Bayer and Webasto Design Largest Polycarbonate Roof

Bayer Material Science, in conjunction with Webasto, recently designed and developed a polycarbonate roof for the popular European Smart Fortwo car, which is slated to deploy in the U.S. in 2008. The car, as its name indicates, is only big enough for two people, which makes it the perfect size for the current limitations of a polycarbonate roof.  The roof of the Smart Fortwo car is, as of now, the largest polycarbonate roof set for production; any bigger and the manufacturers run the risk of added stress, instability and inconsistency.

The roof is made through a two-component injection-compression molding process, which uses both Makrolon AG2677 PC and Bayblend DP T95 MF PC/ABS blend to create the dark transparent roof.  “We are using an advanced injection compression process for the first component and modified injection process for the second component,” says Andreas Lang, product expert – polycarbonate glazing for Webasto AG. “The advanced injection-compression process reduces the internal stress of the molded part to a maximum to avoid stress cracking during the coating process and the usage,” he says.

After the initial molding, the roof is coated using a poly-siloxan coating system. This two-component coating system uses a wet-on-wet primer and hard-coat application method. “The first coating step is the primer application to offer the adhesive to the hard coat itself,” says Lang. “Afterwards there is a flash-off and in the second step follows the flow application of the hard coat.” For the final step, “after the hard coat application follow again a flash-off and a curing step,” he says.

Where Webasto is responsible for the actual production and coating of the roof, Bayer provides a more supportive role. “We provided rheological analysis of the part; support in the design of the mold and part; mechanical simulations to simulate the static as well as the dynamic behavior of the part; development of the tailor-made second component material Bayblend® DP T 95 MF PC/ABS blend resin and support during the start up of the production line,” says Dr. Sven Gestermann, key account manager – automotive glazing for Europe at Bayer MaterialScience AG.

“We are observing a strong trend towards the expansion of transparent areas in vehicle
roofs,” he says. “We believe that polycarbonate, namely Makrolon, will play a significant role here due to its inherent advantages compared to glass.” The implications of this trend are generally aesthetic, but since the material is significantly lighter than glass the roof helps save fuel and increases the vehicle’s handling abilities by lowering the center of gravity.

 “Webasto and Bayer Material Science have been diligently cooperating in the development of polycarbonate panoramic roofs for several years,” says Gestermann, “and are continuing to investigate new and unique applications.”

	Looking down on the Bayer/Webasto polycarbonate roof