Chemical changes in hydrothermal treated wood: FTIR analysis of combined hydrothermal and dry heat-treated wood

Wood specimens of Beech (Fagus silvatica L.) and Scots pine (Pinus sylvestris L.) modified by a hydrothermal treatment process were analysed by means of Fourier transform infra red spectroscopy (FTIR). The chemical transformation of the cell-wall material was studied and associated with improved wood qualities. For this purpose, FTIR spectroscopy was used as since this technique has been found appropriate to determine the intensity of specific bonds and functional groups within the polymeric structure. Cleavage of acetyl groups of the hemicellulose has been found to occur in the first treatment step under moist conditions and elevated temperature. This results in the formation of carbonic acids, mainly acetic acid. Most of the acetyl groups were found to be cleaved during the treatment of wood at high temperature, whereas only partial deacetylation was found to occur at moderate treatment temperature. The concentration of accessible hydroxyl groups was measured by acetylation and found reduced after treating at high temperature. Esterification reactions were found to occur under dry conditions at elevated temperature in the curing step, indicated by the increase of the specific ester carbonyl peak at 1740 cm⁻¹ in the FTIR spectrum. The esters that were formed turned out to be mainly linked to the lignin complex, considering that the newly formed carbonyl groups were found present in heat-treated wood, yet were found to be absent in the isolated holocellulose. Esterification contributes to a decrease of hygroscopicity of wood and consequently improvements of its dimensional stability and durability. However, the role of esterification in the decrease of hygroscopicity in the hydrothermal treatment process examined is believed to be minor compared to the influence of cross-linking reactions known to occur during thermal treatment of wood.