Thermo-electro-mechanical aging and degradation of conductive 3D printed PLA/CB composite

Abstract

Conductive polymeric composites consist of a polymeric matrix filled with conductive particles, providing electrical properties to a naturally insulating material. Their use in material extrusion printers provides great flexibility to geometrical design functional components. However, the degradation of their mechanical behavior during and after the application of electric currents is still absent in the literature. The flow of electric currents induces material heating affecting the strength, and mechanical deformation alters the electric properties leading to bilateral dependences. These processes evolve during the application of electric fields and may degrade the material behavior permanently. In this work, we address these questions taking as baseline material a 3D printed conductive Polylactic Acid (PLA)/ Carbon Black (CB) composite. We performed multi-physical characterizations on samples with different printing orientations and lengths evaluating: i) electric behavior under direct current (DC) regime and a wideband analysis by a Frequency Response Analysis; ii) thermo-electrical behavior studying the temperature field evolution under applied electric fields; iii) thermo-mechanical behavior under uniaxial tension for different testing temperatures; iv) thermo-electrical aging/degradation effects on the mechanical behavior. The study considers a wide range of electric fields (from 180 V/m to 600 V/m) and temperatures (from 25° to 130 °C). The results show enhanced electrical conductivity and mechanical stiffness when using a printing orientation parallel to the electro-mechanical loading. This coupled response is highly influenced by thermal effects due to Joule heating, especially when surpassing the glass transition and cold crystallization temperatures of the composite. The occurrence of such phase transitions governs the material degradation by promoting the growth of mesostructural pores during heating cycles arising from the application of electric fields. The obtained results are essential to unravel changes in mechanical properties when standing for continuous electric conduction and provide an experimental background for the lifetime expectance for the devices and possible thermo-electro-mechanical failure.