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Material · Synthetic

Polyethylene terephthalate

The polyester in every drinks bottle and most clothing — and the only plastic with a genuinely working recycling stream.

PET is the same substance as polyester, and the fact that the packaging trade and the clothing trade use different words for it obscures how much of it there is. It is the drinks bottle and it is the shirt, and by tonnage the fibre is the larger use.

What it is good at is holding carbon dioxide. A PET bottle keeps a soft drink carbonated for months where polyethylene would go flat in days, and it is clear, tough, light and cheap enough to be thrown away — a combination that ended the glass bottle for most purposes within about twenty years.

Processing

Bottles are not moulded as bottles. A thick-walled preform the size of a test tube is injection moulded, shipped or stored, then reheated and blown into shape inside a mould — stretch blow moulding, which orients the polymer in two directions at once and roughly triples its strength.

That orientation is why an empty bottle is far tougher than the same weight of unstretched plastic, and why the base has its distinctive shape: the petaloid form spreads the stress of internal pressure over a curve rather than a flat.

PET is a polyester, meaning the chain is joined by ester links, and those links can be broken by water at high temperature. That makes it hydrolytically degradable if processed wet — a nuisance in manufacturing and the basis of chemical recycling.

Economic significance

PET is the only plastic with a recycling stream that genuinely works at scale, and the reasons are structural rather than moral. It is denser than water so it separates by flotation; bottles are large, clean, uncoloured and consistent; deposit return schemes collect them at high rates; and there is a fibre market that will take the recovered material even when it is not good enough for food contact.

Almost none of those conditions holds for other plastics, which is the honest explanation for why recycling rates differ so wildly between materials that are all described as recyclable.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

contains

  • Carbon — element · the backbone and the aromatic ring that makes the chain stiff enough to hold a bottle's shape
  • Hydrogen — element · on the ring and the chain
  • Oxygen — element · in the ester links that join the units — and the reason hot water can break the chain, which is what chemical recycling exploits

is produced by

  • Polymerisation — process · a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet

is used as

  • Packaging — application · the drinks bottle, which took the market from glass in about twenty years on weight and breakage alone
  • Textiles — application · as polyester, which is the larger of its two uses by tonnage even though the bottle is the famous one

is used in

  • Construction — industry · as insulation board and geotextile, which is where a good deal of recycled bottle material ends up

is an alternative to

  • Soda-lime glass — material · in bottles: a tenth the weight, unbreakable, and a worse oxygen barrier — glass keeps wine and beer, PET took soft drinks and water
  • Nylon — material · as a fibre: nylon resists abrasion and repeated loading better, polyester resists sunlight and stretching better, and that division has held for sixty years
  • Cotton — material · as polyester: the substitution that has run for fifty years and been decided on price and on performance in every measure except comfort
  • PLA — material · as packaging film and rigid containers — and PLA is a contaminant in the PET recycling stream, which is a real cost the substitution imposes elsewhere

is an input to

  • Extrusion — process · as fibre and sheet; the bottle is blow moulded from an injection-moulded preform rather than extruded
  • Mechanical recycling — process · the polymer that recycles best, because it arrives in a reasonably pure stream of identifiable objects — and even so a bottle usually becomes fibre rather than a bottle

is commonly confused with

  • PLA — material · similar enough in appearance and in near-infrared sorting to be mis-sorted, and PLA in a PET recycling batch degrades the whole lot

belongs to the group

  • Plastic — material · the bottle and, in greater tonnage, polyester fibre

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Polyethylene terephthalate back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Polyethylene terephthalate → is produced by (a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as the brine the cell electrolyses, and as the source of the hydrogen that comes off the cathode) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
  • Polyethylene terephthalate → is produced by (a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet) → Polymerisation → takes as input (as tetrafluoroethylene, which is where a substantial share of industrial fluorine chemistry ends up) → Fluorine → is produced by (electrolysis of potassium bifluoride, which is molten and conducts — there is no chemical oxidant strong enough to displace fluorine from a compound, so electricity is the only route and always has been) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite
  • Polyethylene terephthalate → is produced by (a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is extracted from (by electrolysis of brine, which yields chlorine and sodium hydroxide together) → Halite
  • Polyethylene terephthalate → is produced by (a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet) → Polymerisation → takes as input (as tetrafluoroethylene, which is where a substantial share of industrial fluorine chemistry ends up) → Fluorine → is extracted from (the only significant source; hydrofluoric acid is made from it and everything fluorinated follows) → Fluorite
  • Polyethylene terephthalate → is produced by (a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is produced by (in the solar route — the same process, read from the other end) → Brine evaporation
  • Polyethylene terephthalate → is produced by (a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite

These are the most distinct paths back. Polyethylene terephthalate can be traced through others besides.

Downstream — what it becomes