*3.1. Cellulosic Fiber Production and Characterization*

The chemical composition of the OTPB and the cellulosic pulp obtained is shown in Table 1. This composition is similar to that reported in previous work [39]. OTPB was subjected to a soda pulping process to facilitate the deconstruction of the cellulose fiber and the purification of the lignocellulosic components. The soda pulping process showed a yield of 32.0%, similar to other more polluting processes such as kraft pulping (33%) [40].


**Table 1.** Chemical composition of olive tree pruning biomass and cellulosic pulp.

As can be observed, the non-structural elements (Ext. EtOH and Ext. AQ) were drastically reduced after the pulping process. In addition, the lignin content in the fiber was reduced to 14.6%. On the other hand, the cellulosic fraction was purified and concentrated to almost 60% (similar to the value achieved by the kraft process) [40]. The hemicellulose content is a key parameter in the effectiveness of the nanofibrillation process. This component acts as a hydrated steric barrier to microfibril aggregation, preventing the re-agglomeration of the delaminated fiber. Chacker et al. [41] analyzed the role of hemicelluloses in the nanofibrillation process, determining that a hemicellulose content of about 25% in fiber is the ideal value to obtain the maximum efficiency in nanofibrillation. In pulps with a 12% of hemicellulose content, the fibrillation yield decreases by half in comparison with higher hemicellulose content pulps. The OTPB pulp obtained in this work retains most of the hemicelluloses present in the initial raw material, showing a content of 25.68%, higher than that in the OTBP kraft pulp studied in previous work [37]. Compared to other cellulosic pulps successfully used in the production of cellulose nanofibers, OTPB showed a higher hemicellulose content than *Eucalyptus* kraft pulp (19.40%), kraft pine pulp (14%) and other agricultural residues such as corn (20%), wheat (23.30%), barley (18.30%), oat (16.40%), banana leaves (20.28%), tomato (11%) and lime residues (10%), oil palm empty fruit bunches (22%) and Brazilian satintail plants (9%) [40,42–46]. It is therefore concluded that the pulping process carried out produces cellulose pulp with an optimum chemical composition for the production of cellulose nanofibers.

The cellulosic pulp obtained was characterized in terms of thermal stability and crystallinity. Figure 1a shows the thermal degradation behavior of the OTBP cellulosic pulp. The OTBP pulp showed a multi-step degradation process by the presence of several components such as lignin, hemicellulose and cellulose that are degraded at different temperatures in the range studied. The initial weight loss step in the region of 30–120 ◦C is associated with the evaporation of the absorbed and bound water in the fiber. The thermal degradation in the temperature range 120–350 ◦C is related to the breaking of glycosidic bonds, the pyrolysis of polysaccharides and the depolymerization of lignin, hemicellulose and cellulose. In the last region at 350–600 ◦C, the weight loss is due to the pyrolysis of cellulose fibers and the remaining carbonaceous residue [47]. The DTG peak shows that the temperature of the maximum degradation (Tmax) of fiber is observed at 348 ◦C. Figure 1b shows the X-ray diffraction patterns of the fiber structure. It is possible to observe that it presents two major diffractions peaks at 2θ = 16.1◦ and 22.5◦ corresponding to the 110 and 200 reflection planes of cellulose I's structure. The crystallinity index of the cellulosic fiber can be calculated by comparing the reflection intensity of the peak at 22.5◦ (crystalline region) and the valley region between the two peaks associated with the amorphous region [38]. The CI observed for the OTPB pup was 60.26%. Considering that the only lignocellulosic element that can present crystallinity is α-cellulose, it is deduced that all of the cellulose present in the fiber (59.67 ± 0.02) shows a crystalline disposition, compared to the amorphous elements, hemicellulose and lignin, which do not provide crystallinity to the sample.

**Figure 1.** (**a**) Thermogravimetric analysis (TGA) and TGA equivalent derivate (DTG) curves and (**b**) the XRD pattern of OTBP pulp.
