Precipitated calcium carbonate (PCC) by definition is a synthetic chemical material, and the main features that cause it to be useful in paper are derived from its synthetic nature. The ability to reproducibly manufacture PCC made to a particular size, shape or other specifications is called crystal engineering. The combination of chemical synthesis and crystal engineering allows for five distinct characteristics of the product to be tailored to meet customer needs. Specialty Minerals Inc. (SMI) calls these the five degrees of freedom of PCC. They are:
- Shape, or more specifically, morphology. Put simply, morphology is shape in 3-dimensions, which can be plates or cubes or any of several distinct shapes that provide various functionality in paper. Scalenohedral PCC, by virtue of the size of its individual crystallites, is exceptionally efficient at scattering light and lends itself well to applications where high opacity and brightness are required. Prismatic, or barrel-shaped PCC is generally least disruptive to interfiber bonding in the sheet. Other morphologies are useful when sheet bulk, porosity or some other property is paramount.
- Size. Not only can the morphology of PCC be tailored, but its particle size can as well. Smaller particles usually scatter light more efficiently, while larger particles usually provide better sheet bulk. Only a synthetic process like precipitation is capable of producing a particular shape in a particular size. For papermakers, this allows balancing the important properties of the sheet in a way that natural pigments cannot.
- Size Distribution. The average size of PCC particles is important, but so is the distribution of those sizes. Particles that are distributed over a narrow range of sizes are useful in obtaining maximum bulk. Particles that exhibit broad size distributions tend to pack well and thus interfere less with interfiber bonding and sheet strength. Intermediate sizes, or blending of different morphologies of different sizes, allows tailoring of performance properties that cannot be accomplished with other pigments.
- Surface Area. Though specific surface area, expressed as m˛/g, is somewhat dependent on particle size, it can also, within limits, be varied independently of other factors. This is usually less important for paper filled or coated with PCC, but can be very important for other materials, such as filled plastics.
- Surface Chemistry. PCC is a “clean” material that is largely free of impurities. When freshly made, the particles usually carry a slightly positive charge called the zeta potential that can affect PCC's bonding to the paper fibers. However, by using selected chemical agents, this charge can be increased, or neutralized and made negative, making PCC particularly flexible in this regard. PCC is unreactive toward most optical brightening agents (OBAs) used in the paper industry, and this causes OBA to be more effective when used in conjunction with PCC, which in turn lowers the manufacturer’s overall cost of production. PCC is also very compatible with a variety of dispersants and other chemical agents, which makes it useful in both paper and non-paper applications.
Aside from these five degrees of freedom, PCC, as a synthetic material, exhibits brightness, whiteness and chemical purity. These features, along with the five degress of freedom make PCC the calcium carbonate of choice in papermaking operations throughout the world. And, papermakers continue to find ways of leveraging the unique properties of this material.
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