Introduction

Today, production from environmentally friendly, and sustainable raw material sources is gaining importance (Gedik 2021; Chichane et al. 2023; Eryilmaz et al. 2023a; Ovalı and Sancak 2020b). Since the textile industry is an industry that has a negative environmental impact on water, air, and soil, sustainable production in harmony with nature is required to eliminate this negativity. Therefore, the need for new sustainable raw materials in the textile industry is increasing (Gedik 2021).

Manufacturers felt the need to shift from using synthetic fiber-reinforced composites to bio-composite green materials as a result of the global green movement, which also included hydrogen energy. (Eryilmaz et al. 2023b). Therefore, studies on textile and composite fibers, which can be obtained quickly in different industries and used as raw materials, are gaining momentum (Satyanarayana et al. 2009; Yildiz and Eryilmaz 2023; Eryilmaz and Sancak 2021). Cellulosic-based textiles and natural fibers have superior properties such as biodegradability, lightweight, and ability to form effective hydrogen bonds along cellulose chains or within other polymer matrices (Väisänen et al. 2017; Han et al. 2023; Madueke et al. 2023; Sahu et al. 2023). It is therefore important to understand both the physical and chemical structure and properties of new natural fibers (Rajkumar et al. 2016; Saravana Kumaar et al. 2019; Celebi et al. 2022).

The majority of the chemical structure of natural fibers have a hydrophilic structure consisting of cellulose, hemicellulose, which is transparent and colorless, and lignin, which is opaque and dark brown (Han et al. 2023; Udhayakumar et al. 2023; Altun et al. 2021). Fibers can be easily obtained from seeds, roots, stems, leaves, and fruits of plants. The geological characteristics of the regions where the fibers are obtained determine the structure of the fiber. The percentage of cellulose in the fiber structure determines the strength of the structure (Thakur et al. 2014; Ovalı and Sancak 2020a). There is a lot of research on natural fibers such as bananas, palm, bamboo, pineapple, coconut, etc. (Adeniyi et al. 2019). In addition to that Leucaena Leucocephala Tree (Gopinath et al. 2023), Yucca aloifolia L. Leaf (Nascimento et al. 2021), Trachelospermum jasminoides (Gedik 2021), Hierochloe odarata (Dalmis et al. 2020b), Chrysanthemum morifolium (Dalmis et al. 2020a), Carica papaya bark (Saravana Kumaar et al. 2019), Cereus Hildmannianus (Subramanian et al. 2021) such as new fibers have been investigated. With these studies, the use of cellulosic-based fiber-reinforced composites in many fields of industry (aerospace, automotive, railway, food packaging, sports, military, etc.) has been examined (Selvakumar and Meenakshisundaram 2019; Rajak et al. 2019; Sydow and Bieńczak 2019; Jaiswal et al. 2022).

Due to the increasing need for new sustainable fibers, in this study the structural properties of Sorghum vulgare var. Technicus (Sorghum bicolor fiber – SBF) was characterized. Sorghum is one of the four main varieties of sorghum belonging to the Poaceae family and is a cereal crop valued for its grain, stems, and leaves. The International Water Management Institute (IWMI) predicts that by 2025, 25% of the world’s population will experience severe water scarcity, increasing the value of drought-resistant crops such as sorghum (Kumari et al. 2016). The Sorghum bicolor plant originates from central Europe, eastern Europe, and Asia. It is an annual, erect, deep-rooted plant growing 1–1.5 m tall. It is found as a weed in agricultural areas, gardens, and roadsides. The sorghum plant used in the study was obtained from the Adana/Sarıçam region in Türkiye. Sorghum cultivation covers 41.57 million hectares in the tropical region where it originated and also in the temperate zone. Sorghum bicolor forms show wide variation in morphological and physiological characteristics and ecological requirements. “Sorghum vulgare var. Technicus”, this plant, whose panicle branches are almost equal in length and woody in structure, and whose fibers are resistant to breaking, is used primarily for making brooms in many countries. With a growing period ranging between 90 and 120 days. It is a plant that is resistant to drought, can easily grow in places with an annual rainfall of 450–600 mm, and can adapt to places with low summer rainfall and high temperatures (Bakari et al. 2023). Additionally, Sorghum (Sorghum bicolor L.), has the potential to become lignocellulosic biomass for the production of industrial materials such as alcohol, fuel, bioplastics, and reinforcing fiber for composite structures and textiles.

To reveal the morphological structure of sorghum, X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) tests were used to establish the chemical structure of Sorghum bicolor fiber. To define the physical characteristics, density, and mechanical tests were also carried out.

Materials and methods

Fiber extraction of SBF

In this study, the wild Sorghum bicolor plant, which grows from May to July in the mountainous hills of Sarıçam district of Adana province in Türkiye’s Mediterranean region, was studied for its stem fiber. Approximately 50–100 cm tall plants were collected, and the leaves and seeds were cut and removed. The stem parts of the Sorghum bicolor were placed in a 50 cm bucket with only water and kept for two weeks. At this point, the bark began to be affected by microorganisms, causing the dissolution of the chemical bonds holding the bark layers together and the separation of interior fibers from the outer bark layer. In the next stage, the fibers were left to dry in daylight for 48 h, removing the moisture inside. Figure 1 displays the Sorghum bicolor plant and the fibers obtained from the stem part.

Fig. 1
figure 1

a Wild sorghum bicolor plant; b sorghum bicolor harvested from the field; c extracted sorghum bicolor fibers

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Characterization methods

Physical property analysis of SBF 

A tuft of SBF sample was used to randomly select 100 fibers in order to calculate the mean fiber length. After straightening each fiber, a ruler was used to determine its length. The measured SBF length was used to calculate the mean, standard deviation (SD), and coefficient of variation (CV). The density of Sorghum bicolor fiber was determined according to Archimedes’ law ASTM D8171-18 method.

Chemical analysis

The main chemical constituents (cellulose, hemicellulose, lignin, moisture) of the fibers extracted from Sorghum bicolor were determined. The fiber samples were ground into a powder before being subjected to chemical analysis, and they were subsequently dried for four hours at 100 °C in the oven. After the dried fibers were pulverized, the analysis was performed according to the Mylsamy and Rajendran method (Dalmis et al. 2020b). The weight-loss method was used to determine the moisture content of the fiber. Fibers were heated in oven to 105 °C for 48 h. After removing the fibers from the oven, they were weighed, and the moisture content was ascertained (Sarala 2020).

Fourier transform infrared (FTIR) analysis

The Perkin Elmer Spectrum BX was used to get the FTIR spectra of Sorghum bicolor fiber. With a signal-to-noise ratio resolution of 2 cm−1 and a scan rate of 32 scans per minute, infrared spectra in the range of 4000 cm−1 to 400 cm−1 were captured.

Thermogravimetric analysis

The thermal stability of Sorghum bicolor fiber was measured by the Seiko SII TG/DTA 7200 device. Samples weighing 5 mg in nitrogen atmosphere were heated from 30 °C to 700 °C at a heating rate of 10 °C/min.

X-ray photoelectron spectroscopy analysis (XPS)

The Sorghum bicolor fiber’s elemental composition was assessed through X-ray photoelectron spectroscopy, employing a focused beam of 250 µm diameter from a monochromatic Al-Ka X-ray source with energy 1486.7 eV. XPS measurements spanned an energy range of 1361 to 10 eV, utilizing a pass energy of 200 eV and achieving 1 eV resolution. Prior to surface analysis, ionic Ar gas sputtering was performed, followed by 20 scans from a single point to acquire the data.

X-ray diffraction (XRD) analysis

The crystalline structure of Sorghum bicolor fiber was determined by using Rigaku miniflex600 device and the crystallinity index (CI) was calculated as a result of the data generated by the reflection of the X-Ray diffraction (XRD) pattern of the fiber. Before the measurement process, the fibers were pulverized, and the moisture was removed at 105 °C for 24 h. Cu-Kα radiation (λ-Kα = 1.54 Å) was the source of X-ray and the device worked with 40 kV and 15 mA current. The analyses were conducted with a total duration of 67 min, applying a scanning width of 2θ = 5–55 degrees for each sample with scanning at a rate of 0.75° per minute. The crystallinity index was calculated using the formula determined by the Segal equation given below (Segal et al. 1959).

$$CI\left( \% \right) = \frac{{\left( {I_{200} - I_{am} } \right)}}{{I_{200} }}x100$$
(1)

Iam denotes minimum intensity value at 2θ = 18.25°, while I200 relates to the peak with the maximum intensity, related to the (2 0 0) lattice plane at 2θ = 22.13° (French 2014).

Single fiber tensile test of SBF

SBFs were subjected to 30 repetitive tensile tests at a processing speed of 2 mm/min on 10 mm long fibers in the INSTRON 4411 tester shown in Fig. 2 according to ASTM D3822 standards (Ovalı 2023).

Fig. 2
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Tensile test of SBF

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Morphological characterization of SBF

Using a high-resolution SEM (ZEISS/EVO LS10), the surface morphology and fiber diameter of Sorghum bicolor fibers were examined. The specimens were coated with Au to shield them from the electron beam charging during the examination. For the SEM study, an electron beam acceleration of 3 kV was employed.

Results and discussions

Physical property of SBF

The two key elements of any staple fiber that affect its spinnability are its length and fineness. SBF has a length range of 40 to 130 mm and an average length of 62.1 mm. Compared to flax fibers (60–90 mm), it can be deduced that SBF has an average length in a similar range (Pisupati et al. 2021). The measured fibers' SD and CV are 14.88 mm and 23.96%, respectively. SBF has an average diameter of 267.52 micrometers, which falls within the range of flax fiber (40–500 microns) (Bar and Chaudhary 2023). The density of Sorghum bicolor fiber was determined as 1.4525 g/cm3 according to the mentioned standard. Density values of Sorghum bicolor and other fibers are given in Table 1.

Table 1 Density values of SBF and other natural fibers
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When the values in the table are examined, Sorghum bicolor fiber has a density higher than bamboo fiber, similar to sisal and jute fibers, and lower than abaca, ramie, and flax fibers. For this reason, it is advantageous for usage areas where low fiber density is required.

Chemical composition of SBF

Knowing the chemical components of fibers is important because it affects the changes that will occur in the face of heat and mechanical behaviors (Albayrak et al. 2022). Sorghum bicolor fiber consists of 73.6% cellulose, 13.3% hemicellulose, and 12.1% lignin. The moisture content of SBF was measured at 4.26%.

Table 2 shows the cellulose content of SBF and other natural fibers. Sorghum bicolor fiber has a higher cellulose content than other cellulosic-based fibers which are Chrysanthemum morifolium 32.9% (Dalmis et al. 2020a), Conium maculatuma, 49% (Kılınç et al. 2018), Agave americana 68.4% (Mylsamy and Rajendran 2010), Hierochloe Odarata 70.4% (Dalmis et al. 2020b), Linden 61.8% (Seki et al. 2016). The hemicellulose content of SBF (12.1%) is comparable with Chrysanthemum morifolium 13.8% (Dalmis et al. 2020a) Jute 12.6% fibers. It is seen that the SBF lignin ratio is higher than the natural fibers (except Linden fiber) in Table 2 (Seki et al. 2016). The chemical structure of the fiber, hemicellulose, and lignin ratios in the fiber affect the properties such as moisture absorption, thermal degradation, etc. These different cellulose ratios vary based on factors such as plant species, geographical location, and climate of the cultivation area. Although all cellulosic-based fibers share the same fundamental chemical components, the diversity in their ratios results in varied properties and behaviors in the fibers (Feleke et al. 2023).

Table 2 The cellulose content of SBF and other natural fibers
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XPS analysis of SBF

The chemical structure and surface formation of Sorghum bicolor fiber were investigated using the XPS analysis method. The atomic percentages of the elemental compositions forming the Sorghum bicolor fiber surface are given in Table 3.

Table 3 Elemental constituents of SBF
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The main component ratios of the fiber surface were calculated as carbon 58.02%, oxygen 24.74%, nitrogen 2.23%, and silicium 2.34%. Oxygen/Carbon (O/C) and Carbon/Oxygen (C/O) ratios were calculated using asymmetrically centered C (C1s) and O (O1s) signal data from the XPS spectrum to determine whether the fiber surface has hydrophilic or hygroscopic character. From these data, the oxygen/carbon (O/C) and carbon/oxygen (C/O) ratios of Sorghum bicolor fiber were determined as 0.43 and 2.34, respectively, and compared with other cellulosic-based fibers. Since the O/C ratio of Sorghum bicolor fiber was higher than Linden (0.13) (Seki et al. 2016), Flax (0.16) (Csiszár et al. 2013), Conium maculatum (0.21) (Kılınç et al. 2018), Althaea officinalis L. (0.26) (Sarikanat et al. 2014), Hemp (0.27) (Sgriccia et al. 2008), it was found that SBF of the surface hydrophilicity was higher. On the other hand, Sorghum bicolor fiber (2.34) has a higher (C/O) ratio than Centaurea solstitialis (1.83) (Keskin et al. 2020) and jute (2.09) (Sgriccia et al. 2008) fibers and therefore it was found to have a more hydrophobic character. Figure 3 shows (a) C1s and (b) O1s XPS spectra peaks of Sorghum bicolor fiber. The natural functional groups on the fiber surface were identified using the C1s and O1s XPS spectra peaks. Peaks at 285.23, 286.56, and 288.96 eV indicate C–C/C–H, C–O–C, and O–C=O functional groups, respectively (Pandey et al. 2020). The main peak at 285.23 eV, indicating the presence of cellulose or ether, is the most prominent peak representing C–C, and C–H bonds (Pandey et al. 2020). A maximum peak representing C–C/C–C–H and C–O–C bonds may also indicate the presence of cellulose in the fiber (Albayrak et al. 2022).

Fig. 3
figure 3

a C1s and b O1s XPS spectrum peaks of SBF fibers

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In addition, the peak at 532.25 eV represents the C–O–C bond as the cellulose or cyanoethyl cellulose peak and significantly indicates the presence of cellulose in the fiber (Albayrak et al. 2022; Pandey et al. 2020).

Fourier transform infrared spectrum of SBF

The FTIR spectrum of cellulosic-based Sorghum bicolor fiber (SBF) to determine the functional groups and components in its structure is shown in Fig. 4. The absorption peaks of a typical cellulosic FTIR spectrum in Fig. 4 provide information about the functional groups of fiber components such as cellulose, hemicellulose, and lignin (Sukmawan et al. 2016). The vibration in the 3336 cm−1 absorption band is related to the hydrogen bonding and hydroxyl groups of the fibers, and this peak corresponds to cellulose and hemicellulose (Indran and Raj 2015; Porras et al. 2015). The peak at 2924 cm−1 may correspond to C–H stretching vibrations of the methylene group (Indran and Raj 2015), the peak at 1736 cm−1 may correspond to C=O stretching vibrations of carbonyl groups in hemicellulose, and the small peak at 1639 cm−1 may correspond to the liquid absorbed in the fiber (De Rosa et al. 2010; Indran and Raj 2015). The peak at 1510 cm−1 reflects the C=C stretching of benzene rings in lignin (Fackler et al. 2011). The peak at 1422 cm−1 corresponds to the bending vibration of –CH2 in cellulose. The peaks at 1374 and 1326 cm−1 are related to the C–H asymmetric deformation of cellulose and –OH bending vibrations (Albayrak et al. 2022). The peak at 1236 cm−1 corresponds to the C–O bending vibrations of acetyl groups. The peak at 1026 cm−1 is the general major peak of cellulosic structures and reflects the C–O stretching vibration in cellulose. The small peak at 899 cm−1 is associated with β-glycosidic bonds in the structure (Synytsya and Novak 2014).

Fig. 4
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FTIR spectrum of SBF

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XRD analysis of SBF

The graph in Fig. 5 shows XRD pattern. The peak at 2θ = 16.34° is composed of both the (1 − 1 0) and (1 1 0) which are overlapped and the peak at 2θ = 22.13° (2 0 0) indicates cellulose I characteristics for most of the cellulose-based fibers (French 2014). It is important to note that, the (2 0 0) peak includes intensity from both the amorphous and crystalline regions (Segal et al. 1959). The Segal equation assumes a horizontal line at the Iam height going across the 200 peak, and the intensity above that line is considered the crystalline contribution. Segal’s method is attractive to calculate the crystallinity index crystallinity index (CI) due to its simplicity. However, it is now recognized that the crystallinity index (CI) determined by the Segal peak height method is significantly influenced by the varying sizes of the small crystallites of cellulose samples (French and Cintrón 2013).

Fig. 5
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XRD curve of SBF

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Using the data obtained from the analysis of SBF, the crystallinity index (CI) was calculated as 59.34% (Eq. 1). It has a similar crystalline index value to Furcraea foetida (52.6%), Sansevieria ehrenbergii (52.27%) (Zhang et al. 2019), and sisal (57%) (Baskaran et al. 2018). Also, SBF has higher CI value than oil palm fruit fiber (34.1%) (Amroune et al. 2015), Juncus effuses fiber (33.4%) (Raju et al. 2022), Calotropis gigantea (36%) (Narayanasamy et al. 2020), Vachellia farnesiana (13%) (Vijay et al. 2022), Ptychosperma macarthurii (42%) (Loganathan et al. 2022), and Thespesia populnea (48%) (Kathirselvam et al. 2019). High CI value contributes positively to thermal resistance and mechanical properties as a result of the more regular alignment of cellulosic chains (Albayrak et al. 2022).

Thermogravimetric analysis of SBF

It is important to measure the changes that occur in cellulosic-based natural fiber structures against heat. Determining the initial and maximum degradation temperatures of fibers that will be used as reinforcing elements in composites and will be exposed to high temperatures can be advantageous in the production phase (Albayrak et al. 2022; Sahayaraj and Jenish 2023). Figure 6 shows the thermal gravimetric analysis (TGA) and differential thermogravimetry (DTG) curve of Sorghum bicolor fiber. The first thermal decomposition slope is near 100 °C with a mass decrease at 63.17 °C indicating water loss (Tessema et al. 2023). The second degradation, mass loss at 297.38 °C, occurred due to the degradation of both hemicellulose and low proportions of lignin and cellulose (Sahayaraj and Jenish 2023). At the same time, this peak point shows the temperature value at which the cellulose in the fiber structure starts to degrade (Tonset). The highest mass loss due to cellulose degradation occurred between 320 and 400 °C. Also, this peak at 364.22 °C in this temperature range represents the maximum degradation temperature (Tmax) (Albayrak et al. 2022; Sahayaraj and Jenish 2023). When the DTG peaks of Centaurea Solstitialis 360 °C (Keskin et al. 2020), Kenaf 364 °C (Poletto et al. 2015), jute 365 °C (Alvarez et al. 2006) fibers were compared, SBF showed similar results.

Fig. 6
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TGA and DTG curve of SBF

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In addition, when Tmax values were compared with Acacia Leucophloea (346.8 °C) Acacia nilotica (339 °C) (Kumar et al. 2022), Moringa Oleifera (343.5 °C) (Bharath et al. 2020) fibers, it was determined that the thermal resistance of SBF was higher. The analysis process was terminated at 610 °C and approximately 20% ash residue was obtained as a result of the degradation of the fibers. According to the TGA analysis data of SBF, its heat resistance was determined as 297.38 °C.

Morphological characterizations of SBF

Figure 7 shows SEM images depicting the morphology of SBF, showcasing longitudinal and cross-sectional views with magnification of 100X, 500X, and 1000X, respectively. As seen in the longitudinal section image of the fiber in Fig. 7 (a, b), the fiber diameter was calculated as approximately 267.52 ± 3.94 μm, derived from averaging measurements taken from various locations along its length.

Fig. 7
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a, b Longitudinal, and c, d cross-sectional SEM images of the SBF

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Examination of the longitudinal cross-sectional images unveils the presence of both cellulosic and non-cellulosic impurities, a characteristic feature observed in typical cellulosic-based natural fibers (Akintayo et al. 2016). Furthermore, it was observed that the fiber’s cross-section comprises voids and cavities. Therefore, fibers with high surface roughness are better bonded to the polymer (Alavudeen et al. 2015; Indran and Raj 2015). It is seen that the surface structure of the Sorghum bicolor fiber shown in Fig. 7 (a, c, d) is rough. The fiber’s surface structure’s roughness may be impacted by these cavities and voids. The bonding of the fiber/matrix interface is dependent on the surface roughness of the reinforcing fibers used in composite structures. Additionally, the presence of an uneven, rough surface structure may promote good resin penetration into the fibers, thereby enhancing the adhesion of the fibers to the matrix in a composite. The strength of composite laminates is consequently attributed to this enhanced adhesion (Sarala 2020; Shanmugasundaram et al. 2018).

Tensile properties of SBF

Examination of recent studies on cellulosic-based fibers reveals a range of results in mechanical tests applied to the fibers. This variability is directly influenced by the chemical and physical structure of the fiber, environmental conditions affecting this structure, the testing environment, and the application method, including the fiber diameter (Khui et al. 2023; Ovalı 2023). The diameter and mechanical properties of SBF and other natural fibers are compared in Table 4.

Table 4 Diameter and mechanical properties of SBF and other natural fibers
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The tensile strength value of SBF (284.65 MPa) is close to that of hemp fiber (285 MPa) (Maache et al. 2017), but higher than Tridax procumbens (25.7 5 MPa) (Vijay et al. 2019), Manicaria saccifera palm (72.09 MPa) (Porras et al. 2015), Agave (132 MPa) (Bezazi et al. 2014). SBF has a modulus value 4.71 (GPa); similar to Juncus effecus L. (4.38 GPa) (Maache et al. 2017), Phoenix dactylifera L (4.3 GPa) (Amroune et al. 2015), and high modulus values were obtained when it is compared to Banyan tree root (1.8 GPa) (Ganapathy et al. 2019), Tridax procumbens (0.94 GPa) (Vijay et al. 2019). High cellulose content is known to provide better tensile strength and modulus values due to the specific characteristics of cellulose such as a high degree of polymerization and linear orientation (Baskaran et al. 2018). In addition, the mechanical properties of fiber-reinforced composites depend on the fiber-matrix surface compatibility, the distribution of the fiber in the composite, the mechanical properties of the polymer, and the mechanical properties of the reinforcing fiber (Dalmis et al. 2020a).

Conclusions

The seeds of the Sorghum bicolor plant have been studied by researchers in different fields of study, including medicine, due to its natural content. In this study, the fiber obtained naturally from the stem of the Sorghum bicolor plant was characterized and its usability in composite and textile was investigated. As a result of the structural analysis, it was determined that Sorghum bicolor fiber has a density of 1.45 g/cm3, 73.6% cellulose content, and 55.7% crystallinity index. With XPS analysis, it was determined that it has a high C/O ratio, and its high hydrophobic character will positively affect the fiber-matrix adhesion in composite formation. The FTIR results of Sorghum bicolor fiber were found to be similar to the typical results of cellulosic-based fibers and their chemical compositions were confirmed. The initial decomposition temperature of 297.38 °C and the maximum decomposition temperature of 364.2 °C was measured by TGA analysis, indicating that it can be used in composites at relatively high temperatures. When the morphological structure of the SBF is examined, the roughness of the surface without chemical treatment indicates that the polymer and fiber surfaces are more compatible with composites. Mechanical test results suggest that sorghum bicolor fiber was determined to be similar to the commonly used natural stem fiber. These characteristics can make SBF a promising supply of cellulosic fiber for a variety of industries including the fiber-reinforced composite sector which is well-known for its quick production and consumption rates, particularly in applications where lightweight is crucial. On the other hand, the industry uses a lot of synthetic fibers that are utilized such as carbon and glass fibers, which has a considerable environmental effect and negative impact on sustainability. At the same time, SBF fibers can be also utilized in textile industry combining with other synthetic ones which can reduce the consumption of petroleum-based fibers. In addition, SBF can be used as a blended yarn due to its similar properties to flax in terms of fiber length and fineness. Composite industry chains can be formed to extract the roots of the SBF, which can be grown abundantly and used to produce polymer composites for a variety of industrial applications. Therefore, low-cost SBF might benefit society and the environment without depleting global resources.