3.2.1. MFA Results

The calculated MFA for the VS and VF corresponded to 7.49◦ and 7.42◦ , respectively. An average value of 7.46◦ can be considered similar to some MFA values reported for other NLFs such as ramie and hemp [3,28,39,40].

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 7 of 12

**Figure 3.** XRD patterns of ubim fiber-related samples vertical stalk, vertical fiber, horizontal stalk and horizontal fiber. **Figure 3.** XRD patterns of ubim fiber-related samples vertical stalk, vertical fiber, horizontal stalk and horizontal fiber. An average value of 7.46° can be considered similar to some MFA values reported for other NLFs such as ramie and hemp [3,28,39,40].

#### A preliminary evaluation of the percentage of crystalline cellulose by Rietveld re-3.2.2. CI Results 3.2.2. CI Results

vealed values of around 66% for conditions (a) and (b). 3.2.1. MFA Results The calculated MFA for the VS and VF corresponded to 7.49° and 7.42°, respectively. An average value of 7.46° can be considered similar to some MFA values reported for other NLFs such as ramie and hemp [3,28,39,40]. 3.2.2. CI Results The degree of crystallinity of cellulose is an important structural parameter of NLFs. The stiffness of plant fibers is mainly due to the relatively large amount of crystalline re-The degree of crystallinity of cellulose is an important structural parameter of NLFs. The stiffness of plant fibers is mainly due to the relatively large amount of crystalline regions in the cellulose. Figure 4 shows the convoluted peaks obtained from background free XRD patterns to calculate the crystallinity index (CI) according to Equation (6). In this figure the crystallographic planes (2 0 0) and (1 1 0) are associated with the crystalline and amorphous phases of the ubim stalk (VS) and fiber (VF), respectively. As for VS in Figure 4a, the crystalline peak at 2θ = 26.3◦ has an intensity *I*<sup>2</sup> = 402.1 a.u, while the amorphous peak at 2θ = 11.8◦ with *I*<sup>1</sup> = 149.7 a.u allowing to obtain a CI = 63% by Equation (6). Similarly for VF, Figure 4b revealed a crystalline peak at 2θ = 25.6◦ with *I*<sup>2</sup> = 694.5 a.u and an amorphous peak at 2θ = 10.4◦ with *I*<sup>1</sup> = 116.6 a.u, giving CI = 83%, which will be considered the proper one for ubim fiber. The degree of crystallinity of cellulose is an important structural parameter of NLFs. The stiffness of plant fibers is mainly due to the relatively large amount of crystalline regions in the cellulose. Figure 4 shows the convoluted peaks obtained from background free XRD patterns to calculate the crystallinity index (CI) according to Equation (6). In this figure the crystallographic planes (2 0 0) and (1 1 0) are associated with the crystalline and amorphous phases of the ubim stalk (VS) and fiber (VF), respectively. As for VS in Figure 4a, the crystalline peak at 2θ = 26.3° has an intensity *I*2 = 402.1 a.u, while the amorphous peak at 2θ = 11.8° with *I*1 = 149.7 a.u allowing to obtain a CI = 63% by Equation (6). Similarly for VF, Figure 4b revealed a crystalline peak at 2θ = 25.6° with *I*2 = 694.5 a.u and an amorphous peak at 2θ = 10.4° with *I*1 = 116.6 a.u, giving CI = 83%, which will be considered the proper one for ubim fiber.

**Figure 4.** XRD patterns of ubim fiber-related samples of: (**a**) vertical stalk (VS) and (**b**) vertical fiber (VF).

These CIs are within the range of values reported for several common NLFs such as sisal [41], hemp [40], jute [42,43], flax [44], and coir [28], used as reinforcement in polymer composites, which are shown in Table 2. In addition, this table also presents the density, cellulose content, crystallinity index and microfibrillar angle of less known NLFs such

as curauá [43,45], banana [7,37,46], pineapple [7,37,38], bamboo [7,37,38] and sugarcane bagasse [7,37,38]. Also included in Table 2 were cotton [7,36,46] and kenaf [37,46], with relatively higher densities and CI.

**Table 2.** Density, cellulose content, crystallinity index and microfibrillar angle of ubim fibers as compared to other NLFs applied as polymer composite reinforcement.


PW: Present Work.

A close look at the values of density, cellulose content, IC and MFA in Table 2 fails to reveal any direct relationship between the density with either the cellulose content or the CI for the presented NLFs. One possible reason is that the reported densities, like the case of ubim fiber in Table 2, are the apparent ones measured by geometric or Archimedes' methods. Indeed, the apparent density considers the amount of porosity, which is strongly influenced by the internal fiber microstructure (lumen and channels), and the characteristics of the fiber species. On the other hand, cellulose content, IC and MFA correspond to another structural hierarchy, which is more associated with the fiber strength. As such, in several cases a lower cellulose content might be related to higher MFA and weaker fiber like in the coir, banana and sugarcane bagasse fibers. To the knowledge of the authors these relationships have never been fully investigated and are the subject of future research.
