# Thermal and Mechanical Assessment of PLA-SEBS and PLA-SEBS-CNT Biopolymer Blends for 3D Printing
## 4 Conclusions  
From 1 to 5% SEBS thermoplastic elastomer was mixed to PLA, and then the printed specimens were subjected to thermal, mechanical, and optical observations. DSC studies on the printing fiber have shown that SEBS can act as a nucleating agent in PLA by increasing the initial crystallization temperatures regardless of the cooling rates. However, studies on printed specimens have shown that cooling during the printing is so rapid for PLA that it can only produce a minimal crystalline phase, and although the nucleating effect of SEBS can be demonstrated, the amount of crystallites formed is too small to affect the mechanical properties. In the case of an impact test, the impact strength doubles as early as the addition of 1% SEBS and does not change up to 3%. From 4%, partially broken specimens also occur. In the case of tensile testing, the tensile modulus does not change up to 3% SEBS, and from 1%, the phenomenon of neck formation appears, and the elongation of the initially brittle PLA begins. The tensile stress at Yield and tensile strength of the specimens decreased only slightly in the case of the 1% sample, and the values of 4 and 5% were practically the same. In the case of the elongation at break, the increase is small until 3%, and there is a higher change from 4%, where the SEBS content reaches the value where the regions of action of SEBS particles can be connected to PLA chains. The whitening of the drawn specimens, that is, the number of micro-cracks, increased continuously with increasing SEBS concentration, but even at 1%, it went along the entire length of the specimen. In the bending test, the flexural modulus decreased only slightly, then practically did not change between 1-3%, and 4-5% also took almost the same but lower values. During bending, only the PLA broke in a brittle manner, 1-2% SEBS containing samples were partially broken, and from 3%, the specimens were no longer broken, but only bent. The flexural strength decreased slightly but continuously until 4%, with 5% almost equaling  
Figure 22: Samples after bending test (a) PLA, (b) 1% SEBS, (c) 0.06\({}^{\circ}\)CNT, (d) 0.1% CNT. with 4%. The values of deflection measured when the maximum stress was reached were not affected by the SEBS content. In summary, the mechanical tests revealed that in most cases, 1-3% and 4-5% were separated. In all probability, up to 3%, the amount of mixed SEBS is so small, so scattered, that it can dampen the internal tension only at the local level, in its immediate vicinity. From 4%, these damping fields overlapped, connected with PLA chains, thus amplified and extended each other's effect. Thermal studies of the nanocomposites showed that 0.1% CNT was able to modify the crystal structure, and two-peak crystallization became visible on the DSC curves, of which the first peak is clearly the effect of CNT as it precedes SEBS-induced crystallization. In mechanical tests, the reduction in numerical data can be misleading at first glance, especially as it occurs in all studies. The visual inspection of the specimens after testing reveals that CNT increases the impact and bending resistance of the PLA-SEBS blend. While pure PLA and the PLA-1% SEBS blend was broken during the Charpy test, the two nanocomposites were only partially broken or cracked, and the specimens suffered only minor residual deformation. At 0.06% CNT, the samples remained in a slightly bent state, and at 0.1% CNT, they almost regained their original shape. In the bending test, it was found that the pure PLA was completely broken, and the 1% SEBS mixture also suffered a partial break and remained in a bent position at the end of the test. However, the nanocomposites not only did not break, but they did not suffer a partial break, and moreover, almost completely regained their original shape. The carbon nanotube reinforces the mixture to such an extent that the stress during the test does not reach the value causing the residual deformation.