PAPER CHEMISTRY LABORATORY, INC. Spring 2005 Newsletter
Papermaking Nanotechnology Has Multiple Benefits
John Penniman
Papermaking chemistry originated in mid-19th century with the use of alum (aluminum sulphate), followed by rosin size and clay, entailing manufacture at an acid pH, typically in the range 4-5. The period is clearly defined. Earlier paper was archival, but acid paper yellows and degrades to dust.
The scientific rationale for alum use is that the aluminum ion Al+++ neutralizes the negative surface charge of the cellulose fibers. If used in the proper quantity, retention and drainage on the wet end are improved.
The industry never learned how to determine and control the proper quantity. One effort involved measurement of pH as a function of alum concentration. The logarithmic pH scale is too insensitive to be useful; pH sensors of that era were not sufficiently rugged for heavy industrial use.
Dr. Takanori Miyanishi of Nippon Paper did clear up major process problems on an alum newsprint machine using an on-line zeta potential instrument. Nippon is currently converting all newsprint to alkaline.
Origin of Process Additives
During WWII, water from the Columbia River was diverted to cool the nuclear reactors of the Hanford Arsenal, coincidentally raising the river temperature by about 1o F. In due course, scientists began to question the wisdom of producing and discarding thousands of tons of radioactive silt.
Specialty chemicals were developed by Nalco to precipitate the river silt, thereby laying a foundation for the chemical pre-treatment of potable water and municipal waste effluent; and ultimately the first papermaking retention and drainage aids.
Paper machine management welcomed the promise of new chemistry, provided only that runnability was not impaired. Machine stability became the new charge of the chemical suppliers.
In a perfect world, quality would also have been included. Papermaking, however, was viewed as subject to such rapid and extreme excursions in variability as to be beyond the scope of supplier control. This was a major strategic error, a permanent obstacle to process chemistry optimization; and a disabling paradigm that must be eliminated.
Macroflocculation
“Retention Aids” and “Drainage Aids” are high molecular weight, low charge density polyacrylamide resins. Their names apply to the end use; chemistry is the same. They serve as a molecular bridge between particles, creating large flocs that bridge across gaps in the machine wire, in order to improve retention and drainage. Formation is degraded, and product performance pays a huge penalty because intermolecular contact is inhibited.
The active ingredient can be thought of as a relatively large polymer with multiple positively charged sites that are attracted to the negatively charged fines, fillers and fibers, creating a 3-dimensional macro-structure.
Alkaline Papermaking
Beginning just after mid-twentieth century, thrifty Scotsmen determined that the elimination of alum, clay and rosin, in favor of chalk and synthetic size, enabled production of paper that is cheaper, brighter and stronger. Chalk is an amorphous form of calcium carbonate that occurs in England, e. g. the ‘white cliffs’ of Dover. The other principal naturally occurring form is calcite, which is crystalline, found in many other parts of the world.
Two major differences between chalk and clay were in the coarseness of dry ground chalk, which raised retention levels; coupled with its low cost, delivered to Scotland in dry bulk. The buffering pH of calcium carbonate is slightly alkaline, approaching 8.0. Paper is again archival. Strength is up; cost is down.
Transition to alkaline papermaking occurred in many developed countries. In North America, paper machine management defaulted on the chemistry leadership role. General management planned, organized and directed the conversion, a well coordinated and highly successful operation.
Many other lesser issues remained for resolution, including retention of small particle size precipitated calcium carbonate (PCC); use of anionic dispersions of bulk calcium carbonate and/or clay slurry, which greatly reduce retention efficiency; and use of new size chemistry.
Microbiological growth in alkaline media is much more complex. It is most efficiently controlled by maintaining an appropriately high oxidation-reduction potential (ORP), and avoiding dead spots in the white water system. Otherwise, slime holes in the sheet or boil-outs are inevitable.
Alkaline papermaking in less developed countries has not been well received. A typical response is to obtain a quotation on the new size, compare its cost with rosin, and abandon the project forthwith.
Coincident with the use of chalk in Scottish papermaking, laboratory instruments found use to measure the electrostatic charge of papermaking components. Relationships were observed between the surface charge of cellulose fibers and fillers; and the process parameters and physical properties of the finished product. Expressions such as “anionic trash” and “cationic demand”, coined by an author, became industry vernacular.
Nanotechnology
Acceptance of new technology by the paper industry often takes about ten years, and is so gradual that it becomes difficult to assign precise introductory times to novel products. The first public presentation of the Allied Colloids new Hydrocol bentonite process is recalled to be at a Canadian Pulp and Paper Association meeting in the late 70’s. The Eka Compozil colloidal silica process is probably of about the same era.
Initially, the nanoflocculation process was poorly understood, the subject of much trial and error. Nanoflocculation is far superior to macroflocculation, even though usually executed with a chemistry efficiency of only about 10%. Homogeneity at 2 critical junctures is typically not achieved, and the repulsive surface force remains, to forever degrade strength properties.
The principal scientific objective is to increase intermolecular contact, and maximize all the available attractive forces. They are extremely powerful, and they are free! Included are van der Waals Force, hydrogen bonding, preferential wetting, electrostatic attraction and interparticulate coupling. Detailed discussion is beyond the scope of this review.
The first step is to use a mono-functional cationic chemical, such as cationic starch. It should be cost-effective in charge neutralization and high enough in molecular weight to remain on the cellulose surface. Sufficient quantity is added to make the stock positive, with a zeta potential in the range 5-10mV.
The second step is to mix the cationic component thoroughly, so that stock and chemical are homogeneous. In the absence of homogeneity, it is impossible to maximize intermolecular contact and realize its many benefits.
To emphasize this point: stock is often not well mixed, with a high zeta potential standard deviation (SD), 4-5mv, and 6-8 breaks per day. On the other hand, stock mixed by preferential wetting requires 2 orders of magnitude less size, and has a low SD: 0.6mV, with excellent runnability.
Third, the anionic nanoparticle is added at the precise addition rate to fully neutralize the electrostatic charge of the cationic stock. It is mixed to homogeneity. The active chemicals form a 2-dimensional structure, and provide the best possible balance between formation and retention.
The result is that individual particles have been nanoflocculated in a two-dimensional structure. Positive and negative charges have been balanced; residual repulsive charge has been completely eliminated. Intermolecular contact is enhanced, with the counter-intuitive effect of making possible a sheet ash increase of 5-10% accompanied by greater strength.
A major degree of freedom remains open for exploitation. Increasing the amounts of cationic component and nanoparticle in tandem can result in a drainage increase approaching 70%, maximizing productivity. This offers a major opportunity for the producer of super calendared (SC) grades.
Role of the Paper Machine
The traditional management delegation of chemistry responsibility to the supplier created an unintended philosophical conflict that has become deeply divisive. As a direct result, machine management is not qualified to specify in detail the papermaking process chemistry requirements for the machine. Many horrible travesties have resulted.
Multiple headbox machines, of which there are reportedly 117 worldwide, represent one example. In theory, they improve cost-efficiency by employing a cheaper stock in the middle layer. In practice, however, they create a common white water system that enormously complicates the task of thoroughly mixing chemicals with stock, to obtain homogeneity.
Over a 30-year career, one of the authors has been involved in perhaps a dozen instances of poor runnability on multiple headbox machines. In one typical example, a Russian professor was engaged to participate in a comprehensive study, using state-of-the-art instrumentation that monitored 24/7. We determined that the mixing was so poorly done, the zeta potential standard deviation was in the range 4-5mV, far from the target of 0.6mV; and the productivity got as low as 50% on bad days.
After we had devoted 2 weeks to studying the recycle coated boxboard process, the Production Manager called a large meeting of everybody involved. Your author said to the Senior V-P Engineering that the chemicals must be mixed thoroughly with the stock. His response: a stony gaze and stern remarks, “That would cost $10,000,000.” End of discussion, end of meeting, end of project. 10 years later, engineering re-builds were still in progress.
Multiple headbox machines and common white water systems have posed extremely difficult challenges to conventional wet end wisdom, often resulting in poor performance.
However, properly executed papermaking nanotechnology provides the key missing steps of obtaining stock homogeneity and closed loop control. It will be successful every time, even when conventional chemistry yields 8-10 wet end breaks per day, with runnability dropping to the level of 50%.
Metering, Mixing and Controlling
The first requirement of nanoflocculation chemistry is stock homogeneity. It is more than merely important; it is a fundamental requirement, essential to maximizing chemistry efficiency.
The paper machine should be re-designed, and the process modeled on modern paint manufacturing. The traditional wet end is eliminated, and replaced by 2 mixing chambers, in serial order. The first is for the cationic component and all functional additives; the second is for the nanoparticle.
Gone is the vast, capital-intensive, costly-to-maintain white water system. Gone are the dead spots that become anaerobic, turn septic, and create slime. Gone are the holes and wet end breaks. Boil-outs are a dimly remembered, quaint, archaic custom.
Automatic metering and mixing are controlled by a touch-screen. Depiction of the wet end begins with a screen of red rectangles, one for each initial ingredient, including provision for entering control algorithms. A screen of pink circles follows, to enable specification of physical property limits, including the mixing thoroughness of cationic component with stock.
A third screen contains a single yellow square for the anionic microparticle, plus provision for specifying quantity. A fourth screen of pale yellow circles specifies physical performance properties including homogeneity and surface charge neutralization; with set-points that specify quality level, and other set points that control chemical feed rates, so as to match actual production output to machine capacity.
The data is analyzed by neural network, fuzzy logic software to ensure that even the most obscure beneficial relationships are exposed and exploited.
Quest for Perfection
A Process Committee is established by the CEO or his designee, with a membership weighted towards process chemistry competence. Candidates include suppliers of chemicals; sensors; instrumentation; neural network, fuzzy logic software; equipment and paper machines; mill and corporate staff; paper machine, engineering and general management.
The designated leader of a highly qualified team will work towards perfecting nanotechnology and indoctrinating the organization with a new culture.
The ultimate operating objective is to place the wet end under closed loop control, just like the dry end, and expand the responsibilities of the paper machine superintendent to maximize chemistry efficiency.
Restructuring the Economic Incentives
Well executed papermaking nanotechnology has the capability of optimizing end use performance at a high level of uniformity. Value-in-use of the product is significantly increased.
Drainage is measured, and increased or decreased over a wide range in order to maximize productivity and machine operating efficiency. Increase in product strength resulting from inter-molecular proximity is substantial, enabling use of less expensive raw materials, such as filler and recycle fiber.
Raising the bar on quality and performance while simultaneously reducing both operating and raw material cost requires new sensors, greatly enhanced thoroughness of mixing, and continuous closed loop control of chemical feed rates.
Under this modus operandi, the chemical supplier continuously maintains certain supply vessels at prescribed minimum levels of specified product, sets up the control parameters, and applies touch-screen control to chemical feed rates. Effective execution eliminates conventional stock prep labor. Closed loop control minimizes dedicated presence of supplier personnel.
The nanotechnology data is added to the computerized Product Information (PI) data, available to all principal participants on a confidential basis.
Long term, bankable contracts shall be negotiated with all suppliers who have a continuing supply relationship for expendable, replaceable or renewable products. Such contracts will enable the suppliers to fund much or all of the capital investment for which each supplies the technology, receiving in compensation a small share of the increased mill profit.
| John G. Penniman |
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