Coming unstuck

Gluing materials together may be a reliable way of joining them, but it's also a one-way process - if you need to separate the pieces, it's generally better to use a mechanical method.

But that could well change, thanks to a joint project between the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research in Bremen and German adhesives manufacturer Degussa.

They have designed a reversible adhesive system, so that plastic parts which have been glued together can be quickly and easily disassembled. What's more, the system gets around some of the inherent difficulties of gluing plastics.

Most adhesives are hardened by heat, which is a distinct problem with plastics. Many polymers have low melting points or degrade with heat, so if an adhesive has to be warmed to set, it can be completely incompatible or extremely difficult to use. Two-part epoxy adhesives will set hard without extra heat, but the curing is rather gradual, making the bonding a time-consuming process.

The Fraunhofer team, led by Andreas Hartwig, head of the Adhesives and Polymer Chemistry Department, has added a component to the adhesive which makes it much more convenient to use, and also allows the glue to be 'de-bonded' at will. The key is a filler known as MagSilica, which contains iron oxide particles embedded in particles of silicon dioxide.

The iron oxide particles are so small that they have a property known as superparamagnetism. This means that when the particles are heated, their direction of magnetism 'flips over'. The Fraunhofer team actually uses the reverse of this phenomenon: by applying a rapidly-oscillating magnetic field to the particles, they heat up.

'In the high-frequency field, the adhesive hardens immediately without any exposure to external heat sources,' explained Hartwig. 'The heating process is very fast, so we can solidify the adhesive without transmitting any heat to the polymer pieces we are bonding.'

The same system also de-bonds the adhesive, using the same high-frequency magnetic oscillator, but at a higher intensity to produce a higher temperature. 'We integrate an additive that will degrade the adhesive when it gets really hot, or we put in a blowing agent which then decomposes and destroys the connection,' said Hartwig. 'De-bonding doesn't work in all cases - the bonds and adhesive have to be optimised either for de-bonding or bonding, and if you want quick results, it's still quite difficult. But there is a sufficient process window for both.'

The system works with a very wide range of adhesives - single component, multiple component, thermosetting and non-thermosetting - said Hartwig. 'We deliberately made it non-specific to a particular adhesive, because that would not be useful - they vary so much depending on their application and the materials they are bonding, we had to make a very versatile system.'

The system is likely to be most useful where dissassembly is needed for maintenance, or for parts to be removed for re-use or recycling. For the latter, there are likely to be automotive applications, to ensure that the different polymer components can be separated after cars are scrapped, to comply with new legislation on vehicle recycling.