Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention refers to a process and apparatus for the coating of
a web material, particularly a paper or board web, with a thin layer of a
liquid substance.
For the coating of a paper or board web with a thin layer of pigment,
surface size, plastic dispersions or other appropriate chemicals, a number of
different processes and equipment have been developed. These processes
utilize rdlls, bars, brushes, blades etc. or the distribution and the
metering or rationing of the coating material onto the surface of the web. ; -
In these well known processes, two separate stages are involved, for instance,
the coating material is spread with the aid of a rotating roll whereafter the
metering is carried out with the aid of a separate roll, bar, blade etc.,
which wipes off the surplus of coating material and makes the coated surface
smooth. The equipment thus requires a considerable amount of space and in
particular can~ot be applied on-line to a paper machine without special
arrangements.
In accordance with the teaching herein, a paper or board web may be
coated either on-line or off-line with a thin and even layer of slze, plastic
dispersion, pigment or other chemical. The coating can be carried out
either directly on the paper or board machine or in a separate operation.
The herein process can also be used for the moistening of, for instance,
a paper web with water only.
The equipment herein described, is a bar or roll extruder coater. In
this apparatus, the metering and the distribution of the coating solution or
dispersion is carried out in one single stage. As a result, the equipment
itself is very small. It can thus be installed directly in, for instance,
the drying section of a paper or board machine, without making it necessary
to remove drying cylinders from the section. The equipment can be built
cheaply. It is found that the apparatus works well over a large range of
paper web speeds; according to the Applicant's experiments, at least within
the range of 20 - 600 m/min.
Since the amount of coating material distributed on the web can be
kept to a very low value, the amount of water per area unit of the web can
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also be kept low, which decreases the amount of energy needed for drying.
More particularly in accordance with one aspect of the invention there
is provided, a process for coating a web with a layer of a liquid substance
which comprises the steps of, mounting a roll within a feeding chamber with a
portion of said roll and projecting upwardly from said feeding chamber, arrang-
- ing a pair of doctor blades along upper edges of said feeding chamber, project-
ing towards said roll and defining clearance spaces between the respective
blades and the roll, rotating said roll, maintaining a supply of said liquid
substance in said chamber substantially at the level of said clearance spaces,
said rotating roll providing a metered distributed quantity of said liquid on
the projecting portion of said roll, and moving said web into contact with and
over said projecting portion of said roll. The clearance spaces may be ad-
justed to between 0.01 and 3.0mm, preferably 0.05 to l.Omm. The web may be
moved angularly over and at a greater speed than that of the surface of the
roll which it contacts. The metered quantity of liquid may be controlled by
regulating the roll speed or the clearance spaces or both. The output side
clearance space (as determined by the rotation of the roll) may be made greater
than the input side clearance space.
In accordance with a second aspect of the invention there is provided
apparatus for coating a web comprising a housing defining a feeding chamber
for a liquid coating substance, a rotatable roll received in, and with a por-
tion porjecting from, an upper part of the feeding chamber: input and output
side doctor blades positioned along opposite upper edges of the feeding chamber
projecting towards said roll and serving to close said feeding chamber, with
a defined predetermined clearance space between each respective doctor blade
- and said roll, an adjustable means mounting each doctor blade respectively
relative to said roll for respective adjustment of each said blade for setting
its respective said predetermined clearance space, means for maintaining said
coating substance at a level substantially at that defined by said clearance
spaces, means for rotating said roll for metering and distributing said liquid
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substance onto said projecting portion of said roll, and means for contacting ;
said web with said projecting portion of said roll. There may be a pressure
balancing chamber communicating with the feeding chamber through holes in the
housing. Bearings may support the roll at positions along it, preferably
magnetic sliding bearings.
Specific embodiments of the invention will now be described with
reference to the accompanying drawing which is a schematic sectional side
view of an apparatus embodying the invention.
In the Figure, a rotating bar or roll 2, has its ends supported in
ball bearings (not shown). In order to prevent the roll from sagging down-
wards, or in the direction of the web handling machine, it is further supported
by bearings 3, placed at chosen distances along the roll. For this purpose,
magnetic sliding bearings are preferably used. The supporting bearings 3
should be as low and short as possible in order to prevent them from touching
too large a part of the roll periphery, which would disturb an even distribution
~; of the coating material on the surface of the roll. On both sides of the roll,
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here are two inclined doctor blades, ~ and 8. The doctor blade 4, on that
side from which the wcb 1 enters the e~luipment (the input side) is mounted
with the aid of the supporting plate 5 on the wall 6, and together with the
base plate 7, the doctor blade 8 (on output side of the equipment), the
opposite side wall and the end walls form a feeding chamber 9 for the coating
solution or dispersion. Below the feeding chamber 9 is a pressure-balancing
chamber 12, which is connected to the feeding chamber through two rows of
holes 10 and 11 drilled in the base plate 7. The pressure-balancing chamber
12 is connected to a storage tank (not shown). Beyond the output side
doctor blade 8 is a collecting channel 15, which also is connected to the stor-
age tank. The rotation speed of the roll2 can be regulated as can the width
of openings 13 and 14 between the rolland the doctor blades 4 and 8, respect-
ively. The width of the openings 13 and 14 can be regulated for instance,
within the range 0.01 - 3.0 mm, preferably within the range 0.05 - 1.0 mm.
In operation the coating solution or dispersion is pumped from the
storage tank to the feeding chamber 9 in such a way, that the "surface" of
the solution essentially remains at the level of the upper edges of
the doctor blades 4 and 8. The flow from the feeding chamber 9 takes place
via the row of holes 10 and 11 and the pressure-balancing chamber 12. In
this way, an even flow to the feeding chamber is obtained. Overflow of
coating solution or dispersion over the doctor blade 8 enters collecting
channel 15, from which it is fed back into the storage tank.
The paper or board web 1 is passed over the rotating ro~ 2 so that
the web forms an angle over the roll 2. When rotating, ~he ro~ 2 transfers
coating solution or dispersion from the feeding chamber through the opening
13. The amount of a certain coating solution or dispersion transferred to
the rollat a given rotational speed is determined by the width of the opening
13 between the doctor blade 4, and the roll. The control variables of tl-e roll
extruding coater are thus the rotational speed of the roll2 and the width
of openings between the roll and the doctor blades 4 and 8 on each side of it,
i.e., the input opening 13 and the output opening 14. When the ro~ 2 rotates
in the direction of movement of the paper web, the peripheral speed of the
ro~ is generally lower than the speed of the web 1. In this case the width
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of the output side opening 14 is set a~ a larger value than the input side
opening 13, which has been set at a suitahle value. ~len the output side
opening 14 is larger than the input side opening 13, the coating solution or
dispersion fills up the feeding chamber 9 at a smooth rate. If the output
side opening 14 is equal to or smaller than the input opening 13,entrained
air following the rotating ro~ from the output side into the chamber 9 may
disturb the filling of the chamber. The edges of the paper web are kept
clean with the aid of thin trailing metal blades as is common in coating
techniques.
Example
An experimental run was carried out using a paper with a base weight
of about 70 g/m . The width of the web was 60 cm. The web was coated with
starch size. The speed of the web was 500 m/min and the diameter of the roll
30 mm. The rotational speed of the rollwas regulated to 64 rpm, which meant
that its peripherical speed was 6 m/min. The width of the input opening was
regulated to 0.1 mm and the output opening was set at 0.2 mm. A very even
distribution of the starch size into a thin layer on the paper surface was
obtained
The invention is not limited only to the particular apparatus
described above and to the example of application presented, but it may
vary considerably within the limits of the claims below. The rotational
; direction of theroll may also be opposite to that of movement of the web
thus reversing to some extent the functions oE doctor blades 4 and 8. The
coating substance can also be transferred to the roll in some other suitable
way, for instance, by spraying. Instead of having two doctor blades, one on
the input and one on the output side, the equipment may have only one blade,
which then is on the input side.
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