Many people are surprised to learn that the sheet of paper they see and use everyday may actually contain a high percentage of minerals in addition to wood pulp. During paper manufacturing, minerals are added to the paper as a “filler” in the case of uncoated papers, or applied on top, for example on coated paper.
Minerals are added to paper chiefly for two reasons: to improve its printing properties and to reduce the cost of its manufacture. Since paper is primarily a medium for communications, minerals help to enhance paper’s properties such as whiteness, brightness, opacity, gloss, and smoothness. Whether the paper will be used in a copy machine, magazine or company annual report, these properties are essential to good print appearance.
The four most popular minerals used today are calcium carbonate (both precipitated calcium carbonate and ground), kaolin clay, titanium dioxide and talc. Their popularity derives from shared common characteristics: they are all white, opaque and readily available in small particle sizes. And, except for titanium dioxide, all are less expensive than wood pulp.
In uncoated papers, mineral particles are so small they can fit between the tangled mesh of wood fibers and can greatly influence the properties of the sheet. The actual loading level depends on the type of pulp used and the desired paper properties. Minerals are less expensive than wood pulp and can be used to extend or replace a portion of the expensive pulp fiber. In addition, minerals can lower the cost of production in several other ways. They increase water drainage from the paper machine allowing better runnability and speedier production. Minerals also increase paper bulk, which results in more entrained air, increasing opacity and decreasing sheet weight—a benefit as postal rates, based on weight, continue to increase.
On the coated surface, minerals enhance the printed look of paper by controlling ink absorption into the paper. This is especially important in high fidelity four-color printing where the desired effect is to retain the ink on the surface of the paper rather than be absorbed into its interior. Minerals also function to block light from passing through a sheet of paper. This lessens the print show-through when paper is printed on both sides. Minerals can also act as tiny mirrors, reflecting light back to the viewer. This can enhance overall paper brightness and gloss.
Minerals have been used on coated paper at least since the mid-1800’s, but consumption really did not accelerate until after World War II. Today, the global paper industry consumes about 29 million metric tons of all types of mineral products.
Learn more:
