BAYER CORPORATION PAPER DESCRIBES TWO BREAKTHROUGHS IN POLYURETHANE RRIM FOR AUTOMOTIVE EXTERIORS
01 October 2001 - Bayer MaterialScience AG

In his paper, 'Advances in Polyurethane RRIM for Automotive Exteriors,' Michael Hurley, Development Scientist for Bayer Corporation's Polyurethanes Division, describes two breakthroughs in RRIM for automotive body panels. These are the elimination of post cure with the Bayflex® 190 elastomeric polyurethane/polyurea RRIM system, and the achievement of metal-like, isotropic thermal expansion with the Bayflex 180 elastomeric polyurea RRIM system.

The paper is being presented at the Alliance for the Polyurethanes Industry's Polyurethanes Conference 2001 here.

Bayflex® 190 System: No Post-Cure RIM

In the past, RRIM molded parts were required to be baked at 120 degrees Celsius and above to complete chemical reactions, attain complete physical properties, and de-gas parts prior to painting. More recently, RRIM has been used for pickup truck fenders where it replaced SMC (sheet molding compound) or steel. In these cases, the RRIMsmaterial is required to be cured at 190 degrees Celsius prior to prime and assembly in order to improve properties such as modulus and sag and to prevent out-gassing during paint processing.

The Bayflex 190 elastomeric polyurethane/polyurea RRIM system, used for automotive exterior body panels such as car and truck fenders, is capable of replacing SMC and metal in part due to its stability at high temperatures. Because the Bayflex 190 elastomeric polyurethane/polyurea RRIM system exhibits remarkable toughness and is nearly completely cured at demold, it seemed possible to eliminate or significantly reduce the need for post cure, according to Hurley.

'We conducted a series of analytical tests to determine if it might be possible to omit an oven bake prior to prime for automotive body panels, since the material is already cured enough for safe handling,' said Hurley. 'The results of the analytical tests and subsequent testing on an actual paint line confirmed this.'

The omission of the oven bake provides several benefits for manufacturability, including lower energy consumption, decreased handling and lower capital investment for new high-heat body panel applications. 'Overall, this would increase productivity and reduce manufacturing costs,' he continued.

Bayflex 180 System Low CLTE RRIM

In his paper, Hurley explains that molding composite door skins has been a challenge, because unlike fenders or fascia, doors are constrained with regard to thermal expansion. In order to maintain a proper gap between the doors and surrounding body panels, the coefficient of linear thermal expansion (CLTE) in the horizontal direction must match that of the supporting metal frame.

To wit, Bayer has reformulated the Bayflex 180 system to accept high loading of reinforcing filler so that it can control the thermal expansion characteristics in the composite RRIM part.

'Increasing the filler content ofBayflex 180 elastomericacacapolyurea RRIM system to 25 percent and above yields RRIM polyurethane material suitable for automotive door skins,' said Hurley. 'Furthermore, the filled system is molded so that a laminar-like flow is maintained during mold filling, so that the fibers are aligned in the final molded parts,' continued Hurley, who pointed out that gate design and location can be selected to maximize this effect and control the direction of flow.

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