METHOD AND APPARATUS FOR TWO-SIDE COATING OF A THIN PRINTING PAPER WEB MADE OF MECHANICAL PULP

APPAREIL SERVANT A COUCHER DES DEUX COTES UNE BANDE MINCE DE PAPIER D'IMPRESSION OBTENU A PARTIR DE PATE MECANIQUE, ET METHODE CONNEXE

English Abstract

The invention relates to a method and
apparatus for two-side coating of a light
base weight printing paper web containing
mechanical pulp or recycled fiber.
According to the method the first side of
a web (5) is coated in a first coating
station (1), the first-side coat is dried
at least partially in a dryer unit (3),
the second side of the web (5) is coated
subsequent to the drying of the first
side in a second coating station (2), and
the second-side coat is dried at least
partially in a second dryer unit (4).
According to the invention both the
first-side coat and the second-side coat
are formed by applying a required amount
of coating mix onto the perimeter of a
soft film-coating roll (22) and subsequently
transferring said coat film to
the web (5) in a nip (N1) formed between
a backing roll (24) and said film-coating
roll (22).

Claims

9
Claims:
1. A method for two-side coating of a printing paper web
containing mechanical pulp or recycled fiber and having a base
weight of less than 65 g/m3, comprising:
applying a sufficient amount of a coating mix to only a
first side of the web within a first coating station to coat the
first side of the web, wherein the coating mix is pressed onto
the first side of the web by applying the coating mix onto a
perimeter of a first resilient film-coating roll rotated at a
peripheral speed approximately equal to a speed in which the web
travels and subsequently transferring the coating mix to the
first side of the web in a first nip formed between a first
backing rail and the first film-coating roll, the first backing
roll and the first film-coating roll calendering the web;
at least partially drying in a first dryer unit the coating
mix applied to the first side of the web;
applying a sufficient amount of the coating mix to only a
second side of the web within a second coating station to coat
the second side of the web, said step of applying coating mix to
the second side of the web occurring subsequent to said step of
at least partially drying of the coating mix applied to the first
side of the web, wherein the coating mix is pressed onto the
second side of the web by applying the coating mix onto a
perimeter of a second resilient film-coating roll rotated at a
peripheral speed approximately equal to a speed in which the web
travels and subsequently transferring the coating mix to the
second side of the web in a second nip formed between a second
backing roll and the second film-coating roll, the second backing
roll and the second film-coating roll calendering the web; and
at least partially drying in a second dryer unit the
coating mix applied to the second side of the web;

10
wherein said steps of applying coating mix and at least
partially drying of the applied coating mix are performed
continuously in a coating apparatus.
2. The method of claim 1, further comprising chilling the
first and second backing rolls.
3. The method of claim 1, wherein the coating mix has a
viscosity of at least 500 cP.
4. The method of claim 1, wherein the first and second
backing rolls press the coating mix onto the web with a nip
pressure of at least 500 kPa.
5. The method of claim 1, further comprising steam cleaning
the first and second backing rolls.
6. The method of claim 1, further comprising scraper
cleaning the first and second backing rolls.
7. The method of claim 1, wherein the first and second
backing rolls are metal-covered.
8. The method of claim 1, wherein the first and second
backing rolls are ceramic-covered.
9. The method of claim 1, wherein the first and second
backing rolls are polymer-covered.
10. The method of claim 1, wherein coating mix is applied
in the first coating station onto the perimeter of the first
film-coating roll with a first nozzle and in the second coating
station onto the perimeter of the second film-coating roll with
a second nozzle.
11. The method of claim 1, wherein the web travels at a
speed of from 400 to 1500 m/min during said steps of applying

11.
coating mix to the web and said steps of at least partially
drying the applied coating mix.

Description

CA 02107277 2004-04-28
Method and apparatus for two-side coating of a thin printing
paper web made of mechanical pulp
The present invention relates to a method for two-side
coating of a thin printing paper web containing mechanical
pulp, e.g., a paper web made of recycled fiber.
The invention further relates to an apparatus suited to two-
side coating of a thin printing paper web made of mechanical
pulp.
Thin printing paper webs containing mechanical pulp are
conventionally coated on subsequent coating stations
equipped with short-dwell coaters as the coater units. A
combination of two subsequent coater stations is necessary
as low base paper weight and high content of groundwood make
single-run coating on both sides impossible. Two-side
coating with conventional methods would excessively wet the
web and thus impair its runnability. Furthermore, the
measurement of coat weight in a two-side coating operation
is difficult.
Despite their multiple benefits, short-dwell coaters also
have several drawbacks. Air entrapped in the coat paste
easily causes mottling. Owing to the small linear
application pressure and short application distance, wetting
of the base web and subsequent fiber swelling occurs even
after the web has passed the doctor blade. This may impair
the coat smoothness. As a rule, doctor blade coating methods
become critical with thin webs and particularly with light
coat weights.
Paper grades containing a high proportion of mechanical pulp
and a high percentage of coat fillers such as, e.g., SC
paper (wood containing high filler content super-calendered
printing paper) make doctor blade coating impossible owing
to the fragility of the base paper web.

CA 02107277 2004-04-28
2
Web defects leading to brittleness result in low production
yield and inferior runnability.
Base paper grades containing recycled fiber have posed
unexpected problems in doctor blade coaters: The coat is
easily marked during coating by streaks caused by defective
doctor blades.
Recycled-fiber containing base paper grades have a darker
color, thus making the opacifying power of the coat mix more
critical. Owing to its operating principle, doctor blade
coating tends to give a smooth coat, not a coat of uniform
coat weight. As the base paper is not necessarily smooth,
the opacifying power obtained in doctor blade coating is
insufficient, resulting in mottling of the coated web.
It is an object of the present invention to overcome the
disadvantages of the above-described prior-art technology
and to achieve an entirely novel method and apparatus for
two-side coating of a thin printing paper web containing
mechanical pulp such as, e.g., a paper web grade made of
recycled fiber.
The invention is based on performing the coating operation
using a two-step compressive film-lamination technique.
The invention provides significant benefits.

2I~'~2'~'~
3
The technique according to the invention halves the web
wetting relative to single-run two-side coating. Thus,
the invention provides good runnability. Further, good
coat quality is attained at light coat weights.
s Particular benefit is achieved with base paper grades
containing recycled fibers as the formation of streaks
associated with doctor blade coating can be avoided.
Since the compressive film-lamination technique forms a
coat of uniform weight on the web, the coat has a high
to opacifying power. For the same reason, a higher burst
index relative to doctor blade coating is attained.
Moreover, the measurement of coat weight by means of
rupturing testers is easy. The method according to the
invention imposes minimal mechanical stress on the web.
is Two-side single-run coating requires a long path of
unsupported pulling of the web prior to the web support
roll to give the coat a possibility of drying prior to
touching the support roll. In the technique according to
the invention, the uncoated side can be supported by a
zo roll immediately after the coating of the other side,
thus achieving a significant reduction in coater unit
size.
In the following the invention is described in greater
2s detail with the help of exemplifying embodiments
illustrated in the annexed drawing in which
Figure 1 is a side elevation view of a part of a paper
machine incorporating a coater apparatus according to the
3o invention.
Figure 2 shows diagrammatically an alternative coater
apparatus according to the invention.
3s Figure 3 shows in greater detail a nozzle assembly suited
to implement the coater apparatus according to the
invention.

210'7 2'7'7
4
In conjunction with the present invention, the term
compressive film-lamination technique is used to refer to
a technique in which the web during coating is subjected
to a linear pressure in a nip formed between a film-
s coating roll and a backing roll so as maintain the
peripheral speeds of both the film-coating roll and the
backing roll approximately equal to the web speed.
In conjunction with the present invention, the term light
~o web material refers to web materials having a base weight
of less than 65 g/m2.
With reference to Fig. 1, the apparatus according to the
invention comprises a first compressive film-lamination
~s station 1 and a second compressive film-lamination
station 2, having a first drying unit 3 placed between
them. The web path is configured so that subsequent to
the first drying unit 3 on the path of a web 5 is placed
the second coating station 2, followed by a second drying
2o unit 4. Basically, both drying units have a similar
construction. The coating station 1 is comprised of a
film-coating roll 22, a coater bar 20 and a backing roll
24 of the film-coating roll 22. The coater bar 20 serves
for metering a desired amount of coating mix onto the
zs film-coating roll 22, wherefrom the coating mix is
subsequently transferred in the nip N~ to the web 5. To
make the coating mix adhere to the web with the greatest
smoothness free from orange peel effect, the backing roll
24 has advantageously a smaller diameter than the film-
3o coating roll, whereby the angle at which the web 5
conforming to the backing roll 24 exits the nip N~ is
maximized. In an alternative approach the web 5 is guided
toward the film-coating roll 22, whereby also a good coat
surface quality is obtained.
The first drying unit 3 is comprised of infra-red dryers
6 and dryer drums 7.

2 ~. ~'~ ~'~'~
Basically, the coating station 2 has a similar
construction to that of coating station 1. owing to the
routing of the web 5, the station 1 is mirrored from
station 2. The station 2 is analogously followed by the
s second drying unit 4 comprising an infra-red dryer 8 with
subsequent drying cylinders (not shown).
With reference to Fig. 2, the coating bar 20 can be
replaced by a so-called gate roll coater in which the
to coating mix is transferred from a coating mix fountain 34
located above a nip N2, which is formed by a transfer
roll 30 and a metering roll 32, via said nip N2 onto the
perimeter of said metering roll 32 and further via a nip
N3 onto the perimeter of a film-coating roll 36. From the
~s perimeter of the film-coating roll 36 the coating mix is
applied to a web 38 in a nip N4 formed between the film-
coating roll 36 and a backing roll 39. According to the
invention, the diameter of the backing roll 39 can be
smaller than the diameter of the film-coating roll 36.
The film-coating roll 36 has a diameter of 1000 mm
typical, and the backing roll 39 has a diameter of
800 mm, respectively.
2s Characterizingly, in both above-described exemplifying
embodiments the surface of the film-coating roll is
adjusted to carry an approx. 7...15 ~m thick film of the
coating mix, a major portion (typically approx. 80 %) of
which is adhered to the web in the nip between the film-
3o coating roll and the backing roll. Thus, the web is
coated with a coat of 7...15 g/m2 dry weight.
The adsorption of the coating mix paste and the water
contained therein is related to the magnitude of the nip
3s pressure and the duration of said pressure, that is, the
width of the nip. On the other hand, the nip width is
solely determined by the diameters of the rolls and their

CA 02107277 2004-04-28
6
hardness, while the magnitude of the nip pressure is
principally determined by the linear loading of the nip, and
additionally, by the web speed. Accordingly, the good
penetration of the coat into the web is achieved by means of
a high linear application pressure imposed in a relatively
wide nip.
The maximum nip pressure typically is 1000 kPa gauge and the
nip width is in excess of 15 mm. Good results according to
the invention have been obtained by keeping the nip pressure
above 500 kPa and the nip width greater than 10 mm. Such
desirable nip widths can be attained by means of hard rolls
with diameters in excess of 600 mm. Particularly the film-
coating roll must have a diameter greater than 600 mm,
whereby the backing roll must also have a diameter in excess
of 600 mm. To achieve the minimum pressure limit of
500 kPa, the linear nip loading must be at least 20 kN/m for
a typical coating mix paste. In conventional kiss roll
coating, the application pressure is only approx.
50...100 kPa, while a conventional doctor blade coater can
achieve a pressure of 1000 kPa over a nip width of less than
1 mm. In a conventional short-dwell coater the encountered
levels of application pressure are yet lower. The technique
according to the invention is suited to web speeds of
400...1500 m/min typical. The linear nip loading is
typically in the range of 20...50 kN/m, advantageously
approx. 35 kN/m. The coating material of both the backing
roll and the film-coating roll is polyurethane, rubber or
any suitable resilient material. The P&J numbers of the
rolls are typically in the range of approx. 0...40. The
film-coating roll employed in the embodiments according to
the invention is invariably a so-called soft roll with a
surface material of polyurethane, for instance.

CA 02107277 2003-06-23
With reference to Fig. 3, a nozzle assembly for a film-
coating roll 40 is comprised of a water blade 42 tilted
to an acute angle and mounted to a frame structure 41.
Between the frame structure 41 and the blade 42 .is placed
a loading hose 43 suited to controlling the linear
pressure and position of the blade 42. The coating mix 46
is contained in a metering fountain formed between a
front wall 44 and the blade 42, wherefrom the mix is
transferred in a controlled manner onto the perimeter of
to the coating roll 40. The front wa:~:~ 44 is mounted by
means of support elements 4~ onto the chassis of the
apparatus. Such a nozzle assembly is known in 'the art and
its construction is described in greafi:er detail in US
patent 4,839,201, for instance. The blade 42 can
t5 alternatively be replaced by a doctoring bar,
In a preferred embodiment of the invention the backing
roll has a metal surface. The meta:L surface can be of
chromium, for instance. Also ceramic ar polymer covered
zo backing rolls are usable. A metal or ceramic covered
backing roll performs initial calendering of the coated
web. Such coated rolls with a steer core can also be
provided with chilling, whereby condensation of moisture
onto the roll perimeter is attained, which in turn aids
z5 keeping the roll clean. awing to the improved thermal and
wear resistance of a. metal or ceramic covered roll, also
steam and scraper blades can be used for keeping the
rolls clean.
3o According to the invention, the nozzle assembly employed
for metering the coating mix onto the film-coating roll
can also be a slot-orifice die metering assembly or a
spraying apparatus capable of spraying the coating mix
onto the roll surface.

~10'~27'~
8
The table below gives exemplifying compositions of
coating mixes suited for advantageous use according to
the invention:
s Coating color componentCoating mix Coating mix
composition composition
1 2
Calcium carbonate 100 parts
pigment
Kaolin pigment 100 parts
Starch binder 10 parts 6 parts
to Synthetic binder 4 parts 6 parts
Additives 2 parts 0.5 parts
Solids content 55 % 58
Viscosity (Brookfield 500 cP 800 cP
I00)

Representative Drawing