PET

A polyester used in drink bottles and textile fibres.

Also known as
polyethylene terephthalate, poly(ethylene terephthalate), polyester, PET resin

In everyday products

Structure and behaviour

[–O–CH₂–CH₂–O–C(=O)–C₆H₄–C(=O)–]ₙ

Ester links connect ethylene-glycol segments to 1,4-terephthalate units in the backbone. The same chemical repeat unit can produce a clear container, a drawn fibre or a more crystalline article because processing changes arrangement rather than monomer identity.

  • A clear bottle combines a suitable resin with controlled cooling and stretching; transparency is not a property of every PET article.
  • Stretching during bottle forming or fibre drawing orients chains, improving properties along the drawing directions.
  • Crystallinity is the fraction of ordered chain packing. It influences opacity and thermal behaviour alongside orientation.
  • Ester bonds are susceptible to hydrolysis, so moisture control during hot processing matters for maintaining chain length.
  • The aromatic units contribute rigidity, while filament fineness and fabric construction determine whether the product feels soft.

Preparation routes

Terephthalic acid route

Esterification, then polycondensation. The acid and diol first form ester-linked intermediates, releasing water. Further reaction builds longer PET chains while small molecules are removed. Both feedstocks have two reacting ends, enabling repeated links rather than just one small ester. Balanced functional groups and extensive conversion are needed for long chains.

Industrial catalysts and controlled heating assist the reaction; they are not alternative monomers. Removing small-molecule products drives the reversible stages towards chain formation.

  1. Polyesters: manufacture and usesChemical Industry Education Centre, University of York

Dimethyl terephthalate route

Ester exchange, then polycondensation. The ester reacts with ethylene glycol, releasing methanol in the first stage. Further reaction builds PET chains. This complete input pair is an alternative to the acid route. Methanol identifies the initial ester-exchange route; the terephthalate and ethylene-glycol fragments retained in PET match those from the acid route.

Catalysts and heat support ester exchange and chain growth; excess glycol and other small molecules are removed as the process proceeds. Feedstock substitution changes the reaction sequence and by-products, not the required two-ended glycol.

  1. Polyesters: manufacture and usesChemical Industry Education Centre, University of York

From p-xylene to terephthalic acid

  1. p-Xylene + Oxygen from air → Terephthalic acid

    Catalytic air oxidation changes both methyl groups into carboxylic acid groups while retaining their opposite positions on the benzene ring. Acetic acid is the process solvent, not an extra carbon fragment in terephthalic acid. Recovery, purification and crystallisation produce the purified diacid used in polyester manufacture.

  1. Purified terephthalic acid: products and manufacturing, annual report 2023, page 94INEOS Quattro

From ethene through ethylene oxide to ethylene glycol

  1. Ethene + Oxygen + Water → Ethylene glycol

    First, catalytic partial oxidation converts ethene to ethylene oxide, a three-membered epoxide. In the selected thermal route, water then opens the oxide ring to form ethylene glycol. Heavier glycols also form and are separated. The two-carbon unit reaches PET through a diol, unlike the direct addition polymerisation of ethene into polyethylene.

  1. Ethylene oxide and ethylene glycol process technologyShell

Material limits

Bottle, textile, clear-tray and heat-resistant tray grades are not interchangeable specifications. A clear PET tray is not automatically oven-safe. Mechanical recycling keeps the polymer as polymer; chemical recycling breaks chains into smaller chemical feedstocks. Colour, multilayers, fibres blended with other textiles and contamination affect what can be recovered.

Sources

  1. Polyesters: manufacture and usesChemical Industry Education Centre, University of York
  2. PET resin: frequently asked questionsPET Resin Association
  3. Sustainable plastics: the role of chemistryRoyal Society of Chemistry
  4. Beyond processing methods: the impact of the drying technique on PET performanceGabriela Campos et al. · 2025