Mass-produced plastics have been in widespread use for roughly seventy-five years. In that time they have become essential to packaging, medicine, transport and almost every consumer product, and they have also accumulated in enormous volumes in landfills, rivers and oceans. Recycling has been promoted since the 1970s as a partial answer, but the record of ordinary mechanical recycling is modest. Most of what is collected for recycling in high-income countries is shipped, sorted, ground into flakes and remelted, producing a lower-grade material that cannot easily return to its original use. A plastic drinks bottle rarely becomes another drinks bottle; more often it becomes a strapping band or a fibre in a piece of carpet.

The limitation of mechanical recycling is that it does not reverse the chemistry that gave the plastic its properties. When a polymer is melted and recast, its long molecular chains are shortened, trace contaminants accumulate, and colours from different sources mix to a grey. The new material can be useful, but each recycling cycle degrades it further, and after a small number of cycles it is no longer worth collecting. For this reason, most of the plastic produced globally has only ever been used once.

Chemical recycling seeks to go further. Instead of merely reshaping the polymer, it breaks the polymer back down to its original building blocks, or closer to them, so that the result can be used to make new plastic indistinguishable from the virgin material. The most developed of these methods is depolymerisation of polyethylene terephthalate, known as PET, the plastic used in most bottles. Under heat and a suitable catalyst, PET can be broken into its two component monomers, ethylene glycol and terephthalic acid, which can be purified and polymerised again. The resulting plastic is identical to that made from petroleum-derived monomers.

Several pilot plants are now running. A facility in the Netherlands, developed in partnership with the Dutch chemical company Ioniqa, processes coloured and mixed PET waste that mechanical recycling cannot accept and returns high-quality monomer to bottle manufacturers. Another plant, in France, uses an enzyme developed at the University of Toulouse to break down PET under mild conditions. The enzyme, known as LCC and engineered from a soil micro-organism, digests plastic at around 70 degrees Celsius. Early tests showed that it could process a tonne of PET in a few hours, with essentially complete conversion.

Other plastics are harder. Polyethylene and polypropylene, which together account for more than half of the plastic made each year, resist the kind of chemistry that works for PET. Several research groups are experimenting with pyrolysis, in which the plastic is heated in the absence of oxygen until it cracks into a mixture of shorter hydrocarbons. The mixture can be refined to produce fuels or feedstocks for new plastic. Pyrolysis has the advantage of accepting dirty, mixed waste, but the disadvantage of energy intensity: heating large volumes of plastic to several hundred degrees is not free, and if the heat comes from burning fossil fuels, the environmental benefit shrinks.

Economics remain the main obstacle. Virgin plastic is cheap, because the petroleum industry has spent decades optimising its production. Chemical recycling, by contrast, is still scaling up, and its costs are correspondingly higher. Policy is therefore central. The European Union has set targets that require a minimum recycled content in new packaging, and several national governments have introduced taxes on virgin plastic. Chemical engineer Dr. Nkechi Obi has argued that without such policies the chemical-recycling sector will remain small, because no individual company can afford to price its product well above the alternative.

Public attitudes complicate the picture further. Consumers have become used to the idea that recycling is a virtuous personal act. Some of the claims made for chemical recycling are technically correct but easily exaggerated. A bottle labelled as made from 'recycled plastic' may contain only a small percentage of such material, the rest having been added later to dilute impurities. Dr. Obi warns against what she calls 'molecular marketing': claims that are true in principle but misleading in the proportions they imply. The long-term test of chemical recycling, in her view, is not whether particular bottles can be made from it but whether the whole system of plastic production becomes substantially less dependent on new petroleum inputs. That test is still years away from being met.