Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: The most important calcium carbonate products added to the wet end of paper machines have the calcite crystal form. In nature, calcium carbonate is abundantly provided in the form of limestone deposits, marble deposits, and chalk deposits. Of these three, the chalk deposits are by far the softest and most easily converted into suitable fillers. Chalk deposits form as a result of generations of marine organisms (coccoliths). This process results in particles that are rather more uniform, compared to products that result from simple grinding of isotropic solids. High quality chalk is abundant in England and in Denmark, but it is notably absent in North America. Limestone is much harder than chalk and very bright limestone deposits are found in such places as Alabama, Vermont, and British Columbia. Producers of filler-grade calcium carbonate have improved their ability over the years to narrow their particle size ranges and to eliminate large abrasive materials such as quartz sand. A much greater range of particle size, uniformity, and shape can be achieved by chemical means. Precipitated calcium carbonate (PCC) can be produced by "calcining" limestone at very high temperature to drive off CO2, "slaking" the resulting lime by addition of water, and then treating the resulting "milk of lime" suspension with CO2 gas under highly controlled conditions of temperature, pH, and ionic composition. The process results in calcite particles. Depending on the conditions of precipitation they may be either rosette (scalenohedral), blocky (rhombohedral), or various intermediate types. It is also possible to precipitate the aragonite crystalline form of calcium carbonate, and such particles tend to be needle-like (acicular).
Function: CaCO3 is used as a filler for making bright, opaque paper at minimum cost. Calcium carbonate also keeps the pH in the alkaline range (unless controlled with "acid-tolerant" technology). The rosette form of PCC filler tends to produce a bulkier sheet of paper; when calendered it yields improved smoothness at given levels of caliper.
Strategies for Use: Papermakers are most concerned that the filler content of the paper remain within tightly controlled limits. In particular, a momentary increase in the filler content may weaken the paper so that it either breaks or it is too weak to meet all of its specified strength parameters. On-line monitoring of mineral content (or "ash" content) is recommended. Incineration of samples of paper taken from the reel may be adequate for control of filler content in many cases. Considerations regarding the best point(s) of addition include (a) having an ability to rapidly adjust its content in the paper, and (b) avoiding interferences with certain other wet-end additives. In particular, calcium carbonate should never come into direct contact with acids, with alum, or with sizing agents. Acids cause the mineral to dissolve, releasing bubbles of CO2. This does not happen if the pH is kept above about 6.5. Alkaline sizing agents such as ASA and AKD work well under the aqueous conditions associated with calcium carbonate use, but they appear to have an adverse interaction with the mineral surfaces themselves. In particular, bonds formed between AKD and PCC during the drying of paper appear to be unstable during subsequent storage and use of the paper. The problem can be minimized by adding the PCC to the thick stock, well ahead of where the sizing agent is added.
Cautions: Standard precautions need to be followed. Calcium carbonate filler usually poses little danger to anyone. See the MSDS.
|Calcium carbonate products commonly used as fillers 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 .