Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
ANIONIC DISSOLVED AND COLLOIDAL SUBSTANCES
Composition: "Anionic trash" is one of the popular names used to describe a wide range of anionic dissolved polymeric and colloidal materials. Another common name is "DCS" for "dissolved and colloidal substances." Many such substances come from the wood and get released during pulping. Some key components include fatty acid salts, resin acid salts, hemicellulose and its oxidation byproducts, lignin derivatives from kraft pulping, and dispersants or anionic latex and dispersants from coated broke. Various types of bleaching agents tend to oxidize hemicellulose and extractives, further increasing the anionic charge of the dissolved and colloidal fraction of the pulp slurry. Pulp washing of thickening stages after pulping and after major bleaching stages are intended to keep this material from going to the paper machine, but it is common for such devices to be working beyond their rated capacity.
Function: Anionic colloids and dissolved polymeric materials interfere with the performance of cationic retention aids, cationic dry-strength agents such as cationic starch, and wet-strength resins. Also they can play a role in deposit problems.
Strategies for Use: The best strategy for dealing with excess colloidal anionics is to employ highly efficient washing and keep these materials away from the paper machine. Some capital expenses for washing equipment can be justified in terms of chemical savings. That's because another way to deal with anionic trash is to neutralize it. The most common chemicals for neutralization are alum (especially in the case of acidic papermaking), polyethyleneimine of polyaluminum chloride (PEI or PAC, for acidic or neutral systems), or polyamines or poly-DADMAC (for all pH ranges, but especially alkaline pH). The best strategy is to add the cationic material to the pulp stream(s) that is the major source. For example, it makes sense to add a cationic polymer to coated broke. Continuous evaluation of the colloidal charge by means of streaming current titrations makes it possible to keep the treatment level at an optimum point, even if the amount or quality of the broke changes. Other possible strategies include treatment of the process water with enzymes or biological digestion of the process water, i.e. secondary wastewater treatment and its reuse in the process. Because these cationic additives can increase the marginal cost of production, their use and optimum additon levels need to be justified in each application, based on demonstrated improvements in such things as retention, sizing, system cleanliness, or process stabilization, to name a few.
Cautions: In certain chemithermomechanical pulp operations with peroxide bleaching (BCTMP) and in certain neutral sulfite semichemical (NSSC) systems for making corrugating medium the level of anionic trash is so high that no one can afford to neutralize the system. Retention aids (except for PEO) and cationic starch, etc., do not perform well under such highly anionic conditions.
|Examples showing how two types of anionic colloidal materials can hurt the retention of clay.|
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 .