Effects of extracts on the colour of thermally modified Populus tomentosa Carr.

Thermal modification can improve the dimensional stability of wood without the use of biocides, but the process also changes the colour. Changes in relative amounts of chromophores and auxochromes of poplar extracts during thermal modification were investigated by pre-extraction of ground wood in increasingly polar solvents followed by thermal modification. Changes in wood colour and FTIR spectra were used to assess the effects of extraction/thermal modification, while selected extracts were analysed by GC–MS. Colour of non-modified poplar was mainly affected by ethanol extraction, while thermally modified poplar was most affected by 1,4-dioxane and acetone extraction. GC–MS analysis to characterize the main chromophores and auxochromes in the 1,4-dioxane and acetone extracts from non-modified or thermally modified poplar suggested that chromophores and/or auxochromes in extracts including carbonyl, vinyl, benzene ring and hydroxyl groups tended to be much more abundant in thermally modified poplar which accounts for the darkening of the wood.