Influence of the Sampling Area on the Chemical, Physical, and Mechanical Properties of Grewia bicolor(GB) Fiber for Potential Use in the Reinforcement of Tannin Matrix Composites

ABSTRACT

The aim of this study is to chemically, physically, and mechanically characterize Grewia bicolor (GB) fibers harvested in the NGONG locality located in the North Cameroon region for potential use in the elaboration of tannin matrix composites. The fibers are extracted from three zones of the stem (top (T), middle (M), and bottom (B)) by biological retting with stagnant water, and the various standard norms are respected for different tests. Analysis by Fourier transform infrared spectroscopy (FTIR) and chemical composition show that GB fibers consist mainly of cellulose (50.73–53.67%), hemicellulose (20.95–24.73%), and lignin (14.26–17.26%). Physical properties give: true density (1.33–1.35 g/cm³), bulk density (0.61–0.72 g/cm³), porosity (48.37–51.4%), linear density (17.28–29.44 tex), water content (13.16–13.59%), water absorption rate (121.86–134.54%), and a moisture recovery rate at 75% relative humidity (11.12–11.76%). Mechanical parameters give Young’s modulus (17.21–29.56 GPa), stress at break (504.71–936.09 MPa), and strain at break (4.65–5.37%). Analysis of variance (ANOVA) combined with Tukey’s test for mechanical properties show that the bottom and middle fibers are identical at the 95% confidence level. GB fibers can be used in the reinforcement of tannin matrix composites with preferable and indiscriminate sampling in the bottom and middle zones of the stem.

摘要

本研究的目的是对在位于喀麦隆北部地区的NGONG地区收获的Grewia双色（GB）纤维进行化学、物理和机械表征，以潜在地用于制备单宁基复合材料. 通过用死水生物脱胶，从茎的三个区域（顶部（T）、中间（M）和底部（B））提取纤维，并在不同的测试中遵守各种标准规范. 傅立叶变换红外光谱（FTIR）和化学成分分析表明，GB纤维主要由纤维素（50.73–53.67%）、半纤维素（20.95–24.73%）和木质素（14.26–17.26%）组成. 物理性质给出: 真实密度（1.33–1.35 g/cm3）、体积密度（0.61–0.72 g/cm3）、孔隙率（48.37–51.4%）、线密度（17.28–29.44 tex）、含水量（13.16–13.59%）、吸水率（121.86–134.54%）和75%相对湿度下的水分回收率（11.12–11.76%）. 力学参数给出了杨氏模量（17.21–29.56 GPa）、断裂应力（504.71–936.09 MPa）和断裂应变（4.65–5.37%）. 方差分析（ANOVA）结合Tukey的机械性能测试表明，底部和中间纤维在95%的置信水平下是相同的. GB纤维可用于单宁基复合材料的增强，在茎的底部和中部区域进行优选和不加选择的取样.

KEYWORDS:

• Chemical characterization
• physical characterization
• mechanical characterization
• plant fibers
• Grewia Bicolor (GB)
• ANOVA

关键词:

• 化学表征
• 物理表征
• 机械特性
• 植物纤维

Introduction

The use of composite materials in various applications (transport, construction, medical, leisure) has seen considerable progress due to the excellent properties they offer (lightness, mechanical strength, ease of processing) (Djebloun 2018; Mbang et al. 2023; Noah et al. 2020). Plant fibers, which are one of its constituents, are of growing interest from both an economic and environmental point of view (Dallel 2012). They are becoming increasingly attractive thanks to their properties, such as good mechanical strength, low weight, biodegradability, and low cost (Hicham and Abdessalam 2019).

In the context of sustainable development, scientific research is focusing on limiting global warming and its effects by trying to find new, more environmentally friendly production and consumption alternatives (Vidil 2019). These involve not only the development of plant-based composites as substitutes for synthetic fibers (Bureaux 2011) or as an alternative solution (Betene Ebanda 2012), but also the use of eco-labeled resins (Ndiwe et al. 2019).

Synthetic resins made from petroleum waste, which generate toxic gases that have a direct impact on the destruction of the ozone layer (Anderson and Thornback 2012; Mbang et al. 2023; Morgenstern et al. 2018) are being replaced by bio-adhesives (Konai et al. 2015; Pizzi et al. 2009). Several research projects have developed adhesives of animal and plant origin (Rhazi 2015). Resins based on tannins, lignins, or proteins have been developed as adhesives for wood and have been used to develop composite materials with good environmental properties over the years (Mansouri, Pizzi, and Salvado 2007; Nenonene 2009; Ping, Gambier, et al. 2012; Ping, Pizzi, et al. 2012; Pizzi 2006; Santos et al. 2017; Sealy-Fisher and Pizzi 1992; Trosa and Pizzi 2001; Wedaïna et al. 2021; Xi et al. 2020). In addition, tannin-based resins, being the oldest to be marketed, have the advantage over the others of not requiring the addition of synthetic resin derived from petroleum products, as is the case for the others, which is of significant interest (Pizzi 2006; Pizzi et al. 2009; Thébault et al. 2014; Arias et al. 2022).

Various types of plant fibers, such as flax, jute, hemp, kenaf, and other natural fibers, are used as reinforcement in tannin matrix composites (Chupin 2014; Kueny 2013; Nicollin et al. 2012; Pizzi et al. 2009; Saad 2013; Sauget, Nicollin, and Pizzi 2013; Wedaïna et al. 2021). On the other hand, one of the limitations of using plant fibers in composite reinforcement is the wide variability of their properties (Magniont 2010, Ringuette 2011). This non-uniformity in their characteristics may be linked to their origin, the nature of the soil, and natural constraints (rainfall, climate) (Chengoue Mbouyap et al. 2023) the extraction method (Chengoué et al. 2020) plant maturity (Pickering et al. 2007) and experimental conditions (Dumont et al. 2017). Several authors have also shown that the sampling area can have an effect on the physical and mechanical properties of plant fibers (Betené et al. 2022; Charlet et al. 2007; Duval et al. 2011; Libog et al. 2023; Stawski et al. 2020; N. D. Yilmaz et al. 2016).

Grewia bicolor (GB) is a shrub or small tree that can reach 7 m to 14 m in height (Heuzé et al. 2015) producing shoots and branches from the base of the main trunk and belonging to the malvaceae family (Brink 2007). It is widespread in the dry zones of tropical Africa, throughout the Sudano-Sahelian zone and across East Africa. It is also found in Yemen, Saudi Arabia, to India (Fali et al. 2019). In Cameroon, the Grewia bicolor plant is found in the northern zone (Kossoumna Liba’a 2008; Seignobos and Tourneux 2002). This plant has several uses, depending on its parts: GB wood is used for building houses (poles, beams), making tools and edged weapons (tool handles, shepherd’s crooks, pike poles) (Ganaba, Ouadba, and Bognounou 2004) and also as firewood (Elie 2012). The bark is generally used to clarify muddy water and is also used to make ropes, which in some regions are used to make whips because of their strength (Heuzé et al. 2015).The mucilaginous leaves and leaf fibers are used as binders in sauces; fresh leaves are used to make an infusion and also as fodder for domestic animals (Augustino et al. 2011; Brink 2007). The leaves and ash from burnt leaves are sometimes used as soap and for cleaning clothes; nevertheless, the Grewia bicolor plant is a highly prized medicinal plant (Orwa et al. 2009). The Grewia bicolor plant has a wide range of applications in traditional African medicine (Augustino et al. 2011). GB fibers have yet to be exploited in the field of tannin matrix composites, yet they are available in Cameroon and have the advantage of appreciable strength due to their traditional applications. In addition, the work of (Ndiwe et al. 2023) has shown that GB fibers harvested in the Touboro locality (7° 46’8“North; 15° 20’ 49” East) have appreciable properties and have been used to develop polyester resin composites with good physical and mechanical properties. However, certain technical properties, such as the chemical composition, linear density, and moisture absorption of this fiber, remain unknown in the literature. What’s more, only fibers from the central part of the stems, chemically extracted in an aqueous solution containing 5% NaOH, have been used in the elaboration of said composites, due to the variability of fiber properties along the stem. The work of (Jamshaid et al. 2021) showed that it was possible to statistically study this variability in fiber properties at different locations on the plants for a better understanding of their behavior, and tools such as ANOVA (analysis of variance) with Tukey's post hoc test were used. It therefore seems necessary to conduct an experimental campaign in order to provide further information on this fiber and to better define the variability of fiber properties along the stem for potential more rational valorization in the development of tannin matrix composites. Furthermore, the properties of plant fibers are mainly influenced by their physical and chemical characteristics (ArunRamnath et al. 2023; Saba et al. 2016; Wambua, Ivens, and Verpoest 2003). The physical properties are, for example, the apparent density, the absolute density, the porosity, the linear mass, the humidity rate, the water absorption rate, and the humidity recovery (Betené et al. 2022; Libog et al. 2023; Ngoup et al. 2024; Soppie et al. 2023; Stawski et al. 2020; Yilmaz et al. 2016). The main chemical constituents of plant fibers are cellulose, hemicellulose, and lignin, which govern their mechanical properties (Ngoup et al. 2024). For the development of new ecological tannin matrix composites presenting the desired performance and functionality, it is necessary to have knowledge of the chemical, physical, and mechanical properties of the fibers.

The aim of this study was to determine the chemical, physical, and mechanical properties of Grewia bicolor fiber in order to assess its potential for the reinforcement of tannin matrix composites. GB stems were harvested in the NGONG locality, and the fibers were extracted in three zones of the stem (top (T), middle (M) and bottom (B)) by biological retting with stagnant water. Chemical characterization was carried out to determine the content of the main plant fiber constituents, cellulose, hemicellulose, and lignin, and FTIR analysis was used to identify functional groups. Physical characterization involved the determination of water content by gravimetry, moisture recovery, absolute density using a pycnometer, apparent density by application of the Archimedean buoyancy principle, porosity, linear density, and water absorption rate. Finally, the mechanical characterization involved determining the tensile properties of the fiber bundles, followed by an ANOVA statistical analysis.

Materials and methods

Fiber extraction

Grewia Bicolor (GB) stems, one-year-old, length ranging from 80 to 134 cm (Figure 1a), were harvested at Ngong (9°01’00’“North; 13°11’9”’ East), in the North Cameroon region, at a temperature of 23 ± 2°C. Inspired by the techniques of (Libog et al. 2023) as well as (Ndiwe et al. 2023), the stems were cut into three equal zones (Figure 1b): bottom(B), middle(T) and top(T). The diameters of the stems vary between 15 and 40 mm. They are larger at the bottom and decrease as you move toward the top. The stems from each part were stripped of their bark (Figure 1c), and then the bark was soaked in three containers containing 50 L of distilled water for 90 days at an ambient temperature of 25 ± 5°C (Figure 1d). The extraction technique used was that of biological retting with stagnant water. Once the extractives and lignins had decomposed, the bark was washed with distilled water until the fibers were obtained. Finally, the fibers obtained were dried at room temperature of 28 ± 5°C for 4 hours (Figure 1e), then stored in a chamber at 75% relative humidity at 25°C for the various characterizations.

Figure 1. Fibre extraction from Grewia Bbicolor; (a) stem harvesting; (b) cutting into parts; (c) stem bark removal; (d) bark; (e) bark soaking;(f) fibre drying.

Chemical characterization

Fourier transform infrared spectroscopy in attenuated total reflectance spectrometry (FTIRATR)

Fourier transform infrared (FTIR) spectroscopy was used to determine the functional groups and chemical bonds present on Grewia bicolor fiber for each part of the stem (bottom, middle, and top). The infrared spectrum was recorded using a BRUKER ALPHA spectrometer operating in attenuated total reflectance (ATR) mode and controlled by Opus Lab v 7.0 122 software. Acquisitions were made by performing 200 scans in the 4000 to 400 cm⁻¹ frequency range^, with a spectral resolution of 4 cm⁻¹. Fiber bundles were chopped, ground and dried. A mass of 1 g was used for this purpose (Legrand et al. 2020; Libog et al. 2021).

Chemical composition

The composition of fibers such as cellulose, hemicelluloses, and lignin was determined in accordance with ASTM D 1104, ASTM D 1106 and ASTM D 1287 for fibers from the bottom, middle and top of the Grewia bicolor stem. The method used consists of successive solvent extraction of fiber components from a 2 g mass of ground and dried fibers. The first extraction was performed with an alcohol-toluene mixture, which will dissolve waxes and fats; then a second extraction was performed with hot water at 90°C, which will determine the amount of mineral salts as well as the low percentage of tannins, starch, and polysaccharides. Next, a third extraction was carried out for the separation of pectins using an aqueous ammonium oxalate extraction solution.

Lignins were then dissolved in a solution of sodium chlorite and glacial acetic acid. Finally, the hemicelluloses were solubilized with a potassium hydroxide solution, followed by a sodium hydroxide solution. By elimination, it was possible to determine the percentage of cellulose and hemicellulose. The values obtained were the average of three replicates of the fiber constituents (Mewoli et al. 2020).

Physical characterization

Water content

In a non-forced-air oven, ten fiber samples for each part of the stem (bottom, middle, and top) with an initial undried mass m₀ = 5 g ±0.1 g, previously weighed using a digital scale with a sensitivity of 0.001 g, were dried for 24 hours at a temperature of 105^◦C. The mass m₁ is measured after drying, and according to AFNOR B 51,004, the moisture content TH (%) was determined by Equation (1)(1) $\mathrm{TH}\left(\mathrm{\%}\right)=\left(\frac{{\mathrm{m}}_{0-\mathit{\hspace{0.17em}}}{\mathrm{m}}_{1\mathit{\hspace{0.17em}}}}{{\mathrm{m}}_0}\right)\ast 100\mathit{\hspace{0.17em}}$(1) (Libog et al. 2023; Ndiwe et al. 2023).

(1) $\mathrm{TH}\left(\mathrm{\%}\right)=\left(\frac{{\mathrm{m}}_{0-\mathit{\hspace{0.17em}}}{\mathrm{m}}_{1\mathit{\hspace{0.17em}}}}{{\mathrm{m}}_0}\right)\ast 100\mathit{\hspace{0.17em}}$(1)

Moisture absorption rate at 75% relative humidity

Ten fiber samples for each part of the stem (bottom, middle, and top) were oven-dried at 105°C for 3 hours, then weighed using a digital scale accurate to 1/1000^th in order to take the various initial masses (M_i). Samples were conditioned in a sodium chloride chamber (Solubility: 357 g/L, 8 g in 20 ml water at 75% relative humidity at 20°C) for 24 hours (Betene Ebanda 2012) and weighed again to collect the different final masses (M_f). The chamber saturated with sodium chloride is prepared 24 hours before the start of the test in accordance with NF G 08-001-4. Equation (2)(2) $\mathrm{RH}\left(\mathrm{\%}\right)\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}\frac{{\mathrm{M}}_{\mathrm{f}}\mathit{\hspace{0.17em}}-\mathit{\hspace{0.17em}}{\mathrm{M}}_{\mathrm{i}}}{{\mathrm{M}}_{\mathrm{i}}}\times 100\mathit{\hspace{0.17em}}$(2) was used to determine the percentage moisture absorption (RH) of the fibers.

(2) $\mathrm{RH}\left(\mathrm{\%}\right)\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}\frac{{\mathrm{M}}_{\mathrm{f}}\mathit{\hspace{0.17em}}-\mathit{\hspace{0.17em}}{\mathrm{M}}_{\mathrm{i}}}{{\mathrm{M}}_{\mathrm{i}}}\times 100\mathit{\hspace{0.17em}}$(2)

Density

Real density

The pycnometric method using toluene (ρ_a = 0.866 g.cm⁻³ at room temperature of 24 ± 2^◦C) as the immersion liquid, in accordance with ASTM D 2320–98 (2003), was used to determine the true density (${\mathit{\rho }}_{\mathrm{r}}$) of Grewia bicolor fibers on five samples for each part of the stem (bottom, middle, and top) by applying relationship (3). Prior to measurement, the fibers were cut into 5 ± 1 mm lengths, then dehumidified at 105^◦C for 24 h in the non-forced air oven (Ndiwe et al. 2023; Obame et al. 2022). Measurements were taken on a digital scale with a sensitivity of 0.1 mg.

(3) $\mathit{\hspace{0.17em}}{\mathit{\rho }}_{\mathrm{r}}={\mathit{\rho }}_{\mathrm{a}}\frac{{\mathrm{m}}_2-{\mathrm{m}}_0}{\left({\mathrm{m}}_1-{\mathrm{m}}_0\right)-\left({\mathrm{m}}_3-{\mathrm{m}}_2\right)}$(3)

Where: m₀ is the mass of the empty pycnometer; m₁ is the mass of the pycnometer filled with toluene at room temperature; m₂ is the mass of the pycnometer containing the fiber sample; and m₃ is the mass of the pycnometer filled with toluene and fiber at room temperature.

Apparent density

Five fiber clusters for each part of the stem (bottom, middle, and top) with a mass of 5 g were dried at 105°C in an oven for 6 hours in accordance with the ISO 3344 standard (Phung 2018). Density determination then involved weighing the mass $m_0$ of the samples using a 0.001 g precision balance. These samples were soaked in kerosene and weighed again to obtain the mass $m_1$. Then they were immersed in a beaker containing a volume $V_0$ of distilled water. After the water had stabilized, the new volume $V_1$ was read. The density was determined using Equation (4)(4) ${\rho }_a=\frac{m_0}{V_1-V_0+V_P}$(4) (Libog et al. 2021; Mewoli et al. 2020; Ndiwe et al. 2023).

(4) ${\rho }_a=\frac{m_0}{V_1-V_0+V_P}$(4)

With : $V_P=\frac{m_1-m_0}{{\rho }_p}$

Where: ${\rho }_a$ is the bulk density of the fiber in g/cm³; $m_0$ is the initial mass of the sample in g; $m_1$ is the mass of the paraffin sample in g; $V_0$ is the volume of distilled water in cm³; $V_1$ is the volume after immersion of the fiber in cm³; $V_p$ is the volume of the kerosene in cm³, ${\rho }_p$ : kerosene density in g/cm³.

Porosity

The porosity of a material expresses the proportion of voids it contains in relation to its total volume. It is therefore equal to the relative difference between the apparent (${\rho }_a$) and real density ${\rho }_r$. Porosity was determined using Equation (5)(5) $\mathit{p}\left(\mathrm{\%}\right)=\left(1-\frac{{\rho }_a}{{\rho }_r}\right)\times 100$(5) for the fibers of each part of the stem (bottom, middle, and top).

(5) $\mathit{p}\left(\mathrm{\%}\right)=\left(1-\frac{{\rho }_a}{{\rho }_r}\right)\times 100$(5)

Linear mass

ASTM D 1577–96 was used to determine the linear mass of the fibers gravimetrically, using Equation (6)(6) $\mathrm{Linear}\mathrm{mass}\left(\mathrm{tex}\right)\mathit{\hspace{0.17em}}=\frac{\mathrm{Dry}\mathrm{fiber}\mathrm{mass}\left(\mathrm{g}\right)}{\mathrm{fiber}\mathrm{length}\left(\mathrm{Km}\right)\mathit{\hspace{0.17em}}}$(6) . Forty fiber samples for each part of the stem (bottom, middle, and top) were cut to a size of 20 cm ±0.1 cm, and dried in a non-forced air oven at a temperature of 105^◦C for 24 hours (Libog et al. 2023). Each sample was weighed on a 0.1 mg sensitivity balance at an ambient temperature of 24 ± 2^◦C.

(6) $\mathrm{Linear}\mathrm{mass}\left(\mathrm{tex}\right)\mathit{\hspace{0.17em}}=\frac{\mathrm{Dry}\mathrm{fiber}\mathrm{mass}\left(\mathrm{g}\right)}{\mathrm{fiber}\mathrm{length}\left(\mathrm{Km}\right)\mathit{\hspace{0.17em}}}$(6)

Water absorption rate

Ten fiber clusters for each part of the stem (bottom, middle, and top) with an initial dry mass of mi = 100 mg ±5 mg were weighed using a 1 mg sensitivity balance, then immersed in distilled water at a temperature of 32 ± 2°C for 24 hours, in accordance with ASTM D 2402 (ASTM 2001). The fibers were pre-dried in an oven at 105°C. Samples were dried for 3 hours, then conditioned and left to stand for 3 hours to cool. Once removed from the water, a dry cotton cloth was used to wipe excess water from the surface. The method used in this test was gravimetric, and Equation (7)(7) $\mathrm{TA}\left(\mathrm{\%}\right)=\left(\frac{{\mathrm{m}}_{\mathrm{f}}}{{\mathrm{m}}_{\mathrm{i}}}-1\right)\ast 100\mathit{\hspace{0.17em}}$(7) was used to calculate the water absorption rate (TA%).

(7) $\mathrm{TA}\left(\mathrm{\%}\right)=\left(\frac{{\mathrm{m}}_{\mathrm{f}}}{{\mathrm{m}}_{\mathrm{i}}}-1\right)\ast 100\mathit{\hspace{0.17em}}$(7)

Mechanical characterization

The determination of Young’s modulus, stress, and strain at break of the fibers was carried out on thirty samples for each part of the stem (bottom, middle, and top) using a COYON CJ-0820 tensile testing machine with a load capacity of 5 kN as recommended by ASTMD 3822 (Betene et al. 2022; Obame et al. 2022). The fibers were processed at a controlled temperature of 23°C ±1, 50% relative humidity, and a displacement speed of 1 mm/min. The fibers were separated and attached to standard-sized supports, as shown in Figure 2.

Figure 2. Unit fiber support.

The various experimental points are obtained and processed in Microsoft Excel 2019. Equations (8)(8) $\mathrm{Strain}=\frac{\mathrm{Fiber}\mathrm{displacement}\left(\mathrm{mm}\right)}{\mathrm{Initial}\mathrm{fiber}\mathrm{length}\mathrm{under}\mathrm{stress}\left(\mathrm{mm}\right)}.$(8) & (9(9) $\mathrm{Stress}\mathrm{at}\mathrm{break}\left(\mathrm{MPa}\right)\mathit{\hspace{0.17em}}=\frac{\mathrm{Strength}\left(\mathrm{N}\right)}{\mathrm{Fiber}\mathrm{cross}-\mathrm{section}\left(\mathrm{m}{\mathrm{m}}^2\right)\mathit{\hspace{0.17em}}}.$(9) ) were used to plot the stress-strain curves that gave rise to equations of the type Y=Ax+B, where A and B were respectively the slope and the ordinate at the origin of the regression line for each sample in the elastic zone. Young’s modulus (MPa) was calculated by averaging the values of A for each sample.

(8) $\mathrm{Strain}=\frac{\mathrm{Fiber}\mathrm{displacement}\left(\mathrm{mm}\right)}{\mathrm{Initial}\mathrm{fiber}\mathrm{length}\mathrm{under}\mathrm{stress}\left(\mathrm{mm}\right)}.$(8)

(9) $\mathrm{Stress}\mathrm{at}\mathrm{break}\left(\mathrm{MPa}\right)\mathit{\hspace{0.17em}}=\frac{\mathrm{Strength}\left(\mathrm{N}\right)}{\mathrm{Fiber}\mathrm{cross}-\mathrm{section}\left(\mathrm{m}{\mathrm{m}}^2\right)\mathit{\hspace{0.17em}}}.$(9)

Statistical analysis

An analysis of variance (ANOVA) was used to assess the significant or non-significant impact of the extraction zone on fiber tensile mechanical properties using IBM SPSS Statistic 27 software. 95% confidence level was adopted, and Tukey’s test completed this statistical analysis to quantitatively assess significant differences between different fiber extraction zones on the stem (Scarpa, Amroune, and Bezazi 2015).

Results and discussion

Chemical characteristics

Fourier transform infrared spectroscopy in attenuated total reflectance spectrometry (FTIRATR)

The spectrum resulting from the ATR-FTIR analysis of Grewia bicolor (GB) fiber is shown in Figure 3, and the identification parameters are listed in Table 1.

Figure 3. FTIR of Grewia Bicolor fibers.

Table 1. Characteristic positions of the FTIR bands on Grewia Bicolor (GB) fiber and their assignments.

Pics	Assignation	References
3334	OH bond stretching (cellulose hemicellulose)	Kılınç et al. (2018); Legrand et al. (2020)
2919	CH and CH2 Stretching and vibration of cellulose and hemicellulose	Kılınç et al. (2018)
1674	Symmetrical elongation of ester groups (C=O) of carbonyls of hemicelluloses also present in pectins and waxes	Obame et al. (2022)
1618	Shearing of OH bonds from free water	Dai and Fan (2011)
1426	CH deformation (lignin) and CH symmetrical curvature2 (cellulose)	Moonart and Utara (2019)
1317	Flexion in the O-H plane or CH household stirring2	Beldjoud and Ghadbane (2020)
1315	Flexion in the O-H plane or CH household stirring2	Beldjoud and Ghadbane (2020)
1032	O-H and C=O stretching vibration (cellulose and lignin)	Mouhoubi et al. (2012)
889 (892)	Symmetrical stretching of CH and OH bonds in cellulose.	Betene et al. (2020)
774	Out-of-plane deformation of water OH bond	Obame et al. (2022)

All three zones show spectra similar in appearance to flax, sisal, and jute fibers (Del Rio et al. 2007; Garside and Wyeth 2003; Seki et al. 2019). The peak located at 3334 cm⁻¹ is attributed to hydrogen bonds (OH) in the inter- and intramolecular cellulose network of free hydroxyl groups in hemicellulose (Kılınç et al. 2018; Legrand et al. 2020). This broad absorption band is characteristic of the presence of liquid water, more or less bound to the polymeric network constituted by natural fibers (Célino et al. 2014). The one observed at 2919 cm⁻¹ is associated with asymmetric CH and CH₂ stretching vibration in cellulosic and hemicellulosic components (Kılınç et al. 2018). The absorption band centered at 1674 cm⁻¹ corresponds to the symmetrical ester group (C=O) stretching of carbonyl groups in hemicelluloses, also present in pectins and waxes.

The absorption peak at 1619 cm⁻¹ is located at the (O-H) bond shear in free water (Dai and Fan 2011). The absorbance band at 1426 cm⁻¹ is associated with the symmetrical CH bending present in cellulose (Moonart and Utara 2019). The peak located at 1032 cm⁻¹ characteristic of the vibration of OH and C=O stretches is attributed to cellulose and lignin (Mouhoubi et al. 2012). The symmetrical stretching of the CH and OH bonds and the band at 889 cm⁻¹ confirmed the presence of β 1,4 glucosidic cellulose (Betene et al. 2020). Thus, the above analysis clearly proved that GB fibers are mainly composed of cellulose, hemicellulose, lignin, and water. We can clearly see that the spectrum of top fiber is lower in intensity than the other two, which tend to be superimposed. This level of intensity would explain why the properties of the top fiber differ significantly from those of the bottom and middle fibers. This would be due to the lengthwise maturity of the plant. This result is in line with the mechanical characterization of the fibers in the three plant zones.

We can conclude that Grewia bicolor (GB) fiber is made up of cellulose, hemicellulose, lignin, pectin and other extractives. It would therefore be interesting to know the content of each constituent.

Chemical composition

The chemical composition of Grewia bicolor (GB) fibers as a function of the sampling area in the stem, compared with plant fibers from the literature, is presented in Table 2.

Table 2. Chemical composition of plant fibers.

Fibers	Cellulose (wt%)	Hemicellulose (wt%)	Lignin (wt%)	References
Flax	60–81	18.6–20.6	14–19	Pereira et al. (2015)
Triumfetta Cordifolia	44.4	30.8	18.9	Mewoli et al. (2020)
Hemp	70–74	17.9–22.4	3.7–5.7	Jeyapragash, Srinivasan, and Sathiyamurthy (2020)
Banana	37.5	27.6	15	Sango et al. (2018)
Jute	45–84	12–21	13–26	Li, Tabil, and Panigrahi (2007)
Kénaf	31–63.5	17.6–21.5	15–19	Gurunathan, Mohanty, and Nayak (2015)
Sisal	65.8	12	9.9	Youmssi et al. (2017)
Ananas Comosus (AC)	68.11	4.9	12.01	Betene et al. (2020)
Recktophillum camerunence (RC)	65.15	7.42	16.2	Betene et al. (2020)
Neuropeltis Acuminatas (NA)	36.08	15. 33	25.15	Betene et al. (2020)
Top	50.73	24.73	14.26
Middle	52.52	20.95	17.01	Present study
Bottom	53.67	21.99	17.26

The chemical composition of Grewia Bicolor(GB) fibers is similar to that of other plant fibers, with no significant difference in results between Middle and Bottom fibers. The amount of lignin is close to that of Triumfetta cordifolia (TC), banana and Recktophillum Camerunence (RC), while the amount of pectin and cellulose is close to that of TC. Grewia bicolor top fibers have a lower cellulose and lignin content than bottom and middle fibers, so their mechanical strength and stiffness could be lower. Previous studies on plant fibers show that increases in fiber properties such as tensile strength and Young’s modulus are closely linked to the amount of cellulose they contain (ArunRamnath et al. 2023). This cellulose content also has a positive influence on the reinforcement of tannin-matrix composites. Middle and bottom fibers with the highest cellulose values (52.52% and 53.67%) are more recommendable. It should be noted that these proportions of cellulose contained in GB fiber are higher than those of some fibers already used in the reinforcement of tannin matrix composites with good properties, such as Triumfetta cordifolia fiber (Mewoli et al. 2023), banana fiber (Konai et al. 2020), jute fiber (Chupin 2014; Kueny 2013), and kenaf fiber (Pizzi et al. 2009). This is an indicator that Grewia bicolor fiber can effectively reinforce tannin matrices. Hemicellulose has a negative effect on the tensile strength and Youngs’ modulus of the fibers (ArunRamnath et al. 2023); it may also compromise the mechanical strength and dimensional stability of the tannin matrix composite, as it is hygroscopic. Top fibers have a slightly higher hemicellulose content than middle and bottom fibers, making them less suitable than middle and bottom parts for reinforcing tannin matrix composites. On the other hand, the fibers of the three stem zones of Grewia bicolor have a lower hemicellulose content than Triumfetta cordifolia and banana fibers and are within the hemicellulose content range of flax, jute, and kenaf fibers already used in the reinforcement of tannin matrix composites. Nevertheless, alkaline treatment can eliminate amorphous hemicelluloses on the fiber and promote better compatibility and anchoring of GB fiber in tannin matrices, which can improve composite properties (Zhu, Abhyankar, and Njuguna 2013).

Physical properties

Table 3 shows the results for water content, bulk density, true density, porosity, and water absorption rate of Grewia bicolor fibers, and Figure 4 illustrates the different behaviors.

Figure 4. Apparent density, true density and porosity of GB fibers.

Table 3. Physical properties of plant fibers.

Fibers	Water content (%)	Density Apparent (g/cm)^3	Real density (g/cm)^3	Porosity (%)	Water absorption rate (%)	References
GB (Bottom) GB (Middle)	12.08 10.49	0.69 0.63	1.32 1.29	47.73 51.16	54.2 62.3	Ndiwe et al. (2023)
GB (Top)	14.76	0.61	1.30	53.08	64.2
Kenaf	-		-	-		Faruk et al. (2012)
Sisal	11		1.5	-	198.2	Asim et al. (2015; Béakou et al. (2008)
Palm fiber	-		0.7–1.55	-	276.2	Essabir et al. (2016)
Cotton	-		1.5–1.6	-	-	Asim et al. (2015)
Bamboo	8.9		0.6–1.1	-	-	Ramesh, RajeshKumar, and Bhuvaneshwari (2021)
Flax	7		1.5	-	-	Amiri et al. (2017)
Jute	12		1.3	-	-	Asim et al. (2015)
Hemp	9		1.48	-	-	Rohen et al. (2017)
Ramie	9		1.5	-	-	Nam and Netravali (2006)
Banana	-		1.35	-	-	Ronald Aseer et al. (2013)
Pineapple	13		0.8–1.6	-	-	Asim et al. (2015)
E-glass	-		2.5	-	-	Marimuthu et al. (2019)
S-glass	-		2.5		-	Marimuthu et al. (2019)
Top	13.59 ± 0.11	0.61 ± 0.02	1.25 ± 0.04	51.4 ± 0.01	153.58 ± 0.3
Middle	13.16 ± 0.24	0.71 ± 0.01	1.36 ± 0.02	48.07 ± 0.02	134.54 ± 0.3	Present study
Bottom	13.51 ± 0.1	0.72 ± 0,03	1.39 ± 0,07	48.37 ± 0.1	121.86 ± 0.6

Moisture content

Hemicellulose is a key factor in understanding the moisture content of a plant fiber; the higher it is, the more important it is. Table 3 shows the results of the moisture content of Grewia bicolor (GB) fibers in comparison with other plant fibers. Fibers from different parts of the GB plant (bottom (B), top (T), and middle (M)) have moisture contents between 13.16% and 13.59%; these values are within the range of (Ndiwe et al. 2023). Middle(M) fibers from the GB have a lower moisture content than those from the other two zones, which is in agreement with the results of (Ndiwe et al. 2023). This finding can be explained by the fact that they have a lower hemicellulose content compared with the other two zones, as shown in Table 3. GB fibers have a higher water content than the other plants presented in Table 3, as they have a higher hemicellulose content. It would be necessary to remove this moisture from Grewia bicolor fibers before shaping tannin matrix composites, as moisture can adversely affect composite properties.

Moisture recovery at 75% relative humidity

Anhydrous fibers absorbed water in a 75% relative humidity chamber for 24 hours. Values of 11.12% ± 1.62, 11.42% ± 1.38, and 11.76% ± 0.63 were found for top, middle and bottom fibers respectively. The difference was not significant. The same applies to water content. These values are lower than those for Sida rhombifolia (21.02–21.45%) (Ngoup et al. 2024). This difference could be explained by the different nature of the two fibers. So Grewia bicolor fiber can be used in the same field of application as Sida rhombifolia.

Porosity

From Figure 4, we can see that apparent and real densities increase as we move from Top to Bottom, and the values are respectively 0.61 g.cm⁻³ ±0.02 to 0.72 g.cm⁻³ ±0.03 and 1.25 g.cm⁻³ ±0.04 to 1.39 g.cm⁻³ ±0.07. These values are quite close to those of (Ndiwe et al. 2023). However, the behavior is different, which could be explained by the difference in botanical origin of the Grewia bicolor (GB) stems as well as their age and experimental conditions. This evolution according to the zone on the stem is also the opposite of the results of (Libog et al. 2023) who reported an increase from bottom to top for Cavendish bananas. This finding could be explained by the different nature of GB and banana. There was also no significant difference between the bottom and middle densities. The density considered in the industry is the real one, and GB densities are close to those of jute, ramie, banana, hemp, and flax fibers; GB fibers can be used in the same fields of application as the latter (Ndiwe et al. 2023) report that the porosity of GB fibers decreases as one moves from the head to the foot of the stem. The porosities of GB fibers in this study are 51.40%, 48.07%, and 48.37%, respectively for top, middle and bottom. These values are close to those of (Ndiwe et al. 2023) as well as their behavior. It was found that the porosity between the middle and bottom fibers is almost identical and lower than that of the top, which would lead us to say that the top fibers are lighter, resulting in a lower density, and will also absorb more water. These findings also suggest that bottom and middle fibers will have superior mechanical properties (stiffness, deformation, and stress) to top fibers. The bottom and middle fibers have a greater density than the jute fibers (Asim et al. 2015) and banana fibers (Ronald Aseer et al. 2013) used in tannin matrix composites. It would therefore be advisable to reinforce tannin matrix composites with GB fibers from the bottom and middle because a denser plant fiber has the capacity to support mechanical loads, which increases stiffness. Furthermore, they can contribute to the better dimensional stability of the tannin matrix composite by minimizing variations due to humidity and other environmental factors.

Linear mass

The linear density is a very important parameter for the textile application of a plant fiber. The values for Grewia bicolor (GB) for different zones are respectively 29.44 tex ± 1.66, 28.1 tex ±1.37, and 17.28 tex ±1.98 for the bottom, middle and top (Figure 5).

Figure 5. Linear density and water absorption rate of GB fibers.

The linear mass of GB is higher at the bottom and lower at the top. This can be explained by the porosity of the GB fiber, which is greater at the top than at the bottom. Furthermore, the more porous the fiber, the less weight it will have along its length. Regardless of the sampling area on the stem, GB fibers have a higher linear density than pineapple leaf (8–8.4 tex) (Betene et al. 2022) and lower than banana top, middle, and bottom (31.6–34 tex) (Libog et al. 2023). GB fibers can also be used in textiles. It is also observed that the linear density of the bottom and middle fibers is almost identical and higher than that of the top. These observations are made on porosity, density, and water absorption rate (Yilmaz et al. 2016) for corn fibers (Singh et al. 2020), for nettle leaf fibers, and (Libog et al. 2023) for banana fibers, reported instead an increase in linear density from bottom to top. This may be related to plant growth, which causes voids in mature areas to shrink in younger areas of the plant (Betene et al. 2022, Sikame et al. 2014; Libog et al. 2023). The fibers at the top are finer than those at the middle and bottom. Middle and bottom fibers can be an advantage, as they can lead to a more uniform distribution of loads within the tannin matrix composite, not only strengthening its overall structure but also reducing stress concentration zones that could lead to premature failure zones.

Water absorption rate

Plant fibers with a water absorption rate equal to or greater than 100% have a hydrophilic character due to the presence of hemicellulose in their structure (Mewoli et al. 2020). It’s clear to see that Grewia bicolor (GB) fibers are hydrophilic (absorption rate between 121.86% and 153.59%), and the sampling area influences the water absorption rate (Figure 5). This ability to absorb water reduces the material’s rigidity in composite construction (Libog et al. 2021; Mewoli et al. 2020). This characteristic can be corrected by using a surface treatment or a coupling agent (Betene et al. 2020). The absorption rate values in this study are well above those of (Ndiwe et al. 2023). This may be explained by the difference in plant harvesting location and maturity. The porosity and higher hemicellulose content of the top zone explain why it has a higher absorption rate than the other zones (153.59%). An increase in the rate of water absorption from the bottom to the top was also observed. This could be explained by the more efficient capillary effect from the bottom to the top of the GB plant. Such a finding was made by (Libog et al. 2023) for Cavendish banana, (Sikame et al.,2014) for raffia, and (Yilmaz et al. 2016) for maize. Grewia bicolor fibers have a lower absorption rate than banana fibers (347–517%) (Libog et al. 2023), sisal fibers (1982.2%) (Asim et al. 2015; Béakou et al. 2008), and Triumfeta cordifolia fibers (342.3%) (Mewoli et al. 2023). These fibers have already been used to reinforce a tannin matrix composite, so given the low absorption rate of Grewia bicolor fibers, their adhesion to a tannin matrix will be more favorable.

Mechanical properties and statistical analysis

Figure 6 shows that Grewia bicolor (GB) fibers have a type 3 brittle mechanical behavior also encountered in the case of Triumfetta cordifolia fibers (Soppie et al. 2023) This is justified by the fact that the deformations of the GB are of the order of less than 10%; moreover, the first linear zone marks the alignment of the microfibrils on the tensile axis, and the second linear zone is the elastic zone for the determination of Young’s modulus (Soppie et al. 2023). Table 4 shows the mechanical properties of GB fibers in relation to other plant fibers, depending on the extraction area.

Figure 6. Mechanical behavior of Grewia Bicolor fibers.

Table 4. Mechanical properties of study fibers compared with other plant fibers.

Fiber	Tensile Strength (MPa)	Young’s modulus (GPa)	Strain to failure (%)	Reference
GB (Bottom)	1127.1$\pm 58$	32.97$\pm 3.20$	3.15$\pm 0.35$	Ndiwe et al. (2023)
GB (Middle)	783.52$\pm 43$	35.18$\pm 2.25$	2.55$\pm 0.25$
GB (Top)	667.78$\pm 38$	30.40$\pm 1.88$	2.48$\pm 0.20$
Kenaf	930	53	1.6	Faruk et al. (2012)
Sisal	511–635	9.4–22	2–2.5	Asim et al. (2015); Béakou et al. (2008)
Palm fiber	80–248	0.5–3.2	17–25	Essabir et al. (2016)
Cotton	287–800	5.5–12.6	7–8	Asim et al. (2015)
Bamboo	140–230	11–17	-	Ramesh, RajeshKumar, and Bhuvaneshwari (2021)
Flax	345–1035	27.6	2.7–3.2	Amiri et al. (2017)
Jute	393–773	26.5	1.5–1.8	Asim et al. (2015)
Hemp	690	70	1.6	Rohen et al. (2017)
Ramie	560	24.5	2.5	Nam and Netravali (2006)
Banana	355	33.8	5.3	Ronald Aseer et al. (2013)
Top	504.08 ± 142	17.21 ± 7.63	5.35 ± 0.8	Present study
Middle	670.15 ± 289	27.91 ± 5.97	4.65 ± 0.97
Bottom	936.08 ± 363	29.56 ± 5.22	5.37 ± 1.21

The Young’s modulus, stress, and strain at break of GB fibers change with the extraction zone (Table 4). Young’s modulus and stress at break decrease when moving from bottom to top, while strain at break increases (Figure 7). This observation was made by (Ndiwe et al. 2023) for Grewia bicolor extracted at Touboro and also by (Libog et al. 2023; Subramanya, Satyanarayana, and Shetty Pilar 2017) for the case of banana stem. This evolution of mechanical properties on the GB stem could be explained by the fact that the degree of maturity of the plant decreases from bottom to top, as in the case of bananas (Subramanya, Satyanarayana, and Shetty Pilar 2017). The amount of cellulose in many plant fibers also depends on their degree of maturity, which could indicate or justify their high stress and deformation. In addition, the mechanical behavior of GB fibers is in agreement with the chemical composition values in Table 2. The richer the cellulose, the greater the mechanical strength; the less rich the lignin, the less rigid (lower Young’s modulus). The evolution of the mechanical behavior of GB fibers differs from that of pineapple leaves (Betene et al. 2022) and flax (Charlet et al. 2007). This possible difference can be attributed to the origin of the plant.

Figure 7. Mechanical properties of GB fibers.

Compared to the mechanical properties of (Ndiwe et al. 2023) Table 4 shows that GB fibers have weaker properties. This could be explained by the difference in age, which affects the chemical composition of the fiber (Yilmaz et al. 2017) as well as experimental parameters (Obame et al. 2022, 2022).

The descriptive statistics in Table 5 indicate that there is a difference between the mean values of Young’s modulus, stress, and strain at break depending on the location of the fiber on the GB rod. However, dispersions around the mean have been neglected for such an assertion; an ANOVA statistical analysis test (Table 5) will enable us to tell with a 95% confidence level whether there is a significant difference between the values of the mechanical parameters (Mishra et al. 2022).

Table 5. Results of analysis of variance (ANOVA) test for mechanical properties of study fibers.

Mechanical properties	Source of variation	Degree of freedom	sum of squares	Medium square	F value	Significant value
Young’s modulus (GPa)	Extraction zone	2	1470.79	735.39	4.6	0.013
Stress at break (MPa)	Residue Extraction zone	69	11022.39 945046.59	159.74 472823.29	4.24	0.018
	Residue	69	769150.17	111480.44

Here, we observe (Table 5) that there is a significant difference depending on the location of the fiber on the rod for Young’s modulus (p = .013 < 0.05), stress at break (p = .018 < 0.05) and strain at break (p = .018 < 0.05). Unfortunately, the ANOVA test does not allow us to say how this significant difference exists between the different zones for each parameter. For this, a Tukey test with a rejection threshold of 5% (Libog et al. 2023) can clearly solve this problem (Table 6).

Table 6. Results of Tukey’s analysis test for mechanical properties of studied fibers.

Mechanical properties	Extraction zone A	Extraction zone B	Average difference (A-B)	Significant value
Young’s modulus (GPa)	T	M B	−6.07 −11.04	0.02 0.01
	B	M	−4.97	0.37*
Stress at break (MPa)	T	M B	−204.86 −266.31	0.09 0.02
	B	M	61.44	0.8*

*No significant difference.

When the significant value is less than 0.05, there is a significant difference between the data; otherwise, the difference is not significant at 95% confidence. Whatever the pair (A, B), the significant value is different. This result was also found for bananas (Libog et al. 2023). In the case of stress at break, we find that the difference (A-B) is negative between top-middle and top-bottom, and the significant value is less than 0.05; we can conclude that the stress at break of Top fibers has a significant difference with Bottom and Middle, and is lower than the latter. There is no significant difference between the Bottom and Middle fibers (p = .8 > 0.05). There is no significant difference between bottom and middle fibers in the case of Young’s modulus (p = .37 > 0.05). However, there was a significant difference between top-middle and top-bottom. The greater the density of the fibers, the more it is observed that the Young’s modulus and the breaking stress also increase, which corroborates with the evolution of the porosity. The denser the fiber, the less porous it is and the better mechanical properties it has. It can be concluded that the middle and bottom fibers are identical in terms of mechanical properties.

Conclusion

Investigation into the understanding of the chemical, physical, and mechanical properties of Grewia bicolor (GB) fibers according to the location of the parts on the stem points to their possible use in the elaboration of tannin matrix composites. Statistical analysis (ANOVA) using Tukey’s 95% confidence test shows that the mechanical properties of the bottom and middle zones do not vary significantly from those of the top. Indiscriminate sampling of fibers in the bottom and middle parts of the GB stem is more advisable for the development of tannin matrix composites. It would be interesting to continue this study by carrying out a chemical treatment to further improve the properties of the GB fiber, which could improve fiber-tannin matrix adhesion.

Author’s contribution

Mbere Taoga Michel: Interpretation of results, drafting and reading of manuscript; Betene Ebanda Fabien: Supervision of work, validation of tests and reading of manuscript; Pizzi Antonio : Methodology, reading of manuscript; Ndiwe Benoit: Supervision of work, validation of tests and reading of manuscript, Noah Pierre Marcel Anicet: Supervision of work, validation of tests and reading of manuscript; Essome Mbang Jonas Peequeur: Interpretation of results, drafting and reading of manuscript; Kibong Kimbi Kevin and Etinge Tabi Akwo :Data collection, César Segovia: Investigation into methodology, reading of manuscript

Conflicts and interests

The authors declare no conflict of interest regarding the publication of this article.

Ethical approval

The authors confirm that this work has no ethical problems.

The authors declare no conflict of interest regarding the publication of this article.

Major points

-The plant fibers come from NGONG in the North Cameroon region;

-The influence of the physical, chemical and mechanical properties was determined according to the location of the fiber on the Grewia bicolor(GB) stem;

-Statistical analysis by ANOVA was used;

-The study fiber is a major potential for tannin matrix composites.

Acknowledgments

The authors would like to thank Mr. Ngue Alain (Second Grade Technical High School Teacher in Mechanical Manufacturing), Mr. Mansassou Claude (Second Grade Technical High School Teacher in Mechanical Manufacturing) and Mr. Temwa Jean Marc (Engineering student at Ecole Nationale Polytechnique de Douala) for their help during the experimental campaign. We would also like to thank Dr. Mewoli Edwige Armel and Mr. Ndounakon Bissiliken Alphonse (PhD student in mechanical engineering) for their support and practical advice in this work. The authors would like to thank Ms. Biloa Otiti Sandrine (PhD student in mechanical engineering) and Ms. Nga Liliane (PhD student in forestry engineering) for their material contributions. Finally, we thank the reviewers of this manuscript for their critical contributions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work has not been financed; the expenses come from the authors’ own funds.