LUOMI, Seppo (Retiisikuja 4, Järvenpää, FI-04400, FI)
NURMIAINEN, Timo (Kymenmaankatu 1, Järvenpää, FI-04430, FI)
HAAVISTO, Jouni (Oolannintie 6 A 8, Vantaa, FI-01520, FI)
LUOMI, Seppo (Retiisikuja 4, Järvenpää, FI-04400, FI)
NURMIAINEN, Timo (Kymenmaankatu 1, Järvenpää, FI-04430, FI)
| Claims
1. A method of coating a fibrous web with a curtain coater, in which method the coating color is subjected to a degasification treatment before feeding it to the coater, which method comprises using in the degasification process a vacuum degasifier for eliminating harmful defects caused by a gas content of the coating color, said degasifier being supplied with a coating color having its gas content within the range of about 2 to 35%, the degasification being conducted in such a way that the air content of the coating color is reduced to the value of about 0 to about 0,25%, characterized in that the method comprises removing gas bubbles such that the size of gas bubbles possibly remaining in the coating color is less than about 0,1 mm, whereby the size of defects, caused by gas bubbles and possibly occurring in the coating present on top of a fibrous web, is respectively less than about 0,1 mm.
2. A method as set forth in claim 1, said method comprising the use of a vacuum degasifier, in which the coating color is fed to the bottom section of a drum (3), housed in a vacuum tank (1) and rotating around a substantially vertical shaft, whereby a rotating motion of the drum results in the coating color rising along an inner wall of the drum and discharging from a top edge of the drum in a thin film against an inner wall of the vacuum tank (1), wherefrom the coating color flows downwards, characterized in that the method comprises adapting the coating color to rise stepwise along the drum wall such that the coating color forms a thin hazy film on at least two different stages (10, 11, 12), whereby the gas bubbles contained in the coating color break up and/or move out of the coating color to be further discharged out of the tank.
3. A method as set forth in any of claims 1-2, characterized in that the coating is formed of a single layer or a plurality of concurrently applied layers, in which method the coating color constituting each layer is subjected to a separate degasification treatment.
4. A method as set forth in claim 3, characterized in that, as the coating color for at least one coating layer, the method comprises the use of a pigment coating.
5. A method as set forth in claim 3, characterized in that, as the coating color for at least one coating layer, the method comprises the use of a material which forms a liquid- and/or gastight barrier layer on the surface of a fibrous web.
6. A method as set forth in any of claims 1-4 for making coated printing paper or printing-surface board, characterized in that the method comprises forming a multiply coating, at least one coating color layer having a dry content of more than 40% and the coating color having a viscosity of more than 100 (BrIOO) CPs.
7. A method as set forth in any of claims 1-4 or 6 for making coated printing paper, characterized in that the method comprises coating the paper on both sides.
8. Use of a method as set forth in any of claims 1-7 for making coated printing paper or printing-surface board. |
Method of coating a fibrous web with a curtain coater
The present invention relates to a method of coating a fibrous web with a curtain coater, in which method the coating color is subjected to a degasification treatment before feeding it to the coater.
As coated grades of paper and coating as a method grow in popularity, increasing requirements are imposed on coating processes and equipment. In coating, more specifically in pigment coating, the surface of paper is formed with a layer of coating color at a coating station, followed by draining the extra water. A common pigment coating method is so-called blade coating, wherein the amount of coating is controlled by means of a so-called doctor blade. The most common types of blade coating station are a blade coater equipped with an application roll and a blade coater equipped with a nozzle application feature. Also used in coating is a so-called film transfer coater, the popularity of which has increased lately. Likewise, an upcoming new technology in the coating of fibrous webs is the use of curtain coaters.
Curtain coaters can be divided into slot-fed or slide-fed coaters. In a slide-fed curtain coater, coating is fed by means of a nozzle block onto an inclined slide, along which the coating flows towards an edge of the slide, the curtain forming as the coating flows over the slide edge which constitutes a feeding lip. In slot-fed application bars, the coating is pumped through a supply chamber into a narrow vertical slot, the curtain forming at its lip and flowing onto a web. Coating can be applied in one or more layers. The resulting curtain can be controlled by edge guides located, as the name suggests, along the edges of a feeding slot/feeding lip.
There are a multitude of various grades of paper and board and those can be divided by basis weight into two classes: papers, which are in a single ply and have a basis weight of 25-300 g/m 2 , and boards, which are manufactured by multiply technique and have a basis weight of 100-600 g/
m 2 . As noted, there is a flexible dividing line between paper and board, the boards with lightest basis weights being lighter than the heaviest papers. In general, paper is used for printing and board for packing. Papers and boards can be coated or uncoated.
The following descriptions are examples of current values for coatable fibrous webs and considerable deviations from the given values may exist. The descriptions are mainly based on the source publication Papermaking Science and Technology, section Papermaking Part 3, Finishing, edit. Jokio, M., publ. Fapet Oy, Jyvaskyla 1999, 361 p.
Coated magazine paper (LWC = light weight coated) contains mechanical pulp 40-60%, bleached softwood pulp 25-40%, and filling and coating colors 20-35%. General values for LWC paper can be regarded as follows: basis weight 40-70 g/m 2 , Hunter gloss 50-65%, PPS SlO roughness 0.8-1.5 μm (offset) and 0.6-1.0 μm (roto), density 1100-1250 kg/m 3 , brightness 70-75% and opacity 89-94%.
General values for MFC paper (machine finished coated) can be regarded as follows: basis weight 50-70 g/m 2 , Hunter gloss 25-70%, PPS SlO roughness 2.2-2.8 μm, density 900-950 kg/m 3 , brightness 70-75% and opacity 91-95%.
General values for FCO paper (film coated offset) can be regarded as follows: basis weight 40-70 g/m 2 , Hunter gloss 45-55%, PPS SlO roughness 1.5-2.0 μm, density 1000-1050 kg/m 3 , brightness 70-75% and opacity 91- 95%.
General values for MWC paper (medium weight coated) can be regarded as follows: basis weight 70-90 g/m 2 , Hunter gloss 65-75%, PPS SlO roughness 0.6-1.0 μm, density 1150-1250 kg/m 3 , brightness 70-75% and opacity 89- 95%.
HWC (heavy weight coated) has a basis weight of 100-135 g/m 2 and it can be coated even more than twice.
In coated, chemical-pulp based, woodfree printing papers or fine grade papers (WFC), the amounts of coating fluctuate over a wide range according to requirements and an intended application. In the following, typical values are presented for once- and twice-coated, chemical-pulp based printing paper: once-coated basis weight 90 g/m 2 , Hunter gloss 65-80%, PPS slO roughness 0.75-2.2 μm, brightness 80-88% and opacity 91-94%, and for the twice-coated, basis weight 130 g/m 2 , Hunter gloss 70-80%, PPS SlO roughness 0.65-0.95 μm, brightness 83-90% and opacity 95-97%.
Boards make up quite a heterogeneous group, including grades high in terms of basis weight, the basis weight of which can be even more than 500 g/m 2 , and grades low in terms of basis weight, the basis weight of which is approximately 120 g/m 2 , the grades possibly ranging from those based on virgin stock even to those based by 100% on recycled stock and from uncoated to those with multiple coating. Coated boards include as follows:
- folding boxboard (FBB) based on virgin stock, solid bleached board (SBB), liquid packaging board (LPB), coated white top liner, carrier board
- white lined chipboard (WLC) based on recycled stock, coated recycled board.
Curtain coating is basically applicable to the manufacture of all foregoing coatable grades. Compared to blade or film transfer coating, the curtain coater applies a much lesser force on a web, thus resulting in fewer discontinuities caused by a break of the paper web, and hence improving runnability. Curtain coating is not capable of achieving a surface smoothness equal to that obtained by blade coating, but the coverage reached thereby is better than what is achieved by blade coating or film transfer coating.
In process industry, the mixing of gases, such as air, with liquids and compositions used in a process constitutes typically a source of many problems. Especially in the curtain coating process of paper or a corresponding fibrous web material, a gas or gas bubbles present in the coating color are responsible in the coating process for developing irregularity on the surface of paper and even spots which are completely void of coating. In multiply curtain coating, the meaning of degasification is even more pronounced. Thus, if the number of coating layers is for example three or four, the coating used for producing each layer must be degassed as thoroughly as possible.
E.g. various vacuum degasifiers have been developed for removing a gas mixed and dissolved within a coating color, as described e.g. in the present Applicant's earlier patent applications FI20055280, FI20055713 and FI20055704.
Vacuum degasifiers are preferably provided with an absolute pressure of about 1 kPa to 15 kPa. If the chamber is provided with a lower absolute pressure, e.g. the vaporization temperature of a liquid contained in the coating color becomes lower and there is a risk of the coating color degrading in quality as a result of vaporization. On the other hand, if the chamber is provided with a higher absolute pressure, that will not be necessarily enough for increasing the size of gas bubbles contained in the coating color and for removing the same by means of vacuum. In a highly preferred case the vacuum chamber is provided with an absolute pressure of about 3 kPa to 15 kPa. Raising a lower limit for the applied absolute pressure serves to make it even more certain that there is no chance for the coating color to vaporize during degasification. The vacuum degasifier provides a high efficiency, enabling a gas contained in a coating color to be rapidly and thoroughly removed from a large quantity of the coating color.
It is an objective of the present invention to provide an improved curtain coating method for producing a flawless coated surface. In order to achieve this objective, there is provided a method of coating a fibrous web with a curtain coater, in which method the coating color is subjected to a degasification treatment before feeding it to the coater, which method comprises using in the degasification process a vacuum degasifier for eliminating harmful defects caused by a gas content of the coating color, said degasifier being supplied with a coating color having its gas content within the range of about 2 to 35%, the degasification being conducted in such a way that the air content of the coating color is reduced to the value of about 0 to about 0,25%, characterized in that the method comprises removing gas bubbles such that the size of gas bubbles possibly remaining in the coating color is less than about 0,1 mm, whereby the size of defects, caused by gas bubbles and possibly occurring in the coating present on top of a fibrous web, is respectively less than about 0,1 mm.
In the context of this application, the size of a defect present on the surface of a fibrous web refers to an extent of the defect caused by a gas bubble present in the coating color in a direction lateral to the fibrous web advancing direction. As the curtain coating trickles down onto the surface of a fibrous web, the fast moving fibrous web causes the elongation of a gas bubble present in the coating color in the fibrous web advancing direction, whereby the defect occurring in the fibrous web coating becomes oval or elongate. The dimension essential in terms of the defect size is the extent lateral to the web advancing direction, which is preferably less than 0,1 mm, being thus visually undetectable. Furthermore, in printed matter the halftone screen size in printing is typically 0,1 mm, whereby the defect size smaller than that does not compromise the print quality.
The curtain coating method is suitable not only for pigment coating but also for the application of a material providing e.g. a barrier layer on the surface of a fibrous web. Such a barrier layer can be a liquid- and/or gastight layer.
In addition, the method can be used for providing various other functional layers on top of a fibrous web.
The invention will now be described in more detail with reference to the accompanying drawings, in which:
fig. 1 shows in a diagrammatic view of principle one way of implementing a curtain coating process, and
fig. 2 shows one vacuum degasifier applicable for implementing a method of the invention.
Referring to fig. 1, there is merely diagrammatically shown one arrangement applicable for implementing a method of the invention, which is described in more detail in the Applicant's earlier Finnish patent application FI20065055. In the arrangement of fig. 1, the coating color coming from a coating color preparation process is first conveyed into a storage tank 1, from which it is delivered by means of a pump 6A to a degasification process 2, which in the illustrated embodiment is preceded by a pretreatment process 7. The pretreatment process 7 may comprise e.g. subjecting the coating color to heating, cooling, filtering, a preliminary degassing treatment, an ultrasound treatment or various combinations thereof. The pretreatment process can also be omitted. The degasification process 2 is preferably based on the application of vacuum.
The degasification process is followed by passing the coating color by means of a pump 6B into a supply tank 4 for a coating device 5, from which it is fed by means of a pump 6C to the coating device for application on a fibrous web. In the illustrated arrangement, the pump 6C is followed by filter elements 9, which can be of any conventional type.
In addition, the arrangement can be provided with a preferably on-line measuring unit 8 for measuring e.g. the coating color for its gas content as it leaves the degasification process, whereby the coating color deviating from allowed limits in terms of its gas content is denied access to the coating device supply tank 4 and returned back into the storage tank, e.g. in connection with starting the apparatus, in which case the coating color can be recycled, if necessary, several times through the degasification process 2 prior to its delivery into the supply tank 4. The on-line measuring equipment 8 may comprise e.g. measuring instruments for a density of the coating color arriving from the degasification process, the measured density data being useful e.g. for the indirect determination of a gas content remaining in the coating color. The density measurement can be conducted e.g. as a pressure difference measurement. The on-line measuring equipment may also include a direct gas content measuring device, based e.g. on the use of sound or ultrasound, which can be used instead of the indirect gas content determination based on the measurement of density.
The means for passing the coating color not into the supply tank 4 but instead back into the storage tank may comprise appropriate valve elements (not shown), which are also used for passing back into the storage tank 1 the extra coating color to be delivered from the degasification process 2 in normal operation.
With the apparatus in normal operation, coating color is delivered from the storage tank 1 in a standard quantity to the degasification process 2 by means of the pump 6A. This serves to stabilize the operation of a degasifier and provides a more consistent result. From the degasification process is delivered, likewise in a standard quantity, degasification treated coating color by means of the pump 6B into the supply tank 4. The supply tank 4 is provided e.g. with level measuring equipment, on the basis of data provided thereby the supply tank is supplied with the amount of coating color required at any particular time, the extra coating color being guided back into the
storage tank 1. The feeding amount of coating color from the degasification process is preferably within the range of 0 to circa 200% more than what is needed for the coating process. The delivery of coating color from the degasifier can also be adapted to proceed directly to the coating process without a supply tank 4.
Referring to fig. 2, there is shown in a schematic view of principle one embodiment for a vacuum degasifier applicable for implementing a method of the invention. This vacuum degasifier has been described in more detail in the Applicant's earlier Finnish patent application FI 20055280. The apparatus comprises a rotatable drum 18 disposed inside a vacuum tank 13 and driven by a motor 21. The drum has its inside surface in a stepwise configuration, comprising in the present embodiment three stages 10, 11, 12, such that the coating color rising upwards in response to rotary motion forms at each of said stages a thin hazy film. Upon leaving the final stage, the film of coating color is delivered from a top edge of the drum onto an internal wall of the vacuum tank and therealong further to a bottom section of the tank for a transfer to the application site by way of a discharge opening 17. In its upper section, the tank 13 is preferably provided with instruments 20 for imaging the resulting hazy film whereby, provided that the hazy film has remained intact, there are no more air bubbles left in the coating color. The visual information produced by the imaging means 20 can be used for controlling the operating parameters of a degasifier.
As shown e.g. in fig. 2, the stages can be established by successive concave- bottom sections whose forward edge 10a, 11a, 12a defines the elevation of a stage, i.e. the elevation of a resulting thin hazy film. For efficient degasification and in view of providing of a compact apparatus design, the difference in levels between successive stages is preferably within the range of 20 mm to 150 mm, more preferably within the range of 40 mm to 100 mm. The radial distance between stages, i.e. the width of a resulting haze is
preferably within the range of 20 mm to 200 mm, more preferably within the range of 40 mm to 120 mm.
This solution for a degasifier enhances the degasification of a coating color and increases capacity. The solution also enables early tracking of the process and a control over operating parameters of the apparatus on the basis of this tracking. Operating parameters include e.g. a rotating speed of the drum 18, a vacuum level in the tank 13, a liquid level present in the bottom of the tank 13 which is influenced by a feed rate of the coating color entering the drum and by a discharge rate of the degasification treated coating color leaving the tank.
The multistage hazing of a coating color enhances the removal of air, thus enabling the absolute pressure level to be given a lower limit which is higher than in prior solutions, thereby avoiding the degradation of coating color quality, which could result from possible evaporation of some coating color ingredients caused by an excessively low pressure level.
Such a multistage implementation provides sufficient time for gas bubbles to escape from the coating color, whereby even the gas bubbles in the order of less than 0,1 mm in diameter have enough time to leave the coating color and the gas bubbles possibly remaining in the coating color are of such a small size that no visible defect is inflicted thereby in the coating to be laid on top of a fibrous web. The vacuum degasifier may also comprise e.g. solutions disclosed in the present Applicant's earlier Finnish applications FI20055713 and FI20055704, and it can be a multi-operation solution such as the method presented in the application FI20055704, which makes use of a vacuum-operated degasifier formed with two separate compartments for conducting the degasification in two separate operations while using a single apparatus.