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Isotherm moisture absorption kinetics in natural-fiber-reinforced polymer under immersion conditions

In natural-fiber-reinforced polymer, absorption of water or moisture is a significant issue in maintaining strength and stiffness. To enhance the understanding of water and moisture sorption behavior, the kinetics of moisture sorption in natural-fiber-reinforced polymers are investigated under immersion conditions. Samples of hemp-bast-fiber-reinforced polyethylene are prepared using an injection molding technique at different hemp fiber volume fractions (vf). The samples are then immersed in water for 274 days. Moisture content and uptake rate are analyzed at different fiber volume factions and matrix crystallinity percentages. A simplified two-dimensional contraction model is developed to investigate the contraction effect on the moisture uptake; it shows that a matrix with high crystallinity has more stiffness contraction on the reinforcing natural fibers, which limits the maximum amount of the absorbed moisture. The Fickian diffusion is found to be the dominant mechanism, shifting toward pseudo-Fickian or anomalous diffusion depending on the natural fiber volume fraction and the crystallinity percentages of the matrix. The natural-fiber-reinforced polymers diffusivity is evaluated and modeled to characterize the ability of liquid molecules to diffuse into these composites at different hemp fiber volume fractions. Both the crystallinity percentage of the matrix material and the volume fraction of the reinforcing fibers were found to interactively affect the sorption kinetics of the tested natural-fiber-reinforced polymers.