Life cycle assessment of two automobile components: metal versus plastic

Stringent tail pipe emission rules and fuel economy improvements to tackle the climate crisis are driving automakers to produce lightweight vehicles. Although lightweighting is an important strategy to reduce the environmental impacts of vehicles, it may also have disadvantages, as some lightweight materials have higher embodied impacts. Therefore, a life cycle assessment (LCA) is crucial to balance the environmental benefits from lightweighting at the use stage with the burdens of producing lightweight materials. This article presents an LCA to evaluate and compare the environmental impacts of two automobile armrest alternatives –metal (heavier) versus plastic (lighter)–, for two system boundaries: cradle-to-gate vs. cradle-to-grave. The comparison of impacts between the metal and plastic armrests is very different: from cradle-to-gate, the metal armrest presents the lowest impacts in all categories; however, from cradle-to-grave (lifetime travel distance 200000 km), is the opposite, since the higher weight of the steel armrest (+26 % compared to the plastic armrest) leads to higher use impacts due to increased mass-induced fuel consumption. Break-even points (equal impacts for the armrest alternatives) are also calculated as a function of the travel distance: from 14000 km (photochemical oxidant formation) to 120100 km (human toxicity), which emphasizes the need for an LCA not restricted to GHG. This article shows how lightweight materials with higher embodied environmental impacts may result in lower lifecycle impacts, emphasizing the importance of an LCA to evaluate the benefits of lightweighting. A robust LCA (not limited to GHG) is required to successfully curb the environmental impacts of transportation.