Heavy metals, rare earths and conflict minerals can all be found in tires along with trace amounts of naturally occurring, but radioactive, carbon-14 (or 14C). Does this mean that tires, like many other widely used items, are instinctively suspect for environmental or social reasons because they contain such materials? Not so, once relevant facts are known.
For example, the radioisotope 14C exists at minuscule levels (one part per trillion) in all carbon-based molecules except those derived from crude oil, coal or natural gas due to its half-life decay in long-buried fossil fuels. Further, radial tires of all types are principally composed of three innocuous elements – the stable isotope carbon 12C (73%), iron (13%) and hydrogen (6%) – which total 92% of tire weight. More than 20 other chemical elements in lesser amounts make up the remaining 8% of the tire, including the non-metallic elements nitrogen, oxygen and sulfur along with the heavy metals copper, zinc, tin and cobalt; minute amounts of other materials are present. Prescribed in precise quantities, each material has a specific purpose and contributes in different ways to enhance physical properties, facilitate manufacturing and improve tire performance. For example, brass, an alloy of copper and zinc, coats the small diameter steel cord filaments, which aids wire drawing and provides adhesion to the belt compound. Bronze plating, mainly composed of copper and tin, provides a hard protective coating on bead wire for factory operations and ensures adequate bonding to vulcanized rubber.
The term “heavy metal” has a negative connotation to many individuals but has no consensus definition within the scientific community. Humans require varying amounts of six so-called “good” heavy metals (iron, cobalt, copper, manganese, molybdenum and zinc) to support life. However, excessive levels of these needed elements can damage living organisms. Cobalt, not only a heavy metal but a potential conflict mineral, has been used for more than six decades in steel belt skim stocks to promote cord-rubber adhesion, but with adverse effects on rubber properties such as crack growth and aging. Cobalt usage in belt compounds could be eliminated by a Bekaert process (now in pilot tire trials) using a ternary alloy of copper, zinc and 4% cobalt that coats the steel cord. In small PCR tires, this technology could remove a little more than 1g of cobalt from the skim rubber which is replaced with less than 0.1g on the steel cords – a small, but significant, reduction. Larger reductions are possible in TBR tires featuring cobalt containing belt and body ply compounds. Zinc oxide, previously added to all rubber recipes at about 5phr, enhances accelerator performance and shortens cure cycles, but is of potential concern because zinc is contained in tread wear debris; associated leachate may find its way into streams and lakes via rainwater runoff, possibly impacting aquatic health. Thus, for the past decade most tread rubber compounds have featured greatly reduced amounts of ZnO in their formulations.
The polymerization of tire grade polybutadiene involves the use of a small amount of one of the heavy metals – cobalt, nickel, titanium – or the rare earth, neodymium. Polybutadiene catalyzed by neodymium is growing in usage due to its ability to slightly lower tire rolling friction. Such catalytic chemicals remain in cured tires at measurable levels in parts per million. Tires are not alone in using atypical minerals that can cause an overzealous environmental reaction or stir up a social controversy. For example, vehicle electronics are dependent on a multiplicity of rare earths that may be considered to be conflict materials. And, auto emissions systems cannot meet today’s environmental standards without using precious metals – platinum, palladium and rhodium.
Before production release, tire materials require a rigorous review for potential environmental, safety and health-related effects – real or perceived. Importantly, the major tire company CEOs representing approximately 75% of the global tire manufacturing capacity have had an ongoing commitment (since 2005) to identify and address, among other topics, the potential lifecycle impact of chemicals commonly used in tires. This is done under the auspices of the World Business Council for Sustainable Development. Technical reports written to date by reputable, independent third-party organizations treat
tread wear debris, nanoscale materials and end-of-life tire management. Details can be accessed at wbcsd.org. Read on!