# Effect of Heat Treatment on Water Absorption of Chinese fir Using TD-NMR
## 3 Results and Discussion
### Water States and Migration  
Water states in softwood were distinguished with relaxation analysis by several researchers [22; 23; 24; 25; 37]. According to the research results, the T\({}_{2}\) was within 10 ms of bound water and greater than 10 ms of free water. Whereas T\({}_{2}\) of the wood proton was about 10-15 us shorter than the dead time of the spectrometer in this study, thus the spectrometer could not detect the signal of the wood proton. Figure 3 illustrates the states and the relaxation behavior of water in wood during the process of water uptake. In Figure 3a, there are two peaks which correspond to two components in water states in the 160 \({}^{\circ}\)C and 180 \({}^{\circ}\)C heat-treated samples and the untreated control samples. The component with shorter relaxation time showed that the water became more restricted in motion, which belonged to bound water in the wood cell walls, and the other one with longer relaxation time belonged to free water in the rays and tracheid lumens. Besides these two components, a new component with a T\({}_{2}\) value about 20 ms was found in the 200 \({}^{\circ}\)C and 220 \({}^{\circ}\)C heat-treated samples. According to the references [25; 37], this component belonged to free water, which was more restricted in motion than the water in cell lumens. By considering that the high-temperature heat treatment might change the pore structure of timber [38; 39; 40] and that the relaxation behavior of water was significantly affected by wood pore size [41], this component probably located in very small pores or cracks caused by the degradation of wood components due to the heat treatment. Figure 3: _Cont._In addition to the environment where the moisture located, the hydrogen bond strength and water amount also affect the relaxation time of moisture in wood, and hence the change of T\({}_{2}\) could indicate the change of the MC and the bonding form between wood and moisture. Figure 3b shows the variation of T\({}_{2}\) value and the integral peak area of different water states during water uptake process. Of all the test samples, the T\({}_{2}\) value of bound water was small at the beginning of water sorption. In this stage, monomolecular layer adsorption was mainly carried out, and with the process of water absorption, which gradually transferred to multi-molecule layer adsorption manifest as the increase of T\({}_{2}\) value. However, the variation tendency of the T\({}_{2}\) value of free water was different. Figure 3: Changes of the amount and relaxation behavior of water in the wood samples during the water absorption process. **(a)** 3D trajectory and corresponding contour plot of T\({}_{2}\) distribution for the water absorption process (the color bar in the right of each figure reflects the water signal intensity). **(b)** The change of T\({}_{2}\) values and integral peak areas of bound water and free water along with the MC. At lower MC, the T\({}_{2}\) value for all the samples was greater and gradually decreased with the elongation of the immersion time, and then tended to be constant. Moreover, it should be noted that for all the heat-treated samples, the first T\({}_{2}\) values were small due to the very small amount of water. As mentioned previously, the variation tendency of free water was supposed to be related to the water amount and the interaction between wood and water. In the initial stage, water penetrated into the wood by capillary force, which might not be hydrogen bonding with the inner surface of the cell lumen, and the weaker binding force led to larger T\({}_{2}\) value. Then the T\({}_{2}\) gradually decreased with the immersion time because part of the moisture was absorbed on the inner surface of cell lumens and the motion of water was restricted. The T\({}_{2}\) value was an average of water which had similar characteristics, and thus the decrease of T\({}_{2}\) value could be interpreted.