CompositesWorld

JAN 2018

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NEWS CompositesWorld.com 27 What's been tried Fast forward three years and a lot has been accomplished. e team has used iron oxide (Fe 2 O 3 and Fe 3 O 4 ) nanoparticles at loadings of 4-20% in a variety of thermoplastic adhesives. "We characterized the particles in and out of the adhesive, so we could understand their synergy and be able to predict when impact and strength would drop, and what concentration was best so mechan- icals stayed good but the bond was quickly reversible," explains Haq. "For example, at 4% loading, the joint has good impact and strength, but takes 10 minutes to heat. However, at 12-16% loadings, we have a small drop in mechanicals but we can heat the joint in less than 20 seconds. ose bonds still carry loads, prevent fractures, resist fatigue and eliminate corrosion." ey started with polyamide (PA) 6 and 6/6, then tried acrylonitrile butadiene styrene (ABS), polycarbonate (PC), high- impact polystyrene (HIPS) and several olefin-based hot-melt adhesives. anks to Drzal's surface-chemistry expertise and the team's hard work, Haq says they now know how to adjust the surface chemistry of not only the polymers but also the nanoparticles and, therefore, can make the technology work in just about any thermoplastic adhesive system, although he acknowledges that some polymers require more adjustment than others. "is is a fairly tailorable technology, so it doesn't matter if we're using amorphous or crystalline, polar or non-polar resins," he adds. "It's just a matter of establishing application needs, then adjusting the chemistry so functional groups on particles match up with those on the polymer." "It's been part of our technology approach from the beginning to think of this as an enabling technology," adds Day. "We took a fairly broad view of thermoplastics as an adhesive element and tried lots of resins so we weren't limited to one polymer family." He notes that specific commercial applica- tions will still require fine-tuning, regard- less of which package is used. "is is pre-competitive work, so we knew we couldn't figure everything out, but had to focus on proving the concept," Day explains. When resin/particle surface compatibility and substrate surface prep have been rightly done, the team has achieved joint shear strengths in the range of 12-13 MPa (1,741-1,886 psi), which it says is better than epoxy structural adhesives. Interestingly, bond strength reportedly increases after several cycles of bonding/debonding vs. that achieved via convection heating alone. While the initial target was to create a reversible joint that could survive six cycles of bonding/debonding — a goal the team thought a reasonable lifetime expectation — they were pleasantly surprised to find joints surviving 20 and even 100 cycles. Also, electromagnetic bonding is faster than with convection ovens, it does not heat surrounding substrates and is amenable to automation. From the theoretical to the practical When the technology moves from the lab to commercial applica- tion, however, manufacturers will need to be aware of certain limits. One is that regardless of the plastic used in the multi-material mix, it must be a grade without internal mold release, to avoid poten- tial release interference with the bond. Another is that, despite the seemingly limitless theoretical ability to debond/rebond, in practical terms, bondline thickness will limit the number of cycles a joint can survive. e joint is thin to start with and becomes thinner with each rebonding cycle as resin is squeezed out when substrates are forced together. A third concern is whether compos- ites with ferromagnetic particles in the matrix could increase the risk of galvanic corrosion when joined to metal. "We've been thinking a lot about this issue," notes Haq, who says his team has had joints with 18% particle-loading levels sitting in salt-corrosion chambers for more than two months without seeing any corrosion. He feels confident that adhesive and substrates can be tailored for each application in ways that avoid such galvanic reactions. So far, substrate geometry has not presented limitations. e technology has been successfully applied to lap shear joints, out-of- plane T-joints and torsional joints. While exceptionally large electro- magnetic coils probably present a practical size limitation, another MSU professor, Dr. Lalita Udpa, is working on coils to go around very tight geometries. To avoid that late-project falter when it is often discovered that a technology is feasible but not affordable, ACC-PD worked with analysts at the Center for Automotive Research (CAR, Ann Arbor, MI, US) to evaluate the economics of reversible bonding in current automotive assembly operations. "You can do technical research all you like," explains Day, "but in the end, economic reality can really complicate things if you don't address it." Based on benchmarking different joining technologies and comparing those with the new process, the team believes its reversible bonding technology will be cost-effective. Because a trained workforce will be needed to apply a commercially viable technology, MSU undergraduate and graduate students are deeply involved in the research. What's next? e team is looking at coating nanoparticles to increase toughness, reduce corrosion risks, and improve dispersion. ey also are wrapping up simulation work and looking for partners interested in field trials. Reversible Bonds Contributing writer Peggy Malnati covers the automotive and infrastructure beats for CW and provides communications services for plastics- and composites-industry clients. peggy@compositesworld.com Read this article online | short.compositesworld.com/RevMMBonds Any specific commercial applications will still require fine-tuning, regardless of which polymer is used.

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