5 Conclusions
So-called material functions, for instance the shear viscosity and the dynamic
(complex, elastic and viscous) moduli functions, play a key role in polymer science
and technology. By describing how a material behaves with respect to the strain
(or strain rate) and the stress tensors, such complex functions provides information
dealing with the processing behavior and the mechanical properties. The mode of
deformation defines the non-zero components of both the strain (strain rate) and the
stress tensors and, in what polymer systems are concerned the effect of external
parameters such as the time and the temperature deserves a special attention. Time
effects allow understanding how a system behaves with respect to a large time span;
temperature effects concern the usage and processing windows of the material. In
the specific case of rubber compounds, the presence of reinforcing fillers further
complicates the material functions.
Throughout this chapter it was shown that, correctly assessed, the shear viscosity
and the dynamic (complex, elastic and viscous) moduli functions are the necessary
material properties that one needs to know with respect to the processing behavior
of rubber systems and their mechanical properties after vulcanization. Overall shear
viscosity functions, i.e., ηð_ γ, TÞ and ηð_ γ, T, ΦÞ are difficult to obtain through rather
tedious and time consuming experiments, with different type of instruments, not all
commercially available. By nature however, the shear viscosity function encompasses the linear and the nonlinear viscoelastic behavior. In contrast with many
thermoplastic polymers, (filled) rubber systems hardly exhibit a linear behavior
within the practical shear rate window of available rheometers. This aspect severely
restraints the applicability of considerations based on the theory of linear viscoelasticity, at least in what steady shear is concerned. From a practical point of view
however, because most processing operations occur in the nonlinear viscoelastic
region, the fact that the asymptotic high shear viscous behavior of rubber
material reduces to a simple power law is of course of high interest for engineering
purposes.
Multiparametric dynamic functions, i.e. G*(ω, γ, T) and η*(ω, γ, T); G*(ω, γ,
T, Φ) and η*(ω, γ, T, Φ) are relatively easy to obtain with modern dynamic rheometers, providing the correct tests protocols and the appropriate data treatment are
used. This was extensively demonstrated in the chapter. It is however worth noting
that (filled) rubber materials are generally so stiff that open-gap rheometers do not
give reproducible results. Closed-cavity torsional rheometers must be used with
such materials and commercially available instruments are so robust that their use
can be considered on the factory floor. Depending on the test protocol considered,
dynamic rheometers give access either to the linear or to the nonlinear viscoelastic
behavior and it was shown in the chapter that tests must be repeated within the
room-to-curing temperature range for an overall characterization to be obtained.
Frequency–Temperature sweep tests address the linear domain, Strain–Temperature
sweep tests concern nonlinear viscoelasticity. In the former case, all the resources of
the theory of linear viscoelasticity can be used, namely the possibility to derive the
298
J.L. Leblanc
So-called material functions, for instance the shear viscosity and the dynamic
(complex, elastic and viscous) moduli functions, play a key role in polymer science
and technology. By describing how a material behaves with respect to the strain
(or strain rate) and the stress tensors, such complex functions provides information
dealing with the processing behavior and the mechanical properties. The mode of
deformation defines the non-zero components of both the strain (strain rate) and the
stress tensors and, in what polymer systems are concerned the effect of external
parameters such as the time and the temperature deserves a special attention. Time
effects allow understanding how a system behaves with respect to a large time span;
temperature effects concern the usage and processing windows of the material. In
the specific case of rubber compounds, the presence of reinforcing fillers further
complicates the material functions.
Throughout this chapter it was shown that, correctly assessed, the shear viscosity
and the dynamic (complex, elastic and viscous) moduli functions are the necessary
material properties that one needs to know with respect to the processing behavior
of rubber systems and their mechanical properties after vulcanization. Overall shear
viscosity functions, i.e., ηð_ γ, TÞ and ηð_ γ, T, ΦÞ are difficult to obtain through rather
tedious and time consuming experiments, with different type of instruments, not all
commercially available. By nature however, the shear viscosity function encompasses the linear and the nonlinear viscoelastic behavior. In contrast with many
thermoplastic polymers, (filled) rubber systems hardly exhibit a linear behavior
within the practical shear rate window of available rheometers. This aspect severely
restraints the applicability of considerations based on the theory of linear viscoelasticity, at least in what steady shear is concerned. From a practical point of view
however, because most processing operations occur in the nonlinear viscoelastic
region, the fact that the asymptotic high shear viscous behavior of rubber
material reduces to a simple power law is of course of high interest for engineering
purposes.
Multiparametric dynamic functions, i.e. G*(ω, γ, T) and η*(ω, γ, T); G*(ω, γ,
T, Φ) and η*(ω, γ, T, Φ) are relatively easy to obtain with modern dynamic rheometers, providing the correct tests protocols and the appropriate data treatment are
used. This was extensively demonstrated in the chapter. It is however worth noting
that (filled) rubber materials are generally so stiff that open-gap rheometers do not
give reproducible results. Closed-cavity torsional rheometers must be used with
such materials and commercially available instruments are so robust that their use
can be considered on the factory floor. Depending on the test protocol considered,
dynamic rheometers give access either to the linear or to the nonlinear viscoelastic
behavior and it was shown in the chapter that tests must be repeated within the
room-to-curing temperature range for an overall characterization to be obtained.
Frequency–Temperature sweep tests address the linear domain, Strain–Temperature
sweep tests concern nonlinear viscoelasticity. In the former case, all the resources of
the theory of linear viscoelasticity can be used, namely the possibility to derive the
298
J.L. Leblanc