Vibration Characteristics of Tread Stocks

Abstract

Nonlinear vibration characteristics of tread compounds, as evidenced by a dependence of modulus and internal friction on amplitude, were studied in order to reach a n understanding of these unexplained phenomena in terms of plausible structural alterations which may occur in the tread compounds because of vibration. This information is interesting both for a more exact description of the deformation processes in tread compounds, and because of the necessity of dealing with the effects in any dynamic testing procedure. Nonlinear vibration characteristics are readily observed for tread stocks of both Hevea and synthetic rubbers in the mechanical range of frequencies.Experiments were undertaken t o study the extent to which these effects are dependent upon temperature, compounding variables, and type of vibration. I t may be shown that the nonlinearity is not due to temperature rise from vibration, although precise measurements are complicated by the temperature rise. The effects occur for stocks reinforced with fine silica pigment, Pliolite resin, and higher loadings of pigments generally considered to be nonreinforcing, as well as for those compounded with reinforcing blacks. The pigment loading necessary for the same degree of nonlinearity increases with the pigment size. The effects are observed in unvulcanized tread stocks and occur for vibrations in both shear, compression, and tension. Measurements of the nonlinearity for tread stocks over a range of temperatures suggest that the vibration elicits structural changes in the rubber analogous to those observed in the rheology of rubber solutions and raw rubber. The nature of these changes can be interpreted by the rheological criteria of structure developed for non-Newtonian systems. Such recognized mechanisms for structural viscosity as orientation, deformation, and breaking and reforming of bonds of the flow units are useful in explaining the phenomena observed. The effects appear to be of such magnitude that they should be taken into account in any exact study of the effect of modulus on tread wear and performance. S E of the most distinctive features of the vibration of tread 0 stocks is a dependence of the dynamic modulus and internal friction upon the amplitude. This has been consistently observed over a wide range of experimental conditions (4-6, 8, 12, 13, 16, 18-20). For gum stocks these effects are so small that they have usually escaped detection. However, analogous effects with gum stocks of a magnitude of a few per cent have been ieported and it is very probable that they exist. In contrast, nonlinear vibration characteristics for tread stocks are so pronounced that they must be takeh into account in any dynamic testing procedure for evaluating tread compounds, and also in any precise effort to estimate the effective hardness of tire treads on the road.These phenomena with tread stocks are of particular interest because of their puzzling nature, the difficulty in formulating any exact explanation for them, and the possibility that a better understa...

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