The New eco-friendly adhesive that can self-heal too

The New eco-friendly adhesive that can self-heal too

Epoxy is one of the most common adhesives owing to its mechanical strength, durability, and relatively low cost. However, researchers have been looking for alternatives ever since BPA, a key epoxy component, was identified as an endocrine disruptor. Now, a team of researchers at Delft University of Technology, the University of Belgrade, and Technology Innovation Institute (TII) in the UAE have identified an eco-friendly alternative that can also self-heal.

Dr. Nataša Tomić, research associate at the TII, said, “We wanted to replace the BPA in epoxies - which is toxic, so we tried to switch it with a bio-based source using tannic acid.” Tannic acid is a natural substance found in trees and plants. 

Another attractive property of this material allows it to be used to melt and regenerate bonds of parts that have fractured when treated with heat and pressure. This is called self-healing since the original bond can fix itself without needing the application of more glue. In contrast, traditional epoxy just burns when heated. 

The repair challenge

Self-healing approaches reduce the need to directly access materials for repair. For instance, in the case of a traditional bonded joint, a technician might need to cut other materials to gain access, apply the adhesive, and patch the cut. With the new technique, they need to simply place the section in an autoclave to generate the required heat and pressure to start the repair process. 

“You can apply the thermal activation process to restore properties without thinking too much about that accessibility,” said Dr. Mohamed Nasr Saleh, Thermoplastic Composites Lead at Technology Innovation Institute. The new material also appeared to have better adhesion than epoxy when tested mechanically.

Researchers have explored other approaches to self-healing materials in the past. One common method is to include tiny micro- capsules that comprise a special material called Grubbs Catalysts in the adhesive. The micro-capsules can break apart when the bond is stressed and heal the bond automatically. However, the bonds can only heal themselves once, and the capsules are relatively expensive - costing about US$50 to produce as little as 100 mg. 

In contrast, the new adhesive is made from tannic acid, a common ingredient that can be obtained from most trees, and can be used to re-treat the bonds multiple times. Researchers successfully healed fractures and helped the bonds regain 80% of their original strength in early tests. “There is potential for doing it again and again,” Tomić said. 

Promising alternative to BPA

“There is a high potential for the industry to use this component in their formulation,” Saleh said. However, he cautioned that the new material is still in relatively early stages of development. It is currently at technology readiness level (TRL) 3. The TRL scale characterizes the maturity of material processes from TRL 1 to 9. Depending on industry interest, it could be a few years to a decade before the new approach is mass-produced. 

Saleh added that adhesives generally complement mechanical fastening approaches for structural bonding, which adds weight and manufacturing complexity. “We are trying to prove you don’t need as many fasteners, and you can activate some self-healing functionality,” he said. Doing so could help build trust in using adhesives for structural applications in the future. 

Tomić hopes this study will open the doors for further research on how new manufacturing processes could replace BPA with safer alternatives that may be stronger to boot.