## 3 Results
### Thermal Properties of Paper Multilayers  
Thermal stability was determined by TGA to ascertain the application conditions of the resultant multilayer sheets. The TGA curves of each sample, which show the variation of the mass percentage as a function of temperature, are gathered in Figure 5. Table 3 includes the resultant thermal stability parameters. In the case of paper, three relevant mass losses occurred during heating, which approximately took place at temperatures of 100, 350, and 490 \({}^{\circ}\)C. These mass losses have been widely studied in lignocellulosic materials [43]. The first one corresponds to the evaporation of absorbed water in the dried paper sheets, which indicates that the initial moisture content of these samples was 2\(-\)3%. The second and main degradation step, between 310 and 410 \({}^{\circ}\)C, showed an average mass loss of nearly 55%. This is referred to as the \"active pyrolysis zone\" since the mass loss rate is high. This mass loss corresponds to the decomposition of hemicellulose and cellulose, which are main components of paper. Both degradation processes are known to involve complex reactions (e.g., dehydration and decarboxylation, among others) as well as the breaking of C\(-\)H, C\(-\)O, and C\(-\)  
Figure 5: Thermogravimetric analysis (TGA) curves of the paper sheet, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and polyethylene terephthalate (PET) films, and PHBV/paper/PHBV and PET/paper/PET multilayer sheets. \begin{table} \begin{tabular}{l c c c c} \hline \hline
**Sample** & \(T_{onset}\) (\({}^{\circ}\)C) & \(T_{deg}\) (\({}^{\circ}\)C) & \
\begin{tabular}{c} **Mass Loss at** \\ \(T_{deg}\) (\%) \\ \end{tabular} &
\begin{tabular}{c} **Remaining** \\ **Mass (\%)** \\ \end{tabular} \\ \hline Paper & 310.8 \(\pm\) 7.8 \({}^{\text{a}}\) & 353.8 \(\pm\) 1.6 \({}^{\text{a}}\) & 39.4 \(\pm\) 1.0 \({}^{\text{a}}\) & 14.0 \(\pm\) 0.1 \({}^{\text{a}}\) \\ PHBV & 289.7 \(\pm\) 5.2 \({}^{\text{b}}\) & 288.2 \(\pm\) 3.8 \({}^{\text{b}}\) & 60.6 \(\pm\) 0.5 \({}^{\text{b}}\) & 0.1 \(\pm\) 0.0 \({}^{\text{b}}\) \\ PET & 327.3 \(\pm\) 3.7 \({}^{\text{c}}\) & 367.5 \(\pm\) 3.2 \({}^{\text{c}}\) & 49.1 \(\pm\) 8.9 \({}^{\text{c}}\) & 7.2 \(\pm\) 0.8 \({}^{\text{c}}\) \\ PHBV/Paper/PHBV & 275.5 \(\pm\) 4.3 \({}^{\text{d}}\) & 349.6 \(\pm\) 2.9 \({}^{\text{d}}\) & 54.8 \(\pm\) 0.2 \({}^{\text{d}}\) & 12.0 \(\pm\) 1.6 \({}^{\text{d}}\) \\ PET/Paper/PET & 313.6 \(\pm\) 4.1 \({}^{\text{a}}\) & 357.9 \(\pm\) 1.1 \({}^{\text{e}}\) & 55.2 \(\pm\) 0.4 \({}^{\text{d}}\) & 11.4 \(\pm\) 0.9 \({}^{\text{d}}\) \\ \hline \hline \end{tabular}  Mean values and standard deviation. \({}^{\text{a}\,\text{\text{\  
bonds [44]. The onset of the third stage of degradation overlapped with the previous one and continued progressively until temperatures neared 600 \({}^{\circ}\)C. In this step, the mass loss was around 28%, representing the \"passive pyrolysis zone\" since the mass loss rate was much lower than that observed in the previous one. The latter mass loss can be assigned to the thermal decomposition of lignin, which is known to occur slowly and over a wide temperature range. Furthermore, at 800 \({}^{\circ}\)C, the paper showed a remaining mass of 14%. This mass corresponds to both inorganic material, such as silica or titanium dioxide (TiO\({}_{2}\)), which is usually applied to give brightness and whiteness to paper, and ashes generated from the decomposition of the organic material in an inert atmosphere [45]. Furthermore, PHBV thermal degradation occurred through a single and sharp degradation step that occurred from 290 to 320 \({}^{\circ}\)C. Thermal degradation of PHAs generally follows a random chain scission model of the ester bond, involving a cis-type elimination reaction of \(-\)CH and a six-membered ring transition to form crotonic acid and oligomers [46]. The thermal stability of the PET film was significantly higher (\(p<0.05\)) than that of PHBV. This thermal degradation occurred in two stages.