# Carbon Nanotube-Based Composite Filaments for 3D Printing of Structural and Conductive Elements
## 2 Materials and Methods
### Composite Preparation  
The first stage of preparing the composite materials was to break-up the CNT agglomerates. To achieve this, ultrasound mixing of CNT in acetone was performed using a Vibra-Cell VCX130 sonicator with 80% maximum power. Ultrasound mixing was performed for 20 min. After breaking up the nanotubes agglomerates, the polymer was gradually added to the acetone/CNT suspension. The mixing of the ABS/CNT solution in acetone was performed using an MS7-H550-Pro magnetic stirrer for 5 h. After the preparation of a highly homogeneous ABS/CNT mixture, it was poured into a large-area container to obtain a layer of several millimetres in thickness. The container with the mixture was left overnight at room temperature to evaporate the acetone. After that time, the material was granulated. The composite granulate was then placed in a dryer at 100 \({}^{\circ}\)C for 1 h to evaporate the remaining solvent.  
The single screw extruder was used to prepare the filaments with a constant diameter. The extrusion machine was equipped with two independent heating zones, as shown in Figure 2. The rotation speed of the screw is constant (38 rpm), and temperatures may be adjusted in the range of 25 \({}^{\circ}\)C to 300 \({}^{\circ}\)C in both heating zones. To control the extruded filament diameter, the extrusion machine enabled the exchange of the nozzle with different diameters.  
In this paper, the ABS/CNT composites filaments with filler fractions of 0.99, 1.96, 4.76, and 9.09 wt.% were investigated. To obtain filaments with an appropriate diameter, the nozzle of a 1.7 mm diameter was used. The extrusion machine's heated zones were set to 140 \({}^{\circ}\)C and 155 \({}^{\circ}\)C for the first and second zone, respectively. The lower extrusion temperature caused an insufficient flow of the melted matrix material, which resulted in clogging of the extruder nozzle and obtaining heterogeneous filament. On the other hand, higher extrusion temperatures caused the material to flow too rapidly, making it impossible to obtain a fixed diameter of the filament. Additionally, the increased temperature may cause thermal degradation of the polymer. During the filament extrusion process, it  
Figure 1: SEM (scanning electron microscope) micrograph of carbon nanotube functional filler.  
was observed that acetone was not completely evaporated despite drying the composite before the extrusion process. Excessive acetone evaporated in the high temperature of the extrusion process and caused the formation of an irregular-shaped filament with high internal porosity. The extrusion process had to be performed several times to obtain a high-quality material. ABS copolymer decomposes at 368 \({}^{\circ}\)C [35]; therefore, multiple thermal processing of ABS/CNT composite at max 240 \({}^{\circ}\)C does not have a significant impact on the mechanical properties of the material. Additionally, in the literature we found studies presenting the influence of multiple injection molding processes on the properties of ABS, which also confirm no significant effect on the mechanical properties of the polymer despite repeated thermal processing in the temperature range we used for the processing of ABS/CNT composites [36]. All of the resulting composite filaments had to be extruded four times to evaporate the remaining solvent completely. Finally, optimization of the extrusion process allowed us to obtain a continuous filament with a diameter ranging from 1.7 mm to 1.8 mm (Figure 3). The filaments were also characterized by high homogeneity and a lack of porosity.