Interactions in Rotogravure Coating Colours Containing Clay and Alkali-Thickening Latices, 1993 Coating Conference Proceedings

The rheological behaviour of rotogravure coating colours containing a coarse clay and a series of alkali-sensitive latices has been determined as a function of pH and shear rate (30 - 39000 s-l). The colours, and suspensions of the latices alone, exhibited large increases in viscosity between pH 6.0 and 7.5, i.e. in the region of pKa values at which the ionisation of carboxylic groups occurs. Photon correlation spectroscopic studies demonstrate that this effect is associated with an increase in the hydrodynamic size of the latex particles. Unexpectedly, it was observed that the pH at which the viscosity increase occurred was higher in the colours than in the latex suspensions themselves. One possible explanation for this is that the ionisation of the latex polyacid groups is suppressed in the chemical environment of the coating colour. Alternatively, the results point to an interaction between the ionised latex and clay. As expected, the effect is less pronounced at higher shear rates >l000 s-l) where pigment - latex particle packing effects predominate. Finally, oscillation experiments showed that the coating colours (but not the separate components) developed considerable elasticity, as has been seen for offset colours and has been implicated in runnability and post-blade levelling phenomena.