Effect of harvest time and field retting duration on the chemical composition, morphology and mechanical properties of hemp fibers

tThe large variability in the mechanical properties of hemp fibers is an issue in relation to their use inhigh-grade composites. The objective of the present study was to determine the optimal growth stage forharvesting hemp fibers for use in composites and to evaluate the effect of field retting time on mechanicalperformance of the fibers. Reduction in bast content and thickness of the primary bast fiber layer instems were found to be highly significant (P < 0.01) with plant maturity. A significant increase in thesecondary fiber fraction occurred with maturity, reaching a maximum value of 10% at seed maturity.A highly significant reduction in cellulose deposition in fiber cell walls was reflected by reduced fiberwall thickness with plant maturity and was related to the development and ripening of hemp seeds.A statistically significant increase in lignin deposition and a slight decrease in pectins in hemp fibercell walls were also noted with stem maturity. Microscopy observations and histochemical analyzescorroborated the results from the chemical analyzes and revealed variations in morphological aspectsand spatial micro-distributions of carbohydrates and lignin within the cell structure of the hemp stemsbetween early- and late growth phases. Fibers harvested at the beginning of flowering exhibited hightensile strength and strain, which decreased with plant maturity. Reduction in strength was related tothe increase in proportion of secondary fibers and decrease in cellulose deposition leading to inferiorproperties of fibers. A negative effect of field retting occurred only after extended field retting (i.e., 70 days) which was presumably due to accelerated degradation of cellulose by the action of microorganisms.