The American Chemistry Council (ACC) and researchers at Michigan State University (MSU) have demonstrated a new “reversible” adhesive for multi-material joining that becomes stronger after use and repair/rebonding—a significant development for the repairability and end-of-life recyclability of CFRP-intensive (carbon-fiber-reinforced polymer) vehicles, for example.
“What the researchers found this year is when you bond it, unbond it, and then rebond, the ‘healed’ new bond actually becomes stronger than the original bonds. That means that a repaired carbon-fiber frame could be stronger in a crash after the repair. This also means getting an auto frame repaired at 50,000 miles can literally mean the car is better than new,” Sandra McClelland, automotive business development manager, Solvay Specialty Polymers USA and presenting chair, ACC Auto Team, said at the Management Briefing Seminars (MBS) in Traverse City, MI.
McClelland described the latest discoveries in multi-material reversible bonding, in collaboration with MSU’s Composite Vehicle Research Center and Dr. Mahmoodul Haq. The alternative adhesive connector attempts to satisfy several auto-industry goals for a joining technology, apart from working in multi-material applications (plastics, steel, aluminum, and polymer composites): reduced weight, binds as robustly as other connectors (ASTM testing for structural efficiency), applies and adheres in 45 seconds and is reversible—meaning the bond can come apart and be “re-joinable” in a local repair shop or at end-of-life for faster and easier recycling of composites.
This “magic glue,” as McClelland referred to it at MBS, was developed through collaboration between ACC and MSU and appears to satisfy these criteria, she said. The thermoplastic adhesive material grips the entire seam of a joint, not just one spot as with some other types of connectors. Spreading stress results in “considerably stronger” joins, which can enable additional weight savings because parts joined can be subsequently thinner and lighter.
Process for ‘reversibility’
The thermoplastic adhesive can melt apart and rebond in less than a minute, the amount of time required in an assembly plant, McClelland noted. The material contains magnetic nanoparticles—iron microspheres that vibrate with electric current and become very hot, 320 to 800°F within seconds, melting the thermoplastic adhesive before the surrounding material heats up. It then can harden in seconds—essentially becoming fast-acting glue that bonds and rebonds on demand.
After MSU researchers apply warm glue to bond the multi-materials, they test the joints to failure and record structural efficiencies by ASTM standards for joint fatigue, strength, shear, impact, flex, creep and modulus. The researchers then use electromagnetic coils to re-melt the adhesive in 30 seconds and establish a new “healed” bond to conduct parallel duplicate tests. The data shows this new re-bond is stronger.
“By melting the adhesive and giving it all that energy, that material becomes more crystalline than it was initially. A more crystalline polymer has better strength,” McClelland said.
The adhesive will be economically feasible and eventually commercially available, she added, but did not state a specific timeframe.
“Strengthening after repair could stand the automotive notion of joint fatigue on its head,” she said.