Anti-Graffiti Coatings
editGraffiti costs billions of dollars annually to clean up. Many cities have started anti-graffiti programs but vandalism is still a problem. Companies across the globe are attempting to develop novel coatings to prevent taggers from vandalizing public and private property. The coatings being developed can be the paint itself, or a clear coat added on top of existing paint or building facades. Depending on the substrate and the severity of graffiti, different coatings give different benefits and disadvantages [1].
Annual Expenses of Graffiti Worldwide:
| Annual Graffiti Costs | ||||
| Germany | over $700 million | |||
| London | $200 million | |||
| UK | $1.5 billion | |||
| Los Angeles | $140 million | |||
How Paint Works
editThere are two common types of paint used today. The first is water-based paints such as latex and acrylic paint, and the second is oil-based paints. The paint of choice will depend on the substrate to be painted upon and the desired end result. All paints have the same basic structure:
Pigment - This is the part of the paint that is seen by the eye. The pigment gives the paint opacity and color. The pigments of all paints contain a white base composed of titanium dioxide (TiO2) or zinc oxide (ZnO). Dyes are added to the pigment to attain the desired color.
Binder - the binder is the glue that holds the paint together. This is usually a polymer that upon drying will polymerize to keep the pigments homogenous and adhered to the substrate.
Solvent - this is the bulk of the paint, it is used to keep the paint workable when it is wet. After paint is applied to a surface the solvent evaporates, the pigment and binder will coalesce together to form a uniform coating. The solvent is water for water-based paints, and an oil for oil-based paints.
There actually is not a chemical bond between paint and an underlying surface. Paint adheres simply through physical forces like Van der Waals. When paint is first applied to a surface it goes on as a thick wet coating. As the solvent is allowed to evaporate out, the pigment plates which are attracted to one another stack up to form layers. The binder polymerizes essentially locking the pigment plates together. What you are left with is a uniform coating of binder and pigment. Anti-graffiti coatings make paints unable to adhere to the surface.
Different Types of Coatings and How They Work
editAnti-graffiti coatings can be invisible to the naked eye making them ideal for preserving historic buildings. There are two different categories of anti-graffiti coatings. The first, sacrificial coatings, are applied to a surface and then removed when graffiti is applied. The surface underneath will be left clean and a new sacrificial coating can be applied. The other type of coating are permanent coatings that prevent graffiti from adhering to a surface in the first place.
Newer coatings are comprised of charged polymeric materials that form a gel on the surface of the building or substrate. Some of the most important characteristics of anti-graffiti coatings are:
- Sufficient adherence without damage to historical buildings
- Hydrophobicity (water-repellance)
- Environmentally-friendly composition and processing
- Resistance to UV aging and weathering
- Good Cleaning Efficiency
Sacrificial Coatings
editA sacrificial coating forms a clear coat barrier over the wall or surface being protected. If the surface is vandalized the coating can be removed (sacrificed) using a high-pressure washer taking the graffiti with it. The coating then must be reapplied. The materials used to make a sacrificial coating are usually inexpensive optically clear polymers such as acrylates, biopolymers, and waxes. These polymers form weak bonds with the substrate to allow for easy removal.
Permanent Coatings
editPermanent coatings are often more expensive than sacrificial coatings, but if used appropriately only have to be applied once. These work by creating a protective surface that spray paint cannot bond to. After the surface has been vandalized, often all that is needed to remove the paint is a simple solvent (toluene) and some elbow grease. The underlying surface and the protective coating will remain undamaged.
Some of the types of permanent coatings include those based on polyurethanes, nano-particles, fluorinated hydrocarbons, or siloxanes. Polyurethane coatings are useful because of their barrier properties. High chain stiffness and high crosslinking density reduces the ability of the polymer to swell and absorb graffiti paint[3].
Fluorinated coatings are some of the most effective in the field of graffiti prevention. Fluorine is the most electronegative element, meaning that it shows very little affinity for the electrons of other elements. When fluorine is attached to a surface it will decrease surface energy at the interface, minimizing the contact with the graffiti paint. For the same reason that a Teflon-coated pan repels both water and oil, a fluorinated coating will repel water and oil-based paints. These coatings also have the added benefit of being chemically inert as well as very durable. Unfortunately, they are also expensive and can be difficult to apply[4].
Silicon based coatings are hydrophobic, which means the surface repels water. This reduces the effects of photo-oxidation of surfaces.
One of the newer additions to this ever growing market are nanoparticle based coatings. Silica particles are formed using the sol-gel method. The resulting silica particles have both reactive (Si-OH) and nonreactive (Si=O) groups on the surface. The reactive groups provide locations for further chemical processing, which allow you to change the surface properties of the nanoparticles. For anti-graffiti coatings, hydrophobic and oleophobic (oil-fearing) ligands are grafted onto the silica nanoparticles. Hydrophobic ligands are non-polar molecules such as hydrocarbon chains. Oleophobic ligands consist of polar molecules. Normally these two different types of molecules would phase separate in solution, for the same reason that water and oil do not mix. By chemically grafting the ligands onto the silica particles, this effect is counteracted. The effect is a coating that shows an equal dislike for both water-based and oil-based paints [5].
Anti-Graffiti Suppliers
edit| Bayer SEI Chemical |
- ^ Andrew Turley. 9 March 2009. Anti-Graffiti Paint Cleans Up On Historic Buildings. Chemistry and Industry. 9.
- ^ Castano, Victor M., and Rogelio Rodriguez. "A Nanotechnology Approach to High-Performance Anti-Graffiti Coatings." International Journal of Applied Management and Technology 2 (2004): 53-58.
- ^ CBS Interactive. BNET. http://findarticles.com/p/articles/mi_hb3226/is_5_5/ai_n29440435/
- ^ Jurgen Scheeder, N. Visscher, T. Nabuurs, and A. Overbeek. Oct 2005. Novel, Water-Based Fluorinated Polymers With Excellent Antigraffiti Properties. JCT Research. 2:8, 617-625.
- ^ Bayer MaterialScience. Wiping Out Graffiti. Bayer AG. 26 April 2009. <http://www.bayer.com/en/graffiti-resistant-waterborne-coatings.aspx>.