Effect of high-temperature drying on spruce and larch Shape stability, mechanical properties and mould growth

Sammanfattning: Shape stability, the ability of the wood to retain its shape, is one of the most important parameters when using timber as a building material and for other applications. Generally, timber undergoes changes in dimensions and shape when its moisture content is changed. However, this can often not be tolerated in constructions and thus, methods to improve the shape stability have to be found. In this thesis, the influence of high-temperature drying on the immediate shape stability after drying but also on the long-term behaviour under in-service conditions was studied for Norway spruce and different larch species. Special focus was on twist, as this is the predominant distortion type leading to downgrading of Scandinavian timber species. Small-scale specimens were dried at high temperatures (120?C and 170?C) and compared to conventional drying (80?C). All drying schedules, independent of temperature, included pre-steaming and steaming phases at saturated conditions (98?C, 98 % relative humidity). The effect of restrained drying or top-loading was investigated by restraining half of the boards during the drying process. After completed drying, all specimens were moisture-cycled to investigate the permanency of shape stability and distortion reduction attained during restrained drying. The influence of drying temperatures on shape stability of spruce was also compared to commercial heat-treatment. The results showed that high-temperature drying is an alternative for Scandinavian timber species such as Norway spruce and larch. Distortions, especially twist, were successfully reduced by restrained drying and the obtained shape stability was permanent, even in changing humidity conditions. Drying temperature did not influence shape stability, but similar results were found for shape stability for drying at temperatures levels 80, 120 and 170?C. The good shape stability for low-temperature drying (80?C) is most probably achieved by the pre-steaming and steaming phases, which constituted a short high-temperature treatment with corresponding softening and creep behaviour. Excessively twisted boards were heat-treated at 95?C to investigate the possibility to reduce twist. Twist could be effectively reduced by heat-treatment, however, twist increased slightly in subsequent moisture cycling. These results confirm earlier studies in that excessively twisted boards can be straightened by heat-treatment, as mechano-thermal creep helps to attain the desired shape of the board. High-temperature drying influences both the chemical and physical properties of timber. In this study, sorption behaviour, stiffness (static and dynamic modulus of elasticity in bending), bending strength and toughness were investigated. It could be shown that stiffness is more or less unaffected by drying temperature, whereas bending stress decreases with increasing drying temperature. The toughness tests did not show a clear tendency of strength loss with increasing temperature. Finally, the effect of different drying temperatures and heat-treatment on the susceptibility for mould growth was studied. It could be stated that all kiln drying temperatures resulted in about equal mould growth levels, whereas air-drying had lower and heat-treatment substantially lower mould levels. Other factors influencing the amount of mould growth were presence of sapwood or heartwood and the type of surface ? original drying surface or re-sawn surface.