Vascular Networks Enable Repeated Self-Healing in Fiber-Reinforced Composites
by Joseph Bennington-Castro
Materials Research Society | Published: 30 April 2014
In recent years, scientists have made great strides in developing self-healing polymers. However, creating fiber-reinforced composites that heal autonomously has remained a great challenge - an unfortunate reality, considering that delamination damage in such materials is difficult to detect and worse to repair using conventional techniques. Now, in a major breakthrough, researchers at the University of Illinois at Urbana-Champaign have introduced 3D microvascular networks containing healing agents into a fiber-reinforced composite. The unique design, detailed in a recent paper in the journal Advanced Materials , allows the composite to repeatedly self-heal fractures.
"We've shown that you can heal delamination damage in a structural composite multiple times using an autonomous approach - that's a big deal," explains study senior author and materials scientist Nancy Sottos. "We've also shown that you can introduce a pretty complex vascular network into a structural material without the loss of mechanical properties, which is another big step in this field."
Unlike self-healing (pure) polymers, there are major hurdles in designing self-healing composites, Sottos says. For one thing, you have to try to squeeze healing chemistries into a tiny space, because composites contain a high percentage of fibers that are only a few microns apart from one another. Additionally, while self-healing polymers can be processed at close to room temperature, aerospace-grade composites need to be processed at high temperatures of 150 to 200°C.
Sottos and her collaborators, Scott White and Jeffrey Moore, have been working on the problem for more than a decade. Initially, their approach involved integrating capsules containing healing agents into the polymers and composites. But they realized the capsule-based system wasn't the most practical design for composites, especially considering that each capsule can only heal the material once. They later turned to figuring how to introduce vascular networks into the materials - an approach that appeared very promising. "Then we hit a road block," Sottos says. "The methods we were using were not applicable to advanced composites." Eventually, they realized that using so-called sacrificial fibers was the ideal way to implant vascular networks into composites and grant them the ability to heal autonomously.
To create their self-healing fiber-reinforced composites, the team, which included graduate student Jason Patrick, weaved sacrificial fibers - poly (lactic acid) monofilaments - in a precise pattern into an aerospace-grade, woven fiberglass fabric. Next, they integrated a high-quality epoxy matrix around the glass fibers and sacrificial fibers using vacuum assisted resin transfer molding. They then performed a post cure by raising the temperature high enough to vaporize the sacrificial fibers, leaving behind evacuated, undulating microchannels in the composite. Finally, they filled the vascular networks with two healing agents (an epoxy resin and a hardener) using pressurized fluid pumping. Importantly, the two healing agents reside in their own microchannel networks that are interpenetrating, but don't cross paths.
The unique design ensures that delamination damage ruptures the internal vasculature, causing the microchannels to release their healing agents. The epoxy resin and hardener bleed into the crack plane and polymerize when they meet, forming a kind of structural glue that reinforces the composite at the fracture site. In tests, Sottos and her colleagues discovered that their fiber-reinforced composite could heal delamination damage in a single area autonomously and repeatedly (at least five times). What's more, the material had healing efficiencies of more than 100 percent, meaning that the fracture energy increased with each healing cycle.
"This is an excellent work and undoubtedly a very important step in the field of self-healing materials," says Ibon Odriozola, a materials scientist at the IK4-CIDETEC Research Centre in Spain, who wasn't involved in the new study. Before commercialization of the technique, Odriozola suggests there are a few issues to address, including that the method is relatively time-consuming and may not be suitable to fabricate large components. "However, it is an important breakthrough and probably could be successfully applied for specific applications."
Sottos notes that with the current design, there is a healing cycle limit due to the buildup of healing materials, which eventually block up the channels and prevent them from delivering their healing agents. She and her team are hoping to increase the healing cycles by implementing more advanced, bio-inspired architectures for the microvascular networks. They are also looking into ways to heal larger damage volumes, such as holes in the composites. "How do you restore material that has been completely removed in the damage process?" she says.
Read the abstract of the study in Advanced Materials here.
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