The inlhrence of ozone on tensile strength and
elongation at break of vulcanized NR latex Iilms is
also presented in Table 4. The reductions of tensile
strength, in a time of 72 hr of ozone exposure, are as
great as 50%. Vulcanizates obtained by accelerator
DPG maintained, relatively, the best mechanical
properties, after ozone degradation. It may be
speculated that the low crosslink densities of these
vulcanizates may reflect the presence of long
polyisoprene chains between junctions. Under ageing
favorable conditions may arise for crystallization and
strengthening of the vulcanizates.
Although an increase of crosslink density affects
the chain mobility and supposedly reduces the rate of
ozone degradation, this relationship could not be
established here. The specimens exposed to ozone
were in an unstretched state, so ozone is reacting only
with rubber until the surface double bonds are
consumed and the reaction then ceases. For this
reason the characteristic ozone cracks were not
observed. The reaction of ozone with polyisoprene
chains leads to chain scissions. Also, an increase in
elongation at break was observed. Ozone reacts with
sulphur crosslinks, but these reactions are much
slower than the reactions with olefinic unsaturation.
It is known from the literature that the reaction
of ozone with di- and polysulphides is at least 50
times slower than the corresponding reaction with
olefins [19