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 coa ting suspensions, containing 5 to 15wt% of synthetic binder, was studi ed 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 spher ical-like (CaCO3) particles and were used either individually or in bl ends. The effects of five types of co-binders and three types of simpl e 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 f rom pigment blends decreased with CaC03 content. Individual pigments h ave a higher degree of pseudoplasticity and lower thixotropic coeffici ent compared with pigment blends. Coatings containing synthetic binder have lower flow parameters compared with binder-free suspensions. Syn thetic binder in combination with hydrocolloid additives, namely three grades of CMC, HEC, HMC (carboxymethyl, hydroxyethyl and hydroxymethy l celluloses), oxidized starch and casein (as synthetic/synthetic or s ynthetic/natural dual binder) produced higher flow parameters compared with single binder coating formulation. Also, it was found that addit ion of monovalent electrolyte (NaCl) has a smaller effect on flow para meters than divalent electrolytes (CaCl2 and MgCl2).