Impact of thermal modification on the chemical changes and impact bending strength of European oak and Norway spruce wood

This paper deals with the effect of various thermal modification temperatures (160 degrees C, 180 degrees C and 210 degrees C) on changes in the basic chemical components of wood, and the effect of these changes in wood on the impact bending strength (IBS) of European oak (Quercus robur L.) and Norway spruce (Picea abies (L.) H. Karst) wood in comparison with unmodified wood. The IBS was determined according to ISO 13061-10 (2017), and the chemical composition (extractives, lignin, holocellulose, cellulose and hemicelluloses) by NREL methods (wet chemical methods). During the thermal modification the IBS increased in both wood species at first (temperature of 160 degrees C), but at higher temperatures these values gradually decreased. The highest drop in IBS values, found at a temperature of 210 degrees C, was 32.2% for oak and 39.8% for spruce. Thermal modification caused an increase in the relative content of extractives, lignin and cellulose, but the relative content of hemicelluloses in the wood was lower due to the increase in the thermal modification temperature. Changes in IBS correlate with chemical changes in both wood species (especially in the case of cellulose); closer correlations were found in spruce.