*3.3. Mechanical Properties*

Table 9 shows a summary of the tensile properties of both TPS and TPS/ASP biocomposite after each reprocessing cycle. The TPS-1 presented Young's modulus of 1658 MPa and tensile strength of 39.4 MPa. As can be seen in Table 9, the number of reprocessing cycles influenced the tensile properties, decreasing the values progressively. In particular, after the third processing cycle, Young's modulus significantly decreased to 1184 MPa. Regarding the tensile and elongation at break, changes were more noticeable after the fourth processing. That indicated that the material was less ductile.


TPS/ASP-6 1052 ± 47 13.4 ± 0.9 1.5 ± 0.1 13.4 ± 0.9 1.5 ± 0.1

**Table 9.** Mechanical properties, Young's modulus (E), tensile strength (σM), elongation at tensile strength ( **Table 9.** Mechanical properties, Young's modulus (E), tensile strength (σM), elongation at tensile strength (ƐM), tensile strength at break (σR), elongation at break (ƐR), of TPS and TPS/ASP biocomposite subjected to different reprocessing cycles. <sup>M</sup>), tensile strength at break (σR), elongation at break ( **Table 9.** Mechanical properties, Young's modulus (E), tensile strength (σM), elongation at tensile strength (ƐM), tensile strength at break (σR), elongation at break (ƐR), of TPS and TPS/ASP biocomposite subjected to different reprocessing cycles. R), of TPS and TPS/ASP biocomposite subjected to different reprocessing cycles. which indicated similar rigidity. The rest of the parameters (tensile and elongation strength/break) kept similar values to TPS/ASP-1 until the fourth reprocessing. which indicated similar rigidity. The rest of the parameters (tensile and elongation strength/break) kept similar values to TPS/ASP-1 until the fourth reprocessing.

strength/break) kept similar values to TPS/ASP-1 until the fourth reprocessing.

maximum strength, tensile, and elongation at break) [12]. However, in this work, as can be observed in Table 8, Young's modulus of TPS/ASP biocomposite was lower than the unfilled TPS. The addition of ELO increased the flexibility of the biocomposite, reaching values of Young's modulus even lower than those of the as-received TPS. Thus, the tensile and elongation of the TPS/ASP biocomposite decreased due to its progressive lack of capacity to sustain deformation. When it comes to the effect of reprocessing TPS/ASP biocomposite in tensile properties, it did not have a significant effect on Young's modulus, which indicated similar rigidity. The rest of the parameters (tensile and elongation

*Polymers* **2021**, *13*, 1159 14 of 19

*Polymers* **2021**, *13*, 1159 14 of 19

maximum strength, tensile, and elongation at break) [12]. However, in this work, as can be observed in Table 8, Young's modulus of TPS/ASP biocomposite was lower than the unfilled TPS. The addition of ELO increased the flexibility of the biocomposite, reaching values of Young's modulus even lower than those of the as-received TPS. Thus, the tensile and elongation of the TPS/ASP biocomposite decreased due to its progressive lack of capacity to sustain deformation. When it comes to the effect of reprocessing TPS/ASP biocomposite in tensile properties, it did not have a significant effect on Young's modulus, which indicated similar rigidity. The rest of the parameters (tensile and elongation

*Polymers* **2021**, *13*, 1159 14 of 19

maximum strength, tensile, and elongation at break) [12]. However, in this work, as can be observed in Table 8, Young's modulus of TPS/ASP biocomposite was lower than the unfilled TPS. The addition of ELO increased the flexibility of the biocomposite, reaching values of Young's modulus even lower than those of the as-received TPS. Thus, the tensile and elongation of the TPS/ASP biocomposite decreased due to its progressive lack of capacity to sustain deformation. When it comes to the effect of reprocessing TPS/ASP biocomposite in tensile properties, it did not have a significant effect on Young's modulus,

maximum strength, tensile, and elongation at break) [12]. However, in this work, as can be observed in Table 8, Young's modulus of TPS/ASP biocomposite was lower than the unfilled TPS. The addition of ELO increased the flexibility of the biocomposite, reaching values of Young's modulus even lower than those of the as-received TPS. Thus, the tensile and elongation of the TPS/ASP biocomposite decreased due to its progressive lack of capacity to sustain deformation. When it comes to the effect of reprocessing TPS/ASP biocomposite in tensile properties, it did not have a significant effect on Young's modulus,

strength/break) kept similar values to TPS/ASP-1 until the fourth reprocessing.

TPS/ASP-6 1052 ± 47 13.4 ± 0.9 1.5 ± 0.1 13.4 ± 0.9 1.5 ± 0.1

TPS/ASP-3 1057 ± 83 16.8 ± 0.2 2.1 ± 0.1 13.2 ± 0.2 4.3 ± 0.5

. After the second

. The reprocessing cycles did not have a significant effect

. After the second

. Additionally, an evi-

(TPS/ASP-6). This behavior was ob-

. Additionally, an evi-

. After the second

. Additionally, an evi-

(TPS/ASP-6). This behavior was ob-

(TPS/ASP-6). This behavior was ob-

TPS/ASP-3 1057 ± 83 16.8 ± 0.2 2.1 ± 0.1 13.2 ± 0.2 4.3 ± 0.5

*Polymers* **2021**, *13*, 1159 14 of 19

Figure 7 shows the evolution of impact strength in both the reprocessed TPS and TPS/ASP biocomposite. The impact tests of TPS were carried out with notched samples because the material presented a high strength and did not break, even after the applied reprocessing cycles. The as-received TPS (TPS-1) presented an impact strength of 6.0 kJ/m<sup>2</sup> . This value barely decreased after the second injection cycle (TPS-2). After the third and fourth injection cycle, a slight reduction in energy absorption capacity was attained, reaching values around 4.5 kJ/m<sup>2</sup> . The reprocessing cycles did not have a significant effect on the impact strength. This behavior was observed in other grades of starch-based polymer (Solanyl, [16]). The addition of 20 wt% of ASP drastically affected the impact strength. While the TPS samples needed notching to do the test, the TPS/ASP biocomposite did not. Figure 7 shows the evolution of impact strength in both the reprocessed TPS and TPS/ASP biocomposite. The impact tests of TPS were carried out with notched samples because the material presented a high strength and did not break, even after the applied reprocessing cycles. The as-received TPS (TPS-1) presented an impact strength of 6.0 kJ/m<sup>2</sup> . This value barely decreased after the second injection cycle (TPS-2). After the third and fourth injection cycle, a slight reduction in energy absorption capacity was attained, reaching values around 4.5 kJ/m<sup>2</sup> . The reprocessing cycles did not have a significant effect on the impact strength. This behavior was observed in other grades of starch-based polymer (Solanyl, [16]). The addition of 20 wt% of ASP drastically affected the impact strength. While the TPS samples needed notching to do the test, the TPS/ASP biocomposite did not. TPS/ASP-4 1066 ± 17 16.7 ± 0.1 2.3 ± 0.1 16.7 ± 0.1 2.3 ± 0.1 TPS/ASP-5 1028 ± 41 15.1 ± 0.4 1.8 ± 0.1 15.1 ± 0.4 1.8 ± 0.1 TPS/ASP-6 1052 ± 47 13.4 ± 0.9 1.5 ± 0.1 13.4 ± 0.9 1.5 ± 0.1 Figure 7 shows the evolution of impact strength in both the reprocessed TPS and TPS/ASP biocomposite. The impact tests of TPS were carried out with notched samples because the material presented a high strength and did not break, even after the applied reprocessing cycles. The as-received TPS (TPS-1) presented an impact strength of 6.0 kJ/m<sup>2</sup> . This value barely decreased after the second injection cycle (TPS-2). After the third and fourth injection cycle, a slight reduction in energy absorption capacity was attained, reaching values around 4.5 kJ/m<sup>2</sup> . The reprocessing cycles did not have a significant effect TPS/ASP-4 1066 ± 17 16.7 ± 0.1 2.3 ± 0.1 16.7 ± 0.1 2.3 ± 0.1 TPS/ASP-5 1028 ± 41 15.1 ± 0.4 1.8 ± 0.1 15.1 ± 0.4 1.8 ± 0.1 TPS/ASP-6 1052 ± 47 13.4 ± 0.9 1.5 ± 0.1 13.4 ± 0.9 1.5 ± 0.1 Figure 7 shows the evolution of impact strength in both the reprocessed TPS and TPS/ASP biocomposite. The impact tests of TPS were carried out with notched samples because the material presented a high strength and did not break, even after the applied reprocessing cycles. The as-received TPS (TPS-1) presented an impact strength of 6.0 kJ/m<sup>2</sup> . This value barely decreased after the second injection cycle (TPS-2). After the third and fourth injection cycle, a slight reduction in energy absorption capacity was attained, reaching values around 4.5 kJ/m<sup>2</sup> In a previous study, the addition of 20 wt% of ASP produced an increment in Young's Modulus and a reduction in the rest of the tensile parameters (tensile and elongation at maximum strength, tensile, and elongation at break) [12]. However, in this work, as can be observed in Table 8, Young's modulus of TPS/ASP biocomposite was lower than the unfilled TPS. The addition of ELO increased the flexibility of the biocomposite, reaching values of Young's modulus even lower than those of the as-received TPS. Thus, the tensile and elongation of the TPS/ASP biocomposite decreased due to its progressive lack of capacity to sustain deformation. When it comes to the effect of reprocessing TPS/ASP biocomposite in tensile properties, it did not have a significant effect on Young's modulus, which indicated similar rigidity. The rest of the parameters (tensile and elongation strength/break) kept similar values to TPS/ASP-1 until the fourth reprocessing.

The TPS/ASP biocomposite showed an impact strength of 14.13 kJ/m<sup>2</sup> . After the second reprocessing cycle, the impact strength decreased until 12.26 kJ/m<sup>2</sup> . Additionally, an evident reduction in energy absorption capacity was attained when increasing the reprocessing cycles, reaching values down to 9.70 kJ/m<sup>2</sup> (TPS/ASP-6). This behavior was observed in PLA systems with cellulose fibers [17–19]. Finally, this loss in the impact strength can be attributed to the polymer degradation occurring during each thermo-mechanical recycle. The TPS/ASP biocomposite showed an impact strength of 14.13 kJ/m<sup>2</sup> reprocessing cycle, the impact strength decreased until 12.26 kJ/m<sup>2</sup> dent reduction in energy absorption capacity was attained when increasing the reprocessing cycles, reaching values down to 9.70 kJ/m<sup>2</sup> served in PLA systems with cellulose fibers [17–19]. Finally, this loss in the impact strength can be attributed to the polymer degradation occurring during each thermo-mechanical recycle. on the impact strength. This behavior was observed in other grades of starch-based polymer (Solanyl, [16]). The addition of 20 wt% of ASP drastically affected the impact strength. While the TPS samples needed notching to do the test, the TPS/ASP biocomposite did not. The TPS/ASP biocomposite showed an impact strength of 14.13 kJ/m<sup>2</sup> reprocessing cycle, the impact strength decreased until 12.26 kJ/m<sup>2</sup> dent reduction in energy absorption capacity was attained when increasing the reprocessing cycles, reaching values down to 9.70 kJ/m<sup>2</sup> served in PLA systems with cellulose fibers [17–19]. Finally, this loss in the impact strength can be attributed to the polymer degradation occurring during each thermo-mechanical recycle. on the impact strength. This behavior was observed in other grades of starch-based polymer (Solanyl, [16]). The addition of 20 wt% of ASP drastically affected the impact strength. While the TPS samples needed notching to do the test, the TPS/ASP biocomposite did not. The TPS/ASP biocomposite showed an impact strength of 14.13 kJ/m<sup>2</sup> reprocessing cycle, the impact strength decreased until 12.26 kJ/m<sup>2</sup> dent reduction in energy absorption capacity was attained when increasing the reprocessing cycles, reaching values down to 9.70 kJ/m<sup>2</sup> served in PLA systems with cellulose fibers [17–19]. Finally, this loss in the impact strength can be attributed to the polymer degradation occurring during each thermo-mechanical recycle. Figure 7 shows the evolution of impact strength in both the reprocessed TPS and TPS/ASP biocomposite. The impact tests of TPS were carried out with notched samples because the material presented a high strength and did not break, even after the applied reprocessing cycles. The as-received TPS (TPS-1) presented an impact strength of 6.0 kJ/m<sup>2</sup> . This value barely decreased after the second injection cycle (TPS-2). After the third and fourth injection cycle, a slight reduction in energy absorption capacity was attained, reaching values around 4.5 kJ/m<sup>2</sup> . The reprocessing cycles did not have a significant effect on the impact strength. This behavior was observed in other grades of starch-based polymer (Solanyl, [16]). The addition of 20 wt% of ASP drastically affected the impact strength. While the TPS samples needed notching to do the test, the TPS/ASP biocomposite did not. The TPS/ASP biocomposite showed an impact strength of 14.13 kJ/m<sup>2</sup> . After the second reprocessing cycle, the impact strength decreased until 12.26 kJ/m<sup>2</sup> . Additionally, an evident reduction in energy absorption capacity was attained when increasing the reprocessing cycles, reaching values down to 9.70 kJ/m<sup>2</sup> (TPS/ASP-6). This behavior was observed in PLA systems with cellulose fibers [17–19]. Finally, this loss in the impact strength can be attributed to the polymer degradation occurring during each thermo-mechanical recycle.

The fracture of the surface after impact was studied by SEM analysis. TPS surface after the first injection cycle (Figure 8a) showed less roughness than after the fourth injection cycle (TPS/ASP-4) (Figure 8b). This increase in the surface roughness can be directly attributed to the degradation of the TPS matrix. SEM micrographs of the impact fracture surfaces of TPS/ASP biocomposite gave qualitative information about the dispersion of almond shell particles. The fracture surfaces of TPS/ASP after the first injection cycle (Figure 8c) showed that the almond shell was homogeneously distributed in the polymer matrix. As with the TPS surface, it could be observed that the surface roughness and the imperfections on the impact fracture surface increased after the fourth injection cycle (TPS/ASP-4) (Figure 8d).

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**Figure 8.** SEM micrographs of the impact fracture surfaces of the composites: (**a**) TPS after the first injection cycle; (**b**) TPS after the fourth injection cycle; (**c**) TPS/ASP biocomposite after the first injection cycle; (**d**) TPS/ASP biocomposite after the fourth injection cycle. **Figure 8.** SEM micrographs of the impact fracture surfaces of the composites: (**a**) TPS after the first injection cycle; (**b**) TPS after the fourth injection cycle; (**c**) TPS/ASP biocomposite after the first injection cycle; (**d**) TPS/ASP biocomposite after the fourth injection cycle.

#### *3.4. Melt Flow Index 3.4. Melt Flow Index*

of the material.

It is a well-known fact that polymer chain breaking due to polymer degradation results in reduced molecular weight and increased flowability with temperature [20]. MFI is a simple test that can potentially offer detailed information about the degradation a polymer material has undergone [5,20]. Figure 9 shows the evolution of the MFI values after different reprocessing cycles. As can be seen, the MFI suffered a drastic increment in It is a well-known fact that polymer chain breaking due to polymer degradation results in reduced molecular weight and increased flowability with temperature [20]. MFI is a simple test that can potentially offer detailed information about the degradation a polymer material has undergone [5,20]. Figure 9 shows the evolution of the MFI values after different reprocessing cycles. As can be seen, the MFI suffered a drastic increment in the

the value in both TPS and TPS/ASP biocomposite from the second injection process (TPS-2 and TPS/ASP-2). This effect was observed in other grades of commercial Mater-Bi and

cessing [7]. TPS/ASP-1 biocomposite presented an MFI value higher than TPS-1, probably because TPS/ASP was previously subjected to a thermal process to obtain the pellets. However, despite the evident degradation of the material according to the MFI values obtained, this was not reflected in the decrease in the mechanical and thermal properties value in both TPS and TPS/ASP biocomposite from the second injection process (TPS-2 and TPS/ASP-2). This effect was observed in other grades of commercial Mater-Bi and Mater-Bi YI014U/C, in which MFI increased from 9 to 16 g/10 min after the second processing [7]. TPS/ASP-1 biocomposite presented an MFI value higher than TPS-1, probably because TPS/ASP was previously subjected to a thermal process to obtain the pellets. However, despite the evident degradation of the material according to the MFI values obtained, this was not reflected in the decrease in the mechanical and thermal properties of the material.

**Figure 9.** Variation of the melt flow index (MFI) after different reprocessing cycles.
