# Carbon Nanotube-Based Composite Filaments for 3D Printing of Structural and Conductive Elements
## 3 Results  
### Mechanical Properties  
The mechanical properties of the printed samples from developed ABS/CNT composites filaments were measured by preparing a standard tensile properties test using the Cometech QC-506M2 machine. Since there are no standards for testing 3D printed elements, available standards for injection moulding of thermoplastics were used during the research. The methods and conditions for the tensile test were based directly on ISO 527-1:2012 and ISO 527-2:2012 standards. The speed of tensile testing was set to 1 mm/min.  
Five specimens from each composite filament were printed and tested to reduce statistical uncertainty and determine the degree of microstructure and mechanical properties variability between samples. The mechanical properties of fabricated materials were also compared with pure polymer (ABS).  
As shown in Figure 6, the tensile results of the ABS/CNT composites 3D prints demonstrate that tensile strength decreased for lower CNT content, below 5 wt.%. At the same time, specimens with 9.09 wt.% of filler showed higher tensile strength compared with pure ABS samples. Nano-filler was expected to increase the tensile strength of the composite, according to the standard theory of composite materials [37]. However, despite these expectations, the observed composite seemed to have, in some cases, a lower value of tensile strength than in the case of pure material. With low functional phase content, the strengthening effect can be overwhelmed by the printing defects resulting from non-optimal print parameters and the heterogeneity of the composite film.  
Based on the obtained results, we observed an improvement in the mechanical properties of the composites with the increasing content of the CNT functional phase as it is described by the composite theory [38; 39; 40]. On the other hand, we observed the counterintuitive effect of lower values of the tensile strength of samples with low CNT content than for the pure ABS samples. It should be noted that the addition of nanoparticles may cause difficulties in the printing process, which was unknown for the pure polymer samples. Nanomaterials affected the rheological properties of the composite material, causing the non-uniform extrusion of material from the nozzle, and therefore induced voids inside the printed parts, caused the delamination of printed layers, and created problems with the adhesion of individual paths, etc. As a rule of thumb, we have estimated the mechanical percolation threshold for such composites at approximately 9% (a distinct increase of tensile strength value compared to samples with lower CNT content). In high CNT content composites, connected and tangled nanotubes occur, running along the whole length of the printed element and resulting in improved tensile strength of the composite. Below the threshold, too few nanotubes are tangled and connected together, reducing the cross-sectional area of the polymer and therefore acting as inclusions decreasing the tensile strength of the composite. The same effect of the reduced tensile strength of the composite below the percolation threshold compared to a pure polymer was described previously in the literature [41; 42].