# Effect of Heat Treatment on Water Absorption of Chinese fir Using TD-NMR
## 3 Results and Discussion
### MC Profiles and NMR Imaging of Absorption of Free Water  
The change of MC and MC profiles of untreated and heat-treated Chinese fir samples during water uptake are presented in Figure 6. Figure 6a shows the MC change of the samples, along with the immersion time. MC profiles of the samples during water uptake are shown in Figure 6b-f. The two cross sections were located at position x = 0 and x = 50 mm respectively, and the magnetic field gradient was along the longitudinal direction of the sample. After immersion for 2 h, the MC of the untreated sample was about 52.12%, which was almost double the values of 160 \({}^{\circ}\)C and 180 \({}^{\circ}\)C heat-treated samples and triple that of 200 \({}^{\circ}\)C and 220 \({}^{\circ}\)C heat-treated samples. The depth of water reached and the signal amplitude of the same depth in the heat-treated samples was less than the untreated sample, indicating that the heat treatment made it difficult for moisture to enter the wood samples. For the untreated sample, the distance away from the ends that could detect the moisture signals was longer than 15 mm, while it was about 6 mm for samples treated at 160 \({}^{\circ}\)C and 180 \({}^{\circ}\)C, and about 3 mm for samples treated at 200 \({}^{\circ}\)C and 220 \({}^{\circ}\)C. As reported by [PERSON] [41], during the early phase of water absorption, a front was developed both in the untreated and heat-treated samples, indicating that the transport was internally limited. After 9 days, the water signal in the center of the untreated sample was obviously increased. By comparing the signal values, it can be concluded that it was difficult for water to penetrate into the center of the sample treated at 160 \({}^{\circ}\)C and 180 \({}^{\circ}\)C, and it took about 15 days for the water signal to be increased significantly. Moreover, it was more difficult for samples treated at 200 \({}^{\circ}\)C and 220 \({}^{\circ}\)C, because there was only a very small increase in the water signal in the center of the samples, even after 30 days. After 60 days, the water signal of the untreated sample was about 3.6 times the value of the sample heat-treated at 220 \({}^{\circ}\)C. According to the previous studies, after heat treatment, a decrease was observed in the wetting tension while the contact angles were increased, both indicating a more hydrophobic behavior compared to the untreated wood [42; 50]. Also, the variation of the contact angle was most pronounced in the axial direction [42]. In addition, the pits which connecting the wood cells were closed after treated at high temperature and would block the absorption routes of free water appeared to be another significant reason [49]. Therefore, the differences of water absorption properties in the longitudinal direction between untreated and heat-treated samples could be interpreted. The NMR images and three-dimensional reconstruction of the samples heat-treated at 220 \({}^{\circ}\)C, as well as the untreated samples, are presented in Figure 7. Slices from the two ends and the middle of the samples were selected. The three-dimensional reconstruction was based on the NMR images, thus what can be seen in the figure is the water signal instead of the real wood. It should be noted that the echo time employed in the NMR imaging experiments was 9.5 ms longer than the relaxation time of bound water, and as a result, only the signals of free water were reflected in the images. The differences of water absorption properties between untreated and 220 \({}^{\circ}\)C heat-treated samples could be directly reflected according to the images. After 1 day of immersion, water had absorbed into the ends of both the samples, and no signal was detected in the middle of the sample. After 5 days of immersion, a few signals emerged in the middle of the untreated sample and they were stronger after 10 days of immersion. While after 20 days the water signals were still very small in the middle of the 220 \({}^{\circ}\)C heat-treated sample.