# Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft
## 3 Results and Discussion
### Properties of SPU Coatings
For SPU-2 and SPU-3 coatings, it was expected that the silicone branches would behave similarly to the fluorinated chains in terms of surface enrichment. However, due to unknown reasons, XPS analysis of the Si content did not show a clear trend of increasing Si with the increasing MCR-C18 content of SMP. The detection of Si for PU-R and SPU-1 coatings, where no MCR-C18 was used, may be attributed to the presence of a silicone-containing surface additive, namely BYK-306 (BYK Additives and Instruments), in the PU-R formulation at a loading level of ca. 0.3 wt.%, based on total coating formulation. The decreasing Si content from PU-R to SPU-1 and to SPU-2 may be attributed to the strong tendency of SMP-1 to migrate to the surface during the drying and curing processes, which suppressed the enrichment of BYK-306 at the surface. All SPU coatings exhibited good thermal stability with no material softening or degradation after heating at 121 \({}^{\circ}\)C for 24 h. Figure 3 shows the representative second DSC heating scans of the coatings recorded in nitrogen at a heating rate of 20 \({}^{\circ}\)C/min, which revealed no thermal transitions in the temperature range of \(-\)80 \({}^{\circ}\)C to 200 \({}^{\circ}\)C. These coating samples were pre-treated by heating to 200 \({}^{\circ}\)C at 20 \({}^{\circ}\)C/min, cooling to \(-\)50 \({}^{\circ}\)C at a rate of 10 \({}^{\circ}\)C/min, and stabilized at \(-\)80 \({}^{\circ}\)C before the second heating. Neither the first heating nor the cooling scans showed discernible glass or melting transitions, indicating thermoset properties for the coatings and high heat resistance. Table 4 summarizes the tensile properties of the thin film samples, measured according to ASTM Standard D412 at a grip separation rate of 500 mm/min. It can be seen that all coating films exhibited high tensile strength (e.g., 19-27 MPa), high elongation at break (e.g., 640-730%) and low tensile set (e.g., 20-35%). In comparison with the matrix PU-R, the incorporation of SMP-1 did not substantially affect the tensile strength (SPU-1 in Figure 4A), but a slight decrease in modulus, as indicated by stress at 500% strain, was observed (Figure 4B). For SPU-2 and SPU-3, marginally decreased tensile strength and modulus were noticed with the increasing amount of SMP-2 and SMP-3, respectively. Elongation at break increased from 640% for PU-R to 680-730% for SPUs, depending on the SMP content, but no sizable changes in tensile set were observed. The small tensile sets (i.e., 20-35%) recorded within 1 min of sample failure indicate high resilience of the coating films. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Coatings**} & \multicolumn{5}{c}{**Elemental Content (at.\%)**} \\ \cline{2-6}  & **C 1s** & **N 1s** & **O 1s** & **F 1s** & **Si 2p** \\ \hline PU-R & 69.38 & 2.79 & 20.79 & 0 & 7.04 \\ SPU-1\_1\% & 59.11 & 4.31 & 13.14 & 20.08 & 3.37 \\ SPU-1\_3\% & 56.66 & 4.96 & 9.56 & 27.96 & 0.87 \\ SPU-1\_5\% & 56.16 & 4.87 & 9.18 & 28.95 & 0.85 \\ SPU-2\_1\% & 58.56 & 4.00 & 14.28 & 18.21 & 4.95 \\ SPU-2\_3\% & 56.83 & 4.73 & 9.41 & 27.95 & 1.08 \\ SPU-2\_5\% & 56.31 & 4.49 & 11.02 & 25.16 & 3.03 \\ SPU-3\_1\% & 58.76 & 3.39 & 16.91 & 13.05 & 7.89 \\ SPU-3\_3\% & 56.05 & 3.64 & 14.05 & 19.07 & 7.19 \\ SPU-3\_5\% & 56.00 & 4.24 & 11.92 & 23.36 & 4.48 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: XPS analysis of PU-R and SPU coatings. Figure 5A presents the IAS of the SPU coatings measured at \(-14\)\({}^{\circ}\)C. All the detachment occurred at the ice-coating interface due to adhesion failure, no breakage of ice block was observed. The matrix PU-R showed an IAS of about 622 kPa and the SPU coatings showed IAS in the range of 220-480 kPa. No substantial differences in IAS were discovered between the SPU coatings, regardless of the different SMPs and varying SMP loadings, except for SPU-3, which exhibited some lower IAS (e.g., ca. 250 kPa) at SMP-3 loadings of \(\geq\)3 wt.%.