*3.9. Thermogravimetric Analysis of Lignins (TGA)*

The TGA was used to determine the basic behavior and thermal stability of the MWL and PFL. The processes of pyrolytic decomposition can be divided into three stages: the release of water and volatiles (from approximately 100 to 200 ◦C), the rapid decomposition of the bonds that form the structure of lignin (200 to 500 ◦C), and a last slow carbonization to form charry residue (500 to 900 ◦C). The TGA and DTG curves are shown in Figure S3 (Supplementary Material).

The first step in the thermal decomposition was qualitatively similar for both samples, although the process in MWL was faster and produced greater weight loss. The weight loss in this stage came from the release of volatile compounds and water from the samples, and their values were 7.2% and 2.8% for MWL and PFL, respectively. The most important mass change corresponded to the temperatures between 200 and 500 ◦C, approximately, which accounted for the additional weight loss values of 32.2% for MWL and 37.5% for PFL. In this stage, the bonds forming the structure of the lignin molecule were progressively modified, starting with the fragmentation of the interunit links [43] that were broken to release volatile oxygen compounds (CO, CO2, formaldehyde, formic acid, and some simple ethers and alkanes, among others) leaving a solid with a higher degree of unsaturation [44,45]. In this period, the maximum weight loss rate was located at 291 ◦C (0.31%/ ◦C) for MWL and 375 ◦C (0.29%/ ◦C) for PFL. The significant difference between these values clearly reflected the structural changes already mentioned, suffered by the lignin in the process of delignification. The residual phase of pyrolysis (500–900 ◦C) was very slow and generated a very low rate of volatile compounds: in this phase, mass losses of 6.8 and 6.7% were measured for MWL and PFL, respectively. The percentage of non-volatilized material at 900 ◦C was higher in the case of PFL (53.0%) than in MWL (35.2%). A greater amount of non-volatile residue was related to a more branched structure (more condensed) which agreed with the spectroscopic data of the lignins studied in this work [45,46].
