Plastics on point: killer apps for popular polymers

There are many situations where plastics remain the best option because of the properties they can provide - it’s a testament to the extraordinary properties of carbon, though its Group IV neighbour from the Periodic Table, silicon has also proved to be a strong contender. Improved technology, as discussed elsewhere in this issue, may improve sustainability in the future. But for the here and now, here are five reasons why carbon and silicon polymers remain so prevalent.

Mighty ribbons

Most polymers are hydrocarbons of some kind, readily made from the long-chain carbon molecules that make up crude oil. Under the right conditions - and if you stop it oxidising to carbon monoxide or dioxide - carbon makes a strong polymer just by itself. Most of the time we see it only as the powdery black material graphite, but with high temperatures and stresses, the flat sheets of carbon that form graphite can be coaxed into fibres that can be mixed with epoxies and moulded to form lightweight but incredibly strong structures, from sports bicycles frames to the bodies of racing vehicles.

Further changes to production could yield a better carbon fibre by swapping to suspensions of graphene – the single-layer molecular form of graphene – in the resin. Although fundamentally the same chemical composition, the much finer control needed to make graphene sheets removes a key weakness of carbon fibre: its tendency to form loops that lead to cracks forming when the ribbons are fused together in the final fibre.

Low-friction option

Although popularly thought of as being a product of the space race of the 1960s, polytetrafluoroethylene (PTFE) was the pre-war byproduct of an industry that put a large hole in the ozone layer instead. Working in a New Jersey lab operated by DuPont, Roy Plunkett was trying to make a chlorofluorocarbon refrigerant and wanted to work out why the bottles of fluorine-based gas he was using as a feedstock ran out more quickly than he expected and had formed a white powder around the neck. Cutting one open, he found a slippery white layer inside – polymerised perfluoroethylene.

PTFE’s first industrial use was as an internal coating for seals in pipes used to transfer uranium hexafluoride for the atomic-bomb project. It took another ten years for it to turn up as a lining for frying pans. But as possibly the most slippery substance we know and one that practically makes itself, as Plunkett discovered, it remain the leading material for forming low-friction surfaces though some of the chemicals that help get it into place have since been found to be toxic.

Natural options for cars

Some of the best polymers remain natural. Some of the rubber found in car tyres even today is likely to come from natural sources though synthetic variants that come from oil are used for certain sections of the tyre because they make it easier to tune their properties, such as hardness and heat resistance. One issue with tyre manufacture is how much oil you need to make each tyre – close to 20 litres – as the necessary isoprene precursor is just one component of crude. There is a high energy cost on top.

Although it is more energy-efficient to acquire polyisoprene from the Hevea tree, first found in Brazil, the tyre-making industry finds it difficult to obtain enough and has, for many years, tried to find additional sources for polyisoprene. One option is the Guayule plant that can be made to grow in much drier climates, such as its native US southwest, than the rain forests favoured by Hevea. Another is a Russian dandelion. But despite work on Guayule-sourced rubber dating back to the mid-1930s and has shown promise in recent trials, it is not yet a mainstream source.

Strong contender

Fears over a looming oil supply problem for synthetic rubber in tyres in the 1960s led to chemical companies seeking other options. However, the results often did not wind up sitting underneath vehicles. DuPont chemist Stephanie Kwolek’s discovery of poly(paraphenylene terephthalamide), for example, provided a whole new role for plastic: as a steel substitute.

When she first discovered the material forming as a cloudy layer in a solvent, Kwolek had a hunch the polymer would work well in fibre form. A technician ran it through a spinneret and found, unlike most polymer fibre, turned out to be extremely difficult to break. Although it took some time to refine the production process, DuPont was eventually able to introduce it under the brand name Kevlar. Now almost synonymous with body armour, Kevlar and the Twaron alternative made by Japanese company Teijin, is used for products as diverse as loudspeaker cones and brake pads.

Transparent advantage

In the tongue-twisting polymer poly(methyl methacrylate), which normally mercifully abbreviates to PMMA, the methyl group has a critical function. It stops the polymerised chains from crystallising and so the material often winds up as a glass. As a result, PMMA has spawned a number of glass substitutes usually known by much more famous and jealously guarded trademarks. Those uses as glass substitutes extend to highly flexible and rugged optical fibres.

Despite being flammable, PMMA is relatively inert chemically, which makes it a useful material not just for transparent windows and fibres but as a cement for medical bone replacement and for making dentures. One of its more unusual applications is as a carrier medium for fluorescent dyes in tattoo inks that light up under ultraviolet – though approved by the US Food and Drug Administration only for tagging animals and not use on humans. A final attribute in PMMA’s favour is that it can be depolymerised and recycled back to its original feedstock. The bad news is that the high temperatures needed mean this is not often performed.

Carbon’s competition

Plastic is practically synonymous with the element carbon. In the periodic table, only carbon has the ready ability to bond with itself stably even to the extent of forming triple bonds between atoms in the case of chemicals such as acetylene. Even silicon – one row down in the table below carbon – lacks the same ability. It is more prone to form bonds with oxygen than other silicon atoms, giving us various forms of glass. But those silicon-oxygen moieties readily form into long chains themselves and they are chains with a flexibility that is at odds with regular glass.

Thanks to that, we get from silicone – or polysiloxane – all manner of bendy, slippery polymers that do not freeze solid until they reach cryogenic temperatures or melt even in the presence of boiling water. And the highly stable silicon-oxygen bond makes it unlikely to react chemically in most situations or even provide a home from bacteria or fungi, which has seen the material find a huge number of uses in everything from medical implants to bathroom sealants.