PAPER CHEMISTRY LABORATORY, INC.
NEWSLETTER SPRING 2001 Revised
Maximizing the Influence of van der Waals Force, and Increasing Hydrogen Bonding
Maximizing the powerful intermolecular van der Waals force is a key step towards enabling maximum raw material cost efficiency at optimum levels of quality and productivity. This recent accomplishment provides the perspective from which to review a few of the more important changes and accomplishments in papermaking chemistry in the last half of the 20th century.
There have been two major transitions. On the chemistry side, we started adding relatively large lumps of dry ground chalk instead of alum/clay, and switched from rosin size to the alkyl-ketene-dimer (AKD) or alkyl-succinic-acid (ASA). As the use of calcium carbonate spread from the UK, the chalk became (a more finely divided) wet ground calcite, and finally included both calcite and a precipitated version that is economically produced in dedicated plants at the mill site, so-called "satellite plants."
Precipitated calcium carbonate (PCC) provides a large incidental benefit that has not received proper recognition. Because it is pumped over, at relatively low solids content, from the satellite plant next door, it does not carry the contaminating negative charge of the calcite slurry. Therefore it does not contribute a significant component of anionic trash, and is more efficiently retained.
On the physics side, we started by measuring particle charge via microelectrophoresis, assessing the speed and direction of movement of fines, under an applied electric potential. When the typically negative, repulsive charge is reduced to an appropriate range, superior papermaking performance is obtained.
As a chronological marker, the expression "anionic trash" was first coined in 1979. In a Contributing Editor, Chemistry, article for Paper Trade Journal, the undersigned coined the expression to account for the increased cationic demand of recycle stock dispersed in white water of a cylinder machine, as opposed to tap water.
The use of a microscope to measure particle charge was never popular, and was replaced by the purely instrumental measurement of cationic demand. Unfortunately, both techniques have decisive limitations.
Filtering out the fine particles to measure by microelectrophoresis has the disadvantage of not providing a direct measure of the charge on the surface of the large particles that make up the bulk of the stock. On the other hand, the cationic demand measurement is offset by the (variable) amount of anionic trash in the sample. In both cases, the measurement is made at ambient, instead of elevated, temperature; and after an extended time delay.
The use of an on-line, pad-forming streaming potential instrument has been demonstrated to provide reliable measurement of electrostatic charge, and accepted by scientists, although it has not yet been widely adopted by papermakers.
This is, however, a complex subject, and when measuring charge with an off-line streaming potential instrument, cationic decay occurs in the elapsed time required for headbox sampling, transport of sample to lab and measurement in the lab. This can cause the final reading to be significantly more negative than the on-line measurement, by as much as -10 to -15mV zeta potential.
Another major impediment has also been the philosophical difficulty that most papermakers prefer to view papermaking as an exercise in physics, to the exclusion of chemistry. Even when the on-line instrumentation has been installed to accurately measure particle charge, the papermaker often declines to use it.
As we start the 21st century, we have learned that the best combination of chemicals, coupled simultaneously with minimum repulsive negative feedstock charge, enables us to maximize the intermolecular van der Waals force. The door has been opened to maximize stock cost-efficiency at optimum levels of quality and productivity.
A next step is to maximize hydrogen bonding so as to further effect improvement in physical properties. At this early stage, there appear to be manifold opportunities, including: minimizing the presence of anionic trash, modifying the fiber surface during pulping and utilizing natural gums.
John G. Penniman