## 1 Introduction
However, according to previous publications [19; 20; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48], a favorable alternative to these modification processes is low-temperature plasma activation. Preparing the surface for printing involves improving the wettability of the polymer substrate and cleaning it, which is reflected by a decrease in the water contact angle and an increase in the surface free energy value of the material and the unfolding of its surface. This also translates into improved adhesion of the printing ink to the substrate [35; 36; 37]. It is generally recognized that the higher the SFE values, the better the wettability and adhesion should be, and thus the quality of the print [49]. Prior to the printing process, materials in printing houses should be stored under constant conditions (usually temperature 20-25 \({}^{\circ}\)C, relative humidity (RH) 50-55%) close to standard conditions (23 \(\pm\) 0.5 \({}^{\circ}\)C, RH 50 \(\pm\) 1.5%) in accordance with ISO 187:1990. Changing these conditions will lead to problems during the printing process, such as the accumulation of static electricity on the material and improper drying of the print. It is known that during storage the SFE of films modified at the production stage changes; that is, it decreases, while the water contact angle increases toward the pre-modification value. This is related to the fact that during storage the polar functionalities present at the polymer surface disappear at different rates [50]. Moreover, the rate of change occurring over time depends on storage conditions and the type of material [51; 52]. Hence, this article investigates how the type of PLA film affects hydrophobic recovery. The results presented in the article focus on the aspects of aging of PLA film connected to application relevant properties only. The aspect of chemical changes occurring on the surface layer of the material was omitted, as there is continuing research in the field of plasma chemistry for identification of reactive species generated in the plasma [13; 17; 19; 20; 50], and this is beyond the scope of this article. From the point of view of printing and its quality, the main effects of aging of printed films are the changes in the hydrophilicity of the material and its strength after short and long storage periods. However, storage time is usually quite short due to the high turnover of materials in printing houses. We have previously conducted studies on the aging of plasma-modified PLA films [19; 20], but these were carried out for different films while considering only one type of film in each study. By comparison, [PERSON] et al. [22] showed that the type of polymer has a significant effect on the persistence of plasma activation effects. Due to the complexity of plasma activation processes and the complexity of the physicochemical changes that occur, it can be suspected that not only will the type of gas used and the varying activation time have a significant impact on the disappearance of hydrophilic properties of the plasma-modified film, but also the type of substrate (film produced on the basis of polylactide with various additives used at the production stage). Moreover, thermal, thermo-oxidative, or thermo-mechanical degradation of the polymer may occur at the PLA processing stage, which will translate into different properties of the produced material [53]. Therefore, this paper aims to determine the efficiency of the plasma activation process depending on the material, demonstrate whether plasma activation itself leads to the appearance of signs of film aging by examining selected parameters such as color change, gloss, and tensile strength, and determine the effect of storage on the plasma activation durability of the various PLA-based films tested. The results of the study can be useful for better understanding of plasma activation processes, and have application significance, allowing possible prediction of changes occurring in plasma-modified materials.