Vulcanization (curing, crosslinking) is one of the most important processes for rubber technologies. The essence of vulcanization is the creation of crosslinks between rubber macromolecules in which a three-dimensional network of rubber matrix is formed. It is often desired to increase the vulcanization speed of the rubber, thereby reducing the time required to cure the rubber product and the energy required in curing operations. A vulcanization accelerator is defined as the chemical added into a rubber compound to increase the speed of vulcanization and to permit vulcanization to proceed at lower temperature and with greater efficiency. Vulcanization accelerator also decreases the amount of sulfur needed to cross-link the polydiene, which improves the aging properties of the vulcanized rubber. The vulcanization accelerators can be further classified as primary and secondary accelerators. Some representative examples of primary accelerators include thiazoles and sulfenamides. In many cases, the primary accelerators are used in conjunction with another accelerator, called a booster or secondary accelerator, which further increases the rate of sulfur vulcanization. Examples include dithiocarbamates and thiurams.
Classification of vulcanization accelerators
The classification of common vulcanization accelerators, in terms of their examples and speeds of vulcanization, is shown in Table 1.
Table 1. Classification of accelerators and their relative curing speeds
Relative curing speeds
2-Benzothiazyl disulfide (MBTS)
Diphenyl guanidine (DPG)
N, N'-Diorthotolyl guanidine (DOTG)
Dipentamethylene thiourea (DPTU)
Dibutyl thiourea (DBTU)
Tetrabutylthiuram disulfide (TBTD)
Tetraethyl thiuram disulfde (TETD)
Zinc dibutyldithiocarbamate (ZDBC)
Zinc diethyldithiocarbamate (ZDEC)
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.
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).
Guanidines are condensation products of aromatic amines (aniline) and carbondisulfide with subsequent substitution of the thione functionality (>C=S) for a primary ketimine group (>C=NH). The only two guanidines that are used on a commercial scale in vulcanization accelerators are diphenyl guanidine (DPG) and N, N'-diorthotolyl guanidine (DOTG). Guanidines are not recommended for light colored goods because they cause a brown discoloration.
Thiourea are ultrafast primary or secondary accelerators. Some commercially important thioureas include ethylene thiourea (ETU), dipentamethylene thiourea (DPTU), and dibutyl thiourea (DBTU). They are mainly used for the vulcanization of chloroprene rubbers.
Thiurams are very fast and effective sulfur cure accelerators. They can be used for all common unsaturated rubbers. Like some sulfenamides, most thiurams have two or more sulfur atoms in the sulfur bridge. These compounds not only function as accelerators but also act as sulfur donors, i.e., allow for "sulfur-less" cure.
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.
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