Polymer memories activate for quick release

Written by: Tom Shelley | Published:

Tom Shelley reports on an innovation to solve disassembly problems that has the potential to slash the cost of shape memory devices



Most engineering polymers show shape memory effects that might be harnessed for use in mechanisms.

The discovery comes out of research to develop fasteners that could be heated to release their hold when electronic goods reach end of life.

As well as the intended purpose, shape memory devices have many potential applications in mass market temperature controlled devices, presently considered impractical because of the cost of shape memory alloys.

The development of fasteners what would unfasten when heated originally began as a project called, 'Active Disassembly using Smart Materials' initiated by Professor David Harrison and Professor Eric Billett at Brunel University as part of a 'Cleaner Electronics' programme. It commenced in 1996 in the light of the then just published first drafts of the Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS) directives.

The base idea was to use shape memory alloys that would revert to a non fastening shape on heating, so devices could be easily disassembled at end of life for recycling. It was soon realised, however, that these materials are too expensive to be attractive to makers of consumer electronic products, and are hard to form into complex shapes. This led onto trying the shape memory polymers developed by Mitsubishi Heavy Industries which were the basis of the shape changing furniture and car bodies developed by Florian Ortkrass and Hannes Koch, revealed by Eureka in its September 2002 edition.

These materials are block copolymers of two different kinds of polyurethane, one hard and one rubbery. Above their glass transition temperature they are completely flexible and easily formed into shapes that are locked in by cooling them down. Re-heating them allows them to recover their initial shape.

Dr Habib Hussein, technical director of Brunel spinout company Active Fasteners recently explained to Eureka that while the materials work well enough for some applications, such as the shape changing furniture, they are too brittle to allow mobile phones made with such fasteners to survive standard 1m drop tests. The reversion change also occurs with almost no force, unlike metal shape memory alloys.

Dr Hussein then discovered that most engineering polymers exhibit shape memory effects.
The fasteners may then be moulded in the usual way, formed into their fastening form while still hot, and quenched subsequently. The built in stresses which moulders normally carefully avoid to prevent post moulding distortion may then be released by heating at product end of life to permit disassembly. Dr Hussein demonstrated the behaviour of snap and screw fasteners made out of standard polyurethane using hot water from an electric kettle, which showed that the reversion change to non fastening on heating is almost instantaneous. He also demonstrated the effect with a standard test piece made of PET (polyethylene terephthalate), which has a glass transition temperature of 85 deg C, and said ABS produced, "Quite a lot of force" when it reverted to its as moulded shape.

Although the recovery forces are greater than for shape memory polymer, they are still orders of magnitude less than those associated with metals. However, taking up an idea demonstrated in the shape changing car bodies and furniture, it possible to achieve much greater forces by incorporating carbon or glass fibres. The polymer is then used only to lock the fibres in place, which provide the spring force when the polymer is heated above the glass transition temperature and goes floppy. Polymers can be cycled up and down through the glass transition temperature an indefinite number of times, just like metals.

Adding carbon black or carbon fibre also offers a means of applying heat either by induction or microwave heating. The problem then becomes one of how best to recycle composite, but if sufficient heat can be applied, it is possible to burn the plastic off. An alternative approach is to chemically break down composite plastics to monomers, which requires careful identification of polymers used in individual parts and complete disassembly in order to ensure sufficient purity for re-use. Carbon fibre is a commodity whose cost continues to reduce, and even carbon nanotubes can now be purchased from China for about $5 per gram as such or in the form of master batches.

There are alternatives, of course, both for disassembly and recycling generally. TWI, which has been instructed by EPSRC to work together with Brunel and Active Fasteners listed 15 possible disassembly techniques in its initial bulletin on the subject. These were: Removal of fasteners, fracture, fatigue, shearing, ultrasonic, laser cutting, plasma cutting, strong acid, strong alkali, electrochemical, explosive, biological digestion, biochemical, high temperature, low temperature.

Living organisms function until end of life after which they can be relied on to rot and decay. It is possible to do the same with manufactured products, usually by incorporating gelatine, starch, chitin (from insects or maggots) or wood into their construction. Triggering biodecomposition, such as by applying boiling water to break down something that prevents decay is an area of research. Looking after a an SSSI (Site of Special Scientific Interest) woodland, the author has noted that anything made of wood placed in it breaks down completely in a matter of months under the combined effects of bacteria, insects and fungi. All natural products act as food for species that have evolved to live on them, but in this age of genetic engineering, it should be possible to engineer organisms to consume anything derived from organic molecules.

In the shorter term, making use of the shape memory properties of most engineering plastics looks very adequate as a low cost means to aid end of life disassembly. Used to provide the shape memory muscle in consumer products, polymers, possibly reinforced, have the potential to be used to regulate temperature in show mixer heads and taps, and to function as actuators in heating and ventilating systems in a similar manner to shape memory alloys but at much lower cost. They might even be used to work heat driven motors, making use of temperatures on either side of the glass transition temperature.

Research continues. Active Fasteners and Brunel University are looking for partners and collaborators.

Active Fasteners
Dr Habib Hussein

Eureka says: Making use of the shape memory properties inherent in most engineering plastics could make shape memory powered devices much more economically attractive.

Pointers

* Most engineering plastics may be made to exhibit shape memory effects

* The plastic is first moulded, then deformed above the glass transition temperature and quenched to lock in stresses

* On reheating above the glass transition temperature, the material returns to its as moulded shape

* Shape restoring forces are much less than for shape memory alloys

* There is some tendency for quenched in stresses to be relieved below the glass transition temperature over a long time period

* Shape restoring forces could be greatly increased by incorporating reinforcing fibres to act as springs


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