There is no easy way to identify plastics. As well as physically testing the materials it is possible by handling different plastics to develop a feel for their different properties. Many plastics look the same, but after a while small differences can be felt, which together with other clues such as trade marks, smell and function make the identification process a little easier.
A few organisations supply identification kits which contain samples for different polymers, but they do not contain some of the older plastics like celluloid.
Below are some simple non-laboratory tests. They are not conclusive but are intended as a guide towards identifying materials and mouldings.
Please remember that the moulding must be as clean and pure as possible before carrying out any of the following tests, as grease and dirt can affect the result.
The density of most plastics does not allow them to float, but polythene and polypropylene are the two main plastics that do so. If polypropylene has been reinforced with a filler, however, it will not float so well. Foamed plastics, such as expanded polystyrene and polyurethane, will obviously float as their cellular structure contains air, and sometimes a moulding may be a sandwich construction with a foam layer hidden between solid surfaces.
Great care must be taken when performing this test. The surface must be fireproof and fumes should never be inhaled while the plastic is burning, but sniffed quickly afterwards. Professional laboratories use metal tongs for holding the plastic, but small shavings scraped off in powder form with a very sharp knife can be placed on a saucer or in a glass test tube. The shavings can be ignited by a match or cigarette lighter. In a professional laboratory the glass test tube is warmed over a Bunsen Burner.
| Acrylic |
|
burns
with a bluish flame and a floral scent, rather like methylated spirits |
 |
| Casein |
|
derived from milk,
this plastic burns with an unpleasant but typical smell of burnt milk
or cheese. |
|
| Cellophane |
|
burns with the smell
of burning paper or cotton. |
|
| Celluloid |
|
typically the whitish
powder pops with minute explosions and burns with a sooty yellow flame.
Sometimes you can smell camphor before it evaporates. |
|
| Cellulose Acetate Butyrate |
|
smells of acidic,
rancid butter. |
|
| Cellulose Acetate |
|
sparkles when ignited
and smells of vinegar.. |
|
| Hard rubber (vulcanite) |
|
burns with orange-yellow
flame which is not self-extinguishing and gives off the characteristic
smell of burning natural rubber. |
|
| Nylon |
|
burns with a blue
flame. Its aroma has been described as burning "vegetation, celery
or hair". if touched with a metal point it can be drawn out into
threads. |
|
| Phenol formaldehyde
(phenolic, Bakelite) |
|
when burnt gives off
strong smell of carbolic acid, also smells of this when wet. |
|
| Polythene & Polypropylene |
|
both these materials
burn with a smell of paraffin (wax candle), but polypropylene smells
more like hot diesel oil. |
|
| PVC |
|
burns with black smoke
and gives off hydrochloric acid. |
Some plastics are basically transparent (or appear translucent or opaque). If a moulding is transparent it is most likely to be one of the following:
• Acrylic (yellows with age)
• Celluloid (yellows with age)
• Cellulose esters and ethers (eg cellulose acetate butyrate)
• Some nylons
• Polycarbonate
• Cast phenolic
• Polyester
• Polystyrene
• PVC (yellows with age)
The following plastics are only translucent or opaque but never transparent:
•ABS (can be clear in rare cases)
•Casein
•HIPS
•Certain nylons
•Polypropylene
•Polythene
•Shellac
•Natural & hard rubber (ebonite or vulcanite)
If you look at the marks of the moulding process you can often tell whether the plastic is a thermoset or a thermoplastic. For example, a compression moulded box or ash tray will most likely be a thermosetting plastic, and will have a scar of flash line along the part where the two halves of the mould closed together. An injection moulded object, at least, if made before the 1970s, will be thermoplastic and will have a sprue mark (see glossary) at the point where the molten polymer was injected into the mould. It may also bear the scars of ejector pins, which pushed the moulding out of its mould and maybe flow or sink marks.
Polypropylene and polythene both feel very waxy, but polythene can be scored with a fingernail, unlike polypropylene. HDPE is not so easy to scratch as LDPE.
Sometimes the function of a particular object can give a clue to the plastic it is moulded from.
Phenolic (eg Bakelite) will give off a distinct smell of carbolic acid when wet or warmed.