Development of Eco-Sustainable Silica-Reinforced Natural Hybrid Polymer Composites for Automotive Applications

Te increasing demand for eco-friendly materials and technology has made the industry focus on bio-compatible composites. Tis made the researchers explore the potential of eco-friendly, bio-degradable


Introduction
Developing composites for engineering applications such as automotive, aerospace, and structural industries [1][2][3] is the need of the day. Composite materials have two or more chemically distinctive phases. Te discontinuous phase, which is harder, is known as reinforcement [4]. Te continuous phase in the composite is termed the matrix [5,6]. Te composites were prepared using resin reinforced with synthetic and natural fbres [7]. Today, natural fbres have started replacing synthetic fbres. Te environment-friendly characteristic makes the composite material used in automobile and construction industries [6].
Natural fbres are sustainable, nonabrasive, compostable, have a high calorifc value, exceptional mechanical properties, low density, are less expensive, and are less harmful to the environment [8]. At present, banana fbre is a waste product of banana cultivation [9] without any further cost, and banana fbres can be acquired for engineering purposes [5]. Banana fbres can be extracted through retting, mechanical, and chemical extraction methods [9,10].
Generally, fbres concentrated near the outer region are extracted by hand scraping, chemical extraction, retting, or using raspadors. Te extracted leaf sheaths are boiled in a sodium hydroxide solution. Also, hand-scraped fbres represent better quality than other techniques. Te fbres extracted are washed and dried to remove the moisture present [11]. Te use of natural fbre helps in reducing the problem of environmental degradation due to the pollution caused by synthetic fbres. Banana peduncles and leaves are expected to contribute 20% of the plant's total biomass [12].
In composites, the matrix is an essential element responsible for the positioning of fbres, as well as stress transmission from one fbre to another and protecting the fbres. Diferent matrices used are epoxy, polyester, etc. Epoxy resin ofers low viscosity, good fowability, dimensional stability, and better productivity. Composites prepared using epoxy resin are used in numerous applications. Te mechanical properties of the fbre-reinforced composite depend on two signifcant aspects: the fbre-matrix interface and the stress transfer ability of the matrix to all fbres. Enhanced properties can be obtained by using natural and artifcial fbres in the same matrix [13,14].
Some authors have reported comparing the tensile properties of bananas with diferent natural fbres [15]. Te advantage of using banana fbre as a natural fbre for composite preparation is that it has a larger fbre diameter. As a result, the unit area of the composite is high. Hence, the efciency of transferring stress from the matrix to the fbres is greater [16,17]. Terefore, banana fbre is a good substitute for synthetic fbres [18].
Many attempts have been made to manufacture a composite using banana fbre and polyester resin as the matrix [17,19]. An increase in the fbre's weight fraction is reported to increase composites' mechanical properties [8]. Due to the chemical composition of banana fbre, it has an appreciable value in breaking load, breaking extension, and persistence [20]. Te physical, chemical, and mechanical characteristics decide the fbre quality [12]. Increasing the weight fraction of the composite fbres enhanced the mechanical properties [8].
Te selection of suitable fller in the appropriate amount in the composite afects the mechanical, physical, and chemical properties and dimensional stability [21,22]. Te addition of silica particles increased the composite resin's epoxy viscosity [23]. Silica and silicate-based fllers modify the properties of composites, like the hardness and elasticity of components [24][25][26][27]. Te fabrication and application of polymer composites were reviewed [28].
One major disadvantage of using natural fbre in composite preparation is that it absorbs moisture. As a result, the interface bond between the matrix and the fbre gets weakened. Tis ultimately leads to material failure in the loading condition. It leads to the degradation of the mechanical properties. Te problem of water absorption can be reduced by treating the banana fbres chemically before using them for composite manufacturing. Researchers tried to characterise composites by adding diferent fllers to enhance their mechanical properties [29].
Natural fbre composites have many industrial and domestic applications, such as insulators of heat and electricity; they can also be used in freproofng [30]. As a result, natural fbres have again started to gain importance. Te banana stem that ofers the source of fbre is available as waste in the world. Te banana fbre, known for its high strength, can be used as reinforcement in composites for many applications. Similarly, an attempt to test the mechanical properties was reported using banana fbres subjected to a tensile load. Te failure was due to the pull-out of the microfbrillar accompanied by the tearing of cell walls. Te tendency of fbre pull-out can be reduced by increasing the testing speed [31].
In order to study the impact of lesser weight fraction of fbres on mechanical properties [8], the banana fbres with varying proportions (5-15 wt.%) and diferent fbre lengths (10-25 mm) along with silica (15 wt.%) as fller material were reinforced with epoxy resin to make the composites. Te produced composites were tested to fnd the composites' tensile, fexural, impact strength, hardness, and water-resistant properties.

Extraction of Banana
Fibre. Fibre is obtained from the bark of the banana tree through the extraction process. Te extraction of banana fbres was carried out by removing the outer layer of the banana stem. Ten, the core part is cut into half horizontally and vertically, and pressure is exerted on the sliced parts, which get squeezed. Removing water partially from the fbres helps to separate the fbres easily.
After separating the fbres, it is placed in the hot air oven and exposed to a temperature of 80°C for 12 hours to dehydrate the banana fbre completely. In order to increase its stifness, banana fbres were chemically treated with NaOH solution [32]. Fibres were submerged in the NaOH of concentration 1 N for 12 hours. Te dehydration process is repeated by placing it in the hot air oven for 8 hours at 80°C. After, it transforms into strong banana fbres, cut into short lengths of 10, 15, 20, and 25 mm.

Composite Preparation.
Te extracted fbres were used as the reinforcement in the composite. Te resin used as a matrix is commercially available epoxy (ARALDITE LY 554), and a hardener (HY 951) was used for composite preparation. Te composites were processed in the mixing ratio of 100 parts by weight of epoxy and 10 parts of hardener. Hand lay-up is the eldest process of woven composite manufacturing [33]. Te various steps involved in the hand lay-up process are described as follows: frst, the mould surface is coated with the releasing agent to avoid sticking the matrix with the die. Ten, a delicate plastic layer is employed at the mould's top and bottom to make the composite's smooth surface.
Banana fbres (weight fraction 5-15%) were prepared with diferent lengths (10, 15, 20, and 25 mm) with and without the fller material silica four (weight fraction 15%) added to the epoxy. Finally, the hardener was added and uniformly mixed with a stirrer. Ten, the resin mixed with ingredients was poured into a die coated with the releasing agent to avoid sticking the matrix using a helping brush to uniformly spread it and pressured using a roller to get rid of any confned air foams and the excess polymer. Ten, the mould is closed, and pressure is released to acquire a single mat. After curing at room temperature, the mould is opened, and the woven composite is removed from the mould surface. Te schematic of the hand lay-up process and the fabricated specimen is shown in Figure 1.

Mechanical Properties
Testing. Prepared composites were cut to the specimens of the desired shape to perform various mechanical tests. Five specimens were used to study the mechanical properties. Tests such as tensile, fexural, and impact tests were conducted to determine the composites' efectiveness. Te tensile test was done on the bases of ASTM D 638-03 standard [34], and the test speed was 5 mm/min in Universal Testing Machine (UTM) (Make: Shimadzu). Specimen with dimensions [160 (l) × 12.5 (w) × 12 (t)] mm was used for the test. Finally, a comparative study was conducted to fnd the relationship between the fbre weight fraction and the tensile strength.
Te fexural strength was found using ASTM D 790 standard [35], and the cross head was maintained at 1.3 mm/min. Te three-point test was done on the UTM (Make: Shimadzu). Specimen prepared for the test with dimension [100 (l) × 25 (w) × 12 (t)] mm was placed between rollers 64 mm apart in a three-point bending test. Te three-point test is chosen because it is straightforward to fnd the midpoint defection. Te composite's maximum stress and fexural modulus were found using relations in the literature [36]. Te strain rate used in the test was 0.5 mm/min, measured with the help of an electronic tensometer.
Te impact strength was determined by using an impact test. Te standard procedure used for the impact test was ASTM D 256 34 . Impact strength determination by the Izod impact test is preferred due to its simplicity and no threat to its credibility due to its detailed history. Te hardness test was performed in a Vickers hardness tester (Make: Shimadzu). Composites fabricated were cut to 25 × 25 mm, and the load of 0.3 kgf was applied to the composite for 10 seconds.

Water Absorption Test.
According to the ASTM D570 standard, the water absorption test was conducted to fnd the approach toward Fickian behaviour. Te absorption % was calculated [37] using the following equation: where m 1 and m 2 are the weight of the dry and wet specimens.
Te kinetic parameter, the difusion coefcient D (mm 2 /s), is calculated using the following equation: where θ � slope of the linear portion of the sorption curve. h � initial specimen thickness (mm). Te ability of solvent molecules to move among the polymer segments depends on the difusion coefcient. Te sorption phenomenon of the fbre defnes the permeability of water molecules through the composite specimen. Terefore, the sorption coefcient "S" is calculated as follows [38]: where Q ∞ � molar % of water uptake at time t � ∞. Q t � molar % of water uptake at time t. Te permeability coefcient P (mm 2 /s) gives the net impact of sorption and difusion.
Te water absorption kinetics was studied by frst taking the initial measurement of the specimen to be tested after it is dehydrated for one hour. After recording the initial reading of weight, then it is dipped in water and taken out periodically. Te surface is wiped, and then again, weight is measured. Finally, the percentage increase in water absorption is computed and recorded. Te microstructure of the composite sample was observed through SEM to observe the distribution of fbres and internal structure of the tested composite samples.

Mechanical and Water Absorption Properties
Mechanical properties like hardness, tensile strength, fexural strength, impact strength, and water absorption behaviour were examined. Composites' mechanical properties mainly depend on fbre content and length.

Hardness.
Te efect of silica content, banana fbre volume fraction, and fbre length on the hardness of hybrid composites is presented in Figure 2. Te hardness of polymer composite increases by 8.57%, 5.47%, and 10.1% for 15%, 10%, and 5% vol. fraction of banana fbre reinforcement with the addition of 15 wt.% of silica. Te increase in hardness is due to an increase in restricting the movement of polymer chains with silica content. Te hardness of silica-polymer hybrid composite increases by 36.01%, 40.12%, and 47.83%, with a rise in fbre length from 5 mm to 20 mm for 15%, 10%, and 5% vol. fraction of banana fbres reinforcement, respectively. Te hardness increases ascribed to increased interfacial adhesion between polymer and banana fbres and improved surface hardness.
From Figure 2, it was found that the hardness improved with an increase in fbre length, and it is highest at 10 mm fbre length with silica fller and neat composite. However, with the increase in fbre up to 15 wt.%, it was found that Advances in Materials Science and Engineering hardness increased. It is well known that in hybrid composite material, fller weight fraction signifcantly afects the hardness value of the hybrid composite material. Tis was observed from the results of this research, which showed a superior hardness to the neat composites. Te hardness of the specimen with fbre length 25 mm and 15 wt.% of silica fller (15 wt.%) measured 46.74 HV was found to be maximum. Te result obtained in this research may be due to the excellent compatibility between the silica fller and composites [37]. Figure 3 shows the infuence of silica content, fbre volume fraction, and fbre length on the tensile properties of the composites. Te tensile strength increased approximately 12.5%, 10.5%, and 10.2% for 15%, 10%, and 5% vol. fraction of banana fbres reinforced with 15 wt.% of silica. Te increase in tensile strength is attributed to silica content in hybrid polymer composites, which restrain the fow of the polymer chain and consequently improve the tensile strength of the silica-reinforced banana fbre reinforced composite compared with the composites without silica. Te tensile strength of hybrid composite increases by 48.38%, 23.91%, and 41.1%, with a rise in fbre length from 5 mm to 20 mm for 15%, 10%, and 5% vol. fraction of banana fbre reinforcement, respectively. Te percentage increase in the tensile strength in silica-reinforced banana polymer composites with a 10% banana vol. fraction is the lowest. Tis is due to poor adhesive bonding between banana fbre and polymer matrix. Figure 3 shows the results of tensile strength comparisons for specimens of fbre lengths 10, 15, 20, and 25 mm for neat and silica-flled composites. Te results confrmed that the weight fraction and the tensile strength have a linear relationship. As the wt.% fraction of the banana fbre increases in the epoxy matrix, its tensile strength is enhanced, which leads to the desired more robust material. Observed data reveal that composites' tensile strength increases with fbre length and weight fraction. Te tensile strength of the specimen for a 15 wt.% showed an increase from 38.93-54.71 MPa for the silica-flled natural hybrid composite. Te results of this research were consistent as the chemical and silane treatments improved the compatibility between natural fbres and polymer matrices could be the reason for the increase in tensile strength [39]. Te results show that the tensile increased with the increase in fbre length ( Figure 3). Te fbres that take up the load from the matrix are distributed uniformly. Furthermore, a higher amount of reinforcement may lead to agglomeration [40]. Te results reported a higher strength than previous work on a hybrid composite of banana/sisal [39]. Te tensile strength obtained was appreciable and substantial owing to the surface treatment of fbres and adding the fller material as silica four compared with the neat composites. Te tensile strength of the specimen was found to increase by 40%. Te results show that composites possess mechanical features such as mobility, stifness, and modulus. Te role of silica fller in the composite is justifed by the dispersion toughened phase formed observed as micron-sized particles in the composite [36]. Te composite containing microsized fllers reported that the epoxy matrix increased the young's modulus and lowered the % elongation [26,41]. Hence, silica fllers were used to fabricate composites using epoxy resin [42]. Te results conclude that exploring inexpensive natural fbres treated chemically ofers good stifness and strength, which is better than artifcial glass fbre. Te research fndings indicated the growing demand for lighter parts made using natural fbres.

Flexural Properties.
Te fexural strength of the specimen measured showed that 25 mm fbre length with 10 wt.% exhibited the highest value of 127.94 MPa (Figure 4). Similar results were obtained in the previous literature [43]. From the results, it is clear that as the weight fraction of the composite increased, the fexural strength increased up to a fbre weight fraction of 10 wt.%. Beyond this, fexural strength decreased for the 15 wt.% fbre weight fraction. Te reason for the decrease in strength is that as the fbre wt.% increases, water absorption increases and confrms the debonding of fbres and weak fbre interphase. Hence, interfacial strength between the fbre and matrix is vital to achieving efective fbre reinforcement. Tere is an increase in fexural modulus by 36% for the composite prepared.
However, the fexural strength decrease was appreciable owing to the fbres' surface treatment. Silica showed the existence of good adhesion and appropriate bonding between the fller and matrix, which enhances the strength of the composite. A similar trend was found in the literature [44].

Impact Properties.
Te impact energy of material shows its ability to absorb and dissipate energies under sudden loading. Te efect of silica content, fbre volume fraction, and fbre length on the tensile properties of polymer hybrid composites is shown in Figure 5. Te results show that the NaOH-treated banana fbre/epoxy composite's impact strength is signifcant. Te impact energy of hybrid composite increases by 11.11%, 12.32%, and 4.3% for 15%, 10%, and 5% vol. fraction of banana fbre reinforcement with the addition of 15 wt.% of silica. Te restriction of polymer composite chain movement increases the rigidity in the hybrid composite, resulting in improved impact energy absorption.
Te impact strength of the composite material was found to be minimum for the composite specimen with 5 wt.% and a fbre length of 10 mm. Te reason may be due to the presence of fbre in low wt.%, and also the resin due to its brittleness. Similarly, it was found to be maximum for a weight fraction of 15% and fbre length of 25 mm 15.19 kJ/m 2 ( Figure 5). Te previous work reported 13.25 kJ/m 2 for a  hybrid composite [39]. Tere has been an enhancement in the impact strength by 48%, which is appreciable by using reinforcement as banana fbres. Te increase in impact resistance was due to the increased density of fbres in the matrix, and their random orientation as the fbre carried the load. Tis caused an increase in resistance to the impact load. Also, the presence of silica fller increased the rigidity that absorbs more energy could be the other cause for the improved impact strength compared with the neat composites. Te addition of silica creates a noteworthy diference in the impact strength of the banana fbre-reinforced epoxy composites. Hence, this study shows that the impact strength is directly proportional to the weight fraction.

Water Absorption Behaviour.
Composites prepared for application come in contact with water and have fundamental importance attached to the efect of water absorption. Hence the water absorption behaviour depicts the characteristics of composites' usefulness. Te water absorption test results are shown in Table 1 for diferent wt.% for the fbre length of 25 mm. Te percentage moisture absorption calculated shows a minimum of 5 wt.% is 23.57%. It was interesting to note that as the fbre wt.% increases, the weight of the specimen increases by some signifcant amount due to moisture absorption. Te present banana fbres have maximum water absorption capacity compared with silica and epoxy resin. However, as the wt.% of banana fbre increases, the number of free hydroxyls (-OH) groups increases in cellulose. Tus, an increase in moisture absorption is observed. Te OH groups in connection with humidity form hydrogen bonding, resulting in weight gain in the composites. After 24 hours, the water absorption coefcient becomes stable. Te hydrophilic nature of fbres well elucidated this phenomenon, it is because of the fact the cellulose fbres. Furthermore, the swelling and cracks in the fbre increase the water transport by difusion. Te active capillary mechanism makes the water molecules fow through the fbre-matrix interface [45]. Te water absorption behaviour can be decreased by increasing the reinforcement and making a strong adhesive bond between the fbre and matrix, which provides minimal micropores to difuse the water molecules [7].
3.6. SEM Analysis. Figure 6 shows the micrograph for 10-25 wt.% banana fbre-reinforced epoxy composite. Te images revealed the reasons behind the variation in the mechanical properties of the composites. Te study indicated a change in mechanical properties due to changes in fbre length and wt.% and fller dispersion. Te micrograph for 10 wt.% (Figure 6(a)) showed sample voids and nonuniform distribution of fbre leading to agglomeration. Te agglomeration, a collective stacking of fbres together in the matrix, reduces the strength by nonuniform stress transfer [46].  A similar trend was observed in Figure 6(b), which showed less porosity. Te less porosity may be the reason for low mechanical properties, leading to early failure. Te micrograph shown in Figures 6(c) and 6(d) indicated the uniform distribution of fbres and fllers. Te dispersion fbres and fllers were uniform, indicating good bonding between banana fbres and epoxy resin. Also, good fbre-matrix adhesion is visible. Te well-known fact of dispersion toughening of fllers and uniform distribution of reinforcement in resin improves the properties of the composites [47].

Conclusions
Banana fbres are one of the cheapest and most abundantly available from the waste part of banana trees. Hence it is one of the most economical and robust natural fbres. Te extraction process of the banana fbres is simple too. Tis research assisted in discovering the variation in the mechanical properties of the banana fbre-reinforced composite without and with diferent wt.% silica fller and for the varied fbre wt.% and length. From the above-obtained results of the specimens, the conclusion is as follows: (i) Fabrication of banana fbre-based epoxy composites with various wt.% and fbre lengths were successfully fabricated using a hand lay-up procedure.
(ii) Te results revealed that fbre loading, length, and addition of silica fller play a signifcant role in the mechanical characteristics.  (vi) Te water absorption behaviour for the banana fbre-reinforced epoxy follows Fickian and non-Fickian characteristics, and the composite's water absorption becomes stable after 24 hours. Due to the water absorption, the bonding between the fbres and the epoxy resin decreases, and the composite becomes weaker. (vii) Terefore, the environmentally friendly nature of the hybrid composite material remains a substitute for engineering applications.