Preface

After a half-century of intensive development, papermaking wet end chemistry has become too complex for routine personal management by the PM Superintendent. This essay attempts to dispel the complexities by limiting consideration to essential principles.

Introduction

During WW II the Hanford Atomic Energy Facility pumped the Columbia River through its reactors to serve as cooling water. Proof that it worked is that it raised the river temperature by one degree. However, after a period of time concern was raised about making the particulates radioactive and the horrifying possibility of precipitating them at the river mouth in critical concentration.

Mid-century at the Hanford Facility marked the initiation of organic flocculating agent usage in water treatment. It became a significant specialty chemical application, and soon led to the use of similar chemicals on the wet end of the paper machine.

For decades, chemical suppliers tried to keep the technology secret. Chemicals were identified by end use, rather than their formula. They were (and still are) denigrated with the appellation "additives". The industry has made significant progress in understanding the new technology, but it still remains an abstruse and argumentative subject.

In reality, wet end papermaking chemistry need not be as complicated as chemists make out. When examined closely, chemistry is arguably more straightforward than the physics of papermaking. It can be expressed in a few guiding principles, the first of which is to optimize retention and/or drainage.

Principles of Wet End Chemistry

This is a win-win situation, and leads immediately to:

When retention and drainage are optimized, physical properties are maximized.

Discussion: This is the best of all possible worlds, and has a simple explanation. Positively charged chemicals are used to neutralize the negative charge of the stock. Elimination of the repulsive negative charge causes the particles to aggregate, creating a microflocculation.

Microflocculation creates a structure that enables good drainage on the wet end, and good water removal downstream. It causes a high level of formation, and good retention of fillers, fines and the various additives that improve physical properties such as strength and sizing.

Laboratory Implementation

The Dynamic Paper Chemistry Jar, originally the "Britt Jar", has been used in the lab since the 70's to determine what chemicals provide the best results. The Dynamic Paper Chemistry Jar" is helpful in optimizing drainage, providing only that use of conventional MACROflocculation "retention aids" is eliminated, or greatly reduced, in favor of microflocculation charge-neutralizing chemicals.

Optimization springs from use of charge neutralizing chemistry to eliminate the repulsive, negative stock charge so that microflocculation occurs.

Optimization can be done reproducibly by measuring zeta potential. .

Discussion: Zeta potential represents the electrostatic charge of the stock. A positively charged, or "cationic", chemical is added to the negatively charged stock to reduce the final zeta potential close to zero. This ensures that the highly repulsive negative, or "anionic", charge has been completely neutralized.

Systems employing microparticles such as colloidal silica or bentonite have come into wide use because they offer the best balance between retention and formation. However, the paper machine wet end is not designed for mixing efficiency, and the chemicals are added too late in the process to accomplish thorough blending with stock. The better this is accomplished, the more closely will the final conditions approach optimum.

After a charge-neutralizing chemical reduces the repulsive effect, the microflocculation process proceeds, activating extremely powerful intermolecular forces that enable higher sheet ash level accompanied by significantly greater strength. This counter-intuitive effect arises from the same forces that enable the gecko lizard to climb walls and move across ceilings at high speed. For more insight on this subject, refer to the Summer 2000 Newsletter, "Wet End Chemistry and the Gecko Lizard", on the Web site: www.papermaking-chemistry.com

On-Line Implementation

Effective process chemistry control is most difficult during non-equilibrium periods. When in a transition state such as grade change, machine break, upset, or re-start, it is important to use on-line sensors to stabilize stock ash and zeta potential at near-normal levels so the transition can be as smooth and expeditious as possible.

During stable periods of operation the ash level is measured by the dry end scanner, and adjusted to the desired level. Drainage can be increased, to compensate for increased stock ash, by increasing the flow rate of microparticle and cationic scavenger in tandem. Meanwhile, the two flow rates are automatically balanced to maintain optimum zeta potential.

Monitoring, with appropriate feed-rate corrections, can serve to help prevent upsets. Maximum benefit is realized by closing the loop and maintaining simultaneous control of retention, drainage and zeta potential within narrow limits. All machines ideally should harness the powerful intermolecular forces that maximize physical properties.

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John G. Penniman

www.papermaking-chemistry.com