Analysis of temperature-dependent polar and dispersive surface energy components of thermoplastic: Impact on fiber-matrix adhesion and interfacial shear strength

The interfacial interactions between fibers and thermoplastic matrices play a critical role in determining the performance of fiber reinforced composites. While surface energy measurements are commonly performed at room temperature, the surface tension components of thermoplastics under melt processing conditions remain largely unexplored. In this work, a recently developed methodology was employed to determine interfacial tensions using molten polypropylene (PP) and polylactic acid (PLA) over the temperature range of 180 to 240 °C. This enabled, for the first time, the determination of the temperature-dependent polar and dispersive components of the surface tension of molten thermoplastics. In parallel, the surface energy and its polar and dispersive components were determined for sized and unsized carbon fibers and basalt fibers, as well as for solid PP and PLA. Comparison between the solid and molten states of thermoplastics revealed comparable total surface energy values but significantly different polar and dispersive contributions, demonstrating that room temperature surface energy measurements cannot be directly extrapolated to melt processing conditions. These results were further used to evaluate the work of adhesion using the Owens–Wendt approach. Although fiber polarity strongly influenced the work of adhesion at room temperature, its effect became negligible in the molten state due to the predominance of the dispersive component of PP and PLA. Furthermore, interfacial shear strength (IFSS) measurements showed that a reliable prediction cannot rely on work of adhesion alone to predict fiber/matrix interfacial performance. Instead, IFSS is more closely correlated with the compatibility of the surface energy components of fibers and thermoplastics.