Influences of extractives and thermal modification on the UV resistance of Albizia lebbeck wood

Abstract

ABSTRACT Thermal modification is a chemical-free process for improving hydrophobicity, dimensional stability, and biological durability of wood. There are inconsistent reports on the UV resistance of thermally modified wood with the majority of studies indicating its vulnerability to UV degradation. This may be due to the natural variability of chemical constituents, particularly extractives, in different wood species. In this study, the UV resistance of sapwood and heartwood of Albizia lebbeck was evaluated to elucidate the role of extractives on photo-stabilisation. A comparison between the extractive rich untreated and heat-treated sapwood and heartwood was carried out. UV resistance was evaluated by exposing modified wood to UV light in an accelerated weathering tester. Untreated sapwood attained dark colour (photoyellowing) upon UV light exposure, whereas thermally modified sapwood showed contrasting behaviour (lightening). Untreated heartwood showed more resistance to photodegradation. Thermally modified heartwood retained its colour for a considerable extent and exhibited good UV resistance than the untreated samples. Results indicate that extractives present in the heartwood of A. lebbeck act as UV absorber to impart substantial UV stability to wood. Furthermore, the enhanced UV resistance of heat-treated heartwood could be due to the accumulation of photostable compounds evolved during the thermal modification of wood.