A beginner's guide to towel and tissue, Part 1.

Most grades of paper are produced in a similar fashion. While there are differences, paper machines producing most grades of paper all are starting to look remarkably similar. Most modern machines use gap formers, shoe presses, and single tier dryer sections. But one type of paper--towel and tissue--is manufactured quite differently

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--with basis weights from as low as 5 grams per square meter (3 lb/3000 f[t.sup.2]) to as heavy as 35gsm (21.5 lb). Just for reference, newsprint has a basis weight of 40-45 gsm (24.5 to 27.5 lb), so 5 gsm for facial tissue is truly lightweight. And lightweight is not the only difference. Tissue grades are made faster than any other grade of paper, presently over 2000 m/min (6560 feet/min), and likely to be approaching 2500 or even 3000 m/min (8200-9840 feet/min) in the not too distant future.

Annual global production of towel and tissue is around 23 million metric tons or about 6.2% of the total paper and board production, and there are approximately 2000 tissue machines worldwide (Jaakko Poyry). In established markets, demand is increasing for higher quality, softer products. In developing nations, the per capita consumption of tissue is growing extremely rapidly--not something that can be said about many grades of paper.

Tissue machines typically take up a lot less space than machines that produce other grades of paper because the forming, pressing, and drying sections are much more compact. Tissue machines are also not as wide as modern paper machines producing fine papers or linerboard. A modern fine papers machine may be over 11 meters wide while a large tissue machine is only 6 to 7.5 meters wide. What they give up in size, they make up for in speed. A fast tissue machine can make enough paper to go around the world in less than 5 days!

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SHEET FORMING

Although towel and tissue grades were originally all made on standard fourdrinier machines, the one-direction drainage and free-top surface limited the speed. Even though it is old technology, there are still more fourdrinier tissue machines running than any other configuration. However, they cannot compete with newer designs.

The first real new development was the suction breast roll former (Fig. 1), and this is also a very common configuration. In many ways, this was a modified fourdrinier since it only had a single forming wire. However, by extending the lip of the headbox around a multi-chambered suction roll, it was possible to dewater the sheet in a very short distance and speed up the machine. This was the dominant former during the 1960s and '70s, but it also had speed limitations due to water handling and formation.

Twin-wire paper machines were developed in the 1970s, and the same principles that were used for newsprint and fine papers were also applied to tissue grades. Modern tissue machines are all now gap roll formers. Since tissue grades do not have as much water to drain, forming and dewatering the sheet can be accomplished over a single large-diameter forming roll. By gap former, we mean that the headbox jet is deposited in the nip between two converging belts--in some designs these are both forming fabrics or wires. In one design it is a forming fabric and a felt.

Figure 2 shows an "S-wrap" former configuration, named because of the path that the sheet takes. The forming roll may be solid or it can be a suction roll to give higher drainage capacity. The outer wire, as the two wires wrap the drainage zone, is also the conveying wire since it carries or conveys the sheet after the drainage zone. The other wire is called the inner wire. In roll formers, most of the water is squeezed out of the sheet by the tension of the two wires as they wrap the forming roll. If the forming roll is solid, then most of the water leaves the sheet through the outer wire. If the forming roll is a suction roll, at least a portion of the water also leaves through the inner wire.

Figure 3 is a "C-wrap" configuration, again named due to the path that the sheet takes through the forming section. Like the "S-wrap," the forming roll can be solid or have a suction box. Note that the backing wire is now the outer wire, and the conveying wire is the inner wire. Why would you choose an "S-wrap" or a "C-wrap?" The answer depends on several factors. One factor is "stickies." Contaminants in recycled paper such as hot-melts, glues, and other adhesives can form small particles that stick to the wires, felts, and dryers, and many mills that make tissue grades use recycled paper. The first concern is "where will the stickies accumulate?"

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Remember that most of the water drains through the outer wire, so we would prefer that the stickies stay on the inner wire. If they are on the outer wire, they plug the drainage paths and cause pinholes in the sheet. In many mills the contaminants tend to accumulate on the conveying wire. For a C-former, that's OK because the conveying wire is not the drainage wire. But for an S-former, contaminants on the conveying wire also mean they are on the drainage wire. So perhaps an S-former would not be the best choice. However, some mills have more problems with contaminants on the inner wire. If that were the case, then maybe an S-wrap would be better.

The second consideration concerns sheet stratification. To discuss that, we need to talk about headboxes. Modern tissue machines use hydraulic headboxes. Many hydraulic headboxes can be designed to discharge two or more different layers of fiber from the same headbox (Fig. 4). Called "stratification," it is commonly used to make higher quality tissue grades. Note that the vanes extending into the nozzle of the headbox keep the layers separated until the jet leaves the headbox. By separating layers, you can construct a sheet that has different properties on each side. Tissue needs to be strong, so that would suggest that softwood fibers would be needed in the sheet. But softness is also important, so that might mean that hardwood fibers would be useful in the furnish. What about bulk and fiber cost? Maybe mechanical pulp or waste paper is desirable. In stratification, a layer of softwood can be deposited on one side of the sheet with a layer of hardwood or other type of pulp on the other. In converting, the two softwood layers can be laminated together, leaving a two-ply sheet, which is soft on the outside but strong on the inside.

In a two-layer stratified headbox, the longer fiber is deposited on the side against the outer wire and the shorter fiber on the inner wire side. Why? Because the drainage is mostly through the outer wire, and the longer fibers form a better mat that the shorter fibers can then drain through. This arrangement will give the best fiber retention. In the example we have been discussing with softwood and hardwood, on a "C-wrap' machine that means that the hardwood layer ends up against the Yankee dryer. For a "S-wrap" machine that means the softwood layer ends up against the Yankee. Which is better? It depends on the furnish and on the product properties the mill wants to achieve. What if the mill wants to put mechanical pulp on the inside in order to increase the sheet bulk? Or how about a three-layered stratified headbox, which is used to make a single ply product? There is no one right answer for configuration. It depends on what you want to do.

C-formers and S-former tissue machines are true twin-wire, roll/gap formers. There is another roll/gap former machine that is not a twin-wire machine--the Crescent former (Fig. 5). Crescent formers were developed about the same time (the 1970s) as C and S-former machines, but recent developments have made it the configuration of choice. A Crescent former looks like an inverted C-wrap, with the headbox on the top. This is still a roll/gap former, but it is not a twin-wire former because the inner conveying belt is now a felt, not a wire. Since the sheet is conveyed on the felt, there is no need to transfer the sheet from the conveying wire onto another press felt. That means the Crescent former is more compact, has lower operating cost, and better product performance.

PRESSING OPERATION

If the former is a suction breast roll or a C- or S-former, then the sheet must be transferred from the forming fabric to the press fabric, usually with a suction pickup. The sheet is still very wet at this point, usually around 15% solids.

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Many tissue machines use a single or perhaps a double press arrangement (Fig. 6). However, the main difference from other grades is that the press nip is formed between the press roll and the Yankee dryer. The press dewaters the sheet and also sticks the sheet onto the Yankee dryer.

Since it is important to maintain as much bulk as possible in the sheet, press loadings are relatively mild and the press rolls have very soft covers, which produce a wide, gentle pressure profile. The first press is normally a suction roll with an internal vacuum box to assist in water removal. The second roll usually is a "blind-drilled" roll. Sheet dryness after the second press is around 40 to 45% solids. The use of a second press roll improves sheet solids but at the cost of a loss in sheet bulk.

Recently, a few tissue machines have replaced conventional press rolls with shoe presses. Shoe presses use a hydraulic ram with the "shoe" shaped to fit the opposing roll (in this case the Yankee dryer) in order to form a very wide nip (Fig. 7). Shoe presses allow dewatering to occur over a much longer time, so sheet dryness is improved and sheet bulk is preserved. Excellent dewatering without destroying sheet bulk are important goals for tissue pressing. There are 12 shoe presses currently running on tissue machines, and depending on the success of these installations, there likely will be more in the future.

WHAT YOU WILL LEARN

* How tissue papermaking differs from other grades.

* Different tissue machine configurations.

* The basics of tissue forming and pressing.

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 engine: 04JUNS035.

* Part 2 of this article, which will be published in the November 2004 issue of Solutions!, will focus on tissue drying, creping, and calendering as well as the development of Through-Air-Drying (TAD) machines.

Editor's Note: The second part of this article will discuss tissue drying, creping, and calendering as well the development of Through-Air-Drying (TAD) machines. It will be published in the November 2004 issue of Solutions! for People, Processes, and 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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