Technological breakthroughs

Compiled by Bao Hien

A team at Harvard University has found a way to mix natural rubber with silica without breaking its polymer chains, making the material about ten times tougher. A separate team at the University of South Florida has explained why filler particles make rubber stronger, an effect the tyre industry has relied on for nearly a century without fully understanding it.

Tyre rubber is almost never used in its pure form. Manufacturers add reinforcing fillers, most commonly carbon black and silica, so that tyres are stiff enough to carry heavy loads. Two studies published in the Proceedings of the National Academy of Sciences (PNAS) in April 2026 approach this from two sides. One looks at how fillers are mixed in, the other at how they reinforce rubber.

Keeping polymer chains long

Natural rubber is harvested as latex from trees. It is the world’s most widely used bio-based elastomer and the main material in tyres for trucks, buses and aircraft. Its long molecular chains make it elastic and resistant to cracking.

The problem lies in mixing. Conventionally, dry rubber, silica and additives are mixed in high-intensity machines, which break the polymer chains (chain scission). The resulting tyres are stiff enough but less elastic, crack more easily and wear faster, releasing more tyre dust into the environment.

The team led by Professor Zhigang Suo at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) took a different approach. They first dissolved the latex in toluene to separate it into individual chains. Toluene was chosen because its polarity is similar to that of rubber. Silica powder was then blended into the solution at low intensity, so the chains stayed long. The mixture was dried into a solid compound, then moulded and cured. During curing, the long chains form sparse crosslinks with one another and bond to the silica particles.

The dissolving step is essential. If silica is added directly to latex, the particles remain in the water phase and do not penetrate the rubber particles.

According to the researchers, the material’s toughness, the energy it absorbs before a crack spreads, rose about tenfold. Its stiffness remained high, at about 19 MPa. The team also applied the method successfully with carbon black and with several grades of silica.

Co-author Yakov Kutsovsky said electric cars are heavier than conventional cars, so their tyres wear out faster. This makes more durable materials all the more necessary.

In the near term, the results could be applied to truck, bus and aircraft tyres, which already use large amounts of natural rubber. In the longer term, the team hopes to make passenger car tyres mainly from bio-based rubber instead of petroleum-based synthetic rubber, if cost and supply challenges can be solved.

The study measured crack resistance and fatigue but did not test abrasion directly, and no finished tyres have been made yet. The authors say toluene is widely used in industry and can be recovered in closed-loop systems. However, they have not published any cost estimates for scaling up the process.

Answering a question nearly a century old

Carbon black has been mixed into rubber for nearly 100 years, but scientists still debate why it makes rubber stronger. There are three main hypotheses. The first holds that the particles link up into networks inside the rubber. The second holds that the particles stick to and stiffen the surrounding rubber. The third holds that the particles take up space, forcing the remaining rubber to stretch more.

Carbon black particles are nanoscale, which makes them very difficult to observe directly. So a team led by Professor David Simmons at the University of South Florida (USA) used molecular dynamics simulations involving hundreds of thousands of atoms. The team ran 1,500 simulations, totalling about 15 years of computing time, on a computer cluster over many months. The initial models did not match real-world data. The team had to adjust how the structure and dispersion of carbon black were represented before the results agreed with experiments.

According to the researchers, the key is volume. Rubber is almost incompressible. When a rubber band is stretched, it gets longer but also thinner, so its volume stays almost the same. Carbon black particles stop the rubber from thinning as it normally would. So when the material is stretched, it is forced to increase in volume, which rubber strongly resists. This resistance is what makes the material stiffer and stronger.

The team calls this mechanism “Poisson’s ratio mismatch”. The three earlier hypotheses are not ruled out. According to the team, all three effects contribute to this mechanism.

Professor Simmons said tyre companies still mainly rely on “trial and error” when developing new materials. He said the findings lay the groundwork for designing tyres deliberately. The aim is to better balance the tyre’s “magic triangle” of fuel efficiency, grip and durability, three properties that usually have to be traded off against one another.

This was a simulation study. Its benefits for tyres remain potential and have not been tested in products.

References
• Tan M. W. M., Nian G., Chen Z., Bao X., Kutsovsky Y., Suo Z. Amplifying toughness in silica-reinforced natural rubber by preserving long chains. PNAS, 2026. DOI 10.1073/pnas.2530834123
• Kawak P., Bhapkar H., Simmons D. S. Glassy interphases reinforce elastomeric nanocomposites by enhancing volume expansion under strain. PNAS, 13 April 2026. DOI 10.1073/pnas.2528108123