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Control of Wet End Chemistry -- The "Holy Grail" of Papermaking

The desire to operate with well-defined and controlled paper machine wet-end chemistry is not new, but the topic seemed to attract increased attention beginning with the large scale conversion to alkaline papers. The conversion brought changes in fiber furnishes, fillers, retention aids, drainage aids, dyes and manufacturing methods, which resulted in the need for a better understanding of the entire wet-end operation. Under past acidic operating conditions, individual machine systems had been "optimized" based on papermaking experience related to the various grades. And the benefit of a key chemical, alum (aluminum sulfate hydrate), not only for pH control but to adjust many paper properties was widely recognized by papermakers, even though other wet end chemistry interactions may not have been well understood.

I do not mean to imply that wet-end chemistry control is not important in acidic papermaking. In fact, I recall being involved in trials during the early 1970's with attempts to determine the Zeta Potential value at selected sample points for correlation with one pass retention and pH. Zeta Potential gives an indication of the effective outer-surface charge of particles suspended in a liquid, but interpretation of the results can be difficult. We were attempting also to determine how the system responded to alum changes, particularly when alum was increased to reduce the Zeta Potential. The goal was to optimize retention by keeping the Zeta Potential close to zero. We hoped this would help define the "best" operating conditions for machine runnability, and result in cost reduction. Unfortunately, the trial results were inconclusive. In hindsight, this may have been due to:

• The necessity of using off-line measurement
• Stock samples that may have been poorly mixed
• Insufficient knowledge of how to interpret the Zeta Potential variations
• A degree of cationic "decay" that may have occurred from time of sampling until measurements were made

Whatever the reason, our interest in conducting additional Zeta Potential trials waned at the time, turning instead to simply studying the effects that various chemicals, retention aids, fillers, and fiber furnishes had on one-pass retention. These were studied using selected addition levels and machine speeds. However, this did not mean that the desire to learn more about wet end chemistry control was abandoned.

Some of the basics of wet end chemistry were either known or assumed at the time, and others were later identified with the introduction of on-line instruments and different methods of measuring surface chemistry effects. These included micro-electrophoresis, streaming potential, and AC streaming current. The details of using these methods are readily available from literature sources. Suffice it to say that they are useful in accurately defining the cationic or anionic demand of furnishes.

The surfaces of cellulosic papermaking fibers and the fillers in the wet end are generally anionic (negatively charged), but the level of charge can be significantly different. For example, titanium dioxide is more highly charged than kaolin clay, and ground calcium carbonate may even possess a slightly positive charge. Measurable degrees of difference exist between various pulps, and the pulping and bleaching process used can also affect the surface charge. Pulps containing a high residual of resin acids, hemicelluloses, or lignin by-products are likely to be highly anionic.

These are a few of the reasons why a change in furnish can result in wet end "upsets", since the electro-kinetic properties of the system can change rapidly.

• The fiber surfaces obtain their charge from surrounding fluids which contain both positive cations and negative anions. These are adsorbed on the surface, and the resulting net charge (positive or negative) determines the surface electrical property of the fiber. The solution pH can greatly affect the surface charge of fibers. Alum will neutralize the anionic charge on the surfaces of both fibers and fillers.
• An increase in surface charge normally results in a negative effect on filler retention, and can adversely affect properties such as paper opacity and print density.
• In addition to filler retention, control of the surface charge can be used for optimum doses of dispersants, dyes, and polymers used for slime control.
• The adsorption of cationic retention aids can be correlated to the surface charge.
• The AC streaming current measurement does not actually measure Zeta Potential, but the results are very useful in establishing whether a system is cationic or anionic.
• A non-ionic retention aid contains two charged groups, resulting in a neutral system effect.
• Alkaline papermaking has resulted in increased usage of high molecular weight retention aids, such as acrylamide co-polymers, different sizing agents, and drainage aids.

It should be recognized that other parameters in papermaking, such as refining, filler particle size, and sheet basis weight significantly affect first pass retention (defined as difference between headbox and white water consistency, expressed in %). Paperboard can have first pass retention values of up to 90%, while lighter basis weight sheets may be only in the 50% range. Increasing retention is often one of the primary goals on lighter basis weight papers that are produced at high speeds. This results in an increased requirement for better wet end chemistry control. To achieve optimum retention requires consideration of using wet end continuous analyzers as part of the control scheme, as off-line measurements will likely not be sufficient. Although these systems may be expensive initially and require close attention to calibration, they reportedly are used successfully on over 500 paper machines world-wide.

Robert Moore is a retired chemical engineer, and is an experienced technical and fictional writer. His past work experience spanned the chemical, paper and equipment manufacturing industries, including holding management positions at Voith Paper, Scapa plc, and The Mead Paper Corporation. He is also the author of humorous short stories about life in southwest Virginia, circa 1940-1960.



 


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