New polymer coating changes color for early warning of damage

When cracks form, microbeads embedded in the material break open and cause a chemical reaction that highlights the damaged area - image courtesy of Nancy Sottos.
When cracks form, microbeads embedded in the material break open and cause a chemical reaction that highlights the damaged area - image courtesy of Nancy Sottos.

Researchers at the University of Illinois have developed a new polymer coating which can provide an early warning of structural damage and can be applied to coat metals, other polymers and glass.

The early indication system features a polymer coating that will allow engineers to address problem areas before they become more of an issue or lead to major damage of the material.

The early warning system would be of particular use on applications featured in the aerospace, petroleum and infrastructure industries, such as buildings, petroleum pipelines, air and space transport and other vehicles.

The development of the early warning system was led by the research of University of Illinois’ materials science and engineering professor Nancy Sottos, and the universities aerospace engineering professor Scott White. Professor Sottos said the system would help to identify damage early on in materials in which it is notoriously hard to detect.

“Polymers are susceptible to damage in the form of small cracks that are often difficult to detect,” she said. “Even at small scales, crack damage can significantly compromise the integrity and functionality of polymer materials. We developed a very simple but elegant material to autonomously indicate mechanical damage.”

The system works as a damage indicator through the use of a dye which changes color as a reaction to slight damage that is normally hard to detect. The University of Illinois researchers achieved this detection method by embedding tiny microcapsules of a pH-sensitive dye in an epoxy resin.

The capsules break open if the polymer form cracks or is scratched, stressed of fractured. The dye releases from the capsules and reacts with the epoxy to cause a dramatic color change from light yellow to a bright red.

The deeper the scratch or crack, the more microcapsules are broken, resulting in a more intense color which helps to indicate the extent of the damage. Even tiny microscopic cracks are enough to cause a color change, making minute compromises to the materials’ structural integrity apparent.

Professor Sottos said the color changing technique of the new system didn’t take the use of many dye capsules to be an excellent damage indicator.

“A polymer needs only to be 5% microcapsules to exhibit excellent damage indication ability,” she said. “It is cost effective to acquire this self-reporting ability.”

The University of Illinois researchers are now looking at further applications for their damage indicator system, such as applying it to fibre-reinforced composites, as well as integrating it with self-healing systems which could mean future materials could not only notify engineers of minute and potential further damage, but attempt to repair it themselves.

Similar research for head injuries

While University of Illinois researchers have been busy developing a system to detect early damage to certain materials, a team at the University of Pennsylvania has developed a polymer-based material that could be used in headgear to gauge the severity of head trauma and the extent and indication of the injuries.

The polymer-based material developed by the University of Pennsylvania team changes color following a blow, depending on the level of force.

The aim of the development is to provide an immediate indication of head trauma, so evaluation and treatment can be sought in a quick fashion and the decision to remove the player from the field can be made.

The head trauma detection was made by the Pennsylvania team creating a mould using silica particles of various sizes which were made to self-assemble into the desired pattern.

A heated polymer was then introduced into the mould and allowed to set, before the silica mould was removed with the finished material picking up the pattern of the mould to form inverted polymer crystals.

The original version of the team’s material was made by using a technique called holographic lithography which uses lasers to create interference patterns, but was replaced by the polymer-based materials after it was found to be too expensive a process.

The Pennsylvania team’s head trauma development comes at a time when US high school football has been plagued by a number of concussions and even deaths among its athletes.

The new development could see the spate of serious injuries and deaths prevented through the use of the head-trauma detection headgear by high school footballers.