Why are tires made of rubber? Two professors at Zhejiang University have solved a century-old puzzle.

　　In a laboratory on the Yuquan Campus of Zhejiang University, a research group has been studying rubber reinforcement for 17 years and has finally achieved a new breakthrough: the “Song–Zheng” two-phase model theory. In September 2016, the group’s invited review article was published in the prestigious journal Progress in Materials Science. This theory, named after the researchers’ surnames, breaks free from the constraints of Einstein’s viscosity theory, resolving the longstanding inconsistencies among previous theoretical models that had hindered their practical application in production, and holds the promise of significantly impacting the manufacturing of rubber materials.

　　The “necklace” and the “grape” in the tire

　　Professor Song Yihu of the Department of Polymer Science at Zhejiang University is one of the key members of the “Song–Zheng” two-phase model research group. In his office, there is a transparent block of “modeling clay,” which he explains is raw rubber—essential for manufacturing tires. “Rubber has the highest molecular weight among polymeric materials. If we liken a rubber molecule to a string of molecular beads, it takes millions of these ‘pearls’ linked together to form a single rubber molecule. These long molecular chains become entangled with one another, creating a network of interwoven strands. The longer the chains, the better the elasticity.”

　　Why are tires black? Song Yihu explains that this is due to a substance called carbon black. “It is a form of elemental carbon, with a structure resembling clusters of grapes, where each ‘grape’ is a carbon particle about 20 nanometers in diameter. A single speck of soot visible to the naked eye corresponds to a ‘grape cluster’ roughly 100 nanometers (0.1 micrometer) across.”

　　Over more than a century of tire industry development, carbon black has proven to be the optimal filler. In the tread compound of a tire, rubber accounts for roughly 50%, while carbon black makes up over 40%. With the assistance of more than 50 types of additives, these two components are thoroughly blended during the mixing process and, following vulcanization, yield a robust, wear‑resistant tread compound.

　　The combination of “necklaces” and “grape clusters” endows the tire with robust mechanical properties. Professor Zheng Qiang, another key member of the research team, explained: “Numerous carbon black ‘grape clusters’ assemble into a three-dimensional network, while rubber molecules—like ‘necklaces’—wrap around them. Upon vulcanization, these components form permanent cross-links, resulting in an inextricable tangle that we call ‘crosslinking.’ It’s akin to how cooking a raw egg causes its proteins to denature.”

　　At least five major theoretical schools have emerged on the international stage.

　　High strength and excellent elasticity are essential characteristics of a good tire, particularly for heavy‑duty trucks. Moreover, different operating conditions impose distinct performance requirements on tires.

　　“Especially with the heavy trucks that haul ore, when their tires roll over sharp rocks, we must ensure that the tread isn’t punctured,” said Zheng Qiang. Over more than a decade of research, the team has been most intrigued by how the carbon black network interacts with the rubber network and how the filler—carbon black—affects the viscoelastic properties of the rubber network.

　　On this issue, it has been discussed in textbooks and in the scholarly literature. Song Yihu and Zheng Qiang, through systematic analysis, reviewed more than 700 publications and over a hundred theoretical models—both widely and less commonly used—and found that, internationally, at least five major theoretical schools have emerged concerning rubber modulus (note: a parameter related to hardness). However, these models are either internally inconsistent or conflict with existing empirical findings.

　　To this end, the industry and academia operate in parallel—each pursuing its own agenda. This disconnect has kept advanced tire manufacturing firmly within the realm of proprietary “trade secrets” held by the major tire manufacturers, where whoever can crack the formula reaps the profits.

　　A “bias” that has existed for over a century has been discovered.

　　“There is international consensus that carbon black forms a three-dimensional network and provides mechanical reinforcement, and we have confirmed this through experiments,” said Song Yihu. What leaves them rather puzzled, however, is that all existing theoretical models focus solely on the carbon black network, neglecting the role of rubber molecules within it. These models are overly simplified, leading to biases in their interpretation of reality. “In industry, tires are made with rubber as the base material, yet no theory has been able to articulate how rubber contributes to the material’s properties. So we’ve been working while simultaneously reflecting: if a theoretical model lacks even a single parameter capable of capturing rubber’s influence on the material’s elasticity, then that theory is clearly immature.”

　　From an academic standpoint, this research falls within the realm of the viscoelasticity of polymer nanocomposites. The earliest studies on this system are attributed to Albert Einstein, who around 1911 published several papers on the viscosity of suspensions, analyzing the influence of particles on the fluid’s flow field. Although subsequent research has involved changes in the material systems used for tires, the most successful and widely applied theories for predicting viscosity still stem from modifications and refinements of Einstein’s equation.

　　“A fundamental question is: why must tires be made from rubber compounded with fillers, rather than from a water‑based suspension system? Therefore, the influence of rubber on material properties cannot be overlooked,” said Song Yihu. He added that it is time to reexamine this “prejudice” that has persisted for more than a century.

　　It is the rubber molecules that cause the tire to heat up.

　　The research group employed a solvent‑leaching method to extract rubber from uncured compounded rubber, thereby obtaining an intact carbon black network. This network exhibits both strength and elasticity. Additionally, by using a high‑temperature ablation approach, the group also succeeded in obtaining an intact carbon black network.

　　The reporter examined a bulk carbon black network sample obtained via ablation. With a gentle touch, the network crumbled instantly, much like a waffle. Song Yihu explained that the surface of carbon black particles exhibits adsorptive properties: when long rubber molecules approach, the carbon black binds to a specific point along the polymer chain, forming an exceptionally stable interfacial structure. “Even if you pull out a few ‘necklaces’ that haven’t been adsorbed, the structure will still collapse.” However, the carbon black network produced by high‑temperature ablation lacks these rubber‑molecule “necklaces,” rendering it unstable and brittle.

　　“Previous models only accounted for the carbon black network, but the experimental evidence was weak. Our two-phase model, by contrast, takes rubber into consideration,” said Song Yihu. This model offers a new theoretical explanation for the heat generated during tire rolling: it is not the carbon black that produces the heat, but rather the rubber molecules themselves.

　　Along the sides of highways in summer, we often see heavy‑truck drivers carrying buckets of water to cool their tires. Without such cooling, the tires would age more rapidly. Conventional wisdom holds that heat is generated by the friction between carbon black particles within the rubber. “Our experiments, however, reveal the opposite: it is the friction among long rubber polymer chains that produces the heat. Carbon black particles merely accelerate this intermolecular friction.”

　　Expectancy theory can be applied to guide practical production.

　　Zheng Qiang explained that the key to the two-phase model lies in redefining the roles of carbon black and rubber in determining tire performance, thereby elucidating the mechanism of tire reinforcement: filler particles such as carbon black form a network structure that restricts the mobility of rubber molecular chains, thereby enhancing the elasticity of the tire tread compound.

　　The research group also prepared compounded rubber materials using other fillers, including precipitated silica; their rheological behavior all conforms to a two-phase model, which some fellow scientists have dubbed the “Song–Zheng two-phase model.”

　　“I wouldn’t dare claim that our theory has put an end to the existing debates, but one thing is clear: internationally, studies based on earlier models are gradually becoming less common, while papers citing our theory are increasing year by year,” said Song Yihu. He added that the research team plans to conduct further experiments to explore ways of reducing friction between polymer chains. “We hope our theory can be applied to guide industrial production, enabling the development of high‑performance tires with superior strength, excellent elasticity, and low heat generation.”