An Alternative Approach to Characterize Filler Network Dynamics inside Rubbers Under Dynamic Load

Rubbers exhibit viscoelastic properties under dynamic loads, which are quantified and expressed in terms of dynamic storage modulus (E’), dynamic loss modulus (E”) and mechanical damping factor (δ). These properties are primarily governed by the imposed experimental conditions such as temperature, strain amplitude, test frequency, etc. Therefore, understanding the viscoelastic properties is vital for designing any rubber compound. On the other hand, electrically conducting rubbers, i.e. rubbers filled with highly conducting fillers such as carbon nanotubes, conductive carbon blacks, etc., are gaining huge interest in research and development.

In the present case, we have coupled the well-known dynamic mechanical analysis with real-time monitoring of electrical conductivity. This enables us to visualize the strain-dependent electrical conductivity under dynamic loading conditions. A periodic destruction-reformation of the filler networks is expected to occur (corresponding to the dynamic strain) and is also substantiated by the electrical conductivity changes in the rubber composite. Throughout the dynamic conductivity study, electrical conductivity changes arise merely due to the alterations in filler network. Therefore, our experiments are able to provide valuable information regarding the behavior of filler networks under dynamic conditions.

In the course of our dynamic experiments, a relaxation in filler network is evidenced from the electrical conductivity values. The degree of this particular relaxation is dependent on frequency, dynamic strain amplitude, test temperature and, interestingly, also on the crosslink density. Understanding these effects is of significance for practical applications since they are related to load situations experienced during real life performance of the rubber products. Moreover, we also observed a phase shift between the strain and resistance signals which was quantified in terms of tan δ_R-ε. Apparently, the phase shift values between resistance and strain were found to be always higher than the phase shift between stress and strain (tan δ_R-ε > tan δ_σ-ε).

Our novel method of envisioning the filler network behavior inside rubber matrices is easily applicable for all rubber products that undergo repeated loading during their service. This information therefore has the ability to yield more practical and reliable information regarding the dynamic properties for products such as tires, vibration absorbers, dynamic seals, etc. which are permanently under dynamic motion during applications.