PET
Summary: PET (polyethylene terephthalate) is a condensation polyester formed from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Water is eliminated at each ester linkage. Used for drinks bottles, food containers, and polyester clothing fibre (Terylene). Tags: igcse chemistry organic-chem polymers polyester Created: 2026-07-14 Last Updated: 2026-07-23
What Is PET?
PET stands for poly(ethylene terephthalate) — a condensation polyester formed by the reaction between:
- A diol: ethane-1,2-diol (HO—CH₂—CH₂—OH)
- A dicarboxylic acid: benzene-1,4-dicarboxylic acid (HOOC—C₆H₄—COOH)
Each time the monomers link, a molecule of water (H₂O) is eliminated. The polymer chain is held together by ester linkages (—COO—).
PET is one of the most important and widely recycled plastics. When used as a fibre for clothing, it is commonly called Terylene, Dacron, or simply polyester.
Formation of PET
The Reaction
n HOOC—C₆H₄—COOH + n HO—CH₂—CH₂—OH → —[OC—C₆H₄—COO—CH₂—CH₂—O]—ₙ + 2n H₂O
The Monomers
| Monomer | Formula | Functional Groups |
|---|---|---|
| Ethane-1,2-diol | HO—CH₂—CH₂—OH | Two —OH (alcohol) groups |
| Benzene-1,4-dicarboxylic acid | HOOC—C₆H₄—COOH | Two —COOH (carboxylic acid) groups |
Repeating Unit
O O
‖ ‖
—[—O—CH₂—CH₂—O—C—C₆H₄—C—]—ₙ
The ester linkage (—COO—) connects the monomers. This is why PET is called a polyester.
Properties
| Property | Description |
|---|---|
| Strong | Good tensile strength in fibre form |
| Lightweight | Low density (1.38 g/cm³) |
| Transparent | Clear when amorphous (bottles) |
| Gas barrier | Good barrier against CO₂ and O₂ → keeps drinks fizzy |
| Moisture barrier | Resistant to water vapour |
| Shatter resistant | Does not break like glass |
| Thermoplastic | Can be melted and reshaped/recycled |
| Crease resistant | As fibre (Terylene), resists wrinkles |
| Chemical resistant | Resists most solvents and dilute acids |
Uses of PET
| Use | Form | Property Exploited |
|---|---|---|
| Drinks bottles (water, soft drinks) | Moulded plastic | Clear, lightweight, gas barrier, shatterproof |
| Food containers (trays, jars) | Moulded plastic | Food-safe, transparent, moisture barrier |
| Polyester clothing (Terylene/Dacron) | Fibre | Strong, crease-resistant, quick-drying |
| Carpets and upholstery | Fibre | Durable, stain-resistant |
| Sleeping bag filling | Hollow fibre | Lightweight, insulating, compressible |
| Strapping and belts | Film/fibre | High tensile strength |
| Microwave-safe packaging | Moulded plastic | Heat resistant (up to ~70°C) |
PET Recycling
PET is one of the most recycled plastics worldwide (resin identification code #1):
- Bottles are collected, sorted, and cleaned
- Labels and caps are removed
- PET is shredded into flakes
- Flakes are melted and re-extruded into pellets
- Recycled PET (rPET) is used for new bottles, polyester fibre, and strapping
Benefits: Reduces landfill, lower energy than virgin PET production.
PET vs Terylene
Terylene is the trade name for PET when used as a textile fibre. Chemically identical to PET, but processed differently:
- PET: Melted and injection/blow moulded into containers
- Terylene: Melted and extruded through spinnerets into fine fibres, then woven into fabric
Key Facts
- PET = condensation polyester made from diol + dicarboxylic acid
- Ester linkages (−COO−) in the polymer backbone
- Water (H₂O) eliminated at each link (condensation)
- Thermoplastic → can be melted and recycled
- Known as Terylene when used as clothing fibre
- Excellent gas barrier → used for fizzy drink bottles
- Resin identification code: #1 (most recycled plastic)