Method and apparatus for degassing coating material

The present invention relates to a method and apparatus for degassing coating material. In the method, coating material is fed into a vacuum container inside which are means for separating gas from the coating material.

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 material 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 materials than others, but the problem is emphasised particularly with those coating materials that accumulate more gas than others. For example, coating materials 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 material have been developed vacuum deaerators, a known embodiment of which is shown in Figure 1. The apparatus comprises a rotating drum arranged inside a vacuum container, into which drum the coating material is led, whereupon the

coating material 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 materials are unable, even under an extremely high vacuum, that is, low absolute pressure, to grow large enough to be broken or to be 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 material, for example water, vaporises extremely readily, whereupon the quality of the coating material deteriorates, for example, as a result of an increase in the solids content of the coating material following from the vaporisation of the coating material solvent. Another method used involves increasing mixing times, but in that case the operational capacity of the deaerators remains too low, which means that a greater number of deaerators must be acquired. Furthermore, if the separation capacity of known deaerators is increased by increasing the size of the apparatuses, the apparatus size will become excessively large and manufacturing costs will increase markedly.

The aim of the method and apparatus according to the present invention is to provide an improved gas separation solution, by means of which the gas contained in coating material can be removed better than before.

To achieve this aim, the method relating to the invention is characterised in that inside a second container part is arranged a drum rotating around an essentially vertical axis, and that in the method, coating material is fed from the first container part to the bottom part region of the drum, whereupon the rotating motion of the drum causes the coating material to rise up the

inner wall of the drum and to discharge from the top edge of the drum as a thin film against the inner wall of the second container part, wherefrom the coating material flows downwards to be led out of the apparatus by discharge means. The apparatus according to the invention, comprising: a first container part, to which are connected means for providing a vacuum therein, means for feeding coating material inside the first container part, means for degassing the coating material in the first container part, and means for discharging the coating material from the first container part; and a second container part, to which are connected means for providing a vacuum therein, means for feeding coating material inside the second container part, means for degassing the coating material in the second container part, and means for discharging the coating material from the second container part, wherein the coating material 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, is for its part characterised in that inside the second container part is arranged a drum rotating around an essentially vertical axis, in the bottom part region of which is fed coating material from the first container part, whereupon the rotating motion of the drum causes the coating material to rise up the inner wall of the drum and to discharge from the top edge of the drum as a thin film against the inner wall of the container part, wherefrom the coating material flows downwards to be led out of the apparatus by discharge means.

In a preferred embodiment of the present invention, an absolute pressure of approximately 1 kPa - 15 kPa is 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 material will fall and there is a risk of the quality of the coating material 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 material by means of a vacuum. 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 material will not be able to vaporise during degassing.

The greatest advantage of the method and apparatus according to the invention is its efficiency, whereby gas contained in coating material can be removed rapidly and thoroughly from a large amount of coating material.

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

Figure 1 shows diagrammatically a vacuum deaerator of the prior art,

Figure 2 shows a diagrammatic view in principle of the solution according to the invention,

Figures 3 to 5 show diagrammatically some alternative embodiments of the apparatus according to the invention.

Figure 1 shows diagrammatically a vacuum-operated apparatus for removing gas, typically air, from coating material. As shown in the Figure, the system comprises an impermeable vacuum container 1, inside which is arranged a drum 3 rotated by a motor 2. The gas-containing coating material is fed inside the drum via a pipe 4. The container is connected to a vacuum source 5 by means of a vacuum pipe 6. When the drum rotates, the coating material rises up the inner wall of the drum by the effect of centrifugal force and is centrifuged from the top edge of the drum tangentially as a thin film colliding with the wall of the vacuum container, along which it flows downwards,

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discharging through a discharge opening 7 in the bottom part of the container. An air bubble that has entered with the film breaks and the air released discharges through the vacuum pipe via a deaerator connected to the vacuum source.

Figure 2 shows a diagrammatic view in principle of the apparatus according to the invention. The apparatus 10 comprises a first, upper container part 11, and a second, lower container part 12. The coating material to be subjected to degassing is supplied along a pipe 13 into the upper container part, where a first degassing stage is carried out, after which the coating material is fed into the lower container part, where a second degassing stage is carried out, following which the coating material is transferred from the apparatus to the place of use via discharge means 14. At the first stage, or preliminary degassing stage, is preferably removed the major part of the gas contained in the coating material and the remainder is removed at the latter stage. The solution increases the efficiency of degassing substantially, resulting in at least twice, preferably even three or four times the deaeration capacity of a single-stage solution at a relatively lower cost and smaller space requirement than when using, for example, two separate single-stage apparatuses.

Figure 3 shows an embodiment of the apparatus according to the invention. The apparatus comprises a rotating drum 3 arranged inside each container part 11, the said drums being driven by a motor 2. The inner surface of the drum is stepped, comprising, in the embodiment shown, two step levels 20, 21 so that the coating material rising upwards by the effect of the rotary motion will form a thin, veil-like film on each of the said step levels, respectively. When leaving the last level, the paste film is guided from the top edge of the drum to the inner wall of the container part 11 and along that further to the lower part of the container part, from where it is led to the lower container part 12. In the embodiment shown, there is a

corresponding drum in the lower container part, which is also driven by a motor 2. The drum in both container parts 11, 12 corresponds to a solution disclosed in an earlier Finnish patent application FI 20055280, filed on June 2, 2005, by the Applicant of the present application, by means of which is achieved an improved degassing capacity compared with the prior art solution.

In the upper part region of the lower container part 1 are preferably arranged means (not shown) for imaging the veil-like film produced, whereupon when the veil-like film remains intact, there are no air bubbles remaining in the paste. The image information produced by the imaging means may be used for controlling the operational parameters of the degassing apparatus.

The step levels may be made, for example, in accordance with Figure 3 of successive concave-based sections, the front edge 20a, 21a of which determines the elevation of the step level, that is, the elevation of the thin, veil-like film produced. To achieve efficient degassing and for compact dimensioning of the apparatus, the difference in height between the successive step levels is preferably within the range from 20 mm to 150 mm, more preferably within the range from 40 mm to 100 mm. The radial distance between the step levels, that is, the width of the veil produced is preferably within the range from 20 mm to 200 mm, more preferably within the range from 40 mm to 120 mm. The thickness of the final veil-like film leaving the drum 3 is preferably arranged to be adjusted so as to be at most as thick as the coating layer desired.

The coating material feeding means 4, 4a preferably comprise a nozzle part 24, which is arranged in the vicinity of the central region of the bottom part of the drum, and around the nozzle part is arranged an annular protective part 25, which prevents the coating material from splashing sideways and

the formation of thicker sections resulting therefrom in the coating material rising up the wall of the drum, which would lead to irregular degassing of the coating material and thus to faults in the coating formed on the fibrous web.

The solution according to the invention improves the degassing of paste and increases capacity. The solution also makes possible process-time monitoring and controlling of the operational parameters of the process on the basis of such monitoring. The operational parameters include speed of rotation of the drum 3, vacuum levels in container parts 11, 12, height of the fluid level at the bottom of the lower container part, which is affected by the feed rate of the paste fed into the drum, and the discharge rate of the degassed paste led out of the container part.

Multilevel veiling of the coating paste improves deaeration, which means that it is possible to use a higher lower limit for the absolute pressure level than in earlier solutions, thus avoiding the deterioration of paste quality, which might result from the possible evaporation of some of the components of the paste due to a too low pressure level.

Figure 4 shows another embodiment of the apparatus according to the invention, where the lower container part 12 corresponds to the lower container part of Figure 3. The upper container part 11 is made into a cyclone, where preliminary degassing takes place.

Figure 5 shows yet another embodiment of the apparatus according to the invention, where the lower container part 12 corresponds to the lower container part of Figure 3. The upper container part 11 is provided with spraying means 23, from which the coating material to be degassed flows out and forms into drops, from which gas separates relatively easily due to the effect of the vacuum.

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The solution according to the invention may conceivably also be used for degassing other substances than coating material, such as for the deaeration of sludge.