Droplet relaxation of molten metals in polypropylene matrix: Measurement of the interfacial tension

Using the retraction method of a deformed drop, the interfacial tension between polypropylene (PP) and two different molten metals (tin and the eutectic SnAgCu alloy) has been measured at 250 and 230 °C, respectively. Systematic rheological investigations of the materials under study enabled us to measure the viscosity of the polymer matrix and that of the metal liquids. A viscosity of 1.9 and 2 mPa s in the melt is reported for the tin and SnAgCu alloys, respectively. Based on these investigations, the interfacial tension between the molten metals and the polymer matrix is determined for the first time. The influence of the polymer matrix viscoelasticity and liquid metal surface oxidation is analyzed in order to determine the most accurate conditions for measuring the interfacial tension between the polymer and the liquid metal. Values as high as 350 mN m−1 (for PP/SnAgCu) and 140 mN m−1 (PP/tin) are reported, indicating a poor affinity between the molten metal and the polymer matrix. By temperature quenching, the dispersion of the liquid metal drops in the polypropylene under constant shear, and the formation of solid metal ellipsoids is obtained, with a characteristic size that is limited by the critical deformation parameter defined by Taylor. Our study, thus, offers an experimental approach of measuring the interfacial tension between two highly incompatible liquids, opening at the same time a promising perspective for obtaining, by a rheological approach, metal particles with a non-negligible form factor.