Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: By far the major pigment component used in xerographic toner production is carbon black. But from a papermaker's standpoint the behavior of toners is most strongly affected by the binders used in their formulation. The chemistry of these binders is diverse and ever-changing as new laser printers and photocopiers are introduced. The one common feature is that the toners have to be able to melt and fuse onto the rough surfaces of paper. Xerographic printing can cause the toner to be partly interlocked with the roughness of fiber surfaces. Most, but not all of the toner can be released by the high levels of hydrodynamic shear during pulping and dispersing operations when waste paper is de-inked. Most of the detached toner particles are then removed either by washing, floatation, or agglomeration into particles large enough to be screened out of the mixture. The other notable attributes of most toner particles are (a) somewhat hydrophobic character, and (b) usually anionic charge.
Function: For xerographic printing. During papermaking, toners present in the furnish (from wastepaper) tend to reduce brightness of the product, create spots, and possibly deposit on papermaking equipment. The thermoplastic nature of toner binders means that it is likely that there will be a deposit issue in the early dryers.
Strategies for Use: This is another case where the best solution is to remove toner particles before they reach the wet end. Dryer-can adhesion problems are likely to become less severe if the first-pass retention is kept high, keeping the fine material on the fibers rather than in the residual water still in the sheet. The idea is that free water that flashes out from the sheet in the dryer section will deposit any thermoplastic particles not anchored onto the fiber surfaces. The down-side of an efficient retention aid system is that it can slightly decrease the brightness of the sheet by improving retention of toner.
Cautions: Dryer-can surfaces can be extremely hot. Collection of deposit samples from such surfaces can be hazardous.
|Schematic diagram of the xerographic printing process (laser-type xerographic printer shown)|
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 .