Structure formation and thermal conduction in polymer-based composites obtained by fused filament fabrication

Abstract: Fused filament fabrication (FFF) is widely used to obtain polymer-based composites with improved mechanical and thermal conduction properties. The effective properties of the composites are sensitive to its structure including the shape, distribution, and orientation of inclusions in the matrix and FFF-specific defects. The present work aims to study the formation of composite structure and relate it to the measured effective thermal conduction properties. Polymer flow in the hot end and the structure of printed samples are studied by metallography. The effective thermal diffusivity is measured by the laser flash method and analyzed by the Maxwell Garnett theory. Fibers in a polymer-matrix composite visualize polymer flow in the hot end of the FFF printer. In the nozzle, fibers are generally oriented parallel to the flow direction while gas bubbles may locally disturb flow and disorient fibers. Printed composite Carbon fiber/TPU inherits preferential orientation of fibers observed in the filament. Disorientation of fibers relative to the alignment direction increases after printing. In the printed Al2O3/PA composite, there is a lattice of triangle-based cylindrical pores parallel to the extrusion direction. Al2O3 particles are of irregular shape and approximately equiaxed. No segregation or agglomeration are observed. Significant anisotropy of thermal diffusivity is observed in the printed samples of 5%vol. Carbon fiber/TPU composite. The measured effective thermal diffusivity of printed Al2O3/PA composites is essentially isotropic and significantly increases with the volume fraction of inclusions. In the studied FFF-printed composites, the effective thermal diffusivity is proportional to the thermal diffusivity of polymer matrix, which is sensitive to its structure formed in non-equilibrium conditions of rapid quenching from liquid phase.