Review: Natural fiber-based biocomposites for potential advanced automotive applications

Flax fiber

Flax fiber is a naturally occurring cellulosic fiber with a low volume and exceptional tensile force. Fiber possesses favorable environment with energy-saving properties due to its biodegradability, low weight, and reliable thermal insulation properties. In addition to its useful features, flax fibers are utilized for the automobile business to create desired elements that tend to be usable with polyester, epoxy, polyethylene, polypropylene, etc. Some biobased polymers are polylactic acid (PLA), starch, PHB, and oil-based matrix. These materials are used in wall reinforcements, interior panels, bumpers, spoilers, insulation panels, bicycles, tennis rackets, and skis that offer outstanding results at a lower weight.^23-26

Hemp fiber

Sustainable fibers of cellulose called hemp fibers gets obtained through hemp stalks from herbs that were valued to their strength, durability, and sustainability. Hemp fibers are integrated with PLA, polyhydroxyalkanoates (PHA), and starch-based biopolymers to produce entirely recyclable composites. Applications include hoods, trunk lids, seat backs, and acoustic insulation.^27,28

Kenaf fiber

The plant Hibiscus cannabinus is the origin of kenaf fibers, which are highly sought after for their great durability, stiffness, and strength. To create biocomposite products for cutting-edge industrial uses like fenders, bicycles, and surfing boards as well as to lessen noise and vibration in automobiles these fibers can be combined with biopolymers. ²⁹

Jute fiber

Hessian based materials are widely recognized for their excellent strength in tension along with inexpensive value, and biodegradability. Advanced industries including sports equipment, construction, automotive, and aerospace can create sustainable composite materials with great performance by combining jute fibers with biopolymers. ³⁰

Sisal fiber

A high-strength, long-lasting fiber made of Agave Sisalana that is resistant to degradation. Both structural and non-structural applications are made of them. Based on their exceptional fracture toughness along acceptable bending and tensile characteristics, sisal fiber-reinforced composites outperform other NFCs and can be employed in situations where good impact strength is required.^31,32

Abaca fiber

A naturally occurring cellulosic fiber produced by Musa textilis that possesses exceptional strength, elasticity, and resilience against degradation from seawater. Abaca fiber composites are used to create lightweight, fuel-efficient parts that decrease greenhouse gas emissions and increase durability for the aerospace and automotive sectors.^33,34

Banana fiber

Bast fiber is made by the Musa species, which is coveted for its biodegradability, sustainability, and outstanding yield strength. ³⁵ Among many natural fibers, banana fibers helped in attaining numerous properties such as swelling, mechanical, dielectric properties, and thermal degradation. ³⁶

Coconut fiber

A fiber with a high lignin content is taken from the coconut husk and is noted for its stiffness and strength. Following a peculiar property, this fiber can flow at conditions between 92 and 150°C, which is below the point of decomposition Td and above the temperature of transition Tg. In the automobile industry, this fiber offers an environmentally conscious replacement for traditional materials. ³⁷

Cotton fiber

A cellulose fiber that comes from cotton plant seeds is renowned for its high tensile strength, breathability, and softness. Cotton fibers can be mixed with different biodegradable matrices for producing composite materials. Lightweight and fuel-efficient elements have become common in automotive applications, particularly for interiors. ³⁸

Strength properties of natural fibers in automotive components

The automotive sector utilizes a growing number of natural fibers because of their cost-effectiveness, sustainability, and good engineering properties. Table 1 lists the mechanical specifications of typical organic fibers that are pertinent for automotive use. The use of natural fiber reinforcements in place of man-made materials in composites is growing in popularity among automobiles and component vendors as a sustainable alternative. Although natural fiber composites (NFCs) initially limited to unstructured parts, the idea of employing NFCs in more structural and exterior portions is gaining traction quickly due to ongoing, continuing studies. Many natural fibers used in automobiles are vegetable fibers because of their excellent characteristics as well as their substantial amounts of acoustic and thermal insulation. The automobile industry has enormous possibilities for natural fiber-based composites because of growing need of sustainable development and low-mass components. Based on research, using natural fiber composites can help reduce a vehicle part’s weight and expense by 30% and 20%, respectively. Henry Ford incorporated hemp fiber for fabricating pioneering composites of auto parts in the 1940s. Further uses of hemp, a natural fiber, were carried out by East German Trabant’s body, Daimler-Benz, and Mercedes in 1950, 1994, and 1996, respectively. For example, the European automobile industry primarily employs linseed and hempen, conversely sisal, hessian, and hibiscus cannabinus are brought in primarily from Bangladesh, the Republic of Brazil, and the US, and Philippines bananas. Flax fiber has been a particularly significant renewable resource used by the German automotive industry. ¹²

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Table 1.

Strength properties of natural fibers in automotive components.

Natural fiber	Denseness [g/cm3]	Dimensions [µm]	Tensile force [MPa]	Strength density [S/ρ]	Elastic modulus [GPa]	Modulus density [E/ρ]	Break elongation [%]	Refs.
Bamboo	1.2	244–334	503	451	35.91	31.9	1.37	Guillén-Mallette et al. 39
Musa	1.41	53–254	599	439	17.85	12.8	3.24	Kamath et al. 40
Coir	1.20	103–452	498	429	2.5	1.99	19.32	, Yap Thai Ming et al. 41 , Sathishkumar et al. 42
Abaca	1.8	—	400	267	12	—	3–10	Bui et al. 43
Coco	1.4	—	173	145.3	4–6	2.99–4.89	28.7	Yap Thai Ming et al. 41 , Sathishkumar et al. 42
Gossypium	1.7	—	290–596	176–369	5.5–12.6	3.36–6.62	—	Dharmaratne et al. 44
Ananas erectifolius	—	170	158–729	113–521	—	—	5	Kurien et al. 45
Flax	1.5	—	800–1500	535–000	27.6–80	18.4–53	1.2–3.2	Wankhede et al. 46
Hemp	1.48	—	550–900	372–608	70	47.3	—	Neves et al. 47
Hibiscus cannabinus	1.51	73–253	929	638	53	35.73	1.46	Malik et al. 48
Rhea	1.57	52.1	218–931	142–619	44–128	27.5–83.2	1.98–3.54	Patel et al. 49
Agave sisalana	1.50	51–303	528–638	358–438	9.4–22	5.98–14.7	2.95–6.82	Hassan et al. 50
Coniferous wood	1.49	—	998	657	—	26.67	4.39	Ramesh et al. 51
Jute	1.46	40–350	393–800	269–548	10–30	6.85–20.6	1.5–1.8	Senthilkumar et al. 33

The trajectory of organic/natural fibers employed in the automobile sector is shown in Figure 2.

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Figure 2.

Timelines for using natural fibers in the automobile sector.^12,52–54

Automobiles based on natural fibers have acceptable mechanical features and address non-biodegradability and greenhouse gas emissions. The automotive and insulation industries are where composites manufactured from natural fibers are most commonly used. Figure 3 shows the composite fiber revenue by applications. As a result, replacing all the synthetic composites in automobiles with natural fiber posed an additional issue. Therefore, at first, natural fiber-reinforced composites were only utilized for the fabrication of structurally devoid elements like panels, parcel shelves, spare tire covers, wood trims, headliners, and seat fillers. They were also restricted to interior parts.

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Figure 3.

Natural fibers composites revenue by application.^55–57

Thermal properties of different natural fibers relevant to automotive components

Thermal characteristics are important when utilizing natural fibers in automotive applications because they influence how the material behaves at different temperatures, which in turn affects durability, efficiency, and reliability. Organic fibers have worse heat stability than matrix. ⁵⁸ Lignin, cellulose, and hemicellulose are the primary components of organic fibers. ⁵⁹ A total of four stages of natural fiber’s thermal energy disintegration. In the initial phase, water that does not bond chemically to the fiber loses water and low-molecular-weight components break down between 50°C and 150°C. The subsequent phase, occurring within the temperature range of 200°C–350°C, involves the breakdown of hemicellulose. ⁶⁰ Cellulose decomposition is linked to the third phase, which usually occurs between 320°C and 400°C. On the other hand, the breakdown of lignin is connected to the last phase, which may occur at temperatures between 100°C and 900°C. ⁶¹ Despite this, based on an analysis of the literature, it is possible to determine that the hydrogen bonds barely affect the fiber and polymer adherence. Primary constituents of renewable materials, cellulose, hemicellulose together with lignin, have macromolecular structures that prevent convergence between the groups with polarity during treatment. As a result, inadequate adhesion qualities of the composite, fiber pull-out, and issues with fiber dispersion were brought on by the integration of the matrix and the fiber.^62,63 To get over these constraints, considerable chemical processing was performed. The purpose of top layer of material modification was to improve material adherence to the polymer. Compatible substances can be added, or the fiber can be chemically treated to change the polymer structure in conjunction with surface treatment. Poor thermal properties are another concern with utilizing natural fibers in composite materials. Natural fiber processing temperatures vary to the maximum of 200°C. Composites heat tolerance is more significantly impacted by chemical treatment during fiber mixing with biodegradable polymers. ⁶⁴ Table 2 shows how material modification affects various aspects of the heat performance of the biocomposite.

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Table 2.

Treated natural fibers thermal properties for biocomposites.

Material identity	Biopolymer	Surface modification	Heat tolerance	Refs.
Coconut husk	TPS	Sodium hydroxide: 10% w/v	Highest decrease in weight at 317°C	Adekunle 65 , Das et al. 66
Cocos nucifera	TPS	Solution for bleaching: Hydrogen peroxide and 30% w/w sodium hydroxide	The greatest drop in mass at 343°C	Tomczak et al. 67 , Rosa et al. 68
Cellulose nanofiber	Lactide polymer	N-methylmorpholine in water-based solution (20% w/w)	Reduce the heat of desorption by 10°C	John and Thomas 69 , Bledzki et al. 70
Kenaf	Lactide polymer	0.5, 1, 2, and 3 h of acetylation	Degradation started after 160°C	Mamun and Bledzki 71
Banana	PP	Alkali treatment	Maximum weight loss at 455°C	George et al. 72
Jute	PLA	Carding of fluffy jute fiber via needle-punching	Maximum weight loss at 340°C	Ochi 21
Kenaf	PU	Acetylation	Maximum weight loss at 468°C	Zaman and Khan 73
Ramie	PLA	Maleic anhydride (MA)	Maximum weight loss at 378°C	Fang et al. 74
Flax	Bio epoxy	Silane	Maximum weight loss at 330°C–400°C	El-Shekeil et al. 75

Various natural materials have organic biopolymers, mostly composed of polysaccharides and proteins, including chitin, collagen, silk, and cellulose. Meanwhile, derived from petroleum-manufactured biopolymers including polycaprolactone (PCL) and polyvinyl alcohol (PVA) and biomass-based PLA are examples to illustrate. A renewed interest in producing composites from naturally occurring biopolymers has emerged in the past few years. The impact of natural fiber and biopolymer thermal behavior on composites’ thermal decomposition has been thoroughly studied by scientists. The most widely researched biopolymers for producing panel materials for composite applications include starch, PLA, PVA, and PCL. Composites called hybrids are produced by introducing two or more natural fibers to decrease the retention of moisture, increase mechanical characteristics, and improve thermal characteristics. Composites thermal tolerance for materials reinforcement have been demonstrated to be improved by hybridization in numerous papers.^76,77 Table 3 displays a few of the results pertaining to the properties of hybrid biocomposites composed of processed organic fiber strengthened by biomass matrix.

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Table 3.

Composite materials made from biopolymers and natural fibers.

Biopolymer	Natural fiber	Treatment	Thermal stability	Refs.
TPS	Phoenix dactylifera fiber + linen fiber	Alkaline	Maximum weight loss at 356°C	Neto et al. 78 , Neto et al. 79
Maize flour	Maize husk fiber + Arenga fiber	____	Maximum weight loss at >280°C	Ibrahim et al. 80 , Khan et al. 81
PLA	Sisal fiber + coir fiber	Alkaline	Maximum weight loss at >250°C	Ibrahim et al. 82 , Hazrol et al. 83
PLA	Kenaf fiber + nano clay	Alkaline	Maximum weight loss 337°C–355°C	Veerasimman et al. 84
PLA	Aloevera fiber + nano clay	Alkaline	Maximum weight loss 338°C	Duan et al. 85 , Ramesh et al. 86
PLA	Coir fiber + PALF	Alkaline	Maximum weight loss 289°C	Ramesh et al. 87
Heat-moldable sugar palm starch agar	Arenga fiber + marine algae	____	Maximum weight loss at >270°C	Ramesh et al. 88
PBS	palm fiber + cassava starch	____	Maximum weight loss 322°C	Siakeng et al. 89
PLA	Banana + sisal	Benzoyl peroxide (BP)	Maximum weight loss at 378°C	Jumaidin et al. 90

A large number of fibers induction led to an enhancement in thermal properties in thermoplastic starch-based hybrid composites including date palm, flax, cornhusk, sugar palm, seaweed, kenaf, coir, sisal, aloevera, and banana fibers. The natural fibers’ cellulose content is more thermally stable than its starch content. By adding more natural fibers, TPS-based composites’ temperature adaptability is increased, suggesting superior fiber-to-TPS matrix adherence for prospective use in automobiles. By thoroughly evaluating thermal properties, manufacturers can ensure that the natural fibers will perform reliably under the demanding conditions typical of automotive environments, ultimately contributing to safer, more efficient, and sustainable vehicle components. ⁹¹

Automotive parts using natural fibers

The transportation industry is using a greater number of natural fibers because of their cost, sustainability, and lightweight. Some of the most common products are doors sheets, seats support headliners, cargo freighters, as well as other interior decorative elements. These fibers have several advantages, including a lower overall environmental impact on the automobile, increased performance from less weight, and possibly even lower production costs. One of the most important steps in designing automobiles that are more efficient and ecological is the incorporation of natural fibers. For instance, it has been shown that composites based on manmade fibers of glass may meet the dimensional and reliability requirements of vehicle both inside and outside parts. Excellent strength features promoted the automotive industry’s adoption of fiber-glass-built-up polymers. On the other hand, glass-fiber-based plastics exhibit drawbacks such as relatively dense fibers, difficulty within the manufacturing process, awful reuse qualities in addition with medical issues associated using such materials. In response to government requirements, the automotive industry is focusing on the ecological effects of an auto entirety span. More sectors utilizing renewable resources are seeing an increase in their use as well, including construction, aerospace, and marine. German automakers usually utilize natural fibers with polymer bases to build parts for doors, seats, shelves, and dampening and wrapping pieces, among other components. For its interior components, Toyota makes use of composites derived from sugar cane. DaimlerChrysler is part of an innovative technology transfer program that uses green materials to promote sustainability. To promote global sustainability, the company concentrated on using natural resources rather than fossil fuels. In particular, it created a supply network for automobiles centered on renewable resources, which will help producers develop raw materials for the vehicle industry. According to a different study, natural fibers are used in railroads to create berths, dividers, floor and ceiling panels, and modular restrooms. Various natural fibers with different polymers are used by industries like Mercedes-Benz, Ford, Toyota, Volkswagen, General Motors, BMW, Mitsubishi, Fiat, Volvo, Citroen, Honda, and Mazda to fabricate different components such as dashboards, acoustic insulation, ceiling liner, cargo area floor, sliding door inserts, door panels, parcel shelves, engine encapsulations, spare tire wheel cover, luggage compartment, and rear flap lining.^58,92–96 Table 4 lists the kinds of fibers which are frequently used to make various parts for automotive manufacturing.

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Table 4.

Various automotive parts using different natural fibers.^{54,57,97–99}

Automotive parts	Natural fibers
Door trim cover, wrapped, dashboard panels, coated inserts, armrests, seat cushions, rear panels	Flax
Door trims, seat rear panels, control panel	Hemp
Vehicle body sheet, underbody panel	Abaca
Packaging sheets	Banana
Door trims, instrumental panels	Jute
Seat upholstery, seat cushions, floor mats	Coir
Insulating material, acoustic damping, luggage compartment panel	Cotton
Door insides, cabin panels	Sisal/Flax
Door parts, armrest supports	Wood flour
Furniture covering, back seat protectors	Wool
Foam-filled control panels, seat rear panels, fiber in cushion backing, coated inserts and assemblies, insert pieces	Wood

Composites made of natural fibers exhibited ductile crash behavior without abrupt fracturing. Natural fibers replaced carbon fibers, cutting the carbon footprint by 75% and improving the finished products’ sustainability at a 30% cost savings.

The raw materials used in certain transport industries are shown in Figure 4. Automotive composites market is projected to expand as the demand for lightweight materials, fuel efficiency, and compliance with global emissions regulations drive the increased use of composites in passenger vehicles. A recent study estimated the market’s value at $9.5 billion by the end of 2022, with expectations to reach $16.5 billion by 2030, indicating a 7% annual compound growth rate (CAGR) for the duration of the plan.^57,100,101

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Figure 4.

Incorporation of natural fibers in automobile uses: (a) hemp, (b) jute, (c) kenaf, (d) flax, (e) jute, and (f) flax.^25,102,103

Polysaccharides

The most common polymers in nature, polysaccharides (glycans), also called carbohydrates, are vital for the survival of living things. Polysaccharides are naturally occurring compounds with a special mixture that includes environmentally beneficial characteristics and utility. These substances are recyclable, biodegradable, and environmentally friendly. Prominent polysaccharides including amylum, polypeptides, cellulose, N-acetylglucosamine polymer, xanthan polysaccharide, hydrolyzed starch, cellulose, and seaweed extract can be found with their common names. ¹⁰⁹

Proteins

Renewable resources and important macromolecules in biological systems are the basis for protein extraction. Alpha-amino acid chains naturally occur in proteins, which are expanded via the synthesis of amide bonds. Due to their significantly lower breakdown temperature compared to other organic polymers, they are not as commonly used in mixtures. There are two types of proteins: plant-derived (from potatoes, triticum, maize grain, glycine max, as well as peas) and animal-derived (from milk protein, lactoserum, keratin, connective tissue protein along with gelatin protein). Soy protein is appealing due to its biodegradable nature, environmental friendliness, abundance of naturally occurring resources, and desirable functional qualities for industrial use.^111,112

Natural materials

Biodegradable synthetic polymers

Natural fibers compatibility with various matrix materials

Natural fiber is categorized according to where it comes from plants, animals, or minerals. Botanical fibers are structural polysaccharide, whereas protein-based materials such as keratin, sheep fibers, and sericin. Botanical based fibers include cereal straw, sclerenchyma fiber, pericarp, hard-plant fiber, and lignocellulosic fibers.

The fundamental constituent of all fibers made from lignin is structural polysaccharide. The linear macromolecule known as cellulose is composed of β-1,4-glycosidic bonds in link with β-D-glucose at the C1 and C4 positions, leading to a linear polymer structure. Microfibrils, which have transparent and non-transparent regions and are 2.9–5.1 nm in thickness are created when structural polysaccharides chains aggregate. It is insoluble in water yet resistant to hydrolysis due to its compact, exceptionally clear structure, parallel, solid supramolecular fiber with a high breaking force, and restricted entry. A refractory compact structure is formed by microfibrils, which are held together via the oxygen atoms on a single strand or a nearby strand forming a hydrogen link to the hydroxyl groups that compose of those of glucose on that strand. Hemicellulose, the next fastest-growing organism on globe following a substance called structural polysaccharides was thought to contain 2.7 times more moisture that of lignified fibers. Hemicellulose has a complicated structure than cellulose, which is mostly composed of 1,4-β-D-Glucopyranose connections. Trans-4-hydroxy-3-methoxycinnamyl alcohol, trans-4-hydroxy-3,5-dimethoxycinnamyl alcohol, and trans-4-hydroxycinnamyl alcohol are the three main precursors that commence the dehydrogenative polymerization that results in lignin, which is classified as a polymeric natural product. In addition to binding the natural fibers into bundles and influencing their luster and feel, pectin is a crucial component of natural fibers. Plant tissue contains pectins to varied degrees; fruit peel and gum are the main places to find them. There are 2.9%–3.9% peptides in raw linseed. The distinct microstructure along with the minimal dense value of cellulose are primarily responsible for these favorable physical and structural characteristics. Worldwide, plant fibers have a particular tensile force of 1598–2940 MPa and an individual elasticity of 11–128 GPa. Modulus of elasticity for hemicelluloses estimated to be 8 GPa, whereas that of cellulose is approximately 140 GPa. Humidity ratio affects the two values. Moreover, it is well known that lignified fiber acts in the role of compatibilizer amongst both types of cellulose, strengthening the structure of fiber.^114–116 The geometrical arrangement in the primary elements of the fiber cell membrane is shown in Figure 6.

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Figure 6.

Structural layout of the core components in the cell wall of fiber.^117,118

Considering that fiber’s hydrophilic character sometimes causes it to cling easily to matrix polymers. Several approaches to outer layer modification have been tried to resolve this problem. These changes not only make them more wettable with the polymer matrix but also less absorb moisture, occasionally resulting in special qualities and processing simplicity. Surface modification usually involves one of four approaches: procedure based on chemical (alkali treatment, acetylation, silane coupling, etc.); mechanical treatments (irradiation, plasma activation, steam shock); physicochemical processes (solvent removal); and processes based on mechanics (forging, rolling), which have been tried by numerous researchers.^119,120

The inclusion of natural fibers such as poly(1,4-dioxan-2-one) fiber/polylactide composites increased the shear strength at the interface between PLA and plasma-processed PPDO material relative to the untreated condition. The increase in strength was caused by an induced polar functional group. The outer layers of PPDO strand produced oxygen-based functions as a result of the plasma of oxygen processing. Hydroxyl along with carboxyl groups can produce main covalently and subsequent bonds of hydrogen on the PPDO fiber interface with hydrogen in the PLA matrix, which would enhance interface adherence. Considering optimal circumstances, including medium power level, medium chamber pressure, and plasma treatment duration, it has been found that the shear strength of sisal/PP composites was greater for air plasma-treated fibers (3.1 MPa) than for argon plasma-treated fibers (2.6 MPa).

Figure 7 comparison reveals that sisal fiber treated with Ar-plasma exposure for half a minute has more profound curves than its unmodified version, likely because of engraving effect its Ar-plasma’s interface over the strand. The modified fibers show some granules, and the grooves are notably deeper compared to the untreated surface. Additionally, air plasma-treated fibers display prominent cracks on the surface. These cracks increase the interface area, potentially enhancing the bonding among air plasma-modified material and PP, thereby improving the interfacial shear strength. ¹²¹

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Figure 7.

Scanning electron microscope photos of sisal fibers that has been: (a) unmodified, (b) Argon plasma-processed, and (c) atmospheric plasma-processed. ¹²¹

Jute material processing has produced results that are comparable, and biocomposites showed increased performance. The miscanthus fiber experienced a corona discharge, and they noticed that the chemical oxidation and physical etching increased miscanthus/poly (lactic acid) and miscanthus/polypropylene laminates physical characteristics. ¹²² Researchers examined at how sugarcane cellulose processed with alkali affected the strength and stiffness of polyamide composite materials strengthened with bagasse.^123,124 The best characteristics were shown by the biocomposites formed from 1% alkali-modified sugarcane throughout all of the levels of sodium hydroxide evaluated. The best adhesive for organic fiber surfaces was found to be silane. Heat, pH, silane organofunctionalization, and hydrolysis duration are some of the variables that affect its absorption. Silane experiences the phases of breakdown, water retention, and bonds forming during treatment. Post-hydrolysis, silanol groups react with hydroxylated cellulose together with matrix functional groups (Si-matrix). Silane modification enhances the fiber’s water absorption capacity, thereby improving its reactivity with the matrix of polymers chemically. Figure 8 displays the outer alteration of organic fibers together with a schematic illustration of it.

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Figure 8.

Diagrammatic illustration of naturally occurring fiber surface modification. ¹²⁵

For instance, outer layer modification jute materials with a base, chameleon minerals, hydrogen peroxide, and siloxane procedures can improve the bonding of jute and PLA composites. Using specialized prepreg fabrication method combined with these surface treatments resulted in at least a 45% improvement in modulus of elasticity and bending modulus. ¹²⁶ The effects of silane methods on non-woven hibiscus cannabinus/poly (lactic acid) composites and the simultaneous impact of siloxane and alkaline treatments on boehmeria nivea fiber/ poly (lactic acid) composites were investigated by further researchers. Researchers observed notable improvements in their mechanical characteristics throughout, with the best outcomes coming from the combination of siloxane along with alkaline procedures. ¹²⁷ Acetylation modification, an esterification method, used to plasticize natural fibers. In this process, acetyl groups (−CH₃COO) react to the fiber’s hydrophilic hydroxyl molecules, eliminating any water that may already be present. This reaction reduces the fiber’s hydrophilicity, enhancing dimensional stability and improving fiber dispersion in polymer composites. After acetyl the fibers’ ability to absorb water is greatly decreased because acetyl bonds are substituted for hydroxyl chains, making them more resistant to water absorption. C₇H₅ClO is used in benzoylation to cure natural fibers. By interacting with the hydroxyl chains within the cellulose, the benzoyl (C₆H₅CO) molecule decreases the water-loving property of the cellulose fibers. Among various strategies for developing effective coupling agents, adding MHA grafted compatibilizer agents provides strong bond with the external layer of the fiber matrix. Maleic units connect physically with the hydroxyl molecules on the material’s interface through bonds of hydrogen and covalently through precipitation processes. Natural substances like fungus and protease can be used as a substitute for chemical and physical approaches. Among the many benefits of biological alterations are the ability to remove hydrophilic gelatin and hemicellulosic components with a lower energy expenditure. The efficiency of the composites is improved by this association, which increases the bonding among the fiber and the matrix. ¹²⁸

Possibilities and limitations in manufacturing composites enhanced with natural fibers

Biocomposites, or natural fiber reinforced composites, processing offers several opportunities as well as challenges. The inherent variety of natural fiber characteristics poses a significant problem as it can impact the final composite material’s performance and uniformity. Moisture absorption by natural fibers can cause problems with dimensional stability and gradual degradation. Additionally, the chemical reaction among cellulose fibers and the polymer is usually weaker in contrast to manmade fiber composite materials, requiring chemistry of outer layer treatments or a use of binding substances to increase adhesion. Biocomposite processing methods also need to be optimized to balance composite performance and fiber integrity because processing procedures can harm renewable sources and composites. Biocomposites fabrication is always complicated by the low stability of biobased raw materials because these procedures are designed for synthetic materials. Therefore, enhancing the properties as well as materials processability, pretreatment/modification of raw materials becomes an essential component of the fabrication of biocomposite using traditional fabrication techniques. Optimizing water-holding capacity, temperature stability, fiber allocation, material class, quantity, and manufacturing are crucial in biocomposites to achieve desirable properties. Figure 11 represents the specified temperature at which the components of the biofibers break apart. Hemicellulose degradation is the first step in the breakdown of biofibers. Another crucial problem in the production of biocomposites is the fibers splitting throughout the production phase. Fabrication method taken into consideration primarily determines the degree of fiber damage. Furthermore, variables including fiber entangling, length, temperature, pressure, shear rates, and collisions between fibers and mold, among others, also affect the breaking of fibers. Moisture infiltration increases with a larger fiber volume fraction. Furthermore, ductility is changeable. In composites, the fiber dimension is also very important. Inadequate reinforcing dispersion in biocomposite leads to weak or fiber-rich (cracking-prone) regions.

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Figure 11.

The specified temperature at which the components of the biofibers break apart.

Process variables, fiber sizes, fiber orientation, and both chemical and physical treatments (such as bonding agents, treatment with alkali, etc.) can all affect the arrangement of the fibers in the composites. Composite materials components are assembled through machining procedures. Because of their complicated microstructure, biocomposites require substantially more intricate machining as compared to manmade fibers. When damage like peel-up, debonding, or matrix cracking occurs, machining has a significant impact on the material’s mechanical performance degradation. Biocomposite laminates are made of several plies, which can cause delamination. Wear on the tool assembly and the materials’ abrasiveness are challenges while machining biocomposites. It is also crucial to consider the material behavior when machining composites because the material will vary during the machining process. The aforementioned techniques for fabrication have experienced significant development and have demonstrated efficacy in producing biocomposites that exhibit favorable attributes and quality. To create high-performance composites with a range of innovative uses, a great deal of research is required.^133–135

Considering natural fibers are better for the environment, there are still many advantages to using them, regardless of these difficulties. Renewable sources-based fibers are typically environmentally friendly, renewable, and biodegradable. Additionally, they have desirable characteristics like stiffness, high specific strength, as well as minimal density this qualifies them for low-mass purposes. Improvements in surface treatments, chemical treatments, and hybrid composite formulations are improving biocomposites performance and reliability. Moreover, renewable matrices fabrication to combine with organic fibers to create composite systems that are completely sustainable is gaining momentum. The automotive sector is investigating the application of biocomposites for lightweight structural elements and body panels, influenced by the need for more environmentally friendly and fuel-efficient automobiles. The matter of biodegradable composites has the ability to overcome existing constraints and increase its applications with more study and innovation.^119,136

Starc based composites

A variety of reinforcing natural fibers, such as carnauba, timber and coniferous woods, fibroin, araucaria cellulose husk, Phoenix dactylifera and linseed, and recycled paper fiber, are combined using starch-based monomers to increase the polymeric materials’ elasticity or impact on durability and reduce their dependence on water. For instance, a matrix made of plasticized cassava starch and cellulose nanofibrils derived from cassava waste combine to serve strengthening elements. The final performance of the material is characterized due to plasticizer’s bonding along with the availability of residual sugars. The transcrystallization of starch chains surrounding the surface of the nanofibrils is likely the reason why the glycerol/sorbitol mixture prevents stress transfer within polymer adhesion. Another starch composite made of raw wood hemp core as a reinforcement stated the improved strength traits. Rise in SPF (fibers from sugar palm), the biocomposites’ tensile strength and modulus exhibited a rising trend, whereas the percentage of extension dropped from 7.99% to 3.27% after SPF was given. The following resulted from the remarkable intrinsic binding of sugar palm starch (SPS) and cellulose fibers at the material-polymer contact. Thermoplastic starch (TPS’s) force of tension and elasticity value expanded up 260% when fiber from salvaged newspapers was used as a reinforcing material. Moreover, the addition of silk fiber was shown to considerably improve the thermoplastic rice starch (TPRS)/silk biocomposites elasticity as well as greatest force value. These illustrations highlight the possibility for renewable reinforcing to improve the strength inherent in biocomposites based on starch.^138,139

Poly (hydroxy alkanoates) (PHA) based composites

PHA adheres to lingo-cellulose fiber more readily than traditional polyolefins, so its polar feature makes it suitable for biocomposite applications with improved mechanical properties. At a volume percentage of 20%, hemp/biomer 226 samples exhibit the maximum tensile strength (21.4 MPa), modulus (2.4 GPa), and elongation (4.20%). Incorporating hemp fiber enhanced modulus, but decreased force value together with extension compared to typical matrix. Modulus of bending and tension reached 3.64 GPa as well as 44.3 MPa, each, at 19.5% content percentage. In wood fiber-biopol composites, process temperature significantly affects tensile strength, ranging 22.7 MPa at 217°C to 16.7 MPa at 238.6°C. Considering uniform heat, the modulus number ranged between 2.84 and 2.13 GPa. The integration of PHA with diverse renewable sources materials like cannabis sativa, wood, linseed, jute along with agave offers promising route to develop high-performance, eco-friendly biocomposites. These materials meet the rising need for environmentally friendly resources across a range of industries by having improved strength and lowering their environmental impact.^112,138

Poly(lactide) based composites

Research demonstrated that adding more cellulose fiber reinforcements to PLA-based composites enhanced their tensile and flexural properties. Table 6 highlights several biocomposite systems produced with PLA as the matrix using different approaches. ¹⁴⁰

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Table 6.

Mechanical properties for various lactide polymer composites.

Lactide polymer materials	Maximum stress (MPa)	Tensile elasticity (GPa)	Extension at rupture (%)	Processing method	Refs.
PLA-soy protein matrix reinforced with 3% cornhusk and silane coupling agent (5%)	17.22 ± 0.09	0.69 ± 20.8	___	Single-helix extrusion machine	Meereboer et al. 141
Polylactic acid with 30% flax fiber reinforcement	51.2 ± 2.89	7.95 ± 0.43	0.9 ± 0.17	Co-rotating twin-screw extrusion machine	Liu et al. 142 , Saenghirunwattana et al. 143
Polylactic acid matrix with 1% MFC reinforcement	43.5 ± 3.94	4.3 ± 0.19	___	Triple-roll milling followed by injection molding	Oksman et al. 144 , Couture et al. 145
Polylactic acid-ramie fiber composite (30% ramie)	64.5 ± 1.43	___	3.9 ± 0.18	Hot-press sheet molding technique	Okubo et al. 146
PLA reinforced with 20% flax fibers	62.8	5.086	5.77	Extrusion followed by injection molding	Liu et al. 147
PLA-ramie fiber composite with 5% maleic anhydride (30% ramie content)	63.4 ± 0.85	3.97 ± 0.08	4.1 ± 0.2	Twin-helix extrusion system	Teshome Wagaye et al. 148
Polylactic acid-wood flour composite (40% wood flour)	59.19 ± 1.59	6.93 ± 0.38	1.24 ± 0.2	Injected plastic molding	Ketata et al. 149
PLA laminated with bamboo fabric (specific laminate orientation)	76.31	1.62	13.46	Sheet stacking process	Gurunathan et al. 150
Polylactic acid-bamboo fabric composite (warp orientation)	78.82	4.98	___	Press molding	Kianifar et al. 151

The essential function that weaves patterns or weave designs serve in composites made with fibers. As an illustration, hemp/PLA composite with a twill structure fabric as reinforcement showed 15% and 10% greater impact and tensile characteristics, respectively, than the plain weave. The fabricated hemp fabric composites’ coefficient of thermal expansion significantly decreased (between 69.4 × 10⁻⁶ m/°C and ×10⁶⁻ m/°C) when the material’s percentage by volume increased from 5.98% to 19.7%. This suggests in which the materials possess a lot of prospects in components that are used in the automotive and aerospace industries and are exposed to a broad spectrum of heats. Similarly, jute fibers, when combined with PLA, result in composites with notable improvements in strength values. Jute/PLA blends exhibit enhanced reliability and offer environmental advantages, rendering them appropriate for application in diverse sectors such as construction, transportation, and packaging, where mechanical robustness and sustainability are crucial attributes. ¹⁵²

Performance comparison for biocomposites with traditional materials commonly used in automotive applications

When comparing biocomposites to traditional materials commonly used in automotive applications, several performance aspects need to be considered. These include mechanical properties, environmental impact, cost, and processability.