Plastics 201
by cleavage of the NO bonds joining the cellulose ring that are the weakest
bonds in the molecule. Nitrogen dioxide reacts with moisture in air to form
nitric acid, known for its ability to corrode metals on contact. Chain scission
along the backbone between the cellulose monomeric rings follows, resulting
in a considerable reduction in molecular weight.
Cellulose nitrate is particularly susceptible to light of wavelengths between
360–400 nm. Degradation is due to a nitrate ester cleavage in a similar man-
ner to thermal decomposition. At shorter wavelengths, i.e.those with higher
energy, disintegration of the cellulose ring occurs, causing a rapid decrease in
molecular weight. Once started, this process continues even in the dark.
An examination of cellulose nitrate adhesive taken from repairs in cuneiform
(dried clay) writing tablets from the British Museum, where the date of the
repair was known, suggests that the degradation of cellulose nitrate is retarded
substantially by the plasticiser added during manufacture and that such adhe-
sives are stable for at least 30 years. The relationship between loss of plasticiser
and reduced stability of cellulose nitrate is also demonstrated by three-
dimensional objects. Degradation can be divided into three stages. The first
stage involves the evaporation or migration of plasticiser manifested by shrink-
age of the object; the remaining material is highly flammable and burns at a
temperature up to 15 times higher than that achieved by burning paper. As
degradation continues, internal cracks or crazes develop and cellulose nitrate
yellows, as shown in Figure 2. In the final stage, crazing known as crizzling is
so extensive that cellulose nitrate disintegrates. At this point, its flammability
is the same as that of paper.
Some metals, notably copper, accelerate the rate of degradation of cellulose
nitrate. Copper screws and arm wires of cellulose nitrate spectacle frames
Figure 2Poster made from cellulose nitrate in 1960s showing shrinkage due to loss of
camphor plasticiser and cracking