*3.1. Effect of ECO in PLA on the Mechanical Properties*

One of the main disadvantages of PLA is its low ductile property, which gives it a characteristic brittleness. As is shown in Figure 2, neat PLA used in the present study reaches an elongation at break of 8% and a tensile strength higher than 45 MPa. The resulting tensile modulus was higher than 3100 MPa, as plotted in Figure 3. The plasticization effect was clearly observed in the different formulations of PLA with ECO. For example, the addition of 2.5 wt.% of ECO (PLA\_2.5%ECO) provided higher ductile properties, with an elongation at break around 18%. Consequently, mechanical properties like tensile strength and tensile modulus slightly decreased (40.9 and 3040 MPa, respectively), which was attributed to the elastomeric and toughening effect of ECO plasticizer. In Figure 2, it was possible to observe how the slope of the stress–strain curve became lower as the ECO content in the samples increased. This decrease resulted in a lower tensile modulus. At the same time, the elongation at break increased notably, reaching values of 64.5% for samples with 10 wt. % of ECO. As a consequence of this variation in mechanical and ductile properties, the toughness of the samples was clearly improved. This increase represents a 700% rise com-

pared to neat PLA. Therefore, the presence of epoxy groups in ECO interacts with hydroxyl groups present in PLA, decreasing intermolecular forces and, consequently, increasing its ductile properties [30]. These results are in accordance with previous studies on the use of epoxidized vegetable oils like PLA plasticizer. Yu-Qiong et al. reported an increase of 123% employing epoxidized palm oil [30]. This lower increase in elongation values was due to the lower content of epoxy groups in palm oil (3.23%) with respect to chia oil (6.71%), which is one of the main parameters to be taken into account. For example, other authors have reported similar increases of 700% or even higher using epoxidized vegetable oils with content of epoxy groups around 5.8% [36,40,41]. Thus, stronger interactions occurred as epoxy content increased due to the increased presence of reactive groups Therefore, in view of the results obtained, it seems that once epoxy group values of 5.8% or higher were reached, a saturation effect was shown, not reaching further increases in the elongation at break. Where the use of ECO with PLA appeared to have a direct effect was on the attenuating effect of the sharp drop in mechanical properties. For example, Chieng et al. reported that using palm oil in 10 wt.% proportion reduces the tensile strength of the neat PLA by almost 50%. However, this decrease was only 21% when ECO was employed [35]. Therefore, the higher oxirane oxygen content in ECO than, for example in palm oil, can interact more strongly with PLA chains leading to intense polymer-plasticizer interactions allowing to maintain mechanical and ductile balanced properties. As a consequence of these interactions between PLA and ECO, it was remarkable that samples with 7.5 and 10 wt.% ECO content provided constant values in tensile mechanical properties. On the other hand, this attenuation of decrease in mechanical properties also suggested, as other cited authors have pointed out, that a percentage higher than 10 wt.% produces a negative effect on ductile properties due to the excess of plasticizer and a possible phase separation. Some authors like Sanyang et al. and Rizzuto et al. remarked that once all free volume is full of plasticizer, a decrease of elongation at break occurs due to free volume reduction [42,43]. Similar results were reported by Emad et al., who observed a decrease of elongation at break above of 9 wt.% of epoxidized palm oil [44]. Therefore, PLA\_10%ECO obtained the highest elongation at break (64.3%), together with a reduction of 6.7% in tensile modulus (2893 MPa) and 21% in tensile strength (33.4 MPa) in respect to initial value (PLA neat). *Polymers* **2021**, *13*, 1283 7 of 17

**Figure 2.** Plot evolution of characteristic stress-strain curves of PLA with different epoxidized chia **Figure 2.** Plot evolution of characteristic stress-strain curves of PLA with different epoxidized chia seed oil (ECO) contents.

**Figure 3.** Plot evolution of tensile mechanical properties of PLA with different epoxidized chia

Other techniques that provide substantial information about the plasticizing effect of ECO on PLA are the Charpy impact test and Shore D hardness. As shown previously, the addition of ECO provides a plasticizing effect. As a result, tensile strength and tensile modulus decrease slightly with increment of ECO, contrary to the elongation at break. Impact absorbed energy is a parameter related to the ductile properties and toughness.

seed oil (ECO) contents.

seed oil (ECO) contents.

seed oil (ECO) contents.

seed oil (ECO) contents.

**Figure 2.** Plot evolution of characteristic stress-strain curves of PLA with different epoxidized chia

**Figure 3.** Plot evolution of tensile mechanical properties of PLA with different epoxidized chia **Figure 3.** Plot evolution of tensile mechanical properties of PLA with different epoxidized chia seed oil (ECO) contents.

Other techniques that provide substantial information about the plasticizing effect of ECO on PLA are the Charpy impact test and Shore D hardness. As shown previously, the addition of ECO provides a plasticizing effect. As a result, tensile strength and tensile modulus decrease slightly with increment of ECO, contrary to the elongation at break. Impact absorbed energy is a parameter related to the ductile properties and toughness. Other techniques that provide substantial information about the plasticizing effect of ECO on PLA are the Charpy impact test and Shore D hardness. As shown previously, the addition of ECO provides a plasticizing effect. As a result, tensile strength and tensile modulus decrease slightly with increment of ECO, contrary to the elongation at break. Impact absorbed energy is a parameter related to the ductile properties and toughness. Therefore, as expected initially, PLA is a brittle polymer, which has a low impact absorption capacity (37.1 J·m−<sup>2</sup> ), but adding ECO improves absorbed impact energy due to elastomeric and toughing effect (Figure 4). It is possible to observe how an increase from 2.5 to 10 wt.% of ECO provides a gradual gain in impact absorption capacity. Sample ECO\_10%ECO obtained the highest value (68.3 J·m−<sup>2</sup> ), which represented an increase of 85% with respect to non-plasticized PLA. These results were in concordance with Carbonell-Verdu et al., who evaluated the plasticization effect on PLA using a 7.5 wt.% of epoxidized cottonseed oil, obtaining an increase on the impact of absorbed energy of 18% [45]. Again, the greater content of epoxy groups in ECO (6.7%) compared to cottonseed oil (5.8%), provided higher ductile properties. On the other hand, the plasticizing effect of ECO inversely affected the hardness of the samples. As the ECO content increased, the hardness decreased. However, in the same way that the tensile strength analysis showed a stabilization in the decrease of the properties of ECO\_7.5%ECO and ECO\_10%ECO samples, the difference between the hardness of neat PLA and ECO\_10%ECO sample was only less than 6%.

Regarding the morphological changes produced by the incorporation of ECO to PLA, Figure 5a showed a brittle morphology with smooth surface and very low plastic deformation characteristic of neat PLA. With increasing ECO content, remarkable changes can be observed. Figure 5b, which represents PLA\_2.5%ECO, showed a slight change in surface roughness. PLA\_5%ECO, Figure 5c, shows a rougher surface as well as presence of filaments thus indicating an increase in ductility as a consequence of plasticization. So, an increase in roughness and a higher filament density was observed with increasing ECO. However, a higher presence of plasticizer (equal or more than 7.5 wt.%) began to display some spherical voids due to the plasticizer saturation, Figure 5d,e. A similar finding was reported by Ferri et al., who observed spherical voids above 5 wt.% of maleinized linseed oil as a plasticizer for PLA [46]. Phase separation was produced, and a worse miscibility occurred [31]. Finally, in Figure 5f, the presence of high density of filaments and voids in sample PLA\_10%ECO, indicating a plasticizer saturation, can be observed better (2500×).

only less than 6%.

epoxidized chia seed oil (ECO) content.

Therefore, as expected initially, PLA is a brittle polymer, which has a low impact absorption capacity (37.1 J·m−2), but adding ECO improves absorbed impact energy due to elastomeric and toughing effect (Figure 4). It is possible to observe how an increase from 2.5 to 10 wt.% of ECO provides a gradual gain in impact absorption capacity. Sample ECO\_10%ECO obtained the highest value (68.3 J·m−2), which represented an increase of 85% with respect to non-plasticized PLA. These results were in concordance with Carbonell-Verdu et al., who evaluated the plasticization effect on PLA using a 7.5 wt.% of epoxidized cottonseed oil, obtaining an increase on the impact of absorbed energy of 18% [45]. Again, the greater content of epoxy groups in ECO (6.7%) compared to cottonseed oil (5.8%), provided higher ductile properties. On the other hand, the plasticizing effect of ECO inversely affected the hardness of the samples. As the ECO content increased, the hardness decreased. However, in the same way that the tensile strength analysis showed a stabilization in the decrease of the properties of ECO\_7.5%ECO and ECO\_10%ECO samples, the difference between the hardness of neat PLA and ECO\_10%ECO sample was

**Figure 4.** Plot evolution of Shore D hardness and impact absorbed energy of PLA with different **Figure 4.** Plot evolution of Shore D hardness and impact absorbed energy of PLA with different epoxidized chia seed oil (ECO) content.

**Figure 5.** Fracture surface morphology of Charpy test at 1000x by field emission scanning electron microscopy (FESEM): (**a**) neat PLA; (**b**) PLA\_2.5%ECO; (**c**) PLA\_5%ECO; (**d**) PLA\_7.5%ECO; (**e**) **Figure 5.** Fracture surface morphology of Charpy test at 1000× by field emission scanning electron microscopy (FESEM): (**a**) neat PLA; (**b**) PLA\_2.5%ECO; (**c**) PLA\_5%ECO; (**d**) PLA\_7.5%ECO; (**e**) PLA\_10%ECO; and (**f**) PLA\_10%ECO at 2500×.

Storage modulus (G') and damping factor (tan δ) were assessed by dynamic mechanical response. In Figure 6, the viscoelastic behavior of PLA\_ECO formulations were exposed. Two characteristic changes in the storage modulus could be distinguished. The first change, between 50 and 70 °C, was the drop of storage modulus, which was related to glass transition temperature (Tg) at around 60 *°*C, as reported by Yong et al. [47]. The second change, between 80 and 100 *°*C, was recognized as the beginning of cold crystallization process. The addition of ECO to PLA resulted in a loss of storage modulus at lower temperatures. This was due to the plasticizing effect that ECO exerts on the PLA matrix, which increases the free volume between PLA chains before a saturation effect, decreasing the interaction between them [48]. In addition, at room temperature, neat PLA showed a storage modulus value of 1300 MPa, while the plasticized PLA formulations showed a decrease of this modulus up to 1000 MPa as a consequence of the plasticization effect. On the other hand, the beginning of cold crystallization decreased as ECO content increased, obtaining a shift from 87 up to 84 *°*C for plasticized PLA. This effect was due to plasticizer

enabling the rearrangement in packed structure under lower energetic conditions.

PLA\_10%ECO; and (**f**) PLA\_10%ECO at 2500×.
