drying groups R1, R2, R3 that apply single-wire draw. The first group R1 is a
downwards open drying group R1 in which heated drying cylinders 411, 413, 413
are above and reversing suction rolls 414, 415 are below.
The web W is brought to the pre-dryer section 400 on support of the drying
wire
419 of the first drying group Rl aided by the vacuum of a suction box 416
disposed inside said drying wire loop 419. After that, the web W runs along a
meandering path between the drying cylinders 411, 412, 413 and the reversing
suction rolls 414, 415. Above the first reversing suction roll 414 there is a
runnability component 417, which ensures the running of the web W in the
portion between the first reversing suction roll 414 and the upper drying
cylinders
411, 412.
From the last drying cylinder 413 of the first drying group Rl the web W is
passed in a nip between said drying cylinder 413 and a drying wire 429 of the
second drying group R2 onto the drying wire 429 of the second drying group R2
and to a first reversing suction roll 424 of the second drying group R2. From
said
reversing suction roll 424 the web W is passed to a first drying cylinder 421
of the
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second drying group R2 and therefrom further to a large-diameter impingement
and/or through drying cylinder 420, preferably having a diameter of over 4 m,
situated below the floor level of the paper machine hall. The impingement
units of
the impingement drying cylinder 420 are designated by the reference numerals
420a and 420b. From the impingement drying cylinder 420 the web W is passed
to a second drying cylinder 422 of the second drying group R2 and therefrom
further in a meandering fashion over a second reversing suction roll 425 of
the
second drying group R2 to the last drying cylinder 423 of the second drying
group
R2. Runnability components 426, 427 are disposed on the reversing suction
rolls
424, 425 of the second drying group R2. The impingement and/or through drying
cylinder 420 placed below the floor level of the paper machine hall provides a
long drying distance for the web in relation to the machine direction advance
of
the web.
From the last drying cylinder 423 of the second drying group R2, the web W
passes in a nip between said drying cylinder 423 and a drying wire 439 of the
third drying group R3 onto the drying wire 439 of the third drying group R3
and
to a first reversing suction roll 434 of the third drying group R3. After
that, the
web W runs in a meandering fashion between drying cylinders 431, 432, 433 and
reversing suction rolls 435, 436 of the third drying group R3. A runnability
component 437 is disposed on the first reversing suction roll 434 of the third
drying group R3.
Fig. 3 shows the downstream end of the pre-dryer section 400 illustrating
latter
four drying groups R4-R7. The fourth R4 and the sixth R6 drying group
correspond in structure to the second drying group R2. The fifth RS and the
seventh R7 drying group in turn correspond in structure to the third drying
group
R3. The pre-dryer section 400 thus comprises in total seven drying groups Rl-
R7.
The second R2, the fourth R4 and the sixth R6 drying group are provided with
impingement cylinders 420, 440, 460 situated below the floor level and with
associated impingement units 420a, 420b; 440a, 440B; 460a, 460b.
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The run of the web W is closed and supported from the beginning of the forming
section 200 to the end of the pre-dryer section 400.
5 Fig. 4 shows the pre-calender 500, the coating station 600 and the after-
dryer
section 700 situated after the pre-dryer section 400.
From the last drying cylinder 473 of the last i.e. the seventh drying group R7
in
the pre-dryer section 400, the web W is passed as an open draw via a frst
10 measurement device 490 to the pre-calender 500. The cross profile of the
web W
is measured in the first measurement device 490 in order that the variations
found
in it might be taken into account in pre-calendering. The variations in the
cross
profile of the web are sought to be equalized by performing profiled pre-
calendering.
The pre-calender 500 may be a soft calender, a shoe calender or a belt
calender.
The nip of the soft calender is formed between a thermo roll having a hard
surface
and a backup roll having an elastic surface. The nip of the shoe calender is
formed
between a thermo roll having a hard surface and a shoe roll serving as a
backup
roll. The nip of the belt calender can be formed between a thermo roll having
a
hard surface and a metal belt running around a backup roll having an elastic
surface or between two rolls having a elastic surface and metal belts running
around the rolls. In the figure, the pre-calender 500 is a two-nip NE1, NE2
shoe
calender, in which the first pre-calendering nip NE1 is formed by a lower
first
shoe roll 510 and an upper hard surface first thermo roll S 11. The second pre-
calendering nip NE2 is formed by an upper second shoe roll 520 and a lower
hard
surface second thermo roll 521. The upper surface of the web W is thus placed
against the outer surface of the shell of the first thermo roll 511 in the
first pre-
calendering nip NEl and the lower surface of the web W is placed against the
outer surface of the shell of the second thermo roll S21 in the second pre-
calendering nip NE2.
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In connection with the pre-calender 500, there is also shown a first
moisturizing
device 512 of the web W, which is situated before the first pre-calendering
nip
NE1 on the side of the thermo roll 511 of the first pre-calendering nip NE1,
and a
second moisturizing device 522 of the web W, which is situated between the
first
pre-calendering nip NE1 and the second pre-calendering nip NEZ on the side of
the thermo roll 521 of the second pre-calendering nip NE2. When needed, the
moisture content of the web W can be regulated by the moisturizing devices
512,
522 so as to be suitable separately for each pre-calendering nip NE1, NE2. As
the
moisturizing devices 512, 522 it is possible to use steam moisturizing devices
or
water moisturizing devices known in themselves. The first moisturizing device
512 is necessary only in the cases in which the web W has been dried too dry
in
the pre-dryer section 400, in which connection the moisture content of the web
W
must be raised before pre-calendering. This is, of course, not desirable but
the web
W should be dried already in the pre-dryer section 400 to a correct solids
content
appropriate for pre-calendering. If there is no need to enhance the
calendering
effect of the second pre-calendering nip NE2, the second moisturizing device
522
can also be omitted. Moisturizing can also be arranged such that a
moisturizing
device 512, 512a is arranged on both sides of the web W before the first pre-
calendering nip NE1 of the pre-calender 500 and/or such that a moisturizing
device 522, 522a is arranged on both sides of the web W before the second pre-
calendering nip NE2 of the pre-calender 500.
The moisturizing device 512, 512a is arranged at a suitable distance before
the nip
of the pre-calender such that the time of action of water before the nip is in
a
range of 0.05-0.5 s. The aim of an appropriately selected time of action is to
cause
both surfaces of the web to be moistened while the middle part of the web
remains
substantially non-moistened. In a two-nip calender, in which the web runs only
a
short distance between the nips, the web can be guided, when needed, to an
additional loop between the nips in order that moisturizing shall have a
sufficiently long time of action. Another possibility is that moisturizing is
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performed only before the first nip of the pre-calendar. In moisturizing,
water is
applied to the web in an amount of 1-4 g/m2/side in order to achieve a desired
moisture content. The initial moisture of the web, the temperatures of the
thermo
rolls of the pre-calendar and the linear loads of the pre-calendar determine
the
need for moisturizing.
After the pre-calendar 500, the web W is passed via a second measurement
device
590 to the coating station 600. The coating station 600 is a coating station
600
which applies roll application 611,612 and is based on film transfer and in
which
both surfaces of the web W are surface-sized/pigmented simultaneously. This
kind of coating station is very efficient and it becomes relatively short in
the
machine direction. The second measurement device 590 measures the cross
profile of the web W in order that the variations found in it might be taken
into
account in coating. The variations in the cross profile of the web are sought
to be
equalized by performing profiled coating.
After the coating station 600, the web W is passed to the after-dryer section
700.
The after-dryer section 700 is mainly formed of a portion 7I0 applying contact-
free drying and of a short cylinder group 720 applying single-wire draw. The
portion 710 applying contact-free drying comprises an airborne web-dryer 711,
a
contact-free turning device 712 for the web W, and an infrared drying unit
713.
The cylinder group 720 comprises a drying wire 729, heated drying cylinders
721,
722 and a reversing suction roll 723 between them. Between the portion 710
applying contact-free drying and the cylinder group 720, the web W is passed
to a
third measurement device 790. The third measurement device 790 measures the
profile of the web W in order that the variations found in it might be taken
into
account in end calendering. The variations in the cross profile of the web are
sought to be equalized by performing profiled end calendering.
Fig. 5 shows the end calendar 800 and the reel-up 900.
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From the last drying cylinder 722 of the cylinder drying group 720 of the
after-
dryer section 700, the web W is passed to the end calender 800, which is
formed
of a two-nip soft calender. A first calendering nip N1 is formed between a
lower
roll 810 having an elastic surface and an upper thermo roll 811 having a hard
surface, and a second calendering nip N2 is formed between an upper roll 820
having an elastic surface and a lower thermo roll 821 having a hard surface.
As
the end calender 800 it is also possible to use a soft calender having 4 nips
or a
mufti-nip calender having 4-7 nips. In the end calender 800, the gloss of the
surfaces of the web W is mainly increased. From the last calendering nip N2 of
the end calender 800, the web W is passed to the reel-up 900 in which machine
reels 910 are made out of the web.
Fig. 6 shows the PPS-s10 roughness of base paper as a function of bulk. The
filled
squares represent the values measured from an uncalendered base paper, in
which
1S connection PPS-s10 roughness is in a range of 6.2-7.1 ~.m and bulk is in a
range
of 1.95-2.21 cm3/g. The unfilled diamonds represent the values measured from a
base paper pre-calendered by a one-nip machine calender in accordance with the
state of the art, in which connection PPS-s10 roughness is in a range of 4.0-
5.6
~.m and bulk is in a range of 1.7-1.9 cm3/g. The filled circles represent the
values
measured from a base paper subjected to reinforced pre-calendering on a two-
nip
soft calender, in which connection PPS-s10 roughness is in a range of 2.2-3.4
p,m
and bulk is in a range of 1.22-1.52 cm3/g. The filled triangles represent the
values
measured from a base paper subjected to reinforced pre-calendering on a four-
nip
soft calender, in which connection PPS-s10 roughness is in a range of 2.1-2.8
~.m
and bulk is in a range of 1.22-1.32 cm3/g. The unfilled squares represent the
values measured from a base paper subjected to reinforced pre-calendering on a
two-nip shoe calender, in which connection PPS-s10 roughness is in a range of
2.6-3.0 p,m and bulk is in a range of 1.45-1.58 cm3/g. The figure shows that
values
of below 3.5 p,m in PPS-s10 roughness can be achieved with a base paper
subjected to reinforced pre-calendering but, at the same time, some bulk is
lost as
compared with traditional pre-calendering.
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Fig. 7 shows the Cobb-Unger oil absorption of base paper as a function of
bulk.
The filled squares represent the values measured from an uncalendered base
paper, in which connection Cobb-Unger oil absorption is in a range of 16-28
g/m2
and bulk is in a range of 1.95-2.21 cm3/g. The unfilled diamonds represent the
values measured from a base paper pre-calendered by a one-nip machine calender
in accordance with the state of the art, in which connection Cobb-Unger oil
absorption is in a range of 13-26 g/m2 and bulk is in a range of 1.7-1.9
cm3/g. The
filled circles represent the values measured from a base paper subjected to
reinforced pre-calendering on a two-nip soft calender, in which connection
Cobb-
Unger oil absorption is in a range of 6.5-14.5 g/m2 and bulk is in a range of
1.22-
1.48 cm3/g. The filled triangles represent the values measured from a base
paper
subjected to reinforced pre-calendering on a four-nip soft calender, in which
connection Cobb-Unger oil absorption is in a range of 8-13.5 g/m2 and bulk is
in a
range of 1.22-1.32 cm3/g. The unfilled squares represent the values measured
from a base paper subjected to reinforced pre-calendering on a two-nip shoe
calender, in which connection Cobb-Unger oil absorption is in a range of 12-15
g/m2 and bulk is in a range of 1.45-1.59 cm3/g. The figure shows that values
of
below 15 g/m2 in Cobb-Unger oil absorption can be achieved with a base paper
subjected to reinforced pre-calendering but, at the same time, some bulk is
lost as
compared with traditional pre-calendering.
It is seen from Figs. 6 and 7 that a sufficiently low PPS-s10 roughness and a
sufficiently low Cobb-Unger oil absorption are achieved by a base paper which
is
subjected to reinforced pre-calendering in order that the base paper may be
coated
by applying a film coating method or a non-contact coating method for the
manufacture of LWCR paper. The PPS-s10 roughness and Gobb-Unger oil
absorption of the traditionally pre-calendered base paper require a blade
coating
method in order that paper meeting the quality requirements of LWCR might be
manufactured.
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Fig. 8 shows the PPS-s10 roughness of a printing paper coated once as a
function
of bulk for LWC paper. The base paper has been manufactured by pre-calendering
in accordance with the state of the art on a one-nip machine calender, in
which the
temperature of the thermo roll was 60-100 °C and the linear load was 10-
60
5 kN/m. The filled diamonds represent the values measured from an LWCO paper
coated by a film coating method or a non-contact coating method, in which
connection PPS-s10 roughness is in a range of 1.12-1.7 p,m and bulk is in a
range
of 0.815-0.93 cm3/g. The unfilled circles in turn represent the values
measured
from an LWCO paper coated by a blade coating method, in which connection
IO PPS-sI0 roughness is in a range of 0.9-1.57 p,m and bulk is in a range of
0.81-1.0
cm3/g. The unfilled triangles represent the values measured from an LWCR paper
coated by a blade coating method, in which connection PPS-s10 roughness is in
a
range of 0.6-0.92 p.m and bulk is in a range of 0.77-0.92 cm3/g.
15 Fig. 9 shows, as a function of bulk, the roughness of LWCO printing paper
coated
once by a film coating method. The unfilled diamonds represent the values
measured from a printing paper pre-calendered by a one-nip machine calender in
accordance with the state of the art and film-coated once, in which connection
PPS-s10 roughness is in a range of 1.7-1.75 p,m and bulk is about 0.94 cm3/g.
The
20 filled circles represent the values measured from a printing paper
subjected to
reinforced pre-calendering on a two-nip soft calender and film-coated once, in
which connection PPS-s10 roughness is in a range of 1.21-1.39 p,m and bulk is
in
a range of 0.9-0.95 cm3/g. The results show that the PPS-s10 roughness of the
LWCO printing paper manufactured in accordance with the invention is lower and
25 its bulk is almost as high as the corresponding values of the LWCO printing
paper
manufactured by traditional pre-calendering.
Fig. 10 shows, as a function of bulk, the roughness of LWCR printing paper
coated once by a film coating method. The unfilled diamonds represent the
values
measured from an LWCR printing paper pre-calendered by a one-nip machine
calender in accordance with the state of the art and coated once by a blade
coating
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method, in which connection PPS-s10 roughness is in a range of 1.15-1.30 p.m
and bulk is in a range of 0.93-1.02 cm3/g. The filled circles represent the
values
measured from an LWCR printing paper subjected to reinforced pre-calendering
on a two-nip soft calendar and film-coated once, in which connection PPS-s10
roughness is in a range of 0.85-1.15 p.m and bulk is in a range of 0.875-1.02
cm3/g. The filled triangles represent the values measured from an LWCR
printing
paper subjected to reinforced pre-calendering on a four-nip soft calendar and
film-
coated once, in which connection PPS-s10 roughness is in a range of 0.97-1.29
~,m and bulk is in a range of 0.92-1.00 cm3/g. The unfilled squares represent
the
values measured from an LWCR printing paper subjected to reinforced pre-
calendering on a two-nip shoe calendar and film-coated once, in which
connection
PPS-s10 roughness is in a range of 1.1-1.21 ~,m and bulk is in a range of 0.92-
0.97 cm3/g. The results show that the PPS-s10 roughness and bulk of the LWCR
printing paper manufactured in accordance with the invention are on the same
level as the corresponding values of the LWCR printing paper manufactured by
traditional pre-calendering.
The results according to the invention shown in Figs. 6-10 have been obtained
with a base paper that contained at least 70 % mechanical pulp. In the pulp, 0-
40
% of recycled fibres was also used, in which connection the mechanical fibres
contained in the recycled fibres were included in the above-mentioned
proportion
of mechanical pulp. The proportion of chemical pulp was 0-30 %, which also
included the chemical fibres contained in the recycled fibres. The pre-
calendering
in accordance with the state of the art was performed on a one-nip machine
calendar in which the temperature of the thermo roll was in a range of 60-100
°C
and the linear load was in a range of 10-60 kN/m. In reinforced pre-
calendering, a
soft calendar with 2-4 nips and a shoe calendar with 2 nips were used. The
temperature of the thermo rolls of the pre-calendars was in a range of 200-300
°C
and the linear loads were in a range of 50-500 kN/m. The moisture content of
the
base paper before the pre-calendar was 5-20 % and the running speeds used were
in a range of 1500-2200 m/min. As the end calendar, a soft calendar with 2-4
nips
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was used, in which connection the temperature of the thermo rolls was 200
°C,
and a mufti-nip calender with 5-7 nips, in which connection the temperature of
the
thermo rolls was 150 °C.
The results obtained by different pre-calendering arrangements are not
necessarily
quite directly comparable with one another. The largest trial series were
performed on a two-nip soft calender, in which connection it has been possible
to
optimize better the pre-calendering and/or end calendering parameters
affecting
the end result.
The claims are presented in the following and the details of the invention may
vary Within the inventive idea defined by said claims and differ from the
disclosure given above by way of example only.
