# The Impact of Thermal Treatment on Structural Changes of Teak and Iroko Wood Lignins
## 3 Results and Discussion
### FTIR Spectra Analysis  
The most frequent changes in lignin structure are determined in the FTIR spectra around the bands: 1720, 1600, 1500, 1460, 1420, 1370, 1326, 1266, 1220 and 1120 cm\({}^{-1}\)[45; 46]. A wide band in the range of 3300-3500 cm\({}^{-1}\) and bands of about 2940 cm\({}^{-1}\) and 2850 cm\({}^{-1}\) can also be used to detect \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(T\) (\({}^{\circ}\)C) & \(M_{\rm w}\) (g\(\cdot\)mol\({}^{-1}\)) & \(M_{\rm n}\) (g\(\cdot\)mol\({}^{-1}\)) & \(M_{\rm z}\) (g\(\cdot\)mol\({}^{-1}\)) & PD \\ \hline
20 & 7145 \(\pm\) 76 & 1929 \(\pm\) 24 & 44,401 \(\pm\) 2435 & 3.70 \(\pm\) 0.07 \\
160 & 6076 \(\pm\) 159 & 1740 \(\pm\) 37 & 28,668 \(\pm\) 2240 & 3.49 \(\pm\) 0.15 \\
180 & 6632 \(\pm\) 259 & 1691 \(\pm\) 23 & 34,131 \(\pm\) 2232 & 3.92 \(\pm\) 0.15 \\
210 & 7531 \(\pm\) 230 & 1874 \(\pm\) 35 & 35,901 \(\pm\) 1618 & 4.02 \(\pm\) 0.14 \\ \hline \hline \end{tabular} Note: \(M_{\rm w}\) = weight average molecular weight (MW), \(M_{\rm n}\) = number average MW, \(M_{\rm z}\) = z average MW, PD (polydispersity) = \(M_{\rm w}/M_{\rm n}\).  
\end{table} 
Table 4: Results of macromolecular characteristics of lignins from teak wood. Data represent mean \(\pm\) SD. Figure 2: Chromatograms of lignins isolated from untreated and thermally modified iroko wood. changes in lignin. The changes in both wood species, teak and iroko, are similar, thus their FTIR spectra will be evaluated together. Broad bandwidth low absorbance around 3400 cm\({}^{-1}\) (OH stretching vibration in primary or secondary alcohols, as well as in phenolic groups in lignin) [4; 47] predominantly decreases with an increasing temperature. The reduction of hydroxyl groups is accompanied by condensation of lignin and oxidation to C=O groups [48; 49] and was observed in thermally treated teak wood [50]. Bands near of 2937 and 2840 cm\({}^{-1}\) (arising from C\(-\)H stretching vibrations in methyl, methylene and methoxyl groups and tertiary CH groups) [51; 52; 53] show only insignificant changes (especially in teak wood). The band found in the carbonyl region between 1700-1730 cm\({}^{-1}\) (C=O stretching in unconjugated functional groups of aldehydes, ketones, esters and carboxylic acids) [51; 52; 54]. Our results showed an increase in the absorbance at 1715 cm\({}^{-1}\) (teak) and 1712 cm\({}^{-1}\) (iroko) (Figures 3 and 4). The increase may be confirmation of the more pronounced cleavage of the \(\beta\)-alkyl-aryl ether bonds and the subsequent formation of new C=O groups in the lignin structure. This results in an increase in the amount of acetyl and carboxyl groups from the lignin [52; 55]. The mentioned changes are due to splitting of aliphatic side chains in lignin accompanied by cleavage of \(\beta\)-O-4 bonds during its heat treatment [56; 57]. Similarly, the cleavage of the \(\beta\)-O-4 linkages and the splitting of the aliphatic methoxyl chains from the aromatic ring in lignin were observed at steam-heat treated teak wood [58]. Figure 4: FTIR spectra of the untreated and thermally modified iroko wood lignin. Figure 3: Fourier transform infrared (FTIR) spectra of the untreated and thermally modified teak wood lignin. An increase in the band at 1665 cm\({}^{-1}\) (C=O stretching in various conjugated groups in lignin) [59; 60] was observed at leak wood at 210 \({}^{\circ}\)C (this band did not appear at any temperature value for the iroko wood). This trend may be an indicator of the cleavage of \(\alpha\)-alkyl-aryl ether linkages [25]. The peaks near 1600 cm\({}^{-1}\) and 1500 cm\({}^{-1}\) (C=C aromatic skeletal vibrations) [61; 62] show only small changes with increasing temperature (a more pronounced decline is noticeable in the iroko wood). This trend suggests the relative thermal stability of the lignin aromatic structures [63]. At bands of 1460 cm\({}^{-1}\) and 1422 cm\({}^{-1}\) (asymmetric C\(-\)H deformations in lignin and carbohydrates) [51; 64], a slight decrease with increase of used treatment temperature was noted.