## 1 Introduction  
Aging of polymeric materials is an undesirable but inevitable process. It can be spontaneous (natural aging) or forced (accelerated aging). Moreover, aging can be caused by external and internal factors. As a result of the aging processes, polymers undergo thermal, oxidative, chemical, radiation, mechanical, and biological degradation. Furthermore, it should be noted that spontaneous aging of the material is a process that occurs in all plastics, both conventional and biodegradable. The nature of its course is usually very slow, but irreversible and impossible to stop [1; 2; 3]. Since it cannot be stopped, it is important to know its course and take into account changes in polymer properties when processing and applying materials. Aging of polymer films can be manifested by changes in their mechanical properties (including a decrease in tensile strength), chemical properties (the appearance of functional groups indicating degradation of the material), or changes in appearance (a change in or gloss, and, as a result of long-term storage or in extreme weather conditions, even surface cracking associated with a change in the britteless of the material). The course of the aging process and the intensity of the changes occurring in the material depend on the time and conditions under which it takes place. Properties of materials depend on such factors as temperature, humidity, and light, especially in the UV region, as well as the type of gas used [3; 4; 5; 6; 7; 8; 9; 10; 11]. Materials with a plasma-modified top layer are subjected to short but nevertheless intense UV radiation resulting from the plasma process [12] and the interaction of ions, electrons, and other reactive molecules, which can lead not only to the desired changes in the properties of the film--its functionalization--but also its aging, i.e., degradation [13; 14; 15; 16; 17; 18]. Another important issue is the disappearance of properties obtained by plasma activation; with storage time, the chemical changes introduced in the surface layer of the material are altered [19; 20; 21; 22; 23; 24]. Therefore, in this article, two aspects of aging are distinguished and studied:  
1. aging in the context of changing the properties of the modified layer--hydrophobic recovery, 2. additionally, aging (change in mechanical and optical properties) of the film as a result of plasma modification of its surface layer. Hydrophobic recovery is essential from an application perspective. Hydrophilicity, and by extension the process of its loss during storage, is a fundamental parameter determining the printability and processability of the material. The second point complements the first, allowing a more complete evaluation of the impact of the activation process, determining that plasma activation does not lead to a significant change in the material's properties and its degradation, thus preventing its proper application. The study was conducted for biodegradable films, which are an environmentally friendly alternative to films made of traditional plastics. Their similar mechanical and optical properties, as well as analogous processing, make it feasible to use them in the packaging industry, which is one of the main areas of application for plastics [25; 26; 27], and represents a significant environmental problem in the form of packaging waste [28]. One of the most relevant biodegradable polymers with wide applications in packaging, but also used in other areas such as medicine, textiles, and 3D printing, is polylactide (PLA) [29; 30; 31; 32; 33]. Polymer films, as non-absorbent materials, require surface preparation before the printing and finishing process [34; 35]. In industrial settings, this is commonly done using in-line corona activation during the printing process [36] or, alternatively, chemically modified materials at the film production stage are used in printing houses.