Thermoplastic Paint Marks Arizona’s Roads

Thermoplastic paint is a very important part of Arizona's highways. The shiny material marks lane lines, speed limits and even turn lanes. However, it has to meet a certain standard of reflectiveness before it can be used.

That is where ADOT's Chemical Lab comes in.

Part of the Materials Group, the Chemical Lab tests every batch of manufacturer thermoplastic or "hot melt" paint to ensure it is reflective enough and has a certain percentage of other ingredients such as binder and pigment. For example, white paint must have at least 40% glass beads, 20% binder content and a yellowness index of less than 0.12. Yellow paint must have at least 20% binder content and 40% glass beads.

Transportation engineer specialist Matthew Nickday says each batch test takes eight hours. The paint powder is melted in a kettle at 424 degrees. Then, a small portion is placed in a small metal dish and dried. Pieces of the dried paint are baked in an oven in small crucibles, which are then rinsed out with acid in large beakers. (Are you reliving high school chemistry class?) Water is then added to the acid and the resulting liquid is poured into a small tub, which reveals glass beads at the bottom of the beaker.

Those beads are dried in an oven and weighed, and the sample's percentage of glass beads is calculated.

When the paint batch meets standards, a certificate is issued for the lot and the material used throughout the state.

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