Transmitted Off Crossed Polarized Light Illumintation
Tire wear particles are a carbonblack pigmented mixture of natural and synthetic elastomers (rubber) with various mineral
and fiber fillers and reenforcement. They typically contain from 30% to 45% by weight elastomer. The balance of the
weight in the particle is pigment and filler material. Calcite and quartz are two common fillers but wood sawdust, glass,
clay, talc, feldspar and other minerals may also be used. Tire wear particles are generated by abrasion as a result of the
friction caused by the relative motion between a rubber surface (tire, drivebelt, or conveyer belt) and a non-rubber
surface (road, floor, or drive wheel). The shape and size of the particle is determined by the relative properties of the
rubber surface, the non-rubber surface, and the force exterted between the two. Truck tires contain more natural rubber
by design and that results in longer tapered cylinders than is the case with automoble tires under the same conditions.
Very long tapered cylinders are created by the tires of vehicles used in warehouse environments. This is due to the
relative smooth surface, which creates larger uniform microenvironments on the contact surface, and the higher rubber
content in the tires needed to get traction on the smooth floor.
Drivebelt wear is very similar to tire wear but the associated particle assemblage is different (see below).
Significance in the Environment:
The amount of tire wear particles in an environmental sample indicates the relative proximity to vehicular traffic and the
relative ease of transport from that source to the sample site. For indoor environments the population is normalized
against skin flakes, paper fiber, and clothing fiber. For outdoor environments it can be normalized against the plant and
fungal background. High concentrations of tire wear generally indicate a high exposure to vehicle emission.
"Tire wear" particles may also indicate wear of black rubber drive belts in the HVAC system or in other locations where a
black rubber surface rubs against a non-rubber surface.
Tire wear particles are characterized by their very rough texture and "fractile" outline. The particles tend to be tapered
cylinders with the aspect ratio determined by the texture of the non-rubber surface and the amount and type of elastomer
in the tire or rubber belt. Thin edges of the particle are transparent, depending on the concentration and distribution
of the black pigment. Filler minerals typically protude from the edges of the particle at a few locations along the edge
and can be seen between crossed polarizing filters. Off crossed circular polarized light (slight rotation of one of the
polarizing filters) can be very useful in order to see the black particle outline as well as the birefringent filler
particles. Darkfield reflected light is also very useful to characterize the surface texture of the particle.
Tire wear particles can be destinguished from large combustion particles by the presence of small highlight reflectivity
on the tire wear particles caused by the refractive indix difference between the rubber and the mounting medium and the
larger granularity of the rubber particle surface.
Tire wear particles can be destinguished from large cenospheres (carbonized liquid fuel, typically from Bunker C fueled
boilers) by the higher reflectivity of the cenosphere surface due to the graphine structure of the carbon in the cenosphere.
Tire wear particles can be destinguished from charred wood and plant material by the cell morphology and higher reflectivity
of the charred plant material.
The tire wear particles in this image have a high aspect ratio. This is from a tapelift collected in a home along a major
bus route. The high aspect ratio cylinders are from bus tires in this case.
Tire wear is typically associated with a road wear mineral content greater than that of the tire wear component of the
sample. If tire wear exceeds the road wear mineral content then a parking garage environment may be indicated. If that
is not the case then another black rubber wear source must be present. Pollens, spores, plant parts, and other outdoor
particles are generally associated with the tire wear. If they are less evident then mechanical transport of tire wear
from outside is indicated and other mechanically transported particles; such as moss fragments, etc.; should be present.
Drive belt wear particles are typically associated with the HVAC blower in office and school environments. If the drive
belt for the blower is the source then other typical HVAC agglomerates will also be seen in the sample. If they are not
seen then another source of black rubber wear particles should be found.