## 4 Discussion
### Ground-Air-Ground (GAG)  
The Ground-Air-Ground (GAG) test was performed to assess the performance of the material during flight. The test consists of subjecting the specimens to the combined effects of temperature, pressure, and relative humidity, thus reproducing flight conditions. The GAG test was conducted using the same combined environmental chamber used for the moisture uptake test, and the samples were tested for 10 repeated cycles. The test procedure is shown in Figure 9. In accordance with ASTM D5229 [47], samples were subjected to a rapid temperature drop to \(-\)54 \({}^{\circ}\)C, followed by a rapid temperature rise to 70 \({}^{\circ}\)C, under conditions of maximum moisture saturation (95% RH). Specifically, each cycle has a duration of 240 min and follows the following steps:  
1. Parking the aircraft on the ground in a cool environment: lower the chamber temperature to \(-\)54 C\({}^{\circ}\) at a rate of 5 \({}^{\circ}\)C/min and keep these conditions for 45 min.
2. Lowering the chamber pressure to 120 mbar with a minimum temperature of \(-\)54 \({}^{\circ}\)C. This condition represents the take-off up to 15,000 m with a maximum ascent rate. Keep these conditions for 40 min.
3. Increase the pressure to 1013 mbar and raise the temperature to 70 \({}^{\circ}\)C simulating landing in a warm environment and parking on the ground at 70 \({}^{\circ}\)C for 25 min.
4. Increase relative humidity to 95% and remain in this condition (70 \({}^{\circ}\)C and RH 95%) for 60 min (park on the ground in a hot, humid environment).
5. Return chamber conditions to 20 \({}^{\circ}\)C and 50% and remain for 10 min.  
Figure 8: Contact angle measurements on films @70 wt% and 90 wt% f GNPs.  
All panels were subjected to the GAG test. Before starting the test, all samples were dried in a vacuum oven at 80 \({}^{\circ}\)C for 3 weeks and weighed. Further weighing was done after the first and tenth cycles.  
After ten cycles, the maximum absorption in panels coated with the GNP layer was sensibly reduced, as well as their absorption rate. The barrier effect decreases with increasing GNP content: sample P70 showed the lowest value of absorption. Results of the GAG test conducted on panels are reported in Table 8 in terms of moisture uptake.  
During the GAG test, the integrity of the GNP layer on the surface was monitored by measuring the electrical conductivity of the layer. Two electric wires were fastened with a conductive paste on the diagonal of each specimen and the different specimens were connected in series, as shown in Figure 10. The samples were fed with a 100 mA current, and voltage was recorded across each sample.  
The results of the surface resistance measurements of specimens during the GAG test, until the 10 th cycle, are reported in Figure 11. The resistance slightly increased during the test for all samples due to water absorption, as reported in Figure 12.  
By comparing the results of the moisture uptake and GAG tests, similar behaviours between GNP coating compositions are found, as shown in Figure 13. In the case of the moisture uptake test, after 72 h the absorption sensibly reduces in the panels coated with GNP layers compared to the uncoated panel by \(-71\%\) for P70, \(-61\%\) for P80, and \(-41\%\) @90 wt% for P90. Likewise, in the case of the GAG test after 40 h the panel absorption reduces by \(-85\%\) for P70, \(-65\%\) for P80, and \(-63\%\) for P90 compared to P REF.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Panel ID** & **M @ Cycle 1** & **M @ Cycle 10** \\  & **[\%]** & **[\%]** \\ \hline P REF & 0.59 \(\pm\) 0.05 & 1.81 \(\pm\) 0.08 \\ P70 & 0.07 \(\pm\) 0.01 & 0.28 \(\pm\) 0.08 \\ P80 & 0.18 \(\pm\) 0.03 & 0.64 \(\pm\) 0.04 \\ P90 & 0.17 \(\pm\) 0.06 & 0.68 \(\pm\) 0.09 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Moisture uptake during the GAG test for each panel.  
Figure 9: GAG test: temperature, humidity, and pressure profiles.