Reflective glass beads are used for pavement markings on all different types of properties. In fact, thousands of beads per square foot are bonded to our Tampa highways with a powerful binder. This is also true of retroreflective pavement marking materials and paints. Rather than scatter light like traditional paints, retroreflective paints have special glass beads that turn light back around and send it at the headlights of vehicles.
How Glass Beads Perform Retroreflection
In order for beads to retroreflect light, a couple of properties are required. The beads must be round and transparent, in order to reflect light.
If the glass beads are not transparent, light will not pass in and out of the spheres. The light ray is bent as it moves into the bead and down as the bead’s rounded surface takes shape. The light that strikes the back of the bead’s paint-coated surface is reflected off the paint. Only a minor amount of the light moves back to the source of illumination.
The Application of Glass Beads to Pavement Marking Materials
Glass beads can be added to pavement marking materials in three unique ways. They can be added right into marking materials prior to application. They can also be sprayed or dropped in the wet paint behind the sprayer. It is also possible to drop a portion onto the premixed portion of the thermoplastic materials or epoxy. The glass beads and paint should be of high quality so the paint girth and bead coverage creates the opportunity for extensive retro-reflectivity in each direction of road travel.
The light retroreflected from the glass beads is a function of three variables. The first is a combination of bead shape, size, and additional superficial characteristics. The second is the number of beads and exposure to light rays. The final component of reflected light is the glass beads’ index of refraction.
The Refractive Index of Glass Beads
Refractive Index, commonly referred to as “RI”, is a key bead characteristic. A bead with an especially high RI and minimal impurities in its glass material will prove that much more costly to manufacture. RI is a function of bead chemical makeup. A high RI means more light ends up retroreflected.
The beads used in traffic paint usually have a 1.5 RI. There are 1.65 RI beads out there as well as some 1.9 RI beads. Though the especially high refractive index can lead to enhanced illumination, highway and state agencies typically opt for economic efficiency with beads that are 1.5 RI.
These beads are usually 60 micrometers to 850 micrometers in size. The size of the bead is often expressed in terms of sieve number or size of the mesh screen the bead will pass on through. A bead with an 840-micrometer diameter will pass through Sieve Number 20 yet beads of a smaller diameter will not pass through a Number 200 mesh.
It is important to note inconsistencies can result in the size of beads is not always consistent with production runs. The marking material’s dry time alters the bead settlement to the binder. Even alterations in weather can affect drying time.
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