PAPER CHEMISTRY LABORATORY, INC. EDITOR'S NOTE: 'Six Sigma' quality doctrine has been largely ignored by the paper industry, but has become a DNA component of the American military-industrial complex. Application to the papermaking wet end is described in the following Newsletter. Industry acceptance can add billions of dollars to the bottom line. Strong leadership is essential to success. For convenience in reference access, Web site links are provided. JGP
January Newsletter 2004
RAISING THE BAR ON WET END PERFORMANCE
Preface
In 1983, a Motorola engineer realized that decreasing the variability of a manufactured product would reduce the number of defects, accompanied by a reduction in production cost and increase in consumer satisfaction.
The ‘Six Sigma’ quality doctrine emerged. Reducing quality variability to 6X the standard deviation of the parameter being measured results in reducing defective output to 3.4 units per million produced. “Six Sigma’ has been implemented by many large manufacturing companies.
Jack Welch, formerly CEO of GE, credits it with adding literally billions to their bottom line, and says: “It has become part of our DNA.”
The balance of this article contains data collected over a decade of examining wet end chemistry variability. For the first time, wet end data is objectively analyzed and recommendations are made for improvement. The principal thrust is simple and straightforward.
Introduction
Chemists have always realized that modern paper machines do very poorly in mixing chemicals thoroughly with stock. Truth be told, papermaking chemistry is fundamentally dependent on maximizing inter-molecular contact. This can only be accomplished by two simultaneous actions:
- Mix the stock thoroughly, so that fibers, fillers and chemicals are homogeneous.
- Simultaneously, neutralize the repulsive negative surface charge.
Sheet formation with a homogeneous stock at zero zeta potential is essential to obtaining the highest product quality at the lowest raw materials cost, and maximum productivity.
Historical Chronology
Traditional suppliers of water treatment chemicals, like Nalco and Betz, started using their flocculants as “retention and drainage aids” shortly after WWII. This was so terribly foreign to traditional papermaking that machine management delegated the entire chemistry responsibility to the supplier.
Bright young scientists entered the industry and started trying to measure new surface chemistry parameters. The first particle charge methodology was called “microelectrophoresis” and measured the direction and speed of movement of fines in white water, in response to an applied electric charge. The author has introduced two such instruments to the paper industry.
Next, an instrumental titration was developed that measures cationic demand. This is cheap and quick, and persists today. It is based on a concept first introduced by the author in 1979, in which the expression “anionic trash” was introduced. Finally, an on-line measurement emerged, the streaming potential measurement of stock particles formed into a pad. www.papermaking-chemistry.com/zdproduct.htm
The Retrospective View
Viewing the history of wet end chemistry through the objective lens of science, what has really transpired is that the microelectrophoresis technology originally employed, as well as the cationic demand measurement that replaced it, can clearly be helpful in reducing the frequency of “upsets”.
The objective in using microelectrophoresis methodology was to operate the headbox at a small negative charge on the fines, typically in the range 0 to -10mV zeta potential. The cationic demand measurement objective was similar, to maintain a small negative charge in the white water.
What both measurements have in common is the prevention of a significant amount of cationic chemical from building up in the system. Experience has shown that, when the system becomes excessively cationic the efficiency of chemical additives decreases, and can potentially lead to an upset. The prevention of upsets has been the major objective for chemical suppliers.
Optimizing Wet End Chemistry
Careful lab work has been executed that measures an incremental progression of zeta potential in comparison with several performance parameters: drainage, retention, sizing and strength. More than 5000 experiments were done in the author’s lab over a period of six years, using internally developed instrumentation.
The most significant conclusion is that all four performance parameters are maximized within a narrow, slightly positive zeta potential range. We later came to realize that the positive value resulted from cationic decay, a symptom of poor mixing.
The optimization subject is discussed in more detail, supported by data extracted from many experiments, in a Web site article, “Maximizing van der Waals Force in Papermaking”. www.papermaking-chemistry.com/waal.htm
Homogeneous Stock
Data is presented on sizing efficiency in the Web site fall 2003 Newsletter, ‘Six Sigma’ Paper Quality. www.papermaking-chemistry.com/fall2003.htm It is measured with paper made on a modern machine and shows that, in practice, 3X to 15X more sizing is required than is necessary in carefully controlled lab experiments.
Unpublished data from our lab shows that dispersing internal size in a synthetic solvent comparable to kerosene, and applying it quantitatively to the wet web, enables reduction in size usage by as much as two orders of magnitude. The explanation lies in the low surface tension of the synthetic solvent, only one-third that of water, resulting in highly effective water displacement and extremely efficient sizing.
The January 2003 Newsletter www.papermaking-chemistry.com/january2003.htm relates stock homogeneity to zeta potential standard deviation. Data first reported by the author in 1993, www.papermaking-chemistry.com/pm93129.PDF (free viewer, down-load at http://www.adobe.com/products/acrobat/readstep2.html ) and collected over the past decade, shows increasingly poor runnability as the standard deviation increases. With a six sigma value of 1.2mV, the lowest zeta potential obtained on a modern machine is 1.9mV in the case of tissue, a product with modest chemical usage.
The worst case is a coated recycle board machine, with a standard deviation in the range 4 to 5mV and horrible runnability. At times, productivity was down to about 50%.
Multiple headbox machines are generally problemated because they invariably have a mixed white water system, making stock homogeneity virtually impossible of attainment with currently available technology.
Discussion of Results
Controlling wet end chemistry quality of mixing and charge neutralization within fairly broad limits can result in greatly reducing the frequency of upsets.
On the other hand, careful lab work shows that raising the quality of mixing to meet ‘Six Sigma’ criteria and operating the headbox at zero zeta potential can result in an astonishing improvement in quality and cost.
Examples include use of interfacial tension as the mixing force, so as to reduce sizing usage by 2 orders of magnitude; and increasing sheet ash by 5 to 10% with no loss of strength.
The task becomes that of translating the lab work to the machine.
Conclusions
The objective is to mix the chemicals thoroughly with the stock, and neutralize the repulsive negative charge prior to sheet formation.
Multiple on-line sensors must be installed to measure key wet end parameters, including zeta potential, drainage, and their six sigma values.
The paper machine wet end must be re-conceived as a mechanism for thorough mixing of stock and chemicals, and engineered to completely fulfill that function.
In the meantime, chemical addition points must be moved upstream, chemicals added at maximum dilution and mixed by strategically placed static internal mixers. Several less conventional, but promising means of mixing must be evaluated.
The loop must be closed on chemical feed rates of the microparticle and its cationic charge neutralizing component, in order to cope effectively with the well-known vagaries of the pulp and papermaking process. This will result in greatly improved quality.
Closing the loop also has the advantage of enabling the adjustment of drainage over a wide range, and enabling the use of inexpensive, low drainage components such as recycle fiber and larger quantities of filler.
When drainage is stabilized at a high level, retention will be also, with negligible adverse effect on formation. Should retention require further improvement, a small amount of retention aid may be thoroughly blended with the cationic chemical.
Finally, please refer to Figure 1, presenting a generalized image of what a ‘Six Sigma’ zeta potential frequency distribution graph should look like.
Figure 2 represents the current level of industry performance. Thoroughness of mixing is poor, at a standard deviation of about 3.55, about 18 sigma. The zeta potential is far removed from zero. It is atypical to have such a positive value, and is indicative of the lack of interest in closed loop control, even on a well-instrumented machine.
Effective application of the ‘Six Sigma’ doctrine to the wet end will save the industry many billions of dollars per year. The challenge is clear.
