Porous mineral particles as coating pigments

Different grades of precipitated calcium carbonate (PCC) and of ground calc ium carbonate (GCC) were investigated with respect to the rheological prope rties of pigment suspensions and coating colours and their abilities to pro duce coating layers of high porosity with good light-scattering properties. The pigments were used to coat a woodfree paper in a high-speed laboratory coater and also on a pilot scale. The printing performance of the coated p apers was evaluated both with laboratory techniques and using a fun-scale 4 -colour offset printing press. The PCC pigments can be regarded as model sy stems for different types of porous pigments. Coating layers of high porosity were obtained with the porous PCC-pigments. The Light-scattering coefficients of these layers were very high, and coat ed paper with high brightness and high opacity could be produced with these PCC-pigments. The maximum volume fraction of the solid phase in the suspen sion appears in a sense to be a key parameter with regard to the flow behav iour, the immobilisation point, the porosity and the Light-scattering abili ty. The laboratory evaluation of the printing performance of the coated pap ers showed that in general the PCC-pigments gave a lower ink demand, less s et-off tendency and less print mottle than the GCC-grades used. The lower p rint mottle was also confirmed in the full-scale printing. These improvemen ts are considered to be an effect of the higher porosity and improved cover ing ability of the coating layers. However, the surface strength of the coa ted papers decreased substantially with increasing porosity. A possible rou te to reduce this problem is mentioned. The porous character of the pigment s also Limits the solids content of the coating colour that can be used in the coating operation.