The paint protects urban buildings and roads against pollution. The TiO2 particles are able to trigger a photocatalytic activity which releases free radicals to destroy through oxidation organic and inorganic compounds and biological species on the coating surface.
In addition, the paints have a self-cleaning capability. They create uniform water films on their surfaces which prevent the adhesion of both organic and inorganic substances.
As a result, after absorbing air pollutants, particularly during nighttime, and then removing the pollutants by day through oxidations, the catalysts are regenerated by rainfall.
However, the coatings are already revealing the sort of technical and marketing difficulties coatings with a much broader functionality than conventional paints are having in making inroads into the mainstream coatings markets in Europe.
They are also showing that the advent of smart or intelligent paint will open up opportunities in coatings for relative newcomers or to players for whom coatings are not their core business.
The pacesetters in the European photocatalytic market are building material manufacturers. They include Italcementi Group, Milan, Italy, one of the world's largest cement producers which has developed a range of coatings and plaster products with a photocatalytic compound called TX Active.
Two others are the Sto Group of Germany and the Swedish-based maxit Group, part of HeidelbergCement Group of Germany. Maxit recently won an innovation award for its photocatalytic Airfresh product, which is applied both inside and outside buildings.
So far about the only coatings company marketing a photocatalytic paint is Caparol of Germany which has been involved in a development project in the technology with Japanese collaborators.
Europe has fallen behind other major markets in the world in the commercialization of TiO2 photocatalytic paints, particularly Japan which is considered to have the most advanced sector for the coatings. Even China has been quicker off the mark with the paints by taking advantage of silicone-based binder systems for their application.
Academia in Europe has been active in research and development into nanostructured materials and processes in photocatalysis. But European coatings companies and raw material suppliers have been reluctant to invest in photocatalytic technologies which are considered to need improvements to make them commercially viable.
Italcementi and its partners, which included the TiO2 producer Millennium Chemicals, were fortunate in that over half of the €3.4 million of the R&D costs for the final stages of the development of TX Active was provided by the European Union as part of a European research project. This four-year scheme called PICADA (Photocatalytic Innovative Coverings Applications for De-pollution Assessment) was completed at the end of last year.
The Italian company claims that laboratory tests demonstrate that the photocatalytic reaction caused by TX Active reduces the amount of pollutants on the surface of walls by up to 75% after only three minutes of exposure to UV light from the sun.
One of the first tests of TX Active was on a 230-meter stretch of road in the Milan region with a traffic flow of 1,000 vehicles an hour. Nitric oxides on the asphalt surface were reduced by approximately 60%.
Among buildings in which the photocatalytic compound has been applied is the Dives in Misericordia Church at Tor Tre Teste, Rome, which was built in 2003 and designed by the U.S. architect Richard Meier. The church has three huge concrete sails which should remain totally white because of the self-cleaning properties of their TX Active surfaces.
Italcementi, which has also been marketing TX Active in the U.S., admits it could have a tough time raising demand for its product in Europe.
"Although we launched it last year, we are still not finding much demand for the product, even in Italy," said Alberti Ronzi, a commercial manager at Italcementi. "We are having to do a lot of promotional work to expand the market beyond just a small niche. We are having to create awareness of the technology not only among commercial customers but also among municipalities and other public sector organizations."
The company denies that it has been having any technical troubles with the formulation of its coatings, which contrasts with the experience of some coatings manufacturers and their raw material suppliers.
Akzo Nobel has shelved plans for the continued development of photocatalytic coatings for the architectural sector using TiO2 nanoparticles.
"We had certain technical difficulties with the coatings, one of which was that the photocatalytic activity caused the disintegration of the binders," said an Akzo Nobel official. "To make them work properly it appears you have to use certain types of binders which we don't regard as being suitable for our purposes."
TiO2 producers concede that the problems with nanosized particles of the pigments could extend to the whole formulation of the photocatalytic paints.
Nanoscale TiO2 particles are necessary for the coatings in order to maximize the surface area available to generate sufficient energy from the UV light to trigger a catalytic effect. But for the catalysis to operate efficiently the other components of the coatings, especially the binders and resins, have to be photo-resistant.
"It is not possible to use the same formulation as will conventional paints," said Tommy Book, a project manager at Kemira, Pori, Finland, a leading European producer of speciality TiO2 pigments. "Organic binder systems are not really suitable so there is more or less a need for new types of binders which may not be commercially available."
"Some Japanese companies seem to have dealt with these difficulties by reducing the photocatalytic capability of the coatings," continued Book. "The use of silicone binders, which is happening in China, does not appear to be acceptable to European coatings companies."
With Millennium, Kemira is among a number of the producers of the pigment which are, nonetheless, involved in schemes for the commercialization of TiO2-based photocatalytic coatings.
However, some scientists now reckon that TiO2 itself could be the underlying problem because it reacts to only a certain spectrum of UV light, which increases the risk of instability. Academia and industry in Europe have already come together to try to develop more efficient photocatalytic materials.