Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: The major component of "white pitch" sometimes observed in paper machine systems is latex binder from recycled coated broke. This can include styrene-butadiene resin (SBR), acrylic resins, and polyvinylacetate latex components. It also can contain the associated coating pigments, including clay, plastic pigments, calcium carbonate, and titanium dioxide. White pitch problems are often made more difficult due to mixing with wood resins, defoamer components, and other tacky materials in the furnish.
Function: White pitch can cause reduced papermaking efficiency due to felt filling and down-time required to clean up equipment.
Strategies for Use: It is highly recommended to do a chemical analysis. This may make it possible to determine the most likely root causes of the problem. One of the most general strategies for avoiding pitch problems is to keep the tacky materials bound to the fibers. This is one of the ideas behind a common practice of adding alum during or immediately after mechanical pulping. The alum complexes with any soaps of resin acids and fatty acids, and the presence of these complexes can serve to bind the multi-component droplets of pitch to the fiber surfaces. Addition of highly cationic polymers such as polyethyleneimine (PEI) to the process can have a similar effect. The relative turbidity of filtrate obtained from different points in the process can be used as an indication of the probable effectiveness of such treatments with cationic materials. It is very common also to add talc or another detackifying agent to coat tacky particles and minimize their tendency to form large agglomerates, deposits, or spots in the product. An effective retention aid system can help limit the filling of press felts with pitch.
Cautions: Safe procedures must be followed when collecting samples of pitch deposits. Equipment must be locked out and the air quality checked by qualified people before entering enclosed areas such as headboxes.
|Schematic explanation of why treatment of coated broke with a highly cationic polyelectrolyte can decrease pitch deposit problems|
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, firstname.lastname@example.org .