Outdoor exposure and accelerated weathering of polyurethane/polysiloxane hybrid coatings

Abstract

Two Polyurethane/polysiloxane hybrid coatings were compared using both real time exposure and accelerated weathering. The urethane coatings were differentiated by oligoesters selection with one being derived from cyclohexane diacids (CHDA) and 2-butyl-2-ethyl-1,3-propanediol (BEPD) and the other adipic acid (AA), isophthalic acid (IPA), 1,6-hexanediol (HD), and trimethylol propane (TMP). Three sites in Hawaii, Campbell Industrial Park (CIP), Kilauea Volcano, and Marine Corp Base Hawaii (MCBH), were selected for the outdoor exposure test with the increasing severity of the environment. After weathering, the coatings were analyzed by Fourier transform infrared spectroscopy (FTIR) and solid state 13C nuclear magnetic resonance (NMR), Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC), and Scanning electronic microscopy (SEM). Urethanes based on cycloaliphatic oligoesters exhibited much better appearance compared to the aromatic-based polyesters. A degradation of both the carbamate and ester functionality was observed spectroscopically which was accompanied by an increase in Tg, and erosion of the surface of the coatings. It was found that the accelerated weathering test conducted in weatherometer provides harsher conditions than the three outdoor exposure tests (marine, volcano, and industrial park), while marine is the harshest among the three outdoor exposure sites.