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. Thes e interactions induce a microstructure which is time and shear-dependent an d is responsible for the nonlinear viscoelastic behavior. The nonlinear vis coelastic properties of kaolin based colors have been studied through small amplitude oscillations, steady shear and stress growth experiments. The ob served rheological behavior for coating colors containing two different wat er-soluble polymers (a carboxymethyl cellulose and a polyvinyl alcohol) hav e been interpreted in light of microstructure changes. Both the loss and el astic moduli were found to be functions of the strain amplitude. The elasti c modulus was shown to decrease above a critical strain amplitude; the loss modulus, which was much smaller than the elastic modulus, initially increa sed, depicted a maximum and then decreased with strain for values above the critical strain. The rheological behavior of the coating colors studied wa s found to be time-dependent and the properties were found to be similar fo r the coating colors containing either CMC or PVA, but the thixotropic char acter was much stronger for the PVA based colors.