Preparation and performance of hydrophobic two-component polyurethane clearcoats based on perfluoro alcohol modified polyisocyanate hardener

In this study, three perfluoro alcohol with different fluoroalkyl chain length, i.e., 3,3,3-trifluoro-1-propanol (TEOH1), 2-perfluorobutyl ethanol (TEOH4), 2-perfluorohexyl ethanol (TEOH6), were first reacted with polyisocyanate to get hardeners bearing various contents and lengths of fluoroalkyl chain. The fluorinated polyisocyanate hardener was then adopted to fabricate hydrophobic two-component polyurethane (2K PU) clearcoats. The highest water contact angles (WCAs) of the hydrophobic 2K PU clearcoats with no haze and non-sticky surface are 91.9°, 101.6° and 106°, achieved at the perfluoro alcohol dosage of 10 %, 20 % and 20 % for TEOH1, TEOH4 and TEOH6, respectively. Accelerated weathering and outdoor exposure tests were adopted to examine the hydrophobicity durability of the perfluoro alcohol-modified coatings, and dirt pick-up resistance (DPUR) outdoors was also monitored. All coatings showed decreased WCAs in weathering tests. Nevertheless, WCAs of coatings with ≥20 % TEOH4 and ≥5 % TEOH6 remained hydrophobicity after 228-day outdoor exposure. Loss of hydrophobicity is attributed to the oxidation of PU matrix, removal of free perfluoro alcohol-modified polyisocyanate and embedding of silicate dirt. Improved DPUR of 2K PU clearcoat was demonstrated outdoors after incorporation of perfluoro alcohol, especially for the cases with TEOH4 and TEOH6 under an adequate dosage. As a whole, longer fluoroalkyl chain favors the acquisition of both higher hydrophobicity, better hydrophobicity durability, and better outdoor DPUR.