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The tissue segment exhibits several interesting characteristics that makes it unlike other major pulp, paper, and paperboard grades. One primary difference is that it tends to be less cyclical than other commodity products such as market pulp, board, and newsprint. It is also one of the largest paper markets in which a significant portion of production is converted and sold directly to consumers by the manufacturer.
As a result, the tissue segment in North America has seen a steady installation of new machines. U.S. tissue capacity growth is forecast to grow at an annual average rate of 1.6% through 2002 to reach nearly 7.5 million tons, according to the American Paper & Forest Assn. While this growth is slightly below the ten year average annual rate of 2.1%, it is more than double the expected growth rate of just 0.7% for all paper and paperboard grades.
Market shares. Tissue has been a consolidated segment of the industry for many years. Historically, the top five North American producers have been responsible for over 70% of total manufacturing capacity, though the names of the companies have changed due to mergers and acquisitions. Currently, the top five producers account for over 79% of total capacity (Figure 1).
FIGURE 1: Five companies account for 79% of total North American tissue capacity.
Tissue markets are divided into two major sectors —at home (consumer) and away-from-home (commercial and industrial)—and are further composed of several key grades (toilet, towel, facial, and napkin grades). The specific products needs are dependent on the final end use of the product as illustrated in Figure 2.
FIGURE 2 (below): Product needs are dependent on the end use.
FIGURE 3: Familiar names dominate consumer and U.S. tissue sales.
The consumer tissue markets (based on total U.S. sales) are dominated by Procter & Gamble Co. and Kimberly-Clark Corp. (K-C) (Figure 3). Private label brands—products sold primarily as store brands—also account for a significant market share of the consumer tissue arena. The away-from-home segment is dominated by three major players—Fort James Corp., K-C, and Georgia-Pacific Corp. (G-P)—which accounted for 73% of total U.S. sales in 1998.
Wider and faster. As capacity continues to grow, the number of smaller machines continue to dwindle as they are being replaced by wider and faster machines. Traditionally, there have been four available trim widths for tissue machines—102 in., 204 in., 135 in. and 270 in.—to match the widths of the converting lines that convert parent rolls to finished products. Fort James has been a leader in installing the 270 in. former with nearly a dozen of its machines at this size.
However, the first reported installation of a 300+ in. tissue machine is currently underway. Valmet Corp. is installing a 306 in. trim Crescent former at G-P’s Port Hudson mill in Zachary, La. With a design speed of 6,500 fpm, the new machine requires what will be the world’s largest Yankee dryer ever cast. The Yankee will have a diameter of 18 ft. and a shell face of 328 in. and will weigh a massive 188 tons. The new machine is scheduled to begin production later this year.
Tissue machines have been recognized as the fastest paper machines with design speeds typically in the range of 6,000 fpm to 7,000 fpm. However, installations of new machines for newsprint and groundwood papers have started to challenge the upper speed limits of paper manufacturing. In 1999, two newsprint machines and an SC papers machine began operation in Europe, each with a design speed of 5,900 fpm. And the new lightweight coated groundwood papers machine being installed by Cartiere Burgo SpA in Verzuolo, Italy has a design speed of over 6,500 fpm, matching the speed of the new G-P tissue machine.
The continued growth of tissue capacity has been sustained by development of manufacturing technologies to meet the product quality needs. Machine manufacturers are continually working on technology developments to enhance machine performance and product quality. Improvements and advancements are enabling producers to manufacture softer, more absorbent products for both the consumer and commercial and industrial markets.
Tissue forming. One major issue that is driving tissue forming technology is the fiber choice—virgin or recycled. Virgin fiber is typically used for consumer tissue grades, with hardwood fibers—primarily eucalyptus—still being the preferred fiber. Softwood fibers may be used to provide sheet strength.
Virgin fiber is stronger and provides better softness. Recycled fiber has long been used for commercial and industrial tissue grades and has also been used in some consumer products, notably napkins and some paper towel grades. However, recycled fibers tend to lack strength and softness.
The development of multi-layer headboxes is providing producers the opportunity to combine virgin and recycled fibers in a single product, providing greater versatility in engineering the final product. In a multi-layer headbox, two or three layers of fibers are combined to form a single sheet. Typically, only two furnishes are used.
For example, for a single-ply tissue, a three layer headbox can be used to manufacture a sheet consisting of top and bottom layers of eucalyptus and a middle layer of softwood. The eucalyptus provides the product softness while the softwood layer provides necessary sheet strength.
At the present time, few producers are using this technology to manufacture a single ply sheet combining both virgin and recycled fiber but interest is reportedly increasing. For example, use of a recycled softwood fiber as the middle ply and virgin hardwood fiber as the top and bottom layers would provide the sheet with necessary softness and strength while burying the lower quality fibers in the center of the sheet.
Two sheets with varying fiber layers can also be combined to form a two-ply sheet. As an example, two plies could be formed with the top and middle layers of hardwood and a bottom layer of softwood. The two plies would then be mated to join the two softwood layers to form the middle of the two-ply sheet. Thus, the final two-ply sheet structure would consist of six layers in the following order—hardwood, hardwood, softwood, softwood, hardwood, hardwood.
To achieve better quality, another possibility that is being considered is fiber fractionation of a recycled fiber furnish. By applying such technology in the recycled fiber operation, it could allow producers to produce sheets with both a good appearance and feel using recycled fiber. However, this is reportedly not being utilized at the present time.
In addition to the added versatility of multi-layer headboxes, consistency dilution profiling provides the capability to reduce the basis weight variation of the sheet. The technology has only been around for the past two to three years and is still not standard with new tissue machines. However, it can be easily retrofitted to existing machines, especially to multi-layer headboxes. The basis weight profile variation can be reduced from 2% to 2.5% to 1% with consistency dilution profiling.
Reduced basis weight variation provides a more consistent sheet which typically provides less operational problems during the converting operation. Thus, tissue producers who operate their own converting operations have been quicker to realize the benefits of consistency dilution profiling while producers who sell parent or jumbo rolls to converters have not been as quick to adopt the technology.
Drying. An interesting new technology development is being pioneered in the marketplace by Voith Sulzer Paper Technology and Andritz Inc., partners in developing tissue technology. Voith Sulzer and Andritz have introduced TissueFlex technology, applying its NipcoFlex shoe press technology to tissue manufacturing.
The nip pressure profile of a shoe press is wider than a conventional suction press roll which allows it to generate the same linear force at a much lower maximum nip pressure, as shown in Figure 4. The recommended configuration for installing a shoe press for tissue manufacture is with the NipcoFlex roll pressing directly against the Yankee dryer.
FIGURE 4: Higher linear nip forces can be achieved with a shoe press, allowing for higher dryness.
When applied to tissue manufacturing, a number of benefits can be gained with a shoe press. The press shoe is very flexible and able to follow the shape of the Yankee dryer surface when it is deformed by the linear nip force. As a result, Voith claims that there is no longer a need to machine a crown on the Yankee. Additionally, the creping quality is reportedly improved because the sheet is pressed uniformly against the Yankee, developing a good moisture profile.
The quick pressure drop at the outlet of the shoe press in conjunction with the asymmetrical nip minimizes rewetting of the sheet. In a conventional suction press, sheet rewetting can occur, with up to a 10% loss of dryness due to rewetting.
The press can be installed on current machines as a direct replacement for a suction press. In such an installation, it is possible to produce a tissue sheet at the same dryness with more bulk (Figure 5). Bulk can reportedly increased 15% to 30% using a shoe press in place of a conventional suction press with a conventional Yankee. However, a small tensile strength loss may be realized.
FIGURE 5: A shoe press can be used to manipulate sheet bulk and dryness.
The shoe press can be used to allow tissue producers to produce a sheet that more closely resembles the performance of a through-air dried (TAD) sheet without installing TAD technology. For example, a typical tissue machine using a 100% recycled furnish uses two conventional suction press rolls on the Yankee. Using a second suction press provides a 3% to 4% gain in sheet dryness but greatly reduces the bulk of the sheet. By replacing the two presses with a single shoe press, it is possible to gain bulk without sacrificing sheet dryness.
In order to achieve higher dryness, greater linear nip forces than those currently possible are required. As Figure 4 shows, it is possible to triple the linear nip force without exceeding the nip pressure in a conventional suction press. However, current Yankee dryers are not able to be subjected to the required higher linear forces.
To achieve higher dryness, Voith Sulzer and Andritz have developed a modified Yankee dryer that uses a T-shaped rib. Compared to the rectangular ribs currently used, a Yankee with T-shaped ribs can be subjected to higher loading and can withstand the necessary linear force of up to 200 kn/M. The T-shaped ribs impart greater strength as there is more material on the inside of the dryer wall. This also gives the Yankee a larger heat transfer area which allows for a lower condensate film thickness, resulting in greater dryer efficiency as well.
The essence of the through-air drying process is blowing hot air through a specially designed open mesh fabric that contacts the sheet, as shown in Figure 6. This results in a much dryer sheet entering the Yankee dryer, with a sheet dryness of 70% to 80% compared to a conventional tissue machine configuration with a sheet dryness of 18% to 30%. This results in a much bulkier sheet that feels more “cushiony” than can be obtained by crepeing alone. Consequently, the process is used for producing premium grades of consumer tissue which also typically can be sold at premium prices in stores.
Through-air drying is more effective for making heavier weight single sheets than lightweight grades. Because of the greater bulk that through-air drying imparts to the sheet, the process allows producers to make a single-ply tissue sheet instead of a two-ply sheet. The tissue produced for a through-air dried sheet must be strong so 100% virgin fiber is still the main fiber choice. This added strength also aids in the converting process as it should be less prone to breaks.
However, there is growing interest in extending through-air drying into recycled fiber tissue manufacture with research ongoing. One challenge that must be addressed is how to meet the necessary sheet strength requirements using a recycled fiber. Additionally, potential contamination of the through-air dried fabric is of great concern. Any buildup in the fabric mesh from stickies and other possible contaminants from a recycled pulp furnish would restrict air flow which could cause the sheet to develop wet spots where breaks could occur.
FIGURE 6: BY PASSING AIR THROUGH THE SHEET, A TAD SYSTEM PROVIDES MORE BULK AND SOFTNESS THAN A CONVENTIONAL YANKEE DRYER.
Dry end handling. While a paper machine speed increases toward the dry end, a dry crepe tissue machine operates at slower speeds at the dry end. While the wet end speed may be operating at 6,000 fpm, the dry end may be running at only 4,500 fpm to 5,000 fpm, depending on the amount of crepe imparted to the tissue.
A major challenge of manufacturing tissue on wider machines and at higher operating speeds is overcoming the handling of the sheet on the dry end of the machine. Unlike most papers, a tissue sheet is lightweight and has little strength. With the trend towards wider machines, dry end handling becomes even more critical as sheet deflection increases exponentially with machine width.
Because dry end handling is currently the bottleneck to increasing tissue machine operating speed, machine vendors are working on the problem. A number of developments are showing potential in addressing the issue. One possible solution is an air foil design where a negative air pressure is applied to keep the sheet suspended—in other words, a vacuum is induced above the sheet to provide support.
Another dry end operating problem that causes many problems is dust. Dust is not only a runability hazard, but is a fire and health hazard as well. With continuous crepeing, many new machines are finding it possible to run six to seven hours without a break. However, these longer run times provide fewer times to clean the machine both of dust and wet end fiber buildup. And it is possible that the Occupational Safety and Health Administration (OSHA) may introduce standards to minimize the health risks of dust. Therefore, many mills are addressing the issue by installing state-of-the-art dust handling equipment.
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