Plastic

Plastic is a class of material rather than a material, and the class is defined by behaviour rather than by chemistry: a solid made of long-chain molecules that can be softened and formed into a shape.

What the members share is that ability to be moulded, and remarkably little else. Polyethylene and PTFE differ by a factor of two in density. Polystyrene shatters and polyethylene barely can be broken. PEEK works continuously at 250 °C and PLA sags at 60. There is no density of plastic, no melting point of plastic, and no strength of plastic — which is why this entry records no measured property, and why any figure quoted for 'plastic' is either about one polymer or about nothing.

The division that matters most is between thermoplastics, which melt and re-form indefinitely, and thermosets, which cure once into a crosslinked network and cannot be melted again. Almost everything about recycling follows from that one distinction.

Overwhelmingly from oil and gas: crude oil is distilled to naphtha, naphtha is steam cracked to ethylene and propylene, and those are polymerised. Roughly a twentieth of world oil production ends up as plastic, which is small next to combustion and large next to everything else made from it.

Shaped by injection moulding for complex parts, extrusion for anything long and uniform, blow moulding for bottles, thermoforming for trays, and increasingly by additive manufacturing at low volumes.

Bio-based routes exist and are a small share. PLA is fermented from plant sugar; bio-polyethylene is chemically identical to the oil-derived kind and made from sugarcane ethanol. Neither changes what the polymer is, only where its carbon came from.

Packaging takes the largest share — roughly a third by tonnage — followed by construction, textiles, consumer goods, transport and electronics.

That distribution is worth holding onto, because the public argument is almost entirely about the third that is packaging while the construction share sits in buildings for fifty years and the textile share is most of modern clothing.

Semi-synthetics first: cellulose nitrate from the 1860s and cellulose acetate after it, both natural polymers chemically modified. Bakelite in 1907 is the first fully synthetic plastic and the beginning of the industry.

The modern commodity polymers are almost all of the 1930s: polyethylene at ICI in 1933 by accident, PVC, polystyrene, nylon and PMMA all within the decade. Wartime demand scaled them, and the petrochemical cracking industry built for fuel made the feedstock cheap enough for a mass market afterwards.

World production has risen from around two million tonnes in 1950 to several hundred million, which is among the fastest sustained increases of any material in history.

The honest account has four parts and public argument usually reaches only one.

Persistence. Most plastics do not biodegrade meaningfully in the environment. They fragment, and the fragments are the microplastic now found essentially everywhere that has been looked at. Tyre wear is plausibly the largest single source, which is a use emission rather than a disposal one and no recycling arrangement addresses it.

Recycling. Mechanical recycling works, is the only route operating at scale, and reaches thermoplastics only. It degrades the polymer slightly each cycle, so material tends downward in demand. PET and HDPE do well because they arrive in clean identifiable streams; multilayer packaging cannot be separated by any economic means. A thermoset cannot be melted at all.

Emissions. Most of a plastic's carbon footprint is the energy of cracking and polymerising rather than the carbon in the polymer, which stays locked in the solid unless it is burned.

The substitution question. Replacing plastic packaging with glass, metal or paper generally increases mass, freight and often total emissions, while reducing persistence. Which of those matters more is a judgement about which harm is being minimised, and stating it as a settled question in either direction is the commonest dishonesty in this subject.