Calcium carbonate (CaCO3) is one of the most commonly occurring minerals on earth. Calcium carbonate exists in one of three different polymorphs, or spatial arrangements, of its calcium and carbonate ions. Calcium carbonate polymorphs are calcite, aragonite and vaterite. All these polymorphs are observed in nature, and all can be produced from processes that yield synthetic precipitated calcium carbonate, or PCC. It is the arrangement of the atoms and ions in the crystal structure of these polymorphs that determine the actual physical shape of the crystal. We call these shapes morphologies, and a single polymorph, such as calcite, can exhibit several different morphologies.
The vaterite polymorph of calcium carbonate often exhibits nearly perfect spherical shapes. Even at high magnifications under an electron microscope, these spheres may appear smooth, but can be quite porous, as evidenced by techniques such as mercury porosimetry. Vaterite may also exhibit a platy structure, and when this happens, the plates tend to self-assemble into larger structures that may be spheroidal or some other shape. Whether observed in nature or prepared synthetically, vaterite is metastable. This means the atoms in the crystals have a natural tendency to rearrange usually to a calcite structure, as discussed below. For this reason, vaterite is not commonly found in nature and is not a commercially significant form of PCC.
The aragonite polymorph generally exhibits needle-shaped orthorhombic crystals. The needle shape is called acicular, and the ratio of length-to-diameter of the crystals is called aspect ratio. High aspect ratio aragonite is useful in many applications. In paper coatings, this morphology tends to produce high gloss finishes and is better at covering substrates at lower coating thicknesses. A high aspect ratio also can improve strength or impact resistance in polymeric materials that employ this form of calcium carbonate as an additive. Sometimes, clusters of needle-shaped crystals are observed, and these can be very efficient at scattering incident light. These aragonite clusters behave similarly to the scalenohedral clusters discussed below.
Calcite is the most common polymorph of calcium carbonate and the most stable. Specialty Minerals Inc. (SMI) produces calcite in rhombohedral (cubic), prismatic (barrel-shaped) and scalenohedral (triangular) morphologies. The rhombohedral and prismatic forms are useful in paper coating applications and as strength enhancers in polymer matrixes. In filled papers, large prismatic morphologies are useful for improving drainage on the paper machine and providing a bulky finished product. When applied in pigmented size applications, large prismatic PCC can help lower gloss or sheen of the paper surface.
Scalenohedral calcite can be a collection of discrete particles or a cluster of individual crystals arranged in a rosette or starbust pattern. The latter morphology is commonly used in paper filling because this unique shape scatters light efficiently, which significantly enhances paper opacity. Large scalenohedral particles also increase bulk in filled papers.
The chemical and physical properties of calcium carbonate do not vary greatly among the polymorphs. Aragonite has a slightly higher density than calcite and is also slightly more soluble. All PCC polymorphs exhibit high chemical purity, provided the quality of the lime or calcium hydroxide starting material is sufficient. Because of this, PCC whiteness and brightness is often unmatched, and these pigments provide high performance and good value, especially in paper applications. The clean surface resulting from the PCC precipitation process causes PCC to exhibit a slight positive surface charge, typically in the range +10 to +25 mV or higher. This is called the zeta-potential, and a positive zeta potential is particularly useful in paper filling applications. Since cellulose pulp fiber in water carries a slight negative charge and “opposites attract,” using PCC often allows the papermaker to reduce the amount of chemicals needed to cause mineral fillers to be retained in the paper making process. This is a direct cost savings. The clean surface also allows PCC to be easily and efficiently treated with chemicals to enhance its performance. PCC is often more compatible with optical brightening agents or other papermaking additives than other mineral pigments.
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