Water-repellent efficiency of thermally modified wood as affected by its permeability

This study shows how the air permeability of thermally modified wood contributes to its water-repellent efficiency. For this purpose, freshly cut boards of hornbeam ( Carpinus betulus ), poplar ( Populus nigra ), and heartwood of oak ( Quercus castanifolia ) were modified at a steam temperature of 180 °C for 3 h inside a ThermoWood kiln. The porous structure, permeability, and water uptake of wood were affected differently by thermal modification, depending on the wood species. The creation of micro-cracks in the cell walls, due to collapsing of fiber cells, resulted in a noticeable increase in the permeability of hornbeam. Despite checking in the poplar wood structure, its permeability was negatively affected by thermal modification. In contrast to oak and poplar, a negative water-repellent efficiency was observed for the modified hornbeam, caused by an increase in the permeability.