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.
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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 .