1. Introduction
Environmental concerns and increasing demand for lightweight structures by the automotive industries have encouraged these industries to adopt sustainable composites, particularly natural fiber reinforcements in polymer composites, for making vehicles. Natural fibers are effectively used to develop composite material, which shows excellent strength (in some applications) compared to synthetic fibers. Bagasse is a waste product and can be obtained by extracting the juice from sugarcane. Bagasse is frequently used as a combustible material in the sugarcane industries for energy supply. It has other applications, such as being used as pulp in paper industries and for developing fiber-board. It has been shown that the fiber composites’ strength primarily depends on interfacial adhesion and the transfer of stress from the matrix to the fiber. The stress transfer phenomenon played a significant role in evaluating the mechanical properties of fiber-reinforced composites [
1].
Coconut is generally found in humid tropical regions. It is used as an oil, fruit, etc., and is primarily found in Africa, Latin America, and Southeast Asia [
2]. Coconut husk can be used to produce coir fibers [
3]. The approximate primary composition of coconut husk is “cellulose 40%”, “hemicellulose 20%”, and “lignin 30%” [
4]. Most of the present research lacks the effective utilization of coconut husk as a functional material [
5].
As per the Brazilian Institute of Statistics, the production of Brazilian coconut in 2017 was around 1.7 million tons, which generated coconut fiber as a waste. According to previous studies, coconut fiber could have various applications in building materials, as it has tremendous physical, thermal, and mechanical properties [
http://www.abc.gov.br/training/informacoes/InstituicaoIBGE_en.aspx] (accessed on 30 November 2022). The most significant component of coconut fiber is tannin, a natural fungicide that prevents fungal and insect attacks [
6].
The only problem with regard to the mechanics of fiber reinforcement is their hydrophilic nature. Because of this hydrophilic nature, proper interfacial strength is not achieved and this negatively affected the mechanical properties of the composite material. Interfacial adhesion is a significant factor in deciding the adhesion ability between the “fiber” and the “matrix”. The reason behind the deficient bonding is the moisture availability in the fibers. Pendant hydroxyl groups and polar groups are responsible for the moisture. So, to efficiently utilize fiber reinforcements in composites, one must perform fiber surface treatment to get a better interlock or adhesion with the matrix.
To enhance the mechanical properties, one needs to treat the natural fibers to make them hydrophobic. There are two types of treatments available. One is a physical treatment, and the other is a chemical treatment, which ultimately makes the fiber surface rough, removes the moisture from the fiber surface, and improves the adhesion strength with the matrix. On the other hand, bleaching strengthens the alkalized fibers by improving the fiber’s surface energy and crystallinity [
7].
In the case of a hydrophobic polymer matrix, natural fiber reinforcements are challenging because of their high polarity. This is because of the high polysaccharide content, which affects the bondability between the fiber and the matrix [
8]. Chemical treatment showed a positive effect, specifically the interaction of the “fiber and polymer matrix”, “adsorbent and adsorbate”, “enzyme and support”, and “functionalizing agent and support” [
9].
The disruption of hydrogen bonding removes polysaccharides and impurities from the fiber surface, such as hemicellulose, lignin, pectin, wax, and oils. It makes them rough by reducing their hydrophilic nature and is the primary goal of alkali treatment [
10].
Alkalization significantly improves the fibers’ mechanical properties. Different parameters, including fiber length, loading, and orientation, were analyzed in developing the polypropylene/sisal composites. It is observed that a fiber length of 2 mm gives optimum and good results [
11].
So, alkalization ultimately improves the adhesion ability or interfacial bonding strength, leading to improved mechanical properties [
12,
13].
Bagasse fiber and coconut shell particles reinforced cardanol composites show high mechanical properties. Raw bagasse fibers and coconut shell particles are hydrophilic in nature, ultimately affecting the composites’ mechanical properties. However, alkalized bagasse and coconut shell particles showed excellent mechanical properties [
14].
The chemical concentration, processing time, and temperature are the significant factors that must be considered before the mercerization/alkalization of the natural fibers [
15].
However, the Bagasse mainly fuels sugarcane mill furnaces despite its low caloric power. So, its reinforcement into the polymer is a good solution for boosting the problems of bagasse fiber disposal [
16].
Figure 1 shows raw (untreated) bagasse fiber and coconut husk.
It has been observed that two significant factors, entangle inclination and fiber agglomeration, occur during the processing of the polymer composites. This is just because of the fiber–fiber interactions. Alkalization is a suitable surface treatment method to overcome this problem [
17]. Mercerization ultimately improves the fibers’ surface properties, which is useful in fiber-matrix bonding [
18].
Umit and co-workers have performed different treatments on flax fibers, such as acetic acid, sodium hydroxide, and silane. They found that NaOH (alkalization) is a suitable treatment that enhances the adhesion strength between the fibers and the matrix with enhanced mechanical properties [
19].
Alkali-treated bagasse and green coconut fibers were utilized for particle board production with two distinct densities, i.e., 500 and 700 kg/m
3. The particleboards consist of three layers: one layer of bagasse fiber and two layers of green coconut fibers. The boards were bonded with castor oil-PU resin. Fatigue results showed that all the developed particle boards are able to survive 40,000 cycles at a 25% stress level of the fracture load [
6].
PLA-based natural rubber and alkalized bagasse fiber-reinforced composite foams were developed. It has been found that composite foams showed distinct improved mechanical properties of treated bagasse fibers than the untreated ones [
20].
Two types of polypropylene-based composites have been developed using alkalized bagasse fiber and raw bagasse fibers with calcium carbonate (CaCO
3). It has been observed that alkalized bagasse fiber-CaCO
3 composites showed excellent mechanical properties compared to the raw bagasse-CaCO
3 hybrid composite. In addition, good thermal properties of the alkalized bagasse fiber-CaCO
3 composite were observed, as confirmed by the thermogravimetric analysis and derivative thermogravimetric analysis [
16].
Green bagasse fibers were alkalized with 5% NaOH and used for developing polyester resin-based composites. The treated bagasse fiber exhibited improved mechanical properties compared to the untreated ones. In addition, water absorption and chemical resistance tests showed favorable results concerning their applications [
21].
Two distinct types of epoxy-based composites were developed by using raw coconut sheaths and alkalized (5 wt%) coconut sheaths. It was noticed that the treated coconut sheath-epoxy composites showed excellent mechanical properties compared to the raw coconut sheath-epoxy composites. In addition, treated coconut sheath-epoxy composites showed good thermal stability and storage modulus, as confirmed by TGA and DMA analysis [
22].
The reinforcement of CaCO
3 ultimately improved the mechanical properties; precisely, the composites’ compression strength, flexural strength, and impact strength. In addition, the higher melting point of the CaCO
3 showed better thermal stability, as confirmed by TGA analysis [
23].
The present investigation used bagasse fibers, coconut husks (chemically modified/unmodified), and CaCO3 to develop hybrid composites. The hybrid reinforcement is expected to improve the epoxy composites’ mechanical properties and thermal stability. In the present research, the effect of the inclusion of alkalized and non-alkalized fibers with the nano reinforcement, i.e., CaCO3, on the mechanical and thermal properties of the composites was observed, and their comparative analysis was established. Various mechanical properties, including tensile, compression, and flexural properties, were evaluated and reported. Water absorption tests were also performed to investigate the water uptake behavior of the developed composites. Thermal analysis was performed using TGA analysis.