Previously obtained dynamic shear modulus data over a wide amplitude range of rubber vulcanizates loaded with carbon blacks of widely varying structure as well as graphitized black were augmented with dynamic conductivity data at 30° C and at 100° C. Results confirm the existence of reversible transient and irreversible persistent carbon structures, the former being nearly completely destroyed at moderate shear amplitudes (50% strain), but effectively restored at lower amplitudes in a dynamic equilibrium between chain destruction and reformation. Higher structure blacks are characterized by a higher rate of chain rebuilding than lower structure blacks and by marked persistent chain alignment at higher amplitudes. Conductivity as a function of strain and time shows carbon chain behavior during stressing. The rates of destruction and rebuilding of the transient chains and the exact point at which the transient structure effects balance the chain alignment effects on conductivity have been established. The unusual behavior of vulcanizates loaded with graphitized black is explained.

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Measurements have been made of the relaxation of stress and electrical resistivity of carbon black-loaded vulcanizates subjected to shear deformation varying between 0.5 and 20%. Samples were initially brought into an equilibrium state by the application of a sinusoidal shear strain for a period of time long enough to ensure equilibrium condition, established by constancy in the dynamic stress- and resistivity-time cycles at each selected strain amplitude. It could be shown that at the small strains employed all phenomena were completely reversible, indicating the absence of rupture of elastomer chains or crosslinks and of strong bonds between carbon and elastomer, as well as of chemical aging. The shear stress relaxation rates show a small initial decrease with deformation and are fairly constant over the rest of the deformation range. Carbon black loading increases the rate—the more, the smaller the particles and the higher, the higher the loading—but the rate is independent of the carbon structure. Actual initial stress values follow the expected stress-strain relations. Resistivity relaxation rates, on the other hand, show a very rapid increase with shear deformation, more pronounced at higher structures and for smaller particles. Initial resistivity values are in complete accord with the dynamic behavior of vulcanizates. An explanation of the shear stress relaxation rates of the loaded vulcanizates has been made on the basis of reestablishment of previously disrupted weak bonds between particles and elastomer chains. Resistivity relaxation rates could be explained by assuming rebuilding of disrupted transient structures, while the unusual behavior of Graphon, with a zero relaxation rate, appears to be caused by its lack of bonding to elastomer chains. Finally, there are indications that the equilibrium resistivity behavior of a carbon-loaded vulcanizate subjected to shear strain at increasing amplitudes under reversible conditions is closely parallel to the reinforcement characteristics of the carbon black filler. Measurement at higher temperature permits calculation of an activation energy which appears to correspond to physical interaction.

Mild, reversible stress softening of vulcanizates loaded with carbon blacks of widely different structures and reinforcing characteristics was investigated by means of a novel electronic variable shear amplitude dynamic tester. The instrument yields low frequency dynamic data for vulcanizates in the range of double strain amplitudes of 3×10−5 to 1.0, covering nearly five decades. Mild reversible stress softening (up to 100 per cent amplitude) in a variety of vulcanizates investigated is caused by a reversible thixotropic breakdown of transient carbon chains, which contribute to reinforcement by an energy dissipating mechanism. Persistent, “fused” carbon chains contribute to reinforcement by their beneficial influence on the crosslink density of the vulcanizate. Elastomer and carbon chains respond independently to stress.

A method has been developed to separate quantitatively the contributions of filler volume, particle anisometry, and interaction with the elastomer to the dynamic modulus of carbon black loaded vulcanizates. The influence of reduction of transient structure by chemical promotion has been evaluated.