## 3 Results and Discussion
### Feedback's Fabrication and Its Microstructural, Thermal and Rheological Properties
In this way, both low and high shear rates were covered to better understand the rheology of the mixtures. Additionally, for comparison reasons, mixtures of the neat binder with and without the melamine were also prepared and analyzed by oscillatory rheology, to further study the behavior of the melamine additive in the polymeric binder without the influence of the metallic powder. The results regarding the amplitude-sweep oscillatory rheology are given in Table 3 for the neat binders (i.e., without the metallic powder) and for the feedstocks (see Table 1 for the feedstocks' composition). Studying the neat binder, the presence of melamine reduces the linear viscoelastic region (LVE) and the elastic limit (\(\tau_{y}\)), and it produces the increase of the elastic moduli (G\({}^{\prime}\)), which is due to the reinforcement effect of the solid particles (lower and higher values of \(\tau_{y}\) and G\({}^{\prime}\) in FA-MM and FE-MM systems, respectively). The reduction of loss tangent (tan (\(\delta\))) when the melamine is added indicates that the elastic contribution increases, although it remains above 1 (a situation in which the loss modulus G\({}^{\prime\prime}\) is higher than the storage modulus G\({}^{\prime}\) or G\({}^{\prime\prime}\)/G\({}^{\prime}\) > 1). However, if the feedstock is analyzed, the LVE is not affected by the presence of melamine (the \(\tau_{y}\) value remains very low due to the metallic powder), while it unexpectedly reduces G\({}^{\prime}_{y}\) and increases tan (\(\delta\)) (FA-MM and FE-MM). Thus, the fluid becomes less viscous because of the lubricating effect of melamine in these mixtures. Comparing the role of the surfactant in the feedstocks, the increase of tan (\(\delta\)) is more significant with A than with E, which might be related to the interactions created between melamine and the surfactant. As mentioned before, the acid-based surfactant A may form a covalent bond with melamine, resulting in a better  
Figure 4: Yield stress and activation energy (E\({}_{a}\)) rheological parameters calculated from the viscosity measurements at high shear rates (10–7000 s\({}^{-1}\)) and temperatures of 190, 200 and 210 \({}^{\circ}\)C. (For E\({}_{a}\), a shear rate of 1000 s\({}^{-1}\) was considered). integration of the solid additive in the molten fluid and, therefore, more severe viscosity reduction. In the case of surfactant E, the increase of tan (\(\delta\)) is moderate, so it provides lower fluidity to the solid additive, maybe due to the non-covalent H-bonding interactions formed, equivalent to a crosslinking effect. Concerning the frequency-sweep oscillatory tests, the results are gathered in Figure 5. If the binders are analyzed separately (Figure 5a), only the FE-MM mixture exhibits G\({}^{\prime}\)-G\({}^{\prime\prime}\) intersection at 10 rad/s. The other binder mixtures show G\({}^{\prime\prime}\) > G\({}^{\prime}\) in all the frequency ranges studied, even when melamine is incorporated (FA-MM). Thus, the addition of melamine to the mixture containing surfactant E changes it into a viscoelastic solid in the lowest frequency values, while in the case of surfactant A, it remains a viscoelastic liquid during the entire frequency range. This is in accordance with the values of tan (\(\delta\)) previously shown, confirming that melamine is creating a three-dimensional network with surfactant E. In contrast to the traditional surfactant A (acid-based), the use of E when designing the feedstock prevents the melamine from covalently reacting, losing its functional groups. We can conclude that, although this reaction is beneficial from the rheology point of view since it diminishes the effect of the solid additive in the binder, it is not intended for the purpose of the global study, in which keeping the melamine in its nucleophile form is desirable for the next processing steps of the subsequent feedstocks.