Colloid chemistry
The word "colloids" (used in science) describes
materials that have at least one dimension that is smaller than 1 micro-meters.
The word colloid does not give any indication on the chemical makeup of the
particle.
Almost everything the papermakers deal with can be
considered to be colloidal. Although fibers are larger than the classical
definition, the fiber surface is highly porous, and micro fibrils of colloidal
dimensions extend out into solution from the surface of a refined cellulose
fiber.
Other colloidal particles common in papermaking furnish
include fiber fines, filler particles, sizing emulsion particles, and retention
aid molecules (for example; molecules so big that they no longer behave
like regular molecules – high molecular weight polymers).
The average end-to-end distance of a retention aid polymer
(500 nm) is much larger than the size of a typical colloidal particle (2 to 5
nm ).
When papermakers refer to colloids, they usually are most
interested in the colloidal organic materials, including fatty acids, lignin
by-products, and oxidized hemicellulose. These are often called "DSC"
for "dissolved and colloidal materials," or "anionic
trash."
Deposits
form on papermaking equipment due to the “thermodynamic instability” of many
materials suspended in water (oils, pitch, hot melts etc.). We can combat this
by getting those materials to deposit/ bind onto fibers, thereby keeping their
concentration low in the liquid, ergo less deposits on the machine.
Retention
of colloidal materials is best achieved by a combination of coagulation
(treatment to neutralize charges (conductivity), causing the particles to come
out of the suspension) and flocculation (treatment with polyelectrolytes (high
molecular polymers) so large that they can bridge between the surfaces).
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