WO2007085688A1

WO2007085688A1 - Method for processing and supplying a coating colour used for coating a fibrous web to a coating device

Info

Publication number: WO2007085688A1
Application number: PCT/FI2007/050001
Authority: WO
Inventors: Eero Kesti; Esa Forsten
Original assignee: Metso Paper, Inc.
Priority date: 2006-01-26
Filing date: 2007-01-03
Publication date: 2007-08-02
Also published as: EP1976645A1; CN101356015B; FI20065055A; ATE502698T1; EP1976645B1; JP4897823B2; CN101356015A; DE602007013350D1; FI121122B; FI20065055A0; JP2009516582A

Abstract

The invention relates to a method for processing and supplying coating colour used for coating a fibrous web to a coating device (5). In the 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), and from there further to a supply tank (4) arranged in conjunction with the coating device (5).

Description

Method for processing and supplying a coating colour used for coating a fibrous web to a coating device

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

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.

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.

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.

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.

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. 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.

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. 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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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), characterised in that in the 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), and from there further to the supply tank (4) of the coating device (5).

2. A method as claimed in claim 1, characterised in that coating colour is supplied to the degassing stage (2) at a constant feed rate from the storage tank (1).

3. A method as claimed in claim 2, 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.

4. 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).

5. 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.

6. 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).

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

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

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

10. A method as claimed in claim 8 or 9, 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.

11. 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.

12. An apparatus for processing and supplying a coating colour used for coating a fibrous web to a coating device (5), characterised in that the apparatus comprises means for delivering the coating colour from coating colour production to the storage tank (1), means (6a) for supplying coating colour 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).

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

14. An apparatus as claimed in claim 12 or 13, 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.