Green machine: Tackling the plastic menace.

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Green machine: Tackling the plastic menace

* 17:01 28 June 2010 by Helen Knight
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Green machine is our weekly column on the latest advances in environmental technologies

Plastic can take hundreds of years to degrade, so bottles and bags can be a danger to wildlife, strangling birds, mammals and fish, and soaking up toxic chemicals from seawater that can poison any creatures that swallow them.

What's more, plastic is expensive to recycle and requires a significant energy outlay, particularly in sorting and separating the different polymers that may be present.
Mixed plastic

Now Vilas Ganpat Pol at the Argonne National Laboratory in Illinois has developed a technique to convert a mixture of waste plastics into micro-spheres of a form of carbon called carbon black. The micro-spheres can be used in paints, lubricants and tyres, and even incorporated into the anodes of lithium-ion batteries.

To create the spheres, Pol melted a mixture of plastics in a reactor at 700 °C. At this temperature, the pressure in the reactor reaches 34 atmospheres, helping to break down the bonds between the hydrogen and carbon atoms in the polymer chains. The hydrogen gas is siphoned off, leaving behind carbon micro-spheres up to 10 micrometres in diameter (Environmental Science and Technology, DOI: 10.1021/es100243u).

Pol recently used a similar process to convert plastic waste into carbon nanotubes. However, this required the use of a relatively costly cobalt acetate catalyst, which could make the process prohibitively expensive if scaled up. The new technique requires no catalyst at all, says Pol.

Geoffrey Mitchell, a material scientist at the University of Reading in the UK, says the fact that the process uses no catalyst is a major plus, and if the technique can be used to recycle the growing mountain of low-value, mixed plastic waste, it could have a rosy future.
Self-destruct

Meanwhile, Scott Phillips and Wanji Seo at Pennsylvania State University in University Park have developed self-destructing plastics that could lead to packaging that is more easily recycled and friendlier to wildlife. Working with the polymer poly(phthalaldehyde), the team attached one of two chemical end groups, or "triggers" – either a silyl ether or an allyl ether – to each phthalaldehyde building block.

When a square of the polymer was exposed at room temperature to fluoride ions, the central section, where molecules were capped with the silyl ether, underwent rapid depolymerisation and broke down. Those sections capped with the allyl ether remained unchanged (Journal of the American Chemical Society, DOI: 10.1021/ja104420k).

The technique could be modified to develop plastic products that quickly degrade when exposed to triggers in the environment, he says. If a bag made of the right plastic reaches the ocean, for example, microbial enzymes in the water would make the material depolymerise and "the bag just disappears", Phillips says.

By capping all the polymer sections with an end group that responds to a certain chemical, the technique could also be used as a low-energy method for recycling plastic waste, says Phillips. The resulting monomers would have to be re-polymerised to create a new plastic, but this may prove cheaper than separating different polymers before recycling can begin, he says.
Smart materials

So far the team has developed polymers with end groups that react with fluoride ions, palladium and hydrogen peroxide, and they are also hoping to develop polymers that respond to enzymes, he says.

The team cautions that the research is still at the proof-of-concept stage. Work remains to be done to find polymers that break down into substances that are more environmentally friendly than phthalaldehyde. Another problem is that the polymers they have so far made are sensitive to acidity and need to be more stable to be usable. Tweet