Optimizing microwave dielectric heating behavior of natural fiber-reinforced polypropylene composites

Non-uniform microwave heating remains a major limitation in processing natural fiber-reinforced thermoplastics, since most previous studies have focused mainly on dielectric characterization or on microwave heating behavior as a function of fiber content alone, without considering fiber morphology or its combined effect on heating uniformity. This study examines the influence of wood-fiber morphology and content on the microwave dielectric heating behavior of polypropylene (PP) biocomposites reinforced with Canadian maple wood-fibers. Composite sheets were fabricated with different lengths and fiber contents. Differential scanning calorimetry measurements conducted over a temperature range of 22–100°C showed that increasing fiber content reduces the specific heat capacity of the composites by approximately 15%, thereby facilitating a faster temperature rise during heating. Dielectric spectroscopy performed at 2.45 GHz revealed that incorporating fibers increases the dielectric constant from 1.45 for PP to a maximum of 1.69 for 15% content, while the loss tangent varies with fiber length and concentration. These combined changes in dielectric properties modulate the dielectric loss factor, which governs microwave energy absorption and directly influences the heating rate, volumetric heating efficiency, and temperature uniformity. Experimentally measured dielectric and thermophysical properties were implemented in a three-dimensional finite element model based on the enthalpy method and Crank–Nicolson time integration. Numerical results demonstrate that, the composites reach higher temperatures than PP and exhibit improved volumetric heating with reduced through-thickness temperature gradients. The combined experimental–numerical findings highlight the critical role of fiber morphology in tailoring microwave heating efficiency and demonstrate the strong potential of these biocomposites for microwave-assisted thermoforming.