# Ultraviolet-Assisted Modified Delignified Wood with High Transparency
## 3 Results and Discussion
### Fourier Transform Infrared Spectroscopy Analysis  
The infrared spectroscopic images of balsa wood treated under different conditions are shown in Figure 5. Lignin has a variety of complex structures, and the quinone structure of lignin is the main factor that gives lignin its colour [32]. As seen in Figure 5, compared with natural wood, infrared absorption 1733 cm\({}^{-1}\) peak of C=O in the quinone structure of lignin in the four groups of delignified wood essentially disappeared, indicating degradation of chromophoric groups [33]. This is due to a reaction between hydrogen peroxide and the quinone structure of lignin, destroying its structure and changing into other colourless structures. The chromophore groups of lignin are removed to whiten the wood [34]. In addition, by comparing the infrared spectra of DW-B-U and DW-B, we found that the spectral lines of the two groups overlap. Still, the peak value of DW-B-U at 1733 cm\({}^{-1}\) is lower than that of DW-B group, which can indicate that UV irradiation accelerates the removal of chromophore groups in wood to a certain extent. This is also confirmed by the fact that the transmittance of TW-B-U is better than that of TW-B group in the transmittance and haze test mentioned above. In addition, the absorption peak of the aromatic ring skeleton group of lignin decreased slightly at 1504 cm\({}^{-1}\), the reduced absorption peak at 1231 cm\({}^{-1}\) of lignin acetyl and ether, and reduced intensity of the peaks at 1031, 1231, 1423 and 1594 cm\({}^{-1}\). This means that while the chromophore group is removed, a small part of the lignin in the wood is dissolved in the alkaline solution. Compared with the spectra of original wood, the intensity of the infrared spectra of DW-D-U and DW-D samples decreased significantly at 3 332 cm\({}^{-1}\), indicating that the cellulose structure of lignin delignified wood was damaged to a certain extent [15].  
Chemical changes in wood components such as lignin and hydrogen peroxide under alkaline conditions lead to partial hemicellulose and lignin removal, weakening the links between wood tissues. Therefore, although the transmittance of the \"transparent wood\" is expected to be higher as the lignin modification time continues to increase, the mechanical properties will significantly decrease, which limits the manufacture of large structures [35]. Therefore, this study does not pursue too much higher transmittance.  
It can be seen from the infrared spectroscopic images of TW-D-U that the benzene-disubstituted absorption peaks of the benzene-skeleton near 830 cm\({}^{-1}\) and 1606 cm\({}^{-1}\) appear in the epoxy resin-filled wood of balsa wood and the vibration absorption peaks of the benzene-skeleton near 1508 cm\({}^{-1}\) and 1606 cm\({}^{-1}\). There is an epoxy-based symmetric stretching vibration absorption peak near 1243 cm\({}^{-1}\)[36]. The results showed that the vacuum-impregnated delignified wood exhibits mainly the characteristic peaks of the epoxy resin without forming new characteristic peaks, indicating that no chemical bond was formed between the wood and epoxy resin. However, a hydrogen bond was formed between the hydroxyl group and epoxy group in the wood.