Sink, Float, Sort: How a Recycling Plant Tells Plastics Apart

Sink, Float, Sort

[1]A bale of mixed plastic arrives at a recycling plant looking like one kind of stuff: a crushed tangle of bottles, tubs, trays and lids. It is not one kind of stuff. The bale holds at least six different substances, each a different plastic, and a factory that melts them together gets a weak, streaky material that nobody wants to buy. Plastics are synthetic materials made from natural resources such as oil and natural gas, and each kind has its own set of properties. The plant's job is to use those properties to pull the tangle apart, substance by substance.

[2]The first clue is printed on the item. Since 1988, a plastics industry group has stamped most containers with a small triangle and a number from 1 to 7. A 1 means PET, the clear plastic of soda bottles. A 2 means high-density polyethylene, the cloudy plastic of milk jugs. A 5 means polypropylene, used for yogurt tubs and caps. The code is helpful but weak. Labels wear off, lids get separated from bottles, and a sorting line moving thousands of pieces a minute cannot stop to read each one. The plant needs properties it can test on every piece automatically.

[3]The oldest and cheapest test uses density. Shredded flakes drop into a tank of water. Polyethylene has a density of about 0.95 g/cm³ and polypropylene about 0.90 g/cm³, both lower than water's 1.00 g/cm³, so their flakes float and are skimmed off the top. PET, at about 1.38 g/cm³, sinks and is collected from the bottom. The tank does not care about the shape, color or size of the flake. A tiny chip of PET and a large one sink just the same, because density belongs to the substance, not to the piece.

[4]Density also shows where the method fails. Polyvinyl chloride, PVC, the plastic of blister packs and pipes, has a density of about 1.38 to 1.41 g/cm³, nearly the same as PET. In the water tank they sink side by side. This matters because PVC and PET behave very differently when heated: PET melts cleanly at about 250 °C, while PVC begins to break down near 150 °C and gives off hydrogen chloride gas, which corrodes equipment and spoils the batch. A few PVC flakes in a ton of PET can ruin it. When two substances share a property, a plant has to test a second property to tell them apart.

[5]A second test uses light. A near-infrared sorter shines invisible light, just beyond red, onto the flakes and reads the pattern each flake reflects. Every plastic reflects its own pattern, a kind of fingerprint, so the machine can flag PVC among PET and blow it aside with a puff of air. Even this test has a blind spot. Black plastic, colored with carbon black, swallows nearly all the infrared light and reflects almost nothing, so the sorter cannot read it. Black trays often end up in the trash for that reason.

[6]Chemists in the plant's lab use a third property when the machines disagree: solubility. A flake of PET will not dissolve in water or ordinary alcohol, but it dissolves in certain strong solvents in which polyethylene stays solid. A questionable flake dropped into a small dish of the right solvent settles the argument. Like the water tank, this test gives the same answer for a big flake and a small one, because solubility is also a property of the substance.

[7]Notice what the plant never does. It never sorts by mass, because a heavy flake of polypropylene and a light flake of PET are both ordinary. It never sorts by color, because a blue PET bottle and a blue polyethylene jug are different substances in the same disguise. Every reliable test on the line measures something that stays the same for every piece of a substance no matter its size or shape: density, melting behavior, the infrared fingerprint, solubility. And no single test is trusted alone. The water tank, the light sorter and the lab each catch what the others miss, which is how a careful scientist identifies any unknown: several properties, compared against a table of known substances, until one answer is left.