# Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft
## 3 Results and Discussion
### Properties of SPU Coatings  
Table 2, above, displays the static and dynamic water contact angles of the coatings, measured using a goniometer equipped with an automated dispenser. As expected, the unmodified matrix PU-R is hydrophilic and has a static water contact angle (\(\theta\)) of 69\({}^{\circ}\), whereas all SPU coatings comprising the SMPs are hydrophobic, with \(\theta\) being larger than 97\({}^{\circ}\). Water beaded up quickly and slid easily on the coating surfaces. Different SMPs showed no substantial differences in producing hydrophobic surfaces. However, SPU-2 and SPU-3 did give a more slippery feel than SPU-1 when rubbing fingers on the coating, probably due to the presence of PDMS on the surface. No smudge was observed after the finger rubbing, and the surface hydrophobicity could be easily restored for oil-contaminated surfaces by washing with detergent. A dependence of the water contact angle on the SMP content was revealed, as evidenced by the increase in \(\theta\) when the SMP loading was increased from 1 wt.% to 3 wt.%, based on the total resin solid. Further increasing the SMP loadings to 5 wt.% did not lead to additional increase of \(\theta\). The hydrophilicity of PU-R was more clearly seen from its small receding angle (\(\theta_{\text{rec}}\)) of ca. 20\({}^{\circ}\) and large contact angle hysteresis (CAH) of ca. 57\({}^{\circ}\). In contrast, all SPU coatings exhibited high \(\theta_{\text{rec}}\) of ca. 80\({}^{\circ}\) and small CAH in the range of 22-36\({}^{\circ}\). Based on the relationship between water wettability and ice adhesion proposed by [PERSON] et al. [27], as illustrated by Equation (1), where \(\tau_{\text{ice}}\) is the ice adhesion strength, the matrix PU-R is expected to have an IAS of ca. 650 kPa, while the SPU coatings should have reduced IAS of ca. 400 kPa. \[\tau_{\text{ice}}=(340\pm 40\text{ kPa})(1+\text{cos}\theta_{\text{rec}}), \tag{1}\]  
To understand the surface elemental compositions, XPS was recorded for PU-R and the SPU coatings. As shown in Table 3, all the SMP-containing SPU coatings showed a high \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Coatings** & **SMPs** &
\begin{tabular}{c} **wt.\%** \\ **SMPs** \\ \end{tabular} & \(\theta\)**, Water** & \(\theta_{\text{adv}}\)**, Water** & \(\theta_{\text{rec}}\)**, Water** \\ \hline PU-R & - & 0 & 69.0 & 77.4 & 20.4 \\ \hline SPU-1\_1\% & & 1.0\% & 103.7 & 108.0 & 80.2 \\ SPU-1\_3\% & SMP-1 & 3.0\% & 109.7 & 115.5 & 82.1 \\ SPU-1\_5\% & & 5.0\% & 109.6 & 114.0 & 82.0 \\ \hline SPU-2\_1\% & & 1.0\% & 100.0 & 101.2 & 78.3 \\ SPU-2\_3\% & SMP-2 & 3.0\% & 109.0 & 114.0 & 80.5 \\ SPU-2\_5\% & & 5.0\% & 109.3 & 115.9 & 79.4 \\ \hline SPU-3\_1\% & & 1.0\% & 96.9 & 100.7 & 69.4 \\ SPU-3\_3\% & SMP-3 & 3.0\% & 109.5 & 114.8 & 78.8 \\ SPU-3\_5\% & & 5.0\% & 112.0 & 117.5 & 81.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Preparation and water contact angle measurements of SPU coatings. Figure 2: (**A**) FTIR spectra of PU-R and SPU coating films after curing at ambient conditions for 2 days, and (**B**) surface fluorine content (at.%) of SPU coatings from XPS studies. fluorine content in the range of 13-29 at.%, indicating the enrichment of SMPs on coating surfaces. In contrast, the PU-R showed no fluorine on the surface. Figure 2B compares the surface fluorine contents of the SPU coatings. It can be seen that SPU-1 coatings have the highest fluorine contents, compared to SPU-2 and SPU-3, due to the high PFOA content in SMP-1. SPU-3 coatings, which are based on SMP-3, showed the lowest surface fluorine contents. For SPU-1 and SPU-2, the fluorine content seems to reach a plateau at a SMP loading of 3 wt.%, which agrees with the water contact angle measurement, where an increase of static water contact angle occurred from SPU-1_1% and SPU-2_1% to SPU-1_3% and SPU-2_3%, respectively, and leveled off thereafter. The SPU-3 coatings showed a monotonous increase of fluorine content with the SMP-3 loadings.