1. Introduction
Natural fibers (NFs) are alternative materials to environmentally harmful synthetic fiber materials. Nowadays, natural fibers used as alternative reinforcements in polymer composites have received a lot of attention in research and in various industries due to their advantages over glass and carbon fibers because of their low environment impact, light weight, low energy consumption, abundance, and wide range of applications [
1]. Natural fiber-based composites have a wide range of applications in automotive and construction industries [
2]. In addition to the fiber and matrix properties, fiber treatment, fiber orientation, fiber volume and hybridization are the most common factors that govern the properties of natural fiber composites [
3,
4,
5,
6].
Fiber treatment is one way of reducing its hydrophilic tendency for improving compatibility with the matrix materials [
7]. Alkali (mercerization) treatment is the most common fiber chemical treatment method using Sodium hydroxide (NaOH), which is widely employed to modify the cellulosic molecular structure by removing the amorphous contents like lignin, wax, and oils that cover the external surface of the fiber and make a strong link between the fiber and the matrix [
8]. To improve the physical, mechanical, chemical, morphological, and thermal properties of natural fibers, the composites fibers are chemically treated using Alkali (NaOH), Acetic acid, Silane, Benzoyl peroxide, Isocyanate treatment, etc. [
9,
10,
11,
12]. Alkali treatment (mercerization) is the most common chemical treatment method using Sodium hydroxide (NaOH), which is widely employed to modify the crystal structure by modifying the cellulosic molecular structure of fibers. It removes weak components like lignin and hemicelluloses, wax, oil, and other impurities that cover the parts of the fiber surface to increase the roughness of the surface of the fibers [
8,
13]. The effect of treatment on the mechanical property of the Cordia dichotoma fabric was studied and the result shows that the 5% alkali-treated fabric shows better mechanical (tensile) strength than bleached and untreated fabric [
14]. The percentage of alkali affects the fiber surfaces, which determines the mechanical properties of the composite. High percentages of alkali may cause fiber surface damage, leading to a decrease in the mechanical properties. As the percentage of alkali concentration decreases, only a few impurities are removed from fiber surface. This affects the fiber–matrix interfacial and influences the mechanical properties of the composite [
15,
16].
The mechanical behavior (tensile, flexural, and impact strength) of an untreated and a NaOH-treated sisal-jute fiber hybrid epoxy composite was studied and the results showed that the treated-fiber composite improved in all cases of mechanical properties [
17]. In addition to fiber treatment, fiber volume and fiber orientation, a hybrid of two or more different fibers improves the mechanical properties of its composites [
18,
19].
When fiber volume is increased, the composite becomes stiffer and harder leading to decreased elongation at the break point of the composites. With increasing fiber loading, the tear strength of the composites increases due to the fiber, which makes the tear path more difficult for crack propagation. In the article reported in [
20], oil palm empty fruit bunch (EFB)/jute fiber reinforced epoxy bi-layer hybrid composites were fabricated by the hand lay-up technique with a total fiber loading of 40% by weight (1:4) and the effect of jute fiber loading on the tensile, storage modulus, loss modulus, and damping properties were studied in comparison with the pure EFB and jute composite. The result showed that the storage modulus of EFB and jute (4:1) hybrid composite was the lowest. It also showed that when jute fiber loading increased, the effectiveness of stress-transfer increased. Hybrid composite materials exhibit unique properties compared with EFB and jute composite. These properties suggest potential applications of oil palm–epoxy composite with jute fibers [
21]. The effects of hybridization for sisal/glass, jute/glass, and sisal/jute/glass polyester reinforced composites were studied and the hybrid composite (sisal/jute/glass) had better mechanical strength [
22]. The mechanical properties (tensile, flexural, impact, and hardness) of natural fiber-based composites (jute/epoxy, hemp/epoxy, flax/epoxy) and their hybrid composites (jute/hemp/epoxy, hemp/flax/epoxy, and jute/hemp/flax/ epoxy) were characterized and the results showed that hybrid composites had better mechanical properties [
23]. Similarly, in pure jute, jute/sisal, and jute/curaua composites the hybrid composite had better mechanical properties [
24].
Enset provides fiber as a byproduct of decorticating leaf sheaths. In rural areas, the Enset fiber is used to make sacks, bags, ropes, cordage, mats, construction materials (such as tying materials that can be used in place of nails), sieves, etc. Similar to other cellulosic natural fibers, enset fiber is chemically composed of cellulose, hemicellulose, lignin, pectin, moisture, wax, and oils [
25,
26,
27].
Alkali treatment of fiber modifies the interfacial adhesion of fiber and the matrix, which improves the mechanical properties of enset polyester composites. Alkali-treated enset fiber, treated with 5.0% NaOH, exhibited the highest Young’s modulus, flexural modulus, static, and dynamic properties compared to untreated enset fibers [
26]. The promising application of 5.0% alkali-treated enset fiber polyester composite is for automotive components like parcel shelves, dashboard, seat cushion, door trim panel, backrests, and cabin linings. They can also be used in non-structural applications like in packaging industries (egg shelves), consumer products, and sports items (hubbub handle, bicycle frame) [
25,
28]. The mechanical properties of sisal-banana hybridized natural fiber composites with a distinct weight fraction were investigated and the results showed that the flexural strength value could be improved by decreasing the proportion of banana fiber and that the tensile strength improved by adding a greater proportion of banana fiber [
29].
Moisture absorption is a chief hindrance that leads to poor fiber–matrix adhesion, a decrease in the interfacial bonding between the fiber and the matrix, and a decrease in the mechanical properties of the composite. The high cellulose content of natural fibers contributes to higher water absorption of the composite. Greater water absorption leads to fiber swelling, which develops the stresses that cause failure in the composite [
30]. As observed from different literature [
3,
31,
32], hybridization and treatment are the main parameters considered for the enhancement of the mechanical and water absorption properties of natural fiber-based composites.
The most common thermosetting polymer composite fabrication methods are hand lay-up, resin transfer molding, and pultrusion. Composite manufacturing using the hand lay-up technique is the simplest method which has low mold costs, low processing costs, the ability to manufacture complex designs, availability of the tools required for production, and uses molds that are easy to maintain. The process is also known for having long processing times and being labor intensive, which are among the few disadvantages associated with the technique [
33,
34].
Modifications have been major topics in natural fiber reinforced composites to improve the interfacial adhesion and, in turn, to improve the overall properties of the composite product. It is known that poor adhesion between the fiber and matrix affects the mechanical properties of composites. Furthermore, fiber treatment is used to improve the hydrophilic character of natural fibers, which make poor interfacial interactions with hydrophobic polymeric materials that limit the stress transfer between the composite components. In addition, fiber hybridization and fiber orientation are both promising strategies to improve the mechanical and physical properties of the composite. When two or more types of fibers are combined in a matrix of composite materials, the drawback of the type of fibers is mitigated by keeping the benefits obtained from the others.
As observed from the literature, better mechanical properties of the composite can be obtained when the fiber orientation is aligned in the load direction, while the composite is too weak when loaded at an angle that is different from the fiber orientation. Therefore, fiber hybridization is a promising strategy to improve the mechanical properties of natural fiber-based composite to mitigate the drawbacks of the type of fiber, while keeping other benefits. Furthermore, the mechanical properties of hybrid composites are affected by the fiber volume ratio, fiber orientation, and the treatment of fibers. Thus, the aim of the research reported in this article is to investigate the effect of chemical treatment and hybridization on the mechanical- and water-absorption properties of E/S hybrid composite, which are not sufficiently addressed in the existing published works.
2. Materials and Methods
2.1. Materials
For this study, enset (false banana) and sisal fibers were collected from Southwest Ethiopia, where these plants are widely distributed. The leaf parts of the Agava sisalana plant were used to extract sisal fiber, while the pseudostem part of the enset plant was used to obtain enset fiber. Ensete ventricosum is widely cultivated in East Africa and is mostly known as a wild species in Ethiopia; it is abundantly concentrated in the southern highlands and southeastern parts of the country. Enset, however, is usually larger than banana, with the largest plants up to ten meters tall and with a pseudostem up to one meter in diameter. The pseudostems, which may be two to three meters tall, contain an edible pulp and quality fiber. The rest of the materials, such as wax, hardener, and unsaturated polyester resin, were bought from a local supplier called World Fiber Glass in Addis Ababa, Ethiopia.
2.2. Composites Fabrication Methods
The mold used to prepare the composite was prepared from wood and had a dimension of 300 mm × 300 mm × 5 mm. The hand lay-up method was applied to manufacture the E/S hybrid composite. To make the hybrid composite, a total E/S fiber volume ratio of 30% and a total volume of polyester ratio of 70% were used. After calculating the volume and mass of each composite using the rule of mixture, the hybrid composite was prepared considering three cases: (1) untreated, (2) 5% treated, and (3) 10% treated with NaOH, and varying hybridization ratios of E/S (i.e., 100/0, 75/25, 50/50, 75/25, and 0/100) were used with unidirectional and woven orientations. The mechanical (tensile, flexural) and water absorption properties were investigated.
2.3. Composite Mechanical Testing
The test specimens were prepared before undergoing different mechanical testing (i.e., tensile test and flexural test) and moisture absorption per the relevant ASTM standards. Five specimens were tested for each set of samples and the average value was taken for analysis.
2.3.1. Tensile Test
Tensile tests were used to measure the force required to break the test specimens and the extent to which the specimens stretched or elongated up to the breaking point. The specimen dimension used for the tensile tests was 250 mm × 25 mm × 5 mm (length × width × thickness) per the ASTM D3039 standard [
35]. The test was carried out using a universal test machine (Bairoe, Shanghai, China) with a 50 kN capacity and a test speed of 5 mm/min crosshead speed and 150 mm gauge length. The test setup is displayed in
Figure 1a.
2.3.2. Flexural Test
A flexural test was carried out to find the ability of the material to resist the deformation under a three-point bending load, which promotes failure by inter-laminar shear. Rectangular cross-section test specimens were cut from the molded samples. This test was conducted using a UTM (WP 310 universal material tester (Gunt, Germany)) per the ASTM D790 standard [
36] with a test speed maintained between 0.5 and 1 mm/min. For this study, the lowest test speed, i.e., 0.5 mm/min was used. The specimen dimension was 127 mm × 13 mm × 5 mm. The setup for this test is displayed in
Figure 1b.
2.4. Water Absorption
Testing the water absorption properties of enset/sisal hybrid composites in water at room temperature was recommended. A water absorption test was carried out per the ASTM D570 standard. To study the water uptake, the specimens were immersed in water at room temperature. The samples were taken out periodically and weighed immediately, followed by cleaning of the surfaces of the samples with a dry cloth. Then, the samples were weighed using a precise 4-digit balance to find out the content of absorbed water [
37,
38]. All the samples were dried in an oven until a constant weight had been reached before immersing them again in the water. The percentage of water absorption was determined using the formula in Equation (1).
where WA = water absorption, m = 1 and m = 2 are the weight of dry and wet samples of composites respectively.
2.5. Analysis of Variance
Analysis of Variance (ANOVA) is a statistical technique which can infer some important conclusions based on analysis of the experimental data. This method is rather useful for revealing the level of significance of the influence of factor(s) or their interaction on a particular response. Most of the time, a key result of ANOVA is determined by the
p-value (denoted by alpha (α)) or by comparing the F-value and F
crit. value, in which case the factor is considered statistically significant if F is greater than F
crit. [
39].
2.6. Morphological Surface
The morphological characterization of the composite fracture surface was conducted using a scanning electron microscope (SEM), a Gemini SUPRA 35VP (Carl Zeiss, Jena, Germany) equipped with EDAX type Energy Dispersive Spectroscopy (EDS). The fractured surface was examined following the tensile test for woven (untreated, 5%, and 10% NaOH treated) and unidirectional (untreated, 5%, and 10% NaOH treated).