In 2008, John Baldwin and David Bauer, writing for the consultancy firm now known as Exponent, contributed a useful review on steel belt rubber oxidation in tires. They investigated how steel belt rubber oxidizes in practice, how this real-world data is useful for oxidation testing in the laboratory, and how such research may help industry design more-durable tires.
Inerting your tires with nitrogen may help prevent the oxidative damage reviewed by Baldwin and Bauer from occurring. Here, we’ll begin an overview of Baldwin and Bauer’s findings by introducing how scientists and engineers originally studied tire durability, and the limitations of such approaches in light of current knowledge and modern tire construction.
Tire researchers used to commonly believe the only factors limiting tire lifespan are (1) normal wear and tear, and (2) the higher temperatures occurring in a tire while driving.
In other words, rubber oxidation when a vehicle is not in use was not thought to be pertinent to tire durability.
As a result, for decades industrial tests of tire durability tended to focus on tire mechanics, e.g. driving vehicles for a set number of miles and evaluating the subsequent tread separation. This is analogous to how a typical driver inspects a tire for wear, checking for punctures, the integrity of the tread, and so on. However, modern tires have a thin layer of rubber that binds the steel belts together and binds them to the rest of the tire.
This thin rubber layer is built within the tire and a typical driver cannot easily discern when the steel belt rubber is damaged. Nevertheless, damage in this part of the tire can facilitate tread separation on the roadway, and possibly cause a serious accident.
Such tire failures led companies to issue massive tire recalls in 2000/2001 (primarily for sport utility vehicles), and spurred research and regulations of aged tires. In particular, the National Highway Traffic Safety Administration dissected many recalled tires to see why they were failing and what could be done about it.
As you may have guessed by now, the key villain in this story of oxidation is oxygen—present in air-filled tires but not in nitrogen-inerted tires. Stay tuned for a discussion of diffusion-limited oxidation, a topic that proves why tire durability testing protocols required updating.
- M. Baldwin and D. R. Bauer. “Rubber oxidation and tire aging – A review.” Rubber Chem. Technol., 2008, 81(2), 338–358. http://dx.doi.org/10.5254/1.3548213