Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: Despite the fact that colloidal silica has the same chemical formula as quartz sand, SiO2, the two materials could hardly be more different in their effect on paper machine operations. The key difference is size. The subunits of colloidal silica particles are typically in the range of 1 to 5 nm. Whether or not these subunits are joined together depends on the conditions of polymerization. Initial acidification of a water-glass (sodium silicate) solution yields Si(OH)4. If the pH is reduced below 7 or if salt is added, then the units tend to fuse together in chains. These products are often called "silica gels." If the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and they gradually grow. These products are often called silica sols. Hydrogen ions from the surface of colloidal silica tend to dissociate in aqueous solution, yielding a high negative charge. Substitution of some of the Si atoms by Al is known increase the negative colloidal charge, especially when it is evaluated at pH below the neutral point. Because of the very small size, the surface area of colloidal silica is very high.
Functions: Key part of drainage-aid programs marketed by Eka Chemicals and Nalco; also capable of increasing the amount of cationic starch that can be retained as a dry-strength agent.
Strategies for Use: The conventional procedure is to add the colloidal silica 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 additives are expected also to make the molecules of the subsequently added cationic starch or cationic PAM adsorb with more loops and tails extending into solution. The function of the colloidal silica 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 also 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: No significant hazards are reported. Read the MSDS.
|Effect of aqueous conditions on the results of colloidal silica synthesis|
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 .