Keeping the weight off: lower basis weight and lower coat weight can mean lower costs of papermakers. The adoption of careful coating strategies and technologies such as spray coating can help paper mills maintain paper quality while maximizing profits and minimizing costs.
Charles Klass of Klass Associates, Radnor, Pennsylvania, USA, a member of the Solutions! Editorial Board, notes three keys to making ULWC:
* Minimizing print through
* Good coating coverage and print gloss at minimum coat weight
* Use of the minimum practical amount of expensive bleached kraft pulp in the base stock furnish to make ULWC economically.
The need to minimize stress on a base sheet makes metered size press coating (MSP) preferable to blade coating in making ULWC according to Klass. MSP coating provides random pigment orientation that helps to improve opacity at a cost of reduced sheet gloss.
"Higher aspect ratio pigments--platey day or engineered PCC--can help improve opacity and print gloss," Klass--said. "Most ULWC is printed via heatset offset. This makes internal bond strength critical and limits the amount of filler in the base stock. The choice of filler can have a dramatic effect on reducing print through. Certain PCC pigments perform well in this application. The most promising filler for reducing print through is a recently developed high brightness modified zeolite.
SAME COATING EQUIPMENT
Production of ULWC grades uses the same coating equipment as conventionally coated paper. ULWC has different requirements, according to Femi Kotoye, technical service and development leader for Dow Chemical Co., Midland, Michigan, USA. "ULWC papers require more bulk for the same weight of conventional papers. They use more kraft fibers with less filler. Achieving opacity is therefore more difficult with ULWC."
"The coating technology for ULWC requires low solids since the coat weights tend to be low," continued Kotoye. "In North America, starch is usually the thickener of choice. In recent time, structured pigments, platey clays (Capita), narrow PS carbonates, and PCC have had common use for this grade. Titanium dioxide is used to provide opacity and brightness. Almost all producers use hollow pigments to achieve bulk, opacity, and gloss."
As basis weight decreases, the coat weight applied becomes more limited according to Roger Wygant, research fellow at Imerys. "Therefore, higher shape factor pigments are necessary for improved coverage, gloss, and print-ability. This means that clay based coating formulations are better than carbonate based ones for these types of grades. Ultra lightweight fine papers may become a market niche. They would require a high brightness, fine, and platey pigment."
SHIFTS IN COATING TECHNOLOGY
Wygant added that lower coat weights can dictate lower coating solids. This then dictates the formulation and application technology used. In addition to the effect on pigment choice, this may mean future shifts in coater technology to improve efficiency. "Low impact coater technology to improve contouring and coverage may be necessary," he said. "Lower fillers may be required for higher base sheet strength. Functional fillers for improved coating holdout, base sheet strength, and optics will be necessary to improve efficiency further. Nanotechnology pigments could find use to alter the properties for an optimized combination of optics, stiffness, coating holdout, and base sheet strength. Rheological modification through mineral engineering could optimize coverage and operational efficiency beyond the polymer technologies used today."
Douglas Donigian, technical manager, paper coating and printing research, for Specialty Minerals Inc., Bethlehem, Pennsylvania, USA, sees the ULWC challenge as filler synergy and coating coverage. "How can properties like stiffness, strength, opacity, gloss, and smoothness be maintained when the base stock becomes lighter? Synergy refers to processes that allow higher filler loading while maintaining opacity and smoothness with no loss in sheet or web strength. This is likely to require deliberate operations well in advance of the headbox. Coverage refers to surface property maintenance in the lace of lighter weight base stocks with potentially more coarse, long fiber. We have found that PCC coating pigments with narrow particle size distributions and high aspect ratios give better coating coverage."
Engineering a high quality ULWC sheet involves a carefully tested and proven formulation of performance pigments, binders, and additives according to Robin McCann, marketing director, Rohm-Nova LLC. "Binders and additives that build structure in a coating and low density pigments can significantly reduce the weight of a finished product. The application of hollow sphere pigments in these grades has given paper producers a window of operation that delivers excellent optical properties at reduced basis weights."
Michael Altemeier, technical service manager at Omya, noted that in Europe the ULWC market (at approximately 7%) has less significance than in North America (at approximately 20%). "In most cases, the use of jet coaters to replace the applicator roll has resulted in higher efficiency and quality improvements," he said. "A trend seen in LWC and ULWC papers is the use of MSP and advanced multi nip calendering technology. MSP allows better coverage at lower grammages."
"As long as the printing process remains primarily heatset offset, the demands on the surface with regard to roughness remain on the same level," added Michael Trefz, Voith Paper GmbH & Co. "A more or less sufficient coverage of a coated mechanical sheet is achieved at 7 g/[m.sup.2] per side. For such a coat weight, the metered size press is the most successful coating applicator with a sufficiently large potential for machine speed and surface quality. Engineered pigments promise about 1% more opacity, but they are expensive. For commodity grades, the proven mix of calcium carbonate and clay in many variations will likely be most popular."
"Upstream" papermaking processes often influence coating formulations according to Paul Fish, product manager, pulp and paper, with Eka Chemicals, Akzo Nobel. Properties such as opacity sometimes require consideration before coating. "To improve coating, eliminating pinholes from the base sheet with improved information is important," he said. "Polyacrylamides contribute to this problem. Their use can be minimum on high-speed LWC machines with use of a good starch with a silica nanoparticle system. The result is less use of expensive opacifying fillers such as TiO2. Remembering that coating formulation changes often start at the wet end is important.
"The optimized wet end--particularly on lightweight, woodfree grades--is one where the best balance between polyacrylamide, starch and silica nanoparticle technology has been created to generate a uniform sheet with optimum formation and controlled early drainage to provide a sheet with optimum porosity characteristics and free from pinholes," Fish concluded.
SPRAY COATING SHOWS PROMISE
Spray coating is a low impact technology that may have a big impact on coated paper and board production. Early experiences suggest a suite of potential advantages. The most significant is the improvement of low coat weight grades. Some limitations exist (see box on page 25).
Spray coating is an adaptation of spray painting for use in paper and paperboard coating noted Klass. It provides non-contact, low impact coating for a variety of base stocks including low strength base stocks such as newsprint. It allows simultaneous two-side coating in a single coating station. Staggered fan nozzles provide complete coverage in two-side coating. He noted that spray coaters are compact and can be installed in existing machines.
"To obtain uniform coverage, the spray droplets must coalesce into a uniform coating film," said Klass. "The coating is not subjected to the same high shear forces encountered in blade coating. Binders developed for blade coating may not provide the rheology needed to optimize spray coating."
Spray coating provides random pigment particle orientation and a coating layer that follows the contour of the sheet surface according to Klass. "This may make spray coating the technology of choice for prime coating linerboard, SUS, and recycled boxboard by reducing the requirement for large amounts of titanium dioxide in top coating."
One driving force behind non-contact coating methods such as spray coating is their excellent machine runnability according to Time Kiiha, business manager, coaters and reels, for Metso Paper, Appleton, Wisconsin, USA. "Spray coating is optimum because the coating is not forced into the base sheet. It remains on the surface of the base sheet," he explained.
Petri Paloviita, manager, coating heads, Metso Paper, Jarvenpaa, Finland, added that spray coating--a non-contact coating method--provides better machine efficiency and helps reduce production costs. "Improved runnability and good coating coverage can produce significant savings in expensive raw materials such as kraft pulp and coating pigments. Spray coating is suitable for most standard paper grades, including MFC, LWC, coated news, and double coated WF. Various rebuilt layouts can easily include it."
ATOMIZING COATING COLOR
Spray coating atomizes the coating color into small, uniform droplets and applies them to the surface of the sheet. They must retain mobility upon impact with the paper surface to cover the surface uniformly through secondary spreading and leveling. This requires a different coating formulation according to Nick Triantafillopoulos, director, new technologies, RohmNova LLC. "Low solids coatings are easier to spray. They form small droplets and wear the nozzle less. For paper quality and coverage, however, the solids level should be as high as possible. Because of the requirement for low viscosity, no water retention thickeners are necessary. Low-viscosity starch cobinders are only practical at small amounts."
"Besides coating rheology, base-sheet properties are critical for spray coating. Both small- and large-scale formation and surface evenness are necessary to allow sufficient and uniform spreading upon droplet impact," Triantafillopoulos concluded.
Spray coating technology has the potential to improve efficiency and reduce capital investment according to Tony Lyons, director technology platform, Imerys. "Increased efficiency is always welcome, but this technology can also facilitate the use of higher filler contain since a weaker base sheet will not affect its efficiency. The spray coater will apply a contour coat, and the coating holdout will improve compared with the higher impact coating methods. This means that dramatic improvements in optics, coverage, printability, and gloss are possible compared with blade coating for example. All these benefits are possible using traditional clay and carbonate pigments," he said.
Triantafillopoulos noted that spray coating requires a lower initial capital investment, lead time, and installation period. It also has a shorter startup period than other coating technologies. "The total savings achieved with spray coating are about EUR 50 per metric ton of paper compared with metered size press coating and up to EUR 100 per metric ton compared with blade coating," he said. "In comparison with metered size press coating, the advantage of spraying is smaller because of the ash content--less than 15%--requirement with spray. This influences the maximum filler and DIP usage at least when using coating solids less than 55 % with spray."
"Other sources adding to the production cost will be higher for film coating than spray," he added. "Overall, the relatively low base paper strength requirement of spray coating allows mills to upgrade paper products for printing and communication, for instance using newsprint furnishes with only small amounts of kraft pulp content." While blade coating offers a slightly better paper quality, Triantafillopoulos noted that cost savings can make spray coating an attractive alternative for rebuilds and retrofits.
At MD Lang Papier's Albbruck Mill in Germany, a Metso OptiSpray[R] online spray coating unit has use on a prototype basis. The mill installed the unit on an older machine using MSP coating to make groundwood LWC grades. It has been in operation since January 2003. The paper machine has variable widths an produces 80,000 metric tons/year. It makes coated offset printing papers as sheet and reel products.
"With the new process, the coating color is atomized under high pressure and applied simultaneously to both sides of the web. The web is not in contact with any part of the machine," said Triantafillopoulos.
"The machine was becoming too small to make those grades competitively. By using spray coating, the operation has improved their quality level so that they can compete with newer machines," added Kiiha.
Trefz suggested that the future for spray coating has limitations due to the comparatively large droplet size. "With such large droplets and more or less random distribution on the paper surface, a uniform coating layer of the required thickness probably is not possible. Our research showed that color for spray coating needs a significant reduction in solids content. This has a negative influence on the energy consumption and will make adding the additional required drying capacity in rebuild scenario difficult."
Lyons suggested potential for many future applications although the technology is still in the early stages of its development. "The contouring and coating holdout characteristics may make it ideal for coated recycled and coated unbleached kraft liner packaging grades," he said. "it may be replacement for air knife coating used in coating board today. It may make a good pre-coating technology just as MSP coaters are used This may be a facilitating technology for the coating of SC or highly filled papers. This might provide the same or better quality as double or triple coated grades."
Donigian called spray coating a large advance toward low impact surface improvement. "Retrofitting to upgrade the quality of low cost h base stocks is one natural application," he said. "Drawbacks at present include lower coating solids and more coated surface roughness relative to nip operations. This would tend to restrict spray coating to low cost grades or precoats possibly. Spray coating is likely here to stay. As suppliers of coating ingredients develop specialized products for them, sprays will become appropriate for more demanding applications."
IN THIS ARTICLE, YOU WILL LEARN:
* The requirements for making ULWC grades
* Coating formulations needed for ULWC grades
* The pros and cons of spray coating
* Why spray coating can be used in upgrades and rebuilds
* "Kruger builds for the future at Wayagamack," Charles Donnelly Solutions!, May 2003, p. 26
* "Coating 2003: A research review," Solutions!, May 2003, p. 34.
* For books and CD-ROM materials about coating and coated papers, visit www.tappi.org and click on "TAPPI Press "
RELATED ARTICLE: Spray coating pros and cons.
The experts cited in the accompanying article provided the following main benefits and drawbacks of spray coating:
* The low-pressure application allows for improved and uniform coverage at low coat weights.
* Since this coating method puts less stress on the paper it has use for weak basestock, low basis weight paper, highly filled paper, or any paper with low internal strength.
* Paper can undergo coating on both sides at one coating head to save space.
* Spray coating could be a good replacement for air knife coatings or rod particularly for low coat weight applications.
*Sheet pressure pulse during coating does not occur and coating holdout is better.
* Elimination of the nip should remove certain base stock limitations and reduce the chance the web breakage.
* Non-impact application may reduce speed limitations.
* Spray coating usually requires reformulation.
* Because it is a contour coating, spray coating makes it difficult to achieve high smoothness.
* Very high solids coating has not been possible.
* Low coating solids may lead to binder migration.
* Spray coating is not suitable for rotogravure printed grades without further developments in coating materials.
* Abrasive pigments will prematurely wear spray nozzles.
* Development is still in its early stages--only one prototype machine is in commercial operation.
Janice Bottiglieri Is a senior editor of the Solutions! and editor of TAPPPI JOURNAL. Contact her at email@example.com.
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|Publication:||Solutions - for People, Processes and Paper|
|Date:||Jul 1, 2003|
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