ENR/PLA blend samples were filled with commercially available cellulose (used commonly for paperboard production and paper coating), flax fibers, and montmorillonite. Plant-derived natural fibers were expected to enhance the biodegradation potential of the prepared samples and contribute to mechanical performance improvement.
Additionally, montmorillonite (MMT) was employed in the role of a supplementary natural inorganic filler that could possibly affect the degradation characteristic and mechanical performance of the analyzed polymer blends.
As was mentioned above, samples prepared in this research were analyzed regarding the influence of the applied filler on the ENR/PLA blend properties and its potential impact on the specimen degradation process. Therefore, the carried-out measurements were carefully chosen. The static mechanical analysis was carried out to assess the influence of employed fillers on the performance of the prepared ENR/PLA blend specimens. Further, a swelling experiment was carried out to assess the crosslinking density as it was reported that the bonds created between different polymer macromolecules may have an influence on the mechanical properties of ENR/PLA blends and might exhibit a stabilizing effect on the properties of the material during the aging processes [
40]. Moreover, it is inferred that the polymer composites of a higher polar component of surface free energy are more prone to degradation during the aging process regarding the ongoing radical reactions [
41], e.g., oxidation. Therefore, the contact angle measurement, based on which the surface free energy value and its components were calculated, was carried out.
2.1. Characterization of Specimens Before the Accelerated Ageing Process
Regarding the data gathered in
Figure 2a, it is possible to observe some variations in the swelling behaviour of prepared ENR/PLA polymer blend samples considering the filler type and employed crosslinking system. In turn, mass rise after the swelling process was proportional to the amount of solvent trapped in the structure of the analyzed specimen. The more solvent that was accommodated inside the polymer network, the lower the crosslinking degree between the polymer macromolecules. Thus, it could be concluded that natural fibers (cellulose fiber (CF), flax fiber (FF)) addition might contribute to the improvement in the density filler–polymer matrix interactions. This may be caused by both the physical interactions between hydroxyl groups of cellulosic material and oxirane rings of ENR or ester moieties of PLA (
Figure 1) and covalent chemical bonds created during the vulcanization process. Thus, both physical and chemical crosslinking were present in the system described above. Moreover, lower swelling of CF- or FF-loaded specimens may also be affected by the filler’s poor affinity towards the solvent herein toluene.
On the contrary, MMT incorporation into ENR/PLA blends caused an opposite effect—the investigated sample was swollen similarly to the neat ENR/PLA blend. This may indicate that MMT particles, which are in the shape of plates, could behave as a steric blockade and prohibit the creation of crosslinks in the structure of the polymer blend.
Moving forward, the mass loss detected during the swelling process might be attributed to some low molecular weight additives rinsed when the sample was put in the solvent environment. This could be explained by the fact that low molecular weight additives usually migrate to the surface of the polymeric material [
42]. Therefore, washing out of some part of the applied additives is, in most cases, inevitable. Considering the errors accompanying the main bars (
Figure 2a), the mass loss was on the same level throughout the analyzed samples, no matter what the filler was.
The highest tensile strength was evidenced in the case of the lowest swollen sample, namely ENR/PLA + FF. This results in the material stiffening and loss of the ability to elongate (
Figure 2b,c). An intense improvement in tensile strength of the sample filled with FF in comparison to the CF-filled ENR/PLA blend could be caused by the different structures and performances of these two fillers. Flax fibers are composed of various additional substances rich in hydroxyl moieties, e.g., lignin, hemicellulose, which may affect the polymer matrix–filler interface and intensify the predicted interactions (
Figure 1). On the other hand, cellulose is a material with a homogenous chemical composition, simultaneously being capable of establishing hydrogen bonds. Despite some similarities, cellulose and flax fibers may significantly differ considering their surface characteristics, e.g., hydrophilicity, specific surface area, accessibility of surface functional groups, which may affect the filler’s behavior in the polymer matrix and its effect on the properties of the polymer blend [
43].
One should also consider the CF and FF behavior in the analyzed matrixes as natural fibers added to ENR/PLA blends may be concentrated only in a certain polymer domain and, thus, affect the blend properties in varied ways. According to the data gathered in the literature, cellulosic material itself may slightly contribute to the PLA tensile strength improvement—from approximately 65 MPa to 68 MPa (improvement of 5%) [
44]. Yet, when cellulose was added to natural rubber, an increase in tensile strength from 12 MPa to almost 18 MPa (improvement of 50%) could be observed [
45].
Moving forward, the sample filled with both FF and MMT exhibited lower tensile strength, which was on the level of the ENR/PLA + CF specimen. Nevertheless, the ENR/PLA + FF + MMT sample revealed the highest elongation at break of approximately 600%. This phenomenon could be explained with the synergic effect of MMT particles alignment within the structure of the polymer matrix (MMT plates are high aspect ratio particles; their orientation is a significant factor regarding the mechanical performance) and potentially lower crosslinking density (evidenced before), which leads to the material plasticization [
46,
47]. The effect of plasticization observed with MMT may be caused by free surfactant present in MMT.
A similar effect has been observed in work presented by Keawkumay et al. [
48]. According to the results presented in the research, a certain MMT surface treatment may contribute to the lowering of rubber’s crosslinking density and, thus, decrease the mechanical performance of a composite.
Moreover, Wang et al. [
49] underlined the importance of appropriate filler dispersion within the polymer matrix. The authors presented results for MMT contents from 1–5 wt%. A significant increase in the composite tensile strength was only observed in the case of low MMT contents.
On the other hand, Jiang et al. [
50] drew attention to the problem of high specific surface area and shape of MMT particles—with their large L/D ratio, the MMT platelets induced lower stress concentration, which partly contributed to the higher elongation of MMT-filled blend. Similarly, scientists report that the highest values of tensile strength and elongation at break are evidenced for the MMT content up to 2.5 wt%. This indicates that a specific aluminosilicate amount is required to ensure the most satisfying mechanical properties of a final product.
Additionally, it was proven by Papageorgiou et al. [
51] and Mathew et al. [
52] that natural additives, such as MMT and cellulose, might significantly influence the crystallization behavior contributing to some variations in composite mechanical properties. According to the studies, these fillers may behave as nucleating factors and promote the crystalline domain’s formation. Yet, it was reported that MMT incorporation may result in the considerable imperfectness of created crystals, and, thus, it might contribute to the lowering of the stiffness and performance of a PLA-containing composite.
Confirmation for the analyzed ENR/PLA blend stiffening and plasticization can be found in
Table 1, where the tensile tension values at elongation of 100%, 200%, and 300% are presented. It is visible that the specimen filled with FF achieved the tension of (16.2 ± 0.4) MPa at the elongation of only 100%, confirming its high stiffness. Additionally, the lowest value of tensile tension at 300% was attributed to the sample filled with both FF and MMT, which was supposed to be plasticized.
Finally, the surface free energy (SFE) and its components are presented in
Figure 2d. It may be observed that the incorporation of cellulose and flax fibers into the ENR/PLA blend did not significantly affect the surface energy characteristics. Yet, the sample filled with both FF and MMT exhibited lower total SFE and relatively slight polar component.
This phenomenon is also visible regarding the data gathered in
Table 2. ENR/PLA reference blend and specimens filled with cellulose either flax fibers revealed similar water and diiodomethane droplets’ behavior on the surface of the polymer blend. Nonetheless, the MMT-containing sample exhibited the contact angle for water of (97 ± 2)° and in the case of diiodomethane-(71 ± 2)°, which were significantly higher values, when compared to the rest of the specimens.
It is often inferred that materials exhibiting a higher polar part of surface free energy (SFE) could be more affected by the aging processes [
41]. Therefore, taking into consideration data gathered in
Figure 2d and
Table 2, ENR/PLA blend filled with both FF and MMT could exhibit improved resistance to the aging processes. Yet, as FF and MMT are natural additives, the blend should still be prone to the fungi environment similar to the specimens filled only with plant-derived fibers, which exhibit a higher polar part component of surface free energy.
2.2. Characterization of the Aging Impact
Polymer blend samples were subjected to an accelerated aging process to assess their resistance to material degradation induced by temperature (thermo-oxidative aging) and UV light (UV irradiation; 340 nm). Both aging processes indicated radical reactions, which may contribute to the oxidation, crosslinking or chain scission within the polymeric material [
53]. The progress of the degradation processes was tracked via the mass control, color change, and mechanical properties variations over accelerated aging time, which was 200 h. Time intervals between the measurements were established for 50 h (from 0–200 h) as the prepared samples significantly degraded during the relatively short period, 5 h.
A summary of experiments carried out can be found in
Figure 3. In general, the mass loss among the performed aging processes (
Figure 3a) was higher for thermo-oxidative aging, which could be connected with the temperature-induced moisture evaporation from the natural fibers employed as fillers in the investigated polymer blends. The moisture content in plant-derived fibers is not homogenous and repetitive [
54]. Therefore, the mass loss processes might be less regular among the investigated samples for thermo-oxidative aging in comparison with UV aging.
Considering
Figure 3b, the specimens changed their visual features during both aging processes in varied ways. Thermo-oxidation caused a relatively high and quick color change in the PLA/ENR blend filled with both FF and MMT in comparison with the UV aging during which the color variations’ rate was steadier. On the other hand, it could be said that UV irradiation caused more significant changes for different samples after 200 h of the aging process.
What should be underlined is samples of reference PLA/ENR blends became too brittle after just 100 h of thermo-oxidation/UV irradiation. Thus, it was impossible to carry out the color change measurements after this time as it was necessary to press the sample against the measuring instrument.
Additionally, variations of some additional parameters, such as whiteness index, chroma, and hue angle, which contribute to the final color change, are presented in
Table 3. Regarding the gathered data, it might be concluded that UV aging leads to the fading of specimens resulting in less intense color visible for the human eye. Contrary, thermo-oxidative aging results in more complex and less predictable variations.
Considering the changes in the mechanical properties of the analyzed polymer blends, it could be observed that regarding the thermo-oxidative (
Figure 3c,d) and UV (
Figure 3e,f) aging, samples filled with flax fibers exhibited higher resistance to the aging processes in comparison with the reference ENR/PLA blend or CF-filled ENR/PLA specimen. Moreover, as was mentioned before, prepared ENR/PLA blends seemed to be more resistant to UV irradiation, e.g., samples filled with CF and FF were not degrading that fast when subjected to UV aging (the tensile tests were possible to carry out; samples were flexible enough to enable the measurement).
Moving forward and comparing aging processes, it was found that UV irradiation usually caused temporary tensile strength improvement, which indicates that UV irradiation may favor the crosslinking processes, e.g., preliminary tensile strength rise for ENR/PLA + CF or ENR/PLA + FF during the UV aging. On the contrary, the increased temperature probably led to the material’s intense oxidation and polymer chain scission.
As expected, the performed accelerated aging revealed the high aging resistance of the sample filled with both FF and MMT. This could be explained with the lower polar part component of the surface free energy of the analyzed specimen. Yet, barrier characteristics of MMT-filled materials and the presence of an aging-resistant inorganic fraction ought to be considered [
55]. It was reported that MMT might contribute extensively to the reduction in oxygen permeability to the interior of the composite and, thus, prohibit the aging process (oxidation cannot occur) [
56,
57].
In
Table 4, the aging coefficient values, calculated based on ENR/PLA blend tensile strength and elongation at break before and after the performed aging processes, are presented. The lower the
K value, the more degraded the material. According to the gathered data, polymer blend samples filled only with plant-derived fibers were almost fully degraded after only 50 h of performed accelerated aging processes. Nonetheless, the incorporation of an inorganic faction into ENR/PLA blends may successfully lead to the increased aging-resistance of the sample, e.g., when aging lasts for 200 h most of the samples were fully degraded, and it was impossible to carry out the measurement, but when MMT was incorporated, the measurement was possible to conduct and the blend’s mechanical properties’ loss was on the level of 80% (
K was approximately 0.2).
2.3. Microorganism Growth Tests
In the previous subsection, it was proved that the ENR/PLA blend degradation rate in a thermo-oxidative environment and under UV irradiation may be controlled with different natural additives, such as natural fibers or aluminosilicates. MMT incorporation contributed extensively to the improvement in the aging resistance of the investigated polymer blends.
The materials analyzed in this research were also subjected to some tests regarding the resistance of ENR/PLA blends filled with natural additives to molds. Data gathered from this experiment is presented in
Table 5. It is visible that the natural fiber incorporation into the polymer matrix improved the microorganisms’ growth and impaired the material ability to resist molds.
Moreover, MMT presence in the ENR/PLA blend, which was the reason for the delayed material degradation when subjected to increased temperature or UV irradiation, did not affect the molds’ growth on the surface of the investigated polymeric material.
According to the presented molecular growth evaluation (MGE) and information given in the standard, none of the analyzed materials was resistant to microorganism growth. Nonetheless, filling ENR/PLA blends with commercially available neat cellulose fibers and flax fibers may only increase the degradation potential of prepared materials. What is also important is inorganic silicates, which may improve the mechanical properties of polymer blends and contribute to the prolonged time of use in certain conditions, did not have a negative effect on the final product biodegradation.