Stress Relaxation of Natural and Synthetic Rubber Stocks

Abstract

1. The complete decay of stress in the rubbers studied, held at constant elongation, appeared to involve the rupturing of a definite bond, either at some point along the molecular chain or at the cross-linking bond put in by vulcanization. In the case of a Hevea rubber gum stock the data could be fitted very well by ordinary reaction-rate theory, leading to the conclusion that the free energy of activation required for breaking the bond is 30.4 kcal. per mole of bonds. This result was found to be practically independent of the elongation, and of the presence of carbon black in a Hevea rubber tread stock. This is to be compared to a strength of about 90 kcal. per mole for the C—C bond. 2. In the case of other rubbers (Buna-S, Butaprene-N, Neoprene-GN, and Butyl) the activation free energy for breaking the bond did not vary by more than ±2.0 kcal. per mole from that of Hevea rubber. However, these differences were quite definite. For example, the relaxation of stress in GR-S was slower than in Hevea; a small difference in energy corresponding to a 2:1 ratio in the respective times of decay. 3. The effect of temperature on the relaxation of stress appeared to be of the general type characteristic of chemical reactions. By use of the ordinary formula for expressing rate of reaction in terms of energy of activation, one could predict very closely the behavior of the stress-log time curves at different temperatures. 4. Natural rubber and GR-S vulcanized with paraquinone dioxime and lead dioxide showed relaxation curves very similar to those of the sulfur vulcanized stocks. 5. Relaxation experiments in an ordinary air atmosphere and in an atmosphere of commercial nitrogen showed no appreciable differences. 6. Examination of stretched rubber bands in which the stress had decayed nearly completely (at 100° C) gave no evidences of gross oxidation, such as would make the rubber bands sticky or hard, or of surface deterioration. At higher temperatures, however, the rubber could be observed getting sticky, and then brittle. Specimens in which the stress had completely decayed showed very low tensile strength (by hand test). 7. Antioxidant added to a sulfur-stabilized Buna-S stock caused a definite retardation of the rate of relaxation. 8. Comparison of the results of these experiments with previously recorded observations in the literature indicated that the chemical reaction which ruptured the rubber structure and caused the decay of stress in these experiments (and concomitantly a lowering of tensile strength) was an oxidation of the rubber by small amounts of oxygen, the reaction rate being independent of the oxygen pressure in the range between that present in an ordinary air atmosphere and in a commercial nitrogen atmosphere. 9. The tests suggested a convenient and accurate laboratory method of determining the oxidizability of natural and synthetic rubber stock designed for service.

Influence of Thermally Induced Chemorheological Changes on the Inflation of Spherical Elastomeric Membranes

“…Due to the formation of new crosslinks, a network is formed during the interval fromt t tot t þ dt t whose reference configuration is the configuration of the original material at timet t: As suggested by Tobolsky et al [10] and Tobolsky [11], this is assumed to be an unstressed configuration for the newly formed network. During subsequent deformation, the configurations of the newly formed material network coincide with the configurations of the original material network.…”

On the mechanics of elastomers undergoing scission and cross-linking

“…Crosslinks are introduced in a later state, producing a second macromolecular network. It can be formed due to the cross-linking of previously unreacted constituents, or, as postulated by Tobolsky, Prettyman and Dillon [3], by macromolecular cross-links of the initial network that have broken and reformed in a new reference state. This new macromolecular mechanism has two consequences.…”

“…This model, known as the two network theory, was used to describe the phenomena of modified stiffness and permanent set upon unloading. Motivated by the molecular considerations of Tobolsky [2] and Tobolsky, Prettyman and Dillon [3], Fong and Zapas [4] outlined a three dimensional constitutive theory that allows for continuous recruitment of new cross-links, scission of existing cross-links and their reformation or healing. Wineman and Rajagopal [5] and Rajagopal and Wineman [6] then provided a general three dimensional framework that can be used to describe elastomers undergoing microstructural changes.…”

An Experimental Facility to Measure the Chemorheological Response of Inflated Elastomeric Membranes at High Temperature

“…The first work on the microstructural degradation of elastomers was started by Tobolsky et al [2] in 1944. Since then, many researchers [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] have studied elastomers undergoing scission and cross-linking effects.…”