RATION CHARA OF TRE D STO K. E. GUI,C. S.WILKINSON,
J R . , AND S. D. GEHMAN
The Goodyear Tire and Rubber Co.,Akron 16, Ohio 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
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S E of the most distinctive features of the vibration of tread 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 t h a t they must be takeh into account in any dynamic testing procedure for evaluating tread compounds, and also in any precise effort t o 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 understanding of them may lead t o new information on molecular deformation mechanisms and structural details of tread stocks. The more obvious explanations t o account for a decrease of modulus and internal friction with increasing amplitude of vibration of tread stocks have been ruled out by experimental work which has been reported. That it is not due primarily t o a temperature rise of the test piece is indicated by the experiments of Stambaugh (le),Gehman ( 6 ) , and Waring (18). The fact that theeffect occurs for shear vibrations (4-6, IS),as well as for compression vibrations (6, 8, l e , 18),eliminates any association of the effect with nonlinear static stress-strain characteristics. More evidence on these points will be given in the present paper. The occurrence with shear vibrations also excludes any explanation based on volume changes a t small deformations (10). The effect of the time schedule or deformation history on the amplitude effects has been studied in several investigations (6, 7, 18). This aspect of the phenomena points t o structural mechanisms analogous t o those which occur in colloidal systems showing structural viscosity. Additional reasons for this point of view will be presented and an attempt will be made to develop t h e ideas more specifically than has been accomplished heretofore.
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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. METHOD
All compression and shear experiments were made using the Goodyear Vibrotester. Since the apparatus and theory involved in its application and the procedure employed have been discussed in previous articles (3,8 ) , no extended discussion will be given here. While the instrument is capable of being operated over a frequency range of 20 t o 200 cp., all measurements reported here were made in the range of 55 t o 65 cp. unlegs otherwise specified. Variation in modulus and resilience over this limited range of frequencies is negligible. However, as internal friction varies approximately inversely with frequency, all values for internal friction have been reduced t o equivalent values at 60 cp. for purposes of comparison. For several special experiments in which extremely small amplitudes of vibration were used it became necessary t o modify the Vibrotester by replacing the optical system for indicating amplitude with an electrical system. An inertia type of piesoelectric crystal vibration pickup was mounted on the driving axis in such a way that a voltage would be produced as the samples were oscillated. This voltage was amplified and measured by means of an electronic voltmeter This crystal pickup was calibrated against the regular optical system and the curve extrapolated t o the low range desired. COMPOUNDS USED
Most of the work was done using GR-S compounds, although several Hevea compounds were studied for comparison. Formulations are given in Table I. Best cures were used as indicated by ordinary tensile tests. EXPERIMENTAL RESULTS
When a tread-type compound is vibrated over a range of increasing amplitudes and its dynamic modulus plotted as a function of amplitude, a curve such as t h a t in Figure 1is obtained. A part of the softening is due to the increase in temperature of
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No, 4
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ASTOMERS-D y namic Properties
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Table I. Formulas of Compounds Used A 100
GR-S, cold GR-S re ular Srnokkd &eet Carbon black HAF EPC SRF MT Silica (RS-1) Pliolite (S6) Zinc oxide Sulfur Amax Diphenylguanidine Stearic acid Altax Triethanolamine
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100 50
C 100
D
F
E
100
70
100
100
H
G
I 100
J
L
I
