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

Chapter 1
forces of the Van der Waals types. The extent of aggregation and
agglomeration has a marked influence on the reinforcing properties of fillers.
By comparing the reinforcing action of fillers in the same surface area range,
the morphology is the major factor determining reinforcement. Highly
structured aggregates typically have branches that form voids between them,
where polymer can be occluded, which results in higher effective loading of
the carbon black in an elastomeric compound. Skeletonisation method has
been used for the first quantitative and direct measurement of branching in
carbon black aggregates. 59
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The addition of silica to the elastomer results in a strong increase of
the viscosity. Silanol groups, responsible for the strong interparticle forces,
cover the silica surface. Silica particles tend to agglomerate, they are difficult
to disperse and re-agglomerate after mixing. 60 One of the effects of
reagglomeration is the reduction of processability during storage. The
viscosity of a compound increases during storage, with an increasing rate at
higher temperatures. The viscosity increase follows a second order kinetic
law, suggesting that the process is based either on coalescence of filler-bound
rubber entities or re-agglomeration of filler particles. The tendency of re-
agglomeration is influenced by the water content of the silica. Higher water
content results in a lower rate of viscosity increase during storage due to a
shielding effect of the water molecules on the surface of the silica, which
reduces the interparticle forces. 61
Hewitt reported that compound viscosity is directly related to the
surface area of silica. 62 Highly developed filler networks of silica can give rise
to compounds of high viscosities. The difference between types of filler
became less pronounced as the average filler particle size is increased. 63 At
about 20 nm size silica produces significantly higher viscosity than carbon
black of comparable size. 64 Viscosity modification studies of rubbers filled
with smaller particle size silica was reported by Dunnom. 65-66 Use of silane