Abstract
Acetylation of softwoods has been largely investigated to increase the dimensional stability and biological resistance of wood. However, the knowledge of this technology has not been applied to tropical hardwood species up to date. The objective of this work was to study the effect of acetylation on nine tropical hardwood species, from forest plantations in Costa Rica, by applying acetic anhydride in three different treatment times (1 h, 2.5 h, 4 h), as well as to evaluate this by Fourier-Transform Infrared Spectroscopy (FTIR). Results showed that weight percentage gain (WPG) of wood varied from 2.2 to 16.8%, with Vochysia ferruginea species showing the highest WPG, and Gmelina arborea and Tectona grandis species exhibiting the lowest WPGs. Tropical woods such as Enterolobium cyclocarpum, Hieronyma alchorneoides and Samanea saman exhibited statistical differences among treatment times, whereas the rest of the species studied showed no significant differences. In general, the most effective acetylation time was 2.5 h for all the species. The ratio of intensity (RI) from the FTIR spectra was greater at the 1732 cm−1, 1372 cm−1 and 1228 cm−1 peaks for all tropical species, associated with lignin. A good correlation between the RI of those peaks and WPG was found; the same was also found between all RIs and each other. Meanwhile, RI associated to the hemicelluloses and lignin (1592 cm−1 and 1034 cm−1 peaks, respectively) showed no correlation with WPG, nor between each other or with the other RIs. Furthermore, it was suggested that RI at 1732 cm−1 (associated to acetyl groups C=O) can be considered as a reliable indicator of the degree of acetylation for tropical hardwood species. Finally, it was observed that tropical hardwoods having more suitable anatomical features, like larger vessel diameter, higher ray width and frequency, and lesser deposits such as gums and tyloses in the vessels, resulted in significantly higher WPGs.
Highlights
Wood has been used by the humans for centuries, since it is a natural material, easy-to-work, renewable, widely abundant and sustainable [1, 2]
In V. ferruginea, V. guatemalensis, C. alliodora, G. arborea, C. odorata and T. grandis no statistical differences appeared for both parameters amongst the acetylation times (Table 1)
Tropical woods acetylated in this work presented a high variability in the degree of acetylation, in respect to weight percentage gain (WPG) and ratio of intensity (RI) at 1732, 1592, 1372, 1228 and 1334 cm−1 peaks of the Fourier-Transform Infrared Spectroscopy (FTIR) spectrum
Summary
Wood has been used by the humans for centuries, since it is a natural material, easy-to-work, renewable, widely abundant and sustainable [1, 2]. The presence and amount of hydroxyl groups, capable of forming hydrogen bonds with water molecules, are crucial for dimensional stability This takes place in sorption sites (as they are commonly termed) which are mainly present in hemicelluloses, followed by cellulose and lignin [7]. In the case of cellulose, its configuration in microfibril aggregates makes hydroxyl groups on the surface the only possible sorption sites [9], whereas the amount of sorption sites is much greater in hemicelluloses and lignin [7] During adsorption in these sites, the water molecule with two full-strength covalent bonds can become bound by two relatively strong H-bonds with a pair of nearby –OH groups of the amorphous polysaccharide polymers, in low moisture content. Conjunction in the H-bond network increases with increasing moisture content, gradually allowing the coalescence of water vapour molecules with already absorbed water molecules to form water dimers [8] This gain in moisture makes the wood dimensionally unstable
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