## 3 Results and Discussion
### Feedback's Fabrication and Its Microstructural, Thermal and Rheological Properties  
All the components are mixed in a double-rotor mixer (ThermoHaake, Waltham, MA, USA, Haake Rheocord 252p) at 190 \({}^{\circ}\)C to gently homogenize the powder with the binder (FA and FE), and with the binder and the melamine (FA-MM and FE-MM), leading to the four feedstocks produced for this study, as described in Section 3.1. The evolution of the mixing torque of the four feedstocks during the process is shown in Figure 2. After the addition of all the components in each case (from minute 12), the mixing process continues, torque values tend to equilibrate up to the final stabilization time (considered at minute 90) and the resulting feedstocks are removed from the mixer and pelletized. Further mixing time could lead to a steady-state torque value; however, it could also produce the thermal degradation of some binder components and a sudden increase of torque, so a balance at 90 min was achieved. Feedstocks FE and FE-MM show higher torque values after 90 min mixing (8.01 \(\pm\) 0.06 N\(\cdot\)m and 8.37 \(\pm\) 0.07 N\(\cdot\)m, respectively) than feedstocks FA and FA-MM (5.62 \(\pm\) 0.05 N\(\cdot\)m and 6.82 \(\pm\) 0.04 N\(\cdot\)m, respectively). However, the torque evolution observed in Figure 2 shows an increasing tendency in the stabilization of FA and FA-MM feedstocks, while a decreasing evolution in the case of surfactant E. This latter behavior is preferred, since it means that the surfactant is more effective in the homogenization process between powder, binder and the melamine additive, if there is any. Nevertheless, in all cases, the melamine is well homogenized in the feedstocks, as can be seen in the SEM micrographs (JEOL JSM-6610 LV, Tokyo, Japan) displayed in Figure 2.  
It should be mentioned that the melamine powder shows agglomerates of large particle size up to 50 \(\upmu\)m (Figure 2a) that are subsequently broken during mixing into the double-rotor machine, which cannot be detected in the feedstocks' micrographs (see SEM micrographs of FA, FA-MM, FE, FE-MM in Figure 2b-e). Since MM melting does not occur in the range up to 200 \({}^{\circ}\)C, it is hypothesized that homogenization is explained by the wear effect of the powder particles breaking the MM particles apart, favoring the blending with other binder components and powders. Thus, feedstocks exhibit strong homogenization both before and after the incorporation of melamine, confirming that the new surfactant E has a successful role in the feedstock. The torque monitorization is another way to predict the viscosity of the mixtures, which are more viscous at higher values. It is worth noting that the introduction of melamine hardly produces any effect on the FE and FE-MM feedstock properties during the blending process, as can be seen in the torque evolution and torque values, meaning that those mixtures exhibit similar viscosity (Figure 2). The higher torque value exhibited by FA-MM when compared to FA can be associated with the loss of the wettability effect in the case  
Figure 2: Torque evolution vs. time during the mixing process of the feedstocks FA, FE, FA-MM and FE-MM and their microstructural characterization by SEM. (Melamine microstructure also included). Arrows indicate the increase/decrease tendency in the torque curves in each case. of using A (acid-based), as was previously predicted by DSC studies gathered in Figure 1, which affects the rheology of the mixture and causes the torque and viscosity to increase. The thermal properties of the feedstocks were studied by thermogravimetric analysis (TGA) (Labsys EVO Setaram Instrumentation, Caluire-et-Cure, France), and the degradation curves with temperature are shown in Figure 3. In the feedstocks FA and FE, the mass losses at 350 \({}^{\circ}\)C and 440 \({}^{\circ}\)C correspond to POM and HDPE polymers in the binder, respectively. The surfactant decomposition is not detected by this technique due to the low concentration in the mixture.