30.1  How can I simply identify common plastics?. 

Read the recycle code :-). Alternatively, give it to the nearest IR 
spectroscopist who has a polymer library. But if you want some fun, try the
following.
 
There are several simple tests that can be performed in the home that can
assist in separating common plastics, however it is important to realise 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.

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 - moulded, 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, colour 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 colour, formaldehyde smell )
      = polyacetal
 - "thud", sinks, difficult to ignite ( yellow colour, weak smell ), draws
      into long threads = Nylon
 - "thud", sinks, difficult to ignite ( minimal flame, decomposition but no
      charring, cellular structure forms = polycarbonate.
  
30.2  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 soon be introduced.

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