Making and breaking bonds

German scientists have developed a reversible adhesive system, which also boasts faster cure times. The new system could result in more automotive and aeronautical parts being bonded with adhesives – rather than using mechanical fasteners – which would lead to lighter components. Other advantages include fast repair of the bonded parts, and – at the end of a vehicle’s life, for example – an easier way to separate components for recycling. Sound too good to be true? The secret ingredient is the additive Magsilica – a nanoscale iron oxide embedded in silicon dioxide. Adhesives containing Magsilica are being developed by Degussa and the Fraunhofer Institute for manufacturing technology and applied materials (IFAM) in Bremen, Germany. The catch? Degussa is currently producing only 5-10 tonnes of Magsilica per year in a pilot plant. However there are plans to scale up production with market launch scheduled for 2008. Magsilica becomes magnetic if exposed to an external magnetic field. This means adhesives containing the particles can be heated using an alternating magnetic field, which results in rapid curing. The bonds created can also be switched off using a magnetic field. IFAM’s Dr Andreas Hartwig told Eureka: “The particles are a kind of antenna for the high frequency radiation and are warmed up by this energy. This leads to warming up of the adhesive without heating the parts. The heat is used for curing and, at even higher frequency, also for debonding.” This effect could lead to a shorter bonding process and lower process temperatures. Heat-cured structural adhesives used in automotive construction often require curing temperatures of about 150°C – but many thermoplastics, including ABS, deform at lower temperatures than this, according to Degussa. The new filler could allow thermally sensitive polymers, such as ABS, to be bonded using adhesives with high curing temperatures. For instance the research team used a one-part epoxy resin, which contained 10% of Magsilica by weight, to bond ABS. The onset temperature of the resin was 123°C. Curing for 16 seconds resulted in a lap shear strength of 3.8MPa. The adhesive system can also be used to disassemble parts for maintenance, for re-use, or recycling. The researchers showed with selective heating of Magsilica modified adhesive PC parts could be non-destructively debonded within about 30 seconds. “Quick curing is more important than debonding,” says Dr Hartwig. “Examples of applications are all kinds of transportation such as automotive parts – especially plastic parts used in the passenger cabin, and bonding of parts made from fibre reinforced plastics.” Adhesive bonding can cause production process delays because of the time it takes for the adhesive to set. However, the curing speed of two-part adhesive systems can also be boosted without ovens using the filler. For example glass fibre-reinforced unsaturated polyester parts conventionally glued using a two-part epoxy resin and cured at 80°C for 1,800 seconds show a final strength of 4.2 MPa, the same strength can be achieved within 200 seconds using AC magnetic fields and the Magsilica-doped resin. “Bonding is easier, because it is quicker and in some cases few seconds instead of hours in an oven are sufficient,” Dr Hartwig says. So far the research team has worked with epoxy resins, and some hot melt adhesives. At least one of the components to be bonded must be electrically non-conducting for the system to work. Dr Hartwig says the Magsilica filler can be used for any application where at least one non-metallic part is adhesively bonded in a construction. “But preferentially both parts should be non-metallic. Metal by itself is strongly heated in the electromagnetic field and shields the field. In distinct cases one of the two parts to be bonded might also be a metal, preferably aluminium.” The equipment needed to bond and debond materials consists of magnetic coil and a high frequency transmitter, with frequencies in the range of some hundred kilohertz. When asked how to fit the equipment in a production line, Dr Hartwig says: “It depends on the line, but it is easy to fix the coil on the arm of an industry robot. In other cases stand alone machines could be used.” The technique could replace oven curing in the long term. It could make the design and manufacturing of products simpler with fixations – for example additional spot weld points or rivets – for adhesive bonds being replaced. The filler is not in commercial use, though some companies are beginning to develop applications, Dr Hartwig said. Degussa says the automotive and aviation industries could use Magsilica for lightweight construction, with the adhesives being substituted for nuts and bolts. The company says other applications include increased productivity in rubber vulcanisation and reinforced plastics for aviation.