# Effect of Heat Treatment on Water Absorption of Chinese fir Using TD-NMR
## 1 Introduction  
Wood is a porous material which has a complex hierarchical structure, and the hygroscopic nature makes it sensitive to moist conditions. Water generally exists in wood in two basic states, namely bound water and free water. The former is dissolved or absorbed within the hygroscopic cell walls and associated with them by hydrogen bonding, while the latter occupies the cell lumens and cell cavities and is subject to capillary forces. The amount of moisture contained within wood affects its durability, stability, physical properties, and so on [1; 2; 3; 4], for example, wood with a higher moisture content (MC) is more likely to wear and decay. Of all the methods for wood modification, heat treatment is an environmentally friendly method to improve timber performance by changing the wood-water relationship without using chemicals. This method is to treat wood at a high temperature, for example, about 200 \({}^{\circ}\)C, in a nearly oxygen-free environment, inducing chemical and morphological changes of the wood substance [5]. The changes include the acid hydrolysis and decarboxylation reactions of hemicellulose [6; 7], the cleavage of ether linkages and demethylation, as well as the condensation reactions of lignin [7; 8; 9]. In addition,high temperature would result in the increase of cellulose crystallinity [10; 11]. All these changes might affect the amount of accessible hydroxyl sites and the pore structure of wood. Consequently, the water state and water content adsorbed in wood would be influenced when wood is situated in a humid environment or immersed in water. Being utilized for the noninvasive, quick and quantitative characterization, the nuclear magnetic resonance (NMR) technique is known as a powerful tool, which can be used to understand wood structure and the dynamics of water in wood [12]. In previous studies, the signal value from the free induction decay (FID) curve, the first echo amplitude value of the decay curve obtained by the Carr-Purcell-Meiboom-Gill (CPMG) sequence and the integral peak areas of NMR relaxation curves according to the CPMG experiment were applied to describe the MC changes during the wood drying and moisture adsorption processes [12; 13; 14; 15; 16; 17; 18; 19; 20]. Since the decay rate of the NMR signal is relevant to the mobility and local environment of molecules, the moisture components (bound water and free water) can easily be separated by the different spin-spin (or transverse) relaxation time (T\({}_{2}\)) values for either hardwood or softwood [21; 22; 23; 24; 25; 26; 27; 28]. Moreover, bound water that is hydrogen bonded to the wood cell wall components, or in the void spaces between the cellulose chains and in the cell wall micropores, and free water from earlywood or latewood could be distinguished [28]. The spatial distribution of water in wood can be reflected visually by magnetic resonance imaging in a nondestructive way, which is difficult to achieve using other methods [28; 29; 30; 31]. The water absorption performances are extremely important factors defining the dimensional stability, decay resistance and other physical properties of timber. A number of experiments have been carried out in order to investigate the effects of heat treatment on the water absorption performance of wood [32; 33; 34; 35; 36]. Most of them dealt with the changes of water absorption amount and rate, and the dimensional change caused by water absorption. However, the whole progress of changes in the water absorption process needs further investigation. For example, it is still unclear whether the heat treatment caused the absorption rate and amount change of both bound water and free water, or only one of these two water components is influenced. In addition, further research of the water distribution in heat-treated wood is needed.