A beginner's guide to towel and tissue, Part II.

Among the many grades produced by the global paper industry, towel and tissue are unique. In Part I of this article, we discussed how towel and tissue represent a broad line of products--consumer and "away from home" commercial toilet and facial tissue, napkins and toweling, and specialty grades such as sanitary padding. We also discussed the basics of tissue forming and pressing. In this article, we discuss tissue drying and finishing processes.

DRYING

Most tissue machines use a Yankee dryer to remove remaining sheet moisture. The Yankee also has a second purpose. As the sheet is removed from the Yankee, the sheet is creped by a doctor blade to improve softness. Yankee dryers vary in size from around 3.6 meters (12 feet) in diameter for a small tissue machine up to 5.5 meters (18 feet) in diameter for a large machine. A large Yankee may also be 7.75 meters (300") wide and weigh 180 metric tons!

To remove condensate from the inside of the Yankee, the condensate is picked up by a series of small syphons known as "soda straws" (Fig. 1). Since the condensate header pressure is lower than the steam pressure inside the Yankee, the condensate is pushed out of the dryer along with some "blow through" steam. Blow through steam is steam that did not condense inside the Yankee. It is normally re-compressed and re-used in the Yankee or used instead to heat hot water for showers on the paper machine. Although we said that the tissue sheet was dried by the Yankee, this is not quite true. Only about 40% of the drying required is supplied by the steam inside the Yankee--60% of the drying is done by the hot air hood that surrounds the Yankee.

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The hood system consists of the actual hood, a burner, fans, heat exchangers, and ducts that carry hot air to and from the hood (Fig. 2). The burners are usually gas-fired and air temperature may range from 500 to 700[degrees]C or 930 to 1300[degrees]F! The fans blow this hot air through a series of ducts and headers that distribute the air across the surface of the sheet with a series of nozzles. The nozzles are located close to the sheet, around 20mm (0.8"), since drying efficiency drops off quickly as distance from the sheet increases. The two hood sections roll apart for startup and cleanup. Some hoods also have dampers that can be adjusted to reduce drying in certain areas across the sheet to correct the cross-direction (CD) moisture sheet profile.

The hot air blown on the sheet vaporizes water, which cools the air temperature slightly. The exhaust plenum is usually located between the nozzle headers. The cooler, wetter air is sucked into exhaust ducts. Based on the moisture level in the returning air, a portion of the air is recycled back to the burner and a portion is diverted to a heat exchanger to remove as much thermal energy as possible before being released to the atmosphere. Some hoods have a dual burner system so that the hot air from the dry end of the hood is cascaded to the wet end of the hood. Excess moisture is dumped to the heat exchangers from the wet end.

CREPING

Earlier we discussed that the sheet is "stuck" to the Yankee and then peeled off by the creping blade. The creping action puts very small folds or accordions in the sheet that improve softness, bulk, and absorbency of the product. Creping can occur when the sheet is almost completely dry, that is, 92 to 95% solids or when the sheet is wetter, around 70% solids. If the machine is designed to crepe at high dryness, it is referred to as a "dry crepe" machine. Dry creping has the greatest effect on sheet properties because the sheet is more firmly stuck to the Yankee surface.

If the machine is designed to crepe when the sheet is still somewhat wet, it is called a "wet crepe" machine. Wet crepe machines have several conventional dryers following the Yankee to complete the drying process. Wet creping is limited mostly to heavier weight toweling grades.

The adhesion of the sheet to the Yankee is critical because it determines how the sheet crepes at the doctor blade. Sheet adhesion is controlled by coating on the surface of the Yankee. This coating is a mixture of hemicelluloses from the pulp, fiber dust, metal oxide, and synthetic adhesives and release agents sprayed on the Yankee. The coating also serves another important purpose--protecting the metal surface of the Yankee dryer from the abrasive action of the creping blade. Without this coating, the Yankee shell wall thickness would be quickly reduced to the point that the Yankee no longer met the specifications for design steam pressure.

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THROUGH-AIR DRYING

To this point, we have been discussing what might be termed conventional tissue papermaking. A newer technology--"through air drying" (TAD)--alters the pressing and drying process to produce a higher quality product. Compared to a dry creped tissue grade of the same basis weight, a TAD product will be 15 to 75% bulkier, have much better absorbency, and have excellent softness. On the downside, TAD energy use per ton is substantially higher than in conventional processes.

The main difference between a TAD machine and a conventional dry creped machine is that the TAD process replaces the pressing operation with a through-air-drying step (Figure 3). Instead of forming, pressing, and drying, we have forming, air-drying, and more drying. Since water removal by mechanical pressure is much cheaper than thermal drying, you can see why TAD energy costs are substantially higher.

A TAD machine forming section looks like a typical twin-wire C former with the exception that a steambox and vacuum dewatering are added on the conveying wire to improve sheet solids as much as possible (Fig. 4). From the conveying wire, the sheet is transferred to the TAD fabric with a suction pickup box and then passed over the molding box, which is also under vacuum. The molding box is vital because it dewaters the sheet further and also causes the sheet to assume the topography of the TAD fabric.

When the sheet enters the TAD unit, it is around 25% to 30% solids. Much of the sheet drying takes place on the TAD roll (Fig. 5). As the sheet enters the TAD hood area, hot air at slight pressure is blown on the sheet. There is a vacuum inside the TAD roll and the differential pressure from outside the roll to the inside causes the air to be drawn through the sheet and the TAD fabric. The cool moist air on the inside of the roll exits from the end of the roll. Since the moisture is transferred to the air as it passes through the sheet, TAD rolls have very high drying rates. After the cooler air leaves the roll, most of it returns to be re-heated and a portion is sent to a heat exchanger and released to atmosphere to remove moisture.

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After leaving the TAD unit, the sheet is carried on the TAD fabric to the Yankee. The pressure roll sticks the sheet to the Yankee with the aid of coating spray applied at much higher amounts than used on conventional dry crepe machines. Press roll pressure against the Yankee is about 50% of what it would be for a dry crepe machine, and since the sheet is already much drier, the structure of the sheet is set. That is, the sheet now has taken on the surface shape of the TAD fabric--when it is pressed against the Yankee, it only sticks on the high points or "knuckles." The sheet does not lose the bulk built into it on the TAD unit. The Yankee and the hot air hood complete the drying process, and the sheet is removed with a creping blade although the amount of crepe imparted to the sheet is low.

The difference between TAD and conventional dry creped tissue products is in the way that bulk is developed. In the TAD process, bulk structure is molded into the sheet while it is still wet (Fig. 6). In a conventional sheet, bulk is developed as it is dry-creped and then further in the off-machine embossing operation. These products might appear similar but when wet, the TAD sheet does not lose bulk like the dry-creped sheet, making it much more absorbent. For many consumers, the TAD product is simply "better." A TAD towel manufacturer can make a product with one ply instead of two, or two plies instead of three, resulting in substantial fiber savings. For these reasons, TAD is very popular among high quality tissue and toweling manufacturers.

CALENDERING 

The final operation in tissue papermaking is calendering. Although not all grades of towel and tissue may be calendered, many are--especially TAD products. Calendering helps control sheet caliper, improves smoothness, and can be a part of off-machine ply combining and embossing operations. Figure 7 shows a typical two-roll calender stack following a Yankee dryer. A single nip is usually sufficient to control caliper and improve smoothness, and low pressures are used so as not to lose too much bulk. Another approach is to set the gap between the rolls slightly smaller than the desired final sheet caliper to allow for sheet rebound.

CONTINUING EVOLUTION

Although towel and tissue grades of paper were originally made on single-wire four-drinier machines, there has been an evolution to roll type gap formers, and the dominant design for conventional tissue grades is the Crescent former. For most towel and tissue grades, softness, absorbency, and bulk are developed in the sheet by creping after the sheet is dried on a Yankee dryer, sometimes followed by further off-machine embossing and combining of several plies. A growing volume of ultra high-end towel and tissue is being produced on TAD machines where the sheet structure is molded into the sheet during the through-air drying process. TAD grades are more expensive to produce due to higher energy costs, but that cost may be partially or wholly offset by the fiber savings and quality improvements.

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WHAT YOU WILL LEARN

* The differences between conventional drying and through-air-drying (TAD).

* The basics of the creping process.

* Properties imparted in the calendering process.

ADDITIONAL RESOURCES

* "Using new technology to target tissue markets," Solutions!, June 2004, p. 35. To access the article online, go to www.tappi.org and enter this product code in the search field: 04JUNS035.

* Part I of this article, which was published in the September 2004 issue of Solutions! Part I focused on how tissue papermaking differs from other grades, different tissue machine configurations, and the basics of tissue forming and pressing. To access Part I, enter the following product code in the search field on www.tappi.org: 04SEPS032.

* 2005 Tissue Runnability Short Course, Marriott Bay Point Hotel, Panama City Beach, Florida, USA, May 9 to May 12, 2005. For more information, please enter the following product code in the search field on www.tappi.org: 05TISSUE.

Editor's Note: Some of the graphics and text in this article are compliments of the TAPPI Tissue Runnability Short Course and Metso Paper.

ABOUT THE AUTHOR

Jim Atkins is president of Atkins Inc., Flemington, New Jersey, USA, and is a member of the Solutions! Editorial Board. Contact him by phone at + 1 908 806-8689 or [e-mail].

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