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

Chapter 4
sulphur and fatty acids diffuse through the rubber matrix and are
adsorbed onto ZnO with the formation of intermediate complexes. 5,6 Fatty
acids are generally regarded as indispensable activators in conjunction
with zinc oxide. The function of fatty acid activators such as stearic acid ,
is to solubilize the zinc oxide, a secondary effect is an increase in the amount
of zinc sulphide produced. 7 The zinc salts of fatty acids, which are a
type of surfactant , also solubilize insoluble accelerators to form the
actual catalyst. A general scheme of accelerated sulfur vulcanization that
demonstrates the role of zinc oxides in conjunction with fatty acids is
shown in scheme I. 8,9
vulcanization
76
Scheme I: Role of ZnO and fatty acid in accelerated sulfur
X= Accelerator residue. L = ligand
Duchacek 10 noted that increasing zinc oxide concentration increased
the rate and extent of cross linking up to a certain zinc oxide concentration,
this concentration is believed to be the minimum level of zinc oxide needed
for complete conversion of accelerator to the zinc-accelerator sulfur complex.
It was also noted that the optimum zinc oxide content to minimize reversion
was slightly greater than this minimum. Coran 11 also noted that the induction
time had a dependence on the zinc oxide concentration in excess of that