# Effect of Plasma Surface Modification on Print Quality of Biodegradable PLA Films
## 3 Results and Discussion
### Effect of Plasma Activation on Print Quality  
One of the key parameters determining the quality of the print is the adhesion of the ink to the substrate. Without good adhesion, it is impossible to obtain a high-quality print. As a result of plasma activation, irrespective of the type of gas used for modification, very good adhesion of the ink to the film was obtained for each of the tested inks. However, for unmodified films, good adhesion was obtained for solvent-based inks, while it was not fully satisfactory for water-based inks. Water-based inks are more difficult to print than solvent-based inks and can create problems with ink adhesion to the substrate due to their composition and surface tension [13; 15; 58]. Contrast is an alternative to the measurement of tonal value gain. It is determined from the optical density of the fully covered field (100%) and the raster field (80%--recommended for black ink). High quality prints should have the highest possible print contrast, which is possible due to the high optical density of the fully covered field. For all prints made with solvent-based inks, whether the NTSS film was plasma activated or not, the relative contrast was very high [58; 63] (see Figure 3). In contrast, both untreated films printed with water-based inks exhibited very low relative contrast values, with both oxygen and argon plasma activation having a beneficial effect in increasing contrast values. For solvent-based inks, the contrast values are many times higher than those for water-based inks. In addition, for water-based inks, where two different printing substrates were used, the type of film affects the results. The highest contrast values were obtained for printing with solvent ink 3 and water-based ink 1 on NTSS film activated by argon plasma (33 and 28, respectively) and for prints on untreated NTSS films printed with solvent inks 2 and 3 (26 and 32, respectively). The fact that for some prints the contrast decreased as a result of activation and did not increase further can be explained by the fact that an increase in the optical density value is associated with an increase in the thickness of the fused ink layer, which is only possible up to a certain limit and thus limits the further possibility of contrast increase due to halftone dot filling [64]. Furthermore, the change in the thickness of the ink layer transferred to the NTSS film may have been influenced by changes in the substrate roughness due to plasma activation with different gases. \begin{table}
\begin{tabular}{c c c} \hline
**Measured Parameter** & **BCP** & **NTSS** \\ \hline \(\Delta\)E (100\% Ar plasma) & 0.61 & 0.48 \\ \hline \(\Delta\)E (100\% O\({}_{2}\) plasma) & 0.71 & 0.50 \\ \hline \(\mathrm{Gloss}\) difference (100\% Ar plasma) (GU) & \(-27.68\) & 23.82 \\ \hline \(\mathrm{Gloss}\) difference (100\% O\({}_{2}\) plasma) (GU) & \(-31.81\) & 33.47 \\ \hline \end{tabular}
\end{table} 
Table 5: Change in color and gloss of the film due to plasma activation. In summary, the key influence on the relative contrast value is the kind of ink that was used to print the substrate. Films printed with solvent-based inks have significantly higher relative contrast than films printed with water-based inks. In the case of printing with water-based inks, plasma activation with both oxygen and argon positively increases the value, while a significant effect of the substrate on the results is observed. Figure 4 shows the results of the color difference (\(\Delta\)E) between the prints made on the substrate not activated before printing and activated by plasma activation using oxygen and argon as determined by the color coordinate measurements L*, a*, b*. The \(\Delta\)E values obtained for ink 3 are the smallest and imperceptible to the observer. For this ink, the substrate contact angle values were only slightly different from each other, resulting in a similar amount of ink being transferred and fixed to the activated and nonactivated substrates, giving similar color coordinate values and little color difference.