Effects of original wood extract on color, dimensional stability, and decay resistance of thermally modified wood

Thermal modification can improve wood’s dimensional stability and decay resistance, concurrently deepening its color. However, the role of original wood extract in these changes remains unclear. This study aimed to investigate the effects of original wood extract on color, dimensional stability, and decay resistance of thermally modified wood, spruce (Picea asperata) and poplar (Populus tomentosa) wood were extracted by acetone, and then thermally modified at 180, 200, and 220 °C. The color, dimensional stability, and decay resistance of thermally modified wood (TMW) or thermally modified pre-extracted wood (TMEW) were analyzed. The results indicated that the presence of original wood extract contributed to darker coloration, enhanced dimensional stability, and reduced decay resistance in TMW. The total color difference (ΔE∗) of TMEW was lower than TMW, and the extractions had a significant effect on wood treated at 180 °C and 200 °C. The anti-swelling efficiency (ASE) of extracted wood decreased significantly after thermal modification. The ASE of pre-extracted spruce decreased from −6.9, 16.8, 26.3 to −11.5, 9.5, 18.3 after thermal modification at 180, 200, and 220 °C, respectively. TMEW has better decay resistance than TMW, especially for 180 °C TMW against brown-rot fungi, showing an increase of 90.5% and 33.6%. The extract has a certain effect on the properties of TMW, significantly at 180 °C, and can be used to regulate the properties of TMW.