*3.2. TGA*

TGA curves reveal the residual mass of materials with respect to the temperature of thermal degradation and was used to assess the thermal stability of the both polymers PLA and Lignin but also of the biocomposites, PLA/20%Lignin and PLA/40%Lignin. The different chemical bonds present in the lignin molecular structure, leads to a range of degradation temperatures, extending from 100 to 800 ◦C [49]. The results show that 42 wt % of lignin sample still remained at 800 ◦C. This is due to the formation of highly condensed aromatic structures which have the ability to form char (Figure 1). Degradation of lignin sample can be divided into three stages [36]. Firstly, an initial weight loss between 50–120 ◦C, due to water evaporation. Secondly, above 220 ◦C degradation of carbohydrates occurs, which are converted to gasses such as CO, CO2, and CH4 [50]. The last stage occurs to around 420 ◦C, and then continues to lose mass at a slower rate, leading to the formation of gaseous products and phenolics, alcohols and aldehyde acids [50]. PLA, PLA/20%Lignin, and PLA/40%Lignin present a similar thermal degradation behavior. They show a faster thermal degradation between 340–400 ◦C. However, the residual mass at 800 ◦C is higher for biocomposites with lignin due to the carbonaceous composition of the lignin. With respect to the starting temperature of decomposition, the lignin shows a faster degradation (326 ◦C) compared to neat PLA (348 ◦C). As expected, the biocomposites resulting from the mixtures PLA/Lignin present an intermediate degradation start temperature (347 ◦C and 339 ◦C for 20% and 40% lignin content, respectively).

**Figure 1.** Thermo-gravimetric analysis (TGA) plot of Lignin, polylactic acid (PLA), and PLA/Lignin biocomposites.
