Durability of silane-modified Kender-N Fiber/PVC core/biochar reinforced vinyl ester hybrid composites

Abstract

The present study investigates the environmental durability, mechanical performance, creep behaviour, and moisture resistance of MPS silane-treated Kender-N fiber/PVC core reinforced vinyl ester hybrid composites incorporated with nutmeg husk-derived biochar. The composites were fabricated using hand layup followed by compression molding and post-curing to obtain structurally stable laminates. Surface treatment using (3-methacryloxypropyl)trimethoxysilane (MPS) was applied to both the Kender-N fibers and PVC core to enhance interfacial adhesion, moisture resistance, and long-term durability. The developed composites were exposed to different aging environments, including warm water, tap water, distilled water, and seawater immersion for 7 days to evaluate environmental degradation behaviour. The results revealed that moisture exposure significantly affected the mechanical and viscoelastic properties due to matrix plasticization, hydrothermal swelling, interfacial debonding, and microcrack formation. Among all aging conditions, the distilled water-aged composite exhibited superior performance, showing tensile, flexural, and impact strength improvements of approximately 10.3%, 12.1%, and 13.0%, respectively, compared with the warm water-aged composite. In creep analysis, the distilled water-aged composite showed the lowest creep strain value of 0.00791 at 15,000 s, corresponding to nearly 15.9% lower deformation. Water absorption was also reduced by approximately 32.5%, indicating enhanced dimensional stability and resistance to moisture diffusion. FESEM analysis confirmed improved matrix encapsulation and reduced interfacial degradation due to silane treatment. The developed hybrid composites demonstrate strong potential for lightweight marine, automotive, transportation, and sustainable infrastructure applications requiring long-term environmental durability and mechanical reliability.

Keywords

MPS silane treatment vinyl ester composites Kender-N fiber PVC core biochar reinforcement environmental aging creep behaviour water absorption interfacial adhesion hybrid composites

Introduction

Modern engineering applications have seen a dramatic increase in the use of polymer matrix composites due to its low cost, high strength-to-weight ratio, and lightweight nature.¹ Due to their higher mechanical strength and durability, synthetic reinforcements including glass, carbon, and aramid fibres have traditionally been widely used. Nevertheless, there are a number of drawbacks to these materials that need to be considered, including their high production cost, inability to regenerate, and impact on the environment.^2,3 Because of its biodegradability, low density, renewability, and reduced environmental effect, natural fibre reinforced composites have garnered a lot of interest as sustainable alternatives. Natural fibres have a number of advantages, including less processing energy usage, higher specific strength, and less environmental impact.⁴ Natural fibres have its downsides, though, such being able to soak up moisture and not being as durable when exposed to the elements over the long haul.⁵ Table 1 compares the important mechanical, environmental, and durability-related characteristics of natural and synthetic fiber composites.

Table 1.

Comparative analysis of natural and synthetic fibers.

Aspect	Natural fibers	Synthetic fibers	Significance
Density	Low	Higher	Lightweight structures
Tensile strength	Moderate	High	Better load-bearing in synthetic fibers
Fatigue resistance	Moderate	Excellent	Synthetic fibers show better cyclic durability
Moisture absorption	High	Low	Natural fibers require surface treatment
Energy consumption	1–2 MJ/kg	10–15 MJ/kg	Lower processing energy for natural fibers
CO2 emissions	60–80% lower	Higher	Reduced carbon footprint
Renewability	Renewable and biodegradable	Non-renewable	Environmentally sustainable
End-of-life impact	Biodegradable	Persistent waste	Lower environmental pollution

The mechanical and physical qualities of natural fibres such as hemp, areca, flax, jute, and kenaf make them ideal for use in polymer composites. Stiffness, dimensional stability, and stress transmission capabilities are all further improved through hybridisation of natural fibres.⁶ Natural fibres that are routinely utilised and their qualities that are important to polymer composite applications are presented in Table 2.

Table 2.

Common natural fibers and their properties.

Fiber type	Tensile strength (MPa)	Density (g/cm³)	Major advantages
Hemp	550–900	1.48	High stiffness and strength
Jute	300–800	1.30	Economical and biodegradable
Flax	500–1500	1.40	Excellent specific strength
Kenaf	295–930	1.20	Good impact resistance
Areca	150–400	1.10	Lightweight and low cost
Kender-N hybrid fiber	500–950	1.25–1.40	Balanced strength and sustainability

An eco-friendly hybrid composite was created in this study by combining a vinyl ester matrix with silane-treated Kender-N hybrid fibre, recycled PVC core, and biochar from nutmeg roots. Its high cellulose content, low weight, and excellent tensile strength led to the selection of Kender-N fibre.⁷ The purpose of the silane treatment was to decrease hydrophilic behaviour and enhance fiber-matrix interfacial bonding. Reinforced structural rigidity and support for circular economy activities were provided by recycled PVC core material, while thermal stability, stiffness, and wear resistance were improved with the introduction of nutmeg husk biochar, a sustainable carbon-rich filler.⁸ As shown in Table 3, each reinforcement material’s selection criteria and functional purpose are summarised.

Table 3.

Selection and role of reinforcement materials in the composite.

Material	Reason for selection	Role in composite
Kender-N hybrid fiber	High cellulose content, lightweight, biodegradable	Improves tensile and flexural strength
Nutmeg husk biochar	Waste-derived, thermally stable, low cost	Enhances creep and wear resistance
Recycled PVC core	Lightweight and recyclable	Improves structural rigidity and toughness

For example, Agumba et al.⁹ demonstrated the jute fiber and lignin filler reinforced composite and showed the improvement of flexural strength by 37.7% and modulus by 72.9%. Similarly, Muthukumarasamy et al.¹⁰ examined the novel caryota fiber and biochar filler reinforced composite and determined the highest tensile strength of 98 MPa. In related investigation, Elumalai et al.¹¹ examined the silane treated PET core and ixtle fiber reinforced epoxy composite and determined the improved mechanical and fatigue performance of the composite. Likewise, Ci et al.¹² evaluated the ageing effects on pultruded fiber reinforced epoxy composite and determined the improvement of bending strength by 6% and impact strength by 12.8%.

While there are many benefits to using natural fibre composites, one major issue is how well they hold up when exposed to dampness. Swelling, matrix deterioration, fibre debonding, and diminished mechanical performance are all possible outcomes of water absorption. When testing the accelerated environmental durability of composites, warm water ageing is a typical method used. Natural fibre reinforced composites have had their degrading behaviour studied in a variety of ageing conditions, including freshwater, saltwater, and distilled water.

In light of previous work developing a silane-treated Kender-N fibre reinforced vinyl ester composite, the current study seeks to understand how this material reacts to warm water ageing. This research presents a novel approach by utilising a vinyl ester composite system that combines silane surface modified Kender-N hybrid fibre, recycled PVC core, and nutmeg husk biochar. Then, after exposing it to prolonged ageing, its mechanical, creep, and water absorption behaviour are evaluated. Marine, automotive, construction, and lightweight structural applications are anticipated to benefit from the proposed composite’s sustainable and long-lasting performance.

Experimental method

Raw material

Atul Ltd. of India supplied the vinyl ester resin that was utilised as the matrix material. The resin’s high mechanical strength, resistance to corrosion, dimensional stability, and exceptional adhesive properties led to its selection. Vinyl ester resin is commonly used in structural composites due to its high tensile strength (70–85 MPa), flexural strength (120–150 MPa), density (1.04–1.10 g/cm³), elongation at break (4–6%), and tensile modulus (3–5 GPa). The resin’s low water absorption, superior thermal stability, and excellent chemical resistance make it a great choice for composites that are exposed to weather conditions and age. Located in Chennai, India, Core Composites Pvt. Ltd Supplied the 2 mm thick PVC core. Also sourced from Kerala, India’s Kancor Ingredients Ltd, the biochar was made from nutmeg fruit husks. Go Green Products of Tamil Nadu, India, supplied the Kender-N hybrid mat fibre, which has a density of 1.2 g/cm³. The surface modification of the fibre and PVC core was accomplished using ethanol, acetic acid, and a silane coupling agent that were procured from Merck Life Science India, India.

Biochar extraction from Nutmeg husk

Nutmeg husk residues were collected from local processing units, where they are routinely discarded as agricultural waste. In the laboratory, the collected husks were first manually sorted to remove foreign impurities and then thoroughly washed with distilled water to eliminate adhering dust and surface contaminants. The cleaned material was oven-dried in a laboratory hot-air oven at 100°C for 4 h to reduce moisture content to a stable level. After drying, the husks were mechanically size-reduced using a high-speed laboratory grinder to obtain coarse powder. The powdered biomass was then subjected to controlled pyrolysis in a laboratory muffle furnace (with inert atmosphere maintained using continuous nitrogen gas purging) at 700°C for 2 h.¹³ The heating process was carried out under a regulated thermal program to ensure gradual carbonization and minimize oxidation. Following pyrolysis, the resulting biochar was allowed to cool naturally inside the furnace under nitrogen flow to prevent sudden oxidation. The obtained biochar was subsequently re-ground to achieve a uniform fine particulate form suitable for composite applications. Figure 1 illustrates the laboratory-scale conversion process of nutmeg husk into biochar.

Figure 1.

Extraction of biochar particles from nutmeg husk.

The pore structure and nitrogen adsorption-desorption isotherm of the synthesized biochar particles were analyzed using BET analysis and are presented in Figure 2. The biochar exhibited a type IV isotherm with an H3 hysteresis loop, indicating the presence of mesoporous structures with slit shaped pores. The adsorption volume gradually increased with relative pressure and showed a sharp rise at higher relative pressure regions due to capillary condensation within the pores. The biochar particles possessed a heterogeneous mesoporous structure with pore sizes predominantly in the range of 2–20 nm and a specific surface area of approximately 85 m²/g. The relatively high surface area and porous morphology of the biochar promote enhanced interfacial interaction and mechanical interlocking with the polymer matrix, which can contribute to improved mechanical and thermal performance of the composites.

Figure 2.

BET plot of biochar.

Surface modification

Surface modification was performed using (3-methacryloxypropyl)trimethoxysilane (MPS) as a silane coupling agent to improve the interfacial adhesion between the PVC core, Kender-N fibers, and the polymer matrix. The silane solution was prepared by dissolving 2 wt.% MPS in an ethanol–water medium (95:5 ratio), followed by the addition of a small quantity of acetic acid to maintain the solution pH at approximately 4–5, which facilitates hydrolysis of the methoxy groups into reactive silanol groups. The solution was continuously stirred for 30 min to ensure homogeneous hydrolysis and uniform dispersion of silane molecules throughout the treatment medium. To achieve effective and uniform surface coating, the PVC core and Kender-N fibers were completely immersed in the freshly prepared silane solution and maintained under gentle mechanical stirring during treatment. This continuous agitation prevented localized silane accumulation and promoted even adsorption of silane molecules over the entire surface of the reinforcements. The treatment duration of 3 h was selected based on preliminary trials and previously reported studies, where sufficient exposure time was found necessary to maximize silane condensation and interfacial interaction without causing excessive surface deposition or agglomeration.¹⁴ During immersion, the hydrolyzed MPS molecules chemically interacted with hydroxyl-containing sites present on the fiber surface, forming siloxane linkages and creating an active interfacial layer that enhances compatibility with the polymer matrix. After treatment, the modified materials were rinsed thoroughly with distilled water to remove loosely attached silane residues and unreacted species. Finally, the treated PVC core and fibers were dried in a hot-air oven at 70°C for 20 min to remove residual solvents and stabilize the silane coating. Figure 3 illustrates the complete silane surface modification procedure adopted for the fiber and PVC core.

Figure 3.

Surface modification process of the reinforcements.

Fabrication of composites

The hybrid sandwich composites were fabricated through a laboratory-scale hand layup technique combined with controlled compression molding to obtain defect-minimized laminates with improved structural integrity. Prior to fabrication, the mold cavity was thoroughly cleaned, dried, and polished to eliminate surface impurities that could affect laminate quality. A uniform layer of mold-release wax was subsequently applied to facilitate easy demolding and to obtain a smooth surface finish on the fabricated composites. The vinyl ester matrix system was prepared separately by incorporating the required proportion of hardener into the resin under continuous mechanical stirring for 10 min to ensure proper mixing and activation of the curing system. Subsequently, nutmeg husk-derived biochar particles were gradually introduced into the resin mixture at different loading concentrations. To promote homogeneous filler distribution and minimize particle agglomeration, the suspension was magnetically stirred for 20 min at 60°C, resulting in a stable biochar-modified resin system. Composite fabrication commenced with the application of a thin resin-rich layer onto the prepared mold surface, followed by the careful arrangement of Kender-N fiber mats to achieve uniform reinforcement alignment. The PVC core was then centrally positioned within the laminate architecture to function as the structural core layer of the sandwich composite. Additional layers of resin and fiber mats were sequentially deposited above and below the core structure to ensure complete impregnation and encapsulation of the reinforcement phases. Following layup completion, the laminate assembly was subjected to controlled compression to expel entrapped air voids, improve wetting behavior, and strengthen fiber–matrix interfacial bonding. The fabricated composites were initially cured under ambient laboratory conditions for 24 h to facilitate primary crosslinking, followed by post-curing at 140°C for 3 h to enhance the degree of polymerization and improve the thermal and mechanical stability of the composites.¹⁵ The compositional variations of Kender-N fiber, PVC core, and biochar-reinforced vinyl ester composites are summarized in Table 4.

Table 4.

Composition design and reinforcement variations of Kender-N fiber, biochar, and PVC core incorporated vinyl ester hybrid composites.

Composite designation	Vinyl ester resin (Vol.%)	PVC core (Vol.%)	Fiber (Vol.%)	Biochar (vol.%)	Aging condition (50°C for 7 days)
EC0	38	30	30	2	Warm water
EC1	38	30	30	2	Tap water
EC2	38	30	30	2	Distilled water
EC3	38	30	30	2	Sea water

Water aging process

To evaluate the environmental durability of the fabricated composites, systematic water aging studies were performed under different aqueous exposure conditions to investigate the influence of moisture ingress on the mechanical integrity and structural stability of the materials. The aging experiments were designed to simulate practical service environments and to understand the degradation behaviour of the composites during prolonged exposure to moisture-rich conditions. For this purpose, the prepared specimens were subjected to four distinct aging media, namely warm water, tap water, distilled water, and seawater, under controlled laboratory conditions. Accelerated hydrothermal aging was carried out by immersing the composite samples in warm water maintained at 50°C inside a laboratory hot-air oven for a duration of 7 days.¹⁶ In parallel, separate sets of specimens were completely immersed in tap water, distilled water, and seawater for the same exposure period to comparatively evaluate the influence of varying water chemistries on moisture absorption and material degradation. The selected immersion duration provided sufficient time for water diffusion into the composite structure, enabling the initiation of possible interfacial weakening, matrix swelling, and microstructural deterioration. These aging studies offer valuable insight into the long-term performance, dimensional stability, and resistance of the developed composites when exposed to different environmental conditions.

Characterization techniques

Mechanical characterization

The mechanical performance of the developed hybrid composites was systematically evaluated using standardized ASTM testing procedures to ensure reliability, repeatability, and accurate comparison of material behaviour. Tensile characterization was carried out in accordance with ASTM D3039 to determine the resistance of the composites against axial loading. The test provided essential parameters including ultimate tensile strength, tensile modulus, and elongation at fracture. Flexural behaviour was investigated following ASTM D790 using a three-point bending configuration, enabling the assessment of bending strength, stiffness, and load-carrying capability of the fabricated laminates. Surface hardness measurements were performed according to ASTM D2240 using a Shore D durometer to evaluate the resistance of the composite surface against localized indentation. Impact resistance was analysed using the Izod impact method based on ASTM D256, which determines the ability of the composites to absorb sudden impact energy before fracture. To improve experimental consistency and reduce statistical deviation, each mechanical test was conducted on five individual specimens and the average value was reported. To further investigate the fracture behaviour and interfacial characteristics of the composites, scanning electron microscopy (FESEM) analysis was performed on the fractured surfaces of selected specimens after mechanical testing. Prior to examination, the fractured samples were sputter-coated with a thin conductive gold layer to minimize charging effects during imaging. The micrographs were used to analyse fiber–matrix adhesion, fiber pull-out, matrix cracking, void formation, filler dispersion, and interfacial debonding mechanisms associated with different composite formulations.

Creep behaviour

The creep performance of the fabricated composites was evaluated using a Metro Precision testing system equipped with a high-temperature furnace having an internal chamber capacity of 250 mm³ and an operating temperature capability up to 450°C. The creep experiments were conducted under constant loading conditions corresponding to 25% of the ultimate tensile strength (UTS) of the composites, while maintaining the test temperature at 45°C. The deformation behaviour was continuously monitored over an exposure duration of 15,000 s to study the time-dependent strain response of the material under sustained loading conditions. Five specimens from each composite configuration were tested in accordance with ASTM D7337 to ensure reproducibility and reliability of the obtained creep data.

Water absorption behaviour

The moisture absorption characteristics of the fabricated composites were investigated in accordance with ASTM D570 to evaluate the diffusion behaviour and environmental durability of the material. Prior to immersion, the specimens were dried in a hot-air oven to remove residual moisture and their initial dry weights were recorded using a precision electronic balance. The samples were then immersed in different aqueous environments for predetermined durations, after which they were periodically removed, surface-dried using absorbent tissue paper, and reweighed to determine the amount of absorbed moisture. The percentage of water uptake was calculated from the weight difference before and after immersion, providing insight into moisture diffusion behaviour and the susceptibility of the composites to environmental degradation. In the present work, water absorption diffusion was primarily studied along the thickness direction of the composite laminates. To ensure unidirectional diffusion through the exposed thickness surfaces and to minimize edge-induced moisture penetration, the peripheral sides of the specimens were carefully sealed using epoxy resin prior to immersion. This approach enabled more controlled and consistent evaluation of through-thickness water diffusion characteristics.

Results and discussion

Mechanical properties

The environmental aging study revealed that prolonged exposure to different aqueous media significantly influenced the mechanical integrity and moisture resistance of the developed MPS-treated Kender-N fiber/PVC core reinforced vinyl ester composites as illustrated in Table 5 and Figure 4. Among all conditions, the distilled water-aged composite (EC2) exhibited the highest retention of mechanical performance, achieving tensile, flexural, and impact strength improvements of approximately 10.3%, 12.1%, and 13.0%, respectively, compared with the warm water-aged specimen (EC0). The reduced degradation observed in EC2 can be attributed to comparatively lower ionic activity and minimized hydrothermal attack on the fiber–matrix interface.¹⁷ In contrast, the warm water-aged composite showed the highest reduction in mechanical properties. Elevated temperature accelerated moisture diffusion into the composite structure, leading to matrix plasticization, swelling of natural fibers, microcrack initiation, and weakening of the interfacial region. Hydrothermal exposure also promoted partial debonding between the reinforcement and matrix due to differential expansion stresses developed at the interface.

Table 5.

Mechanical properties of the samples.

Composite designation	Tensile strength (MPa)	Flexural strength (MPa)	Impact strength (kJ/m²)	Shore D hardness
EC0	71.84 ± 1.92	108.63 ± 2.44	18.92 ± 0.51	81.42 ± 0.76
EC1	76.35 ± 1.67	116.48 ± 2.13	20.14 ± 0.46	83.17 ± 0.63
EC2	79.28 ± 1.54	121.76 ± 2.06	21.37 ± 0.43	84.65 ± 0.58
EC3	73.46 ± 1.88	111.25 ± 2.37	19.43 ± 0.49	82.04 ± 0.71

Figure 4.

Mechanical performance of the samples.

The seawater-aged specimens exhibited moderate degradation behaviour because dissolved salts and ionic species penetrated the composite network, inducing localized interfacial deterioration and osmotic diffusion effects. Although seawater exposure increased interfacial stresses, the presence of MPS-treated surfaces reduced severe fiber pull-out and restricted excessive moisture-induced debonding compared with untreated natural fiber systems reported in earlier studies. The enhanced durability of the present composites is strongly associated with the MPS silane treatment applied to both the Kender-N fibers and PVC core.1¹⁸ The hydrolyzedmethoxy groups of MPS formed reactive silanol groups, which chemically interacted with hydroxyl functionalities present on the natural fiber surface, resulting in stable siloxane (Si–O–Si) linkages. Simultaneously, the methacrylate functional end of MPS established improved compatibility with the vinyl ester matrix during curing. This dual functionality created a molecular bridge between the reinforcement and matrix phases, thereby improving stress transfer efficiency and suppressing interfacial crack propagation under aging conditions. Furthermore, the silane-treated surfaces reduced the number of accessible hydrophilic sites on the fibers, thereby lowering moisture sensitivity and restricting capillary water penetration along the fiber–matrix interface. SEM observations supported this behaviour by revealing reduced interfacial gaps, lower fiber pull-out, and improved matrix adherence in the treated composites even after environmental aging exposure.¹⁹ These findings confirm that MPS surface functionalization substantially contributes to enhancing long-term durability, dimensional stability, and retention of mechanical performance in moisture-exposed hybrid composites.

The SEM micrographs reveal the influence of different aging environments on the surface morphology and interfacial integrity of the MPS-treated Kender-N fiber/PVC core reinforced vinyl ester composites. Figure 5(a) corresponding to EC0 (warm water aged composite) exhibits severe surface deterioration characterized by large cavities, matrix cracking, and interfacial debonding. The elevated temperature accelerated hydrothermal diffusion, leading to matrix softening, micro-void formation, and weakening of the fiber–matrix interface. Figure 5(b) representing EC1 (tap water aged composite) shows moderate crack propagation and limited surface damage compared with EC0, indicating comparatively lower moisture-induced degradation. The micrograph demonstrates partial interfacial deterioration caused by prolonged water diffusion into the composite structure. Figure 5(c) corresponding to EC2 (distilled water aged composite) displays a relatively smooth and compact morphology with minimal voids and reduced crack formation, confirming improved interfacial adhesion and enhanced structural stability. The lower extent of degradation indicates effective stress transfer and reduced moisture penetration within the composite network. Figure 5(d) representing EC3 (seawater aged composite) reveals localized micro-cracks and minor interfacial cavities caused by salt-ion penetration and osmotic diffusion effects during seawater exposure.²⁰ However, severe fiber pull-out and extensive delamination were not observed, demonstrating the effectiveness of MPS silane treatment in improving interfacial bonding and restricting moisture-assisted degradation. Overall, the SEM observations confirm that the silane-treated composites maintained comparatively good interfacial integrity under different environmental conditions, while the distilled water-aged composite exhibited the least microstructural damage among all tested specimens.

Figure 5.

SEM micrographs of (a) EC0, (b) EC1, (c) EC2, and (d) EC3.

Statistical significance

The statistical significance of the developed composite groups was evaluated using a one-way ANOVA test based on the F-distribution with df (3,16), and the obtained results are presented in Table 6. The analysis revealed a p-value of 0.0017, which is lower than the significance level (α = 0.05), leading to the rejection of the null hypothesis (H0). This confirms that the variation among the mean values of the composite groups is statistically significant. The calculated F-ratio of 8.027 was found to exceed the acceptance region, further validating the presence of significant differences among the developed composites. The probability of committing a Type I error was very low (0.17%), indicating strong statistical reliability of the obtained results. In addition, Levene’s test was performed to evaluate the homogeneity of variances, and the obtained p-value of 0.964 confirmed that the variances among the groups were statistically equal. Since all groups possessed similar sample sizes, the ANOVA model remained robust with respect to the homogeneity assumption. Furthermore, the normality assumption was verified using the Shapiro–Wilk test, confirming that the datasets followed a normal distribution pattern. Although the statistical tool suggested consideration of the non-parametric Kruskal–Wallis ANOVA test, the assumptions required for one-way ANOVA were satisfactorily fulfilled in the present study.

Table 6.

One-way ANOVA was employed to statistically analyze the developed composite groups by evaluating the degree of freedom (DF), sum of squares (SS), mean square (MS), F-ratio, and corresponding p-value to determine the significance of variations among the composite formulations.

Source	DF	Sum of square	Mean square	F statistic	P-value
Groups (between groups)	3	158.95	52.9833	8.0278	0.001728
Error (within groups)	16	105.6	6.6
Total	19	264.55	13.9237

Creep properties

The creep strain of the MPS-treated Kender-N fiber/PVC core reinforced vinyl ester composites increased progressively with testing time, indicating the viscoelastic deformation behaviour of the polymer matrix under sustained loading conditionsas presented in Table 7 and Figure 6. Initially, the composites exhibited rapid strain development due to elastic deformation and molecular chain rearrangement, followed by a slower and more stable creep response as the loading duration increased from 5000 s to 15,000 s. Among all aging conditions, the warm water-aged composite (EC0) exhibited the highest creep strain, reaching 0.00941 at 15,000 s. The elevated temperature accelerated moisture diffusion, matrix softening, and polymer chain mobility, resulting in increased viscoelastic deformation. In addition, hydrothermal exposure weakened the fiber–matrix interface through moisture-induced swelling and localized interfacial debonding, thereby reducing stress transfer efficiency.²¹

Table 7.

Creep properties of the samples.

Composite designation	Creep strain at 5000 s	Creep strain at 10000 s	Creep strain at 15000 s
EC0	0.00528 ± 0.00016	0.00742 ± 0.00022	0.00941 ± 0.00028
EC1	0.00471 ± 0.00014	0.00668 ± 0.00020	0.00852 ± 0.00024
EC2	0.00436 ± 0.00013	0.00617 ± 0.00018	0.00791 ± 0.00022
EC3	0.00503 ± 0.00015	0.00711 ± 0.00021	0.00901 ± 0.00026

Figure 6.

Creep performance of the samples.

The seawater-aged composite (EC3) also showed relatively higher creep strain due to the penetration of dissolved salts and ionic species into the composite structure, which promoted localized interfacial degradation and microstructural instability. In contrast, the distilled water-aged composite (EC2) exhibited the lowest creep strain values throughout the test duration, with nearly 15.9% lower creep deformation at 15,000 s compared with EC0. This indicates improved dimensional stability and better resistance to time-dependent deformation. The enhanced creep resistance is mainly attributed to the MPS silane treatment applied to both the Kender-N fibers and PVC core. The silane molecules formed stable siloxane linkages with the fiber surface while improving compatibility with the vinyl ester matrix, creating a stronger interfacial bonding network. This improved adhesion restricted fiber slippage, delayed crack propagation, and enhanced stress transfer during prolonged loading.^22,23 Furthermore, the silane-treated surfaces reduced moisture sensitivity by limiting accessible hydrophilic sites, thereby minimizing hydrothermal degradation and improving long-term creep stability of the composites.

Water absorption

The water absorption behaviour of the MPS-treated Kender-N fiber/PVC core reinforced vinyl ester composites was significantly influenced by the aging environmentas depicted in Table 8 and Figure 7. Among all specimens, the warm water-aged composite (EC0) exhibited the highest moisture uptake of 4.86%, whereas the distilled water-aged composite (EC2) showed the lowest absorption value of 3.28%. The reduction in water absorption for EC2 was approximately 32.5% compared with EC0, indicating improved resistance against moisture diffusion. The higher water uptake observed in EC0 is mainly attributed to accelerated hydrothermal diffusion caused by elevated temperature exposure. Increased temperature enhanced polymer chain mobility and facilitated faster penetration of water molecules into microvoids, capillary channels, and fiber–matrix interfacial regions. This process promoted fiber swelling, matrix plasticization, and the formation of microcracks, leading to increased moisture retention within the composite structure.^24,25

Table 8.

Water absorption performance of the samples.

Composite designation	Water absorption (%)
EC0	4.86 ± 0.18
EC1	3.94 ± 0.14
EC2	3.28 ± 0.11
EC3	4.41 ± 0.16

Figure 7.

Water absorption of the samples.

The seawater-aged composite (EC3) also exhibited relatively higher moisture absorption due to the presence of dissolved salts and ionic species, which contributed to osmotic diffusion and localized interfacial degradation. In contrast, the distilled water-aged composite demonstrated improved dimensional stability and lower moisture uptake because of reduced ionic interaction and comparatively limited interfacial deterioration. The comparatively low water absorption behaviour of the developed composites is strongly associated with the MPS silane treatment applied to both the Kender-N fibers and PVC core. The silane treatment chemically modified the reinforcement surfaces through the formation of siloxane linkages, thereby improving fiber–matrix adhesion and reducing the availability of hydrophilic hydroxyl groups responsible for moisture attraction. This surface modification minimized capillary water transport along the interfacial region and restricted moisture-assisted debonding.²⁶ Furthermore, the enhanced interfacial compatibility between the silane-treated reinforcements and vinyl ester matrix reduced void formation and improved matrix encapsulation around the fibers and PVC core. As a result, the composites exhibited improved environmental durability and moisture resistance even under prolonged aqueous exposure conditions.

Limitations

Although the developed composites demonstrated improved mechanical performance and environmental durability, certain limitations and processing challenges were observed. Uniform dispersion of nutmeg husk-derived biochar within the vinyl ester matrix remains a critical factor, as excessive filler loading may promote particle agglomeration and localized stress concentration. In addition, achieving consistent wetting and homogeneous resin distribution during the hand layup process can be challenging, potentially leading to void formation and interfacial defects. Furthermore, long-term environmental exposure and large-scale manufacturing feasibility require additional investigation to validate the industrial applicability of the developed hybrid composites.

Conclusions

In this investigation, environmentally aged MPS-treated Kender-N fiber/PVC core reinforced vinyl ester hybrid composites containing nutmeg husk-derived biochar were successfully fabricated and evaluated for their mechanical, creep, and moisture absorption behaviour under different aqueous exposure conditions. The study confirmed that environmental aging significantly influences the long-term durability and structural stability of hybrid composites through hydrothermal diffusion, matrix plasticization, interfacial degradation, and moisture-assisted crack initiation. Among the tested aging environments, the distilled water-aged composite exhibited superior overall performance with tensile, flexural, and impact strength improvements of approximately 10.3%, 12.1%, and 13.0%, respectively, compared with the warm water-aged composite. The same specimen also demonstrated the lowest creep strain value of 0.00791 at 15,000 s, indicating nearly 15.9% improvement in creep resistance. Furthermore, water absorption was reduced by approximately 32.5%, confirming enhanced dimensional stability and moisture resistance. The improved durability and mechanical retention are primarily attributed to the MPS silane treatment applied to both the Kender-N fibers and PVC core. At the molecular level, the hydrolyzed silane molecules formed stable siloxane linkages with hydroxyl groups present on the fiber surface while simultaneously improving compatibility with the vinyl ester matrix. This chemically bonded interphase enhanced stress transfer efficiency, reduced fiber pull-out, minimized interfacial void formation, and restricted moisture-assisted debonding under aggressive aging conditions. SEM analysis further validated the effectiveness of the silane treatment by revealing improved matrix encapsulation, lower interfacial gaps, and reduced microstructural damage in the treated composites. The incorporation of nutmeg husk-derived biochar additionally contributed to structural stability by improving filler–matrix interaction and limiting crack propagation within the composite network. Overall, the developed hybrid composites exhibited promising resistance against environmental degradation and sustained mechanical stability, making them suitable candidates for lightweight structural applications in marine panels, automotive interior components, transportation structures, building materials, and sustainable engineering applications where long-term durability and moisture resistance are essential.

Footnotes

ORCID iD

H. V. Ganvir

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

References

Simões

. High-performance advanced composites in multifunctional material design: state of the art, challenges, and future directions. Materials 2024; 17(23): 5997. https://doi.org/10.3390/ma17235997

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

Jiang

Pan

, et al. Durability of basalt fibers, glass fibers, and their reinforced polymer composites in artificial seawater. Polym Compos 2022; 43(4): 1961–1973. https://doi.org/10.1002/pc.26511

Xian

Tian

, et al. Effect of fiber surface treatment with silane coupling agents and carbon nanotubes on mechanical properties of carbon fiber reinforced polyamide 6 composites. Polym Compos 2025; 46(2): 1267–1283. https://doi.org/10.1002/pc.29035

Barreira-Pinto

Carneiro

Miranda

, et al. Polymer-matrix composites: characterising the impact of environmental factors on their lifetime. Materials 2023; 16(11): 3913. https://doi.org/10.3390/ma16113913

Kumar

Rakesh

Sreehari

. In IOP conference series: materials science and engineering. IOP Publishing 2023; 1291(1): 012017. https://doi.org/10.1088/1757-899X/1291/1/012017

Rakesh

Melese

Gairola

, et al. The mechanical properties of pine needle fiber‐reinforced polymer composites under different environments. Eng Rep 2025; 7(10): e70376. https://doi.org/10.1002/eng2.70376

Rakesh

Davim

(eds). Walter de Gruyter GmbH & Co KG, 2023.

Agumba

Park

Kim

, et al. Mater Lett 2024; 360: 136055. https://doi.org/10.1016/j.matlet.2024.136055

10.

Muthukumarasamy

Karthik

. Experimental analysis of biochar filler effects on the mechanical, water absorption, and viscoelastic properties of novel caryota fiber reinforced polymeric composites: the role of biodegradable reinforcements concentration. Polym Compos 2025; 46: S119–S134. https://doi.org/10.1002/pc.29830

11.

Elumalai

&Asokan

. Silane surface treatment as a failure-mitigation strategy for ixtle fiber–PET core–Elaeocarpus ganitrus husk biochar reinforced epoxy composites under service temperature conditions. J Eng Appl Sci 2026; 73(1): 180. https://doi.org/10.1186/s44147-026-01030-6

12.

Wang

, et al. Effect of ultraviolet aging on properties of epoxy resin and its pultruded fiber-reinforced composite. Polymers 2025; 17(3): 294. https://doi.org/10.3390/polym17030294

13.

Iwuozor

Emenike

Bakare

, et al. A review on the conversion of plant husk-based biomass into biochar. Biofuels 2024; 15(10): 1331–1345. https://doi.org/10.1080/17597269.2024.2376367

14.

Varghese

Cho

Farrar

, et al. Dent Mater 2023; 39(4): 362–371. https://doi.org/10.1016/j.dental.2023.03.002

15.

Zhao

Qin

Piao

, et al. Improving the hydrophobicity, dimensional stability and mold resistance of bamboo by paraffin/microcrystalline wax/stearic acid modification. Constr Build Mater 2024; 414: 134902. https://doi.org/10.1016/j.conbuildmat.2024.134902

16.

Xie

, et al. Effects of immersion in water, alkaline solution, and seawater on the shear performance of basalt FRP bars in seawater–sea sand concrete. J Compos Construct 2022; 26(2): 04021071. https://doi.org/10.1061/(ASCE)CC.1943-5614.0001184

17.

Faizur Rahman

Soundararajan

Raja

, et al. Characterization of epoxy composites reinforced with silane-modified Dracaena reflexa fiber and Citrus medica leaf-derived cellulose particles. J Compos Mater 2026; 20: 00219983261436906. https://doi.org/10.1177/00219983261436906

18.

Rahman

Soundararajan

Ariffuddeen

, et al. High-frequency EMI shielding and load bearing performances of PVA composite reinforced with Cissus quadrangularis fiber and ultra-porous nutmeg husk biochar. Polym Bull 2026; 83(5): 217. https://doi.org/10.1007/s00289-025-06141-3

19.

Kumar

Soundararajan

Raffik

, et al. Mechanical and functional performance of epoxy composites reinforced with cluster bean fibers and balloon Vine stem lignin. Polym Bull 2026; 83(4): 208. https://doi.org/10.1007/s00289-025-06123-5

20.

Narasimharaj

Soundararajan

Vasanth

, et al. Sustainable food packaging materials: PLA composites reinforced with Parthenium hysterophorus lactones and river tamarind bark microfiber. Polym Bull 2026; 83(4): 206. https://doi.org/10.1007/s00289-025-06132-4

21.

Kumar

Soundararajan

Srinivasan

. Effect of surface-treated paddy straw biosilica additive on PET core–pineapple fiber composite subjected to water and temperature aging: a characterization study. Biomass Convers Biorefinery 2025; 15(10): 15985–16001. https://doi.org/10.1007/s13399-024-06431-2

22.

Nathan

Soundararajan

Gandhi

, et al. Mechanical, wear, fatigue, and water absorption behavior of epoxy biocomposite toughened using marine waste Sepioteuthis sepioidea pen chitosan biopolymer and pineapple plain-weaved fiber. Biomass Convers Biorefinery 2025; 15(5): 6699–6712. https://doi.org/10.1007/s13399-024-05762-4

23.

Karthi

Soundararajan

Arunkumar

, et al. SAE technical paper 2022. https://doi.org/10.4271/2022-28-0538

24.

Karthik

Jeyakumar

Sampath

, et al. SAE technical paper 2022. https://doi.org/10.4271/2022-28-0532

25.

Sivakumar

Thangarasu

Soundararajan

, et al. Mechanical and machining behavior of betel nut fiber/leather/chitin-toughened epoxy hybrid composite. Biomass Convers Biorefinery 2023; 13(5): 4365–4372. https://doi.org/10.1007/s13399-022-02994-0

26.

Zhang

Song

, et al. Appl Surf Sci 2025: 164716. https://doi.org/10.1007/s13399-022-02994-0