Damage developing in a material can be difficult to see until something breaks or fails. A new polymer damage indication system automatically highlights areas that are cracked, scratched, or stressed, allowing engineers to address problem areas before they become more problematic.
The
early warning system would be particularly useful in applications like
petroleum pipelines, air and space transport, and automobiles—applications
where one part’s failure could have costly ramifications that are difficult to
repair. Led by University of Illinois (Champaign, Illinois) materials science and
engineering professor Nancy Sottos
and aerospace engineering
professor Scott White,
the researchers published their work in
the journal Advanced Materials.
“Polymers are susceptible to damage in the form of small cracks
that are often difficult to detect. Even at small scales, crack damage can
significantly compromise the integrity and functionality of polymer materials,”
Sottos says. “We developed a very simple but elegant material to autonomously
indicate mechanical damage.”
The researchers embedded tiny microcapsules of a pH-sensitive
dye in an epoxy resin. If the polymer forms cracks or suffers a scratch, stress
or fracture, the capsules break open. The dye reacts with the epoxy, causing a
dramatic color change from light yellow to a bright red—no additional
chemicals or activators required.
The deeper the scratch or crack, the more microcapsules are
broken, and the more intense the color. This helps to assess the extent of the
damage. Even so, tiny microscopic cracks of only 10 micrometers are enough to
cause a color change, letting the user know that the material has lost some of
its structural integrity.
“Detecting
damage before significant corrosion or other problems can occur provides
increased safety and reliability for coated structures and composites,” White
says. White and Sottos are affiliated with the Beckman Institute for Advanced Science and Technology at the
University of Illinois.
The researchers demonstrated that the damage indication system
worked well for a variety of polymer materials that can be applied to coat
different substrates including metals, polymers and glasses. They also found
that the system has long-term stability—no microcapsule leaking to produce
false positives, and no color fading.
In addition to averting unforeseen and costly failure, another
economic advantage of the microcapsule system is the low cost, Sottos says.
“A polymer needs only to be 5 percent microcapsules to exhibit
excellent damage indication ability,” Sottos says. “It is cost effective to
acquire this self-reporting ability.”
Now, the researchers are exploring further applications for the
indicator system, such as applying it to fiber-reinforced composites, as well
as integrating it with the group’s previous work in self-healing systems.
“We envision this self-reporting ability can be seamlessly
combined with other functions such as self-healing and corrosion protection to
both report and repair damage,” Sottos says. “Work is in progress to combine
the ability to detect new damage with self-healing functionality and a
secondary indication that reveals that crack healing has occurred.”
The BP International Centre for Advanced Materials supported
this work. Postdoctoral researcher Wenle Li is the first author of the work,
and graduate students Christopher Matthews, Michael Odarczenko and Ke Yang are
co-authors.
The
paper “Autonomous Indication of Mechanical Damage in Polymeric Coatings” was published in the January 11, 2016 issue of Advanced Materials and is
available online.
Source: University of Illinois,
illinois.edu.