Nonlinear viscoelastic behavior of paper coating colors

Paper coating colors exhibit complex rheological properties resulting from the interactions between the different components in the formulations. These interactions induce a microstructure which is time and shear-dependent and is responsible for the nonlinear viscoelastic behavior. The nonlinear viscoelastic properties of kaolin based colors have been studied through small amplitude oscillations, steady shear and stress growth experiments. The observed rheological behavior for coating colors containing two different water-soluble polymers (a carboxymethyl cellulose and a polyvinyl alcohol) have been interpreted in light of microstructure changes. Both the loss and elastic moduli were found to be functions of the strain amplitude. The elastic modulus was shown to decrease above a critical strain amplitude; the loss modulus, which was much smaller than the elastic modulus, initially increased, depicted a maximum and then decreased with strain for values above the critical strain. The rheological behavior of the coating colors studied was found to be time-dependent and the properties were found to be similar for the coating colors containing either CMC or PVA, but the thixotropic character was much stronger for the PVA based colors.