Fractionation of willow bark for combined production of extracts and fiber bundles

Abstract

This study investigates the coproduction of willow bark sclerenchyma fiber bundles and extracts in order to increase the potential added-value from short-rotation willow biomass. Willow is a fast growing biomass source that is grown mainly for heat and power generation, especially in the Northern Hemisphere on marginal lands. On afforested peatland, willow can be grown to prevent erosion and to remove excess nutrients from the soil. A few sporadic studies investigating useful bioactive phenolic compounds from willow bark have been conducted, but burning bark for energy purposes is prevalent.

The novel introduced biorefinery concept aims to use 1) the bark fraction for the production of fibers and extractives and 2) the wood fraction is hydrolyzed for sugars and lignin recovered for chemical industry. The morphology and chemical constitution of the inner bark is characterized as the first step towards complete willow valorization. The distinguishing features of inner bark are its high ash and extractive content and bundles of relatively long and thick-walled sclerenchyma fibers. We discovered that picein, triandrin and catechin could be extracted at an approximate 14% overall yield rate from bark by hot water (20 min at 80 ºC). 2D–HSQC NMR spectroscopy and wet chemical analyses demonstrate that guaiacyl is the predominant unit in bark lignin over syringyl and p-hydroxyphenyl. Therefore, the bark lignin structure is the key for developing a novel strategy for separating fiber bundles by judiciously using a mild alkali treatment, followed by fabrication of a composite from the fiber bundle layer and polylactic acid (PLA). Effective routes for separating such fiber bundles require a much lower amount of energy and chemicals than does the separation of individual fibers by conventional methods. Moreover, the fiber bundles exhibited the best compatibility (fiber surface lignin up to 40%) with the matrix PLA compared to other reinforcements. This strategy expands novel composite applications of bark fiber bundles, indicating considerable promise for utilizing this otherwise burned bark material. Additionally, bark lignocellulose nanofibrils exhibit higher hydrophobic properties, and its films display higher tensile strength in comparison with wood, which again provides another example of the superior properties of the bark compared to the wood.

This thesis carried out a general, integrative study regarding the morphological structural and chemical characterization of bark and wood, their fractionation, as well as a review of the literature in order to achieve full utilization of fibers and extractives from bark. Gaps in our current knowledge and potentially interesting research approaches are identified and discussed.