PAPER CHEMISTRY LABORATORY,
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EDITOR'S NOTE:
Our lab did more than 5000 experiments to reach a definitive conclusion on the influence of electrostatic charge, as measured by zeta potential, on the papermaking process and product properties.
The procedure was to make up 1.5 liters of furnish in a 2.0 liter beaker, while measuring zeta potential and drainage; then to make hand-sheets from that furnish, and test for sizing, strength, and retention. The latter three parameters were plotted, along with drainage, against an incrementally altered zeta potential, to assess the influence of electrostatic charge.
Because of the real world variabilities, including the influence of anionic trash and cationic decay, the task of interpretation was not simple. Initially more questions were raised than answered. It took literally years to fully appreciate the data significance.
We have finally concluded that, to fully understand and perfectly execute wet end papermaking chemistry, it must be viewed purely as a nanotechnology. The stock must be mixed to homogeneity and the repulsive negative charge eliminated, to bring the particles close enough that intermolecular attractive forces come into play.
It CAN BE a perfect world because these actions maximize both process AND physical property parameters. A more detailed explanation is provided in the following article: Nanotechnology Enables Cost-Effective P&W Paper.
Implementation is economical and straightforward. The 8th generation on-line Zeta Data sensor system is inexpensively available on a lease basis.
Please forward your questions and comments.
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Summer 2006 Newsletter Nanotechnology Enables Cost-Effective P&W Paper |
John Penniman |
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Historical Context Introduction Since WWII organic chemical “additives” have been used in producing paper, in a successful effort to improve both process efficiency, principally retention and drainage; as well as physical properties of the final product. The art of wet end chemistry has grown like Topsy, with incremental improvements of all imaginable kinds, but in the context of a complete absence of a central, dominating and guiding technical concept. The compelling central concept has finally been perceived by a few workers, including the undersigned, to be that of nanotechnology. Physical forces on an inter-molecular scale are dominant. In order to maximize the cost-efficiency of papermaking, their influence must not only be recognized, every effort must be made to maximize them. Fine paper has been selected as the best example with which to make the case. Printing and writing (P&W) grades are among the most complex in terms of chemistry, and most demanding in terms of performance. Nanotechnology Nanoparticulate Process Nanoparticulate processes were introduced about 1980, based on nanoparticles such as colloidal silica, termed by Eka Nobel Compoziltm; or bentonite, by Allied Colloids (now Ciba), and marketed as Hydrocoltm. In principle, the microparticulate process is of particular value in making product containing high filler and/or fines. Properly executed, it increases drainage (or ‘water removal’) without adverse effect on formation, physical properties, printability, etc. Ideally, this is accomplished in two steps. An appropriate charge-neutralizing cationic chemical is added until the entire stock is at a target, positive zeta potential, often in the range +5 to +10mV. At this point, step two takes place. Sufficient nanoparticle (by its nature highly negative) is introduced to precisely reduce the positive charge to zero zeta potential. The electrostatic balance thereby created provides an open structure of the filler + fines + fibers and nanoparticles, all bound together by electrostatic attraction, because of the precisely correct amount of cationic and anionic chemicals. The key word is BALANCE. It is the mutual attraction of the positive and negative particles that creates the structure so conducive to water removal and retention. However, the charges must be precisely balanced, or negative effects occur: first, water removal and retention are impaired by the imperfect structure; and second, excess surface charge exerts a repulsive effect, further impairing retention and also degrading strength. Prevailing papermaking technology, using cationic demand measurement for control, is counter-productive because it always leaves a residual repulsive negative charge. Additionally, some form of “retention aid” is added to achieve retention and drainage by macroflocculation. Process parameters and physical properties are degraded, with major negative influence on retention, drainage, formation, strength and printability. On the other hand, under closed loop control of chemical feed rates, the charge-balanced system is flexible. Flow rates of the cationic chemical and the nanoparticle can be increased or decreased in tandem, while maintaining a final balance of zero zeta potential, in order to continuously maximize process parameters, physical properties, and productivity, despite a wide variation in stock composition. Note that maximizing productivity has a major influence on cost effectiveness. The eighth generation on-line Zeta Datatm technology dramatizes the significance of flexibility in process control. 21st Century Technology makes it highly cost efficient, lease price only $30K for the first year. A running accounting of total product cost-on-the-reel can also be provided. Zeta Potential The author’s laboratory did over 5000 controlled experiments, in which the electrostatic particulate charge, or zeta potential; and specific filtration resistance, or drainage; were measured as process parameters. A handsheet was made, and sheet ash, sizing and Scott Bond were measured from it. Our first exciting conclusion was that maximizing the process parameters, retention and drainage, resulted simultaneously in maximizing the physical property parameters, such as sizing and Scott Bond. We were tempted to think that perhaps it was a perfect world, with all in proper balance. In fact, it was our first major clue as to the importance of intermolecular proximity. Van der Waals Force Our second exciting conclusion occurred when we controlled one experiment close to zero zeta potential, with the second experiment uncontrolled and equilibrating at some negative value. The result in the controlled case was higher sheet ash, accompanied by higher Scott Bond. We concluded that the counter-intuitive influence was van der Waals force, which increases exponentially as intermolecular distance decreases. This was our second nanoscience insight. Thoroughness of Mixing It would seem to follow that the intermolecular distance between chemical and stock would be beneficially decreased by increasing thoroughness of mixing. Chemists are in substantial agreement that wet end mixing has not been sufficient to the task on real world high speed machines. On-line instrumentation quantifies the insufficiency by measuring zeta potential standard deviation at the headbox, as a function of thoroughness of mixing. Modern, high speed machines with complex chemistry have a typical zeta potential standard deviation of 4-5mV accompanied by an unacceptable frequency of machine breaks and excessive chemical usage. A Principal Cause of Machine Breaks is Lack of Thorough Mixing. The production management of a highly filled super-calendared (SC) sheet, used in rotogravure printing, has long complained of poor uniformity in a cross-machine (CD) direction, measured both by print tests and by scanning electron microphotographs. Bearing in mind that uniformity is even more important to consistent print performance than sheet quality, it is surprising that industry has failed to recognize the connection to thorough mixing. It was left to a clever Finnish engineer, Jouni Matula, to develop the TrumpJettm, for which he was first importantly recognized in 2003. TrumpJettm technology has grown into a small family of what Jouni calls “booster pumps.” They are oriented vertically to pipe flow, and introduce a stream of additive at low concentration relative to the stock stream, under great pressure and high speed, ensuring thorough mixing. The TrumpJettm is now so widely used that it obviously performs a cost-effective service. In particular, Jouni claims that some of his coated fine paper mills are saving “millions”. Preferential Wetting We did carefully controlled experiments in which small amounts of the internal size alkyl-ketene-dimer (AKD) were added to a liquid with excellent wetting properties. The quality of dispersion of the AKD in the stock was much improved and we were able to reduce its concentration by one to two orders of magnitude. Similar results were obtained with another functional chemical additive, wet strength resin. There is nothing mysterious about this; the technique is similar to that which is widely used in dispersing silicone defoamer. What is important to recognize is that conventional mixing has poor efficiency. We used the Albany International variable speed press section pilot plant, in Albany, NY, to conduct a preferential wetting efficiency study. Results showed that the low surface tension liquid could greatly reduce water re-wetting in the first press, as a function of machine speed, the faster the better. The amount was surprising: up to a 25% reduction in dryer section energy. Hydrodynamics is a key factor, as the phenomenon is not observed on old, slow recycle board machines. Summary and Conclusion A number of phenomena, all with inter-molecular significance, have each been revealed to exert profound beneficial influence on both the papermaking process and physical property parameters. This is not intended as a comprehensive list. Our investigation did not include the intermolecular phenomenon of hydrogen bonding, which already has a large and enthusiastic following.
We should expect the following improvements to result from the use of nanotechnology as a guiding central concept:
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| John G. Penniman |
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www.papermaking-chemistry.com
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