## 3 Results and Discussion
### Fourier-Transform Infrared Spectroscopy  
The changes in the functional groups are shown in Figure 6. The FTIR spectra of 1-6 show that the peaks of the chemicals were basically the same at the same temperature (Figure 6a), indicating that the addition of ZnCl\({}_{2}\) did not affect the functional groups. When the temperature rose to 103 \({}^{\circ}\)C and 120 \({}^{\circ}\)C, the peaks at 3330 cm\({}^{-1}\) changed slightly in groups 3-6, indicating that the polycondensation reaction of the hydroxyl group (O-H) in the MA and DM was hardly promoted without the wood samples. The band at 1710 cm\({}^{-1}\) exhibited a shift for groups 3-6. The shift was more apparent at 103 \({}^{\circ}\)C, indicating that a O = C-N bond was generated in the chemicals. The absorption at 1457 cm\({}^{-1}\) exhibited no significant change in the peak of the C-N stretching vibration. As demonstrated in previous studies [18], the stretching vibration would only have been strengthened if a crosslink reaction \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Methods** & **103 (\%)** & **ZC-103 (\%)** & **120 (\%)** & **ZC-120 (\%)** \\ \hline
10\% DM & 8.21 \(\pm\) 1.88 & 8.49 \(\pm\) 0.87 & 8.65 \(\pm\) 1.27 & 8.85 \(\pm\) 1.28 \\
10\% DM + 5\% MA & 14.03 \(\pm\) 2.38 & 19.61 \(\pm\) 1.67 & 16.99 \(\pm\) 0.85 & 16.48 \(\pm\) 0.600 \\
10\% DM + 10\% MA & 19.28 \(\pm\) 0.83 & 21.97 \(\pm\) 2.49 & 20.39 \(\pm\) 2.99 & 19.65 \(\pm\) 1.72 \\
10\% DM + 15\% MA & 23.03 \(\pm\) 2.19 & 24.97 \(\pm\) 1.21 & 22.69 \(\pm\) 2.00 & 23.42 \(\pm\) 2.45 \\
10\% DM + 20\% MA & 29.95 \(\pm\) 2.49 & 32.43 \(\pm\) 4.47 & 26.07 \(\pm\) 3.29 & 28.15 \(\pm\) 3.19 \\ \hline \hline \end{tabular} All values are expressed as mean \(\pm\) standard deviation (STDVA). DM: 1,3-dimethylol-4,5-dihydroxyoleureurea; 103: 103 \({}^{\circ}\)C curing; ZC-103: ZnCl\({}_{2}\) + 103 \({}^{\circ}\)C curing; 120: 120 \({}^{\circ}\)C curing; ZC-120: ZnCl\({}_{2}\) + 120 \({}^{\circ}\)C curing.  
\end{table} 
Table 1: WPG of wood for different groups after leaching cycles. Figure 5: Changes in the immersion liquid after the leaching cycles (**a–c**); absorbance of the liquid after the leaching cycles (**d–f**). had occurred between the DM and wood: therefore, the chemicals did not crosslink. The absorption peak of C-O-C at 1157 cm\({}^{-1}\) increased due to the condensation reaction of DM to form an ether bond. The highest absorption peak occurred at 1050 cm\({}^{-1}\) and was caused by the stretching vibration of the alcohol and ether groups in the polysaccharide [11]. The peaks of groups 3-6 were significantly higher than the peaks of the groups that were modified at a normal temperature (group 1 and 2), but at 103 \({}^{\circ}\)C, the peak value was highest. This indicated that there were more alcohol and ether groups in the wood and that more chemicals remained in the wood at the 103 \({}^{\circ}\)C curing treatment. As shown in Figure 6c, as the temperature increased, the color of the chemicals changed from no color to yellow, which further demonstrated that the high temperature contributed to the formation of alcohol and ether groups as a result of the reaction between MA and DM: this resulted in a strong absorption peak at 1050 cm\({}^{-1}\). After the water in the modifier without ZnCl\({}_{2}\) (7) was completely evaporated at 120 \({}^{\circ}\)C, it was found that a large amount of yellow solid particles adhered to the tube wall. In the modifier with ZnCl\({}_{2}\) (8), water droplets adhered to the tube wall, which indicated that the addition of ZnCl\({}_{2}\) decreased the volatilization of the chemicals. This may have occurred because ZnCl\({}_{2}\) facilitated the formation of large-particle polymers in the DM/MA system that did not degrade at the same temperature. Figure 6b shows the FTIR of the modified wood samples and the untreated material. In the experimental group, the stretching vibration was strengthened at 3330 cm\({}^{-1}\), which occurred because DM contains three O-H per molecule: When reacting with poplar wood, O-H was introduced into the samples.