196 Chapter 8
and instrumental techniques. Before examining plastic objects, it is helpful to
have as much historical information about them as possible. The date of manu-
facture or, if unavailable, the date of collection, can give information about
the type of plastic. If an object was manufactured before around 1905, it is
likely to be a semi-synthetic or natural material, rather than a synthetic plas-
tic. Styles are not always helpful in identifying age or material because the
same mould can be used for many years and popular styles are often revived.
3.1 Simple tests
Appearance is only a guide to the type of plastic present because various
fillers and colouring materials can change the appearance of a polymer con-
siderably. However, Bakelite (phenol–formaldehyde) is always brown, black
or red, while urea- and melamine-formaldehydes may have pastel shades.
Polyethylene and polypropylene always appear slightly cloudy as thin films,
while most other colourless plastics, particularly polyesters and polycarbon-
ate, are crystal clear.
Odour of plastics may assist with identification. Since warming plastics
gently increases the concentration of volatile materials, they should be rubbed
with a clean cotton cloth just prior to sniffing close to the surface. The smell
is usually that caused by volatilising a monomer, plasticiser or degradation
product by warming. An odour of vinegar is typical of cellulose acetate while
that of mothballs (camphor) suggests cellulose nitrate and a smell similar to
that of a new car indicates plasticised PVC. A fishy odour is often produced
by warming melamine- or urea formaldehydes.
A rather unusual test is to tap the plastic firmly with a fingernail. If the sound
is metallic, the plastic is likely to contain polystyrene. Hardness is another
property that can be roughly tested with a fingernail applied to the underside of
an object. Plastics which can be marked with a fingernail include polyethylene,
polypropylene, plasticised PVC and polyurethane; other types are not affected.
The ability of plastics to float on the surface of a beaker of tap water at
20°C is related to their density at that temperature. The density of water at
20°C is approximately 1 g cm^3. If a small sample floats on the surface of the
water, it has a density lower or equal to one at the same temperature; if it
sinks, it has a density greater than one. Polyethylene, polypropylene and
polystyrene float on water while other plastics sink, Bakelite and casein hav-
ing the highest densities. The flotation test is a rough method to identify plas-
tics since results are dependent on the physical form of the plastic. Foams
contain cells filled with air, so their densities will be lower than a solid block
of the same type of plastic.
If it is permissible to take a small sample and heat it in a test tube, the
pH of the vapours may be determined by placing a piece of moistened