Novel Insight into the Separation and Composite Utilization of Sclerenchyma Fiber Bundles of Willow Bark

The bark from fast-growing willow crops contains long and strong sclerenchyma fiber bundles, which could potentially replace the pulp and annual plant fibers currently used to reinforce green polymer composites. Here we successfully demonstrate the isolation of fiber bundles of willow bark with a simple alkali treatment under much milder conditions than what pulp fiber separation requires. The fiber bundles separated had hydrophobic surfaces, which made them compatible with polymers, in this case with polylactic acid, without using any additives. The most hydrophobic fiber bundles of willow bark provided the strongest and toughest composites, superior to the corresponding isotropic composites of pulp and flax fibers. Integration of the fiber bundle isolation with a prior recovery of hot water extractable aromatics from the bark and further processing of the debarked willow stems into bioethanol and lignin, for example, could make the full valorization of the willow biomass feasible.