Accelerators control how quickly a rubber material begins to vulcanize and how the vulcanization process proceeds after the initial stage. It is generally the case that several accelerators are added to the rubber blend. Some of these have primary effects and others have secondary effects; the latter often affect the former via reaction synergism.
Standard accelerators
This group comprises all accelerators, that break down sulfur during heating and vulcanization. Here you find e.g sulfenamides, thiazoles, dithiocarbamate, guanidines, and thiophosphates.
Sulfur donors
These accelerators have the ability to release reactive sulfur during vulcanization. Some of them can, when added in sufficient quantities to a rubber blend, function as both accelerator and crosslinking agent. If these accelerators are used in rubber blends, sometimes a small quantity of pure sulfur is added. Such vulcanization systems are often called EV/semi-EV systems, and generate vulcanizates with low compression set, good heat aging, and fast vulcanization. The downside to this is reduced flexibility and less good fatigue behavior on the part of the vulcanizate.
Unfortunately, most of these accelerators generate nitrosamines during the vulcanization process.
Common accelerators in this group are thiurams.
