Road Paint
We drive by it everyday. What is road paint?
Road paint is a chemistry found all over the world across the millions of kilometers of paved roadway. These markings help to control traffic and keep drivers safe. In North America, these familiar white and yellow markings are perfect examples of an everyday chemistry application. Road paint must be durable enough to withstand harsh weather conditions and the normal wear-and-tear caused by vehicles. Road paint must be vibrant and reflective so drivers can see it even in the dead of night and it must dry quickly to avoid any traffic delays.
The first, and perhaps the most important component of a road paint formulation is its colour. Historically, different metals were used to colour paints. White paints were made from Titanium dioxide while yellow colours were made from Lead chromate. While the former is still used today, Lead chromate has been phased out because of its well-documented toxicity. The colouring agents employed today are called pigments, which can be either organic or inorganic molecules. Depending on the structure and bonding within the pigment, the molecule will absorb a certain wavelength of light. The wavelengths of light that are not absorbed are reflected and observed by us. Because of the vast arrangements of bonding and elements used in pigments, there are hundreds of coloured pigments used in the world of chemistry.
The pigments are then incorporated into acrylic paints, which is the most common road paint formulation. These paints, which are also popular amongst artists, use water as a solvent. The pigments are crushed into microscopic pieces and suspended in the solvent forming a colloidal solution. In addition to the pigments, molecules called binders are also found in the water. In acrylic paint, acrylic polymers are used as binders which keep pigments ordered after the paint dries giving the paint a consistent color.
As the white and yellow coloured acrylic paint is sprayed onto roadways, the water rapidly evaporates allowing the paint to dry quickly. This process forces the acrylic polymers and pigments closer together consequently driving out any existing water via capillary action force. As more water is forced out of the paint, acrylic polymers become so crowded with each other that they begin to deform and combine with one another to form a coherent polymeric film. This process creates the road paint we see everyday.
Before the paint dries, glass beads are often added on top to make the road paint more reflective. The glass is rounded to ensure no damage to vehicle tires and allows the glass to reflect light in many directions which makes it easy for the observer to see the paint. These glass beads are typically 100 μm in size which is about the size of a grain of sand!
I would guess many of you have never questioned the chemistry of something so familiar such as road paint. But would you have expected road paint to contain polymers, metals/pigments and glass beads?
