Willow inner bark as a potential source of fibres and chemicals

Abstract

The aim of this thesis was to acquire basic information on the physical and chemical structure of willow inner bark in order to assess its potential as a raw material for chemicals and fibres. Inner bark from four cultivated willow species/hybrids was studied and compared with their wood tissue. The cell and cell wall structure was studied by optical microscopy, SEM and TEM. The fibres were separated with an acid chlorite treatment and analyzed for their dimensions and morphology. The chemical composition was determined by standard methods including quantification of ash, extractives (extraction with acetone), lignin and carbohydrates. FTIR spectroscopy and Raman spectroscopy were applied as additional characterization methods.

In comparison with willow wood fibres the sclerenchyma fibres of inner bark were much longer and they had higher aspect ratio. The inner bark fibres were thick-walled and their lumina were almost nonexistent. The inner bark had almost ten times as high ash and extractives contents as the willow wood had. The lignin-to-polysaccharide ratio was similar in wood and bark although pectin/arabinogalactan was the dominant heteropolysaccharide in inner bark over xylan in wood.

The excellent fibre properties and the high extractives content of inner bark may justify its separation from willow biomass for production high-value special fibres and specific extractive-based chemical compounds. The debarked biomass could be used for production of sugars, lignin and their derivatives.