The factors which influence the life of
an elastomeric product fall into two basic categories which can
be called "product characteristics" and "ageing processes".
There is no chance of retrospectively influencing the product characteristics
since these describe the product as it was made and include such
- which elastomer was used?
- is the product filled or not and if so, is it black, coloured
- what is the vulcanizing system?
- what is the protective system?
- is it properly designed and manufactured or has it built-in
- what is its size and shape?
Once the product exists,
this illustration shows what can happen to it and again
the potential for damage falls into several groups. Abrasion
and external chemical attack may be unavoidable during service
life but should not occur on storage. Temperature, sunlight
and stresses can all accelerate or initiate the chemical
changes which are waiting to happen.
These changes, collectively known as ‘ageing’
involve three distinct and potentially co-synchronous mechanistic
routes for most (sulphur) vulcanizates. These can be identified
- continuing sulphur chemistry
- shelf ageing
- atmospheric ageing
Sulphur chemistry is particularly important
in vulcanized products which are resisting a distorting
force and that will include anything from a stretched rubber
band to a flat tyre. This chemistry does not stop when the
vulcanizing temperature is reduced from some 150oC to ambient
- it just proceeds more slowly. One of the features of vulcanizate
ageing is ‘set’ and this is caused by the polysulphidic
bonds which form the chemical crosslinks between polymer
chains breaking and the loose ends re-attaching to either
their original partner or another loose end (crosslink reversion).
If the vulcanizate is under a distorting force, then, as
the bond breaks, the elasticity of the polymer chain causes
it to relax against the force so that the bond will reform
further down the chain. The result is a slow "slippage"
of the vulcanizate which cannot be reversed although it
can be minimized by the appropriate choice of crosslinking
agents, an option not available to the conservator.
Shelf ageing is basically oxidative degradation and, apart from
the obvious influence of oxygen, one must consider the catalytic
effects of heat, light, internal and external stresses or strains
and pro-oxidant metals. The chemistry of oxidative degradation
is, even now, not completely resolved but, in a simplistic form,
oxidation of a sulphur vulcanized polyolefin such as natural rubber
proceeds via at least one chain reaction sequence which introduces
C-C and C-O-O-C crosslinks between polymer chains as well as C-O-O-C
rings within the same polymer chain, and another set of chain
reactions between oxygen and the sulphur atoms of the crosslinks
or pendent groups. These two sequences of chain reactions can
result in both chain scission and the formation of additional
crosslinks. The reactions between sulphur and oxygen can also,
eventually, lead to sulphuric acid formation, a particular problem
There is therefore a large range of both sequential
(or chain) reactions and competing reactions and that the ones which
predominate depend on factors such as the composition of the vulcanizate
as well as the influences of heat, light and metal catalysis. In
heat ageing we are balancing the rate of reaction of oxygen with
the elastomer and the rate of diffusion of the oxygen into the bulk
material. If the temperature is relatively low, it has been postulated
that for an unprotected vulcanizate diffusion predominates and therefore
there is slow oxidation throughout the product, but as the temperature
rises, the rate of oxidation increases much more than the rate of
diffusion so substantial oxidation occurs on the surface and an
oxidized (hard) surface skin is formed. As oxidation continues the
chain breakdown may become more significant and the hard surface
then softens and turns sticky. To complicate matters further, under
certain conditions this order can be reversed and an initially sticky
degraded surface can harden with further oxidation.
The mechanism for light catalysed oxidative degradation
is that the energy of UV light may be sufficient to break a C-H
bond and generate a radical species which can then react with oxygen
to initiate the same chain reaction sequences as occur in direct
oxidation. Two types of protective agents are available, the ultra
violet absorbers (UVA's) and the hindered amine light stabilizers
(HALS). The UVA's compete with the olefinic double bonds for the
available energy and dissipate it harmlessly. It is important to
remember that the absorption of light follows Beer's law so even
if one has a UVA present it would give no real protection to a material
less than about 0.5 millimetres. HALS operate by a different mechanism
which is still not completely resolved but is believed to be a radical
trap. It thus operates throughout the UV-transparent sample and
is often used in conjunction with a UVA.
Light-catalysed oxidation can result in an inelastic
skin which, as it thickens, cracks in random directions and produces
a pattern known as "crazing". In the early stages, in
thin sheets, the effect has been called "light stiffening"
whilst in highly filled articles the degradation can result in complete
loss of the resinified elastomer and one ends up with the "chalking"
effect mentioned earlier. This has given rise to comments about
'blooming' inorganic fillers which is mechanistically impossible.
Because of the light absorbing properties of black-filled materials,
this effect is not normally seen in them. In many real life vulcanizates
of reasonable bulk under ambient conditions, it seems that the ingress
of oxygen is limited and, apart from the surface few millimetres
or so, the bulk rubber remains in excellent condition.
Atmospheric ageing differs from shelf ageing in
that it is characterized by the attack of ozone on the rubber. It
is essential to be aware that this is not just another form of oxygen-induced
degradation as the mechanism is quite different, with simple bimolecular
ozonolysis of the rubber olefinic double bond being followed by
immediate cleavage to give two carbonyl end groups. If the rubber
is under any sort of stress this will result in 'atmospheric cracking'
in which the cracks are perpendicular to the direction of elongation.
As early as 1931 it was shown that 'ozone cracking' was different
from 'light ageing' and took place more readily in the night than
during the day.
The effect of trace metal contaminants such as
copper, chromium and iron in any of the compounding chemicals is
inevitably to accelerate the rate of degradation although the change
in rate cannot be predicted as it depends on many factors, not least
the chemical state of the metal.