Effect of hydrophilicity and lipophilicity balance on rheological properties of pigment dispersions

Many solvent-based flexographic and gravure inks are prepared by grinding pigments in resin solutions (acrylics, polyamide, polyester, etc.). Polymer chemists and formulators have modified resins with various functional groups in order to achieve optimal ink performance. This paper demonstrates that resin hydrophilicity and lipophilicity balance has a dramatic effect on the rheological properties of liquid inks. Hydrophobic groups adsorb onto the pigment surface, while hydrophilic groups are solvated in solution and stabilize pigment particles (steric/electrostatic stabilization). Both the hydrophobic and hydrophilic groups are important for particle stabilization and an appropriate balance can result in an ink or dispersion with a Newtonian flow. Otherwise, pigment particles may flocculate and the ink becomes shear thinning. The degree of shear thinning strongly depends on the balance of resin hydrophilicity and lipophilicity and may significantly impact the ink performance.