US scientists make rubber 10 times tougher, 4x more crack-resistant under repeated stress
Materials scientists in the U.S. have just given natural rubber a major upgrade by developing a method to make it stronger and significantly more resistant to cracking, without compromising its signature stretchiness, even after repeated cycles of use.
Led by Zhigang Suo, an Allen E. and Marilyn M. Puckett professor of mechanics at materials at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), the study explored crack growth, one of rubber's most persistent weaknesses.
According to Suo and his team, while natural rubber has been used for millennia, initially by the indigenous cultures of Mesoamerica, its ability to resist cracking, particularly under repeated stress, has remained largely unimproved.
"Improving crack resistance will extend the material's service lifetime and therefore improve its sustainability," Guodong Nian, PhD, a former SEAS postdoctoral researcher and first author of the study.
Native to the Amazon basin and sourced from the milky latex of the Hevea tree (Hevea brasiliensis), natural rubber is a durable polymer used in everything from gloves and tires to medical devices, shoes, and conveyor belts.
But the research team has now found a way to modify its traditional high-intensity vulcanization process, which usually creates short polymer chains within the material that are densely crosslinked, or chemically bonded.
This, according to the team, resulted in a novel type of rubber, which they called tanglemer. Filled with long, entangled polymer strands resembling a bowl of spaghetti, the new rubber reportedly boosts durability by absorbing and distributing stress more efficiently.
"We used a low-intensity processing method, based on latex processing methods, that preserved the long polymer chains," Nian explained. According to the scientists the new material is four times more resistant to slow crack growth under repeated stretching, and 10 times stronger overall.
This, according to the scientists, is because when a crack forms in it, the long spaghetti strands spread out the stress by sliding past each other, allowing more rubber to crystallize as it stretches, ultimately making the material more resilient.
"We imagined that the properties would be enhanced maybe twice or three times, but actually they were enhanced by one order of magnitude," Chen concluded in a press release, adding that the key to the discovery lies in replacing the dominance of chemical crosslinks.
Yet, while the research highlights the benefits of preserving long polymer chains, challenges remain as the process requires significant water evaporation, limiting material yield and making it less suitable for larger products such as tires.
This currently makes it less suitable for bulky applications like tires, but better suited for thin rubber products such as gloves, condoms, or other items that require flexibility without large material volume. According to the researchers, the new process also opens up possibilities for applications like flexible electronics and components for soft robotics.
The study was supported by the National Science Foundation's Materials Research Science and Engineering Centers (DMR-2011754) and the Air Force Office of Scientific Research.
It has been published in the journal Nature Sustainability.
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