Pittsburgh, Pennsylvania - Until recently, graffiti-resistant coatings were only available in solvent-based formulations that exhibited a glossy finish, but two-component waterborne polyurethane coatings are now available for concrete, which do a better job environmentally, according to Bayer MaterialScience.
Such ultra-low VOC (volatile organic compound) waterborne polyurethane coatings also achieve an unmet need in the construction market, said BMS: graffiti resistance with a matt finish.
Development of these graffiti-resistant, two-component waterborne polyurethane coatings, their properties, and tests examining the reasons they exhibit good graffiti resistance, was discussed at PACE 2009, the annual conference and exhibition, sponsored by the Painting and Decorating Contractors of America and the Society for Protective Coatings, 15-18 Feb 2009, in New Orleans.
Peter Schmitt, senior technology manager and Kathy Allen, associate scientist, both of BMS llc, presented a paper called Now You See It Now You Don't: Waterborne Polyurethane Graffiti Resistant Coatings.
The authors said it is generally accepted that, to be easily cleaned of graffiti, a surface coating should have a lowersurface energy than that of the graffiti paint. Hence the graffiti finds it difficult to adhere to the coating and is easily removed. But BMS tested waterborne coatings with higher surface energies, which were still easy to clean of graffiti.
Schmitt and Allen brought another factor into the equation: using atomic force microscopy (AFM), they proved that surface roughness plays a role. Graffiti sticks well on a surface with imperfections -- common in coatings with a matt finish. However, without pores to serve as anchors for graffiti paint to stick, "a surface with a roughness between 10 and 300 nm is easily cleaned," they found. Both glossy and matt finishes can be achieved in that range.
A coating needs a delicate balance of characteristics to achieve a matt finish and remaining graffiti resistant, Schmit and Allen found. Important factors are reaction rate and coalescence speed as a coating changes from discrete particles to a continuous film, without the alignment a glossy surface has.
Because two-component waterborne PU coatings are made up of two types of particles, there is competition between the physical change (coalescence) and the chemical reaction (creating urethane groups).
Nevertheless, "We conclude that with waterborne polyurethane coatings, it is possible to combine good cleanability in an environmentally acceptable system," said Schmitt.