Fossil fuels collectively supply about 85% of today’s world energy; of this amount, transportation powered by petroleum produces 25% of man-made atmospheric carbon dioxide – a greenhouse gas contributing to global warming. Consequently, petroleum is facing threats from renewable energy sources, climate change concerns, EV usage and politicians sometimes promoting pseudoscientific environmental policies. It seems that future undertakings directed at decarbonizing transport will be driven mainly by government legislation and mandates, not by consumer preferences or diminished petroleum supplies. The tire industry will not be immune to these pressures, but the movement toward sustainable mobility could be problematic for tire manufacturers and their suppliers.
Petroleum is readily available and serves as a plentiful source of organic molecules for polymer synthesis. Consider that polymeric materials and petroleum-derived substances may comprise 85% of a tire’s weight. Furthermore, tires consume about 60% of the global rubber supply; of this amount, 60% is synthetic. This man-made rubber is totally dependent on hydrocarbons such as naphtha, ethane and benzene, derived from non-renewable oil and natural gas – though this is only a small slice of world petrochemical output.
Take into account that tire elastomers should preferably be capable of high-volume production, have relatively low cost and possess suitable physical properties. Rubbers derived from petroleum that meet these conditions are styrene butadiene (SBR) and polybutadiene (BR) found in most tire components including tread and sidewalls; variants of butyl, although produced at lower volumes, are essential for innerliners. Biomaterials will have high standards to meet or exceed before achieving commercial success in tires.
Biopolymers can be produced from ethanol through fermentation and distillation of corn or sugarcane. Butadiene monomer, a feedstock for SBR and BR, has been made from bioethanol for decades but has not proved to be competitive with crude oil derivatives. BioIsoprene has been developed by Goodyear and Dupont as a replacement for traditional isoprene using renewable feedstocks, but the process has not been scaled beyond the concept tire stage. Decarbonizing challenges remain daunting; for example, recent studies counterintuitively conclude that ethanol as a biofuel is a greater contributor to global warming than gasoline.
Then there are accelerators, antioxidants and antiozonants. All are complex organic molecules, crucial for manufacturing efficiency or extended highway service; all are created from petroleum products and not easily extracted from biomaterials, if at all.
Polyester, nylon and aramid tire cords are all derived from crude-oil feedstocks. Inorganic tire cords, such as steel and fiberglass, have their own undesirable physical properties – plus their carbon footprints. Man-made rayon, a natural polymeric material primarily produced from wood cellulose, has formidable environmental drawbacks. Recently, aramid cords featuring green raw material feedstocks have been developed by Teijin with undisclosed costs. Relatedly, processing oils seem to be readily replaced, often with much fanfare, using plant-based oils. However, replacing all carbon black with non-renewable silica, or other inorganics, still presents unfavorable price or performance trade-offs.
Adhesives, or dips, are required to promote cord-to-rubber bonding. Dips generally contain a toxic resorcinol formaldehyde resin and a rubbery latex incorporating vinylpyridine. Fortunately, resorcinol can be extracted from the distillation of brazilwood. Less hazardous synthetic dips have recently been developed by Kordsa and Continental.
Some materials used in tires do occur naturally, but most are created synthetically from petroleum. Attempting to replace all fossil-fuel tire constituents with biomaterials will be impossible in the near term and extremely difficult in the long term, but will create challenging job opportunities for organic chemists.
It seems that there will always be an important place for oil and gas in the global economy. For example, some sources state that 99% of pharmaceutical feedstocks are derived from petrochemicals. Nonetheless, rules and regulations will continue to promote a carbon-neutral future.
An actual zero-carbon global future is unlikely. Constructing new wind turbines, solar panels and storage batteries will not be carbon free – they call for a prodigious increase in demand for depletable base metals and rare earth minerals. Unfortunately, I surmise that tomorrow’s ‘greener’ tires will cost more and wear out sooner than today’s fossil-fuel tires.
