Under the direction of professors Scott White and Nancy Sottos, researchers Wenle Li, Michael T. Odarczenko and Ke Yang used a microcapsule approach for the system – which White says is the easiest approach with the longest track record of success in damage indication.
The process begins by putting a dye (dichlorofluorescein – or DCF) into a microcapsule. The researchers then dissolve the DCF in a carrier solvent (ethyl phenyl acetate – or EPA). If there’s no damage, the dye remains undetected inside the capsule. Whenever there is some sort of damage, whether a scratch or a mechanical rupture, the microcapsules break and release a liquid curing agent.
“The solvent carries the dye out into the damaged region, and the dye itself comes into contact with the native matrix material,” explains White. “In our case, we targeted epoxy polymers. There are residual amines which trigger a color change in the dye itself. It transforms from this pale yellow to a bright red color.” As a result of that pH trigger, a bright red color is visible wherever there is damage.
A deeper scratch will produce a more intense color, which helps the researchers determine the extent of the damage. The team says the system is incredibly consistent. It took scratched coatings and stored them for eight months. The color of the stored coatings matched the intensity of new scratches.
According to White, the team chose Hexion’s EPON™ 813 epoxy resin and Hexion’s EPIKURE™ 3233 curing agent – commonly used materials – to cover a wide range of applications, including aerospace, construction, civil infrastructure, automotive systems and shipbuilding.
He adds that investing in this type of technology makes sense for customers who want a “failsafe mechanism,” where it is possible to do manual maintenance and inspection of the damaged places to ensure the healing process is taking place.
“I think [our system] is part of a portfolio of different, multifunctional applications for composites that are the wave of the future,” says White. “When you talk about self-healing, you extend the lifetime of composite structures, but at the same time, customers who use these would like to know when there’s damage and when it’s been healed.”
White says the next step is to apply the damage system to CFRP and GFRP systems. Other aspects that have to be addressed are the long-term stability of the microcapsules and microcapsule-infused material when it’s exposed to various environmental conditions, such as temperature and humidity cycles.