Deterioration in building façade coatings can be caused by a wide range of factors, such as soot deposition, biological growth, acid rain corrosion, or a combination of various biochemical/physical effects. New methods for the protection of outdoor façades have attracted attention in order to reduce the huge economic loss that comes with unexpected corrective maintenance. 

Innovation in building façade coatings can address not just the degradation problems but also work as a force for more sustainable cities by helping to ensure more durable and high-performance buildings. The technologies can even contribute to the reduction of CO₂ emissions, improve energy conservation, and increase air quality. 

Some of the latest and most interesting applications are self-cleaning façade covers, lightweight thermal insulating coating mortars with aerogel/EPS for energy conservation, autogenous and biobased self-healing techniques applied to cementitious materials, and thermochromic superhydrophobic coatings that improve building energy conservation.

Photocatalytic façade coatings:

Many research studies have shown the effect of semiconductors such as the titanium dioxide (TiO₂) applied to building materials and its degradation effect on removing volatile organic compounds (VOCs), organic dyes, and biological developments that occur on different surfaces.

Titanium dioxide is a white pigment and is most often used for self-cleaning purposes in the mineral form of nano-anatase. The mechanism occurs due to the adsorption of UV light, when TiO₂ particles receive photons with energy superior to the bandgap energy, which leads to the transfer of electrons from the valence band to the positive holes. If the surface is hydrated (which can be caused by air humidity or rain), the holes react with water molecules, generating hydroxyl radicals (OH⁺), which are strong oxidizing agents that work on turning organic pollutants into less harmful CO₂ components. Titanium dioxide is also nontoxic, stable, zero energy consumption, and is proven to have better photocatalytic performance when used in mean diameters of 5-50 nanometers.

Furthermore, TiO₂ applied to mortars and concrete can effectively contribute to air purification in the area close to the building coatings by converting dangerous gases such as NO_x and toluene into harmless components. Common applications are performed in white coatings, and one example is the use of TiO₂ as an additive in the white concrete of the Jubilee Church in Rome.

In addition to decomposing components that cause building damage and air pollution, photocatalytic treatments have other environmental advantages. For example, a self-cleaning concrete façade will not require the use of solvents that are usual for cleaning treatments of buildings, eliminating another source of pollutants. Moreover, a lighter coating (normally white, caused by the TiO₂ pigmentation) reflects more light, reducing the internal and external temperatures during heat seasons.  

According to European Coatings, the market for titanium dioxide is currently worth €17.82 billion and the global consumption of TiO₂ in 2020 was about 6.45 million tonnes, with the paint and coatings industry being the main consumer. Annual consumption for 2025 is estimated to grow up to  8 million tonnes.

Self-healing mechanism in mortars and concrete:

Fractures in cementitious materials are considered a natural occurrence, since these structures are loaded or are in contact with different exposure environments. The cracks are an easy way for the entrance of aggressive agents such as sulfate and chloride ions, which will later be a source of deterioration in a building’s coating. The effect can be intensively harmful because not just the surface of these materials is affected, but further corrosion may happen when in contact with concrete steel elements, implying an extreme loss of durability that might also lead to catastrophic failure.

Due to these concerns, studies that try to evaluate and improve the self-healing mechanisms in mortars and concrete have been of great interest. The self-healing phenomenon of cementitious materials is categorized into two main processes: autogenous and autonomous healing, which are different in their causes.

Autogenous self-repairing is based on the inherent hydration process of the cement particles, and the mechanism works by continuous hydration of unreacted cement particles in the presence of water or by the precipitation at cracks of calcium carbonate (CaCO₃) through the dissolution of calcium ions (Ca²⁺) present in the matrix that react with carbonate ions (CO₃^2-) from the environment. The efficiency of autogenous self-healing is determined by the environment (e.g., the presence of rain/water), temperature, and mix composition. 

Autonomous healing requires the application of additives, one of the most promising ones being the biobased bacteria species that work on sealing the cracks also by calcium carbonate precipitation. Other common methods are based on embedded capsules or vessels that facilitate the flow of healing agents (usually chemical additives) when there is a crack in the cementitious surface, allowing it to heal even after several days of curing. 

A major research group project that includes studies of self-healing phenomena on ultrahigh-durability concrete (UHDC) to evaluate durability under extreme conditions is the ReSHEALience project, a consortium of 13 countries that has received funding from the European Union Horizon H2020 Research and Innovation Programme. Moreover, the global self-healing concrete market is expected to grow 36.8% in revenue CAGR by 2030.

Thermochromic façade coatings:

According to the International Energy Agency tracking report on building envelopes, in 2020, building construction accounted for 10% of global energy and process-related emissions, which is a significant contributor to the increase in carbon emissions and waste of natural resources. Due to the use of high-energy-consumption cooling equipment during high temperatures, materials such as superhydrophobic solar reflective coatings were successfully developed to fulfill the need for solar reflectance. However, in cold periods, those materials fail to establish the normal temperature inside the buildings, since the solar reflectance is too high — leading to the need for heating equipment.

To address these energy use problems, thermochromic microcapsules (TCM) have been studied in applications to cementitious materials for external walls because of their ability to change their own color. The mechanism is very simple: when the external temperature is high, the TCM will change its color to a lighter/colorless one; when it is cold outside, the color induced by the TCM is darker, decreasing the solar reflectance. This is possible due to thermochromic characteristics that are based on organic pigments in the form of powders, which are encapsulated in organic microcapsules and have a transition temperature that sets their change of color.

The main thermochromic dyes are a mixture of leuco dyes (such as fluorans, spirolactones, and fulgides) with chemicals compounds that are responsible for electron donating, such as cyclic ester, and a solvent (e.g., an acid or an ester). A wide range of colors have been tested for building applications, and in the studies of Yuxuan et al., the color transition temperature of the dyes was 31 °C, which is similar to some of the pigments commercially available by Kolortek.

The applications for building envelopes are still in development, but there is a patent for rendering mortars containing thermochromic microcapsules, and a study in China showed by simulation that the use of a thermochromic superhydrophobic coating for buildings decreased the total annual energy consumption in North China by 13.74% when compared to the traditional white cooling coating.

Lightweight thermal insulating mortars:

Another alternative for reducing the energy consumption in buildings and reducing their carbon footprint is the addition of lightweight materials with thermal properties, replacing the fine aggregates in mortar. These alternative materials improve the thermal performance of buildings by lowering the heat change while at the same time reducing the materials’ weight and increasing their acoustic insulation properties.

Some of the lightweight materials that have been tested as additives to mortars include silica aerogel, plastic waste, rubber, and, most recently, vermiculite and expanded polystyrene (EPS). Some companies, such as Tekto, based in Greece, have developed a commercial pre-mixed mortar with ultralightweight and thermal insulating EPS, and there is a patent for a type of inorganic lightweight aggregate thermal insulation mortar. The mechanism is based on its insulating performance, which significantly reduces the thermal conductivity of the mortars and also increases service life.

Conclusion:

Many alternatives for more intelligent and environmentally friendly building materials are already being utilized, and there are abundant opportunities for companies that further develop and encourage the use of innovative coatings. With economic advantages tied to benefits such as thermal indoor comfort, cleaner and more durable renderings, and even external air purifying, there is a bright future for innovation in building façade coatings.

Featured image: Jubilee Church in Rome, by Vincenzo Pentangelo, CC BY-SA 4.0 

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