# Thermal and Mechanical Assessment of PLA-SEBS and PLA-SEBS-CNT Biopolymer Blends for 3D Printing
## 3 Results
### Results of PLA-SEBS Blends
For PLA, the tensile strength was plotted. In the case of the tensile stress at break, a similar statement can be made as in the case of tensile stress at Yield, with the difference that three separate parts can be observed here. The tensile stress at break of PLA decreases slightly with the addition of 1% SEBS. The values of 2 and 3% are smaller but very similar. The tensile strength of the 4 and 5% samples is even lower, but they also show approximately the same values. The addition of 1% SEBS to PLA increases the impact strength, does not reduce the tensile modulus, and only slightly reduces the tensile stress at break. However, 2 or 3% SEBS further increases the impact strength and does not reduce the tensile modulus but already reduces the tensile stress at break to a greater extent. In the case of 4 or 5% SEBS, even if the impact strength increases even more, the tensile modulus decreases and the tensile stress at break decreases by about half. In the case of the bending test, one of the changes was also that while the brittle PLA was broken, the samples containing SEBS no longer broke or only partial fracture occurred. The flexural strength is thus the maximum flexural stress for mixtures. The blended SEBS grains can elongate on the drawn side and compress on the compressed side due to their flexibility, thus reducing the strength values. As the amount of SEBS increases, the flexural strength gradually decreases until 4%, and then 5% is practically the same as the 4% sample. In the case of the tensile strain at Yield, it can be seen that 1-3% and 4-5% are separated from each other (Figure 6). Although the tensile stress at Yield continuously decreased until 3%, the tensile strain at Yield did not change, remaining at nearly the same value. At 4 and 5%, when the lower strength was associated with less elongation, the forming of the neck started earlier. In the case of PLA, the tensile strain at break was also shown here, due to its brittle behavior. Figure 5: Tensile and flexural stress as a function of SEBS content. In the case of the tensile strain at break or elongation at break, the value of the 1% sample is practically the same as the PLA, but the rupture of the 2 and 3% samples also occurs, only with a slightly higher elongation. The larger change occurs from 4%, where the elongation at break increases severalfold. From 5%, we can measure even higher values. Until 3%, the amount of SEBS mixed is not large enough to form continuous areas; it can be too scattered, and thus only locally, around itself, can it initiate the rearrangement of PLA molecules in the direction of pull--no large elongations are possible. From 4%, the regions of action of SEBS particles can be linked with PLA binding molecules, which can lead to much higher elongations. The flexural deflection measured when the maximum stress is reached practically changes only tenths of a millimeter, and if the standard deviation is taken into account, the change cannot be said to be significant. However, in the case of the mean values, it can be noted that the 1% sample is the same as the PLA, followed by a continuous decrease. Figure 7 depicts optical microscopic images of the impacted specimens at 20\(\times\) magnification. Figure 7a,b show the fractured surfaces and Figure 7c,d show the side views of the partial fractures of the unbroken specimens. Figure 6: Tensile strain and flexural deflection as a function of SEBS content. Figure 7: Microscopic images of fracture surfaces (**a**,**b**) and side views of partial fractures (**c**,**d**), (**a**) PLA-1-2–3% SEBS (left to right), (**b**) 4–5% SEBS (left to right), (**c**) 4% SEBS, (**d**) 5% SEBS. The microscopic images show that the fracture surfaces of the printed specimens are quite similar up to 3% SEBS content, and at 4 and 5%, the separation of the fibers can be observed (Figure 7). From PLA until 3%, the entire cross-section of all specimens was broken, while at 4 and 5%, some partial fractures (shown in Figure 7c,d) also occurred.