Electrical Resistance Assisted Prediction of Stress Relaxation in Rubber Parts

Visco-elastic properties of rubber, namely dynamic storage modulus (E’) and dynamic loss modulus (E”), are influenced immensely by the operating conditions such as duration, temperature, humidity, and extent of stress and deformation, among others. Predicting the viscoelastic properties changes is complex yet vital for designing rubber components with longer service life. Due to the electrically conducting nature of certain fillers that can be used to reinforce rubbers (such as high structured carbon blacks, carbon nanotubes (CNTs), Graphene, etc.) it is possible to obtain current information about mechanical and visco-elastic properties by monitoring the electrical conductivity or resistivity, respectively. Within such a concept, inherent (conductive) ingredients of a rubber formulation can serve as sensors.

In the present case, creep, stress relaxation and dynamic mechanical experiments are coupled with electrical conductivity measurements. This enables us to visualize the strain-dependent electrical conductivity changes under various loading conditions that can refer to different situations of a rubber part in real service conditions. Especially in dynamic experiments, according to the dynamic strain, a periodic destruction-reformation of the conducting filler network is observed; substantiated by the electrical conductivity values. An electrical conductivity change would arise merely due to the changes in filler network and therefore the experiments provide also vital information regarding the destruction behavior of filler networks, which is one of the underlying mechanisms leading to changes in mechanical and visco-elastic properties.

The above mentioned experiments were performed on SSBR samples filled with different either commercial carbon blacks or CNTs. In our experiments, the well-known stress relaxation phenomenon is evidenced from the changes in filler networks, whose rate is dependent on temperature, strain, frequency, and interestingly, also on the crosslink density. The stress values can be theoretically predicted from the electrical resistance values and show good correlation with experimental results. The findings are promising for simple stress relaxation and creep experiments, as well as for complex dynamic experiments. A simple relation that bridges electrical resistance and the mechanical stress is derived in the present study. Stress predictions are performed for various rubber samples under different experimental conditions to affirm the validity of the proposed model.

Our method of envisioning changes in the filler network behavior inside rubber matrices through electrical resistance measurements and thus predicting the mechanical stress can be applicable for monitoring rubber products, such as seals, gaskets, dampers, even during service.