Long-Time Creep in a Pure-Gum Rubber Vulcanizate: Influence of Humidity and Atmospheric Oxygen

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Abstract
The long-time creep of natural rubber cured with a conventional sulfur-accelerator recipe containing no filler can be conveniently shown near room temperature by a plot of ΔE/E1 with a double-abscissa scale, one marked in units of log t and the other in units of t. When experimental data from the present work and from previous studies reported in the literature are plotted in this manner it is noted that invariably the first scale yields a linear relation at short times and the second a linear relation at long times. The limiting linear relations just mentioned suggest the two-constant Equation (2), already proposed as a general creep equation for many materials. In the case of rubber the range of values of t investigated is from about 10 ms as studied by previous investigators to about 70 days in our work and other studies. Any significant deviations from the equation can be noted by inspection of the double-abscissa plot. We found that the equation furnished an excellent representation of almost all our experimental data up to the longest times. In one instance in our work and in a few other cases there was a prerupture increase of ΔE/E1 above the values given by the equation. This behavior can reduce somewhat the upper limit of validity of the general equation. The constants A and B can be evaluated from experimental observations of ΔE/E1 by solving two simultaneous equations obtained from the values at the longest time, at one minute, and at an intermediate time. In the present work, the constant A was essentially the same (about 2.4%/ (unit log t)) when the atmosphere surrounding the specimen was a vacuum, dry nitrogen, or dry air. The value was raised when the atmosphere was room air at 35% relative humidity and became about 4%/ (unit long t) when the air was saturated with water. The constant B was raised tenfold when the atmosphere was dry air instead of dry nitrogen. It was further increased by a factor of about 2, when the air was saturated. The value of B for the specimen in an atmosphere of stagnant room air was still greater than this by another factor of more than 2. It is possible that this atmosphere contained autocatalytic degradation products or other constituents which were removed when the air was bubbled through water or passed over CaCl2. The approximate boundaries of three different regions of time are determinable from the ratios A/B. In the first region, where t is less than 0.1(A/B), ΔE/E1 is approximately linear with log t. In the second region, where t is between 0.1(A/B) and 4.343 (A/B), ΔE/E1 is not linear with either log t or t. In the third region, where t is greater than 4.343 (A/B), ΔE/E1 is approximately linear with t. A fourth region of anomalous increase preceding rupture is sometimes found, especially when B is large.

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