Density

Also called: Specific gravity, relative density

Density is the mass per unit volume of a plastic in g/cm3; it helps identify polymers, detect fillers and separate materials by float-sink.

Density is the mass of a material per unit volume, normally reported in g/cm3. Specific gravity is the same value relative to water. For plastics it is measured by methods such as ASTM D792 or ISO 1183.

Typical values

• PP about 0.90-0.91, LDPE 0.91-0.94 and HDPE 0.94-0.97 g/cm3: all float in water.
• Polystyrene about 1.04-1.05, ABS 1.04-1.06, PC about 1.2 g/cm3.
• PET about 1.38 g/cm3 and PVC about 1.3-1.45 g/cm3: both sink in water.

Why it matters when buying recycled plastic

• Density higher than expected for a polyolefin usually points to fillers such as calcium carbonate or talc; confirm with an ash test.
• Density differences are the basis of float-sink separation in washing lines.
• For HDPE, density also distinguishes blow, film and injection grades.
• Higher density means fewer parts per kilogram, which affects your cost per part.

Ask suppliers to state the density of each lot, and spot-check it yourself with a simple water or immersion test before processing.