# Effect of Plasma Surface Modification on Print Quality of Biodegradable PLA Films
## 3 Results and Discussion
### Effect of Plasma Activation on Film Properties That Can Affect Printability
The change in sample weight can indicate the removal of impurities from the material surface, oxidation of the material and etching of the material [61]. As in other studies [29, 34, 37, 44, 49], the results obtained also confirm a slight change in roughness and significant etching of the sample due to activation with oxygen and a lower degree of etching, but the significant changes in roughness are due to activation with argon. Changing the roughness affects the printing properties of the films by changing the surface wetting by the ink, which is confirmed by the contact angle results. Despite similar SFE values for different films activated with different gases, the obtained ink contact angle results differ, which may be due precisely to the different level of etching of the material layer and differences in surface roughness. The correlation between the substrate roughness and its water contact angle has long been studied [62]. The film roughness results obtained by the author exceed the value of 100 nm, for which the effect of roughness on the contact angle was found to be significant. Since the obtained water contact angle \begin{table}
\begin{tabular}{c c c} \hline \hline
**Measured Parameter.** & **BCP** & **NTSS** \\ \hline Surface roughness (without plasma) (nm) & 130.22 \(\pm\) 37.09 & 107.32 \(\pm\) 13.07 \\ \hline Surface roughness (100\% Ar plasma) (nm) & 150.02 \(\pm\) 16.23 & 244.64 \(\pm\) 14.01 \\ \hline Surface roughness (100\% O\({}_{2}\) plasma) (nm) & 130.07 \(\pm\) 48.19 & 89.62 \(\pm\) 47.61 \\ \hline Weight difference (100\% Ar plasma) (mg) & 0.280 \(\pm\) 0.071 & 0.405 \(\pm\) 0.064 \\ \hline Weight difference (100\% O\({}_{2}\) plasma) (mg) & 0.355 \(\pm\) 0.007 & 0.425 \(\pm\) 0.064 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Changes in roughness and weight of the film due to plasma activation. Figure 2: Changes in surface free energy for (**a**) NTSS film; (**b**) BCP film. values for both films activated with both gases were less than 60\({}^{\circ}\) (they were 37.5\({}^{\circ}\) and 38.3\({}^{\circ}\) for the activation with argon and oxygen, respectively, for the NTSS film, and 43.4\({}^{\circ}\) and 39.6\({}^{\circ}\), respectively, for the BCP film), the roughness causes a decrease in the contact angle. In summary, it can be concluded that plasma activation with oxygen produced slightly better film wettability than with argon. Flexographic printing is carried out on high-speed web machines [8; 14; 55]. In order to ensure the continuity of the printing process and eliminate the problem of tearing of the web of printed material, it is necessary to ensure certain strength properties of the material. It is important that the modification of the substrate does not have a significant effect on these parameters. Table 4 shows the effect of plasma activation on the measurement results for two basic strength parameters: tensile strength and elongation at break in the machine direction (MD) direction. The results obtained were different from those reported in the material specifications from the manufacturers but showed an analogous relationship demonstrating that the strength of NTSS films despite the lower thickness being significantly higher. Both types of plasma activation positively affected the breaking strength of BCP films, increasing it by more than 1.5 times. Such changes may be due to processes such as recombination and cross-linking associated with the formation of free radicals due to plasma activation on the modified material surface [43; 63]. Plasma activation with argon had an equivalent effect on the NTSS film, while activation with oxygen decreased the value of the breaking strength, although it was still relatively high. The BCP film showed lower elongation at break than the NTSS film, which was significantly reduced by plasma activation regardless of the gas type.