A large number of chemicals that belong to different chemical classes are known to accelerate rubber vulcanization. About 50 of these accelerators are used on a commercial scale. Most of these belong to the six classes as described below.
The vulcanization curatives can be further classified as primary and secondary accelerators. Primary accelerators have usually long scorch time and cure fast during the main vulcanization stage. The two main classes of primary accelerators are sulfenamides and thiazoles. Secondary accelerators have a synergistic effect on cure, that is, they boost cure and increase the cross-link density. They are usually used at much lower concentration. The three most important classes of secondary accelerators are guanidines, thiurams, and dithiocarbamates.

Dithiocarbamate

Dithiocarbamates are ultra fast accelerators that have virtually no induction time. Thus when used as primary accelerators, retarders are usually added to avoid scorch. They also require initators for activation such as zinc oxide / fatty acid.
A number of zinc dialkyl dithiocarbamates are commercially available. Three very common ones are dimethyl- (ZDMC), diethyl- ZDEC) and diburyl-dithiocarbamate ZDBC). The scorch safety of these compounds increases with increasing length of the alkyl group: ZDMC < ZDEC < ZDBC. All three have similar high reactivity. However, their solubility in non-polar rubber compounds is rather low due to the very polar nature of the Zn-sulfide bonds and therefore have a tendency to bloom to the surface at higher concentration. The rubber solubility decreases in the order ZDMC < ZDEC < ZDBC.
Dirhiocarbamates are often a good choice for low sulfur cure and low temperature cure and white/transparent or brightly colored rubber goods and as secondary accelerators to speed up cure.

Thiazole

Thiazoles are some of the most widely used primary vulcanization accelerators. They have improved scorch safety and allow for cure at quite high temperatures with short curing time and broad vulcanization plateau. Due to their versatility, they are used for a wide variety of rubber products. The most common commercial thiazoles are bis(2-benzothiazole) disulfide (MBTS), 2-mercaptobenzothiazole (MBT) and the zinc salt of mercaptobenzothiazole (ZMBT). They are only medium fast curatives and are usually activated by zinc oxide / stearic acid. To increase their cure speed, they are often combined with small amounts of basic accelerators such as diphenyl guanidine (DPG) or diorthotolyl guanidine (DOTG). These accelerators are also known for their good scorch delay.

Sulfenamine

Sulfenamides are very popular primary accelerators. They can be synthesized by the reaction of 2- mercaptobenzothiazole with simple amines such as cycloheylamine or tert-butylamine. They are often used in combination with small amounts of basic secondary accelerators like diphenyl guanidine (DPG), diorthotolyl guanidine (DOTG) or tetramethylthiuram mono or disulfide (TMTM, TMTD). These co-accelerators increase the cure rate but also reduce the scorch safety. A drawback of sulfenamides is their limited storage stability. They tend to decompose when exposed to high humidity and heat and thus have to be stored in a cool and dry environment. Furthermore acids and acidic substances cause rapid degradation. Sulfenamides are known for their noticeable scorch delay and can be considered “safe” accelerators that allow for easy processing and molding of rubber compounds. They also provide a relative broad vulcanization plateau, and good aging resistance which explains their popularity despite their short shelf life.