Technological breakthroughs

Many drivers rely solely on tread wear to decide whether to replace their tires. However, a recent review study published in an international scientific journal shows that tires can become dangerous from the inside even when they appear intact on the outside—and most drivers are completely unaware of this.

The tire you are using may be aging day by day—not because of wear, but because of the very air you breathe, the sunlight hitting the road, and summer heat silently degrading the rubber structure from within. This is the key conclusion of a comprehensive scientific review recently published in Engineering Science and Technology by researchers from Politecnico di Milano (Italy) and Pirelli—one of the world’s leading tire manufacturers.

This is the first comprehensive review focusing specifically on tire aging—a factor which, according to the U.S. National Highway Traffic Safety Administration (NHTSA), contributes to approximately 90 deaths and more than 3,000 injuries annually in the southern United States alone.

THE NUMBER ONE ENEMY OF TIRES: OXYGEN AND HEAT

Among all factors that degrade tires, oxygen in the air is identified as the most dangerous. Oxidation occurs continuously, even when the vehicle is parked in a garage. At high temperatures—such as road surfaces in Hanoi or Ho Chi Minh City at midday in summer—the reaction rate increases significantly.

The chemical mechanism proceeds in three stages: initiation (heat or radiation creates free radicals), propagation (free radicals react with oxygen to form hydroperoxides), and termination. The result is two opposing yet harmful effects: polymer chain scission (making the tire softer and less elastic) and excessive cross-linking (making it harder, more brittle, and prone to cracking).

DID YOU KNOW?

• The elastic modulus and stiffness of rubber increase with tire age 
• Elongation at break and tensile strength decrease—older tires are more prone to sudden failure 
• Aged tires significantly increase braking distance in emergency situations 
• Oven tests: after 60 days at 100°C, tensile strength drops by up to 85%, elongation by 50% 
• Tire-related accidents peak between June and September, especially during daytime hours 

FOUR OTHER SILENT ENEMIES

In addition to heat and oxygen, the study identifies four environmental factors that are always present but often overlooked:

UV radiation from sunlight triggers photo-oxidation on the tire surface. The most visible result is fine cracks on the sidewall, often mistaken as cosmetic damage. In reality, these indicate a gradual loss of elasticity from the outside inward.

Ozone—a common pollutant in urban and industrial areas, especially in summer—directly attacks C=C double bonds in rubber polymer chains, causing cracks perpendicular to stress direction. Notably, ozone damage can be more severe than thermal and photo-oxidation combined.

Humidity penetrates the rubber structure and causes hydrolysis, breaking chemical bonds in the polymer network. When combined with high temperatures, moisture significantly accelerates material degradation. At relative humidity above 50%, the degradation mechanism of ozone may even change fundamentally.

“The most dangerous aspect is that these factors do not act independently—they interact synergistically in nonlinear ways that single-factor tests cannot predict.” — Research team, Politecnico di Milano

HOW DOES SCIENCE MEASURE TIRE AGING?

To understand how quickly tires age, scientists use two main approaches:

Long-term field studies—where tires are used in real conditions and monitored over time—provide the most realistic results but take years. One study analyzed over 1,500 tires from various U.S. cities and found that even tires of the same age and location performed differently depending on parking habits (sun vs. shade), tire pressure, load, and driving style.

Accelerated aging tests in climate chambers use elevated temperature, ozone concentration, UV exposure, and humidity to shorten testing time. For example, simulating a 6-year-old tire used in Arizona’s summer conditions requires only about 10 weeks at 70°C. However, if temperatures are too high, degradation mechanisms may change, leading to inaccurate predictions.

To estimate tire lifespan from lab data, scientists use the Arrhenius equation (chemical modeling) and the Williams–Landel–Ferry equation (viscoelastic modeling). Both have limitations, as real-world tires are exposed to multiple interacting factors simultaneously.

MATERIAL CHALLENGES: FROM GRAPHENE TO MICROPLASTICS

The tire industry is actively responding to these findings. Manufacturers are experimenting with nanofillers such as graphene, carbon nanotubes, and nano-silica to improve thermal conductivity, mechanical strength, and resistance to oxidative aging. Hybrid filler systems combining silica and carbon black are also being developed to balance durability and rolling resistance.

However, a more concerning issue lies beneath: tires are a major source of microplastic pollution. As rubber wears, fine particles are released into soil, rivers, and oceans—and even enter the food chain. More critically, the antioxidant 6PPD—widely used in tires—can oxidize into 6PPD-quinone, a compound proven to be acutely toxic to certain aquatic species.

DID YOU KNOW?

• 6PPD-quinone: an oxidation product of a common tire antioxidant, toxic to aquatic life 
• Tire wear particles (TWP) are a significant source of microplastics in urban environments 
• “Green tires” using silica instead of carbon black reduce CO₂ emissions but may have lower wear resistance 
• Most discarded tires are incinerated in cement kilns, landfilled, or ground into materials for road construction 

WHAT REMAINS UNKNOWN?

The study candidly highlights major gaps in current knowledge. Most experiments are conducted on small rubber samples in laboratories—not full tires under real operating conditions. Very few studies simultaneously incorporate mechanical load, temperature, ozone, UV radiation, and humidity—even though these are exactly the conditions tires face daily on the road.

The authors call for the development of next-generation climate chambers that integrate dynamic mechanical loading with environmental variables, the establishment of standardized international testing protocols, and the application of machine learning to build more accurate tire lifespan prediction models from large datasets.

“Future research must integrate multi-factor testing, dynamic loading, and environmental impact assessment—this is the only path toward safer, more durable, and more environmentally friendly tires.” — Stabile et al., 2025

Source:

Pietro Stabile, Gianpiero Mastinu, Massimiliano Gobbi (Politecnico di Milano) and Perla Bardini (Pirelli Tyre S.p.A.), “Tire aging: A state-of-the-art review”, Engineering Science and Technology, an International Journal, Vol. 69, 2025, 102101. DOI: 10.1016/j.jestch.2025.102101. Published online June 28, 2025.