Effect of oxidizing thermal modification on the chemical properties and thermal conductivity of Norway spruce (Picea abies L.) wood

This research work aims to investigate the effects of thermal modification and wood cutting direction (transverse, radial and tangential) on the content of extractives, lignin, cellulose, and hemicelluloses, and on the thermal conductivity of Norway spruce (Picea abies L.) wood. Samples were thermally modified for 3, 6, and 9 h at temperatures of 160 degrees C, 180 degrees C, and 200 degrees C in the oxidizing atmosphere with natural convection. The extractives, lignin, cellulose, and holocellulose contents were determined by wet chemistry methods, the structural changes were evaluated by using Fourier-transform infrared spectroscopy (FTIR). The highest relative content of extractives was determined at the temperature of 200 degrees C and a thermal modification duration of 9 h. The lignin content decreased at the temperature of 160 degrees C and increased at higher temperatures due to the formation of pseudo-lignin. The cellulose content increased, and hemicellulose content decreased with the increased temperature and thermal modification duration. FTIR analyses revealed changes mainly at the temperature of 200 degrees C, primarily a decrease in the intensities of the characteristic absorption bands of lignin due to degradation reactions. Markedly, density and thermal conductivity decreased mostly due to depolymerization of hemicelluloses. Regarding the thermal conductivity, a slightly higher decrease in transversal and tangential sections was determined.