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

Long-time creep of natural rubber cured with a sulfur-accelerator recipe containing no filler can be conveniently represented by a plot of (E - E ₁)/E ₁ = ΔE/E ₁ with a double-abscissa scale showing log t and t. E is the elongation at any time t, after application of the load, and E ₁ its value at unit time. Experimental data conform to the equation except for a more rapid rise preceding rupture. The constants A and B can be evaluated from only three observations-at the longest time (about 70 days), at one minute, and at an intermediate time. ΔE/E ₁ is approximately linear with log t when t is less than 0.1(A/B) and approximately linear with t when t is greater than 4.343(A/B). The observed modulus was about 1.4 MPa and A was about 2.4 percent/(unit log t) when the atmosphere was a vacuum, dry N₂, or dry air. The modulus was lowered very slightly and A became about 4 percent/(unit log t) when the air was saturated with water. B was raised from about 2 × 10^-5 percent/min to about 20 × 10^-5 percent/min when the vacuum or dry N₂ was replaced by dry air and to about 50 × 10^-5 percent/min when the air was saturated with moisture. A is considered to be related to physical relaxation, while B corresponds to a chemical reaction, probably oxidative degradation.