Changes in vehicle materials are made for a variety of reasons, including cost, absolute performance, lightness, (to improve fuel economy), and longevity. The magnitude of fuel savings over an estimated 174,000 km vehicle road life has been computed as 15-25 l/kg of weight saved.5'6 Though improved engineering design may achieve weight reductions, a point is reached where further improvements may be made only through the use of light materials. Considerable effort has therefore been devoted to substitution of steel and cast iron by lighter materials, such as aluminium and polymers.
Polymers are used in manufacturing for financial and technical reasons. In transport they can provide reductions in the first cost.7 Such cost reductions are not necessarily permanent, since technological improvements and shifts in the raw materials prices continually give one material a cost advantage relative to another. Such a case is the substitution of plated ABS for zinc-based die castings in trim. However, plastics also offer other advantages: their use can achieve significant weight savings relative to metals and, whether for this reason or for others, they may offer substantial lifetime energy savings. For example, it has been calculated that although manufacture of a given automotive fuel tank in HDPE typically consumes 7% more energy (taking into account both the energy content of the material and the energy required for fabrication) than does a corresponding tank in steel, (0.738 GJ/tank in plastic compared with 0.685 GJ in steel) 68% of the total energy consumed in the production of the latter is associated with the conversion of sheet steel into tanks and so cannot be reclaimed through recycling processes. By contrast, most of the total energy of manufacture of a plastic fuel tank is inherent in the material and only relatively little (12%) with fabrication into the finished article. Thus, plastics have a low requirement for energy once the material has been manufactured. In addition, weight savings associated with the plastic fuel tank represent a saving of 23.3% in the fuel required to transport the tank during an estimated vehicle lifetime of 150,000 km. If the energy required for recycling is also included in the calculations it may be seen that, relative to the recycled steel fuel tank, the recycled plastic tank can effect a 28.8% energy saving during manufacture, lifetime use, and recycling.8 There is, therefore, some advantage in reclaiming the plastic as material rather than burning it for energy recovery.
So far as recyclability in its widest sense is concerned the most important change has been the reduction in the amount ofsteel used. Ofincreasing significance now, however, is the disposal cost for the non-metallic residuals of vehicle and appliance dismantling and shredding operations; their low density makes them expensive to transport and landfill. This problem is likely to intensify. The 1960 model composite car described by Dean and Sterner2 contained a mere 0.9 wt% polymers; in the model years 1972 and 1973 U.S.-built cars typically contained almost 5% polymers, largely in safety and comfort applications. By 1985 the plastics contents of automobiles produced in various countries were 8% (Japan), 9% (USA), and 10.5% (Federal Republic of Germany). It is estimated that these figures will, by 1995, have risen to 13%, 14.5%, and 15%, respectively, with a corresponding increase in the quantities of non-metallic waste during scrapping.
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