Precipitated Calcium Carbonate (PCC) Pigments
Precipitated calcium carbonate (PCC) is a white, bright, synthetic material that can be produced in a variety of particle shapes (morphologies) and sizes for use as pigments in filled and coated papers. The size and shape of the PCC particles determine many of the important properties of the newsprint, copy paper, and magazines commercially produced in America and, increasingly, throughout the world. Specialty Minerals Inc.’s (SMI’s) talented research and development team has created many of the PCCs considered to be industry standards today. We outline some of these PCC products below.
- Scalenohedral Albacar® HO and LO PCC, exhibit clusters (rosettes) of triangular-shaped crystals emanating from a central core. Albacar® PCC ranges in size from about 1 to 2 µm in diameter and exhibits a specific surface area of about 6 to 12 m²/g. When Albacar® PCC is incorporated into the paper matrix, it increases whiteness, brightness, and opacity, so that the image printed on one side is crisp, exhibits good contrast against the background, and cannot be seen from the opposite side.
Sheet thickness (caliper) is also important in order to maximize paper machine productivity. This is because some paper, especially cut-size copy paper, has fixed dimensions of length, width, and thickness. Anything that causes a sheet to be thicker (bulkier) allows the papermaker to reduce the amount of material in the sheet to maintain a constant thickness. Thickness, or bulk, can be greatly influenced by the size and shape of the mineral particles in the sheet, and Albacar® PCC excels at maximizing caliper.
- Prismatic Albafil® PCC (discrete crystals that exhibit a hexagonal shape) can be used to control the porosity of paper—that is, the rate at which air can pass through the sheet. This is very important, for example, when the paper is used to produce photocopies on a machine that uses air handling to move the paper from the feed tray into the machine. If the sheets in the tray are too porous, multiple sheets will be pulled through simultaneously, causing paper jams. Albafil® PCC exhibits average particle sizes in the range 0.7 to about 2 µm, and specific surface areas from about 4 to 10 m²/g.
Generally, Albafil® PCC does not interfere with paper strength because it allows for better interfiber contact. This is important because the strength of both wet paper (while it is on the paper machine) and the dry finished sheet depend on the ability of the individual cellulose pulp fibers to come into close contact and bond with each other.
- In special batch reactors, SMI obtains hexagonal (prismatic) crystals that are clustered around a central core. These prismatic crystals are called Megafil® 2000 and 3000 PCC. Paper that incorporates Megafil® PCC exhibits a highly desirable balance of optical and physical properties, including opacity, strength, and bulk. The average size of Megafil® PCC ranges from 1 to 2 m²/g and surface areas from 4 to 7 m²/g.
- In a continuous process, SMI manufactures Opacarb® PCC as a fluid dispersion of high-pigment solids. Opacarb® A40, A50 and A60 PCC exhibit individual needle-shaped (acicular) crystals that are long and narrow. The ratio of length to width is called the aspect ratio. The proper aspect ratio allows PCC particles to become aligned during the paper-coating process and leads to good paper gloss and fiber coverage. Paper makers use Opacarb® PCC for coating high end grades of publication papers, such as magazine stock.
Today’s printing and writing papers are manufactured by separately combining wood fibers and mineral fillers. Tomorrow’s papers might be made from a composite material, where the synthetic process that forms PCC is carried out in the presence of the fiber. In such a process, the resulting filler and fiber are attached to each other. We call the resulting materials filler-fiber composites; the challenge for SMI is to develop the conditions and equipment to do this. Filler-fiber composites offer the promise of papers filled at perhaps twice the level common in today’s papers, with better formation, lower chemical demand, good mechanical properties, and lower overall cost. SMI's research and development team continues to investigate new and promising PCC shapes with unique features and beneficial properties for filler-fiber composites.