In the first discussion of wet end starch solids consistency, the cost for failing to consistently control starch solids to a narrow target was discussed. This can amount to tens of thousands if not hundreds of thousands of dollars in excess costs to a paper mill. From this perspective, the cost and control of wet end starch solids is fairly straightforward and relatively easy to remedy.
However, the total excess costs incurred from failure to consistently control cationic wet end starch solids can be much greater and commensurately more difficult to understand and control.
- Due to volume and charge density, cationic wet end starch is typically the largest cationic influence on system charge as the pulp slurry approaches the headbox.
- Retention can be impacted by even small changes in wet end starch flow and concentration. This is particularly true in white paper systems where the pulp furnish is generally well washed and conductivity relatively low, typically less than 10,000 ?S.
Particularly consequential is when the pulp furnish becomes overcharged from excess cationic starch or combination of cationic additives in the system. When this happens the system “charge”, or cationic demand[1], eventually swings out of control resulting in unstable retention, loss of paper tests, harder drying and slower machine speed, deposits and/or web breaks. This phenomenon is demonstrable and ultimately manageable in most systems through regular periodic measurement of the cationic demand and/or Zeta potential.
Machine stability through “charge” management will be the subject of the next discussion.
[1] Cationic demand, as measured by streaming current potential is not exactly the same as electrical charge as measured by Zeta potential, strictly speaking. In common usage the two terms are frequently interchanged. But to really understand what is happening within the pulp furnish, it is necessary to be clear about what and how these measurements differ, the techniques for sample preparation, and the interpretation of the results. These items will be discussed in the next paper.