1. Manufacture of PCC and PCC for coating
Calcium carbonate is the most prevalent mineral on the Earth’s surface, other than quartz. It exists in three different mineral forms: Calcite, Aragonite and Vaterite. Calcite is the most common natural form; aragonite and Vaterite are metastable and will irreversibly convert to calcite. Since calcium carbonate can be crystallized into a variety of forms, it is multifunctional and can be used to modify the physical and chemical properties of filled or coated paper. In practice, this is achieved by controlled synthesis to produce a special form of precipitated calcium carbonate (PCC).
2. Physical properties of PCC for coating
The high whiteness (blue light reflectance) of the paper is extremely important because it creates a high degree of contrast between unprinted and printed parts. Calcium carbonate is known for its higher whiteness than kaolin and talc pigments. Impurities in limestone are usually grey or even black thus reducing whiteness, but as described in the PCC production process, these impurities can be effectively removed from slaked lime. For ground calcium carbonate (GCC), some limestone impurities can be removed by flotation or by ore sorting. If the impurities are naturally occurring, an additional increase in whiteness can be obtained by bleaching. The ISO whiteness of PCC or GCC is 93% to 96%, while that of kaolin and talc is significantly lower, about 84% to 87%.
3. Flow characteristics and running performance of PCC-containing coatings
PCCs with high aspect ratios and narrow particle size distributions, such as GCCs with near-circular and broad particle size distributions, encounter more rheological challenges. Such challenges include not only the pumping possibilities, but also the flow characteristics in the coating equipment. On the other hand, PCC is more likely to rotate about its axis than the layered flaky kaolin, and we can at least assume that needle-shaped PCC particles tend to align along the long axis under shear stress. This can actually be seen as due to the lower viscosity of the coated PCC than the layered kaolin at the same solids content.
4. Application of various grades of PCC in coating
In double or triple coated papers, PCC in fine particle size has been found to be the best choice. In this application, the fine particle size refers to an APS (average particle size) of 0.3-0.4um. At this point it does not matter whether the base paper contains no mechanical wood pulp (only chemical wood pulp) or mechanical wood pulp (at least 10% groundwood). If a plastic pigment is used, it can be replaced by PCC, making the coating economically attractive. If high gloss is not required, a natural high gloss can be achieved with light calendering, which means higher bulk, higher stiffness and better opacity and whiteness compared to other pigments .
The precoat must be a coarser coating pigment. It is recommended to use PCC with APS of 0.6-0.8um in order to make the structure of the precoat more open. This open structure provides a significant improvement in opacity due to the addition of voids for light scattering. However, for this open structure, water retention may be a problem, and it may be necessary to compensate by adding several CMCs or thickeners with longer molecular chains to the coating.
We know that matte papers with good smoothness, low paper gloss, high print gloss and adequate ink curing properties are not easy to manufacture. This combined performance is possible using PCCs with higher APS and narrower particle size distributions.
5. Properties of PCC coated paper
The advantages of coated paper with PCC are: high whiteness; low b-value; high light scattering, i.e. high opacity; high bulk effect; good fiber coverage; adjustable ink curing properties, i.e. good printability; Low bubbling tendency.