# Thermal and Mechanical Assessment of PLA-SEBS and PLA-SEBS-CNT Biopolymer Blends for 3D Printing
## 3 Results
### Results of PLA-SEBS Blends
At a cooling rate of 10 \({}^{\circ}\)C/min, the cooling intensity is already so high that it inhibits the amount of crystals that can be formed, so it does  
Figure 3: The initial extrapolated temperature of crystallization and the enthalpy changing as a function of SEBS content. Figure 2: DSC curves of the first heating of printed specimens. not reach the values measured at slower rates. However, in this case, increasing SEBS content can somewhat compensate the effect of the rapid cooling and increase the number of crystallites formed. The heat of melting of the sample containing 1% SEBS is lower than that measured at PLA, and the value of 2% is practically the same. The growth of crystalline phases starts from 3%, but the differences are not too large. The sample of 5% reaches twice the heat of melting of PLA. However, the measured heat of melting of 1-3 J/g is quite low compared to 30-35 J/g measured in the non-isothermal test, so it can be said that although SEBS behaves as a nucleating agent, the mixtures are practically amorphous due to the cooling conditions of the printing. Figure 4 shows the change in impact strength, the tensile modulus and the flexural modulus as a function of SEBS content. Already, with the addition of 1% SEBS, the impact strength doubles. The growth is minimal between 1 and 3%, practically unchanged; for 4%, it starts to increase further, where it reaches three times the impact strength of the original PLA, and at 5% it is an even higher value. Thanks to the flexibility of SEBS, it absorbs impact energy, thus improving its impact resistance of the originally brittle PLA. Until 3%, the amount is so small that the SEBS particles are at a great distance from each other so that each can locally relieve an internal tension of a small area. The amount may be 4% when the mixture already contains enough SEBS that the spreading impact energy collides into another energy-absorbing SEBS particle. From 4%, the regions of action of SEBS particles can be linked to PLA chains that act as binding molecules, similar to amorphous molecules that bind crystals. Thanks to these overlaps, the energy absorbing effect is amplified and extended. The tensile modulus is practically unchanged until 3% SEBS is added if the standard deviations are taken into account. The 4 and 5% are also nearly the same, but their values are already lower than the values of the original PLA or the 1-3% samples. Adding a small amount of SEBS improves the impact strength but does not reduce the tensile modulus. During the impact test, the specimen receives the full load immediately, while during the tensile test, the software calculates the tensile modulus according to the standard, from the initial stage of the pull (0.05-0.25%), when the load only begins to build up. At this stage, the amount of SEBS has not yet reached the level to influence the original tensile modulus of the PLA. The change occurs only for a SEBS content of 4% or more. The flexural modulus decreases slightly with the addition of 1% SEBS but does not change up to 3%, remaining broadly constant. The values of 4 and 5% are also about the same, but even lower. The bending modulus also applies to the initial phase of the load, but here the stress is different; here, tensile and compressive stresses occur simultaneously at the two edges of the specimen. Additionally, while the original modulus value of PLA has been retained in the tensile test, a smaller value is obtained at bending. However, the differences are similar, with 1-3 and 4-5% separated. Figure 4: Impact strength, tensile and flexural modulus as a function of SEBS content. During the tensile test, the PLA behaves in a brittle manner, and even 1% of the SEBS can change this because the neck appears and begins to extend (Figure 5). The elasticity and low strength of SEBS also affect the strength of the mixture. The amount of the thermoplastic elastomer increases, and the tensile stress at Yield decreases. The decline lasts until 4%, then the 4 and 5% samples are similar.