Self-healing glass fiber reinforced composite after production.
Example of healing of a delamination that is caused by impact.Fiber-reinforced polymer composites (FRPCs) are increasingly finding use as lightweight structural components in the aerospace and automotive industries. But the susceptibility of these materials to impact and fatigue damage is a disadvantage. To overcome this shortcoming, FRPCs that can heal themselves are a very attractive option. To date, however, such self-healing FRPCs have had poor mechanical properties or required high healing temperatures or a combination of both.
Now researchers from Technical University Delft (TU Delft) in the Netherlands and Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have designed a self-healing composite with competitive mechanical properties that can be produced by conventional means [Post et al., Composites Science and Technology 152 (2017) 85-91].
“We have developed, for the first time, an intrinsic healable thermoset polymer for composites that can be used directly in current fiber composite manufacturing processes and healed at mild temperatures,” says Santiago J. Garcia of TU Delft.
The composite is based on an organic-inorganic epoxy thermoset containing disulfide groups reinforced with glass fibers. The inclusion of disulfide groups means that the composite contains both reversible and irreversible covalent bonds.
When the composite is subjected to repeated bending, fracture or low-speed impact, a short heat treatment at relatively mild temperatures of 70-85°C heals the damage. At these temperatures, the reversible covalent bonds open up to allow the repair of cracks and delamination while the irreversible bonds maintain the composite’s structural and mechanical integrity. The matrix material appears to flow into and partially fill any cracks. Moreover, multiple damage events can be repaired with repeated heat treatments with little deterioration in the composite’s mechanical properties.
“The composite can recover its original properties after damage,” explains Garcia. “This enables easy manufacturing and development of repairable fiber composites with the ability to heal multiple times, provided the damage to the reinforcing fibers is at a modest level.”
While the healing process can restore cracks and delamination, it cannot fix broken fibers. Moreover, the approach requires some pressure to be exerted on the damaged surfaces as they self-heal.
“We are not quite there yet for real applications,” admits Garcia. “As this is the first proof-of-concept material there is obvious room for improvement.”
Nevertheless, the composite is the first of its type with mechanical properties comparable to commercial epoxies that can be produced by conventional fiber-composite processing tools and self-heal at mild temperatures.
“We believe that our composites could be of great value in applications requiring lightweight structures where manual repair is difficult or even impossible, or during the manufacturing process itself,” points out Garcia.
The researchers are confident that improving the chemistry of the polymer matrix will get around its current limitations.