PAPER CHEMISTRY LABORATORY,
INC.
October 2006 Newsletter
How Papermaking Nanotechnology Emerged from Wet End Chaos
Maximizing Cost-Efficiency at the Highest Level of Quality
John Penniman
the October issue of the new tappi pima journal GLOBAL 360, on its Web site.
Introduction
Nanotechnology may be a long word with complex innuendoes but, as we begin to think about implementation, one only need remember three things:
1. Van der Waals postulated that the inter-particulate attractive force increases inversely as the 6th power of the distance separating them. Papermaking stock must be mixed with chemicals until it is homogeneous.
2. Homogeneity is achieved by incorporating functional chemicals into a low surface tension liquid that diffuses through the stock after injection. Functional chemical usage can be decreased by 1 to 2 orders of magnitude.
3. Nanoparticles used as process chemicals are high in surface area and total negative charge. When added to precisely neutralize cationized papermaking stock, strength, productivity and quality can be maximized.
The second half of this essay provides more detail in exploring the application of nano principles to papermaking, and explains why cost-efficiency is maximized at the highest level of quality.
We start with the haphazard historical papermaking evolution into the current awkward and chaotic state of the art.
Paper Machines
In the early 19th century a Frenchman named Fourdrinier conceived of a mechanical means of producing paper, via a continuous loop of wire mesh. It was left to the British to produce an early working model. In principle, pulp was dispersed in water at a concentration of 1% or less and extruded onto a coarse wire mesh moving as a continuous belt. Fibers were retained on the surface of the mesh while water drained through
Paper machines are only produced by a few highly specialized firms, which lately have lost the cutting edge of technology. For example, a small team of consultants was recently enlisted to advise a manufacturer in placing an order. Among other recommendations, they specified a more compact and cost-efficient white water system with much improved mixing.
The machinery supplier declared that any changes in the proposal would void the performance warrantee. The proposed order was cancelled.
Introducing Chemistry
By mid-century, alum became the first chemical used in papermaking. “Alum” is short for aluminum sulfate Al2(SO4)3. The aluminum ion Al+++ is intended to neutralize the negative charge of the papermaking stock, causing it to flocculate on the wire, improving retention and productivity.
The only serious process control effort was a mis-guided effort to control the pH by adjusting alum feed rate. pH may be defined as the negative logarithm of the hydrogen ion concentration. This means that there is an order of magnitude shift between each pH value. Effective control would require using a pH sensor with 10 divisions of 0.1, each divided again by 10 divisions of .01. This is beyond the sensitivity, accuracy and repeatability of commercially available pH sensors
The notorious side effect is that the acid paper thereafter deteriorated, turning into grey dust, even on the shelves of the Library of Congress, prompting heroic efforts to re-process and recover historical records. Only archival paper, such as that intended for bibles, was made with calcium carbonate instead of alum.
During WWII scientists at the Hanford Arsenal in Washington became concerned that silt in the water pumped through the atomic reactors for cooling was making too much silt highly radioactive. After all, the temperature of the Columbia River was increased by 1oF. as a result.
Nalco was called in and devised a polyacrylamide resin flocculant system that precipitated silt prior to the cooling stage. Similar chemistry was later introduced for the chemical pre-treatment of potable water and, in a further metamorphosis, became “retention aids” and “drainage aids” to improve the
paper-making process.
This must be viewed as a brute force approach that regrettably remains in wide use today. Macro flocculation sufficient to bridge the gaps of a 40 mesh wire has serious adverse effect, both on the quality of formation and on the physical strength properties. If the chemistry is over-done the system can easily be over flocculated, with major adverse effect on water removal and further degradation in formation and physical properties.
Stock Homogeneity is the Criterion for Thoroughness of Mixing
An important purpose in adding process chemicals, as opposed to functional chemical additives, is to neutralize the repulsive negative charge so that the particles may approach as closely as possible on an intermolecular scale. As we have noted, Van der Waals’ powerful attractive Force states the intermolecular attraction increases inversely as the 6th power of the distance.
Properly executed, this action can enable excellent microparticulate control of formation, significant increase in fine paper ash level, and (contrary to conventional wisdom) increased strength at the higher ash level.
The electrostatic charge is called the zeta potential, measured in millivolts (mV). Papermaking stock is negatively charged or “anionic”, and various positively charged or “cationic” chemicals, such as cationic starch, polyamine, polydadmac (poly-diallyl-dimethyl-ammonium-chloride) can be used to reduce the repulsive negative charge to zero. We have collected machine data that shows a high 0.72 Correlation Coefficient of zeta potential with AKD sizing.
An instrument called the Zeta Data can measure zeta potential to a standard deviation of 0.20mV under controlled laboratory conditions. When measured on a paper machine, zeta potential standard deviation becomes a measure of thoroughness of mixing, or stock homogeneity.
Typical zeta potential standard deviation values obtained on a modern machine are all greatly in excess of 0.20mV: 1.5-2.0 for tissue, 2.5-3.0 for alkaline fine paper, and consistently higher for a 3-headbox board machine, actually as high as 4-5mV with endless breaks and poor productivity.
Another major disruptive factor can be the existence of a common white water system. They are a functional component of 3-headbox machines, and the reason why multiple-headbox machines have poor runnability.
Calcium Carbonate, Clay and Titanium Dioxide
For convenience in handling and economy of shipment, fillers and pigments are usually shipped in slurry form, as high solids aqueous dispersions made with anionic dispersant and water. The anionic dispersing agent on the surface of the filler reacts first with a retention aid, and in fact must be totally precipitated before the retention aid can react with the filler. This is a wasteful and inefficient means of obtaining retention and imposes an unnecessary obstacle to the task of creating an efficient retention process. It would make far more sense to provide the slurry as a cationic dispersion.
This is done in the precipitated calcium carbonate (PCC) plants built as “satellites” to paper mills. The PCC is pumped over with a slightly positive zeta potential, at 30% solids. Unfortunately, no one has been either sufficiently wise or opportunistic to control the zeta potential and maximize both retention and formation. Meanwhile, calcium carbonate and synthetic size have replace alum and rosin in fine paper, in the developed world.
Measuring “Charge”
The prevailing paper industry practice is to measure cationic demand in the lab. The objective is to avoid over-cationization (which could upset the process) by maintaining a small residual cationic demand.
There are two problems with the practice, both of which are decisive:
1. The procedure is not easily reproducible.
An on-line Zeta Data was installed at the headbox of a specialty groundwood machine for a year, with continuous data output. Once a day, a key technician from a major international chemical supplier visited the lab and made a cationic demand measurement. At the end of the year, a Correlation Coefficient was calculated with zeta potential. It was only 0.17.
On another occasion, we invited a well-known cationic demand measurement expert to spend a week in our lab, working alongside Professor Anatoly Makhonin, as he did a series of experiments with the Lab Zeta Data.
The expert appeared daily and appeared to be working, but never produced any data. When he left to return home on Friday, he promised that the calculations would be done over the week-end and reported to us on Monday. We never heard from him again.
The author attended a Stockholm exhibit at which a prominent US on-line cationic demand instrument was being displayed. The president confided that all his instruments gave different results when tested prior to shipment.
2. Even if the data were repeatable, the measurement lacks the capability of ensuring that zero zeta potential is maintained at the time of web formation.
Nanotechnology
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.
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 reduce the positive charge to precisely zero zeta potential.
The electrostatic balance thereby created provides an open structure
Prevailing papermaking technology using cationic demand measurement for control is counter-productive, as we have noted, because it 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 beneficial 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. A running accounting of total product cost-on-the-reel can also be provided, as a real time “what if” tool, to help enable the most cost-efficient operation, given the available stock and chemicals.
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 certainly seems logical to assume that the intermolecular distance between chemical and stock would be decreased by mixing thoroughly to homogeneity. 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, and a functional chemical cost savings upwards of 90% is feasible.
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 established physical chemistry phenomenon of hydrogen bonding.
We should expect the following improvements to result from the use of nanotechnology as a guiding central concept:
1. A significantly higher level of absolute quality and performance
2. A high and impeccable level of quality uniformity
3. Simultaneously maximizing process efficiency,
4. More efficient use of chemicals, leading to significant cost reduction
5. Decrease in dryer energy usage on a high speed machine
6. Productivity increase of 5-15%
It is clear that papermaking can be accomplished at significantly lower cost and much higher quality by observing the principles of nanotechnology
of the filler + fines + fibers and nanoparticles, all bound
together by electrostatic attraction, because of the precisely
correct amount of cationic and anionic chemicals. It is the
mutual attraction of the positive and negative particles that
creates the structure so conducive to water removal, retention and
strength. BALANCE is the key word, as 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.
and either maximizing physical property parameters or reducing basis weight