During the last decades several processes of wood thermal modification have been developed and obtained material properties investigated. By comparison only a few researches are devoted to environmental impact of this quite novel material. It is now known that bio-durable, thermally modified wood cannot be used as load-bearing material and therefore its aesthetic properties have become more significant. Colour of thermally modified wood is a momentous optical property and consequential for its appliance. In order to investigate difference between unmodified and thermally modified wood weathering, colour of three thermally modified hardwood species – aspen (Populus tremulus L.), gray alder (Alnus incana (L.) Moench) and birch (Betula pendula Roth.) besides untreated wood were monitored under four different conditions. Samples were exposed to full spectra of direct sunlight; exposed to direct sunlight through a window-glass shield; kept in a room with indirect sunlight illuminating the samples, and the fourth portion of the samples was kept in the dark. Periodic reflectance spectra measurements were taken during all of the experiment and CIELAB colour system was used to calculate colour parameter` (ΔL*, Δa*, Δb*, ΔEab) changes with time. Results indicate that there are some processes that are not photoinduced, but cause colour changes – thermally modified hardwood as well as untreated samples become 1,5 units darker (ΔL*) even if kept in the dark. When exposed to light thermally modified wood showed greater initial colour changes than untreated wood. Under all conditions, except when kept in dark, lightness parameter ΔL* changes went to opposite directions for modified and untreated wood. Thermally modified wood became about 6 units lighter after receiving full solar spectra and about 5 units lighter through window-glass as well as less than 1 unit lighter without direct sunlight. Meanwhile untreated wood became 8; 3 and 2 units darker, respectively.
Keywords: thermally modified wood, colour changes, UV radiation, hardwood
Authors
Meija-Feldmane A.
Laboratory of Wood Biodegradation and Protection, Latvian State Institute of Wood Chemistry, Riga, Latvia
Cīrule D.
Laboratory of Wood Biodegradation and Protection, Latvian State Institute of Wood Chemistry, Riga, Latvia
Kuka E.
Laboratory of Wood Biodegradation and Protection, Latvian State Institute of Wood Chemistry, Riga, Latvia
Andersons B.
Laboratory of Wood Biodegradation and Protection, Latvian State Institute of Wood Chemistry, Riga, Latvia
Login to download the PDF