Revision as of 12:10, 7 July 2025

1. Introduction

It is estimated that, under the right conditions, ocean energy could contribute around 10% of EU power demand by 2050 [1], thus potentially playing a considerable role in its energy matrix in a future.
Like in wind energy, the blades are the most challenging structural component, so the materials used are usually composites. This poses the problem of end-of-life disposal. It is estimated that the wind blade waste material in Europe, coming either from replacements or decommissioning, will be close to 150 000 tons in 2025 [2]. While much research exists for recyclability of wind turbine blades [3] not much has been done for tidal turbines.
This paper presents an overview of the initial stages of an ongoing research project on this very matter. We will discuss some key aspects of the choice of a suitable sustainable material and the extensive testing necessary to confirm its suitability to this end.

2. Materials selection

Many constraints arise from the application. The material chosen must have particularly good mechanical properties, behave well in the particular environment, allow production methods adapted to the production of large parts and thick walls, and have interesting sustainable aspects.

In the resin selection process, both conventional thermoset systems and emerging sustainable alternatives were evaluated. Their sustainable character was justified by different means: they were either biobased, recyclable, or reprocesable. While bio-based resins offer a renewable origin, they were ultimately excluded due to their inability to be recycled, which limits their alignment with full life-cycle sustainability objectives. Instead, the project selected Elium, is a reactive thermoplastic acrylic resin that combines excellent mechanical performance with recyclability and reprocessability capabilities [7]. Elium can be processed via infusion due to its low viscosity at ambient temperature and can later be reshaped or recycled via pyrolysis or mechanical means. This makes it particularly suitable for circular economy applications, offering a more comprehensive sustainability profile than bio-based systems, which offer limited end-of-life recovery solutions.

For the reinforcement, multiple candidates were considered, including glass, carbon, basalt, aramid, and natural fibers. Choice was based on cost, moisture resistance, mechanical properties, and carbon footprint. While basalt fibers presented promising environmental and mechanical performance, glass fiber was selected as the optimal reinforcement due to its lower cost, widespread use in marine applications, and well-documented hydrothermal stability. Comparative studies indicated that both glass/epoxy and basalt/epoxy composites exhibit similar water absorption and post-immersion mechanical degradation. However, glass fiber offers higher commercial availability and requires less optimization at the fiber-matrix interface, making it the more reliable and practical choice for tidal blade applications [4][5][6]. Also, given its widespread use, it allows for more direct comparison of the resulting composite with known alternatives. Finally, given the characteristics of the resin, this choice does not hinder the sustainable character of the composite.

3. Thermal analysis

The characterization effort began with the study of the resin itself.

Rheological and kinetic studies were carried out to ensure the behaviour of the resin was aligned with the data supplied by the manufacturer with satisfactory results.

The kinetic study was later complemented by comparing cured and postcured unreinforced samples, with the objective of comparing their glass transition temperature (Tg). A differential scanning calorimetry (DSC) was carried out for both cases. The resulting calorimetry curves are shown in Figure 1.

The red curve corresponds to samples that underwent 24 hours of ambient temperature curing. The samples from the black curve were cured in the same fashion, and then postcured for 3 hours at 80ºC.

In the first case, the Tg was found to be relatively low: 60ºC. Also, the exothermal peaks present at temperatures over the Tg give away residual curing. The postcured samples, instead, presented a significantly higher Tg of 92ºC, and no extra exothermal peaks, confirming complete curing was achieved.

4. Mechanical testing

With the resin duly characterized, it’s time to face the main object of this paper, i.e. the mechanical characterization of the composite. Four types mechanical tests were performed (see Table 1):

In order to give a more comprehensive characterization regarding the project this research is framed in, these tests were performed on both virgin and aged samples.

The test specimens have been manufactured under the following conditions to simulate the infusion process:

• Method: Manual lamination followed by curing in a vacuum press.
• Lamination: 5 layers of biaxial glass fibre.
• Conditions: 600 mbar vacuum.
• Cure schedule: ambient cure – 24 h, post-cure: 3 h at 80°C.

4.1. Diffusion Kinetics

Glass fiber composite plastic (GFRP) composite samples were aged by full immersion in deionised water. Full immersion was chosen to replicate the accelerated degradation mechanisms that can occur in the tidal blade due to prolonged seawater exposure. However, since seawater contains salts that hinder diffusion, moisture uptake is lower compared to pure water. For this reason, deionised water was selected in order to simplify the nature of the degradation reactions. It was also considered to represent a more extreme scenario and to enable a more conservative assessment of material degradation [7].

Given that the heat deflection temperature (HDT) of the Elium matrix is 75 °C, ageing was conducted at 70 °C to prevent material distortion while still accelerating the ageing process.

Ageing was continued until the samples reached saturation. Samples with varying geometries were prepared to accommodate the different mechanical tests mentioned before.

After cutting the composite samples, their cross-sections were sealed using Elium resin to ensure material compatibility. A double sealing approach was used, consisting of two thin successive coating layers to ensure a complete and reliable seal.

The ageing process was concluded once the samples reached saturation. All samples exhibited Fickian-like diffusion behaviour [8], as shown in Figure 2. Upon saturation, the composites showed a weight gain of approximately 2.5% to 3%, which was attributed to the relatively high void content, measured between 3% and 5%. Detailed void content measurements are presented in the next section.

Due to this elevated water uptake, it was accepted that the tested laminate represented a worst-case scenario in terms of performance under water. The increased void content further highlighted the necessity of the optimisation of the resin infusion manufacturing process, which could significantly reduce water uptake and increase material performance. In general, voids are known to act as pathways for water ingress, and their presence contributes directly to increased moisture absorption in composite materials [9].

An additional set of samples was aged under the same conditions for 24 hours to verify whether any further absorption occurred post-saturation. The results confirmed a stable, Fickian diffusion curve, with no further mass gain as shown in Figure 3.

After ageing, the samples were subjected to a controlled drying process under vacuum at 45°C to assess any potential weight loss, which would indicate leaching by hydrolysis – a sign of chemical degradation induced by the aqueous environment. As also shown in the desorption curve of Figure 3, no such weight loss was detected, and the samples regained their original mass. The presence of physical (reversible) ageing was further validated by thermal analysis experiments, detailed in the Thermal Analysis vs Ageing section.

4.2. Void Content Study

The void content analysis aimed to assess the relationship between the amount of voids in the glass fibre laminates and the ageing environment to which they were exposed. Hence, void content was measured in composite samples both in their virgin state and after ageing.

The procedure involved determining the fibre weight fraction using the loss on ignition technique, following ASTM D2734 – 94. The analysis was conducted in ceramic crucibles, where samples were heated to 600 °C for 45 minutes. After the thermal cycle, they were allowed to cool in the furnace before weighing.

A high void content was recorded, ranging between 3.5% and 5.2%. This level of porosity did not change with ageing, as seen in Figure 4, despite exposure to elevated temperatures, which typically accelerates degradation, especially in regions with pre-existing defects [8].

A lower void content would likely result in better fibre/matrix interfacial performance, as encountered in later sections. This elevated porosity highlights the need to optimise the manufacturing process, which was identified as one of the main challenges when working with this material.

4.3. Thermal Recovery Post-Ageing

A further DSC test was carried out to compare samples in their virgin state, and during and after ageing, as well as after redrying. Figure 5 shows that material properties are recovered after drying, indicating that the ageing experienced was physical rather than chemical in nature, as all samples exhibited the same Tg of around 91°C. This conclusion is further supported by the mass measurements presented in a previous section, which show that the samples returned to their original weight with no net loss after ageing, as previously discussed. The absence of phenomena such as leaching suggests that no chemical alterations occurred in the matrix, ruling out irreversible degradatio

4.4. Mechanical tests' results

As mentioned before, four types of mechanical tests were carried out: shear, longitudinal and transversal tensile and compression.

Table 2, above, shows a summary of the testing results. The values demonstrate performance that is comparable to – and in some cases exceeds – that of conventional polyester [10] [11] [12] and vinyl ester [13] FRPs used in similar applications. This confirms the material’s ability to perform reliably, despite employing a relatively new resin system with still limited industrial application.

In order to have a view of the progression of the degradation caused by the ageing process, samples were retrieved after 1.5 hours of ageing and tested as well.

Figure 6, below, summarizes the relative degradation observed for each mechanical property tested. For each of the studied properties, the reduction with respect to the baseline of Table 2 is shown as a function of the ageing period.

4.4.1. Longitudinal tensile tests

Following ageing, a 6.5% reduction in the longitudinal Young’s modulus and a 14.5% decrease in tensile strength were recorded. The modest drop in stiffness suggests that the glass fibres largely retained their integrity. However, the more pronounced loss in tensile strength indicates that ageing significantly affected the matrix and the fibre/matrix interface. Moisture absorption and thermal exposure likely caused matrix plasticization, microcracking, or debonding at the interface, reducing the material’s tensile load-bearing capacity.

4.4.2. Transverse Tensile Strength

In the transverse direction, the composite experienced a 28% decrease in Young’s modulus, reflecting a notable loss of stiffness. However, transverse tensile strength dropped by only 8%, suggesting that while the material became less stiff in this direction, its ultimate strength remained largely intact. This implies that ageing mainly affected the matrix and interface, reducing rigidity, without fully compromising the composite's ability to carry loads in the transverse direction.

Longitudinal Compression Strength
A 38% reduction in compressive strength was observed after ageing, representing a significant loss and pointing to multiple possible degradation mechanisms:

• Matrix plasticization: The Elium matrix, upon absorbing moisture, likely softened, losing rigidity. This compromises its ability to laterally support the fibres, which is crucial under compressive loads where fibre buckling is a primary failure mode.
• Interface weakening: Moisture and heat exposure may have degraded fibre/matrix adhesion, limiting load transfer and increasing the risk of localised fibre buckling. This interfacial cohesion loss impacts the structural compressive response.
• Influence of void content: The measured void content (3.5–5.2%) may have facilitated the formation of kink bands or local failures. Voids reduce local stiffness and serve as damage initiation sites under compressive stress.
• Absence of chemical ageing: DSC results and Tg recovery in redried samples confirmed that ageing was physical and reversible, with no chemical matrix degradation. Therefore, the drop in compressive strength is attributed to temporary physical changes in the matrix and interface weakening, rather than irreversible polymer chain scission or leaching.

Taken together, the 38% drop in compressive strength demonstrates that even physical ageing can substantially affect the structural performance of the material, particularly in systems with elevated porosity. With respect to the uncertainty surrounding the failure mode, electron microscopy of the fractured regions is a known method to gain insight into the underlying mechanisms [7].

4.4.3. Short-beam Shear - Interlaminar Shear Strength (ILSS)

A 35% reduction in ILSS was observed after ageing. This significant drop indicates a considerable deterioration of the fibre/matrix interface, which plays a critical role in resisting interlaminar shear stresses. The combined effects of moisture absorption and thermal exposure weakened the adhesion between fibres and matrix, reducing the material's ability to efficiently transfer loads and compromising its shear strength across layers.

Interface weakening is a well-known ageing mechanism, as water tends to accumulate at the fibre/matrix bond, diminishing adhesion. The percentage loss in ILSS following ageing is comparable to that reported for conventional composites such as vinylester- or polyester-based GFRPs, especially considering the severe hydrothermal ageing conditions applied. Moreover, the material’s porosity likely contributed to water uptake, as higher void content is a recognised initiator of interfacial failure, facilitating water storage between fibre and matrix [8].

5. Conclusions

The accelerated ageing study conducted on the glass fibre/Elium resin composite (GF/Elium) provided a comprehensive evaluation of the material’s performance under elevated humidity and temperature conditions. The results reflect a generally strong performance, even under a challenging scenario characterised by high void content (3.5–5.2%) and absence of surface protection.

From a thermal perspective, the DSC analysis showed the recovery of the Tg in the dried samples, which confirms that the ageing process was of a physical and reversible nature, with no evidence of irreversible chemical degradation in the polymer matrix. Physical ageing was also confirmed by redrying the aged specimens under a rigorous drying schedule, which returned to their original, virgin weight, with no weight losses, which may be attributed to irreversible, chemical ageing mechanisms, such as hydrolysis.

In terms of mechanical performance, moderate to significant losses were observed, particularly in compressive strength (−38%) and interlaminar shear strength (ILSS, −35%). These reductions were primarily linked to weakening of the fibre/matrix interface and the role of void content in initiating structural damage. Nevertheless, the longitudinal modulus and fibre integrity remained only marginally affected, indicating that the reinforcement retained its primary structural capacity. This demonstrates that, with proper process control, the material has clear potential for demanding structural applications.

In summary, the GF/Elium composite demonstrated good overall resistance to physical ageing under hydrothermal conditions, reinforcing its suitability as a structural material—provided that key process parameters such as void content are optimised and appropriate surface protection is considered. It can thus be concluded that the Elium system can serve as a more sustainable composite constituent with adequate performance in structural applications and under harsh hydrothermal conditions.

References