dk nkf - Nordisk Konservatorforbund Danmark

Figure 1: The frames of these cellulose nitrate spectacles made in the 1960s, began to show degradation by crazing and by the plastic´s
ability to corrode copper ear wires and screws in the frame in the 1980s (left of image). By 1995, degradation had progressed so
dramatically that the spectacles could no longer function and were hardly recognizable as a cohesive object (right of image). They were
therefore deaccesioned by the National Museum of Denmark
Cellulose nitrate
Cellulose nitrate undergoes thermal, photochemical
and hydrolytic degradation reactions, the latter being
the most important [9]. In addition, breakdown
of the polymer is autocatalytic. This means that,
if not removed from the undegraded material, the
breakdown products catalyse a faster and more
extensive reaction than the primary processes. This
may happen if a pair of spectacles with cellulose
nitrate frames is stored in their closed case, for
example. The major product of thermal degradation
is the highly reactive, oxidising agent nitrogen
dioxide NO 2, identified by its yellow vapour and
distinctive odour.
Nitrogen dioxide reacts with moisture in air to form
nitric acid. Water diffuses into cracks at surfaces,
reacts with nitrogen dioxides evolved in the bulk of
the CN object and produces nitric acid there. The
acid attacks the cellulose polymer chains resulting
in chain scission along the backbone between the
cellulose rings. A reduction in molecular weight
follows which is manifested typically by a network of
cracks which start inside the object before spreading
to surfaces causing brittleness and weakening. As
degradation continues, internal cracks or crazes
develop and cellulose nitrate yellows (Figure 1).
Keynote speech
In the final stage, crazing known as crizzling is so
extensive that cellulose nitrate disintegrates.
Some metals, particularly copper, accelerate the
rate of degradation of cellulose nitrate. Nitric acid
produced by cellulose nitrate corrodes metals. If
the metal is a structural component of the object,
such as the shaft of a knife, corrosion can lead to
destruction of the handle. Because of the corrosion
layer formed, the volume occupied by the metal
increases until the CN cracks and bursts. Nitric acid
may also corrode metals in the vicinity of the CN
object.
Cellulose acetate
Like cellulose nitrate, cellulose acetate (CA) is
deteriorated by both physical and chemical factors
and the physical cause of degradation is plasticiser
loss. Three-dimensional objects moulded from
cellulose acetate comprise 20-40 per cent by weight
plasticiser. Typical plasticisers include triphenyl
phosphate (TPP), a solid with melting point 48.5ºC
which also acts as a flame retardant. Migration and
subsequent evaporation of plasticiser from between
the cellulose acetate chains give rise to shrinkage,
tackiness and increased brittleness (Figure 2).
The major chemical degradation reaction of cellulose
acetate is also similar to that of cellulose nitrate,
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