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Review

Enhancement of Mechanical Properties of Natural Fiber Reinforced Polymer Composites Using Different Approaches—A Review

by
Dharanendra Yachenahalli Thimmegowda
,
Jamaluddin Hindi
,
Gurumurthy Bethur Markunti
and
Muralishwara Kakunje
*
Department of Mechanical & Industrial Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal 576104, Karnataka, India
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(5), 220; https://doi.org/10.3390/jcs9050220 (registering DOI)
Submission received: 1 March 2025 / Revised: 3 April 2025 / Accepted: 5 April 2025 / Published: 29 April 2025
(This article belongs to the Section Polymer Composites)
Browse Figures
Versions Notes

Abstract

:
Natural fibers have become increasingly popular owing to their affordability, environmental friendliness, and renewability. Owing to their abundance and low density, they have gained attention in their use as reinforcements in polymer composites. However, untreated natural fiber composites have several disadvantages, including higher water absorption, low-to-moderate mechanical properties, and challenges with fiber-to-matrix adhesion. To address these drawbacks, various approaches have been employed, such as the chemical treatment of natural fibers, fiber hybridization, and the incorporation of nanoparticles/fillers. Chemical treatment enhances the interfacial bonding with the polymer matrix by different mechanisms. Hybridization enhances the mechanical properties of composites by leveraging the advantages of individual fibers. The incorporation of nanoparticles enhances the mechanical properties and various other properties due to a significant increase in interfacial interaction, which is a result of the increased surface area of nanoparticles.

1. Introduction

Natural-fiber-reinforced polymer composites (NFRPCs) have a wide range of applications that span across different sectors—automotive, construction and manufacturing, defense, and consumer goods. In the automotive industry, NFRPCs have been used as early as 1996 for door panels in one of the Mercedes-Benz cars. In automotive, the application is in interior components, such as dashboards, headliners, door panels, seat backs, decking, noise insulation panels, boot lining, hat racks, trunk panes and roofing, and spare tire covers [1]. In the construction and manufacturing sectors, they are used in buildings and furniture, respectively. In buildings, they are used in door frames, windows, floor matting, partitions, railing systems, fencing, and ceilings. In furniture, they are used for manufacturing tables, chairs, and other kitchen tools. In the defense sector, personal body armor is manufactured out of NFRPs. They are also used in the packaging industry as take-out food containers or to preserve perishable fruits. In the sports industry, since movement requires lightweight components, NFRPs, especially natural–synthetic hybrid polymer composites, are most preferred. Bicycle frames, snowboards, fishing rods, archery, and ski poles are some of the components used. Drug delivery, tissue engineering, orthopedics, medical implants, wound care, and cosmetic orthodontics are among their medical applications [1,2].
Research and engineering departments have shown interest in fiber-reinforced materials instead of monolithic materials over the past several years. Synthetic fibers, such as glass, carbon, and aramids, are commonly used in the aerospace, automobile, construction, and sports equipment industries. Glass fibers are widely used because of their low cost and good mechanical properties. However, synthetic fibers have many drawbacks, such as not being environmentally friendly, being hazardous to health, producing carbon dioxide when burned, and having disposal issues. To overcome these problems, natural fibers have been introduced to manufacture composites [3,4]. Table 1 provides a comparison of the natural fibers and glass fibers.
One of the major reasons for the use of natural fibers in polymer composites is that the increasing population is causing a major shortage of petroleum products; thus, researchers were searching for inexpensive and readily available natural resources. When fruits, vegetables, and plants are grown domestically, the produce feeds the populace, and agro-waste serves as a raw material for biofuels, packaging, textiles, automobiles, aerospace, and sports industries. Every year, over 50 million agricultural wastes are generated, which are utilized as raw materials in the building, sports equipment, packaging, aero, and automobile sectors [4].
Natural fibers are derived from plants or animals and are not artificially created or manufactured by humans. Humans have used these fibers to create a variety of products, such as ropes, construction materials, textiles, and paper. Natural fibers are increasingly considered the best substitute for synthetic fibers in many engineering applications because of their biodegradability, renewability, cost-effectiveness, and light weight. They are an excellent and promising alternative to man-made fibers in composites [3,4,5]. A detailed classification of the fibers is illustrated in Figure 1.
Plant-based natural fibers can be obtained from bast (kenaf, hemp, ramie, and jute), seeds (cotton, coir, and kapok), leaves (abaca, pineapple, and sisal), wood, and roots. Natural fiber performance is affected by fiber extraction techniques, the plant age, and processing. Natural fibers have various drawbacks. First, fiber quality varies according to growth circumstances, harvesting procedures, and processing techniques. Second, they have a high water absorption rate because they absorb moisture from the environment, which causes swelling in the composites and reduces their durability and mechanical properties. The third problem is fiber adhesion to the matrix, which makes it difficult to transmit the load to the fibers ([3,5,7]. Previous studies have been conducted to address the abovementioned issue with natural fibers, and the results revealed that the chemical structure and arrangement of cellulose fibrils directly influence the mechanical properties of the reinforcing fibers. The three common approaches for treating natural fibers are physical, chemical, and biological approaches. The fiber surfaces were altered using corona and plasma treatment during the physical treatment process. This treatment roughened the fiber surface, imparting strong bonding between the fiber and matrix in the polymer composites. Chemical treatment of natural fibers is one of the most widely used and inexpensive materials. Biological treatment is the third most environmentally sustainable and friendly treatment. Enzymes, bacteria, and fungi are used in this process to break down the fiber components and strengthen the fibers [7,8,9].
The chemical composition of natural fibers is important because it directly affects the fiber’s properties. The chemical compositions of the various natural fibers are listed in Table 2.
Natural fiber polymer composites (NFPCs) integrate natural fibers with polymer matrices to prepare high-quality composite materials. The polymer matrix plays a major role in determining the capability of the composite during its continuous phase, which transfers the load to the reinforced fibers and improves adhesion. Moreover, the matrix helps protect the fiber from environmental degradation. Thermosetting and thermoplastic polymers are the most used matrices for preparing polymer composites. Epoxy, Bakelite, phenolic resin, and polyester are commonly used thermosetting polymers to fabricate polymer composites. The thermoplastics used included vinyl ester, polypropylene, unsaturated polyester, low-density polypropylene, and polypropylene. Epoxy is a widely used thermoset resin for preparing composites because it has higher fatigue strength than other thermoset resins. Epoxy polymers possess excellent adhesive, chemical resistance, toughness, and electrical insulation properties [8,9,11].
This review paper begins with a brief description of the properties of various commonly used natural fibers, followed by a description of fiber extraction, fiber surface treatment, and composite fabrication. This paper also addresses the challenges faced or problems with untreated single natural fiber polymer composites and also discusses approaches undertaken to tackle these issues.

2. Properties of Commonly Used Natural Fibers

Jute fiber: Jute fibers were obtained from the bark of the white jute and Tossa plants (Corchorus capsularius). According to the OEC World, USD 205 million of jute was exported from Bangladesh and USD 29.1 million from India in 2022, including important exports from Belgium, the United States, and Tazawa [12]. The chemical composition of jute fiber is approximately 60–71% cellulose, 14–21% hemicellulose, 12–13% lignin, and a small amount of pectin, wax, and other constituents. Compared to other natural fibers, jute plants are the most cost-effective, eco-friendly, lightweight (density of 1.3 gm/cc), biodegradable, possessing outstanding mechanical properties, including high tensile strength of 400–800 MPa, Young’s modulus of 10–30 MPa and elongation at break of 1.5–18%, higher resistance to heat and higher water absorption and maintenance. Owing to their flexibility, jute fibers have numerous applications in many industries, including the textile and packaging industries, composites, agriculture, horticulture, and home decoration [13]. Jute fibers were also used to make Hessian cloth, carpets, ropes, and bags [14].
Coir fiber: Coir fiber was obtained from the outermost husk of the coconuts. India and Sri Lanka are the largest producers of coir. The chemical constituent of the fiber includes cellulose 32–43%, hemicellulose 0.2–0.3%, lignin 40–45%, and a small quantity of pectin. Coir fibers are known for their biodegradability, durability, thermal insulation, and versatility. However, their main drawbacks include water absorption, low tensile strength, and variability [15].
Sisal fiber: Sisal is sourced from the leaves of the Agave sisalana plant and is abundantly available and cultivated in Tanzania and Brazil. These fibers are known for their strength and versatility, making them suitable for various applications. The chemical composition of sisal fiber is 65–70% cellulose, 10–15% hemicellulose, and approximately 2–3% lignin. Sisal fibers have many advantages, including a density of 1.45 g/cc, a tensile strength of 530–640 MPa, an elongation at break of 2.5%, a tensile modulus of 9.4–22 GPa, durability, and biodegradability. However, the primary drawback of sisal fibers is their water absorption, which is subjected to chemical treatment to improve their mechanical properties. Traditionally, fibers are utilized in the preparation of ropes, carpets, mats, and fancy articles [16,17].
Pineapple leaf fiber (PALF) or ananas: The pineapple plant, which is 1–2 m tall and wide, is a member of the Bromeliaceae family, and the fibers are derived from the leaves of the pineapple plant. It is mostly grown for fruit in the coastal and tropical regions of India. The chemical composition of PALF is 70–82% cellulose, 5–12% lignin, and 1.1% ash. PALF has many advantages: it is biodegradable, economical, low density (1.07 to 1.5 g/cc), renewable, low cost, and has mechanical properties, namely, a tensile strength of 413–1627 MPa, a Young’s modulus of 34.5–82.5 GPa, and an elongation % of 1.6–4 [18]. The main disadvantages of PALF include its superior water absorption capacity and insufficient fiber–matrix interfacial contact, which can be chemically treated to enhance its mechanical properties. PALFs are used in furniture, automobiles, and textile industries [19,20].
Bamboo: Bamboo fibers originate from the pulp of bamboo plants, and China is the main source of these fibers. The bamboo fibers have an excellent tensile strength of 540–630 MPa, elongation at break of 2.89%, and Young’s modulus of 11–17 GPa. The fibers have a low density of 0.6–1.1 g/cc, are biodegradable, and are reusable [21]. The disadvantages of bamboo fibers include susceptibility to moisture, pests, and fungal attacks, inconsistent quality, low load-bearing capacity, fire risk, and the cost of chemical treatment. Bamboo is a rapidly growing and renewable resource, making it an eco-friendly choice for textiles, paper, and even as a reinforcement in polymer composites.
Flax: Flax fibers are derived from the stem of the flax plant. Linen fibers are generally familiar and appreciated because of their strength, durability, and natural luster. Flax fibers are broadly grown in Western Europe. The chemical composition of flax is 60–81% of cellulose, 17–20.6% of hemicellulose, 2–3% of lignin, 1.8–5% of pectin, and 1.7% of wax. Flax fibers are enduring, eco-friendly, lightweight (density of 1.5 g/cc), have a tensile strength of 346–1500 MPa, Young’s modulus of 27 GPa, and elongation at break of 2.5–3.1 [22]. A few disadvantages are moisture sensitivity, brittleness, and divergence in quality, which need to be handled through treatments and meticulous processing to fully unleash their potential in advanced applications. Flax fibers are used in the textile industry, automobile industry, and eco-packaging [23].

3. Natural Fiber Extraction, Surface Modification (Chemical Treatments) and Composite Fabrication Techniques

Natural fibers originate from numerous plant sources, including leaves, stems, and seeds, which necessitate specific extraction methods. Leaf fibers, including pineapple leaf fiber (PALF), sisal, and banana, are typically extracted through a process known as decortication. This technique involves crushing, scraping, or soaking the leaves to detach the fibers from the surrounding tissue. For example, sisal and PALF fibers have been obtained from the leaves of Agave and pineapple plants. Water or chemical retting, decortication, and manual techniques are used to extract these fibers. The stem fiber jute, hemp, and flax were extracted through the retting process. The plant stems were submerged in water for a few days to separate the pectin from other impurities. The retting process is widely used for flax plants, where the stem is immersed in water for several days to separate the fibers. Ginning techniques were employed to remove seeds from the cotton fiber. The key challenges in extracting the fibers are uniformity in quality, fiber length, waste disposal in case of chemical retting, and the time required for the manual extraction process [24,25,26].
Natural fibers are extracted through various processes and altered to improve their mechanical properties, strength, durability, and adhesion to the matrices used in composite materials. The fiber surface can be altered through chemical, physical, or a combination of the two. Chemical treatments include alkali (NaOH), acid, and silane treatments, which are the most employed treatments. These treatments aim to increase the fiber adhesion with the matrices, remove impurities, and increase water resistance. For example, alkali treatment enhances the crystallinity and surface roughness of fibers, thereby improving the adhesion capability of polymers. The second surface modification technique is physical treatment, which includes plasma treatment, heat treatments, and UV radiation. It is used to change the fiber surface and increase its wettability, making it more compatible with matrices. This process involves changes in the fiber surface without altering the internal chemical composition. Furthermore, surface coatings can be applied to boost the fiber’s durability and resistance to environmental factors [25,27,28,29].
Chemically treated natural fibers are employed in the preparation of composite materials and are distributed as reinforcements within a polymer matrix. The treated fibers improved their interaction with the matrices and enhanced their performance compared with the untreated fibers. The remarkable advantages of Natural fibers are that they are eco-friendly, strong, and lightweight, making them excellent replacements for synthetic fibers in many industries, including automotive, aerospace, construction, and packaging. Different manufacturing techniques are employed to prepare composites, namely hand lay-up, where layers of fibers are manually arranged in a mold and resin is added to bind them; in compression molding, both the mixture of fibers and matrix are arranged into a mold, and heat and pressure are applied to prepare the composite. Other methods include injection molding and filament winding. NFRC offers many advantages, including eco-friendliness, light weight, minimal environmental impact, and low cost. For example, flax fibers are increasingly utilized in bio-composites for automotive interiors and panels, offering a sustainable substitute for synthetic materials [25,26,30].

4. Challenges and Setbacks of Untreated Single Natural Fiber Polymer Composites

Untreated single natural fiber polymer composites have various disadvantages that reduce their performance and utilization, particularly in demanding structural and functional environments. The main drawback is the weak interfacial bonding between the hydrophilic natural fibers and the hydrophobic polymer matrix. This leads to poor stress transfer, resulting in composites with low tensile and flexural strengths, reduced stiffness, and brittle fracture behavior [31]. The non-cellulose components of natural fibers, such as hemicellulose, lignin, pectin, and waxes, act as unwanted impurities and create stress concentrators, which reduce the strength of the composites [32]. Variations in the chemical composition, diameter, and surface roughness of the fiber contribute to inconsistent mechanical properties, making these composites unreliable for large-scale applications that require uniform performance [33].
One of the noted drawbacks of single natural fibers in composite materials arises from their hydroxyl groups, which render the fibers hydrophilic and prone to moisture absorption [30]. Moisture absorption causes them to swell and become unstable, resulting in delamination of the composites and eventually reducing their mechanical strength, especially in high-moisture environments. The development of voids and defects while making composites, often a result of irregular bonding and non-uniform fiber distribution, further degrades the physical properties such as density and structural integrity [27]. The presence of hemicellulose and lignin in untreated fibers indicates low thermal stability, which decomposes at a low temperature of 200–300 °C. This reduces the processing and working temperature range of the composites and renders them susceptible to thermal degradation during use [34]. In addition, the low thermal conductivity of untreated natural fiber composites limits heat dissipation and is not suitable for high thermal applications [35].
Overall, untreated single natural fiber polymer composites have setbacks in mechanical, physical, and thermal performance, mainly owing to the untreated surface and natural variability of the fibers. These problems highlight the requirements for fiber surface modifications to make them work better with polymer matrices and enhance composite properties for practical applications.

5. Enhancement of Mechanical Properties of NFPCs by Different Approaches

Natural fiber polymer composites (NFPC) have gained significant attention owing to their light weight, eco-friendliness, and cost-effectiveness. However, their mechanical properties are often weaker than those of conventional man-made fiber composites. These limitations arise from the poor interfacial bonding between natural fibers and polymer matrices, high moisture intake, and variability in the fiber structure and composition. The following three approaches have been introduced to address these issues.
1. Fibers can be modified by chemical treatment to enhance their composite’s mechanical properties.
2. Fibers can be hybridized to enhance their composite’s mechanical properties.
3. Filler materials can be used to enhance their composite’s mechanical properties.

5.1. Modification of Fiber Surface by Chemical Treatment

Various chemical treatments have been employed to modify fibers to improve the adhesion between the fiber, the matrix, and the mechanical properties. Different chemical treatments can enrich the surface properties of the fibers and strengthen the matrix, which improves the overall strength, durability, and stiffness. The various chemical treatments include alkali, silane, plasma, acetylation, permanganate, silane coupling agents, and peroxide treatments. Among the chemical treatments listed above, acid or alkali (NaOH) treatment is the most common and simplest method for fiber treatment. When selecting chemical treatments, it is important to consider how the fibers and matrix are affected. This approach is significant from the point of view of the applications of NFRPs in automotive interiors, buildings, decking/furniture, etc. [1]. Different types of chemical treatments are discussed below.

5.1.1. NaOH Treatment

NaOH treatment involves the use of sodium hydroxide to chemically modify the natural fibers. The process involves soaking natural fibers in a NaOH solution at a specific concentration and temperature for a specific period. The mechanism of action of NaOH can be well explained with the help of the chemical Equation (1) given below [28].
Fiber–OH + NaOH → Fiber–O–Na + H2O
The purpose of NaOH treatment is to enhance the compatibility of the natural fiber surface with the polymer matrix in a composite material. The treatment removes the non-cellulosic components, which increases the surface roughness of the fiber, leading to better mechanical interlocking with the polymer. In one of the research works, Pineapple leaf fiber (PALF) was treated with NaOH, which led to enhancement in surface roughness, which is qualitatively evaluated through Scanning Electron Microscope (SEM) images [36]. The SEM image is shown in Figure 2, wherein the surface of untreated fiber is compared with that of treated fiber. Apart from its effect on surface roughness, the treatment also causes a reduction in the microfibril angles, which leads to an enhancement in the load alignment through a load transfer ring close to the axis of the fiber, which, in turn, increases the molecular orientation and enhances the tensile properties of the fiber matrix [37].
In terms of efficiency of the treatment in enhancing the properties, in one of the research works, it is observed that alkali-treated hemp fiber reinforced in epoxy matrix showed an increase of 2.21% in tensile strength, 15.56% increase in flexural strength, and 20.73% in impact strength when compared with untreated hemp fiber composite [38]. Similarly, the Agave fibers treated with alkali and embedded in an epoxy matrix exhibited significant enhancements in tensile strength of 23.23%, flexural strength of 38.25%, and impact strength of 118.23% [39]. The jute fiber was chemically treated using the alkali method with 5% NaOH for two hours, and epoxy composites were prepared. The tensile properties improved by 8% compared to untreated jute composites. Similarly, the flexural strength and its modulus were enhanced by 10 to 15.9%, respectively [40].
In the context of the applicability of the treatment, it is to be noted that NaOH treatment is cheap, fast, relatively eco-friendly, and repeatable, and hence, extensively applied on all natural fibers when compared with other treatments [41,42,43].

5.1.2. Silane Treatment

Compatibility between hydrophilic natural fibers and hydrophobic polymers in a composite material can be enhanced not only by increasing the surface roughness and achieving better mechanical interlocking but also by forming a new chemical linkage between natural fiber and the polymer matrix, called the chemical coupling method. This is true in the case of silane treatment, wherein one end of the alkoxysilane coupling agent interacts with the polymer matrix, and another end interacts with the natural fiber. The coupling agent is hydrolyzed into a silane solution wherein a condensation reaction takes place to form siloxane linkages [44]. These linkages form hydrogen bonds with the cellulosic natural fibers, as shown in Figure 3.
Even though silane treatment aims to enhance bonding between fiber and polymer matrix, it also results in a reduction of diameter and an increase in crystallinity of the natural fiber [28]. An increase in crystallinity is achieved by the removal of non-cellulosic contents. During this process, the well-joined microfibrils separate into individual fibers, which is called fibrillation. This effect of silane treatment was observed in a couple of other literature as well [32,46]. It is to be noted that silane coupling agents are very expensive and are usually carried out after alkaline pre-treatment.
There are several pieces of literature on silane-treated natural fiber polymer composites. In one of the articles, Leucas aspera fibers were chemically treated with 10% Tri-ethoxy-vinyl-silane solution, which enhanced the ultimate tensile strength by 30% and hardness by 7% when compared with untreated fibers [46]. The flax fibers were treated with silane solution, and prepared composites significantly reduced the water absorption by 20% compared with untreated fiber composites [47].

5.1.3. Benzoylation

Benzoylation is usually performed after NaOH pre-treatment. Equations (2) and (3) show the chemical reactions of NaOH pre-treatment and following benzoylation with natural fibers.
Fiber–OH + NaOH → Fiber–O–Na+ + H2O
Fiber–O–Na+ + benzoyl–Cl → Fiber–benzoyl + NaCl
The NaOH treatment replaces the -OH group of cellulose with -ONa. Benzoylation replaces -ONa with -OC6H5. Na and Cl integrate to form NaCl, which is then eliminated by washing with distilled water. The purpose of this treatment is to form organic linkages and enhance compatibility with the polymer matrix [48]. In addition, benzoylation removes non-cellulosic content, thereby leading to fibrillation and an increase in surface roughness [28]. A higher surface roughness results in better wettability and enhanced bonding because of mechanical bonding with the polymer matrix in composite applications. Overall, similar to the alkali and silane treatments, the properties of the fibers were enhanced by benzoylation [49]. This method has a greater environmental impact as the use of acid chloride is an environmental pollutant and, hence, undesirable [43].
The Sugar Palm Fibers (SPF) were subjected to benzoylation treatment post-NaOH treatment, and the results revealed that in the benzoyl-treated SPF, the diameter was reduced by 39.49% and tensile strength increased by 123.23% when compared with untreated SPF [50]. The Acacia pennata natural fibers were treated with benzoyl chloride, and the fibers showed an increased crystallinity index of 72.14% with an increase in tensile strength by 158.1% and a decrease in tensile modulus by 52.2% [51].

5.1.4. Acetylation

The use of an acetic acid (CH3CO) solution for surface treatment is called acetylation. The reaction of natural fibers with acetic acid is shown in Equation (4) [48].
Fibers–OH + CH3–C (= O)–O–C (= O)–CH3 → Fiber–OCOCH3 + CH3 COOH
Acetylation treatment of pineapple leaf fiber increased surface roughness while reducing its hydrophilic nature. Fibers treated for 65 min exhibited an increase in the highest tensile strength of 181.25%, surpassing both untreated and other treated fibers [52]. Acetylation of pre-alkali-treated Banana Bunch Fiber (BBF) enhanced its mechanical properties compared to untreated fibers. The treatment improved fiber–matrix adhesion, increased surface roughness, and reduced moisture absorption, leading to superior tensile strength and durability [53]. Acetic acid treatment of Napier grass fibers led to enhanced tensile properties and thermal stability, even though there was no appreciable increase in the crystallinity index [28]. Acetic acid treatment with varying concentrations: 5, 10, and 15% increases in alpha cellulose and reduces hemicellulose by a small amount. However, the lignin content increased with an increase in acetic acid concentration. The optimum concentration for tensile and thermal stabilities was 10% [54].
From the point of view of sustainability, it is to be noted that, though acetylation leads to better adhesion of natural fibers with polymer matrix and acetic anhydride being a cheap chemical, it is toxic in nature [43].

5.2. Hybridization of Reinforcements

Hybridization, when discussing fiber-reinforced polymer composites, refers to the method of integrating two or more different types of fibers (natural and synthetic fibers) within a single composite to attain a combination of properties that exceed those of composites made from a single type of fiber. This hybridization strategy uses the advantages of individual fiber types to counterbalance their drawbacks. For example, in PALF-GF hybrid composites, glass fibers have high tensile properties, while PALF contributes to light weight and environmental sustainability. Enhancing mechanical properties, environmental sustainability, cost-effectiveness, and lightweight applications are the objectives of hybridization.
The hybridization of fibers in polymer composites requires careful attention to various key factors to achieve optimal performance. Properties such as the mechanical strength, weight, stiffness, and moisture resistance of each fiber play a major role in determining specific applications [2,55]. Bonding between the fibers and the matrix is essential for superior stress distribution and durability of the composites. It is also vital that the manufacturing process aligns seamlessly with the chosen fibers for maximum effectiveness and performance [56].
In one of the research works, varying combinations of coir and bagasse fibers were reinforced in an epoxy matrix to prepare hybrid composites to maintain a total fiber weight percentage of 10%. When compared with coir or a bagasse fiber composite, a hybrid combination of 3 wt.% coir and 7 wt.% bagasse exhibited the maximum tensile strength, with an increase of 20%. Other hybrid combinations resulted in enhanced tensile strength compared to single fibers but were lower than the above combination [57]. In another similar study, composites were prepared by combining bamboo and coconut fibers, each with 15 wt.%, and then compared with composites containing 30 wt.% of each fiber separately with epoxy. The combination of these natural fibers showed an increase in maximum tensile strength of 228.53% and 27.39% when compared to the individual fiber composites [58]. Coir and sisal fibers were used together in an epoxy matrix, and the authors noted an increase in tensile strength and flexural strength by 43.59% and 19.15% in dry conditions and 26.32% and 32.56% in wet conditions, with 40% of fiber reinforcement when compared with 20% of fiber concentration respectively [59]. Researchers have used a combination of banana and sisal fibers with epoxy resin to create polymer composites. The combination of 20 wt.% banana and 15 wt.% sisal showed an enhancement in tensile strength of 76.92%, while a combination of 30% sisal and 5% banana resulted in a flexural strength enhancement of 14.63% [55].
Natural fibers are combined with synthetic fibers to improve their mechanical properties. Glass fibers (GF) are synthetic fibers commonly used with natural fibers. Carbon fibers and aramids are also used in some applications because of their benefits. Composites were created using various natural fibers combined with glass fibers in different sequences and subjected to mechanical characterization. A significant amount of research has been conducted on the utilization of PALF and GF in composites consisting of epoxy, vinyl ester polymers, and polyesters. Researchers have used various combinations of PALF and GF with different weight percentages, orientations, and sequences to achieve improved results. The authors noted significant improvements in tensile strength, flexural strength, and impact strength [60,61,62,63,64]. The composite materials were prepared using natural fiber hemp and GF with epoxy in various wt.%. These hybrid composites showed a significant enhancement in impact properties compared to other types of hybrid composites [65].
Hybrid natural fiber polymer composites are utilized in aircraft, marine, civil construction, automotive, and sports goods applications. Aircraft domes, ship/boat hulls, pedestrian bridges, cross-arm structures in transmission grid systems, automotive bumper beams, hockey sticks, and helmets are some examples of their specific applications [41,42].

5.3. Incorporation of Filler Material/Nanoparticles/Nano-Fillers

Nanotechnology is a promising technology for the development of advanced materials in the future engineering world. Polymer nanocomposites work better than pure polymers owing to their properties, such as low density, easy processing, and toughness. Nanocomposites consist of particles with at least one dimension between 1 and 100 nanometers; these nanoparticles act as fillers in the matrix [66,67]. Nanomaterials are generally classified based on their dimensionality as follows:
Zero-dimensional: The dimensions of the nanoparticles, including width, length, and height, are all nanoscales. Notable examples of such nanoparticles are quantum dots and fullerenes, specifically gold, metal nanoparticles, and silver.
One-dimensional: These materials have one dimension outside the nanoscale, and nanotubes and nanowires are the best examples.
Two-dimensional: These materials have one dimension at the nanoscale (1–100 nm), and the other two dimensions are larger than the nanoscale. Examples include graphene and nanosheets (NSs).
Three-dimensional: These are materials in which all three dimensions exceed the nanoscale but still exhibit nanostructures. Examples of such materials include nanoporous materials, nanocomposites, and polymers with nanostructured fillers [66,67].
Figure 4 shows the classification of the nanomaterials based on their dimensionality.
Incorporating fillers, nano-fillers, and nanoparticles into materials, such as composites, significantly enhances their properties. These additives have unique advantages based on their size, structure, and interactions with the matrix material.
Nanoparticles (NPs) have distinctive properties due to their high surface area-to-volume ratio, quantum effects, and interactions at the nanoscale. The mechanism of the nanoparticles reinforced with a polymer matrix has enhanced the mechanical properties by creating strong interfacial bonding with the matrix, which will improve the load transfer, thermal properties, and barrier properties [69]. The NP can react with the polymer chain, creating crosslinked structures. The mechanism of nanoparticle (NP) interaction can be better understood through the following example: Surface modifiers for nano clay increase the spacing between its layers by means of dispersion and covalent bonding [70]. When these modifiers are mixed with crosslinked epoxy resin, they enhance the crosslinking within the nanoclay matrix [71]. The amine groups in the modifiers bond with free epoxy chains by breaking the epoxy ring, which allows the resin to adhere more strongly and disperse effectively throughout the nanoclay matrix. Figure 5 below shows the reaction mechanism of nano clay with epoxy resin. Similarly, each nanoparticle shows unique properties when interacting with the different polymer matrices, depending on its reaction mechanism and compatibility.
Nanoparticles play a significant role in improving the mechanical properties of polymer composites. The enhancement of properties depends on the size, shape, variants, dispersion, and applications. They act as stress concentrators to maximize the load transfer between reinforcement and the matrix, producing the tortuous path for crack propagation and enhancing the toughness, also increasing the interfacial bonding between the fibers and matrix. A couple of researchers discussed below have used various extents of nanoparticles to enhance the mechanical properties of composites. The incorporation of 4% SiO2 with epoxy resin enhances the mechanical tensile strength by 30.57%, flexural strength by 17%, and modulus by 76% [72]. The coating of nanoparticles of graphene (30 min with 1:10 M:L ratio and air dried) in jute-epoxy composites enhances the mechanical properties of tensile strength by 110% and Young’s modulus by 324% when compared with untreated ones [73]. The nanoparticles titanium dioxide (TiO2), magnesium oxide (MgO), and zinc oxide (ZnO) were incorporated into 6% alkali-treated kenaf-epoxy composites, and results showed that enhancement in tensile strength by 11.2%, modulus by 24.9%, flexural strength by 8.4%, and impact strength by 28.7% [74].
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Figure 5. Reaction mechanism of nanoclay with epoxy resin [75].
Figure 5. Reaction mechanism of nanoclay with epoxy resin [75].
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5.3.1. Fillers

Fillers are commonly used to reduce costs and enhance mechanical properties such as stiffness, hardness, and wear resistance. Despite being larger in size, typically on the micrometer scale, their primary function is to modify bulk properties, such as density, thermal stability, and durability. Examples of fillers include calcium carbonate, silica (SiO2), glass fibers, and talc [76,77].

5.3.2. Nano-Fillers

Nano-fillers, with particle sizes less than 100 nanometers, have a high surface area and interact more strongly with the surrounding matrix than conventional fillers. Nano-fillers enhance mechanical, thermal, and sometimes electrical properties. Even a small amount can significantly enhance the material’s strength, toughness, and thermal stability without increasing its weight or density and can be used in many applications [76,77].
Commonly used nano-fillers are nano-silica, carbon nanotubes (CNTs), boron nitride, and nano clay.

5.3.3. Nanoparticles

Nanoparticles are materials with dimensions in the nanometer scale (1–100 nm). Unlike structural fillers, they are discrete small particles. Because of its tiny particles, it shows unique properties and is useful in various fields, including medicine, environmental science, energy, and electronics. Nanoparticles are commonly used to alter specific physical properties such as conductivity, reactivity, or optical characteristics [76,77].
Examples of nanoparticles include SiO2 Nanoparticles, TiO2 Nanoparticles, ZnO Nanoparticles, and fullerenes.
Incorporating filler materials while preparing natural fiber polymer composites is a key strategy for enhancing their mechanical properties. The selection of filler materials must be compatible with the matrix, and their dispersion within the matrix is a critical factor in determining the effectiveness of the reinforcement. The filler material can effectively transfer the load to the matrix, thereby enhancing its mechanical properties. This helps prevent crack propagation in the composites by acting as reinforcement. Some fillers can also increase the crystallinity of the composites [76,77]. Table 3 lists the different filler materials used and their effects on the mechanical properties.
Nano clay and nano-silica are the most used nano-filler materials in NFPC. Non-clay, also known as nanoscale or nano-sized clay, strengthens the link within the matrix, reinforces composite materials, and functions as a booster. The ideal weight percentage of the nano-filler materials increases the composite modulus and tensile strength [50,84,85]. Because of its high aspect ratio and distinct intercalation properties, nano clay has drawn more attention as a polymer-reinforcing material. Amino groups found in montmorillonite (MMT) clay interact with the matrix to increase the attachment of the fiber to the matrix. The area of the nanoparticles exceeds their volume; in the composites, it increases the surface area of the nanoparticles and the interaction of the matrix [86]. The inclusion of nano clay with polypropylene (PP) increases the thermal stability of the composites [87]. There are some advantages of nano clay, such as enhanced mechanical properties (mechanical strength, stiffness, and durability), excellent barrier properties, reduced weight and barrier properties, and cost-effectiveness.
Graphene is the thinnest material, but it is stronger than steel and is highly conductive. These materials are single-layer carbon atoms placed in a hexagonal honeycomb structure. The characteristics of these materials are strong, light weight, flexible, and transparent. The graphene is reinforced with natural fibers; the surface area of these materials is high and has a large contact area with the matrix. Also, it enhances the mechanical, thermal, crystallinity, and barrier properties. The jute fiber was treated with alkali and carboxylate graphene oxide to improve the tensile properties by 183.14% when compared to acetone alone, 3.09% with silane, and 122.25% with alkali treatments, similarly flexural strength of 61.84% with acetone, 5.95% with silane and 67.92% with alkali treatment and electrical conductivity of 0.0217 mS/m and improved thermal properties [88]. Carbon nanotubes are carbon-based nanomaterials that have very high mechanical, thermal, and electrical properties. These materials are used with different polymer matrices to enhance the physiochemical and mechanical properties [89].
Silica nanoparticles, also called nano-silica, are silicon dioxide (SiO2) particles with nanoscale dimensions. Polymers containing silicon dioxide (SiO2) have outstanding mechanical, physical, and thermal properties. It is extensively employed in the coating, polymer, and chemical industries owing to its high adsorption capacity, large surface area, small size, rich content, and high efficiency. Compared with micro-SiO2, nano-SiO2 coatings are more resistant to corrosion [81]. Compared with other nanoparticles, the amorphous form and high specific surface area of nano-silica particles make them more advantageous for the fabrication of composites. Typically, all composites supplemented with lignocellulosic material have improved mechanical characteristics when the nano-silica particle content is higher. Numerous studies have observed that increased water intake may be the outcome of an increase in SiO2 concentration. And the thickness swelling of the composites [90]. Some of the advantages of nano-silica (SiO2) are listed below: good thermal stability and high melting point, chemical inertness, high hardness and transparency, and versatility in applications (glass manufacturing, coating, filler materials, chemical industries, and cosmetics).
Applications of natural fiber hybrid polymer nanocomposites are in the automotive industry, construction industry, electrical and electronics, food packaging, and cosmetics industry. Interior and exterior parts in automobiles, flame-resistant panels, printed circuit boards, and containers are some specific examples [91].

6. Summary and Future Directions

The review paper started with a review of the properties of common natural fibers and collectively briefed about extraction, surface modification, and composite fabrication techniques. There was a discussion on the drawbacks of untreated natural fiber polymer composites, which highlighted the pitfalls of insufficient adhesion between hydrophilic natural fiber and hydrophobic polymer matrix. The section also suggested that there is a need to overcome these drawbacks in order to use the fibers in polymer composites. The following sections discussed three different approaches to tackle these issues–chemical treatment, hybridization, and use of nanoparticles or nano-fillers.
Different types of chemical treatments were discussed, viz. NaOH treatment, silane treatment, acetylation, and benzoylation. The applicability and limitations of these treatments were also discussed. Future research must focus on finding sustainable treatment methods that are environmentally friendly and non-toxic; they must also be affordable and scalable. There are biological and physical treatments that are either not affordable or not scalable, even though they are easy on nature and human beings. Therefore, there are a lot of research gaps existing in this area in developing the technologies that would help achieve the purpose.
Even though natural fibers have a lot of advantages, they do not possess the mechanical properties of synthetic fibers like glass and carbon fibers, to name a few. Therefore, hybridization, along with synthetic and nanoparticles, is carried out. However, this will not make the composite completely eco-friendly. Also, nanoparticles are expensive since the process of preparation and the technology used in their preparation demands precision. Future direction must focus on developing biobased or biodegradable synthetic fibers and resins. The future must also focus on developing eco-friendly surface modifications of nanoparticles so that, in the process of tackling the problem of compatibility issues with the polymer matrix, new problems are not created. The future must inspire researchers to innovate and develop solutions that are complete.

Author Contributions

D.Y.T.—reviewing the literatures, formal analysis, paper writing and editing. J.H.—conceptualization, supervision and paper editing. G.B.M.—methodology and supervision. M.K.—conceptualization, methodology, supervision and paper editing. All authors have read and approved the final version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are gathered from published research articles which are available in public domain.

Acknowledgments

The authors would like to thank the Manipal Institute of Technology, Manipal, for providing necessary support.

Conflicts of Interest

No potential conflict of interest was reported by the authors.

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Figure 1. Detailed classification of fibers [6].
Figure 1. Detailed classification of fibers [6].
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Figure 2. SEM images of PALF fibers at a calibrated scale of 10 µm (a) The untreated PALF, (b) The NaOH-treated PALF [36].
Figure 2. SEM images of PALF fibers at a calibrated scale of 10 µm (a) The untreated PALF, (b) The NaOH-treated PALF [36].
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Figure 3. Mechanism of chemical reaction between siloxane linkage and natural fiber [45].
Figure 3. Mechanism of chemical reaction between siloxane linkage and natural fiber [45].
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Figure 4. The classification of nanomaterials is based on dimensionality [68].
Figure 4. The classification of nanomaterials is based on dimensionality [68].
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Table 1. Comparison of natural and glass fibers [3].
Table 1. Comparison of natural and glass fibers [3].
Natural FibersGlass Fibers
PriceLowLow, but higher than NF
Density/WeightLowTwice that of natural fibers
RenewabilityYesNo
RecyclabilityYesNo
DistributionWideWide
Energy AbsorptionLowHigh
Carbon-balancedYesNo
Health risk when breathingNoYes
Abrasion to machinesNoYes
DisposalBiodegradableNot biodegradable
Table 2. The chemical components and their composition of various natural fibers [10].
Table 2. The chemical components and their composition of various natural fibers [10].
FiberCellulose
(Weight%)		Hemicellulose
(Weight%)		Lignin (Weight%)Pectin
(Weight%)		Moisture Content
(Weight%)		Waxes
(Weight%)
Hemp70–7417–223.7–5.70.96.2–120.8
Kenaf45–57228–133–5
Flax7118–202.22.38–121.8
Jute61–7114–2012–130.212–140.5
Sisal66–7810–1410–141010–222
Ramie69–7614–170.6–0.71.98–170.3
Cotton85–905.7 0–18–90.6
Henequen774–813.1
PALF70–82 5–12.7 12
Cereal Straw38–4515–3112–208
Abaca56–63 12–1315–10
Oil palm EFB65 19
Banana63–64105 10–12
Oil palm Mesocarp60 11
Coir32–430.2–0.340–453–48
Nettle86 11–17
Table 3. Various filler materials are used and their effect on mechanical properties.
Table 3. Various filler materials are used and their effect on mechanical properties.
Filler MaterialTypeEffect on Mechanical PropertiesReferences
Calcium Carbonate
(CaCo3)		MineralIt enhances stiffness and dimensional stability.[78]
TalcMineralIt will diminish the shrinkage.[78]
Carbon Nanotubes (CNTs)Nano-fillersExcellent mechanical properties, electrical and thermal conductivity.[79]
GrapheneNano-fillersExcellent mechanical properties, electrical and thermal conductivity.[79]
Nano clays
Montmorillanite (MMT)		Nano-fillersHigh barrier properties, enhanced strength and mechanical properties.[80]
Silica NanoparticlesNano-fillersHigh stiffness and strength.[80,81]
Cellulose NanocrystalsNatural Nano-fillersIt is biodegradable, has good tensile strength, and low weight.[82]
Wood FlourParticulateBiodegradable and cost-effective.[83]
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Thimmegowda, D.Y.; Hindi, J.; Markunti, G.B.; Kakunje, M. Enhancement of Mechanical Properties of Natural Fiber Reinforced Polymer Composites Using Different Approaches—A Review. J. Compos. Sci. 2025, 9, 220. https://doi.org/10.3390/jcs9050220

AMA Style

Thimmegowda DY, Hindi J, Markunti GB, Kakunje M. Enhancement of Mechanical Properties of Natural Fiber Reinforced Polymer Composites Using Different Approaches—A Review. Journal of Composites Science. 2025; 9(5):220. https://doi.org/10.3390/jcs9050220

Chicago/Turabian Style

Thimmegowda, Dharanendra Yachenahalli, Jamaluddin Hindi, Gurumurthy Bethur Markunti, and Muralishwara Kakunje. 2025. "Enhancement of Mechanical Properties of Natural Fiber Reinforced Polymer Composites Using Different Approaches—A Review" Journal of Composites Science 9, no. 5: 220. https://doi.org/10.3390/jcs9050220

APA Style

Thimmegowda, D. Y., Hindi, J., Markunti, G. B., & Kakunje, M. (2025). Enhancement of Mechanical Properties of Natural Fiber Reinforced Polymer Composites Using Different Approaches—A Review. Journal of Composites Science, 9(5), 220. https://doi.org/10.3390/jcs9050220

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