Harvard researchers develop crack-resistant natural rubber

Cambridge, Massachusetts – Researchers at Harvard have developed a new way to produce natural rubber (NR) that is up to 10 times tougher than conventional rubber and more resistant to cracking.

The work, led by Zhigang Suo, professor of mechanics and materials at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), was published in Nature Sustainability, Harvard reported 22 May.

Derived from natural rubber latex, rubber is harvested, coagulated, dried, mixed with additives, shaped, and heated to trigger vulcanisation.

This process creates short polymer chains within the material that are densely crosslinked, or chemically bonded, explained the Harvard report.

The research team, it said, “modified the high-intensity process to induce a gentler transformation that retains long polymer chains in their natural state, rather than cutting them into shorter chains.”

The result is a structure filled with long, tangled strands — described as a “tanglemer” — that improves durability by spreading stress over a larger area when a crack forms.

“We used a low-intensity processing method, based on latex processing methods, that preserved the long polymer chains,” said Guodong Nian, first author and former SEAS postdoctoral researcher.

When a crack forms in the new material, the long spaghetti strands spread out the stress by sliding past each other, allowing more rubber to crystallise as it stretches, and overall making the material stronger and more resistant to cracking.

The result, according to Zheqi Chen, was that the rubber became “four times better at resisting slow crack growth during repeated stretching and 10 times tougher overall.”

“We imagined that the properties would be enhanced maybe twice or three times, but actually they were enhanced by one order of magnitude,” Chen said.

The new method could initially be applied to “thin” rubber products such as gloves or condoms and may also be useful in flexible electronics or soft robotics.