Note: Descriptions are shown in the official language in which they were submitted.
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Method and apparatus for treating the surface of a web
The invention relates to a method for treating the surface of a web, in
which method liquid is added on the surface of a paper or paperboard
web, possibly with other constituents. The invention also relates to an
apparatus for implementing the aforementioned method.
The invention relates to the manufacture and finishing of paper, the end
product of which is a finished paper. In particular, the invention relates
to the manufacture of printing papers, intented to be printed at a
separate stage after their surface has been finished and the web has
been reeled.
After the paper or paperboard has been dried, the surface structure of
the web is made suitable by means of a mechanical treatment,
calendering. There are several calendering methods, but it is common
to all of them that the web is passed through one or several nips which
are formed between two surfaces, typically between rotating roll
surfaces. The purpose of the calendering is to improve the paper
quality by pressing the paper into a fixed final thickness, and especially
by smoothing its surface. As is well known, the mouldability of the fibres
contained in the paper or paperboard, the "plasticization" of the web in
connection with the calendering, can be improved by increasing the
temperature and/or moisture. It is often advantageous to restrict the
impact of the temperature and moisture only on either surface layer or
on both surface layers of the web, wherein the mouldability of their
fibres can be improved without affecting the central layer of the web
excessively. As a result of this, a known procedure is the wetting of the
surfaces of paper or paperboard webs before the calendering of the
web.
When wetting is utilized, the problem is that it is difficult to increase the
water content to attain the desired moisture gradient in the thickness
direction of the web. When for example steam is used, water can
condensate poorly on the surface of the web, especially when its
surface temperature is high. The most common problem is the
hydrophobicity of the web, i.e. when the web is wetted by means of a
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known method, the adhesion of the web to the surface is poor. To attain
a sufficient wetting on the surface of the web, it is thus necessary to
use such a dosage which results in an undesirable increase in the
moisture also in the inner parts of the web. The aim is, however, to
attain a steep moisture gradient between the surface and the inner part
of the web. Hydrophobicity may result from the raw materials used in
the pulp, but there is also an apparent hydrophobicity which results
from the air film carried by the rapidly moving web, and the air brought
along by the wetting liquid, which is perceivable especially in spray
wetting.
Another problem occurring in the finishing of the web is the absorption
of the composition used in surface sizing or pigmenting into the paper
or paperboard web. Since the solutions, sizes and pastes used in these
working stages are aqueous, the problems are based on the same facts
as in the wetting.
The purpose of the invention is to eliminate the aforementioned
drawbacks and to present a versatile method by means of which it is
possible to attain a sufficient absorption of liquids, especially water, on
the paper or paperboard web. To attain this purpose, the method
according to the invention is primarily characterized in that the
absorbency of the surface of the paper or paperboard web is increased
before liquid is brought on the surface of the web or simultaneously with
bringing liquid on the surface of the web. The absorbency of the
surface can be improved e.g. by means of a preceding energy
treatment, with which polar groups are produced in the material
constituting the surface of the web, by means of a preceding addition of
steam or an underpressure effecting simultaneously through the
surface of the web.
Papers and paperboards are materials with heterogeneous surface
energy. It has been discovered that the increase in the proportion of
mechanical pulp and/or the act of adding a hydrophobic chemical
increases the amount of fluctuation in the surface energy. The surface
energy of the paper affects the absorbtion of liquids, the wettability of
the surface and the adhesion properties of the surface. The more
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uniform the structure in view of its surface energy is, the more uniform
are the effects of said processes. By means of the above-described
treatments it is possible to reduce and equalize the surface energy.
Especially when water is sprayed from underneath the paper with
determined spraying parameters, the surface energy unequivocally
determines the amount of water attached on the surface of the paper.
The fluctuations in the surface energy appear in an irregular, small-
scale fluctuation in the "adhesion" and moisture profile. This can cause
an uneven calendering result, which causes e.g. mottling in printing. An
uneven wetting of the surface results in that the sections which have a
lower surface energy become wet and absorb water faster than
sections with a higher surface energy. In practice, this can, in addition
to the aforementioned reasons; result from irregularities in the moisture
profile of the paper before wetting.
W ith the oxygen and radical reactions produced by means of an energy
treatment, chemical changes are effected in the surface of the web W
in the form of new polar groups, such as carbonyl, amide, hydroxyl and
nitro groups, in the polymer material of the fibrous raw material of the
web, especially in cellulose, hemicellulose and lignin. Thereafter it is
easier to add the liquid, i.e. to perform the wetting with water, the
surface sizing or the pigmenting.
The energy effecting the aforementioned chemical changes can be
produced by means of a corona treatment, a flame treatment, an ozone
treatment or a combination of these.
Another way of reducing the surface energy is to add steam. Steam can
be brought in the form of hot or cold steam, and by means of a suitable
dosage it is possible to attain a nearly monomolecular liquid layer. The
steam is adsorbed evenly on the surface of the web, and it equalizes
and maximizes the adhesion of a considerably larger amount of liquid
brought at the next stage on the web.
The third way is to remove the apparent hydrophobicity of the web by
means of an underpressure acting on the surface simultaneously with
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the addition of liquid, the underpressure being effected through the web
from the other side. To increase the absorbency of the surface in this
way, it is possible to use a suction box and/or means producing at least
underpressure impulses.
Furthermore, the purpose of the invention is to present an apparatus by
means of which it is possible to produce a better adhesion of the liquid
on the web. To attain this purpose, the apparatus is characterized in
that in addition to the device for adding the liquid it comprises a device
for reducing the hydrophobicity. This device is provided for bringing
energy at least to the immediate vicinity of the surface of the web, to
produce new polar groups on the surface of the web, for applying
steam on the surface of the web, or for exerting direct underpressure
on the surface of the web.
In the following, the invention will be described in more detail with
reference to the appended drawings, in which
Fig.1 shows the principle of the method according to the
invention,
Fig. 2 shows a side-view of an apparatus according to the first
embodiment of the invention,
Fig. 3 illustrates an experiment performed by means of the first
embodiment of the invention,
Fig. 4 shows a side-view of an apparatus according to the second
embodiment of the invention, and
Fig. 5 shows a side-view of an apparatus according to the third
embodiment of the invention.
Fig. 1 illustrates the treatment stages of a paper or paperboard web W
according to the invention. The web in question travels in a paper or
paperboard machine or in a finishing machine for paper or paperboard,
and its surface is subjected to finishing whereafter it is reeled to form a
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reel. Hydrophobicity is reduced at a stage A by means of a treatment
effected on the surface of the web on the same side from which liquid is
added at a stage B. It is also possible to reduce the hydrophobicity at a
stage C by means of a treatment effected from the opposite side of the
5 web W on the surface of the web on which liquid is added. This
treatment stage C can take place simultaneously with the stage B or
slightly thereafter. Finally, the web W is passed to a finishing stage D of
the surface which can follow immediately after the stages A and B or
after the stages B and C.
The need to reduce the hydrophobicity arises after a press at that stage
when a completely formed web is at least partly dry, and it is absorbent
for re-wetting. When the hydrophobicity occurs for the first time, the dry
matter content is approximately 60 to 65%.
The liquid to be added denotes liquid as such or liquid in a formulation
in which other substances are mixed therein, for example substances
used in surface sizing and pigmenting, wherein the liquid functions as a
carrier for these substances. In these cases the liquid is issued in the
form of a surface sizing solution, a surface sizing paste, or a coating
colour. The liquid is any liquid whose adhesion to the web can be
improved by reducing the hydrophobicity on the surface of the web,
especially water. The above-described stage B can thus be wetting,
surface sizing or pigmenting.
In the paper machine or finishing apparatus f.or paper, the treatment is
effected on the web W which moves at a high speed past the treatment
sections. After the stages A and B or B and C the finishing effected on
the web at the stage D can be immediately succeeding calendering, if
the stage B is wetting, or it can be drying and calendering, if the
preceding stage B is surface sizing or pigmenting.
Fig. 2 schematically shows the paper or paperboard web issued from a
drying section in a paper or paperboard machine, which web W is
passed to a calender K. Before the calender in the travel direction of
the web, there is a device 1 for applying energy, by means of which
device 1 energy is issued into the layer of air bordered by the surface of
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the web travelling past the device, the energy also affecting the surface
of the web in such a way that more hydrophilic groups are produced. In
the travel direction of the web W, the device is succeeded by a wetting
apparatus 2 by means of which the surface whose hydrophilicity was
increased at the previous stage, is wetted. Thereafter the web W is
passed to the calender K, which in the drawing is illustrated by two rolls
3 and 4 between which there is a calender nip N through which the web
is passed. The nip in question can be the only nip in the calender, or
there may be several successive calender nips in the travel direction of
the web, wherein the drawing shows the first one of them. The calender
is not restricted to any type, and it can be a machine calender, a soft
calender, a long-nip calender or a multiple-roll calender.
The device 1 applying energy on the web can be a corona treatment
device, which is provided with an electrode or a series of electrodes,
located at a distance of a small air gap from the surface of the web W,
and on the opposite side of the web with a counter-electrode, e.g. a roll.
The corona treatment is based on the fact that the air gap between the
electrode and the paper or paperboard web has a lower dielectric
discharge voltage than the web itself. When a high frequency high
voltage is produced between the electrode and the counter electrode,
the air between the electrode and the web is ionized, and generates a
gaseous conductor which can be seen as a blue corona. The corona
burst generated with the electrode produces the following oxidants:
ozone, atomic oxygen, free oxygen radicals and high-speed electrons,
whose energy in the corona burst often reaches the amount of several
keV, e.g. the value of approximately 10 keV. The energy is sufficient for
breaking the bonds present on the surfaces of the fibres and between
the fibres. When the bonds break, very reactive free radicals are
produced on the surface of the web, which free radicals react with the
oxidants produced in the corona treatment. The surface of the web
becomes oxidized, and new polar groups are produced in the polymer
material of the fibres on the surface, the polar groups being primarily
carbonyl, amide, hydroxyl and nitro groups. These groups provide new
adsorption sites for water, thus increasing the hydrophilicity of the
surface.
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In addition to the aforementioned chemical effect the corona treatment
also has an electrical effect which improves adhesion, because the
corona burst charges the surface of the web.
Alternatively, the device 1 directing energy can be a flame treatment
device. In the flame treatment, the act of subjecting the surface of a
paper or paperboard web W to an oxidizing flame with high thermal
energy produces reactions similar to those described above on the
surface of the paper or paperboard web, as well as an increase in the
hydrophilicity via these oxidizing reactions. A further advantage of the
flame treatment is that it improves the coatability of the web by
removing so-called "standing" fibres from the surface of the web.
In the corona burst ozone is also produced, which has an effect on the
surface of the web. The surface of the web can also be subjected to an
oxidizing effect by means of a special ozone treatment, ozonization, in
which a suitable amount of ozone is directed as such to the web from
an ozone generator. The device 1 can thus be an ozonizer, by means
of which energy is issued on the surface of the web in the form of
chemical energy contained in gas. In this context it is also possible to
use the term energy treatment device.
Furthermore, it is possible to place e.g. two different devices such as a
corona treatment device and a flame treatment device, a flame
treatment device and an ozonizer, or an ozonizer and a corona
treatment device successively before the wetting apparatus 2 on the
same side of the web. With the combined effect of these treatments it is
possible to attain a strong oxidizing treatment. These different
treatments can take place in either order. Furthermore, the invention
does not exclude the possibility that all the three treatments take place
succesively in a suitable order.
The aforementioned treatment is effected on the surface of the web on
which the fibres are exposed. It is also possible, that the web W has
already been surface sized before it is passed to the aforementioned
energy treatment and wetting. Thus, the treatment has the same effects
as on the bare surface of the web. In the starch, modified starch, other
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natural polymers or their derivatives or in corresponding substances,
such as synthetic polymers; used in the surface sizing, polar groups are
produced analogously with the phenomena occurring in the cellulose,
hemicellulose or lignin.
The aforementioned energy treatments are described for example in an
article by S. Greig "Adhesion promoters for extrusion coating & cast film
- a three-pronged attack", in a conference publication "Extrusion coating
of paper and paperboard", 1990 and in the book by Chi-Ming Chan:
"Polymer Surface Modification and Characterization", pp. 265-279.
These publications primarily discuss the increase of adhesion of plastic
coatings in extrusion coating.
The aforementioned treatments or a combination of them can be
performed either on one side or on both sides of the web W. Thus,
there are many alternatives that are possible before wetting, namely:
- The corona treatment only on one side, the opposite side
untreated,
- the corona treatment on both sides,
- the flame treatment only on one side, the opposite side
untreated,
- the flame treatment on both sides,
- the ozonization only on one side, the opposite side untreated,
- the ozonization on both sides,
- the corona treatment on one side and the flame treatment on
the opposite side,
- the corona treatment on one side and the ozonization on the
opposite side,
- the ozonization on one side and the flame treatment on the
opposite side,
- a combination of the corona treatment and the flame treatment
only on one side, the opposite side untreated,
- a combination of the corona treatment and the ozonization only
on one side, the opposite side untreated,
- a combination of the ozonization and the flame treatment only
on one side, the opposite side untreated,
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- a combination of the corona treatment and the flame treatment
on one side, only the corona treatment on the opposite
side,
- a combination of the corona treatment and the ozonization
only
on one side, only the corona treatment on the opposite
side,
- a combination of the ozonization and the flame treatment
only
on one side, only the corona treatment on the opposite
side,
- a combination of the corona treatment and the flame treatment
only on one side, only the flame treatment on the opposite
side,
a combination of the corona treatment and the ozonization
on
one side, only the flame treatment on the opposite side,
- a combination of the ozonization and the flame treatment
on
one side, only the flame treatment on the opposite side,
- a combination of the corona treatment and the flame treatment
on one side, only the ozonization treatment on the opposite
side,
- a combination of the corona treatment and the ozonization
treatment on one side, only the ozonization treatment
on the
opposite side,
- a combination of the ozonization and the flame treatment
on
one side, only the ozonization treatment on the opposite
side
- a combination of the corona treatment and the flame treatment
on both sides,
- a combination of the corona treatment and the ozonization
on
both sides,
a combination of the ozonization and the flame treatment
- on
both sides
- a combination of the corona treatment and the flame treatment
on one side, a combination of the corona treatment and
the
ozonization on the opposite side,
a combination of the corona treatment and the ozonization
- on
one side, a combination of the ozonization and the flame
treatment on the opposite side,
- a combination of the ozonization and the flame treatment
on
one side, a combination of the corona treatment and the
flame
treatment on the opposite side.
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The alternatives which entail a combination of two different types of
treatments on the same side can also contain a treatment of a third
type on the same side.
5 Naturally, the wetting is effected on that side of the web W which has
been subjected to a treatment of adding hydrophilic groups in any of the
aforementioned ways. If the treatment has been conducted on both
sides of the web W, the wetting by means of the wetting device 2 is
effected on both sides of the web, and this two-sided wetting treatment
10 is illustrated with a second wetting apparatus depicted with broken lines
in the drawing. The wetting device can function on any of the following
principles:
- spray wetting
- film transfer wetting (LAS, sym-sizer, etc.)
- moistening by adjusting the relative humidity of air
- steaming
As a result of the treatment conducted before wetting, the contact angle
of water on the surface of the paper is considerably reduced, i.e. the
surface energy is reduced.
The corona, flame and ozonization treatments are the advantageous
alternatives for treating the surface of the web, because they are
efficient enough for producing changes in the structure of the surface of
the web. There are also other treatment alternatives which will be
discussed hereinbelow.
One possible pretreatment method is the production of a surface
charge on the surface of the web on the side which is wetted at a later
stage.
The pretreatment according to the invention is helpful especially in
spray wetting underneath the web, in which the surface energy is the
determining factor in the adhesion of the liquid on the paper. According
to the preferred embodiment, the treatment is conducted at least
underneath the web for a bottom-side spray wetting.
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Similarly, the treatment also makes it possible to increase the amount
of water that can be added e.g. by means of steaming in the web.
Naturally, in the two-sided wetting it is possible to utilize different
wetting principles on different sides of the web.
In the wetting the web obtains the desired surface moisture, and it is
passed in this moisture into the calender nip N of the calender K.
The wetting can also be replaced by adding liquid as a carrier material
contained in the composition to be applied on the surface of the paper
web, such as the surface sizing of the paper web or the pigmenting of
the surface. Thanks to the pretreatment, the aqueous compositions
added at these working stages adhere better on the web.
The pretreatment is especially well suited for coating methods in which
the composition is brought in contact with the web at a low pressure:
spray coating, jet coating as well as spreading roll application.
Fig. 3 shows water sorption as a function of the contact time for fully
sized fine paper with different powers in the corona treatment, and for
unsized and soft sized fine paper. The size used in the surface sizing is
AKD glue. The fully sized sample, whose comparative test effected
without a treatment is also shown, was subjected to the corona
treatment using three different powers. The reference samples were an
unsized fine paper and a fine paper soft surface sized with AKD, which
were not subjected to the treatment.
Fig. 3 shows that the corona treatment clearly increases the water
sorption of the surface sized paper, and the sorption is proportional to
the power used.
Fig. 4 shows a second alternative in which the hydrophobicity of the
surface is reduced by subjecting the surface to a steam treatment. The
treatment device 1 can be a suitable device, such as a steam box,
which doses steam on the surface of the web. The steam can be
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applied in the form of hot or cold steam, and it adsorbs evenly on the
surface. The steam can be dosed in such a small quantity, that nearly a
monomolecular liquid layer is obtained on the surface. Such quantities
do not substantially increase the moisture content of the paper, and
they can be applied e.g. in the form of hot steam on a hot web.
As a result of the treatment, the contact angle of the water on the
surface of the paper is reduced uniformly into zero. This equalizes and
maximizes the adhesion on the web of a considerably larger quantity of
liquid (e.g. "bulk water"), which is introduced at the next stage.
By means of steaming it is possible to apply the maximum quantity of
2g/m3 of water on such a cold paper web whose surface has not been
pretreated to increase the adsorbtion of steam.
The steam treatment is especially advantageous in spray wetting from
underneath the web, because in this wetting method the surface energy
has a great significance in the adhesion of the liquid on the surface of
the paper web. Thus, Fig. 4 illustrates the treatment device 1
underneath the paper web W, and a spray wetting device 2 located
thereafter in the travel direction of the web. The invention is not,
however, restricted to this arrangement, but corresponding devices may
also be located successively above the paper web W. It is also possible
that the aforementioned successive treatments are conducted both on
the upper and the lower side.
After the wetting the web is guided substantially in the same surface
moisture in the calender nip of the calender.
The wetting may be replaced with any of the aforementioned treatment
methods in which the liquid functions as a carrier material in the
composition to be added on the surface.
Fig. 5 shows a third embodiment according to the invention. Here, the
wetting in question is a spray wetting below the web W by means of a
wetting device 2. The web W which is moving at high speed carries
along an air film whose thickness is proportional to the driving speed
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and the roughness of the paper. This air film is ilustrated schematically
with an arrow 11. When liquid is sprayed on the surface of the paper by
means of dispersing air spray nozzles according to Fig. 5, the
dispersing air is typically in the order of 4 bar and the pressure of the
liquid is usually between 0.5 and 2 bar. Thus, the dispersing air
accompanying the spray fills the pores P of the paper with air. In the
appended drawing, in which the size of the pores is exaggerated for the
sake of clarity, this air is marked with the reference 12.
The dispersing air accompanying the spray and the air carried along by
the web W impair the adhesion of the liquid on the web, because the
dispersing air entering the pores decelerates the absorption of liquid in
the paper and the air film already initially impairs the contact of the
liquid and the web.
To reduce the apparent hydrophobicity resulting from the
aforementioned phenomenon and not from the surface structure or
materials of the web, a treatment device 1 is placed on the opposite
side of the web, in the case of Fig. 5 above the web, which treatment
device is substantially across the entire width of the web in contact with
the surface of the web W or with the surface of a highly air-permeable
supporting belt loop, such as a wire 5, supporting the web at this point,
and it produces at least an underpressure through the web on the
surface which is subjected to the wetting. This device improves the
physical adhesion of the liquid and the web by sucking air through the
web W away from the path of the water front. The underpressure can
be produced by suction boxes with a known structure, which suction
boxes are previously known only in connection with the wet end of the
paper machine, in which they suck the water contained in the web away
from the web at a large water content. The device shown in Fig. 5 in
more detail comprises at least one stationary member 1 a extending in
the transverse direction of the web substantially over the entire width of
the web. The member produces at least an underpressure impulse with
a co-operative action of its shape and/or position and the rapid
movement of the web. The member is a foil known from the wet end,
and there are advantageously two or more of them successively in the
travel direction of the web. These foils produce the underpressure in
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the form of underpressure impulses. They also produce the over
pressure impulses which precede the underpressure impulses, and
which are shown schematically by vacuum profiles placed in
corresponding locations in the upper part of Fig. 5, in which the X-axis
corresponds to normal pressure.
The foils may be placed as such against the surface of the web, or
they can be placed as a covering structure of a suction box. The
suction box and the change it produces in the vacuum profiles is
illustrated in Fig. 5 with broken lines.
Naturally, such a placement is also possible that the suction box, foils
or both are located below the web, and the spray wetting is effected
from above.
After the wetting the web is guided substantially in the same surface
moisture in the calender nip of the calender.
When a web which has been surface wetted in any of the above-
described manners is calendered, it is possible to use many known
solutions as the calender K. The calender nip N shown in the drawings
can be a hard nip, a soft nip, or a long nip which has been produced
with a belt or a shoe. The aforementioned nip variations can be
implemented by means of two hard rolls, two soft rolls, a soft and a
hard roll, a roll and a belt, and a roll and a shoe. The calender can also
be provided with several successive nips, of which two or more
successive nips can be of the same type or of a different type. Similarly,
it is possible that the calender is a multiple-roll calender in which
successive nips are formed between the rolls in a stack of rolls.
The calendering advantageously entails the heating of the surface of
the web by means of a heated, smooth calender surface in the calender
nip N. It is also possible that heating and smoothing is effected in the
calender only on the upper side of the web, only on the lower side of
the web or on both sides of the web. Naturally, the aforementioned
treatment is effected at least on that side whose hydrophilicity has been
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initially increased in a way described above, and which has been
wetted thereafter.
The invention is not restricted solely to the aforementioned treatment
5 points in the travel path of the paper or paperboard web, but it can be
used in all points after the occurrence of hydrophobicity. According to
the invention, the reduction in hydrophobicity can also be conducted in
the drying section, in which the web is later rewetted e.g. between
drying groups to adjust the moisture profile. It is a known procedure to
10 re-wet the web in the drying section by controlling the wetting in the
transverse direction of the web from the wetting devices. According to
the invention, the wetting can be profiled by adjusting the pretreatment
reducing the hydrophobicity in the cross machine direction. The effect
of the device producing the corona treatment and/or flame treatment
15 and/or ozonization treatment is thus adjustable in different points in the
cross machine direction, or there may be separate devices adjustable
independently of each other in the cross machine direction. Thus,
certain portions of the web can be rendered sensitive to absorb more
water than others. Thus, the wetting need not necessarily be profiled
separately, i.e. the wetting device can be "evenly" spreading. The
aforementioned profiling possibilities naturally exist in the wetting
effected before calendering as well. The profiling is advantageous
especially in spray wetting, especially in sparay wetting underneath the
web, and in the wetting effected by means of steaming.
The method and the device can be used in connection with a coating
station and/or on-line calender functioning as a direct extension to the
process of making paper or paperboard, or in a separate working stage
when temporarily stored paper or paperboard is coated and/or
calendered, the surface of such paper or paperboard being already
initially dry for re-wetting.
