# Effect of Plasma Surface Modification on Print Quality of Biodegradable PLA Films
## 4 Conclusions  
The wettability of PLA films by flexographic printing inks, both water-based and solvent-based, can be effectively improved by modifying the substrate with cold plasma involving oxygen as well as argon. At the same time, the type of gas used does not have as significant an effect on the wettability improvement as the type of ink and the printing substrate. Nevertheless, the reason for the observed change in ink wetting may be the change in film roughness due to plasma activation. A significant effect of the type of gas used for plasma activation on surface roughness and sample etching was observed. Activation with oxygen leads to minimal change in roughness and more etching of the sample, while activation with argon resulted in significant changes in roughness and less etching of the samples. Moreover, plasma activation, regardless of the gas type, increased the SFE values to 57-60 mJ/m\({}^{2}\), which is well above the minimum value needed for high quality prints. The change in SFE was due to the increase in the polar component of SFE. It is worth noting that a greater increase in the polar component was observed for oxygen plasma-activated films. Plasma activation affected the strength properties of the films, with the effect depending on the type of gas and material. Generally, the breaking strength improved, while the elongation at break decreased. However, the observed changes should not affect the printing process. Regarding the visual alteration of the film, the color of the film did not change significantly due to both oxygen and argon plasma activation. However, the gloss changes were dependent on the type of modified film rather than the gas used for plasma activation. Analyzing all the data for the modified films, it can be concluded that the use of oxygen plasma for activation resulted in slightly better wettability of the films than using argon plasma. The improvement in substrate wettability as a result of the plasma activation of the films translates to the quality of the prints produced. Plasma activation with both oxygen and argon allows for very good adhesion of all printing inks to both substrates. The relative contrast value is mainly influenced by the type of printing ink and, to a lesser extent, by the type of printing substrate and the way it is modified before printing. Plasma activation plays a significant role when printing with water-based inks, while its influence is insignificant in the case of solvent-based inks. As far as the color changes of the print are concerned, here not only the type but also the composition of the ink and the substrate influence the magnitude of the observed  
Figure 5: Gloss of prints measured with different geometry: (**a**) 60\({}^{\circ}\); (**b**) 20\({}^{\circ}\) for very high gloss prints. changes. Plasma activation of the film had a significant effect on the printability with water-based and solvent-based polyamide inks, as indicated by the \(\Delta\)E values obtained. On the other hand, there was no significant effect on the gloss of the prints, which depends mainly on the type of printing ink. In conclusion, the changes occurring in PLA films as a result of plasma activation favorably influence the printability of these substrates. Several parameters simultaneously have a significant influence on the results obtained, namely the kind of material modification, the kind of printing ink and the kind of film. This research was funded by the European Union within the European Social Fund from WUT Development Program. This research was funded by Polish National Agency for Academic Exchange under grant no. PPI/APM/2018/1/00047 entitled \"Industry 4.0 in Production and Aeronautical Engineering\" (International Academic Partnerships Programme). The APC was funded by the Polish National Agency for Academic Exchange. Not applicable.