Influence of the inorganic phase concentration and geometry on the viscoelastic properties of latex coatings through the glass-transition

The mechanical properties of artists' acrylic (latex) paint films containing different volume fractions of TiO2, CaCO3 and kaolin were measured in uni-axial tension over a broad range of temperatures and crosshead speeds. Young's modulus results in the glassy region were first compared with several micromechanics theories for particle-filled composites containing elastic phases. It was found that the Mori-Tanaka theory slightly under-predicted the modulus enhancement, while the Lielens approach provided the most accurate results. A nonlinear viscoelastic material model involving a Prony series and the neo-Hookean hyperelastic function was used to represent the tensile data up to relatively small strains of a few percent. From the experimental data, the material model was calibrated and the required parameters were determined. The derived parameters were then used to re-construct relaxation modulus plots, which were compared with the approximations given by Clements and Mas for the viscoelastic Mori-Tanaka theory in the time-domain. It was found that the experimentally observed modulus enhancement was much stronger than the predicted values in the rubbery region. Mechanisms such as constrained polymer at the inorganic particle interface, and the possible formation of a percolation network are discussed.