The vario roof that collapses fully automatically into the trunk of the Mercedes SLK, transforming it into a convertible in less than 20 seconds, makes driving with the top down an incomparable experience.
To give drivers an unobstructed view to the outside even with the top up, the modular roof system is optionally sold with a transparent panoramic roof element from Webasto. The lightweight component is manufactured from Makrolon AG 2677 polycarbonate.
"It's the first time a transparent polycarbonate is being used in an electrohydraulic hardtop of this kind. Our custom material for automotive glazing sets standards in terms of weight reduction, comfort, surface quality and heat protection," explains Dr. Sven Gestermann, key account manager on Bayer MaterialScience's Automotive Glazing team. The base version of the SLK is equipped with a non-transparent roof panel coated in the same color as the body. It also is made of a high-performance thermoplastic from Bayer MaterialScience, specifically Bayblend T85 XF, a polycarbonate/acrylonitrile butadiene styrene blend (PC+ABS).
High optical quality, 40 percent less weight
The transparent panoramic roof element is manufactured by Webasto in a two-component, injection-compression molding process at its Plastics Competence Center in Schierling, Germany. In the first step, the transparent external polycarbonate surface is fabricated with an average wall thickness of 5.5 mm. In the second step, the interior black margin is back-injected with colored Bayblend T95 MF. The mineral-reinforced PC+ABS blend was customized for this process and its properties precisely adapted to the Makrolon AG 2677. The roof element, measuring 1063 x 727 millimeters, can therefore be manufactured to close tolerances, with extremely low interior stresses and warpage. "This is one reason for the excellent optical quality of the glazing component, which is the first thing you notice because of the SLK's low profile. It enhances the overall appearance of the vehicle," says Dr. Jörg Löffler, who as Webasto's Vice President of Product Group Efficiency Technologies is responsible for innovative technologies and thus the development of the polycarbonate roof element. Another advantage is the component's low weight. "Compared with a similar element made of glass, the polycarbonate roof is about four kilograms or 40 percent lighter, which reduces the SLK's fuel consumption and CO2 emissions. What's more, it improves vehicle handling, because the vehicle's center of gravity is lower," explains Detlef Penczek, project manager for Daimler.
The roof element is darkened with a special IR-absorbing gray tone. The driver and front passenger have an unobstructed, undistorted panoramic view, but are not blinded by the sun. This eliminated the need for a shading system, such as an interior roller blind, which means more headroom for passengers. "The colorant developed for the Makrolon AG 2677 used in this application significantly reduces the transmission of heat to the inside of the vehicle, and is just as effective as similar IR colorants for glass. Light transmission and direct energy transmission each amount to only about six percent," Gestermann states. The car interior does not heat up as much as in the sun, and the radiant heat passengers feel from the sunlight is effectively suppressed.
The panoramic roof element made of polycarbonate has a polysiloxane hardcoat from Momentive Performance Materials GmbH in Leverkusen. "Manufacturing this component marked the first time we ever used the optimized SHP 470 FT2050 primer system on a polycarbonate glazing element in a production vehicle, in addition to the established AS 4700 topcoat. The resulting coating lends the roof element its excellent weather stability," explains Dr. Jörg Löffler of Webasto.