Identify a plastic before you heat it, using the resin code if there is one, then a float test in fresh and salt water, then tests that need no flame, because burning a sample to see how it smells means breathing the fume you are trying to avoid. The float test separates polyethylene and polypropylene, which float, from the rest, but PVC and PET both sink even in salt water, so something else must tell them apart.
Part of Plastics and Polymers
Plastic identification is the process of working out which polymer a salvaged object is made of, so that it can be sorted for remelting or kept out of heat.
Sorting Is A Safety Step Before It Is A Quality Step
Mixed plastics make a weak product, which is the quality reason to sort. The safety reason is stronger: heated PVC gives off hydrogen chloride, PTFE gives an acutely toxic fume, and nitrogen-bearing plastics can give hydrogen cyanide. You cannot protect yourself from a hazard you have not identified. See Fume Safety, which is the same article from the other direction.
Step 1: Read The Resin Code
Most items carry a triangle with a number. The standard meanings, as Thermoplastics gives them: 1 PET, 2 high-density polyethylene, 3 PVC, 4 low-density polyethylene, 5 polypropylene, 6 polystyrene, 7 other. Keep each number separate. Code 7 is a catch-all and tells you nothing, and an item with no code is unidentified. Weathered or worn marks mislead, so confirm a code with the next steps.
Step 2: Float Test
Prepare two containers, one of fresh water (density 1.0) and one of saturated salt water (about 1.2). Wet the sample first so that trapped air does not carry it up.
| Result | Plastic type (carrier) |
|---|---|
| Floats in fresh water | Polyethylene or polypropylene |
| Sinks in fresh water, floats in salt water | Polystyrene, nylon, ABS |
| Sinks in salt water | PVC, PET |
The table is from Plastics Manufacturing. Its limits matter. Foamed polystyrene floats because it is full of gas. Fillers and glass fibre raise density, so a filled polypropylene may sink. Above all, the last row lumps together PVC and PET, which are the pair you most need to separate, because one of them gives off hydrogen chloride.
Step 3: Tests That Do Not Need A Flame
These come from general knowledge, not the corpus, so confirm against known samples before relying on them.
- Form and use. PET drink bottles are thin, clear and ring when tapped. Water pipe and conduit are most often PVC. Jugs and caps are most often polyethylene or polypropylene.
- Feel. Polyethylene is waxy and bends easily; polypropylene is stiffer and springier; rigid PVC is hard and dense and does not feel waxy.
- Scratch. Polyethylene can be marked with a fingernail, harder plastics cannot.
- Cut. Polyethylene and polypropylene shave off in waxy curls.
- Solvent dab. A small drop of acetone softens polystyrene and ABS but not polyethylene or polypropylene. Acetone is flammable: no flame near it.
Step 4, Last Resort: The Burn Test
The traditional field method is to burn a tiny sample and watch the flame and smell the smoke. The carrier’s version reads: polyethylene and polypropylene burn easily, drip and smell like candle wax; polystyrene burns with black smoke and a sweet styrene smell; PVC self-extinguishes with a green flame and an acrid hydrogen chloride smell.
Be honest about what this is. Smelling the smoke is inhaling it, and the test works by producing exactly the gas that makes the material dangerous. Use it only when the earlier steps fail, only outdoors, upwind, with the smallest possible sample held at arm’s length, and never in quantity. Do not sniff. Watch the flame colour and the drip, and step back from the plume. The carrier’s own advice is to treat anything that identifies as PVC as one not to burn in quantity.
A better version for chlorine is the copper wire test, from general knowledge: heat a clean copper wire in a flame until it stops colouring the flame, touch it to the sample so that a trace sticks, and return it to the flame. A green colour indicates a chlorine-bearing plastic such as PVC. It uses a trace of sample rather than a burning piece.
Worked Example: Is It Polypropylene Or PVC?
A grey pipe offcut sinks in fresh water and also sinks in saturated salt water, so the float test puts it in the PVC-or-PET row. It is a thick-walled pipe, not a thin bottle, which points to PVC. A copper wire test that gives a green flame confirms a chlorine-bearing plastic. Result: reuse it whole as pipe and do not melt it.
Common Mistakes
- Trusting the float test alone — foams and fillers fool it, and PVC and PET are indistinguishable.
- Sniffing the smoke — the test becomes the exposure.
- Heating “just a little” of an unknown plastic indoors.
- Believing a mild smell means safe — carbon monoxide has no smell, and many people cannot smell hydrogen cyanide at all.
Burn-and-sniff is a hazard, not a safeguard
Heated PVC releases hydrogen chloride, a corrosive gas that damages the airway. PTFE fume causes polymer fume fever, and nitrogen-bearing plastics can release hydrogen cyanide. Never burn a plastic indoors, never burn it in quantity, and never inhale the smoke to identify it. If an unidentified plastic will not be identified safely, do not heat it.
What’s Next
- Learn next: Remelting — melting each sorted type correctly
- Combine with: Recycling and Reprocessing — the whole workflow