## 3 Results and Discussion
### Optical Properties of Green Composites  
Figure 6 gathers the visual images of the PHBV and its composite films with different contents of MFC and/or the compatibilizers ESAO and TGIC with DCP. The films were obtained by thermo-compression at 200 \({}^{\circ}\)C for 4 min using the doughs resulting from the melt-mixing process. One can observe from the images that the optical properties that the neat PHBV sample, shown as a control in Figure 6a, presented very similar characteristics in terms of transparency or color to the PHBV film samples with MFC without compatibilizers (Figure 6b-d), with compatibilizers without MFC (Figure 6e,f), or with MFC and compatibilizers (Figure 6g,h). In all cases, thermo-compression yielded homogeneous PHBV films, of about 130 \(\upmu\)m thickness, with high opacity due to the inherent crystallinity of PHBV [18]. It is also worth mentioning that, by simple visual observation, it was not possible to determine the presence of MFC due to their micrometer-scale size, as demonstrated previously in Figure 2.  
Figure 6: Films obtained by thermo-compression of poly(3-hydroxybutyrate-\(\alpha\)-3-hydroxyvalerate) (PHBV) with microfibrillated cellulose (MFC) and compatiplified with epoxy-based styrene-acrylic oligomer (ESAO) and with triglyceridel isocyanate (TGIC) and dicumyl peroxide (DCP) as follows: (**a**) PHBV; (**b**) PHBV_2.5%MFC; (**c**) PHBV_5%MFC; (**d**) PHBV_10%MFC; (**e**) PHBV_ESAO; (**f**) PHBV_TGIC+DCP; (**g**) PHBV_5%MFC_ESAO; (**h**) PHBV_5%MFC_TGIC+DCP.  
In order to quantify the visual appearance of the PHBV/MFC composite films, Figure 7 represents their spectral distribution curves in terms of the percentage of Ti as a function of wavelength (\(\lambda\)). High Ti values are related to a higher ability of the sample to transmit light and, therefore, correspond to more transparent film samples, while low Ti values indicates higher opacity and a decrease in light passing through the film [52]. In this case, it can be observed that the films presented very similar Ti values throughout the spectral range, showing low-to-intermediate values. It is also worth noting that a slight reduction was observed for the samples with MFC contents of 5 wt % and, more noticeably, 10 wt %, due to the presence of the cellulose microparticles. In this regard, one of the desired characteristics of packaging materials is that they should protect food from the effects of light, especially ultraviolet radiation [53], for which the presence of MFC can be advantageous.  
As can be observed in Table 2, the green composite films showed very similar values of brightness as PHBV, measured as \(L^{*}\), in the range of 87-89, showing no significant differences (\(p>0.05\)). Regarding the color coordinates \(a^{*}\) and \(b^{*}\), the values that were obtained from all of the samples in the CIE-L*\(a^{*}b^{*}\) space indicated the color of the films as yellow (\(b^{*}\simeq\) 8-12) with a slight green hue (\(a^{*}\simeq-1\)). This corresponds to a \(h_{ab^{*}}\) angle \(\simeq\) 95-99, that is, of a yellow color with a very slightly green hue. On the other hand, the chroma or color saturation was very similar in all of the samples, with \(C_{ab}\)* values ranging between 8 and 12. In relation to the color difference between PHBV and its composites with MFC, these can be only appreciable by an experienced observer (\(\Delta E_{ab^{*}}\)\(\geq\) 1 and <2). The color differences were very similar in all cases, showing no significant differences (\(p>0.05\)) and being slightly lower for the compabilized films. These values confirmed that the presence of the cellulose microstructures did not successfully alter the optical properties of PHBV.  
Figure 7: Spectral distribution curves of the percentage of internal transmittance (Ti) of the films of poly(3-hydroxybutyrate-\(co\)-3-hydroxyvalerate) (PHBV) with microfibrillated cellulose (MFC) and compabilized with epoxy-based styrene-acrylic oligomer (ESAO) and with triglyceride isocyanurate (TGIC) and dicumyl peroxide (DCP).