Many golf clubs and aerospace parts are both strong and light because they are made from a class of materials known as carbon-fiber reinforced polymeric (CFRP) composites. The sturdy plastics, however, do crack and wear down over time.

Researchers from Rensselaer Polytechnic Institute and the University of Washington have created a new type of CFRP composite that reverses fatigue damage — after applying some heat.

The findings from the team, including Aniruddh Vashisth, University of Washington assistant professor of mechanical engineering, and Nikhil Koratkar, Professor of Mechanical Engineering and Materials Science at RPI, were published on Nov. 2 in the journal Carbon.

The material — part of a class of carbon-reinforced structures known as vitrimers — has both solid and fluid properties. Unlike thermosets, which are held together and "set" with epoxy, vitrimers can be linked and unlinked, and therefore recycled.

"By heating these polymers to a certain temperature, we initiate an exchange reaction that leads to re-arrangement in the polymer chain, resulting in a healed polymeric structure," Vashisth told Tech Briefs in an edited Q&A below.

Vashisth and the team believe that re-connecting vitrimers could be a viable alternative for many products currently manufactured from thermosets, which are not recycled and pile up in landfills .

"That re-connection is how the material gets repaired," Vashisth told Tech Briefs.

In the Q&A below, Vashisth explains more about the new materials that are possible when fatigue can be fixed up and reversed.

Tech Briefs: Are the structural properties – the strength – of vitrimers as good as thermosets and thermoplastics?

Prof. Aniruddh Vashisth: The mechanical properties of vitrimers depend on the polymer backbone; this is similar to any other polymer such as thermosets or thermoplastics. In our recent publication, we found that vitrimers made with bisphenol-A epoxide and adipic acid have a strength of 78 MPa; this is comparable to the traditional thermosets and thermoplastics used in structural components.

Tech Briefs: What are the chemical links? Are they molecules?

Prof. Aniruddh Vashisth: The chemical links are the reactive sites in thermosetting polymers, so for example in traditional epoxies, when we mix an epoxide and an amine together, the reactive groups at the end of these polymers interact with each other and form a "chemical link." Through this process you end up with a large polymeric chain.

Similarly, in our case, the chemical links are epoxide and carboxyl groups in bisphenol-A epoxide and adipic acid, respectively. While these "chemical links" are locked in place in thermosets, vitrimers are unique in the sense that they allow these links to interchange even after curing.

Tech Briefs: What is the mechanism that causes the total number of links to remain constant after heating?

Prof. Aniruddh Vashisth: Imagine each of these different types of polymer materials (thermosets, thermoplastics, and vitrimers) as a room full of people. In the thermoset room, all of the people are holding hands and won’t let go. In the thermoplastic room, people are shaking hands and moving all around. In the vitrimer room, people shake hands with their neighbor, but they have the capacity to exchange handshakes and make new neighbors so that the total number of interconnections remains the same.

Prof. Aniruddh Vashisth with the vitrimer material. (Image Credit: Andy Freeburg/University of Washington)

That re-connection is how the material gets repaired, and our recent paper was the first to use atomic-scale simulations to understand the underlying mechanisms for those chemical handshakes. In short, the number of chemical links (or "handshakes") remains the same after heating.

Tech Briefs: Am I correct in assuming that means the strength of the vitrimer reverts to its pre-weakened state?

Prof. Aniruddh Vashisth: This is still an active area of research in my group, and we are taking a deeper dive into the molecular mechanisms driving healing in these polymers and extent of healing that can be achieved.

Tech Briefs: How much heat needs to be applied? Is it always the same? Is it different depending on the percentage of carbon fibers?

Prof. Aniruddh Vashisth: The heat provided to these polymers is the driving mechanism for healing, and it is dependent on the chemistry of the vitrimer polymer. By heating these polymers to a certain temperature, we initiate an exchange reaction that leads to re-arrangement in the polymer chain resulting in a healed polymeric structure.

For example, in a recent paper in Carbon, our collaborators at RPI (Prof Nikhil Koratkar) saw that one hour at 80 °C was sufficient to retain almost ~80% of modulus for bisphenol-A epoxide/adipic acid vitrimers. We haven't explored the heat requirement depending on volume fraction of carbon fibers, but that would certainly be an interesting study!

Tech Briefs: How do you know when the structure needs to be heated?

Prof. Aniruddh Vashisth: Usually commercial composites are regularly tested for their structural health. Any non-destructive evaluation method can be used to figure out the extent of damage in such composites.

Tech Briefs: Would the structure have to be pulled out of service to be repaired? Have you thought about how this would be done practically? I guess it would be different for different products

Prof. Aniruddh Vashisth: For vitrimers, the same polymeric material can be recycled and reprocessed, as compared to conventional polymers where parts of structure need to be cut and replaced.

Specifically, if we talk about carbon fiber composites, we envision using a unique rapid heating method that my lab specializes in, namely Radio Frequency (RF) heating. We use an RF applicator (think of this as a wand) that creates fringing electric fields that can heat carbon fibers as fast as 70°C/s with as low as 25 W of input power.

It should be noted that in practical situations, damaged vitrimer components could be selectively healed by RF heating, while still remaining attached to the structure, without the need to un-assemble the system. Such local healing of damaged vitrimer CFRP is a key advantage of RF heating, since the RF heating can be applied selectively to locations where the stresses are the greatest, and damage is most likely to occur.

Tech Briefs: Where do you think this material might find its first applications? How do you envision the heat being applied in typical use-cases?

Prof. Aniruddh Vashisth: We envision that these materials would have a wide range of applications, especially in applications where we can replace traditional thermosets. These would include aerospace and automotive industry, protective coatings, or adhesives. With significant push towards sustainability, we think automotive and aerospace industries could be the first movers for these materials. Traditional ovens can be used for heating and healing for small components, whereas large composites can be healed using our novel method of RF non-contact heating.

What do you think? Share your questions and comments below.