Note: Descriptions are shown in the official language in which they were submitted.
217D929
METHOD AND INSTALLATION FOR TREATING PAINT
WHEN APPLIED TO A PAPER WEB
BACl~OUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a method of treating paint when applied to a
paper web and an inct~ tion for putting the method into practice.
BACKGROUND ART
In installations of the generic type, the paint is stored in one or several
big agitated tanks of vertical agitator a~is, where it is stirred and,
5 having passed through filters, is supplied in excess to the paint coating
device. The surplus of paint is returned to the storage tank. In the
coating device, paint in an amount of 25 to 40 times the quantity re-
quired for coating (brushing) is applied to the paper web. This enormous
surplus of paint must be removed from the paper web by means of the
20 doctor blade. Consequently, a considerable amount of air is included in
the paint at the doctor blade, which may lead to foaming on the one
hand and as a result of which the recirculated pain~ may contain very
fine air bubbles which burst open after the final coating of the paper
web, which may lead to deterioration of the surface of the coated paper
25 web.
SUMMARY OF THE INVENTION
It is the object of the invention to embody a method and an installation
30 in which problems resulting from the inclusion of air in the paint do not
occur.
In a method of the generic type, this object is attained by the features
of providing paint, intermediately storing the paint, acting on the paint
- 21 7D929
by partial vacuum during the intermediate storage, filtering the paint,
feeding the paint at a high surplus as compared to the quantity of paint
applied to the paper web, applying the paint to the paper web, and re-
turning the surplus of paint to the paint stored intermediately. In an
5 installation for putting the method into practice, this object is attained
by a vacuum mixing tank having a mixing unit that is rotatingly driv-
able, by a conduit for the supply of paint discharging into the vacuum
mixing tank, by a vacuum duct discharging into the vacuum mixing tank
for the production and maintenance of partial vacuum in the vacuum mix-
10 ing tank by a flow pipe discharging from the vacuum mixing tank, by afilter disposed in the flow pipe, by a paint coating device connected to
the flow pipe for the application of paint to a paper web that is passed
through, and by a return pipe connecting the paint coating device with
the vacuum mixing tank.
The gist of the invention resides in that after return from the coating
device, the paint is stirred comparatively siowly in a gastight tank,
while it is subjected to partial vacuum.
20 Further features, advantages and details of the invention will become ap-
parent from the ensuing description of an installation for the treating of
paint in connection with the coating of a paper web by the paint, taken
in conjunction with the drawing.
25 BRIEF DESCRIPTION OF THE DRAWING
The drawing figure shows an installation for treating paint.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The installation illustrated in the drawing comprises a mixer 1, in which
paint for coated paper is continuously produced from numerous constitu-
ents by mixing This ~mished paint is supplied via a conduit 2 to a
vacuum mixing tank 3. The latter comprises a gastight drum 4 resistant
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to pressure and of holi~onlal arrangement, in which a mixing unit 6
drivable by a motor S is disposed centrally.
On the drum 4, provision is made for a vacuum top 7, a so-called dome,
5 which is open towards the interior chamber 8 of the drum 4. A vacuum
duct 9 opens into the upper portion of this vacuum top 7, by way of
which duct 9 the interior chamber 8 of the drum 4 can be acted upon by
partial vacuum. The vacuum pump 10 is driven by a motor 11.
0 A flow pipe 12 discharges from the underside of the drum 4, a feed pump
13 disposed in the flow pipe 12 conveying paint via the latter through a
filter 14 to a paint coating device 15, from where paint is returned
through a return pipe 16 back into the drum 4.
5 In the filter 14, paper particles, for instance paper fibers, and paint
re-agglomerates are separated from the paint for the latter to reach the
coating device 15 in a condition not i~lplllirled. This coating device 15
conventionally comprises a rotatable roll 17, along which a paper web 18
to be coated by paint is guided and conveyed in the direction of trans-
20 port 19. The direction of rotation of the roll 17 corresponds. A rotatablepaint coating roll 20 is located below the roll 17, bearing against the
latter and the paper web 18. The lower side of the coating roll 20 dips
into a paint tank 21, from which it collects paint, and applies the paint
to the paper web 18. Further, a doctor blade 22 is provided, removing
25 paint applied to the paper web 18 at a surplus of 25 to 40, so that the
paper web 18 is only coated by a very thin layer of paint, which is si-
multaneously pressed into the surface of the paper web 18 by the doctor
blade æ. The flow pipe 12 discharges into the paint tank 21; in like
manner, the return pipe 16 discharges from the latter.
In the return pipe 16, a recir~ ting pump 23 can be disposed as a
vacuum stop-work, preventing any penetration of the partial vacuum from
the drum 4 into the paint tank 21. Likewise, the conduit 2 may have a
feed pump 24 equally serving as a vacuum stop-work and precluding any
21 70929
penetration of the partial vacuum from the drum 4 into the mixer 1.
By alternative to the pumps 23, 24 - even taken individually - siphon
connecting pipes 25 or 26 can connect the return pipe 16 or the conduit 2
5 with the lower portion, filled with paint, of the drum 4, the deepest
point of these siphon connecting pipes 25, 26 being distant from the level
27 of the paint in the drum 4 by a measure L which corresponds to the
partical vacuum in the upper portion of the drum 4. This will also help
preclude any penetration of the partial vacuum from the drum 4 into the
0 return pipe 16 and thus into the coating device 15 or, respectively, into
the conduit 2 and thus into the mixer 1. The necessary length is directly
proportional to the partial vacuum prevailing in the upper portion of the
drum 4 and inversely proportional to the density of the paint. The siphon
connecting pipes 25 and 26 are shown by dotted lines in the drawing. In
15 this embodiment, the mixer 1 and the coating device 15 must be disposed
above the level 27 for gravity feed of the paint to be possible.
The vacuum mixing tank 3 is disposed on a balance 28 which ensures
that the degree to which the drum 4 is filled, i.e. the level 27 of paint
20 in the drum 4, fluctuates only within a given range. The balance 28 can
trigger a solenoid valve 29 in the conduit 2, by means of which the sup-
ply of paint from the mixer 1 can be controlled. The balance 28 can also
trigger the motor 30 of the feed pump 13, thereby influencing the circu-
lation of the paint.
A foam probe 31 is disposed in the vacuum top 7, triggering a ventila-
tion valve 32 in the vacuum duct 9 when paint foam penetrates into the
vacuum top 7, whereby the partial vacuum in the interior chamber 8 of
the drum 4 is destroyed in such a case so that the risk of paint foam
30 penetrating into the vacuum duct 9, and thus into the vacuum pump 10,
is elimin:~ted.
The installation works as follows. From the mixer 1, paint is supplied as
required via the conduit 2 to the vacuum rnixing tank 3. The quantity
21 70929
supplied per unit of time corresponds to the quantity of paint applied to
the paper web 18 in the paint coating device 15. The entire stocks of
paint are circlll~te~l in the vacuum mixing tank 3 by the mixing unit 6
which rotates comparatively slowly. This helps ensure good deairing of
5 the paint, owing to the partial vacuum prevailing in the interior chamber
8 of the drum 4, which implies macro deairing as well as micro deairing
corresponding to the amount of the partial vacuum. Macro deairing pri-
marily means the destruction of foam, whereas micro deairing means the
removal of very fine air bubbles from the paint.
By means of the feed pump 13, the paint is conveyed through the filter
14 in the paint tank 21 of the paint coating device 15, where it is ap-
plied to the paper web 18. The paint being deaired to a high degree, the
coat applied to the paper web 18 is at least subst~nti~lly free from mi-
15 croscopical air bubbles which might burst open and negatively affect thesurface of the coated paper web 18. Paint is returned from the paint
tank 21 to the vacuum mixing tank 3. The paint is supplied to the coat-
ing device 15 at a surplus of approxi~ tely 25 to 40, i.e. the coating
device 15 is supplied with paint per unit of time in an amount 25 to 40
20 times as high as the quantity that is applied to the paper web 18 after
the latter has passed the doctor blade 22. So 95 percent of the paint de-
livered by the feed pump 13 to the coating device 15 are recirculated. In
particular when the paper web 18 passes the doctor blade 22, particles
are picked up from the latter and returned to the paint tank 21 along
25 with the surplus of paint; having passed through the tank 3, these par-
ticles are removed in the filter 14.
