## 3 Results and Discussion
### Measurement of the Impact Strength  
The statistical evaluation of the impact strength of the samples is shown in Table 6 and Figure 7. The result of this experiment is the determination of the absorbed kinetic energy per unit area of the sample. There was a Charpy's hammer with a potential energy  
Figure 6: Example of the nature of sample failure after the degradation test (bending test): (**a**) Chemical smoothing (59.07 MPa); (**b**) heat treatment (63.99 MPa); (**c**) epoxy resin (59.73 MPa); (**d**) synthetic coating (59.36 MPa); (**e**) water-based coating (57.92 MPa); (**f**) standard-recycled rPET-G (55.17 MPa). of 50 J. The chosen method differs from others of a similar nature in that samples without notches are tested. However, the absence of a notch is considered advantageous precisely for testing 3D printed materials [56]. Based on the statistical evaluation, it can again be stated that the 60 testing cycles implemented did not bring about significant changes compared with the initial state of the samples. This result is also given by the cross-section of the test specimens, which is identical to the cross-section of the specimens for the bending test. The highest impact strength values were again achieved using heat treatment. A comprehensive assessment of the effect of annealing on the mechanical properties of the PET-G material was also dealt with by the authors [29]. The result of the experiment was an increase in hardness by 7.8%, tensile strength by 8.5%, impact strength by 5.5%, and flexural strength by 9.4% in the case of the annealed samples compared with the samples without annealing treatment. The authors define these improvements as an increase in interlayer diffusion coupling. Furthermore, the results published in the publication [56] point to the fact that the impact strength of the PET-G material is affected not only by the orientation of the sample during printing but also by the thickness of the layer. However, e.g., in the case of PLA \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & \multicolumn{6}{c}{**Quantity**} \\ \hline \multicolumn{1}{c}{\multirow{2}{*}{**Type of PP**}} & \multicolumn{2}{c}{**AM, kJ\(\cdot\)m\({}^{-2}\)**} & \multicolumn{2}{c}{**D, kJ\({}^{2}\).m\({}^{-4}\)**} & \multicolumn{2}{c}{**SD, kJ\(\cdot\)m\({}^{-2}\)**} & \multicolumn{2}{c}{**CV, \%**} \\ \cline{2-10}  & **Before** & **After** & **Before** & **After** & **Before** & **After** & **Before** & **After** \\ \hline
**CS** & 84.80 & 83.41 & 6.9696 & 15.0544 & 2.64 & 3.88 & 3.11 & 4.65 \\
**HT** & 87.93 & 86.72 & 6.2500 & 12.6736 & 2.50 & 3.56 & 2.84 & 4.11 \\
**ER** & 85.28 & 84.54 & 9.7344 & 16.9744 & 3.12 & 4.12 & 3.66 & 4.87 \\
**SC** & 84.19 & 83.57 & 3.1684 & 8.5264 & 1.78 & 2.92 & 2.11 & 3.49 \\
**WBC** & 84.02 & 83.16 & 3.9204 & 9.2416 & 1.98 & 3.04 & 2.36 & 3.66 \\
**WPP (rPET-G)** & 81.54 & 80.06 & 4.9284 & 19.0096 & 2.22 & 4.36 & 2.72 & 5.45 \\
**WPP (PET-G)** & 83.17 & 81.69 & 6.4516 & 15.8404 & 2.54 & 3.98 & 3.05 & 4.87 \\ \hline \hline \end{tabular}  
* PP—Post-processing CS—Chemical smoothing HT—Heat treatment; ER—Epoxy resin; SC—Synthetics coating; WBC—Water-based coating; WPP—Without post-processing; AM—Arithmetic mean; D—Dispersion; SD—Standard deviation; CV—Coefficient of variance.  
\end{table} 
Table 6: Impact strength test evaluation (before and after 60 degradation cycles). Figure 7: Impact strength before and after the degradation (arithmetic mean \(\pm\) standard deviation). or ABS materials, the authors demonstrated, on the contrary, a negligible influence of the layer thickness. The influence of chemical smoothing was further investigated by, for example, the authors [7; 8]. The result was an increase in impact strength in both cases. The nature of the sample failure during the impact test is further evident from Figure 8. The results obtained are comparable to the bending strength test.