200 Chapter 8
of the earliest commercial plasticisers, sublimes at ambient temperatures
and so was lost rapidly from cellulose nitrate formulations. The result of
plasticiser loss is shrinkage and brittleness. Most PVC formulations
contain between 1% and 3% by weight of a lubricant to prevent adhesion
of the object to the mould during production. The most common lubri-
cant is stearic acid, which is incompatible with PVC. With time, excess
stearic acid migrates to the surfaces of the finished product and forms a
white, disfiguring layer.
●Absorption of liquids or vapours. Polyethylene is especially vulnerable
to absorption of liquids. Tupperware food containers often develop tacky
internal surfaces after long-term use due to the absorption of oily mater-
ials from contact with foods.
●Excessive exposure to high or low temperatures or rapid change in
temperature.Chemical causesinclude:●Oxidation. Most degradation reaction paths for plastics involve oxygen,
either by direct reaction of polymers or additives with oxygen or by the
formation of another reactive material from oxygen. Highly unstable
ozone (O 3 ) is the product of the reaction between oxygen and ultraviolet
light. It readily adds across carbon double bonds present in synthetic
rubbers.
●Metal ions. Copper ions act as catalysts to accelerate deterioration reac-
tions of many polymers, particularly synthetic rubbers. Spectacle frames
constructed from cellulose nitrate are often more degraded in the area
around copper-containing screws and around the wires in the arms, than
in the rest of the frame.In addition to physical and chemical factors, all degradation reactions require
energy in the form of heat, light or radiation.
4.1 Degradation of Cellulose Nitrate
The major causes of instability of cellulose nitrate are due to the products of
hydrolytic, thermal and photochemical reactions. Degradation of the polymer
is autocatalytic, that is, the products of breakdown tend to catalyse a faster
and more extensive degradation reaction than the primary processes, if allowed
to remain in contact with degraded cellulose nitrate.
One of the first products of thermal deterioration, i.e.in the absence of
light, is the highly reactive, highly toxic oxidising agent nitrogen dioxide
(NO 2 ), identified by its yellow vapour and distinctive odour. This is formed