dk nkf - Nordisk Konservatorforbund Danmark

Figure 2: A pile of transparent cellulose acetate tracing sheets
from the 1980s illustrate typical physical deterioration. Sheets
have shrunk and droplets of lost plasticiser can be seen at
surfaces
the primary reaction being hydrolysis also known
as deacetylation, during which hydroxyl groups
replace acetate groups (CH 3 COO) on the cellulose
ring, producing acetic acid (CH 3 COOH). Cellulose
acetate undergoes autocatalytic breakdown if
acetic acid is allowed to remain in contact with the
degrading polymer. This happens easily because the
solubility of acetic acid in cellulose acetate is high,
similar to the solubility of acetic acid in water in
atmospheric moisture [10].
Deacetylation is accelerated by water (usually in
the form of moisture in air), acid or base. Because
the loss of acetyl groups from cellulose acetate
results in the formation of acetic acid which
gives a distinct vinegar-like odour to degrading
materials, the process is also known as the ‘vinegar
syndrome’. Because the acidic vapours are mobile,
acetic acid produced by deteriorating cellulose
acetate contributes to degradation of other organic
materials in the vicinity. Metals are corroded by the
acetic acid produced by deacetylation. With time
and loss of acetate groups, the production of acetic
acid lessens and the cellulose acetate is reduced to
its starting raw material, namely cellulose.
28
Plasticised polyvinyl chloride
In many museum collections, degradation of
plasticised PVC materials such as clothing and
footwear, furniture, electrical insulation, medical
equipment, housewares, toys and packaging
materials, is detected only 5-10 years after
acquisition. Degradation is usually manifested first
as tackiness at surfaces accompanied by a glossy
appearance, then by discolouration and sometimes
with white crystals at surfaces. The degradation
pathways exhibited by plasticised PVC are the
result of degradation of the two major components
of its formulation, namely polymer and plasticiser.
Although the two components deteriorate
independently of each other, the resulting products
destabilise the whole.
Plasticisers are the major modifier for PVC
formulations in terms of percentage weight (between
15 per cent for vinyl flooring and 50 per cent for
waterproof boots) and physical properties. Plasticiser
molecules are evenly dispersed throughout the
PVC, and are weakly attached to the surfaces of
polymer chains. The largest single product used as
Figure 3: Plasticised PVC photograph pocket from 1980s
showing liquid plasticiser which has migrated from the PVC
polymer and will contaminate any enclosed materials