Effects of thermal treatment on chemical, mechanical and colour traits in Norway spruce wood

In several different branches of the wood industry heat treatment is a growing application as it changes the chemical, mechanical, physical and biological properties of wood. Investigations using wet chemical analyses, Fourier transform infrared spectroscopy, size exclusion chromatography, and CIELab colour system have been conducted to study the changes in Norway spruce wood subjected to temperature up to 270 °C over a 30 min time period. The results showed that mass loss (ML), total crystallinity index (TCI) of cellulose, total colour difference (ΔE∗), and the content of lignin and extractives increased with the temperature, whereas degree of polymerization (DP) of cellulose, modulus of rupture (MOR), modulus of elasticity (MOE), lightness difference (ΔL∗), and the content of holocellulose, cellulose and hemicelluloses all decreased with the thermal treatment. Relationships between temperature and the examined wood traits were all fitted by exponential curves. Power law relationships were found to fit the trends for DP of cellulose with ΔE∗, ΔL∗, and TCI of cellulose. Also found were power law regressions for the content of hemicelluloses with MOE, MOR, ΔL∗, and ML. Temperatures ranging from 20 to 187 °C formed a compact cluster, clearly separated from the higher examined temperatures in the multivariate wood trait space.