## 2 Materials and Methods  
### Materials  
A three-component binder system was used for the study. Polypropylene-ethylene copolymer (PP), Licocene PP1602, Clariant Produkte GmbH, Frankfurt, Germany was used as the main component. The second component, which was used as a backbone component in the binder was a polypropylene functionalised with maleic acid anhydride (MAPP), Scona TPPP 9212, Byk Chemie GmbH, Germany. The last component of the binder system was thermal stabiliser (TS), Add-Vance TH130, Byk Chemie GmbH, Germany, which was used to prevent thermal degradation of the composites. The binder system was used to prepare four highly filled composites by mixing it with inorganic spherical fillers: aluminium alloy, 316L stainless steel, titanium alloy, and hollow glass beads. For the systematic study, three filler loadings: 30, 45 and 60 vol.% were prepared. The binder composition was adjusted to the filler loading. The amount of MAPP increased from 3 vol.% of main PP for filler loadings of 30 vol.% to 6 vol.% of main PP for filler loadings of 60 vol.%. The amount of MAPP component in the binder system was increased with increasing filler concentration to provide better filler-binder compatibility and was constant in relation to the filler loading (10%). For all composites, 2 wt.% of thermal stabiliser was added to the main PP. The exact binder composition for each filler loading level is presented in Table 1.  
Filler Materials and Filler Size Distributions  
As mentioned above, four spherical filler materials were selected for the investigation. Aluminium alloy spherical particles AlSi10 Mg (denoted as Al) were obtained from IMR Metal Powder Technologies GmbH, Velden am Worthersee, Austria; stainless steel spherical particles Micro-Melt 316L were obtained from Carpenter Powder Products AB, Torshalla, Sweden; titanium alloy spherical particles Ti6 Al4V (denoted as Ti) were obtained from TLS Technik GmbH & Co. Spezialpulver KG, Bitterfeld-Wolfen, Germany, and glass beads 3M\({}^{\text{TM}}\) Glass Bubbles iM16K (denoted as GB) were obtained from 3M, Minneapolis, MN, USA. The particle size distribution of filler materials was determined by using laser diffraction by Particle size analyser (PSA 1190), [PERSON], Graz, Austria. A small amount of powder was fed to the water that was circulating in the analyser. Before each measurement, the particles-water mixture was exposed to ultrasound for 10 s to break up the agglomerated particles and prevent re-agglomeration. The first step was followed by 30 s of particle size measurement. During this time, the particles-water mixture passed through the analyser several times to obtain average size distributions of filler materials. The volume size distribution of filler materials is presented in Figure 1.  
From Figure 1, the size parameters can be obtained for all the materials used in the study. The obtained values with the density of filler materials as provided by material suppliers are presented in Table 2.  
The shape of the particles was investigated using scanning electron microscopy (SEM) JSM-IT100, JEOL, Tokyo, Japan, for which the micrographs are shown in Figure 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Filler Loading (vol.\%)** & **Main PP (vol.\%)** & **Backbone MAPP (vol.\%)** & **Stabiliser TS (wt.\% of Main PP)** \\ \hline
30 & 67.96 & 2.04 (3\% of main PP) & \\
45 & 52.63 & 2.37 (4.5\% of main PP) & \\
60 & 37.74 & 2.26 (6\% of main PP) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of the binder system used for different filler loading levels. Figure 1: The volume size distribution of filler materials. The particle size distribution and SEM images showed that all the particles exhibited spherical shapes and comparable size distributions. Mean values of diameters for Al, 316L and GB ranged from 16.94 to 17.71 \(\upmu\)m, while the values of the diameters for Ti particles slightly deviated with the lowest mean value of 15.19 \(\upmu\)m. A similar trend can be observed for size parameters D10, D50, and D90.