We quickly accept that different grades will probably require different settings for the coating chemistry to achieve the desired properties.

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1 By John Stitt Buckman Laboratories International, Inc. Memphis, TN, U.S.A. Key Words Charge control, creping, retention, adhesion, release, softness, machine efficiency, blade life ABSTRACT Control of charge of tissue furnishes enhances the ability to control and optimize creping, increases machine efficiency and results in more uniform achievement of desired product properties. Uniform sheet adhesion to the Yankee cylinder is influenced by charge. Other operational benefits such as reduced wet strength resin consumption, dye fixation, stickies control, and increased retention can be realized through this application technology with a properly designed treatment program. Introduction We tissue makers have long known and focused on the importance of a proper and uniform Yankee cylinder coating but most of our focus has been on how to control the application of the coating for a uniform profile, or on uniformity around the circumference of the cylinder. One of the most important sources of variation has been mostly overlooked, and viewed as not realistic or just too complex to address, the uniformity of the fiber being formed and applied to the Yankee through time. We quickly accept that different grades will probably require different settings for the coating chemistry to achieve the desired properties. Good operators will tell you that they run the machine where it runs the best, meaning trying to gain smoother operation, or put in their terminology, a simple shift without problems. Perhaps a new measure of machine efficiency should be how many cups of coffee or tea the operator drinks per shift! Unfortunately to achieve this stable state of operation operators often set up the machine to easily handle the variabilities of the system, whether they be streaks of picking or moisture or skippy crepe. Don t slight the operators for this because they have been told, Don t make reject paper! We also all know that the fate of a soft or hardroll is determined by the worst part of the roll. If even a small part of the roll is reject, we can seldom reject just that part of the roll. The entire roll becomes reject and the inherent costs and productivity is lost. 1

2 So if we are to improve the productivity and quality of a machine simultaneously, it makes sense to address the sources of variability to the creping operation. As previously mentioned, much effort is usually and properly focused on obtaining a uniform Yankee coating. The adhesive and release package, and even basis weight and moisture control, are not all the factors of a good coating. Over half the actual coating on a Yankee comes from the tissue sheet. In this paper we will assume that the mill already has a superior coating chemistry package, a well-surfaced Yankee cylinder, and properly adjusted creping blade geometry, for the products they are producing and the mill management s operational strategy. This is often a rather large assumption but it does happen in some mills. Granted this assumption, one still finds that there will be sub-optimization of the creping process to gain operational ease, usually at the sacrifice of sheet quality and often forsaking total operational efficiency. Through work to fine tune creping for the maximum sheet quality by increasing the aggressiveness of the creping conditions, we experienced erratic operational problems even though we had optimized the geometry and produced a superior product for a short time. Quite often the more aggressive creping would run properly for awhile, and then the quality would fall apart. The question became, what source of variation have we not properly addressed. As usual, the machine tenders already knew the answer; Stock prep sent me lousy stock. They recognized that the stock was different from that being run only a short time ago. There still remained a big problem; the machine tenders did not know how to solve this problem. The answer resided in wet end chemistry. Tissue Wet End Chemistry Fiber, fines, fillers, pitch, stickies and many other stock preparation additives are anionic, or carry a negative surface field charge. Secondary fiber systems are extremely anionic while unrefined long fiber virgin fiber systems are usually only mildly anionic. The level of this anionic surface charge is highly dependent on the total surface area of the particles. Although there are several more academic methods, a very effective and practical way of quantifying this charge in the stock system is through cationic polymer demand. The fiber slurry is titrated against a known charge density cationic polymer using as an indicator a streaming current measurement instrument. The results of this titration can then be expressed as kilograms of the polymer per ton of dry furnish. Figure 1 Titrating cationic demand using a BTG PCD4 particle charge detector instrument. 2

3 When we take a number of such measurements over time we find that there is variability in the level of cationic demand, especially in secondary fiber. It should come as no surprise that secondary fiber varies from pulper to pulper. We can look at the results of these titrations on a histogram as in figure 2 and see the level and nature of the variation. It is also useful to look at the nature of this variation as a time series graph as in figure 3. Now we can see that the machine tenders were correct, the stock did change and we see this change through measurement of the cationic demand. In most cases, these shifts in demand can be associated with differences in the furnish, pulper to pulper. The Nature Of The Variations Why would charge variation result in fiber different behavior on the Yankee dryer? First, charge is the primary method of bonding of fines, fillers, and many additives onto the fiber and their retention in the sheet. Control of charge is the mechanism of modifying first pass retention on the wet end. The small and very small particles in a furnish are responsible for a majority of the cationic charge demand. If the percentage of fines and fillers increases then the charge demand Count of Occurances will increase exponentially. Small changes in the percentage of fines and fillers result in large changes in the cationic demand and in the retention of these materials in the sheet. If these materials are retained in the sheet then, since the coating on the Yankee is mostly fines, and fillers, the nature of the coating changes. Increased retention usually results in a harder coating. If dyes, wet strength resins (a cationic resin), optical brighteners (usually anionic), or fillers (usually anionic) are used their retention will vary. These too become part of the Yankee dryer coating. The point is small changes in cationic demand result in a different sheet going to the Yankee Histogram of 5 Stock System Charge Measurements Taken Over Several Days Measured Charge Of The Stock System Time Series Graphic of Charge Demand Data Especially in secondary fiber tissue (PCD charge readings through time) Initial cationic demand Before charge modifier Figure 2Graphic representation of the distribution of 5 charge demand titrations. System has become cationic Figure 3 Time series representation of the same 5 points of charge demand titration data Cationic +1 +2

4 Second, the nature of the adhesion of the sheet to the Yankee is largely one of charge bonding. Almost all dryer adhesives are cationic polymers with a positive charge. Just like magnets, opposites attract and bond the sheet to the Yankee. Stock System Charge Cationic Dryer System has become cationic System goes cationic - When the system goes cationic the crew experiences complete loss of crepe control, Generally, anionic stock Cationic Demand Variation Through Time TIME - Hours, Days, or Pulpers Figure 4Graphic model of charge variation through time on a tissue machine. When a tissue machine stock system becomes cationic the following phenomena are experienced: loss of adhesion control on the Yankee dryer. a reduction in wet strength development. excess foam due to excess wet strength resin free floating and not attached to the fiber. inconsistent formation and retention, especially if a chemical retention system is in use. almost unrecoverable felt filling due to excess wet strength resin bonding with the felt fibers color variation due to ineffective fixation of dyes to the fiber. Obviously, one does not want the stock system to become cationic while using a cationic dryer coating. And, it stands to reason and experience that smaller charge variations 4

5 within the anionic range of fiber also produce significant differences in content of the coating and the attachment of the sheet to the Yankee coating. Controlling Furnish Charge It is possible to control the charge of the furnish system. For Yankee coating stability the objective is to level the curve. For Yankee coating consistency it is not necessary that the charge be brought to near neutral, although such a change is highly desirable if pitch or stickies control is also an objective of the charge modification program.. Addition of a low molecular weight cationic polymer (LMWCP) to the stock system produces a two step change in the charge of the system. LMWCPs find the smaller materials in the system that account for a majority of the cationic demand preferentially without interfering with the bonding of long fibers. The first stage step in charge modification appears to is a buffering or smoothing of the charge variation. Addition of more polymer then produces a shift in total charge demand. Furnish Charge Modification Especially in secondary fiber tissue (charge demand through time) + System is cationic Initial stock charge before charge Result of addition of an initial amount Result of charge sensed variable feed rate of larger amount of Figure 5 Effect of first an initial dosage of LMW P followed by an increased amount of polymer. For increasing Yankee coating stability the curve flattening effect is the most significant shift. 5

6 Charge Modification with Unmodified Virgin Fiber Although beyond the scope of this paper the charge situation with virgin fiber systems should be mentioned because it follows a different scenerio. Because there are few fines and fillers in a virgin fiber system the cationic demand of such a system is usually very low. In fact, often so low that even the addition of cationic wet strength resin can drive the system to be cationic. The problems mentioned with a cationic stock system also apply to virgin fiber systems. Where large amounts of wet strength resins are desired in the system it has become common practice to add an anionic charge modification agent, such as carboxy methyl cellulose (CMC) or a chemical anionic charge modifier. When this is done there still appear to be benefits to creping to the later addition of a LMWCP. Other Benefits of Charge Modification There are other benefits available to the mill through a charge modification program. Although addition point is important in all charge control programs, as one seeks to gain the multiple benefits from one application the addition technology becomes critical. Charge control also has proven to also provide: control of stickies optimization of wet strength resin effectiveness improved first pass retention dye fixation improved felt life fewer breaks and longer blade life Talk with your process control chemistry expert if these benefits are also of interest to you. Bibliography Forbess, Dennis; Wet End Chemistry in Towel and Tissue, published in TAPPI Tissue Runnability Short Course Notes March 3-April 1, 1998 Panama City, FL. Smook, Gary A,; Handbook For Pulp and Paper Technologists, 2 nd edition, Angus Wilde Publications, Vancouver, B.C. and Bellingham, W.A., 1992, Stitt, John B.; Charge Control Helps Tissue Producers Achieve Quality, Productivity Benefits ; Pulp and Paper, May,