Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: Bentonite is a common name for the anionic smectite clay minerals used by papermakers as a retention and drainage additive or "microparticle." Under the scanning electron microscope (SEM), bentonite particles can be almost indistinguishable from kaolin filler clay or coating clay. The main difference is the thickness. The sodium or potassium salts of bentonite exfoliate into extremely thin plates. In theory these plates can be as little as about 1 nm in thickness, yielding a huge surface area per unit mass. In practice, not all of the platelets will become separated from each other.
Functions: Retention and drainage (if added after or before a cationic retention aid); pitch control (if used in combination with highly charged cationic polymers).
Strategies for Use: First let us consider the role of bentonite as a retention and drainage promoter. Following the patented technology of Ciba Specialty Chemicals, bentonite added downstream of a cationic polyacrylamide retention aid causes a marked improvement in dewatering. Best results are achieved when sufficient very-high-mass cationic polymer (e.g. cationic acrylamide) has been added or is about to be added downstream so that the furnish momentarily has a net cationic charge. If the furnish has a high level of anionic dissolved and colloidal materials, then it makes sense to first treat the stock with a highly charged cationic material to neutralize most of this excess charge. The high aspect ratio of bentonite means that it has significant ability to bridge between different particles or fibers covered by the cationic polymer. That explains why bentonite tends to be very effective for retention, as well as for improving drainage. Patented technology from Vinings, Inc. shows that bentonite can be used in combination with highly cationic polymers for adsorption and detackification of pitch.
Cautions: When handling dry materials it is important to take proper precautions against airborne dust.
|The length-to-thickness ratio of fully exfoliated sodium montmorillonite is about 500 to 1.|
PLEASE NOTE: Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification.
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This page is maintained by Martin Hubbe, Associate Professor of Wood and Paper Science, NC State University, email@example.com .