Abstract
The permanent set, γ defined in usual methods of testing material for vulcanized rubber was studied rheologically by identifying it with the residual strain left in a viscoelastic body. After appropriate approximations the following expression was obtained.
(γ/γ0) ≅ (ktan δ)2exp[-(ktan δ)(t/t')] |ω=1/t'
Here γ0 is a constant strain applied to the sample for a period of time, t', by an external force, and t stands for the time elapsed after the removal of the external force. The loss tangent and the angular frequency are symbolized by tan δ and ω, respectively.
The numerical factor, k, expects to be mainly affected by the strain dependence of tan δ in a sample.
The experimental data on several typical rubber vulcanizates checked fairly well the above equation for t'-, t- and tan δ-dependencies of γ.
An average value of 2.2 was found to be replacable for the numerical factor, k irrespective of rubber species at room temperature when t', t and γ0 were fixed as 10 min., 10 min. and the one half of the strain at break in extension, respectively.
