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TiO2–based nanocomposite coatings were fabricated using two silicon-oxo hybrid binders and further treated with UV-irradiation to acquire coatings with a wide range of static water contact angles (SWCA) (from 5° to 118.6°) and different dynamic surface wetting behaviors. The dirt adsorption and desorption behavior of these coatings were measured after applying modified rice-straw ash and washing with a water-spray. The results were correlated with static surface wettability and dynamic surface wettability values. It was found that the coatings with an SWCA of about 75° displayed the highest dirt adsorption regardless of the type of binder used. Lower SWCA and receding water contact angles (RWCA) allowed for better dirt desorption and hence better dirt pickup resistance. The coatings with RWCA values close to 0° exhibited self-cleaning performance even when the surface was not superhydrophilic. In addition, the amount of dirt removed from the surface of the coatings by dirt desorption tests was evaluated and used as an indicator of the ability of the coating to remove dirt from polluted air. Specific SWCA and low RWCA values seem to be critical to the coating’s ability to remove dirt from the air. In addition, the dependence of the dirt removal ability on the hysteresis of water contact angle (HWCA) was different for the two silicon-oxo hybrid binders.

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.

Photocatalytic TiO 2 provides a promising route to produce self-cleaning coating surfaces, but it often causes decomposition of organic substrates and other coating components. An interlayer is often employed to insulate photocatalytic activity from sensitive substrate layers, complicating the application of these self-cleaning coatings. Herein, photocatalytic TiO 2 -based coatings were prepared based on an aqueous TiO 2 dispersion in a hybrid binder synthesized from tetraethyl orthosilicate and methyl trimethoxy silane via a sol-gel process. Coatings with various levels of nano TiO 2 were prepared and found to be transparent to visible light; i.e., the clear coating could be directly cast on architectural latex coatings without changing its original appearance. The photocatalytic coating system was composed of a TiO 2 -based clear top coat and an opaque latex film underneath. The coatings were evaluated through outdoor exposure studies and accelerated weathering tests. Results of these tests showed that the photocatalytic coatings with a TiO 2 content range of 33–45% exhibited excellent self-cleaning performance, while displaying none of the expected degradation. In fact, this nano TiO 2 -based clear coating actually protected the latex film from UV-induced damage. The increased stability of the organic film may have resulted from reduced UV transmission through the photo-catalyst containing clear coat as well as reduced water permeability. It was demonstrated that the combination of appropriate TiO 2 content and a suitable binder are crucial for the fabrication of robust photocatalytic self-cleaning coatings for painted surfaces without the need for an interlayer.

The evaluation of the dirt resistance of organic coatings according to traditional accelerated methods (e.g., the standard method GB/T 9780-2013 in China) usually provides results which deviate from their real-world performance in seriously polluted environments. As an alternative, a novel adsorption method with methyltrimethoxysilane-modified rice-straw ash as the artificial dirt standard was proposed in this paper. With this method, the ash was dispersed into the air in a testing chamber and then allowed to physically adsorb to the coating, which is believed to more accurately simulate the adsorption behavior of floating particulates outdoors. Two groups of latex coatings from Dow Chemical Company were employed to evaluate the adsorption method by monitoring the changes in color, whiteness, and reflectivity. In parallel, outdoor exposure testing of these coatings was conducted. The influences of charging method, of ash type, and panel position on the testing results were carefully investigated. Multicharging of ash (9 g × 3) and parallel panels at different positions in the testing chamber are designed for the generation of repeatable results. It was demonstrated that the adsorption method basically matched the outdoor exposure results and was more predictive of real-world results than GB/T 9780-2013.

The progressive replacement of organic solvent-based coatings by waterborne latex polymer coatings has substantially renovated the coating industry, and generated huge environmental and health benefits. Today, on top of the continuing demand for higher performance and lower costs, the coating industry faces tighter regulation and higher sustainability standards. In addition, the new waterborne coatings have created unique opportunities and challenges in terms of fundamental understanding and research development. To address these challenges, polymer latex binders with diverse particle morphologies have been developed to improve coating performance. Furthermore, colloidal self-assembly has been utilized to help manufacturers make better paint with less cost. In this report, we review the recent progress in both fundamental study and industrial application in the context of developing new generation architectural coating materials. We introduce the basic concepts in coating materials and showcase several key technologies that have been implemented to improve coating performance. These technologies also represent the most important considerations in architectural coating design.