*3.2. Thermal Properties*

Table 7 shows the main thermal degradation parameters, initial degradation temperature (Tonset), and maximum mass loss rate temperature (Tmax1 and Tmax2) obtained by TGA. Figures 3 and 4 show the TGA curves of TPS and TPS/ASP biocomposite, respectively, obtained after different reprocessing cycles. Furthermore, all samples presented a two-step process because the thermogravimetric analysis was carried out under an N<sup>2</sup> and O<sup>2</sup> atmosphere. When it comes to the comparative TGA curves of TPS samples obtained after different reprocessing cycles, those suggested no significant changes in the thermal degradation parameters since the curves overlapped. TPS-1 presented moderate thermal stability with Tonset, and Tmax1 of 326.1 ◦C and 362.1, respectively. It is noticeable that, after the fourth injection cycle, the values remained almost invariable. This indicated that the reprocessing cycles did not have a significant effect on thermal degradation at the current processing temperatures. Regarding the TGA thermograms of TPS/ASP, the addition of ASP reduced the thermal stability of the biocomposite because almond shells degrade faster than polymer matrix [10,12], and Tonset and Tmax1 moved towards a lower temperature. Tmax2 remained practically unchanged. However, in a previous study, which is in the process of publication, it was found that the addition of 10 phr ELO had a positive effect on the thermal stability of this type of biocomposite, increasing Tonset and Tmax1 by 6 ◦C: TPS/ASP-1 biocomposite presented a Tonset of 315.0 ◦C and Tmax1 of 346.0 ◦C. After the second injection cycle, Tonset decreased by 9 ◦C and continued to decrease progressively after each reprocessing cycle. Nevertheless, the Tmax1 remained almost invariable until the fifth injection cycle (340.6 ◦C).

**Table 7.** Thermal properties of the injection-molded samples of TPS and TPS/ASP biocomposite subjected to different reprocessing cycles obtained by thermogravimetric analysis (TGA).


**Figure 3.** (**a**) Thermogravimetric analysis (TGA) thermograms corresponding to different reprocessing cycles of TPS (**b**) first derivative (DTG) curves.

To obtain the main thermal transition of the material a differential scanning calorimetry was used. Figures 5 and 6 show the DSC curves achieved from the cooling and second heating scans of TPS and TPS/ASP biocomposite samples, respectively, obtained after different reprocessing cycles. In addition, Table 8 summarizes the most relevant thermal parameters obtained from the cooling and second heating of the samples subjected to different reprocessing cycles. Bastioli et al. [14] described Novamont's starch-based technology as a process that could destroy amylose and amylopectin from starch. Thus, the main endothermic (Tm1 y Tm2) and exothermic (Tc) peaks were related to the melting and the cold crystallization of the crystalline structure of the synthetic biodegradable polymer presented in Mater-Bi, respectively. After the first injection run (TPS-1), TPS presented a bimodal endothermic peak (Figure 5b). The first one, smaller, at around 161.4 ◦C (Tm1), and the second one, more pronounced, at 168.5 ◦C (Tm2). These are associated with the fusion of the crystalline structure as a result of chain scission caused by different resistance

to heat. The TPS-1 thermogram corresponding to the cooling scan presented a unique main exothermic peak around 109.5 ◦C (Figure 5a). As can be observed, after different reprocessing cycles, the thermal transitions of TPS, such as Tc, Tm, and their respective enthalpy, did not present significant changes. The addition of 20 wt% produced a slight reduction in all parameters. After the first injection run, TPS/ASP-1 also presented a bimodal endothermic peak. The melting point temperature, Tm1, decreased from 161.4 ◦C to 157.1 ◦C and the Tm2 from 168.5 ◦C to 166.4 ◦C. Regarding the normalized melting enthalpy (∆Hm), it decreased from 23.8 to 17.2 J/g. These results indicated that the addition of almond shell to the starch-based polymer decreases the crystallization of the molecular chains [12]. After the second injection cycle (TPS/ASP-2), Tm1 and Tm2 decreased 2–3 ◦C compared to TPS/ASP-1 and continued to decrease progressively after each reprocessing cycle until 144.6 ◦C and 162.7 ◦C (Figure 6b), respectively. The decrease in the values may be attributed to the higher mobility of the polymer chains as a result of the reduction in the molecular weight during the recycling process. In addition, it was noticed that the crystallization temperature (Tc) slightly decreased upon adding ASP, thus indicating that the presence of ASP made crystallization start later, compared to as-received TPS. Furthermore, it was noticed that the crystallization temperature and enthalpy of TPS/ASP decreased progressively after each reprocessing cycle (Figure 6b). This indicated a loss of crystalline structure and a major difficulty in the crystallization process (i.e., starting at a lower temperature) as the number of processing cycles was increased [15].

**Figure 4.** *Cont.*

**Figure 4.** (**a**) TGA thermograms corresponding to different reprocessing cycles of TPS/ASP biocomposite (**b**) first derivative (DTG) curves.

**Figure 5.** Comparative plot of differential scanning calorimetry (DSC) curves of TPS after different reprocessing cycles: (**a**) cooling cycle, (**b**) second heating cycle.

**Figure 6.** Comparative plot of DSC curves of TPS/ASP after different reprocessing cycles: (**a**) cooling cycle, (**b**) second heating cycle.

**Table 8.** Thermal properties of the injection-molded samples of TPS and TPS/ASP biocomposite subjected to different reprocessing cycles obtained by differential scanning calorimetry (DSC).

