These limitations led Haq’s team at the CVRC to create several novel joining technologies, including advanced adhesives with special properties that allow them to be taken apart, repaired and healed. Funding by the U.S. Department of Energy and the American Chemistry Council has allowed the group to research the use of thermoplastic adhesives embedded with electrically conductive nanoparticles for multimaterial joining.
Unlike thermosets, thermoplastic adhesives can be heated and separated at will for repair or reassembly. Haq added conductive metallic nanoparticles to the thermoplastics, then used electromagnetic radiations to activate only the glue – not the substrate. “We can very rapidly just heat the glue and separate the two joints,” says Haq. “In our lab, we have shown that up to a 2 x 2-foot panel can be separated in less than one minute.”
However, there are limitations based on the electromagnetic radiation used for activation of the adhesives. For instance, microwave activation is currently limited by the size of the microwave. Although equipment manufacturers are developing applicators that can go on the end of a robotic arm, Haq has moved forward with other activation techniques research to bypass the problem. For instance, metallic nanoparticles derived from iron ores can be activated using induction systems to heat up the glue.
“We’ve been able to join large areas, as well as dissemble, reassemble and repair large areas of joints,” says Haq. “Furthermore, if we control the amount of electromagnetic radiation – specifically the induction system – we can actually heal the joint.”
The researchers ran samples through thousands of fatigue cycles, simulating the damage done by repeatedly driving on roads with potholes. They healed the joint, then pulled it apart to examine it. “The healed joint exhibited no fatigue,” says Haq. “In other words, we can rearrange polymer chains, heat the adhesive as if it’s brand new, close the cracks and refurbish or refresh the joints as though nothing has happened.”
Haq’s team has tested several thermoplastic materials, including nylon 6, polycarbonate and acrylonitrile-butadiene-styrene (ABS). They are now working with high-impact polystyrene (HIPs).
The group also is in discussion with a couple of OEMs and Tier-1 suppliers to demonstrate feasibility on a composite part in the next few months. In addition, the researchers are developing a coil induction system that can be mounted on a robotic arm for use on an assembly line. Finally, the team is doing a lot of modeling and nondestructive evaluation to better understand the joints.