As glazing plays an increasing role in the overall design of a vehicle, manufacturers are looking for ways to use glass on other areas of the body. This has led to increased research into how sunroofs can be used more effectively as a design feature. The sunroof has subsequently emerged as a key styling feature and can in some cases replace the majority of the roof panel. Matthew Beecham reports.

For example, a Peugeot 304 produced in 1969 carried a total of 2.24m². The Peugeot 305 (produced in 1977) featured some 2.82m², while a Peugeot 306 (produced in 1993) carried 2.96m² and the Peugeot 307 currently being marketed carries 3.3m². Adding a Cielo roof on the latest generation Peugeot 307 SW results in a total glazed surface area of 5.34 m².   Guy Faillat, director, body engineering, PSA Peugeot Citroën, told us: "Although there are limitations of glass forming and production methods, there is a clear trend toward larger sunroofs such as those fitted to our 307 SW and C4 models and the wrap-around windscreen found on our RC concept car."

While the use of roof glazing is becoming an increasingly important development for the automotive glazing industry, what are the real driving forces on roof modularization compared with, say, five years ago?  "For the user, the driving force is a car that appears more spacious and less confining," said John Baxter, senior vice president, sales and marketing, Glasstech Inc.  "I call it 'openity'! For the automobile manufacturer, using increased areas of glass generally and specifically in the roof makes the vehicle more aesthetically appealing to the customer, thereby providing a competitive advantage."

In looking ahead, Baxter sees a vehicle where there will be no metal in view above the waistline.  He added:  "This means that large glass panels will be required for side windows and on the roof. These panels may have to be bent to complex shapes in order to follow the roof contour. One can also envision sliding glass panels within the glass roof. Similar panels are already incorporated in the side windows of commercial vehicles. Because glass is not the best structural material there will be a continued need for a metal frame behind the glass roof. The onlookers [outside the vehicle] may see what appears to be a continuous glass surface, but the occupants will see a typical supporting framework clad of course with protective materials."

While novel panoramic glass roofs flood more light into the cabin, they also introduce more weight, noise, solar control and cost issues. Although toughened glass can meet most of these demands, it can limit the opportunities for adding value and functionality.

Michel van Russelt, Solutia Inc's market development director, said: "One thing that is becoming interesting is to combine a piece of tempered glass with the interlayer and an initial polyester sheet on the other side, which is what we call the bi-layer kind of configuration, which allows to combine the benefits of the tempered glass. It has all the benefits of laminated glass and yet still maintains the weight at a reasonable level."

Webasto has developed this kind configuration for Mercedes-Benz, dubbing it Webasto Glass ProTec. In recalling how Webasto developed ProTec, Andreas Engl programme manager, Webasto ProTecâ International, said: "We have been applying solar cells on tempered glass for the past ten years. Five years ago, we asked ourselves 'what else could we apply on tempered glass?' And we got into this safety issue and discussed if we could have an additional film on a normal tempered glass which would therefore create a new safety feature. This is how we started. Currently, we have a production here in the US in Lexington, Kentucky and we are running two projects for a German customer - the Mercedes R-class and S-class. The safety film itself is a DuPont Spallshield film though that means it is a bi-layer. It is a PVB film - the same material used in laminated glass - and on the top inner surface, it is PET film. So when you cut through the glass, on the inner side of the car you have the PET film and then the PVB film which is the bonding film between the PET and the inner side of the glass."

ProTec basically comprises a glass pane, foil and a special bonding. This means bonding a film onto the glass panel in such a way that no splinter or shards can fall into the car interior in the event of the glass breaking. The Webasto patent to glue ProTec to the frame or the body in white increases the penetration force up to eight times compared to laminated glass.  The roof remains smooth on the inside. Webasto says this "gentle glass" is sturdy but also elastic, too. The company envisages a number or potential further applications.

Some glassmakers believe that the shading on panoramic roofs is an unsatisfied market area. "There are not many good solar solutions for roofs currently on the market," said John Brandmeier, global product marketing manager, PPG Industries.  "We believe this is an area where we can bring some value."

Solutia's van Russelt agrees that that shading on such large roofs is a major topic of interest. He said: "That is one of the big concerns and we need to work on different technical solutions, allowing us to reduce as much as possible the amount of heat getting into the car.  Carmakers are addressing this through a combination of different technologies, including graded shaded bands at the top of the windshield which extend into the roof."

Tomorrow's roof

Given that glass designers have adopted the concept of making glass surfaces as panoramic as possible, by widening the windscreens and side windows, how much further can this trend go? Rob Vandel, director of process management, Guardian Automotive Products, said: "We are seeing in North America that side glazings are becoming a little shorter. The beltline of vehicles is coming up - partly due to the new side impact requirements and partly to styling. What I call the 'gangster look' is in style right now. And you will see that in the Chrysler 300 and you see design starting to follow that because that has been popular. Also, glazing is moving to taller windshields and more in the roof. It is about turning that top part of the greenhouse into one long piece of glass much as possible. Even though it is the same height, you are just raising the belt a little."

Brandmeier added: "We will continue to see OEMs pushing the boundaries of what can be accomplished to create customer 'wow' factors. It may be panoramics. It may be accent roof glazing. Currently we are seeing evidence of high windshields coming up into the roofline in North America.  Obviously cost will remain a driver. Certain vehicle segments will support the costs of more complex roof systems, whereas others will not.  But in general, roofs remain a fertile ground for product innovation.

Solutia's van Russelt concludes: "Based on our discussions with the OEMs and the laminated glassmakers, the only real barrier centres on the ability of the glassmakers to achieve very complex shapes using their bending technology. That is probably the only real limiting factor for the time being."

 

Matthew Beecham