RESUMEN
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.
RESUMEN
Natural rubber mixed with varying amounts of dicumyl peroxide was cross-linked by heating 120 min at 149 °C. The quantitative measure of cross-linking was taken as the amount fp of decomposed dicumyl peroxide, the product of p, the number of parts added per hundred of rubber and f the fraction decomposed during the time of cure. The shear creep modulus G was calculated from measurements of the indentation of a flat rubber sheet by a rigid sphere. The glass transition temperature T g , was raised about 1.2 °C for each part of decomposed dicumyl peroxide. Above (T g + 12) the modulus-temperature relations were linear with a slope that increased with increasing cross-linking. The creep rate was negligible except near the glass transition and at low values of fp. Values of G, read from these plots at seven temperatures, were plotted as a function of fp. The linearity of the two plots permits the derivation of the general relation: G = S(fp + B)T + H(fp + B) + A where A, B, H, and S are constants. The lines representing G as a function of fp at each temperature all intersected near the point, fp = 0.45 phr, G = 2.70 Mdyn cm-2 (0.270 MN m-2). The constants were evaluated as A = 2.70 Mdyn cm-2, B = -0.45 phr, S = 5.925 × 10-3 Mdyn cm-2(phr)-1K-1 and H = 0.0684 (Mdyn cm-2) (phr)-1. This equation represented satisfactorily all the data obtained at temperatures from -50 to +100 °C for values of fp from about 1 to 24 phr.