Heat treatment has shown to be an efficient approach to enhance certain valuable properties of wood. Latest technological developments have enabled the following to be attained; higher hydrophobic properties, improved elasticity, and enhanced dimensional stability, among other things.
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The resulting chemical modifications rely on the heating atmosphere and regimes, and involve hemicellulose degradation, alterations of cellulose and lignin structures and chemical wood composition owing to the loss of wood extractives.
Scientists from Institute of Perspective Research Tatarstan Academy of Sciences, Institute of Physics of Kazan Federal University, and Nanoscience Department of Institut Neel carried out a study on a range of thermally treated wood species that were obtained from the Central European part of Russia using magnetic resonance techniques, and discovered significant changes in the structure of wood which could not be examined by other techniques.
It is well known that magnetic resonance techniques are non-invasive methods that help gain information regarding the processes and structure within the samples.
The choice of sapwood samples comprised Birch (Betula pendula), Scots pine (Pinus sylvestris), Norway spruce (Picea abies), Russian larch (Larix sibirica). Small-leaved lime (Tilia cordata) were treated under vacuum by heating at 220°C temperature with different time intervals up to 8 hours.
Electron paramagnetic resonance experiments showed alterations in the quantity of free radicals in samples that were treated thermally. They demonstrated that free radicals EPR signal amplitude relies strongly on the wood samples’ moisture content and lessens as the latter value increases. Further EPR experiments with absorbed ethanol specify a potential association of this effect with the water molecules’ electric dipole properties.
Microscopic techniques revealed changes in the distribution of pore size which indicate deformation and shrinking of cell walls. This method is indirectly linked to the loss of mass and development of stable free radicals that are identified by EPR method. Given that the connection between the wood hardness and EPR signal amplitude was determined for lime, larch and spruce, EPR method could well be used to evaluate the hardness of wood.