Impact of Processing and Char Feedstock on the Thermal, Mechanical, and Electrical Behavior of PLLA Composites

This work explores the influence of two preparation methods, solvent casting and melt mixing, on the structure-property relationships of poly-L-lactic acid (PLLA) composites reinforced with char derived from different waste feedstocks. Three types of char were produced by slow pyrolysis at 550 °C: olive pruning waste biochar (OC), tyre-derived char (TC), and a 1:1 hybrid co-pyrolyzed char (OTC). Each filler was incorporated into PLLA at 1 and 2 wt.% loadings, and the resulting composites were characterized through physicochemical, thermal, mechanical, and electrical analyses. Raman, FTIR, and SEM analyses revealed distinct structural characteristics for each char, with the hybrid OTC exhibiting the highest structural order due to synergistic interactions during co-pyrolysis. The preparation method affected filler dispersion. Solvent-cast films displayed micrometric agglomerates and interfacial voids, whereas melt mixing ensured a more homogeneous distribution. Thermal characterization showed that char addition did not significantly alter the crystallization or melting behavior of PLLA, although melt-mixed samples exhibited restricted chain mobility. Mechanical tests revealed opposing effects of filler loading depending on processing: in solvent-cast materials, stiffness increased while strength remained nearly unaffected, whereas melt-mixed composites exhibited reduced modulus and strength, attributed to the disruption of the denser amorphous structure generated during melt processing. Electrical resistivity depended on the preparation method. Solvent-cast composites remained insulating, while melt mixing, with OTC at 2 wt.%, led to a resistivity drop (down to 0.02 × 10¹⁵ Ω·cm from 20 × 10¹⁵ Ω·cm for unfilled PLLA), although all materials remained within the insulating regime. Overall, this work provides insight into the role of sustainable char fillers in improving the performance of PLLA composites and highlights the interplay between processing method and material properties. The developed PLLA/char composites are promising candidates for applications in flexible electronics, sensors, and antistatic components, as well as in lightweight structural materials and energy devices.