Plastics 199
Unknown plastics may be identified either by comparing spectra with
those from reference or known samples; or with reference libraries such as
those specific to art, architecture and archaeological materials produced by
the Infrared and Raman Users Group (http://www.irug.org); or by using tables
to correlate infrared absorption energies with specific chemical bonds. Figure 1
shows that polymers give their own distinctive ‘fingerprints’, so they can be
distinguished from each other. However, FTIR spectra reflect a mixture of all
components present in the plastic, including additives and colouring materi-
als, making them complicated to interpret.
4 Degradation of Plastics
A survey of plastics-containing objects in the British Museum and the
National Museum of Denmark showed that 1% of objects were actively deteri-
orating and were in immediate need of conservation, while 12% exhibited
deterioration and required cleaning, stabilising and repair. All the ‘acute’
objects contained cellulose nitrate, cellulose acetate, plasticised PVC or
polyurethane foam. Most objects (60%) were defined as being of low con-
servation priority, that is, they were in a stable condition but needed some
treatment such as cleaning. Only just over one quarter required no conservation
treatment. In general, deterioration of plastic objects in museums is visible
within 5–25 years of collection.
It should be remembered that most museum objects have been used or dis-
played before they are collected. They have an unknown past, usually, which
contributes greatly to the rate and type of deterioration. Instability of the earli-
est plastics, cellulose nitrate and acetate is expected due to their poorly sta-
bilised formulations and because they are the oldest man-made plastics in
museums. However, PVC was first developed in 1926 and is still in use, so its
deterioration is rather unexpected.
Causes of deterioration of plastics may be broadly divided into physical
and chemical.
Physical causesinclude:
●Stress, fatigue and mechanical damage. These are results of the use of
the plastic objects and could comprise frequent bending of a PVC soft
toy leading to its failure; or abrasion of the surface of a vinyl record as it
is repeatedly pulled out of its sleeve to play, resulting in its inability to
produce perfect sound.
●Migration or loss of additives. These are consequences of the properties
of additives used to formulate plastics; their selection is related to the
function, expected lifetime and price of the final product. Camphor, one