# Effect of Plasma Surface Modification on Print Quality of Biodegradable PLA Films
## 1 Introduction  
Biodegradable materials are particularly relevant to modern world economies struggling with waste and environmental issues [1; 2; 3]. One of the key biodegradable plastics is polylactide (PLA) [4; 5; 6], which (as a non-absorbent substrate (biodegradable film)) can be printed using flexographic or gravure printing techniques commonly used in industry. Flexography is one of the varieties of relief printing, where printing is done using flexible printing forms and liquid water-soluble, solvent or UV inks [7; 8]. This technology enables high quality printing with an optimized selection of printing process parameters [9]. Print quality is influenced by many factors, such as the type of printing substrate, its finishing and preparation (coating or smoothing of paper substrates, modification of the surface layer of non-absorbent materials, etc. ), the printing technique and the kind of printing inks [10; 11; 12; 13; 14]. PLA films, like other non-absorbent materials, require surface preparation prior to printing or finishing processes to improve wettability and to impart adequate hydrophilicity [15; 16; 17; 18]. Plasma activation, in comparison to chemical activation, is an environmentally friendly alternative for modifying the surface layer of polymers prior to their further refinement and finishing processes. Other advantages include the high reproducibility and stability of the process and the possibility of obtaining diverse effects due to the use of different gases (O\({}_{2}\), CO\({}_{2}\), N\({}_{2}\), NH\({}_{3}\), air, H\({}_{2}\), He, CF\({}_{4}\) and Ar) [19; 20] or process parameters(activation time, pressure, power, etc.) [21; 22]. Plasma activation results in the formation of functional groups on the surface of the modified material, changes in surface hydrophilicity and surface free energy, changes in surface crosslinking, changes in surface morphology, the removal of impurities or the improvement of biocompatibility [23; 24]. The surface free energy of the film is calculated using the contact angle of various measuring liquids. It is the basic parameter for good wettability of the substrate and its printing [25; 26; 27]. Even if the surface wetting is satisfactory and the ink can be applied, this does not guarantee good adhesion of the ink to the substrate [15]. Wettability is one of the basic characteristics of printing substrates, which depends on the chemical composition and morphology of the material, both of which affect hydrophilicity. On the other hand, the value of adhesion force is influenced by the impurities present on the surface of the material, the development of its surface, the surface free energy and the magnitude of the difference between the surface free energy of the substrate and the printing ink [16; 28]. The effect of plasma activation on PLA properties has been the subject of previous studies [6; 19; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44]. They showed a good effect, using argon and oxygen to increase the surface free energy values and improve the wettability [22; 29; 30; 31]. Upon activation with oxygen, various groups containing oxygen atoms are formed on the surface of the modified material, thus improving the hydrophilicity of the material [45; 46], causing etching of the material surface [47; 48], and slightly changing the surface roughness [34; 44; 49]. With argon activation, in addition to a significant improvement in material hydrophilicity and adhesion, there is a significant increase in surface roughness and the formation of free electron pairs at the carbon atom [29; 37]. As a result of previous in-house studies, it was found that oxygen plasma activation is more effective than argon plasma activation and has a greater effect on improving the wettability of PLA films. Furthermore, it was shown that a longer activation time has no significant effect on the wetting angle in the case of argon plasma activation.