M.Sc Thesis

M.Sc StudentZaiats Gary
SubjectBlends of EPDM and Polybutadiene Rubbers Containing
Electrically Conductive Carbon Black
DepartmentDepartment of Materials Science and Engineering
Supervisor PROF. Michael Silverstein
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Rubber is a non-conductive polymeric material. Rubber can be made conductive by adding conductive fillers such as metal particles or carbon black. The electrical properties of a polymer filled with conductive fillers have been described using percolation theory. Below the percolation threshold the addition of conductive filler has little effect on resistivity. The drastic reduction in resistivity at the percolation threshold results from the formation of continuous pathways of conductive filler particles. Above the percolation threshold the addition of more conductive filler has little effect on resistivity since it simply increases the number of conductive pathways.

Modern engineering systems are often required to supply real-time information about the stress or strain in the parts. It would be advantageous to monitor the changes in the mechanical conditions of a part by measuring the changes in its electrical properties. The objectives of this research were to develop and characterize blends of ethylene propylene diene monomer (EPDM) rubber, polybutadiene rubber (BR) and carbon black (CB) which could be used for the production of rubber products with stress/strain sensor capabilities. Rubber blends containing EPDM and various amounts of BR and CB were prepared. Their morphologies, mechanical properties and electrical properties were characterized. Finally, the resistivity was measured as a function of compressive stress and/or strain using a cyclic stress/strain experiment and a stress relaxation experiment.

The relationship between the electrical properties and the compressive stress depended upon whether the CB content was below or above the percolation threshold. Below the percolation threshold, compressive stress reduces the average distance between the particles yielding a reduction in resistivity. On the other hand, above the percolation threshold, the compressive stress breaks the conductive particle pathways and this produces an increase in resistivity.