Factors affecting wood-bark adhesion for debarking of shrub willow

Debarking has the potential to make short rotation woody crops more economically attractive by both improving the quality of woody material and creating a new product stream from separated bark. Thus, in this research the ultimate strength of the wood-bark interface was studied relative to the development and optimisation of debarking systems and strategies. Four genetically different cultivars of shrub willow, three harvest times and different moisture levels were tested at a quasi-static extension rate of 0.5 mm min⁻¹. Regression models show that moisture content has a negative correlation with wood-bark bond strength, which is similar to observations made on other species. Harvesting time had a significant effect on wood-bark strength of shrub willow (p < 0.01). Shrub willow harvested during the winter had significantly (p < 0.01) higher wood-bark bond strength compared to early-growing with a mean increase of 90% and 108% compared to mid-year. This is thought to be due to seasonal changes in cambium layer morphology. Moreover, wood-bark bond strength was significantly different (p < 0.05) for genetically different cultivars of shrub willow. Cultivar ‘Millbrook’ had higher wood-bark bond strength than ‘Fabius’. Preliminary indications suggest it is possible that cross-linkages of calcium ions, and arabinan and galactan side chains of pectins play a role in cultivars that have high wood-bark bond strength.