# The Impact of Thermal Treatment on Structural Changes of Teak and Iroko Wood Lignins
## 1 Introduction  
Wood is a natural, renewable, mostly inexpensive and easily accessible material. Modification is applied to overcome weak points of the wood material that are mainly related to moisture sensitiveness, hardness, low dimensional stability, low resistance to wear, UV irradiation, to bio-deterioration against fungi, termites, and marine borers [1]. Wood thermal treatment is an economical and environment friendly pretreatment. It can be performed in range of 160 to 260 \({}^{\circ}\)C in different environments, e.g., air, vacuum, nitrogen and oil. Its main objective is to improve dimensional stability and durability, but sometimes thermal treatment is used to change the aesthetic properties of wood. The wood colour becomes darker depending on the treatment temperature, time, and environment [2; 3; 4]. Tropical woods generally have a great resistance to biological agents and mechanical wear, and a good dimensional stability and interesting wood figure. Many species of exotic wood are used for exterior constructions, decking, claddings, garden furniture or special plywood [5; 6]. Teak is one of the most valuable woods, but its use is limited by scarcity and high cost. It is currently used in the construction of boats, flooring, decorative objects and veneer. Because of its colour and durability, iroko is suggested as a substitute for teak. Its durability makes it suitable for boat building, piles, other marine work, and railroad crossties. Other uses include joinery, furniture, and cabinetwork [7; 8; 9]. Extractives in tropical wood allow for exceptional resistance against damage caused by insects and fungi. However, their impact on the weathering process retardation is lower. This process begins with the photodegradation of lignin by adsorption of photons from sunlight with chromophores of lignin macromolecules-conjugated carbonyls, etc. [10]. The authors of the reference paper investigated colour and chemical changes during the natural and artificial weathering of seven tropical wood samples and found a particularly significant decrease of lignin, a decrease of conjugated and unconjugated carbonyl groups, and a decrease of cellulose crystallinity [10]. High-value wood surfaces for exclusive wood-based floorings, furniture and interior outfitting of residences, luxury yachts, aircrafts, and railway vehicles are preferentially made of dark coloured tropical wood species [11]. The thermal modification of tropical wood species can be used to colour homogenization for decorative purposes. Despite the good physical and mechanical properties of the teak wood, it presents important colour changes when exposed to UV radiation. In the case of teak wood, thermal treatment can be an alternative to improve the durability of sapwood, modify the colour of sapwood to match with that of heartwood, providing a more stable wood colour when exposed to UV radiation, thus adding more value to the final product and increasing its competitiveness on the market [12]. Heat modification is expected to improve the physical properties of young (15-years old) teak wood. The results showed that the density of the teak wood slightly decreased after heat treatment. However, its dimensional stability improved substantially, under its reduced water absorption. Furthermore, the surface of the young teak became darker, with a more even brown colour [13]. The lightness of teak wood was the most affected colour attribute during the thermal treatment [14]. The thermowood process of iroko wood caused its density decrease, colour darkening, and a considerable improvement of dimensional stability [15]. Similar changes were observed during thermal treatment of iroko and padauk wood [16]. Lignin is one of the most abundant organic materials. Its environmentally friendly characteristics and physical and chemical properties enable valuable utilisation [17]. Its changes during thermal treatment influence several wood properties, mainly colour, however, exposure to daylight often causes brightening or greying.