Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
GROUND CALCIUM CARBONATE (GCC)
Composition: When papermakers use the term "GCC" they are usually referring to ground limestone, a calcium carbonate material having the calcite crystal structure. It is possible that they might be referring to a type of calcium carbonate called chalk, but the word "chalk" itself is also in common use. Limestone formations are formed by natural precipitation, a process that occurs slowly. This is part of the explanation why the material in limestone products is completely converted to the CaCO3 form. By contrast, depending on the conditions of its production, precipitated calcium carbonate may contain some residual calcium hydroxide. Because of the hardness of limestone, the grinding process is energy-intensive. Anionic dispersants, including phosphates and acrylates, are used as grinding aids and to make the resulting slurry more stable in a colloidal sense. Papermakers commonly use ground limestone products that are "fine-ground" or "ultrafine-ground" with equivalent particle sizes from 60% to 90% smaller than 2 mm based on sedimentation rates.
Function: For filling of paper for cost reduction, brightness, buffering in the alkaline pH range, and some opacity
Strategies for Use: Like any other calcium carbonate filler, GCC is incompatible with acidic conditions. Alum or sulfuric acid, etc., must never be added to a stream of GCC slurry. GCC can be obtained from high-quality limestone sources, yielding a product of very high brightness. Compared to the rosette form of PCC, GCC tends to produce a denser paper sheet with higher tensile strength at a given filler level. The strength advantage of GCC can be further improved if one uses a product whose particle size distribution has been narrowed by separation operations after grinding. PCC is often preferred in grades that require maximization of caliper at a given smoothness value. In other cases there may be better runnability and economy by replacing some or all of the rosette product with either GCC or rhombohedral (blocky shaped) PCC particles. The point of addition for GCC slurry can be more related to process control than chemical issues. Effective control requires that at least a significant fraction is added to the thin-stock, just several seconds ahead of where the furnish begins to become paper. A retention aid system should be used to maintain acceptable Z-directional uniformity, basis-weight uniformity over time, and efficient performance of ASA or AKD sizing agents.
Cautions: Ground limestone is generally non-hazardous. Unstirred tanks and pipes containing stagnant GCC slurry may become the site of dense sedimentation, that is very difficult to resuspend.
|Typical shapes and sizes of ground calcium carbonate products for filling of 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, email@example.com .