# Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft
## 2 Materials and Methods
### Methods
Both the test sample preparation and the push rig test were performed in a cold room that was kept constantly at \(-\)14 \({}^{\circ}\)C.  
Preparation of Surface-Modifying Polymers (SMPs) and Durable Hydrophobic Polyurethane (SPU) Coatings  
Synthesis of SMP-1: 1H,1H,2H,2H-perfluoro-1-octanol (PFOA, 6.5 g) and dibutyltin dilaurate (DBTDL, 0.3 g) were added to a solution of Desmodur(r) Z4470 BA (10.0 g) in 25 mL of anhydrous N,N-dimethylformamide (DMF). The mixture was heated to 60 \({}^{\circ}\)C and stirred under nitrogen for 2 h before Terathane(r) PTMEG-650 (3.7 g) in 10 mL of anhydrous DMF was added, dropwise. The resulting reaction mixture was stirred at 60 \({}^{\circ}\)C for another 3 h. After cooling to room temperature, the viscous reaction solution was precipitated in 300 mL of de-ionized water/methanol (4/1, \(v/v\)). The sticky solid formed was washed twice with warm de-ionized water (300 mL) and once with methanol (200 mL), then dried at 75 \({}^{\circ}\)C in a convection oven for 72 h to yield 16.0 g of a clear rubbery product (yield 80%). Synthesis of SMP-2: PFOA (6.2 g), MCR-C18 (4.1 g) and DBTDL (0.5 g) were added to a solution of Desmodur(r) Z4470 BA (10.0 g) in 30 mL of anhydrous DMF. The mixture was heated to 60 \({}^{\circ}\)C and stirred under nitrogen for 4 h before Terathane(r) PTMEG-650 (3.7 g) in 10 mL of anhydrous DMF was added. The reaction mixture was stirred at 60 \({}^{\circ}\)C for another 3 h. After cooling to room temperature, the viscous reaction solution was precipitated in 300 mL of de-ionized water/methanol (4/1, \(v/v\)) and the resulting sticky solid was washed twice with warm de-ionized water (300 mL) and once with methanol (200 mL), then dried at 80 \({}^{\circ}\)C in a convection oven for 72 h to yield 21.6 g of an opaque gel product (yield 90%). Synthesis of SMP-3: PFOA (5.9 g), MCR-C18 (8.1 g) and DBTDL (0.5 g) were added to a solution of Desmodur(r) Z4470 BA (10.0 g) in 30 mL of anhydrous DMF. The mixture was heated to 60 \({}^{\circ}\)C and stirred under nitrogen for 4 h before Terathane(r) PTMEG-650 (3.7 g) in 10 mL of anhydrous DMF was added. The reaction mixture was stirred at 60 \({}^{\circ}\)C for another 3 h. After cooling to room temperature, the viscous reaction solution was precipitated in 300 mL of de-ionized water/methanol (4/1, \(v/v\)) and the resulting sticky solid was washed twice with warm de-ionized water (300 mL) and once with methanol (200 mL), then dried at 80 \({}^{\circ}\)C in a convection oven for 72 h to yield 23.8 g of an opaque gel product (yield 86%). Preparation of SPU coating and thin film samples: SPU coating solutions were prepared by formulating the SMPs (i.e., SMP-1 to SMP-3) into the erosion-resistant polyurethane coating PU-R at levels of 1.0, 3.0 and 5.0 wt.%, respectively, based on the total resin solid. The coating solutions (ca. 1.50 g) were cast on fiberglass substrates (FR-4, 2 \(\times\) 2 \(\times\) 1/4 in), followed by drying and curing at ambient conditions for 7 days to produce coatings with a dry film thickness of ca. 0.35 mm. The coating samples for water droplet erosion tests were prepared by casting ca. 0.10 g of coating solution on aluminum specimen (Al 2024 alloy,  
Figure 1: Schematic drawings of the push rig for ice adhesion test (**left**) and a picture of test specimens with ice formed in a cavity of PTFE-lined aluminum mold on top of coating surfaces (**Right**). 0.31 \(\times\) 0.85 \(\times\) 0.12 in), pre-coated with a chromate-free Aeeeudur HS 2118 CF primer (ca. 40 \(\upmu\)m thick). The coatings were dried and cured at ambient conditions for 7 days before testing. For thin film samples, ca. 7.62 g of coating solutions were cast into an aluminum mold (4.5 \(\times\) 4.5 in) pre-treated with X-9032/G401 Nix Stix(r) release agent. The coating solutions were allowed to flow and level naturally. After drying and curing at room temperature for 1 day, the resulting thin films (ca. 0.35 mm thick) were removed from the mold and allowed to further cure at ambient conditions for 6 more days before testing.