Bill Bryden posted this on an underwater ROV group.

Thought I would pass it along.

DOC



There are several simple tests that can be performed in the home that
can assist in separating common plastics, however it is important to
realize that formulated products contain large quantities of pigments,
plasticisers, and fillers that can dramatically alter the following
properties. If possible repeat the tests on a reference sample of the
plastic so you get a good feel for the test and can compare directly.

a. Visually examine the sample, looking for recycle codes :-) While
you are at it, you can check for indications of how the plastic was
made - molded, injected, rolled, machined etc.
b. Try assessing the flexibility by bending, and look at the bending
zone - does the material stretch or is it brittle?
c. Test the hardness, try scratching it with pencils of differing
hardness ( B,HB,1-6H ) to ascertain which causes a scratch in the
plastic. Alternatively, attempt to scuff the sample with a fingernail.
d. Cut a small slither with a sharp knife. Does the sample cut cleanly
( thermoplastic ) or does it crumble ( thermosetting )?
e. Hold sample in small flame, note whether it burns,
self-extinguishes on removal from the flame, color of the flame, and
smell/acrid nature of fumes when flame is blown out (Caution - the
fumes are likely to be toxic). Also attempt to press melted sample
against a cold surface, and pull away - does sample easily form long
threads.
f. Drop onto a hard surface, does the sample "ring" or "thud"?
g. Place it in water. Does it float, sink slowly, or sink rapidly? If
it sinks rapidly, it is likely to be halogenated (PVC, Viton, PTFE).
If it sinks slowly, possibly nylon. If it floats possibly
polyethylene or polypropylene. You can ascertain the actual density
by adding measured volumes of a low density solvent like methanol
until the sample neither rises nor sinks.


Cutting thin slivers results in powdery chips ( thermosetting )
- carbolic smell in flame, self extinguishing = phenol formaldehyde
- self extinguishing, black smoke, acrid = epoxide
- fishy smell = urea formaldehyde, or melamine formaldehyde

Cutting thin slivers results in smooth sliver ( thermoplastic )
- metallic "ring", burns (styrene smell) = polystyrene (note that
high impact polystyrene may not give "ring" )
- "thud", floats, hard, glossy surface, burns (paraffin wax smell) =
polypropylene
- "thud", floats, medium-hard surface, burns (sealing wax smell) =
high density polyethylene
- "thud", floats, soft surface, burns (paraffin wax smell) = low
density polyethylene
- "thud", sinks, burns (fruity smell ) = acrylic
- "thud", sinks, burns (burning paper smell ) = cellulose acetate or
propionate.
- "thud", sinks, burns ( rancid butter smell ) = cellulose acetate
butyrate
- "thud", sinks, difficult to ignite ( greenish tinge ) = PVC
- "thud", sinks, difficult to ignite ( yellow color, formaldehyde
smell ) = polyacetal
- "thud", sinks, difficult to ignite ( yellow color, weak smell ),
draws into long threads = Nylon
- "thud", sinks, difficult to ignite (minimal flame, decomposition
but no charring), cellular structure forms = polycarbonate.


What do the plastics recycling codes mean?

The recycle codes for plastics are currently being reviewed, and new
codes (probably inside a totally different symbol ) will probably be
introduced at some point.

1 = PET
2 = High density polyethylene
3 = Vinyl
4 = Low density polyethylene
5 = Polypropylene
6 = Polystyrene
7 = Others, including multi-layer