Processing

Float-Sink Separation

Also called: Sink-float, density separation, flotation tank

Float-sink separation uses water to separate ground plastics by density: polyolefins float while PET, PVC and most other plastics sink.

Float-sink separation is a step in a washing line where ground plastic is fed into a tank of water. Materials lighter than water float and are skimmed off, while heavier materials sink and are collected from the bottom. It works because plastics have different densities.

What floats and what sinks

Floats in water: PP, HDPE, LDPE and LLDPE, all below 1.0 g/cm3.
Sinks in water: PET, PVC, ABS, PC, nylon and most filled plastics.
Polystyrene is close to the density of water, so it may float or sink depending on conditions.

Why it matters when buying recycled plastic

In PET bottle recycling, it removes PP and PE caps and labels from the sinking PET flakes.
It cannot separate PP from PE, since both float, or PET from PVC, since both sink. Other methods are needed for those.
Heavily filled polyolefins can sink and be lost or mixed with the heavy fraction.
Ask suppliers about float-sink and any further sorting to judge likely contamination.

Related terms

Washing Line

A washing line is a set of machines that shreds, washes, separates and dries post-consumer plastic to produce clean flakes or regrind.

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.

PET (Polyethylene Terephthalate)

PET (polyethylene terephthalate) is the clear polyester used for bottles and fibre; recycled PET flakes are among the most traded recycled materials in India.

Contamination

Contamination is any unwanted material in recycled plastic, such as other polymers, metal, paper, dirt or labels, that lowers quality and processability.