Applications in Paper Production

In the last two decades, calcium carbonate has achieved the same production volume as kaolin in the paper industry worldwide. Today, more than 10 million tons of calcium carbonate is used annually in the paper industry. This corresponds to 44% of the minerals consumed in the paper industry worldwide. When this ratio is separated, it evolves to an average of 2.5 million tons of PCC-based calcium carbonate and 8 million naturally ground calcium carbonate.

In the mid-1980s in North America, PCC played an important role as a filler. Although coating applications are not included in this role. As limestone is widespread throughout the world, PCC production around paper mills has allowed the selection of the most price-effects. In this case, PCC occurs at a solid component ratio of 25% and can be used directly as a filler production. A total of 48 PCC factories were built in the Americas in an average of 15 years.

Meanwhile, the general idea of economic division has led natural calcium carbonate producers to devise logical and convincing strategies to secure the supply of many mills. Today, the rate of natural calcium carbonate that the largest production facility produces for the paper industry from only one area is 2.5 million tons per year.

The growth possibilities of calcium carbonate in the paper industry are of no concern. Although coated papers cover a significant portion of the material consumption, the first steps have already been taken in the use of high-whiteness minerals in newsprint and wood-containing papers. Especially for newsprint, a new trend was seen in the mid-1990s. Thanks to the weight of the minerals, the use of 15% is quite common in today’s high quality papers. Due to its natural whiteness, readiness for use and the development of high application technologies, calcium carbonate will continue to play an important role in the future.

The fineness of the calcium carbonate used affects many properties of the paper and has definite and important effects on the paper improvement processes.

Modern paper research is based on the general view that the coating structure is formed before the wet curing process and the internal structure is not affected by calendering. In fact, these are surface layers affected by calendering.

When it is understood that fine calcium carbonate leads to a denser coating structure, the fact is that fine calcium carbonate produces a brighter paper surface under the same calendering conditions. To achieve the same paper gloss as a coarse calcium carbonate, more calendering pressure is required. In this case, the fibrous components under the coating are necessarily affected. It can cause loss of brightness and opacity, which is called calendering scribbling. To avoid this, the paper gloss is limited to coarse calcium carbonate. However, the coarse material is suitable for dull surfaces and for intermediate coating.

The mechanical properties of the paper surface, such as smoothness, porosity, and vapor permeability, also vary according to the grain size distribution of calcium carbonate. With increasing fineness, paper smoothness increases while roughness decreases. Even if very fine calcium carbonates are used, micropores cannot be avoided. This also applies to the subsequent calendering process. These pores keep the coating layers open and facilitate the passage of water vapor. This is true even for high-gloss papers with strongly condensed surfaces.

In heat-set offset printing, the structure of kaolin-based layers at rising drying temperature is a barrier that prevents water from escaping from the fibrous layers. In the same way that synthetic latex bonds hold the fragmented layers together, waterlogged layers form on the paper surface.

This significant effect of calcium carbonate on typical thermoset offset printing speeds was noticed in the early 1980s. During this period, watery bubbles were frequently formed during printing. Here, this is unavoidable if the paper is coated with high kaolin components and the print is dried at high temperatures.

In Europe, this problem has gradually subsided with the increasing transition from kaolin to calcium carbonate on paper surfaces. At the same time, the fineness of the calcium carbonate is increased to maintain the gloss and avoid calender blackening. As a result, the gloss ratio of the European paper market has increased noticeably in the 1980s and 1990s. The fineness of calcium carbonate products has reached its peak. These materials are high-quality, high-gloss carbonates with particles smaller than 1 micron in size and less than 90%. The increase in paper gloss values achieved in this way has been achieved with expensive synthetic minerals. For example, satin white or specially crystallized calcium carbonate.

During printing, printing ink affects the paper surface both mechanically and chemically. The particle size distribution of the calcium carbonate used in the coating plays a very important role in determining these two interactions.

Fine calcium carbonate qualities are better than coarse calcium carbonate in that it increases the porosity of the paper surfaces and contains a wider range of dispersing agents. Similarly, differences can occur between these two qualities. Depending on the characteristics of the printing ink, features such as printing brightness, semi-tone dot spread, uniform printing, drying behavior and scratch resistance of the color can be mentioned.

Such differences occur especially in the printing of low-weight coated rotogravure and four-colour cold-set newsprint. In both printing processes, a colored pigment is used. This dye forms a film on the paper surface following the absorption of organic solutions. In either case, very fine particles of calcium carbonate may interfere with rapid absorption of solutions such as mineral oils. In such cases, better printing results can be obtained by using reduced calcium carbonate by selecting coarse or very fine particles.