# Production of Microfibrilated Cellulose from Fast-Growing
## 3 Results and Discussion
### Film Properties  
#### 3.3.1 Optical and Mechanical Properties  
Films were obtained from MFC samples showing a density of 79.0 g m\({}^{-2}\) and a thickness between 65.0 and 68.0 \(\upmu\)m (Table 4). Optical properties (brightness and opacity) were evaluated (Table 4). The highest brightness value was observed for E-MFC film followed by P-MFC and O-MFC films. Taking into account the fact that all materials presented similar lignin contents (between 1.2% and 1.6%), the high brightness of the E-MFC film could be related to its lower xylan content (19.1%). In addition, xylan-lignin complexes are a source of residual poly-conjugates chromophores [51]. Compared with the initial raw materials (61.5%, 73.2%, and 78.2% brightness values for O-Pulp, P-Pulp and E-Pulp, respectively), a brightness decrease was observed for all MFC films after PFI refining and microfluidization (Table 4). This effect was previously reported by [PERSON] et al. [46], describing a brightness reduction for hardwood and softwood pulps during the refining pretreatment with Valley beater followed by microfluidization. It is \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Brightness** & **Opacity** & **Thickness** & **Tensile Index** & **Elongation at Break** \\  & **(\%ISO)** & **(\%)** & **(\(\upmu\)m)** & **(kN m kg\({}^{-1}\))** & **(\%)** \\ \hline O-MFC & 51.8 \(\pm\) 1.4 & 74.0 \(\pm\) 0.6 & 65.0 \(\pm\) 2.0 & 150.0 \(\pm\) 0.01 & 5.6 \(\pm\) 0.3 \\ P-MFC & 66.2 \(\pm\) 1.5 & 71.0 \(\pm\) 3.1 & 68.0 \(\pm\) 6.0 & 130.0 \(\pm\) 0.02 & 4.2 \(\pm\) 0.0 \\ E-MFC & 73.1 \(\pm\) 1.0 & 73.1 \(\pm\) 0.8 & 65.0 \(\pm\) 2.0 & 150.0 \(\pm\) 0.01 & 5.5 \(\pm\) 0.6 \\ \hline \end{tabular} O-MFC: olive microfibrilated cellulose; P-MFC: poplar microfibrilated cellulose; E-MFC: eucalyptr microfibrilated cellulose.  
\end{table} 
Table 4: Brightness, opacity, thickness, tensile index and elongation at break of MFC films. Figure 2: X-ray diffractograms of O-MFC, P-MFC and E-MFC, respectively. well known that, among other factors, the temperature reached during the refining stage may contribute to a decrease in the brightness of cellulosic pulps [52]. Regarding opacity, the three MFC films showed similarly high values (71.0-74.0%), resulting in translucent films. Taking into account the low nanofibrillation yields of the MFC samples (11.3-13.3%), a high presence of microscopic fibers, instead of nanofibers, remained in the suspension after microfluidization. This may produce high light scattering, which results in the high opacity of the MFC films. In this sense, [PERSON] et al. [43] described a correlation between nanofibrillation yield and the transmittance (inversely related to opacity) observed for the resulting CNF films. Mechanical properties (tensile index and elongation at break) of the MFC films were also characterized (Table 4). Tensile index values (130-150 kN m kg\({}^{-1}\)) of the three MFC films were higher than the typical tensile indices reported for films from commercial Paper (range of 10-100 kN m kg\({}^{-1}\)) [53]. It is known that microfluidization leads to nanofibers with higher specific surface area, which improves structure compaction and hydrogen bonding with the adjacent fibers, resulting in good mechanical properties. In this work, although low nanofibrillation yields were observed for the three samples, the micro- and nanofibrils contained in the three suspensions (as indicated by SEM images) could contribute to improving the tensile index of the resulting MFC films. Moreover, the drying treatment applied in this study during the preparation of the films (i.e., vacuum draining followed by hot-pressing) could also enhance the mechanical properties. In this regard, [PERSON] et al. [54] produced CNF films with twice the tensile strength by pressurel filtration followed by an additional pressing step at about 100 \({}^{\circ}\)C and 1800 Pa. They speculated that when the films are dried at high applied load and elevated temperature, denser films with a higher amount of bonds and fewer voids are produced.