## 3 Results  
In order to define the usability of paper with agricultural residues for secondary packaging and labels, the influence of ink penetration depth and the integral optical density of prints on the print-through was investigated. When analyzing the cross-sections of the printed samples with and without agricultural residues in epoxy resin (Figure 1), the maximum penetration values were recorded at 10 points where ink penetration into the paper substrate was observed. The results are shown as mean values in Figure 2a. Based on the qualitative characteristics of the printed paper substrates, it is clear that there are differences in the prints depending on the color of ink used. Observing the ink penetration depth parameter (Figure 2a), it is evident that the prints obtained with cyan ink show the lowest ink penetration (\(Hp_{m}\) = 25.90-51.42%) inside the paper substrate, while the highest ink penetration was achieved with magenta (\(Hp_{m}\) = 55.50-79.76%) and yellow ink (\(Hp_{m}\) = 53.16-64.08%). Regarding the optical ink density parameter (Figure 2b), which is obtained from the light reflected by the print and describes the proportional value of ink coverage on the paper substrate [10; 31], it is possible to conclude that the yellow prints show the lowest value of optical ink density (\(D_{i}\) = 0.767-0.806), while the cyan prints show the highest value of optical ink density (\(D_{i}\) = 1.286-1.312). The print-through parameter (Figure 2c), which can be reduced by coating and lamination, is very important for packaging and labels, whether they come into direct contact with food or not, where the background must be opaque, that is, able to hide the content printed on the reverse [31; 32]. Looking at the printed paper substrates in terms of the ink used, cyan prints have the highest print values, ranging from 10.06 to 20.67, while the lowest values were measured for the yellow prints, ranging from 3.89 to 9.42. Figure 1: Microscopic cross-sectional images of printed paper substrates (R, 70R30W, 70R30B, and 70R30T) in epoxy resin at 200\(\times\) magnification. An analysis of the data normality was conducted using the Shapiro-Wilk test for normality. The Shapiro-Wilk parametric test is employed for small sample sizes (<50) due to its sensitivity and precision in detecting deviations from normality. Each variable was tested individually owing to the distinct characteristics of the variables, precision in interpretation, sensitivity analysis, and potential decision-making regarding transformation. The \(p\)-value is observed, and if it is >0.05, the null hypothesis is not rejected, leading to the conclusion that the data do not significantly deviate from a normal distribution. If the \(p\)-value is <0.05, the null hypothesis is rejected, suggesting that the data are not normally distributed (Table 4). Furthermore, for the analysis of the relationships among multiple metric variables (_Density_, _Penetration_, and _Print-through_), a multiple regression analysis was employed. This analysis observed the impact of multiple independent variables, or predictors in this case, and types of paper on the dependent variable of _Print-through_. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable** & **Shapiro–Wilk W** & \(p\)**-Value** \\ \hline Density C & 0.97756 & 0.59957 \\ Penetration C & 0.95067 & 0.07995 \\ Print-through C & 0.94642 & 0.05715 \\ Density M & 0.91995 & 0.07670 \\ Penetration M & 0.97869 & 0.64096 \\ Print-through M & 0.95290 & 0.09545 \\ Density Y & 0.64252 & 0.40207 \\ Penetration Y & 0.96502 & 0.24767 \\ Print-through Y & 0.98722 & 0.15908 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The Shapiro–Wilk parametric test results for each variable. Figure 2: Qualitative parameters: (**a**) ink penetration depth, (**b**) optical ink density, and (**c**) print-through of printed paper substrates (R, 70R30W, 70R30B, and 70R30T).