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Bayer Develop Use of Polyurethane for Composite Applications

08 October 2004

Bayer MaterialScience have developed a sprayable polyurethane system called Multitec Short Fibre Spraying, supplementing the existing range of filled reinforcing materials.

Because of its glass fibre reinforcement, parts made from the Multitec Short Fibre Spraying process can absorb considerable forces and ensure long term dimensional stability.

This reinforcement is absolutely essential, because a PMMA film, which is only a few millimeters thick after thermoforming, will find it very difficult indeed to bear the weight of an adult without changing shape. "Shower trays (one of such uses for this technology) must, however, have good shape retention under high concentrated loads – and also, of course, under heat," says Dr. Schütze a polyurethane specialist with Bayer MaterialScience.

To guarantee this stability, many manufacturers still opt for the complicated hand lay-up process with glass fibre-reinforced polyesters (GRP), which need styrene as a solvent and co-reactant. "However, such technology could almost be described as antiquated today," says Schütze. The production of a single shower tray or bathtub using these manual techniques can take up to one hour. Following the manual production step, the product then has to be subjected to an energy- and time-consuming annealing process so that the unsaturated polyester resin can cure completely.

The new polyurethane system has a property level comparable with the GRP polyester resins, but is both economically and ecologically superior. The process involves the production of a reinforcing layer by spraying a fast-curing polyurethane at room temperature – possibly by a robot – onto the back of the thermoformed film. With this efficient process, the properties of the moulded part can be varied to suit the customer's requirements and the specific application. Layers with or without glass fibres, solid or cellular, can be produced individually, seamlessly and in a single process.

"In the case of the Duscholux shower trays, the outstanding material properties of the Multitec Short Fibre Spraying coat result from the addition of glass fibres to the jet of polyurethane material immediately after it leaves the spray head. This makes the new system an environmentally friendly, sprayable GRP alternative, adding another particularly economical variant to the existing polyurethane spray systems. The Multitec Spray System is also faster than conventional reinforcing systems. The excellent properties of the coating are attained immediately as the polyurethane cures, in other words less than 15 minutes after the Multitec reaction mix has been produced. Nor is there any need for an annealing process with this method, "making production of the high-quality spray shower tray up to five times faster than the conventional manual method of GRP hand lay-up," says Dr. Schütze. "Consequently, attractively designed sanitary furniture now goes hand in hand with efficient, ecological production."

Material concepts that have proven effective in the aerospace sector have long since found their way into everyday applications, such as in the motorhomes made by Hymer AG of Bad Waldsee, Germany.

The rear end of its newest vehicle generation is made of a lightweight but very strong sandwich composite element manufactured using the polyurethane spray system Baypreg from Bayer MaterialScience AG. The material offers manufacturers significant processing and cost advantages over the epoxy resins used previously for similar applications.

"The back section of the new Hymer motorhome is a real precision component," says Elke Springer, who helped to develop the Hymer part at Bayer MaterialScience AG, both in the laboratory and in onsite technical service. The product measures 200 x 50 centimetres but weighs only 11 kilograms. Despite its low weight, it still displays all the mechanical properties required for use in a high-end motorhome. For example, its impact and torsional resistance are better than that of similar products made of epoxy resins.

These "inner qualities" come from the product's clever sandwich design, comprising a core made of expanded polystyrene and two natural fibre facings, which reliably absorb all tensile forces. All of the components are joined together in a strong, permanent bond by the Baypreg polyurethane spray system. "To manufacture the composite, natural fibre mats are impregnated with the liquid polyurethane system and placed in an open mould. They are followed by the core layer and another impregnated fibre layer. Once the mould is closed, the components are automatically compressed into the desired shape. The polyurethane cures in about ten minutes," says Elke Springer, describing the production process. "Overall, we beat the cycle time for epoxy processing by a significant margin."

In addition, the Baypreg method enables products with a complex design to be made in a simple manner. "The compression process allows a very high degree of design freedom," explains Springer. "For instance, the rear end manufactured by Formtec has a tricky 'L' shape with exterior grooves and projections." Other aspects demonstrate the flexibility of the process as well. For example, the range of possible core materials is very broad. Apart from paper or thermoplastic honeycombs and rigid foams, sensitive materials such as expanded polystyrene can also be used, thanks to the low processing temperatures. Moreover, if the core layers are not all that critical, the curing time can be cut to three minutes by increasing the mould temperature.

Ms. Springer believes the cost-effectiveness of the Baypreg process offers additional advantages over epoxy resin. "The Hymer rear end is provided with a high-quality coating in the mould by means of in-mould coating. Compared to the epoxy resin method, the lower processing temperatures and the elimination of solvents offer added economic and ecological advantages, which an increasing number of processors have come to value," explains Springer. And the faster cycle times, which permit better mould utilization for example, are another factor in favour of using Baypreg. "Finally, in contrast to other materials, we can dispense with complex after-treatment of the moulded part, such as post-curing. The parts exhibit all their final properties immediately after cooling."





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