Hybrid bio-composites reinforced with kenaf and snake grass fibers and neem gum: Synergistic effects and role of fiber aspect ratio

Abstract

The influence of neem gum powder (NGP) was evaluated relative to the mechanical, physical, and morphological properties of hybrid epoxy composites reinforced with varying ratios of kenaf and snake grass fibers. Six composite samples (KS1 to KS6) were fabricated with a constant epoxy content of 60 wt%. KS1 to KS5 incorporated 10 wt% NGP, while KS6 served as the control sample without gum. The results revealed a substantial improvement in mechanical performance with the inclusion of gum. The KS4 composite, containing 20% kenaf and 10% snake grass fiber, exhibited the highest tensile strength (59 MPa), flexural strength (82 MPa), inter-laminar shear strength (11.8 MPa), hardness (85.5 Shore D), and impact strength (5.23 J), along with the lowest water absorption (27%). In contrast, KS6 showed significantly lower values in all these properties, confirming the reinforcing effect of NGP. Scanning electron microscopic analysis of fractured surfaces revealed enhanced fiber-matrix adhesion in gum-containing composites, with fewer voids, reduced fiber pull-out, and minimal crack propagation, validating the mechanical test results. These findings demonstrated that the synergistic effect of hybrid fibers and gum significantly improved overall performance, making these composites promising for structural and eco-friendly applications.

Full Article

Hybrid Bio-Composites Reinforced with Kenaf and Snake Grass Fibers and Neem Gum: Synergistic Effects and Role of Fiber Aspect Ratio

Nashmi H. Alrasheedi  ,^a Palanisamy Sivasubramanian  ,^b,*

Manickaraj Karuppusamy  ,^c Barun Haldar,^d and Thresh Kumar Durairaj ^e,*

The influence of neem gum powder (NGP) was evaluated relative to the mechanical, physical, and morphological properties of hybrid epoxy composites reinforced with varying ratios of kenaf and snake grass fibers. Six composite samples (KS1 to KS6) were fabricated with a constant epoxy content of 60 wt%. KS1 to KS5 incorporated 10 wt% NGP, while KS6 served as the control sample without gum. The results revealed a substantial improvement in mechanical performance with the inclusion of gum. The KS4 composite, containing 20% kenaf and 10% snake grass fiber, exhibited the highest tensile strength (59 MPa), flexural strength (82 MPa), inter-laminar shear strength (11.8 MPa), hardness (85.5 Shore D), and impact strength (5.23 J), along with the lowest water absorption (27%). In contrast, KS6 showed significantly lower values in all these properties, confirming the reinforcing effect of NGP. Scanning electron microscopic analysis of fractured surfaces revealed enhanced fiber-matrix adhesion in gum-containing composites, with fewer voids, reduced fiber pull-out, and minimal crack propagation, validating the mechanical test results. These findings demonstrated that the synergistic effect of hybrid fibers and gum significantly improved overall performance, making these composites promising for structural and eco-friendly applications.

DOI: 10.15376/biores.21.1.459-481

Keywords: Hybrid bio-composites; Kenaf fiber; Mechanical properties; Neem gum powder; Snake grass fiber; SEM

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, School of Engineering, Mohan Babu University, Tirupati – 517102, Andhra Pradesh, India; c: Department of Mechanical Engineering, CMS College of Engineering and Technology, Coimbatore – 642109, Tamil Nadu, India; d: Industrial Engineering Department, College of Engineering, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh-11432, Saudi Arabia;

e: Department of Mechanical Engineering, P T R College of Engineering & Technology, Thanapandiyan Nagar, Austinpatti, Madurai-Tirumangalam Road, Madurai, 625008, Tamilnadu, India;

* Corresponding authors: sivaresearch948@gmail.com; threshkumar_1234@yahoo.co.in

INTRODUCTION

Increasing environmental concerns and the growing demand for sustainable materials have led to a significant shift in research and industrial practices toward the development of bio-based and eco-friendly composite materials (Guna et al. 2018). Traditional composites reinforced with synthetic fibers such as glass, carbon, and aramid offer excellent mechanical properties but pose serious environmental challenges because of their non-biodegradability, high energy consumption during manufacturing, and difficulties in disposal (Mani et al. 2023). In contrast, natural fiber-reinforced polymer composites (NFRPCs) have emerged as promising alternatives owing to their renewability, biodegradability, low cost, low density, and relatively good mechanical performance (Ramesh 2016; Sumesh et al. 2024).

Among the wide variety of available natural fibers, kenaf (Hibiscus cannabinus) and snake grass (Sansevieria cylindrica) fibers are of particular interest. Kenaf is a bast fiber known for its high tensile strength, high cellulose content, and good fiber-matrix compatibility (Thandavamoorthy et al. 2024). It is widely cultivated and processed for composite applications, paper production, and textiles. Snake grass, on the other hand, is a leaf fiber with a unique tubular structure, good mechanical stiffness, and excellent moisture resistance (Sathish et al. 2024; Vijay et al. 2022). Although not as extensively studied as other common fibers, snake grass offers a high aspect ratio and sufficient interfacial bonding capability, making it suitable for reinforcing polymer matrices (Pachiappan and Santhanam 2023).

Hybridization of fibers—combining two or more types of fibers in a single composite—has proven to be an effective strategy to improve the overall performance by leveraging the unique properties of each fiber type (Vinod et al. 2021; Nanthakumar et al. 2025). In this study, kenaf and snake grass fibers were used in different weight ratios to reinforce epoxy resin. Epoxy is a thermosetting polymer that is widely used in composite manufacturing due to its superior adhesive properties, low shrinkage, chemical resistance, and excellent mechanical strength (Negi et al. 2024). However, like most thermosets, it is inherently brittle and benefits greatly from the reinforcement of both fibers and gum.

In addition to fibers, the introduction of other particulates into fiber-reinforced composites is another effective method to enhance interfacial bonding, improve stress distribution, and reduce moisture uptake (Kumar et al. 2022). In recent years, the use of natural gums derived from agricultural and forest residues has garnered attention due to their availability, renewability, and compatibility with natural fibers (Balaji et al. 2021). In this regard, Neem Gum Powder (NGP) presents itself as a sustainable option for composite formulation. Neem gum is a water-soluble exudate obtained from the Azadirachta indica tree, which contains polysaccharides with excellent adhesive and binding properties (Karuppusamy et al. 2025a). Its application is expected to improve fiber-matrix interaction, reduce voids and micro-cracks, and contribute to the mechanical integrity of the composite (Manimaran et al. 2018; Mishfa et al. 2023).

The core objective of this study was to investigate the combined effect of fiber hybridization and NGP (gum) on the mechanical, physical, and morphological properties of epoxy composites (Rangaraj et al. 2022). Six different composite samples were fabricated: five of them (KS1–KS5) contained 10 wt% NGP with varying kenaf and snake grass fiber ratios, while KS6 was kept as a control sample with no gum but a higher total fiber content (Chandramohan et al. 2024). The specific combination of kenaf and snake grass was chosen to exploit the strength of kenaf and the structural stiffness and moisture resistance of snake grass, while NGP was expected to act as a performance-enhancing component (Zaman and Khan 2022).

Mechanical characterizations included tensile strength, flexural strength, interlaminar shear strength (ILSS), impact strength, and hardness assessments. These properties are critical for evaluating the load-bearing capacity, resistance to deformation, toughness, and surface durability of the composites. In addition to mechanical testing, water absorption behavior was studied to measure the composite’s suitability in humid or aqueous environments (Manickaraj et al. 2023; Ramakrishanan et al. 2025). Water uptake in natural fiber composites is a critical factor that can lead to swelling, debonding, and deterioration of properties over time (Ramesh et al. 2023).

To gain further insight into the failure mechanisms and fiber-matrix interactions, Scanning Electron Microscopy (SEM) was employed to analyze the fractured surfaces of the tested specimens. SEM images can help reveal morphological features such as fiber pull-out, fiber breakage, gum dispersion, resin-rich zones, and voids—factors which directly correlate with the mechanical performance of the composites (Sathishkumar et al. 2012). Enhanced interfacial adhesion observed in SEM micrographs provides a qualitative confirmation of the performance improvement due to gum incorporation (Jenish et al. 2021). The outcomes of the cited research indicate that the addition of bio-based components such as neem gum can significantly improve the mechanical robustness and environmental stability of natural fiber composites (Sathishkumar et al. 2013). This approach aligns with current trends in green materials engineering and supports the development of sustainable, lightweight, and cost-effective composite materials for applications in automotive interiors, furniture, packaging, and low-load structural components (Suriyaprakash et al. 2023).

Furthermore, the novelty of incorporating snake grass fibers, which are relatively underexplored, alongside a naturally-derived gum, provides an original contribution to the existing body of knowledge on bio-composites (Lokantara et al. 2020). The synergy between the selected fibers and gum, when optimized, not only enhances performance but also promotes the utilization of underused biomass, contributing to value-added product development and waste minimization (Rajamanickam et al. 2023). In summary, the current investigation demonstrates a holistic and environmentally conscious pathway for the development of high-performance bio-composites by strategically combining hybrid natural fibers and other green components (Vinodkumara et al. 2019). Future work may include durability studies under weathering conditions, fire resistance evaluation, and life cycle assessment to further validate the industrial applicability of these composites.

EXPERIMENTAL

Materials and Methods

Kenaf fiber

Kenaf fibers (K) used in this study were extracted from plants cultivated in the Negamam area of Coimbatore district, Tamil Nadu, India. The raw fibers were manually cleaned to eliminate dust and impurities and then sun-dried to reduce inherent moisture content (Khan et al. 2023). To ensure consistency during composite fabrication, the fibers were chopped into uniform lengths of approximately 25 mm (Pandiarajan et al. 2025). Figure 1 shows the kenaf fibers.

Snake grass fiber

Snake grass (Sansevieria cylindrica) fibers were also obtained from the Negamam region in Coimbatore district. The fibers were extracted through water retting for 12 days, followed by manual mechanical scraping to separate the strands (Ravichandran et al. 2025). After thorough washing to remove residual matter, the fibers were dried at ambient temperature and then cut into lengths of 25 mm for reinforcement purposes (Ramesh et al. 2018). Figure 2 shows the snake grass fibers.

Fig. 1. Kenaf fiber

Fig. 2. Snake grass fibers

Neem gum powder

Neem gum was sourced from local regions in and around Pollachi, Tamil Nadu. The crude gum was collected and subsequently pulverized using a mechanical grinder. The ground powder was sieved to a small particle size and effective dispersion within the epoxy matrix during mixing (Natarajan et al. 2023). Figure 3 shows the neem gum powder.

Fig. 3. Neem gum powder

Epoxy resin

A bisphenol-A-based epoxy resin (LY 556) and its corresponding hardener (HY 951) were procured from Covai Seenu and Seenu Company, Coimbatore. The resin and hardener were mixed in a 10:1 weight ratio, as recommended by the manufacturer (Jenish et al. 2022). This epoxy system was selected due to its excellent adhesive strength, dimensional stability, chemical resistance, and compatibility with natural fiber reinforcements (Sahoo et al. 2022).

Composite Formulation

Six composite formulations were developed with a constant epoxy resin content of 60 wt%. The remaining 40 wt% consisted of varying proportions of kenaf fiber, snake grass fiber, and NGP (Saravanakumar and Reddy 2022). The formulations are presented in Table 1.

Table 1. Composite Designations

Composite Fabrication

The composite laminates were fabricated using a combination of the hand lay-up method and compression molding. Initially, the required amount of NGP was thoroughly mixed with the epoxy resin using a mechanical stirrer at 600 rpm for approximately 10 min to ensure uniform dispersion (Iyyadurai et al. 2023). Subsequently, the pre-weighed kenaf and snake grass fibers were gradually added to the resin-gum mixture and manually stirred to achieve homogeneous fiber distribution (Thiruvasagam et al. 2017). The prepared mixture was then transferred into a rectangular steel mold of dimensions 300 mm × 300 mm × 3 mm, which was pre-coated with a suitable release agent to prevent adhesion. Compression molding was performed by applying uniform pressure using a hydraulic press to compact the laminate and eliminate entrapped air (Prabhu et al. 2020). The composites were allowed to cure at room temperature for 24 h under compression, followed by post-curing in a hot air oven at 60 °C for 3 h to enhance the degree of cross-linking and improve mechanical properties. After complete curing, the laminated plates were demolded and cut into standard specimens for mechanical and physical testing using a diamond-tipped saw, adhering to the respective ASTM standards. Figure 4 shows the composite fabricated plate (Dev et al. 2025).

Fig. 4. Composite fabricated plate

Mechanical Testing

All mechanical tests were performed at ambient room temperature using specimens prepared according to the respective ASTM standards. Tensile strength was measured following ASTM D638-14 (2022) using a universal testing machine (UTM) at a crosshead speed of 2 mm/min, with dog-bone shaped specimens having a gauge length of 50 mm (Aravindh et al. 2022; Maheshwaran et al. 2022; Manickam et al. 2023; Sathishkumar 2016). Flexural strength was evaluated in accordance with ASTM D790-17 (2017) using the three-point bending method, maintaining a support span of 64 mm and a crosshead speed of 2 mm/min. Interlaminar shear strength (ILSS) was determined by short beam shear testing as per ASTM D2344 2022, with a constant span-to-depth ratio of 5:1. Impact strength was assessed using a Charpy impact tester as per ASTM D256 (2023), employing un notched rectangular specimens. Surface hardness was measured using a Shore D durometer in line with ASTM D2240-15 (2021), with five readings taken from different regions of each specimen to obtain an average value (Balaji et al. 2016; Nithyanandhan et al. 2024; Ramadoss et al. 2024).

Water Absorption Test

Water absorption was tested according to ASTM D570-22 (2022). Specimens were dried in an oven at 50 °C for 24 h, cooled in a desiccator, weighed (W₀), and then immersed in distilled water at room temperature for 72 h. After immersion, the specimens were wiped dry and reweighed (W₁). Water absorption (%) was calculated using Eq. 1 (Vijay and Singaravelu 2016; Dhilipkumar et al. 2025)

 (1)

Scanning Electron Microscopy

The microstructure of the fractured composite surfaces was analyzed using scanning electron microscopy (SEM) with a JEOL SEM (JEOL GmbH, Gute Änger, Germany) set to an accelerating voltage of 15 kV. The SEM imaging offered comprehensive insights into the distribution of components, the bonding at the interface between the matrix and fibers, as well as the mechanisms of failure observed within the composites (Akil et al. 2011). The SEM analysis facilitated an assessment of the morphology, allowing for the identification of voids, cracks, or inadequate filler-matrix interactions that may influence the mechanical performance of the material (Edeerozey et al. 2007).

RESULTS AND DISCUSSION

Tensile Strength

Tensile strength is a primary indicator of a composite material’s capacity to resist axial stretching forces. The tensile behavior of fiber-reinforced polymer composites is influenced by factors such as fiber-matrix adhesion, fiber volume fraction, aspect ratio, and the uniformity of filler dispersion (Aruchamy et al. 2025). In this investigation, a consistent improvement in tensile strength was observed with the incorporation of a hybrid combination of kenaf fiber, snake grass fiber, and NGP. The composite KS4, containing 20% kenaf fiber, 10% snake grass fiber, and 10% NGP, exhibited the highest tensile strength of 59 MPa. This superior performance is attributable to the optimal synergy between the rigid kenaf fibers, which contribute high modulus, and the flexible snake grass fibers, known for their energy-absorbing capabilities. Neem gum powder, with particle sizes below 100 µm, was finely dispersed within the epoxy matrix, enhancing the interface between fibers and matrix by acting as a micro-filler. This led to reduced microvoids and improved stress transfer mechanisms across the matrix and reinforcing phases. The improvement in tensile performance was further validated through SEM analysis of the fracture surfaces. The composite KS4 displayed fewer fiber pull-outs, strong interfacial adhesion, and reduced matrix cracking (Elsaid et al. 2011). In contrast, KS6, which lacked NGP, recorded the lowest tensile strength of 39 MPa. SEM images of KS6 showed poor matrix-fiber bonding, prominent voids, and fiber pull-out, indicating ineffective load transfer and weak cohesion. This underscores the importance of the presence of the particles in facilitating mechanical interlocking and chemical bonding at the interface. The progressive enhancement from KS1 to KS4 indicates that the balanced fiber ratios significantly influence the tensile properties. The initial increase in tensile strength from KS1 to KS3 was attributed to the gradual rise in kenaf fiber content, which added stiffness and strength, while the reduction in snake grass fiber content was offset by the toughening effect of the former. At KS4, the composition reached its optimal point with the best fiber and gum synergy. Beyond this, in KS5, where kenaf fiber was increased to 25% and snake grass was reduced to 5%, a slight drop in tensile strength to 53 MPa was observed. This suggests a possible mismatch in the stress-strain behavior of the two fibers, leading to non-uniform stress distribution and increased localized stress concentrations. The incorporation of 10% NGP across KS1 to KS5 consistently contributed to improved mechanical properties by enhancing the matrix continuity, reducing void formation, and increasing the overall stiffness of the matrix. Neem gum’s hydrophilic nature possibly improved wetting with natural fibers, thereby improving fiber adhesion and crack bridging capability (Sreenivas et al. 2020; Ramakrishnan et al. 2025a). In KS6, the absence of gum compromised the microstructural integrity and load-bearing efficiency, which was evident from both mechanical data and SEM imagery. In conclusion, the tensile strength results highlight the significant role of hybrid fiber reinforcement and gum integration in tailoring the tensile behavior of epoxy composites. KS4, with an optimal composition of 20% kenaf, 10% snake grass, and 10% NGP, demonstrated a strong, ductile, and well-integrated microstructure, making it the most suitable formulation for high-performance structural applications involving tensile loading. Figure 5 shows the tensile results of the specimens.

Fig. 5. Tensile strength of composites

Flexural Strength

Flexural strength determines the composite’s ability to resist deformation under bending loads. Such resistance is critical for structural applications subjected to transverse stresses (Guo et al. 2019). The flexural properties of the developed composites exhibited a trend similar to that of tensile strength, with KS4 showing the highest flexural strength of 82 MPa. This superior bending resistance is a result of the optimal balance between the stiff kenaf fibers and the tough, flexible snake grass fibers. Kenaf’s higher cellulose content contributes significantly to load-bearing capacity during bending, while snake grass enhances energy absorption and crack deflection. The matrix-fiber interaction plays a pivotal role in determining flexural behavior (Karuppiah et al. 2020). The NGP, acting as a micro-filler, fills voids within the matrix and provides micro-scale reinforcement that enhances stiffness. Its integration improves interfacial bonding by bridging microcracks and distributing stress effectively across the fiber and matrix interface. The presence of neem gum also reduces stress concentrations around fiber ends, delaying crack initiation and propagation under bending loads.

Fig. 6. Flexural strength of composites

SEM micrographs of KS4 confirm minimal fiber pull-out and strong matrix encapsulation around both kenaf and snake grass fibers. This well-bonded interface contributes to effective stress transfer and delayed failure, which is evident in the flexural testing results. The samples KS3 and KS5 also demonstrated relatively high flexural strength values (78 MPa and 79 MPa, respectively), indicating their strong matrix integrity, although slightly lower than KS4 due to imbalances in fiber synergy. The lowest flexural strength was recorded for KS6 (64 MPa), which lacked NGP. SEM images of KS6 revealed prominent voids and fiber debonding, leading to premature failure under bending stress. These observations highlight the significance of the gum phase in reinforcing the composite’s bending stiffness and enhancing damage resistance. Overall, the improved flexural strength of hybrid composites with optimized gum and fiber proportions underscores the synergistic effect of hybrid reinforcement in achieving high performance. KS4 once again emerged as the best formulation, suggesting its suitability for applications requiring high flexural rigidity such as automotive panels, flooring, and load-bearing structural components (Hassan et al. 2017; Ramakrishnan et al. 2025b). Figure 6 shows the flexural strength of the composites.

Interlaminar Shear Strength

Interlaminar shear strength (ILSS) measures the resistance of laminated composites to shearing stresses between layers and is a crucial parameter in evaluating the bonding quality between plies (Malik et al. 2021). The highest ILSS value was recorded for KS4 (11.8 MPa), which confirms the strong interfacial bonding and cohesive integrity of the composite. This enhancement in ILSS can be attributed to the optimal hybridization of kenaf and snake grass fibers with NGP, effectively reinforcing the resin-rich interlaminar regions. The addition of 10% NGP across KS1 to KS5 consistently improved ILSS values compared to the control sample KS6 (8.62 MPa), which lacked gum. The gum’s fine particle size allowed it to occupy the voids between fibers and resin, creating a denser microstructure that improved the load-bearing capacity in shear. Moreover, the hydrophilic nature of neem gum improves compatibility with natural fibers, enhancing matrix-fiber adhesion and reducing delamination tendencies. SEM analysis of KS4 revealed excellent resin-fiber wetting and fewer resin-starved regions at the interlaminar zones. The well-anchored fibers were surrounded by a dense and continuous matrix with gum particles visibly embedded, which is indicative of improved shear resistance. Conversely, KS6 showed pronounced fiber debonding and microcracks at the interfacial boundaries, which serve as stress concentrators under shear loading.

Fig. 7. The ILLS values of specimens

The gradual improvement in ILSS from KS1 to KS4 is consistent with the increasing kenaf fiber content, known for its higher mechanical stiffness and adhesion compatibility. The optimized 20% kenaf to 10% snake grass ratio in KS4 facilitates effective stress distribution between stiff and tough fiber types, thus minimizing interfacial mismatch and internal stress concentrations. These findings suggest that the presence of NGP and the proper balance between different fiber types are essential for enhancing the shear integrity of natural fiber-reinforced epoxy composites. The improved ILSS of KS4 makes it particularly well-suited for applications requiring reliable interlaminar performance, such as marine structures, wind turbine blades, and aircraft interiors (Manickaraj et al. 2025b). Figure 7 shows the ILLS of the specimens.

Impact Strength

Impact strength reflects the ability of a material to absorb and dissipate energy under sudden loading. The maximum impact energy was observed in KS4 (5.23 J), demonstrating the superior toughness of this composition (Gurusamy et al. 2025). This result suggests that the hybrid combination of kenaf and snake grass fibers, along with the presence of NGP, contributed to enhanced energy absorption and crack arrest mechanisms. The structure-property relationship in KS4 is evident from the morphology of the fracture surface observed under SEM.

Fig. 8. Impact strength of specimens

The presence of long fiber bridges, extensive matrix deformation, and rough fracture paths indicated a more ductile failure. The intertwined nature of the two fiber types promoted crack deflection and fiber bridging, which enhanced energy dissipation. The NGP further contributed by filling the interstitial spaces, impeding crack propagation, and creating micro-barriers within the matrix. KS6, with an impact strength of only 3.28 J, exhibited a brittle failure with clean fracture surfaces and widespread fiber pull-out under SEM. The absence of gum resulted in poor matrix continuity and a reduced energy-absorbing interface. As the kenaf content increased and snake grass decreased beyond the optimal point (as in KS5), the impact resistance slightly dropped due to reduced flexibility and increased stiffness, causing a decline in the material’s ability to undergo plastic deformation. These results emphasize that a well-balanced ratio of stiff and flexible fibers, combined with an appropriate gum particle system, is essential for maximizing toughness in hybrid composites. The significant energy absorption capability of KS4 highlights its potential for applications in protective gear, automotive bumpers, and packaging where impact loading is a concern (Sekar et al. 2025). Figure 8 shows the impact strength of the specimens.

Hardness

Hardness reflects the material’s resistance to surface indentation and wear. Among the tested composites, KS4 showed the highest Shore D hardness of 85.5, indicating its superior surface integrity and structural compaction.