Energy-absorbing foams to protect pedestrians
Driving a car is becoming an ever safer pursuit nowadays what with crumple zones, effective restraint systems, airbags and shock-absorbing padding. Car occupants are increasingly likely, with every new generation of vehicles, to walk away from a crash having suffered nothing more than a nasty shock.
Recently, the attention of policymakers has also focused more intensively on the safety of pedestrians. The idea behind new bumper designs is also to protect the most vulnerable road-users should the worst happen. The energy-absorbing polyurethane foam system Baysafe®, developed by Covestro in cooperation with automotive experts, can be expected to play a key role in the implementation of these new safety systems.
Even when used in thin layers, this semi-rigid foam effectively cushions any impact, should a pedestrian be hit, and enables the designers to develop bumper systems that are up to the demanding safety requirements of the future without having to compromise too much on design. The passenger compartments in a wide variety of cars - including Daimler-Chrysler, BMW, Volkswagen and Audi - have long since included Baysafe® safety padding as standard. This padding has a well-balanced foam structure composed of open and closed cells which upon impact - e.g. in the event of a person's head hitting an upholstered C pillar - are compressed and open up.
This effectively converts the kinetic impact energy into deformation energy. However, whilst safety padding within the car itself merely has to cope with high stresses on a one-off basis and can therefore be brittle, cushioning the impact immediately and without any rebound, the requirements when it comes to designing bumpers are a bit different: the material must be resilient enough to deflect repeated smaller impacts so as to avoid minor damage. This means it has to be much more elastic but must still avoid causing any excessive rebound.
Baysafe® can conform with both these profiles, since the compressive strength and elasticity and thus the force/deformation curve of this polyurethane foam can be finely tuned, not simply by adjusting the density, but also by varying the chemical components in a very specific manner. In this way Baysafe® provides a precisely measurable safety margin in bumper design too.
Tests commissioned by Covestro at the British Motor Industry Research Association MIRA in Nuneaton, Warwickshire, showed that bumpers made from Baysafe® and the flexible polyurethane Bayflex® 180/190 slowed the impact of a model leg moving at 40 km/h with a negative acceleration of only approximately 130 g. This is already well below the strict limit currently being pursued by the policymakers - bumpers already used in volume production today may produce four times this level of acceleration under some circumstances.
Yet at the same time, the mono-material solution based on polyurethane fulfills all the necessary criteria in terms of elasticity and resilience in the event of minor bumps. And it goes without saying that Baysafe® also offers all the benefits of polyurethane technology in this application. Strengthening and design elements are easily integrated into the foam during the foaming process itself, as are sensors to measure distance and distance warning devices. The dimensional stability of the semi-rigid foams guarantees "wobble-free" positioning for many years.
Compared with energy-absorbing foams made from thermoplastics, the mechanical properties of Baysafe® foam backings display significantly lower dependence on ambient temperature and atmospheric humidity. Above all, the material does its job: the force/deformation curve of Baysafe® foams is close to the physical optimum with the result that energy-absorbing layers made from this material can be significantly thinner but just as effective as, for example, EPP equivalents. And so the future requirements with regard to pedestrian protection can be turned into reality without an unattractive "spare tire" around the car's middle.