dk nkf - Nordisk Konservatorforbund Danmark

Figure 1: The EWO dosimeter holder with the PPO polymer
covered glass sensor chip (inset).
EWO is a PPO (Polyphenyloxide) polymer that
is suggested to be degraded by photo oxidation in
several steps [2]
, common in polymer degradation,
such as UV absorption and excitation of a conjugated
species, oxidation and peroxide formation, and
radical formation followed by chain scission or
cross linking depending on conditions. The result of
the degradation in the presence of oxygen or other
oxidizers is a decrease in molecular weight and
increase in opacity of the film that can be measured
with a spectrophotometer. Although PPO is degraded
by the similar environmental influences as many
other modern synthetic polymers the exact steps and
rates of degradation reactions are different. For most
polymers many different degradation mechanisms
are proposed. E.g. for polycarbonate, which is a much
used modern synthetic polymer, initial UV absorption
and excitation of either carboxyl groups followed by
CO or CO abstraction and molecular rearrangement
2
or chain scission, or of methylene groups followed by
cross linking or further oxidation, or direct hydrolysis
without radiolysis, are proposed [2] .
The environmental effect on the EWO was measured
and it was calibrated against the effective environment
(UV, O 3 , NO 2 , T / RH) in a field test in 10 European
museums during the EU project MASTER [3] . Tolerable
levels of environmental parameters and related
effect levels for the dosimeter, representing organic
cultural heritage objects in indoors locations with
60
five levels of environmental control, from archives
to open structures with no control, were determined
by conservation scientists. As the dosimeter is itself
made from a modern synthetic polymer it should be
well suited to assess the potential for environmental
degradation of modern synthetic materials in
collections.
To use PPO/EWO as an effect dosimeter for modern
synthetic polymers ideally one would like to know the
dose-response functions not only for the PPO/EWO,
but for the different specific synthetic materials in
museum objects and collections. However detailed
environmental dose-response functions do not exist
for most modern synthetic polymer materials. In
addition, many collections include a range of different
modern synthetic materials, and it may be more
useful to determine levels of tolerability for such
collections as a whole, with additional guidelines
for particular materials, such as e.g. cellulose nitrate,
where mechanisms and reaction rates are more known.
The EWO could be calibrated and threshold levels
determined for collections including modern synthetics
by comparing its rate of degradation with that of a
range of modern polymers in museums. Alternatively,
or in addition, the degradation rate of the EWO can
be compared to known effects of environmental
parameters on other specific modern polymers.
Effects calibration and results
reporting for the EWO
The calibration of the environmental effect observed
on the EWO was performed by statistical correlation
of the values for the environmental parameters and
EWO effects measured in the MASTER project.
A non linear dose-response formulation found
to represent degradation processes in a range of
economically important polymers; polyurethane
(PUR), polyvinylchloride (PVC), fibre reinforced
polyester (PES) and resin based lacquer [4] was
used. The following dose-response functions
were determined for the EWO, for a situation in
showcases (Eq. 1) where UV light was absent, and
in museum rooms (Eq.2)
EWO-G effect (x1000) = 4.5+√T(0.3NO 2 +0.1O 3 ) (1)
EWO-G effect (x1000) = 8.7+√UV+√T(0.2NO 2 +0.3O 3 ) (2)
With units: T (°C), UV (mW/m 2 ) and NO 2 and O 3 (ppb).