Modulus of Natural Rubber Cross-Linked by Dicumyl Peroxide. I. Experimental Observations

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 , was raised about 1.2 °C for each part of decomposed dicumyl peroxide. Above (T + 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.