dk nkf - Nordisk Konservatorforbund Danmark

diisocyanate such as diphenylmethane diisocyanate
(MDI) are highly vulnerable to photooxidation
whereas polyester-based polyurethanes are more
resistant to ultraviolet radiation [13].
Photooxidation results in chain scission, in which
energy breaks polymer chain bonds to create a
polymer with two or more shorter chains and is
manifested by collapse and crumbling of foams.
Scission of the urethane link ( -NHCOO-) results
in the formation of amino and carbonyl groups with
the evolution of carbon monoxide and dioxide.
Crumbling often starts at surface skins of foams and,
when the surfaces crumble and fall away from the
object, fresh, undegraded foam is exposed to light,
perpetuating degradation to the point of complete
failure (Figure 5).
Conservation - the nightmare
challenge
Once initiated, degradation of plastics cannot be
prevented, reversed or stopped, but only inhibited
or slowed. Slowing the rate of degradation is
achieved by storing objects in conditions which
exclude the main factors causing degradation,
particularly oxygen and moisture. Interventive
conservation involves adhering broken sections,
cleaning and strengthening. It is poorly developed
compared to inhibitive conservation due to the
high sensitivity of plastics to cleaning agents,
Figure 5: Photo-oxidation of polyurethane ether foam frequently
results in loss of structure and crumbling
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adhesives and consolidants. There is a high risk of
causing irreversible damage to plastics, a nightmare
scenario for conservators. However, pressure from
professionals who study plastics, the commercial art
market, private collectors and exhibition organisers
is accelerating progress in this field [14].
Inhibiting degradation using
adsorbents
Adsorbents, also known as molecular traps or
scavengers, can be used to inhibit the rate of
degradation of plastics by minimising those factors
which either initiate or accelerate deterioration.
Adsorbents may either be installed in an active filter
system, simply placed in a dish or polyethylene
bag inside the storage area or incorporated into
packaging materials.
Activated carbon is widely used for industrial,
domestic and medical applications and is effective
at adsorbing large non-polar molecules. It has a huge
surface area of 300-2000m 2 per gram and contains
millions of microscopic pores each 1-5 molecular
diameters across. Molecules at surfaces of activated
carbon are retained by physisorption, primarily via
weak dispersion forces. Activated carbon is used to
reduce the rate of degradation of cellulose nitrate
CN. When used as a filter in an active system,
carbon has been shown to remove as much as 90%
nitrous oxides, CN’s primary degradation products,
from air in a single pass [15].
Because of activated carbon’s poor specificity,
water molecules and other pollutants in air compete
with nitrous oxides for sites (Figure 6). Activated
charcoal is not self-indicating so the first sign
that the adsorbent is exhausted may be renewed
deterioration of an object.
Zeolites are hydrated silicates of metallic ions,
most frequently calcium and aluminium, which
contain micropores of pre-determined diameters.
The dimensions of micropores determine which
gases, vapours or liquids particular zeolites are able
to trap. Whereas activated carbon retains molecules
weakly by physisorption, molecules are held onto
zeolite’s walls by chemisorption or covalent bonds
which contain about 10 times greater energy.