Influence of processing temperatures on fiber dimensions and microstructure of polypropylene/hemp fiber composites

Abstract

The processing characteristic of polypropylene/hemp fiber composite, at various melt-compounding temperatures between 180 and 230°C, was characterized by torque rheometry. Hemp fibers were extracted from polypropylene/hemp fibers composites (melt-compounded at 180, 200, and 230°C, respectively), in a Soxhlet extractor with xylene. The length distribution, aspect ratio distribution, and the diameter of hemp fibers were measured, and the relationship between fiber dimensions and the processing temperature was investigated. Scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction were employed to characterize the changes in fiber morphology and microstructure after melt-compounding. Fiber dimensions statistical results show that fiber length and aspect ratio distribution become narrow and shift to short average values as the processing temperature is raised. In addition, elevated processing temperature induces a remarkable decrease of fiber average diameter. Fourier transform infrared spectroscopy and X-ray diffraction results reveal that heat and shear forces applied to hemp fibers cause the reduction of cellulose relative content and the formation of amorphous components. When the processing temperature is higher than 200°C, irregular fracture morphology of hemp fibers and remarkable decrease in fiber average diameter indicate that hemp fibers’ amorphous components, such as hemicellulose and lignin, undergo apparent decomposition at higher processing temperature.