Abstract
Viscoelastic properties of disperse systems of pigments, especially of carbon black in linseed oil, were investigated. The dynamic modulus and dynamic viscosity decrease with increasing amplitude of strain. The decrease of the former is more marked. The difference may be consistent with the concept that the stress is supported by the network structure formed by the particles and that the structure is destroyed with increasing amplitude. The second plateau modulus, the equilibrium value of the dynamic modulus in the low frequency region, continues to be observed up to quite a high amplitude.
The effects of varying magnitude of shear strain and rate of shear on dynamic viscoelastic properties and the stress relaxation can be described using the box type relaxation spectrum, which varies with deformation state. The long time portion of the relaxation spectrum becomes lower in deformation; the critical relaxation time is determined by the rate of shear and the degree of lowering depends also on the period of flow. The behavior is interpreted by taking into account the process of network formation and breaking down.
Shear stress development after sudden initiation of steady shear flow showed a remarkable overshoot phenomenon. The elastic properties of the network structure of dispersed particles can also be evaluated from the creep and creep recovery experiments. The critical strain for breaking down of the structure, determined from the relationship between shear stress under steady flow and recoverable strain, is approximately equal to the strain at which the stress attains maximum at the start of shear flow.
