Studies on the use of nano zinc oxide and modified silica in NR, CR ...

Chapter 1
silica by mass with minor amounts of metallic elements. Extensive research
has been carried out in order to explore the possibility of using silica from rice
husk.
1.4.5 Antioxidants
32
Natural and synthetic polymers deteriorate on ageing in varying
degrees as a result of the combination of a number of factors such as heat,
light, oxygen and ozone. Thermal and light initiated oxidation in polymers
proceeds by a free radical chain mechanism involving the formation of
hydroperoxides, which can be summarized as below.
Initiation
H
( Polymer)
under oxidative
Conditions
. .
R ⎯⎯⎯⎯⎯→ R + H …………………………….(1.3)
. .
o oo
R + → R
…………………………….(1.4)
2
Propagation
. .
..
. .
oo oo
oo o o
oo o o
R + RH → R H + R
……………………………. (1.5)
R H → R + H
……………………………. (1.6)
R H + RH → R + R + H
……………………………. (1.7)
Termination
. .
oo o
2
R + R → Inert products ……..……………...(1.8)
Impurities such as a low concentration of certain transition metal ions
can greatly accelerate initiation through reduction and oxidation of
hydroperoxides.
oo
.
o
oo
+ ++ −
M + R H → M + R + OH …………………… (1.9)
++ + +
M + RooH → M + R + H
..…………………. (1.10)