dk nkf - Nordisk Konservatorforbund Danmark

Degradation of modern synthetic polymers in
museums and environmental assessment with
EWO dosimetry
Introduction
The initial production of modified and synthetic
polymers was a part of the major scientific progress in
the 19 th and first part of 20 th century. Modification of
cellulose to produce cellulose nitrate was discovered
around 1850 and cellulose acetate about 15 years
later. The first truly synthetic polymer, Bakelite, was
discovered in 1907. The advances in material science
in the second part of the 20 th century resulted in a
great increase in the use of synthetic polymers for
all kinds of industrial products, but also in the work
of artists and conservators. Museums and galleries
today possess objects made from the thousands of
different plastics that have been produced and also
use a large range of plastics for objects conservation
purposes. It has been observed that some plastics
deteriorate faster than many other items in museums
collections and have useful lifetimes of 5 – 25 years [1] .
The degradation mechanisms of synthetic polymers
differ, but generally it involves photochemical
reactions with UV excitation of molecules in the
polymer and following quenching and chemical
conversions when the conditions are “favourable”. [2]
The mechanisms of the chemical conversions often
include acidic or oxidative agents, such as e.g. O 3
for the initial oxidation of elastomers or “rubbers”,
and reaction rates usually depend on humidity, which
can participate in the degradation process in complex
chemical and physical ways, and on temperature.
Guidelines for the conservation of different polymeric
materials have been formulated based on knowledge
about their degradation mechanisms, e.g. the removal
of autocatalytic NO 2 by absorbers in the packing of
cellulose nitrate materials or the removal of oxygen,
and other oxidants, from storage bags with objects
made from rubbers and polyurethane foam [1] . In
exhibition or storage rooms preventive environmental
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control can be more difficult and / or costly. Whichever
level of control is possible it is important to be able
to assess the expected environmental degradation
effects on exposed objects. The easiest way to do this
is by using a sacrificial material, or dosimeter, which
reacts much faster than the museum object and for
which the amount of degradation after exposure at
the location of interest can be measured. To be able to
assess the effects of the environment on the objects of
interest the effect on the dosimeter must be calibrated
towards the effect on the real objects. When the
dosimeter consists of a similar kind of material as the
objects, calibration may be performed by comparing
the reaction rates of dosimeter and objects. For all
dosimeters calibration can be performed by comparing
the specific effects of the environmental parameters
on the dosimeter and on objects in the collection.
The PPO/EWO – Early Warning
Organic dosimeter
One such existing dosimeter that can be used to
evaluate the environmental effect on modern synthetic
polymers is the EWO (Early Warning Organic)
dosimeter (Figure 1) developed by the Norwegian
Institute for Air research in the previous EU project
MASTER.
This dosimeter measures the actual effect of the
environment on a synthetic polymer and can be
exposed in any small (e.g. a sachet for a small
object) or large (e.g. an exhibition room) location.
The dosimeter is simply placed at the location to be
evaluated for three months and returned to NILU
who will provide a results diagram and report that
shows the measured condition compared to tolerable
levels for the location as generally evaluated by
conservation scientists. The active substance in the
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