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01/01/2011
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Plastic windscreens are still a distant dream, as car manufacturers look to commercialise more promising glazing applications. Lou Reade reports
The uptake of plastics in cars continues to increase and one of the fastest growing areas is in automotive glazing. Just as designers inspect every metal component to see if it could be replaced with plastics, the same is true for glass.
Side windows, panoramic roofs and headlamps can all now be made in plastics – usually polycarbonate. A recent report from Chemical Market Associates Inc (CMAI) says that plastic glazing is the fastest growing area for polycarbonate.
When glass first began to be replaced by plastics more than 10 years ago, the industry had its sights firmly set on a plastic front windscreen – which would allow huge weight savings at a stroke. But this still remains a distant dream. The technical and legislative challenges of developing a plastic windscreen are huge: scratch-resistant coatings need further development; major chassis redesign may also be necessary, as the whole body of the car relies on the stiffness of the front windscreen; and the cost of glass is relatively low.
This means that other plastic glazing applications – such as rear window modules – will see commercialisation much sooner.
"Right now, people are working on these other areas of glazing," says Michael Fischer, sales manager technologies at injection moulding machinery manufacturer Engel. "Windscreens are a long-term option."
The major benefits of plastics are weight saving, design flexibility and the potential for component integration. All of these advantages are combined in 'rear window modules' – a new concept in which a conventional hatchback rear window is redesigned in plastic, so that it incorporates other parts, such as
rear lights, registration plate and radio antenna. According to Fischer, this has huge potential to deliver savings in weight and cost, through parts integration. "For a front windscreen, that kind of integration would not be possible."
Polycarbonate supplier Bayer Material Science (BMS) – which has bought a large injection moulding machine from Engel to test out glazing concepts – showed a plastic rear window module at the recent K2010 plastics exhibition in Germany.
Joachim Simon, head of the automotive and transportation segment of BMS's polycarbonates business, says that the prototype tailgate offers new styling options for car manufacturers. Instead of a conventional metal carrier with glass window, the part has an outer skin of coated polycarbonate that incorporates all rear lights, including one for the licence plate. "Everything is included in a single injection-moulded part," he says.
The company is expecting to move ahead with several development projects based on this concept, which explains why it has little interest in front windscreens. "I think that, as long as cars remain unchanged at the front end, there's not much emphasis on replacing glass in the windscreen," Simon comments.
"Maybe when things change – and you have an electric motor on each wheel and a different construction concept – then why not?"
For all its advantages, polycarbonate has one serious downside, in glazing terms: its scratch resistance. However, this can be overcome with the latest coatings. "The front windscreen is the most demanding, in terms of performance," says Stephen Shuler, chief technology officer of Exatec – the plastic glazing arm of polycarbonate supplier Sabic Innovative Plastics. "We're not particularly focused on it right now – we see greater opportunities elsewhere."
Exatec recently signed a deal with Ulvac to improve its offering in plasma coatings for plastics. Shuler explains that all plastic glazing needs a 'wet coat' – which gives around six years of weathering performance. "Adding a plasma coating on top will extend that to 10 years," he says.
In common with other developers, he sees rear windscreens as a huge market. Sabic's Lexan polycarbonate has been specified in one of the few commercial applications – the Honda Civic in Europe. While this only uses a wet coat, he says that a plasma coating would be needed for applications in North America. "The challenge is to look at wiper performance," he points out. "We need to know how many cycles it can withstand."
Germany's Evonik comes to plastic glazing from a slightly different perspective: it supplies Poly(methyl methacrylate (PMMA), rather than polycarbonate, but says the material has some potential advantages.
"PMMA has excellent weatherability, and a naturally hard surface – but, for normal passenger cars, it would still need a scratch-resistant layer," says Rudolf Blass, director of business development for automotive and surface design.
The material is commonly used in external building applications and has also proved itself in demanding environments, such as aerospace. Evonik is developing glazing applications using both injection moulding and thermoformed sheet.
"Glass will be used for years to come, but the doors are open for alternative materials," states Blass. "Lightweight design is becoming so important that this will increase the use of plastics in glazing."
One potential area is in movable side windows. Again, these need scratch resistance – but probably less than for a front or back windscreen. And while there is a need for part redesign, this opens up the chance of part integration.
"Moving side windows are possible in principle, but some smart solutions will be needed in the construction of the door itself. You cannot just replace glass with plastic."
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Author
Lou Reade
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