Leverkusen, Germany - Sandwich composites with high stiffness, good impact strength and low weight can be made using honeycomb or corrugated paper cores between glass-fibre mats, bonded with a polyurethane spray system such as those supplied by Bayer MaterialScience.
But while paper honeycombs have predominantly been used for the lightweight core, BMS now suggests that thermoplastics, and specifically polycarbonate, are "admirably suited to the task" of extending the property spectrum of this type of polyurethane composites.
"Polyurethane composites with a thermoplastic core have considerable advantages over their paper honeycomb equivalents. Above all, they are insensitive to moisture and humidity, making them a logical choice for exposed applications inside and outside the vehicle, such as seat-backs, seat shells and wind deflectors for trucks," said Dr Stephan Schleiermacher, an expert in polyurethane car components at Bayer MaterialScience, in a BMS release.
Typical automotive applications for such composites include trunk floors, floor and spare-wheel covers and sunroof panels.
To make a sandwich part of this kind, the thermoplastic core is embedded between the two glass-fibre layers, and impregnated on both sides with a heat-activated, two-component polyurethane spray system such as Baypreg F.
The construction is then compression moulded at 130° to 140°C to give it the desired shape. All parts of the composite are bonded permanently to one another with the low-foaming PU. Demoulding can take less than a minute.
The result is a stable sandwich composite with excellent strength and stiffness. "This means that economical production with short cycle times is possible even for large mouldings," commented Schleiermacher.