Thermowood ® vs Termovuoto process-comparison of thermally modified timber in industrial conditions

The study aimed at treating Scots pine (Pinus sylvestris L.), spruce (Picea abies Karst.) and two hardwoods, metil (Sterculia appendiculata K. Schum) and neem (Azadirachta indica A. Juss) timbers in industrial conditions by steam (Thermowood ®) and vacuum (Termovuoto) thermal modifications. Matched boards were treated identically and alterations in spectral characteristics and chemical composition of wood were compared. Mass loss, selected mechanical properties and wood durability were also traced and related to the chemical alterations. The physical nature of the two processes induced similar changes in chemistry and structure of softwoods but at different magnitudes for the hardwoods. The applied vacuum removed partly the acetic acid, which catalysed carbohydrate degradation, i.e. heat applied under vacuum was less destructive. Wood permeability was found important in the thermo-vacuum process. Mass loss was significantly higher after Thermowood ® process namely, 14.1% vs. 9.9% for metil and 14.2 and 12.1% for neem wood. No significant mass loss differences were found between pine and spruce wood after the two treatments. Mass loss correlated with the decrease of shear strength, rupture and elasticity moduli and increase of wood decay resistance. As more permeable, metil demonstrated significant difference between the properties above; thermo-vacuum process was less destructive but ensured lower improvement of durability compared to Thermowood ® treatment. The entire study allows concluding that Thermowood ® and thermo-vacuum treatment according to Termovuoto technology produce similar final products with regard to chemical, physical-mechanical properties and durability with clear difference in the appearance (colour). Thermo-vacuum modification of wood can be advantageous where the end use of the product demands retaining the mechanical properties closer to those of the untreated material.