Previously published in Plastics Engineering and posted with permission from the Society of Plastics Engineers.
All right car and science fans, time to get your geek on…
Today’s cars and trucks are technological marvels. Due to modern innovations, we now drive the safest, best-designed, most fuel-efficient cars. Ever.
To meet consumer demand for these stylish, safer, high-performing cars—as well as federal standards to increase fuel efficiency—carmakers increasingly are relying on multiple advanced materials: “military grade” aluminum, “high strength low alloy” steel, composites that combine plastics and high tech materials such as carbon fiber, and more.
Many of these materials choices are driven by a focus on weight reduction. As the Ford Motor Company said: “Few innovations provide a more wide-ranging performance and efficiency advantage than reducing weight. All factors of a vehicle’s capabilities—acceleration, handling, braking, safety, efficiency—can improve through the use of advanced, lighter materials.” That’s one reason why use of lightweight plastics and plastic composites has grown so much over the past few decades. So much so that today’s cars are generally comprised of about 50 percent plastics/composites by volume… but only 10 percent by weight.
Challenges of multiple materials
But do all these advanced materials play nice together? How do carmakers join disparate materials to create critical components? And does using multiple materials make repairs a costly headache? Just how do they fit together, stay together, and get repaired together?
It’s complicated. And very important. For safety reasons, components must fit and stay together for the car’s entire useful life. But we also need to separate them for repair… and then rejoin them. And then separate them again for recycling at the car’s end-of-life.
And while these multi-material components can improve safety and performance, they also can be costlier. To help get us get back on the road sooner and reduce expense after that inevitable accident, it’s important to be able to readily repair damage instead of replacing expensive components and tossing large broken chunks of our cars in landfills.
New research may show a path forward to dramatically increase our ability to join, unjoin, and rejoin car parts more efficiently and economically. And to further drive down the weight of cars using more lightweight materials and components.
Carmakers typically join materials in three ways: welding, fastening, and bonding. Each has advantages and disadvantages.
- Welding can produce very strong joints but also permanently links components, making repair difficult. And welding is not an option for certain materials.
- Fasteners (e.g., screws, bolts, rivets) can be unfastened to disconnect and then reconnect replacement/repaired components. But fasteners often don’t work with plastics/composites, and they can corrode, disrupt aesthetics, add weight, and concentrate stresses.
- Adhesive bonds are lightweight and often stronger than surrounding materials, protect aesthetics, and spread out stresses over the entire bonded area, unlike fasteners. However, bonding typically uses thermoset polymers, making repair or replacement impractical. Or… is it?
Two projects on the same track
Two research projects—one focused on finding new ways to join multi-material car components and another on increasing adoption of composites in cars—combined forces a few years back.
Researchers at Michigan State University (MSU) had set out to leverage existing knowledge of the behavior of certain nanoparticles (graphene) mixed with plastics: Microwave energy applied to the nanoparticles causes thermoplastics to heat and melt. Question: Could researchers harness this phenomenon to create reversible adhesive bonds that would allow us to readily join, unjoin, and rejoin car parts?
At the same time, the plastics division of the American Chemistry Council (ACC) was conducting a technology review to identify technological barriers to increased use of lightweight plastics/composites. ACC discovered the MSU work and proposed collaboration.
Research leads to reversible bonding
The collaboration initially was stymied by a simple mechanical issue. Existing commercial microwave units were no bigger than the home versions, far too small to be practical for large structural car components. So researchers switched to ferromagnetic nanoparticles that could be triggered by electromagnetic energy from small coils. This led to three years of research on various mixtures of iron oxide nanoparticles and thermoplastic polymers that can create bonds that carry loads, prevent fractures, resist fatigue, and eliminate corrosion—while being readily reversible.
Researchers say they can adjust the chemistry to work with just about any thermoplastic adhesive and achieve better strength ranges than epoxy/thermoset structural adhesives. Interestingly, research showed that the bond strength actually increased after numerous cycles of bonding and debonding… and the joints survived up to 100 cycles. Researchers say this type of electromagnetic bonding/debonding can be done quickly and is amendable to automation. And separating components for recycling at a car’s end-of-life could be made more efficient and cost-effective.
Researchers note some limitations that manufactures and repair shops will need to consider. For example, while bonding/debonding can be done multiple times, the joint becomes a bit thinner each cycle, which might limit the number of practical cycles. And some questions remain, such as whether an iron-containing composite material is at risk of corrosion (the answer appears to be no, so far, but obviously practical experience is limited).
Lab to field
At this point, researchers have successfully tested the technology with multiple joints and do not foresee any insurmountable technical limitations.
But moving from lab to field will require surmounting a common tripping point for new technologies: real-world practicality. ACC has worked with the Center for Automotive Research in Ann Arbor, MI, to evaluate the feasibility of using reversible bonding in current car assembly processes. Based on comparisons with existing joining technologies, it appears the new technology could be viable and cost-effective.
And MSU undergraduate and graduate students are deeply involved in the research, as well, which could help create a trained workforce to apply the technology, leading to additional efficiencies.
Next step: researchers are actively seeking partners interested in field trials.
Connect, fix, recycle… repeat
If successful, reversible bonding could pave the way for significantly more efficient and economical joining, repair, and recycling of car components.
Plus lead to increased use of plastics/composites in even safer, lighter weight cars.
And potentially drive down repair delays and costs… and the environmental footprint of our cars.
See reversible bonding in action. Check out this video: