METHOD FOR PROCESSING AND SUPPLYING A COATING COLOUR USED FOR COATING A FIBROUS WEB TO A COATING DEVICE

Description

[0001] The present invention relates to a method for processing and supplying a coating colour used for coating a fibrous web to a coating device.

[0002] In the processing industry, the mixing of gases, such as air, with the liquids and compositions used in a process typically causes several problems. Especially when coating paper or a similar fibrous web material, gas and gas bubbles in the coating colour result in roughness on the surface of the paper in coating, and even areas where there is no coating at all. This problem is greater with some coating colours than others, but the problem is emphasised particularly with those coating colours that accumulate more gas than others. For example, coating colours containing talc typically contain large amounts of gas due to the properties of talc. The significance of the problem is further affected by the coating method used. For example, in curtain coating, the gas content of the coating may be at most 0-0,25% by volume. Otherwise the gas bound by the coating may result in uncoated areas in the material to be coated, such as paper or board.

[0003] In multilayer curtain coating, the significance of degassing is even greater. Thus, if there are, for example, three or four coating layers, the coating used to produce each layer must be degassed as carefully as possible.

[0004] For removing the gas mixed with or dissolved in coating colour have been developed, for example, various types of vacuum deaerators, a known embodiment of which comprises a rotating drum arranged inside a vacuum container, into which drum the coating colour is delivered, whereupon the coating colour rises up the inner wall of the drum by the effect of centrifugal force and is discharged from the drum as a thin film colliding with the wall of the vacuum container. The problem with prior art vacuum deaerators is their insufficient deaeration capacity, especially with highly viscous substances. This is due to the fact that the small air bubbles contained in highly viscous coating colours are unable, even under an extremely high vacuum, that is, low absolute pressure, to grow large enough to be broken or distinguished due to their specific rising rate.

[0005] Attempts have been made to eliminate this problem by increasing the vacuum, but as a result, the solvent used in the coating colour, for example water, vaporises extremely readily, whereupon the solids content of the coating colour increases. Another method used involves increasing processing times, but in that case the operational capacity of the deaerators falls, which means that a greater number of deaerators must be acquired or their size increased.

[0006] DE 102 60 593 A1 describes a method and an arrangement for coating a material web, especially a paper or board web. The coating material which is applied to the web is fed by means of a pump through a duct to an aeration and filtering means. The aeration and filtering means is followed by a valve and a spread chamber from which the coating material is directly supplied to the web.

[0007] A coating method and apparatus for air bubble-free deposition of a coating solution to a substrate is known from GB 2 333 476 A. The solution is supplied from a container to a delivery module and then to a means for supplying the coating to a substrate. The solution is recirculated via lines until the delivery module, which includes de-bubbling apparatus, is substantially free of bubbles. A valve is then switched to allow the solution to pass from the delivery module via a line to the means, thus minimising wastage of the solution. If the coating process is interrupted, the delivery module is switched into recirculation mode to keep the solution bubble-free until coating can be resumed.

[0008] One solution to this problem has been presented by the applicant of the present application in their earlier patent application FI 20055713 disclosing a method and apparatus, where oscillatory action is applied to the coating colour flow to be fed into the degassing apparatus, which causes the gas content to fall and/or the gas bubbles to integrate or their size to increase in the coating colour flow fed into the degassing apparatus. The oscillating frequency is preferably within the range from about 16 to about 60 kHz, which affects the gas bubbles in the coating colour by making them bigger and thus easier to remove, for example, by means of a vacuum-operated degassing apparatus, or which break when they grow large enough. Due to the oscillation treatment, the efficiency of degassing increases considerably compared with using a known degassing apparatus alone. The oscillation treatment may also take place at frequencies markedly lower than the ultrasound frequency. Coating colour subjected to oscillation treatment may be at underpressure, overpressure or normal atmospheric pressure.

[0009] Another solution to this problem is a method disclosed in a previous application FI 20055704 by the applicant of the present application, which utilises a degassing apparatus in which are formed two separate compartments, so that degassing is carried out in two different stages by using a single apparatus. The apparatus comprises a first container part, to which are connected means for providing a vacuum therein, means for feeding coating colour inside the first container part, means for degassing the coating colour in the first container part and means for discharging the coating colour from the first container part, and a second container part to which are connected means for providing a vacuum therein, means for feeding coating colour inside the second container part, means for degassing the coating colour in the second container part and means for discharging the coating colour from the second container part, wherein the coating colour is arranged to be first fed into the first container part, where the first degassing stage takes place, and then from the first container part into the second container part, where the second degassing stage takes place.

[0010] An absolute pressure of approximately 1 kPa -15 kPa is preferably provided in the vacuum container parts. If a lower absolute pressure is provided in the chamber, this will mean, among other things, that the evaporating point of the solvent contained in the coating colour will fall and there is a risk of the quality of the coating colour deteriorating as a result of degassing. On the other hand, if a higher absolute pressure is provided in the chamber, this will not necessarily suffice to increase the size of gas bubbles in the coating colour by means of a vacuum. Providing an absolute pressure of about 3 kPa - 15 kPa in the vacuum container is highly preferable. By raising the lower limit of the absolute pressure used is ensured even better that the coating colour will not be able to vaporise during degassing. By means of this solution is achieved high efficiency, whereby gas contained in the coating colour can be removed rapidly and thoroughly from a large amount of coating colour.

[0011] The solutions presented above are as such useful for making the degassing of coating colour more efficient. In a prior art solution, this degassing stage is, however, usually carried out immediately before supplying the coating colour to the coating apparatus, which causes the problem that the degassing capacity will vary according to the amount supplied to the apparatus, which means that the degassing apparatus will operate in unstable conditions and produce coating colour having a varying gas content. This may cause the finished coating forming on the surface of the fibrous web to contain unwanted gas bubbles. Figure 1 shows diagrammatically a prior art arrangement of this type, where the coating colour is delivered from the production line into the supply tank 1, after which it is delivered, for example, by means of feeder pump 6A to the degassing stage 2, which is preferably vacuum-operated 3. From the degassing stage 2, the treated coating colour is taken, for example, by means of feeder pump 6B, to the coating apparatus 5, by means of which it is applied on the surface of the fibrous web to be coated. The amount of coating colour supplied to the degassing stage 2, 3 depends on the amount of coating colour required by the coating apparatus 5 at a given time.

[0012] It is, therefore, an aim of the present invention to provide a method and apparatus for avoiding the supply of coating colour having a varying gas content to the coating device. To achieve this aim, the method according to the invention is characterised in that in the method, the coating colour from coating colour production is delivered into a storage tank, from where it is taken to the degassing stage, and from there further to the coating colour supply tank.

[0013] In the method according to the invention, coating colour is supplied to the degassing stage preferably at a constant feed rate, the feed rate being, for example, 0 to about 50% more than the amount required for coating, whereby the excess is returned to the storage tank preceding the degassing stage. The feed rate is preferably 5 to 15% more than the amount required for coating.

[0014] From the degassing stage, degassed coating colour is supplied, again at a constant feed rate, to a supply tank arranged in the coating device, whereupon in the supply tank of the coating device is only supplied the amount required each time by the coating device and the excess is returned to the storage tank. In previous solutions, the coating device has not had a supply tank of its own, which means that the amount of coating colour fed to the degassing stage has been dependent on the amount of coating colour required by the coating device each time. The solution according to the invention, where the coating device is provided with its own supply tank and the supplying of coating colour to the degassing stage and the coating colour supply tank takes place at constant quantities, provides stable conditions at the degassing stage and thus the production of coating colour of uniform quality as regards degassing. The coating colour flow discharging from the degassing stage at a constant feed rate is typically smaller than the coating colour flow entering the degassing stage due to the discharging of gases from the coating colour. Also on the coating device, any excess coating colour from the coating stage is preferably returned to the storage tank preceding the degassing stage instead of to the machine cycle.

[0015] Some preferred further developments of the method according to the invention are disclosed in dependent claims 2 to 11.

[0016] The apparatus relating to the invention is in turn characterised in that the device comprises means for delivering the coating colour from coating colour production to the storage tank, means for supplying coating colour to the degassing stage and means for supplying the coating colour further from the degassing stage to supply tank arranged in conjunction with the coating device.

[0017] The invention is described in greater detail in the following, with reference to the accompanying drawings, in which:

Figure 1

shows diagrammatically a prior art arrangement for performing the degassing of coating colour and its supply to the coating device, and

Figure 2

shows diagrammatically an arrangement according to the invention for performing the degassing of coating colour and its supply to the coating device.

[0018] Figure 2 shows only diagrammatically an arrangement according to the invention, where the coating colour from the coating colour production process is first delivered to a storage tank 1, from where it is led by means of pump 6A to the degassing stage 2, which is preceded by a pre-treatment stage 7 in the embodiment disclosed. The pre-treatment stage may include, for example, heating or cooling or filtering of the coating colour, pre-degassing treatment, ultrasound treatment or different combinations of these. The degassing treatment 2 is preferably based on the use of a vacuum and may include one or more stages, such as, for example, the method disclosed in the applicant's earlier application FI 20055704, which uses a vacuum-operated degassing apparatus in which are formed two separate compartments, so that degassing is carried out in two different stages by using a single apparatus.

[0019] After the degassing stage, the coating colour is delivered by means of pump 6B to the supply tank 4 of the coating device 5, from where it is supplied by pump 6C to the coating device to be spread on the fibrous web. In the arrangement shown, after the pump 6C are located filter means 9 which may be of any type known as such.

[0020] The arrangement further uses on-line measuring equipment 8, for example, for measuring the gas content of the coating colour discharging from the degassing stage, thus preventing coating colour having a gas content deviating from the allowed limits from entering the coating colour supply tank 4 and returning it to the storage tank, for example, in connection with the start-up of the device, which means that the coating colour may be recirculated several times through the degassing stage 2 if necessary, before feeding it into the supply tank 4. The on-line measuring equipment 8 may include, for example, means for measuring the density of the coating colour coming from the degassing stage, in which case the measured density data can be used, for example, for indirectly determining the remaining gas content in the coating colour. Density measurement may be carried out, for example, as differential pressure measurement. The on-line measuring equipment may also include, for example, a direct measuring device for gas content based on the use of sound or ultrasound, which may be used instead of indirect gas content determination based on density measurement.

[0021] The means for directing the coating colour back to the storage tank instead of the supply tank 4 may be any appropriate valve means (not shown), which direct excess coating colour fed from the degassing stage 2 back to the storage tank 1 also in normal operation.

[0022] During the normal operation of the device, coating colour is supplied from the storage tank 1 at a constant rate to the degassing stage 2 by means of pump 6A. This stabilises the operation of the degassing device and gives a more uniform end result. From the degassing stage is likewise supplied degassed coating colour at a constant rate by means of a pump 6B to the coating colour supply tank 4. The supply tank 4 comprises, for example, liquid-level measuring means providing data on the basis of which the amount of coating colour required each time is led into the supply tank, excess coating colour being directed back to the storage tank 1. The supply rate of coating colour from the degassing stage is preferably within the range from 0 to about 50% more than is required for coating.

[0023] The solution according to the invention may also be realised as an integrated degassing apparatus for two or more coating heads, in which there may be a separate pump 6B for each coating head for supplying coating colour from the degassing stage 2 to the coating heads or the pump 6B may be shared by all coating heads, feeding coating colour at a constant rate, whereby the amount of coating colour required each time is directed to the supply tank of each coating head, respectively, and the remainder is returned to the shared storage tank 1.

Claims

1. A method for processing and supplying coating colour used for coating a fibrous web to a coating device (5), in which method the coating colour from coating colour production is delivered to a storage tank (1), from where it is taken to the degassing stage (2), characterised in that the coating colour is supplied to the degassing stage (2) at a constant feed rate from the storage tank (1), and that the coating colour is taken from the degassing stage (2) further to the supply tank (4) of the coating device (5).

2. A method as claimed in claim 1, characterised in that 0 to about 50% more coating colour is fed to the degassing stage (2) than is required for coating, whereby the excess is returned past the supply tank (4) of the coating device to the storage tank (1) preceding the degassing stage.

3. A method as claimed in claim 1, characterised in that the excess coating colour from the coating stage on the coating device is returned to the storage tank (1) preceding the degassing stage (2).

4. A method as claimed in any of the above claims, characterised in that in the method, the density of the coating colour is measured by on-line measurement

5. A method as claimed in any of the above claims, characterised in that in the method, on-line measurement (8) is used for measuring the gas content of the coating colour discharging from the degassing stage (2), and that in the method are used means for preventing coating colour having a gas content deviating from the allowed limits from entering the coating colour supply tank (4).

6. A method as claimed in claim 5, characterised in that measurement based on sound or ultrasound is used for measuring gas content.

7. A method as claimed in any of the above claim, characterised in that degassing is carried out in a single or several stages.

8. A method as claimed in claim 7, characterised in that degassing is carried out by utilising a vacuum.

9. A method as claimed in claim 7 or 8, characterised in that the degassing stage involves one or more pre-treatment stages selected from a group including heating or cooling of the coating colour, filtering of the coating colour, pre-degassing stage, ultrasound treatment or different combinations of these.

10. A method as claimed in any of the above claims, characterised in that the coating colour is supplied from the degassing stage (2) to two or more coating heads.

11. An apparatus for processing and supplying a coating colour used for coating a fibrous web to a coating device (5), wherein the apparatus comprises means for delivering the coating colour from coating colour production to a storage tank (1), means (6A) for supplying coating colour from the tank (1) to a degassing stage (2), characterised in that the apparatus comprises means (6A) for supplying the coating colour at a constant feed rate to the degassing stage (2) and means (6B) for supplying the coating colour further from the degassing stage to a supply tank (4) arranged in conjunction with the coating device (5).

12. An apparatus as claimed in claim 11, characterised in that the apparatus comprises on-line measuring means arranged after the degassing stage for measuring density and/or gas content.

13. An apparatus as claimed in claim 11 or 12, characterised in that the apparatus comprises, before the degassing stage (2), coating colour pre-treatment means (7) selected from a group including coating colour heating or cooling means, coating colour filtering means, pre-degassing means, an ultrasound device and different combinations of these.

Ansprüche

1. Verfahren zur Verarbeitung und Zufuhr einer Streichfarbe, welche zur Beschichtung einer Faserbahn verwendet wird, zu einer Beschichtungsvorrichtung (5), wobei in dem Verfahren die Streichfarbe von der Streichfarbenproduktion zu einem Speichertank (1) gefördert wird, von wo sie zu dem Entgasungsschritt (2) gebracht wird,
dadurch gekennzeichnet, dass die Streichfarbe dem Entgasungsschritt (2) mit einer konstanten Förderrate von dem Speichertank (1) zugeführt wird, und dass die Streichfarbe von dem Entgasungsschritt (2) weiter zu dem Zuführtank (4) der Beschichtungsvorrichtung (5) gebracht wird.

2. Verfahren nach Anspruch 1,
dadurch gekennzeichnet, dass 0 bis ungefähr 50 % mehr Streichfarbe zu dem Entgasungsschritt (2) geführt wird als zum Beschichten gebraucht wird, wobei der Überschuss vor dem Entgasungsschritt über den Zuführtank (4) der Beschichtungsvorrichtung zu dem Speichertank (1) zurückgeführt wird.

3. Verfahren nach Anspruch 1,
dadurch gekennzeichnet, dass die überschüssige Streichfarbe vor dem Entgasungsschritt (2) von dem Beschichtungsschritt in der Beschichtungsvorrichtung zu dem Speichertank (1) zurückgeführt wird.

4. Verfahren nach einem der vorhergehenden Ansprüche,
dadurch gekennzeichnet, dass bei dem Verfahren die Dichte der Streichfarbe mittels einer Online-Messung gemessen wird.

5. Verfahren nach einem der vorhergehenden Ansprüche,
dadurch gekennzeichnet, dass bei dem Verfahren eine Online-Messung (8) zum Messen des Gasgehalts der von dem Entgasungsschritt (2) ausgegebenen Streichfarbe verwendet wird, und dass in dem Verfahren eine Einrichtung verwendet wird, um zu verhindern, dass die Streichfarbe einen Gasgehalt aufweist, welcher von den erlaubten Grenzen zum Eintreten in den Zuführtank (4) für die Streichfarbe aufweist.

6. Verfahren nach Anspruch 5,
dadurch gekennzeichnet, dass eine auf Schall oder Ultraschall basierende Messung zum Messen des Gasgehalts verwendet wird.

7. Verfahren nach einem der vorhergehenden Ansprüche,
dadurch gekennzeichnet, dass die Entgasung in einem einzelnen oder mehreren Schritten durchgeführt wird.

8. Verfahren nach Anspruch 7,
dadurch gekennzeichnet, dass die Entgasung unter Verwendung eines Vakuums durchgeführt wird.

9. Verfahren nach Anspruch 7 oder 8,
dadurch gekennzeichnet, dass der Entgasungsschritt einen oder mehrere Vorbehandlungsschritte aufweist, welcher aus der Gruppe ausgewählt ist, die Erwärmen oder Abkühlen der Streichfarbe, Filtern der Streichfarbe, Vorentgasungsschritt, Ultraschallbehandlung oder unterschiedliche Kombinationen davon aufweist.

10. Verfahren nach einem der vorhergehenden Ansprüche,
dadurch gekennzeichnet, dass die Streichfarbe von dem Entgasungsschritt (2) an zwei oder mehr Beschichtungsköpfe zugeführt wird.

11. Vorrichtung zur Verarbeitung und Zufuhr einer Streichfarbe, welche zur Beschichtung einer Faserbahn verwendet wird, zu einer Beschichtungsvorrichtung (5), wobei die Vorrichtung eine Einrichtung zum Zuführen der Streichfarbe von einer Streichfarbenproduktion zu einem Speichertank (1) und eine Einrichtung (6A) zum Zuführen der Streichfarbe von dem Tank (1) zu einem Entgasungsschritt (2) aufweist,
dadurch gekennzeichnet, dass die Vorrichtung eine Einrichtung (6A) zum Zuführen der Streichfarbe mit einer konstanten Förderrate zu dem Entgasungsschritt (2) und eine Einrichtung (6B) zum Zuführen der Streichfarbe weiter von dem Entgasungsschritt zu einem Zuführtank (4) aufweist, der in Verbindung mit der Beschichtungsvorrichtung (5) angeordnet ist.

12. Vorrichtung nach Anspruch 11,
dadurch gekennzeichnet, dass die Vorrichtung eine nach dem Entgasungsschritt angeordnete Online-Messeinrichtung zum Messen der Dichte und/oder des Gasgehalts aufweist.

13. Vorrichtung nach Anspruch 11 oder 12,
dadurch gekennzeichnet, dass die Vorrichtung vor dem Entgasungsschritt (2) eine Streichfarben-Vorbehandlungseinrichtung (7) aufweist, welche aus einer Gruppe ausgewählt ist, die Streichfarbenheiz- oder -kühleinrichtungen, Streichfarbenfiltereinrichtungen, Vorentgasungseinrichtungen, eine Ultraschalleinrichtung und unterschiedliche Kombinationen davon aufweist.

Revendications

1. Procédé de traitement et d'amenée, jusqu'à un dispositif de couchage (5), d'un lait de couchage utilisé pour coucher une bande fibreuse, procédé dans lequel le lait de couchage provenant d'un lieu de production de lait de couchage est distribué à un réservoir de stockage (1), d'où il est prélevé à l'étape de dégazage (2), caractérisé en ce que le lait de couchage est amené à l'étape de dégazage (2) à une vitesse d'alimentation constante à partir du réservoir de stockage (1), et en ce que le lait de couchage est tiré de l'étape de dégazage (2) à la suite du réservoir d'alimentation (4) du dispositif de couchage (5).

2. Procédé tel que revendiqué dans la revendication 1, caractérisé en ce que 0 à environ 50% de lait de couchage de plus que ce qui est requis pour le couchage est fourni à l'étape de dégazage (2), le surplus étant retourné au-delà du réservoir d'alimentation (4) du dispositif de couchage au réservoir de stockage (1) précédant l'étape de dégazage.

3. Procédé tel que revendiqué dans la revendication 1, caractérisé en ce que le lait de couchage excédentaire provenant de l'étape de couchage sur le dispositif de couchage est retourné au réservoir de stockage (1) précédant l'étape de dégazage (2).

4. Procédé tel que revendiqué dans l'une quelconque des revendications ci-dessus, caractérisé en ce que, dans le procédé, la densité du lait de couchage est mesuré par une mesure en ligne.

5. Procédé tel que revendiqué dans l'une quelconque des revendications ci-dessus, caractérisé en ce que, dans le procédé, la mesure en ligne (8) est utilisée pour mesurer la teneur en gaz du lait de couchage déversé à partir de l'étape de dégazage (2), et en ce que, dans le procédé, des moyens sont utilisés pour empêcher le lait de couchage ayant une teneur en gaz s'écartant des limites permises de pénétrer dans le réservoir d'alimentation (4) de lait de couchage.

6. Procédé tel que revendiqué dans la revendication 5, caractérisé en ce qu'une mesure basée sur le son ou l'ultrason est utilisée pour mesurer la teneur en gaz.

7. Procédé tel que revendiqué dans l'une quelconque des revendications ci-dessus, caractérisé en ce que le dégazage est effectué en une ou plusieurs étapes.

8. Procédé tel que revendiqué dans la revendication 7, caractérisé en ce que le dégazage est effectué en utilisant le vide.

9. Procédé tel que revendiqué dans la revendication 7 ou 8, caractérisé en ce que l'étape de dégazage nécessite une ou plusieurs étapes de pré-traitement choisies parmi un groupe comprenant le chauffage ou le refroidissement du lait de couchage, la filtration du lait de couchage, l'étape de pré-dégazage, le traitement par ultrasons ou différentes combinaisons de celles-ci.

10. Procédé tel que revendiqué dans l'une quelconque des revendications ci-dessus, caractérisé en ce que le lait de couchage est amené de l'étape de dégazage (2) à deux ou plusieurs têtes de refroidissement.

11. Appareil pour traiter et amener, à un dispositif de couchage (5), un lait de couchage utilisé pour coucher une bande fibreuse, dans lequel l'appareil comprend des moyens pour distribuer le lait de couchage du lieu de production du lait de couchage à un réservoir de stockage (1), des moyens (6A) pour amener le lait de couchage du réservoir (1) à une étape de dégazage (2), caractérisé en ce que l'appareil comprend des moyens (6A) pour amener le lait de couchage à un débit d'alimentation constant à l'étape de dégazage (2) et des moyens (6B) pour amener en outre le lait de couchage de l'étape de dégazage à un réservoir d'alimentation (4) ménagé conjointement avec le dispositif de couchage (5).

12. Appareil tel que revendiqué dans la revendication 11, caractérisé en ce que l'appareil comprend des moyens de mesure en ligne disposés après l'étape de dégazage pour mesurer la densité et/ou la teneur en gaz.

13. Appareil tel que revendiqué dans la revendication 11 ou 12, caractérisé en ce que, avant l'étape de dégazage (2), les moyens de pré-traitement (7) du lait de couchage sont choisis parmi un groupe comprenant des moyens de chauffage ou de refroidissement du lait de couchage, des moyens de filtration du lait de couchage, des moyens de pré-dégazage, un dispositif à ultrasons et différentes combinaisons de ceux-ci.

Drawing