The Distribution of Combined Sulfur in Vulcanized Rubber and Its Bearing on the Sulfide Linkage Theory of Vulcanization

Abstract

Abstract No direct evidence exists to support the sulfide linkage theory of vulcanization, which assumes that cross-linking of the molecules with sulfur would retain the elasticity and decrease the plasticity of the rubber. The following facts do not favor the sulfide linkage theory of vulcanization : (1) Vulcanized rubber can be peptized under conditions which do not appear to rupture a sulfide linkage. (2) No direct relation exists between the physical properties of vulcanized rubber and the amount of combined sulfur. (3) No direct relation exists between the ease of peptization and the physical properties of the vulcanizate. (4) Combined sulfur appears to assist the solvation of rubber. Rubber vulcanized with thiuram disulfides in the absence of free sulfur has poor physical properties but peptizes with difficulty. An increase in combined sulfur without a corresponding improvement in physical properties assists solvation. (5) Rubber having the best physical properties is also the most heterogeneous with respect to sulfur. The sulfide linkage theory would suggest that an even distribution of sulfur would produce the best physical properties. (6) Fractions of peptized vulcanizates, on evaporation of their solutions, leave insoluble vulcanized films, which shows that linkage by means of primary forces is unnecessary for producing the vulcanized condition.