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Made-to-measure semi-rigid polyurethane foam for laser perforation

Bayer MaterialScience AG : 07 March, 2005  (Company News)
Automobile instrument panels are becoming increasingly complex and have to offer high levels of safety, comfort and design. This certainly applies to the instrument panel on the new A Class from DaimlerChrysler.
The designers faced a number of challenges during development including how to integrate the passenger airbag invisibly in the component while giving the instrument panel a soft, pleasant feel and meeting the high emission requirements of the DaimlerChrysler specification. The fact that all these requirements could be met is down to the polyurethane (PU) foam which connects the instrument panel to the decorative skin.

'In particular, we managed to configure the semi-rigid foam in such a way that, using laser perforation technology from JENOPTIK, a weakened line for the safe release of the passenger airbag could be created that doesn't damage the decorative skin,' says Josef Beuth, an expert in polyurethane instrument panels at Bayer MaterialScience AG. The foam is manufactured from the polyurethane raw materials Bayfill« VP.PU 54IF25 and Desmodur« VP.PU 58IF06. The instrument panel is manufactured by JR Interiors GmbH & Co.KG of Unterriexingen near Markgroeningen, Germany.

The component is produced using the innovative wet-in-wet method, involving the initial step of introducing the polyurethane decorative molded skin to the mold with a special spray technology developed by Recticel. As soon as the skin-forming reaction is complete, the semi-rigid foam system is introduced into the mold. The mold closes, connecting with the second part of the mold to which the instrument panel made of long glass fiber-reinforced polypropylene has already been applied. As it cures, the foam forms a stable, resilient bond between the skin and the panel. 'The wet-in-wet process works with far higher mold temperatures so we had to adapt the polyurethane system to ensure that a foam with a uniformly fine cell structure formed under these conditions too,' says Beuth.

One particular challenge involved developing a polyurethane foam with a characteristics profile that combined the specified softness with sufficient stability and optimal elongation behavior. The thermal foam stability is particularly important for laser perforation of the airbag break point using the method largely developed by JENOPTIK.

'The minute perforation channels that occur in the foam during laser processing using our micro perforation method should be as small, neat and precisely formed as possible. The improved thermal foam stability results in small holes and means that they do not show through on the decorative layer,' says Norbert Preuss, Head of the Laser Technology Customer Application Center at JENOPTIK Automatisierungstechnik.

In the event of the airbag being released, the optimized elongation behavior ensures that the foam tears evenly and reliably along the perforation line at both low (-30 ░C) and high temperatures (80 ░C). The high tensile strength also prevents foam particles being jettisoned when the airbag is released.

The semi-rigid filling foam meets DaimlerChrysler's stringent specification in terms of fogging and VOC emissions (volatile organic compounds). 'This is thanks to special catalysts that ensure a chemical bond with the polyurethane matrix. Ultimately, this class of catalyst enabled us to make a key contribution to optimizing the characteristics of composite systems for automobile instrument panels,' says Beuth.
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