EFFECTS OF SHEAR HISTORY ON THE MECHANICAL, ELECTRICAL, AND MICROSTRUCTURAL PROPERTIES OF PARTICLE-REINFORCED RUBBER

The extent and nature of networks of carbon black particles in rubber compounds play a key role in determining the mechanical hysteresis and conductivity of rubber goods. It is well known that in uncrosslinked compounds, such networks display transient and time-dependent behavior when subjected to steps or ramps in shear and temperature (often called flocculation). This study probes the observed structural recoveries of carbon black networks following various levels of imposed shear strain histories. It is demonstrated that the level of shear experienced by the compound immediately before vulcanization can have a dramatic effect on the final dynamic mechanical properties of the subsequently vulcanized materials. Significant reductions in Payne effect occur when the timescales of shear-induced structural recovery, determined from rheological experiments, exceed the kinetics of vulcanization. Electrical conductivity/resistivity is also affected, especially for compounds formulated in the electrical percolation transition region. Furthermore, the microstructure of carbon black networks is tracked at different extents of recovery by using transmission electron microscopy thin section analysis and atomic force microscopy methodologies for particle network microstructure quantification. Evidence is found that relates flocculation to the progressive relaxation of shear-induced anisotropy of the carbon black micro dispersion.