# Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft
## 3 Results and Discussion
### Properties of SPU Coatings
However, as expected, all these IAS values are in the range of hydrophobic fluorinated polymers (e.g., 150-350 kPa for PTFE [28]) and silicones (200-400 kPa [20]) and agree well with the predicted values according to Equation (1) by [PERSON] et al. [27]. Figure 5B displays the IAS test results for coating samples that were subjected to repeated icing/ice removal cycles. As it can be seen, the IAS of matrix PU-R fluctuated in the range of 606-715 kPa \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Tensile** & **Stress at** & **Stress at** & **Elongation** & **Tensile Set** \\  & **Strength** & **300\% Strain** & **500\% Strain** & **at Break** & **(\%)** \\  & **(MPa)** & **(MPa)** & **(MPa)** & **(\%)** & **(\%)** \\ \hline PU-R & 26.8 & 5.4 & 12.2 & 640\% & 25\% \\ SPU-1\_1\% & 27.2 & 5.2 & 11.2 & 680\% & 20\% \\ SPU-1\_3\% & 26.7 & 5.2 & 11.0 & 700\% & 20\% \\ SPU-1\_5\% & 23.9 & 4.9 & 9.9 & 720\% & 30\% \\ SPU-2\_1\% & 25.2 & 5.2 & 11.3 & 690\% & 20\% \\ SPU-2\_3\% & 18.9 & 4.5 & 8.9 & 690\% & 20\% \\ SPU-2\_5\% & 20.1 & 4.2 & 8.5 & 730\% & 30\% \\ SPU-3\_1\% & 23.0 & 5.1 & 10.7 & 690\% & 25\% \\ SPU-3\_3\% & 22.9 & 5.0 & 10.6 & 700\% & 35\% \\ SPU-3\_5\% & 18.7 & 4.5 & 8.8 & 700\% & 30\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Tensile properties of PU-R and SPU coating films. Figure 4: Plots of tensile strength (**A**) and modulus (**B**), as indicated by stress at 500% strain of SPU coatings against the SMP contents. Figure 3: Representative second DSC heating scans of PU-R and SPU coatings in nitrogen at a heating rate of 20 \({}^{\circ}\)C/min. while the IAS of SPU-3_3% varied in the range of 220-370 kPa, which are in the error range based on Equation (1). The fact that no substantial increase in IAS was observed for SPU-3_3% indicates a good stability against multiple icing/ice removal cycles. Although hydrophobic PTFE and commercial silicones showed reduced ice adhesion strength, they were found to have neither good erosion durability against high-speed particles nor good applicability for aircraft leading edge protection applications. To address these shortcomings, the erosion durability of the SPU coatings against both solid particles and water droplets were evaluated according to ASTM Standards G76 and G73, respectively. Figure 6 shows the solid particle erosion rates and maximum particle loads of the matrix PU-R and representative SPU coatings (i.e., SPU-1_3% and SPU-2_3%), by blasting the coatings with angular alumina particles of 50 \(\upmu\)m at an impinging velocity of 150 m/s and an impinging angle of 30\({}^{\circ}\). Figure 6 compares the erosion rates and maximum particle loads between the coatings. It can be seen that both SPU-1_3% and SPU-2_3% exhibited low erosion rates of about 130-140 \(\upmu\)g/g and high particle loads of 66-73 g/cm\({}^{2}\). In comparison with the matrix PU-R, only a slight increase in erosion rate was observed for the SPU coatings due to the incorporation of SMPs; the maximum particle loads remained substantially unchanged. As comparative references, commercial aircraft exterior coatings such as Alumigrip 4200 polyurethane topcoat (Akzo Nobel) and Aeroglaze(r) 1433 polyurethane midcoat (Socomore) were subjected to the same erosion test and failed, with extensive coating removal and substrate exposure at particle loads of <20 g/cm\({}^{2}\). The water droplet erosion (WDE) resistance of the SPU coatings was evaluated using a water spin rig test facility, at an impact velocity of 175 m/s and an impingement angle of 90\({}^{\circ}\). The coating configuration is Metaflex(r) SP 1050 pretreatment/Aerodur HS 2118 primer (ca. 30 \(\upmu\)m)/PU-R or SPU (ca. 0.35 mm). Figure 7 presents the pictures of the coatings  
Figure 5: Ice adhesion strength (in kPa) of SPU coatings measured using a static push rig at \(-14\) °C (**A**) and ice adhesion strength of PU-R and SPU-3_3% coatings that were subjected to repeated icing/ice removal cycles (**B**). Figure 6: Sand erosion resistance of PU-R and SPU coatings.