Impact of thermal loading on selected chemical and morphological properties of spruce ThermoWood

Abstract

Physico-chemical changes to ThermoWood produced from Norway spruce (Picea abies L.) with the dimensions 20 mm × 20 mm × 10 mm were evaluated as a function of thermal loading at a flame ignition temperature (FIT) and spontaneous ignition temperature (SIT). The extractives, cellulose, holocellulose, and lignin contents were determined by wet chemistry methods. Carbohydrates and cellulose changes were investigated by high-performance liquid chromatography and size exclusion chromatography. The dimensional characteristics of the fibers (length, width, and shape factor) were measured. After thermal loading, an increase in both the cellulose (FIT - 33.1%, SIT - 64.6%) and lignin (FIT - 28.3%, SIT - 63.4%) contents and a decrease in the extractives content (approximately 8% to 9%) were observed. The increase in the cellulose content was attributed to structural changes (carbonization and crosslinking). The lignin content increased because of condensation and its greater thermal stability compared with carbohydrates. After the thermal treatment, saccharides content was lower due to degradation of non-glucosic saccharides (mainly pentosans - 51.3%). The cellulose degree of polymerization decreased by 40.8% during thermal loading at the FIT and by up to 75.4% at the SIT. Thermal loading of ThermoWood samples caused a significant decrease of fibers average width.