PAPER CHEMISTRY LABORATORY, INC.
Process Chemistry Optimization, Coated Free Sheet (CFS)
A laboratory study of the coated free sheet (CFS) process explores the effect of varying the zeta potential on drainage, sizing, sheet ash and Scott Bond, and at increasing levels of PCC filler. We obtain the counter-intuitive result that an optimized process has both higher sheet ash and higher strength. A possible explanation is the influence of van der Waals force.
We have established that, in the case of the microparticle process, drainage is maximized at a small positive zeta potential. In the present report, the study is expanded to include the results obtained for retention, sizing and strength.
The coated free sheet process, at a broke content of 30%, was selected for experimentation. Designated Series I, cationic scavenger incremental addition experiments were conducted in order to vary the zeta potential and determine its influence on drainage, sizing, sheet ash and Scott Bond. For this particular microparticle system the optimum zeta potential was determined to fall within the range +1 to +6mV.
We conducted a second and third series of experiments in which dry precipitated calcium carbonate (PCC) was added in 5% increments from 0 to 30% to replace fiber in the stock, in order to determine its influence on process and physical property parameters. In the second series we tried to obtain a fmal zeta potential in the +1 to +6mV range by titrating the stock to +5mV zeta potential with cationic scavenger before adding functional and process chemical additives. The target turned out to be too low, as we only managed to obtain a fmal average zeta potential of -0.6mV. The third series was uncontrolled. We added no cationic scavenger and obtained an average zeta potential of -14.2mV.
When we quantify the difference in results obtained from our effort at optimization in comparison with the uncontrolled process, we show that a controlled process with 14% sheet ash has equivalent strength to an uncontrolled process with 4% sheet ash (derived from coated broke). On a 10% filler for fiber substitution basis, this represents a potential savings of $15. per ton of product. Additionally, drainage is far superior. Retention is about double. Sizing efficiency is superior by an order of magnitude.
In Series IV the optimized average zeta potential was +4.5mV and the uncontrolled -15.2mV. The sheet ash levels ranged from 21 to 29%. In every case, the optimized sheet ash and Scott Bond both are higher. The data shows that sheet ash can be increased by 5% with equal or superior strength. Sizing and drainage of the optimized system are of course superior.
On the paper machine, we would implement the following strategy: employ an on-line Zeta Data to measure zeta potential and drainage, with output to a Distributed Control System. Zeta potential would be controlled at the value which maximizes drainage, thereby stabilizing physical properties at maximum levels in the most cost effective way.
After each addition, the stock is mixed for 10 minutes on a magnetic stirrer. A print-out is Made of zeta potential, drainage, conductance and temperature in cycles of about 1½ min. Three Handsheets are prepared, using the Dynamic Paper Chemistry Jar ® Mark V; two at 70 g/m² for sizing tests and one at 100 g/m² for Scott Bond.
