Measurements of stress decay as afunction of time made at constant elongation on thin bands of gum and tread type natural rubber (Hevea), Neoprene, Butyl, Buna S, and Butaprene N stocks indicate that both secondary and primary bond relaxation occur. Practically complete relaxation is observed to take place in the experimental time (about 100 hours) at temperatures at and above 100 degrees C. The manner in which the rate of relaxation depends on temperature and the fact that the rate is independent of elongation and of the presence of carbon black in the vulcanizate indicate that stress decay is caused by a definite chemical reaction which deteriorates the rubber structure, and oxidative scission is suggested as the mechanism of deterioration of the primary bonds. The stress relaxation data, obtained over a temperature range from -50°C to +150°C, appear to verify modern concepts of the structure of elastomers. Theoretical equations are derived which give very good agreement with the observed relaxation data at high temperatures. Thefree energy of activation for the oxidative scission isfound to be 30.37 kcal.per mole for the Hevea gum stock, and differs from this value by less than ±2.0 kcal. per mole for all other stocks, indicati.ng the same general reaction for all. However, these small differences in free energy of activation correspond to considerable differences in times of decay, which fact might be significant in evaluating the resistance of rubber stocks to deterioration.

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.

An extrusion plastometer operating at rates of shear comparable with those existing in rubber tubing machines (10 to 1000 sec.−1) is described. The relation between efflux rate and pressure at constant temperature for various types of rubber stocks was determined. For highly compounded rubber stocks, such as tread stocks, the efflux rate vs. pressure curves attain linearity at low rates of shear. Both the slope of the curves and the extrapolated pressure intercept vary rapidly with the temperature, indicating that both the mobility and the yield stress are functions of temperature for highly compounded stocks. For lightly compounded stocks and for crude rubber, the curves are of the power function type at the lower rates of shear, but appear to attain linearity at the very high rates. These linear portions of the curves, for a given stock at different temperatures, are approximately parallel. Elastic recovery was determined as a function of rate of efflux. The slope of the recovery vs. efflux rate curves decreases with increasing rates of efflux. The relation between efflux rate at constant temperature and pressure, and time of milling, is approximately linear. The extrusion plastometer is shown to be more sensitive to overmilling than is the Williams plastometer. The partial failure of the compression-type plastometer to correlate with the factory extrusion machine is explained on the basis of the much lower rates of shear employed in the compression-type instrument than those existing in the extrusion machine.

The nature of hysteresis in products such as pneumatic tires, solid tires, and transmission belts is analyzed and the requirements of a laboratory test for evaluating the relative hysteretic characteristics of natural and synthetic rubber stocks are developed. The significance of tmrious definitions of the "hysteresis defect" in rubberlike materials is discussed. A forced resonance vibrator in which rubber samples are deformed in shear at frequencies of 20 to 300 cycles/sec., shear strains of 0.05 to 0.35, and temperatures of -20 to +120°C is described. Experimental results obtained with natural rubber and GR-S gum and tread stocks are presented. The hysteresis index w'1 is found to be nearly independent of dynamic shear strain while the dynamic modulus G decreases moderately with increasing dynamic strain. Neither w'1 nor G depends upon the height to diameter ratio of cylindrical samples. These results are at variance with those obtained by previous investigators who, employing compressive vibrations, have reported marked dependences of both modulus and friction upon dynamic strain and the "shape factor" of tread type stocks. In agreement with previously reported work, G isfound to be independent of frequency and w'1 only slightly dependent upon frequency, for tread type stocks. Results are presented for stocks based on Buna S type copolymers with varying monomer ratio and on N-type Butaprenes, Neoprene, and butyl rubbers.

No abstract