Abstract
National Highway Traffic Safety Administration (NHTSA) is mandated to promote improvements in automotive occupant protection by specifying vehicle crash-worthiness regulations in terms of forces and accelerations measured during test crashes. A primary focus of these Federal Motor Vehicle Safety Standards (FMVSS) has been in the area of energy-management during the "second collision"—the collision between the occupant and the automotive interior. As relevant to energy-absorbing materials, regulations are continuously being revised and upgraded: FMVSS 201, head impact [HIC(d)] for pillars, roof rails, visors and instrument panels; FMVSS 208, passive restraint for head, upper-thorax and upper-leg protection; FMVSS 213, for child safety seats; FMVSS 214, for side impact protection; and FMVSS 218, for motorcycle helmets. In addition, manufacturers need to be concerned about the potential product liability for enhanced injuries due to the "second collision" when superior alternate materials are technologically available and commercially practical. As a result of these demands to improve occupant protection through energy-management materials, it was necessary to examine those that may find use in these applications, including: expanded polypropylene (EPP), expanded polystyrene (EPS), expanded polyphenylene oxide/polystyrene (EPPE/PS), and polyurethanes (PU). The areas of examination and analysis are dynamic impact and static compression testing; -physical properties; flammability; dimensional stability and energy-absorption as a function of temperature; and chemical resistance. The advantages of polyurethane technology are in its versatility, diversity and adaptability to customer and application needs. Energy-absorbing polyurethanes have properties (e.g., low weight, moldability, relative temperature insensitivity) which allow the automobile manufacturers greater design flexibility at lower cost. Whereas thermoplastics have the one-dimensional ability to vary energy-absorption profiles only through density adjustment, polyurethanes can be customized to meet an infinite variety of energyabsorption profiles and performance characteristics through adjustments in either density and/or polymer structure. Energy-management polyurethanes can be produced as: rigid foams, friable or crushable; recoverable semi-rigid foams; or visco-elastic foams. The object of this paper is to compare the physical properties and impact performance characteristics of EPP, EPS, EPPE/PS, and rigid and recoverable polyurethane foams. The results will show that energy-management polyurethane foams have performance properties which make them particularly suitable for occupant protection applications.
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