Spring 2006 Newsletter

John Penniman

Historical Context

“Maximizing retention/drainage in pulp slurries”, published by the author in Paper Trade Journal on March 1, 1978, was notable for two reasons: It represented the first use of the expression ‘anionic trash;’ and it showed that satisfying the cationic demand maximized drainage.

This was the best we could do with wet end chemistry until the introduction of nanoparticles. Towards the end of the article, we described the progressive effects of adding a charge-neutralizing cationic chemical.

It first reacts with the soluble anionics, including hemi-celluloses, lignins and humic acids. The next most reactive component, according to Tom Lindstrom and Christer Soremark, (1) is the fines. Continuing addition of cationic chemical causes them to be flocced, maximizing fines retention.

As chemical addition progresses, increasingly large fibers are flocked, and an interlaced structure is formed because the cationic chemical bridges between the fibers. That structure accounts for the achievement of superior retention, drainage and physical properties.

Finally, a cationic chemical of sufficiently low molecular weight will sink into the fiber structure, creating a dynamic that requires hours or days to equilibrate. The equilibration process is called ‘cationic decay’.

Flash forward two decades. We are operating the headbox of a large CFS machine at a zeta potential in the range 0 to +2mV. A headbox sample is expeditiously taken to the lab and measured on the lab streaming potential instrument. A zeta potential of -9 to -11mV is obtained. The difference between the two measurements represents the magnitude and significance of the dynamic ‘cationic decay.’

Modern Technology

We now have the technology, as exemplified by TrumpJettm chemical mixing stations, to mix chemicals and stock far more thoroughly than previously. We also have the on-line Zeta Datatm System to measure zeta potential and control chemical feed rates to obtain continuous process and product quality optimization.

Question: What can we do to incorporate the time-dependent ‘cationic decay’ phenomenon into the execution of our plans? Please refer to the exhibit shown on the link: www.papermaking-chemistry.com/max_waals_force.ppt#5  (Page 5 of presentation.)

The last named curve: cationic starch and the nanoparticle colloidal silica, orange in color, exemplifies the cost efficiency of nanotechnology. It represents an excellent balance between a high level of retention and good formation. This is the ideal we constantly strive to achieve.

The first listed chemical, polyethylenimine (PEI) B, in purple, has a linear curve that maximizes at about 50% retention, with a degradation of about 70% in formation. (2) But what creates the strange looking hook at the end?

The initial sharp inflection point represents zero zeta potential, maximum retention. The vertical continuation means improving formation, and no loss of retention; increasing cationicity simply makes the flocs smaller. The second, less pronounced, inflection point indicates commencement of a loss in retention caused by a cationic redispersion of the stock components.

Support for this interpretation is found in the practice of some who are highly skilled in management of the wet end plastic laminate base sheet process, in which formation quality and good retention are paramount considerations. They have learned to operate the headbox at a zeta potential, measured off-line by microelectrophoresis, of about +10mV.

Conclusion

Stock homogeneity is mandatory to achieve nanoflocculation. The TrumpJettm can mix thoroughly, but cannot address the time-related issue of cationic decay. Therefore, it would be prudent, after maximizing TrumpJettm efficiency, to control the headbox at a positive zeta potential. The precise optimum value will depend upon the chemistry specifics, such as the molecular weight of the cationic, and be determined empirically.

References

     1. Lindstrom, T., and Soremark, Ch., J Colloid Interface Sci 55:305 (1976)
     2. Pierre, Christian, CTP, Grenoble. Private communication

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