Mechanical and viscoelastic properties of novel polymer composites reinforced with cellulosic fibers from Hylocereus undatus agro-waste and snake plant

Abstract

Natural-fiber-reinforced epoxy composites are attracting increasing attention as lightweight and sustainable alternatives for structural and semi-structural applications. In this study, epoxy composites reinforced with snake plant (SP) fiber and agro-waste-derived Hylocereus undatus (HU) fiber were fabricated at different fiber ratios (100/0, 75/25, 50/50, 25/75, and 0/100) by hand lay-up. Increasing HU content enhances tensile and flexural modulus, hardness, storage modulus, loss modulus, and damping because the higher intrinsic stiffness, higher lignin/hemicellulose content, and lower elongation of HU fibers restrict matrix deformation and improve resistance to elastic and viscoelastic strain. In contrast, increasing SP content improves tensile, flexural, and impact strengths, as well as the glass-transition temperature, because its higher cellulose content, finer morphology, and better wettability with epoxy promote stronger interfacial adhesion, more efficient stress transfer, and greater energy absorption during loading and fracture. Thus, HU fiber mainly acts as a stiffness- and damping-dominant reinforcement, while SP fiber mainly acts as a strength- and toughness-dominant reinforcement. Among the hybrids, 25SP:75HU shows the maximum stiffness, while 75SP:25HU demonstrates the highest strength. FTIR analysis of the 50SP:50HU composite confirms the coexistence of lignocellulosic and epoxy functional groups, while the broad O-H band suggests hydrogen-bond-assisted interfacial compatibility between the fibers and the epoxy matrix. These findings demonstrate that hybridizing SP and HU fibers provides a sustainable pathway to tailor the balance between stiffness, strength, impact resistance, and viscoelastic performance in epoxy composites.

Graphical Abstract

Keywords

snake plant agro-waste plant fiber cellulosic fiber mechanical properties viscoelastic behavior

Highlights

• Novel hybrid polymer composites were fabricated using HU and SP cellulosic fibers.

• Higher HU fiber content in the polymer increases stiffness, hardness, and damping behaviors.

• Higher SP fiber content in the polymer improves strength, impact resistance, and moisture resistance.

• The 25% SP/75% HU composite exhibited maximum stiffness; 75% SP/25% HU showed maximum tensile strength.

Introduction

Natural fibers (NFs) have emerged as promising reinforcement materials for polymer matrices, offering the dual benefits of environmental compatibility and performance efficiency.¹ Compared with conventional synthetic fibers such as carbon, glass, and basalt, natural fiber-reinforced composites offer advantages including low density, renewability, biodegradability, and lower production costs, making them attractive for sustainable material development.² Although synthetic fiber composites generally exhibit superior mechanical strength, fatigue resistance, and long-term durability, natural fiber composites have gained increasing attention due to their favorable specific strength-to-weight ratios and reduced environmental impact. These composites are therefore being explored for a wide range of lightweight and semi-structural applications in automotive, construction, and consumer product sectors.^3–5 Among the various natural fibers available, cellulosic fibers derived from plants have demonstrated significant potential, owing to their high cellulose content, low density, and relatively good mechanical properties.^6,7 Plant-based cellulosic fibers, commonly referred to as lignocellulosic fibers, are NFs primarily made of cellulose, hemicellulose, and lignin. These structural biopolymers play a vital role in determining the fiber’s mechanical strength, flexibility, and resistance to environmental degradation.^3,6,8 Cellulose provides rigidity and strength through its crystalline microfibrils; hemicellulose contributes to the matrix structure and water absorption behavior; and lignin, a complex aromatic polymer, imparts stiffness and acts as a natural binder, offering protection against microbial attack and moisture.^9,10 The unique combination of these components makes lignocellulosic fibers an attractive option for reinforcing polymer composites.

Numerous studies have reported that incorporating natural fibers into polymer matrices can significantly influence the mechanical, thermal, and durability properties of composites. Natural fibers such as jute, flax, sisal, and pineapple leaf fibers have demonstrated improvements in tensile and flexural properties due to effective stress transfer between fibers and the resin. Additionally, the presence of lignocellulosic constituents enhances damping characteristics and reduces the density of the composites.^11,12 However, natural fiber composites may exhibit higher moisture absorption due to cellulose’s hydrophilic nature, which can affect long-term durability and interfacial bonding. Surface treatments and hybridization strategies have therefore been widely explored to improve fiber–matrix compatibility and overall composite performance.¹² For instance, Dev et al.¹³ investigated jute/Rosa hybrida (RH) fiber composites and reported that incorporating a lower proportion of RH fibers alongside jute can significantly enhance the mechanical properties. Chandramohan et al.¹⁴ studied jute/aloe vera epoxy composites, both individually and in combination. Their findings indicated that the hybrid composite (HC) incorporating both fibers outperformed composites reinforced solely with jute or aloe vera in terms of mechanical properties. Jenish et al.⁷ evaluated the mechanical behavior of HCs reinforced with mudar and snake grass fibers at varying fiber loadings. The results demonstrated that a 30% fiber content yielded the most favorable mechanical properties, outperforming the composites with 10%, 20%, and 40% fiber reinforcement. Kumar et al.¹⁵ assessed the mechanical properties of SP/kenaf/epoxy composites and observed that the HC, consisting of SP and kenaf fibers at 12.5% by weight, exhibited improved mechanical performance.

The damping behavior of jute/sisal/epoxy composites was studied by Gupta¹⁶ at various frequencies, and the results showed that the storage modulus, loss modulus, and glass transition temperature (T_g) increase with increasing frequency. Pappu et al.¹⁷ investigated the mechanical, thermal, and water absorption properties of sisal/hemp/poly (lactic acid) composites and reported that the HCs exhibit superior tensile, flexural, and impact strength, enhanced thermal stability, and reduced water absorption. Jawad et al.¹⁸ investigated the thermal, dynamic-mechanical, and thermo-mechanical properties of date palm fiber (DPF)/bamboo fiber (BF) HCs and reported that the HCs exhibit improved thermal stability and a higher T_g compared to the BF-reinforced composites, highlighting the benefits of fiber hybridization.

To meet the growing demand for advanced green materials, further innovation is needed in terms of fiber selection, hybridization, and performance optimization. Less-utilized NFs can be hybridized with agro-waste-derived fibers to produce novel polymer composites with optimized mechanical, viscoelastic, and water absorption properties for potential applications. Among them, snake plant (Sansevieria trifasciata, SP) fibers are known for their high tensile strength and cellulose content, making them suitable for reinforcing polymer matrices. Native to tropical and subtropical regions, the snake plant is a hardy, fast-growing species with minimal cultivation requirements, making it a sustainable and accessible fiber source.^7,19,20 On the other hand, agricultural residues, often discarded or incinerated after harvesting, can be transformed into high-value reinforcement materials, thereby contributing to waste valorization and resource efficiency. The valorization of agricultural waste for material development represents an equally promising pathway to sustainability. Dragon fruit (Hylocereus undatus (HU)) is a widely cultivated tropical crop, primarily valued for its edible fruit. However, the plant’s fibrous stem, which constitutes a large volume of post-harvest residue, is typically discarded as waste. This stem material contains substantial amounts of cellulose and can serve as a low-cost, renewable reinforcement for polymer composites, as first explored by Shibly et al.²¹ HU fiber, containing over 50% cellulose, offers a lower density than traditional fibers (e.g., jute, banana, and hemp), along with a modulus of 11–14 GPa and a tensile strength of 240–270 MPa, making it a suitable reinforcement for composite manufacturing. Utilizing such agro-waste not only addresses disposal issues but also adds economic and environmental value through material upcycling.

However, achieving optimal performance depends on carefully selecting and proportioning the constituent fibers to overcome compatibility challenges, such as fiber-matrix debonding and inefficient stress transfer. Considering the individual potential of both SP and HU fibers, this study aimed to evaluate the mechanical and viscoelastic properties of SP/HU fiber-reinforced epoxy HCs fabricated by hand lay-up and compression molding at five fiber ratios (100/0, 75/25, 50/50, 25/75, and 0/100). Comprehensive testing has been employed to assess their mechanical and viscoelastic properties, failure modes, water absorption behavior, and Fourier transform infrared spectroscopy (FTIR) characteristics. The primary objective was to investigate how varying fiber compositions affect the performance of SP/HU hybrid composites and to evaluate their suitability for lightweight, eco-friendly materials for structural and semi-structural applications.

Materials and methods

Materials

The natural fibers used in this study were snake plant (Sansevieria trifasciata, SP) and dragon fruit (Hylocereus undatus, HU). These fibers were selected due to their local availability, sustainability, ease of extraction, and favorable mechanical properties suitable for polymer composite reinforcement. Both plants are readily grown as agricultural or ornamental plants, enabling sustainable collection without affecting food resources. These two fibers were selected because SP offers high cellulose content, good tensile strength, and sustainable local availability, whereas HU is an underutilized dragon-fruit agro-waste with relatively high stiffness and strong potential for value-added composite reinforcement.

SP and dragon fruit plants were sourced locally from Gazipur and Tangail, Bangladesh, respectively (Figure 1). The plants were first cut into suitable lengths to aid fiber extraction. The SP leaves and the central stem portions of HU plants were immersed in water for 3 and 5 days, respectively, for retting. Following retting, the fibers were manually extracted. Finally, the fibers were oven-dried at 70°C for 3 h. Table 1 presents the properties of SP and HU fibers. The epoxy resin (Araldite LY556) was used as the matrix with a density of 1.1 g/cm³ and a viscosity of 12,000–13,000 cP, while the hardener (Aradur HY951) was employed as the curing component.

Figure 1.

Plants used in this study and their extracted fibers: (a) Dragon fruit (Hylocereus undatus) plant, (b) collected stems of HU plants, (c) obtained middle portion of HU stems containing cellulosic fiber, (d) extracted HU fiber after water retting, (e) snake plant, (f) collected snake plants, and (g) extracted SP fiber after water retting.

Table 1.

Chemical compositions, physical and mechanical characteristics of SP and HU fibers^22–25.

Properties	SP fiber	HU fiber
Cellulose (%)	70–80	55.21
Hemicellulose (%)	10–12	16.97
Lignin (%)	5–8	15.48
Pectin (%)	1–2	–
Average diameter (µm)	45–250	173.53
Density (g/cm³)	0.88	1.08
Lumen size (µm)	4	–
Microfibril angle (°)	19	–
Moisture content (%)	10.25	14.28
Tensile modulus (GPa)	9.7	11–14
Tensile strength (MPa)	270–545	240–270
Elongation at break (%)	2.87	1.90–2.80

Fabrication of fiber preform

The SP and HU fibers were conditioned at 25 ± 2°C and 65 ± 2% relative humidity for 3 h. After conditioning, the fibers were combed using a hand-combing tool and then cut into 175 mm lengths to prepare preforms. Unidirectional (UD) preforms of SP and HU fibers were produced for the fabrication of SP/HU fiber-based hybrid composites (175 mm × 110 mm), following the procedure described in the authors’ earlier work.²³ In this process, the ends of the SP and HU fibers were clamped to preserve the UD orientation, and the fibers were oriented in a single direction within a wooden frame. The aligned fibers were lightly dampened and subsequently pressed at 2 MPa and 120°C for 10 min. Five types of preforms were developed with different SP-to-HU weight ratios: 100SP, 75SP:25HU, 50SP:50HU, 25SP:75HU, and 100HU. Figure 2(a)–(e) shows the schematic representation of the preform fabrication process, while Table 2 summarizes the corresponding fiber content, dimensions, thickness, and density of the prepared preforms.

Figure 2.

Fabrication process of SP/HU fiber-reinforced epoxy composites: (a) combing, (b) combed fiber, (c) UD alignment, (d) hot-pressing of fibers for preform fabrication, (e) manufactured preform sheet, (f) epoxy resin, (g) fabrication of composites, and (h) manufactured composite laminate.

Table 2.

Fiber content and properties of fabricated preforms.

Preform	Fiber content (%)		Preform size (mm²)	Thickness (mm)	Areal density (g/m²)
Preform	SP fiber	SA fiber	Preform size (mm²)	Thickness (mm)	Areal density (g/m²)
100SP	100	0	175 × 110	2.43	1210.71
75SP:25HU	75	25		2.78	1231.23
50SP:50HU	50	50		2.82	1242.12
25SP:75HU	25	75		2.98	1296.56
100HU	0	100		2.88	1318.17

Fabrication of composites

The composites in this research were fabricated by hand lay-up (Figure 2). A mold measuring 175 mm × 110 mm × 3 mm was used for stacking, and a plastic sheet was first placed on the mold surface. The epoxy resin and hardener were mixed in a 10:1 weight ratio. Half of the prepared mixture was spread over the plastic surface, after which the preform was positioned on the resin layer. The remaining resin-hardener mixture was then uniformly applied over the top surface of the preform. A hand roller was used to ensure proper fiber impregnation and eliminate trapped air bubbles. Afterward, another plastic plate was kept on top of the laminate, and a compressive load of 25 kg was applied for 24 h. Following this, the load was released, and the composite laminates were carefully demolded. The produced composites were identified by their corresponding preform type. Table 3 presents the fiber and matrix weight percentages and properties of the fabricated composites.

Table 3.

Fiber weight and volume percentages, thickness, density, and void contents of fabricated composites.

Composite	Composite thickness (mm)	Fiber weight percentage, wt_f (%)	Fiber volume percentage, v_f (%)	Theoretical density, ρ_ct (g/cm³)	Experimental density, ρ_ce (g/cm³)	Void content (%)
100SP	3.21	27.24	31.19	1.03	1.01	1.94
75SP:25HU	3.63	28.76	32.03	1.05	1.02	2.85
50SP:50HU	4.02	29.12	28.32	1.08	1.01	6.48
25SP:75HU	4.05	29.86	27.58	1.10	1.05	4.54
100HU	3.62	28.65	27.08	1.14	1.10	3.50

Characterization of composites

Measurement of density and void content

The theoretical density of composites was measured based on the weight fractions of their individual constituents, as expressed in equation (1).

ρ_{c t} = \frac{1}{(\frac{w_{f s p}}{ρ_{f s p}}) (\frac{w_{f h u}}{ρ_{f h u}}) + (\frac{w_{m}}{ρ_{m}})}

(1)

where ρ_ct, ρ_fsp, ρ_fhu, and ρ_m are the densities of the composite, SP fiber, HU fiber, and matrix, respectively, while w_fsp, w_fhu, and w_m are their corresponding weight fractions. The experimental density (ρ_ce) of the composites was measured using Archimedes’ water immersion technique in accordance with ASTM D570-98. Subsequently, the void content was evaluated according to ASTM D-2734-70, as expressed in equation (2).

Void content (%) = \frac{ρ_{c t} - ρ_{c e}}{ρ_{c t}} \times 100

(2)

Tensile test

Tensile testing was performed using Type 1 dog-bone specimens in accordance with BS EN 2747:1998. Using a 10 kN load cell and a constant crosshead speed of 5 mm/min until failure, tests were conducted on a Zwick/Roell Z010 (Ulm, Germany) UTM. All mechanical test results represent the mean and standard error obtained from five specimens.

Flexural test

According to ASTM D790, flexural (three-point bending) tests were performed on composite specimens measuring 125 mm × 12.7 mm × 3 mm using a universal testing machine (AG-X Plus, Japan) equipped with a 10 kN load cell. Throughout the test, the crosshead speed was kept at 1.4 mm/min, and the span length between the supports was held at 50 mm.

Impact test

Charpy impact testing was performed in accordance with ASTM D256. The manufactured composite laminates were used to create V-notched specimens measuring 63.5 mm × 12.7 mm × 3 mm. The samples were struck by a 2.634 kg pendulum that was released at a 150° angle. The contact angle between the pendulum and the specimen was precisely recorded using an indicator. Using the reference chart that came with the impact tester, the absorbed impact energy was calculated.

Hardness test

Hardness measurements were performed on composite samples measuring 35 mm × 15 mm × 3 mm using a Shore D durometer, according to ASTM D2240. The test employed a hardened steel indenter rod (1.25 mm in diameter) with a 30° conical tip and a 0.1 mm tip radius. An approximate force of 45 N was applied during each test.

Scanning electron microscopy (SEM) characterization

A scanning electron microscope (SEM, ZEISS EVO 10, Germany) was used to analyze the fracture surfaces of the tensile-fractured specimens. Cross-sectional samples were prepared to reveal the inner microstructure. To increase electron conductivity, each specimen was placed on brass or aluminum stubs, and an AGAR sputter coater was used to apply a thin layer of gold for approximately 20 min. SEM images were then acquired for analysis.

DMA

A Q800 dynamic mechanical analyzer (TA Instruments, New Castle, DE, USA) in dual cantilever mode was used to evaluate the composites’ thermo-mechanical behavior. Testing was carried out in air at a heating rate of 5°C per minute over a temperature range of 20-170°C. A strain of 0.01% and an oscillation frequency of 1 Hz were utilized. Using cuboid-shaped specimens with dimensions of 80 mm × 10 mm × 2 mm, the composites’ storage modulus (E′), loss modulus (E″), and damping factor (tan δ) were measured.

Water absorption measurements

Water absorption (WA) testing of the composite specimens (64 mm × 12.7 mm × 3 mm) was performed as per ASTM D570-98. The samples were first oven-dried at 65°C, and their initial weight (W_b) was recorded using a high-precision electric balance. They were then submerged in distilled water at room temperature. At 10-h intervals, the specimens were removed, surface water was gently removed with a dry cloth, and the samples were weighed (W_a) before being returned to the water. This procedure was continued for a total duration of 120 h. The WA rate was determined using equation (3).

Water absorption, M (%) = \frac{W_{a} - W_{b}}{W_{b}} \times 100

(3)

FTIR test

FTIR was used to examine the chemical composition of epoxy resin, SP fibers, and HU fibers. A part of the composite was ground into powder and then mixed with potassium bromide (KBr) to make tablet samples. The spectra of these samples were measured using a Fourier transform infrared spectrophotometer (L1600400 Spectrum, Two FT-IR/DTGS, PerkinElmer, UK) with scanning ranging from 400 to 4000 cm⁻¹. A total of 34 scans per sample were obtained at a spectral resolution of 4 cm⁻¹.

Results and discussion

Mechanical properties

Tensile properties

Tensile properties of the tested samples are presented in this section. Figure 3 depicts the tensile behavior of SP/HU/epoxy composites. The tensile modulus increases with increasing HU fiber content, while the tensile strength decreases. The maximum tensile modulus of 5.57 GPa is obtained for the composite containing 100% HU fiber, which is approximately 47% higher than the modulus of the 100% SP fiber composite (3.79 GPa). Among the HCs, the tensile modulus rises from 2.67 GPa for 75SP:25HU to 3.64 GPa for 50SP:50HU, reaching 5.06 GPa for the 25SP:75HU formulation. The increasing stiffness with greater HU fiber content can be linked to the higher intrinsic modulus of HU fiber (11-14 GPa) compared to SP (9.7 GPa) (see Table 1), and to the stiffer microstructure provided by HU’s higher lignin (15.48%) and hemicellulose (16.97%) contents.^21,23 Additionally, HU fibers exhibit lower elongation at break (1.90–2.80%) than SP fibers (2.87%), thereby enhancing the rigidity of the composite structure. Reinforcement with a higher proportion of such low-ductility fibers results in limited tensile deformation, contributing to a higher tensile modulus.^26–28

Figure 3.

(a) Tensile modulus and (b) tensile strength of manufactured SP/HU fiber-based composites.

In contrast, the tensile strength behavior shows an opposite trend. The maximum tensile strength of 89.05 MPa is observed in the 100SP composite, while the 100HU composite demonstrates a much lower tensile strength of 61.96 MPa. In HCs, tensile strength decreases steadily from 85.60 MPa (75SP:25HU) to 59.72 MPa (50SP:50HU) and reaches a minimum of 54.81 MPa in 25SP:75HU. The reduction in tensile strength with increasing HU content in hybrids is due to reduced interfacial compatibility between HU and epoxy. The surface chemistry and rougher morphology of HU fibers may hinder proper wetting, leading to interfacial debonding and inefficient stress transfer.^21,29 Additionally, fiber dispersion can be less uniform with increasing HU content, particularly when mixed with SP fibers, which have distinct structural and chemical properties. Moreover, HU fiber possesses comparatively lower strength than SP fiber, resulting in low tensile strength of composites with higher HU fiber content, as listed in Table 1. The chemical constituents and physical properties of fibers also play a significant role in terms of the tensile properties of fiber-reinforced composites. The higher cellulose content (70-80%) in SP fibers compared to HU (55.21%) supports improved load transfer and tensile resistance.^23,30 Cellulose, being the most crystalline component in natural fibers, governs strength, and its high content in SP promotes efficient stress distribution across the composite.^31,32 Moreover, the comparatively smaller lumen size (4 µm) and a narrower diameter range (45-250 µm) of SP fibers likely facilitate better resin impregnation, thereby enhancing interfacial adhesion with the epoxy matrix.²³ In contrast, HU fibers, although stiffer, are more brittle due to their high lignin content and lower elongation, leading to premature fiber breakage and stress concentration during tensile loading.

The findings indicate that SP fibers are more effective at enhancing tensile strength due to their favorable strength, high elongation at break, elevated cellulose content, and superior fiber-matrix bonding. In contrast, HU fibers significantly contribute to composite stiffness due to their inherent rigidity, which stems from their higher lignin content. The observed differences in tensile behavior can be explained by the chemical composition, morphological characteristics, and fiber-matrix interfacial interactions of the two fibers, emphasizing the importance of fiber selection and hybridization strategy in natural fiber-reinforced composites.

Tensile stress-strain curves

Figure 4 depicts the tensile stress-strain behavior of SP/HU/epoxy composites. Among all samples, the 100SP composite shows the maximum stress of 89.05 MPa at approximately 2.1% strain, indicating superior load-bearing capacity and moderately ductile behavior. This performance is attributed to the higher cellulose content (70-80%), lower lignin concentration, and finer morphology of SP fibers, which promote strong interfacial bonding and efficient stress transfer across the matrix, as explained in Section 3.1.1. The 75SP:25HU hybrid shows the maximum stress among hybrids (85.60 MPa) with a strain of ∼2.0%. This indicates that limited HU fiber addition enhances stiffness without severely compromising ductility or interfacial compatibility. The inclusion of HU fibers, which are stiffer due to higher lignin (15.48%) and hemicellulose (16.97%) content, contributes to increased modulus while maintaining adequate strength due to the dominant SP fiber fraction. As the HU fiber content increases, a notable decline in stress and ductility is observed. The 50SP:50HU composite reaches 59.72 MPa with a reduced strain of around 1.85%, indicating more brittle behavior. The 25SP:75HU sample, with a tensile stress of 54.81 MPa, exhibits a comparatively higher strain at break (∼2.1%, similar to 100SP) than the 50SP:50HU sample, reflecting slightly improved deformability despite reduced strength. This trend in fracture toughness can be attributed to the interplay between fiber type and interfacial bonding. Higher HU content increases brittleness due to stiffer fibers and weaker fiber-matrix adhesion, generating stress concentration points that promote crack initiation and propagation.²¹ In contrast, SP-rich regions provide better stress transfer and energy dissipation, enhancing toughness. Hybrid compositions exhibit intermediate fracture behavior, in which the balance between SP and HU fibers governs crack deflection, fiber pull-out, and overall energy absorption.^15,21 This increase in strain may result from a less compact microstructure or localized SP fiber effects within the HU-dominant hybrid.

Figure 4.

Tensile stress-strain curves of SP/HU fiber-reinforced composites.

The 100HU composite exhibits moderate stress (61.96 MPa) and a minimum strain of ∼1.5%, confirming its brittle nature. The reduced tensile performance can be attributed to poor interfacial bonding between HU fibers and the epoxy resin, higher fiber brittleness, and the lower cellulose content (55.21%) in HU compared to SP. The stiffer yet more fragile nature of HU fibers leads to early microcracking and fiber pull-out under tensile loading. These stress-strain characteristics clearly reflect that SP fibers contribute significantly to strength and ductility because of their better wetting, higher cellulose content, and fine structure, while HU fibers mainly enhance stiffness due to their high lignin content and low elongation at break. Therefore, the hybridization strategy should be carefully tailored to strike a balance between strength, stiffness, and ductility for targeted composite applications.

Flexural properties

The flexural modulus and strength of SP/HU fiber-reinforced epoxy composites display a similar trend to their tensile behavior, with a generally positive correlation between HU fiber content and flexural modulus, while the flexural strength shows a peak at an intermediate fiber ratio and then declines (Figure 5). The composite reinforced with 25SP:75HU exhibits the maximum flexural modulus of 7.20 GPa, which is significantly higher than both the 100SP (4.02 GPa) and 100HU (5.82 GPa) composites by 79.1% and 23.7%, respectively. A gradual improvement is also observed among hybrids, from 4.71 GPa for 75SP:25HU to 3.55 GPa for 50SP:50HU, followed by a sharp rise at 25SP:75HU, indicating a strong stiffening effect of HU fiber when its content exceeds 50%. This enhancement in flexural modulus can be primarily attributed to the higher intrinsic stiffness of HU fiber compared to SP fiber, along with its higher lignin (15.48%) and hemicellulose (16.97%) content, which contribute to increased structural rigidity and reduced deformation during bending. Moreover, HU’s lower elongation at break compared to SP contributes to its better resistance against bending-induced strain, thereby increasing the stiffness of the resulting composite, as detailed in Section 3.1.1.

Figure 5.

(a) Flexural modulus and (b) flexural strength of fabricated SP/HU fiber composites.

However, the flexural strength trend differs notably, with a maximum at 75SP:25HU (236 MPa), approximately 62.4% higher than the 100SP composite (145.25 MPa) and more than twice that of the 100HU composite (114 MPa). The strength then decreases with a higher HU content, dropping to 185.81 MPa in the 50SP:50HU and 185.02 MPa in the 25SP:75HU composites, indicating that the optimal reinforcement efficiency occurs when HU is used moderately. The superior flexural strength of the 75SP:25HU hybrid may be due to a favorable balance between the ductility and high cellulose content of SP fibers (70–80%) and the stiffness provided by HU, allowing effective stress distribution and energy absorption under bending. This composition may also facilitate improved fiber dispersion and interfacial bonding, thereby minimizing stress concentrations and enhancing the composite’s resistance to crack propagation.^21,33 As HU content exceeds 50%, flexural strength declines, likely due to the inherently lower strength and cellulose content of HU fibers, which diminish their reinforcing potential. Additionally, the higher rigidity and brittle nature of HU fibers may contribute to early microcrack formation and lower energy dissipation during flexural loading, as explained in Section 3.1.1.

The observed reduction in flexural strength at higher HU content may also be related to poor fiber-matrix compatibility and insufficient wetting due to the rougher surface morphology and higher lignin content of HU fibers. These factors can limit effective load transfer at the interface and create weak zones, especially in fiber-dense regions, which act as failure initiation points under flexural stress. Furthermore, HU’s lower aspect ratio and larger average diameter than SP may negatively affect its dispersion and interlocking efficiency within the matrix.^21,32,34 As a result, composites with higher HU proportions (e.g., 25SP:75HU and 100HU) may exhibit reduced homogeneity, leading to non-uniform stress distribution and decreased flexural performance. Overall, these findings highlight the reinforcing potential of both SP and HU fibers in flexural applications, where an optimal ratio, such as 75SP:25HU, can harness the strengths of each fiber type to produce composites with both high stiffness and bending strength.

Flexural stress-strain curves

Figure 6 illustrates the flexural stress-strain behavior of SP/HU fiber-reinforced epoxy composites with different fiber ratios. The curves indicate that hybridization significantly influences both the load-bearing capacity and deformation behavior of the composites. The 75SP:25HU composite exhibits the maximum flexural strength (236 MPa) but fails at a relatively low strain, indicating a strong yet comparatively brittle response under bending. In contrast, the 50SP:50HU and 25SP:75HU composites sustain higher strain levels (∼4–4.5%) before failure, demonstrating improved deformation capability and gradual stress buildup during bending. Among them, the 50SP:50HU composite shows a high stress level (185.81 MPa) combined with extended strain, reflecting enhanced stiffness and resistance to bending deformation, which aligns with its previously reported highest flexural modulus. The 100SP composite reaches about 145.25 MPa with moderate strain, suggesting relatively balanced stiffness and ductility due to the higher cellulose content of SP fibers. The 100HU composite shows the lowest flexural strength (114 MPa) and fails at comparatively low stress, indicating weaker reinforcement efficiency when HU fibers are used alone. The hybrid systems, therefore, demonstrate a synergistic effect, in which moderate HU incorporation enhances stress transfer and bending stiffness, while excessive HU content slightly reduces strength due to its lower intrinsic strength and greater brittleness.^21,33 Overall, the flexural stress-strain curves confirm that optimal mechanical performance under flexural loading is achieved at intermediate SP/HU ratios, particularly 75SP:25HU for strength and 25SP:75HU for stiffness.

Figure 6.

Flexural stress-strain curves of SP/HU fiber-reinforced composites.

Impact strength

The impact strength of SP/HU/epoxy composites shows a decreasing trend with rising HU fiber content, as illustrated in Figure 7. The composite made solely from SP fibers exhibits the maximum impact strength of 28.19 kJ/m², whereas the 100% HU fiber composite shows the minimum at 16.24 kJ/m². HCs demonstrates intermediate values, with 75SP:25HU yielding 24.71 kJ/m², 50SP:50HU at 23.52 kJ/m², and 25SP:75HU at 18.37 kJ/m². This gradual reduction in impact strength suggests that HU fibers are inherently less able to absorb energy during sudden impacts than SP fibers. SP fibers possess higher toughness and ductility, enabling them to deform more under impact and dissipate more energy before failure. In contrast, HU fibers are stiffer and more brittle, which makes them less effective at hindering crack propagation and absorbing impact energy.²¹ The nature of the fiber-matrix interaction also plays a crucial role; SP fibers bond more effectively with epoxy resin due to their surface properties, thereby improving stress transfer and enhancing impact resistance. HU fibers, however, may exhibit weaker adhesion at the fiber-matrix interface, leading to premature debonding or fiber pull-out during impact loading, thereby reducing the composite’s energy absorption capability.^15,21

Figure 7.

Impact strength of SP/HU fiber-reinforced composites.

The differences in chemical composition and microstructural characteristics between the two fibers further explain the observed trend. HU fibers contain more lignin and hemicellulose, which contribute to their rigidity but reduce their toughness. SP fibers, by contrast, have a higher cellulose content and a greater microfibril angle (19°), allowing greater internal deformation and enhancing impact resistance. Fibers with a higher microfibril angle tend to absorb more energy due to the inclined orientation of their microfibrils.^10,35 Therefore, the gradual substitution of SP fibers with HU fibers compromises the composite’s toughness, indicating that SP fibers are more effective in resisting impact-induced damage. The findings are consistent with the study of jute/snake plant/epoxy composites in Ref. 32.

Hardness behaviour

The Shore D hardness results of the fabricated SP/HU fiber-reinforced epoxy composites demonstrate a clearly increasing trend with higher HU fiber content (Figure 8). The 100SP composite exhibits a moderate hardness of 65.51, while the 100HU composite reaches a significantly higher hardness of 71.42 Shore D. Among the hybrids, the 25SP:75HU composite depicts the maximum hardness of 73.26 Shore D, suppressing both the 100HU and the other hybrids. The 50SP:50HU composite records 66.87 Shore D, while the 75SP:25HU composite shows the minimum (i.e., 62.34 Shore D) among the hybrids. This trend indicates that the hardness of the composites increases notably as the HU fiber content rises, especially beyond the 50% HU threshold. The increased hardness in HU-rich composites can be explained by the intrinsic stiffness and rigidity of HU fibers, which are generally higher than those of SP fibers. HU fibers likely possess a denser structure and greater resistance to surface deformation, contributing to the improved surface hardness of the composites.^21,23 Because hardness is closely related to a material’s resistance to localized plastic deformation, the presence of more HU fibers results in composites that are stiffer and less compliant under applied pressure. Additionally, HU fibers may have a higher lignin content than SP fibers, further enhancing their stiffness and surface resistance, resulting in higher Shore D values. On the other hand, SP fibers, while beneficial in improving tensile and impact performance, are relatively softer and more ductile. Their incorporation in large proportions results in composites with lower rigidity and hence lower hardness. Moreover, fiber-matrix interactions also play a role in the hardness behavior. Although SP fibers show better interfacial adhesion with the epoxy matrix, this advantage does not necessarily contribute to higher hardness. HU fibers, despite potentially weaker bonding, still contribute to higher surface resistance due to their inherent material properties.²³ Thus, the hardness enhancement in HU-rich composites is more a function of fiber stiffness than interfacial bonding. The sharp increase in hardness from 50SP:50HU to 25SP:75HU suggests threshold behavior, where a dominant phase of HU fiber leads to a more rigid, plastic-resistant composite surface, consistent with the known characteristics of stiff lignocellulosic fibers.

Figure 8.

Hardness of SP/HU fiber composites.

Surface morphology

The SEM micrographs of tensile-fractured specimens in Figure 9 illustrate failure morphologies consistent with their tensile properties. The 100HU composite (Figure 9(a)) displays extensive fiber pull-out, interfacial gaps, and fractured HU fibers, indicating poor fiber-matrix adhesion. The smooth surfaces of the pulled-out fibers indicate inadequate resin wetting, resulting in weak bonding and inefficient stress transfer, which explains the lower tensile strength despite its high stiffness. In contrast, the 100SP composite (Figure 9(b)) shows residual fiber breakage and minimal fiber pull-out, indicating strong fiber-matrix adhesion. Despite the presence of voids, the dominance of fiber rupture over debonding implies that the matrix-fiber interface is held firmly under load, requiring higher tensile stress to initiate fracture. Such morphology confirms efficient load transfer from matrix to fiber, supporting the maximum tensile strength observed (89.05 MPa).

Figure 9.

SEM micrograms of tensile-fractured specimens: (a) 100HU, (b) 100SP, (c) 75SP:25HU, (d) 25SP:75HU, and (e) 50SP:50HU.

The 75SP:25HU HCs (Figure 9(c)) present well-adhered fiber surfaces with visible shearing and matrix cracks, indicating that failure occurred predominantly through fiber rupture rather than interfacial separation. The presence of sheared fibers indicates that the fibers underwent plastic deformation and rupture under applied stress rather than simply detaching, implying strong interfacial adhesion. This behavior is attributed to the higher cellulose content, finer structure, and better resin wettability of SP fibers. The matrix tightly grips the fiber surface, restricting pull-out and forcing failure through internal shearing and fracture. This morphology implies improved interfacial bonding and more effective stress transfer compared to composites with higher HU content. On the other hand, the 25SP:75HU composite (Figure 9(d)) reveals widespread fiber pull-out, debonding gaps, and minimal fiber breakage, indicating poor matrix-fiber adhesion. The weaker interfacial interaction, due to the high HU content, results in limited stress transfer and premature failure, which corresponds to its minimum tensile strength among the hybrids. The 50SP:50HU composite (Figure 9(e)) shows a mix of fractured fibers and interfacial separation. This suggests intermediate fiber-matrix bonding, where partial load transfer occurs before failure. The coexistence of fiber breakage and pull-out indicates a mixed failure mode, consistent with its moderate tensile modulus and strength. Overall, increasing HU content enhances stiffness but reduces interfacial compatibility, while higher SP content improves adhesion and tensile performance.

Viscoelastic properties

Storage modulus (E′)

Figure 10 depicts the viscoelastic properties of manufactured SP/HU fiber composites. As shown in Figure 10(a), all the composites demonstrate a high E′ in the glassy region (30-80°C), followed by a significant drop as the temperature approaches the glass transition region (∼90-110°C), and finally a plateau in the rubbery region (>120°C). Among the samples, the 100HU composite exhibits the highest storage modulus, reaching approximately 6200 MPa at 30°C, indicating a significant increase in stiffness due to the inclusion of HU fibers. This is followed by 25SP:75HU (4910 MPa), 50SP:50HU (∼5750 MPa), 100SP (3998 MPa), and 75SP:25HU (4000 MPa). This trend of increasing modulus with increasing HU content suggests that HU fibers make a significant contribution to the composite’s structural rigidity. This enhancement is attributed to the intrinsically higher modulus of HU fibers, supported by their higher lignin and hemicellulose contents, which impart rigidity to the composite matrix.^36–38 Conversely, SP-rich composites exhibit relatively lower storage moduli, reflecting the lower intrinsic stiffness of SP fibers. In the glassy region (30-80°C), all composites exhibit high stiffness, with the modulus largely governed by fiber content and rigidity. HU-rich composites show the maximum E′ due to the intrinsic stiffness of HU fibers, while SP-rich composites are less stiff. Hybrid composites display intermediate behavior, reflecting the combined contributions of both fibers.³⁹ The increase in storage modulus from 75SP:25HU to 25SP:75HU hybrid composites further confirms that increasing the proportion of rigid HU fibers reinforces the composite’s elastic behavior. When the temperature exceeds 80-90°C, the storage modulus of all composites drops sharply due to enhanced molecular mobility of the epoxy matrix chains, indicating the onset of the glass transition region. This sharp reduction in E′ reflects the transition from a stiff, glassy phase to a more flexible, rubbery phase, where polymer chains gain mobility, and the material’s resistance to deformation decreases. The pronounced reduction in E′ around T_g reflects a shift from a stiff, glassy phase to a more flexible, rubbery phase, where polymer chains have sufficient thermal energy to move freely, thereby decreasing the material’s resistance to deformation.^40–42 However, HU-rich composites retained comparatively higher moduli in the rubbery region, indicating enhanced dimensional stability at elevated temperatures, which is critical for structural applications under thermal stress.^9,18

Figure 10.

Dynamic mechanical analysis of different composites: (a) Storage modulus, (b) loss modulus, and (c) damping factor.

Loss modulus (E″)

Figure 9(b) demonstrates the variation in loss modulus (Eʺ) of the SP/HU fiber-reinforced epoxy composites as a function of temperature. The incorporation of HU fibers significantly increased the Eʺ values, indicating enhanced energy dissipation capabilities of the hybrid and HU-rich composites. All samples exhibit a distinct single peak in their Eʺ curves, occurring between ∼90°C and 105°C, which corresponds to the T_g of the composites. The 100HU composite records the maximum peak value of approximately 790 MPa, followed by 25SP:75HU (710 MPa), 50SP:50HU (580 MPa), 100SP (490 MPa), and 75SP:25H (450 MPa), confirming a gradual increase in viscoelastic energy absorption with increasing HU content. This progressive increase in Eʺ correlates with the stiffness trend observed in the tensile modulus results, where the maximum tensile modulus (5.57 GPa) was also recorded for the 100HU composite.²¹ The inherently greater stiffness and lower elongation at break of HU fibers limit the mobility of polymer chains, thereby enhancing internal friction and increasing viscous energy dissipation within the composite. Furthermore, HU fibers, with higher lignin and hemicellulose content, are inherently more rigid and less ductile than SP fibers.²¹ Such rigidity restricts matrix deformation under dynamic loading and generates localized frictional losses, which are observed as increased Eʺ values.^21,36 Additionally, the increased heterogeneity introduced by HU fibers likely contributes to stress concentration and micro-yielding mechanisms, enhancing damping and frictional losses. However, it is essential to note that while the HU-rich composites show improved stiffness and energy dissipation, their tensile strength decreases due to their relatively lower intrinsic fiber strength, as demonstrated in the tensile test.^16,41 These findings suggest that although HU fibers contribute positively to viscoelastic damping and modulus, their contribution to ultimate tensile strength is limited due to their brittleness and potential interfacial incompatibility. The loss modulus data thus support the conclusion that HU fibers enhance the dynamic mechanical performance in terms of stiffness and damping but at the expense of tensile strength, reinforcing the trade-off between strength and stiffness in hybrid natural fiber composites.^40,43

Damping factor (tan δ)

Figure 9(c) illustrates the variation in the damping factor for the SP/HU fiber-reinforced epoxy composites, highlighting the viscoelastic behavior and the quality of interfacial bonding between the fiber and matrix. The tan δ peak (tan δ_max) represents the ratio of energy dissipated to energy stored during cyclic loading and is commonly used to estimate the T_g. In this study, the T_g was determined from the temperature corresponding to the tan δ_max value, ranging from 89.72 to 108.13°C across the composites. Among all the samples, the 25SP:75HU hybrids exhibit the maximum tan δ_max of 0.56, indicating the greatest damping ability and energy dissipation under dynamic loading. This elevated tan δ_max is often associated with a higher degree of polymer chain mobility and internal friction, possibly due to microstructural heterogeneities or interfacial slippage between the SP and HU fibers and the epoxy matrix.^43,44 It is followed by 100SP (0.49), 100HU (0.46), 50SP:50HU (0.44), and 75SP:25HU (0.38). The relatively lower tan δ values for SP-rich composites suggest stronger fiber-matrix interactions and reduced molecular mobility, which correlate with their improved dimensional stability. In contrast, the higher tan δ in 100SP indicates a more pronounced viscous response, despite its high SP content, possibly due to interfacial mismatches or stress concentrations introduced by fiber mixing at this ratio.^37,40,42 Moreover, the variation in T_g values supports the tan δ trend, with the maximum T_g observed for the 75SP:25HU hybrid (108.13°C), indicating delayed chain relaxation due to effective stress transfer. The minimum T_g (89.72°C) is observed for the 25SP:75HU composite, consistent with its higher tan δ, which suggests early molecular motion and lower thermal stability. Overall, the tan δ analysis confirms that increasing SP fiber content enhances the composite’s elastic recovery and thermal resistance by reducing energy dissipation, while HU-rich systems exhibit higher damping but at the expense of mechanical and thermal performance. Table 4 lists the key parameters of DMA for fabricated SP/HU composites.

Table 4.

Key factors obtained from DMA for SP/HU composites.

Parameters	100SP	75SP:25HU	50SP:50HU	25SP:75HU	100HU
tan δ_max	0.49	0.38	0.44	0.56	0.46
T_g (°C)	100.54	108.13	99.78	89.72	90.14

Note. T_g was determined from the peak value of tan δ (tan δ_max).

Water absorption behavior of fabricated composites

The water absorption behavior of the fabricated SP/HU fiber composites over time reveals a clear dependence on the fiber composition, with HU-rich composites absorbing significantly more moisture than SP-rich ones (Figure 11). The 100SP composite exhibits the minimum water uptake, beginning at 3.7% and gradually increasing to 11.3% after 120 h, while the 100HU composite shows the maximum absorption, reaching 17.6%. Regarding HCs, water absorption increases proportionally with HU content. For instance, 75SP:25HU starts at 4.8% and reaches 12.3%, whereas 25SP:75HU begins at 8.4% and ends at 15.8% after 120 h. This trend is primarily due to the inherent differences in moisture affinity between the two fibers. HU fiber, with a higher initial moisture content of 14.28%, is more hydrophilic than SP fiber, which has a moisture content of 10.5%. HU fibers contain more hemicellulose and exhibit a more open, porous microstructure, facilitating greater water uptake.^21,32

Figure 11.

Water absorption behavior of different composites.

Additionally, weaker interfacial bonding in HU-epoxy may allow more interfacial voids or capillaries, promoting easier water ingress. In contrast, SP fibers, with their relatively lower moisture content and better fiber-matrix adhesion, restrict water diffusion and reduce the extent of water absorption.^13,27 The epoxy matrix itself resists water penetration to some extent, but the presence and proportion of hydrophilic NFs significantly influence the overall water uptake. The water absorption curves display a typical Fickian diffusion behavior, with rapid initial uptake followed by a slower approach to equilibrium after ∼60 h, reflecting saturation of accessible fiber and matrix sites. The diffusion rate is influenced by fiber proportion, fiber microstructure, and void content within the composites. Hybrid composites exhibit intermediate absorption characteristics, demonstrating that fiber hybridization can effectively tune moisture resistance.^13,45,46 The results clearly show that the higher the HU fiber content, the higher the water absorption, making SP-rich composites more dimensionally stable and better suited for moisture-sensitive applications.

FTIR analysis

The FTIR spectrum of the fabricated composite specimen (50SP:50HU), shown in Figure 12, exhibits several characteristic absorption bands corresponding to the functional groups of the lignocellulosic fibers and the epoxy matrix. A broad and intense absorption band appearing in the region of approximately 3300–3500 cm⁻¹ is attributed to the O-H stretching vibration of hydroxyl groups present in cellulose, hemicellulose, and lignin of the natural fibers.^47,48 This broad peak also indicates the presence of intermolecular hydrogen bonding within the composite structure. The absorption band observed around 2900–2950 cm⁻¹ corresponds to the C-H stretching vibration of aliphatic -CH₂ and -CH₃ groups associated with the lignocellulosic components of the fibers and the epoxy matrix.⁴⁹ A noticeable peak around 1630–1650 cm^-1 can be attributed to the O-H bending vibration of absorbed moisture in the natural fibers and may also correspond to the aromatic C = C stretching of lignin. Furthermore, the absorption band observed near 1500–1520 cm-1 is associated with aromatic skeletal vibrations of lignin, confirming its presence in the SP and HU fibers.^50,51 The peak appearing around 1240–1260 cm⁻¹ corresponds to the C-O stretching vibration of lignin and hemicellulose and may also correspond to ether linkages present in the epoxy network. In addition, a strong absorption band around 1020-1060 cm^-1 is attributed to the C-O-C stretching vibrations of cellulose and hemicellulose present in the natural fibers. The bands observed below 900 cm⁻¹ are generally associated with β-glycosidic linkages of cellulose.⁵¹ Overall, the FTIR spectrum of the 50SP:50HU composite confirms the coexistence of lignocellulosic fiber constituents and the epoxy matrix, with the identified O-H, C-H, C-O, C-O-C, and aromatic lignin-related bands indicating successful incorporation of the natural fibers and possible intermolecular interactions within the composite structure.

Figure 12.

FTIR analysis of the fabricated composite specimen (50SP:50HU).

Comparison of mechanical properties of the current study with existing literature

Table 5 compares the mechanical properties of the SP/HU fiber composites fabricated in this study with those reported in the literature. The 100HU composite has a tensile modulus of 5.57 GPa, which is about 678% greater than banana/jute/epoxy (0.72 GPa),⁵² 271% higher than snake plant/banana/epoxy (1.5 GPa),²³ and surpasses even the stiff date palm/kenaf/epoxy composite (3.7 GPa)⁵³ by 50%. Among hybrids, the 25SP:75HU composite also exhibits a high modulus of 5.06 GPa, outperforming many natural fiber systems reported in the literature. In terms of tensile strength, the 100SP composite exhibits a maximum value of 89.05 MPa, which is significantly 371% higher than banana/jute/epoxy (18.96 MPa),⁵² 143% higher than snake plant/banana/epoxy (36.65 MPa),²³ and even outperforms high-strength systems (e.g., hemp/flax/epoxy (44.17 MPa))⁵⁴ and jute/hemp/epoxy (42.19 MPa)⁵⁴ by over 100%.

Table 5.

Comparison of mechanical properties with reported literature.

Types of composites	Fiber weight percentage (wt%)	Tensile modulus (GPa)	Tensile strength (MPa)	Flexural modulus (GPa)	Flexural strength (MPa)	Impact strength (kJ/m²)	Refs.
Banana/jute/epoxy	50/50	0.72	18.96	9.1	59.84	18.23	⁵²
Snake plant/banana/epoxy	28.57	1.50	36.65	0.26	60.70	19.32	²³
Hemp/flax/epoxy	25	1.56	44.17	0.74	46.6	4.18	⁵⁴
Jute/hemp/epoxy	25	1.49	42.19	0.78	86.6	6.93	⁵⁴
Date palm/kenaf/epoxy	50	3.7	21.71	4.4	50.26	-	⁵³
Coir/glass/epoxy	10/20/70	1.7	17	-	65.27	-	⁵⁶
S-glass/jute/epoxy	35/15/50	0.06	18.65	-	-	30	⁵⁵
100SP	27.24	3.79	89.05	4.02	145.25	28.19	This study
75SP:25HU	28.76	2.67	85.60	4.71	236	24.71	This study
50SP:50HU	29.12	3.64	59.72	3.55	185.81	23.32	This study
25SP:75HU	29.86	5.06	54.81	7.20	185.01	18.37	This study
100HU	28.65	5.57	61.96	5.82	114	16.24	This study

The 25SP:75HU hybrid exhibits the maximum flexural modulus of 7.20 GPa, followed by 100HU with 5.82 GPa, both surpassing all literature-reported composites listed here except banana/jute/epoxy.⁵² The flexural strength of the 100SP composite (145.25 MPa) is markedly 143% higher than jute/hemp/epoxy (86.6 MPa),⁵⁴ and over 142% greater than banana/jute/epoxy (59.84 MPa),⁵² indicating enhanced bending resistance. Likewise, 75SP:25HU and 50SP:50HU composites exhibit flexural strengths of 236 MPa and 185.81 MPa, respectively, which are approximately 3-4 times higher than reported values in the literature, demonstrating their superior structural integrity and performance. Regarding impact strength, 100SP achieves 28.19 kJ/m², which is over 54% higher than snake plant/banana/epoxy (19.32 kJ/m²),²³ 6.7 times greater than hemp/flax/epoxy (4.18 kJ/m²).⁵⁴ The hybrid composites, especially 75SP:25HU (24.71 kJ/m²) and 50SP:50HU (23.32 kJ/m²), also exceed the impact performance of nearly all the composites in the literature except for S-glass/jute/epoxy (30 kJ/m²),⁵⁵ highlighting their excellent energy absorption capabilities.

Overall, the results reveal that both 100SP and hybrid SP/HU composites, particularly 75SP:25HU, demonstrate superior mechanical properties compared to conventional natural and natural-synthetic fiber composites. Their balanced combination of high tensile stiffness and strength, flexural resistance, and impact energy absorption suggests their potential for structural and semi-structural applications such as automotive parts, protective gear, and sustainable composite panels.

Conclusions

This study fabricated SP/HU fiber-reinforced epoxy composites with varying fiber contents and evaluated their mechanical, viscoelastic, and morphological properties. The conclusions based on the key findings of this study are summarized as follows.

• The tensile and flexural moduli of the HCs increase with higher HU fiber content, while greater SP fiber loading enhances their tensile and flexural strengths. Among hybrids, the 25SP:75HU composite has the maximum tensile (5.06 GPa) and flexural modulus (7.20 GPa), whereas the 75SP:25HU composite achieves the maximum tensile (85.60 MPa) and flexural strength (236 MPa). Additionally, the 100SP composite shows a notable tensile strength of 89.05 MPa, and the 100HU composite records a prominent tensile modulus of 5.57 GPa.

• The impact strength of the composites improves with increasing SP fiber content. The 100SP composite exhibits the maximum impact strength (28.19 kJ/m²), while the 75SP:25HU hybrid also shows a significant impact strength of 24.31 kJ/m² among the hybrids.

• With the increase in HU fiber content, the hardness of hybrids is enhanced, and 25SP:75HU has the maximum hardness of 73.26 Shore D, followed by the 100 HU composite (71.42 Shore D).

• Fractographic analysis reveals that composites with greater SP fiber content demonstrate enhanced fiber-matrix bonding, more fiber shearing, and less fiber pull-out than those containing higher proportions of HU fiber.

• DMA results indicate that composites with higher HU fiber content exhibit increased storage modulus, loss modulus, and damping factor, whereas those with higher SP fiber content display an improved T_g.

• HU-rich composites absorb considerably more moisture than SP-rich ones, with the 100SP composite showing the minimum water uptake of 11.3% after 120 h.

• FTIR verified the successful incorporation of SP and HU fibers into the epoxy matrix and indicated good interfacial interaction through characteristic lignocellulosic bands and broad O-H absorption.

Composites with higher HU fiber content offer enhanced stiffness, hardness, and damping, making them suitable for structural applications where rigidity and dimensional stability are essential, such as automotive interior panels, wall claddings, and furniture components. On the other hand, composites with increased SP fiber content demonstrate superior strength, impact resistance, and lower moisture absorption, rendering them ideal for load-bearing applications such as protective casings, helmets, sporting goods, and construction panels.

For future work, further optimization through surface treatments (e.g., alkali, silane, or acetylation), compatibilizer incorporation, and hybridization with bio-based fillers (e.g., eggshell, fish scale, mollusk shell, and plant seed-derived fillers) may further improve interfacial adhesion, thermal stability, and barrier properties. In addition, thermal analysis (TGA/DTG and DSC), elemental characterization (EDS), life cycle assessment, biodegradability studies, and performance evaluation under real-service environmental conditions would provide a more comprehensive validation of their practical applicability for commercial use.

Footnotes

ORCID iD

Md Zillur Rahman

Author contributions

Barshan Dev: Conceptualization, Methodology, Investigation, Supervision, Writing-Original Draft; Md Ashikur Rahman: Methodology, Investigation, Visualization, Writing-Original Draft; Md Zillur Rahman: Methodology, Validation, Supervision, Writing-Review and Editing; Shanta Debnath: Investigation, Formal Analysis; Bithi Islam Mou: Resources, Data Curation; Hamim Bin Rashid: Resources, Data Curation.

Funding

The authors declare that no funds, grants, or other support were received for this research.

Declaration of conflicting interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data Availability Statement

Data will be made available on request.*

References

Saha

Das

Rahman

. Hybridization in natural fiber composites: enhanced performance and sustainability. Compos Part B Eng 2025; 308: 112986. https://doi.org/10.1016/j.compositesb.2025.112986

Jiang

Pan

, et al. Durability of basalt fibers, glass fibers, and their reinforced polymer composites in artificial seawater. Polym Compos 2022; 43: 1961–1973. https://doi.org/10.1002/pc.26511

Jagadeesh

Puttegowda

Boonyasopon

, et al. Recent developments and challenges in natural fiber composites: a review. Polym Compos 2022; 43: 2545–2561. https://doi.org/10.1002/pc.26619

Rahman

Jayaraman

Mace

. Impact energy absorption of flax fiber‐reinforced polypropylene composites. Polym Compos 2018; 39: 4165–4175. https://doi.org/10.1002/pc.24486

Rahman

. Damping under varying frequencies, mechanical properties, and failure modes of Flax/Polypropylene composites. Polymers 2023; 15: 1042. https://doi.org/10.3390/polym15041042

Mulla

Norizan

Rawi

NFM

, et al. A review of fire performance of plant-based natural fibre reinforced polymer composites. Int J Biol Macromol 2025; 305: 141130. https://doi.org/10.1016/j.ijbiomac.2025.141130

Jenish

Sahayaraj

Suresh

, et al. Analysis of the hybrid of Mudar/Snake grass fiber-reinforced epoxy with Nano-Silica Filler composite for structural application. Adv Mater Sci Eng 2022; 2022: 1–10. https://doi.org/10.1155/2022/7805146

Mylsamy

Shanmugam

SKM

Aruchamy

, et al. A review on natural fiber composites: polymer matrices, fiber surface treatments, fabrication methods, properties, and applications. Polym Eng Sci 2024; 64: 2345–2373. https://doi.org/10.1002/pen.26713

Jayaseelan

Mathivanan

Xavier

, et al. Effect of palm sprout fiber and palm kernal de-oiled cake cellulose on mechanical, fatigue, and DMA properties of toughened vinyl ester composites. Biomass Convers Biorefinery 2025; 15: 3215–3222. https://doi.org/10.1007/s13399-023-04918-y

10.

Dev

Rahman

Repon

, et al. Mapping the progress in natural fiber reinforced composites: preparation, mechanical properties, and applications. Polym Compos 2023; 44: 3748–3788. https://doi.org/10.1002/pc.27376

11.

Xin

Zhang

Guo

, et al. Design of novel glass fiber reinforced polypropylene cable-anchor component and its long-term properties exposed in alkaline solution. Case Stud Constr Mater 2024; 20: e03383. https://doi.org/10.1016/j.cscm.2024.e03383

12.

Tazwar

Antora

Rahman

. Functionalization strategies for sustainable plant fiber composites: a comprehensive review of techniques, performance, and future directions. J Mater Res Technol. 2025; 38: 1083–1102.

13.

Dev

Rahman

, et al. Fabrication, properties, and morphologies of hybrid polymer composites reinforced with jute and Rosa hybrida fibers. J Mater Sci 2024; 59: 15676–15694. https://doi.org/10.1007/s10853-024-10119-3

14.

Chandramohan

Sathish

Kumar

, et al. Mechanical and thermal properties of jute/aloevera hybrid natural fiber reinforced composites mechanical and Thermal properties of jute/aloevera hybrid natural fiber reinforced composites. Proc Int Conf Recent Trends Mech Mater Eng AIP Conf Proc 2020; 2283, 020084–1–020084–11.

15.

Kumar

Muthukrishnan

Sahayaraj

. Experimental investigation on jute/snake grass/kenaf fiber reinforced novel hybrid composites with annona reticulata seed filler addition Experimental investigation on jute/snake grass/kenaf fi ber reinforced novel hybrid composites with annona re. Mater Res Express 2022; 9: 1–13.

16.

Gupta

. Effect of frequencies on dynamic mechanical properties of hybrid jute/sisal fibre reinforced epoxy composite. Adv Mater Process Technol 2017; 3: 651–664. https://doi.org/10.1080/2374068X.2017.1365443

17.

Pappu

Pickering

Thakur

. Manufacturing and characterization of sustainable hybrid composites using sisal and hemp fibres as reinforcement of poly (lactic acid) via injection moulding. Ind Crops Prod 2019; 137: 260–269. https://doi.org/10.1016/j.indcrop.2019.05.040

18.

Jawaid

Awad

Fouad

, et al. Improvements in the thermal behaviour of date palm/bamboo fibres reinforced epoxy hybrid composites. Compos Struct 2021; 277: 114644. https://doi.org/10.1016/j.compstruct.2021.114644

19.

Kiruthika

. A review of leaf fiber reinforced polymer composites. J Eng Appl Sci 2024; 71: 24. https://doi.org/10.1186/s44147-024-00365-2

20.

Wantahe

Bigambo

. Review of Sansevieria Ehrenbergii (SE) leaf fibers and their potential applications. Cellulose 2023; 30: 9241–9259. https://doi.org/10.1007/s10570-023-05481-5

21.

Shibly

MAH

Islam

Hoque

MMU

, et al. Hylocereus undatus plant's stem agro-waste: a potential source of natural cellulosic fiber for polymer composites. Sustain Chem Pharm 2024; 41: 101692. https://doi.org/10.1016/j.scp.2024.101692

22.

Sathishkumar

Navaneethakrishnan

Shankar

, et al. Mechanical properties of randomly oriented snake grass fiber with banana and coir fiber-reinforced hybrid composites. J Compos Mater 2013; 47: 2181–2191. https://doi.org/10.1177/0021998312454903

23.

Dev

Rahman

, et al. Mechanical properties of unidirectional banana/snake plant fiber-reinforced epoxy hybrid composites: experimental and numerical analyses. Polym Bull 2025; 82: 3145–3174. https://doi.org/10.1007/s00289-025-05658-x

24.

Singh

Rani

. Extraction and processing of fiber from Sesbania aculeata (Dhaincha) for preparation of needle punched nonwoven fabric. Natl Acad Sci Lett 2013; 36: 489–492. https://doi.org/10.1007/s40009-013-0166-7

25.

Venkatesan

Vignesh

Nagarajan

, et al. Extraction and characterization of agricultural discarded Sesbania Aculeata stem waste as potential alternate for synthetic fibers in polymer composites. J Nat Fibers 2022; 19: 10601–10615. https://doi.org/10.1080/15440478.2021.2002756

26.

Zahrotin

Juwono

Siregar

, et al. Analyzing the influences of fabrication methods on the mechanical properties of Sumberejo kenaf/epoxy sandwich composites with polyurethane foam core. In: ITM Web of Conferences. EDP Sciences, 2024, p. 1021.

27.

Mohammed

Jawad

AJM

Mohammed

, et al. Challenges and advancement in water absorption of natural fiber-reinforced polymer composites. Polym Test 2023; 124: 108083. https://doi.org/10.1016/j.polymertesting.2023.108083

28.

Sathishkumar

Navaneethakrishnan

Shankar

. Tensile and flexural properties of snake grass natural fiber reinforced isophthallic polyester composites. Compos Sci Technol 2012; 72: 1183–1190. https://doi.org/10.1016/j.compscitech.2012.04.001

29.

Lai

Sapuan

Ahmad

, et al. Mechanical and electrical properties of coconut coir fiber-reinforced polypropylene composites. Polym Plast Technol Eng 2005; 44: 619–632. https://doi.org/10.1081/PTE-200057787

30.

Jenish

Sahayaraj

Suresh

31.

Ravishankar

Arya

Dhakal

, et al. Assessing the mechanical performance of natural fiber thermoplastic composite sandwiches for advanced applications. Results Mater 2024; 23: 100600. https://doi.org/10.1016/j.rinma.2024.100600

32.

Dev

Khan

Rahman

, et al. Mechanical and thermal properties of unidirectional jute/snake plant fiber-reinforced epoxy hybrid composites. Ind Crops Prod 2024; 218: 118903. https://doi.org/10.1016/j.indcrop.2024.118903

33.

Hosseini

Raji

. Improved double impact and flexural performance of hybridized glass basalt fiber reinforced composite with graphene nanofiller for lighter aerostructures. Polym Test 2023; 125: 108107. https://doi.org/10.1016/j.polymertesting.2023.108107

34.

Turjo

SMKS

Hossain

Rana

, et al. Mechanical characterization and corrosive impacts of natural fiber reinforced composites: an experimental and numerical approach. Polym Test 2023; 125: 108108. https://doi.org/10.1016/j.polymertesting.2023.108108

35.

Jawaid

Khalil

HPSA

Bakar

, et al. Effect of jute fibre loading on the mechanical and thermal properties of oil palm-epoxy composites. J Compos Mater 2013; 47: 1633–1641. https://doi.org/10.1177/0021998312450305

36.

Rahman

Ndiaye

Weclawski

, et al. Dynamic mechanical analysis of Borassus husk fiber reinforced epoxy: evaluating suitability for advanced aerospace and automotive applications. Eng Reports 2025; 7: e70102. https://doi.org/10.1002/eng2.70102

37.

Kaliappan

Velumayil

Natrayan

, et al. Mechanical, DMA, and fatigue behavior of Vitis vinifera stalk cellulose Bambusa vulgaris fiber epoxy composites. Polym Compos 2023; 44: 2115–2121. https://doi.org/10.1002/pc.27228

38.

Guo

Cao

Ren

, et al. Mechanical, dynamic mechanical and thermal properties of TiO2 nanoparticles treatment bamboo fiber-reinforced polypropylene composites. J Mater Sci 2021; 56: 12643–12659. https://doi.org/10.1007/s10853-021-06100-z

39.

Vinu Kumar

Senthil Kumar

Siddhi Jailani

, et al. Mechanical, DMA and Sound Acoustic behaviour of flax woven fabric reinforced Epoxy composites. Mater Res Express 2020; 7: 085302. https://doi.org/10.1088/2053-1591/abaea5

40.

Venkategowda

Manjunatha

Anilkumar

. Dynamic mechanical behavior of natural fibers reinforced polymer matrix composites–A review. Mater Today Proc 2022; 54: 395–401. https://doi.org/10.1016/j.matpr.2021.09.465

41.

Parameswaranpillai

Dominic

CDM

Muthukumar

, et al. Characterization of viscoelastic properties of biocomposites by dynamic mechanical analysis: an overview. Vib Damping Behav Biocomposites. 2022; 19: 157–169.

42.

Dev

Rahman

Repon

, et al. Recent progress in thermal and acoustic properties of natural fiber reinforced polymer composites: preparation, characterization, and data analysis. Polym Compos 2023; 44: 7235–7297. https://doi.org/10.1002/pc.27633

43.

Ashok

Srinivasa

Basavaraju

. Dynamic mechanical properties of natural fiber composites—a review. Adv Compos Hybrid Mater 2019; 2: 586–607. https://doi.org/10.1007/s42114-019-00121-8

44.

Rahman

. Mechanical and damping performances of flax fibre Composites–A review. Compos Part C Open Access 2021; 4: 100081. https://doi.org/10.1016/j.jcomc.2020.100081

45.

Dev

Rahman

Nabi

. Eggshell bio-filler integration in jute/banana fiber-reinforced epoxy hybrid composites: fabrication and characterization. J Appl Polym Sci 2024; 141: 1–14. https://doi.org/10.1002/app.56155

46.

Guo

Xian

, et al. Hygrothermal resistance of pultruded carbon, glass and carbon/glass hybrid fiber reinforced epoxy composites. Constr Build Mater 2022; 315: 125710. https://doi.org/10.1016/j.conbuildmat.2021.125710

47.

Babu

Singh

Prabha

, et al. Mechanical, machinability and water absorption properties of novel kenaf fiber, glass fiber and graphene composites reinforced with epoxy. Sci Rep 2024; 14: 1–21. https://doi.org/10.1038/s41598-024-81314-0

48.

Pang

Ismail

Abu Bakar

. Mechanical, morphological, and thermal properties of kenaf filled linear low-density polyethylene/poly(vinyl alcohol) composites: effect of chemical treatment. J Vinyl Addit Technol 2018; 24: E164–E171. https://doi.org/10.1002/vnl.21622

49.

Rahman

Chowdhury

Shuvho

MBA

, et al. Fabrication and characterization of jute/cotton bio-composites reinforced with eggshell particles. Polym Bull 2023; 80: 931–957. https://doi.org/10.1007/s00289-021-04049-2

50.

Merzoug

Bouhamida

Sereir

, et al. Quasi-static and dynamic mechanical thermal performance of date palm/glass fiber hybrid composites. J Ind Text 2022; 51: 7599S–7621S. https://doi.org/10.1177/1528083720958036

51.

Dev

Rahman

Nipu

, et al. Hybrid bamboo-banana-cotton sandwich composites for lightweight roofing: mechanical, thermal and microstructural analyses. Hybrid Adv 2025; 11: 100537. https://doi.org/10.1016/j.hybadv.2025.100537

52.

Boopalan

Niranjanaa

Umapathy

. Study on the mechanical properties and thermal properties of jute and banana fiber reinforced epoxy hybrid composites. Compos Part B Eng 2013; 51: 54–57. https://doi.org/10.1016/j.compositesb.2013.02.033

53.

Ghori

Rao

Rajhi

. Investigation of physical, mechanical properties of treated date palm Fibre and Kenaf Fibre reinforced epoxy hybrid composites. J Nat Fibers 2023; 20: 2145406. https://doi.org/10.1080/15440478.2022.2145406

54.

Chaudhary

Bajpai

Maheshwari

. Studies on mechanical and morphological characterization of developed Jute/Hemp/Flax reinforced hybrid composites for structural applications studies on mechanical and morphological characterization of developed Jute/Hemp/Flax reinforced hybrid Co. J Nat Fibers 2018; 15: 80–97. https://doi.org/10.1080/15440478.2017.1320260

55.

Sriranga

Kirthan

Ananda

. The mechanical properties of hybrid laminates composites on epoxy resin with natural jute fiber and S-glass fibers. Mater Today Proc 2021; 46: 8927–8933. https://doi.org/10.1016/j.matpr.2021.05.363

56.

Bhagat

Biswas

Dehury

. Physical, mechanical, and water absorption behavior of coir/glass fiber reinforced epoxy based hybrid composites. Polym Compos 2014; 35: 925–930. https://doi.org/10.1002/pc.22736