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Wet End Chemistry Measurement

Over the past decade it has become generally recognized that some kind of chemistry monitoring, accompanied by appropriate adjustments, is helpful in keeping the machine running "well". The most widely used measurement has taken the form of charge titration, in which a low concentration of cationic chemical is added until the cationic demand is neutralized, and something close to the zero point of charge is reached.

Early on, a colorimetric indicator identified the end point. This has been superseded by the less labor-intensive practice of filtering out the solids and measuring the end point instrumentally. The streaming current detector (SCD) has become widely used for this purpose. Its reproducibility has been increased by computer-controlled automatic titration, and in on-line use. As we approach the centennial, use of a SCD such as the Mutek has become pervasive.

A method that we consider superior is now available, measurement of the electrostatic charge on the fibers, fines and fillers, called the "zeta potential". We shall attempt to show that chemistry optimization can best be attained by going further than monitoring. We propose to control the zeta potential on-line within a narrow range, somewhat removed from the zero point. On-line and lab Zeta Data technology is now available which measures zeta potential, drainage, conductance and temperature repeatedly in cycles of about two minutes.

Limitations of the SCD Methodology

Use of the SCD technology has been largely successful in achieving the important but limited objective that the machine continues to run "well". Its wide acceptance, however, has masked a major limitation.

It is well recognized, and reflected in Miyanishi's pioneering papers, that conventional wet end chemistry (i. e. pre-Compozil [Eka Nobel] and pre-Hydrocol [Allied Colloids]) is optimized at a low negative zeta potential (rather than the point of zero charge), typically someplace between 0mV and -10mV zeta potential. Furthermore, that the more modern Compozil or Hydrocol processes, which employing a microparticle such as colloidal silica or bentonite, are optimized at a low positive zeta potential as demonstrated elsewhere on this Web Site.

The SCD measurement of cationic demand is, by definition, only capable of assessing the zero point of charge. This is important because maintaining it can help achieve the objective of stabilizing the process and, for example, eliminating upsets caused by over-cationization. However, the inherent limitation in SCD capability precludes the possibility of fully optimizing the process, for the following reasons:

Optimization Sensitivity

"Process Chemistry Optimization, Coated Free Sheet" is the title of a Web Site paper that provides data for the next part of this exposition. We first refer to the exhibit entitled "Series I", with the sub-title, "Determining the optimum zeta potential range," Slide 1. It is important to note that all the properties measured, drainage, sizing, ash and Scott Bond, are at maximum in a narrow range, in this instance between +1 and +6mV.

x Note that both the process parameters and the physical property parameters fall off sharply on either side of optimum zeta potential.

It is appropriate to explain why optimization occurs at a positive zeta potential. It is an apparent contradiction of the conventional wisdom created by cationic demand measurement, which determines when the system is over-cationized and therefore may not be running well. The explanation is straightforward.

Microparticles have in common a high surface area and a high negative charge. They are, therefore, most highly attracted to a positively charged system. Since inter-mixing of the microparticles with positively charged fibers, fines and fillers is less than 100% efficient, microparticulate process optimization consistently occurs at a small residual positive charge.

The graph in Slide 3 entitled "Sizing Comparison, Series II vs Series III" shows that sizing in Series II, measured at a controlled zeta potential which averaged -0.6mV, was far superior to that measured in Series III. The uncontrolled process with no added cationic scavenger, Series III, had a relatively high negative zeta potential of -14.2mV, which accounts for the exceptional contrast in sizing.

x Sizing superiority of Series II is better by about an order of magnitude. This leads to a more efficient, less variable, less costly process.

When we turn to the Series II vs Series III comparisons of ash and Scott Bond, Slides 4 & 5, it is noteworthy that the Scott Bond of the highest filler loading optimized process was 58lb/in² at an ash level of 14%. The lowest filler level uncontrolled process had a sheet ash of 4% and also a 58lb/in² Scott Bond. There is an almost incredible difference between the controlled and uncontrolled processes of 10% ash at equal strength!

The Winning Game Plan

Monitoring does not optimize process chemistry because it is inherently limited to preventing upsets. Process chemistry optimization is accomplished by continuously maintain a precise, narrowly defined zeta potential. The prospective efficiency improvement is substantial. Zeta Data technology can help accomplish this objective by enabling headbox operation at the zeta potential that maximizes drainage. Closing the loop by computer control of chemical feed rates not only effectively eliminates wet end variability, it maximizes runnability and optimizes quality at maximum chemical cost-effectiveness.

Some believe that this presages the dawn of the CHEMISTRY ERA in papermaking.

John G. Penniman,