Mechanical Behavior of Epoxy Reinforced by Hybrid Short Palm/Glass Fibers

Abstract

Natural fibers (NFs) have recently been the center of attention among researchers due to their low cost, availability, ease of manufacture, and potential environmental friendliness as reinforcing agents in composites. The present work deals with the mechanical behavior of palm fiber-reinforced epoxy-based composites with different weight percentage (Wt.%) ratios, ranging from 6% to 31.6%. Glass and hybrid fiber-reinforced epoxy-based composites were also examined. The indirect tensile test, i.e., diametral tensile test (DTT) and the small punch test (SPT), were used in the present work to determine the mechanical properties of the epoxy reinforced with discontinuous random oriented short fibers. Furthermore, short glass fibers were used to compare with palm fiber-reinforced epoxy. In addition, morphology observations of epoxy residue clinging to the natural fibers were carried out using the optical microscope and Scanning Electron Microscopy (SEM). The results showed that the natural fiber has a better adhesion bonding between the palm fiber/epoxy than that of glass fiber/epoxy. Therefore, adding palm fibers improves epoxy’s mechanical properties compared with synthetic glass fibers. The composite with high Wt.% of NF showed the highest diametral tensile strength (DTS), 21.74 MPa, over other composites. The DTS of composites with medium and low Wt.% of NF was lower than that of the high Wt.% by 14% and 30%, respectively.

1. Introduction

Nowadays, green composite materials dominate the world of material engineering because of the growing need for more environmentally friendly materials to reduce environmental degradation and pollution and save the earth. Green composite materials consist of natural materials. Natural fibers are one of the abundant resources that exist in nature. Natural fibers have many advantages including recyclability, ease of manufacturing and shaping, and acting as a cost-effective alternative to synthetic fibers. Plant-based natural fibers are becoming more popular as reinforcing elements for polymer composites because of their environmental benefits, low cost, easy availability, and biodegradability. Several types of natural fibers are frequently used as reinforcing elements in composites: Jute, sisal, kenaf, flax, hemp, and palm [1,2,3], which have comparatively high tensile strengths, among others.

The mechanical properties such as tensile strength, flexural strength, and impact strength of those types of composite material were listed in [2]. Obviously, natural fibers are good candidates to use instead of synthetic fibers. Moreover, the mechanical characteristics of composites fabricated with natural fiber reinforcement are influenced by several factors, such as moisture absorption, fiber treatment, fiber distribution, and the use of additives [4]. Abdal-hay, et al. [5] used date palm fibers (DPFs) to reinforce polymer matrices. They studied the effects of changing DPFs diameters and alkali treatments on the tensile properties of date palm fibers/epoxy composites with discontinuous random-oriented short fibers. They concluded that alkali treatment of DPFs improved the tensile strength and elastic modulus and the fiber-matrix interaction of DPFs reinforcement composites. Helaili, et al. [6] used the natural Alfa short fiber to reinforce Epoxy resin by randomly distributing the fibers. They concluded that reinforcing the resin with a 10% volume of Alfa fibers improved the elastic modulus and failure stress of the epoxy. In addition, Berthet, et al. [7] used milled natural fiber wastes extracted from the agribusiness of manufacturing peach palm hearts to reinforce the epoxy with (50–70 Wt.%). They used the manual mixed method and concluded that the natural fiber improved the mechanical behavior by improving flexural strength, flexural modulus, and impact resistance of the epoxy-based composite.

Furthermore, some applications use natural fibers with synthetic fibers in hybrid composite preparations. Ramasamy, et al. [8] studied the mechanical properties of Kenaf and Kevlar fiber-reinforced epoxy-based composites. They found that Kenaf and Kevlar fiber hybridization improved epoxy composites’ mechanical properties. In addition, Ganesh, et al. [9] used the hand lay-up technique to manufacture four different tri-layer composites by varying the stacking sequence of areca sheath and palm leaf sheath fibers with epoxy as a matrix. They assessed the hybrid polymeric composites using tensile, flexural, compression, and impact tests. Furthermore, many researchers [10,11,12,13,14,15,16,17,18,19,20] have studied the tribological behavior of natural fiber (including agave, bamboo, Borassus fruit, flax, hemp, jute, kenaf, ramie, and sisal fibers)-reinforced epoxy-based composites. They concluded that the natural fiber improved epoxy-based composites’ wear and friction performance. The use of natural fiber can improve the tribological behavior of the composite material more than glass fiber [21]. It can be summarized that some researchers have studied the effect of the natural fiber diameter, alkali treatment [5], fiber weight fraction [6], stacking sequence [9], and fiber size [22] on the mechanical properties of the natural fibers epoxy-based hybrid composite.

The main objective of the present work is to enhance the mechanical properties of epoxy by reinforcing it using short palm fibers, glass fibers, and a hybrid of them. The effect of weight percentage, Wt.% of the natural fiber, 0, 6%, 15.8%, and 31.6%, on the mechanical properties of palm fiber-reinforced epoxy-based composites are considered and then compared with synthetic fibers. The novelty of the present work is the study of the synergistic effect of palm-glass fibers in the epoxy-based hybrid composite. Moreover, the microstructural examination was performed to investigate interfacial adhesion between epoxy and short palm/glass fibers of both tested and untested specimens using the optical microscope and SEM. The mechanical properties obtained from both the diametral tensile test and the small punch test were implemented to obtain the value of the material-dependent empirical constant (β_UTS) suggested by ASTM E3205-20 [23].

2. Experimental Work

2.1. Raw Materials and Preparations

The raw materials used in the present study to manufacture the composite materials were natural fibers, epoxy (LR 1110), hardeners (RC932), and glass fibers. The natural fibers were palm fibers (gathered from the base surrounding the stem of the palm tree plant) that can be found in abundance throughout Asia, South America, and Africa, as shown in Figure 1a. The short palm fiber was exposed to surface treatment by immersing the palm fiber in ethanol at room temperature (see Ref. [22]) to clean it from sand and dust and then immersed in hot water to remove other sediment attached to the surface. After being dried in sunlight for 24 h, the palm fibers were divided into single bundles. The palm fibers were trimmed so that the fibers had an average length of around 3 to 5 mm, with a diameter of 1 mm, as shown in Figure 1c. Subsequently, the short palm fibers were immersed in polyethylene to remove moisture, as recommended by Abdal-hay et al. [5]. The glass fibers were cut from the long continuous fiber, as seen in Figure 1b, to create short fibers with lengths of 3 to 5 mm, as shown in Figure 1d.

The commercial epoxy (LR 1110) and hardeners (RC932) were used in the present work to manufacture epoxies reinforced with natural and glass fibers. The physical and mechanical properties of glass fibers and epoxy are listed in

Table 1. The physical and mechanical properties of the used materials *.

Material	Physical Properties	Value **
Epoxy	Density	1.11 ± 0.02 Kg/L
	Temperature resistance	Humid 90 °C/dry 140 °C
	Modulus of elasticity, E, (GPa)	3.7
	Poisson’s ratio, ν	0.36
E-glass fiber	Filament diameter	13 µm
	Linear density	2400 tex
	Modulus of elasticity, E, (GPa)	82
	Poisson’s ratio, ν	0.22

* The raw materials used in this research were provided by Amiantit Group and Jubail Chemical Industrial Company, KSA. ** According to the manufacturer’s datasheet.

, and the physical properties of date palm fiber were reported in [24].

The short palm fibers and glass fibers were mixed with the epoxy resin in a random orientation using a hand lay-up technique at room temperature. After that, the hardener was added to the mixture. Four different weight ratios, Wt.% (0%, 6%, 15.8%, and 31.3%), were used for epoxy reinforced with palm fibers, while a 15.8 Wt.% was used for epoxy reinforced with glass fibers, as shown in

Table 2. The mixing ratios of various epoxy composites.

Specimen, ID	Description	Short Palm Fiber, Wt.%	Short Glass Fiber, Wt.%
PE	Pure/Net Epoxy	0	0
LNF	Low percent Natural Fiber	6	0
MNF	Medium percent Natural Fiber	15.8	0
HNF	High percent Natural Fiber	31.6	0
GF	Glass Fiber	0	15.8
HyF	Hybrid Fiber	15.8	15.8

. The current ratios were adopted according to the previous work found in the literature [5,10,14], ranging from 5% to 43%. Furthermore, hybrid palm–glass fibers with 15.8 Wt.% each were used to produce a hybrid composite, as shown in Table 2.

2.2. Small Punch and Diametral Tensile Tests

The hand layup technique was used to manufacture the natural/epoxy composite. A sharp blade was used to cut the short fiber. The short fibers were added to epoxy at room temperature; after that, the 12% hardener was added with soft mechanical steering (Economic mechanical stirrers AM90L-P series with speed rotation range 50 to 1400 r.p.m) to remove the bubble formations. Subsequently, the mixture was poured into the foam mold, whose geometry depended on the test type. The mold was left for 24 h to complete the solidification process then the specimen was ready to test. The mechanical behavior of the epoxy reinforced with the short palm fibers can be determined in the present work by using two types of tests, the indirect tensile test, i.e., the diametral tensile test (DTT); and the small punch test (SPT). The SPT specimens had a diameter of 17 mm, with a thickness of 2 mm, while the DTT specimens had cylindrical diameters and heights of 10 mm and 40 mm, respectively, as shown in Figure 2. The diametral tensile strength (DTS), (σ_DS) can be determined from DTT by using the following equation [25,26]:

$${\sigma }_{DS}= \frac{2F}{\pi dt}$$

(1)

where F represents the fracture load, d is the cylindrical diameter of the specimen, and t is the cylindrical height of the specimen.

The SPT and DDT investigate the effect of the fiber’s weight ratio, Wt.%, on the stiffness, and diametral tensile strength of short palm fiber-reinforced epoxy-based composite specimen, as shown in Figure 3. The SPT and DTT were carried out using a 100 kN Universal Testing Machine INSTRON-8801 with a 0.75 mm/min crosshead speed for applying axial load-displacement. All experiments acted at room temperature. The load-displacement response until full damage was traced for each test. Five specimens were used for each test to confirm the results.

2.3. Microstructural Examination

The microstructural examination investigated interfacial adhesion between epoxy and short palm/glass fibers of both tested and untested specimens. The optical microscope with a zoom of 2000× was used firstly for morphological analysis of the interfacial adhesion between epoxy and palm/glass fibers. Scanning Electron Microscopy (SEM) was employed for morphological analysis, using a field emission gun SEM of model: JEOL JSM-6380 LA) at an accelerating voltage of 10 kV. To improve the image quality, the samples were coated with chromium using a sputter-coater. The SEM instrument was fully embedded with an energy dispersive X-ray spectrometer (EDS) employed to quantitatively map the impurities in the natural palm fiber.

3. Results and Discussion

3.1. The DTT Results

The DTT describes the effect of short palm fiber’s weight ratio, Wt.%, on the diametral strength of epoxy-based composite specimens reinforced with short palm fibers. Figure 4 shows that the DTS of the palm fiber specimens is increased by increasing the weight percentage of short palm fibers. Whereas the maximum value of DTS was recorded for the HNF specimen as 21.74 MPa, the MNF and LNF specimens were reduced by 14% and 30%, respectively, more than that of the HNF specimen, as shown in Figure 4. Moreover, when introducing short glass fibers to the epoxy resin in a Wt.% of 15.8%, the DTS was 5.87 MPa, which means the DTS was less than that of the LNF specimen by 60%. The DTS of epoxy reinforced by short palm is higher than that of epoxy reinforced by short glass fiber; this may be attributed to the strong adhesion between the epoxy and palm fibers. Although the DTS of the hybrid palm/glass fiber specimen is lower than that of the HNF specimen, it is higher than those of single fiber composite of the same Wt.% ratio. This means that some synergic effects can be observed. It is worth noting that LNF, MNF, and Gf have lower DTS than PE. Nascimento et al. [20] found similar results, where epoxy-reinforced with Piassava fiber content of Vol.% up to 20% has a lower flexural strength than neat epoxy. However, they found that epoxy reinforced with Piassava fiber content of Vol.% equal to 30% and 40% significantly improved the epoxy’s flexural strength. In general, they found that the flexural strength of epoxy reinforced with Piassava fiber increased with increasing fiber volume fraction. On the other hand, Sbiai et al. [22] concluded that the neat polymer and the long palm fiber/epoxy composite showed ductile behavior. In contrast, in the case of using short palm fiber, it displayed a brittle behavior.

Figure 5 illustrates the damaged surface of the epoxy-based composite specimens reinforced with short palm fibers and glass fibers. The specimen with a low short palm fiber weight ratio has more fracture debris than the specimen with a high weight ratio, as shown in Figure 5. Furthermore, the glass fiber specimen was damaged into a powder-like structure. From Figure 5, it can be concluded that the specimens are fractured into many pieces under the indirect tensile test. These pieces increased by decreasing the weight ratio of short palm fibers, which may be due to the increase in the weight ratio of the short fibers increasing the integrity of the composites, i.e., the transition from brittle to ductile. It is worth noting that Mahmud et al. [27] fabricated ductile and toughen Polylactide (PLA) composites by melt blending epoxidized soybean oil (ESO) and frangible powder form of cellulose nanocrystals (CNCs) to prepare the ternary composite system (PLA/CNC/ESO). They found that their synthesis approach enabled a mechanical turning of the PLA’s brittle phenomenon into ductile.

3.2. Small Punch Test Results

The effect of short palm fiber’s weight ratio on the load-displacement curve extracted from the small punch test was investigated and compared with those of pure/net epoxy (PE) and GF, as shown in Figure 6. The behavior of the load-displacement curves of the PE, LNF, MNF, HNF, and GF specimens is relatively similar and different from the common load-displacement curves of metallic materials, as shown in ASTM E3205-20 [23]. The results revealed several points of interest, as the natural fiber-reinforced epoxy showed superior mechanical behavior compared with the glass fiber-reinforced epoxy. HNF is the only natural fiber-reinforced epoxy with greater strength than PE. Based on the small punch test results, Figure 7a,b shows the maximum values of force, F_m, and its deflection, U_m, for a short fiber composite sample with varying weight percentages of palm and glass fibers (PE, LNF, MNF, HNF, and GF). From Figure 7a, for the specimens reinforced with short palm fibers, the maximum load of HNF was 1.85 times higher than that of the LNF specimen and 1.3 times higher than the MNF specimen. In addition, the palm-reinforced specimens provided more resistance than glass fibers, regardless of the palm fibers’ weight ratio. Furthermore, the specimens with a 0% weight ratio of fiber (PE) show higher load resistance than other reinforced specimens except for HNF specimens. On the other hand, the deflection at the maximum load decreased with increasing the palm fiber weight ratio, as shown in Figure 7b. Furthermore, the deflection of the PE specimen is lower than all other reinforced specimens. For the specimens reinforced by natural fibers, it can be concluded that the maximum load increased with increasing the palm fiber weight ratio, while the opposite trend was observed for deflection at maximum load, i.e., the stiffness of the composite increased with increasing the palm fiber weight ratio. HNF specimens showed the highest mechanical properties over the other specimens, i.e., F_m = 258 N, U_m = 0.2 mm, and stiffness = 258/0.2 = 1290 N/mm.

According to ASTM E3205-20 [23], the modulus of elasticity, E, of short palm and glass fiber composite specimens can be determined by using the following empirical correlations:

E = λ × Slope

(2)

where the slope is the slope value of the elastic region of the load-displacement curve in Figure 6 and λ is the correlation factor which can be computed by using the modulus of elasticity of the epoxy in Table 1. The result of the previous equation is plotted in Figure 8 for the various specimens (PE, LNF, MNF, HNF, and GF). As shown in Figure 8, it can be concluded that no attitude can be shown for the modulus elasticity of short fiber composite specimen with different Wt.% of palm fibers and fiber types.

Furthermore, it can be predicated on the ultimate tensile strength (UTS), σ_u, as described in ASTM E3205-20 [23], by using the following empirical correlations equation:

$${\sigma }_u= {\beta }_{UTS}\frac{F_m}{t{u}_m}$$

(3)

where F_m and U_m are the maximum values of applied force and the specimen deflection corresponding to the maximum force F_m; t is the specimen thickness; and β_UTS is the material-dependent empirical constants. To determine β_UTS, it may be assumed that the ultimate tensile strength (UTS), σ_u, of Equation (3) is equal to the diametral tensile strength shown in Figure 4.

The values of β_UTS based on the above assumption are shown in Figure 9. Figure 9 shows that β_UTS decreased from 0.29 to 0.09 by increasing the Wt.% of palm fibers from 6% to 31.6%. It is worth noting that this trend is similar to the trend observed for the deflection at maximum load in SPT.

3.3. Morphological Analysis

3.3.1. Optical Microscope

Morphological analysis was conducted to examine the interfacial adhesion between epoxy and short palm/glass fibers of the tested specimen. The surface treatment of the natural fiber by ethanol seems to be a crucially important process for cleaning the outer surface to remove many impurities and incomplete grown fibers. Figure 10 shows the natural, glass, and natural/glass fibers after fracturing at different positions in the tested samples. The diametric size of a single natural fiber compared with a single glass fiber can be noticed in the hybrid fracture composites in Figure 10d. It can be noticed that natural fibers were hard to pull out through the matrix compared with glass fibers. In addition, natural fibers in Figure 10 have a rough surface compared with glass fibers that directly affect the bonding with the epoxy matrix. In general, Figure 10 predicts a failure in fiber-matrix adhesion, which resulted in the breaking of fibers at the point of loading in the small punch test.

3.3.2. Scanning Electron Microscopy (SEM)

SEM micrographs at the interfacial adhesion between epoxy and palm and glass fibers are shown in Figure 11. The samples were examined before and after diametral tensile tests. Figure 11a,b shows the glass fiber surrounded by epoxy before and after the test. They indicate that interfacial adhesion between the glass fiber and the epoxy is poor as the glass fiber is completely pulled out from the epoxy in the fractured sample. The lack of physical contact between the glass fiber and the epoxy in the tested sample is possibly due to the loss of interactions at the interfaces. For palm fibers, however, Figure 11d,e exhibit evidence of a better adhesion bonding between the palm fiber and the epoxy. It can be seen that most of the surface regions around the palm fiber are directly attached to the surrounding epoxy. No differences in interfacial adhesion were found due to the change in the mixing ratios of various epoxy composites.

Various researchers have studied the factors influencing the adhesive bonding properties in palm fibers. Delzendehrooy et al. [28] investigated the effect of type, size, treatment method, and density of fibers. They concluded that improved adhesion bonding of palm fibers led to a slowing down of crack propagation, absorbing energy, and load endurance. Kaddami, et al. [29] investigated the thermoset matrix composites on adhesive and mechanical properties for short palm fibers. They concluded that esterification and chemical treatments can be used to improve the interactions between the fiber and the epoxy, leading to improved adhesion and mechanical properties.

The fiber/matrix adhesion bonding is essential in determining the composite performance under all loadings as the applied force is exchanged from matrix to fibers via fiber/matrix interfaces [30]. Thus, the difference in interfacial adhesion between the glass and palm fibers observed in this study may be attributed to the distinction in the physicochemical interactions at the fiber/epoxy interfaces. Furthermore, silica bodies have been observed on the surfaces of all-natural fibers. This can be clearly seen in Figure 12, where two EDS spectra obtained from two different positions from the fiber surfaces exhibit the high silicon-rich particles with a round, spiky shape. The silica bodies were found to be relatively uniformly distributed over the fiber’s surface, with sizes ranging from 1 to 10 μm. In the literature, the presence of silica bodies on the fiber surfaces has been directly linked with interfacial bonding and mechanical properties [20,31,32]. In work by Nascimento et al. [20], for example, a relatively higher amount of silica on the surface of a palm fiber was found to be responsible for the reinforcement of the epoxy composites. A similar observation was also concluded by other authors [32]. In the current study, the presence of the silica bodies on the surface of the natural fiber may assist the interfacial bonding between the fiber and the epoxy. From the morphological analysis, it can be concluded that the adhesion bonding between palm fiber/epoxy is better than that of glass fiber/epoxy. Accordingly, the mechanical behavior of the palm/epoxy composite is better than that of the glass/epoxy composite, regardless of the fiber weight ratio.

4. Conclusions

In the present work, experiments were carried out to investigate the mechanical properties of epoxy-based composite specimens reinforced with short palm fibers. The following conclusions can be drawn:

• Palm fibers can be used to improve the ductility of epoxy;
• The use of palm fibers in an epoxy-based composite may improve the strength of the epoxy according to the weight ratio of fiber used;
• For the same Wt.%, epoxy reinforced with short palm fibers has a higher DTS than epoxy reinforced with short glass fibers;
• The composite with high Wt.% of NF showed the highest DTS, 21.74 MPa, over other composites. The DTS of composites with medium and low Wt.% of NF was lower than that of the high Wt.% by 14% and 30%, respectively;
• The material-dependent empirical constants (β_UTS) decreased from 0.43 to 0.06 by increasing the Wt.% of palm fibers from 6% to 41.6%;
• SEM analysis showed that the natural fiber has a better adhesion bonding between the palm fiber/epoxy than in the glass fiber/epoxy case;
• The better adhesion bonding of the natural fiber is possibly attributed to silica bodies attached to the surfaces and the differences in the physicochemical interactions at the fiber/epoxy interfaces.