Polyurethane film composites systems for lightweight automotive components

Polyurethane film composites are being used more and more frequently in the production of lightweight automotive roof modules and antenna covers.

Now these parts can be produced with a Class A finish in a color to match the vehicle, as illustrated by the antenna covers on the Mercedes-Benz R and M Class models. The covers are made of a polycarbonate based composite film that is provided with a special coating composition and back-injected with Baydur* STR, a long glass fiber-reinforced polyurethane system from Bayer MaterialScience. Designing the antenna as a polyurethane film composite offers clear advantages.

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

“Compared with a part manufactured from sheet molding compound, the surface quality is significantly better while part costs are lower,” explains Dr. Stephan Schleiermacher, polyurethane automotive parts specialist at Bayer MaterialScience. The neutral-color Senotop** composite carrier film is manufactured by Senoplast Klepsch & Co. GmbH in Priesendorf, Austria, a globally operating manufacturer of extruded plastic sheets und films for thermoforming. Senco specializes in the coextrusion of plastic films for the automotive industry. The film finishing process and production of the complete antenna cover is carried out by the system supplier Carplast, which is headquartered in Altbach, Germany and belongs to Magna Exteriors & Interiors. The company, which is part of the Canadian automotive supplier Magna International, will be presenting the series production version of the component at the VDI conference “Plastics in Automotive Engineering”. Other applications featuring this technology are said to be in the pipeline at Carplast.

Antenna cover requires no complex wet paint application
To produce the antenna cover, the composite film is first treated at Magna with a base coat and a clear coat, provided with a protective film and trimmed. Although the clear coat is physically dried, it is not yet chemically cured. To avoid damaging the clear coat, curing is carried out by UV radiation after the film has been thermoformed. Another advantage of this specially coated film surface is the color matchability of the film and its outstanding resistance to scratching and chemicals, which meets the performance requirements of new bodywork finishing systems. The last step in producing the antenna cover is to back-inject the film insert with Baydur STR. This method of preliminary film coating and processing minimizes the logistical and technical effort otherwise required to subsequently spray-coat a finished plastic part.

Large film composite parts are the trend
One advantage of using Baydur STR is the possibility of manufacturing large polyurethane film composite parts with complex geometries and sudden changes in wall thickness. The product not only adheres well to a wide variety of thermoplastic films, but also lends a part high stiffness and good dimensional stability thanks to its low coefficient of thermal expansion. It can be processed at low temperatures and low cavity pressures, which minimizes stress on the film surface. Metallic anchoring elements and inserts can easily be molded into a part. Because the polyurethane system's formulation contains special demolding additives, the shot-by-shot use of external release agents can be eliminated virtually entirely during manufacturing, thus contributing to high process reliability and productivity.

Bayer MaterialScience markets special formulations of Baydur STR for the various technologies used to back-inject thermoplastic films, such as S-RIM (Structural Reaction Injection Molding) and long fiber polyurethane processes (LFI, InterWet, CSM). “Under the umbrella brand BaySystems* – which represents our global polyurethane systems business – we sell these formulations individually tailored to the needs and demands of our customers, so as to guarantee optimum finished parts,” observes Dr. Schleiermacher.

* Trademarks or registered trademarks of Bayer MaterialScience.
** Registered trademark of Senoplast Klepsch & Co. GmbH.

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