The necessity as well as possibility of application of polymer rheology to rubber technology was first discussed, mainly occasioned by the research for development of new rubber products. The most effective and economical testing methods and standards were suggested therein as workable in practice.
Adopting the theory of linear viscoelasticity, the time-temperature superposition principle, the factorization of the time-and strain-dependences of stress at large deformations, the qualitative similarity between the dynamic and the steady-flow viscosities, the qualitative similarity between the failure envelope and the isochronal stress-strain curve and others as the working hypotheses, a set of material functions was newly proposed. Those were the relaxation spectra, the nonlinear factor for the elasticity, the strain-at-break vs. temperature diagram and the shift factor as a function of temperature, and they are tabulated in Table I in the text.
The analytical expressions of those material functions are here presented, listed also in Table II. Twelve material parameters involved in those expressions are summarized in Table III, and this rheological model is named as the "JSR 12 constant model."
An approximate method of formulating each item listed in the present testing standards as a function of those material parameters and the parameters describing the excitation factor is recommended, and the significance of this type of study is stressed. The expression of the heat buildup with a Goodrich flexometer is given as an example and the result of the application of this expression to the experimental data on various sample specimens is illustrated in Fig. 1.
The importance of correlating quantitatively the material parameters with the parameters characterizing the internal structure of the material is emphasized herein from an industrial point of view and also in consideration of the present situation. An illustration is given in Fig. 2 of the relationship between the two material parameters, n and λ
bmax, which are supposed to have the same molecular structural origin.
Finally, the estimation of optimum values of rubber products including unvulcanized synthetic rubbers are discussed with respect to the material parameters of each species. After reviewing the history of rubber industry, it will be expected that the optimization of products in quality in the market at present in general will proceeds to a considerably high degree. This expectation is experimentally verified when the commercial products from different factories are compared with respect to the shapes of the material functions of particular species where all their curves fall within a band of fairly narrow width irrespective of their chemical composition or of any other physical factors. The results of this experimental verification are demonstrated graphically in the relaxation spectra of various commercial synthetic rubbers in Fig. 3 and also in the strain-atbreak vs. temperature diagrams of the tread portions of automobile tires (Passenger car) from different factories in Fig. 4.
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