Influence of the Extraction Location on the Physical and Mechanical Properties of the Pseudo-Trunk Banana Fibers

ABSTRACT

The specific properties and availability of banana pseudo-trunk fibers make them a promising alternative for the development of green composites. However, the wide dispersion of their properties can hinder their use. In this study, the influence of the sampling area of the banana pseudo-trunk on the physical and mechanical properties of the fibers was evaluated. Prior to retting, the trunk was sampled longitudinally (bottom, middle and top) and transversely (periphery, intermediate and heart). Gravimetric tests were carried out and revealed variations in water absorption (347.1–517.4%), density (0.92–1.45 g.cm⁻³) and linear mass (25 -34tex). Tensile tests were also performed and showed a significant effect of fiber location on Young’s modulus (6.60–34.6GPa), tensile strength (91-350MPa) and elongation at the break (0.9–2.6%). Due to diameter scatter, variations of 42% were found for fibers in the same area. In a region, the physical properties increase from the periphery to the core, and the mechanical properties decrease in the same direction, except for elongation. The results of this study showed good agreement with those of other natural fiber types. However, we recommend the peripheral areas of the pseudo-trunk to extract reinforcing fibers from composites because of their low density (0.9 g.cm⁻³) and their high stiffness (34GPa).

摘要

香蕉假纤维的特殊财产和可用性使其成为开发绿色复合材料的一个很有前景的替代品. 然而，其财产的广泛分散可能会阻碍其使用。在本研究中，评估了香蕉假丝取样面积对纤维物理和机械财产的影响. 在脱胶之前，对树干进行纵向(底部、中间和顶部)和横向(外围、中间和心脏)采样. 进行了重量测试，发现吸水率(347.1-517.4%)、密度(0.92-1.45 g.cm-3)和线性质量(25-34tex)发生了变化. 还进行了拉伸试验，结果表明纤维位置对杨氏模量(6.60-34.6GPa)、拉伸强度(91-350MPa)和断裂伸长率(0.9-2.6%)有显著影响. 由于直径分散，在同一区域内发现纤维有42%的变化. 在一个区域内，物理财产从外围到核心都在增加，而机械财产则在相同的方向上减少，除了伸长率. 这项研究的结果与其他天然纤维类型的结果显示出良好的一致性. 然而，由于其低密度(0.9 g.cm-3)和高刚度(34GPa)，我们建议使用伪主干的外围区域从复合材料中提取增强纤维.

KEYWORDS:

• Pseudo-trunks
• banana fibers
• dispersion of properties
• statistical analyses
• mechanical properties
• ANOVA

关键词:

• 伪树干
• 香蕉纤维
• 财产分散
• 统计分析
• 力学财产

Introduction

Recently, many sources such as seeds, fruits, leaves, stems, pseudo-trunks, lianas, and roots have been used as raw materials to extract lignocellulosic fibers that can be used in composite materials (Obame et al. 2022, Ramesh et al., 2020). These composites can be used in transportation, construction, sports and leisure, arts, and packaging (Mewoli et al. 2020, Jamshaid et al. 2021, Libog et al. 2021). Plant fibers are abundant, accessible, renewable, and biodegradable, making them an attractive biosourced material and an alternative to less environmentally friendly synthetic glass fibers (Ramesh et al. 2020, Betene et al., 2020). The addition of fibers to polymer and ceramic matrices reduces their weight and production cost, and allows the development of environmentally friendly biomaterials. However, the isotropy and performance of the resulting biocomposites are mainly related to those of the fibers, which requires a better understanding of the fiber properties before their use for reinforcement (Yilmaz et al. 2017).

The physical and mechanical properties of a plant fiber are dispersed due to their natural character and chemical composition (Duval et al. 2011, Bezazi et al. 2014). This dispersion is also related to the plant variety, growth conditions and defects, extraction method (biological, chemical or mechanical) and conditioning (temperature and humidity) of the fiber, test conditions and parameters adopted (Indira et al. 2013). Romhany et al, (2003) found for flax fiber (density 1.52 g.cm⁻³), a Young’s modulus of 50 GPa and a tensile strength of 1500 MPa, while Blos and Donald (1999) obtained 100 GPa and 840 MPa. In addition to these parameters, the fiber diameter also has a significant effect on the mechanical properties. It is generally observed that the tensile strength of the fiber increases with decreasing diameter (Obame et al. 2022, Ndoumou et al., 2022). In addition, the location of the fiber in the plant (bottom, middle, head, periphery, middle and heart) also leads to a dispersion of the fiber properties. Yilmaz et al., (2017), observed that the size of extracted fibers decreases from the periphery to the heart of corn husk leaves and from the bottom to the top of the leaves. But, the breaking strength, fracture toughness and elongation at break of the fibers increase from the top to the bottom of the husk leaf. For flax fibers of the varieties Herms and Agatha, Charlet et al. (2007, 2009) showed that the fibers in the middle of the stem are finer, stiffer, and stronger than those in the bottom and top of the stem. Yet, Duval et al. (2011) observed small differences for the fibers that make up the entire hemp stem. The influence of fiber location can be analyzed by sampling the plant (Betené et al. 2022, Charlet et al. 2009). Studies on the analysis of the dependence between banana fiber properties and their location in the pseudo-trunk are scarce (Efeze et al. 2020, Yilmaz et al. 2016). However, a better understanding of this dependence is crucial to optimize the conditions of use and classification of these fibers.

The valorization of agricultural by-products has led to the use of banana pseudo-trunks as raw material for the extraction of vegetable fibers. The banana tree is an annual monocotyledonous plant (height 0.8–16 m) of the Musaceae family native to tropical countries, which is cultivated mainly for its edible fruits. In Cameroon, approximately 32 million tons of banana fruits are harvested annually (Faostat, 2016). The harvesting of banana fruits produces large amounts of organic waste, only a small portion is used for compost and the rest, especially the pseudo-trunk is discarded. The banana pseudo-trunk that is the subject of this study is composed of 90% water, and 10% plant matter Sango et al., (2018). It is possible to extract fibers from the banana pseudo-trunk. These fibers are used for eco-friendly textiles and single-use food paper packaging (Efeze et al. 2020, Subramanya et al. 2017). Recently, many studies have shown the strengthening potential of banana fiber (Ramesh et al. 2014, Yilmaz et al. 2017, Sango et al. 2018, Chengoué et al. 2020). It is a secondary bast fiber, which can be extracted by hot or cold-water retting (Libog et al. 2021) and by chemical retting with soda (Sango et al. 2018, Chengoué et al. 2020). But the properties of the fibers obtained by chemical retting depend significantly on the conditions (temperature, time and concentration) of extraction. It is reported in the literature (Sango et al. 2018, Raghavendra et al., 2017, Subramanya et al., 2017), that the fiber extracted in bulk from banana pseudo-trunk has a density of 1.15–1.35 g.cm⁻³, a diameter between 125–250 µm, a tensile modulus between 13–32 GPa, a tensile strength between 500–600 MPa, and an elongation at break between 1–3.5%. In addition, the fiber is composed of cellulose (63–64%), hemicellulose (20–30%), lignin (15–25%), pectins (4–8%), extractables (8%) and ash (15%) (Jordan and Chester 2017, Sango et al., 2018). As with all natural fibers, the structure of banana fiber is composed of a primary wall that thickens during the growth of the plant and a secondary wall formed by the successive deposition of layers of cellulose. The cellulose layer is divided into three sub-layers (S1, S2 and S3), of which the middle layer is thicker and ensures the rigidity and strength of the fiber. The microfibrils that make up this middle layer wrap around the fiber axis at an angle of 11° (Ramesh et al. 2014). In a previous study (Libog et al. 2021), the thermal stability of banana fibers was observed between 100 and 200°C. Below this range, the fiber dehydrates and above this range, its main components (hemicellulose, pectin, lignin and cellulose) progressively degrade until the formation of residues (20%) at 900°C. This high residue content (>10%) indicates a good thermal resistance of banana fibers. Overall, there is a dispersion in the physical and mechanical properties of banana pseudo-stem fibers. Sango et al. (2018) showed that this dispersion was partly related to fiber diameter. However, the trunk is composed of a central cylinder and concentric layers of sheaths. The color of these sheaths tends to be darker as one moves toward the periphery. In addition, the pseudo-trunk is long and narrow. This variability in the shape and organization of the banana pseudo-trunk structure can lead to differences in the properties of banana fibers. Therefore, it is important to perform biaxial sampling of the pseudo-trunk and evaluate the properties of each zone. The first axis is longitudinal, following the height of the pseudo-trunk and the second axis is transverse, following the diameter of the trunk.

The objective of this study is to determine the influence of the pseudo-trunk extraction area on the physical and mechanical properties of banana fibers. Gravimetric analyses to evaluate density, water absorption and linear density have been performed. Fiber diameter measurements have been performed and tensile tests have been carried on fibers extracted from nine distinct zones according to the height and diameter of the pseudo-trunk.

Materials and methods

Sampling of the banana trunk and fibers extraction

The banana trunk used in this study is of the Cavendish variety and was collected in Penja (Moungo, Cameroon). The study area (Figure 1a) is 3.3 meters long, and has a geometry that can be likened to a cone trunk.

Figure 1. a) Banana plant with indication of the study region; b) Schematic of the sampling principle of the banana pseudo-stem in the longitudinal direction A- Base, B-Middle and C- Head; c) Cross-section of the banana pseudo-stem with the schematic of the sampling principle in the transverse direction C- Heart, I- Intermediate and P- Periphery.

The samples extraction zones were taken according to the position of the fiber in the pseudo-trunk. Thus, three regions A (bottom, close to the roots: 1/3), B (middle: 2/3$)$ and C (top, near the leaves 3/3) were distinguished in the longitudinal direction from the roots to the leaves Figure 1b). Any section to any region shows three regions on the radial direction P (Periphery), I (Intermediary), and C (Heart) as shown in Figure 1c. The codings for the nine sample areas obtained are shown in Table 1.

Table 1. Coding of fiber samples by considered extraction area.

Regions Sections	A (Bottom)	B (Middle)	C (Top)
Periphery (P)	PA	PB	PC
Intermediary (I)	IA	IB	IC
Center (C)	CA	CB	CC

Fiber extraction

Fibers were extracted from each sample area by retting in water (Chengoué et al., 2020). Stipes were cut into small pieces 20–30 cm long and immersed in fresh water for 30 days in an ambient environment of 25°C and 50% relative humidity. After decomposition of the stipe, the fibers were dehulled manually, washed in fresh water, and dried in a vacuum oven at 60°C for 24 hours.

Physical characterization of the studied fibers

Water absorption test

Water absorption of fibers previously dried at 105°C for 24 hours was estimated gravimetrically (Obame et al., 2022). Ten samples for each fiber type were made and then immersed in distilled water at room temperature (28°C). After 48 hours of immersion, the samples were wiped to remove surface water and then weighed on a micrometer balance. Equation (1)(1) $W_{Abs}=\frac{m_f-m_i}{m_i}\times 100$(1) was used to calculate the water absorption of the fiber.

(1) $W_{Abs}=\frac{m_f-m_i}{m_i}\times 100$(1)

where $m_i$ = 100 ± 5 mg is the initial sample mass and $m_f$ is the sample mass after 48 hours of immersion.

Determination of the density

The density ${\rho }_m$ was determined using the ASTM D3800–99 and D 792 standards. The fibers are immersed in Benzene (${\rho }_b$ = 0.876 g.cm⁻³). Ten samples of each extraction zone were tested. The density of each studied fiber is obtained by equation (2)(2) ${\rho }_m=\frac{{\rho }_b\ast m_f}{m_f-m_{fb}}$(2) .

(2) ${\rho }_m=\frac{{\rho }_b\ast m_f}{m_f-m_{fb}}$(2)

where $m_f$ is the mass of the fiber sample and $m_{fb}$ is the mass of the fiber sample in benzene.

Determination of the diameter

The diameter was determined using the ASTM 2130–90 standard. With a digital caliper, the diameter of the fiber is measured. The measurements are taken at five different points of the fiber (at each end, at the middle, and each third). Twelve samples of each extraction zone were tested.

Determination of the linear density

The linear density (in tex, equation 3(3) $\mathrm{L}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{a}\mathrm{r}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}=\frac{M_f}{{\mathrm{L}}_f}$(3) ) of the studied fibers was determined gravimetrically (Betené et al. 2022) according to NF G 07–007. Ten samples of each type of fiber were considered.

(3) $\mathrm{L}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{a}\mathrm{r}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}=\frac{M_f}{{\mathrm{L}}_f}$(3)

where $M_f$ (in g) is the mass of the fiber sample and ${\mathrm{L}}_f$ is the total length (in km) of the fiber sample.

Tensile properties of the banana fibers

The tensile strength, Young’s modulus, and elongation at break of banana fibers were determined by tensile tests in a controlled environment of temperature 23 ± 1°C and relative humidity 50%. For each sample area, 25 fiber specimens were prepared and tested. For specimen fabrication, the fiber was glued to a paper frame so as to have a gauge length of 10 mm as recommended in NF T25 501–2 (Betené et al. 2022). The tests were performed using an LDW-5 universal machine equipped with a load cell with a capacity of 100N. The specimen was held by the jaws of the machine and deformed at a constant speed of 1 mm.min⁻¹ (Ndoumou et al. 2022) until rupture.

Statistical analysis

The analysis of variance (ANOVA) method was performed using R Core Team (2018) software to further analyze the dependence between the results obtained for mechanical properties and fiber extraction area. In addition, Tukey’s test was performed to make a comparison of the mean values adopting a 95% confidence level (p < .05).

Results and discussion

Physical properties

The physical properties of the fibers studied, i.e. water absorption, density, diameter and linear density are presented in Table 2. Figure 2 shows the evolution of these physical properties in function of the extraction zone.

Figure 2. Influence of fiber extraction area on physical properties a) water absorption, b) density, c) linear density and d) diameter.

Table 2. Physical and mechanical properties of banana fibers from the nine extractions zones.

			Location of the fiber in the pseudo-trunk of the banana
			PA	PB	PC	IA	IB	IC	CA	CB	CC
Physical properties	Water absorption (%)	Mean	347.1	357.0	394.0	382.5	432.7	449.9	467.4	491.5	517.4
		σ	70.0	24.8	20.5	26.0	6.5	23.6	14.75	28.4	20.8
		CV	20.1	6.9	5.2	6.8	1.5	5.2	3.2	5.8	4.0
	Density (g.cm^−3)	Mean	0.917	0.919	0.983	0.925	1.002	1.002	1.065	1.140	1.145
		σ	0.089	0.014	0.055	0.025	0.017	0.025	0.089	0.014	0.016
		CV	9.7	1.5	5.6	2.7	1.7	2.4	8.9	1.2	1.4
	Diameter (µm)	Mean	116.7	156.7	183.3	203.3	206.7	213.3	223.3	243.3	246.6
		σ	5.8	5.8	60.3	35.1	37.9	25.2	37.9	37.9	20.8
		CV	5.0	3.7	32.3	17.3	18.3	11.8	16.9	15.6	8.4
	Linear density (tex)	Mean	8.3	14.0	16.0	18.0	19.3	19.8	22.3	31.6	34.0
		σ	0.7	2.8	3.2	0.4	1.1	1.1	2.8	0.7	1.1
		CV	8.4	20	20	22.2	5.7	5.5	12.5	2.2	3.2
Mechanical properties	Young modulus (GPa)	Mean	34.6	25.3	20.4	17.4	12.6	11.9	9.7	9.3	6.6
		σ	1.6	4 .2	3.3	3.0	1.5	3.4	2.4	2.6	1.7
		CV	4.6	16.6	16.7	17.2	11.9	28.6	24.7	27.9	25.7
	Tensile strength (MPa)	Mean	349.3	222.5	179.8	173.7	162.0	160.0	157.5	130.8	90.9
		σ	53	34	45	40	27	18	29	35	38
		CV	15.2	15.2	25.0	23.0	16.7	11.25	18.4	26.7	41.8
	Elongation at break (%)	Mean	0.92	0.91	0.88	1.52	1.07	1.35	0.99	1.89	2.61
		σ	0.0015	0.0015	0.0022	0.0039	0.0032	0.0023	0.0023	0.0033	0.0036
		CV	0.16	0.22	0.25	0.25	0.30	0.17	0.23	0.17	0.11

σ, Standard deviation ; CV, Coefficient of variation (%).

Water absorption

Banana fibers are hydrophilic and can absorb 3.5 to 5.2 times more water than their dry weight after 48 hours of immersion. This water absorption capacity is related to the presence of hemicellulose in plant fibers Betene et al., (2020) Water uptake increased continuously from the base to the head (347.1% to 467.4%) of the trunk and from the periphery to the core (347.1% to 449.9%) of the trunk (Figure 2a). This can be explained by a more efficient capillary effect from the top to the bottom of the trunk and from the core to the periphery of the trunk. Such a scenario was observed by Sikamé et al. (2014) for raffia vinifera fibers, and by Yilmaz et al. (2016) for corn leaf fibers. The water uptake values obtained in this study are in the wide range of values obtained for raffia vinifera fibers (Table 3). They are lower than the values obtained for fibers extracted from the upper zone of corn leaves (Yilmaz et al., 2016). However, they are higher than those obtained for fibers from the lower, middle and upper pineapple leaf (197–255%, Betené et al. 2022). This water absorption functionality may lead to embrittlement of the fiber and fiber/matrix interfaces in the composites intended for structural applications. Surface treatments could be considered to reduce the hydrophilic functionality of banana fibers. However, this water absorption capacity is beneficial for the development of functional textile yarns.

Table 3. Comparison of the water absorption values of the studied fibers with that of natural fibers in the literature.

Type of fiber	Water absorption (%)	Soaking duration	Temperature (°C)	References
Africa wood	102	25 days	25	(Khazaei, 2008)
Ojan lesh wood	54	25 days	25	(Khazaei, 2008)
Roosi wood	120	25 days	25	(Khazaei, 2008)
Hemp	62	13 h	27–67	(Saikia, 2010)
Okra	64	13 h	27–67	(Saikia, 2010)
Betel nut	38	13 h	27–67	(Saikia, 2010)
Raffia vinifera	303–662	25 days	23	(Sikamé et al., 2014)
Banana stem	347.1–517.4	48 h	25	(Present study)

Density

The density values (Table 2) obtained for the nine banana fibers are close to those available (1.02–1.35 g.cm⁻³) in the literature for fibers from the same banana source (Chengoué et al., 2020, Yilmaz et al. 2017, Indira et al., 2013). Furthermore, these values are lower than those reported for fibers commonly used for composite reinforcement such as sisal, flax, hemp and kenaf (1.2–1.5 g.cm⁻³, Chengoué et al. 2020, Ramesh et al. 2014). This can be attributed to the different plant species, and the 30-day extraction time which must have reduced the fiber mass by removing non-cellulosic material. According to Bezazi et al, (2014), the dissolution of amorphous materials that make up the fibers increases when the extraction time is extended. This difference can also be attributed to the test conditions (Ramesh et al. 2020). The highly volatile nature of benzene may lead to a slight underestimation of the mass of the fiber-benzene system. In addition, the porosity of the fiber is also a factor that can make it lighter (Sango et al. 2018, Yilmaz et al. 2017). The low standard deviations found for fibers from the same extraction area indicate a low dispersion of the density.

Diameter

Fibers diameters (Table 2) vary between 116. 7 μm and 246.7 µm. These values are similar to those found in literature (Biswal et al. 2011; Indira et al. 2013; Sango et al. 2018; Satyanarayana et al. 2007; Subramanya et al. 2017). The fibers of the zone PA have the smallest values of diameter among the others of the nine extraction zones (116.7 ± 20.8 μm) and the fibers of the CC zone has the highest one (246.7 ± 35.1 μm). This can be due to the fact that cells of PA (periphery 3/3) zone are more mature in the plant, tend to be thinner and the ones of CC (center- base three third), the youngest and coarser. Figure 2d presents the evolution of the diameters based on the extraction zone. It is observed that these diameters increase from the periphery toward the center and from the Base one-third toward the Base three third. This can be explained by the diameter growth mechanism of monocotyledon banana plant as explained earlier.

Linear density

The linear densities found (Table 1) range from 8.3 ± 1.1 tex to 34 ± 2.8 tex. The lower values in this range are very similar to those found by Efeze et al. (2020) for fibers of the same banana variety. The finest fibers were obtained in the lower peripheral zone (PA, 8.3 ± 0.7 tex), while the largest were in the upper central part (CC, 34 ± 1.1 tex). The lowest value is of the same order as found for raw fibers at the top of the pineapple leaf (8.0–8.4, Betené et al., 2022), but lower than sisal fiber (12–24, Efeze et al., 2020). This can make them fibers of choice for textile manufacturing in the same way as pineapple, jute and nettle fibers (Stawski et al. 2020). In addition, the median periphery (PB) and upper periphery (PC) fibers have linear densities in the same order as reported for the Turkish banana stem fibers in Yilmaz et al. (2017). Slight deviations may be associated with the location of the raw material harvest. The effect of the extraction area on the linear density is shown in Figure 2c. Linear densities increase from the periphery to the core (center) and from the bottom to the top. These variations are similar to those observed by Yilmaz et al. (2016) for corn and nettle leaf fibers (Singh et al. 2020). They also correspond well to the variations obtained for diameter (Figure 2d), water absorption (Figure 2a) and density (Figure 2b). This may be related to the growth of the plant, which causes the voids of mature areas to shrink into younger areas of the plant (Charlet et al. 2007, 2009, Sikamé et al. 2014, Yilmaz et al. 2016, Betené et al. 2022).

Tensile properties

The mechanical properties, namely the Young’s modulus, the breaking strength and the elongation at break of the fibers studied are reported in Table 2. Figure 3 shows the evolution of these mechanical properties in function of the extraction zone.

Figure 3. a) Typical stress-strain curves of banana pseudo-branch fibers. Influence of fiber extraction area on mechanical properties b) Young’s modulus, c) Tensile strength and d) Elongation at break.

Typical shape of the tensile curve

The typical stress-strain curves of obtained banana pseudo stem fibers (diameter between 116.7 and 246.6 μm) are shown in Figure 3(a). The fiber breaks suddenly and exhibits linear elastic behavior. This sharp fracture of the fiber is related to the low winding angle of microfibrils in banana fibers (Ramesh et al., 2014). According to Subramanya et al., (2017), upon stretching the fiber, its microfibers gradually align with the fiber axis and break abruptly if their winding angle is small. This type of behavior has already been observed by Sango et al., (2018) for banana trunk fibers and for other types of plant fibers such as okra (De Rosa et al., 2010), hemp (Duval et al., 2011) pineapple (Betené et al., 2022) for the pineapple leaf fiber. Young’s modulus is determined as the slope of the stress-strain curve.

Young’s modulus, tensile strength and elongation at break

The effect of the extraction zone on Young’s modulus, tensile strength and elongation at break is shown in Figures 3(b-d) respectively. The differences between fibers in the same area are less. The observed variations may be related to the variable morphology inherent in natural fibers (Yilmaz et al., 2017). It is only observed that the values found for Young’s modulus and tensile strength are higher for the fibers that were extracted in the peripheral areas. These values decrease progressively from the periphery to the core, and from the bottom (region A) to the top (region C) of the trunk. These variations are different from those observed for elongation at break (Figure 3c), which increases from the periphery to the core of the trunk for the middle fibers (Region B) and the top fibers (Region C°). For the bottom fibers (Region A), a peak of 1.5% is observed (Figure 3d) in the middle crown (I). The observed variations can be justified by the degree of maturity of monocot banana fibers, which gradually decreases from the bottom to the top and from the periphery to the core (Subramanya et al., 2017). This indicates that more mature fibers are richer in cellulose, justifying their high strength and stiffness. This result agrees well with those observed in the previous sections for diameter, linear density, and density. Interestingly, the variations in mechanical properties observed in these studies are similar to those observed for banana fibers by Efeze et al. (2020) and Stawski et al., 2020). However, in the longitudinal direction (from bottom to top), these variations are different from those observed for hemp stalk (Duval et al. 2011) and flax (Charlet et al., 2007), and pineapple leaf (Betené et al. 2022) fibers. This difference is possibly attributed to the origin of the plants.

A comparative study of the mechanical properties in this study with those of banana fiber in the literature is made in Table 4. The observed differences indicate that the properties of banana fiber depend on the harvesting area and the variety studied. This dependence may also be related to test conditions and parameters (Obame et al. 2022, Charlet et al., 2007). Moreover, the low elongation at break (0.9–2.6%) of the banana fibers in this study is slightly lower (average of 4.9 ± 1.2) than that obtained for the same fibers (of Turkish origin) by Yilmaz et al. (2017). This could limit their use in the field of spinning which requires the availability of supple fibers. Alkaline chemical treatments (Yilmaz et al. 2017) and bleaching with sodium hypochlorite (Betené et al. 2022) could improve suppleness function.

Table 4. Comparison of the mechanical property values of the studied fibers with that of banana fibers in the literature.

Young’s modulus (GPa)	Tensile strength (MPa)	Elongation at break (%)	Fiber diameter (µm)	Density (g.cm^−3)	References
13	161	1.27	167	-	(Subramanya et al., 2017)
27–32	700–800	2.5–3.7	10–34	-	(Satyanarayana et al., 2007)
29	540	3	80	-	(Indira et al., 2013)
29–32	500–600	1–3.5	125–250	1.35	(Biswal et al., 2011)
7	90	-	-	-	(Jordan and Chester, 2017)
5–26	200–750	1.5–4	40–140	-	(Sango et al., 2018)
-	816.6	2.83	-	1.02	(Chengoué et al., 2020)
6.6–34.6	91–343	0.88–2.61	117–247	0.91–1.14	(Present study)

In order to show an overall variation in the experimental data obtained, box plots have been drawn in Figure 4 for each mechanical property as a function of fiber location. It is interesting to note that the mean values (Table 2) of the mechanical properties are very close to the median value, which itself lies in the rectangular area of the box. This result indicates that about 50% of the values in each area are close to the average value found. In addition, the trends observed in Figure 3 for Young’s modulus and tensile strength in the longitudinal direction (from bottom to top) corroborate those in Figure 4. It can be seen that the Young’s modulus box (Figure 4a) and the tensile strength box (Figure 4b) associated with the fibers in the PA zone are located higher, while the first quartile of the CC zone boxes for these two properties is located lower. This arrangement confirms that the largest values of Young’s modulus and tensile strength were obtained for the PA fibers, while the smallest values are found in at least 25% of the CC zone fibers. It should also be noted that about 50% of the elongation at break values (Figure 4c) of the CC fibers are higher. Although a clear trend was observed for the mean values (Figure 3) in the transverse direction (periphery to heart), a clear trend did not emerge for the statistical series of all mechanical properties. Usually, the interquartile range (length of the rectangle) gives indications of the homogeneity of the statistical series studied, and the lower its value, the more homogeneous the series (Ndako et al. 2020, Moeini et al. 2021). This shows that the fiber series of (i) CC of Young’s modulus, (ii) CA of tensile strength and (iii) PA of elongation at break are more homogeneous.

Figure 4. Box plots of (a) Young’s modulus, (b) Tensile Strength and (c) Elongation at break.

Overall, the physical (density, water absorption, linear density, and diameter) and mechanical (Young’s modulus, strength at break, and elongation at break) properties of the nine banana fibers are relatively close to those obtained for fibers available in the open literature (Ramesh et al. 2014, Sango et al. 2018, Chengoué et al. 2020). Furthermore, their average mechanical properties combined with their low density make them a promising bio sourced reinforcement for composite development. Due to their low density and high tensile strength and Young’s modulus, fibers in peripheral areas (bottom to top of trunk: PA>PB>PC) are believed to provide better reinforcing effect by compared to those of the intermediate zones (IA>IB >IC) and the central zones (CA>CB>CC).

ANOVA statistical analysis

The results of the descriptive statistics (Table 2) indicate that there are differences in the mean values of Young’s modulus, tensile strength, and elongation at break depending on the location of the fiber in the pseudo-trunk of banana. After verifying (Sig.>0.05) that the variances are homogeneous with Levene’s test (Sudha Mishra and Mohapatra 2022), a one-way ANOVA was performed with a 95% confidence level to determine the significance level of the observed differences. Thus, we can observe (Table 5) that the location of the fibers significantly affects the Young’s modulus (p = 2.04 × 10⁻⁶<0.05), the tensile strength (p = 3.11 × 10⁻⁴<0.05) and the elongation at break (p = 1.88 × 10⁻⁷<0.05). However, the ANOVA does not reveal from which area of the sample this difference exists.

Table 5. Results of the variance testing (ANOVA) made on the mechanical properties of the studied fibers.

Mechanical properties	Source variation	Degree of freedom	Sum square	Mean square	F value	Significance value
Young’s modulus	Extraction zone	8	8.98 × 10^9	1.12 × 10^9	160.2	2.04 × 10^−6
	Residue	156	1.09 × 10^9	7.01 × 10^6
Tensile strength	Extraction zone	8	6.53 × 10^5	8.17 × 10^4	64.57	3.11 × 10^−4
	Residue	156	1.97 × 10^5	1.27 × 10^3
Elongation at break	Extraction zone	8	6.53 × 10^5	8.17 × 10^4	6.526	1.88 × 10^−7
	Residue	156	1.97 × 10^5	1.27 × 10^3

Number of fibers per zone: PA-10, PB-14, PC-13, IA-23, IB-22, IC-14, CA-23, CB-21 and CC-25.

To clearly show the differences between the areas, the Tukey posthoc test was performed with a significance level of 5% (Jamshaid et al. 2021, Belaadi et al. 2020). For this multiple comparison test, only the results for tensile strength and Young’s modulus with positive mean differences (I-J) were reported in Table 6. From one pair (I, J) to another, it can be observed that the Sig. or p-values are different. A similar result was obtained for flax fibers (Belaadi et al. 2020). Sig. values below 0.05 indicate a significant difference between the two zones compared, while those above 0.05 suggest that the difference is not significant for both zones. On the other hand, the positive result I-J indicates that the average value of zone I is higher than that of zone J. For tensile strength, the pairwise comparison of the different zones shows that there is no statistically significant difference between the fibers in zone PB and IC, IC and IA, IC and PC, CC and IB, and CA and IB, as the Sig. value of these pairs is above the 5% significance level. Similarly, for Young’s modulus, no statistically significant difference was obtained for the fibers in zone IC and IA, IA and CA, CA and CB, and CB and IB. However, the difference in elongation at break for the CC zone was significant (Sig. value less than 0.05) compared to all other fibers, except for the CA fiber with a Sig. value of 0.22 greater than 0.05.

Table 6. Tukey’s test: statistical quantification of the level of significant difference in tensile strength and Young’s modulus as a function of fiber location in the pseudo-banana trunk.

Tensile strength (MPa)
Location of the fiber		Mean Difference (I-J)	Sig.
(I)	(J)
PA	CA	217.3	0.0000
	CB	283.5	0.0000
	CC	214.3	0.0000
	IA	197.9	0.0000
	IB	243.2	0.0000
	IC	169.7	0.0000
	PB	145.8	0.0000
	PC	199.1	0.0000
PB	CA	71.6	0.0000
	CB	137.8	0.0000
	CC	68.6	0.0000
	IA	52.1	0.0009
	IB	97.4	0.0000
	IC	23.9*	0.6943
	PC	53.3	0.0045
IC	CA	47.6	0.0037
	CB	113.8	0.0000
	CC	44.6	0.0073
	IA	28.2*	0.3274
	IB	73.5	0.0000
	PC	29.4*	0.4472
IA	CA	19.5*	0.6450
	CB	85.7	0.0000
	CC	16.4*	0.8036
	IB	45.3	0.0011
	PC	1.2*	1.0000
PC	CA	18.2*	0.8639
	CB	84.5	0.0000
	CC	15.2*	0.9433
	IB	44.1*	0.0149
CC	CA	3.0*	1.0000
	CB	69.2	0.0000
	IB	28.9*	0.1311
CA	CB	66.2	0.0000
	IB	25.8*	0.2721
IB	CB	40.4	0.0082
Young’s modulus (GPa)
Location of the fiber		Mean Difference (I-J)	Sig.
(I)	(J)
PA	CA	22.7	0.0000
	CB	24.9	0.0000
	CC	28.0	0.0000
	IA	21.9	0.0000
	IB	25.5	0.0000
	IC	19.8	0.0000
	PB	9.3	0.0000
	PC	14.2	0.0000
PB	CA	13.3	0.0000
	CB	15.6	0.0000
	CC	18.7	0.0000
	IA	12.5	0.0000
	IB	16.1	0.0000
	IC	10.4	0.0000
	PC	4.9	0.0001
PC	CA	8.5	0.0000
	CB	10.7	0.0000
	CC	13.8	0.0000
	IA	7.7	0.0000
	IB	11.3	0.0000
	IC	5.6	0.0000
IC	CA	2.9	0.0411
	CB	5.1	0.0000
	CC	8.2	0.0000
	IA	2.1*	0.3347
	IB	5.7	0.0000
IA	CA	0.8*	0.9830
	CB	3.1	0.0057
	CC	6.2	0.0000
	IB	3.6	0.0003
CA	CB	2.3*	0.1162
	CC	5.4	0.0000
	IB	2.8	0.0140
CB	CC	3.1	0.0037
	IB	0.6*	0.9989
IB	CC	2.5	0.0335

*No significant difference.

Overall, fiber location contributes significantly to the dispersion of tensile strength and Young’s modulus of banana pseudo-trunk fibers. Because of this strong dependence, sampling as described in this study is a solution for classifying banana fibers to select them for specific structural applications.

Conclusion

An original sampling of the banana pseudo-stem was performed to analyze the influence of fiber location on their physical and mechanical properties. Considering three regions A (bottom), B (middle) and C (top) in the cross direction of the trunk, fibers were extracted by wet retting in three crowns P (periphery), I (middle) and C (core). The nine fiber types obtained were subjected to standardized gravimetric and tensile tests. The ANOVA and Tukey’s test results revealed that fiber location in the banana pseudo-trunk significantly affects Young’s modulus, tensile strength and elongation at break. In addition, density, linear density, water absorption, fiber diameter and elongation at break have been shown to increase from the periphery to the core and from the bottom to the top of the pseudo-trunk. In contrast, tensile strength and Young’s modulus decrease from the periphery to the core. Some variations were also observed in the fibers within the same zone. This was mainly attributed to the variation in their morphology inherent in natural fibers. Although significant variations were observed in different areas of the sample, it is interesting to note that the properties determined in this study are relatively very close to those of fibers in the literature such as sisal, pineapple, hemp and kenaf. For this reason, banana pseudo-branch can be used as a whole for the production of reinforcing fibers. However, if a better reinforcing effect is desired, it is recommended to select the fibers from the periphery due to their lower density, and higher stiffness and tensile strength.

Highlights

• Biaxial sampling of 3 × 3 from the base to the top and from the periphery to the heart of the banana pseudostem to distinguish its fibers.

• Increased physical properties and elongation at break of the fibers from the periphery to the heart and from the base to the top of the pseudo-trunk.

• Decreased tensile strength and Young’s modulus of fibers from the periphery to the heart and from the base to the top of the pseudotrunk.

• ANOVA and Tukey’s test present significant differences between the mechanical properties of fibers extracted from different locations in the pseudotrunk.

• The results show that the physical and mechanical properties of the fibers in the peripheral zone of the banana pseudotrunk make them relevant biosourced reinforcements for structural applications.

Acknowledgements

The authors would like to acknowledge Pr Alexandre Teplaira Boum for the assistance in data analysis and verification of mechanical characteristics. The authors also thank the LAMMA laboratory of the ENSET of Douala and the CECAM technical center for the technical support in the physical and mechanical characterization of the studied fibers.

Disclosure statement

The authors have no financial or proprietary interests in any material discussed in this article.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/15440478.2023.2204451

Additional information

Funding

No funds, grants or other support was received