Polypropylene

At 905 kg/m³ it floats in water, which is how recyclers separate it — polyethylene is the only other common plastic that does. Its unique property is the living hinge: a thin folded section of polypropylene survives millions of flexes without cracking, which is why every flip-top bottle cap is made from it. It resists boiling water and dishwashers, but almost nothing sticks to it — glue and paint both fail, and joining means welding or a mechanical fixing.

Properties

Density

905 kg/m³ (895–920) — 0.91 g/cm³

Melting range

160–170 °C

Tensile strength

25–40 MPa

Young's modulus

1.5 GPa

Thermal conductivity

0.15 W/(m·K)

Thermal expansion

100 µm/(m·K)

Floats in water

Yes

Magnetic

No

What it is used for

• Food tubs, kettles and microwave containers
• Car bumpers and battery cases
• Living hinges on flip-top caps
• Rope, twine and woven sacks

How it compares

Does it rust?

Polypropylene contains no iron, so it cannot rust. Whether it degrades depends on the material, not on oxidation.

Is it magnetic?

Polypropylene is not a metal and has no ferromagnetic response.