Changes in the content and composition of the extractives in thermally modified tropical hardwoods

Abstract

Chemical composition of wood is known to change during thermal treatments. Two species grown in Turkey, afrormosia ( Pericopsis elata ) and duka ( Tapirira guianensis ) were heat treated according to Thermowood® method. Lignin, cellulose, hemicelluloses and extractives in dichloromethane, ethanol and water were determined. Wood extracts were analysed by gas chromatography with mass detection and existing compounds were identified by NIST17 database. Results show that hemicelluloses and cellulose content decreased for both heat-treated woods along the treatment while lignin percentage increased. The analysis of extractives has shown several compounds normally associated to lignin thermal degradation that increased along the treatment. At the same time several compounds associated to carbohydrate thermal degradation were found in all the extracts for both heat-treated woods. These founding have allowed the understanding of the degradation pattern of wood during thermal modification. There was not much difference between afrormosia and duka woods structural compounds behaviour along thermal modification. However, the variation of the amount of extractives along the treatment depended on the species. due to water and ethanol extractives. While the extractives increased along the treatment for afrormosia, in duka wood there was an initial increase followed by a decrease with the increase of heating time, which was probably due to the high amount of initial ethanol extractives in duka wood that are degraded along the treatment. The new formed compounds that increased along the thermal modification found in dichloromethane extract are vanillin, syringaldehyde, vanilic acid and syringic acid. For the most severe treatment (2 h), coniferaldehyde, sinapaldehyde and acetovanillone were also detected. All these compounds have been associated to lignin heat degradation showing that although the percentage increases, there is still some lignin thermal degradation. The compounds identified as resulting from lignin degradation in dichloromethane extracts still represent a significant amount in ethanol extracts of both heat-treated woods. Additionally, several other compounds like hydroxy acids, deoxy-pentonic acids, deoxy-hexonic acids γ-lactone and levoglucosan were found in ethanol extract. On heat-treated duka some carbohydrates were also found. All these compounds have been associated to C5 and C6 carbohydrate thermal decomposition. The results have contributed to the understanding of the chemical degradation path of modified wood by monitoring extractive content transfor mations and have shown the importance of optimizing the treatment for each wood species in order to make the best utilization of this material.