Polyurethane automotive components save weight on roof modules

Polyurethane automotive components concept opens up completely new opportunities for saving weight in the roof area for car manufacturers.

With climate change and the current situation regarding energy and environment policy dominating the agenda, car manufacturers are faced with the complex problem of drastically lowering the fuel consumption of their vehicles in order to reduce CO2 emissions. One way of tackling the problem is to pursue a consistent strategy of lightweight construction. A concept developed by Bayer MaterialScience for automotive roof modules based on polyurethane opens up completely new opportunities for saving weight in the roof area.

A prototype component has now been produced for a vehicle that is already in series production. "It combines polyurethane sandwich technology with the polyurethane composite technique, and reduces weight by 25 percent compared with conventional roof modules made of steel," says Dr. Stephan Schleiermacher, an expert in polyurethane automotive components at Bayer MaterialScience.

The necessary polyurethane materials and processing techniques are now so far advanced that parts can be developed for production-line manufacture. Bayer MaterialScience is therefore cooperating extensively with major car manufacturers, focusing on roof modules for mid-size and top-of-the-range vehicles where the calls for weight reduction are particularly vociferous.

The new concept is based on the Multitec* polyurethane spray system. First of all, the system is sprayed without reinforcement into an open compression mold. Later, it also forms the solid facing layer and, as such, is responsible for the quality of the part's surface. The next step involves producing a sandwich prepreg by combining a lightweight paper honeycomb core with two glass fiber mat facing layers. The core layer deliberately stops short of the edges. The entire package is then sprayed with the heat-activated two-component polyurethane spray system, Baypreg* F, with long chopped strands being additionally applied to the edges of the glass fiber mats during the spraying process. After that, the entire sandwich composite package is compression molded at 130 – 140 °C. In this process, the Baypreg F expands slightly, penetrates the honeycomb structures and permanently bonds the glass fiber mats and the facing layers of Baytec* to the core layer. At the same time, the geometry of the mold gives the part its shape.

High torsional and flexural stiffness
The result is an automotive roof module reinforced with a strong glass fiber mat / chopped strand structure at the edges, and a glass fiber mat / honeycomb sandwich structure in the middle. The high glass fiber content at the edges gives the roof module a high level of torsional stiffness, opening up the possibility of integrating elements for fixing the roof or inserts for handles, for example. “The sandwich section with the paper honeycomb is responsible for the low weight of the roof module and also gives it its high flexural stiffness,” says Schleiermacher.

Class A surfaces
The facing layer of Multitec prevents the glass fibers and honeycomb structures from showing through on the surface of the finished part. With its high heat resistance and excellent surface quality, it also provides the basis for the Class A coating surface. Bayer MaterialScience is also working on integrating the In-Mold Coating (IMC) process into the production procedure to permit the manufacture of a pre-coated part. The IMC step would be at the beginning of the production process and precede the spraying on of the Multitec facing layer.

Various machinery manufacturers now offer suitable polyurethane spray units with a 4-component mixing head, opening the door to the use of this new automotive roof module concept in series production. With these units, two different polyurethane systems can be applied in rapid succession. Furthermore, suitable metering units are also available for mixing in the chopped strands.

Single supplier
Through its global BaySystems* organization for the polyurethane systems business, Bayer MaterialScience is able to offer automotive manufacturers of roof modules all over the world customized polyurethane formulations that comply fully with the relevant specifications concerning the materials and finished parts. They also allow the economical production of large series. “Our partners benefit from the fact that, as a systems supplier, we can offer them all the necessary polyurethane materials for the new roof module concept from a single source, and that we also have a wealth of know-how in the processing technologies that we can contribute to the joint project,” notes Schleiermacher.

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