Abstract
This study investigates the enhancement of mechanical characteristics of hybrid polymer composites reinforced with Palmyra Palm Leaflet (PPL) and Coconut Sheath Leaf (CSL) fibers by integrating Tamarind Shell Powder as a filler material. The composites were fabricated with varying ratios of PPL and CSL fibers, and their tensile strength, flexural strength, interlaminar shear strength (ILSS), impact strength, hardness, and water absorption were evaluated. Results indicated that the composite with 20% PPL and 10% CSL exhibited superior mechanical performance, achieving the highest tensile strength of 42.22 MPa, flexural strength of 94.35 MPa, ILSS of 7.52 MPa, and impact strength of 5.98 J. Hardness values peaked at 84.12 SD for the same composition. Moreover, the integration of Tamarind Shell Powder significantly improved the mechanical properties compared to composites without filler, which showed lower values across all parameters. Water absorption tests revealed an increase in water uptake with filler incorporation, though within acceptable limits for practical applications. Scanning Electron Microscopy (SEM) analysis further supported these results by revealing enhanced fiber-matrix bonding and better dispersion of the filler, resulting in fewer voids and defects. This research highlights the potential of bio-based fillers in optimizing the mechanical performance of hybrid composites for sustainable engineering applications.
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Enhancement of Mechanical Properties of Hybrid Polymer Composites Using Palmyra Palm and Coconut Sheath Fibers: The Role of Tamarind Shell Powder
Karthik Aruchamy 
,a,* Manickaraj Karuppusamy,b Sivasankari Krishnakumar,c Sivasubramanian Palanisamy ,d,* Manivannan Jayamani,e Kumar Sureshkumar,f Syed Kashif Ali,g,h and Saleh A. Al-Farraj i
This study investigates the enhancement of mechanical characteristics of hybrid polymer composites reinforced with palmyra palm leaflet (PPL) and coconut sheath leaf (CSL) fibers by integrating tamarind shell powder as a filler material. The composites were fabricated with varying ratios of PPL and CSL fibers, and their tensile strength, flexural strength, interlaminar shear strength (ILSS), impact strength, hardness, and water absorption were evaluated. The composite with 20% PPL and 10% CSL exhibited superior mechanical performance, achieving the highest tensile strength of 42 MPa, flexural strength of 94 MPa, ILSS of 7.52 MPa, and impact strength of 5.98 J. Hardness values peaked at 84 SD for the same composition. Moreover, the integration of tamarind shell powder significantly improved the mechanical properties compared to composites without filler, which showed lower values across all parameters. Water absorption tests revealed an increase in water uptake with filler incorporation, though within acceptable limits for practical applications. Scanning electron microscopy supported these results by revealing enhanced fiber-matrix bonding and better dispersion of the filler, resulting in fewer voids and defects. This research highlights the potential of bio-based fillers in optimizing the mechanical performance of hybrid composites for sustainable engineering applications.
DOI: 10.15376/biores.20.1.698-724
Keywords: Mechanical characteristics; Filler incorporation; Hybrid polymer composites; Tamarind shell powder; Palmyra palm leaflet; Coconut sheath leaf
Contact information: a: Department of Mechatronics Engineering, Akshaya College of Engineering, and Technology, Kinathukadavu, Coimbatore – 642109, Tamil Nadu, India; b: Department of Mechanical Engineering, CMS College of Engineering and Technology, Coimbatore – 641032, Tamil Nadu, India; c: Department of Electronics and Communication Engineering, Akshaya College of Engineering, and Technology, Kinathukadavu, Coimbatore – 642109, Tamil Nadu, India; d: Department of Mechanical Engineering, PTR college of engineering and technology, Austinpatti, Madurai – Tirumangalam road, Madurai – 625008, Tamil Nadu. India; e: Department of Mechanical Engineering, Kalasalingam Academy of Research and Education, Krishnankoil-626126, Tamil Nadu, India; f: Department of Electronics and Communication Engineering, Koneru Lakshmaiah Education Foundation Vaddeswaram, Guntur District – 522 302 Andhra Pradesh, India; g: Department of Physical Sciences, Chemistry Division, College of Science, Jazan University, P.O. Box. 114, Jazan 45142, Kingdom of Saudi Arabia; h: Nanotechnology Research Unit, College of Science, Jazan University, P.O. Box. 114, Jazan 45142, Kingdom of Saudi Arabia; i: Department of Zoology, College of Science, King Saud University, Riyadh, Saudi Arabia;
* Corresponding Authors: sivaresearch948@gmail.com; akarthikme86@gmail.com
INTRODUCTION
The growing global concern for environmental sustainability and the urgent need to reduce reliance on non-renewable resources have significantly influenced research and development in the materials science field (Iroegbu and Ray 2021; Kamarudin et al. 2022; Manickaraj et al. 2024a; Sumesh et al. 2024). Traditional synthetic fiber-reinforced polymer composites, such as those reinforced with glass, carbon, or aramid fibers, have long been the materials of choice for various high-performance applications due to their excellent mechanical properties, including superior strength, stiffness, and durability (Alam et al. 2022; Gurusamy et al. 2024). These materials are widely used in industries such as aerospace, automotive, and construction, where performance under demanding conditions is critical (Karuppiah et al. 2022; Karthik et al. 2023b; Palanisamy et al. 2023b). However, synthetic composites come with several drawbacks that undermine their long-term sustainability, especially in the context of environmental preservation and resource management (Prabhu et al. 2020; Ead et al. 2021; Wan and Lee 2021). One of the most significant concerns with synthetic fiber-reinforced composites is their dependence on petrochemical-based materials, both in the fibers and the polymer matrices. The production of these materials is energy-intensive, contributing to greenhouse gas emissions and other environmental pollutants (Mikulčić et al. 2016; Govindarajan et al. 2024; Palanisamy et al. 2024). Furthermore, the non-biodegradable nature of synthetic fibers and polymer matrices poses a significant waste management challenge. Once these materials reach the end of their useful life, they often end up in landfills or are incinerated, leading to further environmental degradation (Gutowski et al. 2013; Chen et al. 2020). The high cost of production and limited recyclability of synthetic composites further exacerbate these issues, making it imperative to seek sustainable alternatives (Rashid et al. 2023; Karthik et al. 2024).
In response to these challenges, natural fiber-reinforced polymer composites have emerged as a viable and environmentally friendly alternative. Natural fibers, which are derived from renewable sources such as plants, animals, or minerals, offer several advantages over synthetic fibers (Zhao et al. 2018; Mahir et al. 2019; Zhao et al. 2020; Thapliyal et al. 2023; Deshmukh and Palanisamy 2024). They are biodegradable, renewable, and have a lower environmental footprint throughout their life cycle, from production to disposal. In addition, natural fibers of plants are abundantly available and cost-effective, making them attractive for large-scale applications (Rajeshkumar et al. 2021). These fibers are typically composed of lignocellulosic materials, which are a combination of cellulose, hemicellulose, and lignin. This composition gives natural fibers their desirable mechanical properties, such as good tensile strength, low density, and high specific strength (Karimah et al. 2021).
Despite the environmental and economic benefits of natural fibers, they often have mechanical limitations compared to synthetic fibers (Ahmad and Zhou 2022; Aruchamy et al. 2024; Palaniappan et al. 2024b). Natural fibers generally exhibit lower tensile strength, lower thermal stability, and higher moisture absorption, which can compromise the durability and performance of composites in demanding environments (Azwa et al. 2013; Palaniappan et al. 2024a). These limitations have spurred extensive research into improving the mechanical properties of natural fiber-reinforced composites, leading to the development of hybrid polymer composites (Asyraf et al. 2022; Kumar et al. 2022b; Sumesh et al. 2023).
Hybrid polymer composites represent a significant advancement in materials engineering, as they combine two or more types of fibers within a single polymer matrix. This hybridization approach allows for the synergistic exploitation of the complementary properties of different fibers, resulting in composites with enhanced mechanical performance (Deshmukh 2022; Asyraf et al. 2023). By carefully selecting and combining natural fibers with varying mechanical properties, it is possible to create materials that are stronger, stiffer, and more durable than those reinforced with a single type of fiber (Lotfi et al. 2021; Nurazzi et al. 2021). For example, one fiber may provide high tensile strength, while another may offer better impact resistance or moisture resistance. By blending these fibers, hybrid composites can achieve a balance of properties tailored to specific application requirements (Safri et al. 2018).
The simultaneous usage of two or more types of natural fibers (which is sometimes called “hybridization”) can also address the moisture absorption issue that plagues many natural fiber composites. Some natural fibers have better water resistance due to their higher lignin content or waxy surface layers (Hajiha et al. 2014; Manickaraj et al. 2019). By incorporating such fibers into a hybrid composite alongside fibers with higher strength but lower moisture resistance, it is possible to mitigate the negative effects of moisture absorption while maintaining the desired mechanical properties (Bahrami et al. 2020). This makes hybrid composites more suitable for applications in environments where exposure to moisture or humidity is a concern, such as in outdoor structures, marine environments, or automotive components (Mayandi et al. 2020). In addition to enhancing mechanical properties, hybrid composites also offer the potential for improved processability and manufacturability. This flexibility in manufacturing makes hybrid composites suitable for mass production and scalable industrial applications, further enhancing their appeal as a sustainable alternative to traditional materials (Bahrami et al. 2020; Goutham et al. 2023).
Among the various natural fibers used in hybrid polymer composites, palmyra palm leaflet (Borassus flabellifer) and coconut sheath leaf fibers have shown considerable promise due to their unique mechanical properties and availability (Manickaraj et al. 2022; Thirupathi et al. 2024). Both of these fibers are considered agricultural waste, making their use in composites an excellent example of waste valorization and resource efficiency. Palmyra palm leaflet fibers are derived from the leaflets of the palmyra palm, a tropical plant widely cultivated in Asia and Africa. The fibers are lightweight, biodegradable, and possess moderate tensile strength, making them suitable for reinforcement in polymer matrices (Ain et al. 2016; Khan et al. 2018). Similarly, coconut sheath leaf fibers are obtained from the sheath of coconut leaves, a byproduct of the coconut industry. These fibers are known for their high lignin content, which gives them better rigidity and resistance to moisture compared to many other natural fibers (Hasan et al. 2021; Manickaraj et al. 2023; Thapliyal et al. 2023).
The combination of palmyra palm leaflet fibers and coconut sheath leaf fibers in a hybrid composite offers the potential for a balanced mechanical performance. Palmyra palm leaflet fibers provide good flexibility and tensile strength, while coconut sheath leaf fibers offer rigidity and better moisture resistance (Manickaraj et al. 2024b; Thirupathi et al. 2024). This complementary nature makes these hybrid composites well-suited for applications requiring a combination of strength, toughness, and environmental durability.
To further enhance the mechanical properties of palmyra palm leaflet fibers and coconut sheath leaf fiber-based composites, the integration of bio-fillers has emerged as a promising approach. Bio-fillers are natural materials added to the composite matrix to improve fiber-matrix bonding, reduce void content, and enhance mechanical performance (Ghori et al. 2018; Kumar et al. 2022a; Mylsamy et al. 2024). Tamarind shell powder (TSP), derived from the hard outer shell of the tamarind fruit (Tamarindus indica), is one such bio-filler that has shown great potential in improving the performance of natural fiber-reinforced composites (Stalin et al. 2019). TSP is rich in cellulose and hemicellulose, which provide strength and rigidity to the filler. When incorporated into a polymer matrix, TSP can improve the dispersion of fibers, enhance fiber-matrix adhesion, and reduce the presence of voids and defects that can weaken the composite (Mehdikhani et al. 2019; Lal and Mhaske 2021; Niang et al. 2021).
The addition of tamarind shell powder to palmyra palm leaflet and coconut sheath leaf hybrid composites is expected to have several beneficial effects. First, TSP can improve the tensile and flexural strength of the composite by reinforcing the matrix and providing additional load-bearing capacity. Second, the presence of the filler can enhance the interlaminar shear strength (ILSS) by improving the bonding between the layers of fibers, reducing the likelihood of delamination or failure under shear loads. Third, TSP can increase the composite’s hardness and impact strength, making it more resistant to wear and sudden impacts (De Cicco et al. 2017; Dattu et al. 2022; Kasinathan and Rajamani 2022). However, one potential trade-off is the increase in water absorption, as natural fillers like TSP tend to be hydrophilic. Proper surface treatment of fibers and fillers, as well as careful control of the composite formulation, can mitigate this issue and maintain acceptable levels of moisture resistance (Mohammed et al. 2022).
A novel feature of this work is its focus on a hybrid composite using palmyra palm leaflet and coconut sheath fibers, reinforced with tamarind shell powder (TSP), a bio-filler that remains largely unexplored in composite research. Unlike commonly used natural fibers and fillers, the combination of these agricultural waste-derived materials provides a unique synergy: Palmyra offers tensile strength and flexibility, while coconut sheath adds moisture resistance and rigidity. The addition of TSP further enhances mechanical properties by improving fiber-matrix adhesion and reducing voids, resulting in increased tensile strength, interlaminar shear strength, and hardness. This comprehensive mechanical profile, combined with the sustainability benefits of using low-cost, eco-friendly materials, sets this work apart from existing studies, providing an innovative and practical alternative to synthetic composites for structural applications.
Overall, hybrid polymer composites reinforced with palmyra palm leaflet and coconut sheath leaf fibers, along with tamarind shell powder as a filler, offer a sustainable and high-performance alternative to synthetic composites. By leveraging complementary properties of these natural fibers and enhancing them with bio-fillers, these hybrid composites can achieve the mechanical strength, durability, and environmental resistance needed for a wide range of structural applications (Fragassa et al. 2024). This approach not only addresses the mechanical limitations of individual natural fibers but also contributes to the broader goals of sustainability, resource efficiency, and waste reduction. As industries continue to seek eco-friendly materials for future applications, hybrid polymer composites made from natural fibers and bio-fillers represent a promising solution for the development of greener, more sustainable products. The hybrid polymer composites offer several advantages, including enhanced mechanical properties such as increased tensile strength, flexibility, and moisture resistance. They are cost-effective and eco-friendly, utilizing agricultural waste materials to reduce environmental impact. Furthermore, these composites provide a sustainable alternative to synthetic materials, aligning with the growing demand for greener and more resource-efficient solutions in industrial applications.
EXPERIMENTAL
Palmyra Palm Leaflet Fibers
The leaflets from the palmyra palm were collected from local agricultural waste. The fibers were extracted using the water retting process, followed by manual separation. After extraction, the fibers were washed thoroughly with distilled water to remove impurities and dried under sunlight (Karthik et al. 2023a). The dried fibers were cut to a uniform length (10 to 20 mm) for composite fabrication. Figures 1A and 1B show palm leaflets and fibers.
Fig. 1A. Palmyra palm leaflet; 1B. Palmyra palm leaflet fiber; 1C. Coconut leaf sheath with coconut tree; 1D. Coconut leaf sheath
Coconut Sheath Fibers
Coconut sheath fibers were sourced from the outer sheath of coconut leaves, which is another agricultural byproduct. The fibers were extracted using a mechanical decortication process, cleaned with water, and dried in a hot air oven at 60 °C to remove moisture (Sathish et al. 2021). The fibers were then cut to lengths similar to palmyra palm leaflet fibers for consistency in the composite manufacturing process. Figures 1C and 1D show coconut sheath leaves.
Bio-Filler
The bio-filler, tamarind shell powder, was prepared by grinding the shells of tamarind fruit into a fine powder. The powder was sieved to obtain particles of uniform size for use in the polymer matrix. TSP was selected due to its high cellulose and hemicellulose content, which enhances the strength and rigidity of the composites. Figures 2 and 3 show tamarind seeds and their powder.
Matrix Material
Epoxy resin (LY556) and the corresponding hardener (HY951) were used as the polymer matrix (Palanisamy et al. 2023a). The epoxy resin was chosen for its excellent mechanical properties, good adhesion, and ease of processing in composite fabrication.
Surface Treatment of Fibers
Both palmyra palm leaflet and coconut sheath leaf fibers were treated with alkali to improve fiber bonding and reduce water absorption. The fibers were soaked in 5% sodium hydroxide (NaOH) solution at room temperature for 4 h. After treatment, the fibers were washed thoroughly with distilled water to remove excess NaOH and neutralized with dilute acetic acid solution (Rajeshkumar et al. 2016; Murugesan et al. 2022). The treated fibers were dried at 60 °C for 24 h to obtain a moisture content below 5%. Alkali treatment with sodium hydroxide (NaOH) improves the bonding between natural fibers (palmyra palm leaflet and coconut sheath leaf) and the polymer matrix by breaking down lignin and hemicellulose, exposing cellulose’s hydroxyl groups. This enhances the fiber-matrix adhesion, improving mechanical properties such as tensile and flexural strength. The treatment also reduces water absorption by modifying hydrophilic groups, which helps prevent swelling and degradation. After washing with distilled water and neutralizing with acetic acid, the fibers are dried to remove excess moisture, stabilizing the material. This process results in a stronger, more stable fiber-matrix complex, improving the overall performance and durability of the composite.
Preparation of Hybrid Composites
The hybrid composites were prepared using a hand lay-up technique followed by compression molding. The weight fractions of PPL fibers, CSL fibers, and TSP were varied according to the composite designations as shown in Table 1. The total fiber content varied between 30% to 60% of the composite’s weight, while the tamarind shell powder filler content was kept constant at 10%, except for the last combination (30PPL30CSL), which had no filler content (Kumar et al. 2022b). The specific combinations of fiber and filler content were adjusted to assess their impact on the mechanical properties.
Table 1. Hybrid Composite Designations
Incorporation of Tamarind Shell Powder
For composites that included tamarind shell powder, the powder was mixed with the epoxy resin at a fixed weight percentage of 10%. The resin-hardener mixture (in a 10:1 ratio) was stirred thoroughly to ensure uniform dispersion of the bio-filler. For composite designation 30PPL30CSL, no tamarind shell powder was added.
Lay-Up Process
A mold release agent was applied to the mold surface to prevent the composite from sticking. A layer of epoxy resin was first poured into the mold, followed by a layer of fibers (a mixture of palmyra palm leaflet and coconut sheath leaf fibers). Another layer of epoxy resin was applied, and this process was repeated to achieve the desired thickness. tamarind shell powder was uniformly distributed throughout the resin layers.
Compression Molding
The laminate was then placed in a hydraulic press and compression molded under a pressure of approximately 2 MPa. The mixture was cured at room temperature for 24 h and then cured in an oven at 80 °C for 2 h to improve the bonding of the epoxy matrix.
Mechanical Testing
The prepared composite samples were cut according to ASTM standards for mechanical testing.
Tensile Strength (ASTM D638-14 2022)
The tensile strength of hybrid composites was evaluated using a universal testing machine (UTM). This test measures the maximum tensile stress that the composite can withstand before failure. Test specimens were cut into dumbbell shapes in accordance with ASTM D638 (2022) to ensure uniform stress during testing (Singh et al. 2014; Laureto and Pearce 2018). The machine applied a uniaxial tensile force to the specimen at a rate of 5 mm/min until it fractured (Karuppiah et al. 2020; Carmona and Colorado 2021). Tensile strength, deformation, and Young’s modulus (hardness) were recorded. These results provide insight into the ability of the composite to withstand tensile strength and show how the fiber-matrix bond behaves under tension. Figure 4 shows the tensile specimens.
Fig. 4. Tensile specimen
Flexural Strength (ASTM D790 2017)
Flexural strength was assessed through a three-point bending test, which evaluates the material’s capacity to withstand deformation when subjected to an applied load. Rectangular composite specimens were supported at two ends, and a load was applied at the center, as per ASTM D790 (2017) (Anggraini et al. 2017). This setup mimics real-world bending scenarios, such as those encountered in beams or structural components. The force required to bend the composite before failure, along with the maximum deflection, was recorded (Vinod et al. 2021). Flexural modulus (stiffness during bending) was also calculated. This test helps in understanding how well the composite performs under flexural or bending stresses, particularly in applications like panels or beams.
Impact Strength (ASTM D256 2023)
The impact strength of the composite was measured using an Izod impact tester, which assesses the material’s toughness and its ability to absorb energy during a sudden impact (Karuppiah et al. 2020; Koffi et al. 2021). Notched specimens (which create a stress concentration point) were subjected to a pendulum strike, and the energy absorbed by the specimen during fracture was recorded. This test provides information on the composite’s resistance to sudden, high-energy impacts, making it relevant for applications where the material may experience shocks or impacts, such as in automotive or protective gear. Figure 5 shows the test specimens.
Fig. 5. Impact specimen
Interlaminar Shear Strength (ASTM D2344 2022)
To evaluate the bonding strength between fiber layers and the matrix, short-beam shear tests were performed. Composite samples were loaded in a three-point bend configuration with a shorter span-to-depth ratio than flexural tests. The goal was to induce shear failure between the layers. Interlaminar shear strength (ILSS) (ASTM D2344 2022) (Kotik and Ipina 2021) was calculated from the maximum load the composite could carry before delamination or shear failure occurred. This test is essential for evaluating the quality of the interface between the fibers and the matrix, which plays a vital role in the overall durability and performance of the composite when subjected to shear forces.
Hardness (ASTM D2240 2021)
The surface hardness of the composites was assessed using a Shore D durometer accordance to ASTM D2240 (2021), a tool designed to measure the resistance of the composite surface to indentation. Higher hardness values signify increased resistance to surface wear and indentation. This characteristic is particularly crucial for applications where the material is subjected to abrasive conditions or requires enhanced surface durability (Arockiasamy 2022).
Water Absorption Test (ASTM D570 2022)
The water absorption (ASTM D570 2022) (Hassan et al. 2019) behavior of the hybrid composites was tested to assess their moisture resistance, an important factor for materials exposed to humid or wet environments (Barjasteh and Nutt 2012; Maslinda et al. 2017). The composite samples were first dried and weighed before being completely immersed in distilled water at room temperature. At 48-h intervals, the samples were removed from the water, wiped dry, and reweighed. The percentage of water absorption was then calculated based on the increase in weight of the samples. This test provides insights into the hydrophilic nature of the fibers and fillers used in the composite, and how they might affect the mechanical performance when exposed to moisture. The goal is to ensure that the composites maintain acceptable moisture resistance, minimizing the risk of degradation over time. These mechanical and environmental tests provide a comprehensive understanding of the hybrid composite’s structural and functional performance, ensuring suitability for a range of applications (Nurazzi et al. 2021; Sumesh et al. 2021).
Scanning Electron Microscopy (SEM)
To investigate the microstructural characteristics of the hybrid composites, SEM was performed using a Zeiss EVO 18 scanning electron microscope (Alaneme and Sanusi 2015; Sathish et al. 2021). SEM analysis elucidates the fiber-matrix interface, distribution of fibers and fillers, and identifying potential defects such as voids, fiber pull-out, and matrix cracking, which influence the overall mechanical properties.
RESULTS AND DISCUSSION
Tensile Test
The tensile strength results of the hybrid composites, which include varying amounts of palmyra palm leaflet (PPL) fibers, coconut sheath leaf (CSL) fibers, and a fixed amount of tamarind shell powder (TSP) filler, revealed important insights into the relationship between fiber content, filler inclusion, and mechanical performance. Initially, as the content of PPL fibers increased from 5% to 20%, the tensile strength of the composites showed a steady improvement. This trend can be attributed to the strengthening effect of PPL fibers, which are known for their high tensile strength (Reddy et al. 2014). These fibers act as load-bearing components within the matrix, providing resistance to tensile forces and improving the composite’s ability to withstand stress without failure. The gradual increase in strength reflects the contribution of PPL fibers to the overall structural integrity, enhancing the composite’s performance under load. The composite with 20% PPL and 10% CSL (20PPL10CSL) exhibited the highest tensile strength at 42.2 MPa, indicating an optimal balance between PPL and CSL. The PPL fibers provided flexibility and strength, while the CSL fibers contributed rigidity and moisture resistance. Together, these fibers work synergistically, enhancing the composite’s mechanical properties. This balanced fiber ratio ensures effective load distribution and minimizes the chances of fiber misalignment, which could lead to weak spots in the composite. Moreover, the presence of TSP further enhanced tensile strength by improving fiber-matrix adhesion, filling voids, and reducing the occurrence of defects that could act as stress concentrators. The filler likely also contributed to better dispersion of the fibers within the matrix, preventing clumping or uneven distribution that could weaken the material (An et al. 2024; Sonar et al. 2024). However, when the PPL content increased beyond 20% (as seen in the 25PPL5CSL composite), the tensile strength slightly decreased to 40.5 MPa. This suggests that there is an optimal amount of PPL fibers that maximizes the composite’s tensile strength, and excess PPL content may lead to reduced performance. Excessive fiber content can lead to overcrowding, poor wetting of the fibers by the matrix, and the formation of voids or air pockets that compromise the structural integrity of the composite. Additionally, with more fibers packed into the matrix, the alignment and dispersion of the fibers may become less uniform, leading to local areas of weakness where cracks could propagate more easily (Mohammed et al. 2023). Figure 6 shows the tensile characteristics. The composite without TSP filler exhibited the lowest tensile strength at 32.2 MPa, highlighting the reinforcing role of TSP. The presence of TSP significantly contributed to enhancing the mechanical properties by improving the interaction between fibers and the matrix. TSP, being a bio-filler, fills voids, enhances fiber-matrix bonding, and helps reduce defects in the composite, resulting in a more durable material. Overall, these results underscore the importance of optimizing both fiber content and filler addition to achieve the best mechanical performance. The synergistic effect of the PPL, CSL, and TSP combination not only improved tensile strength but also provided an environmentally friendly alternative to traditional synthetic composites. The careful balancing of these components is crucial for creating a composite that is both strong and durable, suitable for structural applications, and sustainable due to the use of natural fibers and bio-fillers.
Fig. 6. Tensile Characteristics
Flexural Strength
The flexural strength data highlights the effectiveness of hybrid composites reinforced with palmyra palm leaflet (PPL) fibers, coconut sheath leaf (CSL) fibers, and tamarind shell powder (TSP) as a filler. As the PPL fiber content increased from 5% to 20%, a significant improvement in flexural strength was observed, with the 20PPL10CSL composite showing the highest value of 94.4 MPa. The improvement can be attributed to the high cellulose content of PPL fibers, which enhances the material’s load-bearing capacity. The cellulose helps transfer stress more efficiently between the matrix and fibers, strengthening the composite. This interaction increases the composite’s ability to resist bending deformation, resulting in superior flexural strength. The uniform distribution of the fibers within the matrix also helps optimize stress transmission, reducing the likelihood of failure or crack propagation under bending stress. However, when the PPL content was increased to 25%, the flexural strength slightly decreased to 89.6 MPa, suggesting that an optimal balance exists for fiber content. This reduction may be due to fiber agglomeration, which can interfere with matrix bonding. Agglomerated fibers reduce the effectiveness of fiber-matrix adhesion, which weakens the stress transfer and creates potential weak spots in the composite. These weak spots can lead to failure under flexural loads. The result underscores the importance of carefully optimizing fiber content to avoid negative effects on the composite’s mechanical properties, as excessive fiber concentration can disrupt the uniformity of the material (Blokhin et al. 2020). The addition of TSP as a filler significantly enhanced the flexural strength, as seen by the much lower strength (52.3 MPa) of the composite without filler. TSP helps improve the fiber-matrix bonding by filling voids in the matrix and contributing additional reinforcement. This leads to better distribution of stress under flexural loads and reduces the risk of matrix cracking. Furthermore, the combination of PPL and CSL fibers offers a balanced approach, where PPL provides tensile strength and CSL adds rigidity and moisture resistance. The synergy between these fibers ensures that the composite exhibits both strength and flexibility, making the 20PPL10CSL composite ideal for structural applications. By optimizing fiber ratios and incorporating bio-fillers such as TSP, these hybrid composites present a sustainable and high-performance alternative to synthetic materials, offering enhanced mechanical properties with environmental benefits (Kasinathan and Rajamani 2022; Guo et al. 2021). Figure 7 shows the flexural characteristics.
Fig. 7. Flexural characteristics
Interlaminar Shear Strength (ILSS)
The chart displays the interlaminar shear strength of hybrid composites with varying amounts of PPL and CSL fibers, along with a fixed 10% TSP filler. ILSS is crucial for assessing a composite’s resistance to shear forces between its layers. As the PPL content increased from 5% to 20%, the ILSS improved, peaking at 7.52 MPa for the 20PPL10CSL composite. This enhancement can be attributed to the superior stiffness and load-bearing capacity of PPL fibers, which provide better fiber-matrix adhesion, allowing for effective stress distribution and resistance to shear forces (Aisyah et al. 2021). However, the ILSS decreased slightly to 6.34 MPa for the 25PPL5CSL composite. This reduction may result from fiber overcrowding, which can impair the resin’s ability to wet the fibers, leading to weaker bonding and reduced shear resistance (Clifton et al. 2020). The composite without TSP filler showed the lowest ILSS at 4.62 MPa, highlighting the reinforcing effect of TSP. As a micro-filler, TSP enhances the matrix’s bonding capability and increases stiffness, improving interlaminar shear strength (Gao et al. 2022). Overall, the findings indicate that optimizing the balance of PPL and CSL fibers with TSP filler is essential for maximizing the interlaminar shear properties of hybrid composites. Figure 8 shows the interlaminar shear strength characteristics.
Fig. 8. Interlaminar shear strength characteristics
Impact Strength
Figure 9 presents the impact strength of hybrid composites made with varying amounts of PPL and CSL fibers, along with a fixed 10% TSP filler. Impact strength measures a material’s ability to withstand sudden forces without fracturing. As PPL content increased from 5% to 20%, impact strength improved, peaking at 5.98 J for the 20PPL10CSL composite. This increase is likely due to the toughness and flexibility of PPL fibers, which enhance energy absorption during impacts. However, the impact strength decreased slightly for the 25PPL5CSL composite (5.56 J), suggesting that exceeding a certain fiber content may not further enhance toughness, possibly due to fiber overcrowding or reduced bonding (Osterberg et al. 2023; Qureshi et al. 2024). The composite without TSP filler showed the lowest impact strength at 4.03 J, highlighting TSP’s role in improving energy absorption and fiber-matrix interaction. Overall, optimizing the ratios of PPL and CSL fibers, along with TSP filler, is essential for maximizing the impact strength of hybrid composites and enhancing their performance against sudden loads. Figure 9 shows the impact characteristics.
Fig. 9. Impact characteristics
Shore D Hardness
The graph presents the Shore D hardness values of hybrid composites made with varying amounts of PPL and CSL fibers, along with a fixed 10% TSP filler. Shore D hardness is a measure of a material’s resistance to indentation, indicating its rigidity and durability (Pintaude 2023). As the PPL content increased from 5% to 20%, the Shore D hardness improved, reaching a maximum of 84.1 SD for the 20PPL10CSL composite. This increase can be attributed to the high cellulose content and structural integrity of PPL fibers, which reinforce the epoxy matrix, enhancing its overall rigidity and resistance to deformation (Nurazzi et al. 2021). However, the hardness value slightly decreased for the 25PPL5CSL composite (82.8 SD). This reduction may indicate that excessive PPL content can lead to fiber agglomeration or uneven dispersion within the matrix, potentially creating weak spots that lower hardness. The composite without TSP filler showed the lowest Shore D hardness at 60.6 SD, emphasizing the significant role of TSP in enhancing the mechanical properties of the composite (Neitzel et al. 2011). TSP acts as a reinforcing agent, improving the bonding between the fibers and the matrix, and contributing to a more uniform force distribution throughout the composite. The findings suggest that optimizing the balance of PPL and CSL fibers with TSP filler is essential for maximizing Shore D hardness in hybrid composites (Nurazzi et al. 2021). The combination of natural fibers and fillers enhances the material’s resistance to indentation and wear, making these composites suitable for applications requiring superior mechanical performance. Figure 10 shows the hardness results.
Fig. 10. Hardness characteristics
Water Absorption
Water absorption is an important indicator of a material’s resistance to moisture, which can affect its mechanical properties and durability. The water absorption increased with higher concentrations of PPL and CSL fibers. The composite with 10% PPL and 20% CSL exhibited the highest water absorption percentage at 51.4%, whereas the composite with 20% PPL and 10% CSL showed a lower absorption of 42.6%. This rise in water absorption can be attributed to the porous structure of the natural fibers, which allows for greater moisture absorption, thus increasing the overall weight of the composite. Generally, a higher fiber content results in more voids and gaps within the matrix, facilitating water penetration. In contrast, the composite without any filler had a water absorption of 42.2%, which is relatively low compared to the composites containing tamarind shell powder. The presence of TSP likely enhances the interfacial bonding between the fibers and the epoxy matrix, thereby reducing the amount of water that can permeate the composite structure (Chen et al. 2021; Zaghloul et al. 2023).
Overall, these findings indicate that while natural fibers contribute to improved mechanical properties, they also lead to increased water absorption. Careful optimization of fiber and filler content is essential to achieve a balance between mechanical performance and moisture resistance in hybrid composites (Nurazzi et al. 2021). Figure 11 shows the water absorption test result.
Fig. 11. Moisture characteristics
Scanning Electron Microscopy
The SEM micrographs presented in Figures 12A, 12B, and 12C depict the fractured surfaces of PPL and CSL fiber-reinforced epoxy composites with varying weight percentages of TSP filler. In Figures. 12A, a significant number of fibers are detached from the matrix in various directions, indicating poor bonding between the fibers and the epoxy. This weak adhesive interaction points to insufficient fiber-matrix interface engagement, which may compromise the overall integrity of the composite (Shakil et al. 2020). Such inadequate bonding could result from insufficient surface treatment of the fibers or improper mixing techniques, leading to a lack of interfacial adhesion essential for effective load transfer during mechanical stress (Teklal et al. 2018; Marques et al. 2020). Conversely, Figures 12B illustrates strong adhesion between the fibers and the matrix, marked by reduced fiber pullout and enhanced resistance to crack propagation. This improved adhesion is likely attributed to the presence of TSP, which serves as a reinforcing agent, enhancing interfacial bonding between the fibers and the epoxy matrix (Kasinathan and Rajamani 2022). The addition of TSP may increase the resin’s viscosity and fill any voids, resulting in a denser and more uniform matrix that effectively adheres to the fibers, thereby improving the overall strength properties of the composites (Shahari et al. 2021). Figure 12 C presents a mixed scenario, revealing some fibers pulling out along with visible cracks on the fractured surface, indicating an intermediate level of bonding effectiveness (El-Abbasy 2023).
The presence of agglomerations indicates potential issues with matrix curing, which could negatively affect the mechanical performance. Inadequate curing can lead to incomplete polymerization, resulting in a weaker matrix that is less effective in supporting the fibers (Mostafa et al. 2017; El-Abbasy 2023). Compared to unfilled composites, Fig. 12 C reflects a noticeable improvement in fiber-matrix adhesion, suggesting that while challenges remain, the addition of TSP has positively influenced the overall composite integrity by promoting better fiber distribution and interaction within the matrix (Binoj et al. 2016). These observations highlight the critical role of filler content in optimizing fiber-matrix adhesion and mechanical performance in hybrid composites.
Fig. 12. (A) Fiber pullout; (B) good bonding; (C) crack and agglomerations
Overall, the study emphasizes the significant potential of PPL and CSL fibers, combined with TSP filler, for developing high-performance hybrid composites. The results indicate that careful optimization of fiber and filler content is essential for maximizing the mechanical properties while minimizing water absorption, making these composites suitable for various applications in the fields of construction, automotive, and consumer products. Future work should focus on further refining the processing techniques to enhance fiber distribution and adhesion, ultimately improving the overall performance of these composite materials.
CONCLUSIONS
- The results demonstrated that increasing palmyra palm leaf (PPL) content enhanced tensile strength, reaching a peak at 42.2 MPa for the 20PPL10CSL composite. This suggests that an optimal balance of fiber content is crucial for achieving maximum strength, as excessive PPL may lead to decreased performance.
- The flexural tests indicated a similar trend, with a maximum value of 94.4 MPa observed for the 20PPL10CSL composite. The improvements in flexural strength can be attributed to the effective reinforcement provided by PPL fibers and the adhesive properties of tamarind shell powder (TSP), contributing to better load distribution.
- The impact strength improved with increased fiber content, peaking at 5.98 J for the 20PPL10CSL composite. The ability of the fibers to dissipate energy during impact is vital for applications requiring toughness and durability.
- ILSS values showed a positive correlation with PPL content, with the highest strength of 7.52 MPa observed in the 20PPL10CSL composite. This highlights the importance of fiber-matrix adhesion in resisting shear forces, essential for the structural integrity of layered composites.
- Hardness values improved significantly with increased PPL content, peaking at 84.1 Shore D hardness (SD). This enhancement reflects the contribution of PPL fibers to the composite’s rigidity and wear resistance.
- The water absorption tests revealed a tendency for increased moisture uptake with higher fiber content, particularly for the 10PPL20CSL composite, which had the highest absorption at 51.4%. This underscores the porous nature of natural fibers and the need for careful optimization to balance mechanical properties with moisture resistance.
- The scanning electron microscope (SEM) images provided insights into the fiber-matrix interactions, revealing both the benefits and challenges of using natural fibers. Improved adhesion in composites with TSP filler highlighted the role of fillers in enhancing mechanical performance, although some challenges, such as fiber agglomeration and incomplete curing, were noted.
ACKNOWLEDGMENTS
This project was supported by Researchers Supporting Project Number (RSP-2025R7) at King Saud University, Riyadh, Saudi Arabia.
Data Availability Statement
Data are available on request from the authors.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
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Article submitted: October 11, 2024; Peer review completed: November 2, 2024; Revised version received and accepted: November 10, 2024; Published: November 21, 2024.
DOI: 10.15376/biores.20.1.698-724