Properties of solid wood responses to drying and heat treatment

Abstract: The hygro-thermal processes that wood is exposed to during drying and heat treatment lead to different reactions or responses in the wood material. The objective of this thesis has been to study the impact of different drying and heat-treatment strategies on various responses, such as strength, sorption/desorption behaviour, dimensional stability and colour changes. A decrease of shear strength along grain direction was found for high- temperature dried pine that was dried at temperatures exceeding 100°C, compared to boards dried at lower temperatures. No unambiguous decrease of surface hardness, cleavage strength or toughness was found for clear wood samples when high-temperature dried material was compared to material dried at lower temperatures. A decrease of hygroscopicity for wood exposed to increased temperatures was found. The higher the temperature, the greater was the decrease in equilibrium moisture content, EMC. The desorption isotherm of dried pine wood compared to initial desorption isotherm for fresh, green wood also showed lower EMC. Noticeable colour-change responses to heat treatment were found when different wood constituents such as pine and spruce sap and extractives from pine heartwood were heat-treated separately. Colour changes increased with time and temperature. An accelerated colour change was found for pine sap and extractives at temperatures exceeding 70°C. Studies of material properties such as extractive content and its influence on diffusivity show that density has greater influence than extractive content on diffusivity in pine and spruce. Pine shoved lower diffusivity than spruce, but when extractives were removed from pine heartwood, no difference was found in diffusivity compared to pine sapwood or spruce heartwood in a comparison of average levels between unpaired groups. The relation between diffusion coefficients in tangential, radial and axial direction in solid pine sapwood were found to be 1:1.8:7 respectively. Phenomena within the area of process dynamics were also studied. Calculations of thickness of a thin, dry outer shell formed in pine sapwood boards early in the capillary phase of drying were done based on temperature and mass flux measurements. Comparison with dry shell thickness analysed in a computer tomography scanner showed fairly good agreement.