More than 30% of the materials used in cars today could be products that originate from chemical-company laboratories, according to Evonik Degussa. The use of plastic materials, structural foams, related coatings, additives (such as organosilanes for tires), matting agents, and adhesives is instrumental in limiting the weight gain of vehicles—as are other measures such as new powertrains. However, some plastic materials have been considered too costly for volume production in the automotive industry, as they are currently more expensive to manufacture than their steel counterparts.
The structural foam Rohacell is one example. It is a closed-cell rigid foam based on polymethacrylimide (PMI) that is used as a core material for sandwich components. According to Evonik, it offers the best ratio of weight and mechanical properties as well as the highest heat-distortion temperature in comparison to all other foams.
The fully machinable structural foam is currently manufactured in 11 grades. Its standard quality 71 IG, for instance, has a density of 75 kg/m3 (4.7 lb/ft3), a compressive yield strength of 1.5 MPa (218 psi), a flexural strength of 2.5 MPa (363 psi), a tensile strength of 2.8 MPa (406 psi), and an elasticity modulus of 92 MPa (13.3 ksi). Also, Rohacell resists heat distortion: It offers up to 180°C (356°F) of deflection temperature under (flexural) load and could be used in the autoclave process for 2 h at 130°C (266°F) with more than 0.3 MPa (44 psi).
Rohacell has an over 20-year-long career in the aerospace industry and is currently being used, e.g., in belly fairings, pressure domes, engine cowlings, radoms, and helicopter rotor blades. On account of the higher production costs of composite parts vs. steel parts, its automotive use has so far been restricted to small-volume cars such as super sports cars, racing cars such as those in Formula One, or the Lotus Exige RS from RED, where the clam shells in front and rear and the rear airfoil, made from Rohacell, help to reduce weight significantly.
Rising fuel prices and continually increasing vehicle weight, primarily due to higher safety and comfort levels, may make such materials more attractive. With advanced curing processes at higher temperatures (to shorten cycle times), Evonik calculates that for production scales of 10,000 units annually, the costs for fiber-reinforced sandwich composites with Rohacell are actually lower than that of steel as there is no need for expensive metal forming.
Two Volkswagen Golf V models demonstrate the weight-reduction potential. For instance, the standard steel tailgate of a Golf V has a mass of 21.9 kg (48.3 lb). The modified 1.4-L TSI car, presented by the German Institute of Automotive Engineering (ika), has a tailgate made from structural foam and has a mass of 3.6 kg (7.9 lb). Replacing the conventional 4.6-kg (10.1-lb) glazing with 3.4 kg (7.5 lb) of PMMA (Plexiglas) reduced the weight even further.
The Vox TV Golf V show car presented at the FISITA 2008 World Automotive Congress in Munich, Germany, is an example of more aggressive weight reduction: The car normally has a curb weight of 1360 kg (2998 lb) and is powered by a four-cylinder, 1.9-L TDI engine. Looking only at the weight reduction achieved by using the lightweight materials Rohacell and Plexiglas, and by replacing the conventional 16-kg (35-lb) lead-acid starter battery with a 3-kg (6.6-lb) lithium-ion battery, this amounted to a savings of 102 kg (225 lb). Hood and tailgate are made from Rohacell foam, which is covered by carbon-fiber layers.
“By taking out 102 kg in total, the plastic materials and new battery alone account for 3 to 5% higher fuel efficiency," said Sven Augustin of the Evonik automotive industry team. “Custom-made tires with particularly low rolling resistance further reduced fuel consumption by 5 to 8%. Special blended engine and gear oil with friction-reducing polymers save another 3 to 4% of diesel.”
Replacing the original engine with a VW Lupo three-cylinder TDI, which saved 50 kg (110 lb), together with the many other mass-reduction measures in the interior and chassis, brought the vehicle mass down to 989 kg (2180 lb)—an impressive reduction of 371 kg (818 lb).
Based on NEDC measurements, this increased the car’s fuel economy to 60.3 mpg (3.9 L/100 km), which compares to 41.3 mpg (5.7 L/100 km) for the standard version. The CO2 emissions were thus reduced from 150 g/km to 103 g/km.