# Thermal and Mechanical Assessment of PLA-SEBS and PLA-SEBS-CNT Biopolymer Blends for 3D Printing
## 3 Results
### Results of PLA-SEBS-CNT Nanocomposites  
From the study of PLA-SEBS blends, it can be concluded that the greatest potential is the blend containing 1% SEBS, because it does not or only slightly reduces the strength values of PLA, while doubling its impact resistance and mixing as little foreign matter as possible to PLA. For this reason, the sample containing 1% SEBS was chosen to mix carbon nanotubes in 0.06 and 0.1%. The conditions for blending, fiber production and printing are the same as those used for PLA-SEBS blends. DSC measurements were performed on the fiber and mechanical tests were performed on the printed specimens (Figure 10). The labeling of the samples has been simplified, always containing only the name of the last additive, so that in the following, the names of the four samples examined are PLA, 1% SEBS, 0.06% CNT and 0.1% CNT. Examining the DSC curves of the nanocomposites, it can be said that the initial temperatures of crystallization (Figure 11) increase with increasing CNT content, which is evidence of nucleation. Thus, crystallization begins earlier, but the heat of crystallization (Figure 12) does not increase, which is likely to mean a modification of the crystal structure. The two peaks in the 0.1% CNT crystallization curve refer this. The first peak, as it precedes SEBS nucleation, is clearly the crystal structure formed by CNT. Figure 11: Initial extrapolated temperature of crystallization. Figure 10: Crystallization DSC curves of samples using different cooling rates. The heat of crystallization may also have been reduced because SEBS has been shown to be a good nucleating agent, so that at higher cooling rates, the addition of CNT rather inhibits the nucleation of SEBS. There is too much nucleating agent while less time is available for crystallization, so too much growing crystallite inhibits each other. In the case of slow cooling, however, it is possible to prevail the positive effect of carbon nanotubes. Figure 13 depicts the results of the Charpy impact testing of nanocomposites compared to the original PLA and 1% SEBS sample. Comparing the results of the nanocomposites with the original PLA and the samples containing 1% SEBS, it can be seen that the impact strength of the PLA doubled under the influence of SEBS and then decreased back to its original value when CNTs were added. Increasing the content of CNTs slightly increased the impact strength (Figure 13). However, looking at Figure 14, it is important to note that both PLA and 1% SEBS samples were broken in their full cross-section during the study. The direction of fracture was perpendicular to the longitudinal axis of the specimen. In contrast, for samples containing CNTs, only partial fiber separation occurred, or the fracture traveled longitudinally along the specimen and not perpendicularly. The magnitude of the force acting on the specimens did not reach the energy required for complete fracture but was large enough to cause a slight permanent deformation, and the 0.06% CNT specimens remained in a slightly bent state at the end of the test. For 0.1% CNT samples, the impact energy could cause fiber separation, but the specimens almost regained their original shape. Numerical values show a decrease, while in reality, the carbon nanotube changes the impact resistance. It is likely that this effect is caused by an improvement in the adhesion of the layer in the impact behavior of the material. Figure 12: Heat of crystallization of PLA, 1% SEBS and the nanocomposites. Figure 13: Impact strength of PLA, 1% SEBS, 0.06 and 0.1% CNTs specimens. The results of the tensile test show that with the addition of carbon nanotubes, the results decrease. The tensile modulus and the tensile strength are halved, and the elongation at break is slightly reduced.