Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: Chalk is a form of calcium carbonate, having the same chemical composition as ground calcium carbonate, limestone, marble, and precipitated calcium carbonate (PCC). In fact, all of the calcium carbonates listed in the previous sentence have the same crystal form, calcite. What makes chalk different is its origin as the shells of little "coccolith" organisms. Generations of coccoliths gradually built up deposits. Since most of us have experience with chalk-board chalk it should come as no surprise that the deposited mineral is quite easy to grind. The particles released by grinding the chalk have a highly desirable property; almost all of them are in an excellent particle size range for use at relatively high levels as a filler. The moderately large size and low surface area (about 4 m2/g) relative to other common fillers means that chalk has less adverse effect on strength than most kinds of filler at any given filler content in paper. Especially abundant and high-quality chalk deposits are found in England and Denmark - but not in North America. This fact is probably part of the reason that the European paper industry was earlier in its widespread adoption of alkaline papermaking technology. The modest brightness of chalk (typically 80 to 86%) can be an issue, especially when it is compared against ground limestone and precipitated calcium carbonate products (both about 95 to 99% brightness). Also, chalk cannot be expected to achieve a high opacity. Opacity can be improved more by using something with a high surface area, bulking ability, and open structure, e.g. scalenohedral PCC. Chalk typically is dispersed with anionic materials such as phosphates or polyacrylates.
Function: Paper filler for cost reduction, pH buffering (alkaline), and superior ability to maintain acceptable paper strength as a function of increasing filler content.
Strategies for Use: Chalk filler is a relatively forgiving additive, as long as one avoids direct contact with certain other additives that can be adversely affected by it. These include acidic materials such as alum and sulfuric acid. Sizing agents typically should be well separated from calcium carbonate filler in the scheme of addition; usually the filler should be added early, and the size should be added late in the process. However, chalk is probably the most sizing-tolerant of all of the calcium carbonate filler varieties. Either a cationic or anionic retention aid can work well with chalk due to the presence of Ca2+ sites on its surface, and due to the presence of anionic materials (including filler dispersants) in the white water system. The amount of filler in the paper (if all of it can assumed to be chalk) can be determined by ashing the paper at 900 oC in a tared crucible, then dividing the "percent ash" by 0.56. This coefficient comes from the reaction of CaCO3 to CaO + CO2. In the presence of other minerals one needs to do a second ashing analysis for a standardized length of time at 500 oC or some other suitable temperature. Under such conditions almost all of the calcium carbonate remains as CaCO3.
Cautions: Since chalk is often delivered as dry powder, it is important to take precautions against breathing of the dust in the filler preparation area.
|Illustration of the typical size, shape, and partially fused nature of chalk as used for paper.|
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 .