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7.10 Coating Machines 7.10.1
Overview
Coating machines are separate machines in which a previously produced dry base paper is unwound, coated, dried, and then reeled again (Fig. 7.24). In contrast to coating applicators placed on-line in a paper machine (Section 6.8), they are usu¬ally called “off-line coaters” or “OMC” (off-machine coaters).
In a paper mill, one off-line coater is usually associated with one paper machine. Sometimes, however, one OMC receives paper from different paper machines. In most cases, the off-line coater has the same width as the corresponding paper machine (except for the edge trim). Some coaters with half width are in operation, where the jumbo reels from the paper machine are split, and the two halves are coated separately.
Off-line coating has the advantage that the paper machine can continue produc¬tion during machine stops on the off-line coater. These stops can be for instance due to coating color changes, which is especially important for machines with a very broad product range. Since off-line coaters operate faster than the paper ma¬chine to which they belong, the time efficiency of the entire line is higher than for on-line concepts.
7.10.2
Applicators
The actual coating process usually comprises two steps: (i) the application of the coating medium onto the paper or board and (ii) the metering of the coating medium to the desired quantity (or coat weight). If the metering is done directly on the paper after the application, the process is called direct coating or blade coating. If, however, metering is done before the transfer of the coating medium to the paper or board, the process is called indirect coating or film coating.
7.10 Coating Machines
7.10.2.1 Direct Coating
In direct coating, the web is supported by a backing roll and the coating medium is fed in excess onto the web. After a certain distance, defining the “dwell time”, the coat layer is metered down to the final thickness, being the desired coat weight.
In the “roll applicator” or “LDTA” (long dwell time applicator), feeding is done with a roll, which draws the coating medium from a pan onto the web (Fig. 7.25). The applicator roll and the backing roll form a nip. The nip pressure and, conse¬quently, the nip gap are adjustable, determining the amount of coating medium fed to the web. The nip load also yields an external penetration pressure which, together with the capillary pressure, causes penetration of some coating medium or components thereof into the paper or board web. This is to a certain extent desirable, because it anchors the coating to the base paper and gives surface strength. Excessive penetration, however, would reduce the gain in surface proper¬ties, such as smoothness or gloss.
Penetration of the coating into the base paper can be controlled with the “water retention” of the coating medium. At elevated machine speeds, e. g. above approx¬imately 1500 m min–1, the splitting of the coating color between applicator roll and paper at the nip exit shows irregularities, which negatively influence the homoge¬neity of the coat layer. Also, the relatively short nip and the considerable penetra¬tion caused by the nip load cause runnability and quality problems. To overcome these limitations, the application of the coat medium to the paper can be per¬formed with a free jet. In a “free jet applicator” (Fig. 7.26), the jet usually has a thickness of less than one millimeter and a length of a few centimeters. The result¬ing coat layer (still before final metering) is much more homogeneous than that of a roll applicator, and, since the external pressure at the impingement point is considerably less than in the roll nip, the penetration of the coating medium into the base paper is reduced. This improves the so-called “coating holdout”, which results in a better surface quality of the coated paper.
The final metering of the applied coating is often performed with a blade. With this blade, most of the coating medium is removed and returns to the working tank. The coat layer remaining on the paper or board is evenly distributed. The blade pressure ensures that the surface voids of the web are filled with coating. Due to the blade geometry, the coated web has a good smoothness. The blade usually has a thickness of less than one millimeter, typically approximately
0.4 mm. The stick-out length (or unsupported length) is usually a few centimeters.
The operating angle between blade and paper web is usually 20–35°. Under these conditions, the blade is considered a “stiff blade”, because its bending is negligible with respect to the operating principle. If, however, a blade geometry is chosen where the bending of the blade results in an operating angle close to 0°, the mode is called “bent blade”. The bent blade gives – in comparison to the stiff blade – an even smoother surface but it is more prone to surface defects, such as blade scratches, and is more difficult to operate. Since the web has a considerably re¬duced roughness after coating, the coat layer itself must be nonuniform in caliper to even out the base paper roughness. On certain grades, especially board, this nonuniform layer thickness can be seen as inhomogeneous coverage or mottling. If coverage is desired rather than smoothness, metering can be done with a rod rather than with a blade. Rod metering is typical for the pre-coat of board and for specialty papers where the base web has to be evenly covered with a specialty coat.
In most cases, the metering blade (or rod) is placed against the same backing roll as the coat applicator. The angle between point of application and point of meter¬ing is then approximately 60°. This results in a dwell length of 400–600 mm, de¬pending on the backing roll diameter. The resulting “dwell time” depends strongly on the machine speed. It is in the range of 10–200 ms.
A “short dwell time applicator” (or SDTA) was tried in order to reduce the dwell time to much shorter values. This SDTA consists of a closed chamber which incor¬porates the coating feed and the metering blade. However, vortices in the chamber caused a streaky appearance of the coat layer at elevated machine speeds. There¬fore, the SDTA has not received much attention outside North America.
7.10.2.2 Indirect Coating
As an alternative to direct coating, a film press can be used for the application of coating media. The film press was derived from the size press, which is used for the application of starch or size solutions. Today, a wide variety of coating media are applied, including pigment dispersions with high solids content. In a film press, a film of the coating medium is formed and metered on a large diameter roll. This roll forms a nip with another roll. The paper or board web passes this nip
7.10 Coating Machines
and picks up a certain portion of the film. Application can be – but does not have to be – simultaneous on both sides. A schematic drawing of a film press is shown in Section 6.7.
Metering is performed using metering rods. These can be either smooth or profiled. Profiled rods provide a certain volume of coating medium due to the open cross section in the profile. A deeper or coarser profile gives a higher coat weight than a fine profile. The application weight is mainly adjusted by choosing an ade¬quate profile. Fine tuning of the application weight can be done by modifying the rod pressure. Profiled metering rods are mainly used for low viscosities (i. e. low solids contents of the coating medium) and low machine speeds. With increasing viscosity and speed, the rod loses contact with the roll due to the hydrodynamic forces of the coating medium, comparable to aquaplaning. Then, the application weight depends less on the profile and more and more on the hydrodynamic conditions, such as rod pressure, rod diameter, viscosity and speed. Consequently, a smooth metering rod is used. Typical rod diameters are 14–38 mm for smooth rods. Larger diameters yield higher application weights. Higher viscosities and machine speeds require smaller rod diameters for the same application weight than lower ones. The coating medium is not transferred completely to the web. A certain amount remains on the roll and returns to the application unit where it is mixed with fresh coating medium. The transfer ratio depends on the acceptance behavior of the web, on the properties of the coating medium and – to a limited degree – on the surface properties of the roll. For starch, where the absorptivity of the web is high, the transfer ratio can be more than 90 %. For pigment coats, it can be as low as 50 %.
The pre-metered films usually have thicknesses of 7–20 mm (or ml m–2). Lower values would require very high rod pressures and also coverage of the web would be insufficient. Higher values are not meaningful since the web has a limitation with respect to coating acceptance. If the amount of pre-metered coating medium is too high, the surface of the coated web appears uneven, with an “orange peel” character. At elevated machine speeds, the film split at the nip exit can create a fine mist of coating medium. This mist deposits on machine parts or even on the paper or board web. Since this misting increases with film thickness, it is the major limitation for the application weight at high speed. For typical applications, mist¬ing becomes a limitation above 1500 m min–1. At 1800 m min–1, for example, coat weights above 7 g m–2 are difficult to achieve without disturbing misting.
7.10.2.3 Curtain Coating
Besides film and blade coating, curtain coating is used for the application of coat¬ing media onto paper surfaces. Although curtain coating is a very old application technique, going back to the end of the 19th century, it has only been used for coating of paper since the 1990s. Curtain coating is mainly used for specialty papers. Specialty coats are usually very expensive. Therefore, the application amount must be kept to an absolute minimum. On the other hand, a certain amount is required to achieve the desired function of the coat. In a curtain coater, a thin film of coating medium is formed which falls by gravity onto the paper surface. The only metering device for the coating color is the nozzle slice. The slot opening is about 20 to 50 times larger than the actual coating color thickness. Thickness is reduced by gravity forces while the jet is moving towards the web and by the shear forces exerted by the high speed difference between the jet and the running web (Fig. 7.27). Due to the homogeneity of the film, the coverage of the paper by the coating medium is very uniform.
Unlike film and blade coaters, the application is contact-free. This considerably reduces the risk of web breaks and, therefore, improves runnability. It is expected that the significance of curtain coating will increase.
7.10.3
Application Concepts
Typical coat weights for graphic papers are given in Fig. 7.28, plotted against the basis weight of the base paper. The coat weight is typically 25–40 % of the final basis weight of the paper.
Wood-containing papers are mainly single-coated (LWC). Coat weights are usu¬ally between 6 and 10 g m–2 per side. Traditionally, the coat is applied with a blade coater. More modern installations use a film press for the single coat.
At higher basis weights (MWC), two coat layers may be applied per paper side. Here, the top coat is usually a blade coat. The pre-coat may be either a film or a blade coat.
Woodfree papers cover a very wide basis weight range. Depending on the basis weight, the application can be single coat, double coat or triple coat. The coat weight per layer can vary in a wide range between approximately 8 and 20 g m–2.
For board grades, the coat weight depends on the requirements of the final product and on the quality of the base board. Coverage, brightness and smooth¬ness are the main properties that are improved by the coating. Depending on the required coat weight, two or three coat layers are applied.
Due to their functions, different formulations are used for the individual coat layers. For instance, pigments with high opacity are used in the first layers, whereas very fine pigments with good smoothness potential are used for the top coat.
Drying
After its application, the wet coat must be dried. Three different drying principles are used: radiation (infrared), convection (hot air), and contact (cylinders).
Infrared drying yields a very good energy transport, whereas mass transfer is inferior. Therefore, infrared is especially suitable for heating the paper or board web to temperatures where considerable drying occurs. Heating of the infrared radiators can be by electricity or gas. Gas-heated infrared dryers reach higher tem¬peratures and are usually cheaper. Electrical infrared dryers are easier to control and are, therefore, often used for moisture profiling in the cross machine direc¬tion. It depends very much on the local conditions at the mill site, such as availabil¬ity and prices, which heating principle is used for infrared drying.
Air flotation dryers are widely used for the drying of coat. In an air-flotation dryer, hot air is blown onto the paper surface. Air temperature can be as high as 350 °C, sometimes even higher. The impingement velocities of the air can exceed 40ms–1. The impingement effect gives a very effective heat and mass transfer between air and paper surface. The blowing nozzle sections in the dryer are alter¬nately placed on one side of the paper and on the other side. This gives a sinusoidal web run through the dryer with a web-stabilizing and wrinkle-avoiding effect.
Cylinder drying of coat is possible, as soon as the freshly coated surface is dry enough that the coating color does not stick on the cylinder surface. Therefore, drying cylinders are only found after a certain amount of contact-free drying, i. e.
towards the end of the dryer section after a coater station. In modern coating machines, cylinder drying contributes only a small amount to the total drying capacity, sometimes less than 20 %. Contact drying has usually the lowest specific costs. Furthermore, the cylinder group is an excellent fix point for the web run. When no cylinders are used, a separate fix point between the coater stations has to be included, such as an S-wrap or a pull stack.
In some machines, the last cylinders are used to cool down the web rather than for drying. After drying, the web can reach temperatures well above 70 °C and does not cool sufficiently before the next coater station. Too high a web temperature causes excessive penetration of the coating color into the web so cooling provides a higher product quality.
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