Nanoscale mechanism behind tire degradation revealed

Tires used for automobiles, trucks, buses, agricultural machinery, and construction vehicles have steel cords embedded inside them for durability and handling stability during driving. If separation occurs between these steel cords and the tire rubber, it leads to tire damage or blowouts, so extremely high adhesion and long-term degradation resistance are required.

The surface of the steel cords is coated with brass (an alloy of copper and zinc) plating. During the vulcanization process, this brass plating reacts with sulfur compounded into the rubber to form a complex adhesion interface layer consisting of copper sulfide and zinc sulfide, achieving strong adhesion with the rubber.

In the conventional fabrication process organic cobalt salts added to rubber suppresses both the excessive growth of the adhesion interface layer, and the decline in adhesion under high-temperature and high-humidity environments. Their addition to tire rubber has become widely popularized. However, organic cobalt salts presented a dilemma. In the long term, they trigger metal-induced damage that promotes rubber degradation through oxidation, becoming a factor that impairs the durability of the entire tire. Furthermore, because cobalt has raised environmental and health concerns and is a scarce resource with high supply risks, there is a demand to optimize its usage and develop alternative substances.

To achieve this, it is essential to understand at the nanoscale what state the organic cobalt salts are in and by what mechanism they function at the adhesion interface, but this had not been clarified until now.

A joint research group consisting of Associate Professor Yohei Sato and Associate Professor Tomohiro Miyata of the Institute of Multidisciplinary Research for Advanced Materials at Tohoku University, and Principal Researcher Katsunori Shimizu of Yokohama Rubber prepared samples in which brass-plated steel cords were embedded in natural rubber added with cobalt stearate, which is a representative organic cobalt salt, and vulcanized. They specifically analyzed the rubber-brass adhesion interface structure using several observation and analysis methods based on scanning transmission electron microscopy.

As a result, they successfully observed for the first time in the world that cobalt is localized at a high concentration at the adhesion interface after vulcanization. Furthermore, they clarified that cobalt exists as divalent cobalt sulfide nanoparticles.

The group quantitatively evaluated the relationship between the spatial distribution of these cobalt sulfide nanoparticles and the thickness of the adhesion interface layer. As a result of this evaluation, they found that the cobalt sulfide nanoparticles function as a barrier layer that suppresses the elution of copper and zinc from the brass into the rubber, thereby suppressing void formation inside the brass. Furthermore, they analyzed the adhesion interface after degrading the same sample under a high-temperature and high-humidity environment. In this analysis, they found that copper and zinc eluted into the rubber mainly from regions where the cobalt sulfide barrier layer was sparse, expanding the voids inside the brass.

From preceding research, it is known that voids inside brass serve as initiation points and propagation paths for cracks. Therefore, it can be said that the suppression of void expansion through the formation of the barrier layer is the microscopic mechanism by which the addition of organic cobalt salts improves the adhesion and degradation resistance at the interface between rubber and brass-plated steel cords.

The mechanism by which the addition of organic cobalt salts improves the adhesion and degradation resistance at the rubber-brass-plated steel cord interface, which had been unexplained for many years, was clarified at the nanoscale for the first time.

The insight that organic cobalt salts form a cobalt sulfide barrier layer at the adhesion interface is expected to accelerate the development of alternative substances with low environmental impact and supply risks, leading to the realization of next-generation high-durability tires that also contribute to safety. The study was published online in Rubber Chemistry and Technology.

Journal Information
Publication: Rubber Chemistry and Technology
Title: NANOSCALE DISTRIBUTION AND ROLE OF COBALT AT THE INTERFACE BETWEEN BRASS-COATED STEEL WIRE AND RUBBER
DOI: 10.5254/rct.25.00050