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]. As a lignocellulosic complex, wood does have a structure of cellular walls made of biopolymers such as cellulose and hemicelluloses as well as phenolic polymers like lignin [1, 3, 4]. Despite its positive characteristics, wood can easily be affected by the increasing presence of moisture in its hierarchical structure, which can lead to its dimensional instability and/or biological deterioration, or degradation by fungi, insects or bacteria [1, 4]. Polymeric constituents of wood, e.g., lignin, cellulose and hemicelluloses, contain a large amount of free hydroxyl groups [1, 2]. Hemicelluloses, accessible or non-crystalline cellulose, and lignin are mainly responsible for moisture uptake [3, 5]. In these polymers, free hydroxyl groups (OH− anions) adsorb and release water depending on the changes in temperature and relative humidity causing cell walls—and all of the structure of wood—to adjust to the presence (or absence) of other OH− anions, such as those of water, thus giving way to changes in dimensional stability of wood, or to potential biological attacks by fungi [1, 3, 6].
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 . Accessibility of these sorption sites in wood has gained a high interest as methods to improve wood performance by chemical treatments have been implemented . In the case of cellulose, its configuration in microfibril aggregates makes hydroxyl groups on the surface the only possible sorption sites , whereas the amount of sorption sites is much greater in hemicelluloses and lignin . 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. In meanwhile, 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 . Thus, this gain in moisture makes the wood dimensionally unstable.
It is due to the dimensional instability that current research sets the objective of implementing chemical modification to wood for lower water uptake, aiming at achieving higher dimensional stability and increased biological resistance [1, 10,11,12,13,14]. Amongst these chemical modification techniques, a typical one is that of acetic anhydride [5, 15,16,17,18], wherein the OH− anion group in wood polymeric components becomes chemically bound to a residue of the acetate (CH3COO–) of an acetic anhydride molecule [(CH3CO)2O]; this is well known as acetylation of wood . In this process, the OH− anion group is reduced, decreasing hygroscopicity of the wood and thus, increasing its dimensional stability and biological resistance to fungi [4, 19,20,21]. Thybring  has clearly stated that decay of acetylated wood cannot progress below the level of 25% moisture content.
As a matter of fact, the composition and distribution of the polymeric constituents in hardwoods differ from those in softwoods, which cause species groups to vary in their sorption sites. Besides these variances, it is true that hardwood species have their own proportion of structural polymeric constituents; therefore, hydroxyl groups are present in different amounts. In hardwood species, hydroxyl groups were reported to be present at percentages of 2.0 to 4.5%, whereas in softwoods, these vary from 0.5 to 1.7% (Rowell, 2016). This difference is attributed to hemicelluloses and lignin compositions: in hardwood species, hemicelluloses contain mostly xylans, whilst hemicelluloses of softwoods contain mostly glucomannans as well as lignin in a lesser amount [10, 22]. Thus, they possess sorption sites at varying proportions [7, 21].
Studies on wood acetylation have indicated that acetylated softwoods achieve a much higher weight gain, as compared with hardwood species , despite the fact they are more abundant in hemicelluloses. Nonetheless, hardwoods contain a higher amount of xylans, which do not have a primary hydroxyl group in which to react . Moreover, softwood species contain a higher percentage of lignin, the polymeric component in which the higher percentage of acetylation typically takes place [1, 3].
In addition to the differences in the type and proportion of hemicelluloses, the anatomical structure differs considerably between wood groups: hardwoods are characterised by the presence of conducting elements such as vessels, whereas softwoods are made of tracheids . This distinction causes the flow of liquids to vary largely, between the wood groups , thus affecting the acetylation reaction (associated with the liquid flow in wood) as well as other processes performed in the wood cell walls. In fact, most of the research on acetylated wood has been focused on softwoods , primarily on pine species [16, 17, 26,27,28].
In spite of these differences, some research work on hardwood species has been carried out. Though, the tropical species remain scarcely known concerning the processes that can be employed to improve their physical and biological properties. One of the few studies is that of Matsunaga et al.  on wood species like Paraserianthes falcata, Alstonia macrophylla, Pinus caribaea and Heveabrasiliensis which confirmed that acetylation can increase the dimensional stability of wood, up to 60%. Because of the lack of knowledge on the effects of acetylation on tropical hardwoods, there must be a focusing effort to expand the knowledge of the acetylation on such woods, thus widening their potential applications , especially for hardwood species of forest plantations.
In Central America, Costa Rica has been implementing reforestation programs with fast-growing plantations that utilise a variety of tropical hardwood species for lumber production . In these programs, early-age tree harvesting yields juvenile wood , which is characterised by dimensional instability. Therefore, acetylation provides a way to improve dimensional stability, increase durability and advance other material properties of tropical hardwood species [4, 5, 11, 20, 32]. Given that plantation species in the tropical regions are valuable, research efforts to upgrade the quality of wood and wood products are very important .
However, because of the variations in the anatomical structure of tropical hardwoods, which differ from that of softwoods , there is insufficient technical information respecting the potential of tropical species for chemical modification, e.g., acetylation [4, 20, 32, 35]. Hence, the aim of this study was to evaluate the effects of acetylation, using acetic anhydride in liquid phase, on nine tropical hardwood species, commonly cultivated in forest plantations in Costa Rica (Cedrela odorata, Cordia alliodora, Enterolobium cyclocarpum, Gmelina arborea, Hieronima alchornoides, Samanea saman, Tectona grandis, Vochysia ferruginea and Vochysia guatemalensis), and the analysis of key parameters such as solution uptake and weight percentage gain by Fourier-Transform Infrared Spectroscopy (FTIR).