## 3 Results and Discussion
### Contact Angle Measurements  
#### 3.4.1 Water  
As shown in Figures 7 and 8, fluorinated paper resulted far more hydrophobic than other samples, possibly because of the epichlorohydrin treatment. Contact angle values of first set samples resulted to decrease as number of layers and coating amount increase. This result could be explained by the Wenzel theory [44], assuming the effect of the relative roughness of coatings, and in particular, assuming a raise of the measured hydrophilicity as roughness of the material increases: as noted in SEM images, double layer coatings showed a higher roughness compared with single layers and resulted to be the most hydrophilic. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Code** & \multicolumn{3}{c}{**Time (minutes)**} \\ \hline UCP & 10 & - & 0 \\ Fluo & \textgreater{}4320 & \textgreater{}4320 & \textgreater{}4320 \\ Poli & \textgreater{}4320 & \textgreater{}4320 & \textgreater{}4320 \\
80s80 & 20 & 20 & 20 \\
80s50 & 10 & 30 & 20 \\
80d & 10 & 10 & 10 \\
100s & 10 & 10 & 40 \\
100d & 10 & 20 & 20 \\ PHBV & 10 & 20 & 20 \\ PHBVpeg5 & 40 & 60 & 40 \\ PHBVpeg10 & 20 & 20 & 20 \\ PHBVpeg20 & 20 & 10 & 40 \\ PCL & 240–720 & 240–720 & 240–720 \\ PCLpeg5 & 240–720 & 240–720 & 240–720 \\ PCLpeg10 & 240–720 & 720–1440 & 240–720 \\ PCLpeg20 & 240–720 & 240–720 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Grease resistance times. Figure 7: Average water contact angle values for first set samples. Error bars report standard deviation. Different letters indicate statistically significant differences (\(p<0.05\)). “UCP” refers to base paper, “Fluo” to fluorinated paper, while “Poli” to polyethylene-coated paper. See Table 1 for other samples codification. Calendered paper showed a good hydrophobicity. The relatively high value detected for this paper could be related to a behavior that comes closer to the Cassie-Baxter model, assuming the contribution of air bubbles entrapped between interfiber gaps, given the high porosity of this material, but it is difficult to verify such a hypothesis (Figure 8) [44,45]. It is possible to notice a general upward trend of the contact angle (or a downward trend of wettability), by adding plasticizer. Given the relative hydrophilicity of PHBV and PEG, this result could be explained considering the Wenzel theory and the effect of surface roughness [44]. The use of plasticizer in the coating solution formulation could be associated with a better solution spreadability and a more uniform coating distribution. The SEM images seemed to support the effectiveness of PEG in this regard, as the coated surface appeared more uniform and less rough with increasing plasticizer concentrations. PCL contact angle values were higher than the ones found in literature for films based on the same PCL type used in this work (always at 5% \(w/v\), but in trichloromethane and differently processed), equal to 64.5\({}^{\circ}\)[46]. Even if the trend is not as linear as for PHBV, the Wenzel theory could explain the significantly higher contact angle values detected for PCL with PEG at 10% and 20% samples, considering the progressive increase of smoothness observed for these coatings by the SEM images. Although PCL with PEG at 5% sample showed a contact angle value lower than pure PCL, the difference was not statistically significant. PHBV and PCL both with PEG at 20% samples showed the lowest wettability (highest contact angle values) among coated samples. In particular, PCLpeg20 reached a wettability level significantly lower compared to reference polyethylene-coated paper (Figure 9). Figure 8: Average water contact angle values for the second set samples. Error bars report standard deviation. Different letters indicate statistically significant differences (\(p<0.05\)). “UCP” refers to base paper, “Fluo” to fluorinated paper, while “Poli” to polyethylene-coated paper. See Table 1 for other samples codification.