Common Accelerators for Sulfur Vulcanization

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.

Thiazoles

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.

Thiurams

Thiurams are very fast and effective sulfur cure accelerators. They can be used for all common unsaturated rubbers including NR, IR, SBR, BR, and NBR. 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. They are often used in systems with low or no sulfur content and for rubbers with low unsaturation like IIR and EPDM. Often metal oxides are added which bind the released dithiocarbamic acid and prevent accelerated oxidative degradation.1 The three most popular thiurams are tetramethyl thiuram monosulfide (TMTM), tetramethyl thiuram disulfide (TMTD) and dipentamethylene thiuram tetrasulfide (DPTT). Rubber products cured with these curatives have excellent heat and water vapor resistance and thus, are often a good choice for rubber goods that are exposed to hot water and steam. However, these curatives are more expensive and also have a tendency to bloom to the surface.

Sulfenamides

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.2,3 

Guanidines

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 are diphenyl guanidine (DPG) and N, N'-diorthotolyl guanidine (DOTG). Both guanidines by themselves have a rather slow cure rate and require the use of zinc oxide for activation. They are a good choice for thick walled rubber products. However, they are most often used as secondary accelerators in combination with thiazoles. This combination is frequently used to cure rubber goods. The vulcanization in the presence of guanindes results in relative high crosslink density and good physical-mechanical properties such as high modulus and good compression set. However, guanidines are not recommended for light colored goods because they cause a brown discoloration. 

Dithiocarbamates

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.
Dithiocarbamates 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.

Xanthates

Are zinc salts of xanthic acids. They are ultra fast primary vulcanization accelerators and are mainly used for vulcanization of rubber latex and rubber in solution. These compounds are rather polar and some are even soluble in water. Since they result in super fast cure, they are usually employed for low vulcanization temperature applications. The most common xanthates are zinc (ZIX) and sodium isopropyl xanthate (NaIX). The later is water soluble, and ideal for latex vulcanization.

Thioureas

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.

  • Summary

    Vulcanization Accelerator

    is a compound that increases the speed of vulcanization and that enables vulcanization to proceed at lower temperature and with greater efficiency.

  • Vulcanization accelerators can be classified as primary and secondary accelerators. Important primary accelerators include thiazoles and sulfenamides.

  • Secondary accelerators are used to activate primary accelerators. They also greatly increase the speed of vulcanization but reduce scorch safety. The dosage of secondary accelerators is typically only a fraction of that of the primary accelerators.

  • Most sulfur-based vulcanization systems require activators. The most common activator is zinc stearate. It is often formed in-situ by reaction of stearic acid with zinc oxide.

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