Coating for survival

Tough times mean coating has to be that little bit better to justify a full order book

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It has not been the best of years for coated paper producers so far and as a general rule at present, the lighter your coatweight, the slimmer your order books. Times like these bring out the worst, or best in paper buyers, depending on your standpoint. They get picky and paper quality is scrutinized more closely than some might find comfortable. But expectations are increasing, whatever the economic climate.

Printers, for example, want more 'pop' in the photos and graphics produced on their high-speed 10 color presses, and it is here that the combination of retention/drainage and surface enhancement technologies can make the difference. In addition to nanoparticle based retention/drainage systems to improve base sheet uniformity, many coated paper producers are boosting sheet performance with 'nano' surface treatments. These include latex formulations enhanced with zirconium-based crosslinkers, and engineered pigments. Much of the innovation at the size press revolves around nanocombinations applied there, such as surface sizing and specialty coating additives to improve sheet properties.

Eka Chemicals has patented such a nano surface treatment system, in which zirconium crosslinkers have the ability to react with the starch, surface size and fibers. This allows the size press solution to remain on the surface and not dive into the sheet where it is of no use. This ability makes it easier to use lower-grade starches, which are not good film formers, without sacrificing print quality and low dusting.

Shape-engineered kaolins

The eye is very sensitive to imperfections especially when looking at color images. The ability to produce a very flat surface to lay the image on the sheet is critical to meet current demands. Recognizing that minerals make a significant contribution to these coated paper value properties, pigment producers face the challenge to develop new pigment platforms that can further enhance the paper surface.

Meeting these new pigment challenges is tough. It means looking at mineral deposits and mining processes in fundamentally different ways. It also requires taking a new look at how pigments are applied and used in coating formulations to maximize their potential contribution. Effective total delivered cost is also a critical development boundary.

IMERYS says R&D efforts in minerals science have resulted in "shape-engineered" kaolins, a novel pigment design platform that offers a new paper coating formulation tool to change the contour of the coating basestock for a smoother surface with exceptional print properties. These new shape-engineered kaolins are said to deliver a unique combination of surface gloss and superior print properties - ideal for lightweight publication grades and coated board.

The combination of a rotogravure latex from Omnova and a new binder coated hollow sphere pigment from Rohm and Haas is claimed to have achieved significant improvements in sheet gloss while delivering excellent Helio missing dot

Shape-engineered kaolins are the first to combine the coverage and print benefits derived from European clays, while maintaining the glossing characteristics of Georgia clays. This patent-pending technology has been made possible through the development of radically different pigment characterization methods and improved mining and processing techniques. It is also the entry point for clayto-paper products into mainline nanotechnology. Shape-engineered kaolins are a good example of the previously untapped potential of minerals, which is now readily available for the upgrade of existing and new paper and paperboard grades - or to reduce overall cost without sacrificing quality.

One way to differentiate product in a difficult market for publication papers is to deliver a higher level of sheet gloss than the competition. With a combination of a rotogravure latex from Omnova and a new binder coated hollow sphere pigment from Rohm and Haas, significant improvements have been reported in sheet gloss along with excellent Helio missing dot.

For coated woodfrees, Omnova has introduced a line of high performance latices. While the particular attributes of these binders are most effectively demonstrated in a customer formulation, one general feature of this product line is improved strength. Other advantages of these new binders are engineered into the product according to customer needs. Performance latices consistently provide higher binding efficiency (reduced binder level possible), reduced print mottle and increased blister resistance, as well as excellent runnability, according to Omnova. In practice, binder levels have been reduced as much as 15% with these latices, and blister temperatures as much as 45°C higher than the control product have been achieved.

For the coated solid bleached sulfate (SBS) producer, a cost-effective way to optimize a coating formulation for cost and performance is to use styrene-butadiene latices in the basecoat for strength and styrene acrylic latices in the topcoat for improved printability and decreased mottle. Rohm and Haas has added a new styrene acrylic latex, Rhoplex P-332, to its acrylic product line. It is claimed to improve printability and decrease mottle; it also offers increased stiffness and improvements in coating gloss.

Coating technology

Brand new coating technologies may be a rarity but variations on a theme can sometimes have just as much impact. So while an OptiSizer might be familiar, who has ever seen a vertical one? Nobody, is the answer, unless they have visited M-Real Kyro in Finland in the last couple of months.

Metso Paper completed a rebuild of the mill's coated and uncoated wallpaper base PM in March and installed an OptiSizer metering size press, which is its first delivery of the technology in a vertical setup. This idea behind this format is to increase the flexibility of the coating section's overall layout.

The rebuild will improve the quality of the base paper, especially its dimensional stability and base formation, and the surface properties of the coated wallpaper.

Metso says there is a clear trend towards all online concepts. The best example must be the UPM-Kymmene (Haindl) Augsburg PM 3, where all PM-line machines are online. A corresponding line is the recently started Burgo Verzuolo PM 9. The only exception is that at Burgo, the multinip calendering happens offline.

The first all-online concept for fine papers was M-real Kangas's PM 4 in Finland.

On-line coating is challenging but space savings, lower investment costs/tonne, lower operating costs and higher production efficiency are all very persuasive.

Updated pre-metering

It seems doubtful that printers will ever make their own paper but one particular technology for papermakers certainly owes its origins to the printroom. With updated features for Billblade and Twin-HSM coaters, BTG has expanded its gravure roll pre-metering technology. This "flexographic" principle is being used in BTG's FlexiFlexo coaters and now it can be adapted to Billblade and Twin-HSM coaters, too.

Benefits of the gravure roll system include that fact that it is easily controllable over a wide range of coatweights. At a set coating color solids level, the gravure roll speed determines the coatweight.

It also has high solids capabilities. Coatweights of 5-6 g/m² can be applied at solids content up to 65%.

Coating color distribution and surface quality are said to be excellent and tests report color distribution to be extremely good. The gravure roll is said to provide an excellent surface quality.

As hinted before, the gravure roll system is essentially an adaptation of the rotogravure printing technique. The gravure roll surface consists of small depressions or cells. The cells are completely filled with fluid when the gravure roll passes the pond. Excess fluid is doctored off using a DB doctorblade that has a metering tip of a soft polymer material. The coating fluid is transferred to the backing roll in the roll nip.

The pre-metering with the gravure roll system is 100% volumetric. The wet amount pre-metered onto the backing roll surface is related to the volume of the cell and the speed of the gravure roll. Once the gravure pattern has been chosen, the applied wet amount is directly related to the speed of the gravure roll.

BTG has installed a gravure roll system on a Billblade differential coater. In this case the Billblade is used for applying CF coating with the blade and starch on the roll side. Following installation of the gravure roll system, it is now possible to apply starch from high to low solids without pattern, resulting in improved quality from high to low pickups.

BTG's gravure roll pre-metering technology adapted to Billblade coaters

Another recent installation shows that the twin gravure reportedly makes possible the application of low coatweights at a high solids content as well as over a very wide range of coatweights.

Packaging strength

Sizing and coating have in many ways proceeded hand in hand in terms of technology. But a new departure from GL&V sees a coating technology, which traditionally finds itself further downstream, applied way up in the wet end. The aim is to improve strength in packaging grades.

The Hydra-Sizer uses curtain-coating technology to impregnate paper or board with additives, such as starch, on the forming section of the paper machine. Varying starch slurry solids controls the application rate. The applicator and its catwalk assembly can be positioned along the wire to change the consistency at the point of application. The retention mechanism for the process is physical entrapment, so applying the slurry at low sheet consistencies of 2-3% disperses the starch through the thickness of the sheet. Application at higher sheet consistencies, typically 6-8%, applies the starch generally to the sheet surface.

This technology can be used in place of other application methods such as wet end additives in the headbox, sprays on the fourdrinier, or a size press in the dryer section. GL&V claims Hydra-Sizer can achieve significant strength improvements with no loss in drying capacity, making it an ideal retrofit for an existing machine. The first installation, on a corrugating medium and linerboard machine in Mexico, increased strength properties, allowing the mill to reduce refining and speed up the machine. A second installation, recently started up at Solvay Paperboard in New York, on a two-ply recycled linerboard machine, produced similar results. This new technology has produced higher strength, reduced drying and has allowed this machine to speed up by an average of 3%. The mill has ordered Hydra-Sizers for two other machines at this facility.

Installed costs are said to be considerably lower than for a size press, and the unmodified corn or wheat starch used is less expensive than modified and cationic starches typically added in the headbox. The starch is applied on the fourdrinier table and cooks in the dryer section.

Speed records

And finally, no coating report would be complete without reference to speed. One of the notable achievements during the past year in this respect was at UPM-Kymmene's Kymi Paper mill in Kuusankoski, Finland. For its UPM Finesse coated woodfree fine paper grade, the mill chose Metso Paper's new generation OptiCoater with two OptiCoat Jet coating stations. In December last year, the OptiCoater was run on a trial basis at 2,101 m/min and the paper produced met the mill's quality targets. The unwinding and flying splicing of parent reels with diameters of about 4,000 mm at high speeds are a challenge. At the moment the mill can splice at approximately 1,850 m/min without problems, which of course requires a lot of care, the right splicing tapes and splicing style, as well as precise unwind adjustments.

Metso claims its new OptiSpray coating technology, which hit the headlines last year, has been run in trials at up to 2,500 m/min. The first installation is the subject of "final negotiations". Now that will be a story.