202 Chapter 8
from the 1930s accelerate degradation of surrounding cellulose nitrate and are,
in turn, corroded by the nitric acid formed as a degradation product.
4.2 Degradation of Cellulose Acetate
The major degradation reaction of cellulose triacetate is similar to that of
cellulose nitrate, the primary reaction being deacetylation (also known as
hydrolysis) during which hydroxyl groups replace acetyl groups (COCH 3 )
on the cellulose ring. Deacetylation is accelerated by water (usually in the
form of moisture in air), acid or base. Because the loss of acetate groups from
cellulose acetate results in the formation of acetic acid (CH 3 COOH) that
gives a distinct vinegary odour to degrading materials, the process is also
known as the ‘vinegar syndrome’.
Cellulose acetate undergoes autocatalytic breakdown if acetic acid is allowed
to remain in contact with the degrading polymer. This is a common situation
because the solubility of acetic acid in cellulose acetate is high, comparable
with the solubility of acetic acid in water, for example in atmospheric mois-
ture. Degradation does not follow a linear rate, but is slow during the initial,
induction period and more rapid after the onset of autocatalysis. As deacety-
lation progresses, chain scission takes place with bonds breaking between the
cellulose monomeric units, dramatically reducing the molecular weight,
tensile strength and solubility of the polymer.
Shrinkage, tackiness and increased brittleness due to the migration and sub-
sequent evaporation of plasticiser from between the cellulose acetate chains, is
also a frequent cause of degradation. The degradation of some plasticisers has
been shown to increase acidity of cellulose acetate-containing materials.
Triphenyl phosphate, used as a plasticiser for cellulose acetate since the 1940s,
decomposes to form diphenyl phosphate and phenol. Diphenyl phosphate is
a strong acid so it is likely to accelerate the deacetylation of cellulose acetate.
4.3 Degradation of Plasticised PVC
In many international museum collections, degradation of plasticised PVC
materials, in the form of clothing and footwear, furniture, electrical insula-
tion, medical equipment, housewares, vinyl records and cassette tapes, toys and
packaging materials used to store objects, has been detected as early as five
years after acquisition.
Degradation of plasticised PVC materials in museums is frequently observed
as migration of the plasticiser from the bulk phase to surfaces. From there,
plasticiser evaporates at a rate dependent on its vapour pressure. This process
may be detected as a tacky feel to the plastic, increasing brittleness and sub-
sequent discolouration of the PVC polymer itself. The mechanism by which