The role of expandable graphite particle size and microstructure on the dielectric and thermal properties of...

This work demonstrates the decisive role of expandable graphite (EG) particle size and processing route in tailoring the microstructure and multifunctional performance of linear low density polyethylene (LLDPE)-based composites. In the studies, the EG of low: 90 ml/g (EG096), medium: 350 ml/g (EG350), and high: 500 ml/g (EG500) expansion rates were chosen, which are related to the shortest, up to ~ 200 μm; medium, up to ~ 1 mm; and the longest vermicular structures, up to over 1 mm, after expansion. The main finding is that the EG096, with the shortest vermicular structure, produced the most homogeneous morphology, characterised by minimal voids and agglomeration. The highest dielectric performance was achieved by the sample containing 15 wt% of EG096, the most microporous system with the largest pore volume, combining an enhanced dielectric constant (ε' = 13 at 3.6 GHz) and dielectric loss. We propose that the microporosity of the EG enhances electromagnetic wave dissipation via multiple reflections and local dielectric losses, with short vermicular EG structures supporting a connected network that boosts microwave-frequency dielectric performance. Processing route strongly influenced dielectric behaviour, with hot pressing shown to preserve the vermicular structure better than extrusion. While the effect on thermal diffusivity and conductivity was less pronounced, the homogeneous microstructure remained attractive for thermal management, with EG096_20 extruded samples reaching λ = 0.86 W m^-1 K^-1. Unlike previous studies, this work systematically correlates EG expansion rate, vermicular morphology, and microporosity with both dielectric and thermal properties, establishing key design parameters for next-generation dielectric and thermal management materials.