## 3 Results and Discussion
### DSC Analysis  
With constant pressure, the heat flow (Q) is equal to the enthalpy change (\(\Delta\)H) as Equation (1).  
\[\Delta\mathrm{H}\;=\;\mathrm{Q} \tag{1}\]  
The melting temperature (Tm) and melting enthalpy (\(\Delta\)Hm) for thermoplastic polyurethane are 145.8 \({}^{\circ}\)C and 9.7 \(\mathrm{J/g}\) and for polypropylene 164.5 \({}^{\circ}\)C and 92.7 \(\mathrm{J/g}\). On the other hand, the crystallization temperature (Tc) and crystallization enthalpy (\(\Delta\)Hc) for thermoplastic polyurethane are 62.5 \({}^{\circ}\)C and 6.2 \(\mathrm{J/g}\), and for polypropylene 118.1 \({}^{\circ}\)C and 91.3 \(\mathrm{J/g}\).  
Figure 6 shows the melting behaviors, including Tm and \(\Delta\)Hm of T/P/MA blends, while Figure 7 shows the crystallization behaviors, including crystallization temperature (Tc) and crystallization enthalpy (\(\Delta\)Hc). Figure 6A-C shows that with a rise in polypropylene content, T50 has the maximal melting enthalpy for polypropylene, which is a semi-crystalline polymer. However, thermoplastic polyurethane is an amorphous polymer, and hence, the yielded maximal melting enthalpy is primarily attributed to the presence of polypropylene. Based on the employment of melt-recycling, the melting enthalpy of samples undergoing one-off hot-pressing (Figure 6A) is higher than the melting enthalpy of the two batches of samples undergoing post-2 nd-recycling (Figure 6B) and post-3 rd-recycling (Figure 6C). Despite being thermoplastic polymers, both T and P still trigger a greater number of interactions among molecular chains due to the enthalpy variation. Subsequently, the molecular chains are exerted with influences or even damage, which adversely affects the stability of the entire structure [36].  
Figure 6: Melting behaviors in the differential scanning calorimetry (DSC) performance of T/P and T/MA blends as related to melt-recycling: (**A**) one-off hot-pressing, (**B**) the post-2 nd-recycling, and (**C**) the post-3 rd-recycling. Tm = melting temperature and \(\Delta\)Hm = melting enthalpy.  
From Figure 7A,B, the crystallization curves of the T90 and T90/MA groups show small exothermic peaks around 60-70 \({}^{\circ}\)C, which are related to the disordered arrangement of rigid segments in thermoplastic polyurethane block copolymers [17]. Figure 7A-C shows that the T90 and T90/MA groups demonstrate insignificant crystallization peaks regardless of whether MA is added. On the other hand, increasing the polypropylene content provides the T70 and T50 groups with increasingly complete crystallization peaks, followed by a rise in crystallization enthalpy. Being an amorphous polymer, thermoplastic polyurethane has rather low intrinsic crystallization behavior. Thus, the deductive comparison confirms that the occurrence of the crystallization peak is owed to the presence of polypropylene. The interactions between polypropylene and thermoplastic polyurethane polymer chains lead to an increment in heat transition. Furthermore, as an increasing consequence of the presence of polypropylene crystallites in the thermoplastic polyurethane matrix, the crystallization rate of the T/P blended group increases [37]. Moreover, the addition of MA broadens the shapes of both melting peaks and crystallization peaks, which also suggests that either the heating process or cooling process demands more time to melt or generate crystals. In addition, MA-containing blends also have lower melting enthalpy or crystallization enthalpy than the MA-free blends. The urethane groups located in the rigid segment of thermoplastic polyurethane exhibit dissociation, and then have grafting reactions with the anhydride groups of MAs. As a result, the molecular chains from some crystal phases are entangled and, as such, limit the composition of the entire crystals, which eventually interferes with the difference in enthalpy.