*3.3. Effect of MBNO and MHO on Thermal Properties of Plasticized PLA Formulations*

**Figure 7.** Graphic representation of impact strength and hardness (Shore D) of PLA formulations plasticized with various contents of MBNO (**a**) and MHO (**b**). Thermal stability of PLA and plasticized formulations with different content of MBNO and MHO was performed by thermogravimetry analysis (TGA). Figure 8 show the weight loss versus temperature curves for each sample (TG) and their corresponding first derivative curves (DGT). Table 2 shows some characteristic thermal parameters of the thermograms, such as the degradation onset temperature (T5%), which indicates the temperature at which a 5% weight loss occurs, and the maximum degradation temperature (Tmax), which corresponds to the peak of the first derivative curve. Neat PLA possesses good thermal stability with a T5% of 347.9 ◦C and a Tmax of 386 ◦C. As can be seen, adding MBNO or MHO to PLA causes a slight decrease in T5% as they are incorporated in higher quantities. In this case, the addition of 10 phr MNBO results in a T5% reduction of 4.5 ◦C, while with 10 phr MHO, the reduction is 10.1 ◦C. Regarding the maximum degradation temperature, it can be observed that both plasticizers lead to a decrease in temperature, obtaining the lowest value at 10 phr, with a reduction of 16.4 ◦C for both plasticizers. Similar behavior was observed by Garcia-Campo et al. [44] for PLA/PHB/PCL blends compatibilized with epoxidized soybean oil (ELO). The authors observed how the addition of ELO into the blend resulted in a reduction in T5% by 19.1 ◦C and Tmax by 20 ◦C.

*3.3. Effect of MBNO and MHO on Thermal Properties of Plasticized PLA Formulations* 

ELO into the blend resulted in a reduction in T5% by 19.1 °C and Tmax by 20 °C.

Thermal stability of PLA and plasticized formulations with different content of MBNO and MHO was performed by thermogravimetry analysis (TGA). Figure 8 show the weight loss versus temperature curves for each sample (TG) and their corresponding first derivative curves (DGT). Table 2 shows some characteristic thermal parameters of the thermograms, such as the degradation onset temperature (T5%), which indicates the temperature at which a 5% weight loss occurs, and the maximum degradation temperature (Tmax), which corresponds to the peak of the first derivative curve. Neat PLA possesses good thermal stability with a T5% of 347.9 °C and a Tmax of 386 °C. As can be seen, adding MBNO or MHO to PLA causes a slight decrease in T5% as they are incorporated in higher quantities. In this case, the addition of 10 phr MNBO results in a T5% reduction of 4.5 °C, while with 10 phr MHO, the reduction is 10.1 °C. Regarding the maximum degradation temperature, it can be observed that both plasticizers lead to a decrease in temperature, obtaining the lowest value at 10 phr, with a reduction of 16.4 °C for both plasticizers. Similar behavior was observed by Garcia-Campo et al. [44] for PLA/PHB/PCL blends compatibilized with epoxidized soybean oil (ELO). The authors observed how the addition of

**Figure 8.** TGA (**a**,**b**) and DTGA (**c**,**d**) of unplasticized PLA and PLA plasticized with different content of MBNO and MHO. **Figure 8.** TGA (**a**,**b**) and DTGA (**c**,**d**) of unplasticized PLA and PLA plasticized with different content of MBNO and MHO.

Table 2 shows the main thermal properties obtained by DSC. On the other hand, Fig-

ure 9 shows the plot representation of the dynamic curve of DSC obtained with the unplasticized PLA and PLA plasticized with different contents of MBNO and MHO. As can **Table 2.** Summary of the TGA and DSC thermal parameters of PLA unplasticized and plasticized with different amounts of MBNO and MHO content.


[a] T5%, calculated at 5% mass loss.

Table 2 shows the main thermal properties obtained by DSC. On the other hand, Figure 9 shows the plot representation of the dynamic curve of DSC obtained with the unplasticized PLA and PLA plasticized with different contents of MBNO and MHO. As can be seen, both maleinized oils have a direct effect on some thermal properties of PLA. Unplasticized PLA has a glass transition temperature (Tg) located at 61 ◦C, a cold crystallization temperature (Tcc) at 123.6 ◦C and a melting temperature located at 150.7 ◦C. Both plasticizers, MBNO and MHO, decrease the T<sup>g</sup> and Tcc of PLA, which is indicative of an increase in the mobility of the polymer chains at lower temperatures, evidencing the plasticizing effect of both maleinized oils [45]. The same evolution was reported by Dominguez-Candela et al. [46], who employed

epoxidized chia oil (ECO) in PLA. On the other hand, no significant variations were observed in Tm, showing that after the addition of the plasticizers, the T<sup>m</sup> remained at values around 150–153 ◦C for all the samples. As it is possible to observe in Figure 9, the addition of lower contents of maleinized oils provides two small melting temperature peaks, which are associated to the formation of two regions with different crystallinities, a result of the compatibilization process between the PLA matrix and the plasticizer. The addition of higher oil contents broadens these two peaks, producing an overlap between them. Regarding crystallinity, it is observed that PLA without plasticizing has a crystallinity of 1.7%, whereas as MBNO or MHO is added, the crystallinity increases progressively. As can be seen in Table 2, with a 10 phr MBNO, the crystallinity of PLA increases to 7.3%, while with the 10 phr MHO, it increases to 10.6%. A similar increase was reported by Carbonell-Verdu et al. [5], who observed that the addition of 10 phr commercial MLO and MCSO to PLA increased its crystallinity up to 11.6% and 19.1%, respectively. The increase in crystallinity with the addition of plasticizers, in this case MBNO and MHO, is caused by the facilitation of the laminar rearrangement of the amorphous zones of PLA, which is caused by the increased mobility of the polymer chains [47]. phous zones of PLA, which is caused by the increased mobility of the polymer chains [47]. **Table 2.** Summary of the TGA and DSC thermal parameters of PLA unplasticized and plasticized with different amounts of MBNO and MHO content. **Samples TGA Parameters DSC Parameters T5% (°C) [a] Tmax (°C) Tg (°C) Tcc (°C) ΔHc (Jg<sup>−</sup>1) Tm (°C) AHm (Jg−1) XPLA (%)**  PLA 347.9 386.2 61.0 123.64 1.2 150.7 3.2 1.7 PLA + 2.5%MBNO 346.6 372.1 58.0 100.8 24.9 153.3 26.8 2.1 PLA + 5%MBNO 346.5 384.6 57.9 120.8 9.2 150.3 15.0 5.8 PLA + 7.5%MBNO 344.5 379.3 58.7 123.7 3.3 151.2 10.0 6.8 PLA + 10%MBNO 343.4 369.6 57.8 123.5 2.6 151.0 3.2 7.3 PLA + 2.5%MHO 346.9 373.1 57.8 103.1 21.0 153.3 22.6 1.6 PLA + 5% MHO 342.9 370.6 56.8 101.8 20.8 153.6 28.4 7.5 PLA + 7.5% MHO 342.2 370.6 57.3 113.9 13.5 150.5 19.5 6.0 PLA + 10% MHO 337.8 369.6 56.9 109.9 9.7 149.2 20.3 10.6 [a] T5%, calculated at 5% mass loss.

plasticized PLA has a glass transition temperature (Tg) located at 61 °C, a cold crystallization temperature (Tcc) at 123.6 °C and a melting temperature located at 150.7 °C. Both plasticizers, MBNO and MHO, decrease the Tg and Tcc of PLA, which is indicative of an increase in the mobility of the polymer chains at lower temperatures, evidencing the plasticizing effect of both maleinized oils [45]. The same evolution was reported by Dominguez-Candela et al. [46], who employed epoxidized chia oil (ECO) in PLA. On the other hand, no significant variations were observed in Tm, showing that after the addition of the plasticizers, the Tm remained at values around 150–153 °C for all the samples. As it is possible to observe in Figure 9, the addition of lower contents of maleinized oils provides two small melting temperature peaks, which are associated to the formation of two regions with different crystallinities, a result of the compatibilization process between the PLA matrix and the plasticizer. The addition of higher oil contents broadens these two peaks, producing an overlap between them. Regarding crystallinity, it is observed that PLA without plasticizing has a crystallinity of 1.7%, whereas as MBNO or MHO is added, the crystallinity increases progressively. As can be seen in Table 2, with a 10 phr MBNO, the crystallinity of PLA increases to 7.3%, while with the 10 phr MHO, it increases to 10.6%. A similar increase was reported by Carbonell-Verdu et al. [5], who observed that the addition of 10 phr commercial MLO and MCSO to PLA increased its crystallinity up to 11.6% and 19.1%, respectively. The increase in crystallinity with the addition of plasticizers, in this case MBNO and MHO, is caused by the facilitation of the laminar rearrangement of the amor-

*Polymers* **2021**, *13*, x FOR PEER REVIEW 11 of 18

**Figure 9.** Dynamic DSC curve of PLA unplasticized and plasticized with different amounts of MBNO (**a**) and MHO content (**b**). **Figure 9.** Dynamic DSC curve of PLA unplasticized and plasticized with different amounts of MBNO (**a**) and MHO content (**b**).
