A material for very lightweight and stiff car roofs with excellent thermal insulating properties has been developed by polyurethanes experts at Bayer MaterialScience. This was made possible by the Baypreg and Multitec polyurethane systems from BaySystems, the company's global polyurethane business. All kinds of automobiles can benefit from the roof modules' outstanding properties, but they are particularly beneficial for electric vehicles powered by batteries or fuel cells, and for solar-powered vehicles. This is because their practicality and range depend to a great extent on the drive energy being used efficiently. An essential part of this is the weight saving achieved by the new-generation roof modules with their low weight per unit area of just 4.5 kg/m2. The modules' low thermal conductivity also reduces heating energy requirements, especially in winter. A coated Makrofol film gives the roof modules a high-quality surface. The modules also provide superior drive comfort to conventional designs. They boast excellent acoustic properties and are highly resistant to hailstones and mechanical influences. "State-of-the-art polyurethane processing technologies enable highly cost-effective production in both small and large volumes, despite the multilayer structure," explains Detlef Mies from Bayer MaterialScience. The polyurethane experts work with automakers and component manufacturers in projects aimed at series application.
The sandwich construction of the roof module's support structure delivers excellent flexural strength and dimensional stability. It is made up of two glass fiber mats with a lightweight core layer between them - a polycarbonate or paper honeycomb, for example. The mats and the core layer are bonded to the Baypreg polyurethane system from BaySystems by compression molding. At the edges of the roof, the sandwich construction's support function is taken over by the Baypreg polyurethane spray system, which is processed in solid form and reinforced with long glass fibers. The solid area at the edges is used to create a bond to the roof frame of the bodywork. It is also easy to integrate inserts such as handles, and the entire roof module is able to withstand even higher mechanical loads.
The support structure is followed by a multifunctional intermediate layer of the highly reactive two-component polyurethane system Multitec. This prevents glass fibers or cavities from appearing on the roof surface. The material's beneficial properties include very low linear expansion and excellent tensile strength. It also ensures good adhesion between the support structure and the Makrofol polycarbonate film forming the roof's surface. In addition to its impressive brilliance, the film is highly resistant to scratches, cleaning agents and chemicals. Bayer MaterialScience develops coated films for the external bodywork in a contrasting color (black). With transparent coating films it is also possible to use printing processes for colored designs or to create functional prints. The coatings applied to the films then only undergo physical drying. Having been cut to size, the films can be stored until the next processing step. Only when the roof panel has been shaped by thermoforming is the coating fully hardened by UV radiation. The films' clear-coat structure gives them an excellent deep gloss finish