## 3 Results and Discussion
### Melt Processing of Green Composites  
Figure 4 shows the torque vs. time curves of the different PHBV formulations with MFC and/or the different compatibilizing agents during melt-mixing at 50 rpm and 170 \({}^{\circ}\)C. Figure 4a shows the torque evolution for a processing time of 25 min for the neat PHBV sample, which was conducted to ascertain the stability of the biopolyester in terms of melt shear-thinning strength at high temperatures. One can observe that, during processing, the torque sharply increased for about 30 s until the biopolymer pellets were plasticized and then it decreased, showing a peak centered at approximately 1.2 min. The latter peak can be related to the complete melting of the PHBV crystals and spanned up to 2 min. Thereafter, the torque of the liquid PHBV mass progressively reduced, this effect was more noticeable from the processing times that were longer than 5 min. Finally, a plateau was nearly reached after approximately 20 min. The lower torque values indicate that a progressive decrease in viscosity was achieved in the samples, which can be ascribed to thermo-mechanical degradation of PHBV due to chain scission and resultant M\({}_{\text{W}}\) reduction, confirming the low processing window of this biopolyester. Indeed, the inherently poor thermal stability of PHB, and also of PHBV with low 3 HV contents, is a major concern since thermal decomposition by chain scission starts at temperatures close to the melting point, making its processing in the melt state a considerable challenge [39]. However, a minimum residence time would be needed to both facilitate a correct mixing and dispersion of the microparticles in the PHBV matrix during melt compounding [40] as well as to achieve or promote the reactivity of the compatibilizing agents [41]. In particular, the dispersion of the cellulose microfibrils depends on time and shear force that will open the physical entanglements. According to this result, a residence time of 5 min was selected to process all of the formulations as a trade-off solution between stability and mixing. Thus, Figure 4b-d show the curves of torque vs. mixing time for all of the formulations, which were processed at 50 rpm and 170 \({}^{\circ}\)C for 5 min. As can be seen in the curves for the uncompatibilized green composite samples, shown in Figure 4b, the incorporation of the microstructures of cellulose yielded very similar curves to the neat PHBV sample, with a progressive increase in the torque during the first \(\sim\)2 min. This effect can be ascribed to the mechanical friction increase due to the MFC presence in the mixer prior to melting since the dispersion and disaggregation of particles in the biopolymer matrix mainly occurred during this period. In the case of the unfilled compatibilized PHBV samples, included in Figure 4c, one can observe that these samples presented a second increase in torque, approximately after 1-1.5 min, which corresponds to the increase in viscosity associated with the chemical reaction of the compatibilizing agents. In the case of the PHBV sample that was processed with TGIC, this reaction took place at a slightly shorter time than for ESAO, which may be associated with the presence of the highly reactive DCP. Finally, Figure 4d shows the composite samples that were prepared with the compatibilizing agents, where a much higher viscosity increase can be observed than in the rest of the samples, especially noticeable in the PHBV/MFC composite samples that were processed with TGIC. This viscosity increase can be ascribed to the formation of new bonds in the PHBV chains  
Figure 3: Field emission scanning electron microscopy (FESEM) micrographs of the atomized microfibrilated cellulose (MFC) observed at different magnifications, as follows: (**a**) taken at 1000\(\times\) with a scale marker of 30 \(\upmu\)m; (**b**) 5000\(\times\) and 6 \(\upmu\)m; (**c**) 20,000\(\times\) and 900 nm. or between PHBV and MFC.