## 3 Results and Discussion  
The water, formamide, and diiodomethane equilibrium contact angles measured on the examined samples are presented in Figure 3. On the initial (non-modified) sample, the water contact angle was equal to about 67.2 degrees. The air plasma modification caused a  
Figure 2: Scheme of the adhesive strength test. decrease in the contact angle value, and this effect can be seen in all samples. For the one-second plasma treatment, the water contact angle decreased by about 20 degrees, while for longer modification times its value decreased to less than 10 degrees and the surface became more hydrophilic. However, the effect obtained for modification times longer than 10 s was found to be comparable. This is consistent with the results obtained by [PERSON] et al., who reported that, in the case of plasma hydrophobization of paper, modification times longer than 5 s did not affect the material's surface properties significantly [8]. A similar tendency can be observed in the case of the second polar liquid (formamide). However, Figure 2 shows that the plasma modification of the samples did not affect the diiodomethane contact angles significantly. This was due to the different types of interactions between the test liquid and the surface. [PERSON] et al. reported that the diiodomethane surface tension only has a non-polar (dispersive) component; thus, the liquid interacts with the surface only in a dispersive way [14]. Based on this, it can be concluded that the air plasma modification did not affect this kind of interaction. Figure 4 shows that the surface free energy of the material increased as the plasma modification time increased regardless of the approach used to estimate its value. In the case of the CAH theory and the use of water as a probe liquid, the surface free energy increased as the modification time increased up to 10 s; longer treatment times did not affect the energy values. Similar results were obtained using formamide as the test liquid. In the case of diiodomethane, no significant changes in the energy between the samples were observed. The LWAB approach shows increasing surface free energy values that are compatible with the CAH results for the polar liquids (water and diiodomethane). More details are provided in Figure 5, which shows the dispersive and polar components of the surface free energy. It can be observed that the air plasma treatment only changed the polar component of the material's surface free energy significantly. A similar effect was observed in the case of air plasma modification of a polymer [15]. This might be due to the introduction of hydrophilic groups, such as C=O, -OH, and -COO, on the surface [16], which was  
Figure 3: Surface wettability (water, formamide, and diiodomethane equilibrium contact angles) versus modification time. confirmed by the XPS analysis. According to the LWAB theory, the polar component of the surface free energy is composed of acidic and basic constituents [17]. In practice, the acidic constituent (the electron-acceptor parameter) achieves a negligible value, so it seems that air plasma modification only influences the electron-donor parameter (the basic constituent) [10]. On the other hand, studies have reported that, in the case of the LWAB approach, some of the major problems are the systematic overestimation  
Figure 4: Surface free energy calculated using the LWAB and CAH approaches. Figure 5: Components of the surface free energy calculated using the LWAB approach. of the electron-donor component of the surface free energy [18] and the fact that the method does not provide comprehensive values of the free energy components [19]. These results lead to the conclusion that the modification of the material with air plasma caused significant changes in the surface chemical groups that can interact in a polar way. However, more details were provided by the X-ray photoelectron spectroscopy (XPS) analysis, which is discussed in the next section of this paper.