Glass ceiling

Polycarbonate could replace glass in almost all car windows, reducing weight and improving safety, according to engineering plastics producer Bayer MaterialScience.

The material, most familiar as the transparent shell of CDs and DVDs, is already in production for some niche automotive applications, and demand is expected to rise sharply in the next few years, said MaterialScience chairman Hagen Noerenberg. ‘We estimate that up to 30 million car windows per year could be made from polycarbonate within the next three to five years,’ he said.

European and other regulations still prevent the use of any material other than glass for windscreens, but all other windows could be – and, in some cases, are being – replaced with the polymer material, said Noerenberg.

Cars contain an average of 44kg of glass. But as polycarbonate is around half the density of glass, this could be cut to 26kg if it were replaced entirely with polycarbonate. Moreover, polycarbonate is stronger and stiffer than glass, and is more resistant to impact, leading to improved safety performance.

In particular glass side-windows tend to shatter when cars roll over, which is a major cause of injury and death in car accidents. Polycarbonate would remain intact, keeping passengers safer. It is also easier to mould and shape than glass, requiring lower temperatures to process.

Despite this, glass still has three major advantages over polycarbonate: scratch resistance, stability in ultraviolet light and cost. Car glazing has to withstand punishing conditions, with flying stone chips, friction from windscreen wipers and even the scraping that occurs every time the windows are wound up and down.

But Bayer believes it has solved this problem, and some manufacturers are already using the company’s polycarbonate, Makrolon, in its higher-end models.

The company has overcome the problem of scratch resistance by developing a method for coating the polycarbonate with a tough film.

Collaborating with the Institute for New Materials in Saarbrucken, Bayer researchers have developed a method for coating polycarbonate sheets with a liquid silicate solution which is hardened in an oven at moderate temperatures to produce a silicate coating containing microscopic ceramic particles. Known as the ‘sol-gel’ process, this technique is also being used to add further layers to filter out UV and infrared radiation.

Increasing sizes of polycarbonate production plants have reduced the cost of the material to the point where it is now seen as a commodity rather than a speciality product, although it is still more expensive, weight for weight, than glass.

The increasing complexity of car body shapes has worked in the material’s favour, however, according to Ian Paterson, responsible for R&D at Bayer. ‘When manufacturing panorama roofs, there is now little difference in cost between Makrolon and laminated glass,’ he said.

Car manufacturers have been quick to catch on. In the latest generation of Smart-type cars, the rear side-window and panorama roof are made from Makrolon; Mercedes Benz also uses the material for the transparent hatch door of its sports coupé.

‘Designers are already considering light domes featuring 360 degree vision,’ said Paterson.

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