Physicochemical and Mechanical Characterization of Raffia vinifera Pith

Mechanics and Adapted Materials Laboratory (LAMMA), ENSET-University of Douala, Douala, Cameroon Research Unit in Engineering of Industrial Systems and the Environment (UR-ISIE), IUT/FV Bandjoun-University of Dschang, Dschang, Cameroon Research Unit in Mechanics and Modeling of Physical Systems (UR-2MSP), Faculty of Science-University of Dschang, Dschang, Cameroon Mechanics Laboratory (LM), ENSET-University of Douala, Douala, Cameroon Université de Haute-Alsace, LPMT UR 4365, F-68100 Mulhouse, France Université de Strasbourg, Strasbourg, France


Introduction
Waste management in recent years has led researchers to take an interest in the development of biodegradable materials for the emergence of biodegradable composites based on natural fibers [1][2][3]. Indeed, works have been carried out on plant fibers through treatments to replace synthetic fibers in composites materials [4]. Raffia vinifera pith (RVP) is a material from the stem of Raffia vinifera (RV) belonging to the Arecaceae family according to the authors of [5,6]. is plant mostly grows in marshy areas in Asia, South America, and tropical zone in Africa, especially in Cameroon. e RVP and RV stem are used by the populations of these regions as a raw material in the realization of structures (ceilings) and handicrafts such as baskets and mats, according to Sikame et al. [7].
Scientific works have been carried out on RV fibers. e density, the water absorption rate, and the diffusion coefficient were studied in the kinetics of water absorption and drying of RV fibers from the stem, as studied by the authors of [8][9][10]. In addition, the effective diffusion coefficient of the RVP along the stem was also determined in the kinetics of water absorption [11]. Mechanically, studies on the delayed behavior of RV stem versus load and temperature were performed, which resulted in the determination of Burger model parameters unlike those of Schapery, by the authors of [6,[12][13][14][15]. Also, Young's modulus of the fiber along the RV stem and its evolution has been evaluated [16]. Similarly, the study of creep/recovery behavior has enabled to propose a rheological model and to determine the viscoelastic parameters of RV fibers [7]. Finally, from the thermal point of view, the thermal conductivity of RVP has been determined [17,18].
Considering the works carried out so far on the RV, we have noticed that it has interesting properties, although products from RVP such as baskets used in transportation of livestock and agricultural products deteriorate rapidly due to bad weather. One of the solutions could be the setting up of a biodegradable composite material based on RV fibers as reinforcement. In addition, we noted that pith from RV stem has undergone very little work. Extraction of RV fibers reveals the presence of other constituents, suggesting that the RVP itself is a macroscopic composite. e objectives of this study are firstly to confirm after observations that RVP is a natural composite, secondly to determine some of its physicochemical and mechanical properties, and finally to characterize the natural matrix resulting from it (RVP). Such results may help in selecting the type of matrix to be used in biodegradable composite material making.  Figure 1. RV stems were collected from the swampy areas around Fotso Victor University Institute of Technology in Bandjoun, Cameroon. A digital caliper of precision 0.001 mm was used to determine the dimensions of each sample. e RVP cross-sectional images were observed using an optical microscope of 16x magnifications from the Laboratory of Textile Physics and Mechanics (LPMT) in Mulhouse, France. e study of the bulk density of RVP was carried out using a 0.001 g precision digital scale for the weighing of the samples.

Experimental
Axial dynamic tensile and three-point bending tests were carried out using the MTS M/20 tensile machine at a speed of movement of the jaws of 5 mm/min at the LPMT.
Approximately 25 samples were used in each study area for each parameter explored.

Determination of Surface Fractions.
e determination of the surface fractions of RV fibers was made using images from various cross sections and ImageJ software. e surface fraction is calculated using the following equation: where S fi , S RVP , and S f are, respectively, the cross-sectional area of each fiber and RVP and the surface fraction of the RV fibers.

Determination of the Chemical Elements of RVP along the RV Stem.
e chemical elements of RVP were determined using the JEOL scanning electron microscope (SEM) of the LPMT in Mulhouse based on the analysis of energy dispersion spectrometer (EDS).
is approach has been applied in the evaluation of the chemical composition of certain natural fibers such as Passiflora foetida, Roselle, palm, or Carica papaya bark fibers by the authors of [19][20][21][22].

Determination of Bulk Density
(1) Case of RVP along the Stem. e bulk density of RVP was evaluated using the gravimetric method based on Archimedes' principle from ASTM-D3800-99 [23], also used for the determination of the density of kenaf or RV [16,24]. Equation (2) enables us to determine the density of the RVP: where V p , V i , V t , and V nut , respectively, represent the paraffin, initial volume of the tube, the final volume of the tube, and the final volume of the nut with V p � (m t − m RVP )/ρ p ; then, m RVP and m t are the respective masses of each sample of RVP and the sample RVP made up with paraffin, and finally ρ p is the density of paraffin, which was also applied to evaluate the density of oil palm fruits or RV fibers [16,25].
(2) Case of the RVP Matrix. e RVP is a composite whose bulk density of fiber along the stem is known for each study area (twelve) and that of the natural matrix remains unknown. e bulk density of RVP considered here as composite is given by the law of mixtures in the following equation: where ρ RVP , ρ f , and ρ m are, respectively, the densities of the RVP, the fiber, and the RVP matrix taken in each study area.
Moreover, S f represents the surface fraction of fiber in a sample of RVP. is formula of equation (3) was used to evaluate the bulk density of polypropylene fiber composites [26]. Determination of the bulk density of the RVP matrix is performed using equation (4) obtained from equation (3):

Determination of Young's Modulus of RVP (Composite) by Tensile
Test. e approach used in determining Young's modulus of RVP in the tensile test is the ASTM D 3822-01 [27] standard method. Data were obtained from the tensile test with an imposed speed of 5 mm/min, namely, the elongation (Δl) as a function of the effort (F) allowed us to find the strain (ε) using equation (5), which was also used in mechanical characterization of RV fibers [16]: where l 0 is the initial length of the specimen. e stress (σ) was calculated using equation (6), which was also used in determining the mechanical properties of sugarcane, coconut, or RV fibers [16,28,29]: where S represents the cross section of the specimen. Young's modulus was considered to be the slope of regression line in the elastic domain of the stress-strain diagram plotted in MATLAB R2015a software.

Determination of Young's Modulus of the Matrix from
RVP. e calculation of Young's modulus of the RVP matrix along the stem was made from Young's moduli of the RVP considered as the composite, and that of the RV fiber admitting the interface between the matrix and the fiber is perfect. By the law of mixtures of Young's modulus given in equation (7), also used for composites based on flax fibers [30,31], we deduced Young's modulus of the matrix: where E f , E m , and E RVP , respectively, represent Young's moduli of the fiber, the matrix, and the RVP and S f represents the surface fraction of the fibers in a sample of RVP. Equation (8) was obtained from equation (7) and is to calculate Young's modulus values of the matrix in each sampling zone:

Calculation of the Tensile Strength of the Matrix from the RVP.
e determination of the tensile strength of RVP matrix along the stem was made using RVP and RV fiber tensile strengths obtained graphically from experimental curves. From the law of mixtures of stress in equation (9), also used for composites based on flax fibers [31], we obtained the tensile strength of the RVP matrix: where σ f , σ m , and σ RVP , respectively, represent the tensile strength of the RV fiber, the matrix, and the RVP and S f represents the surface fraction of the fibers in a sample of RVP. Equation (10) was obtained from equation (9) to determine the tensile strength values of the RVP matrix in each sample area:

Determination of Young's Modulus of RVP (Composite) by the ree-Point Bending
Test. e evaluation of RVP Young's modulus using the three-point bending test was performed according to the ASTM D790-07 [32] standard method. e tests have been carried out using the MTS 20/M tensile machine set at a speed of 5 mm/min for the displacement of the movable jaw with the adaptation of a threepoint bending device. From the bending force F and the deflection Δl data, we plotted the curve of the force with respect to the deflection. We know that equation (1) gives the expression of the deflection as a function of the bending force and it has been used for the evaluation of Young's modulus of some samples of wood or composites based on flax fibers [31,33]. Δl � where E fl and I, respectively, are Young's modulus in the bending test and the quadratic moment in the cross section of the specimen. We can deduce Young's modulus of RVP in the bending test as the slope of the elastic zone of the curve of the pressure force with respect to the following equation: where α � F/Δl is the slope of the curve and I � (bh 3 /12) is the quadratic moment of the section. en,

Determination of Surface Fractions of the RVP.
Figures 2(a) and 2(b) show the cross section of a sample of RVP from the center and at the base (PL-1/4-R3) of the RV stem, respectively, from the optical microscope and ImageJ software. Similar approach of calculating the area fraction has been used to identify wood species by Gurau et al. [34] and for bacterial adhesion to polymeric substrates by Yuan et al. [35]. We observe the fibers and the material which makes it possible to organize them. is material gives shape to the RVP and also to the RV stem. It can be considered as the matrix of our sample. Figure 3 illustrates the evolution of the surface fractions of the RV fibers in a sample of RVP. We notice that these values vary globally in the interval of 35.80%-54.50%. ey are smaller than those found on wood during the study of the surface fractions which varied between 26% and 74% by Poorter et al. [36] and from 20% to 74% by Ziemińska et al. [37].
Considering the obtained values, we observe that, at any longitude along the RV stem, the surface fraction of the RV fiber increases from the center to the periphery. However, if we are at any radial position, the variation of the surface fraction will not be significant. is can be justified by the fact that fiber is continuous from the base to the top of the stem and the plants are at maturity. e surface fraction of the matrix constituting the pith can be deduced from the following formula: where S m is the surface fraction of the RVP matrix. We noted that the surface fraction of the RVP matrix oscillates in between the interval of 45.50%-64.20%. ese values are close to the volume fraction assimilated to surface fraction in our case during the realization of natural fiber-reinforced polymer composites [38]. . Figures 4(a) and 4(b) highlight the structure of different chemical elements constituting the RVP. e atoms of carbon, oxygen, and silicon can be observed. Figures 5(a) and 5(b) show peaks emerging from the chemical elements contained in a RVP sample extracted from the 1/4 center area, respectively, on the fiber/matrix boundary and on the matrix. We noted that each sample generally contains a majority of carbon followed by oxygen and then traces of silicon. Nevertheless, some areas of study show us the presence of calcium in a very small percentage, as shown in Table 1. Note also that, in the synthesis of the chemical elements of the sampling zones along the RV stem, the percentage of carbon is in between 51% and 57% while that of oxygen varies in the interval of 41%-48%. Such an observation was also made for Roselle, palm, and hemp fibers [20,39].

Determination of the Chemical Elements of the RVP along the Stem
Finally, we notice the traces of calcium and silicon with mass percentages not exceeding 2.5%. Figure 6 shows the evolution of the bulk density of the RVP along the stem. We noted that, in any transversal position, the bulk density evolves increasingly from the center to the periphery and in any position of the cross section of the stem and decreases from the base to the top of the stem.

Case of the RVP.
We find that the values are between 0.116 g/cm 3 and 0.343 g/cm 3, and it is comparable to the bulk density of RV fibers which evolves from 0.128 g/cm 3 to 0.237 g/cm 3 during the study conducted by Sikame et al. [16]. e RVP can be considered as a light material.

Case of the RVP Matrix.
e bulk density of the RVP matrix is evaluated using equation (4). Figure 7 shows the evolution of the bulk density along the RV stem. We noted also that the values increase from the center to the periphery at any cross section along the stem and they are between 0.1 g/cm 3 and 0.46 g/cm 3 . e density of the matrix of RVP is less than that of polymer generally used as matrix during the elaboration of composite [38,40]. en, we observed that the maximum values are at the base of the stem for the bulk density of RVP and its matrix.
By comparing them to the synthetic matrices given in Table 2, we found that they are the lightest bulk density. Figure 8 shows the curve of the stress with respect to the deformation of a sample of RVP from the zone (PL-1/4-R3).

Determination of Young's Modulus of RVP in Tensile Test.
is curve has a linear tendency. is appearance was observed for all the samples tested and enables us to conclude that the RVP seems to be a fragile composite material. Young's modulus was obtained from the linear zone of the curve as they have been done for the RV fibers [16]. Figure 9 illustrates the evolution of Young's modulus in the tensile test along the twelve sampling zones of the RV stem. We observe firstly that it increases from the center to the periphery in any cross section and secondly from the top towards the base of the stem when it is situated in a radial position. A similar result was found when studying the distribution of mechanical properties of RV fibers along the stem [16]. Finally, it appeared that Young's modulus of RVP is between 0.79 GPa and 3.23 GPa. ese values are close to those of wood species varying from 2.79 GPa to 16.03 GPa [40], which means the RVP can be resistant when used as raw material in building construction.
Furthermore, the tensile strength of RVP was determined by the curves in Figure 8 of the samples along the RV stem. It indicates that the values of this stress lie in the range of 11.7-44.5 MPa. Figure 10, Young's modulus of RVP matrix increases from the center to the periphery in any transversal position along the stem. Similarly, it is observed that the zone with the highest Young's modulus is the base of the stem (PL-1/4). is remark is similar to that of RVP. It is in contradiction with the results of the work carried out on the mechanical characterization of RV fibers along the stem Advances in Materials Science and Engineering [16]. e values of Young's modulus of the RVP matrix are in the range of 1.20-5.5 GPa and are in the same order compared to those of some polymers [41,42].

Determination of Young's Modulus of the Matrix from RVP. As shown in
In addition, the tensile strength of RVP matrix and RV fiber was evaluated using equation (1). e values obtained are between 12.82 MPa and 75.16 MPa.

Determination of Young's Modulus of RVP in ree-Point
Bending Test. Figure 11 illustrates the force versus deflection curve of a RVP sample. Note that this curve has two zones, namely, a linear zone (I) and a nonlinear zone (II). To determine Young's modulus of the RVP samples from the bending tests, we used the linear part. is approach has been applied for the determination of Young's modulus of flax, abaca, or RV fibers [16,43,44].   Similarly, Figure 12 shows the evolution of Young's modulus of RVP along the stem at the end of the three-point bending test. It is clear that Young's modulus evolves from the center to the periphery in any cross section along the stem. e values obtained are between 1.02 GPa and 4.20 GPa, and the highest values are at the base of the stem (PL-1/4). ey are more important than Young's modulus of some wood species [42]. Table 2 regroups the values of physical and mechanical properties of some polymers, wood species, and materials derived from the RV plant. We noted that Young's modulus is higher in flexion than in tensile tests. We observed that RVP has Young's modulus comparable to some wood varieties. Moreover, the specific mechanical properties show that RVP is more resistant than some wood species given in Table 2. Similarly, the RVP matrix has Young's modulus value in the same order as that of the polymer matrix. Also, it has one of the lowest densities, conferring its high specific mechanical properties. It emerges that the physical and

Conclusion
In this study, the physicochemical and mechanical characterization of RVP is investigated. Samples were taken from twelve sampling zones along the stem and were characterized using the following techniques: the observation of images using an optical microscope and ImageJ software, SEM for chemical element analysis, Archimedes' principle for bulk density determination, and tensile tests and threepoint bending tests for Young's modulus determination. e conclusion of these experiments is as follows: (i) Surface fraction values of RV fibers evolve from the center to the periphery in the radial position and vary between 35.80% and 54.50%. (ii) RVP is constituted essentially of carbon (C) and oxygen (O) with a percentage of mass, respectively, in the intervals 51%-57% and 41%-48%. (iii) e bulk density values of RVP and RVP matrix increase from center to periphery and range, respectively, from 0.114 g/cm3 to 0.343 g/cm3 and 0.10 g/cm3 to 0.46 g/cm3. (iv) Young's modulus of RVP and RVP matrix rises from the center to the periphery and varies, respectively, in the intervals 0.    Advances in Materials Science and Engineering make a better choice of matrix. In this perspective, we can consider the development of biocomposites based on RV fibers and the study of thermal properties of RVP.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.