unsuitable for intercalation by polymer chains. This is because bulk graphene has a
pronounced tendency to agglomerate in the polymer matrix [64].
Graphene dispersions can be prepared in organic or aqueous media using either
covalent or noncovalent methods of functionalisation. Noncovalent functionalisation involves wrapping of graphene sheets with surfactants like poly ethylene
glycol, CTAB, SDS or through π-π interaction with certain organic molecules [65].
3 Reinforcing Effects of Fillers in Rubber Blends
Elastomers in general are not used in their pure form, but are reinforced by fillers.
The reinforcement of rubber by active fillers is a well-recognized phenomenon but
the term ‘reinforcement’ is not well defined. Reinforcement means the marked
increase in tensile strength, tear resistance, abrasion resistance and modulus far
beyond the values expected on the basis of the Einstein-Guth and Goldtheory [66],
taking into account the effects caused by colloidal spherical particles (hydrodynamic effect) and occlusion of rubber. The reinforcement of elastomers by fillers
has been studied in depth in numerous investigations [67] and it is generally
accepted that this phenomenon is dependent, to a large extent, on polymer properties, filler properties and processing.
The addition of fillers fundamentally changes the properties of rubber: For
unfilled rubbers there is increase in modulus with increasing temperature, as
predicted by the kinetic theory of rubber elasticity. The addition of fillers significantly changes the temperature coefficient of the modulus; it can even alter the sign
of the coefficient resulting in a decrease of the modulus with increasing temperature. Another effect of blending fillers with rubber is the transition to non-linear
behavior. The use of reinforcing fillers gives the material unique properties: a
combination of high elasticity with high strength. Figure 9 illustrates the influence
of the addition of increasing amounts of reinforcing fillers on various properties of
an elastomer.
A condition for filler reinforcement is the interaction between the filler particles
and the polymer. These interactions can be strong, for example in the case of
covalent bonds between functional groups on the filler surface and the polymer,
or weak as in the case of physical attractive forces. When carbon black is blended
with a polymer, the level of physical interaction is high. In contrast to this, the
interaction between silica particles and the polymer is very weak, and only by the
use of a coupling agent a bond is formed between the filler and the polymer.
Besides the interaction between the polymer and the filler, an interaction
between filler particles occurs, predominantly above a critical concentration threshold, the percolation threshold. The properties of the material change drastically,
because a filler-filler network is established. For example; an over proportional
increase of electrical conductivity of a carbon black filled compound. But even at
Non-linear Viscoelastic Behaviour of Rubber-Rubber Blend Composites and. . .
99
pronounced tendency to agglomerate in the polymer matrix [64].
Graphene dispersions can be prepared in organic or aqueous media using either
covalent or noncovalent methods of functionalisation. Noncovalent functionalisation involves wrapping of graphene sheets with surfactants like poly ethylene
glycol, CTAB, SDS or through π-π interaction with certain organic molecules [65].
3 Reinforcing Effects of Fillers in Rubber Blends
Elastomers in general are not used in their pure form, but are reinforced by fillers.
The reinforcement of rubber by active fillers is a well-recognized phenomenon but
the term ‘reinforcement’ is not well defined. Reinforcement means the marked
increase in tensile strength, tear resistance, abrasion resistance and modulus far
beyond the values expected on the basis of the Einstein-Guth and Goldtheory [66],
taking into account the effects caused by colloidal spherical particles (hydrodynamic effect) and occlusion of rubber. The reinforcement of elastomers by fillers
has been studied in depth in numerous investigations [67] and it is generally
accepted that this phenomenon is dependent, to a large extent, on polymer properties, filler properties and processing.
The addition of fillers fundamentally changes the properties of rubber: For
unfilled rubbers there is increase in modulus with increasing temperature, as
predicted by the kinetic theory of rubber elasticity. The addition of fillers significantly changes the temperature coefficient of the modulus; it can even alter the sign
of the coefficient resulting in a decrease of the modulus with increasing temperature. Another effect of blending fillers with rubber is the transition to non-linear
behavior. The use of reinforcing fillers gives the material unique properties: a
combination of high elasticity with high strength. Figure 9 illustrates the influence
of the addition of increasing amounts of reinforcing fillers on various properties of
an elastomer.
A condition for filler reinforcement is the interaction between the filler particles
and the polymer. These interactions can be strong, for example in the case of
covalent bonds between functional groups on the filler surface and the polymer,
or weak as in the case of physical attractive forces. When carbon black is blended
with a polymer, the level of physical interaction is high. In contrast to this, the
interaction between silica particles and the polymer is very weak, and only by the
use of a coupling agent a bond is formed between the filler and the polymer.
Besides the interaction between the polymer and the filler, an interaction
between filler particles occurs, predominantly above a critical concentration threshold, the percolation threshold. The properties of the material change drastically,
because a filler-filler network is established. For example; an over proportional
increase of electrical conductivity of a carbon black filled compound. But even at
Non-linear Viscoelastic Behaviour of Rubber-Rubber Blend Composites and. . .
99