LATEST EXPERIENCES OF FILM COATING AND INTRODUCTION OF A NEW CONTACTLESS OPTISPRAY COATING METHOD

Author

Juha M. Virtanen

Company and address

Metso Paper Inc., Coaters and Reels, Wärtsilänkatu 100, 04400 Järvenpää, Finland

email

juha.m.virtanen@metso.com

Keywords

coating, MSP, layout, C2S, 2C1S, runnability, misting, web stealing, base paper, coating coverage, spray coating, paper quality

ABSTRACT

The recent announcements of new high-speed film coating capacity in Europe have caused a lot of interest in the Metered Size Press coating technology. The MSP technology is quickly becoming the leading process especially in LWC production. Furthermore, the choice of layout, e.g. C2S or 2C1S, for high-speed coating is also offering new alternatives since C2S is now introduced even at high machine speeds.

As the result of years of intensive development work and customer cooperation, Metso Paper has developed its new OptiSpray coating technology to the market. The coating process is based on a controlled, high-pressure spray application of coating or surface size directly on to the surface of paper or board without any direct machine contact.

Spray coating is able to achieve an excellent coverage of the base paper. The spray-coating station can be situated flexibly on the paper machine. The process will significantly lower running costs as cheaper raw materials can be used maintaining still high production efficiency, because the contactless process allows for lower machine directional paper strength. The new technology can be used for high speed coating and for a wide coatweight range.

1 LATEST EXPERIENCES OF FILM COATING

1.1 Background
Today, we can look back at a tremendous development of the Metered Size Press (MSP) technique, which took it far beyond its original target. The initial aim was only to solve speed problems related to pond size presses. But since then, the MSP technique has proven possible to be adapted for coating of both paper and board grades.

Until the recent years, the coating concepts for high-speed papermaking lines have mainly consisted of blade coating units. There have been attempts with film coating, but with lower machine speeds and special grades. The basic film-split mechanism and contour type coating with the MSP technique has been accused of producing a too rough paper with a too low gloss. Through the new production capacity in Europe, there are projects for 800,000 ton/year of single coated LWC paper with the MSP technique. What has happened during the last years, which makes it possible to utilize the MSP technique for production of high quality coated paper? It is a fact that only a further development of the base paper and the coating process has made it possible. These possibilities to produce MSP coated paper even at high machine speeds are discussed in this paper.

1.2 Coating process
A basic issue concerning coating concepts is the difference between the MSP and the blade technique. When should one or the other technique be selected to efficiently reach a desired coverage and quality at a specific coat weight? There are also different philosophies regarding the MSP concept, whether the coating should be done simultaneously in one unit or separately in two units. What need to be considered when deciding about the MSP concept? Today, the runnability issues need to be almost considered as history, because the process and the paper parameters influencing and controlling the colour mist tendency are known. By considerably increasing the nip length, coating has been possible without colour mist at very high machine speeds. Another positive effect is the increased solids contents, which has been possible due to the smooth metering rods. Now, the most critical issues are web release at the nip exit, coating strike-through and coating drying on applicator rolls.

1.3 Differences between blade and MSP coating concepts
There are different issues to consider when deciding about a blade or MSP coating concept. The layouts shown in Figure 1 illustrate the difference in space requirement, where the simultaneous MSP coating is considerably more compact than the two blade stations. High process efficiency is also required for a coating concept. Here, the MSP concept allows quite weak webs to be coated resulting in fewer web breaks.

Figure 1

Figure 1. High-speed layouts for C2S MSP (top picture) and 2C1S blade (bottom picture) coaters (OptiConcept, Metso Paper Corporation).

Blade coaters with short contact time and low application pressure have traditionally been used when single coating LWC papers. The target for these grades is to achieve sufficient fibre coverage at quite low coat weights, but the blade coaters run into restrictions due to an uneven coating layer thickness at rather high blade loads. Smoothness is more easily achieved with blade coating, but a higher coat weight is required to reach the same coverage and optical evenness as with MSP. A better fibre coverage is achieved with MSP at lower basis and coat weights on the expense of smoothness as a result of contour type coating (Figure 2 and 3). Despite of a superior coverage, a higher tendency for gloss variation is apparent for MSP coated paper with large density variations in the base paper (i.e. formation issue).

Figure 2

Figure 2. Comparison of LWC paper coated with MSP (left) and blade (right) to a coat weight of 5 g/m² [2].

Figure 3

Figure 3. The principle of coverage by alternate coating methods

Production and pilot machine experiences also indicate that MSP is less demanding than blade coating in terms of drying strategies, which allows a more compact and less costly dryer section. In particular, blade coating can easily create a mottled surface, if the drying strategy is not optimized properly. MSP is less sensitive to the influence of incoming moisture on the coating profiles or runnability, compared to blade coaters. For a paper machine producing LWC, the incoming moisture content is typically 4-5 % in MSP and 2-3 % in blade coating. It is also a fact that MSP produces fewer scratches and defects than blade, which in turn might cause deposits on the roll surfaces in an on-line calender. In the on-line multinip calendering, the coating colour sticking has arisen as a critical issue.

1.4 MSP Coating Concepts
Investments in coating technology have to assure a higher coated paper quality without sacrificing production efficiency. There are many different issues to consider when deciding on a coating concept for a specific paper grade. The layout of the coating-line can consist of C2S and 2C1S concepts depending on the space available for the production line (Figure 5). In a C2S MSP layout, both sides of the paper web are treated simultaneously in one compact unit. A two-sided simultaneous coating of the paper makes the paper machine more compact and naturally reduces the investment cost. More compact coating concepts are attractive especially for rebuilds, where the available space is usually restricted. For example, an inclined or a vertical MSP configuration allows further possibilities to fit the unit in the layout of a papermaking line. Furthermore, more reliable coat weight measurement for each side separately in C2S units and compact solutions with low flow circulation are attractive solutions. But, the demands on the base paper and paperboard also change. A reduced basis weight certainly increases the importance of surface treatment processes that minimally stress the base sheet. At the same time, an excellent coating coverage is still required, to maintain or even improve quality.

A 2C1S MSP concept is based on two one-sided MSP units, which was originally developed for single coating of wood-containing LWC offset papers. Currently, both 2C1S and C2S concepts are also utilized to improve the fibre coverage in precoating prior to applying the top coating with blade.

Figure 5

Figure 5. High-speed layouts for C2S (top picture) and 2C1S (bottom picture) MSP coaters (OptiConcept, Metso Paper Corporation).

The concept is applied in the new high-speed paper machines with on-line coating capacity, like UPM-Kymmene Augsburg (Haindl) mill and Cartiere Burgo Verzuolo mill. The speeds of these machines above or near 1700 m/min. In pilot scale, coating with the C2S concept has been successfully done up to 2000 m/min without runnability drawbacks such as misting and web stealing.

Some issues to consider when deciding about a MSP layout are listed in Table 1. The web release at the roll nip exit is perhaps the most critical issue for a C2S concept, which is not a problem in a 2C1S concept. For the 2C1S concept, the colour strike-through is critical when coating the first side of the paper web. Another important feature is the possibility to measure the coat weight separately for each side, which has been a challenge for the C2S concepts. By measuring the coat weight accurately for each side, there are possibilities to improve the coat weight control and add automatic profiling systems.

Table 1

Table 1. A comparison of differences between a C2S and 2C1S coating concept [9].

1.5 Runnability issues in MSP coating

1.5.1 Coating colour mist at the nip exit
Runnability of a MSP unit has been widely discussed and especially when the machine speeds exceeded 1300 m/min. The general opinion was that colour mist restricts the possibility for this technique to be used at high coat weights and machine speeds [11,12]. Today, the parameters to control the process are rather well identified and it is possible to avoid the colour mist up to relatively high machine speeds (> 2000 m/min) and coat weights (Figure 6).

Figure 6

Figure 6. Parameters to consider when eliminating the colour mist at the roll nip exit.

The coating film is generally accepted to split in the non-immobilized layer due to a low z-directional resistance, as illustrated in Figure 7. At this point, film-split of an excessive amount of non-immobilized coating causes colour mist. The misting amount is observed to increase exponentially as a function of the film amount, which mostly occurs at a transfer ratio below 40 % [11]. As a result of the recent development work regarding the whole process, the colour mist can be controlled up to very high machine speeds [2]. The progress in respect to MSP runnability has been made possible by increasing the solids contents of coating colours, i.e. without viscosity increase, and additionally the use of smaller rod diameters [1]. Additionally, a mechanical development has also allowed introduction of increased nip contact between the colour film and the base paper.

Figure 7

Figure 7. Principle of coating colour transfer in the roll nip of a C1S MSP unit [11].

The colour mist at the nip exit generally increases exponentially as a function of both the machine speed and the coat weight (Figure 8). To avoid such an exponential behaviour, the solids content need to be increased, which reduces the thickness of the non-immobilized layer and the risk for severe colour mist. It is, generally, possible to add 1 g/m² more coating when increasing the solids content with 1 %.

Figure 8

Figure 8. Influence of coating colour solids content on the mist amount at a coat weight of 10 g/m²/side and different machine speeds. A visual colour mist level occurs at 10 g/60 s/A4.

The base paper also influences the colour mist, where a more dense base sheet (i.e. low absorptivity and porosity) restricts colour penetration. A thicker layer of the non-immobilized colour is then available for the film-split. A twosided base paper also increases the risk for colour mist on the denser side.

1.5.2 Web stealing at the nip exit
The web release is especially critical in a MSP process, where both sides are coated simultaneously. An uncontrolled web release, or web stealing, occur as web flutter between rolls when the process conditions are similar on both sides of the paper and the web does not know which roll surface to follow (Figure 9).

Figure 9

Figure 9. Web stealing at the nip exit in a C2S MSP unit [9].

The web stealing can either appear on the whole web width or just on the edges. However, it immediately causes streaks or pattern in the coated surface. The streaks are usually 10 -20 mm wide or wider and oriented in the machine direction. The length of the streaks depends on the stability of the web flutter, where coat weight variation appears across the streaks in the cross direction of the web. The most sensitive situation for web stealing occur when coating at high-speed with equally hard rolls at a coat weight on a symmetric base sheet. Then the web will not know which roll to follow without creating a twosided process. The parameters to consider when eliminating the web stealing and controlling the web release are presented in Figure 10. Another issue to consider regarding web release is colour mist, where the colour mist is easier to control when releasing on the bottom side. There are possibilities to control the web release by changing the base paper properties, the process parameters or the coating colour formulation. The paper web generally follows the paper side with the thicker colour film or the film with the higher cohesive resistance (i.e. solids content or viscosity).

Figure 10

Figure 10. Parameters to consider when eliminating web stealing at the web release [9].

The properties of the base paper, such as sheet openness and smoothness, are most dominant regarding the web release at coat weight below approximately 10 g/m². A more open base paper surface allows more coating to penetrate and a thinner non-immobilized film then remain on the surface with less resistance toward web release. When there is no remarkable absorption twosidedness, the smoothness dominates.

With a difference in roll hardness and a certain speed difference between the rolls, it is possible to control the web release to some degree. The harder roll dominates the web release due to less deformation and a better surface contact. The softer roll is expected to deform more in the roll nip resulting in less surface contact due to a varied surface velocity. A slight speed difference between the rolls of about 0.5 % can also be used to control the web release. The speed of one roll is reduced to make the web to follow the faster roll due to the more uniform contact between the faster roll and the paper. The speed difference is, however, quite difficult to use for web control on a production machine.

A certain coat weight is required to be able to influence the web release by changing the coating colour composition, such as solids content and water-soluble thickeners. The coat weight need to be high enough to cover the base sheet and to allow a uniform split in a non -immobilized layer. In practice, the coat weight needs to be at least 7-8 g/m² to avoid a direct influence of the base sheet properties. Again, the coat weight is highly dependent on the base paper properties. The most practical way of controlling the web release in a C2S unit is to induce a coat weight or a solids content difference between the top and bottom coating films. A higher coat weight on the bottom side with a thicker non-immobilized coating layer would force the paper to follow the bottom roll. The same can be done with a solids content difference between the films, where a higher solids content dominates the web release. However, a coat weight or a solids content difference might induce a twosidedness of the coated paper. A third possibility is to use different coating colour formulations for top and bottom sides, which might be possible to utilize with certain restrictions. The pigment types or ratios cannot be changed without too large optical differences, but the amount of water-soluble thickeners could be feasible to vary and thereby achieve viscosity differences (or cohesive strength). [10]

1.5.3 Coating colour spitting from the metering rod
With coating colour spitting we intend a situation where small colour droplets are released from the nip between the metering rod and the roll. The mechanism generating the droplets is somewhat similar to that of colour misting at the roll nip exit, but the parameters influencing it are different. Generally speaking, spitting occurs when running low coatweight amounts with high speeds. With small droplet size, this phenomenon does not cause paper quality defects, but as the spitting increases also its' effect on paper quality increases. The parameters to consider when eliminating colour spitting from the metering rod are presented in Figure 11.

Figure 11

Figure 11. Parameters controlling the possibility for coating colour spitting from the metering rod

1.5.4 Coating drying on applicator rolls
The trend towards increased solids contents has brought forward the problems with coating "drying" on the roll surface. As a general rule, the drying phenomenon, where the coating film is completely immobilized in the nip, is reversed to colour misting [11]. The coating layer dried on the roll surface accumulates and becomes thicker for each roll revolution. Process and coating colour parameters, which increases the tendency for colour mist reduces the possibility for buildup on the roll surface. There are certain parameters such as base paper properties, coating colour properties and process conditions, which can be used to avoid the coating colour drying on the roll surface. The most important parameters to avoid or eliminate colour drying on rolls are mentioned in figure 12.

Figure 12

Figure 12. Parameters controlling the possibility for coating colour drying on the roll surface.

The buildup of a dry coating layer on the roll can initially appear as streaks (Figure 13, left), which gradually can develop to a thick layer (Figure 13, right). These situations have been observed under process situations, where the following issues have had a combined or a sole effect:

  • high solids content of the coating colour,
  • low machine speed ( <1200 m/min),
  • low water retention of the coating colour,
  • high content of plately pigments or soft polymer binders,
  • low coat weight ( <5 g/m²) combined with absorbent base paper,
  • high linear nip pressure or roll hardness, and
  • high base paper and/or coating colour temperature.

Figure 13

Figure 13. Coating colour drying on the roll surface. Partial drying as streaks (left) and severe drying on the whole width of the coating film (right).

When the coating film is completely immobilized from the paper to the roll surface, there is no stable film split in a non-immobilized layer [10,11]. The film-split then takes place at the paper-coating interface and the dry coating layer rapidly accumulates on the roll surface. To avoid the situation, the solids content and the water retention of the coating colour need to be optimized. The pigment choice and ratio is the most critical issue in respect to the immobilization properties of the coating colours. Delaminated and engineered clays are always critical due to a low immobilization point, which then reduces the maximum solids content to avoid coating drying on the roll surface. Here, the CaCO3 pigments are not as restrictive in respect to the solids content.

A dry streak or a dry coating film gradually growing in thickness is definitely a danger to both the rod and the roll cover. Most important, the surface of coated paper is detoriated depending on the degree of buildup. The friction between the roll cover and the rod increases dramatically, which can destroy the surface of the roll and create scratches in the rod. To prevent the roll surface from being destroyed, the dried layer or streaks need to be removed immediately.

1.6 Summary
The development of MSP coating has been tremendous during the past few years. Initially, the possibility to coat at high coat weights and machine speeds was shadowed by process related runnability problems. Colour mist at the roll nip exit was considered as the most difficult problem to overcome. However, today speeds above 2000 m/min are possible to run without mist problems even at coat weights of 10 g/m²/side. The possibility to coat with very high solids content coatings has been a very important issue to avoid colour mist. To reach even higher machine speeds without severe colour mist, the answer is a longer contact between the paper and the coating colour. The development of coating colour formulations and especially solids contents made it possible to coat without split pattern. The layout recommendations for MSP processes have also been varying during this time. Initially, the 2C1S layout was recommended for high-speed operations due to a better control of the web runnability and colour mist. This is not necessary true anymore since better knowledge has been gained to overcome the mentioned restrictions. Thereby, a considerable reduction of space required is achieved even for high-speed MSP operations with a C2S layout instead of 2C1S. Qualitywise, there is no difference between the concepts, if the paper is dense enough to avoid coating strike-through in a C1S unit.

2. CONTACTLESS OPTISPRAY COATING METHOD

2.1 Introduction
Over time we have seen a decrease in the trend of paper product prices. In recessionary times, low value products lose their profit marginal quite easily. To achieve good profitability, the value of the product must increase while at the same time production costs decrease. The dream of every papermaker is to produce a high-value product with economic raw materials while maintaining good runnability at high speeds.

The demand for lower impact in the paper coating process is well known, as are the difficulties in blade coating. Film transfer technology has taken a big step in the right direction. Weaker base papers can be coated. Still there is heavy web contact with many dependencies and interactions between coater and paper in the film transfer nip. The coat weight variations caused by unevenness of the base paper and changes in the cross directional tensile profile over the coater are ready examples.

Thanks to film transfer technology, the coated and improved newspaper grades are already available. With the help of an improved print image, four-colour newspaper printing will gradually attract the commercial advertising business. Even traditional newspapers will be four-colour printed on light-weight coated news grade, making them as attractive for product advertising as the newspaper inserts are today.

2.2 Principle of OptiSpray
The theory behind OptiSpray has long been known in paint spraying applications. The coating colour is conducted through nozzles at a high pressure.

The nozzle tip geometry is such that the liquid comes out as a thin film, which spreads continuously. When this spreading high-speed film hits the air, it atomizes to small droplets.

Figure 14

Figure 14. Atomization according to Rayleigh's and wave theory.

Typically 90% of the droplets are in the size range of 20 - 60 µm.

The diameter of the colour drop is bigger than the thickness of the coating layer. Therefore the spreading of the drop onto the surface plays a major role in spray coating.

2.3 Paper quality
The spray-coated surface is much more contoured than the surface coated by traditional blade or film transfer coating. This can be clearly seen in small-scale coat weight distribution measured by the laser induced plasma spectrography. In spray coating there are far fewer low and high coat weight spots. With traditional coating methods, the "hills" have gotten less coating and the "valleys" have gotten much more coating. With spray coating, the small-scale coat weight distribution is sharper. This is beneficial for fibre roughening and  ink mottling as well as for brightness and opacity -- not to mention the print image.

Figure 15

Figure 15. The principle of coverage by alternate coating methods

In visual evaluation of the printed samples, the spray-coated printed sheet has very good graduation. Less fibre roughening, better appearance, good touch and not so much mottle have been noted as better than many existing commercial magazine papers.

In a visual evaluation of the printed samples, the spray-coated printed sheet has very good graduation. Less fibre roughening, better appearance, good touch and less mottle have been noted as an improvement over many existing commercial magazine papers.

Some trends compared with traditional coating methods can be clearly seen. With the spray coating, higher brightness and opacity, as well as stiffness, can be reached.  A more open coating structure allows for higher moisture without any risk of blistering.

Figure 16

Figure 16. The impact of the coating method on the blistering of double coated fine paper at 130 °C ink drying temperature

The porous structure leads to slightly higher ink consumption, but at the same time gives good set off which is very advantageous, especially in the cold-set process.

The spray coating method is well suited for surface sizing. Both wood containing cold-set and heat-set grades have been produced that are readily accepted by the printers. The latest studies clearly show that spray coating also has potential for pre- and top coating of woodfree and wood containing multi coated grades.

The optimal base paper for the spray process has a hydrophobic inner structure and a hydrophilic surface. Too much absorbency leads to binder migration to the base paper and decreases surface strength. The biggest issue in base paper development will be the administration of low viscosity liquid penetration to the inner structure.

The coating colour window is smaller than in traditional coating application methods. Low viscosity and surface tension, as well as the abrasiveness of the colour lead to a round pigment particle shape. The solids content has to be lower also. However, with calcium carbonate and suitable latex a solids content of over 60 % is realistic.

2.4 Spray coating possibilities

Newspaper Grades
Numerous studies have been performed in order to test the possibilities of OptiSpray. As described earlier, the porous structure of the coating layer gives a clear benefit for cold-set printing. The very important set of properties can be adjusted so that the paper product will be robust for many types of processes and printing inks. This technology provides a good opportunity to expand cold-set printing to the commercial field. Typical newspapers may become completely four-colour printed, with high-quality advertisements throughout.

By sizing, newspaper linting can be eliminated and consequently the paper can be printed using the heat-set process.

Magazine Papers
The porous coating structure gives good blistering resistance during the heat-set process. Furthermore, a higher final product moisture can be used and lower air temperatures will be needed in the ink drying process.

Good coverage gives an excellent brightness-opacity ratio and fibre roughening is at a lower lever than traditional coating methods when used with wood-containing grades. The paper "touch" is good and the visual impact is fascinating.

Many SC-paper grades need surface treatment to bind the filler completely onto the paper surface for better offset printability. OptiSpray is a compact solution for this.

Board
The spray process doesn't smooth the rough surface so much as cover it well. Thus the spray coated layer is extremely well-suited in the intermediate layer of high quality board coating as a replacement for the air knife. As seen in the case of car painting, a smooth base surface leads to a smooth finished surface. In the future the whole process must be thought of in new ways. Perhaps the product should be pre-calendered and then coated afterwards.

Because of the porous nature of the coating, the barrier products will not be very easily handled via the spray process, but further development of chemicals may make this possible in the future.

Woodfree
OptiSpray's benefits are not so evident for woodfree grades, because the base paper properties are already at a high level. However spray sizing, pre-coating and top coating just after calendering of the precoating are areas to be studied in the near future.

2.5 Summary
As base paper properties affect the coating process less than with traditional methods, the requirements for base paper can be smaller. The nozzles consistently spray the same amount of coating colour onto the paper, regardless of base paper variations.

Due to low impact, the base paper strength can be lower than traditionally required. More economical raw materials can be used and base paper faults are better tolerated. Higher base paper moisture can be used, providing greater flexibility when it comes to locating the coater in the paper machine.

A non-contact process is beneficial in high-speed solutions as well. At high speeds the colour window of existing coaters narrows, which will partly decrease the coating colour formulation differences in the future.

Blade or rod streaking is no longer an issue. Coarser, cheaper pigments can be used. The functional criteria of the spray itself is the colour coming from the couple of hundred microns diameter nozzle hole.

The spray coater doesn't limit the coat weight range, with the exception of the minimum coat weight at low speeds. The range can be e.g. 0.1 to 30gsm. Paper wetting and the down-flowing coating colour on the vertical paper surface are the main limiting factors for the maximum coat weight.

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