METHOD AND DEVICE FOR IMPREGNATING AND DRYING A CONTINUOUS WEB

PROCEDE ET INSTALLATION D'IMPREGNATION ET DE SECHAGE D'UNE BANDE CONTINUE

English Abstract

In conventional methods absorbent base paper is impregnated by being passed
through an aqueous solution of the impregnating agent and then conveyed to a
drier after the excess has been squeezed off by means of metering rolls. The
quantity of resin applied and the residual moisture are checked at set
intervals using sample pieces cut out from the web and in the case of
deviations the setting of the metering roll or the throughput speed are
corrected. The inventive method uses an automatic regulating device. The basis
weight of the freshly impregnated, wet web is measured and adjusted to a
predetermined control input. The residual moisture of the dried web is
measured and adjusted to a set value, for example by influencing the speed of
the drying air which is circulated during the last drying phase. The quantity
of resin applied is calculated from the basis weight and the residual moisture
of the dried web and the basis weight of the untreated web. This is then
compared to a set value to give the control input for metering. The method is
especially suitable for producing cosmetic films for refining surface of chip
board for the furniture industry.

French Abstract

L'invention concerne un papier support absorbant. Selon des procédés d'imprégnation connus, le papier passe dans une solution aqueuse de l'agent d'imprégnation puis est introduit dans un séchoir après expression de l'excédent par un cylindre doseur. A intervalles réguliers, on contrôle la quantité de résine appliquée et l'humidité résiduelle sur des spécimens découpés et, au besoin, on procède au réglage du cylindre doseur ou de la vitesse de défilement. Le nouveau procédé introduit un réglage automatique. Le grammage de la bande mouillée et fraîchement imprégnée est mesuré et réglé à une grandeur de commande déterminée. L'humidité résiduelle de la bande séchée est mesurée et réglée en fonction d'une valeur de référence, par exemple, en modulant, dans la dernière phase du séchage, la vitesse de l'air de séchage pulsé. La quantité de résine appliquée est calculée à partir du grammage de l'humidité résiduelle de la bande sèche et du grammage de la bande non traitée. Et la grandeur de commande est déterminée par comparaison avec une valeur de référence. Ce procédé est particulièrement adapté à la fabrication de films décoratifs destinés à recouvrir les panneaux de particules destinés à l'industrie du meuble.

Claims

Claims
1. Method for impregnating and drying a continuously running
absorbent paper web, with an aqueous solution of a synthetic
resin, such as urea resin or melamine resin,
having the following features:
a) The synthetic resin dissolved in water is applied in metered
amounts onto a continuously running web;
b) the web is guided in a suspended manner through heated
recirculated air and dried to a predetermined residual
moisture;
c) the basis weight of the freshly impregnated, wet web is
measured and adjusted to a predetermined reference value
through regulated metering of the resin amounts;
d) the residual moisture of the dried web is measured and
regulated to a target value, by influencing at least one of
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the following values:
d1) the air velocity at least in a last segment of the drying
process;
d2) the air temperature at least in a last segment of the drying
process;
d3) the throughput speed of the web;
e) from the basis weight and the residual moisture of the dried
web and the basis weight of the untreated web the applied
amount of resin is calculated, and by comparing it with a
target value, the reference value for the metering is found.
2. Method according to claim 1, characterized in that the basis
weight of the untreated web is detected continuously or at short
time intervals and is used for calculating the applied resin
amount.
3. Method according to claim 1 or 2, characterized in that the
web passes at first at least one drying segment, wherein its
temperature is measured and adjusted to a target value, whereby
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at least one of the two following values is changed:
a) the velocity of the recirculated air
b) the temperature of the recirculated air.
4. Device for impregnating and drying a continuously running
absorbent paper web, with an aqueous solution of a synthetic
resin, such as urea resin or melamine resin, according to the
method of one of the claims 1 to 3,
with an impregnating device (3) which has an adjustable metering
element (6, 7);
with a suspension dryer (9) equipped with nozzles (10) for
blowing hot air, with blowers (13, 14) for recirculating the hot
air and with heating devices (17, 18),
with a conveying device for transporting the web (1) through the
impregnating device (3) and the suspension dryer (9),
and with a regulating system which has at least the following
components:
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a first control circuit comprising a device (21), arranged
between the impregnating device (3) and the suspension dryer (9),
for measuring the basis weight of the wet web (1) and a first
regulating device (25) which under the action of the metering
unit (6, 7) adjusts the basis weight to a predetermined reference
value;
a second control circuit comprising a moisture measuring device
(23) arranged at the outlet of the suspension dryer and a
regulating device (27) adjusting the residual moisture to a
target value while influencing at least one of the following
parameters:
a) the rotational speed of the blower (14) in the outlet zone
(12) of the suspension dryer (9);
b) the heat supply through the heating device (18) of the
outlet zone (12) of the suspension dryer (9);
c) the speed of the conveying device (5, 6, 7, 19, ...);
a process computer (26) which calculates the amount of applied
15

resin from the basis weight of the dried web measured with the
device (22), the residual moisture and the basis weight of the
untreated web, and compares it with a target value and
corresponding to the found deviation gives the reference value
for the regulating device (25) of the metering unit (6, 7).
5. Device according to claim 4, characterized by a further
device (20) arranged upstream of the impregnating device (3) and
connected with the process computer (26) for measuring the basis
weight of the untreated web (1).
6. Device according to claim 4 or 5, characterized in that at
least one zone (11) is arranged upstream of the outlet zone (12)
of the suspension dryer (9) which is equipped with a measuring
instrument (24) for the temperature of the passing web and with a
regulating device (28), which adjusts the temperature to a target
value, by influencing at least one of the two following
parameters:
a) rotational speed of the blower (13);
b/ heat supply to the heating device (17).
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7. Device according to one of claims 4 to 6, characterized in
that the devices (20, 21, 22) are arranged to be transversely
movable.
8. Device according to claim 7, characterized in that the
metering device (6, 7) can be unevenly adjusted over the width.
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Description

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METHOD AND DEVICE FOR IMPREGNATING AND DRYING
.A CONTINUOUS WEB
The invention relates to a method for impregnating and
drying of a continuous web of absorbent material, particularly a
paper web with a synthetic resin dissolved in water, such as urea
resin or melamine resin, according to the preamble of claim 1.
The invention also relates to a device suitable to implement the
method according to the preamble of claim 4.
Papers impregnated with synthetic resin, particularly urea
resin or melamine resin are used for instance as decorative films
for chip boards and. the like, particularly for the furniture
industry. The absorbent base paper is impregnated in
conventional processes by being passed through an aqueous
solution of the impregnating agent and then conveyed to a drier,
after the excess has been squeezed off by means of metering
rolls. During drying the hardening reaction of the resin is
triggered. However this reaction should not be allowed to be
completed, since during further process steps the resin has again
to become fluid. The residual moisture of the web (after drying)
is a measure for evaluating the hardening reaction.
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After drying the web is cooled and cut into sheets by means
of a cross cutter or wound up.
During the production process it is necessary to check at
certain intervals whether the prescribed characteristics, such as
residual moisture and the amount of applied resin, are observed.
Up to now for this purpose a narrow strip of foil is cut out with
the cross cutter. From this strip surface pieces of the size of
1 dm~ are punched out. This surface pieces are weighted and
dried out in a dry box. The drying in the dry box takes
approximately 5 to 10 minutes. The difference between the weight
measured before drying and the weight measured after drying
indicates the residual moisture. If with the aid of these tests
it is found that for instance the applied resin amount is not
correct, then the attempt is made to correct the weight through a
different setting of the nip of the metering rolls. However if
an unacceptable deviation. of the residual moisture is found,
which as a rule must be at 6.5 % ~ 0.1 %, then the moisture is
corrected by modifying the web speed or by changing the drying
capacity.
The described method has among others the disadvantage that
the taking of the sample strips leads to losses of material. It
can be practically performed only when the dried web is cut into
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sheets. But when the dried web is wound up, a sample taking is
almost impossible. A particular drawback consists in that the
information about the quality of the product is available only
after 5 to 10 minutes. If there is a deviation from the required
values, in this time span the production of defective material
continues. Frequently impregnating installations are operated at
reduced speeds, in order to insure that the required quality
characteristics are observed. In order to prevent the applied
resin amount from falling below a minimal level, often the
process is run with a somewhat excessive resin content.
From Swiss Patent 535 074 a method of continuous and even
application of treatment liquors on a textile material is known,
wherein the basis weight of the running web is automatically
measured in a total of three locations, namely before
impregnation, between impregnation and drying and after drying.
With the found data the amount of treatment liquor applied is
adjusted, and by in:Eluencing the drying temperature and the
throughput speed the residual moisture is kept constant.
A method for the production of so-called prepregs and the
like on the basis o:E paper is known from the publication PRINTED
CIRCUIT HANDBOOK, published by Clyde F. Coombs, Jr., 3rd Edition,
McGraw-Hill Book Company, 6.3 to 6.5, wherein the web is passed
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at first through an. impregnating system consisting of a vat
filled with resin and a set of metering rolls, and after that
through a dryer. Measuring instruments working with beta rays
compare the basis weight of the base paper with the semi-hardened
final product and automatically correct the metering rolls, so
that the prescribed ratio between resin and base material is
maintained. The polymerization degree is controlled through
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11.98
temperature and thE: velocity of the drying air, as well as
through the throughput speed of the paper web.
It is the object of the invention to continuously and
automatically adjust the applied resin amount as well as the
residual moisture in a process according to the preamble of claim
1. This also includes the object to equip a device according to
the preamble of claim 4 with the elements suitable to carry out
the adjustment according to the invention.
The first part: of the object is achieved due to the
characteristic features of claim 1, the second part due to the
characteristic features of claim 4. The dependent claims refer
to advantageous embodiments of the invention.
The drawing sE:rves for the clarification of the invention
with the aid of a :~chemar_ic and highly simplified representation
of an embodiment eacample.
A paper web 1 to be impregnated is wound from a roller 2
supported on a delivery mechanism not shown in the drawing and
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fed to a an impregnating device 3. The paper web has a basis
weight of approximately 20 to 200 g/m2 , preferably of
approximately 60 to 120 g/m~. The impregnating device 3
comprises a vat 4 filled with an aqueous resin solution, through
which the web is passed aver deflection rollers 5, and a metering
unit in the form of a pair of squeezing rollers 6, 7. The
squeezing roller 6 is adjustable in horizontal direction as
indicated by the double arrow 8, by means of an actuating drive,
so that the width of the nip between the two squeezing rollers 6,
7, through which the wet paper web 1 passes, can be very
sensitively changed. Fram the impregnating device 3 the paper
web 1 reaches a suspensian dryer 9. The latter is equipped with
a great number of finger-like air-cushion nozzles 10, which are
arranged in rows above arid below the paper web 1 and extend
transversely with respect to the paper web 1. The suspension
dryer 9 is built of several zones in a building block manner. In
the drawing only two zones are visible, namely the inlet zone 11
and the outlet zone 12. In between there can be - as indicated -
several intermediate zones. Each zone is equipped with at least
one blower 13, 14 for maintaining
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a circulating air flow. Each blower 13, 14 has a frequency-
controlled drive 15, 16, and can therefore be driven at various
rotational speeds. In each zone a heating device 17, 18, for
instance a burner, is arranged. Downstream of the suspension
dryer, a cooling roller pair 19 is arranged. From there the
dried paper web reaches for instance a cross cutter not shown in
the drawing or a winder.
The delivery mechanism wherein the roll 2 is supported and
the various rollers over which the paper web 1 is guided,
particularly the deflection rollers 5, the squeezing rollers 6, 7
and the cooling roller 19 are provided with adjustable drives,
which can be run with rotational speeds attuned to each other.
Together they form a conveying apparatus on which the paper web 1
can be transported at low tension and adjustable speed.
The installation has a total of three devices 20, 21, 22 for
measuring the basis weight, the first between the roll 2 and the
impregnating device 3, the second between the impregnating device
3 and the suspension dryer 9, the third immediately downstream of
the suspension dryer 9. The devices operate on the principle
that the absorption. of a radioactive beam is measured as a
criterion for measuring the basis weight. In the vicinity of the
device 22 a moisture-measuring device 23 operating on a microwave
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basis is provided, for measuring the residual moisture of the
dried paper web leaving the suspension dryer 9. The inlet zone
11, as well as possible intermediate zones not shown in the
drawing, are equipped with an infrared sensor 24 for measuring
the temperature of the paper web 1.
During operation the basis weight of the wet paper web 1,
i.e. freshly impregnated with an aqueous resin solution of
approximately 30 to 60 % content of resin, is measured
continuously, or at short intervals, with the device 21. It
corresponds as a rule to 2 to 5 times the basis weight of the
base paper. A corresponding measuring signal is fed to the
regulating device 25 and there compared to the reference value.
Corresponding to the found difference, the regulating device 25
gives an actuating signal to the actuating drive of the squeezing
roller 6. Thereby the roller nip is changed in the sense that
the basis weight is adjusted to the reference value. The
reference value is given to the regulating device 25 by a process
computer 26. The latter processes measuring signals of the
devices 20 and 22, as well as the moisture measuring device 23.
The device 20 measures the basis weight of the dry, untreated
paper web 1, the device 22 measures the basis weight of the
impregnated and dried paper web. The latter is a sum of the
basis weight of the base paper, of the amount of resin applied
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per surface unit and of t:he residual moisture detected by the
moisture measuring device 23. As a rule the basis weight of the
dried, impregnated paper web ranges between 1.5 and 3 times the
basis weight of the base paper. The residual moisture ranges
between 5 and 8 %, i.e. up to 6.5 % ~ 0.1 %.
From the measuring signals the process computer 26
calculates the amount of applied resin (without residual
moisture). It compares it with a predetermined target value and
in the case of a deviation influences the reference value of the
regulating device 25 with the opposite sign, so that the
deviation is reduced. A regulating device 27 compares the
measured residual moisture with the predetermined target value
and, corresponding to the found deviation, gives an actuation
signal to the actuating motor of the blower 14 of the outlet zone
12.
In the inlet zone 11 a signal of the infrared sensor 24
corresponding to th~~ web temperature is transmitted to a
regulating device 2.8. The latter compares the web temperature
with an empirically established target temperature based on
tests, by means of which the optimal course of drying has been
established. In case of deviation, through a correction signal
the regulating device 28 causes the drive of the blower 15 to
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correspondingly modify the rotational speed. Possible
intermediate zones are also equipped with corresponding control
circuits. In a suspension dryer consisting of a total of four
zones the optimal web temperature ranges for instance for the
inlet zone 11 between 70oC and 80oC, for the two following
intermediate zones between 100oC and 110oC. When the optimal
temperatures are observed in the inlet zone 11 and in-the
intermediate zones, it is insured that at the moisture measuring
device 23 only minimal deviations from the prescribed residual
moisture can occur. Therefore the correcting action of the
regulating device 27 can be limited to the outlet zone 12.
In a deviation from the described embodiment example, for
the purpose of adjusting the temperature of the paper web 1,
instead of changing the rotational speed of the circulating air
blower 13, 14 - under certain circumstances also additionally -
it is possible to change the heat supply to the heating devices
17 of the inlet zone and optionally of the intermediate zones.
Furthermore it is possible to adjust the residual moisture at an
unchanged rotational speed of the blower 14 of the outlet zone
12, by changing the throughput speed at which the paper web 1
traverses the suspension dryer 9. In this case the regulating
device 27 produces an actuation signal for the drives of the
various rollers 5, 6, 7, 19..., over which the paper web is

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guided. It is also possible to control the residual moisture by
changing the heat supply to the heating device 18 of the outlet
zone 12. When the paper to be impregnated is very uniform, the
device 20 between the roll 2 and the impregnating device 3 can be
eliminated. Instead the basis weight is directly entered in the
process computer 26.
In a variant of the invention, the devices 21, 22 for
measuring the basis weight of the wet, respectively dried paper
web 1 are arranged on rails extending in transverse direction.
They can be moved back and forth by means of a drive. They
measure the basis weight alternately close to the two edges and
optionally also at additional locations. Correspondingly the
metering device can be adjusted unevenly over the width. In the
simplest case, the lateral bearings of the squeezing roller 6 are
independently adjustable, so that at different settings of the
two bearings a wedge-like nip results. If during operation it
appears that on one side more resin solution is applied than on
the other side, the defect can be at least approximately
corrected through a unilateral adjustment of the squeezing roller
6. An even finer compensation of irregularities is possible when
the squeezing roller 6 is designed as a roller with controllable
deflection.
11

Representative Drawing