## 3 Results
### Thermal Stability  
Figure 4 shows the TGA curves of the neat GA powder and the PLA films, whereas Table 3 summarizes the values obtained from these curves. On the one hand, it can be seen that the thermal decomposition of GA presented four peaks. The first mass loss occurred in the 70-130 \({}^{\circ}\)C range, which has been attributed to the degradation of volatile compounds of low molecular weight [39] as well as to the mass-loss event attributed to the water content [40]. The second and third peaks were centered at ca. 291 \({}^{\circ}\)C and 338 \({}^{\circ}\)C, respectively, which correspond to the main chain scission of GA and the loss of hydroxyl groups (-OH) [41]. Finally, minor and progressive degradation phenomena of lower intensity were observed at temperatures above 400 \({}^{\circ}\)C, which are related to the residual decomposition of GA at high temperature [39; 40]. On the other hand, one can observe that all PLA films presented a practically identical pattern of mass loss, similar to that of the neat PLA film. In particular, PLA decomposed in a single step, starting at \(\sim\)340 \({}^{\circ}\)C, with a main degradation temperature (T\({}_{\text{deg}}\)) slightly above 375 \({}^{\circ}\)C [42]. The incorporation of GA provided a thermal stability improvement, even at the lowest GA content, due to the intrinsic antioxidant activity of the polyphenol. The onset degradation temperature, measured at the 5% of mass loss (T\({}_{5\%}\)), was delayed for up to 10 \({}^{\circ}\)C, whereas the values of T\({}_{\text{deg}}\) also increased up to 5 \({}^{\circ}\)C. It is also worthy to mention that the thermal stability of the bilayer films was slightly better than that observed for the multilayer film, indicating that the capacity of GA to counteract the thermal effect was more effective when it was incorporated onto the film surface. For all film samples, the residual mass was nearly zero. \begin{table}
\begin{tabular}{l c c c} \hline
**Film** & **T\({}_{5\%}\)**(\({}^{\circ}\)C)** & **T\({}_{\text{deg}}\)**(\({}^{\circ}\)C)** & **Residual Mass (\%)** \\ \hline GA & 261.5 \(\pm\) 1.1 & 290.6 \(\pm\) 1.1 / 337.5 \(\pm\) 1.2 & 5.32 \(\pm\) 0.12 \\ Monolayer & 338.1 \(\pm\) 1.3 & 375.1 \(\pm\) 1.4 & 0.12 \(\pm\) 0.04 \\ Bilayer 1h & 348.2 \(\pm\) 1.2 & 375.2 \(\pm\) 1.1 & 0.11 \(\pm\) 0.05 \\ Bilayer 2h & 348.9 \(\pm\) 1.0 & 378.7 \(\pm\) 1.2 & 0.12 \(\pm\) 0.03 \\ Bilayer 3h & 349.1 \(\pm\) 1.4 & 379.5 \(\pm\) 1.3 & 0.19 \(\pm\) 0.04 \\ Multilayer 1h & 345.3 \(\pm\) 0.9 & 375.1 \(\pm\) 1.1 & 0.14 \(\pm\) 0.03 \\ Multilayer 2h & 346.4 \(\pm\) 1.2 & 377.4 \(\pm\) 1.0 & 0.20 \(\pm\) 0.05 \\ Multilayer 3h & 347.3 \(\pm\) 1.4 & 377.5 \(\pm\) 1.2 & 0.18 \(\pm\) 0.04 \\ \hline \end{tabular}
\end{table} 
Table 3: Thermal degradation properties in terms of the onset degradation temperature (T\({}_{5\%}\)), degradation temperature (T\({}_{\text{deg}}\)), and residual mass at 650 \({}^{\circ}\)C of gallic acid (GA) and the GA-containing polylactic (PLA) films. Figure 3: Visual appearance and contact transparency of the gallic acid (GA)-containing polylactic (PLA) films of: (**A**) bilayer 1 h; (**B**) bilayer 2 h; (**C**) bilayer 3 h; (**1**) monolayer; (**2**) multilayer 1 h; (**3**) multilayer 2 h; and (**4**) multilayer 3 h.  
A similar behavior has been previously observed for other GA-containing films. For instance, [PERSON] et al. [41] recently described that the addition of 5 wt% GA delayed thermal degradation of poly(ethylene-_co_-vinyl alcohol) (EVOH) for up to about 20 \({}^{\circ}\)C respect to the neat EVOH film. In another study, [PERSON] et al. [43] developed an active film based on gelatin with green tea extract (GTE), which contained 294.59 mg GA/g. It was observed that the thermal stability of the GTE-containing gelatin films was higher than that of the neat gelatin film and improved with increasing the concentration of GTE in the 0.3-0.7 wt% range. The increase of GTE did not only cause the T\({}_{\text{deg}}\) values of the gelatin films to increase but also decreased the maximum rate of mass loss.