Note: Descriptions are shown in the official language in which they were submitted.
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S P E C I F I C A T I O N
The present invention relates to processes for
coating papers and cardboards. It is addressed more
particularly at a continuous process for regulating and
regularizing the thickness of the coating of an aqueous
5 composition deposited in excess on a conti llUOUS strip of
paper or cardboard supported by the lateral surface of a
cylinder, whereby the excess coated subst~nce is removed
by scraping. It also relates to an apparatus permit~ing
the performance o~ said process and to the coated papers
and cardboards obtained by said process.
It is known that, in principle, aqueous composi-
tions for coating paper and cardboard contain pigments and
binders. The most freguently used pigment is mineral clay.
In a small proportion comparad with the mineral clay, it is
1~ also possible to use other pi~ments such as calcium car-
bonate, titanium oxide, hydrargillite~ talc~ barium sù~phate,
etc. Thes~ pigments are dispersed in water, generally in an
alkaline medium and in the presence of dispersing agents,
the most important of which are tetrasodium pyrophosphate,
2~ sodium hex~netaphosphate and low molecular weight polyacry-
lates in quantities between 0.2 and 0.5% by weight of the
pigments. These pigment~ are fîxed to the paper or card-
board by means of bindexs. Generally, the binders consist
of an aqueou.s dispersion of a synthetic polymer, such as a
styrene/butadieile copolymer, an acrylic polymer or a vinyl
acetate polymer, either used alone or mixed with natural
binders such as ~tarches, proteins or casein or synthetic
~inders, such as polyvinyl alcohols. It is gener2Lly ~os-
sible to mix the agueous dispersions to includè ingre~ients
3 ~llich are able to improve the water retention properties o~
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the roating compositions, such as carboxymethyl cellulos~s
`~ or alginates~ Finally, the coating compositions can contain
various other ingredients, such as cross-linkin~ ayents,
anti-foaming agents, lubricants, optical briglltening agents
and coloring agents.
A paper or cardboard coating process is known in
which, on one of the faces of a paper or cardboard strip,
an excess auantity of the a~ueous composition is continuously
deposited, generally by means of a coating roller. The
1~ excess material is then removed by scrapin~ by means of a
so-called smoothing roller. The coating speed is generally
between 300 and 1500m/min. The quantities of aqueous compo~
~ition deposited on the paper or cardboard by the coating
roller are gen~rally between 200 and 800 g/m2. Depending
on the rigidity of the blade, the pressurs which is applied
thereto, the distance between the blade and the paper or
cardboard, the compressibility of the paper or cardboard,
; and the rheological properties of the coating kath, the
quantity of material deposited on the paper or cardboard
a~ter drying the coating can vary between 6 and 15 g,~m2.
When coating in accordance with the described
processO straaks can occur on the coating as a result of
particles having a size greater than the distance between the
blade and the paper or cardboard, whereby said particles jam
between the blade and the paper or cardboard. These particles
may come from the coating bath or may comprise various aggre-
gates, such as clots or sticks present on the surface of
the paper or cardboard. It is also possible to observe
defects in the uni~ormity of the coating spread on the sur-
face of the pap~r or cardboard, such as furrows and projection~
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often due to irregularities in the paper or cardboard. They
are made all the more readily visible due to -the fact that,
during the coating opexation, turbulence occurs in -the excess
coa-ting which the blade is required to remove by scraping.
The process forming the object of the present
invention makes it possible, in an unexpect:ed manner~ signific-
antly to reduce these disadvantagesO
According to the process of the invention, scraping
is performed by at least two successively acting blades. The
final blade is called the smoothing blade and the other blade
or blades are called preme-tering blades. In practice, the
number oE blades is at the most equal to four.
Accordingly the present invention provides a process
for applying a smooth~ uniform coating onto a continuous
strip of paper or cardboard, the improvement wherein the thick-
ness of the coating is continually regulated by depositing an
excess of the coating composition onto the strip while the
back side of the strip is being supported, and rPmoving excess
coating by engagement of the coated surface with at least two
successive scraper blades which extend crosswise of the strip.
The present invention also provides papers and
ca~boards having a coating of an aqueous composition in which
the coating is characterized by a regular thickness, is free
from furrows and streaks and corresponds -to a ~uantity of dry
solids between 15 g/m2 to 5 g/m2.
q~o prevent a reduction in the quantity of water by
absorption through the paper or cardboard or by evaporation in
the air when passing from the first to last blade, the distance
between said two blades should be as small as possible. Such
reduction in the quantity of water could also lead to streaks
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caused by expansion in con-tact with the final blade. In
practice, the distance between the free ends of the first and
last blades applied against the coating of aqueous composition
is generally between 0.2 and 50 centimeters.
In order to bring about a maximum improvement of spread-
ing and reduce to the greatest possible extent the risks of
streaks being formed, the distance betweell the first blade
and -the strip is xegulated in such a way that the weight
of the coating left on the strip by said blade, expressed in
dry solids, is between 10 and 50 g/m .
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In order to permit a completely satisfactory
perormance of the invention, each or the hlades i~ inclined
relative to the strip ~y an angle within the range of 5 to
60.
The process according to the inv~ntion makes it
po~sible to obtain coatings of regular thi.ckness, which are
frae ~rom streaks and furrows, in which the quantity of dry
solids is generally below 15 ~/m~ and which go as low as
5 g/m2 witho~t decreasing the viscosity of the coating baths.
It is ound that through the use of 3 or 4 blades, a coating
: with a better appearance is generally obtained than when
using only two blades.
A machine for realizing the process of the inven-
tion in the production of coated paper or cardboard com-
prises at least one assemb3.y having a supporting cylinder
(or more commonly a supporting surace~ on the outer surface
of which is displaced a continuous strip of paper or card-
board~ and a wetting or coating roller which serves to
transfer to said paper or cardboard strip a coating of an
aqueous composition supplied from a suitable reservoir.
The apparatus Eorming the object of the invention
: ~or regulating and regularlzing the thickness of the coating
of aqueous composition deposited in excess on a continuous
paper or cardboard strip suppcrted by the la eral surface o~
a cylinder o~ a paper or cardboard coating machine comprises
at least two successively arranged blade~, of which one of
the ends lS free and appli2d to the aqueous composition
coating. It a~so advantageously comprises means for regu-
lating the slope of each of the blades relat.ive to the strip
3 and means .or regulating the distance of each o~ the blades
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from the strip. The number of blades ls generally at
the rnost equal to four.
For a satisfactory performance of the invention,
the distance ketween the free end~ of the first and final
blades applied against the aqueous ~omposition coating is
~etween 0.~ and 50 centimeters.
A detailed clescription of an em~odiment of the
apparatus of the invention will hereinaf-ter be described
with reference to Figs. 1 to 4, in which:
Fig. 1 is a schematic elevation view of a paper
or cardboard coating machine partially in section, equipped
with the apparatus according to the invention.
Fig. 2 is a side view of the apparatus mounted
on the coating rllachine.
Fig. 3 is a ront view of the apparatus mounted
on the coating machine.
Fig. 4 is a plan view of the apparatus mounted
on the coating machine.
The apparatus forms part of a conventional
paper or cardboard coating machine, whereof the main compo-
nents are shown schematically in Fig. 1. This machine
substantially comprises a supporting cylinder 1 which
rotates in the direction of the arrow about axis 0. On
; the outer surface of supporting cylinder 1 is wound a
2~ continuous strlp of paper or cardboard 2 supplied from a
reel (not shown~ This strip is caused to bear on the
supporting cylinder in accordance with a ~iven windir.g arc
by an appropriate conventional device, for example one
or several tension rollers 3.
A rotary coating rcller 4 is part~ally submerged
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in a bath 5 of an a~ueo-ls composition ~ontained in a
reservoir 6 while the uppar surface tarlgentially engages
th~ strip in such a way that during its rotational movement,
it transfers a coating 4a of a~ueous composition, to the
surface o strip 2~ Fig. 1 shows an appropriate relative
position of coating roller 4 below supporting cylinder 1,
~ut it is obvious that any random appropriate relative
position can be adopted. Moreover, a number of coating
rollexs can be associated with the supporting cylinder 1,
whereby each of them is able to transfer a given quantity
of aqueous composition to strip 2
The aqueous composition can be any of a number :~
of well known composition suitable for giving a suitable
coating to stxip 2 and normally contains one or more
pigme`nts, such as for example kaolin, cal~ium carbonate, ~;:
titanium oxide, hydrargil-lite, talc,barium sulphate and
one or more binders, such as for example synthetic poly-
mers, starches, proteins or casein. The solids content
of the composition is between 50 and 65% by weight.
The regulating apparatus 7 according to the
invention is located on the periphery of strip 2 downstream
of coating roller 4. It has a vertical plane of symmetry
perpendicular to rotation axis 0 of supporting cylinder 1.
This apparatus essentially comprises two scraping
blades 8 and 9, whereby one of the ends thereof is applied
against strip 2, respectively at points P a~d Q. Blades
8 and 9 are respectively fixed at their other end to
blade holders 10 and 11 by means of screws such as 12.
Blade holders 10 and 11 are respectively articulated ~ith
3 supports 13 and 14 in such a way that each of thei.r two
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ends, provided t~ith a guidance me~ber 15, can move in a
direction orthogonal to axis 0 within a slot 16 made in
the corresponding support. I~nurled buttons 17 and 18
mounted respectively on supports 13 and 14 permit, by means
of conventional mechanical means not shown, the sliding
and maintalning in position of guidance members 15 in
windows 19 made in slots 16 and thus regulate the distance
b~tween each of the blades 8 and 9 on the one hand and the
strip 2 on the other.
Each support is provided with two trunnions 20
engaged in recesses 21 made in side plates 22, permitting
the pivoting thereof about an axis parallel to axis 0.
In this way, it i5 possihle to adjust the blades 8 and 9
to the d~sired angle of inclination relative to strip 20
Side plates 22 are interconnected by bars 23 and 24 maln-
tained in position by nut members 25 and 26~
Side plates 22 are mounted for pivotal movement
about shaft 27 parallel to axis 0 and are carrie~ by
mambers 28 integral ~ith the rame 29 of the coating machine.
Slde plates 22 are fixed in position by assembly with mem-
bers 28 by means of screws 30. Stops 31, carried by side
plates 2 , prevent blades 8 and 9 from striking supporting
cylinder 1 during the assembly o~ the apparatus on the
coating machine. For reasons of clarity9 bars 23 and 24,
nuts 25 and 26, members 28 and frame 29 are not shown in
Fig~ 4.
Locking pins 32 and 33, located in recesses,
~uch as 34, in support3 13 and 14, make it possible to
maintain said supports and consequently blades ~ and 9 in
the selected inclined position corresponding to an angle
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~f 30, 35, 40 or 45 degrees re7ative to strip 20 For
this purpose, the locking plns 32 and 33 are respectively
engaged and maintained in the engagement position i.n
recesses 35 and 36 in side plate~ 22 by means of knurled
buttons 37 and 38 which are able to slide in openings 39
and 40 provided in recesses, such as 34~
Hereinafter, several examples for performing
the process of the invention are given.
Examples l to 5
An aqueous coating compos-ition is prepared
containing the constituents and amounts by weight shown
hereinafter:
Kaolin.O.............. O........................... 100
Sodium hexametaphosphate.~................ .Ø4
Car~oxylated butadiene/styrene copo'ymer.. 4 14
Carboxymethyl cellulose.... `.... ~................. .Ø4
The coating composition is brQught to a p~ of
8.5 by adding ammonia and is a~justed in such a way that it
contains 60% by weight of dry solids.
~ A quantity in excess of the coating composition is
deposited continuously on one of the faces of a strip of
paper, o~ 74 g/m~, by means of a coating roller then, by
scraping by means o one or several blades whose inclina-
tion angle relative to the strip i5 regulated to 30 degrees,
~5 the excess coating is removed in such a way that a roating
layer of g g/m2, expressed in dry s~bstance is obtained.
The thus coated paper is dried in a tunnel furnace
at a temperature of 100C, then undergoes calendaring con-
sisting of being passed four times in succession between
two cylinders under a load of 80 kg/cm.
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In example l, which i5 given for comparison
purposes, a single scraping blade is used.
In examples 2, 3, 4 and 5, illustrative of the
practice of khis invention, an apparatus is used which has
a premetering blade and a smoothing blacle. The distance
between the free ends of the premeterinq blade and the
smoothing blade i5 regulated to 2cm and the position
o~ the premetering blade is regulated in such a way that
the weight of the coating let by said blade, expressed
in dry solids is respectively-12, 20, 25 and 4~ g/m2~
The spreading quality is evaluated by means of
the so-called "Microcontour test", whose principle is as
follows: An ink formed from a course grained pigment is
dispersed in an oil of average viscosity, marketed by
Etablissements ~or illeux Lefranc under the name "ink
for microcontour test", is deposited in excess on the
paper. ~s the plgment particle dimensions are greater
than the diameter of the paper pores, the pigment cannot
penetrate into the paper~ After appropriate wiping, the
quantlty of pigment Ieft on the paper increases in propor-
tion to the roughness of the paper.
The coloring intensity obtained thus makes it
possible to evaluate roughness and also the size of the
furrows.
~5 Table l gives the results obtained in each of
the ex~mples.
Table 1
Example l¦Example ~ Example 31 xample 4 Example 5
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Coloriny Relatively Small Small Small Relatively
Intenslty High _ _ Small
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~t can he seen that the use of pre~etering
blade leads to coa~ed papers with reduced roughness.
Exam~les 6 to 9
The operation is carried out under the same
conditions as in examples 2 to 5, but using an apparatus -~
having two premetering blades and one smoothing blade.
The distance between the free ends of the two premetering
blades is regulated to 2cm and the distance between the
free ends of the second premetering blade and the smoothing
blade is also regulated to 2cm. The position of the `7
premetering blade is regulated in such a way that the
weight of the eoating le~t be~ind by said blades, expressed
in dry material is as indicated in Table 2.
- Table 2
15 - ¦Ex~mple 5 ExamplP 7 Example 8iExample 9
. . , .. . . - _ .
Coating weight le~t . .
by first premetering .
blade (g/m2) 20 30 45 65
_
Coating weight left . ¦
by seccnd premetering . I
blade (g/m2) 1~ ¦ 20 25 48
.
The microcontour test shows that the coloring
intensity and consequently the roughn ss of the coated papers
obtained are smaller than those of the coated papers obtained
in examples 2 to 4.
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