Leverkusen, Germany - Polyurethanes experts at Bayer MaterialScience have developed a material for making very light but stiff car roofs with excellent thermal insulating properties.
The properties were achieved by using Baypreg and Multitec polyurethane systems from BaySystems, the company's global polyurethane business, with a coated Makrofol film giving the roof modules a high-quality surface, BMS said in a 20 Oct announcement.
The combination gives the new-generation roof modules a low weight per unit area of just 4.5 kg/m2. The modules' low thermal conductivity also reduces heating energy requirements, especially in winter, BMS pointed out.
All vehicles can benefit from these properties, but weight-saving is 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. The modules' low thermal conductivity also reduces heating energy requirements, especially in winter.
The modules also provide superior drive comfort to conventional designs as they boast excellent acoustic properties. They are also highly resistant to hailstones and other mechanical influences, BMS claims.
"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.
Module structure
The sandwich construction of the roof module's support structure delivers excellent flexural strength and dimensional stability. It is made up of two glass-fibre 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 by compression moulding. 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 fibres, BMS continued.
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, according to the BMS statement.
Working outward, the support structure is followed by a multifunctional intermediate layer of the highly reactive two-component polyurethane system Multitec. This prevents glass fibres or cavities from appearing on the roof surface and has 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 colour (black). With transparent coating films it is also possible to use printing processes for coloured designs or to create functional prints.
At this stage, the coatings applied to the films only undergo physical drying and, once 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.
"Starting with a thermoformed film and having prepared the sandwich support structure, the entire roof module can be produced in a single manufacturing step using just one mould," explains Mies. He qualified this statement by saying that it is not necessarily the most cost-effective production method. "The larger the volume, the more worthwhile it is to think about separating the production steps," he concluded.
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