Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: The word "colloids" says nothing about chemical composition. Rather, the word implies materials that have at least one dimension that is smaller than 1 micro-meters. 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 microfibrils 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 (i.e. molecules so big that they no longer behave like regular molecules). The average end-to-end distance of a retention aid polymer (e.g. 500 nm) is much larger than the size of a typical colloidal silica microparticle (e.g. 2 to 5 nm per subunit). When papermakers refer to colloids, they usually are most interested in the colloidal organic materials, including fatty acids, lignin byproducts, and oxidized hemicellulose. These are often called "DSC" for "dissolved and colloidal materials," or "anionic trash."
Function: By considering the colloidal properties of all of the materials that go into the wet end, it is possible to develop more efficient addition strategies and quality improvements in some cases.
Strategies for Use: Papermaking objectives that are most dependent on colloidal principles include deposit control, retention, drainage, and sizing. Let's start with deposit control. Deposits form on papermaking equipment due to the thermodynamic instability of many materials suspended in water. Papermakers can combat this by getting those materials to deposit onto fibers, thereby keeping their concentration low in the liquid phase. Retention of colloidal materials is best achieved by a combination of coagulation (i.e. treatment to neutralize charges, causing the particles to come out of suspension) and flocculation (i.e. treatment with polyelectrolytes so large that they can bridge between the surfaces, even in the presence of net electrostatic repulsion between them). In contrast, exceptionally poor retention of fine materials can occur if the surfaces are coated with water-loving molecules with long loops and tails extending from their surfaces, i.e. steric stabilization. Drainage usually improves as the colloidal charge of the system is made closer to the neutral point and as fiber fines are retained to a greater degree onto the surfaces of longer fibers. Sizing requires that the water-loving (hydrophilic) surfaces of cellulosic materials be converted into water-hating (hydrophobic) surfaces. This requires treatment with molecules having hydrophobic groups, an ability to be well dispersed in the solution, an efficient mechanism of retention, and a means of becoming anchored and oriented at the fiber surface either during formation or when the paper is dried.
Cautions: Colloidal particles cannot be seen, except by viewing scattered light from a beam passing through the solution. Liquids that appear to be pure water might possibly be colloidal silica (which happens to be non-toxic, but the same cannot be assumed for all colloidal materials).
|Schematic illustration showing how surface area is expected to increase in inverse proportion to the diameter of primary particles (assumed to be nonporous)|
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.
RETURN TO INDEX PAGE OF ENCYCLOPEDIA
This page is maintained by Martin Hubbe, Associate Professor of Wood and Paper Science, NC State University, firstname.lastname@example.org .