Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: The word "amphoteric" implies that a chemical product has both positive and negative ion groups when it is dissolved in solution. When papermakers refer to amphoteric starches they are usually referring to products derived from corn starches. The least expensive starch products are based on dent corn, a corn variety that is also the source of most corn sweetener. Amphoteric starches also are prepared from waxy maize, a variety of corn that produces only the branched type of starch, amylopectin. The cationic groups are the same quaternary ammonium substituents used in preparation of cationic starches. The anionic groups usually are phosphates. Strictly speaking all cationic potato starches are really amphoteric starches since potato starch contains about 0.08% phosphorous. The cationic content of amphoteric starches is typically in the range of 0.2 to 0.3% nitrogen. All of these products are delivered to the mill as a dry powder having a moisture content of 10 to 20%.
Function: Enhancement of dry strength. Possibly useful for improving drainage and retention, especially in systems where alum is also used.
Strategies for Use: As in the case of other wet-end starches, the first step is "cooking." The batch procedure takes about 20 to 30 minutes at about 95 oC at atmospheric pressure. Much more rapid cooking can be achieved in continuous "jet cooking" procedures at higher temperature and pressure. The performance of starch as a dry strength agent is only moderately affected by its point of addition. Addition to the thick stock has both potential advantages, and disadvantages. Advantages include getting a higher proportion onto long fibers, ensuring good retention and a relatively high effect on overall dry-strength. Also, addition to the thick stock ensures that any flocculation caused by the starch has been completely reversed before the headbox - making it possible to get a very uniform sheet (depending also on what is done with retention aids). Disadvantages of early addition include breakdown of starch by hydrodynamic shear and possible disappearance of some of the starch into the interior of pores in the fiber cell walls. For these reasons it is common to add at least part of the starch to the thin stock with only a few seconds of exposure to the furnish before it is made into paper. The performance of amphoteric starch can be enhanced if alum or polyaluminum chloride (PAC) is also being used. Amphoteric starches have a reputation for performing well under conditions of high electrical conductivity. Also, they can be an effective strength agent for systems that would tend to become overly cationic if a straight cationic starch was used.
Cautions: Starch products can be very slippery when spilled. Steam and steam lines in the vicinity of starch preparation areas can cause burns. The dry powder should not be breathed.
|Diagram illustrating a typical ratio of sites on amphoteric starch|
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 .