*3.2. Effect of MBNO and MHO on Mechanical Properties of Plasticized PLA Formulations*

The results obtained from the mechanical properties of the different PLA formulations with both MBNO and MHO show a significant reduction in PLA stiffness, indicating that both oils are effective as renewable PLA plasticizers. Figure 4 shows the tensile mechanical

properties of PLA with MBNO and MHO. Unplasticized PLA has a Young's modulus close to 3000 MPa (2977 ± 21 MPa) and a tensile strength of 35.8 ± 7.3 MPa, relatively high values in the thermoplastic commodity range. However, its toughness is low because it shows an elongation at break of 7.4 ± 7%. By incorporating MBNO into the PLA matrix, an increase in elongation is observed from 2.5 phr, reaching the maximum value of 52% ± 3.0% at 7.5 phr, which represents an increase of 643% with respect to neat PLA. From this concentration of MBNO, a decrease in elongation occurs, probably due to plasticizer saturation, a phenomenon that some authors call an anti-plasticization effect [38]. On the other hand, the effect of MHO on the elongation at break is observed from 7.5 phr MHO where it increases drastically up to 42% ± 6% and even increases up to 61% ± 3.0% with the addition of 10 phr MHO, i.e., 771% more than neat PLA. These results obtained are close to the results reported by Ferri et al. [28], in which an elongation of 78.4% was obtained by incorporating commercial MLO into PLA. In Figure 5, it is possible to observe the changes in the appearance of the different tensile specimens after the test. Figure 5a shows specimens made with different MBNO contents. It can be seen how from 7.5 phr of MBNO content, the specimen has a lower final elongation. Figure 5b shows the specimens with different MHO contents after the tensile test. The main difference with respect to MBNO is observed in the PLA + 10% MHO sample, whose elongation is higher than that obtained with 7.5 phr MHO. This increase in elongation at break is due to the enhancement of molecular mobility, which is explained by several plasticization theories. The first of these, the lubricity theory, holds that the plasticizer functions as a molecular lubricant of the polymer. On the other hand, gel theory, applied to amorphous thermoplastics such as the grade of PLA used in this work, suggests that the plasticizer molecules are placed between the polymer chains and weaken the interactions between them. Finally, the free volume theory argues that the plasticizer increases the free volume and thus decreases the interactions between the polymer chains [39]. *Polymers* **2021**, *13*, x FOR PEER REVIEW 8 of 18

**Figure 4.** Plot of evolution of tensile properties of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**). **Figure 4.** Plot of evolution of tensile properties of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**).

In addition, with the incorporation of both oils, an increase in tensile strength is observed with 2.5 phr, with respect to neat PLA, and then a decreasing trend as the percentage of oil incorporation increases. Several authors report a similar effect, but with lower strength values than PLA as an oil content increases. Garcia-Garcia et al. [40] reported a decrease in strength from 46.5 MPa to 42.2 MPa with the addition of 10% epoxidized karanja oil—EKO. In the case of MBNO and MHO, by incorporating between 2.5 and 10 phr, higher strengths were obtained than those of neat PLA, counterbalanced, in turn, by an improvement in its elongation. This phenomenon can be explained by the fact that MHO and MBNO, by improving the mobility between PLA chains, also facilitate crystallization [41], which at the same time generates an increase in its stiffness. Finally, Young's modulus follows a decreasing trend in both cases, obtaining the lowest value

**Figure 5.** Plot of evolution of tensile properties of PLA formulations plasticized with various con-

In parallel, Figure 6 shows a decrease in strength and flexural modulus as MBNO and MHO content increases. In the case of MBNO, a maximum reduction in flexural strength of 37.7% and flexural modulus of 20% is obtained with 10 phr and 39.6% and 18.4%, respectively, with 10 phr MHO content. However, as with the tensile properties, it is at 5 phr MBNO and 7.5 phr MHO that the decrease in strength and modulus tends to stabilize, suggesting plasticizer saturation. Ferri et al. [28] reported a decrease in the flexural strength of PLA with commercial MLO of up to 24%, and also a saturation around 15 phr MLO. The antiplasticizing effect of polymers depends, above all, on the molecular weight and the concentration of the diluent, so that in each formulation it is produced

tents of MBNO (**a**) and MHO (**b**).

with a specific percentage of plasticizer [42].

MHO (**b**).

with the incorporation of 5 phr MBNO of up to 45.2% and with 7.5 phr MHO of up to 47%. From these contents in phr, the modulus tends to increase as the elongation tends to decrease. This decreasing evolution is in line with the results reported by authors such as Carbonell-Verdu et al. [24], who with the incorporation of ECSO to PLA obtained a reduction in the tensile modulus of 47.5% with respect to neat PLA. The high decrease in tensile modulus obtained with MBNO and MHO is related to the decrease in molecular interactions that facilitate movement. **Figure 4.** Plot of evolution of tensile properties of PLA formulations plasticized with various contents of MBNO (**a**) and

*Polymers* **2021**, *13*, x FOR PEER REVIEW 8 of 18

**Figure 5.** Plot of evolution of tensile properties of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**). **Figure 5.** Plot of evolution of tensile properties of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**).

In parallel, Figure 6 shows a decrease in strength and flexural modulus as MBNO and MHO content increases. In the case of MBNO, a maximum reduction in flexural strength of 37.7% and flexural modulus of 20% is obtained with 10 phr and 39.6% and 18.4%, respectively, with 10 phr MHO content. However, as with the tensile properties, it is at 5 phr MBNO and 7.5 phr MHO that the decrease in strength and modulus tends to stabilize, suggesting plasticizer saturation. Ferri et al. [28] reported a decrease in the flexural strength of PLA with commercial MLO of up to 24%, and also a saturation around 15 phr MLO. The antiplasticizing effect of polymers depends, above all, on the molecular weight and the concentration of the diluent, so that in each formulation it is produced with a specific percentage of plasticizer [42]. In parallel, Figure 6 shows a decrease in strength and flexural modulus as MBNO and MHO content increases. In the case of MBNO, a maximum reduction in flexural strength of 37.7% and flexural modulus of 20% is obtained with 10 phr and 39.6% and 18.4%, respectively, with 10 phr MHO content. However, as with the tensile properties, it is at 5 phr MBNO and 7.5 phr MHO that the decrease in strength and modulus tends to stabilize, suggesting plasticizer saturation. Ferri et al. [28] reported a decrease in the flexural strength of PLA with commercial MLO of up to 24%, and also a saturation around 15 phr MLO. The antiplasticizing effect of polymers depends, above all, on the molecular weight and the concentration of the diluent, so that in each formulation it is produced with a specific percentage of plasticizer [42].

On the other hand, Figure 7 summarizes the energy absorbed by Charpy's impact test and the Shore D hardness of neat PLA and formulations. The energy absorbed by Charpy's impact test is closely related to the toughness of the material; therefore, it is also representative for evaluating the effectiveness of MBNO and MHO as a plasticizer. Due to its brittleness, neat PLA has a relatively low absorbed energy (around 35.5 kJ·m−<sup>2</sup> ). By adding plasticizers, an increase in absorbed energy is observed, in the case of MBNO up to 20% higher and in the case of MHO up to 46% higher. The energy absorbed by a material depends both on the deformation capacity linked to the ductility properties and on the breaking strength, which is related to the mechanical properties [28]; therefore, the results are in full agreement with the obtained values of elongation, modulus and breaking strength. As for hardness, the value decreases progressively as more plasticizer is incorporated, although it is not as strongly visualized as with other properties. With MBNO the Shore D hardness decreased from 80 to 71.7 at 7.5 phr MBNO, and with 10 phr MHO to 72.9. Other studies reported a similar decrease from 75.6 to 59.6 with the addition of 22.5% commercial ELO to PLA/hazelnut shell flour (HSF) blends [43].

*Polymers* **2021**, *13*, x FOR PEER REVIEW 9 of 18

**Figure 6.** Plot of evolution of flexural properties of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**). **Figure 6.** Plot of evolution of flexural properties of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**). to 72.9. Other studies reported a similar decrease from 75.6 to 59.6 with the addition of 22.5% commercial ELO to PLA/hazelnut shell flour (HSF) blends [43].

22.5% commercial ELO to PLA/hazelnut shell flour (HSF) blends [43]. **Figure 7.** Graphic representation of impact strength and hardness (Shore D) of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**). **Figure 7.** Graphic representation of impact strength and hardness (Shore D) of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**).
