Durable Self-Cleaning Coatings for Architectural Surfaces by Incorporation of TiO₂ Nano-Particles into Hydroxyapatite Films.

Enrico Sassoni,Elisa Franzoni,George Scherer,Norberto Roveri,Eros D’Amen

doi:10.3390/ma11020177

Enrico Sassoni, Elisa Franzoni + Show 3 more

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https://doi.org/10.3390/ma11020177

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Abstract

To prevent soiling of marble exposed outdoors, the use of TiO2 nano-particles has been proposed in the literature by two main routes, both raising durability issues: (i) direct application to marble surface, with the risk of particle leaching by rainfall; (ii) particle incorporation into inorganic or organic coatings, with the risk of organic coating degradation catalyzed by TiO2 photoactivity. Here, we investigated the combination of nano-TiO2 and hydroxyapatite (HAP), previously developed for marble protection against dissolution in rain and mechanical consolidation. HAP-TiO2 combination was investigated by two routes: (i) sequential application of HAP followed by nano-TiO2 (“H+T”); (ii) simultaneous application by introducing nano-TiO2 into the phosphate solution used to form HAP (“HT”). The self-cleaning ability was evaluated before and after prolonged exposure to simulated rain. “H+T” and “HT” coatings exhibited much better resistance to nano-TiO2 leaching by rain, compared to TiO2 alone. In “H+T” samples, TiO2 nano-particles adhere better to HAP (having flower-like morphology and high specific surface area) than to marble. In “HT” samples, thanks to chemical bonds between nano-TiO2 and HAP, the particles are firmly incorporated in the HAP coating, which protects them from leaching by rain, without diminishing their photoactivity and without being degraded by them.

Highlights

Architectural surfaces exposed outdoors are subject to soiling, i.e., darkening originated by accumulation of particulate matter, mainly fine carbonaceous particles rich in dark elemental carbon (Figure 1) [1]
In the case of marble and other carbonatic substrates, darkening can occur by two processes: (i) particles can deposit over the surface, by gravity and/or electrostatic attraction; (ii) particles can be embedded in a layer of gypsum formed over marble surface by sulfation caused by atmospheric SOx [2]
We investigated the combination of TiO2 nano-particles with the HAP-based treatment with three objectives: (i) preventing nano-TiO2 leaching by rain, by incorporation into the HAP coating; (ii) enhancing the photocatalytic activity of TiO2, thanks to combination with HAP; (iii) enhancing the development of HAP layers over the surface and inside cracks, thanks to the presence of TiO2 nano-particles

Summary

Introduction

Architectural surfaces exposed outdoors are subject to soiling, i.e., darkening originated by accumulation of particulate matter, mainly fine carbonaceous particles rich in dark elemental carbon (Figure 1) [1]. (ii) particles can be embedded in a layer of gypsum formed over marble surface by sulfation caused by atmospheric SOx [2]. In the latter case, the so-called “black crusts” are formed [3]. Materials 2018, 11, x FOR PEER REVIEW their coefficient, compared to because the substrate, dark layers canand enhance favorsdifferent sulfationthermal [1] andexpansion thermal degradation [5]. Of their darker color their marble microcracking (the so-called “sugaring”), leading grain detachment and [5,6]. Of their darker color their marble microcracking (the so-called “sugaring”), leading grain detachment and [5,6]. marble different thermal expansion coefficient, compared to the to substrate, dark layers canloss enhance microcracking (the so-called “sugaring”), leading to grain detachment and loss [5,6]

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