Extended hydrophobicity and self-cleaning performance of waterborne PDMS/TiO 2 nanocomposite coatings under accelerated laboratory and outdoor exposure testing

It has been shown that incorporation of TiO2 nanoparticles into hydrophobic coatings can show self-cleaning performance. Accelerated laboratory testing indicated that the coats retain their hydrophobic nature for an extended time period. In this paper, hydrophobic polydimethylsiloxane (PDMS)/TiO2 nanocomposite coatings with a TiO2 content of 0–40% were fabricated by simple blending of a PDMS dispersion with an aqueous TiO2 nanoparticle dispersion. Their long-term hydrophobicity and self-cleaning performance were investigated both in laboratory and real-world outdoor testing. As expected, TiO2 nanoparticle-based coatings exhibited better self-cleaning relative to the TiO2-free PDMS control coating as measured by methylene blue degradation testing. Excellent long-term hydrophobicity was observed in accelerated weathering testing when they contained the appropriate levels of TiO2 nanoparticles (i.e., 0–30%). However, the same PDMS/TiO2 coatings did not show self-cleaning performance, and instead, exhibited improved dirt pickup resistance, in outdoor exposure testing. Sustained hydrophobicity was observed in outdoor exposure testing for the clear films except when TiO2 levels were at 40%. The hysteresis of water contact angle (HWCA) significantly increased for the PDMS control coating, and water beading was lost as the film surface picked up dirt. In contrast, the TiO2-based coatings with appropriate TiO2 levels maintained a relatively low HWCA after outdoor exposure and no water sheeting on rainy days was observed. This result demonstrates that while photocatalytic TiO2 nanoparticles can maintain coating hydrophobicity upon outdoor exposure, long-term self-cleaning performance in polluted environments has not yet been achieved with this type of coating under real-world conditions.