BIO PUR Composite Manufacturing and Testing

BIO-PUR composite was characterized before subjecting the composite to marine exposure, to ensure its quality and avoid any unexpected behaviour due to unusual defects. The obtained test results are presented in Table 2. The ILSS value of the composite samples, extracted at various plate lengths from that shown in Fig. 1b, showed minimal variability, indicating consistent quality throughout the material. Moreover, the void content in the samples remained consistently low, with values below 0.72% in all cases. Furthermore, the fibre volume content remained constant and converged with the theoretical value of 57.1%, underscoring the composite’s uniformity.

Table 2 BIO-PUR composite properties

As shown in Table 3, the candidate BIO-PUR matrix met the mechanical and physical properties of the conventional resins used for offshore structural applications.

Table 3 BIO-PUR properties vs. conventional offshore materialsMarine Environment Performance

The BIO-PUR composite test samples were exposed for 3 and 5 months in the HarshLab during the period from July to December to cover all the expected annual conditions. Reported seawater temperature in this period on the Bay of Biscay were quite representative of the mean historic values as shown Fig. 5 [33].

Fig. 5

Seawater temperature during immersion exposures

The exposure zones were selected considering the potential final applications. Wind blades in the W-SW part of the atmospheric zone (A) and tidal blades or floating structures in the E-SE of immersion zone (I). Two replicates were analysed in each zone (A and I) and time (3 and 5 months). The designation and exposure details of the BIO-PUR composite samples are summarized in Table 4.

Table 4 Designation of BIO-PUR composite samples vs. exposure time

In order to study the worst-case scenario, no protective coating was applied, and the trimmed zones edges were not sealed. As expected, the quantity of biofouling increased progressively with the exposure time, particularly in the immersion zone, where marine organisms tend to accumulate more rapidly due to the constant contact with seawater (Fig. 6). The biofouling not only added visible mass to the composite surfaces but also could have potentially interfered with subsequent property measurements. To ensure accurate characterization, the samples were carefully cleaned prior to testing, following a standardized procedure to remove fouling and any residual contaminants (Fig. 6c). This cleaning process was essential to isolate the material properties from any extraneous biological effects, allowing us to assess the true impact of marine exposure on the BIO-PUR composites. The samples were analyzed from a physic-chemical, thermal, dynamic-mechanical and mechanical point of view using the experimental techniques summarized in Table 4.

Fig. 6

Samples after their exposure in the HarshLab: (a) atmospheric zone, (b) immersion zone

Water Absorption

Water absorption is a crucial factor that significantly influences the final properties of composite materials [34]. To evaluate this further, the weight change of the composite parts exposed to different offshore conditions was evaluated (Table 5).

The average moisture uptake Mt was calculated as Osa-Uwagboe et al. [35]:

$$\:Mt=\frac_-_}_}100\%$$

Where Mt is the percentage of moisture gained, while M0 is the initial mas of the sample and M1 is the mass of the sample at a specific time.

The results reveal that the water uptake in all cases remains below 0.90%. Notably, no statistically significant differences were observed between plates exposed in different zones or those with varying exposure times. Thus, it can be inferred that the samples exhibit minimal water absorption after immersion in the sea water.

Table 5 Water uptake vs. exposure time of the samples exposed to marine environment in the different zonesThermogravimetric Analysis

In Fig. 7, the weight loss as a function of temperature and its derivative curves, obtained by thermogravimetric analysis, are presented. The decomposition of BIO-PUR takes place in two main stages. The initial region, between 200 and 350 °C, corresponds to the dissociation of urethane bonds and polyol decomposition, and the second subsequent region (370–500 °C) refers to the degradation of residues formed during the previous degradation stage, which eventually lead to the formation of carbon residues [15]. For the BIO-PUR composites, exposed both in the atmospheric and in the immersion zones, three distinct decomposition processes are observed. A first peak coinciding with the first observed in BIO-PUR, a second, almost undetectable, also coinciding with the one observed in the BIO-PUR, and a third, between 500 and 600 °C, which could correspond to the release of the degraded BIO-PUR trapped between the glass fibre reinforcement, which is stable in the range analysed [36, 37].

Fig. 7

Weight loss with temperature of composites: (a) atmospheric zone and (b) immersion zone, and their derivative curves: (c) atmospheric zone and (d) immersion zone

Consistent with the earlier findings reported in this study, the BIO-PUR composite samples exposed to both immersion and atmospheric conditions show no signs of water absorption, with no significant weight loss observed at around 100 °C. This agreement across multiple analyses reinforces the conclusion that BIO-PUR composites exhibit minimal water uptake under marine exposure conditions.

Moreover, no differences in thermal decomposition were observed between the composite samples exposed to both atmospheric and immersion zones. Compared to the reference sample, the exposed BIO-PUR composites exhibit identical decomposition peaks and corresponding mass losses at the same temperatures, as shown in Fig. 6. Finally, the average residue content observed in the composite samples, 54% ± 6, is consistent with the reinforcement content. The fibre volume of the samples was determined taking into account the carbon residue contributions from the matrix (12%) and the reinforcement (98%) and relating these values to the total fibre volume of each sample.

Dynamic Mechanical Analysis

The thermomechanical behaviour of all BIO-PUR composites exposed to different zones and times was analysed by DMA. Figure 8a and b show the average storage modulus and tan δ curves as a function of temperature of a BIO-PUR (each curve represents the average of two replicate tests) composite exposed in different zones and times. The glass transition temperature (Tg) of the material was determined as the temperature corresponding to the peak of tan δ. The Tg values for the samples exposed to atmospheric conditions are equivalent to those exposed to immersion conditions. Furthermore, exposure time did not have a significant effect on the Tg values, indicating the stability of the BIO-PUR composites. In all the cases, the values, including the reference unexposed sample, were in the range of 154 ± 3 °C, within experimental error. In addition, the behaviour of the storage modulus observed in the thermomechanical curves is also similar. Notably, the composite modulus remains unchanged after exposure to the offshore environment, with values also aligning with the reference unexposed sample, recorded at of 33 ± 2 GPa. Overall, these findings underscore the robust thermal and mechanical stability of BIO-PUR composites, demonstrating their resilience under various environmental conditions.

Fig. 8

Average loss factor (tan δ, dashed line) and storage modulus (E’, continuous line) vs temperature of composites: (a) atmospheric zone and (b) immersion zone

Mechanical Properties

Figure 9 presents the average results of interlaminar shear strength tests for the BIO-PUR composite plates (in this case, each curve represent the average of five individual test results). Notably, after exposure to the HarshLab environment, both in atmospheric and immersion zones for 3 and 5 moths, Fig. 9a and b, respectively, no significant change is observed in the ILSS properties. The three curves display a high degree of similarity, indicating that the BIO-PURs did not undergo degradation due to the marine environment, neither in the atmospheric nor in the immersion zones. These mechanical test results corroborate the findings obtained from the DMA tests, leading to the conclusion that exposure to a marine environment for several months does not compromise the final properties of the BIO-PUR composite, thereby maintaining the required performance for its intended applications.

Fig. 9

Average ILLS results of composites before and after their exposure to the offshore environment: (a) atmospheric zone and (b) immersion zone

Fourier Transform Infrared Spectroscopy

Finally, FTIR-ATR spectroscopy was used to study any chemical changes occurring on the composite surface due to the different degradation processes identified in marine environment, including hydrolytic, thermo-oxidative, mechanical, biological, and photodegradation [38].

The spectra of the BIO-PUR samples before and after exposure to the marine environment did not show significant differences except for the disappearance of the residual isocyanate peak at 2270 cm−1 (Fig. 10). This is because the residual isocyanate groups react with the moisture present in the environment. However, this is an expected behaviour that doesn’t affect the BIO-PUR properties.

Fig. 10

FTIR-ATR spectra of for the BIO-PUR composites: (a) atmospheric zone and (b) immersion zone

Regarding changes in the absorption bands that could indicate degradation by UV irradiation, it should be noted that no alterations were detected. In degraded PUs quinone products accumulate due to chain scission, leading to the formation of coloured compounds. This is normally indicated by the reduction in intensity of FT-IR signals at 1597 cm⁻¹ and 814 cm⁻¹, corresponding to the stretching vibration of double bonds in the aromatic ring and the out-of-plane C–H bending vibration in 1,4-disubstituted aromatic rings [39, 40]. In the spectra presented in Fig. 10, these changes were not found. Another observed effect is the replacement of the signal at 3328 cm⁻¹, which is typical of the stretching vibration of the N–H group, by a prominent peak featuring two characteristic peaks at 3435 cm⁻¹ and 3251 cm⁻¹. This change indicates the degradation of urethane structures due to UV exposure [39]. However, none of these changes were observed in the spectra of the BIO-PUR composites.

Based on these results, it can be concluded that there was no degradation or alterations of BIO-PUR composites final properties after 3- and 5-months exposure in offshore environment in atmospheric or immersion zones.