All recent coated groundwood expansions use metering size press coating due to its lower cost, but coating formulation, notably pigment selection, is critical for quality and runnability
	Production of LWC with MSP Coaters Requires Proper Coating Formulation
	By HAROLD CODY, Senior Editor

Blade coaters are used to produce most of the coated groundwood and coated free-sheet paper in the world. However, in the last two years, the metering size press (MSP) coater has emerged as the dominant choice on new lightweight coated (LWC) groundwood production lines and for rebuilds to produce LWC. Lower capital and operating costs for MSP coaters and the ability to use weaker furnishes provide advantages over traditional blade coating. However, the process also requires different coating formulations and produces a rougher sheet. Pulp & Paper spoke with experts in coating equipment, raw materials, and coating formulation to assess the current status and future direction of technology for these grades.

Among the key areas addressed in this article are the pros and cons of MSP versus blade coating, formulation and raw material requirements, and runnability and coating rheology considerations.

Opinions on many subjects varied widely, which is likely due to the limited experience with this technology on groundwood grades in North America. In addition, it is noted that only limited coating formulation information could be published due to confidentiality.

HISTORY OF MSP. The MSP transfers coating to the sheet by contact with an applicator roll. Thus, the process is also called film or roll coating. The coating is normally metered using a grooved or smooth rod. All new installations coat both sides simultaneously as the sheet passes between two applicator rolls. Some older installations coated one side at a time, but this approach is no longer favored.

The MSP has been around for many years and is used for surface sizing, pigmented sizing, and coating. There are about 90 to 100 MSP units in North America, but many simply apply starch. During the 1980s, improved MSP designs were developed that could apply size at much higher speeds on uncoated free-sheet grades. It wasn't long before papermakers began experimenting with pigmented applications using these systems.

Initially, the technology attempted to use metering rod designs that weren't practical, and it appeared that MSP pigment coating would not be successful at normal coatweights. However, papermakers persisted and were able to overcome the limitations posed by wire-wound or grooved rods through use of larger diameter solid metering rods and higher solids coating colors that allowed coat weights of up to about 6-lb/side (10 g/m2).

Both coated groundwood and coated free-sheet grades use MSP coating. However, blade coating is used to apply the final coat on high-quality coated free-sheet grades where heavy coatweights and very high sheet smoothness and gloss are required. In many cases a pre-coat is applied using an MSP.

M-Real's mill in Kirkniemi, Finland, was the first to use the modern MSP for coated groundwood grades when they produced film-coated offset paper in 1993. As shown in Figure 1, MSP capacity grew modestly in Europe during most of the 1990s, but capacity has taken a major leap forward in the last two years following the startup of two new large, high-speed lines. North American capacity has also increased considerably in the last two years, most notably in late 2001 following the rebuild of an existing newsprint mill.

Figure 1: The recent startup of two major lines has increased metered size press lightweight coated capacity to more than 2 million tons in Western Europe.

MAJOR CAPACITY EXPANSIONS. Capacity based on MSP coating expanded considerably once advances in equipment and coating formulations allowed for operating speeds to approach 5,900 fpm. In turn, this meant that the annual capacity of these lines could reach 400,000 tpy, thus providing attractive economics. UPM Kymmene, formerly Haindl Papier, started up the first of this generation of high-speed lines in 2000 in Augsberg, Germany. Cartiere Burgo installed the second major line in 2001 in Verzuolo, Italy. Perlen installed a third but considerably smaller line at about the same time in Switzerland. Currently, MSP-based capacity accounts for about 35% of European coated groundwood offset capacity.

The technology was adopted more rapidly in Europe than in North America. In late 2001, Madison Paper installed the first system in North America to combine MSP coating with online multi-roll calendering at its mill in Alsip, Ill. Norske Canada's mill in Port Alberni, B.C., was the first to produce film-coated LWC in North America, but it uses two single-nip hot-soft calenders.

However, these two existing installations will soon be joined by three major new expansions. The first will be Bowater's rebuild of an existing machine in Catawba, S.C., which is set to start up in early 2003. In Canada, Gaspesia Paper will rebuild an existing machine at the Chandler, Que., mill, while Kruger Wayagamack will install a new MSP production line at the Trois-Rivieres, Que., mill. Design speed on the Gaspesia project is reported to be 4,400 fpm.

By 2003, about 750,000 tpy of film-coated LWC grades will be produced in North America. This estimate includes tonnage produced by Bowater's Nuway converting plants, which also use the online concept to coat a newsprint-type base stock.

However, blade coating is hardly a dead technology due to several factors, including its inherent ability to produce a smoother sheet combined with the benefits derived from decades of effort to fine tune the process. Blade coaters can achieve high operating efficiencies and very fast speeds, running, for example, at nearly 6,000 fpm. Nevertheless, as shown in Figure 2, MSP coating is accounting for a growing share of capacity in North America and Europe.

Figure 2: European mills adopted film coating more rapidly, but capacity in North America will expand during the next three years, reaching almost 20% of total coated groundwood capacity.

ROLL VS. BLADE COATING. Mills contemplating the use of MSP technology have to consider several pros and cons compared to blade coating. While many mills have successfully produced film-coated papers, there can be a substantial learning curve involved in operating the coater to avoid patterning or misting, in obtaining adequate coverage on rough and poorly designed base sheets, and in efficiently running a complicated 8-roll or 10-roll calender on line. In addition, the process places different demands on the base sheet and coating color.

The MSP coater, in conjunction with online calendering, is expected to take an increasing share of the LWC market because it is fundamentally cheaper to run and to build than a blade-coated line. "You only have one dryer section and one coating station, and the machine is smaller, thus requiring less space," notes Timo Kiiha, business manager, coaters and calenders, at Metso.

In North America, capital-strapped paper companies continue to run a lot of outdated equipment. "About two-thirds of the 47 or 48 coated groundwood lines in North America have capacity less than 400 tpd," notes George Ionides, president of Temanex Consulting. These older coated mills are vulnerable not only to offshore imports, but to competition from local mills that install MSP technology.

What will happen to all of the mills currently operating blade coaters? The ones that survive still have an advantage in that they can make rotogravure grades, something MSP mills have yet to accomplish. Also, for many higher-cost newsprint and groundwood mills, the lower-cost threshold of MSP coating makes it an attractive upgrade for competitive purposes, and there remain quite a few such larger, older machines operating in North America.

Operating costs are lower because online operation requires fewer people and because weaker, and thus cheaper, furnish can be used in comparison to what is required in blade coating. "The favorable economics possible due to less bleached kraft means MSP paper can be sold at a lower price while maintaining the net $/hour of machine time, thus offering a better return on assets employed," explains Charles P. Klass, president, Klass Associates. For example, Madison Paper uses deinked pulp-a low-cost furnish.

Beyond cost, there are other advantages. "One of the key considerations in MSP coating is that it can provide the capability to run lower coatweights than a blade coater," notes Dennis Philp, technical service director for RohmNova. This might be a key advantage in producing lighter-weight grades. Ionides concurs with this, noting a recent market study he authored on MSP coating that concluded that "the higher bulk of an MSP sheet provides added stiffness, which is a key advantage as basis weights go lower."

ROTOGRAVURE GRADES REMAIN ELUSIVE. In North America, about 80% of coated groundwood is printed offset while the remaining 20% is printed rotogravure. Can MSP coating produce an acceptable quality coated groundwood sheet for rotogravure printing? This question produced several answers ranging from "no" to "it is possible." The experts did agree that no one to date has been able to successfully produce a competitive coated groundwood sheet for the rotogravure market. Many have tried and all failed.

The two major equipment suppliers agreed on this point as well. "The inherent sheet roughness produced using contour coating simply makes it difficult to produce rotogravure grades," notes Dr. Michael Trefz, vice president of coating technology for Voith. Metso's Kiiha concurs, noting that "mills in Europe have tried for five years, but not one has produced an acceptable sheet."

The MSP process, since it is a contour coating, produces a rougher sheet than blade coating, which levels the coating with a blade. For example, MSP offset grades typically have a smoothness of about 1.5 to 1.7 microns Parker Print Surf (PPS), compared to an average PPS of 1.2 to 1.3 microns for blade-coated grades.

However, smoothness is not the only quality that is important in offset. "The sheet may be rougher, but the printability and quality overall are comparable or close enough to blade coating that it doesn't make a difference in Europe," says Trefz.

The smoothness limitation is a key factor for mills already equipped with traditional blade coater/off-machine supercalender configurations. While low-cost MSP mills will capture their share of the offset market, blade-coated mills are likely to continue to dominate the rotogravure market.

However, other experts expressed the view that it is possible a rotogravure sheet could be developed with additional research. In fact, promising trials are reportedly underway in Europe. As one pigment supplier noted, "It might be possible to make the smooth sheet required using exotic pigments, but I'm not sure that this makes sense. Low cost is a key rationale for the MSP, and if you add plastic pigments or other premium products, it raises costs."

The new installations in North America will employ calendering before the coater, and several people observed that this could help produce a rotogravure grade. However, Metso's Kiiha noted that "all the new machines in Europe pre-calender the sheet with a hard, steel roll calender prior to coating. This is mainly done for caliper control." Current North American installations do not calender prior to coating.

The project to convert the Gaspesia mill to coated paper production will take the technology and sheet quality to a new level since it will incorporate two MSP coaters supplied by Metso and a 10-roll calender. Reportedly, this double-coated grade is not targeted at the LWC market. Instead, the goal is to make a No. 4 or even No. 3 quality offset sheet using what is believed to be a high-quality groundwood pulp. Some experts speculated that this approach might be another way to address the smoothness needed for rotogravure grades.

BASE SHEET REQUIREMENTS. Base sheet properties have a major influence on MSP coating and are key in successfully applying the technology. Several suppliers observed that they have run trials using base stocks that range from good to terrible quality. Designing the coating for each specific base stock is key to success.

For example, sheet porosity has a major impact on water retention in the coating. Sizing of the base sheet can be an important aspect to control base sheet absorption according to some. "Off-machine blade-coated sheets are sized, while on-machine grades are normally not sized," notes Allan Brown, technical services manager for J.M. Huber. Other experts pointed out how some of the major coated paper producers long ago concluded that sizing had no effect on coating and sheet quality. Information on base sheet makeup and properties is mainly the realm of the paper producers.

RAW MATERIALS AND FORMULATION. The rheology of an MSP coating is different than a blade coating. For example, leveling is very important in order for the coating to flow and even out the surface. However, it cannot be too fluid, or misting will occur as the sheet exits the applicator nip. Solids content is pushed as high as possible to minimize coating color misting. Most MSP coating solids are in the low 60% range, with mills in Europe running slightly higher. This represents a lower solids level than is run on most blade coaters.

The specific pigments and binders used vary considerably from mill to mill. In general, a mixture of calcium carbonate and clay is used for MSP coatings worldwide. In North America, more clay, including 100% clay formulations, has been used. However, both ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) are currently in use, according to various suppliers. Opinions on carbonate usage levels vary, with 20 or 30 parts carbonate most commonly reported. Work by Omya has studied usage at 60 parts carbonate and 40 parts clay. Premium pigment use is reported to be minimal.

Brazilian clays are used quite a bit because of their narrow particle size distribution, which provides optical advantages. "Kaolin, including both hydrous and calcined, GCC, and PCC can be used, but speed is the deciding factor on which ones and in what quantities," explains Paul Meizanis, Midwestern technical service manager for Imerys. "Typically, a higher level of delaminated clay might be used in MSP coatings, while a latex-starch binder system may work better than a latex-CMC [carboxymethylcellulose] combination," describes Klass.

In addition to the base sheet, the difference between how coating is applied in the MSP versus a blade coater also impacts coating color formulation. For example, water retention in the coating is a very important parameter owing to the greater pressure present in the longer nip of the MSP.

In most MSP applications, both sides of the sheet are coated simultaneously. This is a key factor driving the requirements of the base sheet and the coating itself. "Coating both sides of the sheet at the same time is a challenge," explains Bruce Nelson, regional technical service manager at Omya. "This makes base sheet factors like porosity critical, as well as coating color parameters such as water retention. You need to balance out these factors and adjust water retention to where it isn't so high that you get roll buildup or so low as to cause misting."

Certain aspects of formulating coatings for MSP are similar to blade coating. "Some benefits in terms of coating formulation strategy, such as the use of select high-aspect ratio pigments for gloss development, carry over from blade to MSP coating," says Jim Allen, senior technical service representative with Specialty Minerals. "Since gloss is harder to develop for an MSP application relative to a blade application, then particle shape, size, and distribution become especially important factors in pigment selection for MSP grades."

But there are major differences as well. "The MSP is more forgiving of high shear rheology than a blade coater," explains Engelhard's Midwestern technical manager Dave Lewis. Huber's Brown agrees, noting that "the visco-elasticity of the coating is very important in an MSP application compared to the high shear rheology critical for blade coaters."

The coating binder is a key component in any formulation. Mills can use "off the shelf" binders that are essentially the same used in blade coating. However, binders designed specifically for MSP coating are available and are used.

"MSP coatings in North America use latex and starch binders together, and the latex has to provide enough stability to survive in the nip and avoid having the coating plate out on the rolls," notes Femi Kotoye, technical service manager for Dow Chemical. "The coating also has to be designed so that it won't mist or produce orange peel. As we learn to design coatings to deal with these issues, the trend is toward higher coating solids, because those provide better properties."

Steve Abell, director of research at Penford Products, observes that "starch use varies widely, but in general, as solids increase, starch decreases."

The MSP is normally used in conjunction with online calenders that run at very high temperatures, which has a major influence on binder choice. "Due to the high temperatures generated in hot-soft or multi-roll calenders, the binder must exhibit stability at temperatures of up to 400æ F," according to Nick Triantafillopoulos, director of polymer innovation for Omnova.

Operating speed has a major impact on the design of the coating color. At lower speeds of 3,000 to 4,000 fpm, the process is more forgiving and more tolerant of rheology and raw materials. This changes considerably at the 5,000 fpm or even higher speeds run in Europe. At such speeds, the design of the formulation and its components is critical. "It can take two years of research and development work to develop a coating that runs at these speeds on a specific base stock," notes Voith's Trefz.

Many others concurred with this observation. Graham O'Neill, manager of technology concepts and market communication for Imerys Paper Europe, explains that "at the very highest speeds, the options to overcome rod spitting and misting are a limiting factor, and this may restrict the specific pigments that can be used." Most North American installations are rebuilds and run considerably below the extremely high speeds attained on the new lines in Europe.

MARKET ACCEPTANCE. As with any new paper grade, it can take time for the market to fully embrace something that is different. In Europe, the MSP sheets are widely accepted. However, in North America, some printers still perceive a quality difference.

There are disadvantages when switching from a blade-coated clay sheet to a clay/carbonate MSP sheet, such as having to adjust printing press settings. However, advantages include the fact that the even coating layer produced in the MSP results in more consistent ink-drying rates, which can reduce the print-mottling effect caused by different ink density.

OPERATING EFFICIENCY. Most new MSP lines have all operations online due to the lower cost of this approach, but this does present some disadvantages. For example, having both coating and calendering on-machine means that a problem in either will shut down the paper machine. This is a key reason that Burgo chose off-machine calendering for its Verzuolo mill in Italy. It provides the capability to run faster on the paper machine and provides better quality control.

"Operating efficiency must be considered when designing a new coated mechanical production line," explains Trefz. "For the most efficient operation, locating the calender offline must be considered."

FUTURE TRENDS. Improved knowledge of how to overcome misting, orange peel, and spitting problems will lead to better productivity in MSP applications. As more and more mills gain experience running clay and carbonate, it seems likely that carbonate use will increase in order to increase brightness, just as it has in blade-coated LWC grades. For MSP grades, it is expected that a blend of clay and carbonate will be used in future coating formulations.

The use of double coating offers promise for developing another niche of the coated paper market. This is similar to what occurred with high-quality SC-A grades, where their substantially lower cost, coupled with print quality near that of LWC, led to major market gains at the expense of LWC in areas such as the Sunday insert business. Similarly, double-coated MSP grades could be positioned as a lower cost alternative to existing No. 3 or No. 4 coated grades.