Impact of drying and heat treatment on physical properties and durability of solid wood

Sammanfattning: During drying and heat treatment, wood is exposed to a hygrothermal process during which wood properties are chemically and physically affected, resulting in responses on a microscopic as well as on a macroscopic level. The main objective of this thesis is to build knowledge on the interaction between the wood material properties and the drying and heat treatment process in terms of material responses. Hopefully, the benefit of the compiled conclusions in this thesis will contribute to the knowledge base on which decisions are made regarding choice of material and control process parameters in order to attain desired quality in end products. The results are summarized as follows: In this study of material properties such as extractive content and its influence on diffusivity, it is shown that density has greater influence than extractive content on diffusivity in pine and spruce. Pine showed 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. The relation between diffusion coefficients in tangential, radial and axial directions in solid pine sapwood was 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 and was supported by SEM studies of the dark-coloured shell zone in pine sapwood. The following responses to drying and heat treatment process were studied: strength, sorption/desorption behaviour, dimensional stability, colour changes, capillary water absorption capacity and natural durability. A decrease of shear strength along grain direction was found for high-temperature dried pine i.e. at temperatures exceeding 100°C, compared to boards dried at lower temperatures. No unambiguous decrease of surface hardness, cleavage strength or toughness was found at the same temperature comparison. 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. Computer tomography studies of capillary water absorption in heat-treated and dried pine, spruce and birch showed that heat treatment results in a decreased ability to transport free water in the longitudinal direction in all wood types studied except for pine sapwood. The differences in absorption capacity between spruce sapwood, spruce heartwood and pine heartwood were small. SEM studies of the anatomical microstructure were done with focus on the crossfield pits between horizontal ray parenchyma and longitudinal tracheids in pine and spruce. Crossfield pits in heat-treated and dried pine sapwood were found to be considerably more open than those of not artificially dried sapwood, with partly loose or ruptured membranes. No difference in share of open crossfield pits was found between the material dried at 60°C and material heat-treated at 170°C and 200°C. In pine heartwood, the open, ruptured structures found in heat-treated pine sapwood were rare. A hypothesis involving the emptying of the parenchyma cells has been proposed to explain this observation. The open crossfield structures between ray parenchyma and longitudinal tracheids in dried and heat-treated pine sapwood are believed to play an important role in explaining the differences in water absorption between pine and spruce sapwood. The impact of drying on the natural durability of pine lumber was studied, with mass loss in a rot test as a measure of durability. Air-dried heartwood showed the best durability compared to kiln-dried at 70°C, 90°C and 110°C. Lowest durability wasfound when drying was performed at 90°C with high material temperature early in the capillary regime of drying with high moisture content. The interpretation is that the duration of high material temperature at high moisture content is a critical state for decay resistance in heartwood. Steam conditioning after drying was found to diminish the durability of sapwood. The relation between mass loss in rot test and concentration of total phenolics-compounds known to contribute to the natural durability of pine heartwood-showed a weak negative correlation, as did the relation between mass loss and density. Heating of extractive-rich green sawdust caused a reduction of phenolics with temperature and time.