Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCEDURE AND APPARATUS FOR GLAZING A PAPER OR
CARDBOARD WEB
The present invention concerns a procedure for
glazing a paper or cardboard web, a coating being ap-
plied on one side of the web, the web being dried and
the coating glazed.
The invention further concerns apparatus for
glazing a paper or cardboard web, comprising a coating
means and a drying means.
The methods used to perform glazing of paper
and cardboard can be classified by two main groups. To
the first main group belong those procedures in which
the paper is coated in conventional manner and the
coating, dried close to its ultimate dry matter con-
tent, is glossed. This group includes several greatly
different techniques, such as e.g. brush glossing and
high-gloss calendering.
The second main group consists of those pro
cedures in which the coating is dried and glazed on one
glossy drying cylinder. This process is known as pour
coating.
No very good gloss is achieved with mere brush
glossing, and this method is therefore mostly applied
in supplementation of calendering. However, calendering
has invariably an adverse effect on bulk and stiffness
of the cardboard.
In the glossing methods, the pigment coating
is first dried, using convection and radiation drying
procedures of prior art, and thereafter glossed, using
brush glossing or high-gloss calendering. It is however
usually necessary to varnish such cardboard which is
meant to be printed on, owing to insufficient gloss.
Pour coating gives substantially better gloss
than, for instance, brush glossing and high-gloss
calendering. Moreover the bulk is not lessened in pour
coating. However, the pour coating technique in present
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use cannot be applied as an on-line process on paper or
cardboard machines. This is because the highest running
speeds of modern cardboard machines are on the order of
400 to 600 m/min, and paper machines have running
speeds up to the order of 1300 m/min. In contrast, the
highest feasible running speed in pour coating is on
the order of about 100 m/min. The low speed of pour
coating is due to the fact that the water to be evapo-
rated from the coating has to be removed on one cylin-
der through the base web, that is, the wet coating is
conducted against a polished cylinder and dewatered
through the opposite side of the web. When pour coating
is used, the largest possible cylinder diameter is on
the order of about 5 m, and this imposes a limit on the
region that is available for evaporation, and thus on
the drying capacity of the entire process.
The object of the present invention is to
eliminate these drawbacks. It is a particular object of
the invention, to further develop the pour coasting
process so that its effectiveness, and the capacity of
the pour coating machine, can be raised to be higher
than before, retaining at the same time'the advantages
that can be gained with pour coating.
~2egarding the features characterizing the
invention, reference is made to the claims section.
It has been found in investigating the factors
affecting the rate of evaporation in connection with
the drying of pigment coating on paper or cardboard,
that the evaporation rate is affected by the partial
pressure of the evaporating substance, that is water,
in the close neighbourhood of the evaporatian eurface
compared with the pressure of saturated vapour corres-
ponding to the temperature in the case, and to the par-
tial pressure of any other gases in the region. If dry-
ing takes place under atmospheric pressure, the,~artial
pressure of air will be 'substantially higher than the
partial pressure of water vapour. Presence of air re-
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tards the evaporation of water substantially. The evap-
oration rate can be multiplied by removing the air.
Heretofore the so-called Condebelt technique
has been employed in connection with papermaking to dry
the web. The Condebelt technique is based on the expe
dient of reducing the air pressure in the pores of the
web prior to drying, in addition to which the water
vapour produced by evaporation is condensed by cooling,
in order to keep the partial pressure of water vapour
as low as possible in the evaporating region. The
Condebelt technique has not heretofore been used in
connection with drying a coating, to say nothing of
glazing a coating.
The present invention is based on utilization
of the Condebelt technique in connection with coating
paper and/or cardboard, and in particular with its
glazing. As taught by the invention, the coated side of
the paper or cardboard web, dry matter content of the
coating less than 80~, is contacted with a heated,
smooth, endless and thermally well conductive surface,
such as a burnished metal belt, and the opposite side
of the web is contacted with a cooled belt, such as a
drying felt or equivalent. The moisture present in the
coating will then move through the web and it will
depart from the web through the opposite side of the
web while the coating is in contact with the smooth
surface of the belt, so that the gloss of the coating
increases during the drying process.
It is thus understood that in the procedure of
the invention the Condebelt technique, known in itself
in the art in connection with paper drying, is utiliz
ed, the coating being, in addition to drying, glazed
with the aid of said technique in contact with a smooth
belt, e.g. a metallic belt, the coating thus being
simultaneously glazed in conjunction with drying. In
the procedure, the evaporation rate is mux~tiplied as
compared with traditional techniques, by lowering the
CA 02051579 2003-O1-10
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partial pressure of air and water vapour in the pores of the web. The specific
novelty of the procedure is seen in the simultaneous drying and glazing of the
coating. Production of gloss is based, in this procedure, on the fact that the
exceedingly smooth and glossy metal surface is replicated on the surface of
the wet, and malleable, surface of the coating.
Thanks to the invention, the running speeds can be substantially
increased in conjunction with paper coating and glazing. Thanks to the
invention, a paper or cardboard web can even be coated in an on-line
process adjoined to a paper or cardboard machine. The power applied in
io drying the coating can be raised to desired level because in the Condebelt
technique a metallic belt is used for heat transmitter, instead of a cylinder,
and this belt can be made as long as seems desirable, without its imposing
any limits on the evaporation distance. If desired, and whenever the drying
capacity requires, it is also possible to join a plurality of Condebelt units
in
i5 succession so that the desired drying capacity will be achieved.
It is particularly essential in the procedure of the invention that the
coated side of the web, that is the coating, is contacted with the surface of
a
heated, smooth and endless belt with good thermal conductivity,
immediately after applying the coating and while the coating is moist,
2 o dry matter content of no more than 80% by weight, advantageously
less than 70°r6 but at least about 45°~ by weight. Thus, a novel
embodiment of
the pour coating technique is now concerned. Upon drying the web, the
moisture being removed through the web and through the side of the web
which is opposed to the coating, the web is released from contact with the
25 smooth belt. The drying technique here employed, the Condebelt technique,
is in itself previously known e.g. through the Finnish Patents FI-54514,
FI-55539 and FI-59635. In the procedure and apparatus of the invention
5
the drying technique and associated vacuum technique
known through the references cited, or otherwise, may
be applied.
The invention is described in the following in
detail with the aid of an embodiment example, referring
to the attached drawing presenting an apparatus accord
ing to the invention fox implementing the procedure.
In Fig. 1 is depicted an apparatus according
to the invention for glazing a cardboard web. The
apparatus comprises a coating means 4 and a drying
means 5. The coating means 4 comprises a conventional
applicator roll 15 for spreading the coating material
on the surface of the web l, a doctor blade 16 for lev-
elling the coating, and a backing roll 17, which the
web is conducted to lap, running in the direction
indicated by arrow 18. The coating means may be any
kind of coating means applied in itself in connection
with coating paper and/or cardboard.
The drying means 5 comprises an endless, ther
mally well conductive, polished metallic belt 2, and a
heating means 7 for heating this belt e.g. with the aid
of steam. The belt 2 is disposed immediately after the
coating means 4 in that the web 1 has been conducted
onto the belt immediately after the coating means, with
the coating against the polished face of the belt so
that the dry matter content of the coating is less than
80~ by weight when the coating is being conducted
against the belt 2.
Furthermore, the drying means 8 comprises a
drying felt 1' with rolls 12 and drying apparatus i3.
The drying felt is conducted to run around the rolls 12
in the travelling direction of the web with a speed
consistent with that of the web, i..e., of the belt 2.
The drying means 5 further comprises and endless, and
thermally well conductive, belt 2' with associated
rolls and cooling member 8; the belt 2' is conducted to
run around the rolls 6' in the direction of travel of
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the web. The belts 2 and 2' constitute a pair of belts
in which the belts run in parallel between the rolls 6
and 6', respectively, pressed against each other. The
drying felt 1' and the web disposed upon this drying
felt are conducted in between the belts 2,2'.
The polished belt 2 lying against the coating
is heated with the aid of a heating means 7; the drying
felt under the web is continuously cooled with the
cooling means 8 of the belt 1'. The moisture will then
move from the coating through the web 2 into the drying
felt, and it will condense therein. The drying felt is
continuously dried with the aid of the drying means 13,
as it makes its circuit of the rolls 12. After passing
through the drying means 1, the web is released from
the drying felt and conducted to another drying means
5' operating according to the Condebelt technique.
Prior to being conducted into contact with the
belt 2, the web has been subjected to vacuum treatment
for air removal from the web, that is for reducing the
air pressure within the web and the drying felt, in
order to enhance the drying process. Web and drying
felt are conducted in between the belt 2 immediately
following vacuum treatment. The belts 2 and 2' are im-
permeable to air, and the vacuum treatment of web and
felt lowers the air pressure in the web and in the
felt, this promoting the drying of the web as the par-
tial pressure of air in the felt is lower than before
and thereby the partial pressure of water vapour is en-
abled to be higher than normal.
The heating means 7 of'the belt 2 may be any
heating means whatsoever, e.g: electric, gas flame-
operated, steam-operated, etc. The heating means may
further comprise e.g. a liquid removal means far elim3.--
nating the condensed steam.
The cooling means serving the belt 2' and the
felt 1' may be any kind of cooling means known in
itself in the art, e:g. one operating with a cooled
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fluid. Furthermore, the drying means 13 serving the
felt 1' may be any kind of drying means known in itself
in the art, for instance one based on pressure rolls, a
heating appliance, vacuum apparatus, etc. Furthermore,
the vacuum means 10 may be of any kind whatsoever,
based e.g. on vacuum chambers connected with pipelines
20 to a vacuum pump, and sealed against the web 1
and/or the felt 1'.
In the embodiment here presented, the web 1 is
carried in between two belts 2,2' impermeable to air,
e.g. of metal, upon a felt 1'. The web may alternative
ly be conducted to lie against the belt 2, merely upon
the felt 1', in which case the cooling means 8 is dis
posed to cool the felt directly. Z'he felt may in that
case be either impermeable or permeable to air.
The embodiment example is meant to illustrate
the invention, and the invention is not meant to be
confined to the embodiment example; in contrast, its
embodiments may vary within the scope of the claims
following below.
