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Papermaking: Film press for pigment coating: A review

Intense R&D efforts have led to a better understanding of the coating process

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By: X. Zou and D. Vidal
2003-03-02
Film press coating has emerged as a versatile technology for pigment coating of paper and board. The need for value-added products with low investment cost, and high production efficiency with low cost furnish has been the main driving force behind the rapidly increasing use of film press coaters, also called metered size presses (MSP). A few reviews have previously been published in the area of film press coating, but almost all of them have focussed on process and runnability-related issues [1-5]. With the maturing of film press coating technology, however, interests have recently been shifting towards improving the quality of film-coated paper, resulting in a significant amount of information on this subject being published. In this paper, we review the most recent developments, with a particular focus on the quality of film-coated papers and the potential of using this technology for product development.

FILM PRESS COATING PROCESS

Film coating with a metered size press consists of applying a very thin premetered film of coating colour onto a rubber-covered roll (the so-called transfer roll), and then transferring this film onto the paper web by means of a two counter-rotating roll nip. There are runnability and quality issues related to the film press coating process for MSP applications with a rod-metering element. For example, spitting and ribbing appear at the rod-metering nip whereas misting, ribbing and orange peel occur at the transfer nip. Furthermore, coating build-up may take place at both nips and lead to uneven coating films. A more detailed description of these issues is given in Ref. [6]

FILM SPLITTING AT THE TRANSFER NIP

Film splitting at the transfer nip is very important because it affects the transfer ratio (coat weight achievable), and can create runnability issues such as misting and orange peel formation. Film splitting is a complex phenomenon depending on many parameters, such as microscopic gap, speed, rate of dewatering, colour rheology, roll deformation, and surface tension. The current concept of film splitting is summarized in Fig. 1, and is largely borrowed from offset printing presses where a premetered ink film is transferred to a moving web at a rolling nip between two rolls. The pressure profile in the nip is characterized by a pressure peak slightly before the centre of the nip, and under certain critical conditions (mainly high speeds), a small but significant negative pressure at the nip exit where the film splits may appear. The occurrence of this negative pressure zone results in vortices that grow with increasing inertial forces. This type of pressure profile is well-documented in the case of printing nips [7-8]. The pressure in the nip is thought to generate liquid penetration and, to a certain extent, solid penetration into the paper, and it is now widely accepted that an immobilized layer (often called filter-cake) is formed on top of the paper. In paper coating, the rate of coating colour immobilization on the paper in the nip is considered very important for film splitting. The divergent flow at the exit is assumed to split along the remaining non-immobilized layer. In the absence of a sub-ambient pressure zone at the exit, it should form a nice smooth free meniscus. In the case of very high negative pressure at the exit, especially if the pressure falls below the vapour pressure of the liquid phase, vapour bubbles are generated, causing a phenomenon known as cavitation. This leads to the creation of filaments that elongate as the rolls move away from each other, and eventually break up. The break-up of these filaments is believed to create an orange peel pattern and misting [9-10], which will be discussed later.

To describe the coating colour transfer to the paper, a transfer ratio has been defined as the ratio of the amount of coating colour transferred to paper to the amount of premetered coating. Pilot-scale film press coating trials have shown that the transfer ratio depends strongly on the amount of premetered film, the rate of dewatering and immobilization of the coating colour at the transfer nip, and to a lesser extent, the coating speed [11-12]. A higher amount of premetered film leads to a lower transfer ratio. Faster dewatering and colour immobilization results in a higher transfer ratio [11]. Too high a transfer ratio could result in an uneven split and drying of the coating colour remaining on the transfer roll surface, while at low transfer ratios (<0.4) a serious increase in misting starts to occur. Grön et al. suggested transfer ratios between 0.6 and 0.8 as the optimum range [11].

RUNNABILITY ISSUES

Misting and orange peel are two common runnability issues in film press coating. Misting, a phenomenon originating from film splitting at the transfer nip, has been regarded as a limiting factor for MSP application for coating at high coating speeds. The formation of misting can result in the waste of coating colour, cause operational difficulties, and also create quality problems when misting droplets drip back to the coated surface. Similarly, orange peel is not only a runnability problem related to film splitting, but also a quality issue as it affects the appearance of the coated paper surface. Two other runnability problems can be encountered at the transfer nip, namely web stealing and breaks, although they are less frequently reported and studied.

Web Stealing: Web stealing (i.e., web fluttering between two rolls) is an undesirable phenomenon, particularly when coating both sides of the paper simultaneously. Because of the two-sidedness nature of the paper, using the same coating conditions for both sides makes the web flutter uncontrollably between the rolls, creating quality problems such as cross-directional marking of the coated surface. Web stealing is mainly a problem with wood- containing papers and is related to the point of web release from the transfer roll. To avoid web stealing, an even release line and a small release angle on the film transfer roll surface should be targeted. Web threading at the roll nip exit is suggested as a way of physically changing the point of web release [13]. By guiding the paper web to follow either the top or bottom roll, the risk of uncontrolled web release and web stealing can be eliminated. Besides such physical means, the web release point and web stealing could be controlled by adjusting parameters such as the premetered film amount, the roll cover hardness, the speed difference of the rolls, the solids content and viscosity of the coating colour, and the basestock properties [13-14]. The choice of pigments also has a significant impact on web release. For example, clay-based colour has been found to provide an earlier web release than GCC-based colour [11]. This is attributed to the higher immobilization solids for GCC-based coating colours.

Web Breaks: It has often been mentioned in the literature that film press coating can run weak basestocks with fewer web breaks, and thus gives higher machine efficiency than blade coating. This is attributed to the premetering process (i.e. no direct contact with the paper web) and the reduced mechanical stress on the paper web in the film transfer nip [9, 11, 15-17]. Increased production efficiency has been considered as a significant advantage for film press coating. However, only a small amount of data on the number of paper breaks during film press coating has been documented, particularly in comparison with other traditional coating processes such as blade coating. Ryder reported that for a low basis weight wood-free basestock, the number of breaks was reduced from 20 per week to five, after a pond-type size press was replaced with a film press coater [18]. McCaig and Harala reported a start-up run of 52 hours without a break for a commercial production of LWC using MSP with a C2S (simultaneously coated on both sides) [14]. When coating ULWC grades, Grön and Rautiainen claimed that film press coating could achieve 50% fewer web breaks than blade coating [19]. Considering the need for blade changes during blade coating, the estimated improvement in machine efficiency for film press coating could be as high as 2-5% [16, 19].

Misting: Misting usually occurs at high speeds during film press coating. Pilot coating trials showed that severe misting is always accompanied by low transfer ratios (<0.4) [11]. Although misting is generally an issue in high-speed operations, it can also be important at lower speeds if the coat weight is high enough or the solids content is low.

The mechanisms leading to the formation of misting in roll application of coating colours and printing inks have been the subject of a number of previous studies [9, 11, 20-23]. One proposed mechanism, originating from printing nip studies, involves filament formation and its subsequent break-up. As already explained in the previous section, the negative pressure at the exit zone of the nip may cause cavitation and the expansion of the resulting vapour bubbles as the pressure drops, leading to filament formation. Eventually, the filaments break up into small droplets as they elongate at the roll nip exit, causing misting. The second mechanism involves the air entrained in the coating colour that could form air bubbles. The bursting of these air bubbles at the nip exit generates misting [24]. This proposed mechanism is supported by the results that increasing the air content in coating colour significantly increases the formation of misting [21]. The third mechanism is based on the centrifugal and surface tension forces. Mist droplets can be generated by centrifugal forces acting on the fluid film on the roll surface after film splitting [9]. At high coating speeds, if the size of the fluid bumps (caused by the break-up of the filaments) on the roll surface is large enough, the centrifugal forces may be strong enough to overcome the surface tension forces and fling the fluid from the rotating roll, creating mist droplets. This is supported by the results from Roper et al., who examined the size distribution of mist droplets collected from a pilot scale trial [9]. They found that misting was made of small droplets of approximately 100 µm in diameter that could be explained by filament break-up. However, under certain conditions, they also noticed the appearance of a few very large droplets, on the order of 1 mm in diameter. The authors believed that these large mist droplets were generated by the mechanism based on the centrifugal and surface tension forces.

Extensive experimental work on the pilot-scale has recently been carried out to investigate the effect of process, formulation, and basestock variables on misting formation [9, 11, 13, 21, 25-28]. Some of the main factors affecting misting are summarized in Fig. 2 (note that "+" indicates an increase while "-" indicate a decrease). Among them, machine speed and solids content have been found to be the most important factors.

Orange Peel: Orange peel formation has not been studied as extensively as misting. This phenomenon is caused by non-uniform film splitting (or filament break-up) at the transfer nip and poor levelling of the coating colour after splitting. Good coating levelling may partially or completely eliminate orange peel.

Orange peel is a real problem only at high coat weights. It has been shown that higher coat weight, lower solids content, rougher basestock surface and higher water absorbency lead to more orange peel. In a comprehensive pilot-scale coating study, Roper et al. evaluated orange peel subjectively and observed that serious orange peel formation occurred at coat weights above 13 g/m2 [29]. The air present in coating colour was found to cause a significant increase in orange peel formation. This was attributed to the burst of air bubbles at the transfer nip, as discussed in the previous section on misting. Pre-calendering of a LWC basestock reduced orange peel formation.

Both misting and orange peel are related to film splitting process at the transfer nip, and are sometimes affected by the process and formulation parameters in a similar way. However, it should be pointed out that a higher degree of misting does not necessarily lead to more orange peel formation. For example, Glittenberg and Hemmes found that for both LWC and wood-free basestock, as water retention of the coating colour increased by using different types of starches, the misting tendency decreased but the orange peel pattern increased [30]. Roper et al. observed instances of high levels of misting accompanied by low levels of orange peel pattern, as well as low levels of misting with high levels of orange peel pattern [29]. This suggests that the factors controlling orange peel formation are sometimes independent of those controlling misting.

Strategies to Eliminate Misting and Orange Peel: Most of the previous studies have focussed on modifying the coating formulations to eliminate misting, since the coating formulation is more convenient to modify than the process and basestock parameters [11, 15, 21, 25-27, 30]. This approach has achieved success under certain operating conditions by improving coating immobilization. Pilot studies have shown that modifying the coating formulation combined with the use of a high solids content allows a film press coater to run at a coating speed of 2,000 m/min without misting [11, 19]. However, different formulation strategies are required depending on the coat weight, coating speed and basestock, making optimising the coating formulation difficult. On the other hand, as the coating speed increases to a certain level (>1,800 m/min) or the coat weight increases to above 10 g/m2, the role of coating immobilization diminishes due to the short residence time at the nip. For example, Gane et al. reported cases where attempting to achieve rapid immobilization failed to reduce misting [31]. Therefore, modifying the coating formulation is only part of the solution.

APPLICATION OF FILM PRESS COATING TECHNOLOGY

Recent technological improvements have enabled film press coating to apply a wide range of coat weights (4-20 g/m2) with high solids content (up to 70%). As a result, film press coating has become a flexible and versatile technology that is suitable for developing new and speciality products. Film press coating can be used for pre-coating of wood-free grades, high-speed coating of light-weight and ultra light-weight coated papers (LWC and ULWC), as well as for coating of speciality paper [4, 32-33].

One popular application of MSP is for upgrading newsprint and producing LWC offset papers. The significantly reduced stress on the paper during film press coating is the major reason. Film-coated LWC has similar quality attributes to blade-coated, but is produced at a much lower cost. Most of the savings come from the use of cheaper fibres, because less kraft pulp is needed and more TMP, GWD, CTMP, or even recycled fibres can be used. Installing an on-line film press coater with a C2S concept, combined with soft-nip calendering also requires less capital cost and takes less space than other coating processes [14].

QUALITY OF FILM- COATED PAPERS

Short-dwell-type blade coating has traditionally been used for producing LWC and ULWC grades. However, the dominance of blade coating in producing these two grades has been challenged with the introduction of film press coating. Our review on quality issues will therefore focus on the LWC and ULWC grades, particularly in comparison with blade coating.

LWC Offset Grades: In the area of film-coated offset grades, significant progress has been made through the adjustment of process conditions and basestock properties. Film press coating can now achieve offset quality targets comparable to those of blade coating [15, 33-34].

-- Coating coverage and coat weight uniformity: It is generally believed that film press coating produces a "contour" coating layer while blade coating tends to give a smooth (levelled) surface [1, 35]. This has been considered as a significant advantage for film-coated paper because the coat weight uniformity is very important for the print quality of LWC offset grades. However, surprisingly very little data has been published in comparing the coat weight uniformity of film-coated paper to that of the blade-coated paper. A very interesting result recently published by Grön and Rautiainen indicates that the coating layer from film press coating is far from the ideal contour coating, with complete coverage being achieved only at coat weights above 10 g/m2 [19]. The surface-covering ability of film press coating for a 38 g/m2 basestock is compared with that of blade coating in Fig. 3. They divided the coating coverage curves into three regions [19]:

• (A): At coat weights below 8 g/m2, film press coating is most advantageous due to its superior coverage.

• (B): At coat weights of 8-10 g/m2, there is no significant difference in coverage.

• (C): At coat weights higher than 10 g/m2, both blade and film coating gives almost complete coverage. However, blade coating gives better coating surface smoothness and gloss.

In another study, Suontausta compared the surface quality of film-coated and blade-coated papers at a coat weight of 10 g/m2 [36]. The surface profiles of uncoated and coated sheets were measured with a laser surface profilometer, and the surface roughness at different wavelengths was obtained using frequency analysis. Figure 4 compares film-coated and blade-coated papers in terms of surface roughness at different wavelengths. Since the same basestock and the same coating formulation were used for both samples, the difference in roughness is solely due to the difference produced by the coating processes. As can be seen, in the wavelength range of 5-10 µm, there is a similar small reduction in roughness by both film and blade coating. In the wavelength range of 10-80 µm, roughness is significantly reduced to a similar degree by both film and blade coating, indicating "smoothening" effect. On the other hand, in the wavelength range of 80-1,280 µm, both film coating and blade coating reduce roughness, but to a much less extent for film coating. In the wavelength range of 1,280-5,120 µm, at the so-called fibre floc level, the roughness reduction by film coating is again less than that of blade coating. These results suggest that film press coating produces a high degree of contour coating only at large scales. In other words, film-coated paper has the same performance as blade coating in reducing the roughness at scales of less than 80 µm. Most recently, a benchmarking comparison at Paprican using direct measurements showed that film-coated paper has a slightly better overall coating thickness uniformity than blade-coated paper, but the difference is mainly at larger scales [36].

-- Gloss and smoothness: Gloss and smoothness are important quality parameters for coated papers. Lower gloss used to be a major problem for film-coated LWC grades (45-50% for film-coated vs. 50-55% for blade-coated). A recent study by Suontausta compared the surface quality of film-coated and blade-coated papers using the same basestock and the same formulation [36]. The results showed that high gloss comparable to blade coating could be achieved by soft-nip calendering with low pressure (60-80 kN/m), and high temperature (150°C) [36]. There was still, however, a difference in surface smoothness of approximately 0.5 µm at the same gloss level, as shown in Fig. 5. It is interesting to note that gloss is linearly related to surface roughness, but film-coated and blade-coated papers have two distinct lines. At the same gloss, film-coated paper always has a higher roughness. A careful examination of the roughness at different wavelength ranges (Fig. 4) indicates that the major difference in roughness between film-coated and blade-coated paper is on the scale of 80-1,280 µm, where film-coated paper has a significantly higher roughness. Suontausta found that even calendering with high pressure and temperature could not remove the roughness of film-coated paper at these wavelengths [29]. This is the reason why film-coated paper always has a lower smoothness than blade-coated paper. It should be pointed out, however, that the higher roughness of film-coated paper is not necessarily a problem for offset print quality, which is mainly controlled by coat weight uniformity.

For the visual appearance of the coated paper, not only is the average gloss important, but also the local variation of gloss and the size of glossy spots. For a good visual impression, it is important to avoid a high gloss variation combined with large gloss spots. Suontausta showed that film-coated paper always has a higher intensity of gloss variation than blade-coated paper [36], but the size of glossy spots in film-coated paper is smaller. In the case of film-coated paper, the size of glossy spots was found to increase with coat weight, which can be attributed to the orange peel effect discussed earlier. A higher calendering temperature was found to cause more gloss variation at the same final average gloss.

-- Print quality: In heat-set offset printing, print mottle, print density and print gloss are important quality parameters. Although it is widely believed that film-coated LWC paper has as good print quality as blade-coated LWC, very few studies have done a direct comparison. Using the same basestock and coating formulation, Drage et al. compared the offset print quality of papers coated by both film press and blade coaters [38]. The blade-coated paper was supercalendered while the film-coated paper was calendered by a soft-nip calender. They found that at a coat weight of 9 g/m2, the film-coated paper had the same print gloss and print density as blade-coated paper. In another study, Ahlroos et al. reported that film-coated LWC had slightly less print mottling than a typical blade-coated paper in heat-set offset colour printing [34]. This was attributed to the more uniform coating thickness of film-coated papers at the macro-scale. It is known that for LWC grades, coating thickness uniformity is the most important factor for print quality [39-40]. The print gloss of film-coated paper was found to be comparable to that of blade-coated paper [26, 34].

ULWC Grades: The production of ULWC grades requires a lower basestock basis weight and also a lower coat weight. To achieve a sufficient fibre coverage at low coat weights, the blade coating process may have a problem due to an uneven coating layer thickness at high blade loads. In this situation, film press coating is more suitable since it provides a better coating coverage at low coat weights (Fig. 3) [19]. A low mechanical stress is also beneficial with respect to runnability (e.g. paper breaks). Pilot trials have indicated that by using a high solids content, a Hunter gloss of 40 to 45% and a Park-Print Surf (PPS10) roughness of 1.5 µm can be achieved at a coat weight of 4 g/m2 (each side) on a 30-g/m2 wood-containing basestock [19]. However, the production of ULWC with film press coating is still relatively new and very few results have been published.

Even with the benefits of film press coating, there still remain some challenging issues in the production of ULWC grades, such as low opacity, poor dimensional stability and the occurrence of pinholes. A low basis weight basestock is more likely to have pinholes, particularly when coarse mechanical pulp fractions are included. This may result in the coating colour penetrating through the basestock and attaching to the backing roll, called strike-through. From a coating point of view, strike-through can be eliminated by increasing the solids content of the coating colour, using pigments with a higher aspect ratio or reducing the transfer nip pressure through the use of a softer roll cover or lower linear load. From a basestock point of view, reducing the fibre-wall thickness and improving the forming section to eliminate air bubbles have been the approaches used to reduce the formation of pinholes. The choice of these strategies will likely depend on the furnish and the flexibility of the process units.

BASESTOCK EFFECTS:

-- Effect of surface roughness: Suontausta showed that basestock surface roughness still has a significant effect on the quality of coated paper in film press coating [36]. In particular, the small-scale roughness (10-80 mm) significantly affects the smoothness and gloss of film-coated paper. From a papermaking point of view, the use of less coarse and thinner-walled fibres to obtain a smoother basestock surface is as important in film press coating as in blade coating. Reducing the roughness of wood-containing basestock by pre-calendering may also be very effective in achieving a more uniform coat weight uniformity and better surface quality [34]. This is somewhat different from the case of blade coating where pre-calendering is claimed to be less effective, due to surface roughening during coating application [42]. In film press coating, surface roughening is less significant due to a shorter contact time and the use of a coating colour with a higher solids content.

-- Effect of formation: Drage et al. compared the quality of film-coated papers with different basestock formations [38]. They changed the sheet formation by adjusting the dosages of a three-component retention/drainage/formation aid, while the paper machine parameters were kept constant. The corresponding Kajaani formation number varied from 63 to 76 for an LWC basestock. In the case of film press coating, the results from a pilot scale trial (at a coat weight of 9 g/m2) showed no clear effect of formation change on coated gloss, smoothness, print density and print gloss. This is quite different from blade coating where in the same study, improved formation was found to slightly improve coated gloss, print density and print gloss. It was suggested that film press coating covers large-scale variations to a greater extent than blade coating. In other words, film press coating is probably more forgiving to the basestock visual formation (i.e. fibre flocs) problem than blade coating.

-- Effect of pre-calendering and filler addition: Both pre-calendering and the addition of filler can be used as ways of making a smoother and more consolidated basestock. In a pilot-scale trial, Grön and Ahlroos found that pre-calendering improved the coat weight uniformity measured on the scale of 1 to 8 mm [34, 42]. By indirect measurements (burn-out test), they claimed that coating penetration was also reduced, however, with no significant improvements in the surface smoothness or gloss of the coated paper. In terms of print quality, although there was no significant impact of pre-calendering on print gloss, fibre roughening during heatset offset printing was reduced. This effect of pre-calendering on roughening was more pronounced when filler content in the basestock was low (<10%). They suggested that pre-calendering could be used to reduce fibre roughening in offset colour printing.

Increasing the filler content in a GWD-based basestock was found to have a positive effect on coat weight uniformity, smoothness/gloss, print gloss and print mottle of film-coated paper [43]. Ahlroos et al. found that 20% filler addition, combined with pre-calendering, could increase the gloss of coated paper from 45% to 55%, and reduce the roughness from 1.9 µm to 1.3 µm. Slightly improved print mottle and much less fibre roughening were observed during heat-set offset printing. This was attributed to better fibre coverage, caused by a smoother basestock surface with filler addition.-- Groundwood (GWD) and TMP as basestock furnishes: GWD and TMP are the two most common mechanical pulps used for LWC basestock. GWD (both stone and pressurized) generally gives a denser and a more closed basestock structure, with more fines on the surface than with TMP. There are more longer and coarser fibres in TMP pulp, which increases the tendency of fibre roughening upon water application. Because of these differences, GWD is generally believed as a better furnish for LWC basestock. For example, Liimatainen et al. found that in blade coating, it is easier to achieve the targeted LWC quality with pressurized GWD than with TMP [45]. In film press coating, Ahlroos and Grön compared the quality of coated paper between the stone GWD and TMP basestocks with 20% kraft content [45]. They observed that the coated TMP had a poorer coverage and higher coat weight variations than coated GWD. This led to more print mottle for the coated TMP. The coated TMP also had lower print gloss and print density than the coated GWD, despite the fact that the coated TMP was smoother after super-calendering. The coated GWD also had a higher surface strength than the coated TMP.

In a more direct comparison, Suontausta measured the surface profile and the gloss of film-coated paper using two commercial GWD and TMP basestocks [36]. These two basestocks had almost the same initial surface roughness, but the GWD basestock had a somewhat lower porosity. Pilot trials were carried out using the same coating formulation and coating conditions. The analysis of coated sheets indicated that at a coat weight of about 10 g/m2, coated papers from both the GWD and TMP had almost the same coated roughness before calendering. The gloss and smoothness were also identical after soft-nip calendering at lower temperatures (90-120°C). At a higher calendering temperature of 150°C, however, the coated TMP had a gloss of approximately three points lower than that of the coated GWD.

Although TMP generally gives poorer coated paper quality than GWD, the difference depends on how the TMP is processed. In particular, the fibre wall thickness of the TMP long fibre fractions is very important. For example, Hallamaa et al. found that when a TMP pulp with a large proportion of long fibres was further refined to gain more flexibility and thinner fibre walls, it gave a coated smoothness equal to that of conventional GWD [46]. After heat-set web offset printing, roughening was also very similar between the coated TMP and GWD. This suggests that if the TMP pulp (particularly its long-fibre fractions) is appropriately refined, it is possible to achieve a quality comparable to that of coated GWD.

-- Effect of recycled fibres: Varsa et al. reported the effect of deinked pulp (DIP) on the quality of coated LWC paper by both blade and film press coating [47]. During blade coating, the basestock containing DIP was found to cause streaks in the coating layer as a result of released contaminants or fibres. The maximum DIP content in the basestock used for blade coating was found to be about 15-20%. According to their results, at a DIP content higher than 20% [47] film press coating seems to have been more suitable for coating LWC grades than blade coating, due to better runnabilty and quality of the coated sheets. With an increased DIP content from 0 to 60%, the porosity of the basestock was reduced [47] and it was speculated that coating coverage is improved as a result of reduced coating penetration [19].

SUMMARY

Through recent intensive research and development efforts, progress has been made in understanding the film press coating process, particularly the film transfer at the transfer nip. The main runnability issues such as misting and orange peel formation can now be solved through modifications of the basestock and the coating formulations. However, these solutions are mostly based on experience and results from pilot-scale trials. There is still a lack of fundamental understanding of film splitting dynamics at the transfer nip. Another challenge is to solve runnability problems without sacrificing the quality of coated paper.

In terms of product quality, film-coated LWC offset grades with gloss and print quality comparable to blade coating can now be achieved by the modification of basestock structures and the selection of appropriate calendering conditions after coating. Film-coated paper has a more uniform coating thickness than blade-coated paper, but mostly on the large scale. The lower smoothness of the film-coated paper is not a problem for achieving coated gloss and offset print quality because the higher roughness of film-coated paper is mostly large-scale. With LWC grades, the challenge is to reduce gloss variations whereas with ULWC grades, the challenge is to reduce coating penetration.

LITERATURE

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10. ERCAN, S.N. and BOUSFIELD, D.W., "Influence of Process Parameters on Filament Size Distribution", Proceedings of the 1998 Pan-Pacific and International Printing and Graphic Arts Conference, pp. 111-115, 1998.

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17. HOSTETLER, R.E., FREUNDSCHUH, U., MECK, M.S.D. and RUCKERT, H., "The Film Press as a Precoater for Paper Board: An Alternative to the Air Knife Coater?", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 211-215, 1998.

18. RYDER, D.J., "Practical Experiences with a Metered Size Press", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 197-204, 1998.

19. GRON, J. and RAUTIAINEN, P., "Coating Solutions for Wood-Containing and Wood-free Paper Grades", Proceedings of the 1999 TAPPI Coating Conference, TAPPI Press, pp.457-467, 1999.

20. NINNESS, B., BOUSFIELD, D.W. and TRIANTAFILLOPOULOS, N.G, "Fluid Mechanics Model of the Film-fed Rolling Nip with a Porous Web", Proceedings of the 1998 TAPPI Coating/Papermakers Conference, TAPPI Press, pp. 515-530, 1998.

21. ROPER III, J.A., MOORE, E., SALMINEN, P and URSCHELER, R., "Optimisation of Formulation Parameters to Reduce Misting and Orange Peel Formation on Metered Film Coater", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 37-55, 1998.

22. VOET, A., "Ink Misting and Its Prevention", American Ink Maker 32(2): 32 (1956).

23. BLAYO, A., FANG, S.W., GANDINI, A. and LE NEST, J.F., "Study of Ink Misting Phenomena" American Ink Maker 76(5): 54-61 (1998).

24. SCRIVEN, L.E., Personal Communication, 1998.

25. GRON, J., ANAS, P. H. and MOLARIUS-MAYRANEN, S., "Improving the Process Runnability and FCO Quality by Optimizing the Coating Colour Composition", Proceedings of the 1997 TAPPI Coating Conference, TAPPI Press, pp. 23-41, 1997.

26. SALMINEN, P., ROPER III, J.A., URSCHELER, R., and CHASE, D., "Optimizing the Coating Formulation to Reduce Misting in High-Speed Film Coating", Proceedings of the 1996 TAPPI Metered Size Press Forum, TAPPI Press, pp. 51-55, 1996.

27. HIRONS, A.G., OAK, G., COGGON, L. and ENGLEY, M., "The Runnability of High Speed MSP when Coating Mechanical Paper with High Solids", Proceedings of the 1996 TAPPI Metered Size Press Forum, TAPPI Press, pp.161-169, 1996.

28. HIRONS, A.G. and COGGON, L., "The Use of Neural Network Modeling to Determine the Degree of Misting Based on Pigment Morphology", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 175-185, 1998.

29. ROPER III, J.A., SALMINEN, P., URSCHELER, R. and BOUSFIELD, D.W., "Studies of Orange Peel Formation in High-speed Film Coating", Tappi J. 82(1): 231-238 (1999).

30. GLITTENBERG, D. and HEMMES, J., "Improved Runnability of Film Press Coating", Paper Technology (3): 35-41 (1998).

31. GANE, P.A.C., BURRI, P., SPIELMANN, D., DRECHSEL, J. and REIMERS, O., "Formulation Optimization for Improved Runnability of High Speed Pigmented Coatings on the Metered Size Press", Proceedings of the 1997 TAPPI Coating Conference, TAPPI Press, pp.15-22, 1997.

32. AARNIKOIVU, P., "Creating New Surface-Sized Grades on the Film Press", Paper Technology (3): 29-34 (1998).

33. TREFZ, M., "New Developments in Metering Size Press Technology", Proceedings of the 85th PAPTAC Annual Meeting, Volume B, PAPTAC, pp. 121-125, 1999.

34. AHLROOS, J., ALEXANDERSSON, M. and GRON, J., "Influence of Base Paper Filler Content and Pre-calendering on Metered Film Press Coating -- Part II: Paper and Print Quality", Proceedings of the 1998 TAPPI Coating/Papermakers Conference, TAPPI Press, pp. 915-924, 1998.

35. FREUNDSCHUH, U. and MECK, D., "Now the Future is Captured on Film", Paper and Paper Europe (11): 11-12 (1996).

36. SUONTAUSTA, O., "The Influence of Calendering and Coating Variables on the Smoothness and Gloss of LWC Paper Grades". Proceedings of the International Symposium on Paper Coating Coverage, Training Centre of Finish Forest Industry and the Finish Paper Engineers' Association, Helsinki, Finland, pp. 1-19, 1999.

37. ALLEM, R., ZOU X. and SUONTAUSTA, O., "A Benchmarking Analysis of Film and Blade Coated Papers", to be published.

38. DRAGE, G., VAUGHAN, C., HENDERSON, K., PARSONS, J. and HIRON, T., "The Influence of Freesheet and Groundwood Basepaper Formation on Coated and Printed Paper Properties", Proceedings of the 1999 TAPPI Coating Conference, TAPPI Press, pp. 469-479, 1999.

39. ENGSTROM, G., "Forming and Consolidation of a Coating Layer and Their Effect on Offset Print Mottle", Proceedings of the 1993 Advanced Coating Fundamental Symposium, TAPPI Press, pp. 43-50, 1993.

40. ALLEM, R., "Characterization of Paper Coating by Scanning Electron Microscopy and Image Analysis", J. Pulp Paper Sci. 24(10): 329-336 (1998).

41. GRON, J. and AHLROOS, J., "Influence of Base Paper Filler Content and Pre-calendering on Metered Film Press Coating -- Part I: A Coating Process Study", Proceedings of the 1998 TAPPI Coating/Papermakers Conference, TAPPI Press, pp. 899-914, 1998.

42. ENGSTROM, G. and LAFAYE, J.F., "Precalendering of the Base Paper and Its Effect on Paper-Coating Interaction", Tappi J. 75(8) 117-122 (1992).

43. FERNANDO, R.H., "Rheological Aspects of Misting Mechanism in Roll Applied, Non-Newtonian Paper Coating and Inks", Proceedings of the 1999 TAPPI Advanced Coating Fundamental Symposium, TAPPI Press, pp. 99-110, 1999.

44. LIIMATAINEN, H., HAIKKALA, P. and TUOVINEN, O., "LWC Paper from PGW and TMP Pulps with Gap Former", Paper and Timber 80(6): 434 (1998).

45. AHLROOS, J. and GRON, J. "A Comparison of SGW and TMP as Fibre Raw Materials for Film-coated LWC", Proceedings of the 1999 TAPPI Coating Conference, TAPPI Press, pp. 481-502, 1999.

46. HALLAMAA, T., HEIKKURINEN, A. and FORSSTROM, U., "The Effect of Fibre Properties on LWC Paper Structure and Printability", Proceedings of the International Mechanical Pulping Conference, TAPPI Press, pp. 1-8, 1999.

47. VARSA, P., TOPPILA, M. and ANTTIA, T., "The Quality of High Deinked Pulp Content Magazine Paper Produced with Various Coating Methods", Proceedings of the 1997 TAPPI Coating Conference, TAPPI Press, p. 281, 1997.

Abstract: Film press coating has emerged as a versatile technology for paper surface treatment. With the progress in understanding the film press coating process, particularly the film splitting at the transfer nip, interests have recently been shifting towards improving the quality of film-coated paper. This has resulted in a significant amount of information on this subject being published. In this paper, we review the most recent developments, with a particular focus on the quality of film-coated papers and the potential of using this technology for product development.

Résumé: Le couchage sur presse est devenu une technologie polyvalente pour le traitement en surface du papier. On connaît maintenant mieux ce procédé et particulièrement la séparation de la pellicule à la pince de transfert, et l'attention a récemment davantage porté sur l'amélioration de la qualité du papier enduit d'une sauce de couchage. On a ainsi publié de nombreux documents sur ce sujet. Dans la présente communication, nous passons en revue les plus récents développements, en mettant un accent particulier sur la qualité des papiers couchés et sur la possibilité d'employer cette technologie pour le développement de produits.-- Groundwood (GWD) and TMP as basestock furnishes: GWD and TMP are the two most common mechanical pulps used for LWC basestock. GWD (both stone and pressurized) generally gives a denser and a more closed basestock structure, with more fines on the surface than with TMP. There are more longer and coarser fibres in TMP pulp, which increases the tendency of fibre roughening upon water application. Because of these differences, GWD is generally believed as a better furnish for LWC basestock. For example, Liimatainen et al. found that in blade coating, it is easier to achieve the targeted LWC quality with pressurized GWD than with TMP [45]. In film press coating, Ahlroos and Grön compared the quality of coated paper between the stone GWD and TMP basestocks with 20% kraft content [45]. They observed that the coated TMP had a poorer coverage and higher coat weight variations than coated GWD. This led to more print mottle for the coated TMP. The coated TMP also had lower print gloss and print density than the coated GWD, despite the fact that the coated TMP was smoother after super-calendering. The coated GWD also had a higher surface strength than the coated TMP.

In a more direct comparison, Suontausta measured the surface profile and the gloss of film-coated paper using two commercial GWD and TMP basestocks [36]. These two basestocks had almost the same initial surface roughness, but the GWD basestock had a somewhat lower porosity. Pilot trials were carried out using the same coating formulation and coating conditions. The analysis of coated sheets indicated that at a coat weight of about 10 g/m2, coated papers from both the GWD and TMP had almost the same coated roughness before calendering. The gloss and smoothness were also identical after soft-nip calendering at lower temperatures (90-120°C). At a higher calendering temperature of 150°C, however, the coated TMP had a gloss of approximately three points lower than that of the coated GWD.

Although TMP generally gives poorer coated paper quality than GWD, the difference depends on how the TMP is processed. In particular, the fibre wall thickness of the TMP long fibre fractions is very important. For example, Hallamaa et al. found that when a TMP pulp with a large proportion of long fibres was further refined to gain more flexibility and thinner fibre walls, it gave a coated smoothness equal to that of conventional GWD [46]. After heat-set web offset printing, roughening was also very similar between the coated TMP and GWD. This suggests that if the TMP pulp (particularly its long-fibre fractions) is appropriately refined, it is possible to achieve a quality comparable to that of coated GWD.

-- Effect of recycled fibres: Varsa et al. reported the effect of deinked pulp (DIP) on the quality of coated LWC paper by both blade and film press coating [47]. During blade coating, the basestock containing DIP was found to cause streaks in the coating layer as a result of released contaminants or fibres. The maximum DIP content in the basestock used for blade coating was found to be about 15-20%. According to their results, at a DIP content higher than 20% [47] film press coating seems to have been more suitable for coating LWC grades than blade coating, due to better runnabilty and quality of the coated sheets. With an increased DIP content from 0 to 60%, the porosity of the basestock was reduced [47] and it was speculated that coating coverage is improved as a result of reduced coating penetration [19].

SUMMARY

Through recent intensive research and development efforts, progress has been made in understanding the film press coating process, particularly the film transfer at the transfer nip. The main runnability issues such as misting and orange peel formation can now be solved through modifications of the basestock and the coating formulations. However, these solutions are mostly based on experience and results from pilot-scale trials. There is still a lack of fundamental understanding of film splitting dynamics at the transfer nip. Another challenge is to solve runnability problems without sacrificing the quality of coated paper.

In terms of product quality, film-coated LWC offset grades with gloss and print quality comparable to blade coating can now be achieved by the modification of basestock structures and the selection of appropriate calendering conditions after coating. Film-coated paper has a more uniform coating thickness than blade-coated paper, but mostly on the large scale. The lower smoothness of the film-coated paper is not a problem for achieving coated gloss and offset print quality because the higher roughness of film-coated paper is mostly large-scale. With LWC grades, the challenge is to reduce gloss variations whereas with ULWC grades, the challenge is to reduce coating penetration.

LITERATURE

1. STRANGER, K.M., "The Film Press -- A Versatile Coating System", Paper Age (5): 12-16 (1995).

2. KLASS, C.P. and AKESSON, R., "Development of the Metering Size Press: A Historical Perspective", Proceedings of the 1996 TAPPI Metered Size Press Forum, TAPPI Press, pp. 1-12, 1996.

3. SMITH, M.K., "MSP Coating: Recent Developments and New Challenges", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 233-239, 1998.

4. AKESSON, R., "The Future for Metered Size Press Applications", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 261-266, 1998.

5. TRIANTAFILLOPOULOS, N.G. and SMITH, M.K., "Troubleshooting Rheological Problems in Metered Size Press", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 13-35, 1998.

6. ZOU, X., VIDAL, D., "Film Press for Pigment Coating: A Review"

7. CARVALHO, M.S. and SCRIVEN, L.E., "Capillary and Viscoelastic Effects on Elastohydrodynamic Lubrication Flow and Film Splitting in Roller Nip", Proceedings of 1994 International Printing and Graphic Arts Conference", CPPA, pp. 259-266, 1994.

8. ZANG, Y.H., ASPLER, J.S., BOLUK, M.Y. and DE GRACE, J.H., "Direct Measurement of Tensile Stress ("tack") in Thin Ink Films", J. Rheology 35: 3-10 (1991).

9. ROPER III, J. A., BOUSFIELD, D.W., URSCHELER, R. and SALMINEN, P., "Observations and Proposed Mechanisms of Misting on High-speed Metered Size Press Coaters", Proceedings of the 1997 TAPPI Coating Conference, TAPPI Press, pp. 1-14, 1997.

10. ERCAN, S.N. and BOUSFIELD, D.W., "Influence of Process Parameters on Filament Size Distribution", Proceedings of the 1998 Pan-Pacific and International Printing and Graphic Arts Conference, pp. 111-115, 1998.

11. GRON, J., SUNDE, H. and NIKULA, E., "Runnability Aspects in High Speed Film Transfer Coating", Tappi J. 81(2): 157-165 (1998).

12. TREFZ, M. and SEIZ, R., "Investigation of Film Transfer in Metered Size Presses as Related to Base Paper and Coating Colour Properties", Proceeding of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 85-105, 1998.

13. GRON, J., NIKULA, E. and SUNDE, H., "Influence of Coating Composition on Web Release in High Speed Film Transfer Coating", Tappi J. 81(1): 216-225 (1998).

14. McCAIG, R.G. and HARALA, E., "Manufacture of No. 5 Light Weight Coated Paper Using On-line Film Press Coating -- Experience from MacMillan Bloedel PM5", Proceedings of the 83rd CPPA Annual Meeting, pp. A57-A61, 1997.

15. BALZEREIT, B., DRECHSEL, J., BURRI, P., and NAYDEWSKI, C, "Blade versus Metering Size Press", Tappi J. 78(5): 182-188 (1995).

16. MA, H., "Film-coated Grades", Papermaker (6): 39-41(1996).

17. HOSTETLER, R.E., FREUNDSCHUH, U., MECK, M.S.D. and RUCKERT, H., "The Film Press as a Precoater for Paper Board: An Alternative to the Air Knife Coater?", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 211-215, 1998.

18. RYDER, D.J., "Practical Experiences with a Metered Size Press", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 197-204, 1998.

19. GRON, J. and RAUTIAINEN, P., "Coating Solutions for Wood-Containing and Wood-free Paper Grades", Proceedings of the 1999 TAPPI Coating Conference, TAPPI Press, pp.457-467, 1999.

20. NINNESS, B., BOUSFIELD, D.W. and TRIANTAFILLOPOULOS, N.G, "Fluid Mechanics Model of the Film-fed Rolling Nip with a Porous Web", Proceedings of the 1998 TAPPI Coating/Papermakers Conference, TAPPI Press, pp. 515-530, 1998.

21. ROPER III, J.A., MOORE, E., SALMINEN, P and URSCHELER, R., "Optimisation of Formulation Parameters to Reduce Misting and Orange Peel Formation on Metered Film Coater", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 37-55, 1998.

22. VOET, A., "Ink Misting and Its Prevention", American Ink Maker 32(2): 32 (1956).

23. BLAYO, A., FANG, S.W., GANDINI, A. and LE NEST, J.F., "Study of Ink Misting Phenomena" American Ink Maker 76(5): 54-61 (1998).

24. SCRIVEN, L.E., Personal Communication, 1998.

25. GRON, J., ANAS, P. H. and MOLARIUS-MAYRANEN, S., "Improving the Process Runnability and FCO Quality by Optimizing the Coating Colour Composition", Proceedings of the 1997 TAPPI Coating Conference, TAPPI Press, pp. 23-41, 1997.

26. SALMINEN, P., ROPER III, J.A., URSCHELER, R., and CHASE, D., "Optimizing the Coating Formulation to Reduce Misting in High-Speed Film Coating", Proceedings of the 1996 TAPPI Metered Size Press Forum, TAPPI Press, pp. 51-55, 1996.

27. HIRONS, A.G., OAK, G., COGGON, L. and ENGLEY, M., "The Runnability of High Speed MSP when Coating Mechanical Paper with High Solids", Proceedings of the 1996 TAPPI Metered Size Press Forum, TAPPI Press, pp.161-169, 1996.

28. HIRONS, A.G. and COGGON, L., "The Use of Neural Network Modeling to Determine the Degree of Misting Based on Pigment Morphology", Proceedings of the 1998 TAPPI Metered Size Press Forum II, TAPPI Press, pp. 175-185, 1998.

29. ROPER III, J.A., SALMINEN, P., URSCHELER, R. and BOUSFIELD, D.W., "Studies of Orange Peel Formation in High-speed Film Coating", Tappi J. 82(1): 231-238 (1999).

30. GLITTENBERG, D. and HEMMES, J., "Improved Runnability of Film Press Coating", Paper Technology (3): 35-41 (1998).

31. GANE, P.A.C., BURRI, P., SPIELMANN, D., DRECHSEL, J. and REIMERS, O., "Formulation Optimization for Improved Runnability of High Speed Pigmented Coatings on the Metered Size Press", Proceedings of the 1997 TAPPI Coating Conference, TAPPI Press, pp.15-22, 1997.

32. AARNIKOIVU, P., "Creating New Surface-Sized Grades on the Film Press", Paper Technology (3): 29-34 (1998).

33. TREFZ, M., "New Developments in Metering Size Press Technology", Proceedings of the 85th PAPTAC Annual Meeting, Volume B, PAPTAC, pp. 121-125, 1999.

34. AHLROOS, J., ALEXANDERSSON, M. and GRON, J., "Influence of Base Paper Filler Content and Pre-calendering on Metered Film Press Coating -- Part II: Paper and Print Quality", Proceedings of the 1998 TAPPI Coating/Papermakers Conference, TAPPI Press, pp. 915-924, 1998.

35. FREUNDSCHUH, U. and MECK, D., "Now the Future is Captured on Film", Paper and Paper Europe (11): 11-12 (1996).

36. SUONTAUSTA, O., "The Influence of Calendering and Coating Variables on the Smoothness and Gloss of LWC Paper Grades". Proceedings of the International Symposium on Paper Coating Coverage, Training Centre of Finish Forest Industry and the Finish Paper Engineers' Association, Helsinki, Finland, pp. 1-19, 1999.

37. ALLEM, R., ZOU X. and SUONTAUSTA, O., "A Benchmarking Analysis of Film and Blade Coated Papers", to be published.

38. DRAGE, G., VAUGHAN, C., HENDERSON, K., PARSONS, J. and HIRON, T., "The Influence of Freesheet and Groundwood Basepaper Formation on Coated and Printed Paper Properties", Proceedings of the 1999 TAPPI Coating Conference, TAPPI Press, pp. 469-479, 1999.

39. ENGSTROM, G., "Forming and Consolidation of a Coating Layer and Their Effect on Offset Print Mottle", Proceedings of the 1993 Advanced Coating Fundamental Symposium, TAPPI Press, pp. 43-50, 1993.

40. ALLEM, R., "Characterization of Paper Coating by Scanning Electron Microscopy and Image Analysis", J. Pulp Paper Sci. 24(10): 329-336 (1998).

41. GRON, J. and AHLROOS, J., "Influence of Base Paper Filler Content and Pre-calendering on Metered Film Press Coating -- Part I: A Coating Process Study", Proceedings of the 1998 TAPPI Coating/Papermakers Conference, TAPPI Press, pp. 899-914, 1998.

42. ENGSTROM, G. and LAFAYE, J.F., "Precalendering of the Base Paper and Its Effect on Paper-Coating Interaction", Tappi J. 75(8) 117-122 (1992).

43. FERNANDO, R.H., "Rheological Aspects of Misting Mechanism in Roll Applied, Non-Newtonian Paper Coating and Inks", Proceedings of the 1999 TAPPI Advanced Coating Fundamental Symposium, TAPPI Press, pp. 99-110, 1999.

44. LIIMATAINEN, H., HAIKKALA, P. and TUOVINEN, O., "LWC Paper from PGW and TMP Pulps with Gap Former", Paper and Timber 80(6): 434 (1998).

45. AHLROOS, J. and GRON, J. "A Comparison of SGW and TMP as Fibre Raw Materials for Film-coated LWC", Proceedings of the 1999 TAPPI Coating Conference, TAPPI Press, pp. 481-502, 1999.

46. HALLAMAA, T., HEIKKURINEN, A. and FORSSTROM, U., "The Effect of Fibre Properties on LWC Paper Structure and Printability", Proceedings of the International Mechanical Pulping Conference, TAPPI Press, pp. 1-8, 1999.

47. VARSA, P., TOPPILA, M. and ANTTIA, T., "The Quality of High Deinked Pulp Content Magazine Paper Produced with Various Coating Methods", Proceedings of the 1997 TAPPI Coating Conference, TAPPI Press, p. 281, 1997.

Abstract: Film press coating has emerged as a versatile technology for paper surface treatment. With the progress in understanding the film press coating process, particularly the film splitting at the transfer nip, interests have recently been shifting towards improving the quality of film-coated paper. This has resulted in a significant amount of information on this subject being published. In this paper, we review the most recent developments, with a particular focus on the quality of film-coated papers and the potential of using this technology for product development.

Résumé: Le couchage sur presse est devenu une technologie polyvalente pour le traitement en surface du papier. On connaît maintenant mieux ce procédé et particulièrement la séparation de la pellicule à la pince de transfert, et l'attention a récemment davantage porté sur l'amélioration de la qualité du papier enduit d'une sauce de couchage. On a ainsi publié de nombreux documents sur ce sujet. Dans la présente communication, nous passons en revue les plus récents développements, en mettant un accent particulier sur la qualité des papiers couchés et sur la possibilité d'employer cette technologie pour le développement de produits.

Photos

X. ZOU, Paprican Pointe-Claire, QC
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D. VIDAL, Paprican, Pointe-Claire, QC
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FIG. 1. Film splitting at the transfer nip
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Caption: FIG. 1. Film splitting at the transfer nip
FIG. 2. Parameters affecting misting.
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Caption: FIG. 2. Parameters affecting misting.
FIG. 3. Comparison of blade-coated and film-coated papers on coating coverage [19].
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Caption: FIG. 3. Comparison of blade-coated and film-coated pape...
FIG. 4. Surface roughness at different wavelengths [36].
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FIG. 5. Gloss vs. roughness for blade-coated and film-coated papers [36].
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