## 3 Results
### Morphology of Paper Multilayers  
Figure 2 shows the cross-sectional morphology observed by FESEM of the three components of the multilayers, that is, the paper sheet and the PHBV and PET films. As can be seen in Figure 2a, the paper sheet presented an average thickness of 291 \(\pm\) 6 \(\upmu\)m and it was composed of micrometer-sized fibers, giving rise to a cross-section with a rough surface. These cellulosic fibers, which showed an average diameter of approximately 20 \(\upmu\)m, are responsible for the high level of porosity of the paper [33]. In contrast, one can observe in Figure 2b that the PHBV film showed a continuous section of 12 \(\pm\) 2 \(\upmu\)m, with a brittle fracture, because there were no macroscopic plastic deformations. The observed inorganic microparticles embedded in the biopolymer matrix may correspond to boron nitride, which is habitually employed by the manufacturer as a nucleating agent during the fabrication of PHAs [34]. Finally, Figure 2c shows the section of the commercial PET film, which presented a multilayer structure having an average thickness of 104 \(\pm\) 1 \(\upmu\)m. The external layer, shown at the bottom with a lower thickness, can be ascribed to a sealing layer of polyolefin (e.g., LDPE or PP) or a modified PET (e.g., polyethylene terephthalate glycol, PET-G) with a low sealing temperature. Both the thickest internal layer and the upper external layer would correspond to PET. Moreover, the inner layers with low thickness can be related to EVOH. The layer structure attributed herein is based on that previously observed for a polyolefin-based multilayer, supplied by the same manufacturer for this product range (Cryovac(r) Darfresh(r) VST200P TOP WEB), which contained two inner layers of EVOH of nearly 5 \(\upmu\)m to provide the film with high oxygen barrier capacity [35].  
\begin{table}
\begin{tabular}{c c c c} \hline
**Temperature (\({}^{\circ}\)C)** & **Time (s)** & **Multilayer** & **Comments** \\ \hline
160 & 4 & The PHBV film partially melted in several areas and the surface still showed some wrinkles and bubbles. Further times at this temperature were ruled out of the study. \\ \hline
170 & 4 & The PHBV film partially melted in several areas and the film still showed some wrinkles and bubbles. Further temperatures and times were ruled out of the study. \\ \hline \end{tabular}
\end{table}
Table 1: _Cont._On the other hand, Figure 3 shows the morphology of the paper-based multilayer structures obtained from the different monolayers, presented in Figure 2, by the heat-sealing process. Thus, Figure 3a shows the resultant three-layer structure based on the paper substrate coated, on both sides, with PHBV films. The average thickness of the multilayer sheet, measured after heat sealing, resulted in a value of 297 \(\pm\) 8 \(\upmu\)m. Likewise, a similar morphology was observed for the paper coated by two PET-based multilayer films, the so-called PET/paper/PET sheet, shown in Figure 3b. This multilayer sheet presented an analogous structure with a higher average thickness, of 473 \(\pm\) 3 \(\upmu\)m, due to the higher thickness of the outer layers of the PET-based films. Both multilayer sheets, that is, PHBV/paper/PHBV and PET/paper/PET, showed good adhesion between the polyester layers and paper substrate, suggesting adequate mechanical resistance for handling and transport in food packaging. Furthermore, it is also worth noting that both multilayers were slightly thinner than the sum of the thicknesses of the individual monolayers. This is due to the radial flow of the polyester monolayers and to the reduction of the free volume of the paper (highly porous) during the heat-sealing step.