For nearly a century, engineers have relied on reinforced rubber without fully knowing why it works so well. Now, researchers at the University of South Florida say they have pinned down the mechanism that makes rubber much stronger when mixed with carbon black particles.
The team, led by engineering Professor David Simmons, published the findings in the Proceedings of the National Academy of Sciences after running 1,500 molecular dynamics simulations that added up to roughly 15 years of computing time.
“How is it that we’ve been using this for 80, 90, 100 years and haven’t really known how it works?” Simmons said. “It’s been through enormous trial and error. The tire companies can purchase many different grades of carbon black, basically fancy soot, and they just have to use trial and error to figure out what’s worth paying more for and what isn’t.”
Reinforced rubber is used in car and airplane tires, industrial machinery, medical devices and garden hoses. Manufacturers have long mixed microscopic particles, usually carbon black, into rubber to make it tougher, longer-lasting and more resistant to wear.
Scientists had proposed several explanations for the effect. Some said the particles formed chain-like structures through the rubber. Others said they stiffened the surrounding material like glue. Another theory said the particles mainly took up space and forced the rubber to stretch differently.
The USF researchers said those ideas each captured part of the picture.
Working with USF postdoctoral scholar Pierre Kawak and doctoral student Harshad Bhapkar, Simmons used computer simulations to model how hundreds of thousands of atoms behave inside reinforced rubber. The researchers also refined earlier models to better represent the shape and distribution of carbon black particles.
“It’s not that we literally had a simulation running for 15 years,” Simmons said. “What it means is if you ran a calculation using your laptop for one hour and it used up the whole laptop with six cores, it would be six computing hours. We used USF’s large computing cluster with many, many cores for many months.”
The team said the key lies in how rubber responds when stretched. Ordinary rubber gets thinner while mostly keeping the same overall volume. Carbon black particles change that by acting like tiny structural supports inside the material, stopping it from thinning as much.
That forces the rubber to expand in volume, something it strongly resists. According to the researchers, the rubber effectively “fights against itself,” which sharply increases stiffness and strength.
The researchers said particle networks, adhesive interactions and space-filling effects all contribute to that resistance to volume change. They described the result as a unified explanation for rubber reinforcement.
Early versions of the simulations did not match experimental results. The team said it improved the model by incorporating insights from earlier studies until the simulations reproduced observed behavior.
Simmons said the findings could help tire makers move beyond costly trial and error. He pointed to what engineers call the “Magic Triangle” of tire design, balancing fuel efficiency, traction and durability.
“The struggle always is to get more than two of the three to be good, and this is where trial and error only gets you so far,” Simmons said. “With these findings, we’re laying a new foundation for rationally designing tires.”
The researchers said reinforced rubber is also widely used in power plants, aerospace systems and other infrastructure where failures can be serious. Simmons pointed to the 1986 Space Shuttle Challenger disaster.
“If you remember, the reason the Challenger failed was a rubber gasket that got too cold,” Simmons said. “A lot of energy systems, power plants have rubber parts. Everybody’s had a garden hose that started leaking because a rubber gasket failed. Now imagine that happening in a power plant or a chemical plant.”
The research was supported by the U.S. Department of Energy Office of Science.
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