# 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 parameter that enables printing of the film is its appropriate wettability by the printing ink. The smaller the contact angle is, the better the liquid wettability is. Figure 1 shows the variation of the contact angle of the two tested films, before and after plasma activation, with the three flexographic inks used subsequently for printing. The BCP film had a higher contact angle before modification than the NTSS film, which was 72.6\({}^{\circ}\) and 62.5\({}^{\circ}\), respectively. Activation with both oxygen and argon plasma resulted in a significant reduction in the water contact angle for both materials (24\({}^{\circ}\) and 25\({}^{\circ}\) for NTSS films, and 33\({}^{\circ}\) and 29\({}^{\circ}\) for BCP film, respectively), indicating a significant improvement in the wettability of both films. Analyzing the data obtained, it can be concluded that plasma activation, both oxygen and argon, had a beneficial effect on the wettability of BCP film by printing inks, while for the other film the effect is not conclusive. This may be due to the fact that the NTSS film is a corona-modified production material by the manufacturer, which affects its wettability and printability, and despite an SFE value comparable to the film without surface pretreatment, the film morphology, roughness and surface development may be altered. The type of gas used for activation has only a minor effect on ink wettability, although slightly better results as in previous studies performed for another PLA film [22], were obtained using oxygen activation. By far the more important factors determining wettability are the kind of ink and substrate. Obviously, solvent-based inks show better wettability than water-based inks due to the surface tension of these inks. In water-based ink, the solvent is water, which has a high surface tension, which translates into ink properties [60]. PLA films, like all films made of biodegradable or plastic materials, must have a surface free energy higher than the surface tension of the printing ink in order to be flexographically printed. The higher this difference, the better the wettability and adhesion of the ink to the substrate [15; 16; 28]. Based on the film contact angle studies, before and after activation using water and diiodomethane as two measuring fluids with different polar and dispersion properties, the values of SFE and its polar and dispersion components were calculated (see Figure 2). Figure 1: Changes in contact angles for inks. Regardless of the type of gas used for activation, very high SFE values were obtained for both films. It is worth noting that the increase in the SFE value was due to the increase in the value of its polar component, which more than doubled. Moreover, the obtained SFE values are much higher than those considered to enable satisfactory print quality given in the literature [13, 57, 59]. In general, the higher the SFE value and the lower the water contact angle value, the more hydrophilic the film is and will be more susceptible to ink wetting. While the gas type did not significantly affect the total SFE value, it did affect its components. Higher values of the polar component are possible with oxygen plasma, which should have a beneficial effect on print quality. Table 3 shows the results of the surface roughness and weight change of the samples due to film activation. From these, it was found that the surface roughness of both films decreased slightly with oxygen plasma activation, with the surface uniformity also decreasing. With plasma activation with argon for both films, an increase in surface roughness was observed. The obtained roughness changes are consistent with observations obtained in other studies [29, 34, 37, 44, 49]. From the results obtained (Table 3), it can be concluded that the type of gas used for plasma activation has a significant effect on the surface roughness and weight change of the sample.