Potential of natural fiber based polymeric composites for cleaner automotive component production A comprehensive review

. It was found that cellulosic/natural ﬁber based polymeric composites can be an efﬁcient alternative for man-made synthetic ﬁbers in the automotive sectors. © 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).


Introduction
Initially, traditional materials, whether metal or synthetic fiber, that were being used in the automotive industry were found to be very poor in terms of recyclability and biodegradability, and hence were causing several types of pollution and severe disposal complications that needed to be eliminated as soon as possible [1,2].
The automotive industry all over the world was focusing on strong and rigid materials, for which weight was not considered a crucial factor; however, as time passed, it was discovered that decreasing the weight of vehicle bodies has extraordinary positive impacts on vehicle performance, and much more efficient vehicle operation can be achieved by also considering weight as a crucial factor along with rigidity and strength [3,4]. Weight reduction provided an impressive increase in vehicle mileage, and steering stiffness could also be controlled for longer periods of time [5]. To obtain these results and to control pollution and disposal problems, the main factor that was to be considered was the material that was being used in various parts and then finding an alternative material in their place that was comparatively lighter and easier to recycle than the material that was being used at that time [6,7].
Natural fibres proved to be the best possible alternate in many areas where there was unnecessary placement of metallic parts [8]. Gradually, major automotive companies like BMW, Mercedes, Audi, Ford, and General Motors, along with other companies, started investing more and more capital for research and development and the application of these natural fibres into their vehicles [9,10]. Tests and various experiments conducted for material validation concluded that natural fibres were successfully capable of being used in areas requiring low performance, i.e. predominantly in interior parts [11,12]. Much more research and improvement has yet to be done to utilise the capabilities of these fibres in areas that require high performance like brakes and engine parts, etc. Fig. 1 shows the evolution of natural fibres from their inception to the automobile giants that adopted and researched on these fibres in order to improve the performance of their products. Table 1 shows the mechanical properties of natural fibres being used in the automotive industry [13e16].
The demand for better material has been going on for ages. Doctors and scientists have been working on making highquality artificial materials for a long time, and they have been very successful at it too. So the question arises: is there a need to shift towards natural composites or green materials? To answer that question, there are many reasons as to why natural composites are becoming necessary now. These lightweight materials, if used in the right way, yield many more mechanical properties than artificial materials; they increase strength, flexibility, toughness, elasticity, etc.
R. Shen et al. [18] mentioned the following characteristics that make a nanocellulose composite an exemplary nanofiller: low particle size, high modulus, high strength, and ease of modification. It was also mentioned that if the ongoing preparation process can be improved in terms of pollution and energy consumption, a natural biomass can be yielded with all its environmentally friendly qualities, eventually also helping to retain the carbon footprint. The advantages of nanocellulose composites in the automotive industry are given in the following chart (Fig. 2).
N. Ramli et al. [3] investigated that kenaf fibre was showing exceptional performance characteristics and also leading to an appreciable increase in the absorption of discomfort produced by engine noise, exhausts, and other electrostatic charges. The nonabrasive characteristics of natural fibres make them exceptional in areas like the formation of sharp edges that are usually observed on glass fibres in cases of deformation.
According to Sendhil Kumar et al. [19], the following are the benefits of natural fibre reinforced composites: 1) the production of natural fibres leads to CO2 absorption, which in turn provides oxygen to the environment.2) These fibres have low density, ultimately leading to a reduction in weight along Fig. 1 e History of Natural Fibers based automotive products (with permission) [17]. with an appreciable specific strength.
3) The amount of toxic fumes produced by heating it is very low, and these fibres are manufactured using low-risk processes.4) Production of these fibres is possible at a low cost and with a relatively low initial investment. These fibres are also skin-friendly and do not cause any type of irritation to the skin.
According to Oludaisi Adekomaya et al. [20], promoting the use of natural fibres in automotive sectors can greatly benefit farmers; it can generate numerous new opportunities for farmers and help them to expand their work beyond just growing economic and cereal crops and can get them great trade opportunities in automotive sectors, resulting in huge profits for both farmers and the country's economy. Fogorasi M et al. [21] mention other factors that tend to validate their use in automotive sectors, as shown in the below-given Fig. 3.
Tests were performed to observe the usage of these bioresourced composite fibres in place of petroleum-based material, and natural fibres were able to show better mechanical properties, and even a higher thermal stability was also observed, but again, when processing temperatures crossed 200 C, there was a decrease in the homogeneity of the fibre material, which could cause major accidents if used in critical parts of the automobile body. Moreover, they could not be used even in exterior parts of the vehicle body, as fibres would undergo changes in their matrix composition and would hence not be able to bear the harsh and moist weather conditions in which they were expected to be used.
O Faruk et al. [22] discovered that when natural fibres were not properly treated with chemicals, they produced a bad odour when used in components such as PU foam and other parts that lie inside the vehicle's body, and their water vapour gain index was found to be quite high, causing structural imbalance and reducing their use in inner parts of the vehicle.
When these fibres were used as components in making PU foam, a hindrance was observed at the time of fibre expansion, and it was also noticed that there was a delay in the foaming time, and the cell size obtained after the addition of natural fibres into the foam material was smaller than the overall size of the foam cell without natural fibre reinforcement.
Because of their extremely small size, these micro-and nanocellulosic fibres in foam material could not be seen with a standard electron microscope. Trials were performed to properly identify the exact cell structure with the help of a transmission electron microscope, but those were also unsuccessful. The maximum information that could be obtained about structure was that when inserting these nano-cellulosic fibres into foam, many fibres get framed in the cell wall. J. Njuguna et al. [23] discussed how natural fibres were starting to make a comeback in the automotive industry in the form of reinforced fibers, but that there were still loopholes that were opposing the use of natural fibers. One of the major disadvantages of reinforced natural fibres is poor interfacial adhesion between polar (hydrophilic) and non-polar (hydrophobic) matrices.It was observed that there was a need for chemical modification as these fibres had poor wetting properties, which was causing difficulties in mixing the fibre with the matrix. Hence, extended research was still needed to find the Table 1 e Natural fibers being used in the automotive industry and their properties [9,10].  ideal structural modification and best-suited coupling agent to improve the adhesion properties of natural fibres and make them feasible for mass production. According to the present situation of natural fibre research, executing a plan to improve the adhesion properties of fibres is a very challenging task. Natural fibres have been observed to be prone to attacks by micro-biological species that lead to the weakening of their molecular structures and cause a significant decrease in their life span. On applying the usual processing techniques that were used for processing glass fibers, it was noted that natural fibres were very weak in terms of resisting applied heat as compared to glass fibers. Table 2 shows the natural fibres and their applications in the automobile industry and other sectors.

Fibers
In this article, we have briefly discussed the application of natural fibres in automotive bodies, bumper beams, anti-roll bars, gears, and other areas of application, along with their scope in the future.      Cellulosic fiber/nanocellulose-based polymeric composites for automotive body With the passage of time, the importance of the usage of natural fibres in automotive industries gradually came to light as it was realised how important it was to dispose of the material safely after its use. Table 3 shows the cellulosic fibers used in automotive body.
Claire S. Boland et al. [24] investigated cellulosic-based natural fibres and concluded that with the help of natural fibers, industries successfully reduce the weight of an automobile body without affecting any of its major performance characteristics. When Jute Fiber composites were compared to Glass Fiber unsaturated polyester, it was discovered that Jutebased fibres had a much lower environmental impact than glass fibers. After conducting an analysis, it was discovered that by combining 50% Kenaf fibre and 37% Glass fibre for interior components of an automotive body, one can save approximately 23 MJ/kg (Mega Joules per kilogram) of nonrenewable energy demand. It was also discovered that Kenaf fibre had quite high nitrogen and phosphorus emissions, but after a life cycle assessment of these bio-based polymeric reinforced materials, the energy demand could be reduced, potentially leading to lower greenhouse gas emissions [25,26].
Zewdie Alemayehu et al. [27] analysed the use of cellulosic fibres in automotive fenders by creating a prototype of the fender. After performing various performance tests, it was found that sisal-based fibre having a fibre orientation of 0e90 was best suited for panels of automotive bodies, especially fender applications, as it was experiencing very little stress. The hand layup method was used to create the fender prototype, and materials were mixed with resin and binder material.After curing the moulded prototype for 24 h at room temperature with a sufficient amount of weight, it was taken for testing and gave good strength results in most of the mechanical property tests [29,30].
N. C. Loureiro et al. [31] studied bio-fibers such as cellulosic fibres and inferred that these fibres are nonabrasive while mixing, and moreover, these fibres come up with high thermal conductivity and have much better sound-absorbing attributes than traditionally used material. These fibres have a constructive effect on nature, and the amount of energy required to produce them is much less when compared to other petroleum resources or glass fibre-made materials. These fibres have very high sustainability, as well as easy recyclability and biodegradability, in addition to excellent mechanical properties and lower ecotoxicity.These fibres can be easily disposed of, and the residue produced is almost negligible when compared to other artificial glass-based fibers [28].
Dawit et al. [32] investigated hybrid reinforced natural fibres and concluded that hybrid natural fibres are worthwhile, possess excellent mechanical properties, and are also thermally stable. Due to these outperforming properties, they can be a replacement for traditionally used glass fibre and other synthetic materials. Presently, the use of materials fully prepared by using natural fibres is not found to be practical due to the flaws of natural materials, like their hydrophilic nature and a little shorter life span compared to artificial fibers, but glass fibres mixed with natural fibres and natural fibres with polymer matrix composites are able to find numerous new applications due to rising concern about environmental degradation and the usage of excess energy for the production of artificial fibers. Natural fibre materials like kenaf, sisal, and jute, etc. are rapidly replacing artificial materials because of their high specific strength, lower weight, and easy disposability; in addition, they have high tensile toughness and a high ratio of strength to weight. Moreover, hybrid composites, when tested, proved to be more resistant to moisture and showed better overall properties than separately used artificial fibers. These are all characteristics that make natural fibre composites more viable for use in exterior parts of automotive bodies rather than just internally, as this will have a significant impact on the ergonomics and overall performance of the vehicle.Orientation of fibre is also an important factor in automotive bodies, as it majorly affects the tensile and compressive strengths of natural fibre materials. 0 fibre-oriented composites provided both tensile and compressive strength to a greater extent than 90 fibre-oriented composites and bidirectional (0/90) fiber-oriented composites. Instead of using a material that is fully made of one single fibre composite, a mixture of two or more fibres can also be a good replacement for getting even more refined material for superior vehicle performance. According to the current state of natural fibre development, hybrid composites can be used for vehicle body parts that do not require high mechanical efficiency because they offer benefits such as light weighting and ease of recycling the used material [33,34].
Parikh et al. [35] examined lignocellulosic fibres like jute, hemp, kenaf, and flax for their application in the manufacturing of automotive body parts. Natural (cellulosic) fiber-based material can be a major cause of vehicle weight reduction and can also lead to a decrease in noise levels. It was concluded that addition of these fibres in body components like door panels and car boots, mainly between soundproducing components (like vibrations in steel panels), can lead to suppression of noise levels in cars to a great extent as they are capable of dissipating sound wave energy to absorb unwanted sounds. It can be concluded that the use of natural fibre-reinforced material in automotive bodies can absorb a significant amount of unwanted sound without adding much weight to the vehicle body. Natural fibre materials can be used in blends with other fusible fibres like polypropylene; the material obtained after mixing is known as non-woven fabric and gives satisfactory results with different compositions when used for components like boot liners, door panels, internal body mats, and in some other padding areas, especially those that do not require stability at high temperatures (like exterior door lining) [36,37].
Dilpreet S. Bajwa et al. [38] inspected natural fibre composites and concluded that these fibres are being principally used in automotive body applications in parts like backrests, door trims, car boots, break shoes, seat foam, etc. These fibers, which mainly cover the interior area of cars, can also be used in exterior components of automotive bodies like the covers for spare wheels and spoilers, etc. These fibres are getting greater attention with each passing day because of the realisation that they can help to get an increased output and a lower fuel consumption because they are lightweight and have good mechanical and thermal properties. Their blends can be used to replace a number of metallic components in a vehicle's body. A shift from traditional metallic material to natural fibre-reinforced material has been observed in some of the enormous automotive companies like Mercedes, Mitsubishi, Toyota, Ford, and Audi for replacing parts like the rear parcel tray, pillar trims, and in some other parts for insulation purposes and cabin noise reduction. Even when creating high precision parts, fewer finishing processes are required than with metal components, resulting in even better impact performance.Ramie fibres were discovered to have very good thermal conductivity along with excellent tensile strength; these fibres also offer good antibacterial and ventilation functions [39,40].
Yachmenev [41] examined the use of natural fibre nonwovens for automotive applications and observed that about 20 square metres of synthetic nonwoven fibres were predominantly used in making interior parts and trunk linings of automotive bodies. After their use, the metal components are scrapped and recycled, but the synthetic fibre components have to be put into waste landfills for their degradation, and it is well known that under normal atmospheric conditions, degradation of these fibres takes a very long time and mostly creates severe environmental issues. By replacing synthetic fibres in parts such as wall panels, trunk and roof liners, parcel shelves, and hood insulation liners with natural fibres instead, the biodegradability of these fibres can be greatly improved while also ensuring that the quality of the part to be manufactured is not compromised [42,43].
Alper Kiziltas et al. [44] studied the teamwork of the Ford biomaterial team and a giant pulp company, Weyerhaeuser, which were working to maximise the use of cellulosic fibres in place of glass or other mineral-reinforced fibers. Weyerhaeuser was able to create a thermoplastic composite known as THRIVETM, which consists of engineered palm cellulose fused with polypropylene. This composite product creation by Weyerhaeuser was found to be very cost effective, and the material was also readily available.When compared to short abrasive glass fibers, this composite could be manufactured with less wear and tear on production equipment and lower production energy.These composite fibres were able to meet the standards of the automobile industry in terms of hardness, durability, and temperature resistance and weighed about 10% less than a similar glass fibre sample, which could have a great impact on vehicle performance. These savings of weight and energy in production made this composite even more economical and helped Ford Motor Company reduce their carbon footprints without affecting their production or performance. Besides Ford Motor Company, many other automobile giants like BMW Motors, Fiat, General Motors, Toyota, Volkswagen, etc. are widely using natural fibre material in their vehicle interior components such as seat back rests, headliners, rear panel shelves, door trim panels, dashboard components, etc. But there are still some challenges for even more widespread use of these cellulosic fibers, as a lot of energy is required to produce them at the micro-and nanoscale. These fibres sometimes also have odour issues, show increased affinity to moisture, and are occasionally not chemically stable with some hydrophobic polymers. These all-encompassing pitfalls limit their usage only in parts that require low performance characteristics [45,46]. Fig. 4 shows how Mercedes used natural fibres in various parts of their different car models, one of which is shown here.

3.
Cellulosic fiber/nanocellulose-based polymeric composites for automotive anti-roll bar The anti-roll bar is one of the most important parts of the automobile's suspension system. An anti-roll bar, also known  as a sway bar, links together the right and left tiers with the help of the torsion springs. The anti-roll bar helps provide stability and reduces the sway or body roll of the vehicle during cornering or irregular road conditions [48]. Also, the automobile industry's strict regulations on environment and health sections suggest that natural fibre reinforced composites (NRFC) could be a potential material [49].
These anti-roll bars help stabilise the vehicle. The U-shaped bar that connects both the front wheels plays an important role. When the vehicle is rolling, swaying, cornering, or turning and the body weight shifts to one side, this bar assists in keeping the front tyres level for proper vehicle balancing. Bharane et al. asserted [50] that the material for manufacturing an anti-roll bar is steel; anti-roll bars are most likely to be manufactured from SAE class 550 and class 700 steel. The specific codes for the steels included in this class are G5160 to G6150 and G1065 to G1090, respectively. As per the research for the bars produced from these materials, the 700 MPa mark should be exceeded in terms of operating stress [51].
The purpose of the current design, or in general, the purpose of any design for the anti-roll bar, is to obtain the required stiffness to increase the vehicle's stability and handling performance while keeping in mind the mechanic limitations of the bar's material. Mastura et al. stated [52]. The automobile, a mechanical invention, is a very physical product. It has to keep up with a lot of mechanical loads, which is not possible without the components holding that automobile together. Awad et al. [53] working on a design have used optimization techniques that have provided advantageous results, offering a solution to geometrical and material constraints like load. Bayrakceken et al. [54] One such component is an anti-roll bar; it has to hold up against very strong mechanical forces caused by bending and torsion while the automobile is in operation. Due to its strong tolerance, an antiroll bar's primary material for its construction has been steel till now [55]. Now the disadvantage of using steel as the primary material is that it makes it heavy, and the tubular design makes it prone to breakage. Furthermore, in the event of an extremely critical condition, anti-roll bars absorb 30e40% of the force, making them vulnerable to severe roll [56].
In a recent study, it was noticed that for many passenger cars, anti-roll bars tend to break after 100,000 km of travel. It was also noticed that the fractures are taking place in a very similar region. The major reasons for the same are: (1) fatigue under combined bending and torsional stresses is highly reversible. (2) The crack of the fracture is initiated at the highly stressed region of the bar. (3) The fracture is taking place in a ductile manner. (4) The production process could have affected the initiation region of the failure.
Mastura et al. examined [52] Now, the issues with the current design of the anti-roll bar can be solved with a new material that works appropriately with design requirements. Furthermore, natural reinforced fibre composite material has its own distinct properties, implying that a different or newer design than conventional is required [49]. The primary goals of implementing a new material are not only to reduce costs, but also to improve performance and overcome previous issues.The structure of the new anti-roll bar should be designed while keeping in mind the functions, mechanical  loads, and potential failure modes. Also, keep in mind the most basic demands of the sway bar, i.e., restrict the sway movement and maintain an adequate stiffness value. While suggesting and thinking of new and efficient designs, environment consciousness comes to mind, which indirectly leads us towards green materials, i.e., natural fiberreinforced composites. The substitution of metal materials with natural fibre-reinforced materials should be highly considered for a better design. B Ravishankar et al. studied [57] that, while designing an automotive component, linking an anti-roll bar with the green material in substitution of the metal material led to very high energy consumption, which in turn ended up affecting the environment itself. As stated by Wu et al. [58,59] in order to design and develop an ecologically sound anti-roll bar design, the design engineers need to take into consideration not only the voice of customers (VOC) but the voice of the environment (VOE) as well. Consequently, QFD (Quality Function Development) will no longer be the only tool used for material selection. Environment-driven aspects could also be considered for material selection for environmentally friendly design using the QFDE (Quality Function Deployment for Environment) tool. The QFDE tool will play a major role in determining the range of materials that can be best used as eco-friendly in designing the anti-roll bar.
M. Sapuan et al. [60] working on hybrid bio-composite material selection, examined that one of the best fibres that can satisfy the design requirements is sugar palm. After comparing, it turned out that sugar palm has a 10% lower environmental impact because of its lower energy consumption and carbon footprint; hence, it was chosen as the material for the hybrid bio-composite. Koronis G et al. [61] also found that midrange Audi A2 cars use flax or sisal, and Mitsubishi motors engulf bamboo fibres in some of their interior components.
Furthermore, after reviewing numerous studies, we concluded that natural materials such as coir, jute, kenaf, hemp, sisal, and flax are dominating the automobile marketing. FB Dilek et al. [62] learned that in the entire process of 87 manufacturing steel-based products, making steel affects the environment the most; Seppala et al.'s [63] review also supports these claims. It is not only harmful to the environment and climate, but it is also hazardous to human life, making clear the need for green materials and industries. To support the cause, Tata Motors shifted to 100% bioplastic material for some of its components. Material based on sugarcane, sweet potato, and kenaf can be found in the covers of their spare tyres. Table 4 shows the cellulosic fibers used in automotive antiroll bar.

3.1.
New inventions for anti-roll bar in terms of green material C. Schulz et al. [64] invented an anti-roll bar with bent rotating arms that are linked to the wheel suspension by the ends. This ARB consists of a cord made of resin; a rope yard was used to carry out the procedure. It is laid into the closed tool and filled with resin. This invention has been helpful in reducing the manufacturing costs.
Krahl M et al. [65] found a solution for the issue of weight in an anti-roll bar. He invented an ARB made of fiberreinforced plastic composite. In order to strengthen this fiber-reinforced plastic, they designed this ARB with various different orientations. This was created with the goal of improving force distribution and lowering the risk of failure. Klauke et al. [66] developed a composite material in the form of flat, superimposed laminated material to ensure the stiffness of the ARB. It consists of at least one layer of fiberreinforced composite and at least one layer of metal. This invention helped to reduce the weight as well as strengthen the component. Fig. 5 is a description of the anti-roll bar used in the suspension system of a car and all its components.

4.
Cellulosic fiber/nanocellulose-based polymeric composites for automotive bumper OT Adesina et al. [67] mechanically tested natural fibres by reinforcing them into bumper beams because of their excellent corrosion resistance and superior thermochemical properties and durability. These fibres have self-lubricating properties, which let them prevail over the traditional metals and metal alloys. The decrease in body weight and vibration and noise absorption properties of these fibres tend to be some of the major reasons for the selection of natural fibres in bumper beams. Use of natural fibre composites in bumper beams leads to absorption of more collision energy, excellent torsional stiffness, and improved fatigue resistance properties, along with an attractive appearance [68,69]. Fig. 6 shows where the bumper beam is placed in a vehicle. A blue bumper beam has been marked and energy absorbers have been installed to protect the people inside the vehicle from severe damage.
Firstly, pure natural fibre material was used for making bumper beams, and mechanical properties along with its flexural and impact properties were investigated by tensile testing and several other test methods. Finally, it was concluded that pure natural fibre alone was not able to deliver the required standards of mechanical and impact strength properties needed in the automotive industry. It effectively contributes to the light-weighting factor for vehicle bodies, leads to more carbon neutrality, and improves recyclability and eco-friendliness, but these factors are not enough to be considered for using pure natural fibre in automotive as mechanical and thermochemical stability are some predominant factors to be taken into account as vehicles might be used in harsh conditions and would also be operated at high speeds. This problem could be resolved up to an extent by using hybrid fibre materials that contain natural fibre reinforced with some synthetic fiber, as they have improved both thermal and mechanical properties over those of pure natural fibre materials, but it still remains a challenging task to achieve the best possible industry standard properties from fibre material [71,72].
Muthalagu R et al. [73] compared traditionally used glass fibre material with epoxy fibre material reinforced with natural fibers, namely Kevlar and date palm fibers. By using the hand layup method and finite element analysis, many tests were conducted and data was obtained for tensile strength and modulus of tensility. Epoxy fibre was also tested with different weight percentages of Kevlar and Date Palm fibre reinforced in it. Through testing of various compositions of natural fibre-reinforced epoxy resin, it was observed that improved ductility could be obtained by increasing the composition of both Kevlar and Date Palm fibre by 50%. Testing results concluded that Epoxy composite reinforced with Kevlar and Date Palm fibres had very similar or better stress, strain, and deformation resistance properties than the traditionally used glass fibre material, and this also confirmed the increased passenger and driver safety, as well as pedestrian safety. Thus, conclusions were made in favour of natural fibre-reinforced epoxy resin, which was considered a better alternative to be used in bumper beams than glass fibre materials [74,75]. Fig. 7 depicts the step-by-step process of compression molding bio composites.
Daz-lvarez et al. [76] studied the behaviour of PLA/flax composite bumper beams using a low-velocity impact test. It was found that both the composite and matrix had good biodegradability. As separation of subsequent layers did not take place, the composite was able to provide exceptional after-impact results. It was observed that the damage generated by the impact mainly reduced the residual stiffness, but the residual strength remained almost unaffected [77].
M. M. Davoodi et al. [78] analysed the use of reinforced Kenaf fibres with materials like glass and epoxy resins in automotive bumper beams by performing tensile tests and comparing it with fibre material purely made of 100% Kenaf.  They concluded that the use of 100% Kenaf fibre material in terms of its mechanical and economic feasibility requires a lot more time and research, whereas its reinforcement with glass fibres makes it much more mechanically, chemically, and economically stable. As the bumper beam is an exterior component of the vehicle body and is a very important part that is responsible for absorbing kinetic energy during high impact collisions and providing the required bending stresses in a low impact collision, it is very important that the material being selected to be used in the bumper meet the standards of deflection and crash solidity criteria. A tensile test was performed on Kenaf reinforced fibers; about six samples were made with the provided dimensions and tested on a hydraulic testing machine for maximum yield strength, and the Young's modulus of Kenaf reinforced epoxy fibres was found to be almost double in amount than that of GMT material. Higher yield strength and Young's modulus values were most likely caused by the amount of pressure that epoxy fibres had to endure in order to become denser and have much better adhesion properties than GMT Fibers. An ASTM D 790-03 standard test was conducted on about five specimens to compare the flexural strength of both specimens. Both the flexural modulus and flexural strength of epoxy fibres were found to be higher than those of the GMT material.
Even in terms of density and impact properties, GMT material lagged behind Epoxy fibers. These critical properties of Kenaf-reinforced epoxy fibres aided in their adoption for use in automotive bumper applications.These fibres were perfectly able to be considered an alternative to GMT material, too, with much better mechanical and economic stability [58,79].
The given figure consists of an exploded view of a bumper beam which displays how low impact energy absorption takes place in real bumper system. Absorbers are often kept on a less stiffer side so that so that it can respond in case of lower impact collision but in order for absorption of higher loads, bumper beam themself are kept at a much high stiffness [78].

5.
Natural fibers for automotive leaf spring K. Umanath et al. [80] investigated the use of pineapple leaf fibre mat as the material for leaf springs. Low density, low processing cost, wide availability, and minimal impact on health and the environment all encouraged their use in a widely used and critical component like the leaf spring.-Because pineapple fibre is not carcinogenic, it does not cause cancer risk, industries have used it to make an increasing number of components, most notably leaf springs. Another material that was able to be used as a substitute for traditionally used synthetic materials was epoxy resin. It was observed that when mixed with a catalysing agent, epoxy resin gave a reaction product that was generally higher in viscosity but could be further worked upon in order to decrease the viscosity accordingly. This characteristic of epoxy resin contributes to the overall viscosity and shrinkage Rajendran et al. [82] examined that on the basis of factors like tensile strength, Young's modulus, etc. Pineapple fibre topped the table among its competitors and was found to be appropriate for being used in automotive applications such as leaf springs [83].
MM Shokrieh et al. [84] analysed the use of epoxy composite reinforced with glass fiber, collectively known as "E glass/epoxy fibers," in place of steel material that was traditionally being used for manufacturing leaf springs on a large scale. Experimentation on a steel leaf spring was done on the basis of two methods of finite element analysis, and then it was replaced by an optimised composite spring, with the geometry of the leaf spring being the most important. Results of experimentation on composite fibre showed that slight modifications in spring width and thickness can lead to a composite spring that can bear the same loading conditions as a steel leaf spring, and that too with about 80% less weight. It was also checked that resonance wouldn't hinder the change of material, as the natural frequency of the composite leaf spring was found to be much higher than that of the steel leaf spring and was also not near the range of road frequency that would lead to resonance [85].
Ravi Kumar V et al. [86] tested leaf springs under static loading circumstances and calculated the stresses and deviations, and then finite element analysis techniques were applied. Natural fibre epoxy material was taken into consideration for reducing the vehicle weight, and a single composite leaf spring was designed with natural epoxy and e-glass fibre material and was subjected to limitations like different loading conditions, varied orientation angles of the ply and laminate thickness, etc. Through the results of the experiments, it could be concluded that all the factors and stresses that could affect the material were within permitted limits. As this natural fibre epoxy was used to replace a steel leaf spring, a 75% weight reduction was also observed [87,88]. Fig. 8 shows a leaf spring made of a composite material consisting of jute, e-glass, and epoxy. Each of these materials has some unique  quality, and now they can be used together by mixing them in some calculated composition.

6.
Natural fibers for automotive helical spring Among the various types of springs available today, the helical spring is one of those springs that plays an important role in many different mechanical systems [90]. The helical springs are important to act against and reduce the constraints or loads or attributes like impacts, vibrations, weight, bumps, etc. [91]. This spring is being used in many automobile mechanical systems because it helps reduce the weight, which eventually helps save energy and improve vehicle performance [92]. Kumar MS et al. [93] examined using composite polymer material to make helical spring structures reduces their own weight, making the entire system lighter than before, but it doesn't reduce the weight or the load it can carry in a given mechanical system.
Renugadevi K et al. [94] compared E-Glass fibered helical springs to Calotropis gigantea fibered helical springs and discovered that high fibre volume improves mechanical properties while low fibre volume decreases performance. This research also found out that 35% of the volume of fibre is apt for producing desirable mechanical properties. CG fibre helps reduce weight, which is an advantage over the E-glassfibered spring. Helical springs being one of the essential parts of the automobile mechanical system, this will be a very interesting and wide area to work on for its improvement with polymeric composites. Fig. 9 shows composite helical springs treated with NaOH in order to modify and enhance their structural quality and performance.

Natural fibers for automotive gears
Gear is one of the essential parts of the automobile ecosystem; it is generally used for power, torque, and motion transmission under various loads and speeds. CM Meenakshi et al. [95] found out that gears are one of the most promising applications of natural fibre polymers. Polymeric gears are preferred over metal gears for mechanical systems that require low power, such as gear pumps, billeted roller cams, car drive shafts, and so on, because they offer numerous benefits such as light weight, low cost, good damping resistance, better performance under tensile loads, noiseless operations, minimal lubrication, and so on. Several natural fibres have several advantages and help improve different types of properties in a gear. Xia et al. [96] found that using kenaf fibre-reinforced polyester with powder activated carbon directly has an effect on electromagnetic signal absorption but also has the downside of resulting in low mechanical strength. But he noticed that using hybrid boron nitride with natural fibres would improve thermal conductivity.
Ramesh et al. [97], after research, found that performance under tensile load can be increased with the use of sisal or glass fibers. Also, Ramnath et al. [98] found that the banana and jute hybrid composite showed an increment in performance. Lastly, Saxena et al. [99], after extensive review, found that plant fibre is a better substitution for synthetic fibres in terms of environment safety (low CO2 emission), cost, density, and renewability. Fig. 10 contains gears made up of different composite materials, each having unique properties that can help in replacing traditional materials.

8.
Nano cellulose based polymeric composites for automotive applications Natural fibre nanocellulose has such great potential that it has applications not only in the automobile industry but also in other industries such as medicine, building materials, packaging, and so on.
MNF Norrahim et al. [100] discovered that these nanocellulose composites are mostly used as replacements for fibres like glass, carbon fibre polymer, steel, aluminum, etc., which are used in automotive parts like door panels, trunk liners, anti-roll bars, bumpers, seat covers, etc.
It is true that natural fibers are the future in the automotive industry, but Abu Bakar et al. [101] found out that as of now, we have seen a spike in the use of polymer reinforced with natural fibers, also R Masoodi et al. [102] investigated both types of polymer composites, thermoset and thermoplastic, and discovered a 15% and 10% increase in their application in the automotive sector, respectively. MJ Mochane et al. [103] studied how fusing these two, polymers and natural fibers, has given many advantages to the industry in terms of engineering and the environment. T. Abt et al. [104] analysed that these polymers reinforced with natural fibres showed an increment in abilities like strength, biodegradability, sustainability, and stiffness. According to Bajwa et al. [38], another important factor that can be improved by utilising the lightweight properties of this fused material is fuel efficiency. MNF Norrahim et al. [105] found that PP is one such thermoplastic polymer that is in the most demand. PP has many properties like recyclability, design flexibility, low cost, low density, etc., which makes it best suited for auto parts like bumpers, gas cans, carpet fibers, cable insulation, etc.

9.
Natural fibers from amazon that can be used in automotive sector in future There are several natural fibers extracted from Amazon plants have the potential to be used in Automotive applications. Belayne Zanini Marchi et al. [106] investigated physical properties of the fibers extracted from the stem of Brazillian Amazon plant, also known as Ubim fiber. It was found that these fibers had desirable properties, were cost effective and also gave positive results for application as reinforced polymer composites which are eco friendly and can be used in civil construction material, with this conclusion it can be inferred that Ubim fibers can also be used in automotive applications in parts that experience relatively lesser vibration levels.
Thuany E. S. de Lima et al. [107] studied Amazon natural fibers and it was found that these fibers were comparatively less dense and due to which final weight of the composite was reduced. It was also found that these fibers have superior tensile strength which confirms their usage in automotive applications.
Andressa Teixeira Souza et al. [108] compared neat Epoxy fibers with Caranan fiber reinforced epoxy composites and it was observed that reinforced composites showed a noticeable increase in impact energy than neat samples. Afonso R.G. Azevedo et al. [109] investigated use of Guaruman fiber when mixed with cement based mortars and obtained results suggest that they decrease the density of material when mixed. It can indirectly be used in automotive sector in the form of molds that are used to produce some parts used in vehicles. There is huge scope of research on this fiber in relation with automotive sector.
Raphael Henrique Morais Reis et al. [110] used Guaruman fiber to check ballistic performance and found improved results, this indicates that Guaruman fiber is able to be used in areas which experience higher vibrations in a vehicle.
Raphael Henrique Morais Reis et al. [111] studied Guaruman fiber and experimental outcomes showed that Guaruman fiber has higher much more desirable tensile strength and were much more coherent due to comparatively low microfibril angles. These results clearly point toward usage of Guaruman fibers in automotive sector.
Andressa Teixeira Souza et al. [112] explored Caranan Fiber reinforced Epoxy composites and it was found that by mixing 30% volume of Caranan fiber in epoxy composite, mechanical properties like tensile strength, modulus of elasticity, total elongation and tensile roughness tend to increase in a positive way, hence final composite had much more superior properties which can be an excellent alternative to traditionally used natural fibers in automotive sector.
Edwillson Gonç alves de Oliveira Filho et al. [113] investigated Raffia fiber and it was found that mixing of Raffia fiber with polyester increased Young's modulus and overall stiffness of the composite, this result can be a very strong basis for research on Raffia fiber and it's reinforcement with other fibers being used in automobile sector.

Future
The advancements provided by cellulose nanofiber (CNF) are limitless but come with their own set of challenges. It typically requires an excruciatingly high production cost and energy consumption at industrial levels. There have been many attempts to address this issue, but none like the usage of superheated steam pretreatment, which would ideally reduce the production cost and time of cellulose nanofiber by miles [114]. Another issue is its demand for compatibilizers. In theory, LCNF (lignin-containing CNF) is the best alternative to CNF, as it takes the least toll on the environment while providing a high yield and low production cost. Melt compounding followed by injection molding without compatibilizers and surface treatment can be used to make cellulose fibre composites in high-melting technical thermoplastics [115]. Using a wet, high-solid CNF solution in conjunction with refined melt blending methods to create improved PEbased nanocomposite materials. The CNF does not need to be pre-dried, which saves time and money in the process [44]. Functionalization of CNF can also be done using surface functionalization techniques like etherification, j o u r n a l o f m a t e r i a l s r e s e a r c h a n d t e c h n o l o g y 2 0 2 3 ; 2 5 : 1 0 8 6 e1 1 0 4 esterification, etc. in order to utilise the characteristics of CNF polymer composites.
Jorge s et al. [116]stated that the percentage of weight loss, the degradation temperature, Tg, and viscoelastic properties (storage modulus, loss modulus, and the damping factor) are the most common thermal properties. There are different factors affecting these properties such as fiber and matrix type, the presence of fillers, fiber content, and fiber orientation, the chemical treatment of the fibers, manufacturing process, and type of loading.
As per the TGA analysis performed by J. Neto et al. hybrid -composites based on epoxy are more stable in terms of thermal properties as compared to the polyester based composites [117].
To solve the existing issues, future research on nanocellulose-based materials must incorporate many factors, such as biomimetics, mathematical modelling, artificial intelligence (AI), and machine learning-based algorithms. For homogenous and on-demand nanocellulose use, novel production approaches such as additive manufacturing or 3D printing should be studied [118,119].
The exhaustive study called Life Cycle Assessment (LCA), as suggested in Ref. [5], could be used to gauge the environmental performance of the material. a) Natural fibre manufacturing has fewer environmental consequences than glass fibre production. b) In the composite, a higher content of natural fibre replaces the matrix (synthetic polymer). c) Weight reduction improves fuel economy and lowers usage-phase emissions. d) End-of-life burning of natural fibres generates energy and carbon credits [120].

Conclusion
This review clearly aims to exhibit the use of natural fibres in automotive bodies and major automotive parts such as bumper beams and antiroll bars. Factors that play a crucial role in deciding the use of natural fibres in automotive body parts, bumpers, and antiroll bars have been discussed, along with various tests conducted for validation of the use of natural fibres in automotive applications. Through the above article, it can be concluded that the use of natural fibres till now is only viable for internal parts or less performancerequiring regions of vehicles, such as trunks and internal door panels, and also for sound proofing in some internal areas due to their excellent sound absorbing properties. The review of current applications of cellulose-based fibres clearly showed that the pure form of these fibres is still not competent enough to be used in place of traditionally used materials, as its limitations are far more numerous than its advantages. Reinforcement with synthetic material can be taken as an alternative, but it also has limitations, including not being rigid enough to be used in high-performance areas and not completely eliminating the problem of recyclability and proper degradability. As a result, extensive research on the use of pure forms of fibre in both low-and highperformance areas of the automotive industry is required.

Declaration of competing 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.