JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 52

matrix

FTIR microspectrophometer

moisture regain, stress relaxation,
stress cracking), it is more often
chemical in origin. In environmental
conditions, i.e. at moderate temperature and wavelengths above 300
nm (solar radiation at the Earth's
surface), the main mechanism is a
radical chemical reaction promoted
by impurities or by-products from
the converting process (e.g. metal or
peroxide impurities). The diagram on
the previous page shows the chain of
reactions for the mechanism.
The radicals are symbolized by a dot
( l) that represents an unpaired electron. Because they are highly reactive,
these radicals are short-lived and can
be observed only at low temperatures.
Their reaction both with the polymer
chain and with the oxygen in the air
finally results in what are called oxidation products, involving the insertion
of the oxygen atoms into the macromolecular chain (hydroperoxides/
POOH, peroxides/POOP, alcohols/
POH, ketones/P(P)CO, carboxylic
acids/PCOOH, esters PCOOP').

Photoageing room of the CNEP

Exposure chamber of ATLAS-SEPAP 12-24 unit

Some of these organic functions
decompose thermally at moderate
temperature (POOP, POOH), others are UV light sensitive (especially
ketones and hydroperoxides); these
produce (still through radical chemical reaction) oxidation products that
are more stable, such as alcohols,
carboxylic acids or esters.
The oxidation chain that generates
ageing, therefore, is both autocatalysed and autoaccelerated (autooxidation), and constitutes a chain
reaction that leads sooner or later
to a deterioration of the material's
physical properties, chemically taking
the form of cleavages or (more rarely)
crosslinking in the macromolecule.

Analytical approach
and predicting ageing
The appearance of such oxidized
functions in the polymer can be
detected by infrared spectroscopy,
an analytical technique that is both
sensitive and non-destructive, with a
wide range of detection modes that
are suitable for the inspection of heavily formulated industrial materials. It
also allows obtaining a molar distribution for the oxidation products and
comparing the effects of natural ageing (e.g. obtained in an ageing station)
with those of artificial ageing as simulated or accelerated in a laboratory.
When there is a match, it is possible
to determine the acceleration factors
and predict ageing times in utilization

52 jec composites magazine / No94 January - February 2015

Exposure chamber of SEPAP-50-24 unit (prototype
for ultra-acceleration)

conditions. Thanks to this approach,
the CNEP is able to predict a year's
worth of in situ ageing in the South
of France (with southern exposure),
within a few weeks (or even days) of
ageing in the accelerated-photoageing
units that it has developed. As a function of the specific materials, this
predictive approach can be extrapolated up to in situ exposure times of
20-25 years. Another advantage is that
it helps manufacturers to optimize the
lifespan of their materials as a function of the expected duration of use,
thanks to a thorough knowledge of
the stabilization systems.
A similar approach can be taken for
thermal ageing, as long as the Arrhenius equation can be applied.
When that is the case, determining
the activation energy for the oxidation reaction enables predicting the
material's lifespan as a function of the
conditions for its use (exposure times

JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015