*3.1. Visual Appearance and Color Properties*

One of the most important requirements of packaging materials for consumers' acceptance is seeing the packed food through the packaging material. Figure 1 shows the visual appearance of the obtained films. It is possible to observe the high transparency of the films suggesting their potential application in the food packaging field. The addition of 20 wt.% of PBAT showed some loss in transparency. The further incorporation of GR also led to a partial decrease in the high transparency of PLA, which was more marked with the increasing amount of GR. In the case of the neat PBAT formulation, it showed less transparency than neat PLA and the incorporation of GR was practically imperceptible on the transparency of the sample (PBAT\_10GR).

The film color properties were determined in the CIEL\*a\*b\* space and the results are summarized in Table 2. PLA showed the highest lightness (L\*), in good accordance with the visual appearance of the films. Meanwhile, the lowest lightness was observed for the PLA/PBAT blend with the higher amount of GR (PLA/PBAT\_20GR). The a\* values (which correspond to red-green coloration), although significant (*p* < 0.05), did not highly change its values in any of the studied formulations. In contrast, the b\* coordinate and the yellowness index (YI) significantly (*p* < 0.05) and considerably increased with the resin content, since the blend became more yellow due to the inherent characteristics of the GR.

**Table 2.** Color change of the studied formulations.


a–g Different letters within the same property show statistically significant differences between formulations (*p* < 0.05).

*Polymers* **2021**, *13*, 1913 7 of 19

**Figure 1.** The visual appearance of PLA, PBAT, PBAT\_10GR, PLA/PBAT, and PLA/PBAT with 5, 10, 15, and 20 phr GR resin films. **Figure 1.** The visual appearance of PLA, PBAT, PBAT\_10GR, PLA/PBAT, and PLA/PBAT with 5, 10, 15, and 20 phr GR resin films.

#### The film color properties were determined in the CIEL\*a\*b\* space and the results are *3.2. Microstructural Characterization*

formulations (*p* < 0.05).

summarized in Table 2. PLA showed the highest lightness (L\*), in good accordance with the visual appearance of the films. Meanwhile, the lowest lightness was observed for the PLA/PBAT blend with the higher amount of GR (PLA/PBAT\_20GR). The a\* values (which correspond to red-green coloration), although significant (*p* < 0.05), did not highly change its values in any of the studied formulations. In contrast, the b\* coordinate and the yellowness index (YI) significantly (*p* < 0.05) and considerably increased with the resin content, since the blend became more yellow due to the inherent characteristics of the GR. **Table 2.** Color change of the studied formulations.  **Color Change Formulation L\* a\* b\* YI**  PLA 41.2 ± 0.7 a −1.3 ± 0.4 a -2.7 ± 0.6 a -12.3 ± 2.4 a PLA/PBAT 87.5 ± 0.3 b −0.7 ± 0.1 b 2.1 ± 0.3 b 3.8 ± 0.6 b PLA/PBAT\_5GR 86.2 ± 0.6 c −1.7 ± 0.1 c 8.0 ± 0.8 c 14.5 ± 1.5 c PLA/PBAT\_10GR 82.7 ± 0.9 d −1.4 ± 0.1 a 17.7 ± 1.8 d 33.5 ± 3.4 d Figure 2 shows the effect of the GR resin on the partially miscible PLA/PBAT blends, as well as the effect on the neat PBAT, taken as reference. Neat PLA (Figure 2a) showed a flat surface with small prominences, characteristic of a brittle break of the material under cryofracture conditions. The blend of PLA with 20% PBAT (PLA/PBAT, Figure 2b) showed a smoother surface with PBAT domains sizing less than 0.5 µm, a characteristic size that shows that the components have partial miscibility, although it is very poor [22]. The incorporation of a 5 phr of GR (Figure 2c) showed significant differences in the morphology of the cryofracture surface. Specifically, the PBAT domains were larger (between 0.5–1.5 µm), with an average size of 1 µm. Moreover, PBAT domains turned from presenting irregular shapes (Figure 2b) to almost perfect spherical shapes. This is indicative of the loss of affinity and miscibility between PLA and PBAT when adding GR. However, with a 5 phr of GR, there was still some interaction between the PLA and the PBAT matrices since the PBAT domains broke through the plane of the fracture (in other words, the PBAT spheres broke through the crack of the fracture). This means that the PLA-PBAT interaction was greater than the cohesion forces of PBAT. Moreover, it should be highlighted that, although higher, the PBAT domains showed good adhesion with the PLA matrix at the interface.

PLA/PBAT\_15GR 82.4 ± 0.6 d −0.9 ± 0.3 b 20.7 ± 1.2 e 39.1 ± 2.1 e

a–g Different letters within the same property show statistically significant differences between

PBAT 83.7 ± 0.7 f −0.6 ± 0.2 b 6.3 ± 0.5 g 12.5 ± 1.1 c PBAT\_10GR 76.8 ± 0.5 g 1.3 ± 0.3 e 20.9 ± 0.7 e 43.4 ± 1.4 g

53.1 ± 2.1 f

PLA/PBAT\_20GR 79.5 ± 0.8 e −0.1 ± 0.2 d 28.2 ± 1.1 f

*Polymers* **2021**, *13*, 1913 9 of 19

**Figure 2.** FESEM images at 5000 X of studied materials: (**a**) PLA, (**b**) PLA/PBAT, (**c**) PLA/PBAT\_5GR, (**d**) PLA/PBAT\_10GR, (**e**) PLA/PBAT\_15GR, (**f**) PLA/PBAT\_20GR, (**g**) PBAT, and (**h**) PBAT\_10GR. **Figure 2.** FESEM images at 5000 X of studied materials: (**a**) PLA, (**b**) PLA/PBAT, (**c**) PLA/PBAT\_5GR, (**d**) PLA/PBAT\_10GR, (**e**) PLA/PBAT\_15GR, (**f**) PLA/PBAT\_20GR, (**g**) PBAT, and (**h**) PBAT\_10GR.

*3.3. Mechanical Properties of the PLA/PBAT/GR Formulations*  Table 3 shows the main values of the mechanical properties, maximum tensile and flexural strength, Young's moduli, elongation at break, impact absorption energy (Charpy), Shore D hardness, and the HDT temperature of each obtained formulation, as well as the neat PLA as a reference. This type of fracture did not persist with higher concentrations of GR. Figure 2d,e show the PLA/PBAT formulation with 10 and 15 phr of GR, respectively. In these images, the larger size of the PBAT domains (1.5–4 µm) can be observed. Although there were slight differences in PBAT domain sizes, their fractures were not similar to Figure 2c. In fact, in these formulations, the PBAT domains were not broken and showed complete PBAT spheres with small (nanoscale) domains of GR. The non-breakage of the PBAT spheres was due to the lower miscibility and interaction between the PLA and the PBAT domains, which is due to the phobic effect that exists between the PLA matrix and the GR resin [48]. This lack of interaction generated points of zero interaction around the PBAT spheres that

prevented their breakage, although it improved the impact energy absorption, probably due to the PBAT domains still showing good adhesion with the PLA matrix at the interface, since there was not a gap between both polymeric matrices. Finally, Figure 2f shows the formulation with a 20 phr of GR, where the PBAT domains had an approximate average size of 4–5 µm and exhibited signs of GR saturation within them. These nanodomains, which were less evident in the other formulations, generated an important reduction in the PLA/PBAT interactions and resulted in a decrease of the impact energy absorption. Therefore, the formulations with PBAT domain sizes of 2–3 µm were those that improved the impact energy absorption or the toughness.

Additionally, Figure 2g,h show the cryofractured surface of neat PBAT and PBAT with 10 phr of GR. It is possible to verify, along with the mechanical properties, that GR acted as a plasticizer for the PBAT. There were no appreciable differences in the morphology of the cryofractured surfaces of both materials since PBAT is a soft material that became even more softer by the addition of GR, demonstrating the plasticization effect of the GR.
