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 = Δ E/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. Δ E/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 N 2 , 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 N 2 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.