## 3 Results and Discussion
Furthermore, it should be kept in mind that both chemical (functional groups) and physical (topography) factors influence the surface wettability [20]. The two samples (unmodified (S-0) and modified for 10 s (S-10)) with the most visible changes were taken for further investigations. The surface height maps are presented in Figure 6. Based on them, the roughness parameters of the samples were calculated and are shown in Table 2.  
The air plasma modification of the material surface caused an increase in the roughness (Table 2). This may improve the gluing effect because surface development promotes adhesion [21]. The X-ray photoelectron spectroscopy analysis showed differences in the surface groups and their chemistry. After the air plasma modification, there was an increase in the oxygen content on the sample surface. This was due to the generation of C=O and C=OH species during the plasma process, which was confirmed by the data obtained from the high-resolution XPS spectra (Table 3), in which an increasing number of carbonyl and hydroxyl groups can be seen. Despite the reduction in the nitrogen content (because of the oxidation properties of air plasma), amines appeared. The O/C ratio of the non-modified sample is equal to 0.291, which corresponds to the range 0.31 to 0.4, which is typical of lignin [22]. In the case of the sample treated by the air plasma for 10 s, the O/C ratio increased almost 2-fold, which confirms the strong surface oxidation mostly to unsaturated hydrocarbons, in particular to produce the C=O and O=C=O species [23]. However, it should be emphasized that despite the relative simplicity of the modification process, the interactions between the plasma and the material surface are rather complex [24], particularly in the case of the composite material. Therefore, it is not possible to propose a scheme for the specific reaction that occurs during plasma surface modification and only the states before and after the treatment were compared without taking into account the course of the process itself. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Ra (nm)** & **Rq (nm) 1** & **Rt (\(\mu\)m)** \\ \hline S-0 & 313.5 \(\pm\) 75.5 & 404.8 \(\pm\) 94.1 & 5.8 \(\pm\) 0.4 \\ S-10 & 372.9 \(\pm\) 29.3 & 480.7 \(\pm\) 29.9 & 8.2 \(\pm\) 1.2 \\ \hline \hline \end{tabular} 1 Root mean square (RMS) deviation of the roughness.  
\end{table} 
Table 2: Roughness parameters of the selected samples. Figure 6: Roughness map of the selected samples: (A) non-modified sample (S-0) and (B) sample modified for 10 s (S-10). The force values necessary to detach the samples are shown in Figure 7. In the case of the two unmodified sheets glued together (at 0 s--the reference test), the joint was broken at about 0.3 N.  
The tests confirm that the air plasma modification improved the adhesion of the examined materials, which is consistent with the surface free energy analysis results because the relation between the SFE of the paper and the surface tension of the glue is decisive in a good adhesive bond--an increase in the energy will improve the adhesion [25]. The one-second treatment did not produce a significant change, while the ten-second treatment increased the durability of the joint more than 2-fold. The force necessary to unglue the materials was equal to 0.8 N. In the case of using the paper in the technological process for the production of packaging, this is a sufficient and satisfactory change. A