## 3 Results and Discussion
### Characterization of Nanocellulose Gels and Films
#### 3.2.5 Grammage, Thickness, Density and Transparency of CNF Films  
CNF films were characterized according to their grammage, thickness, density and transparency, being the results reported in Table 1. Although the same grammage was intended for all the produced films, some differences were observed in their experimental values. The actual grammage of the CNF\({}_{1}\) films is lower than that of the CNF\({}_{2}\) films. This discrepancy may be attributed to differences in the structure of the suspensions. So, if CNF\({}_{1}\) retains more free water, the mass measured in the wet state could be overestimated. However, after drying, the actual dry mass would be lower, resulting in a reduction in grammage.  
Higher grammage may imply a denser material; the CNF\({}_{2}\) film has a thicker fibrous matrix and, consequently, higher apparent density, and probably the sulfate pulp fibers compact better than the sulfite pulp fibers due to their morphology [19] and crystallinity [29].  
The thickness of the films, coated and uncoated, is approximately the same and close to 38 \(\upmu\)m. It is worth noting that the film with the highest grammage (CNF\({}_{2}\)) is not the thickest, but rather CNF\({}_{1}\). These results allow us to conclude that the CNF\({}_{1}\) fiber matrix must include a greater amount of air, which can be reinforced by the apparent density values. Regarding transparency, the values vary between 79.6% for CNF\({}_{1}\) films and 94.1% for CNF\({}_{2}\) films. The greater transparency of the CNF\({}_{2}\) film may be a result of the size of the cellulose microfibrils, the degree of homogenization and surface roughness [30]. Fiber morphology and alignment reduce light scattering and increase light transmission. Furthermore, the CNF\({}_{2}\) film has a lower density and, consequently, less air in the fibrous matrix, which can also influence transparency, since air has a lower refractive index compared with cellulose. These results are corroborated by other published works [31].  
Regarding the WC CNF films, it can be highlighted that the wax coating does not result in a significant increase in grammage and thickness, as only an increase of less than 1% in apparent density is obtained. However, regarding the transparency of coated films, a slight decrease is recorded compared with uncoated films. The transparency of CNF films decreases by 1.7% and 2.7%, respectively, for CNF\({}_{1}\) and CNF\({}_{2}\) after wax coating.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Properties** & **CNF\({}_{1}\)** & **WC CNF\({}_{1}\)** & **CNF\({}_{2}\)** & **WC CNF\({}_{2}\)** \\ \hline Grammage & 40.18 \(\pm\) 0.32 & 40.68 \(\pm\) 0.21 & 44.65 \(\pm\) 0.37 & 45.12 \(\pm\) 0.17 \\ Thickness & 38.25 \(\pm\) 1.39 & 38.39 \(\pm\) 0.79 & 37.98 \(\pm\) 1.82 & 38.19 \(\pm\) 0.68 \\ (\(\upmu\)m) & 1.05 & 1.06 & 1.17 & 1.18 \\ (\(\upmu\)m) & 79.6 \(\pm\) 1.51 & 78.24 \(\pm\) 0.98 & 94.1 \(\pm\) 1.10 & 91.6 \(\pm\) 0.90 \\ \hline \end{tabular}
\end{table}
Table 1: Structural properties of CNF films (results presented as mean \(\pm\) standard deviation).  
Natural wax has a higher refractive index [32] compared with nanocelluloses; consequently, it increases light reflection while reducing transmission.