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TINTA FOTOCATALÍTICA

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Self Cleaning Paint: Introduction of Photocatalytic Particles into a Paint System

PhD Thesis Sverrir G. Gunnarsson

” ABSTRACT

Photocatalysts are generally known to have adverse effects on organic coatings due to the highly reactive chemical species created by the photocatalytic reaction, which can damage the coating itself. The novel strategy for integrating the photocatalyst into the coating proposed in this work however offers most of the benefits of such self-cleaning coatings without the disadvantages.

It has been demonstrated that TiO2 coated microspheres give organic paint films self-cleaning properties without causing severe degradation of the binder. The coated microspheres improve the durability of the paint films both in accelerated weathering test and during outdoor exposure. The self-cleaning films are found to increase the films resistance to micro-organisms due to the photocatalytic activity and through improved crack resistance and lower water permeability.

Heat treating the microspheres before coating them was found to drastically improve the adhesion of the coating to the glass. It was concluded that this was due to physically adsorbed water occupying the main adsorption sites for the titanium precursor. Removing the adsorbed water allows the precursor to react with hydroxyl groups on the glass surface and form a covalent bond with the surface. Furthermore it was found that during the crystallisation of the coating the calcining temperature should be as close to the temperature tolerance of the microspheres as possible to optimise the photocatalytic activity.

The photocatalytic activity of the paint films was found to be highly dependent on the concentration of microspheres in the surface. Pigmented films containing coated microspheres exhibited excellent UV stability which caused the surface concentration to remain relatively low compared to the volume concentration for the entire duration of the QUV testing period. The surface concentration in transparent films on the other hand quickly becomes high due to their lower UV stability. Coated microspheres are however not found to be suitable for use in transparent paint systems due to negative effects on film colour after extended periods of exposure. Uncoated microspheres are however found to impart better properties mainly due to the improved cracking resistance of the films and are therefore suggested as a substitute in transparent films.

The particle size of the microspheres is one of the key properties when designing the paint system and the optimal size is in the range of 5-20μm. If the spheres are too large the permeability of the film will be too high and if the particles are too small they may separate from the film prematurely. Small particles however have the advantage of giving better surface coverage relative to volume concentration as compared to large particles.

The effect of the microspheres on the viscosity profiles of both solvent borne and water borne paint systems is found to be minimal and it should therefore be possible to adjust the viscosity to that of the standard by minor adjustments of the formulations.

The mechanical properties of films containing microspheres are found to generally become poorer with increased concentrations of spheres. At high concentrations the films become less elastic, the elongation to break is less, stress relaxation becomes slower and the creep recovery is significantly poorer. Nevertheless the mechanical performance of the films during exposure is improved due to a different mechanism for crack propagation in the film.

The microspheres are found to reduce the gloss of paint films which makes incorporation into high gloss systems difficult or altogether impossible.”

http://orbit.dtu.dk/fedora/objects/orbit:127743/datastreams/file_ba7cd53e-c311-4366-8d2f-a696d6f9c8f3/content

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