Innovation: Graffiti artists on the skids with non-wettable coating
Sunday 06 March 1994
The Dow product - as yet nameless - forms a hard, clear film that can be easily applied to all manner of surfaces. When a marker pen is drawn across it, the ink beads up, leaving most of the surface uncontaminated. The ink can then be washed away with water.
Moreover, the Dow scientists claim they have yet to find a glue that will stick to their product.
Applications of the material are likely to extend well beyond anti-graffiti coatings.
Because hardly anything will stick to it, the coating would be useful in any situation in which a surface needs to be kept free.
Barnacles could be deterred from the hulls of ships, eggs from the surface of frying pans, and grime from the bodies of cars.
The material, which is described in the latest issue of the science journal Nature, has been licensed to 3M Corporation for commercial exploitation.
The spreading of liquid on a surface, or the adhesion of contaminants, results from the surface's relatively high energy. Because they are exposed, surface molecules have a higher energy than those buried within the bulk of the material.
Liquids will spread, or particles of dirt stick if, by covering up the exposed surface molecules, they can reduce the overall energy.
So the ideal non-stick material would have a very low surface energy to start with, ensuring that there is nothing to be gained by covering the surface with other matter.
The polymer Teflon, currently the most familiar non- stick agent, derives its low surface energy from the presence of fluorine atoms at the surface.
But because a film of the polymer is actually porous on the microscopic scale, grime can get down into the cracks and lodge there.
The new material also derives its low-energy surface from fluorine atoms. It consists of short strands of Teflon-like polymer chains, which are attached to a backbone of a different polymer, like the legs of a centipede.
When laid down on a surface, the fluorine-coated chains all line up, pointing up from the surface like an array of bristles, to present a highly orderly, very low-energy outer layer.
The polymer backbone contains appendages, bearing an electrical charge, that make it soluble in water.
Once a film of the material has been deposited from solution on to the surface, a 'cross linking agent' is added to make the film robust.
When cured by moderate heating to between 30 and 120C , this agent forms links between neighbouring polymer chains, joining the whole assembly into a tough network of strands.
As both the film-forming molecules and the cross-linking agent are water-soluble, the coatings can be prepared in a safe and convenient way.
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