SOME ASPECTS OF THE RHEOLOGICAL PROPERTIES OF PAPER COATING SUSPENSION AND ITS APPLICATION .2. INFLUENCE OF PIGMENT COMPOSITION, BINDER LEVEL, CO-BINDER AND SIMPLE ELECTROLYTES ON FLOW PROPERTIES

The effect of pigment composition on the theological properties of coating suspensions, containing 5 to 15wt% of synthetic binder, was studied by using a low-shear rheometer, 0.5 to 100rpm, at 22 degrees C. The pigments used differed in shape from plate-like (China clay) to spherical-like (CaCO3) particles and were used either individually or in blends. The effects of five types of co-binders and three types of simple electrolytes on the flow parameters of clay-based coating suspension were also presented. The results showed that the flow properties (tau(0), tau(pl), Delta eta and eta(i)) of the coating suspension formed from pigment blends decreased with CaC03 content. Individual pigments have a higher degree of pseudoplasticity and lower thixotropic coefficient compared with pigment blends. Coatings containing synthetic binder have lower flow parameters compared with binder-free suspensions. Synthetic binder in combination with hydrocolloid additives, namely three grades of CMC, HEC, HMC (carboxymethyl, hydroxyethyl and hydroxymethyl celluloses), oxidized starch and casein (as synthetic/synthetic or synthetic/natural dual binder) produced higher flow parameters compared with single binder coating formulation. Also, it was found that addition of monovalent electrolyte (NaCl) has a smaller effect on flow parameters than divalent electrolytes (CaCl2 and MgCl2).