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Tensile testing of PLA specimen loaded with different amounts of carbon particles revealed that both tensile strength and failure strain decreased with the addition of carbon particles to a certain extent ([Figure 10](#polymers-08-00205-f010){ref-type="fig"}). The tensile strength dropped from 25 MPa for PLA without lignin particles to 13 MPa for PLA filled with 5% carbonaceous lignin powder. For PLA filled with 5%--35% of the carbonaceous powder, the tensile strength is almost steady. Failure strain also decreased. Without carbon filling it was about 1.5%. After adding only 5% lignin derived carbon, the failure strain fell to 0.8%. Between 5% and 35% carbon loading, the failure strain was almost steady. Other studies found that the tensile strength increases with low loadings and decreases with high loadings. It was also found that the influence of carbon black particles differs a lot depending on the polymer matrix material \[[@B48-polymers-08-00205]\]. In this case, agglomerates of the carbon particles can lead to imperfections in mechanical testing. The largest carbon particles, which were measured with 60 µm, can also lead to breakages.

In contrast to the decrease of tensile strength and failure strain, the Young modulus E seems to increase slightly at higher loadings. Overall, the effects of carbon filler on the mechanical performance of PLA are in agreement with other studies \[[@B49-polymers-08-00205],[@B50-polymers-08-00205]\]. As outlined in a review on mechanics of particulate-filled polymers \[[@B51-polymers-08-00205]\] an increase in composite stiffness in parallel with a reduction in tensile strength and extensibility is principally observed upon addition of particulate fillers. However, as shown in [Figure 10](#polymers-08-00205-f010){ref-type="fig"}, the PLA-carbon composites produced in the present study do not show a distinctly proportional relationship between filler content and change in mechanics, which one would expect. As wi