1. Introduction
The intensive development of science and various technologies resulting from the constantly growing demand for innovative materials as well as the growing human awareness and legal changes related to the replacement of common raw materials with those that come from renewable sources are currently the main factors of progress in modern science [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10].
Petroleum-based epoxy resin is a thermosetting polymer that is most commonly used in composite materials [
11]. Due to high mechanical strength, good adhesiveness, low cost, high thermal stability, good chemical resistance, and ease of use of epoxy resins, they are successfully used in load-bearing applications (aerospace, automotive, construction, and marine). In addition, they are also utilized in the production of coatings, adhesives, insulation, and high-performance composites [
4,
12,
13,
14,
15,
16]. However, due to the insolubility and non-fusibility of epoxy resins, they are difficult to recycle, which results in a waste of resources and limits their applications [
17]. This problem was solved in 2011 by Ludwik Leibler and co-workers [
18], who discovered a new class of materials, namely epoxy vitrimers, which are reversible covalently cross-linked networks and, in addition to having excellent mechanical behavior, are characterized by good reprocessability and recyclability [
19,
20,
21]. An interesting approach to obtaining bio-based epoxy vitrimers was demonstrated by Liu et al. (2021) [
22], who synthesized two dynamic imine bond curing agents using m-xylylenediamine, 1,6-hexanediamine, and biomass energy vanillin as raw materials, which were then cured with DGEBA. The authors observed that the functional properties of these composites were comparable or even better than those for conventional epoxy resin; however, they exhibited degradable characteristics due to the hydrolysis of imine bonds. Another disadvantage of cured epoxy materials is that they can be stiff and brittle, depending on the hardener used [
23]. Therefore, scientists are constantly working on methods related to improving their physical performance [
24].
One of the common ways to refine their physical properties is the addition of fillers to the epoxy matrix, which not only reduce the production costs but also improve the mechanical properties of these composites [
25,
26]. However, in many cases, an additional functionalization of the filler is necessary to improve the interphase between the polymer matrix and the additive and, consequently, the overall composite properties. There are many examples in the available literature in which fillers of natural (jute flour, coconut shell, waste peanut shell, banana bark, etc.) or synthetic origin (aramid and graphite fibers, etc.) were used for the epoxy resin [
8,
27]. Natural fibers are lighter, cheaper, and better in terms of natural environment protection [
28]. For example, Salasinska et al. [
29] prepared the epoxy composites modified with ground walnut shell in the amounts of 20, 30, 40 and 50 wt%. It was found that the incorporation of these organic waste fillers improved the thermal stability, stiffness, and hardness of epoxy composites; nonetheless, their tensile strength and impact resistance decreased in comparison to the unmodified resin. Similarly, Barczewski et al. (2019) [
30] observed a reduction in the mechanical performance of epoxy materials filled with sunflower husk. Another example of the use of ecological fillers in the form of hemp fibers for epoxy resin was presented by Gargol et al. (2021) [
31], whose test results showed that 30 wt% of these fibers causes a decrease in strength from 40.2 MPa to 4.2 MPa compared to the unfilled sample. In addition, the hardness of these composites also decreased. Therefore, currently, the most commonly used fibers in the epoxy matrix are glass and carbon fibers in the form of mats or short-cut fibers due to their outstanding mechanical behavior [
27].
Another important aspect is the service life of epoxy resin products. During operation, these materials may be exposed to various environmental factors (solar radiation, elevated temperature, and many others) [
32]. For example, epoxy resin composites are commonly used in the aerospace industry where they are exposed to high-energy gamma radiation [
33]. Therefore, it is important to design the products so that they are safe and non-defective throughout their lifetime. For this purpose, simulated aging tests are usually carried out in appropriate chambers [
34]. Moreover, due to the flammability of epoxy resin-based materials and the high fire hazard, appropriate chemicals are required to ensure their safe use and to provide them with flame-retardant properties. For example, Wang et al. (2021) [
35] prepared an effective fire retardant system for epoxy resin by introducing 15 wt.% silica and a 5 wt.% phenethyl-bridged 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivative. The limiting oxygen index increased from 21.8 to 30.2% compared to the pure epoxy resin sample.
Exposure to solar radiation and elevated temperature causes photochemical damage, which results in the degradation of composite elements and deterioration of their performance properties. In order to prevent the undesirable changes in polymer products caused by aging processes, there are many ways to improve their aging resistance [
36]. Currently, the most common method to mitigate the photooxidation effects is to introduce antioxidant compounds or a small amount of inorganic fillers to the polymer matrix [
27,
37,
38,
39]. Antioxidants are active compounds that are able to reduce the rate of photooxidation by removing or inactivating free radicals through donating hydrogen atoms, which in turn interrupts the chain reactions [
40]. However, some researchers observed that these substances added to the polymer matrix cause its relaxation or affect the glass transition temperature (Tg) [
41]. Siddiqui et al. (2021) [
42] demonstrated that thymol, carvacrol, limonene, and cinnamaldehyde introduced into the polylactide (PLA) matrix resulted in a decrease in the Tg value due to the plasticizing effect of these compounds. On the other hand, in the case of the second group of additives, i.e., inorganic fillers, Khotbehsara et al. (2020) [
43] showed that the incorporation of hydrated alumina powder and fly ash can significantly improve the UV resistance of epoxy resin. In another study, Wang et al. (2023) [
44] explored the role of graphene in enhancing the resistance to radiation of epoxy resin. Their results showed that as a result of gamma irradiation, the glass transition temperature, nano-indentation depth, and hardness of graphene-modified epoxy resin composites decreased by 20.32%, 416.3 nm, and 16.00%, respectively, whereas for unmodified epoxy resin, they decreased by 30.34%, 502.1 nm, and 41.82%, respectively. It was proved that the addition of graphene nanoparticles can reduce the free radical content in epoxy resin and inhibit its aging process. According to Zhang et al. (2015) [
45], this phenomenon can be related to the adsorption of oxygen-containing free radicals by the surface defects of graphene oxide, which are then consumed by surface oxygen-containing groups (e.g., quinones) via single-electron reduction, ultimately quenching oxygen.
Taking into account all the aspects described above, the aim of this work was to design composites based on epoxy resin which, as a result of appropriately selected additives of natural origin, in addition to increasing their recycling potential and being more environmentally friendly, will also retain their functional properties (e.g., durability, hardness, and thermal resistance) and will be characterized by improved resistance to various environmental factors during their operation. Therefore, polylactide in the powder form has been proposed as a biofiller, and natural quercetin was used to provide the epoxy matrix an antioxidant effect. The material compositions proposed in this article may be an excellent alternative to solutions commonly used in the epoxy materials industry, which additionally solve the problems related to the use of compounds of natural origin in epoxy composites described by other authors. Moreover, such an approach has not been described in the literature so far; therefore, it is definitely a novelty in science.
2. Materials and Methods
2.1. Materials
An epoxy resin containing flame retardants (ASSET 1011) supplied by New Era Materials (Modlniczka, Poland) was used as a polymer matrix. According to the MSDS provided by the resin producer, this mixture contained about 73 wt.% pure epoxy resin, less than 18 wt.% poly(ammonium phosphate), and about 9 wt.% expanded graphite. The second polymer used in this study was an eco-friendly polylactide in the powder form (PLA RXP 7501, MFI = 75 g/10 min) obtained from Resinex (Warsaw, Poland). Moreover, natural quercetin (quercetin hydrate, ≥95%) supplied by Sigma-Aldrich (Steinheim, Germany) was incorporated into the epoxy resin containing polylactide. Fiberglass fabric (RXT 350 g/cm2) obtained from Rymatex (Rymanów, Poland) was used as a reinforcement in the produced composites.
2.2. Preparation of Composites
The process of producing composites consisted of 3 stages: preparation of the prepreg and its plasticization, cross-linking, and hardening. First, all ingredients were weighed on a laboratory balance by sieving them through a strainer directly into a beaker (resin) or a plastic cup (polylactide and quercetin). Then, the additives were added to the resin through a strainer and homogenized for about 3 min. The weight composition of the prepared samples is shown in
Table 1. The next step was the prepreg preparation. Each prepreg consisted of 3 layers of epoxy resin with or without additives and 3 layers of fiberglass fabric. Ready prepregs were processed (plasticization, cross-linking, and hardening) in accordance with the conditions presented in
Table 2.
2.3. Accelerated Aging Test
Fabricated composites were exposed to the accelerated aging test in an Atlas SC 340 MHG solar simulator climate chamber from AMETEK Inc. (Berwyn, IL, USA) that was equipped with a 2500 W MHG lamp. A unique range of solar radiation (UV, Vis, and IR) was provided by a rare-earth halogen lamp. The samples were aged for 800 h at a temperature of 70 °C with maximum solar radiation throughout the aging process.
2.4. Surface Wettability and Determination of Surface Energy
Surface energy is a measure of the interaction of a solid substrate with the liquid that wets the tested material, and its determination consists in determining the contact angles by appropriately selected liquids, the surface tensions of which are known, including their dispersion and polar parts. In this case, distilled water, diiodomethane, and ethylene glycol were used. In addition, the Owens–Wendt–Rabel–Kealble (OWRK) model was applied, thanks to which the surface energy was calculated including its polar and dispersion components:
The ratio of polar and dispersion constituents affects the wetting and adhesion of the two tested phases. The polar component of surface energy is the sum of acid–base, hydrogen, and inductive forces, while the dispersion part is the magnitude of intermolecular forces such as Van der Waals forces. The more similar the ratio of these two components, the greater the interactions and the stronger the adhesion between both phases [
46,
47].
The test was carried out using a goniometer (OCA 15EC) equipped with a camera from DataPhysics Instruments GmbH (Filderstadt, Germany), a Braun DS-D 1000 SF syringe, and SCA20 software and it consisted of placing 6 drops of each measuring liquid with a volume of 1 µL on the substrate of each test sample from the side where the last layer was the resin. The obtained results from the static contact angles were used to determine the surface energy of the tested materials by the SCA20 program (version 1.0).
2.5. FTIR Spectroscopy
Fourier transform infrared spectroscopy (FTIR) was performed using a Nicolet 6700 FT-IR spectrometer from Thermo Scientific (Waltham, MA, USA), which was equipped with a diamond Smart Orbit ATR sampling equipment. The FTIR spectra were recorded in the range of 4000–400 cm
−1 using 64 scans and at a resolution of 4 cm
−1. The test was performed for all tested materials before and after 800 h of solar aging. On the basis of the spectra obtained, the carbonyl index was calculated, which can be the basis for the assessment of the material degradation progress [
48,
49,
50]:
2.6. Color Change Measurements
The color change analysis of samples based on the epoxy resin was carried out according to the PN-EN ISO 105-J01 standard by applying the UV-VIS CM-3600d spectrometer from Konica Minolta Sensing, Inc. (Osaka, Japan). Each sample was tested in at least three different places. Then, the obtained results were interpreted in the CIE-Lab space, thanks to which a color description was obtained by designating three coordinates (L*, a*, and b*). The first of them is L—a lightness indicator with a value range from 0 to 100, where 0 means black and 100 means diffuse white. The second coordinate is the a* parameter, whose positive value means red and negative value means green. For the third parameter (b*), a positive value means yellow, and a negative one means blue [
51,
52]. The color change of the tested materials was calculated with the formula below [
53]:
2.7. Hardness Tests
Hardness measurements were conducted according to the PN-71/C-04238 standard. Two digital hardness meters were used, one on the Shore “C” scale and one on the Shore “D” scale (Zwick GmbH&Co, Ulm, Germany, pressure 50 N). Hardness tests were performed for all samples before and after 800 h of aging.
2.8. Optical Microscopy
Images of microstructures obtained on cross-sections were made using a Keyence VHX-950F microscope equipped with VH-Z100R lens. Hybrid lighting mode (bright and dark field) at 200× magnification was used.
2.9. Thermogravimetric Analysis
Thermogravimetric analysis (TGA) allowed for the assessment of the influence of applied additives incorporated into the epoxy resin on its thermal degradation process, which involves the mass change of a material as a function of raising temperature. During the analysis, a Mettler Toledo TGA/DSC 1 STARe device equipped with a GC10 gas controller (Greifensee, Switzerland) was used. The measurement was carried out in the oxidizing atmosphere of synthetic air in the temperature range of 25–1000 °C, at a heating rate of 20 °C/min, and an air flow of 60 cm3/min. The test samples were placed in crucibles made of alumina.
2.10. One-Directional Tensile Test
The tensile tests were carried out on the basis of standard UNE EN ISO 527-1:2020-01 using an INSTRON testing machine. The constant speed of the moveable traverse was assumed to be stable and equal to 2 mm/min. As a result of tests, the diagrams of force vs. elongation were obtained. The samples were cut out from the plate according to the scheme shown in
Figure 1a, where the symbols mean the following: MD—main direction; PD—perpendicular direction; and 45—angle orientated to the plate edges. The dimensions of the samples were based on the aforementioned standard: L = 170 mm, b1 = 40 mm, w1 = 10 mm, w2 = 20 mm, and mean thickness t = 3 mm (
Figure 1b). The Young’s modulus of the composite was determined by using an extensometer with a gauge length of 50 mm.
4. Conclusions
The following paper proposes the use of a biofiller in the form of polylactide and quercetin for epoxy resin composites intended for seat elements in bulk transport. Their processing was carried out in three stages: prepreg preparation and plasticization, cross-linking, and hardening. The aim of this research was to investigate the effect of the selected additives on the aging behavior of the epoxy matrix. Therefore, the produced samples were subjected to solar aging at a temperature of 70 °C for 800 h.
Microstructure analysis of these composites showed that the introduction of both additives did not contribute to an increase in the matrix defectivity, e.g., in the form of porosity. The proper integration of additives into the composite matrix and low porosity could have a beneficial effect on good mechanical properties, which were significantly better compared to other bioadditives described in the literature. Moreover, based on the TGA results, it was observed that the incorporation of PLA and quercetin allowed for the maintenance of the thermal resistance of the epoxy resin. In the case of one-directional tensile tests, a slight growth in the strength and stiffness was noted for the composite containing polylactide.
Based on the contact angle and color change measurements, a high susceptibility of quercetin to oxidizing was observed; however, it is possible that by its oxidation and reaction with free radicals, the polymer matrix was protected during exposure to elevated temperature and solar radiation. This was confirmed by the FTIR analysis, where the value of the CI for the composite with the addition of polylactide and quercetin was lower than for the reference sample. In addition, the hardness of the tested composites did not deteriorate as a result of aging tests. Therefore, it can be concluded that the samples produced were characterized by good compatibility and no structural defects. The obtained composite structures may be a good alternative to traditionally used systems as seat elements in rail vehicles, which are not only characterized by high aging resistance but are also more eco-friendly.