composites are materials made by combining two or more different types of fillers
in a common matrix. The hybridization of two types of mineral fillers having
different sizes and shapes offers some advantages over the use of single filler
alone in a rubber matrix [19]. Hybrid fillers are also used to enhance the thermal
conductivity of polymer composites. Generally, carbon fillers are used to improve
the conductivity of polymer composites. However, individual filler [viz graphene
nanoplatelets (GNPs) or carbon nanotubes (CNTs)] limit the realization of the
desirable thermal conductivity of the composite. But the composites derived from
carbon hybrid fillers composed of CNTs directly grown on the GNP support show
enhancement in thermal conductivity than the individual fillers [36].
2.8 Effect of Hybrid Fillers on Non-Linear Viscoelasticity
of Rubber Nanocomposites
In this section, the effect of hybrid fillers on nonlinear viscoelastic properties of
rubber composites/nanocomposites is discussed. The viscoelastic properties of
rubber nanocomposites include different properties like cure behavior, rheology,
creep and dynamic mechanical analysis etc. The properties measured under
dynamic mechanical analysis are storage and loss moduli and tan δ. Storage
modulus is a measure of the maximum energy stored in the material during one
oscillation cycle which represents the elastic nature of the material whereas the loss
modulus measures the energy dissipated as heat, representing the viscous portion of
the material [37] (http://en.wikipedia.org/wiki/Dynamic_mechanical_analysis).
The ratio of loss modulus to storage modulus is referred to as internal damping or
loss tangent (tan δ) [37]. The effect of hybrid fillers on these properties is thoroughly discussed under this section. The significant improvement of various properties of rubber/polymer composites derived from different hybrid fillers are studied
by many researchers [38–43]. The dynamic mechanical properties of natural rubber
(NR) filled with fly ash silica (FASi) and precipitated silica (PSi) is studied by
S. Thongsang et al. Different weight fractions of FASi:Psi used in this investigation
are 100:0, 75:25, 50:50, 25:75 and 0:100. They measured different mechanical
properties with respect to Psi content and observed increase in mechanical properties with increasing Psi content, resulting in a decrease in tan δ max value. The
optimum mechanical properties were achieved at 75 % Psi loading. So the dynamic
mechanical analysis was done on composites containing 75 % Psi with different fly
ash particle sizes and at two different silica loadings (10 phr and 40 phr). The
variation of viscoelastic properties (storage modulus, loss modulus, and tan δ) with
temperature of NR vulcanizates filled with 75 % Psi fraction in FASi/Psi hybrid
filler at total silica contents of 10 and 40 phr is shown in Fig. 18 [37]. It is observed
from Fig. 18 that storage modulus of composites containing 40 phr silica is higher
than the composite containing 10 phr silica both in glassy and rubbery regions. But
this is more pronounced in the rubbery region which is due to the higher stiffness of
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in a common matrix. The hybridization of two types of mineral fillers having
different sizes and shapes offers some advantages over the use of single filler
alone in a rubber matrix [19]. Hybrid fillers are also used to enhance the thermal
conductivity of polymer composites. Generally, carbon fillers are used to improve
the conductivity of polymer composites. However, individual filler [viz graphene
nanoplatelets (GNPs) or carbon nanotubes (CNTs)] limit the realization of the
desirable thermal conductivity of the composite. But the composites derived from
carbon hybrid fillers composed of CNTs directly grown on the GNP support show
enhancement in thermal conductivity than the individual fillers [36].
2.8 Effect of Hybrid Fillers on Non-Linear Viscoelasticity
of Rubber Nanocomposites
In this section, the effect of hybrid fillers on nonlinear viscoelastic properties of
rubber composites/nanocomposites is discussed. The viscoelastic properties of
rubber nanocomposites include different properties like cure behavior, rheology,
creep and dynamic mechanical analysis etc. The properties measured under
dynamic mechanical analysis are storage and loss moduli and tan δ. Storage
modulus is a measure of the maximum energy stored in the material during one
oscillation cycle which represents the elastic nature of the material whereas the loss
modulus measures the energy dissipated as heat, representing the viscous portion of
the material [37] (http://en.wikipedia.org/wiki/Dynamic_mechanical_analysis).
The ratio of loss modulus to storage modulus is referred to as internal damping or
loss tangent (tan δ) [37]. The effect of hybrid fillers on these properties is thoroughly discussed under this section. The significant improvement of various properties of rubber/polymer composites derived from different hybrid fillers are studied
by many researchers [38–43]. The dynamic mechanical properties of natural rubber
(NR) filled with fly ash silica (FASi) and precipitated silica (PSi) is studied by
S. Thongsang et al. Different weight fractions of FASi:Psi used in this investigation
are 100:0, 75:25, 50:50, 25:75 and 0:100. They measured different mechanical
properties with respect to Psi content and observed increase in mechanical properties with increasing Psi content, resulting in a decrease in tan δ max value. The
optimum mechanical properties were achieved at 75 % Psi loading. So the dynamic
mechanical analysis was done on composites containing 75 % Psi with different fly
ash particle sizes and at two different silica loadings (10 phr and 40 phr). The
variation of viscoelastic properties (storage modulus, loss modulus, and tan δ) with
temperature of NR vulcanizates filled with 75 % Psi fraction in FASi/Psi hybrid
filler at total silica contents of 10 and 40 phr is shown in Fig. 18 [37]. It is observed
from Fig. 18 that storage modulus of composites containing 40 phr silica is higher
than the composite containing 10 phr silica both in glassy and rubbery regions. But
this is more pronounced in the rubbery region which is due to the higher stiffness of
152
S. Nayak and T.K. Chaki