Polymers mean business

Some notable progress

Some of my predictions were accurate. New composites are becoming increasingly important, particularly in transport. The Boeing Dreamliner and the Airbus A380 have a significant amount of metal replaced by such materials, and the amount of plastics and composites in cars, for example the Smart car and the Tata Nano, is steadily growing.

Likewise metallocene catalysts, such as zirconocene in combination with organoaluminium compounds, have rejuvenated the polyolefin industry. The improved control over molecular weight and tacticity that these catalysts bring compared with earlier Ziegler–Natta catalysts has shifted the cost-performance balance back towards polyolefins compared with other polymers.

And there have been plenty of interesting niche plastics, textiles, coatings and packaging materials. Shape-memory polymers are now in the marketplace, supplying, for example, the gripping ‘fingers’ for robotic hands. They are also found as heat-shrinking tapes, surgical splints, and safety and sports helmets. In textiles, the liquid crystalline polyester Vectran, from Kuraray, was used for the airbags on Nasa’s Mars Rovers. These ‘polyarylates’ have been known in the literature since the 1980s but have only found significant application relatively recently. Vectran, a copolyester of 2-hydroxybenzoic acid and 6-hydroxynaphthalene-2-carboxylic acid, has a rigid rod structure. In the melt, the molecules form domains of parallel molecules, giving a liquid crystalline structure. This highly extended conformation is carried over into the fibres, resulting in extremely high strength and stiffness.

There has also been significant progress in anti-graffiti coatings. There are several different approaches. Sacrificial coatings are cheap, easily removable topcoats – a good example in the market is a polysaccharide, which is transparent and resistant to rain, UV and mildew, but which when disfigured can be easily removed, along with the graffiti, with steam. There are also permanent coatings, some based on perfluoroacrylates – a sort of non-stick coating – and others made of tightly crosslinked polymers with good surface hardness and chemical resistance, and a high contact angle for water, rather like glass. And an interesting arrival in packaging has been MXD6 from Mitsubishi Gas. This highly crystalline nylon-based on m-xylylenediamine is used in drink bottles where its resistance to oxygen ingress and carbon dioxide egress is competitive with older, more expensive multi-layer technologies.

However, what I failed to predict were all the new, low volume, high margin polymer businesses based on nanotechnology and new polymerisation methods. Rubber nanoparticles, carbon nanotubes, living radical polymerisation, mini-emulsion polymerisation, conductive and light emitting polymers have all made progress. And in the past decade green issues, energy security and oil prices have become increasingly important.

Growing areas

Core-shell rubber nanoparticles, and indeed other nanoparticles, such as silica, are now widely used as tougheners for epoxy resins in composites and adhesives for transport applications. Heat-cured epoxy resins can be very strong, but unless they are carefully toughened they become brittle and fracture. The nanoparticles function as ‘holes’ in the resin – as a crack tip approaches one of the holes, the resin deforms around it and prevents the crack from propagating. A good example of a toughening additive is KaneAce MX 120 from Kaneka, which consists of nano-sized particles with a polysiloxane core and a polymethylmethacrylate shell, dispersed in a liquid epoxy resin.

The price of single-walled and multi-walled carbon nanotubes has been coming down rapidly in recent years, to the extent that composites containing them are now available commercially for high-end applications. The nanotubes have strengths and stiffnesses an order of magnitude higher than conventional carbon fibres. For instance, the Piranha USV (unmanned surface vehicle) is a 15m demonstration vessel built from Arovex nanotube-based composite from Zyvex Performance Materials.

Another breakthrough in industrial polymer chemistry over the past decade has been the progress of living radical polymerisation (LRP) from a laboratory curiosity to genuine bulk applicability. By preventing unwanted termination of growing radical chains, it is now relatively straightforward to prepare on the tonne scale telechelic – reactively functionalised at both ends – polyacrylates with precisely controlled molecular weights, and with the various blocks having controlled glass-rubber transition temperatures and solubility parameters. The reactive end groups can be double bonds, epoxy or silane groups. The most commercially advanced approach to LRP is the variant of ATRP (atom-transfer radical polymerisation) developed by Krzysztof Matyjaszewski at Carnegie-Mellon University, US, in the 1990s and some related techniques that use copper or copper salts and alkyl halides, such as methyl-2-bromopropanoate. Early applications of this approach are high-performance coatings, and high-temperature automotive gaskets.

Emulsion polymerisation has been used for decades to make latexes either for direct use – PVA glues, emulsion paints – or for preparing bulk polymers. However, an interesting variant of this technique is ‘mini-emulsion polymerisation’. Unlike conventional emulsion polymerisation where initiation takes place in the continuous phase, all the monomers are held in stabilised 50–300nm droplets, in which the radicals are also generated. In principle, the technique could be used to make bulk latexes or polymers, but that is not where the real interest lies.

The main value of emulsion polymerisation is the creation of spherical polymer particles of uniform size, with a wide variety of chemical architectures, for example, with hydrophilic, biocompatible outer shells. And it is possible to incorporate other hydrophobic materials inside the particles, notably drugs. The particles can then be fine-tuned so that they can release their contents under certain environmental stimuli – pH, temperature, hydrophilic/lipophilic balance. The dream, which is rapidly becoming reality, is that such particles could be used for controlled-release drug delivery – ideally, for instance, only releasing a highly toxic anti-tumour agent inside a cancer cell.

Using this approach and other polymer encapsulation methods, there are now many drug/nano- or microcapsule combinations under clinical trial. Progress is slow, not least because it is necessary to find out what happens to the particle fragments in the body once they have released their payloads. But a few have already received US approval and there is no reason to believe that many more won’t follow soon.

Light emitting polymers – and their reverse, polymeric photovoltaics – and semiconducting polymers, based on doped conjugated polymers, have been known since the 1980s, but didn’t find much application because of their poor processability. This has been improved in recent years through choice of substituents, novel polymerisation methods and co-polymerisation.

Just as the discovery of light emission via electron–hole recombination in inorganic semiconductors led to the development of light emitting diodes (LEDs), light emission from semiconducting polymers in the Cavendish Laboratory in Cambridge, UK, in 1989, led to the relatively new business of polymeric organic LEDs or P-OLEDs, led by Cambridge Display Technology, now part of Sumitomo. In P-OLEDs, two layers of semiconducting polymers, typically poly-p-phenylene vinylene and PEDOT or F8BT – variants of polythiophene and polyfluorene – are sandwiched between a transparent anode (indium tin oxide) and a metallic cathode.

To date, most OLED displays are still of the small molecule variety rather than P-OLEDs, but the latter are certainly finding application. The first industrial use was in 2002 by Philips in the display of a shaver – used by James Bond in Die Another Day. And a related new business is that of polymeric photovoltaics – ‘plastic solar panels’. Konarka’s Power-Plastic, which incidentally also uses a solubilised fullerene derivative, can already be found, for example, on the outside of computer bags, charging the laptop via solar energy.

There has also been significant progress in the fields of biodegradable plastics and polymers from renewable resources. Probably the best known polymer that is both renewable and biodegradable is polylactic acid (strictly polylactide), -(CHMeCO₂-)_n. This was originally discovered by Carothers in the 1930s and was first marketed in the early 1990s. But it is only in the past few years that its consumption has taken off, thanks to the availability of inexpensive lactide from biorefineries. Similarly, polyamide 11, for example, Rilsan from Arkema, is made from castor oil and has been sold for decades because its performance is similar to nylon12, but it is now attracting more attention owing to its renewability.

However, possibly the most important recent development in renewable polymers is the availability, at competitive prices, of bio-sourced chemical feedstocks traditionally made from petrochemicals. Bioethylene, made by dehydrating bioethanol, was the first to appear, and it is already possible to buy polyethylene made purely from bioethylene. Biobased acrylic acid, methylmethacrylate and terephthallic acid are likely to follow soon.

Of course, ethylene from petrochemical sources is chemically identical to ethylene from a biorefinery, and the same will need to be true to the newer feedstocks, so it will be necessary to certify how renewable is a polymer. One approach being considered is radio-carbon dating since petroleum contains virtually no radioactive ¹⁴C, while plant-derived materials have relatively high levels of ¹⁴C.

Crystal-ballgazing

In this survey of recent developments in polymer and materials chemistry, I have limited myself to discoveries already finding industrial application. But beyond this it would be possible to write whole books on what is happening in academe in the areas of carbon nanotubes and graphene, polymeric nano- and microcapsules for drug delivery, living radical polymerisation and conductive polymers. And the books would be out of date before they were published, so busy are these fields.

So what do I see happening in the next decade or so? Drug delivery is likely to be revolutionised, electronic paper will almost certainly become important, at least some of the many applications proposed for graphene will become reality, we will be well on the way to the all-plastic and composite car, and a substantial proportion of bulk plastic production will come from bio-sourced feedstocks.

No doubt I will have failed to predict again some even more interesting developments, but if they genuinely do make all of the previous developments look mundane, then they will have to be pretty jaw-dropping. I am looking forward to finding out.