Protective Action of Antioxidants of the Phenolic Type on the Aging of Natural Rubber

Abstract Oxidation of natural rubber in solution by atmospheric oxygen at 100° C in the presence of antioxidants and iron palmitate was studied. On the basis of the relative viscosity of the oxidized rubber solutions, the following conclusions can be drawn. 1. Addition of 1 per cent of certain antioxidants (pyrogallol, pyrocatechol, hydroquinone, 2-naphthol, resorcinol, Neozone D, and Oxynone) retards the oxidation of rubber in solution. However, this quality of antioxidant is not sufficient to stabilize the solution. 2. Comparison of the effectiveness of antioxidants (according to the data of relative viscosity) showed that the best antioxidant is pyrogallol, followed by pyrocatechol, hydroquinone, and Oxynone. The weakest are Neozone D and especially resorcinol. 3. Pyrogallol A, which is not shown in the table, does not protect a 1 per cent rubber solution from deterioration, showing a relative viscosity in solution of 4.08 at the beginning of the experiment and 1.40 after 7 hours, which is equal to that of the unprotected rubber solution. 4. The cresol and xylenol fractions of peat phenols were also used as antioxidants. The addition of 1 per cent of peat fractions to a rubber solution retards the drop in relative viscosity of oxidized solutions, but less than does the standard antioxidant, Neozone D. 5. The structure viscosity of solutions prepared from natural-rubber films which were stored 400 days, both with and without antioxidants, was measured. 6. Antioxidants of the phenol type protect rubber during storage in the form of films in darkness in air; in this case the best antioxidant is pyrogallol, and the next best pyrocatechol. 2-Naphthol is more active than hydroquinone. 7. When rubber is stored for a long time in air in darkness in the presence of certain antioxidants of the phenol type, its solubility decreases considerably; this shows the polymerizing action of some phenols on rubber. 8. Pyrogallol has a particularly strong polymerizing effect on rubber films. After 40 days of storage in air, these films dissolve in benzene in 25–30 days; if they are stored longer, their solubility is still less.