Compounding

Vulcanization or curing is the process in which the chains are chemically linked together to form a network, thereby transforming the material from a viscous liquid to a tough elastic solid. Strength and modulus increase, while set and hysteresis decrease. Various curing systems are used to vulcanize different types of elastomers,The most widely used vulcanizing agent is sulfur. For sulfur to effectively crosslink a rubber, an elastomer must contain double bonds. General purpose diene elastomers such as BR, SBR, NR, and IR meet this basic requirement.
Two forms of sulfur are used in vulcanization: soluble (crystals of S8 rings) and insoluble (amorphous, polymeric sulfur). Sometimes, in compounds containing high levels of sulfur, insoluble sulfur is used to prevent sulfur blooming, a process by which the sulfur migrates to the surface of a compound and crystallizes there. Blooming can occur when large amounts of soluble sulfur are used, because at high mixing temperatures, the solubility of S8 is high, enabling large amounts to dissolve, but upon cooling the solubility decreases. When the solubility limit is reached, excess sulfur blooms to the surface. Sulfur bloom reduces the "tack" of a rubber compound, a necessary property if layers of rubber are to be plied up to make a composite structure, such as a tire. Insoluble sulfur does not bloom because it disperses in rubber as discrete particles, which cannot readily diffuse through the rubber. However, above 120°C, insoluble sulfur transforms into soluble sulfur. Thus, mixing temperatures must be kept below 120°C to take advantage of the bloom resistance of insoluble sulfur.
Crosslinking with sulfur alone is quite inefficient and requires curing times of several hours. For every crosslink, 40 to 55 sulfur atoms are combined with the rubber. The structure contains polysulfide linkages. Much of the sulfur is not involved in crosslinks between chains. Moreover, such networks are unstable and have poor aging resistance.
To increase the rate and efficiency of sulfur crosslinking, accelerators are normally added. These are organic bases
Often, a combination of accelerators is used to obtain the desired scorch resistance and cure rate. Generally, if two accelerators of the same type are combined, then cure characteristics are approximately the average of those for each accelerator alone. However, there is no general rule when combining accelerators of different types. Moreover, the type of accelerator is much more important than the level of accelerator in controlling scorch time. Although increased levels of accelerator increase the degree of crosslinking attained, generally accelerator concentration has only a small effect on scorch time.