**1. Introduction**

Composites made of synthetic materials have become a class widely applied in various technological sectors [1]. However, their production is associated with a relatively high consumption of energy and non-degradable wastes. Indeed, synthetic materials not only demand greater manufacturing energy but also produce after service life wastes that are highly polluting and cause serious environmental damage [2]. The search for new materials that are environmentally friendly, sustainable and offer similar mechanical properties at a lower cost, has been arousing the interest of industry and researchers since the end of 20th century, to replace synthetic components by cellulose- based ones [3].

Natural fibers and related fabrics are known to be a renewable resource, and have gained considerable interest in different sectors of the industry, from the production of ropes, handcrafts and textiles, to automobile and construction industries [4–7]. In recent decades, the search for new natural lignocellulosic fibers (NLFs) as reinforcement in composite materials has increased, as reported in several review papers [8–12]. Consequently, these NLFs are currently applied to reinforce polymeric composites, used in construction materials, food packaging, and automotive parts [13–15]. Indeed NLFs such as jute, coir,

**Citation:** Marchi, B.Z.; Oliveira, M.S.; Bezerra, W.B.A.; de Sousa, T.G.; Candido, V.S.; da Silva, A.C.R.; Monteiro, S.N. Ubim Fiber (*Geonoma baculífera*): A Less Known Brazilian Amazon Natural Fiber for Engineering Applications. *Sustainability* **2022**, *14*, 421. https:// doi.org/10.3390/su14010421

Academic Editor: Mostafa Ghasemi Baboli

Received: 30 November 2021 Accepted: 29 December 2021 Published: 31 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

bamboo, hemp and sisal have been reinforcing polymer composites as eco-friendly civil construction materials for panels, doors, roofing tiles and even low cost buildings [15]. In particular, NLFs are currently being considered as part of multilayered ballistic armor [16] for personnel protection against being shot by a rifle with 7.62 mm ammunition.

NLFs stand out in general as they are biodegradable, non-polluting, have a low energy consumption and are more economically viable. In addition to being lighter than synthetic fibers, they present similar chemical and mechanical properties [7]. The properties of NLFs vary according to their chemical composition, diameter, arrangement of constituents in the fiber, degree of polymerization, crystalline fraction of cellulose, part of the plant organism (stem, leaf, root, seed, among others) and cultivation aspects (age, climatic conditions, degradation processes) [17].

The NLFs extracted from the stem have good quality, mainly due to the presence of crystalline cellulose with a high degree of polymerization [18]. The structure of NLFs is basically composed of two different phases: one crystalline and the other amorphous [3,17,18]. Cellulose is the main chemical constituent of these fibers and is responsible for stiffness. A higher cellulose content in the fiber represents better mechanical properties [19].

Most of the crystalline structure of an NLF is restricted to the cellulose. In fact, cellulose is found not to be uniformly crystalline. However, as indicated by Eichhorn et al. [20], the cellulose ordered regions are extensively distributed throughout the material. According to these authors, the amorphous part of an NLF is composed of hemicellulose, as well as lignin, pectin and waxes. As for the crystalline part, the cellulose consists of a threadlike structure called the microfibril. Although the individual densities of cellulose, hemicellulose and lignin might be ~1.60 g/cm<sup>3</sup> [21–23], the density of an NLF measured by the geometric method (mass/volume) or the Archimedes' scale is found to be lower due to the fiber's porosity. Notably, the cellulose films were reported as having densities of 1.49–1.51 g/cm<sup>3</sup> [24].

Cellulose microfibril, as the most crystalline phase in an NLF, is known as the reinforcement phase incorporated into lignin, which is considered to be an amorphous matrix. The cell wall of a fiber is not a homogeneous membrane: it presents an order of cells composed of distinct walls, with different helicoid angles between the fiber axis and the microfibrils. The strength and ductility of cellulosic fibers depend on the angle, at which these microfibrils are curled in relation to the fiber axis, which is also referred to as the microfibrillar angle (MFA) [25]. The primary wall is the first layer where cellulose fibrils are deposited during cell development. Since fibril orientation is more oblique, MFAs are randomly oriented exhibiting varying degrees of alignment. The primary layer involves a secondary layer, which in turn is composed of three layers S1, S2 and S3. The S1 and S3 layers exhibit microfibrils in a smooth helical orientation, whereas the S2 layer is the thickest on the cell wall and has axial orientation. The orientation of these microfibrils is the main responsible for the fiber's tensile strength, it can be said that a low MFA implies superior mechanical properties [26].

Among the most diverse NLFs with potential application as polymer composite reinforcement, are those from the Amazon region [27]. A fiber extracted from the ubim plant found in the Amazon, Brazil, is not well known, has been scientifically denominated *Geonoma bacculifera*, and is a promising reinforcement for composites. In fact, ubim fiber is not mentioned in a study on Brazilian natural fibers with potential for reinforcing polymer composites used in engineering applications [28]. The ubim fiber is extracted from a plant with multiple stems, which is smooth with elongated leaves and unbranched. The plant has a height varying between one to four meters and diameter from one to three centimeters as well as erect or partially crawling stipe, with seven to twelve leaves, little branched inflorescences and globose or ovoid fruits [29]. Ubim fibers have been used in the north of Brazil in hut covers, boat awnings, and rain protection lining. The ubim stem is used in fishing corral walls [30].

Some basic characteristics show that ubim fiber appears as a promising NLF for reinforcing polymeric matrix composites. Thus, the present study aims to investigate the ubim fiber through density, X-ray diffraction (XRD), microfibrillar angle, crystallinity index and scanning electron microscopy. These characteristics may be relevant for composites manufacturing, evaluating their potential for future applications as low cost civil construction materials. sites manufacturing, evaluating their potential for future applications as low cost civil construction materials. **2. Materials and Methods** 

the north of Brazil in hut covers, boat awnings, and rain protection lining. The ubim stem

Some basic characteristics show that ubim fiber appears as a promising NLF for reinforcing polymeric matrix composites. Thus, the present study aims to investigate the ubim fiber through density, X-ray diffraction (XRD), microfibrillar angle, crystallinity index and scanning electron microscopy. These characteristics may be relevant for compo-

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 3 of 12

#### **2. Materials and Methods** The ubim fiber (Figure 1a) was extracted from the stem of the plant, scientifically called *Geonoma baculifera*, purchased in Belém do Pará, Brazil. The stem of the ubim plant

is used in fishing corral walls [30].

The ubim fiber (Figure 1a) was extracted from the stem of the plant, scientifically called *Geonoma baculifera*, purchased in Belém do Pará, Brazil. The stem of the ubim plant was divided into longitudinal splints, which are then manually separated into finer fibers. was divided into longitudinal splints, which are then manually separated into finer fibers. Figure 1 illustrates the ubim plant (a), as well as the split splints of the stalk (b) and final split fibers (c).

**Figure 1.** Ubim (**a**) plant, (**b**) mechanically divided splints form the stalk, and (**c**) bunch of final separated and isolated fibers. **Figure 1.** Ubim (**a**) plant, (**b**) mechanically divided splints form the stalk, and (**c**) bunch of final separated and isolated fibers.

The characterization of the ubim fiber, in terms of density, microfibrillar angle and degree of crystallinity was preliminarily evaluated. The fiber was shredded and subse-Figure 1 illustrates the ubim plant (a), as well as the split splints of the stalk (b) and final split fibers (c).

quently measured for the dimensions and length of the fiber cross-section. *2.1. Characteristic Density*  The characterization of the ubim fiber, in terms of density, microfibrillar angle and degree of crystallinity was preliminarily evaluated. The fiber was shredded and subsequently measured for the dimensions and length of the fiber cross-section.

### 2.1.1. Geometric Method *2.1. Characteristic Density*

#### To obtain the average diameter, each fiber was divided into five distinct points uni-2.1.1. Geometric Method

formly separated, and then rotated by 90°. A new reading was carried out at the same five previous points. A total of 3 measurements were taken at each point, thus obtaining a statistical average. The equipment used was an Olympus microscope, model BX53M with 5x magnification. The length of each fiber was measured using a 0.01 mm precision caliper, and its mass was weighed on a Gehaka electronic scale, model AG-200, with 0.0001 g precision. To obtain the average diameter, each fiber was divided into five distinct points uniformly separated, and then rotated by 90◦ . A new reading was carried out at the same five previous points. A total of 3 measurements were taken at each point, thus obtaining a statistical average. The equipment used was an Olympus microscope, model BX53M with 5x magnification.

The fiber cross-section was considered to be ellipsoidal in shape, and its approximate area calculated according to Equation (1), where *a* and *b* represent the major and minor axes of the ellipsoid, respectively: The length of each fiber was measured using a 0.01 mm precision caliper, and its mass was weighed on a Gehaka electronic scale, model AG-200, with 0.0001 g precision. The fiber cross-section was considered to be ellipsoidal in shape, and its approximate area calculated according to Equation (1), where *a* and *b* represent the major and minor axes of the ellipsoid, respectively:

$$A = \frac{\pi ab}{4} \tag{1}$$

Density results for the five intervals were statistically evaluated using Weibull Analysis (WA) software. This statistical analysis is based on the cumulative Weibull frequency distribution function (F(x)).

$$\mathbf{F}(\mathbf{x}) = \exp\left[-\left(\frac{\mathbf{x}}{\theta}\right)^{\beta}\right] \tag{2}$$

where *x* is measured density, *β* is the Weibull modulus and *θ* is the scaling parameter. Weibull modulus (*β*) is associated with data uniformity, *θ* has precision adjustment given by R<sup>2</sup> . Equation (2) can be modified to fit a linear graph:

$$\ln\left[\ln\left(\frac{1}{F(\mathbf{x})}\right)\right] = \beta \ln \mathbf{x} - (\beta \ln \theta) \tag{3}$$
