Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use

MICROPARTICLES

Composition: The main types of microparticles used by papermakers consist of colloidal silica, bentonite (montmorillonite), and certain organic products that serve similar functions. All of the commercially successful microparticle products have a negative colloidal charge and very high surface area. The high surface area is easy to understand when one considers the primary particle size of the colloidal silica additives (1 to 5 nm) or the thickness of a fully defoliated sodium montmorillonite platelet (1 nm).

Function: Microparticles are a key part of certain drainage-aid programs marketed by Eka Chemicals, Nalco, Ciba Specialty Chemicals, among others. An anionic "micropolymer" product developed by Cytec Industries and also sold by Buckman, Inc. performs a similar role.

Strategies for Use: Microparticles such as colloidal silica and bentonite are usually added downstream of a cationic polyacrylamide retention aid or cationic starch. The technology is protected by various patents. The combined treatment causes a marked improvement in dewatering. Best results are achieved when sufficient very-high-mass cationic polymer (e.g. cationic acrylamide) has been added to flocculate the fibers. 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. Microparticles or micropolymers usually are added very late in the approach flow to a paper machine, typically just after a set of pressure screens. A drainage rate increase is expected only if the furnish already has been treated with a suitable high-mass cationic polymer such as cationic starch or cationic poly-acrylamide. The effect is most pronounced when the net amount of cationic additives is enough to render the system at least slightly cationic before the addition of the micro-particle. For this reason it can be helpful to treat highly anionic furnish with a highly cationic material such as alum, poly-aluminum chloride (PAC), polyamine, or polyethyleneimine (PEI). In addition to scavenging excess anionic colloidal charge, such additive are expected also to make the subsequently added cationic starch or cationic PAM molecules adsorb with more loops and tails extending into solution. The function of the microparticle or micropolymer appears to involve (a) release of water from polyelectrolyte bridges, causing them to contract, and (b) acting as a link in bridges that involve macromolecules adsorbed on different fibers or fine particles. These effects create more streamlined paths for water to flow around the fibers. The tendency of microparticles to boost first-pass retention will tend to have a positive effect on initial dewatering rates. It has been reported that paper produced by means of a microparticle retention and drainage program has a more open, porous structure, though the effect may become obscured by subsequent wet-pressing and calendering operations. Papermakers often are able to "trade away" chemical-induced drainage improvements in favor of improving formation uniformity. This is possible by either (a) decreasing the headbox solids by increasing the amount of white water recirculated, (b) increasing the proportion of hardwood fiber relative to softwood, or (c) increased refining.

Cautions: Usual safe practices should be followed and the MSDS should be consulted.

Synthesis of colloidal silica    

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, m_hubbe@ncsu.edu .