The largest single user of kaolin is the paper industry because when kaolin is used, paper products print better and are made whiter and smoother. Kaolin used as a filler in the interstices of the sheet adds ink receptivity and opacity to the paper sheet. Kaolin used to coat the surface of the paper sheet makes possible sharp photographic illustrations and bright printed colors. Kaolin constitutes nearly one-third the weight of today’s slick sheet magazines.
The significant properties of kaolin of greatest value to the paper industry are whiteness, low viscosity, non-abrasiveness, controlled particle sizes, and flat hexagonal plates. Opacity is an extremely important property to the paper industry. Flow properties or rheology of kaolin clays, especially the kaolin coating clays used in the paper industry, are very important because of their influence on coat weight, smoothness, texture, and other properties.
If one were going to design in a laboratory a coating pigment for the paper industry, it would be white, disperse readily in water, and have low viscosity, be soft, have a fine particle size, and be a thin plate-shaped particle. Nature produced a mineral which has essentially all the above properties, and that mineral is kaolin. It can be readily seen why kaolin is an ingredient essential to the paper industry.