Development of Super Olefin Bumper for Automobiles 920525
The EMT (Elastomer Modified Thermoplastics) currently used in passenger car bumper fascia are limited in retaining low CLTE (Coefficient of Linear Thermal Expansion) and impact resistance, although they are highly rigid, which allows a reduction in weight, and also have high flowability during injection molding.
We have developed a new bumper material called “Super Olefin Polymer” using a unique theory based upon a reversal of the current concept. The current polymer design concept of the EMT material is to compound and disperse the EPR (Ethylene Propylene Rubber) into the resin matrix such as polypropylene. We reversed the domain and the matrix, and treated the resin phase as the filler and the elastomer phase as the matrix. The function of the resin as a filler is thus only to maintain the surface hardness and molding flowability, while hard segments are introduced into the matrix elastomer phase to maintain the necessary properties of the bumper fascia by forming a micro-scale IPN (Inter-Penetrating Network) through the control of crystallization speed.
The polymer established using this concept has a molecular-composite structure, and allows realization of a lower weight, higher surface quality bumper material with extremely low CLTE value, good impact resistance and paintability. It also increases the physical properties required for a bumper material by 50%.
Plastic bumpers began to be used after FMVSS 215 was proven effective in the U.S.A in 1972. In Europe around 1976 EMT caught peoples' attention as a new material for plastic bumpers when Italian and German car manufacturers began utilizing them. In Japan, Toyota Motor Corporation developed a RIM (Reaction Injection Molding) urethane bumper fascia and foam system of an energy absorption type, and adopted it on Toyota Celica in 1977. In 1981, an easy-paintable EMT developed by Toyota captured the notice of automotive industry. In the early period of bumper material development, since the key performance of EMT bumper material was its mechanical strength, an embossed surface was commonly employed to hide surface waviness and scratched marks. The requirements for flow marks and surface smoothness were not so severe at that time. However, as the bumper is one of the outer body parts of a vehicle, it must be painted. Today's bumpers without an embossed finish must have high surface smoothness to achieve the quality desired after painting. Other types of materials for the bumpers are PC/PBT and PA/PPO. Most of all bumpers on recent cars are made from plastics.
Though RIM urethane offers good impact resistance and good distortion recovery it is being replaced by R-RIM (Reinforced Reaction Injection Molding) because of the latter's low CLTE value. EMT has benefits of being light weight and low in material cost, but it is easily scratched. PC/PBT and PA/PPO have lower processability and are higher in material cost than EMT. In the early stage plastic bumpers were small and simple, and there was no significant difference in design between plastic bumpers and ones of steel. Then, plastic bumper design enjoyed some freedom of expression when the injection molding process was utilized.
Since then, the bumpers have become more complex in styling and in their consolidation with other parts. They have also become larger, as the bumper was consolidated first with an air-intake and then with fenders. Current bumpers act as a part of the vehicle's exterior panels.  With the expansion in size which causes a greater weight, but a reduction in weight is now necessary from the stand point of fuel economy.
Table 1 shows the physical properties of typical plastic bumper materials.