# Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft
## 3 Results and Discussion  
### Synthesis of SMPs and SPU Coatings  
To mitigate icing and ice accretion on the leading edge surfaces of aircraft wings and helicopter rotor blades without sacrificing erosion durability against high-speed solid particles and rain droplets, it would be straightforward and ideal to modify the existing erosion resistant coatings to render the surface with icephoicity. To this end, effort was first made by introducing various commercial surface modifying agents such as polyether-modified polydimethylsiloxane (e.g., BYK-306, BYK), fluorinated copolymers (e.g., CapstoneTM FS-83, Chemours), hydrophobic funnel silica (e.g., Aerosil(r) R 812, Evonik), PTFE micropowders (e.g., Zonyl(r) MP 1100, Chemours) and PEGylated polydimethylsiloxane (e.g., DBE-224, Gelest) into the erosion resistant PU-R coating at different levels in hoping to achieve hydrophobic and slippery surfaces that can delay ice formation or reduce ice adhesion strength. Unfortunately, limited success was achieved. None of these additives led to coatings with high surface hydrophobicity. Furthermore, foam stabilization and phase separation were observed when some of the additives (e.g., FS-83 and DBE-224) were incorporated at high amounts (e.g., \(>\)1 wt.%, based on total resin solid). Thus, to achieve durable icephobic coatings for leading edge icing protection, new surface-modifying polymers (SMP-1 to SMP-3) were designed and synthesized in this study, which bear low surface tension fluorinated and polydimethylsiloxane (PDMS) branches, and have good compatibility with the polyurethane matrix, to enable high PU-R surface hydrophobicity. Scheme 1 shows the synthetic scheme of the SMPs, where the multifunctional poly-isocyanate Z4470 BA (f \(\approx\) 3.5) first reacts with the mono hydroxyl compounds (i.e., PFOA and MCR-C18) to produce a reaction mixture, followed by polymerizing with the diol PTMEG-650. The intermediate reaction mixture consists of a distribution of reaction products of Z4470 with the mono hydroxyl compound (e.g., unreacted, mono-, di-, tri- and fully reacted Z4470), depending on the molar ratio of Z4470/(PFOA+MCR-C18). Structure-wise, it is preferred to have a minimum amount of unreacted and fully reacted Z4470 in the intermediate reaction mixture. The former has high functionality and tends to lead to crosslinking when polymerizing with PTMEG-650, while the latter may contribute to incompatibility of SMP with matrix PU-R due to poor inter-chain interactions. At the same time, the di-reacted Z4470 product that has a remaining isocyanate functionality of about 2 should be maximized so as to produce high molecular weight SMPs with branched structures. For the reaction system of this study, it was experimentally found that a suitable molar ratio of Z4470/(PFOA+MCR-R18) was about 1/2.2, where highly branched polymers could be obtained from the subsequent polymerization reaction with PTMEG-650 without crosslinking. **Scheme 1. ** Synthetic scheme of the surface-modifying polymers. Table 1 summarizes the syntheses of the SMPs using different combinations of PFOA and MCR-C18 as the mono hydroxyl compounds for the purpose of examining the effects of fluorinated branches and silicone branches of the SMPs on coating properties. The reaction of N4470 with PFOA and MCR-C18 at 60 \({}^{\circ}\)C in the presence of DBTDL as a catalyst gave clear solutions, although MCR-C18 was initially immiscible with the reaction mixture. The addition of PTMEG-650 quickly increased the solution viscosity, suggesting the build-up of molecular weight. FTIR investigations showed the disappearance of the characteristic NCO absorption at around 2265 cm\({}^{-1}\) in about 2 h of reaction time, indicating the completeness of the reactions. SMP-1 was obtained as a clear rubbery gel, while SMP-2 and SMP-3 were white soft gels. The white color of SMP-2 and MSP-3 may be attributed to the phase separation of PDMS segments within the SMPs.