## 4 Discussion
Similarly, FTIR results for Ti composite suggest physical interaction between the filler and MAPP component of the binder system. The Ti-O surface groups reacted with the carbonyl groups of the MAPP component. No vibration peak shifts between the binder and filler were observed for Ti composite material. Thus, no chemical bonding of Ti filler material with the binder system could be clearly confirmed. On the other hand, Al and 316L composite materials showed the adsorption of Al and Fe surface groups to the MAPP component indicating chemical bond formation. In addition, for both fillers (at all concentrations) a vibration peak shifts of Al, 316L, and carbonyl groups of MAPP were observed suggesting the formation of a covalent bond between the filler and the binder. From the presented results, we can conclude that the chemical bonding between filler and binder causes stronger polymer-filler interactions which result in higher values of storage and loss modulus at low frequencies where material behaviour is governed by the particles (except for GB material at 60 vol.%). The same was also applied to amplitude tests, where materials 316L and Al were showing higher G\({}^{\prime}\) values at all strains and concentrations (again apart from GB material at 60 vol.%). Moreover, the results suggest that if the shape and size of the particles are similar and the binder is the same, the chemical interaction between the binder and the filler determines the composite behaviour, regardless of filler material. This is clearly seen for the examples of 316L and Al materials which showed the similar strain and frequency behaviour since they both formed bonds with the binder. To further investigate the unexpected behaviour of GB composite materials, SEM micrographs were obtained for GB composites at all three concentrations (Figure 14). Figure 14a shows the GB composite material at 30 vol.% loading. We can see that the glass beads were embedded in the binder matrix, and no considerable damage of the glass particles could be observed. However, at the 45 vol.% loading, shown in Figure 14b, we can already see some broken glass beads, some of them presented by a red arrow. Furthermore, at 60 vol.%, more glass beads were broken (Figure 14c). Glass beads are fragile, and at high enough concentration, they start to break during the processing in the kneader and in the press. With the presented SEM micrographs, we could now understand that the increase of dynamic rheological properties compared to other materials was related to the broken glass particles, and as the concentration of filler increased, the number  
of broken glass beads was increased, too. As reported in the literature [66], the glass pieces show much higher viscosity at the same filling grade than the undamaged spheres. Moreover, the frictional forces between irregular particles are higher and cause an increase in viscosity and the occurrence of shear-thinning behaviour. The results of the presented research proved that the rheological behaviour of highly filled materials strongly depends on the interactions between the filler and matrix material, assuming that the same matrix material is used and that the filler material size and shape are the same. Therefore, in this particular case, it can be stated that steel and aluminium particles are forming chemical bonds between the particles and the matrix, and as such the rheological values are higher than for fillers forming physical interactions only (i.e., Ti and GB). As the results give an insight into the particle-polymer interaction, it would be reasonable to assume that using materials that exhibit a stronger particle network and stronger particle-filler interaction would result in a more homogeneous material flow. This is of particular importance, especially in the SDS processes (Shape--Debind--Sinter), where it is crucial to obtain a homogeneous distribution of particles. The main benefit lies in the production of improved feedstocks, highly filled polymers, which will be the base for better products.