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a general purpose plasticiser worldwide since the
1950s is di (2-ethylhexyl) phthalate (DEHP) and
this has been identified in many museum plastics
[11]. The tackiness and development of high gloss
in deteriorating PVC indicate that liquid plasticiser
has migrated to surfaces (Figure 3). From there,
plasticiser evaporates at a rate dependant on its
vapour pressure. DEHP has a boiling point of 386ºC
and evaporates slowly under ambient conditions.
The PVC polymer is susceptible to degradation
when exposed to heat, light and oxygen. The
pathway by which degradation of the PVC polymer
takes place is complex but comprises one major
reaction involving the evolution of hydrogen
chloride (dehyrochlorination). Dehydrochlorination
occurs at imperfections in the PVC structure and
starts with the breaking of a C-Cl bond. This leads
to the progressive ‘unzipping’ of neighbouring
chorine and hydrogen atoms to form a conjugated
polyene system (alternate single and double carbon
bonds), accompanied by the formation of hydrogen
chloride. After between 7 and 11 repeat polyene
units have formed, absorption of light shifts to
longer wavelengths until the deteriorated PVC is
absorbing in the violet, blue and green parts of the
spectrum. The rate of degradation can therefore
be followed using colour changes from white to
yellow to orange to red, brown and, ultimately
black. Dehydrochlorination is an autocatalytic
reaction so if the hydrogen chloride produced is not
removed from the environment surrounding PVC,
degradation continues at an accelerated rate.
The rate and extent of degradation of the PVC
polymer and the migration and loss of plasticiser,
particularly phthalates, are related. Because DEHP
inhibits the degradation of the PVC polymer, when
it either migrates to surfaces or is absorbed by other
materials, PVC discolours, becomes tacky and brittle.
Addition of phthalate plasticisers to PVC has been
shown to reduce the rate of dehydrochlorination by
the polymer, by inhibiting the growth of the polyene
sequence [12].
Like all esters, phthalate plasticisers are susceptible
to hydrolysis (addition of water) when exposed to
strongly acidic or alkaline environments. Acidic
conditions develop when PVC polymers undergo
Keynote speech
Figure 4: Star Wars figures are made from plasticised PVC and
are highly collectible. Darth Vader has remained in his original
packaging with the result that the migrating plasticiser has
hydrolysed in the acid environment. White crystals of phthalic
anhydride are visible on his once-black head
dehydrochlorination and results in acid hydrolysis of
plasticisers to form phthalic acid, a white crystalline
solid acidic hydrogen chloride. Although phthalic
anhydride disfigures objects, it does not accelerate
degradation (Figure 4).
Polyurethane foams
The many pores or cells in polyurethane foams make
the polymer highly accessible to oxygen, light and
water in air. In addition, the processing of foams may
involve blowing air through polyurethanes in liquid
form, providing perfect conditions for oxidation.
The extent of sensitivity of polyurethane foams to
chemical degradation factors is dependant on the
polyol base used. Polyester-based polyurethanes
are more readily hydrolysed than polyether-based
materials. The degradation products catalyse further
hydrolysis.
Polyurethane ether (PUR-ether) foams are thought
to degrade primarily by oxidation, particularly in
the presence of light, resulting in discolouration
and a loss of mechanical properties. Polyurethanes
synthesised from a polyether polyol and an aromatic
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