*3.3. Effect of ECO in PLA on the Thermal Properties of PLA*

The addition of ECO led to a change in the main thermal transitions of neat PLA. Neat PLA presented a T<sup>g</sup> at 62 ◦C, cold crystallization temperature (Tcc) at 119.4 ◦C, and melt temperature T<sup>m</sup> at 150 ◦C, as can be seen in Table 3. As percentage of ECO increased in PLA matrix, a remarkable decrease of T<sup>g</sup> values (62 ◦C from neat PLA up to 56.8 ◦C for PLA\_10%ECO) was observed. It was seen that PLA\_7.5%ECO and PLA\_10%ECO showed similar T<sup>g</sup> values (56.3 and 56.8 ◦C, respectively), demonstrating the plasticizer saturation effect. Additionally, the Tcc values presented a slight decrease with respect to neat PLA, obtaining in all plasticizer formulations lower values than neat PLA. Melt temperature (Tm) presented also a slight decrease with the incorporation of ECO, concluding that the presence of plasticizer also affected the melt temperature due to the increase of chain mobility. The same evolution was reported by Garcia-Garcia et al., who employed epoxidized Karanja oil in PLA [31]. Addition of ECO allowed to increase the free volume of polymer chains and thus provide better movement at lower temperatures [48]. The changes in the degree of crystallinity (Xc) also indicated the chain motions of PLA. It was clearly observed that X<sup>c</sup> of PLA increased with the rise of ECO content, confirming that plasticizer enables the mobility of chains to form stable crystallites at lower energy conditions. Specifically, above 5 wt.% ECO, more evident changes were observed, where the highest crystallinity was found at PLA\_10%ECO with 11.5%. This value was almost 67% higher than neat PLA, indicating the enhancement of chain mobility.

**Table 3.** Main thermal parameters of PLA plasticized with different ECO contents obtained using DSC.


<sup>1</sup> Glass transition temperature; <sup>2</sup> Cold crystallization temperature; <sup>3</sup> Cold crystallization enthalpy; <sup>4</sup> Melt temperature; <sup>5</sup> Melt enthalpy; <sup>6</sup> Degree of crystallization.

Thermogravimetric analysis (TGA) was assessed for PLA formulations with different contents of ECO. Temperature at 5% weight loss (T5%) and temperature at maximum degradation (Tmax) showed an important increase in thermal stabilization. CO presented higher thermal stability at elevated temperatures than neat PLA, obtaining in T5% around of 320 ◦C and Tmax around of 425 ◦C, as was reported by Timilsena et al. [33]. On the other hand, in order to study the influence of the epoxidation process in thermal stability of the vegetable oil, the authors compared an epoxidized linseed oil [51] and virgin linseed oil [52], obtaining very similar results. Linseed oil is characterized by an epoxy content very similar to ECO. For this reason, thermal stability of CO could be considered the same as ECO.

Thus, as can be seen in Figure 7a, higher contents of ECO provided an improvement of thermal stabilization with respect to neat PLA. Regarding weight loss derivate (Figure 7b), it was observed that samples up to 5 wt.% ECO showed an increase of T5%, while higher contents caused a decrease, possibly due to the first evidence of plasticizer saturation [31]. On the other hand, Tmax of neat PLA was 390.4 ◦C and increased up to 404.9 ◦C for PLA formulations with a 2.5–5% ECO, showing a slight decrease when the saturation effect was beginning. Typical plasticizers employed in PLA as citrate esters or ATBC reduce thermal stability of PLA when their content increases [53,54]. However, in this report, different trends were observed in T5% and Tmax. Addition of ECO produced an evident delay of degradation temperature, as was reported by Garcia-Garcia et al., who used epoxidized oils as plasticizers [20]. The reason for this behavior was due to the presence of epoxy groups of ECO that allowed the scavenging of acid groups, obtaining a better thermal stabilization [55]. *Polymers* **2021**, *13*, 1283 12 of 17 of epoxy groups of ECO that allowed the scavenging of acid groups, obtaining a better thermal stabilization [55].

**Figure 7.** Thermal parameters of degradation of PLA with different epoxidized chia seed oil (ECO) contents. (**a**) Weight loss; (**b**) derivative thermogravimetry. T5% is temperature at 5% weight loss; Tmax is temperature at maximum degradation. **Figure 7.** Thermal parameters of degradation of PLA with different epoxidized chia seed oil (ECO) contents. (**a**) Weight loss; (**b**) derivative thermogravimetry. T5% is temperature at 5% weight loss; Tmax is temperature at maximum degradation.

#### *3.4. Disintegration under Composting Conditions 3.4. Disintegration under Composting Conditions*

Disintegration process under compost soil of neat and plasticized PLA is shown in both Figure 8 and Figure 9, in which the visual appearance and the weight loss in respect to the initial mass, respectively, are plotted. After 3 days of incubation, film samples changed their visual appearance from translucid to opaque due to increased crystallinity and possible water absorption. It is important to remark that this experiment was conducted at thermophile conditions, with constant temperature of 58 *°*C and 50% relative humidity. The proximity to the Tg, as was studied by DSC, can induce an increment of chain mobility and thus the crystallization that causes the increasing opacity [31]. After 7 days buried in controlled compost soil, neat PLA and the sample with less amount of ECO, started the embrittlement process and slight weight loss, as plotted in Figure 9. However, samples with higher amounts of ECO did not show any signs of disintegration. After 14 days buried, visual changes and weight loss were evident in all samples. Neat PLA disintegrates faster than plasticized PLA with ECO. Neat PLA obtained the highest weight loss with 60.2%, while PLA\_10%ECO lost 35% of its initial weight. Above 21 days buried in controlled conditions, samples showed a physical inconsistency and disintegration. According to ISO 20200, a disintegrable material was considered when the degree of disintegration achieved 90%. Neat PLA and samples with 2.5 and 5 wt.% of ECO achieved more than 90% of weight loss in respect to their initial value. However, samples with 7.5 and 10 wt. % needed 3 more days to reach this value, indicating a delay of the disintegration process provided by an increasing amount of ECO. These results are in concordance with Disintegration process under compost soil of neat and plasticized PLA is shown in both Figures 8 and 9, in which the visual appearance and the weight loss in respect to the initial mass, respectively, are plotted. After 3 days of incubation, film samples changed their visual appearance from translucid to opaque due to increased crystallinity and possible water absorption. It is important to remark that this experiment was conducted at thermophile conditions, with constant temperature of 58 ◦C and 50% relative humidity. The proximity to the Tg, as was studied by DSC, can induce an increment of chain mobility and thus the crystallization that causes the increasing opacity [31]. After 7 days buried in controlled compost soil, neat PLA and the sample with less amount of ECO, started the embrittlement process and slight weight loss, as plotted in Figure 9. However, samples with higher amounts of ECO did not show any signs of disintegration. After 14 days buried, visual changes and weight loss were evident in all samples. Neat PLA disintegrates faster than plasticized PLA with ECO. Neat PLA obtained the highest weight loss with 60.2%, while PLA\_10%ECO lost 35% of its initial weight. Above 21 days buried in controlled conditions, samples showed a physical inconsistency and disintegration. According to ISO 20200, a disintegrable material was considered when the degree of disintegration achieved 90%. Neat PLA and samples with 2.5 and 5 wt.% of ECO achieved more than 90% of weight loss in respect to their initial value. However, samples with 7.5 and 10 wt. % needed 3 more days to reach this value, indicating a delay of the disintegration process provided by an increasing amount of ECO. These results are in concordance with Balart et al., who observed

Balart et al., who observed a reduction of disintegration capacity by increasing the epoxidized linseed oil content in PLA matrix [56]. This delay was directly related with the fact

microorganism in the soil compost [57] and this microorganism acts faster in amorphous domains [58,59]. Thus, addition of ECO leads to a delay in the disintegrations process but, in general terms, PLA films developed with ECO can be considered as biodegradable ac-

cording to ISO 20200.

a reduction of disintegration capacity by increasing the epoxidized linseed oil content in PLA matrix [56]. This delay was directly related with the fact that samples plasticized with ECO possess a higher degree of crystallinity as reported in Table 3. Biodegradability is usually done by lipases, proteases, and esterase secreted from microorganism in the soil compost [57] and this microorganism acts faster in amorphous domains [58,59]. Thus, addition of ECO leads to a delay in the disintegrations process but, in general terms, PLA films developed with ECO can be considered as biodegradable according to ISO 20200. *Polymers* **2021**, *13*, 1283 13 of 17 *Polymers* **2021**, *13*, 1283 13 of 17


**Figure 8.** Visual appearance of disintegration under composting conditions. **Figure 8.** Visual appearance of disintegration under composting conditions. **Figure 8.** Visual appearance of disintegration under composting conditions.

**Figure 9.** Weight loss recorded during disintegration test. **Figure 9.** Weight loss recorded during disintegration test.

**Figure 9.** Weight loss recorded during disintegration test.

#### *3.5. Migration of ECO by Solvent Extraction Test* PLA is considered a safe polymer for food contact applications [60]. For this reason,

*3.5. Migration of ECO by Solvent Extraction Test* 

*Polymers* **2021**, *13*, 1283 14 of 17

PLA is considered a safe polymer for food contact applications [60]. For this reason, the development of a new plasticizer, as ECO, needs to be studied due to plasticizer migration being an important drawback [61,62]. Plasticizer migration is defined as the capacity of transferring plasticizer molecules from the surface of the matrix to the contact medium [63] and the assay developed by solvent extraction is a quite aggressive test to provide information about the potential use at industrial scale. In Figure 10, the percentage of migration of plasticizer from PLA matrix using *n*-hexane as dissolvent at different temperatures is plotted. Neat PLA, taken as control sample, presented a similar value below 0.02% between 30 and 60 ◦C due to absence of plasticizer. Regarding plasticized PLA with ECO, an increment of percentage of migration is appreciated as temperature increases achieving a highest migration of 0.108% with PLA\_7.5%ECO and PLA\_10%ECO at 60 ◦C. These results were lower than those reported by Carbonell-Verdu et al., who employed epoxidized cottonseed oil, obtaining values up to 0.12% [45]. A higher content of epoxy groups in ECO provided stronger interactions with the hydroxyl groups of PLA matrix and, as a consequence, lower migration levels in respect to other plasticizers with less reactive groups, indicating a correct functionality to be employed at an industrial scale. On the other hand, the high molecular weight characterizing the vegetable oils (around 900 g·mol−<sup>1</sup> ) could be another positive effect to minimize the migration level, compared with industrial plasticizers with lower molecular weight, as for example tributyl citrate plasticizers (350–400 g·mol−<sup>1</sup> ) [48]. the development of a new plasticizer, as ECO, needs to be studied due to plasticizer migration being an important drawback [61,62]. Plasticizer migration is defined as the capacity of transferring plasticizer molecules from the surface of the matrix to the contact medium [63] and the assay developed by solvent extraction is a quite aggressive test to provide information about the potential use at industrial scale. In Figure 10, the percentage of migration of plasticizer from PLA matrix using *n*-hexane as dissolvent at different temperatures is plotted. Neat PLA, taken as control sample, presented a similar value below 0.02% between 30 and 60 *°*C due to absence of plasticizer. Regarding plasticized PLA with ECO, an increment of percentage of migration is appreciated as temperature increases achieving a highest migration of 0.108% with PLA\_7.5%ECO and PLA\_10%ECO at 60 *°*C. These results were lower than those reported by Carbonell-Verdu et al., who employed epoxidized cottonseed oil, obtaining values up to 0.12% [45]. A higher content of epoxy groups in ECO provided stronger interactions with the hydroxyl groups of PLA matrix and, as a consequence, lower migration levels in respect to other plasticizers with less reactive groups, indicating a correct functionality to be employed at an industrial scale. On the other hand, the high molecular weight characterizing the vegetable oils (around 900 g·mol−1) could be another positive effect to minimize the migration level, compared with industrial plasticizers with lower molecular weight, as for example tributyl citrate plasticizers (350–400 g·mol−1) [48].

**Figure 10.** Migration of epoxidized chia seed oil (ECO) plasticizer in PLA matrix by n-hexane solvent extraction. **Figure 10.** Migration of epoxidized chia seed oil (ECO) plasticizer in PLA matrix by n-hexane solvent extraction.

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

Epoxidized chia seed oil (ECO) was applied in a PLA matrix to evaluate its effect as a new bio-based plasticizer. The low elongation at break of neat PLA (8%) was improved up to values of 64.5% using 10 wt.% ECO, obtaining an improvement of 700%. Morphological images showed spherical voids at equal or higher percentage of 7.5 wt.% ECO in PLA matrix, indicating the beginning of plasticizer saturation. With regard to absorbed impact energy, an almost twice as high value was obtained with 10 wt.% ECO in respect to neat PLA, and, as a consequence, a decrease of hardness. Then, ECO led to enhanced chain mobility of PLA, which induced an increase in free volume and a reduction in intermolecular forces. This lubrication of chains also led to reduce the glass temperature around 4.7 **°**C with 7.5–10 wt.% ECO and slightly the cold crystallization temperature. Thermal stability was highly improved up to 14.0 **°**C in respect to neat PLA. The disintegration ability of PLA up to 5 wt.% ECO content was not affected, meanwhile at 7.5–10 Epoxidized chia seed oil (ECO) was applied in a PLA matrix to evaluate its effect as a new bio-based plasticizer. The low elongation at break of neat PLA (8%) was improved up to values of 64.5% using 10 wt.% ECO, obtaining an improvement of 700%. Morphological images showed spherical voids at equal or higher percentage of 7.5 wt.% ECO in PLA matrix, indicating the beginning of plasticizer saturation. With regard to absorbed impact energy, an almost twice as high value was obtained with 10 wt.% ECO in respect to neat PLA, and, as a consequence, a decrease of hardness. Then, ECO led to enhanced chain mobility of PLA, which induced an increase in free volume and a reduction in intermolecular forces. This lubrication of chains also led to reduce the glass temperature around 4.7 ◦C with 7.5–10 wt.% ECO and slightly the cold crystallization temperature. Thermal stability was highly improved up to 14.0 ◦C in respect to neat PLA. The disintegration ability of PLA up to 5 wt.% ECO content was not affected, meanwhile at 7.5–10 wt.%, it was slightly delayed, being considered, in general terms, biodegradable formulations. Finally, very low migration of plasticizer was detected in migration test by using *n*-hexane. The maximum migration recorded was 0.108% with 10 wt.% ECO at 60 ◦C, while lower migration

was obtained (<0.06%) at 30 ◦C. Therefore, ECO is a promising bio-based plasticizer with potential to be applied in the packaging industry.

**Author Contributions:** Conceptualization, I.D.-C. and V.F.; methodology, I.D.-C.; validation, I.D.-C., J.M.F.; formal analysis, S.C.C. and J.M.F.; investigation, I.D.-C.; resources, V.F and J.L.; data curation, S.C.C.; writing—original draft preparation, I.D.-C. and V.F.; writing—review and editing, V.F. and J.M.F.; visualization, J.L.; supervision, J.L. and V.F.; project administration, 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).

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

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** Authors want to thank the support of the Cátedra FACSA-FOVASA of Water, Waste and Circular Economy, 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.
