PET under heat
How polyethylene terephthalate behaves from storage to moulding: glass transition, blowing window, crystallisation and melting. Move the temperature and watch the preform.
The repeating unit of polyethylene terephthalate: ethylene glycol and terephthalic acid residues. Thousands of them in a chain give PET its strength and clarity.
- ≈1.33 g/cm³density of a clear preform
- code 1PET recycling mark
- ≈75 °Cglass transition: PET softens above it
PET properties in numbers
Typical values for bottle-grade PET. Exact figures depend on the resin grade.
| Property | Value | What it means |
|---|---|---|
| Density | ≈1.33-1.34 g/cm³ | clear amorphous PET; crystalline PET is denser |
| Glass transition, Tg | ≈70-80 °C | PET softens above this temperature |
| Blowing window | ≈95-110 °C | the preform body is pliable and stretches |
| Crystallisation | above ≈120 °C | PET turns white and loses clarity |
| Melting | ≈250-260 °C | crystals melt and PET turns liquid |
| Preform moulding | ≈270-290 °C | melt temperature in the moulding machine |
| Viscosity (IV) | about 0.8 dL/g | typical of bottle-grade PET |
| Recycling code | 1, PET | PET is recycled into secondary raw material |
Why PET is clear
In a preform, PET is amorphous: the molecular chains are randomly arranged and do not scatter light. Held too long in the crystallisation range, the chains form crystals and PET turns white. That is why preforms are cooled fast after moulding and not overheated during blowing.
What happens during stretching
During blowing the molecular chains line up along the stretch directions. This oriented PET is stronger and holds gas better than the original preform material. That is why a thin bottle wall can hold the pressure of a carbonated drink.
Moisture and drying
PET absorbs moisture from the air. Pellets are dried before moulding: moisture in the melt breaks the molecular chains and makes the material brittle. Finished preforms are less sensitive, but they are still best stored in a dry room.
Acetaldehyde and taste
Melt processing creates a little acetaldehyde in PET. In larger amounts it gives a sweetish taste, most noticeable in water. That is why water uses bottle-grade PET and careful moulding settings.
