Oxidation of Vulcanized Rubber. Effect of Temperature, State of Cure, and Thickness
Abstract 1. A previous investigation of the rate of oxidation of rubber over the temperature range of 60° to 80° C. has been extended to 90°, 100° and 110° C. 2. The rate of oxidation over the range which accounts for substantial deterioration of tensile strength appears to be a straight-line function of the time of aging from 60° up to an including 90° C. The rate of oxidation at 100° C. appears to diverge somewhat from a straight-line function of time decreasing as oxidation proceeds. 3. The rate of oxidation increases with increased temperature. Over the range of 60° to 110° C, the rate is doubled for each 7.5° increase in temperature, corresponding to a temperature coefficient of oxidation of 2.5. 4. Tensile strength decreases as a straight-line function of the time of aging and of the quantity of oxygen absorbed. 5. The quantity of absorbed oxygen, determined by weight gain corresponding to a 50 per cent decrease in tensile strength, varies with the temperature of oxidation. At 60° C. the oxygen requirement for 50 per cent deterioration is about 1.30 per cent, and at 110° C. about 0.65 per cent on the rubber hydrocarbon content. 6. Tensile strength increases, probably due to a mild curing effect, when specimens of the compound are heated in an atmosphere where the oxygen is replaced with carbon dioxide for the same periods of time at 80° and 110° C, were required to cause a 50 per cent deterioration in tensile strength in an oxygen atmosphere. 7. With increased time of vulcanization, the rate of oxidation increases. Oxidation appears to be autocatalytic in the case of overvulcanized rubber. 8. As time of vulcanization increases, there is a decrease in the quantity of oxygen required to cause a given decrease in tensile strength. 9. Thickness variations of 0.43 to 1.90 mm. in test-specimens of a compound containing an antioxidant do not affect quantity of oxygen absorbed or degree of tensile strength deterioration when aging is conducted at 80° C. and 3.5 kg. per sq. cm. oxygen pressure. 10. When tetramethylthiuram disulfide is used for vulcanization in place of sulfur, there appears to be no change in the mechanism of oxidation, but the rate of oxidation is reduced.