In the rubber and tire industries, it is common knowledge that rubber is a living material. During mechanical loading, rubber changes its mechanical behavior due to inner structural changes on the molecular scale. It is therefore important to have information about the structure of rubber, the molecular mobility, the size of carbon black clusters within the rubber during dynamic-mechanical treatment.
The simultaneous combination of high-force dynamic-mechanical and dielectric analysis determines these important criteria and generates a better understanding of molecular dynamics within rubber systems.
How does simultaneous dynamic-mechanical and dielectric analysis work?
Dynamic-mechanical analysis (DMA) provides information about global mechanical bulk properties like stiffness and damping. Dielectric analysis (DEA) gives insights into the material’s intrinsic structure on a molecular level by determining its permittivity and dielectric conductivity.
The benefit of coupling and synchronizing the two technologies is the following: The mechanical load (generated by DMA) varies the DEA signal, which monitors the changes in the material’s structure in real time during mechanical loading.
Dielectric percolation threshold
Figure 1 clearly illustrates that increasing the carbon black content increases the dielectric conductivity of the HNBR system. The dielectric conductivity is attributed to the carbon black, which disposes of enough free electrical charge carriers on its surface area.
At 50 phr carbon black, the dielectric percolation threshold is reached. An interconnected filler network is built within the rubber matrix. Carbon black clusters come close to each other offering the electrical charge carriers sufficient conductive paths to migrate through the entire sample along the electrical field lines.
What are carbon black clusters?
Carbon black is an allotrope of carbon produced by incomplete combustion processes or thermal decomposition of gaseous or liquid hydrocarbons. Primary particles have a size of 20 to 50 nm. The primary particles agglomerate to aggregates with a size of 100 to 200 nm. The aggregates themselves build clusters with a size of 10.000 to 1.000.000 nm.
Carbon black clusters are mechanically not stable and can be destroyed by means of mechanical loads.
Changes in dielectric conductivity under dynamic load
With increasing dynamic-mechanical load from 5 N to 25 N, continuous destruction of the carbon black network takes place. Consequently, the density of conductive paths within the rubber matrix decreases, and hence the dielectric conductivity of the material.
A wide range of information
Simultaneous DMA and DEA can be used to generate information about the filler network dynamics within the rubber samples. The reinforcement potential of the filler network, and thus the sample can be deduced.
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Source: Kastner, A. (2002): Dielelektrische Charakterisierung rußgefüllter Elastomere. Technischen Universität Darmstadt.