Reaction coordinates for the healing reaction; a polymer chain with diglycidyl ether of bisphenol-A/adipic acid goes through transesterification reaction, where the adipic acid switches from one epoxy molecule to the other.
Reaction coordinates for the healing reaction; a polymer chain with diglycidyl ether of bisphenol-A/adipic acid goes through transesterification reaction, where the adipic acid switches from one epoxy molecule to the other.
Photograph of a pure vitrimer dogbone-shaped sample and a carbon-fiber reinforced vitrimer (vCFRP) composite.
Photograph of a pure vitrimer dogbone-shaped sample and a carbon-fiber reinforced vitrimer (vCFRP) composite.

Demand for lightweight, high-performance carbon-fiber reinforced polymer composites (CFRPs) is growing for structural applications in aerospace, automotives, marine transport, sports equipment, and renewable energy. Although these materials exhibit better specific strength and stiffness than many metals, they are prone to fatigue failure. While CFRPs could represent a market worth $31 billion by 2024, the costs of structural health monitoring systems to detect fatigue damage could reach upwards of $5.5 billion. To combat this issue, nano-additives to block the spread of cracks through the material and self-healing polymers are being explored.

Now, however, researchers from Rensselaer Polytechnic Institute, University of Washington, and Beijing University of Chemical Technology have come up with a CFRP featuring a glass-like vitrimeric polymer matrix that can reverse fatigue damage [Kamble et al., Carbon 187 (2022) 108-114, https://doi.org/10.1016/j.carbon.2021.10.078].

“We developed a carbon-fiber composite material in which the epoxy matrix that binds together the carbon fibers is a specialized material called a vitrimer,” explains last author of the study, Nikhil Koratkar. “The vitrimer is similar to a regular epoxy, but with one crucial difference – it has the ability to heal itself when heated above a critical temperature.”

Fatigue in CFRPs originates in the thermoset polymer matrix, which is made up of a network of molecular chains that become cross-linked when cured. During use, these cross-links can snap or rupture, triggering the growth of cracks that ultimately lead to failure. This type of failure is irreversible: nano-additives block cracks but cannot remove them, while self-healing polymers that release curing agents to seal microcracks get used up.

Instead, Koratkar and his colleagues led by Dong Wang and Catalin R. Picu, developed a vitrimeric CFRP in which fatigue damage is repeatedly reversed by heating to a temperature above a specific point known as the ‘topology freezing transition temperature’ where reversible crosslinking reactions occur. The composite is made up of a vitrimeric matrix synthesized from a conventional epoxy and carboxylic acid in the presence of a suitable catalyst into which carbon fiber prepregs are embedded.

Fatigue can be reversed in the composite by heating to just over 80°C for an hour periodically, even after 100,000 damage cycles. Exploiting the propensity of carbon materials to heat up when exposed to radio frequency (RF) electromagnetic fields, moreover, could replace the use of conventional heaters. RF heating could be used to heal components selectively, while part of a larger structure, or be applied to the most vulnerable parts.

The approach addresses the ‘irreversible’ nature of fatigue damage and could enable near-indefinite reversal or postponement of fatigue-induced damage in vitrimeric CFRPs.

“Structural materials could last much longer than they presently do, resulting in substantial improvement in maintenance and operating costs,” points out Koratkar.