# Production of Microfibrilated Cellulose from Fast-Growing
## 3 Results and Discussion  
### Pulp Production and Characterization  
It is well known that the composition of the lignocellulosic biomass plays an important role in assessing its revalorization, as it has a great influence on the characteristics of the obtained products [32]. Table 1 shows the composition of the raw materials and their corresponding bleached pulps used in this work to produce cellulose microfibers by mechanical means. The composition of the used materials shows typical hardwood values [16; 22; 33] and is more similar between poplar and olive. Several authors have studied the effect of hemicellulose and lignin content on the nanofibrillation process to obtain cellulose micro/nanofibers. For instance, [PERSON] et al. [34] observed the highest efficiency in the nanofibrillation process when the hemicellulose content of the starting material was about 25%. The same authors concluded that halving the hemicellulose content also halved the efficiency of fibrillation. The reason for this effect is that hemicelluloses (mainly xylans) stabilizes the aqueous suspension of the fibers, preventing the aggregation of the microfibrils and thus, favoring the fibrillation process [35]. On the other hand, it has been observed by other authors that when the starting material contains more than approximately 20% of lignin, the efficiency of the fibrillation process decreases [36]. Nonetheless, excessive delignification has also been shown to decrease the fibrillation process [37]. Therefore, it seems that a residual amount of lignin in the starting material could increase the efficiency of the fibrillation process, as the antioxidant capacity of such residual lignin could prevent the re-bonding of formerly fractured covalent bonds [38]. In view of the above, a kraft pulping process was applied to reduce the lignin content and increase the glucan amount without eliminating the hemicellulose content of the raw materials, thus encouraging the subsequent fibrillation process to obtain cellulose micro/nanofibers. After applying the kraft pulping process, an almost complete elimination of extractives, a decrease in lignin content (resulting in a total residual lignin content of around 1.2-1.7%) and an increase in carbohydrates (more pronounced for glucan (increase of 68-103%) than for xylan (increase of 7-17%) were observed in all cases. The hemicellulose contents obtained in the resulting bleached kraft pulps were 23.6% and 24% for O-Pulp and P-Pulp, respectively. These values are very close to the optimum content to ensure the efficiency of the nanofibrillation process, as mentioned above. However, the E-pulp (reference sample) showed a slightly lower value (19.1%). In general, these hemicellulose content values are similar to or even higher than those observed by other authors for pulps used to successfully obtain nanofibrillated cellulose from different raw materials [20; 22; 39]. On the other hand, the cellulose content observed in this work was higher than those reported by other studies using eucalypt and olive tree pruning for cellulose nanofibers, which could improve, in our case, the yield of the microfibrillated cellulose production [20; 22].