Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
PRECIPITATED CALCIUM CARBONATE (PCC)
Composition: Most PCC added to the wet end of paper machines consists almost entirely of the calcite crystal form of CaCO3. The calcite crystal can have several different macroscopic shapes. These depend on the conditions of production. The most economical way to get PCC at a paper mill requires a satellite facility. The starting material may consist of limestone, but this is usually calcined (heated to drive off CO2) at the mine, producing burnt lime, CaO. The great advantage of CaO is that is has only 56 percent the mass of the original CaCO3, reducing the cost of shipping. Water is added to "slake" the lime at the satellite plant next to the paper mill. The resulting "milk of lime," a suspension of Ca(OH)2, is then exposed to bubbles of CO2 gas. Often it is possible to collect a stream of CO2 gas from a kraft recovery operation or power generating operation at the mill site. Cool temperatures during addition of the CO2 tend to produce rhombohedral (blocky) PCC particles. Warmer temperatures during addition of the CO2 tend to produce scalenohedral (rosette-shaped) PCC particles. In either case it is critical to end the reaction at an optimum pH where the milk of lime has been effectively converted to CaCO3, and before the concentration of CO2 becomes high enough to acidify the suspension and cause some of it to redissolve. In cases where the PCC is continuously agitated and stored no more than a few days, it may be unnecessary to add more than a trace of such anionic dispersants as polyphosphates. On-site PCC usually has a weak cationic colloidal charge. By contrast, dried PCC can be similar to most ground CaCO3 products in having a negative charge, depending on whether dispersants have been used.
Function: Filling of paper, especially in cases requiring high brightness. Rosette-type PCC is used in applications requiring opacity improvements and maintenance of caliper at specified levels of weight and smoothness. Rhombohedral PCC products have effects more similar to those of ground CaCO3 products.
Strategies for Use: Many of the considerations for use of PCC filler are the same as those that apply in the case of other forms of calcium carbonate. The fact that PCC tends to be weakly cationic, and the other forms of the mineral tend to be anionic, can have a subtle effect on retention, but usually there is no significant problem. Most well-designed retention aid programs are affective with either type of CaCO3 filler. Because scalenohedral (rosette) PCC tends to lower the density of paper (especially before it is calendered) it can be more difficult to press water out of the paper made with that form of PCC. Sizing agents such as AKD do not form permanent bonds to the surface of PCC. When AKD and PCC are both used in the same paper machine system it is recommended to add them at widely separated points, usually with the filler added first. Evidence suggests that this practice allows colloidal material including starch to coat the surfaces of the filler particles, giving the AKD something secure to attach to. Also, the coating may keep the AKD from coming into contact with residual Ca(OH)2 in the PCC. In certain cases the weakly cationic surface of PCC helps it to be effective in the collection of tacky materials from the furnish.
Cautions: Usually there are no significant hazards associated with PCC use in the paper mill.
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 .