PAPER CHEMISTRY LABORATORY, INC.
.....the acknowledged leader in papermaking chemistry Instrumentation

WET END CHEMISTRY NEWS LETTER FOR DECEMBER, 1998

It is not sufficient merely to neutralize the anionic trash by satisfying the cationic demand. It is important, as we shall demonstrate, to use the particular scavenger that is simultaneously most effective in improving the process. We have added a technical paper to the web site that illustrates how we accomplish this, entitled "SCAVENGER EVALUATION" . It represents work done about four years ago with the Lab Zeta Data, for a coated free sheet (CFS) process.

In principle, our approach is to use the candidate cationic scavengers with bentonite, which has a highly negative charge. This system represents an important form of the microparticulate process. We add sufficient scavenger, with a fixed amount of bentonite, to obtain a final zeta potential close to zero. Then, having leveled the playing field, we compare the final drainage results.

We do this at three different levels of anionic trash. At each of the three trash levels, the system with the highest drainage represents the most efficient scavenger for that particular form of anionic trash. The test becomes more stringent as the level of anionic trash increases.

Candidate cationic scavengers are distinguished by at least three specific characteristics:

1. Cationic Demand represents the amount of active scavenger required to bring the zeta potential close to zero, and is expressed in the referenced paper as "Cationic Demand, %".
2. Hydrophobic Content. A significant amount of anionic content is hydrophobic, representing such species as resin, lignin, pigment dispersant, etc. Therefore, part of the scavenger molecule should also be hydrophobic, in order to maximize the affinity for such anionic species. A commonly used example is the well-known "polyDADMAC", or poly-diallyl-mono-ammonium-chloride, where the diallyl portion is hydrocarbon, which is hydrophobic by definition.
3. Molecular Weight. As a general rule, the higher the molecular weight, the more surface active, and the more effective is the scavenger in being adsorbed on, and thereby permanently neutralizing, the anionic species.

In the study entitled "SCAVENGER EVALUATION", we used ground calcium carbonate (GCC) slurry as the source of anionic trash, at three concentration levels. GCC and clay slurry employ an anionic dispersant, which we understand to be a polyacrylate polymer. Coating color contains a high level of such fillers, and is a well-known source of anionic trash in the CFS process. Therefore, we consider that the use of GCC slurry provides a fair approximation of anionic trash specific to the CFS process.

The Lab Zeta Data is ideal for this experiment because it requires no sample preparation with stock of headbox consistency. Also, it directly measures a key electrokinetic parameter, zeta potential; as well as drainage, which correlates with Specific Filtration Resistance.

One candidate chemical, polyaluminum chloride (PAC), was disqualified at the lowest level of GCC slurry, viz 15% GCC, because it had a cationic demand well into double digits, as opposed to a small fraction of one per cent for all others. Also, it was the only candidate scavenger that decreased drainage, rather than increasing it.

As we increased the GCC level, from 15% to 25% and finally 50%, the amount of scavenger required to neutralize the anionic trash and reach zero zeta potential increased, and the effect on drainage was increasingly adverse. In this particular series, designed for the CFS process, a modified polyethylenimine, Polymin SKA from BASF, and a polyDADMAC, Alcofix 110 from Allied Colloids, were consistently the most effective.

We extended this work, as shown on slides 41 and 42 of the Zeta Data Presentation on the web site. Levels of 2%, 25% and 50% GCC were employed. Because of its wide commercial acceptance we used Nalco 7607, which we understand to be a polyamine, as the scavenger. The results are shown graphically. As the amount of anionic trash increases, cationic scavenger usage increases, and process efficiency decreases.

Anionic trash is clearly bad for process chemistry efficiency. These studies attempt to quantify precisely how bad, and how best to deal with it.

John G. Penniman

Email your experience and suggestions, and I'll put them on the web site with attribution, so that others can respond.