*3.4. Effect of MBNO and MHO on Thermomechanical Properties of Plasticized PLA Formulations*

The evolution of the storage modulus (G') as a function of temperature is shown in Figure 10a,b. Neat PLA, at room temperature, has a storage modulus above 1000 MPa and this remains constant up to 60 ◦C. With the incorporation of MBNO and MHO in the PLA matrix, no significant changes are observed in terms of G' at room temperature, since in all of them the values remain between 900 and 1100 MPa, although it is observed that with the incorporation of the plasticizers, the G' tends to decrease slightly, showing its plasticizer effect. This is followed by a drop in G' of up to three orders of magnitude, a change that is related to Tg. In this case, a significant difference can be observed between the plasticized samples and neat PLA. As can be seen after the addition of 10 phr MBNO, the T<sup>g</sup> decreases from 60 ◦C to 48 ◦C and to almost 50 ◦C with 7.5 phr MHO. The almost 10 ◦C shift in T<sup>g</sup> with both plasticizers is due, as described above, to the fact that the plasticizer generates an increase in the free volume, which improves the mobility of the chain due to a lower interaction between them, being able to move with lower energy [45]. Neat PLA above 75 ◦C behaves like an elastomer up to 95–100 ◦C. Above this temperature, the energetic conditions favor and promote crystallization, improving the elastic behavior and leading to an increase in the storage modulus up to 60 MPa. Here, again, differences are observed when incorporating the plasticizers into PLA. As can be seen, this temperature decreases by around 10 ◦C in all the MBNO-plasticized formulations and by around 5 ◦C in the MHO-plasticized samples with respect to neat PLA. This decrease in temperature is due to the higher mobility of the chains, which allows their rearrangement into a packed structure with lower energy [48].

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

**Figure 10.** Storage modulus (**a**,**b**) and damping factor (**c**,**d**) of unplasticized PLA and PLA plasticized with different content of MBNO and MHO as a function of temperature. **Figure 10.** Storage modulus (**a**,**b**) and damping factor (**c**,**d**) of unplasticized PLA and PLA plasticized with different content of MBNO and MHO as a function of temperature.

*3.5. Disintegration under Composting Condition of PLA Formulations*  The disintegration process of PLA formulations with MBNO and MHO in compost soil is shown visually in Figures 11 and 12, respectively. Initially, the samples were translucent in all formulations; however, after 3 days of incubation, a change in visual appearance to opaque was observed. This may be due to several factors. One cause can be attributed to the 50% relative humidity test condition, since possible hydrolytic degradation due to water absorption affects the refractive index [50]. On the other hand, taking into account the fact that the test was carried out under thermophilic conditions at 58 °C, the opacity may also be due to crystallization, since this temperature is close to the Tg obtained The damping factor (tan *δ*), which represents the energy lost due to viscous behavior relative to the energy stored due to elastic behavior, is shown in Figure 10c,d. The peak tan *δ*, is a way to obtain the T<sup>g</sup> value of the materials and, as can be seen, while that of neat PLA is around 73 ◦C, in the formulations plasticized with MBNO and MHO, this temperature decreases to 58 ◦C and 60 ◦C for samples plasticized with 7.5 phr MBNO and MHO, respectively. Santos et al. [49] observed a similar decrease in T<sup>g</sup> with the addition to 10 wt.% and 20 wt.% of oligoesters obtained from sunflower oil biodiesel in PLA. The authors observed that the T<sup>g</sup> of PLA decreased from 62 ◦C to 52 ◦C and 44 ◦C, respectively.

#### in the thermal study by DSC. As can be seen in Figure 13, where the weight loss with respect to the initial mass at different periods of incubation for two PLA formulations with *3.5. Disintegration under Composting Condition of PLA Formulations*

MBNO (a) and MHO (b) after 7 days was buried, the samples began to lose mass, which led to increased embrittlement, which is observed in the images with the appearance of cracks. However, it was not until day 14 that significant weight loss and inconsistency of the samples was observed. In the case of neat PLA, a faster degradation than in the plasticized PLA was observed, since on day 17 it had already exceeded 90% mass loss, the degree of disintegration determined by the ISO 20200 standard for considering a material to be disintegrable. In the case of plasticized PLA, in the formulations with 2.5 phr and 5 phr of both MBNO and MHO, 90% mass loss was reached at day 27, while with the 7.5 phr and 10 phr formulations, it was reached at day 24. Although the difference in the disintegration time of PLA formulations with plasticizer is small, and in all cases, the time is longer than with neat PLA, and a slight increase in the disintegration rate is observed when more plasticizer is added. This delay in disintegration with respect to neat PLA when introducing plasticizer is due to the fact that the PLA grade used in this work is very amorphous, as can be seen in the thermal analysis; when introducing plasticizer, crystallinity increases, making it difficult for microorganisms to act in the degradation, which act faster in amorphous domains [51,52]. A similar trend was reported by Balart et The disintegration process of PLA formulations with MBNO and MHO in compost soil is shown visually in Figures 11 and 12, respectively. Initially, the samples were translucent in all formulations; however, after 3 days of incubation, a change in visual appearance to opaque was observed. This may be due to several factors. One cause can be attributed to the 50% relative humidity test condition, since possible hydrolytic degradation due to water absorption affects the refractive index [50]. On the other hand, taking into account the fact that the test was carried out under thermophilic conditions at 58 ◦C, the opacity may also be due to crystallization, since this temperature is close to the T<sup>g</sup> obtained in the thermal study by DSC. As can be seen in Figure 13, where the weight loss with respect to the initial mass at different periods of incubation for two PLA formulations with MBNO (a) and MHO (b) after 7 days was buried, the samples began to lose mass, which led to increased embrittlement, which is observed in the images with the appearance of cracks. However, it was not until day 14 that significant weight loss and inconsistency of the samples was observed. In the case of neat PLA, a faster degradation than in the plasticized PLA was observed, since on day 17 it had already exceeded 90% mass loss, the degree of disintegration determined by the ISO 20200 standard for considering a material to be disintegrable. In the case of plasticized PLA, in the formulations with 2.5 phr and 5 phr of both MBNO and MHO, 90% mass loss was reached at day 27, while with the 7.5 phr and 10 phr formulations, it was reached at day 24. Although the difference in the disintegration time of PLA formulations

with plasticizer is small, and in all cases, the time is longer than with neat PLA, and a slight increase in the disintegration rate is observed when more plasticizer is added. This delay in disintegration with respect to neat PLA when introducing plasticizer is due to the fact that the PLA grade used in this work is very amorphous, as can be seen in the thermal analysis; when introducing plasticizer, crystallinity increases, making it difficult for microorganisms to act in the degradation, which act faster in amorphous domains [51,52]. A similar trend was reported by Balart et al. [53], who observed an increase in disintegration time upon the incorporation of ELO into the PLA matrix. Therefore, in view of the results, as a conclusion of the disintegration study, it has been demonstrated that the maleinized oils slightly retard the disintegration of PLA samples. However, PLA compounds plasticized with MBNO and MHO can be considered equally biodegradable by composting. *Polymers* **2021**, *13*, x FOR PEER REVIEW 14 of 18 al. [53], who observed an increase in disintegration time upon the incorporation of ELO into the PLA matrix. Therefore, in view of the results, as a conclusion of the disintegration study, it has been demonstrated that the maleinized oils slightly retard the disintegration of PLA samples. However, PLA compounds plasticized with MBNO and MHO can be considered equally biodegradable by composting.

**Figure 11.** Visual appearance of disintegration under composting conditions of PLA and PLA plasticized with different contents of MBNO. **Figure 11.** Visual appearance of disintegration under composting conditions of PLA and PLA plasticized with different contents of MBNO.

**Figure 12.** Visual appearance of disintegration under composting conditions of PLA and PLA plasticized with different contents of MHO. **Figure 12.** Visual appearance of disintegration under composting conditions of PLA and PLA plasticized with different contents of MHO.

**Figure 13.** Weight loss recorded during disintegration test of PLA formulations with MBNO (**a**) and MHO (**b**). **Figure 13.** Weight loss recorded during disintegration test of PLA formulations with MBNO (**a**) and MHO (**b**).

#### **4. Conclusions 4. Conclusions**

This research work has developed for the first time a maleinized Brazil nut oil (MBNO). This, as well as a maleinized hemp oil (MHO), were introduced into the PLA matrix to study and analyze their effect as bio-based plasticizers. In addition, the results obtained show MBNO and MHO provided a similar performance compared with a commercially available maleinized linseed oil (MLO), demonstrating its potential. For example, the elongation at break of neat PLA is 7.4%, which is quite low given its brittleness. With the addition of MBNO and MHO, an improvement of 643% and 771%, respectively, was achieved. On the other hand, with values higher than 7.5 phr MBNO, a decrease in the elongation at break was observed due to the anti-plasticizing effect produced by saturation in the mixture. In terms of absorbed impact energy, a 20% higher value was obtained with the addition of 7.5 phr MBNO and 46% higher with 10 phr MHO compared to unplasticized PLA. In addition, mechanical properties such as tensile strength do not decrease, as both maleinized oils provide an improvement in the mobility of PLA chains and an increase in free volume, increasing the degree of crystallinity and thus counteracting the plasticizing effect that would decrease these properties. Thermal parameters, such as Tg, show a decreasing influence with the presence of these bio-plasticizers developed, but not drastically. Finally, the disintegration test under composting conditions showed that the addition of MBNO and MHO, although slightly delaying the process, did not lead to a loss of the biodegradability of PLA. Therefore, both MBNO and MHO are shown as potential bio-plasticizers of organic origin to increase the ductility of PLA without affecting its mechanical properties and not affecting its biodegradability, which makes them two bio-plasticizers of interest for industrial applications. This research work has developed for the first time a maleinized Brazil nut oil (MBNO). This, as well as a maleinized hemp oil (MHO), were introduced into the PLA matrix to study and analyze their effect as bio-based plasticizers. In addition, the results obtained show MBNO and MHO provided a similar performance compared with a commercially available maleinized linseed oil (MLO), demonstrating its potential. For example, the elongation at break of neat PLA is 7.4%, which is quite low given its brittleness. With the addition of MBNO and MHO, an improvement of 643% and 771%, respectively, was achieved. On the other hand, with values higher than 7.5 phr MBNO, a decrease in the elongation at break was observed due to the anti-plasticizing effect produced by saturation in the mixture. In terms of absorbed impact energy, a 20% higher value was obtained with the addition of 7.5 phr MBNO and 46% higher with 10 phr MHO compared to unplasticized PLA. In addition, mechanical properties such as tensile strength do not decrease, as both maleinized oils provide an improvement in the mobility of PLA chains and an increase in free volume, increasing the degree of crystallinity and thus counteracting the plasticizing effect that would decrease these properties. Thermal parameters, such as Tg, show a decreasing influence with the presence of these bio-plasticizers developed, but not drastically. Finally, the disintegration test under composting conditions showed that the addition of MBNO and MHO, although slightly delaying the process, did not lead to a loss of the biodegradability of PLA. Therefore, both MBNO and MHO are shown as potential bio-plasticizers of organic origin to increase the ductility of PLA without affecting its mechanical properties and not affecting its biodegradability, which makes them two bio-plasticizers of interest for industrial applications.

**Author Contributions:** Conceptualization, A.P.-N. and V.F.; methodology, A.P.-N., A.L.-C. and I.D.-C.; validation, D.G.-G., J.M.F. and V.F.; formal analysis, A.P.-N., A.L.-C. and I.D.-C.; investigation, A.P.-N. and A.L.-C.; resources, D.G.-G. and V.F.; data curation, D.G.-G.; writing—original draft preparation, A.P.-N., J.M.F. and D.G.-G.; writing—review and editing, V.F.; visualization, D.G.-G.; supervision, J.M.F. and V.F.; project administration, V.F.; funding acquisition, J.M.F., D.G.-G. and V.F. All authors have read and agreed to the published version of the manuscript. **Author Contributions:** Conceptualization, A.P.-N. and V.F.; methodology, A.P.-N., A.L.-C. and I.D.-C.; validation, D.G.-G., J.M.F. and V.F.; formal analysis, A.P.-N., A.L.-C. and I.D.-C.; investigation, A.P.-N. and A.L.-C.; resources, D.G.-G. and V.F.; data curation, D.G.-G.; writing—original draft preparation, A.P.-N., J.M.F. and D.G.-G.; writing—review and editing, V.F.; visualization, D.G.-G.; supervision, J.M.F. and V.F.; project administration, V.F.; funding acquisition, J.M.F., D.G.-G. and V.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** I.D.-C. wants to thank Universitat Politècnica de València for his FPI grant (PAID-2019- SP20190013) and Generalitat Valenciana (GVA) for his FPI grant (ACIF/2020/233). J.M.F. thanks the postdoc contract (APOSTD/2019/122) Generalitat Valenciana (2019–2021). **Funding:** I.D.-C. wants to thank Universitat Politècnica de València for his FPI grant (PAID-2019- SP20190013) and Generalitat Valenciana (GVA) for his FPI grant (ACIF/2020/233). J.M.F. thanks the postdoc contract (APOSTD/2019/122) Generalitat Valenciana (2019–2021).

**Institutional Review Board Statement:** Not applicable. **Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Acknowledgments:** The authors want to thank the Cátedra FACSA-FOVASA of Water, Waste and Circular Economy for their support, which promotes and supports training, dissemination, innovation, social responsibility and entrepreneurship activities in the field of circular economy and has kindly provided the necessary chemical reagents.

**Conflicts of Interest:** The authors declare no conflict of interest.
