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Article

Seawater Ageing Effects on the Mechanical Performance of Basalt Fibre-Reinforced Thermoplastic and Epoxy Composites

by
Mohamad Alsaadi
1,2,3,*,
Tomas Flanagan
2 and
Declan M. Devine
1
1
PRISM Research Institute, Technological University of the Shannon, N37 HD68 Athlone, Ireland
2
ÉireComposites Teo, An Choill Rua, Indreabhán, H91 Y923 Galway, Ireland
3
Materials Engineering Department, University of Technology, Baghdad 10066, Iraq
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(7), 368; https://doi.org/10.3390/jcs9070368
Submission received: 31 May 2025 / Revised: 5 July 2025 / Accepted: 11 July 2025 / Published: 15 July 2025
(This article belongs to the Special Issue Advances in Continuous Fiber Reinforced Thermoplastic Composites)
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Abstract

This research paper employed the recently developed Elium thermoplastic resin and basalt fabrics as an alternative to thermoset/synthetic fibre composites to reduce their environmental impact. Elium® 191 XO/SA and Epoxy PrimeTM 37 resin were reinforced with mineral-based semi-unidirectional basalt fibre (BF). Physical, chemical, tensile, and flexural performance was investigated under the effect of hydrothermal seawater ageing at 45 °C for 45 and 90 days. The results show that the BF/Elium composite exhibited superior tensile and flexural strength, as well as good stiffness, compared with the BF/Epoxy composite. Digital images and scanning electron microscope images were used to describe the fracture and failure mechanisms. The tensile and flexural strength values of the BF/Elium composite were 1165 MPa and 1128 MPa, greater than those of the BF/Epoxy composite by 33% and 71%, respectively. The tensile and flexural modulus values of the BF/Elium composite were 44.1 GPa and 38.2 GPa, which are 30% and 12% greater than those of the BF/Epoxy composite. The result values for both composites were normalised with respect to the density of each composite laminate. Both composites exhibited signs of resin decomposition and fibre surface degradation under the influence of seawater ageing, resulting in a more recognisable reduction in flexural properties than in tensile properties.

1. Introduction

The growing use of composite materials is driving a sharp increase in composite waste. For example, onshore wind turbine blades alone are projected to generate over 43 million tons of composite waste by 2050 [1,2]. In the past decade, the circular economy has emerged as a key focus in sustainability discourse, aiming to achieve full circularity and net-zero greenhouse gas emissions by 2050. In response, EU member states established ambitious goals through the 2015 Circular Economy Action Plan. Among the priority areas, the plastics sector has received particular attention due to its central role in EU policy [3,4]. Due to their heterogeneous nature, composite materials, particularly thermoset-based polymer composites, are a significant challenge for recycling, resulting in a substantial amount of composite waste. The sustainable end-of-life disposition of polymer composites is one of the most significant issues confronting the industrial and academic composite communities today. Consequently, much research is being focused on developing novel recyclable materials to avoid landfilling and incineration, which are the standard methods of dealing with composite waste [5,6].
Traditional thermoplastic composites are generally unsuitable for manufacturing large structures due to their high melting viscosity and melting temperature. These properties necessitate the application of positive pressures that exceed atmospheric pressure to create void-free laminates. Such pressures are typically applied using presses or autoclaves, which makes the process economically unviable for large-scale components, such as ship sections or tidal energy blades [7,8]. In contrast, liquid acrylic resins such as Elium (developed by Arkema) combine the low viscosity and room-temperature processability of thermosets with the recyclability of thermoplastics. These resins, primarily made from methyl methacrylate monomers, are infused into fibre reinforcements and polymerised in situ [9]. Being thermoplastics, they can be recycled using methods such as crushing and remelting, the dissolution-based separation of fibres and matrix, pyrolysis, or the thermoforming of continuous fibre composites. Their ease of processing, recyclability, and competitive mechanical properties position Elium® resins as a viable alternative to conventional epoxy infusion systems [10,11].
Basalt fibre (BF) has lower CO2 emissions than other fabrics, most notably glass fibre (GF) and carbon fibre (CF) [12]. For instance, life-cycle analysis (LCA) studies have shown that BF-reinforced polymer (BFRP) bars exhibit significantly lower global warming potential (excluding transportation), measured in kg CO2eq, by approximately 74%, 49%, 88%, and 44% compared with carbon steel, galvanised steel, stainless steel, and GF-reinforced polymer, respectively [13]. Industries such as automotive, aerospace, and construction are increasingly using basalt fibre to improve performance while reducing weight, enhancing fuel efficiency and overall effectiveness. As a result of this growing demand, the global basalt fibre market is projected to grow from USD 282 million in 2023 to USD 758 million by 2032, achieving a compound annual growth rate (CAGR) of 11.8% [14].
In recent years, significant attention has been directed toward studying the mechanical properties of Elium-based composites to understand their behaviour better and improve their performance under various loading conditions. For instance, Yaghoobi et al. [15] used Elium 150 resin with a semi-unidirectional (0/90) non-crimp fibre (NCF) basalt fibre (BF) fabric having an areal density of 450 g/m2, manufactured using the VARTM process. The study reported that the resulting BF/Elium composite achieved a tensile strength of 440 MPa, 24% higher than that of the BF/Epoxy composite, while its tensile modulus was 17.55 GPa, 6% lower than that of the BF/Epoxy composite. The BF/Elium composite exhibited a 28% lower flexural strength than the BF/Epoxy version, but it had a flexural modulus of 20.72 GPa, which is 5% higher. Chilali et al. [16] examined the behaviour of GF/Epoxy and GF/Elium composites, with fibre volume fraction (FVF) values of 44% and 52%, respectively, and found that both composites exhibited similar maximum tensile strengths of approximately 380 MPa. Davies et al. [6] employed semi-UNI (0/90) NCF GF and BF with areal densities of 373 g/m2 and 416 g/m2, respectively, using the VARTM manufacturing process, and the VF was 44% for both composites. The authors found that the BF/Epoxy composite exhibited flexural strength and modulus of 698 MPa and 38.4 GPa, which were 15% and 1% higher than GF/Epoxy, respectively [17]. Another study employed the same type of BF, and the manufacturing process was performed by Chowdhury et al. [18]. Their study showed that the flexural strength and modulus of the BF/Epoxy composite were 618 MPa and 30.1 GPa, respectively. A recent work by Bahatia et al. [19] employed two different types of Elium: Elium® 188 O, compatible with CF, and Elium® 188 XO, compatible with GF. CF and GF were semi-UNI NCF with areal density values of 438 g/m2 and 1182 g/m2, respectively. The results showed that the tensile strength and modulus values of CF/Elium were 1711 MPa and 106 GPa higher than those of GF/Elium, representing 47% and 57% increases, respectively. While the flexural strength of CF/Elium was 931 MPa, it was 937 MPa for GF/Elium.
Hydrothermal ageing can lead to moisture absorption in composites, where water molecules penetrate the layers and trigger hydrolysis, breaking covalent bonds in the polymer chains. This process causes swelling, debonding, and the weakening of interlaminar bonds, leading to fibre surface degradation, resin decomposition, and ultimately, interfacial failure [19,20]. Therefore, understanding the impact of hydrothermal ageing on the mechanical performance of composite materials is essential to revealing the underlying degradation mechanisms. Numerous studies have demonstrated that Elium-based composites offer excellent recyclability and durability, underscoring their strong potential for use in marine environments. Davies et al. [17] showed that BF/Epoxy composites initially have higher flexural strength but show a greater decrease after seawater ageing, resulting in similar final properties to GF/Epoxy composites. The findings suggest that BF/Epoxy offers marine durability comparable to E-glass/Epoxy, with the potential for performance enhancement through reinforcement design optimisation. The reduction in flexural strength due to water absorption and matrix swelling is attributed to the reactivity of oxides present in basalt fibres, which enables interactions with surrounding ions. Similar findings were reported by Davies et al. during seawater immersion at 20 °C [18,21]. Davies et al. [22] demonstrated that biaxial woven GF/Elium samples exhibited reductions in tensile modulus of 11% and in tensile strength of 50% when subjected to seawater ageing at up to 60 °C over 18 months. Bel Haj Frej et al. [23] reported a 6% reduction in tensile modulus and a 10% decrease in tensile strength for non-woven 0/90° CF/Elium samples after six months of ageing in distilled water at 70 °C. In another study, Barbosa et al. [24] observed only a 4% decrease in tensile modulus and a slight 3% increase in tensile strength in woven 0/90° CF/Elium samples aged in distilled water at 80 °C for eight weeks.
To the best of the authors’ knowledge, no prior studies have investigated the tensile and flexural performance of semi-UNI NCF BF reinforced with the recently developed Elium® 191 XO/SA resin and Epoxy PrimeTM 37 resin under the effect of seawater ageing. This paper aims to analyse the effect of seawater ageing at 45 °C for 45 and 90 days on the tensile and flexural properties of the BF/Elium and BF/Epoxy. The failure modes were analysed using a digital camera and a scanning electron microscope (SEM). The chemical behaviour was analysed using Fourier transform infrared spectroscopy (FTIR).

2. Materials and Methods

2.1. Materials and Manufacturing Process

Elium® 191 XO/SA resin from Arkema, Colombes, France, and Epoxy PrimeTM 37 resin from Gurit, Newport, UK, were used to manufacture composite panels using the infusion process. Elium® 191 XO/SA resin is compatible with large composite structures due to extended gel time and low exothermic reaction. Table 1 outlines the physical and mechanical properties of Elium, as provided in the manufacturer’s datasheet.
The reinforcement fabrics used were semi-UNI NCF BFs labelled BAS-UNI 550 and BAS-UNI 350, with multi-compatible sizing, suitable for Elium matrix and Epoxy matrix, respectively. These fabrics were supplied by Basaltex (Belgium) in their original form. Table 2 details the composition of the composite laminates. Polyester stitching maintains structural stability and helps maintain fibre alignment.
The fabrication process began by placing dry BF layers onto a steel plate, followed by resin impregnation at RT using the VARTM method under vacuum pressures of 700 mbar for BF/Elium and 900 mbar for BF/Epoxy panels, due to the low viscosity of the Elium resin. The VARTM setup consisted of a vacuum pump, infusion mesh, a vacuum bag, peel ply, and a resin delivery system, including a pot, silicone tubing, spiral wire, and connectors (Figure 1). Resin mixing ratios, gel time, and the curing and post-curing steps are summarised in Table 1 and Table 2.

2.2. Physical Testing

The thickness of each sample was assessed at four different points. The coupon thickness values were 2.46 ± 0.05 mm for the BF/Elium laminate and 2.29 ± 0.07 mm for the BF/Epoxy laminate. The density of the BF/Elium composite and BF/Epoxy composite for the control and aged coupons was measured in accordance with ASTM standard 792 [27]. Density measurements of coupons were taken using a digital solid density meter, and the average values were recorded. The mass of the dry sample was measured to the nearest 0.0001 g. The FVF of the composites was determined using a matrix burn-off method in accordance with ASTM D3171 [28], employing an oven at 600 °C for 2.5 h. Six coupons were used, and the average values of FVF and ρc were determined.
The coupons of both composites were immersed vertically in synthetic seawater at 45 °C using a Memmert water bath for 0, 45, and 90 days until complete saturation to assess the hydrothermal accelerated ageing performance. The mass of the water uptake (weight gain, Mm) was calculated according to the ASTM D5229 standard [29] from the change in the weight of the coupons as follows [30,31,32]:
Mm = (Wt − Wo)/Wo × 100%
The weight of each specimen after a specified immersion period is recorded as Wt, while Wo represents the initial weight, which is measured after the sample has been oven-dried for 2 h at 45 °C. The specimens were weighed to the nearest 0.0001 g before and after each hydrothermal ageing interval. The results presented reflect the average values of three specimens. The effect of hydrothermal ageing on the chemical composition of the BF/Elium and BF/Epoxy composites was characterised by FTIR (ATR-FTIR, PerkinElmer Spectrum Inc., Norwalk, CT, USA) with a scanning range of 1000 cm−1 to 4000 cm−1 wavenumber and a resolution of 4 cm−1.

2.3. Tensile and Flexural Testing

The mechanical tests were conducted at RT using the Zwick Roell machine, equipped with a 45 kN load cell (GmbH & Co. KG, Baden-Württemberg, Germany). The tensile test samples, with fibre directions of 0° and 90°, were manufactured and tested according to ASTM D3039 [33], with dimensions of 250 mm × 13 mm and 180 mm × 25 mm, respectively. The samples for the flexural tests were prepared according to ASTM D 790 [34], with a size of 130 mm × 13 mm. In order to control and reduce the thickness difference between both laminates, the laminates were manufactured with seven plies of BAS-UNI 350 for the BF/Epoxy composites and six plies of BAS-UNI 550 for the BF/Elium composites. The crosshead speeds were set to 2 mm/min for the tensile tests and 4 mm/min for the flexural tests. At least four samples were tested for each test, and the average value was dependent. The flexural span-to-thickness ratio was 32 to generate a strain rate of 0.01, as preferred by the ASTM D790 standard [34], according to Equation (2) below:
Z = 6 R h / L 2
where L and h are the span and depth of the specimen (mm), R is the rate of crosshead speed (mm/min), and Z is the straining rate of the outer fabric surface of the sample (mm/mm/min). On the other hand, Poisson’s ratio (µ) values were determined from the tensile test using an extensometer to measure the transverse and longitudinal strains. The shear modulus (G) values were determined from the E and µ values according to the equation below:
G = E/(2(1 + µ))

3. Results and Discussion

3.1. Physical and Chemical Performance

The physical properties of BF/Elium and BF/Epoxy for 0-, 45-, and 90-day ageing at 45 °C are presented in Table 3. The ρc and FVF were determined as 1.96 g/cm3 and 53% for the BF/Elium composite and 1.84 g/cm3 and 48% for the BF/Epoxy composite, respectively. Thus, the BF/Elium composite exhibited 7% higher density and 12% higher FVF compared with the BF/Epoxy composite. These increases are mainly attributed to the 39% higher areal density of the BAS-UNI 550 fabric used in the BF/Elium laminate compared with the BAS-UNI 350 fabric in the BF/Epoxy laminate, along with the influence of the matrix densities. Hydrothermal ageing led to increases in the thickness and density of BF/Elium of 1.22% and 2.4%, respectively, while those of BF/Epoxy were increased by 1.75% and 3.2%, respectively, after 90-day ageing at 45 °C. Hence, hydrothermal ageing leads to increased thickness due to moisture uptake and swelling, while density increases mainly due to the post-curing of the thermosets and the crystallisation phenomena of thermoplastics. Similar behaviour was found in the literature: as a result of hygrothermal ageing, the thickness and density of the GF/Elium composite increased by 3.13% and 4.14%, respectively [35]. The BF/Elium coupons exhibited lower water uptake, with Mm values of 1.12% at 45 °C, compared with BF/Epoxy, which had Mm values of 1.48% at 45 °C. On the other hand, the Mm of the BF/Elium samples increased by 22.9% at the hydrothermal ageing temperature of 45 °C, while that of the BF/Epoxy samples increased by 35.9%.
FTIR experiments were conducted to examine the FTIR spectral alterations of the Epoxy-based network in the 3DPd composites under the accelerated hydrothermal ageing of the BF/Elium and BF/Epoxy composites for 0, 45, and 90 days at 45 °C (Figure 2). As shown in this figure, the peak intensity levels of the thermoplastic BF/Elium composites decreased after hydrothermal ageing due to microstructural relaxation. The opposite behaviour occurred for the thermoset BF/Epoxy composites, as the peak intensity levels increased after seawater ageing due to plasticisation. The chemical bond alterations of the BF/Elium composite included the key bands (~900 cm−1) Si–O–Si due to the hydrolysis of the fibre. The acrylic-specific C=O peaks near ~1730–1750 cm−1, and C–O peaks near ~1140 cm−1, which may shift or change intensity due to hydrolysis. The same trend was observed for the C=C peak at 1200 cm−1, as well as the symmetric stretching of CH2 and CH3 at peaks between 2850 cm−1 and 2950 cm−1, respectively, indicating moisture uptake and hydrolysis [36]. On the other hand, an increase in broad absorbance of the BF/Epoxy composites around 3423 cm−1 was observed, which corresponds to the O–H stretching vibration of hydroxyl groups. This increase is attributed to the hydrolysis reaction, which breaks the ether bonds; these bonds are known to be the most sensitive to hydrolysis within the Epoxy network. Consequently, phenol groups are formed, leading to a higher concentration of hydroxyl (OH−) groups [37,38].

3.2. Tensile Performance in 0° Direction

The tensile stress–strain curves for both composites in the 0° direction are shown in Figure 3a for control and aged coupons. The tensile strength, modulus, and their normalised values with the effect of seawater ageing are presented in Table 4. As shown in this figure, the tensile strength is 1165 MPa, which is 33% greater than that of the BF/Epoxy composite. Maybe the main difference is due to the higher fibre volume fraction of the BF/Elium composites. As noted in Table 1, the bulk Epoxy matrix exhibits a tensile strength 54% higher and a modulus 20% higher compared with the bulk Elium matrix. On the other hand, a statistically significant reduction was observed in the tensile strength of the BF/Elium composite after 45 days of seawater exposure, from 1165 ± 49 MPa to 1009 ± 38 MPa (p < 0.05). The p-values were calculated for statistical significance using the t-test [39,40]. The BF/Epoxy composite exhibited a more pronounced reduction in tensile strength following ageing, with a statistically significant drop from 880 ± 57 MPa to 622 ± 14 MPa (p < 0.01).
A notable decrease was also seen in modulus (−4.2%), from 34.9 ± 0.2 GPa to 33.4 ± 0.2 GPa. The tensile strength values exhibited reductions of approximately 13% and 29% for both composites after 45 days of ageing, and these reductions increased to approximately 19% and 38% after the samples reached a seawater saturation state (90 days).
The tensile modulus (E) of the BF/Elium composite was 44.1 GPa, which is 30% higher than that of the BF/Epoxy composite. It is noteworthy that the superior tensile properties of the BF/Elium composite reflect the good interfacial adhesion between the BF fabrics and the Elium 191 XO/SA resin. Both composites exhibited a similar stiffness reduction of approximately 8% when the coupons reached saturation after 90 days of seawater ageing. The behaviour of the stress–strain curves indicates that the strain-at-fracture values show slight variation between both composites for the control coupons. This variation increased after hydrothermal ageing. Hence, the strain-at-fracture values were reduced significantly for the BF/Epoxy composites. This behaviour was due to seawater ageing acting as a plasticiser for polymer matrices, affecting both thermoplastic and thermoset composites. It lowers the glass transition temperature, reduces the stiffness and strength of the matrix, and leads to deformation occurring at lower loads, resulting in earlier failure [41,42]. Furthermore, seawater penetrated the fibre–matrix interface, causing the weakening of the interfacial bond, which led to microcracks that reduced the load transfer efficiency between the fibre and matrix [31,43].
The selection of materials in most lightweight structures, such as those used in renewable energy, aerospace, and automotive applications, can be based on the density values of the components produced by specific materials. Figure 3b presents the normalised tensile strength and modulus of each composite. Accordingly, the normalised values were adjusted with respect to the density of each composite (Table 4). As shown in Figure 3b, even when tensile strength was normalised by density, BF/Elium maintained higher specific strength after ageing (507 MPa/(g/cm3)) than BF/Epoxy (332 MPa/(g/cm3)), reinforcing the improved performance retention of the thermoplastic matrix system and this possibly due to enhanced resistance to microcrack formation. Hence, the BF/Elium composite displays greater normalised tensile strength and modulus values, with increments of 11% and 16%, respectively, compared with the BF/Epoxy composite. Despite increasing the density due to hydrothermal ageing, the normalised values of the BF/Elium composites are still greater than those of the BF/Epoxy composites. There is a statistically significant difference between the BF/Elium and BF/Epoxy composites (p < 0.0001) after 90 days of seawater ageing, indicating that BF/Elium exhibits superior tensile stiffness after prolonged exposure to seawater.
Figure 4 presents the failed tensile samples in the front view and the side view of the BF/Elium and the BF/Epoxy coupons. The high interfacial strength was characterised by an explosion-like bursting of the composites, as presented in the failure behaviour of the 0° tensile coupons (Figure 4). Since most fibres of both types of BF are aligned at 0°, i.e., they are semi-UNI (Table 4), a sudden failure of the fibres occurred for both composites, accompanied by the strong release of stored elastic energy. Observations from the images of the failed samples indicate that the size of the diffused fibre in BF/Elium is larger than that in BF/Epoxy for the control and aged coupons due to the higher amount of stored elastic energy released. Similar behaviour was noticed in the literature by David et al. [44]. As shown in this figure, the degree of diffusion in fabrics is reduced due to the effect of seawater ageing, resulting in a reduction in the release of stored elastic energy and, consequently, a decrease in tensile strength and stiffness.
The Poisson’s ratio values of the control coupons, µ, were determined with tensile tests to be 0.202 and 0.189 for the BF/Elium and BF/Epoxy composites, respectively. Using the µ values, the shear modulus (G) values were calculated by Equation (3) to be 18.4 GPa and 14.3 GPa for the control coupons of the BF/Elium and BF/Epoxy composites, respectively. Since µ is a ratio of lateral strain to axial strain, as shown in Table 4, these values were slightly increased for both composites due to the reduction in axial strain under the hydrothermal ageing effect. The G values were more affected by the E reduction than by the increase in µ values resulting from hydrothermal ageing, and both parameters led to a decrease in the G values.

3.3. Effect of Manufacturing Method and Reinforcement on Tensile Performance

The mechanical properties, such as tensile and flexural strength and modulus, of fibre-reinforced polymer composites can vary depending on the manufacturing method and the type of reinforcement material. For instance, fabric characteristics, including their properties and performance, are significantly influenced by various factors, such as the fibre type and the fabric’s weave or structural configuration [45]. Figure 5 shows the tensile strength values of BF-based composites and GF-based composites, which varied from 299 MPa for the NCF BF/Epoxy (fibre areal density of 516 g/m2, Vf of 32%, and Vv of 6.5%) using the VARTM manufacturing process, as stated by Farid et al. [46], to 1310 MPa for the UNI prepreg BF/Epoxy reported by David et al. [44]. No studies in the literature reveal that the tensile strength value of BF- or GF-reinforced thermoset or thermoplastic polymer composites was higher than the value reported in the present study (except for the UNI prepreg). For instance, the tensile strength values of the semi-UNI (0/90) NCF BF/Elium and NCF BF/Epoxy composites were 440 MPa and 371 MPa, respectively. The VARTM manufacturing process was used with a fibre areal density of 450 g/m2 and Vf of 49% for both composites by Yaghoobi et al. [15]; i.e., the tensile strength of semi-UNI NCF BF/Elium in the present study was greater by 134% and 178%, respectively.

3.4. Tensile Performance at 90° Fibre Orientation Angle

On the other hand, the 90° tensile stress–strain curves for both composites are shown in Figure 6a. The average 90° tensile strength and strain-at-fracture values of the BF/Epoxy composite were 139.3 MPa and 4.56, i.e., 57% and 152% higher than those of the BF/Elium composite. These values were dependent on the matrix properties. Hence, only 50 g/m2 fibres are available in 90° for both composites. For instance, the UNI prepreg BF/Epoxy composite exhibited a 90° tensile strength of 49.8 MPa, with matrix failure being the dominant mode in the 90° UNI laminate [44].
Additionally, Young’s modulus value of BF/Elium was 95.8 MPa, which is 58% higher than that of the BF/Epoxy composite. The difference in the strength values can be attributed to the high tensile strength of the Epoxy resin. Hence, the matrix bears most of the load in the 90° direction. The SEM images (Figure 6b) show the fibres of both composites being pulled out of the matrix, with matrix fracture being the dominant mode of failure. Hence, the failure at the interface between the BF and matrix was primarily due to shear loading, which led to matrix cracking in a direction parallel to the 0° orientation of the fibres. Additionally, the BF/Epoxy composite’s fibres tend to separate from the matrix, resulting in further ductile failure. The SEM images also show good interference adhesion between the matrix and fibre.

3.5. Flexural Performance

The flexural properties and the normalised values with the effect of seawater ageing are presented in Table 5. The flexural stress–strain curves of both composite laminates are presented in Figure 6a. The average flexural strength and modulus of the BF/Elium composite were 1128 MPa and 36.6 GPa, respectively, which are 71% and 12% higher than those of the BF/Epoxy composite. According to the datasheet of both resins (Table 3), the flexural strength and modulus of the bulk Epoxy matrix are greater than those of the bulk Elium by 40% and 9%, respectively. The flexural strength values showed decreases of approximately 12% and 41% for BF/Elium and BF/Epoxy composites, respectively, after 45 days of ageing. These reductions increased to 15% and 63% once the samples reached seawater saturation in 90 days. The good flexural properties of the BF/Elium composites align with their tensile performance and indicate strong interfacial bonding between the basalt fibres and the Elium matrix. This robust adhesion contributed to improved resistance against hydrothermal ageing compared with the BF/Epoxy composites. After 90 days of seawater exposure, the flexural modulus of the BF/Elium composite decreased by only 2.4%, whereas the BF/Epoxy composite exhibited a significant reduction of 29.8% upon reaching saturation.
The analysis of the flexural stress–strain behaviour showed that the fracture strain of BF/Elium was higher than that of BF/Epoxy under the unaged (control) condition. Hence, the BF/Elium composites demonstrate significantly more ductile behaviour, with a failure strain 73% higher than that of the BF/Epoxy composites. In addition, this difference became significantly higher following the hydrothermal ageing of 45D and 90D, with the BF/Epoxy composite showing a significant reduction in strain-at-fracture values. The t-test was conducted to evaluate the statistical significance of the flexural strength difference after 90 days of seawater ageing. The results showed a statistically significant difference between the BF/Elium and BF/Epoxy composites (p < 0.001), confirming that the BF/Elium retained significantly higher strength than the BF/Epoxy under ageing conditions. This degradation is attributed to the seawater-induced plasticisation of the polymer matrices in thermoplastic and thermoset composites, which reduces stiffness and strength. Consequently, the matrix deforms under lower loads and fails at an earlier stage. Additionally, seawater ingress at the fibre–matrix interface induces swelling stresses, weakening interfacial bonding and promoting microcrack formation that diminishes load transfer efficiency between the fibre and matrix [47,48,49].
Figure 7b presents the normalised flexural strength and modulus values of both composites. The BF/Elium composite exhibited higher normalised flexural strength and modulus, with increments of 60% and 6%, respectively, compared with the BF/Epoxy composite values. Despite the increase in density caused by hydrothermal ageing, the normalised properties of the BF/Elium composites remain superior to those of the BF/Epoxy composites (Table 5).
The failed flexural samples in the front view and side view of the BF/Elium and the BF/Epoxy coupons are shown in Figure 8. This enhanced flexural performance is primarily attributed to the strong interfacial bonding between the basalt fibres and the Elium matrix. Figure 8 shows the fracture surfaces of the flexural specimens, where both fibre and matrix breakage are observed, followed by delamination in both composite systems. Unlike the sudden failure seen in tensile tests, the flexural fractures occur more gradually after the peak load. Furthermore, Elium, as a thermoplastic acrylic resin, exhibits semi-crystalline behaviour with potential for better resistance to water diffusion compared with fully amorphous Epoxy. It was noted that lower water uptake in thermoplastics can reduce the effects of plasticisation and hydrolysis. Also, thermoplastics like Elium typically have higher fracture toughness, which can contribute to greater resistance to microcrack initiation and propagation during ageing or mechanical loading cycles. This performance can be considered a factor in maintaining mechanical integrity over time. The fracture surface observations suggest that the BF/Elium interface may remain more stable under ageing conditions compared with BF/Epoxy. This behaviour may be attributed to better stress distribution and energy dissipation at the interface.

3.6. Effect of Manufacturing Method and Reinforcement on Flexural Performance

The flexural strength and modulus can vary significantly based on the material type and manufacturing method. Figure 9 shows the flexural strength values for both BF-based and GF-based composites, ranging from 257 MPa for NCF BF/Elium (with a fibre areal density of 450 g/m2 and a fibre volume fraction (FVF) of 49%) produced via VARTM, as reported by Yaghoobi et al. [16], to 1170 MPa for UNI prepreg BF/Epoxy, as reported by Wang et al. [18]. Aside from the UNI prepreg system, no other studies in the literature have reported higher flexural strength values for BF- or GF-reinforced thermoset or thermoplastic composites than those in the present study. For instance, Davies et al. [6] reported flexural strengths of 595 MPa and 698 MPa for semi-UNI (0/90) NCF GF/Epoxy and BF/Epoxy composites, respectively. Compared with these, the NCF BF/Elium composite in this study demonstrated 102% and 62% higher flexural strength, respectively. Their composites were manufactured using VARTM with fibre areal densities of 373 g/m2 (GF) and 416 g/m2 (BF) and an FVF of 44% for both.

4. Conclusions

BF/Elium and BF/Epoxy composites were manufactured using the VARTM process and cured at RT. Physical, tensile, and flexural performance was investigated under hydrothermal seawater ageing at 45 °C for 45 and 90 days. The main conclusions from this work can be summarised as follows:
  • The density and fibre volume fraction values of BF/Elium were 1.96 g/cm3 and 53%, while those of the BF/Epoxy composite were 1.84 g/cm3 and 48%, respectively. Hydrothermal ageing increased the thickness and density of both thermoset and thermoplastic composites, attributed to moisture uptake and swelling.
  • The peak intensity levels of the thermoplastic BF/Elium composites decreased after hydrothermal ageing due to microstructural relaxation. The opposite behaviour occurred for the thermoset BF/Epoxy composites, as the peak intensity levels increased after seawater ageing due to plasticisation.
  • Hydrothermal seawater ageing acted as a plasticiser for the polymer matrices in both thermoplastic and thermoset composites, leading to deformation under lower applied loads. As a result, the stiffness, strength, and failure strain of both BF/Elium and BF/Epoxy composites were reduced during tensile and flexural testing.
  • The tensile and flexural strength values of the BF/Elium composite were 1165 MPa and 1128 MPa, which are 33% and 71% higher than those of the BF/Epoxy composite, respectively. The strength values exhibited a reduction for both composites after the coupons reached saturation in seawater.
  • The tensile and flexural modulus values of the BF/Elium composite were 44.1 GPa and 38.2 GPa, respectively, representing increases of 30% and 12% compared with the BF/Epoxy composite. Both composites exhibited stiffness reduction when the coupons reached saturation after 90 days of seawater ageing.
  • The strain-at-fracture values of the tensile and flexural tests were significantly reduced under the effect of hydrothermal ageing.
  • The failed tensile samples indicated that the size of the fibre diffused in BF/Elium is greater than that in BF/Epoxy, attributed to the higher stored elastic energy.
  • The G values were affected more by the stiffness reduction than by the increase in Poisson’s ratio values due to hydrothermal ageing, and both parameters resulted in a decrease in the G values.
  • The 90° tensile strength was 57% higher and the modulus 57% lower compared with the BF/Elium composite. In both composites, matrix fracture was dominant due to the effect of the 10 g/m2 BF density in the 90° direction.
  • The strength and modulus values of the tensile and flexural tests for both composites were normalised with respect to the density of each composite laminate. Even after normalising, the strength and stiffness values of the BF/Elium composite were higher than those of the BF/Epoxy composite.
  • In general, the flexural properties were degraded more than the tensile properties due to hydrothermal seawater ageing at 45 °C for 45 and 90 days.
The findings of this study demonstrate the superior mechanical performance of the novel BF/Elium composite, reinforcing the importance of sustainable materials for advanced applications such as the marine and tidal energy sectors.

Author Contributions

Conceptualisation, M.A.; methodology, M.A.; resources, M.A., D.M.D. and T.F.; writing—original draft preparation, M.A.; writing—review and editing, M.A., T.F. and D.M.D.; visualisation, M.A.; supervision, T.F. and D.M.D.; project administration, M.A., T.F. and D.M.D.; funding acquisition, M.A., T.F. and D.M.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research study was funded by a research project with the financial support of Research Ireland (Formerly Science Foundation Ireland, SFI) grant number 23/IRDIFB/12098.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to express their gratitude to the lead engineer, Conor Kelly, and project engineer, David Murray, at EireComposites for their guidance and contributions during the manufacturing and testing processes of the GreenCompos research project. The corresponding author would like to acknowledge EireComposites and the CTL laboratory for their support.

Conflicts of Interest

The authors declare no conflicts of interest. Author Tomas Flanagan was employed by the company ÉireComposites Teo, An Choill Rua, Indreabhán. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. VARTM setup: (1) inlet tube, (2) outlet tube, (3) spiral wire, (4) peel ply, (5) infusion mesh, (6) vacuum bag, (7) sealant tape, and (8) BF.
Figure 1. VARTM setup: (1) inlet tube, (2) outlet tube, (3) spiral wire, (4) peel ply, (5) infusion mesh, (6) vacuum bag, (7) sealant tape, and (8) BF.
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Figure 2. FTIR spectra of the BF/Elium and BF/Epoxy composites with the effect of hydrothermal ageing.
Figure 2. FTIR spectra of the BF/Elium and BF/Epoxy composites with the effect of hydrothermal ageing.
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Figure 3. (a) Tensile stress–strain 0° direction curves with the effect of hydrothermal ageing and (b) normalised tensile strength and modulus.
Figure 3. (a) Tensile stress–strain 0° direction curves with the effect of hydrothermal ageing and (b) normalised tensile strength and modulus.
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Figure 4. Failed tensile samples: (i) Front view and (ii) side view of BF/Elium and BF/Epoxy, the arrows refer to the level of diffused fibre.
Figure 4. Failed tensile samples: (i) Front view and (ii) side view of BF/Elium and BF/Epoxy, the arrows refer to the level of diffused fibre.
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Figure 5. Comparison of tensile strength among BF/Elium, BF/Epoxy, GF/Elium, and GF/Epoxy composites: 1. Present study; 2. present study; 3. David et al. [44]; 4. Farid et al. [46]; 5. Farid et al. [46]; 6. Farid et al. [46]; 7. Farid et al. [46]; 8. Yaghoobi et al. [15]; 9. Yaghoobi et al. [15]; 10. Chilali et al. [16]; 11. Chilali et al. [16]; and 12. Bandaru et al. [35].
Figure 5. Comparison of tensile strength among BF/Elium, BF/Epoxy, GF/Elium, and GF/Epoxy composites: 1. Present study; 2. present study; 3. David et al. [44]; 4. Farid et al. [46]; 5. Farid et al. [46]; 6. Farid et al. [46]; 7. Farid et al. [46]; 8. Yaghoobi et al. [15]; 9. Yaghoobi et al. [15]; 10. Chilali et al. [16]; 11. Chilali et al. [16]; and 12. Bandaru et al. [35].
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Figure 6. (a) Stress–strain curves of the 90° tensile sample. (b) SEM images of the failed 90° tensile samples of BF/Elium and BF/Epoxy.
Figure 6. (a) Stress–strain curves of the 90° tensile sample. (b) SEM images of the failed 90° tensile samples of BF/Elium and BF/Epoxy.
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Figure 7. (a) Flexural stress–strain curves and (b) normalised flexural strength and modulus of control composites.
Figure 7. (a) Flexural stress–strain curves and (b) normalised flexural strength and modulus of control composites.
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Figure 8. Failed flexural samples: (i) front view and (ii) side view of BF/Elium and BF/Epoxy, arrows refer to the fracture surface.
Figure 8. Failed flexural samples: (i) front view and (ii) side view of BF/Elium and BF/Epoxy, arrows refer to the fracture surface.
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Figure 9. Comparison of flexural strength among BF/Elium, BF/Epoxy, GF/Elium, and GF/Epoxy composites: 1. Present study; 2. Present study; 3. Wang et al. [50]; 4. Chowdhury et al. [51]; 5. Davies et al. [17]; 6. Davies et al. [17]; 7. Lopresto et al. [52]; 8. Yaghoobi et al. [15]; 9. Yaghoobi et al. [15]; 10. Bandaru et al. [35]; 11. Bandaru et al. [53]; and 12. Hussnain et al. [54].
Figure 9. Comparison of flexural strength among BF/Elium, BF/Epoxy, GF/Elium, and GF/Epoxy composites: 1. Present study; 2. Present study; 3. Wang et al. [50]; 4. Chowdhury et al. [51]; 5. Davies et al. [17]; 6. Davies et al. [17]; 7. Lopresto et al. [52]; 8. Yaghoobi et al. [15]; 9. Yaghoobi et al. [15]; 10. Bandaru et al. [35]; 11. Bandaru et al. [53]; and 12. Hussnain et al. [54].
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Table 1. Physical and mechanical characteristics of Elium®191 XO/SA resin [25] and Epoxy PrimeTM 37 resin [26].
Table 1. Physical and mechanical characteristics of Elium®191 XO/SA resin [25] and Epoxy PrimeTM 37 resin [26].
PropertyElium®191 XO/SAEpoxy PrimeTM 37
Viscosity (cP) at 25 °C100181
Density (g/cm3) at RT1.011.10
Gel Time (min) at 25 °C210–260240
Curing Process at RT *24 h infusion and curing24 h infusion and curing
Post-Curing 2 h at 80 °C16 h at 50 °C
Tensile Strength (MPa)47.1072.50
Tensile Modulus (GPa)2.683.21
Flexural Strength (MPa)80.51113
Flexural Modulus (GPa)2.773.01
* RT is room temperature.
Table 2. The structure of the BF/Elium and BF/Epoxy composites in this study.
Table 2. The structure of the BF/Elium and BF/Epoxy composites in this study.
CompositeBasalt Fibre StructurePolymer Matrix
BF/EliumBAS-UNI 550:
  -
0° 520 g/m2
  -
90° 50 g/m2
  -
Stitching 8 g/m2
Elium®191 XO/SA, three parts:
191 XO acrylic resin (50 wt%), 191 SA accelerator (50 wt%), and methyl ethyl ketone peroxide (MEKP) initiator (2 wt%)
BF/EpoxyBAS-UNI 350:
  -
0° 357 g/m2
  -
90° 50 g/m2
  -
Stitching 9 g/m2
Epoxy PrimeTM 37, two parts:
Epoxy resin (100 wt%) and slow hardener Ampreg 3X (29 wt%)
Table 3. Physical properties of BF/Elium and BF/Epoxy for 0-, 45-, and 90-day ageing at 45 °C.
Table 3. Physical properties of BF/Elium and BF/Epoxy for 0-, 45-, and 90-day ageing at 45 °C.
CompositeFVF (%)h (mm)Increment
(%)		ρc (g/cm3)Increment
(%)		Mm
(%)
BF/Elium-0D53.5 ± 0.012.46 ± 0.05-1.96 ± 0.02--
BF/Elium-45D-2.47 ± 0.020.411.99 ± 0.031.60.85 ± 0.03
BF/Elium-90D-2.49 ± 0.091.222.01 ± 0.022.41.12 ± 0.01
BF/Epoxy-0D47.8 ± 0.012.29 ± 0.07-1.89 ± 0.01--
BF/Epoxy-45D-2.32 ± 0.041.311.87 ± 0.031.751.20 ± 0.02
BF/Epoxy-90D-2.33 ± 0.061.751.90 ± 0.043.21.48 ± 0.03
Table 4. The tensile properties and the normalised values with the effect of seawater ageing.
Table 4. The tensile properties and the normalised values with the effect of seawater ageing.
Compositeσt
(MPa) 		Change%Et (GPa) Change%σtn
(MPa/(gr/cm3)) 		Etn
(GPa/(gr/cm3)) 		µ
(-)		G
(GPa)
BF/Elium-0D1165 ± 49-44.1 ± 03-59422.50.202 18.4
BF/Elium-45D1009 ± 38−13.440.5 ± 02−8.350720.30.20716.8
BF/Elium-90D933 ± 53−19.940.3 ± 02−8.646520.10.20816.7
BF/Epoxy-0D880 ± 57-34.9 ± 02-47817.90.189 14.3
BF/Epoxy-45D622 ± 14−29.333.4 ± 02−4.233218.10.19314.0
BF/Epoxy-90D538 ± 18−38.931.9 ± 01−8.528316.80.19713.3
Table 5. The flexural strength, modulus, and their normalised values with the effect of seawater ageing.
Table 5. The flexural strength, modulus, and their normalised values with the effect of seawater ageing.
Compositeσf
(MPa) 		Reduction
(%)		Ef (GPa) Reduction
(%)		σfn
(MPa/(gr/cm3)) 		Efn
(GPa/(gr/cm3))
BF/Elium-0D1128 ± 06-36.6 ± 1.49-57618.7
BF/Elium-45D992 ± 24−1234.8 ± 1.66−4.849917.8
BF/Elium-90D952 ± 84−15.635.7 ± 0.46−2.447418.2
BF/Epoxy-0D662 ± 43-32.6 ± 3.08-36017.7
BF/Epoxy-45D390 ± 33−41.126.2 ± 1.44−19.420814.0
BF/Epoxy-90D244 ± 49−63.122.9 ± 1.07−29.812918.7
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MDPI and ACS Style

Alsaadi, M.; Flanagan, T.; Devine, D.M. Seawater Ageing Effects on the Mechanical Performance of Basalt Fibre-Reinforced Thermoplastic and Epoxy Composites. J. Compos. Sci. 2025, 9, 368. https://doi.org/10.3390/jcs9070368

AMA Style

Alsaadi M, Flanagan T, Devine DM. Seawater Ageing Effects on the Mechanical Performance of Basalt Fibre-Reinforced Thermoplastic and Epoxy Composites. Journal of Composites Science. 2025; 9(7):368. https://doi.org/10.3390/jcs9070368

Chicago/Turabian Style

Alsaadi, Mohamad, Tomas Flanagan, and Declan M. Devine. 2025. "Seawater Ageing Effects on the Mechanical Performance of Basalt Fibre-Reinforced Thermoplastic and Epoxy Composites" Journal of Composites Science 9, no. 7: 368. https://doi.org/10.3390/jcs9070368

APA Style

Alsaadi, M., Flanagan, T., & Devine, D. M. (2025). Seawater Ageing Effects on the Mechanical Performance of Basalt Fibre-Reinforced Thermoplastic and Epoxy Composites. Journal of Composites Science, 9(7), 368. https://doi.org/10.3390/jcs9070368

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