Alkaline papermaking nanotechnology: the ideal digital imaging and printing surface, Solutions!, Online Exclusives, September 2004

Nanotechnology underpins the basic principles of papermaking. It is a hot new sector for investors because it so often brings great and unexpected benefits. We believe that is also the prospect for papermaking.

Stock market followers are aware of the new Merrill Lynch "Nanotechnology Index," which is so volatile its structure must be altered quarterly. It is listed under the symbol NNZ on the American Stock Exchange.

About two decades ago papermakers serendipitously began to use nanoparticles. They were called "bentonite" and "colloidal silica", and can be characterized by their large surface area per unit of weight. A nanoparticle weight of 5 grams, equivalent to the weight of a five-cent piece, could represent up to an acre of surface area.

Properly used, these particles can form nanoflocculation loci. Functional chemical additive usage could be reduced by 2 orders of magnitude, or 99%. Sheet ash could be increased by 5-10% at a higher level of sheet strength. Press section re-wetting on a modern, high speed machine can be significantly reduced.

These are extraordinary claims that will be more fully described in the following text. The reason for citing them at the introduction is to ask for the reader’s patience. While a detailed explanation of effective nanotechnology implementation calls for recitation of multiple apparently unrelated segments, a considerable amount of inductive reasoning will make the relationships integrally clear at the end of this article.

Papermaking nanotechnology
To use nanotechnology effectively in the paper industry, we must strictly follow three guiding principles:

1. Employ the most efficient chemistry.
   Nanoflocculation provides the best possible balance between retention and drainage on one hand; and formation on the other.^[1] It becomes the process of choice because high retention and drainage are important cost and performance considerations, while excellent formation is fundamental to strength properties.
2. Maximize intermolecular contact.
   Van der Waals force increases exponentially with proximity. Mixing of stock with chemicals must be so thorough that homogeneity is obtained. Application of the "Six Sigma" quality doctrine enables quantification of this key parameter.
3. The repulsive negative surface charge must be neutralized.
   Sheet formation must take place at zero zeta potential in order to maximize the benefit of van der Waals force.

At the conclusion of this article, readers will understand why nanotechnology makes feasible continuously optimized quality and productivity at minimum raw materials cost under closed loop control of chemical feed rates.

Runnability and quality problems
An independent research organization^[2] has reported that there are 117 currently operating multi-headbox machines. In the authors’ view, the concomitant employment of a mixed white water system enormously complicates the task of obtaining stock homogeneity. Similarly, many large paper mills have considered it economical to install a common white water system, again making the task of achieving homogeneity much more difficult, if not virtually impossible.

We have often received requests to investigate and help eliminate unaccountable runnability and quality issues, often for political reasons because the local staff had given up. It is a great relief to finally understand exactly what causes them.

For more than a decade, one of the authors has monitored zeta potential on-line, often during investigation of runnability problems. There is a high correlation between zeta potential standard deviation, which can be as high as 3-4mV, and poor runnability.

The "Six Sigma" quality doctrine can quickly quantify thoroughness of mixing. The zeta potential standard deviation measured in the lab of 0.2mV is multiplied by 6, to obtain a Six Sigma target value of 1.2mV. An on-machine zeta potential standard deviation must not exceed 1.2mV in order to maximize thoroughness of mixing and thereby obtain stock homogeneity. To ensure a uniform high quality of product performance and to maximize productivity, a higher standard deviation is unacceptable.

History
For two decades, paper chemists did experiments that we, in our wisdom, regarded as informative and significant. In retrospect we made small progress. As a result, we were regarded by papermakers (and still are) as working outside the mainstream of the process.

Papermaking chemicals are called "additives"; experiments on the machine are called "trials". The TAPPI Course originally carried the simplistic name, "Retention and Drainage." In some cases our best efforts were merely successful in improving machine runnability. Basically, we lacked sufficient breadth and depth of understanding to effectively trouble-shoot or optimize. As a result, we floundered (and still are.)

One important activitity of this primitive era was "deposit analysis". It required costly instrumentation, highly trained manpower to do lab work, it took time and patience, and (importantly) gave management an impression of motion and progress. The contribution to process optimization is negligible. In retrospect, it is clear that reverse engineering of nanotechnology is impossible.

The first on-line zeta potential instrument was installed on the headbox of a paper machine in the summer of 1989 by one of the authors, with the sponsorship of a young Japanese scientist, Takanori Miyanishi of Jujo Paper.^[3]

In late 1991, an author’s lab initiated a series of experiments with a Russian professor, Anatoly Mikoyan, which extended into 1998. The research finally numbered more than 5000 lab and on-line experiments, and was a major learning experience.

Early in the period, Professor Al Springer of Miami University suggested adding the measurement of "drainage" (later amended to a more scientific expression: Specific Filtration Resistance, or SFR) to our on-line zeta potential measurement instrumentation. Dr. Springer helped greatly with the task, and with its documentation in TAPPI JOURNAL.^[4] A copy of his paper is posted on the internet: www.papermaking-chemistry.com/94aug121.pdf.

First comprehensive effort to optimize chemistry
One of the more difficult papermaking chemistry processes seemed to be "CFS", or coated free sheet, so we focused on it. Our objectives were to stabilize quality and reduce the incidence of coated broke, stated by workers in the field to comprise 20-40% of production.

The initial experimental plan to improve the CFS process was to vary the electrokinetic parameter, zeta potential, over a wide range and measure the effect on process and physical property parameters. Lynden Stryker of Specialty Minerals mentored the early work. His firm was pioneering in the installation of satellite precipitated calcium carbonate (PCC) production facilities that produce filler free from anionic trash (in the form of dispersant); a more effective filler in highly filled sheets.

In the lab, handsheets were made from each 1.5 liter sample that had been produced while monitoring zeta potential and drainage, using an 8” diameter dynamic hand sheet mold. The handsheets were used to assess sheet ash, internal sizing and Scott Bond. The first “eureka” experiment is accessed by referring to page 10 of www.papermaking-chemistry.com/waal.htm^[5]

The process parameters, retention and drainage as well as the physical property parameters, sizing and Scott Bond, are all maximized by operating in a final zeta potential range of +1 to +6mV. Eureka!

This work just cited is basically nanotechnology, in the sense that intermolecular contact must be maximized for maximum efficiency. In fact the underlying theory indicates that efficiency increases exponentially with intermolecular proximity. Supporting data is provided in the paper: www.papermaking-chemistry.com/procchem.htm^[6]

When CFS producers quote 20-40% broke generation, they include product rejected because of errors in the coating process, as well as cuttings returned from converting operations. Recycling of coated broke is an exceptionally sensitive, difficult procedure because the anionically dispersed pigment and latex in the coating make it highly negative and therefore disruptive to the wet end, especially in the variable amounts that are commonly used.

It is ironic that, if the latex paper coating itself were produced according to nanotechnological principles, it would not only cease disrupting the wet end, it would have infinitely greater adhesion to nanotechnological paper, and provide an impeccable surface for printing.

The fact is that digital inks are universally based on nanotechnology. It makes little sense to apply modern inks to a primitive coating, resting on an archaic substrate. The perfect digital imaging sandwich will comprise nano paper, nano coating and modern ink. The paper and coatings industries clearly have new homework assignments.

Thoroughness of mixing/cationic decay
A world class machine at a coated free sheet (CFS) mill, for which one of the authors was technical director, has two on-line zeta potential sensors: one at the headbox and a second at the blend chest. On one occasion, when the headbox zeta potential was in the range +2 to +4mV zeta potential, a headbox sample was taken, and measured promptly on the lab zeta potential instrument.

The result obtained by the lab measurement, completed in about half an hour, was -7 to -9 mV zeta potential. The zeta potential had become 10 to 12 mV more negative in a short period of time. The sharp change is caused by the classical phenomenon of cationic decay with time, as a consequence of imperfect mixing of chemicals and stock.

Too much cationic chemical (including cationic starch) was being added too close to the headbox. The headbox zeta potential standard deviation was measured at 3.5mV, far exceeding the Six Sigma target value of 1.2mV. One can safely conclude that off-line measurement of imperfectly mixed stock is of questionable value.

Incidentally, the reason our lab experiments appeared to be optimized in the initially rather puzzling range of +1 to +6mV is caused by the same phenomenon: cationic decay. The particular magnetic stirrer used with the Mark V Dynamic Paper Chemistry Jar is insufficient to homogenize the stock in the time allowed.

The expression "cationic decay" is itself the subject of debate, and deserves explanation. Cationic chemicals are used to improve either physical properties, in which case they are called "functional" additives; or to improve process parameters such as retention or drainage. In either case they are supplied in a positive, or cationic, form in order to be attracted to the negatively charged cellulose fibers.

The scenario then becomes more complex, because the cationic chemicals react first with the most negative stock component, usually the fines, possibly sometimes in part the fillers. Then they migrate to the next most negative surface, the fibers, and spread out in a symmetric way. Last, small molecular weight cationic components migrate into the fiber and neutralize the negative internal charge.

This is a dynamic process that requires initial thorough mixing and then can take considerable time for completion. Workers at the Swedish Forest Products Institute in Stockholm once estimated that it took a full week to reach complete equilibrium.

The nanoparticles used to maximize retention and drainage are all highly negative in current technology and are among those least thoroughly mixed. New Finnish technology is understood to be in development to address the issue.[7]

The last half of the 20th Century
For the past 50 years, paper industry efforts to achieve process chemistry optimization have been futile:

• Paper mill management fails to implement new technology
• Papermakers focus on runnability, ignore optimization
• Chemical suppliers’ dominant objective is to profit
• Paper machine design ignores process chemistry
• Paper science academics publish technically excellent papers in prestigious journals at the expense of real industry needs.

The defining issue of the era, alkaline papermaking, is symptomatic. In about 1850 papermakers started adding aluminum sulfate (alum) at the wet end. The acid pH turns paper yellow and degrades strength so greatly that the product eventually turns to dust.

Scotland pioneered in alkaline papermaking, using dry ground chalk, because customers prefer a whiter product of archival quality. The mill is better off because greater strength permits higher filler loading, leading to lower cost, even though the internal size is more expensive than rosin.

The dissolved bivalent calcium ion precipitates organic solubles and colloidals, reducing BOD/COD of the waste effluent by about 20%. This can be a mixed blessing, however, because dissolved calcium carbonate has been known to precipitate and form nasty deposits.

In short, alkaline process chemistry was so radically new and different that it was not embraced by paper machine management. Guided by a perception of superior quality at lower cost, many American CEOs made a command decision to convert, and a large wave of conversions began in the 1980s.

Nanotechnology represents another conceptual break with the past. The CEO may again be forced to intervene, in order to realize the benefits.

Stock homogeneity: Breaking the mold
One of us was invited by Roger Campbell to help improve the Weyerhaeuser process for making export newsprint. The consulting plan was to organize the lab investigation and then prowl the premises, clipboard in hand, trying to unearth information of value.

In chatting with a paper machine superintendent, an early occasion was recounted in which the kerosene emulsifying equipment malfunctioned, and raw kerosene was applied to the felt for cleaning purposes. Amazingly, the first drying section steam pressure fell by half. This was unexpected and therefore unwelcome, so the normal process was quickly resumed.

Philosophically, it is the occasional unexplained anomalies that are the most important learning experiences (in addition to our mistakes.) As soon as we returned to the home lab we attempted successfully to reproduce the phenomenon. More than three years later, we had learned two important new things.

Application of a small quantity of an innocuous synthetic hydrocarbon with a low surface tension can reduce water re-wetting in the first press section of high speed machines. The higher the speed, the greater the reduction in re-wetting. Secondly, laboratory process chemistry development work revealed that wet end application of internal size dissolved in the hydrocarbon can reduce internal size usage by two orders of magnitude. A comparable efficiency increase was also demonstrated with wet strength resin emulsified in the hydrocarbon.

The CEGEP pilot plant machine in Trois Rivieres, Quebec was helpful in sorting out some early issues. The old cylinder board machine in Lafayette, Indiana showed no effect, nor did the recycle board BRDA member machine in Milwaukee. A Syracuse University pilot plant machine with plugged felts showed no response.

However, a meticulously supervised pilot plant study by Ivan Pikulik and Larry Allen of PAPRICAN provided excellent results in reducing the dryer energy requirement. The Albany International felt company variable speed pilot plant showed increasing effect with machine speed. We conclude that the dynamics is a key factor.

Finally, the phenomenon occurred again serendipitously on a modern Canadian board machine during a defoamer trial. We have reason to believe that an important component of the silicone defoamer is a higher molecular weight homolog of the same synthetic hydrocarbon that we use.

Consulting engineers advise that recycling the hydrocarbon will be straightforward, and involve collection, separation by specific gravity, and automatic decantation. (Further discussion of the Penniman Process is beyond the scope of this paper.)

In the "Maximizing the van der Waals Force" paper previously cited, the HST sizing results vary from 3X to 15X the required values, and that is the real world. Contrast this with a potential reduction of sizing usage of 99%!

The use of preferential wetting, instead of brute force mixing, in conjunction with the other nanotechnology benefits waiting for exploitation, can save the paper industry billions of dollars/year. Among the major beneficiaries will be industry customers who currently must incorrectly assume that major players are fully and effectively collaborating in optimizing the process.

Implementation
Use the most efficient (which will likely also be the most cost-effective) chemicals. Add them as far upstream and as dilute as feasible, with static in-line mixing as necessary to obtain homogeneity at sheet formation.
Control feed rates of the charge-neutralizing cationic component and the anionic microparticle so that that the drainage necessary to maximize productivity is maintained, while balancing feed rates, so that the zeta potential of homogeneous stock, at sheet formation, is controlled at zero.

Conceptual conclusion
Papermaking chemistry is re-defined as a nanotechnology. Effective application of the described principles can reduce industry costs by billions of dollars per year, while providing customers with products of superior performance and more consistent quality.

The upper limit on cost savings and quality improvement will rest on the quality of execution of three simple principles.

References

1. Huber, P., Pierre, C., Bermond, C., Carre, B. TAPPI J. 3 (4); 19 (2004)
2. Fisher International, South Norwalk, CT private communication www.fisheri.com
3. Miyanishi, Takanori, "On-Line Zeta Potential Analyses of a Fine Paper Machine and a Newsprint Machine” Presented at the Papermakers Conference, San Francisco, CA., April, 1994
4. Springer, A., Penniman, J., Pires, Eduardo TAPPI J. 77 (8); 121 (1994)
5. www.papermaking-chemistry.com
6. Opus cited
7. www.wetend.com