Effect of the Addition of Oil Palm Mesocarp Fibers on the Physical and Mechanical Properties of a Polyester Matrix Composite

This work focuses on the assessment of oil palm mesocarp fibers as reinforcement in a composite material with an unsaturated polyester matrix. Several volume ratios of OPMF reinforcement (0 to 15%) were used, the fibers being distributed randomly. The resulting composite was characterized on the physical and mechanical aspects. Physically, the true and apparent densities were determined as well as the porosity rate. It appears that the addition of fibers further lightens the composite and increases the porosity. The water absorption rate of the different composites samples was evaluated. The more fibers the composite contains, the higher its water absorption rate. On the mechanical aspect, the bending modulus of elasticity, bending stress at break, and breaking strain were evaluated through a three-point bending test on all combinations. The same parameters were also evaluated for certain combinations by a unidirectional tensile test. It appears from this mechanical characterization that the volume fraction of 5% reinforcement has the highest specific modulus. Impact tests were performed on samples of this combination using several sizes of reinforcing fibers. Impact resistance is enhanced as the size of the inclusion increases. The Halpin-Tsai micromechanical model for randomly distributed short fiber composites was used for the inverse approach determination of the theoretical moduli of the matrix and OPMF, then in a direct approach to determine the elastic modulus of the composite at 7.5% reinforcement.


Introduction
The use of plant fibers to replace synthetic fibers in the reinforcement of composite materials has really taken off in the last decade. In addition to being environmentally sustainable, these composites in some cases have technical and usage properties close to synthetic fiber composites [1]. If plant fibers are criticized for their great hydrophobicity, their great lightness and their mechanical properties acceptable for use in many fields can be recognize [2][3][4][5]. In addition, many treatments, although most of them are chemical, improve their resistance to water and humidity and promote their use [6][7][8]. The fields of use for plant fiber composites are varied. They are found in the automotive industry, sport, aeronautics, energy, furniture, construction, and many other fields [9]. These new construction materials made from recycled plastic waste, mining waste or plant fibers, represent one of the best ways to solve environmental challenges according to the logic of sustainable development [10][11][12]. They reduce greenhouse gas emissions, limit energy consumption, and optimize the use of nonrenewable natural resources. The use of agricultural residues therefore becomes a reliable alternative to solve current technical as well as environmental problems such as the depletion of primary resources, the improvement of certain technical properties by the use of conventional materials (density, rigidity, thermal conductivity, and dimensional stability), and pollution by agricultural and industrial by-products [11,13].
The plant fibers used for these composites come from various origins (leaves, fruit, trunks, roots, etc.), and many plants are the subject of study. Jute, sisal, cotton, coconut, and ramie fibers are widely tested and used in the manufacture of biocomposites. The choice of fibers is often justified by their availability, their technical properties, and their compatibility with the matrix, which can be a thermosetting or thermoplastic polymer [1,14]. In some cases, the fibers are woven to produce fabrics serving as reinforcement for the composites. The influence of weaving strategies is also discussed in the literature [15]. Some plant fibers, such as oil palm mesocarp fibers (OPMF), although abundantly available because of the industrial production of palm oil, are still poorly used for the reinforcement of biobased composites.
The main works in the literature on the particular use of OPMFs in the manufacture of composites are those of Okafor [16] in the reinforcement of a ceramic matrix for tiles production. The results show a considerable improvement in the wear properties of the tile; however, the resistance to humidity and water is strongly influenced. Epoxy resin is also reinforced by OPMF by Yussof et al. [17]. Results show improvement in modulus of elasticity for 5% volume reinforcement in OPMF, but continued decrease in flexural stress and elongation with fiber inclusion. Other resins such as polypropylene (PP), polyvinyl chloride (PVC), Phenol formaldehyde (PF), and polyurethane (PU) [18] are also reinforced with fibers from oil palm. These composites have acceptable physical and mechanical performances for their use in fields such as aerospace, automotive parts, sports and recreation equipment, boats, office products, machinery [19]. Whatever the biocomposite, the challenges lie in the choice of volume fractions, architecture, and sizes of particles or fibers to optimize the physical and mechanical performance [20][21][22].
This research work focuses on the effect of the rate of reinforcement in OPMF on the physical properties (density, porosity, and water absorption rate) [23], and mechanical (bending, tensile, and impact test) of a polyester matrix biocomposite. The experimental approach is discussed here. It is then coupled with a theoretical micromechanical approach based on the Halpin-Tsai composite homogenization theory on the mechanical properties of composites.

Materials and Methods
2.1. UP-OPMF Composite Formulation. Fibers from a palm oil production factory in the city of Nkongsamba-Cameroon are washed in a 5% sodium hydroxide (NaOH) solution. They are then dried for three days away from the sun. Seven (07) mixtures corresponding to different reinforcement volume rates are then formulated. They are referred to as UP-OPMF-X. X is the volume fraction of fiber in the mixture and UP-100 to the samples without fiber used as reference as shown in Table 1.
In practice, due to the difficulty of measuring fiber volume, the law of mixtures has been used to convert volume fractions (ϑ f and ϑ m ) to mass fraction (m f and m m ).
where ϑ f and ϑ m , respectively, the volume fractions of the fiber and matrix, ρ f and ρ m their densities, ρ f = 1206 kg/m 3 [24], and ρ m = 1300 kg/m 3 [25]. Cold compression contact molding [26] was used for the manufacture of composites in the form of composite plates of dimension 10 × 90 × 150 mm which will subsequently be machined to produce the specimens for the various tests.  Figure 1). For each combination, six specimens were used, and the average density of the combination is calculated.

Experimental
The apparent density is calculated using where ρ a is the bulk density of the sample in kg/m 3 , m is the mass of the sample in kg, and V ac is the apparent volume of the composite in m 3 calculated from the dimensions of the specimen.

Determination of the True Density of the Composite.
The specimens used for the determination of the true density of the composite have a geometry similar to those in Figure 1. The gravimetric method was used to evaluate the volume of a column of water displaced in a beaker. The sample is weighed before being immersed in water, and its mass is noted m. it is then immersed in water and causes a variation ΔV of the volume in the beaker. The true density is then determined by Equation (3). For each of the combinations, 06 specimens are used, and the average density of the combination is calculated.
where ρ r is the true density of the sample in kg/m 3 , m is the mass of the sample in kg, and V rc is the volume of the composite in m 3 corresponding to the change in the volume of water.

Determination of the Porosity Ratio of the Composite.
Depending on the manufacturing process, the porosity rate of composites is between 1 and 10%. A high porosity rate essentially results in significant drops in mechanical properties. A low level of porosity reflects a good compactness of the composites.

International Journal of Polymer Science
The porosity pð%Þ of the UP-OPMF composite is calculated by Equation (4) [27].
where ρ r and ρ a , respectively, are the true density and the apparent density in kg/m 3 .

Determination of the Water Absorption
Ratio of the UP-OPMF Composite. Specimens ( Figure 1) are initially weighed and subsequently immersed in distilled water at room temperature. The gravimetric method is used. Initially, the test pieces are weighed every 5 min, and the new masses are noted. After observing that the mass does not vary considerably after 5 minutes, the duration of immersion in distilled water is increased to 10 minutes, 30 minutes, 60 min, and 120 min then every 1440 min. The operation is repeated until the mass no longer changes (saturation). At the time of the mass measurement, the samples are taken out of the bath and lightly wiped with an absorbent paper to remove the film of water present on the surface. The samples are then weighed and again immersed. A balance with an accuracy of 1 mg is used for these measurements, which are necessary for calculating the water absorption ratio using Equation (5) [28].
where F is the force applied in (N), L is the distance between the supports in (mm), h and l are, respectively, the thickness and the width of the test piece in (mm), and f the deflection corresponding to the load F in (mm). The ratio ðΔF/Δf Þ corresponds to the slope of the linear zone (elastic domain) of the force-displacement curve.

Mechanical Characterization of the UP-OPMF
Composite by Unidirectional Tensile Test. The specimens of dimensions 7 × 15 × 150 mm are used for the unidirectional tensile test. The BS EN ISO 527-5 [31] standard relating to the determination of the tensile properties of plastics/composites has been implemented. An average of five specimens per combination are tested. Only the combinations with 5, 7.5, and 10% volume ratio of reinforcement were studied. Relations (7), (8), and (9) are used for the calculation of the tensile stress σ Rt , the tensile modulus of elasticity E tr , and the ultimate strain of the composite ε Rt .
where Fr is the breaking force in (N), L 0 is the distance between the strain gauges in (mm) at the start of the test, h   Designation     International Journal of Polymer Science and l are, respectively, the thickness and the width of the test piece in (mm), and L u the length of the test specimen at break in (mm).

Determination of the Impact Strength of the Composite.
Impact strength characterizes the material's ability to withstand shocks [32,33]. The Charpy pendulum test [34] is performed here. This test is carried out in accordance with the ASTM D256 standard. Prismatic specimens of rectangular sections were used for this test [35]. Only the 5% combination of fiber reinforcement rate was investigated. The influence of the size of the reinforcing inclusions on the impact resistance of the composite was evaluated. For this purpose, the fibers were crushed and then sieved to obtain different sizes (0.5, 1.5, 2.5, 4, and 5 mm). 10 specimens per formulation were tested, and the average value of the impact strength was calculated.
During the test, the specimen is placed between two jaws, then the pendulum is moved from its equilibrium position by an angle θ i , and released without initial speed. As it passes vertically, it strikes the specimen and breaks it, then rises at an angle θ f . The energy absorbed by the specimen is obtained by subtracting from the energy stored at position θ i that which remains after rupture of the specimen and allows the lift (residual energy). Equation (10) allows the calculation of the fracture energy W a ðJÞ and the impact strength K ðJ/m 2 Þ.
where m p is the mass of the pendulum (100 g), g the     The theoretical modulus of elasticity of the composite was determined using micromechanical homogenization tech-niques [36,37] valid for the short-fiber composites randomly dispersed from Halpin-Tsai. The Halpin-Tsai model uses the mechanical properties of the fiber and the matrix to calculate the mechanical properties of the randomly dispersed fiber composite. The objective is to obtain results that can serve as a basis for discussion on the molding protocol, to speculate on the structural defects that may result from this forming technique as well as the compatibility between the polyester matrix and the OPMF. Equations (11) and (12) present this formulation [16].
P, P f , and P m are the respective moduli (longitudinal, transverse, or shear) of the composite, the fiber, and the matrix. ζ is a constant defining the aspect ratio of the fiber. These parameters are detailed in Table 2.
The Halpin-Tsai model was used in the case of this work in reverse approach. For this purpose, the longitudinal modulus of elasticity obtained in unidirectional traction (E tr ) of two combinations of the composite (5 and 10%) is used to determine the theoretical modulus of elasticity of the matrix (E mth ) and of the fiber (E fth ). These theoretical moduli are reused in the direct homogenization approach to predict the theoretical modulus of elasticity of the sample at 7.5% and compare it to those obtained experimentally in unidirectional tension and in three-point bending. This approach is implemented by a system of two equations where ϑ f i ϵ h5 ; 10i is the volume ratio of composite i and E ci is the unidirectional tensile modulus of elasticity of the composite having the volume fraction ϑ f i .

Results and Discussions
3.1. Composite UP-OPMF. At the end of the contact molding, the composite obtained has a light brown appearance indicative of the presence of the OPMFs. It is rigid and easily machinable, and the fibers are dispersed randomly but uniformly. The plates of Figure 2(a) have in fact been sawn on an edger and then surfaced with a milling machine (Figure 2(b)) to obtain the beam specimens of Figure 2(c). The parts likely to undergo conventional machining (drilling, sawing, surfacing, etc.) can, if the usage properties allow it, be molded and then machined in UP-OPMF biocomposite.

Apparent, True, Theoretical Densities and Porosity
Rate of the UP-OPMF Composite. The apparent and true densities obtained experimentally are presented in Table 3. There is also the theoretical density obtained by calculation of homogenization from the densities of the constituents as well as the porosity rate of the various samples. Table 3 highlights the difference between true densities and theoretical densities significant for the existence of porosities within the material. The variations of the different densities and the rate of porosity with the ratio of volume of reinforcement in fibers can be appreciated in Figure 3.
All densities decrease as the percentage of fiber increases. This result is in agreement with the specific literature on composites. However, the variation is more pronounced on the experimental values compared to those obtained by the homogenization theories. This calls into questioning the validity of theoretical homogenization models. A comparison of the true and theoretical densities of the composite makes it possible to demonstrate the porous nature of the material. It is observed that the presence of the fibers increases the level of porosity of the material. These porosity values are however lower than those obtained for other composites in the literature [38,39] and are well below the maximum permissible porosity threshold for large diffusion (GD) composites which is approximately 10% [22].

Water Absorption Rate in the UP-OPMF Composite.
Saturation is observed for all samples after the fourth hour of immersion. The average absorption rates as well as the standard deviations are presented in Table 4. Table 4 highlights the fact that adding fibers to the composite increases its water absorption capacity and may affect its durability. Figure 4 shows the evolution of the absorption rate as a function of the reinforcement ratio.
The water absorption rate of UP-OPMF composites is correlated with the porosity rate although the trend of the curves is not exactly the same. The increasing of the absorption can be linked to hydrophilicity character of lignocellulosic fibers. By including 15% of OPMF in the matrix, there is an approximately 17% increase in water absorption ( Figure 4). This suggests the need to consider treatments to reduce the hydrophilicity of OPMFs prior to their insertion into such composites. However, this absorption edge is acceptable for the materials of the front building industry serve interior trim, ceiling, or partition walls. Moreover, these composites can undergo the surface treatments (coating, varnish) commonly applied to wood to reduce their absorbent character.

Mechanical Bending Properties of the UP-OPMF
Composite. The variation of the mechanical bending properties of the UP-OPMF composite is presented in Table 5.
From this table, it can be said that the addition of OPMF affects the mechanical flexural properties of polyester. The evolution of the flexural strength of the composite, the breaking strain, and the flexural modulus of elasticity as a function of the reinforcement rate is presented in Figure 5.
The addition of OPMFs in the polyester matrix reduces the flexural breaking stress as well as the flexural breaking strain by almost 60%. This reduction is certainly due to the presence of reinforcing fiber which generates areas of stress concentration, the low adhesion at the interface, and the inherent mechanical properties of the fibers which are weaker than those of the matrix. Since these fibers are low in number, there is a low charge transfer capacity and therefore a low level of resistance of the composite. This level of resistance improves with the increase in the number of fibers and therefore the charge transfer capacity; this results in an increase in the bending breaking stress between 2.5 and 10% of fiber reinforcement. A similar result was also obtained in the literature [40] for the hybrid composite PLA/PCL/nanoclay/OPMF. Beyond this reinforcement  International Journal of Polymer Science value, although the elongation capacity of the material before rupture continues to increase, the bending breaking stress drops, indicating that the reinforcement limit threshold has been reached. The longitudinal modulus of elasticity increases up to 5% reinforcement. Beyond this value, it drops continuously with increasing fiber content. In view of the bending results, the composite with 5% reinforcement seems to present the best compromise between the different properties.

Tensile Mechanical Properties of UP-OPMF Composite.
Unidirectional tensile tests were performed on specimens of the 5, 7.5, and 10% combinations. For each of these combinations, the longitudinal modulus of elasticity, the breaking stress as well as the tensile breaking strain is determined. Table 6 summarizes the different parameters obtained for each of the combinations tested.
In view of the tensile results, it is observed that the modulus of elasticity of the UP-OPMF-5 composite is higher than that of the other combinations. In addition, its tensile strength at break as well as its breaking strain is the highest. A similar behavior is also described in the literature by Zuhri et al. [17]. Figure 6 shows the distribution of the different tensile properties as a function of the reinforcement ratio.
The breaking strength and the breaking strain evolve in a similar way. An unexplained drop is observed for the 7.5% fiber combination. The modulus of elasticity decreases with the addition of fibers. While the magnitude is the same for the tensile modulus of elasticity with those of the threepoint bending test, we can see that it is higher in the tensile case. This would come from the fact that the fibers, although distributed randomly, have a particular distribution plane resulting in a plane orthotropy of the composite. Similar behavior was reported by Eng et al. [41] in the case of reinforcement of hybrid composites PLA/PCL/nanoclay/OPMF.

Impact
Behavior of the UP-OPMF Composite. Due to the fact that the 5% composite presented the best mechanical properties, only specimens reinforced with 5% fibers of different sizes (0.5, 1.5, 2.5, 4, and 5) were tested as well as an unreinforced control specimen. It is observed on the one    hand an increase in impact strength by the addition of OPMF fibers and on the other hand by an effect of the size of the reinforcing particles on the impact strength of the composite. Figure 7 shows this variation.
Although the rate of change appears to be low, impact strength increases with the size of the fibers. It can therefore be said that the higher the aspect ratio of the fibers, the more energy the composite absorbs before breaking. This would be due to the greater attachment surface of the fibers in the matrix preventing their loosening. A similar result is also described in the literature for a polyester/banana fiber composite [42]. A comparison of the impact strength values with other composites is given in Table 7.
The impact strength values of the UP-OPMF composite are much higher than those obtained for poly (L-lactic acid)/ jute composites as well as all polyester matrix composites except the polyester/sisal-banana-PALF hybrid composite. However, they remain low compared to the values obtained for the various epoxy matrix composites.

Theoretical Modulus of Elasticity of the UP-OPMF
Composite by Halpin-Tsai Theory. Applying the Halpin-Tsai homogenization theory, the theoretical moduli of elasticity of the fiber (E fth ) and matrix (E mth ) were determined by an inverse approach. For this, the tensile results of UP-OPMF composites with 5 and 10% fiber reinforcement were used as baseline data. Table 8 shows the results obtained.
Obviously, although being of the same order of magnitude, it is observed that the theoretical modulus of elasticity of the matrix is greater than that of the composite. A comparison between the theoretical and actual moduli of elasticities of the fiber and the matrix is made in Table 9.
There is a significant difference between the theoretical and experimental moduli of elasticity of the matrix. This difference would be due on the one hand to the fact that the modulus of elasticity in bending is underestimated compared to that in tension and on the other hand to the possible defects which would exist in the structure of the molded composite (porosities, foreign inclusions, and interface faults). For against, the theoretical modulus of the fiber is 4.6% almost identical to that obtained experimentally.
By direct homogenization approach, theoretical fiber and matrix values was used to determine the theoretical modulus of elasticity of the composite at 7.5% fiber reinforcement rate. Table 10 presents the results as well as the error made in the prediction process.
The theoretical modulus of elasticity is in the same order of magnitude (within 5.8%) as the experimental modulus of elasticity. The Halpin-Tsai homogenization technique therefore makes it possible to effectively predict the tensile modulus of elasticity of the UP-OPMF composite.
3.7. Specific Modulus of the UP-OPMF Composite. The specific tensile and flexural moduli of the composite were calculated. They are summarized in Table 11.
The 5% fiber-reinforced UP-OPMF composite has the highest specific modulus. It therefore has the best strengthto-weight ratio.

Conclusion
This work focused on the formulation, manufacture, and characterization of several combinations of an UP-OPMF composite with differential volume fractions of OPMF reinforcement and having as matrix the polyester resin. The different combinations of this composite have undergone physical characterization tests (density, porosity, and water absorption) and mechanical (flexion, traction, and impact strength). It appears that the density increases with the rate of fiber reinforcement. In addition, the true density is higher than the bulk density. This is due to the presence of the pores in the material. The porosity rate was therefore determined; it varies from 2.22 to 3.46%. Obviously, the more porous the material, the more absorbent it is. The average absorption rate ranges from 0.13 to 17%. Then, by a threepoint bending test, the flexural modulus of elasticity of the UP-OPMF composite was determined. The modulus of elasticity obtained in three-point bending varies from 1218 to 2489 MPa with a peak for the reinforcement at 5%. The tensile modulus of elasticity, the breaking stress, and the breaking strain for the composites at 5, 7.5, and 10% was determined. As in the case of bending, the 5% composite is the one with the highest tensile modulus of elasticity (3318 MPa) as well as the stress and strain at break. By applying the Halpin-Tsai homogenization theory, the theoretical prediction of Young's modulus was made. It appears that the composite with 5% volume reinforcement is the one with the best specific modulus. Although some analyses are necessary to clarify the final use of the composite (analysis of the thermal behavior for example), the current results allow to conclude that this composite can be used in the building industry for the interior lining, the ceiling, or the partition walls.

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

Conflicts of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.