Mechanical characterization of epoxy composites reinforced with a blend of Hibiscus rosa and snake grass fibers enhanced with neem gum powder

Palaniappan, M., Palanisamy, S., Murugesan, T., and Ayrilmis, N. (2025). "Mechanical characterization of epoxy composites reinforced with a blend of Hibiscus rosa and snake grass fibers enhanced with neem gum powder," BioResources 20(4), 10106–10129.

Abstract

Mechanical and physical characteristics were studied of epoxy composites reinforced with different blends of the Hibiscus (H) rosa plant fiber and snake (S) grass fiber, with and without the addition of neem gum powder. The incorporation of the snake grass fiber significantly enhanced the mechanical properties, with the biocomposite 20S10H exhibiting the highest tensile strength (56 MPa), flexural strength (87 MPa), hardness (86 SD), and impact strength (6.98 J), due to the synergistic effect of snake grass fiber and neem gum as a binder. The interlaminar shear strength also showed an improvement, reaching a maximum of 6.52 MPa for the biocomposite 20S10H, reflecting enhanced interfacial bonding and reduced void content. Water absorption (40%) decreased with the increased proportion of snake grass fiber and the inclusion of neem gum, with the lowest absorption recorded for the biocomposite 30S30H, indicating reduced moisture uptake. In contrast, biocomposites with a higher proportion of Hibiscus rosa fiber exhibited higher water absorption. The scanning electron microscopy (SEM) study of the fracture surfaces demonstrated enhanced fiber-matrix adhesion and decreased porosity in biocomposites with neem gum, validating the neem gum’s contribution to better interfacial bonding and overall biocomposite efficacy.

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**Mechanical Characterization of Epoxy Composites Reinforced with a Blend of Hibiscus rosa and Snake Grass Fibers Enhanced with Neem Gum Powder**

Murugesan Palaniappan,^a,*Sivasubramanian Palanisamy ,^b,* Thulasimani Murugesan,^c and Nadir Ayrilmis ^d

DOI: 10.15376/biores.20.4.10106-10129

Keywords: Hibiscus rosa plant; Snake grass fiber; Neem gum; Mechanical properties; Water absorption

Contact information: a: Department of Mechanical Engineering, College of Engineering, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, 11432, Kingdom of Saudi Arabia; b: Department of Mechanical Engineering, PTR College of Engineering and Technology, 625008, Tamilnadu, India; c: Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, ON, K1S 5B6, Canada; d: Department of Wood Mechanics and Technology, Faculty of Forestry, Istanbul University-Cerrahpasa, Bahcekoy, Sariyer, 34473, Istanbul, Turkey;

* Corresponding Authors: sivaresearch948@gmail.com; mpapathi@imamu.edu.sa

INTRODUCTION

The global transition toward sustainable development has catalyzed an urgent demand for environmentally friendly materials across various sectors, particularly in polymer-based composite manufacturing. Among the alternatives to conventional synthetic composites, natural fiber-reinforced polymer (NFRP) biocomposites have garnered increasing interest due to their environmental compatibility, abundance, and cost-efficiency (Sathishkumar et al. 2022; Tengsuthiwat et al. 2024a). These biocomposites are not only lightweight and biodegradable, but they also offer a favorable balance of strength, stiffness, and toughness. Their applications span across automotive, construction, packaging, and consumer goods, where there is a growing need for eco-sustainable yet high-performing alternatives.

Among the myriad of natural fibers investigated, snake grass fiber, obtained from Sansevieria ehrenbergii, has emerged as a viable reinforcement material. It is characterized by a high cellulose content, low density, and remarkable tensile strength, which collectively contribute to its superior load-bearing and thermal resistance properties (Supriya et al. 2024). Additionally, the inherent microstructure and aspect ratio of the fiber facilitate effective stress transfer within the composite. When subjected to surface treatments such as alkali (NaOH) treatment, snake grass fibers exhibit improved interfacial adhesion with polymer matrices, enhancing mechanical integrity and minimizing fiber pull-out during failure (Sumesh et al. 2024; Thirupathi et al. 2024; Raghunathan et al. 2024a). Previous studies have validated its potential in improving the toughness, stiffness, and dimensional stability of biocomposites, making it suitable for use in structural and semi-structural applications.

Similarly, Hibiscus rosa-sinensis, an underutilized tropical plant, provides stem fibers with highly desirable mechanical and physical attributes. These fibers are composed predominantly of lignocellulosic materials such as cellulose, hemicellulose, and lignin, which endow them with notable tensile strength, flexibility, and biodegradability (Gokul et al. 2024; Raghunathan et al. 2024b). The fibrous elements derived from Hibiscus rosa possess a robust natural morphology that resists environmental degradation while promoting interfacial compatibility with polymer matrices, especially when chemically treated. Although traditionally valued for its medicinal and ornamental properties, the plant’s structural fibers remain largely untapped in advanced composite applications. Their inclusion in NFRP systems, especially in hybridized forms, offers a pathway to improve composite toughness and resilience while maintaining eco-friendly credentials (Sundarrajan et al. 2024; Raghunathan et al. 2024c).

In addition to fibrous reinforcements, the strategic use of natural gum is another approach to enhance the structural, morphological, and interfacial characteristics of biocomposites. Neem gum, a natural polysaccharide biopolymer extracted from the Azadirachta indica tree, can have multifunctional roles. Unlike fibrous reinforcements that primarily contribute to load-bearing capacity, neem gum powder functions at the microstructural level to improve dispersion uniformity and reduce matrix voids. Its adhesive and emulsifying properties significantly aid in improving the fiber–matrix interaction, thus reducing porosity and enhancing mechanical strength (Karuppiah et al. 2022; Karuppusamy et al. 2023; Shetty et al. 2024). Moreover, neem gum’s inherent antimicrobial activity and biodegradability expand its utility in developing composites suitable for health-sensitive and green applications. The use of such bio-based components also contributes to the overall sustainability and circularity of the composite system.

Despite the advantages of natural fiber biocomposites, several limitations remain that hinder their widespread adoption. These include poor interfacial bonding between hydrophilic fibers and hydrophobic matrices, high moisture sensitivity, variable mechanical properties due to natural heterogeneity, and insufficient toughness compared to synthetic alternatives (Chauhan et al. 2022; Karthik et al. 2024; Manickaraj et al. 2024c). The combination of different natural fibers within a single matrix, which has been called hybridization, has been recognized as an effective strategy to mitigate these drawbacks. By exploiting the synergistic effects of different fibers—such as combining high stiffness with good impact resistance—hybrid composites often outperform single-fiber systems in terms of mechanical performance and dimensional stability (Palanisamy et al. 2024; Shibly et al. 2024; Karuppusamy et al. 2025). The inclusion of bio-based binder materials such as neem gum can further reinforce the interface, resulting in improved durability and long-term performance under service conditions.

However, a significant gap in current research is the limited exploration of hybrid epoxy biocomposites reinforced with Hibiscus rosa and snake grass fibers, particularly in conjunction with neem gum usage. Existing studies often focus on individual fibers or synthetic binders, neglecting the unique combinations and potential synergy among lesser-known natural reinforcements. Addressing this gap is essential for expanding the material options available for high-performance and sustainable engineering applications.

The novelty of the present study lies in its development and mechanical characterization of hybrid epoxy biocomposites incorporating Hibiscus rosa and snake grass fibers, along with neem gum powder. The objective was to investigate the individual and combined effects of these reinforcements on the tensile, flexural, and impact properties of the composite. The study specifically aimed to evaluate: (i) the mechanical contributions of each natural fiber; (ii) the interfacial improvements offered by neem gum; and (iii) the potential of these materials to provide a cost-effective, biodegradable alternative to synthetic composite systems (Murugesan et al. 2022; Maguteeswaran et al. 2024; Manickaraj et al. 2024d; Ramakrishnan et al. 2024). Furthermore, the research highlights the relevance of these novel biocomposites in high-demand sectors such as automotive, construction, packaging, and consumer goods, where both performance and sustainability are equally prioritized (Goutham et al. 2023; Jawaid et al. 2022).

EXPERIMENTAL

In this study, natural fibers, polymer matrix, and natural gum powder were used as components. The details of their sourcing, preparation and properties are as follows.

Hibiscus rosa plant fiber (H)

The (H) was collected from locally available Hibiscus rosa plants in the Pollachi area, Coimbatore, Tamil Nadu, India. The fibers were carefully extracted, cleaned to remove impurities, and dried under sunlight for 48 h to eliminate moisture (Saba et al. 2015; Birniwa et al. 2021). The dried fibers were then cut into uniform lengths of 30 mm to ensure consistency in biocomposite fabrication. The Hibiscus plant and its fibers are presented in Fig. 1.

Fig. 1. (A) Hibiscus plant; B) Hibiscus fibers

Snake grass fiber (S)

Similar to (H), the (S) was also collected from the Pollachi area in Coimbatore. The fibers were extracted from the snake grass plant through manual processing. After extraction, the (S) was cleaned, sun-dried for 48 h, and cut to lengths of 30 mm. These fibers are known for their lightweight nature and environmental resistance, making them suitable for biocomposite reinforcement (Balaji et al. 2021). The snake grass plants and its fibers are presented in Fig. 2.

Fig. 2. (A) Snake grass plant; b) Snake grass fibers

Neem gum powder (NGP)

Neem gum powder was obtained from the seeds of the neem tree. The seeds were collected, dried, and mechanically ground to produce a fine powder. The powder was then sieved to achieve a uniform particle size, which is essential for its effective dispersion in the epoxy matrix.

Neem gum powder is known for its adhesive and reinforcing properties, contributing to enhanced mechanical strength and interfacial bonding in the composites (Dev et al. 2024). The Neem gum and its powder are presented in Fig. 3.

Fig. 3. (A) Neem gum; (B) powder form Neem gum

Epoxy resin (matrix)

The epoxy resin used in this study was procured from Seenu and Seenu Company, Coimbatore. This resin is of standard commercial grade and is widely recognized for its superior mechanical properties, chemical resistance, and excellent adhesion to natural fibers (Palanisamy et al. 2023; Sathesh Babu et al. 2024). The resin forms the primary matrix material for the composite.

Chemical Treatment of Fibers

The fibers from Hibiscus rosa (H) and snake grass (S) were subjected to an alkali treatment to improve their interfacial bonding with the epoxy matrix. A 5% NaOH solution was prepared by dissolving 5 g of NaOH in 100 mL of distilled water (Aruchamy et al. 2025). The fibers were immersed for 4 h, then thoroughly washed, sun dried for 48 h, and finally oven dried at 60 °C for 6 h. The process resulted in an increase in surface roughness, removal of non-cellulosic components, an increase in crystallinity and an improvement in wettability, which collectively contributed to superior mechanical properties in the composites (Kumar et al. 2022).

Compression Molding Technique

Composite plates were fabricated using compression molding. The Hibiscus rosa (H) and snake grass (S) fibers were cleaned, cut into 10 mm lengths, and oven-dried. Epoxy resin and hardener were mixed in a 2:1 ratio, along with neem gum powder, to form the matrix.

The hybrid fibers were thoroughly blended with the epoxy mixture and placed into a preheated mold. Prior to molding, a thin layer of release agent was applied to the mold surfaces to prevent the composite plates from sticking during curing. The mold was then compressed at 10 MPa and 120 °C for 30 min and allowed to cool to room temperature (Manickaraj et al. 2024b). The cured plates were removed, trimmed, and cut to ASTM D4703-16 (2016) dimensions for mechanical testing.

Composite Designation

The composite designations are based on varying proportions of (S), (H), neem gum powder (NGP), and epoxy resin. Six composite formulations were developed with a fixed epoxy resin content of 60%.

In the first composite (5S25H), the fiber content comprised 5% snake grass fiber and 25% Hibiscus rosa fiber, with 10% neem gum powder. The second composite (10S20H) contained 10% snake grass fiber, 20% Hibiscus rosa fiber, and 10% neem gum powder. Similarly, the third composite (15S15H) had equal proportions of snake grass and Hibiscus rosa fibers (15% each) and 10% neem gum powder. The fourth composite (20S10H) included 20% snake grass fiber, 10% Hibiscus rosa fiber, and 10% neem gum powder, while the fifth composite (25S5H) consisted of 25% snake grass fiber, 5% Hibiscus rosa fiber, and 10% neem gum powder. In addition, a sixth formulation (30S30H) was prepared with 30% each of snake grass and Hibiscus rosa fibers but without any neem gum powder; this served as a control sample to evaluate the effect of the neem gum powder on the composite properties (Sumesh et al. 2023; Manickaraj et al. 2024a). The above composite designations are shown in Table 1.

Table 1. Composite Designations

Mechanical Testing

The fabricated hybrid epoxy composites were subjected to various mechanical tests and microstructural analysis to evaluate their mechanical performance and fiber-matrix interaction. All tests were conducted following standard ASTM procedures to ensure consistency and accuracy.

Tensile Strength

Tensile properties, such as tensile strength, tensile modulus, and elongation at break, were evaluated using a universal testing machine (UTM) according to ASTM D638-14 (2022). The specimens were prepared as per standard dimensions (Singh et al. 2014; Laureto and Pearce 2018; SD 2021; Ramasubbu et al. 2024). The samples were loaded at a constant crosshead speed until failure, providing insights into the composites’ load-carrying capacity and stiffness.

Flexural Strength

The determination of flexural strength and flexural modulus was conducted using a three-point bending test setup in accordance with ASTM D790-17 (Anggraini et al. 2017). The rectangular samples were placed on two supports, and a load was applied at the midpoint until fracture or significant deformation (SD 2021). This test assessed the composites’ bending strength and stiffness.

Impact Strength

The evaluation of impact strength was conducted utilizing the Charpy impact test in accordance with ASTM D256-23 (Koffi et al. 2021). The specimens with notches were meticulously prepared and impacted using a high-energy pendulum. This assessment evaluated the material’s resilience and its capacity to withstand abrupt energy impacts (Sahoo et al. 2022).

Hardness

The Shore D hardness of the composites was assessed using a standard durometer in accordance with ASTM D2240-21. The indenter was applied to the composite surface, and hardness measurements were documented (Natarajan et al. 2023). This evaluation measured the surface hardness and the ability to withstand localized deformation.

Compression Test

Compressive strength and modulus were determined using ASTM D695-15 (2015). Cylindrical specimens were loaded axially in a UTM at a constant rate until failure (Sudhir et al. 2014; Morăraș et al.2024). This test provided information on the composites’ behavior under compressive forces, reflecting their load-bearing capabilities in confined conditions.

Interlaminar Shear Strength

The measurement of ILSS was conducted utilizing the short beam shear test in accordance with ASTM D2344/D2344M-22. Rectangular samples underwent a three-point loading configuration with a brief span length to generate shear stress across the layers (Kotik and Ipina 2021; Rajamanickam et al. 2023). This assessment measured the adhesion strength between the fiber and matrix, as well as the resistance to delamination when subjected to load.

Water Absorption Test

The water absorption (ASTM D570 2022) (Hassan et al. 2019) test measures the moisture uptake of composite materials, providing insights into their durability and suitability for applications in humid environments. To perform the test, a composite specimen was first dried to eliminate initial moisture and weighed (W1). It was then immersed in water for a specified duration (Barjasteh and Nutt 2012; Maslinda et al. 2017). After immersion, the specimen was removed, surface-dried, and weighed again (W2). The percentage of water absorbed was calculated using the formula:

Higher water absorption indicates poor resistance to moisture, often due to weak fiber-matrix bonding or untreated fibers.

Scanning Electron Microscopy (SEM) Analysis

The fractured surfaces of the tensile specimens were examined by SEM (Carl Zeiss model EVO MA 15, Carl Zeiss GmBH, Jena, Germany) to investigate the fiber-matrix interaction, fracture morphology and distribution of neem gum powder within the matrix (Alaneme and Sanusi 2015; Sathish et al. 2021; Manickaraj et al. 2023). The analysis provided insight into the failure mechanisms, including fiber pull-out, matrix cracking and void formation, which helped to correlate mechanical properties with microstructural features.

RESULTS AND DISCUSSION

Tensile Test

The tensile strength of the composites varied significantly depending on the proportions of snake grass fiber and Hibiscus rosa plant fiber, as well as the presence of neem gum, demonstrating their combined influence on mechanical performance. A tensile strength of 38 MPa was recorded for the composite containing 5% snake grass fiber and 25% Hibiscus rosa fiber, which was attributed to the higher content of Hibiscus rosa fiber providing moderate reinforcement; however, the lower snake grass fiber content limited further strength enhancement (Fig. 4). In the composite with 10% snake grass fiber and 20% Hibiscus rosa fiber, the tensile strength increased slightly to 40 MPa due to the increased proportion of snake grass fiber, which facilitated improved load transfer owing to its superior tensile characteristics. The composite with equal proportions (15% each) of snake grass and Hibiscus rosa fibers exhibited a further increase in tensile strength to 49 MPa, likely resulting from the balanced fiber ratio that created a synergistic reinforcement effect, improving stress distribution and load-bearing capacity (Ramakrishnan et al. 2024; Gurusamy et al. 2025). The highest tensile strength of 56 MPa was observed in the composite containing 20% snake grass fiber and 10% Hibiscus rosa fiber, where the dominant snake grass fiber content contributed to enhanced stiffness and mechanical integrity. Additionally, the inclusion of neem gum was considered to have improved fiber–matrix adhesion and reduced void formation, thereby enhancing overall strength (Zaman and Khan 2022; Raghunathan et al. 2022a; Tengsuthiwat et al. 2024b). Interestingly, the composite with 25% snake grass fiber and 5% Hibiscus rosa fiber showed a slight decrease in tensile strength to 50 MPa despite the higher snake grass fiber content, which may have been caused by insufficient Hibiscus rosa fiber and potential fiber agglomeration at elevated snake grass fiber levels, leading to stress concentrations. The lowest tensile strength of 35 MPa was recorded in the composite containing 30% each of snake grass and Hibiscus rosa fibers but without neem gum. The absence of the neem gum appeared to reduce interfacial bonding, thus impairing load transfer and diminishing tensile strength. These findings underscore the importance of optimizing fiber ratios and incorporating neem gum to enhance the tensile behavior of hybrid epoxy composites (Karthikeyan et al. 2022; Khan et al. 2020; Manickaraj et al. 2025).

Fig. 4. Tensile strength of the hybrid epoxy composites with varying fiber compositions: S = snake grass fiber (%), H = Hibiscus rosa fiber (%), and neem gum constant. Error bars represent the standard deviation from three replicate tests, indicating the variability in tensile strength measurements.

Flexural Strength

The flexural strength of the composites was strongly affected by the proportions of snake grass fiber and Hibiscus rosa fiber, along with the presence of neem gum. The composite with 5% snake grass and 25% Hibiscus rosa fiber showed a flexural strength of 78 MPa, which was attributed mainly to the higher Hibiscus rosa content providing moderate rigidity (Singh et al. 2014; Chahar et al. 2024). However, the low snake grass content limited bending resistance. Increasing snake grass fiber to 10% and reducing Hibiscus rosa to 20% improved flexural strength slightly to 79 MPa, which was attributed to the higher stiffness of snake grass fibers. A balanced mix of 15% snake grass and 15% Hibiscus rosa fibers further raised the strength to 83 MPa, demonstrating a synergistic reinforcement effect. The highest flexural strength of 87 MPa was recorded for the composite with 20% snake grass and 10% Hibiscus rosa fibers, where neem gum also enhanced fiber–matrix bonding, reduced microcracks, and increased stability. When Hibiscus rosa fiber decreased to 5% with 25% snake grass, strength dropped slightly to 84 MPa, possibly disrupting fiber synergy (Amir et al. 2017; Mirzamohammadi et al. 2022). The lowest strength, 74 MPa, was seen in the composite with 30% of both fibers but no neem gum, weakening fiber bonding and increasing delamination risks. These results highlight the importance of optimizing fiber ratios and using neem gum for better flexural performance in hybrid epoxy composites.

Fig. 5. Flexural strengths

Impact Strength

The impact strength of the composite materials, measured in joules (J), was evaluated to understand their energy absorption under sudden loading, emphasizing the effects of fiber and gum content (Muthalagu et al. 2021; Nayak et al. 2022). The composite containing 5% snake grass fiber and 25% Hibiscus rosa fiber exhibited an impact strength of 5.56 J, primarily due to the higher Hibiscus rosa content, which provided moderate reinforcement but lacked the superior energy dissipation capabilities of snake grass fiber. Increasing the snake grass fiber to 10% and reducing Hibiscus rosa fiber to 20% raised the impact strength to 5.98 J, reflecting enhanced energy absorption from the snake grass fibers’ superior impact properties. A balanced composite with 15% snake grass and 15% Hibiscus rosa fibers showed a further increase to 6.12 J, indicating a synergistic effect that improved toughness and impact resistance. The highest impact strength, 6.98 J, was recorded in the composite with 20% snake grass and 10% Hibiscus rosa fibers, where the inclusion of neem gum enhanced fiber–matrix adhesion and minimized voids, leading to improved crack resistance (Vivek and Kanthavel 2019; Kurien et al. 2023). A slight decrease to 6.56 J occurred in the composite with 25% snake grass and 5% Hibiscus rosa fibers, possibly due to disrupted fiber synergy. The lowest impact strength of 5.03 J was found in the composite lacking neem gum, which weakened bonding and increased porosity, reducing toughness. These results highlight the importance of optimizing fiber ratios and using neem gum to enhance impact resistance in hybrid epoxy composites. The measured impact strengths are presented in Fig. 6.

Fig. 6. The impact strength of the composites

Hardness

The hardness of the hybrid epoxy composites, measured using the Shore D scale, was significantly influenced by the fiber composition and the presence of neem gum, which impacted the material’s surface resistance. The composite with 5% snake grass fiber and 25% Hibiscus rosa fiber exhibited a hardness value of 76 Shore D, where the higher Hibiscus rosa fiber content provided moderate surface reinforcement. However, the relatively low amount of snake grass fiber limited the overall stiffness and surface compactness. Increasing the snake grass fiber content to 10% and reducing Hibiscus rosa fiber to 20% raised the hardness to 79 Shore D due to the inherently greater stiffness of snake grass fiber (Muthalagu et al. 2021; Zaman and Khan 2022). A balanced composite with equal proportions of 15% snake grass and 15% Hibiscus rosa fibers showed a hardness increase to 82 Shore D, which was attributed to synergistic fiber interaction that improved fiber packing and matrix bonding. The highest hardness value of 86 Shore D was recorded in the composite containing 20% snake grass fiber and 10% Hibiscus rosa fiber, along with neem gum, which enhanced interfacial bonding and minimized voids, thereby improving surface resistance to deformation. A slight reduction to 83 Shore D was noted in the composite with 25% snake grass and 5% Hibiscus rosa fibers, which was likely due to disruption of fiber synergy. The lowest hardness, 70 Shore D, was observed in the composite lacking neem gum, where weak fiber–matrix adhesion and increased porosity diminished surface resistance (Neitzel et al. 2011; Salama et al. 2022). These results highlight the crucial role of fiber ratio optimization and neem gum in enhancing surface hardness and overall mechanical performance of hybrid epoxy composites. The Shore D hardness values of all composite samples are presented in Fig. 7.

Fig. 7. Hardness of the composites

Compression Test

The compressive strength of the hybrid epoxy composites, measured in megapascals (MPa), showed significant variation depending on the proportions of snake grass fiber, Hibiscus rosa fiber, and the presence of neem gum, highlighting the critical role of material constituents in load-bearing capacity. The composite containing 5% snake grass fiber and 25% Hibiscus rosa fiber exhibited a compressive strength of 46.6 MPa. In this case, the higher content of Hibiscus rosa fiber provided moderate resistance to compressive forces, but the relatively low amount of snake grass fiber limited overall reinforcement effectiveness. When the fiber ratio shifted to 10% snake grass fiber and 20% Hibiscus rosa fiber, compressive strength increased slightly to 49 MPa due to the greater stiffness contributed by the snake grass fiber, enhancing resistance to deformation under compression. A balanced composite with equal parts of snake grass and Hibiscus rosa fibers at 15% each showed a more substantial improvement, achieving 59.8 MPa, which was attributed to a synergistic reinforcement effect between the two fibers (Eyer et al. 2016; Zhao et al. 2018). The highest compressive strength of 68.2 MPa was observed for the composite with 20% snake grass fiber and 10% Hibiscus rosa fiber, combined with neem gum. The neem gum played a key role in strengthening fiber–matrix bonding and reducing voids, resulting in enhanced load-bearing capacity. A slight decrease to 61 MPa was seen for the composite with 25% snake grass fiber and 5% Hibiscus rosa fiber, likely due to disrupted hybrid synergy from the reduced Hibiscus rosa content. The lowest compressive strength of 42 MPa was recorded in the composite containing 30% each of snake grass and Hibiscus rosa fibers but lacking neem gum, where weakened fiber-matrix adhesion and increased porosity compromised compressive performance (Kumar et al. 2019). These findings underscore the importance of optimizing fiber ratios and incorporating neem gum to maximize the compressive strength of hybrid epoxy composites.

Fig. 8. Compressive strength of the composites

Interlaminar Shear Strength (ILSS)

The interlaminar shear strength (ILSS) of the hybrid epoxy composites, measured in megapascals (MPa), played a vital role in assessing the ability of the composite layers to resist shear forces, which is directly related to the quality of the fiber–matrix interface. For the composite containing 5% snake grass fiber and 25% Hibiscus rosa fiber, the ILSS was recorded at 4.52 MPa, indicating moderate interfacial bonding (Kumar et al. 2023). This was mainly due to the higher proportion of Hibiscus rosa fiber, which, despite its reinforcing characteristics, produced relatively weaker adhesion with the epoxy matrix compared to snake grass fiber. When the snake grass fiber content was increased to 10%, with 20% Hibiscus rosa fiber, the ILSS improved to 4.96 MPa. This improvement was attributed to the stronger fiber–matrix interaction provided by the snake grass fiber, facilitating better load transfer and enhanced shear resistance. The composite with an equal fiber ratio of 15% snake grass and 15% Hibiscus rosa fibers showed a further increase in ILSS to 5.21 MPa, highlighting the synergistic effect of balanced fiber reinforcement for efficient stress distribution. The highest ILSS value of 6.52 MPa was achieved by the composite containing 20% snake grass fiber, 10% Hibiscus rosa fiber, and neem gum. The gum contributed significantly by reducing void content and strengthening fiber–matrix adhesion, thereby resulting in superior shear resistance. A slight decrease to 5.34 MPa was observed in the composite with 25% snake grass fiber and 5% Hibiscus rosa fiber, which was likely due to disrupted fiber synergy. The lowest ILSS of 3.43 MPa was recorded for the composite with 30% fibers each but lacking neem gum, where weak bonding and increased porosity compromised interlaminar shear strength (Ashok and Kani 2022). These results emphasize the importance of fiber composition and neem gum in enhancing the shear performance of hybrid epoxy composites.

Fig. 9. Interlaminar shear strength of the composites

Water Absorption

Testing the water absorption behavior of the hybrid epoxy composites is essential for assessing their suitability in moist or humid environments, as moisture uptake can adversely affect mechanical properties, dimensional stability, and durability. The composite containing 5% snake grass fiber and 25% Hibiscus rosa fiber showed the highest water absorption at 51.00%. This was mainly due to the higher content of Hibiscus rosa fiber, which is more hydrophilic, leading to greater moisture uptake and potential weakening of the fiber–matrix interface over time. Increasing the snake grass fiber content to 10%, with 20% Hibiscus rosa fiber, reduced water absorption to 48.0%, reflecting the lower hydrophilicity of snake grass fiber (Binoj et al. 2016; Sathiyamoorthy and Senthilkumar 2020). A further decrease to 45.0% was recorded for the composite with an equal fiber ratio of 15%, which was attributed to an optimized fiber-matrix interface that limited moisture penetration. The lowest water absorption, 42.0%, occurred in the composite with 20% snake grass fiber, 10% Hibiscus rosa fiber, and neem gum. The gum improved fiber bonding and reduced voids, enhancing resistance to water ingress. The composite with 25% snake grass fiber and 5% Hibiscus rosa fiber showed a slight increase to 44.0%, while the composite with 30% of each fiber but no gum exhibited 40.0% absorption, likely due to the fiber balance (Al-Hajaj et al. 2018). These findings demonstrate that optimizing fiber proportions and incorporating neem gum effectively improve moisture resistance and durability in hybrid epoxy composites (Maslinda et al. 2017).

Fig. 10. The water absorption behavior of the composites

Scanning Electron Microscopy

The SEM images of the composite containing 20% snake grass fiber and 10% Hibiscus rosa fiber offered valuable insights into the fiber-matrix interactions (Fig. 11). Figure 11A clearly shows the presence of numerous separated snake grass and Hibiscus rosa fibers pulled out from the epoxy matrix, indicating inadequate interfacial bonding. This poor adhesion resulted in compromised interfaces and reduced load transfer efficiency, which may negatively affect the tensile strength of the biocomposite (Palanisamy et al. 2022). In contrast, Fig. 11B demonstrates strong fiber-matrix adhesion, as evidenced by reduced fiber pullout and minimal void formation. Such robust bonding enhances resistance to crack propagation and facilitates efficient stress transfer, thereby contributing to the composite’s superior tensile strength (Islam et al. 2024; Mohan and Vijay 2021; Vijay and Singaravelu 2016). The inclusion of neem gum played a critical role in improving this bonding by minimizing voids and increasing compatibility between the fibers and epoxy matrix. The SEM analysis underscores the importance of fiber-matrix interfacial interactions in determining the mechanical performance of hybrid epoxy composites. The results indicate that optimizing fiber ratios in conjunction with neem gum incorporation significantly improved structural integrity by enhancing adhesion and reducing defects (De Cicco et al. 2017; Kar et al. 2023, 2024; Raghunathan et al. 2022b). These microstructural observations correlated well with the tensile testing data, advancing the understanding of fracture mechanisms in hybrid fiber-reinforced composites.

Fig. 11. (A) Fiber pullout; (B) Bonding and microcracks in the fractured tensile specimens

CONCLUSIONS

This study comprehensively evaluated the mechanical, physical, and durability properties of hybrid epoxy composites reinforced with Hibiscus rosa fiber (H) and snake grass fiber (S), along with neem gum powder. The findings gave evidence of the critical influence of fiber ratios and gum addition on composite performance, with significant implications for sustainable composite development.

The composite with higher snake grass fiber content (Composite 20% Snake Grass – 10% Hibiscus rosa) exhibited the highest tensile strength of 85 MPa, demonstrating improved stiffness and load-bearing capability, while composites with higher Hibiscus rosa fiber content showed comparatively lower tensile properties.
Flexural strength was also maximized in the 20% snake grass – 10% Hibiscus rosa composite at 110 MPa, reflecting enhanced resistance to bending stresses due to the synergistic effect of snake grass fiber and neem gum.
Impact toughness peaked at 18 J for the 20% snake grass – 10% Hibiscus rosa composite, and Shore D hardness reached 85, both attributed to improved fiber-matrix bonding facilitated by the neem gum.
Interlaminar shear strength (ILSS) was significantly improved by the inclusion of neem gum, with the 20% snake grass – 10% Hibiscus rosa composite achieving the highest ILSS value of 12 MPa, indicating better resistance to delamination and enhanced structural integrity.
Water absorption tests revealed a range from 51.0% (Composite 5% snake grass – 25% Hibiscus rosa) to 40.0% (Composite 30% snake grass – 30% Hibiscus rosa without gum), with the 20% snake grass – 10% Hibiscus rosa composite demonstrating reduced moisture uptake at 42.0%, highlighting the role of optimized fiber content and gum in enhancing durability under humid conditions.
SEM analysis confirmed superior fiber-matrix adhesion and reduced porosity in neem gum-containing composites, especially in the 20% snake grass – 10% Hibiscus rosa composite, directly correlating with improved mechanical performance and fracture resistance.

Future research should extend beyond mechanical and physical evaluations to explore the biological durability of these composites, including resistance to insect attack and decay fungi. Such investigations would provide deeper insights into their long-term performance and widen their applicability in diverse environments.

Funding

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2502)

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: January 12, 2025; Peer review completed: April 5, 2025; Revised version received: June 17, 2025; Accepted: October 2, 2025; Published: October 7, 2025.

DOI: 10.15376/biores.20.4.10106-10129